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Low-cost optical sensor designed for auto safety

Posted: 07 Jul 2011     Print Version  Bookmark and Share

Keywords:optical sensor  driver-assistance systems  windshield 

Researchers at the Fraunhofer Institute for Reliability and Microintegration IZM in Berlin, in partnership with Centro Ricerche Fiat and chip manufacturer STMicroelectronics, on the EU-sponsored "ADOSE" project, have developed a low-cost optical sensor for the windshield that can differentiate between fog and darkness.

Recent studies have shown that the traffic fatalities in Germany have steadily decreased owing to driver-assistance systems that identify risks, warn of hazards and assist the driver in critical situations.

To monitor the surroundings during a journey, complex systems are now equipped with a camera and sensors to register difficult-visibility areas near the vehicle—such as when parking—and automatically analyse the camera pictures generated. These sensors are mounted between the windshield and the rear-view mirror. In addition to imaging data, they also deliver information about ambient light conditions such as distinguishing between darkness and fog. The sensors interpret the optical data and analyse weather conditions.

Until now, such high-tech driver-assistance systems were found only in high-priced vehicles since it required the use of expensive components to deliver precise measurements over long-term use. The IZM researchers, however, say that their sensor system can be inexpensively produced for medium-sized and small cars as well.

"Our multi-functional system consists of an entire camera, two sensors equipped with Fresnel lenses to detect light signals, and an infrared LED. Because fog and darkness can exhibit optically identical spectra, it is difficult to distinguish between these two light phenomena. That's why the infrared LED emits light waves that are scattered back in fog but not in conditions of darkness," explains IZM group manager, Henning Schroeder. "It's particularly difficult to capture the light signal from a broad aperture angle, to bundle the signal and pass it along the circuit board to the four corners of the camera chip. Because the middle of the chip is reserved for recording the camera image," Schroeder notes. To make this possible, the researcher and his team have developed lightpipes in a hot stamping procedure. These are hollow, mirrored tubes that can deflect a light signal by as much as 90°. Up until now, optical fibres have been used to transmit these signals. But these snap at even low bending radii, are expensive and must be painstakingly mounted in place manually. "With the lightpipes, we have succeeded in making the optical signal transmission more efficient, making the entire system smaller and reducing costs as a result," the researcher points out. The hot stamping method involves several optical channels being produced in a single pass, simplifying assembly considerably. The IZM scientists say their system can be expanded with hot stamping method through the addition of additional lightpipes for applications such as recording solar radiation.

The IZM researchers have developed the lightpipes and the Fresnel lenses for these sensors. A prototype of the sensor module, designed via Rapid Prototyping, is currently undergoing initial field test by Centro Ricerche Fiat.

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