Removing Large Initial Offset for Load Cell or Strain Measurements with SCXI-1520 or PXI-4220
Hardware: SCXI>>Modules>>SCXI-1520
Problem: Why do I get a large initial offset error on my load cell or strain measurements with the SCXI-1520 or PXI-4220?
Solution: The voltage excitation circuit on the SCXI-1520 and PXI-4220 can introduce an offset error on the input channels. There are two ways to remove this error:
- The best method is to use the DAQmx Perform Bridge Offset Nulling Calibration function in DAQmx to remove offset errors. When you create a Strain Task or Global Channel in MAX, you can perform an Offset Null Calibration by clicking on the "Device" tab, and then clicking on "Calibrate". There is also a shipping example in LabVIEW that demonstrates how to programmatically perform a offset null offset calibration 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.
- If you are using a load cell or some other type of sensor that prevents an offset null calibration from being practical, then you can instead set the Auto-Zero Mode to Once for your channel. This will remove any offset that is associated with the channel. This can be done either in a Task/Global channel in Measurement and Automation Explorer under the Device Tab. Or it can be programmatically set in your programming environment through the AI.AutoZeroMode DAQmx channel property. Consult the DAQmx documentation for your API for more information on this property.
Warning: Auto-Zero Mode should not be used at the same time as an Offset Null calibration for any channel. Doing this will generally result in a offset error due to over-compensation in software.
Related Links: SCXI-1520 User Manual
Setting the Auto Zero Mode of an SCXI Module in Traditional DAQ
Attachments:
Report Date: 01/31/2006
Last Updated: 02/01/2006
Document ID: 3TUIIKI8
