The stability of a system is determined by its response to inputs or disturbances. Intuitively, a stable system is one that will remain at rest unless excited by an external source and will return to rest if all excitations are removed. The definition of a stable system can be based upon the response of the system to bounded inputs. Bounded inputs have magnitudes that are less than some finite value for all time. A relaxed system is said to be bounded-input bounded-output (BIBO) stable if every bounded input produces a bounded output. The stability that is defined in terms of the input—output description is applicable only to relaxed systems. This stability is referred to as the input—output stability. A zero-input system is said to be asymptotically stable if the response approaches zero asymptotically as time t approaches infinity. If the response remains bounded for t > to, we speak about the bounded stability. There are several methods (criteria) for determining system stability: Routh, Hurwitz, Liénard—Chipart, Nyquist, Lyapunov,
and so on.
Consideration of the stability of a system provides valuable information about its behavior and is an important issue in system design. Often, it is desirable to determine a range of values of a particular system parameter for which the system is stable. The concept of stability is extremely important, because almost every workable system is designed to be stable. If a system is not stable, it is usually of no use in practice.