AC to DC conversion of a LVDT signal

[AnsjANSJns87]

Hello,

I'm working with an LVDT sensor and DAQ USB6001, what I try to do is getting the signal from the LVDT with the DAQ, then convert the AC signal into a DC and finally if necessary filter the signal so it's smooth as possible.

Until now I have this (I get the AC signal from the LVDT): (The graph from the left is the one from the LVDT plus an absolute value  function)

I saw a solution in another forum where they advice to use "Array Max & Min Function" but when running, the sample time is to much compared to the sample clock of the DAQ. 

Also I tried to use an AC&DC Estimator PtbyPt  and an AC&DC Estimator but no results appeared in any case.

I runned out ideas to convert the signal to a DC (without using any more hardware). 

I'll aprecciate any help 

Thanks

[Henrik_Volkers]

Usually you drive the LDVT with a sine and use synchonious rectification followed by a low pass ...

so you need to sync your sine exitation with your LVDT output.

And I expect a sine signal at the input (LVDT output) if not in perfect zero position.

You can use two channels of your 6001 and read the (maybe attanuated) exitation and the LVDT output at the same time ... followed by two tone detections to get amplitude and phase of both signals. the amplitude ration will be your displacement from zero and the phase relation will tell the direction.

(oh and you can use the soundcard instead of your 6001 😉 ) 

However different other methods are possible...

what LVDT are you using?

[AnsjANSJns87]

Hello,

I tried tone measurement, but when running it appears nothing in the graph.

I'll try synchronous rectification. I guess I would have to use some pyshical mosfets for the rectifier, right? Or it can be make purely in LABVIEW?

Actually I'm using an Tinius Olsen lvdt extensometer, the model is kind of old it says S40 - 2. (In the end I attach a photo from it)

Best regards

[Henrik_Volkers]

Show the circuit how you connect the LDVT

What  source and frequency do you use to drive the LVDT?

can you measure the drive voltage with your 6001?

[AnsjANSJns87]

I attach a photo of the circuit at the end with the name "circuit".

To energyze the LVDT I use a 220 VAC - 12 VAC transformer, the transformer is connected at the same time with an outlet of 220 VAC and 60Hz (Those are the standar values for Perú).

I can measure the drive voltage with the USB6001. The amplitude of VAC sign is 12.

Thanks

[Henrik_Volkers]

Do know the pin out ? internal schematic ?

Have you measured the resistance between the pins?

50 Hz is a bit low for the LVDT i know ...   400 Hz to 20 kHz are the frequencies I have seen ...

Unless you see the 'same' AC at input and output (output not 12V .. more in the 1V or below , while sensing element is not in center )

something can be considered wrong in your connection.

Better use the soundcard headphone output ( with 33 Ohm in series if you are afraid of magig white smoke  😄 ) and output 2kHz to 8kHz sine ... see LabVIEW Help exsamples ...   you don't need that power of the big transformer

try it at both endpositions of the LVDT .. maybe they originally used only half of the range and simply measured the rectified AC value...