FFT Power Spectrum

[smalhao]

I have a magnetoresistive sensor with two Wheastone bridges conected to my cRIO 9040, thie sensor is powered by 1.25VDC, I have the output voltage values ​​of each bridge, what I wanted to do was the FFT Power Spectrum of each bridge so as to detect slight variations when approaching a small magnet. But not appears anything.

I attached my VI.

[GerdW]

Hi smalhao,

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@smalhao wrote:

But not appears anything.

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Because of heavy overuse of ExpressVIs!

Why do you convert your scalar AI readings into arrays using those ConvertFromDDT ExpressVIs?

Which content do you expect in the resulting arrays? What is the actual content? (And why?)

Why do you need ConvertToDDT, followed by ToDBL, to convert arrays to scalar values?

What do you expect from applying a filter (ExpressVI) on scalar values?

Which frequencies do you want to detect when you read your AI values at 100Hz?

Simplify your VI similar to this:

[smalhao]

hello When I apply the previous scheme, the spectrum that appears is that of the Hanning window or another one that I can choose, ie, it makes me filter the analog signal, however I want to convert to the frequency spectrum as in the image as an example. I need detect small variations on frequency in a sensor of vibrations.

[GerdW]

Hi smalhao,

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@smalhao wrote:

I want to convert to the frequency spectrum as in the image as an example. I need detect…

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There is no question in your message, so I can only reply with: You need to implement whatever you "want to" or "need to"…

[smalhao]

I need obtain the spectrum of frequencies through the filtering signal obtained through the Hanning window and showing in a frequencies spectrum for analyze de variations of frequency in the vibration sensor in a machine.

[cbutcher]

It sounds as though you perhaps want to examine the change of your FFT over time?

To do so, you'll probably want something like a difference (i.e. subtraction) of two spectra, but be aware that this may be very sensitive and produce a lot of not very useful information...

You could look at some increased ensemble averaging (although I believe this is the functionality offered by the windowing, to a certain extent) to try and mitigate the 'noisiness' of this process - YMMV.

Shift Registers might help you if you do indeed wish to look at variations of a single sensor over time. If you wish to look at the difference between the two sensors, that probably isn't necessary (the SR).

[smalhao]

This is a one sensor only, the two entries mak a subtration of tensions and show only one tension. And i need to view de spectrum of frequencies of that tension, the sensor is supply by 1.25VDC and the output of sensor is 11mV because the sensor have a bridge Wheastone , the reason of i need see the spectrum of frequencies is to detect small variations when the machine vibrates or detect when broken an wire. The objective is apply preditive manutention. When i apply the Hanning window that window makes a filtering of signal, and i need see the central frequence and the others frequencies, the output should be something like a Sinc function