Control device for four-wheel drive vehicle

Abstract

To make it possible to appropriately control the distribution of driving force according to the friction coefficient of a road surface in a four-wheel drive vehicle capable of traveling in a driving state through a control method in which the distribution of driving force to sub-driving wheels is increased when the occurrence of a slip is detected.SOLUTION: A control device 100 includes a slip determination section 102. The control device 100 increases the distribution of driving force transmitted to front wheels 14L and 14R relative to driving force transmitted to rear wheels 16L and 16R on condition that the occurrence of a slip is determined by the slip determination section 102. The slip determination section 102 determines the occurrence state of the slip while making it part of or the whole of a criterion that a wheel slip ratio S in a vehicle 10 exceeds a threshold L. The slip determination section 102 sets the threshold L such that the threshold L becomes higher as lateral acceleration acting on the vehicle 10 becomes higher.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a control device for a four-wheel drive vehicle. [Background technology]

[0002] Conventionally, a control system for an all-wheel drive vehicle, such as that disclosed in Patent Document 1 below, has been provided as a control device for a four-wheel drive vehicle. The control system of Patent Document 1 controls the connection between a first axle that is temporarily driven by a prime mover, a second axle that is constantly connected to the prime mover via a propeller shaft, and a sub-shaft that drivingly connects the propeller shaft and the first axle. This control system includes a first clutch that disconnectably connects the propeller shaft and the sub-shaft, a second clutch that disconnectably connects the sub-shaft and the first axle, a synchronization determination means that determines whether the second clutch is connectable, and an engagement determination means that determines whether the second clutch is engaged. By configuring this control system, Patent Document 1 aims to enable rapid switching between 2WD mode and AWD mode while preventing shock or vibration fluctuations in transmission torque. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2020-032773 Summary of the Invention [Problem to be solved by the invention]

[0004] Here, the inventors have studied the problems that can be expected when a control system is used in a four-wheel drive vehicle equipped with a first clutch that selectively disconnects or connects the power transmission path between the driving force source and the power transmission member, and a second clutch that selectively disconnects or connects the power transmission path between the power transmission member and the auxiliary drive wheels, to detect the occurrence of slippage and increase the distribution of driving force to the auxiliary drive wheels.

[0005] As a result, it was found that even on so-called high μ roads, where the friction coefficient is high and slippage is unlikely to occur, slippage may occur when making a sharp turn because the ground load on the wheels on the inside of the turning radius (inner wheels) becomes lighter.In addition, it was found that in such cases, if slippage is detected and control is performed to increase the distribution of driving force to the auxiliary drive wheels, while slippage will be reduced, there is a concern that the so-called tight corner braking phenomenon may occur due to the difference in rotation between the front and rear wheels.

[0006] Therefore, the present invention aims to provide a control device for a four-wheel drive vehicle that can appropriately control the distribution of driving force in accordance with the friction coefficient of the road surface in a four-wheel drive vehicle that can run in a driving state using a control method that detects the occurrence of slippage and increases the distribution of driving force to the auxiliary drive wheels. [Means for solving the problem]

[0007] After extensive research, the inventors have discovered that the lateral acceleration acting on a vehicle can be an index that accurately reflects road surface conditions, and that using lateral acceleration as an index allows for appropriate drive force distribution control in accordance with road surface conditions. Specifically, since it is assumed that the greater the lateral acceleration, the higher the road surface condition, making it less likely for slip to occur due to a higher friction coefficient, the inventors have discovered that using the magnitude of lateral acceleration as an index allows for accurate understanding of road surface conditions and appropriate drive force distribution control.

[0008] (1) A control device for a four-wheel drive vehicle of the present invention, which is provided based on such findings, includes a first clutch that selectively connects or disconnects a power transmission path between a driving force source and a power transmission member, and a second clutch that selectively connects or disconnects a power transmission path between the power transmission member and auxiliary drive wheels, and has the following modes: a two-wheel drive mode in which driving force is transmitted from the driving force source to left and right main drive wheels by disengaging at least one of the first clutch and the second clutch; a four-wheel drive mode in which driving force is also transmitted from the driving force source to the left and right auxiliary drive wheels by engaging the first clutch and the second clutch, respectively; and a four-wheel drive mode in which the degree of engagement of the first clutch is changed while the second clutch is engaged, on the condition that slippage is detected in the four-wheel drive vehicle, to adjust the relationship between the driving force transmitted to the main drive wheels and the auxiliary drive wheels. and a variable drive distribution state in which drive control is performed using a standby control method to increase the distribution of drive force transmitted to the auxiliary drive wheels. In the variable drive distribution state, the slip determination unit determines whether slip has occurred using, as part or all of the conditions for determination, that the slip ratio of a wheel in the four-wheel drive vehicle exceeds a threshold value. The slip determination unit increases the distribution of drive force transmitted to the auxiliary drive wheels relative to the drive force transmitted to the main drive wheels, using, as part or all of the conditions, that slip has occurred. The slip determination unit sets the threshold value using, as part or all of the indicator, the lateral acceleration acting on the four-wheel drive vehicle, and sets the threshold value so that it increases as the lateral acceleration increases.

[0009] Based on the above findings, the control device for a four-wheel drive vehicle of the present invention sets a threshold value using lateral acceleration as part or all of an index. Furthermore, the control device for a four-wheel drive vehicle of the present invention sets a threshold value serving as a criterion for slip determination higher as the lateral acceleration increases. Therefore, the control device for a four-wheel drive vehicle of the present invention sets a higher threshold value the higher the road surface condition, where the friction coefficient is high and slip is less likely to occur. This allows the control device for a four-wheel drive vehicle of the present invention to suppress slip determination when traveling on a so-called high-μ road. As a result, the control device for a four-wheel drive vehicle of the present invention suppresses control to increase the distribution of drive force transmission to the auxiliary drive wheels when traveling on a high-μ road, thereby suppressing the occurrence of tight corner braking.

[0010] Here, when a vehicle makes a sharp turn, the inner wheels (inner wheels) tend to slip before the wheels (outer wheels) on the outer side in the turning radius direction. Furthermore, when the control device for a four-wheel drive vehicle of the present invention performs the above-described control, it is preferable that the occurrence of slippage can be detected as early as possible. Therefore, in the control device for a four-wheel drive vehicle of the present invention, the slip determination unit preferably determines the occurrence of slippage based on the slip ratio of the inner wheels.

[0011] (2) Based on this knowledge, the above-mentioned control device for a four-wheel drive vehicle of the present invention may be configured such that the slip determination unit determines the occurrence of slip based on the slip ratio of the wheel on the inside of the turning radius direction of the four-wheel drive vehicle.

[0012] Based on the above findings, the control device for a four-wheel drive vehicle of the present invention determines whether slip has occurred based on the slip ratio of the wheel on the inside of the turning radius of the four-wheel drive vehicle. As a result, the control device for a four-wheel drive vehicle of the present invention can quickly and appropriately determine whether slip has occurred and perform appropriate control.

[0013] In the above-described four-wheel drive vehicle control device of the present invention, in order to more accurately grasp road surface conditions and set the threshold value serving as a slip determination criterion to an appropriate value, it is preferable to add, as an index, other factors suitable for grasping road surface conditions in addition to lateral acceleration. For example, the greater the force acting on the four-wheel drive vehicle due to the influence of lateral acceleration and wheel driving force, the higher the friction coefficient and the less likely slip will occur in the road surface conditions. Therefore, by setting the threshold value using the force acting on the four-wheel drive vehicle due to the influence of lateral acceleration and wheel driving force as an index, the four-wheel drive vehicle control device of the present invention is thought to be able to more accurately control the distribution of driving force that reflects road surface conditions.

[0014] (3) Based on this finding, the above-described control device for a four-wheel drive vehicle of the present invention is configured such that the slip determination unit uses the magnitude of the force acting on the four-wheel drive vehicle due to the influence of the lateral acceleration and wheel driving force as an index for setting the threshold value, instead of or in addition to the magnitude of the lateral acceleration acting on the four-wheel drive vehicle, and the slip determination unit determines whether the force acting on the four-wheel drive vehicle due to the influence of the lateral acceleration and wheel driving force is large. Naruno Preferably, the threshold value is set so that it increases with increasing temperature.

[0015] The control device for a four-wheel drive vehicle of the present invention sets the threshold value using the magnitude of the force acting on the four-wheel drive vehicle due to the influence of lateral acceleration and wheel driving force as an index, instead of or in addition to the magnitude of the lateral acceleration acting on the four-wheel drive vehicle. Furthermore, the control device for a four-wheel drive vehicle of the present invention sets the threshold value higher as the force acting on the four-wheel drive vehicle due to the influence of lateral acceleration and wheel driving force increases, assuming that the road surface condition has a high friction coefficient and is less likely to cause slippage. Therefore, the control device for a four-wheel drive vehicle of the present invention more reliably prevents control that increases the transmission allocation of driving force to the auxiliary drive wheels when traveling on a high μ road, thereby preventing the occurrence of tight corner braking.

[0016] After extensive research, the inventors have found that, in addition to the lateral acceleration described above, the yaw rate, for example, can also be used as an index that accurately reflects road surface conditions. Specifically, since it is assumed that the larger the yaw rate, the higher the friction coefficient and the less likely slippage will occur on the road surface, the inventors have come to the realization that if the magnitude of the yaw rate is used as an index, the road surface condition on which the vehicle is traveling can be accurately determined and the distribution of driving force can be appropriately controlled.

[0017] (4) In the control device for a four-wheel drive vehicle of the present invention described above, the slip determination unit sets the threshold value using yaw rate as part or all of an index instead of or in addition to the lateral acceleration, and it is preferable that the threshold value be set so that the threshold value increases as the yaw rate increases.

[0018] The control device for a four-wheel drive vehicle of the present invention sets a threshold for determining whether slip has occurred using yaw rate as part or all of an index in addition to or instead of lateral acceleration. As a result, the control device for a four-wheel drive vehicle of the present invention can suppress the occurrence of tight corner braking by increasing the drive force transmission allocation to the auxiliary drive wheels when traveling on a so-called high μ road.

[0019] (5) In the control device for a four-wheel drive vehicle of the present invention, the slip determination unit determines whether or not the magnitude of the force acting on the four-wheel drive vehicle, which is grasped as an index using either or both of the wheel driving force and the vehicle speed, in addition to the yaw rate, is large. Naruno Preferably, the threshold value is set so that it increases with increasing temperature.

[0020] Because the control device for a four-wheel drive vehicle of the present invention is configured in this way, it can grasp the magnitude of the force acting on the four-wheel drive vehicle by taking into account the effects of wheel driving force and vehicle speed in addition to yaw rate, and based on the results, can control the transmission and allocation of driving force according to the road surface conditions. [Effects of the Invention]

[0021] According to the present invention, a control device for a four-wheel drive vehicle can be provided that can appropriately control the distribution of driving force in accordance with the friction coefficient of the road surface in a four-wheel drive vehicle that can run in a driving state using a control method that detects the occurrence of slippage and increases the distribution of driving force to the auxiliary drive wheels. [Brief explanation of the drawings]

[0022] [Figure 1] 1 is an explanatory diagram showing a control device according to an embodiment of the present invention and a vehicle equipped with the control device; [Figure 2] 4 is a flowchart showing a control flow that is carried out when the driving state of the vehicle shown in FIG. 1 is a variable drive distribution state. [Figure 3] 3 is a timing chart corresponding to the control flow according to FIG. 2. [Figure 4] 10 is a timing chart showing the transition of the slip ratio and the change in the drive distribution to the auxiliary drive wheels when the threshold value is kept constant, together with the transition of the vehicle speed, steering angle, and lateral acceleration. DETAILED DESCRIPTION OF THE INVENTION

[0023] Hereinafter, a four-wheel drive vehicle control device (control device 100) according to an embodiment of the present invention will be described with reference to the drawings, taking a four-wheel drive vehicle (vehicle 10) that employs the same as an example. In the following description, before describing the specific configuration of the control device 100 and the control performed by the control device 100, a general configuration of the vehicle 10 will be described.

[0024] FIG. 1 is a diagram illustrating the schematic configuration of a vehicle 10 to which the present invention is applied. The vehicle 10 employs a front-engine, rear-wheel-drive drive system. As shown in FIG. 1, the vehicle 10 includes a drive power source 12, a pair of left and right front wheels 14L, 14R, a pair of left and right rear wheels 16L, 16R, a power transmission device 18, and a control device 100. The vehicle 10 is a four-wheel drive vehicle that can be switched between a two-wheel drive state in which drive power is transmitted to the rear wheels 16L, 16R, and a four-wheel drive state in which drive power is transmitted to the front wheels 14L, 14R in addition to the rear wheels 16L, 16R.

[0025] The driving force source 12 generates driving force for the vehicle 10. The driving force source 12 can be configured, for example, by an engine or a motor. The front wheels 14L, 14R constitute auxiliary driving wheels of the vehicle 10. The front wheels 14L, 14R function as driving wheels in four-wheel drive mode and as driven wheels in two-wheel drive mode. The rear wheels 16L, 16R constitute main driving wheels of the vehicle 10. The rear wheels 16L, 16R function as driving wheels in both four-wheel drive mode and two-wheel drive mode.

[0026] As shown in FIG. 1, the power transmission device 18 includes a speed change device 20, a transfer case 22, a front propeller shaft 24 (power transmission member), a rear propeller shaft 26, a front wheel differential gear device 28, a rear wheel differential gear device 30, a pair of left and right front wheel axles 32L, 32R, and a pair of left and right rear wheel axles 34L, 34R.

[0027] The power transmission device 18 can transmit the power generated in the driving force source 12 to the rear wheels 16L, 16R via a power transmission path that sequentially passes from the transmission 20 to the transfer 22, the rear propeller shaft 26, the rear wheel differential gear device 30, and the rear axles 34L, 34R, etc., before reaching the rear wheels 16L, 16R. The power transmission device 18 can also configure a power transmission path that distributes and transmits a portion of the driving force transmitted from the driving force source 12 to the transfer 22 to the front wheels 14L, 14R. That is, by adjusting the engagement state of the front wheel drive clutch 46, which will be described in detail later, the power transmission device 18 can transmit a portion of the power generated in the driving force source 12 to the front wheels 14L, 14R via a power transmission path that sequentially passes from the transmission 20 to the transfer 22, the front propeller shaft 24, the front wheel differential gear device 28, the front axles 32L, 32R, etc., before reaching the front wheels 14L, 14R.

[0028] The transmission 20 operates by receiving output from the driving force source 12, and is configured by, for example, a conventionally known MT (manual transmission), AT (automatic transmission), CVT (continuously variable transmission), or the like.

[0029] The transfer case 36 houses an input shaft 38, a rear-wheel-side output shaft 40, a front-wheel-drive drive sprocket 42, and a front-wheel-drive clutch 46 (first clutch) around a first rotational axis C1. The transfer case 36 also houses a front-wheel-side output shaft 48 and a front-wheel-drive driven sprocket 50 around a second rotational axis C2 extending in a direction along the first rotational axis C1 (substantially parallel to the first rotational axis C1 in this embodiment). The transfer case 22 also houses a front-wheel-drive chain 52 wound around the front-wheel-drive drive sprocket 42 and the front-wheel-drive driven sprocket 50.

[0030] The input shaft 38 is connected to the transmission 20. This allows the input shaft 38 to receive power transmitted from the driving power source 12. The rear-wheel output shaft 40 is connected to the rear propeller shaft 26 so as to be capable of transmitting power. A front-wheel drive drive sprocket 42 is supported on the rear-wheel output shaft 40 so as to be rotatable relative to the rear-wheel output shaft 40.

[0031] By engaging the front-wheel drive clutch 46, the front-wheel drive sprocket 42 can rotate integrally with the rear-wheel output shaft 40, and can transmit power to the front-wheel output shaft 48 via the front-wheel drive chain 52. Therefore, by engaging the front-wheel drive clutch 46, a portion of the driving force transmitted from the driving power source 12 to the rear propeller shaft 26 via the rear-wheel output shaft 40 can be distributed and transmitted to the front-wheel output shaft 48 via the front-wheel drive drive sprocket 42 and the front-wheel drive chain 52. On the other hand, by disengaging the front-wheel drive clutch 46, the driving force transmitted from the driving power source 12 to the rear-wheel output shaft 40 can be transmitted to the rear propeller shaft 26 without being transmitted (distributed) to the front-wheel drive drive sprocket 42.

[0032] The front-wheel drive clutch 46 is configured as a wet multi-plate clutch. The front-wheel drive clutch 46 is capable of adjusting the transmission torque transmitted from the rear-wheel output shaft 40 to the front-wheel drive drive sprocket 42 by adjusting the degree of engagement (engagement pressure). In other words, the front-wheel drive clutch 46 functions as a clutch (first clutch) for selectively disconnecting or connecting the power transmission path between the driving power source 12 and the front propeller shaft 24, which functions as a power transmission member to the front wheels 14L, 14R. The front-wheel drive clutch 46 is actuated by the action of hydraulic pressure, and the degree of engagement can be adjusted by controlling the magnitude of the hydraulic pressure.

[0033] The front-wheel-side output shaft 48 is connected to the front propeller shaft 24 so as to be able to transmit power. The front-wheel-drive driven sprocket 50 is provided so as to be able to rotate integrally with the front-wheel-side output shaft 48. In addition, a front-wheel-drive chain 52 is wound around the front-wheel-drive drive sprocket 42 and the front-wheel-drive driven sprocket 50, so that power can be transmitted between the two sprockets.

[0034] The front wheel differential gear device 28 includes a differential case 80, a pinion shaft 82, a pair of side gears 84L, 84R, a pair of pinions 86a, 86b, and a ring gear 90. The pinions 86a, 86b are attached to the differential case 80, with the pinions 86a, 86b disposed at both ends of the pinion shaft 82. The side gears 84L, 84R are disposed opposite each other within the differential case 80 and mesh with the pinions 86a, 86b, respectively. The side gears 84L, 84R are connected to the front wheels 14L, 14R via the front axles 32L, 32R. The ring gear 90 is attached integrally to the differential case 80. A front drive pinion 25 connected to the front propeller shaft 24 meshes with the ring gear 90. The front wheel differential gear device 28 also includes a mesh clutch 94 (second clutch). The mesh clutch 94 can be engaged by applying pressure (negative pressure). The mesh clutch 94 functions as a second clutch that selectively connects or disconnects the power transmission path between the front propeller shaft 24, which functions as a power transmission member to the front wheels 14L, 14R, and the front axles 32L, 32R, which are auxiliary drive wheels.

[0035] The rear wheel differential gear device 30 includes a differential case 120, a pinion shaft 122, a pair of side gears 124L and 124R, a pair of pinions 126a and 126b, and a ring gear 130. The pinions 126a and 126b are attached to the differential case 120, with the pinions 126a and 126b disposed at both ends of the pinion shaft 122. The side gears 124L and 124R are disposed opposite each other within the differential case 120 and mesh with the pinions 126a and 126b, respectively. The side gears 124L and 124R are connected to the rear wheels 16L and 16R via rear wheel axles 34L and 34R. The ring gear 130 is attached integrally to the differential case 120. A rear drive pinion 27 connected to the rear propeller shaft 26 meshes with the ring gear 130.

[0036] Because the vehicle 10 is configured as described above, when both the front-wheel drive clutch 46 and the dog clutch 94 are connected (engaged) to transmit torque, the vehicle 10 is in a state (four-wheel drive state) in which power generated in the driving force source 12 can be transmitted not only to the rear wheels 16L, 16R but also to the front wheels 14L, 14R. On the other hand, when at least one of the front-wheel drive clutch 46 and the dog clutch 94 is disconnected (disengaged), the power transmission path to the front wheels 14L, 14R is cut off, and the vehicle 10 is in a state in which torque transmission is disabled. As a result, the vehicle 10 is in a state (two-wheel drive state) in which power generated in the driving force source 12 can be transmitted to the rear wheels 16L, 16R but cannot be transmitted to the front wheels 14L, 14R.

[0037] The vehicle 10 is equipped with a selector switch 140 for switching the drive state. The selector switch 140 can be set to any one of three drive states: "2WD," "4WD LOCK," and "4WD AUTO." When the selector switch 140 is set to "2WD," one or both of the front-wheel drive clutch 46 and the dog clutch 94 are disengaged. This puts the vehicle 10 in a state where it drives in two-wheel drive mode. When the selector switch 140 is set to "4WD LOCK," the dog clutch 94 is engaged and the front-wheel drive clutch 46 is fully engaged. This puts the vehicle 10 in a state where it drives in four-wheel drive mode.

[0038] Furthermore, when the selector switch 140 is set to "4WD AUTO," the drive control is enabled by a standby control method (a drive distribution variable state) in which the drive distribution distribution of the drive force transmitted to the front wheels 14L, 14R relative to the drive force transmitted to the rear wheels 16L, 16R is increased by changing the engagement degree of the front-wheel drive clutch 46 while the mesh clutch 94 is engaged, provided that slippage is detected in the vehicle 10. Specifically, when the selector switch 140 is set to "4WD AUTO," the mesh clutch 94 is engaged, and the front-wheel drive clutch 46 is engaged at a predetermined standby engagement degree S in preparation for slippage. When the selector switch 140 is set to "4WD AUTO," if slippage occurs in the rear wheels 16L, 16R, which are the main drive wheels, the drive state of the vehicle 10 is switched by fully engaging the front-wheel drive clutch 46 while the mesh clutch 94 is maintained engaged.

[0039] The control device 100 is capable of controlling the connection state (engagement state) of the front-wheel drive clutch 46 and the dog clutch 94. The control device 100 is configured to include a microcomputer equipped with, for example, a CPU, RAM, ROM, an input / output interface, etc. The control device 100 includes a slip determination unit 102, a first clutch control unit 104, and a second clutch control unit 106.

[0040] The slip determination unit 102 determines whether a slip has occurred in the vehicle 10. In this embodiment, the slip determination unit 102 determines whether a slip has occurred using, as a determination condition, the slip ratio S of the rear wheels 16L, 16R out of the front wheels 14L, 14R and rear wheels 16L, 16R that are the wheels of the vehicle 10. The slip determination unit 102 may determine whether a slip has occurred based on the slip ratio S of one or both of the rear wheels 16L, 16R. In this embodiment, in consideration of the fact that the wheel (inner wheel) on the inside in the turning radius direction tends to slip before the wheel (outer wheel) on the outside when the vehicle 10 turns, the slip determination unit 102 determines whether a slip has occurred based on the slip ratio S of the rear wheel 16L, 16R that is on the inside in the turning radius direction of the vehicle 10.

[0041] Here, the "slip ratio S" is a value derived as follows: when the speed of the vehicle 10 is Vb, and the speed (wheel speed) calculated from the rotational speed of the wheels is Vw, S={Vb-Vw) / Vb}×100[%] when the vehicle 10 is decelerating, and S={(Vw-Vb) / Vw}×100[%] when the vehicle 10 is accelerating.

[0042] In the drive distribution variable state (when the selector switch 140 is set to 4WD AUTO), the slip determination unit 102 determines whether slip has occurred based on the determination condition that the slip ratio S of the rear wheels 16L, 16R, which are the main drive wheels, exceeds a threshold value L. Furthermore, when the vehicle 10 is turning, the slip determination unit 102 determines whether slip has occurred based on the determination condition that the slip ratio S of the inner wheel of the rear wheels 16L, 16R exceeds a threshold value L.

[0043] The slip determination unit 102 sets the threshold value L using the lateral acceleration acting on the vehicle 10 as a part or all of the index. In this embodiment, as can be seen by referring to the timing chart in Fig. 3, the slip determination unit 102 sets the threshold value L so that the threshold value L increases as the lateral acceleration increases.

[0044] The first clutch control unit 104 controls the transmission torque (degree of engagement of the first clutch) transmitted to the front propeller shaft 24 via the front-wheel drive clutch 46. As described above, in this embodiment, the front-wheel drive clutch 46 is a wet multi-plate clutch whose degree of engagement (engagement pressure) can be adjusted by controlling the magnitude of hydraulic pressure. Therefore, the first clutch control unit 104 controls the magnitude of the torque transmitted to the front propeller shaft 24 by controlling the magnitude of hydraulic pressure acting on the front-wheel drive clutch 46.

[0045] The second clutch control unit 106 controls the operation of the dog clutch 94 to control the torque transmission from the front-wheel drive clutch 46 to the front-wheel differential gear unit 28. As described above, the dog clutch 94 can be engaged by applying pressure (negative pressure). Therefore, the second clutch control unit 106 can switch the dog clutch 94 between an engaged state and a disengaged state by controlling the magnitude of the pressure acting on the dog clutch 94.

[0046] Here, the above-described control device 100 is characterized in that, when the selector switch 140 is set to "4WD AUTO" and drive control is performed using the above-described standby control method, it performs control (driving force distribution control) to enable appropriate control of driving force distribution in accordance with the friction coefficient of the road surface. Below, the control performed by the control device 100 when the selector switch 140 is set to "4WD AUTO" will be described in detail in accordance with the flowchart in Fig. 2 and with reference to the timing chart in Fig. 3.

[0047] (Step 1) In step 1, the control device 100 checks whether the vehicle 10 is in a state where drive control is performed using the standby control method (drive distribution variable state) by checking whether the selector switch 140 of the vehicle 10 is set to "4WD AUTO." If it is confirmed that the drive state of the vehicle 10 is in the drive distribution variable state, the control flow proceeds to step 2.

[0048] (Step 2) In step 2, the control device 100 acquires an index value that serves as an index of the threshold value L (threshold setting index) in preparation for setting the threshold value L used by the slip determination unit 102 to determine whether the vehicle 10 is in a slipping state. In this embodiment, the magnitude of the lateral acceleration acting on the vehicle 10 is used as the threshold setting index. Therefore, in step 2, the control device 100 acquires the magnitude of the lateral acceleration as an index value (threshold setting index value) by acquiring an output signal from a sensor or the like provided on the vehicle 10. Thereafter, the control device 100 advances the control flow to step 3.

[0049] (Step 3) In step 3, the slip determination unit 102 of the control device 100 sets the threshold value L based on the threshold setting index value obtained in step 2. In this embodiment, the threshold value L is set based on the magnitude of the lateral acceleration acting on the vehicle 10. Here, the slip determination unit 102 sets the value of the threshold value L based on a predetermined function, a predefined map, or the like, so that the threshold value L increases as the lateral acceleration increases, as shown in the timing chart of FIG. 3. Once the setting of the threshold value L is complete, the control device 100 advances the control flow to step 4.

[0050] (Step 4) In step 4, the control device 100 causes the slip determination unit 102 to derive the slip ratio S of the rear wheels 16L, 16R of the traveling vehicle 10 based on the speed Vb and wheel speed Vw of the vehicle 10. When the vehicle 10 is turning, the slip determination unit 102 derives the slip ratio S of the inner one of the rear wheels 16L, 16R. Thereafter, the control device 100 advances the control flow to step 5.

[0051] (Step 5) In step 5, the control device 100 determines whether or not slippage has occurred in the vehicle 10 using the slip determination unit 102. Specifically, the slip determination unit 102 compares the slip ratio S derived in step 4 described above with the threshold value L set in step 3. If the slip ratio S is greater than the threshold value L (S>L), the slip determination unit 102 determines that slippage has occurred, and the control flow proceeds to step 6. On the other hand, if it is confirmed that slippage has not occurred, the control flow returns to step 1.

[0052] (Step 6) When the control flow proceeds from step 5 to step 6, slippage has occurred in the vehicle 10. Therefore, under the control of the first clutch control unit 104, the control device 100 increases the engagement degree of the front-wheel drive clutch 46 from the standby engagement degree S. This increases the distribution of driving force transmitted to the front wheels 14L, 14R, which are the auxiliary drive wheels.

[0053] The control device 100 mounted on the vehicle 10 described above has the following characteristic configurations (A) and (B), and therefore can exhibit characteristic effects.

[0054] (A) The control device 100 of this embodiment includes a front-wheel drive clutch 46 (first clutch) that selectively connects or disconnects the power transmission path between the driving force source 12 and the front propeller shaft 24 (power transmission member), and a mesh clutch 94 (second clutch) that selectively connects or disconnects the power transmission path between the front propeller shaft 24 and the front wheels 14L, 14R (auxiliary drive wheels). By disengaging at least one of the front-wheel drive clutch 46 and the mesh clutch 94, the control device 100 can be set to a two-wheel drive state in which driving force is transmitted from the driving force source 12 to the left and right rear wheels 16L, 16R (main drive wheels); The control device 100 is used in a vehicle 10 that can switch between a four-wheel drive state in which driving force is transmitted from the driving force source 12 to the left and right front wheels 14L, 14R by engaging the front-wheel drive clutch 46 and the dog clutch 94, respectively, and a variable drive distribution state in which drive control is performed using a standby control method in which the degree of engagement of the front-wheel drive clutch 46 is changed to increase the distribution of driving force transmitted to the front wheels 14L, 14R relative to the driving force transmitted to the rear wheels 16L, 16R while the dog clutch 94 is engaged, provided that slippage is detected in the vehicle 10. The control device 100 has a slip determination unit 102 that, in the variable drive distribution state, determines whether slippage has occurred using, as part or all of the determination criterion, a slip ratio S of the wheels of the vehicle 10 exceeding a threshold L. Furthermore, the control device 100 increases the distribution of driving force transmitted to the front wheels 14L, 14R relative to the driving force transmitted to the rear wheels 16L, 16R, using as a partial or complete condition that the slip determination unit 102 has determined that slip has occurred. The control device 100 is characterized in that the slip determination unit 102 sets the threshold value L using the lateral acceleration acting on the vehicle 10 as a partial or complete index, and sets the threshold value L so that it increases as the lateral acceleration increases.

[0055] The control device 100 of this embodiment sets the threshold value L, which is the determination criterion for slip determination, higher as the lateral acceleration increases. Therefore, the control device 100 of this embodiment sets the threshold value L higher as the road surface condition becomes higher, with a higher friction coefficient making slip less likely to occur. This allows the control device 100 of this embodiment to suppress slip determination when traveling on a so-called high μ road. As a result, the control device 100 of this embodiment suppresses control to increase the distribution of driving force transmission to the front wheels 14L, 14R when traveling on a high μ road, thereby suppressing the occurrence of tight corner braking.

[0056] More specifically, if the threshold value L, which is the criterion for slip determination, is kept constant even when the lateral acceleration changes, as in the comparative example shown in Figure 4, control is performed to increase the distribution of driving force transmitted to the front wheels 14L, 14R during the period when the slip ratio S exceeds the threshold value L as the lateral acceleration increases, even though the vehicle 10 is able to make sharp turns without slipping. If such control is performed, there is a concern that the tight corner braking phenomenon may occur.

[0057] However, in the control device 100 of this embodiment, the threshold value L, which is the criterion for slip determination, is set higher as the lateral acceleration increases. Therefore, by performing control using the control device 100, it is possible to prevent control that increases the distribution of driving force transmitted to the front wheels 14L, 14R, which are the auxiliary drive wheels, even if the slip ratio increases, as shown in Fig. 3. This makes it possible to prevent slip determination from being made when traveling on a high μ road, and to prevent the tight corner braking phenomenon from occurring.

[0058] In this embodiment, the threshold L is set using the lateral acceleration acting on the vehicle 10 as the sole indicator. However, the present invention is not limited to this example, and any appropriate indicator can be used as the basis for setting the threshold L without departing from the technical spirit of the present invention. That is, when the vehicle speed increases at the same lateral acceleration on a road surface with a high friction coefficient, the turning radius of the vehicle 10 increases compared to when the vehicle speed is low. Therefore, the slip ratio S of the inner rear wheel 16L, 16R when traveling without slipping becomes smaller than when the vehicle speed is low. Therefore, the control device 100 should lower the threshold L as the vehicle speed increases. From another perspective, the difference in turning radius between the front and rear wheels decreases at high vehicle speeds, making the tight corner braking phenomenon less likely to occur. Therefore, when slip occurs on the inner wheel during a sharp turn on a high vehicle speed and high μ road, there is no problem in increasing the driving force distribution, and there is no need to increase the threshold L. Therefore, the control device 100 should lower the threshold L for slip determination as the vehicle speed increases.

[0059] Based on the above findings, as in a modified example described below, the control device 100 can use other factors, such as the driving force acting on the wheels (wheel driving force), yaw rate, and vehicle speed, in addition to or instead of the lateral acceleration as indicators for setting the threshold value L. By setting the threshold value L using a plurality of indicators in this way, it is possible to set the threshold value L to a more appropriate value.

[0060] Furthermore, in this embodiment, an example has been shown in which the slip ratio S of the wheels of the vehicle 10 exceeding the threshold L is the only determination condition for determining whether a slip has occurred, but the present invention is not limited to this, and it is also possible to add other elements as determination conditions for determining whether a slip has occurred. If elements other than the slip ratio S are taken into consideration when determining whether a slip has occurred, it becomes possible to grasp the occurrence of a slip more accurately.

[0061] (B) In the control device 100 of the present embodiment described above, the slip determination unit 102 determines whether or not a slip has occurred based on the slip ratio S of the wheel on the inner side of the vehicle 10 in the turning radius direction.

[0062] The control device 100 of this embodiment is configured as described above in (B) in consideration of the fact that slip occurs earlier on the inner wheels than on the outer wheels when the vehicle 10 turns. Therefore, the control device 100 of this embodiment can quickly and appropriately grasp the state of slip occurrence and perform appropriate control.

[0063] In this embodiment, the occurrence of slippage while the vehicle 10 is turning is determined by focusing on the slip ratio S of the inner wheel (in this embodiment, the inner one of the rear wheels 16L, 16R), but the present invention is not limited to this. The control device 100 may, for example, determine the occurrence of slippage by comprehensively determining the slip ratios S of both the inner and outer wheels.

[0064] The above-described control device 100 is merely an example of the present invention, and various modifications are possible without departing from the spirit of the present invention. For example, the above-described control device 100 can achieve a characteristic effect that cannot be achieved by the prior art even by configuring it as in (C) below.

[0065] (C) In the control device 100 of the present embodiment described above, the slip determination unit 102 uses the magnitude of the force acting on the vehicle 10 due to the influence of the lateral acceleration and the wheel driving force as an index for setting the threshold L, instead of or in addition to the magnitude of the lateral acceleration acting on the vehicle 10. When the force acting on the vehicle 10 due to the influence of the lateral acceleration and the wheel driving force is large, the slip determination unit 102 Naruno The threshold value L can be set so that it becomes higher as the temperature increases.

[0066] The control device 100 according to (C) above is based on the finding that, in order to more accurately grasp road surface conditions and set the threshold L, which is the criterion for determining slip, to an appropriate value, it is effective to add, as an index, other factors suitable for grasping road surface conditions in addition to lateral acceleration. The control device 100 according to (C) above assumes that the greater the force acting on the vehicle 10 due to the influence of lateral acceleration and wheel driving force, the higher the friction coefficient and the road surface conditions that make slip less likely to occur. Therefore, instead of or in addition to the magnitude of the lateral acceleration acting on the vehicle 10, the control device 100 sets the threshold L using the magnitude of the force acting on the vehicle 10 due to the influence of lateral acceleration and wheel driving force as an index. Therefore, by configuring the control device 100 as described in (C) above, it is possible to more reliably prevent control that increases the distribution of driving force transmission to the front wheels 14L, 14R when traveling on a high-μ road, thereby suppressing the occurrence of tight corner braking.

[0067] Furthermore, the above-described control device 100 can also obtain a characteristic effect that cannot be achieved by the conventional technology by adopting the following configuration (D).

[0068] (D) In ​​the above-described control device 100, the slip determination unit 102 sets the threshold value L using the yaw rate as part or all of the index instead of or in addition to the lateral acceleration, and the threshold value L can be set so that the threshold value L increases as the yaw rate increases.

[0069] Here, it is assumed that the greater the yaw rate when the vehicle 10 is turning, the higher the coefficient of friction and the less likely the road surface condition is to cause slippage. Therefore, as described above in (D), the control device 100 may set the threshold L for determining the occurrence of slippage using the yaw rate as part or all of an index in addition to or instead of the lateral acceleration. With this configuration, the control device 100 can suppress the occurrence of tight corner braking by performing control to increase the distribution of driving force transmission to the front wheels 14L, 14R when traveling on a so-called high μ road.

[0070] The present invention is not limited to the above-described embodiments and variations thereof, and other embodiments may be possible within the scope of the claims. The components of the above-described embodiments may be arbitrarily selected and combined. Furthermore, any component of the embodiments may be arbitrarily combined with any component described in the Summary of the Invention or any component embodying any component described in the Summary of the Invention. The present invention intends to obtain rights to these as well through amendments to this application or divisional applications, etc. [Industrial Applicability]

[0071] The present invention can be suitably used in control devices for four-wheel drive vehicles in general, which are equipped with a first clutch that selectively disconnects or connects the power transmission path between a driving force source and a power transmission member, and a second clutch that selectively disconnects or connects the power transmission path between the power transmission member and an auxiliary drive wheel. [Explanation of symbols]

[0072] 10: Vehicle (four-wheel drive vehicle) 12: Driving force source 14L, 14R: Front wheels (auxiliary drive wheels) 16L, 16R: Rear wheels (main drive wheels) 18: Power transmission device 24: Front propeller shaft (power transmission member) 46: Front wheel drive clutch (first clutch) 94: Dog clutch (second clutch) 100: Control device 102: Slip determination unit 104: First clutch control unit

Claims

1. a first clutch that selectively connects or disconnects a power transmission path between the driving power source and the power transmission member; a second clutch that selectively connects or disconnects a power transmission path between the power transmission member and the auxiliary drive wheels, a two-wheel drive state in which driving force is transmitted from the driving force source to the left and right main drive wheels by disengaging at least one of the first clutch and the second clutch; a four-wheel drive state in which the first clutch and the second clutch are engaged to transmit driving force from the driving force source to the left and right auxiliary drive wheels; a variable drive distribution state in which drive control is performed using a standby control method in which the degree of engagement of the first clutch is changed while the second clutch is engaged, on the condition that occurrence of slippage is detected, thereby increasing the distribution of drive force transmitted to the auxiliary drive wheels relative to the drive force transmitted to the main drive wheels; and A control device for a four-wheel drive vehicle used in a four-wheel drive vehicle capable of switching a drive state, a slip determination unit that determines whether a slip has occurred in the four-wheel drive vehicle when the drive distribution is variable, using a slip ratio of a wheel of the four-wheel drive vehicle exceeding a threshold as part or all of the determination conditions; a drive force distribution control device for increasing a ratio of the drive force transmitted to the auxiliary drive wheels to the drive force transmitted to the main drive wheels, the drive force distribution control device being configured to increase ... A control device for a four-wheel drive vehicle, characterized in that the slip determination unit sets the threshold value using lateral acceleration acting on the four-wheel drive vehicle as part or all of an index, and sets the threshold value so that the threshold value increases as the lateral acceleration increases.

2. 2. The control device for a four-wheel drive vehicle according to claim 1, wherein the slip determination unit determines whether a slip has occurred based on a slip ratio of a wheel on the inside in a turning radius direction of the four-wheel drive vehicle.

3. the slip determination unit uses, instead of or in addition to the magnitude of the lateral acceleration acting on the four-wheel drive vehicle, the magnitude of a force acting on the four-wheel drive vehicle due to the influence of the lateral acceleration and wheel driving force as an index for setting the threshold value, 3. The control device for a four-wheel drive vehicle according to claim 1, wherein the threshold value is set so that it increases as the force acting on the four-wheel drive vehicle due to the influence of the lateral acceleration and wheel driving force increases.

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