Interference in data acquired by SCB-68

[Varun Agrawal]

    Hi,

I'm using SCB-68 with PCI-6221 DAQ card. I'm trying to measure:

1. Voltage due to a load cell, and
2. voltage across 1ohm resistor (basically current flowing) in a  circuit where a motor is connected to a controller card.

Both of the measurements are being taken as differential inputs.
As soon as I enable the controller card for the motor, the voltage reading for the load cell shoots to an arbitrary (but fixed) value. I've cross-checked that the problem is not with the load cell as the same phenomenon was observed for a DC power supply too. I've no idea why this is happening. Can somebody help me get over this. A possible reason might be of the signal source type, but I tried using floating signal type for the load cell and grounded signal for the resitor voltage, and vice versa, since I'm not quite sure if the sources are floating point or grounded.

Waiting for response.
Thanks,
Varun

[Brian_Beal]

Please tell us what voltage levels you are trying to read, and what voltage level is being reported by the DAQ system.  Are you using a signal conditioner for the load cell?

[Varun Agrawal]

    Yes, my load cell has an in built signal conditioner. The voltage levels I'm trying to read is 2-4 Volts. And due to interference it reads a constant value around 4.5 V. The reading jumps up to this value irrespective of the voltage across the resistor.

[Brian_Beal]

Have you checked the voltage with a voltmeter?  If you are measuring voltage across a resistor, I would use differential mode.  Only wire one signal at a time and check them that way first.  Are you checking the readings using MAX?

[Varun Agrawal]

    Hi Brian,

I'm checking the readings using DAQ-mx. I guess that's what you mean by MAX. And indeed with one signal at a time both of them work perfectly fine. But while connected together they show this abrupt change. I tried checking with Voltmeter and found another interersting pheonmenon. As soon as I activate the motor the readings are:

                    Ideal reading      DAQ reading      Voltmeter reading
Resistor         0                          2.5                     0
Load Cell      4.8                        2.6                     10.5


This indeed is strange. as soon as I remove the one of the DAQ connections the readings come back to normal on both the DAQ and the voltmeter. And yes I'm measuring everything in differential mode. And I believe the resistor voltage should be floating signal, though I've tried it bothways.

Thanks,
Varun

[Brian_Beal]

You appear to be exceeding the common mode voltage rating.  Tell me more about your circuit.  Where does the signal conditioner get its power from?  How many wires does it have?  How is the motor wired into the circuit.  I assume you are trying to measure the motor voltage and not the current.  Is the motor AC or DC?  Any other information would help.

[Varun Agrawal]

According to the readings that I expect (refer the ideal readings in my last post), I don't think that I'm exceeding the -10V to 10V range. Perhaps that's what you're refering by common mode voltgae range.

I don't think that the problem is with the load cell, since the measurement of DC power supply of 5V also showed the same response with the resistor voltage. The motor I'm using is a DC motor of rating 48V, and is connected to a controller card. I'm attaching a rough sketch of the ckt I'm using. And yes I'm trying to measure voltages in both the cases.

[Varun Agrawal]

I somehow couldn't attach the ckt image. It's basically controller card, resistor and motor in series forming a closed ckt. Motor is being powered by the controller card, which in turn is connected to a 48V DC power supply. The voltage across resistor is being measured by channels AI1, AI9.

[Elijah_K]

Hey Varun,

So if I understand you correctly, you're attempting to take two differential measurements and they only work if you read one signal at a time?  Brian's suggestion to check the signal using Measurement and Automation Explorer (MAX) is a good place to start.  If you open MAX and use a TestPanel, you can see the values taken directly from the card's inputs, which can narrow down the cause of the problem further.  See if those values don't correspond to the same readings taken from the Voltmeter.

My guess is that you were having charge build up on the analog to digital converter due to the noise created by the motor.  Running the test I described above will help us determine this by eliminating the possibility for any ghosting between channels. 

Also, are you using LabVIEW to write your program? 

[Varun Agrawal]

Hi Elijah,

I tried using Automation Explorer (MAX) and it was showing the same measurements. I tired measuring only the voltage across the resistor and it was showing the value of 2.6V in the MAX also as compared to some 30 mV in the digital voltmeter (which looks more reasonable, since the motor on enabling is supposed to have a minimal current). So I guess problem is not with my program. My program is in LABVIEW 8.0 using NI-Daq-mx.

And the possibility of charge building up at the DAQ card seems reasonable to me, though I'm not sure how to avoid that.

Thanks,
Varun