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opening and closing of a valve through which a neutral solution is circulat-
ed. The amount of neutral solution added is relative to the opening of the
valve, which is directly proportional to the magnitude of difference between
the mixing tank pH and the set point.
Control of pH implies a complex adjustment. The relation between the added
reagent and the process pH is logarithmic. There is the possibility to intro-
duce large errors in the process due to over dosing or under dosing reagent
creating an oscillation effect. PID control can be used to reduce the possibility
of overshoot and large oscillations in the process by creating control output
proportional to the magnitude of deviation from the set point (P), time integral
of error (I), and rate of change of the measurement (D). Proportional, Integral,
and Derivative (PID) control can be used individually (typically Proportional
control only) or in combination such as PI, PD or PID. How these control
actions are used depends upon the requirements of the process.
Set point (reference value) is the desired value of the measurement. The
error is defined as the difference between the set point and measurement:
Error = Setpoint – Measurement
The descriptions and definitions of the individual control actions are as follows:
PROPORTIONAL ACTION (P):
The simplest continuous control mode is proportional control, so called
because the controller output is proportional to the magnitude of error.
However, the 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. For this reason the proportional control by itself it is
used primarily for slow, consistent processes that can tolerate high control-
ler gain, which minimizes the steady state offset. Consequently, high gain
control action can throw the process into oscillation if the process variable
becomes unstable and begins to change rapidly.
INTEGRAL ACTION (I):
To eliminate offset droop and tighten the control of the process, the integral
action is introduced in conjunction with proportional control (PI). Integral control
produces control action proportional to the time integral of the error. As long
as the error exists (steady deviation from the set point), the integral term will
continue to increase, adding more control action, driving the error toward zero.
DERIVATIVE ACTION (D):
With derivative action, the controller output is proportional to the rate of change
of the measurement and is primarily used to avoid overshoots. Derivative action