Trying to measure and scale +/- 10v signal. Output is different than currently used DAQ hardware

[OKors]

I'm using an SCXI-1302 feedthrough to acquire a +/-10volt signal coming from a servo/hydraulic controller. The signal is pre-calibrated and then amplified at the controller so the +/- 10v is proportional to the range of the transducers being used. In our old DAQ hardware we scale this within the software. If we're using a 20kip load cell and 12" actuator 10v is always the full-scale range of the transducers.

We're trying to integrate this new SCXI hardware into our lab so I'm doing a quasi MSA on the two DAQ devices to verify the data is the same.

My problem is, when taking data simultaneously with both systems, each from the same signal (tee'd from BNCs), and export the data to excel, when I overlay both graphs on top of eachother they are not the same as seen in the attachment.

I realize plotting and overlaying a load/deflection plot may not be the most accurate way to do this, but my main question is this:

Will someone tell me if my connections and test setup are correct? I'll attempt to explain:

• Signals coming in are +/-10v through BNC which are terminated as follows;
  CH 1 and 2 go to pin 5 and 7 on the 1302 terminal block. The grounds for these two channels are shared at terminal 2.
  This shows up as Dev1_ai1 and Dev1_ai2 in the DAQmx Acquire step and sampled at the default sample rate of :Continuous/Samples to Read=100/Sample Rate=1k
  Termination Configuration is RSE (Differential gives me some strange results which I may have to address later).

• I've already made a library of custom scales based on commonly used transducer ranges.
  Ch1 is a 20kip load cell scaled as: +10v=+20k / -10v=-20k
  Ch2 is our displacement channel which is a pre conditioned (pre-cal'd) LVDT and scaled as: +10v=+10" / -10v=-10"

• The second Step in Signal Express is an Amplitude and Levels step. I do this because it's only by passing the signal through this step that I can actually "see" and "record" my values as the scaled values defined in the custom scaling configuration and also this is the only step (that I know of) that will let me drag the signal into the display and view as a strip or scope chart as well as plot an XY graph (this option is very important for us in our lab).
  In Amplitude and Levels I've selected to Export DC Value. This is the value I'm watching in the display as well as passing to the 3rd step which is Save to ASCII.

• Save to ASCII is pretty straight forward. I'm saving as .txt with overwrite and append to file selected. I open the .txt in Excel to plot my graph.

So before I get any deeper, does anyone have questions or see any problems with this setup?

At this point I'm not sure which signal to believe.  

Message Edited by OKors on 08-04-2009 05:15 PM

Message Edited by OKors on 08-04-2009 05:17 PM

Message Edited by OKors on 08-04-2009 05:20 PM

[Jason_D]

OKors,

2 suggestions:

1) The best way to tell is to use a known external source and see how they both compare to that source.  This can resolve the uncertanity and allow us to do some more accurate troubleshooting.

2)  Is your source floating or grounded?  I couldn't exactly tell but remember if it is grounded this is not a recommended connection as per this document.