Abstract:
A variable steering angle mechanism for a steering apparatus is comprised of a differential gear unit which includes first through fourth bevel gears, an input shaft, an output shaft and a differential gear casing. The output shaft is aligned with the input shaft. The differential gear casing supports the third and fourth bevel gears meshed with the first and second bevel gears and is rotatable on the axis of the input shaft. A motor rotates the differential gear casing around the axis of the input shaft to vary a ratio between a rotation angle of the input shaft and a rotation angle of the output shaft.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates to a vehicle steering control apparatus which is equipped with a variable steering angle mechanism using a differential gear unit, and more particularly to a variable steering angle mechanism using a differential gear unit which enables a steering operation as is similar to that of a normal state even when a steering angle increasing/decreasing shaft of the differential gear unit is put in a rotation disabled state, and which improves safety without applying a strange feeling to a driver.  
         [0002]     There have been proposed various steering control apparatuses each of which is equipped with a variable steering angle mechanism for varying a ratio between a rotation angle of a steering wheel and a turning angle of steered wheels according to a traveling mode of a vehicle. Japanese Published Utility Model Application No. (Showa)64-6177 discloses a typical variable steering angle mechanism of a differential gear unit  101  as shown in  FIG. 6 . The differential gear unit  101  is arranged such that a first shaft  102  is used as an input shaft, a second shaft  103  is used as a steering angle increasing/decreasing shaft, and a differential gear casing  104  is used as an output shaft. A worm wheel  106  is provided on the second shaft  103  functioning as a steering angle increasing/decreasing shaft. The worm wheel  106  is rotated by a motor through a worm gear  105 , and a rack shaft  108  is driven by a gear portion  107  integrally connected to the differential gear casing  104  functioning as an output shaft. An increasing and decreasing control of a turning angle of the steered wheels is executed by changing a gear ratio of the differential gear unit  101 . The changing of the gear ratio is executed by controlling a motor for rotating the steering angle increasing/decreasing shaft  103  according to a traveling state indicative signal.  
       SUMMARY OF THE INVENTION  
       [0003]     However, when the second shaft  103  functioning as the steering angle increasing/decreasing shaft is put in a rotation disabled state due to a trouble such that the motor is stopped due to failure or is put in an uncontrollable state, the differential gear casing  104  functioning as the output shaft rotates at an average speed of the first shaft  102  and the second shaft  103 , and therefore a rotation angle of the output shaft becomes a half of the rotation angle of the input shaft. Therefore, in case that the second shaft  103  functioning as the steering angle increasing/decreasing shaft is stopped, a driver is required to steer a steering wheel by a very large angle in order to achieve a small radius turn of the vehicle at a low speed.  
         [0004]     Since the rotation speed of the differential gear casing  104  functioning as the output shaft becomes an average speed of the input shaft (the first shaft  102 ) and the steering angle increasing/decreasing shaft (the second shaft  103 ), the motor attached to the steering angle increasing/decreasing shaft is required to rotate at a very high rotation speed when a vehicle turns with a small radius at a low speed, such as in case of putting the vehicle in a garage. Further, the variable steering angle mechanism having a plurality of gears generates play during a neutral state or a steering back (turning back) state, due to backlash of the gears. This play tends to degrade the steering feeling.  
         [0005]     It is therefore an object of the present invention to provide a variable steering angle mechanism using a differential gear unit which facilitates a steering operation as is similar to that under a normal state, even when a steering angle increasing/decreasing shaft of the differential gear unit is not rotated due to a failure of a motor and the like.  
         [0006]     An aspect of the present invention resides in a variable steering angle mechanism for a vehicle steering control apparatus, which comprises: a differential gear unit which comprises first, second, third and fourth bevel gears, an input shaft, an output shaft and a differential gear casing, the input shaft being integrally connected to the first bevel gear, the output shaft being integrally connected to the second gear, the output shaft being aligned on an axis of the input shaft, the differential gear casing supporting the third and fourth bevel gears meshed with the first and second bevel gears, the differential gear casing being rotatable on the axis of the input shaft; and a motor which is disposed outside the differential gear casing, the motor rotating the differential gear casing around the axis of the input shaft to vary a ratio between a rotation angle of the input shaft and a rotation angle of the output shaft.  
         [0007]     Another aspect of the present invention resides in a steering control apparatus which comprises: a steering wheel; a variable steering angle mechanism comprising a differential gear unit which comprises first, second, third and fourth bevel gears, an input shaft, an output shaft and a differential gear casing, the input shaft being connected to the first bevel gear and the steering wheel, the output shaft being integrally connected to the second gear, the output shaft being aligned on an axis of the input shaft, the differential gear casing supporting the third and fourth bevel gears meshed with the first and second bevel gears, the differential gear casing being rotatable on the axis of the first shaft; a motor which is disposed outside the differential gear casing, the motor rotating the differential gear casing around the axis of the input shaft to vary a ratio between a rotation angle of the input shaft and a rotation angle of the output shaft; and a rack-and-pinion mechanism which is connected to the output shaft.  
         [0008]     The other objects and features of this invention will become understood from the following description with reference to the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]      FIG. 1A  is a longitudinal cross-sectional view showing a differential gear unit of a variable steering angle mechanism according to an embodiment of the present invention.  
         [0010]      FIG. 1B  is a lateral cross-sectional view showing essential parts of the differential gear unit.  
         [0011]      FIG. 2  is a lateral cross-sectional view showing a modification of  FIG. 1B .  
         [0012]      FIG. 3  is a schematic view explaining the variable steering angle mechanism.  
         [0013]      FIG. 4A  is a schematic view explaining operations of parts of the variable steering angle mechanism according to the embodiment shown in  FIGS. 1A and 1B .  
         [0014]      FIG. 4B  is a schematic view explaining operations of parts of the variable steering angle mechanism according to the embodiment shown in  FIG. 2 .  
         [0015]      FIG. 4C  is a schematic view explaining operations of parts of a variable steering angle mechanism of a prior art.  
         [0016]      FIG. 5A  is a graph showing a relationship between a steering-wheel rotation angle (an input-shaft rotation angle) and a tire steered angle in each drive mode.  
         [0017]      FIG. 5B  is a graph showing a relationship between the input-shaft rotation angle and an output-shaft rotation angle in each drive mode.  
         [0018]      FIG. 6  is a cross-sectional view showing a differential gear unit of a variable steering angle mechanism according to a prior art. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0019]     Referring to  FIGS. 1A through 5B , there is shown an embodiment of a variable steering angle mechanism  1  used in a vehicle steering control apparatus according to the present invention.  
         [0020]      FIG. 1  shows a differential gear unit  10  of the variable steering angle mechanism  1  according to the embodiment of the present invention. The differential gear unit  10  comprises four spiral bevel gears  12 ,  14 ,  16  and  17 . The spiral bevel gear  12  integrally connected to an end of a first shaft  11  functioning as an input shaft. The other end of the first shaft  11  is connected to a steering wheel  2 . A second shaft  13  is aligned with the first shaft  11  so that it are disposed on an axis of the first shaft  11 , and functions as an output shaft. An end of the second shaft  13  integrally connected to the spiral bevel gear  14 , and the other end of the second shaft  13  is connected to a rack-and-pinion mechanism  51  shown in  FIG. 3 .  
         [0021]     The spiral bevel gears  16  and  17  are disposed on an axis perpendicular to the axis of the first shaft  11 , and mesh with the spiral bevel gears  12  and  14 . The spiral bevel gears  16  and  17  are rotatably supported by a differential gear casing  20 . The differential gear casing  20  is rotatably supported by the first and second shafts  11  and  13  respectively through bearings  29 . The differential gear casing  20  functions as a steering angle increasing/decreasing (varying) shaft.  
         [0022]     A thrust shaft  15  is disposed between the first shaft  11  and the second shaft  13  in order to maintain the set positions of the spiral bevel gears  12  and  14 . A thrust bearing  15   a  is disposed between the thrust shaft  15  and the first bevel gear  12  to receive a thrust force along the axis of the first shaft  11 .  
         [0023]     A worm wheel  18  is disposed in the differential gear casing  20  functioning as a steering angle increasing/decreasing shaft. A worm gear  19  for rotating the worm wheel  18  is disposed around the differential gear casing  20 . The worm gear  19  meshes with the worm wheel  18 . The worm gear  19  is connected to a motor M as shown in  FIG. 3 . The motor M rotates the worm gear  19  to control the rotation of the differential gear casing  20 . The differential gear casing  20  is supported by a housing  41  of the differential gear unit  10  through bearings  45  so as to be rotatable around the axis of the first shaft  11 . The housing  41  is constructed by left and right housings  42  and  43  and screws  44  for integrally connecting the left and right housing  42  and  43 . The input shaft (first shaft)  11  and the output shaft (second shaft)  13  are supported by the differential gear casing  20  through the bearings  29 .  
         [0024]     The differential gear casing  20  comprises a pair of circular wall portions  22  and  23  for supporting respectively the first shaft  11  and second shaft  13 , and a pair of connecting portions  31  and  34  for connecting with the wall portions  22  and  23 . As shown in  FIG. 1B , each of the connecting portions  31  and  34  has an arc-shaped outer peripheral surface and a flat inner peripheral surface. The connecting portion  31  and  34  are disposed so that the flat inner peripheral surface of one of the connecting portions  31  and  34  is parallel to the flat inner peripheral surface of the other. A pair of gear supporting shafts  24  and  25  protrude respectively from the connecting portions  31  and  34  toward the common axis of the first and second shafts  11  and  13  while being perpendicular to the flat inner peripheral surfaces.  
         [0025]     Flanges  24   a  and  25   a  are formed at end portions of the gear supporting shafts  24  and  25 , respectively, to prevent the gear supporting shafts  24  and  25  from being drawn out from the wall portions  31  and  34 . The spiral bevel gears  16  and  17  are rotatably supported by the gear supporting shafts  24  and  25 , respectively, through bearings  26   a ,  26   b  and  26   c . The gear supporting shafts  24  and  25  for supporting the spiral bevel gears  16  and  17  are inserted into installation holes  31   a  and  34   a  of the connecting portions  31  and  34 , respectively, and are fixed to the connecting portions  31  and  34  by means of pins  28 . Annular plates  32  functioning as shim members are disposed between the connecting portion  31  and the bearing  26   c  and between the connecting portion  34  and the bearing  26   c , respectively.  
         [0026]      FIG. 2  shows another gear supporting structure of the differential gear casing which is also employed in the differential gear unit  10  of the variable steering angle mechanism according to the embodiment of the present invention. The spiral bevel gear  16  in the differential gear casing  20  is supported by the gear supporting shaft  24  as is similar to that shown in  FIG. 1 . The spiral bevel gear  17  opposite to the spiral bevel gear  16  is supported by a gear supporting shaft  25 ′ which is formed so that a base end portion of the gear supporting shaft  25 ′ is slightly smaller than the other portions of the gear supporting shaft  25 ′. After the spiral bevel gear  17  is installed on the gear supporting shaft  25 ′ through bearing  26   a ,  26   b  and  26   c , the thin base end portion of the gear supporting shaft  25 ′ is engaged with an annular plate  35  and a cylindrical adjust screw  36 , and the gear supporting shaft  25 ′ is then integrally connected with the adjust screw  36  by means of a pin  38 . The annular plate  35  is made by spring member and performs an elastic characteristic. The cylindrical adjust screw  36  is for adjusting the axial position of the spiral bevel gear  17 . The connecting portion  34  has a screw hole  37 , into which the cylindrical adjust screw  36  can be screwed.  
         [0027]     The gear supporting shaft  25 ′, on which the spiral bevel gear  17  and the annular plate  35  is installed, is set by screwing the adjust screw  36  from the inner side of the differential gear casing  20  into the screw hole  37  of the of the connecting portion  34 , so that the annular plate  35  contacts with the flat inner peripheral surface of the connecting portion  34 . Thereafter, the differential gear casing  20  is assembled by installing the wall portions  22  and  23 , in which the spiral bevel gears  12  and  14  and the thrust shaft  15  are installed, at both sides of the connecting portions  31  and  34  in which the spiral bevel gears  16  and  17  are installed.  
         [0028]     Subsequently, clearances of the spiral bevel gears  12  and  14  relative to the spiral bevel gear  17  are adjusted by moving the gear supporting shaft  25 ′ toward the center axis of the differential gear casing  20 . The movement of the gear supporting shaft  25 ′ is executed by screwing the adjust screw  36  with the screw hole  37  toward the center axis. Then, the gear supporting shaft  25 ′ is fixed to the connecting portion (gear supporting shaft installation portion)  34  by locking the cylindrical adjust screw  36  by means of a lock nut  39 . During the clearance adjustment of the spiral bevel gears  12  and  14 , the plate  35  is deformed into a coned-disc shape and functions as a disc spring. Therefore, the plate  35  biases the spiral bevel gear  17  into the center direction of the differential gear casing  20 , and the spiral bevel gear  17  is put in no-backlash state relative to the spiral bevel gears  12  and  14 . As is apparent from the comparison between  FIGS. 1B and 2 , a structure of an upper portion of the differential gear unit  10  shown in  FIG. 2  is different from that shown in  FIG. 1 .  
         [0029]      FIG. 3  shows a schematic construction of the variable steering angle mechanism  1 . The steering wheel  2  is connected to the input shaft  11  of the differential gear mechanism  10 . The rack-and-pinion mechanism  51  for steering steered wheels  3  is connected to the output shaft (second shaft)  13 . The motor M for varying the steering angle is connected to the worm gear  19  at an outside of the differential gear casing  20 . The output shaft  13  rotates in the inverse direction relative to the rotation of the input shaft (first shaft)  11 . Therefore, the engagement between a rack  53  and a pinion  52  of the rack-and-pinion mechanism  51  is arranged such that the rotational direction of the output shaft  13  becomes inverse relative to the steering direction of the wheels  3 .  
         [0030]     Subsequently, there is discussed an operation of the variable steering angle mechanism  1  used in the vehicle steering control apparatus according to the embodiment of the present invention.  
         [0031]     When the steering wheel  2  is steered, the motor M is rotated in reply to a rotation command according to a vehicle traveling condition. The gear ratio of the differential gear unit  19  is variably controlled by the rotation control of the differential gear casing  20 . Since the second shaft  13  of the differential gear unit  10  is used as the output shaft  13 , the rotational direction of the input shaft  11  becomes opposite to that of the output shaft  13 . However, the engagement between the rack  53  and the pinion  52  of the rack-and-pinion mechanism  51  is inversely changed, the steering direction of the steering wheel  2  corresponds with the steering direction of the steered wheels  3 , as shown in  FIG. 4A .  
         [0032]     Instead of changing the engagement between the rack  53  and the pinion  52  of the rack-and-pinion mechanism  51 , a connection of each knuckle arm  4  for transforming the axial displacement of the rack into the steering angle may be changed as shown in  FIG. 4B , so as to correspond the steering direction of the steering wheel  2  with the steering direction of the steered wheels  3 .  
         [0033]      FIG. 4C  shows a conventional steering control apparatus where the rotational direction of the input shaft corresponds to that of the output shaft, and wherein a rack-and-pinion mechanism  51   b  is arranged such that the rack  53  is disposed under the pinion  53 .  
         [0034]     The steering angle of the variable steering angle mechanism  1  according to the embodiment of the present invention is assisted by the differential gear unit  10 . More specifically, an steering assist angle is set to be changed according to the vehicle speed, and more specifically to become different in a medium speed range, a low speed range and a high speed range. In the medium speed range, the rotation of the motor M is stopped and therefore the steering assist angle is 0 degree. In the embodiment according to the present invention, a rotation angle ratio between the rotation angle of the input shaft and the rotation angle of the output shaft  13  during the medium speed range is set at a ratio of 1:1. The rotation angle ratio during the high speed range is set at a ratio 1.2:1.0. The rotation angle ratio during the low speed range is set at a ratio 1.2:1.0. The change of the ratio may not be limited to these three steps, and may be set in a larger number of steps according to the vehicle speed.  
         [0035]      FIG. 5A  shows a relationship between a steering-wheel turn angle (input-shaft rotation angle) and a tire (wheel) steered angle in each of the medium speed range, the low speed range, the high speed range, and a motor stop state.  FIG. 5B  shows a relationship between the steering-wheel turn angle (input-shaft rotation angle) and an output-shaft rotation angle in each of the medium speed range, the low speed range, the high speed, and the motor stop state. TABLE 1 shows rotation angles of the output shaft (second shaft)  13  and the differential gear casing  20  in case that the input shaft (first shaft)  11  is rotated by 180 degrees in each of the medium speed range, the high speed range, the low speed range and the motor stop state, in accordance with the embodiment of the present invention. TABLE 2 shows rotation angles of the output shaft (second shaft)  13  and the differential gear casing  20  in case that the input shaft (first shaft)  11  is rotated by 180 degrees in each of the medium speed range, the high speed range, the low speed range and the motor stop state in accordance with the conventional mechanism.  
                                                             TABLE 1                                   medium           motor stop           speed   high speed   low speed   state                                        input   180°   180°     180°   180°           output   −180°     −150°     −600°    90°           casing    0°    15°   −210°    0°                        
         [0036]    
       
         
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                   
               
               
                   
                 medium 
                   
                   
                 motor stop 
               
               
                   
                 speed 
                 high speed 
                 low speed 
                 state 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 input 
                 180° 
                 180° 
                 180° 
                 180° 
               
               
                   
                 output 
                 180° 
                 150° 
                 600° 
                  90° 
               
               
                   
                 second 
                 180° 
                 120° 
                 1020°  
                  0° 
               
               
                   
                 shaft 
               
               
                   
                   
               
             
          
         
       
     
         [0037]     Since the conventional differential gear unit is arranged such that a differential gear casing is used as an output shaft, the ratio between the rotation angles of the input shaft and the output shaft becomes 1:0.5 under a condition that no motor assist is obtained (the motor is stopped), as shown in TABLE 2.  
         [0038]     In contrast, the differential gear unit  10  according to the embodiment of the present invention is arranged such that the second shaft  13  is used as an output shaft. Therefore, even when the rotation of the differential casing  10  is stopped due to a failure of the motor M and the like, the ratio between the rotation angles of the input shaft and the output shaft becomes 1:1. In case that the motor M is stopped due to failure or put in a trouble such as a uncontrollable condition, even when the turn radius of the vehicle is small under a low speed condition, it is not necessary for a driver to steer the steering wheel  2  by a very large turn angle, as is apparent from a comparison between the relationship a according to the present invention and the relationship b of the prior art shown in  FIG. 5A . Further, in case of the differential gear unit  10  according to the embodiment of the present invention, even when the vehicle turns with a small radius at a low speed, such as in a case of putting the vehicle in a garage, the motor M is not required to operate at a very high rotation speed, as is apparent from the comparison between the relationship of the differential gear casing  20  in the low speed case shown in TABLE 1 according to the present invention and the relationship of the second shaft in the lower speed shown in TABLE 2.  
         [0039]     In case of the convention differential gear unit, when the second shaft  103  is stopped due to a failure of a motor, it is may be proposed to employ a differential gear unit of doubling the rotation angle of the output shaft, in order to set the ratio between the rotation angles of the input shaft and the output shaft is set at 1:1. However, such an arrangement of the differential gear unit will derive problems of increasing the number of parts and of increasing backlash. In such a case, the rotation angle of the second shaft, which is necessary for obtaining the rotation angle as same as shown in TABLE 1, becomes double in high speed and in low speed, as shown in Table 3. In this conventional variable steering angle mechanism, it is necessary to quickly rotate the steering angle increasing/decreasing shaft according to the increase of the operation speed of the steering wheel. In case of the conventional structure shown in TABLE 3, it is necessary to rotate the second shaft at a speed more than double of the rotation speed of the steering wheel.  
         [0040]     In contrast, the present invention is arranged to rotate the differential gear casing  20  at a speed slightly higher than the speed of the steering wheel  2 . This obtains an advantage in a following-up ability.  
                                                             TABLE 3                                   medium           motor stop           speed   high speed   low speed   state                                        input   180°   180°   180°   180°           output    90°    75°   300°    90°           second    0°   −30°   420°    0°           shaft                      
 
         [0041]     Further, the bevel gears  12 ,  14 ,  16  and  17  of the differential gear unit  10  according to the embodiment of the present invention are spiral bevel gears. Therefore, in case that the gear supporting structure shown in  FIG. 2  is employed, the plate  35  functioning as biasing means keeps a condition that the spiral bevel gear  17  does not generate backlash relative to the spiral bevel gear  12  of the first shaft  11  and the spiral bevel gear  14  of the second shaft  13 . This improves the differential gear unit  10  in a level of suppressing a play during a neutral state or a steer-back (or turn-back) of the steering wheel  2  at a normal feeling level. Further, the backlash may be adjusted only by the adjusting means of moving the spiral bevel gear  17  in the axial direction, or only the biasing means may be employed.  
         [0042]     Although the spiral bevel gears  12 ,  14 ,  16  and  17  have been employed in the differential gear unit  10  according to the embodiment of the present invention, the gears are not limited to them. The gears of suppressing the backlash may be employed. Further although the backlash adjustment of the gears has been executed using the plate  35  and the adjust screw  36 , the invention is not limited to this.  
         [0043]     With the thus arranged variable steering angle mechanism according to the embodiment of the present invention, the second shaft (output shaft)  13  of the differential gear unit  10  functions as an output shaft. Therefore, even when the rotation of the differential gear casing  20  functioning as a steering angle increasing/decreasing shaft is stopped, the input and output shafts  11  and  13  rotate at a speed ratio of 1:1. This enables a driver to steer the steering wheel  2  with a normal feeling even when a small radius turn is carried out at a low speed. Further, even when the steering wheel  2  is quickly turned during a low speed turn such as a case of putting a vehicle in a garage, it is not necessary to rotate the motor M at a very high speed.  
         [0044]     Further, since a backlash adjusting means is provided for the differential gears of the differential gear unit  10 , the engagements of the gears are always put in no-backlash state. Since the gears smoothly rotate while keeping the normal level as to a play during a neutral state or steering back state, the steering feeling is improved.  
         [0045]     This application is based on Japanese Patent Application No. 2004-340004 filed on Nov. 25, 2004 in Japan. The entire contents of this Japanese Patent Application are incorporated herein by reference.  
         [0046]     Although the invention has been described above by reference to certain embodiments of the invention, the invention is not limited to the embodiments described above. Modifications and variations of the embodiments described above will occur to those skilled in the art, in light of the above teaching. The scope of the invention is defined with reference to the following claims.