Every control arrangement−from the simplest, self-contained valve to the most complicated, computerized control system-shares certain basic elements.
A sensor monitors and measures a variable (something that changes). For example: temperature, humidity, pressure.
A controller receives information from the sensor, selects a portion of that input for control, and then produces an intelligent output signal.
A controlled device will act upon the signal from the controller.
A source of energy is required to power the control system. The most common sources of power for automatic control systems are compressed air and electricity.
Excerpt from Technical Institute Manual #3, available at the online store at RSES, Refrigeration Service Engineers Society, The HVACR Training Authority