Abstract:
A steering system includes a differential transmission having an input shaft and an output shaft, a rotary input device attached to the input shaft, a movable steering effector in operable communication with the output shaft, an angular-to-linear converter which converts a rotative motion of the output shaft to a linear motion that acts to move the steering effector. The motion of the steering effector enables path alteration of a vehicle. The steering system also includes an electromotive actuator in operable communication with the differential transmission. An activated electromotive actuator provides an active alteration of a speed change between a speed of the input shaft and a speed of the output shaft and a deactivated electromotive actuator provides a default speed change between the speed of the input shaft and the speed of the output shaft.

Description:
BACKGROUND OF THE INVENTION 
     The present disclosure relates generally to vehicle steering control systems and, more particularly, to a mechanically linked active steering system. 
     Conventional vehicular steering systems have an articulated mechanical linkage connecting an input device (e.g., steering wheel or hand-wheel) to a steering actuator (e.g., steerable road wheel). Even with power assisted steering in an automobile, for example, a typical hand-wheel motion directly corresponds to a resulting motion of the steerable road wheels, substantially unaffected by any assist torque. 
     However, for a vehicular steering system with active steering, such as that used in an automotive front-controlled steering system, a given motion of the hand-wheel may be supplemented by an additional motion, such as that from a differential steering actuator, which translates into a motion of the steerable road wheels that does not necessarily correspond to the given motion of the hand-wheel. Consequently, when the differential steering actuator is inactive, the motion of the steerable road wheels directly corresponds to the hand-wheel motion due to the articulated mechanical linkage, just as in conventional systems. 
     The term “active steering” relates to a vehicular control system, which generates an output that is added to or subtracted from the front steering angle, wherein the output is typically responsive to the yaw and/or lateral acceleration of the vehicle. It is known that, in some situations, an active steering control system may react more quickly and accurately than an average driver to correct transient handling instabilities. In addition, active steering can also provide for continuously variable steering ratios in order to reduce driver fatigue while improving the feel and responsiveness of the vehicle. For example, at very low speeds, such as that which might be experienced in a parking situation, a relatively small rotation of the hand-wheel may be supplemented using an active steering system in order to provide an increased steering angle to the steerable road wheels. 
     Prior devices act to modify the relationship between driver input and steering output by providing a supplemental power source within the steering system that actively augments the position of the wheels or acts to augment the control of the primary steering power source. Examples include (1) the addition of a second axially actuated device in addition to the primary axial translating device (e.g., hydraulic assisted steering rack), and (2) addition of a motor driven differential device between the operator and the steering valve of a typical hydraulic power steering system. In each case, additional power is added to the system through the added component to affect steering augmentation and in each case a portion of that power is transmitted to the operator as secondary feedback. Also noteworthy is the requirement in each case that the driver provide the upstream reaction to the system input in order for the desired steering change to be realized. Additionally, in the example number two, any lash in the differential will be directly felt by the operator. 
     Without operator reaction, most of the system input will be directed to the operator input device (i.e., steering wheel) and result in no change to the vehicle path. Conversely, steer-by-wire systems have the ability to directly control the primary steering actuator to affect the operator-to-steerable device kinematic relationship. However, steer-by-wire systems do not maintain a full-time mechanical link between the operator and the steerable device. 
     Thus, it is desirable to provide active steering orientation of the steerable device directly, as in by-wire systems, and maintain a mechanical link between the operator input and steerable device, as in prior active steer systems, while isolating the operator to some degree from such steerable device orientation modifications and associated feeback. 
     BRIEF DESCRIPTION OF THE INVENTION 
     Disclosed herein is a steering system including: a differential transmission having an input shaft and an output shaft; a rotary input device attached to the input shaft; a movable steering effector in operable communication with the output shaft; an angular-to-linear converter which converts a rotative motion of the output shaft to a linear motion that acts to move the steering effector, wherein the motion of the steering effector enables path alteration of a vehicle; and, an electromotive actuator, in operable communication with the differential transmission, wherein an activated electromotive actuator provides an active alteration of a speed change between a speed of the input shaft and a speed of the output shaft and a deactivated electromotive actuator provides a default speed change between the speed of the input shaft and the speed of the output shaft. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Referring to the exemplary drawings wherein like elements are numbered alike in the several Figures: 
         FIG. 1  shows a top level block diagram depicting an exemplary embodiment of an active steering system for a vehicle; and 
         FIG. 2  is a cross-section view to show the structure of an exemplary embodiment for a steering apparatus employed in the active steering system of  FIG. 1 . 
         FIG. 3  is a cross-section view of an alterative embodiment of the steering apparatus of  FIG. 2 . 
         FIG. 4  is a cross-section view of an alterative embodiment of the steering apparatus of  FIG. 2 . 
         FIG. 5  is a cross-section view of an alterative embodiment of the steering apparatus of  FIG. 2 . 
         FIG. 6  is a cross-section view of an alterative embodiment of the steering apparatus of  FIG. 2 . 
         FIG. 7  is a top schematic view of a parallel axis gear train of the steering apparatus of  FIGS. 5 and 6 . 
     
    
    
     DETAILED DESCRIPTION 
     Referring now to  FIG. 1 , an active steering system  100  is shown and discussed. The system utilizes an electromechanical actuator, in this instance, a steering actuator  110 , and a control unit  150  executing control algorithms, responsive to input signals including but not limited to vehicle speed, yaw and lateral acceleration sensors, hand wheel position, and motor position sensors to provide both variable ratio and stability control. Control unit  150  communicates with a storage medium through a data signal  62 . In an exemplary embodiment, variable ratio is a control algorithm configured to: reduce driver workload to maneuver vehicle, improve steering feel at various speeds and driving conditions, and change steering feel at all speeds based on the amount and rate of steering input. In yet another exemplary embodiment, stability control is a control algorithm configured to: reduce oversteer by leading the driver with countersteer, start vehicle correction before brake-based stability system is activated to reduce obtrusiveness of brake based stability systems, integrate with brake-based stability systems to allow optimization of brake and steering systems to reduce stopping distances. 
     The following description refers directly to the embodiment of  FIG. 1  and is not intended to limit the possible embodiments to this specific configuration. The active steering system  100  includes, but is not limited to a steering actuator  110 , with an electric motor  140  and gear train or differential shown generally at  130 . An input device, such as a steering wheel  120 , for operator input is coupled to a mechanical input of the steering actuator  110  to facilitate combination with the output of the electric motor  140 . Active steering system  100  further includes control unit  150  and various sensors, shown as A-E, interfaced with the control unit  150 . The various sensors are operatively coupled with control unit  150  to measure and transmit respective sensed parameters to the control unit  150 . 
     The steering actuator  110  is further coupled with a steering mechanism  160 , which includes a power assist mechanism  162  that transfers inputs to output shafts  164  providing a force assistance to oppose system and vehicle loads to aid an operator in achieving the desired output. The output shaft  164  is operatively connected to an output device, such as a steerable wheel (not shown), to direct the vehicle. Operator input is provided through steering wheel  120  that is connected to a steering column  122 . The steering column  122  is connected to the steering actuator  110  through an intermediate shaft  124 . The steering actuator  110  is connected to steerable wheels (not shown) through tie rods or output shafts and corresponding steering knuckles operably communicating with respective steerable wheels (not shown). 
     It will be appreciated that while in an exemplary embodiment as disclosed herein the power assisted steering mechanism utilizes a hydraulic configuration to provide assist torque, other configurations are possible for example an electric power assist could also be employed. Such a differential steering actuator is disclosed in U.S. patent application Ser. No. 09/812,240, U.S. Patent Publication No. 2002-0029922 A1 the contents of which are incorporated by reference herein in their entirety. An illustrative configuration that employs hydraulic assist may be found in U.S. Pat. No. 4,871,040 the contents of which are incorporated by reference herein in their entirety. An illustrative electric power steering system that provides assist torque to aid the driver is disclosed in U.S. Pat. No. 5,704,446 the contents of which are incorporated by reference herein in their entirety. 
     The steering actuator  110  is essentially a hydraulically assisted rack and pinion gear with an electric motor driven differential  130  embedded within the hydraulic circuit. The hydraulic system can be activated by both operator and motor inputs. Embedding the differential within the assist loop allows friction associated with the differential to have little impact on the steering torque felt by the operator as the assist function reacts most of this friction. Any lash present in the differential and any vibration generated by the differential during motor activation will be attenuated and filtered by the damping and compliance properties of the assist servo located between the differential and the operator. The differential could also be employed between the operator and the assist loop, however the added friction associated with the differential will have a direct affect on steering feel, any lash within the differential will be felt by the operator, and motor generated vibration can be more readily transmitted to the input device and operator. 
     The hydraulic valve details are as described in U.S. Pat. No. 4,871,040 with a differential input gear substituted in place of the helical pinion gear portion of the aforementioned valve assembly. 
     A hydraulic pump and a hydraulic reservoir (both not shown) are included to feed hydraulics in both the steering valve  182  and power assist steering mechanism  162 . An electronically controlled valve torque supplementing device may be included but not shown for control of steering valve  182  and power assist steering mechanism  162 . For example, U.S. Pat. No. 5,119,898, issued 9 Jan. 1992 describes a hydraulic power steering system manufactured by General Motors Corporation, and identified by the tradename MAGNASTEER™ including a steering gear in which an electromagnetic mechanism is selectively operable to vary the performance characteristics of a conventional proportional control valve of the steering gear. 
     Vehicle state sensors to monitor vehicle dynamic conditions (velocity, yaw, and the like) are in communication with actuator  110  via controller  150  and power distribution equipment to implement system functionality while a computer based database and algorithms define kinematic and kinetic relationship between operator input and steering orientation/effort based on vehicle state and sensor data. 
     An exemplary embodiment of steering actuator  110  includes a valve activated hydraulic power assist device  180  for orienting the steerable wheels and reacting to external loads. Differential device  130  is disposed between a steering valve  182  and the power assist device  180 . The differential  130  acts as a positive mechanical link between the operator and the steerable device. 
     Referring now to  FIG. 2 , steering actuator  110  is illustrated in greater detail. A related mechanism is disclosed in U.S. Pat. No. 6,135,233, the contents of which are incorporated by reference herein in their entirety. U.S. Pat. No. 5,265,019 differential discloses a specific simple epicyclic differential having a speed and torque change at the output with respect to the input, however, the differential is not delashed. 
     Steering actuator  110  includes a spool shaft  202 , which is connected to the intermediate shaft  124  (shown in  FIG. 1 ). Spool shaft  202  is connected to input shaft  204  over torsion bar  206 . An upper input shaft bearing  208  and a lower input shaft bearing  209  support input shaft  204 . Input shaft  204  is connected to input gear  210 , which in this embodiment is a ring gear. Input gear  210  is meshed with planetary gears  212  that rotate about planet shaft  214 . Planetary gears  212  are supported by planet bearings  216 , which can incorporate a delash gear. The configuration of an exemplary embodiment of the delash gear is also disclosed in copending U.S. patent application Ser. No. 10/868,612, entitled DOUBLE FLANK DELASH GEAR MECHANISM, filed Jun. 15, 2004 and incorporated herein by reference in its entirety. Low friction details at each end of a corresponding planet shaft are optionally included. 
     Planetary gears  212  are also meshed with a sun gear  220 , which in this embodiment is also operably connected to a worm gear  221 . Sun gear  220  and worm gear  221  are supported by bearings  222  and  224 . Worm gear  221  meshes with a worm  226  that is operably connected to motor  140 . Steering actuator  110  also includes a carrier  230  that carries planet pins  214  of planetary gears  212  and is connected to output shaft  231 , which includes a pinion  240 . Output shaft  231  is supported by bearings  242  and  244 . 
     During the default operation, which means there is no motor input, but with hydraulic assist, there is a default kinematic relationship between the input shaft  204  and the output shaft  231  in which there is a speed change between the input shaft and the output shaft. Referring to  FIGS. 1 and 2 , as the driver turns the hand wheel  120 , intermediate shaft  124  turns and so does spool shaft  202 . Spool shaft  202  and input shaft  204  are rotatable relative to each other over torsion bar  206 . Input shaft  204  rotates input gear  210 , which rotates planet gear  212 . Since motor  140  is not in operation, sun gear  220  is fixed in its current location. As planet gear(s)  212  are rotating about pins  214  and traversing along the outside of sun gear  220 . The rotation pushes pins  214  and carrier  230  which imparts rotation to output shaft  231  and pinion  240 , which then translates rack (not shown). 
     In one exemplary embodiment, the input to output ratio is about 3:2 and thus the output speed is reduced to about ⅔ of the input speed. In order to normalize the hand wheel motion to rack translation, pinion  240  is increased in size by a reciprocal proportion to the speed change. Thus, pinion  240  is about 1.5 times the size of a standard pinion to compensate for the 3:2 ratio through differential actuator  130 . The embodiment can further be described as the steering system including a planetary gear set having a gear ratio and an angular-to-linear converter, which for example may be a rack and pinion gear, having a translational ratio wherein the translational ratio neutralizes the effect of the planetary gear ratio. In addition, the input to output ratio can be any ratio between the theoretical limits 2:1 and 1:1. However, the larger the ratio, the larger the pinion will need to be to compensate for the default differential ratio. The smaller the ratio the greater the relative gear set size and motor speed required for any given ratio. Thus, in an exemplary embodiment, the ratio ranges from about 1.3 to about 1.7, which means that the speed reduces from the input shaft to the output shaft to about 0.8 to about 0.6 times. 
     In normal, powered operation, the kinematic relationship between the operator and the steerable wheels is continuously variable. Operator input (torque), position, and speed, derived from sensor(s), are input into controller  150  along with vehicle speed and yaw rate. A database of predefined relationships relating vehicle speed, and operator input position and speed are used to establish the desired steering ratio. A second database relating vehicle speed to maximum stable yaw rate may be used to invoke stability correction when the stable value is exceeded. The stability correction is in addition to the variable ratio to define the desired instantaneous overall steering ratio. The steering valve geometry and compliance establishes the basic relationship between operator torque and steering assist power. This basic relationship is modified whenever either a ratio modification and/or a stability correction, and/or supplemental torque is invoked. In this situation the power fluid flow to the power assist is either increased or decreased, per the controller, to establish the required steering modification. The steering modification is monitored by comparison of signals from position sensor A between the handwheel  120  and the differential  130 , and sensor B at the motor  140 . In addition, a position sensor C may also be located at the rack for diagnostic purposes or redundancy. This sensor may be a rotary or linear sensor activated by the rotation of the pinion or translation of the rack. 
     When the signals differ from the predefined relationship, the differential motor  140  is commanded to activate in proportion to the steering modification such that the motor driven differential input augments the valve output to the power assist while simultaneously modifying the input to output ratio. Supplemental torque is commanded in proportion to the motor command to counter the valve torque associated with motor input, resulting in reduction or elimination of torque modification feedback to the operator. Steering input position is also monitored with respect to motor commanded position to determine if the operator is providing reaction torque at the input device. This improves the overall control of the system and eliminates the possibility of a “run away” steering input device if the operator is not actively controlling the input, where the system could otherwise continue to command steering modifications that are not realized because the motor power is converted into steering input motion with no vehicle path change. 
     Referring to  FIG. 3 , an alternative embodiment of differential actuator  130  is illustrated. This embodiment is similar to the embodiment illustrated in  FIG. 2 ; however, the bearing positions are different. For instance, upper sun bearing  222  and lower sun bearing  224  are shown in a different location with respect to sun gear  220 . An optional bearing  250  redundantly keeps planetary gears  212  in the right position over carrier  230  and planetary pins  214 . 
     Referring to  FIG. 4 , an alternative embodiment of differential actuator  130  is illustrated. This embodiment is similar to the embodiment illustrated in  FIG. 2 ; however, the valve  182  is replaced with an electric power steering actuator  300 . Electric power steering actuator  300  has a motor  302  with a worm  304  and a worm gear  306 , which is operably joined to output shaft  231 . In addition, a torque sensor  310  is located at shaft  202 . All other reference numbers are the same as  FIG. 2 . 
     During a default operation, which means there is no motor  140  output, but with electric power assist from motor  302 , the actuator  110  operates the same as the actuator in  FIG. 2 . During assist operation, when shaft  202  rotates, torsion bar  206  twists and torque sensor  310  sends a signal to the control unit  150  (see  FIG. 1 ). The control unit  150  provides electric power to motor  302 , which rotates worm  304 , worm gear  306 , output shaft  231 , and pinion  240 , which then translates the rack (not shown). In the non-default case, sensors send signals to the control unit  150  and the control unit  150  then signals the motor  140  to turn on, which rotates worm  226  and sun gear  220  for rotation augmentation. 
     Referring to  FIGS. 5-7 , alternative embodiments of differential actuator  130  are illustrated. These embodiments are similar to the embodiment illustrated in  FIGS. 2 and 4 , respectively; however, the transmission mechanization between the motor  140  and the differential actuator  130  is different.  FIG. 5  illustrates the valve  182  (like  FIG. 2 ) and  FIG. 6  illustrates the torque sensor  310  (like  FIG. 4 ).  FIGS. 5-7  illustrate a parallel axis gear train  400  that conveys the desired torque and speed from the motor  140  to the sun gear  220 . The train  400  may employ spur gear or helical gear forms and may be configured to reduce lash. The gear train embodiment as shown comprises a motor shaft (not shown) with a motor shaft gear  402 . A stepped shaft  404  is supported by bearings  406  and has an upper gear  408  that meshes with the motor shaft gear  402  and a lower gear  410  that meshes with an idler gear  412 . Shaft assembly  414  is supported by a bearing  416  and retained with a nut  418 . An external gear  420  is fixed to the sun gear  220  and meshes with idler gear  410 . The idler gear  410  and stepped shaft gears are supported within the housing assembly by bearings. 
     Exemplary embodiments of the invention include an active steering system with variable assist. The system includes a differential actuator that has an input rotation through an input shaft and an output rotation through an output shaft. There is a default kinematic relationship between the input shaft and the output shaft such that there is a speed change. The speed change is a reduction in speed between the input shaft and the output shaft. In addition, there is a pinion in operable communication with the output shaft and the pinion is increased in size by a reciprocal or nearly reciprocal proportion to the speed change ratio. 
     In an exemplary embodiment, the speed of the output shaft to the speed of the input shaft ranges from about 0.6 about 0.8 times. The input gear is a ring gear. The differential actuator has rotatable mechanisms to vary the ratio between an input device, such as a hand wheel, and an output device, such as the steerable wheels. The differential actuator is disposed between the input device and the steering mechanism. A steering valve disposed between the input device and the differential actuator. The system further includes a controller in communication with the differential actuator, the steering valve and the controller are configured to provide a supplement torque to the input device, and the supplemental torque is in proportion to a motor current command. The system further includes a power assist mechanism coupled to the differential actuator and the output device, the power assist mechanism provides an assist torque responsive to activation of a valve or an electric power assist system. The system further includes a power assist mechanism coupled to the differential actuator and the output device, the power assist mechanism provides an assist torque responsive to a torque sensor. In addition, the input gear is delashed with a planet gear. The system further includes a controller that is configured to control the ratio between the input device and the output device. 
     There are numerous advantages to incorporating differential actuator  130 , which is a normalized speed changing differential, into an active steering system. Typically steering geometry is defined based on road load management, and a compromise between steering responsiveness and steering safety. Optimum steering component geometry is chosen such that road loads are efficiently and effectively reacted with minimal structure within available space and with a compromise responsiveness. An overall steering ratio (OSR) results that is a ratio of handwheel angle over corresponding roadwheel angle. In relative terms a low OSR results in a quick reacting steering system with high load reaction, and a high OSR results in a slower reacting steering system with lower load reaction. Active steering systems allow the OSR to be variable by providing additional steering inputs actively by, for example, an electric motor input to the differential. When this input is absent, the OSR will be a default relationship of (OSR without differential x differential ratio). Application of a speed changing differential with this optimized steering geometry will either alter the default steering behavior away from the optimum, or require modifications to the optimized steering geometry to retain equivalent default steering behavior. 
     The range of active OSR is defined to provide reduced operator work at low vehicle speed, and enhanced safety at high vehicle speed, with sufficient bandwidth for stability correction. The choice of default OSR has performance implications over the active OSR range as well as the default condition. Choosing a low default OSR will require minimum motor generated steering augmentation for low vehicle speed operation (low OSR), but significant motor generated augmentation for high vehicle speed operation (high OSR) and stability correction. Low default OSR choices are limited by the sensitivity of the operator to OSR changes associated with any system fault condition, where the OSR will change from some active OSR to the default OSR. 
     Choosing a high default OSR will require minimum motor generated steering augmentation for high vehicle speed operation (high OSR), but significant motor generated augmentation for low vehicle speed operation (low OSR). An active steering system with a speed changing differential with a pinion upsized with respect to an optimized non-active steering system pinion, by a ratio reciprocal to the speed change ratio, will normalize the effect of the differential ratio and provide a combination of optimum OSR and balanced actuator performance demands over the active steering OSR range. 
     It will be appreciated that the use of first and second or other similar nomenclature for denoting similar items is not intended to specify or imply any particular order unless otherwise stated. 
     While the invention has been described with reference to an exemplary embodiment(s), it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.