Thermic Influence on the Dynamics of Pneumatic Servosystems

Abstract

The gain values that can be imposed in pneumatic system controllers are bounded to the restricted actuator bandwidth. That limitation, with low damping and stiffness due to the air compressibility, seriously affects accuracy and repeatability when varying payloads or supply pressures. For modeling and control intents, a correct characterization of the pneumatic actuator natural frequency is indispensable. The aim of the paper is to evaluate how heat exchange process affects the proper characteristics of pneumatic drivers and, in particular, their pneumatic stiffness. To this purpose dynamic stiffness had been studied both by imposing in the cylinder’s chambers a polytrophic transformation of the fluid with a prefixed index and by employing energy equations. Numerical results obtained by implementing the two formulations for different working conditions are reported and compared in order to point out the ranges in which they overlap, and hence both approaches produce accurate results, or the ones in which there is a difference, and then it is necessary to consider the temperature dynamics.