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 -----
Sent: Thursday, March 27, 2008 =
9:48=20
AM
Subject: Re: Channel, 1,2 and =
3?
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
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