How to perform shunt calibration and offset correction to 9237 module using cDAQ

[NSWC Ben]

I am measuring pressure data from a full-bridge pressure gage using the 9237 module plugged into the cDAQ chassis.  My voltage readings react to changes in pressure but the values are not correct, so I suspect a shunt calibration could help determine the correct gage factor to multiply my readings by in order to get the correct values.   I have not been able to figure out how to activate the "software" controlled shunt calibration or offset correction as is mentioned very briefly on pg. 13 of the operating instructions at the link below: 

http://digital.ni.com/manuals.nsf/websearch/5759E6CBA6DF1AC9862571CA0068B5D6

I understand somehow through software the shunt cal. circuit is enabled and disabled which shunts the gage with a 100 k Ohm precision resistor.  However, I have no idea how to enable this feature through software.  I found this lengthy post on the 9237 when used with the RIO but this technique (using a property node) I don't think applies to cDAQ (I could be wrong but I don't think this is the case). 

http://forums.ni.com/ni/board/message?board.id=280&message.id=3055&requireLogin=False

I have found hints that maybe this shunting is done in MAX?  I really have no clue, and there is absolutely no information on how to actually perform this in the documentation.  Any help would be greatly appreciated. 

Ben

[J_Thomas]

Hi NSWC Ben,

Welcome to the NI Discussion Forums!  You pose a great question and you are right in saying that the operating instructions for the NI-9237 do not explain how to enable the shunt calibration in software.

There are two ways to enable/disable the shunt calibration:

1. In Measurement & Automation Explorer (MAX) using a strain task:

If you already have a strain task configured, you can skip to step 6.

1. Open MAX
2. Right-click on NI-DAQmx Tasks and select Create New NI-DAQmx Task.
3. Use the Create New NI-DAQmx Task window to select a Acquire Signals >> Analog Input  >> Strain
4. Select a channel on the NI-9233
5. Give it at name and select Finish
6. On the Configuration tab, go to the Device tab under Strain Setup and select Strain Calibration.
7. The Strain Gauge Calibration window has options to enable or disable the shunt calibration

2. In LabVIEW:

DAQmx 8.7 has added the DAQmx Perform Shunt Calibration VI to enable/disabled shunt calibration.  If you have DAQmx 8.7 installed this VI can be found on the functions palette under Measurement I/O >> NI-DAQmx >> Advanced >> Calibration >> DAQmx Perform Shunt Calibration VI.

There is an example of this in the example finder as well.  Help >> Find Examples.  Navigate to Hardware Input and Output >> DAQmx >> Analog Measurements >> Strain >> Cont Acq Strain Samples (with Calibration) – NI 9237 VI.

In earlier version of NI-DAQmx, the shunt calibration is enabled with DAQmx Property Node. I have attached a screenshot of how to get to the property you need in case you have an earlier version of NI-DAQmx.

[NSWC Ben]

This is great information.  Thanks for your response.  I am now able to perform the shunt calibration and it works great. 

On a seperate but closely related topic, is there a way to null the offset with the 9237 or is the only way simply to subtract the offset out using software?  i.e. if the no load signal is 0.001 V then simply subtract 0.001 V from all measurements

Ideally there is a way to perform this offset nulling without just subtracting it although this works okay too.  Maybe this is what the remote sense lines are used for?

Thanks Again,

Ben

[J_Thomas]

The best method is to use the DAQmx Perform Bridge Offset Nulling Calibration VI  in DAQmx to remove offset errors. There is also a shipping example in LabVIEW that demonstrates how to programmatically perform a offset null offset calibration. It  can be found from the NI Example finder: Browse According to Task » Hardware Input and Output » NI-DAQmx » Analog Measurements » Strain » Acq Strain Samples (with Calibration).vi.  The example I mentioned in my previous post also has this functionality.  When you create a Strain Task or Global Channel in MAX, you can also perform an Offset Null Calibration by clicking on the "Device" tab, and then clicking on "Calibrate".

Message Edited by J_Thomas on 04-16-2008 01:59 PM

[DBerry]

Jared,

Are you aware of any problems with the DAQmx Perform Bridge Offset Nulling Calibration VI  and Custom Scaling using an NI-9237 ?

Thanks

Dan

[Margaret F.]

Hello Dan,

I have looked through our database for any reported problems and have not come accross any. Is there a particular problem that you are having?

[DBerry]

Margaret,

I found a workaround to my problem through a different post:

"Why do I receive Error 200378 "Custom scale specified does not exist"?

Thanks

Dan