Abstract:
A method directed to improving the stability of a motor vehicle having front and rear steering capabilities includes determining a coefficient of friction of the road surface with which the motor vehicle is engaged and adjusting a phase gain function of a rear steering mechanism to compensate for the steerability of the motor vehicle over the road surface. A system for improving the stability of a motor vehicle having front and rear steering capabilities includes a control unit, a front steering mechanism in informational communication with the control unit, and a rear steering mechanism in informational communication with the control unit. The rear steering mechanism is responsive through the control unit to road conditions of the road surface.

Description:
BACKGROUND 
     Motor vehicles having four-wheel steering systems are typically defined by a set of front wheels that are steerable in unison with each other and a set of rear wheels that are likewise steerable in unison with each other. The control of the direction and angle of the steering movement of the rear steerable wheels is independent of the control of an operator of the motor vehicle and dependent upon the speed of travel of the motor vehicle. When steering the motor vehicle at low speeds, the rear wheels are generally steered out-of-phase or in the opposite direction of the front wheels, thereby enabling the motor vehicle to articulate turns of a small radius. When steering the motor vehicle at high speeds, the rear wheels are generally steered in-phase or in the same direction of the front wheels in order to impart increased stability to the motor vehicle during high speed maneuvering such as lane changes. 
     SUMMARY 
     A method and a system for improving the stability of a motor vehicle having front and rear steering capability is described herein. The method includes determining a coefficient of friction of a road surface with which the motor vehicle is engaged and adjusting a phase gain function of a rear steering mechanism to compensate for the steerability of the motor vehicle over the road surface. The system includes a control unit, a front steering mechanism in informational communication with the control unit, and a rear steering mechanism in informational communication with the control unit. The rear steering mechanism is responsive through the control unit to road conditions of the road surface with which the motor vehicle is engaged. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic representation of a motor vehicle having an anti-lock braking system and front and rear steerable wheels. 
     FIG. 2 is a schematic representation of an anti-lock braking system of a motor vehicle. 
     FIG. 3 is a graphical representation of the out-of-phase angle of a motor vehicle having front and rear steerable wheels as the out-of-phase angle relates to the velocity of the motor vehicle. 
    
    
     DETAILED DESCRIPTION 
     Referring now to FIG. 1, a steering system is shown generally at  10  and is hereinafter referred to as “system  10 .” System  10  is a incorporable into a motor vehicle  12  to provide directional control to motor vehicle  12  and is configurable to enable motor vehicle  12  to adapt to limit handling conditions caused by understeering and oversteering. System  10  comprises a front steering mechanism, shown generally at  14 , a rear steering mechanism, shown generally at  16 , and a control unit  18  disposed in intermediate and informational communication with front steering mechanism  14  and rear steering mechanism  16 . Although system  10  is described as being applicable to a rack and pinion steering arrangement, system  10  can be incorporable into other steering arrangements including, but not being limited to, integral gear steering systems. 
     Front steering mechanism  14  comprises a rack shaft  20 , a rack  22  disposed intermediate opposing ends of rack shaft  20 , a tie rod  24  disposed on each opposing end of rack shaft  20 , a knuckle arm  26  connected to each tie rod  24 , and a front steerable wheel  28  rotatably disposed on each knuckle arm  26 . Rack shaft  20 , tie rods  24 , and knuckle arms  26  are configured such that front steerable wheels  28  can be angled in unison relative to a body of motor vehicle  12  to steer or to effect a change in the direction of travel of motor vehicle  12  while motor vehicle  12  is moving. 
     Front steering mechanism  14  further comprises a mechanism through which an operator of can effectuate a desired change in the direction of travel of motor vehicle  12 . Such a mechanism comprises a steering column  30  disposed in operable communication at one end thereof with rack  22  through a pinion  32  and at an opposing end thereof with a steering device  34 . Steering device  34  may be a hand steering wheel. Manipulation of steering device  34 , e.g., rotation of the hand steering wheel, causes the axial rotation of steering column  30 , which in turn causes the rotation of pinion  32 . Rotation of pinion  32 , through the engagement of rack  22  and pinion  32 , effectuates the lateral translation of rack  22  relative to the body of motor vehicle  12 . The lateral translation of rack  22  causes front steerable wheels  28  to angle relative to the body of motor vehicle  12 , thereby altering the direction of travel of motor vehicle  12  while motor vehicle  12  is moving. 
     Rear steering mechanism  16  comprises a rack shaft  36 , a rack  38  disposed intermediate opposing ends of rack shaft  36 , a tie rod  40  disposed on each opposing end of rack shaft  36 , a knuckle arm  42  connected to each tie rod  40 , and a rear steerable wheel  44  rotatably disposed on each knuckle arm  42 . Rack shaft  36 , tie rods  40 , and knuckle arms  42  are configured such that rear steerable wheels  44 , like front steerable wheels  28 , can be angled in unison relative to the body of motor vehicle  12  to steer motor vehicle  12  upon lateral translation of rack  38 . 
     Rear steering mechanism  16  further comprises a mechanism through which rear steerable wheels  44  can be angled. Such a mechanism comprises a motor  46  operably connected to rack  38  through a drive mechanism  48 . Drive mechanism  48 , through a pinion  49 , transfers the rotational motion of a rotor shaft  47  of motor  46  to linear motion of rack  38 , which effectuates the lateral motion of rack shaft  36  and, ultimately, the angling of rear steerable wheels  44 . 
     Motor vehicle  12  is further provided with a steering sensor  50  for detecting an angular position of steering column  30 , a vehicle speed sensor  52 , and a rear rack shaft displacement sensor  58 . Rack shaft displacement sensor  58  detects the displacement of its corresponding rack shaft  36  from a reference position, which is the position in which each rear steerable wheel  44  is aligned and rotatable. Wheel rotation speed sensors  60  are disposed at each steerable wheel  28 ,  44  to detect the various angular speeds at which each steerable wheel  28 ,  44  rotates. The various angular speeds of steerable wheels  28 ,  44  are then used to apply braking pressure to each individual steerable wheel  28 ,  44  through a brake system, shown below with reference to FIG.  2 . 
     Control unit  18  is disposed in informational communication with the various systems of motor vehicle  12 . Control unit  18  receives informational signals from each of the systems, quantifies the received information, and provides an output response signal to rear steering mechanism  16  through motor  46 . The output to rear steering mechanism  16  is such that the out-of-phase gain is altered, thereby increasing motor vehicle stability on road surfaces having low coefficients of friction. A low coefficient of friction can be inferred by the actuation of the brake system or by differences in the rates of wheel spin as determined by wheel rotation speed sensors  60 . 
     Referring now to FIG. 2, the brake system and its incorporation into the motor vehicle is shown generally at  62 . Brake system  62  is configured to receive an input  64  from the operator of the motor vehicle through an anti-lock braking system (ABS) control unit  72  and control unit  18  and to provide an appropriate response to the motor vehicle. In brake system  62 , each front steerable wheel  28  and rear steerable wheel  44  is provided with a braking device, shown generally at  66 . Braking device  66  includes a rotor  68  disposed in a co-rotational relationship with its respective steerable wheel  28 ,  44  and an associated caliper  70 . Upon an application of hydraulic pressure to effectuate the actuation of calipers  70 , rotational motion of rotors  68  is arrested. Each individual caliper  70  can be selectively actuated by ABS control unit  72 . Moreover, each individual caliper  70  can be actuated in varying degrees, thereby providing improved braking ability and increased stability to the motor vehicle in the yaw directions. 
     Referring back to FIG. 1, upon actuation of ABS control unit  72 , the magnitude of the control signal transmitted to rear steering mechanism  16  is altered accordingly. In particular, upon actuation of ABS control unit  72 , control unit  18  infers that a road surface over which motor vehicle  12  is traveling has a surface having a low coefficient of friction. Control unit  18  then reduces the magnitude of the control signal to rear steering mechanism  16  in order to reduce the low speed out-of-phase gain function of the control signal, thereby reducing the effect of yaw moments on motor vehicle  12  and increasing its stability. Upon disengagement of the brakes under ABS control unit  72 , control unit  18  augments the magnitude of the control signal to rear steering mechanism  16  to restore the low speed out-of-phase gain function of the control signal to its pre-reduced state. 
     Alternately, control unit  18  may be configured to maintain the reduction of the magnitude of the control signal to rear steering mechanism  16  upon disengagement of the brakes under ABS control. In such an instance, motor vehicle  12  operates in an “enhanced control” mode wherein the low speed out-of-phase gain function of the control signal is maintained beyond the point at which motor vehicle  12  reacts to the limit handling condition that initially triggers operation of the directional control function provided by system  10 . The operation of motor vehicle  12  in the enhanced control mode effectuated by the reduction of the magnitude of the control signal to rear steering mechanism  16  can be maintained for a specified period of time or until system  10  is re-triggered. Control unit  18  can further be configured such that operation of motor vehicle  12  in the enhanced control mode can be cancelled at the option of the operator. 
     System  10  may also be configured such that a road surface defined by a low coefficient of friction could be inferred from variations in the wheel rotation speeds. Excessive variations in the speeds of rotation of steerable wheels  28 ,  44  is indicative of the existence of a potential slip condition in which motor vehicle  12  may be subject to undesirable yaw moments. In such an instance, signals received into control unit  18  from wheel rotation speed sensors  60  are interpreted such that the magnitude of the control signal to rear steering mechanism  16  is altered in order to properly adjust the low speed out-of-phase gain function. The inference of a surface having a low coefficient of friction by the detection of excessive variations in the wheel rotation speeds may be made in conjunction with the actuation of ABS control unit  72 . 
     Referring now to FIG. 3, a graphical interpretation of the rear-to-front ratio (R/F ratio) of the steerable wheels as it relates to the velocity of the motor vehicle is shown generally at  74  and is hereinafter referred to as “graph  74 .” The R/F ratio is defined as the ratio of the angle of the rear wheels to the angle of the front wheels. During out-of-phase steering (in which the rear wheels are steered in a direction opposite the direction of the front wheels), the R/F ratio is negative, and during in-phase steering (in which the rear wheels are steered in the same direction as the front wheels), the R/F ratio is positive. 
     Graph  74  is applicable to limit handling conditions in which the motor vehicle travels over surfaces having low coefficients of friction, as are indicated by the actuation of the ABS and/or the detection of excessive wheel spin. As stated above, travel of the motor vehicle across such surfaces while being subject to high amounts of out-of-phase gain may result in conditions of reduced yaw stability. In order to combat reduced yaw stability, the magnitude of out-of-phase steering at lower velocities is decreased and the velocity at which the change from out-of-phase steering to in-phase steering occurs is correspondingly reduced. The point at which such change occurs is defined as a crossover point  80 . At increasingly higher velocities, the reduced yaw stability is countered by increasing the magnitude of the in-phase gain. 
     In graph  74 , the R/F ratio as a function of the velocity of the motor vehicle is indicated by a curve, shown generally at  76 . At low velocity (on a surface of reduced friction), the out-of-phase steering gain as represented by the R/F ratio is generally large, as is shown by a portion  78  of curve  76 . At crossover point  80 , the R/F ratio is zero and for all angles of the front steerable wheels, the rear steerable wheels remain parallel to a longitudinal centerline of the motor vehicle. Upon operation of the motor vehicle at speeds on curve  76  beyond crossover point  80  and in an area defined by a portion  82  of curve  76 , the rear steerable wheels are angled to be in-phase, and improved yaw stability is realized. By reducing the out-of-phase gain as the motor vehicle travels over a surface having a reduced coefficient of friction at low velocities, particularly over portion  78  of curve  76 , the yaw stability of the motor vehicle can be improved during limit handling conditions. By increasing the amount of in-phase gain as the motor vehicle travels over a surface having a reduced coefficient of friction at higher velocities, the improved yaw stability of the motor vehicle can be further realized. 
     While preferred embodiments have been shown and described, various modifications and substitutions may be made thereto without departing from the spirit and scope of the invention. Accordingly, it is to be understood that the present invention has been described by way of illustration and not limitation.