Thanks, Chris for your reply. Most helpful

   I am in the process of building a sensor bases on your prototype, and =
great drawings.   I have most of the parts located, again with your help =
and others on the mailing list.  I will send pictures as I go along, if =
you like, and if you tell me how to send them.

I regards to the angle between the bottom and top hinge, a fraction of a =
degree, how do you measure for that? Or do you adjust and measure the =
period, instead?

Chris, I see on your drawing you did not need to brace the vertical arm =
of your device. Looks very strong.  How did you attach that vertical to =
the cross member?

Many Thanks, Ted


  ----- Original Message -----=20
  From: ChrisAtUpw@..........
  To: psn-l@.................
  Sent: Thursday, October 05, 2006 1:26 PM
  Subject: Re: Building a lehman seismometer


  In a message dated 2006/10/05, tchannel@.............. writes:


    1 Does your coil and damper assemblies just set on the aluminum =
base, free to move for adjustment, or are they fixed to the base?


  Hi Ted,

         My arm has a support wire fixture and then a mass right at the =
end of the arm.
         The support fixture carries a damping tongue in the direction =
of the hinge and a coil on the mass side.
         Both the sensor and damper magnets slide on the base strip, for =
mounting and adjustment of their position. They have small guide strips =
underneath which hold them parallel.  =20


    2 You cleaned the plates, which hold the magnets, and then painted =
the exterior, but not the surface the magnets touch.  Would the paint =
interfere with something on that surface?


         I suggest that you paint everything with Hammerite or similar, =
but try to get it smooth and level. I etch coated the magnet position =
with phosphoric acid, allowed it to react and dry overnight and then put =
CL anticorrosion car grease on it. I was not certain that I could get =
enough damping initially, but I found that I could overdamp the system =
quite easily, so I increased the damper magnet spacing.


    3 This is a hard question. Assuming I have finished my sensor, and =
levelled it, what is the procedure for adjusting the period? Maybe you =
could walk me through this procedure?


         You first lower the base plate / strip at the mass end. Then =
you mount the arm and make any necessary solder connections for the coil =
wiring. Then you put the magnet units on the base and adjust the top =
wire so that both the coil and the damping plate are central within the =
magnet spacings. The first time, you may need to make adjustments of the =
magnet block heights, so that the arm is parallel to the base.
         Then you slide the damper free and adjust the cross levels so =
that the arm is stable in the central position. If you can't get a =
stable central position, you need to adjust the suspension so that the =
top hinge is nearer the mass end - or the bottom fitting is further =
away. You may mark the end of the arm with an inkline and also put a =
central mark on the base. You deflect the arm and time a swing. It may =
be about 5 sec initially. Then you slowly raise the base at the mass end =
of the arm with the adjusting screw, testing to determine the period, =
until you get the period you want. As the period is lengthened, you will =
probably need to trim the cross level slightly.
         Then you slide the damping magnet over edge of the tongue, =
deflect the arm 10 mm and release it. It may help to stick a bit of =
graph paper to the base to be able the measure the position of the =
damping block. 0.7 damping is obtained when the arm swings 0.5 mm past =
the zero position and then falls back again.
         The amount of damping decreases as the set period increases, so =
it needs to be easily adjustable. If the critical position is obtained =
with the tongue less that half covering the magnets, increase the damper =
magnet spacing.=20


    4 I get confused with many of these terms, so I hope you understand =
this question.  If the target period is say 10 seconds, would the arm =
move, when pulled 10mm, very slowly, taking 10 seconds to complete one =
cycle?


         That is correct. Assuming that the arm is not damped, you =
deflect it and then release it. It takes 2.5 sec to get to zero, =
overshoots and comes back to zero in 5 sec, overshoots again and comes =
back to zero from the initial direction in another 5 sec. Total 5 + 5 =
=3D 10 sec. The period is the time for one complete oscillation cycle. =
You want at least twice this.


    5  After it is leveled and the period is set, Is it then that you =
can check the damp, by moving the arm 10mm and releasing it?


         Correct, you set up the period first and then you slide the =
damper further over the tongue in small steps until it is just a bit =
under critical.


        I know what an underdamped situation would look like.  But is =
there way to tell if it is over damped?


         As you increase the damping from zero, the time that the arm =
takes to fall back through / to the balance position increases slightly. =
If you overdamp the arm, it never swings through the zero position and =
the more you damp it, the longer it takes to get there. Look for the arm =
swinging just past the zero line, but without another oscillation.

         Regards,








Thanks, Chris for your reply. Most=20
helpful
 
   I am in the process of =
building a=20
sensor bases on your prototype, and great drawings.   I have =
most of=20
the parts located, again with your help and others on the mailing =
list.  I=20
will send pictures as I go along, if you like, and if you tell me how to =
send=20
them.
 
I regards to the angle between the =
bottom and top=20
hinge, a fraction of a degree, how do you measure for that? Or do you =
adjust and=20
measure the period, instead?
 
Chris, I see on your drawing you did =
not need to=20
brace the vertical arm of your device. Looks very strong.  How did =
you=20
attach that vertical to the cross member?
 
Many Thanks, Ted
 
 

  
----- Original Message ----- 
  From:=20
  ChrisAtUpw@....... 
  
To: psn-l@.............. 
  
Sent: Thursday, October 05, =
2006 1:26=20
  PM
  
Subject: Re: Building a lehman=20
  seismometer
  

In a=20
  message dated 2006/10/05, tchannel@..............=20
  writes:


  1 Does your coil and damper assemblies just set on the =
aluminum=20
    base, free to move for adjustment, or are they fixed to the=20
    base?

Hi=20
  Ted,

       My arm has a support =
wire=20
  fixture and then a mass right at the end of the=20
  arm.
       The support fixture =
carries a=20
  damping tongue in the direction of the hinge and a coil on the mass=20
  side.
       Both the sensor and =
damper=20
  magnets slide on the base strip, for mounting and adjustment of their=20
  position. They have small guide strips underneath which hold them=20
  parallel.   


  2 You cleaned the plates, which hold the magnets, and =
then=20
    painted the exterior, but not the surface the magnets touch.  =
Would the=20
    paint interfere with something on that surface?

       I=20
  suggest that you paint everything with Hammerite or similar, but try =
to get it=20
  smooth and level. I etch coated the magnet position with phosphoric =
acid,=20
  allowed it to react and dry overnight and then put CL anticorrosion =
car grease=20
  on it. I was not certain that I could get enough damping initially, =
but I=20
  found that I could overdamp the system quite easily, so I increased =
the damper=20
  magnet spacing.


  3 This is a hard question. Assuming I have finished my =
sensor,=20
    and levelled it, what is the procedure for adjusting the period? =
Maybe you=20
    could walk me through this procedure?

       You=20
  first lower the base plate / strip at the mass end. Then you mount the =
arm and=20
  make any necessary solder connections for the coil wiring. Then you =
put the=20
  magnet units on the base and adjust the top wire so that both the coil =
and the=20
  damping plate are central within the magnet spacings. The first time, =
you may=20
  need to make adjustments of the magnet block heights, so that the arm =
is=20
  parallel to the base.
       Then you =
slide=20
  the damper free and adjust the cross levels so that the arm is stable =
in the=20
  central position. If you can't get a stable central position, you need =
to=20
  adjust the suspension so that the top hinge is nearer the mass end - =
or the=20
  bottom fitting is further away. You may mark the end of the arm with =
an=20
  inkline and also put a central mark on the base. You deflect the arm =
and time=20
  a swing. It may be about 5 sec initially. Then you slowly raise the =
base at=20
  the mass end of the arm with the adjusting screw, testing to determine =
the=20
  period, until you get the period you want. As the period is =
lengthened, you=20
  will probably need to trim the cross level slightly.
      =20
  Then you =
slide the=20
  damping magnet over edge of the tongue, deflect the arm 10 mm and =
release it.=20
  It may help to stick a bit of graph paper to the base to be able the =
measure=20
  the position of the damping block. 0.7 damping is obtained when the =
arm swings=20
  0.5 mm past the zero position and then falls back=20
  again.
       The amount of damping =
decreases=20
  as the set period increases, so it needs to be easily adjustable. If =
the=20
  critical position is obtained with the tongue less that half covering =
the=20
  magnets, increase the damper magnet spacing. 


  4 I get confused with many of these terms, so I hope you =

    understand this question.  If the target period is say 10 =
seconds,=20
    would the arm move, when pulled 10mm, very slowly, taking 10 seconds =
to=20
    complete one cycle?

       That is=20
  correct. Assuming that the arm is not damped, you deflect it and then =
release=20
  it. It takes 2.5 sec to get to zero, overshoots and comes back to zero =
in 5=20
  sec, overshoots again and comes back to zero from the initial =
direction in=20
  another 5 sec. Total 5 + 5 =3D 10 sec. The period is the time for one =
complete=20
  oscillation cycle. You want at least twice this.


  5  After it is leveled and the period is set, Is it =
then=20
    that you can check the damp, by moving the arm 10mm and releasing=20
    it?

      =20
  Correct, you set up the period first and then you slide the damper =
further=20
  over the tongue in small steps until it is just a bit under=20
  critical.


      I know what an underdamped situation =
would=20
    look like.  But is there way to tell if it is over=20
  damped?

       As =
you=20
  increase the damping from zero, the time that the arm takes to fall =
back=20
  through / to the balance position increases slightly. If you overdamp =
the arm,=20
  it never swings through the zero position and the more you damp it, =
the longer=20
  it takes to get there. Look for the arm swinging just past the zero =
line, but=20
  without another =
oscillation.

      =20
  Regards,

       Chris =
Chapman=20