Experimental Investigation of a Displacement-controlled Hydrostatic Pump/Motor by Means of Rotating Valve Plate

Abstract

Interest in the control of variable fluid power pumps/motors has increased in recent years. The actuators used are inefficient and expensive and this reduces the variable units' usability. This paper introduces displacement control of pumps/motors by means of a rotating valve plate. By changing the angle of the valve plate, the effective use of the stroke is changed. The rotating valve plate is experimentally verified by a modified in-line pump. In the prototype, the valve plate is controlled with a worm gear connected to an electric motor. The results show potential for this kind of displacement control. However, the rotating valve plate creates pressure pulsations at part-displacement due to the commutation being performed at high piston speeds. If the piston speed and hence the flow from each piston is low, the pressure pulsation is acceptable.

A New Degree of Freedom for Variable Axial Piston Pumps with Valve Plate Rotation

“…In order to reduce compressible flow pulsations in fluid power machines, pre-compression takes place before the displacement volume connects to the HP kidney and de-compression takes place before the displacement volume connects to the LP kidney. The angle at bottom dead center (BDC) is larger than the angle at top dead center (TDC), as the fluid volume at BDC is larger [2]. The principle design for a valve plate depending on the operating quadrant can be seen in fig.…”

“…In [2], the concept of displacement control through valve plate rotation has been summarised. Figures 4 and 5 visualise the principle: When the commutation is moved away from TDC and BDC, the kinematic flow while connected to the HP kidney becomes negative for some degrees of shaft rotation, and thus the effective stroke is reduced.…”

“…Figure 4: A valve plate in full motor-mode (ε=-1), no-flow-mode (ε=0) and full pump-mode (ε=1). Red/light grey rectangles symbolise the high-pressure kidney and blue/dark grey rectangles symbolise the low-pressure kidney for one full shaft rotation from 0 to 2π [2]. Figure 5: Normalised kinematic flow from one cylinder.…”

“…Figure 5: Normalised kinematic flow from one cylinder. Blue/dark grey areas are the volumes entering the cylinder from the low-pressure kidney and red/light grey are the volumes leaving the cylinder to the high-pressure kidney [2].…”

“…Conventionally, variable axial piston machines vary displacement by adjusting the length of the piston stroke, which can be done by adjusting the angle of the swash plate in swash plate machines or the angle of the cylinder block in bent axis machines [1]. Another way to achieve variable displacement is to adjust the effective use of the piston stroke, which can be done by rotating the valve plate [2]. It is beneficial that valve plate rotation requires smaller control forces and provides easier controllability in comparison to conventional displacement adjustment [3].…”