9237 output is always 0.025011

[CalPoly_Drew]

Hi All-

I'm brand new to labview and DAQ in general, so sorry for the newbie question.  I'm using a 9014 on a 9101 chassis with a 9237 module and a full bridge load cell, trying to run the 9237 getting started example.  The problem is that the ouput from the module is stuck at 0.025011v.  I don't think it's a problem with the bridge, I checked it with a standalone strain indicator and it works fine.  Any ideas?

Thanks

-Drew

[Doug_L._Bear]

I have a few questions to help narrow down the issue.

Is it a problem with all channels on the 9237 or just one of them? 

Does it happen in all slots of the 9101 or just one of them? 

Where are you reading in this voltage, in Measurement & Automation Explorer, or in LabVIEW?

Do you have a power supply that you can use to send a constant voltage into the 9237 and then try to read that voltage?  (be careful about the max/min voltage of the 9237 listed in the spec)

[CalPoly_Drew]

Hi Doug, thanks for the speedy reply-

I have a bridge wired into channel 0, the others are empty.  They read -0.024980, -0.02366, and -0.02498 respectively.  I'm using labview to read this voltage.

I just tried moving the module to slot 2- channel 0 still reads the same, but the other channels are all over the place now.  Can i disregard these channels when there's no bridge attachced or does this indicate a problem? 

I'll see if I can dig up a power supply to hook up to the 9237. 

Thanks

-Drew 

[CalPoly_Drew]

Also- does the 9237 require the shunt calibration and remote sense terminals to be connected, or can i disconnect them for simplicity?

Thanks

-Drew 

[Doug_L._Bear]

It sounds like the voltage is railing to the ADC because the max voltage read in by the card is usually 25 mV/V.  What kind of bridge is this (full bridge, half bridge, quarter bridge) and how is it wired up the pins (include excitation please).  Also what excitation level are you using and what is the nominal resistance of your bridge, as there is a maximum supply power that the 9237 can supply.

The shunt resistor does not need to be connected unless you are performing shunt callibration.  The remote sense is also not required but can be used to ensure accuracte excitation voltage supply.

[CalPoly_Drew]

It's a full bridge, wired as follows:

Pins 1, 10: shunt calibration

Pins 4, 5: RS+ and RS-

Pin 2: AI+

Pin 3: AI-

Pin 6: Ex+

Pin 7: Ex- 

The gages are 350 ohm, and I left the excitation voltage at 3.3v. 

Thanks

-Drew 

[Doug_L._Bear]

It seems you are not overloading the power supplied by the 9237 so our next step would be to calibrate the device.  Look into using the offset nulling VI and see if that helps for you.  It is located under DAQmx»DAQmx Advanced»DAQmx Calibration»DAQmx Perform Bridge Offset Nulling Calibration.

[CalPoly_Drew]

I don't have that VI (this computer doesn't have DAQmx), but I do have the shuntoffset example (http://zone.ni.com/devzone/cda/epd/p/id/5816).  When I run that example, I get simulated ue = 0, ideal (ue)  = 0.0017139, % Error = -100, and Bridge Offset (V/V)  = 0.  I'm not sure how to apply these results to fix the other VI.

[StephenB]

Hey everyone,

Drew called in and we figured this one out on the phone. Turned out to be a bad solder joint on his connections.

For future reference:

• Shunt calibration in scan mode: http://digital.ni.com/public.nsf/allkb/F1AA4688B1E7F832862574AB005AA72B?OpenDocument
• Shunt calibration in FPGA mode: http://zone.ni.com/devzone/cda/epd/p/id/5816
• Null offset with cRIO can be done with just measuring the voltage with no strain, remembering that value, and then using that value to subtract from all subsequent values.