95
CONTROL MODES
The Proportional Control (Low mode) is modeled as follows:
CO
error
SP PV
error
if SP PV
DEV if SP PV
DEV
CO
DEV
error
t
CP CO
t
CP t
0
0
0
<
n
n
on
n
off
on
1
:
2
$
=
-
-
-
=
=
=
-
-
)
CO
error
SP PV
error
if SP PV
DB
DEV if SP PV
DEV
CO
DEV
error
t
CP CO
t
CP t
0
0
<
>
n
n
on
n
off
on
1
:
=
=
-
-
-
=
=
=
-
-
)
CO – Control Out
CP – Control Period
PV – Process Value
t
On
– Time Relay is On over CP
SP – Set point
t
Off
– Time Relay is Off over CP
DB – Dead Band
t
n-1
– Time at n-1 CP
DEV – Deviation
t
n
– Time at n CP
error = SP - PV
Proportional control of a batch pH process using a pump as external dosing device
Same as with On/Off control, for Proportional control, a dosing solution can be an acid or a base
depending on the desired results; and control mode can be set High or Low.
With Proportional control enabled in Setup, the dosing time depends on the Deviation, the Control
period as well as how far the measurement is from the Set point. The controller will vary the On and
Off times in the defined control period.
Once enabled, and within the Deviation, the duration of the activated control is proportional to the
variance; as the measurement approaches set point, the On (relay energized) period diminishes.
Note:
When configuring the Setup values for this control, it is important to understand the
dynamics of the process. This can be determined by manually adding chemicals to the process
and seeing how long they take to react. The Control period should be approximately 1½ times
it takes the system to react. If this time is too short an additional dose causes overshooting the
desired Set point, if it is too long, the Set point may never be reached.
