Selection of Controlled Output Variables

Guideline 2. Choose output variables that would exceed the equipment and operating constraints without control. Clearly, when safety or operational constraints are imposed, it is important to measure and control these output variables to comply with the constraints. 

Guideline 3. Select output variables that are a direct measure of the product quality or that strongly affect it. Examples of variables that are a direct measure of the product quality are the composition and refractive index, whereas those that strongly affect it are the temperature and pressure. This guideline helps the control system to ensure that the product specifications are regulated and met. 

Guideline 4. Choose output variables that exhibit significant interactions with other output variables. Plantwide control must handle the potential interactions in the process. Improved closed-loop performance is achieved by stabilizing output variables that interact significantly with each other. 

Guideline 5, Choose output variables that have favorable static and dynamic responses to the available manipulated variables. All other things being equal, this guideline should be applied. 

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