Calibration method for inter-axle differential control system

1. In a vehicle having steerable front wheels, an interaxle differential for transmitting torque from an engine to the front wheels and to rear wheels, the differential having a controllable device operable to control a ratio of front wheel speed to rear wheel speed, a front wheel speed sensor, a rear wheel speed sensor, a steering angle sensor, and a control unit for controlling the controllable device as a function of sensed steering angles and stored information including steering angles and corresponding front to rear wheel speed ratio values, a method of automatically calibrating the control unit comprising:

determining if the vehicle has been in a straight ahead steering mode for at least a certain time period;

determining if a speed of the vehicle is greater than a threshold speed;

replacing at least a portion of the stored information if the vehicle has been in said steering mode for at least said certain time period and if the speed of the vehicle is greater than said threshold speed;

generating an average steering angle value derived by averaging an old steering angle value with a current steering angle value, if the vehicle has been in a straight ahead travel mode for at least a certain time period; and

preventing replacement of the stored information if a magnitude of the average steering angle value is greater than a certain value.

2. The method of claim 1 , wherein:

the replaced information relates to a front to rear wheel speed ratio.

3. The method of claim 1 , further comprising:

periodically generating front to rear wheel speed ratio values from the speed sensors, the portion of the stored information being replaced comprises a new front to rear wheel speed ratio value which is an average value derived by averaging an old front to rear wheel speed ratio value with a current front to rear wheel speed ratio value.

4. The method of claim 1 , wherein:

periodically generating front to rear wheel speed ratio values from the speed sensor, the portion of the stored information being replaced comprises a new front to rear wheel speed ratio value which is a weighted average value derived by averaging a plurality of old front to rear wheel speed ratio values with a current front to rear wheel speed ratio value, wherein the old ratio value is weighted heavier than the current ratio value.

5. The method of claim 1 , wherein:

the predetermined steering mode is a turning steering mode.

6. The method of claim 5 , further comprising:

preventing said replacing if the controllable device is being operated.

7. The method of claim 5 , further comprising:

preventing said replacing unless a certain time period has expired.

8. The method of claim 5 , further comprising:

preventing said replacing unless a certain time period has expired; and

preventing said replacing if the controllable device is being operated.

9. In a vehicle having steerable front wheels, an interaxle differential for transmitting torque from an engine to the front wheels and to rear wheels, the differential having a controllable device operable to control a ratio of front wheel speed to rear wheel speed, a front wheel speed sensor, a rear wheel speed sensor, a steering angle sensor, and a control unit for controlling the controllable device as a function of sensed steering angles and stored information including steering angles and corresponding front to rear wheel speed ratio values, a method of automatically calibrating the control unit comprising:

periodically obtaining steering angle values, from the steering angle sensor;

periodically obtaining front and rear wheel speed values from the speed sensors;

periodically generating new front to rear wheel speed ratio values from the obtained speed values;

replacing one of the stored front to rear wheel speed ratio values with one of the new ratio values if the steering angle values indicate that the vehicle has been in a straight ahead travel mode for at least a certain time period; and

preventing replacement of the stored front to rear wheel speed ratio value unless the vehicle is traveling faster than a certain threshold speed.

10. The method of claim 9 , wherein:

the replaced value is a stored front to rear wheel speed ratio value corresponding to straight ahead travel.

11. The method of claim 9 , further comprising:

preventing replacement of the stored front to rear wheel speed ratio value if the controllable device is being operated.

12. In a vehicle having steerable front wheels, an interaxle differential for transmitting torque from an engine to the front wheels and to rear wheels, the differential having a controllable device operable to control a ratio of front wheel speed to rear wheel speed, a front wheel speed sensor, a rear wheel speed sensor, a steering angle sensor, and a control unit for controlling the controllable device as a function of sensed steering angles and stored information including steering angles and corresponding front to rear wheel speed ratio values, a method of automatically calibrating the control unit comprising:

determining if the vehicle has been in a predetermined steering mode for at least a certain time period;

determining if a speed of the vehicle is greater than a threshold speed;

replacing at least a portion of the stored information if the vehicle has been in said steering mode for at least said certain time period and if the speed of the vehicle is greater than said threshold speed; and

periodically generating front to rear wheel speed ratio values from the speed sensor, the portion of the stored information being replaced comprises a new front to rear wheel speed ratio value which is a weighted average value derived by averaging a plurality of old front to rear wheel speed ratio value with a current front to rear wheel speed ratio value, wherein the old ratio value is weighted heavier than the current ratio value.

13. In a vehicle having steerable front wheels, an interaxle differential for transmitting torque from an engine to the front wheels and to rear wheels, the differential having a controllable device operable to control a ratio of front wheel speed to rear wheel speed, a front wheel speed sensor, a rear wheel speed sensor, a steering angle sensor, and a control unit for controlling the controllable device as a function of sensed steering ankles and stored information including steering angles and corresponding front to rear wheel speed ratio values, a method of automatically calibrating the control unit comprising:

determining if the vehicle has been in a turning steering mode for at least a certain time period;

determining if a speed of the vehicle is greater than a threshold speed;

replacing at least a portion of the stored information if the vehicle has been in said steering mode for at least said certain time period and if the speed of the vehicle is greater than said threshold speed; and

preventing said replacing if the controllable device is being operated.

14. In a vehicle having steerable front wheels, an interaxle differential for transmitting torque from an engine to the front wheels and to rear wheels, the differential having a controllable device operable to control a ratio of front wheel speed to rear wheel speed, a front wheel speed sensor, a rear wheel speed sensor, a steering angle sensor, and a control unit for controlling the controllable device as a function of sensed steering angles and stored information including steering angles and corresponding front to rear wheel speed ratio values, a method of automatically calibrating the control unit comprising:

determining if the vehicle has been in a turning steering mode for at least a certain time period;

determining if a speed of the vehicle is greater than a threshold speed; and

replacing at least a portion of the stored information if the vehicle has been in said turing steering mode for at least said certain time period and if the speed of the vehicle is greater than said threshold speed; and

preventing said replacing unless a certain time period has expired.

15. In a vehicle having steerable front wheels, an interaxle differential for transmitting torque from an engine to the front wheels and to rear wheels, the differential having a controllable device operable to control a ratio of front wheel speed to rear wheel speed, a front wheel speed sensor, a rear wheel speed sensor, a steering angle sensor, and a control unit for controlling the controllable device as a function of sensed steering angles and stored information including steering angles and corresponding front to rear wheel speed ratio values, a method of automatically calibrating the control unit comprising:

determining if the vehicle has been in a turning steering mode for at least a certain time period;

determining if a speed of the vehicle is greater than a threshold speed;

replacing at least a portion of the stored information if the vehicle has been in said turning steering mode for at least said certain time period and if the speed of the vehicle is greater than said threshold speed;

preventing said replacing unless a certain time period has expired; and

preventing said replacing if the controllable device is being operated.

16. In a vehicle having steerable front wheels, an interaxle differential for transmitting torque from an engine to the front wheels and to rear wheels, the differential having a controllable device operable to control a ratio of front wheel speed to rear wheel speed, a front wheel speed sensor, a rear wheel speed sensor, a steering angle sensor, and a control unit for controlling the controllable device as a function of sensed steering angles and stored information including steering angles and corresponding front to rear wheel speed ratio values, a method of automatically calibrating the control unit comprising:

periodically obtaining steering angle values, from the steering angle sensor;

periodically obtaining front and rear wheel speed values from the speed sensors;

periodically generating new front to rear wheel speed ratio values from the obtained speed values;

one of the stored front to rear wheel speed ratio values with one of the new ratio values if the steering angle values indicate that the vehicle has been in a straight ahead travel mode for at least a certain time period; and

preventing replacement of the stored front to rear wheel speed ratio value if the controllable device is being operated.