Improper load cell excitation value from 9219

[NicW]

With the components below, I am not able to get reliable readings from Interface Force load cells. All wiring connections have been verified, as have the bridge resistances on all load cells. The calibration checks on the load cells are okay. Readings are very erratic and much lower than expected.

Using a multi-meter and also the 9215 module various voltages were measured, representative values are:
  * Futek Load cell
      - Excitation: 2.69 V
      - Signal: 1.720 mV (1.6090 mV expected)
  * Interface Load cell
      - Excitation: .080 V
      - Signal: 93 uV to 1.041mV

The problem, obviously, is that the 9219 is not supplying the load cell with the proper excitation voltage, and hence the readings are down in the noise level. Each load cell is configured the same in the same NI-DAQmx Task:
  * Excitation Source = Internal
  * Bridge Type = Full Bridge
  * Ex Value = 2.5 (Note: this is not user-selectable)
  * Terminal Configuration = <Let NI-DAQ Choose>
  * Continuous Samples, 2 Samples to Read, 2 Hz]

The question is: Why is the 9219 module not supplying the correct excitation voltage to the load cell?

System Components

Chassis:
  * NI cDAQ 9172
Modules:
  * NI 9215
  * NI 9263
  * NI 9219
Instruments
  * Interface Force load cell model 1210, 350-ohm full bridge
  * Interface Force load cell model 1220, 350-ohm full bridge
  * Futek load cell model LCM 500, 350-ohm full bridge

[mallorim]

Hi NicW,

Thanks for posting. So as I understand the situation, you have three full bridge load cells connected to three channels of the 9219 Unniversal Input module. You have connected the EX+ and EX -  leads and the IN+ and IN- leads, to all three sensors. While you are able to get the correct reading from the Futek load cell, for the two Interface load cells, it looks like the excitation value is too low to return a proper signal. Is this correct?

While I doubt that this is the case, since it is two Interface sensors that are having problems, something to check might be to switch channels, connect the Interface load cell to the channel that the Futek was connected to previously. If the Interface sensors have problems no matter the channel they are connected to, we can rule out the 9219 being broken and instead look for some reason that these particular sensors are having problems.

You are right, the internal excitation voltage of the 9219 is nominally 2.5 V, but it can vary based on the resistance of the sensor. For a 350 Ohm load cell, the configured task should have an excitation of about 2.7 V, which is correctly appearing in the Futek task.

When you say that "the calibration checks on the load cells are okay" does this mean that you are able to successfully perform offset nulling and shunt calibration, or did you mean something else?

Just doing a quick search of our database, I haven't found any previous issue with Interface sensors. This leads me to believe we should be able to work with these fine. I would like to hear the results of changing channels and I will do some research here to see if I can find any reason that the 9219 wouldn't be outputting the proper excitation values.

Regards,

Mallori M.