Connecting Strain Gauges and Shunt Resistors to the NI 9237Hardware: CompactDAQ>>Analog Input Modules>>NI 9237
Problem: What external connections do I need to make in order to take readings from my strain gauge and perform shunt calibration? 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. You can also purchase and use the NI 9945, a Quarter Bridge completion module. Shunt calibration is a process used to correct for system gain error of the 9237 (gain error is introduce by the analog-to-digital convertor). A strain is simulated by placing shunt resistor of known value (Rs) across one leg of a bridge sensor, as seen below in Figure 1. Typically, the shunt resistor is connected across R3. 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 compensating for the gain errors.
Figure 1: A Wheatstone Bridge with a Shunt Resistor applied across R3 Below we illustrate not only the external connections necessary to take measurements from various types of strain gauges, but also the external connections needed for shunt calibration. The NI 9237 has 10 pins for each channel. These pins correspond to:
Full Bridge
Figure 2: Connection Diagram for a Full Bridge Strain Gauge and the NI 9237 NOTE: Here 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. Half Bridge
Figure 3: Connection Diagram for a Half Bridge Strain Gauge and the NI 9237 NOTE: Here we can see the NI 9237’s internal bridge completion resistors. EX+ , EX-, and AI+ will still need to be connected by the user. NOTE: Quarter Bridge II configuration is connected to the NI 9237 as if it were a Half Bridge. R4 is the active quarter bridge strain gauge. 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
Figure 4.Connection Diagram for a Quarter Bridge Stain Gauge and the NI 9237 and NI 9945 NOTE: Here we can see the connections and R3 resistor internal to the NI 9945. No external connections are necessary for shunt calibration because the NI 9945 makes those connections internally. Related Links: Developer Zone Example: NI 9237 Example for Shunt Cal and Offset Cal Developer Zone Tutorial: Measuring Strain with Strain Gages Developer Zone Tutorial: Strain Gauge Configuration Types KnowledgeBase 47SGM5CY: NI 9945 or NI 9944 with an NI 9237 Connection Guide Product Page: USB-9237 Attachments:
Report Date: 01/23/2009 Last Updated: 09/18/2009 Document ID: 4TMG659G |
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