Modeling and analysis of active valve control of a digital pump-motor

This work involves the development of a hydraulic pump/motor that incorporates actively controlled high speed on/off valves connected to each piston cylinder displacement chamber. The fluid is commutated between the ports and the displacement chamber by the on/off valves and there is no longer a need for the valve plate. Unit displacement is electronically controlled by on/off valve timing, not by a swash plate or other typical means. Pump/motors of this design can have increased efficiency due to reduction of friction, leakage, and compressibility losses as well as increased displacement control bandwidth. A unique contribution of this work is the development of a coupled dynamic model of a digital hydraulic pump/motor that is crucial for understanding the design tradeoffs and operating characteristics. The simulation model is used to characterize and predict pump/motor efficiency, define the dynamic response and flow requirements of on/off valves required to provide significant improvements in efficiency and dynamic response over traditional pump/motors, and perform design optimization studies. Different operating strategies have been analyzed to characterize the effects on pump/motor efficiency and flow ripple (valve timing effects, partial fill methods, etc.). A three-piston pump/motor unit was used to experimentally validate the model, design, and operating strategies of a digital pump/motor. For a digital pump/motor with direct acting valves, sequential flow-limiting showed the best efficiency followed closely by sequential flow-diverting and partial flow-limiting. The partial flow-diverting method had the worst efficiency results. The key enabling technology for the digital pump/motor is the on/off valves. The largest simulated losses were due to the throttling across the valves and the electrical energy consumption required too actuate the valves.