Testing for tyre safety with machine vision
21 December 2016
Millions of people rely on the reliability of rubber tyres for their safety in vehicles such as cars, buses, airplanes and trucks.
Four measuring heads mounted at 90°
Tyre inspection systems based on a non-destructive optical measurement technique called shearography are now standard in the automotive industry and mandatory by law in the aviation sector.
With over 1000 INTACT tyre measurement systems installed worldwide, Carl Zeiss Optotechnik is one of the suppliers in this sector. Machine vision is at the heart of its four camera measurement technique and Stemmer Imaging has worked closely with the company for many years to supply cameras and other optical and electromechanical components. Since, a number of different measuring heads have been developed over the years, it was decided to standardise on a new measuring head that could not only be used for all systems but also accommodate smaller tyre sizes down to 12in.
Tyre inspection method
Shearography is an interferometric testing method which allows the reliable and reproducible detection of material deformations in the micron range by inspecting a material both in its normal state and under stress. The test object is illuminated with a laser light and is inspected with a CCD camera equipped with a special ‘shearing’ lens. These lenses project the image onto the camera sensor twice. Each point from the material is therefore represented twice, which results in an interferogram. When the test material is put under load, the reflected laser light changes and by comparing the two images, any changes can be detected. This method ensures the high-precision quality control essential for a consistent product quality and the highest safety standards.
For testing tyres, the required load is produced by mounting the tyre in a pressure chamber and reducing the ambient pressure by about 50mbar between recording the two images. Any air bubbles enclosed in the tyre expand as a result of the negative pressure causing the surface of the tyre to deform at any weak points. Conclusions on defect type and size can be made by comparing and evaluating the resultant images.
Developing a standardised system
The system is used to inspect the tyre tread and side walls for internal detachments and air inclusions that would not be visible to a human inspector. Each measuring system consists of four measuring heads mounted at 90° to each other to reduce the throughput time. Each head contains a camera and corresponding lens and inspects two sectors of 45°. Once the tyre is loaded into the pressure chamber, the measuring head assembly is lowered precisely inside the tyre to inspect the tread from the inside. The heads are then automatically repositioned to inspect the side wall from the top. Finally the tyre is flipped to allow inspection of the second side wall. The process is fully automated and allows a cycle time of less than a minute for a complete tyre test.
Systems have been developed which have allowed the inspection of tyres up to 4,300mm diameter which covers Formula 1 racing tyres as well as tyres for passenger cars, motorcycles, trucks, aircraft and mining vehicles. However due to the size of the heads, the lower limit was tyres of 1,600mm diameter. The existing head design made use of Allied Vision Prosilica GC1380 cameras equipped with a Sony ICX285 CCD sensor having a resolution of 1388 x 1038 pixels. This particular sensor is important since it is optimised for use with infrared illumination and is matched to the optics Zeiss had developed for shearography measurement.
Positioning the measuring heads to measure tyre tread
The new camera chosen for the application was a high speed camera utilising the same sensor, the Allied Vision G145B which has a frame rate of up to 30 frames per second compared to 20.2fps from the previous model. While this new camera offered improved performance benefits without the need for an optical redesign, the camera was still too large for use with 1,200mm diameter tyres. This problem was overcome through a special hardware modification developed by Stemmer Imaging. This involved a revised camera housing in which the cable connections are arranged at an angle of 90° to the image sensor, reducing the overall length of the Manta camera from 80 to 64mm, thus allowing the four head measurement geometry to be maintained for 1,200mm diameter tyres. In addition Stemmer Imaging are able to supply the cameras with preset apertures and sealed screws on each individual lens, as well as help with the selection and long-term testing of cables. Another major benefit is that a mechanically identical version of the camera is available with a CMOS sensor, so that when the CCD sensor is eventually discontinued by Sony in 2020 there will be an easy transition to the new model.
Benefits of using the system
These easy-to-use tyre measurement systems can be used in the production of both new and retreaded tyres and can be readily adapted to the specific production environment. With no sample preparation required, they provide an indispensable tool for the advancement of state-of-the-art manufacturing processes to support ever new emerging requirements and materials. In addition to providing fully documented product quality assessment for finished products, the inspection technique can provide early and accurate detection of deviations following any process modifications or adaptations, including:
• Exchange of components
• Temperature changes, fluctuations in the vulcanisation temperature
• Changes to the tool design/profile
• Series production/new products
The importance of being able to detect defects that are not visible to the human eye is highlighted in the fact that experience shows that about 20 percent of retreaded truck tyres that have passed a visual inspection are found to be unsafe when tested using shearography. One hundred percent safety is the goal by using this technology.
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