Asymptotic stability
From Wikipedia, the free encyclopedia
- See also Lyapunov stability for an alternate definition used in dynamical systems.
In control theory, a continuous linear time-invariant system is asymptotically stable if and only if the system's transfer function has poles (or, equivalently, eigenvalues) only with strictly negative real parts. That is, the poles are in the left half of the complex plane.
A discrete linear time-invariant system is asymptotically stable if and only if the poles (or eigenvalues) of its transfer function lie strictly within the unit circle centered on the origin of the complex plane.
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[edit] Practical consequences
An asymptotically stable system is one that, if given a finite input, will not "blow up" and give an unbounded output. Moreover, if the system is given a fixed, finite input (that is, a step), then any resulting oscillations in the output will decay, and the output will tend asymptotically to a new final, steady-state value. If the system is instead given an Dirac delta impulse as input, then induced oscillations will die away and the system will return to its previous value. If oscillations do not die away, or the system does not return to its original output when an impulse is applied, the system is instead marginally stable.
[edit] Example asymptotically stable systems
The graph on the right shows the impulse response of two similar systems. The green curve is the response of the system with impulse response 
, whilst the blue represents the system 
. Although one response is oscillatory, both return to the original value of 0 over time.
[edit] Real-world example
Imagine putting a marble in a ladle. It will settle itself into the lowest point of the ladle and, unless disturbed, will stay there. Now imagine giving the ball a push, which is an approximation to a Dirac delta impulse. The marble will roll back and forth but eventually resettle in the bottom of the ladle. Drawing the horizontal position of the marble would give a gradually diminishing sinusoid rather like the blue curve in the image above.
A step input in this case requires supporting the marble away from the bottom of the ladle, so that it cannot roll back. It will stay in the same position and will not, as would be the case if the system were only marginally stable or entirely unstable, continue to move away from the bottom of the ladle under this constant force equal to its weight.
It is important to note that in this example the system is not stable for all inputs. Give the marble a big enough push, and it will fall out of the ladle and fall, stopping only when it reaches the floor. For some systems, therefore, it is proper to state that a system is asymptotically stable over a certain range of inputs.
