State-of-the-art of Variable Displacement Technologies for Radial Piston Hydraulic Machines

Abstract The hydraulic motor is one of the main parts of hydrostatic transmission implemented in some off-road vehicles for instance. It uses hydraulic power to convey energy from the generator of the machine to the wheels and other actuators. As the hydraulic circuit may contain several hydraulic actuators, each of them needs to be controlled independently since the only control of the pump displacement may impact the entire circuit and all its actuators simultaneously. To properly drive the vehicle, the hydraulic actuators have to be accurately controlled. This paper aims to present a state-of-the-art of the different ways of changing hydraulic pumps/motors displacement ratios because it is one of the most important parameter to control the torque/speed (resp. pressure/flow rate). Some hydraulic machines enable a continuous change of this displacement ratio thanks to their specific architectures, whereas other machines use hydraulic valves to pilot this ratio. Most of the technologies, from variable displacement axial solutions to new digitally modulated displacement machines will be presented and compared. Digital Displacement solutions and their valve timing control solutions will then be depicted. Finally, technological locks in Digital Displacement will be exposed based on the modeling of a radial piston motor.