This is one for the electronics buffs. I purchased a probe gauge that
uses a Linear Variable Differential Transformer to sense the probe
position. I understand the concept of how these work and was wondering
how hard it would be to make my own reader to use this probe. I only
paid $9.00 for the thing and bought it just to see if I could make
something to read it. I was thinking of using an analog readout. Can
Depends how fussy you are trying to be. I just checked over the notes
for a project I did with an LVDT, and for all intents and purposes an AC
voltmeter on a secondary would do the job. Preferably one that you can
easily open up and put a paper face on with markings that have something
to do with your reading. The primary coil input needs to be consistent
for this to work...
If you can get some specs, it would help greatly. First, you need a
oscillator, to excite the driving coil. Then, you need two simple
amplifiers. You could use regular op amps, but the AD620 makes a much
better instrumentation amp. You then compare the amplified signal level
from the two sense coils. A good circuit would be an active rectifier (look
in any op-amp circuits book) on each of the AD620 outputs, after a DC
capacitor. Then, just feed the two active rectifiers into a difference
or rig one of the active rectifiers to provide the revers polarity, and
Feed this to a range selector switch and a big analog meter.
Finding the right frequency and drive voltage would be real helpful in
work best. The frequency is likely to be somewhere around 800 Hz to 3 KHz
for the smallest LVDTs.
FWIW there are LVDTs that only require a DC input and they in turn deliver a
DC output swinging from a minus level through zero to a positive level, thus
the output is calibrateable/readable on a DV voltmeter.. These types
contain the AC oscillator and the rectifier/demodulator built in. The
Series-240LVDTs work on input voltages from 6V DC to 30V DC. I'm sure
tightly regulated input voltages are required. For example, one of the
Series-240 LVDTs has a working range of +- 0.050" and with an input voltage
of 6V DC, it gives a full range output swinging from - 1.2 VDC to +1.2 VDC.
Another in the same series powered with 15V DC input has a working range of
+- 1.00" and the output swings from -10.8 V DC to +10.8 V DC.
I suggest you find some data sheets on LVDTs. My information on the
Series-240 LVDT is over 10 years old so you may find better versions in new
you will get your best accuracy with a synchronous demodulator - basically
you want to cancel out variations in the modulation voltage - if you can
find the schematics for any of hte old Collins flight control stuff you will
see the circuit. You will need to know the excitation freq for your unit -
many are 400hz, some are higher, and drive it with the proper freq - make a
simple sine wave source to do that. You can get really excellent accuracy
and linearity, they are great for servo systems
If it is an aircraft LVDT, the exc. voltage will be 26vac, 400 hz. There
are LVDT's all over Boeing, Lockheed, and Airbus autopilot flt. control
actuators. But they are_never_used for anything but feedback to the flight
control computers. All position indications to the cockpit surface position
indicators are always from RVDT's. Rotary variable displacement
transformers. I don't know why this is, it just is. One of those questions
I'd like to ask a Collins engineer someday. But I see no reason that an
LVDT couldn't be used in a linear position circuit.
On Wed, 29 Sep 2004 02:04:18 -0400, "Garrett Fulton"
I don't know squat about the electronics, but did use an LVDT in a
servo-hydraulic circuit to control position within .001". Worked
great. It required a scaled down servo valve for the low flow. The
Vickers sales guy said they were originally developed for the Disney
animatronic critters. By the way, I really didn't like dealing with
Vickers. If I ever need to do that again (unlikely), I'll go with a
(sorry about threading- my ISP changed their news setup)
Check out these:
The PLCC version is USD 27 1-off the Cerdip version is USD 80, so
you'll want to pick up a PLCC socket and use the former.
"it's the network..." "The Journey is the reward"
firstname.lastname@example.org Info for manufacturers: http://www.trexon.com
On Wed, 29 Sep 2004 06:59:41 -0400, Spehro Pefhany
It looks like there are many solutions to this. I am going to see if
Edmunds, the gauge company that made it, will give me the exitation
frequency for the thing. Their web site doesn't list this probe. I
think it is now obsolete.
I've seen a digital system that was pretty ingenious....
it started with a clock. The clock drove a free running
counter maybe 0-1024. The output of the counter fed a
pair of roms, one for Sine lookup, and one for Cosine
lookup. The roms fed a set of dacs and those fed the
sine and cosine coils, as drive coils.
The pickup was off the center coil....
Depending where the slug was, the pickup coil is going to
be closer to Sine or Cosine in terms of phase relationship.
The pickup coil fed a zero crossing cicuit. The zero crossing
time captured the value of the free running counter.
If the slug was near the sine drive coil, the zero cross
would be closer to the sine wave output for zero crossing.
If it was closer to the Cosine drive coil, it would be closer
to 90 degrees phase shift before the zero crossing would occur,
and the value captured from the free running counter would reflect
the position of the slug.
This circuit is fairly clever for a noisy environment. Since the
sensing circuit does not have a varying amplitude, it is not as
sensitive to noise.
The traditional method is to drive the center coil, and measure
the _realtive_ amplitudes of the Sine, and Cosine coils, so with
one coil delivering a low level signal, noise immunity is a concern.
The method here is an all digital approach.
And NOT that complicated really. It could likely be all done inside
a small micro processor.
It's harder for me to explain than it is...
The LVDT has three coils. the center coil, and the
outer coils. The slug in the middle moves, and either couples more
from the left coil to the center or, at the other end couples
better from the right coil to the center.
You could drive the center coil, and then compare the feedback
from the left and right coil to see which one is getting a
stronger signal. The coil with the strongest signal is where
the slug (movable part) is.
A quick and DIRTY way to cheat is to drive the center coil,
and look at the signal level from just one coil, to guess where
the slug is at. Without comparing the relative strengths of
the two end coils, you are subject to noise, and varying
signal levels. The accuracy, and long term stability might
not be real great.
Both of those methods require you to measure the signal level
of the sensing coils.
In my previous post, a completely different method of finding
the position is described. In that method, the two outer coils
are driven with a sine/cosine signal. The center coil is used
for sensing position. Driving the two outer coils with sine/cosine
signals will genreate a signal response from the center winding
that will vary from a Sine to a Cosine depending on where the
slug is located. If the slug is to the left, the sine drive coil
will couple more strongly to the sense coil, and the sense (center)
coil will match the sine wave in phase angle.
If the slug is to the right, the cosine drive coil will couple
into the sense coil, and the sense coil will have a phase angle
closer to the cosine drive signal.
This sensing method does not rely on the signal level, but rather
the phase angle.
So, all that is needed is a sine/cosine gernator, such as a ring
oscillator if done in analog, and a zero crosing detector to
find where the sense signal crosses zero.
The zero crossing detector <maybe> could drive a sample/hold
circuit to capture the sine wave drive value, to give an indication
of the position.
I hope expaining it another helps to clarify the concept.
Depends on the accuracy you need. For a "Science Fair" demonstration
you can do it with an oscillator and an AC voltmeter. For real
measurement, find, scrounge or buy an LVDT module from Burr-Brown or
Analog Devices. They appear simple, but they're prone to all sorts of
inaccuracies -- best dealt with by someone else's clever work on a
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