In Measurement & Automation Explorer (MAX), there is a Connection Diagram
tab when configuring a task that will dynamically update and display a diagram similar to that seen in figure below, which varies depending on the strain gauge configuration selected (i.e. Quarter Bridge, Half Bridge, etc). This connection diagram shows how to correctly wire a strain gauge to the NI 9944 or NI 9945. Note that CH+ in the MAX diagram is the same as IN+ on the module and in documentation.Note:
The 9944 and 9945 quarter bridge completion modules have 120 Ohm and 350 Ohm resistors, respectively. The resistor value in the accessory must match the resistance of the strain gauge or the readings will be incorrect.
When using an NI 9944/9945 with the NI 9237, you can connect a two lead strain gauge by connecting one lead to pin 0 (EX+) of the 9944/9945 and the second lead to pin 1 (CH+ / IN+). You will also need to provide a jumper wire to connect pins 1 and 2 together (CH+ / IN+ and QTR). In the case of a three wire strain gauge, no jumper wire is needed. Instead, you can connect the third lead of the strain gauge to pin 2 (QTR).
CH+ or IN+: Channel/Signal input
QTR: Quarter Bridge Completion Terminal
As shown below, there is a diagram in the NI 9944/9945 Quarter Bridge Completion Accessory User Guide
that labels the four pin connections. Notice the internal resistor between EX+ and QTR that completes the bridge.
To configure the NI 9944/9945, create a global virtual channel within MAX by selecting Analog Signals » Analog Input » Strain. Select the NI 9237 module and appropriate channel, and name the task. Now you can configure the task using the window shown below.
Note that if you hold your mouse cursor over the Strain Configuration field, MAX will display the circuit diagram (seen below) in MAX's bottom right help window.
In the case above, R4 is the strain gauge, and R3 is the completion resistor provided by the NI 9944/9945. All circuitry, with the exception of the strain gauge (R4), is internal.
To access the connection diagram click on the Connection Diagram tab at the bottom of the window. Make the physical connections illustrated in the diagram.
When setting up a strain task, you might consider calibrating the strain gauge. You can do this by returning to the Configuration window by clicking on the NI-DAQmx Global Channel tab at the bottom of the page. Here you can select the Device tab under Strain Settings.Now click on the Strain Calibration button. The window seen below will appear.
Note: When performing shunt calibration, the NI 9237 contains a built in shunt resistor, which the NI 9944/9945 internally hard wires across R3. You will therefore want to select R3 as the Location.
For more details about each calibration type and instructions on completing the calibration, click on the Help button at the bottom.