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From: meredith lamb mlamb1@..........
Date: Sun, 13 Feb 2000 19:19:44 -0800

RADIOTEL@....... wrote:

> Meredith, et all
> I recently visited Meredith Lambs's web page and viewed his SG seismo with a
> hall effect sensor.  Is the SG with hall effect sensor as sensitive as the
> original SG configuration?

Theres alot of variables here that determine sensitivity.  Technically
I've never tried to measure it on a actual operating SG.  Earlier
methods used by Bob Lamb showed variations of roughly 25mv on up too ~
40+mv per each .001", which is dependent on the gain of the amplifier
AND, how close the Hall is too the center of the 4 pole magnet/s used. 
The Hall device itself has a amplifier which I'am guessing the gain is
about 350.  The circuit on the web site and I use has a gain of 5 which
with the amplifier gain ends up around ~1800.  I have no idea of how
sensitive the capactive model of SG really is; but I wouldn't be
surprised if the Hall versus the capacitive model isn't fairly similar.
The Hall device is technically overdriven in the circuit, however the
linearity is still (?+ - 5%?) very good for this use.

> What is the period range for an SG with hall
> effect sensors?

The physical period is always going to be short, as they are simply a
hanging pendulum.  However, they are subject to seismic wave tilt like
any seismometer, and here is where with the amplifier and its "period"
adjusting capacitors, one can pick what they want.  Betcha this is
confusing to alot of people.  The Hall device can be operated in its
Tilt mode (relatively linear displacement) or, Frequency mode (with its
capacitor, resistor bleed off function).  The circuit on the web site
uses both aspects, one for mechanical adjustment, and the other
frequency mode for normal operation with a null voltage point.  At the
moment, I use 30mfd mylar caps on the amplifiers; which roughly equates
to some 30 seconds max.  The output roughly matches the output of the
Sprengnether coil & magnet affairs I also operate, and compare too. 
So....while the physical period is some 1.05s, the actual seismic wave
tilt of the seismometer will be followed, up to the value of the
capacitors used.  I don't think any higher than 30mfd is really

>Is the electronics easier to make ( it certainly seems so).

Quite abit easier, but, some of the components like the zenors, mylar
caps, and precision pots used could rack up the cost...but, it depends
on what you want to financially settle for.  Being totally biased for
the Hall, it is probably the simpliest, and cheapest route to a
non-precision seismometer I've ever seen.  I got virtually all the parts
via surplus outlets cheaper than commercial outlets prices. One
important note...I'am not very technically qualified for much more 
than simple details, nor has any effort been made to "go for any max"
seismo output, nor any extensive followup on alot of specifics.

Operationally the 2 Hall S-G's I have been working well over a 
year now.  Once the pier and mechanics settle down, the adjustments
have been infrequent, and often done via the amplifier which
is away from the seismometers. 

The hairest part of building the seismometer is the .002" brass
hinges and installing them without crimping/bending them.  They
need to be thin to flex.  Thicker hinges didn't work with the
~3.5 pound mass.  None have broken from the weight yet.

The eddy current damping of the mass is the most unorthodox
feature I use.  Its probably overdamped but it works well.  
This is probably the most difficult part to simulate, but with
neodymium magnets and certain aluminium, or copper plates it
can be done.  Its probably easier to do so on a hanging
pendulum like this as the movement is so slight, than if it
were a normal gatehinge type seismo.

The Hall S-G's don't seem to have the range as the Sprengnether
coil and magnet affairs.  To be fair though, the Sprengnethers
do have huge signal generating coil magnets.  Being as I have
to operate in a city environment....they do very well overall.

Big quakes can saturate the Halls output on the SDR program.
Roughly 4000 bits is about the limit with the Allegro 3515,
but the 3503 could possibly extend this limitation to perhaps
twice that capacity; as it has roughly half the sensitivity.  

The Hall circuit shouldn't be construed as being only useful
in a S-G.  I've used one on a horizontal Sprengnether for a
day or two, and with the Allegro 3503, it did well, as a
additional output besides only the coil/magnet output.  For
its lesser movement range, the seismometer has to be 
mechanically stable and on a seasoned pier.  For all 
practical purposes the Hall circuit could even serve as a
zero point indicator with other sensors on the same seismo;
if one wished to do so.

Robert Lamb did very well developing this circuit; its a
very good one.

> Any comments anyone has would be appreciated.
> Jim Allen
> Cerritos, California

Take care,

Meredith Lamb


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