UNDER THE HOOD OF OVERRIDE CONTROL, PART 1 - All the techniques for preventing reset windup of non-selected controllers will perform comparably to reset feedback if the dynamics of the various loops aren't too dissimilar

摘要

Live to override. Override to live. While the old motorcycle mantra can be adapted for the process industries, override control refers to applications where one process variable controls the process in normal operation, but during abnormal operation another variable assumes control to prevent encroachment on safety, process or equipment limits. A key element of override control, or selector control, is a hardware or software-based selector function, which selects the lower or higher of two or more controller outputs for passage to the final actuator. When applying override control to a fired heater (Figure 1), the heater outlet temperature normally controls the process. Excessive tube heat causes its temperature controller overrides to prevent encroachment of the tube's high limit. If the non-selected controller contains reset action, it will tend to windup unless special provisions are made.