5 ms·

The video is terrifying, I never understood why cars are able to go so much over the speed limit instead of being hardware limited to ~150kph in the first place

The video is terrifying, I never understood why cars are able to go so much over the speed limit instead of being hardware limited to ~150kph in the first place Having 500+ hp in a software limited car than can all of a sudden decide to go pedal to the metal is pure insanity

addaon 4y ago
What would a hardware limit even look like in an electric car? A centrifugal governor on the axle that physically interrupts a kiloamp-scale conductor? A clock comparator between a Hall sensor on the wheel and a calibrated oscillator that keeps a contactor closed when the former is slower than the latter? These seem like heavy, error-prone systems that add more failure modes than they remove, compared to just having a software limit...
- lm28469 4y ago
  > What would a hardware limit even look like in an electric car? A properly sized motor ? My electric scooter can't reach 200kph, even without the 25kph software throttle
  - addaon 4y ago
    A motor sized to be have a maximum speed of, say, 100 mph means by definition that the power the motor can send to the wheels at 100 mph exactly equals the power lost to drag etc. by the vehicle at 100 mph (under the design conditions, presumedly dry flat road). Which means that the time to accelerate to 100 mph is infinite; and, more importantly, the time to accelerate from 70 mph to 80 mph is going to be ridiculously long. In other words, a vehicle that is power-limited to a "reasonable" speed is unsafe for normal highway driving because of insufficient acceleration to pass and avoid hazards. There's a reason that even a modern econobox can easily exceed reasonable speed. If you want to limit speed, you have to do it based on (wheel) rpm, not based on power.
    - lm28469 4y ago
      Still, do you need to do 0-60 on 2.9s in your 3 tonnes tesla ? There is a whole spectrum between 0-100 in an "infinite time" and 0-100 faster than a supersport car of the 90s
bob1029 4y ago
> can all of a sudden decide to go pedal to the metal Perhaps we should focus on this root cause? Why is the car even capable of this in the first place? Maybe we should consider removing software from certain areas of automobiles. More is definitely not making things any safer. Hydraulics and cable linkages are extremely robust. Even if the throttle on your 90s car gets stuck wide open, it would be trivial to pop the vehicle into neutral. A modern vehicle with the digital gear selector will give you a much more likely issue for such a bold action, especially if the computer is already misbehaving per acceleration. The steering wheels aren't even real anymore. What would stop a gamma ray from glitching my steering program full left turn while driving at 80mph on the freeway? How much of our safety do we really need to make the computer responsible for?
- addaon 4y ago
  > What would stop a gamma ray from glitching my steering program full left turn while driving at 80mph on the freeway? The processor running this (ASIL D) software almost certainly has hardware mitigations for SEUs. The most common such mitigation is lockstep cores (see e.g. TI Hercules, Infineon Aurix) where the same computation is run through two copies of the core (usually physically displaced and rotated to reduce common cause errors), and the output cross-checked between the two. Handling a mismatch has various approaches, ranging from pure hardware (three lockstep cores with voting) to pure software (on detected SEU issue a reset and try again) to pure analysis (show that failing to issue a steering command in this time slice has no safety impact as long as a correct one is issued in the next time slice), with many middle grounds.
- GoToRO 4y ago
  Different techniques to deal with gamma rays and other scenarios: every variable is stored in two places, the second is the complement value; plausability tests, can the value jump from 10 to 1000 in 10ms? and so on. Source: me
  - bob1029 4y ago
    And if something directly impacts the mosfet gate drivers, what then? Is there some sort of N+1 strategy all the way up to the mechanical steering linkage?

addaon 4y ago

What would a hardware limit even look like in an electric car? A centrifugal governor on the axle that physically interrupts a kiloamp-scale conductor? A clock comparator between a Hall sensor on the wheel and a calibrated oscillator that keeps a contactor closed when the former is slower than the latter? These seem like heavy, error-prone systems that add more failure modes than they remove, compared to just having a software limit...

lm28469 4y ago
> What would a hardware limit even look like in an electric car? A properly sized motor ? My electric scooter can't reach 200kph, even without the 25kph software throttle
- addaon 4y ago
  A motor sized to be have a maximum speed of, say, 100 mph means by definition that the power the motor can send to the wheels at 100 mph exactly equals the power lost to drag etc. by the vehicle at 100 mph (under the design conditions, presumedly dry flat road). Which means that the time to accelerate to 100 mph is infinite; and, more importantly, the time to accelerate from 70 mph to 80 mph is going to be ridiculously long. In other words, a vehicle that is power-limited to a "reasonable" speed is unsafe for normal highway driving because of insufficient acceleration to pass and avoid hazards. There's a reason that even a modern econobox can easily exceed reasonable speed. If you want to limit speed, you have to do it based on (wheel) rpm, not based on power.
  - lm28469 4y ago
    Still, do you need to do 0-60 on 2.9s in your 3 tonnes tesla ? There is a whole spectrum between 0-100 in an "infinite time" and 0-100 faster than a supersport car of the 90s

bob1029 4y ago

> can all of a sudden decide to go pedal to the metal Perhaps we should focus on this root cause? Why is the car even capable of this in the first place? Maybe we should consider removing software from certain areas of automobiles. More is definitely not making things any safer. Hydraulics and cable linkages are extremely robust. Even if the throttle on your 90s car gets stuck wide open, it would be trivial to pop the vehicle into neutral. A modern vehicle with the digital gear selector will give you a much more likely issue for such a bold action, especially if the computer is already misbehaving per acceleration. The steering wheels aren't even real anymore. What would stop a gamma ray from glitching my steering program full left turn while driving at 80mph on the freeway? How much of our safety do we really need to make the computer responsible for?

addaon 4y ago
> What would stop a gamma ray from glitching my steering program full left turn while driving at 80mph on the freeway? The processor running this (ASIL D) software almost certainly has hardware mitigations for SEUs. The most common such mitigation is lockstep cores (see e.g. TI Hercules, Infineon Aurix) where the same computation is run through two copies of the core (usually physically displaced and rotated to reduce common cause errors), and the output cross-checked between the two. Handling a mismatch has various approaches, ranging from pure hardware (three lockstep cores with voting) to pure software (on detected SEU issue a reset and try again) to pure analysis (show that failing to issue a steering command in this time slice has no safety impact as long as a correct one is issued in the next time slice), with many middle grounds.
GoToRO 4y ago
Different techniques to deal with gamma rays and other scenarios: every variable is stored in two places, the second is the complement value; plausability tests, can the value jump from 10 to 1000 in 10ms? and so on. Source: me
- bob1029 4y ago
  And if something directly impacts the mosfet gate drivers, what then? Is there some sort of N+1 strategy all the way up to the mechanical steering linkage?