A cruise control system basically consists of the following devices: vehicle speed sensor, control computer, actuator to set throttle, cancellation switches, and control buttons for the driver. Obviously, real systems in today’s vehicles are extremely complex, but they still consist of devices that perform these necessary functions that allow a cruise control system to exist. The following paragraphs give a quick summary of the purpose of each component.
Vehicle speed sensor: the vehicle speed sensor measures how fast the vehicle is traveling. This is central to the control of speed, because in order to maintain a constant speed, the actual desired speed of the car must be known. When a cruise control system is activated, the vehicle speed sensor sends data of how fast the car is traveling to a computer where the value is stored as a reference. Most vehicles use a rotating mechanism connected to the transaxle shaft to create a pulse signal that changes proportional to the vehicle speed. Some vehicles also utilize the actual speedometer itself to sense the vehicle speed.
The control computer is where the brains of the cruise control system are located. The control computer receives signals from the vehicle speed sensor, and then sends signals out that adjust the throttle to maintain the desired speed that is determined when the system is first activated. The computer uses control logic to continually adjust the throttle as the forces acting on the car attempt to change the vehicle’s speed. The computer is also responsible for receiving input from the driver that allows the cruise control to increase or decrease the vehicle speed, as well as receiving signals that turn the cruise control off.
The actuator receives electronic signals from the computer and turns them into mechanical motion that controls how much the throttle is open. The actuator is attached to a linkage that pushes the throttle farther open or closed, depending on what the computer tells it to do. Most actuators are operated by vacuum pumps. The electronic signals from the control computer are sent to small motors that control the vacuum pump. The vacuum pump sucks air out of a chamber to create a low pressure chamber; the greater the amount of power supplied to the motors, the more the vacuum pump sucks, and the lower the pressure becomes in this chamber. As the pressure lowers, a diaphragm at one end of the chamber is pulled farther into the chamber. This diaphragm is linked to the throttle cable, and thus pulls the throttle farther open as it slides into the chamber.
Cancellation switches turn the cruise control system off. A switch is always installed on the brake pedal that breaks the cruise control circuit when the brake is depressed; likewise, for cars with manual transmission, a switch of the same sort is installed on the clutch pedal. The reasons for this are obvious—it would be very dangerous and harmful for the cruise control system to stay activated while the driver is trying to stop or slow down the car. These switches allow the cruise control system to shut off as soon as the driver desires to slow down without the driver actually having to consciously turn it off.
The user control buttons allow the driver to turn the cruise control on and off. Furthermore, most modern systems allow the driver to change the speed of the vehicle one the cruise control is on through use of accelerate and decelerate buttons. These buttons each send a signal to the control computer.