Specifications

4 Control and Automation Solutions Guide
Basic Process Control
How simple can process control be?
Consider a common household oven.
The oven’s components are enclosed
inside one container, so no long distance
communication is necessary. When the
user sets the thermostat to the desired
temperature, the oven maintains the
internal temperature at the set point.
When the thermostat setting senses that
the oven temperature is low, the switch
is closed, completing the circuit to open
the gas valve to the main burner. Once
the thermostat detects that the oven
has reached the set point, the switch
opens, the gas valve closes, and the
main burner shuts o. The cycle repeats
as needed. The pilot light provides a
fail-safe function while also providing
an ignition source for the main burner. If
the pilot light were to go out, no voltage
would be created by the thermocouple,
so the main valve would not open.
Consider a process-control system that
goes beyond what is needed for such a
small, simple system like a household
oven. What controls and congurations
are necessary in a factory? For a fully
automated bakery, for example, many
subsystems are needed such as weigh
scales, valves, ow gauges, mixers,
yeast-rising warming chambers, ovens,
conveyer belts, fans, and packaging
equipment. If this bakery is to be
fully automated, a process-control
system would be needed to manage
and coordinate all of the time-critical
events between these subsystems.
More complex communications
capability is required when the
controllers and the controlled elements
are separated by a signicant distance.
In a complex control environment,
a PLC spends signicant time
communicating signals and process
events to other components of the
system. For more information on
communications needs, see the Fieldbus
Functions section in this chapter.
THERMOCOUPLE
350°F
GAS LINE IN
MAIN
BURNER
ELECTRICALLY OPERATED
DIAPHRAGM VALVE
PILOT VALVE
THERMOSTAT
HOME OVEN
A household oven serves as a simple example of process control.