Design of a nonlinear adaptive controller for an electrohydraulic actuator

This paper presents a new implementation of indirect model reference adapti ve (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 u nstable pole-zero cancellation. The robustness of, the parameter adaptation is achieved by employing the recursive least-squares algorithm in combinat ion with a dead-zone in the adaptive law. It is shown that while the contro ller 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 resultin g nonlinear MRA controller is shown to have improved performance over its l inear counterpart. Step-by-step experiments are presented to confirm the ef fectiveness and performance improvement of the proposed method.