Patent Publication Number: US-2010121544-A1

Title: Driving power distribution control apparatus, differential limiting control apparatus, method for controlling torque coupling, and method for controlling differential apparatus

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a driving power distribution control apparatus, a differential limiting control apparatus, a method for controlling a torque coupling, and a method for controlling a differential apparatus. 
     Conventionally, driving power distribution control apparatuses have been known that are located in a driving power transmission system for transmitting the torque of an engine to wheels and have a torque coupling. Based on the engaging force of a clutch mechanism, the torque coupling changes transmittable torque amount, that is, torque transmission amount, from an input side to an output side. For example, Japanese Laid-Open Patent Publication No. 2005-3167 discloses a torque coupling that includes a cylindrical first rotating member and a shaft-like second rotating member. The second rotating member is rotatably and coaxially arranged relative to the first rotating member. The torque coupling includes a clutch mechanism, which is located between the first rotating member and the second rotating member. The clutch mechanism couples the first rotating member and the second rotating member to each other so that torque can be transmitted therebetween. 
     When a four-wheel drive vehicle equipped with a driving power distribution control apparatus is running on a road surface that is partially frozen and thus includes high μ road surface and low μ road surface, the vehicle may skid if one of the wheels enters the low μ road surface. When the slipping wheel exits the low μ road surface and enters the high μ road surface, the wheel starts holding the road surface, which abruptly increases the reaction from the road surface. The speed of the wheel thus abruptly drops. This can apply a shock to the driving power transmission system (for example, the propeller shaft). 
     Accordingly, the driving power distribution control apparatus disclosed in, for example, Japanese Laid-Open Patent Publication No. 2003-320857 reduces the torque transmission amount of the torque coupling when the deceleration of a wheel is greater than or equal to a predetermined value. This prevents torque that is greater than or equal to the torque transmission amount from being transmitted to a portion of the transmission system that is located beyond the torque coupling as seen from a slipping wheel. 
     Typically, when a vehicle is moving without skidding, each wheel is holding the road surface. In this case, torsion is generated in the driving power transmitting members forming the driving power transmission system. Thus, when a four-wheel drive vehicle is running on a road surface having high μ road surface and low μ road surface, the torsion of the driving power transmitting members is released if a wheel slips on the low μ road surface, which produces torsional vibration. However, according to the configuration of the above described prior art structure, since the torque transmission amount is reduced by detecting deceleration of the wheels, the torsional vibration generated during slipping cannot be prevented. 
     This problem is not limited to a case where a torque coupling is provided in a driving power transmission system, but also occurs in a four-wheel drive vehicle having a differential apparatus that has a limited slip differential. The differential apparatus distributes torque to vehicle wheels while permitting the left wheels and the right wheels to rotate at different speeds or permitting the front wheels and the rear wheels to rotate at different speeds, and the limited slip differential limits the speed difference between the left wheels and the right wheels and between the front wheels and the rear wheels. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an objective of the present invention to provide a driving power distribution control apparatus, a differential limiting control apparatus, a method for controlling a torque coupling, and a method for controlling a differential apparatus that reduce shock applied to a driving power transmission system due to slipping of wheels. 
     To achieve the foregoing objective and in accordance with a first aspect of the present invention, a driving power distribution control apparatus for controlling driving power that is distributed from a driving power source of a vehicle to each of a plurality of wheels is provided. The apparatus includes a torque coupling, a torque transmission amount controller, and slip detection means. The torque coupling is provided in a driving power transmission system that transmits, as the driving power, torque of the driving power source to each of the wheels. Based on an engaging force of a clutch mechanism that transmits the driving power, the torque coupling is capable of changing the amount of torque transmission, which amount is torque that is transmittable from an input side to an output side. The torque transmission amount controller controls the operation of the torque coupling based on the driving state of the vehicle. The slip detection means detects slip of wheels. When the slip detection means detects that at least one of the wheels has slipped, the torque transmission amount controller reduces the torque transmission amount of the torque coupling. 
     In accordance with a second aspect of the present invention, a differential limiting control apparatus including a differential apparatus, a differential limiting force controller, and slip detection means is provided. The differential apparatus transmits torque of a driving power source of a vehicle to a first drive shaft and a second drive shaft, while permitting the first and second drive shafts to rotate at different speeds. The differential apparatus has a limited slip differential that limits the speed difference between the first drive shaft and the second drive shaft. The differential limiting force controller controls a differential limiting force of the limited slip differential. The slip detection means detects slip of any of wheels that are coupled to the first drive shaft or the second drive shaft. When the detection means detects slip of a wheel coupled to the first drive shaft or the second drive shaft, the differential limiting force controller reduces the differential limiting force of the limited slip differential. 
     In accordance with a third aspect of the present invention, a method for controlling a torque coupling provided in a driving power transmission system that transmits, as a driving power, torque of a driving power source to each of a plurality of wheels, is provided. Based on an engaging force of a clutch mechanism that transmits the driving power, the torque coupling is capable of changing the amount of torque transmission, which amount is torque that is transmittable from an input side to an output side. When it is detected that at least one of the wheels has slipped, the torque transmission amount of the torque coupling is reduced. 
     In accordance with a fourth aspect of the present invention, a method for controlling a differential apparatus that transmits torque of a driving power source of a vehicle to a first drive shaft and a second drive shaft, while permitting the first and second drive shafts to rotate at different speeds, is provided. The differential apparatus has a limited slip differential that limits the speed difference between the first drive shaft and the second drive shaft. When it is detected that at least one of the wheels has slipped, the differential limiting force of the limited slip differential is reduced. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram showing a four-wheel drive vehicle equipped with a driving power distribution control apparatus according to a first embodiment of the present invention; 
         FIG. 2  is a block diagram showing an ECU; 
         FIG. 3  is a flowchart showing a switching process of the control mode of the ECU; 
         FIG. 4  is a flowchart showing a process of switching determination from normal control to protection control; 
         FIG. 5  is a flowchart showing a process of return determination from the protection control to the normal control; 
         FIGS. 6A to 6C  are diagrams showing a condition in which a four-wheel drive vehicle runs on a road partially having a low μ road surface; 
         FIG. 7  is a diagram showing a four-wheel drive vehicle equipped with a differential limiting control apparatus according to a second embodiment of the present invention; and 
         FIG. 8  is a diagram showing a four-wheel drive vehicle equipped with a differential limiting control apparatus according to a third embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Embodiment 
     A first embodiment of the present invention will now be described with reference to the drawings. 
     As shown in  FIG. 1 , a vehicle  1  is a front drive-based four-wheel drive vehicle. An engine  2  serving as a driving power source is mounted in a front portion (a left portion as viewed in  FIG. 1 ) of the vehicle  1 . A transaxle  3  is attached to the engine  2 . The transaxle  3  includes a transmission, a transfer case, and a front differential. A pair of right and left front axles  4 R,  4 L are coupled to the transaxle  3 . A propeller shaft  5  is coupled to the transaxle  3 . The propeller shaft  5  can be coupled to a pinion shaft (drive pinion shaft)  7  with a torque coupling  6 . The pinion shaft  7  is coupled to a pair of right and left rear axles  9 R,  9 L with a rear differential  8  in between. The rear differential  8  is configured to permit the left rear axle  9 L and the right rear axle  9 R to rotate at different speeds and to distribute the torque of the engine  2  transmitted through the pinion shaft  7  to the right and left rear axles  9 R,  9 L in accordance with the speed difference of the rear axles  9 L,  9 R. A differential carrier  11  is fixed to a frame (not shown) of the vehicle  1 . The torque coupling  6 , together with the rear differential  8 , is accommodated in the differential carrier  11 . 
     That is, the torque of the engine  2  is constantly transmitted to the right and left front wheels  12 R,  12 L via the transaxle  3  and the right and left front axles  4 R,  4 L. When the propeller shaft  5  and the pinion shaft  7  are coupled to each other by the torque coupling  6  so that torque can be transmitted therebetween, the torque of the engine  2  is transmitted to right and left rear wheels  13 R,  13 L through the propeller shaft  5 , the pinion shaft  7 , the rear differential  8 , and the right and left rear axles  9 R,  9 L. 
     Therefore, in the first embodiment, the right and left front wheels  12 R,  12 L function as main drive wheels, to which the torque of the engine  2  is always transmitted, and the right and left rear wheels  13 R,  13 L function as auxiliary drive wheels, to which the torque of the engine  2  is transmitted as necessary. Driving power transmitting members, which include the transaxle  3 , the right and left front axles  4 R,  4 L, the propeller shaft  5 , the torque coupling  6 , the pinion shaft  7 , the rear differential  8 , the right and left rear axles  9 R,  9 L, form a driving power transmission system that transmits the torque of the engine  2  to the wheels  12 R,  12 L,  13 R,  13 L. 
     The torque coupling  6  includes an electromagnetic clutch  16 , which serves as a clutch mechanism. The electromagnetic clutch  16  includes an electromagnetic coil  15  and a plurality of clutch plates located in the vicinity of the propeller shaft  5  and the pinion shaft  7 . In accordance with the amount of current supplied to the electromagnetic coil  15 , the frictional engaging force the clutch plates is changed. Based on the frictional engaging force of the electromagnetic clutch  16 , the torque coupling  6  inputs torque from the propeller shaft  5  on the input side and outputs the torque to the pinion shaft  7  on the output side away from the engine  2 . That is, the torque coupling  6  (the electromagnetic clutch  16 ) adjusts the torque that can be transmitted to the right and left rear wheels  13 R,  13 L, which serve as auxiliary drive wheels. In other words, the torque coupling  6  adjusts the torque transmission amount. 
     The electrical configuration of the vehicle  1 , which is constructed as described above, will now be described. 
     The torque coupling  6  is connected to an ECU (electronic control unit)  21 , which functions as a torque transmission amount controller and slip detection means. As shown in  FIG. 2 , the ECU  21  includes a microcomputer  22  and a drive circuit  23 . 
     The microcomputer  22  includes a CPU  25 , which performs various computations, a ROM  26 , which stores control programs, a RAM  27 , which functions as a working area of the CPU  25 , and an input-output circuit (I/O)  28 , which inputs and outputs signals from and to various types of sensors and the drive circuit  23 . The CPU  25 , the ROM  26 , the RAM  27 , and the input-output circuit (I/O) circuit  28  exchange data with each other through a bidirectional bus. The CPU  25  also includes a timer  29 . The timer  29  measures time based on a command from the CPU  25 . 
     Through operations of the microcomputer  22  and the drive circuit  23 , the ECU  21  supplies drive current to the electromagnetic coil  15  of the electromagnetic clutch  16  in accordance with the driving state of the vehicle  1 . Through the supply of current, the ECU  21  controls the operation of the torque coupling  6 , thereby changing the torque transmission amount. That is, the torque coupling  6  and the ECU  21  form a driving power distribution control apparatus. 
     Specifically, as shown in  FIGS. 1 and 2 , the ECU  21  is connected to an accelerator pedal position sensor  31  and a vehicle wheel speed sensors  32   a  to  32   d.  Based on the right front wheel speed Vfr and the left front wheel speed VFl, and the right rear wheel speed Vrr and the left rear wheel speed Vrl detected by the wheel speed sensors  32   a  to  32   d,  the ECU  21  computes the vehicle speed V and a front-rear wheel speed difference AW between the front wheels  12 R,  12 L and the rear wheels  13 R,  13 L. In the first embodiment, the ECU  21  sets, as a vehicle speed V, the average value of the right rear wheel speed Vrr and the left rear wheel speed Vrl, and sets, as front-rear wheel speed difference ΔW, the difference between the average value of the right front wheel speed Vfr and the left front wheel speed Vfl and the average value of the right rear wheel speed Vrr and the left rear wheel speed Vrl. The ECU  21  computes a control target value (target torque τp) based on the vehicle speed V, the front-rear wheel speed difference ΔW, and the accelerator pedal depression degree Sa. 
     Specifically, by referring to a torque map stored in the ROM  26 , the ECU  21  computes a first torque based on the vehicle speed V and the accelerator pedal depression degree Sa, and a second torque based on the vehicle speed V and the front-rear wheel speed difference ΔW. Next, the ECU  21  adds up the first torque and the second torque to compute the target torque τp, which corresponds to the current vehicle speed V, the accelerator pedal depression degree Sa, and the front-rear wheel speed difference ΔW. The torque map is configured such that the lower the vehicle speed V and the greater the accelerator pedal depression degree Sa, the greater the first torque becomes, and that the lower the vehicle speed V and the greater the front-rear wheel speed difference ΔW, the greater the second torque becomes. 
     The ECU  21  the supplies an electric current to the electromagnetic clutch  16  so as to generate a frictional engaging force that corresponds to the determined target torque τp. Accordingly, the ECU  21  controls the operation of the torque coupling  6 , or the distribution of the drive force between the right and left front wheels  12 R,  12 L and the right and left rear wheels  13 R,  13 L. 
     The ECU  21  executes protection control for reducing a shock applied to the driving power transmission system when one of the wheels slips. 
     The ECU  21  executes the protection control when only one of the wheels  12 R,  12 L,  13 R,  13 L slips, so as to reduce the target torque τp to a predetermined torque τth or lower. The predetermined torque τth is a value at which it is possible to prevent torsional vibration generated when torsion of a specific driving power transmitting member (for example, the left front axle  4 L) is released from being transmitted to the other driving power transmitting members. For example, the predetermined torque τth is set to zero. 
     In contrast to the protection control, control mode in which the target torque τp is determined based on the driving state of the vehicle  1  (the vehicle speed V, the front-rear wheel speed difference ΔW, and the accelerator pedal depression degree Sa) is referred to as normal control. 
     The normal control and the protection control will now be described. The ECU  21  computes differences between the wheel speed of one of the wheels  12 R,  12 L,  13 R,  13 L (for example, the right front wheel  12 R) and the speeds of the other wheels (the wheels  12 L,  13 R,  13 L). Then, the ECU  21  performs the same computation for all the wheels  12 R,  12 L,  13 R,  13 L, and determines whether the vehicle is skidding based on the wheel speed differences between the wheels  12 R,  12 L,  13 R,  13 L (between the four wheels). 
     In the normal control, the ECU  21  determines whether slip is taking place by taking into consideration whether the wheel speed of any one of the wheels is greater than or equal to a value calculated by adding a first threshold amount K 1  to an average wheel speed of the other three wheels, and whether all the wheel speed differences between the latter three wheels are less than or equal to a second threshold value K 2 . When the wheel speed of one of the wheels is greater than or equal to the value calculated by adding the first threshold amount K 1  to the average wheel speed of the other three wheels, and all the wheel speed differences between the latter three wheels are less than or equal to the second threshold value K 2 , the ECU  21  determines that only the first wheel has slipped and switches the control mode from the normal control to the protective mode. 
     In the protection control, the ECU  21  determines whether the slipping of only the one wheel has continued for a predetermined period (for example, 200 msec). That is, the ECU  21  determines whether a state has continued for a predetermined period Tth in which state the wheel speed of one of the wheels is greater than or equal to the value calculated by adding the first threshold amount K 1  to the average wheel speed of the other three wheels, and all the wheel speed differences between the latter three wheels are less than or equal to the second threshold value K 2 . If slipping of only one of the wheels has continued for the predetermined period Tth, the ECU  21  determines that torsional vibration has been damped and the shock applied to the driving power transmission system has been decreased. In this case, the ECU  21  switches the control mode from the protection control to the normal control. 
     During the protection control, the ECU  21  determines whether the wheel speed differences between the four wheels are all less than or equal to the second threshold value K 2 . When the wheel speed differences between the four wheels are all less than or equal to the second threshold value K 2 , the ECU  21  determines that slipping of any of the wheels has stopped, and switches the control mode from the protection control to the normal control. 
     Further, in the protection control, the ECU  21  determines whether the accelerator pedal depression degree Sa is less than or equal to a predetermined depression degree Sath. The predetermined depression degree Sath corresponds to the depression degree when the driver is substantially not depressing the accelerator pedal (not shown). When the accelerator pedal depression degree Sa is less than or equal to the predetermined depression degree Sath, the ECU  21  determines that slipping of any of the wheels has stopped because the output from the engine  2  is substantially stopped, and switches the control mode from the protection control to the normal control. 
     Next, an operation of the driving power distribution control apparatus according to the first embodiment will be described with reference to the flowcharts of  FIGS. 3 to 5 , which represent a procedure executed by ECU  21 . 
     While the vehicle  1  is running on a road  33  as shown in  FIGS. 6A to 6C , the ECU  21  repeats the procedure of steps S 1  to S 5  shown in the flowchart of  FIG. 3  at a predetermined cycle. To facilitate illustration, a low μ road surface  33   b  is illustrated with hatching so as to be distinguished from a high μ road surface  33   a  in  FIGS. 6A to 6C . 
     First, at step S 1 , the ECU  21  obtains various state quantities (the accelerator pedal depression degree Sa, the wheel speeds Vfr, Vfl, Vrr, Vrl) from the accelerator pedal position sensor  31  and the wheel speed sensors  32   a  to  32   d . Subsequently, based on the accelerator pedal depression degree Sa and the wheel speeds Vfr, Vfl, Vrr, Vrl, the ECU  21  computes the wheel speed differences between the four wheels (step S 2 ). 
     After obtaining the wheel speed differences between the four wheels, the ECU  21  determines whether the current control mode is the normal control (step S 3 ). If the current control mode is the normal control mode (YES at step S 3 ), the ECU  21  executes a switching determination process for determining whether to switch the control mode from the normal control mode to the protection control mode (step S 4 ). 
     In the switching determination process, the ECU  21  determines whether the wheel speed of any one of the four wheels is greater than or equal to the value calculated by adding the first threshold amount K 1  to the average wheel speed of the other three wheels as shown in  FIG. 4  (step S 4 - 1 ). That is, based on the computation results obtained at step S 2 , the ECU  21  determines whether the wheel speed of the slipping one of the four wheels is greater than or equal to the value calculated by adding the first threshold amount K 1  to the average wheel speed of the other three wheels, which are not slipping. 
     When the vehicle  1  is running on the high μ road surface  33   a  of the road  33  as shown in  FIG. 6A , the ECU  21  determines that the wheels  12 R,  12 L,  13 R,  13 L are not slipping and none of the wheels  12 R,  12 L,  13 R,  13 L is rotating at a wheel speed greater than or equal to the value calculated by adding the first threshold amount K 1  to the average wheel speed of the other three wheels (NO at step S 4 - 1 ). The ECU  21  returns to step S 1  while maintaining the control mode at the normal control mode. At this time, since the wheels  12 R,  12 L,  13 R,  13 L hold the high μ road surface  33   a,  torsion is occurring in the driving power transmitting members such as the left front axle  4 L. 
     In contrast, when the vehicle  1  advances and the left front wheels  12 L enters the low μ road surface  33   b,  the left front wheel  12 L slips. Thus, the wheel speed of the left front wheel  12 L becomes greater than or equal to the value calculated by adding the first threshold amount K 1  to the average wheel speed of the other three wheels (the wheels  12 R,  13 R,  13 L) (YES at step S 4 - 1 ). Thus, the ECU  21  determines whether the wheel speed differences between the other three wheels are all less than or equal to the second threshold value K 2  (step S 4 - 2 ). 
     At this time, since the other three wheels (the wheels  12 R,  13 R,  13 L) are holding the high μ road surface  33   a  in the state shown in  FIG. 6B , the wheel speed differences between the three wheels are all less than or equal to the second threshold value K 2 . Accordingly, the ECU  21  determines that only the left front wheel  12 L is slipping. 
     When determining that only the left front wheel  12 L is slipping (YES at step S 4 - 2 ), the ECU  21  switches the control mode from the normal control mode to the protection control mode (step S 4 - 3 ). After switching to the protection control mode, the ECU  21  returns to step S 1 . If, for example, two of the four wheels are slipping (NO at step S 4 - 2 ), the ECU  21  returns to step S 1  while maintaining the control mode at the normal control mode. 
     After switching to the protection control mode, the ECU  21  continues the protection control until the control mode is switched to the normal control mode. 
     That is, the ECU  21  reduces the target torque τp to value less than or equal to the predetermined torque τth, thereby preventing the torsional vibration produced by the release of torsion of the left front axle  4 L from being transmitted to a portion of the driving power transmission system that is located beyond the torque coupling  6  as seen from the left front wheel  12 L, that is, to the pinion shaft  7 , the rear differential  8 , and the right and left rear axles  9 R,  9 L. 
     When the protection control mode is started, the ECU  21  determines that the control mode has been switched from the normal control mode to the protection control mode at step S 3  (NO at step S 3 ). Then, the ECU  21  executes a return determination process for determining whether to return the control mode from the protection control mode to the normal control mode (step S 5 ). 
     In the return determination process, the ECU  21  determines whether the wheel speed of any one of the four wheels (in this case, the left front wheel  12 L) is greater than or equal to the value calculated by adding the first threshold amount K 1  to the average wheel speed of the other three wheels as shown in  FIG. 5  (step S 5 - 1 ). That is, based on the computation results obtained at step S 2 , the ECU  21  determines whether the wheel speed of the slipping left front wheel  12 L is greater than or equal to the value calculated by adding the first threshold amount K 1  to the average wheel speed of the other three wheels, which are not slipping. 
     In the state shown in  FIG. 6B , the left front wheel  12 L continues slipping, and the wheel speed of the left front wheel  12 L is greater than or equal to the value calculated by adding the first threshold amount K 1  to the average wheel speed of the other three wheels (the wheels  12 R,  13 R,  13 L) (YES at step S 5 - 1 ). Thus, the ECU  21  determines whether the wheel speed differences between the other three wheels are all less than or equal to the second threshold value K 2  (step S 5 - 2 ). 
     At this time, the other three wheels (the wheels  12 R,  13 R,  13 L) are holding the high μ road surface  33   a.  Thus, the wheel speed differences between the three wheels are all less than or equal to the second threshold value K 2 . Accordingly, the ECU  21  determines that only the left front wheel  12 L is slipping (YES at step S 5 - 2 ) and proceeds to step S 5 - 3 . 
     At step S 5 - 3 , the ECU  21  increments a count value T of the incorporated timer  29  by one. Thereafter, the ECU  21  determines whether the count value T has become greater than or equal to a predetermined value (the predetermined period Tth). The predetermined period Tth is a period required for torsional vibration to be damped after the left front wheel  12 L starts slipping and for shock applied to the driving power transmission system to become small. At this point, since the left front wheel  12 L has just started slipping, the ECU  21  determines that the predetermined period Tth has not elapsed. 
     When determining that the left front wheel  12 L has not continued slipping for the predetermined period Tth (NO at step S 5 - 4 ), the ECU  21  determines whether the accelerator pedal depression degree Sa is less than or equal to the predetermined depression degree Sath (step S 5 - 5 ). Specifically, the ECU  21  determines whether the driver has released the accelerator pedal to substantially stop the driving power output from the engine  2 , thereby decreasing the slipping of the left front wheel  12 L. 
     At this time, since the left front wheel  12 L has just started slipping, and the elapsed time is less than or equal to the predetermined period Tth, the driver is still stepping on the accelerator pedal. Therefore, at this point, the ECU  21  determines that the driver has not released the accelerator pedal and the accelerator pedal depression degree Sa has not become less than or equal to the predetermine depression degree Sath. In this case, the ECU  21  proceeds to step S 5 - 6 . 
     At step S 5 - 6 , the ECU  21  determines whether the wheel speed differences between the four wheels are all less than or equal to the second threshold value K 2 . Here, the ECU  21  determines whether the left front wheel  12 L has exited the low μ road surface  33   b.  That is, after the left front wheel  12 L exits the low μ road surface  33   b,  since the wheels  12 R,  12 L,  13 R,  13 L hold the high μ road surface  33   a,  the wheel speed differences between the four wheels are all less than or equal to the second threshold value K 2 . That is, by determining whether the wheel speed differences between the four wheels are all less than or equal to the second threshold value K 2 , the ECU  21  determines whether the left front wheel  12 L has exited the low μ road surface  33   b.    
     At this point, since the left front wheel  12 L has not exited the low μ road surface  33   b,  the ECU  21  determines that the wheel speed differences of the four wheels are all less than or equal to the second threshold value K 2  (NO at step  5 - 6 ). Then, the ECU  21  returns to step S 1  and repeats the protection control mode. 
     When the vehicle  1  advances further, the left front wheel  12 L exits the low μ road surface  33   b.  Then, when determining that the wheel speed differences between the four wheels are all less than or equal to the second threshold value K 2  (YES at step  5 - 6 ). At step S 5 - 6 , the ECU  21  switches the control mode from the protective mode to the normal control mode (step S 5 - 7 ). After switching to the normal control mode, the ECU  21  returns to step S 1  and continues the normal control until the control mode is switched to the protection control mode. 
     If the ECU  21  determines that the accelerator pedal depression degree Sa has become less than or equal to the predetermined depression degree Sath (YES at step S 5 - 5 ), that is, if the ECU  21  determines that the driving power output from the engine  2  is substantially stopped and that the slipping of the left front wheel  12 L has subsided, the ECU  21  moves to step S 5 - 7  and switches the control mode from the protective mode to the normal mode. 
     Further, if the ECU  21  determines that the left front wheel  12 L has been slipping for the predetermined period Tth at step S 5 - 4  (YES at step S 5 - 4 ), the ECU  21  resets the count value T of the timer  29  and switches the control mode from the protective mode to the normal control mode (step S 5 - 7 ). 
     Further, when the wheel speed of the left front wheel  12 L is less than the value calculated by adding the first threshold amount K 1  to the average wheel speed of the other three wheels (NO at step S 5 - 1 ), or when the wheel speed differences between the other three wheels are all not less than or equal to the second threshold value K 2  at step S 5 - 2  (NO at step S 5 - 2 ), the ECU  21  determines that the slipping of only the left front wheel  12 L has subsided. At this time, the ECU  21  resets the count value T of the timer  29  (step S 5 - 9 ) and proceeds to step S 5 - 5 . 
     As described above, the first embodiment has the following advantages. 
     (1) The driving power distribution control apparatus of the present invention includes the torque coupling  6  and the ECU  21 . The torque coupling  6  is provided in the driving power transmission system for transmitting the torque of the engine  2  of the vehicle  1  to each of the wheels  12 R,  12 L,  13 R,  13 L. Based on the frictional engaging force of the electromagnetic clutch  16 , the torque coupling  6  is capable of changing the amount of torque that can be transmitted to the right and left rear wheels  13 R,  13 L, which serve as auxiliary drive wheels. The ECU  21  controls the operation of the torque coupling  6  based on the driving state of the vehicle. The ECU  21  reduces the torque transmission amount of the torque coupling  6  to a value less than or equal to the predetermined torque τth when only one of the wheels  12 R,  12 L,  13 R,  13 L is slipping. When any one of the four wheels (the left front wheel  12 L) slips, the torsion in a driving power transmitting member (the left front axle  4 L) is released. This generates torsional vibration. The configuration of the first embodiment prevents the torsional vibration from being transmitted to a portion of the driving power transmission system that is located beyond the torque coupling  6  as seen from the slipping left front wheel  12 L. 
     (2) When the wheel speed of any one of the wheels  12 R,  12 L,  13 R,  13 L is greater than or equal to the value calculated by adding the first threshold amount K 1  to the average wheel speed of the other three wheels, and all the wheel speed differences between the latter three wheels are less than or equal to the second threshold value K 2 , the ECU  21  determines that only one of the four wheels has slipped. Thus, for example, when the wheel speed of the left front wheel  12 L is greater than or equal to the value calculated by adding the first threshold amount K 1  to the average wheel speed of the other three wheels (the wheels  12 R,  13 R,  13 L), and the wheel speed of the right front wheel  12 R is greater than or equal to the value calculated by adding the first threshold amount K 1  to the average wheel speed of the other two wheels (the wheels  13 R,  13 L), in other words, when two wheels are slipping, the ECU  21  does not detects that only one of the wheels is slipping. Thus, slipping of only one wheel is reliably detected. 
     (3) When only one wheel slips for a predetermined period, the ECU  21  switches the control mode to the normal control. That is, when only one wheel slips for a predetermined period and torsional vibration is damped, the ECU  21  switches the control mode to the normal control. Accordingly, sufficient torque is distributed to the right and left rear wheels  13 R,  13 L in accordance with the driving state of the vehicle, which improves the traction performance. 
     (4) The ECU  21  switches the control mode to the normal control when determining that the wheel speed differences between the four wheels are all less than or equal to the second threshold value K 2 . That is, when a slipping wheel exits the low μ road surface  33   b  and holds the road  33  (the high μ road surface  33   a ) so that the slipping has subsided, the ECU  21  switches the control mode to the normal control. Accordingly, sufficient torque is distributed to the right and left rear wheels  13 R,  13 L in accordance with the driving state of the vehicle, which improves the traction performance. 
     (5) When the accelerator pedal depression degree Sa is less than or equal to the predetermined depression degree Sath, the ECU  21  switches the control mode to the normal control. That is, when the accelerator pedal depression degree Sa is less than or equal to the predetermined depression degree Sath, and slipping of a wheel has subsided because the driving power output from the engine  2  is substantially stopped, the ECU  21  switches the control mode to the normal control. Accordingly, sufficient torque is distributed to the right and left rear wheels  13 R,  13 L in accordance with the driving state of the vehicle, which improves the traction performance. 
     Second Embodiment 
     A second embodiment of the present invention will now be described with reference to the drawings. 
     For purposes of illustration, like or same reference numerals are given to those components that are like or the same as the corresponding components of the first embodiment and detailed explanations are omitted. 
     As shown in  FIG. 7 , a rear differential  8  serving as a differential apparatus is coupled to a right rear axle  9 R and a left rear axle  9 L, each of which serves a first drive axle, as in the case of the first embodiment. 
     The rear differential  8  includes an electromagnetic clutch  42 , which serves as a limited slip differential. The frictional engaging force of the electromagnetic clutch  42  is changed in accordance with the amount of current supplied to an electromagnetic coil  41 . The electromagnetic clutch  42  is configured to change differential limiting force (frictional engaging force), which limits the speed difference between the right rear axle  9 R and the left rear axle  9 L, in accordance with the amount of current supplied to the electromagnetic coil  41 . 
     Also, the rear differential  8  (the electromagnetic clutch  42 ) is connected to the ECU  21 , which functions as a differential limiting force controller. The ECU  21  supplies drive current to the electromagnetic coil  41  in accordance with the driving state of the vehicle  1  to control the operation of the electromagnetic clutch  42 , thereby controlling the differential limiting force. Therefore, in the second embodiment, the rear differential  8 , the electromagnetic clutch  42 , and the ECU  21  form a differential limiting control apparatus. 
     When only one of the right rear wheel  13 R coupled to the right rear axle  9 R and the left rear wheel  13 L coupled to the left rear axle  9 L slips, the ECU  21  executes a protection control to reduce the differential limiting force of the electromagnetic clutch  42  to a value less than or equal to a predetermined differential limiting force. The predetermined differential limiting force is a value at which it is possible to prevent torsional vibration generated when torsion of a driving power transmitting member (for example, the left rear axle  9 L) is released from being transmitted to the other driving power transmitting members. For example, the predetermined differential limiting force is set to zero. 
     Accordingly, when only one of the right and left rear wheels  13 R,  13 L slips, the ECU  21  reduces the differential limiting force of the electromagnetic clutch  42  to a value less than or equal to the predetermined differential limiting force. This configuration prevents shock from being transmitted to a portion of the driving power transmission system that is located beyond the electromagnetic clutch  42  as seen from the slipping wheel (the right rear wheel  13 R or the left rear wheel  13 L). 
     Specifically, for example, when the left rear wheel  13 L slips, the torsion in the left rear axle  9 L is released. This generates torsional vibration. The configuration prevents the torsional vibration from being transmitted to the transaxle  3 , the right and left front axles  4 R,  4 L, the propeller shaft  5 , the torque coupling  6 , the pinion shaft  7 , and the right rear axle  9 R. 
     The second embodiment provides the same advantages as the first embodiment. 
     Third Embodiment 
     A third embodiment of the present invention will now be described with reference to the drawings. 
     For purposes of illustration, like or same reference numerals are given to those components that are like or the same as the corresponding components of the first embodiment and detailed explanations are omitted. 
     As shown in  FIG. 8 , a vehicle  1  is a rear drive-based four-wheel drive vehicle. A transmission  51  is attached to the engine  2 . The transmission  51  is coupled to a center differential  53 , which function s as a differential apparatus, through an input shaft  52 . The center differential  53  is coupled to a first propeller shaft  54  serving as a first drive shaft and a second propeller shaft  55  serving as a second drive shaft. The first propeller shaft  54  is coupled to a pair of right and left front axles  4 R,  4 L with a front differential  56  in between. The second propeller shaft  55  is coupled to a pair of right and left rear axles  9 R,  9 L with a rear differential  8  in between. 
     The center differential  53  allows the first propeller shaft  54  and the second propeller shaft  55  to rotate at different speeds, and distributes torque transmitted through the input shaft  52  to the first and second propeller shafts  54 ,  55  in accordance with the speed difference. 
     The center differential  53  includes an electromagnetic clutch  58 , which serves as a limited slip differential. The frictional engaging force of the electromagnetic clutch  58  is changed in accordance with the amount of current supplied to an electromagnetic coil  57 . The electromagnetic clutch  58  is configured to change differential limiting force (frictional engaging force), which limits the speed difference between the first propeller shaft  54  and the second propeller shaft  55 , in accordance with the amount of current supplied to the electromagnetic coil  57 . 
     Therefore, the torque of the engine  2  is first transmitted to the center differential  53  from the transmission  51  through the input shaft  52 . Then, the torque of the engine  2  is transmitted from the center differential  53  to the right and left front wheels  12 R,  12 L via the first propeller shaft  54 , the front differential  56 , and the right and left front axles  4 R,  4 L. Also, the torque of the engine  2  is transmitted from the center differential  53  to the right and left rear wheels  13 R,  13 L via the second propeller shaft  55 , the rear differential  8 , and the right and left rear axles  9 R,  9 L. 
     In the third embodiment, the driving power transmitting members, which include the transmission  51 , the input shaft  52 , the center differential  53 , the right and left front axles  4 R,  4 L, the first and second propeller shafts  54 ,  55 , the front differential  56 , the rear differential  8 , the right and left rear axles  9 R,  9 L, form a driving power transmission system. 
     Also, the center differential  53  (the electromagnetic clutch  58 ) is connected to the ECU  21 , which functions as a differential limiting force controller. The ECU  21  supplies drive current to the electromagnetic coil  57  in accordance with the driving state of the vehicle  1  to control the operation of the electromagnetic clutch  58 , thereby controlling the differential limiting force. Therefore, in the third embodiment, the center differential  53 , the electromagnetic clutch  58 , and the ECU  21  form a differential limiting control apparatus. 
     When only one of the right and left front wheels  12 R,  12 L coupled to the first propeller shaft  54  and the right and left rear wheels  13 R,  13 L coupled to the second propeller shaft  55  slips, that is, when only one of the four wheels slips, the ECU  21  executes a protection control to reduce the differential limiting force of the electromagnetic clutch  58  to a value less than or equal to a predetermined differential limiting force as in the first embodiment. 
     Accordingly, when only one of the four wheels slips, the ECU  21  reduces the differential limiting force of the electromagnetic clutch  58  to a value less than or equal to the predetermined differential limiting force. This configuration prevents shock from being transmitted to a portion of the driving power transmission system that is located beyond the electromagnetic clutch  58  as seen from the slipping wheel. 
     Specifically, for example, torsional vibration that is generated when torsion of the left front axle  4 L is released by slipping of the left front wheel  12 L is prevented from being transmitted to the second propeller shaft  55 , the rear differential  8 , and the right and left rear axles  9 R,  9 L. 
     The third embodiment provides the same advantages as the first embodiment. 
     The above described embodiments may be modified as follows. 
     In the first and second embodiments, the torque coupling  6  is located between the propeller shaft  5  and the pinion shaft  7 . However, the torque coupling  6  may be located elsewhere in the driving power transmission system. For example, the torque coupling  6  may be located between the rear differential  8  and the right rear wheel  13 R and between the rear differential  8  and the left rear wheel  13 L. 
     In the first and second embodiments, the present invention is applied to the vehicle  1  in which the right and left front wheels  12 R,  12 L function as main drive wheels. Instead, the present invention may be applied to a vehicle  1 , in which the right and left rear wheels  13 R,  13 L function as main drive wheels. Also, in the third embodiment, the present invention may be applied to a vehicle in which the right and left front wheels  12 R,  12 L function as main drive wheels. 
     In the second embodiment, the electromagnetic clutch  42  serving as a limited slip differential is provided in the rear differential  8 . Instead, an electromagnetic clutch serving as a limited slip differential may be provided in a front differential, and the same control as the second embodiment may be executed. 
     In the above illustrated embodiments, if the slipping of only one of the wheels has continued in the protection control, the ECU  21  determines that torsional vibration has been damped and the shock applied to the driving power transmission system has been decreased, and switches the control mode to the normal control. However, the ECU  21  does not necessarily need to switch the control mode to the normal control even if slipping continues for a predetermined period. In this case, when the slipping wheel (for example, the left front wheel  12 L) exits the low μ road surface  33   b  and holds the road  33 , torque the amount of which is greater than or equal to the torque transmission amount of the torque coupling  6  is reliably prevented from being transmitted to a portion of the driving power transmission system that is located beyond the torque coupling  6  as seen from the left front wheel  12 L. 
     In the illustrated embodiments, when the wheel speed differences between the four wheels are all less than or equal to the second threshold value K 2 , the ECU  21  determines that slipping of a wheel has subsided, and switches the control mode to the normal control. Instead, the ECU  21  does not necessarily need to switch the control mode to the normal control when slipping of a wheel subsides. 
     Further, in the illustrated embodiment, when the accelerator pedal depression degree Sa is less than or equal to the predetermined depression degree Sath, the ECU  21  determines that slipping of a wheel has subsided, and switches the control mode from the protection control to the normal control. Instead, the ECU  21  does not necessarily need to switch the control mode to the normal control when the accelerator pedal depression degree Sa is less than or equal to the predetermined depression degree Sath. 
     In the protection control, the ECU  21  may switch the control mode to the normal control when a condition other than those presented above is met. 
     In the illustrated embodiments, when the wheel speed of any one of the wheels is greater than or equal to the value calculated by adding the first threshold amount K 1  to the average wheel speed of the other three wheels, and all the wheel speed differences between the latter three wheels are less than or equal to the second threshold value K 2 , the ECU  21  determines that only the first wheel has slipped. However, the present invention is not limited to this. For example, the ECU  21  may determine that only one wheel has slipped on condition only that the wheel speed of one wheel is greater than or equal to the value calculated by adding the first threshold amount K 1  to the average wheel speed of the other three wheels. Besides this, the ECU  21  may detect slipping by other methods, for example, by using acceleration of the wheels. 
     In the illustrated embodiments, the ECU  21  switches the control mode to the protective mode when only one of the wheels  12 R,  12 L,  13 R,  13 L slips. However, the ECU  21  may switch the control mode to the protective mode when two or more of the wheels  12 R,  12 L,  13 R,  13 L slip.