Abstract
Multi-sensor coordinate measuring machines (CMM) have a potential performance advantage over existing CMM systems by offering the accuracy of a touch trigger probe with the speed of a laser scanner. Before these systems can be used, it is important that both random and systematic errors are evaluated within the context of its intended application. At present, the performance of a multi-sensor CMM, particularly of the laser scanner, has not been evaluated within an automotive environment. This study used a full-scale CNC machined physical representation of a sheet metal vehicle body to evaluate the measurement agreement and repeatability of critical surface points using a multi-sensor horizontal dual arm CMM. It was found that there were errors between CMM arms and with regard to part coordinate frame construction when using the different probing systems. However, the most significant effect upon measurement error was the spatial location of the surface feature. Therefore, for each feature on an automotive assembly, measurement agreement and repeatability has to be individually determined to access its acceptability for measurement with a laser scanner to improve CMM utilisation, or whether the accuracy of a touch trigger probe is required.
Highlights
Accurate measurements are important in being able to monitor production processes and ensure conformity to design specifications [1]
The error compared to the calibrated diameter of the ceramic sphere was 9.6 μm (2σ = 3.9 μm) with the LH coordinate measuring machine (CMM) arm fitted with the touch trigger probe and 16.7 μm (2σ = 2.1 μm) with the RH CMM arm
A full scale automotive artefact with the same geometry and similar surface properties as a vehicle body was used. It has been found, taking the worst case, that there was a systematic error of 66.7 μm between the touch trigger probe and the laser scanner
Summary
Accurate measurements are important in being able to monitor production processes and ensure conformity to design specifications [1]. A touch trigger probe has been employed as the CMM sensing mechanism because of well-established calibration processes and knowledge of measurement uncertainties [4, 5]. While laser scanning is well-established in the reverse engineering field and in the inspection of freeform surfaces, lack of knowledge about measurement uncertainties has meant their use in the dimensional control of mechanical parts has been more limited [4, 10, 11]. The different data acquisition methods of the touch trigger and laser scanning probes cause systematic errors which affect the agreement between the measurement results and random errors which affect individual probe repeatability
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