Siemens SIMATIC S7-1200 Function Manual page 431

State / Mode Description of operating mode
1 Pretuning
The pretuning determines the process response to a jump change of the output value and searches for the
point of inflection. The PID parameters are calculated from the maximum rate of rise and dead time of the
controlled system. You obtain the best PID parameters when you perform pretuning and fine tuning.
PID_Temp offers different pretuning types depending on the configuration:
Pretuning heating:
•
A jump change is output at the output value heating, the PID parameters for heating are calculated (Re-
tain.CtrlParams.Heat structure), and control to the setpoint then takes place in automatic mode.
If the process behavior strongly depends on the operating point, an adaptation of the delay time can be
activated at the setpoint with PIDSelfTune.SUT.AdaptDelayTime.
Pretuning heating and cooling:
•
A jump is output at the output value heating. As soon as the process value is close to the setpoint, a
jump change is output at the output value cooling. The PID parameters for heating (Re-
tain.CtrlParams.Heat structure) and cooling (Retain.CtrlParams.Cool structure) are calculated. Then,
control to the setpoint takes place in automatic mode.
If the process behavior strongly depends on the operating point, an adaptation of the delay time can be
activated at the setpoint with PIDSelfTune.SUT.AdaptDelayTime.
Depending on the effect of the cooling actuator compared to the heating actuator, the quality of tuning
can be influenced by whether or not the heating and cooling outputs are operated simultaneously during
tuning. You can specify this with PIDSelfTune.SUT.CoolingMode.
Pretuning cooling:
•
A jump change is output at the output value cooling and the PID parameters for cooling are calculated
(Struktur Retain.CtrlParams.Cool). Then, control to the setpoint takes place in automatic mode.
If you want to tune the PID parameters for heating and cooling, you can expect a better control response
with "Pretuning heating" followed by "Pretuning cooling" rather than with "Pretuning heating and cooling".
However, carrying out pretuning in two steps takes more time.
General requirements for pretuning:
The PID_Temp instruction is called in a cyclic interrupt OB.
•
Inactive (State = 0), manual mode (State = 4), or automatic mode (State = 3)
•
ManualEnable = FALSE
•
Reset = FALSE
•
The setpoint and the process value lie within the configured limits.
•
PID control
Function Manual, 03/2017, A5E35300227-AC
Instructions
9.3 PID_Temp
431