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CHAPTER 29. CLOSED-LOOP CONTROL |
29.13.3SCADA and telemetry systems
A similar control system architecture to Direct Digital Control (DDC) – assigning a single microprocessor to the task of managing multiple control functions, with digital communication between the microprocessor units – is used for the management of systems which are by their very nature spread over wide geographical regions. Such systems are generally referred to as SCADA, which is an acronym standing for Supervisory Control And Data Acquisition.
The typical SCADA system consists of multiple Remote Terminal Unit (RTU) devices connected to process transmitters and final control elements, implementing basic control functions such as motor start/stop and PID loop control. These RTU devices communicate digitally to a Master Terminal Unit (MTU) device at a central location where human operators may monitor the process and issue commands.
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Operator |
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SCADA system with |
workstation |
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radio communication |
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MTU
Radio interface
Radio interface
Radio interface
RTU
RTU
4-20 mA signal
Com NC NO |
Com NC NO |
Control valve
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Process switches |
Process transmitters |
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L H
L H
Process transmitters
29.13. DIGITAL PID CONTROLLERS |
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A photograph of an RTU “rack” operating at a large electric power substation is shown here:
Some RTU hardware, such as the substation monitoring system shown above, is custommanufactured for the application. Other RTU hardware is more general in purpose, intended for the monitoring and control of natural gas and oil production wells, but applicable to other applications as well.