Voltage shift in Load Cell output

[ntaINC]

We have a PCI-6220 with the SCC-2345 having a SCC-SG24 and SCC-LP02 dual stage modules to read a load cell. We can configure the load cell so that it works wonderfully, less than 1% error of the value from 1% full scale to maximum captivity. We ran a few tests using the load cell without any problems. About a month has passed and we need to use the load cell again. However, the offset voltage with no load has shifted an equivalent of 80-lbf. The load cell hasn’t been used since the previous test, when there was no shift. This is a problem we have had with other load cells on the same acquisition system as well as the same load cells on different systems. The load cell is a physical device and only a change in the stain gauges would cause a shift; to the best of me knowledge the load cell has not experienced a force near it’s capacity. This leads me to believe there is something wrong with the acquisition system or the way we are preparing the load cell. Has anyone else experience this or have any advice on how to eliminate this.

Thanks for any help in this matter,

Tessa Parshall

NTA Testing Laboratories, Inc

[K C]

Hi Tessa,

Can you tell us how you connected the DAQ system ?

[ntaINC]

I've attached the set-up from MAX.

[Chris_K_]

Hi Tessa,

Thank you for using our discussion forums.

It sound like you just have to perform some offset nulling.  This procedure is outlined in the SG24 manual linked here.  Have a look at page 49 of 50.

Also have a look at the following link.  It looks like the offset nulling resistors are not installed in the SG24 as a factory default so you may need to install them in order to remove this offset. 

Chris_K

[ntaINC]

We nulled it through software, using a custom scale in MAX. The problem is that the zero load voltage is not the same as when it was initially nulled. A shift like this could happen if the load cell was overloaded, the resistance in the bridge changing from deformation, but I don't think this happened. I can't think of what else could change the zero load voltage.

[La-Z-Man]

I am experiencing the very same thing with 9237 using signal express.....

[Broken_Arrow]

Applying offsets through a custom scale is not the same as using Offset nulling with DAQmx (or whatever Express VI or MAX equivalent). Offset nulling literally changes the potentiometers in the signal conditioning circuit to accommodate for a zero balance (notice I didn't say offset). I've had brand new torque sensors read hundreds of lbf just sitting on the table. DAQmx Offset Nulling does a great job balancing the equation.