Optical Inspection Muscles Up
Optical and camera-based systems are bumping their capabilities to the next level
By Michael Tolinski
Over the last several decades, few inspection tools have been quicker and easier to use than the optical comparator. In years past, for immediate go/no-go inspection, virtually anyone could compare a magnified part profile projected on a shadowgraph screen against a scaled-up Mylar drawing that showed the part's shape and tolerances. Although this straightforward use of comparators has become outmoded by improved electronics that allow more precise measurements of geometric features, comparators continue to be the most common visual inspection tool brought onto the shop floor.
Meanwhile, other, higher-tech inspection tools continue to have their capabilities extended. Video-camera-based vision inspection systems reduce operator measurement inconsistency, and make minute part features visible and measurable, although their full capabilities aren't needed in all manufacturing situations.
There are times, however, when it's valuable to have more inspection detail or an additional dimension of measurement. For situations when a part feature can't be adequately illuminated by using the base mode of a comparator or vision system, developers are adding features to each type of tool to allow the next-higher level of inspection.
Reports about the eventual death of optical comparators have not only been exaggerated, they're flat wrong. In fact, there's been a recent upsurge of comparator purchases and use, says Simon Cosham of L.S. Starrett Co. (Athol, MA). Even companies that own over a dozen comparators are bringing in more, he states.
Their popularity is based on simplicity and familiarity. "People like them, people know them, they're easy to use, [and] they go on the shop floor and do the job," says Cosham. Comparator sales are keeping up with or exceeding those of vision inspection systems (even though you might expect a younger generation of workers to be more comfortable with a video camera and monitor).
The measurement area of Micro-Vu's Excel system is being extended, allowing it to measure multiple parts in one setup.
Comparators benefit from ruggedness and a relatively low-tech approach. "There's a workstation, a lens, a light source, a screen, and a mirror. The image gets projected and you measure it." Someday, perhaps their shop-floor role will be taken over by video-camera systems, which are often kept in the quality lab and used for final inspection. Yet video systems' capabilities could sometimes be useful to have near the comparator. So why not combine the two?
To that end, Starrett has redesigned its OV2 Optical-Video Adaptor for easier use with its horizontal projectors/comparators. After first developing the OV2 a few years ago, the company now anticipates greater interest in the use of video inspection at the comparator itself. So it has cleaned up the system's original hardware connections, and combined the video image output and comparator readout into one QC300 touchscreen display. The system can be toggled between optical and video inspection mode by switching two lenses on a sliding mount on Starrett's HD 400 projector.
The potential value of the idea seems obvious enough for cases when a part must routinely be brought to another piece of equipment, like a vision system or microscope, for more detailed inspection. A user might be able to do a majority of measurements with the comparator's low-mag lenses, observes Cosham, but the OV2 covers the things the comparator lenses can't see.
The QC300 touch-screen illustrates the trend towards more useful and accurate comparator readouts. It shows X-Y counter and measurement data, while a built-in video card supports the live video-camera image. It also allows video edge detection—a common feature on CNC vision systems—that automatically detects and targets edge points by scanning the image area within a circle around the display's crosshairs. "So any subjectivity that was there with an operator trying to place the crosshair on an edge is now eliminated," says Cosham.
The video-camera add-on addresses a basic magnification limitation of optical inspection, he explains. Comparators generally have lenses that magnify from 25 to 100x, but higher magnifications require smaller lenses, reducing the light that can pass through them. "If you have small parts or you're trying to look inside some part feature, the comparator can't always do the job, just because it's a very dark image." The video camera reveals smaller features with greater illumination. "With the zoom lens, we can go up to 250x magnification, and we don't lose any light."
Similarly, vision inspection systems are being equipped with tools that allow even more different part details to be checked, without switching to a different measuring machine. These multisensor machines, in which a touch probe and other probes are added to a video-based system, have been around for years. Gradually, high-end systems are starting to become all-in-one machines that combine the attributes of CMMs and laser scanners for multidimensional measurement.
"A touch probe and/or laser can add capabilities that allow more measurements to be done in a single setup than by video alone," says Fred Mason of Optical Gaging Products (OGP; Rochester, NY). "Video excels at measuring edges. Touch probes, scanning probes, and lasers can provide surface contours as well. Accumulating all that sensor information in a single system software application can make it easier to compare manufactured parts with CAD files."
Several multisensing options are integrated with OGP's SmartScope video measuring machines. The SmartScope Flash 500 is a relatively small-footprint multisensor system that can be equipped with a through-the- lens laser for noncontact contour measurement. The system has a 500 x 450 x 200-mm measurement volume, with an optional 300-mm extended Z axis. Alternatively, the SmartScope Quest 600's sensors include a microprobe with milligram-level probing pressure for measuring small features on fragile or sensitive parts. This system covers 450 x 610 x 200 mm of X,Y,Z stage travel.
But software upgrades are key to managing these otherwise overwhelming multisensor capabilities. "In fact, it's possible to program measurement routines from part CAD files, speeding and simplifying the total process," says Mason. To this end, CAD software has been developed for OGP systems to streamline the creation and manipulation of these routines. The company's SmartCAD 3-D software allows users to base the programming of the routine on the selection of features from the part's CAD file. Programming can be done either at the measurement machine or at an offline PC, freeing up the machine.
For CAD integration and overall vision system programmability, a "Vision" software module has been developed by Wilcox Associates Inc., a Hexagon Metrology company (North Kingstown, RI). Part of the company's PC-DMIS 4.0 software release, this module is designed with an interface that's said to be familiar to users of CMMs, while incorporating vision-specific tools. To make off-line programming more realistic, these tools simulate and then compensate for the vision system's actual lighting, magnification, and focus control. The company says the software can be retrofitted to different vision systems. And on multisensor machines, it reportedly allows users to switch between touch and vision probes without recalibration.
Multisensor systems come in small packages. Optical Gaging Products' SmartScope Flash 200 is a benchtop video system with a through-thelens laser and a touch-probe to measure surface contours and hard-to-image features.
The software helps fit a part's CAD model into the video world of vision inspection, says Hexagon product manager Gary Hobart. Bringing the CAD model into the programming allows vision inspection results that otherwise would be something less than truly 3-D.
One software feature the company points to is AutoShutter. It's something like an automatic guide to help users of manual machines perform inspection routines along a given field of view. "It gives the operator a target to follow in the form of an arrow, a distance until that target is met, a direction which dynamically updates itself, and autocapture [of the target feature]," says Hobart. "The operator never needs to let go of the cranks/handles to process the measurement."
Another trend in vision/multisensor systems can be stated simply: They're getting bigger. "In the last couple of years we've seen a shift from smaller to larger measurement systems," says Greg Chatfield of Micro-Vu Corp. (Windsor, CA). Larger measuring envelopes, coupled with automatic inspection routines, allow big parts or rows of small parts to be inspected on one machine.
Chatfield points to the company's Excel multisensor measuring machines, which start at a 600 x 600 x 150 mm measuring volume, but which are now available with stage lengths to 2500 mm. He says manufacturers are using the long Excel machine to measure 50" (1300-mm) LCD screens and housings. The tolerances are tight for these TV screen parts, as they are for another of the system's large measurement applications: the hole locations in appliance panels.
The large machines aren't just for large parts; users can also measure several different smaller parts in one automated run. An operator might be able to load 20 six-inch parts on the unit, "and while the parts are being measured, the operator can measure other parts or do something else." This approach increases productivity and cuts labor costs by allowing operators to multitask.
One extreme example is the measuring of soda-can lids. Forty-eight lids at a time are being measured on the Excel, and Chatfield says each lid has a surprisingly high number of inspection points to ensure its good fit with the can and proper function of the tab opening. "The lid manufacturer can continuously measure samples of parts from production, with very little operator input.
"This kind of high-volume automated inspection wasn't widely embraced until relatively recently," he adds. About five years ago, there was a shift from manual to automated inspection. The automation now requires operators to do little more than load parts and start the program. So, as with easy-to-use optical comparators, vision inspection is becoming less dependent on the technical expertise of operators—and less dependent on their subjective judgement as well.
Digital Camera Enables Quick Inspection
One alternative to traditional visual measurement tools is based on a relatively recent invention: the digital camera. A lot of measurement data can be acquired with a system built around a single high-definition camera, says Danyel Benoit, president of The PMST Group Inc. (Granby, Quebec, Canada). These measurements can then be used to inspect, reverse engineer, or measure the thickness or flatness of mainly 2-D components.
Called Planar, the system captures images of parts placed on its glass measurement surface in approximately 1/8 sec, says Benoit. Then, in the next 25 sec, software converts the image to millions of measurement points with 0.001-0.004" (0.025-0.1-mm) accuracy, or higher if needed.
It's also rugged enough to be placed next to production machinery on the shop floor. It reportedly functions even with deep scratches or cracks in its glass measurement surface (damage that's inevitable in production environments).
As a 2-D system for measuring relatively flat parts, it may be an alternative to more conventional optical or camera-based systems. Other tasks are possible with the system, Benoit adds. It can convert printed CAD drawings to electronic DXF files, measure bent tubes and, using a laser, create 3-D models of a component. "Laser triangulation is used to measure the deviation of a spot point, allowing you to measure the height of any part of the object, including those places where gages and calipers can't reach."
This article was first published in the July 2006 edition of Manufacturing Engineering magazine.