"Open Wiring" vs. Ratiometric LVDTs

Out there in the wilds of industrial application how common are the "open wiring" type of LVDT vs. the ratiometric type (see http://www.efunda.com /
DesignStandards/sensors/lvdt/lvdt_theory.cfm for definitions if you need to).
I had thought that the ratiometric types were pervasive, but recently a client got blindsided by an LVDT that appeared to be suitable for either open wiring or ratiometric, but in reality only worked as a ratiometric type. I'm helping them cope, but I'm wondering how hard they'd have to look for a 'proper' ratiometric type.
Thanks in advance.
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Tim Wescott
Control systems and communications consulting
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Tim Wescott wrote:

So open wiring consists of connecting the output windings in series opposing? What is the advantage of ratiometric operation? I can imagine that for large displacement the sum of A and B might vary. Does the ratiometric connection extend the linear range?
(I have never gotten good performance at low displacements from an LVDT without using a synchronous detector to eliminate unwanted quadrature that creates a soft null. Quadrature injection is just too touchy.
Jerry
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On Sun, 10 Feb 2008 20:30:07 -0500, Jerry Avins wrote:

Ratiometric leaves your measurement insensitive to changes in excitation such as may happen with long cable runs and/or temperature variations.
Synchronous detection is definitely the way to go (and it's aided by ratiometric detection -- you use the phase of the sum to time the synchronous detection, thereby rendering your application insensitive to phase shift within the detector or on long cable runs).
If you check the Analog Devices web site for LVDT signal conditioners you'll find white papers extolling the virtues of ratiometric detection.
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Tim Wescott
Control systems and communications consulting
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Tim Wescott wrote:

I did that by using the open wire connection (with synchronous detector) and dividing the signal by the excitation voltage. (There's no need to actually divide in analog implementations. Using a controlable-gain amplifier -- an OTA, for example -- simply normalize the excitation reading before reading the displacement. Maybe that will work for you?
Jerry
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