In a message dated 31/05/2009, rog@.......... writes:

What is  the thinking on the optimal position of the magnetic damper (MD) 
along the  lower boom?
It seem by shifting the MD position towards or away from the  lower pivot 
one can tune the damping strength.



Hi Rob,
 
    You need to have as low a vertical force on the  bottom suspension as 
practicable. I suggest that you put the mass on the end of  the arm and then 
slide the arm over a round horizontal rod until the weight of  the mass on 
one side balances the weight of the arm on the other. Mark this  point to 
attach the top suspension. Check the photos at 
_http://jclahr.com/science/psn/chapman/school/MKII/index.html_ 
(http://jclahr.com/science/psn/chapman/school/MKII/index.html)  
    You need the damping fairly close to the mass, but  this is not 
critical. But the damping plate DOES need to be lie on the  OFFSET line joining the 
centre of mass to the bottom suspension, hence the  offset mounting hole in 
the mass. This prevents a quake motion from trying  to rotate the mass as 
well as pushing it to one side. Please note that the  next modification was 
to fit the sensor block with a V wire top suspension. See  
_http://jclahr.com/science/psn/chapman/2008%20lehman/index.html_ 
(http://jclahr.com/science/psn/chapman/2008%20lehman/index.html)  This  30 lb fishing trace with looped 
ends worked very well indeed, preventing any  rotation of the mass around the 
long axis of the arm. 
    Have a look at the damper drawings at  
_http://jclahr.com/science/psn/chapman/lehman/index.html_ 
(http://jclahr.com/science/psn/chapman/lehman/index.html)  These  use a quad NdFeB magnet block. You only have to slide the 
block about 1/2" to  3/4" over the 1" magnets to get the correct damping. You 
adjust the separation  of the top and bottom mild steel backing plates to 
set the damping in this  range. But you also need to be able to remove the 
damping easily when setting  the period and to set the damping afterwards to 
0.7 critical, hence this layout  where the damping block simply slides along 
the baseplate. 
    This is a good, compact and easily adjusted design.  I can set a period 
of 30 seconds, no problem. The trimming of the  height of the arm, setting 
up the period and then setting the damping are all  sequential and non 
interacting adjustments.
 
    Regards,
 
    Chris Chapman






In a message dated 31/05/2009, rog@.......... writes:
What is 
  the thinking on the optimal position of the magnetic damper (MD) 
alo=
ng the 
  lower boom?
It seem by shifting the MD position towards or away from=
 the 
  lower pivot 
one can tune the damping strength.


Hi Rob,
 
    You need to have as low a vertical force on=
 the 
bottom suspension as practicable. I suggest that you put the mass on the=
 end of 
the arm and then slide the arm over a round horizontal rod until the weigh=
t of 
the mass on one side balances the weight of the arm on the other. Mark thi=
s 
point to attach the top suspension. Check the photos at http=
://jclahr.com/science/psn/chapman/school/MKII/index.html 
    You need the damping fairly close to the mass=
, but 
this is not critical. But the damping plate DOES need to be lie on th=
e 
OFFSET line joining the centre of mass to the bottom suspension, hence the=
 
offset mounting hole in the mass. This prevents a quake motion from=
 trying 
to rotate the mass as well as pushing it to one side. Please note tha=
t the 
next modification was to fit the sensor block with a V wire top suspension=
.. See 
 ht=
tp://jclahr.com/science/psn/chapman/2008%20lehman/index.html This=
 
30 lb fishing trace with looped ends worked very well indeed, preventing=
 any 
rotation of the mass around the long axis of the arm. 
    Have a look at the damper drawings at  <=
A 
href=3D"http://jclahr.com/science/psn/chapman/lehman/index.html">http://jc=
lahr.com/science/psn/chapman/lehman/index.html These 
use a quad NdFeB magnet block. You only have to slide the block about 1/2"=
 to 
3/4" over the 1" magnets to get the correct damping. You adjust the separa=
tion 
of the top and bottom mild steel backing plates to set the damping in this=
 
range. But you also need to be able to remove the damping easily when sett=
ing 
the period and to set the damping afterwards to 0.7 critical, hence this=
 layout 
where the damping block simply slides along the baseplate. 
    This is a good, compact and easily adjusted=
 design. 
I can set a period of 30 seconds, no problem. The trimming of th=
e 
height of the arm, setting up the period and then setting the damping are=
 all 
sequential and non interacting adjustments.
 
    Regards,
 
    Chris Chapman