Chris;

Do you really mean Q of 0.7
Which is a Damping of 1/0.7 ?

If I make a filter with a damping
of only 0.7 I get a peak in
the roll off response instead of a Butterworth
profile ?

Regards;
geoff

----- Original Message ----- 
From: 
To: 
Sent: Sunday, May 31, 2009 9:45 AM
Subject: Re: optimal position of magnetic dampter


> 
> In a message dated 31/05/2009, rog@.......... writes:
> 
> What is  the thinking on the optimal position of the magnetic damper (MD) 
> along the  lower boom?
> It seem by shifting the MD position towards or away from the  lower pivot 
> one can tune the damping strength.
> 
> 
> 
> Hi Rob,
> 
>    You need to have as low a vertical force on the  bottom suspension as 
> practicable. I suggest that you put the mass on the end of  the arm and then 
> slide the arm over a round horizontal rod until the weight of  the mass on 
> one side balances the weight of the arm on the other. Mark this  point to 
> attach the top suspension. Check the photos at 
> _http://jclahr.com/science/psn/chapman/school/MKII/index.html_ 
> (http://jclahr.com/science/psn/chapman/school/MKII/index.html)  
>    You need the damping fairly close to the mass, but  this is not 
> critical. But the damping plate DOES need to be lie on the  OFFSET line joining the 
> centre of mass to the bottom suspension, hence the  offset mounting hole in 
> the mass. This prevents a quake motion from trying  to rotate the mass as 
> well as pushing it to one side. Please note that the  next modification was 
> to fit the sensor block with a V wire top suspension. See  
> _http://jclahr.com/science/psn/chapman/2008%20lehman/index.html_ 
> (http://jclahr.com/science/psn/chapman/2008%20lehman/index.html)  This  30 lb fishing trace with looped 
> ends worked very well indeed, preventing any  rotation of the mass around the 
> long axis of the arm. 
>    Have a look at the damper drawings at  
> _http://jclahr.com/science/psn/chapman/lehman/index.html_ 
> (http://jclahr.com/science/psn/chapman/lehman/index.html)  These  use a quad NdFeB magnet block. You only have to slide the 
> block about 1/2" to  3/4" over the 1" magnets to get the correct damping. You 
> adjust the separation  of the top and bottom mild steel backing plates to 
> set the damping in this  range. But you also need to be able to remove the 
> damping easily when setting  the period and to set the damping afterwards to 
> 0.7 critical, hence this layout  where the damping block simply slides along 
> the baseplate. 
>    This is a good, compact and easily adjusted design.  I can set a period 
> of 30 seconds, no problem. The trimming of the  height of the arm, setting 
> up the period and then setting the damping are all  sequential and non 
> interacting adjustments.
> 
>    Regards,
> 
>    Chris Chapman
>
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