Thread by @wk057

I want to drive home how impossible "Sudden Unintended Acceleration" is in a Tesla, why any issue with the autopilot/FSD system can't do this, and a few other myths and technical aspects of this whole thing in a thread.🧵 (Which ended up longer than expected.)

Claim: The brakes get disabled and the driver can't stop!

Reality: False. The brakes are still a completely mechanical system in all Teslas, and function even with no power if you press the brake pedal. The brakes are also more than strong enough to overpower the motors...

... and the brakes can't be "disabled." The self-driving features can only command to apply braking. They can't command to "unapply" braking. Brake return is a spring mechanism, as all FSD/AP can do is press the pedal just like you can as the driver. Return is 100% mechanical.

Claim: Some error in the motor/AP/FSD/CAN bus/etc can make the car accelerate out of control!

Reality: No, it definitely can not. First, the self drive system commands movement by telling the rest of the car essentially "I'd like to get X far in Y amount of time." It doesn't...

... actually command a pedal amount or anything like that. This way the AP system doesn't need to know anything about the dynamics of the car (S/3/X/Y, doesn't matter), and the rest of the vehicle determines the best way to handle the request only if it passes all crosschecks...

Then the motors look at this information, check if it's sane (no launches, nothing that the AP system wouldn't normally ask for, etc) and calculates how much torque (or braking) is needed to meet the request. If it would result in unusual G-load, power use, etc... it rejects it.

Let's say there's some error, though, AND somehow the motor accepts it. Well, then there's a half-dozen other modules throughout the vehicle that are also crosschecking these commands. If any of them think something is amiss, things will just not happen. There are multiple...

... levels of preventing things from happening, too, in these instances. They range from as simple as "hey motor, that doesn't look right stop right there!" to the BMS literally cutting off all HV power with the one-time use pyrotechnic HV fuse. Don't mess with the crosschecks!

Claim: Well, the logs can be faked or wrong or otherwise not prove anything.

Reality: Firstly, the number of people (outside Tesla) who have any clue how to even interpret the proprietary vehicle logs I can probably count on less than a hand. Second, out of those and the ...

... folks at Tesla who understand them, faking them is a whole different story. Impossible? Wouldn't go that far. Monumental? Yeah, definitely.

Because of the way the data is serialized in the log, and entry formats and such are firmware version specific (and change every ...

... update, which is annoying)... tampering is difficult.

Without getting two technical, the log has to be processed in sequence, and is self referencing (to save space and such). Modifying any of it breaks other sections, which then have to be modded, etc etc etc.

Modifying the log to show something that didn't occur, while still fitting with the rest of the log... would be so tedious that it might as well be impossible.

As for the validity of the data in the logs at all... acceleration, for example, is logged probably 15 different ways.

You have pedal press (logged by two sensors read by 2-4 different systems, depending on the car). You have accelerometer logging, autopilot command logging, torque output logging, brake pedal logging, etc etc. All from multiple modules feeding into the logging system.

It's super easy to see how any acceleration was commanded.

For ex: If you mash the accelerator pedal, all of the relevant readings will respond accordingly. The motor will report "commanded torque". Pedal position reported by multiple modules. It's super obvious... moving on...

I've personally tried to use the AP/FSD interface to do some unusual things.

First, there's no way to get significant acceleration out of it at all even abiding by all of the crosscheck limitations.

Next, disobeying any of these limitations ranges from annoying to $$$ fixes.

(Like the HV pyrodisconnect.)

I recently did an April fools video of "Fuller Self Driving" where my plan was to drive the car from the back seat with a game controller and make a video of the car doing things that FSD just couldn't do.

My thought was, if FSD can drive the car, surely I can just do those inputs manually and be fine.

Boy was I wrong!

To make this video required modifying almost every component in this crosscheck chain. And that was a complete nightmare.

Even then, the car would still give up randomly and I'd discover YET ANOTHER crosscheck I hadn't seen previously.

In the end, I got steering working perfectly, braking working great, and acceleration I gave up on trying to use the FSD systems for. I ended upbuilding a ...

... completely custom accelerator pedal emulator... which was STILL almost a bust. The tolerances the drive unit enforces on the data from the accelerator pedal are super tight, and emulating it successfully with DACs proved tricky.

Long story short, any failure or error is going to result in nothing happening.

There's no failure of the accelerator pedal, for example, that can result in uncommanded acceleration. Multiple systems just reject it.

In a last ditch effort for my April fools video, before giving up and moving to a pedal emulator, I modified the drive unit firmware to accept a completely new command to accept a raw torque command as if I'd pressed the pedal. Deleted tons of safety code.

Hmm... seems twitter lost some messages here... dangit. :(

Well anyway, suffice it to say I ran into more issues. Car actually opened the contactors under load to prevent what it interpreted as uncommanded acceleration.

So this stuff just isn't happening on an unmodified car with human error.

Tesla's seem overrepresented here, probably due to the obvious media bias (everything "Tesla" gets clicks!) and because human error in these cases is probably more likely to cause damage when it happens in an EV than in an ICE due to the unforgiving response time to the command.

Feel free to ask questions and I'll do my best to answer. Seems a good 20% of my thread got lost somehow. er.

*without human error. yay typos.

Alright, so... I'll definitely try to get in as many responses to questions and such as possible, but might take me a bit!

"Police are currently seeking a third party appraisal agency to identify the truth behind this accident and we will actively provide any necessary assistance,"

Happy to be a third party here and help out the authorities with log deciphering!

My Twitter notifications per second is slowing a bit... haha

I think I've kept up alright, though!

Again, feel free to ask whatever questions about this. Will try my best to answer.