PSN-L Email List Message

Subject: Re: STM 8 (previous Sprengnether subject email)
From: meredithlamb meredithlamb@.............
Date: Fri, 15 Nov 2002 11:56:54 -0700


Hi Karl and all,

Barry wrote:

> Hi Meredith     I have wanted to put a web site together like several
> of you folks have so nicely done but I seem to have a problem with
> documentation.  I have made some photos but not to many yet. Written
> descriptions----well  not yet. I can briefly describe what I have done
> wrt the SMT8 style verticals.     I think of the SMT8  in four
> parts.              1. the mechanical - boom, leaf spring, hinges.On
> my first sensor I tried as closely as possible to duplicate the the
> SMT8 geometry. The only change I made was to remove part of the leaf
> spring support where it attaches to the end pins so I could slightly
> squeeze the ends of the bent leaf spring together and quickly remove
> it from the assembly w/o tools.
>
> Sounds interesting on the mechanical change, although I don't
> totally understand it.
>
>
> On the second SMT8 I reproduced the first sensor but made the boom
> about 10 " long to try to make it more portable (for possible field
> use in an ammo box).              2. the displacement sensorIn the
> first sensor,  I installed a homemade LVDT with phase shift oscillator
> and the demodulator that Karl Cunningham has described.  In the second
> sensor I installed the VRDT hardware as STM described but with phase
> shift oscillator, instrumentation amp to sum and Karl's demodulator.
> Both work well and are quite linear. Side note: I calibrate the
> displacement sensors with a "diving board" type device that I
> calibrated with precision weights. I found it was easier to apply a
> precise weight than a small deflection directly. All the circuitry for
> either can be crammed on a 2X6.5 size breadboard. Not too complicated
> of circuitry, mostly just dual opamps.
>
> Most interesting on the LVDT switch from the LRDT original design
> on one of your instruments!  Am not really "up" to either; but its
> interesting on the switching sensor capability you've already done
> and the success.
>
> 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 layed device.  Yet.....according to Seans website, his
> LRDT is sensitive to a incredible .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.  Interesting.

>
>                3. the  feedback circuitryI used the Mathcad equations
> STM suggested.              4. mechanical adjustment control.There are
> some adjustments I found necessary as STM had described in depth.  The
> vertical is not as sensitive to tilting but is sensitive to
> temperature and the initial relaxation of the leaf spring. For
> temperature correction I installed a bimetal thermometer coil near the
> boom. With some trial and error positioning and thermal insulation I
> was able to remove a substantial amount of this temperature variation.
> For zero adjustment I used the Edmond Scientific low rpm motor and
> threaded rod and sliding  weight. Works  well.
>
> Quite a nice additon on the bi-metal thermometer coil!
>
> Take care, Meredith Lamb



Hi Karl and all,

Barry wrote:

Hi Meredith     I have wanted to put a web site together like several of you folks have so nicely done but I seem to have a problem with documentation.  I have made some photos but not to many yet. Written descriptions----well  not yet. I can briefly describe what I have done wrt the SMT8 style verticals.     I think of the SMT8  in four parts.              1. the mechanical - boom, leaf spring, hinges.On my first sensor I tried as closely as possible to duplicate the the SMT8 geometry. The only change I made was to remove part of the leaf spring support where it attaches to the end pins so I could slightly squeeze the ends of the bent leaf spring together and quickly remove it from the assembly w/o tools.

Sounds interesting on the mechanical change, although I don't
totally understand it.
 

On the second SMT8 I reproduced the first sensor but made the boom about 10 " long to try to make it more portable (for possible field use in an ammo box).              2. the displacement sensorIn the first sensor,  I installed a homemade LVDT with phase shift oscillator and the demodulator that Karl Cunningham has described.  In the second sensor I installed the VRDT hardware as STM described but with phase shift oscillator, instrumentation amp to sum and Karl's demodulator. Both work well and are quite linear. Side note: I calibrate the displacement sensors with a "diving board" type device that I calibrated with precision weights. I found it was easier to apply a precise weight than a small deflection directly. All the circuitry for either can be crammed on a 2X6.5 size breadboard. Not too complicated of circuitry, mostly just dual opamps.

Most interesting on the LVDT switch from the LRDT original design
on one of your instruments!  Am not really "up" to either; but its
interesting on the switching sensor capability you've already done
and the success.

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 layed device.  Yet.....according to Seans website, his
LRDT is sensitive to a incredible ..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.  Interesting.

 
               3. the  feedback circuitryI used the Mathcad equations STM suggested.              4. mechanical adjustment control.There are some adjustments I found necessary as STM had described in depth.  The vertical is not as sensitive to tilting but is sensitive to temperature and the initial relaxation of the leaf spring. For temperature correction I installed a bimetal thermometer coil near the boom. With some trial and error positioning and thermal insulation I was able to remove a substantial amount of this temperature variation. For zero adjustment I used the Edmond Scientific low rpm motor and threaded rod and sliding  weight. Works  well.

Quite a nice additon on the bi-metal thermometer coil!

Take care, Meredith Lamb


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