Controllers:

Controllers are the controlling element of a control loop.  Controllers function is to maintain a procedure variable (temperature, pressure, level and many more.) at a few desired value.  This value might or might not be constant.

The function is completed through comparing a setpoint signal (desired value) along with the actual value (controlled variable).  An error signal is produced if the two values differ.

The error signal is amplified (increased in strength) to generates a controller outcome signal.  An output signal is sent to a last control element that alters a manipulated variable and returns the controlled variable to setpoint.

That chapter will describe two controllers generally found in nuclear facility control rooms. While plants might have other categories of controllers, information presented here will commonly apply to those controllers as well.