Switch-mode hydraulic control is a compact and theoretically efficient alternative to throttling valve control or variable displacement pump control. The virtually variable displacement circuit created by a pulse-width modulated high-speed valve has drawbacks, such as large energy losses due to throttling and compressibility during valve transitions. Hydraulic soft switching was proposed as method of reducing the throttling energy loss, by absorbing, in a small variable volume chamber, the flow that would normally be throttled across the transitioning high speed valve. An active locking mechanism was presented by Van de Ven that overcomes the main challenge with soft switching, which is a lock mechanism that releases quickly and with precise timing. A numerical model is developed for a switch-mode pumping circuit, utilizing the proposed soft switch. The model is then used as a means of designing a proof of concept prototype to further verify the model and provide a future optimization tool. The prototype design includes methods for controlling the soft switch spring preload, travel distance, piston displacement required to unlock the soft switch, valve command duty ratio, switching cycle length, and load pressure. Testing demonstrated that the soft switch circuit performed as expected within a set of parameter values, defining normal operation. High control valve switching periods, high load pressures, high flow rates, low soft switch unlocking distances, and low spring preloads caused the soft switch to unlock prematurely in the switching cycle. This resulted in the soft switch to failing to absorb the switched volume pressure spike during control valve transitions, leading to low efficiencies. Under normal operating conditions, the soft switch circuit was calculated to be 60.9% efficient, which is lower than the 62.1% efficiency for same circuit without a soft switch, and also lower than the 70.5% efficiency predicted by the numerical model. The discrepancies between the numerical and experimental model can be attributed to poor resolution in the measurement instruments, as well as the model not accurately capturing flow dynamics through the check valves behind the soft switch piston. Since the soft switch parameters were chosen to validate the lock release concept, and not to maximize efficiency, it was not unexpected that the baseline soft switch case had lower efficiency than the control case. Using the insights gained from these model comparisons, an efficient locking soft switch can be designed for use in a dual soft switch circuit scheme, shown by Van de Ven to reduce energy losses by 66% compared to a control circuit. A successful hydraulic soft switch circuit will allow for the use of slower switching valves, which will reduce component cost as well as energy consumption.
University of Minnesota M.S.M.E. thesis. June 2015. Major: Mechanical Engineering. Advisor: James Van de Ven. 1 computer file (PDF); vii, 109 pages.
Design and Validation of a Soft Switch for a Virtually Variable Displacement Pump.
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