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Humans relate acceleration of a car with change in pedal angle over t

  1. Dec 27, 2013 #1
    This is in relation to the modelling of pedal input([itex]\theta[/itex]) to vehicle velocity 'v' by using controllers.

    In this video() the teacher says that humans accelerate the car NOT by sensing the pedal angle([itex]\theta[/itex]),but rather by sensing the change in the pedal angle over time([itex]\frac{d \theta}{dt}[/itex]).

    I do understand that in daily drives I wouldn't relate 20kph to 10 degrees in pedal angle,but why is it easier to relate 20kph to pushing the pedal 10degrees/1sec in my brain?
    *Pushing the pedal 10degrees/1sec just relates to the acceleration of the car and not the velocity.The video gives the output as the velocity.Is he wrong?*

    Can someone explain it to me in a more simpler sense?
     
    Last edited by a moderator: Sep 25, 2014
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  3. Dec 27, 2013 #2

    Pythagorean

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    I always perceived that a constant petal position gave you a constant final velocity (given a flat straightaway) after the acceleration transient, so it makes sense to me (if you only think of steady state and ignore transient). Of course, if you floor it, you're going to spend a lot of time accelerating before the car reaches that steady-state velocity.
     
  4. Dec 27, 2013 #3
    Yes,a lot of the time is spent accelerating than maintaining a constant velocity.But,by presuming that humans sense 'change in pedal angles w.r.t time' rather than just the pedal angle([itex]\theta[/itex]),I would have to add an extra controller to my design. What's the proof that we sense acceleration of the car this way? I might go as far to say that some people might relate acceleration of the car with respect to how far the pedal is from the floor.Right?
     
  5. Dec 27, 2013 #4

    Pythagorean

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    That probably depend on driving habits and the responsiveness of the vehicle. In my old manual transmission, taking my foot of the gas would almost always slow me down (of course, depending on the gear I was in) and there wasn't much of an acceleration transient when speeding up (unless I floored it). But that's where driving habits come in. I don't floor it, I depress the pedal slowly enough to where I feel like the transients are able to keep up with me.
     
  6. Dec 27, 2013 #5
    Makes sense now.
     
  7. Dec 27, 2013 #6

    cjl

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    I would say that's not true at all for me. I don't drive by the rate of change of pedal angle - I would say I sense pedal angle more directly, but it's only one of many inputs I use to control how I want the car to behave. This is probably largely related to the fact that I have always driven manual cars - I can accelerate slowly and gently while flooring the gas pedal, as long as I shift appropriately (at relatively low rpm). On the other hand, I can accelerate briskly without flooring the gas, just by allowing the rpms to rise more between shifts.

    In other words, it works fine as a hypothetical example of a simple PID controller, but I doubt anyone really drives entirely that way (and some people probably don't drive that way at all).
     
    Last edited: Dec 27, 2013
  8. Dec 27, 2013 #7
    I think most people decide to accelerate or slow down based on their visual perception of how quickly the world is passing them by. Based on that visual perception, they either press the pedal further down or release it a bit and use the feedback from the pedal position change compared to the speed they see the world passing by to decide if more pedal change is required.

    This is why someone can easily go from one vehicle to another vehicle that requires completely different amounts of pedal movement and can still drive the new vehicle exactly the same as they would in the previous vehicle.
     
  9. Dec 27, 2013 #8
    I never paid much attention to the pedal angle, unless I run out of travel mashing it to the floor. I think the seat of the pants feel and engine noise are more informative in terms of feedback.
     
  10. Dec 28, 2013 #9
    You need to be careful, because the point of this isn't really about cars its about controls.

    The video uses driving as an analogy for visualisation of control principles. It's not meant to be an exact reflection of how one drives and feels a car.

    For the purposes of an analogy, the video works well. Its just that noone drives like that in the real world.



    At the base of it, the throttle is a torque demand switch. Its why its called an accelerator. So an angle is most related to acceleration. However angles can then become related to speed based on losses.

    The way the video is describing driving style would be a fixed acceleration curve to a speed everytime you pulled away from the lights.

    Most people drive like the 'too much gain' example, though with more finesse. They have a high gain, but instead of overshooting the speed and braking, modulate the throttle to reducde the acceleration to settle at the speed they desire.

    If the engine is down on power, or the cars alignment is out. You need to push the pedal slightly more. Most people wont notice a few % drop in acceleration. However they will feel the increase in travel for a given speed.
     
  11. Dec 30, 2013 #10
    Interesting how each driver has their own perspective of relating the acceleration of the car to the pedal angle/other parameters.

    Chris: Very true that the video is just an analogy. I didn't notice the part of constant acceleration though,good spot! The problem though is that I did actually solve a similar exercise in my controls class last semester, and the teacher did assume(quite confidently)that humans sense the rate of pedal angle change. Quite frustrated with myself that I did not question this in the past. Thanks to you guys,I now know its just a teaching philosophy...I guess.
     
  12. Jan 7, 2014 #11

    Baluncore

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    When I take my foot off the accelerator pedal, the pedal rises again. That is because it has a healthy return spring. I sense the pressure of the spring, not pedal position or rate of change of pedal position. The angle of my ankle makes no difference, nor if I drive with my toe or heal on the pedal, so long as the pressure is correct. The proprioceptors in my legs are really useful when walking, or riding a bicycle, but proprioceptors can also be very useful when sitting in the driving seat of a vehicle.

    If I am driving at a particular speed and want to accelerate I will push the pedal down gently to a point I estimate will give me the power needed at the higher speed. I judge the pedal movement required partly by the return spring pressure and partly by the change in the sound of the engine.

    With diesel engines that have a centrifugal governor in the fuel pump, the engine RPM is determined by the pedal position. The difference between RPM and pedal position decides the fuel injection volume.
     
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