Bad Readings or Noise Measuring Load Cell or Force Sensor

Updated Oct 24, 2019

Reported In

Hardware

  • NI-9237
  • NI-9218
  • NI-9949

Software

  • SignalExpress
  • LabVIEW

Driver

  • NI-DAQmx

Issue Details

I have a load cell connected to my bridge input module like the NI 9237 or NI 9218 in either half or full bridge configuration, and am seeing strange/wrong readings like unexpected measurements or high noise levels in my software. What could be causing this?

Solution

  • Use a LabVIEW shipping example (Bridge-Continuous Input) or Test Panels in NI MAX to verify that the raw values coming into your software are correct.
    • If test panels still show the incorrect values, test the measurement from the sensor output with an oscilloscope or DMM to verify the sensor is outputting the expected data.
  • Ensure the channel is appropriately connected. Refer to Connecting Strain Gauges and Shunt Resistors to the NI-9237 or to your module documentation for additional information.
  • Move your sensor to another channel in the module to verify the channel is working properly.
  • Move the module to a different slot in the chassis to verify your chassis is working properly.
  • With the 9237 it is important to make sure you are frequently calibrating the device in NI MAX.
    • If you are seeing a constant offset, this step will help to adjust the readings.
  • If the sensor and module are located in an area with a noisy environment, test the measurement in a different location to check if noise in the environment is causing the problem.
It is important with Force (bridge) tasks to ensure your NI-DAQmx task has been configured correctly to ensure proper readings:
  • Please follow the Configuring a Load Cell in MAX with the NI 9237 tutorial to ensure every aspect of the task has been configured correctly.
    • When configuring the sensor in the NI-DAQmx Task, make sure you enter the correct sensor specifications (for example, the bridge resistance). Entering wrong values can lead to incorrectly scaled data.
  • If you are seeing erratic or noisy measurements in a test panel in NI MAX, you may want to refer to our article on Incorrect Test Panel Readings on the NI 9237.
  • If. you using an external power supply on NI 9237, ensure you have the Remote sensing inputs (RS+ and RS-) connected to the load cell to compensate any power supply instability or noise and/or any gain error caused by long cables. Even if your sensor does not have the Remote sensing wires, connect the NI 9237 Remote sensing inputs to its excitation ports (RS+ connected to Ex+ and RS- connected to EX-). In that case, the compensation will be just for power supply instability or noise.
  • Finally, refer to our Field Wiring and Noise Considerations for Analog Signals white paper for general best practices on improving analog input readings with NI hardware.