Method and system for controlling the stability and yaw response of a vehicle by locking a differential at elevated speeds

1. A method for increasing medium to high speed stability and for controlling a transient yaw response of a road vehicle as a result of steering input, said vehicle being equipped with a front axle, a rear axle, a controllable differential and a control unit arranged for locking and unlocking said differential, said method comprising:

selectively locking or unlocking said differential depending on an operation of said vehicle, including at least the sub-steps of:

measuring at least the longitudinal vehicle speed;

comparing the measured vehicle speed with a predetermined first reference speed; and

locking said differential if said measured vehicle speed exceeds said first reference speed;

comparing said measured vehicle speed with a predetermined further reference speed if the differential is in a locked condition, said further reference speed being lower than said first reference speed; and

unlocking said differential if the measured vehicle speed is lower than said further reference speed.

2. The method according to claim 1 , further comprising:

setting said first reference speed to a value within the interval 80-110 km/h.

3. A method for increasing medium to high speed stability and for controlling a transient yaw response of a road vehicle as a result of steering input, said vehicle being equipped with a front axle, a rear axle, a controllable differential and a control unit arranged for locking and unlocking said differential, said method comprising:

selectively locking or unlocking said differential depending on a steering operation of said vehicle, including at least the sub-steps of

measuring at least the longitudinal vehicle speed,

comparing the measured vehicle speed with a predetermined first reference speed, and

locking said differential if said measured vehicle speed exceeds said first reference speed;

and in the event that the measured vehicle speed does not exceed said first reference speed,

comparing the measured vehicle speed with a predetermined second reference speed,

measuring a front wheel steer angle of the vehicle,

comparing the front wheel steer angle with a predetermined reference value of the front wheel steer angle value,

measuring a front wheel steer angle rate of the vehicle,

comparing the front wheel steer angle rate with a predetermined reference value of the front wheel steer angle rate value, and

locking said differential if the measured vehicle speed exceeds said second reference speed and also any one of the following conditions:

i) the measured front wheel steer angle exceeds said predetermined reference value of the front wheel steer angle, or

ii) the measured front wheel steer angle rate exceeds said predetermined reference value of the front wheel steer angle rate, or

iii) the measured front wheel steer angle exceeds said predetermined reference value of the front wheel steer angle and the measured front wheel steer angle rate exceeds said predetermined reference value of the front wheel steer angle rate.

4. The method according to claim 3 , further comprising:

setting said second reference speed to a value within the interval 60-80 km/h.

5. The method according to claim 3 , further comprising:

setting said reference value of the front wheel steer angle rate to a value within the interval 20-50 degrees/s.

6. The method according to claim 3 , further comprising:

measuring the longitudinal vehicle speed and comparing said speed with a predetermined further reference value if the differential is in a locked condition;

unlocking said differential if the measured vehicle speed is lower than said further reference value and if a front wheel steer angle is higher than a reference front steer angle.

7. The method according to claim 6 , further comprising:

setting said further reference value to a value within the interval 55-80 km/h.

8. A system for increasing medium to high speed stability and controlling a transient yaw response of a road vehicle as a result of steering input, where said vehicle is equipped with a front axle, a rear axle, a controllable differential and actuation means arranged for locking and unlocking said differential, the system comprising:

a control unit configured to control said actuation means for selectively locking or unlocking said differential depending on a steering operation of said vehicle,

wherein the control unit is configured to measure at least the longitudinal vehicle speed to compare the measured vehicle speed with a predetermined first reference speed, and to lock said differential if said measured vehicle speed exceeds said first reference speed,

wherein the control unit is also configured to, in the event that the measured vehicle speed does not exceed said first reference speed, compare the measured vehicle speed with a predetermined second reference speed,

wherein the control unit is further configured to measure a front wheel steer angle rate of the vehicle, compare the front wheel steer angle rate with a predetermined reference value, and measure a front wheel steer angle of the vehicle and comparing the front wheel steer angle with a predetermined reference value, and

wherein the control unit is yet further configured to lock said differential if the measured vehicle speed exceeds said second reference speed and also any one of the following conditions:

i) the measured front wheel steer angle exceeds said predetermined reference value of the front wheel steer angle, or

ii) the measured front wheel steer angle rate exceeds said predetermined reference value of the front wheel steer angle rate, or

iii) the measured front wheel steer angle exceeds said predetermined reference value of the front wheel steer angle and the measured front wheel steer angle rate exceeds said predetermined reference value of the front wheel steer angle rate.

9. A vehicle comprising a system according to claim 8 .

10. A non-transitory computer readable medium having recorded thereon a computer program comprising program code that, upon execution by a processor of a computer, causes the computer to perform the steps of claim 1 .

11. A control unit for controlling the stability and yaw response of a vehicle, the control unit being configured to perform the steps of the method according to claim 1 .