## 1 Introduction
established static and dynamic simulation models of proportional relay valves of a pneumatic EBS (Electronic Braking System) for commercial vehicles with MATLAB/Simulink; then, the simulation models were verified to be correct on a test bench and, thus, can be used for the development of hardware and control algorithms of a pneumatic EBS for commercial vehicles [19; 20]. [PERSON] designed a new pneumatic valve to improve the response of air-actuated brakes for heavy vehicles to demand pressures generated during electronically controlled braking [21]. [PERSON] et al. used a double voltage driving circuit to reduce the switching time and delay time of the high-speed on/off valve, and hence, the response characteristics and the control performance were optimized [22; 23]. [PERSON] designed and manufactured an electronic valve driving circuit with fast response characteristics by using a three-power source. The new circuit shortened the switching delay time from 5 ms to 1.55 ms. Therefore, the hydraulic system with the new circuit showed excellent position tracking control performance [24]. The above research was mainly aimed at the improvement of the structure of the existing APRV or the optimization of the control strategy. The working conditions were relatively fixed in this research, and there has been no in-depth study on whether the structure of the APRV can cope with the driving conditions under special circumstances. In order to adapt to the development of driving automation and intelligization of the brake system, the brake system in commercial vehicles is in urgent need of an APRV that can adapt to the development of driving automation and an intelligent brake system. Therefore, this paper studies the structure configurations of the APRV to ensure that the structure configuration of the APRV can adapt to more dangerous driving conditions, so as to better guarantee the driving safety of vehicles and the development of driving automation. Based on the classification of driving automation proposed by the SAE (Society of Automotive Engineers) and the working principle of the ECPBS of commercial vehicles, four structural configurations of APRV are proposed. The simulation model for the APRV of different structural configurations is established in AMESim, and the working condition in which the driver fails to apply sufficient braking force is simulated. Then, the pressure regulation characteristics of the APRV under different control conditions and during the switch between these conditions is studied. The correctness and the reliability of the simulation model are verified through the test bench for the pressure regulation characteristics of the APRV of different structural configurations. Finally, through simulation and tests, the pressure regulation characteristics of the APRV of different structural configurations under different control conditions are studied, and the suitable levels in the SAE automated driving classification for the APRV of different structural configurations are determined; thus, the theoretical underpinning to improve driving safety and develop driving automation is provided. 2 Pressure Regulating Requirements and Control Modes of the APRV in the ECPBS of Commercial Vehicles  
The ECPBS of commercial vehicle is a new type of brake system that meets the requirements of automated driving. The electronic control mode is added on the basis of retaining the traditional braking circuit, and the ECPBS provides decent solutions to many problems in the traditional pneumatic brake system [25], thus ensuring driving safety and driving comfort. As depicted in Figure 1, the APRV is the core pressure-regulating component in the ECPBS. As the key component to ensure driving safety, its structure configuration and the pressure-regulating function must meet the braking requirements of the driver and the automated driving system. Moreover, the APRV should be able to operate under different control modes to ensure driving safety under different working conditions.