Abstract
Variable-delivery high-pressure hydraulic pumps are usually controlled by a small servo actuator fed from the pump-generated pressure. The actuator consists of a three-way valve and differential-area jack or a four-way valve and equal-area jack.
This paper analyses the effect of perturbations of the generated pressure when the pump is part of a system incorporating a relief valve. It is demonstrated that, because the load on the servo jack can be a function of the generated pressure, the ‘closed loop’ formed can be unstable.
In particular, the three-way-valve actuator is shown to be sensitive to perturbations in its supply pressure even when the load is independent of pressure.
Frequency-response diagrams are presented, the assumptions made in the analysis are clearly discussed, and design recommendations are made in the light of the analytical results.
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