Cascade control
system
The basic principle of cascade control is that if the secondary variable responds
to the disturbance sooner than the primary variable, then there is a
possibility to capture and nullify the effect of the disturbance before it
propagates into the primary variable.
In
cascade control configuration, we have one manipulated variable and more than
one measurement. It is an alternative to consider if direct feedback control using
the primary variable is not satisfactory and a secondary variable measurement
is available. Cascade control uses the output of the primary controller to
manipulate the set point of the secondary controller.
The
two measurements are taken from the system and used in their respective control
loops. In the outer loop, the controller output is the set point of the inner
loop. Thus, if the outer loop dynamic variable changes the error signal affects
a change in set point of the inner loop. Even though the measured value of the
inner loop has not changed, the inner loop experiences an error signal and
produces a new output by virtue of the set point change. Cascade control
generally provides better control of the outer loop variable than is
accomplished through a single variable system. The primary objective of cascade
control is to divide an otherwise difficult control process into two portions;
whereby a secondary control loop is formed around major disturbances, leaving
only minor disturbances to be controlled by the primary controller.
The
outer controller is also known as primary or master controller and inner
controller is also known as secondary or slave controller.
Features
of cascade control
The
main features of cascade control are:
● The output of the master
controller serves as the set point for the slave controller.
● More than one measurement,
but one manipulated variable.
● Two feedback loops are nested
together.
● Inner loop has the effect of
reducing time lag in the outer loop, so that cascade control responds very
quickly and improves dynamic performance.
● Decrease variation in primary
variable.
● Enhances stability
characteristics.
● Insensitive to modeling
errors.
● The secondary controller
corrects the disturbances arising within the secondary loop before they affect
the primary variable.
Rules for designing and tuning cascade control
- Let the secondary loop be the input point of the most serious disturbance. Secondary loop should reduce the effect of one or more disturbances.
- Make the secondary loop fast by including only minor lags of complete control system. Secondary loop must be at least 3 times faster than the primary loop.
- Choose a secondary variable
which will provide stable performance with narrow proportional band.
The
correct sequence of operation while tuning the controller that work in cascade
is as follows:
- Set the primary controller to
manual
- Tune the secondary controller
- Put the secondary controller in
automatic mode
- Tune the primary controller
Advantages
of cascade control
● Better control of the primary
variable
● Primary variable is less
affected by disturbances
● Faster recovery from
disturbances
● Increase the natural
frequency of the system
● Reduce the effective
magnitude of a time-lag
● Improve dynamic performance
● Provides limits on the
secondary variable
Disadvantages
of cascade control
● It requires an additional
measurement to work
● The control strategy is more
complex
● Required an additional
controller and it has to be tuned.
Examples
of cascade control system
Cascade
control of a jacketed CSTR
Consider the CSTR shown in the figure. The
reaction is exothermic and the heat generated is removed by the coolant, which
flows in the jacket around the tank. The control objective is to keep the
temperature of the reacting mixture constant at the desired value. Possible
disturbances to the reactor include the feed temperature and the coolant
temperature.
Now the next figure illustrates how the cascade control is implemented on jacketed CSTR. The response of simple feedback control to the change in coolant temperature can be improved by measuring the change in coolant temperature, and taking control action before its effect has been felt by the reacting mixture. If the coolant temperature goes up, the control action increases the flow rate of the coolant.
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