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12 Apr 2006
A laser-based, non-contact measurement system is helping Continental Tyres check all of its products for defects that could affect vehicle safety.
Continental Tyres of Germany has solved an in-line inspection problem with a laser-based, non-contact profile-measurement technique. Rather than just sampling the production line, the system rapidly checks every tire for the slightest bulge, recess or other surface defect that could compromise vehicle safety.
The rig inspects individual tires in less than 1 s, enabling Continental to carry out 100% inspection of the tires from its German factory.
The new inspection system is based on three ScanControl 2800 2D/3D laser profile sensors from Micro-Epsilon, of Ortenburg, Germany. The combined laser transmitter and sensor units are mounted on a test rig developed by systems integrator and machine builder MicroStep, based in Bratislava, Slovakia.
Rather than using multiple laser-based point sensors or capacitive sensors, the ScanControl sensors measure the profile of the tire with a non-contact laser line, on which multiple measurement points - up to 256,000 per second - can be acquired and analysed.
"Our breakthrough with the three ScanControl 2800 sensors is combining the laser source and receiver into a single unit," Chris Jones, managing director of Micro-Epsilon UK, told optics.org. "The key to this is our solid state triangulation technology, which the company has been working on for the past 12 years.
"The problem we have overcome is to correct, in real time, for changes in target geometry because of the trapezoid nature of the laser line output. What we have developed is a precision measurement system rather than just a vision system."
Previously, Continental employed an inspection method based on capacitive sensors and laser-based optical point sensors. Compared with the new laser line scan method, capacitive sensing detects over a relatively large measurement locus meaning that patterns, symbols or legends on the tire might not be distinguished from real defects.
With laser point or spot sensors, not only would multiple sensors have to be used to acquire the same surface defect data, but the sensors could not cope with the high throughput rates of tires on the test rig. The sensors also had problems with the reflective properties of newly-produced, shiny rubber tires.
How it Works
The ScanControl 2800sensor has an integrated, sensitive CMOS array, which allows inspection of almost any shiny, reflective or transparent surface, independent of reflections. Using customised software algorithms, logos and symbols on the tire can be removed when the data are analysed.
Each ScanControl transmitter/sensor features a class 2M 15 mW solid state visible laser that projects a red 675 nm beam onto the tire surface from a distance of about 250 mm. The measurement system consists of three sensors and a controller. Measurement data is output from the controller via a Firewire interface to the manufacturer's in-house developed analysis/evaluation software.
Along with distance information (z-axis), the controller also calculates the true position along the laser line (x-axis) from the camera image and outputs both values in the sensor's 2D coordinate system. A moving target or a traversing sensor generates a 3D representation of the object being measured.
MicroStep's test rig receives tires automatically from the production line. These are held in place by a fixture or gripper, which rotates each tire through 360° in under 1s. During this time, the three sensors inspect the three sides of the tire walls for defects, taking measurements in real time.
Other potential applications for the ScanControl 2800 include contour measurement, robot guidance, door gap alignment, position measurement, rail profile, groove measurement, co-planarity measurement and flatness measurement.
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