Siemens SIMATIC Manual page 45

The schematic below illustrates the overshoot of the process variable when the
excitation is too strong (process type III):
PHASE
PHASE = 1
= 0
Temp.
LMN
TUN_DLMN
Cold process state
In typical temperature processes, aborting 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.
Errors estimating the lag or order
The lag (STATUS_H = 2x1xx or 2x3xx) or the order (STATUS_H = 21xxx or 22xxx)
could not be obtained correctly. The tuning then continues with an estimated value
that cannot lead to optimum controller parameters.
Repeat the tuning and make sure that there is no disturbance of the process
variable.
Note
The special situation of a purely PT1 process is also indicated by
STATUS_H = 2x1xx (TU <= 3*CYCLE). It is then not necessary to repeat the
experiment. Weaken the controller parameters if the control oscillates.
PID Temperature Control
A5E00125039-02
PHASE = 2
PHASE = 7
LMN
SP
75 % SP
Controller Tuning in FB 58 "TCONT_CP"
Tuning stopped at 75 %
Point of inflection
SP
LMN
TUN_ON
PHASE = 0
PV
have r esulted
from unchanged
LMN
PV
Warm process state
(operating point)
t
t
3-13