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Parallel Cascade
Control
Another popular
solution to the measurement problem is to
apply a control strategy known as parallel cascade control (as
opposed to the more usual series cascade scheme). A schematic
of the parallel cascade control strategy is shown below.
The scheme
is quite often employed in the product composition control of distillation
columns. It employs two control loops. In a distillation column application,
the fast secondary loop controls a tray temperature, which is
an easily measured variable that is related to product composition, the
primary variable. The slow loop operates on the time scale dictated by
the delay of a measurement device, such as a gas chromatograph for example.
As with the series cascade strategy, inner loop control could be
achieved using a proportional controller. The rationale behind this scheme
is quite simple: if the secondary variable is kept at some value, then
the primary variable should as a result, also be kept at some level since
the two are related. The slow loop is then used to correct (trim) the
set-point of the faster secondary loop whenever the delayed measurements
of the primary variable become available.
Unlike the
manual control solution to the measurement
problem, this parallel cascade strategy uses only one piece of process
information. It assumes that changes made to the secondary variable will
affect the primary one. However, there may be cases where disturbances
affecting the secondary variable may not affect the primary variable and
vice versa. Thus controlling the secondary variable could cause deviations
of the primary variable away from its desired value. Another assumption
being made is that the steady state relationship between the two is known
In practice, the relationship between the primary and secondary variables
is non-linear, and can change depending on the operating conditions. Thus,
tuning the two loops can prove to be a difficult task.
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