PSN-L Email List Message

Subject: Re: Size of a lehman type sensor ?
From: ChrisAtUpw@.......
Date: Sun, 29 Apr 2007 15:38:49 EDT


In a message dated 2007/04/29, jonfr@......... writes:

> Next month I finally can start building a lehman type sensor. But I need
> to know what size / height I need to use for the sensor.

Hi Jon,

       Take a look at http://jclahr.com/science/psn/chapman/lehman/index.html
and http://jclahr.com/science/psn/chapman/school/MKII/index.html
       This is a modern type with NdFeB magnetic damping and sensing.
       There is quite a lot of helpful information on pendulums at 
http://jclahr.com/science/psn/epics/links.html

       I know that you are short of space, but the absolute minimum length 
for the arm is about 35 cm from the bearing on the upright to the CENTRE of the 
mass. It is preferable to use about 60 cm, but lengths of 1 m are OK. Larger 
than this and you tend to get problems with the enclosure and thermal 
stability. The whole arm and base need to be a bit longer.
       It is usual to use about a 30, 60, 90 triangle for the suspension, 
with the 30 deg at the mass end, but you can use 45 deg OK.
       Make the seismometer as a single unit on a base frame.
       Use NdFeB  magnetic damping. It is clean, effective and dead easy to 
adjust. 
       The arm needs to be light but rigid - preferably tube, not solid rod. 
I use 15 mm SS water pipe. It is cheap, rigid and you can get compresssion 
fittings to fit it. The mass may be ~1 kg. Keep any magnets or ferromagnetic 
material OFF the arm. Put the sensor and damping magnets on the baseplate.
       Design the vertical position of the damping plate and the centre of 
the sensor coil to lie ~on the line from the centre of mass to the lower bearing 
- offset the mass below the arm if necessary.
       What sort of suspensions were you considering? Use a V wire from the 
mass to the top of the upright. 
             Do NOT use either a point in a cup or a knife edge suspension. 
They are both unsatisfactory.
       A SS ball bearing in a Tungsten carbide triangle, or a hardened 
Martensitic SS plate works fine. Crossed Tungsten carbide or Martensitic SS 
cylinders work fime. You can buy TC needle bearings, TC drills and also 415 grade SS 
shoulder bolts.
See http://pages.prodigy.net/fxc/JC.html 
       I use 3" x 1" U channel Aluminum for the frame with either large joint 
plates or cross bracing to make it really rigid / strong. The upright must 
not bend / flex / oscillate as the arm swings.
       I have made suggestions for magnetic dampers and sensors at 
http://jclahr.com/science/psn/chapman/lehman/index.html The sensors give a higher output 
than the large coils and U Alnico magnets pictured elsewhere.
       You will need an airtight insulating cover to house the seismometer. 
See http://pages.prodigy.net/fxc/
        Al covered Cellotex is usually used.

       Hope that this helps.
       
       Chris Chapman

   
In a me=
ssage dated 2007/04/29, jonfr@......... writes:

Next month I finally can start=20= building a lehman type sensor. But I need
to know what size / height I need to use for the sensor.


Hi Jon,

       Take a look at http://jclahr.com/scienc= e/psn/chapman/lehman/index.html
and http://jclahr.com/science/psn/chapman/school/MKII/index.html
       This is a modern type with NdFeB magnet= ic damping and sensing.
       There is quite a lot of helpful informa= tion on pendulums at http://jclahr.com/science/psn/epics/links.html

       I know that you are short of space, but= the absolute minimum length for the arm is about 35 cm from the bearing on=20= the upright to the CENTRE of the mass. It is preferable to use about 60 cm,=20= but lengths of 1 m are OK. Larger than this and you tend to get problems wit= h the enclosure and thermal stability. The whole arm and base need to be a b= it longer.
       It is usual to use about a 30, 60, 90 t= riangle for the suspension, with the 30 deg at the mass end, but you can use= 45 deg OK.
       Make the seismometer as a single uni= t on a base frame.
       Use NdFeB  magnetic damping. It is= clean, effective and dead easy to adjust.
       The arm needs to be light but rigid - p= referably tube, not solid rod. I use 15 mm SS water pipe. It is cheap, rigid= and you can get compresssion fittings to fit it. The mass may be ~1 kg. Kee= p any magnets or ferromagnetic material OFF the arm. Put the sensor and damp= ing magnets on the baseplate.
       Design the vertical position of the dam= ping plate and the centre of the sensor coil to lie ~on the line from the ce= ntre of mass to the lower bearing - offset the mass below the arm if necessa= ry.
       What sort of suspensions were you consi= dering? Use a V wire from the mass to the top of the upright.
             = Do NOT use either a point in a cup or a knife edge suspension. They are both= unsatisfactory.
       A SS ball bearing in a Tungsten car= bide triangle, or a hardened Martensitic SS plate works fine. Crossed Tungst= en carbide or Martensitic SS cylinders work fime. You can buy TC needle bear= ings, TC drills and also 415 grade SS shoulder bolts.
See http://pages.prodigy.net/fxc/JC.html
       I use 3" x 1" U channel Aluminum for th= e frame with either large joint plates or cross bracing to make it really ri= gid / strong. The upright must not bend / flex / oscillate as the arm swings= ..
       I have made suggestions for magnetic da= mpers and sensors at http://jclahr.com/science/psn/chapman/lehman/index.html= The sensors give a higher output than the large coils and U Alnico magnets=20= pictured elsewhere.
       You will need an airtight insulating co= ver to house the seismometer. See http://pages.prodigy.net/fxc/
        Al covered Cellotex is usually us= ed.

       Hope that this helps.
      
       Chris Chapman


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