PSN-L Email List Message

Subject: RE: My lehman type sensor is ready
From: "Gary Lindgren" gel@.................
Date: Sat, 10 May 2008 11:40:21 -0700

Hello Jon,
I'm in the process now in building my first Lehman, but here are my
observations looking at the picture.=20
1. I agree with the comments of Chris. I highly recommend going to the
Chapman magnet assembly for the sensor. Your coil seems too small in
diameter and the highest flux area of the magnet is over the coil wires =
are in the direction of the magnet motion. In order for a voltage to be
generated, you need to have the wires perpendicular to the magnet =
The problem with the small coil is that a voltage is generated from the =
field broken on the far side of one polarity and then the opposite =
voltage is generated on the near side coil wires. The 2 voltages will =
offset each other. If you want to use your coil and magnet, then rotate =
magnet 90 degrees and then move the coil so that only one side of the =
moves through the magnet flux field. You can purchase Neodymium magnets =
from  They are very reasonable in cost.

2. The damping is probably not working at all. It looks as if you took =
speaker magnets and have either the north poles opposite or the south =
opposite each other. If this is what you have then you have zero flux in =
middle where you have the damping plate. You must have a flux field set =
That means that a north pole must face a south pole magnet. If you have
opposite poles facing each other then you need to have a better return =
path. Now you have only the 1/4 inch bolts are the return path. You =
have magnetic flux spread all over. Think of your magnet assembly just =
an electrical circuit. The 1/4 inch bolts are just like a high =
path. Again go with the Chapman magnet assembly.


-----Original Message-----
From: psn-l-request@.............. [mailto:psn-l-request@............... =
Behalf Of J=F3n Fr=EDmann
Sent: Friday, May 09, 2008 5:12 PM
To: psn-l@..............
Subject: Re: My lehman type sensor is ready

Hi all

The lehman type sensor is up and running. However I have found one bad
bug in it. Where the arm as attached to the arm where the mass is. I
need to make it shorter (I think, at least so I can get it to hold the
mass up properly) and attach it to the mass arm it self to hold it up.
But the current attachment can't hold up the mass, that did destory the
hoding inside of it.

The second issue that I might have is that I am not sure if the damping
is correct or not.

Using Winquake FFT feature it appears that the seismomter has a natrual
frequancy of around 1Hz but it goes down to 10 seconds. This is a second
bug that I also need to fix somehow. As I am looking for up to 30 second

I hope to fix the first bug on monday, I might also do some other
modifactions to the sensor to make it more sensitive.

J=F3n Fr=EDmann


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