How Cruise Control Works

Driving is not always the delightful and entertaining treat we'd like it to be. Sometimes it's downright boring. When the road stretches on to the horizon and traffic is nil, cruise control is one of those little luxuries that helps the miles melt away more comfortably.

Function
Exactly how do these silent sentinels of speed work? By monitoring time, the vehicle's speed and throttle position, the cruise control's Electronic Control Unit (ECU) commands an actuator to open or close the throttle to maintain the speed that the driver has selected. As anybody who's driven with a particularly bad cruise control will tell you, this is a surprisingly difficult technical challenge. With changes in road grade, headwind or tailwind, a cruise control has to be pretty smart to stay smoothly locked on its set speed.

Cruise control is one of those little luxuries that helps the miles melt away more comfortably.
The components of the system are innocuous enough. The actuator is usually a vacuum powered device that uses engine manifold vacuum to provide the energy to set the throttle correctly. In newer throttle-by-wire cars, the cruise control logic is simply integrated into the electronics that command the throttle position, so there are no visible components. All cruise control systems monitor brake pedal position and turn off automatically if you put your foot on the brake. (Incidentally, even if the cruise control should fail in the on position and not turn off with a tap of the brake, the brakes themselves have more than enough authority to bring a modern vehicle to a complete stop even with the throttle fully open.)

Logic
Typically, almost all of the controls you need to operate the cruise control system are mounted on the steering wheel within easy reach. Once you accelerate the vehicle up to speed, pressing the "set/accel" button locks in your velocity. Press it again if you need minor upward adjustments in speed, or tap the "coast" (sometimes labeled "decel") button to reduce the set speed. If traffic forces you to touch the brake, the system will shut off. Tap the "resume" button, and the car will accelerate up to the most recently set speed.


The operating logic that makes for smooth and accurate speed control is necessarily complex. Most current systems use something called Proportional-Integral-Derivative (PID) control. From a practical standpoint this means that the system uses three different types of calculations to come up with the proper throttle setting required from moment to moment. The Proportional aspect compares the vehicle's actual speed with the set speed. If the error is large, the system makes a big change in throttle setting to get the velocity back to the set speed. As the velocity nears the set speed the throttle adjustment is proportionally reduced so that the vehicle doesn't overshoot the set speed. The Integral factor calculates time and distance, comparing the set velocity (and calculated distance traveled over time) versus the actual velocity and distance traveled. This Integral logic is particularly good at correcting the long duration, small reduction in speed you get on a gradual hill, where the previously mentioned Proportional logic lacks sufficient authority to get the speed where it belongs. The final logic aspect, Derivative, watches for fast deviations from the set speed that indicate a sudden and steep hill. Based on the rate of velocity change, this logic starts a correction with the throttle more decisively than the two other types of logic.