Due to the complexity and large number of variables in many process control
systems, microprocessor based Programmable Logic Controllers (PLC) are used
for decision making. The PLC can be configured to receive a small number of
inputs (both analog and digital) and control a small number of outputs or the
system can be expanded with plug-in modules to receive a large number of signals
and simultaneously control a large number of actuators, displays, or other
types of devices. In very complex systems, PLCs have the ability to communicate
with each other on a global basis and to send operational data to and be
controlled from a central computer terminal. Figure 13.13 shows a typical controller
setup, monitoring a single variable. The output from the sensor is conditioned
and transmitted to the input module of the controller; if the signal is
an analog signal it is converted to a digital signal and compared to a reference
signal stored in the computer. A decision can then be made and the appropriate
control signal sent via the output module to the actuator.