Difference between Single–Point and Area Scanners

There is a fundamental difference in philosophy between full-field non–contact scanners and mechanical touch probes on CMMs or mechanical arms. A full–field scanner can capture millions of measurements, revealing intricacies of freeform shapes that the point–based measurement system (such as a CMM or laser tracker) is simply too slow to acquire. However, when inspecting a feature such as a hole, a CMM's x–y resolution means that it may only need three or four points to measure the hole diameter – a full–field system would have to capture many more points to rival this precision.

This is illustrated by the object in Figure 2, which has a feature that we wish to inspect – a large-diameter hole. The CMM can inspect this by gathering perhaps four points, and estimating the hole diameter – touch points that might be used for measuring the hole are shown.

In contrast, a full–field scanner would gather many more measurement points. Its z–accuracy could be similar to that for the CMM, but the x–y resolution per point would be significantly less – see Figure 3.

Providing the full–field scanner can gather enough points, it will give as good an estimate for the hole diameter as the CMM – but in addition, it can estimate many other parameters of the object using the same data. For example, the roundness of the hole, the flatness of the plane around the hole, or the locations of the mounting boltholes for the casting, can all be determined from data which was gathered in a few seconds. As this can often include the reference datums for the part, the need for complex fixturing – and part location procedures – can also be reduced.

With sufficient measurement points, area based scanner accuracy can be indistinguishable from measurements made with older, slower technologies such as CMM or lasers – and that is often the case, where measurements are distributed across a freeform component or an aerofoil. While it is unlikely that the accuracy of a single point measured with an area scanner will rival a single point measurement from a CMM for several years to come, with sufficient pixels on each feature, the full–field scanner can offer the same accuracy – while also delivering free–form shape measurement and significant benefit in throughput.