Contact vs. Noncontact Measurement
Understanding the target is as important as hitting it. Your operation’s requirements on accuracy, speed and price will work as opposing forces in the decision-making process. In the early days of personal computing, there was a popular phrase: “Better, faster, cheaper; choose any two.” In CAI this factor still remains: Fast scanners will not be as accurate, more accurate scanners will cost more money.
Accuracy of Scanning
Establish what is the tightest accuracy required by the design specifications and work to them. Further establish if future products might have tighter restrictions on accuracy or if it will remain consistent—same production methods, similar product and materials, similar assemblies—in the known future. Tolerances are a key part to this discussion. Not all tolerances are created equally and some may be extremely rigid versus others where more flexibility is acceptable. That will need to be defined by engineering and needs to be commensurate with known process capabilities for the operation. Sometimes design engineers can add in particularly tight tolerances and accuracy requirements that are simply not practical in the production world. An accuracy of 5 microns is not needed if the production process being used will only be accurate to plus or minus 0.5 millimeter. It is important to clearly identify which tolerances are arbitrarily tight and which accuracy requirements are practical.
Fast measurements are not usually highly accurate measurements, and that holds true for contact and noncontact methods for inspection. Fast measurement is typically required if you have a scenario for in-line measurement, and then has to be balanced by the tolerance requirements in place. Sampling parts is a viable alternative to faster measurement, which can buy the time required to ensure that measurements are being adequately checked; i.e. if you measure one part in every 10, then you have gained 10x the time required to check the production quality.
Price
Greater accuracy and greater speed will result in a higher cost of the equipment required. Conversely, if you want to reduce the cost, you will forfeit either speed, or accuracy, or both. This means that you will need to choose up to two out of the three available factors: accuracy, speed or cost.
A contact device can be as cheap as $8,000 while industrial CT scanners will set you back a cool $150,000 to $500,000. Other technologies fall in between those two extremes, and be aware, even while measurement technology has dropped in price while gaining in accuracy and speed, metrology equipment can still cost some serious cash. Add into that equation the cost of inspection software, training and maintenance and you could be on the way to blowing through budgets.
But setting the expected return on investment (ROI) is also useful here. Measuring the price of quality is not always easy but the cost of bad quality can become alarmingly clear very quickly. Schneider Electric, an electrical product manufacturer that invested in an industrial CT scanner last year, managed to identify and solve a quality problem on a production line in one hour, using the CT scanner and inspection software. Prior to that investment, the problem encountered would have required at least 21 days of downtime at easily a cost of about $480,000—a single example of how the company easily achieved return on investment.
Contact solutions are a more traditional and widely accepted technology, and come in two main varieties: stationary CMMs and portable CMMs.
Stationary CMMs
Stationary coordinate measuring machines (CMMs) are typically very large installations—gantry, bridge and horizontal systems—that are highly accurate, expensive and much slower compared to other methods. These CMMs have zero portability and the part being measured has to be ported to the CMM itself, not vice versa.
Portable CMMs
Portable CMMs (PCMMs), which typically are stationed on an arm or are observed by a tracking device, are, as the name implies, highly portable and can be moved to a part rather than the other way around as with stationary CMMs. They are manually operated, and lower accuracy than stationary CMMs, but also come at a much reduced cost. Use of portable CMMs requires a lot less training, can be used on very large parts without requiring complex set up, and it is easy to add additional portable CMMs to increase throughput.
Drawbacks with Contact Measurement
Stationary CMMs have the disadvantage of being fixed in one place, being very slow and quite costly. But otherwise, the advantage of accuracy always needs to be considered. Probe compensation is the one that needs to be mostly considered. The probe is set to assume that the first part being measured is perfect, which it is not, and all compensation is based on that assumption. The measurements are offset from the center of the probe, based on the initial assumption of perfection. The more complex the part, the more those assumptions will come into play. While this can be managed, the assumptions have to be realistic. The most accurate probe based device may still give ambiguous results on a complex form.
Secondly, data capture is slow. Capturing 100 points can take ten minutes, but those tend to be some very valuable points.
The noncontact solutions are a relatively new approach in a time-honored discipline. They range across laser scanners, structured light scanners and industrial CT scanners. All of them capture the “shape” of the part so that measurements can be made and analyzed using inspection software. The advantages of noncontact scanners over contact devices include fast data collection, and more comprehensive collection of points means a clearer view of the entire part. They are (except for CT scanners) highly portable and flexible to use, with no or very few compensations for the data being collected.
Laser Scanners
Laser scanners use laser light to create the 3-D shape of the part as a point cloud. They tend to be very flexible—you can mount them to CMMs and PCMMs—and can be hand-held or mounted on tripods. Prices range from the low thousands to above $100,000, again with the advantage being greater accuracy at higher prices. Even then, laser light causes inherent measurement noise and diffusion from the laser light limits the resolution possible, and laser based systems typically have difficulties measuring highly reflective surfaces. Camera resolution is always better than laser coherence. Your use of these scanners should be made based on the available resolution of a scanner compared to your tolerance requirements on the shop floor.
Structured Light Scanners
A structured light scanner uses projected light patterns and a camera system to record the deviations of the light to record the 3-D shape of the part. Available in multiple shades of light, these scanners are usually more accurate than laser scanners, due to a markedly lower measurement noise. In addition, they can deal with shiny parts. The combined light and camera technology delivers much greater accuracy and also means much heavier datasets to deal with. They are also less flexible, needing to be mounted and calibrated on tripods or robots.
Industrial CT Scanners
CT scanners have the advantage of being able to capture both internal and external geometry, even to the point of being able to see and identify cracks and fissures in a material almost down to the atomic level if you have the right system. This kind of accuracy demands much higher prices, and CT scanners are far less mobile—you bring the part to the scanner unit. CT scanners also have limitations on what materials can be scanned so that needs to be carefully checked against requirements.
Automation Using Noncontact Devices
Noncontact systems in particular can be easily automated for constantly repetitive measurement procedures. With the right software, the measurement analysis can also be easily automated so that go/no-go reports can be quickly created to help identify a problem almost as soon as it begins.
There are several inspection software products on the market, all of which have their own advantages and disadvantages. Inspection software bridges the gap between the point collection and the design requirements. Obviously, not all software is the same, and you need to be on the lookout for the ease-of-use versus training requirements. You also need to choose your software based on the capture device(s) you are using. Always require a test run of the software using one of your typical production parts, and if you have budget available go for a full pilot test of the complete system. This will allow you to make considered and well-researched decisions. Consider how you want to be given results of the measurements, as reporting types can vary across software products.
Contact-centric software
This software is targeted primarily at contact-based measurement: it typically cannot handle large datasets provided by noncontact scanning as well as products developed for the noncontact solutions. If you are certain you will only be using contact measurement, look to FARO CAM2, Hexagon PC-DMIS, Verisurf, BuildIt!, and Delcam PowerInspect, among others.
Noncontact Software
Inspection software for noncontact measurement is a wholly different experience from the traditional methods, and is aimed at the new generation of measurement. Consider if you have reference data available—normally a CAD file—for comparison purposes, which is the best solution, even though most of the products available also work without CAD reference data. Define if you might need automation, and also be on the lookout for software that has been certified by NIST for accuracy. Products include Geomagic Qualify, Rapidform XOV, Innovmetric Polyworks, and GOM Inspect.
Reporting Tools
Once you calculate results, you are going to need to share them with someone—or many people. Define how those results should be shared—in a 3-D PDF, by 2-D spreadsheet, etc. As a further detail, define what data is needed in the reports by management, so that the information is available to those who need it. Then make sure that the software meets those requirements.
The right choices of capture device and software can mean the difference between a production line being closed for weeks—or hours. The increase in quality is less easily measured but can be significant and not recognized until those product recalls cease to be an issue. Choose carefully, and if necessary get an expert in to help.