JP7743271B2 - Control device for four-wheel drive vehicle - Google Patents

Description

The present invention relates to a control device for a four-wheel drive vehicle.

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 in Patent Document 1 below controls the connection between the prime mover and the first axle in an all-wheel drive vehicle equipped with a first axle that is driven temporarily by the 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. This configuration of the control system in Patent Document 1 aims to enable rapid switching between 2WD mode and AWD mode while preventing shock or vibration fluctuations in the transmitted torque.

Japanese Patent Application Laid-Open No. 2020-032773

The inventors here investigated potential problems that may arise when a control system is used to detect slippage and increase the distribution of driving force to the auxiliary drive wheels in a four-wheel drive vehicle equipped with a first clutch that selectively connects or disconnects the power transmission path between the drive power source and the power transmission member, and a second clutch that selectively connects or disconnects the power transmission path between the power transmission member and the auxiliary drive wheels.

As a result, it was found that even on so-called high-μ roads, where the friction coefficient is high and slippage is unlikely, slippage may occur when making a sharp turn because the ground contact 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 exercised to increase the distribution of driving force to the auxiliary drive wheels, while slippage may 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.

The present invention aims to provide a control device for a four-wheel drive vehicle that can appropriately control the distribution of driving force according to 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.

After careful consideration, the inventors have discovered that the lateral acceleration acting on a vehicle can be an indicator that accurately reflects road surface conditions, and that using lateral acceleration as an indicator will allow 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 friction coefficient and the less likely slippage will occur on the road surface, the inventors have discovered that using the magnitude of lateral acceleration as an indicator will allow for an accurate understanding of the road surface conditions on which the vehicle is traveling and will allow for appropriate drive force distribution control.

(1) The control device for a four-wheel drive vehicle of the present invention, provided based on such findings, comprises a first clutch that selectively disconnects or connects a power transmission path between a driving force source and a power transmission member, and a second clutch that selectively disconnects or connects a power transmission path between the power transmission member and auxiliary drive wheels, and has the following configurations: 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 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 control device for controlling the relationship between the driving force transmitted to the main drive wheels and the auxiliary drive wheels by changing the degree of engagement of the first clutch while engaging the second clutch, provided that slippage is detected in the four-wheel drive vehicle. This device is used in a four-wheel drive vehicle that can switch between a variable drive distribution state, a standby control 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, 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, a slip determination unit is provided that determines whether slip has occurred, using as part or all of the conditions a slip ratio of a wheel in the four-wheel drive vehicle that exceeds a threshold. The slip determination unit determines that slip has occurred, and increases the distribution of drive force transmitted to the auxiliary drive wheels relative to the drive force transmitted to the main drive wheels. The slip determination unit sets the threshold using as part or all of the indicator the lateral acceleration acting on the four-wheel drive vehicle, and sets the threshold so that it increases as the lateral acceleration increases.

Based on the above-mentioned 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 higher threshold value, which serves as the criteria for slip determination, 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 friction coefficient and the road surface conditions are, making slip 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 so-called high-μ roads. As a result, the control device for a four-wheel drive vehicle of the present invention suppresses control to increase the distribution of driving force transmission to the auxiliary drive wheels when traveling on a high-μ road, thereby suppressing the occurrence of tight corner braking.

When a vehicle makes a sharp turn, the inner wheels (inner wheels) tend to slip before the wheels (outer wheels) on the outer side of the turning radius. Furthermore, when the control device for a four-wheel drive vehicle of the present invention performs the control described above, it is preferable to be able to detect the occurrence of slip as early as possible. Therefore, in the control device for a four-wheel drive vehicle of the present invention, it is preferable that the slip determination unit determine the occurrence of slip based on the slip ratio of the inner wheels.

(2) Based on this knowledge, in the control device for a four-wheel drive vehicle of the present invention described above, the slip determination unit may determine 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.

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 accurately determine whether slip has occurred and perform appropriate control.

In the four-wheel drive vehicle control device of the present invention described above, in order to more accurately grasp road surface conditions and set the threshold value used to determine slip to an appropriate value, it is advisable to add other factors suitable for grasping road surface conditions as indicators in addition to lateral acceleration. For example, the greater the force acting on a 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 a four-wheel drive vehicle due to the influence of lateral acceleration and wheel driving force as indicators, the four-wheel drive vehicle control device of the present invention is believed to be able to more accurately control the distribution of driving force to reflect road surface conditions.

(3) Based on this knowledge, the above-mentioned control device for a four-wheel drive vehicle of the present invention is configured so 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 indicator 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 sets the threshold value so that the threshold value increases as the force acting on the four-wheel drive vehicle due to the influence of the lateral acceleration and wheel driving force increases .

The control device for a four-wheel drive vehicle of the present invention sets a 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 a higher threshold value the greater the force acting on the four-wheel drive vehicle due to the influence of lateral acceleration and wheel driving force, assuming that the road surface conditions have a high friction coefficient and are 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 distribution of driving force transmission to the auxiliary drive wheels when traveling on a high μ road, thereby preventing the occurrence of tight corner braking.

After careful consideration, the inventors discovered that in addition to the lateral acceleration mentioned above, yaw rate, for example, can also be used as an indicator that accurately reflects road surface conditions. Specifically, since it is assumed that the higher the yaw rate, the higher the friction coefficient and the less likely slippage will occur on the road surface, they discovered that if the magnitude of the yaw rate is used as an indicator, it is possible to accurately grasp the road surface conditions on which the vehicle is traveling and appropriately control the distribution of driving force.

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

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. Therefore, the control device for a four-wheel drive vehicle of the present invention controls the transmission distribution of driving force to the auxiliary drive wheels when traveling on so-called high μ roads, thereby preventing the occurrence of tight corner braking.

(5) In the control device for a four-wheel drive vehicle of the present invention described above, the slip determination unit may set the threshold value so that the threshold value increases as the magnitude of the force acting on the four-wheel drive vehicle, which is grasped as an indicator of either or both of wheel driving force and vehicle speed in addition to the yaw rate, increases.

The four-wheel drive vehicle control device of the present invention is configured in this way, so it can grasp the magnitude of the forces 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 can control the transmission and allocation of driving force according to road surface conditions based on the results.

This invention provides a control device for a four-wheel drive vehicle that can appropriately control the distribution of driving force according to the friction coefficient of the road surface in a four-wheel drive vehicle that can travel 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.

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; 4 is a flowchart showing a control flow performed when the drive state of the vehicle shown in FIG. 1 is in a variable drive distribution state. 3 is a timing chart corresponding to the control flow shown in FIG. 2; 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.

The following describes a control device for a four-wheel drive vehicle (control device 100) according to an embodiment of the present invention, taking a four-wheel drive vehicle (vehicle 10) that employs the same as an example, with reference to the drawings. In the following description, the general configuration of vehicle 10 will be explained before discussing the specific configuration of control device 100 and the control performed by control device 100.

Figure 1 is a diagram illustrating the general 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 Figure 1, the vehicle 10 is equipped with 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.

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.

As shown in FIG. 1, the power transmission device 18 includes a transmission 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.

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 runs from the transmission 20 through the transfer case 22, the rear propeller shaft 26, the rear wheel differential gear unit 30, the rear axles 34L, 34R, etc., in that order, to 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 case 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 runs from the transmission 20 through the transfer case 22, the front propeller shaft 24, the front wheel differential gear unit 28, the front axles 32L, 32R, etc., in that order, to the front wheels 14L, 14R.

The transmission 20 operates by receiving output from the driving force source 12, and is composed of, for example, a conventionally known MT (manual transmission), AT (automatic transmission), CVT (continuously variable transmission), etc.

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

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 also connected to the rear propeller shaft 26 so that power can be transmitted. The front-wheel drive drive sprocket 42 is supported on the rear-wheel output shaft 40 so that it can rotate relative to the rear-wheel output shaft 40.

By engaging the front-wheel drive clutch 46, the front-wheel drive sprocket 42 can rotate integrally with the rear-wheel output shaft 40, enabling power transmission 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 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 sprocket 42.

The front-wheel drive clutch 46 is composed of 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 operated by the action of hydraulic pressure, and the degree of engagement can be adjusted by controlling the magnitude of the hydraulic pressure.

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

The front wheel differential gear device 28 comprises 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. The front drive pinion 25 connected to the front propeller shaft 24 meshes with the ring gear 90. The front wheel differential gear unit 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 the auxiliary drive wheels.

The rear wheel differential gear unit 30 comprises 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 located at both ends of the pinion shaft 122. The side gears 124L and 124R are positioned 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 the rear axles 34L and 34R. The ring gear 130 is attached integrally to the differential case 120. The ring gear 130 meshes with the rear drive pinion 27, which is connected to the rear propeller shaft 26.

Since 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 is in a state (four-wheel drive state) in which power generated in the drive power 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 torque transmission is disabled. This allows the vehicle 10 to transmit power generated in the drive power source 12 to the rear wheels 16L, 16R, but not to the front wheels 14L, 14R (two-wheel drive state).

The vehicle 10 is equipped with a selector switch 140 for switching the drive mode. The selector switch 140 can be set to any of three drive modes: "2WD," "4WD LOCK," and "4WD AUTO." When the selector switch 140 is set to "2WD," either the front-wheel drive clutch 46 or the dog clutch 94, or both, are disengaged. This places the vehicle 10 in a state where it can be driven 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 places the vehicle 10 in a state where it can be driven in four-wheel drive mode.

Furthermore, when the selector switch 140 is set to "4WD AUTO," the dog clutch 94 is engaged, and, provided that slippage is detected in the vehicle 10, the engagement degree of the front-wheel drive clutch 46 is changed to enable drive control using a standby control method that increases the distribution of drive force transmitted to the front wheels 14L, 14R relative to the drive force transmitted to the rear wheels 16L, 16R (variable drive distribution state). Specifically, when the selector switch 140 is set to "4WD AUTO," the dog clutch 94 is engaged, and the front-wheel drive clutch 46 is engaged at a predetermined standby engagement degree S in preparation for the occurrence of slippage. When the selector switch 140 is set to "4WD AUTO," if slippage occurs on 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 dog clutch 94 remains engaged.

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 described above. The control device 100 includes 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.

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

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

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 when the slip ratio S of the rear wheels 16L, 16R, which are the main drive wheels, exceeds the threshold value L. Furthermore, when the vehicle 10 is turning, the slip determination unit 102 determines whether slip has occurred when the slip ratio S of the inner wheel of the rear wheels 16L, 16R exceeds the threshold value L.

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

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.

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

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 standby control method described above, it performs control (drive force distribution control) to appropriately control the distribution of drive force according to 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 Figure 2 and with reference to the timing chart in Figure 3.

(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.

(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 slipping. 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.

(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.

(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.

(Step 5)
In step 5, the control device 100 determines, via the slip determination unit 102, whether or not slip has occurred in the vehicle 10. 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 slip has occurred, and the control flow proceeds to step 6. On the other hand, if it is confirmed that slip has not occurred, the control flow returns to step 1.

(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.

The control device 100 mounted on the vehicle 10 described above has the following characteristic configurations (A) and (B), and is therefore able to achieve distinctive effects.

(A) The control device 100 of this embodiment is equipped with a front-wheel drive clutch 46 (first clutch) that selectively disconnects or connects 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 disconnects or connects the power transmission path between the front propeller shaft 24 and the front wheels 14L, 14R (auxiliary drive wheels), and is configured to provide 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) by releasing at least one of the front-wheel drive clutch 46 and the mesh clutch 94; 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 while the dog clutch 94 is engaged, provided that slippage is detected in the vehicle 10, thereby increasing the distribution of driving force transmitted to the front wheels 14L, 14R relative to the driving force transmitted to the rear wheels 16L, 16R. In the variable drive distribution state, the control device 100 has a slip determination unit 102 that determines whether slip has occurred, using as part or all of the determination criterion that the slip ratio S of the wheels of the vehicle 10 exceeds 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, when the slip determination unit 102 determines 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.

The control device 100 of this embodiment sets the threshold value L, which is the criterion for slip determination, higher as the lateral acceleration increases. Therefore, the control device 100 of this embodiment sets the threshold value L higher the higher the friction coefficient and the road surface conditions where slip is 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 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.

More specifically, if the threshold value L, which is the criterion for determining slip, is kept constant even when the lateral acceleration changes, as in the comparative example shown in Figure 4, control will be 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.

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

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, and any appropriate indicator can be used as the basis for setting the threshold L as long as it does not deviate from the technical spirit of the present invention. In other words, when the vehicle speed is increased 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, at high vehicle speeds, the difference in turning radius between the front and rear wheels becomes smaller, making tight corner braking 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 with increasing the drive 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.

Based on the above findings, 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, as indicators for setting threshold L in addition to or instead of lateral acceleration, as in the modified example described below. By using multiple indicators to set threshold L in this way, it is possible to set threshold L to an even more appropriate value.

In addition, in this embodiment, an example has been shown in which the only condition for determining whether a slip has occurred is that the slip ratio S of the wheels of the vehicle 10 exceeds the threshold value L. However, the present invention is not limited to this, and other factors can also be added as conditions for determining whether a slip has occurred. If factors other than the slip ratio S are taken into account when determining whether a slip has occurred, it will be possible to grasp the state of slippage more accurately.

(B) In the control device 100 of the present embodiment described above, the slip determination unit 102 determines whether slip has occurred based on the slip ratio S of the wheel on the inside of the turning radius direction of the vehicle 10.

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 determine the state of slippage and perform appropriate control.

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. For example, the control device 100 may determine the occurrence of slippage by comprehensively determining the slip ratios S of both the inner and outer wheels.

The above-described control device 100 is merely one 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 distinctive effects not achievable with conventional technology by adopting the configuration shown in (C) below.

(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 lateral acceleration and wheel driving force as an indicator for setting the threshold value L, instead of or in addition to the magnitude of the lateral acceleration acting on the vehicle 10, and the slip determination unit 102 can set the threshold value L so that it increases as the force acting on the vehicle 10 due to the influence of lateral acceleration and wheel driving force increases .

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 serves as the slip determination criterion, to an appropriate value, it is beneficial to add, as an indicator, 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 less likely slip will occur on the road surface. 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 indicator. 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.

Furthermore, the above-mentioned control device 100 can also achieve distinctive effects not achievable with conventional technology by adopting the configuration described below in (D).

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

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 conditions are to cause slippage. Therefore, as described above in (D), the control device 100 may set the threshold L for determining whether slippage has occurred using yaw rate as part or all of an index in addition to, or instead of, lateral acceleration. By configuring in this way, the control device 100 can reduce the occurrence of tight corner braking by increasing the distribution of driving force transmission to the front wheels 14L, 14R when traveling on a so-called high μ road.

The present invention is not limited to the above-described embodiments and variations, and other embodiments may be possible within the scope of the claims, based on the teachings and spirit of the invention. 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 that embodies any component described in the Summary of the Invention. We intend to obtain rights to these as well through amendments to this application or divisional applications, etc.

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 connects or disconnects the power transmission path between a driving power source and a power transmission member, and a second clutch that selectively connects or disconnects the power transmission path between the power transmission member and an auxiliary drive wheel.

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 (3)

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. The control device for a four-wheel drive vehicle described in claim 1, characterized in 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. 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 the threshold value increases as the force acting on the four-wheel drive vehicle due to the influence of the lateral acceleration and wheel driving force increases.