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What is adaptive cruise control (acc)?

Adaptive Cruise Control (ACC) intelligently controls the vehicle's set speed as well as the selected safe distance from the vehicle in front. Modern adaptive cruise control systems can control the vehicle up to a speed of more than 210 km/h and can also automatically brake the vehicle to a standstill.

Adaptive cruise control keeps danger at a distance

The semi-autonomous electronic system known as Adaptive Cruise Control (ACC) can help to avoid many rear-end collisions. According to a study carried out within the framework of the European Road Safety Action Programme from 2011 to 2020, the number of severe braking manoeuvres was reduced by 67 percent. Scientists from the euroFOT project (European Field Operational Test) were also able to prove that the number of instances of vehicles driving at a critical distance from the vehicle in front also fell by 73 percent.

Automatic cruise control reacts when there is insufficient distance from the vehicle in front. It warns the driver or even automatically intervenes in the driving process by automatically braking the vehicle. When activated, ACC can be deactivated by the driver at any time. For example, drivers can do this by accelerating strongly or touching the brake pedal. Modern vehicles usually employ radar-based sensors to measure the distance from the vehicle in front and its relative speed. These sensors continuously monitor the area in front of the vehicle.

ACC uses the Doppler effect

In order to determine the values required for the adaptive cruise control system, the vehicle is equipped with an ACC radar antenna with several radar sensors that are installed in the front of the vehicle. The measurement is physically based on the Doppler effect. The relative speed is determined by measuring the different wavelengths between the transmitted and reflected signal. If an object is moving towards the vehicle, for example, the frequency of the reflected wave increases. If the object moves away from the transmitter, the frequency decreases. A comparison of the two signal curves takes place in the sensor control unit (SCU), which is responsible for controlling the adaptive cruise control system. The SCU also houses the compact radar sensor unit and the electronic control unit (ECU) for the ACC system.

Whereas the first generation of adaptive cruise control monitored the measured area using three radar beams, with the centre radar beam being directed precisely forwards and the two side sensors each directed to the side at an angle of 2.5 angular degrees, second-generation ACC systems use four radar beams. This improves the entire horizontal measurement range of the radar sensor from ± 4 to ± 8 angular degrees. With a transmission frequency of 76 to 77 Gigahertz, the ACC sensor continuously scans an area of between 2 and 120 metres in front of the vehicle with a power of around 10 mW.

ACC for semi-autonomous functions

For adaptive cruise control systems that are fitted in the small car segment, less expensive radar sensors with a frequency range of 24 GHz are available. Some vehicles also use lidar (light detection and ranging) systems, which operate on the basis of non-visible light. These are much cheaper and provide a sharper image with a better resolution than radar sensors. However, because they transmit with lower power in the infrared range, they have a significantly lower range than radar-based systems. What is more, lidar systems tend to fail in weather conditions that produce poor visibility, such as fog or heavy rain.

These systems are not sufficient for driving functions such as those already offered by semi-autonomous driving today. For that reason, luxury-class vehicles use modern third-generation radar sensors that measure an area of between 0.5 and 250 metres in front of the vehicle. In addition, ACC is equipped with additional sensors for shorter ranges. The short range radar (SRR) scans an area of up to 14 metres. This makes stop-and-go driving possible, for example in the Porsche Panamera. However, the ACC system must be combined with an automatic transmission or a dual-clutch transmission (DCT).

City emergency braking function with traffic jam assist

A further development of advanced ACC systems is the city emergency braking function, as offered, for example, by Volkswagen in the Golf VII. The latest VW Passat B8 also includes a Traffic Jam Assist system, which enables the car to follow the vehicle ahead semi-autonomously at speeds of up to 60 km/h. In addition to acceleration and braking functions, the electronic control unit also performs steering functions. The latest development stage offers autonomous emergency braking with the maximum braking force. The system became available in the facelifted version of the BMW 7 Series in 2012. The BMW engineers integrated the sensor functions for the camera in the foot of the rear-view mirror and in the ACC sensor.

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Bertrandt AG

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RWTH Aachen, WKM

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Volkswagen AG

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Mahle Behr GmbH & Co. KG

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Burkhard Goeschel Consultancy

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Schaeffler AG

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Robert Bosch GmbH

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MAN Truck & Bus AG

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Ford-Forschungszentrum Aachen GmbH

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Audi AG

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Continental Teves AG & Co. oHG

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Valeo Schalter und Sensoren GmbH

Dr.-Ing. Harald Naunheimer
ZF Friedrichshafen AG

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Edag GmbH & Co. KGaA

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Hochschule München

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Adam Opel AG

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Daimler AG

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Wabco GmbH

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