Control system for distributing drive torque between front and rear wheels of vehicle

• US 5,752,211 A
• Filed: 05/17/1996
• Issued: 05/12/1998
• Est. Priority Date: 11/24/1993
• Status: Expired due to Term

• Current Assignee: Nissan Motor Co., Ltd.
• Original Assignee: Nissan Motor Co., Ltd.
• Inventors: Hara, Tomoyuki, Takasaki, Toshiharu
• Primary Examiner(s): LOUIS JACQUES, JACQUES H
• Application Number:
  US08/649,475
• Time in Patent Office: 725 Days
• Field of Search: 364/426.01, 364/426.2, 364/426.03, 364/426.041, 364/426.04, 364/431.07, 364/424.1, 180/178, 180/233, 180/248, 180/247, 180/249, 180/197, 180/24, 180/24.09, 180/250, 192/60, 60/437, 475/86, 475/150, 475/89, 74/650, 477/36
• US Class Current: 701/69
• CPC Class Codes: B60K 23/0808 for varying torque distribution between driven axles, e.g. by transfer clutch

Abstract

In a control system for distributing a driving force from an engine between the front and rear wheels of a vehicle, a controller is designed to decrease the driving force transmitted to the secondary drive wheels which may be the front wheels by modifying a driving force distribution control characteristic when the vehicle is in a predetermined constant speed steady state, in order to improve the fuel economy in the steady state operation while maintaining a vivid vehicle response to a driver'"'"'s accelerator input. Preferably, the controller determines an offset quantity L which has at least a cubic term proportional to the third power of a vehicle speed when the predetermined steady state exists, and increases the driving force to the secondary wheels as an excess of the wheel speed difference over the offset quantity increases from zero.

Claims

1. A driving torque distribution control system for a vehicle, comprising:

a drive system for distributing a driving torque from a prime mover of the vehicle between a primary drive axle and a secondary drive axle of the vehicle, said drive system comprising an actuator for varying a driving torque distribution ratio between the primary drive axle and the secondary drive axle in response to a control signal;

first sensing means for determining a wheel speed difference between a primary drive wheel speed of the primary drive axle and a secondary drive wheel speed of the secondary drive axle;

second sensing means for detecting a constant speed running state of the vehicle, the second sensing means comprising a vehicle condition sensor for sensing a vehicle operating condition, wherein the second sensing means detects the constant speed running state in accordance with the sensed vehicle operating condition; and

controlling means for receiving the wheel speed difference as an input signal from the first sensing means, for producing the control signal for the actuator in response to the input signal in accordance with one of a normal control characteristic and a modified control characteristic, and for selecting one of the normal and modified control characteristics in response to a condition signal supplied from the second sensing means, wherein in the modified control characteristic a share of the driving torque to the secondary drive axle is decreased, with respect to the normal control characteristic,

wherein said second sensing means includes a means for producing the condition signal which is in a first signal state when the vehicle is accelerated during a straight ahead operation, and in a second signal state when said constant speed running state is detected during straight ahead operation of the vehicle, and said controlling means produces said control signal according to said normal control characteristic when said condition signal is in said first signal state, and according to said modified control characteristic when said condition signal is in said second signal state, and

wherein a magnitude of said control signal is increased to increase the share of the driving torque to the secondary drive axle as the wheel speed difference increases, and a rate of increase of the magnitude of said control signal with respect to said wheel speed difference is smaller in said modified control characteristic than in at least a portion of said normal control characteristic.

2. A control system according to claim 1, wherein the controlling means comprises a control characteristic modifying means for determining a modification quantity in accordance with the vehicle operating condition sensed by the vehicle condition sensor, and for modifying the normal control characteristic in accordance with the modification quantity to provide the modified control characteristic.

3. A control system according to claim 2, wherein the vehicle operating condition sensed by the vehicle condition sensor is one of a vehicle speed, a time rate of change of the vehicle speed, and an operating condition of one of an accelerator system and a brake system of the vehicle for increasing and decreasing a vehicle speed of the vehicle.

4. A control system according to claim 1, wherein the controlling means comprises a control characteristic modifying means for determining a modification quantity, and for modifying the normal control characteristic in accordance with the modification quantity to provide the modified control characteristic, and wherein the controlling means includes a means for holding the control signal at a minimum value when the wheel speed difference is lower than the modification quantity, and increasing the control signal from the minimum value in accordance with the wheel speed difference when the wheel speed difference is higher than the modification quantity.

5. A control system according to claim 4, wherein the vehicle operating condition sensed by the vehicle condition sensor is one of a vehicle speed of the vehicle, a wheel speed, a longitudinal acceleration of the vehicle, a condition of an accelerator system of the vehicle and a condition of brake system of the vehicle.

6. A control system according to claim 4, wherein the vehicle condition sensor is a longitudinal acceleration sensor for sensing a longitudinal acceleration of the vehicle, and the second sensing means includes a means for determining that the constant speed running state exists when the longitudinal acceleration is lower than a predetermined acceleration threshold value.

7. A control system according to claim 1, wherein said rate of increase of the magnitude of said control signal with respect to said wheel speed difference in said modified control characteristic is smaller than that of said normal control characteristic in a low wheel speed difference range in which said wheel speed difference is positive and equal to or smaller than a predetermined threshold, said wheel speed difference being a difference resulting from subtraction of said secondary drive wheel speed from said primary drive wheel speed.

8. A control system according to claim 7, wherein said rate of increase of the magnitude of said control signal with respect to said wheel speed difference in said modified control characteristic is equal to zero in said low wheel speed difference range.

9. A control system according to claim 1, wherein said controlling means comprises a control signal generating means for producing said control signal according to said one of the normal control characteristic and the modified control characteristic, and for holding the magnitude of said control signal equal to or lower than a predetermined control level until said wheel speed difference exceeds a predetermined wheel speed difference threshold, and a control characteristics modifying means for increasing said predetermined wheel speed difference threshold when said condition signal is in said second signal state.

10. A control system according to claim 9, wherein said control characteristic modifying means includes a means for determining a modification quantity in accordance with the vehicle operating condition sensed by the vehicle condition sensor so that said modification quantity is increased when a vehicle speed of the vehicle is increased, and for setting said predetermined wheel speed difference threshold equal to said modification quantity when said condition signal is in said second signal state.

11. A driving torque distribution control system for a vehicle, comprising:

a drive system for distributing a driving torque from a prime mover of the vehicle between a primary drive axle and a secondary drive axle of the vehicle, said drive system comprising an actuator for varying a driving torque distribution ratio between the primary drive axle and the secondary drive axle in response to a control signal;

first sensing means for determining a wheel speed difference between a primary drive wheel speed of the primary drive axle and a secondary drive wheel speed of the secondary drive axle;

second sensing means for detecting a constant speed running state of the vehicle, the second sensing means comprising a vehicle condition sensor for sensing a vehicle operating condition to detect the constant speed running state of the vehicle, and a constant speed running condition detecting means for detecting the constant speed running condition in accordance with the vehicle operating condition;

controlling means for receiving the wheel speed difference as an input signal from the first sensing means, producing the control signal for the actuator in response to the input signal by using a predetermined control characteristic between the control signal and the input signal, and modifying the predetermined control characteristic to decrease a share of a driving torque to the secondary drive axle when the constant speed running state is detected, the controlling means comprising a control characteristic modifying means for determining a modification quantity, and for modifying the predetermined control characteristic in accordance with the modification quantity, and a means for holding the control signal at a minimum value when the wheel speed difference is lower than the modification quantity, and increasing the control signal from the minimum value in accordance with the wheel speed difference when the wheel speed difference is higher than the modification quantity,

wherein the vehicle condition sensor is a longitudinal acceleration sensor for sensing a longitudinal acceleration of the vehicle, and the constant speed running condition detecting means includes a means for determining that the constant speed running state exists when the longitudinal acceleration is lower than a predetermined acceleration threshold value; and

a vehicle speed sensor for sensing a vehicle speed of the vehicle, and the control characteristic modifying means having a means for determining the modification quantity which comprises a cubic term proportional to the third power of the vehicle speed.

12. A control system according to claim 11 wherein the modification quantity is equal to a sum of the cubic term and a linear term proportional to the vehicle speed.

13. A control system according to claim 12 wherein the controlling means comprises a means for determining a control magnitude which is equal to a product obtained by multiplying a difference resulting from subtraction of the modification quantity from the wheel speed difference, by a control gain, and the modifying means includes a means for determining a basic offset quantity which is equal to a sum of the cubic term and the linear term, and making the modification quantity equal to the basic offset quantity when the longitudinal acceleration is equal to or lower than the predetermined acceleration threshold value, and equal to zero when the longitudinal acceleration is higher than the acceleration threshold value.

14. A control system according to claim 11 wherein the controlling means comprises a means for determining a control magnitude which is equal to a product obtained by multiplying a difference resulting from subtraction of the modification quantity from the wheel speed difference, by a control gain, wherein the modification quantity is equal to a product obtained by multiplying the third power of the vehicle speed by a predetermined first parameter, and wherein the modifying means comprises a learning means for determining the parameter by dividing an average wheel speed difference by the third power of an average vehicle speed while the longitudinal acceleration remains equal to or lower than the acceleration threshold value.

15. A control system according to claim 14 wherein the learning means comprise an average updating means for updating the average vehicle speed, an average primary wheel speed and an average secondary wheel speed while the longitudinal acceleration is equal to or lower than the threshold value, and a parameter updating means for updating the parameter by dividing the average wheel speed difference obtained by subtracting the average secondary wheel speed from the average primary wheel speed, by the third power of the average vehicle speed while the longitudinal acceleration remains equal to or lower than the threshold value.

16. A control system according to claim 11 wherein the drive system comprises a transfer comprising an input shaft connected with the prime mover, a first output shaft which is always connected with the input shaft, and further connected with the primary drive axle for driving primary drive wheels, and a second output shaft connected with the secondary drive axle for secondary drive wheels, and the actuator comprises a transfer clutch which is disposed between the second output shaft of the transfer and the input shaft of the transfer and which increases a clutch engagement force of the transfer clutch continuously from a fully disengaged state to a fully engaged state as the control signal increases.

17. A control system according to claim 16 wherein the primary drive wheels are left and right rear wheels of the vehicle, and the secondary drive wheels are left and right front wheels of the vehicle.

18. A vehicle comprising:

an engine;

a pair of primary drive wheels, and a pair of secondary drive wheels;

a drive system for transmitting a driving torque from the engine to the primary drive wheels and the secondary drive wheels, the drive system comprising a transfer comprising a transfer input member for receiving a driving torque from the engine, a first output member for driving the primary drive wheels, a second output member for driving the secondary drive wheels, and a transfer clutch disposed between the transfer input member and the second output member, the first output member being always drivingly connected with the transfer input member, the second output member being connected with the transfer input member through the transfer clutch;

an actuator for varying a clutch engagement force of the transfer clutch in response to a control signal;

a sensor group for determining a primary wheel speed of the primary drive wheels, a secondary wheel speed of the secondary drive wheels, and a vehicle speed of the vehicle; and

a controller for determining a wheel speed difference by subtracting the secondary wheel speed from the primary wheel speed, determining an offset quantity which is zero when the vehicle is out of a predetermined running state in which a time rate of change of the vehicle speed is equal to or lower than a predetermined threshold, and which is set equal to a quantity comprising a cubic term proportional to a third power of the vehicle speed when the vehicle is in the predetermined running state, determining a control magnitude which is a product obtained by multiplying, by a predetermined control gain, a difference resulting from subtraction of the offset quantity from the wheel speed difference, and producing the control signal representing the control magnitude.

19. A vehicle according to claim 18 wherein the vehicle further comprises a vehicle speed operating system for increasing and decreasing a vehicle speed of the vehicle, the vehicle speed operating system comprising an accelerator system for varying a vehicle speed of the vehicle by varying a supply of fuel to the engine, and a brake system for decreasing the vehicle speed;

the sensor group comprises a first sensor for sensing the primary wheel speed, a second sensor for sensing the secondary wheel speed, a third sensor for sensing the vehicle speed, and a fourth sensor for sensing a vehicle operating condition which is one of a longitudinal acceleration of the vehicle, and a condition in the vehicle speed operating system; and

the controller comprises a detecting means for monitoring an output signal of the fourth sensor to determine whether the predetermined running state exists.

20. A driving torque distribution control system for a vehicle, comprising:

a drive system for shifting a driving torque from a prime mover of the vehicle between a primary drive axle and a secondary drive axle of the vehicle in response to a control signal;

a constant speed detector for detecting a constant speed, straight ahead running state of the vehicle and for discriminating between the constant speed, straight ahead running state and an acceleration, straight ahead running state of the vehicle;

a controller for providing the control signal to the drive system, wherein the controller is configured to operate in one of a normal mode and a modified mode, and wherein a rate of shifting the driving torque to the primary drive axle from the secondary drive axle is greater in the modified mode than in the normal mode, and wherein the controller is configured to enter the modified mode in response to the constant speed detector detecting the constant speed, straight ahead running state, thereby improving a fuel economy of the vehicle; and

a wheel speed difference sensor for sensing a wheel speed difference between a first wheel speed corresponding to primary drive wheels and a second wheel speed corresponding to secondary drive wheels,

wherein, in the normal mode, the controller is configured to provide the control signal in accordance with a predetermined control characteristic based on the wheel speed difference, wherein, in the modified mode, the controller is configured to shift the predetermined control characteristic by an offset quantity in response to the constant speed detector detecting the constant speed, straight ahead running state, and wherein the offset quantity defines a dead band in which the controller increases the rate of shifting the driving torque to the primary drive axle from the secondary drive axle.

21. A control system according to claim 20, wherein the offset quantity is determined in accordance with a third power of the vehicle speed.

22. A control system according to claim 20, wherein the controller is configured to determine the offset quantity without regard to yaw rate of the vehicle.

23. A control system according to claim 20, wherein the controller is configured to determine the offset quantity only in accordance with a speed of the vehicle.

24. A control system according to claim 20, wherein the controller is configured to determine the offset quantity only in accordance with a speed of the vehicle and the wheel speed difference.

25. A method of controlling a vehicle, comprising the steps of:

(A) providing a drive system for distributing a driving torque from a prime mover of the vehicle between a primary drive axle and a secondary drive axle of the vehicle in response to a control signal;

(B) determining a wheel speed difference in accordance with a first wheel speed of primary drive wheels and a second wheel speed of secondary drive wheels;

(C) determining whether the vehicle is in one of a constant speed, straight ahead running state or an acceleration, straight ahead running state;

(D) determining an offset quantity in accordance with a first coefficient if the vehicle is determined to be in the constant speed, straight ahead running state and in accordance with a second coefficient if the vehicle is determined to be in the acceleration, straight ahead running state; and

(E) providing the control signal to distribute the driving torque in accordance with the offset quantity and the wheel speed difference,

wherein, in step (D), said first coefficient is one and said second coefficient is zero, and wherein the offset quantity is determined in accordance with a product of one of the first coefficient and the second coefficient, thereby defining a dead band in which the driving torque is fully shifted to the primary drive axle to improve a fuel economy of the vehicle.

26. A method of controlling a vehicle, comprising the steps of:

(A) providing a drive system for distributing a driving torque from a prime mover of the vehicle between a primary drive axle and a secondary drive axle of the vehicle in response to a control signal;

(B) determining a wheel speed difference in accordance with a first wheel speed of primary drive wheels and a second wheel speed of secondary drive wheels;

(C) determining whether the vehicle is in one of a constant speed, straight ahead running state or an acceleration, straight ahead running state;

(D) determining an offset quantity in accordance with a first coefficient if the vehicle is determined to be in the constant speed, straight ahead running state and in accordance with a second coefficient if the vehicle is determined to be in the acceleration, straight ahead running state; and

(E) providing the control signal to distribute the driving torque in accordance with the offset quantity and the wheel speed difference,

wherein, in step (D), the offset quantity is determined in accordance with a third power of the vehicle speed.

27. A vehicle comprising:

an engine;

a pair of primary drive wheels and a pair of secondary drive wheels;

a drive system for transmitting a driving torque from the engine to the primary drive wheels and the secondary drive wheels, the drive system having a transfer including a transfer input member for receiving a driving torque from the engine, a first output member for driving the primary drive wheels, a second output member for driving the secondary drive wheels, and a transfer clutch disposed between the transfer input member and the second output member, the first output member being always drivingly connected with the transfer input member, the second output member being connected with the transfer input member through the transfer clutch;

an actuator for varying a clutch engagement force of the transfer clutch in response to a control output signal;

a first sensor for sensing a first vehicle operating condition representing a primary drive wheel speed of the primary drive wheels;

a second sensor for sensing a second vehicle operating condition representing a secondary drive wheel speed of the secondary drive wheels;

a third sensor for collecting information on a longitudinal movement of the vehicle by sensing at least one third vehicle operating condition indicative of the longitudinal movement of the vehicle;

a controller for producing a control input signal representing a wheel speed difference between the secondary drive wheel speed and the primary drive wheel speed in response to signals supplied from the first and second sensors, for detecting a constant speed running state by monitoring the information supplied from the third sensor, for producing a constant speed condition signal when the constant speed running state is detected, for producing the control output signal representing the clutch engagement force of the transfer clutch in response to the control input signal in accordance with a control characteristic for increasing the clutch engagement force with increase in the wheel speed difference, and for decreasing the clutch engagement force in response to the constant speed condition signal by shifting the control characteristic by an offset quantity which is independent of a lateral acceleration of the vehicle.

28. A vehicle according to claim 27, wherein the controller determines, from the information supplied from the third sensor, a vehicle operating variable representing a longitudinal vehicle speed of the vehicle, and increases the offset quantity in accordance with the vehicle operating variable so that the offset quantity is increased above zero when the vehicle is moving straight ahead.