Preparations - Siemens SIMATIC Manual

Controller Tuning in FB 58 "TCONT_CP"
3.5

Preparations

SIMATIC and Controller
Tuning is started by the in/out parameters TUN_ON, TUN_ST or SP_INT. You can
set the parameters in the following ways:
•
With the parameter assignment user interface
•
With an operator control and monitoring device
•
From the user program
You write to the in/out parameters only for one cycle since FB 58 "TCONT_CP"
resets the parameters.
Warning
!
Death, serious injury, or considerable damage to property may occur.
During tuning, the MAN_ON parameter is not effective. As a result, the
manipulated variable or process value can achieve unacceptable, extreme
values.
The manipulated variable is set by the tuning functions. To stop the tuning, you
must first set TUN_ON = FALSE. MAN_ON is then effective again.
Ensuring a Quasi Steady State Initial Situation (Phase 0)
If there is low-frequency oscillation of the controlled variable, for example due to
bad controller parameters, the controller should be changed to manual prior to
starting the tuning and you should wait until the oscillation dies down. As an
alternative, you can change to a PI controller with less aggressive settings (low
loop gain, high integral time).
You must now wait until a steady state is achieved; in other words, until the
process value and value of the manipulated variable have settled. Asymptotic
settling or slow drift of the process value is also permitted (quasi steady-state, see
diagram below). The manipulated variable must be constant or fluctuate around a
constant mean value.
Note
You should avoid changing the manipulated variable shortly before starting the
tuning. A change in the manipulated variable can also be brought about
accidentally when setting up the conditions required for tuning
(for example closing a furnace door)! If this does happen, wait at least until the
process variable begins to settle asymptotically towards the steady state. You
will, however achieve better controller parameters if you wait until the transients
have decayed completely.
3-6
PID Temperature Control
A5E00125039-02