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Measuring a Full-Bridge Absolute Transducer with the SCXI-1520 or PXI-4220

Hardware: SCXI>>Modules>>SCXI-1520, Multifunction DAQ (MIO)>>SC Series>>PXI-4220

Problem:
I have a full-bridge transducer (such as a pressure transducer, load cell, torque transducer, etc). I want to excite the transducer and monitor the response with an SCXI-1520. How do I use Traditional NI-DAQ or NI-DAQmx 7.x to configure the SCXI-1520 (or PXI-4220) and create virtual channels, tasks, or global channels to measure my transducer?

Solution:
The SCXI-1520 and PXI-4220 were designed for all bridge-based transducers. To measure a non-strain gage full-bridge transducer, you will need to follow a slightly different procedure than if you were measuring a strain gage.

NI-DAQmx 7.0 or later
  1. Add the SCXI chassis, and autodetect the SCXI-1520 and SCXI-1314 terminal block in MAX (or install the PXI-4220).

  2. Create a Custom Voltage with Excitation analog input Task or Global Channel.

  3. Under the Settings tab configure the Excitation Source for internal and Excitation Value as appropriate for your transducer. Configure the Bridge Type as Full Bridge. If you already have a custom scale created, use the pull-down menu to select the custom scaling for this channel. Otherwise, configure your custom scale by clicking on the wrench symbol to the right of the pull-down menu.

  4. In your application, you will likely run into large offset errors from the transducer and the inputs to the SCXI module. There are two methods to solve this: Offset Bridge Nulling and Auto-Zero:

    Offset Bridge Nulling: Offset null compensation and shunt calibration can be performed programmatically using task or channel properties and the DAQmx Perform Bridge Offset Nulling Calibration VI. This can only be done through use of the DAQmx VIs; the DAQ Assistant does not provide a way to do offset nulling calibration. A shipping example for LabVIEW can be found by browsing the Example Programs using the Example Finder as follows:
    Browse According to Task » Hardware Input and Output » NI-DAQmx » Analog Measurements » Strain » Acq Strain Samples (with Calibration).vi.
    This program contains subVIs that perform offset null compensation and shunt calibration.

    Auto-Zero: If you are already performing an offset null calibration, then Auto-Zero must not be enabled. If both of them are enabled at the same time, then the software will overcompensate for the offset errors. Only enable Auto-Zero mode if you do not plan on performing an offset null calibration. Auto-zero mode is available in the DAQ Assistant under the Device Tab and in the DAQmx Channel Property Node (Analog Input»General Properties»Advanced»High Accuracy Settings»Auto Zero Mode)

Traditional NI-DAQ (SCXI-1520 only, not PXI-4220)
  1. Add the SCXI chassis and SCXI-1520 in the Measurement & Automation Explorer (MAX) Configuration Tree to Devices and Interfaces as directed in the SCXI-1520 User Manual or the SCXI Quick Start Guide (see related links below).

  2. Configure the SCXI-1520 Module
    1. Right-click on the SCXI-1520 and select Properties from the pop-up menu.
    2. In the Channel window, set the bridge configuration to Full-Bridge.
    3. Set the appropriate excitation voltage and filter setting.
    4. Select a gain that will ensure the amplified voltage is within the input limits of the Data Acquisition (DAQ) board.
    5. Verify that the SCXI-1314 is selected in the Accessories window. If not, select SCXI-1314 from the pull-down menu.

  3. The hardware is now correctly configured.

  4. To use a virtual channel, complete the following steps:
    1. Right-click on Data Neighborhood in MAX, and select Create New....
    2. Select the appropriate type of channel, Traditional NI-DAQ or DAQmx. (The steps below will be for Traditional NI-DAQ but DAQmx will have the same basic steps)
    3. Select Analog Input
    4. Enter a name and then a description (the description is optional).
    5. From the next menu, you will want to select Voltage measurement (not strain gage).
    6. Enter the units (for example, lbs.) and then the range of your sensor. If you have a 50 lb. load cell, then we will want to enter -50 and 50 as our range.
    7. The next screen will ask us to enter a scale. To scale data into a unit other than volts (such as psi), you must create a custom scale. Usually the conversion is linear, and slope and intercept values should be provided by the transducer vendor. You can also choose Map Ranges. For example, if you have a load cell with a +/- 20 mV output, select Map Ranges and then enter 0.02 and -0.02 so that the virtual channel will automatically apply the necessary gain to the readings.
      Note: If you create a strain gage virtual channel, all data will be scaled into microstrains.
    8. In the next window, select the SCXI-1520 module from the list and choose the channel of your incoming signal.
    9. Click Finish and you are now done creating the virtual channel.


  5. Before taking measurements, you will want to perform a nulling operation to compensate for any voltage offset. You must choose a non-zero excitation voltage in MAX because the nulling circuitry on the SCXI-1520 is internally connected to the excitation voltage.
    1. If you are using the VI, Getting Started with Load Cells Using the SCXI-1520, then skip step b.
    2. Connect the signal input terminals (S+ and S- on the SCXI-1314) to chassis ground. The SCXI-1314 does not have a screw terminal for AIGND, but you can access chassis ground at the case screw located at one end of the strain-relief bar.
    3. You are now ready to perform the nulling operation. Nulling must be performed by making the appropriate Strain Null DAQ function call (see the SCXI-1520 User Manual for more information). In LabVIEW, use the SCXI Strain Null VI located on the Data Acquisition » Calibration and Configuration palette. Enter your virtual channel name for the SCXI Channel String and run the VI.
    4. Once the nulling operation is complete, simply connect S+ and S- to your transducer. Any voltage offset has now been hardware-compensated and you are ready to take measurements.



Related Links:
NI-DAQmx Related Links:
Product Manuals: SCXI-1520 User Manual (NI-DAQmx version)
Product Manuals: SCXI Quick Start Guide (NI-DAQmx version)
Traditional NI-DAQ Related Links:
Developer Zone Example: Getting Started with Load Cells Using the SCXI-1520
Product Manuals: LabVIEW Measurements Manual
Product Manuals: SCXI Quick Start Guide (NI-DAQ 6.9.x version)
Product Manuals: SCXI-1520 User Manual


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Report Date: 02/08/2002
Last Updated: 10/03/2007
Document ID: 2I7EBBJ3

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