Research On Thermal Hydraulic Model And Control Strategy Of Hydraulic Hub-motor Drive System For Heavy Vehicle

As an important means of delivery,heavy vehicle plays an irreplaceable role in China’s modernization.The working condition of heavy vehicle is complex and it often travels on poor road surfaces such as mine roads and mud and snow roads,which is prone to wheel slippage and insufficient driving force.A set of Hydraulic Hub-motor Drive System(HHDS)can be installed on the traditional heavy vehicle to improve traction performance and passability under low adhesion coefficient pavement.In HHDS,the hydraulic hub-motors which are installed in the non-drive wheel hubs are driven by the hydraulic pump to provide auxiliary driving so that the vehicle can enter the four-wheel drive mode.For the vehicle of HHDS,how to coordinate the power output of the mechanical path and the hydraulic path during the auxiliary drive mode is the key to ensure the performance of the system.And the temperature rise problem of the hydraulic system is also an important factor affecting the performance.The fluctuation of temperature will have a greater interference to the control accuracy.Therefore,this thesis aims to study the coordinated control of the multi-power system of the HHDS vehicle considering the temperature change problem.The main content of this thesis includes the following elements:Firstly,the vehicle power system is divided into a mechanical transmission system and a hydraulic transmission system and the modeling research is carried out separately.The mechanical transmission system model is built on MATLAB/Simulink software,including vehicle dynamic model,engine model,clutch and transmission models and tire model.The hydraulic transmission system model is an AMESim thermal hydraulic model of HHDS.The thesis analyzes the oil properties,explores the influencing factors of pump and motor efficiency and their influence laws,expounds the heat generation and heat dissipation mechanism of each hydraulic component and establishes the system heat balance relationship.With the software interface,the mechanical transmission system model and the hydraulic transmission system model are integrated to establish a co-simulation platform.Secondly,according to the characteristics of different working modes,considering the influence of temperature change,the temperature compensation strategy is formulated for the creep mode and the assist mode respectively.According to the speed regulation characteristics of the hydraulic closed circuit,HHDS in the creep mode can be regarded as a Hydraulic Continuously Variable Transmission(HCVT).Based on the target of optimal engine operating point,the HCVT control strategy considering temperature compensation is designed.The assist mode control strategy,which is based on the wheel speed following idea,includes the pump displacement control rules with temperature compensation considering various factors such as temperature and pressure.In addition,for the high temperature and high pressure limit states that the system may reach,the pump displacement limit control is proposed to further ensure the reliability of the system.The simulation of the proposed control strategy on the co-simulation platform shows that the controller can achieve good control results.Finally,in order to optimize the control effect of the assist mode,the time-varying efficiency caused by the temperature is considered and a model predictive feedback controller is designed.By analyzing the dynamic relationship of the transmission system and combining the characteristics of the time-varying efficiency system,the system state space equation is established.Then according to the model predictive control theory and the target that the front wheel speed follows the rear wheel speed quickly and the system impact is reduced,the engine torque and pump displacement are considered as the control objects,the feedback control law is derived and the model predictive controller is designed.The effectiveness of the "temperature compensation + model predictive control" composite algorithm is verified by simulation analysis on the co-simulation platform.The research of this thesis on the HHDS control strategy considering the temperature change problem has guiding significance for the subsequent actual vehicle experiments.It also provides a reference for the similar hydraulic system to compensate the temperature effect from the control perspective.