Fully automatic x-ray inspection for tire manufacturers--part 1.

Focus on tire quality has never been more intense. Pressure is on manufacturers to insure the quality of their product while reducing costs in order to survive in these tough economic times. This article will discuss the benefits and potential cost savings to tire manufacturers who incorporate today's state-of-the-art x-ray inspection techniques into the production process.

Automatic x-ray inspection automates decisions that, in the past, were made by operators, and offers repeatable, dependable analysis of tires according to specific pre-set parameters. Operators can now supervise a number of production lines instead of being tied to the monitor of only one, resulting in significant labor savings.

Topics to be examined will include the workings of automatic x-ray inspection systems, including high resolution, digital detectors, powerful computer processors and highlights of software capability, possible labor savings and the future of automatic x-ray inspection.

Tire manufacturers have used x-rays to inspect prototype, green and finished tires for decades. Over the years, x-ray inspection systems have evolved from x-ray tubes coupled with direct view fluorescent screens to today's linear diode array imaging systems, coupled with panoramic x-ray tubes. One part of the process has not changed in all these years; pass/fail decisions, in most cases, are made by an operator viewing tire images presented on a viewing screen.

Automatic x-ray inspection incorporates technological advances made through the years, and offers an alternative to the labor intense process of one operator per station analyzing tire images on a viewing screen. One operator can now oversee multiple lines as the automatic x-ray system analyzes tires according to specific pre-set parameters and makes pass/fail decisions without operator intervention.

Why is today the right time for automatic x-ray inspection of tires?

Analog versus digital

Electric signals generated in the real world are analog in nature. They vary continuously. Digital data are a representation of the analog data in a format that can be used by a computer. Advances in digital technology are part of the reason automatic x-ray inspection is now possible.

Isocon tubes/cameras--disadvantages

For years, the isocon-based camera was the most widely used x-ray imaging system for tire inspection (figure 1). While it provided the best images possible for its time, there were some serious deficiencies inherent in the system.

[FIGURE 1 OMITTED]

The imaging portion of the tire x-ray system started out with an NTSC video signal. Gathering this information required multiple tire revolutions for a complete inspection. Multiple revolutions translate into longer cycle times.

Cameras and other traditional imaging devices are flat screen devices. Flat screen devices result in distortion of contoured areas of the tire. Extremely accurate positioning is required to make useful relational measurements.

Analog data received from the camera are equivalent to 8 data bits. The resulting image is displayed on a monitor where the operator may distinguish 256 gray shades.

High resolution digital detectors As the years passed, technology improved to a point where the deficiencies of the isocon-based camera could be overcome with new imaging systems. Today's x-ray inspection systems incorporate state-of-the-art imaging systems such as amorphous silicon flat panel and linear diode array detectors.

As with the traditional cameras, LDA (linear diode array) imaging systems produce an analog signal. This is immediately converted to a digital signal for computer processing. Some data loss is inherent in the conversion, but this loss is minimized by using short cables between the LDA and ADC (analog to digital converter).

To minimize image distortion and generate the highest possible image resolution, a horseshoe or c-shaped linear diode array detector is often chosen for tire inspection (figure 3). The c-shape closely simulates the shape of the tire and wraps around the tire's surface to enable a bead-to-bead inspection in a single rotation of the tire.

[FIGURE 3 OMITTED]

Additional benefits of the system include modular design with field replaceable 64-element detector modules with advanced signal processing chip; x-ray sensitive energy range of 20-160 keV; 16-bit data acquisition; wide dynamic range >4,000:1 ; digital input control unit provides simple interface to a PLC or user pendant; automatic correction of pixel variations; easy to use bad detector correction software; customized look-up tables for image enhancement; integral collimator/shielding; RS-422 data output; meets ASTM F1035 pie disk standard; and industry standard mounting.

In a typical application, a panoramic x-ray source that emits a fan beam of x-rays in an angle >180[degrees] is inserted inside of the tire under inspection and aligned with the internal collimator aperture opening. The tire is rotated at constant speed, and the x-ray photons are integrated in synchronization with the rotation speed for a period of time equal to one pixel width of tire motion. Each integration period produces one line of the image. The detector signals are digitized and transmitted to the PC while the next integration period is occurring.

At the PC, the incoming data are automatically corrected for offset (dark current) and gain (x-ray sensitivity) for each detector element. There are also customizable look-up tables that can be applied to the data at this time that can be used to significantly enhance image quality. The resulting corrected and enhanced image scrolls down the TV monitor in real time as the tire rotates through 360[degrees]. The offset and gain correction tables are stored on the PC's hard disk during a calibration cycle. Multiple correction tables can be stored for different kV setting and integration times to accommodate random tire sizes. Likewise, multiple lookup tables can be stored on the hard drive for the same reason. All correction tables and look-up tables are easily activated through the digital input interface unit.

270[degrees] panoramic x-ray tube

Deficiencies were inherent in some past x-ray tube designs. The lighthouse tube, for example, used moving parts to spread the x-ray beam over the shape of the tire. With the development of new panoramic x-ray tubes, better signal disbursement across the 270[degrees] beam was achieved. The beam geometry is optimized for use with horseshoe or c-shaped x-ray detectors. They include better collimation for cleaner signal and better cooling for extended life.

This article will be continued in next month's Rubber World (June 2005), where automatic image analysis will be explored in depth, including its many benefits.