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Chapter 2 PID settings
16 CNT-APG002-EN
Example
In this scenario, we want to find the sampling frequency for a PID loop
controlling a heating application.
1. Fully close the output.
2. The stabilized temperature is 60°F (16°C).
3. Fully open the output.
4. The stabilized temperature is 105°F (41°C).
5. The change in temperature is 105°F – 60°F = 45°F (41 – 16 = 25°C).
6. Two-thirds of the change in measured variable 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).
7. Close the output. The temperature stabilizes.
8. Fully open the output. The time to reach 90°F (33°C) is 54 seconds (so
the system time constant is 54 seconds).
9. Divide the system time constant by ten, resulting in 54 ÷ 10 = 5.4.
The best initial sampling frequency is 5 seconds.