Level 1:(single operational task by system)：ACC/LKS
Level 2:(combined operational tasks by system and supervision by driver): integrated cruise assist
Level 3:(operational and tactical tasks done by system, driver can be reengaged if needed): Traffic jam pilot, park pilot,highway pilot
Level 4:(operational and tactical tasks done by system in defined enviornment): auto pilot in defined enviornment
Level 5:(operational and tactical tasks done by system everywhere): auto pilot
Different levels of automated driving
There is an important dividing point in this classification, which is L3. Before L3 (including L3), the braking system enters the back-up mode after a failure, requiring the driver to take over the vehicle, which is often called fail-safe. The driver actually becomes the most reliable back-up for assisted driving functions. . After reaching the L4 and L5 stages, the vehicle control system is responsible for the vehicle status after the braking system fails. At this time, all actions originally taken over and completed by the driver need to be completed by the vehicle, which is the so-called fail-operational.
Standing at the node of 2020, most mainstream car companies are in the transition stage from L2 to L3, such as the strengthening and popularization of functions such as APA and TJA. A few pioneer car companies have already occupied the high ground of L3 assisted driving and are making efforts to take a "thrilling leap" towards L4 level autonomous driving. With this leap, what are the new requirements for the braking system at the executive level?
2.1 Brake system architectures (IPB working principle)
The following figure is a simplified diagram of the IPB braking system, which is mainly divided into three parts: pedal-master cylinder-pedal simulator part, pressure building part and pressure adjustment part.
The IPB systemis a decoupled system. During normal operation, valves 1, 4, and 5 are connected and valves 2 and 3 are disconnected. Once the driver steps on the pedal, brake fluid enters the master cylinder and pedal simulator, building pressure. The pedal force-pedal stroke curve is determined by the characteristics of the master cylinder and pedal simulator. At the same time, the IPB ECU recognises the pedal displacement signal and controls the motor to build pressure based on the calibrated pedal displacement-system pressure curve to generate vehicle deceleration. In the longitudinal and yaw motion control, the wheel cylinder pressure of each wheel is adjusted through the ABS/ESC hydraulic modulation module. Therefore, for the IPB braking system, the pedal displacement-deceleration curve can be changed by refreshing the calibration parameters.
The IPB degradation mode is relatively complex, with different failure types corresponding to different degradation modes. This article focuses on power-assisted failures, such as IPB power outages. In this mode, the IPB enters the mechanical backup mode, valves 1, 4, and 5 are closed, and valves 2 and 3 are opened. The pressure established by the driver's pedalling directly enters the wheel cylinder and generates vehicle deceleration. In accordance with the stipulations of the ECE R13-H regulation, the system must be capable of producing a braking deceleration of at least 2.44 m/s².
Given that the brake pedal is decoupled, it is unnecessary to consider the impact of the master cylinder's fluid demand on pedal displacement. The IPB's master cylinder bore can be smaller than that of a traditional brake application system. In mechanical backup mode, the system generates higher pressure under the same pedal force. During the braking system matching design process, it is important to consider the brake fluid volume (corresponding to the master cylinder stroke and bore). The following factors should be taken into account during the design process:
In backup mode, a braking deceleration of 2.44 m/s2 is required (involving pedal and basic braking system matching).
System changes occur over the vehicle's lifetime, including friction plate wear, system stiffness changes, friction coefficient changes, and so on. This article will not provide a detailed explanation.
Schematics of an integrated power brake (IPB)
2.2 From driver assistance towards highly automated driving
As previously stated, the current design of driver assistance systems places the driver at the core of the operation, with the system acting as a supplementary feature. In some high-level L3 driver assistance systems, such as park pilot and highway pilot, drivers can hand over control of the vehicle in specific scenarios. However, the driver is still required to remain in the driving seat. In the event of a braking system failure, the driver must assume control at any time and park the vehicle in a safe area in mechanical backup mode.
From L3 to L4/L5, the driver gradually relinquishes responsibility and is no longer required to sit in the driving seat. This ensures that, even in the event of a malfunction, the vehicle control system will bring the vehicle to a safe area. For the braking system, the question of how to complete the operations originally performed by the driver by the vehicle control system is a new challenge posed to the braking system by high-level autonomous driving.