> If the PID cycled fast enough, couldn't you do that with one loop and
DAC?

Perhaps you could.  The problem I was worrying about is that the force on
the ball is highly dependent on its position due to the non-linear nature
of magnetic fields.  The PID constants that worked well at one point would
probably not work well at a different point because of this.  That's why I
suggested providing a linear interface to force that the PID drives.  That
should allow one set of control contstants to work over the entire
positional range of the ball.


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