On the Development of a Modified On-Off Controller for Pneumatic Servo Mechanisms

A low cost pneumatic servo positioning system which utilizes a modified on-off controller is developed to ease with system complexity as well as to enhance positioning accuracy. The feature of this servo system is that it utilizes additional reference signal band termed "pulse band" outside a deadband. When an error signal is outside the deadband but inside the pulseband, the valve is operated like a dither according to a periodic pulse train generated by a modified controller, whereas in other region the valve is operated in a conventional on-off mode. To evaluate the effectiveness of this controller a series of experiments was conducted for various controller design parameters such as pulse band, pulse period and deadband. The experimental results are compared with those obtained by a theoretical stability prediction. Both results show that higher positioning accuracy can be achieved by this simple control configuration, as compared to the accuracy that can be achieved by a conventional on-off controller.