Sensor Integration
In conjunction with all the different sensor modalities for environmental perception, hardware integration, and calibration are other important topics.
Integration is not only a matter of packaging, space claim, fixation, and precise adjustment of the various sensors but also needs to consider the lowest possible impact on signal influence and interference, and should ensure possibilities for cleaning and heating.
Two of the most obvious constraints are size and power consumption. The smaller the sensor can be designed, the better it is for integrating it into the vehicle. However, power consumption determines heat dissipation, significantly impacting the integration concept. Heat sinks, highly conductive interface materials, and, in the worst case, active cooling are concepts for maintaining a reasonable ambient temperature around the sensors.
Mounting locations
It is evident that optical sensors such as cameras and LiDARs should be positioned at a high position on the vehicle. Traffic infrastructure is designed for human vision and, thus, for the human eye's perspective. The highest possible position of optical sensors ensures a preferred perspective that is less likely to be obstructed by surrounding objects. Only then can the perception system gain a better overview of the environment and provide crucial data for the environment model. An impressive comparison of high and low sensor placements is presented in Sascha Jannik Steyer's dissertation, using LiDAR data[1].
In contrast, radars and ultrasonic sensors are suitable for installation in the bumper area. This is primarily due to their intended purpose. The processing of radar and ultrasonic data focuses on detecting nearby objects. An obstruction of the radar sensor by a vehicle in front does not limit its functionality. After all, the radar is meant to detect the vehicle ahead and determine its speed. It is also worth mentioning that radar sensors can sometimes detect vehicles in a second row through ground reflections. However, such detections' stability and repeatability are subject to variability.
Mounting on the roof or behind the windshield are common positions for cameras and Lidars. First-generation Lidars were also installed in the grille, but obstructions are expected at this position. Additionally, installing the sensor in the grille requires higher protection ratings (IP) for the sensor. When mounted behind the windshield, lower protection ratings can be used. However, mounting behind the windshield introduces other challenges, such as the need for special glass materials and effective temperature management.
Overall, there is a trend towards installing sensors behind the vehicle's exterior surface. Current-generation radars are mostly integrated behind emblems or bumpers. Due to their compact size, cameras are discreetly incorporated into the vehicle design, with only their lens visible. Lidars are still an exception to this trend when mounted on the roof or in the grille. However, future generations will also be integrated behind a covering material, as is already the case with windshield installations.
Lidar installation in headlights or taillights has been carried out multiple times in development projects. The protected installation and the possibility of cleaning and integration into the vehicle's existing control network are compelling arguments for installation in the headlights. For intersection scenarios, Lidar sensors in the headlights positioned at the front of the vehicle would be beneficial for detecting crossing traffic participants.
Cleaning
Contamination of lenses leads to reduced visibility for cameras and LiDAR sensors. Therefore, the cleaning of optical sensors must be planned. Fluid and air spray nozzles are suitable for this purpose. However, this naturally entails additional effort on the vehicle side, as the cleaning hardware, cleaning fluid, compressor, and fluid pipes must be designed in. When mounted behind the windshield, these efforts are eliminated because the windshield wiper covers the cleaning.
The radome of the radar sensor can tolerate a certain degree of contamination, primarily due to the sensor's wavelength. However, a thick layer of ice or snow can cause limitations.
Cover materials
As mentioned before, the trend of integrating sensors into the vehicle is to hide them behind the styling surface and cover materials. Many aspects must be considered when a cover material is placed in front of a sensor.
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The transmissivity of the cover material should be ensured for the corresponding wavelengths of the sensors (usually between 840 nm and 1550 nm for lidar sensors and approximately 3.9 cm wavelength for a 77Ghz radar). For radar sensors, a polycarbonate or ABS substrate is common. This can also be applied to lidar sensors.
Glass is another option for lidar cover material. Metallic material in front of the sensor could disturb the radar. However, a very thin metallic structure can support the transmission of radar waves while avoiding disturbing reflections, which allows metallic paint.[2]
In any case, the cover material should have a homogenous thickness and surface. The cover's geometry and shape must be precisely aligned with the sensor manufacturer to avoid blockage, unintended reflection, or interference with the sensor signal path.
The material has to be automotive-grade in terms of durability (e.g., stone chip resistance, aging, sunlight, temperature exposure, etc.). Finally, the surface of the cover might include a special coating to prevent water and dirt from sticking to it. The surface should also allow easy cleaning by fluid and/or air pressure.
Read more about Environmental Perception here:
[1] Grid-Based Object Tracking, Dr.-Ing. Sascha Jannik Steyer, Fortschritt-Berichte VDI, Reihe 08, 12.04.2021 - mediaTUM - Media and Publication Server
[2] Handbuch Fahrerassistenzsysteme, 3. Auflage, Hermann Winner et al, Springer Vieweg, March 2015
Very informative with valuable insights