Chapter 9 SCXI—Signal Conditioning

Figure 9-9. Measuring Temperature Using Information from the DAQ Channel Wizard

The VI in Figure 9-9 continually acquires data until an error occurs or you stop the VI from running. To perform a continuous hardware-timed acquisition, you must set up a buffer. In this example, the buffer is 10 times the number of points acquired for each channel. For each acquisition, your device averages the temperature data. After completing the acquisition, always clear the acquisition by using the AI Clear VI.

If you are not using the DAQ Channel Wizard, you must use the RTD Conversion VI in addition to specifying additional input parameters, as shown in Figure 9-10. The Convert RTD Reading VI, available on the Functions»Data Acquisition»Signal Conditioning palette, converts the voltage read from the RTD to a temperature representation.

Note Use the RTD conversion function in LabVIEW only for platinum RTDs. If you do not have a platinum RTD, the voltage-temperature relation is different, so you cannot use the LabVIEW conversion function.

Chapter 9 SCXI—Signal Conditioning

Figure 9-10. Measuring Temperature Using the Convert RTD Reading VI

The VI in Figure 9-10 continually acquires data until an error occurs or you stop the VI from executing. For continuous hardware-timed acquisition, you need to set up a buffer. In this example, the buffer is

10 times the number of points acquired for each channel. After your device averages the voltage data from the AI Read VI, it converts the voltage values to temperature. After completing the acquisition, remember to always clear the acquisition by using the AI Clear VI.

Measuring Pressure with Strain Gauges

Strain gauges give varying voltages in response to stress or vibrations in materials. Strain gauges are thin conductors attached to the material to be stressed. Resistance changes in parts of the strain gauge to indicate deformation of the material. Strain gauges require excitation (generally voltage excitation) and linearization of their voltage measurements. Depending on the strain-gauge configuration, another requirement for

using strain gauges with SCXI is a configuration of resistors. As shown in Figure 9-11, the resistance from the strain gauges combined with the SCXI hardware form a diamond-shaped configuration of resistors, known as a Wheatstone bridge. When you apply a voltage to the bridge, the

Chapter 9 SCXI—Signal Conditioning

differential voltage (Vm) varies as the resistor values in the bridge change. The strain gauge usually supplies the resistors that change value with strain.

Supplied by signal conditioning hardware

Physical strain gauge

Rg is value at rest

Figure 9-11. Half-Bridge Strain Gauge

Strain gauges come in full-bridge, half-bridge, and quarter-bridge configurations. For a full-bridge strain gauge, the four resistors of the Wheatstone bridge are physically located on the strain gauge itself. For a half-bridge strain gauge, the strain gauge supplies two resistors for the Wheatstone bridge while the SCXI module supplies the other two resistors, as shown above. For a quarter-bridge strain gauge, the strain gauge supplies only one of the four resistors for a Wheatstone bridge.

The SCXI-1520 is a dedicated strain measuring module, with software configurable bridge-completion, excitation, and resistance shunt switches, as well as filter and gain, on each of the 8 channels.

The SCXI-1121 and the SCXI-1122 modules are also commonly used with strain gauges because they include voltage or current excitation and internal Wheatstone bridge completion circuits. You also can use the signal conditioning device SC-2043SG as an alternative to SCXI modules. The device is designed specifically for strain-gauge measurements. For more information on this device, refer to your National Instruments catalog.

You can set up your SCXI module to amplify strain-gauge signals or filter noise from signals. Refer to your Getting Started with SCXI manual for the necessary hardware configuration and For information about setting up the excitation level, gain, and filter settings.

To build a strain-gauge application in LabVIEW, you can use the Easy I/O analog input VIs. If you are measuring multiple transducers on several

Chapter 9 SCXI—Signal Conditioning

different channels, you need to scan the necessary channels as quickly as possible. Because the Easy I/O VIs reconfigure your SCXI module every time the VI is called, you should use the Intermediate analog input VIs as well as the Strain Gauge Conversion VI, as shown in Figure 9-12.

The Convert Strain Gauge Reading VI, available on the Functions»Data Acquisition»Signal Conditioning palette, converts the voltage read by the strain gauge to units of strain.

Using the DAQ Channel Wizard to configure your channels simplifies the programming required to measure your signal, as shown in Figure 9-12. LabVIEW configures the hardware with the appropriate input limits and gain, measures the strain, and scales the measurement for you. Enter the name of your configured channel in the channels input. You do not need to wire the device or input limits input. The acquired data is in the physical units you specified in the DAQ Channel Wizard.

Figure 9-12. Measuring Pressure Using Information from the DAQ Channel Wizard

The VI in Figure 9-12 continually acquires data until an error occurs or you stop the VI from running. In order to perform continuous acquisition, you need to set up a buffer. In this example, the buffer is 10 times the number of points acquired for each channel. After your device averages the voltage data from the AI Read VI, it converts the voltage values to strain values.

After completing the acquisition, always remember to clear the acquisition by using the AI Clear VI.