Problem with measure with CRIo NI 9215

[zabloros]

Hello,

I have project of control DC motor in LV with CRIo in SCAN MODE and I want to scan voltage and current by analog input modul NI 9215. But I have wrong values if check it by volt-ampermeter. I suppose that is some problem with sampling of A/D converter. Because voltage and current changes very quickly and it sample very diffrent values. So the result is not correct. I just need measure effective values and make a graphes.

Thanks for advices.

Ross.

[adamcseh]

Hi zabloros,

could you please describe a little more in detail what and how you are measuring with the 9215 and especially with the 'volt-ampmeter'? Is it a multimeter with RMS display option?

What is your sampling frequency when using 9215? In what range and how does the sampled data change?

I suspect that you see the AC content of a signal when sampling with 9215, which is fitlered out by the multimeter when measurign with that one.

You could also try to use the Basic Averaged DC - RMS vi from the Signal Processing->Waveform Measurements palette and see if it helps.

Regards,

[zabloros]

Thank you for reply. RIght now I´m working on it in lab. Sorry, my description was a bit confused. Yes, I checked voltage and current by multimetr. I have voltage and current probe connected to DC motor and NI 9215. I want to measure these two values. The voltage is twice higer then on multimetr and sometimes jumps to zero. The Basic Averaged DC - RMS doesn´t help. I just tried it. Where I can change sampling frequency? I think that I need lower frequency because it samples every disturbances.

Thank you. 

[zabloros]

Settings of time engine I have 100 ms. I´m adding the grapgh of the voltage. It should be constant 15 V, which I measure by multimetr.

Thank you.

[adamcseh]

Well, the attached screenshot looks good to me, this is how your signal actually looks like. With the multimeter you obtain different values, most probably because of the low-pass filtering (and perhaps rectifying too) characteristic of the device.

Sampling with lower speed will not help to obtain better results. I think, you should rather do some processing on the digitized signal (depending on your application of course).

Regards,

[falcon98]

HI,

I am measuring 2 voltage  signals using 9215. The cRIO is working in scan mode. I have observed that if one signal affects other signal. The rise in signal 1 bring a drop in signal 2.There are no grounding issues with the sustem.

Can you please tell me how can I prevent this issue.

Regards,

falcon