Hanna Instruments HI 8050 - Manual (Page 11)

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circulated. The amount of additive solution added is relative to the open-
ing of the valve, which is directly proportional to the magnitude of differ-
ence between the “water pool” pH and the set point.
Set point (reference value) is the desired value of the measurement.
Error is define as the difference between set point and measurement:
Error = setpoint – measurement Error = setpoint – measurement Error = setpoint – measurement Error = setpoint – measurement Error = setpoint – measurement
The descriptions and definitions of the individual control actions are as
follows:
Proportional control so called because the controller output is propor-
tional to the magnitude of error. However, proportional control is subject
to one major limitation, steady state offset (steady deviation from the set
point). Increasing the sensitivity of the controller (controller gain) can
reduce the steady state offset but only with slow processes. Because of this
proportional control by itself is used primarily for slow, consistent pro-
cesses that can tolerate high controller gain (like the control of pH and EC
in irrigation applications), which minimizes steady state offset. Both pH
and EC are controlled by using the same proportional control method.
The proportional control acts on the closing and opening timings of the
motorized valves.
The control cycle used for the pH and EC control can be set by using
panel S60 (SET PROP CTRL). To regulate the aperture of the motorized
valves the proportional control will control the opening and closing times.
These timings are in fact pecentages from the control cycle length.
The sensivity factor simply divides the times used for the actual command
of the motorized valves. It can be used for a fine time of the proportional
control. For each program and for each of the controls (pH or EC) two
parameters define the behavior of thecontroller around a designated set-
point:
- the dead band is the area around the setpoint where the dosage is “in
limits” therefore the actuators need not to change their position.
- the proportional band is the an area around the setpoint where the
proportional control takes place. Outside of these two areas the controller
will open or close the motrized valve with 80% of the control cycle.