Semi on topic LVDT

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.

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is the wiki page which is okay but after you have read that, go to
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and watch the animation that makes it clear as a bell.

I learned something today and wanted to share it.

Wes

Reply to
Wes
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device. I had

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.

Reply to
Tim Wescott

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ent device. =A0I had

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.

Reply to
Cross-Slide

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

Reply to
Wes

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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

Reply to
Wes

They can be, when they're used right.

Reply to
Tim Wescott

uent this list it is

rement device. =A0I had

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.

Reply to
Cross-Slide

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

Reply to
Larry Jaques

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)

Reply to
Gunner Asch

centers just made

Which reminds me of the joke Q: What sound does a pubic hair make just before it hits the floor?

A:

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

Reply to
Larry Jaques

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