Flywheel/Encoder patterns

[tom cloud]

>Chris Morriss wrote:
>> 
>> A neat and very reliable solution to taking up the backlash is to mount
>> an eddy current brake on the shaft.  Much easier than providing viscous
>> drag.  All you need is a plain aluminium or copper disk rotating in the
>> gap of a horse-shoe magnet.  Like a old electricity meter. (If you had
>> the rotating disk meters in the US)  The principle was also used as the
>> speed control a#on the old but revered Garrard 401 record turntable.
>
>	Pardon my ignorance, but what effect does a magnet have on a non
>metallic element like aluminum or copper.
>	How's it work?
>	And what kind of drag can you expect from such a device?
>
>Todd Knighton

Todd, I think 'analog' speedometers are based on this principle, as
is my reloading scale.

 ... speedo: round brass disk attached to speedo pointer and to return
sproing.  disc magnet connected to speedo shaft.  rotating magnet
induces cemf (counter emf) in brass disk, which acts as a shorted
turn thereby inducing a magnetic field opposing the rotating magnet,
making the speedo pointer move.

[understand that this is what I think happens -- comes from working
with the speedo on my /90 Triumph 650 Bonneville -- those dratted
things vibrated the instruments apart after 10 or 20 thousand miles.
When I rebuilt the speedo, I only found a brass disk and circular
magnet as described above.  All else is my conjecture.]

 ... reloading scale -- and every triple beam balance I've seen in
chemistry labs.  An aluminum tab is hung on the end of the balance
beam in the field of a magnet mounted to the balance frame.  The
movement of the tab through the magnet works like the speedo above,
effectively dampening the movement of the beam so readings can
be taken quicker.


Tom Cloud <cloud at peaches.ph.utexas.edu>