A pressure-based, velocity independent, friction model for asymmetric hydraulic cylinders

Modelling and experimental identification of friction has been carried out for an asymmetric hydraulic cylinder. A practical model of friction for this class of actuators is described. The model captures the magnitude and direction of friction forces without explicitly including a velocity-dependent term. Instead, it uses values of fluid pressure at the two cylinder ports, to yield a model that is less sensitive to load variation. Experimental parameter identification was undertaken in parallel for a classical viscous-Coulomb friction model. Two distinct friction regimes have been revealed, corresponding to "low" and "high" velocities. A comparison between the two models shows that the loss in accuracy in the pressure-based model is less than 1% when compared with the conventional one. Stiction analysis has also been performed. It has been found in this case that it is reasonable to assume invariance of stiction with piston position and rate of applied input. Friction was found to be highly dependent on the direction of piston motion in asymmetric hydraulic cylinders. Finally, computer simulation results show agreement with experimental data.