Intelligent Vehicles > Collision Avoidance > Longitudinal Avoidance

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• Introduction. 
• Technological Solutions 
  • Principles of operation 
  • External sensors 
• Systems in Development 
• Intelligent Cruise Control Systems
• Collision Avoidance Systems
• References 

INTRODUCTION

Longitudinal Collision Avoidance Systems (CAS) involve maintaining safe vehicle headways in an attempt to avoid rear-end collisions and backing collisions. Rear-end accidents are quite prevalent, and major contributing causes of these accidents are driver inattention and following too close. The National Highway Traffic Safety Administration (NHTSA) estimates that about 88% of rear end collisions in the United States are caused by driver inattention or by vehicles following too closely (see Figure 1). In turn, rear-end collisions represent about 28% of all vehicle collisions nationwide (Cerrelli, 1996), and 36% of fatal and injury collisions in California alone (California Highway Patrol, 1996).

Longitudinal CAS is also concerned with backing collisions, which occur more frequently on smaller streets and arterials. Backing accidents usually involve two distinct conflicts; stationary or slow moving objects (pedestrians, stopped vehicles), and moving into the crossing path of a moving vehicle.

A reliable longitudinal collision avoidance system is the first step towards achieving a reliable and safe system for controlling a platoon of automated vehicles for "smart" highway usage. This paper discusses Longitudinal Collision Avoidance Systems technology and current research projects.

TECHNOLOGICAL SOLUTIONS

Principles of Operation

A collision avoidance system generally operates in the following manner: a sensor installed at the front end of a vehicle constantly scans the road ahead for vehicles or obstacles. When found, the system determines whether the vehicle is in imminent danger of crashing, and if so, a collision avoidance maneuver is undertaken. Most CAS are non-cooperative, that is, detection is independent of whether other vehicles on the road are equipped with collision avoidance devices. An alternative technology relies on vehicle-to-vehicle communications to exchange information on vehicles' presence, location, lane of travel, and speed among other factors. In addition to the front end sensor, vehicles require a rear end transponder as well, since communication, and therefore detection, only occurs among equipped vehicles. Researchers at California Partners for Advanced Transit and Highways (PATH), the California DOT, and the University of California have been experimenting with cooperative adaptive cruise control (CACC) since the 1990's. Trials have entailed three test vehicles, which use a communication protocol in which the lead car broadcasts information about its speed and acceleration to the rest of the group every 20 milliseconds. Each car transmits information about its speed and acceleration to the car behind it. Such a system can allow two or more cars to communicate and work together to avoid a collision.

The criteria for activation of collision avoidance is usually one of these two: 

The time-to-collision criterion: the system determines whether a collision is likely to happen at prevailing speeds and distances, within a certain time interval. In a car-following situation, the time-to-collision is the time taken for the two vehicles to collide if they maintain their present speed and heading. 

The worst-case criterion: the system assumes that the vehicle preceding the CAS-equipped vehicle could brake at full braking power at any time. In essence, it operates on a "critical headway distance", that is, the minimum distance necessary for the CAS-equipped vehicle to come to a stop in the event the leading car suddenly brakes.

Collision avoidance maneuvers include one or more of the following: 

Headway distance control: the system warns the driver whenever his/her car is following the leading car too closely. Some systems include automatic speed control, i.e., the CAS-equipped car would automatically reduce its speed in order to maintain a safe headway with the leading vehicle. 

Hazard warning: the system warns the driver of an object (moving or stationary) within its projected path, so that the driver has sufficient time to avoid a crash. 

Automatic vehicle control: the system controls the vehicle's brakes and steering wheel, and applies them automatically when it deems it necessary. In actuality, there are currently no systems that use automatic steering to prevent collisions, although the technology has been developed and tested experimentally.

Warning Devices

Visual head-up displays: warnings are displayed on the windshield in the driver's field of view, so that their content can be assimilated in conjunction with the driving scene ahead. These displays are intended to minimize distraction from driving tasks, in addition to ensuring that the warning does not go undetected. 

Audio/Voice signals: in comparison to visual signals, auditory signals appear to be less intrusive on driving tasks. They are also insensitive to external conditions such as poor light, bad weather, or a dirty windshield. Two different auditory warnings have been developed: speech (synthesized voice) or non-speech (buzzer) displays. 

Haptic devices: a good CAS design should provide redundant information via alternative sensory modalities, given that the primary visual or auditory channel may be degraded or overburdened. Research suggests that one possibility is to increase the force needed to push the gas pedal (Janssen and Nilsson, 1991).

External Sensors

These sensors fulfill the tasks of headway control and obstacle detection, which are the basis of CAS Sensing techniques can be classified in three main groups, according to Stobart and Upton, 1995 (see Tables 1 and 2 for a more detailed description of the performance of particular sensors): 

Optical techniques (Passive infrared, laser radar and vision): they all suffer from the disadvantage of being sensitive to external environmental conditions. Passive infrared and vision cannot provide a direct measurement of distance to an object. Laser radar (lidar) appears as the most useful of these techniques, despite its high cost. 

Electromagnetic techniques (FMCW radar, impulse radar and capacitive): unlike the optical techniques, they perform well under adverse environmental conditions. Despite its relatively high cost, FMCW radar seems to be the best technique for long-range distance measurement. It could also be used at short and medium range, rendering a quite flexible technique. 

Acoustic techniques (ultrasonics): well-suited in applications where only short-term relative distance measurements are required, because they are able to provide high resolution for a relatively low cost.

Researchers are considering combinations of several of these technologies (sensor fusion), to overcome some of the disadvantages. In March 2003, of Michigan Transportation Research Institute (UMTRI) announced the beginning of its forward collision warning and adaptive cruise control systems field tests. The key methodology underlying this test will be its use of sensor fusion. In this case, sensor fusion entails the use of a GPS digital map to locate the vehicle and its direction of travel on a map, a forward-looking machine-vision system that uses lane markings to estimate the road geometry ahead of the vehicle, and radar tracking that uses the trajectories of tracked vehicles ahead to determine if there is a pattern that may indicate the upcoming road geometry (read more about Sensor fusion in Michigan field test). Sensor performance may also be improved by using cooperative techniques, such as having vehicles equipped with front sensors and rear passive transponders. As this requires wide system implementation, it is unlikely to happen in the near future. 

Table 1 below presents a brief description of the principles of operation of different types of sensors, and their main advantages and disadvantages. Sensors should be able to determine the headway distance, the relative speed between the preceding vehicle or obstacle and the equipped vehicle, or both. How well this is achieved can measured with the following indicators: 

• Sensing range: the maximum range over which the technique can be used 

• Resolution: the relative change in distance that can be measured 

• Directionality: the width of the beam over which the sensor is sensitive 

• Response time: how quickly the sensor can respond to a change in distance 

Source: Stobart and Upton, 1995. 
 

Source: Stobart and Upton, 1995

SYSTEMS IN DEVELOPMENT

Following is a description of particular Intelligent Cruise Control and Collision Avoidance Systems, by manufacturer or developer. Additional detail on technical specifications can be found in the listed references. 

Intelligent Cruise Control Systems (ICCS)

The typical functions of intelligent cruise control systems are headway control and speed control. These systems are considered precursors of CAS because they use forward obstacle and headway detection technology to maintain a constant distance behind a lead vehicle. 

A.D.C. GmbH (a joint venture of Temic GmbH in Nuremberg and Leica AG in Heerbrugg)

Fiat
Fiat has developed an Autonomous Intelligent Cruise Control (AICC) system, called ALERT. The system uses a combination of sensors (laser radar, microwave radar, and a camera for blind spot monitoring), to detect obstacles in the road ahead. Vehicle control is performed through electronic braking and a throttle actuator. ALERT uses the Laser radar (lidar) as the distance sensor, while the microwave radar, which guarantees visibility in adverse conditions, is used as the collision avoidance sensor. As long as no target is detected, the AICC system provides speed control. If a target is detected, the AICC system switches automatically to distance control: it keeps the same speed as the target vehicle at a safe distance, which in turn is continually updated to account for changes in the vehicle's speed and in road conditions. 

Honda

Honda Motor Company has developed a new safety system designed to predict and help prevent rear-end collisions. Honda's Collision Mitigation Brake System (CMS) anticipates a collision based on driving conditions, distance to the vehicle ahead and relative speeds. It then uses visual and audio warnings to prompt the driver to take preventative action and also initiates braking if the driver fails to respond to the warnings. The system works in conjunction with the "E-Pretensioner" seatbelt retraction system and is due to be introduced into the Japanese domestic market with the June release of the new Inspire. The CMS and E-Pretensioner systems use millimeter-wave radar to scan the road 100 meters ahead and calculates the likelihood of a collision by analyzing the distance between the vehicles, the relative vehicle speeds, and the anticipated vehicle path.

If a collision is likely, the system has three staged modes to prevent or lessen the impact of a rear-end collision:

• An audible warning,
  
• An audible warning, light braking and light seat-belt retraction,
  
• An audible warning, strong braking and strong seat-belt retraction.
  

If a collision is unavoidable, the system also has a number of functions to reduce impact on occupants, including a brake assist function that compensates for insufficient pedal pressure to reduce the speed of impact and seatbelt control that increases seatbelt tension to hold the driver more securely in place.

Honda is currently investing in the research and development of 'Honda Pre-crash Safety Technologies', which are designed to predict collisions and minimize impacts. CMS and the E-Pretensioner, which warn the driver of impending collisions and reduce impact when collisions are unavoidable, represent the first stage in the practical application of these technologies.

Overview of CMS and E-Pretensioner systems

If a collision is likely, the CMS and E-Pretensioner systems use the following three modes to prevent or lessen the impact of a rear-end collision:

1. Primary warning - When there is a risk of collision with the vehicle ahead or if the distance between the vehicles has become too short, a buzzer sounds and the message 'BRAKE' appears on the multi-information display in the instrument panel, prompting the driver to take preventative action.

2. Secondary warning - If the distance between the two vehicles continues to diminish, CMS applies light braking, and the E-Pretensioner retracts the seatbelt gently two or three times, providing the driver with a tactile warning. At this point, if the driver applies the brakes, the system interprets this action as emergency braking, and activates the brake assist function to reduce impact speed.

3. Collision damage reduction - If the system determines that a collision is unavoidable, the E-Pretensioner retracts the seatbelt with enough force to compensate for seatbelt slack or baggy clothing, providing even more effective driver retention than conventional seatbelt pretensioners, which only begin to operate once the collision has occurred. The CMS also activates the brakes forcefully to further reduce the speed of impact. The E-Pretensioner is designed to operate whenever the driver brakes suddenly and the brake assist functions, tightening the seatbelt to secure the driver even if the CMS has not predicted a collision.

CMS & E-Pretensioner System Configuration

Millimeter-wave radar: Detects vehicles within a range of about 100 meters ahead, in a 16-degree arc.

Sensors: The system determines driving conditions using a range of sensors that detect factors such as yaw rate, steering angle, wheel speed, and brake pressure.

CMS Electronic Control Unit (ECU): Based on distance to the vehicle ahead and relative speed obtained from radar information, and on the anticipated vehicle path as determined based on sensor information, the ECU calculates the likelihood of a collision, and warns the driver, and in some cases activates the braking function. The ECU exchanges information as required with the E-Pretensioner, the Variable Signal Analyser (VSA) and the Meter Unit (see below).

VSA-ECU integrated hydraulic unit: Receives information from the various sensors, and sends this information to the CMS ECU and other control units. Also controls the brake hydraulic unit to activate the brakes based on instructions from the CMS ECU.

E-Pretensioner ECU: Sends instructions to the motorized E-Pretensioner to retract the seatbelt, based on braking instruction signals from the CMS ECU and electronically controlled brake assist signals.

E-Pretensioner: Retracts the seatbelt using an internal motor, based on instructions from the E-Pretensioner ECU. Used in combination with conventional pretensioners.

Meter unit: Receives signals from the CMS ECU, and warns the driver of potential danger using a buzzer and a visual warning.

Source: May 2003 article from Honda.com.au

Delco Electronics

Delco Electronics developed the FOREWARN system. This system warns the driver of an impending collision, but does not automatically take control of the vehicle. The integrated warning system incorporates four basic steps: road object sensing, collection of vehicle data, data processing and threat assessment, and driver warning execution. To fulfill these tasks, vehicles are equipped with two front sensors (laser radar and microwave radar), a rear sensor, and driver warning devices (head-up display, audio, and a brake pulse). 

Mazda
Beginning in 1995, Mazda has been testing its Advanced Safety Vehicle (ASV). Its most recent version of the ASV bears the following features:

• Full speed range adaptive cruise control system with brake control
  With this feature, the distance between two vehicles is maintained by adjusting the speed within the range set by the driver based on information from a Laser radar (lidar). This enables control in all speed ranges, including high speed driving on motorways, and mid-low range daily driving, and even when stopping. This releases the driver from frequent speed adjustments helping to alleviate driver fatigue. 
  
  
• Advanced front-lighting system
  This safety feature controls distribution of luminous intensity emitted from headlamps during the night according to driving conditions, road conditions, and steering angle. The distribution area has been enlarged to improve driver visibility and recognition of traffic signs and pedestrians.
  
  
• Forward obstacle information/warning system (pedestrian warning)
  This safety feature judges the degree of danger based on the distance between the vehicle and an object in front measured by a laser radar, and the vehicle's speed/braking status. In accordance with the degree of danger, appropriate warning is given to the driver using visual and sound information to reduce the number of collisions with vehicles ahead and pedestrians who are in the vehicle's path.
  
  
• Neck injury mitigation system for rear-end collision
  With this system, the seatbelt restrains passengers in an appropriate posture just before a rear-end collision. It can predict a rear-end collision by detecting the distance from and the speed of a following vehicle using a laser radar mounted at the rear of the vehicle. When the system predicts a rear-end collision, the seatbelt is wound up by a motor to pull passengers back into the seatback. As a result, it reduces the distance between the passengers' head and headrest, minimizing the severity of whiplash injuries.

Source: December 2002 article from Mazda.com

Mercedes-Benz
First initiated in 1998, the DISTRONIC system is now installed in more than 40,000 passenger cars (in the CLK, E, S, CL and SL-Classes). A radar sensor behind the radiator grille enables the system to maintain a constant distance to the vehicle in front. The proximity and cruise control system can also improve road safety levels. The DISTRONIC system uses a micro-computer to process signals from the radar sensor; within this sensor, three transmission and receiving units scan the full width of a three-lane motorway over a distance of approximately 100 marts and recognize any moving vehicles ahead. The reflection of the radar impulses and the change in their frequency enables the system to calculate the correct distance and the relative speed between the vehicles.

If the distance to the vehicle in front reduces, the proximity and cruise control system immediately reduces acceleration or – should this be necessary – applies the brake. Once the distance increases again, DISTRONIC steps in again as a conventional cruise control system and, at speeds of between 30 and 180 km/h, will maintain the desired speed as programmed. If more drastic action should become necessary, the system will alert the driver with a warning light in the instrument cluster and with an acoustic signal. In this case the driver must apply the brakes her or himself. The findings of the DaimlerChrysler research division show that the reaction time of drivers using DISTRONIC is up to 40 per cent faster than that of those without this assistance system.
Source: article from new-cars.com

Mitsubishi, Nissan, Hino
In 1999, Mitsubishi introduced its Driver Support System in Japan, which supplements Adaptive Cruise Control with lane-departure warning and side and rear monitoring through machine vision. The millimeter wave radar sensor directs the car to slow down if the distance to the leading car is too short, or if it detects an obstacle. The forward-facing camera and a lane tracking system alert the driver if he or she inadvertently wanders within the lane or drifts out of the lane. If necessary, it will correct the steering and slow the vehicle. The rear-facing stereo camera in the tailgate displays its view on the large central monitor, eliminating rear blind spots.

REFERENCES

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Authors: Francois Granet, Rosella Picado, Lauren Smith. Last update: 08/05/03