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Airbag physics and Calc1?

  1. Dec 1, 2014 #1
    Today, I am an adult who is re-learning some Calculus. But, I ask a lot of questions about anything and everything.

    Has anyone ever explored the concepts and/or Calculus/Physics behind auto airbag deployment? Computer feeds real time velocity points and the ECU is monitoring for a sudden drop in rate of change / sudden spike in negative acceleration?

    What is the basic idea at work?
    Got any links that would drive the concept home?
  2. jcsd
  3. Dec 1, 2014 #2


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    Gold Member

    I'm not sure, but I assume that it's based upon a pendulum system the same as a seat-belt locking reel, with possibly a compression-bumper-strut connection as well. I'll snoop around some to see what's out there. I recommend that you do some serious Googling yourself, though. The info should be readily available.
  4. Dec 1, 2014 #3
    Car crash finish in 80 ms. in time 30 ms, head impact to steering. for explosion of airbag roughly we need 10~15 ms (for full filled)
    so we have something about 15 ms (golden time) to decide for firing or not.
    there is a ACU in car, with acceleration sensor in different direction(for detecting front crash, rolling,..)
    when you start car, ACU turn on and start for data sampling and calculating. there is some algorithm inside of ACU software that decide for firing or not.
    this software is modified by some coefficient which we adjust them trough airbag development process and test.
    this test are full front crash with rigid barrier, rolling, deer test, mis use test,.....
  5. Dec 1, 2014 #4
    Pretty sure it's not a sensor in the bumper, but a computer algorithm that monitors speed.

    This sounds right.
    Is there any way to describe what the algorithm is looking for?
    ACU is fed constant speed values from speedometer.
    ACU can calculate acceleration based on time elapsed b/w 2 speed data points.

    eg: v(t) values
    t=0 (milliseconds), s=55 (mph)
    t=5 (milliseconds), s=60 (mph)
    t=10 (milliseconds), s=70 (mph)
    t=15 (milliseconds), s=50 (mph)
    t=20 (milliseconds), s=0 (mph) <== Has hit into wall and stopped suddenly

    Let's calculate some rate of change of velocity using deltaY/deltaX:
    60-55 / 5-0 = 5/5 = 1 mph
    70-60 / 10-5 = 10/5 = 2 mph
    50-70 / 15-10 = -20/5 = -4 mph
    0-50 / 20-15 = -50/5 = -10 mph

    During some interval, maybe the airbag is triggered once the acceleration exceed some negative threshold ?
  6. Dec 1, 2014 #5


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    Yeah. I thought that I remembered seeing something like that, but I realize now that it was a shock-absorber system.
  7. Dec 2, 2014 #6
    Anyone care to verify my basic thought process?
  8. Dec 2, 2014 #7
    please see attachment.

    Attached Files:

  9. Dec 2, 2014 #8


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  10. Dec 2, 2014 #9
    Nothing mathematical there, but did mention about the concept of deceleration.

    Because airbag sensors measure deceleration, vehicle speed is not a good indicator of whether an airbag should have deployed. Airbags can deploy due to the vehicle's undercarriage striking a low object protruding above the roadway due to the resulting deceleration.

    The airbag sensor is a MEMS accelerometer, which is a small integrated circuit with integrated micro mechanical elements. The microscopic mechanical element moves in response to rapid deceleration, and this motion causes a change in capacitance, which is detected by the electronics on the chip that then sends a signal to fire the airbag.
  11. Dec 2, 2014 #10
    mmeftahpour, thanks for the Word doc. That also has a small description of the acceleration data.
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