## PSN-L Email List Message

Subject: Re: Channel, 1,2 and 3?
From: ChrisAtUpw@.......
Date: Thu, 27 Mar 2008 11:48:56 EDT

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In a message dated 27/03/2008, tchannel1@............ writes:

I think I understand.  However could someone explain why ch1 works  okay, and
ch3 does not work at all.

Ch1 is a 42" vertical pendulum, just hanging there like a clock pendulum,
same coil, magnet and hinge as the sensor in question.
Ch3 'THE LOUSY SENSOR' is a 36" below the pivot pendulum and about 36"  above
the pivot, similar to an airplane propeller.  It has an adjustable  mass
which can go up or down the shaft above the pivot to extend the  period......and
this it does very well, as I can get very long periods up to  53 seconds.

Hi Ted,

As the ground moves horizontally, the mass on Ch1  hardly moves at all,
so there is a large difference between the motions which  you sense with the
magnet + coil.
However, the centre of mass of Ch3 is close to the  same height as the
suspension. Horizontal vibrations in the Earth move the  whole pendulum + mass +
sensor almost equally so there is very little  difference for the magnet +
coil to pick up.
The pendulum has a large moment of inertia and  tends to stay still, so
if the Earth motion is rotational, you will sense a  large signal. Very local
quakes, nearby road works and you climbing the stairs  will all tend to
generate rotational motion well as translational motion.
As I said, the nearly balanced pendulum can have a  long period and can
make a fairly good clock, but it  has very little  sensitivity to ground
motions.

Regards,

Chris Chapman

In a message dated 27/03/2008, tchannel1@............ writes:
<=
FONT=20
style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size=
=3D2>
I think I understand.  However could someone explain why ch1 wor=
ks=20
okay, and ch3 does not work at all.

Ch1 is a 42" vertical pendulum, just hanging there like a clock pendu=
lum,=20
same coil, magnet and hinge as the sensor in question.
Ch3 'THE LOUSY SENSOR' is a 36" below the pivot pendulum and about 36=
"=20
above the pivot, similar to an airplane propeller.  It has an adjusta=
ble=20
mass which can go up or down the shaft above the pivot to extend the=20
period......and this it does very well, as I can get very long periods up=20=
to=20
53 seconds.

Hi Ted,

As the ground moves horizontally, the mass on C=
h1=20
hardly moves at all, so there is a large difference between the motions whic=
h=20
you sense with the magnet + coil.
However, the centre of mass of Ch3 is close to=20=
the=20
same height as the suspension. Horizontal vibrations in the Earth move=20=
the=20
whole pendulum + mass + sensor almost equally so there is very little=20
difference for the magnet + coil to pick up.
The pendulum has a large moment of inertia and=20
tends to stay still, so if the Earth motion is rotational, you will sense a=20
large signal. Very local quakes, nearby road works and you climbing the stai=
rs=20
will all tend to generate rotational motion well as translational motion.
As I said, the nearly balanced pendulum can hav=
e a=20
long period and can make a fairly good clock, but it  has very little=20
sensitivity to ground motions.

Regards,

Chris Chapman
```