Electro-hydraulic brake control method for hybrid electric vehicle during process of downhill assist control

Abstract: In order to improve vehicle safety and fuel economy of the whole control system for hybrid electric vehicles (HEV), this paper presents an electro-hydraulic braking control method for the downhill auxiliary braking process. First, through the analysis of the downhill auxiliary braking process and dynamic change of the braking torque, the appropriate time for electro-hydraulic braking and the distribution principle of the braking torque are proposed. Experiment platform and typical control signals are adopted to test the electronic vacuum brake, of which the results illustrate that the braking torque of the electronic vacuum brake is sufficiently large to finish the braking process. However, if the target pressure is low (for example, below 0.15 MPa), the respond of electronic vacuum brake to the pressure is also slow. Increasing target pressure can slove the problem of start-up delay and the response time of the brake. Besides, response errors appearing in the control process of the electronic vacuum brake cannot be eliminated as well. On the contrary, the drive motor has high response speed and high control precision though the maximum driving/braking torque is limited. This provides the possibility of combining the advantages of both devices. Thus, based on the response data of electric vehicle drive motor, the complementarity of braking capacity and response characteristics of the electro-hydraulic system is specifically analyzed and the control inclination of the electro-hydraulic system is obtained. Then, the distribution principle of the downhill auxiliary braking torque is established, which can maximize the breaking torque of the motor in the prerequisite of assuring the total breaking torque. Based on the blending control framework of forward feed and feedback, the hydraulic system response under low pressure is realized using the proposed minimum pressure maintaining method. Meanwhile, by increasing the objective start-up pressure, time delay of the hydraulic system is controlled within the objective minimum range all the time. The response speed is also enhanced by increasing the torque proportion of the hydraulic system when total torque varies. System response errors are also offset by making use of the high response speed and high control precision of the drive motor. The above control strategy can sufficiently take advantage of the electro-hydraulic system and realize the coordinated torque control, achieving the high efficiency of the entire braking system. Finally, based on the slope data of a highway in a certain mountain area (with the slope of between 1% and 13%), the control algorithm model for the electro-hydraulic braking method is built with the software of MATLAB/Simulink using the elementary parameters of a hybrid vehicle with a displacement of 1.6 liters. The effectiveness of this control logic is precisely verified by the simulation results. Furthermore, the control commands related to the hydraulic system are extracted to conduct hardware-in-the-loop test, of which the results greatly coincide with the simulation results. Therefore, the proposed approach can not only reduce the start-up time delay of the hydraulic system effectively, ensuring the overall response accuracy of the downhill auxiliary system, but also provide a new idea for further research of the HEV compound braking.