From: Dave Nelson 
To: psnlist 
Sent: Sat, 6 Aug 2011 23:47
Subject: Re: new seismo


hi guys
**** Am still getting nowhere with this system  !!  :(
**** Rebuilt the dampening system and it works well for the=20
natural period of the seismo. That is, have the boom swinging=20
naturally, ~ 7 sec period. Slide up the dampening magnets to=20
the aluminium vane and the natural osc stops within a cycle.=20
That part looks ok.
.
Hi Dave,=20
    I would not call getting your damper actually operating, from=20
being totally useless, exactly 'going nowhere' ?
    Now you need to increase the period to about 25 seconds and re=20
set the damping to 0.7 critical. A period of 7 seconds is only=20
useful if you just want to observe microseisms ! You need to be=20
able to pick up the ~20 second surface waves at their full=20
amplitude.
.

    To set the damping accurately, I stick a label with a vertical=20
line on it onto the mass and view the line with a magnifying glass=20
through a transparent plastic ruler. I deflect the mass 10 mm and=20
release it. The mass moves back toward the balance point and=20
usually past it. I check how far it moves past the balance point.=20
I increase / decrease the damping until the mass goes 1/2 mm past=20
the balance point and then moves slowly back to zero without any=20
oscillations. The arm is then set to 0.7 critical damping. The=20
amount of damping needed decreases as the set period is increased.
   =20

****The problem is when viewed on SDR there is an ongoing osc of ~ 10 Hz=20
that slowly dies off over another ~ 30 sec till the trace is basically=20
latlining. (of course you can't see the 10Hz osc visibly.)
***With the dampening in place, the seismo is VERY responsive to=20
hi freq (>1Hz) noise and any of this noise, walking around the=20
house near the seismo, vehicular traffic out on the road, etc etc=20
just shows up as a large amplitude of this 10Hz. osc that then dies=20
off back to the flatline ( + - a small data count).
****If I remove the sensor coil from the arm there is NO ~ 10 Hz osc=20
visible. That sort of indicates that its not some extraneous signal=20
being picked up. But on the other hand, I can't imagine how a=20
supposed long period device is even able to osc at ~ 10 Hz ??
 pic of the Lehman http://www.sydneystormcity.com/IMGP2023a.jpg

city.com/IMGP2023a.jpg

     Can you show us a new photo of the modified Lehman, please ?=20
    As shown, the coil will be very sensitive to vertical motions=20
of the mass. You can eliminate most of this with a quad magnet in=20
a soft iron frame. GUESS : the oscillations are coming from the=20
vertical motion of the mass bending the long baseplate - you have=20
something like a 'tuning fork' construction.
    If you have only one magnet disk, or one top and bottom, you=20
need to offset the edge of the magnets to the centre of the coil.=20
Otherwise you WON'T get positive and negative signals from the=20
coil - only same polarity signals for movements in BOTH directions,=20
with a null when the magnets are centralised !!=20
.
    Regards,
    Chris Chapman






























=
From: Dave Nelson <dave.nelson@...............>

To: psnlist <psnlist@..............>

Sent: Sat, 6 Aug 2011 23:47

Subject: Re: new seismo





hi guys

**** Am still getting nowhere with this system  !=
!  :(

**** Rebuilt the=
 dampening system and it works well for the 
natural period of the =
seismo. That is, have the boom swinging 
<=
FONT size=3D2>naturally, ~ 7 sec period. Slide up the dampening magnets to 
the<=
/FONT> aluminium vane and the natural osc stops within a cycle. 
That part looks ok.
.
Hi Dave, 
    I would not call getting your damper actuall=
y operating, from 
being totally useless, exactly 'going nowhe=
re' ?
   =
 Now you need to increase the period to about 25 seconds and re 
set the damping to =
0.7 critical. A period of 7 seconds is only 
useful if you just want to observe micr=
oseisms ! You need to be 
able to pick up the ~20 sec=
ond surface waves at their full 
amplitude.
.
=
    To set the damping accurately, I stick a label wi=
th a vertical 
line on it onto the mass and view the line with a magnifying glass 
through a transparent plastic ruler.=
 I deflect the mass 10 mm and 
release it. The mass moves back toward the balance point and 
usually past =
it. I check h=
ow far it moves past the balance point. 
I increase /=
 decrease the damping until the mass goes 1/=
2 mm past 
the balance point and then moves slowly back to zero without any 
oscillations. The arm is then set to 0.7 c=
ritical damping. The 
amount of damping <=
TT>needed decreases as the set period is increased.
<=
FONT size=3D3>    
****The problem is when viewed on SDR there is an ongoi=
ng osc of ~ 10 Hz 
that slowly dies off over =
another ~ 30 sec till the trace is basically=
=20
flatlining. (of course you can't see the 10Hz osc visibly.)
****With the dampening in place, the seismo is VERY responsive to 
=
hi freq (>1Hz) noise and any of this =
noise, walking a=
round the 
house near the seismo, vehic=
ular traffic out on the road, etc etc 
just shows=
 up as a large amplitu=
de of this 10Hz. osc that then dies 
off <=
FONT size=3D2>back to the flatline ( + - a small data count).

****If I remove =
the sensor coil from the arm there is NO ~ 10 Hz osc 
visible. That=
 sort of indicates that its not <=
FONT size=3D2>some extraneous s=
ignal 
be=
ing picked up. =
But on the other hand, I can't imagine how a 
supposed l=
ong period device <=
TT>is even able<=
/FONT> to osc at=
 ~ 10 Hz =
??
A pic of the Lehman http://www.sydneystormcity.com/IMGP2023a.jpg


     Can you show us a new photo of t=
he modified Lehman, please ? 
    As shown, the coil will be very sensitive to vertical motio=
ns 
of the mass. Y=
ou can eliminate most of this with a quad magnet in 
a soft iron frame. GUESS : the oscillati=
ons are coming from the 
vertical motion of the mass bending the long baseplate - you have 
something like a 't=
uning fork' construction.
    If you have only one magnet disk, or one top and bottom, you 
need to offset the =
edge of the magnets to the centre of the coil. 
Otherwise you WON'T get positive and negative=
 signals from the 
coil - only same polarity signals for movements in B=
OTH directions, 
w=
ith a null when the magnets are centralised !! 
.
<=
FONT size=3D2>    Regards,
    Chris Chapman