Design of a Nonlinear Adaptive Controller for an Electrohydraulic Actuator1

Abstract

This paper presents a new implementation of indirect model reference adaptive (MRA) control scheme for positioning of hydraulic actuators that operate by low-cost proportional valves. A proper linear discrete-time plant model is used which has dead-time and no zeros, eliminating the possibility of unstable pole-zero cancellation. The robustness of the parameter adaptation is achieved by employing the recursive least-squares algorithm in combination with a dead-zone in the adaptive law. It is shown that while the controller is adequate for hydraulic valves with linear flow characteristics, it exhibits low performance in the presence of deadband and nonlinear orifice opening characteristics of low-cost proportional valves. The linear plant model is therefore augmented by adding a static nonlinearity. The resulting nonlinear MRA controller is shown to have improved performance over its linear counterpart. Step-by-step experiments are presented to confirm the effectiveness and performance improvement of the proposed method.