Using Bridge-based C Series Modules in Scan Engine Mode for Strain Measurements

Updated Sep 11, 2019

Reported In

Hardware

  • NI-9218
  • NI-9219
  • NI-9235
  • NI-9236
  • NI-9237
  • C Series Strain/Bridge Input Module
  • PXI Strain/Bridge Input Module

Issue Details

I am using a bridge-based C series module for strain measurements (NI 9218, NI 9219, NI 9235, NI 9236, or NI 9237) with the cRIO scan engine mode.  Where I can specify the gauge factor and gauge resistance to yield measurements in microstrain units?

Solution

Unlike normal DAQmx programming or cRIO FPGA programming, you can not directly specify gauge factor and gauge resistance values to be used in the cRIO's scan mode. Subsequently, there is no inherent data acquisition mode in the scan engine mode to perform strain measurements. It is recommended to use DAQmx or FPGA mode instead of the Scan Interface Mode to make bridge based measurements. 

If Scan Interface Mode is required for your application, you will have to use LabVIEW's Convert Strain Gauge Reading function in the scalar mode to perform this calculation manually on data acquired as a voltage.
 
The value that the bridge-based NI 9218, 9219, 9235, 9236, 9237 returns is in ratiometric form (Volts/Volts), as can be seen from the LabVIEW Help file for each module. Therefore use this function with caution as it assumes a plain voltage in its calculations. In fact, it performs this initial calculation first to turn a plain voltage (Vsg) into a ratiometric form (Vr):
 
Vr= (Vsg – Vinit) / Vex
 
Where Vinit is an initial offset voltage and Vex is the excitation voltage.
 
Because of this, you can either wire a constant of 1V to Vex to make this calculation independent of it, or you can refer to the Developer Zone Community example linked below to see an alternative conversion VI.

The Convert Strain Gauge Reading defaults to the waveform polymorphic type. To use it with voltage readings, you will need to change it to the scalar polymorphic type. To do this, you will need to Right-click on Convert Strain Gauge Reading's icon and select Open Polymorphic VI. In the dialog box first select the Convert strain gauge reading (scalar).vi, then click the Up Arrow on the right side of the dialog to push the selection to the top of the list. Select Close from the File menu, and click Save when the dialog prompts. With the polymorphic VI now in the scalar mode you can now wire up the configuration inputs, as well as the voltage input, and the VI will perfrom the strain calculations and yield results in microstrain.