A non-linear model of a rotary pneumatic servo system

Abstract

A simulation model of a rotary pneumatic servo system has been developed and verified against an experimental system. The double-acting rotary actuator is a novel device utilizing flexible inflatable bladders. The device differs from conventional pneumatic cylinders in that it exhibits low levels of friction, as it has no sliding seals, but it does suffer from hysteresis caused by bladder distortion. The objective of this study was to develop a tool with which to investigate the effect of various forms of hysteresis and other parameters on system performance. The dynamic model utilizes a linearized analysis of pneumatic components based on earlier studies. The principal advance described in this paper relates to the inclusion of non-linear elements. These are hysteresis caused by friction or the distortion of flexible components, and a digital controller implementation. The simulation successfully predicted all major modes of behaviour. Simplified models of rubber and coulomb friction-induced hysteresis modified simulation performance in line with experimental results. An investigation in simulation showed that coulomb friction-induced hysteresis had a greater damping effect than that caused by distorting the bladders. This result applies only to position control, as hysteresis has been shown not to provide damping in trajectory control applications.