PSN-L Email List Message

Subject: Re: Geophone questions
From: ChrisAtUpw@.......
Date: Sun, 27 May 2007 21:58:27 EDT

```In a message dated 2007/05/27, tchannel@.............. writes:

> I have a few questions about magnets and coils:
>
> 1  I have seen a cross section of a Geophone, magnet, spring and mass etc.
> When used in a Seismic Sensor, how is it dampened, or is it do
> electronically?

Hi Ted,

Geophone coils are usually wound on a copper former, but this usually
gives only maybe 40% of the critical induced damping required. It is usual to
solder a carefully chosen resistor across the output terminals, which adds the
rest of the damping. PSN seismic amplifiers have a 10 K input resistor fitted
already and this needs to be taken into account when calculating the value of
the required resistor.

>  2 I am making a new vertical sensor, and this question has to do with the
> coil and magnet.  The coil is a spool with a 1" dia. hole in the center.  The
> thickness of the coil is 3/4"   The value of the coil is 500ohms.   The
> magnet will be suspended from a rod and centered in this 1" hole. Left to Right
> and Top to Bottom.
>
> I can choose from three magnets.  One magnet 1/4" thick x 3/4" dia. centered
> leaving 1/4" of coil above and below the magnet......... OR Two magnets 1/4"
> thick, together to equal 1/2" x 3/4", leaving 1/8" of coil above and below
> the two magnets when centered........... Lastly I could use One magnet 1/2"
> thick x 3/4" dia. leaving 1/8" above and below.
>
> This is a basic question about the magnet's surface and the N/S poles,  I do
> not know the science, but just curious about just these three scenarios,
> which arrangement is the best?.....I could try all three and measure the result,
> but in theory is it better one way or the other?

Putting a magnet on a seismometer arm will enable you to pick up
transients from the building Utility Power Wiring, passing trucks and cars, changes
in the Earth's magnetic field.... If you really want too monitor any of
these, OK, otherwise put the coil on the arm and the magnet on the frame ! !

None of them will be satisfactory. The centralised magnet position will
give you an output minimum.

Consider how a loudspeaker is made. You have an external cylindrical
magnet with poles at either end. One end A has a flat iron plate with a large
hole in it. The other end B is fitted with another flat plate with a central
solid iron rod which projects a xially to the top of the hole in the plate A.
The coil moves in the radial annular field between plate A and the central iron
rod.

Can you get a mild steel tube which will surround the coil, a flat
1/4" mild steel plate and a stub of 3/4" mild steel rod with flat machined ends?
Lap the end of the tube flat with emery paper on the plate. Sit the magnets on
top of the rod and the rod on the centre of the plate. Put the iron tube
centrally around the magnets and lower in the coil so that the top face of the
magnets is about half way down it.
Alternatively, get three strips of 1/16" mild steel, bend them in a U
shape and mount the mild steel column in the centre, with the strips at 120
degrees. Cut the strips to length so that when stacked they end level with the
top of the magnets. You could also use a taller stack of magnets, but 2 off
1/4" ones won't reach the centre of the coil and allow for any free motion. You
will probably need to stick the strips together with acrylic glue to form a
rigid cup structure. Then give it a coat of anti rust paint.

Regards,

Chris Chapman
In a me=
ssage dated 2007/05/27, tchannel@.............. writes:

I have a few questions about ma=
gnets and coils:

1  I have seen a cross section of a Geophone, magnet, spring and mass=20=
etc.  When used in a Seismic Sensor, how is it dampened, or is it do el=
ectronically?

Hi Ted,

Geophone coils are usually wound on a c=
opper former, but this usually gives only maybe 40% of the critical induced=20=
damping required. It is usual to solder a carefully chosen resistor across t=
he output terminals, which adds the rest of the damping. PSN seismic amplifi=
ers have a 10 K input resistor fitted already and this needs to be taken int=
o account when calculating the value of the required resistor.

2 I am making a new vertical=20=
sensor, and this question has to do with the coil and magnet.  The coil=
is a spool with a 1" dia. hole in the center.  The thickness of the co=
il is 3/4"   The value of the coil is 500ohms.   The mag=
net will be suspended from a rod and centered in this 1" hole. Left to Right=
and Top to Bottom.

I can choose from three magnets.  One magnet 1/4" thick x 3/4" dia. ce=
ntered leaving 1/4" of coil above and below the magnet......... OR Two magne=
ts 1/4" thick, together to equal 1/2" x 3/4", leaving 1/8" of coil above and=
below the two magnets when centered........... Lastly I could use One magne=
t 1/2" thick x 3/4" dia. leaving 1/8" above and below.

This is a basic question about the magnet's surface and the N/S poles, =
; I do not know the science, but just curious about just these three scenari=
os, which arrangement is the best?.....I could try all three and measure the=
result, but in theory is it better one way or the other?

Putting a magnet on a seismometer arm w=
ill enable you to pick up transients from the building Utility Power Wiring,=
passing trucks and cars, changes in the Earth's magnetic field.... If you r=
eally want too monitor any of these, OK, otherwise put the coil on the arm a=
nd the magnet on the frame ! !

None of them will be satisfactory. The centra=
lised magnet position will give you an output minimum.

Consider how a loudspeaker is made. You=
have an external cylindrical magnet with poles at either end. One end A has=
a flat iron plate with a large hole in it. The other end B is fitted with a=
nother flat plate with a central solid iron rod which projects a xially to t=
he top of the hole in the plate A. The coil moves in the radial annular fiel=
d between plate A and the central iron rod.

Can you get a mild steel tube which wil=
l surround the coil, a flat 1/4" mild steel plate and a stub of 3/4" mild st=
eel rod with flat machined ends? Lap the end of the tube flat with emery pap=
er on the plate. Sit the magnets on top of the rod and the rod on the centre=
of the plate. Put the iron tube centrally around the magnets and lower in t=
he coil so that the top face of the magnets is about half way down it. =

Alternatively, get three strips of 1/16=
" mild steel, bend them in a U shape and mount the mild steel column in the=20=
centre, with the strips at 120 degrees. Cut the strips to length so that whe=
n stacked they end level with the top of the magnets. You could also use a t=
aller stack of magnets, but 2 off 1/4" ones won't reach the centre of the co=
il and allow for any free motion. You will probably need to stick the strips=
together with acrylic glue to form a rigid cup structure. Then give it a co=
at of anti rust paint.

Regards,

Chris Chapman
```