Torque distribution control system for a four wheel drive motor vehicle

Abstract: A four wheel drive vehicle is provided with a central differential for distributing the output torque of the transmission to the front and rear wheels, a larger proportion of the torque being applied to the rear wheels. A fluid-operated restricting device is provided for restricting the differential operation of the central differential. The slip ratio of the rear wheels is calculated based on rear-wheel speed and vehicle speed. The fluid-operated restricting device is operated in accordance with the slip ratio, in such a manner that differential operation is reduced with an increase of the slip ratio.

Description: The present invention relates to a torque distribution control system for a four-wheel drive motor vehicle having a central differential, and more particularly to a control system in which the output torque of a transmission is unequally distributed to front wheels and rear wheels in accordance with driving conditions. In a motor vehicle with permanently engaged four-wheel drive, and having a central differential, a fluid operated multiple-disk friction clutch is provided for restricting the differential operation. The torque distributed to the front wheels and the rear wheels is controlled by the torque of the friction clutch, thereby improving straight-ahead stability, acceleration and driveability. In order to control the distribution of torque over a wide range, it is necessary that the central differential is arranged to unequally distribute the torque to the front wheels and the rear wheels. There are two methods of controlling such a distribution system, one of which uses a front-weighted distribution while the other uses a rear-weighted distribution. In the front-wheel weighted system, a large torque is transmitted to the front wheels. Accordingly, although the vehicle has good straight-ahead stability, it is inferior in the steering because it tends to drift off course. In the rear-wheel weighted system, the larger torque is transmitted to the rear wheels. The vehicle has good steering and driveability on a dry road. However, the vehicle is liable to spin on a slippery road. On the other hand a general disadvantage of the four-wheel drive vehicle is that all of the four wheels may slip (slipping spin) at the same time, causing difficulty in driving. In order to ensure stable driving of the vehicle, the torque transmitted to the rear wheels is usually set to a larger value than that to the front wheels, so that the rear wheels may slip first. Thus, the vehicle can then be safely driven by the front wheels at small torque while the rear wheels idle. Since the driving force (acceleration force) is distributed to all four wheels, each wheel can sustain a larger side force. Thus, in order to ensure the driving stability in the rear-wheel weighted system, it is necessary to keep the sustainable side force of the rear wheels large. The driving force and the side force change with the slip ratio of the wheel to the ground (slip ratio is described hereinafter in detail). The slip ratio changes in accordance with the condition of the road surface and the friction coefficient of the tyre. In particular, if the slip ratio is larger than a predetermined value (10 to 20%) the driving force and the sustainable side force are dramatically reduced, which results in deterioration of driving stability. Accordingly, the slip ratio of the rear-wheel is controlled so as not to exceed the predetermined value. If the system operates to increase the torque distributed to the front wheels in accordance with the reduction of the sustainable side force, slipping of the rear wheels is prevented to ensure the driving force. Therefore, it is desirable to control the torque distribution based on the rear-wheel slip ratio. Japanese Laid Open Patent Applications Nos. 62-55228, 62-261539 and 63-8026 disclose four-wheel drive control systems in which a central differential comprising a simple planetary gear device is provided. The standard torque to the front and rear wheels is unequally distributed at a ratio determined by a difference between pitch circles of gears of the planetary gear device. The torque-capacity of the clutch is controlled in accordance with the difference between the speeds of the front wheels and the rear wheels, or with a predetermined value corrected by vehicle speed and steering angle. However, in conventional systems, the standard torque distribution ratio determined by the ratio of the pitch circles cannot be changed, unless the diameters of the gears are changed. However, it is difficult to change the diameters because of limitation of space. Since the distribution of torque to the rear wheels cannot be set to a large value, the control range of the torque distribution becomes small. Further, the torque distribution is controlled by restricting the differential operation of the central differential. Therefore, the slip condition of the wheels on the road surface cannot be detected so that it is impossible to actively control the torque distribution in accordance with the slip conditions. In addition, on a road surface having a low coefficient of friction, if the vehicle is driven under slipping conditions and the speed difference of the front wheels and the rear wheels is small, the torque distribution cannot be controlled. The present invention seeks to provide a torque distribution control system for a four-wheel drive motor vehicle in which a large torque is distributed to rear wheels, thereby enabling control of a wide torque distribution range. The present invention also seeks to provide a system which may ensure good driveability, driving stability and steering in accordance with slip conditions. The reader will be further enlightened as to the scope of the prior art by reference to EP-A-0 313 371, with reference to which the present invention, according to claim 1, has been characterised. According to the present invention, there is provided a system for controlling torque distribution to the front and rear wheels of a motor vehicle, as defined in claim 1. In a preferred embodiment of the invention, the slip ratio of the rear wheels is calculated based on velocity of the rear wheels and the ground speed of the vehicle. During slipping of the front wheels, the slip ratio is calculated based on the angular velocity of the front wheels before the slipping and acceleration of the vehicle. Some embodiments of the invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram showing a power transmission system for a four-wheel drive motor vehicle according to the present invention; Figure 2a is a graph showing characteristics of driving force and side force corresponding to slip ratios; Figure 2b is a graph showing differential operation restricting clutch torque; Figure 3 is a block diagram of the embodiment; Figure 4 is a flowchart showing the operation of the embodiment; Figure 5a is a graph showing a relationship between front-wheel torque and rear-wheel torque; Figure 5b is a graph showing characteristics of the differential operation restricting clutch torque in accordance with a rear-wheel slip ratio. Figure 1 shows a power transmission system for a four-wheel drive motor vehicle having a central differential according to the present invention. An engine 1 is mounted on a front portion of the vehicle. A clutch 2 and an automatic transmission 3 are disposed at the rear of the engine 1 in the longitudinal direction of the vehicle. An output of the automatic transmission 3 is transmitted to an output shaft 4 which is aligned with a central differential 20 for rearwardly transmitting the torque. The output shaft 4 is connected to a front drive shaft 5 which is arranged parallel to and beneath the automatic transmission 3 through a pair of reduction gears 25 and 26 of the central differential 20. The front drive shaft 5 is connected to front wheels 9 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 rear wheels 13 through a propeller shaft 10, a rear 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 planet pinion 23 comprising a first planet pinion 23a meshed with the first sun gear 21, and a second planet 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, the 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 pinions 23a, 23b at predetermined respective torque distribution ratios. The difference between the rotational speeds of the carrier 24 and the second sun gear 22 is absorbed by the rotation and revolution of the first and second planet pinions 23a and 23b. In the central differential 20, the first sun gear 21 is engaged with the first planet pinion 23a and the second sun gear 22 is engaged with the second planet pinion 23b. Consequently, a standard torque distribution for a front torque T F and a 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 9 and the rear wheels 13 is determined, for example as T F : T R = 34 : 66 A large standard torque can be distributed to the rear wheels 13. A fluid operated multiple-disk friction clutch 27 is provided adjacent the central differential 20 for restricting 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 9, 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 a front static weight WF and a rear static 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 62 : 38 which is approximate to the weight distribution. Thus, the torque distribution is controlled in the range between the standard torque distribution weighted to the rear wheels 13 and the torque distribution on complete engagement of the clutch in accordance with the differential operation restricting clutch torque Tc. The hydraulic circuit of the control system for the clutch 27 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 the pressure of oil supplied from the oil pump 30 driven by the engine 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 downstream of the orifice 36, and with one 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 27 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 the drain of 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 27 so as to control the clutch pressure (torque) and hence the differential operation restricting clutch torque Tc. The principle for controlling the torque distribution is described hereinafter, with reference to the operation of the preferred electronic control system. The torque distribution is controlled in accordance with a feedback signal on the slip ratio of the rear wheels because the torque to the rear wheels is set to a value larger than that to the front wheels by arranging the central differential 20 so that the rear wheels-slip first. Fig. 2a shows relationships between the driving force T or the sustainable side force F corresponding to the slip ratio S. When the slip ratio is zero, the sustainable side force F is maximum and gradually reduces as the slip ratio S increases. The driving force T increases with the slip ratio S and reduces when the slip ratio exceeds a predetermined slip ratio Sa (10 to 20%). Thus, it will be seen that if the slip ratio S is controlled in the range of S ≦ Sa, a large sustainable side force F is ensured, thereby providing driving stability by rear-wheel weighted driving. The slip ratio S of the rear wheels is represented as follows with the vehicle ground speed V, the radius of the tire r, and the rear-wheel angular velocity ωR. S = (r · ωR - V)/(r · ωR) If the rear-wheel slip ratio S is controlled in a linear range of S<Sa at torque distribution 3:7, the front-wheel slip ratio S becomes small to be approximately equal to the vehicle speed. Namely, the ground speed V is V ≒ r · ωF where ωF is the front-wheel angular velocity and r is the radius of the tire. Thus, the slip ratio S of the rear wheels is represented as follows: S = (r · ωR - r · ωF)/(r · ωR) = (ωR - ωF)/ωR In order to prevent the malfunction of the central differential at cornering, the range smaller than the slip ratio Sb (for example 3%) is determined as a dead zone where apparent slip ratio produced in accordance with the difference between the speed of the front wheels 9 and the speed of rear wheels 13 at a maximum steering angle is included. Thus, the control range becomes Sb<S<Sa. The slip ratio S is calculated within the control range to control the differential operation restricting clutch torque To as an increasing function of the slip ratio, so that a part of the torque to the rear wheels 13 is transmitted to the front wheels 9. Consequently, the sustainable side force F of the rear wheels 13 can be increased. The control unit 50 is supplied with output signals from a front-wheel speed sensor 40 and a rear-wheel speed sensor 41. The control unit 50 has a slip ratio calculator 51 to which a front-wheel angular velocity ωF and a rear-wheel angular velocity ωR from the sensors 40 and 41 are applied. In the slip ratio calculator 51, a slip ratio S is calculated in accordance with the above mentioned equations. The slip ratio S 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 the clutch torque Tc from a look up table. Fig. 2b shows the clutch torque Tc stored in the look up table. The clutch torque Tc is an increasing function of the slip ratio S in the control range Sb<S<Sa. The clutch torque Tc is applied to a control quantity providing section 53 in which duty ratio D corresponding to the derived clutch torque Tc is provided. A 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 Figure 4. The power of the engine 1 is transmitted to the transmission 3 through the clutch 2 at which the transmission ratio is automatically controlled. The output of the transmission 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 gears of the central differential 20. The torque is transmitted to the reduction gear 25 through the carrier 24 at a ratio of 34% for example and to the second sun gear 22 at the ratio of 66%. The front-wheel angular velocity ωF and the rear-wheel angular velocity ωR are detected by the front-wheel speed sensor 40 and the rear-wheel speed sensor 41. If the measured front-wheel angular velocity ωF is used at slipping of the front wheels 9, the rear-wheel slip ratio S becomes a smaller value than the actual slip ratio because the front-wheel angular velocity ωF becomes higher than the ground speed V. Referring to Figure 3, the control unit 50 is provided with an acceleration sensor 43. A vehicle body acceleration a detected by the acceleration sensor 43 is applied to a front-wheel slipping determining section 57. The rear-wheel angular velocity ωR detected by the sensor 41 is applied to the rear-wheel slip ratio calculator 51. The front-wheel angular velocity ωF is applied to a changing rate calculator 58 where a changing rate dωF/dt of the front-wheel angular velocity ωF is calculated. The changing rate dωF/.dt is applied to the front-wheel slipping determining section 57 in which the changing rate dωF/dt is compared with the vehicle body acceleration α. If dωF/dt > α , slipping of the front wheels 9 is determined. The result of the comparison and the acceleration a are applied to a front-wheel angular velocity correcting section 59 to which the front-wheel angular velocity ωF is also applied. In a slipping state, an actual front-wheel angular velocity Fa is calculated as follows, using a front-wheel angular velocity ωF' before slipping, the detected acceleration α, and a calculating time Δt. ωFa = ωF' + α · Δt In a no slip state, the front-wheel angular velocity ωF is applied to the rear-wheel slip ratio calculator 51 through the section 59. The operation of the system is shown in the flowchart of Figure 4. At acceleration, the vehicle body acceleration α and the changing rate dωF/dt of the front-wheel angular velocity ωF are compared. When dωF/dt > α , the front-wheel angular velocity ωF is corrected to the value corresponding to the actual vehicle speed in accordance with the angular velocity ωF' before slipping and the vehicle body acceleration α. Thus, the rear-wheel slip ratio S is accurately calculated, thereby properly controlling the torque distribution. If no slip state (S≦Sb) is detected in the control unit 50 while the vehicle is driven on the dry road having a high friction coefficient µ, the clutch torque Tc is determined to zero. 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 27. The clutch 27 is disengaged and the clutch torque Tc becomes zero so as to render the central differential 20 free. Accordingly, the torque of the reduction drive gear 25 is transmitted to the front wheels 9 through the reduction driven gear 26, the front drive shaft 5 and the front differential 7 at the ratio of 34%. The torque at distribution ratio of 66% is transmitted to the rear wheels 13 through the second sun gear 22, the rear drive shaft 6, the propeller shaft 10 and the rear differential 11. Thus, a full-time four-wheel driving is established at a point P1 of Fig. 5a. When the vehicle makes a turn, the rotating difference in the speeds of the front and the rear wheels 9, 13 is absorbed by the rotation and revolution of the first and second pinions 23a and 23b, so that the vehicle smoothly negotiates a sharp corner owing to the differential operation of the central differential 20. If the vehicle is driven on a slippery road, the rear wheels 13 slip first because the larger amount of the torque is distributed to the rear wheels 13. If the calculated rear-wheel slip ratio becomes a value S2 in the control range, the driving of the rear wheels 13 is unstable. Therefore, a differential operation restricting clutch torque T2 is determined corresponding to the slip ratio S2. Consequently, the clutch torque Tc₂ is produced in the clutch 27. The clutch 27 is provided in parallel with the carrier 24 and the second sun gear 22 of the central differential 20. Accordingly, the torque is transmitted from the second sun gear 22 to the carrier 24 to increase the torque to the front wheels 9. Thus, the distribution ratio of the front torque and the rear torque T F :T R becomes T F2 :T R2 at a point P2 of Fig. 5a. On the other hand, torque to the rear wheels 13 is reduced to eliminate slipping, thereby improving driveability to ensure good operability and safe driving. When the slip ratio S becomes larger than the value Sa (S≧Sa) and the driving is extremely unstable, the differential operation restricting torque Tc becomes α maximum. Thus, the carrier 24 is directly engaged with the second sun gear 22 to lock the central differential 20. Thus, the entirely coupled four-wheel driving is established in accordance with the torque ratio distribution ratio 50:50 corresponding to the axle loads of the front and the rear wheels 9, 13 at a point P3 of Fig. 5a. Thus, the torque distribution is continuously controlled in accordance with the slipping condition for preventing the wheels 9, 13 from slipping. In accordance with the present invention, since large torque is transmitted to the rear wheels through the central differential, driving stability for the four-wheel drive system as well as cornering stability are improved. The rear-wheel slip ratio is calculated to determine the differential operation restricting clutch torque for controlling the torque distribution. Since a necessary minimum torque is transmitted to the front wheels for preventing slipping, thereby enhancing stability and steerability of the vehicle. The control range in accordance with the slip ratio is properly determined by the characteristics of the driving power and the sustainable side force. Thus, the tight corner braking is effetively prevented to ensure the driveability. The rear-wheel slip ratio is calculated in accordance with the front-wheel speed as the vehicle speed, so that the calculation is simplified and corrections of various factors can be easily achieved. Since the rear-wheel slip ratio is corrected an accurate slip ratio is obtained corresponding to the actual driving conditions. Thus, the torque distribution is properly controlled.

Claims: A system for controlling the torque distribution to the front and rear wheels (9,13) of a motor vehicle having a transmission (3), a rear wheel speed sensor (41), a front wheel speed sensor (40), a control unit (50) including a rear wheel slip calculator (51) for calculating the slip ratio (S) of the vehicle rear wheels (13) based on the front and rear wheel speeds, a central differential gear (20) for distributing an output torque of the transmission (3) between the front and rear wheels (9,13), and a variable restricting device for restricting the differential operation of the central differential gear (20) in accordance with an increase of the rear wheel slip ratio (S), characterised in that said central differential gear (20) is configured to distribute a larger torque to the rear wheels (13) than to the front wheels (9) in unrestricted operation; and in that said system further comprises an acceleration sensor (43) to sense the actual acceleration of the vehicle, a changing rate calculator (58) to calculate the acceleration from the rate of change of the front wheel speed sensed by the front wheel speed sensor (40), a front wheel slip determining section (57) responsive to the actual acceleration and the calculated acceleration to determine the front wheel slip, and a front wheel angular velocity correcting section (59) responsive to the front wheel speed and the front wheel slip to apply a correction to a rear wheel slip ratio calculator (51) for said calculation of the rear wheel slip ratio (S). A system according to claim 1 in which the said restricting device is an hydraulically operated clutch (27) connected in parallel with said differential gear (20), so that differential operation is restricted as the degree of engagement of the clutch (27) is increased. A system according to claim 1 or claim 2, wherein said slip ratio (S) of the rear wheels (13) is calculated from the velocity of the rear wheels and the calculated ground speed of said vehicle. A system according to any preceding claim wherein at slipping of said front wheels said rear wheel slip ratio (S) is calculated based on the angular velocity of the front wheels before the slip and on the acceleration of the vehicle. A system according to claim 3, wherein said ground speed is calculated from the angular velocity of the front wheels. A system according to any preceding claim, wherein said central differential gear (90) comprises a planetary gear device including a first sun gear (21) connected to an output shaft (4) of the transmission (3), a carrier (24), first and second planet pinions (23) integral with each other and rotatably supported on the carrier (24), and a second sun gear (22) coupled to the rear wheels (13), the first planetary pinion (23a) being engaged with the first sun gear (21), and the second planetary pinion (23b) being engaged with the second sun gear (22).

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: B60K23, B60K17