Automatic compensation measurement and .dlls function node

[Asel.G]

Hello,

I'm a totally new user in LabVIEW and I began with an example program, which is made to acquire a measurement from and Analog - Digital converter.

I have a problem with peaks and an automatic offset ? / compensation ? (Sorry I'm a total beginner, I'm not sure what is the right term).

The signal should stay horizontal (like the drawed blue line).

I think something isn't right configured in the Sub.vi's which call .dlls. (circled in red) I would like to see what the program do in the .dlls. How can I read that ?

 

some of the sub.vi's look like that:

some of other look like that:

 

The program can't run, if I delete even the easy one, which looks like to only give an error code.  

 

Thank you in advance for your help. 🙂

[GerdW]

Hi Asel,

ask the manufacturer of your device and the programmer of those device driver VIs...

Do you mind to name the exact type of device and provide a link to the device driver?

[rolfk · Accepted solution]

That definitely doesn't look like a software error. Is your hardware by any chance an audio device? Because audio is usually defined to be between 20 Hz to 20 kHz, those analog inputs are usually always AC coupled with a high pass frequency of around 20 Hz. Your signal looks like it has a time constant Tau of around 2 to 3 seconds which would indicate a high pass AC coupling of around 0.3 to 1 Hz.

The function name vib_InitMeasure that the first Call Library Node calls would also hint at a vibration measurement device, which would be one more reason to use an AC coupled analog input! If you want to measure a steady state signal as you indicate, your hardware needs to be able to have DC coupled analog inputs. Maybe your hardware allows to select AC or DC coupling, otherwise you need to use different hardware.

[Asel.G]

Thank you for your help. 😄

The Problem was the coupling. My hardware is a strain gauge connected to an A/D converter.

I need to measure the influence of vibration on the result of the measurement from the stain gauge.

I am really grateful for the explanation about the time constant Tau, I have something to read and learn, and that's make me rally happy. (I have no ideas about what I'm doing, I learn by pressing buttons and observing what it does. It's very helpful to have keywords to google.)

Thank you so much !

[Asel.G]

Hello GerdW,

The device ist a A/D converter : Vibpilot 4 from m+p international and the software come from dydaqtec.

Unfornuatlly I can't give you a link to the driver, I seems it isn't on the website. https://www.dydaqtec.de/

I have aked dydaqtec for my problem but the support didn't give me a reply yet.

Thank you for your help.

[rolfk]

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@Asel.G wrote:

Thank you for your help. 😄

The Problem was the coupling. My hardware is a strain gauge connected to an A/D converter.

I need to measure the influence of vibration on the result of the measurement from the stain gauge.

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Since many strain gauges are actually in the form of half bridges (two resistors forming a voltage devider) they all have the problem of parasitic offset voltages that can be much greater than the measurement signal itself. With DC coupling and the unavoidable signal amplification, the resulting signal after the amplifier would often end up against the power rails and make measurements impossible. The most simple and cost effective method to solve that is to simply add a capacitance to the analog imput which cuts off that DC offset voltage and only allows the AC signal (change in strain) to pass through.

Full bridge sensor with 4 resistors mitigate the problem with DC offsets somewhat but only if the 4 resistors are on the same substrate, build the same way and usually laser calibrated somehow to make the resistors match each other almost perfectly. That of course results in a rather expensive sensor too.