Elements of SCADA Systems
SCADA monitors, controls and alarms the plant and/or regional facilities’ operating systems from a centralized location. It includes the communication of information between a SCADA central host computer, many scattered units and/or Programmable Logic Controllers. For example, in a water filtration plant, the remote units measure the pressure in pipes and report the readings to the central computer located somewhere in the control tower. In case of any anomaly, the SCADA system would alert the main station of the problem apprising it of other details like the severity of the anomaly and measurement values in an organized fashion. The systems may vary from simple, like temperature reporting in a building to complex like monitoring the traffic on many traffic lights. The system consists of the following elements:
1. SCADA Master Station Computer Systems: It is the repository of the real-time or near real-time reported data collected from the remote terminal units connected to it. It is generally standard computer hardware equipment and very few SCADA system suppliers have ventured out to make their own computer equipment. A few companies like IBM and CDC did try making hardware for it, but the effort was short lived and commercial off-the-shelf computer products continue to be the main stay. The back end SCADA software must be able to repeatedly poll the RTUs for data values, should have software for their retrieval, storage and processing. The processing may include unit conversion, cataloguing into tables etc.
2. Human-Machine Interface: This is the eye candy part on the host station. The values that have been stored in the host computers are presented to the human operator in an understandable and comprehensible form using HMIs. These may provide trending, diagnostic or management information and detailed schematics and animations representing the current states of the machines under its control. Pictorial representation being more understandable to humans is the preferred form in SCADA HMIs.
3. Remote Terminal Units (RTUs): An RTU is a normally a transducer or a sensor which allows the electrical circuitry to interface with the process instrumentation and control equipment. The physical parameter like pressure, temperature etc. are measured through a change in electrical property of some component in the transducer which is indicative of the physical change. A single RTU may measure many different types of parameters. Depending on the values of the measurements, the Input/Output circuitry of a RTU can be analog or digital. Analog corresponds to measurements with a numeric range of continuous values which are later converted using an ADC, like a temperature scale, while digital have limited number of states (generally two) mainly used for flagging. Specific signals can be generated to control process equipment. These days, RTUs are microprocessor based devices and these conversions are primarily internal to them.
4. Programmable Logic Controllers: The use of microprocessors on RTUs has helped RTUs become smarter with increased functionality. PLCs have been built around the philosophy of automation. Reprogrammability being the biggest asset, PLC based RTUs can be debugged and fixed on the field itself along with adding new features like support for multiple polling, exception reporting, time-tagging etc. This also enables them to execute simple logical processes without involving the master station. Vendors using different type of communication and coding on these equipment has led to standardization of protocols and languages for RTUs too, for example the standardized control programming language, IEC 61131-3. These languages require very less training and are based on intuitive approach unlike procedural languages like C and FORTRAN.
5. SCADA Communication: The conveying of data from an RTU to the master station and commands from the host to the RTU need to be done over a communication system. Also, since a SCADA system might not be localized to just a single plant, the vastness of the network also has to be catered to along with speed, accuracy, security and performance being among other important issues. Before the computer networking solutions were made available, most systems for communication were voice communication based. SCADA communication systems were also built using the same infrastructure and had the same bandwidth limitations. But, with the corporate now wanting to include the SCADA information network into their core networks over security concerns, SCADA systems have also embraced LANs and WANs for seamless integration with everyday office computer networks. This has an advantage for the corporate users that they would not need a separate parallel network for SCADA systems.
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