Siemens SIMATIC S7-1200 Function Manual page 232

Using PID basic functions
7.3 TCONT_CP
The following schematic illustrates the overshoot of the process variable when the excitation
is too strong (process type III):
In typical temperature processes, cancelation shortly before reaching the point of inflection is
not critical in terms of the controller parameters.
If you repeat the attempt, reduce TUN_DLMN or increase the setpoint step change.
Principle: The value of the manipulated variable used for tuning must be suitable for the
setpoint step change.
Error estimating the delay time or order
The delay time (STATUS_H = 2x1xx or 2x3xx) or order (STATUS_H = 21xxx or 22xxx) were
not acquired correctly. Operation continues with an estimate that can lead to non-optimum
controller parameters.
Repeat the tuning procedure and ensure that disturbances do not occur at the process
value.
Note
The special case of a PT1-only process is also indicated by STATUS_H = 2x1xx (TU <=
3*CYCLE). In this case, it is not necessary to repeat the attempt. Reduce the controller
parameters if the control oscillates.
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Function Manual, 03/2017, A5E35300227-AC
PID control