Connecting Strain Gages and Shunt Resistors to the NI-9237

Updated Aug 9, 2018

Reported In

Hardware

  • NI-9237
  • NI-9944
  • NI-9945

Issue Details

  • What external connections do I need to make in order to take readings from my strain gage or load cell?
  • How do I perform shunt calibration with NI-9237?

Solution

The NI-9237 is a bridge and strain measurement module. As such, a shunt resistor and half bridge completion resistors are built into the 9237 module.
 

It is important to note that the numbering of the Wheatstone bridge arms is not arbitrary.  The resistor values are used in complex equations to calculate the strain or expected voltage.  The resistor name definition can be summarized as follows:

  • R1 is defined between EX + & AI - 
  • R2 is defined between EX - & AI -
  • R3 is defined between EX - & AI +
  • R4 is defined between EX + & AI +

Below we illustrate not only the external connections necessary to take measurements from various types of strain gages, but also the external connections needed for shunt calibration.

The NI-9237 has 10 pins for each channel. These pins correspond to:
 

Pin 1SCShunt Resistor end 1
Pin 2AI+Channel Input +
Pin 3AI- Channel Input -
Pin 4RS+Remote Sense +
Pin 5RS-Remote Sense -
Pin 6EX+Excitation +
Pin 7EX-Excitation -
Pin 8T+TEDS+
Pin 9T-TEDS-
Pin 10SCShunt Resistor end 2

Full Bridge:
We can see the internal shunt resistor. It does not matter which SC pin is connected to which side of R3. When shunt calibration is being performed, the internal switch closes to connect the resistor. During normal operation of the module, the switch remains open.

Note: If your strain gage only has 4 wires, then you do not need to connect the two wires for shunt calibration.

Half Bridge:

We can see the NI-9237’s internal bridge completion resistors.  EX+ , EX-, and AI+ will still need to be connected by the user.

Quarter Bridge II configuration is connected to the NI-9237 as if it were a Half Bridge. R4 is the active quarter bridge strain gage. R3 is a dummy resistor placed perpendicular to the direction of strain, and will therefore exhibit changes in resistance only due to temperature drift.  When configuring the strain measurement task in software, you will need to select Quarter Bridge II in the task set up.

Quarter Bridge:

Additional Information

Shunt calibration is a process used to correct for the system gain error of the 9237, which is introduced by the analog-to-digital converter. A strain is simulated by placing a shunt resistor of known value, Rs across one leg of a bridge sensor. The change in total resistance of that leg will create a change in the voltage difference between the legs of the bridge, AI+ and AI-. The measured signal levels can then be compared with the expected signal levels, allowing software adjustments to be made to compensate for the gain errors. Typically, the shunt resistor is connected across R3, however it is possible to use any of the Wheatstone bridge arms as long as it is specified as an input for the shunt calibration VI. See Shunt Calibration (Gain Adjustment) for more information.

The 9237 has an internal shunt resistor of 100 kohm

You can also purchase and use the NI-9944 and NI-9945, two Quarter Bridge completion modules at 120 and 350 Ohm nominal resistance respectively. For the NI-9944 and NI-9945, no external connections are necessary for shunt calibration because the NI-9944 and the NI-9945 make those connections internally. 

WAS THIS ARTICLE HELPFUL?

Not Helpful