Active cruise control with Stop&Go function

Active cruise control with Stop&Go function

Note! Optional extra 5DF depending on the country, only in conjunction with certain optional extras.

The active cruise control with Stop&Go function is currently only available with optional extra 205 ”Automatic gearbox” or with optional extra 2TB ”Sports automatic gearbox”.
In some markets, ACC is offered without the Stop&Go function due to official approval restrictions.
The only difference between this and ACC without the Stop&Go function is that there are no short-range sensors.
In Europe, optional extra 5DF is only offered in conjunction with the navigation system. Reason: legally prescribed shutdown when approaching radio-astronomy stations.

Notes for Service department

Observe the following information in Service:

• The diagnosis of the long-range sensor (LRR) takes place via the LDM control unit. If the LDM control unit is defective or not connected, no LRR diagnosis can be carried out: the control unit does not respond.
  
• After replacement, the LDM control unit must be recoded.
  
• The long-range sensor must be recoded after it has been replaced.
  
• The short range sensors are automatically coded via the assignment of pins of the connector (no separate coding after replacement of a short-range sensor).
  
• After replacement of the long-range sensor, this must be readjusted. Use the corresponding service function.
  
• On replacement of a short-range sensor, observe the exact installation location: in accordance with the repair instructions. A setting similar to that for the long-range sensor is not necessary. Instead, the installation angle of the sensor must be stored again in the LDM control unit: Use the corresponding service function.
  
• In the event of a conversion of the front bumper (from series to sports or vice versa), the corresponding installation instructions must be followed without fail. The position of the short-range sensors must be changed. Moreover, the fitting angles of the short-range sensors must be stored again in the LDM control unit.
  
  

Introduction

The following text describes the new active cruise control with Stop&Go function (optional extra 5DF).

Active cruise control with Stop&Go function is an extension of the familiar active cruise control (ACC).
As a supplement to the familiar active cruise control, the Stop&Go function enables:

• Distance and cruise control up to a standstill through engine or brake intervention
  
• Automatic driving off from a standstill after a short immobilisation period (1 to 3 seconds)
  
• Adaptive brake assist
  

The required speed can be selected in the range from 30 km/h to 180 km/h: in convenient steps of 10 or fine steps of 1. The required speed is displayed in the instrument cluster. Furthermore, it is also possible to choose between 4 distances.

With the new optional extra, a new sub-bus is also used: Sensor CAN (S-CAN).

Brief description of components

The following components are described for ACC:

Short-range sensor, left and right

The short-range sensors are radar sensors (Short Range Radar).

For the ACC with Stop&Go function, 2 identical short-range sensors have been installed. The short-range sensors are mounted on the front bumper bracket with a bracket made of plastic.

The short-range sensors are connected via the sensor CAN (S-CAN) with the LDM control unit.

The short-range sensors have the following task:

• Calculating position and movement data
  
• Sensing objects in the short range up to 30 metres ahead of the vehicle
  

The object data of the short-range sensors is only used for the ACC Stop&Go function (not for adaptive brake assist).

LRR: long-range sensor

LRR stands for Long Range Radar.

The long-range sensor (former ACC sensor) is a radar sensor. At the same time, the long-range sensor is a control unit. This means the long-range sensor has a control unit address and can be diagnosed and programmed.

A long-range sensor senses the distance, angle as well as speed of moving objects. The area covered is up to 150 metres ahead of the vehicle. The data is processed in the LDM (longitudinal dynamics management) control unit.

LDM: Longitudinal dynamics management

The control unit for longitudinal dynamics management is installed on the carrier behind the glove compartment.

For ACC with Stop&Go function, the LDM control unit performs the following tasks:

• Collating the data transferred by the radar sensor regarding objects
  
• Evaluating the objects and selecting the object relevant to distance control
  
• Evaluating the control signals and generating display signals
  
• Regulating speed and distance
  
• Generating and sending the target values to the actuators drive and brake via the PT-CAN
  
• Monitoring all input signals, the control unit (hardware) as well as the vehicle response characteristics for faults or implausible states
  

The LRR and LDM control unit are supplied by the CAS control unit with their own wake-up line that is electrically decoupled from the usual wake-up line. This solution was selected, as the long-range sensor is fitted in a zone that is subject to the risk of accident (vehicle front). If the wake-up line of the LRR were to be damaged in the event of an accident (e.g. short circuit to earth), this solution limits the effects on other control units.

S-CAN: sensor CAN

The new sensor CAN (S-CAN) connects:

• the control unit for longitudinal dynamics management (LDM)
  
• the long-range sensor (LRR)
  
• the short-range sensors
  

The S-CAN was necessary due to the large data volume from the radar sensors.
This data volume would have exceeded the available transfer capacity on present bus systems.

Characteristics of the sensor CAN:

• Data transfer rate 500 kBit/s
  
• 2 terminal resistors each with 120  (in the long-range sensor and in the LDM control unit)
  
• Two-wire connection
  
• Own wake-up line for long-range sensor and LDM control unit (electrically decoupled from the wake-up line that is used for the other control units).
  
  

System functions

The following system functions are described:

• Functional networking
  
• Adaptive brake assist
  
• Unexpected system behaviour
  
  

Functional networking

For implementation of the Stop&Go function, a complex composite system with distributed functions in other control units is necessary. The LDM control unit is connected across data bus with these other control units.

The following control units are networked for the overall function:

Adaptive brake assist

The adaptive brake assist is a function of the DSC. The LDM control unit is the gateway from the S-CAN to the PT-CAN.

The adaptive brake assist provides its greatest benefit in emergency braking situations. If the driver of the vehicle ahead brakes suddenly, the long-range sensor detects this situation.
The brake assist supports the driver in optimising the brake operation, preventing a run-up collision in the best case.

The following functions are available:

• Pre-filling of the brake system
  
• Adaptation of a number of parameters for the braking functions in the DSC according to the situation
  

The adaptive brake assist is always switched on when the ACC is active.

The adaptive brake assist is also function in the case of ACC without Stop&Go function.

Unexpected system behaviour

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

• Great differences in speed between the vehicle and the vehicle in front cannot be regulated by the system. In this case, brake intervention on the part of the driver is necessary.
  
• The active cruise control does not react to stationary traffic obstructions or oncoming traffic.
  
• The distance control is limited by the detection range of the radar sensors: in the event that vehicles cut in very closely and quickly in front of the vehicle, intervention on the part of the driver can be necessary.
  
• The leading vehicle may be lost when cornering due to the limited side field of vision of the radar sensors. In this case, the acceleration to the required speed is briefly suppressed.
  Reason: this will prevent the vehicle from accelerating too close to the vehicle ahead, which is possibly only briefly not being monitored.
  
• If the radar sensors are ”blinded”, for example by heavy snow, the ACC switches off. A Check Control message is output to the driver. The ACC can be reactivated after cleaning the radar sensors.
  Important: Multiple layers of paint on the bumper in the area of the short-range sensors can also ”blind” the short-range sensors.
  
• If ASC or DSC intervene for longer periods (with critical coefficients of friction), ACC switches off.
  
• Rain, fog and snow absorb the radar beams. This can significantly reduce the range of ACC. 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 sensed with incorrect measured values (usually the angle). For example, due to reflection of the radar beams from tunnel walls and guarding rails.
  
  

No liability can be accepted for printing or other errors. Subject to changes of a technical nature