Aiming at the problems, bad low velocity performance and slow response, existing in variable speed hydraulic systems, this study proposes a variable speed hydraulic system in valve-pump parallel variable structure control, in which control structures vary with control requirement. The leaking parallel valve control is applied to improve low-speed performance at the start and stop stages by increasing damping ratios and compensating the reduction of damping ratios due to the friction negative slop; the replenishing parallel valve control is applied to achieve fast response to load disturbance at the uniform stage with high speed; the variable speed pump control is to make full use its high efficiency at other stages. During the regulation process, these control structures switch smoothly and a good speed tracking is achieved, in addition, the pump provides majority of required flow, so the proposed system still can work efficiently as pump control systems. Therefore, the valve-pump parallel variable structure control establishes a flexible control mechanism by two channels of valve control and pump control, and could improve comprehensive performances of variable speed hydraulic systems with large power, such as low-speed performance, rapid response and high efficiency
