Single-loop control Subject

A processor does not have control of all the steps from basic material to finished product. Material processing capability is limited by what is received and by when inspection is made or required so selection tests are important and must be subject to change (see Chapter 10). In turn, the process line has many variables that must be coordinated. A practical procedure today, with all closed-loop process control systems, is to subdivide the controls into distinct subsystems (Fig. 3-33). They can then be controlled within single control loops or by simple intermeshed circuits. A single-loop feedback circuit has one input and one output signal disturbances that affect the process are registered by the controller directly, by an ad-... 

One final comment should be made about model-based control before we leave the subject. These model-based controllers depend quite strongly on the validity of the model. If we have a poor model or if the plant parameters change, the performance of a model-based controller is usually seriously affected. Model-based controllers are less robust than the more conventional PI controllers. This lack of robustness can be a problem in the single-input-single-output (SISO) loops that we have been examining. It is an even more serious problem in multi-variable systems, as we will find out in Chaps. 16 and 17. 

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