In a message dated 07/05/2009, jonfr@......... writes:

But the  arm that I was trying to use always fell off because of it's 
weight. So I  switched to the aluminum one. Using that arm now gives me 5 seconds 
easily and  a clear signal.
However, the signal is at interestingly narrow band, I am  unsure why that 
is.



Hi there,
 
    When deciding where to mount the top suspension on  a Lehman arm, 
balance the arm + mass on a round rod and slide it along until the  mass balances 
the weight of the rest of the arm. Mount the top suspension at  this point 
and you should get zero vertical loading on the bottom  suspension. This 
greatly reduces any tendency to slip while it is in  operation. Mount both 
balls / vertical rollers on the frame for greatly improved  stability.
 
    On a Lehman, the swing axis is at a very small  angle to the local 
vertical. Changing this angle alters the natural period.  Some values are shown 
at _http://jclahr.com/science/psn/swing/index.html_ 
(http://jclahr.com/science/psn/swing/index.html)    To get a period of 20 seconds instead of 10 
seconds, you need a swing angle 1/4  that in the table. The various sorts of 
pendulum are shown at _http://www.phy.mtu.edu/~suits/PH3110/pendulums.html_ 
(http://www.phy.mtu.edu/~suits/PH3110/pendulums.html)  See  the Lehman diagram.
 
    The swing angle theta is usually set using a  vertical screw on the 
base frame close to the mass, which tilts the whole  frame. Using a single 
frame enables you to set up the sensor clearance  first, then the period and 
finally the damping without the adjustments  effecting one another. 
 
    When you have set up the natural period, you then  need to set the 
damping to about 0.7 critical to get a broad band response. Note  the balance 
position and mark it using a fibre pen. Using a ruler or similar  lever, 
gently move the mass 10 mm to one side and then release it. The mass  should 
swing 1/2 mm beyond the balance position and then slowly return to the  balance 
position. Adjust the damping until you get about this response.
 
    If you use very little damping, the  sensor will give a narrow band 
response, peaked about it's  natural period, as Jon described. I use a ~1 mm 
thick sheet of Copper for the  damping blade, but you can also use pure 
Aluminum. See _http://jclahr.com/science/psn/chapman/lehman/index.html_ 
(http://jclahr.com/science/psn/chapman/lehman/index.html)  Note  that the damping 
blade is larger than the damping magnets and can be positioned  so that it only 
partly covers the central N/S magnet junction. The damping is  increased by 
moving the magnet block so that the damping plate covers more of  the N/S 
magnet junction. It is possible to use Alnico magnets as described  in the 
1979 Sci Am article, but NdFeB rectangular block magnets are  both cheaper and 
give a stronger field. 
 
    Mounting the damping plate horizontally as  shown in the photo makes it 
much easier to adjust the  damping. See photo 
_http://jclahr.com/science/psn/chapman/2008%20lehman/lehman_prototype.jpg_ 
(http://jclahr.com/science/psn/chapman/2008%20lehman/lehman_prototype.jpg)  The  left hand red damping 
magnet block partly covers the Copper damping plate. The  right hand red magnet 
block covers the sensor coil attached to the square Al  block on the arm. 
Note also the rigid V wire suspension made from 7 core SS  fishing trace and 
the wind on wire adjustment at the top of the vertical column.  This Lehman 
seems to be a good design. I can set it for stable periods of  up to 60 
seconds, but it then becomes very sensitive to ground  tilts. Photos of an 
earlier prototype before the top suspension was  modified are shown at 
_http://jclahr.com/science/psn/chapman/school/MKII/index.html_ 
(http://jclahr.com/science/psn/chapman/school/MKII/index.html)  
 
    Jon, if you look at the lehman_prototype photo, you  will see a 
vertical adjusting screw with a black knob on the right hand end of  the frame. 
This controls the tilt of the whole frame and sets the period. 
 
    Regards,
 
    Chris Chapman






In a message dated 07/05/2009, jonfr@......... writes:
But the 
  arm that I was trying to use always fell off because of it's weight. So=
 I 
  switched to the aluminum one. Using that arm now gives me 5 seconds easi=
ly and 
  a clear signal.
However, the signal is at interestingly narrow band,=
 I am 
  unsure why that is.


Hi there,
 
    When deciding where to mount the top suspensi=
on on 
a Lehman arm, balance the arm + mass on a round rod and slide it along unt=
il the 
mass balances the weight of the rest of the arm. Mount the top suspension=
 at 
this point and you should get zero vertical loading on the bottom 
suspension. This greatly reduces any tendency to slip while it is in=
 
operation. Mount both balls / vertical rollers on the frame for greatly im=
proved 
stability.
 
    On a Lehman, the swing axis is at a very smal=
l 
angle to the local vertical. Changing this angle alters the natural period=
.. 
Some values are shown at http://jclahr.com/=
science/psn/swing/index.html  
To get a period of 20 seconds instead of 10 seconds, you need a swing angl=
e 1/4 
that in the table. The various sorts of pendulum are shown at http://www.ph=
y.mtu.edu/~suits/PH3110/pendulums.html See 
the Lehman diagram.
 
    The swing angle theta is usually set using a=
 
vertical screw on the base frame close to the mass, which tilts the=
 whole 
frame. Using a single frame enables you to set up the sensor clearanc=
e 
first, then the period and finally the damping without the adjustment=
s 
effecting one another. 
 
    When you have set up the natural period, you=
 then 
need to set the damping to about 0.7 critical to get a broad band response=
.. Note 
the balance position and mark it using a fibre pen. Using a ruler or simil=
ar 
lever, gently move the mass 10 mm to one side and then release it. The mas=
s 
should swing 1/2 mm beyond the balance position and then slowly return to=
 the 
balance position. Adjust the damping until you get about this response.
 
    If you use very little damping, the 
sensor will give a narrow band response, peaked about=
 it's 
natural period, as Jon described. I use a ~1 mm thick sheet of Copper for=
 the 
damping blade, but you can also use pure Aluminum. See http://jc=
lahr.com/science/psn/chapman/lehman/index.html Note 
that the damping blade is larger than the damping magnets and can be posit=
ioned 
so that it only partly covers the central N/S magnet junction. The damping=
 is 
increased by moving the magnet block so that the damping plate covers more=
 of 
the N/S magnet junction. It is possible to use Alnico magnets as desc=
ribed 
in the 1979 Sci Am article, but NdFeB rectangular block magnets are 
both cheaper and give a stronger field. 
 
    Mounting the damping plate horizontally=
 as 
shown in the photo makes it much easier to adjust the 
damping. See photo http://jclahr.com/science/psn/chapman/2008%20lehman/lehman_prototy=
pe.jpg The 
left hand red damping magnet block partly covers the Copper damping plate.=
 The 
right hand red magnet block covers the sensor coil attached to the square=
 Al 
block on the arm. Note also the rigid V wire suspension made from 7 core=
 SS 
fishing trace and the wind on wire adjustment at the top of the vertical=
 column. 
This Lehman seems to be a good design. I can set it for stable period=
s of 
up to 60 seconds, but it then becomes very sensitive to ground 
tilts. Photos of an earlier prototype before the top suspension was=
 
modified are shown at http=
://jclahr.com/science/psn/chapman/school/MKII/index.html 
 
    Jon, if you look at the lehman_prototype phot=
o, you 
will see a vertical adjusting screw with a black knob on the right hand en=
d of 
the frame. This controls the tilt of the whole frame and sets the period.=
 
 
    Regards,
 
    Chris Chapman