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.
In this paper, certain theoretical results due to Porter concerning the probability of stability of a class of linear dynamical systems are generalized significantly. These results are compared with the theoretical results obtained by Porter and also with certain empirical results obtained by Ashby.
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