Siemens SIMATIC S7-1200 Function Manual page 227

Remedy: Reduce the setpoint value during the inflection point search.
Note
If processes are extremely sluggish, it is advisable during tuning to specify a target setpoint
that is somewhat lower than the desired operating point and to monitor the status bits and
PV closely (risk of overshooting).
Tuning only in the linear range:
The signals of certain processes (e.g., zinc or magnesium smelters) will pass a non-linear
area at the approach of the operating range (change in the state of aggregation).
By selecting a suitable setpoint step change, tuning can be limited to the linear range. When
the process value has passed 75% of the setpoint step change (SP_INT-PV0), tuning is
ended.
At the same time, TUN_DLMN should be reduced to the extent that the point of inflection is
guaranteed to be found before 75% of the setpoint step change is reached.
Fine tuning
During this tuning, the process with a constant setpoint is activated through a output value
jump.
PID control
Function Manual, 03/2017, A5E35300227-AC
Using PID basic functions
7.3 TCONT_CP
227