## 1 Introduction
### Structural Configurations of the APRV  
Based on the working principle of the ECPBS of commercial vehicles shown in Figure 1, the pressure-regulating requirements and the control modes of the APRV, four structural configurations of the APRV (shown in Figure 2) are proposed. First of all, the APRV must ensure independent pressure regulation by the driver or the automated driving system, namely support for the manual control mode and electronic control mode. Additionally, through the optimization of the structural configuration and the control strategy, the coupled control mode and the switch between the two control modes can be realized. As shown in Figure 2, the lower valve bodies in the four structural configurations are the same, and the electronic control circuits are similar. In these configurations, the pressure regulation is done by the control of high-speed solenoid valves, which serve as switch valves or high-speed inlet/exhaust valves, and the difference is that different configurations apply different types of solenoid valves, namely normally open (NO) ones and normally  
Figure 2: Principle drawing of the different structural configurations of the APRV. closed (NC) ones. Correspondingly, the manual control circuit of each configuration differs. In configuration I, the manual control circuit and the electronic control circuit are in parallel. In configurations II and III, the switch between the control modes is done by the control of the switch valves, which are, respectively, a two-position three-way solenoid valve in configuration II and a three-position three-way solenoid valve in configuration III. In configuration IV, a high-speed inlet valve is applied between the inlet port and the lower valve body on the basis of configuration II. Therefore, two electronic control circuits are set up to implement the coupled control mode. The configurations are elucidated in Table 1.  
Test Bench for the Pressure Regulation Characteristics and Control Conditions of the APRV of Different Structural Configurations in the ECPBS of Commercial Vehicles  
To analyze the pressure regulation characteristics of the APRVs of different structural configurations under different control conditions, a test bench for the pressure regulation characteristics of the APRVs of different structural configurations was built. As shown in Figure 3, a pedal valve, the equivalent models, air tanks, a computer, an NI acquisition card, etc. were included in the test bench, and the specific piping and components of the test bench were consistent with that of the ECPBS (Figure 1) to ensure compliance with the requirements of the braking system. The pressure signal and the control signal were acquired by the NI acquisition card and processed in real time, and thus, the test for equivalent models of the APRV of different structural configurations could be carried out. \begin{table}
\begin{tabular}{c} two-position three-way \\ solenoid valve \\ \end{tabular} & 1 NO and 2 NCs & Yes & Yes & Yes \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Features of the different structural configurations of the APRV. Figure 3: Test bench for the pressure regulation characteristics of the APRV of different structural configurations. The test covers a pressurization-decompression test under manual control, a pressurization-decompression test under electronic control, a switch test between the manual and electronic control modes during pressurization and a switch test between the manual and coupled control modes during pressurization. Considering the significant effect of the control strategy on the pressure response, the equivalent model is set to operate at the full open condition or full close condition to avoid such influence. The switch tests between modes are planned to simulate the conditions in which the brake force is insufficient owing to the false operation of the driver or a malfunction of the pedal valve so that the automated driving system takes over and switches the control mode to the electronic control mode or coupled control mode.