Optical device for the improvement of positioning accuracy in large machine tools

摘要

The very large machine tools now available for applications ranging from aerospace to composite material castings present a new set of challenges when trying to match the traditional machining accuracies of the mechanical workshop. An optical system for the automatic recalibration of the machine during the machining process has been developed. While conventional linear and rotary optical encoders are used to control axes positioning, and their resolution is already more than compliant with the needed accuracies, our optical system provides a further error signal during operation in order to compensate for structural deformations of axis and sliding parts. Those signals are used in order to reach a global positioning error vector under 50 microns on a 3-axis translation stage. The system has been installed on a test machine, with a total range on the 3-axis of 3, 1.6 and 1.2 meters. The device increases ranging measurement accuracy by decreasing the dependence of the position to the temperature variation and other deformations. To achieve such results, collimated diode lasers and 2D position-sensing devices have been installed to the machine. In order to provide signals which are least affected by electrical noise, transimpedance amplifiers and analog to digital converters have been integrated close to the detectors. The tests performed on the prototype demonstrate the capability of mapping the actual distance from the ideal linear translation with an error of 25 μm along the full axis travel for a tracing capability of ±3.5 mm in both directions on each of the three detectors. This result is within the requirements of the end users, manufactures of travelling column type boring and milling machines.