High speed web inspection for laser-machined holes

The vision system checks that each hole is present and measures it to ensure it meets specification, and must be capable of working on pre-printed or plain films.

Given the size of the holes, the speed of the web and fluctuations in the speed of the web, 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 over 5mm of film passes through the field of view of each camera every millisecond. The system must process the data from the 4 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. However a number of other factors influence the exact triggering position. 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. This algorithm 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.