Control Modes & Algorithms 86
On/Off control of a batch pH process using a pump as external dosing device
Dosing solution can be an acid or a base, depending on the desired results. Control mode can be set as High or Low.
With On/Off control type enabled in Setup, the algorithm uses configured “Set point” and “hysteresis” parameters.
See 6.3 (Process) Control Settings & Alarm Settings section for further details.
With High mode control, the hysteresis is below the Set point. With Low mode control, the hysteresis is above
the Set point.
•
When in High control mode the controlled process value is too high. The dosing pump runs (adding an acid
to bring down the pH) until the process value decreases to the Set point minus hysteresis value. Above the
Set point, the relay is activated. The dosing pump turns off and remains off until process value reaches
Set point value.
•
When in Low control mode, the controlled process value is too low. The dosing pump starts running (adding
a base to bring up the pH) until it reaches the Set point plus hysteresis. The pump remains off until the
process value decreases to a value equal to Set point.
High control mode
Relay statusRelay status
On
Off
Low control mode
On/Off Control
Set point
Measured value
Set point
Measured value
Hysteresis
Hysteresis
On
Off
Figure 24: On/Off Control, High /Low Control Mode
Following graphs exemplify how the input parameters work.
Here’s an example of hystersis‑free control output.
SP
Control mode High
Process value
Control output
Control output
On
Off
On
Off
Figure 25: General On/Off Control
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