Measuring mV with a NI 9221

[SergeArmz]

Hello All I need help on this matter,

I'm trying to measure a voltage variation using the NI 9221 module, my task basically consist in to energyze a motor with a 5V DC source, however besides reading that constant voltage, I need to read the current (A) generated by the motor whe it is subjected to a load. Fot that I'm using a shunt resistor that gives the next relationship (10A = 50mV), but the problem is when I got the postitive and negative lines from the shunt and try to read the voltage drop between the two terminals using the NI 9221 I have no signal, it looks like if both cables were unplugged, but when using a multimeter I can see all the voltage variations when applying the load to the motor. The question here is: Does any body has an idea on how to deal with this problem? Does the NI 9221 has the capability to read such a little values (0.2-0.5 mV). Why the multimeter shows the voltage variation increments when applying the load to the motor and the NI module is not? what am I doing wrong?

Please let me know your thoughts

I'm attaching a little diagram on how I'm plugging my setup

Thanks a lot for your help

SergeArmz 

[johnsold]

According to the specifications the NI 9221 has a +/-10 V input range (nominal) and 12 bit resolution. Using the typical input range of +/-10.53 V results in a resolution of ~5.1 mV. So you will not be able to get anywhere close to sub-millivolt measurements with this device.

Lynn

[SergeArmz]

Yes it seems that way you mentioned, actually the readings I'm getting with the Multimeter are voltage drops of 0.15 mV and when you apply load to the motor they get increased for up to 4.5 mV, however they still being very small values to be read with that module according to what you say. However, I would like to know what do you recommend me to use to perform these readings? Is there any other module capable to catch such small values? Do I need to go and research for a current module?

thank you very much for your valuable help!

Serge Armz

[Hooovahh]

There are many modules that fit your needs but without knowing the other constraints like number of channels I will simply guess.

Here is a module made for measuring thermocouple voltages (very small).  It has a range of +/-80mV and a resolution of 24 bit.  But it is very slow because it's made to measure temperature not current.

http://sine.ni.com/nips/cds/view/p/lang/en/nid/208787

With these settings of +/-80mV and 24bit that means each bit represents 0.0000000095367431640625V

But a better solution is for you to scale up your amps per voltage gain so you aren't trying to read a signal that is in the mud.  I mean the noise in any environment might make your reading not correct.

[SergeArmz]

Thanks Hooovahh,

I reviewed the module you just mentioned and actually its reading speed will not work for my application, I took into account what you said regarding to the inherent noise of the signal trying to be very cautious on the appropriate module. I'm gonna try this:

NI 9205 32-Ch ±200 mV to ±10 V, 16-Bit, 250 kS/s Analog Input Module

The resolution and the speed for this module at +/- 200 mV seems to be good for my application giving resolution of  0.0030517578125 mV

I'll let you know how the things work out

Cheers!