THE DIGITAL DISPLACEMENT PUMP AS A FREQUENCY GENERATOR

Digital Displacement® pumps are a type of variable displacement, radial piston hydraulic pump currently being developed for use in efficient mobile hydraulic systems. The
pump displacement is controlled by selective enabling of solenoid valves at the inlet of each cylinder and since the pump displacement can be controlled directly by software it is possible to use the pump as a flow source with any arbitrary demand. By enabling precise flow control and reducing leakage and throttling losses they can provide a significant improvement in
efficiency over conventional pumps; however, this also leads to a decrease in the overall sources of damping in the system and may result in increased transmission of vibration and fluidborne noise.

This paper presents a method for characterising vibration sensitivity of a hydraulic system, using the pump as a frequency generator whose flow output follows a sinusoidal ‘chirp’ demand. Simulation results are presented of the pump open-loop frequency response, which show the control bandwidth and demonstrates that the pump can modulate its output flow at
frequencies into the audible range. This enables the possibility of using the pump to identify potential sensitivities in a downstream hydraulic system up to 200 Hz. A method is described for characterising the noise and vibration of the connected system within this frequency range. Test data from a hydraulic excavator are presented and analysed to create a characteristic transfer function for the system, relating pump output flow to pressure ripple and vibration in the downstream system. These system transfer functions can be used to develop control methods to reduce the impact of vibration, either by active damping, filtering of the control signals or choice of cylinder enabling strategy. Test data are presented also showing the effect of some mitigation strategies in the same hydraulic excavator, leading to a reduction of overall vibration in the vehicle cabin.