Audi A4 Adaptive cruise control (ACC)

Adaptive cruise control (ACC) - The adaptive cruise control system available for the new Audi A4 enables the driver to preselect a suitable road speed between 19mph and 124mph, and to maintain this selected speed for as long as the distance from the preceding vehicle permits; this can be up to 180 metres. The radar sensor detects when the car is approaching the vehicle ahead too closely.

If the driver decides to overtake, ACC is temporarily overridden by pressing the accelerator pedal, but remains active and later restores the pre-selected road speed and the corresponding distance that the system maintains from the preceding vehicle. The system is de-activated whenever the brake pedal is pressed, however, and then has to be reset using the control lever. This restores the previously chosen settings.

The radar sensor is located to the left of the single-frame grille, in an exposed housing with a covering lens. The scanning angle is about eight degrees, and the range approximately 180 metres.

The radar sensor’s four transmitting/receiving units operate in the 76.5 GHz frequency range, which is licensed for radio signals. They calculate the differences in frequency between the transmitted and received radar signals. With the aid of the Doppler effect and the signal transmission/reception time, the speed of the vehicle in front and its distance from the car can be determined.

The system is integrated into the CAN network, which extends through the entire car; it can therefore communicate within a few thousandths of a second with other control units, such as those for engine management, the automatic transmission and the brakes.

The driver selects his or her preferred speed according to the speedometer calibrations: between 19 and 50mph in 3mph steps, and above this in 6mph steps. The preselected speed is shown by LEDs on the speedometer dial.

The system’s status (on/off − vehicle identified) is also visible on the central display. The time interval between the car and the vehicle in front can be varied in four stages, from 1.0 to 2.3 seconds: the driver information system displays this graphically. There are also three settings for the dynamic character of the system: dynamic/standard/comfort. By accelerating or braking, the system adjusts the car’s speed until it matches the programme selected by the driver. The ‘comfort’ setting has a maximum retardation rate of 3 m/s2.

The ACC sensor also has an additional safety function. If there is a risk of colliding with the vehicle ahead, the system warns the driver in two successive steps. In the first, a gong signal is heard and a signal is displayed on the instrument panel.

As a precaution, during the first step the ESP builds up a degree of pressure in the hydraulic system, so that full braking performance is available if the driver reacts by applying the brakes. If the driver does not react, the system applies the brakes for a brief period. This only slows the car down slightly, but is sensed by the driver as a sudden jolt and is of course intended as a ‘wake-up’ reminder. If the driver then applies the brakes, full stopping power is immediately available.

The braking guard remains active even if ACC is de-activated. However, either the early warning or the complete braking guard function can be switched off separately through the MMI.

Published 9 February 2008 Melanie Carter

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