usb-6008 to measure/display force

[towne107]

Hello all,

         I am trying to perform some force measurements for a personal project. I have limited labview experience. I have an omega lcl-040 load cell and am acquiring data using the NI usb-6008. What I would like to do is measure the voltage coming from the load cell and then display the voltage signal, resulting force (based on the force-voltage relationship), and the mass in labview. I have run some tests using some pre-loaded VIs and VIs I found online, but want to create my own VI. What I would like to do is be able to set a load on the sensor, read the signal for about 10 seconds or so, then have labview take the average voltage from the readings taken in that time. One issue I am having is that the load cell puts out approximately 2mV when unloaded and I was wondering if there was a way to offset this so that the readings will start at zero when unloaded. If anyone has any suggestions on where I should start it would be greatly appreciated.

Thanks,

Aaron

[johnsold]

Unfortunately load cells are not complete precision instruments. They drift. The have offsets. They may have hysteresis, although it is low in good load cells. And the have a small output signal riding on a large common mode signal.  Other than that, they are wonderful.

From the Omega specification the zero balance is 0.3 mV/V. So a 2 mV offset is near the maximum with 5 V excitation and well below maximum for 12 V excitation.

To take advantage of the limited capabilites of the USB-6008 you should add a signal conditioning circuit between the load cell and the USB-6008. It should include a differential amplifier with >60 dB CMRR, gain of 100-500, and an offset adjustment. Depending on your application some filtering may be appropriate.

Lynn

[Diego101]

What is the CMRR of the analogue inputs in the usb-6008 board?