Driving torque distribution control system for vehicle and the method thereof

• US 5,742,917 A
• Filed: 06/13/1995
• Issued: 04/21/1998
• Est. Priority Date: 06/27/1994
• Status: Expired due to Fees

• Current Assignee: Fuji Jukogyo KK (Subaru Corp.)
• Original Assignee: Fuji Jukogyo KK (Subaru Corp.)
• Inventors: Matsuno, Koji
• Primary Examiner(s): ZANELLI, MICHAEL J
• Application Number:
  US08/489,785
• Time in Patent Office: 1,043 Days
• Field of Search: 364/426.029, 364/426.033, 364/424.098, 364/426.027, 364/426.034, 364/426.035, 180/197
• US Class Current: 701/69
• CPC Class Codes: B60G 2400/822 Road friction coefficient determination affecting wheel traction
• CPC Class Codes: B60G 2800/21 Traction, slip, skid or slide control
• CPC Class Codes: B60G 2800/24 Steering, cornering
• CPC Class Codes: B60G 2800/952 Electronic driving torque distribution
• CPC Class Codes: B60K 17/3462 with means for changing distribution of torque between front and rear wheels
• CPC Class Codes: B60K 23/04 for differential gearing
• CPC Class Codes: B60K 23/0808 for varying torque distribution between driven axles, e.g. by transfer clutch
• CPC Class Codes: B60K 28/165 acting on elements of the vehicle drive train other than the propulsion unit and brakes, e.g. transmission, clutch, differential acting on brakes B60T8/17
• CPC Class Codes: B60T 2230/02 Side slip angle, attitude angle, floating angle, drift angle
• CPC Class Codes: B60T 8/172 Determining control parameters used in the regulation, e.g. by calculations involving measured or detected parameters B60T8/17551 takes precedence
• CPC Class Codes: B60T 8/1755 Brake regulation specially adapted to control the stability of the vehicle, e.g. taking into account yaw rate or transverse acceleration in a curve road vehicle drive control systems for control of driving stability otherwise than by controlling a particular sub-unit B60W30/02
• CPC Class Codes: B60T 8/1769 specially adapted for vehicles having more than one driven axle, e.g. four-wheel drive vehicles
• CPC Class Codes: B60W 2050/0033 Single-track, 2D vehicle model, i.e. two-wheel bicycle model
• CPC Class Codes: B60W 2510/1005 Transmission ratio engaged
• CPC Class Codes: B60W 2520/14 Yaw
• CPC Class Codes: B60W 2530/16 Driving resistance
• CPC Class Codes: B60W 2540/18 Steering angle
• CPC Class Codes: B60W 2552/40 Coefficient of friction
• CPC Class Codes: B60W 2720/14 Yaw

Abstract

In a torque distribution control system of a four wheel drive vehicle, it is important to distribute a driving force properly between front and rear wheels according to friction coefficient of road surface. Especially when a vehicle runs on roads with low friction coefficients of road surface, it is very important to estimate friction coefficients of the road surface which vary every moment and to control the driving force according to the estimated friction coefficients. The present invention provides the torque distribution control system with means for estimating friction coefficients of road surface every moment based on data from a steering angle sensor, a vehicle speed sensor and a yaw rate sensor and for reflecting them on the calculation of torque distribution between front and rear wheels. Furthermore, yaw moment calculating means are provided to prevent the vehicle from tack-in phenomenon by properly controlling left and right wheels.

Claims

1. A torque distribution control system of a vehicle for distributing an input torque to wheels, comprising:

a yaw rate sensor for detecting a yaw rate;

a steering angle sensor for detecting a steering angle;

a vehicle speed sensor for detecting a vehicle speed; and

road friction estimating means for estimating a friction coefficient of road surface based on said yaw rate, said steering angle and said vehicle speed;

torque distribution ratio calculating means for calculating a torque distribution ratio by using said friction coefficient; and

a torque distributing mechanism for distributing said input torque to said wheels based on said torque distribution ratio.

2. The torque distribution control system according to claim 1, wherein

said friction coefficient is estimated by comparing a cornering power of a wheel which has been calculated based on said steering angle, said vehicle speed and said yaw rate according to equations of motion of said vehicle with a reference cornering power of said wheel on a high friction coefficient road.

3. The torque distribution control system according to claim 1 is for the purpose of distributing said input torque between front and rear wheels.

4. A torque distribution control system of a vehicle for distributing an input torque to wheels, comprising:

a yaw rate sensor for detecting a yaw rate;

a steering angle sensor for detecting a steering angle;

a vehicle speed sensor for detecting a vehicle speed;

a lateral acceleration sensor for detecting a lateral acceleration;

road friction estimating means for estimating a friction coefficient of road surface based on said yaw rate, said steering angle, said vehicle speed and said lateral acceleration;

torque distribution ratio calculating means for calculating a torque distribution ratio by using said friction coefficient; and

a torque distributing mechanism for distributing said input torque to said wheels based on said torque distribution ratio.

5. The torque distribution control system according to claim 4, wherein

said road friction estimating means comprise yaw rate and lateral acceleration calculating means for calculating a yaw rate and a lateral acceleration according to equations of motion of said vehicle based on said steering angle, said vehicle speed and cornering powers, deviation calculating means for calculating a deviation of said detected yaw rate from said calculated yaw rate and a deviation of said detected lateral acceleration from said calculated lateral acceleration, tire characteristic control means for estimating cornering powers of wheels based on said deviation of said detected yaw rate and said deviation of said detected lateral acceleration and friction coefficients determining means for determining a friction coefficient of road surface based on said cornering powers and for outputting said friction coefficient to said torque distribution ratio calculating means so as to calculate said torque distribution ratio based on said friction coefficient.

6. The torque distribution control system according to claim 4 is for the purpose of distributing said input torque between front and rear wheels.

7. A torque distribution control system of a vehicle for distributing an input torque to wheels, comprising:

an engine speed sensor for detecting an engine speed;

an accelerator pedal opening angle sensor for detecting an accelerator pedal opening angle;

a gear position sensor for detecting a gear position;

a steering angle sensor for detecting a steering angle;

a vehicle speed sensor for detecting a vehicle speed;

a yaw rate sensor for detecting a yaw rate of said vehicle;

input torque estimating means for estimating said input torque based on said engine speed, said accelerator pedal angle and said gear position;

target yaw rate determining means for determining a target yaw rate based on said steering angle and said vehicle speed;

yaw rate gain determining means for determining a yaw rate gain based on said vehicle speed;

target steering characteristic determining means for determining a target stability factor based on said target yaw rate, said yaw rate, said vehicle speed and said yaw rate gain;

road friction estimating means for estimating a friction coefficient of road surface based on said steering angle, said vehicle speed and said yaw rate;

torque distribution ratio calculating means for calculating a torque distribution ratio based on said input torque, said target stability factor, said vehicle speed, said yaw rate and said friction coefficient; and

a torque distributing mechanism for distributing said input torque to said wheels based on said torque distribution ratio.

8. A torque distribution control system of a vehicle for distributing an input torque to wheels, comprising:

a yaw rate sensor for detecting a yaw rate;

a steering angle sensor for detecting a steering angle;

a vehicle speed sensor for detecting a vehicle speed;

target yaw rate calculating means for calculating a target yaw rate based on said steering angle and said vehicle speed;

deviation calculating means for calculating a deviation of said yaw rate from said target yaw rate based on said yaw rate and said target yaw rate;

yaw rate gain determining means for determining a yaw rate gain based on said vehicle speed;

yaw moment calculating means for calculating a yaw moment based on said deviation of said yaw rate from said target yaw rate and on said yaw rate gain; and

a torque distributing mechanism for distributing said input torque to said wheels according to said yaw moment.

9. The torque distribution control system according to claim 8 is for the purpose of distributing said input torque between left and right wheels.

10. A torque distribution control system of a vehicle for distributing an input torque to wheels, comprising:

an engine speed sensor for detecting an engine speed;

an accelerator pedal opening angle sensor for detecting an accelerator pedal opening angle;

a gear position sensor for detecting a gear position;

a steering angle sensor for detecting a steering angle;

a vehicle speed sensor for detecting a vehicle speed;

a yaw rate sensor for detecting a yaw rate of said vehicle;

a lateral acceleration sensor for detecting a lateral acceleration of said vehicle;

input torque estimating means for estimating an input torque based on said engine speed, said accelerator pedal angle and said gear position;

target yaw rate determining means for determining a target yaw rate based on said steering angle and said vehicle speed;

target steering characteristic determining means for determining a target stability factor based on said target yaw rate and said yaw rate;

road friction estimating means for estimating a friction coefficient of road surface based on said steering angle, said vehicle speed and said yaw rate;

torque distribution ratio calculating means for calculating a torque distribution ratio based on said input torque, said target stability factor, said vehicle speed, said yaw rate, said lateral acceleration and said friction coefficient; and

a torque distributing mechanism for distributing said input torque to said wheels based on said torque distribution ratio.

11. A method of distributing an input torque to wheels of a vehicle, comprising the steps of:

detecting a yaw rate;

detecting a steering angle;

detecting a vehicle speed;

estimating a friction coefficient of road surface based on said yaw rate, said steering angle and said vehicle speed;

calculating a torque distribution ratio by using said friction coefficient; and

distributing said input torque among said wheels based on said torque distribution ratio.

12. A method of distributing an input torque to wheels of a vehicle, comprising the steps of:

detecting a yaw rate;

detecting a steering angle;

detecting a vehicle speed;

detecting a lateral acceleration;

estimating a friction coefficient of road surface based on said yaw rate, said steering angle, said vehicle speed and said lateral acceleration;

calculating a torque distribution ratio by using said friction coefficient; and

distributing said input torque to said wheels based on said torque distribution ratio.

13. A method of distributing an input torque to wheels, comprising the steps of:

detecting an engine speed;

detecting an accelerator pedal opening angle;

detecting a gear position;

detecting a steering angle;

detecting a vehicle speed;

detecting a yaw rate of said vehicle;

estimating said input torque based on said engine speed, said accelerator pedal angle and said gear position;

determining a target yaw rate based on said steering angle and said vehicle speed;

determining a yaw rate gain based on said vehicle speed;

determining a target stability factor based on said target yaw rate, said yaw rate, said vehicle speed and said yaw rate gain;

estimating a friction coefficient of road surface based on said steering angle, said vehicle speed and said yaw rate;

calculating a torque distribution ratio based on said input torque, said target stability factor, said vehicle speed, said yaw rate and said friction coefficient; and

distributing said input torque to said wheels based on said torque distribution ratio.

14. A method of distributing an input torque to wheels of a vehicle, comprising the steps of:

detecting a yaw rate;

detecting a steering angle;

detecting a vehicle speed;

calculating a target yaw rate based on said steering angle and said vehicle speed;

calculating a deviation of said yaw rate from said target yaw rate based on said yaw rate and said target yaw rate;

determining a yaw rate gain based on said vehicle speed;

calculating a yaw moment based on said deviation of said yaw rate from said target yaw rate and on said yaw rate gain; and

distributing said input torque to said wheels according to said yaw moment.

15. A method of distributing an input torque to wheels of a vehicle, comprising the steps of:

detecting an engine speed;

detecting an accelerator pedal opening angle;

detecting a gear position;

detecting a steering angle;

detecting a vehicle speed;

detecting a yaw rate of said vehicle;

detecting a lateral acceleration of said vehicle;

estimating said input torque based on said engine speed, said accelerator pedal angle and said gear position;

determining a target yaw rate based on said steering angle and said vehicle speed;

determining a target stability factor based on said target yaw rate and said yaw rate;

estimating a friction coefficient of road surface based on said steering angle, said vehicle speed and said yaw rate;

calculating a torque distribution ratio based on said input torque, said target stability factor, said vehicle speed, said yaw rate, said lateral acceleration and said friction coefficient; and

distributing said input torque to said wheels based on said torque distribution ratio.

16. A torque distribution control system of a vehicle having, a center clutch for distributing a torque from an engine to a front and a rear wheel respectively, input torque estimating means for estimating an input torque based on engine operating conditions and for producing an input signal, a steering angle sensor for detecting a steering angle and for generating a steering angle signal, a vehicle speed sensor for detecting a vehicle speed and for producing a vehicle speed signal, a yaw rate sensor for detecting a yaw rate of said vehicle and for outputting a yaw rate signal, comprising:

target yaw rate determining means responsive to said steering angle and said vehicle speed signals;

yaw rate gain determining means responsive to said vehicle speed signal for determining a yaw rate gain and for generating a yaw rate gain signal;

road friction estimating means responsive to said yaw rate signal, said vehicle speed signal and said steering angle signal for estimating a friction coefficient of road surface and for outputting a friction coefficient signal;

torque distribution ratio calculating means responsive to said input torque, said target stability factor, said vehicle speed, said yaw rate and said friction coefficient signals for calculating a torque distribution ratio and for producing a torque distribution ratio signal;

torque distribution ratio calculating means responsive to said torque distribution ratio signal and said input torque signal for calculating a torque distribution ratio by using a friction coefficient and for producing a torque signal; and

a torque distributing mechanism responsive to said torque signal for distributing said input torque to said respective wheels based on said torque distribution ratio so as to prevent said vehicle from a tack-in phenomenon.

17. A torque distribution control system of a four-wheel drive vehicle for distributing engine torque to each wheel of the vehicle having an engine mounted on said vehicle, a transmission connected to said engine via a clutch for changing engine speed, a center differential operatively and mechanically connected to said engine and said wheels and for absorbing a speed difference between front wheels and rear wheels thereof, a solenoid valve operatively and hydraulically connected to said center differential, an engine speed sensor for detecting said engine speed and for generating an engine speed signal, a throttle sensor for detecting an opening degree of a throttle valve and for producing a throttle signal, a gear position sensor mounted on said transmission for detecting a gear position thereof and for producing a gear position signal, a steering sensor mounted on said vehicle for sensing a steering angle of said front wheels and for generating a steering angle signal, a vehicle speed sensor mounted on said vehicle for detecting vehicle speed and for producing a vehicle speed signal, and a yaw rate sensor mounted on said vehicle for detecting an actual yaw rate of said vehicle and for generating an actual yaw rate signal, comprising:

torque estimating means responsive to said engine speed signal, said throttle signal and said gear position signal for calculating an input torque to said center differential by estimating an outputted torque of said engine by referring to an engine output characteristic map stored in a memory and for producing an input torque signal;

target yaw rate determining means responsive to said steering angle signal and said vehicle speed signal for setting a target yaw rate based upon a standard turning characteristic corresponding to a friction coefficient of a road surface and for producing a target yaw rate signal;

road friction estimating means responsive to said steering angle signal, said vehicle speed signal and said actual yaw rate signal for setting a predetermined value of a friction coefficient corresponding to a road condition by using a predetermined equation of motion of said vehicle and for producing an estimated friction signal;

target steering characteristic setting means responsive to said target yaw rate signal and said actual yaw rate signal for setting a target stability factor in accordance with a deviation between said actual yaw rate and said target yaw rate and for outputting a target steering characteristic signal;

torque distribution ratio calculating means responsive to said input torque signal, said target steering characteristic signal, said vehicle speed signal, said actual yaw rate signal and said estimated friction signal for calculating a torque distribution ratio and for generating a torque distribution ratio signal; and

duty ratio deciding means responsive to said input torque signal and said torque distribution ratio signal for calculating a limiting torque of said center differential by a predetermined equation in relation with a standard torque distribution ratio and for transmitting an operating signal to said solenoid valve so as to perform an optimum control of said vehicle and to improve stability and driveability when said vehicle is negotiating a tight corner on a slippery road.

18. The torque distribution control system according to claim 17, wherein:

said road friction estimating means produces said estimated friction signal by selecting a higher friction coefficient from those of said front and rear wheels.

19. The torque distribution control system according to claim 17, wherein:

said target steering characteristic setting means changes said target stability factor in accordance with said deviation between said actual yaw rate and said target yaw rate.

20. A torque distribution control system of a four-wheel drive vehicle for distributing engine torque to each wheel of the vehicle having an engine mounted on said vehicle, a transmission connected to said engine via a clutch for changing engine speed, a center differential operatively and mechanically connected to said engine and said wheels and for absorbing a speed difference between front wheels and rear wheels thereof, a solenoid valve operatively and hydraulically connected to said center differential, an engine speed sensor for detecting said engine speed and for generating an engine speed signal, a throttle sensor for detecting an opening degree of a throttle valve and for producing a throttle signal, a gear position sensor mounted on said transmission for detecting a gear position thereof and for producing a gear position signal, a steering sensor mounted on said vehicle for sensing a steering angle of said front wheels and for generating a steering angle signal, a vehicle speed sensor mounted on said vehicle for detecting vehicle speed and for producing a vehicle speed signal, a yaw rate sensor mounted on said vehicle for detecting an actual yaw rate of said vehicle and for generating an actual yaw rate signal, and a lateral G-sensor mounted on said vehicle for detecting a lateral acceleration of said vehicle and for outputting an actual lateral acceleration signal, comprising:

torque estimating means responsive to said engine speed signal, said throttle signal and said gear position signal for calculating an input torque to said center differential by estimating an outputted torque of said engine by referring to an engine output characteristic map stored in a memory and for producing an input torque signal;

target yaw rate determining means responsive to said steering angle signal and said vehicle speed signal for setting a target yaw rate based upon a standard turning characteristic corresponding to a friction coefficient of a road surface and for producing a target yaw rate signal;

calculating means responsive to said steering angle signal and said vehicle speed signal and for calculating a yaw rate and a lateral acceleration with a vehicular motion model equation and for producing a calculated yaw rate signal and a calculated lateral acceleration signal;

deviation calculating means responsive to said actual yaw rate signal, said actual lateral acceleration signal, and said calculated yaw rate signal and said calculated lateral acceleration signal for deriving a yaw rate deviation value of said calculated yaw rate signal from said actual yaw rate signal and a lateral acceleration deviation value of said calculated lateral acceleration signal from said actual lateral acceleration signal and for producing a deviation signal;

tire characteristic control means responsive to said deviation signal for deriving a cornering power of both said front and rear wheels and for generating a cornering power characteristic signal;

road friction estimating means responsive to said cornering power characteristic signal for setting a predetermined value of a friction coefficient corresponding to a road condition by using a predetermined equation of motion of said vehicle and for producing an estimated friction signal;

target steering characteristic setting means responsive to said target yaw rate signal and said actual yaw rate signal for setting a target stability factor in accordance with a deviation between said actual yaw rate and said target yaw rate and for outputting a target steering characteristic signal;

torque distribution ratio calculating means responsive to said input torque signal, said target steering characteristic signal, said vehicle speed signal, said actual yaw rate signal and said estimated friction signal for calculating a torque distribution ratio and for generating a torque distribution ratio signal; and

duty ratio deciding means responsive to said input torque signal and said torque distribution ratio signal for calculating a limiting torque of said center differential by a predetermined equation in relation to a standard torque distribution ratio and for transmitting an operating signal to said solenoid valve so as to perform an optimum control of said vehicle and to improve stability and driveability when said vehicle is negotiating a tight corner on a slippery road.

21. A torque distribution control system of a four-wheel-drive vehicle for distributing an engine torque to each wheel of the vehicle having an engine mounted on said vehicle, a transmission connected to said engine via a clutch for changing engine speed, a center differential operatively and mechanically connected to said engine and said wheels for absorbing a speed difference between front wheels and rear wheels thereof, a rear differential operatively and mechanically interposed between a left rear wheel of said wheels, and a right rear wheel of said wheels for absorbing a left and right rear wheel speed difference, a rear solenoid valve operatively and hydraulically connected to said rear differential, a steering sensor mounted on said vehicle for sensing a steering angle of said front wheels and for generating a steering angle signal, a vehicle speed sensor mounted on said vehicle for detecting vehicle speed and for producing a vehicle speed signal, and a yaw rate sensor mounted on said vehicle for detecting an actual yaw rate of said vehicle and for generating an actual yaw rate signal, comprising:

target yaw rate determining means responsive to said steering angle signal and said vehicle speed signal for setting a target yaw rate based upon a standard turning characteristic corresponding to a friction coefficient of a road surface and for producing a target yaw rate signal;

calculating means responsive to said steering angle signal and said vehicle speed signal and for calculating a yaw rate with a vehicular motion model equation and for producing a calculated yaw rate signal;

deviation calculating means responsive to said actual yaw rate signal and said calculated yaw rate signal for deriving a yaw rate deviation value of said calculated yaw rate signal from said actual yaw rate signal and for producing a deviation signal;

yaw rate gain setting means responsive to said vehicle speed for determining a yaw rate gain corresponding to a present vehicle speed by referring to a yaw rate gain map and outputting a yaw rate gain signal;

yaw moment calculating means responsive to said deviation signal and said yaw rate gain signal for deriving a yaw moment by using a predetermined moment equation of said vehicle and for producing a moment signal; and

rear differential control means responsive to said moment signal for calculating a limiting torque of said rear differential by a predetermined equation to transmit an operating signal to said rear solenoid valve so as to perform optimum control of said vehicle by distributing torque to said left and right rear wheels and to improve stability and driveability when said vehicle is negotiating a tight corner on a slippery road.

22. A torque distribution control method of a four-wheel-drive vehicle for distributing engine torque to each wheel of the vehicle having an engine mounted on said vehicle, a transmission connected to said engine via a clutch for changing engine speed, a center differential operatively and mechanically connected to said engine and said wheels for absorbing a speed difference between front wheels and rear wheels thereof, a solenoid valve operatively and hydraulically connected to said center differential, the method comprising the steps of:

sensing an engine speed;

detecting an opening degree of a throttle valve;

indicating a gear position of said transmission;

measuring a steering angle of said front wheels;

calculating vehicle speed;

deriving an actual yaw rate of said vehicle;

computing an input torque from detected values of said engine speed, said opening degree of said throttle valve and said gear position to said differential by reference to an engine output characteristic map stored in a memory;

first determining a target yaw rate from said steering angle and said vehicle speed by using a standard turning characteristic corresponding to a friction coefficient of a road surface;

secondly determining a predetermined value of a friction coefficient corresponding to a road condition from said steering angle, said vehicle speed, and said actual yaw rate by using a predetermined equation of motion of said vehicle;

determining a target stability factor in accordance with a deviation between said actual yaw rate and said target yaw rate;

deriving a torque distribution ratio from values of said input torque, said target stability factor, said vehicle speed, said yaw rate and said predetermined friction coefficient;

calculating a limiting torque of said center differential from said torque distribution ratio and said input torque by a predetermined equation in relation to a standard torque distribution ratio; and

determining and transmitting an operating signal based on said calculated limiting torque to said solenoid valve so as to perform optimum control of said vehicle and to improve stability and driveability when said vehicle is negotiating a tight corner on a slippery road.

23. The torque distribution control method according to claim 22, wherein:

said secondly determining step further comprises producing said estimated friction signal by selecting a higher friction coefficient from those of said front and rear wheels.

24. The torque distribution control method according to claim 22, wherein:

said first determining step further comprises changing said target stability factor in accordance with said deviation between said actual yaw rate and said target yaw rate.

25. A torque distribution control method of a four-wheel-drive vehicle for distributing engine torque to each wheel of the vehicle having an engine mounted on said vehicle, a transmission connected to said engine via a clutch for changing engine speed, a center differential operatively and mechanically connected to said engine and said wheels for absorbing a speed difference between front wheels and rear wheels thereof, a solenoid valve operatively and hydraulically connected to said center differential, the method comprising the steps of:

sensing an engine speed;

detecting an opening degree of a throttle valve;

indicating a gear position of said transmission;

measuring a steering angle of said front wheels;

calculating a vehicle speed;

deriving an actual yaw rate of said vehicle,

detecting an actual lateral acceleration of said vehicle by a lateral G-sensor mounted on said vehicle;

computing an input torque from detected values of said engine speed, said opening degree of said throttle valve and said gear position by referring to an engine output characteristic map stored in a memory;

determining a target yaw rate from said steering angle and said vehicle speed by using a standard turning characteristic corresponding to a friction coefficient of a road surface;

calculating a yaw rate and a lateral acceleration from said steering angle and said vehicle speed by making use of a vehicular motion model equation;

calculating a yaw rate deviation value of said calculated yaw rate from said actual yaw rate and a lateral acceleration deviation value of said calculated lateral acceleration from said actual lateral acceleration;

calculating a tire characteristic from said yaw rate and acceleration deviation values representing a cornering power of both said front and rear wheels;

calculating a predetermined value of a friction coefficient corresponding to a road condition by using a predetermined equation of motion of said vehicle;

calculating a target steering characteristic of a target stability factor in accordance with a deviation between said actual yaw rate and said target yaw rate;

calculating a torque distribution ratio from values of said input torque, said target steering characteristic, said vehicle speed, said actual yaw rate and said estimated friction;

calculating a duty ratio of a limiting torque of said center differential from said calculated torque distribution ratio and said input torque by a predetermined equation in relation to a standard torque distribution ratio to transmit an operating signal to said solenoid valve so as to perform optimum control of said vehicle and to improve stability and driveability when said vehicle is negotiating a tight corner on a slippery road.

26. A torque distribution control method of a four-wheel-drive vehicle for distributing an engine torque to each wheel of the vehicle having an engine mounted on said vehicle, a transmission connected to said engine via a clutch for changing engine speed, a center differential operatively and mechanically connected to said engine and said wheels for absorbing a speed difference between front wheels and rear wheels thereof, a rear differential operatively and mechanically interposed between the left rear wheel and a right rear wheel of said wheels for absorbing a left and right rear wheel speed difference, and a rear solenoid valve operatively and hydraulically connected to said rear differential, the method comprising the steps of:

sensing a steering angle of said front wheels;

detecting vehicle speed;

detecting an actual yaw rate of said vehicle;

setting a target yaw rate based upon a standard turning characteristic corresponding to a friction coefficient of a road surface from said sensed steering angle and said detected vehicle speed;

calculating a yaw rate from said sensed steering angle and said vehicle speed with a vehicular motion model equation;

deriving a yaw rate deviation value of said calculated yaw rate from said actual yaw rate from said actual yaw rate and said calculated yaw rate;

determining a yaw rate gain corresponding to a present vehicle speed by referring to a yaw rate gain map from said detected vehicle speed;

deriving a yaw moment by using a predetermined moment equation of said vehicle from said derived deviation and said yaw rate gain; and

calculating a limiting torque of said rear differential from said yaw moment by a predetermined equation and transmitting an operating signal to said rear solenoid valve so as to perform optimum control of said vehicle by distributing torque to the left and right rear wheels and to improve stability and driveability when said vehicle is negotiating a tight corner on a slippery road.