Pretuning - Siemens SIMATIC S7-1200 Function Manual

Using PID_Temp
6.3 Commissioning PID_Temp
6.3.2

Pretuning

The pretuning determines the process response to a jump change of the output value and
searches for the point of inflection. The tuned PID parameters are calculated as a function of
the maximum slope and dead time of the controlled system. You obtain the best PID
parameters when you perform pretuning and fine tuning.
The more stable the process value is, the easier it is to calculate the PID parameters and the
more precise the result will be. Noise on the process value can be tolerated as long as the
rate of rise of the process value is significantly higher compared to the noise. This is most
likely the case in operating modes "Inactive" or "Manual mode". The PID parameters are
backed up before being recalculated.
PID_Temp offers different pretuning types depending on the configuration:
● Pretuning heating
A jump is output at the output value heating, the PID parameters for heating are
calculated and then the setpoint is used as the control variable in automatic mode.
● 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 (Retain.CtrlParams.Heat structure) and cooling
(Retain.CtrlParams.Cool structure) are calculated and then the setpoint is used as the
control variable in automatic mode.
● Pretuning cooling
A jump is output at the output value cooling.
The PID parameters for cooling are calculated and then the setpoint is used as the
control variable 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
● The PID_Temp instruction is called in a cyclic interrupt OB.
● ManualEnable = FALSE
● Reset = FALSE
● PID_Temp is in one of the following modes: "Inactive", "Manual mode", or "Automatic
mode".
● The setpoint and the process value lie within the configured limits (see Process value
monitoring (Page 170) configuration).
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Function Manual, 03/2017, A5E35300227-AC
PID control