High Speed Rotary Pulse Width Modulated On/Off Valve

A key enabling technology to effective on/off valve based control of hydraulic systems is the high speed on/off valve. High speed valves improve system efficiency for a given PWM frequency, offer faster control bandwidth, and produce smaller output pressure ripples. Current valves rely on the linear translation of a spool or poppet to meter flow. The valve spool must reverse direction twice per PWM cycle. This constant acceleration and deceleration of the spool requires a power input proportional to the PWM frequency cubed. As a result, current linear valves are severely limited in their switching frequencies. In this paper, we present a novel fluid driven PWM on/off valve design that is based on a unidirectional rotary spool. The spool is rotated by capturing momentum from the fluid flow through the valve. The on/off functionality of our design is achieved via helical barriers that protrude from the surface of a cylindrical spool. As the spool rotates, the helical barriers selectively channel the flow to the application (on) or to tank (off). The duty ratio is controlled by altering the axial position of the spool. Since the spool no longer accelerates or decelerates during operation, the power input to drive the valve must only compensate for viscous friction, which is proportional to the PWM frequency squared. We predict that our current design, sized for a nominal flow rate of 40l/m, can achieve a PWM frequency of 84Hz. This paper presents our valve concept, design equations, and an analysis of predicted performance. A simulation of our design is also presented.