Civil Engineering Reference
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calibrated for each operating condition. The stability and performance of the
system are typically evaluated by simulation. Particular attention is given to
the transition between different operating conditions. The number of entries
in the scheduling table can be increased if necessary. However, there is no
feedback from the performance of the closed-loop system to the controller
parameters.
One drawback of gain scheduling is that it is an open-loop compensa-
tion. There is no feedback to compensate for an incorrect schedule. Another
drawback of gain scheduling is that the design may be time-consuming. The
controller parameters must be determined for many operating conditions,
and the performance must be checked in tests and commissioning.
Gain scheduling has the advantage that the controller parameters can be
changed very quickly in response to process changes. Since no estimation of
parameters occurs, the limiting factors depend on how quickly the auxiliary
measurements respond to process changes.
7.8.3 Self-tuning controller
Development of a control system involves many tasks such as modelling,
design of a control law, implementation and validation. The
self-tuning
regulator
(STR) or self-tuning controller attempts to automate several of
these tasks. This is illustrated in Figure 7.17, which shows a block diagram
of a process with a self-tuning controller. It is assumed that the structure of a
process model is specified. Parameters of the model are estimated online, and
the block labelled '
Estimation
' in the figure gives an estimate of the process
parameters, which is the essential function block of a self-tuning controller.
It is a recursive estimator. The block labelled '
Controller design
' contains
computations that are required to perform a design of a controller with a
specified method and a few design parameters that can be chosen externally.
Self-tuning regulator
Specification
Process parameters
Controller
design
Estimation
Controller
parameters
Reference
Controller
Process
Input
Output
Figure 7.17
Conceptual diagram of self-tuning controller.
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