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Feature request: one-shot speed limiter

Discussion in 'Model S: Driving Dynamics' started by jgs, Sep 11, 2015.

  1. jgs

    jgs Member

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    Probably almost everyone here has occasionally succumbed to the temptation to peel out when you have pole position at a stoplight. And probably almost everyone has experienced how easy it is to exceed the speed limit when you do that, even when you're trying not to.

    I'd like some way to tell the car that I'm going to floor it, and I want it to get me to the speed limit as fast as possible, without exceeding the limit. Then I want it to clear that option until the next time I invoke it.

    Naturally such a feature would be no substitute for using your judgment, but that's the same with any driver-assistance feature.
     
  2. Max*

    Max* Autopilot != Autonomous

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    Turn on speed limit chime, it'll beep when you exceed speed limit. Turn it off once you're done.

    Yes it's not a limiter, but I don't see a limiter being a... much requested feature, so I really don't see them implementing this.
     
  3. CmdrThor

    CmdrThor Member

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    A feature that promotes irresponsible acceleration? I can just imagine the lawsuits now.
     
  4. jgs

    jgs Member

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    Try it sometime. You'll find yourself a good 10 mph over the speed limit by the time the first chime sounds and you get your foot off the go pedal and onto the brake. It's not like on the rare occasions when I do this, I'm not watching the speedo. I'm just not accomplished enough to both punch the acceleration as much as I want, for as long as the car is capable, and back off sufficiently before crossing the line. I either taper acceleration before I have to (almost always that), or exceed the limit and have to decelerate (and waste energy). For that matter, don't get the wrong idea, I seldom punch the acceleration out of a stoplight at all, I just pull out sedately (or sedately for a Model S, anyway :).

    As with other maneuvers of this sort, I think it's almost indisputable that the car's software is potentially more capable than all but the most accomplished human driver of executing it without violating either the "don't exceed the limit" or "give it all you've got" parameters.

    And yeah, I don't really expect the feature to be implemented, but I bet I'm not the only one who would find it fun.

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    That's a really good point. For exactly the same reason, I can't imagine Tesla shipping a vehicle that can greatly exceed the speed limit in the region it's sold in. That's why no Model S can exceed 100 mph in North America. And it would take a real madman to imagine a car company shipping a vehicle with an "insane" or "ludicrous" acceleration mode. Talk about features that promote irresponsible acceleration! No, you're right.
     
  5. ra-san

    ra-san Member

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    I admit that I've thought something like this would be a cool feature. I'd actually thought of it more as a triggered stomp, accelerating to the set point, defeatable by touching the brake. Sort of a launch control for the S. Sure stomping is easy and fast, but software is even faster, and having the max speed set would be cool. I'm sure that almost all of us have done launches not on the track. I think it'd improve safety vs. promote irresponsibility to have a function to limit the launch, as well as prime you for the launch with you able to focus completely on your surroundings with your foot posed to touch the brake rather than switching from the stomp to the brake and trying to watch your speed.

    I'd rather them add / fix other stuff in the software, but things aren't always an either/or. If the appropriate engineer(s) fit this in (20% type time, personal project, or just while some higher priority item was blocked by other groups), I'd be impressed and happy.
     
  6. scottf200

    scottf200 Active Member

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    So if speed limit is 40 then set it to 30 (or 27, etc).
     
  7. jgs

    jgs Member

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    I already addressed this, or tried to, in my reply to Max*: I am but a human with human reaction times. Furthermore my mental model of the car's performance envelope only vaguely approximates what the actual hardware can do. That means, as I said earlier, the only two realistic outcomes when I do this are either I overshoot my goal speed, or I undershoot my goal acceleration. I don't think the speed limit chime changes this in any meaningful way – it's a crutch for when one isn't paying attention to the speedometer, which is very much not the case in this instance.

    I believe automation could allow the car to hit both goal speed and goal acceleration quite accurately. I probably erred in putting "speed limiter" in the thread title. It sounds boring and nannyish. I probably should have called it "acceleration maximizer" or something. There's a reason I'm not in marketing.

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    Yes! Exactly! Thank you! There you go, I should have called the thread "launch control".

    Agreed.
     
  8. Dashdog

    Dashdog Member

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    You realize that for the car to stop accelerating at the speed limit, then it must begin to reduce acceleration well before that- especially if you're talking about the P85D. Don't really know the math, but to go from say 0-45 MPH, the car must start reducing by, say 35 MPH. If you floor it all the way to 45, then you can't help but go over. Is going over the limit by 10 MPH really that big of a deal that you need a feature to prevent it?
     
  9. Johan

    Johan Took a TSLA bear test. Came back negative.

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    I promise you wouldn't like the feature. The only reason 0-55 mph launches feel good is you go way past 55 and then quickly come down to 55. If the car were to calculate it and time it, the 0-55 time would be way longer since you'd be doing the last 40-55 at a lot lower velocity to hit but don't go over 55.
     
  10. CmdrThor

    CmdrThor Member

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    Fair point, ludicrous mode is similar, but Tesla could argue that they meant for that option to be used on a track. The sole purpose of this proposed feature would be to avoid breaking a speed limit which means you are on a public road, and although you are avoiding breaking one law by limiting your speed, you are in turn likely breaking another law depending on what state you live in (street racing, demonstration of speed, etc). Point was is that it is a silly request because using the feature you are still breaking the law.
     
  11. Max*

    Max* Autopilot != Autonomous

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    10mph reduced speed, 15mph reduced speed? I wouldn't expect it to be that much, but I'm not going to do the math.

    It's a computer on wheels, you can easily program a feedback loop, that cuts power a few mph before you reach your desired speed, while you're still accelerating. Then it could kick regen on full blast to reduce your acceleration and bring you up to desired speed just right. No need to taper acceleration.

    Yes, it wont be a typical launch as Johan pointed out, because
    1. You'll go from crazy acceleration to crazy deceleration VERY fast
    2. It'll take longer to hit your desired speed.
     
  12. Johan

    Johan Took a TSLA bear test. Came back negative.

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    Dude you're crazy if you think going max acceleration then hard deceleration is going to get you faster to 55 mph (let me remind you; without ever passing 55 mph speed and coming back down) than just throttling acceleration from max to less than max and then just have a slighter deceleration at the very end????
     
  13. Max*

    Max* Autopilot != Autonomous

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    Do the math, prove me wrong :p
     
  14. Johan

    Johan Took a TSLA bear test. Came back negative.

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    I did in my head. Just imagine the two curves you'd have (I don't have the time to do it). Have acceleration on the Y-axis and time on the X-axis. Speed will be a function of this. Imagine how your and my curve would look. Then imagine integrating these two to and see at which X value (time) you hit 55 mph speed as a function. Now, with your curve the way you're likely imagining it your're going to hit and go past 55 mph which is not allowed. The only way to avoid this is to reduced the amplitude of your curve until - guess what - it looks exactly like my curve did from the beginning (I'm describing the optimum function).
     
  15. Max*

    Max* Autopilot != Autonomous

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    Think of it in a different way, what'll get you to 55mph faster
    1. Accelerate from 0-40*mph, then start tapering acceleration to get to 55mph
    2. Accelerate from 0-53*mph, then slam the brakes to reduce your acceleration to avoid going above 55mph

    In both cases you're doing pretty much the same thing, but one will get you there first...

    *made up numbers

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    My curve doesn't have to get to me above 55mph -- you're thinking of a classical feedback system which oscillates until it hits steady state (55mph), to which I agree it would hit above 55mph.

    My regen comment would "taper" acceleration too. But it'd get you to 55mph a lot faster than the way I assumed you meant to taper acceleration (going from max accel of 480kw to 0kw).

    I'm just suggesting going from 480kw to -60kw as close to the end as possible, and then shooting back up to whatever kw number is required to maintain 55mph.
     
  16. Johan

    Johan Took a TSLA bear test. Came back negative.

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    This can not be done. Imagine all the inertia and movement energy that is in the car when it's doing 53 while accelerating as fast as possible. It will take the car 0,05 seconds to go from 53-55, but you're going to brake it in that time? Please.

    Maybe the reason we're disagreeing too is that I imagine a mode where after your reach 55 mpg you want to keep going 55 mph whereas in your model, where I would argue you'd have to start slamming the brakes at around 45-50 mph after hitting 55 mph you're going to what - reengage acceleration? That will be one snatch you're going to feel in the drive unit.

    Edit: I see now you don't mean mechanical braking, but regen braking which is basically the exact same thing I'm saying: Loosen up on the accelerator.
     
  17. Max*

    Max* Autopilot != Autonomous

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    I think it's semantics.

    I see tapering as reducing to diminish, i.e. going from X to zero.
    I don't see tapering as going from X to negative Y.
     
  18. Johan

    Johan Took a TSLA bear test. Came back negative.

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    Then of course we are talking about the same mode/curve/function - the optimal one. It was just a communications issue :)

    But let's agree it won't feel as good as most people have experienced their "0-55" launches (since in realilty they have likely gone way past 55).
     
  19. Max*

    Max* Autopilot != Autonomous

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    :).

     
  20. Cyclone

    Cyclone Active Member

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    U.S.-spec Model S cars can definitely exceed 100 mph
     

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