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High speed web inspection of laser-machined holes

Olmec-UK and Universal Converting Equipment have co-operated to produce a vision system capable of real-time measurement of laser machined holes in film.

Olmec-UK and Universal Converting Equipment have co-operated to produce a vision system capable of real-time measurement of laser machined holes in film.

The system is mounted directly onto the web, which runs at 350m/min. Lasers are used to perforate the extruded web material with hole sizes from 30-120 micron, depending on the particular requirement. The vision system checks that each hole is present and measures it to ensure it meets specification. It is capable of working on pre-printed or plain films.

Given the size of the holes, the speed of the web and fluctuations in its speed, the correct triggering of the camera, imaging and illumination is extremely challenging. Four lasers are spaced across the web, with a camera at each laser position to make a measurement.

The speed of the web means that more than 5mm of film passes through the field of view of each camera every millisecond. The system must process the data from the four cameras in less than 20ms to keep up with the web manufacturing speeds.

The time at which the laser fires to make a hole is locked against a high resolution encoder to allow a specific distance to be counted before triggering the camera and light.

Key to the success of the project was the development of a proprietary algorithm which searches for the hole but disregards imaging locations with no hole features. It also provides the operator freedom to add or reduce web tension, which also impacts the trigger location.

An algorithm was also developed to find the holes in ‘noisy’ images based upon the target hole size entered in to the laser system. This helps avoid detecting background noise as a hole feature.