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{{short description|Control theory for nonlinear or time-variant systems}}
[[Image:Feedback_loop_with_descriptions.svg|thumb|upright=2.0|A feedback [[control system]]. It is desired to control a system (often called the ''plant'') so its output follows a desired ''reference'' signal. A ''sensor'' monitors the output and a ''controller'' subtracts the actual output from the desired reference output, and applies this error signal to the system to bring the output closer to the reference. In a nonlinear control system at least one of the blocks, system, sensor, or controller, is nonlinear.
'''Nonlinear control''' theory is the area of [[control theory]] which deals with systems that are [[nonlinear system|nonlinear]], [[time-variant system|time-variant]], or both. Control theory is an interdisciplinary branch of engineering and [[mathematics]] that is concerned with the behavior of [[dynamical system]]s with inputs, and how to modify the output by changes in the input using [[feedback]], [[Feed forward (control)|feedforward]], or [[filter (signal processing)|signal filtering]]. The system to be controlled is called the "[[plant (control theory)|plant]]".
Control theory is divided into two branches. [[Linear control theory]] applies to systems made of devices which obey the [[superposition principle]]. They are governed by [[linear equation|linear]] [[differential equation]]s. A major subclass is systems which in addition have parameters which do not change with time, called ''[[linear time invariant]]'' (LTI) systems. These systems can be solved by powerful [[frequency ___domain]] mathematical techniques of great generality, such as the [[Laplace transform]], [[Fourier transform]], [[Z transform]], [[Bode plot]], [[root locus]], and [[Nyquist stability criterion]].
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