Need help understanding LVDT sensor circuitry

[Roman oh]

I have recently acquired a (vintage) Mercer 155 comparator unit which uses an LVDT (at least I think that”s what it must be) to make precise (sub micron) comparative length measurements. The UK-based company that made it (Thomas Mercer Ltd) went out of business in 1984. The unit appears to work well, despite its age and the fact that no componentry at all appears to have been touched.

After lots of fruitless searching on the web, I am trying to reverse engineer a schematic against the day some repair may be needed (and a BIG element of simple curiosity!). Sketching out the (discrete component) PCB is simple enough, but my big stumbling block is understanding the LVDT front end.  The sensor has only 3 wires, one of which is ground. The other two are driven by a 6kHz sine wave which, at balance, is of equal amplitude (about 2V) on both legs, goes up and down in opposite directions by about 50% as the sensor moves. Changing the display polarity simply involves swapping these two lines. There is no separate “primary” winding as I would expect in a conventional LVDT. The unit will handle two LVDTs (I only have one).

The problem is that the leads from both LVDT channels, along with a few wires from the PCB all disappear into a single multi-winding pot core transformer where all the magic happens. Characterising that transformer is a major disassembly/un-soldering task which I am hesitant to do.

Does anyone have any info or can point me at something that might give insight into the theory of how this thing works?

Thanks in advance.

[moffy]

The following document shows a 3 wire LVDT schematic on page 4: https://www.strainsense.co.uk/client/uploads/10/lvdt-manual-a-05-23.pdf
Hope it helps.

[Roman oh]

Thanks, Moffett,
That 3-wire schematic on P4 is clear and straightforward but that configuration would not appear to me to be reversible. In the Mercer unit I have, neither of the two “active” pins is a constant excitation; they both move in opposite directions as the plunger moves. And polarity reversal is a simple changeover switch of these two pins.
I’m still not seeing something…..

[Renate]

This is all very interesting and since I know absolutely nothing about it I'll venture an opinion.

How about this?

[moffy]

If the excitation is a sinusoidal current source then the voltages varying together would make sense, as one impedance increases the other decreases, and you could do that with 3 wires. Effectively the sense coils are also the excitation and the coil inductance change is what is being measured.

[Roman oh]

Renate, your diagram is a very plausible suggestion! Wish I had thought of it! It certainly addresses the symmetry question, and moffy’s bit about coil inductance change is quite credible. I can even imagine then how this configuration could easily be extended to cater for a second channel yielding a single A +/- B channel, all in the one transformer, which is what this device appears to do. To validate this, I will probably now have to disassemble the unit to characterize the transformer. Not too keen to rush into that, but I’ll certainly think a lot more about how this architecture matches the circuitry I have mapped so far.

Still a long shot, but maybe there is someone out there who is actually familiar with these sorts of devices. Mitutoyo, Brown and Sharpe, Federal, lots of people made them back in the 80’s, so I’ll keep hoping for a little while.
 
Thanks, guys, for your suggestions so far!

[Roman oh]

In fact, this opened up a whole new search path and I discovered Linear Variable Inductance Transducers (LVIT) which are a real thing that I didn’t know existed - exactly as Renate suggested. So know I’m sure I’m on the right path.