load cell differential circuit USB 6009

[elixirnova]

Hello,

I have read through the great information available on hooking up my load cell to my USB 6009 DAQ. I do not have the $$ to purchase a better DAQ so I am going to amplify my signal to the range of 0-10Vdc FS and want to read it through analog input on my DAQ. I am currently having a problem where my circuit configuration is giving me 5.6V reading in my analog input 0 (AI0). In this setup I have a AC to DC regulated power supply at 12V output that is grounded via my AC outlet. My DAQ is hooked up to my laptop and I  hooked up the +out wire from my load cell to the AI0 and –out to GND. After  reading through the field wiring page

http://zone.ni.com/devzone/cda/tut/p/id/3344#toc1

It seems like hooking the (-)out to GND is my problem since that 5.6V could likely be due to Difference in ground voltages.

Looking down the link I posted there is a table of configurations and I believe the setup I am using is the one with a big X through it table.

So I am wondering if I can do the differential ground referenced measurement system pictured in the top right hand corner of the table where I would keep my setup but hook the (-) out to a second analog input terminal instead of GND. Based on that link that seems like the best setup for me. I am not next to my DAQ and software but is this possible for me to perform a measurement with the +/- outputs going to AI0 and AI1 respectively on my USB6009. If so how do I setup the reading in Measurement & Automation or Labview?

Hope I’m on the right track!

Thanks for any help.

-William Broch

[elixirnova]

... while I'm at it, would it be better for me to use a battery to power the load cell rather than a AC to DC regulated power supply?

I think my big problem is I'm just not understanding all the groundings in the diagrams in that table.

[BROhan]

Hi William,

I think you are getting on the right track if you want to measure differentially. However, for a differential measurement,  if you want AI0, you will wire to AI0+ and AI0-, which are pins 2 and 3, respectively on your USB 6009. Take a look at the user guide for more details.

[j_J_j]

For your information that to measure the load cell output, it normally generate very small signal in mV range for full scale load applied.

Which depends on the excitation voltage that you have generated for the bridge circuit.

The supplier usually inform the specification of the output in mV/V unit. I think that you can also check the recommended excitation voltage from the sheet.

After that you will need to scale it with multiple good techniques to avoid the noise and inaccurate measurement.

This article will be useful for you then http://zone.ni.com/devzone/cda/tut/p/id/6186

Best,

EJ

[elixirnova]

Thanks for the help.

I ordered an INA122 instrument amplifier and hooking it up as mentioned in differential worked well. I managed to have a zero load voltage of around 0.00045V granted with +/-2lbf of noise at around 20Hz. I'll read through the link you gave and try amplifying the signal without generating much more noise hopefully. I'll give an update once I get a chance to try the instrumentation amplifier, although I wasn't too sure which one to choose. So hopefully the INA122 works well!

[j_J_j]

INA122 will absolutely simplify your acquisition system.

The key of this kind of signal conditioning is to amplify the low-level voltages near the signal source to reduce the effects of noise.

Please keep update your track, thanks!

EJ

[elixirnova]

I got an amp circuit set up. I used a 620ohm resistor to amplify the signal about 327x. I am now running on the recommended excitation voltage with a 10V regulator putting out about 10.25Vdc.

Right now I have my USB6009 reading the INA122 output to the first differential input and reading the excitation voltage to the 2nd differential input. Right now I'm getting around 1V output at 50lb load (500lb FS capacity) but am still experiencing significant noise since my calculated lbf output is jumping between 0.2 and -0.2 at zero load. I have not performed any software filtering and am using the excitation voltage reading for the calculation of the lbf output. I have not trimmed my resistors or anything yet so maybe that will help.

Another thing is I am powering my voltage regulators with a regulated 12Vdc wall power supply (from 120VAC line in house). Since I will be using this at a swim pool I plan on using series 9V batteries to supply 18V to the circuit instead of the wall power supply. Maybe this will also reduce the noise...

I plan on going through the information in this post http://forums.ni.com/t5/Multifunction-DAQ/USB-6009/m-p/784834 and hopefully get the noise level down.

Two things that I am curious about still... I see on some load cell circuit schematics that the people are takign the +10V and -10V signals that power the  INA122 and wiring them to a small capacitor and then to ground. Will this help with system noise as well or what does that accomplish, here is an example of what I'm talking about

http://www.nakka-rocketry.net/strainlc.html

My other inquiry is that in tension my labview program is reading beyond 50lbf but in compression it stops at around 45lbf or just under -1Vdc output. What is the maximum differential voltage I can input to the USB 6009? Am I limited to +/-1Vdc? I thought it was +/-10Vdc.

[BROhan]

Hi William,

The capacitor from V+ to ground is meant to filter out noise. You can check out the specs for the USB-6009 on it's product page. It does have a max voltage range of +/-10V.

[elixirnova]

Well that makes alot of sense and it worked well. I have it all working well now. Might go ahead and switch to battery power for convinience at the pool in the end.