Hierarchical digital control of a proportional electro-hydraulic valve

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. 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 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 dead-band hysteresis. The digital conversion provides the necessary derivatives through a state predictor, in order to withstand the 5-kHz Nyquist limit of the power supplier. The digital control law is fed by state predictions and repeats the analogue control law with some improvement. Preliminary experiments prove that the conversion repeats and improves analogue performance. Some flaws of the resulting digital controller are outlined and discussed in view of a model-based conversion.