VI to calibrate LVDT

[wik]

I am using LabVIEW 7.1 to develop a gauging application.  I am using a SCXI-1600 for the DAQ and a SCXI-1540 with a SCXI-1315 terminal block to connect to my LVDTs.  I would like a way to make a VI to allow the operator to calibrate the LVDTs.  Is there any way that I can do this without granting the operator access to the block diagram?  I guess I would like to make a VI that would walk the user through the calibration process, instead of accessing the DAQ Assistant on the block diagram.

Thanks for the help,

Chad

[Laura F.]

Hi Chad,

It would be possible to make a VI that does a calibration on an LVDT without the user having access to the block diagram.  You could programmatically have a subVI that pops up and asks the user for specific input about the measurement and then apply that data to a scaling formula of some sort to be used for the remaining measurements.  I am assuming that you will want to move the LVDT a certain distance, measure its voltage and then apply that knowledge to future measurements with that LVDT.  If you want to provide more information about how you are trying to calibrate the LVDT, I'd be happy to help you further.

Laura   

[wik]

Laura,

I am new to all of this, but here is my idea so far.  I would like to measure two know values, and use these two values to calcluate the slope and offset of the LVDTs for calibration.  Other than that, I haven't got much farther (I don't really know what else the calibration VI needs to do).  Also, after I measure these values, I'm not sure how to apply them to the future measurements.  Thanks a lot for the help.

Chad

[Laura F.]

Hi Chad,

I think you have a good start on how to do the calibration.  You can make it as difficult as you want I suppose, but basically want you need to do is calculate the slope between two or more points and the offset.  Once you have those values, you can apply them to adjust the distances that you acquire with your SCXI system.  There is a DAQmx example that will help you get started with acquiring from your SCXI-1540.  In LabVIEW if you go to help>>Find Examples you will open the example finder.  Browse into the folder Hardware Input and Output >>DAQmx>>Analog Measurements>>Position and you will find examples to help you get started.  There is one specifically for LVDTs.  You can first acquire data to get the slope and offset and then send that data to the main part of your program that does the data acquisition.  You can use a formula node or even set up a custom scale for that channel using property nodes to adjust the data you acquire according to the offset and slope. 

I hope this helps you get started!

Laura 

[wik]

Good morning Laura,

Thanks for the advice so far.  Yesterday, (7/13/05), I developed a way to calculate and apply the offset to zero the LVDT.  I had already used DAQ Assistant to calibrate the LVDT however.  I think what you described is what I need, but I'm not sure how to apply it. 

Here is what I want to do

For example, if I have the LVDT in a calibrator at zero, I tell LabVIEW "This is zero".  I then move the calibrator to .025" and tell LabVIEW "This is .025"".  I then need some way to calculate and apply this slope (Perhaps DAQmx Channel Properties?).

I can then use the VI that I have to calculate the offset to zero the probes.

Thanks again for your help,

Chad

[Laura F.]

Hi Chad,

The best property node to use is the DAQmx scale property node where you can choose to specify a linear scale with a slope and a y-intercept that you have calculated from your first two measurements.  After you have used this property node, all of your future measurements will be scaled accordingly until you change those values the next time you run that VI.  Please let me know if you have any questions about how to do this.

Thanks,

Laura

[wik]

Good morning Laura,

Thanks a lot for your help.  I was able to figure out the scale property node this morning and properly apply it to my application.  I think that I have this issue solved.  I will post back if any other issues come up.

Thanks again for you help,

Chad