The Theoretical Frequency Response of an Over-Center, Pressure-Controlled, Axial Piston Pump

Axial piston pumps are ubiquitous in the hydraulic systems employed on various off-highway machines such as hydraulic excavators or wheel-loaders. These pumps exhibit a fairly slow response to the commanded input resulting in a slow response of the work tool (implement or transmission) as well, which is undesirable to machine operators and also slows down the machine’s productivity. Hence it is very important to understand the factors affecting the dynamic pump response. Much of the earlier work aims at understanding the steady state behavior of the pump control system using linear transfer function analysis. A closed-form solution for the dynamic frequency response has not been reported in previous research. This work presents the analysis of a variable displacement pump with an adjustable swash-plate for the purposes of identifying parameters that contribute to the response characteristics of the pump. The full model of the pump involves a seventh order model including a large number of non-linear terms. Hence a reduced order model has also been derived for calculating the frequency response of the pump in the closed form and it is shown that the design parameters that impact the frequency response most heavily are the actuator area, the swash-plate moment arm, and the flow gain of the four-way valve. As it turns out, an increased frequency response of the pump may be most readily achieved by increasing the charge pressure which in turn reduces the required actuator area and increases the flow gain of the valve.