Chapter 10 Built-in PID Functions
10 - 20
Note
Transient and Normal States
1) Transien State : A state during which the control system starts its control operatoin and
reaches the desired control state; there often occurs an instantaneous output
variation and, while the integral value approaches stability, there may occur a
vibration or overshoot.
2) Normal State : A state during which the control system reaches the desired state after
passing through the transient state; vibration is eliminated, there may occur
an offset and there is little change in the output value.
Upper / Lower Deadbands
When the analog output of the sensor is converted into a digital signal by means of an AD
device, much of the signal is mixed with noise. The PID control instruction executes Auto-
tuning using this converted value; it enables PV to increase and dedcrease 3 times up and
down at SV. In the course of this process, if noise is input at the time when SV equals PV, a
single up and down conversion is incorrectly perceived as multiple conversions. This is the
same effect as the chattering of the digital switch. To overcome this, the PID controller uses a
unidirectional deadband (Hysteresis); when the PV value of the system increases toward SV,
the set deadband value operates in the upper part of SV and when the PV value passes SV
and decreases, the set deadband value operates only in the lower part of SV.
Time
SV
PV
Mode
Transition
Mode
Transition
Upper
Deadband
Lower
Deadband
Upper
Deadband
