Accurate position control of a pneumatic actuator

A control strategy is proposed for on-off valve-controlled pneumatic actuators and robots, with emphasis on position accuracy. A mathematical model incorporating pneumatic process nonlinearities and nonlinear mechanical friction was developed to characterize the actuator dynamics. This model, with a few simplifications, was used to design the controller. In this control scheme, one valve is held open and the other is operated in pulsewidth-modulation mode to simulate proportional control. An inner loop utilizing proportional-plus-integral control is formed to control the actuator pressure, and an outer loop with displacement and velocity feedbacks is used to control the load displacement. A two-stage feedforward force is implemented to reduce the steady-state error due to nonlinear mechanical friction. Experimental results on a single-degree-of-freedom pneumatic robot indicate that the control system is better than the conventional on-off control strategy as it is effective in achieving the desired position accuracy without using any mechanical stops in the actuator.