Active cruise control

The active cruise control system ACC is an expansion of the conventional cruise control system FGR. A new feature is the convenient distance and cruise control by means of automatic engine management and brake intervention.

ACC system functions

A radar sensor determines the distance, angle and speed of moving objects in front of the vehicle. The driver can preselect a required speed in the range from 30 km/h to 180 km/h in steps of 10 km/h. This setting is shown in the instrument cluster. Furthermore, it is also possible to choose between three fixed time-based distance stages. This means that the distance from to the vehicle in front changes with speed.

ACC is a convenience system. This means driver interventions always have higher priority than the ACC control.

ACC provides the following functions to the driver:

• Cruise control (FGR) with preset required speed, LED indication in the speedometer
  Cruise control with adaptation to the speed of the vehicles in front
  Maintaining a distance to the vehicle in front - set by the driver
  Avoiding distance ranges that are subject to fines
  Quick change of required speed in 10 km/h steps with rapid response of vehicle by way of brake intervention
  Specific brake intervention ensures the set speed is not exceeded even when driving downhill
  

ACC function limits

The following variables are limited for the purpose of ensuring reliable ACC operation:

• ACC operation only possible from 30 km/h to 180 km/h
  
• Limitation of ACC deceleration by way of brake intervention to max. 2.0 m/s2
  
• Limitation of ACC acceleration to max. 1.2 m/s2. This prevents coming critically close to other vehicles and limits the acceleration when cornering
  
• Limitation of transverse acceleration to max. 3 m/s2 for comfort reasons
  
• Under stationary conditions, the time gap to a vehicle in front is not less than 1 s. This time gap may be less than 1 s for a short time under certain conditions, e.g. when a vehicle cuts in closely.
  
• ACC should only be used under adequate visual conditions.
  
• The detection range of the radar sensor is limited.
  
• A vehicle cutting in can cause delayed responses in the ACC. For this reason, do not use ACC on winding roads or where it is necessary to change lanes frequently.
  
• Large speed differences, e.g. quickly approaching a heavy goods vehicle, cannot be regulated.
  
• Driver intervention will be necessary if the rate of deceleration of the ACC is not sufficient.
  
• ACC cannot be used in stop-and-go traffic. The driver must take control of the vehicle by braking in good time.
  
• The ACC will no longer accelerate after intervention of the traction control systems ASC or DSC, i.e. it is necessary to switch on the system again.
  

If the ACC is no longer able to maintain the selected distance, a signal is sent indicating that the driver should take control. The ”object detected” lamp begins to flash.

Unexpected system behaviour

A system situation that requires explaining may occur if the function limits of the system are reached in ACC mode. Such situations are described in the following:

- Limited range and deceleration

On the one hand, the radar sensor has a limited range of 120 m while, on the other hand, the ACC system is provided a maximum deceleration of only 2.0 m/s2 by the DSC. The ACC can therefore automatically control only a limited relative speed. When the system has reached its functional limits, the driver is requested to assume control by the flashing ”object detected” lamp.

- Side field of vision

The leading vehicle may be lost when cornering due to the limited side field of vision. When cornering, the ACC vehicle is not accelerated to the required speed for approx. 2 s in order not to drive up too close to the leading vehicle which may be not detected for only a short time.

When driving straight ahead, the response of the ACC to a vehicle cutting in close may be delayed. The vehicle that cuts in is not detected before it is positioned on the same lane as the ACC vehicle.

- Switching off

The system switches off if ’blinded’ by extremely thick snow cover. It can be reactivated after cleaning the radar sensor.

The system switches off in the event of prolonged control interventions by the ASC or DSC, i.e. at critical coefficients of friction. It can, however, be reactivated.

In both cases, after being reactivated the ACC resets the long following distance of 2 s.

- Special situations

The driver recognises such situations when the ”object detected” lamp is not lit up but the vehicle remains below the required speed and does not accelerate.

• In tight corners, the vehicle reduces the speed such that, for comfort reasons, the maximum transverse acceleration is 3 m/s2. This applies both in open-road and follow driving modes
  
• If there is a stationary object in the area directly in front of the driver and the speed has already been reduced for other reasons, the current speed will be maintained.
  
• Rain, fog and snow absorb the radar beams. The range can be reduced quite considerably and in extreme cases ACC can no longer be used.
  
• Brows of hills and depressions may obstruct detection, causing objects to be lost
  
• In very rare cases objects can be detected with incorrect measured values (mainly angles), e.g. by reflection of the radar waves on tunnel walls and crash barriers.
  

- Lane prediction

Lane allocation is a central function in the ACC system. Object detection alone is not sufficient for the ACC to effectively interpret the situation. The detected objects must be brought into a relation with the driver's own intentions. Vehicles are then relevant to the control system when they are located in the same lane as the ACC vehicle.

In effect, the ACC does not have any predictive knowledge of the lane progression so that absolutely reliable object/lane allocation is not possible with the ACC. In addition to the limited field of vision, the reliable situation interpretation within restrictions is the most significant system limit of the ACC.

Lane prediction can therefore only be based on the current driving situation. Since the ACC cannot recognise the lane progression, it must depend on the current own driving status.

ACC must calculate the future lane in advance in order to be able to select the correct object for the purpose of distance control. For the next 2 - 4 seconds, the driven radius of the bend is assumed to continue to be valid for the lane progression, an assumption that virtually always applies on motorways and on trunk roads. The lateral deviation of every detected object with respect to the precalculated lane can be determined with the lane prediction facility. The following information is used for this estimate:

• Knowledge of the state of movement of the ACC vehicle (speed, yaw speed, steering angle, etc.). This information has the largest influence in determining the likely course of the lane.
  
• The joint lane movements of all stationary or driving objects are used to improve lane prediction. If the objects all move in the same direction (flock of birds for instance) the probability is then very high that a bend starts here. This procedure can only play a supportive role in lane prediction.
  

However, uncertainties in lane prediction and thus in object allocation should always be assumed, particularly in the transition from driving straight ahead to cornering.

The received radar signals cannot differentiate between living creatures, vehicles and road signs. Road signs or parked vehicles next to the driven lane may therefore be mistakenly allocated to the own lane. Stationary objects are therefore ignored to a large extent in order to avoid incorrect responses.

ACC sensor

The ACC control unit is installed at the front of the vehicle under the bumper. Only one type of control unit is used which can be coded to specific variants. It is not possible for the workshop to conduct repairs on the control unit.

The radar sensor contains a transmitter and a receiver. The signal is emitted via an antenna that also serves to receive the reflected signals. The radar beam has the shape of 3 cones. One of the cones points in the direction of travel, the other two are offset horizontally by 2.5 degrees.

Technical data ACC sensor:

• Transmission frequency 76 - 77 GHz (wavelength approx. 4 mm)
  Mean transmission power 1 mW
  Detection range: Distance 2 m - 170 m (120 m is used)
  Detection range: Relative speed 60 m/s
  Detection range: Horizontal +/- 4 degrees
  Detection range: Vertical +/- 2 degrees
  

The ACC control unit is protected against short circuits, positive or ground supply, polarity reversal and open circuits. The responses of the radar sensor at undervoltage and overvoltage are listed in the following:

ACC composite system

The ACC is a complex system network with distributed functions in various partner control units.

The partner control units are connected via the data bused with the ACC sensor.

Important partner control units are: