Optical inspection goes the distance

30 Dec 2009

Manufacturers could benefit from a non-contact distance sensor that can operate on rough or transparent objects

As components zip by at high speed on production lines, they require careful monitoring for quality control and position measurement. There are several non-invasive optical methods in use in today's factories to do just this – usually by bouncing laser light off the object to calculate its coordinates.

On the line, however, certain surfaces pose more of a problem than others. The object's roughness and transparency, for example, can disrupt the amount and direction of the reflected light, which in turn leads to inaccurate sensing.

Now, scientists at the University of Kuopio, Finland, have exploited a well known optical phenomenon to measure the distance to almost all kinds of rough, semitransparent, highly transparent and opaque objects, including objects travelling at high velocities (100 m/s and faster) (Appl. Opt. 48 5266).

"We have developed a final product that is almost ready for industrial use," Dmitry Semenov, a researcher in Kuopio's department of physics, told optics.org. "Our device can be used to measure the surfaces of metals, papers, plastics, woods and even objects with arsenic-deposited coatings – for which no other technique can be used."

The Finnish team bases its approach on an optical effect known as a speckle pattern, which is created when any optically rough surface is illuminated with coherent light. Put simply, light is reflected differently at different points on the surface of the rough object. As this reflected light interferes, it creates a nonuniform distribution of light intensity – a speckle pattern.

This speckle pattern can be used to deduce important information about the object. For example, when the object's surface and the illuminating beam move with respect to one another, the speckle pattern becomes dynamic. And it is the dynamic speckle pattern that the Finnish team uses to calculate the relative speed and distance to the object.

"By measuring the speckle velocity, we can calculate the distance to the object," explained Semenov. "We use a simple diffraction grating with a grating period approximately the size of a speckle to filter the pattern. As the speckles move across the grating, the light intensity is modulated and registered by a photodiode, from which we can find the distance to the object."

According to Semenov, because rough objects are more widely used than polished ones, the speckle sensor will be useful in a many fields. "Most manufacturers want to see a ready device tailored for their particular needs," he explained. "While we cannot develop appropriate electronics for every single case [for example, dark-coloured materials absorb most of the laser light, thus more powerful lasers and more sensitive photodiodes are required], we can demonstrate a universal sensor prototype to manufacturers."

Until recently, Semenov and colleagues had agreements to develop the sensor with several paper and protective coatings firms. The recession, however, has halted commercialization in its tracks.

"We are now actively looking for new industrial partners," Semenov added.