Position control of X-Y table at velocity reversal using presliding friction characteristics

This paper aims at precision position control of the X-Y table of a computerized numeric control (CNC) machining center at velocity reversal. The characteristics of presliding friction are analyzed, and a simple and effective method is proposed to compensate the friction on the basis of these characteristics. A large position tracking error occurs at velocity reversal due to the sudden transition of friction between presliding regime and sliding regime. This paper investigates the transition time to reduce the tracking error, and derives a relationship between the transition time and the acceleration at zero velocity. This paper also proposes a method of estimating the transition time using this relationship, without having to measure velocity. Experimental observations confirm this correlation holds over a large dynamic range. In addition to friction, there is a large change in a torsional displacement at velocity reversal in a two-inertia system with finite stiffness like an X-Y table linked to a motor through a ballscrew. The proposed friction compensation scheme can be easily incorporated with the compensation method for torsional displacement to achieve good tracking performance. The experimental results are described to show the effectiveness of the proposed method.