Real-time compensation of time-variant volumetric error on a machining center.

Abstract

This dissertation is concerned with the enhancement of the volumetric accuracy of a horizontal machining center through off-line calibration, on-line sensing, and real-time compensation techniques. Geometrically-induced machine errors are calibrated off line with the use of a laser interferometer system, while thermally-induced errors are predicted on line based on the measurement of temperature changes on various locations on the machine. The prediction of the thermal errors is made by both multiple regression analysis and artificial neural network that correlate the errors to measured temperature changes. Both methods adopt a multi-input approach to accommodate the different thermal time constants of various machine components. Test results show that both models can accurately predict new observations of thermal errors under varying spindle speed and temperature history. A comprehensive model has been developed to predict the spatially-variant and time-variant volumetric error by synthesizing the geometrically-induced and thermally-induced errors existing at machine elements. In addition, cross-coupling effects of linkage errors between slides have been considered and added to the model by both off-line curve-fitting and on-line identification approaches. The off-line method relies on a grid like measurement for the coupling effect, while the on-line method uses an in-process laser measurement system to identify the linkage errors with coupling effects. A compensation controller is developed using an IBM/PC to upgrade an existing CNC controller for volumetric error compensation. The compensation controller constantly sends out correction signals to the CNC controller to shift the NC reference points in real time so that complex contour compensation can be achieved without any hardware modification to the machine servo-drive loops. The compensation scheme implemented on the machining center has improved the machine accuracy by one order of magnitude. The actual dimensional errors of a cut part with a nominal dimension of 404 x 310 mm$\sp2$ were reduced from 92.4 $\mu$m to 18.9 $\mu$m and could be further reduced to 6.1 $\mu$m after the correction of non-rigid body kinematic effects. The depth variation of milled surfaces was also reduced from 196 $\mu$m to 8 $\mu$m.