Laser maps tire-sidewall flaws

With increased reliability and longer lifetimes, today's automobiles are used 50% longer than those of three decades earlier.

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With increased reliability and longer lifetimes, today's automobiles are used 50% longer than those of three decades earlier. Automobile components have grown more dependable, too; for example, some tires now come with 100,000-mile warranties. Making better car parts, however, requires better ways to inspect them. An online tire sidewall inspection system developed at Commercial Time Sharing (CTI; Akron, OH) combines laser triangulation with data-analysis software to catch sidewall deformities as small as 0.025 mm in size.

A sidewall-inspection system should detect suspect deformities while minimizing false rejects, which require the intervention of a human to do labor-intensive hands-on inspections. "Bulge heights of significance have now been specified to be about 0.3 mm," says Ron Symens, president of CTI. "Customers are asking to resolve bulge heights of 0.2 mm. They are doing this because many bulges are not cord-related, but are air blisters and are peanut-shaped (smaller on the end)—so to find, for instance, a 1.5-mm bulge when you have only sampled the end where the bulge is 0.5 mm, you need better resolution."

Symens notes that this required precision over the whole sidewall is one reason the industry is looking at "sheet-of-light" (line-laser) systems, which examine a large area of the side wall. But multiple test paths can do the same thing, notes Symens. "The advantage of not using line lasers is the reduction of cost and complexity; multiple paths give the same results," he says. Other sidewall-profile measurement techniques include capacitive and contact-mechanical sensing.

The laser sensors, which are made by LMI Selcom (Detroit, MI), have a 200-µm spot size and are sampled at a rate of 16 kHz. In use, a tire-uniformity machine rotates a tire at up to 60 rpm, with the laser-spot scanning the sidewall all the while. The return signal from the sensor is not affected by sidewall surface-color differences (motor oil on the tire, for example, or contrast due to black or white lettering), texture, speed of the tire rotation, or differing ambient light conditions. Software provides an x-y plot of bulges, dents, depressions, and the location of lettering on the sidewall (see figure). In addition, radial runout (out-of-roundness of the tire) can be measured.

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A tire-uniformity machine measures sidewall deformities on both sides of a rotating tire at the same time with two noncontact laser-based sensors (top). Each sensor terminates in a "whisker" safety switch (a spring-shaped device that protects the laser sensor from damage by shutting the machine down if the tire gets too close; a third can be seen at the left of the photo). The signal is processed to produce a plot showing the sidewall surface profile (bottom). The successions of regula features in the plot are lettering on the tire.

Symens notes that one tire manufacturer took tires inspected by another system, ran them through the laser-triangulation system, and found that two of every three tires rejected by the other system were not defective. "False-positive tests are generally referred to as alpha misses," he says. "The alpha-miss rate is higher for capacitive probes, and also for tires that do not have a clear path on the sidewall (one unobstructed by letters, logs, or pin vents)." Hard-to-test tires may produce an alpha-miss rate as high as 15% to 20% for some tire-inspection systems.

The technology developed at CTI has been integrated into tire-uniformity machines built by several companies, including Akron Special Machinery, CTI's parent company. Laser triangulation is not new, even as applied to the characterization of tires; CTI's efforts have gone into more-tightly integrating the laser-sensor-based system with the testing and optimization control unit, which replaces existing controllers on four-post tire-test machines. To speed up measurements, engineers at CTI are in the process of increasing the sample rate of the system to 32 kHz.

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