DCS (Distributed Control System)INTRODUCTION Digital Control Systems(DCS) also known as Distributed Control System is the brain of the control system. It is generally, since the 1970s, digital, and normally consists of field instruments, connected via wiring to computer buses or electrical buses to multiplexer/de multiplexers and A/Ds or analog to digital and finally the Human-Machine Interface (HMI) or control consoles. A DCS is a process control system that uses a network to interconnect sensors, controllers, operator terminals and actuators. A DCS typically contains one or more computers for control and mostly use both proprietary interconnections and protocols for communications. Control Systems are collectively named as "ICSS" Integrated Control and Safety System. Distinctly identified as "BPCS" Basic Process Control System. "SIS" Safety Instrumentation System. "F&G" Fire and Gas System. DCS is employed in BPCS as well as used and prevalent control system.
DCS (Cont..)The DCS is a control system which collects the data from the field and decides what to do with them. Data from the field can either be stored for future reference, used for simple process control, use in conjunction with data from another part of the plant for advanced control strategies.A distributed control system (DCS) is part of a manufacturing system.Distributed control systems (DCS) are used in industrial and civil engineering applications to monitor and control distributed equipment with remote human intervention.
What must be in the DCS for it tobe able to do so much? Operator Console These are like the monitors of our computers. They provide us with the feedback of what they are doing in the plant as well as the command we issue to the control system. These are also the places where operators issue commands to the field instruments. Engineering Station These are stations for engineers to configure the system and also to implement control algorithms. History Module This is like the hard disk of our PCs. They store the configurations of the DCS as well as the configurations of all the points in the plant. They also store the graphic files that are shown in the console and in most systems these days they are able to store some plant operating data.
Cont.. Data Historian These are usually extra pieces of software that are dedicated to store process variables, set points and output values. They are usually of higher scanning rates than that available in the history module. Control Modules These are like the brains of the DCS. Specially customized blocks are found here. These are customized to do control functions like PID control, ratio control, simple arithmetic and dynamic compensation. These days, advanced control features can also be found in them. I/O These manage the input and output of the DCS. Input and output can be digital or analogues. Digital I/Os are those like on/off, start/stop signals. Most of the process measurements and controller outputs are considered analogue. These are the points where the field instruments are hard-wired to. All above mentioned elements are connected by using a network, nowadays very often used is Ethernet.
How does a DCS work?In the field you have sensors and gauges that give and receive information. They convert this information into a electric signal that is sent to a control room somewhere in the field. This control room has programmed logic that is able to converts the signal into a pressure, flow rate, concentration, temperature, or level. This logic also contains the information that controls the process and takes the signal compares it with the set point sent from the operator may or may not be in the field and sends a signal to the manipulated variables in the field. The DCS covers all of the computer logic from the operator screen to the field box that contain the logic.
Shutdown systemsShutdown system are the emergency setting of the logic to make sure the process can be contained and is environmentally safe. These setting are important for emergency response of the system. It is the job of the DCS to contain the logic for the shutdown system and be able to operate when a process exceed a certain limit.
DCS APPLICATIONSDCS is a very broad term that describes solutions across a large variety of industries, including: * Electrical power grids and electrical generation plants * Environmental control systems * Traffic signals * Water management systems * Refining and chemical plants * Pharmaceutical manufacturing.
SCADA (Supervisory Control And DataAcquisition)INTRODUCTION As the name indicates, it is not a full control system, but rather focuses on the supervisory level. As such, it is a purely software package that is positioned on top of hardware to which it is interfaced, in general via Programmable Logic Controllers (PLCs), or other commercial hardware modules. In reality, the primary purpose of SCADA is to monitor, control and alarm plant or regional operating systems from a central location. While override control is possible, it is infrequently utilized; however control set points are quite regularly changed by SCADA. SCADA systems have made substantial progress over the recent years in terms of functionality, scalability, performance and openness such that they are an alternative to in house development even for very demanding and complex control systems.
Cont..A SCADA application has two elements: The process/system/machinery you want to monitor a control - this can be a power plant, a water system, a network, a system of traffic lights, or anything else. A network of intelligent devices that interfaces with the first system through sensors and control outputs. This network, which is the SCADA system, gives you the ability to measure and control specific elements of the first system.You can build a SCADA system using several different kinds of technologies and protocols. This white paper will help you evaluate your options and decide what kind of SCADA system is best for your needs.
Where is SCADA Used? You can use SCADA to manage any kind of equipment. Typically, SCADA systems are used to automate complex industrial processes where human control is impractical - systems where there are more control factors, and more fast-moving control factors, than human beings can comfortably manage. Around the world, SCADA systems control: • Electric power generation, transmission and distribution: Electric utilities use SCADA systems to detect current flow and line voltage, to monitor the operation of circuit breakers, and to take sections of the power grid online or offline. • Water and sewage: State and municipal water utilities use SCADA to monitor and regulate water flow, reservoir levels, pipe pressure and other factors. • Buildings, facilities and environments: Facility managers use SCADA to control HVAC, refrigeration units, lighting and entry systems.
Cont.. • Manufacturing: SCADA systems manage parts inventories for just-in-time manufacturing, regulate industrial automation and robots, and monitor process and quality control. • Mass transit: Transit authorities use SCADA to regulate electricity to subways, trams and trolley buses; to automate traffic signals for rail systems; to track and locate trains and buses; and to control railroad crossing gates. • Traffic signals: SCADA regulates traffic lights, controls traffic flow and detects out-of-order signals. As Im sure you can imagine, this very short list barely hints at all the potential applications for SCADA systems. SCADA is used in nearly every industry and public infrastructure project - anywhere where automation increases efficiency. Whats more, these examples dont show how deep and complex SCADA data can be. In every industry, managers need to control multiple factors and the interactions between those factors. SCADA systems provide the sensing capabilities and the computational power to track everything thats relevant to your operations.
Cont..SCADA systems are used not only in industrial processes: e.g. steel making, power generation (conventional and nuclear) and distribution, chemistry, but also in some experimental facilities such as nuclear fusion. The size of such plants range from a few 1000 to several 10 thousands input/output (I/O) channels. However, SCADA systems evolve rapidly and are now penetrating the market of plants with a number of I/O channels of several 100 thousands I/Os.SCADA systems used to run on DOS, VMS and UNIX; in recent years all SCADA vendors have moved to NT, Windows XP, Windows Server 2003 and some also to Linux.
Cont…Hardware ArchitectureOne distinguishes two basic layers in a SCADA system: the "client layer" which caters for the man machine interaction and the "data server layer" which handles most of the process data control activities. The data servers communicate with devices in the field through process controllers. Process controllers, e.g. PLCs, are connected to the data servers either directly or via networks or field buses that are proprietary (e.g. Siemens H1), or non- proprietary (e.g. Profibus). Data servers are connected to each other and to client stations via an Ethernet LAN.
Cont…The products are multi-tasking and are based upon a real-time database (RTDB) located in one or more servers. Servers are responsible for data acquisition and handling (e.g. polling controllers, alarm checking, calculations, logging and archiving) on a set of parameters, typically those they are connected to.However, it is possible to have dedicated servers for particular tasks, e.g. historian, data logger, alarm handler. The figure above shows a generic SCADA software architecture.
How SCADA Systems Work?A SCADA system performs four functions: Data acquisition Networked data communication Data presentation Control These functions are performed by four kinds of SCADA components: Sensors (either digital or analog) and control relays that directly interface with the managed system. Remote telemetry units (RTUs). These are small computerized units deployed in the field at specific sites and locations. RTUs (Remote Telemetry Units) serve as local collection points for gathering reports from sensors and delivering commands to control relays.
Cont… SCADA master units. These are larger computer consoles that serve as the central processor for the SCADA system. Master units provide a human interface to the system and automatically regulate the managed system in response to sensor inputs. The communications network that connects the SCADA master unit to the RTUs in the field.
Cont…The simplest possible SCADA system would be a single circuit that notifies you of one event. Imagine a fabrication machine that produces widgets. Every time the machine finishes a widget, it activates a switch. The switch turns on a light on a panel, which tells a human operator that a widget has been completed.Obviously, a real SCADA system does more than this simple model. But the principle is the same. A full- scale SCADA system just monitors more stuff over greater distances.
Data Acquisition First, the systems you need to monitor are much more complex than just one machine with one output. So a real-life SCADA system needs to monitor hundreds or thousands of sensors. Some sensors measure inputs into the system (for example, water flowing into a reservoir), and some sensors measure outputs (like valve pressure as water is released from the reservoir). Some of those sensors measure simple events that can be detected by a straightforward on/off switch, called a discrete input (or digital input). For example, in our simple model of the widget fabricator, the switch that turns on the light would be a discrete input. In real life, discrete inputs are used to measure simple states, like whether equipment is on or off, or tripwire alarms, like a power failure at a critical facility. Some sensors measure more complex situations where exact measurement is important. These are analog sensors, which can detect continuous changes in a voltage or current input. Analog sensors are used to track fluid levels in tanks, voltage levels in batteries, temperature and other factors that can be measured in a continuous range of input..
Cont…For most analog factors, there is a normal range defined by a bottom and top level. For example, you may want the temperature in a server room to stay between 60 and 85 degrees Fahrenheit. If the temperature goes above or below this range, it will trigger a threshold alarm. In more advanced systems, there are four threshold alarms for analog sensors, defining Major Under, Minor Under, Minor Over and Major Over alarms.
Data CommunicationIn our simple model of the widget fabricator, the "network" is just the wire leading from the switch to the panel light. In real life, you want to be able to monitor multiple systems from a central location, so you need a communications network to transport all the data collected from your sensors.Early SCADA networks communicated over radio, modem or dedicated serial lines. Today the trend is to put SCADA data on Ethernet and IP over SONET. For security reasons, SCADA data should be kept on closed LAN/WANs without exposing sensitive data to the open Internet.Real SCADA systems dont communicate with just simple electrical signals, either. SCADA data is encoded in protocol format. Older SCADA systems depended on closed proprietary protocols, but today the trend is to open, standard protocols and protocol mediation.
Cont..Sensors and control relays are very simple electric devices that cant generate or interpret protocol communication on their own. Therefore the remote telemetry unit (RTU) is needed to provide an interface between the sensors and the SCADA network. The RTU (Remote Telemetry Unit) encodes sensor inputs into protocol format and forwards them to the SCADA master; in turn, the RTU (Remote Telemetry Unit) receives control commands in protocol format from the master and transmits electrical signals to the appropriate control relays.
Data Presentation The only display element in our model SCADA system is the light that comes on when the switch is activated. This obviously wont do on a large scale - you cant track a light board of a thousand separate lights, and you dont want to pay someone simply to watch a light board, either. A real SCADA system reports to human operators over a specialized computer that is variously called a master station, an HMI (Human-Machine Interface) or an HCI (Human-Computer Interface). The SCADA master station has several different functions. The master continuously monitors all sensors and alerts the operator when there is an "alarm" - that is, when a control factor is operating outside what is defined as its normal operation. The master presents a comprehensive view of the entire managed system, and presents more detail in response to user requests. The master also performs data processing on information gathered from sensors - it maintains report logs and summarizes historical trends. An advanced SCADA master can add a great deal of intelligence and automation to your systems management, making your job much easier.
Control Unfortunately, our miniature SCADA system monitoring the widget fabricator doesnt include any control elements. So lets add one. Lets say the human operator also has a button on his control panel. When he presses the button, it activates a switch on the widget fabricator that brings more widget parts into the fabricator. Now lets add the full computerized control of a SCADA master unit that controls the entire factory. You now have a control system that responds to inputs elsewhere in the system. If the machines that make widget parts break down, you can slow down or stop the widget fabricator. If the part fabricators are running efficiently, you can speed up the widget fabricator. If you have a sufficiently sophisticated master unit, these controls can run completely automatically, without the need for human intervention. Of course, you can still manually override the automatic controls from the master station. In real life, SCADA systems automatically regulate all kinds of industrial processes. For example, if too much pressure is building up in a gas pipeline, the SCADA system can automatically open a release valve. Electricity production can be adjusted to meet demands on the power grid. Even these real-world examples are simplified; a full-scale SCADA system can adjust the managed system in response to multiple inputs.
A Brief Note on Sensors andNetworksSensors and control relays are essentially commodity items. Yes, some sensors are better than others, but a glance at a spec sheet will tell you everything you need to know to choose between them.An IP LAN/WAN is the easiest kind of network to work with, and if you dont yet have LAN capability throughout all your facilities, transitioning to LAN is probably one of your long-term goals. But you dont have to move to LAN immediately or all at once to get the benefits of SCADA. The right SCADA system will support both your legacy network and LAN, enabling you to make a graceful, gradual transition.
What to Look for in a SCADA RTU(Remote Telemetry Unit)? Your SCADA RTUs need to communicate with all your on-site equipment and survive under the harsh conditions of an industrial environment. Heres a checklist of things you should expect from a quality RTU: Sufficient capacity to support the equipment at your site … but not more capacity than you actually will use. At every site, you want an RTU (Remote Telemetry Unit) that can support your expected growth over a reasonable period of time, but its simply wasteful to spend your budget on excess capacity that you wont use. Rugged construction and ability to withstand extremes of temperature and humidity. You know how punishing on equipment your sites can be. Keep in mind that your SCADA system needs to be the most reliable element in your facility. Secure, redundant power supply. You need your SCADA system up and working 24/7, no excuses. Your RTU (Remote Telemetry Unit) should support battery power and, ideally, two power inputs.
Cont… Redundant communication ports. Network connectivity is as important to SCADA operations as a power supply. A secondary serial port or internal modem will keep your RTU (Remote Telemetry Unit) online even if the LAN fails. Plus, RTUs with multiple communication ports easily support a LAN migration strategy. Nonvolatile memory (NVRAM) for storing software and/or firmware. NVRAM retains data even when power is lost. New firmware can be easily downloaded to NVRAM storage, often over LAN - so you can keep your RTUs capabilities up to date without excessive site visits. Intelligent control. As I noted above, sophisticated SCADA remotes can control local systems by themselves according to programmed responses to sensor inputs. This isnt necessary for every application, but it does come in handy for some users. Real-time clock for accurate date/time stamping of reports. Watchdog timer to ensure that the RTU (Remote Telemetry Unit) restarts after a power failure.
What to Look for in a SCADAMaster? Your SCADA master should display information in the most useful ways to human operators and intelligently regulated your managed systems. Heres a checklist of SCADA master must-haves: Flexible, programmable response to sensor inputs. Look for a system that provides easy tools for programming soft alarms (reports of complex events that track combinations of sensor inputs and date/time statements) and soft controls (programmed control responses to sensor inputs). 24/7, automatic pager and email notification. Theres no need to pay personnel to watch a board 24 hours a day. If equipment needs human attention, the SCADA master can automatically page or email directly to repair technicians. Detailed information display. You want a system that displays reports in plain English, with a complete description of what activity is happening and how you can manage it. Nuisance alarm filtering. Nuisance alarms desensitize your staff to alarm reports, and they start to believe that all alarms are nonessential alarms. Eventually they stop responding even to critical alarms. Look for a SCADA master that includes tools to filter out nuisance alarms.
Cont…Expansion capability. A SCADA system is a long-term investment that will last for as long as 10 to 15 years. So you need to make sure it will support your future growth for up to 15 years.Redundant, geo diverse backup. The best SCADA systems support multiple backup masters, in separate locations.. If the primary SCADA master fails, a second master on the network automatically takes over, with no interruption of monitoring and control functions.Support for multiple protocols and equipment types. Early SCADA systems were built on closed, proprietary protocols. Single-vendor solutions arent a great idea - vendors sometimes drop support for their products or even just go out of business. Support for multiple open protocols safeguards your SCADA system against unplanned obsolescence.
Why is SCADA so popular? The major attraction of SCADA to a municipality is the ability to significantly reduce operating labor costs, while at the same time actually improve plant or regional system performance and reliability. Information gathering within a plant no longer requires personnel to spend time wandering all over the site, and correspondingly the frequency of field site inspections required in a regional system can be minimized. Costly after-hours alarm call-outs can often be avoided since a SCADA system will indicate the nature and degree of a problem, while the ability to remotely control site equipment may permit an operator at home to postpone a site visit till working hours. SCADA based alarming is also very reliable since it is in-house and tied directly to process control. A significant feature of a SCADA system, often not fully appreciated, is the trending of data and nothing comes close for speed and ease of operation. When graphically displayed, accumulated operating data often will indicate a developing problem, or an area for process improvement. Reports can easily be generated from this data utilizing other common software programs. It should be appreciated that while a SCADA system is often complex to configure - it is extremely easy to operate!
What is involved?There are five phases to creating a functional SCADA system:Phase 1 The DESIGN of the system architecture. This includes all important communication system, and with a regional system utilizing radio communication often involves a radio path survey. Also involved will be any site instrumentation that is not presently in existence, but will be required to monitor desired parameters.Phase 2 The SUPPLY of RTU, communication and HMI equipment, the latter consisting of a PC system and the necessary powerful graphic and alarm software programs.
Cont…Phase 3 The PROGRAMMING of the communication equipment and the powerful HMI graphic and alarm software programs.Phase 4 The INSTALLATION of the communication equipment and the PC system. The former task is typically much more involved.Phase 5 The COMMISSIONING of the system, during which communication and HMI programming problems are solved, the system is proven to the client, operator training and system documentation is provided.
Why You Need Help With YourSCADA Implementation?Implementing an SCADA system can seem deceptively easy - you just look on the Web, find a few vendors, compare a few features, add some configuration and youre done, right?The truth is, developing a SCADA system on your own is one of the riskiest things you can do. Here are some of the typical problems you might face if you dont get expert advice when youre designing your system:1. Implementation time is drawn out: Its going to take longer than you think. Network monitoring is a highly technical subject, and you have a lot to learn if you want a successful implementation. And anytime you are trying to do something youve never done before, you are bound to make mistakes - mistakes that extend your time and your budget beyond their limits.
Cont…2. Resources are misused: If youre not fully informed about your options for systems integration, you may replace equipment that could have been integrated into your new system. Rushing into a system wide replacement when you could have integrated can cost you hundreds of thousands of dollars.3. Opportunities are missed: If you install a new SCADA system today, youre committing your company to that system for as long as 10 to 15 years. Many companies design what they think is a state-of-the-art SCADA system - and then find that their technology is actually a generation behind.
DCS vs. SCADA in ModernEnvironmentsThere is considerable confusion today about the difference between DCS ("Distributed Control Systems") and SCADA ("Site Control And Data Acquisition") systems. As you can tell from expanded acronyms above, SCADA includes "Data Acquisition" in addition to "Control". DCS, on the other hand, contains only "Control".Understanding why this difference exists requires a 15-second history lesson. Historically, when computer networks either did not yet exist or had very low bandwidth, a SCADA system was the top-level controller for many lower-level intelligent agents. It was simply impractical to have a single system controlling every minute aspect of a system. In this technical environment, DCS devices did most of the detail work and simply reported to (and took high-level orders from) the SCADA system.
Cont…Today, computer networks have become so fast that theres no practical reason for SCADA and DCS to be separate. Thats why they have blurred together into a single monitoring and control system. The choice of name - SCADA vs. DCS - largely depends on the region where you work. Some areas favor SCADA, others favor DCS. Occasionally, some people who worked with the systems before they effectively merged or who have moved from another region will use a term different than their coworkers. This again leads to confusion when new employees must learn to manage SCADA/DCS.
SUMMARY(SCADA vs. DCS) DCS is process oriented, while SCADA is data acquisition oriented.DCS is process state driven, while SCADA is event driven.DCS is commonly used to handle operations on a single locale, while SCADA is preferred for applications that are spread over a wide geographic location. DCS operator stations are always connected to its I/O, while SCADA is expected to operate despite failure of field communications.