Design and Characterization of a Rotary Actuated Hot Gas Servovalve

This paper describes the design and characterization of a unique three-way hot gas servovalve designed for the flow control of steam at 235°C. The valve incorporates a pressurebalanced rotary spool coupled to a servomotor/gearbox/encoder combination to achieve high-bandwidth and high-precision operation, and also incorporates several design elements in order to accommodate the high temperatures associated with the working fluid. The spool and sleeve entail geometries of low aspect ratio and are mechanically isolated from the manifold with Viton O-rings to ensure uniform thermal expansion and contraction. To thermally isolate the DC motor, a PEEK motor mount is used to connect the motor housing with the valve manifold. Additionally, the motor shaft is coupled to the spool with an Oldham coupling that incorporates a PEEK center disk to further insulate the motor from the high temperature spool. The design is presented, along with experimental data that characterizes the dynamic performance and flow characteristics of the valve. INTRODUCTION Just as electrically-powered servo systems utilize power transistors to provide high-bandwidth real-time control of electrical power, so does a fluid-powered system utilize servovalves to provide high-bandwidth real-time control of fluid power. Specifically, a servovalve is designed to proportionally throttle a fluid flow, thus imposing a desired pressure drop across, or flow rate through, the valve (depending on the boundary conditions). Typically, servovalves are used in a three-way or four-way configuration. A three-way configuration incorporates two inlet ports and one outlet (or vice-versa), and connects either one inlet or the other to the outlet with a controllable orifice area. A four-way configuration, which is the most common for fluid-powered servo control applications, has two inlet ports and two outlet ports, and connects either inlet with either outlet with controllable orifice area, while also connecting the other inlet/outlet pair with the same area. Figures 1 and 2 illustrate the use of three and four-way servovalves for the control of a pneumatic servoactuator. Note that, while control via single four-way servovalve is typical, the configuration utilizing a pair of three-way valves can offer a greater degree of control flexibility, as described in [1, 2].