Chapter 9 PID control
66 CNT-SVX12C-EN
measured value. The system may miss important information and reach
setpoint slowly or not at all.
Figure 21 and Figure 22 show how aliasing can affect system response. In
Figure 21 the sampling frequency is too slow. Because of this, many of the
actual changes in duct static pressure are missed. In Figure 22 the sam-
pling frequency is fast enough that the changes in static pressure are
tracked accurately.
Figure 21. Sampling too slowly
Figure 22. Sampling at the correct rate
Problems also arise from sampling too quickly. Some systems have natu-
rally 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 vari-
able (measured value) changes slowly, then the error will remain constant
for an extended period of time. If the process variable is sampled repeat-
edly 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 pro-
portion to the reality of the situation, which can destabilize the system.
Sampling points
Changes missed
by system
Time
Duct static pressure
Time
Duct static pressure
Sampling points
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