Cereals processing technology

38 Cereals processing technology
Process control equipment/methods may be divided into two principal subcategories,
namely conventional controls and advanced controls. The details of
these two types and their differences are as follows.
3.6.1 Conventional controls
Conventional controls are generally Single-Input Single-Output (SISO) Proportional
Integral Differential (PID) controllers. These have been in use for many
years and are especially effective when process dynamics are reasonably
constant, where there is minimal lag in the process and where the objective is to
adhere to some measurable set point. Other control systems have been
developed subsequently to handle complex process dynamics. These include:
• Feed forward control, which compensates for up stream disturbances before
the controlled variable is influenced.
• Ratio control, which is able to hold one process variable at a fixed ratio to
another.
• Cascade control, which is the application of one controller to adjust the set
point of a second controller.
• Constraint control, which is used to move a process variable towards one or
more constraint values.
Dead-time or lagging compensated controllers are ones that have algorithms that
allow for delays in a process or slow sensor responses, an example being discrete
sampling, on-line analysers.
In addition to these major groups, controllers incorporating specialist features
are also available. Robust control techniques add functionality to controllers that
are designed to work under all operating conditions. These are applied especially
in safety critical applications. Overrides are functions employed to ensure a
control system does not violate the limits of the process and gain scheduling is
used to adjust automatically the tuning parameters of a controller to take account
of different operating regions.
The type of controller employed in mills today fits into one or more of the
above categories. They are used for operations such as water-addition to wheat
and micro-ingredient addition to flour.
It may be seen from the list above that many different types of conventional
controller exist to cope with most simple process requirements. However, whole
process control does not fit into this category of control requirement and so
advanced control techniques must be investigated in the future in order to
increase process efficiency.
3.6.2 Advanced control techniques
As stated, advanced control techniques are considered when conventional
approaches cannot develop adequate performance. The types employed are as
follows: