Cross-Slide wrote:

On Sep 16, 4:30*pm, Wes wrote:
I had to troubleshoot one of these things today. *For some that frequent this list it is
something they likely have run into or wondered about, expecially if they work in
manufacturing.

LVDT Linear Variable Differential Transformer. *An interesting measurement device. *I had
assumed it was something like an inductosyn but I was mistaken.

http://en.wikipedia.org/wiki/Linear_..._transformeris the wiki page
which is okay but after you have read that, go tohttp://www.rdpe.com/ex/hiw-lvdt.htm

and watch the animation that makes it clear as a bell.

I learned something today and wanted to share it.

Wes

There is an even neater and much more reliable way to read the
position of an LVDT.
The way shown is dependent on the amplitude for position. With DC
drift you don't know where you are.
And DC offset is a real problem.

You can also use a digital Sine / Cosine generator. You drive the two
coils with the Sine and Cosine signals.
The phase angle if the third (pickup) coil is now dependent on the
position. If the Sine/Cosine generator is driven by a digital counter,
you can look for the Zero Crossing point of the sense signal and latch
up the current count of the free running counter that is driving the
Sine/Cosine digital waveform synthesizer.
This is how the Old Bendix CNC units measured the position. They were
driving Resolvers. But the same principles apply to resolvers, LVDT od
inductosyns. as far as decoding the position. Send out Sine/Cosine and
read the phase angle of the sense signal. It is not sensitive to DC
drift, it always a full strength signal and not drifting down to zero
volts where you do not know if you are reading noise or the intended
signal. And being full strength you can discriminate against noise
much better.

That sorta sounds like an inductosyn?

Wes
--
"Additionally as a security officer, I carry a gun to protect
government officials but my life isn't worth protecting at home
in their eyes." Dick Anthony Heller