PSN-L Email List Message

Subject: Re: My lehman type sensor is ready
From: jonfr@.........
Date: Thu, 8 May 2008 18:28:19 -0400 (EDT)

Hi Chris

The arm moves freely in both direction (Horizontal). But that direction is
the assumed default one, rather the vertical one, that also exist on the
arm, it was not unexpected behavior as I did see it in the test sensor
too. But it doesn't do so easy. I will know tomorrow what movement is the
default one. I know about the axis moving strongly to the sides. But the
damping magnets limit that movement greatly, it needs a strong force (at
least a push of few cm) to move it, that type of push should not happen
during a earthquake detection.

I did put the coil in the base, rather then it's arm because the coil fits
there better, given the U type magnet that I use (this also gives me
voltage of 0.135 as it is on the picture). A equipment is magnetic
sensitive is going to stay that way regardless of the setup, most setups I
have seen show this type of background noise (there plots show it).

I will see tomorrow how well this design is going to perform. The
helicorder for the sensor is going online after I have done the basic
working tests on him. The sensitivity test is going to be when the next
big earthquake happens.

To see a bigger picture of the seismometer you can choose the maximum size
of 1200x1600 in the gallery. I will post better pictures tomorrow of the

Jón Fríman.

> In a message dated 2008/05/08, jonfr@......... writes:
>> My lehman type sensor is ready. I hope that it gives me up to 30 seconds
>> peroid. But I won't know if that is the case of not until tomorrow when
>> I
>> put it online at home.
> Hi Jon,
>        Sorry, but you have made a couple of design mistakes with your
> Lehman.
>        The coil as shown in the photograph WOULD be sensitive IF the
> magnet
> were able to move in the direction of the arm, but NOT at right angles to
> it!
>        Turn the magnet at right angles and put the coil out to one side.
> You
> get a true linear signal as the coil approaches or recedes from the open
> jaws
> of the magnet, NOT when the magnet jaws stay at ~the same distance from
> the
> coil, moving parallel to the sufaces of the jaws.
>        The setup as shown will only have a very small and non linear
> sensitivity to small movements. It will give a signal of double the
> frequency. You
> definitely need to change it.
>        Putting the magnet on the arm and the coil on the frame allows the
> arm
> to be sensitive to changes in the Earth's magnetic field, to surges in the
> electricity supply cables, to passing trucks and cars....  With the
> existing
> setup, you will get quite a noisy signal. It is VASTLY PREFERABLE to put
> the coil
> on the arm and the magnet on the base frame!
>        Using a U magnet and a coil will only give about 1/10 the signal
> that
> you can get from quad NdFeB magnets on mild steel backing plates.
>        The arm will be sensitive to rocking motions about it's axis. I put
> a
> cross bar on the arm close to the mass and a V suspension up to the top
> hinge
> to avoid this problem.
>        I can't see from the photo how you have constructed the bottom
> bearing, or the top suspension. The top suspension is as important as the
> bottom
> bearing in getting a long period.
>        Regards,
>        Chris Chapman


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