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Obligatory Newton to g force question, accelerometers drop

  1. Apr 30, 2015 #1
    To summarize, I am doing some drop test by hanging an object off of a fork lift forks, raising it, dropping it onto either concrete floor or some foam/dampening material. an accelerometer is placed on the object being dropped (epoxied on).

    if i am getting 46403.666 Newtons as the force, how much G force is that equivalent too? the mass of the object is 31.07 kg, the drop distance is 0.1524 m

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    details

    i have a 3/4" aluminum plate that is say 20x20", underneath of that i have bolted a 3/4" piece of particle board.

    i drop it using a quick release mechanism (after leveling it to the best i can using a digital bulls eye level, within +/- 1°)

    From a height of 6" it is landing on a pile of polyethylene extruded foam, 4" worth, pretty flat. beneath that is concrete floor. I was originally dropping it right on the concrete, but the deceleration spike was so high and the sample rate of my accelerometer was so low that i was aliasing and getting different results each time. The peaks were always different since my sample rate is less than 2 ms per sample. the foam has slowed it down enough to where i can actually see what is going on now. i can achieve higher peak G's by simply dropping from a higher distance.

    so the problem is i am still not 100% sure that i have an adequate sample rate (you know fs/2 and all that fun stuff). I made pretty good assumptions by doing a drop test repeatedly from x height and getting the same result every time, and then another test where i did 6 inch, 5, 4, 3, 2, 1, and have got a decline in the peak G force - as you can see from the graphs i attached. another test you can see (test 1c and test 1d) i have dropped two times from 1" (as close as i could measure) and got exactly the same peak within two decimal places and similar damping oscillations after.

    so again i think its safe to assume i am actually sampling the right thing now.

    I would like to calculate this to prove that i am close to the real world results. however there are a few assumptions that have to be made. one of them would be the distance traveled after impact. on concrete i can assume this is a very small number, since it would be the compression of the particle board only, which cant be more than a couple millimeters. the concrete floor and the 3/4" aluminum block are presumably negligible.

    i am using this site as a guidline for calculations

    http://hyperphysics.phy-astr.gsu.edu/hbase/flobi.html

    if i am getting 46403.666 Newtons as the force, how much G force is that equivalent too?

    Any input is greatly appreciated.
     

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    Last edited: Apr 30, 2015
  2. jcsd
  3. Apr 30, 2015 #2

    CWatters

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    Apply Newton's second law to work out the deceleration.
     
  4. Apr 30, 2015 #3

    CWatters

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    As for the sampling rate... You could estimate the "stopping time" by measuring the stopping distance and applying the equations of motion. You will need to sample somewhat faster than that.
     
  5. Apr 30, 2015 #4

    CWatters

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    One question... Why does the accelerometer give the output as a force not an acceleration?
     
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