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You are a manufacturer. Whether electronics, pharmaceuticals, soda, it doesn't really matter as long as you need to measure parts as they move along the assembly line. Plenty of machine vision systems can provide non-contact measurements. But do they give you enough accuracy?
The highest-resolution machine vision lenses can't provide accurate measurements if the distance between the piece and lens changes, or if the piece tilts or vibrates. To some extent, software can compensate, but a hardware solution exists: telecentric lenses.
WHAT ARE THEY?
All lenses have some magnification, but it usually changes with distance. Objects near the lens look larger while objects far from the lens appear smaller. We are so used to this effect that it takes a minute to remember that our eyes— and most lenses—have variable magnification: the perceived size of an object depends not only on its actual size but also on its distance from the lens.
This is a huge advantage for estimating how far away an object is. The classic example used to illustrate parallax is two railroad tracks: When standing between the metal rails looking down at our feet, we can easily estimate that the rails are about a yard apart. When we lift our head and look off into the distance, the rails seem to converge and meet. Yet common sense and experience remind us that the distance between the rails stays constant.
For some applications, this perspective is the last thing we want. That's where telecentric lenses come in. They don't work like human vision. The magnification stays constant for a telecentric lens: when an object moves from far away to near the lens, it goes into and out of sharp focus, but its image size is constant.
For example, consider using software to measure the distance between rails. A normal 16mm lens provides an image in which the crossbar width varies by 27 pixels and the track width varies by 103 pixels. A similar setup that instead uses a .08x telecentric lens yields measurements of both the track width and crossbar width that vary only by 1 pixel.
There are a host of gauging applications that can use this property. Suppose you have a three-dimensional part with stepped surface, and you need to measure the distance from one edge to the other, but they aren't in the same plane. A normal lens would make the nearer plane appear larger—but a telecentric lens can provide an accurate measurement. In a similar application, suppose you want to measure the label stuck on a squashed cylinder (think underarm deodorants that come in oval packages) but the edges aren't in the same plane and the tilt of the cylinder may vary. As long as the locations fall within a given working range, a telecentric lens can provide the measurements.
USES
Glenn Archer, Director of Business Development at EPIC Vision Solutions (St Louis, Mo.) uses both PC-based standard and smart cameras. Both sometimes need telecentric lenses or lenses with telecentric properties over a small range. "The difference is cost," he explains. A number of companies make telecentric lenses. But some applications are difficult: "If you're looking at a very large object, the lens has to be big." This can be a problem. "If you need to look at a 6-inch-diameter part, there's only one company that makes something like that."
Many applications that Archer sees are for measuring parts with surfaces that slope away or toward the camera, with FOVs roughly around 1-inch in diameter. Precision measurements of these parts would fail under those conditions without telecentric lenses. "If you want to measure how parallel two surfaces of a part are, but the surface is sloped," Archer says, "then a telecentric lens provides a real benefit because you don't have to rotate the part."
COST
Shawn Campion, Principle Application Engineer at Integro Technologies (Chambersburg, Pa.), says that one of the main misconceptions that users have about telecentric lenses is that they are too expensive. Prices have fallen considerably. "A lens that was $5,000 five years ago is now $750 to $1,000."
Cost is still an issue, but not as much. "If you can use a standard lens then use it," Archer says. "But if you have to worry about the Z axis, how you accommodate it, then a telecentric lens is useful. Frequently, people will have spent a lot on a vision solution, $100K. Sometimes ... a $700 telecentric lens can solve the problem."
The lenses are expensive partly because of size: the first element must be as large as the field width. Potential users must figure out which lens will provide the field of view (FOV) needed and how to match the lens' FOV with the camera's. "You have to know exactly the size of the CCD or CMOS" imager, explains Archer. "But hardly any manufacturers publish an exhaustive list of FOV with a chip size."
Also, consider the speed of the lens—i.e., how well does it gather light? Paul Tungseth at Braas Company (Eden Prairie, Minn.) recommends monochromatic backlights (if allowed by the application), and often suggests a polarized light source and filter in front of the lens. Using this illumination and a telecentric lens, his company developed a system for a large tier-1 auto industry manufacturer that met the measurement goals "right out of the box."
Campion tends to use strobed monochromatic LEDs. Strobing reduces blur, allowing the system to deal with lines running at 2,000 parts per minute or more.
OTHER ISSUES
For in-line process measurements, users need to take into account not just the lens and illumination, but also the mounting needed to properly support the system weight and reduce vibration. Vibration of the system can limit precision.
Some users attempt to use a normal lens and correct for distortion using software. But using a telecentric lens has advantages: "The larger depth of focus makes a telecentric lens more reliable with less calibration," says Campion. "I would rather put money into hardware to keep software simple as possible." Alternatively, robust hardware allows users to tighten tolerances later, using software tools.
"The application bar starts to rise the more precise you want to be," says Tungseth.
author: By Yvonne Carts-Powell