West Control Solutions KS98-1 - Manual (Page 263)

Process-value tracking
It can occur that the is far distant from the momentary process-value (e.g. when starting a plant).
In order to prevent the jump developing here, the function process-value tracking can be used.
Process-value tracking causes a take-over of the process-value on the internal setpoint , when changing over Wext
r W. When shifting (W back r Wext) Wext is started with the attitude of Grw+/- (see rFigure below).
If the controller shall follow process value or setpoint-tracking is determined with the configurationword Wtrac. The
digital input "track" enables Tracking. Tracking can be activated via interface or by operating the switchover Wext r
W . Tracking is evaluated with priority.
a
If a signal source switches to tracking, switching from another source is not possible!
Setpoint and correcting variable behaviour after setpoint switch-over
After setpoint and correcting variable switch-over, control behaviour or start-up behaviour has priority. The PID charac -
teristic must be partly suppressed. The previous history which is important for the integral action and especially for the
derivative action is largely insignificant with setpoint change due to the new target setpoint.
Switch-over operations which might affect the control behaviour are:
1 Manual -> automatic Switch-over from manual to automatic mode
2 Off -> start-up Start-up after off-line (power failurel/configuring)
3 W
old
-> W
new
Setpoint change
4 W -> W2 Switch-over to 2nd setpoint
5 W2 -> W Switch-over from 2nd setpoint to normal setpoint
6 We -> Wi, without tracking Switch-over from external to internal setpoint without tracking
7 Wi -> We Switch-over from internal to external setpoint
8 We -> Wi with tracking Switch-over from external to internal setpoint with tracking
The approach to a new setpoint may be affected by further parameters. Parameters Grw+ (positive setpoint gradient),
Grw- (negative setpoint gradient) and Grw2 (setpoint gradient during the approach to W2) can be used for gradual ap -
proach to a new target setpoint via a ramp function.
Unless a gradient is defined (Grw = off), approach to the new setpoint starts with a step change at the previous set -
point or at the actual process value.
To influence the correcting variable when switching over, any after-effect of the derivative action is eliminated inter -
nally or the integral action is adapted to avoid correcting variable bumps.
The following table gives a survey of the controller switch-over behaviour implemented from operating version 8.
9499-040-82711
Setpoint functions III-263
internal
setpoint W
internal setpoint W
upper setpoint limit W100
effective setpoint
x
W
Wext
Time
Setpoint, process value
Process value
Grw -
lower setpoint limit W0
Switch over Switch over
Wext W r W Wext r
Wext
Fig. 82: