1. Technical Field of the Invention
The present invention relates to a steering apparatus for a vehicle that directs steerable wheels in response to operator input in which the steerable wheels are not mechanically coupled to the manually steerable member.
2. Description of the Prior Art
A typical automotive vehicle is steered by transmitting operations of a manually steerable member, such as a steering wheel, to a steering mechanism for directing steerable wheels for steering. Generally, the manually steerable member is located inside the vehicle passenger compartment, and the steerable wheels are located at the front of the vehicle. Thus, a suitable steering mechanism is necessary to couple the manually steerable member and the steerable wheels.
A representative steering mechanism is a rack-and-pinion type steering mechanism. In a rack-and-pinion steering mechanism, the rotational motion of the steering wheel is communicated through a steering column to a pinion gear at its distal end. The pinion gear is engaged with a rack gear disposed laterally between the steerable wheels, which in turn are coupled to the rack gear by knuckle arms and tie rods. In this manner, rotation of the steering wheel is translated into the lateral movement of the rack gear, which causes the steerable wheels to pivot in the desired direction. In general, mechanical steering mechanisms are power-assisted by hydraulic or electrical assist units.
In order to overcome limitations presented by mechanical steering systems, it has been proposed to utilize a steering system in which the manually steerable member is not mechanically coupled to the steerable wheels and steering movement is achieved by an electrically controlled motor, a so-called steer-by-wire system. In a steer-by-wire system, a road wheel actuator operates in response to detected values of various steering parameters, such as for example steering wheel angle, vehicle speed, vehicle lateral acceleration, and road wheel angle. The detected values are communicated electronically to the road wheel actuator from sensors, or alternatively, from a centralized controller. Upon receipt and processing of the steering command, the road wheel actuator orients the steerable wheels in the desired direction in accordance with the vehicle steering parameters.
In order to provide a steering xe2x80x9cfeelxe2x80x9d to the vehicle operator, a typical steer-by-wire vehicle will also utilize a reaction torque generator that synthesizes and generates a reaction torque in the manually steerable member. For example, if the manually steerable member is a steering wheel, then the reaction torque generator will generally rotate a shaft coupled to the steering wheel in order to give the vehicle operator a resistive or assisting torque. In general, the magnitude and direction of the reaction torque will be determined by a control system cooperating between the reaction torque generator, the road wheel actuator, and the various vehicle sensing systems.
The adaptability of steer-by-wire systems to myriad situations provides a great number of advantages not apparent in a mechanically steered vehicle. In spite of these advantages, steer-by-wire vehicles are not dominating the current automotive marketplace. It is believed that a hybrid-type steering system is in order to ease the transition in the automotive market from mechanically coupled steering systems to steer-by-wire steering systems.
Accordingly, the present invention comprises a steering system selectively operable in one of three modes: steer-by-wire, electronic power assist steering (EPAS), and manual steering. The steer-by-wire system includes a driver interface system (DIS), a road wheel actuator system (RWAS), and a controller for monitoring and implementing the preferred control strategy. The control architecture of the present invention reduces the total number of sensors necessary to operate a steer-by-wire vehicle, consequently reducing the overall cost of the vehicle steering system.
The DIS includes a steerable member that is rotatable about a shaft. The rotation of the steerable member and the shaft is measured by a steering wheel angle sensor, disposed about the shaft. The shaft is coupled to a reaction torque generator for generating a steering feel based upon the applicable steering parameters, such as the vehicle speed, steering wheel angle, yaw rate, rack load, and lateral acceleration.
The RWAS includes a road wheel actuator responsive to control commands from the controller. The road wheel actuator is operatively coupled to a rack and pinion type steering system. Through the controller, the road wheel actuator rotates the pinion gear, which in turn causes the lateral motion of the rack gear thereby steering the road wheels. Like the DIS, the performance of the RWAS is monitored by a plurality of sensors.
The steering system of the present invention operates normally in a steer-by-wire mode, in which information regarding the angular position of the steerable member is combined with other pertinent information to compute a control signal, which the controller directs to the road wheel actuator. As noted, the road wheel actuator then mechanically steers the road wheels through the rack and pinion mechanism of the RWAS. However, the steering system is also adapted for operation in an electronic power assist steering (EPAS) mode and a manual mode in response to a malfunction in any part component of the DIS or RWAS subsystems.
In each of the EPAS mode and manual mode, the controller causes a clutch mechanism to engage, thus creating a mechanical linkage between the steerable member and the rack and pinion system. In the EPAS mode, one of the road wheel actuator or the reaction torque generator is available to assist in the steering operation. Alternatively, in the manual mode, both the DIS and the RWAS are deactivated and the vehicle is steerable through entirely mechanical means. In the event that the system power is cut off or the vehicle is not running, the steering system of the present invention is operable in the manual mode.