6 1 Introduction
1.2 System Stabilities
An essential issue in control-systems design is the stability. An unstable sys-
tem is of no practical value. This is because any control system is vulnerable
to disturbances and noises in a real work environment, and the effect due
to these signals would adversely affect the expected, normal system output
in an unstable system. Feedback control techniques may reduce the influence
generated by uncertainties and achieve desirable performance. However, an
inadequate feedback controller may lead to an unstable closed-loop system
though the original open-loop system is stable. In this section, control-system
stabilities and stabilising controllers for a given control system will be dis-
cussed.
When a dynamic system is just described by its input/output relation-
ship such as a transfer function (matrix), the system is stable if it generates
bounded outputs for any bounded inputs. This is called the bounded-input-
bounded-output (BIBO) stability. For a linear, time-invariant system mod-
elled by a transfer function matrix (G(s) in (1.2)), the BIBO stability is guar-
anteed if and only if all the poles of G(s) are in the open-left-half complex
plane, i.e. with negative real parts.
When a system is governed by a state-space model such as (1.1), a stability
concept called asymptotic stability can be defined. A system is asymptotically
stable if, for an identically zero input, the system state will converge to zero
from any initial states. For a linear, time-invariant system described by a
model of (1.1), it is asymptotically stable if and only if all the eigenvalues of
the state matrix A are in the open-left-half complex plane, i.e. with positive
real parts.
In general, the asymptotic stability of a system implies that the system
is also BIBO stable, but not vice versa. However, for a system in (1.1), if
[A, B, C, D] is of minimal realisation, the BIBO stability of the system implies
that the system is asymptotically stable.
The above stabilities are defined for open-loop systems as well as closed-
loop systems. For a closed-loop system (interconnected, feedback system), it is
more interesting and intuitive to look at the asymptotic stability from another
point of view and this is called the internal stability [20]. An interconnected
system is internally stable if the subsystems of all input-output pairs are
asymptotically stable (or the corresponding transfer function matrices are
BIBO stable when the state space models are minimal, which is assumed in
this chapter). Internal stability is equivalent to asymptotical stability in an
interconnected, feedback system but may reveal explicitly the relationship
between the original, open-loop system and the controller that influences the
stability of the whole system. For the system given in Figure 1.1, there are
two inputs r and d (the disturbance at the output) and two outputs y and u
(the output of the controller K).
The transfer functions from the inputs to the outputs, respectively, are
T
yr
= GK(I + GK)
−1