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  • 1. wwwControl Network
  • 2. Distributed Control System and Programmable Logic Control Course Aim The aim of this training course is to build up the procedural and declarative knowledge required to be recognized by projects engineer that don not have past background of DCS or PLC. This will help them to supervise projects dealing with control systems with a strong background. In this course, the training cycle is divided in five steps that necessitate the cooperation between the instructor and the trainees. These steps are shown in figure below, they are summarized as follows: 1. Define the knowledge and skills required to be developed. 2. Define the elements of each knowledge or skill. 3. Formulate a verbal phrase for the learning objective of each element. 4. Choose an adequate instructional activity to present each element. 5. Set up an indicator to measure the outcomes of the course and modify the training skills to adapt the vocational needs. Define Knowledge Determine & Skills Elements Measure Learning & Correction Objectives Instruction Activity Training Cycle. 1
  • 3. Knowledge and Elements  Illustrate DCS & PLC Benefits, Usage and History.  Overview of control system history.  Control system benefits and usage.  Types of control  Develop Knowledge of DCS Components (Hardware & Software).  Infrastructure [Communication Bus, Interfaces, Controllers, Gateways, RTU, Others].  Hardware and technologies.  Software [Configuration, Graphics, Alarming, Trending, System Management, Others].  Extend Knowledge of DCS installation and Maintenance.  Site Installation, Commissioning and Startup.  Diagnostics, Spares, Tools and Power Distribution.  Maintenance [Backup, Replacements and System Installation].  Develop Knowledge of PLC Components.  PLC fundamentals.  PLC Logic.
  • 4. Table of Contents Section IChapter 1 IntroductionChapter 2 Regulatory Control Section IIChapter 3 DCS InfrastructureChapter 4 DCS HardwareChapter 5 DCS Software Section IIIChapter 6 InstallationChapter 7 MaintenanceChapter 8 Power Distribution Section IVChapter 9 PLC Fundamentals.Chapter 10 Ladder Logic And SFC Appendices A Electrical Relay Diagram And P&ID Symbols B Serial Communication C Networking D Software Engineering
  • 5. Distributed Control System and Programmable Logic Control 4
  • 6. Distributed Control System and Programmable Logic Control Chapter 1 Control Systems 1.1 Automation System Structure Although applications differ widely, there is little difference in the overall architecture of their control systems. Why the control system of a power plant is not sold also for automating a brewery depends largely on small differences (e.g. explosion-proof), on regulations (e.g. Food and Drug Administration) and also tradition, customer relationship. The ANSI/ISA standard 95 defines terminology and good practices Level 4 Business Planning & Logistics Enterprise Resource Plant Production Scheduling Operational Management, etc. Level Manufacturing 3 Operations & Control Dispatching Production, Detailed Product Manufacturing Execution Scheduling, Reliability Assurance,... Level 2,1,0 Batch Continuous Discrete Control & Command Control Control Control 1.1.1 Large Control System Hierarchy • Administration: Production goals, planning • Enterprise: Manages resources, workflow, coordinates activities of different sites, quality supervision, maintenance, distribution and planning. 5
  • 7. • Supervision: Supervision of the site, optimization, on-line operations. Control room, Process Data Base, logging (open loop)• Group (Area): Control of a well-defined part of the plant. closed loop, except for intervention of an operator) o Coordinates individual subgroups o Adjusting set-points and parameters o Commands several units as a whole• Unit (Cell): Control (regulation, monitoring and protection) of a small part of a group (closed loop except for maintenance). o Measure: Sampling, scaling, processing, calibration. o Control: regulation, set-points and parameters o Command: sequencing, protection and interlocking• Field: Sensors & Actors, data acquisition, digitalization, data transmission, no processing except measurement correction and built-in protection.4 Planning, Statistics, Finances administration3 Workflow, Resources, Interactions enterprise SCADA supervisi on Supervisory Supervisory2 And Data Group Control Unit Control1 Field Sensors T & Actors A V0 Primary Figure 1.1 Large control system hierarchy
  • 8. 1.1.2 Response Time and Hierarchical Level Planning ERP (Enterprise Level Resource Planning) MES Execution (Manufacturing Level Execution System) SCADA (Supervisory Control Supervisory and Data Acquisition) Level DCS (Distributed Control System) Control Level PLC (Programmable Logic Controller) ms seconds hours days weeks month years Figure 1.2 Response Time And Hierarchical Level1.2 What is DCS?  A DCS is an integrated set of modules with distributed functions. – Multi-loop controllers (10’s-100’s) that connect to field devices – Supervisory coordinating controllers – Multi-loop operator stations and engineering stations – Servers for system data management – Control network for intercommunication – External connections
  • 9. Supervisory Operator System Stations Remote Users Controller www Server Engineering Station Remote Server Control Network Multi-loop Controller Direct I/O Module Other Industrial Devices Figure 1.3 DCS Hierarchy  A DCS, throughout the whole system, must provide: – Performance: control must be faster than the process. – Determinism: control must always take the same time. – Fault tolerance: redundancy; must fail to a known state. – Security: must have access restrictions/controls. Even though performance, ease of use, and interoperability are keyevaluation criteria for any control system software package, the followingis intended to provide the manufacturing engineer with a concise list ofcontrol system software evaluation criteria. 1. INTEROPERABILITY. This refers to the interaction of all control system hardware and software components at all levels. 2. INTERCONNECTIVITY. This criterion is concerned with the transmission medium, which is constrained by the network topology and how efficiently the system’s components communicate with each other.
  • 10. 3. DISASTER PROCESSING.This component is defined by the efficiency with which thesoftware provides the operator with system failure information andthe ease at which the operator is permitted to bring the system backto maximum operation after system failure.4. DATABASE.This refers to the software’s ability to maintain the system’sdatabase.5. PROCESSES/DATA.This criterion is concerned with the variety of processes and datathat can be controlled by the SCADA package.6. DIAGNOSTICS.The SCADA package’s ability to assist in the resolution of systemfailures is evaluated by this diagnostic utility.7. SECURITY.This component is concerned with the levels of security providedby the software.8. MONITORING/CONTROLMonitoring of a given process in real-time and control of thatprocess, within preset parameters, is evaluated by this criteria.9. ALARM MANAGEMENT/LOGGING.This is the category for detecting, annunciating, managing, andstoring alarm conditions.10. STATISTICAL PROCESS CONTROL.This is the portion of the SCADA package that evaluates theprocess data. Production and quality is greatly effected by this data.12. OPERATOR INTERFACE.The graphical user interface (GUI) is evaluated using this criterion.
  • 11. 13. TRENDING. The software’s ability to display trending plots using historical and current data is considered in this category. 14. REPORT GENERATION. The production of logs and reports using current real-time data and data retrieved from historical files is evaluated under this category. Due to the advancements in computer technology and low cost, apersonal computer-based distributed control system can be installed for afraction of the cost required just a few years ago. However, prior toselecting any piece of DCS equipment, first examine the existingequipment, in particular the smart controllers, for network compatibility.Then, examine and select the software to be employed.1.3 What is PLC? A programmable logic controller, also called a PLC orprogrammable controller, is a computer-type device used to controlequipment in an industrial facility. The kinds of equipment that PLCs cancontrol are as varied as industrial facilities themselves. Conveyorsystems, food processing machinery, auto assembly lines…you name itand there’s probably a PLC out there controlling it. In a traditional industrial control system, all control devices arewired directly to each other according to how the system is supposed tooperate. In a PLC system, however, the PLC replaces the wiring betweenthe devices. Thus, instead of being wired directly to each other, allequipment is wired to the PLC. Then, the control program inside the PLCprovides the “wiring” connection between the devices.
  • 12. The control program is the computer program stored in the PLC’smemory that tells the PLC what’s supposed to be going on in the system.The use of a PLC to provide the wiring connections between systemdevices is called soft-wiring. Lets say that a push button is supposed to control the operation ofa motor. In a traditional control system, the push button would be wireddirectly to the motor. In a PLC system, however, both the push button andthe motor would be wired to the PLC instead. Then, the PLCs controlprogram would complete the electrical circuit between the two, allowingthe button to control the motor. Figure 1.4 PLC developmentA PLC basically consists of two elements: • The central processing unit • The input/output system1.3.1 The Central Processing Unit The central processing unit (CPU) is the part of a programmablecontroller that retrieves, decodes, stores, and processes information. Italso executes the control program stored in the PLC’s memory. In
  • 13. essence, the CPU is the “brains” of a programmable controller. Itfunctions much the same way the CPU of a regular computer does, exceptthat it uses special instructions and coding to perform its functions. TheCPU has three parts: • The processor • The memory system • The power supplyThe processor is the section of the CPU that codes, decodes, andcomputes data. The memory system is the section of the CPU that storesboth the control program and data from the equipment connected to thePLC. The power supply is the section that provides the PLC with thevoltage and current it needs to operate. Figure 1.5 Microprocessor Hardware1.3.2 The input/output (I/O) system It is the section of a PLC to which all of the field devices areconnected. If the CPU can be thought of as the brains of a PLC, then theI/O system can be thought of as the arms and legs. The I/O system is whatactually physically carries out the control commands from the programstored in the PLC’s memory.
  • 14. The I/O system consists of two main parts: • The rack The rack is an enclosure with slots in it that is connected to the CPU. • I/O modules I/O modules are devices with connection terminals to which the field devices are wired.Together, the rack and the I/O modules form the interface between thefield devices and the PLC. When set up properly, each I/O module is bothsecurely wired to its corresponding field devices and securely installed ina slot in the rack. This creates the physical connection between the fieldequipment and the PLC. In some small PLCs, the rack and the I/Omodules come prepackaged as one unit. Figure 1.6 I/O Racks
  • 15. 1.4 How is a DCS different from a PLC system? DCS PLCMfr sells a complete system of integrated Mfr sells some components; an SIcomponents. acquires others and engineers the system.Mfr supports the system. Mfr supports the components.On-line repair/ maintenance are the norm. Off-line repair/ maintenance are the norm.System management built-in. System management designed per project.Users expect to evolve/upgrade/expand a System is a one-off project (like a house).system over 10/20/30 years. Upgrades / expansions are new projects.1.5 Redundancy and Fault Tolerance1.5.1 Redundancy • Hardware redundancy – add extra hardware for detection or tolerating faults • Software redundancy – add extra software for detection and possibly tolerating faults1.5.2 Fault Tolerance • Error Detection • Damage Confinement • Error Recovery • Fault Treatment1.5.2.1 Error Detection • Ideal check – Check should be independent from system – Check fails if system crashes
  • 16. • Acceptable check – Cost – Reasonable check, e.g. monitor rate of change • diagnostics – Performed “by system on system components” – E.g. power-up diagnostics1.5.2.2 Damage Confinement • Error might propagate and spread • Identify boundaries to state beyond which no information exchange has occurred1.5.2.3 Error Recovery • Backward recovery – State is restored to an earlier state – Requires checkpoints – Most frequently used – Recovery overhead • Forward recovery – Try to make state error-free – Need accurate assessment of damage – Highly application-dependent1.5.2.4 Fault Treatment • If transient fault: restart system, goto error-free state • System repair – On-line, no manual intervention, (automatic) – Dynamic system reconfiguration – Spare (hot or cold)
  • 17. 1.5.2.5 Fault Coverage • Measure of system’s ability to perform: – Fault detection – Fault location – Fault containment – (and/or fault recovery) • Note: – Recovery implies that the system as a whole is operational – This does not imply that a “repair” occurred – E.g. duplex system with benign fault can recover to continue operation on one non-faulty processor1.5.2.6 Hardware Redundancy • Passive (static) – Uses fault masking to hide occurrence of fault – No action from the system is required – E.g. voting • Active (dynamic) – Uses comparison for detection and/or diagnoses – Remove faulty hardware from system => reconfiguration • Hybrid – Combine both approaches – Masking until diagnostic complete – Expensive, but better to achieve higher reliability1.5.2.7 Passive Hardware Redundancy • N-Modular Redundancy (NMR) – N independent modules replicate the same function
  • 18. • Parallelism – Results are voted on requirements: N >= 3 • TMR (Triple Modular Redundancy)1.5.2.8 Fault tolerant structures Fault tolerance allows continuing operation in spite of a limitednumber of independent failures. Fault tolerance relies on workredundancy.1.5.2.9 Static redundancy: 2 out of 3 • Workby of 3 synchronised and identical units. – All 3 units OK: Correct output. – 2 units OK: Majority output correct. – 2 or 3 units failure: Incorrect output. – Otherwise: Error detection output. Process input sync sync Voter Process output Figure 1.7 (2 out of 3) Redundancy1.5.2.10 Dynamic Redundancy • Redundancy only activated after an error is detected. – Primary components (non-redundant) – Reserve components (redundancy), standby (cold/hot standby)
  • 19. Input Primary unit Standby unit Switch Output Figure 1.8 Dynamic Redundancy1.5.2.11 Workby and Standby Workby Hot standby Cold standby sync sync on-line workby on-line standby =? Both computers are doing Standby is no operational Standby is not computing the same calculations Error detection needed. Error detection needed. at the same time Long switchover period Easy switchover in case Comparison for easy with loss of state info. of failure. error detection. Easy repair of reserve unit. No aging of reserve unit. Comparator needed. Non- redundant continuation in case of failure? Figure 1.9 Workby and Standby1.5.2.12 Workby Fault-Tolerance for Integrity and Persistency input input synchronization synchronization Worker Co E Worker Co E W-orker D W-orker D Matching Matching Output Output comparator commutator disjunctor output output INTEGER PERSISTENT Figure 1.10 Workby Fault-Tolerance for Integrity and Persistency
  • 20. 1.5.2.13 Hybrid RedundancyMixture of workby (static redundancy) and standby (dynamic redundancy). work- work- work- stand- stand- by by by by by voter Reconfiguration work- work- work- stand- failed (self-purging by by by by redundancy) voter Figure 1.11 Hybrid Redundancy1.6 Microprocessor Control For simple programming the relay model of the PLC is sufficient.As more complex functions are used the more complex VonNeumanmodel of the PLC must be used. A computer processes one instruction ata time. Most computers operate this way, although they appear to bedoing many things at once. Consider the computer components shown inFigure 1.12. Figure 1.12 Simplified Personal Computer Architecture
  • 21. Input is obtained from the keyboard and mouse, output is sent to thescreen, and the disk and memory are used for both input and output forstorage. (Note: the directions of these arrows are very important toengineers, always pay attention to indicate where information is flowing.)This figure can be redrawn as in Figure 1.13 to clarify the role of inputsand outputs. Figure 1.13 An Input-Output Oriented Architecture In this figure the data enters the left side through the inputs. (Note:most engineering diagrams have inputs on the left and outputs on theright.) It travels through buffering circuits before it enters the CPU. TheCPU outputs data through other circuits. Memory and disks are used forstorage of data that is not destined for output. If we look at a personalcomputer as a controller, it is controlling the user by outputting stimuli onthe screen, and inputting responses from the mouse and the keyboard. A PLC is also a computer controlling a process. When fullyintegrated into an application the analogies become; • Inputs - the keyboard is analogous to a proximity switch input circuits - the serial input chip is like a 24Vdc input card
  • 22. • Computer - the 686 CPU is like a PLC CPU unit • Output circuits - a graphics card is like a triac output card • Outputs - a monitor is like a light • Storage - memory in PLCs is similar to memories in personal computersIt is also possible to implement a PLC using a normal Personal Computer,although this is not advisable. In the case of a PLC the inputs and outputsare designed to be more reliable and rugged for harsh productionenvironments.1.7 Role PlayEach trainee should act a role play on the following: 1. Automation system structure. 2. What DCS and PLC and their differences? 3. Redundancy and fault tolerance.
  • 23. Chapter 2 Regulatory Control2.1 Learning Objectives• Introduce Regulatory Control.• Understanding PID control.• Differentiate between various control loops.2.2 Introduction Most of the applications of industrial control process used simpleloops which regulated flows, temperatures, pressures and levels.Occasionally ratio and cascade control loops could be found. There aremany benefits for using regulatory control. One of the most important issimply closer control of the process. Process control is one part of anoverall control hierarchy that extends downwards to safety controls andother directly connected process devices, and upward to encompassprocess optimization and even higher business levels of control such asscheduling, inventory management. Most control engineers would recognize the form of responseshown in figure 2.1. Actually the response could be determined bysolving a differential equation. It is more important to have a goodunderstanding of the physical response than to be able to predict thesolution by solving the differential equation.
  • 24. Figure 2.1 Response of simple dynamic process to step input change Instrumentation, control and process engineers abstract the pictorialform of the process into an iconographic diagram called "Piping andInstrumentation Diagram", i.e. P&ID. Figure 2.2 is an example of theP&ID. Figure 2.2 Control loop representation used on P&IDs. For description and analysis of a control loop, without referring towhether it is implemented with analog or digital hardware, a blockdiagram as shown in figure 2.3 is beneficial. Figure 2.3 Simplified block diagram representation of process control loop.
  • 25. 2.3 PID Control2.3.1 Feedback Control The principle of feedback is one of the most intuitive conceptsknown. An action is taken to correct a less satisfactory situation then theresults of the action are evaluated. If the situation is not corrected thenfurther action takes place. Feedback control can be classified by the formof the controller output. One of the simplest forms of output is discreteform, also called on-off or two position control. An example of this is thehousehold thermostat, which activates heating unit if the temperature isbelow the setting, or deactivates the unit if the temperature is above thesetting. Figure 2.4 On-Off Control. The idea of two position control can be extended to multi-positioncontrol; an example is commercial air-conditioning refrigerationequipment which is operated by loading and unloading compressorcylinders. The ultimate extension is infinite number of positions which iscalled modulating control; an example is the process controller outputthat can drive a valve to any position between 0 and 100 percent, asshown in figure 2.5.
  • 26. Figure 2.5 Flow versus position, infinite position Control.2.3.2 Modes of Control Feedback controllers use one, two, or three methods to determinethe controller output. These methods, called the modes of control,including the following:• Proportional (P)• Integral (I)• Derivative (D)In general these modes can be used singly or in combination.2.3.2.1 Proportional Mode With a controller containing only the proportional mode, thecontroller output is proportional to the measurement value only. Neitherhistory of the measurement value nor consideration to the rate of changeis utilized. Adjustment, i.e. tuning, of the controller is simple becausethere is only one adjustment as shown in figure 2.6.
  • 27. Figure 2.6 Relationship between input and output for proportional control. Figure 2.7 illustrates a proportional control system. The rate offluid flow into the tank represents the load. To be in equilibrium, theoutflow must be the same as the inflow. The outflow is achieved by aparticular valve position where the fixed mechanism between the float,pivot and link attain. Figure 2.7 Proportional control.2.3.2.2 Integral Mode An integrator is the ideal device for automating the procedure foradjusting the controller output bias. It is called the automatic reset.2.3.2.3 Derivative Mode The derivative is used to anticipate the effect of load changes byadding a component to the controller output that is proportional to the rateof change of the measurement. See figure 2.8.
  • 28. Figure 2.8 PID control.2.3.3 Control Loop Structure For microprocessor control system, control strategy is configuredby a series of software function blocks. Just like a set of hardwaremodules require interconnections to form a complete control system, a setof software function blocks also acquire interconnections, i.e. soft-wiring.Figure 2.9 shows a simple feedback loop with the software portionconsists of three function blocks:• An analog input block that causes the analog to digital converter to convert the incoming 4-20mA signal to an analogous value. The value is deposited in a memory register.• A PID control block which obtains the measurement value from the analog input block and compares it with the setpoint then it executes a PID algorithm to calculate the output.• An analog output block that obtains from the PID block the required valve position value. The value is converted by a digital to analog converter to 4-20mA signal.
  • 29. Figure 2.9 Control loop hardware/software structure.2.3.4 Control Loop Tuning The power of PID control is that by good choice of controlparameters the controller can be adjusted to provide the desired behavioron a wide variety of process applications. Determining acceptable valuesof these parameters is called tuning the controller. A good criterion foracceptable performance is the "quarter cycle decay" shown in figure 2.10. Figure 2.10 quarter cycle decay criterionMost loops are tuned by experimental techniques, i.e. trial and error.Figures 2.11 and 2.12 give a tuning map for adjusting control parameters.
  • 30. Figure 2.11 Gain and Reset effects. Figure 2.12 Derivative effects.2.4 Control Loop Types2.4.1 Ratio Control Figure 2.13 shows the P&ID of a process heater in which the fuelflow is measured and multiplied by the required air-to-fuel ratio; thisresults in the required air flow rate, which is introduced as a setpoint ofthe feedback controller. The required air-to-fuel ratio is automaticallyadjusted as the output of the stack O2 controller.
  • 31. Figure 2.13 ratio Control..2.4.2 Cascade Control In figure 2.14 the temperature controller cascades a steam flowcontroller. The temperature controller would react to outlet temperaturedrop by increasing the setpoint of the steam flow controller, which in turnwould increase the signal to the valve. The flow will quickly respond toincreased demand from the temperature controller and thus reaching thedesired setpoint of the outlet temperature stream. Figure 2.13 Cascade Control.2.4.3 Feedforward Control With feedforward control, the objective is to drive the controllingdevice from a measurement of the disturbance that is affecting theprocess, rather than from the process variable itself. In figure 2.14, the
  • 32. application was analyzed the variation in process inlet temperature wasthe principle of disturbance. Hence, a feedforward controller is used todrive the fuel flow controller by sensing the inlet temperature. Figure 2.14 Feedforward Control.2.4.4 Selector (Override) Control There are several ways of using selector switches in controlstrategies. One way is to select the higher (or lower) of severalmeasurement signals to pass the process variable to a feedback controller.For example, the highest of several process temperatures may be selectedautomatically to become the controlling temperature as shown in figure2.15. Figure 2.15 Override Control.
  • 33. 2.4.5 Split Range Control Split range control when one process variable such as plant inletpressure is used to manage two different output devices such as plantbypass control valve and flow control loop for fractionation area. The 4-12 mA signal is used to control the flow control loop. If the plant cannothandle all incoming feed, the 12-20 mA signal control the plant bypassvalve to direct extra feed to the outside of the plant.2.5 Role PlayThe trainees are required to play roles about: 1. Introducing regulatory control. 2. Introducing modes of control. 3. Intruding control loop types.
  • 34. Distributed Control System and Programmable Logic Control 33
  • 35. Distributed Control System and Programmable Logic Control Chapter 3 DCS Infrastructure 3.1 Learning Objectives • Introduce system infrastructure interoperability and interconnectivity. • Illustrate system components of level 2 control. 3.2 Communication Bus Figure 3.1: Communication Bus The communication bus, i.e. the Nodebus, interconnects stations (Control Processors, Application Processors, Application Workstations, and so forth) in the system to form a process management and control node. Depending on application requirements, the node can serve as a single, stand-alone entity, or it can be configured to be part of a more extensive communications network. Operating in conjunction with the Nodebus interface electronics in each station, the Nodebus provides high-speed, redundant, peer-to-peer communications between the stations. The high speed, coupled with the redundancy and peer-to-peer characteristics, provide performance and security superior to that 34
  • 36. Distributed Control System and Programmable Logic Control provided by communication media used in conventional computer-based systems. Station interfaces to the Nodebus are also redundant, further ensuring secure communications between the stations. The Nodebus can be implemented in a basic, non-extended configuration or it can be extended through the use of Nodebus Extenders and Dual Nodebus Interface Extenders. 3.2.1 Nodebus Interface The Nodebus Interface is a module which allows direct connection of a personal workstation (PW), with appropriate Nodebus connector card and software, to the Nodebus figure 3.2. In this configuration, the PW functions as a station on the node. The Nodebus Interface allows connection of a station application workstation hosting an Ethernet configuration to Nodebus. See figure 3.2. Figure 3.2 Nodebus Interface Implementation (Typical) 35
  • 37. Distributed Control System and Programmable Logic Control An Attachment Unit Interface (AUI) cable, connects the PW or an Ethernet hub configuration to the Nodebus via a Nodebus Interface. A coaxial cable (ThinNet) connects an Ethernet daisy chain configuration to the Nodebus via a Nodebus Extender. The Nodebus Interface is non- redundant, and can be used in any of the Nodebus configurations described. 3.2.2 Dual Nodebus Interface The Dual Nodebus Interface (DNBI) is a module which allows direct connection of stations to the appropriate Nodebus. Connection between the DNBI and station is made via an AUI cable. For data transmission security, a separate (RS-423) control cable connects between the station and the DNBI to allow switching between the two redundant Nodebus cables. Switching of the Nodebus cables is controlled by the station, which transmits commands to the DNBI via the control cable. Figure 3.3 shows connection of a station to the Nodebus using a DNBI. Figure 3.3 Local Connection of Station 3.2.3 Dual Nodebus Interface Extender The Dual Nodebus Interface Extender (DNBX) is functionally similar to the DNBI, but provides a greater cabling distance. The 36
  • 38. Distributed Control System and Programmable Logic Control principal transmission medium used is a coaxial Ethernet cable directly connected to the station end by a standard Ethernet transceiver. Figure 3.4 remote connection of a station to the Nodebus using a DNBX. Figure 3.4 Remote Connection of Station 3.3 Control Processor The Control Processor performs regulatory, logic, timing, and sequential control together with connected: • Fieldbus Modules (FBMs) • Fieldbus Cluster I/O Cards (FBCs) It also performs data acquisition (via the Fieldbus Modules), alarm detection and notification, and may optionally serve as an interface for one or more Panel Display Stations. The non-fault-tolerant version of the Control Processor is a single-width processor module. The fault-tolerant version consists of two single-width processor modules. 3.3.1 Enhanced Reliability The Control Processor offers optional fault- tolerance for enhanced reliability. The fault-tolerant control processor configuration consists of two parallel-operating modules with two separate connections to the Nodebus and to the Fieldbus. 37 37
  • 39. Distributed Control System and Programmable Logic Control The two control processor modules, married together as a fault- tolerant pair, are designed to provide continued operation of the unit in the event of virtually any hardware failure occurring within one module of the pair. Both modules receive and process information simultaneously, and the modules themselves detect faults. One of the significant methods of fault detection is comparison of communication messages at the module external interfaces. Upon detection of a fault, self-diagnostics are run by both modules to determine which module is defective. The non-defective module then assumes control without affecting normal system operations. To further ensure reliable communications, the fault-tolerant control processor performs error detection and address verification tests in its Nodebus and Fieldbus interfaces. For enhanced reliability during maintenance operations, the Control Processor is equipped with a recessed reset button. This feature provides for manually forcing a module power off and on (reboot) without removing the module from the enclosure. 3.3.2 Diagnostics The Control Processor uses three types of diagnostic tests to detect and/or isolate faults: • Power-up self-checks • Run-time and watchdog timer checks • Off-line diagnostics Power-up self-checks are self-initiated when power is applied to the control processor. These checks perform sequential tests on the various control processor functional elements. Red and green indicators at 38
  • 40. Distributed Control System and Programmable Logic Control the front of the control processor module reflect the successful (or non- successful) completion of the various phases of the control processor startup sequence. The run-time and watchdog timer checks provide continuous monitoring of control processor functions during normal system operations. The operator is informed of a malfunction by means of printed or displayed system messages. Off-line diagnostics are temporarily loaded into the system for the purpose of performing comprehensive tests and checks on various system stations and devices. Using the off-line diagnostics, a suspected fault in the control processor can be isolated and/or confirmed. 3.4 Engineering Interface The engineering interface, i.e. Application Processor, is microprocessor-based application processor/file server stations. They perform two basic functions: • As application processor (computer) stations, they perform computation intensive functions. • As file server stations, they process file requests from tasks within themselves or from other stations. Bulk storage devices used with the Application Processors include floppy disk drives, hard disk drives, streaming tape drives, and CD-ROMs. The Application Processors operate in concert with other system stations (such as communication processors, workstation processors, and control processors), which provide the necessary means for data input/output and operator interfacing. A smaller system can utilize a single Application Processor, while a larger system can incorporate 39
  • 41. Distributed Control System and Programmable Logic Control several Application Processors, each configured to perform specific functions. Some functions can be performed by individual Application Processors, while others can be shared by two or more Application Processors in the same network. For all models of the Application Processor, applications range from minimal functions, such as the storage of memory images, alarm events, and historical data, to larger-scale applications such as database management and program development. 3.4.1 Application Processor Functions The following sections describe the major functions performed by the Application Processors. 3.4.1.1 System and Network Management Functions The Application Processors perform system management functions, which include collecting system performance statistics, data reconciliation, performing station reloads, providing message broadcasting, handling all station alarms and messages, and maintaining consistent time and date in all system stations. The Application Processor also performs network management functions, which comprise that portion of system management functions which deal with the network. 3.4.1.2 Database Management Database management involves the storage, manipulation, and retrieval of files containing data received and/or produced by the system. The Application Processors utilize the industry-standard Relational Data Base Management System. 40
  • 42. Distributed Control System and Programmable Logic Control 3.4.1.3 File Requests Each Application Processor contains a file manager, which manages all file requests associated with bulk memory attached to the Application Processor. Each Application Processor also supports a remote file system that allows tasks in one station to share files in another. 3.4.1.4 Historical Data The Application Processors can be configured to contain the Historian function, which maintains a history of application messages and continuous and discrete I/O values. These values may represent any parameters such as measurements, setpoints, outputs, and status switches from stations that have been configured to collect data and send it to a Historian. In addition, the Historian computes and stores a history of averages, maximums, minimums, and other derived values. This information is maintained for display, reporting, and access by application programs. An archiving facility saves the data on removable media, where applicable. The Application Processors can be configured to maintain a history of errors, alarm conditions, and selected operator actions. The occurrence of errors, alarms, and events in other stations can be stored (for later review and analysis) by sending a message defining the event to the Historian in one or more Application Processors. 41
  • 43. Distributed Control System and Programmable Logic Control 3.4.1.5 Graphic Display Support The Application Processor supports graphic displays by storing and retrieving display formats, by providing access to objects stored on the Application Processor, and by storing tasks which execute in a workstation processor. Application Processors not only provide storage of information and file management for displays, but also execute programs that perform display and trend service. 3.4.1.6 Production Control Software Production control software represents a large range of packages that require varied Application Processor resources. The following is a list of packages provided: • DBMS • Historian • Spreadsheet • Physical Properties Library • Mathematics Library • BATCH The operation and performance of the production control software are determined by the particular Application Processor configuration. 3.4.1.7 Configuration Configuration refers to the process of entering or selecting parameters to define what a software package does, or to define the environment for a software package. The Application Processors support configuration functions by providing bulk storage for configuration parameters and by executing some of the configuration processes. 42
  • 44. Distributed Control System and Programmable Logic Control 3.4.1.8 Application Development Facilities Application development tools are provided to build programs for all system stations. These include tools to document, enter, translate, link, test, and maintain programs written in several programming languages. The Application Processor supports program development for all stations (workstation processors, control processors, and so forth). Assembly language, FORTRAN, and C programs can be written on the Application Processor using standard operating system tools. An optional package is available including text editors, debuggers, linkers, revision control, and compilers, plus execution statistics functions. 3.4.1.9 User Application Program Execution The Application Processors also execute user application programs. These may be application packages such as special optimizations, test data collections, special data reductions, or other packages that you may have already developed. The allocation of resources reserved for user application varies with each Application Processor. 3.4.2 Diagnostics The Application Processors utilize three types of diagnostic tests to detect and/or isolate faults: • Power-up self-checks • Run-time and watchdog timer checks • Off-line diagnostics Power-up self-checks are initiated when power is applied to the Application Processor. These checks perform sequential tests on the 43
  • 45. Distributed Control System and Programmable Logic Control various Application Processor functional elements. Any malfunction detected during the power-up self-checks is reported by means of messages printed or displayed on a directly connected printer or terminal. The run-time and watchdog timer checks provide continuous monitoring of Application Processor functions during normal system operations. For any processor model, you are informed of a malfunction by means of printed or displayed system messages. Off-line diagnostics are temporarily loaded into the system for the purpose of performing comprehensive tests and checks on various system stations and devices. Using the off-line diagnostics, a suspected fault in the Application Processor can be isolated and/or confirmed. 3.4.3 Workstation Components The workstation components provide user interface to all System CRT display functions. A selection of workstation components is available for command and data entry, along with CRT pointer manipulation and control. These components interact with software resident in versions of the system Workstation Processors (WPs) and Application Workstation Processors (AWs). Many of these components (displays and keyboards) are "common" and allow interchangeability and simplicity in mixed technology configurations. Workstation components include: • Alphanumeric Keyboard • Annunciator and Annunciator/Numeric Keyboards • Workstation Display (with/without Touchscreen) • Mouse • Trackball • Industrial Pointing Device 44
  • 46. Distributed Control System and Programmable Logic Control • Workstation Processor or Application Workstation Processor • Personal Workstation • Modular Industrial Console Selection of the touch screen, mouse, trackball or industrial pointing device is required for picking display objects on the CRT. The touch screen has sufficient resolution for all functions normally associated with a process operator. Only the mouse or trackball provides the picking resolution necessary for engineer-related functions (for example, building graphic displays). The touch screen associated with Workstation Display and the annunciator type keyboards connects to a Graphics Controller Input Output (GCIO) interface unit located beneath the workstation display. The GCIO interfaces to the Workstation Processor and/or Application Workstation that provide secure, high- speed, bidirectional data flow. The alphanumeric keyboard and trackball connect together in a functional grouping via a serial communications link to the processors. Personal Workstations (PW) utilize separate serial communication links for alphanumeric keyboard and mouse/trackball. These buses allow a variety of component connections. Figure 3.5 Table- Workstation Components. 45
  • 47. Distributed Control System and Programmable Logic Control 3.4.3.1 Alphanumeric Keyboard The alphanumeric keyboard is used any time text is entered into the system. It consists of the full set of alphanumeric keys plus punctuation and special symbol keys laid out in the standard format, and a numeric data entry pad (with cursor control). Figure 3.6 Alphanumeric Keyboard 3.4.3.2 Annunciator Keyboard The Annunciator Keyboard Figure 3.7 is an array of LED/switch pairs. It also contains a horn silence switch and a lamp-test switch. Each LED, under control of the processor software, may be ON, OFF, or FLASHING as determined by the process conditions. The LEDs, when used in conjunction with the units audible annunciator, form an effective means of calling a users attention to specific areas of the system. The switch associated with each LED can be used to invoke any pre- configured displays or operator responses.. Figure 3.7 Annunciator Keyboard 3.4.3.3 Workstation Display with/without Touchscreen The workstation display is an analog cathode ray tube (CRT) color monitor supporting ultra-high resolution applications. The monitor is suitable for mounting onto a Modular Industrial Workstation or on a 46
  • 48. Distributed Control System and Programmable Logic Control desktop. The monitor can include a touchscreen optional feature. Figure 3.8 shows the monitor with a tilt and swivel base mounted on the GCIO interface unit. The GCIO interface supports the touchscreen, annunciator and annunciator/ numeric keyboard, and audible horn options. Figure 3.8 Table-Top Workstation Display The optional touch screen is bonded to the front surface of the CRT monitor. The user selects display objects by touching them on the screen. The touch screen senses the action and sends a data signal to the workstation processors software indicating the position of the selection. 3.4.3.4 Trackball The trackball is a stationary component that contains a rotatable sphere. The trackball can be located on a table top. Rotation of the sphere causes CRT pointer movement analogous to the mouse action. Buttons are also provided for user selections/manipulations. See Figure 3.9 Figure 3.9 Trackball 3.4.3.5 Modular Industrial Console Modular Industrial Consoles provide flexible mounting arrangements of components. They allow users to configure centralized or distributed control centers tailored to the functional requirements of each interaction point in the plant. The modular console furniture 47
  • 49. Distributed Control System and Programmable Logic Control described herein may incorporate a mix of equipment - console displays, input devices, processors, Fieldbus Modules, data storage devices, and so on. Alternately, only display-specific equipment can be incorporated. Modular Industrial Consoles (MICs) are ideal for supporting powerful multiple-screen, real-time display software interactions. This combination allows console resources to be optimally allocated to meet changing day- to-day needs. 3.5 Operator Interface Operating in conjunction with human interface input/output components, the workstation processors serve as a link between the operator and other distributed processor modules. They receive graphic and textual information both stored internally or from application processors and generate signals to display the information on a workstation display. Display formats and data files are available from bulk storage. Live display information (distributed data objects) is available from any control -processor, or from shared system global data. The video information displayed can include free form combinations of text, graphic illustrations, charts, and control displays. The workstation processors display textual information as 80 text characters per line, with four fonts. The processors provide resizable and restackable windows. Displays for all of the workstation processors may also be developed using the system software running in a compatible personal computer. A workstation processor, together with its workstation monitor and input components, can be configured with combinations of peripherals to suit functions and user preferences. 48
  • 50. Distributed Control System and Programmable Logic Control 3.6 Gateways The architecture of the DCS permits it to be connected to other foreign systems using a gateway module for adapting different communication protocols. See figure 3.10. Figure 3.10 Field Automation Subsystem 3.7 Role Play Each trainee should introduce one of the main components: 1. Communication Bus 2. Control Processor. 3. Application Processor 4. Operator Interfaces and Gateways 49
  • 51. Distributed Control System and Programmable Logic Control Chapter 4 DCS Hardware 4.1 Learning Objectives • Define fieldbus communication. • Illustrate system components of level 1 control. • Demonstrate interconnection between different components. • Develop knowledge base of foundation fieldbus technology. 4.2 Fieldbus Modules Fieldbus Modules provide connection of digital I/O, analog I/O, and Intelligent Transmitters to control processors. There are two types of Fieldbus Modules: Main and Expansion. Some main modules can be expanded using an expansion module. A wide range of Fieldbus Modules is available to perform the signal conversion necessary to interface the control processor with field sensors and actuators. 4.3 Fieldbus Interconnection The Control Processor is used in three different configurations, which provide broad flexibility in Fieldbus implementation: • Local Fieldbus (Figure 4.1) - Used only within the enclosure. Fieldbus Modules attach directly to the redundant local bus. 50
  • 52. Distributed Control System and Programmable Logic Control Figure 4.1 Local Fieldbus • Twinaxial (Dual-Conductor Coaxial) Fieldbus Extension (Figure 4.2) - Using twinaxial cable, the Fieldbus can optionally extend outside of the enclosure. Fieldbus Modules attach to the extended bus through Fieldbus isolators. The twinaxial Fieldbus extension may be redundant. Figure 4.2 Twinaxial Fieldbus Extension • Fiber Optic Fieldbus Extension (Figure 4.3) - The fiber optic Fieldbus can optionally extend the distance as well as add application versatility and security. Figure 4.3 Fiber Optic Fieldbus Extension 51
  • 53. Distributed Control System and Programmable Logic Control All three Fieldbus configurations use serial data communication complying with Electronic Industrial Association (EIA) Standard RS-485. 4.4 Cluster I/O Subsystem Interfacing The Control Processor interfaces with the Fieldbus Cluster Input/Output Subsystem that consists of the Fieldbus, a multi-slot chassis configuration of a Fieldbus Processor, analog/digital Fieldbus Cards (FBCs), and power supply and power monitor card. These Cluster I/O subsystems meet the needs of applications where a high number of channels per card are required. Figure 4.4 shows a typical twinaxial Fieldbus configuration. Figure 4.4 Twinaxial Fieldbus Cluster I/O Subsystem Interface Configuration 4.5 Fieldbus Cluster I/O Subsystem The Fieldbus Cluster Input/Output Subsystem provides full support for analog measurement, digital sensing, and analog or discrete control capabilities. The Subsystem integrates with the Control Processor or Personal Workstation via the Fieldbus, and includes a multi-slot chassis configuration made up of a Fieldbus Processor, Analog/Digital Fieldbus Cards (FBC), subsystem main power supply, and power monitor card. 52
  • 54. Distributed Control System and Programmable Logic Control The Fieldbus Cluster I/O Subsystem is configurable, gathering analog measurements, while simultaneously handling analog and digital input and output channels. The Fieldbus Cluster I/O Subsystem is offered in both non-redundant and redundant configurations. Each in a redundant pair is individually addressable on the Fieldbus with a unique logical address. In a redundant configuration, the FBPs provide switchover from the primary FBP to the redundant FBP and back again automatically. The FBCs are suitable in applications where a high number of channels per card are required. They are ideal for non-isolated and isolated input signal gathering and data acquisition systems where high quantities of "points per cluster" areas are desired. The FBCs may be optionally connected as redundant pairs. Various input cards are available with one of the following three levels of isolation: • Non-isolated - Each channel is referenced to ground and the card itself is referenced to ground. • Group-isolated - Electrically separate card-to-card but not channel- to-channel on the same card. • Isolated - Each channel is electrically separated from any other channel, card, group, building, site, etc. 4.6 Fieldbus Processor The Fieldbus Processor (FBP) module provides communication between the Fieldbus Cards (FBCs) and the Control Processor. Optionally available is redundancy for the FBP module. Each FBP module is individually addressable via the Fieldbus. If the primary FBP fails or is taken off-line, the secondary FBP automatically assumes control. It remains in control until the primary FBP returns on-line (figure 4.5). 53
  • 55. Distributed Control System and Programmable Logic Control Figure 4.5 FBP Overview 4.7 Fieldbus Cards The Fieldbus Cards support a variety of analog and digital I/O signals. The FBCs convert electrical I/O signals used by field devices to permit communication with these devices via the Fieldbus. The FBCs can be connected in a redundant configuration via the hardware. The redundant FBCs must be in adjacent slots and they are connected via a hardware adapter at the interface to the field devices. In an FBC redundant configuration, the FBP determines which FBC of the redundant pair is to supply the data to the Control Processor. This is done in the software by a predetermined set of conditions. 4.7.1 Analog FBCS The analog FBCs support analog signal types and control functions equipped with accurate signal conditioning circuitry, the analog cards interface between process sensors and actuators. To input an analog voltage (into DCS) the continuous voltage value must be sampled and then converted to a numerical value by an A/D 54
  • 56. converter. Figure 4.6 shows a continuous voltage changing over time.There are three samples shown on the figure. The process of sampling thedata is not instantaneous, so each sample has a start and stop time. Thetime required to acquire the sample is called the sampling time. A/Dconverters can only acquire a limited number of samples per second. Thetime between samples is called the sampling period T, and the inverse ofthe sampling period is the sampling frequency (also called sampling rate).The sampling time is often much smaller than the sampling period. Figure 4.6 Sampling an analog voltage Analog outputs are much simpler than analog inputs. To set ananalog output an integer is converted to a voltage. This process is veryfast, and does not experience the timing problems with analog inputs.But, analog outputs are subject to quantization errors. Figure 4.7 gives asummary of the important relationships. These relationships are almostidentical to those of the A/D converter. Assume we are using an 8 bit D/Aconverter that outputs values between 0V and 10V. We have a resolutionof 256, where 0 results in an output of 0V and 255 results in 10V. Thequantization error will be 20mV. If we want to output a voltage of6.234V, we would specify an output integer of 159, this would result inan output voltage of 6.235V. The quantization error would be 6.235V-6.234V=0.001V. The current output from a D/A converter is normallylimited to a small value, typically less than 20mA.
  • 57. Figure 4.7 D/A converter4.7.2 Digital FBCS The digital FBCs consist of 32- and 64-channel types. Inputs canbe either voltage monitoring or contact sensing. Contact inputs must convert a variety of logic levels to the 5Vdclogic levels used on the data bus. This can be done with circuits similar tofigure 4.8. Basically the circuits condition the input to drive anoptocoupler. This electrically isolates the external electrical circuitry fromthe internal circuitry. Other circuit components are used to guard againstexcess or reversed voltage polarity. Figure 4.8 Contact input circuitry. Contact outputs must convert the 5Vdc logic levels on the DCSdata bus to external voltage levels. This can be done with circuits similarto figure 4.9. Basically the circuits use an optocoupler to switch externalcircuitry. This electrically isolates the external electrical circuitry from
  • 58. the internal circuitry. Other circuit components are used to guard againstexcess or reversed voltage polarity. Figure 4.9 Contact output circuitry.4.8 Other Modules • 0 to 20 mA Input/Output Interface • Pulse Input, 0 to 20 mA Output Interface • Thermocouple/ Millivolt Input Interface • RTD Input Interface • High Power Contact/dc Input/Output Interface4.9 Foundation Fieldbus Technology FOUNDATION fieldbus is an all-digital, serial, two-waycommunications system that serves as the base-level network in a plant orfactory automation environment. Figure 4.10 Foundation Fieldbus Network
  • 59. Figure 4.11 Historical development of field devices technology. Its ideal for applications using basic and advanced regulatorycontrol, and for much of the discrete control associated with thosefunctions. Two related implementations of FOUNDATION fieldbus havebeen introduced to meet different needs within the process automationenvironment. These two implementations use different physical mediaand communication speeds. • H1 works at 31.25 Kbit/sec and generally connects to field devices. It provides communication and power over standard twisted-pair wiring. H1 is currently the most common implementation and is therefore the focus of these courses. • HSE (High-speed Ethernet) works at 100 Mbit/sec and generally connects input/output subsystems, host systems, linking devices, gateways, and field devices using standard Ethernet cabling. It doesnt currently provide power over the cable, although work is under way to address this. Figure 4.12 Field Device Capacity.
  • 60. Conventional analog and discrete field instruments use point-to-point wiring: one wire pair per device. Theyre also limited to carryingonly one piece of information -- usually a process variable or controloutput -- over those wires. As a digital bus, FOUNDATION fieldbusdoesnt have those limitations. • Multidrop wiring. FOUNDATION fieldbus will support up to 32 devices on a single pair of wires (called a segment) -- more if repeaters are used. In actual practice, considerations such as power, process modularity, and loop execution speed make 4 to 16 devices per H1 segment more typical.That means if you have 1000 devices -- which would require 1000 wirepairs with traditional technology -- you only need 60 to 250 wire pairswith FOUNDATION fieldbus. Thats a lot of savings in wiring (andwiring installation). Figure 4.12 Fieldbus wiring diagram.
  • 61. • Multivariable instruments. That same wire pair can handle multiple variables from one field device. For example, one temperature transmitter might communicate inputs from as many as eight sensors -- reducing both wiring and instrument costs.Other benefits of reducing several devices to one can include fewer pipepenetrations and lower engineering costs. • Two-way communication. In addition, the information flow can now be two-way. A valve controller can accept a control output from a host system or other source and send back the actual valve position for more precise control. In an analog world, that would take another pair of wires. • New types of information. Traditional analog and discrete devices have no way to tell you if theyre operating correctly, or if the process information theyre sending is valid.But FOUNDATION fieldbus devices can tell you if theyre operatingcorrectly, and if the information theyre sending is good, bad, oruncertain. This eliminates the need for most routine checks -- and helpsyou detect failure conditions before they cause a major process problem. • Control in the field. FOUNDATION fieldbus also offers the option of executing some or all control algorithms in field devices rather than a central host system. Depending on the application, control in the field may provide lower costs and better performance -- while enabling automatic control to continue even if theres a host-related failure.
  • 62. FOUNDATION fieldbus is covered by standards from three majororganizations: • ANSI/ISA 50.02 • IEC 61158 • CENELEC EN50170:1996/A1 The technology is managed by the independent, not-for-profitFieldbus Foundation, whose 150+ member companies include users aswell as all major process automation suppliers around the globe.Some suppliers have even donated fieldbus-related patents to the FieldbusFoundation to encourage wider use of the technology by all Foundationmembers. Interoperability simply means that FOUNDATION fieldbusdevices and host systems can work together while giving you the fullfunctionality of each component.4.10 Role PlayEach trainee should introduce one of the main components: 5. Fieldbus Module and Interconnection 6. Fieldbus Processor and Clusters. 7. Foundation Fieldbus technology
  • 63. Chapter 5 DCS Software5.1 Learning objectives• To be familiar with main software components of DCS.• Understand main tasks for each application.5.2 Standard Application Packages5.2.1 System ManagementFeatures include: • Display of equipment information for the station and its associated input/output devices, buses, and printers. • Capability for change actions directed to the associated equipment. • Processing of station alarm conditions and messages.5.2.2 Database ManagementFeatures include: • Storage, retrieval, and manipulation of system data files. • A run-time license for the embedded use of the Relational Database Management System. • A spreadsheet package.5.2.3 HistorianFeatures include: • Maintenance of a history of values for process-related measurements that have been configured for retention by the Historian.
  • 64. • Maintenance of a history of application messages that have been sent to the Historian. • Maintenance of a history of alarms and error conditions which generate messages for the Historian. • Access to all Historian data by display and report application programs.5.2.4 View Display ManagerFeatures include: • Presentation of the operating environment. • Setting of the overall operating environment according to the type of user. Process engineers, process operators, and software engineers have access to specialized functions and databases suited to their specific requirements and authorizations. • Dynamic and interactive process graphics. • Display and processing of current process alarms. • Group and default displays for control blocks. • Execution of embedded trending within displays.5.2.5 Draw Display BuilderFeatures include: • Graphical display configuration for viewing and control of process operation. • Access to graphical object palettes allowing easy inclusion of pumps, tanks, valves, ISA symbols, and similar complex objects. • Ready modification of existing displays using a mouse pointer, menu items, and quick-access toolbars. • Association of process variables with objects in the displays.
  • 65. • Dynamic variation of object attributes such as fill level, color, position, size and visibility with changes in the associated process variable. • Inclusion of operator control elements such as pushbuttons and sliders into displays. • A library of faceplates which may be configured by simply specifying the compound and block name of the block to which the faceplate is to be connected.5.3 Alarm System Figure 5.1 Alarm manger Alarm Manager provides an easy-to-use graphical interface ofpreconfigured alarm displays for viewing and quickly responding toprocess alarm conditions. The alarm display windows present alarmmessages initiated by the control blocks and related to digital input, statechange, absolute analog, deviation, rate of change, device statusmismatch, and other alarm conditions.Accessible from any environment, the Alarm Manager Display windowsprovide:
  • 66. • Quick, easy access to the most recent alarm messages via the Most Recent Alarm display or Current Alarm display • Alarm status and value information dynamically updated from the control station • Color-coded priority and status indicators that allow you to quickly focus in on critical alarms • Summary displays for different views of the alarm database based on alarm status • An historical list of alarms • The capability to view subsets of alarms based on specific user- defined criteria • The capability to silence or temporarily mute workstation and annunciator horns. • Secured access to alarming functions dependent on user or system responsibilityThis set of resizable alarm displays providing a variety of current andhistoric views of the process alarm database includes: • A multi-page list of all the current alarms • A single page of the most recent, active, unacknowledged alarms with dynamically updating value and status fields • Three summary displays specific to alarm status also with updating values and statuses: o all active, unacknowledged alarms o all unacknowledged alarms that have returned to normal o all active, acknowledged alarms • A list of historized alarms related to the selected historian database • An operations display for silencing horns, temporarily muting horns, changing environments
  • 67. These displays allow you to respond to alarm conditions, filter andanalyze specific alarm data, and maintain alarm message files forreporting purposes. The Process or Alarm button in the Display Manager (DM) windowindicates the presence of alarms (both acknowledged andunacknowledged) and provides access to Alarm Manager Displays.Initially, the Current Alarm Display (CAD) appears and the otherdisplays are easily accessible from the CAD via its default Displaysmenu: • Most Recent Alarm display (MRA) • New Alarm display (NEWALM) • Unacknowledged Alarms display (UNACK) • Acknowledged Alarms display (ACKALM) • Alarm History display (AHD) • Operations display (OPR)These easy-to-use displays support the following features: • A pre-configured number of alarms per screen or page • Pre-configured alarm message information and formatting per alarm type • A status area for indication of current Alarm Manager and display status, such as horns muted, match active, display paused, initial call-up time • Buttons for responding to alarm conditions, such as acknowledging or clearing alarms, and for accessing additional alarm information and process displays • Pull-down menus for editing, viewing, and filing functions • A pull-down menu for accessing other displays • Pop-up menus for quick access to commonly used functions
  • 68. • A scroll bar and Go To Page option for moving easily through the alarm listAlthough a preconfigured set of alarm displays is provided, many aspectsof the displays and alarm message content are user configurable toaccommodate different process control applications and operationalneeds. See the section on Alarm/Display Manager Configurator.5.4 Historian The Historian collects, stores, processes, and archives process datafrom the control system to provide data for trends, Statistical ProcessControl (SPC) charts, logs, reports, spreadsheets, and applicationprograms. The Historian software is an easy-to-use data collection toolthat allows the user to organize and enforce a plant data collectionphilosophy. The Historian provides extensive data collection andmanagement functions, and data display functions for use by processengineers or operators. Typical historical data are process analog and/or digital variables(points). The Historian can also collect and display application generatedmessages. You can use the Historian to collect data in support of thefollowing production control functions: • Cost accounting • Equipment performance analysis • Historical trending • Information retrieval • Inventory management • Legal record maintenance • Lost time analysis
  • 69. • Maintenance reporting • Material accounting • Process analysis • Production reporting • Quality controlThe Historian can: • Retrieve variables from process databases or accept data from production control databases maintained by user application programs. • Perform built-in calculations on the collected data. • Store calculated (reduced) data in a real time, relational database.Application software in a plant-wide control system can access theHistorian database to obtain historical data for process control, productioncontrol, and management information reporting.You can use SPC chart displays of Historian data to monitor processvariables on-line via the Statistical Process Control Package (SPCP).You can build displays for trending historical data via the Display Builderand Display Configurator with Trending software.Using the Report Writer, you can generate detailed reports of historicaldata for management information.Examples of Industrial Software that interface with the Historian are: • Batch Plant Management • Data Validator • Display Manager • Display Configurator with Trending • Object Manager (for process data histories) • Operator Action Journal • Operator Message Interface • Real-Time Data Base Manager
  • 70. • Spreadsheet • Statistical Process Control Package • System Monitor • Report Writer5.5 Draw Figure 5.2 Draw Draw is a display builder and configurator that allow you to createand maintain dynamically updating process displays. Displays canrepresent the plant, a process area or a detailed portion of the process.You can draw basic objects using Draws toolbars, menu items andshortcut keys. You assign graphic attributes such as color and line style tothe objects, and then configure them to reflect process variable changes oroperator actions. Draw includes numerous palettes of objects such asoperator buttons, pumps, tanks, pipes, motors, valves and ISA symbols.You can also create your own palettes for storing complex objects andcompany-standard symbols. Displays can include faceplates, trends andbitmapped images. You can easily edit your displays to reflect changes inthe process control scheme or to maximize operating efficiency andsecurity.
  • 71. 5.5.1 ConfigurationThere are two ways of configuring a display object. You can: 1. Choose the Dynamic Update tab to connect one of the objects attributes, such as visibility or fill level, to a process variable or a file. With this type of configuration, changes in an attribute are triggered dynamically by changes in the process variable. No operator intervention is necessary. 2. Choose the Operator Action tab to connect the entire object to an action, such as opening a display or executing a command. An operator triggers the action by selecting the object.An individual object can have both types of connections, although it canhave only one operator action.5.5.2 Operator Actions In a display configured for operator action, an operator can triggerevents by selecting an object (typically a button), moving a slider, ortyping text or a numeric value. In response to an operator action,variables can be modified, a new display can open or an overlay canappear. While you can configure only one operator action for each displayobject, you can trigger two or more events with a single operator actionby configuring an object with a View display command script.Operator Actions include: • Open Display • Open Overlay • Close Display/Overlay
  • 72. • Display Command • Relative Pick • Momentary Contact • Ramp • Connect Variable • Move Horizontal or Vertical • Numeric/Text Entry5.5.3 Faceplates A faceplate is a dynamic representation of control blockparameters. Draw provides a complete library of faceplates, ready to beconnected to any control block in the control database. In addition, youcan build your own faceplates using the standard Draw drawing tools.To configure a faceplate, you need only define the Compound:block towhich the faceplate is connected. Draw automatically determines theproper configuration attributes for the associated Compound:block.5.5.4 Trends Trend areas represent changing data values from the real-timedatabase and historian database. A data is displayed as a series of plottedpoints connected by straight lines and scaled according to the high andlow limits configured for each trend line.5.5.5 Group Displays Group displays allow you to group faceplates and trends intounique layouts to meet changing operational needs.
  • 73. 5.6 View Figure 5.3 View View is a window into the system software, providing a user-friendly interface to the total process. You can interact with any or all ofthe real-time plant, field, and process data available in the system.View provides: • Direct access to dynamic process displays. • Entry into user-configurable operating environments specific to each user - the process engineer, process operator, and software engineer. • Execution of embedded real-time and historical trending. • Service and display of process alarms via the Alarm Manager. • An overview of the compounds and blocks in the control database and access to block default detail displays via Select. • Access to other applications, such as: o Draw software for building and configuring dynamic user graphics. o System Management Displays for monitoring system equipment health. o Integrated Control Configurator for configuring the control database. o Historian for configuring the historization of data and system messages.
  • 74. o Access to the four most recently used displays.Additionally, with View you have: • Flexibility in customizing environments to conform to your site requirements. • Rapid access to View while in other applications. • Screen print utility. • Window sizing options.The multi-window capability of Solaris and Windows NT operatingsystems allows you to monitor the information on a process controldisplay as well as access other applications without closing any window.5.6.1 View WindowView Window contains the following features: • A top menu bar for accessing displays, configurators, and other applications as specified by the environment. • A display bar of named display buttons or eight "thumbnail" mini- display buttons for directly accessing process displays. • A system bar with System and Process alarm buttons indicating system and process health; a message bar with a dropdown list of the latest messages; display of the current date and time. • A status bar indicating the current display name, current operating environment, Operator Action Journal logging name, printer logging name, Historian name.Using the control window menu, you can: • Resize the View window automatically or manually. • Move the window.
  • 75. 5.6.2 Operating Environments A collection of programs, utilities, and displays related to usertasks is provided for each of the following: process operator, processengineer, and software engineer. These environments, including menubars, menu content, and Display Bar content, can be modified to conformto your site requirements. You can easily switch from one configuredenvironment to another. To secure environments against unauthorizeduse, environment passwords can be configured and menu entries disabledbased on the environment.5.7 Operator Action Journal The Operator Action Journal is a record of specific operator actionstaken during process control operations. These actions generally consistof manipulating certain Control Processor, and gateway parameters aswell as Application Processor, Application Workstation, and WorkstationProcessor shared variables. Actions of this type are the ramping or directdata entry of point values, toggling points, changing block statuses,acknowledging block alarms, and horn muting. Operator action reportingis limited to operator actions from the Display Manager, View, andAlarm Manager. Also logged are environment change actions, scripts,applics, and invoking other applications such as configuration. When the Operator Action Journal feature is enabled, all operatoractions within the Display Manager, View, and the Alarm Manager thatchange parameters in the process database are logged to a printer and/orto the specified Historian database. These operator actions includetoggling points, ramping or direct data entry of new point values,
  • 76. changing block statuses, acknowledging block alarms, and other actionssuch as horn muting.Information logged as a result of each database change includes: • Name of the Display Manager, FoxView, or Alarm Manager that requested the database change. • Compound:Block.Point for which the change was made. • The "old value" TO "new value" text for non-packed Boolean. • Current mask and data value for packed Boolean/long.Following is an example of an Operator Action Journal Report.Operator Action Journal ReportTue Aug 1 1997 17:04:05 Page 108-02-97 07:57:08 GC3E31 SCRIPT /usr/fox/hi/init.cmds08-02-97 07:57:15 GC3E31 ChgEnv Init_Env ->Init_env08-02-97 07:58:19 GC3E31 ChgEnv Init+Env ->Proc_Eng_Env08-02-97 08:00:34 CG3E31 UC01_LEAD :SINE .OUT 16.18 to 46.1808-02-97 08:00:54 GC3E31 UC01_LEAD :SINE .MA Manual to Auto08-02-97 08:00:57 GC3E31 UC01_LEAD :SINE .LR Remote to Local08-02-97 08:01:01 GC3E31 UC01_LEAD :SINE .MA Auto to Manual5.8 Control Configuration Process control for DCS is based on the concepts of compoundsand blocks. A compound is a logical collection of blocks that performs acontrol strategy. A block is a member of a set of algorithms that performsa certain control task within the compound structure. Figure 7.4 showsthe compound/block relationship.The compound provides the basis for the integration of: • Continuous control • Ladder logic • Sequential control.
  • 77. Within this structure, any block in any compound can be connected toany other block in any other compound in the system. The entirecompound structure can be viewed through the workstation display.The block contains parameters that have values of the types: Real,Boolean, Packed Boolean, Boolean Long, Integer, or String. Figure 5.4 Compound/Blocks relationship5.8.1 Compound FunctionsThe compound supports the following functions for the related blocks: • Process alarm priority, alarm inhibiting, and alarm grouping • Sequence status notification (see Sequential Control section) • Phasing for execution load leveling at execution time.5.8.2 Compound/Block Process Alarming Alarms and status messages are generated by specific alarm blocksand by alarm options in selected blocks. Alarms have five levels ofpriority, 1-5, (where 1 = highest priority) that enable you to quickly focuson the most important plant alarm conditions. An alarm priority of 0indicates the absence of any alarm. These are summarized in a singlealarm summary parameter for each compound. This parameter containsthe priority of the highest current alarm in that compound. To reduce
  • 78. nuisance alarms, alarms can be inhibited at the compound level on apriority level basis. Alarms can also be inhibited at the block level, oneither an alarm type basis, or an overall basis. Alarms are initiated by the blocks within the compound. Alarmmessages are then sent to groups of stations or applications (for example,Workstations, Historians, Printers) according to configured alarm groups.The UNACK alarm acknowledge output parameter allows the user topropagate alarm acknowledge actions to all blocks in a compound.Stations, applications, and devices corresponding to various alarmdestination groups are configured at the compound level or at the stationlevel in the case of station compounds.Group numbers for individual block alarm types are configured at theblock level.5.8.3 Compound/Block Phasing A user-defined phase number can be assigned to each compoundusing a range of integer values that varies with assigned period. Phasingallows the starting time of one compound/block to lead or lag the startingtime of another compound/block, thereby leveling the block processorload.5.8.4 Compound AttributesThe compound has the following attributes: • Name: User-defined name that must be system-unique and no more than 12 characters in length. The name can be any mix of numeric (0 to 9), upper case alphabetic (A to Z), and the underscore (_). • Descriptor: 32-character field for user-defined identification. • On/Off: Parameter that enables or disables the execution of all blocks within the compound, where: 1 = on; 0 = off.
  • 79. 5.8.5 Compound/Block Parameters Compound and block parameters contain values that are of one ofthe types Real, String, Integer, Short Integer, Long Integer, Boolean,Packed Boolean, Packed Long, or Character. Additionally, parameters aredefined as being configurable, and either connectable/settable, notconnectable/not settable, or a combination that is dependent upon thecompound, block, and state.5.8.5.1 Configurable Parameters Configurable parameters are those parameters that can be definedthrough the Integrated Control Configurator. They can be displayableonly, or displayable and editable.5.8.5.2 Connectable Parameters Connectable parameters are those parameters of the user interfacein which secured, change-driven connections may be made betweennetwork stations, or as local direct connections within the same station.Each connection consists of a connectable source and a connectable sink.Output parameters (all outputs are connectable) are sources, while aconnectable input may be a sink or a source, or both.Certain parameters that may be considered functional inputs (such as SPTin the PID blocks, and RATIO in the RATIO block) are settable but notconnectable. A connectable parameter has a value record that contains theparameters value, its status, and its designated value type (Real, Boolean,or Integer).
  • 80. 5.8.5.3 Input Parameters Input parameters are connectable types that are the receivers ofdata from other connectable parameters via a path connection.If no source path is specified during configuration, then the resident dataof the value record is the actual "source" of data. It can be either theinitial default or configured value, or a new value through a SET call tothe input parameter.If a source path is specified, then the data value is an output parameter ofthe same or another block, or a shared variable, thereby securing theinput. By linking a shared variable to a block input during configuration,the user can establish a long-term secured connection between a remoteapplication program and the block input.5.8.5.4 Output Parameters All output parameters are connectable data sources that have valuerecords. There are two types: settable and nonsettable. The settability of asettable output is controlled by the secured status of the value record. Thesecured status is dependent on whether the blocks operational mode is inAuto or in Manual. In either Auto or Manual, nonsettable outputparameters cannot be written by any other source under any conditions. Settable outputs may be conditionally released by the blockalgorithm in the Manual mode. In Manual, the block unsecures settableoutput parameters. They can then be written by other tasks via SET calls.When the block switches to Auto, the block secures and updates itsoutput parameter(s).
  • 81. 5.8.5.5 Nonconnectable Parameters Nonconnectable parameters have no value records and are notlinkable. They mainly consist of string-type variables like NAME, ornonsettable parameters that are used in the configurator only, forexample, block options. Local algorithm variables are alsononconnectable. Nonconnectable parameters are generally accessiblethrough GET calls.There is also a class of nonconnectable input parameters that comprise theblock user interface which can be manipulated through SET calls. Anexample is an alarm deadband.5.9 Role PlayEach trainee should introduce one of the main applications: 8. System Management. 9. Historian 10.Graphics Applications. 11.Control Configurator. 12.Operator Journal 13.Alarm System.
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  • 83. Distributed Control System and Programmable Logic Control Chapter 6 Installation 6.1 Learning objectives • To be able to define installation procedure for each component. 6.2 Modular Industrial Console The Modular Industrial Console (MIC) provides flexible mounting arrangements for components. The MIC can incorporate a mixture of equipment: console displays, input devices, processors, Fieldbus modules, data storage devices, and so on. Modular Industrial Consoles support powerful multiple-screen, real-time display software interactions. This hardware/software combination allows console resources to be allocated with the flexibility to meet changing day-to-day needs. Multi-screen consoles enable comprehensive handling of more plant information in a coordinated fashion. The MIC product line (Figure 6.1) allows a highly flexible packaging configuration of console equipment. Individual MIC modules are joined on-site to provide a customized configuration using standard components. This modular approach offers you combinations of single-screen and multi-screen real-time display software interactions as required at a given console. There are, however, specific allocations for mounting equipment within configurations. 82
  • 84. Distributed Control System and Programmable Logic Control Figure 6.1 MIC Arrangement The MIC is built up from four basic pieces of equipment, each of which is individually configurable: • MIC bay - basic full bay unit, full height, 27-inches wide, with bay module • Spacer module - storage space between MIC full bay units • Desktop/printer bay - a rear bay similar to the full bay units with a flat tabletop • Free standing table - a basic multipurpose table. 6.3 System Equipment 6.3.1 Unloading The system units must be designed to withstand vibration and shock normally encountered during shipping and installation; however, extreme shocks and vibration should be avoided. The system units may be moved from the transportation vehicle to their intended locations by forklift or manual jack truck. If practical, all major movements of the units should be accomplished before the units are unpacked. 83
  • 85. Distributed Control System and Programmable Logic Control 6.3.2 Unpacking Procedure The following unpacking procedure applies, in general, to all system units: • Inspect the exterior of the shipping carton for obvious damage. (Any noticeable damage should be indicated in the shippers bill of lading.) • Verify that the equipment received is that described in the bill of lading. • Remove shipping straps, shipping shroud, and other packing materials, such as polyethylene bags and Styrofoam cushioning materials. • If the unit is attached to a skid, remove all shipping hardware and hold-down bolts used to fasten the unit to the skid. Separate the skid from the unit. • Ensure that the appropriate interconnecting cables are present, by comparing the cable part numbers and quantities with those listed in the bill of lading. 6.3.3 System Power Checks Perform the following checks before you install the equipment: • Check that all the required ac or dc power distribution network lines are installed. • Check that the appropriate number of ac power outlets are installed and spaced appropriately. • Switch on main system power. • Using a multimeter, check that the appropriate operating voltage exists at each ac outlet or connection point. • Switch off main system power. 84
  • 86. Distributed Control System and Programmable Logic Control 6.3.4 Industrial Enclosures Mounting Procedures Figure 6.2 shows a single dual-height modular mounting structure area for containing processors and modules in an Industrial Enclosure. Figure 6.2 Industrial Enclosure Mounting Structure Area Enclosures are designed for floor mounting, and accept processor modules, Fieldbus modules, and data storage devices. Wires, cables, and conduits can enter either the bottom or the top of the enclosure. Side doors provide access to the wiring areas. Additionally, the doors can be mounted to open from left-to-right or right-to-left. Industrial Enclosures are available in two configurations, vented and sealed. The vented configuration has openings at the top and bottom to provide ventilation, and has a metal plate, with gasket, at the bottom for electrical protection purposes. A sealed enclosure has metal plates, with gaskets, at the top and bottom to provide a watertight seal. 1. Check that mounting holes have been drilled in floor. If they have not, proceed as follows. (If below-floor cabling is to be employed, refer to the Site Planning document for information on the recommended size and placement of the floor cutout.) a. Place enclosure in desired location. b. Mark hole locations. c. Move the enclosure away from the markings. d. Drill holes in floor. 85
  • 87. Distributed Control System and Programmable Logic Control 2. If the enclosure is the vented type and conduit entry is to be from the bottom: a. Drill or punch the bottom conduit enclosure plate, and provide appropriate conduit fittings. b. Place the conduit enclosure plate on the floor, in the precise location that the enclosure is to be mounted. c. Go to Step 6. 3. If the enclosure is the vented type, and conduit entry is to be from the top: a. Remove the vent cap and top conduit enclosure plate(s). b. Drill or punch the conduit enclosure plate(s). c. Replace the vent cap and conduit enclosure plate(s). d. Place the enclosure plate on the floor, in the precise location that the enclosure is to be mounted. e. Go to Step 6. 4. If the enclosure is the sealed type and conduit entry is to be from the bottom: a. Drill or punch the bottom conduit enclosure plate, and provide appropriate conduit fittings for a watertight seal. b. Place the conduit enclosure plate on the floor, in the precise location that the enclosure will be mounted. c. Go to Step 6. 5. If the enclosure is the sealed type and conduit entry is to be from the top: a. Remove the top conduit enclosure plate. b. Drill or punch the conduit enclosure plate, and provide appropriate conduit fittings for a watertight seal. c. Replace the conduit enclosure plate. 86
  • 88. Distributed Control System and Programmable Logic Control 6. Position the enclosure, with mounting gasket and enclosure plate, so that the holes in the enclosure base, gasket, and enclosure plate are aligned with the mounting holes in the floor. 7. Install two bolts, with flat washers and lockwashers, in diagonally opposite mounting holes. (Do not tighten.) 8. Install two more bolts, with flat washers and lockwashers, in the other two diagonally opposite mounting holes. (Do not tighten.) 9. Install the remaining bolts, with flat washers and lockwashers, in the center mounting holes. 10.Tighten all bolts evenly and equally, working from center to outside bolts, being careful not to overtighten. Maximum torque should be applied carefully. 6.4 Software Installation The Installation Phase performs the installation of software packages. Installation of software packages is performed by vendor representative on target stations. 6.5 Discussion Initiate a dialogue between trainees to discuss their own experiences and notes about different installation phases related to the text in this chapter. 87
  • 89. Distributed Control System and Programmable Logic Control Chapter 7 Maintenance 7.1 Learning objectives • Understand maintenance philosophy and procedures. 7.2 Maintenance Philosophy The maintenance approach is oriented toward module replacement. The use of diagnostics, fault location tables, and troubleshooting guides described in system document, as well as the presence of status lamps (LEDs) on each module, enables isolation of problems to the module level. In addition, any module can be replaced without affecting the operation of any other module, including the module of a fault-tolerant pair. 7.3 Preventive Maintenance The design of DCS equipment and associated peripheral devices is such that scheduled preventive maintenance on the equipment is limited to visual inspections, periodic cleaning procedures, and adjustment of system modules if necessary. While performing these routines, you should check for damaged cables, loose connections, inoperative fans and indicator lamps, wear or binding of drives and fan motors, and take appropriate corrective action. 88
  • 90. Distributed Control System and Programmable Logic Control 7.3.1 Enclosures Perform a general visual inspection and exterior cleaning of each enclosure after the first six months of service. Approximately every 12 months thereafter perform the same, depending on local environmental conditions. Preventive maintenance procedures for enclosures include the following: 1. Wipe down the exterior of the enclosure with a soft cloth. A damp cloth and/or a nonabrasive cleaner can be used for hard-to-remove spots. 2. Clean any dust buildup from module heat fins. Use a soft cloth. If heat fins are accessible from rear of enclosure, they can be cleaned during normal operation. Otherwise, modules can be removed and cleaned from front of enclosure during routine equipment shutdowns. 3. Check fans (if installed) for proper operation. 4. Check module status indicators for proper operation. Green light indicates normal operation. Red light indicates faulty operation. 7.3.2 Enclosures Air Filters The vented configurations of all metal enclosures have an air filter located inside the door, behind the vents. Periodically check the condition of the filter for dust/dirt accumulation. Perform the following steps to check the condition of the filter: 1. Locate the plastic assembly that retains the filter that is on the inside of the door behind the vents. 2. Unsnap the plastic assembly from the vents and remove the filter. 3. Wash and replace the filter, or if desired, install a new filter, and snap the filter retainer assembly back onto the vent assembly. 89
  • 91. Distributed Control System and Programmable Logic Control 7.3.3 Modular Industrial Workstations Perform a general visual inspection and exterior cleaning of each workstation as often as necessary to ensure proper operation of the equipment. Preventive maintenance procedures for the workstations should include the following: 1. Wipe down the exterior of the enclosure with a soft cloth. A damp cloth and/or a nonabrasive cleaner can be used for hard-to-remove spots. 2. Clean any dust buildup on disk drives (especially the signal connection areas), keyboards, control panels, and monitors. Use a soft cloth. 3. Check fans (if installed) for proper operation. 4. Check module status indicators for proper operation. Green light indicates normal operation. Red light indicates faulty operation. 7.3.4 Monitor-Based Peripheral Devices As a rule, preventive maintenance on these devices should be limited to cleaning only and should be performed as often as necessary, or at least every twelve months. Wipe down the exterior of the device (excluding the monitor) with a soft cloth. A damp cloth and/or nonabrasive cleaner can be used for hard-to- remove spots. To clean the monitor, proceed as follows: 1. Select a screen that does not have direct access to the process, for example, the Initial display. 2. Remove power from the GCIO unit (annunciators are also deactivated). 90
  • 92. Distributed Control System and Programmable Logic Control 3. Turn the monitors power off. Do not move the mouse or depress any keys while the monitor is off. 4. Dampen - do not saturate - a clean, lint-free cloth with liquid glass cleaner. 5. Clean the screen by wiping with damp cloth, using circular wiping motion to avoid streaks. 6. Carefully dry the screen by wiping with a second clean, lint-free cloth. 7. Restore power to the monitor and GCIO. 7.3.5 Printers All printers should be serviced every six months (or after 300 hours of operation), whichever occurs first. Refer to the associated printer users guide (packed with the printer) and perform the following: 1. Perform a general visual inspection and cleaning of the printer. 2. Remove printer cover and inspect internal moving parts for signs of wear, broken or loose parts, frayed cables, and so on. 3. Take a clean, dry, soft cloth and dust the area around carriage shaft and platen. Remove any loose particles of paper and dust. 4. Lubricate printer as described in associated service instructions. 5. Restore printer power. 7.3.6 Keyboard A keyboard should be cleaned at a frequency determined by the environment in which it is used. 1. Use a soft, lint-free cloth dampened with a mild detergent solution to clean the keys and large surfaces. 2. Clean confined areas between the keys with a vacuum cleaner equipped with a fine brush attachment. 91
  • 93. Distributed Control System and Programmable Logic Control 7.3.7 Mouse The following care and cleaning procedure applies to both the inner and outer area of the mouse: 1. The mouse is a very precise mechanical device, so handle it with care. Do not drop, hit, or otherwise subject it to shock. 2. Do not pull on the cable. It may cause damage to both the cable and connector. 3. Do not carry the mouse by holding onto the cable. 4. Be sure to place a clean sheet of paper or use a mouse pad between the mouse and the flat surface. Dirt and grit could collect on the ball. Try not to touch the ball on the bottom. 5. Do not use the mouse in extreme temperatures or in direct sunlight. 6. Do not allow the mouse to come in contact with liquid spills (water, solutions, and so forth). 7. The mouse housing should be cleaned with a lint-free cloth using a mild detergent. Use an unsoiled lint-free cloth to dry housing. 8. Do not disassemble the mouse. If the ball in the unit needs to be cleaned, remove it from the lower case by detaching the cover to the housing. Do not remove all the screws to remove the ball. 9. Use a lint-free cloth with mild detergent to clean the ball, and an unsoiled cloth to dry it. 7.3.8 Data Storage Devices 1. Blow away any lint or dust accumulation on or near the face of the floppy disk and streaming tape drive casings. 2. Clean the outer plastic surface of the drive with a lint-free cloth or a sponge slightly dampened with water. Wipe off residue and dry with soft, lint-free cloth. Do not use abrasive cleaners, solvents, or strong detergents. 92
  • 94. Distributed Control System and Programmable Logic Control 3. Blow away any lint or dust accumulation on the signal and power connectors at the rear of the drive. 4. For the streaming tape drive, clean the head using only Freon TF and polyurethane swabs, commonly available with VCR head cleaning kits. Wet the swab with the Freon TF solution, and wipe the head using an up and down motion. Use a dry swab to clean any remaining residue from the head. 7.4 Fault Analysis Through the System Management facility, you can monitor the health of the system and perform diagnostic tests on all the system stations and associated peripheral devices. 7.4.1 Startup Diagnostics Startup diagnostics are invoked automatically as a result of a power-on reset, an error, or an off-line diagnostic command. The diagnostics exist in each station at all times and are of two basic types: • Reportable diagnostic - Tests a station function which, if faulty, does not prevent the error from being reported over the network. • Nonreportable diagnostic - Tests a station function which, if faulty, inhibits the station from communicating over the network. 7.4.2 On-line Diagnostics On-line diagnostics consist of Carrierband LAN LI (LAN Interface) Cable Tests and Nodebus Cable Tests. These tests are either operator-initiated or automatically invoked to isolate faults and to check the integrity of the communication path. 93
  • 95. Distributed Control System and Programmable Logic Control 7.4.3 Off-line Diagnostics Off-line diagnostics are used to check for, or verify the proper "independent" operation of a stations internal components. These tests do not verify any external reason for failure, thus they can be individually bench tested without regard to the stations subsystem configuration. 7.5 Corrective Maintenance 7.5.1 Module Status Indicators All power modules, Processor modules, LAN modules, and Fieldbus Modules have red and green status indicators that operate in accordance with the maintenance manual codes. 7.5.2 I/A Series Module Replacement The maintenance approach is oriented toward module replacement. Fault analysis provides assistance with isolating station and peripheral faults. The presence of status lamps (LEDs) on each module enables an initial detection of problems that can exist on the module level. In addition, any module can be replaced without affecting the operation of any other module, including the other module of a fault-tolerant pair. Replacement of modules is similar to installation, which is described in the System Equipment Installation. 7.6 Discussion Exchange of ideas with trainees to talk about their own experiences and comments about maintenance related to the text in this chapter. 94
  • 96. Distributed Control System and Programmable Logic Control Chapter 8 Power Distribution 8.1 Learning objectives • Understand power distribution of control systems. 8.2 Power Connections Main power consists of primary and secondary power. Note the voltage and main power distribution requirements for each enclosure before you connect main power. The power should be connected through an uninterruptible power supply. 8.3 Connection Procedure To connect the power lines proceed as follows: 1. Switch off main system power. 2. Open the right side door of the enclosure to access the junction boxes. (Two junction boxes are located in the field termination area.) 3. Place the junction box power switches in the OFF position. 4. Remove the bottom cover from each junction box. 5. Route the power lines to the junction boxes. 6. Connect the power lines. 7. Replace the junction box covers. 8. Switch ON the main system power. 95
  • 97. Distributed Control System and Programmable Logic Control 8.4 Earth Connections To make earth connections to the metal enclosures, locate one of the tapped holes along the bottom interior of the enclosure (see Figure 8.1). Use a ring type solderless crimp connector appropriate for the size of wire used, and use a star-type lock washer between the connector and the enclosure chassis. Figure 8.1 Metal Enclosures, Earth Connection 8.5 Discussion Discuss power distribution schemes. 96
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  • 99. Distributed Control System and Programmable Logic Control Chapter 9 PLC Fundamentals 9.1 Learning objectives • Know general PLC issues • Understand the operation of a PLC • Understand the different types of inputs and outputs. 9.2 Introduction Control engineering has evolved over time. In the past humans were the main methods for controlling a system. More recently electricity has been used for control and early electrical control was based on relays. These relays allow power to be switched on and off without a mechanical switch. It is common to use relays to make simple logical control decisions. The development of low cost computer has brought the most recent revolution, the Programmable Logic Controller (PLC). The advent of the PLC began in the 1970s, and has become the most common choice for manufacturing controls. PLCs have been gaining popularity on the factory floor and will probably remain predominant for some time to come. Most of this is because of the advantages they offer. • Cost effective for controlling complex systems. • Flexible and can be reapplied to control other systems quickly and easily. • Computational abilities allow more sophisticated control. • Trouble shooting aids make programming easier and reduce downtime. 98
  • 100. Distributed Control System and Programmable Logic Control • Reliable components make these likely to operate for years before failure. 9.3 Hardware Many PLC configurations are available, even from a single vendor. But, in each of these there are common components and concepts. The most essential components are: • Power Supply - This can be built into the PLC or be an external unit. Common voltage levels required by the PLC (with and without the power supply) are 24Vdc, 120Vac, 220Vac. • CPU (Central Processing Unit) - This is a computer where ladder logic is stored and processed. • I/O (Input/Output) - A number of input/output terminals must be provided so that the PLC can monitor the process and initiate actions. • Indicator lights - These indicate the status of the PLC including power on, program running, and a fault. These are essential when diagnosing problems. The configuration of the PLC refers to the packaging of the components. Typical configurations are listed below from largest to smallest as shown in Figure 9.1. • Rack - A rack is often large (up to 18” by 30” by 10”) and can hold multiple cards. When necessary, multiple racks can be connected together. These tend to be the highest cost, but also the most flexible and easy to maintain. • Mini - These are similar in function to PLC racks, but about half the size. 99
  • 101. Distributed Control System and Programmable Logic Control • Shoebox - A compact, all-in-one unit (about the size of a shoebox) that has limited expansion capabilities. Lower cost, and compactness make these ideal for small applications. • Micro - These units can be as small as a deck of cards. They tend to have fixed quantities of I/O and limited abilities, but costs will be the lowest. • Software - A software based PLC requires a computer with an interface card, but allows the PLC to be connected to sensors and other PLCs across a network. Figure 9.1 Typical configuration of PLC 9.4 Inputs And Outputs Inputs to, and outputs from, a PLC are necessary to monitor and control a process. Both inputs and outputs can be categorized into two basic types: logical or continuous. Consider the example of a light bulb. If it can only be turned on or off, it is logical control. If the light can be dimmed to different levels, it is continuous. Continuous values seem more intuitive, but logical values are preferred because they allow more certainty, and simplify control. As a result most controls applications (and PLCs) use logical inputs and outputs for most applications. Hence, we will discuss logical I/O and leave continuous I/O for later. 100
  • 102. Distributed Control System and Programmable Logic Control Outputs to actuators allow a PLC to cause something to happen in a process. A short list of popular actuators is given below in order of relative popularity. • Solenoid Valves - logical outputs that can switch a hydraulic or pneumatic flow. • Lights - logical outputs that can often be powered directly from PLC output boards. • Motor Starters - motors often draw a large amount of current when started, so they require motor starters, which are basically large relays. • Servo Motors - a continuous output from the PLC can command a variable speed or position. Outputs from PLCs are often relays, but they can also be solid state electronics such as transistors for DC outputs or Triacs for AC outputs. Continuous outputs require special output cards with digital to analog converters. Inputs come from sensors that translate physical phenomena into electrical signals. Typical examples of sensors are listed below in relative order of popularity. • Proximity Switches - use inductance, capacitance or light to detect an object logically. • Switches - mechanical mechanisms will open or close electrical contacts for a logical signal. • Potentiometer - measures angular positions continuously, using resistance. • LVDT (linear variable differential transformer) - measures linear displacement continuously using magnetic coupling. 101
  • 103. Distributed Control System and Programmable Logic Control Inputs for a PLC come in a few basic varieties, the simplest are AC and DC inputs. Sourcing and sinking inputs are also popular. This output method dictates that a device does not supply any power. Instead, the device only switches current on or off, like a simple switch. • Sinking - When active the output allows current to flow to a common ground. This is best selected when different voltages are supplied. • Sourcing - When active, current flows from a supply, through the output device and to ground. This method is best used when all devices use a single supply voltage. This is also referred to as NPN (sinking) and PNP (sourcing). PNP is more popular. 9.5 Operation Sequence All PLCs have four basic stages of operations that are repeated many times per second. Initially when turned on the first time it will check its own hardware and software for faults. If there are no problems it will copy all the input and copy their values into memory, this is called the input scan. Using only the memory copy of the inputs the ladder logic program will be solved once, this is called the logic scan. While solving the ladder logic the output values are only changed in temporary memory. When the ladder scan is done the outputs will updated using the temporary values in memory, this is called the output scan. The PLC now restarts the process by starting a self check for faults. This process typically repeats 10 to 100 times per second as is shown in Figure 9.2. 102
  • 104. Distributed Control System and Programmable Logic Control Figure 9.2 PLC Scan • Self test - Checks to see if all cards error free, reset watch-dog timer, etc. (A watchdog timer will cause an error, and shut down the PLC if not reset within a short period of time - this would indicate that the ladder logic is not being scanned normally). • Input scan - Reads input values from the chips in the input cards, and copies their values to memory. This makes the PLC operation faster, and avoids cases where an input changes from the start to the end of the program (e.g., an emergency stop). There are special PLC functions that read the inputs directly, and avoid the input tables. • Logic solve/scan - Based on the input table in memory, the program is executed 1 step at a time, and outputs are updated. This is the focus of the later sections. • Output scan - The output table is copied from memory to the output chips. These chips then drive the output devices. The input and output scans often confuse the beginner, but they are important. The input scan takes a snapshot of the inputs, and solves the logic. This prevents potential problems that might occur if an input that is used in multiple places in the ladder logic program changed while half ways through a ladder scan and thus changing the behaviors of half of the ladder logic program. This problem could have severe effects on complex programs. One side effect of the input scan is that if a change in input is too short in duration, it might fall between input scans and be missed. 103
  • 105. Distributed Control System and Programmable Logic Control When the PLC is initially turned on the normal outputs will be turned off. This does not affect the values of the inputs. 9.5.1 The Input and Output Scans When the inputs to the PLC are scanned the physical input values are copied into memory. When the outputs to a PLC are scanned they are copied from memory to the physical outputs. When the ladder logic is scanned it uses the values in memory, not the actual input or output values. The primary reason for doing this is so that if a program uses an input value in multiple places, a change in the input value will not invalidate the logic. Also, if output bits were changed as each bit was changed, instead of all at once at the end of the scan the PLC would operate much slower 9.5.2 The Logic Scan Ladder logic programs are modeled after relay logic. In relay logic each element in the ladder will switch as quickly as possible. But in a program elements can only be examines one at a time in a fixed sequence. The ladder logic will be interpreted left-to-right, top-to-bottom. The ladder logic scan begins at the top rung. At the end of the rung it interprets the top output first, and then the output branched below it. On the second rung it solves branches, before moving along the ladder logic rung. 9.5.3 PLC Status The lack of keyboard and other input-output devices is very noticeable on a PLC. On the front of the PLC there are normally limited status lights. Common lights indicate; • Power on - this will be on whenever the PLC has power. 104
  • 106. Distributed Control System and Programmable Logic Control • Program running - this will often indicate if a program is running, or if no program is running. • Fault - this will indicate when the PLC has experienced a major hardware or software problem. These lights are normally used for debugging. Limited buttons will als o be provided for PLC hardware. The most common will be a run/program switch that will be switched to program when maintenance is being conducted, and back to run when in production. This switch normally requires a key to keep unauthorized personnel from altering the PLC program or stopping execution. A PLC will almost never have an on-off switch or reset button on the front. This needs to be designed into the remainder of the system. 9.6 Role Play Conduct role plays for: 1. Introduce PLC and benefits. 2. Describe PLC hardware. 3. Introduce various inputs and outputs. 4. Describe PLC scan sequence. 105
  • 107. Distributed Control System and Programmable Logic Control Chapter 10 Ladder Logic and SFC 10.1 Learning objectives • To be able to write simple ladder logic programs • Understand basic functions for calculations and comparisons. • Be able to develop SFCs, sequential flow charts, for a process. 10.2 Ladder Logic Ladder logic is the main programming method used for PLCs. As mentioned before, ladder logic has been developed to mimic relay logic. Relays are used to let one power source close a switch for another (often high current) power source, while keeping them isolated. An example of a relay in a simple control application is shown in Figure 12.1. In this system the first relay on the left is used as normally closed, and will allow current to flow until a voltage is applied to the input A. The second relay is normally open and will not allow current to flow until a voltage is applied to the input B. If current is flowing through the first two relays then current will flow through the coil in the third relay, and close the switch for output C. This circuit would normally be drawn in the ladder logic form. This can be read logically as C will be on if A is off and B is on. 106
  • 108. Distributed Control System and Programmable Logic Control Figure 10.1 Simple Relay Control. The example in Figure 10.1 does not show the entire control system, but only the logic. When we consider a PLC there are inputs, outputs, and the logic. Figure 10.2 shows a more complete representation of the PLC. Here there are two inputs from push buttons. We can imagine the inputs as activating 24V DC relay coils in the PLC. This in turn drives an output relay that switches 115V AC, which will turn on a light. Note, in actual PLCs inputs are never relays, but outputs are often relays. The ladder logic in the PLC is actually a computer program that the user can enter and change. Notice that both of the input push buttons are normally open, but the ladder logic inside the PLC has one normally open contact, and one normally closed contact. Do not think that the ladder logic in the PLC needs to match the inputs or outputs. Many beginners will get caught trying to make the ladder logic match the input types. Figure 10.2 PLC with Relays. 107
  • 109. Distributed Control System and Programmable Logic Control Many relays also have multiple outputs (throws) and this allows an output relay to also be an input simultaneously. The circuit shown in Figure 10.3 is an example of this; it is called a seal in circuit or latch circuit. In this circuit the current can flow through either branch of the circuit, through the contacts labeled A or B. The input B will only be on when the output B is on. If B is off, and A is energized, then B will turn on. If B turns on then the input B will turn on and keep output B on even if input A goes off. After B is turned on the output B will not turn off. Figure 10.3 Latch circuit 10.2.1 Ladder Logic Inputs PLC inputs are easily represented in ladder logic. Below there are two types of inputs shown, normally open and normally closed inputs. 10.2.2 Ladder Logic Outputs In ladder logic there are multiple types of outputs, but these are not consistently available on all PLCs. Some of the outputs will be externally connected to devices outside the PLC, but it is also possible to use 108
  • 110. Distributed Control System and Programmable Logic Control internal memory locations in the PLC. Five types of outputs are shown below. The first is a normal output, when energized the output will turn on, and energize an output. The circle with a diagonal line through is a normally on output, when energized the output will turn off. This type of output is not available on all PLC types. When initially energized the OSR (One Shot Relay) instruction will turn on for one scan, but then be off for all scans after, until it is turned off. The L (latch) and U (unlatch) instructions can be used to lock outputs on. When an L output is energized the output will turn on indefinitely, even when the output coil is deenergized. The output can only be turned off using a U output. 10.2.3 Programming The first PLCs were programmed with a technique that was based on relay logic wiring schematics. This eliminated the need to teach the electricians, technicians and engineers how to program a computer - but, this method has stuck and it is the most common technique for programming PLCs today. An example of ladder logic can be seen in Figure 10.4. To interpret this diagram, imagine that the power is on the 109
  • 111. Distributed Control System and Programmable Logic Control vertical line on the left hand side, we call this the hot rail. On the right hand side is the neutral rail. In the figure there are two rungs, and on each rung there are combinations of inputs (two vertical lines) and outputs (circles). If the inputs are opened or closed in the right combination the power can flow from the hot rail, through the inputs, to power the outputs, and finally to the neutral rail. An input can come from a sensor, switch, or any other type of sensor. An output will be some device outside the PLC that is switched on or off, such as lights or motors. In the top rung the contacts are normally open and normally closed. This means if input A is on and input B is off, then power will flow through the output and activate it. Any other combination of input values will result in the output X being off. Figure 10.4 Simple Ladder Logic Diagram The second rung of Figure 10.4 is more complex, there are actually multiple combinations of inputs that will result in the output Y turning on. On the left most part of the rung, power could flow through the top if C is off and D is on. Power could also (and simultaneously) flow through the bottom if both E and F are true. This would get power half way across the rung, and then if G or H is true the power will be delivered to output Y. 110
  • 112. Distributed Control System and Programmable Logic Control 10.2.4 Move Functions The simple MOV will take a value from one location in memory and place it in another memory location. Examples of the basic MOV are given in Figure 10.5. When A is true the MOV function moves a floating point number from the source to the destination address. Figure 10.5 MOV function 10.2.5 Mathematical Functions Mathematical functions will retrieve one or more values, perform an operation and store the result in memory. Figure 10.6 shows an ADD function that will retrieve values from N7:4 and F8:35, convert them both to the type of the destination address, add the floating point numbers, and store the result in F8:36. The function has two sources labelled source A and source B. Figure 10.6 Mathematical Functions 111
  • 113. Distributed Control System and Programmable Logic Control 10.2.6 Block Operations A basic block function is shown in Figure 10.7. This COP (copy) function will copy an array of 10 values starting at N7:50 to N7:40. Figure 10.7 Copy Function 10.2.7 Comparison of Values Comparison functions are shown in Figure 10.8. Previous function blocks were outputs, these replace input contacts. The example shows an EQU (equal) function that compares two floating point numbers. If the numbers are equal, the output bit B3:5/1 is true, otherwise it is false. Figure 10.8 Comparison Functions 10.2.8 Boolean Functions Figure 10.9 shows Boolean algebra functions. The function shown will obtain data words from bit memory, perform an AND operation, and store the results in a new location in bit memory. These functions are all oriented to word level operations. The ability to perform Boolean operations allows logical operations on more than a single bit. Figure 10.9 Boolean Functions 112
  • 114. Distributed Control System and Programmable Logic Control 10.3 Sequential Flow Charts Sequential Function Charts (SFCs) have been developed to accommodate the programming of more advanced systems. These are similar to flowcharts, but much more powerful. The example seen in Figure 10.10 is doing two different things. To read the chart, start at the top where is says start. Below this there is the double horizontal line that says follow both paths. As a result the PLC will start to follow the branch on the left and right hand sides separately and simultaneously. On the left there are two functions the first one is the power up function. This function will run until it decides it is done, and the power down function will come after. On the right hand side is the flash function; this will run until it is done. These functions look unexplained, but each function, such as power up will be a small ladder logic program. This method is much different from flowcharts because it does not have to follow a single path through the flowchart. Figure 10.10 SFC Simple example 113
  • 115. Distributed Control System and Programmable Logic Control The basic elements of an SFC diagram are shown in Figure 10.11. Figure 10.11 Basic Elements of SFC 114
  • 116. Distributed Control System and Programmable Logic Control A simple SFC for controlling a stamping press is shown in Figure 10.12. (Note: this controller only has a single thread of execution, so it could also be implemented with state diagrams, flowcharts, or other methods.) In the diagram the press starts in an idle state. When an automatic button is pushed the press will turn on the press power and lights. When a part is detected the press ram will advance down to the bottom limit switch. The press will then retract the ram until the top limit switch is contacted, and the ram will be stopped. A stop button can stop the press only when it is advancing. (Note: normal designs require that stops work all the time.) When the press is stopped a reset button must be pushed before the automatic button can be pushed again. After step 6 the press will wait until the part is not present before waiting for the next part. Without this logic the press would cycle continuously. Figure 10.12 SFC for Controlling a Stamping Press 115
  • 117. Distributed Control System and Programmable Logic Control 10.4 Case Study Each Trainee should try to develop the following: 1. Ladder Logic for pump operation connected to the suction of a tank where two level switches are available for automatic operation and two push buttons are for start and stop. 2. SFC for loading three tanks through different valve. Tank 1 is load first, and then tanks 2 and three are loaded simultaneously. If the pressure switch on pump discharge line is alarming then tank 2 stops loading from pump and tank 1 would transfer to tank through different line. Tank 3 continues to load from pump. T1 T2 T3 PSL . 116
  • 118. Distributed Control System and Programmable Logic Control 117
  • 119. Distributed Control System and Programmable Logic Control Appendix A Electrical Relay Diagram And P&ID Symbols 118
  • 120. Distributed Control System and Programmable Logic Control 119
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  • 122. Distributed Control System and Programmable Logic Control Appendix B Serial Communication B.1 Introduction Multiple control systems will be used for complex processes. These control systems may be PLCs, but other controllers include robots, data terminals and computers. For these controllers to work together, they must communicate. This chapter will discuss communication techniques between computers, and how these apply to PLCs. The simplest form of communication is a direct connection between two computers. A network will simultaneously connect a large number of computers on a network. Data can be transmitted one bit at a time in series, this is called serial communication. Data bits can also be sent in parallel. The transmission rate will often be limited to some maximum value, from a few bits per second, to billions of bits per second. The communications often have limited distances, from a few feet to thousands of miles/kilometers. Data communications have evolved from the 1800’s when telegraph machines were used to transmit simple messages using Morse code. This process was automated with teletype machines that allowed a user to type a message at one terminal, and the results would be printed on a remote terminal. Meanwhile, the telephone system began to emerge as a large network for interconnecting users. In the late 1950s Bell Telephone introduced data communication networks, and Texaco began to use remote monitoring and control to automate a polymerization plant. By the 1960s data communications and the phone system were being used together. In the late 1960s and 1970s modern data communications techniques were developed. This included the early version of the 121
  • 123. Distributed Control System and Programmable Logic Control Internet, called ARPAnet. Before the 1980s the most common computer configuration was a centralized mainframe computer with remote data terminals, connected with serial data line. In the 1980s the personal computer began to displace the central computer. As a result, high speed networks are now displacing the dedicated serial connections. Serial communications and networks are both very important in modern control applications. An example of a networked control system is shown in Figure B.1. The computer and PLC are connected with an RS-232 (serial data) connection. This connection can only connect two devices. Devicenet is used by the Computer to communicate with various actuators and sensors. Devicenet can support up to 63 actuators and sensors. The PLC inputs and outputs are connected as normal to the process. Figure B.1 Communication example B.2 Serial Communication Serial communications send a single bit at a time between computers. This only requires a single communication channel, as opposed to 8 channels to send a byte. With only one channel the costs are lower, but the communication rates are slower. The communication 122
  • 124. Distributed Control System and Programmable Logic Control channels are often wire based, but they may also be can be optical and radio. Figure B.2 shows some of the standard electrical connections. RS- 232c is the most common standard that is based on a voltage change levels. At the sending computer an input will either be true or false. The line driver will convert a false value in to a Txd voltage between +3V to +15V, true will be between -3V to -15V. A cable connects the Txd and com on the sending computer to the Rxd and com inputs on the receiving computer. The receiver converts the positive and negative voltages back to logic voltage levels in the receiving computer. The cable length is limited to 50 feet to reduce the effects of electrical noise. When RS-232 is used on the factory floor, care is required to reduce the effects of electrical noise - careful grounding and shielded cables are often used. Figure B.2 Serial data standard 123
  • 125. Distributed Control System and Programmable Logic Control The RS-422a cable uses a 20 mA current loop instead of voltage levels. This makes the systems more immune to electrical noise, so the cable can be up to 3000 feet long. The RS-423a standard uses a differential voltage level across two lines, also making the system more immune to electrical noise, thus allowing longer cables. To provide serial communication in two directions these circuits must be connected in both directions. To transmit data, the sequence of bits follows a pattern, like that shown in Figure B.3. The transmission starts at the left hand side. Each bit will be true or false for a fixed period of time, determined by the transmission speed. A typical data byte looks like the one below. The voltage/current on the line is made true or false. The width of the bits determines the possible bits per second (bps). The value shown before is used to transmit a single byte. Between bytes, and when the line is idle, the Txd is kept true, this helps the receiver detect when a sender is present. A single start bit is sent by making the Txd false. In this example the next eight bits are the transmitted data, a byte with the value 17. The data is followed by a parity bit that can be used to check the byte. In this example there are two data bits set, and even parity is being used, so the parity bit is set. The parity bit is followed by two stop bits to help separate this byte from the next one. 124
  • 126. Distributed Control System and Programmable Logic Control Figure B.3 a serial data byte Some of the byte settings are optional, such as the number of data bits (7 or 8), the parity bit (none, even or odd) and the number of stop bits (1 or 2). The sending and receiving computers must know what these settings are to properly receive and decode the data. Most computers send the data asynchronously, meaning that the data could be sent at any time, without warning. This makes the bit settings more important. Another method used to detect data errors is half-duplex and full- duplex transmission. In half-duplex transmission the data is only sent in one direction. But, in full-dup transmission a copy of any byte received is sent back to the sender to verify that it was sent and received correctly. (Note: if you type and nothing shows up on a screen or characters show up twice you may have to change the half/full duplex setting.) 125
  • 127. Distributed Control System and Programmable Logic Control The transmission speed is the maximum number of bits that can be sent per second. The units for this are baud. The baud rate includes the start, parity and stop bits. For example a 9600 baud transmission of the data in Figure B.3 would transfer up to 800 bytes each second. Lower baud rates are 120, 300, 1.2K, 2.4K and 9.6K. Higher speeds are 19.2K, 28.8K and 33.3K. (Note: When this is set improperly you will get many transmission errors, or garbage on your screen.) Serial lines have become one of the most common methods for transmitting data to instruments: most personal computers have two serial ports. The previous discussion of serial communications techniques also applies to devices such as modems. B.3 RS-232 The RS-232c standard is based on a low/false voltage between +3 to +15V, and an high/true voltage between -3 to -15V (+/-12V is commonly used). Figure B.4 shows some of the common connection schemes. In all methods the txd and rxd lines are crossed so that the sending txd outputs are into the listening rxd inputs when communicating between computers. When communicating with a communication device (modem), these lines are not crossed. In the modem connection the dsr and dtr lines are used to control the flow of data. In the computer the cts and rts lines are connected. These lines are all used for handshaking, to control the flow of data from sender to receiver. The null-modem configuration simplifies the handshaking between computers. The three wire configuration is a crude way to connect to devices, and data can be lost. 126
  • 128. Distributed Control System and Programmable Logic Control Figure B.4 Common RS-232 Connection Schemes Common connectors for serial communications are shown in Figure B.5. These connectors are either male (with pins) or female (with holes), and often use the assigned pins shown. The DB-9 connector is more common now, but the DB-25 connector is still in use. In any connection the RXD and TXD pins must be used to transmit and receive data. The COM must be connected to give a common voltage reference. All of the remaining pins are used for handshaking. 127
  • 129. Distributed Control System and Programmable Logic Control Figure B.5 Typical RS-232 Pin Assignments and Names The handshaking lines are to be used to detect the status of the sender and receiver, and to regulate the flow of data. It would be unusual for most of these pins to be connected in any one application. The most common pins are provided on the DB-9 connector, and are also described below. TXD/RXD - (transmit data, receive data) - data lines DCD - (data carrier detect) - this indicates when a remote device is present RI - (ring indicator) - this is used by modems to indicate when a connection is about to be made. CTS/RTS - (clear to send, ready to send) DSR/DTR - (data set ready, data terminal ready) these handshaking lines indicate when the remote machine is ready to receive data. COM - a common ground to provide a common reference voltage for the TXD and RXD. 128
  • 130. Distributed Control System and Programmable Logic Control Appendix C Networking C.1 Introduction A computer with a single network interface can communicate with many other computers. This economy and flexibility has made networks the interface of choice, eclipsing point-to-point methods such as RS-232. Typical advantages of networks include resource sharing and ease of communication. But, networks do require more knowledge and understanding. Small networks are often called Local Area Networks (LANs). These may connect a few hundred computers within a distance of hundreds of meters. These networks are inexpensive, often costing $100 or less per network node. Data can be transmitted at rates of millions of bits per second. Many controls system are using networks to communicate with other controllers and computers. Typical applications include; • Taking quality readings with a PLC and sending the data to a database computer. • Distributing recipes or special orders to batch processing equipment. • Remote monitoring of equipment. Larger Wide Area Networks (WANs) are used for communicating over long distances between LANs. These are not common in controls applications, but might be needed for a very large scale process. An example might be an oil pipeline control system that is spread over thousands of miles. 129
  • 131. Distributed Control System and Programmable Logic Control C.2 Topology The structure of a network is called the topology. Figure C.1 shows the basic network topologies. The Bus and Ring topologies both share the same network wire. In the Star configuration each computer has a single wire that connects it to a central hub. Figure C.1 Network Topologies In the Ring and Bus topologies the network control is distributed between all of the computers on the network. The wiring only uses a single loop or run of wire. But, because there is only one wire, the network will slow down significantly as traffic increases. This also requires more sophisticated network interfaces that can determine when a computer is allowed to transmit messages. It is also possible for a problem on the network wires to halt the entire network. The Star topology requires more wire overall to connect each computer to an intelligent hub. But, the network interfaces in the computer become simpler, and the network becomes more reliable. Another term commonly used is that it is deterministic; this means that performance can be predicted. This can be important in critical applications. 130
  • 132. Distributed Control System and Programmable Logic Control For a factory environment the bus topology is popular. The large number of wires required for a star configuration can be expensive and confusing. The loop of wire required for a ring topology is also difficult to connect, and it can lead to ground loop problems. Figure C.2 shows a tree topology that is constructed out of smaller bus networks. Repeaters are used to boost the signal strength and allow the network to be larger. Figure C.2 The Tree Topology C.3 OSI Network Model The Open System Interconnection (OSI) model in Figure C.3 was developed as a tool to describe the various hardware and software parts found in a network system. It is most useful for educational purposes, and explaining the things that should happen for a successful network application. The model contains seven layers, with the hardware at the bottom, and the software at the top. The darkened arrow shows that a 131
  • 133. Distributed Control System and Programmable Logic Control message originating in an application program in computer #1 must travel through all of the layers in both computers to arrive at the application in computer #2. This could be part of the process of reading email. Figure C.3 The OSI Network Model Application - This is high level software on the computer. Presentation - Translates application requests into network operations. Session - This deals with multiple interactions between computers. Transport - Breaks up and recombines data to small packets. Network - Network addresses and routing added to make frame. Data Link - The encryption for many bits, including error correction added to a frame. Physical - The voltage and timing for a single bit in a frame. Interconnecting Medium - (not part of the standard) The wires or transmission medium of the network. 132
  • 134. Distributed Control System and Programmable Logic Control The Physical layer describes items such as voltage levels and timing for the transmission of single bits. The Data Link layer deals with sending a small amount of data, such as a byte, and error correction. Together, these two layers would describe the serial byte shown in the previous chapter. The Network layer determines how to move the message through the network. If this were for an internet connection this layer would be responsible for adding the correct network address. The Transport layer will divide small amounts of data into smaller packets, or recombine them into one larger piece. This layer also checks for data integrity, often with a checksum. The Session layer will deal with issues that go beyond a single block of data. In particular it will deal with resuming transmission if it is interrupted or corrupted. The Session layer will often make long term connections to the remote machine. The Presentation layer acts as an application interface so that syntax, formats and codes are consistent between the two networked machines. For example this might convert ’’ to ’/’ in HTML files. This layer also provides subroutines that the user may call to access network functions, and perform functions such as encryption and compression. The Application layer is where the user program resides. On a computer this might be a web browser, or a ladder logic program on a PLC. Most products can be described with only a couple of layers. Some networking products may omit layers in the model. C.4 Networking Hardware The following is a description of most of the hardware that will be needed in the design of networks. • Computer - (or network enabled equipment) 133
  • 135. Distributed Control System and Programmable Logic Control • Network Interface Hardware - The network interface may already be built into the computer/PLC/sensor/etc. These may cost $15 to over $1000. • The Media - The physical network connection between network nodes. 10baseT (twisted pair) is the most popular. It is a pair of twisted copper wires terminated with an RJ-45 connector. 10base2 (thin wire) is thin shielded coaxial cable with BNC connectors. 10baseF (fiber optic) is costly, but signal transmission and noise properties are very good. • Repeaters (Physical Layer) - These accept signals and retransmit them so that longer networks can be built. • Hub/Concentrator - A central connection point that network wires will be connected to. It will pass network packets to local computers or to remote networks if they are available. • Router (Network Layer) - Will isolate different networks, but redirect traffic to other LANs. • Bridges (Data link layer) - These are intelligent devices that can convert data on one type of network, to data on another type of network. These can also be used to isolate two networks. • Gateway (Application Layer) - A Gateway is a full computer that will direct traffic to different networks, and possibly screen packets. These are often used to create firewalls for security. Figure C.4 and C.5 shows the basic OSI model equivalents for some of the networking hardware described before. 134
  • 136. Distributed Control System and Programmable Logic Control Figure C.4 Network devices and the OSI model Figure C.5 The OSI network model with a router 135
  • 137. Distributed Control System and Programmable Logic Control Appendix D Software Engineering D.1 Introduction A careful, structured approach to designing software will cut the total development time, and result in a more reliable system. D.2 Fail Safe Design It is necessary to predict how systems will fail. Some of the common problems that will occur are listed below. Component jams - An actuator or part becomes jammed. This can be detected by adding sensors for actuator positions and part presence. Operator detected failure - Some unexpected failures will be detected by the operator. In those cases the operator must be able to shut down the machine easily. • Erroneous input - An input could be triggered unintentionally. This could include something falling against a start button. • Unsafe modes - Some systems need to be entered by the operators or maintenance crew. People detectors can be used to prevent operation while people are present. • Programming errors - A large program that is poorly written can behave erratically when an unanticipated input is encountered. This is also a problem with assumed startup conditions. 136
  • 138. Distributed Control System and Programmable Logic Control • Sabotage - For various reasons, some individuals may try to damage a system. These problems can be minimized preventing access. • Random failure - Each component is prone to random failure. It is worth considering what would happen if any of these components were to fail. Some design rules that will help improve the safety of a system are listed below. Programs • A fail-safe design - Programs should be designed so that they check for problems, and shut down in safe ways. Most PLC’s also have imminent power failure sensors; use these whenever danger is present to shut down the system safely. • Proper programming techniques and modular programming will help detect possible problems on paper instead of in operation. • Modular well designed programs. • Use predictable, non-configured programs. • Make the program inaccessible to unauthorized persons. • Check for system OK at start-up. • Use PLC built in functions for error and failure detection. People • Provide clear and current documentation for maintenance and operators. • Provide training for new users and engineers to reduce careless and uninformed mistakes. 137
  • 139. Distributed Control System and Programmable Logic Control D.3 Debugging Most engineers have taken a programming course where they learned to write a program and then debug it. Debugging involves running the program, testing it for errors, and then fixing them. Even for an experienced programmer it is common to spend more time debugging than writing software. For PLCs this is not acceptable! If you are running the program and it is operating irrationally it will often damage hardware. Also, if the error is not obvious, you should go back and reexamine the program design. When a program is debugged by trial and error, there are probably errors remaining in the logic, and the program is very hard to trust. Remember, a bug in a PLC program might kill somebody. D.4 Troubleshooting After a system is in operation it will eventually fail. When a failure occurs it is important to be able to identify and solve problems quickly. The following list of steps will help track down errors in a PLC system. Look at the process and see if it is in a normal state. i.e. no jammed actuators, broken parts, etc. If there are visible problems, fix them and restart the process. 1. Look at the PLC to see which error lights are on. Each PLC vendor will provide documents that indicate which problems correspond to the error lights. Common error lights are given below. If any off the warning lights are on, look for electrical supply problems to the PLC. a. HALT - something has stopped the CPU b. RUN - the PLC thinks it is OK (and probably is) c. ERROR - a physical problem has occurred with the PLC 138
  • 140. Distributed Control System and Programmable Logic Control 2. Check indicator lights on I/O cards, see if they match the system. i.e., look at sensors that are on/off, and actuators on/off, check to see that the lights on the PLC I/O cards agree. If any of the light disagrees with the physical reality, then interface electronics/mechanics need inspection. 3. Consult the manuals, or use software if available. If no obvious problems exist the problem is not simple, and requires a technically skilled approach. 4. If all else fails call the vendor (or the contractor) for help. D.5 Forcing Most PLCs will allow a user to force inputs and outputs. This means that they can be turned on, regardless of the physical inputs and program results. This can be convenient for debugging programs, and, it makes it easy to break and destroy things! When forces are used they can make the program perform erratically. They can also make outputs occur out of sequence. If there is a logic problem, then these don’t help a programmer identify these problems. 139
  • 141. Distributed Control System and Programmable Logic Control References 1. I/A series Foxboro documentation. 2. HoneyWell Experion process knowledge system, "Honeywell Training.ppt" 3. "Automation Hierarchy", By: Prof. Dr. H. Kirrmann, ABB Research Center, Baden, Switzerland, " AI_14_Hierarchy.ppt" 4. http://newton.ex.ac.uk , By: C.D.H. Williams 5. "Electrical Relay Diagram And P&ID Symbols", From Industrial Text and Video Co. The Leader in Electrical, Motor Control and PLCs Video Training Programs (www.industrialtext.com). 6. "A PLC Primer", " www.industrialtext.com". 7. "Automating Manufacturing Systems with PLCs", By:" Hugh Jack" (jackh@gvsu.edu). 8. "Regulatory and Advanced regulatory control system development", By: Harold L. Wade, Instrumentation society of America. 9. Rosemount Measurement Catalog. 140
  • 142. Distributed Control System and Programmable Logic Control References 1. I/A series Foxboro documentation. 2. HoneyWell Experion process knowledge system, "Honeywell Training.ppt" 3. "Automation Hierarchy", By: Prof. Dr. H. Kirrmann, ABB Research Center, Baden, Switzerland, " AI_14_Hierarchy.ppt" 4. http://newton.ex.ac.uk , By: C.D.H. Williams 5. "Electrical Relay Diagram And P&ID Symbols", From Industrial Text and Video Co. The Leader in Electrical, Motor Control and PLCs Video Training Programs (www.industrialtext.com). 6. "A PLC Primer", " www.industrialtext.com". 7. "Automating Manufacturing Systems with PLCs", By:" Hugh Jack" (jackh@gvsu.edu). 8. "Regulatory and Advanced regulatory control system development", By: Harold L. Wade, Instrumentation society of America. 9. Rosemount Measurement Catalog. 140
  • 143. Distributed Control System and Programmable Logic Control References 1. I/A series Foxboro documentation. 2. HoneyWell Experion process knowledge system, "Honeywell Training.ppt" 3. "Automation Hierarchy", By: Prof. Dr. H. Kirrmann, ABB Research Center, Baden, Switzerland, " AI_14_Hierarchy.ppt" 4. http://newton.ex.ac.uk , By: C.D.H. Williams 5. "Electrical Relay Diagram And P&ID Symbols", From Industrial Text and Video Co. The Leader in Electrical, Motor Control and PLCs Video Training Programs (www.industrialtext.com). 6. "A PLC Primer", " www.industrialtext.com". 7. "Automating Manufacturing Systems with PLCs", By:" Hugh Jack" (jackh@gvsu.edu). 8. "Regulatory and Advanced regulatory control system development", By: Harold L. Wade, Instrumentation society of America. 9. Rosemount Measurement Catalog. 140