Hanna Instruments HI 504 - Manual (Page 23)

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The following graph describes the pH process controller be-
havior. Similar graph may apply to the mV controller.
During proportional control the process controller calculates
the relay activation time at certain moments t
0
, t
0
+T
c
, t
0
+2T
c
etc. The ON interval (the shaded areas) is then dependent to
the error amplitude.
With the integral function (reset), the controller will reach a
more stable output around the setpoint providing a more
accurate control than with the ON/OFF or proportional ac-
tion only.
The derivative function (rate action) compensates for rapid
changes in the system reducing undershoot and overshoot of
the pH value.
During PID control, the ON interval is dependent not only to
the error amplitude but even to the previous measurements.
Definitely PID control provides more accurate and stable con-
trol than ON/OFF controllers and it is best suitable in system
with a fast response, quickly reacting to changes due to ad-
dition of acid or base solution.
An example of how the response overshoot can be improved with
a proper rate action setting is depicted in the following graphic.
t
0
t
0
+T
c
t
0
+2T
c
t
0
+3T
c
pH
RATE ACTION COMPENSATES FOR RAPID CHANGES t
sure falls below setpoint value minus hysteresis.
Such a behavior is suitable to control an acid dosing pump.
A relay enabled as a low setpoint, is energized when the pH
value is below the setpoint and is de-energized when the pH
value is above the sum of setpoint and the hysteresis. This
operational mode may be used to control an alkaline dosing
pump.
P.I.D. CONTROL MODE
PID control is designed to eliminate the cycling associated
with ON/OFF control in a rapid and steady way by means of
the combination of the Proportional, Integral and Derivative
control methods.
With the proportional function, the duration of the activated
control is proportional to the error value (Duty Cycle Control
Mode); as the measurement approaches setpoint, the ON
(relay energized) period diminishes.
ON
OFF
Setpoint 14 Setpoint –
Hysteresis
ON
OFF
14 Setpoint +
Hysteresis Setpoint