Chris,

Thank you as always the advice and pictures are wonderfully helpful.

I am experimenting with a pair of 2 X 1 X 0.25 inch NdFeB magnets with 
spacers at each end and a 1.5 inch Al damping plate.

Rob

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