Cruise control is a feature of many vehicles that maintains a desired speed for the driver. It's best used on straight, flat highways. While it may seem like a fairly simple task to maintain a given speed, it actually takes quite a few components and a rather smart "brain". Listed below are some of the essential parts of the system.

Buttons - These are different depending on the vehicle manufacturer, but include a way to activate the system, a way to deactivate it, a way to resume a previously set speed, and often a way to tap the speed up or down in one mph increments.

Safety Switches - These switches deactivate the system whenever the driver steps on the clutch or the brake. This is to prevent the system from counteracting the stop (in the case of the brake) and to prevent the system from over-revving the engine (in the case of the clutch). Usually the driver can resume the set speed by pressing a "Resume" or "Recall" button.

Note that the cruise control does not check the position of your transmission, or even the fact that you are in gear at all.

Throttle Actuator - This device uses the vacuum generated by your engine to adjust the throttle position. In some vehicles, a small motor is used in lieu of vacuum power. Either way, the effect is the same. This device is used by the cruise control to apply power as necessary to maintain a speed.

Vehicle Speed Sensor - This device, usually mounted on the output shaft of your transmission, sends pulses to your computer and speedometer representative of your speed. The higher the frequency of the pulses, the higher the speed. A cruise control system needs to know the speed of the vehicle to adjust the throttle position.

Computer - Sometimes this is a part of your vehicle's main computer, but more often (especially in the case of aftermarket installations) it is a separate unit. This is easily the most complicated part of the cruise control system.

The computer in a cruise control system will most often utilize a proportional integral derivative control system to determine the amount of throttle to give the vehicle based on the speed. "Wait," you say, "integral calculus in my car's computer?" Yeah, pretty sweet, huh? PID systems are used in hundreds of applications where pumps, valves, engines, and motors need to be controlled because they are incredibly responsive to both quick and gradual changes. Cruise control systems that use this type of computer respond well to hills, and maintain speed with incredible accuracy.

To wrap up, here a few safety issues of which you should be aware when using cruise control:

  • Don't use cruise control in icy, slippery, or off-road conditions. The system is not designed to deal with limited traction, and will try to accelerate the vehicle more as soon as you start to slip.
  • Don't use cruise control if you are tired or fatigued. You always need to be giving 100% of your attention to the task of driving, and often removing the burden of throttle control can make you get a little too comfortable and sleepy.
  • Don't set cruise control and then recall it in a lower gear unless you are full aware of what you are doing. The system has no transmission position sensor, and it will not shift for you. Running the engine at a high number of RPMs for extended periods of time can cause expensive damage and overheating.
  • Don't recall or otherwise activate the cruise control in neutral. Doing so will usually cause the computer to go to 100% throttle (it assumes you're going up a really steep hill, since there is no change in speed) and your engine to bounce off the safety rev limiter. This is not in any way good for the engine, so try to avoid it.

Cruise control is a very nice feature to have in any kind of driving, and takes a great deal of stress off the driver on long trips. As long as it's used properly, it's a very safe and convenient way to drive. These systems will eventually be the basis for cars that drive themselves.

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