| 3D Coordinate Measuring Machines (CMMs) are universal, flexible, and adequate instruments for measuring dimensions of complex products. So far, many types of CMM configurations have been developed, each for a specific range of manufactured products. Basically, a CMM is a robotic machine that positions a sensing probe in three-dimensional working space. The probe contacts a sample of locations on the part surface and the CMM records corresponding three-dimensional point coordinates. The inherent ability to measure almost an endless variety of geometrically complex parts in a rapid and accurate manner has led to widespread use of CMMs in industry.; Using a CMM to closely inspect complex contours, forms, shapes and holes, a large number of points can thus be quickly probed. Collected coordinates of sampled data points are then reduced to workpiece geometric characteristics that are further compared to predefined tolerances. However, errors imbedded in the CMM hardware and software undermine the accuracy of point measurement data collected to provide acceptance of manufactured parts. Indeed, computations to reduce measured point coordinates to feature parameters can be a source of error due to both flaws in the CMM software and limitations of fitting algorithms. Introduced by the mechanical structure, deviations from the probe desired path are, on the other hand, considered to be highly detrimental to the CMM accuracy. These errors in motion are caused by geometric imperfections as well as thermally-induced deformations of the machine structural elements.; It is rather impossible to completely eliminate errors induced by the CMM mechanical structure through better design and improved manufacturing process. Even a small geometric error associated with a given structural element can be amplified at the probe tip through slides motion over long travel lengths and probe offsets. On the other hand, improved structural design to reduce thermal expansion may dramatically increase costs of manufacturing without totally eliminating thermal deformations of machine elements.; Efforts directed towards improving the performance of multi-axes machines have focussed mainly on identifying and tackling structure-related problems. In this context, the growing need for enhanced multi-axes machines accuracy without incurring additional costs imposed a software-based alternative approach to provide real-time prediction and correction of geometric, kinematic, and thermally-induced errors. (Abstract shortened by UMI.)... |
