Crank Position via Hall Effect

John S Gwynne jsg
Thu May 12 15:04:11 GMT 1994


   In message <9405121251.AA05991 at> , you write:
| The magnet on the f'wheel will give a narrower pulse and, with an inductive
| pickup, a larger signal. A Hall device will not see a larger amplitude,
| just a shorter width. 

Yes, I agree...

| The angular precision will be greater for a given magnet diameter as the
| radius increases. 

No, I don't agree. 

inductive pickup -------/  \  /------------
Hall device      -------|      |-----------
(w/ high gain)
                           | center of magnet

For an inductive pickup we look for the zero crossing (in voltage) with
either a positive or negative slope depending on the coil connection. The
larger the magnet, the longer the rise and fall times. But the location of
the zero crossing has not changed! It's *always* in the center were the B
field through the device changes slope and starts decreasing.  For a Hall
effect device we look for a rising or falling edge. In either case the size
of the magnet makes *no* difference; however the placement does. You can also
buy ring magnets with multiple pole pairs. Each pole pair would produce one

                                       John S Gwynne
                                          Gwynne.1 at
               T h e   O h i o - S t a t e   U n i v e r s i t y
    ElectroScience Laboratory, 1320 Kinnear Road, Columbus, Ohio 43212, USA
                Telephone: (614) 292-7981 * Fax: (614) 292-7292

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