FFT window and sample

I use Signal Express but this issue applies to any software. I hope someone can help us with that. I use a card OR-9233 to measure acceleration. I put my 25K sampling rate and put a filter of 10K. I use the software to trigger.

My setup is the accelerometer mounted at the back and the back of the handle of a hammer (down where you would hold it). I was asked to measure the outcome when a small steel ball fell (slightly) to the shock of the hammer head. The accelerometer is a PCB brand 1000g pezioresistive accelerometer, (mechanical BW around 10 kHz).

My question is this: is it possible to perform an FFT when you are just going to "hit" the hammer and measure the result? I'm not measuring the real impact, only the vibrations after it is hit. When I taste, let's dire.5 seconds, I get a nice time signal, and then I apply a FFT. When I shorten the time of acquisition to 40 milliseconds, I'm obviously only enter the beginning and my looks different FFT (highest magnitude), so I started to wonder what is "real"? Operational entities designated and required a repetitive signal of the FFT? Is there a limit to how many "small" or "large" of a time signal that I have?
Looking for some general guidance as my name of field time - frequency domain skills are so right now.
I have the ability to use a sensor to measure impact energy impact, but it is a large sensor. My accelerometer is small. I'm up to the side of the accelerometer AFAIK. I'm wasting my time with a FFT?

Kind regards

Jeff Scharpf

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Jeff,

Ok.  You have a definition of the problem: say the guy marketing you can measure a difference between the two handles.

I guess that both use the same head and the head is solid metal and the handles are of different materials.

You have a way to generate a coherent stimulus.  If you always to clear the ball from the same point, you should get a fairly coherent excitation of the hammers.  It is possible that the way in which the hammers are hung in the framework could affect the results, but my guess is that this will be a minor effect.

I guess that the answer of the head is a complex resonance excited by the impulse produced by the ball.  It will be complicated because the form of the hammer head is asymmetric (unless it's a hammer) and due to the load produced by the handles.

I'll try to measure the reaction of the accelerometer in a few places on the head of the hammer (places where he could not get hit by the ball!).  If you can get something that's fairly consistent between the two hammers here, so you have a reference for comparison purposes joint entry.  If the answers of the heads are very different, so it will be harder.

Then, I would look at the response of the handles in comparison to the heads.  Two accelerometers, an on the head and the other on the handle, measured at the same time are ideal.  I would look for differences in depreciation.  In other words, the ring die down more quickly on a handful than the other?  It is a measure of time, but it may require that the filter to select a unique resonance at a time.  I think you want to get as much data as possible, the second half or more as you mentioned earlier.  Short segments may be more difficult to interpret and not have al the information you want.

If you can post data, a person may be able to give some other ideas.

Lynn

Tags: NI Software

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