Torque distribution control device for four-wheel drive vehicle

A torque distribution control device for a four-wheel drive vehicle includes a rotational speed differential responsive type control coupling combined with a front differential of a front wheel drive line. The control coupling has a variable orifice for varying its torque transfer characteristic and thereby varying distribution of engine torque between front and rear wheels. The orifice opening of the variable orifice varies depending upon a variation of a lateral acceleration of the vehicle, e.g., the orifice opening increases as the lateral acceleration increases.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a torque distribution control device for a 
four-wheel drive vehicle, i.e., a device for controlling distribution of 
engine torque or power between the front and rear wheels of a four-wheel 
drive vehicle on the basis of a vehicle running condition. 
2. Description of the Prior Art 
An example of a prior art torque distribution control device for a 
four-wheel drive vehicle is disclosed in Japanese Patent Provisional 
Publication No. 61-169326 or 62-50231. 
Such a prior art control device includes a multi-disc clutch incorporated 
in a front wheel drive line or rear wheel drive line for producing a 
desired transfer torque through control of a hydraulic pressure supplied 
thereto. The clutch engaging force, which is determined by the hydraulic 
pressure, increases as a detected rotational speed differential between 
the front and rear wheels increases. The ratio of the clutch engaging 
force (i.e., transfer torque) relative to the rotational speed 
differential between the front and rear wheels increases as the lateral 
acceleration of the vehicle increases. 
A problem of the prior art torque distribution control device is that a 
control performed thereby is complicated. That is, with the prior art 
control device, it is necessary to obtain the rotational speed 
differential between the front and rear wheels by computation on the basis 
of the signals from vehicle wheel speed sensor, and vary the clutch 
engaging hydraulic pressure in response to a variation of the rotational 
speed differential so that a desired torque distribution is obtained. 
Accordingly, the control device is required to have a rapid responsiveness 
without causing any hunting. 
Another problem is that the device is expensive since it requires a 
microcomputer having a high responsiveness, in order to exercise a 
complicated control and reduce a computation time. 
A further problem is that the device is heavy and bulky, i.e., it requires 
a large space due to the provisions of a hydraulic pump, accumulator, etc. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, there is provided a novel and 
improved torque distribution control device for a four-wheel drive vehicle 
having a front wheel drive line and rear wheel drive line. The control 
device comprises a rotational speed differential responsive type control 
coupling disposed in one of the drive lines and having a rotor and cam 
ring rotatable relative to each other, hydraulic fluid discharge means 
responsive to a rotational speed differential between the rotor and cam 
ring for discharging a quantity of hydraulic fluid proportional to the 
rotational speed differential, and variable orifice means for restricting 
discharge of hydraulic fluid by the discharge means and thereby producing 
a transfer torque between the rotor and cam ring. 
The control device further comprises actuator means for actuating the 
variable orifice means and thereby changing an orifice opening of the 
variable orifice means, lateral acceleration detecting means for detecting 
a lateral acceleration of the vehicle and producing a signal 
representative thereof, and control means for controlling an operation of 
the actuator means in response to the signal from the lateral acceleration 
detecting means for thereby controlling the orifice opening of the 
variable orifice means on the basis of the lateral acceleration of the 
vehicle. 
The above structure is effective for solving the above noted problems 
inherent in the prior art device. 
It is accordingly an object of the present invention to provide a novel and 
improved torque distribution control device for a four-wheel drive vehicle 
which is simple in structure, compact, light in weight and economical, but 
can attain an optimum torque distribution between front and rear wheels. 
It is a further object of the present invention to provide a novel and 
improved torque distribution control device of the above described 
character which can reduce the load on a computer and shorten the 
computation time performed thereby. 
It is a further object of the present invention to provide a novel and 
improved torque distribution control device of the above described 
character which can perform a torque distribution control based on a 
rotational speed differential between the front and rear wheels and a 
lateral acceleration of the vehicle without having to actively detect the 
rotational speed differential between the front and rear wheels. 
It is a further object of the present invention to provide a novel and 
improved torque distribution control device of the above described 
character which does not require any additional hydraulic devices.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1, a brief description of the invention will first be 
made. 
A torque distribution control device of this invention includes a 
rotational speed differential responsive type control coupling or 
so-called orifice coupling generally designated by 11. The control 
coupling 11 includes a rotor 12 and cam ring 13 which are disposed 
concentrically with each other and adapted to cause a discharge flow of 
hydraulic fluid in response to a rotational speed differential 
therebetween. The control coupling 11 is incorporated in a front wheel 
drive line or rear wheel drive line of a four-wheel drive vehicle and 
further includes a variable orifice means 14 for restricting the above 
described discharge flow of hydraulic fluid and thereby producing a 
transfer torque between the rotor 12 and cam ring 13 in response to a 
rotational speed differential between the rotor 12 and cam ring 13. 
The torque distribution control device further includes an orifice actuator 
30 for actuating the variable orifice means 14 to vary the orifice opening 
response to an actuation signal supplied thereto, a lateral acceleration 
detecting means 41 for detecting a lateral acceleration of an associated 
vehicle and producing a control signal representative thereof, and an 
orifice control means 40 for supplying the actuation signal to the orifice 
actuator 30 and thereby causing the orifice opening variation in response 
to the control signal from the lateral acceleration detecting means 41. 
The torque distribution control device 11 operates as follows. 
In the event of a wheel slippage due to driving or in the event of 
cornering or the like, there is caused a rotational speed differential 
between the front and rear wheels. By this rotational speed differential, 
the rotor 12 and cam ring 13 of the control coupling 11 incorporated in 
the front wheel drive line or rear wheel drive line of the four-wheel 
drive vehicle are caused to rotate relative to each other, thus causing a 
quantity of hydraulic fluid to be discharged under restriction by means of 
the variable orifice means 14. 
On the other hand, in the event of the occurrence of a lateral acceleration 
such as caused during cornering or the like, the orifice opening of the 
variable orifice means 14 is caused to vary under control of the orifice 
control means 40 and orifice actuator 30 on the basis of the lateral 
acceleration detected by the lateral acceleration detecting means 41. 
Accordingly, in the event of the occurrence of a rotational speed 
differential between the front and rear wheels and a lateral acceleration 
of the vehicle, a transfer torque responsive not only to the rotational 
speed differential but also to the lateral acceleration is caused by the 
control coupling 11 through control of the flow restriction by means of 
the variable orifice means 14. 
For example, the ratio of the transfer torque to the rotational speed 
differential increases as the orifice opening of the variable orifice 
means 14 decreases. Thus, the larger the transfer torque becomes, the more 
the torque distribution is directed toward an equal four-wheel 
distribution mode, i.e., a distribution mode in which the torque is 
equally distributed to four wheels. 
Referring to FIGS. 2 to 4, the torque distribution control device of this 
invention will be described more in detail. 
In FIG. 2, a four-wheel drive vehicle in which the torque distribution 
control device of this invention is incorporated, is shown by way of 
example as being of the type derived from an F--F (front engine-front 
drive) vehicle and of the permanent rear-drive type. The vehicle has at 
the front a transverse engine 2, transmission 3, front differential 4 and 
transfer gearing 5. The torque or power of the engine 2 is transmitted to 
the rear wheels 1L and 1R by way of the transmission 3, a differential 
case 4a of the front differential 4, transfer gear 5, propeller shaft 6, 
rear differential 7 and rear drive axles 8L and 8R. On the other hand, the 
torque of the engine 2 is transmitted to a left front wheel 9L (when 
viewed from the rear end of the vehicle) by way of the transmission 3, a 
left side gear 4L of the front differential 4, control coupling 11 and a 
left front drive axle 10L and to a right front wheel 9R by way of the 
transmission 3, a right side gear 4R of the front differential 4 and a 
right front drive axle 10R. 
In this instance, since the differential case 4a of the front differential 
4 distributes an engine torque equally to the left and right side gears 4L 
and 4R, the engine torque distribution to the front wheels 9L and 9R 
entirely depends upon the transfer torque transmitted through the control 
coupling 11. 
The above described control coupling 11 is built in or combined with the 
front differential 4, or more specifically, disposed between the left side 
gear 4L of the front differential 4 and the left front wheel drive axle 
10L for producing a transfer torque in response to a rotational speed 
differential .DELTA.N between the rotor 12 and cam ring 13. 
As shown in FIG. 3, the control coupling 11 includes the aforementioned cam 
ring 13 integral with the left side gear 4L and formed with a rise and 
fall cam surface 13a at the inner periphery thereof, the aforementioned 
rotor 12 accommodated concentrically within the cam ring 13 and splined to 
the left front drive axle 10L to rotate therewith, six radial pistons 15 
installed in the rotor 12 for reciprocation while being driven by the cam 
surface 13a in response to a rotational difference between the rotor 12 
and cam ring 13, pressure chambers 16 defined by the pistons 15 and 
variable in volume in response to reciprocations of the pistons 15, radial 
discharge passages 17 in communication with the respective pressure 
chambers 16, variable orifices 14 disposed at the radially inner ends of 
the respective discharge passages 17 and cooperating with a spool 18 so as 
to be variable in opening in response to axial movement of the spool 18, 
regulator passages 19 extending between the respective pressure chambers 
16 and an accumulator chamber 21 for providing communication therebetween 
by way of check valves (no numeral), and a spool chamber 20 disposed 
between the variable orifices 14 and the accumulator chamber 21 for 
providing communication therebetween. 
The rise and fall cam surface 13a, pistons 15, pressure chambers 16, and 
discharge passages 17 constitute a hydraulic fluid discharge means for 
discharging a quantity of hydraulic fluid proportional to a rotational 
speed differential between the rotor 12 and cam ring 13. 
The construction and operation of the control coupling 11 are basically 
similar to those described in U.S. Pat. Nos. 4,921,085; 4,957,473; and 
4,958,711 and therefore, a further description thereof is omitted for 
brevity. 
Assuming that the rpm of the front wheel is Nf, the rpm of the right front 
wheel 9R is Nfr, the rpm of the left front wheel 9L is N.sub.fl, the rpm 
of the rear wheels is N.sub.r, and the rpm of the left side gear 4L is 
N.sub.h, the following expressions are obtained. 
##EQU1## 
From this, N.sub.h -N.sub.fl =2 (N.sub.r -Nf). 
Thus, two times the rotational speed differential between the front and 
rear wheels (N.sub.r -N.sub.f) is supplied as an input to the control 
coupling 11 to cause a coupling rotational speed differential .DELTA.N. 
By this, it becomes possible to control the torque distribution to the 
front wheels by controlling the torque distribution to the left front 
wheel, thus making it possible to reduce by half the necessary torque 
transfer capacity of the control coupling as compared with, for example, 
the case in which the control coupling is incorporated in a front wheel 
side propeller shaft. 
A stepping motor 30 is employed to constitute the aforementioned orifice 
actuator for varying the degree of opening of the variable orifices 14 in 
response to an external signal. A control motion transmitting mechanism 28 
is provided between a shaft 30a of the stepping motor 30 and the spool 18 
for transmitting a control motion of the stepping motor 30 to the spool 
18. To this end, the control motion transmitting mechanism 28 includes a 
fork 31 secured at one end to the shaft 30a of the stepping motor 30 to 
rotate together therewith, slide ring 34 axially moveably installed on the 
left front axle 10L, thrust plate 32 installed on the slide ring 34 by way 
of a needle bearing 33 and held in contact with a free end of the fork 31 
to move therewith, transverse pin 35 extending transversely of the left 
front wheel drive axle 10L and secured to the slide ring 34 to move 
therewith, and a push rod 36 interposed between the transverse pin 35 and 
the spool 18 for transmitting motion of the transverse pin 35 to the spool 
18. 
A control unit 40 mainly consisting of a microcomputer is employed to 
constitute the aforementioned orifice control means for supplying a 
control signal to the stepping motor 30 and thereby controlling the 
orifice opening of the variable orifices 14. A lateral acceleration sensor 
41 is employed to constitute the aforementioned lateral acceleration 
detecting means for detecting a lateral acceleration Y.sub.G of the 
vehicle and supplies a signal representative thereof to the control unit 
40. A potentiometer 42 is installed on the stepping motor 30 for detecting 
an angular position or rotation of the shaft 30a of the stepping motor 30 
and supplying a signal representative thereof to the control unit 40. 
The operation will now be described. 
FIG. 5 shows a routine of control operations executed by the control unit 
40 repeatedly with a predetermined control cycle (e.g., 10 msec). 
At step 50, a lateral acceleration Y.sub.G detected by the lateral 
acceleration sensor 41 is read. 
At step 51, an optimum orifice opening table such as shown in FIG. 6 is 
used to look up an optimum orifice opening .theta..sub.T therein on the 
basis of the lateral acceleration Y.sub.G obtained at step 50. 
In order that an identical turning characteristic is attained irrespective 
of the coefficients of the friction .mu. of a road surface and that both a 
high driveability and stability are attained at starting and at 
straight-ahead acceleration, the optimum orifice opening table is fixed, 
as shown in FIG. 6, such that when the lateral acceleration Y.sub.G is 
zero or small the orifice opening .theta..sub.T is reduced to zero, i.e. 
the variable orifices 14 are fully closed for thereby increasing the 
torque distribution to the front wheels 9L and 9R. As the lateral 
acceleration Y.sub.G increases beyond a predetermined small value, the 
orifice opening .theta..sub.T is increased for thereby reducing the torque 
distribution to the front wheels 9L and 9R, and as the lateral 
acceleration Y.sub.G increases beyond a predetermined large value 
(corresponding to a vehicle running condition on a high-.mu. road 
surface), the variable orifices 14 are fully open. 
At step 52, a control signal capable of attaining an optimum orifice 
opening .theta..sub.T is supplied to the stepping motor 30. 
Next, a torque distribution between the front and rear wheels will be 
described with respect to those attained at straight-ahead running and 
turning separately. 
(A) In the event of straight-ahead running: 
In the event of straight-ahead running, the variable orifices 14 are 
completely closed since the lateral acceleration Y.sub.G under this 
running condition is normally zero and further since even when a lateral 
wind blows or the road surface on which the vehicle is running is slanted, 
only a small lateral acceleration Y.sub.G of the vehicle is caused. 
When, during straight-ahead running, there is caused a small rotational 
speed differential between the front and rear wheels, such as at 
straight-ahead, constant-speed running on a high-.mu. road surface, there 
is not caused any rotational speed differential between the rotor 12 and 
cam ring 13 of the orifice coupling 11, thus not causing any transfer 
torque to the front wheels 9L and 9R and maintaining the rear wheel drive. 
In the event a rotational speed differential between the front and rear 
wheels is caused by a rear wheel slippage due to driving such as caused by 
starting, climbing, acceleration, etc., there is caused between the rotor 
12 and cam ring 13 of the orifice coupling 11 a rotational speed 
differential .DELTA.N corresponding to two times the rotational speed 
differential between the front and rear wheels, thus causing the orifice 
coupling 11 to produce a transfer torque which is equal to the engine 
torque distributed to the front wheels, thus attaining a four-wheel drive 
mode in which the engine torque is nearly equally distributed to the four 
wheels and therefore making it possible to improve the driving efficiency 
and running stability. 
The torque distribution control device of this invention has a transfer 
torque characteristic as shown in FIG. 7. That is, as the rotational speed 
differential .DELTA.N between the rotor 12 and cam ring 13 increases, the 
transfer torque increases along the various curves of the second order, 
which curves are selected through control of the orifice opening .theta.. 
As seen from FIG. 7, when the orifice opening .theta. is zero, i.e., the 
variable orifices 14 are fully closed, a slight rotational speed 
differential .DELTA.N can cause a four-wheel drive mode in which the 
engine torque is nearly equally distributed to the four wheels. 
(B) In the event of turning: 
In the event of turning, there is caused a lateral acceleration Y.sub.G in 
response to a turning radius, vehicle speed and a coefficient of friction 
.mu. of a road surface. 
For example, assuming a constant turning in which the turning radius and 
vehicle speed are constant but the coefficient of friction .mu. of the 
road surface varies, a high-.mu. road causes a large lateral acceleration 
Y.sub.G, thus causing the variable orifices 14 to increase in orifice 
opening, and a low-.mu. road causes a small lateral acceleration, thus 
causing the variable orifices 14 to reduce in orifice opening. 
Accordingly, in the event of a constant turning on a high-.mu. road, the 
torque distribution to the front wheels is small, thus causing the vehicle 
to exhibit a strong understeer and therefore making it possible to attain 
such a good turning ability as is attained by the F-R (front engine-rear 
drive) vehicle. Further, in the event of a constant turning on a low-.mu. 
road surface, the distribution of the engine torque to the front wheels is 
increased, thus causing the vehicle to exhibit an understeer and thereby 
making it possible to attain a good turning stability. 
In the event of an accelerated turning, the lateral acceleration Y.sub.G 
gradually increases from the entrance of a corner to the exit of same. At 
the entrance of the corner, a torque transfer characteristic of a large 
ratio of the transfer torque to the rotational speed differential is 
selected out of the torque transfer characteristics of FIG. 7, thus 
increasing the torque distribution to the front wheels and making it 
possible to attain a tendency to understeer and therefore a turning 
stability. At the exit of the corner, a torque transfer characteristic 
having a small ratio of the transfer torque to the rotational speed 
differential is selected out of the characteristics of FIG. 7, thus making 
it possible to gradually increase the torque distribution to the front 
wheels when the rotational speed differential between the front and rear 
wheels is caused by a wheel slippage due to driving in response to an 
accelerating operation, for thereby preventing a rapid increase of the 
intensity of oversteer and improving the controllability of the vehicle. 
From the foregoing, it will be understood that according to the present 
invention the control coupling 11 is incorporated in the front wheel drive 
line and the orifice opening .theta. is controlled in response to a 
lateral acceleration Y.sub.G only, thus making it possible to attain an 
optimum torque distribution between the front and rear wheels with a 
simple control and low cost and without causing any substantial increase 
in weight and space. That is, the torque distribution control in response 
to a rotational speed differential between the front and rear wheels 
solely depends upon the control coupling 11. Thus, differing from the 
prior art device, it is not necessary to detect the rotational speed 
differential between the front and rear wheels. Further, it is not 
necessary to execute a control in response to a variation of a rotational 
speed differential between the front and rear wheels unless the lateral 
acceleration Y.sub.G varies, thus making it possible, substantially 
similarly to the prior art device, to attain a torque distribution control 
in response to a rotational speed differential between the front and rear 
wheels and a lateral acceleration with a reduced load on the microcomputer 
and a reduced computation time. It will be further understood that the 
control coupling 11 which is combined with the front differential 4, is 
compact in size and light in weight, without requiring additional 
hydraulic devices such as a hydraulic pump, accumulator, etc. 
While the present invention has been described and shown as being applied 
to a four-wheel drive vehicle of the type having at the front a transverse 
engine, transmission, differential and transfer, and based on a permanent 
rear drive vehicle, this is not for the purpose of limitation. For 
example, the present invention may otherwise be applied to a vehicle of 
the type derived from a permanent front drive vehicle or a vehicle having 
at the front a longitudinal engine and transmission and derived from a 
permanent front or rear drive vehicle.