Bridge Offset Nulling Doesn't Seem to Work With SCC-SG24

[dhruser1]

Does anyone know if the DAQmx "Perform Bridge Offset Nulling Calibration" vi works with the SCC-SG24 modules?  We have used our NI-9237 module in the past to acquire dual-channel load cell (thrust & torque) measurements, and calling that vi does a fine job of zeroing out the values before applying the load.  However, it doesn't seem to work when we try to take the same measurements using an SCC-SG24 module.

We are able to zero the values manually, by using the nulling resistors and potentiometers in the SCC-SG24.  But, whether the nulling resistors are installed or not, we are not able to zero the values using the "Perform Bridge Offset Nulling Calibration" vi.  Running that vi does induce a significant offset in the values, but it is not the correct offset needed to bring the values to zero.  For example, with no load applied, a torque reading before running the vi may be +1090 in-lbs, and be "zeroed" to -465 in-lbs after running the vi.  Running the vi multiple times (to check repeatability) always brings us back to approximately -465 in-lbs.

An additional issue (not sure if it's related or not) is that if we try to run this vi to offset just one of the channels in the task (such as just thrust, but not torque) it will still induce a small offset in the other channel, which doesn't seem right.

I guess I'm not clear on whether the subject vi is performing an operation on the hardware, the software, or both, so I'm not sure whether the SCC-SG24 modules may not be compatible with this vi?

Thanks for any help you can provide.

[Brooks_C]

Hello Dhruser1,

I think that the DAQmx Perform Bridge Offset Nulling Calibration should work for the SCC-SG24 module.  Take a look at this article:  Removing Large Initial Offset for Load Cell or Strain Measurements with SCXI-1520 or PXI-4220 in particular it mentions that using this VI in addition to the auto zero can introduce an offset.  If you're doing both these is likely the cause.

Also, when you specify specific channels using the channels input are you also setting the "skip unsupported channels to false?  If not this may be why it affects other channels.

I hope this helps, and if not please feel free to post back with an update after trying these things.

Cheers,

[dhruser1]

Thanks for your input.  We just discovered this afternoon that on our SCC-68 (that the SCC-SG24 is plugged in to) AISENSE was not grounded to AIGND, as is recommended when SCC modules are installed.  After grounding AISENSE the offset nulling seems to work properly, and it does not affect the other channel if you only null one.

Thanks for the provided link, too.  I will look into the AutoZero property to see if that might be an alternate way of setting up our system.