3D measurement demystified - a guide to specification

Ralph Weir, CEO of Phase Vision sheds some light on the various 3D measurement technologies available in the marketplace today.

The ever-increasing range of equipment available is based around three main system types – point-based systems, consisting mainly of contact probes mounted on either a co-ordinate measuring machine (CMM) or measurement arm, and laser trackers; stripe-based systems; and area-based measurement systems.

Modern systems of each type generate results compatible with latest CAD software. However, each represents a significant investment so it is a decision which companies cannot afford to get wrong.

The eventual decision depends on many factors including measurement speed, accuracy needed, size of objects being measured, and the conditions under which measurement will take place. For example, many items expand as the ambient temperature increases, meaning measurements taken over a long period in an inconsistent temperature may not be completely reliable. This effect must be considered, and even where the system itself is not affected, will limit the accuracy achievable in real production environments.
Point-based systems

CMMs are the longest established 3D measurement system. Traditionally, CMMs have a granite base on which the product to be measured is placed, but this restricts their scope, as no item overhanging the base can be accurately measured, meaning even a very large CMM cannot cater for all requirements. Meanwhile, their lack of portability means objects to be measured must always be brought to them, irrespective of size, weight and fragility. They must also be operated in a temperature controlled environment – and the component to be measured must be allowed to stabilise at that temperature – to gain the CMM's accuracy.

Measurements via a touch probe can be taken manually using a measurement arm, or with a CMM – which can be operated manually or programmed to move automatically.

Laser trackers typically consist of a spherical mounted retroreflector, containing a complex arrangement of mirrors, in front of a tracker. The measurement created is always to the sphere's centre, rather than the surface, while the sphere must be moved manually each time.

Point-based systems are highly accurate, but measure relatively few points over the same amount of time compared with other system types, making them less valuable when measuring items where a detailed understanding of surface characteristics is required. However, point-based systems come into their own in applications where knowledge of the exact position of a few points in relation to each other is required. A typical example is on aircraft wing jigs, which can be up to 20m long but with only 30-40 crucial ‘pick-up' points for where wing ribs will be located. The same principle applies on large metal panels used in shipbuilding to ensure they will fit together in the right places – both ideal applications for laser trackers.

The accuracy of point-based systems is also highlighted when measuring highly polished or reflective surfaces. High specification lenses, for example, require very accurate measurement to ensure they are within tolerance. Point-based systems also give an excellent idea as to the nature of holes, where relatively few measurements can provide detail on diameter, and only a few extra measurements can start to determine roundness.

Where any form of system using a touch probe is not ideal, however, is in measuring items whose surfaces may be affected by contact with the probe. The humidity of the measurement environment must also be considered, as extended high humidity levels can impact on the performance of laser trackers, as can air currents.

Stripe-based systems

Stripe-based systems, such as laser scanners, can be either handheld or mounted on an arm or CMM. They can usually measure ‘stripes' up to 100mm long, taking many thousands of measurements to create point clouds which must then be patched together to create a complete representation.

The patching process can be highly time-consuming unless a high specification system is used which can perform this process automatically, but in any case, the patching process introduces errors. For this reason, combined with their small stripe length, they are not best suited to measuring very large areas. Where they do offer advantages is in measuring where access is restricted, for example in car footwells. Handheld scanners are also well suited to measuring longer items such as large turbine blades, where a measuring arm cannot be used as it is not long enough to scan the entire item without being moved.

The accuracy of laser scanners, however, relies heavily on operator expertise. They are normally designed to operate at a specific distance from the object being measured and any deviations can impact significantly on results. Unfortunately, inaccuracies generated in this way are not always apparent at the time.

Materials which either absorb light or can be penetrated by light also do not lend themselves well to laser scanning. Casting wax and Rohacell – commonly used in carbon fibre manufacture – are just two examples. Effectively what happens is that the laser goes through the surface and measures points underneath it, again generating potentially misleading results. Meanwhile, factors such as inconsistencies in laser focus, laser detune and laser speckle all mitigate against fast scanning of larger areas.

Area-based scanning

Area-based measurement systems such as white light scanners offer advantages over laser scanners, not least in terms of the area they can measure at once – up to several metres across if required – although systems exist to measure very small objects as well. As with laser scanners, a point cloud is generated but there is less reliance on operator skill once the system is calibrated, while the size of area scanned greatly reduces the need to align patches.

These systems do, however, again pose challenges in terms of patching in instances where object's shape cannot be relied upon to dictate the patching process – as is the case with large, smooth surfaces with few features, such as aircraft wings. In these instances, either a rotating table (for smaller objects) or photogrammetry targets in the form of adhesive labels must be employed to facilitate patching. The presence of holes where precise detail is needed on depth and shape can also pose problems best solved by a touch-based system.

Again, white light scanners can experience problems when measuring highly reflective or light absorbent surfaces, although they do offer some control here. However, their speed and ability to measure a complete surface within a few seconds mean white light scanners lend themselves ideally to high throughput environments and full integration into production lines where any inaccuracies or irregularities can be immediately detected and production stopped if required to allow process rectification. This is simply not achievable with point-based or stripe-based systems.

Meanwhile, their robustness and portability makes them ideal for highly demanding environments – even dirty production areas, aircraft hangars and docksides.

Developments continue however and each system type will always have its proponents. For those companies able to employ dedicated in-house measurement staff, or where accuracy of just a few points is needed, a CMM or laser-based system may be the best option. However, the rapid pace of developments in white light scanning, its ease of use and robustness, means it is more frequently becoming the norm especially where speed is of the essence or large object measurement is required.

Phase Vision
www.phasevision.com