Our task is to design a digital controller that can
 be used to provide accurate positioning of the
 read/write head. Conceptually, this is very similar
 to moving (at very high speed) the tone-arm of
 a record-player to a certain location on the record.
 The task is tricky because there is always some
 flexibility in the mechanism.  Bad control design
 can lead to poor performance, or even instability, like
 if the read/write head hopelessly flails back and forth,
 never succeeding in attaining the desired position.