Model-Based Friction COIllpensation in Hy-draulic Servo Sy-steIlls

In the general framework of tracking control of hydraulic servo systems, this paper presents a comparison between two main possibilities to include friction information in the picture: experimental identification and estimation. Experiments were first conducted in order to find a suitable representation for friction in the system. We then looked at two observers reported in the literature (known as FriedlandMentzelopoulou and Tafazoli) for simultaneous on-line estimation of friction and velocity. Additionally, we developed a new variablestructure observer for the same purpose. In order to compare results, a model-based friction compensator using acceleration feedback control has been used. The best tracking performance has been achieved using the identified friction model, at the expense of considerable time spent with the experimental identification. Friction estimation using nonlinear observers are a viable alternative as far as model-based friction compensation is concerned, but a loss of accuracy occurs.