Starting Tuning (Phase 1 -> 2) - Siemens SIMATIC Manual

Controller Tuning in FB 58 "TCONT_CP"
In Phase 1, both the controller sampling time CYCLE and the pulse generator
sampling time CYCLE_P are measured and written to the relevant in/out
parameters at the beginning of Phase 2. In the control mode without the pulse
generator, CYCLE_P = CYCLE.
Note
If you call the pulse controller using SELECT = 0 or 1, you must specify the
required ratio CYCLE/CYCLE_P with the parameters CYCLE and CYCLE_P
before setting TUN_ON.
3.6 Starting Tuning (Phase 1 -> 2)
Tuning by Approaching the Operating Point with a Setpoint Step Change
The tuning manipulated variable (LMN0 + TUN_DLMN) is applied by changing the
setpoint (transition Phase 1 -> 2). The setpoint, however, becomes effective only
when the point of inflection is reached (the controller switches to automatic only
when this point is reached).
The user is responsible for selecting the magnitude of the manipulated variable
change (TUN_DLMN) according to the permitted process variable change. The
sign of TUN_DLMN must be set depending on the intended process variable
change (take into account the direction in which the control is operating).
The setpoint step change and TUN_DLMN must be suitably matched. If
TUN_DLMN is too high, there is a danger that the point of inflection will not be
found within 75% of the setpoint step change.
TUN_DLMN must, however, be high enough so that the process variable reaches
at least 22% of the setpoint step change. Otherwise, you would remain in the
tuning mode (Phase 2).
Remedy: Reduce the setpoint while the tuning function is attempting to detect the
point of inflection.
Note
With extremely sluggish processes, it is advisable to set a somewhat lower target
setpoint than the desired operating point during tuning and to monitor the status
bits and PV (risk of overshoot).
Tuning only in the linear range:
Certain controlled processes (for example zinc and magnesium smelters) have a
non-linear range shortly before the operating point (change in the state of the
material).
By selecting a suitable setpoint step change, the tuning can be limited to the
linear range. When the process variable has passed 75% of the setpoint step
change (SP_INT-PV0), tuning is terminated.
At the same time, TUN_DLMN should be reduced so that the point of inflection is
guaranteed to be found before reaching 75% of the setpoint step change.
3-8
PID Temperature Control
A5E00125039-02