Design of digital pump/motors and experimental validation of operating strategies
Abstract
Most fluid power systems involve conversion of mechanical work to fluid work through a pump. Similarly, many fluid power systems also convert the fluid work to mechanical work through a hydraulic motor. Thus, the overall efficiency of hydraulic pumps and motors is critical to the efficiency of fluid power systems. Current state-of-the-art axial piston units can operate at high efficiency over a limited range of speed, pressure, and displacement. The emerging field of digital hydraulics is providing opportunities to widen the operating conditions where a pump/motor can operate at high efficiency by incorporating high-speed on/off valves to decouple the unit shaft position from the commutation of the displacement chambers to the unit working ports. This freedom permits operating strategies unique to digital pump/motors, but the main advantage is that leakage and friction losses can scale more closely with pump flow. This work defines novel operating strategies of a digital pump/motor in the context of both pumping and motoring. Unit displacement can be continuously varied by switching valves during the piston stroke. Alternatively, digital operation can be implemented by enabling or disabling individual displacement chambers over a sequence of pumping or motoring events. These continuous and sequential methods can be implemented simultaneously. In pumping, the flow-diverting operating strategy controls effective displacement by delivering demanded fluid to the high pressure port. Remaining fluid is returned to the low pressure port. In the flow-limited operating strategy, effective displacement is controlled by the volume of fluid allowed to enter the displacement chamber. Operation in the flow-limited operating strategy leads to a unique condition called displacement chamber voiding where the geometric volume of the displacement chamber has grown beyond the volume of the fluid admitted into the chamber. Displacement chamber voiding was visually observed through the use of a clear hydraulic cylinder and high-speed camera. A single-piston and a three-piston digital pump/motor were fabricated to experimentally evaluate the operating characteristics of the digital pump/motor and to investigate tradeoffs in operating strategies and in the overall configuration. Algorithms were established for continuous and sequenced variation of displacement in flow-diverting and flow-limited operating strategies for both pumping and motoring. These algorithms were implemented through a model operating on a real-time controller with execution through a field programmable gate array. Over the tested range of displacements, digitally sequenced implementation of the flow-limited operating strategy yielded the highest efficiency. Digitally sequenced operation in either operating strategy and in both pumping and motoring exhibited significantly less torque ripple. Furthermore, operation of the unit in pure digital sequenced displacement control is less challenging because valve transitions occur when piston velocity is lowest, resulting in lower error in actual displacement. With two normally closed valves installed at each displacement chamber, electrical energy consumption to operate the on/off valves scales more closely with displacement in sequenced flow-limited operation because the frequency of valve transitions and valve on duration both decrease with displacement. This research forms the basis for the development of a true four-quadrant digital pump/motor that is capable of self starting and is capable of maintaining high efficiency over a wide range in operating displacement, speed, and differential pressure, especially when compared to state-of-the-art units. Furthermore, this work is the first to present a set of data for a digital pump/motor operating in flow-diverting and flow-limited operating strategies as a pump and as a motor.
Degree
Ph.D.
Advisors
Lumkes, Purdue University.
Subject Area
Agricultural engineering|Mechanical engineering
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