For most drivers, keeping a car clean is a laborious task performed primarily for aesthetic reasons. However, for an autonomous vehicle that is covered in vision sensors, keeping those screens clean is safety critical.
Just as windscreen washers provide clear vision for a human driver, tiny washer nozzles are also being implemented to keep the lenses of individual sensors clear of muck, dust and other nasty substances found on the road. Any optical sensor on the vehicle is susceptible to becoming obscured, with camera and LiDAR at particularly high risk. Infrared sensors—which detect heat signals—also need cleaning, and fast.
“Any time a sensor is obstructed by debris, it needs to get cleared right away,” explained Russell Hester, Director, Business Development at dlhBOWLES. The Columbia, Maryland headquartered firm has been in the fluid dynamics space for more than five decades, and is tailoring its existing spray technology for AV sensors. “We’re customising our sprays to clean those lenses rather than the windshield,” explained Hester.
High-tech, simple solution
The technology is fairly rudimentary on paper—a spray of fluid removes muck from the obscured lens—but certain parameters require tweaking.
For instance, the frequency with which sensors must be cleaned has an environmental impact, and considerations must be made to ensure the nozzles are efficient as possible. Water is not guaranteed to remove all dirt, for example, nor is it safe to use in icy conditions. Traditional washer fluid with anti-freeze and surfactants—compounds commonly found in detergents—is ideal for cleaning efficiency, but less so from an environmental standpoint. Then there is the additional weight that comes with a sensor washing system, which could theoretically impact an electric vehicle’s driving range.
“If you’re frequently cleaning various sensors, all of this washer fluid is being sprayed on the vehicle and then eventually into the environment,” said Hester. “That’s where our low fluid-consumption design will afford smaller capacity systems. The technology will be light and dispense less liquid into the environment—it’s better than simply dousing the lenses with copious amounts of washer fluid.”
While sensor cleaning is vital for fully autonomous vehicles, it is also relevant for passenger cars on sale today. Advanced driver assistance systems (ADAS) such as forward collision warning or lane keep assist can be easily blocked even in moderate rain. In some cases, the driver is given an abrupt prompt to regain control of the vehicle, or pull over and clean the sensor manually—hardly the premium experience expected from such advanced technologies.
“If you’re driving a premium vehicle that is equipped with these ADAS features, once they are compromised due to debris they force the driver to re-gain control of the vehicle. It’s not an unsafe condition, but if you’re spending up to US$100,000 on a vehicle, it causes user dissatisfaction,” explained Hester. “I’m concerned that if this issue becomes more prevalent, then the adoption of those ADAS features will be slower on more affordable mass-market vehicles.”
The bonus bottle
Most ADAS functions are enabled via a couple of sensors, often a radar and front-facing camera. Keeping them clean is relatively straightforward, but for a fully autonomous vehicle with an array of sensors, the task is quite different. “In some of the vehicles we’re working on, you could need to clean eight or more individual sensors,” explained Hester. “Some don’t necessarily require cleaning, but with sensor fusion and the need for redundancy from a safety standpoint, that’s something that has to be considered.”
Level 4 and Level 5 autonomous vehicles require what is known as a ‘bonus bottle’, a standalone supply of fluid for the sensor cleaning system. Depending on the use case, this bottle can range anywhere from five litres to ten litres in volume; a fully autonomous vehicle can see that depleted in as little as 400 miles or so. “We’ve seen these bottles empty in a surprisingly short duration,” affirmed Hester. “Because of the aerodynamics of a vehicle, sensors located near the door mirrors are not as dramatically soiled during that same timeframe. Nonetheless, when you’re dealing with safety there will be some thresholds that require even those sensors to be cleaned frequently.”
The frequency of washing also varies depending on geographical location. Largely dry and sunny, Silicon Valley is far more favourable for optical sensors than Detroit, for example, where heavy snow and rain during the winter can lead to muddy, salty roads. “We see vehicles being fit for certain regions; vehicles that might be running around Arizona may not have the same system as those that would operate in Scandinavia or northern US states,” explained Hester.
Given how many variables are in play, it is difficult to say exactly how often the fluid needs to be replenished. In the worst-case scenario, that bonus bottle would need to be refilled every other day, suggested Hester. “But if the conditions present themselves and the sensors are being cleaned constantly, there’s really no alternative other than to consume fluid that frequently.”
With a human driver behind the wheel, recovering from a sensor blockage is rarely more than an inconvenience, provided the driver is paying attention to the road. In a self-driving vehicle with no option for human control, that sensor blockage is a big deal. It may come as a surprise, then, that something as small as a fly could render such a vehicle useless.
“If a bug hits the lens of a forward facing sensor, that bug strike could potentially create a 25 millimetre obstruction and pretty much blind the sensor,” warned Hester. “That’s of grave concern, and if that debris is left on the lens for more than 30 seconds, it will be difficult to restore it to 100% through normal cleaning—it’s going to require some elbow grease.”
In preparation for Level 4 and Level 5 autonomous vehicles, dlhBOWLES is developing an automotive-grade system that can be installed as a retrofit. Looking ahead, the company is investigating how this technology could determine the type of cleaning required, based on the type of blockage. A proprietary method to determine the minimum level of sensor cleanliness has also been formulated. “If you ask a safety engineer, these sensors must be 100% clean, 100% of the time,” concluded Hester. “That scenario is just never achieved once the vehicle is out on the road, so we’re getting down to the practicalities of when and how it must be cleaned, and what is truly ‘clean enough’.”