Development and performance evaluation of real-time geometric error compensation through position feedback modification in 5-axis machining

Abstract

Geometric errors in amachine tool structure are mainly responsible for the volumetric error in theworkspace. They occur at the attachment of each link between axis joints, but also along each axis in their joint frame. Reducing the impact of these errors is a key factor in guaranteeing the functional requirements of high value-added parts. Unlike mechanical correction, software compensation strategies are often chosen for their ease of implementation and versatile nature. In this study, a correction method by modifying the position measurement in real time is introduced and compared to compensation tables. The reaction response of the numerical controller (NC) to the modification of its position feedback is studied, and a 5-axis machining experiment to validate the proposed solution is performed. The principle of the experiment is to impose a virtual volumetric error in the workspace by modifying a machining program, then to test separately the ability of compensation tables and the proposed method to correct the chosen virtual geometric errors. The aim is to obtain a corrected workpiece similar to the one machined with a nominal program. In this way, it is not necessary to identify the geometric errors of the machine’s structure to test the performance of software compensationmethods. The machined workpieces feature geometries that are easy to control, but the tool paths generated to produce them were complex enough to challenge the compensation methods. The ability of the proposed solution to correct the virtual volumetric error introduced by a modified machining program is evaluated at 98%. Indeed, roundness measurements show that over 99% of the added error has been corrected, with residuals lower than 5 μm. Furthermore, the joint trajectories monitored during machining are studied through a contouring error estimation. Nominal and compensated trajectories are 98% similar with the proposed solution, compared with 35% for compensation tables.