Active Cruise Control (ACC) uses a three-beam radar sensor to monitor the road ahead of the car and to adjust the speed of the vehicle by slowing the engine or by gently applying the brakes.

Note: Active Cruise Control is commonly referred to in automotive technology as Adaptive Cruise Control.

Unlike conventional cruise control systems, BMW's ACC system is actually able to adjust the car's speed to suit the surrounding traffic conditions.

Suddenly a car blocks your path. What happens next? 

The system uses a three-beam radar sensor. This electronic eye monitors the road ahead of the car - up to 120 meters. When a car appears in the lane ahead - as in the above picture - the system calculates its location, movement and relative speed, using the reflected radar waves. The system is able to recognize whether a particular vehicle is occupying the lane ahead. As soon as the car ahead changes lanes again, your vehicle picks up speed again, accelerating to the same constant speed as before. ACC adjusts your car's speed to the changing flow of the surrounding traffic.

If a vehicle is detected in the lane ahead, the ACC system adjusts the speed of the vehicle by slowing the engine or by gently applying the brakes, and then holds the appropriate constant distance. The driver is able to choose between three different settings for constant distance.

Here's how it works:

The radar eye of ACC recognizes moving vehicles up to 120m ahead, using three overlapping radar beams. These beams reflect off the vehicles ahead. Sensors pick up the reflection and the system immediately calculates whether the car's speed needs to be adjusted, or if the road ahead is clear. 

Here's a sample block diagram for an adaptive system:

Active Cruise Control is not limited simply to using data from its radar sensors. Other sensors in the chassis control system - such as ABS, ASC+T or DSC - constantly send data on wheel revolutions and levels of vehicle pitch and centrifugal force to the central on-board computer. On the basis of the car's current path and the data supplied, the computer is able to calculate the approaching curve path. 

The computer then compares the current path with the position of the vehicle ahead and can determine whether the curve ahead could effect the vehicle's path. In this case, ACC reduces the speed accordingly. Application of the brake or accelerator immediately deactivates the ACC system. To re-activate ACC, you simply call up the previously set speed and turn ACC on again.

In the case of applying the accelerator when ACC is activated, the car will accelerate as normal, simply going over the ACC pre-set speed. When the accelerator is released, the ACC again holds the car to the pre-set speed. So, braking de-activates ACC, but accelerating does not. 

The sensors effectively regulate speed and constant distance when the car is traveling above 30 km/h. For any speed slower than this, an expanded sensor system is needed to detect the area directly in front of the car. 

When driving up-hill, it can be necessary to change into a lower gear. ACC does this automatically. The same applies when driving down-hill: through automatic application of the brakes, speed is held constant. In order to reduce brake wear, ACC is able to lower the gear. With assistance from engine braking, wheel brake wear is minimized. 

ACC is able to function properly in poor weather conditions, such as rain or light snow, because BMW has built-in heaters in the radar sensor apparatus. In conditions of very poor visibility, such as heavy rain or fog, or in the case of slippery roads, the system should be switched off. In such cases, only the driver is able to accurately judge the situation and react accordingly.

BMW advises against using ACC in fog or situations of poor visibility. The driver needs to be completely responsible and should not place all responsibility onto the system. The driver must always drive in a manner appropriate to the weather conditions.

The automatic constant distance control influences the chosen speed, depending on the traffic flow. It's effective across the whole range from 30 km/h to 180 km/h.

On an open road, the system functions like a conventional cruise control. In heavier traffic, however, the constant distance control is activated and the speed of the car is adjusted, depending on the speed of the cars ahead.

ACC was designed not only for use on the freeway. It can also be used on secondary or tertiary roads. However, potential problems arise on roads that are extremely winding.

The ACC functions at its best on freeways and highways. On roads that are more winding, it is possible that the sensors' beams will lose contact with the vehicle ahead. In order to prevent the car from accelerating to the pre-set speed when this happens and, for example, coming too close to the car ahead, the ACC system calls up data from other electronic systems in the car.

For instance, the Dynamic Stability Control system provides data on the centrifugal forces and degree of yaw, as the car rounds the bend. These two values can then be used to calculate how tight the curve is and how much the pre-set speed needs to be adjusted to maintain a constant distance. Active Cruise Control changes the car's speed to suit the flow of surrounding traffic, and maintains a constant distance, which is chosen by the driver from three distance options. The system can lower speed by reducing engine speed or by gently applying the brakes.

ACC is designed for flowing traffic and adjusts the speed according to that. That means that non-moving objects, such as vehicles in a traffic jam, are not picked up by the system.

If the car ahead happens to brake suddenly, the driver has to apply the brakes. And because the radar beams are cone-shaped, and thus not very wide directly ahead of the car, the driver needs to be aware of other vehicles that pull out suddenly in front.

In contrast to conventional cruise control systems, ACC holds the speed steady when going downhill. This is because it's linked to the brake system, and even the gears. If necessary, ACC will change to a lower gear in downhill driving to minimize wear on the brakes and ensure that a constant distance is always kept.

External links to information on conventional and adaptive cruise control systems:

How Cruise Control Systems Works from How Stuff Works.

Adaptive Cruise Control article from EE Times describes how adaptive systems work. Uses the Mercedes system as an example.

AC-Delco: The Engine includes info on the Vehicle Speed Sensor (VSS) and other cruise control concepts. With many, high-quality illustrations.

Cruise Control Block Diagram is a technical description of how cruise control is implemented in software. Click here for another example, implemented as a Java simulation.

Troubleshooting Your Cruise Control is written for the Mercedes, but many of the principles apply to BMW's.

Cruise Control Basics page is written in clear, concise language.

Cruise Control Service Tips are consumer questions answered by mechanics. Mostly American cars.

Cruise Control Installers' Instructions.

Installing a Cruise Control System describes how to install a system DIY. Principles may apply to BMW's:

• Miata
• Neon

TRW to Show Adaptive Cruise Control is a press release describing the TRW system.

Ford Cruise Control describes principles of operation for a conventional system. Click here for Ford's Adaptive Cruise Control page.

Microprocessor Cruise Control provides history and background of the cruise control system designed by SCS/Frigette.