Modeling and Control of a Miniature Servo Pneumatic Actuator

Pneumatic actuators are low-cost, safe, clean, and exhibit a high power to weight ratio. In this paper a novel servo pneumatic system based on a miniature cylinder with a 9.5 mm bore size is presented. Four low-cost 2-way proportional valves are incorporated to provide greater design flexibility than the traditional single 4-way servo valve solution. A nonlinear system model is developed and validated using open-loop experiments. The use of bipolynomial functions to model the valve flow rates provides a more accurate solution than the commonly used nozzle flow equations. A novel multiple-input single-output nonlinear position control law is designed using the backstepping method. The stability analysis includes the effects of friction modeling error and valve modeling error. In experiments with a 1.5 kg moving mass, the new control law produced maximum tracking errors of $\pm {\bf 0.5}{\bf mm}$ for a 1 Hz sine wave trajectory, and steady-state errors within $\pm {\bf 0.05}{\bf mm}$. The tracking errors are 82% less than those produced by a linear controller.