Collision Avoidance and Parking Assist Systems: Description and Operation
Parking Aid
COMPONENT LOCATION
Parking Aid:
INTRODUCTION
The parking aid system provides an audible warning to the driver when any obstacles are in the path of the vehicle during a forward (if front sensors fitted) or reversing manouevre. The purpose of the system is to assist the driver when parking or manoeuvring in restricted space. It is not designed as a crash avoidance system or a replacement for visual interpretation by the driver.
All vehicles are fitted with rear parking aid sensors. Higher specification vehicles may be fitted with both front and rear sensors.
Audible warnings to the driver are given by the entertainment system speakers. A visual representation is shown on the Touch Screen Display (TSD).
The control module controls operation of the speakers via the audio head unit. A gateway module is incorporated into the system to convert medium speed Controller Area Network (CAN) bus messages from the control module into Media Orientated System Transport (MOST) messages for use by the audio head unit.
If an obstacle is sensed by the rear parking aid sensors, the rear audio system speakers will sound. If an obstacle is sensed by the front parking aid sensors (if fitted), the front audio system speakers will sound.
The parking aid system operates using ultrasonic signals which are transmitted by the sensors. The reflected echo from this output is received by the sensors and used by the control module to calculate the distance from an object.
The roof console mounted control switch allows the driver to deactivate the parking aid system if operation is not required.
NOTE: The control switch is only fitted to vehicles with front parking aid sensors.
CONTROL MODULE
The parking aid control module is located in the passenger footwell, behind a protective panel. The control module uses a single microprocessor to perform the following tasks:
- Control of the ultrasonic sensors
- Monitoring of the sensors
- Evaluation of received echo signals from the sensors
- Noise and disturbance suppression
- Control of the audio system speakers
- Control and monitoring of the switch status Light Emitting Diode (LED) and associated wiring
- Evaluation and monitoring of the control inputs
- Management of diagnostic and test functions
- Monitoring of power supply
- Communication via diagnostic link.
The control module is connected to the vehicle electrical system by either 3 (vehicles fitted with front and rear sensors) or 2 (vehicles fitted with rear sensors only) multiplug connectors.
Inputs and Outputs
The table below shows control module inputs and outputs
Diagnostics
The control module has a diagnostic connection via the medium speed CAN bus to enable faults to be retrieved using the Integrated Diagnostic System (IDS). Additionally an on-board diagnostic routine within the control module constantly monitors the system and alerts the driver to a system fault by emitting a 5 seconds continuous tone through the audio system speakers when the ignition is switched on and a gear is selected. If front parking aid sensors are fitted, the control switch LED will also illuminate.
PARKING AID SENSORS
Four sensors are positioned in the rear bumper and four in the front bumper (if fitted). The front sensor housings are glued into position; the rear sensor housings are welded into position. The sensor bodies are then clipped into the housings from the rear.
Each sensor has a 3 pin connector which connects into a common harness linking all 4 sensors. The 3 pins of each connector provide the sensor a negative feed, a positive feed and a signal line. The rear parking aid harness connects to the main vehicle body harness. The front parking aid harness connects to the Left-Hand (LH) engine compartment harness.
Each sensor comprises a plastic housing which contains a piezoelectric disc. The disc resonates at a frequency of approximately 50 kHz, producing an ultrasonic signal output. The disc also receives the reflected echo signal.
Each sensor has 2 modes of operation; combined transmitter and receiver mode or receiver mode only. The control module controls the operating mode of each sensor by the output of a digital signal on the signal line.
In the combined mode, the sensor emits a series of ultrasonic impulses and then switches to receiver mode to receive the echo reflected by an obstacle in the detection range. These echo signals are amplified and converted from an analogue signal to a digital signal by the sensor. The digital signal is then transmitted to the control module and compared with preprogrammed data stored in an Electrically Erasable Programmable Read Only Memory (EEPROM) within the module.
The module receives this data via the signal line from the sensor and calculates the distance to the obstacle according to the elapses time between the transmitted and received impulse. The duration of the impulse transmission is determined by the control module. The frequency of the impulse is determined by the sensor.
In the receiver mode, the sensor will receive impulses that were emitted by adjacent sensors. The control module uses this information to precisely determine position and distance of the obstacle.
CONTROL SWITCH
The control switch is located in the roof console. The switch is non-latching with an integral LED. The switch receives a 12 V output from the control module, and is also connected to ground. When the switch is operated, a momentary ground is completed. This is interpreted by the control module as a signal to enable or disable the parking aid system.
The switch also receives a 12 V output from the control module to drive the LED when the parking aid system is on.
NOTE: The control switch will only switch off the front parking aid sensors. The rear parking aid sensors will remain operational.
CONTROL DIAGRAM
PRINCIPLES OF OPERATION
If a forward drive gear is selected when the vehicle ignition is first switched on, the front parking aid sensors (if fitted) will not be operational. If reverse (R) is the first gear selected after the ignition is switched on, both the front and rear parking aid sensors will become operational. If a forward drive gear is subsequently selected, the rear parking aid sensors be switched off by the control module but the front parking aid sensors will remain operational until vehicle speed increases above 16 km/h (10 mph).
Parking aid will only become active if the following conditions exist:
Front and rear sensors fitted
- Ignition on
- Park is not selected
- Parking brake is released
- Vehicle speed drops below 7 km/h (4.5 mph) - rear sensors only
- Vehicle speed drops below 7 km/h (4.5 mph) and control switch pressed - front sensors only.
Rear sensors only
- Ignition on
- Reverse gear selected for more than 1 second.
Audible and Visual Warnings
The control module processes the distance readings from the ultrasonic parking aid sensors to determine if there are any objects within the detection areas. If there are no objects no audible warning is produced. If an object is detected, repeated audible warnings are produced via the audio system speakers. The time delay between the audible warnings decreases as the distance between the detected object and the vehicle decreases until eventually a continuous tone is emitted from the speakers.
A visual interpretation of the detection areas is given in the TSD.
The detection ranges of the sensors are shown in the table below.
After the initial detection of an object, if there is no decrease in the distance between an object and the central sensors, the time delay between the audible warnings remains constant. If an object is detected by one of the corner sensors only, the audible warnings stop after approximately 5 seconds if there is no change in the distance between an object and the corner sensor.
Detection Calculation
When operating in the combined transmitter and receiver mode, the sensor outputs a number of ultrasonic pulses and receives the reflected echo signal. The control module amplifies the received echo signals and compares them with a preprogrammed threshold to calculate the distance to the object. This is achieved by determining the elapsed time between the transmission and reception of the ultrasonic signal.
When operating in receiver mode, the sensor receives echo signals transmitted by an adjacent sensor. This mode is used to improve the accuracy of the system.
The detection cycle consists of the control module operating 1 sensor in the combined transmitter and receiver mode and transmitting a number of pulses. The control module then switches the transmitting sensor and the adjacent sensor(s) to receiver mode. After a short time delay this sequence is repeated using a different sensor to transmit the ultrasonic signal. This sequence is completed in 100 ms. The control module uses several measurements of the same sensors to remove errors from the calculation.
If the detected object is directly behind a sensor, the distance is calculated using the time between the transmission and reception of the ultrasonic signal. If the object is positioned between 2 sensors, the control module uses both signals to determine the correct distance using triangulation.
To perform the triangulation calculation, the control module must know the distance between the individual sensors in the bumper. This information is stored in the control module memory. From the received distance signal from each sensor and using the known distance between adjacent sensors, the control module can calculate the minimum distance from the vehicle to the detected object.
When approaching several objects, the control module recognizes the distance from the vehicle to the nearest object.