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process instrumentation, both measuring and controll-
ing, is provided by the customer, as are the set-point
positioners, to alter the set-point adjustments on con-
trolling instrumentation. The controllers regulate the
actuators (valve motors and valves, dampers, relays,
solenoids, etc.) that implement process control.
The Analog Output feature has been designed so
that the customer has maximum control over his out-
put functions.
Controlling Instrumentation
A home furnace thermostat is a controlling instrument.
Depending on feelings of coolness or warmth, the
householder adjusts the thermostat (set-point) up or
down. The difference between the room temperature
and the temperature specified by the set-point causes
a signal that controls the fuel supplied to the furnace.
The resulting change in
BTU
output from the furnace
corrects the room temperature to the temperature spec-
ified by the set-point.
Industrial controlling instruments are generally
more complex, but contain essentially the same ele-
ments:
A device for measuring the value of variable.
An adjustable set-point.
A control mechanism for maintaining the meas-
ured variable at the value specified by the set-
point.
In addition to these basic elements, a controlling in-
strument mayor may not have associated with it an
indicator and/or a recorder. An indicator provides a
continuous visual indication of the measured variable,
e.g., the thermometer mounted on the home thermostat.
I
Set Point
Flow Recorder Controller
}'igure 12. Automatic Control Loop
20
A recorder provides a continuous record of the meas-
ured variable. Controlling instruments are generally
named according to the scope of their functions - re-
cording, indicating, and controlling. For example, an
instrument consisting of all three units is referred
to
as an Indicator/Recorder Controller; an instrument
consisting of a recorder and a controller is referred to
as a Recorder-Controller, etc.
A basic automatic control loop for regulating fluid
flow is shown in Figure 12. The variable is measured
and transmi tted to the Flow Recorder-Con troller by
Flow Transmitter 1. The
controller~
which mayor may
not be mounted inside the instrument, acts as an analog
computer in that it determines the difference between
the measured variable and the set-point value. This
difference, if present, results in a corrective signal to
the actuator or controlling valve, V. Thus, the fluid flow
is maintained at the value specified by the position of
the set-point. When the process requires a change in
flow rate, the process operator repositions the set-point
up or down.
Most process instrumentation set-points are adjusted
by the operator. Some control problems, however, re-
quire that the set-point of an instrument be automati-
cally adjusted by the controller of another instrument.
This form of control is designated as Cascade or Mul-
tiple-Loop control. Cascade control may be employed
to regulate the supply of fuel to a steel-mill furnace.
The fuel, coke oven gas, is obtained as a byproduct of
another operation and as a result its pressure in the
fuel line varies considerably. The demand for furnace
heat also varies and Cascade control is employed to
compensate for both variables (Figure 13). Fluctua-
tions in furnace temperature are sensed by the thermo-
couple (T / c) . The temperature Indicator /Recorder, T,
v
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