In a message dated 13/03/2010, matthew.zieleman@......... writes:

I've  been doing some conceptual design work on a vertical broadband
sensor, and  I have a few questions. By reading past archives of the
PSN-L I noticed  that Sean-Thomas Morrissey was a contributor to this
list, and there was  some discussion about his STM-8 vertical
leaf-spring  design.
Hi Matthew,
 
    Go to _http://psn.quake.net/infoequip.html_ 
(http://psn.quake.net/infoequip.html)  and  download Sean's file on the STM-8. There are also files on 
the St Louis Website  _http://www.eas.slu.edu/People/STMorrissey/index.html_ 
(http://www.eas.slu.edu/People/STMorrissey/index.html) 
 
    Also look at Erhard Wielandt's downloads on  seismometers. And
    
_http://www.geophys.uni-stuttgart.de/~erhard/downloads/textfiles/Seismometry/BroadbandDesign.doc_ 
(http://www.geophys.uni-stuttgart.de/~erhard/downloads/textfiles/Seismometry/BroadbandDesign.doc) 

When I  plug this all in I get a somewhat simpler expression.that what
Sean-Thomas  had. So I went back on including that fact that the
integrator pole is not  zero, and the back EMF generated in the
feedback coil, and wound up with  something way more complicated that I
am not going to type out here. Does  anybody have any hints?

2.) My second question is about the hinge. Is  foil suitable or should
I look at something like a knife edge or ball  bearing hinge? I'm
leaning strongly towards foil. How much of an impact  does the
flexibility of the foil really have on the period? Given that  the
spring constant of the main spring is probably much much  larger.


Foil is perfectly satisfactory, but avoid brass.  The spring in the foil 
has very little effect on the performance provided it is  flat and has a 
single flex.
 
    Avoid knife edge bearings like the plague !!
 
    The main problem in constructing a vertical  seismometer is how to 
compensate for the large temperature coefficient of the  spring. You are limited 
to periods of less than 6 seconds in practice with steel  springs. 
Otherwise the mechanism simply collapse as the instrument  temperature changes. You 
can use Elinvar of Ni-SpanC low coefficient alloys, but  obtaining / forming 
/ heat treating these are likely to give problems. 
    The second alternative is to use three component  force feedback to 
stabilise the operation. 
    The third option is to use a powerful coil + magnet  sensor and put 
this into a negative input impedance amplifier. This prevents the  arm from 
moving and you measure the feedback voltage required to  compensate. You get a 
voltage output proportional to frequency. 
    See _http://www.lennartz-electronic.de/PDF_documents/Seismometers.pdf_ 
(http://www.lennartz-electronic.de/PDF_documents/Seismometers.pdf)  
    The fourth option is to make a vertical with ~2  second period and use 
a low frequency boost amplifier to compensate for the  sensitivity decrease 
at greater periods. You can get a x10 period increase this  way with a coil 
+ magnet sensor, but you need low noise electronics. It may be  easier to 
use a position sensor followed by an integrator, but much more  electronics is 
required. This can be analogue, or digital as in the Volksmeter  on psn and 
the CMG-EDU from Guralp.
 
    Atmospheric noise tends to dominate the other  input noises of 
unshielded vertical sensors by a factor of up to x100.  Current commercial sensor 
technologies mostly seal small sensors inside metal or  glass containers. It 
is also possible to use a float to provide  compensation for a 'bare' sensor.
 
    I hope that this is of some help.
 
    Regards,
 
    Chris Chapman





In a message dated 13/03/2010, matthew.zieleman@......... writes:
I've=20
  been doing some conceptual design work on a vertical broadband
sensor=
, and=20
  I have a few questions. By reading past archives of the
PSN-L I notic=
ed=20
  that Sean-Thomas Morrissey was a contributor to this
list, and there=
 was=20
  some discussion about his STM-8 vertical
leaf-spring=20
design.
Hi Matthew,
 
    Go to http://psn.quake.net/infoequip.html and=20
download Sean's file on the STM-8. There are also files on the St Louis We=
bsite=20
http://w=
ww.eas.slu.edu/People/STMorrissey/index.html
 
    Also look at Erhard Wielandt's downloads on=
=20
seismometers. And
    http://www=
..geophys.uni-stuttgart.de/~erhard/downloads/textfiles/Seismometry/Broadban=
dDesign.doc
When I=20
  plug this all in I get a somewhat simpler expression.that what
Sean-T=
homas=20
  had. So I went back on including that fact that the
integrator pole=
 is not=20
  zero, and the back EMF generated in the
feedback coil, and wound up=
 with=20
  something way more complicated that I
am not going to type out here.=
 Does=20
  anybody have any hints?

2.) My second question is about the hinge=
.. Is=20
  foil suitable or should
I look at something like a knife edge or ball=
=20
  bearing hinge? I'm
leaning strongly towards foil. How much of an impa=
ct=20
  does the
flexibility of the foil really have on the period? Given tha=
t=20
  the
spring constant of the main spring is probably much much=20
  larger.


    Foil is perfectly satisfactory, but avoid bra=
ss.=20
The spring in the foil has very little effect on the performance provided=
 it is=20
flat and has a single flex.
 
    Avoid knife edge bearings like the plague !!<=
/DIV>
 
    The main problem in constructing a vertical=
=20
seismometer is how to compensate for the large temperature coefficient of=
 the=20
spring. You are limited to periods of less than 6 seconds in practice with=
 steel=20
springs. Otherwise the mechanism simply collapse as the instrument=20
temperature changes. You can use Elinvar of Ni-SpanC low coefficient alloy=
s, but=20
obtaining / forming / heat treating these are likely to give problems. 
    The second alternative is to use three compon=
ent=20
force feedback to stabilise the operation. 
    The third option is to use a powerful coil +=
 magnet=20
sensor and put this into a negative input impedance amplifier. This preven=
ts the=20
arm from moving and you measure the feedback voltage required to=20
compensate. You get a voltage output proportional to frequency. 
    See http://www.lennartz-electronic.d=
e/PDF_documents/Seismometers.pdf=20

    The fourth option is to make a vertical with=
 ~2=20
second period and use a low frequency boost amplifier to compensate for th=
e=20
sensitivity decrease at greater periods. You can get a x10 period increase=
 this=20
way with a coil + magnet sensor, but you need low noise electronics. It ma=
y be=20
easier to use a position sensor followed by an integrator, but much more=
=20
electronics is required. This can be analogue, or digital as in the Volksm=
eter=20
on psn and the CMG-EDU from Guralp.
 
    Atmospheric noise tends to dominate the =
other=20
input noises of unshielded vertical sensors by a factor of up to x100=
..=20
Current commercial sensor technologies mostly seal small sensors inside me=
tal or=20
glass containers. It is also possible to use a float to provide=20
compensation for a 'bare' sensor.
 
    I hope that this is of some help.
 
    Regards,
 
    Chris Chapman