Measurement of the envelope of the AC

Hi guys,.

I have a cDAQ-9188 with a NI9205 analog input module. I am able 3 AC 10 kHz differential signals and two of them are amplitude modulated at a low frequency of<1Hz through="" mechanical="" movement="" of="" a="" rotary="" transformer.="" does="" anyone="" know="" a="" way="" to="" measure="" the="" envelope="" of="" the="" modulated="" signals="" in="" a="" simple="">

I was thinking about sampling to ~ 25 kHz and and calculate RMS values each 500 samples, however, in time, I get the 500 samples the mechanical system moves a few (2) so the data are useless.

The brute force approach would be to sample at 25 kHz for the complete tour of the mechanics, but gives me a large amount of data and my tampons could get cluttered with her. Even with these data, I'd still need to extract the envelope in some way.

I look forward to any entries.

Thanks, Felix

Felix,

Let's look at some numbers: 20 RPM is equivalent to 0.333 tr/s or 1200 (intervals of 0.1 °) / s. So there are 833 US for unit rotation of 0.1 degree. It's time 83 10 kHz signal cycles.

The next question is: Can you get information good range of 83 cycles?

During the sampling of a 10 kHz to 25 kHz signal you get 2.5 samples per cycle, or samples are apartment 144 °. The image below shows that sampled data resembles a 25050 sampling frequency. I adjusted the sampling frequency slightly so that the sampling points are not repeated exactly every cycle, as they do at exactly 10 and 25 kHz. Unless the two signals is derived from the same clock, there will be a slight frequency shifts.

Notice that none of the points shown sample place exactly on the summits.

Extract only your Information.vi accepts an input signal and returns the amplitude, frequency and phase of the signal. I ran some tests to see how much data is needed to get good results.  The data displayed in the image above - 21 samples - set Gets the amplitudes with lower to +/-1% error. He treats (simulated) 3 seconds of data in a little more than half a second.

I would like to know what version of LV, you use so I can post an example for you.

Lynn

Tags: NI Hardware

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