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Three-dimensional monitoring of high-speed fabrication

09 June 2013

If you want to gauge the quality of castings on a production line with any degree of accuracy, you need high-speed systems.

A new type of measurement system developed by Fraunhofer IOF researchers in Germany delivers a quick 40 images per second – three-dimensional images, that is. Researchers showed the system for the first time at the Laser World of Photonics 2013 trade fair in Munich last month.

Capturing images of moving objects is difficult. Photographers know this as well as engineers who have to inspect industrial products on fast-moving production lines. Take, for example, castings transported on conveyor belts through assembly facilities that must be examined while in transit.

Does the fabrication quality of the castings meet the requirements? Are they correctly shaped? This is a tall order for any vision inspection system, but now a new three-dimensional measurement process has been developed that can carry out these kinds of analyses at speed.

Researchers at the Fraunhofer Institute for Applied Optics and Precision Engineering (IOF) in Jena jointly developed the technology with their colleagues in the neighbouring Institute of Applied Optics at Friedrich Schiller University. According to IOF group leader, Peter Kühmstedt, this non-contact, three-dimensional measurement process works so fast that it is possible to produce high-quality imaging of dynamic processes.

Optical measurement of itself is nothing new. With previous systems, certain kinds of images, such as stripes or multiple sequences of patterns, were projected onto the object to be measured. If the object was malformed, the image was correspondingly distorted. Experts can create a three-dimensional representation of the object from about ten different images projected sequentially.

If a digital video projector is used to create digitised images for these projections, however, the imaging frequency is quite limited. Usually a digital video projector can project about 60 images per second; extracting 180 2D images per second is about the best that you might expect.

This may be sufficient for slow processes, but the technique reaches its limits at higher speeds such as real-time quality control of components moving quickly past the monitoring station on a production conveyor belt. Fraunhofer’s new process is claimed to cope with these demands, as Dr Kühmstedt explains:

“We are able to project 400 images per second with our gauging technique and create 40 three-dimensional images per second. To achieve this goal, you only need a single photographic slide illuminated with an LED source.

The resultant projection falls on a rotating mirror that directs it at right angles onto the object. The neat trick is that the mirror rotates on a skewed, tilted axis, so that it always directs the identical image onto the object, but at slightly differing angles.”

The scientists have already constructed a prototype that uses LED illumination. In the next phase, the researchers want to move to a laser-based model that is expected to be able to operate at even higher speeds.

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