Mechanical "watchdogs" called controllers are installed in a system to maintain
process variables within a given parameter.
STATE the purpose of the following components of a
typical control station:
Manual-automatic transfer switch
Manual output adjust knob
DESCRIBE the operation of a self-balancing control
Controllers are the controlling element of a control loop. Their function is to maintain a process
variable (pressure, temperature, level, etc.) at some desired value. This value may or may not
The function is accomplished by comparing a setpoint signal (desired value) with the actual value
(controlled variable). If the two values differ, an error signal is produced.
The error signal is amplified (increased in strength) to produce a controller output signal. The
output signal is sent to a final control element which alters a manipulated variable and returns
the controlled variable to setpoint.
This chapter will describe two controllers commonly found in nuclear facility control rooms.
Although plants may have other types of controllers, information presented here will generally
apply to those controllers as well.
Control stations perform the function of a controller and provide additional controls and
indicators to allow an operator to manually adjust the controller output to the final control
Figure 32 shows the front panel of a typical control station. It contains several indicators and
controls. Each will be discussed.