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Comment by potatochup | Hacker News Reader

The brake pedal is physically connected to a hydraulic cylinder. You need to be very strong to fully engage the brake pedal, such that many vehicles, when presented with a sharp brake pedal stab, will electronically increase the amount of brake pressure given (because the system assumes you want to stop, but the driver may not be strong enough/have the right feeling for where maximum pedal application is). If you did this at speed, you'd probably engage ABS which tends to mess with the force-feedback though.

On the other hand, the accelerator pedal is just a sensor, it doesn't require as much force to fully depress. I'd estimate the force required to fully depress the accelerator pedal to be 1/3 of the brake pedal.

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But if it's just a sensor, couldn't it be possible that sensor generates an erroneous signal such that it tells the car to accelerate when nobody actually pressed it?

Acceleration pedal sensors are ASIL D and by ISO 26262 should have a probability of failure per operational hour of <10^-8. So yes, but it should be incredibly unlikely for sensor failures to occur and there's a very thick document somewhere inside Tesla detailing all the considered failure modes and how they've addressed them.

I believe the pedal sensor has two totally redundant position sensors. If one fails or if they disagree, the car will go into limp mode and turn on the MIL. Maybe not all of them are like that, but I remember AvE (the YouTube channel) having to replace the pedal sensor on one of his cars a number of years ago and he talked about how they worked.

For those as confused as I was, MIL in this context is malfunction indicator light, not mother in law.

It is my understanding that Teslas have regenerative braking. Are you sure that the brake pedal is physically connected to a hydraulic cylinder?

Regen braking works by letting off the accelerator, not by pressing the brake.

Yes and no. A lot of EVs can do two different modes, where it "coasts" and mostly only starts the regen through the brake pedal. You're right though, from what I understand Tesla kind of forces the single-pedal driving mode.

You can't disable one-pedal driving? No wonder I see so many people in Teslas randomly flashing their brake lights on the freeway.

You can

Eh, from what I understand Tesla has made a number of changes to the configurability to regen. I don't drive a Tesla so I'm not entirely sure what that means in terms of single pedal and the overall driving feel.

https://teslamotorsclub.com/tmc/threads/21-myp-how-to-turn-o...

The parent comment specifically asked about a Tesla.

Yes. Regen is only capable of what the battery can take in, which is limited at high SoC or cold temps. E.g. see https://electrek.co/2022/05/25/tesla-updates-car-software-re...

You still need friction brakes for everything else, which in Teslas (and most other cars) are hydraulically actuated.

Tesla is brake by wire and most of them use electric brakes (I'm not talking about regen)

To most people, brake-by-wire means totally electronic control. Tesla's current models are only electronically assisted (in that there is an electrically powered hydraulic pump). There is still a physical connection between the brake pedal and the master cylinder. This means you are still available to provide some braking force if you loose 12v entirely.

See https://www.evcreate.nl/wiring-the-tesla-ibooster/ as an example

I was worried about the idea of a fully electric braking system because of the failure mode you mentioned. It looks like hydraulic brakes are not ubiquitous.

"Some Tesla vehicles use hydraulic brakes, but not all. Recently, Tesla has equipped its cars with a full self-driving system. Tesla’s latest car comes with full-electric Brembo brakes. These brakes use electric actuators instead of hydraulic fluid pressure to actuate."[1]

I'd be curious what mitigations they put in place regarding an electrical system failure.

[1] https://www.mech4cars.com/tesla-model-s-braking-systems-all-...

The mech4cars.com article is conflating the service brakes and the parking brake. The service brakes (the primary, foot-pedal actuated brakes) in a Tesla are hydraulic. The mech4cars.com article uses a link to [1] as evidence for the "electric brake system", but this is the _parking_ brake (which is indeed an electric motor, but is an entirely different system, separate caliper, etc). The parking brake will remain engaged even if you lose 12V, so you won't roll away if power cuts out (which you'll know if you've ever tried to tow a Tesla that went totally dead while parked). An electronic park brake is a setup seen in many modern cars.

[1] https://teknisimobil.com/mobil-listrik/penjelasan-komponen-e...

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