Pid Loop Action; Direct Action; Reverse Action - Trane Tracer ZN517 Installation And Operation Manual

Chapter 9 PID control
68

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
Process variable
(temperature)
Setpoint

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
Setpoint
Process variable
(temperature)
Error
Time
Time
Error
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