This invention relates to methods and systems for independently electronically controlling the steering of vehicle wheels.
Today""s steering systems need improvements in several areas. For example, the front wheels of a vehicle are typically linked together mechanically so that each wheel turns at the same radius as the other wheel. One known disadvantage of this kind of system is the occurrence of tire scrub. That is, during turning of the vehicle the outer wheel rolls over the ground while the inner wheel rubs against the ground. Such action accelerates wear on the tires.
Another disadvantage of this kind of system is the occurrence of toe-in. Toe-in changes occur with time due to compliance and wear and results in the front wheels not being properly aligned with the rear wheels. That is, the front wheels are not both pointed straight ahead. This symptom can only be adjusted by taking the vehicle into a vehicle care center for manual adjustments.
Furthermore, steering systems employing hydraulic power assist also have a disadvantage. The hydraulic pump only needs to be activated when turning corners at slow speed, which is a relatively small proportion of the driving time. However, fuel is consumed continually to drive the hydraulic pump during power-on of the vehicle. Thus, a substantial amount of hydraulic loss occurs.
It is desirable to overcome the above listed disadvantages with a steering system that minimizes hydraulic loss, tire scrub, and toe-in to optimize tire life.
A method and system for independently controlling the steering of at least two wheels of a vehicle is achieved by sensing a desired amount and direction of turning of the vehicle via a steering angle sensor. An electronic control unit (ECU) determines a first desired turning angle for one of the wheels and a second desired turning angle for the other one of the wheels based on the desired amount and direction of turning of the vehicle. These turning angles may or may not be the same. The ECU then controls a turning mechanism associated with each of the wheels based on the first and second desired turning angles so that each wheel is turned independently of the other wheel.
A vehicle speed sensor attached to a transmission output shaft senses a speed of the vehicle for use in determining desired power assist to be applied to each of the wheels along with the desired amount of turning of the vehicle. Again, these torque inputs to the wheels may or may not be the same.
Furthermore, each of the wheels may include a valve stem attached thereto having a one-way check valve disposed therein for allowing air to flow into the wheel. A pressure sensor is also disposed in the valve stem for sensing a pressure in each of the wheels. If the pressure in either of the wheels is less than a desired amount of pressure, the ECU controls the one-way check valve to enable air flow from an air supply into either of the wheels.
Tire pressure, along with wheel speed, may be monitored as part of the alignment algorithm. Upon determining that the pressure between each of the wheels match and that the wheel speed between each of the wheels over a relatively short distance also match, an alignment is performed if the wheel speed starts to differ between each of the wheels over a longer distance. If an alignment is needed, the ECU controls the distance between each of the wheels via the corresponding turning mechanisms.
The turning mechanism may include a rack and pinion steering system having a rack and pinion as well as a motor, in which case the ECU controls the distance between the wheels by controlling a position of the rack relative to the pinion via the motor. Alternatively, in a worm-gear steering system having a lead screw, the ECU controls the distance between the wheels by controlling a position of the lead screw.