Adaptive Lost Motion Compensation Using Disturbance Observer

Abstract

Lost motion is a major disturbance to the contouring accuracy of NC machine tools. Although NC machine tools with plain bearing guide ways perform high alignment accuracy, their lost motion sometimes varies according to motion condition such as position, velocity and moving length, so on. Conventional backlash compensation with constant parameter applied to that lost motion, causes contouring error due to inadequate compensation. This paper presents a model of a two-body system with a position dependent spring and motion condition dependent friction that exhibits varying lost motion, and an adaptive compensation method for that lost motion introduced from the model. Tuning method for the compensation parameters is also proposed. Stiffness of the position dependent spring is calculated from specs of mechanical parts, such as a ball screw, bearings and a coupling, so on. The varying friction is estimated by disturbance observer. Contouring accuracy in a circular test of NC machine tools demonstrates the improved performance of the proposed method.