In a message dated 2007/06/04, tchannel@.............. writes:

> Hi Everyone, I wanted to ask some questions about choosing a spring for a 
> vertical spring sensor.
>  
>    Pictures three situations, three different springs being pulled straight 
> down by a mass. No triangular modifications, just straight down.
>  
> 1 One is a strong spring like a screen door spring, pulled down by a large 
> mass.
>  
> 2 One is a weak spring being pulled down by very little mass.
>  
> 3 One is a rubber band being pulled down by a small mass.
>  
> Just for comparisons, say they all had the same period of 1.5 seconds.  Is 
> one of the three better for recording earthquakes?  Two questions here: Is 
> there an advantage in using a weak spring, or strong spring, if the resulting 
> period is the same.  

    You will only get this sort of period with quite large extensions. You 
need a certain mass for the motion not to be swamped by thermal or environmental 
agitation. 

    It is the product M x T x Q, which is important for noise considerations, 
not just the mass. 

    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....

And Secondly is a rubber band spring every used?

       No. Rubber is highly temperature sensitive and it also has a lot of 
loss (hysteresis) associated with the extension.
 
> What prompted the question is seeing how small the spring in a geophones 
> is. Is the idea to have the smallest spring which would move under the smaller 
> stimulus.

       The coil springs used for seismometers have very specific 
characteristics. They are designed to have a zero or negative length when extended. The 
wire is twisted as it is wound and this makes the coils clamp tightly together. 
If you plot the length versus the load, the length stays constant up to a 
certain load and you then get an extension proportional to the load. A plot of the 
load / length graph can be extended back to zero load and the 'zero load 
extension' can be negative. You need a net zero length to get an 'infinite period' 
on a vertical seismometer.

       The changes in metal properties with temperature give a practical 
limit of about 5 seconds for ordinary steel springs and you may only have ~a 5 C 
Deg temperature range before re-balancing becomes necessary. The 'easy' way 
around this is to extend the period electronically, but the noise performance 
then becomes critical. A maximum period extension of about x10 max is 
practicable. It is not very difficult to extend a 2.5 second period sensor to 25 seconds. 
The use of Sm-Co or NdFeB magnets has enabled the output to be considerably 
increased over Alnico magnet systems, reducing problems with noise.

       Regards,

       Chris Chapman
          
In a me=
ssage dated 2007/06/04, tchannel@.............. writes:



Hi Everyone, I wanted to ask so=
me questions about choosing a spring for a vertical spring sensor.

 

   Pictures three situations, three different springs being pulle=
d straight down by a mass. No triangular modifications, just straight down.<=
/FONT>=


 

1 One is a strong spring like a screen door spring, pulled down by a large=20=
mass.

 

2 One is a weak spring being pulled down by very little mass.

 

3 One is a rubber band being pulled down by a small mass.

 

Just for comparisons, say they all had the same period of 1.5 seconds. =
; Is one of the three better for recording earthquakes?  Two questions=20=
here: Is there an advantage in using a weak spring, or strong spring, if the=
 resulting period is the same.  




    You will only get this sort of period with quite large ex=
tensions. You need a certain mass for the motion not to be swamped by therma=
l or environmental agitation. 



    It is the product M x T x Q, which is important for noise=
 considerations, not just the mass. 



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



And Secondly is a rubber band spring every used?



       No. Rubber is highly temperature sensit=
ive and it also has a lot of loss (hysteresis) associated with the extension=
..

 

What prompted the question is=20=
seeing how small the spring in a geophones is. Is the idea to have the small=
est spring which would move under the smaller stimulus.




       The coil springs used for seismometers=20=
have very specific characteristics. They are designed to have a zero or nega=
tive length when extended. The wire is twisted as it is wound and this makes=
 the coils clamp tightly together. If you plot the length versus the load, t=
he length stays constant up to a certain load and you then get an extension=20=
proportional to the load. A plot of the load / length graph can be extended=20=
back to zero load and the 'zero load extension' can be negative. You need a=20=
net zero length to get an 'infinite period' on a vertical seismometer.



       The changes in metal properties with te=
mperature give a practical limit of about 5 seconds for ordinary steel sprin=
gs and you may only have ~a 5 C Deg temperature range before re-balancing be=
comes necessary. The 'easy' way around this is to extend the period electron=
ically, but the noise performance then becomes critical. A maximum period ex=
tension of about x10 max is practicable. It is not very difficult to extend=20=
a 2.5 second period sensor to 25 seconds. The use of Sm-Co or NdFeB magnets=20=
has enabled the output to be considerably increased over Alnico magnet syste=
ms, reducing problems with noise.



       Regards,



       Chris Chapman