pH control is the process of adding acid or base
to opposing solutions aiming to achieve a
solution of pH 7, defined as neutral solution.
The function of the controller is to achieve a
neutral output of the system. This in itself is
not an easy problem to solve as pH control of a
neutralisation process is a difficult non-linear
control problem. This is largely due to the
inherent non-linear behavior of pH
measurement, and is further complicated by
variable time delays in the process. The
relationship between pH measurement and
volume ratio of acid to base is non-linear and
characteristically described as an S-type curve
relationship. Therefore the pH control
problem can be considered as posing three
different regions of sensitivity for control. For
this reason alone conventional methods of
control provide unsatisfactory control results, as
can only truly accommodate one region
scenario.
This paper presents both simulated and
practical results of applying a unique fizzy
logic method to control the pH process,
utilising the novel concept of mutli-regional PD
type fuzzy logic control. Each region of
sensitivity is assigned a unique fuzzy logic
model and set of rules. The results are placed
in context with the results of conventional
means and traditional PD type fuzzy logic
control combined with a variable
transformation technique. The results
demonstrate the accuracy of the methods in
achieving the objective, that is an output of pH
