RE: Calibration problems

From: Bill Christie <b.christie_at_linkeng.com> · Date: Mon, 17 Jul 2006 08:38:20 -0400

Heather,

I'm new to comedi myself, and so can't comment on the "comedi" side of things, but I would recommend that you first rule out a
problem with the DAQ card itself.  If all is well, there is no reason that your voltage readings shouldn't be accurate.  I don't
know your setup either (e.g. sampling rate, how many samples are being averaged, etc.) but I'll offer a few suggestions:

First of all, if you are doing single-ended measurements (not differential) measurement, make sure that your "AI-sense" terminal (if
there is one) is connected to ground.  Another item to check is the settling time between A-to-D conversions.  If you don't allow
enough settling time, you can get erratic readings.

Finally, another idea you might try is to put the card into a Windows box, and run the NI Measurement and Automation explorer
software (download NI-DAQmx from ni.com - it's free).  There are test panels that you can run which will allow you to measure the
voltage present at the input terminals.  If the NI software also shows incorrect readings, then I would suspect faulty hardware.
If the NI readings are good, then at least you know you can trust your hardware.

-bill

> -----Original Message-----
> From: comedi-admin_at_comedi.org [mailto:comedi-admin_at_comedi.org] On Behalf Of hhumphre
> Sent: Thursday, July 13, 2006 4:37 PM
> To: comedi_at_comedi.org
> Subject: Calibration problems
> 
> I am using a NI-6036E board connected to a simple op-amp amplifier circuit to
> measure forces from FlexiForce resistive force sensors.  I have Comedi version
> 0.7.70.  My data acquisition code is probably most similar to the sample code
> tut2.c.  I am having some trouble with the calibration.
> 
> I took a small data set to give an idea of the magnitude of the error that I'm
> getting, and here are my results:
> At zero load, I measure between 0.058V and 0.075V at the terminal board, but
> my reading from Comedi is anywhere in the range of 0.008V to 0.071V, and the
> larger voltages at the terminal board don't correspond to larger values from
> Comedi.  When I load the sensor such that I measure between 4.12V and 4.18V at
> the terminal board, I get readings between 3.50V and 3.51V from Comedi.  I'm
> set on the -10V to +10V range.
> 
> I am running the calibration function at startup.  I have submitted some
> questions about this before.  I appreciate the replies, but none of the
> suggestions that I tried seemed to help.  Does anyone know what kind of error
> I should expect for this kind of measurement?  Is this reasonable?  I could
> deal with a DC offset if it was a consistent offset across the entire
> measurement range, and if it was the same every time I re-boot and
> re-calibrate.  Any more ideas?
> 
> Has anyone tried bypassing the Comedi calibration and working directly from
> the integer raw data output?  Would it likely be more reliable than what I
> have now?
> 
> Thanks,
> 
> Heather
> 
> 
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