Performance Assessment: User-Specified Benchmark

Abstract

Control loop performance assessment has been extended to many situations, and many approaches have been developed as discussed in the earlier chapters, e.g., performance assessment of: 1) SISO feedback control systems (Desborough and Harris 1992; Stanfelj, Marlin, and MacGregor 1993; Kozub and Garcia 1993; Lynch and Dumont 1993; Tyler and Morari 1996); 2) feedback control of nonminimum phase SISO systems (Tyler and Morari 1995); 3) MIMO feedback control systems (Huang, Shah, and Kwok 1995; Huang, Shah, and Kwok 1996; Harris, Boudreau, and MacGregor 1995; Harris, Boudreau, and MacGregor 1996). The portion of a process output that is feedback controller invariant determines the minimum variance achievable theoretically and characterizes the most fundamental performance limitation of a system owing to the existence of time-delays or infinite zeros. Practically, however, there are many other limitations on the achievable control loop performance. The existence of nonminimum phase or poorly damped zeros, sampling rate, amplitude and/or rate constraints on control action, robustness constraints etc. are examples of such limitations; therefore, a feedback controller that indicates performance reasonably close to minimum variance control does not require further tuning (if the variance is the most important issue) while a feedback controller that indicates poor performance relative to minimum variance control is not necessarily a poor controller. Further analysis of performance limitations and comparison with more realistic benchmarks are usually required. Performance assessment with minimum variance control as a benchmark requires minimum effort (routine operating data plus a priori knowledge of time-delays), and serves as the most convenient first-level performance assessment test, therefore, (if the variance is the main point of interest). Only those loops that indicate poor first-level performance need to be re-evaluated by higher-level performance assessment tests. A higher-level performance test usually requires more a priori knowledge than just a knowledge of time-delays. This chapter addresses practical issues that are of interest for such a higher-level performance assessment test.