Hanna Instruments HI 504, Инструкция по эксплуатации

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44
The following graph describes the pH process controller
behavior . 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
action 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
control than ON/OFF controllers and it is best suitable in
system with a fast response, quickly reacting to changes
due to addition 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 .