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📘 How cruise control holds speed

Understand how feedback, speed sensors, throttle adjustment, and control error let cruise control hold a target speed.

4
lessons
~10 min
to learn
Adults
level
Start the course →

What you’ll learn

  1. A goal, not a locked pedalExplain that SET stores a speed target for a feedback loop, not a frozen accelerator position.A locked pedal cannot hold speed when the load changes. Cruise stores a set point and keeps revising the command.
  2. The gap the computer can seeShow that the loop acts on measured vehicle speed minus the set point, usually with PI action on a cleaned-up signal.Wheel-speed sensors, with a possible shaft fallback, feed a signed error. Åström and Murray teach the textbook law as PI.
  3. The actuator has a ceilingConnect electronic throttles, older vacuum servos, and hills to a bounded propulsion request.The request becomes force only within powertrain and traction limits. A steep grade can beat SET without breaking the loop.
  4. You stay in the loopName the driver exits, the temporary accelerator override, and what adaptive cruise adds.Brake, Cancel, and Off drop the request. Adaptive cruise adds a distance goal. The driver remains responsible.

Questions this course answers

What does pressing SET actually store?

SET stores a set point — a desired vehicle speed — not a locked pedal. The loop then compares that target with measured speed.

What is control error in a cruise loop?

Åström and Murray: the controller acts on the error between current and desired speed.

Where does a modern car usually get the speed the cruise loop needs?

NHTSA recall 23V-220 describes a vehicle-speed quality check that can switch from a wheel-speed sensor to the transmission output shaft.

Why can a car still lose speed on a steep hill with cruise on?

The loop can ask for more torque, but throttle, motor, and traction all have ceilings. Bosch: interventions stay within defined limits.

What does adaptive cruise add to ordinary cruise?

NHTSA: adaptive cruise automatically adjusts speed to keep a pre-set distance from the vehicle ahead. IIHS: the driver still stays engaged.

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