Sometimes called contour projectors or shadowgraphs, the venerable optical comparator has existed in some form dating to the 1940s, and the latest comparators can offer significant improvements in accuracy and speed enabled through enhanced software and edge-detection capabilities.
Comparators essentially have morphed into two-axis CMMs that when equipped with digital readouts and SPC software can handle data much more like advanced optical systems, according to Colin Robinson, senior manager for the L.S. Starrett Co.'s (Athol, MA) Metrology Systems Division (Atlanta, GA). "Today's comparators are basically two-dimensional measurement systems," notes Robinson. "They're more widely used in the industry today than CMMs.
"Using X-Y readouts makes measurements of objects very easy, and those systems are also equipped with edge detection, so they're very accurate," Robinson adds. "They're taking points and measuring in a similar way to a CMM."
Simpler to use than CMMs, comparators are much more ubiquitous in the market and less expensive than the more sophisticated CMM. "You can get them cheaper, for about $4000 - $5000, compared to about $10,000 for the least-expensive CMMs," Robinson adds. "Cost, ease of use, and reliability are the key advantages of comparators. There's hardly anything to wear out on them."
Updating technology in the machines helps comparators hold their value on the factory floor, where manufacturing personnel typically first encounter comparators in their introduction to shop-floor metrology. Older systems can be updated in a variety of ways, such as with Starrett's OV2 Optical-Video Adaptor, which converts the Starrett 400 Series benchtop comparator for use as both an optical comparator and as a video-measuring machine with a video camera attached to a color monitor.
Most of the comparators built today are specifically designed to operate right on the shop floor, notes Ron Gardner of Brown & Sharpe (North Kingstown, RI), manufacturer of the Tesascope line of comparators. "Comparators are being designed to go out in a cell environment for the operator to very easily use the tool to perform first-run inspection right at the machine," Gardner says. "Comparators have been around for many years, but the machines have been enhanced with new capabilities in software and digital readouts over the years, as well as automatic edge detection."
Optical sensors for comparators enable the automatic edge detection critical to fast, accurate decisions on whether parts meet manufacturers' requirements, adds Gardner. "The sensor is looking for the difference, for the most contrast, between light and dark," he explains. "So when this sensor crosses from light to dark, at that instant the sensor takes that point."
Such systems not only are more accurate than the naked eye, but also help speed up the process of gaging parts. "It allows you to take points and get the distance between them much more accurately than you would get with cross-hairs, and a lot quicker," Gardner notes.
While edge detection has been around for years, the edge-detection technology offered by comparator suppliers also has gotten much better over time. With its Tesascope models, Brown & Sharpe offers optional software with its TS-200 Tesascope box that Gardner says allows manufacturers full 2-D geometrical featuring of angles and polar coordinates on parts like bolt-hole patterns.
CAD-based software technology has helped optical comparators make the leap into 21st century metrology. With Optical Gaging Products Inc.'s (OGP, Rochester, NY) optional GageFit software, comparator users gain electronic overlay chart capabilities, eliminating the need for plastic charts typically used with optical comparators. OGP's latest family of optical comparators is the Focus line of benchtop models.
"There are a lot of little things in the Focus that probably are transparent to most users, engineering designs that we've incorporated," says Tad Davis, OGP's contour projector product manager. "They're easy to use, and that's the draw of the comparator--you plug it in, turn it on, and you're off and measuring parts within a few minutes. The other thing is that you're actually seeing your part up there on-screen. You also do that on other systems like video measurement machines, but it's more of a processed image."
Seeing parts via shadows on a comparator's ground-glass screen enables users to better and more quickly understand whether parts meet tolerances. Seeing a part's shadow enables comparator users to understand their part in a matter of seconds, Davis notes. "You throw it up there and you can see where you may need to make improvements right away, by just looking at the image, let alone being able to make accurate measurements as well."
Although many comparators are used manually, automatic modes allow a more automated process in which manufacturers can collect and analyze SPC information for improving manufacturing processes, he adds. Davis estimates that somewhere between 30 - 50% of comparators are used in a fully automatic fashion today."In the past, when you have profile tolerancing on a comparator, that was done with a chart," Davis says. "The chart has a min and a max zone, and you visually jiggle the part or chart and see if that profile fits within the zone. Today, you can use edge detection on an automatic system, collect all the point data, and then send it to the computer, which will do that analysis for you automatically."
What system should you buy? When considering an optical comparator, a video-measuring machine, or a measuring microscope, manufacturers should carefully consider several critical factors, including the system's superstructure, stage, and lens quality. "We have measuring microscopes, optical comparators, and vision systems," notes Michael W. Metzger, department manager, Measuring Instruments, Nikon Instruments Inc. (Melville, NY). "All three of those products attempt to solve the same industrial problem, and that is noncontact measuring, so there is a very blurred line between which tool you use. It's really difficult to tell which is the best tool, because all three of those tools can basically do the same job.
"In industry, you need to look at the wider purpose of why you are buying the tool, the part size you're working with, what accuracy you need, what kind of throughput, and the variety of different things you need to measure within the scope of the tools' capabilities," Metzger points out. "There is a definite choice, a cost versus value decision, that must be made between the optical comparator and the lower-priced video measuring tools."
Lower-priced measuring tools currently have become much more common due to increased demand for metrology tools from manufacturers of all sizes, he adds. "Everybody has these $10,000 - $20,000 simple optical or video measuring tools, because the industry, from the mom-and-pop manufacturers up to the large manufacturers, needs to buy these tools," notes Metzger.
Video measuring tools were spawned from the technology mix between the optical comparator and the measuring microscope, he adds. "Once computers and CCD cameras could be married to those noncontact optical tools, you had video measuring tools, and so now, the market has become saturated with video measuring. When you look at the needs of the people in inspection and in noncontact measuring, they need low cost and they need ease of use."
High-end optical comparators and low-cost video measuring tools currently are fighting for the same turf while offering two distinctly different technologies, Metzger contends. "The optical comparator uses a ground-glass screen, and the video system uses a video camera and a monitor. You have a vertical optical system, just like a vertical optical comparator, but that optical comparator uses fixed magnification lenses on a turret and the low-end video measuring system uses a zoom optical system."
Among optical comparators' advantages, is the fact that the user views parts without looking through an eyepiece while standing in an ergonomically comfortable position. "That's why people like to look at an optical comparator--you stand up straight, your ergonomics are correct, the part is on a stage, the optics are vertical," Metzger says. "So the optical comparator is exactly comparable to low-end video measuring systems or measuring microscopes. But the difference is that you're looking at an easy-to-look-at screen, and that technology is really hard to replace, because it's easy, quick, and two or three people can look at the same thing at the same time.
"For the same price, you can get an optical comparator that is built with the finest optics that money can buy and the finest stages that will last for 20 or 30 years, plus the ground glass, which uses your eyeballs as the discriminating resolving power. Our eyes are pretty good at that, much better than video measuring."
Superstructure, stages, and lens quality play key roles in determining what systems to buy, Metzger says. "The first thing to look for is the superstructure that the optics are mounted to. Underneath the skin of an optical comparator should be a very rock-solid foundation. On the lower end, we see that people mount the lens to the sheetmetal housing that is the skin of the product. On the more expensive systems, there will be a cast steel superstructure underneath the housing of the instrument, and that's the foundation that the optics should all be mounted and aligned to."
Compared to low-end video systems, which are usually scaled-down with aluminum stages, an optical comparator of good quality will be built with a cast Meehanite steel stage that has precision-ground ways, just like a milling machine or a production lathe, Metgzer notes. "There is a definite choice, a cost versus value decision, that must be made between the optical comparator and the lower-priced video measuring tools," he says, adding that cast Meehanite is used because its coefficient of expansion is similar to that of the glass measuring scales that are mounted on these precision platforms.
"You want to see a stage that's a precision-ground stage," Metzger says. "The converse, or the low-cost stage, would be an aluminum stage that has little sleeves of steel the ball bearings ride against, or steel guide wires that the ball bearings ride against. Let's face it, optical comparators that were built in the 1940s or the 1950s still operate today because they did it the old way, with precision ways and precision steel construction."
Lastly, lens quality is paramount to maintaining manufacturing quality. "Optics are expensive to make, and it's difficult to achieve a distortion-free image," Metzger states. "That does not come cheaply, because optics are always good in the center of the field of view. If you look at a comparator, you have a screen 12 or 20" [305 or 508-mm] wide and in any comparator from low cost to very expensive, the very center of the field will be the sweet spot. But low-cost optics want to fall apart at the perimeter, and the bigger the optics or the bigger the screen, the more they fall apart. An overlay chart would fill the entire field of view. If you have distortion in your image at the periphery, what good is that overlay chart? It's no good. So, it's important for customers to know that there's a quality versus price relationship at the lens level."
Fixed magnification lenses instead of zoom lenses also perform differently, according to Metzger. "Most microscope manufacturers most likely will have fixed lens magnification lenses on the turret underneath that microscope, because it's really hard to get a distortion-free image in a zoom system," he says. "With Nikon measuring microscopes, we don't have variable magnification, because when you introduce a variable at that objective lens, you take the risk of introducing distortion in the field that you're going to do your measurements in. The fixed optical lens, where nothing moves in that lens, is the most precise system that you can get."
This article was first published in the July 2004 edition of Manufacturing Engineering magazine.
|An Alternative To Comparators
Manufacturers looking for a lower-cost, high-quality noncontact measuring system can consider systems such as the Kestrel two-axis noncontact measuring microscope from Vision Engineering Inc. (New Milford, CT).
Measuring microscopes from Vision Engineering allow easier viewing of parts on larger screens.
Measuring microscopes like the Kestrel model offer an alternative to comparators, featuring superior edge detection, crisp surface detail, good repeatability, and image capture capabilities in a compact, worktop unit, notes Janice Lea of Vision Engineering.
Microscopes give users quality control laboratory accuracy, high repeatability, and optical quality with ease of use in one low-investment system. "It's a huge advantage because you're getting an optical image, as opposed to just a shadow," Lea says. "A comparator will only give you a shadow, not dissimilar to a shadow on the wall when you're walking in the sun. You'll only get the black shadow outline of the part, whereas with the Kestrel system, you get true optics. You can see the colors, you can see the edges, you can clearly see every distinctive feature about a specific part."
Instead of using an eyepiece, the Kestrel line of microscopes features Vision Engineering's easy-to-use Dynascope optical projection technology, which projects an image onto a screen offers users ergonomic benefits.
"It's like the old toolmakers' microscope," Lea adds. "You can do two-axis measurement, get all the optics of a microscope, but with the ease of use of a comparator because you have the screen. The difference is we have true optics versus the shadow."