PSN-L Email List Message
Subject: Re: Long Period Pendulums
From: "Geoff" gmvoeth@...........
Date: Mon, 25 Feb 2008 22:39:15 -0700
Is it possible to use a laser pointer and
surface reflective mirrors to
obtain a mechanical amplification
of motion then simply use a solar cell
as a pickup ?
You might locate a light source in a way
you can not locate any mechanical thing.
To get the most response with the least
mass getting in the way ?
Say using the laser pointer as the mass
and a power wire as the pendulum wire.
Something like that ??
Or the laser pointer attached to
the rod at the top of the pendulum ?
get the mirrors on each side so
you set up a bouncing beam of maybe
50 feet ?
The mirrors are the biggest problem
I bet quality surface silvered
mirrors are expensive.
----- Original Message -----
From:
To:
Sent: Monday, February 25, 2008 9:20 PM
Subject: Re: Long Period Pendulums
> 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
>
>
>
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