Abstract
Lost motion is a major disturbance that affects the contouring accuracy of NC machine tools. Although NC machine tools with plain bearing guideways are capable of high alignment accuracy, their lost motion sometimes varies according to motion conditions such as position, velocity and moving length. Conventional backlash compensation with a constant parameter applied to the 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 the lost motion is derived from the model. A tuning method for the compensation parameters is also proposed. Stiffness of the position-dependent spring is calculated from the specifications of mechanical parts, such as a ball screw, bearings and a coupling. The varying friction is estimated by a disturbance observer. Contouring accuracy in a circular test of NC machine tools demonstrates the improved performance enabled by the proposed method.
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