Hydrostatic or closed-circuit pump-controlled hydraulic systems are attractive due to their high energy efficiency; their lack of throttling losses and ability to recover energy onto the pump shaft have made them a common design choice in systems using rotary actuators such as ground drive transmissions. However, the natural asymmetrical flow of typical hydraulic cylinders have prevented the widespread adoption of closed-circuit systems for linear actuators. Some hydrostatic linear actuator systems have been developed, but these suffer from a large dead volume and reduced force if using a symmetrical dual rod cylinder, or increased cost and complexity if using a specialized cylinder geometry or flow balancing circuits. This paper presents a concept system which uses a pair of common single-rod hydraulic cylinders to achieve the efficiency of a pump-controlled hydrostatic system with the opportunity for energy recovery. The system’s available force is equal to a standard valve-controlled system with a greater maximum velocity. This is achieved using only commercially-available components. The paper will present an analysis of the theoretical energy recovery potential over a representative work cycle for a hydraulic excavator. It will also present a controller design analysis and experimental verification.

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