A cost effective solution for high speed web inspection

We have recently devised a solution for web inspection (for example the print industry).

We needed to inspect a web spool there are labels printed on the web with multiple columns.  The line speed demanded a maximum cycle time of <20 ms for the inspection of one complete frame in the web.

Problem Analysis & Solution Setup:

Although this could easily have been done with a line scan system, we chose to minimize cost by using three high speed area scan cameras. Three 120 FPS firewire cameras (one for each column) along with a high speed 2nd generation desktop PC seemed to suit the purpose. To illuminate the area covered by 3 labels (240 mm x 15 mm), 2 linear lights were used.

With the use of Teledyne Dalsa’s Sherlock machine vision software and the above computing hardware the desired speed and accuracy was achievable. A Photo electric sensor acted as an external trigger to trigger all the three cameras simultaneously for every frame.

Trial and Installation Challenges:

The first challenge was with simultaneously handling 3 cameras on a single PC. Timing was key given the speed requirements and a lot of hardware trials and analysis was performed in our lab to ensure that bus and image transfer speeds were optimized. The use of a lower fps showed that we were eating into a lot of the available cycle time. Using a faster (120 fps) Point Grey camera avoided image queuing and buffering.

Due to high speed triggering we had to design a specialized electrically isolated triggering circuit to provide a stable and fast trigger to the three cameras.

Another used was to eliminate one complete image capture cycle by adopting a counter trigger circuit. This enabled us to capture a bigger area with a single frame.

For the inspection results, we added the option of storing the failed images in a folder & also the error log file generation using Dalsa’s Sherlock software.

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