Hardware:
- 4-wire full bridge load cell - Known good unit, tested with other instruments and a DMM and power supply
- NI 9237 Half/Full Bridge Analog Input
- C-DAQ 9171
- Cables:
- NI 192190A-01
- Custom cable to screw terminals for plugging in load cell.
- Note that sense and teds leads are not being used.
Software: DAQexpress 5.1
- Signal type: custom voltage with excitation
- Unit: Volt
- Configuration: Full Bridge
- Excitation Source: Internal
Reading the signal with no load applied to the load cell, I get very confusing signal indicated. I’ve designed a lot of load cells and used a lot of load cell instruments, so I’m not green to this, but what I’m seeing in DAQexpress is counterintuitive to me.
As I increase the excitation voltage in DAQexpress, the indicated output decreases. What is indicated is labeled as volts, not a ratiometric scale (like V/V or mV/V). Recording the data, exporting, and opening in excel, the files list Volts as the unit, not V/V.
So I recorded the no-load signal (null balance) at each level of possible excitation, and here are the results:
|
Excitation Voltage Applied( V )
|
indicated output ( V )
|
Indicated Output / Excitation
|
Indicated Output x Excitation
|
|
2.5
|
-0.0057
|
-0.0023
|
-0.0143
|
|
3.3
|
-0.0043
|
-0.0013
|
-0.0143
|
|
5
|
-0.0029
|
-0.0006
|
-0.0143
|
|
10
|
-0.0014
|
-0.0001
|
-0.0143
|
Now, in my mind, the indicated output in volts ought to be the raw volts read by the card, it ought to be the number you would see if you measured with a multimeter.
It would then follow that if the output in DAQexpress is in volts, then the indicated null balance should be increasing with increased excitation. If the indicated output is the ratiometric output, then the units should be V/V or mV/V.
Either way what is presented doesn’t make sense.
Does anybody out there have any idea what the heck the indicated output is?
As a side note: I tried it in Force bridge mode as well, with the same sort of results. The Newton signal at 10V is half that indicated at 5V and a quarter that indicated at 2.5V. This makes zero sense.
