Manual push buttons. These buttons are located below each end of the output meter and
are used in the manual mode of operation. Buttons are labeled to indicate their effect on
the final control element. The labels are "open-close" for valves and "slow-fast" for
variable speed devices. The left push button decreases the output signal. The right push
button increases the output signal. Either button can be depressed at two different
positions, half-in and full-in. At the half-in position, the output signal changes slowly.
At the full-in position, the output signal changes about ten times faster.
Mode indicating lights. Located directly below the manual push buttons, these lights
indicate the operating mode of the controller. When in manual mode, the left light,
labeled "M", will be lit; when in the automatic mode, the right light, labeled "A", will be
Mode selection buttons. Located directly under each mode indicating light, each button
will select its respective mode of control. If the button below the "M" mode light is
depressed, the controller will be in the manual mode of operation; if the button below the
"A" mode light is depressed, the controller will be in the automatic mode of operation.
As previously discussed, a particular plant will probably have controllers different from the two
described here. Although most information provided can be generally applied, it is extremely
important that the operator know the specific plants controllers and their applications.
Final control elements are devices that complete the control loop. They link the output of the
controlling elements with their processes. Some final control elements are designed for specific
applications. For example, neutron-absorbing control rods of a reactor are specifically designed
to regulate neutron-power level. However, the majority of final control elements are general
application devices such as valves, dampers, pumps, and electric heaters. Valves and dampers
have similar functions. Valves regulate flow rate of a liquid while dampers regulate flow of air
and gases. Pumps, like valves, can be used to control flow of a fluid. Heaters are used to
These devices can be arranged to provide a type of "on-off" control to maintain a variable
between maximum and minimum values. This is accomplished by opening and shutting valves
or dampers or energizing and de-energizing pumps or heaters. On the other hand, these devices
can be modulated over a given operating band to provide a proportional control. This is
accomplished by positioning valves or dampers, varying the speed of a pump, or regulating the
current through electric heater. There are many options to a process control.
Of the final control elements discussed, the most widely used in power plants are valves. Valves
can be easily adapted to control liquid level in a tank, temperature of a heat exchanger, or flow