## PSN-L Email List Message

Subject: Re: folded pendulum
From: tchannel1@............
Date: Fri, 29 Jun 2007 21:38:17 -0600

```Hi Chris,  All good suggestions!    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
----- Original Message -----=20
From: ChrisAtUpw@..........
To: psn-l@.................
Sent: Friday, June 29, 2007 6:45 PM
Subject: Re: folded pendulum

In a message dated 2007/06/29, tchannel1@............ writes:

Hi Folks, I am still experimenting with the basic structure of the =
folded Pendulum linkage. One normal pendulum, one inverted pendulum and =
a horz connecting arm.

1 I am assuming the length of the two pendulums should be the same =
and as long a practical.  I am thinking of 12 to 18" which would make =
the sensor about 24" to 36" tall.  I may go shorter........

Hi Ted,

It very much depends on what maximum period you want? This is =
often 25 to 30 seconds for amateur equipment?

I suggest that the shortest you consider should be 1 second at =
about 9.8". Above that, what is the maximum framesize that you can =
easily make? Maybe 2 ft square with 3" U channel? With your trial setup, =
you got a period extension of x10. You could probably get x20 fairly =
easily.

2 I gather the length of the horz connecting arm, should be a long a =
practical.  In that, a longer arm would allow more room along the horz =
plane, on which to tweak the balance.  A too short arm, would make the =
balance point too small.
I would be using a fine threaded rod with a threaded mass and a =
threaded smaller mass, for extra fine adj.

I suggest that you make the horizontal connecting arm out of =
brass to form the main mass. Then mount a small 'trim' mass just off =
centre. I don't think that the horizontal arm needs to be more than the =
pendulum length. You are critically dependant on the position of the =
mass to extend the period. Mount the main mass rgidly fixed. Don't mount =
it on threaded rod.

3 Would such a device still require the same type of enclosure as a =
Lehman or Vertical?

Yes. It may be convenient to use 1/8" or 3/16" Al alloy plate =
to form the two sides and connect them together with either square rod =
or U channel to support the arms and to give an enclosed box structure. =
Maybe cut a hole in the centre of one plate to mount and adjust the =
sensor and damping components and provide a cover plate?

4 Would the size of the mass be critical?  I would be using brass or =
SS shim stock at all points for hinges.

Don't use Brass shim. SS or bronze should be fine. You are very =
dependant on getting good bearings to be able to extend the period. The =
load bearing capacity of the shim stock should be considered when =
deciding on the mass. Don't go above 1/10 the max. rated load stress. =
Consider the handling stresses. Don't make the arms very long and narrow =
- max about 5:1, maybe less? Design in stops to prevent the arm moving =
more than about 1/2" either way.

Regards,

Hi Chris,  All good=20
suggestions!    If I understand you would use =
the Arm for=20
the Mass or the Arm also acts as the mass,
With the mass already centered, you =
would only need=20
a small amount of mass on a threaded rod to tweak the =
period?
Thank You, Ted

----- Original Message -----
From:=20
ChrisAtUpw@.......
To: psn-l@..............
Sent: Friday, June 29, 2007 =
6:45 PM
Subject: Re: folded =
pendulum
In a=20
message dated 2007/06/29, tchannel1@............=20
writes:
Hi Folks, I am still experimenting with the basic =
structure of=20
the folded Pendulum linkage. One normal pendulum, one inverted =
pendulum and=20
a horz connecting arm.1 I am assuming the length of the two pendulums =
should be the=20
same and as long a practical.  I am thinking of 12 to 18" which =
would=20
make the sensor about 24" to 36" tall.  I may go=20
shorter........Hi=20
Ted,       It very much depends =
on what=20
maximum period you want? This is often 25 to 30 seconds for amateur=20
equipment?       I suggest that =
the=20
shortest you consider should be 1 second at about 9.8". Above that, =
what is=20
the maximum framesize that you can easily make? Maybe 2 ft square with =
3" U=20
channel? With your trial setup, you got a period extension of x10. You =
could=20
probably get x20 fairly easily.
2 I=20
gather the length of the horz connecting arm, should be a long a=20
practical.  In that, a longer arm would allow more room along =
the horz=20
plane, on which to tweak the balance.  A too short arm, would =
make the=20
balance point too small.I would be using =
a fine=20
threaded rod with a threaded mass and a threaded smaller mass, for =
extra=20
suggest that you make the horizontal connecting arm out of brass to =
form the=20
main mass. Then mount a small 'trim' mass just off centre. I don't =
think that=20
the horizontal arm needs to be more than the pendulum length. You are=20
critically dependant on the position of the mass to extend the period. =
Mount=20
the main mass rgidly fixed. Don't mount it on threaded =
rod.
3 Would such a device still require the same type of =
enclosure=20
as a Lehman or Vertical?       Yes. It=20
may be convenient to use 1/8" or 3/16" Al alloy plate to form the two =
sides=20
and connect them together with either square rod or U channel to =
support the=20
arms and to give an enclosed box structure. Maybe cut a hole in the =
centre of=20
one plate to mount and adjust the sensor and damping components and =
provide a=20
cover plate?
4 Would the size of the mass be critical?  I would =
be using=20
brass or SS shim stock at all points for hinges.       Don't use =
Brass shim.=20
SS or bronze should be fine. You are very dependant on getting good =
bearings=20
to be able to extend the period. The load bearing capacity of the shim =
stock=20
should be considered when deciding on the mass. Don't go above 1/10 =
the max.=20
rated load stress. Consider the handling stresses. Don't make the arms =
very=20
long and narrow - max about 5:1, maybe less? Design in stops to =
prevent the=20
arm moving more than about 1/2" either=20
way.      =20
Regards,       Chris =
Chapman=20

```

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