Assistant Safety Control Of HEV During Downhill Driving

The downhill safety assistant control system can effectively reduce drivers’workload and prevent vehicular overspeeding, which is of great significance to improvevehicle safety during downhill driving. The current studies on downhill safety assistantcontrol were mostly focused on assist braking systems and hill descent controltechnologies for conventional vehicles. However, as prolonged intensive braking usingthe hydraulic braking system may lead to thermal recession of the brake componentsand then affect vehicle safety, only constant-velocity assistant control in low-speedranges can be achieved by the downhill safety assistant control system for conventionalvehicles. Regarding these limitations, the active downhill safety assistant control forhybrid electric vehicles (HEVs) becomes a promising solution due to its advantages offast torque response, high control accuracy and stable capability. In this paper, based onthe analysis of the driving habit and driver’s intention during downhill driving as well asthe characteristics of the braking system of HEVs, a downhill safety assistant controlmethod for HEVs is proposed considering vehicle safety, fuel economy and ridecomfort, and the key technologies are studied.Based on the analysis of driver’s behavior and the characteristics of the brakingsystem of HEVs, the architecture of the downhill safety assistant control is developed,the driver’s intention identification method and the coordinated control technology ofmulti-systems and multi-objectives are discussed during the assistant braking process,and active safety assistant control for HEVs during downhill driving is realized.With respect to the proposed architecture, key technologies are studied. Fordriver’s intention identification, the control timing and control objectives for downhillsafety assistant control are determined considering the driving habit and drivers’intention. In order to realize smooth switch between the assistant control mode and thedriver control mode, ensure vehicle driving safety during control mode switching andenable driver’s intervention of the vehicle velocity in the assistant control mode, an exitcontrol strategy of the accelerator pedal aiming at the respective acceleration during flatroad driving and an exit control strategy of the braking pedal aiming at non-increase ofthe vehicle velocity are proposed. For coordination and torque distribution of the braking system, the distribution method of the assist braking torque is developed toimprove braking safety and fuel economy with respect to the capacity of each brakingsystem. Regarding the ride comfort deterioration resulting from intervention of theengine braking system, the dynamic coordinated control strategy of the motor brakingsystem and the engine braking system is proposed, which takes the advantage ofaccurate adjustment of motor speed in the speed control mode. Furthermore, afeedfoward-and-feedback based dynamic coordinated control strategy ofmotor-hydraulic-engine braking systems is determined to deal with the shortcomings ofcontrol accuracy and torque response of hydraulic braking systems, benefited from fasttorque response and high control accuracy of the motor braking system.In order to validate the effectiveness of the active safety assistant control for HEVsduring downhill driving, a simulation platform and a real vehicle test bench areconstructed. Simulation and experimental results show that the proposed control methodnot only improves vehicle safety and fuel economy, but also reduces drivers’ workloadand betters ride comfort of HEVs during downhill driving.