The gains values that can be imposed in pneumatic systems 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 payload or supply pressures. For modelling and control intent a correct characterisation of the pneumatic actuator natural frequency is indispensable. The aim of this paper is to evaluate how heat exchange process affects the proper characteristics of pneumatic drivers, and in particular their hydraulic 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.

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