Abstract:
The four-wheel-drive vehicle has a torque transmission device configured to distribute torque transmitted from an engine to a first set of wheels to a second set of wheels. Throttle-opening of the engine is detected. A first torque command which depends on the throttle-opening is obtained. A rotational speed difference between the first set of wheels and the second set of wheels is detected. A second torque command which depends on the rotational speed difference is obtained. The torque transmission device is controlled based on the first and the second torque command so as to transmit the driving force to the second set of wheels according to the throttle-opening and the rotational speed difference. Then, a signal that corresponds to the throttle-opening is monitored. When the signal is judged being abnormal, the first torque command is ignored so that the torque transmission device is controlled based on the second torque command.

Description:
INCORPORATION BY REFERENCE 
   The present application claims priority under 35 U.S.C. §119 to Japanese Patent Application No. 2002-120134, filed on Apr. 23, 2002. The contents of that application are incorporated herein by reference in their entirety. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates to a driving force distribution method and a driving force distribution apparatus for four-wheel-drive vehicle that has a torque transmission device. 
   2. Discussion of the Background 
   At a four-wheel-drive vehicle of recent years, front wheels are always driven, and rear wheels are selectively driven depending on its necessity by arranging a torque transmission device on the way of a propeller shaft that connects an engine and the rear wheels. Such torque transmission device is controlled so as to transmit torque to the rear wheels, which is calculated based on rotational speed differences between the front and the rear wheels, for instance. As for a fail-safe of the torque transmission device, if some abnormality happens, the torque transmission device is controlled not to transmit any torque, i.e. the vehicle becomes a two-wheel-drive. However, it is possible that the vehicle sticks if the abnormality happens when the vehicle is on a rough road such as snow-covered road, because of two-wheel-drive. 
   SUMMARY OF THE INVENTION 
   In view of the foregoing, it is an object of the present invention to provide an improved driving force distribution method and apparatus. In order to achieve the above and other objects, a driving force distribution method for a four-wheel-drive vehicle is provided as a first aspect of the present invention. The four-wheel-drive vehicle has a torque transmission device configured to distribute torque transmitted from an engine to a first set of wheels to a second set of wheels. Throttle-opening of the engine is detected. A first torque command which depends on the throttle-opening is obtained. A rotational speed difference between the first set of wheels and the second set of wheels is detected. A second torque command which depends on the rotational speed difference is obtained. The torque transmission device is controlled based on the first torque command and the second torque command so as to transmit the driving force to the second set of wheels according to the throttle-opening and the rotational speed difference. Then, a signal that corresponds to the throttle-opening is monitored. When the signal is judged being abnormal, the first torque command is ignored so that the torque transmission device is controlled based on the second torque command. 
   A driving force distribution apparatus is provided as a second aspect of the present invention. The driving force distribution apparatus for a four-wheel-drive vehicle has a torque transmission device configured to distribute torque transmitted from an engine to a first set of wheels to a second set of wheels. The apparatus comprises a throttle sensor, plural speed sensors and an electrical control unit. The throttle sensor detects throttle-opening of the engine. The speed sensors detect rotational speed of the first set of wheels and the second set of wheels. The electrical control unit determines a first torque command which depends on the throttle-opening detected by the throttle sensor and a second torque command which depends on a rotational speed difference between the first set of wheels and the second set of wheels detected by the each speed sensor. The electrical control unit controls the torque transmission device based on the first torque command and the second torque command so as to transmit the driving force to the second set of wheels according to the throttle-opening and the rotational speed difference. Then, the electrical control unit monitors a signal which is detected by the throttle sensor, and when the signal judged being abnormal the first torque command is ignored so that the torque transmission device is controlled based on the second torque command. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Various other objects, features and many of the attendant advantages of the present invention will be readily appreciated as the same becomes better understood by reference to the following detailed description of the preferred embodiments when considered in connection with the accompanying drawings, in which: 
       FIG. 1  is a schematic diagram showing a four-wheel-drive vehicle according to a first embodiment of the present invention; 
       FIG. 2  is a cross-sectional view of a torque transmission device equipped to the four-wheel-drive vehicle of the first embodiment; 
     FIG.  3 A and  FIG. 3B  are enlarged cross-sectional views showing a cam mechanism of the torque transmission device; 
       FIG. 4  is a flowchart of a main program that is executed by an electric control unit for the torque transmission device; 
       FIG. 5  is a flowchart of a main program for a second embodiment of the present invention; and 
       FIG. 6  is a flowchart of a main program for a third embodiment of the present invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Embodiments according to the present invention will be explained hereinafter with reference to the drawings. 
   [First Embodiment] 
   A first embodiment of the present invention will be described with reference to  FIGS. 1-4 .  FIG. 1  is an explanatory view showing a main configuration of a power train of a four-wheel-drive vehicle according to the first embodiment. At a front side of the vehicle, there is installed a transaxle  11  adjacently to an engine  10 , into which a transmission, a transfer and a front differential  12  are assembled uniformly. A driving force generated by the engine  10  is transmitted to a pair of front driven shafts  13  from the transmission of the transaxle  11  through the front differential, to drive a pair of front wheels  14 . Namely, the driving force is directly transmitted to the front wheels  14 , so that the front wheels  14  are always driven thereby. 
   Further, a front end of a front side propeller shaft  20  is connected with the transmission by the transfer of the transaxle  11  through its gear. On the other hand, rear end of the front side propeller shaft  20  is fixed with an input portion  31  (refer to  FIG. 2 ) of a torque transmission device  30  which is disposed at an intermediate position of the vehicle. Therefore, the input portion  31  of the torque transmission device  30  is always rotated together with the front wheels  14 . 
   On an expansion line of the front propeller shaft  20 , there is disposed a rear side propeller shaft  21  so that the torque transmission device  30  is sandwiched between the both propeller shafts  20  and  21 . A front end of the rear propeller shaft  21  is fixed with an output portion  32  (refer to FIG.  2 ). The other rear end of the rear propeller shaft  21  is connected with a rear differential  17 . Further, rear wheels are attached onto end portions of rear driven shafts  16  which are provided with the rear differential  17  so as to extend therefrom in left and right directions, respectively. With this configuration, the rear wheels  15  are not always driven by the driving force of the engine  10 , but are selectively driven according to operation of the torque transmission device  30 . 
   Besides, the front wheels  14  correspond to “a first set of wheels”, while the rear wheels correspond to “a second set of wheels” in the claim descriptions. 
     FIG. 2  shows a basic construction of the torque transmission device  30 . 
   The input portion  31  of the torque transmission device  30  provides an outer case  33  consisting of a bottom wall  33 T and a shaft portion  33 J. In the bottomed cylindrical outer case  33 , the bottom wall  33 T is arranged at the front side, while its opening is provided at the rear side. The front propeller shaft  20  is connected with the shaft portion  33 J (refer to FIG.  1 ). A rear cover  35  is screwedly fitted into the outer case  33  so as to close the opening of the outer case  33 . An inner shaft  34  that consists of the output  32  of the torque transmission device is liquid-tightly protruded into a through hole  35 A formed in the center of the rear cover  35 . 
   The inner shaft  34  is rotatably supported by the outer case  33  and, is however restricted to move in an axial direction of the inner shaft  34  relative to the outer case  33 . One end of the inner shaft  34  extends by the vicinity of the bottom wall  33 T of the outer case  33 , and the other end of the inner shaft  34  is connected with the rear propeller shaft  21  so as to protrudes from the outside of the torque transmission device  30  (refer to FIG.  1 ). 
   A first cam disk  36  is rotatably supported with the inner shaft  34  at a portion close to the rear cover  35  in the outer case  33 . Plural ring-shaped inner pilot clutch plates  37  are spline-engaged with a circumferential surface of the first cam disk  36 . That is, the inner pilot clutch plates  37  are movable in the axial direction, but is restricted to rotate relative to the first cam disk  36  in a rotational direction. Plural outer pilot clutch plates  38  that are ring-shaped are engaged with the inner surface of the outer case  33  by splines at a portion close to rear cover  35 . Therefore, the outer pilot clutch plates  38  are movable in the axial direction, but are restricted to rotate relative to the outer case  33  in the rotational direction. The inner pilot clutch plates  37  and the outer pilot clutch plates  38  are opposingly disposed from each other alternately of the axial direction. A ring-shaped armature  49  is spline-engaged with the inner surface of the outer case  33  at a position where the rear cover  35  binds the inner pilot clutch plates  37  and the outer pilot clutch plates  38  therewith. 
   Inside of the outer case  33 , an electromagnetic coil  39  is disposed at a rear side of the rear cover  35 . The electromagnetic coil  39  is accommodated into an annular groove formed on the front surface of a yoke  39 A which is rotatably supported with the rear cover by a bearing (not shown). In a case that the electromagnetic coil  39  is not excited, the inner pilot clutch plates  37  can rotate relatively to the outer pilot clutch plates  38  (referred to as “separated operation”). On the other hand, the armature  49  is attracted toward the rear cover  35  with excitation of the electromagnetic coil  38 , the inner pilot clutch plates  37  and the outer pilot clutch plates  38  are attracted to the rear cover  35  together with the armature  49 . Therefore, the inner pilot clutch plates  37  are frictionally engaged with the outer pilot clutch plates  38 , respectively (referred to as “frictional engage operation”). When the electromagnetic coil  39  is further excited, are further attracted to the outer pilot clutch plates  38 . Finally, the inner pilot clutch plates  37  are completely coupled to the outer pilot clutch plates  38  with the furthest excitation of the electromagnetic coil  39  (referred to as “directly coupled operation”). The first cam disk  36  is rotated by torque which is transmitted from the outer case  33  through the frictional engagement or the complete coupling between the inner pilot clutch plates  37  and the outer pilot clutch plates  38 . In the claim description, the above described inner pilot clutch plates  37 , outer pilot clutch plates  38 , electromagnetic coil  39  and armature  49  constitute a pilot clutch mechanism. 
   A second cam disk  41  is disposed in front of the first cam disk  36 , and is spline-engaged with the inner shaft  34 . That is, the second cam disk  41  can move in the axial direction, but its relative rotation to the inner shaft  34  is restricted. Plural V-shaped concavities  40 ,  42  are symmetrically formed on a front surface (left in FIG.  2  and  FIG. 3 ) of the first cam disk  36  and a rear surface (right in FIG.  2  and  FIG. 3 ) of the second cam disk  41  so as to face with each other, respectively. As shown by FIG.  3 (A) that is a cross-sectional view in the circumferential direction of the first cam disk  36  and the second cam disk  41 , each V-shape concavity  40 ,  42  is concaved so as to become gradually deep toward the center of the V-shape concavity  40 ,  42  in its circumferential direction (vertical direction in FIG.  3 (A)). Plural cam balls  43  are held corresponding to between the V-shape concavities  40  of the first cam disk  36  and the V-shape concavities  42  of the second cam disk  41 , respectively. When the first cam disk  36  is rotated relative to the second cam disk  41 , each cam ball  43  moves to a shallow position of both V-shape concavities  40 ,  42 . As a result, such a force occurs that makes the first cam disk  36  and second cam disk  41  be away from each other. In the claim description, the above described first cam disk  36 , second cam disk  38  and cam balls  43  constitute a cam mechanism. 
   As shown in  FIG. 2 , plural ring-shaped inner main clutch plates  44  are disposed in the outer case  33  more forward than the second cam disk  41  to spline-engaged with the outer surface of the inner shaft  34 . That is, the inner main clutch plates  44  can move in the axial direction, but the relative rotation of the inner clutch plates  44  to the inner shaft  34  can be restricted. Plural ring-shaped outer main clutch plates  45  are spline-engaged with the inner surface of the outer case  33 . That is, the outer main clutch plates  45  can move in the axial direction, but the relative rotation of the outer main clutch plates  45  to the outer case  33  can be restricted. The inner main clutch plates  44  and the outer main clutch plates  45  are disposed alternately to face from each other. In a case that the second cam disk  42  does not move toward the bottom wall  33 T, the inner main clutch plates  44  can rotate relatively to the outer main clutch plates  45 . On the other hand, the second cam disk  42  is moved forward by the magnetic attraction of the electromagnetic coil  39 , the second cam disk  42  pushes the inner main clutch plates  44  and the outer main clutch plates  45  to the side of the bottom wall  33 T (to the left in FIG.  2 ). As a result of pushing, the inner main clutch plates  44  and the outer main clutch plates  45  are frictionally engaged with each other, so that the torque can be transmitted from the outer case  33  to the inner case  34  through the inner main clutch plates  44  and the outer main clutch plates  45 . In the magnetic attraction of the electromagnetic coil  39  becomes further large due to further excitation thereof, the pushing force of the second cam disk  41  is further increased, so that the inner main clutch plates  44  and the outer main clutch plates  45  are completely coupled. In the claim description, the above described inner main clutch plates  44  and outer main clutch plates  38  constitute a main clutch mechanism. 
   The aforementioned torque transmission device  30  can selectively take three different states. One of state is “separate state” which no torque is transmitted from the input portion  31  to the output portion  32  (i.e., the input portion  31  are freely rotatable relatively to the output  32 ). Another state is “intermediate state” which a part of torque is transmitted from the input  31  to the output  32  (i.e., the rotational difference between the input portion  31  and output portion  32  can be permitted. The other state is “complete coupling state”, which all torque of the input portion  31  is completely transmitted to the output portion  32  (i.e., the input  31  can rotated with the output portion  32 ). 
   The electromagnetic coil  39  is controlled by an ECU (electronic control unit)  50 . Besides, a driving force distribution control system  90  according to the present invention is composed of the torque transmission device  30  and the ECU  50 , as shown in FIG.  1 . 
   The ECU  50  controls quantity of current supplied to the electromagnetic coil  39  in order to adjust torque that is transmitted through the torque transmission device  30 . As shown by  FIG. 1 , throttle-opening degree signal St representing a degree of throttle opening of the engine  10  which is detected by a throttle sensor  60  is inputted to the ECU  50 . To the ECU  50 , there are also inputted front wheel speed signals N 1 , N 2  representing rotational speed of each front wheel  14  and rear wheel speed signals N 3 , N 4  representing rotational speed of each rear wheel  15  which are respectively detected by rotational speed sensors  61 . 
   The ECU  50  reads out a main program M shown by  FIG. 4  from a ROM (not shown) to execute at a predetermined time interval period. First, the ECU  50  receives the throttle-opening signal St from the throttle sensor  60  and the wheel speed signals N 1 -N 4  from the rotational speed sensors  61  (in step S 1 ). A vehicle speed V is calculated based on the wheel speeds N 1 -N 4  (in S 2 ). It is preferable that The vehicle speed V is calculated as an average value of the wheel speeds N 3  and N 4  of the both rear wheels  15 . Because the rear wheels  15  generally intend to slip compared with the front wheels  14  in a case that the front wheels  14  are always driven and the rear wheels  15  are not always driven.
 
 V =( N   3 + N   4 )/2
 
   Next, a rotational speed difference ΔN between the front wheels  14  and the rear wheels  15  is calculated based on the wheel speeds N 1 -N 4  as a difference between an average value of the wheel speeds (N 1 , N 2 ) of the front wheels  14  and an average value of that (N 3 , N 4 ) of the rear wheels  15  (in step S 3 ).
 
Δ N =( N   1 + N   2 )/2−( N   3 + N   4 )/2=( N   1 + N   2 − N   3 − N   4 )/2
 
   In next step, it is judged whether the throttle-opening degree signal St is abnormal or not (in step S 4 ). In other words, it is judged whether the throttle-opening degree signal St exists within a predetermined range or not. When the throttle-opening degree signal St exists within a predetermined lower limit C 1  and a predetermined upper limit C 2  in the predetermined range, it is judged that the throttle-opening degree signal St is normal. Concretely, it is requested to satisfy the following expression.
 
 C   1 = m=C   2 
 
   On the other hand, it is judged that the throttle-opening degree signal St is abnormal when the throttle-opening degree signal St is out of the predetermined range, i.e., when the above-mentioned expression is not satisfied. 
   Herein, in a case that it is judged that the throttle-opening degree signal St is not abnormal (S 4 =No), a pre-torque Tp corresponding to a first torque command of the claim description is obtained from the throttle-opening degree signal St and the vehicle speed V (in step S 6 ). The pre-torque Tp is obtained by referring to a map that is stored in the ROM and defines values of the pre-torque Tp according to the throttle-opening degree signal St and the vehicle speed V. For example, the map in the form of a three-dimension defines values of the pre-torque Tp so as to increase according to the increase of the throttle-opening degree signal St in a state that the vehicle speed V is less than a predetermined speed, while the map defines it so as to be constant independent of the throttle-opening degree signal St in a state that the vehicle speed V is larger than the predetermined speed. On the other hand, when it is judged that the throttle-opening degree signal St is abnormal. (S 4 =Yes), the pre-torque Tp is set to be zero in its value. Sequentially, a torque relative to a rotational speed difference torque (referred to as “RSD-torque” hereinafter) Tn corresponding to a second torque command of the claim description is obtained from the rotational speed difference ΔN and the vehicle speed V (in step S 7 ). The RSD-torque Tn is obtained by referring a map that is stored in the ROM and defines values of RSD-torque according to the rotational speed difference ΔN and the vehicle speed V (in step S 7 ). For example, the map in the form of a three-dimension defines the values of the RSD-torque Tn so as to increase according to the increase of the rotational speed difference ΔN in a state that the vehicle speed V is less than a predetermined speed, but a proportion of the increase of the values of the RSD-torque Tn decreases according to the increase of the vehicle speed V in a state that the vehicle speed V is larger than the predetermined speed. Finally, an actual torque command Ta is calculated by the sum of the pre-torque Tp and the RSD-torque Tn, and a current value supplied to the electromagnetic coil  39  is controlled so that the torque transmission device  30  transmits from the input portion  31  to the output portion  32  the torque according to the actual torque command Ta (in step S 8 ).
 
 Ta=Tp+Tn 
 
   Namely, the torque transmission device  30  is controlled corresponding to the pre-torque Tp and the RSD-torque Tn when the throttle-opening degree signal St is normal, while it is controlled corresponding to only the RSD-torque Tn when the throttle-opening degree signal St is abnormal. 
   According to the above-described four-wheel-drive vehicle, the throttle-opening degree signal St corresponding to how much a driver presses a throttle pedal is always detected by the throttle sensor  30  while the vehicle travels. The throttle-opening degree signal St is inputted to the ECU  50 . The torque transmission device  30  is usually controlled corresponding to the pre-torque Tp and the RSD-torque (referred to as “a normal control mode” hereinafter). Then, the torque transmission device  30  is controlled according to the pre-torque Tp mainly under a state that the vehicle runs normally, because the rotational speed difference ΔN is small or zero. Under a state that a wheel spin happens because of a rough road such as snowy road, since the rotational speed difference ΔN between the front wheels  14  and the rear wheels  15  increases, effect of the RSD-torque Tn to occupy in the actual torque command Ta increases. As a result, the torque transmitted to the rear wheels  15  through the torque transmission device  30  increases. Therefore, the wheel slip is prevented. 
   Meanwhile, it happens that the throttle-opening degree signal St becomes to be abnormal because of a failure of the throttle sensor  60 , a wire breaking or effect of some noise. Then, since the throttle-opening degree signal St becomes to be out of the predetermined range, the pre-torque Tp is made zero. Therefore, the torque transmission device  30  is controlled according to only the RSD-torque Tn (referred to as “a RSD control mode” hereinafter). Even if the throttle-opening signal degree St becomes to be abnormal when the vehicle travels on the rough road, since the torque transmission device  30  is controlled according to the RSD-torque Tn, the vehicle can run on the rough road in accordance with the RSD command mode. That is, if a wheel spin happens, since the rotational speed difference ΔN increases, the RSD-torque Tn increases. As a result, torque transmitted to the rear wheels  15  through the torque transmission device  30  increases. Therefore, the wheel slip is prevented. In a case that cause of the abnormality is, for example, temporary disconnection, the throttle-opening degree signal St returns to be a normal state immediately. Then, the control of the torque transmission device  30  can be returned to the normal control mode. 
   According to the first embodiment of the present invention, even if the throttle-opening degree signal St becomes to be abnormal, the vehicle can retain function as four-wheel-drive vehicle. 
   [Second Embodiment] 
   Since the second embodiment is same as the first embodiment except that a main program M′ is used for the second embodiment instead of the main program M in the first embodiment, description for other constructions are omitted. 
   In the second embodiment, even if the abnormality of the throttle-opening degree signal St returns to be a normal state, the control of the torque transmission device  30  does not return to the normal control mode immediately. In the main program M′ shown by  FIG. 5 , step (S 11 ) for judging whether a flag F is one (active) or not is disposed in front of step (S 4 ) for judging whether the throttle-opening signal degree St is or not the abnormal state. The flag F is set as zero initially just after an ignition is turned on, for example. The flag F keeps one (active), if it is set to one once unless the driver presses a reset button arranged in a cabin of the vehicle. In other words, the flag F is initialized to be zero when the reset button is turned. Therefore, while the flag F is zero, the torque transmission device  30  is controlled similar to that of the first embodiment. That is, since the judgement in S 11  is No, the process advances to S 4 . If the throttle-opening degree signal is normal, the torque transmission device  30  is controlled in the normal control mode in S 6  to S 8 . On the other hand, when the abnormality of the throttle-opening degree signal St happens (S 4 =Yes), the flag F is set to one (in S 12 ). The torque transmission device  30  is controlled according to only the RSD-torque Tn similar to that of the first embodiment. Since the flag F is set to one, the RSD control mode is kept even if the throttle-opening degree signal St returns to the normal. 
   According to the second embodiment of the present invention, even if the throttle-opening degree signal St is hunted between the normal and the abnormality, it is prevented that the normal control mode and the RSD control mode are repeatedly changed. Therefore, the vehicle is controlled stably. 
   [Third Embodiment] 
   Since a third embodiment is same as the first embodiment except that a main program M″ is used for the third embodiment instead of the main program M in the first embodiment, description for other constructions are omitted. 
   Similar to the second embodiment, even if the abnormality of the throttle-opening degree signal St returns to a normal state, the control of the torque transmission device  30  does not return to the normal control mode immediately in the third embodiment. In the main program M″ shown by  FIG. 6 , step (S 10 ) judging whether a predetermined time passes or not after the throttle-opening degree signal St returned to the normal state is disposed between step (S 4 ) for judging the abnormality of the throttle-opening degree signal St and step (S 6 ) for calculating the pre-torque Tp. When the throttle-opening degree signal St becomes an abnormality, a software timer is started It is judged whether a value of the timer exceeds the predetermined time or not in step (S 10 ). The value of the timer is initially set as a larger value than the predetermined value, because it prevents that the pre-torque Tp is set to zero in step (SS) at the first scan of the main program M″ after the ignition is turned on. 
   According to the third embodiment, if the throttle-opening degree signal St becomes the abnormality once, the control of the torque transmission device  30  is not returned to the normal control mode unless it passes the predetermined time after the throttle-opening signal degree St returns to the normal. Therefore, it is prevented that the normal control mode and the RSD control mode are repeatedly changed in a short period. Therefore, the vehicle is controlled stably. In addition, the reset button is not needed in comparison with the second embodiment. 
   [Modifications] 
   The front wheels  14  are always driven and the rear wheels  15  are selectively driven in the first, second and third embodiments. However, the present invention can be also applied to a four-wheel-drive vehicle that the rear wheels  16  are always driven and the front wheels  15  are selectively driven. 
   The abnormality of the throttle-opening degree signal St is judged according to whether the throttle-opening degree signal St is in the predetermined range in the first, second and third embodiments. However, other ways can be applied to judge the abnormality of the throttle-opening degree signal St. For example, it is possible that the abnormality of the throttle-opening degree signal St is judged by using lengths of its drift cycle. Further, in a case that the throttle-opening degree signal St is serial signal, it is possible that the abnormality is judged by whether the serial signal is received or not during a predetermined receiving cycle. 
   The RSD-torque Tn is calculated by the same way, i.e., by using the same map, in both of the normal control mode and the RSD control mode in the first, second and third embodiments. However, different way or different map can be applied. Further, a value that is obtained so as to multiply the RSD-torque calculated as same way with the normal control mode by some gain can be used as RSD-torque in the RSD control mode. 
   Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is thereby to be understood that within the scope of the appended claims, the present invention may be practiced otherwise than as specifically described herein.