Speed control of an overcentered variable-displacement hydraulic motor with a load-torque observer

This study deals with the speed control of an overcentered variable-displacement hydraulic motor on a constant pressure network, which is noted for its high system efficiency, fast dynamic response and energy recovery capability. To obtain robust speed control despite torque disturbances and variations in system parameters, the load torque is estimated by an observer based on a mathematical model and compensated for by a feedforward loop. It is shown by experiment that robust speed control may be obtained with the proposed controller, in spite of torque disturbances and variations in system parameters. The experimental data agree fairly well with the theoretical results. The frequency and time responses of a simplified first-order model for the regulating piston, driven by a servo valve, are in good agreement with those of an exact fifth-order model. It is also shown that the proposed controller gives better speed control and response characteristics than the conventional cascade PI controller.