Re:Former Tesla Firmware Engineer Discusses the System
by Cyph0n on Friday August 24, 2018 @09:25PM
(#17839040)
It's not a pressure sensor, but an induction loop. Basically, there is a coil placed on the road that has a small AC current passed through it. When a car (metal) sits on top of the coil, the two "coils" couple, changing the overall inductance. A simple sensor can detect this change.
Re:Former Tesla Firmware Engineer Discusses the System
by TheKarateKid on Saturday August 25, 2018 @01:24AM
(#17839681)
The current is even able to pass through the tires?
Re:Former Tesla Firmware Engineer Discusses the System
by Cyph0n on Saturday August 25, 2018 @02:26PM
(#17841873)
No, the current passes through the coil, and the coil has no physical contact with the vehicle.
Look up inductive coupling. The basic idea is that a changing (AC) current in a conductor generates a changing magnetic field (Ampere's law). This changing magnetic field then induces a voltage in the second conductor (Faraday's law). This is the principle behind how transformers work.
Re:Former Tesla Firmware Engineer Discusses the System
by Dreami on Saturday August 25, 2018 @03:12AM
(#17839943)
As far as I understand, it's just electromagnetic waves. No current passing through to the car, but the coil can "register" a change in its magnetic field and can determine that it's a car and how fast it goes
Re:Former Tesla Firmware Engineer Discusses the System
by vageli on Saturday August 25, 2018 @11:56AM
(#17841297)
Can confirm. Also interesting to note that motorcycles often have trouble triggering these sensors (a common trick is to stick a heavy duty magnet underneath).
Re:Former Tesla Firmware Engineer Discusses the System
by BinaryIdiot on Monday August 27, 2018 @03:00AM
(#17849873)
Really interesting! Thanks for the correction!
Re:Former Tesla Firmware Engineer Discusses the System
by mmt on Saturday August 25, 2018 @04:42AM
(#17840132)
As a sibling comment noted, it's not a pressure sensor [1] but an induction loop.
The Wikipedia article https://en.wikipedia.org/wiki/Induction_loop#Vehicle_detecti... has fairly extensive details on modern implementation.
[1] These do exist for weight-in-motion systems, however.
When I was a kid there was one light that, when you drove over the pressure sensor, it wouldn't really do much. But if you backed up and drove over it again it must have registered an additional car coming through and the light would almost immediately go through its light cycle to change. It was really interesting to see!
Nowadays I think it's mostly cameras? We have a light near my home and the left signal will literally never trigger unless someone is in one of the left lanes.