This paper concerns the energy efficiency of a special class of pump-controlled hydraulic cylinders utilizing two prime movers. The performance of such circuits has been studied previously motivated by their capability of providing an actuator stiffness similar to that of servo valve-controlled systems. This characteristic may improve performance and robustness in applications requiring feedback control. In this paper, the presence of losses similar to that of fluid throttling, in the sense that they occur even in the absence of component inefficiencies, are demonstrated for such circuits and shown to degrade the overall energy efficiency of the system. The conditions under which such losses occur are derived analytically as a function of system parameters and operating conditions and two solutions for their elimination are proposed and verified analytically and numerically. Several implementation options are compared in terms of energy efficiency and component sizing and benchmarked to a conventional servo valve solution. It is shown that with the appropriate implementation, an energy efficiency up to ten times greater than that of a conventional servo valve system may be expected.

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