Searching For The Point Of Inflection (Phase 2) And Calculating The Control Parameters (Phase 3, 4, 5); Checking The Process Type (Phase 7) - Siemens SIMATIC Manual

Controller Tuning in FB 58 "TCONT_CP"
3.7 Searching for the Point of Inflection (Phase 2) and
Calculating the Control Parameters (Phase 3, 4, 5)
In Phase 2, the tuning function attempts to detect the point of inflection with the
manipulated variable remaining constant. This method prevents the point of
inflection being found too early as a result of process variable noise.
With the pulse controller, the process variable is averaged over N pulse cycles and
then made available to the controller stage. There is a further averaging of the
process variable in the controller stage: Initially, this averaging is inactive, in other
words, averaging is always over 1 cycle. As long as the noise exceeds a certain
level, the number of cycles is doubled.
The period and amplitude of the noise are calculated. The search for the point of
inflection is canceled and Phase 2 is exited only when the gradient is always
smaller than the maximum rise during the estimated period. TU and T_P_INF are,
however, calculated at the actual point of inflection.
Tuning is, however, only terminated when the following two conditions are met:
1. The process variable is more than 2*NOISE_PV away from the point of
inflection.
2. The process variable has exceeded the point of inflection by 20%.
Note
When exciting the process using a setpoint step change, the tuning is terminated
at the latest when the process variable exceeds 75% of the setpoint step change
(SP_INT-PV0) (see below).
Phases 3, 4 and 5 are then run through once. The process type is then checked in
Phase 7.The tuning mode is then terminated and FB 58 "TCONT_CP" is once
again in Phase 0. The controller now always starts in the automatic mode with
LMN = LMN0 + 0.75*TUN_DLMN (even if you were controlling in manual mode
prior to the tuning).
3.8

Checking the Process Type (Phase 7)

Phase 7 therefore checks whether or not the process type is correct. This check is
made in automatic mode with the controller parameters that have just been
calculated and is completed at least 0.35*TA (recovery time) after the point of
inflection. If the process order highly differs from the estimated value, the controller
parameters will be recalculated and STATUS_D will be increase by 1 otherwise,
the controller parameters remain unchanged.
Note
If Phase 7 is aborted by TUN_ON=FALSE, the controller parameters that have
already been obtained are retained!
3-10
PID Temperature Control
A5E00125039-02