CNT-SVX08F-EN 37
PID Control
30. Fully open the output.
31. The stabilized temperature reads 105°F (41°C).
32. The change in temperature is 105°F - 60°F = 45°F (41 - 16 = 25°C).
33. Two-thirds of the change in measured value is 0.66 × 45°F = 30°F, so two-thirds of the total
change has occurred when the temperature is 60°F + 30°F = 90°F (0.66 × 25°C = 17°C; 16 + 17
= 33°C).
34. Close the output. The temperature stabilizes.
35. Fully open the output. The time to reach 90°F (33°C) is 54 seconds (so the system time constant
is 54
seconds).
36.
Divide the system time constant by ten, resultin
g in 54 ÷ 10
= 5.4. The best initial sampling
frequency is 5 seconds.
PID Loop Action
The action of a PID loop determines how it reacts to a change in the process variable (such as a
room temperature) through either:
• Direct action
• Reverse action
Direct Action
A controller using direct action increases the output when the process variable increases.Figure 19
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 combination 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 19.
Error
Process variable
(temperature)
Setpoint
Time
Temperature
Cooling a space
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