GB2194928A

GB2194928A - Power transmitting apparatus for four-wheel-drive vehicle

Info

Publication number: GB2194928A
Application number: GB08721549A
Authority: GB
Inventors: Yasuji Shibahata
Original assignee: Honda Motor Co Ltd
Current assignee: Honda Motor Co Ltd
Priority date: 1986-09-12
Filing date: 1987-09-14
Publication date: 1988-03-23
Anticipated expiration: 2007-09-14
Also published as: GB8721549D0; DE3730653A1; FR2603847A1; DE3730653C2; GB2194928B

Description

1 GB2194928A 1

SPECIFICATION

Power transmitting apparatus for four-wheel drive vehicle The present invention relates to a power transmitting apparatus for use in a four-wheel-d rive 5 vehicle.

U.S. Patents Nos. 3,760,922 and 4,605,087, for example, disclose a viscous shear coupling as a device for transmitting torque dependent on the difference between the rotational speeds of input and output shafts. As shown in FIG. 29 of the accompanying drawings, the disclosed viscous shear coupling comprises an outer tube coupled to a power source through an input shaft, an inner tube coupled to an output shaft, outer and inner plates fixed to the outer and inner tubes, respectively, and a highly viscous liquid such as silicone oil sealed between the outer and inner tubes. Based on the fact that the viscous shearing resistance of a fluid is proportional to the shear rate, the torque is transmitted from the input shaft to the output shaft dependent on the difference between the rotational speed W1 of the input shaft and the rotational speed W2 of the output shaft. The relationship between the speed difference (W1 - W2) of the input and output shafts and the transmitted torque T is generally illustrated in FIG.

30.

FIG. 31 shows, by way of example, the power train of a full-time fourwheel-drive vehicle employing such a viscous shear coupling.

Since the differential-speed vs. torque characteristic curve illustrated in FIG. 30 is determined by the number and diameter of the outer and inner plates, the viscosity of the fluid, and other factors, the characteristic curve which has once been determined cannot easily be varied. The fou r-wheel-d rive vehicle shown in FIG. 31 has different optimum differential-speed vs. torque characteristics according to different road conditions such as dry, wet, snowy, and rough road 25 conditions. However, the actual differential-speed vs. torque characteristic is of a compromising nature as it cannot easily be varied according to the various road conditions.

In two-wheel-drive vehicles, the rear wheels must not be locked for braking stability when the vehicle is braked. This is because if the rear wheels were locked when braked, the lateral gripping forces of the rear wheels would suddenly be reduced, and the vehicle would be understeered.

Therefore, ordinary vehicles are required to apply greater braking forces to the front wheels than braking forces applied to the rear wheels. The braking forces to be imposed on the front wheels should be considerably great especially in a front-wheel-drive vehicle in which the load on the front wheels is larger than the load on the rear wheels. With such a braking force distribu- 35 tion, the rear wheels are prevented from being locked when the front wheels are locked, and the vehicle remains stable when braked, FIG. 32 shows a vehicle in which the load on the front wheels is larger than the load on the rear wheels. Assuming that the loads on the front and rear wheels are indicated by Wf, Wr, respectively, and the coefficient of friction of the road surface is represented by ji, the maximum 40 braking forces Ff, Fr that can be generated on the front and rear wheels, respectively, are expressed by Ff uWf, Fr = uWr. Therefore, the ratio of front and rear braking forces (Bf/Br) for locking the front and rear wheels simultaneously is given by:

M Ff W Br Wr Therefore, in order to lock the front wheels earlier than the rear wheels, the following relationship must be met:

Bf, Wf Br Wr FIG. 33 shows a four-wheel-drive wheel in which front and rear wheels are directly coupled.

Since the front and rear wheels rotate at the same speed, the slip ratio (S) of each wheel is 55 expressed by:

2 GB2194928A 2 S = V - 27RN V where V: the road surface speed, R: the radius of the tire N: the rotational speed of the tire, and the slip ratios of the front and rear wheels are equal to each other insofar as the radii of the tires of the front and rear wheels are the same.

Since the braking force B is expressed by B cc S x W where W is the load on the tire, the four-wheel-drive vehicle with the front and rear wheels directed coupled to each other does not 15 meet the relationship:

Bf, Wf E _r Wr but meets the equation:

g = Wf Br Wr at all times.

This means that the rear wheels are locked at the same time that the front wheels are locked, making the vehicle unstable, and a desired level of braking stability is not achieved.

FIG. 34 shows a general two-wheel-drive vehicle (i.e., rear-wheel-drive vehicle). While the vehicle is making a turn at a low speed, the rotational speeds (N1 through N4) of the respective wheels, and the distances (131 through RA) between the wheels and the center of the turning circle are related as follows:

11 Rl + R2 > R3 + R4 2 2 N1 + N2 > N3 + N4 2 Thus, during low-speed travel of the vehicle, the front wheels follow turning circles greater than those followed by the rear wheels, and the rotational speeds of the front wheels are higher than 40 those of the rear wheels.

In the four-wheel-drive vehicle with the front and rear wheels being directly coupled to each other as shown in FIG V. 35, however, the rotational speeds of the front and rear wheels are forcibly equalised to each other as indicated by:

N1 + N2 N3 + N4 2 2 Therefore, upon low-speed cornering, the rear wheels are subjected to driving forces D whereas the front wheels undergo braking forces D as shown.

Consequently, in order for the vehicle to make a turn, it is necessary to apply driving forces to overcome the braking forces on the front wheels in addition to the driving forces required to move the vehicle along the turn.

The additional driving forces cause a power loss, and if the power generated by the vehicle engine is smaller than the power loss, the vehicle may not make the turn dependent on the speed of the vehicle. This phenomenon is called---tightcorner braking-, which has been a significant shortcoming of the four-wheel-drive vehicle with the front and rear wheels directly coupled to each other.

It is a major object of the present invention to provide a power transmitting apparatus for a four-wheel-drive vehicle, which is capable of easily and widely varying the differentia 1-speed vs. 60 torque characteristic of the vehicle by utilizing the dynamic braking of an electric motor, and which preferably prevents unnecessarily excessive torque and current from being generated by limiting the maximum torque and current of the motor, so that the capacity of the motor is reduced to make the otor liaht eiaht and comoact.

According to the present invention, a power transmitting apparatus includes a main transmitt- 65 f,Z 3 GB2194928A 3 ing mechanism for transmitting torque of a power unit to main drive wheels, an auxiliary transmitting mechanism having a power transmitting shaft for transmitting torque of the power unit to auxiliary drive wheela, an electric motor disposed in the auxiliary transmitting mechanism and having a rotor and a stator, one of the rotor and the stator having output terminals, and a 5 resistor connected to the output terminals.

In a preferred embodiment there is provided a power transmitting apparatus for a four-wheeldrive vehicle, which releases front and rear wheels from a rotational restraint or interlocked rotation when the vehicle is braked, for thereby preventing the rear wheels from being locked so that braking forces can well be distributed to the wheels.

In a further preferred embodiment there is provided a power transmitting apparatus for a four- 10 wheel-drive vehicle, which releases front and rear wheels from a rotational restraint or interlock rotation while the vehicle is making a turn at a low speed, for thereby eliminating the phenomenon of tight corner braking to allow the vehicle to make the turn smoothly.

Certain embodiments of the invention will now be described, by way of example only, with reference to the drawings, wherein:

FIG. 1 is a schematic view of a power transmitting apparatus according to a first embodiment of the present invention; FIG. 2 is a diagram of differential speed vs. torque characteristic curves achieved by the power transmitting apparatus shown in FIG. 1; FIG. 3 is a schematic view of a power transmitting apparatus according to a second embodi- 20 ment of the present invention; FIG. 4 is a diagram of speed vs. torque characteristic curves achieved by the power transmitt ing apparatus shown in FIG. 3; FIG. 5 is a schematic view of a power transmitting apparatus according to a third embodiment of the present invention; FIG. 6 is a diagram of speed vs. torque characteristic curves achieved by the power transmitting apparatus shown in FIG. 5; FIG. 7 is a schematic view of a power transmitting apparatus according to a fourth embodiment of the present invention; FIG. 8 is a diagram of speed vs. torque characteristic curves achieved by the power transmitt- 30 ing apparatus shown in FIG. 7; FIG. 9 is a schematic view of a four-wheel-drive vehicle incorporating a power train according to a fifth embodiment of the present invention; FIG. 10 is a diagram of differential-speed vs. torque characteristic curves achieved by the power train shown in FIG. 9; FIG. 11 is a schematic view of a four-wheel-drive vehicle incorporating a power train according to a sixth embodiment of the present invention; FIG. 12 is a schematic view of a four-wheel-drive vehicle incorporating a power train according to a seventh embodiment of the present invention; FIG. 13 is a schematic view of a four-wheel-drive vehicle employing a power train according to 40 an eighth embodiment of the present invention; FIG. 14 i's a diagram of differential-speed vs. torque characteristic curves achieved by the power train shown in FIG. 13; FIG. 15 is a schematic view of a four-wheel-drive vehicle employing a power train according to a ninth embodiment of the present invention; FIG. 16 is a schematic view, partly in block form, of a control device for the power train illustrated in FIG. 15.; FIGS 17 and 18 are diagrams of differential-speed vs torque characteristic curves and differnetial-speed vs power characteristic curves obtained by a control mode in which the output current from the motor is kppt at a constant level when the rotational speed of the motor is higher than 50 a preset level; FIGS. 19 and 20 are diagrams of differential-speed vs. torque characteristic curves and differnetial-speed vs. power characteristic curves obtained by a control mode in which the output current from the motor is proportional to the reciprocal of the rotational speed of the motor; FIG. 21 is a schematic view, partly in block form, of a control device for a power train according to a tenth embodiment of the present invention; FIG. 22 is a schematic view of a four-wheel-drive vehicle employing a power train according to an eleventh embodiment of the present invention; FIG. 23 is a schematic view, partly in block form, of a control device for the power train illustrated in FIG. 22; FIG. 24 is a schematic view of a four-wheel-drive vehicle employing a power train according to a twelfth embodiment of the present invention; FIG. 25 is a schematic view of a four-wheel-drive vehicle employing a power train according to a thirteenth embodiment of the present invention; FIG. 26 is a schematic view of a four-wheel-drive vehicle employing a power train according to 65 4 GB2194928A 4 a fourteenth embodiment of the present invention; FIG. 27 is a schematic view of a four-wheel-drive vehicle employing a power train according to a fifteenth embodiment of the present invention; FIG. 28 is a schematic view of a four-wheel-drive vehicle employing a power train according to 5 a sixteenth embodiment of the present invention; FIG. 29 is a schematic view of a power transmitting apparatus employing a viscous shear coupling; FIG. 30 is a diagram of a differential-speed vs. torque characteristic curve achieved by the power transmitting apparatus shown in FIG. 29; FIG. 31 is a schematic view of a four-wheel-drive vehicle having a power train incorporating a 10 viscous shear coupling; FIG. 32 is a side elevational view of a vehicle, showing a disgribution of braking forces; FIG. 33 is a schematic view showing the power train of a four-wheel-drive vehicle having front and rear wheels directly coupled to each other; FIG. 34 is a view showing the manner in which a two-wheel-drive vehicle makes a turn; and FIG. 35 is a view showing the manner in which a two-wheel-drive vehicle with directly coupled front and rear wheels makes a turn.

Like or corresponding parts are denoted by like or corresponding reference numerals through out several figures.

Prior to describing embodiments of four-wheeldrive vehicles, fundamental concepts or prin- 20 ciples of the present invention will first be described with reference to FIGS. 1 through 8 for an easier understanding of-the present invention.

FIG. 1 schematically shows a power transmitting apparatus according to a first embodiment of the present invention. An electric motor 1 has a rotor 2 and a stator 3 serving as a housing accommodating the rotor 2 therein and coupled to a power source. The rotor 2 has output terminals 4 electrically connected to a resistor 5.

The stator 3 serves as an input shaft and the rotor 2 as an output shaft. In operation, the electric motor 1 effects dynamic braking or regenerative braking dependent on the load resistance R of the resistor 5. The output torque T of the motor 1 is produced in proportion to the differential speed (W1 - W2) between the input and output shafts and also to the reciprocal of 30 the load resistance R.

As the load resistance R varies from 0 to infinity, the output torque T of the motor 1 varies from the maximum torque level to substantially 0.

FIG. 3 shows a power transmitting apparatus according to a second embodiment of the present invention. The power transmitting apparatus of the second embodiment includes a differential 11 having a side gear 12 coupled to a power source and an opposite side gear 13 coupled to the rotor 2 of an electric motor 1. The stator 3 of the motor 1 is fixed to a vehicle body B and electrically connected to a resistor 5. The differential 11 also has a ring gear 15 supporting pinions 14 meshing with the side gears 12, 13, the ring gear 15 meshing with a gear 16. The ring gear 15 and the gear 16 have respective numbers N1, N2 of gear teeth.

When the sided gear 13 is braked, there is a difference developed between the rotational speeds of the side gears 12, 13, causing the ring gear 15 and hence the gear 16 to rotate through the pinions 14. The output torque (T2 = 2.(N2/Nl)-TM) of the gear 16 is produced in proportion to the rotational speed (WM = 2.(N2/Nl).W2-Wl) of the side gear 13 and the load resistance R, as shown in FIG. 4.

Therefore, the stator 3 is fixed, and the output torque can be controlled by the motor torque which is smaller than the torque of the power source.

FIG. 5 shows a third embodiment of the present invention. A differential 21 has a ring gear 22 supporting a pair of pinions 23 meshing with a pair of opposite side gears 24, 25, respectively. The side gear 24 is connected to the rotor 2 of an electric motor 1, the stator 3 50 of which is fixed to a vehicle body B and electrically connected to a resistor 5. The ring gear 22 is held in mesh with a gear 20 coupled to a power source.

Upon braking of the side gear 24 coupled to the motor 1, a difference is developed between the rotational speeds of the side gears 24, 25, causing the pinions 23 to rotate the side gear 25. The output torque (T2 = - TIVI) of the side gear 25 is generated in proportion to the rotational speed (WIVI = 2.(N2/Nl).Wl + W2) of the side gear 24 and the loa resistance R, as illustrated in FIG. 6.

According to a fourth embodiment shown in FIG. 7, a differential 31 has a pair of opposite side gears 32, 33, the side gear 32 coupled to a power source. A ring gear 35 supports pinions 34 meshing with and between the side gears 32, 33. The ring gear 35 is held in mesh 60 with a gear 36 connected to the rotor, 2 of an electric motor 1. The stator 3 of the motor 1 is fixed to a vehicle body B and electrically connected to a resistor 5.

When braking the gear 36 coupled to the motor 1, a difference is developed between the rotational speeds of the side gears 32, 33, causing the pinions 34 to transmit rotative power to the side gear 33. The output torque (T2 = (1/2).(N2/Nl)-TM) of the side gear 33 is generated 65 c 1 GB2194928A 5 in proportion to the rotational speed (WM = (W1 + W2)/2-Nl/N2) of the side gear 36 and the load resistance R, as shown in FIG. S.

While the differential is shown as comprising bevel gears in each of FIGS. 3, 5, 7, it may comprise a planetary gear mechanism.

Various embodiments in which the principles of the present invention are incorporated in fourwheel-drive vehicles will be described hereinbelow.

FIG. 9 shows a fifth embodiment in which the first embodiment of FIG. 1 is combined with a four-wheel-d rive vehicle. The four-wheel-drive vehicle has a power unit 41 comprising an engine and a transmission, a- propeller shaft 42 coupled to the power unit 41, a front differential 43 coupled to the power unit 41, two front wheels or main drive wheels 44 coupled to the front 10 differential 43 by drive axles 45, a rear differential 46, and two rear wheels or auxiliary drive wheels 47 coupled to the rear differential 46 by drive axles 48. The propeller shaft 42 is divided into an input shaft member and an output shaft member.. The input shaft member is connected to the stator 3 of a motor 1, and the output shaft member is connected to the rotor 2 thereof with its output terminals 4 electrically coupled to a variable resistor 6. 1 The drive torque T for driving the rear wheels 47 is as shown in FIG. 10. The torque vs.

differential-speed characteristic can easily be varied by adjusting the load resistance R of the variable resistor 6.

FIG. 11 shows a sixth embodiment in which the fourth embodiment of FIG. 7 is combined with a four-wheeldrive vehicle. The torque of a power unit 41 is transmitted through gears 51, 20 52 to a propeller shaft 42, which is divided between input and output shaft members with a central differential 31 coupled therebetween. A side gear 32 of the central differential 31 is connected to the input- shaft member, and a side gear 33 thereof is connected to the output shaft member. A gear 36 meshing with a ring gear 35 supporting pinions 34 is connected to the rotor 2 of a motor 1. The stator 3 of the motor 1 is fixed to a vehicle body and electrically connected to a variable resistor 6.

FIG. 12 shows: a seventh embodiment in which no rear differential is employed. Electric motors 1 are coupled to drive axles 48, respectively, connected to rear or auxiliary drive wheels 47. More specifically, the motors 1 have rotors 2 connected to the drive axles 2, respectively, and stators 3 joined to each other. A gear 53 is fixed to and extends around the joined stators 30 3 and is held in mesh with a gear 54 lying perpendicularly to the gear 53 and connected to the rear end of a propeller shaft 42 that is operatively connected to a power unit 41 through meshing gears 51, 52.

In the embodiment shown in FIG. 12, no rear differential is employed, and a differential locking mechanism is provided.

According to an eighth embodiment shown in FIG. 13, two front wheels 44 and two rear wheels 47 have tires of radii R1, R2, R3, R4, respectively, and front and rear differentials 43, 46 have respective gear ratios N1, N2, these tire radii R1, R2, R3, R4 and the gear ratios N1, N2 - being of the relationship: (131 + 132) x 1/N1 < (R3 + 114) x 1/N2. With this relationship, a differential speed (W1 - W2) is developed by the motor 1 even when the vehicle runs straight 40 under no load.

As shown in FIG. 14, the arrangementof FIG. 13 produces an offset of torque vs. differential speed characteristic curves from the origin of the differential speed axis, so that even when there is no differential speed between the -front and rear wheels, a drive force can be developed on the rear wheels 47 by adjusting the load resistance R of the motor 1.

The above scheme can be applied to the other embodiments, and can also be carried out by employing the seocnd and third embodiments. A variable resistor may be used in each of the seventh and eighth embodiments.

FIG. 15 shows a power train in a fou r-wheel-d rive vehicle according to a ninth embodiment of the present invention. The fou r-wheel-d rive vehicle has a power unit 101 comprising an,engine. 50 and a transmission, a propeller shaft 102 coupled to the power unit 101, a front differential 103 coupled to the power unit 101, two front wheels or main drive wheels 104 coupled to the front differential 103 by drive axles 105, a.rear differential 106, and two rear wheels or auxiliary drive wheels 107 coupled to the rear differential 106 by drive axles 108. The propeller shaft 102 is divided into an input shaft member and an output shaft member between which an electric motor 111 is interconnected. As illustrated in FIGS. 15 and 16, the motor 111 has a rotor 112 and a stator 113 serving as a housing in which the rotor 112 is disposed. The rotor 112 has output terminals 114 electrically connected to a resistor 115 (FIG. 16). The stator 113 is connected to the input shaft member of the propeller shaft 102 which is connected to the power unit 101, the output shaft member of the propeller shaft 102 being connected to the 60 rotor 112 and the rear differential 106. The stator 113 serves as the input shaft of the motor 111, and the rotor 112 as the output shaft thereof. The motor 111 effects dynamic braking and produces output torque T generally in proportion to the differential speed (W1 - W2) between the input and output shafts of the motor 111.

The motor 111 is controlled by a control unit 121 comprising, as shown in FIG. 16, a 65 6 GB2194928A microcomputer 122, a digital-to-analog converter 126, a constant-current regulated circuit 127 including an amplifier 128, a fieldeffect transistor 129, and the resistor 115, and a differential amplifier 116 for amplifying a voltage v 1 between the output terminals of the motor 111. The microcomputer 122 has a ROM 123, a CPU 124, an input/ouput circuit 125, and other ele ments. The terminal voltage v1 is applied from the differential amplifier 116 through the inpu t/output circuit 125 to the microcomputer 122.

The terminal voltage A of the motor 111 is substantially proportional to the differential speed (W 1 - W2) between the input and output shafts of the motor 111.

The constant-current regulated circuit 127 is responsive to a control voltage v2 applied from the microcomputer 122. through the digital-lo-analog converter 126 fQr controlling the Qutput current i of the motor 111. Control voltages v2 are stored as a map in the ROM 123. A control voltage v2 is accessed by a signal corresponding to the terminal voltage v1 and output to the constant-current regulated circuit 127. The control voltage v2 is established such that the constant-current regulated circuit 127 keeps the output current i at i = v1/R until the terminal voltage v1 reaches a voltage vO corresponding to the differential speed WO at the time the output torque T of the motor 111 becomes undesirably excessive, and the constant-current regulated circuit 127 limits the output current i to i = vO/R when the terminal voltage v1 is higher than the voltage vO.

Since the output torque T of the motor 111 is in proportion to the output current i and the control voltage v2, an output-torque vs. differential-speed characteristic curve as shown in FIG. 20 17 is obtained. The power P generated by the motor 111 is given by P = T x (W1 W2) and has a characteristic as shown in FIG. 18.

When v1 < vO, the output torque T is proportional to the terminal voltage v1, and the motor power P is proportional to the square of the terminal voltage v1. When v1 -: vO, the output torque T is maintained at a constant level, and the power P is proportional to the terminal 25 voltage v1. Denoted at k1, kO in FIGS. 17 and 18 are constants.

According to the above control, when v1 i-!: vO, the output torque T is constant so that limitations can be imposed as to mechanical strength, but the motor 111 may be overheated since the power P is increased in proportion to the differential speed. Where no torque is required at differential speeds higher than vO, the current i may be controlled in proportion to 30 the reciprocal of the rotational speed in a speed range higher than vO, as shown in FIG. 19, so that the power P can be kept at a constant level as shown in FIG. 20.

FIG. 21 shows a tenth embodiment which is acutally directed to a modification of the control unit 121 shown in FIG. 16. In the tenth embodiment, no differential amplifier is employed, but the rotational speeds of the input and output shafts of the motor 111 are detected by speed detectors 117, 118, respectively, for detecting a differential speed between the input and output shafts. The manner in which the motor 111 is controlled is the same as described above with reference to the ninth embodiment.

The specific structures of the power train of the four-wheel-drive vehicle and the input and output shafts of the motor are not limited to the illustrated embodiments.

As described above, the control unit is provided for limiting the output current of the motor below a preset level when the rotational speed is higher than a preset speed. Therefore, unnecessarily excessive torque is prevented from being generated, and the motor may be small in capacity and lightweight and compact.

FIG. 22 shows the power train of a four-wheel-drive vehicle according to an eleventh embodi ment of the present invention. The four-wheel-drive vehicle has a power unit 201 comprising an engine and a transmission, a propeller shaft 202 coupled to the power unit 201, a front differential 203 coupled to the power unit 201, two front wheels or main drive wheels 204 coupled to the front -differential 203 by drive axles 205, a rear differential 206, and two rear wheels or auxiliary drive wheels 207 coupled to the rear differential 206 by drive axles 208. The 50 propeller shaft 202 is divided into an input shaft member and an output shaft member between which an electric motor 211 is interconnected. The motor 211 has a rotor 212 and a stator 213 serving as a housing in which the rotor 212 is disposed. The rotor 212 has output terminals 214 electrically connected to a resistor 215. The stator 213 is connected to the input shaft member of the propeller shaft 202 which is connected to the power unit 201, the output 55 shaft member of the propeller shaft 202 being connected to the rotor 212 and the rear differential 206. The stator 213 serves as the input shaft of the motor 211, and the rotor 212 as. the output shaft thereof. The motor 211 effects dynamic braking dependent on the load resistance R of the resistor 215 and produces output torque T generally in proportion to the differential speed (W1 - W2) between the input and output shafts of the motor 211.

A brake pedal switch 216 is connected in series with the resistor 215, the brake pedal switch 216 and the resistor 216 jointly constituting a control unit 221. The brake pedal switch 216 has a movable contact 216a operatively coupled to a brake pedal 217. The brake pedal switch 2.16 is normally turned on or closed, but when the brake pedal 217 is depressed by the driver, the movable contact 216a is moved out of contact with a fixed contact 216b to turn off the brake 65 1Q is V 11 7 GB2194928A 7 pedal switch 216.

While the vehicle is running under normal conditions, the brake pedal switch 216 remains turned on to allow the motor 211 to perform dynamic braking. When the driver depresses the brake pedal 217 to brake the vehicle, the brake pedal switch 216 is turned off to eliminate the output current i of the motor 211, whereupon the motor 211 stops its dynamic braking. Therefore, the front wheels 204 and the rear wheels 207 are released from a rotational -restraint or interlocked rotation, and therear wheels 207 are free to rotate. Therefore, an optimum distribution of braking forces to the front and rear wheels can be achieved on the basis of the relationship:

Elf > Wf Br wr The control unit 221 which incorporates a microcomputer will be described with reference to FIG. 23. The control unit 221 comprises a microcomputer 222, a digital-to- analog converter 15 226, a constant-current regulated circuit 227 including an amplifier 228, a field-effect transistor

229, and the resistor 215, a differential amplifier 218 for amplifying a voltage A between the output terminals 214 of the motor 211, and the brake switch 2 1 S. The microcomputer 222 has a ROM 223, a CPU 224, an input/output circuit 225, and other elements. The terminal voltage v1 is applied from the differential amplifier 218 through the input/output circuit 225 to the microcomputer 222. The brake switch 219 is operated by detecting the pressure of a braking fluid. The brake switch 219 normally remains turned on, and is turned off when the brake is applied. A signal indicative of the ON/OFF condition of the brake switch 219 is also applied to the microcomputer 222.

The constant-current regulated circuit 227 controls the output current i of the motor 211 25 flowing through the resistor 215 in response to a control voltage v2 output from the microcom puter 222 through the digital-to-analog converter 226. The control voltage v2 is accessed by a signal corresponding to the terminal voltage v1 and applied to the constant-current regulated circuit 227. The control voltage v2 is selected such that when the brake switch 219 is turned on, the output current i is kept at i = vi/R, and when the brake switch 219 is turned off, the output current i is limited to a zero or a nearly zero level.

The brake-responsive switch may be replaced with an acceleration detector for detecting the acceleration G of the vehicle in its fore-and-aft direction, so that the motor current can be controlled according to the vehicle acceleration G.

As described above, the control unit is provided for limiting the output current of the motor to 35 a substantially zero level when the vehicle is braked. Thus, when the vehicle is braked, the front and rear wheels are released from a rotational restraint or interlocked rotation to apply an optimum distribution of braking forces to the front and rear wheels.

FIG. 24 shows the power train of a four-wheel-drive vehicle according to a twelfth embodi ment of the present invention. The four-wheel-drive vehicle has a power unit 301 comprising an 40 engine and a transmission, a propeller shaft 302 coupled to the power unit 301, a front differential 303 coupled to the power unit 301, two front wheels or main drive wheels 304 coupled to the front differential 303 by drive axles 305, a rear differential 306, and two rear wheels or auxiliary drive wheels 307 coupled to the rear differential 306 by drive axles 308. The propeller shaft 302 is divided into an input shaft member and an output shaft member between 45 which an electric motor 311 is interconnected. The motor 311 has a rotor 312 and a stator 313 serving as a housing in which the rotor 312 is disposed. The rotor 312 has output terminals 314 electrically connected to a resistor 315. The stator 313 is connected to the input shaft member of the propeller shaft 302 which is connected to the power unit 301, the output shaft member of the propeller shaft 302 being connected to the rotor 312 and the rear differential 306. The stator 313 serves as the input shaft of the motor 311, and the rotor 312 as the output shaft thereof. The motor 311 effects dynamic braking dependent on the load resistance R of the resistor 315 and produces output torque T generally in proportion to the differential speed (W 1 - W2) between the input and output shafts of the motor 311.

A diode 316 is connected in series with the resistor 315, the diode 316 and the resistor 316 55 jointly constituting a control unit 321. The diode 316 serves to cut off a reverse current generated in the motor 311 when the rotational speed of the rear wheels 307 exceeds the rotational speed of the front wheels 304.

Braking forces which will be applied to the front wheels 304 are selected to be greater than those which will be imposed on the rear wheels 307.

While the vehicle is running normally, the diode 316 allows a current i to flow in a forward direction, and hence the motor 311 effects dynamic braking. When the vehicle is braked, the rotational speed of the rear wheels 307 becomes higher than that of the front wheels 304 because of the preselected braking force distribution. At the time the front wheels 304 are locked, a reverse current generated by the motor 311 is cut off by the diode 316, and no 8 GB2194928A 8 output current is produced by the motor 311. Thus, the motor 311 does not effect dynamic braking, releasing the front and rear wheels 304, 307 from a rotational restraint or interlocked rotation. Since the rear wheels 307 are free to rotate, they are prevented from being locked because of an optimum distribution of braking forces based on the relationship:

Bf 5 Wf ii-r wr The above control mode can be carried out by using a microcomputer. In such microcomputer based control, the control voltage v2 in the circuit shown in FIG. 16 is selected in advance such 10 that when the terminal voltage A of the motor is positive, the output current i is ket at i = v1/R, and when the terminal voltage v1 is negative, the output current i is limited to a zero or a nearly zero level.

Alternatively, the differential amplifier 116 shown in FIG. 16 may be replaced with the speed detectors 117,118 (FIG. 21) for detecting the rotational speeds of the input and output shafts 15 of the motor 111 to determine a diferential speed for effecting the above control mode.

With the control arrangement of FIG. 24, no driving forces will be applied to the rear wheels 307 when the vehicle is moved backwards. One solution to this problem is provided by a thirteenth embodiment illustrated in FIG. 25 in which a changeover switch 331 is connected to the diode 316 upstream thereof and has a movable contact 331a that can be connected to a 20 fixed contact 331b coupled to the diode 316 (as indicated by the solid line) when the vehicle is moved forward and can be connected to a bypass 332 parallel to the diode 316 (as indicated by the broken line) when the vehicle is moved backwards. When the vehicle is moved forward, therefore, a forward current if flows through the diode 316, and when the vehicle is moved backwards, a reverse current ir flows in bypassing relation to the diode 316. Consequently, 25 driving forces can be applied to the rear wheels 307 even when the vehicle is moved backwards.

With the above arrangement, since the control unit is provided for limiting the output current of the motor to a substantially zero level when the rotational speed of the rear wheels exceeds that of the front wheels, the front and rear wheels are released from a rotational restraint or 30 interlocked rotation thereby to prevent the rear wheels from being locked at the time the vehicle is braked. Accordingly, braking foreces are applied to the front and rear wheels in an optimum distribution.

FIG. 26 shows the power train of a four-wheel-drive vehicle according to a fourteenth embodi- ment of the present invention. The four-wheel-drive vehicle has a power unit 401 comprising an 35 engine and a transmission, a propeller shaft 402 coupled to the power unit 401, a rear differential 403 coupled to the power unit 401, two rear wheels or main drive wheels 404 coupled to the rear differential 403 by drive axles 405, a front differential 406, and two front wheels or auxiliary drive wheels 407 coupled to the front differential 406 by drive axles 408, The propeller shaft 402 is divided into an input shaft member and an output shaft member 40 between which an electric motor 411 is interconnected. The motor 411 has a rotor 412 and a stator 413 serving as a housing in which the rotor 412 is disposed. The rotor 412 has output terminals 414 electrically connected to a resistor 415, The stator 413 is connected to the input shaft member of the propeller shaft 402 which is connected to the power unit 401, the output shaft member of the propeller shaft 402 being connected to the rotor 412 and the front differential 406. The--stator 413 serves as the input shaft of the motor 411, and the rotor 412 as the output shaft thereof. The motor 411 effects dynamic braking dependent on the load resistance R of the resistor 415 and produces output torque T generally in proportion to the differential speed (W1 - W2) between the input and output shafts of the motor 411.

A diode 416 is connected in series with the resistor 415, the diode 416 and the resistor 416 50 jointly constituting a control unit 421. The diode 416 serves to cut off a reverse current generated in the motor 411 when the rotational speed of the front wheels 407 exceeds the rotational speed of the rear wheels 404 and the front wheels 407 develop braking forces.

While the vehicle is running normally, the diode 416 allows a current i to flow in a forward direction, and hence the motor 411 effects dynamic braking. When the vehicle makes a turn at 55 a low speed, the front wheels 407 rotate faster than the rear wheels and the front wheels 407 genrate braking forces. A reverse current generated by the motor 411 at this time is cut off by the diode 416, and no output current is produced by the motor 411. Thus, the motor 411 does not effect dynamic braking, releasing the front and rear wheels 404, 407 from a rotational restraint or interlocked rotation, Since the rear wheels 407 are free to rotate, the phenomenon 60 of tight corner braking is eliminated and the vehicle is allowed to make a smooth turn.

The above control mode can be carried out by using a microcomputer. In such microcomputer based control, the control voltage Q in the circuit shown in FIG. 16 is selected in advance such that when the terminal voltage v1 of the motor is positive, the output current 1 is ket at I = v1/R, and when the terminal voltage v1 is negative, the output current i is limited to a zero or a 65 1 1 Z 1 1 1 i 1 9 GB2194928A 9 nearly zero level.

As an alternative, the differential amplifier 116 shown in FIG. 16 may be replaced with the speed detectors 117, 118 (FIG. 21) for detecting the rotational speeds of the input and output shafts of the motor 111 to determine a diferential speed for effecting the above control mode.

Where the control arrangement of FIG. 26 is relied upon, no driving forces will be applied to 5 the rear wheels 407 when the vehicle is moved backwards. According to fifteenth embodiment shown in FIG. 27, a changeover switch 431 is connected to the diode 416 downstream thereof and has a movable contact 431a that can be connected to a fixed contact 431b coupled to the diode 416 (as indicated by the solid line) when the vehicle is moved forward and can be connected to a bypass 432 (as indicated by the broken line) parallel to the diode 416 when the 10 vehicle is moved backwards. When the vehicle is moved forward, therefore, a forward current if flows through the diode 416, and when the vehicle is moved backwards, a reverse current ir flows in bypassing relation to the diode 316. Consequently, driving forces can be applied to the rear wheels 407 even when the vehicle is moved backwards.

The arrangement of FIG. 27 causes tight corner braking when the vehicle makes a turn while 15 moving backwards. FIG. 28 shows a sixteenth embodiment in which another diode 433 is connected parallel to the diode 416 for cutting off a forward current if when the front wheels 407 produce braking forces at the time the vehicle is moved backwards. Upon reverse move ment of the vehicle, the movable contact 431a of the changeover switch 431 is connected to a fixed contact 431c of the diode 433 as indicated by the broken line. Since the current if 20 produced when the vehicle makes a turn on reverse movement while producing driving forces on the front wheels 407 is cut off the diode 433, the problem of tight corner braking can be eliminated during a turn on reverse movement.

In addition, tight corner braking can be eliminated while producing driving forces on the front wheels 404 in both forward and backward movements withtout switching between the forward and backward modes. More specifically, in the control unit 121 shown in FIG. 21, when the absolute value of the rotational speed W2 of the front wheels 407 (FIG. 28) as it is detected by the speed detector is greater than the absolute value of the rotational speed W1 of the rear wheels 404 (FIG. 28) as it is detected by the speed detector, the current i is cut off. The current is controlled according to the following control table in which the rotational speed is 30 associated with a positive sign (+) when the vehicle is moved forward and with a negative sign (-) when the vehicle is moved backwards:

1 Control table

Direction W2 Wl W2-Wl jW21-1W11 i Function Forward + High + Low + + 0 No tight corner brakinq + Low + High + 4W Reverse - High - Low + 0 No tight Low I- Highl + corner brakin2 45 I - I 4WD With such current control effected, the vehicle can be in the 4WD mode while running normally in the forward and reverse directions, and the problem of tight corner braking can be eliminated when making a turn at a low speed without switching between the forward and 50 backward modes.

By thus providing the control unit for limiting the output current of the motor to a substantially zero level when the front wheels produce braking forces, the front and rear wheels can be released from a rotation restraint or interlocked rotation when the vehicle makes a turn at a low speed, so that the vehicle can make a smooth turn while eliminating tight corner braking. 55 With the preferred embodiments, as described above, power is transmitted from a power unit to auxiliary drive wheels through an electric motor which allows differentia I-speed vs. torque characteristics to be easily varied in a wide range. Since unnecessarily excessive torque is prevented from being generated by the motor, the capacity of the motor is reduced and the motor is lightweight and compact.

When the vehicle is braked, the front and rear wheels are released from a rotational restraint or interlocked rotation to free the rear wheels from a locked condition. Therefore, braking forces can be applied to the front and rear wheels in an optimum distribution.

Further, the front and rear wheels are also released from interlocked rotation when the vehicle makes a turn at a low speed, with the result that the phenomenon of tight corner braking is 65 l'i GB2194928A 10 eliminated, thus allowing the vehicle to effect smooth cornering.

Although there have been described what are at present considered to be the preferred embodiments of the present invention, it will be understood that the invention may be embodied in other specific forms without departing from the,,.P,, or essential characteristics thereof. The present embodiments are therefore to be considered in all aspects as illustrative, and not restrictive.

Claims

1. A power transmitting apparatus in a four-wheel-drive vehicle having a vehicle body, a power unit, main drive wheels, auxiliary drive wheels, and braking means, said power transmitt- 10 ing apparatus comprising:

a main transmitting mechanism for transmitting torque of said power unit to said main drive wheels; an auxiliary transmitting mechanism having a power transmitting shaft for transmitting torque of said power unit to said auxiliary drive wheels; 1 an electric motor disposed in said auxiliary transmitting mechanism and having a rotor and a stator, one of said rotor and the stator having output terminals; and a resistor connected to said output terminals.

2. A power transmitting apparatus according to claim 1, wherein said auxiliary transmitting shaft is divided into shaft members, one of said rotor and said stator being coupled to an end 20 of one of said shaft members, and the other of said rotor and said stator being coupled to an end of the other shaft member.

3. A power transmitting apparatus according to claim 1, wherein said auxiliary transmitting mechanism comprises a differential gear mechanism disposed in said power transmitting shaft and having a ring gear and a pair of side gears, said rotor being rotatable in response to 25 rotation of said ring gear, said stator being fixed to said vehicle body.

4. A power transmitting apparatus according to claim 1, wherein said auxiliary transmitting mechanism comprises a differential gear mechanism disposed in said power transmitting shaft and having a ring gear and a pair of side gears, said rotor being coupled to one of said side gears, said stator being fixed to said vehicle body.

5. A power transmitting apparatus according to any preceding claim further including a control unit for limiting an output current of said motor below a prescribed level when the rotational speed of said motor exceeds a preset speed.

6. A power transmitting apparatus according to claim 5, wherein said control unit has means for keeping said output current at a constant level when said rotational speed of the motor 35 exceeds the preset speed.

7. A power transmitting apparatus according to claim 5, wherein said control unit has means for controlling said output current in proportion to the reciprocal of the rotational speed of the motor when said rotational speed of the motor exceeds the preset speed.

8. A power transmitting apparatus according to any preceding claim further including a detec- 40 tor for detecting actuation of said braking means, and a control unit for limiting an output current of said motor to a substantially zero level when the actuation of said braking unit is detected by said detector.

9. A power transmitting apparatus according to claim 8, wherein said detector comprises a brake pedal switch. 4

10. A power transmitting apparatus according to claim 8, wherein said detector comprises a detector for detecting a braking fluid.

11. A power transmitting apparatus according to claim 8, wherein said detector comprises a detector for detecting acceleration of said vehicle in a fore-and-aft direction thereof.

12. A power transmitting apparatus according to any preceding claim wherein said main drive 50 wheels comprise rear wheels and said auxiliary drive wheels comprise rear wheels, further including a control unit for limiting an output current of said motor to a substantially zero level when the rotational speed of said rear wheels exceeds the rotational speed of said front wheels.

13. A power transmitting apparatus according to claim 12, wherein said control unit includes means for limiting said output current dependent on the direction in which said output current 55 flows.

14. A power transmitting apparatus according to claim 12, wherein said control unit includes means for limiting said output current dependent on whether a voltage generated by said motor is positive or negative.

15. A power transmitting apparatus according to claim 12, further including a detector for detecting the rotational speeds of said front and rear wheels, said control unit including means for limiting said output current dependent on the difference between the rotational speeds of said front and rear wheels.

16. A power transmitting apparatus according to claim 15, wherein said control unit includes means for cancelling the limitation of or reversing said output current when the vehicle is moved 65 i 1 51 j 11 1GB2194928A backwards.

17. A power transmitting apparatus according to any of claims 1 to 11, wherein said main drive wheels ccmprise rear wheels and said auxiliary drive wheels comprise front wheels, further including a control unit for limiting an output current of said motor to a substantially zero level 5 when the rear wheels produce driving forces and the front wheels produce braking forces.

18. A power transmitting apparatus according to claim 17, wherein said control unit includes means for limiting said output current dependent on the direction in which said output current flows.

19. A power transmitting apparatus according to claim 17, wherein said control unit includes means for limiting said output current dependent on whether a voltage generated by said motor 10 is positive or negative.

20. A power transmitting apparatus according to claim 17, further including a detector for detecting the rotational speeds of said front and rear wheels, said control unit including means for limiting said output current dependent on the difference between the rotational speeds of said front and rear wheels.

21. A power transmitting apparatus according to claim 20, wherein said control unit includes means for cancelling the limitation of or reversing said output current when the vehicle is moved backwards.

22. A power transmitting apparatus substantially as herein described with reference to any of Figures 1 to 28 of the accompanying drawings.

Published 1988 at The Patent Office, State House, 66/71 High Holborn, London WC 1 R 4TP. Further copies may be obtained from The Patent Office, Sales Branch, St Mary Cray, Orpington, Kent BR5 3RD. Printed by Burgess & Son (Abingdon) Ltd. Con. 1/87.