9.2.2. Sensor for ACC Stop & Go
The sensor for ACC Stop & Go is a radar-based sensor for detecting the area ahead of the vehicle. Both the short and long range are detected by a sensor using internal electronic and mechatronic measures. The transmission frequency is 76-77 GHz.
The sensor sends out focused electromagnetic waves. The echoes reflected from objects are received and evaluated by the sensor. In this way, the sensor can gain information about objects in front of it. This information includes size, distance and the speed.
The sensor is located behind a removable grille on the front apron.
| Index | Explanation |
|---|---|
| 1 | Sensor for ACC Stop&Go |
| 2 | Removable grille |
In order to achieve a low overall height for the sensor, complex measures have been taken inside the sensor. There is a continuously rotating roller inside the sensor. The roller has various mouldings and emits electromagnetic waves with various characteristics, depending on the range to be detected. Thus both the short and long range are detected with each revolution of the roller. The roller turns at 900 RPM using a brushless motor Since the sensor becomes functional only upon reaching this speed, in cold temperatures, it can sometimes take a certain time for the sensor to become available. The cover of the sensor is coated with a transreflective layer. This is permits the penetration and passage of waves at a certain angle to allow beams to enter and exit. The beams are led through the waveguide and reflected on the transreflective layer. At a shallower angle, it is reflective in order to guide beams within the housing to the various components. They strike the mirror, upon which the waves have their phase rotated and are reflected. Then they leave the housing through the transreflective layer. If these beams strike objects, they are reflected from these and travel back to the sensor. The beams are captured and measured in the waveguide. The evaluation electronics within the sensor evaluates these beams and forwards detected objects via the CAN bus to the Integrated Chassis Management.
| Index | Explanation |
|---|---|
| 1 | Transreflective layer |
| 2 | Mirrors |
| 3 | Roller (phase-controlled radar) |
| 4 | Waveguide (antenna) |
| Index | Explanation |
|---|---|
| 1 | Mirrors |
| 2 | Connector |
| 3 | Waveguide (antenna) |
| 4 | Roller |
The sensor for ACC Stop & Go is connected by a waterproof plug connection to the sensor CAN, terminal 15 N, and ground. The sensor for ACC Stop & Go receives its power supply via terminal 15N, so it is switched on and off with this supply. Terminal 15N is tapped off at the front fuse carrier. There is a fault memory in the sensor for ACC Stop & Go. The fault memory can be read out in diagnosis via Integrated Chassis Management. If the sensor is defective, replace it and calibrate the new sensor.
The following graphic shows the working range of the sensor with a large opening angle for short range and a small opening angle for long range detection.
| Index | Explanation |
|---|---|
| 1 | Short-range sensing |
| 2 | Long-range sensing |
| 3 | Distance at which the short-range radar can detect objects over the full width of a three-lane highway (approx 10.5 m) |
| 4 | Range of short-range radar (approx. 50 m) |
| 5 | Distance at which the long-range radar can detect objects over the full width of a three-lane highway (approx. 40 m) |
| 6 | Range of radar (approx. 200 m) |
The sensor uses the data on the objects detected to calculate their speeds relative to our car. These data are pre-processed in the sensor, prepared for post-processing and filtered.
If ACC Stop & Go brings the car to a standstill on account of an object it has detected, the ACC sensor switches to a reduced-power mode (FCC requirement). When driving resumes or automatic drive off is active (set speed LED in the speed reading is green) it is switched back to normal power. Short and long range are permanently scanned and the detected objects are combined.
The following is an overview of faults that can occur when using the ACC Stop & Go.
- If the ACC Stop & Go Sensor is dirty
- External disruption of radar-signal processing
- Temporary faults
- Control unit fault
- Misaligned Sensor
The sensor can no longer function reliably if the antenna is obscured by snow, slush or ice. If this condition is detected, a corresponding signal is sent to Integrated Chassis Management (ICM) and the ACC Stop & Go function is deactivated as a result A Check Control message informs the driver about this special case. At the same time a fault entry is stored in the ACC sensor.
Radar sensors used by other automobile manufacturers can disrupt the signal evaluation of the ACC sensor. If such a problem is detected, the ACC Stop & Go is deactivated. It can be switched on again by the driver as soon as the car is far enough away from the vehicle causing the interference. This fault is stored in the fault memory of the Integrated Chassis Management and of the ACC sensor. This does not, however, necessitate any repair measures. Instead, the customer should be informed of the cause of the fault, which is external interference.
The potential causes of this type of fault include communication faults, overvoltage, undervoltage and thermal overloads in the ACC sensor. In these cases it is necessary to proceed as instructed by the test plan in the diagnostic system. Do not replace the ACC sensor until the test schedule prompts you to do so.
If the ACC sensor is affected by a control unit fault, the only way to rectify the fault is to replace the defective sensor.
The interaction between the ACC Stop & Go sensor and the Integrated Chassis Management can detect misalignment caused by an accident (e.g. by bumping something while parking or being bumped when parked). If the computed maladjustment drift exceeds a certain threshold the ACC Stop & Go function is switched off. A fault entry indicates the cause of the fault. To correct the fault, observe the instructions in the diagnostic system and Repair Instructions.
If the ACC Stop & Go sensor is replaced, the diagnosis system must be used to perform a start-up procedure to initialize the sensor. In the course of the start-up procedure, the installation position is entered in the newly installed sensor and it is adjusted. An adjustment may also be required after an accident without damage to the sensor.
It is essential to observe the following important notes in order to adjust the ACC Stop & Go sensor properly:
- Make sure the measurement setup is exactly in accordance with the repair instructions
- Park the vehicle on a perfectly smooth, level surface
- Connect the ISTA diagnosis system
- Make sure that the reflector for adjusting the ACC Stop & Go sensor is correctly positioned
- Perform the adjustment in accordance with the action plan in diagnosis
- Complete the adjustment and clear fault memory, etc.
| Index | Explanation |
|---|---|
| 1 | Reflector, right |
| 2 | Slotted cover, right |
| 3 | Laser pointer right |
| 4 | Longitudinal axis of the vehicle |
| 5 | Laser pointer, left |
| 6 | Slotted cover, left |
| 7 | Reflector, left |
| 8 | Reflector for adjustment |
| 9 | Sensor for ACC Stop&Go |