Patent Publication Number: US-2003230448-A1

Title: Vehicle steering system, steer feel control system and method for providing steer feel

Description:
FIELD OF THE INVENTION  
       [0001] This invention relates to a vehicle steering system, in particular, a steer-by-wire system having steer feel control, and to a method for providing steer feel.  
       BACKGROUND OF THE INVENTION  
       [0002] Traditionally, a driver steers a vehicle using a steering wheel that is mechanically linked to the steerable wheels of the vehicle. With the mechanical linkage, the driver, while turning the steering wheel, senses a steer feel that opposes the turning of the steering wheel. The steer feel provides the driver with a sense of the road conditions, such as the traction of the vehicle wheels with the road surface, and with some sense of the condition of the components of the steering system.  
       [0003] Recently, it has been proposed that direct mechanical linkages be replaced by steer-by-wire systems. In a steer-by-wire system, there is no mechanical linkage between the steering wheel and the steerable vehicle wheels. Rather, the angular position of the steering wheel, i.e., how much the steering wheel is turned, is determined by a sensor, and the sensor sends a signal to a controller to control an actuator, which steers the steerable wheels by a predetermined amount in accordance with the angular position of the steering wheel and often with other parameters related to the operating condition of the vehicle.  
       [0004] Steer-by-wire systems have a number of advantages. For example, a steer-by-wire system allows the elimination of a number of bulky and heavy mechanical components. In addition, the relationship between the angular position of the steering wheel and the steering angle of the steerable wheels can be adapted and optimized for different operating conditions of the vehicle.  
       [0005] In a steer-by-wire system, however, the driver does not sense the mechanical steer feel since there is no mechanical linkage between the steering wheel and the steerable vehicle wheels. Most drivers, accustomed to mechanical steering system, may find the lack of steer feel objectionable, since steer feel, as stated above, provides the driver with information on the operating condition of the vehicle, allowing the driver to make proper vehicle maneuvers.  
       [0006] A number of systems have been proposed to provide steer feel to the steering wheel in a steer-by-wire system. For example, an actuator, connected to the steering wheel, may be provided to apply a torque to the steering wheel in accordance with the operating condition of the vehicle. The torque is controlled to provide the driver with a sensation similar to the one he would experience while driving a vehicle having a conventional mechanical steering system.  
       SUMMARY OF THE INVENTION  
       [0007] Because of the importance of steer feel to the driver, it is essential that steer feel control systems operate with high reliability. The steer feel control systems, as well as methods for providing steer feel, of the present invention have a higher reliability than conventional steer feel control systems. In a conventional steer feel control system, there is no steer feel at all if the control system fails. In the steer feel control system of the present invention, on the other hand, a back-up system still provides some steer feel even if the control system fails to operate in the normal manner. In other words, with the steer feel control system of the present invention, it is much less likely that a driver would experience the disruption of the complete loss of steer feel.  
       [0008] In accordance with one aspect of the invention, a steering system for a vehicle includes a steering handle having a center position, a steer-by-wire system, and a steer feel control system. The steer-by-wire system senses the position of the steering handle and controls the steering angle of a steerable vehicle wheel as a function of the position of the steering handle. The steer feel control system applies a force to the steering handle and includes a spring that biases the steering handle towards to the center position.  
       [0009] In accordance with another aspect of the invention, a steering system for a vehicle includes a steering handle having a center position, a steer-by-wire system, and a steer feel control system. The steer-by-wire system senses the position of the steering handle and controls the steering angle of a steerable vehicle wheel as a function of the position of the steering handle. The steer feel control system includes a motor operatively associated with the steering handle and has first and second modes of operation. In the first mode, the steer feel control system applies a force to the steering handle as a function of the vehicle&#39;s operating condition. In the second mode, the motor is shorted.  
       [0010] In accordance with yet another aspect of the invention, a system, used to provide steering feel to a steering handle of a vehicle having a steer-by-wire system, includes a spring that is operatively associated with the steering handle. The spring biases the steering handle towards to the center position of the steering handle.  
       [0011] In accordance with still another aspect of the invention, a system, used to provide steering feel to a steering handle of a vehicle having a steer-by-wire system, includes a motor that is operatively associated with the steering handle and has first and second modes of operation. In the first mode, the steer feel control system applies a force to the steering handle as a function of the vehicle&#39;s operating condition, and in a second mode, the motor is shorted.  
       [0012] In accordance with a further aspect of the invention, a method, used to provide steering feel to a steering handle of a vehicle having a steer-by-wire system, includes the act of biasing the steering handle towards its center position using a spring.  
       [0013] In accordance with a yet further aspect of the invention, a method is used with a vehicle having a steer-by-wire system and a steer feel control system, to provide steering feel to the steering handle when the steer feel control system fails. The method includes shorting a motor of the steer feel control system to provide steer feel to the steering handle.  
       [0014] Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0015]FIG. 1 is a schematic drawing of a vehicle steering system of the present invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
     [0016]FIG. 1 illustrates a vehicle steering system of the present invention. The vehicle steering system  10  includes a steering handle  12 , a steer-by-wire system  20 , and a steer feel control system  30 . The steering system  10  controls the steering angle  23  of the vehicle&#39;s steerable wheel or wheels  22  in accordance with the position of the steering handle  12  and, preferably, other parameters related to the vehicle&#39;s operating condition.  
     [0017] The steering system  10  can be used in a vehicle of any suitable type. For example, it can be used in a passenger car, a truck, a recreational vehicle, an off-road vehicle, or a military vehicle. The vehicle may have any number of steerable wheels, such as one, two or four steerable wheels.  
     [0018] The steering handle  12  of the steering system  10  is used by the driver to steer the vehicle. The steering handle  12  has a center position  14 , and when the steering handle  12  is at the center position  14 , the steering angle  23  of the steerable wheels  22  is zero, and the vehicle travels along a straight line. A steering handle of the present invention can be a steering wheel that rotates with respect to its axis, and the angular position of the steering wheel is used to control the steering angle of the steerable wheels. Other types of steering handles may also be used in a steering system of the present invention, including a steering handle that moves linearly.  
     [0019] The steer-by-wire system of the present invention may be any suitable steer-by-wire system. The steer-by-wire system  20  illustrated in FIG. 1, for example, includes a position sensor  24  that senses the position of the steering handle  12 , an actuator  26  for adjusting the steering angle  23  of the steerable wheels  22 , and a controller  28  that controls the actuator  26  to adjust the steering angle  23  of the steering wheels  22  in accordance with the position of the steering handle  12 . The actuator  26  may be an electric, hydraulic or pneumatic actuator, and it may also be a linear or rotary actuator. Preferably, other parameters related to the vehicle&#39;s operating condition are also used by the controller  28  to control the steering angle  23 . These parameters can be provided by one or more suitable sensors (not shown). In some cases, the actuator  26  does not directly steer the steerable wheels  22 . Instead, a gearbox  27 , or a similar mechanism, is placed between the actuator  26  and the steerable wheels. The gearbox  27  can be used to increase the force or torque generated by the actuator  26 , allowing the use of a smaller actuator.  
     [0020] The steer feel control system  30  includes one or more sensors  32  for sensing parameters related to the operating condition of the vehicle, one or more motors  34  operatively associated with the steering handle  12 , and a controller  36  that receives the signals from the one or more sensors  32  and controls the one or more motors  34  to apply a force to the steering handle  12 . The force is controlled to provide steer feel. The term “force” is broadly defined. If the steering handle is a steering wheel that rotates about its axis, the term “force,” as defined herein, means “torque.” If the steering handle has a linear movement, the term “force” has its conventional meaning.  
     [0021] The parameters sensed by the sensors  32  may include any parameters that may be useful to provide a realistic steer feel, such as the torque applied to the steerable wheels  22  by the steer-by-wire system  20 . The system  30  may include one motor, or two motors to provide for increased system safety. The two motors may be used at the same time, or alternatively, the second motor may be used only if the first motor fails. The motors may be the same type or different types and may have the same or different power ratings. The motors preferably are permanent magnet electric motors, although each motor may be of any suitable type, such as an electric motor of another type, a hydraulic motor, or a pneumatic motor.  
     [0022] The steer feel control system  30  preferably has two modes of operation. In the first mode, the controller  36  receives the signals from the sensors  32  and controls the motor  34  to apply a force to the steering handle  12  to provide steer feel. The system  30  enters the second mode in case of system failure when the system  30  can no longer provide proper steer feel. The system  30  may fail for a number of reasons. For example, one or more of the sensors  32  and motors  34  may fail, or the controller  36  may fail. However, if it has more than one motor, the system preferably does not enter the second mode when at least one motor is working properly to provide steer feel even if the other motors have failed.  
     [0023] In the second mode, the steer feel control system  30  preferably is configured in a suitable manner to provide some steer feel even though the system  30  can no longer control steer feel as it does in the first mode. In a preferred embodiment of the invention, this “some” steer feel is provided by shorting the motor  34  to provide “braking” (or damping). If the steer feel control system  30  uses a permanent magnet electric motor or a hydraulic or pneumatic motor, the definition of the term “shorting” is generally defined as follows. With respect to a permanent magnet electric motor, the term means shorting a winding (or windings) of the motor with or without a resistive circuit. If the electric motor is a three-phase motor, two windings of the motor may be shorted against each other. With respect to a hydraulic or pneumatic motor, the term generally means that a direct connection is provided between the inlet and outlet of the motor, using a tube having an orifice, for example, so that fluid flows out of the outlet, through the tube and the orifice, and into the inlet. Of course, any other means for creating a “braking” (damping) effect using the motor would also be considered a “shorting” of the motor.  
     [0024] In the second mode when the steer feel control system  30  fails, the steer feel provided by the system  30  is not as realistic as that provided in the first mode. Using a steer feel control system having a shorted permanent electric motor as an example, the steer feel provided in the second mode is a force that is proportional to the speed at which the steering handle  12  is moved (or a torque that is proportional to the speed at which the steering wheel is turned). In other words, the steer feel control system is functioning as a damper. Although this steer feel is not as realistic as that provided in the first mode, it is still a significant improvement over conventional steer feel control systems which provide no steer feel at all when the system fails.  
     [0025] In a steer feel control system that uses a permanent magnet electric motor, the motor is preferably shorted using a resistive circuit, and the resistance of the resistive circuit may be adjusted during operation in accordance with the operating condition of the vehicle to provide a more realistic steer feel. If the steer feel control system uses a three-phase motor, two windings of the motor preferably are shorted against each other with a resistive circuit, and the resistance of the resistive circuit may be similarly adjusted during operation. If the steer feel control system uses two or more motors, only the windings of the motor that failed last preferably is shorted, although the windings of more than one motor may be shorted.  
     [0026] In a steer feel control system that uses a hydraulic or pneumatic motor, the motor may be shorted in the second mode by connecting the inlet and outlet of the motor using a tube having an orifice. The size of the orifice can be adjusted during operation in accordance with the operating condition of the vehicle to provide a more realistic steering feel.  
     [0027] In addition to, or as an alternative to, the shorted motor, a solution is proposed to arrange a spring in series or in parallel with the motors. In other words, the steer feel control system  30  may include a spring  38  connected to the steering handle  12 , which spring may be arranged in series or in parallel with the motors  34  of the steer feel control system  30 . The solution to arrange the spring in parallel is a good approach. The solution to arrange the spring in series, in some cases, may be more difficult technically, because the motor may need to constantly work against the flexible spring during normal operation. The spring  38  applies a force to the steering handle  12  that biases the steering handle  12  to its center position  14 . In other words, the spring force is proportional to the position of the steering handle  12  and tends to return the steering handle  12  to its center position  14 . In this way, the spring  38  provides a steer feel even when the steer feel control system  30  fails. The spring  38  preferably generates sufficient torque to overcome any friction in the system. In some embodiment, the type of spring may be any mechanical combination of mechanical spring along with transmission to attain the required level of force or moment.  
     [0028] In addition to the spring  38 , the steer feel control system  30  may include a mechanical damper  40  that is arranged in parallel with the spring  38 . The mechanical damper  40  may include a certain amount of hysteresis, such as coulomb friction, to simulate the effect of hysteresis in a conventional steering system. The existence of hysteresis in a conventional steering system is evidenced by the fact that when it is turned in one direction and then released, the steering wheel will move towards the center position but will stop short of the center position. This phenomenon is especially evident when the vehicle is operated at a low speed.  
     [0029] In operation, the steer feel control system  30  normally operates in the first mode, in which the controller  36  of the system  30  receives signals related to the operating condition of the vehicle from the sensors  32  of the system  30  and controls the motor  34  of the system  30  to provide steer feel to the steering handle  12 . If the system  30  fails, the system  30  enters the second mode, in which the motor  34  of the system  30  may be shorted to provide some steer feel to the steering handle  12 . The shorted motor  34  provides a force to the steering handle  12 , which force is proportional to the speed, at which the steering handle  12  is moved. The motor  34  may be shorted using a resistive circuit, and the resistance of the circuit may be adjusted in accordance with the operating condition of the vehicle. The system  30  may include a spring  38  that is operatively associated with the steering handle  12  and biases the steering handle  12  towards to the center position  14 . In addition to the spring  38 , a mechanical damper  40  may be provided, which provides a damping force to the steering handle  12 . The system  30  may return to the first mode, when the system  30  recovers and begins to operate properly.  
     [0030] The foregoing description constitutes the preferred embodiments devised by the inventors for practicing the invention. It is apparent, however, that the invention is susceptible to modification, variation and change that will be obvious to those skilled in the art. Inasmuch as the foregoing description is intended to enable one skilled in the pertinent art to practice the invention, it should not be construed to be limited thereby but should be construed to include such aforementioned obvious variations and be limited only by the proper scope or fair meaning of the accompanying claims.