# Any cunning way to zero-shift an LVDT output?

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This is an LVDT
http://www.lvdt.co.uk/howtheywork.html

I have an application where I need to basically shift the zero point
(the position where the two outputs cancel out).

In most applications this is obviously done by physically moving the
two pickoff coils :)

I can design an electronic solution but it is not simple. One needs to
convert the two AC signals into a DC voltage first, level shift that
as required, and re-synthesise the two AC voltages. It's quite a lot
of circuitry...

Is there some cunning way to do this, without doing the AC-DC-AC
conversion? I am vaguely thinking of using something like a resolver
i.e. an iron core with some windings on it.

It would be a fun job to do with a DSP, or actually any half decent
microcontroller given that the frequency is only 400Hz. But I am
looking for an analog circuit.

I do want the AC output BTW, not DC.

BTW Don Y - if you are reading this, I wouldn't mind your current

Re: Any cunning way to zero-shift an LVDT output?

<snip>

Four wire LVDT and you want this adjustable?

Take a second LVDT, connect the LVDT primaries in parallel and the
secondaries in series.  Adjust the second LVDT core in/out to zero the
first.

Cheers
--
Syd

Re: Any cunning way to zero-shift an LVDT output?

That is probably the simplest and most bust proof method.

?-)

Re: Any cunning way to zero-shift an LVDT output?

But hideously unstable compared with any kind of transformer-based
solution, where the zero shift is only going to depend on turns ratio
(plus some minor imperfections that you can pull down to the one part
in ten million level if you are really careful).

--
Bill Sloman, Nijmegen

Re: Any cunning way to zero-shift an LVDT output?

Without thinking this through,...add a small amount of AC to the
sensor coil's output? It's fixed frequency, so you can shift phase as
needed.

Re: Any cunning way to zero-shift an LVDT output?
How about a transformer in parallel;
just couple excitation into output ?
A few resistors to adjust offset...
Let us know how you do it,
Best Regards, Dave

Re: Any cunning way to zero-shift an LVDT output?

Stick in a small transformer, like a MET-37, and a trimpot maybe. Pick
off the excitation, scale as required, add in series with the
secondary signal.

What is the configuration? 5-wire? I could sketch it up if I knew the
details. I'm deep into an LVDT simulation project right now, been

http://dl.dropbox.com/u/53724080/Circuits/V545_A.JPG

--

John Larkin, President       Highland Technology Inc
www.highlandtechnology.com   jlarkin at highlandtechnology dot com

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Re: Any cunning way to zero-shift an LVDT output?

Great suggestion, THANK YOU. Should have thought of that ;)

I think you are interfacing to slightly more than one LVDT ;)

My one is here
http://i101.photobucket.com/albums/m74/peterh337/temp/lvdt.gif

The two AC signals come out from the LM358 outputs.

Re: Any cunning way to zero-shift an LVDT output?

That, however, appears to do just a zero shift in one direction.

If I wanted to shift the zero in both directions around the current
zero, then I need to have a way of adding variable amounts of the
excitation signal to one output or the other.

It seems easy enough with op-amps, and done with just a trimpot, no
transformers.

Re: Any cunning way to zero-shift an LVDT output?

Make two copies of the excitation, one the other inverted,
and connect the ends of the pot to the copies, take the
correction signal from the middle.

(Just wondering what is so difficult here ...)

--

Tauno Voipio

Re: Any cunning way to zero-shift an LVDT output?

Trimpots are fine, until you start needing real stability and
accuracy, and you can't put a turns counting dial on the front to find
out where you have set them. Ten-turn servo-quality potentiometers are
much better, if still not as good as ratio transformers - but they are
much cheaper, if not exactly cheap.

http://uk.farnell.com/vishay-spectrol/534b1103jlb/potentiometer-2w-10k/dp/1 =
144786?Ntt3D%1144786

--
Bill Sloman, Nijmegen

Re: Any cunning way to zero-shift an LVDT output?

In this application, one would not want to shift the full scale
values. One would want to do a very small zero adjustment, of the
order of 1-2% of FSD.

I think a transformer, whose secondary is in series with one of the
output coils, and whose primary is fed from the wiper of a trimpot
connected across excitation and /excitation, will do this fine.

Re: Any cunning way to zero-shift an LVDT output?

Only if the impedance of the trimpot (or potentiometer) is a lot lower
than that of the transformer. You could buffer the output of the
trimpot with an op amp, but the offset in the op amp then applies a
persistent DC voltage across the transformer and if you don't take
care to deal with this, the current through the transformer can build
up to a level which can force the op amp output into saturation.

Once the op amp output is close to saturation, the current through the
transformer will clip at one extreme or the other, compensating for
the off-set, so you may not notice, but it's not a good way of dealing
with the problem.

--
Bill Sloman, Nijmegen

Re: Any cunning way to zero-shift an LVDT output?

Connect a trimpot across the secondary in the obvious manner. Try 10K
to start.

--

John Larkin, President       Highland Technology Inc
www.highlandtechnology.com   jlarkin at highlandtechnology dot com

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Re: Any cunning way to zero-shift an LVDT output?

We do it on sensors at work, a pot across the works with the center
tap (wiper) to slightly drain off one side which offsets the balance.

All units I've used is doing a basic level null mix from a single
frequency source, you only need to offset the high sides of either
the primary or secondary and we do it with a pot with some added wing
R's so that the effects are only marginal and the wiper hits the common.

Jamie

Re: Any cunning way to zero-shift an LVDT output?

You want to go both ways? A transforrmer with two secondaries would
work. They are common too.

Sure, if you have power available. I was thinking that you might want
a "black box" hack to the LVDT.

--

John Larkin, President
Highland Technology, Inc

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Re: Any cunning way to zero-shift an LVDT output?

No; that won't shift the zero.

Imagine you are at zero output already and you want to shift it. The
pots won't help. One has to inject an AC signal in there.

Re: Any cunning way to zero-shift an LVDT output?

Are you sure? Skewing the gains will simulate core motion. LVDTs
usually operate at a small fraction of their total motion transfer
curve, so that things stay linear, so a gain skew should work. It's
not much different from injecting a pair of AC voltages, at least for
modest simulated offsets.

--

John Larkin, President       Highland Technology Inc
www.highlandtechnology.com   jlarkin at highlandtechnology dot com

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Re: Any cunning way to zero-shift an LVDT output?

You want a ratio transformer, as used at National Standards Labs. The
individual taps can be identical to about one part in 10^7 and you can
use them to slice and dice AC signals with remarkable precision

http://reviews.ebay.com/AC-Ratio-Transformers-Inductive-Voltage-Dividers?ug =
id3D%10000000001625986

If you've got access to a good technical library, you may be able to
get hold of a copy of Rayner and Kibble's book

For your application you'd set up the ratio transformer to give you a
well controlled fraction of the AC signal being used to excited the
LVDT and you'd use this signal to back off or augment the signal
coming out of the sensing winding on the LVDT.

--
Bill Sloman, Nijmegen

Re: Any cunning way to zero-shift an LVDT output?

Basically you inject an AC signal into the mix, to add to one pickoff
coil and (possibly) subtract from the other.  How you do this depends
entirely on -- you.

But you need to be very careful about linearity.  LVDTs come in a variety
of flavors, designed for specific methods of converting to a position
measurement, and the result may not be linear when you go and mess with

(This came as quite a surprise to me when I discovered it: the two main
flavors of 5- (or 6-) wire LVDTs are one where x = k * (|A| - |B|) / (|A|
+ |B|), where |A| and |B| are the rectified and averaged coil outputs,
and another where x = k * (|A| - |B|).  In general, each of these is only
linear in the service for which it is designed.  Much careful reading of
LVDT data sheets is necessary to get the desired outcome.)

--
My liberal friends think I'm a conservative kook.
My conservative friends think I'm a liberal kook.
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