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_...al_transformer is the wiki page
which is okay but after you have read that, go to
http://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

On 09/16/2010 02:30 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_...al_transformer is the wiki page
which is okay but after you have read that, go to
http://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.

They're nice non-contact sensors. They'll get you down to the point
where you have to start worrying about differential temperatures messing
up your measurements, and arms cocked over by milliradians, and other
really smoky stuff.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" was written for you.
See details at http://www.wescottdesign.com/actfes/actfes.html

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.

Tim Wescott wrote:

They're nice non-contact sensors. They'll get you down to the point
where you have to start worrying about differential temperatures messing
up your measurements, and arms cocked over by milliradians, and other
really smoky stuff.

This thing is part of a system that measures a part one of our turning centers just made
and also provides wear offsets to bring the next part into tolerance.

We are not doing angstroms. I take it they are pretty accurate?

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

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

On 09/16/2010 03:29 PM, Wes wrote:
Tim wrote:

They're nice non-contact sensors. They'll get you down to the point
where you have to start worrying about differential temperatures messing
up your measurements, and arms cocked over by milliradians, and other
really smoky stuff.

This thing is part of a system that measures a part one of our turning centers just made
and also provides wear offsets to bring the next part into tolerance.

We are not doing angstroms. I take it they are pretty accurate?

They can be, when they're used right.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" was written for you.
See details at http://www.wescottdesign.com/actfes/actfes.html

On Sep 16, 5:31*pm, Wes wrote:
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_...ansformeristhe 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

An inductosyn is VERY similar to a LVDT in that there is three
circuits. The LVDT is magnetically coupled, the inductosyn is
inductively coupled. The electronics to drive and decode them can be
quite similar. Same for the Resolver. In all cases you can either
drive the Sine and Cosine and read the pickup or drive the center and
decode Sine and Cosine signals from the two circuits.

On Thu, 16 Sep 2010 18:29:46 -0400, Wes
wrote:

Tim Wescott wrote:

They're nice non-contact sensors. They'll get you down to the point
where you have to start worrying about differential temperatures messing
up your measurements, and arms cocked over by milliradians, and other
really smoky stuff.

This thing is part of a system that measures a part one of our turning centers just made
and also provides wear offsets to bring the next part into tolerance.

We are not doing angstroms. I take it they are pretty accurate?

Ayup, they beat the venerable RCH all to hell, in all ways except
style and, um, scent.

--
"A patriot must always be ready to defend his country
against his government." --Edward Abbey

On Thu, 16 Sep 2010 20:52:21 -0700, Larry Jaques
wrote:

On Thu, 16 Sep 2010 18:29:46 -0400, Wes
wrote:

Tim Wescott wrote:

They're nice non-contact sensors. They'll get you down to the point
where you have to start worrying about differential temperatures messing
up your measurements, and arms cocked over by milliradians, and other
really smoky stuff.

This thing is part of a system that measures a part one of our turning centers just made
and also provides wear offsets to bring the next part into tolerance.

We are not doing angstroms. I take it they are pretty accurate?

Ayup, they beat the venerable RCH all to hell, in all ways except
style and, um, scent.

And taste.




I am the Sword of my Family
and the Shield of my Nation.
If sent, I will crush everything you have built,
burn everything you love,
and kill every one of you.
(Hebrew quote)

On Fri, 17 Sep 2010 23:42:42 -0700, Gunner Asch
wrote:

On Thu, 16 Sep 2010 20:52:21 -0700, Larry Jaques
wrote:

On Thu, 16 Sep 2010 18:29:46 -0400, Wes
wrote:

Tim Wescott wrote:

They're nice non-contact sensors. They'll get you down to the point
where you have to start worrying about differential temperatures messing
up your measurements, and arms cocked over by milliradians, and other
really smoky stuff.

This thing is part of a system that measures a part one of our turning centers just made
and also provides wear offsets to bring the next part into tolerance.

We are not doing angstroms. I take it they are pretty accurate?

Ayup, they beat the venerable RCH all to hell, in all ways except
style and, um, scent.

And taste.

Which reminds me of the joke

Q: What sound does a pubic hair make just before it hits the floor?

A: ptui

--
Some people are like Slinkies ... not really good for
anything, but you can't help smiling when you see one
tumble down the stairs.