The team from the Intelligent Vehicles Group at Warwick University published its study in the IEEE Sensors Journal, which details how the researchers specifically simulated and evaluated the performance of LiDAR sensors in the rain.
High-level autonomous vehicles (AVs) are promised by Original Equipment Manufacturers (OEMs) and technology companies to improve road safety as well as bringing economical and societal benefits. All high-level AVs rely heavily on sensors to operate.
Using the WMG 3xD simulator, researchers tested an autonomous vehicle’s LiDAR sensors in different intensities of rain, driving around a simulation of real roads in and around Coventry. The simulator is used as a safe environment to test autonomous vehicles, as they are required to have been on several million miles of road.
LiDAR sensors work by emitting numerous narrow beams of near-infrared light with circular/elliptical cross sections. These can reflect off objects in their trajectories and return to the detector of the LiDAR sensor.
One issue with LiDAR sensors is the degradation of its performance in the rain: if a LiDAR beam intersects with a raindrop at a short distance from the transmitter, the raindrop can reflect enough of the beam back to the receiver, therefore detecting the raindrop as an object. Droplets can also absorb some of the emitted light, degrading the range of performance for the sensors.
According to the team, different probabilistic rain models (none, to different intensities) were used to make it ‘rain’ in the WMG 3xD simulator. The researchers then measured the LiDAR sensors’ responses, making a record of false positive and false negative detections.
The study found that as rain intensity increased it became more difficult for sensors to detect objects. In a short range from the vehicle (up to 50m), several rain drops were erroneously detected. In medium range (50m-100m) this had decreased, but as rainfall increased to up to 50mm per hour the sensors detection of objects decreased in conjunction with a longer range in distance.
Warwick’s Dr Valentina Donzella said that future research will have to investigate how to ensure LiDAR sensors can still detect objects sufficiently in noisy environments.
“The developed real-time sensor and noise models will help to further investigate these aspects, and may also inform autonomous vehicle manufacturers’ design choices, as more than one type of sensor will be needed to ensure the vehicle can detect objects in heavy rain.”