Crank-slider Spool Valve for Switch-mode Circuits

A key component of switch-mode hydraulic circuits is a highspeed two-position three-way valve with a variable duty cycle. This paper presents a new valve architecture that consists of two valve spools that are axially driven by crank-slider mechanisms. By phase shifting the two crank links, which are on a common crankshaft, the duty cycle of the valve is adjusted. The two spools split and re-combine flow such that two switching cycles occur per revolution of the crankshaft. Because the spools move in a near-sinusoidal trajectory, the peak spool velocities are achieved at mid-stroke where the valve land transitions across the ports, resulting in short valve transition times. The spool velocity is lower during the remainder of the cycle, reducing viscous friction losses. A dynamic model is constructed of this new valve operating at 120 Hz switching frequency in a switch-mode circuit. The model is used to illustrate design trade-offs and minimize energy losses in the valve. The resulting design solution transitions to the onstate in 5% of the switching period and the combined leakage and viscous friction in the valve dissipate 1.7% of the total power at a pressure of 34.5MPa and volumetric flow rate of 22.8L/min. INTRODUCTION Switch-mode hydraulics, analogous to switch-mode converters from the field of power electronics [1], is an emerging method of controlling hydraulic circuits. This concept utilizes a high-speed valve to switch between efficient on and off states, while temporarily storing energy in inductive and capacitive elements. The mean flow or pressure is controlled by the duty cycle, defined as the time in the on position divided by the switching period. Switch-mode hydraulics have been proposed for buck/boost converters [2], pumps [3-6], linear actuators [7], engine valves [8], and multiple actuators [9]. The benefits of this approach are low cost, low weight, good response time, and improved efficiency over throttling valve control [3, 9]. The valve in a switch-mode hydraulic circuit has a demanding set of competing requirements for the circuit to achieve good performance and high efficiency. A high performance circuit, defined by a fast response time and a low flow ripple, requires a fast valve switching frequency. However, a fast switching frequency creates three main challenges. First, high frequency valves typically use a low mass switching element to minimize the inertial actuation forces. The low mass typically correlates to a small flow area, requiring a balance between fully-open throttling loss and inertial force. Second, each switching event results in throttling across the partially-open transitioning valve. This energy loss can be minimized through soft switching [10, 11] or by reducing the valve transition time, at the expense of increasing the velocity of the switching element. Finally, each switching cycle incurs losses due to compressing and decompressing the fluid in the switched volume. The compressible energy loss can be minimized by reducing the switched volume between the valve and the inductive element.