PSN-L Email List Message

Subject: Re: Resistive Damping
From: ChrisAtUpw@.......
Date: Wed, 23 Aug 2006 22:15:09 EDT


In a message dated 23/08/06, Benji.Johnson@.................. writes:

> I would like to update my Lehman style seismometer from that of using oil 
> damping to using resistive damping

Hi Benji,

       Electromagnetic damping is easy to adjust and it is not temperature 
sensitive, or messy.

> 1) Is there a technique to cutting through the insulation to get to the 
> wires?  I get nervous thinking about working with the fine #34 wire, and having 
> to rewire the RCA jack due to a careless mistake.  Is it just a matter of 
> careful scraping to get to the wire?

       You can use a pad of wire wool to remove insulation. However, most 
fine wires now have polyurethane insulation, which simply melts when you put a 
hot soldering iron on it. This makes the job a lot easier! You can also burn 
insulation off with a gas lighter, with some care, but this leaves the wire very 
soft and you need to clean it with wire wool. 
       It is probably best to solder a suitable 20 turn pre-set cermet 
potentiometer across the terminals on the baseplate. You need to adjust the damping 
whenever you disassemble the arm and then reset the period. 
 
       However, putting a resistance in parallel with the coil does reduce 
the signal. It also depends on your having a strong even magnetic field around 
the coil. Commercial seismometers may have a magnet / coil setup similar to a 
loudspeaker. Sean used this type, but he wound his own coils. This requires 
quite a bit of skill, time and effort, in case you haven't tried it. If you are 
just using a U Alnico magnet and a relay coil, you are very unlikely to get 
enough output. I don't think that inexpensive NdFeB magnets were so plentiful 
back in 1999, so I would now question Sean's assertion that this is the easiest / 
most satisfactory method.

       It is quite easy to fit the mass with a horizontal copper tongue, 
maybe 2.5" long by 2" wide and use a variable position damping magnet. You can 
probably 'get away' with using 32 thou Cu plate, although I use 1/16" thick for 
preference. You buy 4 off NdFeB bar magnets, 1" x 1/2" x 1/4". See 
http://www.kjmagnetics.com/ You need 2 off bright mild steel backing plates 2" wide x 3.5" 
long x 1/4" thick and four 2.5" long zinc plated mild steel 1/4" set screws, 
each with a washer and three nuts. Set screws are completely threaded. The 
backing plate is sold as 2" wide rolled strip. 
       You mark out the 4 drill holes 5/16" in from the corners on one plate, 
clamp the plates together and then drill 1/4" holes through both. This gets 
the holes perfectly aligned. Mark the plates so that you know which way to 
reassemble them. You put a washer on one screw, push it through a hole and fix it 
in place with a nut. Then you screw on another nut to maybe 1" from the plate. 
Repeat at the other corners and assemble with the magnets as shown below.

       XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX Backing plate
                             NNNNNSSSSS                     Magnets
          <--cucucucucucucucucucucucucucucu-->  Damping tongue
                             SSSSSNNNNN
       XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

       The Cu tongue moves perpendicular to the 1" magnet joints. You adjust 
the damping both by varying the separation of the magnets with the adjusting 
nuts and by sliding the magnet block further over the Copper tongue. The Cu 
tongue overlaps the magnets laterally at all times. This avoids any diamagnetic 
'edge effects'.
       Note that the amount of damping required decreases, as you increase 
the set period. If you displace the mass by 10 mm and then release it, 0.7 
critical damping allows the mass to swing ~0.5 mm past the centre balance position 
before falling back. 
       The magnets can pick up dust particles from the air. To clean them, I 
press on Gaffer tape and then peel it off. The rubbery adhesive seems to 
remove dust particles very well. 

       Regards,

       Chris Chapman
In a me=
ssage dated 23/08/06, Benji.Johnson@.................. writes:

I would like to update my Lehma= n style seismometer from that of using oil damping to using resistive dampin= g


Hi Benji,

       Electromagnetic damping is easy to adju= st and it is not temperature sensitive, or messy.

1) Is there a technique to cutt= ing through the insulation to get to the wires?  I get nervous thinking= about working with the fine #34 wire, and having to rewire the RCA jack due= to a careless mistake.  Is it just a matter of careful scraping to get= to the wire?


       You can use a pad of wire wool to remov= e insulation. However, most fine wires now have polyurethane insulation, whi= ch simply melts when you put a hot soldering iron on it. This makes the job=20= a lot easier! You can also burn insulation off with a gas lighter, with some= care, but this leaves the wire very soft and you need to clean it with wire= wool.
       It is probably best to solder a suitabl= e 20 turn pre-set cermet potentiometer across the terminals on the baseplate= .. You need to adjust the damping whenever you disassemble the arm and then r= eset the period.

       However, putting a resistance in parall= el with the coil does reduce the signal. It also depends on your having a st= rong even magnetic field around the coil. Commercial seismometers may have a= magnet / coil setup similar to a loudspeaker. Sean used this type, but he w= ound his own coils. This requires quite a bit of skill, time and effort, in=20= case you haven't tried it. If you are just using a U Alnico magnet and a rel= ay coil, you are very unlikely to get enough output. I don't think that inex= pensive NdFeB magnets were so plentiful back in 1999, so I would now questio= n Sean's assertion that this is the easiest / most satisfactory method.

       It is quite easy to fit the mass with a= horizontal copper tongue, maybe 2.5" long by 2" wide and use a variable pos= ition damping magnet. You can probably 'get away' with using 32 thou Cu plat= e, although I use 1/16" thick for preference. You buy 4 off NdFeB bar magnet= s, 1" x 1/2" x 1/4". See http://www.kjmagnetics.com/ You need 2 off bright m= ild steel backing plates 2" wide x 3.5" long x 1/4" thick and four 2.5" long= zinc plated mild steel 1/4" set screws, each with a washer and three nuts.=20= Set screws are completely threaded. The backing plate is sold as 2" wide rol= led strip.
       You mark out the 4 drill holes 5/16" in= from the corners on one plate, clamp the plates together and then drill 1/4= " holes through both. This gets the holes perfectly aligned. Mark the plates= so that you know which way to reassemble them. You put a washer on one scre= w, push it through a hole and fix it in place with a nut. Then you screw on=20= another nut to maybe 1" from the plate. Repeat at the other corners and asse= mble with the magnets as shown below.

       XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX B= acking plate
            &nbs= p;            &n= bsp;   NNNNNSSSSS        &= nbsp;            Magn= ets
          <--cucucucucucucuc= ucucucucucucucu-->  Damping tongue
            &nbs= p;            &n= bsp;   SSSSSNNNNN
       XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
       The Cu tongue moves perpendicular to th= e 1" magnet joints. You adjust the damping both by varying the separation of= the magnets with the adjusting nuts and by sliding the magnet block further= over the Copper tongue. The Cu tongue overlaps the magnets laterally at all= times. This avoids any diamagnetic 'edge effects'.
       Note that the amount of damping require= d decreases, as you increase the set period. If you displace t= he mass by 10 mm and then release it, 0.7 critical damping allows the mass t= o swing ~0.5 mm past the centre balance position before falling back.
       The magnets can pick up dust particles=20= from the air. To clean them, I press on Gaffer tape and then peel it off. Th= e rubbery adhesive seems to remove dust particles very well.

       Regards,

       Chris Chapman

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