Process controllers are components which basically have an input of the
error signal, i.e. tile difference between the required value signal and the
feedback signal, and an output of a signal to modify the system output.
The simplest form of controller is an on--off device which switches on
some correcting device when there is an error and switches it off when
the error ceases. However, such a method of control has limitations and
often more sophisticated controllers are used. While there are many ways
a controller could be designed to react to an error signal, a form of
controller which can give satisfactory control in a wide number of
situations is the three-term or PID controller. The term control mode is
used for the type of response a controller gives to an error signal and the
three basic modes that are used are proportional (P), integral (I) and
derivative (D); the three-term controller is a combination of all three
modes.
The chapter is a discussion of process controllers and the modes of
control law used, also including a brief consideration of programmable
logic controllers and embedded microprocessor-based controllers.