How do I assign a unit (inches) to a voltage input, and calibrate accordingly?

[NotaTech]

I've rigged five poor man's LVDTs, or more precisely linear potentiometers, using the 5 volt output of the DAC board.  The wipers are on ai1 through ai5.  I can aquire a signal of 2.5 to 5 volts easily enough, but I want to make the unit inches and calibrate accordingly.

This has to be simple.  Where's the button?

[apriltalksrobots]

Hey there, 

You can create a custom scale in SignalExpress. This should allow you to set a certain voltage step to correspond to a set number of inches. I'm assuming you'll want to set a linear scale, but you'll have several options. This KB should walk you through the process of setting that up in SignalExpress. 

How Do I Create a Custom Scale in LabVIEW SignalExpress? 

http://digital.ni.com/public.nsf/allkb/31A3EF18B6141E2C862579DE0077ABA7

Let me know if you have further questions! 

aprillest

[OKors]

There's a lot to be assumed here in your setup: are you wired into your modules properly, are you supplying excitation, how are you setting these up in DAQmx (ai voltage, custom voltage w/ exc.. etc)?

Lets just assume you have good signals when you actuate your (im assuming stick pots maybe???). If you know their effective measuring range you can just create a linear slope based on that. For instance, I use 1.5" stick pots a lot. At 10Vex my slope would be 1.5 However this yields an accuracy of about 0.02 over the full range of the pot. Not NASA accurate but good enough for some of the stuff we do. If I want more accuacy I'll create a custom scale- table. From here I'll use calibrated digital calipers and record the voltages at different positions within the stroke of the stick pot. For best results you would want your working range to be just inside end-to-end stops to account for any dead spots at the extreme ends. So for instance, compress the stick pot with the calipers just a little, record your voltage. Call that zero. Compress again not quite bottoming out (avoid the dead spot). Record your voltage and the value on the calipers. That would be your min/max ranges for the custom scale.

I use stick pots as an example here but you can apply the same method to string pots.
Your accuracy will depend on your reference measuring device, beit a tape measure, calipers or gage blocks.

[NotaTech]

Unfortunately I've been pull off that project for now.  However, when I get back to it next week, I expect to be up and running in a matter of minutes thanks to your help.

[NotaTech]

Wow!  I almost feel dumb.  Thanks for the help.

[OKors]

Nonsense...
Signal Express is great for many things but it's never been the most intuitive at setting up sensors that dont have metrology or cal cert info.

good luck with your project.