Abstract:
A hydraulic control circuit for a vehicle steering system is provided. The hydraulic control circuit is responsive to the rotational position of a steering wheel and controls the actuation of a steering gear assembly in response thereto. The hydraulic circuit includes a first actuator coupled to the vehicle steering wheel and a second hydraulic actuator coupled to the vehicle steering gear assembly. The first and second hydraulic actuators are coupled together by a pair of fluid conduits. Rotation of the steering wheel selectively causes the first actuator to force hydraulic fluid through one of the plurality of fluid conduits to cause the second hydraulic actuator to produce a corresponding output motion. The corresponding output motion is operable for actuating the steering gear assembly in a predetermined direction. The preferred embodiment includes a control system for improved monitoring and control of the hydraulic circuit, as well as an assist pump and compensation valve for gain control.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Technical Field  
           [0002]    The present invention relates generally to vehicle steering systems and more particularly to a vehicle steering system which is hydraulically actuated.  
           [0003]    2. Discussion  
           [0004]    The steering system of modern vehicles includes a steering gear assembly, a steering wheel and an intermediate shaft. The steering gear assembly may be of the manual type wherein the energy for turning the vehicle wheels is supplied by the vehicle operator through the steering wheel, or a power-assisted type where the energy for turning the vehicle wheels is provided by mechanical or hydro-mechanical devices. The intermediate shaft extends through the vehicle firewall and couples the steering wheel to the steering gear assembly.  
           [0005]    The placement of the steering gear assembly relative to the steering wheel and the vehicle engine typically complicate the packaging of the intermediate shaft into a vehicle. Frequently, one or more universal joints or knuckles are required to route the intermediate shaft through the engine compartment to the steering gear assembly. This design strategy, however, has several significant drawbacks.  
           [0006]    These drawbacks relate primarily to the effort required to package the intermediate shaft into a vehicle. In addition to the engineering efforts required to both identify potential interferences and to re-design the intermediate shaft or other vehicle components to eliminate the interference, the use of an intermediate shaft renders the installation and servicing of the engine and its related components more difficult.  
           [0007]    Another drawback inherent in the use of intermediate shafts is their tendency to transmit noise and vibration from the vehicle suspension system to the steering wheel. The drawbacks of the intermediate shaft are further magnified where the vehicle is to have a right-hand steering system option wherein the steering wheel is positioned on the right side of the vehicle. While the design of several of the steering system components may simply be mirrored, this is typically not true for intermediate shafts as interference points with the vehicle engine change.  
           [0008]    Consequently, there remains a need in the art for an improved vehicle steering system which eliminates the intermediate shaft.  
         SUMMARY OF THE INVENTION  
         [0009]    It is therefore one object of the present invention to provide a vehicle steering system which does not utilize an intermediate shaft to couple the vehicle steering gear assembly to the vehicle steering wheel.  
           [0010]    It is another object of the present invention to provide a vehicle steering system which includes a hydraulic circuit which controls the position of an output member of the steering system in relation to the rotational position of a steering wheel.  
           [0011]    It is a more specific object of the present invention to provide a vehicle steering system which utilizes hydraulic circuit having a pair of hydraulic actuators to control the steering gear assembly.  
           [0012]    A hydraulic control circuit for a vehicle steering system is provided. The hydraulic control circuit is responsive to the rotational position of a steering wheel and controls the actuation of a steering gear assembly in response thereto. The hydraulic circuit includes a first actuator coupled to the vehicle steering wheel and a second hydraulic actuator coupled to the vehicle steering gear assembly. The first and second hydraulic actuators are coupled together by a pair of fluid conduits. Rotation of the steering wheel selectively causes the first actuator to force hydraulic fluid through one of the plurality of fluid conduits to cause the second hydraulic actuator to produce a corresponding output motion. The corresponding output motion is operable for actuating the steering gear assembly in a predetermined direction. The preferred embodiment includes a control system for improved monitoring and control of the hydraulic circuit, as well as an assist pump and compensation valve for gain control.  
           [0013]    Additional advantages and features of the present invention will become apparent from the subsequent description and the appended claims, taken in conjunction with the accompanying drawings.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    [0014]FIG. 1 is a partial cut-away perspective view of a vehicle having a steering system constructed in accordance with the preferred embodiment of the present invention;  
         [0015]    [0015]FIG. 2 is a schematic diagram of the steering system of the present invention according to a presently preferred embodiment;  
         [0016]    [0016]FIGS. 3A-C are schematic diagrams of a portion of the hydraulic circuit of the present invention showing several configurations of the steering valve;  
         [0017]    [0017]FIGS. 3D-F are schematic diagrams of a portion of the hydraulic circuit of the present invention showing several configurations of the drive member;  
         [0018]    [0018]FIG. 4 is a schematic diagram of the steering system of the present invention according to a first alternate embodiment of the present invention;  
         [0019]    [0019]FIG. 5 is a schematic diagram of the steering system of the present invention according to a second alternate embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0020]    With reference to FIG. 1 of the drawings, the hydraulic steering system of the present invention is generally designated by reference numeral  10 . Hydraulic steering system  10  is shown in operative association with the suspension system  12  of vehicle  14 . Suspension system includes a frame rail  16 , control arms  18 , axle hub assemblies  20 , strut assemblies  22  and wheels  24  which are conventional in design. Wheel hub assemblies  20  are pivotably coupled to control arms  18  and operable between a first position and a second position. Operation of the wheel hub assemblies  20  between the first and second position controls the relative position of the wheels  24  to the frame rail  16  to thereby control the direction in which vehicle  14  is propelled or steered.  
         [0021]    With additional reference to FIG. 2, hydraulic steering system  10  is shown to include a steering gear assembly  28 , a hydraulic circuit  30 , a steering wheel assembly  32  and preferably, a control system  34 . Steering gear assembly  28  is shown to include an input member  36  and an output member  38 . Output member  38  is coupled to wheel hub assemblies  20   a  and  20   b  and operable between a first steering position and a second steering position for selectively positioning wheel hub assemblies  20  between the first and second positions, respectively. Input member  36  is operable for selectively positioning output member  38  between the first and second steering positions.  
         [0022]    As illustrated, steering gear assembly is a rack-and-pinion type steering gear assembly  40 . It will be understood, however, that the reference to a rack-and-pinion steering gear assembly is merely exemplary and that the present invention has applicability to other types of steering gear assemblies. Rack-and-pinion steering gear assembly  40  includes a housing  42 , a rack member  44  having a plurality of rack teeth  46  and an input shaft  48  having a pinion  50  with a plurality of pinion teeth  52 . Housing  42  is operable for axially supporting rack member  44 , rotatably supporting input shaft  48  and aligning rack member  44  relative to pinion  50  such that pinion teeth  52  meshingly engage rack teeth  46 .  
         [0023]    Rotation of input shaft  48  in a first rotational direction is operable for causing pinion  50  to move rack member  44  relative to housing  42  (e.g., in a direction which extends rack member  44  from housing  42 ) to position rack-and-pinion steering gear assembly  40  toward the first steering gear position. Continued rotation of input shaft  48  in the first rotational direction in a predetermined amount causes rack-and-pinion steering gear assembly  40  to be positioned in the first steering gear position. Similarly, rotation of input shaft  48  in a second rotational direction is operable for causing pinion  50  to move rack member  44  relative to housing (e.g., in a direction which retracts rack member  44  into housing  42 ) to position rack-and-pinion steering gear assembly  40  toward the second steering gear position. Continued rotation of input shaft  48  in the second rotational direction in a predetermined amount causes rack-and-pinion gear assembly to be positioned in the second gear position.  
         [0024]    Hydraulic circuit  30  includes a steering valve  66 , a drive member  68 , a reservoir  70 , an assist fluid pump  72 , a compensation valve  74  and a plurality of fluid conduits  78 . Steering valve  66  is operable for selectively regulating the flow of hydraulic fluid to drive member  68  in response to the rotational position of steering wheel assembly  32 . With brief additional reference to FIGS. 3A-C, steering valve  66  may include any type of hydraulic actuator which selectively transmits hydraulic fluid through fluid conduits  78   a  and  78   b  in response to the rotation of steering wheel assembly  32  such as a hydraulic motor  80 , a hydraulic cylinder  80 ′ or a rotac (rotary actuator)  80 ″.  
         [0025]    Preferably, steering valve  66  includes a hydraulic motor  80  having a housing  80   a  fixed to a stationary portion of the vehicle and a rotor portion  80   b  which is coupled for rotation with the shaft  32   a  of steering wheel assembly  32 . Rotation of steering wheel assembly  32  causes rotor portion  80   b  to rotate relative to housing  80   a . The relative rotation of rotor portion  80   b  creates a pumping action which forces hydraulic fluid through one of the fluid conduits  78   a  and  78   b  depending on the direction in which steering wheel assembly  32  is rotated. The amount of fluid pumped to drive member  68  is dependent upon the amount by which rotor portion  80   b  is rotated relative to housing  80   a.    
         [0026]    Drive member  68  includes an output drive member  82  which is operable in a first mode and a second mode. Output drive member  82  and input member  36  are coupled together and cooperate to position output member  38 . Operation of output drive member  82  in the first mode causes input member  36  to position output member  38  toward the first steering position and operation of output drive member  82  in the second mode causes input member  36  to position output member  38  toward the second steering position.  
         [0027]    Drive member  68  preferably includes a hydraulic motor  86  having a body portion  86   a  fixed to a stationary portion of the vehicle and an rotor portion  86   b  coupled for rotation with input shaft  48 . It will be understood, however, that the reference to a hydraulic motor is not intended to be limiting and as such, the scope of the present innovation includes drive members which utilize other hydraulic mechanisms such as a hydraulic cylinder  86 ′ or a rotac (rotary actuator)  86 ″ as shown in FIGS. 3D-F. Hydraulic motor  86  is operable in a plurality of modes which are selected in response to the direction of hydraulic fluid into fluid conduits  78   a  and  78   b  by steering valve  66 .  
         [0028]    When hydraulic fluid is directed into fluid conduit  78   a , hydraulic motor  86  is caused to operate in the first mode wherein rotor portion  86   b  is rotated in the first rotational direction. Since rotor portion  86   b  is coupled for rotation with input shaft  48 , operation of hydraulic motor  86  in this manner causes rack-and-pinion steering gear assembly  40  to position wheel hub assemblies  20  in a direction toward the first position. Hydraulic fluid in an amount approximately equal to that input to hydraulic motor  86  through fluid conduit  78   a  is discharged from hydraulic motor  86  to fluid conduit  78   b  and directed back to steering valve  66 . As such, hydraulic fluid is continually circulated between steering valve  66  and drive member  68  in response to the rotational position of steering wheel assembly  32 .  
         [0029]    Similarly, when hydraulic fluid is directed into fluid conduit  78   b , drive member  68  is caused to operate in the second mode wherein rotor portion  86   b  is rotated in the second rotational direction. Since rotor portion  86   b  is coupled for rotation with input shaft  48 , operation of hydraulic motor  86  in this manner causes rack-and-pinion steering gear assembly  40  to position wheel hub assemblies  20  in a direction toward the second position. Hydraulic fluid in an amount approximately equal to that input to hydraulic motor  86  through fluid conduit  78   b  is discharged from hydraulic motor  86  to fluid conduit  78   a  and directed back to steering valve  66 .  
         [0030]    Reservoir  70  is operable for storing fluid in an unpressurized state for use by hydraulic circuit  30 . Fluid conduits  78   c  and  78   d  permit hydraulic fluid to be returned to reservoir  70  from steering valve  66  and compensation valve  74 , respectively. Fluid conduit  78   e  permits hydraulic fluid to be supplied to assist pump  72 .  
         [0031]    Assist pump  72  is operable for increasing the pressure of the hydraulic fluid in hydraulic circuit  30 . Hydraulic fluid exiting assist pump  72  is directed through fluid conduit  78   f  to compensation valve  74  which is operable for selectively supplementing the flow of hydraulic fluid to steering valve  66  and fluid conduits  78   a  and  78   b.    
         [0032]    Control system  34  is shown to include a steering sensor  90 , a drive member sensor  92  and an electronic controller  96 . Steering sensor  90  is operable for sensing the position of steering wheel assembly  32  and producing a steering sensor signal in response thereto. Drive member sensor  92  is operable for sensing the position of rotor portion  86   b  and producing an output portion sensor signal in response thereto. Electronic controller  96  is coupled to steering sensor  90 , drive sensor  92  and compensation valve  74 . Electronic controller  96  is operable for monitoring the steering sensor and drive sensor signals and controlling compensation valve  74  in response thereto. Electronic controller  96  is therefore operable for maintaining the rotational relationship between steering wheel assembly  32  and vehicle  14  regardless of any leakage of hydraulic fluid from compensation valve  74 , steering valve  66 , drive member  68  or fluid conduits  78 . Such control causes the steering wheel portion  32   b  of steering wheel assembly  32  to be returned to a consistent position relative to vehicle  14  to permit the vehicle operator to access any controls which may be mounted on steering wheel assembly, including controls for the vehicle radio system, cruise control system and horn.  
         [0033]    Electronic controller  96  maintains the rotational relationship between steering wheel assembly and vehicle  14  through a selective supplemental flow of hydraulic fluid between compensation valve  74  and the fluid conduits  78   g  and  78   h . For example, where additional hydraulic fluid was needed in fluid conduit  78   a  to correct the rotational relationship of steering wheel assembly  32 , electronic controller  96  controls compensation valve  74  to permit a predetermined amount of supplemental fluid to enter into fluid conduit  78   a  from fluid conduit  78   g  and drain a predetermined amount of fluid from fluid conduit  78   b  through fluid conduit  78   h . Similarly, where the rotational relationship is skewed in an opposite direction, electronic controller  96  controls compensation valve  74  permit a predetermined amount of supplemental fluid to enter fluid conduit  78   b  from fluid conduit  78   h  and drain a predetermined amount of fluid from fluid conduit  78   a  through fluid conduit  78   g.    
         [0034]    Control system  34  also includes a load sensor signal in response to the position of rotor portion  80   b . In the particular embodiment illustrated, load sensor signal is transmitted from steering valve  66  to compensation valve  74  through hydraulic fluid in fluid conduit  78   i . The signal preferably consists of the pressure of the fluid in fluid conduit  78   i . Alternatively, the load sensor signal may also be an electronic sensor signal generated by an electronic sensor which directly or indirectly senses the position of rotor portion  80   b  relative to housing  80   a . In response to the load sensor signal, compensation valve  74  is operable for selectively supplementing the flow of fluid to steering valve  66  through fluid conduit  78   j  with hydraulic fluid provided by assist pump  72 . Operation of compensation valve  74  in this manner effectively amplifies the torque input hydraulic steering system  10  through steering wheel assembly  32  to provide a predetermined level of gain or power-assistance. Compensation valve  74  may be configured to provide an incremental level of power-assistance where the fluid output through fluid conduit  78   j  is proportional to the position of rotor portion  80   b . Compensation valve  74  may also be configured to provide a variable level of power-assistance where the amount of supplemental fluid supplied to drive member  68  through fluid conduit  78   j  is varied depending upon the position of rotor portion  80   b.    
         [0035]    A first alternate embodiment of the present invention is schematically illustrated in FIG. 4. Hydraulic steering system  10 ′ is substantially similar to hydraulic steering system  10  described above except that control system  34 ′ includes only the load sensor signal from steering valve  66  to compensation valve  74 ′. Consequently, hydraulic steering system  10 ′ may provide a predetermined level of gain or power-assistance, but cannot monitor the relative relationship between steering wheel assembly  32  and the position of rotor portion  86   b  to actively compensate for any discrepancies in the rotational position of steering wheel assembly  32 . Those skilled in the art should understand that hydraulic steering system  10  preferably functions in a manner similar to hydraulic steering system  10 ′ in the event that electronic controller  96  fails.  
         [0036]    A second alternate embodiment of the present invention is schematically illustrated in FIG. 5. Hydraulic steering system  10 ″ is substantially similar to hydraulic steering system  10 ′ described above except that the control system, the compensation valve and the fluid pump which provides pressurized hydraulic fluid directly to steering valve  66  are eliminated. Consequently, enhanced system features such as gain or power-assistance and the monitoring of relative relationship between steering wheel assembly  32  and the position of rotor portion  86   b  are not available. Those skilled in the art should understand that hydraulic steering systems  10  and  10 ′ preferably function in a manner similar to hydraulic steering system  10 ″ in the event that compensation valve  74  or assist pump  72  fails.  
         [0037]    While the invention has been described in the specification and illustrated in the drawings with reference to a preferred embodiment, 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 as defined in the claims. 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 illustrated by the drawings and described in the specification as the best mode presently contemplated for carrying out this invention, but that the invention will include any embodiments falling within the description of the appended claims.