TBR Technical Corner: Braking Requirements for Optimizing Autonomous Emergency Braking (AEB) Performance (Part 2 out of 3)

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Source: Applus IDIADA

ADELANTO, Calif. – This is the second 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.

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

The aim of this study is to investigate the means of improving AEB performance in terms of efficiency and driver acceptance. The first part of this article has explained the behavior of average and professional drivers in emergency situations, which have been then compared with the limits of vehicle’s braking capabilities.

Improving AEB systems performance by increasing brake jerk application

Different types of braking applications have been compared in previous sections and the focus has been put on the brake jerk and deceleration. Eventually, both parameters will determine the braking or stopping distance. By means of simulation the stopping distance over the initial velocity has been calculated for a standard hatchback vehicle and the result is shown in Figure 5.

  • Average driver: Jerk=11,3m/s3, deceleration=7.2 m/s2
  • Baseline AEB: Jerk=20m/s3, deceleration=10.5 m/s2
  • Professional test driver: Jerk=80m/s3, deceleration=10.5 m/s2
Figure 5. Comparative of different stopping distances

The first impression is that the stopping distance in all the cases shows a quadratic growth with the velocity. Using this information from an AEB system this confirms again that AEB systems lose effectiveness with velocity and other alternatives, such as AES systems may become a complementary solution.

It is beyond any doubt that AEB systems are improving the safety on road vehicles as it brings many benefits. Compared to an average driver, apart from decreasing the reaction time, it clearly reduces the stopping distance. Nevertheless, the braking distance of an AEB system still has room to be reduced and thereby the likely crash velocity. A professional driver could achieve a shorter distance than an automatic emergency braking system. In figure 5 the benefit seems not to be much, but looking in detail it is found to be a great gain. For this study, the main difference between the professional driver brake application and the baseline AEB is the brake jerk.

Figure 6 highlights the consequences of increasing the brake jerk from 20 m/s3 to 80 m/s3. The graph shows the velocity and distance difference between both types of applications for an initial velocity of 30 km/h.

Figure 6. Distance and velocity reduction

The car with the higher jerk stops 1.55 m before the other car and when it stops, the car with the lower jerk is still at 21 km/h.

Figures 7 and 8 show with a larger range of velocities the distance gap and speed gap for both type of braking applications:

Figure 7. Distance gap
Figure 8. Impact speed reduction

Performance of AEB systems in 2018 Euro NCAP tests protocols

Cars with the highest performance in Euro NCAP active safety tests are able to avoid crashes in most of scenarios. In other scenarios, the most difficult ones, some of the best cars are only able to lessen the impact speed. Such challenging scenarios are exposed in this section and could be the target scenarios where to use an improved brake profile application.

Car-to-car tests

Euro NCAP is assessing safety assist in different potential car-to-car rear end collision scenarios where the target vehicle is stationary, moving at low speed and braking. These tests are assessed considering full overlap between both cars and from 2018 it asks for 50 percent and 75 percent overlap situations also as displayed in figure 9.

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The impact with 50 percent of overlap is considered to be one of the hardest in the car-to-car test protocol. The reason is not only that it could be hard for detection systems to be sure whether the target is in front or at a side. It is as well that some drivers would override the AEB function with a steering action to overtake the target. Therefore delaying the activation time to avoid miss-detections and driver overrides is recurrent here. With an improved AEB system with a brake jerk of 60-80 m/s3 these mentioned issues could be better controlled.

Figure 9. Car-to-car rear end collision with multiple overlaps

Car-to-car braking test is another scenario covered by test protocols. In particular, the test with the shortest distance between cars -12 m-and the maximum braking deceleration -6m/s2 is the most critical scenario here. Some cars with an overall excellent performance are crashing in these tests and with an improved braking performance the result could be better.

Figure 10. Car-to-car rear braking scenario

Upcoming test protocols will increase the difficulty and the demands on braking requirements. New versions include the addition of AEB/AES Junction Assist systems and AEB/AES Head On collisions.

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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