Investigation of digital pump/motor control strategies
A pump is the heart of fluid power systems, it has a significant impact on the efficiency of many fluid power systems. Motors are the most common rotary actuators in fluid power systems. State-of-the-art pump/motor units can achieve efficiencies higher than 90% when operating at maximum displacement; however, as the displacement drops, the efficiency of these units drops to below 50%. A new digital pump/motor design aims at increasing these efficiencies by utilizing two electrically controlled high speed on/off valves per displacement chamber; these valves provide the ability to achieve variable displacement and allow freedom in choosing operating strategies. Such a design reduces the compressibility and leakage losses since the chamber would only be pressurized during the working cycle. A simulation model was developed to predict the effects of the valve timing on the behavior of the digital pump/motor. Simulation and experimental testing showed that valve timing is crucial for the success of digital pump/motors. This work addressed the valve limitations on the experimental test stand by investigating a new set of valves which could deliver faster switching times. When tested at 103 bar differential pressure, 500 rpm at 120 F, these valves provided up to 50% improvement in switching times, resulting in up to 15% simulated increase in the digital pump/motor’s overall efficiency and up to 12% increase in overall efficiency experimentally. The model was also used to investigate different valve timing algorithms, showing that the pressure ripples on the high and low pressure lines could be used to predict the optimal valve timing. A code was developed to calculate the simulated delays, and it was experimentally validated with a real-time valve timing correction algorithm. A mode switching algorithm was investigated. Each operating strategy (partial flow diverting/limiting and sequential) has its advantages and disadvantages which vary depending on the operating conditions and system parameters. An algorithm was written to actively select the most efficient operating strategy. Experiments were conducted at speeds of 300, 500, and 700 rpm, differential pressures of 34.5, 103.4, and 172.4 bar, and displacements of 50, 75, and 100 %. For a 3-piston digital pump/motor unit with accumulators installed on both ports, sequential operating strategies yielded the highest efficiencies and lowest ripples in most of the operating parameters.
Lumkes, Purdue University.
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