Torque split control system for 4WD vehicle

Abstract: A driving force distribution control system for a 4WD vehicle includes a torque distributing clutch for varying a driving force transmitted to secondary drive wheels such as front wheels in response to a control signal, sensors for determining a vehicle speed (108) and a wheel speed difference (107) between the sensed speed of the primary drive wheels (103) such as the rear wheels and the sensed speed of the secondary drive wheels (105), and a controller for producing the control signal. The controller determines a, correction quantity which increases as the vehicle speed increases, further determines a corrected wheel speed difference by subtracting the correction quantity from the measured wheel speed difference in order to eliminate an undesired influence of a tire diameter difference between the front and rear wheels, and produces the control signal in accordance with the corrected wheel speed difference.

Description: BACKGROUND OF THE INVENTION The present invention relates to a torque split control system for controlling a driving force distribution between front and rear wheels of a four wheel drive vehicle, and more specifically to a torque split control system which can control the driving force distribution adequately in accordance with a wheel speed difference between front and rear wheels without receiving undesired influences of a tire diameter difference between the front and rear wheels. Japanese Patent Provisional (unexamined, KOKAI) Publication No. 63-13331 shows a conventional driving force distribution control system which controls a driving force distribution between front and rear wheels by controlling an engagement force of a torque distributing transfer clutch in accordance with a wheel speed difference between front and rear wheel speeds detected by wheel speed sensors. However, this conventional control system tends to increases the clutch engagement force excessively especially in a high vehicle speed range when the front and rear wheel tire diameters become different from each other. As shown in Fig. 7, the measured wheel speed difference DVw between the front and rear wheels is equal to a sum of a wheel speed difference DVs due to drive wheel slip, and a wheel speed difference DVt due to a difference between the front wheel tire diameter and the rear wheel tire diameter, and this conventional control system increases the clutch engagement force as shown by a line Tm′ in Fig. 7, in accordance with the measured wheel speed difference DVw, without considering the wheel speed difference DVt due to the tire diameter difference. As a result, there arise undesired up-and-down vibrations and an excessive increase in differential oil temperature due to excessive heat generated by clutch slippage (= DVs + DVt) . The tire diameters of the front and rear wheels are more or less unequal because of difference in operating condition such as tire air pressure and wheel load, inequality in tire wear, intentional installation of tires of different sizes, or usage of a temporary spare tire. A driving force distribution control system for a vehicle according to the preamble of claim 1 is known from DE-C-3 608 059. This known system includes a controllable torque distributing means for distributing the torque to primary drive fields and to secondary drive fields of the vehicle. Further, the control system comprises sensor for detecting the engine speed, the throttle valve angle, the steering wheel angle and the rotation of speed of all four wheels. The torque distribution is calculated using a characteristic field on the basis of the changing of the vehicle speed. A further driving force distribution control system for a vehicle is disclosed in EP-A-0 336 055 which is a document in the sense of Article 54(3) EPC. SUMMARY OF THE INVENTION It is the object of the present invention, to provide a driving force distribution control system which can control the driving force distribution adequately in accordance with the wheel speed difference between the front and rear wheels without receiving undisturbed influences of tire diameter difference between the front and rear wheel speed. This object is solved according to the invention by the subject matter of claim 1. Preferred embodiments of the invention are the subject matter of the subclaims. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic view for showing, as an example, a driving force distribution control system according to the claimed invention. Fig. 2 is a schematic view of a 4WD vehicle equipped with a driving force distribution control system of one embodiment of the invention. Fig. 3 is a block diagram showing a control unit of the embodiment shown in Fig. 2. Fig. 4 is a flow chart showing a control procedure performed by the control unit of the embodiment. Fig. 5 is a graph showing a characteristic of a desired clutch engagement force which can be used in the embodiment. Fig. 6 is a graph showing a characteristic of a correction quantity with respect to a vehicle speed, which is employed in the embodiment. Fig. 7 is a graph of relationships of the desired clutch engagement force and wheel speed difference versus the vehicle speed, for illustrating the operations of the control system of the embodiment. Fig. 8 is a schematic view of a 4WD vehicle for illustrating influences of a tire diameter difference between the front and rear wheels. DETAILED DESCRIPTION OF THE INVENTION One embodiment of the present invention is shown in Figs. 2-6. Fig. 2 shows a 4WD vehicle equipped with a torque split control system (driving force distribution control system). The vehicle has a drive system which includes an engine 1, a transmission 2, a transfer input shaft 3, a rear propeller shaft 4, a rear differential 5, rear wheels 6, a transfer output shaft 7, a front propeller shaft 8, a front differential 9, front wheels 10, and a transfer 11. In ths embodiment, the rear wheels 6 are primary drive wheels, and the front wheels 10 are secondary drive wheels. From the transmission 2, engine torque is transmitted to the rear wheels 6 through a rear drive line which serves as a primary drive means for transmitting engine torque directly to the primary drive wheels, and transmitted to the front wheels 10 through a front drive line which serves as a secondary drive means having a torque distribution clutch (or a transfer clutch). In this embodiment, the torque distribution clutch is a wet type multiple disc clutch provided in the transfer 11. Therefore, the engine torque is transmitted directly to the rear wheels 6. On the other hand, the engine torque is transmitted to the front wheels 10 through the torque distributing clutch which is capable of continuously varying the torque transmitted to the front wheels, and completely disconnecting the front wheels 10 from the engine 1. The transfer and its torque distributing clutch are illustrated and explained more in detail in commonly assigned United States Patents Numbers 4,754,834; 4,757,870; 4,773,500; 4,776,424; 4,846,298; 4,874,056, 4,887,689, 4,941,541, 4,966,250, 4,890,685, and 4,966,249 The torque split (driving force distribution) control system further includes a hydraulic system 20 for producing a control oil pressure Pc supplied to the distributing clutch, a group of various sensors 30, and a torque split control unit 40. The control unit 40 produces a dither current i* in accordance with output signals of the sensors 30, and controls the clutch engagement force of the torque distribution clutch of the transfer 11 by sending the dither current to a solenoid valve 28 of the hydraulic system 20. The hydraulic system 20 includes a relief switch 21, a motor 22, an oil reservoir tank 23, an oil pressure pump 24, a check valve 25, an accumulator 26, and the above-mentioned solenoid valve 28. The motor 22 is turned on and off by the relief switch 21, and drives the oil pump 24 which sucks the oil from the reservoir tank 23. The oil pressure discharged from the pump 24 (primary oil pressure) is supplied through the check valve 25, and stored in the accumulator 26. The solenoid valve 28 receives a line pressure (secondary pressure) from the accumulator 26, and produces the control oil pressure Pc in accordance with the dither current i* sent from the control unit 40. The control oil pressure Pc is supplied to the torque distributing clutch of the transfer 11 through an oil pipe 29. In this way, the control unit 40 can continuously vary the engagement force of the torque distributing clutch. As shown in Fig. 3, the sensor group 30 of this embodiment includes a front left wheel rotation sensor 30a for sensing the number of revolutions Nfl of the front left wheel 10, a front right wheel rotation sensor 30b for sensing the number of revolutions Nfr of the front right wheel 10, a rear left wheel rotation sensor 30c for sensing the number of revolutions Nrl of the rear left wheel 6, a rear right wheel rotation sensor 30d for sensing the number of revolutions Nrr of the rear right wheel 6, a first lateral acceleration sensor 30e, and a second lateral acceleration sensor 30f. The torque split control unit 40 includes a circuit 40a for receiving the signal of the front left wheel rotation sensor 30a and determining a front left wheel speed Vwfl, a circuit 40b for receiving the signal of the front right wheel rotation sensor 30b and determining a front right wheel speed Vwfr, a circuit 40c for receiving the signal of the rear left wheel rotation sensor 30c and determining a rear left wheel speed Vwrl, and a circuit 40d for receiving the signal of the rear right wheel rotation sensor 30d and determining a rear right wheel speed Vwrr. The torque split control unit 40 further includes a circuit 40e connected with the circuits 40a and 40b, for determining a front wheel speed Vwf, a circuit 40f connected with the circuits 40c and 40d, for determining a rear wheel speed Vwr, and a circuit 40g connected with the circuits 40e and 40f, for determining a measured front and rear wheel speed difference DVw. The front wheel speed Vwf is an average (arithmetic mean) of Vwfl and Vwfr, and the rear wheel speed Vwr is an average (arithmetic mean) of Vwrl and Vwrr. In this embodiment, the measured front and rear wheel speed difference DVw is a difference obtained by subtracting the front wheel speed Vwf (i.e. a secondary wheel speed) from the rear wheel speed Vwr (i.e. a primary wheel speed). The torque split control unit 40 of this embodiment further includes a circuit 40h for computing a desired clutch engagement force Tm, a Tm-i converting circuit 40i for converting the output signal of the circuit 40h to a current signal i, an output circuit 40j for outputting the dither current i*, a circuit 40k for determining a correction quantity Qi, a circuit 401 for determining a lateral acceleration Yg of the vehicle by using an output signal Yg1 of the first lateral acceleration sensor 30e and an output signal Yg2 of the second lateral acceleration sensor 30f, a circuit 40m for determining a gain Kh in accordance with the lateral acceleration Yg, a circuit 40n for detecting an abnormal condition such as a broken connection which breaks the continuity of each rotation sensor, a circuit 40o for detecting a short-circuit of each rotation sensor, a circuit 40p for detecting an abnormal condition in the lateral acceleration sensors, and a fail-safe circuit 40q. The control unit 40 further includes analog-to-digital converters A/D and a digital-to-analog converter D/A. The fail-safe circuit 40q is connected with a warning lamp 50. The torque split control unit 40 of this embodiment controls the driving force distribution according to a control procedure shown in Fig. 4. At a step 80, the control unit 40 reads the front left wheel speed Vwfl, the front right wheel speed Vwfr, the rear left wheel speed Vwrl, the rear right wheel speed Vwrr, the first lateral acceleration Yg1, and the second lateral acceleration Yg2. At a step 81, the control unit 40 determines the front wheel speed Vwf, the rear wheel speed Vwr, the lateral acceleration Yg and the vehicle speed Vi. In this embodiment, the front wheel speed Vwf is the average of the front left and front right wheel speeds Vwfl and Vwfr, the rear wheel speed Vwr is the average of the rear left and rear right wheel speeds Vwrl and Vwrr, and the lateral acceleration Yg is the average of the first and second lateral accelerations Yg1 and Yg2. In this embodiment, the vehicle speed Vi is set equal to the front wheel speed Vwf. At a step 82, the control unit 40 determines the measured wheel speed difference DVw which is a difference obtained by subtracting the front wheel speed Vwf (that is, the secondary wheel speed) from the rear wheel speed Vwr (that is the primary wheel speed). At a step 83, the control unit 40 determines a proportionality factor (or proportionality constant) Ki in accordance with the reciprocal of the lateral acceleration Yg. In this embodiment, the proportionality factor Ki is inversely proportional to the lateral acceleration Yg, and determined by using the following equation. Ki = αi/Yg (provided that Ki ≦ βi) For example, αi = 0.001 and βi = 0.01. At a step 84, the control unit 40 determines the correction quantity Qi by using the proportionality factor Ki and the vehicle speed Vi. In this embodiment, the correction quantity Qi is proportional to the vehicle speed, and determined by the following equation. Qi = Ki x Vi Fig. 6 shows a relationship between the correction quantity Qi and the vehicle speed Vi, employed in this embodiment. The correction quantity Qi increases as the vehicle speed increases, and the rate of increase of the correction quantity is decreased as the lateral acceleration Yg increases. At a step 85, the control unit 40 determines the corrected wheel speed difference CDVw which is a difference resulting from subtraction of the correction quantity Qi from the measured wheel speed difference DVw. That is, CDVw = DVw - Qi At a step 86, the control unit 40 determines a control gain Kh in accordance with the reciprocal of the lateral acceleration Yg. In this embodiment, the control gain Kh is inversely proportional to the lateral acceleration Yg, and determined by using the following equation. Kh = αh/Yg (provided that Kh ≦ βh) For example, αh = 1 and βh = 10. At a step 87, the control unit 40 determines the desired clutch engagement force Tm by using the control gain Kh and the correctedd wheel speed difference CDVw. In this embodiment, the desired clutch engagement force Tm is determined by multiplying the corrected wheel speed difference CDVw by the control gain Kh. That is, Tm = Kh x CDVw However, it is possible to employ a control characteristic shown in Fig. 5. The control unit 40 of this embodiment holds the desired clutch engagement force equal to zero when the measured wheel speed difference DVw is negative, and when the corrected wheel speed difference CDVw is negative. However, it is possible to increases the clutch engagement force as the absolute value of the measure wheel speed difference increases or to control the clutch engagement force in any known manner in the negative range in which the measured wheel speed difference is smaller than zero. At a step 88, the control unit 40 converts the signal representing Tm determined at the step 87, into the solenoid drive current i by using a predetermined Tm-i characteristic table. At a step 89, the control unit 40 delivers the dither current i* (for example, i ± 0.1A, 100 Hz), to the solenoid valve 28. The control system of this embodiment is operated as follows: When the vehicle is moving straight ahead on a road surface of a low friction coefficient, or the vehicle is accelerated during straight ahead operation, there is arises an increase in the measured wheel speed difference DVw, which contains a front and rear wheel speed difference DVs due to drive wheel slip, and a front and rear wheel speed difference DVt due to tire diameter difference between the front and rear wheels, as shown in Fig. 7. In the case of Fig. 7, the front tire diameter is slightly greater than the rear tire diameter. The wheel speed difference DVt due to tire diameter difference increases as the vehicle speed increases, as shown in Fig. 7. In this case, the control system of the embodiment subtracts the correction quantity Qi increasing with the vehicle speed, from the measured wheel speed difference DVw, and employs, as the input to the controller element, the corrected wheel speed difference CDVw which is the result of the subtraction. The correction quantity Qi corresponds to the wheel speed difference DVt due to tire diameter difference, and the corrected wheel speed difference CDVw becomes approximately equal to the wheel speed difference DVs due to drive wheel slip. In this way, the control system of the invention increases the clutch engagement force of the torque distributing transfer clutch adequately in accordance with the drive wheel slip by increasing the magnitude of the control signal in accordance with the corrected wheel speed difference, and by so doing prevents an excess in the clutch engagement force due to the tire diameter difference between the front and rear wheeels. As shown by a line indicated by Tm in Fig. 7, the control system of the invention holds the clutch engagement force Tm relatively low even in a high vehicle speed range. Therefore, the control system of the invention can prevent unpleasant up-and-down vibrations and severe clutch slippage resulting in increase in the temperature of the differential oil. The unpleasant vibrations in the high vehicle speed range have been confirmed experimentally, and it is considered that these vibrations are caused not by fluctuation in the clutch engagement force, but by a resonance in the drive system which is produced by the forced connection through the slippingly engaged transfer clutch between the front and rear wheels rotating at different speeds because of the tire diameter difference. In a cornering operation on a road surface of a high friction coefficient, there arises a front and rear wheel speed difference DVc due to a difference between the turning radius of the front wheels and the turning radius of the rear wheels, and the measured wheel speed difference DVw amounts to the sum of the wheel speed difference DVs due to the drive wheel slip, the wheel speed difference DVt due to the tire diameter difference, and the wheel speed difference DVc due to the turning radius difference, so that it is not possible to accurately detect the wheel speed difference DVt due to the tire diameter difference. Therefore, the control system of the embodiment determines the correction quantity Qi by multiplying the vehicle speed Vi by the proportionality factor Ki which decreases as the lateral acceleration Yg increases, and thereby decreases the correction quantity Qi as the lateral acceleration Yg increases. A vehicle body roll during cornering weakens the feeling to vibrations, so that the up-and-down vibrations are not so annoying, and the cornering operation does not continue long time, so that the temperature of the differential oil is not increaed too much. Accordingly, the control system of this embodiment decreases the correction quantity Qi with increase of the lateral acceleration Yg, as shown in Fig. 6, and thereby refrains from correcting the measured wheel speed difference during a cornering operation. Furtheremore, the control system of the embodiment efficiently prevents the undesired effect of tight corner braking on a high friction coefficient road by decreasing the control gain Kh with increase in the lateral acceleration, and minimizes the drive wheel slip on a low friction coefficient road by increasing the control gain Kh with decrease in the lateral acceleration. The present invention is applicable to a 4WD vehicle in which the front wheels are the primary drive wheels, and the rear wheels are the secondary drive wheels.

Claims: A driving force distribution control system for a vehicle, comprising: a torque distributing means comprising a primary drive means (102, 4, 5) for transmitting a driving force from an engine (101, 1) of the vehicle to primary drive wheels (103, 6) of the vehicle, and a secondary drive means (104, 7, 8, 9) for transmitting a driving force from the engine to secondary drive wheels (105, 10) of the vehicle through a torque distributing clutch (106) capable of varying the driving force transmitted to the secondary driving force transmitted to the secondary drive wheels in response to a control signal (Tm, i, i*), a first sensing means (107, 30a∼30d, 40a∼40g) for determining a measured wheel speed difference (DVw) between a primary wheel speed (Vwr) of said primary wheels and a secondary wheel speed (Vwf) of said secondary wheels, a second sensing means (108) for determining a vehicle speed (Vi) of the vehicle, and a controlling means (109, 110, 111, 113, 40, 40h, 40k, 40m, 83∼89) for producing the control signal, characterized in that the controlling means comprises a subtracting means (110, 85) for computing a corrected wheel speed difference (CDVw) which is a difference (DVw-Qi) resulting from subtraction of a correction quantity (Qi) determined in dependence on the vehicle speed, from said measured wheel speed difference (DVw), and a control signal generating means (111, 86-89) for receiving a signal representing said correction quantity from said subtracting means, and computing a desired clutch engagement force (Tm) represented by said control signal, in accordance with said corrected wheel speed difference so that said desired clutch engagement force (Tm) increases continuously from a minimum value to a maximum value as said corrected wheel speed difference increases. A control system according to Claim 1 characterized in that said desired clutch engagement force (Tm) determined by said control signal generating means increases linearly as said corrected wheel speed difference increases. A drive force distribution control system according to claim 2 wherein said control system further comprises a third sensing means for determining a lateral acceleration of the vehicle, and said controlling means includes a means for determining said correction quantity by multiplying said vehicle speed by a factor which decreases as said lateral acceleration increases. A driving force distribution control system according to claim 3 wherein said controlling means includes a means for determining said factor which is inversely proportional to said lateral acceleration. A driving force distribution control system according to claim 4 wherein said system further comprises a control gain determining means for determining a control gain which decreases as said lateral acceleration increases, and said controlling means includes a means for determining said desired clutch engagement force by multiplying said corrected wheel speed difference by said control gain. A driving force distribution control system according to claim 5 wherein said primary drive wheels are rear wheels (103) of the vehicle, and said secondary drive wheels are front wheels (105) of the vehicle.

Inventors: ETO YOSHIYUKI, ETO, YOSHIYUKI
Applicants: NISSAN MOTOR, NISSAN MOTOR CO., LTD.
Application year: 1990
Main classification: B60K23
IPC codes: B60K17, B60K23