Failure conditions for inverse-based multivariable controllers: internal model control structure

Abstract

Properties of inverse-based controllers under failure conditions are addressed. The inverse-based controllers considered are parameterized, designed and implemented under the internal model control (IMC) structure. One important property is that the integral action is lost when failure in actuator and/or sensor occurs. The steady-state errors (offsets) under failure conditions are discussed in detail from aspects of the set-point tracking and disturbance rejection. The steady-state errors for set-point tracking can be characterized by the well-known interaction measures such as RGA and BRG. The overall performance under failure status is assessed based on the sum of squares of steady-state errors ( SSE). An amplification factor (AF) is defined as the ratio of SSE with IMC control over the SSE without control. An amplification that, under failure modes. AF can be greater than one in some cases. This implies IMC feedback system may lead to inferior performance than a system without control. A distillation example is used to illustrate the behavior of IMC system under failure modes. The results presented here provide a better understanding of IMC and give useful information in making on-line decision under failure conditions.