controllability can be defined as the ease with which a continuous
process can be held at a specified steady state. The
other states that controllability may be viewed as a property
of the process, which indicates how easy it is to control the
process to achieve the desired performance. Generally, the
concept of controllability can be qualitatively defined as follows:
controllability is an inherent property of the process
that accounts for the ease with which a continuous plant can
be held within a specified operating regime despite bounded
external disturbances and uncertainties.
Zero dynamics
Zero dynamics13,14 is analogous to the right half plane
(RHP) zeros of a linear system and can be characterized as
the remaining dynamics of a nonlinear system in the case
where the output remains at zero (constant) for all times.
Methodologies for Controllability Analysis
To prevent a process from failing to meet the required
performance specifications, it is important to analyze the
controllability at a design stage when modifications of the
process are still possible. Controllability analysis includes
the assessment of the attainability of a given process and the
improvement of the dynamic performance of the process. In
this section, an overview will be given on existing methods
for controllability assessment. Particularly, the switch from
one operating point to another is analyzed. The major work
on controllability analysis and recent advances are summarized
in Table 1.
In general, these measures can be classified into two main
sets: linear model-based approaches and nonlinear modelbased
approaches. It can be seen from the above table that
most tools relied on the use of steady states or linear
dynamic models before 2000. In recent years, more and
more tools based on nonlinear dynamic models have
appeared. Next, a brief overview based on the sets of linear
model-based approaches and nonlinear model-based
approaches for controllability assessment is given and the
limitations of these methods are also pointed out.