Drive power transmission equipment

Front and rear wheels Tf, Tr of a motor vehicle driven by an engine 10 are connected through a differential control device 15 which is capable of varying the torque to transmit. When the turning radius of a predetermined one wheel or the motor vehicle is equal to or smaller than a predetermined turning radius, the transmission torque of the differential control device is decreased by a control device to prevent the tight-corner braking phenomenon from occurring. When the motor vehicle is stopped momentarily with the transmission torque being lowered, the control device maintains the transmission torque as it is, and the vehicle is then restarted with the transmission torque being lowered. Thus, when the motor vehicle which was stopped momentarily with a large steering angle is then restarted, the tight-corner braking phenomenon can be prevented from occurring from the beginning of the restarting.

TECHNOLOGICAL FILED

The present invention relates to a drive power transmission device capable of preventing a tight-corner braking phenomenon from occurring when the turning radius is small in a four-wheel drive vehicle.

BACKGROUND ART

In motor vehicles, during the traveling along a curve, the turning radiuses of front wheels become larger than those of rear wheels, so that the traveling distance of the front wheels become longer than those of the rear wheels. For this reason, in a four-wheel drive vehicle of the type that front and rear wheels are drivingly connected to be driven by an engine, a problem arises in that since the difference between the traveling distances of the front and rear wheels becomes large during the traveling along a curve with a small radius, a tight-corner braking phenomenon to act as if braking has been applied to the front wheels occurs to exert an adverse influence on the steerability. As measures to obviate the problem, there is known a technology which is disclosed in Japanese unexamined, published patent application No. 8-2278. This technology is to prevent the occurrence of the tight-corner braking phenomenon in the following way. That is, the wheel speeds of four wheels are detected by wheel speed sensors, and then, the speed difference is calculated between the outside front wheel and the inside rear wheel. When the speed difference is equal to or larger than a judgment value which corresponds to a speed difference made on a curve with a turning radius causing the tight-corner braking to occur and when the speed difference is smaller than another judgment value which corresponds to another speed difference made on a curve with the minimum turning radius, the motor vehicle is judged to be in a turning state with a large steering angle, so that the transmission torque of a limited differential clutch (serving as differential control device) for coupling the front and rear wheels is decreased to prevent the occurrence of the tight-corner phenomenon.

However, the foregoing technology involves the following problems. That is, as shown inFIG. 7, when the motor vehicle M which was traveling along a curve with a turning radius equal to or smaller than a turning radius (hereafter simply as “tight-corner turning radius) which causes the tight-corner braking phenomenon to occur, is stopped temporarily at a position (A) and then is restarted, the tight-corner braking phenomenon occurs to adversely influence the steerability until the motor vehicle reaches another position (B) advanced somewhat from the stop position (A). Further, the motor vehicle suffers a shock when the tight-corner braking phenomenon is released at the position (B).

The causes for this are explained as follows. The rotational speeds of the wheels detected by the wheel speed sensors decrease to zero when the motor vehicle is stopped temporarily at the position (A), and since the judgment that the vehicle is in a turning state along a curve with a radius smaller than the tight-corner turning radius is reversed, the transmission torque of the differential control device is increased to bring the front and rear wheels into direct connection. Accordingly, by detecting the rotational speeds of the wheels by the wheel speed sensors after the restarting and then by calculating the speed difference between the outside front wheel and the inside rear wheel, it is judged that the vehicle is turning with a large steering angle to travel along a curve of a smaller radius than the tight-corner turning radius, and until so judged, the tight-corner braking phenomenon occurs to accumulate its stress in a power transmission mechanism between the front and rear wheels due to the competing rotations of the front and rear wheels depending on the difference between the traveling distances. Then, since it is judged at the position (B) that the vehicle is turning with a large steering angle along a curve of a smaller radius than the tight-corner turning radius, the transmission torque of the differential control device is decreased to release the tight-corner baking phenomenon. At this time, the stress accumulated in the power transmission mechanism is released instantaneously, so that the shock is given. This problem occurs not only when the motor vehicle M is restarted after being stopped temporarily, but also when the vehicle speed is increased again after being lowered to decrease the wheel speeds of the respective wheels to lower speeds than those which cannot be detected by the wheel speed sensors.

The object of the present invention is to solve the aforementioned various problems by performing a control of preventing the tight-corner braking phenomenon from occurring from the beginning of a restarting when the vehicle is restarted after being stopped temporarily in a control state that the occurrence of the tight-corner braking phenomenon is being prevented by decreasing the transmission torque of a differential control device for coupling front and rear wheels.

DISCLOSURE OF THE INVENTION

The present invention resides in a drive power transmission device provided with a differential control device for distributing the transmission torque from an engine to front and rear wheels, the differential control device being capable of varying the torque distribution ratio between the front and rear wheels, and with a control device for executing a tight-corner braking prevention control under which the transmission torque of the differential control device to the front wheels or the rear wheels are decreased so as to prevent the tight-corner braking phenomenon from occurring when the turning radius of a predetermined one wheel of the wheels or the vehicle is equal to or smaller than a predetermined turning radius. The control device is constructed so that when the vehicle under the tight-corner braking prevention control being executed is restarted after being stopped momentarily or is accelerated from a lower vehicle speed than a predetermined vehicle speed to a higher vehicle speed than the predetermined vehicle speed, the control device executes the tight-corner braking prevention control from the beginning of the restarting or the acceleration.

Therefore, according to the present invention, since the control device is constructed so that when the vehicle under the tight-corner braking prevention control being executed is restarted after being stopped momentarily or is accelerated from a lower vehicle speed than the predetermined vehicle speed to a higher vehicle speed than the predetermined vehicle speed, the control device executes the tight-corner braking prevention control from the beginning of the restarting or the acceleration, the tight-corner braking phenomenon does not occur to give an influence on the steerability after the restarting or the commencement of the acceleration toward the predetermined vehicle speed or higher. Further, since stress is not accumulated in a power transmission mechanism between the front and rear wheels, any shock which may otherwise be given upon the release of such stress is not given.

Further, the present invention resides in the drive power transmission device improved as mentioned above, wherein when the vehicle is in the state that it is stopped or the vehicle speed is equal to or lower than the predetermined vehicle speed, the control device makes a tight-corner flag for determining whether to execute the tight-corner braking prevention control or not, remain in the state that the tight-corner flag was right before the stopping or right before the vehicle speed is decreased to the predetermined vehicle speed or lower. Thus, since the control device is not required to execute any special operation but to make the tight-corner flag remain in the state that the tight-corner flag was right before the stopping or right before the vehicle speed is decreased to the predetermined vehicle speed or lower, the control device can execute accurate control in a simplified construction.

Furthermore, the present invention resides in the drive power transmission device improved as mentioned above, wherein the motor vehicle is further provided with wheel speed sensors for detecting the rotational speeds of at least one pair of right and left wheels of the front and rear wheels. The control device executes the tight-corner braking prevention control when the turning radius of the predetermined one wheel or of the vehicle, which is calculated based on the rotational speed difference between the right and left wheels detected by the wheel speed sensors, is equal to or smaller than the predetermined turning radius. With this configuration, since the wheel speed sensors can be substituted by those wheel sensors for antiskid brake systems as provided in most of the motor vehicles, the increase in the manufacturing cost in practicing the present invention becomes small.

Where the turning radius of the predetermined one wheel or the vehicle is calculated based on the rotational speed difference between right and left wheels, the slip of each wheel on the road surface causes the calculated turning radius to involve some errors. Therefore, a problem arises in that the tight-corner braking prevention control cannot be executed reliably by the use of the turning radius involving some such errors.

In order to obviate this problem, the present invention resides in the drive power transmission device improved as mentioned above, wherein the motor vehicle is further provided with wheel speed sensors for respectively detecting the rotational speeds of the right and left front and rear wheels. The control device includes first turning radius calculation means for calculating the turning radius of the predetermined one wheel or the motor vehicle based on the rotational speed difference between either one pair of the right and left wheels of the front and rear wheels, and second turning radius calculation means for calculating the turning radius of the predetermined one wheel or the motor vehicle based on the rotational speed difference between the other pair of the right and left wheels of the front and rear wheels or based on the rotational speed difference between each one pair of the right and left wheels of the front and rear wheels. And, the control device executes the tight-corner braking prevention control when the difference between the turning radiuses which are calculated respectively by the first and second turning radius calculation means is equal to or smaller than a predetermined value and when at least any one of the turning radiuses which are calculated respectively by the first and second turning radius calculation means is equal to or smaller than the predetermined turning radius.

With this construction, of the calculated turning radiuses, there is used one obtained when the difference between the two turning radiuses calculated by the two turning radius calculation means exceeds a predetermined value, that is, there is used one turning radius which is left by eliminating the other turning radius which is larger in the foregoing slip and hence, larger in error. Therefore, it can be realized to execute precise tight-corner braking prevention control.

When the motor vehicle under the tight-corner braking prevention control being executed is restarted after being stopped momentarily or is accelerated from a lower vehicle speed than the predetermined speed to a higher vehicle speed than the predetermined speed, the drive wheels may slip because the friction coefficient (μ) of the road surface is small. In this case, the execution of the tight-corner braking prevention control has caused the differential control device to distribute the drive power to the secondary drive wheels at a small transmission torque distribution rate, and therefore, it may be the case that the motor vehicle cannot be started due to the lack of the drive power at the side of the secondary drive wheels.

In order to solve this problem, the present invention resides in the drive power transmission device improved as mentioned above, wherein the control device is further provided with slip judging means for judging whether or not the drive wheels are slipping when the motor vehicle under the tight-corner braking prevention control being executed is restarted after being stopped momentarily or is accelerated from a lower vehicle speed than the predetermined speed to a higher vehicle speed than the predetermined speed. And, the control device discontinues the execution of the tight-corner braking prevention control when the slip judging means judges that the slip is occurring.

Therefore, when the motor vehicle is restarted after being stopped momentarily or is accelerated from the state that the vehicle speed is equal to or lower than the predetermined value, the drive wheels may slip because the friction coefficient (μ) of the road surface is small, in which case, the motor vehicle is returned from under the tight-corner braking prevention control to under an ordinary control, so that the transmission torque is distributed also to the secondary drive wheels to enable the vehicle to be restarted.

Further, the present invention resides in the drive power transmission device improved as mentioned above, wherein the slip judgment means judges that the slip is occurring when the rotational speed of any one of the drive wheels is equal to or higher than a first threshold value and when the rotational speed of any one of the secondary drive wheels is equal to or lower than a second threshold value. Thus, the occurrence of the slip can be judged correctly.

PREFERRED EMBODIMENT TO PRACTICE THE INVENTION

Hereinafter, an embodiment of a drive power transmission device for practicing the present invention will be described with reference to the drawings. As shown mainly inFIG. 1, the drive power transmission device according to this embodiment is composed of a differential control device15capable of varying the transmission torque for connecting front wheels Tf and rear wheels Tr of a motor vehicle M driven by an engine10, and an electronic controller (control device)20for controlling the transmission torque of the differential control device15. The drive power from the engine10is transmitted to a transfer12through a transaxle11incorporating a transmission therein, and at the transfer12, the drive power distributed to the front wheel side is transmitted to right and left front wheels Tf as drive wheels through a front differential (not shown), while the drive power distributed to the rear wheel side is transmitted to right and left rear wheels Tr as secondary drive wheels through a propeller shaft13provided with the differential control device15at mid position thereof as well as through a rear differential14. The differential control device15is a multi-plate friction clutch of electromagnetic or hydraulic type and is capable of varying the transmission torque in dependence on a control output from the electronic control device20. Wheel speed sensors S1, S2, S3and S4for detecting the rotational speeds are provided respectively in association with right and left front wheels Tf and right and left rear wheels Tr.

The electronic control device20is provided with a CPU21for calculating a control value of the engaging force which value is to be output to the differential control device15in dependence on the traveling state of the motor vehicle M, a ROM22having stored control programs which are necessary for such calculation and outputting, a RAM23used as a working area for such calculation, and an input/output circuit24for performing outputting and inputting to and from each of the differential control device15and the wheel speed sensors S1, S2, S3, S4. The CPU21of the electronic control device20calculates the turning radius of any of the wheels or the motor vehicle M based on the rotational speeds of the wheels Tf1, Tf2, Tr1, Tr2which are detected by the wheel speed sensors S1, S2, S3, S4and judges whether or not the calculated turning radius is equal to or smaller than a predetermined tight-corner turning radius. The CPU21then executes an ordinary control under which the engaging force of the differential control device15is increased thereby to increase the transmission torque, unless the calculated turning radius is equal to or smaller than the tight-corner turning radius, but executes a tight-corner braking prevention control under which the engaging force of the differential control device15is weakened thereby to decrease the transmission torque, if the calculated turning radius is equal to or smaller than the tight-corner turning radius.

To start with, calculation expressions for calculating the turning radiuses based on the wheel rotational speeds detected by the wheel speed sensors S1, S2, S3, S4will be explained with reference toFIG. 2. First, various symbols are defined as follows:L: Wheel BaseLf: Front TreadLr: Rear Treadr: Radius of wheels Tf, TrΩ: Turning Speed of Motor Vehicle MR1, ω1: Turning Radius and Rotational Speed of Inside Front Wheel Tf1R2, ω2: Turning Radius and Rotational Speed of Outside Front Wheel Tf2R3, ω3: Turning Radius and Rotational Speed of inside Rear Wheel Tr1R4, ω4: Turning Radius and Rotational Speed of outside Rear Wheel Tr2

Calculated in this embodiment are the turning radius R3of the insider rear wheel Tr1which is derived from the rotational speeds of the right and left rear wheels Tr and the turning radius R3′ of the inside rear wheel Tr1which is derived from the rotational speeds of the right and left front wheels Tf and the right and left rear wheels Tr. The reason for this will be described hereafter. It is to be noted that the turning radiuses R3and R3′ calculated herein represent those values obtained in the case that any of the wheels is not slipping.

Description will be made concerning the calculation procedure for the turning radius R3of the inside rear wheel Tr1which is based on the rotational speeds of the right and left rear wheels Tr. First, the following expression 1 holds true between the turning speed ω of the motor vehicle M and the rotational speed ω1, ω2, ω3, ω4of each wheel.
R1Ω=rω1
R2Ω=rω2
R3Ω=rω3
R4Ω=rω4(Expression 1)

This derives the following expression 2.
R1/ω1=R2/ω2=R3/ω3=R4/ω4(Expression 2)

Further, the following expression 3 holds true among the turning radius R3of the inner rear wheel Tr1, the turning radius R4of the outside rear wheel Tr2and the rear tread Lr.
R4=R3+Lr(Expression 3)

The following expression 4 for calculating the turning radius R3of the inside rear wheel Tr1can be derived from this expression 3 and the foregoing expression 2.
R3=Lr/{(ω4/ω3)−1}  (Expression 4)

Next, description will be made with respect to the calculation procedure for the turning radius R3′ of the inside rear wheel Tr1which is based on the rotational speeds of the right and left front wheels Tf and the right and left rear wheels Tr. Since the front tread Lf between the right and left front wheels Tf is approximately the same as the rear tread Lr between the right and left rear wheels Tr, the following expression 5 holds true.
R12=R32+L2
R22=R42+L2(Expression 5)

The following expression 6 for calculating the turning radius R3′ of the inside rear wheel Tr1can be derived from this expression 5 and the foregoing expression 2.
R3′2={1−(ω2/ω1)2}L2/{(ω2/ω1)2−(ω4/ω3)2}  (Expression 6)

As mentioned earlier, the two turning radiuses R3and R3′ which are calculated based on the rotational speeds of the right and left front and rear wheels Tf, Tr are those values obtained when any of the wheels Tf, Tr is not slipping on the road surface and hence, ought to become the same with each other. However, they take a smaller value than the actual turning radius when the outside front and rear wheels Tf2and Tr2are slipping on mud or mire, but take a larger value than the actual turning radius (or take the value obtained in the case of a reverse curve) when the inside front and rear wheels Tf1and Tr1are slipping on mud or mire. In either of these cases, it results that the detected turning radius includes an error. Since, with any of the wheels slipping, the slip rates of the wheels do not have the same values with one another, the two turning radiuses R3and R3′ become different values. Accordingly, by comparing the two turning radiuses R3and R3′ with each other, it becomes possible to detect whether or not the error involved in the detected turning radius R3has exceeded a tolerable error due to the slipping of the wheels Tf, Tr on the road surface.

Next, detecting the tight-corner braking phenomenon and controlling the engaging force of the differential control device15will be described with reference to flowcharts shown inFIGS. 3 through 5. The CPU21of the electronic control device20executes the flowchart shown inFIG. 3at predetermined short time intervals. The CPU21first takes thereinto the rotational speeds ω1, ω2ω3, ω4of the right and left front and rear wheels Tf, Tr from the wheel speed sensors S1, S2, S3, S4(step100) and judges whether or not any of the rotational speeds ω1, ω2, ω3, ω4is zero, that is, whether or not the motor vehicle M is traveling (step101). When any of the rotational speeds ω1, ω2, ω3, ω4is not zero because the motor vehicle M is traveling, the CPU21advances its control operation to step102. It is to be noted that at step101, instead of judging whether or not any of the rotational speeds ω1, ω2, ω3, ω4is zero, a judgment may be executed of whether or not all the rotational speeds ω1, ω2ω3, ω4are larger than a predetermined value δ which is the slowest rotational speed detectable by the wheel speed sensors. By the foregoing expression 4, the turning radius R3of the inside rear wheel Tr1is calculated based on the rotational speeds ω3, ω4of the right and left rear wheels Tr (step102), and it is judged whether or not the absolute value of the turning radius R3is equal to or smaller than a predetermined turning radius a, i.e., tight-corner turning radius (step103). When the turning radius R3is equal to or smaller than the tight-corner turning radius α, the CPU21advances its control operation to step104, at which by the foregoing expression 6, it calculates the turning radius R3′2of the inside rear wheel Tr1based on the rotational speeds ω1, ω2, ω3, ω4of the right and left front and rear wheels Tf, Tr. Then, the CPU21advances its control operation to step105, at which it judges whether or not the absolute value of the subtraction R32−R3′2is equal to or smaller than another predetermined value β. The predetermined value β is set so that the error of the detected turning radius R3from the actual turning radius of the inside rear wheel Tr1does not exceed a tolerable value. If the absolute value of the subtraction R32−R3′2is equal to or smaller than the predetermined value β, the CPU21sets the tight-corner flag at step106. Thereafter, the CPU21at step110judges whether or not the tight-corner flag has been set. In this case, since it has been set, the CPU21executes a tight-corner braking prevention control (step130) as referred to later in detail and terminates the control operations according to the flowchart shown inFIG. 3. As mentioned above, the CPU21sets the tight-corner flag and executes the tight-corner braking prevention control when the calculated turning radius R3is equal to or smaller than the tight-corner turning radius a and when the absolute value of the subtraction R32−R3′2is equal to or smaller than the predetermined value β, that is, when the error of the turning radius R3is equal to or smaller than the tolerable value.

When the turning radius R3is not equal to nor smaller than the tight-corner turning radius αat step103, or when the absolute value of the subtraction R32−R3′2is not equal to nor smaller than the predetermined value β, the CPU21advances its control operation to step107to clear the tight-corner flag, and judges at step110whether or not the tight-corner flag has been set. In this case, since it has not been set, the CPU21executes an ordinary control (step120) as referred to later in detail and terminates the control operations according to the flowchart shown inFIG. 3. In short, when the calculated turning radius R3is not equal to nor smaller than the tight-corner turning radius α, or when the error of the turning radius R3is not equal to nor smaller than the tolerable value, the CPU21clears the tight-corner flag and executes the ordinary control.

When any one of the rotational speeds ω1-ω4is judged to be zero or smaller than a predetermined value δ at step101because the motor vehicle M has been stopped or substantially stopped, and when the drive wheels are judged at step108to be not slipping, the CPU21skips its control operation up to step110without changing the state of the tight-corner flag which was held when the control cycle at that interval began to be executed, and executes the tight-corner braking prevention control (step120) or the ordinary control (step110) in dependence upon the state of the tight-corner flag which remains at that moment. Accordingly, when the motor vehicle M under the tight-corner braking prevention control being executed is restarted after being stopped momentarily or is accelerated from a lower speed than the predetermined value to a higher speed than the same, it becomes possible to execute the tight-corner braking prevention control from the beginning of the restarting or the acceleration.

In the state that the tight-corner braking prevention control is being executed, the motor vehicle M is restarted after being stopped momentarily or the motor vehicle M is accelerated from the state that any of the wheel speeds is lowered than the predetermined value δ. In either case, the drive wheels slip if the friction coefficient (μ) of the road surface is small. In this case, when it is judged at step101that the rotational speeds of the secondary drive wheels are zero or smaller than the predetermined value δ, and then when it is judged at step108constituting slip judging means that the drive wheels are slipping, the tight-corner flag is cleared at step109. Therefore, if the drive wheels slip due to the friction coefficient (μ) of the road surface being small when the vehicle is restarted after being stopped momentarily or is accelerated from the state that any of the wheel speeds is decreased to the predetermined value δ or lower, the tight-corner braking prevention control is discontinued to be returned to the ordinary control, whereby the motor vehicle M can be restarted with the secondary drive wheels being also given a sufficient transmission torque.

Where the motor vehicle M is started from the stop state as it makes turn along a curve of the minimum turning radius without slipping, the rotational speed of the outside front wheel as drive wheel becomes the maximum value γ when the rotational speed of the inside rear wheel as secondary drive wheel decreases to the predetermined value δ which is the lowest rotational speed detectable by the wheel speed sensor therefor. Therefore, at step108, in order to correctly judge whether or not the slipping has occurred, the slipping is judged to have occurred when the rotational speed ω1or ω2of any one of the drive wheels is equal to or higher than a first threshold value which is set larger than the maximum value γ and when the rotational speed ω3or ω4of any one of the secondary drive wheels is equal to or lower than a second threshold value which is set smaller than the predetermined value δ. Step108may be changed to judge that the slipping has occurred when the rotational speed difference between the drive wheels and the secondary drive wheels exceeds a predetermined value.

In a motor vehicle of the type that the drive power from the engine10is directly transmitted to the right and left rear wheels Tr, but to the right and left front wheels Tf through the differential control device15, the rear wheels Tr work as the drive wheels and the front wheels Tf work as the secondary drive wheels. In this case, it is judged at step108that the slipping has occurred when the rotational speed ω3or ω4of any one of the drive wheels is equal to or higher than the first threshold value which is set larger than the maximum value γ and when the rotational speed ω1or ω2of any one of the secondary drive wheels is equal to or lower than the second threshold value which is set smaller than the predetermined value δ.

In connection with the claimed invention, of those control programs stored in the ROM22, the part for executing the calculation at step102corresponds to the first turning radius calculation means, and the part for executing the calculation at step104corresponds to the second turning radius calculation means. Instead of calculating the turning radius R3of the inside rear wheel Tr1, the first and second turning radius calculation means may be modified to calculate the turning radiuses R4, R1, R2of the outside rear wheel Tr2or the inside or outside front wheel Tf1or Tf2, or may be modified to calculate the turning radius of the motor vehicle M itself. In either case, the tight-corner turning radius is set to correspond to the calculated turning radius in that case. Further, the first and second turning radius calculation means may be constituted to calculate the foregoing respective turning radiuses based on the rotational speeds of the different wheels. For example, the first turning radius calculation means may be constituted to calculate the turning radius based on the rotational speeds of the both front wheels Tf, while the second turning radius calculation means may be constituted to do so based on the rotational speed of the both rear wheels Tr. Alternatively, the first turning radius calculation means may be constituted to calculate the turning radius based on the rotational speeds of the both front wheels Tf and the both rear wheels Tr, while the second turning radius calculation means may be constituted to do so based on the rotational speed of the both front wheels Tf. In either case, as the slip of each wheel on the road surface increases, the difference increases between the respective turning radiuses which are calculated by the first and second turning radius calculation means.

Next, the ordinary control at step120will be described with reference to the subroutine shown inFIG. 4. In the beginning, the CPU21calculates the rotational number difference between the front wheels Tf and the rear wheels Tr based on the rotational speeds of the right and left front and rear wheels Tf, Tr which are received from the wheel speed sensors S1, S2, S3, S4(step121) and then, calculates the vehicle speed from the average of the rotational speeds of the rear wheels Tr as the secondary drive wheels (step122). Compensation coefficients for engaging forces each to be given to the differential control device15under the ordinary control have been set in correspondence to the rotational number differences as well as to the vehicle speeds and have been stored in the ROM22in the form of an ordinary control first map. A compensation coefficient for an engaging force which corresponds to a calculated rotational number difference and a calculated vehicle speed is read out from the ordinary control first map to be determined (step123). In the case that the rotational number difference is large with the vehicle speed being low, it is judged that the wheels are slipping on mud, mire or the like. Therefore, the compensation coefficient in this case has been set to strengthen the engaging force in proportion to the increase in the rotational number difference in order to heighten the traction performance for enabling the vehicle to escape from a bad road such as mud, mire or the like, but has been set to weaken the engaging force in the case that the vehicle speed is high.

Subsequently, the CPU21receives a throttle valve opening degree from a throttle valve opening degree sensor attached to a throttle valve of the engine10(step124). Compensation coefficients for engaging forces each to be given to the differential control device15under the ordinary control have been set in correspondence to throttle valve opening degrees and vehicle speeds and have been stored in the ROM22in the form of an ordinary control second map. A compensation coefficient for the engaging force which corresponds to the input throttle valve opening degree and the input vehicle speed is read out from the ordinary control second map to be determined (step125). In order to heighten the starting performance and the acceleration performance, the compensation coefficients for the engaging forces in this case have been set to increase the engaging force as the throttle valve opening degree becomes large with the vehicle speed being low. Further, based on the compensation coefficient determined at steps123,125, the CPU21determines a control value for giving the differential control device15an appropriate engaging force under the ordinary control and outputs the determined control value to the differential control device15(step126), whereby the transmission torque of the differential control device15can be controlled to control the torque distribution ratio between the front wheels Tf and the rear wheels Tr.

Next, the tight-corner braking prevention control at step130will be described with reference to the subroutine shown inFIG. 5. First, the CPU21calculates the rotational number difference between the front wheels Tf and the rear wheels Tr based on the rotational speeds of the right and left front and rear wheels Tf, Tr which are input from the wheel speed sensors S1, S2, S3, S4(step131), and then, calculates the vehicle speed from the average in rotational speeds of the rear wheels Tr as the secondary drive wheels (step132). Compensation coefficients for the engaging forces to be given to the differential control device15under the tight-corner braking prevention control have been set in correspondence to the rotational number differences and the vehicle speeds and have been stored in the ROM22in the form of a tight-corner braking prevention control first map. Thus, a compensation coefficient for the engaging force which corresponds to the calculated rotational number difference and the calculated vehicle speed is read out from the tight-corner braking prevention control first map to be determined (step133). In order to prevent the tight-corner braking from occurring, the compensation coefficients for this case are set to be smaller than those for the ordinary control so that the transmission torque to be transmitted to the rear wheels Tr as the secondary wheels are made small. The general characteristic or inclination with which the compensation coefficients are set is such that the engaging force is strengthened as the rotational number difference between the front and rear wheels increases with the vehicle speed being low, but is weakened to enhance the steerability as the vehicle speed increases, and the general inclination is the same as that for the ordinary control.

Then, the CPU21receives the throttle valve opening degree input from the throttle valve opening degree sensor which is attached to the throttle valve of the engine10(step134). Compensation coefficients for the engaging forces to be given to the differential control device15under the tight-corner braking prevention control have been set in correspondence to the throttle valve opening degrees and the vehicle speeds and have been stored in the ROM22in the form of a tight-corner braking prevention control second map. Thus, a compensation coefficient for the engaging force which corresponds to the input throttle valve opening degree and the vehicle speed is read out from the tight-corner braking prevention control second map to be determined (step135). In order to prevent the tight-corner braking from occurring, the compensation coefficients for this case are set to be smaller than those for the ordinary control. The general characteristic or inclination with which the compensation coefficients are set is such that the engaging force is strengthened as the throttle valve opening degree becomes large with the vehicle speed being low, and the general inclination is the same as that for the ordinary control. Thereafter, based on the compensation coefficients determined at the both steps133,135, the CPU21determines a control value for giving the differential control device15an appropriate engaging force which prevents the occurrence of the tight-corner braking and outputs the determined control value to the differential control device15(step136), whereby the transmission torque of the differential control device15can be controlled to control the torque distribution ratio between the front wheels Tf and the rear wheels Tr.

According to this embodiment, the tight-corner flag is kept set while the tight-corner braking control is executed because the turning radius R3of the inside rear wheel Tr1is equal to or smaller than the right-corner turning radius, and when the motor vehicle M is stopped or substantially stopped in this state, the tight-corner flag remains as it is without being cleared. Accordingly, when the motor vehicle M traveling along a curve W as shown inFIG. 6is stopped temporarily at a position A and then is restarted or is accelerated from the state that any of the wheel speeds is decreased to be equal to or lower than the predetermined value δ, the tight-corner braking prevention control is executed to decrease the transmission torque of the differential control device15from right after the restarting or the acceleration. As a result, the tight-corner braking phenomenon does not occur after the restarting or the acceleration, so that any adverse influence is not exerted on the steerability. In addition, since no stress is accumulated in the power transmission mechanism between the front and rear wheels, it does not take place that the release of such stress gives rise to a shock.

Further, when the motor vehicle M under the tight-corner braking prevention control is stopped temporarily and then is restarted, or when the motor vehicle M under the tight-corner braking prevention control is accelerated from the state that any of the wheel speeds is decreased to the predetermined value δ or lower, the drive wheels may slip because the friction coefficient (μ) of the road surface is small. In this case, the tight-corner flag is cleared at step109if the rotational speeds of the secondary drive wheels are zero or equal to or lower than the predetermined value δ. As a consequence, in the case that the drive wheels slip because the friction coefficient (μ) of the road surface is small when the motor vehicle M is stopped temporarily and then is restarted or is accelerated from the state that any of the wheel speeds is decreased to the predetermined value δ or lower, the tight-corner braking prevention control is discontinued to be returned to the ordinary control, and a sufficient drive torque is distributed also to the secondary drive wheels, so that the motor vehicle M can be started.

Also in this embodiment, when the motor vehicle M is stopped or substantially stopped as mentioned earlier, the electronic control device20does nothing but to hold the tight-corner flag for determining whether to execute the tight-corner braking prevention control or not, in the state that the flag was right before the stopping, and is not needed to execute any other particular operation. Therefore, the electronic control device20is not complicated in construction.

Moreover, in this embodiment, the CPU21of the electronic control device20calculates the two turning radiuses based on the rotational speeds of different wheels at step102and step104in the flowchart shown inFIG. 3, compares these turning radiuses, and executes the tight-corner braking prevention control by the use of the turning radius which is left by eliminating the other turning radius representing a larger slip and hence, a larger error as a result of such comparison. By doing so, it becomes realized that the tight-corner braking prevention control can be executed precisely. However, the present invention is not limited to this way of control and rather may be practiced in such a way as to calculate only one turning radius and to execute the tight-corner braking prevention control based on such one turning radius.

Also in this embodiment, description is directed to a four-wheel drive vehicle of the type that is not provided with any center differential. However, the present invention is applicable to a four-wheel drive vehicle of the type having such center differential, in which case the present invention may be practiced in such a form that the center differential is incorporated into, e.g., the transfer12.

Additionally, the wheel speed sensors S1, S2, S3, S4may be substituted by those wheel speed sensors which most of motor vehicles have for anti-skid brake systems. By doing so, the increase in the manufacturing cost for practicing the present invention becomes small. Further, the present invention may be practiced in such a form that the substances in calculation of the first turning radius calculation means and the second turning radius calculation means in the foregoing embodiment are exchanged with each other.

INDUSTRIAL APPLICABILITY

The drive power transmission device according to the present invention is suitable for use as a drive power transmission device for controlling the transmission torque distribution ratio between front and rear wheels through a differential control device so that the tight-corner braking phenomenon does not occur in a four-wheel drive vehicle of the type that the transmission torque from an engine is distributed to be transmitted to the front and rear wheels.