Bullet with mounting accelerometer hits the load cell and does not provide the same readings

[StathPol]

Hi guys,

I am hitting directly a load cell (force sensor), from a short distance to check whether the impact force generated from the load cell is the same with the accelerometer (mass times acceleration). I find that there is a difference in the readings with a factor of 3. The acceleration is configured in MAX. The gain of the single axis accelerometer is provided from the signal conditioning. Does anyone please know why there is this difference?

Trial 1:

Load cell = 1000 Newton

Accelerometer = mass x acceleration = 2.64 kg x (120 g x 9.81) = 3107 Newtons

Trial 2:

Load cell = 700 Newton

Accelerometer = mass x acceleration = 2.64 kg x (90 g x 9.81) = 2330 Newtons

[Henrik_Volkers]

My guess: The dynamic response of the load cell. 

Load cells are usually calibrated static . Your measurement is dynamic.

Some key paramenters are the resonance frequencies of your load cell and your accelerometer (later usually much higher)

Take a look at the power spectrum of your accelerometer signal .... compare it with the spec of your load cell. 🙂

Another test to try: Place a mousepad  (or similar platic/rubber moderator) between your mass and the cell ...  measure again....    longer pulse, lower frequencies ( and lower amplitude)

See your load cell as a damped spring with two masses ....  and more coupled masses/springs;)

[StathPol]

Hi Henrik, 

Thanks for your reply.

I used a rubber between the mass and the cell. I agree that the pulse of the graph will be longer with lower amplitudes.

The resonant frequency of the accelerometer is 100 kHz, with a bias voltage of 10,8 V

Sensitivity = 0.000516 V/g

Gravity = Measured Voltage / Sensitivity 

+/- 2.58 Volts = +/- 5000 g

Is something wrong?

[Henrik_Volkers]

well, since you calculate in SI why not only use SI ... ..

The Sensors transmit the foce and acceleration they feel.... that may or may be not the information you expect 😉

How do you drop?  How do make shure all is lined up?  Modes of the mass?     You can't fool physics, but physics always fools you 😄

[StathPol]

Nice 😄

Well, I try to balance the bullet as straight as possible in the vertical direction. Instead of using a rubber pad, why not to use a steel plate in order to not absorb the energy too much?

I did not alter the signals like removing the dc bias or filtering.

I think, high speed camera would give us an additional comparison, but this is an indirect method.

[Ben]

Stepping outside my confort-zone...

Would sample rate also be a consideration?

Sampling too slow and miss the peak?

Ben

[StathPol]

Ben -- the sampling rate of the acquisition is quite fast, 100 kHz.

[StathPol]

Here are some trials

[Ben]

With a difference of X5 between the mac and min (assuming they are the same test repeated)...

Can you put a fast scope on the signal set for single sweep and compare the variation in the displayed signal?

As i said above, walking around outside my comfort zone.

Ben 

[Alexander_Sobolev]

Can you show the graph of the impact? How do you calculate the force, simple maximum? Is bullet round - can it hit with different sides?