DISTRONIC PLUS with Steer Assist, function - GF30.30-P-0016FLM

MODEL 212 as of model year 2014 with CODE 266 (Steer Assist) 

Fig 1: Distronic Plus With Steer Assist Function Diagram

Courtesy of MERCEDES-BENZ USA

Function requirements, general 

• Engine running or drivetrain operational
• DISTRONIC PLUS activated
• Hands on the steering wheel while driving ("Hands-Off Detection")
• Steer Assist switched on the menu "Assistant" of the IC
• Vehicle standstill at least t = 500 ms after engine start or "Drivetrain operational" signal (radar sensor system standstill comparison must be complete)
• Gear range "D" selected
• Active or passive safety systems inactive
• No driving with a trailer recognized (with code (550) Trailer hitch)
• Sufficiently high tire inflation pressure

If the general function requirements have been fulfilled, the Steer Assist is switched on but remains passive until the additional function requirements of the lane or object detection are fulfilled.

Steer Assist, general 

The units and values given in the following text for the speed must be assigned to the possible national versions. The details in "miles per hour (mph)" refer to vehicles with a miles indicator.

The Steer Assist supplements DISTRONIC PLUS (distance and speed control) by assisting the driver in his/her task of lateral vehicle guidance (steering movement) within the vehicle speed range of v = 0 to 200 km/h (v = 0 to 120 mph). The Steer Assist does not relieve the driver of his/her task of keeping the vehicle in the lane, it is more for relieving the strain on the driver and providing additional convenience as a result. The assistance is also only provided within defined steering torque limits, i.e. no assistance can be provided for large steering angles at low speeds, for example those caused by driving on roads going over passes.

The Steer Assist is implemented in the radar sensors control unit. The course of the road in front of the vehicle and the vehicles ahead are recorded via the stereo multifunction camera and the radar sensor system. The steering movements that are requested are carried out by the electric steering.

Information about distance and speed control can be found in the separate subfunction "DISTRONIC PLUS, function" (for code (233) DISTRONIC PLUS).

Technical implementation 

The stereo multifunction camera and the radar sensors control unit evaluate input factors from which appropriate initial parameters or requests from the Steer Assist are calculated.

The following input factors are evaluated:

• Engine running or drivetrain operational
• Vehicle speed/direction of travel
• Acceleration/yaw rate
• Combination switch position

Engine running or drivetrain operational

The CDI control unit or the ME-SFI [ME] control unit sends the "Engine running" or "Drivetrain operational" signal to the stereo multifunction camera and the radar sensors control unit via chassis CAN 1, the chassis gateway control unit and chassis FlexRay.

Vehicle speed/direction of travel:

The direction of travel is defined using the wheel rotation direction. The vehicle speed is calculated on the basis of the wheel speeds. The Electronic Stability Program control or the RBS control unit transmits appropriate information to the stereo multifunction camera and the radar sensors control unit via the chassis CAN 1, chassis gateway control unit and chassis FlexRay.

Acceleration/yaw rate:

The acceleration and the yaw rate of the vehicle are recorded by the yaw rate sensor for lateral and longitudinal acceleration. The yaw rate sensor for lateral and longitudinal acceleration sends corresponding values via the vehicle dynamics CAN, Electronic Stability Program control unit or RBS control unit, chassis CAN 1, the chassis gateway control unit and chassis FlexRay to the stereo multifunction camera and the radar sensors control unit.

Combination switch position:

The steering column tube module control unit directly reads in the switch position of the combination switch and sends this via the chassis CAN 1, the chassis gateway control unit and chassis FlexRay to the stereo multifunction camera and the radar sensors control unit.

The radar sensors control unit also analyzes the following input factors: 

• "DISTRONIC PLUS" status
• "Steer Assist" status
• Lane data/object recognition
• Gear range engaged
• Steering torque from the driver and wheel angle (Hands-Off Detection status)
• Steering angle
• Run Flat Indicator status (with code (475) Tire pressure monitoring (Premium) or code (477) Tire pressure loss warning)
• Trailer recognition status (with code (550) Trailer hitch)

"DISTRONIC PLUS" status:

The driver can switch the DISTRONIC PLUS on and off using the cruise control lever. The steering column tube module control unit directly reads in the control signals from the CRUISE CONTROL lever directly, and sends them via the chassis CAN 1, the chassis gateway control unit and chassis FlexRay to the radar sensors control unit. "Steer Assist" status:

The Steer Assist can be switched on or off over the "Assistance" menu of the IC. The status is sent by the instrument cluster via the chassis CAN 2, the chassis gateway control unit and chassis FlexRay to the radar sensors control unit.

After being switched on or off, the message "Steer Assist on/off" is displayed for t = 5 s in the instrument cluster multifunction display. Lane data/object recognition:

The stereo multifunction camera records the lane and objects in the lane and sends corresponding information via suspension FlexRay to the radar sensors control unit. The front long-range radar sensor and the DISTRONIC (DTR) sensors on the front bumper also detect objects in front of the car. The front long-range radar sensor sends corresponding signals via chassis FlexRay to the radar sensors control unit. The DISTRONIC (DTR) sensors on the front bumper transmit appropriate signals via the radar CAN 1 to the radar sensors control unit. he radar sensors control unit checks the plausibility of the object data from the MFK stereo with that of the radar sensor system.

The plausibility of the recorded lane data is checked by the stereo multifunction camera. Values such as the condition of the lane markings at the side and their distance from the vehicle are used for this purpose.

Engaged gear range:

The fully integrated transmission control unit transmits information about the engaged gear range to the radar sensors control unit via the drive train CAN, CDI control unit or ME-SFI [ME] control unit, chassis CAN 1, chassis gateway control unit and chassis FlexRay.

Steering torque of the driver and wheel angle (Hands-Off Detection status):

The electric power steering torque sensor records the steering torque applied by the driver and the wheel angle. The electrical power steering control unit directly reads in the signals from the electric power steering torque sensor and sends these via chassis CAN 2, the chassis gateway control unit and the chassis FlexRay to the radar sensors control unit. On the basis of these values, the radar sensors control unit determines whether and at which time intervals the driver is making steering movements (Hands-Off Detection).

Steering angle:

The steering angle is needed as an additional parameter for checking the plausibility of the wheel angle and is recorded by the steering angle sensor. The steering column tube module control unit directly reads in signals from the steering angle sensor and sends them via the chassis CAN 1, the chassis gateway control unit and chassis FlexRay to the radar sensors control unit.

Run Flat Indicator (for code (475) (Tire pressure monitor (Premium)) or code (477) Tire pressure loss warning):

Vehicles with code (475) Tire pressure monitor (Premium):

The tire pressure monitor control unit sends the information about the respective tire pressure to the radar sensors control unit via chassis CAN 2, the chassis gateway control unit and chassis FlexRay.

Vehicles with code (477) Tire pressure loss warning (resultant): The tire pressure is determined using the rolling circumference. In terms of its function, the RDW is integrated into the Electronic Stability Program control unit or into the regenerative braking system control unit. The Electronic Stability Program control unit or the RBS control unit transmits information about the calculated tire pressure via the chassis CAN 1, chassis gateway control unit and suspension FlexRay to the radar sensors control unit.

Information about the tire pressure monitoring can be found in the separate function description "Tire pressure monitor, function" (with code (475) Tire pressure monitor (Premium)) or "Tire pressure loss warning, function" (with code (477) Tire pressure loss warning). Trailer recognition status (with code (550) Trailer hitch):

When a trailer is detected, the trailer recognition control unit sends the "Trailer detected" status to the radar sensors control unit via the interior CAN, from SAM control unit, chassis CAN 1, the chassis gateway control unit and chassis FlexRay.

The radar sensors control unit issues the following requests: 

• Steering request
• Output acoustic warnings, display system or warning messages

Steering requests:

The radar sensors control unit transmits appropriate steering requests via chassis FlexRay, the chassis gateway control unit and chassis CAN 2 to the electrical power steering control unit. The electrical power steering control unit actuates the electric power steering actuator motor directly.

Output acoustic warnings, display system or warning messages:

The radar sensors control unit sends the "Output acoustic warnings" or "Display system or warning messages" request via the chassis FlexRay, the chassis gateway control unit and chassis CAN 2 to the instrument cluster. The instrument cluster displays the respective system and warning messages on the multifunction display and actuates the warning buzzer directly to output an acoustic warning.

The stereo multifunction camera outputs the following initial parameters: 

Lane data and object recognition:

The stereo multifunction camera records the lane and objects in the lane and sends corresponding information (e.g. size of object or distance from object) via suspension FlexRay to the radar sensors control unit.

The Steer Assist contains the following subfunctions:

• Function sequence for track guidance 
• Function sequence for object guidance 
• Function sequence for recognition of driver activity/steering movement 
• Function sequence for Steer Assist steering power assistance 

Additional function requirements for track guidance 

• Recognizable lane markings present at both sides (for initializing the control)
• Lane width ≤ 4.2 m (lane width ≥ 4.5 m (hysteresis), system passive)
• Offset to center of lane may be a max. of s = 40 cm when control starts
• Vehicle speed > 0 up to 200 km/h (v > 0 to 120 mph)
• Turn signal not set

Function sequence for track guidance 

The stereo multifunction camera records pictures of the lane markings on the road regularly with a fixed sampling rate. The integrated control unit calculates a lane from the picture sequences and the measurements resulting from the pictures of the road markings. The stereo multifunction camera sends the lane data via the suspension FlexRay to the radar sensors control unit.

For multiple markings on the traffic lane, the inner marking is always taken as the reference point. The function is not ready for control in construction areas because of possible multiple markings (yellow/white). Also, no lane information is sent if unfavorable weather influences (such as extreme glare from the sun or snow) are present or the markings are covered in dirt. Markings with insufficient contrast to the road surface also lead to readiness for control being interrupted.

Lane detection by the stereo multifunction camera 

Fig 2: Stereo Multifunction Camera Lane Detection

Courtesy of MERCEDES-BENZ USA

Additional function requirements for object guidance 

• Vehicle immediately in front within the detection range of s ≥ 1.00 to 40.00 m (speed-dependent)
• Width of the vehicle traveling immediately in front ≥ 1.30 to 2.80 m
• Center line offset of vehicle to vehicle in front: s ≤ 35 cm (when control starts)
• Vehicle speed > 0 up to 60 km/h (v > 0 to 37 mph)
• Turn signal not set

Function sequence for object guidance 

In urban traffic or in a traffic jam on a highway, the lane markings may not be recognizable due to the short distance to the vehicle in front. An additional calculation basis is therefore needed to determine the lane. In this case, raised objects (vehicles) in the same lane as your vehicle are analyzed. Because of the parallax shift effect (disparity), the stereo multifunction camera is in a position to not just visually detect the vehicle in front but also to determine the distance to the vehicle and its dimensions.

The stereo multifunction camera consists of two high-resolution mono cameras and an integrated control unit for evaluation and a plausibility check of the supplied picture data. The laterally offset location of the two mono cameras creates a difference in perspective (parallax displacement) between the left and right optical path. The difference of the picture data relative to each other enables the distance to the vehicle traveling ahead along with other variables such as, e.g. vehicle height to be determined.

The image data from the two mono cameras is prepared by the integrated control unit and sent to the radar sensors control unit. The radar sensors control unit compares the image data with the signals from the radar sensor system. The transition from lane-related to object-related lateral vehicle guidance takes place automatically.

Object detection by the stereo multifunction camera 

Fig 3: Stereo Multifunction Camera Object Detection

Courtesy of MERCEDES-BENZ USA

The shorter the distance to the car in front, the greater the effect of the parallax shift and the more accurate the evaluation (without taking possible interfering variables into consideration). From a distance of approx. s = 40 m (image data differences between the mono cameras is extremely small), evaluation is no longer possible.

Function sequence for recognition of driver activity/steering movement 

The electrical power steering control unit continuously evaluates the steering torque of the driver with the aid of the electric power steering torque sensor. If the electrical power steering control unit detects zero or insufficient torque at the steering wheel for t > 10 s ("Hands-Off Detection"), it sends corresponding information via the chassis CAN 2, chassis gateway control unit and suspension FlexRay to the radar sensors control unit and a message (gray icon with red hands) via the chassis CAN 2 to the instrument cluster. The multifunction display shows the message.

If the driver does not react to the message within t = 5 s, an additional acoustic message is issued over the warning buzzer. In this case the radar sensors control unit renders the Steer Assist passive. The "Hands-Off Detection" is suppressed by the radar sensors control unit at vehicle speeds of v < 10 km/h (v < 6 mph). The status of the Steer Assist is continuously shown to the driver on the multifunction display of the instrument cluster.

The individual states can be seen in the following diagram:

Shown in multifunction display of the IC, shown on model 207 

Fig 4: Steering Assist Activation Display

Courtesy of MERCEDES-BENZ USA

Additional function requirements for Steer Assist power steering 

• Track or object detection active

Function sequence for Steer Assist steering power assistance 

The radar sensors control unit calculates the additive toothed rack force on the basis of factors such as the steering torque of the driver, the lateral acceleration and the vehicle speed, and sends the relevant steering requests via the chassis FlexRay, chassis gateway control unit and chassis CAN 2 to the electrical power steering control unit.

The electrical power steering control unit receives and checks the plausibility of the incoming steering requests and actuates the electric power steering actuator motor accordingly. The steering force assistance is transmitted from the electric power steering actuator motor to the rack-and-pinion steering gear, thereby supporting the steering moment applied by the driver. The additive toothed rack force is only available up to a certain threshold value dependent on the above variables and thereby limits the characteristic of the steering power assistance.

The steering power assistance of the Steer Assist is reduced as the speed increases, when the vehicle is driven on narrow streets or with a limited field of view of the stereo multifunction camera.

Further information on the electrical steering power assistance can be found in the separate function description "Electric power steering (ES)".

The following traffic situations lead to object guidance being aborted or Steer Assist being rendered passive:

If the vehicle in front changes lanes during object guidance, the vehicle aborts object guidance and automatically switches from object-related to lane-related lateral vehicle guidance, provided that a lane has been detected. If lane detection by the stereo multifunction camera is not possible, the vehicle in front is followed.

If there is another vehicle in the blind spot when the vehicle in front is in a lane change, Steer Assist is rendered passive and object guidance is aborted in order to avoid a collision.

If a vehicle is being followed that, for example, narrowly misses a parked vehicle, for example, object guidance is aborted and Steer Assist is rendered passive in order to avoid a collision with the parked vehicle.