impact measurement

[Henrik_Volkers]

You used a M16 (mass) screw reduced to M6 (spring) ... that thing will vibrate ... bending and/or longitude

[Ian_Ren]

Another possibility, could the negative value be a result of a low pass filter? The filter may be in the charge amplifier or be the anti-aliasing filter in the dynamic signal acquisition (DSA) module?

If you use an impact hammer with a DSA, after the real (positive) impact peak, you always get a negative peak with a much smaller amplitude, even though the real impact force, which is in compression, is always positive.   

[Henrik_Volkers]

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@Ian Ren wrote:

Another possibility, could the negative value be a result of a low pass filter? The filter may be in the charge amplifier or be the anti-aliasing filter in the dynamic signal acquisition (DSA) module?

If you use an impact hammer with a DSA, after the real (positive) impact peak, you always get a negative peak with a much smaller amplitude, even though the real impact force, which is in compression, is always positive.   

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That's usually an effect of a high pass filter (mixed it up some posts abowe too 😉 )

[Ian_Ren]

Please see attached.

[Nolimit]

Hello guys,

Thanks for all you contributions.I changed  the charge amplifier low pass filter settings from 3030Hz to 20kHz.The attached file is the result.I got lots of resonance.

[Nolimit]

I am still working on taking out the taking out the low pass filter on the charge amplifier.

[johnsold]

I agree with Henrik that mechanical resonance is likely a big part of your signal.  Certainly your signal processing can have an effect on the wavefroms as well.  Think about the energy in the system.  You have gravitational potential energy in the ball waiting to be dropped.  You have spring energy in the preload. 

When the impact occurs some of the potential energy has been converted to kinetic energy and compresses both the preload spring and the sensor itself. After the ball bounces away, some of that stored energy will be released in the form of expansion of the compressed parts.  That expansion may be producing the negative-going signal.

You have not stated why you are so concerned about the negative portion of the signal. Why is this a problem for you?

Lynn