Dynamic behaviour of pneumatic linear actuators

The use of pneumatic linear actuators is generalised in engineering applications because of their many advantages, but modelling the force they supply may become more of a challenge due to their nonlinear behaviour and the hysteresis their energy losses cause. The authors propose a straightforward model to accurately predict force-displacement behaviour using as a basis experimental observations for several pressures and harmonic displacements of the rod. The model proposed includes two dissipative terms: one due to Coulomb friction and another due to structural damping. The force is proportional to relative pressure when acting as an actuator but nonlinear (modelled as a polytropic transformation) when acting as a pneumatic spring (with a closed pressurised chamber). The model accurately reproduces experimental results (Normalised Root Mean Square Errors lower than 2.5%) and may be used in control systems as well as in adaptive stiffness systems.