Parallel Compensation with a Secondary Measurement

A secondary measurement is usually used in the form of cascade control to effectively reject disturbances to the manipulated variable. In the cascade control system, the inner loop is tuned first and then the outer loop is tuned with the inner loop closed. When the dynamics of the inner process is slow, this tuning procedure does not provide the best control performance. Here, for slow inner processes, parallel compensation similar to the Smith predictor and the internal model control is shown to be more effective than the cascade control. A very simple tuning rule and a simple anti-reset windup are feasible in this control structure. It can be used in its own structure or to tune the cascade control system systematically. For automatic tuning of the proposed control system, an identification method which finds two process models with the relay feedback and P-control tests is also proposed.