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Static and quasi-static errors measurement and compensation in milling machines (SEC & TEC)

Deliverables

The measuring device for the SEC application, consisting of three linear CCD cameras, has been obtained by improvement and optimisation of the existing Krypton K600 type. For brevity's sake it will be referred to, from now on, as K600. Error correction of temperature influences has been studied. This 3D measuring system is capable of tracking multiple targets (LED's) in space. For each target, X, Y and Z co-ordinates are calculated. Good measurement resolution and accuracy, combined with low system weight make it an interesting tool for 3D co-ordinate measurements in the field (shop floor applications).
A similar system has been developed and optimised for the TEC application. It consists of two CMOS matrix cameras and is based upon Krypton's type K100. From now on, it will be simply referred to as K100. The K100 has a lower absolute accuracy than the K600, but still has the good repeatability that is the essential requirement of the TEC application and is considerably less costly, which makes it better suited as a component to be integrated on a machine in a permanent fashion.
The PhD thesis of Dipl.-Ing. Peter Hirsch which is expected to be completed about three years after the SEC&TEC project is concerned with the compensation of thermo-elastic displacements at machine tools. Based on internal data of the machine's controller an indirect compensation model is developed which abandons external sensors for cost reduction. After a reduced pre-calibration procedure the mathematical compensation model calculates the correction values by the input of electric current and speed of the moved axes and the main spindle, which is kept by the NC controller. Furthermore, it is tried to integrate a volumetric approach to calculate the correction values for arbitrary points within the working volume of the machine. Here within the SEC&TEC project first experiences were made.
The PhD thesis of Liang Zhou, which is expected to be completed about two years after the SEC&TEC Project. Higher accuracy machines are always a growing industrial demand, which is continuously drawing researchers' efforts for cost-effective solutions. The progress gained in machine design, production technology, measurement and modelling technology have greatly improved machine performance, especially since the computer and their software tools began to be adopted for machine tool applications. However, the machining errors caused by temperature changes still remain a big topic for industry and researchers, because 30% to 70% of the errors on machine are thermally caused. The major work of this thesis research is to study and enhance the machines' performance by eliminating/compensating the thermal errors. By optimisation of machine design, part of the thermal errors can be eliminated or reduced, but the research will be based more on application of measuring devices and software error compensation. The activities involved in error compensation will cover error measurement techniques, error modelling techniques and compensation algorithms. The results of the research will be passed to relevant industrial partners or published in form of technical paper, conference article and thesis text.
Flexibility demands and time restrictions in modern manufacturing require that dimensional metrology is done close to the production process. As a result, an increasing number of coordinate measuring machines (CMMs) are placed in the harsh shop floor environment. This shift necessitates managing the CMM's thermal behaviour, because non-standard temperature conditions can compromise the measurement accuracy. The dissertation presents a software compensation based on the temperature distribution of the machine. The goal is to widen the temperature restrictions under which the CMMs can achieve their specified accuracy. The research strives for a workable approach for thermal measurement errors. To this end, the number of temperature sensors is kept to a minimum. Preference is given to deformation measurement equipment that is part of the common, everyday gear for CMM manufacturers. The software compensation is build up gradually. A series of laser interferometer measurements enabled to correct the temperature dependency of the machine scales. In a next step, a parametric model is drawn up via the well-considered superposition of the thermal behaviour of individual machine elements. This model enables to correct for the thermal displacements of the measurement probe during temperature transitions. The efficiency of the thermal correction for the targeted (realistic) thermal loads is shown through various tests on two different CMMs. A pragmatic attitude towards external heat sources must ensure that the thermal effects remain manageable for the software correction. Drawing up a parametric thermal model, against popular tendencies, resulted in an added value for the research. The gained insight in the thermal behaviour enabled suggesting design improvements for CMMs, hereby increasing the relevance of the research. The feasibility of the developments is validated through a prototype implementation by an industrial partner. The used measurement methodology in conjunction with the parametric model facilitated the transfer of the research results to industry. Design suggestions have already been implemented on newer CMMs. C. Van den Bergh, Reducing thermal errors on CMM located on the shop-floor, November 2001, ISBN 90-5682-319-1.