Obligatory Newton to g force question, accelerometers drop

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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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CWatters
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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....
Apply Newton's second law to work out the deceleration.
 
  • #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.
 
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CWatters
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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
One question... Why does the accelerometer give the output as a force not an acceleration?
 

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