PSN-L Email List Message

Subject: Re: STM 8 (LVDT subject)
From: meredithlamb meredithlamb@.............
Date: Sat, 16 Nov 2002 16:53:12 -0700


Hi Chris and all,

ChrisAtUpw@....... wrote:

>> > Agreed, but there are several distance transducer approaches which
>> > can be successful. To my mind, there is a distinct shortage of
>> > amateur 'off the shelf' sensor designs capable of giving near
>> > professional resolution and stability. There is only one LVDT Kit
>> > sensor available with a PCB, to my knowledge, which can give about
>> > 7 nano metres resolution at 10 Hz over +/-6 mm, with a max range of
>> > +/-12 mm.
>>
>>
>> Presume its the Italian, Nuova Elettronica unit (?), you've been
>> working with / on over time?
>
> Hi Meredith,
>
> I have done some experiments modifying / improving my Nuova
> Elettronica LVDT board, reducing the two pole detector filter to 10
> Hz, which is more suitable for seismic use,  changing the output opamp
> to a low noise type and increasing the time constant of the high pass
> filter, along with several other small changes. NE seemed to have used
> a circuit designed by Philips for wider bandwidth commercial LVDT
> applications. I have also been working on other magnetic sensor
> systems using the Philips NE5521 LVDT chip, including modified
> transformers, somewhat like Sean's, some using ferrite components and
> lately, a capacitative system. I have a full write up nearly finished.

Your full write up should be VERY interesting!!

>> Perhaps the greatest curiosity item I wonder about their transformer
>> part, is whether its really a precision wound unit like mentioned as
>> a (professional) necessity as on other web sites?  It appears to be
>> much like other general looking transformers. Without any access I'd
>> guess its somewhat linearly wound but no where near precision.
>
> If you want to measure several inches of movement and require a linear
> response, you may need a precision linear winding. If you only want a
> few mm and the field of the sensor rod can be sensed over a physically
> much larger sensor winding, you just need the windings to stay in a
> fixed position. There will of course be thermal expansion and changes
> of resistance with temperature, but these effects are very small over
> the 10 Hz bandwidth and the 5 to 30 sec of the high pass filter time
> constant.
>
>
>> Perhaps the biggest thing I "seem" to see, is that Seans original
>> LRDT transformers are in likelyhood kind of a crude nonlinear wire
>> wrapping; whereas, your LVDT, is a painstaking relatively precision
>> wire layered device.
>
> With Sean's variable reluctance transformer sensor, the magnetic
> circuit is defined by the position of the flat iron armature plate and
> the iron polepieces. In a LVDT, the interaction is between the
> position of a magnetised armature and the varying distance it moves
> inside the sensor coil, so the coil needs to be quite uniformly wound.
> For a description of an LVDT, see
> http://www.msiusa.com/schaevitz/pdf/lvdt/LVDT_Intro.pdf  It is not too
> difficult to make even windings, so long as you do not use extremely
> fine wire, do not try to wind too fast, slow up right at the end of
> each layer and, most importantly, use very thin paper, like cigarette
> or tissue paper, for interleaving each layer. This makes it easy to
> spot and correct any overwinding or incorrectly spaced turns. It also
> makes winding easier and helps to prevent shorted turns in between
> layers. I made two matched 40,000 turn precision differential
> transformer coils this way, for measuring paramagnetic susceptibility.
> A very small plastic tube makes a good wire guide. It is also possible
> to just layer wind two LVDT secondaries end to end on a sensor tube
> and use a ferrite or powder iron cored choke to provide the magnetic
> excitation. This avoids having to wind a central primary magnetising
> section in between, or over the top of the secondaries.
>
> Yet.....according to Seans website, his LRDT is sensitive to a
> incredible 0.1nm.  I've
>
>> zero experience but it makes me wonder further about coils
>> themselves between
>> the two.  It would seem the LRDT is much simpler to make and more
>> sensitive overall.
>
> Coupled with the very high sensitivity of Sean's transducer, the
> movement range is small, perhaps +/-0.5 mm in a 2 mm total gap, and
> the output is only linear for a limited range around the zero
> position. This is just fine for a feedback seismometer, where you try
> to hold the armature position constant. The out of range signal
> increases dramatically before saturating, which can be bad news if the
> seismometer feedback system gets out of lock.

Seans reluctance sensor sounds like it could be used in a "S-G" also,
although one might have a devil of a time especially when any
kind of small height S-G feedback system, might influence the sensor.
Perhaps a fixed eddy current damping system might be workable,
if far enough away from the sensor itself.

>> All this is leading up to the question of whether a fairly linear
>> wound (but not exactly precisely layered) coil "could" somewhat work
>> simarily (with less sensitivity) with the same general ferrite core
>> type setup.  Such coils are rather scarce I'am sure.
>
> If you have an E core transformer type winding, the magnetic flux is
> linked strongly to the winding, so great winding precision is not
> required, only a good physical stability. The magnetic field
> interactions are controlled by the positions and separations of the
> iron laminations, so these need to be firmly fixed. These E core types
> can be either modified audio transformers similar to Sean's, ferrite
> cup transformers with DIY windings, a linear differential reluctance
> system of the magslip type, or two physically large coils pairs
> bridged by a ferrite rod, like the NE type.
>
>> Just some (unexperienced) thoughts.  I think the Nuova item costs
>> around $103 US dollars (PCB and LVDT) plus whatever
>> shipping/handling.  Sometimes I think that a homebrew LVDT with
>> perhaps a Phillips or Analog IC (and other parts) might be a cheaper
>> alternative......but probably more costly for any R&D overall.
>
> The two Analogue Devices LVDT chips have a relatively high noise floor
> and are not suitable for our use. The Philips NE5521 device is about
> 1/3 the price and you can get right down to the intrinsic opamp noise
> level.
>
> The LX1358 Kit price is € 51.65 + Carriage. Looking up today's
> exchange rate, this is just US $51.23 !! There will be a charge on
> your Credit Card for the currency conversion and the exchange rate
> will be lower than the International Banking Rate quoted. Air Mail
> packets are reasonably priced and may take 10 days to arrive. Be
> warned that International Carriers may have a minimum charge of near
> $50 for any packet! They seem to be only relatively reasonably priced
> within the USA.

Am missing something here....whats the difference between a "air mail
packet", and a, "International Carriers"?  Did Nuova offer the
distinction
when you got your LVDT?  Airmail v/s "ground" (ship)?

> With a LX1358, I get ~7 nano metre resolution over a +/-6 mm
> accurately linear range, with 65 mV / micron output. The output is 10%
> down at +/-12 mm and 20% down at +/-15 mm, since the ''out of range''
> response is of the usual S shape. This wide range was designed for
> Lehman type systems, but the sensor will work equally well on simple
> pendulums.

Thats quite a large sensor mm/mv output!  Thanks for all the "feedback"
info!

Take care, Meredith



Hi Chris and all,

ChrisAtUpw@....... wrote:

Agreed, but there are several distance transducer approaches which can be successful. To my mind, there is a distinct shortage of amateur 'off the shelf' sensor designs capable of giving near professional resolution and stability. There is only one LVDT Kit sensor available with a PCB, to my knowledge, which can give about 7 nano metres resolution at 10 Hz over +/-6 mm, with a max range of +/-12 mm.


Presume its the Italian, Nuova Elettronica unit (?), you've been working with / on over time?

Hi Meredith,

I have done some experiments modifying / improving my Nuova Elettronica LVDT board, reducing the two pole detector filter to 10 Hz, which is more suitable for seismic use,  changing the output opamp to a low noise type and increasing the time constant of the high pass filter, along with several other small changes. NE seemed to have used a circuit designed by Philips for wider bandwidth commercial LVDT applications. I have also been working on other magnetic sensor systems using the Philips NE5521 LVDT chip, including modified transformers, somewhat like Sean's, some using ferrite components and lately, a capacitative system. I have a full write up nearly finished.

Your full write up should be VERY interesting!!
Perhaps the greatest curiosity item I wonder about their transformer part, is whether its really a precision wound unit like mentioned as a (professional) necessity as on other web sites?  It appears to be much like other general looking transformers. Without any access I'd guess its somewhat linearly wound but no where near precision.
If you want to measure several inches of movement and require a linear response, you may need a precision linear winding. If you only want a few mm and the field of the sensor rod can be sensed over a physically much larger sensor winding, you just need the windings to stay in a fixed position. There will of course be thermal expansion and changes of resistance with temperature, but these effects are very small over the 10 Hz bandwidth and the 5 to 30 sec of the high pass filter time constant.
 
Perhaps the biggest thing I "seem" to see, is that Seans original LRDT transformers are in likelyhood kind of a crude nonlinear wire wrapping; whereas, your LVDT, is a painstaking relatively precision wire layered device.
With Sean's variable reluctance transformer sensor, the magnetic circuit is defined by the position of the flat iron armature plate and the iron polepieces. In a LVDT, the interaction is between the position of a magnetised armature and the varying distance it moves inside the sensor coil, so the coil needs to be quite uniformly wound. For a description of an LVDT, see http://www.msiusa.com/schaevitz/pdf/lvdt/LVDT_Intro.pdf  It is not too difficult to make even windings, so long as you do not use extremely fine wire, do not try to wind too fast, slow up right at the end of each layer and, most importantly, use very thin paper, like cigarette or tissue paper, for interleaving each layer. This makes it easy to spot and correct any overwinding or incorrectly spaced turns. It also makes winding easier and helps to prevent shorted turns in between layers. I made two matched 40,000 turn precision differential transformer coils this way, for measuring paramagnetic susceptibility. A very small plastic tube makes a good wire guide. It is also possible to just layer wind two LVDT secondaries end to end on a sensor tube and use a ferrite or powder iron cored choke to provide the magnetic excitation. This avoids having to wind a central primary magnetising section in between, or over the top of the secondaries.

Yet.....according to Seans website, his LRDT is sensitive to a incredible 0.1nm.  I've

zero experience but it makes me wonder further about coils themselves between
the two.  It would seem the LRDT is much simpler to make and more sensitive overall.
Coupled with the very high sensitivity of Sean's transducer, the movement range is small, perhaps +/-0.5 mm in a 2 mm total gap, and the output is only linear for a limited range around the zero position. This is just fine for a feedback seismometer, where you try to hold the armature position constant. The out of range signal increases dramatically before saturating, which can be bad news if the seismometer feedback system gets out of lock.
Seans reluctance sensor sounds like it could be used in a "S-G" also,
although one might have a devil of a time especially when any
kind of small height S-G feedback system, might influence the sensor.
Perhaps a fixed eddy current damping system might be workable,
if far enough away from the sensor itself.
All this is leading up to the question of whether a fairly linear wound (but not exactly precisely layered) coil "could" somewhat work simarily (with less sensitivity) with the same general ferrite core type setup.  Such coils are rather scarce I'am sure.
If you have an E core transformer type winding, the magnetic flux is linked strongly to the winding, so great winding precision is not required, only a good physical stability. The magnetic field interactions are controlled by the positions and separations of the iron laminations, so these need to be firmly fixed. These E core types can be either modified audio transformers similar to Sean's, ferrite cup transformers with DIY windings, a linear differential reluctance system of the magslip type, or two physically large coils pairs bridged by a ferrite rod, like the NE type.
Just some (unexperienced) thoughts.  I think the Nuova item costs around $103 US dollars (PCB and LVDT) plus whatever shipping/handling.  Sometimes I think that a homebrew LVDT with perhaps a Phillips or Analog IC (and other parts) might be a cheaper alternative......but probably more costly for any R&D overall.
The two Analogue Devices LVDT chips have a relatively high noise floor and are not suitable for our use. The Philips NE5521 device is about 1/3 the price and you can get right down to the intrinsic opamp noise level.

The LX1358 Kit price is ⬠51.65 + Carriage. Looking up today's exchange rate, this is just US $51.23 !! There will be a charge on your Credit Card for the currency conversion and the exchange rate will be lower than the International Banking Rate quoted. Air Mail packets are reasonably priced and may take 10 days to arrive. Be warned that International Carriers may have a minimum charge of near $50 for any packet! They seem to be only relatively reasonably priced within the USA.

Am missing something here....whats the difference between a "air mail
packet", and a, "International Carriers"?  Did Nuova offer the distinction
when you got your LVDT?  Airmail v/s "ground" (ship)?
With a LX1358, I get ~7 nano metre resolution over a +/-6 mm accurately linear range, with 65 mV / micron output. The output is 10% down at +/-12 mm and 20% down at +/-15 mm, since the ''out of range'' response is of the usual S shape. This wide range was designed for Lehman type systems, but the sensor will work equally well on simple pendulums.
Thats quite a large sensor mm/mv output!  Thanks for all the "feedback" info!

Take care, Meredith
 


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