PSN-L Email List Message

Subject: Re: folded pendulum
From: tchannel1@............
Date: Sat, 30 Jun 2007 06:51:42 -0600

```Chris,    This may be hard to answer.  In this case, the mass would only =
be  an amount required to do the job.
I have read that some people use very little mass, others a lot.  On a =
vertical and on a horz one gets the idea of how much to use.   This a =
bit different, and here is where I could use some direction.

1  If I added no mass, simply the connecting arm, made of the same =
material as the pendulums, and the same length or maybe longer.   This =
arm is the mass, and as such is not much of a stress on the SS? shims =
used as hinges.
My mock up was made like this.   Then I added a large mass, everything =
seemed to work the same. But using a large mass would  put a lot of =
strain on the hinges.

2  About the only advantage I can see with a large mass is that air =
currents would be less likely to move it.

3 It would be easy to construct it either way,  Perhaps no addition mass =
is too little, and an amount which stressed the hinges is too much?  Is =
1/2 that amount proper?    I understand your suggest about the 5% trim =
weight.

Thanks, Ted

----- Original Message -----=20
From: ChrisAtUpw@..........
To: psn-l@.................
Sent: Saturday, June 30, 2007 12:13 AM
Subject: Re: folded pendulum

In a message dated 2007/06/30, tchannel1@............ writes:

Hi Chris,  If I understand, you would use the Arm for the Mass or =
the Arm also acts as the mass.
With the mass already centered, you would only need a small amount =
of mass on a threaded rod to tweak the period?
Thank You, Ted

Hi Ted,=20

That is correct. Making the horizontal arm fairly heavy to act =
as the mass should give it a very high position stability and symmetry. =
There should be minimal thermal expansion problems. You would then just =
move the period trim mass, which might be only a few % of the total =
mass. A small error in the position of the trim mass should be tiny in =
comparison to the total mass.=20

The lateral forces from the two pendulums subtract, hence you =
need very high stability. I suggest that you first make the frame, the =
pendulums and the arm and then slide a trim block along the arm. You =
reduced. You will likely need to fit a magnetic damper from the start. =
Remember that the damping required decreases as the period increases.

If you make the main mass, the trim mass, the mounting fittings =
and the threaded adjuster all out of brass, all the thermal expansions =
should match. The Australians chemically machined a complete square form =
seismometer, including the flexures, out of a solid block of bronze to =
get very high dimensional stability.=20

There are several interesting papers on seismometers and =
flexures in the publications list at http://www.gravity.uwa.edu.au/ =
There used to be some on line both there and a LIGO in the USA.

Regards,

Chris,    This may be =
hard to=20
answer.  In this case, the mass would only be  an amount =
required to=20
do the job.
I have read that some people use =
very little=20
mass, others a lot.  On a vertical and on a horz one gets the idea =
of how=20
much to use.   This a bit different, and here is where I could =
use=20
some direction.

1  If I added no mass, simply the =
connecting=20
arm, made of the same material as the pendulums, and the same length or =
maybe=20
longer.   This arm is the mass, and as such is not much of a =
stress on=20
the SS? shims used as hinges.
My mock up was made like =
this.   Then I=20
added a large mass, everything seemed to work the same. But using a =
large mass=20
would  put a lot of strain on the hinges.

see with a=20
large mass is that air currents would be less likely to move =
it.

3 It would be easy to construct it =
either=20
way,  Perhaps no addition mass is too little, and an amount which =
stressed=20
the hinges is too much?  Is 1/2 that amount =
proper?    I=20

Thanks, Ted

----- Original Message -----
From:=20
ChrisAtUpw@.......
To: psn-l@..............
Sent: Saturday, June 30, 2007 =
12:13=20
AM
Subject: Re: folded =
pendulum
In a=20
message dated 2007/06/30, tchannel1@............=20
writes:
Hi Chris,  If I understand, you would use the Arm =
for the=20
Mass or the Arm also acts as the mass.With the mass =
centered, you would only need a small amount of mass on a threaded =
rod to=20
tweak the period?Thank You, TedHi Ted,=20
That is correct. Making =
the=20
horizontal arm fairly heavy to act as the mass should give it a very =
high=20
position stability and symmetry. There should be minimal thermal =
expansion=20
problems. You would then just move the period trim mass, which might =
be only a=20
few % of the total mass. A small error in the position of the trim =
mass should=20
be tiny in comparison to the total mass.=20
The lateral forces from =
the two=20
pendulums subtract, hence you need very high stability. I suggest that =
you=20
first make the frame, the pendulums and the arm and then slide a trim =
block=20
it could be reduced. You will likely need to fit a magnetic damper =
from the=20
start. Remember that the damping required decreases as the period=20
increases.       If you make the =
main=20
mass, the trim mass, the mounting fittings and the threaded adjuster =
all out=20
of brass, all the thermal expansions should match. The Australians =
chemically=20
machined a complete square form seismometer, including the flexures, =
out of a=20
solid block of bronze to get very high dimensional stability.=20
There are several =
interesting=20
papers on seismometers and flexures in the publications list at=20
http://www.gravity.uwa.edu.au/ There used to be some on line both =
there and a=20
LIGO in the USA.      =20
Regards,       Chris =
Chapman=20

```