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Chapter 2 PID settings
12 CNT-APG002-EN
Sampling frequency
The sampling frequency is the rate at which the input signal is sampled
and the PID calculations are performed. Using the right sampling fre-
quency is vital to achieving a responsive and stable system. Problems can
arise when the sampling frequency is too slow or too fast in comparison to
time lags in the system.
Sampling too slowly can cause an effect called aliasing in which not
enough data is sampled to form an accurate picture of changes in the
measured variable. The system may miss important information and
reach the setpoint slowly or not at all.
Figure 10 and Figure1 1 show how aliasing can affect system response.
In Figure 10 the sampling frequency is too slow. Because of this, many of
the changes in duct static pressure are missed. In Figure11 the sampling
frequency is fast enough that the changes in static pressu re are tracked
accurately.
Figure 10: Sampling too slowly
Figure 11: Sampling at the correct rate
Sampling point Changes missed
by system
Time
Duct static pressure
Time
Duct static pressure