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SCXI-1520 Hardware Offset Nulling Fails with 'out of range' Error

Primary Software: Driver Software>>NI-DAQmx
Primary Software Version: 7.5
Primary Software Fixed Version: 7.5
Secondary Software: N/A
Hardware: SCXI>>Modules>>SCXI-1520

Problem: I tried to use the offset nulling feature of my SCXI-1520, but it fails with an error that says the measurement is too far out of range. How else can I null my measurement offset?

Solution: If hardware offset nulling on the SCXI-1520 fails because the channel offset falls out of range (see related link), an alternative solution is to apply a constant scale offset to the channel. This solution is applied at the software level, and is a workaround to the preferred hardware offset nulling.

To create a constant measurement scale that acts as a null offset:

  1. Create a DAQmx strain measurement task. You can add channels and specify timing information using either Measurement & Automation Explorer (MAX) or the DAQmx API.
  2. Read a single sample from each channel in the task prior to the actual measurement. Make sure that the strain gauges are in a resting state, with no force applied to them. This sample will be used as the offset value.
  3. Knowing that the offset should be zero for each channel, negate its value and input that as the y-intercept of a DAQmx Linear Scale.
  4. Apply a slope of 1 to the Linear Scale to create the following input transformation: Measured value = Actual value – Offset value
  5. Continue to use your DAQmx task with the applied scale.

Note:
It is important to remember that the actual acquired signal has not been corrected, so the practical measurement range is altered. Consider a strain measurement with input limits -100 to 100 (microns per meter). If the range of a strain signal is 200 and its offset is at 20, then the maximum measured strain will be 80, which is incorrectly reported by the device as 100. Using this software offset nulling technique will shift the reported value down to 80 (which is correct), but this will remain the maximum reported input value, instead of 100.

Related Links:
KnowledgeBase [2IOB2QB5 ]: What Is the Maximum Range of Offset I Can Null with the SCXI-1520?

Attachments:


Software Offset Nulling.vi

Report Date: 10/14/2005
Last Updated: 11/22/2006
Document ID: 3QD7N4D3

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