Initial friction compensation by disturbance observer based on rolling friction model

This paper presents an initial friction compensation by a disturbance observer in the fast and precise positioning of ball screw-driven table systems, on the basis of a rolling friction model. The rolling friction behaves as a nonlinear elastic element in the table drive mechanism, especially in the micro-displacement region, deteriorating the fine positioning performance. Effects of the rolling friction on the positioning, therefore, should be compensated to achieve the desired control performance. In the controller design, a feedback control with a disturbance observer allows the plant system to behave as a nominal one with robust stability, and compensates for effects of nonlinear friction on the positioning performance. However, the disturbance observer has an estimation delay for the initial friction behavior at the starting motion in positioning due to a low pass filter and delay components. In this paper, therefore, a rolling friction model is adopted as an initial compensation of the disturbance observer to compensate for the initial friction behavior, and provides the delay-free estimation. The proposed compensation method has been verified by experiments using a prototype of industrial positioning devices.