Consider for a moment the problem of liquid level in a tank. a tank with a valve that controls flow of liquid into the tank and some unspecified flow out of the tank. A transducer is available to measure the level of liquid in the tank. Also the control system has to maintain the level of liquid in the tank at some preset or setpoint value. The controller will operate according to some mode of control to maintain the level against the variations induced from external influences. If the outflow increases, the control system will increase the opening of the input valve to compensate the flow. The level is thus regulated. This is a continuous variable control system because both the level and the valve setting can vary over a range.Even if the controller is operating in ON/OFF mode, there is still variable regulation, although the level will now oscillate as the input valve is opened and closed to compensate for output flow variation. This Continuous control.
Discrete State Control
Consider the same situation with variables like level, setpoint and the valve are discrete
since they can take only two values. this means that the valves can be open or close,
level is either below or above. Now the objective is to fill the tank to a certain level with
no outflow for whcih we specify and event sequence:
1. Close the output valve.
2. open the input valve and let the tank fill to the desired level, as indicated by some
3. Close the input valve.
the level is certainnly not going to change until, at some later time, the output valve is
opened to let the liquid flow out. Notice that the variablesare only two state quantities.
there is no continuous control over a range.
Source: Control Instrumentation topics covered by CD Johnson
The evolution of digital computers, having higher speed, higher reliability and smaller and reduced cost has brought about increased use in process control. The computer these days perform certain kinds of reduction of data using control equations and even indicated the type of action if any, which should be taken to tune a process for maximum operating efficiency. All of the loops in the process were still analog and for the most part, independent except for manual adjustment of set point under guidance of process engineer.
A natural extension of this concept led to the development of the technique where the computer itself performs adjustments of the loop set points and provides a record of process parameters. The loops are still analog, but the set points that determine the overall process performance are set by a computer on the basis of equations solved by the computer, using measured values of process parameters as input.
Advantages of Distributed Control System
The cost of distributed system is lower than the cost of a centralized system which performs the same functions. Small computers are relatively inexpensive and when linked together they provide more computing power than the larger mainframe system of the same cost. The cost of microcomputers is still lower and the actual cost of implementing the module at a later stage may further come down.
The cost of upgrading is also lower
Remote terminal units
RTU is device which interfaces objects in the physical world to a distributed control system or SCADA system by transmitting telemetry data to the system and /or altering the state of connected objects based on control messages received from the system.
RTU sends data in the form of data packets using IEC 870-5 protocol via PLCC, VAST or microwave communication media. RTU also supports and auxillary port for master communication. This facility can be used as a failback line or communication to two master as the case may be.
RTU collecrs data automatically and connect directly to sensors, meters, loggers or process equipment. They serve as slave units to supervisory controllers or SCADA masters. RTU is located near the monitored process and transfer data to the controller unit on command.
Types of RTUs
Transmission Substation RTUs
These large RTUs are deployed at substations and generation facilities where a large number of controls and data points are required. They are usually enclosed in a floor standing NEMA enclosure which has card cages allowing for multiple microprocessor cards as well as multiple ADC, DAC, digital input and output cards. Some transmission RTUs have a distributed IO design whereby IO modules are connected to a data highway which loops through the substation.
Distribution Automation RTUsThese similar RTUS are usually enclosed in a rain tight NEMA 4 enclosures and are used to control air switches and VAR compensation capacitor banks on utility poles, pad mounted switches, monitoring and automating feeders and underground networks and in smaller substations. DA RTUs are typically single board computers that have all inputs and outputs on the main board.