adaptive cruise control

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ADAPTIVE CRUISE CONTROL

1. INTRODUCTION
In this report the structure and functionality of ACC systems are discussed in detail. Although development of ACC systems begin in the late seventies, marketing is now possible after significant progress in both sensor and signal processing technologies. Sensors, on one hand, need to measure accurately the distance and relative speed to preceding vehicles, be easily and cheaply producible, and must meet the strict requirements of the automotive market. On the other hand, the signal processing must guarantee the desired resolution of the processed data while keeping rigorous timing constraints. Progress in these areas was partly triggered by the PROMETHEUS project, an European project involving all European research project involving all European car manufacturers, initializing similar activities in Japan and U.S. In this report, the main components, which typically make up such an ACC system, are described. The control strategy and the system architecture of ACC system are explained. Then the principal limits, safety assured, as well as the simulated effects on the traffic flow are pointed out.

1.1 HISTORICAL SURVEY
At the dawn of 21century, the enormous increase of vehicles has made the prevention of vehicle collision important. At the same time, with the prosperous of highway systems, drivers need only keeping the same distance and speed with respect to the preceding vehicle once they are on the highways. Cruise control offers the advantage of reducing fatigue for drivers, since they can change their position and not necessarily hold one foot on the acceleration pedal. Automated transmission slows the vehicle if it comes within a three-second range of a vehicle in front of it. The controlled declaration alerts the driver to take additional corrective action in order to maintain safe operation of the vehicle. In addition, the steady speed avoids inadvertent acceleration, enhancing both safety and

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compliance with speed limit. Furthermore, such an operation is more economical. The Adaptive Cruise Control and the Collision Warning System work together to supplement a driver's own sense of road conditions. Crowded highways and inclement weather compound the complexity of driving while monitoring all the onboard systems in a commercial truck. The Collision Warning System assists the driver by advising when a vehicle encroaches upon the truck's safety zone. The Adaptive Cruise Control goes a step farther by automatically slowing the vehicle, giving the driver additional time to act. With all these advantages, adaptive cruise control becomes more important at present time. The adaptive cruise control is a critical technology for future intelligent transportation systems. It can be integrated with steering, brakes, suspension, sensors to provide more comfortable and advanced vehicle dynamics and safety. Due to the overwhelming processing speed of microprocessor, the control system underlying adaptive cruise control is chosen digital control system. The digital control systems not only have great ability two integrate several-sensed input signal with complex algorithm to process them but also have relative competitive cost.

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2. ACC COMPONENTS
OVERVIEW:
The ACC system is built as a distributed system using common Electronic Control Units (ECUs)plus one additional ECU, which contains the Range Sensor and the ACC controller. The different ECUs are connected via a CAN data bus. The actual components are displayed in fig. 1. The combined Range Sensor and ACC controller is mounted in front of a car. The curve Sensor helps to predict the future course of vehicle. A man Machine-Interface (MMI) includes all the interfaces of ACC system to the driver: the operation switches, the display and the pedals (accelerator, brake). The actuators in the ACC System are slightly modified standard ECUs for engine control (EGAS, EDC), transmission control and active brake control ( small addition to ASR or Vehicle Dynamic Control (VDC)). An electronic brake control system is optional depending on the characteristic traffic conditions. In unsteady traffic conditions, a significant gain of functionality is achieved by the active brake control.

2.1 RANGE SENSOR
The range sensor is the key component of ACC. It has to supply the ACC controller with data about the position and speed of preceding vehicle relative to the ACC equipped vehicle. The considered ACC system contains a mm-wave Radar sensor in the 77 GHz frequency range. With the FMCW modulation (frequency modulated continuous wave) resolutions of better than 1 m distance and 0.5 m/s in relative speed are achieved. Not only this but also the lateral position or the angle relative to the vehicles longitudinal axis is important to select the relevant preceding vehicle. An angular resolution of better than 1 deg. Within a range between + & -0.5 deg can be achieved using multi-beam techniques. This results in better detection on curved roads. Millimeter wave radar

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satisfies many of the criteria required of a sensor used in outdoors-autonomous vehicle navigation. The short wavelength, compared to microwave units provides a highresolution measurement and radar gives improved performance in inclement whether compared to optical sensors such as lasers. With modern digital signal processing (DSP) techniques, it is possible to locate target reflectors at greater distances than other sensor with a high degree of accuracy. The high bandwidth radar intermediate frequency (IF) gives a measurement of sufficient range and resolution.

2.2 ACTUATOR
A number of methods are used to control the throttle position. A motor can be used to control the throttle cable or, in many cases, a vacuums operated diaphragm is used which is controlled by three simple valves. This is as shown in figure no. 6 below When the speed need to be increased, valve 'x' is opened allowing low pressure from the inlet manifold to one side of diaphragm. The atmospheric pressure on the other side will move the diaphragm and hence the throttle. TO move the other way, valve 'x' is closed and valve 'y' is opened allowing atmospheric pressure to enter the chamber. The spring moves diaphragm back. If both valves are closed then the throttle position is held. Valve 'x' is normally close and valve 'y' is normally open; thus in the event of electrical failure cruise control will not disturb. Valve 'z' provides extra safety and is controlled by the brake and clutch pedals. The following figure shows an electronically control actuator which controls the length of cable that is connected to throttle valve and hence the speed of vehicle. Many cars use actuators powered by engine vacuum to open and close the throttle. These systems use a small, electronically- controlled valve to regulate the vacuum in a diaphragm. This works in a similar way to the brake booster which provides power to your brake system.

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In the figure above, you can see two cables connected to a pivot that moves the throttle valve. One cable comes from the accelerator pedal, and one from the actuator. When the cruise control is engaged , the actuator moves the cable connected to the pivot, which adjusts the throttle; but it also pulls on the cable that is connected to the gas pedal; this is why your pedal moves up and down when the cruise control is engaged. 2.3 .1 Engine Control There are several suitable means of controlling the engine: [1]Separate or integrated electronic throttle control (EGAS), [2]Combined throttle control with engine management, [3]Pneumatic or electric cruise control actuators and [4] EDC systems for diesel engines These possibilities combined with the different engine types result in large variety of ACC system. Application expenses are reduced, however, by using modular control concepts and general interfaces based on physical quantities e.g., engine torque. As per the signal from the controller, LOC gives signal to the actuator to change the throttle position so as to decelerate or accelerate.Actuator,thus, according to its type performs the required operation. 2.3.2 Transmission Control Electric Transmission Control in vehicles with automatic transmission improves the overall ACC comfort significantly. Furthermore, in cars without electronic brake control, it is possible for the ACC to shift down to decelerate with the engine. Hence, the number of necessary driver interventions due to the "normal" engine deceleration capability is reduced. The current gear limits acceleration and deceleration capabilities in vehicles with manual transmission. If the clutch in disengaged, the ACC system is either temporarily or permanently switched off.

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2.4 ACCUMULATOR
The accumulator is a vacuum storage tank that stores vacuum to maintain the satisfactory cruise operation when the manifold vacuum is low at wide throttle opening. A cruise vacuum system is shown in figure as follow.

2.5 CRUISE CONTROL MODULE [CCM]
The cruise control module has to do three things. First it remembers the speed you set. It stores this set speed until you change it or turn off the ignition. Next it takes the speed signal from the vehicle speed sensor and compares it to the set speed. Lastly it sends pulse signals to the actuator. The actuator will move the throttle linkage to bring the vehicle up to the set speed and then modulate vacuum to maintain the speed. The brain of a cruise control system is a small computer that is normally found under the hood or behind the dashboard. It connects to the throttle control seen in the previous section, as well as several sensors. The diagram no. 10 below shows the inputs and outputs of a typical criuse control system.

2.6 TRANSDUCER
Transducer receives vehicle speed signal through the speedometer cable. Electrical signals from the control switch or clutch switch are sent to transducer. In addition the transducer receives engine manifold vacuum. It regulates the vacuum to the servo though the electrical signal received.

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2.7 SERVO
The servo controls throttle plate position. It is connected to the throttle plate by rod, bead chain, or Bowden cable. The servo maintains the set speed by receiving a controlled amount of vacuum from transducer. When the vacuum is applied to servo, the spring is compressed and throttle plate moves to increase speed. When the vacuum is released, the spring returns to the throttle plate to reduce engine speed.

2.8 BRAKE CONTROL:
Active break control in based on the hydraulic systems used for standard traction control(4WD-ASR) or VDC and does not require a smart booster. This system allows quite and comfortable deceleration control, but still can automatically switch to ABS. The brief description about ABS is given:

2.8.1 ANTILOCK BRAKING SYSTEM (ABS) It was designed to prevent wheel lock up if the new brake pedal is operated with too great force, regardless the condition of the road way surface. Thus, a primary purpose of antilock braking system for vehicle is: 1) Wheel lock up-When wheels are locked, frictional heat softens tire thread and traction reduces by approx. 40% at 100% slip. 2) Steering-Steering control is lost and vehicle continues to travel in a straight line even if wheels are steered. 3) Slippery surface-Traction is lost during move on slippery surfaces.

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4) Differential µ between wheels-When one set of tires of the vehicle move on the different surface traction is lost on one side. 5) Skid-when one or both rear wheels lock before the front wheels, rear of vehicle moves faster than front resulting in skidding. 2.8.2 BASIC COMPONENTS OF AN ABS 1) Wheel speed sensors- Provide wheel speed signal to ECU. 2) Electronic Control Unit (ECU)- Translate wheel speed signals to op actuators. 3) Electromagnetic Hydraulic unit/actuators. Control brake line pressures to prevent wheel lock up. 2.8.3 WORKINGABS uses pulse rings attached to the insides of the wheels to monitor their individual rotational speed. As they rotate, the pulse rings generate pulses in stationary wheel sensors, which are fed to wheel’s rotational speed. These signals are fed to ECU, which uses the change in wheel-speed that accompanies braking to determine the wheel's deceleration, acceleration and brake slip. These provide basis for calculations to determine the brake pressure that will supply maximum retardation without leading to lock wheels. Thus ABS provides better braking. The control signal from controller unit is given to the LOC, which via actuator sends signal to brake system. Thus complete loop is formed.

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3. OPERATION AND DISPLAY
Operation of the ACC system is similar to the conventional cruise control:
Switches are used to turn the system on or off, to resume and to set the speed. Additionally the desired time gap representing a speed dependent following distance is main operation variable. To establish the base principle of total driver priority, the driver is also able to intervene in the system by applying the brakes, turning the ACC off, on by using the gas pedal to increase the delivered engine power. In vehicles with manual transmission, the driver can pause or shut off the ACC by pushing the clutch. After engaging the clutch ACC could be automatically resumed. This depends on the preferences of the car manufacturer. The information which is displayed in front panel includes(as shown in fig--). [1] The set speed, since the driver might forget the set value after a longer period of following slower traffic, [2] The set time gap, and [3] The actual ACC mode, i.e. whether the car is in Following Control mode or Speed the Control mode. The last feature is intended especially to support learning and supervision of the ACC system. Another interface element is a warning device. It gives warming sound when ACC cannot brake the vehicle effectively. This situation arises when vehicle preceding is so close that

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by the time radar sensor detects it may collide or in other words insufficient deceleration. In this case driver takes control of vehicle. The system pictured above has five buttons: On, Off, Set/Accel, Resume and Coast. It also has sixth control-- the brake pedal, and if your car has a manual transmission the clutch pedal is also hooked up to the cruise control. The operation of each switch is as follows:  ON/OFF :- The on and off buttons don't actually do much. Hitting the button does not do anything except tell the car that you might be hitting another button soon. The off button turns the cruise control off even if it is engaged. Some cruise controls don't have these buttons; instead ,they turn off when the driver hits the brakes, and turn on when the driver hits the set button.  SET/ACCEL:- With the vehicle traveling at the desired speed , the driver presses the 'SET' button. The ECU's speed control microprocessor the energizes the vacuum pump to move the throttle valve actuator diaphragm until the 'SET' speed is maintained without use of the accelerator pedal and so the driver may remove his foot from the pedal. By continuously monitoring the vehicle speed signal, the microprocessor constantly makes changes to the throttle position to take the account of variations in road gradients, wind resistance and so on, so that the memorized cursing speed is held. If the button is tapped quickly with the cruise control engaged , the cruise control set speed will increase 1mph(1.6 kph). This action may be repeated up to 10 times to achive a 10-mph(16 kph)cruise set speed increase. After 10 taps of this switch has been completed , the system must be to a new speed , the speed remains constant until we press any other button or brake or clutch. This function is referred to as "tap-up" function.  RESUME:- If you recently disengaged the cruise control by hitting the brake pedal, hitting the resume button will command the car to accelerate back to the most recent speed setting.

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 COAST:- When the cruise control system is at a set speed if the coast button is tapped , the cruise set speed will decrease 1 mph (1.6 kph). This action may be repeated until the vehicle speed decreases to 25 mph (40 kph) this function is referred to as "tap down."  The brake pedal and the clutch pedal each have a switch that disengages the cruise control as soon as the pedal is pressed, so you can shut off the cruise control with a light tap on the brake or clutch.

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4. SYSTEM ARCHITECTURE
4.1 OVERVIEW:
The ACC function can be split into three major parts [1]ACC Controller computing how the vehicle should accelerate. [2]Longitudinal Control(LOC) which manages the actuator' systems two achieve the desired acceleration and [3] Man-Machine-Interface enables the driver to operate, supervise and intervene. The functional structure has to be implemented in real vehicles bringing up questions concerning integration and interfaces.

4.2 ACC CONTROLLER
The first control step is to select the relevant preceding vehicle. The expected course of vehicle will be determined using speed and the course curvature. The system measures the distance to a prceding vehicle and the reletive velocity of the vehicles. This can be done by the millimeter-wave radar unit which transmits a radio wave pulse and computes the distance to a forward object from the time it takes for the reflected wave to be received. The relative velocity is calculated from the difference in frequency between the transmitted and reflected waves. Based on this information, it automatically controls the host vehicles speed by giving respective signal to Longitudinal control which in turn sents signal to actuator so as to maintain the set distance between the two vehicles. In the event a preceding vehicle decelerates or another vehicle cuts in front of the host vehicle so that the headway distance is shorter than the value set by the driver, the radar sensor sends signal to controller to decelerate so as to maintain the safe distance which is set. When the situation necessities even grater deceleration, the system also automatically

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applies the brakes. Once the headway distance becomes longer than the set distance, as a result of the preceding vehicle or the host vehicle changing lanes , for example , the system automatically accelerates the host vehicle until the set distance is reached. It then acts again to maintain the desired headway distance to preceding traffic. The driver can override the operation of the ACC system by braking or accelerating the vehicle manually. In this case , the control system is released and precedence is given to the drivers action. 4.2.1 STOP and GO-ACC

As a next step following the series introduction of ACC systems, everybody expects it includes the STOP and GO functions. It assists the driver in traffic jams on highways and in urban areas, which particularly benefit people in congested areas. STOP and GO extension necessities the complete coverage of the near range of up to 30 m. This consists of an electronically controlled brake as well as automatic transmission or coupling device. No special requirements are necessary for latter devices, but an automatic idle position during the stop phase would be advantageous. The controlled brake has to obey stiffer requirements. On one side, higher deceleration values are needed at lower speeds. On the other side, the control strategy evidently has to consider the unsteady deceleration behavior during stopping maneuvers resulting in possible changes of control variables and concepts. 4.2.2 SPEED CONTRLLER The speed controller is based on wheel torque management technique. The wheel torque manager selects from an electrically controlled throttle, an automatic transmission or an electronically controlled brake to send commands depending upon required wheel torque. The driver or the speed controller chooses the required wheel torque alternatively. In manual driven system, the throttle valve opening and transmission ratio are calculated from the required wheel torque Trd that is determined from the accelerator pedal angle, vehicular speed and vehicular acceleration(deceleration).According to required wheel

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torque Tr the throttle valve opening and the transmission ratio are calculated and valve is operated. The wheel torque manager actualizes wheel torque based on the required wheel torque. Then, the vehicle attains the speed so that the actual wheel torque is balanced with the resistance of driving vehicle. 4.2.3 HEADWAY CONTROLLER The headway controller sets itself a desired headway distance, which corresponds to the speed of installed vehicle. The mm-wave radar sensor can measure headway distance and relative speed of the installed and preceding vehicle.

4.3 LOGITUDINAL CONTROL (LOC)
The longitudinal control has to manage the actuator systems in order to achieve the desired acceleration as calculated by ACC controller. Depending on the set value of the acceleration either the drive train or the brakes have to be activated. An estimated road inclination is considered in this selection. In addition , limits of the actuation range of the driven train and the brake system must be known or calculated. After the active actuation branch is selected, the corresponding set values are computed. The active brake system can either be: [1] A deceleration control with an autonomous, closed control circuit using wheel speed sensors. The set value for deceleration is directly derived from the set acceleration of the ACC controller( e.g, ABS), or [2] A brake force control mostly based on a pressure control loop with a pressure sensor. In this case the deceleration control happens on a higher level and will be a part of the Longitudinal Control( e.g, 4WD-ASR).

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Different functional structures of the drive train control are used if the car has manual or automatic transmission for manual transmission the Longitudinal Control in demanding an engine torque. This can also be done with automatic transmission, but a coordination of the Longitudinal Control and the transmission control is more comfortable. In this case the Longitudinal Control sends a drive train torque command to the transmission control. This by itself computes a suitable gear and engine torque corresponding to the current gear shift strategy.

4.4 MAN-MACHINE-INTERFACE
In spite of all the technical aspect of ACC, the driver will remain the master of the system retaining full responsibility for driving the car. The driver has to activate, operate, supervise and, if necessary, intervene or switch off the ACC. He also selects the major control parameters set speed and set time gap. At least one switch is necessary to start the ACC function. This may be separate switch or could have further functionality. As in conventional cruise control, the driver can adjust the desired maximum speed. Additionally, the driver must be able to control the set distance value. This set distance is proportional to vehicle speed. It represents a constant time gap between the preceding vehicle and the ACC vehicle under stationary conditions. Consequently, this time gap is the set value. Again these two functions can be combined in one operation device . Implementing ACC without a driver adjustable set time gap leads to unsolvable dilemma: if the ACC time gap is very large, then ACC control will not be accepted by the customer because of the uncommon long distances. If on the other hand the ACC time gap my is well tuned for the majority of ACC users under normal conditions, then this time gap will be very small in bad weather conditions for example. Thus, the set time gap must be adjustable so that the driver can take on full responsibility for driving the car when using ACC. The driver is still part of the control circuit even after activating ACC.However, instead of operating the gas and brake pedal he has to supervise ACC. This requires some

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kind of information feedback. The most important feed back in the actual vehicle acceleration and acceleration rate of change to which the driver is highly sensitive. Via this information channel the driver is well informed about the state of the ACC controller. Other necessary information for the driver is the value of the set speed and net time gap. Very helpful for supervising especially during the learnings phase is a "vehicle detected" signal. It indicates that a relevant vehicle for following is detected. An indication of insufficient deceleration capability during the actual traffic situation is helpful. The driver is warned if the preceding car decelerates too rapidly or the speed difference is too high while approaching. Such a signal, e.g. as an acoustical information, tells the driver to take over control. Another cause for the driver to take over control can be automatic switch off the ACC function , which must be clearly indicated. This can occur. In case of system failure or in case of an intended switch off e.g., to avoid an engine stall in manually shifted vehicles.

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5. IMPLEMENTATION
For the different functions within the entire ACC system a concept for integration in the vehicle was developed. Evaluation of potential concepts was done taken the following criteria into account: [1] No impact on vehicle safety, [2] No reduction of availability of the basic functions of the vehicle, [3] Low hardware effort, [4] Low application effort, [5] Only small component variations. The output of the ACC controller block, the desired acceleration, is an invarient

command, which can be used for every vehicle. The next functional block, the Longitudinal Control, is more dependant on the vehicles actual equipment. However, at least for the introduction phase of ACC, while it will be sold as an option, the Longitudinal Control will be computed within the ACC/Sensor ECU The commands to drive train control or brake control, the output of the Longitudinal Control, can be different quantities. For example, brake deceleration or brake torque, drive train torque or indicated engine torque could be used depending on the actual components. These commands are sent via a data network like CAN to the actuator subsystems to achieve the active automatic intervention. The standard ECU's, i.e., and brake control will receive these by the ECU for traction control or Vehicle Dynamic Control. The engine powers control has to carry out the torque demand of the ACC

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Longitudinal Control. In all these cases the actuator control systems are fully functional even when ACC is not working properly. Therefore, their availability, reliability and safety are not affected by the ACC systems. This assumes that the driver must have the normal access to these sub-systems not only via ACC.

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6. LIMITS
6.1 RANGE OF OPERATION
A lower speed limit is introduced for safety reasons to prohibit an automatic (positive) acceleration at low speed. This should avoid a dangerous approach to on obstacle that is outside the lateral sensor range, but well inside the collision reign of the vehicle. Usually, lower speed limits of about 40 km/hr are used. As mentioned above there are upper and lower limits for the set time constant. The upper limit of 2s is of no importance for the safety of the system, but it makes no sense extending this limit to higher limit because very few drivers will select a larger time constant. The lower limit in far wore relevant for the system safety. A minimum value must allow the driver to safely take over control of the vehicle in emergency situations. It must also be acceptable for the majority of drivers. Experiences made during the testing phase of ACC systems have shown that values of about 1.0s are reasonable. Active braking must be restricted to moderate values to prevent the driver from getting into an unexpected situation caused by an ACC system reaction. Either the deceleration or the deceleration rate of the active brake has to be limited. Tests have shown that reasonable limits are 2.5 m/s^2 for the maximum deceleration and 1.0m/s^2 for the deceleration rate of change. The latter meaning a change in deceleration of 1/10 g in 1 s. By this "smooth" braking the driver is never surprised by the car's reaction on changing traffic situations, he will not be overstrained in case he has to intervene during automatic braking. An a consequence, however, the ACC system is not capable of making an emergency braking. Mainly for the same reasons as with braking, but also in order to make the control of the car make comfortable, the maximum positive acceleration is limited to a value of 1.0m/s^2.

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6.2 TECHNICAL LIMITS
All ranging sensors-independent of their basic principle-have only limited range, as well as Longitudinal as in lateral direction. Typical range values under ideal environmental conditions are 150 m for Longitudinal and 10 for lateral range. However, these values will be further reduced in two ways: Either by weather conditions or by geometrical obstruction. The longitudinal range of optical sensors is far more influenced by weather conditions than those of Radar Sensors. They are physically limited in the same way as the human eye, e.g. the. by rain, fog or snow. This causes major problems because the driver does not expect the system to be "blind" in this situation. This almost never happens with Radar Sensor. But even those are limited under certain conditions, especially if an absorbing or reflecting medium like snow covers the sensor. This also means detection drop onto with optical sensors. Limitations due to geometrical obstructions are characteristics for all autonomous ranging sensors existing today. Tops of hills and bottoms of valleys naturally limit the longitudinal range. Crash barriers, walls and other side obstacles act in the same way in curves . Vehicles in the same or adjacent lanes can reduce both the longitudinal and the lateral range by hiding objects in hiding objects in front of them. Other limitations of today's systems arise from difficulties in predicting the course far in front of the ACC equipped car. This is mainly due to two reasons: [1] Errors in determining the actual value of the road curvature, and [2] Parts of roads with non-constant curvature.

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Whereas it is difficult to improve the determination of the actual road curvature ( better filtering of the sensor signals usually implies increased filtering delays) the latter difficulty might at least partially be overcome by integrating video or navigation systems.

6.3 PRINCIPAL LIMITS
Although as mentioned above there are many ways to predict the course of the ACC car, one major problem can hardly be overcome. A Lane change intended by the driver is only predictable with a low probability. In the given sample situation the subsequent dilemma for the ACC arises: Approaching vehicle in the same lane with high velocity on a multi-lane highway will, with high probability, lead to a passing maneuver. If this were desired, automatic braking to adjust to the preceding car would be undesirable. On the other hand, if the ACC waits until the decision is obvious, the distance to the car ahead may already be too small. Therefore it is too late to decelerates the own car to time to follow its predecessors. The situation gets far more complicated, if the car in front begin to pass or following traffic does not allow the intended passing. The described situation is typical for principal limits of a technical system. It cannot take on the responsibility of directing the vehicle to the general traffic . This results in restricting the functionality of the system. These limitations must be transparent to the driver. If the driver always understands the system behavior, he is able to decide on his interventions in time.

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7. ABOUT ACC SYSTEM
7.1 ADAPTIVE CRUISE CONTROL [ACC]
There is a new type of cruise coming onto the market called adaptive cruise control. Two companies, TRW and Delphi Automotive Systems are developing a more advanced cruise control that can automatically adjust a car's speed to maintain a safe following distance. This new technology uses a forward-looking radar, installed behind the grill of a vehicle; to detect the speed and distance of a vehicle ahead of it. Adaptive cruise control is similar to conventional cruse control that it maintains the vehicles pre-set speed. However, unlike conventional cruise control, this new system can automatically adjust speed in order to maintain a proper distance between vehicles in the same lane. This is achieved through a radar head headway sensor, digital signal processors and longitudinal controller . If the lead vehicle slows down, or if another object is detected, the system sends a signal to the engine or braking system to decelerate. Then when the road is clear, the system will reaccelerate the vehicle back to the set speed. The Autocruise radar system has forward-looking range of up to 500 feet, and operates at vehicle speeds ranging from 18.6 miles per hour to 111 mph.Delphi's system can also detect objects as far away as 500 feet, and operate at speeds as low as 20 mph. These systems will be enhanced to include collision warning capabilities that will warn drivers through visual and/or audio signals that a collision is imminent, and that braking or evasive steering is needed.

7.2 COOPERAING ADAPTIVE CRUISE CONTROL [CACC]
Through conventional ACC is still an expensive novelty, the next generation, called cooperative adaptive cruise control, or CACC, is already being tested in California and elsewhere. While ACC can only respond to a difference between its own speed and the speed of the car ahead, cooperative systems will allow two or more cars to communicate

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and work together to avoid a collision. Ultimately, experiments say, the technology may let cars follow each other at intervals as short as a half second. At 100 km/hr, which would amount to a distance between cars of less than 14 meters(roughly two car lengths). The car communicate with one another by exchanging radio signals, much as portable electronic devices talk to each other using the Bluetooth wireless protocol. When one car pulls up behind another, the two will scan to determine whether the other is equipped for CACC . The cars will then work out a safe following distance on the basis of their actual performance characteristics --for example, the condition of the brakes of the trailing vehicle.

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CONCLUSION
Man has always desired for coziness. Probably this is the reason why luxury cars are becoming more and more popular these days. Adaptive Cruise Control system provides this and more, what with driver safely being one of the most important criteria in these days of congested traffic. Although it is very efficient, its cost effectiveness is something to be worked out, such that it can be introduced in Indian cars. Also it needs to be technically more sound so as to become foolproof and user friendly. These trifle arguments notwithstanding, ACC has proved its worth over the past few months making driving a child's play. A day will come when a mere verbal order will drive your car.

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REFERENCES
1. WEBSITE VISITED a. www.howstuffworks.com b. www.nissanauto.com c. www.ukcar.co.in d. www.sae.org 2. TECHNICAL PAPERS REFERRED a. Adaptive cruise control system aspects and development trends (SAE paper no. – 961010 ) b. An adaptive cruise control using wheel torque management technique ( SAE paper no. – 980606 )

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