System for controlling distribution of torque to left and right wheels of a motor vehicle

Abstract: A differential (11) is provided between a left (13L) and a right (13R) wheel of a motor vehicle for distributing an output torque of a transmission (3) to the left and right wheels (13). A fluid operated multiple-disk clutch (14) is provided for restricting the differential operation of the differential (11). A speed difference (Δω) between the left and right wheels (13) is calculated. The fluid operated multiple-disk clutch (14) is operated in accordance with the speed difference together with the vehicle speed (V) and its steering angle (ψ) so that a restricting torque (T D ) applied by the clutch increases with an increase of the speed difference (Δω).

Description: The present invention relates to a control system of a torque distribution to the left and right driving wheels of a motor vehicle, and more particularly to a control system in which output torque of a transmission is distributed to the left and right rear driving wheels in accordance with driving conditions of the vehicle. A differential of the motor vehicle is provided with a differential restricting device such as a fluid operated multiple-disk clutch which operates to lock the differential when one of the left and right wheels slips. Thus, it is possible to drive the vehicle without rendering the vehicle stuck. On the other hand, when the differential is locked, steerability of the vehicle decreases. In order to solve such a problem it has been proposed to change the clutch torque for restricting differential operation. However, the change in the clutch torque causes a change in distribution ratio of torques to the left and right wheels, thereby affecting running behavior of the vehicle. Namely, if the clutch torque is zero, the fluid operated clutch is free, so that in the case of a bevel gear differential, a torque TRL transmitted to the left wheel and a torque TRR transmitted to the right wheel are equal (TRL:TRR=50:50) irrespective of a friction coefficient µ against road surface. Hence, stable driving is enhanced while maintaining the steerability. On the other hand, as the clutch torque for restricting the differential operation increases, the torques are distributed at a ratio in accordance with a left dynamic weight WTL, a right dynamic weight WTR, a friction coefficient µL at the left wheel, and a friction coefficient µR at the right wheel as follows. TRL:TRR = WTL · µL:WTR · µR Accordingly, if the dynamic weight WTL and WTR are equal to each other, a larger torque is distributed to the wheel which runs on the surface having, a larger friction coefficient µ. As a result, a vehicle body swings laterally, causing unstable driving as well as a decrease in the steerability. Thus, it is necessary to control the torque distribution ratio when driven on a road having a small friction coefficient µ in dependency on steering angle and vehicle speed. Moreover, it is necessary to determine the differential operation restricting clutch torque in consideration of the steerability and the driving stability. Japanese Patent Application Laid-Open 61-132420 discloses a system for controlling the torque distribution where the differential operation restricting clutch torque is increased with increases of the vehicle speed and the steering angle. However, although the slipping of the wheels in a high vehicle speed range can be prevented by locking the differential, the torque distribution deviates to one of the left and right wheels in accordance with the friction coefficient µR. Thus the driving stability of the vehicle deteriorates when driven straight ahead and when cornering at high speed. In addition, at a large steering angle the differential restriction is slightly effected, thereby decreasing the steer ability. A viscous coupling may be provided for restricting the differential operation. However, since the viscous coupling has differential restricting characteristics which change with the speeds of the left and right wheels the desired restricting characteristic cannot be obtained. The reader will. be further enlightened a sto the state of the art by reference to the prior art Patent Application EP-A-0 288 665 which discloses the same elements as are mentioned in the precharacterising clause of claim 1 of this document and with reference to which claim 1 is characterised. The object of the present invention is to provide a system for controlling a torque distribution to the left and right wheels of a vehicle to ensure steer ability and driving stability in accordance with driving conditions. Accordingly the present invention provides a system in accordance with claim 1. A system for the distribution of torque to the left and right wheels of a motor vehicle which embodies the present invention, will now be described, by way of example only, with reference to the accompanying drawings, in which:- Figures 1a to 1c show a power transmission system for a four wheel drive motor vehicle; Fig. 2a is a graph showing characteristics of differential operation restricting clutch torque in accordance with a speed difference; Figs. 2b and 2c are graphs showing characteristics of differential operation restricting clutch torque corrected in accordance with steering angle and vehicle speed, respectively; and Fig. 3 is a flowchart showing an operation of a control unit in the system. Figs. 1a to 1c show a power transmission system for a four-wheel drive motor vehicle having a differential according to the present invention. An engine 1 is mounted on a front portion of the vehicle. A clutch 2 and a transmission 3 are disposed at a rear of the engine 1 in a longitudinal direction of the vehicle. An output of the transmission 3 is transmitted to an output shaft 4 which is aligned with a central differential 20. The output shaft 4 is connected to a front drive shaft 5 which is parallelly disposed under the transmission 3 through a pair of reduction gears 25 and 26 of the central differential 20. The front drive shaft 5 is connected to left and right front wheels 9L and 9R through a front differential 7 and axles 8. The output shaft 4 is connected to a rear drive shaft 6 through the central differential 20. The rear drive shaft 6 is connected to the left and right rear wheels 13L and 13R through a propeller shaft 10, a rear axle differential 11 and axles 12. The central differential 20 is a complex planetary gear device and comprises a first sun gear 21 integrally formed on the output shaft 4, a second sun gear 22 integrally formed on the rear drive shaft 6, and a combined planetary pinion 23 comprising a first planetary pinion 23a meshed with the first sun gear 21, and a second planetary pinion 23b meshed with the second sun gear 22, and supported on a carrier 24. The carrier 24 is connected to the reduction drive gear 25. Thus, an output torque from the output shaft 4 of the transmission 3 is transmitted to the carrier 24 and the second sun gear 22 through the first sun gear 21 and the pinions 23a, 23b at predetermined respective torque distribution ratios. A difference between rotating speeds of the carrier 24 and the second sun gear 22 is absorbed by rotation of the first and second planetary pinions 23a and 23b. In the central differential 20, the first sun gear 21 is engaged with the first planetary pinion 23a and the second sun gear 22 is engaged with the second planetary pinion 23b. Consequently, a standard torque distribution for front torque T F and rear torque T R can be set to various values by changing radii of pitch circles of the sun gears 21 and 22 and the pinions 23a and 23b. Thus, the torque distribution ratio of the front wheels 9L, 9R and the rear wheels 13L, 13R is determined, for example as T F : T R = 34 : 66 A large standard torque can be distributed to the rear wheels 13L, 13R. A fluid operated multiple-disk friction clutch 27 is provided adjacent the central differential 20 for restricting the differential operation of the central differential 20. The clutch 27 comprises a drive drum 27a secured to the carrier 24, and a driven drum 27b secured to the rear drive shaft 6. When a differential operation restricting clutch torque Tc is produced in the clutch 27, a part of the output torque of the second sun gear 22 is transmitted to the front wheels 9L, 9R, thereby changing the distribution of the torque. The carrier 24 is coupled with the second sun gear 22 when the clutch 27 is entirely engaged, thereby locking the central differential 20. In the vehicle with the front-mounted engine, static weight distribution of front dynamic weight WF and rear dynamic weight WR are determined for example as WF : WR ≒ 62 : 38 When the clutch is directly engaged, the distribution ratio of front torque and rear torque is set in accordance with the weight distribution. Thus, the torque distribution is controlled in a range between the standard torque distribution of 34:66 weighed to the rear wheels 13L, 13R and a torque distribution of 62:38, weighted to the front wheels 9L, 9R at an entire engagement of the clutch 27 in accordance with the differential operation restricting clutch torque Tc. The rear axle differential 11 comprises a bevel gear differential device mounted in a differential case 11a. A fluid operated multiple-disk friction clutch 14 is provided adjacent the rear axle differential 11 for restricting the differential operation of the differential 11. The clutch 14 comprises a drive drum 14a secured to the differential case 11a, and a driven drum 14b secured to one of the axles 12 connected to a side gear 11b of the differential 11. When the clutch 14 is released, the torques are equally distributed to the left and the right wheels 13L and 13R at the ratio of 50:50. When the clutch 14 is engaged, thereby generating a differential operation restricting clutch torque T D , the differential operation of the rear differential 11 is restricted. The torque distribution ratio of the left and right wheels are determined as TRL:TRR = WTL · µL:WTR · µR where WTL and WTR are left and right dynamic weights on the left and right wheels, respectively, and µL and µR are friction coefficients of the road surface under the respective wheels 9L, 9R, 13L and 13R. A hydraulic circuit of a control system for the clutch 14 comprises an oil pump 30, a pressure regulator valve 31, a pilot valve 35, a clutch control valve 33 and a solenoid operated duty control valve 38. The regulator valve 31 operates to regulate a pressure of oil supplied from the oil pump 30 driven by the engine 1 to produce a line pressure and the line pressure is applied to a line pressure conduit 32. The conduit 32 is communicated with a passage 37 through the pilot valve 35. The passage 37 is communicated with the solenoid operated duty control valve 38 at downstream of an orifice 36, and with an end of the clutch control valve 33 through a passage 39. The conduit 32 is communicated with the clutch control valve 33 through a passage 32a. The clutch control valve 33 is communicated with the clutch 14 through a passage 34. The solenoid operated valve 38 is operated by pulses from a control unit 50 at a duty ratio determined therein, thereby controlling to drain the oil to provide a control pressure. The control pressure is applied to the clutch control valve 33 to control the oil supplied to the clutch 14 so as to control the clutch pressure (torque) and hence the differential operation restricting clutch torque T D . The fluid operated multiple-disk clutch 27 of the central differential 20 is operated through a hydraulic circuit (not shown) similar to the rear axle differential 11 so as to control the differential operation restricting clutch torque Tc. The control unit 50 is supplied with output signals from a left rear-wheel speed sensor 40L, a right rear-wheel speed sensor 40R, a vehicle speed sensor 41 and a steering angle sensor 42. The control unit 50 has a speed difference calculator 51 to which a left rear-wheel speed ωL and a right rear-wheel speed ωR from the sensors 40L and 40R are applied. In the speed difference calculator 51, a difference Δω between the speed ωL and ωR is calculated as, Δω = |ωR - ωL| . The difference Δω is applied to a differential operation restricting clutch torque setting section 52. In accordance with the input signal, the clutch torque setting section 52 retrieves a clutch torque T D from a look-up table. The effective diameters of the left and right wheels 13L and 13R may not be quite the same, and the friction coefficient µL at the left wheel 13L may slightly differ from the friction coefficient µR at the right wheel 13R, even on a dry road surface. Thus, the speed difference Δω is apt to increase, particularly in a high vehicle speed range. Hence, the clutch torque T D stored in the look-up table is set as an exponential function of the speed difference Δω which varies like an exponential curve as shown in Fig. 2a. More particularly, when the speed difference Δω is small, the clutch torque T D is also small. As the speed difference Δω increases, the clutch torque T D largely increases so as to sufficiently restrict the differential operation. A vehicle speed V from the vehicle speed sensor 41 and a steering angle ψ from the steering sensor 42 are further fed to the differential operation restricting clutch torque setting section 52 to correct the clutch torque T D . As shown in Fig. 2b, the clutch torque T D decreases with an increase of the steering angle ψ at the same speed difference Δω, thereby increasing steerability. The clutch torque T D also decreases with an increase of the vehicle speed V at the same speed difference Δω as shown in Fig. 2c, thereby decreasing the difference between the torques distributed to the left and right wheels to ensure stable driving. The clutch torque T D is applied to a control quantity providing section 53 where a duty ratio D corresponding to the clutch torque T D is provided. The duty ratio D provided at the section 53 is applied to the solenoid operated duty control valve 38 through a driver 54. The operation of the system will be described hereinafter with reference to the flowchart of Fig. 3. The power of the engine 1 is transmitted to the transmission 3 through the clutch 2 at which the transmission ratio is controlled. The output of the transmission 3 is transmitted to the first sun gear 21 of the central differential 20. The front torque T F and the rear torque T R are determined in accordance with the radii of the gears of the central differential 20 and the clutch torque Tc of the clutch 27 to distribute a larger torque to the rear wheels 13L, 13R than to the front wheels 9L, 9R. The torque is transmitted to the front wheels 9L and 9R through the carrier 24, the reduction drive gear 25, the reduction driven gear 26, the front drive shaft 5 and the front differential 7. The torque is transmitted to the rear wheels 13L, 13R through the second sun gear 22, the rear drive shaft 6, the propeller shaft 10 and the rear differential 11. The left rear-wheel speed ωL and the right rear-wheel speed ωR are detected by the left rear-wheel speed sensor 40L and the right-wheel speed sensor 40R. The speed difference Δω is calculated in the speed difference calculator 51. When the calculated difference is small owing to the friction coefficient µ of the road and the difference between effective wheel diameters of the left and the right rear-wheels, a small clutch torque T D is determined as shown in Fig. 2a. A signal corresponding to a large duty ratio is applied to the solenoid operated duty control valve 38. Thus, the clutch control pressure becomes zero and the clutch control valve 33 operates to close the passage 32a, thereby draining the oil from the clutch 14. The clutch 14 is disengaged and the clutch torque becomes zero so as to render the rear differential 11 free. Accordingly, the torque is equally transmitted to the left rear-wheel 13L and the right rear-wheel 13R, thereby improving steerability and driving stability. If the vehicle is driven on a slippery road and one of the left and right rear-wheels 13L, 13R slips due to the difference between the friction coefficients µL and µR at the left and the right wheels, the speed difference Δω becomes large. Thus, a large clutch torque T D is set so that the small duty ratio D is applied to the solenoid operated valve 38. The clutch control pressure increases to operate the clutch valve 33, thereby opening the passage 32a. The clutch 14 is thus supplied with the oil so that the clutch torque increases so as to restrict the differential operation of the rear differential 11. Consequently, the slip is supressed, thereby preventing the vehicle from becoming stuck. Thus, running performance of the vehicle is improved. When the steering angle ψ increases at cornering, the clutch torque T D is decreased so that good steerability is ensured. In a high vehicle speed range, the clutch torque is similarly decreased with the increase of the vehicle speed V so that the torques are equally distributed to the left and right rear-wheels 13L, 13R, irrespective of the difference between the left and right friction coefficients µL and µR. Therefore, although the wheels may slightly slip, the swinging of the vehicle body is prevented, so that the vehicle is stably drived in the straight-ahead running. Although an embodiment of the present invention is described above, the present invention may be applied to a two-wheel drive vehicle and other four-wheel drive vehicles than the described four-wheel drive vehicle. In accordance with the present invention, the differential operation restricting clutch torque for controlling the torque distribution to the left and right rear-wheels is controlled in such characteristics as an exponential function with respect to the difference between the two wheels to effectively restrict the differential operation at the slipping of the wheels. The clutch torque is further determined as a decreasing function of the vehicle speed and the steering angle, thereby enhancing stability at high vehicle speed and steerability on slippery roads. While the presently preferred embodiment of the present invention has been shown and described, it is to be understood that this disclosure is for the purpose of illustration and that various changes and modifications may be made without departing from the scope of the invention as set forth in the appended claims.

Claims: A system for controlling the torque distribution to the left and right drive wheels (13) of a motor vehicle comprising: a control unit (50) for determining a restricting torque (TD) to be applied by a restricting device (14) to a differential (11) whereby torque is distributed to the right and left drive wheels (13), said control unit (50) being responsive to the speed of the vehicle and a steering angle of the vehicle to determine the restricting torque (TD) and including means (53 54) to control the restricting device (14) to the restricting torque (TD), calculator means (51) for calculating a difference in speed (Δω) between the left and right wheels and the restricting torque (TD) being determined according to the difference in wheel speed (Δω) in addition to the steering angle (ψ) and the vehicle speed (V) so that the restricting torque (TD) is an increasing function of the difference in wheel speed (Δω), characterised in that the restricting torque (TD) is a decreasing function of the steering angle and the vehicle speed (V). A system according to claim 1 wherein the restricting device (14) is a fluid operated device and a control means (53) responds to the restricting torque (TD) for operating the fluid operated restricting device (14). A system according to claim 2 wherein the restricting torque (TD) s set to change exponentially with respect to the speed difference. A system according to claim 3 wherein the fluid operated restricting device (14) is a fluid operated multiple disk clutch.

Inventors: TEZUKA KAZUNARI C O FUJI JUKOG, TEZUKA, KAZUNARI, C/O FUJI JUKOGYO K. K.
Applicants: FUJI HEAVY IND LTD, FUJI JUKOGYO KABUSHIKI KAISHA
Application year: 1990
Main classification: B60K23
IPC codes: F16H48, F16D48, B60K23