How to judge correct rap in impact test

Hi,guys
   I get a trouble during my impact test project.
   I use PCI-4472 and one module hammer connect to channel 0 and one accelerometer connect to channel 1.
   hammer as trigger of accelerometer to get impact response.
   My question is
   1) I need make sure the rap of hammer only once in each impact action. if yes, then record related response from accelerometer, if no, I need repeat impact action without record response. how can i check that?
   2) for this case, if I need average yet? ( I saw the SVXMPL_impact test(DAQmx).vi, it take some average computed for data, but for my application,since the impact only once each time, then if I can ignore the average function? I have few knowlegae about average theory.)
   3)how much samples I should use?
Thanks for any help
Tim

To 1)
How do you see a double rap?
I assume you take a look at the hammer signal and look for more than one peak. Well do the same in LabVIEW. Hint:
Waveform Peak Detection VI
Owning Palette: Waveform Monitoring VIs
Requires: Full Development System
Finds the locations, amplitudes, and second derivatives of peaks
and valleys in Signal In. Wire data to the Signal
In input to determine the polymorphic instance to use or manually select the
instance.
This VI is similar to the Peak Detector VI.
If you search the overall max or min with
Waveform Min Max VI
Owning Palette: Analog Waveform VIs and
Functions
Requires: Base Package
Determines the maximum and minimum values and their associate time
values for a waveform.
you can determine a useful relative threshold.
To 3)
What is the lowest frequency you want to detect? You should at least capture 1 period. Together with pretrigger and windowing go for 2 periods
What is the highest frequency you expect? For impact choose your sample rate at least 10x better 100x times higher. (Some say choose the highest you can get to avoid phaseshifts due to the aliasing filter)
Both values will give you the number of samples.
Greetings from Germany
Henrik
LV since v3.1
“ground” is a convenient fantasy
'˙˙˙˙uıɐƃɐ lɐıp puɐ °06 ǝuoɥd ɹnoʎ uɹnʇ ǝsɐǝld 'ʎɹɐuıƃɐɯı sı pǝlɐıp ǝʌɐɥ noʎ ɹǝqɯnu ǝɥʇ'

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