Abstract:
A traction distribution control system for a 4WD vehicle is constructed to calculate a difference gain in accordance with a difference in spinning state between main driving wheels, and determine a control signal for controlling traction distribution to driven wheels by multiplying a control amount by the difference gain.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to a traction distribution control system for a four-wheel drive (4WD) vehicle, which distributes traction of a power source to a pair of main driving wheels or one of pairs of front wheels and rear wheels and a pair of driven wheels or another pair of wheels. 
   Typically, a traction distribution control system for a 4WD vehicle comprises engaging means arranged in a path transferring power of an engine to the main driving wheels and for transferring power to the driven wheels when the engaging means are in engagement, wherein the engagement state of the engaging means is controlled to control traction distribution to the driven wheels. 
   This traction distribution control system is constructed such that a difference is obtained between an average speed of the front wheels and an average speed of the rear wheels, wherein as the difference is greater, engaging force of the engaging means is increased to enhance the ratio of traction distribution to the driven wheels, and wherein as the difference is smaller, engaging force of the engaging means is decreased to lower the ratio of traction distribution to the driven wheels. Therefore, when torque slip occurs in the main driving wheels, the ratio of traction distribution to the driven wheels is increased to enhance the vehicle running-ability and stability while reducing torque slip of the main driving wheel. 
   With the above traction distribution control system, the average speeds of the front wheels and the rear wheels are obtained, respectively, to determine the ratio of traction distribution to the driven wheels, i.e. engaging force of the engaging means, in accordance with a difference between the average speeds. Thus, when the vehicle runs on a road having different road-surface friction coefficients (hereafter refer to friction coefficient as “μ”) for right and left wheels, i.e. a split-μ road, for example, and when one of the main driving wheels is on the low μ road surface and causes a slip, but another main driving wheel does not cause a slip, an average speed of the right and left wheels, which serves as a reference for control, has slower increase than the speed of the slip wheel, and less increasing amount. As a result, the ratio of traction distribution to the driven wheels determined in accordance with a difference between the average speed of the front wheels and the average speed of the rear wheels becomes smaller than an optimum value for stable vehicle running with slip of the main driving wheel restrained. This renders not only slipping of the main driving wheel uncontrollable, but also enhancement in vehicle start-ability and stability difficult. 
   Moreover, the above problem due to slipping of only one of the main driving wheels can occur not only when the vehicle runs on the split-μ road, but also when the cornering inner wheel has smaller load during cornering. 
   SUMMARY OF THE INVENTION 
   It is, therefore, an object of the present invention to provide a traction distribution control system for a 4WD vehicle, which contributes to earlier convergence of slip of the main driving wheels, and enhancement in the vehicle start-ability, stability and the like. 
   The present invention provides generally a four-wheel drive vehicle, which comprises: a source of traction; a device which transfers traction; main driving wheels which receive traction from the source; driven wheels which form at least a state of receiving smaller traction than traction transmitted to the main driving wheels through the device, the driven wheels receiving traction which can be changed through the device; and an ECU which controls distribution of traction to the driven wheels, wherein the ECU is so programmed as to: form a control signal for operating the device in accordance with an input signal, calculate a difference in spinning state between the main driving wheels, and determine the control signal in accordance with the difference. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The other objects and features of the present invention will become apparent from the following description with reference to the accompanying drawings, wherein: 
       FIG. 1  is a block diagram showing a traction distribution control system for a 4WD vehicle, to which the present invention is applied; 
       FIG. 2  is a flowchart showing the operation of a first embodiment of the present invention; 
       FIG. 3  is a map illustrating a difference gain characteristic; 
       FIGS. 4A-4F  are time charts illustrating examples of operation of the first embodiment; 
       FIG. 5  is a diagrammatic view showing a splits road on which the front left wheel of the vehicle turns free; 
       FIG. 6  is a map similar to  FIG. 3 , showing a second embodiment of the present invention; and 
       FIG. 7  is a map similar to  FIG. 6 , showing a third embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring to the drawings, a description is made about a traction distribution control system for a 4WD vehicle embodying the present invention. 
   Referring to  FIG. 1 , the 4WD vehicle to which the present invention is applied is a front-drive vehicle having front wheels as main driving wheels and rear wheels as driven wheels. Traction of an engine  1  is transmitted to a transfer  3  through an automatic transmission  2  having a torque converter, which is distributed to a front propeller shaft  4  and a rear propeller shaft  5  by transfer  3 . Traction transmitted to front propeller shaft  4  is always transferred to front wheels WFR, WFL through a front differential  6 , whereas traction transmitted to rear propeller shaft  5  is transferred to rear wheels WRR, WRL through a clutch or traction transfer means  7  and a rear differential  8 . 
   Thus, in the state that clutch  7  is in full disengagement, the ratio of traction distribution between front wheels WFR, WFL and rear wheels WRR, WRL is 100:0, whereas in the state that cutch  7  is in full engagement, the ratio is 50:50. Therefore, by changing the engaging state of clutch  7 , traction distribution between wheels WFR, WFL and rear wheels WRR, WRL can arbitrarily be changed in the range of 100:0 to 50:50. 
   The engaging state of clutch  7  is changed in accordance with the magnitude of a control output current I derived from an electronic control unit (ECU) or traction distribution control means  10 . ECU  10  is connected to wheel-speed sensors  11  for sensing speeds Vw of wheels WFR, WFL WRR, WRL When required, ECU  10  may be connected to a longitudinal acceleration sensor  12  for sensing longitudinal acceleration of the vehicle and a lateral acceleration sensor  13  for sensing lateral acceleration of the vehicle. Sensors  11 - 13  can serve as sensors arranged in an ABS control system for preventing wheel lock during braking. 
   Referring to  FIG. 2 , traction distribution control carried out by ECU  10  is described. At a step S 201 , a speed difference ΔV between front and rear wheels is obtained. Wheel-speed difference ΔV is obtained by subtracting an average speed of rear wheels WRR, WRL from an average speed of front wheels WFR, WFL such that:
 
Δ V={(VwFR+VwFL )/2}−{( VwRR+VwRL )/2}
 
   At a step S 202 , a difference gain KDF is calculated. In the first embodiment, difference gain KDF is calculated in accordance with a KDF characteristic map corresponding to a predetermined speed difference between front right and left wheels WFR, WFL, i.e. |VwFR−VwFL|. Referring to  FIG. 3 , in this map, when the speed difference is smaller than a predetermined value V01, KDF=1.0. When the speed difference is larger than a predetermined value V02, KDF=2.0. And between predetermined values V01 and V02, KDF increases proportionally from 1.0 to 2.0 in accordance with a value of the speed difference. 
   At a step S 203 , a wheel-rpm difference control amount or first control amount TΔV is calculated by TΔV=k×ΔV, where k is a constant. Wheel-rpm difference control amount TΔV corresponds to engaging force of clutch  7  in accordance with wheel-speed difference ΔV, and also to traction to be distributed to rear wheels WRR, WRL 
   At a step S 204 , a driving-torque distribution control amount or second control amount TENG, which is an appropriate traction distribution amount to rear wheels WRR, WRL in response to engine output torque of engine  1 , is obtained by TENG=TROD×(WR/W), where TROD is torque of an output shaft of the toque converter of automatic transmission  2 , and can be obtained from the engine speed. When rear wheels WRR, WRL serve as main driving wheels to achieve traction distribution to front wheels WFR, WFL as driven wheels, TROD is multiplied by WF/W corresponding to a load of front wheels WFR, WFL. 
   At a step S 205 , a control-torque calculated value T_ETS is obtained, which corresponds to optimum torque to be transmitted from clutch  7  to rear wheels WRR, WRL Control-torque calculated value T_ETS is obtained by multiplying greater one of wheel-rpm difference control amount TΔV calculated at step S 203  and driving-torque distribution control amount TENG calculated at step S 204  by difference gain KDF. 
   At a step S 206 , control output current I is obtained in accordance with a current-torque conversion characteristic preprogrammed in response to control-torque calculated value T_ETS. At a step S 207 , processing is carried out to provide control output current I to clutch  7 . And a step S 208 , it is determined whether or not 10 ms has elapsed, which is a control cycle of this control flow. If it is determined that 10 ms has elapsed, flow is returned to step S 201 . 
   Referring to  FIGS. 4A-4F , examples of operation of the first embodiment are described. Referring to  FIG. 5 , those examples are taken when the vehicle starts from a standstill on a splits road having highs for right wheels WFR, WRR and low-p for left wheels WFL, WRL, and front left wheel WFL turns free. Specifically,  FIGS. 4A-4F  show a change in speed VwFL of front left wheel WFL from a beginning of free turning with respect to speeds VwFR, VwRR, VwRL of the other three wheels which are substantially equal to a vehicle-body speed V to a convergence thereof. 
   The examples as shown in  FIGS. 4A-4F  are described in order. Referring to  FIG. 4A , free turning of front left wheel WFL starts at point “t1”, so that only its wheel speed VwFL becomes greater than the speeds of the other three wheels. In response to this free turning, at step S 201  in  FIG. 2 , wheel-speed difference ΔV is produced as a positive value, and varies as shown in FIG.  4 C. Due to wheel-speed difference ΔV produced in such a way, at step S 202  in  FIG. 2 , difference gain KDF is formed having a value greater than 1.0, which is produced as shown in FIG.  4 B. Therefore, a value obtained by multiplying wheel-rpm difference control amount or first control amount TΔV obtained in accordance with wheel-speed difference ΔV at step S 203  by difference gain KDF varies as shown in FIG.  4 D.  FIG. 4D  shows an optimum control amount determined in accordance with wheel-speed difference ΔV. 
   At step S 204  in  FIG. 2 , driving-torque distribution control amount or second control amount TENG is calculated in accordance with the output state of engine  1  resulting from the starting operation. A value obtained by multiplying driving-torque distribution control amount TENG by difference gain KDF is produced as shown in FIG.  4 E. Since this value corresponds to the driving state of engine  1 , it can rise immediately before beginning of free turning of the wheel.  FIG. 3E  shows an optimum control amount determined in accordance with engine output torque. Then, control-torque calculated value T_ETS obtained at step S 205  is as shown in FIG.  4 F. 
   ECU  10  provides control output current I in accordance with control-torque calculated value T_ETS. In the first embodiment, as compared with a variation in wheel-speed difference ΔV which serves as a base of formation of control output current I in the prior art, control output current I of a higher value is formed in a feedforward-control way in accordance with an increase in engine output torque immediately after point “t1”. With this, traction distribution at a greater ratio is carried out to driven wheels WRR, WRL at an early stage, making free turning of front left wheel WFL trend to convergence. Then, with an increase in difference gain KDF, control output current I is formed having a higher value obtained by multiplying further by difference gain KDF the feedback-control-like first control amount obtained by multiplying wheel-speed difference ΔV by constant k. With this, traction distribution at a greater ratio is also carried out to driven wheels WRR, WRL at an early stage, making free turning of front left wheel WFL trend to convergence. This leads to free turning of front left wheel WFL in a short time. 
   In such a way, in the first embodiment, even if wheel spin occurs at one of front wheels WFR, WFL as main driving wheels, difference gain KDF corresponding appropriately to the wheel-spin amount is formed to generate appropriate engaging torque in clutch  7 , transmitting traction to rear wheels WRR, WRL as driven wheels, thus allowing enhancement in vehicle start-ability and stability. Moreover, difference gain KDF corresponding appropriately to the wheel-spin amount is formed to generate appropriate engaging torque in clutch  7 , so that even if wheel spin occurs at a cornering inner wheel during cornering and cornering acceleration, appropriate traction distribution is carried out to rear wheels WRR, WRL as drive wheels, thus allowing enhancement in vehicle stability. 
   Referring to  FIG. 6 , the second embodiment of the present invention is described, which is substantially the same as the first embodiment except the method of calculating difference gain KDF at step S 202  shown in FIG.  2 . In the second embodiment, difference gain KDF is formed in accordance with a difference between accelerations ΔVwFR, ΔVwFL of front wheels WFR, WFL as main driving wheels. The second embodiment allows enhancement in sensibility with respect to a difference in road-surface friction coefficient between the right and left wheels. 
   Referring to  FIG. 7 , the third embodiment of the present invention is described, which is substantially the same as the first embodiment except the method of calculating difference gain KDF at step S 202  shown in FIG.  2 . In the third embodiment, difference gain KDF is formed in accordance with a lateral acceleration YG generated in the vehicle. The third embodiment allows appropriate setting of traction distribution to rear wheels ERR, WRL in accordance with a load difference between front wheels WFR, WFL. In place of being detected by lateral acceleration sensor  13  as shown in  FIG. 1 , lateral acceleration YG can be calculated in accordance with wheel speed Vw such that:
 
 YG =(4 p   2 /9.807)× r×V   2  
 
wherein r is a cornering radius, which can be obtained by:
         r=(tread×inner-wheel Vw)/(outer-wheel Vw−inner-wheel Vw).       

   Having described the present invention with regard to the illustrative embodiments, it is noted that the present invention is not limited thereto, and various changes and modifications can be made without departing from the scope of the present invention. By way of example, the present invention is applied to a vehicle having front wheels as main driving wheels, alternatively, it can be applied to a vehicle having rear wheels as main driving wheels. Further, in the illustrative embodiments, the traction transfer means include clutch  7  for transmitting to the driven wheels traction taken from the traction transfer path to the main driving wheels. Optionally, separate and distinct driving sources may be used for the main driving wheels and the driven wheels to transmit tractions thereto independently. Specifically, the main driving wheels can be driven by main driving means, whereas the driven wheels can be driven by auxiliary driving means. In this variation, therefore, the traction transfer means are not limited to clutch  7 , but may include auxiliary driving means and means for transmitting traction from the auxiliary driving means to the driven wheels. Alternatively, the traction transfer means may include a clutch arranged in the power transfer path between the auxiliary driving means and the driven wheels. 
   Furthermore, in the illustrative embodiments, the power source includes engine  1 , alternatively, it may include a motor, for example. Further, in the illustrative embodiments, two control amounts are formed: control amount (TΔV) based on an average-speed difference between the front and rear wheels and control amount (TENG) based on engine output torque. Optionally, one of the two control amounts may be formed. The control amount based on engine output torque may be determined by other input such as accelerator opening. Still further, in the illustrative embodiments, traction distribution between front wheels WFR, WFL and rear wheels WRR, WRL is carried out in the range of 100:0 to 50:50, alternatively, it can be changed in a narrower range. 
   The entire contents of Japanese Patent Application P2002-034413 filed Feb. 12, 2002 are hereby incorporated by reference.