Liquiline CM44P Operation
Endress+Hauser 107
Menu/Setup/Additional functions/Mathematical functions
• MF1: Formula
• Tracking = On
• Source A = Time switch 1
• Source B = pH value of a Memosens pH input
• Formula = A AND(B<4.0)
3. Use the formula as the data source of a relay.
Menu/Setup/Outputs/Relay[x:y]
• Function = Formula
• Operating mode = Static signal
• Source of data = MF1: Formula
The formula yields a logical value (TRUE or FALSE) and is thus suitable to trigger a relay
directly in the static operating mode. Time switch 1 provides a TRUE value for 10 minutes
every 2 hours but only if the pH value has dropped to below 4 at the same time.
10.6.3 Controller
Controller structure in Laplace representation
A0015007
92 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%).
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