Using PXI-4071 for measuring current with high sampling rate

[rozenman]

Hi, I am trying to use PXI-4071 to measure current with sampling rate of 10 micro-second. Well, for doing it I'm using the waveform measuring. So I learned from the labview examples given with the driver how to make it work. The problem is that the current measured is unstable (although with other measuring technologies it is). Meaning, something is incorrectly defined or missing. I added DC-Noise rejection and Auto-Zero - but still I am getting the unstable result. Instead of getting about 14.5mA (+/-0.4mA), I get sometimes 14.5 and sometimes 17.2/10.3/12.7/18.6 etc (and the current drops to zero inconsistently).

The reason of the measure is to watch a Brushed-Motor behavior (the time when the brushes are disconnected and the motor current drops to zero, and then goes back to the stable current).

Does someone know anything about this problem, or got any other option of measuring in this high sampling rate with the PXI-4071...?

Thanks! 

[Steve_B]

Hi rozenman,

What other devices have you used to successfully measure the current waveform that you have been successful with? Also, what frequency content are you looking for in your signal? With the digitizer mode configured for a current measurement your bandwidth is limited and could be affecting your resulting data. It would seem to me that such a current signal would be noisy as the brushes are connecting and disconnecting there could be inrush currents as the cabling can be inductive. Is it possible for you to post your code to get a better idea of your 4071 configuration? Also what physical connections are there between the DMM and your motor?

As a test for the DMM it would be good to see if you are able to correctly measure a static current using the same code and physical connections.

[logic_cube]

I am also trying to measure current using the PXI-4071 at 10 microsecond intervals. I have been unsuccessful so far as my sampling rate is still at 7 ms. I'm currently using Read Multi Pont. Should I try to acquire a waveform instead?

[GarPar]

Hi logic_cube -

     You say you have been unsuccessful - does that mean you are missing data points?  If you try the waveform data acquisition VI, you may indeed get better results.  Try it and lets see what you get.