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02-11-2016, 12:58 PM
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ADAPTIVE CRUISE CONTROL


.pdf  1463503582-mechADAPTIVECRUISECONTROLreport.pdf (Size: 620.38 KB / Downloads: 4)



INTRODUCTION
Everyday the media brings us the horrible news on road accidents. Once a report said
that the damaged property and other costs may equal 3 % of the world’s gross domestic
product. The concept of assisting driver in longitudinal vehicle control to avoid collisions has
been a major focal point of research at many automobile companies and research organizations.
The idea of driver assistance was started with the ‘cruise control devices’ first appeared in
1970’s in USA. When switched on, this device takes up the task of the task of accelerating or
braking to maintain a constant speed. But it could not consider the other vehicles on the road.
An ‘Adaptive Cruise Control’ (ACC) system developed as the next generation assisted the
driver to keep a safe distance from the vehicle in front. This system is now available only in
some luxury cars like Mercedes S-class, Jaguar and Volvo trucks the U.S. Department of
transportation and Japan’s ACAHSR have started developing ‘Intelligent Vehicles’ that can
communicate with each other with the help of a system called ‘Co operative Adaptive Cruise
Control’ .this paper addresses the concept of Adaptive Cruise Control and its improved
versions.


ADAPTIVE CRUISE CONTROL (ACC)
PRINCIPLE OF ACC
ACC works by detecting the distance and speed of the vehicles ahead by using either a
Lidar system or a Radar system [1, 2].The time taken by the transmission and reception is the
key of the distance measurement while the shift in frequency of the reflected beam by Doppler
Effect is measured to know the speed. According to this, the brake and throttle controls are
done to keep the vehicle the vehicle in a safe position with respect to the other. These systems
are characterized by a moderately low level of brake and throttle authority. These are
predominantly designed for highway applications with rather homogenous traffic behavior. The
second generation of ACC is the Stop and Go Cruise Control (SACC) [2] whose objective is to
offer the customer longitudinal support on cruise control at lower speeds down to zero velocity
[3]. The SACC can help a driver in situations where all lanes are occupied by vehicles or where
it is not possible to set a constant speed or in a frequently stopped and congested traffic [2].
There is a clear distinction between ACC and SACC with respect to stationary targets. The
ACC philosophy is that it will be operated in well structured roads with an orderly traffic flow
with speed of vehicles around 40km/hour [3]. While SACC system should be able to deal with
stationary targets because within its area of operation the system will encounter such objects
very frequently.
CONSTITUENTS OF AN ACC SYSTEM:
1. A sensor (LIDAR or RADAR) usually kept behind the grill of the vehicle to obtain the
information regarding the vehicle ahead. The relevant target data may be velocity, distance,
angular position and lateral acceleration.
2. Longitudinal controller which receives the sensor data and process it to generate the
commands to the actuators of brakes throttle or gear box using Control Area Network (CAN) of
the vehicle.


SENSOR OPTIONS
Currently four means of object detection are technically feasible and applicable in a
vehicle environment [2]. They are
1. RADAR
2. LIDAR
3. VISION SENSORS
4. ULTRASONIC SENSOR
The first ACC system used LIDAR sensor.
LIDAR (Light Detection and Ranging)
The first acc system introduced by Toyota used this method. By measuring the beat
frequency difference between a Frequency Modulated Continuous light Wave (FMCW) and its
reflection


RADAR (Radio Detection and Ranging):
RADAR is an electromagnetic system for the detection and location of reflecting objects
like air crafts, ships, space crafts or vehicles. It is operated by radiating energy into space and
detecting the echo signal reflected from an object (target) the reflected energy is not only
indicative of the presence but on comparison with the transmitted signal, other information of
the target can be obtained. The currently used ‘Pulse Doppler RADAR’ uses the principle of
‘Doppler effect’ in determining the velocity of the target .
PULSE DOPPLER RADAR:
The block diagram of pulse Doppler radar is as shown in figure.2.
The continuous wave oscillator produces the signal to be transmitted and it is pulse
modulated and power amplified. The ‘duplexer’ is a switching device which is fast-acting to
switch the single antenna from transmitter to receiver and back. The duplexer is a gas-discharge
device called TR-switch. The high power pulse from transmitter causes the device to
breakdown and to protect the receiver. On reception, duplexer directs the echo signal to the
receiver. The detector demodulates the received signal and the Doppler filter removes the noise
and outputs the frequency shift ‘fd’.


EFFECT OF DOPPLER SHIFT:
The transmitter generates a continuous sinusoidal oscillation at frequency ‘ft’which is
then radiated by the antenna. On reflection by a moving object, the transmitted signal is shifted
by the Doppler Effect by ‘fd’.
If the range to the target is ‘R’, total number of wavelength is ‘λ’ in the two way- path is
given by,
n = 2R/ λ
The phase change corresponding to each λ =2π
So total phase change, p=2n П
=2(2R/ λ) π
So, if target moves, ‘R’ changes and hence ‘φ’ also changes.
Now, the rate of change of phase, or the ‘angular frequency’ is
W=dφ/dt =4 π (df/dt)/ λ
Let Vr be the linear velocity, called as ‘radial velocity’
http://www.studymafia.org
Wd = 4 πVr/ λ =2πfd.
Fd=2Vr / λ
But λ = ft, the transmitted velocity.
Fd= (2c Vr)/ ft
So by measuring the shift, Vr is found. The ‘plus’ sign indicates that the target and the
transmitter are closing in. i.e. if the target is near, the echoed signal will have larger frequency.
RADAR ANTENNA SCHEMES:
Radar systems employ a variety of sensing and processing methods to determine the
position and speed of vehicles ahead. Two such important schemes are:
1. mechanically steered antenna
2. electronically steered antenna
1. Mechanically steered antenna:
A parabolic reflector is used as mechanically steered antenna. The parabolic surface is
illuminated by the source of energy placed at the focus of the parabola. Rotating about its
axis, a circular parabola is formed. A symmetrical beam can be thus obtained. The rays
originating from focus are reflected parallel to the axis of parabola.


Electronically steered phased array radar antenna
A phased array is a directive antenna made up of a number of individual antennas, or
radiating elements. The radiation pattern is determined by the amplitude and phase of current at
each of its elements. It has the advantage of being able to have its beam electronically steered in
angles by changing phase of current at each element. The beam of a large fixed phased array
antenna is therefore can be rapidly steered from one direction to another without mechanical
positioning [1, 5].
Consider the following figure with ‘N elements placed (equally separated) with a
distance‘d’ apart. Suppose they have uniform response to signals from all directions. Element
‘1’ is taken as reference with zero phase.



SPACE OF MANEUVERABILITY AND STOPPING DISTANCE
The space of maneuverability is the space required by the driver to maneuver a vehicle.
An average driver uses larger sideways acceleration while vehicle speed is low. If the curve
radius of a possible trajectory is ‘r’ for a given velocity ‘v’ and sideways acceleration ‘ay’ ,then
r= / ay [2].so to get the required ‘r’ ,when ‘v’ is low, ‘ay’ is also to be low correspondingly.
The stopping distance is given by, Ds = .5 u /ax + td u, where ‘u’ is the initial speed ‘td’ is the
time taken by the system to receive and process the sensor data and ‘ax’ is the acceleration of
the vehicle .the figure shows the detection of edges of the preceding vehicles.



CONTROLLER
The controller translates the situation into appropriate actions through brake and pedal
and throttle control actions.
Depending on the present traffic situation, two types of controls are possible.
1. Speed control
2. Headway control
If there is no vehicle presently in front, then the speed is controlled about a set
point just as in conventional cruise control. But in order to keep a safe distance between the
vehicle s, the headway control is required.
ARTIFICIAL COGNITION
The conversion of raw information from sensors to control actions by the two steps:-
1. Analyzing the traffic conditions
2. Deciding on a particular situation
The controller translates the desired situation into appropriate control action through
brake and throttle actuation.[2]. The controller concept is simplified in the flow-diagram:


CO OPERATIVE ADAPTIVE CRUISE CONTROL [CACC]
Though conventional ACC and SACC are still expensive novelties, the next generation
called Cooperative ACC is already being tested. While ACC can respond to the difference
between its own behavior and that of the preceding vehicle, the CACC system allows the
vehicles to communicate and to work together to avoid collision.
Partners of Advanced Transit Highways (PATH) –a program of California Department
of Transportation and University of California with companies like Honda conducted an
experiment in which three test vehicles used a communication protocol in which the lead car
can broadcast information about its speed, acceleration ,breaking capacity to the rest of the
groups in every 20ms.
PATH is dedicated to develop systems that allow cars to set up platoons of vehicles in
which the cars communicate with each other by exchanging signals using protocols like
Bluetooth.
MAIN POSTULATIONS ABOUT CACC:
1. In CACC mode, the preceding vehicles can communicate actively with the following
vehicles so that their speed can be coordinated with each other.
2. Because communication is quicker, more reliable and responsive compared to autonomous
sensing as in ACC.
3. Because braking rates, breaking capacity and other important information about the
vehicles can be exchanged, safer and closer vehicle traffic is possible.


ADVANTAGES
1. The driver is relieved from the task of careful acceleration, deceleration and braking in
congested traffics.
2. A highly responsive traffic system that adjusts itself to avoid accidents can be developed.
3. Since the breaking and acceleration are done in a systematic way, the fuel efficiency of the
vehicle is increased.
DISADVANTAGES
1. A cheap version is not yet realized.
2. A high market penetration is required if a society of intelligent vehicles is to be formed.
3. Encourages the driver to become careless. It can lead to severe accidents if the system is
malfunctioning.
4. The ACC systems yet evolved enable vehicles to cooperate with the other vehicles and
hence do not respond directly to the traffic signals.



CONCLUSION
The accidents caused by automobiles are injuring lakhs of people every year. The safety
measures starting from air bags and seat belts have now reached to ACC, SACC and CACC
systems. The researchers of Intelligent Vehicles Initiative in USA and the Ertico program of
Europe are working on technologies that may ultimately lead to vehicles that are wrapped in a
cocoon of sensors with a 360 –degree view of their surroundings. It will probably take decades,
but car accidents may eventually become as rare as plane accidents are now, even though the
road laws will have to be changed, upto an extent since the non-human part of the vehicle
controlling will become predominant.
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