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Noncontact 3D-based Machine Vision in Metrology
by Nello Zuech, Contributing Editor - AIA Posted 10/25/2005
Today one finds a variety of ways to capture 3D data. Still the most used tool is the coordinate measurement machine (CMM). While the tactile probe is still the workhorse in capturing the data on a point-by-point basis, the CMM industry appears to finally be embracing noncontact sensors. Most CMM manufacturers now offer one or more noncontact probes. While some essentially substitute a noncontact point probe for the tactile probe, some capture a point cloud of data, either a line at a time or over a region at one time.
Another approach to capturing 3D data mechanically is using an articulating arm. While these also are generally offered with mechanical probes, today one finds 3D-based noncontact probes as an option as well. In addition, there are many noncontact systems that are targeted at specific applications – reverse engineering, CAD input, Computer-Aided-Inspection (CAI), etc. Another characterization of these 3D machine vision-based noncontact metrology tools can be made based on measurement envelope: small - < .5M, medium - <2M and large - >2M.
There are any number of approaches that are being used: stereo photogrammetry, structured light, triangulation, tracking interferometry, phase shift interferometry, laser radar, etc. The ability to collect the data using these approaches has been refined over the years yielding competitive performance. The ability to handle the point cloud of data generated by these approaches has also become more rigorous over the last couple of years with the advances that have been made in microprocessor and other compute capabilities.
Recognizing that these 3D machine vision-based noncontact systems are becoming increasingly more relevant to the metrology world, this article is written to address the issues specific to metrology. We asked virtually all companies known to be participating in this market to address the following questions. The following responded to the questions:
- Dr. Juergen Dold - VP Business Development, Metrology Division, Leica Geosystems
- Giles Gaskell - Director of Business Development, NVision Inc.
- Andy Cumming – VP Marketing, Metris
- Tobby Y. Li – Opton (parent company of MiiC America, Inc.)
- Roland Hoefling - ViALUX GmbH
1. Can you provide a brief general description of your noncontact 3D-based machine vision technology that addresses metrology/computer-aided inspection applications? Basic theory, brief implementation description (cameras, optics, lighting, material handling, need for cooperative targets, etc.), volume measurement envelope, speed, etc.
[Dr. Juergen Dold – Leica Geosystems] Our fundamental approach is based on a local positioning technology that utilizes a tracking interferometer combined with a digital camera. This technology enables the user to walk around in measurement volumes of up to 30m with a hand-held probe and hand-held scanner to inspect and digitize with accuracies of below 100-microns. The hand-held scanner - Leica T-Scan - is a triangulation scanner that captures millions of points in minutes.
[Giles Gaskell – NVision] Our product - the ModelMaker - is a 3Dimentional Laser scanning system that collects point clouds. It uses a Laser strip up to 140mm wide and collects 25,000 points per second to an accuracy of 0.1mm. It will work in all lighting conditions and will scan all colors and opaque materials including shiny metal. It fits on a portable CMM and has a measurement envelope that depends on the length of the arm.
[Andy Cumming – Metris] Metris scanners are mounted utilizing the Renishaw PH10 interface on bridge CMM’s, horizontal arm CMM’s and articulated arms. As such they turn a standard touch probe measuring system into an optical 3D measuring device. The Metris laser scanners working principle is based on triangulation. A laser beam is projected onto the objects surface and a camera simultaneously captures 25 lines per second or 19200 points per second. The measured point cloud data is used for inspection and reverse engineering purposes. The overall accuracy of the scanner is up to 8 microns.
[Tobby Li - Opton] OPTON 3D sensors are based on the white-light structured lighting method. A grating (parallel black/white lines) pattern is projected onto a 3D surface by using a light source and a projection lens. The original parallel black/white lines will deform according to the 3D shape of the surface. In other words, the 3D shape information of the surface is encoded into this deformed grating image, which is taken into a computer via a digital camera. The Fourier Transform method is used to decode the 3D shape information from the deformed grating image. The result is digitizing the surface into a set of points with known X, Y, & Z coordinates (so called point cloud data).
[Roland Hoefling – Vialux] In the Vialux approach the object surface is imaged into a camera and patterns are projected onto the surface under an angle of incidence that differs from the imaging direction. There are various approaches to extract the height change) Z from the virtual shift between the patterns projected at P and P´, respectively. In general, the surface has to be encoded by the projected patterns in such a way that the camera can read out this code reliably, unambiguously, and with high spatial resolution. Based upon the active fringe projection methodology, the system takes advantage of a series of patterns projected onto the object surface and observed by a camera to provide reliable, accurate and highly resolved 3D data from any scattering object surface.
2. What have been some applications for your 3D noncontact metrology systems?
[Andy] The main application is dimensional quality control in the automotive and aerospace manufacturing industries. Examples include inspection of features on sheet metal parts, inspection of automotive interior parts, inspection of turbine blades, etc. Non-contact laser scanning is also used for reverse engineering throughout manufacturing industries.
[Tobby] Reverse engineering and inspection.
[Roland] The system is used in i) 3D recording of human beings (face and hand for biometry, medical applications) and ii) in in-line inspection systems for industry.
[Giles] Reverse Engineering Aerospace and Automotive components and inspection thereof. Scanning Racing Drivers to help design cars that fit them. Scanning bodies to help design clothes. Reproducing Ancient Sculptures. Inspecting for corrosion in Box-section girders plus many others.
[Juergen] Inspection of products (such as car or aerospace parts) or tools (such as jigs, dies and molds).
3. Are there specific volume measurement envelopes for which your approach is more suitable? Why?
[Roland] The typical measuring volume is < 0.5x0.5x0.3 m³ at ½ m working distance, smaller volumes (up to 1x1x1 cm³) are not a problem.
[Giles] Yes, anything that’s too big to fit within the frame of a CMM is obviously better handled by a portable optical system such as ours.
[Juergen] Our T-Scan is suitable for measurements volumes starting from several centimeters up to 30 meters. Our hand-held Leica T-Scan with Leica Tracker is very unique, because it achieves a length measurement accuracy of less the 100 microns within a 30-meter measurement volume.
[Andy] As laser scanners are mounted on CMM’s, they operate with the same working volumes as touch probes and are only limited by the size of the CMM.
[Tobby] Not really, we can work on almost any size from as small as ½ inch to a full-size car model.
4. Is it the case today that various noncontact approaches to 3D measurements are viable competitors of coordinate measuring machines? What has made this possible?
[Giles] There is no real need to compare CMMs and portable non-contact measurement systems. CMMs today are only of use where sub 0.1 mm accuracies are required on geometrical rigid components. On practically anything else, sheet metal, plastics, anything with a complex form or made of a non-rigid material, CMMs are hopelessly impractical and out of date compared with the portable optical system of today.
[Juergen] Yes, our solution is a very viable competitor to the large volume CMM, because we achieve the same accuracy without the need for the part to be brought to the measurement machine and without the need for heavy and costly infrastructure as CMMs need (foundation, stable concrete floor). The accuracy of less then 100 microns in 30 meter measurement volume is only possible because we have created a measurement system that combines laser tracker technology with a very unique camera technique.
[Andy] The main advantage of Metris laser scanner is that it digitizes full parts in a rapid way. As such they complement the touch probes that are more suitable for feature inspection. A recent innovative Metris scanner, the XC50 Cross Scanner is designed to measure features by scanning simultaneously from three directions. This scanner is fast and measures many more points of a feature than a touch probe giving many advantages for form measurement.
[Tobby] Yes, at least in our case since we make CMM-like machines (Surftizer series machines), which are based on structured lighting sensors. In addition we can also integrate a conventional touch probe onto our Surftizer machines as an option. These Surftizer machines are programmable (just like a CMM) to carry out un-attended measurement. Therefore, our Surftizer machines are competing with conventional CMMs. Technology advances in the past few years makes this competition possible.
[Roland] In my opinion, this is not the case in general. Optical systems are superior when it comes to:
- mobile applications
- critical measuring times, in-line testing
- digitizing with high density of points
- production environment
- live science applications
- objects with small dimensions.
Otherwise, CMMs are ahead.
5. What has been the impact of third-party software that can take point cloud data and analyze it against a CAD data file? Why is this important?
[Juergen] Depending on the application, different software is required. For reverse engineering applications it is required to collect large point cloud data and to organize the data for the export to reverse engineering packages. For inspection applications, effectively the inspection software shall compare the measured point-cloud in real-time with the CAD reference. Leica's software T-Scan Collect and T-Scan Inspect are able to perform both.
[Andy] The ability to do full part inspection using point cloud data is now considered a necessary tool in the metrology world. For this reason, Metris also provides integrated solutions that handle the whole inspection workflow from scanning including the comprehensive software packages to enable point cloud manipulation and analysis against CAD.
[Tobby] A data processing software package is very important since nobody wants only the point cloud data.
[Roland] Third party software is an important added-value for laboratory systems, not so much for industrial testing.
[Giles] Obviously the impact has been great since without it the optical point gathering systems wouldn’t be of any use.
6. How do noncontact techniques compare to coordinate measuring machines in terms of resolution, accuracy and repeatability – based on what standards B89? ISO10360? Others?
[Roland] There are many approaches to bring the accuracy of optical system to that of leading CMMs, they are successful – but only under certain conditions! The SURFACE properties are the crucial point for non-contacting optical measurements (more difficult to manage than ambient light conditions). All standards that I know are based upon dull, bright, opaque test objects.
The problems arise if the surface is translucent or high-gloss or deep-black or in the worst case: all together. A CMM measures almost everything - except jelly - an optical systems does not! At least not without expertise, care and skill. It is very common today to overcome the surface problems by spraying the object white – this is not a desirable solution for several reasons: e.g. spray particle size is > 5µm and the removal is not solved. There are better solutions on the way (e.g. the deep blue LED of z-Snapper) but are not yet commonly exploited.
[Andy] In general non-contact techniques are slightly less accurate and repeatable, but provide a much higher resolution. The standards are currently being investigated by various committees (such as OSIS) that are defining the standards for comparing touch probes with non-contact technology.
[Tobby] Data has more spatial resolution but is less accurate and repeatable than CMMs.
[Juergen] Today there is no standard that relates to Laser Tracker/Local Positioning System based accuracy. The closest to the standards is the description how accurate a length can be measured within a certain measurement volume. However, the accuracy definition for hand scanner or any other scanner is much more complex. New standards must be developed that consider the important influences of different surface geometries, surface reflexivity characteristics and also different measurement volumes.
7. Are there specific volume measurement envelopes where noncontact metrology techniques are more competitive? Why?
[Andy] Noncontact laser scanners can be more competitive in all working volumes where large quantities of data are required.
[Tobby] No. What to measure is the most important factor. Machined features (hole, plane, and cylinder...) are more suitable for a CMM.
[Giles] None that we are aware of.
[Juergen] The Leica T-Scan solution is the only measurement solution that allows one to measure with only one setup within a measurement volume of up to 30m. This is important as it reduces measurement time significantly, especially in applications where large tools, parts or multiple large dies are to be measured in one setup. This greatly reduces the down times of parts.
8. Can you comment on the relevance to noncontact metrology approaches of the following standards: ASME Y14.41, DMIS – Dimensional Measuring Interface Standards, STEP – Standard for the Exchange of Product Model Data?
[Juergen] As said, no standard fits right now to our solution with the hand-held walk-around scanner. We have defined the accuracy so that it gets somehow comparable with existing standards. However, we are looking forward that one of the relevant organizations will take up the initiative to start the definition process.
[Giles] The standards don’t apply to non-contact measurement, do they?
[Tobby] The existing standards are not fully applicable to non-contact 3D measurement products. Lack of an industrial standard is one of the tasks which the optical 3D measurement community needs to solve.
[Andy] Laser scanners will certainly be integrated in the DMIS environment. As such they will act as a standard probe that will deliver point clouds rather than individual points.
However the current DMIS standard has not been extended to optical measurement, which has led to a variety of different definitions by the CMM software manufacturers.
9. What are some differences between how actual feature measurements are made by coordinate measuring machines and noncontact metrology approaches?
[Juergen] the major difference is that with the Leica solution you can setup the system in minutes at the place were the part is located and start measurements without any preparation such as preprogramming, without powdering (like many line and patch scanners), without photogrammetric targets (like many photogrammetry scanning systems), without any transport of parts to the CMM. Today’s software packages still have some challenges for high precision feature extraction from point clouds. However, a system that provides tactile functionality and non-contact functionality ensures that the operator can always do his job. The Leica T-Scan combined with the T-Probe ensures exactly this flexibility.
[Tobby] Non-contact approach is more like 'measure the shape first and then use software to extract actual features.' Contact approach is a feature-based measurement.
[Andy] The main difference is that the Cross Scanner measures in one single move a much higher number of points in a short time. The number of points that define the feature will, therefore, be much larger leading to better information and accuracy on the form of a feature.
[Roland] Example 1: The measure of a distance can frequently be taken from the distance of two planes – and many measured points of an optical system contribute to the plane fit. This gives a high accuracy and reliability.
Example 2: The diameter of a hole can be conveniently measured by a CMM, this is much more difficult for optical systems where just the edges are demanding to get.
[Giles] You measure in 3D dimensions and about 50,000 times as fast.
10. What industries are embracing noncontact 3D-based metrology systems and what are the drivers for that?
[Giles] Across the range of industries led by Automotive and Aerospace. The drivers are speed of obtaining results and the demand for quality.
[Andy] Mainly automotive and aerospace industries where fast and accurate inspection is necessary. The full part-to-cad inspection requirement, practically impossible to determine using touch probes, with the possibility of calculating 3D sections is the main driver.
[Tobby] Automotive and Aerospace. Simply for more measurement points and cover more surface.
[Roland] Automotive industry is important always. Medical applications are gaining more and more acceptance, and security is well on track.
[Juergen] Automotive and aerospace push scanners because of faster and more complete measurements and to reduce down times in design processes and production processes.
11. What are the specific application issues that one must be attentive to when deploying machine vision-based, noncontact 3D metrology systems?
[Andy] Laser scanning is suitable for almost all applications. However, laser scanning is not suitable for transparent or very shiny mirror-like parts.
[Tobby] Automation and price.
[Roland] (See above), surface and ambient light, shadowing for all triangulation setups.
[Juergen] We have overcome the need for controlling the environmental light and the need for photogrammetry targeting. Therefore, our solution is:
- Fast in large measurement volumes and reliable.
- Ease of use. Walking around with a Leica T-Scan without setting out a photogrammetric network is much easier than many other portable scanning solutions.
- Flexibility. The measurement systems should allow both tactile and non-tactile approaches in one go with one system. This reduces measurement time and increases accuracy and decreases training cost.
[Giles] Make sure the subject doesn’t move when you are scanning it.
12. What is it that you require from a prospective buyer of a noncontact metrology system for an industrial application to assure you will deliver a system that will satisfy them?
[Roland] Sample objects, environmental conditions, testing speed and accuracy, surface variations, data interface.
[Juergen] Cost consideration should include not only initial investment. If the system can make precise contact measurements and scanning measurements it can do both with one system. Other systems may need an additional system for tactile measurements. This increases training cost and also increases the source of errors.
They should also take into account how fast an untrained user can learn the system to make reliable and accurate measurements. A photogrammetry system typically requires highly skilled operators to generate data that are in the entire measurement volume (also at the edges of the measurement volume) accurately. CMM operators typically need to be highly skilled to pre-program a measurement machine.
[Giles] The prospective buyer needs to be able to understand what an appropriate measurement technology for his product is, i.e. there’s no need for measuring in thou’s if your product is sheet metal or foam rubber! Apart from that: payment.
[Andy] The customer must understand that this is a comparatively new technology compared to coordinate measuring machines. This means that there are no definitive standards yet to guide him during the purchase process.
13. Are there specific application issues that must be understood insofar as selecting the correct noncontact approach?
[Juergen] Yes, the best is if the customers - if possible - have reference parts that are very close to what he wants to measure. The customer should be aware that the surface characteristics have an influence on the measurement accuracy.
[Giles] How big is the object, what’s it made of (Hard or soft) how quick do you need it measuring?
[Andy] Over many years cooperation with our customers has resulted in the development of Metris scanners for different applications:
- The Metris LC15 is a dedicated scanner for digitizing small parts with very high-resolution and accuracy (e.g. small plastic injection parts or turbine blades.
- The Metris LC50 is used for fast inspection of larger parts such as car panels, dashboards.
- The XC50 Cross Scanner is a dedicated scanner for feature measurements.
[Tobby] Yes. Such as how you want the object measured and reported, fully automatically or manually.
14. Are there some emerging technology changes associated with the underlying technology embodied in your 3D-based noncontact metrology systems that will positively impact your product line? What will those impacts be?
[Giles] Yes, our optical Scanner the ModelMaker will scan Shiny Metal objects in any lighting condition. Nothing else can; so this will impact us very positively.
[Andy] Faster (digital) cameras with higher resolution will continue to drive increasing speed and accuracy.
[Tobby] Yes. Speed, Accuracy, and Automation.
[Roland] We recently have seen a very strong positive impact by DMD Discovery Micromirror light modulator technology (Texas Instruments) and this is continuing. LED lighting is a second technology to name. PC processor and RAM capacity progress, finally, have a positive impact on the evaluation time.
[Juergen] The combination of our Leica Local Positioning technology with the Leica T-Scan will change current approaches to how surface measurements will be done. Now it is possible to measure up to 50% faster and up to 2-3 times more accurate than other comparable measurement solutions with no need to prepare surfaces with powder and photogrammetric targets, or without the need to relocate an articulated arm to measure objects that are larger then 1-2m, or without the need to bring the part to a CMM.
15. Any advice you can give to someone investigating a noncontact 3D metrology system application?
[Juergen] Yes, show the vendors your application and let them show how they would solve your application. Don’t ask everyone to do it the same way. Let them do it how they believe the systems will be best used and check at the end the time required and the accuracy. And finally let your operator repeat the measurement in the way the vendor did it and check whether you get the same results and what type of training efforts were required. If the training is short and the results are good, you will get fast return from your purchase. And yes, make sure that the vendor is able to provide you with local after sales support because you certainly want to learn more from your vendor where else you can use the new scanning technology. And last but not least, it’s certainly easier for you if one vendor provides you with the complete solution from probing to scanning to software solution - so you are sure, that everything works together… When it has to be right!
[Andy] Dare to radically change your inspection processes, this technology is now mature and the benefits are great. Partnering with a market leading company like Metris with many years of experience and expertise in the field of laser scanning will allow the realization of these benefits to be seamless.
[Tobby] Try the real part to verify everything.
[Roland] Start with 3D cameras that are already available instead of setting up a dedicated development (as it was common in the past).
[Giles] Yes. Shop around and benchmark under the exact same conditions you will experience in reality. If you need to measure outside - benchmark outside! Also, be realistic about your measurement needs; match your product to the measurement technology.
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