Abstract:
In a steering system for a vehicle incorporated with a rear drive electric motor ( 5 ) for driving a pair of rear wheels ( 3   rl,    3   rr ) that can function as a regenerative brake for the rear wheels, a steering torque control unit ( 38 ) reduces a steering assist torque provided by the power steering assist unit ( 21 ) when the rear drive electric motor is providing a regenerative braking. When the rear wheels are braked by a regenerative braking action without substantially applying a brake to the front wheel, the vehicle may acquire a temporal oversteer tendency. However, by increasing the effort required to steer the front wheels, such a tendency can be canceled or compensated, and the vehicle operator is enabled to control the vehicle without experiencing any unfamiliar feeling or discomfort. The effort required to steer the front wheels can be increased by decreasing the assist steering torque or providing a reactive steering torque at such a time.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a vehicle steering system that provides an assisting/reactive steering torque, and in particular to a vehicle steering system that adjusts the assisting steering torque according to the condition of the vehicle. 
       BACKGROUND OF THE INVENTION 
       [0002]    A vehicle steering system is required to provide a steering reaction that reflects the vehicle condition and road condition for the vehicle operator to operate the vehicle in a comfortable way. The electric power steering system is increasingly widely used in current automobiles, and is often equipped with a control system that provides a reactive torque to the steering wheel according to various parameters such as the steering angle, yaw rate, vehicle speed and so on at the same time as providing an assisting steering torque. 
         [0003]    According to a known vehicle steering system, when a disturbance such as a side wind is applied to a vehicle, an assisting steering torque that counteracts the disturbance is produced. This system comprises a manual steering means for allowing the steerable road wheels to be manually steered, an electric motor for applying a assisting steering torque to the steerable road wheel and a control means for adjusting the assisting steering torque produced by the electric motor according to an output of a vehicle condition detecting means. The control means detects changes in the output values of the vehicle condition detecting means for each unit time, and actuates the electric motor so as to cancel the changes. See Japanese patent No. 3110891 (patent document 1). 
         [0004]    In case of an electric motor vehicle such as a pure electric motor vehicle and a hybrid vehicle using both an electric motor and an internal combustion engine, the rear wheels may be connected to an electric motor, and the electric motor may he used as a regenerative brake. However, when the regenerative brake is activated, it may affect the behavior of the vehicle to such an extent that the vehicle operator may experience an unfamiliar feeling or even a discomfort. This problem may be particularly acute when the regenerative brake is applied only to the rear wheels without using a disk brake or other mechanical brake to brake the front wheels. When the rear wheels are braked without braking the front wheels, the vehicle temporarily acquires an oversteer tendency. A steering system incorporated with a means for canceling external disturbances may detect such a behavior of the vehicle as a disturbance, and may be able to control the vehicle behavior. However, as the steering system would be able to detect the disturbance only after the yaw rate and other conditions of the vehicle have changed as a result of the actuation of the regenerative brake, the response of the steering system is necessarily delayed, and the unfamiliar response of the vehicle may not be adequately controlled. 
       BRIEF SUMMARY OF THE INVENTION 
       [0005]    In view of such problems of the prior art, a primary object of the present invention is to provide a vehicle steering control system for a vehicle equipped with an electric motor operable as a regenerative brake which can avoid changes in the behavior of the vehicle when the regenerative brake is activated, and thereby improve the handling of the vehicle. 
         [0006]    According to the present invention, such an object can be accomplished by providing a steering system for a vehicle incorporated with a rear drive electric motor for driving a pair of rear wheels that can function as a regenerative brake for the rear wheels, comprising: a steering mechanism for steering front wheels of the vehicle; a power steering assist unit configured to assist a manual effort to steer the front wheels; and a steering torque control unit configured to reduce a steering assist torque provided by the power steering assist unit when the rear drive electric motor is providing a regenerative braking. 
         [0007]    When the rear wheels are braked by a regenerative braking action without substantially applying a brake to the front wheel, the vehicle may acquire a temporal oversteer tendency. However, by increasing the effort required to steer the front wheels, such a tendency can be canceled or compensated, and the vehicle operator is enabled to control the vehicle without experiencing any unfamiliar feeling or discomfort. The effort required to steer the front wheels can be increased by decreasing the assist steering torque or providing a reactive steering torque at such a time. 
         [0008]    According to a preferred embodiment of the present invention, the steering torque control unit reduces a steering assist torque according to a regenerative power of the rear drive electric motor. Preferably, the rear wheels are driven by individual rear drive electric motors, and the steering torque control unit reduces a steering assist torque according to a difference between the regenerative powers of the rear wheels. Thereby, a particularly favorable control of the vehicle behavior can be achieved. 
         [0009]    According to a certain aspect of the present invention, the steering torque control unit includes an assist torque setting unit for setting an assist torque that assists a steering effort and a reactive torque setting unit for setting a reactive torque that increases the steering effort, a final steering assist torque provided by the power steering assist unit being determined as a combination of the assist torque provided by the assist torque setting unit and the reactive torque provided by the reactive torque setting unit. 
         [0010]    Typically, the reactive torque is determined as a mathematic function of a vehicle speed, a yaw rate of the vehicle and a regenerative power of the rear drive electric motor. 
         [0011]    According to a particularly preferred embodiment of the present invention, the rear wheels are driven by individual rear drive electric motors, and the reactive torque is given as a mathematical function of a difference between the regenerative powers of the rear drive electric motors for the rear wheels. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    Now the present invention is described in the following with reference to the appended drawings, in which: 
           [0013]      FIG. 1  is a schematic diagram of a motor vehicle equipped with an electric power steering system embodying the present invention; 
           [0014]      FIG. 2  is a block diagram of a control system of the electric power steering system shown in  FIG. 1 ; 
           [0015]      FIG. 3  is a detailed block diagram of a reactive torque setting unit in  FIG. 2 ; 
           [0016]      FIG. 4  is a graph representing data maps defining a relationship between a base value T 1  of the reactive torque, vehicle speed V and yaw rate γ in  FIG. 3 ; 
           [0017]      FIG. 5  is a detailed block diagram of a reactive torque correcting unit in  FIG. 3 ; 
           [0018]      FIG. 6  is a graph representing data maps defining a relationship between a base value T 2  of the reactive torque, vehicle speed V and regenerative power in  FIG. 3 ; and 
           [0019]      FIG. 7  is a flowchart showing the control process for determining a reactive torque correction value by the reactive torque setting unit in  FIG. 2 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0020]    In the following description, each wheel is denoted with numeral  4  and a suffix indicating the position of the wheel such as  4   fl ,  4   fr ,  4   rl  and  4   rr  appended thereto. When the wheels are generally referred to, only the numeral  4  may be used to denote each wheel. 
         [0021]    Referring to  FIG. 1 , a vehicle  1  having a vehicle body  2  is provided with a pair of front wheels  4   fl  and  4   fr  each fitted with a pneumatic tire  3   fl ,  3   fr  and a pair of rear wheels  4   rl  and  4   rr  each fitted with a pneumatic tire  4   rl ,  4   rr , and these wheels are supported by a vehicle body  2  via a corresponding wheel suspension system. The vehicle  1  consists of a four-wheel drive vehicle in which the front wheels are driven by an internal combustion engine not shown in the drawings and the rear wheels are driven by corresponding rear drive motors (rear wheel drive electric motors)  5   l  and  5   r  via corresponding gear mechanisms  6   l  and  6   r , respectively. 
         [0022]    The vehicle  1  is provided with a front wheel steering system  12  which allows the front wheels  4   fl  and  4   fr  to be directly steered from a steering wheel  11 . The front wheel steering system  12  includes a rack and pinion mechanism comprising a pinion  15  integrally attached to a steering shaft  14  and a rack shaft  16  extending laterally with respect to the vehicle body  2  in a slidable manner and meshing with the pinion  15 . The two ends of the rack shaft  16  are connected to knuckles  18   l  and  18   r  of the front wheels  4   fl  and  4   fr  via tie rods  17 . Thereby, by turning the steering wheel  11  integrally attached to the steering shaft  14 , the front wheels  4   fl  and  4   fr  can be steered in a corresponding manner. The front wheel steering system  12  further comprises an electric power steering system (EPS)  21  including an electric assist motor  22  that provides an assist torque for reducing the effort required for the vehicle operator to steer the front wheels. 
         [0023]    Adjacent to the pinion  15  are provided a steering torque sensor  31  for detecting the manual steering torque T applied to the pinion  15  and a steering angular speed sensor  32  for detecting the angular speed ω of the steering wheel  11 . A vehicle speed sensor  33  for detecting a vehicle speed and a yaw rate sensor  34  for detecting a yaw rate γ of the vehicle body are provided in appropriate parts of the vehicle body  2 . 
         [0024]    The vehicle  1  is further incorporated with a main ECU  37  responsible for the overall control of various onboard systems, an EPS CPU  38  for controlling the EPS  21  according to a control command from the main ECU  37  and a power control unit (PCU)  39  for controlling the actuation and regenerative braking action of the rear drive motors  5   l  and  5   r  according to the control command from the main ECU  37 . The ECUs  37  and  38  and PCU  39  each essentially consist of a CPU, ROM, RAM, a peripheral circuit, an input/output interface and various drivers, and are mutually connected to one another via a communication system such as CAN (controlled area network) so as to be able to monitor one another with respect to various control values and states. 
         [0025]    The main ECU  37  receives detection signals from the sensors  31  to  34 , and forwards various control commands to the EPS ECU  38  and PCU  39  according to the received detection signals. The PCU  39  is connected to a nickel hydrogen rechargeable battery  40  for supplying electric power to the rear drive motors  5   l  and  5   r . The rear drive motors  5   l  and  5   r  are provided with the function of a regenerative brake so that the electric power produced by the braking of the vehicle is charged to the battery  40 . 
         [0026]    The PCU  39  detects the regenerated electric power which is charged to the battery  40 , and this information is forwarded to the EPS ECU  38  via the main ECU  37 . Based on this information, the EPS ECU  38  controls the reactive torque as will be described hereinafter. According to an alternate embodiment, the PCU  39  computes a target regenerative power of the rear drive motors  5   l  and  5   r  according to the vehicle condition such as the engine rotational speed and battery charge state, and this target regenerative power may be used as the information to be forwarded to the EPS ECU  38 , instead of the actual regenerated power. 
         [0027]    In the EPS ECU  38  which is illustrated in  FIG. 2 , an assist torque setting unit  41  receives the detection signals (steering torque T, steering angular speed ω and vehicle speed V) from the steering torque sensor  31 , steering angular speed sensor  32  and vehicle speed sensor  33 . Based on the received information, the assist torque setting unit  41  sets an assist torque target value T 0  for a normal steering assist torque control by using prescribed data maps and formulas. A reactive torque setting unit  42  receives detection signals (vehicle speed V and yaw rate γ) from the vehicle speed sensor  33  and yaw rate sensor  34 , and a data signal on the regenerative powers Reg_Vl and Reg_Vr of the left and right rear wheels from the PCU  39 . From the received information, the reactive torque setting unit  42  computes a reactive torque target value TA by using prescribed data maps and formulas. The reactive torque target value TA corresponds to a steering resistance that reduces the assist steering torque. The reactive torque target value TA is added to the assist torque target value T 0  in an adder  43 , and the sum is forwarded to a target current setting unit  44 . 
         [0028]    The target current setting unit  44  sets a target current It for the assist motor  22  according to the output of the adder  43  (sum of the assist torque target value T 0  and reactive torque target value TA), and forwards the target current It to an output current control unit  45 . According to the target current It, the output current control unit  45  controls a drive circuit  46  and drives the assist motor  22 . The output current control unit  45  performs a feedback control based on the actual current la and target current It of the assist motor  22 . 
         [0029]    As shown in  FIG. 3 , a reactive torque computing unit  51  of the reactive torque setting unit  42  computes a base value T 1  of the reactive torque according to the vehicle speed V and yaw rate γ. 
         [0030]    The reactive torque computing unit  51  is configured to obtain the base value T 1  of the reactive torque from data maps relating the base value T 1  to the yaw rate γ for different vehicle speeds V as shown in  FIG. 4 . The data maps are configured such that the base value T 1  gets greater as the vehicle speed V is increased for a given yaw rate γ. When the vehicle speed V is zero, the base value T 1  is zero without regard to the value of the yaw rate γ. 
         [0031]    In the reactive torque setting unit  42 , the reactive torque correcting unit  52  sets a correction value T 2  of the reactive torque. More specifically, as shown in  FIG. 5 , a regenerated power computing unit  61  computes a difference Reg_Vd between the left rear wheel regenerated power Reg_Vl and the right rear wheel regenerated power Reg_Vr, and forwards the difference to a correction value computing unit  62 . The correction value setting unit  62  sets a correction value T 2  of the reactive torque according to the difference Reg_Vd. 
         [0032]    The correction value setting unit  62  may provide the correction value T 2  of the reactive torque from data maps relating the correction value T 2  to the difference Reg_Vd for different vehicle speeds V as shown in  FIG. 6 . The data maps are configured such that the correction value T 2  gets larger as the vehicle speed V is increased for a given yaw rate γ. The correction value T 2  is zero when the difference Reg_Vd is zero, and is increased at least when the difference Reg_Vd is not zero (or when a regenerative braking is performed). 
         [0033]    When a brake pedal not shown in the drawing is not depressed, because only the rear wheels  4   rl  and  4   rr  are braked by the rear drive motors  5   l  and  5   r , a greater change in the behavior of the vehicle is likely to occur as compared to the time when the brake pedal is depressed (or when both the front and rear wheels are braked). Therefore, the correction value setting unit  62  may be configured to set the correction value T 2  of the reactive torque only when the vehicle  1  is decelerating (such as when the depression of an accelerator pedal not shown in the drawings or the throttle opening is small or zero) and it is detected that the brake pedal is not depressed from the brake pedal signal (See  FIG. 5 ). 
         [0034]    Referring to  FIG. 3 , after the base value T 1  of the reactive torque set by the reactive torque computing unit  51  and the correction value T 2  of the reactive torque set by the reactive torque correcting unit  52  are added in the adder  63 , the sum is forwarded to a limiter  64 . The limiter  64  limits the reactive torque target value TA obtained as the sum of the base value T 1  and correction value T 2  within a prescribed range defined between a maximum value Tmax and a minimum value −Tmax, and the limited value of the reactive torque target value TA is produced. 
         [0035]    The process of setting the correction value T 2  of the reactive torque by the reactive torque setting unit  42  is described in the following with reference to  FIG. 7 . First of all, the reactive torque computing unit  51  computes the base value T 1  of the reactive torque from the data maps shown in  FIG. 4  according to the received vehicle speed V and yaw rate γ (step S 101 ). 
         [0036]    In the reactive torque correcting unit  52 , the regenerated power computing unit  61  computes the difference Reg_Vd between the left rear wheel regenerated power Reg_Vl and the right rear wheel regenerated power Reg_Vr (step S 102 ). Thereafter, the correction value setting unit  62  computes the correction value T 2  of the reactive torque from the data maps shown in  FIG. 6  according to the difference Reg_Vd and vehicle speed V (step S  103 ). 
         [0037]    According to an alternate embodiment, the correction value T 2  of the reactive torque may be obtained from the sum Reg_Vt of the left rear wheel regenerated power Reg_Vl and the right rear wheel regenerated power Reg_Vr. In such a case, the two rear wheels  4   rl  and  4   rr  may be driven by a single common electric motor via a differential device. 
         [0038]    Thereafter, the adder  63  computes the reactive torque target value TA by summing the base value T 1  and correction value T 2  (step S 104 ). The limiter  64  then compares the reactive torque target value TA with the prescribed maximum value Tmax (step S 105 ). If the reactive torque target value TA is greater than the maximum value Tmax (step S 105 : Yes), TA is set as Tmax (TA=Tmax) (step S 106 ). The limiter  64  then compares the reactive torque target value TA with the prescribed minimum value −Tmax (step S 107 ). If the reactive torque target value TA is smaller than the minimum value −Tmax (step S 105 : No), TA is set as −Tmax (TA=−Tmax) (step S 108 ). When the reactive torque target value TA has a value intermediate between the maximum value Tmax and minimum value −Tmax, the value of the reactive torque target value TA is not altered. 
         [0039]    In the electric power steering system described above, when the regeneration by the rear drive motors is in progress, the reactive torque setting unit  42  increases the reactive torque output value TA (or increases the correction value T 2 ). By increasing the reactive torque (or reducing the assisting torque) during a regenerative braking, the change in the behavior of the vehicle during the regenerative braking, and the handling of the vehicle can be improved. 
         [0040]    In particular, because the reactive torque setting unit  42  is configured to set the correction value T 2  according to the difference in the regenerative power between the left and right rear wheels, the electric power steering system  21  is enabled to favorably control the changes in the behavior of the vehicle. 
         [0041]    The reactive torque setting unit  42  is configured to compute the base value T 1  of the reactive torque according to the yaw rate y and vehicle speed V, and correct the base value T 1  with the correction value T 2  during a regenerative braking. Therefore, the reactive torque can be favorably controlled during a regenerative braking by performing a relatively simple control process. 
         [0042]    The correction value T 2  of the reactive torque was added to the base value T 1  of the reactive torque in the foregoing embodiment, but other modes of correction can be used without departing from the spirit of the present invention as long as the correction results in the increase in the reactive torque output value TA during a regenerative braking. For instance, a correction coefficient computed in dependence of the regenerative power may be multiplied to the base value T 1  of the reactive torque. The electric motors were used for driving the rear wheels in the foregoing embodiment, but the present invention can be applied to other types of vehicles where electric motors are used for driving front wheels instead of or in addition to the rear wheels. 
         [0043]    Although the present invention has been described in terms of a preferred embodiment thereof, it is obvious to a person skilled in the art that various alterations and modifications are possible without departing from the scope of the present invention which is set forth in the appended claims. 
         [0044]    The contents of the original Japanese patent application on which the Paris Convention priority claim is made for the present application, as well as the contents of any prior art mentioned in the present application, are incorporated in this application by reference.