9237 Null Offset with TEDS

[Crocker84]

I have a 9237 with a TEDS enabled load cell.  I can put the calibration values from the eprom into a task manually and use null offset just fine.  However, if I set the task up as a TEDS load cell and let it pull the cal data automatically, my null offset doesn't move my values to 0.  For instance, say I read .3 lbs without nulling, then I do the nulling and I read -.2 lbs.  The offset is always repeatable, it never jumps around from run to run.  I have 3 different load cells and have the same results.  Has anyone had this trouble before?

Running LV 2010SP1, Daqmx 9.2.3, Interface SML load cell

[Frank-L]

Crocker84

Does this occur only when setting the task up as a TEDS task? If you were to set this up as a typical load cell and use the DAQmx example located in the example finder under "Hardware Input and Output>>Analog Measurements>>Strain>>Cont Acq Strain Samples (with Calibration).vi" does the offset go away when offset nulling is enable? I am trying to figure out whether the offset nulling is the issue or if the TEDS part of the task is the issue. 

[Crocker84]

Actually, the same thing happens when you set it up manually, and when you set it up as TEDS.  I think that I may have figured out part the the mystery.  My load cells are calibrated at -1000 lbs and 1000 lbs.  The mv/v value for positive and negative 1000 are not exactly the same.  This means that the calibration line doesn't go through the origin (imagine a line that is drawn from the 3rd quadrant across -x and +y axis to the first quadrant).  When the zero offset function is used it simply subtracts the initial voltage reading (x axis) from subsequent readings.  Essentially shifting the calibration curve left or right on the x axis.

So, what I believe happens is this:

My calibration values are (-2.07058 mv/v, -1000 lbs) and (2.08607 mv/v, 1000 lbs).  Which boils down to y=481.16x -3.7266.

As you can see the x intercept is -3.7266.  So when I null offset a load cell with no load it will actually report -3.7266 lbs.  I want to tare the load cell, no matter what the applied load, so I have written the following VIs which I believe address the issue.  Is there an easier way to offset all the readings by the yintercept value, than whay I have currently done by subtraction?

[Frank-L]

The current method that you have created should do what you are asking. The zero offset will simply remove the values on the bridge when it is at zero load, and will not do the tarring at any load that you need. The method you have devised should function properly, and as far as I know is the only way that you can TAR a TEDS bridge device.

[Brad_K]

Hi Crocker84,

DAQmx offset nulling zeroes out the voltage (or voltage ratio), not the scaled value in lbs/N/whatever. Since your load cell's TEDS scaling info has an offset, zeroing the electrical value causes the offset to be reflected in the result of the offset nulling operation.

Improved support for subtracting tare weight (as opposed to electrical offset) sounds like a great candidate for the Data Acquisition Idea Exchange.

Brad

[Crocker84]

Yes, I agree totally.  This would make TEDS load cells extremely easy to use with Labview.  The best part is, almost all the framework is there, it is just a matter of pulling the right data out and putting it to use automatically.