Test system aligns radar and visual cameras

September 16, 2021 // By Nick Flaherty
Test system aligns radar and visual cameras
An active radar reflector developed by Fraunhofer FHR and Image Engineering in Germany overcomes misalignment problems between cameras and radar in ADAS test systems

Fraunhofer FHR has worked with Image Engineering in Cologne on an active radar system that overcomes misalignment problems in testing radar and camera systems.

Advanced  drivers’  assistance  systems  (ADAS) often use a camera with an AI trained object detection system backed up by an additional radar system. This is especially important in adverse weather conditions when the visibility for the visual camera is poor.

However, if the camera and radar ‘look’ in different directions, as a result of imperfect alignment, the processor will be misled, creatinga potential f alse judgment of the scene, which can ultimately lead to an accident.

This puts the focus on calibrating and testing the orientation of both systems in relation to each other.

Image Engineering approached the Fraunhofer Institute for High Frequency Physics and Radar Techniques (FHR) to develop  a  system  that  combines  a  visual  test  pattern  with  a  newly developed active radar reflector.

This uses a “golden” radar reflector mounted directly behind the center mark of the visible test chart.

This visual test chart usually consists of 2D printed structures that are visible to cameras in UV, visible or near-infrared light but not to the radar, requiring a well-aligned radar reflector is needed. This combination has to work at relatively short distances, such as in the lab, on production lines, or in repair shops (for example after an accident). Conventional radar  reflectors consisting of  three  orthogonal  plates,  known  ascorner reflectors, are unsuitable because of their depth. Since the reflection occurs in the very corner of the reflector, the reflection is displaced from the visual calibration mark on the test chart if the corner reflector is placed directly behind the chart.

Sensors located at a distance of 1 m in front of a screen and a corner reflector of 10 cm depth can experience angular errors of up to 3° under certain viewing angles and this can cause a vehicle to be assigned to the wrong highway lane at a distance of 75m, a significant problem for the safety of ADAS functions.

Instead, the active radar reflector system developed by Fraunhofer ensures the reflection occurs under precisely the same angle as seen in the visual calibration mark.

“We expected this task to be much simpler than it was in the end, but we are very happy with the system we developed and its excellent performance,” said Dr. Thomas Dallmann from Fraunhofer FHR.

The reflector image is 10 x 10 cm and is mounted in the centre or at various positions in a frame behind a radar transparent visible test chart. The chart can incorporate target markers as well as other structures needed for  alignment and  testing. Even  custom  designs  for  individual  camera  manufacturers  are possible.

The operational distance for the alignment depends on the individual requirements, the field of view of the cameras and the radar systems. Behind the test setup,a radar-absorbing wall is installed to avoid reflections by surrounding materials that may cause errors.

Image Engineering is producing the final system and will act as the contact point.

www.fhr.fraunhofer.de; www.image-engineering.de

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Picture: 
An active radar reflector (right) prevents misalignment with cameras in ADAS test systems

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