User Guide

45
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