Publications

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A mesoscale dimensional artifact based on silicon bulk micromachining fabrication has been developed with the intention of evaluating the artifact both on a high precision Coordinate Measuring Machine (CMM), and on a video-probe based measuring system. A high accuracy touch-probe based CMM can achieve accuracies that are as good as the 2-D repeatability of video-probe systems. While video-probe based systems are commonly used to inspect mesoscale mechanical components, a video-probe system's certified accuracy is generally much worse than its repeatability. By using a hybrid artifact where the same features can be extracted by both a touch-probe and a video-probe, the accuracy of video-probe systems can be improved. In order to use the micromachined device as a calibration artifact, it is important to understand the uncertainty present in the touch-probe measurements. An uncertainty analysis is presented to show the potential accuracy of the measurement of these artifacts on a high precision CMM.

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CMMs equipped with non-contact probes, such as video probes, are becoming popular for a variety of 2-D or 2.5-D objects. The advantages of a video (or vision) probe include the ability to measure features which are either too small or too delicate for a touch probe. Unfortunately, vision-based probing systems do not have the same measurement accuracy as touch probe equipped machines. For example, a Moore M48 coordinate measurement machine has an expected measurement uncertainty of 0.2 μm (plus a scale dependent term) when using a touch probe (the actual repeatability is on the order of 0.03 μm). When the probe is changed to a Leitz LS1 vision system, the expected measurement uncertainty is 1.2 μm plus a scale dependent term. The decreased accuracy is due entirely to the change in probing method. Components of the error budget include environmental effects, choice of lighting, lens distortions, and stage 2-D accuracy. Lighting is a major contributor to the measurement error budget, especially when a bidirectional measurement needs to be made (for example, the width of a line, rather than the center location of a line). We report on the effect of the sensitivity of vision probing on an OGP Avant Apex 200 to different lighting conditions, both for unidirectional and bidirectional measurements.

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