Siemens SIMATIC S7-1200 Function Manual page 331

State Description of operating mode
2 Fine tuning
Fine tuning generates a constant, limited oscillation of the process value. The PID parameters are recalcu-
lated based on 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_3Step 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.
The setpoint is frozen in the CurrentSetpoint tag. Tuning is canceled when:
Setpoint > CurrentSetpoint + CancelTuningLevel
•
or
Setpoint < CurrentSetpoint - CancelTuningLevel
•
The PID parameters are backed up before fine tuning. They can be reactivated with LoadBackUp.
Requirements for fine tuning:
The motor transition time has been configured or measured.
•
The setpoint and the process value lie within the configured limits.
•
ManualEnable = FALSE
•
Reset = FALSE
•
Automatic (State = 3), inactive (State = 0) or manual (State = 4) mode
•
Fine tuning proceeds as follows when started from:
Automatic mode (State = 3)
•
Start fine tuning from automatic mode if you wish to improve the existing PID parameters through tuning.
PID_3Step controls the system using the existing PID parameters until the control loop has stabilized
and the requirements for fine tuning have been met. Only then will fine tuning start.
Inactive (State = 0) or manual mode (State = 4)
•
If the requirements for pretuning are met, pretuning is started. The determined PID parameters will be
used for control until the control loop has stabilized and the requirements for fine tuning have been met.
If PIDSelfTune.TIR.RunIn = TRUE, pretuning is skipped and an attempt is made to reach the setpoint
with the minimum or maximum output value. This can produce increased overshoot. Fine tuning then
starts automatically.
The controller switches to automatic mode following successful fine tuning. If fine tuning is unsuccessful,
the switchover of operating mode is dependent on ActivateRecoverMode and ErrorBehaviour.
The fine tuning phase is indicated with the TIR.State tag.
3 Automatic mode
In automatic mode, PID_3Step controls the controlled system in accordance with the parameters specified.
The controller switches to automatic mode if one the following requirements is fulfilled:
Pretuning successfully completed
•
Fine tuning successfully completed
•
Changing of the Mode in-out parameter to the value 3 and a rising edge at ModeActivate.
•
The switchover from automatic mode to manual mode is only bumpless if carried out in the commissioning
editor.
The ActivateRecoverMode tag is taken into consideration in automatic mode.
PID control
Function Manual, 03/2017, A5E35300227-AC
Instructions
9.2 PID_3Step
331