Techniques for measuring and compensating for servo mismatch in machine tools using a laser tracker

Abstract

Servo mismatch, which affects positioning accuracy in computer numerical control (CNC) machine tools, is an important source of error. Dynamic measurements of simultaneous movements in two axes are essential in determining servo mismatch. A circular test using a double ball-bar (DBB) is used commonly to assess such errors. However, servo-mismatch measurements of some machine tools, such as miniaturized machine tools, remain difficult due to structural restrictions of the measuring devices. In this study, a technique for servo-mismatch measurement via a bi-directional circular test using a laser tracker is described. The laser tracker is easy to set up experimentally and has fewer measuring range restrictions than other measuring devices. In order to ignore the effects of quasi-static errors, a bi-directional circular test is conducted using the laser tracker. Additionally, a compensation procedure, with a proportional equation, is proposed. The validity of the technique described was confirmed by comparison with the DBB result. In a machining test after servo-mismatch compensation, by adjusting the position loop gain, the roundness of a cylinder part was improved by ∼47.9%.