See It. Measure It.
A study by Robert J. Bettinardi Golf (Tinley Park, IL) showed that a putter face that is 0.2 inch off from true can result in a 4-inch deviation from the intended line over a 20-foot putt. In other words, with everything else accounted for, a golfer might still miss his putt because of an out-of-tolerance condition that's invisible to the naked eye. Jeff Counter, quality control manager at Bettinardi, wants to eliminate such factors--in the putters and in other products his company makes.
So what mechanical gage or coordinate measuring machine (CMM) does Counter use to ensure that the vertical machining process his company uses delivers a uniformly flat across-the-face result? He doesn't use either.
In 1995, Counter went shopping for a more efficient way to measure an ordnance housing that his company makes for the military. He needed a way to reduce the three-hour per part inspection time it took to manually measure and calculate the tolerances on the housing. He debated between a CMM and a video measurement system. Video won out. He chose an ROI Omis II video measurement system.
The company's successful use of video measurement means that it successfully overcame some problems that are inherent with video systems.
Solving light problems
"The target of video measurement is to have a system capable of measuring 10 Arial better than what the task demands. In 1998, the industry was only at about four times better than the task required, but in 2001, we really are able to reach toward that 10 times mark," says Mike Metzger, department manager of measuring instruments for Nikon Instruments Inc. (Melville, NY).
Metzger points to advancements in software, lighting and optics as reasons for the technological improvement. Continued improvement is challenged by the push to move measurement tools to the production floor. Metzger believes the critical issue in moving video measurement to the production environment is increasing throughput while improving accuracy.
Adequate lighting is the single biggest factor in using video measurement systems on the shop floor, says Robin Swain, Ph.D., engineering manager for CE Johansson (Irvine, CA). The geometry of light includes where the light comes from, its color and brightness, and all of these factors are critical, says Swain. A manufacturer must be able to detect the edges of the features he needs to measure. Backlighting helps accomplish this. Illumination from an obliquely angled LED, where the light is "skimmed" across the side of the part, helps expose low-profile part features that might otherwise go unseen.
Implementing such lighting solutions in a controlled environment is easier than on a shop floor, where ambient and incidental lighting can be more challenging. However, Swain says that delivering the correct lighting doesn't require exotic lighting solutions. "We used to customize this [lighting]. Now it comes in fairly standard ring lights," says Swain.
Some lighting problems can be addressed through software. According to Fred Mason, communications manager for Optical Gaging Products Inc. (OGP, Rochester, NY), the powerful algorithms that power video measurement software can assist in identifying and analyzing faint edges and features where lighting has done all it can.
Bettinardi Golf solved the lighting problem with backlighting. Counter says his operators can backlight a small connector placed in a V-block, rotate the connector and instantly detect burrs, debris and other malformations. Counter says he can also adjust the backlight to highlight the edges on a correctly made piece, capture the image and use that correct image to compare other parts. "This only works on parts, like threaded connectors, that I can backlight," he says.
The camera eye
While lighting has been fairly standardized, manufacturers face a continual challenge with optics. The goal is to have an optical system with a versatile range of magnification that is quick and precise. Manufacturers address this with either a set of fixed-length optical lenses or a zoom lens--most opt for the latter. "The zoom lens is always a compromise," says Metzger.
The compromise of a zoom lens is that there are "multiple lenses" in one unit. To obtain the desired magni-fica-tion, the zoom lens is extended or retracted. However, this movement creates two challenges: to offset the twisting motion that naturally occurs as the lens is extended and retracted; and to keep the magnification consistent across the field of view.
"We use a zoom lens package instead of a fixed," says Craig Smith, product manager for optical products at The L.S. Starrett Co. (Athol, MA). This choice of lenses means that the ma-chine must compensate for the "cork-screw" effect of the lens as it twists up and down. The result of corkscrewing is that the optics finish in a different position than it started in. This slight deviation is often imperceptible to the naked eye. Smith says Starrett drives the optics in the same manner. This doesn't eliminate the deviation in positioning, but it keeps it consistent, and then software can be programmed to compensate for the change.
"Optics need to be telecentric," says Metzger. By this he means that the magnification above and below the focal plane, at the center and the outer edges of the field of view must be the same. Often, especially in zoom lenses or low-cost optics systems, there is a slight difference in the magnification between these two points. This leads to inaccuracy in measurement. Metzger says manufacturers, especially those manufacturing fiber optics, want measurement-microscope precision and accuracy in video system optics. Numerical apertures are being pushed higher at longer working distances. "We're getting back to pure optics," says Metzger.
Software powers advances
One point that Bettinardi's Counter emphasizes in his use of video measurement is the software. He says he had little computer knowledge when Bettinardi purchased the ROI system six years ago, but the software was easy to learn. The system stores programs that can check all the parts that the company produces. He says that when he was deciding between a CMM and video measurement system, he was impressed with the capabilities of the CMM, but programming it seemed too complex.
Software has to be powerful to process all the data gathered by the video system, as well as to control the lighting, positioning and optics. "We are collecting more data with video, especially in electronics applications," says Ken Parlee, vision product manager for Mitutoyo America Corp. (Aurora, IL). Video systems software, he says, has to control the optics, magnification, pixelization, error mapping and edge acquisition. He admits that CMM software has the edge in solid modeling and 3-D analysis. This doesn't make CMM software more sophisticated than video software, or vice versa, just different, says Parlee. But Starrett's Smith thinks that vision equipment suppliers have an edge in software.
OGP's Mason sees the real demand on software in the nature of control. "The manufacturer has to drive the sensor to an optimum point so that the sensor can gather the best data," says Mason. That, he says, is an important issue, regardless of whether that sensor is a laser, camera or touch probe. But all this power doesn't require more sophisticated machine operators. The people in the company with the highest skill sets continue to be responsible for programming the machine, not running it. "The operator just needs to know about fixturing and part cleanliness," Mason notes.
Counter found this to be the case at his company. "We use oil coolants and have ultrasonic cleaning systems. Operators have to make sure the parts are clean before they put them under the camera," he says.
Increasing the sensors
Beside advancements in lighting, optics and software, equipment suppliers are incorporating more sensors in video measurement systems. Multisensor systems may include a laser, touch probe and a video lens in any combination. Because these hybrid systems borrow from both noncontact and contact measurement technologies, there are two different ap-proaches to creating the systems.
"Those in the CMM world are skilled in working with bigger platforms," says Starrett's Smith. Video measurement systems typically have a smaller Z-axis height range than those found on CMMs. OGP recently introduced a video measurement machine with a 12-inch Z-axis that is considered fairly high, but is small compared to that of a traditional CMM.
To get needed platform or software expertise, many companies are partnering with, or acquiring, suppliers with those strengths. Starrett is partnering with Metronics Inc. (Bedford, NH), a company that has software and controls expertise, and the former Ram Optical Inc. (ROI) merged with CE Johansson, experts in large platform structures.
While expertise in large platform structures would seem to open up more multisensor applications, this doesn't mean that CMM manufacturers have an edge in multisensor technology. Touch probes are considerably slower in collecting data points than are video and laser sensors. This can affect production throughput speeds, a critical issue in today's manufacturing environment. Mitutoyo's Parlee believes that video and laser provide a better multisensor combination than systems that incorporate contact probes. "A touch probe can do Z-axis measurement, but it has to touch the product, which could be a problem if that part is sensitive to pressure. You can get the same Z-axis measurement using a laser," says Parlee.
Using a sensor to its fullest extent is also a challenge on a multisensor machine. Mason points out that in multisensor systems there is always some degree of compromise. A multisensor machine has more overall capabilities than a dedi-cated machine, but the dedicated machine will provide more accurate results for the measurement for which it was built.
CE Johansson's Swain agrees. "There really are no true multisensor systems," Swain says. "The problem is that it's more than just strapping a camera on a CMM or a touch probe on a video system."
More multisensor challenges
Lighting is a critical issue in integrating video to a CMM platform. Because there is no way to effectively backlight an articulated video camera, multisensor ma-chines are limited in the lighting they can use. Most are equipped with some type of LED ring that lights from above. This reduces what the video system can be used to measure. The advanced optics that would improve video measurement capability on a CMM-based multisensor machine can't be integrated, says Metzger, because of their weight and the stress that would place on the machine structure.
Likewise, a touch probe integrated to a video measurement machine presents a set of challenges. According to Swain, switching between the video camera and touch probe would require the probe to be recalibrated because it would be in an offset position. This, he says, could be accounted for in the software, but it requires software to be even more powerful than if it was a video measurement system only. Parlee says a Mitutoyo study showed that it would take 20 person-years for a software package to be developed that would effectively integrate contact and noncontact measurement. He says that Mitutoyo has a program that integrates both, but the software emphasizes contact technology with some noncontact attributes.
Smith is more confident about overcoming these hurdles. "The progress [in true multisensor systems] is exponential." He admits though, that there is a long learning curve to be traveled in the areas of software and controls.
The change from seeing what is being inspected on camera to not seeing what the probe is touching can be unsettling for those operators accustomed to working with a video system. "My operators like to see the part as it's being measured," says Counter. He says it makes them more aware of what they've produced and allows them to spot machining trends.
"Because the operators can see the part as it's measured, they also notice deformities caused by tool wear. They are able to change out tools before they break, preventing far worse quality problems," says Counter.
Who needs multisensor?
"Many manufacturers are looking for the 'ultimate measuring ma-chine,' like a microwave. You put the part in and you get the results, it doesn't matter how," says Mason about multisensor machines. That type of machine doesn't exist.
Indeed, most equipment sup-pliers admit that the market for multisensor machines is limited and that these systems are not a replacement for dedicated video, laser or touch-probe machines. The multisensor machine is best seen as a dedicated machine with some added capability, says Mason.
Smith says it's hard for him to find someone who doesn't want multisensor technology, and Swain says he sees applications across all industries.
Parlee cautions manufacturers not to rush too quickly to multi-sensor technology. "The part drives what (technology) the customer needs," says Parlee.
Indeed, Counter's video mea-surement system can be equipped with a probe, but it is not. "Everything we make is so small we don't need a probe. If we make some-thing large, then we have bigger, dedicated gages to use," he says.