Error Situations And Remedies - Siemens SIMATIC Manual

Controller Tuning in FB 58 "TCONT_CP"
3.11

Error Situations and Remedies

Point of inflection not reached (only with excitation by setpoint step change)
Tuning is terminated at the latest when the process value has exceeded 75% of
the setpoint step change (SP-INT-PV0). This is signaled by "inflection point not
reached" in STATUS_H (2xx2x).
The currently valid setpoint always applies. By reducing the setpoint, it is possible
to achieve an earlier termination of the tuning function.
In typical temperature processes, terminating the tuning at 75% of the setpoint step
change is normally adequate to prevent overshoot. In processes with a greater lag
(TU/TA > 0.1, process type III) caution is advised. If the excitation of the
manipulated variable is too high compared with the setpoint step change, the
process variable can overshoot considerably (up to factor 3).
In higher order processes, if the point of inflection is still a long way off after
reaching 75% of the setpoint step change, there will be significant overshoot. In
addition to this, the control parameters are too aggressive. You should then
weaken the controller parameters and repeat the attempt.
3-12
PID Temperature Control
A5E00125039-02