The Revolution in Optical Comparators

Digital optical comparators offer higher accuracy, lower operating costs, digital storage, and more.

Published: Monday, February 15, 2010 - 05:30

Optical comparators, also referred to as profile projectors and contour projectors, were first introduced in the 1940s and they are still widely used today in a broad range of industries to verify that manufactured parts are within tolerance. These versatile instruments are easy to use and the fact that they are very robust makes them well suited for use on the shop floor as well as in the metrology and quality control labs. They are well suited to complex geometries (i.e., shapes not easily described by simple elements like lines and circles) and, up until now, they have been the easiest way to quickly compare a part to its drawing to allow the operator to make an overall pass/fail determination.

And while optical comparators are generally considered to be a cost effective measuring tool, it is also widely recognized that they rely on old technology and suffer from a number of shortcomings. The most important of these is probably the need to use an overlay, also called a template or a Mylar. With traditional optical comparators, the part’s drawing is scaled to match the comparator’s optical magnification (typically: 5X, 10X, 20X, 50X, and 100X) and printed on a transparent overlay. This overlay is then placed on top of the comparator’s screen and aligned with the image of the part. The operator can then ascertain if the part is within tolerance. There are many problems related to the use of these overlays, including the following:

• The cost of the overlays
• The cost and maintenance of the equipment required to print and calibrate the overlays
• Using overlays necessarily introduces an error when printing the part’s drawing onto the overlay (no printer is perfect).
• Overlays can tear, get damaged, stained, worn, etc.
• Overlays need to be physically stored and retrieved. This takes both time and storage space.
• There is a risk that the operator selects the wrong overlay.
• Overlays can only be used by one person at a time.
• If you want your suppliers to verify with an overlay the parts that they are sending to you, you need to send the overlays to them, which results in costs and delays.
• Setting up overlays on a comparator is a slow and operator-intensive process.
• It’s impossible to “recall settings” for an overlay (e.g., position and orientation). The operator needs to re-align every time that he sets up a part.

A digital optical comparator solves all of these problems and many others as well. For instance, the VisionGauge Digital Optical Comparator from VISIONx uses a high-resolution digital camera and a low distortion telecentric lens to capture a high resolution and geometrically exact image of the part. The VisionGauge software at the heart of this new instrument then projects this image, along with the part’s CAD overlay, onto a high-resolution quad monitor display. This approach produces images with an on-screen resolution better than that of traditional 30-inch optical comparators. It also allows for a number of benefits over traditional optical comparators, including the following:

• Produces a very high contrast image so that there is no problem viewing it in full daylight
• More accurate than traditional optical comparators
• Allows the user to be more productive
• Works directly with the CAD data so that no overlays, templates, or Mylars are required
• Can be used to collect images (either with or without the CAD data overlay or annotations), measurements, and data
• Can carry out fully automated measurements, as with a video coordinate measuring machine (CMM)
• Has a smaller footprint and uses less floor space than a traditional optical comparator
• Can be moved much more easily and without requiring recalibration (i.e., “rolling cart” configuration is standard)
• Has a much greater depth of field
• Has a longer working distance, thus more clearance between the part and the lens
• Allows you to compare a part to its CAD data beyond the optical field-of-view (because the CAD data tracks the part and follows the stage motion)
• Has LED illumination for very stable illumination with a life cycle of more than 10 years. This means that there are no more bulbs to change.

This new instrument is the ideal tool when you need to compare a part to its CAD drawing. It is appropriate for a wide range of industries including orthopedics, medical device manufacturing, automotive, aerospace, energy, and precision mechanical components, and assemblies. It is also appropriate for higher magnification and very high accuracy applications such as microelectromechanical systems (MEMS) device manufacturing, electronics, semiconductor, and so forth.

In the orthopedics industry, 100 percent of parts must be inspected and compared to their CAD data during the manufacturing process, directly on the shop floor. These are implants with complex geometries and tight tolerances. In this application, the VisionGauge’s higher accuracy provides an obvious benefit. Also, because parts can be made in small batches, there is a constant need to change the overlay. With traditional optical comparators, this involves removing the overlay, walking over to the overlay storage area, finding the correct overlay, walking back to the machine and positioning the overlay. All of these operations can easily take a few minutes. With a digital optical comparator, the operator only needs to scan in the work order’s barcode or press a button on the screen to automatically call up the correct overlay, which comes up correctly positioned over the image of the part. The productivity gains are immediate and all possible errors are eliminated.

The digital optical comparator overlays the part’s CAD drawing onto the live video image of the part. In this case, the part is a bone screw in the orthopedics industry.

In the automotive industry, this technology is used to check flexible parts. Comparing flexible parts, such as door and window trim, to their CAD data is notoriously difficult. A digital optical comparator’s ability to quickly and easily align the part and the drawing using an intuitive 3-axis, 3-speed joystick makes the job much simpler.

A typical application in the automotive industry

Like traditional optical comparators, the digital optical comparator uses collimated back illumination to produce very clear and crisp images with sharp edge profiles. However, while traditional optical comparators typically use halogen bulbs that need to be replaced on a regular basis, this product uses LEDs that provide very stable illumination with life cycles of more than 10 years. As is the case with traditional optical comparators, front illumination is also available (to carry out surface inspection, for example).

Left: Image of a part (an orthopedic hip implant) using traditional transmitted (i.e., back) illumination. This produces the maximum edge/profile sharpness. Right: Same part using both transmitted and reflected (i.e., front) illumination. In this case we can also inspect the surface of the part.

There are two main reasons why this technology is more accurate than traditional optical comparators. The first has to do with the size of the lens. As with all things manufactured, it is impossible to produce a perfect lens and all lenses have defects and imperfections. Furthermore, the difficulty in producing a lens increases rapidly with the size of the lens. Traditional optical comparators need to use very large lenses to project the image of the part onto their 30-inch screen. The VisionGauge, on the other hand, only needs to project the image of the part onto the camera’s sensor, which measures slightly less than one inch. This lens can be manufactured much more accurately and the raw image that is projected onto the camera sensor is thus much more geometrically accurate than the image projected onto the 30-inch screen of a traditional optical comparator.

Another reason for the better accuracy is that after the camera image is sent to the system’s on-board computer, the software can then carry out further corrections on the image to eliminate any geometric distortions and inaccuracies. Overall, this system is roughly 10 times more accurate than traditional optical comparators.

The use of a small camera sensor, instead of a large 30-inch projection area, also allows a much greater depth of field and a longer working distance. It is also the principal reason why these systems have a much smaller footprint and use up less manufacturing floor space than traditional optical comparators.

And, of course, because the system works with a digital image, it is possible to carry out fully-automated measurements, with sub-pixel accuracy, just like a video CMM. The digital image can also be saved to disk (with or without the CAD data overlay or annotations), along with measurements and other data.

Another significant benefit to this novel approach is that a digital optical comparator can compare a part to its CAD drawing beyond the optical field of view. With traditional optical comparators, the overlay is fixed on the screen and doesn’t move when the stage carrying the part moves. With the VisionGauge, the overlay “tracks the part.” This means that if you have a part that is larger than the field of view, you can move the stage to see another portion of the part and the overlay will follow the stage. The CAD drawing moves with the part.

Because these systems are completely software-driven, it is possible to automate all of the operations. As a result, it is possible to operate the the system using only a barcode reader and a joystick. This results in very significant productivity gains. Customers typically report a doubling of their productivity with this instrument, i.e., they can get twice as much work done with a single machine.

The system is easy to use and perfectly suited for the shop floor and the metrology lab. The VisionGauge is available in horizontal and vertical configurations and with industry-standard 5X, 10X, 20X, 50X and 100X magnifications.

About The Author

Patrick Beauchemin

Patrick Beauchemin is the president of VISIONx, a company that develops software and systems for visual inspection and high-accuracy measurement. There are more than 3,500 licenses of VISIONx’s VisionGauge software in use worldwide. Beauchemin has published dozens of scientific and technical articles and is the holder of four patents.