Zeroing a load cell

[manchmal2]

Hello,

I'm new to LabView so I'm sorry if this is a pretty simple question. I have a Futek load cell (5lb), which has a maximum output with 5Vdc excitation of around 2mV/V, and the signal always has an offset when I start taking data. The load cell hasn't been calibrated by anyone other than the manufacturer, but we bought the load cell last November so it should be calibrated fairly accurately. I am using the DAQ Assistant (Measuring Custom Voltage with Excitation) and would like to know the best way to zero the signal. Ideally, I would like the user to be able to push a button and have the signal zero (minus some constant from the signal thereafter). Also, I've read on some posts that one way to do this is through specifiying a linear custom scale, but I hesitate to do this because the voltage is off by a different amount each time. 

Thanks!

[MattBradley]

Here's what you need to do.

1) Instruct the user to remove any and all loads from the load cell AND put it in the orientation it will be used (if possible) -- orientation does matter more in some load cells than others.

2) Take a measurement.  Ideally, a lot and take the average.  This is your zero voltage.

3) Subtract the zero voltage from all of your subsequent voltage measurements before applying the cal.

There are other ways to do it as well, but mathematically they work out the same (assuming you have a linear load cell).

[manchmal2]

Hi Matt,

Right, I understand that those are the steps I have to take, but I'm having problems writing the .vi to do this. I have attached a simplified .vi showing what I am trying to do. However, right now I am using two DAQ Assitants, one to continuously read the signal and one to read the signal for a couple seconds to get an offset. This is obviously not allowed. How should I change this?

Thanks.

[Coq_Rouge]

Hi

The idea from Matt Bradley is good. To add some extra you could do a linear fit. The slope should be very close to zero This check that the signal is stable and not drifting. You could also check the standard deviation. During the zeroing the signal shall be very stable. 
I think you need a state machine for this task. Go to http://books.google.com and search for Labview. Then you search for state machine in the Labview books.

Good luck 😉 

[johnsold]

You comment that the offset is different each time.  How large are these differences? Is there any pattern to the offsets over time?  How are you measuring these values?

I wonder if you have some mechanical problems with the way the load cells are mounted or connected to the device you want to measure.  I once spent months trying to tame the mounting system for a load cell.  Not fun!

Lynn