By combining CMOS technology with avalanche photodiodes, researchers at the Fraunhofer Institute for Microelectronic Circuits and Systems IMS (Duisburg, Germany) have developed a potentially cost-effective sensor prototype that aims to support driverless car applications. The “Flash LiDAR” could play a valuable role alongside the cameras, radars, and other components within autonomous vehicles.
Breaking from Tradition
Traditional LiDAR, or Light Detection and Ranging, methods require a rotating mechanical mirror. The glass steers a laser beam to an individual point within a given scene. Google's self-driving car, in fact, features a protruding LiDAR device on the top of the vehicle.
The LiDAR's pulsed laser beams reflect on objects near the vehicle, like surrounding cars, cyclists, or pedestrians. The reflected light comes back to the mirror and detector. To calculate distance, position, and relative speed, such systems measure the time taken for the light to travel to and from any objects.
Google's device — initially a Velodyne 64-beam laser — rotates 360 degrees to achieve readings of its surroundings.
The team at Fraunhofer IMS, led by Werner Brockherde, head of the institute's CMOS Image Sensors business unit, developed a sensor system that captures a scene with one burst of laser pulses.
The appropriately named “Flash LiDAR” detector is composed of single photon avalanche diodes, or SPADs. The solid-state photodetectors are fabricated in a standard high-voltage CMOS process, and placed on the same chip as the electronics.
The Flash LiDAR has limited range — up to 100 meters — but the integrated technology components are simple, with no moving mechanical parts: a solidstate laser diode, optics, and a CMOS sensor with many pixels.
“From a production cost, Flash LiDAR is much less expensive than a scanning mirror LiDAR,” said Brockherde.
Unlike standard LiDAR, which focuses on one point at a time, the Fraunhofer technology illuminates a rectangular shaped area. Distance is then determined by performing time-of-flight measurements for an array of pixels simultaneously.
Fraunhofer's technology places the detector and electronics on a single device, meaning that the “flashy” devices are more compact than the bulkier LiDAR systems with rotating mirrors perched on vehicles like Google's driverless car.
“You can't imagine that a BMW is going to drive with such a thing on top,” said Brockherde.
The Benefits of Flash LiDAR
Because the Flash LiDAR does not have a rotating mirror, automakers can choose a desired field of view or angle. A 90-degree option, for example, would help drivers detect cyclists or pedestrians on the sides of vehicles. Similarly, four sensors could be featured on each corner of a car, to provide surrounding coverage. Brockherde also said the sensors could someday be used to perform functions such as lane departure or parking assistance.
Fraunhofer's SPAD devices have been tested and prototyped in labs. The team has designed various arrays and test chips, investigating different object reflectivities and illumination conditions. The first sensor systems will go into production in 2018, according to Brockherde.
The sensors are also of interest for other fields, such as medicine, analytics, microscopy, and applications that feature a low light intensity.
With video, radar, and ultrasound sensors finding a place in today's cars, and each providing a piece of the autonomous driving puzzle, the Fraunhofer lead sees the Flash LiDAR as a valuable complementary component.
“Redundancy in sensors will be needed in autonomous vehicles,” said Brockherde.
“Therefore, this kind of LiDAR will play an important role.”
The Fraunhofer Microelectronic Circuits and Systems group, based in Duisburg, Germany, is one of 67 Fraunhofer Society Institutes. Fraunhofer, headquartered in Munich, Germany, is an application-oriented research organization. For more information, visit Here .