DISCRETE PISTON PUMP / MOTOR USING A MECHANICAL ROTARY VALVE CONTROL MECHANISM

Hydraulic pumps and motors are desirable for high power density and ruggedness, but they typically exhibit lower efficiency than competing technologies. In conventional devices, high pressure is maintained on each piston for its full stroke, and displacement is varied by changing the stroke length. Maintaining high pressure on all pump/motor interfaces causes leakage and friction losses to remain nearly constant as displacement is reduced, resulting in low efficiency, particularly at low displacements. A different approach is to vary the displacement by removing high pressure from unneeded pistons. This discrete piston control approach has been proposed by a number of researchers, but it is typically accomplished using two electrohydraulic valves per piston, which can increase cost, complexity, and reduce robustness. In this paper a method of discrete piston control using hydromechanical valves is described. A two degree-of-freedom valve that can rotate and translate axially controls the enabling and disabling of the individual pistons and adjusts the displacement. Several strategies for creating a discrete piston device are described, along with the structure of the control system. Preliminary experimental results from a prototype pump/motor are also presented.