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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