HGV and car tires today have a fixed cord geometry which is embedded in rubber. The cords are either formed as metal wires or from plastic fibres. In order to be able to comment on the quality of an individual tire, it must be determined for this tire on an individual basis whether the predetermined geometry of the cords is also realized or if there are deviations from this. Both the relative position of the individual cords in relation to each other and the absolute position of the cords in the rubber, thus for example their distance from the tread of the tire, are relevant for assessing the quality. For technological reasons, in different tires the individual components of the tires are represented on different scales in the X-ray image. The inspection specifications applicable in the X-ray inspection of tires sometimes include measurement indications. In order to be able to determine the deviation from these measurement indications, it is necessary to calibrate the X-ray inspection system, used for the inspection, uniquely for the relevant tires. An X-ray radiography procedure using a line detector which is formed U-shaped and which thus surrounds the tire over its two side walls and the tread is routinely implemented today for the inspection of the cords.
If a specific tire type is inspected, it is necessary to calibrate the inspection system, which routinely comprises an X-ray tube and an X-ray detector, between which a movable manipulator is arranged, on which the tire to be inspected is clamped. A two-dimensional X-ray image only results when the tire turns, while the line detector is read in a clocked manner.
In order to be able to carry out a meaningful assessment of the quality of the tires to be inspected, of the new tire type, in the calibration the internal and external geometry of this tire type must either be estimated, which is very imprecise, or be detected in the X-ray radiography beam using strip-shaped test pieces which are placed over the internal and external walls. From the results obtained thereby, in the subsequent inspection of a specific tire, the position of the cords can only be estimated. Moreover, this type of calibration is very time-consuming. In addition, such a calibration must be carried out by an experienced operator, because he has to carry out a visual check.
After the calibration of the X-ray inspection system to a new tire type, the individual tires are inspected, according to EP 1 867 971 B1 for example, as follows. The X-ray tube is arranged in a first position relative to the tire and radiography of the tire is carried out. The beams that pass through two particular points on the tire are detected in a detector with respect to their respective coordinate. After that, the X-ray tube is moved from its first position into a second position in the axial direction. There, X-ray beams passing through the two points already detected previously in the first position are detected again by the X-ray detector with respect to their coordinates. The actual coordinates of the first and the second point within the tire, the imaging of which is carried out by the detector, are obtained via geometric observations, in that the respective lines of intersection of the X-ray beams which pass through them are obtained from the two positions of the X-ray tube and the respectively mapped coordinates on the detector. Following this, the actual distance between these two points within the tire is calculated and the length between the first and the second point on the image is converted into an actual distance between the first and the second point.