In a number of process control problems, we may have more measurements (controlled variables) than manipulated variables. Hence, it is impossible to eliminate errors in all the controlled variables for arbitrary setpoint changes of disturbances by using only simple (single-input/single output) controllers. Thus selectors are used to share the manipulated variables among the controlled variables. We can therefore define selective control as a scheme whereby a selector is used to choose the appropriate measurement variable from among a number of available measurements. Selective control is used to improve the system operation and to protect the system from unsafe operation.
One type of selector device has an output that is the highest (or lowest) of two or more input signals. On instrumentation diagrams the symbol (HS) denotes high selector and (LS), a low selector. For instance, a high selector can be used to detect a hot spot temperature. If several temperature transmitters are connected to the selector it chooses the maximum temperature and feeds it to the temperature controller. A median selector calculates an average temperature from several sensors and provides increased reliability due to redundant measurements.
You may also read: What is Process Control?
The use of high or low limits for process variables is another type of selective control, called an override. At these limits the normal controller operation is overridden by alarm procedures. Anti-windup features in controllers form another type of override.
Also read: Cascade-control
To sum up, selectors are non-linear elements that can be added to the control loops and are easily implemented in digital systems.
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