PSN-L Email List Message
Subject: Re: Long Period Pendulums
From: ChrisAtUpw@.......
Date: Mon, 25 Feb 2008 23:20:34 EST
In a message dated 2008/02/23, tchannel1@............ writes:
> Hi Folks, http://www.phy.mtu.edu/~suits/PH3110/pendulums.html This is a
> description of different long period pendulums. My goal in building sensors
> is to obtain the long period, like 20 seconds, In most cases I have found
> this hard to do. I know it can be done, but even with the Lehman the setup to
> get to 20 second is, for me difficult. I usually give up at about 15
> seconds.
Hi Ted,
You WON'T get long periods unless you use a good suspension system and
magnetic damping. Knife edge and point in a cup suspensions will PREVENT you
from getting long periods.
> After thinking about the "Rocking Chair Pendulum" recently discussed, I
> saw a similarity between it and one of these Long period pendulums, so I went
> back to the shop and made another mock up of the "Nearly Balanced Pendulum"
> This time I took a little more time. Using a 48" dowel, I drilled a pivot
> hole in the center, inserted a finishing nail into the pivot hole, attached
> the nail to a table top, so the dowel would spin like a propeller. Next I
> threaded a 1/2" machine bolt, nut onto one end of the dowel. I timed the
> period at 2.5 seconds. I threaded another nut on to the opposite end of the
> dowel, this time one half the distance, from the end to the pivot. I timed the
> period at about 3.5 seconds. I keep moving the second nut closer to the
> end of dowel, to match the first nut which remained at the opposite end of the
> dowel. As I move the nut the period got longer and longer. When I reached
> the end of the dowel, I got a 20 second period, with very little effort.
>
> All this may be old news to some of you, but I found it to be amassing.
> Now I need to build another sensor, using this approach.
What do you want to achieve? You can get long periods this way, but it
is near useless as a seismic sensor.
If you drill the dowel and fit a horizontal round rod at right angles
rolling on two horizontal round rods, you can get very long periods with very
low hysteresis / offset.
Regards,
Chris Chapman
In a me=
ssage dated 2008/02/23, tchannel1@............ writes:
Hi Folks, http://www.phy.mtu.edu/~sui=
ts/PH3110/pendulums.html This is a description of different long p=
eriod pendulums. My goal in building sensors is to obtain the lo=
ng period, like 20 seconds, In most cases I have found this hard to do=
.. I know it can be done, but even with the Lehman the setup to g=
et to 20 second is, for me difficult. I usually give up at about=
15 seconds.
Hi Ted,
You WON'T get long periods unless you u=
se a good suspension system and magnetic damping. Knife edge and point in a=20=
cup suspensions will PREVENT you from getting long periods.
After thinking about the "Roc=
king Chair Pendulum" recently discussed, I saw a similarity between it and o=
ne of these Long period pendulums, so I went back to the shop and made anoth=
er mock up of the "Nearly Balanced Pendulum" This time I t=
ook a little more time. Using a 48" dowel, I drilled a pivot hole in t=
he center, inserted a finishing nail into the pivot hole, attached the nail=20=
to a table top, so the dowel would spin like a propeller. Next I=
threaded a 1/2" machine bolt, nut onto one end of the dowel. I=20=
timed the period at 2.5 seconds. I threaded another nut on to th=
e opposite end of the dowel, this time one half the distance, from the=
end to the pivot. I timed the period at about 3.5 seconds. =
; I keep moving the second nut closer to the end of dowel, to match th=
e first nut which remained at the opposite end of the dowel. &nbs=
p; As I move the nut the period got longer and longer. When I reached=20=
the end of the dowel, I got a 20 second period, with very little effort.
All this may be old news to some of you, but I found it to be amassing.&nbs=
p; Now I need to build another sensor, using this approach.
What do you want to achieve? You can g=
et long periods this way, but it is near useless as a seismic sensor.
If you drill the dowel and fit a horizo=
ntal round rod at right angles rolling on two horizontal round rods, you can=
get very long periods with very low hysteresis / offset.
Regards,
Chris Chapman
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