Control theory is an interdisciplinary branch of engineering and
mathematics that deals with the behavior of
dynamical systems with inputs, and how their behavior is modified by
feedback. The usual objective of control theory is to control a system, often called the
plant, so its output follows a desired control signal, called the
reference, which may be a fixed or changing value. To do this a
controller is designed, which monitors the output and compares it with the reference. The difference between actual and desired output, called the
error signal, is applied as
feedback to the input of the system, to bring the actual output closer to the reference. Some topics studied in control theory are
stability (whether the output will converge to the reference value or oscillate about it),
controllability and
observability.