1、1The innovative design for automobile EPS system Based on Active frontwheel steering technologyAbstract. To improve vehicle driving safety and steering performance， analyzing the mechanical structure of the electric power steering （EPS） systems and active front steering （AFS） system and working prin
2、ciple. The new electric power steering system integration active steering technology is presented； the conventional electric power steering system is used in active front steering system. The new EPS system was composed of traditional electric power steering installation dual planetary gear and moto
3、rs around the corner. Not only needed to provide power steering， but also did through the front wheels compensation angle in driving conditions to change the angle transmission ratio， eliminate the interference between the active front steering and driver. Keywords： Electric power steering； active s
4、teering； planetary gear； variable transmission ratio 1. Introduction Driving safety， environmental protection， adjustment assembly simple and low cost are Electric Power Steering 2（EPS） advantages， it represents the future development direction of automotive power steering technology. Active front s
5、teering system is developed by BMW （Active Front Steering， AFS） can independent drivers operating. The front wheel angle and the steering gear ratio are changed by additional angle for the front wheels 1. The stability control of vehicle is achieved by turning interfere， the vehicle safety， flexibil
6、ity and driving pleasure are improved a whole new level. For a new electronic control unit， using some intelligent control strategies and control algorithms and other electronic integrated control systems to improve its performance significantly， it is a bite problem to be solved in the automotive a
7、ctive safety research. Considering steering portability， stability and security， in addition to， the AFS system is collocated， the cost is increased. How to make EPS integrate active steering function is studied based on control characteristics of EPS power， damping， and return. A new type EPS syste
8、m integration active steering is designed. On the basis of traditional EPS structure， a set of dual planetary gear mechanism is assembled turbine worm and AFS actuators is equipped. The new system can provide power， then applying a secondary angle for front wheel 3in the process which does not depen
9、d on the drivers and changed rotation ratio in real time. The dynamic control signals yaw rate and sideslip angle are introduced in the new EPS system ， combining it with the ideal reference model， using robust control， fuzzy control theory to get steering torque correction ， Thus， the better perfor
10、mance is achieved rely on traditional EPS power steering motor， while the Active Steering Function is achieved by active steering angle motor .improving the stability and active safety of the vehicle in separate road adhesion coefficient， large lateral forces and cornering brake extreme operating co
11、nditions. 2. Vehicle attitude control strategy based on AFS 2.1 Phase delay compensation strategy based on yaw rate The 2-DOF motion state of the vehicle is shown in Figure 1， It consist of the center of gravity velocity V， the vehicle body slip angle of gravity . the tire model of front and rear is
12、 simple linear. Figure 1 four wheel model and two wheel model Steering characteristics and suspension performance will affect the control characteristics of the vehicle. As shown in Figure 2， cyclical fluctuations at speed V = 60km /h about 1000kg， 2000 kg respectively. 4From Figure 2 we can know， t
13、he body weight increase would have a negative impact on car yaw rate， heavy body make left and right steering unresponsive. So the front corner is needed compensation in real-time. Figure 3 shows that the AFS system compensates control charts for yaw rate based on the feedback delay. Figure 3 AFS co
14、ntrol system schematic Dashed line in Figure 2 shows that the frequency characteristics， which uses the AFS system and quality of 2000kg， the feedback coefficient1. The figure shows that the speed of vehicle steering response and cornering stability are improved significantly. Therefore， EPS and AFS
15、 integrated control can provide assistance and change steering ratio. At the same time， it is not depend on the drivers to provide additional angel for the front wheels， the steering portability and stability are coordinated reasonable， driving safety and comfort are improved. 2.2 Model EPS mechanic
16、al structure integration of AFS function Electric Power Steering system is power-assisted steering systems which depend on the electric motor directly. Driving safety， environmental protection， adjustment assembly 5suppleness and low cost are its Advantages. It represents the future direction of aut
17、omotive power steering technology. First practical was equipped on a light vehicle in 1998， input torque were enlarged. Then， with the speed is change， the gain ratio is change. Later， some intelligent algorithms which develop from modern control theory can get a better sense of the road， back to th
18、e positive damping. Figure4 EPS schematic Internal construction of EPS steering apparatus is shown in Figure 4. The steering system is mainly composed of mechanical system， the torque sensor， a vehicle speed sensor， a speed reduction mechanism， motor and power electronic control unit （ECU）. When the
19、 vehicle is steering， the electronic signals are detected by torque sensor and vehicle speed sensor， which sent to the electronic control unit ECU. Then the control unit analyzes and computes the data to determine voltage signal of the booster motor， a directive is issued to the motor， the output to
20、rque of motor is achieved power steering2. 1） The relationship between the compensation steering angles is provided AFS and the corner is inputted wheel is introduced. The joint control system consists of three different structures EPS and AFS is designed， as shown in Figure 5. 62） The power steerin
21、g apparatus are installed only one motor for providing additional power steering and front wheel angle. The module is integrated in EPS controller which controls the front compensation corner. 3） On the basis of the mechanical structure， a planetary gear and active steering motor are connected； comb
22、ining with the regulation of power torque in the EPS system and the angel is attached to front wheel. Steering torque and additional steering angle of the front wheels are regulated， so the active steering is achieved. The mechanical connection of the original structure is canceled. Transmission of
23、power， angle are rely on sensors to achieve power steering， it is called steering by wire. Figure5 EPS Schematic diagram integration AFS 3. The control performance of EPS and AFS joint based on single-motor 3.1 Taking AFS brakes as the controller Without changing the original structure of the EPS sy
24、stem，this structure is used mechanical parts connect， high reliability. Firstly， the new controller joint EPS and AFS is conducted performance tested. Considering the rigidity K of steering system， through the 7transfer function test only considers the steering torque increase input driver operation
25、 experience. In addition， AFS control module whether or not it can calculate the additional front wheel angle timely and accurate at high speed conditions. The model of joint control system is shown， which the dotted line show the drivers steering operation. C1 module of EPS controller determines th
26、e operating feel of the system. EPS was originally designed； it makes the output of the auxiliary torque times the input of torque sensor. if the transfer function from Tm to f is linear， output angle of AFS controller C2 and EPS power motor generated by the torque will make it achieve active steeri
27、ng .Assuming that the speed， type of road surface condition， tires raised under different parameters will not cause large changes， the linear model is built. However， the actual controller model is non-linear， correctly identification each parameter is indispensable condition to calculate the attach
28、 angle. 3.2 The non-interference problem of AFS and the reaction force In addition to the steering wheel torque input when the car turned， the steering wheel angle and damping are feedback input for control. Qualitative considerations additional output steering angle of AFS effect the drivers 8steer
29、ing. Whether the feedback torque changes caused by AFS action or irregular surface can interfere with the drivers steering input， which usually require restraining. Therefore， AFS may influence the operation of the driver， or because instability of the steering wheel， the output of AFS is designed m
30、ust be distinguished from the drivers operation， and non-interference. The transfer function from to the angle of the handle is Noninterference formula AFS control system with the operation of the driver steering； （1） However， the transfer characteristic of the controller is 0， it is not the designe
31、d. For decoupling， the additional steering torque and steering angle are needed controlled independently. Therefore， an independent motor will not provide power separately. 4. The new EPS is designed based on differential mechanism and two motors 4.1 Variable transmission ratio of new EPS system is
32、designed The studies have shown that the steering compensation angle of AFS system will produce interference with help power. New 9EPS system based on active front steering technology requires two motors in structural design， which provide front wheel compensation angles and power. Two motors and pl
33、anetary gear mechanism are used in conventional steering system， analyzing the gear mechanism and designing the nonlinear variable transmission ratio. Usually， the characteristic curve of gear ratio with the speed change can indicate the variable transmission ratio features of AFS. It divides into t
34、wo properties of linear and nonlinear models. Nonlinear transmission ratio can fully coordinate the relationship of speed， different steering input angle and corner； it is an ideal aid corner characteristic curve. The formula of additional front-wheel angle based on non-linear variable ratio： For th
35、e transmission ratio with the speed change process is smooth and natural， fitting the curve to obtain the desired angle characteristic curve. （3） Where， ig = g / f is the gear ratio of rack and pinion steering； Kr is stability factor； R is steering gear ratio； s is the input angle of steering wheel.
36、 The following figures show the simulation curve before and after optimization： 10Figure 7 shows that the curve of the corner additional angle， steering wheel angle and drive ratio in the actual steering process before optimized. Form the figure can be see ， the drive motor speed is changes frequent
37、ly within a given range of parameters （10 R 20，0s180） ， the variation range of front wheel additional angle which come from motor is -3，6. Figure 8 shows that a change in surface area in the actual driving process， the speed of the drive motor is in the frequent change. Within a given range of param
38、eters （ ） ， the range of front additional angle which provide is -20， 30. 4.2 EPS controller is designed The control schematics of new EPS system： Fig.9 EPS control with two motors and a planetary gear There is a difference between the wheel turning angle and the operation angle because the internal
39、 gear （C） angel. Actually， the angle is boosted by planetary gear 1： N gear ratio， as shown in equation （13） ； （4） Usually， controlling the motor to achieve steering ， the differences of the control angle and steering angle is changed， so C = 0. The motor 1 and the distortion rod of Figure 6 have the same effect. Through a PI controller to
