A robust control of the pressure in a control-cylinder for the variable displacement axial piston pump

In order to achieve energy-efficient hydraulic systems, an accurate flow rate and pressure required by a load should be supplied to the systems. The discharge flow from the variable displacement swash-plate type axial piston pump is mainly determined by the angle of the swash-plate and it is adjusted by controlling the pressure in the control-cylinder which is a mechanical regulator for the swash-plate. In this paper, a robust pressure control system of the control-cylinder using a direct drive valve is proposed to control the discharge flow from the variable displacement swash-plate type axial piston pump precisely. In order to design a robust pressure control system, a mathematical model of a pressure control system is derived and system parameters are identified by using the signal-compression method. Also, the sliding mode controller is designed based on the identified mathematical model to guarantee the control performance of the pressure control system. Experiments verified the satisfactory performance of the proposed pressure control system that uses a direct drive even with non-linearity and uncertainties such as flow characteristics, unknown discharge coefficient of the valve orifice, variation of the bulk-modulus, leakage and disturbance induced by the pressure fluctuation in the pressure control system.