Compensation algorithms for the real-time correction of time and spatial errors in a vertical machining centre

Abstract

This paper describes research into a machine tool error compensation system for universal application. Based on an indirect identification precalibrated technique, it utilizes a unique algorithm, which allows the compensation system to compensate for the geometric error components of any normal orthogonal machine tool configuration. The movement and position of the machine tool axes can affect individual machine tool axis error components (such as yaw, pitch, roll and straightness). The level of this axis coupling is dependent on the machine tool configuration and the rigidity of the machine tool structure. Also, thermal effects can affect the machine tool axis error components. The kinematic (rigid body) model will be modified to allow for the non-rigid effects, and a novel technique for reducing workpiece errors caused by the thermal distortion of a computer numerical control (CNC) machine tool is introduced. This new approach to thermal error reduction is based on an indirect measurement technique where temperature/distortion relationships are developed by splitting the problem into two parts, a thermal model and a distortion model. Thermal imaging has been used extensively, and research into its usefulness for developing models has been under investigation. The use of separate thermal and distortion models allows analytical techniques, such as finite element analysis, to be used to verify performance. Novel techniques for the fast, accurate and detailed geometric calibration of CNC machine tools were also investigated. Although the wide availability of modern metrology equipment has provided the means for accurate measurement of machine errors, geometric calibration of machine tools is a time consuming, labour intensive and therefore costly process. The methodology and validation of the universal rigid body compensation model with its machine-specific non-rigid body and thermal effects suitably integrated have been demonstrated. Other elements such as calibration methodology, dynamic measurement, non-rigid effects/compensation and thermal distortion models will be the subject of further papers leading to the overall objective.