Microscanning mirror enhances lidar sensors
In the next few years, emergency systems such as the evasion assistant will be mandatory in new cars and pave the way for autonomous driving. However, in coming vehicle generations, human drivers will also be obliged to continuously monitor the vehicle systems and the surroundings in order to be able to intervene in dangerous situations. Only in autonomy levels 4 and 5, this will change – the driver can turn to other activities. Surveillance of the environment will then mainly be carried out by lidar sensors, which measure the distance between the vehicle and the object. They recognise the environment and in a way replace the driver’s eye. Thus, they represent a decisive building block on the way to safe autonomous driving.
A team of researchers at the Fraunhofer IPMS in Dresden (Germany) has now developed a new type of microscanning mirror, an important component for lidar systems that enables digital vision in three dimensions. The component contributes to the three-dimensional measurement of the environment by steering the scanning laser. The company AEye is already using Fraunhofer’s microscanning mirrors: the specialist in light detection and distance measurement systems for autonomous vehicles uses the novel mirrors in its 4Sight lidar sensor.
“With our technology platform, we can meet the respective design needs for the development of microscanning mirrors for lidar. Lidar systems can detect the environment in three dimensions and are thus able to detect pedestrians, cyclists or other vehicles. Our MEMS mirror distributes laser beams in two dimensions and bundles the light in the currently measured position. With the travel time of the reflected light, the distance to the object is measured as the third dimension,” explains Dr Jan Grahmann, scientist at Fraunhofer IPMS.
The light sent from the laser source in the laser scanner first hits the microscanner mirror, which is located on the transmitting unit of the lidar system. The mirror scans the scenery in two dimensions. The third dimension is captured by a lidar sensor on the receiving side based on the light reflected from the object. The more light that hits the sensor, the more precisely the distance can be determined – a task that is taken over by an evaluation algorithm. The distance information for each scanned position in the scenery results in a 3D point cloud that represents the visual range of the lidar.
The MEMS scanner is made of monocrystal silicon. This makes it fatigue-free and robust – in contrast to mechanical scanners. In addition, this technology is characterised by high shock and temperature resistance. There is a reflective coating on the silicon that enhances the reflection of light. Thanks to a position detection system integrated into the chip, it is possible to determine at any time where the mirror is directing the laser light and which position in the image is being measured. This in turn enables corrections to be made to the working point. In the vehicle, the lidar sensor is usually located behind the rear-view mirror and scans the scenery directly through the windscreen. In addition to the normal perception by the passenger’s or driver’s eye, 3D measurement in the infrared range can be realised in this way.
These MEMS mirrors are typically up to about five millimetres in size. Larger mirrors are possible in special cases, but they lose the advantages of MEMS with increasing size. In addition to the microscanning mirrors, Fraunhofer IPMS also offers the development of the necessary packaging as well as the integration of the drive electronics. All components can be customised to ensure optimal integration into the various lidar systems.
The microscanning mirrors at hand work with the wavelengths typical for lidar, from 905 to 1550 nanometres, and their aperture width has a significant influence on the range. Built into the smart lidar sensor from AEYE, high ranges of more than 200 metres are possible. The company’s first driving tests with the MEMS scanners from Fraunhofer IPMS used in the lidar system have been successfully completed. In the next step, the microscanner mirrors are to be produced in larger quantities and transferred to large-scale series production.
More information: https://www.ipms.fraunhofer.de/en.html
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