Sampling frequency
CNT-SVX12C-EN 65
Figure 20. Integral output added to proportional output
Derivative calculation
The derivative calculation responds to the change in error. In other
words, it responds to how quickly the measured value is approaching set-
point. The derivative calculation can be used to smooth an actuator
motion or cause an actuator to react faster.
However, derivative control has several disadvantages:
• It can react to noise in the input signal.
• Setting derivative control requires balancing between two extremes;
too much derivative gain and the system becomes unstable, too little
and the derivative gain has almost no effect.
• The lag in derivative control makes tuning difficult.
• Large error deadbands, common in HVAC applications, render deriv-
ative control ineffective.
Because of these disadvantages, derivative control is rarely used in HVAC
applications (with the exception of steam valve controllers, which use
derivative control to force the steam valve to react faster to changes).
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
Output
Time
Error ≠ 0
Error = 0
Proportional + integral
output
Proportional + integral
output if proportional
output has gone to zero
1
2
Proportional-only
output
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