Siemens SIMATIC S7-1200 Function Manual page 478

Instructions
9.4 PID basic functions
Control zone (CONZ_ON, CON_ZONE)
If CONZ_ON = TRUE, the controller operates with a control zone. This means that the
controller operates according to the following algorithm:
● If process value PV exceeds the setpoint SP_INT by more than CON_ZONE, the value
LMN_LLM is output as the manipulated variable.
● If the process value PV falls below setpoint SP_INT by more than CON_ZONE,
LMN_HLM is output.
● If the process value PV is within the control zone (CON_ZONE), the output value takes its
value from the PID algorithm LMN_Sum.
Note
Changing the manipulated variable from LMN_LLM or LMN_HLM to LMN_Sum occurs
under compliance of a hysteresis of 20% of the control zone.
Note
Before enabling the control zone manually, make sure that the control zone band is not
too narrow. If the control zone band is too small, oscillations will occur in the manipulated
variable and process value.
Advantage of the Control Zone
When the process value enters the control zone, the D-action causes an extremely fast
reduction of the manipulated variable. This means that the control zone is only useful when
the D-action is activated. Without a control zone, only the reducing P-action would
essentially reduce the manipulated variable. The control zone leads to faster settling without
overshoot or undershoot if the output minimum or maximum manipulated variable is a long
way from the manipulated variable required for the new operating point.
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Function Manual, 03/2017, A5E35300227-AC
PID control