CNT-SVX08F-EN
35
PID Control
of the situation, which can destabilize the system. The control system should always wait to
process the result of a change before making another change.
shows the process variable if sampling times are too fast, acceptable, and barely
acceptable. If the sampling frequency is too fast (2 seconds), the process variable begins to oscillate
and finally destabilizes because the PID loop output drives the actuator to extremes.
Figure 17.
System stability with different sampling times
Sampling freq. = 10 s
Sampling freq. = 20 s
Sampling freq. = 2 s
(system destabilizes if
sampling freq. is too fast)
Time
Process variable
Calculating the Sampling Frequency
shows suggested program execution frequencies for common applications. These are
good initial values, but it may take some trial and error to find the best frequency.
Table 8.
Suggested starting sampling frequencies
Application
Suggested Execution Frequency
Duct static pressure
5 seconds
Discharge-air temperature
10 seconds
Space temperature (typical comfort zone)
60 seconds
Space temperature (high air change zone)
30 seconds
Duct humidity
10 seconds
Space humidity
30–60 seconds
It is possible to manually calculate the starting point for the application sampling frequency.
To calculate the sampling frequency:
19. Manually control the analog output to 0%. For example, control a heating valve closed.
