## PSN-L Email List Message

Subject: Re: Channel, 1,2 and 3?
From: tchannel1@............
Date: Thu, 27 Mar 2008 10:52:06 -0600

Chris,  This explains it clearly to me.  Thanks, again
Ted
----- Original Message -----=20
From: ChrisAtUpw@..........
To: psn-l@.................
Sent: Thursday, March 27, 2008 9:48 AM
Subject: Re: Channel, 1,2 and 3?

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. =20
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.=20
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,  This explains it clearly to me.  Thanks, = again
Ted
----- Original Message -----
From:=20 ChrisAtUpw@.......
Sent: Thursday, March 27, 2008 = 9:48=20 AM
Subject: Re: Channel, 1,2 and = 3?

In a message dated 27/03/2008, tchannel1@............ = writes:
I think I understand.  However could someone explain why = ch1 works=20 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"=20 above the pivot, similar to an airplane propeller.  It has an=20 adjustable mass which can go up or down the shaft above the pivot to = extend=20 the period......and this it does very well, as I can get very long = periods=20 up to 53 seconds.
Hi Ted,

As the ground moves horizontally, the = mass on Ch1=20 hardly moves at all, so there is a large difference between the = motions which=20 you sense with the magnet + coil.
However, the centre of mass of Ch3 is = close to=20 the same height as the suspension. Horizontal vibrations in the=20 Earth move the whole pendulum + mass + sensor almost equally = so=20 there is very little 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 stairs=20 will all tend to generate rotational motion well as translational=20 motion.
As I said, the nearly balanced pendulum = can have=20 a long period and can make a fairly good clock, but it  has very = little=20 sensitivity to ground motions.

Regards,

Chris=20 Chapman