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Quality Scan: How Good Is Your Scan?

 Chuck Pfeffer



By Chuck Pfeffer
Vice-Chair
Coordinate Metrology Society
Benbrook, TX




Noncontact measurement is the fastest growing segment in the metrology world, with new methods and technologies being introduced every year. As with all new solutions, end users want to know how to evaluate the performance of these new products, and for that they turn to standards organizations.

The catch is that standards take time, so with all new technology there is typically a period where suppliers and end users struggle to find common ground. The suppliers want to highlight their benefits; the end users want to ensure they get the data they need.

The one existing standard for 3-D scanners is the German VDI/VDE 2634, which was published in August 2002, but it’s not recognized as an international standard. There are currently two committees working on standards for scanners. The ASTM E57 committee is developing standards for midrange scanners, but has abandoned its effort for short-range 3-D imagers. The ISO Technical Committee 213 Working Group 10 is developing international standards for scanners attached to the end of Cartesian CMMs, but does not include articulating-arm CMMs. They may possibly work on standards for other types of scanners in the future. These standards are likely several years from publication.

So now is the time to get involved in the creation of standards that will define how we evaluate existing and future imaging technologies. This past year, I wrote a white paper on this subject, which was presented at the 2010 Coordinate Metrology Systems Conference. The paper explores the complexities in defining a standard or set of standards for short-range 3-D imaging and scanning systems. It starts with a review of different scanning methods and characteristics, and continues with a discussion of proposed test methods and evaluation criteria.

Currently, the ISO Technical Committee 213 Working Group 10 is working on a draft of ISO/CD 10360-8 Geometrical Product Specifications (GPS)—Acceptance and reverification tests for coordinate measuring machines (CMM)—Part 8: CMMs with optical distance sensors. As
contributions are coming from all over the world, the details of this standard are still in flux. The latest version proposed was met with resistance from some users and manufacturers. One of the difficulties for this group is finding a methodology that will make it possible to apply CMM procedures to scanners.

The goal of any standard is to give end users a way to evaluate product performance to see if it meets their needs. To accomplish this goal, a system must be tested as it will be used. This means that for laser-line scanners, they need to be tested on their carrying device. For area imagers, they need to be tested over their full volume and, in many cases, over multiple images that are registered together.

Both standards discussed here include tests on spheres and flat artifacts, but the reality is that most users want to measure something else. Users want to know how well a system can measure a feature, an edge, a corner, or down in a hole. These are all complex evaluations that have not yet been addressed.

Other factors that need to be considered are surface finish and lighting, which often play a part in data quality and quantity. Many systems have settings that reduce noise, but at the expense of resolution. The ISO standard draft gives the manufacturer the opportunity to prescribe settings for the tests, but it is possible that different settings yield different results. If the user’s application cannot use the same settings as tested, then their results may be different.

Even though there are no international standards, there is a way that users can get involved. The manufacturers of 3-D scanning and imaging equipment provide very little in the way of test results, especially when it comes to accuracy. Customer demand is the best way to remedy this situation. I urge metrology users to take it upon themselves to ask for data from manufacturers to help them understand how the products made by those builders will perform in the users’ applications.

These issues will continue to be deliberated in the coming years. I am interested in feedback from users of noncontact scanners, and encourage you to send your thoughts to vicechair@CMSC.org. To familiarize yourself with the Coordinate Metrology Society and our work, please visit
www.CMSC.org. ME

 

This article was first published in the January 2011 edition of Manufacturing Engineering magazine.  Click here for PDF.

 


Published Date : 1/1/2011

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