Proportional electro-hydraulic valves: from analogue to digital control

Proportional electro-hydraulic valves are ubiquitous as flow actuators in hydraulic systems. Flow regulation is the result of the accurate positioning of a spool driven by a solenoid and a position sensor. The overall control consists of two hierarchical loops: the inner loop is the solenoid current regulator with a closed-loop bandwidth close to 1 kHz. A modelbased digital regulator of this kind has been presented elsewhere. The outer loop is a position tracking control, in charge of an accurate positioning of the spool with respect to the valve openings. The paper addresses the outer loop and concentrates on the conversion of an existing industrial analogue controller into a digital one. The analogue controller is a nonlinear proportional, integrative and derivative controller including a second-order derivative, and is capable of recovering a deadband hysteresis. The digital conversion provides the necessary position derivatives through a state predictor, in order to withstand the 5-kHz Nyquist limit of the power supplier. As such it departs from traditional conversions dating back to more than ten years ago. The digital control law is fed by the state predictions and repeats the analogue control law with some improvements, as shown by the reported experiments.