Siemens SIMATIC S7-1200 Function Manual page 434

Instructions
9.3 PID_Temp
State / Mode Description of operating mode
2 Fine tuning
Fine tuning generates a constant, limited oscillation of the process value. The PID parameters are tuned for
the operating point from the amplitude and frequency of this oscillation. PID parameters from fine tuning
usually have better master control and disturbance characteristics than PID parameters from pretuning. You
obtain the best PID parameters when you perform pretuning and fine tuning.
PID_Temp automatically attempts to generate an oscillation greater than the noise of the process value.
Fine tuning is only minimally influenced by the stability of the process value.
PID_Temp offers different fine tuning types depending on the configuration:
Fine tuning heating:
•
PID_Temp generates an oscillation of the process value with periodic changes at the output value heat-
ing and calculates the PID parameters for heating (Struktur Retain.CtrlParams.Heat).
Fine tuning cooling:
•
PID_Temp generates an oscillation of the process value with periodic changes at the output value cool-
ing and calculates the PID parameters for cooling (Struktur Retain.CtrlParams.Cool).
Temporary tuning offset for heating/cooling controllers
If PID_Temp is used as heating/cooling controller (Config.ActivateCooling = TRUE), the PID output value
(PidOutputSum) at the setpoint must meet the following requirements for a process value oscillation to be
generated and fine tuning to be successful:
Positive PID output value for fine tuning heating
•
Negative PID output value for fine tuning cooling
•
If this requirement is not met, you can define a temporary offset for fine tuning which is output at the output
with the opposite effect:
Offset for cooling output (PIDSelfTune.TIR.OutputOffsetCool) with fine tuning heating.
•
Define a negative tuning offset cooling which is less than the PID output value (PidOutputSum) at the
setpoint in the steady state before you start tuning.
Offset for heating output (PIDSelfTune.TIR.OutputOffsetHeat) with fine tuning cooling.
•
Define a positive tuning offset heating which is greater than the PID output value (PidOutputSum) at the
setpoint in the steady state before you start tuning.
The defined offset is balanced by the PID algorithm so that the process value remains at the setpoint. The
height of the offset allows the PID output value to be adapted correspondingly so that it fulfills the require-
ment mentioned above.
To avoid larger overshoots of the process value when defining the offset, it can also be increased in several
steps.
If PID_Temp exits the fine tuning mode, the tuning offset is reset.
434
Function Manual, 03/2017, A5E35300227-AC
PID control