SCADA systems are widely used in industry for Supervisory Control and Data Acquisition of industrial processes.
Functionality, scalability, performance and openness such that they are an alternative to in house development even for very demanding and complex control systems as those of physics experiments.
SCADA stands for Supervisory Control And Data Acquisition. 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.
SCADA systems are used not only in most 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.
SCADA systems used to run on DOS, VMS and UNIX; in recent years all SCADA vendors have moved to NT.
The supervisory control system is a system that is placed on top of a real-time control system to control a process that is external to the system (i.e. a computer, by itself, is not a SCADA system even though it controls its own power consumption and cooling).
This implies that the system is not critical to control the process in real-time, as there is a separate or integrated real-time automated control system that can respond quickly enough to compensate for process changes within the time-constants of the process.
A SCADA system includes input/output signal hardware, controllers, HMI, networks, communication, database and software.
The term SCADA usually refers to a central system that monitors and controls a complete site or a system spread out over a long distance (kilometres /miles). The bulk of the site control is actually performed automatically by a Remote Terminal Unit (RTU) or by a Programmable Logic Controller (PLC). Host control functions are almost always restricted to basic site over-ride or supervisory level capability.
For example, a PLC may control the flow of cooling water through part of an industrial process, but the SCADA system may allow an operator to change the control set point for the flow, and will allow any alarm conditions such as loss of flow or high temperature to be recorded and displayed.
The feedback control loop is closed through the RTU or PLC; the SCADA system monitors the overall performance of that loop.
ARCHITECTURE Hardware Architecture
One distinguishes two basic layers in a SCADA system:
the "client layer" which caters for the man machine interaction
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 fieldbuses 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.
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. datalogger a SCADA architecture that is generic for the products that were evaluated.
Server-client and server-server communication is in general on a publish-subscribe and event-driven basis and uses a TCP/IP protocol, i.e., a client application subscribes to a parameter which is ‘owned’ by a particular server application and only changes to that parameter are then communicated to the client application.
Access to Devices
The data servers poll the controllers at a user defined polling rate. The polling rate may be different for different parameters. The controllers pass the requested parameters to the data servers. Time stamping of the process parameters is typically performed in the controllers and this time-stamp is taken over by the data server. If the controller and communication protocol used support unsolicited data transfer then the products will support this too.
The products provide communication drivers for most of the common PLCs and widely used field-buses, e.g., Modbus.
A single data server can support multiple communications protocols: it can generally support as many such protocols as it has slots for interface cards.
Application Interfaces / Openness
The provision of OPC client functionality for SCADA to access devices in an open and standard manner is developing.
an Open Data Base Connectivity (ODBC)interface to the data in the archive/logs, but not to the configuration database,
an ASCII import/export facility for configuration data,a library of APIs supporting C, C++, and Visual Basic (VB) to access data in the RTDB, logs and archive. The API often does not provide access to the product’s internal features such as alarm handling, reporting, trending, etc.
The PC products provide support for the Microsoft standards such as Dynamic Data Exchange (DDE) which allows e.g. to visualise data dynamically in an EXCEL spreadsheet, Dynamic Link Library (DLL)
Scalability is understood as the possibility to extend the SCADA based control system by adding more process variables, more specialised servers (e.g. for alarm handling) or more clients. The products achieve scalability by having multiple data servers connected to multiple controllers. Each data server has its own configuration database and RTDB and is responsible for the handling of a sub-set of the process variables (acquisition, alarm handling, archiving).
Users are allocated to groups, which have defined read/write access privileges to the process parameters in the system and often also to specific product functionality.
The products support multiple screens, which can contain combinations of synoptic diagrams and text. They also support the concept of a "generic“ graphical object with links to process variables. These objects can be “dragged and dropped” from a library and included into a synoptic diagram.
Most of the SCADA products that were evaluated decompose the process in “atomic” parameters (e.g. a power supply current, its maximum value, its on/off status, etc.) to which a Tag-name is associated. The Tag-names used to link graphical objects’ to devices can be edited as required. The products include a library of standard graphical symbols, many of which would however not be applicable to the type of applications encountered in the experimental physics community. Standard windows editing facilities are provided: zooming, re-sizing, scrolling... On-line configuration and customization of the MMI is possible for users with the appropriate privileges.
Links can be created between display pages to navigate from one view to another.
the parameters to be trended in a specific chart can be predefined or defined on-line a chart may contain more than 8 trended parameters or pens and an unlimited number of charts can be displayed (restricted only by the readability) real-time and historical trending are possible, although generally not in the same chart historical trending is possible for any archived parameter zooming and scrolling functions are provided parameter values at the cursor position can be displayed
The trending feature is either provided as a separate module or as a graphical object (ActiveX), which can then be embedded into a synoptic display. XY and other statistical analysis plots are generally not provided.
Alarm handling is based on limit and status checking and performed in the data servers. More complicated expressions (using arithmetic or logical expressions) can be developed by creating derived parameters on which status or limit checking is then performed. The alarms are logically handled centrally, i.e., the information only exists in one place and all users see the same status (e.g., the acknowledgement)
logging can be thought of as medium-term storage of data on disk, whereas archiving is long term storage of data either on disk or on another permanent storage medium.
Logging is typically performed on a cyclic basis, i.e., once a certain file size, time period or number of points is reached the data is overwritten.
Logging of data can be performed at a set frequency, or only initiated if the value changes or when a specific predefined event occurs.
Logged data can be transferred to an archive once the log is full. The logged data is time-stamped and can be filtered when viewed by a user.
The logging of user actions is in general performed together with either a user ID or station ID. There is often also a VCR facility to play back archived data.
One can produce reports using SQL type queries to the archive, RTDB or logs. Although it is sometimes possible to embed EXCEL charts in the report, a “cut and paste” capability is in general not provided. Facilities exist to be able to automatically generate, print and archive reports.
The majority of the products allow actions to be automatically triggered by events.
A scripting language provided by the SCADA products allows these actions to be defined. In general, one can load a particular display, send an Email, run a user defined application or script and write to the RTDB.
The concept of recipes is supported, whereby a particular system configuration can be saved to a file and then re-loaded at a later date.
Sequencing is also supported whereby, as the name indicates, it is possible to execute a more complex sequence of actions on one or more devices. Sequences may also react to external events.
POTENTIAL BENEFITS OF SCADA
The benefits one can expect from adopting a SCADA system for the control of experimental physics facilities can be summarised as follows:
a rich functionality and extensive development facilities. The amount of effort invested in SCADA product amounts to 50 to 100 years
the amount of specific development that needs to be performed by the end-user is limited, especially with suitable engineering.
reliability and robustness. These systems are used for mission critical industrial processes where reliability and performance are paramount. In addition, specific development is performed within a well-established framework that enhances reliability and robustness.
Distributed Control System (DCS)
A refers to a control system usually of a manufacturing system, process or any kind of dynamic system, in which the controller elements are not central in location (like the brain) but are distributed throughout the system with each component sub-system controlled by one or more controllers. The entire system of controllers are connected by a network for communication and monitoring.
DCS is a very broad term used in a variety of industries, to monitor and control distributed equipment.
Electrical power grids and electrical generation plants
Environmental control systems
Water management systems
Oil Refining plants
A DCS typically uses computers (usually custom designed processors) as controllers and uses both proprietary interconnections and protocols for communication. Input & output modules form component parts of the DCS. The processor receives information from input modules and sends information to output modules.
The input modules receive information from input instruments in the process (a.k.a. field) and output modules transmit instructions to the output instruments in the field. Computer buses or electrical buses connect the processor and modules through multiplexers/demultiplexers.
Buses also connect the distributed controllers with the central controller and finally to the Human-Machine Interface (HMI) or control consoles.
Elements of a distributed control system may directly connect to physical equipment such as switches, pumps and valves or may work through an intermediate system such as a SCADA system.
DCSs are dedicated systems used to control manufacturing processes that are continuous or batch-oriented, such as oil refining, petrochemicals, cement production, steelmaking, and papermaking.
DCSs are connected to sensors and actuators and use set-point control to control the flow of material through the plant. The most common example is a set-point control loop consisting of a pressure sensor, controller, and control valve. Pressure or flow measurements are transmitted to the controller, usually through the aid of a signal conditioning Input/Output (I/O) device.
When the measured variable reaches a certain point, the controller instructs a valve or actuation device to open or close until the fluidic flow process reaches the desired set-point.
A typical DCS consists of functionally and/or geographically distributed digital controllers capable of executing from 1 to 256 or more regulatory control loops in one control box. The input/output devices (I/O) can be integral with the controller or located remotely via a field network. Today’s controllers have extensive computational capabilities and, in addition to proportional, integral, and derivative (PID) control, can generally perform logic and sequential control.
DCSs may employ one or several workstations and can be configured at the workstation or by an off-line personal computer. Local communication is handled by a control network with transmission over twisted pair, coaxial, or fiber optic cable. A server and/or applications processor may be included in the system for extra computational, data collection, and reporting capability.
Distributed Control System Architecture
Engineering & Operator Workstations
The Engineering Workstation (EWS) is for project development, including configuration of graphics, logic, alarms, security, etc. Typically, the EWS is a PC running Windows 2000/XP.
The Operator Workstation (OWS) provides the operator interface, including color graphics, faceplates, alarms, logging, trends, diagnostics, etc. The EWS includes an OWS for testing and troubleshooting. Typically, the OWS is a PC running Windows 2000/XP.
The Process Historical Archiver (PHA) stores and retrieves historical data collected by the FCU, microFCU, SDS, or any other intelligent device in the system. The PHA can run standalone or can share an OWS workstation. Typically, the PHA is a PC running Windows 2000/XP.
The Field Control Unit (FCU) executes sequential and regulatory logic and directly scans I/O. Depending on the FCU's configuration, you can scan multiple brands of I/O from one unit. The FCU runs QNX, a real-time operating system, and is typically a PLC or a ruggedized industrial computer available in a variety of form factors. The I/O Subsystem supports I/O from all the standard industry suppliers. In a UCOS configuration, you don't necessarily need PLCs – just PLC I/O.
The SCADA Data Server (SDS) interfaces UCOS to PLCs, Fieldbus technologies, RTUs, PLC I/O, and other third-party devices. The SDS can execute sequential and regulatory logic and directly scan supported I/O. It can also act as a data gateway allowing UCOS to work with just about any device you can think of. Typically, an SDS is a ruggedized industrial computer running Windows 2000/XP, although direct I/O scanning is run under QNX, the leading real-time operating system.
The UCOS microcosm is a small, low-powered PLC that executes sequential and regulatory logic and directly scans onboard I/O. It can replace RTUs at a significant reduction in cost and power consumption – plus it can provide local intelligent control of devices, which RTUs can't do.
Networking and Communications
supports redundant and non-redundant fiber optic and Ethernet local networks using the TCP/IP networking protocol for standardized, advanced application connectivity.
The LAN/WAN can be extended to other sites inside or outside the plant using such remote communications technologies as satellite, radio, microwave, and dial-up running such standard protocols as TCP/IP, Modbus, OPC, DDE, etc.
Data acquisition begins at the RTU or PLC level and includes meter readings and equipment statuses that are communicated to SCADA as required. Data is then compiled and formatted in such a way that a control room operator using the HMI can make appropriate supervisory decisions that may be required to adjust or over-ride normal RTU (PLC) controls. Data may also be collected in to a Historian, often built on a commodity Database Management System, to allow trending and other analytical work.
SCADA systems typically implement a distributed database, commonly referred to as a tag database , which contains data elements called tags or points .
A point represents a single input or output value monitored or controlled by the system. Points can be either "hard" or "soft". A hard point is representative of an actual input or output connected to the system, while a soft point represents the result of logic and math operations applied to other hard and soft points.
Most implementations conceptually remove this distinction by making every property a "soft" point (expression) that can equal a single "hard" point in the simplest case.
Point values are normally stored as value-timestamp combinations; the value and the timestamp when the value was recorded or calculated.
A series of value-timestamp combinations is the history of that point. It's also common to store additional metadata with tags such as: path to field device and PLC register, design time comments, and even alarm information.