Discrete Proportional‐Integral Control with Constrained Adjustment

SUMMARY It is well known that discrete feed‐back control schemes chosen to produce minimum mean‐squared error at the output can require excessive manipulation of the compensating variable. Also very large reductions in the manipulation variance can be obtained at the expense of minor increases in the output variance by using constrained schemes. Unfortunately, however, both the form and the derivation of such schemes are somewhat complicated. The purpose of this paper is to show that suitable ‘tuned’ proportional‐integral (PI) schemes in which the required adjustment is merely a linear combination of the two last observed errors can do almost as well as the more complicated optimal constrained schemes. If desired, these PI schemes can be applied manually by using a feed‐back adjustment chart which is no more difficult to use than a Shewhart chart. Several examples are given and tables are provided that allow the calculation of the optimal constrained PI scheme and the resulting adjustment variance and output variance. Methods of tuning such controllers by using evolutionary operation and experimental design are briefly discussed.