Across all fields of metrology, there has been a marked increase in the use of non-contact measuring systems. While contact probing systems are often regarded as the gold standard in terms of accuracy, non-contact systems offer many advantages. Simple and rapid collection of vast datasets are obviously well suited to measuring complex freeform surfaces but can also be helpful with unknown or variable items, or when geometric tolerances need to be evaluated.
Non-contact technologies such as laser line scanners and structured light systems are mainly associated with portable measuring systems including portable arms and handheld scanners. They are also used as a non-contact sensor head on some static CMMs – in the right application these can offer the flexibility of non-contact measurement with the accuracy of a traditional CMM.
The Data “Arms Race”
Since rapid, detailed datasets are an advantage of non-contact scanning, it is not surprising that there is an apparent “arms race” among vendors to offer ever greater data capture rates. Terrestrial scanners capture millions of data-points per second, and established systems also tend to upgrade regularly on the resolution and speed of capture. This can be useful – gathering points faster can speed up inspection and increasing resolution can improve performance when measuring small features – but progress comes at a cost. Computer processing speeds do not keep pace with dataset sizes, and there are often limits to data transfer rates that can slow down scanning or cause crashes and data loss.
UK based INSPHERE delivers regular portable metrology good practice training and these days find they spend a lot of time discussing appropriate methods for filtering datasets for practicality. Users generally demand less data rather than more!
Automating The Process
Using a traditional, programmable CMM, it takes time to establish good robust measurement routines, but they can then operate with very little human intervention. Scanners offer increased flexibility and ease of use but can become laborious in production settings if many parts must be measured by an operator performing a repetitive manual task. There is a growing trend towards automation in scanning and a variety of “scan box” configurations are available to allow a repeat job to be programmed and then performed automatically. These inspection cells may use structured light or laser line scanning technologies, the benefits of which depend on the intended application of the system.
INSPHERE is currently seeing a lot of current interest in its HYPERSCAN cell which uses a robot-mounted Leica T-Scan in combination with other metrology to deliver a complete, automated inspection, well-suited to challenging surfaces and tight tolerances.
With all non-contact systems there are challenges in interpreting specifications, determining system accuracy, performing calibrations, and assessing capability for any particular task.
At the heart of the problem is perhaps the fact that systems use fundamentally different technologies and they perform differently on different parts – a metrology system that measures matt metallic surfaces with great accuracy and precision may perform spectacularly badly on a shiny carbon-fibre surface.
Vendors naturally publish specification data in formats that flatter their equipment. This can make initial down-selection of equipment difficult, and it is hard to maintain a good awareness of all the available technologies as they are proliferating so rapidly.
Calibration is governed by standards, but it is hard for standards agreed by international committees to keep pace with technology developments. The ISO10360 standards covering acceptance and re-verification tests for coordinate measuring systems has grown and grown. It now has 12 published parts and part 13 (Optical 3D CMS) is under development.
Scanners and other non-contact systems can be particularly susceptible to bias and errors due to the influences of operator skill, environmental conditions and their use in relatively uncontrolled measurement settings, and this emphasizes the need for applied capability assessment. Appropriate forms of Measurement Systems Analysis (MSA), including Gauge Repeatability and Reproducibility (GR&R) and Evaluating the Measurement Process (EMP) studies are invaluable in building confidence that new measurement systems are fit for their intended purpose.
Advances in Scanning Technology
At the recent CONTROL exhibition in Stuttgart the mind-boggling variety of available systems are testament to the rise of the scanners. It also serves as a good reminder that metrologists are now faced with an ever-increasing array of tools with which to solve measurement challenges. This is positive news – it shows the optimism in the industry – but it also makes clear that capturing data is the easy part, valid interpretation of that data is the great challenge we still face.
Industry 4.0 continues to promise a better future of SMART factories with more controlled processes and step changes in productivity. Metrology systems will undoubtedly be pivotal in driving advanced manufacturing forwards, and metrology skills will remain in high demand to make sense of the vast complexity offered by new datasets.
For more information: www.insphereltd.com
This article was authored by Craig Davey – Chief Operating Officer at Insphere