The present invention relates to a system and method of controlling vehicle steer-by-wire systems to produce an adjustable steering feel for driver of a vehicle.
Steer-by-wire systems considered in this invention replace mechanical linkages between the steering wheel and the front road wheels with electrical wires and electronic components. Moreover, the mechanical linkages between the two front road wheels are eliminated. Instead, two independent actuators are installed on the vehicle, wherein each actuator independently actuates one of the front road wheels. This allows two front road wheels be able to move independently from each other. A steer-by-wire system such as described in this invention can be regarded as having two parts: a steering wheel sub-system and a road wheel sub-system. The electrical signals are translated through the wires to link the steering wheel sub-system to the road wheel sub-system with two independent front road wheels.
The steer-by-wire systems with two independent front road wheels are expected to provide a more flexible application environment to realize different left and right road wheel angle requirements such that the vehicle dynamics and safety can be improved. The expected steering functions including different left and right road wheel angle requirements can be realized by utilizing the control system design.
The control system designer for such steer-by-wire systems experiences several serious challenges. One of these challenges is to provide not only a familiar steering feel to the vehicle driver like that in the conventional steering wheel system with mechanical connection, but also flexible adjustable steering feels. The steering feel provides information of force (or torque) at the road wheel tire-road surface contact. This force information should be fed backed to the steering wheel to produce steering wheel torque felt by the vehicle driver. The vehicle driver relies on the steering feel to sense the force of road wheel tire-road surface contact and maintain control of the direction of the vehicle. Thus, the steering feel has been becoming one of most important vehicle attributes to maintain vehicle directional control and keep the vehicle stability.
Steering feel is determined by the torque reaction of the steering wheel. In the conventional steering system with mechanical connection, the vehicle driver holding the steering wheel can directly produce the torque reaction by steering wheel shaft to obtain information about contact conditions between the road wheel ties and road surface. In steer-by-wire system without mechanical connection, the electric motor is utilized to provide the reaction torque. In order to produce the required familiar and adjustable steering feels, the electric motor torque should be controlled. The steering wheel sub-system in steer-by-wire system is such an electric motor-based control system to implement the reaction torque control. Thus, a further challenge for the control system designers is how to describe the reaction torque quantitatively as the control system design specification and how to obtain the adjustable reaction torque based on the steering wheel sub-system assembly.
Furthermore, the steering wheel is desired to return to the center with the different rates if the hands of vehicle driver leave the steering wheel when the vehicle is driving with a certain speed. The steering wheel is also desired to stop in the certain angle if the hands of vehicle driver leave the steering wheel when the vehicle is driving without speed, such as in the parking state. A road wheel direction command also is required to implement in steering wheel sub-system. All required steering functions including the steering feel, steering wheel return and steering wheel stop, and a road wheel direction command should be achieved by using the steering wheel subsystem control design.
The present invention generally involves a system and method of controlling a steer-by-wire system to produce steering feel for the vehicle driver, wherein the system also provide the active steering wheel return and the steering wheel directional reference angle to road wheel sub-system. The steer-by-wire system in accordance with the present invention may be regarded as having two parts: a steering wheel sub-system and a road wheel sub-system with two independent front road wheels. Electrical signals are transmitted via electrical wires to link the steering wheel sub-system to the road wheel sub-system. Main functions of the steering wheel sub-system are to provide a steering directional reference angle, active steering wheel return, and produce an appropriate steering feel to a driver of the vehicle. Main functions of the road wheel sub-system are to establish tracking between road wheel angles and a steering wheel angle reference input which is provided by the steering wheel sub-system. These two subsystems are integrated in the steer-by-wire system to maintain alignment between the steering wheel and the road wheels of the vehicle and to implement vehicle steering functions.
A control system framework of the steering wheel sub-system is introduced to implement all required steering functions including the steering feel, active steering wheel return rate and steering stop, and steering wheel directional reference angle to road wheel sub-system. Such steering wheel control system includes three feedback loops: the inner torque loop, steering wheel rate feedback loop and steering wheel position feedback loop. In this invention, the driver""s action to turn the steering wheel is regarded as an equivalent external disturbance torque to the steering wheel control sub-system. The steering wheel control sub-system with the torque, rate and position feedback loops is designed according to control system stability and performance requirements. By automatically adjusting parameters of the steering wheel control sub-system related with vehicle variables, such as vehicle speed, the reaction torque will change with the vehicle variables and the loop feedback variables including the steering wheel angle, steering wheel rate and road wheel torque. As a result, the steering wheel control sub-system can produce a variable steering feel by generating the required variable reaction torque as feedback to the vehicle driver. At the same time, the steering wheel control sub-system also can provide the steering wheel return with the adjustable rate and steering wheel directional reference angle.
In present invention, the steering function requirements for the steering feel, steering wheel return and the road wheel directional command are involved into steering wheel system control design. In order to design the steering wheel control sub-system and satisfy the required adjustable steering feels, the steering wheel reaction torque in accord to steering feel is described quantitatively. The expected steering wheel reaction toque based on the steering wheel control sub-system variables which are related with steering wheel angle, road wheel torque, and vehicle speed can be obtained and determined using the quantitative steering reaction toque description. Thus, the steering feel can be treaded as a control system performance specification which may be used in the steering wheel control sub-system design.
The steering wheel control sub-system receives measured and estimated variables including left and right road wheel angles, road wheel torque, steering wheel angle, and vehicle speed. These variables are processed in the steering wheel controller to determine a reaction torque on the steering wheel. It is noted that there is no requirement to measure steering wheel torque on the steering wheel to produce the steering feel.
In the present invention, the expected adjustable steering feels, such as feels in sport cars or luxury cars, can be realized in the same vehicle with steer-by-wire system. The active steering wheel return with the different rotation rate and steering wheel stop in the parking state, and the directional angle reference generation to road wheel sub-system are also implemented by using control system design in the steer-by-wire system.