Override control auto selectors

Frequently, a situation is encountered where two or more variables must not be allowed to exceed specified limits for reasons of economy, efficiency, or safety. If the number of controlled variables is greater than the number of manipulated variables, then a selection must be made for control purposes (SISO). A selector is used to accomplish this. Selectors are available in both electronic and pneumatic versions. The only difference between selectors is the number of inputs a particular hardware implementation may be able to accommodate. In this section, specific examples of such selectors will be discussed. It must be kept in mind that these are only a few examples of such auto selectors [4], 

Let us investigate some typical applications of these selectors in four areas  

3 redundant instrumentation (used commonly with process analytical equipment)  

As Fjn begins to drop, the output m i will also decrease (note increase/increase action on pressure controller). The output m i, will be selected by the first and second low selectors and will be passed through as the manipulated variable, causing the valve to close. 

If either the pump temperature or outlet pressure begins to increase, both outputs m2 and m3 begin to decrease (note increase/decrease action on both of these controllers). The smallest value will be chosen and passed through to manipulate the valve. In general, the smallest output from either of the controllers will always he operating the valve. 

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