Hi,

I was concerned about how you will be able to easily adjust the level of 
the base.  This will need to be done from time to time.  Usually there 
is some adjustment mechanism protruding above the base.

Cheers

Ian

ChrisAtUpw@....... wrote:
> 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     



  


Hi,



I was concerned about how you will be able to easily adjust the level
of the base.  This will need to be done from time to time.  Usually
there is some adjustment mechanism protruding above the base.



Cheers



Ian



ChrisAtUpw@....... wrote:
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