"Open Wiring" vs. Ratiometric LVDTs

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Tim Wescott's Avatar (by Gravatar)Tim Wescott posted on
February 10, 2008, 7:09 pm
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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.

--
Tim Wescott
Control systems and communications consulting
http://www.wescottdesign.com

Need to learn how to apply control theory in your embedded system?
"Applied Control Theory for Embedded Systems" by Tim Wescott
Elsevier/Newnes, http://www.wescottdesign.com/actfes/actfes.html

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Jerry Avins's Avatar (by Gravatar)Jerry Avins posted on
February 10, 2008, 8:30 pm
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Re: "Open Wiring" vs. Ratiometric LVDTs



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
--
Engineering is the art of making what you want from things you can get.
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Tim Wescott's Avatar (by Gravatar)Tim Wescott posted on
February 10, 2008, 9:49 pm
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Re: "Open Wiring" vs. Ratiometric LVDTs



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.

--
Tim Wescott
Control systems and communications consulting
http://www.wescottdesign.com

Need to learn how to apply control theory in your embedded system?
"Applied Control Theory for Embedded Systems" by Tim Wescott
Elsevier/Newnes, http://www.wescottdesign.com/actfes/actfes.html

Jerry Avins's Avatar (by Gravatar)Jerry Avins posted on
February 10, 2008, 10:59 pm
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Re: "Open Wiring" vs. Ratiometric LVDTs



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
--
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

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