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If you study control theory you will encounter so-called block diagrams, which are schematic representations of control systems and help one to understand how they work. Below is the block diagram for a cruise control system.

Feedback loop

Vin  is the desired speed, set by the driver.
Vout  is the actual speed on the speedometer.
The pale blue device calculates the error: e = Vin - Vout  
The processor represents the engine and braking system.

Action of the controller:

  • if e > 0, then accelerate
  • if e < 0, then decelerate