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CNT-SVX08F-EN
PID Control
Sampling Frequency
The sampling frequency is the rate at which the input signal is sampled and PID calculations are
performed. Using the right sampling frequency is vital to achieving a responsive and stable
system. Problems can arise if 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 value. The system may miss important
information and reach setpoint slowly or not at all.
show how aliasing can affect system response. In
, the sampling
frequency is too slow. Because of this, many of the actual changes in duct static pressure are
missed. In
, the sampling frequency is fast enough that the changes in static pressure are
tracked accurately.
Figure 15.
Sampling too slowly
Sampling points
Changes missed
by system
Time
Duct s
tatic pressure
Figure 16.
Sampling at the correct rate
Time
Duct static
pressure
Sampling points
Problems also arise from sampling too quickly. Some systems have naturally slow response times,
such as when measuring room temperature. Slow response times can also be caused by
equipment lags. Since PID loops respond to error and changes in error over time, if the process
variable (measured value) changes slowly, then the error will remain constant for an extended
period of time. If the process variable is sampled repeatedly during this time, the proportional
output remains about the same, but the integral output becomes larger (because it is the sum of
past errors). When the control system does respond, the response is out of proportion to the reality
