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MODEL 54e pH/ORP SECTION 6.0
THEORY OF OPERATION
6.9 PID CONTROL (CODE -20)
PID Control
The Model 54e pH/ORP current outputs can be pro-
grammed for PID control. PID control is used with a
control device which is capable of varying its output
from 0 to 100 percent in response to a changing sig-
nal in milliamps. Automated control valves or vari-
able volume pumps are commonly used. These
types of devices are referred to as modulating con-
trol devices because of their 0 to 100% adjustability.
PID control is typically used where greater accuracy
than is achievable with an on/off device is required,
or where it is desirable to have the pump or valve
"on" continuously, or where the existing or preferred
pump or valve is of the modulating type.
Any process control system must manually or auto-
matically hold the controlled variable (pH, conductiv-
ity, temperature) in a steady condition at selected set
point values. For manual control, the operator looks
at the value of the process variable, decides
whether or not it is correct, and makes necessary
adjustments. He decides the amount, direction, rate
of change and duration of the adjustment. With auto-
matic control, the controller does all of this. The
operator only adjusts the set point of the controller to
the selected value of the measured variable.
Automatic process control such as PID is usually
feedback control; it eliminates the deviation between
measurement and set point based on continuous
updates (feedback) from the process itself.
Measurement and Set Point (Feedback Control)
The Model 54e pH/ORP controller is given two items
of information: measurement and set point. The con-
troller reacts to the difference in value of these two
signals and produces an analog output signal to
eliminate that difference. As long as the difference
exists, the controller will try to eliminate it with the
output signal. When measurement and set point are
equal, the condition of the controller is static and its
output is unchanged. Any deviation of measurement
from set point will cause the controller to react by
changing its output signal.
PID Control Mode Combinations
All PID controllers have several control modes which
can be used in various combinations: proportional
plus integral (reset), proportional plus derivative
(rate) and a combination of proportional (P), integral
(I) and derivative (D). Each control mode produces a
response to the deviation of measurement from set
point that is the result of a specific characteristic of
the deviation, and each control mode is separately
adjustable. D, the derivative, or rate mode, is seldom
used in water treatment and is beyond the scope of
this manual.