Chapter 9 PID control
68 CNT-SVX12C-EN
PID loop action
The action of a PID loop determines how it reacts to a change in the pro-
cess variable (such as a room temperature). A controller using direct
action increases the output when the process variable increases. A con-
troller using reverse action decreases the output when the process vari-
able increases.
Direct action
Figure 24 shows the temperature when a system is cooling a space. If the
error is large and the PID output is at 100%, the actuator and valve com-
bination are fully open. As the process variable (room temperature)
decreases, the error becomes smaller, and the controller closes the valve
to reduce or stop cooling. Since the PID output and process variable move
in the same direction (both decreasing), the loop is direct acting.
Figure 24. Cooling a space
Reverse action
Figure 25 shows the temperature when a system is heating a space. If the
error is large and the PID output is at 100%, the actuator and valve com-
bination are fully open. If the process variable (room temperature)
increases, reducing the error, the controller closes the valve to reduce
heating. Since the PID output and process variable move in opposite
directions, the loop is reverse acting.
Figure 25. Heating a space
Error
Process variable
(temperature)
Setpoint
Time
Temperature
Error
Setpoint
Temperature
Time
Process variable
(temperature)
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