Retrospective-cost-based model reference adaptive control of nonminimum-phase systems

This paper presents a novel approach to model reference adaptive control inspired by the adaptive pole-placement controller (APPC) of Elliot and based on retrospective cost optimization. Retrospective cost model reference adaptive control (RC-MRAC) is applicable to nonminimum-phase (NMP) systems assuming that the NMP zeros are known. Under this assumption, the advantage of RC-MRAC is a reduced need for persistency. The present paper compares APPC and RC-MRAC under various levels of persistency in the command for minimum-phase and NMP systems. It is shown numerically that the model-following performance of RC-MRAC is less sensitive to the persistency of the command compared to APPC at the cost of knowledge of the NMP zeros. RC-MRAC is also shown to be applicable for disturbance rejection under unknown harmonic disturbances.