Any cunning way to zero-shift an LVDT output?

[Peter]

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
email address

 

[Syd Rumpo]

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).

<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

 

[Don Y]

Hi Peter,

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

Are you *only* interested in the null point, thereafter? Or, do
you care about the quality and range of the modified signal?

Tricky question/observation: is this a *static* adjustment that you
want to make? Or, a *dynamic* one (e.g., to null the *current*
position of a control surface, in flight?)

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

Alternatively, the slug floating between them! Presumably, this isn't
something you can mechanically adjust?

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.

Why not a simple transformer excited with the same primary
and mix "some amount" of the secondary in to the LVDT's output.
(you would have to selectively pick which side of the coil to use
based on which side of "true null" you were on)

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
email address

Sorry, I've been ***really*** busy and neglectful of keeping up
with my email. :< Is the (ahem) "2000" address still valid?

 

[Peter]

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.

Great suggestion, THANK YOU. Should have thought of that

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
thinking about this stuff a lot.

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

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.

 

[Peter]

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.

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.

 

[Tauno Voipio]

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.

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 ...)

 

[Paul E. Bennett]

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.

Have you tried looking at the circuit on Page 337 of Walter G Jung's "IC Op-
Amp Cookbook" (third edition). This is an instrumentation amplifier with a
zero off-set adjust and differential input. A bit more digging and you will
get the differential output configuration as well.

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.

If the eventual destination is into a micro-controller, you might also
consider some mathematical corrections. That I will leave as your exercise.

--
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Mob: +44 (0)7811-639972
Tel: +44 (0)1235-510979
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********************************************************************

 

[Jamie]

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.

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

 

[Ralph Barone]

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.

Add a DPDT switch to the BOM.

 

[Peter]

Your schematic already has two pots. Just turn them!

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.

 

[Peter]

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/1144786?Ntt=1144786

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.

 

[josephkk]

<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

That is probably the simplest and most bust proof method.

?-)