Requirements For An Optimization - Siemens SIMATIC S7-1200 Function Manual

Using PID basic functions
7.3 TCONT_CP
7.3.3.2

Requirements for an optimization

Transient response
The process must have a stable, asymptotic transient response with time lag.
The process value must settle to steady state after a step change of the manipulated
variable. This therefore excludes processes that already show an oscillating response
without control, as well as processes with no recovery (integrator in the control system).
This may result in death, severe injury or considerable property damage.
During an tuning, the parameter MAN_ON is ineffective. Through this, the output value or
process value may take on undesired - even extreme - values.
The output value is defined through the tuning. To cancel the tuning, you first have to set
TUN_ON = FALSE. This makes MAN_ON effective again.
Guaranteeing a stationary initial state (phase 0)
With lower-frequency oscillations of the process value, for example, due to incorrect
controller parameters, the controller must be put in manual mode before the tuning is started
and wait for the oscillation to stop. Alternatively, you could switch to a "soft" set PI controller
(small loop gain large integration time).
Now you have to wait until the stationary state is reached, this means, until the process
value and output value have a steady state. It is also permissible to have an asymptotic
transient oscillation or slow drifting of the process value (stationary state, see the following
image). The output value must be constant or fluctuate by a constant average.
Note
Avoid changing the manipulated variable shortly before starting the tuning. A change of the
manipulated variable can occur in an unintended manner through the establishment of the
test conditions (for example, closing an oven door)! If this does happen, you have to at least
wait until the process value has an asymptotic transient oscillation in a stationary state again.
Better controller parameters can be reached if you wait until the transient effect has
completely subsided.
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WARNING
Function Manual, 03/2017, A5E35300227-AC
PID control