TBR Technical Corner: Braking Requirements for Optimizing Autonomous Emergency Braking (AEB) Performance (Part 1 out of 3)

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Source: Applus IDIADA

This is the first of three posts by Alvaro Esquer, Project Manager, ADAS in Applus IDIADA, about a study to determine the means of improving autonomous-emergency braking (AEB) performance in terms of efficiency and driver acceptance.

Vehicle technology new developments have contributed to improve vehicle structural performance and therefore passive protection, but also the inclusion of electronic control units has provided new opportunities to expand active safety systems. This is the case for systems like anti-lock braking systems (ABS), electronic stability control (ESC) and brake assist (BA) among others. A more advanced generation of active systems includes sensorial units that monitor vehicle’s surrounding and detect potential hazards, such as an imminent collision, and performs an automatically and commanded emergency braking to lessen or mitigate the consequences of the impending accident. For this latest system, the so-called autonomous emergency braking (AEB), various consumer testing protocols, such as Euro NCAP protocols, propose and periodically update test catalogues in order to evaluate the performance of such systems and later to inform potential consumers.

The aim of this study is to investigate the means of improving AEB performance in terms of efficiency and driver acceptance. For this, performance of current AEB system will be studied and compared with the limits of vehicle’s braking capabilities.

Related post:
AEB: These Automakers Are Ahead of The Curve


Advanced Driver Assistance Systems (ADAS) supporting the driver to avoid collisions have been over the last few years being available in series production vehicles. These systems are focused on the avoidance of car-to-car rear-end collisions and car-to-pedestrian by fully using the braking capabilities of vehicles or by warning the driver with audio-visual signals after the detection of accidents about to occur. This understanding of the environment is accomplished with the aid of a network of devices such as camera and/or radar sensors constantly monitoring the vehicle’s surroundings.

The first generation of such systems Braking Assist (BA), which assists the driver increasing the braking force in panic braking situations. Such system may also notify the driver of a potential hazard by warning with audio-visual or haptic signals by means of the FCW (Forward Collision Warning) system. The next generation of these systems offered automatic braking in imminent collisions situations, a system like this is the AEB (Autonomous Emergency Braking) system. Next step on AEB technology is to be operational in more demanding situations, for instance on crossroads where ego vehicle turns or target vehicle crosses from one the roadsides. Another system that in the near future will be available on production cars is the Autonomous Emergency Steering (AES), such systems will be able to avoid crashes by applying a controlled torque on the steering wheel and producing a lane change manoeuver on the ego vehicle. This function is potentially effective for pedestrians and bicyclist.

Collision avoidance systems will face newer and harder challenges in the next years. Apart from the addition of more advanced intelligent detection sensors the braking system still can be optimized and upgraded to improve overall system performance. First section in this study aims to quantify in terms of braking efficiency how current AEB systems support average drivers in critical situations and how this support could be improved. For this last point, stopping distance tests with professional test drivers are evaluated to set the limits of AEB performance. In section two, it is provided a solution to improve braking efficiency and the benefit is quantified with the reductions in distance and impact speed. Finally, performance and limitations of current AEB systems in Euro NCAP test cases, and the potential advance of improved braking efficiency is described.

Average drivers in emergency situations

Some experimental studies reveal that most of drivers in emergency situations do not apply a quick an effective braking action. The mean maximum deceleration is close to 7 m/s2 and brake jerk is around to 11 m/s3 in general terms. Same study confirms that users’ reaction time after issuing collision warning ranges from 0.8 seconds –this one is for the brake jerk warning– to 1.3 seconds for a HUD warning. Other experimental studies with a generic sample of drivers in driving simulators have been in the past conducted to evaluate driver behavior as a result of the activation of a pre-crash warning. The study brings the conclusion that 25 percent drivers brake before 0.78 seconds after the pre-collision warning and 75 percent will do it prior to 1.81 seconds after the warning was issued. In the study, it was finally stated that an audio-visual plus brake pulse warning resulted to be far more effective than other kind of warnings in helping drivers to react quicker. Euro NCAP test protocols consider an average reaction time of 1.2 seconds for drivers. This time is contemplated in FCW test cases, where driver reaction is substituted by a brake pedal robot. Further, Euro NCAP scores additional points if a brake jerk pulse is found as a part of the collision warning system. This brake pulse is measured by GNSS/INS units installed in cars during official tests and it is registered between audio warning and main AEB intervention.

These studies indicate that drivers need support in emergency situations. Reduction of reaction time and improvement of braking efficiency are the two factors that AEB systems correct.

Professional test driver in emergency situations

Professional test drivers are able to execute a very strong braking application in stopping distance tests, which is a common test in tires and brakes testing. The tests are conducted by experienced and professional test driver, who apply the brakes as strong and quick as possible.

Stopping distance tests from 5 different drivers and 3 different cars have been selected in this study and the brake jerk and deceleration were analyzed. The average brake deceleration is around 10.5 m/s2 and the mean brake jerk is 80 m/s3.

Figure 1 displays one of the most representative tests according to the average values.

Figure 1. Brake application from a professional driver: Brake deceleration and brake jerk.

AEB activations in imminent accident situations

Braking strategy executed by AEB systems does not always consist of the application of the quickest and highest brake pressure to reduce the stopping distance as much as possible. Generally speaking, anything is regulated or stated with regard to the type of braking on an AEB system. A certain AEB system can execute a medium braking deceleration and still be capable of safely avoiding the possible crash and this is on account of the timing of hazard recognition and AEB activation. For instance, if the AEB control unit triggers the braking commands very early the automatic braking profile may not use all the braking capabilities to prevent the crash. On the other hand, if the braking command is triggered close to the last point to brake (LPTB), the only way to avoid the collision would be by applying maximum braking force. Figure 2 illustrates this situation. It shows the emergency braking profiles of 4 different vehicles at the same test conditions. The progression of braking is from right to left, thus the impact point is located at the left side where distance to impact is 0 m.

Figure 2. Different braking profiles from AEB systems in the market

Car 1 is braking at the closest point to the impact and it is triggering the highest braking profile possible. On the contrary, car 2 is braking at the furthest point and with the lowest average deceleration. Both cars are able to avoid the crash and consequently are equally safe.

Driver comfort could be affected with early collision warnings or with early automatic braking intervention. Such approach can be considered a conservative hazard assessment and will probably reduce the rate of crashes; however the rate of false reactions would consequently increase and will likely disturb active drivers. The opposite strategy would be a system that aims to decrease the amount of false active rates by postponing as much as possible the trigger of the automatic braking while still being a safe and effective active safety system.

The baseline braking profile used in this study is the strongest one found on many car-to-car and car-to-pedestrian test scenarios with market vehicles. A total of 15 vehicles have been selected for this with the greatest brake jerk and deceleration and is depicted in figure 3 and 4:

Figure 3. Brake jerk distribution
Figure 4. Brake deceleration distribution

On figure 3, it is deduced that at least 50% of the cars deploy a brake jerk of 20 m/s3, this is represented by the green area and its attached whisker. As well, same green section in figure 4 is indicating that 50 percent of cars brake with a deceleration superior to 10,5 m/s2. The read areas with the lower whisker represent the 50% as well. For figure 3 it is showed that half of the cars do not exceed a brake jerk of 20 m/s3.

About Applus IDIADA

With more than 25 years’ experience and 2,450 engineers specializing in vehicle development, Applus IDIADA is a leading engineering company providing design, testing, engineering, and homologation services to the automotive industry worldwide.

Applus IDIADA has locations in California and Michigan, with further presence in 25 other countries, mainly in Europe and Asia.

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