Adaptive Cruise Control (ACC)

Adaptive Cruise Control (ACC)

• In the presence of a preceding vehicle, ACC follows that vehicle
• When no vehicle is ahead, ACC maintains a desired cruising speed
• Cruise control is a basic form where the driver picks a speed
• Cruise control has minimum functionality for ACC systems
• Radar is the standard technology for distance measurement

Bosch ACC

• Bosch has its own adaptive cruise control

ACC Practical Use

• ACC systems will speed up to catch up
• ACC systems brake when approaching another vehicle
• ACC systems will follow other cars
• ACC systems will accelerate to return to the desired speed if the car in front moves
• ACC will allow for free driving when there are no other cars

ACC Technical Structure

• ACC use is currently possible on highways or well-developed country roads
• Planned areas of use include traffic jams with improved sensors, and cities

ACC Architecture

• Operation involves vehicle movement, object detection, and preceding vehicles being tracked
• This data is used in distance controllers
• Distance data is used to adjust a car's engine, gearbox, and braking

Ranging Radar

• Distance can be measured by propagation time
• The formula for propagation time is τ = 2d/c, where c is the speed of light
• Relative speed is determined by the Doppler effect
• The Doppler effect can be calculated as fD = -2 fc Vrel/c
• The frequency is multiplied by a factor of -2
• Suitable modulation methods increase frequency when ranging
• Angles are determined by sending and assessing multiple radar lobes

Radar Transceiver

• Radar transceivers use voltage-controlled oscillators and harmonic mixers
• Radar transceivers measure at 12.65 GHz

ACC Sensor and Control Unit

• ACC sensors combine data from multiple sensors and sources

Mitsubishi ACC

• Mitsubishi also makes adaptive cruise control

ACC System Architecture

• System architecture involves the ACC core
• The ACC core connects to sensing equipment through the power train, with ESP, CAN, and other protocols
• Displays and controls let users determine speed and time gaps

ACC in Composite Systems

• The interface of conventional cruise control is used to control the power train
• Knowledge of engine maps are not needed
• Active brake boosters, hydraulic brake actuators (when available), and brake-by-wire systems are used to control the brakes
• The brake light is activated when brakes are in use
• Sensors for yaw rate, steering wheel angle, acceleration, and wheel speed are needed

ACC Safety

• Failsafe measures include shutting off radar emissions, disabling the ACC and storing a fault memory entry
• Hardware and software redundancy is needed for safety
• Safety demands component monitoring, function monitoring, and mutual control

ACC Reactions and Faults

• ACC control may continue with a fault code entry for later diagnosis
• ACC restriction or termination could occur with a fault code for diagnosis
• Reversible faults will disable ACC until detection passes
• Irreversible faults disable ACC until the next ignition cycle
• The car can still be used if the ACC fails

ACC Display Options

• Large amounts of design leeway exist for display options
• Display options vary by manufacturer
• Displays usually show information with typical elements and functions

ACC Setting Options and Operation

• Settings extend options from conventional cruise control
• Drivers can set speed, resume to a prior speed, and increase/decrease speed
• Adjustments can be made in increments of 1, 5, or 10 km/h
• In the presence of a vehicle ahead, ACC becomes active

ACC Control Elements

• The resume function allows the last, desired cruising speed to be set
• The "+" button increases the displayed set cruising speed
• The "-" button decreases the displayed set cruising speed
• The I/O button switches ACC systems on and off

ACC Activation

• ACC usually activates when driving at a desired cruising speed with "set" buttons
• Activation can also involve resume, deactivation, or adjustments that are specific to a manufacturer

ACC Setpoints

• ACC setpoints involve establishing a desired distance
• ACC also requires setting a desired time gap

ACC Time Gap

• The time gap is expressed as wΔ = th v
• In Germany, th is typically 1.8 s
• The desired distance in meters is half the speedometer display in km/h

ACC Distance and Time Gap Controls

• Small knobs and step switches are used as controls

ACC Control Displays

• Displays show "green" for large distance, "yellow" for medium, and "red" for short

Radar Signal Processing

• Indirect propagation time measurements are used in most cases
• For FMCW radar, frequencies of transmitted and received signals are compared
• A fast Fourier transform (FFT) is used to analyze signal frequency

Radar Signal Processing

• Signal peaks are identified to find objects
• Noise is eliminated
• Noise, thresholds, and values are all considered
• Echoes are processed for distance, velocity, and angular position
• Measurement data is compared over time
• Range uncertainty caused by acceleration or deceleration is accounted for

Object Selection

• Object selection assesses lateral position
• Lateral position (dyc) is calculated as dyv-dyvCourse, where dyvCourse = kyd^2/2
• Lane probability is assessed
• Both position, movement, accuracy, and frequency must be considered for plausibility

Course Prediction

• Incorrect object selection is minimized by course prediction
• Curvature is determined from ESP sensor data, including steering wheel angle, lateral acceleration, and wheel speeds

ESP Sensor

• Data from yaw rate and steering wheel angle sensors work best
• Sensor data is enhanced when mutual comparison can be achieved

ACC Architecture

• The ACC has different levels of function from basic sensing to control
• Sensing data provides plausibility
• Plausibility assessments yield output like Distance, relative speed, lateral position
• The ACC then chooses the appropriate decision
• This results in desired vehicle acceleration, whether "drive, slight deceleration", or "deceleration"

ACC Controller Functions

• The vehicle speed must be controlled to match settings desired by the driver Automatic speed is maintained based on a safety distance during cruise control
• Speed is reduced as necessary in a curve
• An appropriate control mode is set

ACC Control Loops

• Systems actuate to manage wheel speed, yaw rate, steering angle, and acceleration data
• This informs the ACC controller to select a cruise, tracking or curve control mode based on speed and distance measurements

Cruise Control

• Set accepts the actual speed as the desired cruising speed
• Resume returns to the last stored cruising speed
• Step will increase or decrease desired cruising speed while cruise control is active
• Acceleration can be temporarily overridden

Tracking Control

• Objects are searched to predict the ACC vehicle course
• The most decisive, next vehicle is tracked
• A feedback loop links controller values to target vehicle selection
• Distance and relative speed affect control
• The time gap between vehicles is in the 1 to 2 second range

Stationary Behavior

• Relative speed vref is 0
• Distance = Desired distance wΔ
• The ACC more sensitive to changes in relative speed

Curve Control

• Curve control is uncritical on expressways and highways
• However, it can be uncomfortable on winding roads with longitudinal accelerations
• Adaptation to limited visibility in tight curves is possible

Lower Level Controller Modules

• Suitable actuator systems are selected
• Power trains and braking systems are both used
• Compensation is needed to control driving uphill or downhill
• Torque or acceleration interfaces can be used
• Components such as ESP and SBC are already available in modern cars

Functional Limits

• Sensors cover an area of 40m or more
• The technical concept performs well on highways and expressways
• Performance is more limited on roads with tight curves, and in cities
• Speed range is limited to 30-200 km/h

Dynamics Limits

• There are limits on vehicle acceleration
• These limits correspond to those for conventional cruise control
• Active braking occurs for ACC with 2.5 m/s²
• This braking corresponds to 25% of maximum possible deceleration on dry roads
• A limited sensor range results in a maximum differential speed
• A larger sensor range is currently not feasible
• Overly large ranges creates challenges due to correct track assignments, changes in lane, and construction

Stationary Objects

• Detection of stationary objects relies comparing relative speed with the ACC vehicle’s own speed
• ACC is primarily a comfort system - not designed for stationary objects
• Strategies primarily rely on a low-speed range
• ACC will stop a vehicle from accelerating if it detects stationary object

New Developments

• Developments are reducing the size of sensors and control units
• Expansion of the evaluable angle doubles degrees and lowers size of the lens antenna
• Development focuses on new environment sensors

Perfected Functions

• New components include long-range radar, video camera, and close-range sensors
• Operation occurs at all speeds, and can stop without assistance
• ACC2 incorporates faster response and collision detection