PSN-L Email List Message

Subject: Re: Different pendulum design
From: Karl Cunningham karlc@..........
Date: Tue, 10 Feb 2004 17:54:04 -0800

John --

I haven't examined the geometry of this design in detail, but could it be
changed in a way to make it work upside down?  If the two side plates were
hanging from a support above, they would be in tension and could be made
from something much much thinner.  Perhaps even wire trusses that would
have rigidity in off-axis directions.  The cross piece would still have to
be strong and thick.  Or maybe the weight could be supported by more wire
trusses with a cross-piece to keep the junctions separated.  One could let
the bending of the wires act as mostly-friction-free flexures.

The geometry would have to change in such a way that it still has a
positive period (negative period being unstable so that it falls to one
side and stays there).  I'm not sure this is possible but maybe.  The
support would have to attach very near to the center of mass.  A bit like
the basket of a hot-air balloon.

   //          //             //             //            // 
       \                                              /
        \                                            /
         \                                          /
          \                                        /
           \                                      /
            \                                    /
             \                                  / wire trusses 
              \                                / for angled
               \       thicker strut          / supports
                 \                          /
                   \                      /
                     \                  /
                       \              /
                         \ -------- /
                           |      |
                           |\ M / |
                           |      |

Karl Cunningham

--On Tuesday, February 10, 2004 3:59 PM -0700 John or Jan Lahr

> At 11:42 AM 2/10/2004, you wrote:
> Hi Chris,
> Thanks for the feedback.  The pivots are made from safety pin point
> embedded in the Plexiglas sheeting.  They rest in dents in the aluminum
> and in dents in the plastic. All of the pivots are in compression.  I
> think I'll try the "ball on plane" joint next.
> I think I need the weight to offset the weight of the sheets of plastic.
> Maybe lighter material would help to reduce their effect.
> The coil is mounted as you suggest.  The axis of symmetry is horizontal
> and although the axis of symmetry moves in a only a slight arc, the coil
> does rotate during this movement, as it remains perpendicular to the
> threaded rod.
> Cheers,
> John
>> Hi John,
>>        I suspect that the problem lies a) in the suspension of the 
>> plates, b) in the 'springyness' or hysteresis of this suspension and c) 
>> in the flexure of the pendulum threaded rod. I can't quite see how you 
>> have made your suspension joints. Three types of suspension give few 
>> problems: 1 flexing foils or wires, preferably crossed for rigidity, 2 
>> ball on plane, 3 circular cylinder crossed on circular cylinder. Point
>> on  plane types are just asking for trouble.
>> >> Romberg-Style Horizontal Pendulum
>> >> February 7, 2004.  Mw 7.1This sensor is my first attempt to make the 
>> compact pendulum described 
>> here.  I have 
>> not been able to obtain a period longer than 2 seconds, possibly due to 
>> slight flexure of central threaded rod that carries the lead 
>> weight.  When adjusted for a  longer period, the pendulum will drift to 
>> one side or the other and is stable at either extreme.
>>        You should be able to get a wide range of periods. I suspect that 
>> the suspension is being stabilised by the spring flexing and that this 
>> has a slightly different force law to the geometrical path relationship.
>>        This is a 'general case' of the folded pendulum type suspension. 
>> It is quite easy to get periods of over 45 sec with folded pendulum 
>> suspensions.
>> >> The magnet/coil pickup is from a PC hard drive.  It seems to have 
>> sufficient gain.  One problem is that in this design the coil rotates as 
>> it moves back and forth so that the amount of free travel is quite
>> limited.
>>        You would probably find it much more satisfactory if you
>>        suspended  the coil with it's axis of symmetry horizontal,
>> perpendicular to the  plane of motion of the pendulum and in between
>> horizontal magnet pairs. I  doubt if you really need all that lead. Plan
>> view:
>>                                      NNNNSSSS
>>                                         C--O--I--L    <--- movement --->
>>                            SSSSNNNN
>>        Regards,
>>        Chris Chapman
> __________________________________________________________
> Public Seismic Network Mailing List (PSN-L)
> To leave this list email PSN-L-REQUEST@.............. with the body of
> the message (first line only): unsubscribe See
> for more information.

Karl Cunningham


Public Seismic Network Mailing List (PSN-L)

[ Top ] [ Back ] [ Home Page ]