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# Shaker Tables - frequency precision for the inverse DFT

## Shaker Tables - frequency precision for the inverse DFT

(OP)
What frequency precision for the inverse DFT do shaker tables normally use in order to convert an input PSD into an excitation time history?

My guess is that the frequency precision they use is far less than 1 Hz. Since there is an inverse relationship between the iDFT frequency precision (in Hz) and duration of created time history (in seconds), a frequency precision of 1 Hz means that the length of created time history is only 1 second, and so the shaker table software needs to repeat this calculation every 1 second which does not seem very efficient.

### RE: Shaker Tables - frequency precision for the inverse DFT

The shaker itself is just an actuator, driven by an amplifier. The input waveform is typically calculated in advance, so can easily be an hour long. If you are trying to create a time history to match a PSD profile then it is often an iterative process to find a 'good' signal - simply IFFTing the PSD does not guarantee you'll create a practical waveform, in particular the crest factor needs to be limited.

BT>=1 is what you are talking about, and it isn't 'inefficient' it is very fundamental maths.

Cheers

Greg Locock

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### RE: Shaker Tables - frequency precision for the inverse DFT

(OP)
Thanks Greg for you quick response.

Can you tell me any rough estimate for the frequency precision (aka frequency bin size), df, that shaker tables normally use in order to discretize the input PSD when they are trying to convert an input PSD to a time-history (is it in the order of df = 1 Hz, 0.1 Hz, 0.01 Hz, 0.001 Hz, etc.)?

Even a rough number or range would be helpful. Thanks!

Cheers,
B

### RE: Shaker Tables - frequency precision for the inverse DFT

Hmm, not really, I don't run durability tests. I do generate the time histories collected from the car on the track, but I know they are reprocessed before being used to do a rig test, otherwise it would only be possible to test in real time.

Cheers

Greg Locock

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