PMA KS 98, Руководство пользователя

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Override (limiting) control
using a three-point
stepping output
Override control is also possible by
means of a classical three-point
stepping controller. The positioning
signals of the limiting controller
must be connected as shown in the
example Fig.: 23.
Which one of the two controllers in-
fluences the process is decided in
the slave controller logic. The first
“close” pulse coming from the limit-
ing controller switches over to over-
ride control. The limited controller
automatically retrieves the position-
ing authority, when it first tries to
close the actuator further.
Bumpless auto/manual switch-over
Sudden process interventions by control mode switch-over are usually not desired. Excepted is purposeful switch-over y
r
Y2.
A
r
M switch-over is always bumpless; the last correcting value is frozen and can be changed manually. M
r
A
switch-over is different. Correcting value differences are compensated as follows: when switching over, the controller
integral action is set to correcting value Y
M
output last plus correcting variable portions of the controller P and D action
running in the background
(Y
I
= Y
M
+ Y
PD
). Now, only the integrator, which adapts the correcting variable slowly to the stationary value according
to the actual control deviation, is active. Until the D action has decayed completely, the adaptation can be delayed or
accelerated
9499 040 50611
Controller
201 Process value calculation
M
Limitation
controller
OPEN
CLOSE
positioner
e.g. flow
e.g. pressure
repeater
reply
Fig.:23 Override control using 3-point stepping controllers
A r M
Time
correcting value y
M r A
upper correcting value limit y+
lower correcting value limit y-
Manual AUTOMATIC AUTOMATIC
Y
I
Ypid
Y
M
Ypid
Y = Y - Y
I M PD
Automatic-
mode Y
PD
Fig.:24 Bumpless switchover