I am very new to Labjack, Labview, and using load cells. I have an Omega S-type load cell that outputs 0 to 30 mV as it is loaded 0 to 10000 lbs. It is power by 10 VDC from a Omega Far-1 power supply. I have the outputs from the load cell going into FI0 and GRD of my Lackjack U3-LV. I am collecting and analying the output with LabVIEW. I took the example analog read program and had it run continously with a while loop at a sample rate of 500 ms.
The issues I am having are that at a near zero pound load the Labjack is either reading 1.616 mV, 1.901 mV, and .496 mV. It jumps around to these exact values, looking kind of like noise but at times the value holds for several seconds and then jumps quickly between them. When I put my DMM leads on the DAQ inputs it reads .1 mV the whole time (the out put from the load cell is correct and not suffering from external noise). I then put is under a 1640 lb load which should read 5 mV. Instead, LabVIEW shows anywhere from 3 mV to 7 mV. Again, it is going to exact values and again sometimes holding at those values for several seconds at a time. I again put my DMM on the inputs to the DAQ and it reads 5 mV exactly the whole. I have attached several pictures showng the hardware, wiring, and code.
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The U3 is a 12-bit device, that gives LV lines a resolution of about 600 µV. So, you are seeing about 1-bit of noise.
The 30 mV signal from that loadcell is too small for the U3 to measure without amplification. We either need to use a device with built in amplificaiton (U6 or T7) or we need to add an amplifier like the InAMP.
Thank you for the quick response. I will add an ampilifier and let you know how it goes. Thanks again.
Bridge Circuits App Note
https://labjack.com/support/app-notes/bridge-circuits
My LJTIA has arrived. I have connected + Signal to INA+ and - Signal to INA-. As a quick reminder, the output signal from the load cell is 0 to 30 mV. I am confused on what configuration of switches I should set to on (Voffset = 0.4V or Voffset = 1.25V, gain of 1, 11, 51, 201, etc.) and what Vouts I should expect to read from my U3.
I did read the example that very much applies to my setup:
"A common example would be a Wheatstone bridge made of 350 ohm strain gauges. If VREF/GND is used to provide the excitation voltage for the bridge, it will draw about 2.5/350 = 7 mA, and the common mode voltage (Vcm) of the differential signal will be about 1.25 volts. The outputs would be connected to IN+ and IN-. If Voffset is set to 1.25 volts and the gain is set to 201, the allowable input range for the LJTIA is –0.00616 to +0.00622 volts (with 0-2.5 volt output)."
but the exitation voltage for the load cell is provided externally and is 10 Vdc.
I apologize in advance if this is a stupid question. I am not a smart person. Any help is greatly appreciated.
With a 10 volt excitation, both signals are sitting at around 5 volts (versus excitation ground). If you connect excitation ground to LabJack GND (the normal way of referring an external supply to the LabJack) and then connect your signals to the LJTIA your common mode voltage is 5 volts which is too much for the LJTIA.
1. Use the Vref from the LJTIA as your excitation rather than the external 10V supply.
2. Define the negative signal as LabJack GND and thus the signal voltage will be very close to 0 versus LabJack GND (Vcm=0). So connect your power supply to +EXC and -EXC on the bridge. Then connect +/- signal from the bridge to INA+/- on the LJTIA. You will then also add a jumper from INA- to any GND terminal. Pull on both wires in INA- to make sure both are securely clamped. Connecting like this is valid as long as your 10V supply is isolated from the LabJack. AC->DC supplies are always isolated, so assuming that is what you have it should be isolated. You will use Voffset=1.25, and then use the highest gain possible per Table A1.2.
https://labjack.com/support/datasheets/accessories/ljtick-inamp/appendix-a