Study On Differential Drive Assist Steering Technology For Electric Vehicle With Independent-Motorized-Wheel-Drive

As traditional transportation means, the global energy crisis and air pollution directly threaten the sustainable development of automotive industry. But as representatives of alternative-fuel vehicles, electric vehicles (EVs) are the main approach to settle this crisis. Furthermore, in the traditional fields of automobile, China lags behind developed countries for few decades, and has lost the chance to catch up with them. Therefore, it is significant that how to impel our automotive industry to sustainable development path and technology innovation of our electric vehicles. As generative products at this background, independent-motorized-wheel drive electric vehicles have injected new vitality into the development of EV. Motorized-wheel drive can remove the traditional mechnical transmission device and set a motor in each wheel. This structure makes the wheels be driven independently and realize various kinematic controls easily, which is often applied on traditional internal combustion engine vehicle (ICEV). With the development of technology of power electronics, independent-motorized-wheel-drive electric vehicle shows more particular advantages and brings some new technologies and functions which can be not realized on traditional ICEV. The concept of differential drive assist steering (DDAS) is presented at this background. This paper is the whole and systematic dissertation on the theory basic, feasibility validation and application prospects of DDAS. The validation procedure consists of three steps, which are theory analysis, computer off-line simulation and real vehicle road test. Composing the whole procedure of theory study and feasibility analysis, all of them complement and boost each other.Based on the independent drive characteristics of motorized-wheel electric vehicles and inspired by others'similar research, we present the concept and structure of differential drive assist steering. First of all, we analyze the basic theory and feasibility of DDAS system. The basic theory is states as following. According to the steering behavior of the driver, the system controls the drive torque difference of two-side wheels and generates the assit steering torque about the kinpin by the interaction of the tire and the road surface. The sum of the driver's steering torque transferred by the steering gear and the assist torque steer the wheels in the light of driver's desire, according to steering geometry, against the aligning torque, friction resistance and inertia of the steering system. Using this method, DDAS system can reduce steering efforts of the driver like other power assist steering system. Similar with the procedure of assist steering, we can also utilize the differential drive torque to assist the steering wheel of return-to-center automatically with active damping control in order to improve the vehicle's poor returnability.From the point of view of verifying the feasibility of DDAS technology, the dynamics model with 18 degrees of freedom (DOFs) for electric vehicle with four motorized wheels is estabilished, inclusive of the 4 DOFs of the steering system model, where we have considered about the nonlinear effects of dry friction and steering trapezium effects. In this paper, the famous"Magic Formula"nonlinear tire model is also used. We also construct the mathematics model of Permanent Magnet Brushless Motor. This nonlinear model platform has the characteristics of more DOFs, higer accuracy and various drive condition simulation. Based on this model platform, we carry out comprehensive open-loop simulation tasks to validate the feasibility of DDAS method, inclusive of parking maneuver, steering handiness at medium or lower vehicle speed, road feel holding at higer vehicle speed and trace performance of the DDAS controller. In order to verify the erery function of DDAS and validate feasibility of the basic theory of DDAS, the co-simulation based on specialized commercial software AMESim and Simulink software was also performed to validate the DDAS system further. The initial conclusion of the simulating validation is shown here. One hand, the differential drive assist steering, which is based on the characteristics of independent drive of motorized-wheel-drive electric vehicle, is not noly like other power assist steering to supply steering assist torque when the driver steers the vehicle, but also minize the road feel lost and improve the stability and safety when it assists steering at higher vehicle speed with the help of variable steering assist force, changing with the vehicle speed. That is to say, the DDAS sytem reaches the tradeoff between the steering handiness and road feel feedback. On the other hand, the simulation results of hands-off maneuver with active return-to-center control operated by DDAS system shows that, with the help of active control, the returnability of the steering wheel is improved to some extent. It not only assists the steering wheel return-to-center fast and accurately, restricts excessive overshoot and oscillation, improve the ability of travel straight line and stability at higher vehicle speed, but also reduce the physical load of the driver when the vehicle is traveling from the cornering to straight line frequently. That means the DDAS system plays a assisting role while returning-to-center.In our initial validation work, we choose the drive torque difference control strategy which is based on the assist torque characteristic curve like electric power assist steering way. But this mehod can only control the steering wheel torque indirectly. After studying the ideal driver's effort when steering, we formulate the direct control strategy to make steering wheel torque follow the reference steering wheel torque. This method can set reference steering wheel torque according to the driver's preference, conveniently. Furthermore, this method utilizes the torque difference between the right and left side of the front axle to minimize the error between the actual steering wheel torque and the reference one. This method needs a steering wheel torque sensor to measure the steering wheel torque signals as a feedback variable. So, this method has many advantages such as clear control ideas, explicity control objects, strong adaptability to kingpin positioning parameters and steering system parameters and easy regulation on the characteristic of steering wheel torque. After that, under the premise of researching the mechanism of steering assist force generation for DDAS system deeply, an anti-windup variable structure proportional integrative (PI) control algorithm is porposed for assist steering control. And the simulation results show that the control algorithm is effective to avoid integral windup generated by the current saturation characteristic of the in-wheel motor. The problem how to assist the steering wheel return to center actively is also studied for DDAS system. Followed several actively return simulations at higher or lower vehicle speed, a proportional-fuzzy-PI multi-mode segmentation control algorithm for assist returning control is also proposed. The simulation results show that this active control method is effective to restrict overshoot or oscillation of steering wheel when returning at higher vehicle speed or poor returnability when returning at lower vehicle speed.Unlike other traditional power asssit steering, differential drive assist steering system is an indirect assist steering method which is based on the drive force difference of the two-side tires to provide steering assist force. That is to say, the steering assist force is not exerted on the steering system directly. So when the tire is slipping excessively, the tire friction force will reach saturation. This situation will impact the DDAS system to produce appropriate steering assist torque and the driving torque. Furthermore, there is a positive yaw moment exerted on the vehicle body, which is performed by the DDAS system. One hand, when the vehicle shows understeer performance, this unexpected yaw moment can enhance the cornering performance of the vechicle and reduce the steering wheel torque further indirectly. On the other hand, this excessive yaw moment will result in over steering, especially at higher vehicle speed. For this reason, it is necessary to research the torque coordinated control for DDAS system. In this paper, a global drive torque control structure is proposed, which is composed of an upper layer and lower layer control subsystem. The upper layer control subsystem comprises DDAS control system for front axle and direct yaw moment control system for rear axle. And the lower control subsystem is mainly composed of an optimal slip ratio control system. Both of them are studied carefully. We analyse the stability compensation of the direct yaw moment control system to DDAS system, as well as the structure, control idea and the assistance on broadening the work fields of DDAS system. After that, a analytical driver model for arbitrary path following is estabilished and several driver-vehicle closed-loop simuliation are performed for this coordination control system mentioned above. The simulation results not only validate the feasibility and effectiveness of DDAS system on steering handiness improvement and road feel keeping once more, but also shows that the global coordinated control system can ensure the stable normal working and enlarge the range of working of DDAS system, while improving the stability, cornering performance and trace performance of the vehicle itself simultaneously. Furthermore, the results show that the global coordinated control system regulates the effect of DDAS system on vehicle stability and moving state, and make the DDAS sytem work well. This research work provides an overall and systematic technology route for the application of DDAS system on a real car.Finally, a motorized-wheel test vehicle with DDAS function is made for the research content of this paper to validate the DDAS method by road tests for evaluating steering handiness and road feel keeping. These road tests mainly comprise constant radius circle cornering maneuver at a certain constant vehicle speed, double lane changing maneuver, parking maneuver and so on. The test results show that the proposed and researched DDAS system can not noly reduce the steering efforts of the driver and improve steering handiness obviously at lower vehicle speed, as well as reduce the steering load of the driver for obstacle avoidance maneuvering at medium vehicle speed, but also keep more road feel feedback and make the driver learn more road information and moving state of the vehicle. So it improves the driving safety of the vehicle. Furthermore, the test results are in accordance with the simulation results, which means the accuracy of the model and credibility of the computer simulation is high. The success of the test on a real car provides strong proofs to validate the feasibility and possibility of DDAS system on its future application. Along with the theory analysis and the computer simulating validation procedure, the real car test validation constitutes a whole theoretical system for the differential drive assist steering system.