The State of The Art of Braking Systems II
---Requirements for highly automated driving
According to current braking system design requirements (ECE R13-H), the driver is responsible for stopping the vehicle in the event of a braking system failure. Therefore, the system must have a purely mechanical backup mode to ensure that the driver can decelerate the vehicle by depressing the brake pedal.
1-Brake system requirements derived from state of the art
For high-level autonomous driving, when a single-point failure occurs in the braking system, what functions should the system have and what kind of performance should it achieve? Currently, based on the best level that the current vehicle braking system can achieve (state of the art), deduction is a better way. This is understandable. After a failure occurs, the level of automatic processing of the system must be equal to or higher than the current level of driver operation.
From this, it can be deduced that the functional and performance requirements for the automatic driving brake system backup are:
① Provide sufficient braking force (braking deceleration greater than 6.4 m/s^2, covering 99% of braking conditions).
②The time to decelerate to 6.4 m/s^2 shall not exceed 1.5 s (taking into account the driver's reaction time and execution time).
③ Anti-lock performance (allowing the vehicle to maintain its steering ability). When ESP fails, the automatic driving system must be equivalent to the anti-lock level achieved by human operation. Regarding lateral stability, under the current state, after ESP fails, human power cannot guarantee lateral stability. safety, so there is no such requirement for autonomous driving systems.
2- Brake system requirements for highly automated driving
Through the analysis of braking system requirements, when designing high-level autonomous driving braking systems, functional degradation is the most difficult problem. It is necessary to consider the design of brake redundancy, that is, the establishment of a secondary brake control unit in addition to the primary brake control unit. In addition, the system must also have advanced functions such as system state detection, redundancy control, and backup state vehicle stability control.
3- Requirements for stabilization in HAD backup
This is a controversial topic. Do autonomous vehicles still need to have lateral stability control capabilities in braking degradation mode? The answer is NO for two reasons:
3.1 When ESP fails in conventional vehicles, the driver takes over the vehicle and cannot provide lateral stability control.
3.2 Assume that the distance from the failure of the autonomous vehicle's braking system to reach a safe location is 5 km. Based on the analysis of the current ESC after-sales market data, during this 5 km distance, the vehicle was driving in the brake degradation mode because there was no lateral stability control. The probability of injury to members is very low.
4- Brake system requirements to E/E architecture
The functional redundancy of the braking system is based on the electronic architecture. For electronic architecture, it is not simply achieved by duplicating all relevant components, which leads to a sharp increase in system complexity and cost. Critical systems must have redundant backups, which are mainly reflected in the following parts:
①HAD guiding unit backup
②Communication backup and power backup of primary and secondary brake control unit and HAD guiding unit
③Key sensors, such as wheel speed sensor WSS backup
5- Requirements related to integrated brake system
As mentioned earlier, the braking system of an autonomous vehicle requires primary and secondary brake control units. The solution proposed by Bosch is to use the IPB as the primary braking system to execute braking requests in most cases, and to use the RBU (redundant brake unit) as a redundant brake in case of IPB failure.
In addition, the design of the RBU should have no additional impact on the functionality, safety, and serialization of the IPB. The RBU should use as many ESP components as possible to achieve low cost.