Steering system and method for independent steering of wheels

1. A steering system comprising:

a steering wheel adapted to receive a driver's rotational input;

left and right wheel units adapted to rollingly interface a vehicle to a ground for driving movement of the vehicle, each wheel unit being rotatable along a steering angle for adjusting a direction of the vehicle during the driving movement;

a steering mechanism for converting the rotational input to a variation of the steering angle of the wheel units, the steering mechanism comprising:

a steering shaft connected to the steering wheel,

gear steering units for each said wheel unit, the gear steering units connected to the steering shaft and each comprising a planetary gear system for receiving the rotational input from the steering shaft and for mechanically converting the rotational input to steering outputs for both said wheel units to concurrently vary the steering angle of said wheel units, and

independent steering units for each said wheel unit, the independent steering units each adjusting the steering angle of a respective one of the wheel units independently from the rotational input and from the other wheel unit by directly actuating a rotation of a ring of the planetary gear system.

2. The steering system according to claim 1 , wherein the gear steering units each comprise a rack and pinion assembly connected to the planetary gear system for transmitting the steering output to a respective wheel unit in a translational degree of freedom.

3. The steering system according to claim 2 , wherein a sun gear of the planetary gear system is secured to the steering shaft, and a pinion of the rack and pinion assembly is secured to a planet carrier of the planetary gear system.

4. The steering system according to claim 3 , wherein the steering shaft, the sun gears and the pinions rotate about a coincident axis.

5. The steering system according to claim 2 , wherein the independent steering units each have an electric motor outputting rotational motion to an independent-steer gear, with the independent-steer gear being meshed with the ring of the respective planetary gear system to adjust the steering angle of the respective wheel unit.

6. The steering system according to claim 2 , wherein the planetary gear systems are symmetrically similar.

7. A method for adjusting a steer angle of wheels of a vehicle comprising:

measuring a steering input from the driver of the vehicle and a vehicle velocity;

calculating a reference yaw rate for the vehicle from the steering input and from the vehicle velocity;

measuring an actual yaw rate for the vehicle;

determining a corrective value for a first wheel of the vehicle as a function of the reference yaw rate and the actual yaw rate, and of the steering input;

adjusting the steer angle of said first wheel by said corrective value, independently of an adjustment of steer angle of any other wheel of the vehicle.

8. The method according to claim 7 , wherein determining a corrective value comprises determining a corrective value for at least a second wheel, and adjusting the steer angle comprises adjusting the steer angle of said first wheel and said second wheel independently from one another.

9. The method according to claim 7 , wherein adjusting the steer angle comprises actuating an electric motor to mechanically adjust the steer angle of said first wheel.

10. The method according to claim 7 , wherein determining a corrective value comprises identifying said first wheel from a direction of said steering input and from an understeer/oversteer characteristic of the vehicle.

11. The method according to claim 10 , wherein identifying said first wheel comprises identifying the outer wheel as said first wheel for an understeer characteristic for the vehicle.