By Joe Feeley, Editor in Chief
We've been fortunate to develop several in-depth machine vision case histories that focus on machine builders, system integrators and discrete manufacturers. We'll print these stories later in 2011. In the meantime, we extracted some of what they said for a quick look at the state of machine vision today, as well as to whet your appetite for things to come.
The inclusion of machine vision in precision parts manufacturing and/or assembly operations moved from an expensive luxury to an essential step a while ago. Demands for faster line speeds, better yields, and tolerances measured in microns affect manual as well as fully automated systems today.
The quality requirements for a precision parts supplier are illustrative of both stress and high opportunity for a parts maker.
Prime Engineered Components (www.primeeci.com) manufactures precision turned products for industries ranging from aerospace to automotive. In its Watertown, Conn., facility, Prime makes about 800,000 parts per week. Each part has to be flawless or Prime runs the risk of costly returns, as well as strained relations with its customers.
"We know that our machines aren't 100% perfect," says Kathy Dibble, director of quality for Prime. "Since our parts are produced too quickly for manual inspections, we had to put vision in place for a multitude of dimensional inspections. We would much rather remedy any quality problems in-house before product ships to our customers."
ThyssenKrupp System Engineering (www.thyssenkrupp-systemengineering.com) in Auburn Hills, Mich., is a global designer and manufacturer of turn-key assembly and test systems for automotive powertrain components including engines, transmissions and axles. A manufacturer of construction and heavy equipment recently commissioned ThyssenKrupp to build an engine assembly line for a new plant. In addition to its role as a line builder, ThyssenKrupp had to provide a machine vision solution for the engine assembly line.
The company was a new customer for ThyssenKrupp and was skeptical about using vision-based inspection as part of its assembly line. It questioned the reliability of the results as well as the dependability and maintainability of vision. "But, working with the customer, we determined that the part complexity, combined with the extensive teardown and rework required if an error is made, justified the use of in-station inspection," explains ThyssenKrupp program manager Pat Coughlin. "A camera was a perfect fit because it could handle the part variation and be located completely out of the operator's workspace."
The complexity of machine-vision inspection applications can vary greatly depending on the speed of the operation and the detection tolerances required to ensure quality.
A machine-vision application to gauge 20 cylindrical parts per second that are 2 or 3 in. long and 9 mm in diameter to micron tolerances at line speeds of 52 in. per second falls in the complex category, even for an experienced machine-vision integrator such as MoviMed (www.movimed.com), based in Irvine, Calif.
The metal cylinders in question are made out of a brass alloy. The parts are made from small metal "cups" that are drawn into a hollow cylinder. The resulting part has small features, such as a little lip, a slant angle and an undercut. "All of these features are critical for proper functioning of the system since a precise mechanical fit is of the utmost importance," says David Ritter, scientific technologist—real time systems at MoviMed.
Why Change Now?
"The replacement of the older vision system was due to changes in critical, high-tolerance dimension requirements from Prime Screw customers," explains Adam Farley, product specialist for integrator and solutions provider Action Automation & Controls (www.actionauto.com), which partnered with Prime Engineered on the new vision system. "Before using any inspection equipment, all QC inspections were done by the quality department using hand inspection tools." The existing system had problematic issues with accuracy, reliability and repeatability.
The machine MoviMed worked on was equipped with a 1980s-era optical-gauging system. Besides obsolescence issues, the system was not measuring to desired tolerances. The company performed some calculations and optical simulations to approximate the old system's performance as a baseline from which to work. "It turned out that the old system was limited primarily by the optical path—mainly a custom lens design that could not reproduce details better than about 100 µm at a contrast level of at least 50%," explains Markus Tarin, president and CEO. "Second, the image sensor used was a line scan sensor with only 128x1 pixels. The system used an averaging technique to try to overcome some of its limitations."
Another flaw in the original design was that critical measurements were based on indirect references derived from the tooling and part fixture and not actual measurements. This invariably led to measurement drift caused by mechanical wear and tear.
The first steps of the ThyssenKrupp vision solution were developed using six Matrox Iris GT smart cameras. As an engine moves down the line, it goes through multiple smart camera-based vision inspection stations. These applications vary from absence/presence to gauging/measurement.
Three applications on this new line used machine vision for inspections. One smart camera verifies that the crank gear and water pump gear are aligned properly. Another application uses four Iris GTs. Cameras 1 and 2 both verify the presence and location of eight water seals. Camera 3 determines the presence and location of a drainback seal, another gasket and eight water seals. Camera 4 verifies that only a single head gasket is installed.