To comply with internal safety regulations, a leading manufacturer of aircraft engines needed to measure the length of jet engine blades after every 7,000 hours of use. Constant exposure to centrifugal force and extreme heat causes blades to stretch, and if blade length exceeds tolerance, it could cause engine damage.

Determining blade "growth" is a traditionally manual process. Human inspectors center a blade under a microscope, measure distance from the root end of a blade to the center of a reference mark, and read the micrometer display. The results are then compared to a record of the blade's original size. Blade surfaces can become so severely degraded, however, that inspectors can't precisely identify the exact center of the reference mark and make an accurate measurement.

The company evaluated several noncontact photoelectric sensors and laser scanners for the job. However, these approaches weren't powerful enough. "Inconsistent metallic reflections and the round shape of the blade itself were problematic," says Mark Granahan, president of Avtex LLC (Avon, Conn.), a system integrator called in to help provide a solution. "We felt an advanced vision system would be better able to handle these degradations and make precise, repeatable measurements."

Mr. Granahan and his team put together a turnkey solution based on a Cognex (Natick, Mass.) Checkpoint machine vision system. The entire system, which Avtex calls the Gage-300B, is housed in an industrial enclosure and includes a Checkpoint 900 vision system, a Sony XC-75 CCD camera, a rugged industrial mounting fixture for the vision camera, a high-magnification lens and LED focused ring light, and an IBM Pentium PC with a 17-in. Sony monitor.

"Once a blade is placed under the vision camera," says Mr. Granahan, "the vision system acquires an image of the blade and uses robust pattern location software algorithms to precisely locate the center of its reference mark. Upon finding the true center, which happens in milliseconds, the system automatically gages the distance between the reference mark and root end of the blade. It then triggers a ball-peen marker via serial communications to inscribe the measurement on the blade's root end. If it has stretched within acceptable limits, it can be ground down and reused. Blades that have stretched beyond tolerance are discarded."

The system, according to Mr. Granahan, produces measurements that are 40 times more reliable than those made manually. "The Gage-300B measures blade length to an accuracy of ±0.001 in. and achieves a "Six-Sigma-percent-of-tolerance" repeatability of 10% or better," adds Mr. Granahan. "Now, instead of straining to examine the blades through a microscope, operators can see the part being automatically measured right in front of them on a large monitor."

The display also features image graphics, giving operators instant visual feedback as to whether the blade was successfully measured. Operators know instantly when a blade has not been properly measured.

By producing measurements that are 40 times more reliable than those made manually, the Gage-300B has helped the manufacturer consistently comply with internal safety regulations. "It's more precise, easier to use, and more objective than older manual methods. It also eliminates the chance of error associated with performing the measurements manually," says Mr. Granahan.

Comments? Send e-mail to: mdrakulich@cahners.com

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