Conventional hydraulic drive systems are generally of two types; those with valve resistance control and those with pump displacement control.In valve controlled systems, the pressure drop due to the oil flow through a valve orifice causes an energy loss and a decrease in system efficiency. In pump controlled systems, the increase in oil volume in the hydraulic line results in a relatively long hydraulic time constant due to the compressibility of the oil. A recently introduced displacement control of an overcentered variable displacement hydraulic motor(VDHM) is noted for its small energy loss, fast dynamic response and energy recovery capability. Although VDHM control system has many good aspects, the dynamic characteristics of this system are complex and nonlinear. This system may be expressed simply as double integrater and easily becomes unstable. This necessitates an accurate evaluation of dynamic characteristics and anlysis of the system stability.
The objective of this thesis is to provide the basis for the design and control of the VDHM speed servo system. To achieve this, a nonlinear model was formulated and verified. Upon this model, theoretical investigations were made for the analysis of the system dynamics, stability and design of robust controller. The results of theoretical analysis were verified by a series of experiments.
This thesis is composed of three main parts. Firstly, the effects of the parameter variation were investigated by parameter sensitivity analysis. The results of analysis show that supply pressure, inertia load and motor displacement are most critical parameters, and parameters related to swash plate positioning dynamics become important for the applications requiring especially fast dynamic response. Secondly, the stability characteristics was analyzed through investigation of closed loop pole location for single and double loop control systems. The effects of inertia load, inner and outer loop gains to the system stability were investigated, and inner loop was found to increase the system damping. Finally, a feedforward control with load torque observer was suggested to obtain robust speed control despite torque disturbances and variation of system parameters such as inertia load and supply pressure. The effectiveness of the proposed controller was verified by comparing with the conventional cascade PI controller.