High Speed Rotary PWM On/Off Valves for Digital Control of Hydraulic Pumps and Motors

Abstract

The research described in this dissertation focuses on the development of innovative on/off valves for high performance, high efficiency control of fixed displacement hydraulic pumps and motors. On/off valves, the hydro-mechanical equivalent of transistors, enable the application of digital control techniques found in electrical systems to hydraulics. These techniques, such as pulse-width-modulation (PWM), have the potential of combining the low cost, high bandwidth characteristics of valve control with the efficiency of variable displacement machines. Effective control of hydraulic systems using PWM requires that the on/off valve simultaneously exhibits fast switching speed, large flow area, and low actuation power. The valves developed in this dissertation exploit continuous rotary motion to achieve the desired, and traditionally competing, operating characteristics. A helical land is used to mechanically embed the desired PWM functionality into the valve spool. The rotary motion of the valve performs the switching functionality while its axial motion determines the PWM duty ratio. Several unique rotary valve concepts are presented in this dissertation for switched-mode pump and pump/motor circuits. An analysis framework is developed that predicts valve performance and typical losses which can be used for design and optimization. Physics based dynamic models of switched-mode pump and motor circuits are also developed for simulating system pressures and flow rates and for validating the analytical models. In addition, guidelines for sizing the valve sleeve based on fatigue considerations are formulated to aid prototype design. Prototype hardware is fabricated and extensively tested to validate the analysis, performance, and predicted efficiency of the proposed valves. The research in this dissertation verifies that helical land rotary valves used in switched-mode hydraulic circuits are capable of exceeding the efficiency of comparable metering valve circuits at moderate PWM frequencies. In two comparable systems, the switched-mode circuit achieved 84% efficiency at 50% output flow compared to 50% efficiency in the bleed off circuit. Analysis also shows that substantial gains in efficiency and switching frequency can be attained with improvements in valve configuration, circuit configuration, and valve geometry. Additional suggestions for further improving efficiency in switched-mode hydraulic systems are also discussed.