Endress+Hauser Liquiline System CA80HA, Operating Instructions Manual

Page: 125 / 214

Liquiline System CA80HA Operation
Hauser 125
10.8.3 Controller
Controller structure in Laplace representation
A B
A0015007
 70 Block diagram of the controller structure
A Neutral zone I Integral value
B Output limiting D Derivative value
K
p
Gain (P ‐ value) αT
V
Damping time constant with α = 0 to 1
T
n
Integral action time (I ‐ value) e Control deviation
T
v
Derivative action time (D ‐ value) w Set point
T
w
Time constant for set point damping x Controlled variable
T
wu
Time constant for anti ‐ windup feedback y Manipulated variable
P Proportional value
The controller structure of the device comprises set point damping at the input to prevent
erratic changes in the manipulated variable if the set point changes. The difference between
the set point w and the controlled variable (measured value) X results in the control deviation
which is filtered by a neutral zone.
The neutral zone is used to suppress control deviations (e) that are too small. The control
deviation thus filtered is now fed to the actual PID controller which divides into 3 parts based
on the P (proportional), I (integral) and D (derivative) values (top-down). The integral section
(middle) also comprises an anti ‐ windup mechanism for limiting the integrator. A low-pass
filter is added to the D ‐ section to damp hard D ‐ terms in the manipulated variable. The sum of
the 3 sections results in the internal controller manipulated variable which is limited
according to the settings (for PID
‐
2s to
‐
100% to +100%).
The graphic does not illustrate a downstream filter for limiting the rate of change of the
manipulated variable (can be configured in the menu in Max y change rate ).
In the menu do not configure the gain K
p
. Instead configure the reciprocal value, the
proportional band X
p
(K
p
=1/X
p
).