Automating Manufacturing Systems - Process Control and ...

plc pid - 25.4
to a process, and tune it through trial and error. This is suitable for simpler systems, but
these systems are less efficient and prone to instability. In other words it is quick and easy,
but these systems can go out-of-control.
25.2 CONTROL OF LOGICAL ACTUATOR SYSTEMS
Many continuous systems will be controlled with logical actuators. Common
examples include building HVAC (Heating, Ventilation and Air Conditioning) systems.
The system setpoint is entered on a thermostat. The controller will then attempt to keep
the temperature within a few degrees as shown in Figure 25.3. If the temperature is below
the bottom limit the heater is turned on. When it passes the upper limit it is turned off, and
it will stay off until if passes the lower limit. If the gap between the upper and lower the
boundaries is larger, the heater will turn on less often, but be on for longer, and the temperature
will vary more. This technique is not exact, and time lags will often lead to overshoot
above and below the temperature limits.
upper
temp.
limit
set temp.
(nominal)
lower
temp.
limit
room
temp.
overshoot
time
heater on heater off heater on heater off heater on
Note: This system turns on/off continuously. This behavior is known hunting. If the limits
are set too close to the nominal value, the system will hunt at a faster rate. Therefore, to
prevent wear and improve efficiency we normally try to set the limits as far away from
nominal as possible.
Figure 25.3
Continuous Control with a Logical Actuator
Figure 25.4 shows a controller that will keep the temperature between 72 and 74
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