Hanna Instruments HI520 - Page 89

89 Control Modes & Algorithms
Control Period is the time interval required for updating control output. High dynamic processes require frequent
control updates, meaning shorter control periods.
Dead Band represents an area where the error between Set point and process value is considered 0. Dead Band
area is unidirectional, for Control mode Low is below the Set point, for control mode High is above the Set point.
Outputs
Proportional control block
Set point
Control mode
Deviation
Control period
Minimum On timeOvertime
Control output
Control enabled
Dead Band
• Control output as 0 to 100 %
Update rate = Control period
Enabled by
• Settings
• Controller status
Figure 29: Proportional Control Block
The Proportional Control (Low mode) is modeled as follows:
CO
errorSPPV
error
if SP PV
DEVifSPPVDEV
CO
DEV
error
tCPCO
tCPt
0
00
<
n
n
on n
offon
1
:
2
$
=-
-
-
=
=
=-
-
)
CO
errorSPPV
error
if SP PV DB
DEVifSPPVDEV
CO
DEV
error
tCPCO
tCPt
0
0<
>
n
n
on n
offon
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. Control mode can be set High or Low.
With Proportional control enabled in Setup, the dosing time depends on the Deviation, the Control period, and
how far the measurement is from the Set point. The controller will vary the On and Off times in the defined
control period.