COMPLEX CONTROL SCHEMES
Feedback control advantages
◼ It requires minimal knowledge about the process to be
controlled. In particular, a mathematical model is not
necessary, although it is useful for control system
design.
◼ The classical PID controllers are versatile and robust. If
process conditions change, re-tuning the controller
usually produces satisfactory response.
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Feedback control weaknesses
◼ The feedback controller acts only after the process feels the upset. It
therefore can never yield perfect control where the controlled variable
does not deviate from the set point during load or set point changes.
◼ Poor feedback tuning may cause instability
◼ PID controller does not always provide the best possible control for all
processes especially for processes with large dead times and/or cascade
processes with large time constant.
◼ In some applications the controlled variable can not be measured on line
and consequently feedback control is not feasible.
◼ Feedback control does not provide predictive control action to
compensate
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of known or measurable
Process Control disturbances. 3
Possible configurations
◼ Additional process output measurements are used
(e.g. cascade , inferential)
◼ Additional process inputs measurements are used
(e.g. feed-forward)
◼ Use explicit modeling in control calculations
(e.g. inferential)
◼ Use a different control algorithm than PID
(e.g. feed-forward)
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Cascade Control
In a cascade control configuration we have one manipulated variable
and two measurements
Feedback Control Cascade Control
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Open loop Process
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Feedback Control
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Open loop Process
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Cascade Control Block Diagram
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Implementation issues
Cascade controller modes and tuning:
◼ The secondary loop is normally P or PI controller.
Derivative modes are not advised in the secondary loop.
The primary loop is usually PI or PID controller.
◼ The cascade controller is tuned in a sequential manner.
The secondary controller is first tuned satisfactorily and
the primary is then tuned.
Cascade controller is desired when:
◼ Single loop does not provide satisfactory performance
◼ A measured secondary variable is available
◼ The secondary loop should be three times as fast as the
primary.
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Advantages
◼ Disturbances felt by the secondary variable, is significantly corrected
by the secondary controller before it is felt by the process.
◼ The dynamics of the secondary loop are much faster than those of
the primary loop. This allows the use of higher gains in the
secondary controller to suppress more effectively the effect of the
disturbance occurring in the secondary loop without affecting the
stability of the system.
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Design of Cascade Control
Case I: Consider the block diagram without cascade
The dynamic of the inner loop is
The dynamic of the outer loop is
The characteristic equation for the overall closed-loop system is given by
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Contd…
Case II : Consider the block diagram of a cascade system
The closed-loop dynamics of the secondary loop
are
The overall closed-loop transfer function for the primary is
The stability of the primary closed-loop is determined by the characteristic
equation
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Example
Let
In conventional feedback
This means that the inner loop has a time constant of 0.1
minute
The characteristic equation of the overall system is
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Contd…
In Cascade control
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