feedback control

    A control system which monitors its effect on
   the system it is controlling and modifies its output
   accordingly.  For example, a thermostat has two inputs: the
   desired temperature and the current temperature (the latter is
   the feedback).  The output of the thermostat changes so as to
   try to equalise the two inputs.

   Computer disk drives use feedback control to position the
   read/write heads accurately on a recording track.  Complex
   systems such as the human body contain many feedback systems
   that interact with each other; the homeostasis mechanisms that
   control body temperature and acidity are good examples.

   (1996-01-02)