PSN-L Email List Message

Subject: Re: Springs again
From: ChrisAtUpw@.......
Date: Tue, 17 Jul 2007 22:26:18 EDT


In a message dated 2007/07/17, Paulc@........ writes:

> My vertical sensor is the type with a lever arm that is 26" long with a 
> mass
> at one end.
> The other end of the lever arm has a pivot point about 2" from the end, and
> a spring on the end to a base plate.

Hi Paul,

       This is a bit difficult to visualise. Can you make a rough drawing 
using text characters?

> This arrangement certainly not a new idea, is new to me.,
> This arrangement yields a 1.1 approx. sec period.
> By careful adjustment of the pivot point and spring location, I can vary the
> period and the mass needed to achieve balance.

       You can get out to 4 or 5 seconds, but beyond this you need NiSpanC 
springs to keep the system stable with ambient temperature variations. However, 
you can extend this period either electronically or using software up to ~x10. 
See the Roberts circuit for geophones in psn references and on John Lahr's 
website. It may be easier to make a system for 2.5 seconds and then extend this 
to 25 seconds.
       The spring attachment point needs to be maybe 1/2" to 1" above the 
horizontal arm.

> 2 questions.
> 
> 1) What is the relationship between spring position, pivot location, and
> mass weight.
> As I would like to try to optimize this design , if indeed this is a valid
> path to follow.

       How good is your applied maths? This is a fairly simple triangle of 
forces problem. You usually start with a coil spring and adjust the arm, mass 
etc to suit.

       See the spring calculator at 
http://jclahr.com/science/psn/mcclure/springcalc/index.html and also 
http://jclahr.com/science/psn/mcclure/vert1/vert2.html

       Is your seismometer design anything like 
http://jclahr.com/science/psn/hill/index.html ? I would strongly advise you to use electromagnetic damping 
as opposed to oil.
 
       Have a look at http://quake.eas.gatech.edu/Instruments/LPVERT0.htm

> 2) If a rule of thumb....the longer the spring, the greater the period,
> within the obvious constraints applies.
> Has or why not has a vertical sensor been tried with a watch spring,
> mainspring,  arrangement been tried.

       This is just how long period verticals are designed, but using long 
triangular shaped leaf springs. Clock type coil springs have too many vibration 
modes. 'Mouse trap' type torsion coil springs have been used.

        For a simple spring, the extension E = g x T^2 / (2 x Pi)^2, where T 
is the period. Thus to get a period of say 10 sec, you need an extension of 
~25 metres....

> Sort of a torsion spring, but one with many turns and greater length.
> Perhaps my assumption that the main spring arrangement equates to a longer
> spring is faulty?

       No, but this cannot be extended very far, or you start to see the 
spring vibration modes. 

       You can also make a very good horizontal seismometer using a U tube 
water manometer.

       Regards,

       Chris Chapman   
In a me=
ssage dated 2007/07/17, Paulc@........ writes:

My vertical sensor is the type=20= with a lever arm that is 26" long with a mass
at one end.
The other end of the lever arm has a pivot point about 2" from the end, and<= BR> a spring on the end to a base plate.


Hi Paul,

       This is a bit difficult to visualise. C= an you make a rough drawing using text characters?


This arrangement certainly not=20= a new idea, is new to me.,
This arrangement yields a 1.1 approx. sec period.
By careful adjustment of the pivot point and spring location, I can vary the=
period and the mass needed to achieve balance.


       You can get out to 4 or 5 seconds, but= beyond this you need NiSpanC springs to keep the system stable with ambient= temperature variations. However, you can extend this period either electron= ically or using software up to ~x10. See the Roberts circuit for geophones i= n psn references and on John Lahr's website. It may be easier to make a syst= em for 2.5 seconds and then extend this to 25 seconds.
       The spring attachment point needs to be= maybe 1/2" to 1" above the horizontal arm.


2 questions.

1) What is the relationship between spring position, pivot location, and
mass weight.
As I would like to try to optimize this design , if indeed this is a valid path to follow.


       How good is your applied maths? This is= a fairly simple triangle of forces problem. You usually start with a coil s= pring and adjust the arm, mass etc to suit.

       See the spring calculator at http://jcl= ahr.com/science/psn/mcclure/springcalc/index.html and also http://jclahr.com= /science/psn/mcclure/vert1/vert2.html

       Is your seismometer design anything lik= e http://jclahr.com/science/psn/hill/index.html ? I would strongly advise yo= u to use electromagnetic damping as opposed to oil.

       Have a look at http://quake.eas.gatech.= edu/Instruments/LPVERT0.htm


2) If a rule of thumb....the lo= nger the spring, the greater the period,
within the obvious constraints applies.
Has or why not has a vertical sensor been tried with a watch spring,
mainspring,  arrangement been tried.


       This is just how long period verticals= are designed, but using long triangular shaped leaf springs. Clock type coi= l springs have too many vibration modes. 'Mouse trap' type torsion coil spri= ngs have been used.

        For a simple spring, the extensio= n E =3D g x T^2 / (2 x Pi)^2, where T is the period. Thus to get a period of= say 10 sec, you need an extension of ~25 metres....

Sort of a torsion spring, but o= ne with many turns and greater length.
Perhaps my assumption that the main spring arrangement equates to a longer spring is faulty?


       No, but this cannot be extended very fa= r, or you start to see the spring vibration modes.

       You can also make a very good horizonta= l seismometer using a U tube water manometer.

       Regards,

       Chris Chapman

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