The operating point of a hydraulic pump (1) in a hydrostatic system in which the pump supplies a hydraulic servo motor (12) is determined by the pump displacement being varied by the position of a double acting cylinder actuator (2). The cylinder is supplied by a 4/3 valve (5) with solenoid input stages (6,7) that respond to commands from a logic circuit (10) that has position feedback (18) of the actual output state of the pump actuator. The command or required output is provided by a joystick input (17). The signals to the valve solenoids are in a PWM form and are related to the system error in an uncertain form. The controller (10) is configured on fuzzy logic rules that are based upon a knowledge based experience of the system behaviour.
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