Hanna Instruments HI520 - Page 85

85 Control Modes & Algorithms
Navigation:
• Press the
m from the Measure mode.
• Select Setup from Channel.
• Select Setup with Control Settings highlighted.
• Press the
ud keys to move between parameters.
• Select parameter to be controlled.
• Assign the Set point value and select control mode: On/Off (constant), Proportional, PID.
18.2.1. On/Off Control Algorithm
On/Off Control is the simplest type of feedback control. The controller drives the relay On or Off, and the Analog Output
at the maximum or at the minimum value depending on the position of the controlled variable relative to the Set point.
The control mode can be set High or Low. High control mode is recommended if the process value is too high
and users want to decrease it using an acid. Low control mode is recommended if the process value is too low
and users want to increase it using a base.
Inputs
• Set point as an absolute controlled parameter value
• Control mode as High or Low
• Hysteresis as a relative parameter, one‑side only
Outputs
On/Off control block
Set point
Control mode
Hysteresis
Update rate
Minimum On timeOvertime
Control output
Control enabled
• Control output as either 0 or 100 %
Update rate = 1 second
Enabled by
• Settings
• Controller status
Figure 23: On/Off Control Block Algorithm
On/Off control (Low mode) is modeled as follows:
CO
CO
if PV SP Hysteresis
if PV SP Hysteresi
s
1
1
0
n
n
1
2
#
=
+
+
-
)
CO
CO
if PV SP
if PV SP
0
1
0
n
n
1
1
$
=
-
)
CO – Control Out
PV – Process Value
SP – Set point