Ultrasonic sensors and radar in vehicles; How do many vehicles operate together? - Page 1
Forum section: electronics / general

author: Halcyon
author_rank: Unknown
timestamp: November 14, 2022, 07:59:13 am
content: This is something that I've wondered when driving my own car, which is fitted with both radar and ultrasonic sensors all around. The front firing radar can detect vehicles and other objects up to 150 metres in front of the car. These systems are obviously coupled with the optical camera systems to further enrich the data.But obviously as these modern technologies becomes more and more common in everyday passenger vehicles, even the cheap entry-level models, how do vehicles detect their own ultrasonic and RF emissions, say in heavy traffic and reject the noise from other vehicles?Presumably they are all using the same approved frequency band (from what I can see, Volvo uses 76 GHz FM, continuous wave in all directions around the vehicle).

author: tom66
author_rank: Super Contributor
timestamp: November 14, 2022, 08:12:44 am
content: Do the ultrasonic sensors get used at speed? It was my understanding these are for parking only - though perhaps they're used for blind spot monitoring on some vehicles too? I've had my car parking sensors activate in traffic for unknown reasons, usually it thinks you're moving at a low speed so parking mode on, but they seem to be a bit erratic when in traffic, nothing is too close but they beep randomly. Maybe that's the reason?For the radar, I would guess it is some kind of repeating code. I've had a pondering for a while as to whether you could "virtually" brake check another vehicle by driving in front of it, picking up its regular radar pulse, then simulating a return pulse that indicated you were braking hard, without actually braking. This would of course have implications for safety as accidents could be triggered remotely by a malicious individual with nearly no evidence. If the radar manufacturer has any sense, the code will be generated by an encryption algorithm that's hard to predict, making this kind of attack impractical; something similar to how the remote key fobs work. If they're not so smart, then the code will be fixed for each vehicle, so at least code collisions are unlikely. If they're clinically stupid, it'll be fixed for all models of that vehicle or radar module, and the vehicle will respond only to the strongest signal. But I really can't see a product like that shipping as there would be too many incidents.If you have a varying code, you can use something like CDMA to separate your code from what otherwise appears as noise. And the radar modules are already doing a great deal of DSP on the signal to use the return pulse. I don'tthinkit'll be as simple as FM radar, as that would probably defeat something like CDMA (you couldn't distinguish your pulse from others) and you're more interested in the time measurement.Incidentally, the radar modules have to filter out all stationary objects (as road signs, manhole covers, etc.) have a relative speed of zero, but this makes ACC systems vulnerable to completely stopped traffic. (i.e. coming up to stopped cars in fog, or driver not paying attention...) I don't know if any manufacturer has solved that yet.

author: Halcyon
author_rank: Unknown
timestamp: November 14, 2022, 08:28:09 am
content: Quote from: tom66 on November 14, 2022, 08:12:44 amThere are separate ultrasonic sensors on the sides of the vehicles which operate continuously. Those enable blind-spot detection and cross-traffic detection.The sensors directly at the front and rear probably turn on/off as required as to not trigger needlessly if you drive through a puddle. But I'm not 100% certain. I know at low speeds they are active for maneuvering.Quote from: tom66 on November 14, 2022, 08:12:44 amYou raise a really interesting point.

author: tszaboo
author_rank: Super Contributor
timestamp: November 14, 2022, 09:06:47 am
content: These are mm-wave radars, that have directional antennas. If you assume the range of 150m, then the signal is only <1us time of flight, and the signal is not continuous, rather it is sent few times a second. Let's say 10. Then the chance of two cars (driving alongside each other) have "collision" of the signal is 1/100000. The received signal goes through a down-mixer, and in case the two signals differentiate from each other, the mixer's output will be out of bands. Even setting a slightly different frequency will result signals that are not processed by the second car. That being said, probably we will end up with some interference. I know my car get's freaked out driving on the highway under some of the low overpasses, prompting me to emergency break.

author: TERRA Operative
author_rank: Super Contributor
timestamp: November 14, 2022, 09:18:35 am
content: I'm not sure if the rear ultrasonic sensors in my car only operate in reverse or at all times, but the front are always on at low speed at least. It will beep if pedestrians walk too close to the front when I'm moving slowly or stopped. I'm not volunteering to test this at high speeds.I'm not sure about interference from other cars, but I have noticed that my front sensors will sometimes seem to beep sooner if I come up close behind another car that is running.Usually when we are inching closer to each other to make space in the small Japanese streets when dropping my son off at daycare, my front beepers will seem more sensitive than at other times and tell me I am closer to the car in front than normal. This might be due to interference from the sensors in the car in front possibly.I'm not sure how easy it would be to make the sensor's output pulse coded to prevent interference like a TV remote, due to the nature of sound wave propagation. Maybe they already do this?I believe this multi-vehicle sensor interference is why many autonomous cars are moving away from LIDAR and to camera based systems, imaging trying to sort out who's LIDAR signal is who's in peak out traffic even with unique coding in the signals........

author: tom66
author_rank: Super Contributor
timestamp: November 14, 2022, 03:25:17 pm
content: Yes, it does seem like a bit of an unresolved question as to how well LIDAR cars would work, being an actively lit sensor, when there are 10 autonomous vehicles next to it, covered in 4-10 individual sensors pointing in all sorts of angles. And then combine that with other LIDAR sources, e.g. traffic monitoring systems, speed camera lasers, some bloke surveying a building nearby...That is the major advantage cameras have, although it's not to say that processing the visual data from the cameras is even remotely trivial (and perhaps LIDAR filtering is easier!)

author: Marco
author_rank: Super Contributor
timestamp: November 14, 2022, 07:37:40 pm
content: One advantage of scanned LIDAR over depth cameras is that it's quite easy to just digitize the signal at high bandwidth and do a correlation with a pseudo-random sequence to only detect the desired reflection.There are some papers on the FMCW radars for adaptive cruise control and their interference, but the real solutions seem mostly ad-hoc, proprietary and without industry cooperation. It reminds me of radio altimeters, they seem to work only slightly by design when there are others present and largely by accident.