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3-lead LVDT?

Is there such a thing as a 3-lead LVDT?

I picked up a Starrett model 712-2 gaging head at a swapmeet, without the electronics.

This ancient gadget measures linear displacement very accurately and repeatably over a small range of travel.

Googling suggests it's an LVDT, but the connections seem strange--it only seems to have three active leads. Does that make any sense?

The connector looks like this:

3 . - . . O . / \\ 2 . O O . 4 . O . . 6 . \\ O O / 5 1 . . ._n_.

And the continuity measurements are:

  1. ------- --- ^ 2. n/c 43 ohms V 3. ------- --- ^ 4. n/c 43 ohms V 5. ------- ---
  2. shield, 2 ohms to LVDT casing

I opened the connector to confirm 2 and 4 are definitely not connected, the cable only has 3 wires + braided shield, the shield does connect to the 712-2 case, and nowhere else.

Inside, a plunger moves a ferrite-looking rod in and out of a bobbin, presumably the LVDT coils, but I dare not peek--the tiny windings must be micro-fine.

Are there LVDTs like this needing only three wires, or are they possibly measuring just the differential inductance to get displacement?

TIA, James Arthur

Reply to
James Arthur
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At work we use something they call a DCDT, a DC version of an LVDT. Evidently the electronics is built in the unit, I'm wondering if that's what you have.

Perhaps it's like one of the GCD-SE gage heads?

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RogerN

Reply to
RogerN

They're probably measuring differential inductance. Put 2500Hz or so across pins 1 and 5, and see what pin 3 does as you move the plunger in and out.

--

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
Reply to
Tim Wescott

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Yep, that's the physical look of the thing. This eBay auction has a picture of a later sensor + electronics:

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I don't think mine's a DC gage--the cavity inside the case is too tiny, and I read that Starrett's manual for the electronics is dated 1966, long before SMT. That would've been sweet though.

Thanks for the info. This is for fun, for learning about LVDTs. They're a lot cooler than I thought.

James Arthur

Reply to
James Arthur

Good advice, but it would be a nasty way to wire up an LVDT - you end up with the excitation current flowing through one of your signal leads. It's nominally 90 degrees out of phase with the voltage you want to measure, but it wouldn't help if you were trying to measure small displacements.

-- Bill Sloman, Nijmegen

Reply to
bill.sloman

Thanks for the thoughts Tim. I suspect you're right, and I'm off to my lab to figure it out. I'll report back.

This sure is a neat gadget...

Cheers, James Arthur

Reply to
James Arthur

Hmmm. It's still a little unclear what all's inside there, but I measure 1.5nF from pin 3 to pin 6. It looks like pins 1 and 5 take the drive, with output a.c. coupled on pin 3.

Doing that, anyway, with a 1v @ 5kHz sine gives a sinewave on pin 3 that shifts first in amplitude, then phase, then amplitude again as the ferrite core plunges into and through the winding bobbin.

I'm doing this manually. The ferrite needs gluing back onto the plunger mechanism, so I can't tell which range of motion they're using, amplitude-varying, or phase. I think phase, but either mode should be a snap to demodulate.

Cheers & thanks all, James Arthur

Reply to
James Arthur

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