A servo system includes a first member and a second member that is
positionable relative to the first member. A position error signal causes
the second member to be positioned at a desired location relative to the
first member. The position error signal includes a position error due to
a disturbance in the servo system, and the position error is reduced by
attenuation of a disturbance signal as a non-linear function of the
amplitude of the disturbance signal.