Friction compensation for servo systems

In order to compensate for the effects of friction, an effective observer-based compensation scheme is proposed. Friction can lead to tracking errors, limit cycles, and undesired stick-slip motion. The popular LuGre model and the friction model with clamping were compared. It pointed out that friction is a natural phenomenon and cannot be simply depicted by an unidirectional signal flow relationship from velocity (v) to the friction force (F). In this sense the LuGre model is not a real model of the friction, and the results of friction compensation with the LuGre model-based observer are not always satisfactory. The disturbance observer (DOB)-based friction compensation was then proposed and analyzed in the paper. The stick-slip motion and the stick-slip limit cycle are typical behaviors for systems with friction, so the effectiveness of the DOB-based compensation was verified through examinations of these nonlinear behaviors under compensation. It shows that the amplitudes of limit cycles are greatly reduced with the DOB-based compensation, and the stick-slip motion at low velocities is also eliminated.