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Control Strategy: Selective Control Systems
There must be one final control element for each process variable to be controlled in a system. In many systems however, the process variables to be controlled outnumber the final control elements. In these applications, the control system must automatically decide how to share the final control elements. When this is the case, selective controls can be employed to switch easily and smoothly between the variables to be controlled. The purpose of this Protuner System Analysis Note is to provide:
1. How the various selective control strategies are properly implemented and configured 2. Guidance on the implementation of external anti-reset windup to insure the unselected controller integral action is disabled when the controller is unselected 3. Information on potential control problems that might be encountered when the digital control system, being used to implement the selective control strategy, employs the incremental or velocity and not the positional form of the PID
4. Protuner control system analysis test techniques to trouble shoot the field equipment and determine the optimum tuning parameters for the controllers
Selective controls involve the use of signal selectors which choose either the lowest, median, or highest control signal from two or more signals. Selective controls are employed in five basic application areas:
Protection of equipment
Valve position control
Each of these selective control application types has its own unique implementation, control equipment requirements, and test procedure for optimization.
Protection of Equipment
In many process control systems, there is a primary process variable that needs to be controlled along with a second process variable that must not be exceeded for reasons of economy, efficiency, or safety. The following example illustrates how a typical selective control strategy is Control Strategy: Selective Control Systems © Techmation Inc. 1999
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implemented for the protection of equipment. In this example, the control of the discharge from a compressor is controlled by manipulating the setpoint of the motor speed controller. The two variables that require control are the discharge airflow and pressure. The control strategy uses two separate controllers and a low signal selector to decide which controller output signal will be used for control of motor speed.
Figure 1 illustrates how the low signal selector is used to select the lower output of either the pressure or flow controller to manipulate motor speed.
Under normal operating conditions, the discharge pressure is below its setpoint and the flow controller output is less than the output from the pressure controller, and therefore selected to change the motor speed to control the discharge flow out of the compressor. Thus the pressure is allowed to drift below its setpoint during normal or high load conditions. During conditions of low loads on the compressor, the pressure controller reaches its setpoint and its output becomes lower than the output of the flow controller, and is allowed to assume control, thereby lowering the flow and controlling the pressure at its setpoint. Decreasing the motor speed decreases both the flow and the pressure, use of the low signal selector guards the system against an excess of either.
Figure 1—Motor speed is manipulated by whichever controller has the lower output
Protection Against Windup
When one controller is selected from two or more, the others are in an open loop condition. If the unselected controllers...
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