QMT Features: September 2015
How does your 3D optical scanner perform on the shop floor?
On 30 September 2015 NPL will be launching a performance verification facility to support the increasing use of 3D optical scanners in industrial environments. Here Dr Martin Dury describes what the facility has to offer and why you need to use it.


Optical scanners provide point-cloud data collection, portability and speed and will play a significant role in the future of quality control for production line metrology. Being readily available and using fast-developing technology the capabilities of 3D optical scanners are not yet fully understood and scanners are known to have difficulties measuring certain surfaces and geometries.

Sales demonstrations often employ artefacts optimised to give results that show the best capability of the instrument; the artefacts may be finished with particularly co-operative surfaces such as ’battleship grey‘ paint and comprise unchallenging shapes. For an end-user measuring production engineering objects in a real-world environment, the spurious and unreliable dimensional data that an inappropriate scanner can produce may be both time consuming to correct and may damage user confidence in the technology.

As optical scanners become more commonplace, end-users will need help with identifying suitable instruments for their needs, while scanner manufacturers will need independent verification to support and demonstrate their instrument capability claims. To encourage industry’s growing use of 3D scanning technology the National Physical Laboratory (NPL), the UK’s National Measurement Institute, is establishing a 3D optical scanner performance verification facility which is due to be launched in September 2015.

Fringe projectors, articulating arm CMMs incorporating laser scanners and other types of portable 3D optical scanners are quickly taking the place of Cartesian-based tactile probing CMMs because of their advanced metrology benefits. Having been developed in the 1950s, Cartesian CMMs are well understood, trusted and methods of verifying their performance are well established. In contrast, optical scanners, whose development has accelerated with increasing access to low cost, powerful computers and cheaper optics, are relatively new and are still being developed.

While optical scanners are unlikely to give Cartesian CMM-level accuracy, they offer the potential for great positives, such as massively increased measurement speeds, equipment portability and relative ease when measuring freeform surfaces. However, their limitations are still being understood and international performance verification standards, like ISO 10360 for CMMs, that describe suitable tests and procedures for their acceptance and use are yet to be developed. Currently available guides such as VDI/VDE 2634, the German guideline for optical 3D measuring systems, address neither freeform surfaces, surface finish nor instrument measurement performance while operating in unfavourable environments.

In 2009, NPL established the National FreeForm Centre to support the growing industrial take up in the use of non-contact 3D coordinate measuring systems. Scientists at the Centre have since developed artefacts with known dimensions such as the NPL dimensional freeform standard (Figure 1), which are now available and supplied complete with traceable measurement data together with associated measurement uncertainties.
A purpose-built dimensional verification laboratory is now being developed to assess many different aspects of 3D optical scanner performance combined with their environmental sensitivities. The aim of this new facility is to provide global industry with greater measurement confidence when using 3D scanners in various environmental conditions.

The portability and fast operation of optical systems make them particularly attractive to use away from the metrology laboratory and in environments where lighting and temperature conditions can quickly vary, such as on the production line, or outside at excavation sites and crime scenes.

Temperature is of critical importance when performing accurate dimensional measurements, not only affecting the object being measured, but also known to influence the performance of optical scanners. Being devices that use projected light to make dimensional measurements, it should be of no surprise to discover that ambient light can also affect these instruments. Understanding the limitations of a piece of equipment and how its environment can affect the measurements is vital when trying to performing accurate metrology in non-ideal surroundings. The purpose built laboratory (Figure 2) has adjustable lighting and temperature to allow 3D optical scanner performance testing over a range of typical use conditions.

Some 3D optical scanners have difficulty in accurately measuring objects of particular colours, materials or surface finishes and subsequently give results that contain anomalous data or regions where very little data are measured; while additional measurements may help to resolve these issues, they can be time consuming and costly.

Knowing the limitations of a potential scanner before committing resources would certainly benefit the end-user and also help scanner manufacturers to manage their customers’ expectations. Multi-faceted test artefacts have been built at the National FreeForm Centre to quantify the ability of optical scanners to measure different surfaces and test pieces are being developed for businesses that would benefit the most from 3D optical scanner uptake, such as the aerospace, automotive, manufacturing, medical and heritage industries.

In addition to temperature, lighting and surface finish, articulating arm CMMs incorporating laser scanners may also be affected by scan speed and produce higher quality data and more dimensionally accurate scans when moved slowly, which costs time. The National FreeForm Centre has therefore developed a system (Figure 3) to find the optimum balance between scan velocity and measurement quality as well as examining the effects of scan height and joint encoder angle on the measurements.

The 3D optical scanner verification facility combines a unique mix of staff with world quality expertise in dimensional metrology from decades of accumulated knowledge, a purpose-built laboratory, test artefacts and procedures designed to challenge 3D optical scanners to their limit. It has been developed for optical scanner users who already have or are thinking of buying a system and want to independently verify that it will work in the environment where they intend to use it and in the way that they want to use it, and that it will measure the objects that they want to measure to the accuracy that they require. The facility is also aimed towards optical scanner manufacturers who want to have an advantage over their competitors by getting their instrument capability claims independently verified at the National Physical Laboratory.
The verification facility will be launched on 30th September 2015.
For further information contact Dr Martin Dury (martin.dury@npl.co.uk)
www.npl.co.uk/freeform
  
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