Trying to Read Load Cell w/ USB-6251

[mckesea]

Hello All,

I have a pancake load cell Cooper LGP310, a USB-6251 DAQ, and a labview program trying to read the measurement.

The output of the load cell is 3mV/V, 10V excitation, lb range is 5 lbs to 150,000 lbs. I will be recording an impact force.

Ive tried hooking up the signal wires differentially and RSE. I cannot see a change in the sensor output when the load cell is exposed to tension. Though when it is hooked up to a trancell ti-500e it outputs the force just fine.

Is this a conditioning issue? Can the DAQ not see changes this small?

I'm a ME with VERY LITTLE exposure to this type of project. Dont think less of me....

Thoughts? Additional info?

[Brad_K]

Hi mckesea,

How much force are you applying to the load cell when you run your program? With 150000 lb capacity and 10 V excitation, it takes at least 30 lb to produce a voltage change that the USB-6251 can even detect, and that's ignoring accuracy. (The USB-6251's sensitivity in the +/-100 mV voltage range is 6 uV; 6 uV * (150000 lb / 30 mV) = 30 lb.)

Also, the USB-6251 doesn't have excitation outputs, so what type of external 10 V excitation source do you have connected?

Brad

[mckesea]

Hey Brad,

I pull up to 10,000 lbs with no change.

I have a 9V battery hooked up for excitation.

[Brad_K]

Hi mckesea,

How do you have the signals wired to your USB-6251? (i.e. which terminal is connected to which?)

Brad

[mckesea]

Differentially

I have the Signal + (white wire) to the AI0

               Signal - (green wire) to the AI8

RSE

I have the Signal + (white wire) to the AI0

               Signal - (green wire) to the AIGROUND

[Sarci1]

Hey mckesea,

If you are wiring differentially, I would recommend using pull-down resistors to AI GND.  Take a look at the DAQ M Series User Manual specifically on page 4-12 (58) fo proper wiring techniques.

Lynn

[mckesea]

I tried the pull down resistors yesterday. Does it matter what size resistor or just that they are the same?

I've learned we have a voltage amplifier, Cooper DCM 460, around here somewhere I have not been able to find it. I understand the need for this piece though I thought I would see some change in output voltage from the load cell even though it is so small a voltage.

Im afraid when I find the amplifier or get a new one, hook it up, and try to read a signal I will get the same results. Nothing.

[Sarci1]

Hi mckesea,

Let's start from the beginning. Are you looking at these measurements in Measurement & Automation Explorer?  How have you configured your task?  Please include any screenshots necessary.

Lynn

[mckesea]

I have not attempted to look at it in Measurement and Automation Explorer. I went straight to a vi. I included this vi in my original post. Are you able to see how I configured my task in this vi or should I take some screen shots?

[Sarci1]

Hey mckesea,

You should always attempt hardware trouble with Measurement & Automation Explorer (Start Menu>>All Programs>>National Instruments>>Measurement & Automation).  This will allow us to isolate the hardware from the software.  Within this software, I would create a task and continue testing from there.

Lynn