JPS63141832A - Control method for four-wheel drive vehicle - Google Patents

Abstract

PURPOSE:To enable production of the maximum traction at all times, by feedback controlling a means for modifying front and/or rear wheel driving torque and setting a driving torque distribution rate. CONSTITUTION:Output rotation of an engine 1 is transmitted through a transmission 2 to a center differential 4 and divided into two, then the divided torques are transmitted through front and rear continuously variable transmissions 5, 6 to front and rear wheel driving shafts. When such four-wheel drive vehicle is controlled through a controller 28, load bearing ratio between front and rear wheels is estimated based on the acceleration and gradient so as to determine a target driving torque distribution ratio between front and rear wheels based on said load ratio. Furthermore, difference of rotation between front and rear wheel driving shafts is calculated based on the rotations of the transmission 2 and respective transmissions 5, 6. Thereafter, a target driving torque distribution ratio is set based on said torque distribution ratio, difference of rotation, etc. so as to feedback control respective flow control valves 27F, 27R.

Description

[Detailed description of the invention] [Industrial application field]

The present invention relates to a control method for a four-wheel drive system, and is particularly suitable for use in a four-wheel drive vehicle that uses a continuously variable transmission 111t74 to control the drive torque distribution between the front and rear wheels. The present invention relates to an improvement in a control method for a four-wheel drive vehicle having a drive torque distribution ratio control means that allows the drive torque distribution ratio between front and rear wheels to be arbitrarily set by changing one or both of the drive torques.

[Conventional technology]

2. Description of the Related Art In recent years, 41a drive (hereinafter referred to as 4WD) vehicles, which drive by dispersing the driving force of an engine between front wheels and rear wheels, have become popular. In 4WD vehicles, the drive torque distribution ratio between the front and rear wheels has traditionally been fixed at, for example, 1:1, but in recent years, models that can set the drive torque distribution ratio to any desired value have become available. Proposed. One of them is disclosed in Japanese Patent Application Laid-Open No. 59-151661, in which the driving force of the engine is distributed to the front wheels and the rear wheels through a differential T8I structure. One or both wheels are provided with a continuously variable transmission wheel, and the speed ratio between the front and rear wheels is differentiated, thereby arbitrarily setting the drive torque distribution ratio. On the other hand, in Japanese Patent Application No. 61-186495, the applicant
A differential mechanism that distributes and outputs the output from the engine into two systems, two continuously variable transmission systems connected to the two distributed output systems, and one of the two continuously variable transmission systems. means for connecting the other to the front wheel and the other to the rear wheel;
A gear train for a four-wheel drive vehicle characterized by comprising means for independently controlling continuously variable transmission mechanisms of the system to simultaneously achieve a target speed ratio and a target front and rear wheel drive torque distribution ratio. We are proposing a continuously variable transmission. In any case, torque distribution control is performed using signals from a running state detection device that detects various running states, such as starting, accelerating, decelerating, high-speed running, climbing up, and dismounting (Japanese Patent Application Laid-Open No. 59-151661). Or, by a signal from an acceleration sensor (Patent Application No. 6l-186495),
The ratio of the shared load between the front and rear wheels is indirectly determined, and the gear ratio of the continuously variable transmission mechanism is determined so that the drive torque of the front and rear wheels is also in this ratio, and open-loop control is performed to generate the most efficient and large traction. I'm aiming to make it happen.

[Problems to be solved by the invention]

However, the methods of determining the front and rear wheel load sharing ratio from various vehicle running conditions and vehicle acceleration are all indirect methods, and considering the complex structure of the vehicle, high accuracy in the required shared load ratio cannot be expected. No, that is, the optimal torque distribution is
Originally, it should be determined by the slippage state of the tires, but instead of directly detecting this amount, the relationship between the tires and the road surface is indirectly estimated using some kind of driving state detection means, and control is performed based on this. As a result, control accuracy decreases. Furthermore, even if the shared load ratio can be determined with great accuracy, if the road surface condition is not uniform or if the road surface condition changes from moment to moment, it may not be possible to control the driving force distribution to an optimal value. Furthermore, there are many disturbance factors for optimal control, such as differences in the wear conditions of front and rear tires and loading conditions.
The oven loop control described above cannot cope with widely varying operating conditions and road surface conditions, and cannot be said to have completely achieved its original purpose.

[Purpose of the invention]

The present invention was made in order to solve the above-mentioned conventional problems, and the present invention is designed to adjust the front wheels so that maximum traction is always generated according to the driving condition of the vehicle, even if the loading condition of the vehicle or the road surface condition changes. It is an object of the present invention to provide a control method for a four-wheel drive vehicle that can appropriately control the drive torque distribution ratio of rear wheels.

[Move F to solve the problem! i】

The present invention provides a 4Va drive system having a drive torque distribution ratio control means that makes it possible to arbitrarily set the drive torque distribution ratio between the front and rear wheels by changing either or both of the front wheel drive torque and the rear wheel drive torque. In a vehicle control method, a rotation difference between a front drive shaft and a rear wheel drive shaft is detected, and the drive torque distribution ratio control means is feedback-controlled so that the rotation difference becomes a target value. The above object has been achieved by setting the drive torque distribution ratio of the rear wheels to an optimum value. Further, in an embodiment of the present invention, the target value of the rotational difference is set to zero. Further, in another embodiment of the present invention, when performing feedback control of the drive torque distribution ratio control means, the target drive torque distribution ratio obtained in advance from the running state of the vehicle is corrected by a feedback term including a rotation difference. This is what I did.

[Function] In the present invention, in a four-wheel drive vehicle as described above,
A rotation difference between the front wheel drive shaft and the rear wheel drive shaft is detected, and the drive torque distribution ratio control means is feedback-controlled so that the rotation difference becomes a target value (zero or a small value). Therefore,
For example, if the rotation speed of the front wheel drive shaft is higher than the rotation speed of the rear wheel drive shaft, the drive torque distribution ratio between the front and rear wheels is changed so that the drive torque is transferred from the front wheels to the rear wheels; If the rotation speed is lower than the rotation speed of the rear wheel drive shaft,
Since the drive torque distribution ratio of the H rear wheels is changed so that the drive torque is transferred from the rear wheels to the front wheels, the rotation difference between the front and rear RI drive shafts is controlled to the target value in any case. Therefore,
Unnecessary wheel slip is reduced and vehicle traction is increased. Therefore, the optimum drive torque distribution ratio can always be achieved depending on the driving condition of the vehicle, regardless of road surface conditions or loading conditions. Further, when the target value of the rotational difference is set to zero, control is simple. Further, when performing feedback control of the drive torque distribution ratio control means, if the target drive torque distribution ratio determined in advance from the running state of the vehicle is corrected by a feedback term including a rotational difference, the M control Highly responsive.

【Example】

Embodiments of the present invention will be described in detail below with reference to the drawings. FIG. 1 schematically shows the power train of a 4WD vehicle with variable driving force distribution in which a first embodiment of the present invention is adopted. In this first embodiment, the rotational speed and torque of the output shaft of the engine 1 are adjusted via the transmission 2 to values suitable for driving. These are transmission 2
The output shaft 3 transmits the signal to the center differential 4 and divides the signal into two. The torque divided by the center differential 4 is further converted by the front wheel side continuously variable transmission mechanism 5 and the rear wheel side continuously variable transmission mechanism 6, but at this time, the front wheel side continuously variable transmission mechanism 5 and the rear wheel side continuously variable transmission mechanism 5 Due to the difference in the set speed ratio of the transmission mechanism 6,
A distribution ratio occurs in the drive torque transmitted to the front and rear wheels. That is, if the speed ratio of the front wheel continuously variable transmission mechanism 5 is eF and the speed ratio of the rear wheel continuously variable transmission mechanism 6 is eR, the ratio of the Tfrj transport drive torque to the rear wheel drive torque is eR:eF. In this way, if there is a difference in the speed ratio of both continuously variable transmission R mechanisms 5.6, a rotation difference will occur in each input shaft 7.8 of the continuously variable transmission M5.6, but this is due to the center differential 4.
absorbed by. Each output shaft 9.10 of the continuously variable transmission mechanism 5.6 is transmitted to a front wheel WF and a rear wheel WR of the vehicle via a front wheel differential 11 and a rear wheel differential 12, respectively. In the figure, 13 is a propeller shaft for transmitting the rotation of the output shaft 10 of the rear wheel continuously variable transmission tM6 to the rear wheel differential 12, and 32 and 33 are front wheel drive shafts, respectively, for the front wheel continuously variable transmission. The rotation speed of the output shaft 9 of the mechanism 5 and the rear wheel fil! which is the rear wheel drive shaft. I continuously variable speed 'a tn
This is a rotation speed sensor for detecting the rotation speed of the output shaft 10 of No. 6. The continuously variable transmission mechanism 5.6 is a well-known continuously variable transmission mechanism of the so-called 1-Ri belt type, in which a conical disk-shaped pulley moves in the axial direction and changes the belt diameter. This is to obtain the speed ratio. However, the continuously variable transmission mtg targeted by the present invention is
The present invention is not limited to the pulley/belt type, and the essential effects will remain the same even if other types are used. FIG. 2 shows a control system for the two continuously variable transmission mechanisms 5.6 shown in FIG. The thrust of the input shaft pulley 17 and the output shaft pulley 18 of the front wheel side continuously variable transmission 'D15 is given by the hydraulic pressure of the hydraulic oil in the input shaft hydraulic cylinder 19 and the output shaft hydraulic cylinder 20. 25F is a pump driven by engine 1,
This discharge pressure is determined by the pressure control valve 26F and becomes the line oil pressure PLF. This line hydraulic pressure PLF acts directly on the output shaft hydraulic cylinder 20 to ensure the pulley thrust necessary to transmit torque without the belt slipping. Further, the hydraulic oil that has become the hydraulic pressure PLF is also guided to the flow rate control valve 27F, and this valve controls the required amount of oil to the input shaft hydraulic cylinder 19.
or drain from the input shaft hydraulic cylinder 19. As a result, a hydraulic control system is created that adjusts the torque capacity and speed ratio of the front wheel side continuously variable transmission 5 using zero or more elements that can set an arbitrary speed ratio. Hydraulic pressure 1 for adjusting the torque capacity and speed ratio of the rear wheel side continuously variable transmission mechanism 6! Similarly, the control system is composed of a pump 25R1, a pressure control valve 26R1, a flow rate I11, a control valve 27R'''C', and the functions of each element are the same as those for the front wheels.The line oil pressure here is PLR. The pressure control valves 26F, 26R and the flow rate control valve 27F,
27R are proportional solenoid valves, and these are driven by electric signals issued from the controller 28. The signals input to the controller 28 are the throttle opening θth and the input shaft rotation speed N i of the front wheel side continuously variable transmission IR mechanism 5.
nF, input shaft rotation speed NinR1 of the rear wheel side continuously variable transmission mechanism 6
Electric signals corresponding to the front wheel side continuously variable transmission mechanism power shaft rotation speed NoutF, the output shaft rotation speed NoutR1 of the rear wheel side continuously variable transmission mechanism 6, the vehicle acceleration and gradient G, and the shift position z3 of the transmission 2, etc. These are the throttle sensor 29 and the rotation speed sensor 30, 31, 32, and 33, respectively.
, the acceleration sensor 34, and the shift position sensor 35. (The front and rear wheel load ratio varies depending on not only the acceleration of the vehicle but also the slope of the road surface.) Therefore, the acceleration sensor 34 does not simply detect acceleration, but also generates a signal that takes into account the inclination of the vehicle in the longitudinal direction. It is said that the structure generates The controller 28 includes a CPU, ROM, and RAM.
It has a built-in so-called microcomputer, which processes input signals according to a program stored in advance in the ROM while utilizing the storage function of the RAM, and outputs control signals. Hereinafter, with reference to FIGS. 3(A), (B), and (C),
The control method in the embodiment will be explained in detail. First, in step S1, the throttle opening θth, the input shaft rotational speed N1nF of the front wheel side continuously variable transmission mechanism 5, the input shaft rotational speed NinR1 of the rear wheel side continuously variable transmission mechanism 6, the upper shaft rotation of the front wheel side continuously variable transmission mechanism 5 The number Nout F (regarded as the rotation speed of the front wheel drive shaft in this example), the output shaft rotation speed Nout R of the rear wheel side continuously variable transmission mechanism 6 (regarded as the rotation speed of the rear wheel drive shaft in the wooden example) The vehicle acceleration, gradient G, and shift position z3 of the transmission (T/M) 2 are determined by the controller 2.
The data is read into the microcomputer built into the 8. Next, in step $2, the output shaft rotation speed Nt of the transmission 2 is calculated. This output shaft rotation speed Nt is determined by the center differential 4 which is a simple differential 111fH.
As a result, the front wheel side continuously variable transmission mechanism 5 and the rear wheel side continuously variable transmission mechanism 6
Since the input shaft rotational speeds are divided into N1nF and N1nR, respectively, it is determined by the following formula. Nt=(NinF+N1nR)/2-(1) Next, proceed to step S3, and set the output shaft rotation speed N.

[and T/M shift position S, engine rotation speed Ne is calculated by the following formula. Ne=Nt −fl (S) ・・・・・・(2)
Here, fl (S) is the total reduction ratio of the transmission 2 determined from the shift position signal S. Next, the process proceeds to step S4, where the output shaft torque Tt of the transmission 2 is calculated from the throttle opening degree θ[h, the engine speed Ne, and the shift position 13 by the following equation. Tt=fz(θth-Ne)・ft(S
L・ (3) Here, fz (θth−Nf3) represents the engine torque obtained from the relationship stored in advance as a map in the ROM of the controller 28, and the 12 reduction ratio f of the transmission 2 is added to this engine torque.
By multiplying +(S), transmission 2
The output shaft torque T (of the front wheel side continuously variable transmission mechanism 5 is calculated from the input shaft rotation speed N1nF of the front wheel side continuously variable transmission mechanism 5 and the output shaft rotation speed NoutF of the front wheel side continuously variable transmission mechanism 5 by the following formula. The speed ratio eF is calculated. eF=Nout F/N1nF --- (4)
Next, the process proceeds to step $6, where the speed ratio eR of the rear wheel continuously variable transmission mechanism 6 is calculated from the input shaft rotation speed N1nR and the output shaft rotation speed NoutR of the rear wheel side continuously variable transmission mechanism 6 using the following equation. eR=Nout R/N1nR-...-(5) Next, the process proceeds to step S7, and from the acceleration and gradient G, the target front and rear fa ill dynamic torque distribution ratio (front wheel drive torque/rear wheel drive Torque) γP (pre-programming) is regulated. γp=fx(G) ・・・・・・・・・(6
) Here, the load ratio between the front and rear wheels is estimated from the vehicle acceleration and the gradient G, and this load ratio is used as the preprogram type γP of the front and rear wheel drive torque distribution ratio. By determining the drive torque distribution ratio in this manner, the vehicle should be able to obtain maximum traction. However, as described in the section on the problems to be solved by the invention, simply determining the drive torque distribution ratio in this way results in oven loop control, and in an fft-tight sense, it is difficult to achieve maximum traction. You may not be able to obtain it. Therefore, in step S8 according to the present invention, the continuously variable transmission mechanism 5
．． From the output shaft rotational speeds N0Ut F and N0LIt R of the transmission 2 and the output shaft rotational speed Nt of the transmission 2, the rotational difference Z between the front and rear @ drive shafts is calculated using the following formula. Z= (Nout R-Nout F) /Nt-(
7) Here, the reason why the difference in the rotation speed of the front and rear wheel drive shafts is divided by the transmission output shaft rotation speed N is because the effect of feedback in the next step S9 is greatly influenced by the transmission output shaft rotation speed Nt. This is to prevent this from happening. Next, the process proceeds to step S9 according to the present invention, and as shown in the following equation, feedbacks are added to the preprograms γP of the target drive torque distribution ratio calculated by equation (6) in step S7, and the target drive torque is newly set as γC. The allocation ratio is set. rc-rp+KD (Z+ (fZdt)/Ti+T
d (dz/dt) l ...... (8) The feedback according to equation (8) performs so-called PID operation (proportional, integral, differential operation), where Kp is the proportional gain and Ti is the integral The time, Td, is the differential time. Among these, the proportional term KD is the most important, and if there is a rotational difference between the front wheel drive shafts, the drive torque distribution ratio can be changed using only this term Kp to make the rotational difference almost equal. In addition, if there is even a slight difference in rotation between the front and rear 1'12 drive shafts, the integral term (Ti) adds this difference and changes the drive torque distribution ratio, which ultimately eliminates the difference in rotation. This makes it possible. Although the integral term is expressed mathematically in equation (8), in reality, the rotational difference rate Z is added each time the control loop turns, and the differential term (Td) has the effect of quickly matching the rotational speed of the front and rear @ drive shafts, and quickly damping and stabilizing the fluctuations and movements of the controlled variable. In equation (8), this differential term is also expressed mathematically, In reality, the rotational difference rate Z obtained each time the control loop rotates is sampled at two or more points, and the rate of change in Z is calculated from these. Note that although the feedback type in equation (8) performs a PID operation, the present invention does not necessarily require a PID operation, and various operations such as a P operation, a PI operation, and a PD operation can be considered. Through steps SIO to S16 after step S9, the drive torque distribution ratio of the vehicle is set to the target drive torque distribution ratio γ.
However, each time the control loop shown in FIG.
Assume that the set distribution of drive torque is biased so that more torque flows to the rear wheels, and the rear wheels are slipping more than the front wheels. In this case, NoutR>N1n
R, so the rotational difference rate Z takes a positive value. Therefore, (8
), the target drive torque distribution ratio γC increases. That is, the target drive torque distribution ratio is modified to increase the distributed torque on the front wheel side. As a result, the slip on the rear wheel side decreases, NoutR approaches NoutF, and finally, N0Ut
A drive torque distribution ratio such that R=NoutF is achieved. Conversely, if a larger slip occurs on the front wheel side than on the rear wheel side, Nout R<Nout F, so
The rotational difference rate Z has a negative value. At this time, (8)
Since the target drive torque distribution ratio γC decreases according to the formula, the torque distribution rate on the rear wheel side increases and the torque distribution rate on the front wheel side decreases. As a result, the large slip on the front wheel side becomes smaller, and finally Nout R=Nout
It becomes F. In summary, steps S8 and S9 according to the present invention are as follows:
By lowering the torque distribution ratio on the side where the rotation speed of the front and rear wheel drive shafts is higher, that is, on the side where greater slip occurs, and increasing the torque distribution ratio on the opposite side, the rotation speed difference between the front and rear wheel drive shafts is reduced to zero. If the rotation speeds of the front and rear wheel drive shafts are the same, it can be considered that the maximum traction is generated with the least loss, so the above If such control is performed, an optimal drive torque distribution ratio will be achieved. Hereinafter, steps after step S10 will be explained in detail. In step 5IO2Sll, target speed ratios eFx, eRx of the front wheel side continuously variable transmission mechanism 5 and the rear wheel side continuously variable transmission Ia mechanism 6 are set.
is determined by the following equations (9) and (10) based on the target drive torque distribution ratio γC obtained in step S9. eFx=C-(γc+1)/2γc-(9)eRx=C
-(7C+1)/2 = (10) Here, equations (9) and (10) are derived by the following procedure. Now, the input shaft torque of the front wheel side continuously variable transmission mechanism 5 is T i
nF, the input shaft torque of the rear wheel side continuously variable transmission mechanism 6 is T i
r+R1 The output shaft torque of the front wheel side continuously variable transmission tiv5 is T.
aut F, output IFIII of rear wheel side continuously variable transmission mechanism 6
When the torque is Tout R, the torque distribution ratio γ is expressed by the following equation. γ=Tout F/Tout R = (eR-TinF)/(eF-TinR)...
・・・・・・(11) Here, the center differential 4 causes TinF
= T inR, so the following relationship holds true. γ= eR/eF ・・・・・・・・・
(12) Therefore, if one of the speed ratios eF or eR is arbitrarily determined, and the other speed ratio is set so that the target drive torque distribution ratio γ can be achieved using equation (12), It will be a good thing. However, in such a method, the reduction ratio of the entire drive train of the vehicle changes every time the drive torque distribution ratio γ is changed. Therefore, the reduction ratio of the entire drive train is changed by the following method. Therefore, the target drive torque distribution ratio γ is achieved. That is, the center differential 4 establishes the following relationship. Nt = (NinF+N1nR)/2= ((Nou
tF/eF) + (Nout R/eR) l/2-(13) Now, the rotation speeds of the front and rear va drive shafts are equal (NoutF=No
ut R=Nout ), the following relationship holds true. Nt = ((1/eF)+(1/eR)1xN
out/2 = (eF+'eR)Nout/(2eF eR)
(14) Therefore, the speed ratio of the combined portion of the two continuously variable transmission mechanisms 5.6 is expressed by the following equation. Nout /Nt = 2 eF eR/ (eF+ e
R)・・・・・・・・・(15) In order to change the drive torque distribution ratio γ, the speed ratios eF, e
When R is changed, Nout/N of this equation (15)
If the value of t is not kept constant, it is inconvenient because the reduction ratio of the entire drive train of the vehicle will change.Therefore, the speed ratio Nou of the entire continuously variable transmission 1a structure is changed.
If t/Nt is a constant value C, then the speed ratio eF,
eR is regulated by the following equation. C=2 eF eR/ (eF+ eR)=-(16)
Equations (9) and (1o) are the speed ratios eF and eR obtained by simultaneously using equations (12) and (16). Therefore, when the target drive torque distribution ratio γC is given, the target speed ratio eF x,
If eR' is determined and controlled to this value,
The target drive torque distribution ratio can be achieved without changing the reduction ratio of the entire drive train. Next, in step S12, the difference (f
fla difference) is the control value V of the flow control valve 27F.
o F. VoF=K(eFX eF)/eF- (17) In response to this control @V o F, the flow rate control valve 27F functions to make the speed ratio eF match eFx. Next, in step S13, similarly to step S12, the control value V o of the flow rate control valve 27H of the rear wheel side continuously variable transmission mechanism 6 is determined.
R is determined by the following formula. Vo R=K (eRx-eR)/eR-・-(18
) In step S14, the control values V and F of the pressure control valve 26F of the front wheel side continuously variable transmission a5 are given as a function of the output shaft torque T of the transmission 2 and the speed ratio eF, as shown in the following equation. . V1F= f4 (Tt, eF) ++ (19
) This is a formula that provides the oil pressure in the output shaft cylinder 20 necessary to transmit the transmission torque divided into two by the center differential 4 at the speed ratio eF without the belt slipping. In step S15, similarly to step S14, the rear wheel side continuously variable transmission I! The control value VfR of the pressure control valve 26H in the treetop 6 is determined by the following equation. VIR=f5(Tt, eR)−(20
) Finally, step S16 is executed, and each control value Vo
F, Vo R, VlF, and VlR are output. The operation of the first implementation row will be explained below. Now, suppose that the engine 1 generates a large torque on a snowy road when the drive torque distribution ratio is not appropriate. At this time, the wheel whose distribution rate is larger than the Et-friendly distribution rate causes a larger slip than the other wheel, and the traction of the vehicle becomes smaller than the value originally obtained. Therefore, the torque distribution ratio is higher than necessary for the side with larger slip, and the larger the rotation difference ratio Z between the front and rear wheel drive shafts, the larger the deviation from the optimum value. The torque distribution ratio is controlled so that the drive torque moves toward the smaller one according to the rotational difference. Therefore, even if the loading status of the vehicle or road surface conditions change, feedback is applied to eliminate the rotation difference between the front and rear drive shafts, and the torque distribution ratio is controlled to always generate maximum traction. . In the device of this first embodiment, means for detecting the rotation speed of the output shaft 9.10 is provided in advance in order to control the continuously variable transmission slides 5.6 on the front and rear wheels. Since the input signal required for the drive torque distribution ratio control method is the rotation speed of the front and rear 112 drive shafts, when the present invention is applied to the device shown in FIG. 1, there is no need to provide a new detection mechanism. The present invention can be applied relatively inexpensively. If the control device and control method of the first embodiment described above are used, feedback is always provided so that the drive torque distribution ratio is MfJi regardless of road surface conditions or loading conditions. Various variations of the method exist. First, a second example, which is a modification of the control method, will be described. As in the first embodiment, steps S8 and S in FIG.
When the feedback shown in 9 is performed, the rotational difference rate Z
= 0, that is, the rotation speed NoutR on the rear wheel side and the rotation speed NoutF on the front wheel side are controlled to be equal. this is,
This is based on the idea that the greatest traction can be generated when the difference in rotation speed between the front and rear drive shafts is zero. however,
In reality, driving performance may be improved if one of the wheels slides a little more. In this case, the following equation (7') according to the second embodiment may be used instead of the equation (7) in step S9. Z= (Nout R-Nout F ) /Nt −
ε・・・・・・・・・(7') Here, ε is the target rotational difference rate, and when the feedback according to the second embodiment is performed, the rear wheel WR slips by the amount of this rotational difference rate ε. You will have to drive while doing so. Next, a third embodiment, which is also a modification of the control method, will be described. In the first embodiment, the transmission output shaft rotation speed Nt, which is the denominator of equation (7) in step S8, is such that the rotation difference between the front and rear wheel drive shafts to be controlled depends on the output shaft rotation speed of the transmission 2, in other words, the vehicle speed. It is used to make it dimensionless so that it is not significantly affected. However, controllability may be better if the vehicle speed is intentionally influenced, rather than performing control completely unrelated to the vehicle speed. Therefore, in such a case, step S
9, the feedback coefficients Kp, Ti, in equation (8),
Td is not set to a constant value, but is set to Nt as in the following third embodiment.
It can be a function of Kp = f(Nt) ・・・・・・・・・
(21) Ti = f(Nt)...
...(22) Td = f(Nt) ・
(23) Next, a fourth embodiment, which is also a modification of the control method, will be described. Equation (8) in step S9 of the first embodiment is
It has a form in which feedbacks are added to the briprograms γP obtained by equation (6). That is, this is a method in which a rough target drive torque distribution ratio γP is determined using pre-programs, and this is corrected using a feedback term. It is desirable to use such flash programs in order to improve control responsiveness. However, in such a system, it is necessary to provide a sensor (acceleration sensor 34 in the first embodiment) for detecting the running state.Feedback on the rotation speed of the front and rear wheel drive shafts in the present invention includes pre-programming. However, as in the fourth embodiment below,
A method that does not use this is also fully possible. In this case, the acceleration sensor 34 in FIG. 2 is unnecessary, and step S7 in FIG. 3 is also unnecessary. Also, step $9 (8
) instead of, for example, the following equation (8') or (8°°)
The formula can be used. 7C=1+K11(Z+ (/Zdt)/Ti+Td
(dZ/dt> 1 ・・・・・・・・・(8'
) γ = A'Kg...
...(8") However, A > 1. If the drive torque distribution ratio is controlled only by feedback, as in this fourth embodiment, the This eliminates the need for the acceleration sensor 34, making it possible to introduce the present invention at a relatively low cost.Next, a modification of the control device will be described.The flowchart in FIG. 3 of the first embodiment is similar to that in FIG. Although the transmission 2 is created as a gear-meshing multi-stage transmission with a mechanical clutch, the type of the transmission 2 is not limited to this, and may be an automatic transmission including a fluid coupling, a torque converter, etc. In this case, an engine rotation speed sensor is required to determine the output shaft torque Tt of the transmission 2, or step S4
Although it is necessary to change fl(s) in equation (3) to f('S, Ne. Nt), the control method is basically the same. In addition, in the first embodiment, as shown in Fig. 1, the power train consists of engine 1 + transmission 2 + continuously variable).
Although the configuration was that of the Xr machine vi5.6, it is also possible to omit the transmission and use only the clutch 41, as in the fifth embodiment shown in FIG. In this case, 2
The continuously variable transmission mechanisms 5 and 6 determine the speed ratio of the vehicle. The control method in this case is as described in Japanese Patent Application No. 61-186495, but the method of determining the drive torque distribution ratio by feedback control of the front and rear drive shaft rotation speeds according to the present invention can be applied here as well. That is, in the configuration shown in FIG. 4, the target speed ratio eF',
eR' is determined, and at this time, the target drive torque distribution ratio may be determined by feedback as in the first embodiment. Note that in both the first and fifth embodiments, the continuously variable transmission mechanism 5.6 is used to change the drive torque distribution ratio between the front and rear wheels, but other methods are also possible. . FIG. 5 shows the engine 1, transmission 2, transfer 51, and wet multi-disc clutches 52 on the front and rear wheels.
．． 53 and differentials 11 and 12 form a power train according to a sixth embodiment of the present invention. In FIG. 5, 54 and 55 are rotational speed sensors that detect the rotational speed of the front wheel drive shaft and the rear wheel drive shaft, and 56 is a propeller shaft on the front wheel side. The wet multi-disc clutches 52, 53 have a hydraulic cylinder piston (not shown), and a friction material is pressed by the hydraulic pressure applied to the cylinder. Therefore, depending on the applied oil pressure,
The maximum transmission torque (torque capacity) of the clutch can be determined. With such a configuration, the torque distribution ratio can be set to an arbitrary value by changing the applied oil pressure. In this case, it is more efficient to change the transmission torque for several people on the side where the distribution ratio is smaller compared to A0, since there is less loss in the friction material. The control flow of this sixth embodiment is shown in FIG. Step S21 is the applied hydraulic pressure PF of the front wheel side clutch 52.
The two clutches 52 and 53, which is a step for determining the magnitude of the hydraulic pressure PR applied to the rear wheel side clutch 53, have the same specification, and if the applied hydraulic pressure is the same, they have the same torque capacity. Therefore, step S21 can also be regarded as a step for determining the magnitude of the torque distribution ratio between the front and rear wheels. If it is determined in step S21 that PF≧PR, the process proceeds to step S22. In step S22, in order to apply hydraulic pressure to the side with a larger torque distribution ratio, that is, the M wheel side clutch 52, to prevent the clutch 52 from slipping. The hydraulic pressure PF applied to the front wheels is determined based on the transmission output shaft torque T by the following equation. PF= f(Tt) River (24
) In the next step 323, the oil pressure of the rear wheel clutch 52 is determined by the following equation. PR=αPF+K (NF-NR)/Nt...
...(25) Here, α is a constant smaller than 1, NF is the rotation speed of the front wheel drive shaft (propeller shaft 56), and NR is the rear WR drive shaft (
The rotation speed of the propeller shaft 13), N is the transmission output shaft rotation speed, and K is the proportional gain. This (2
According to equation 5), a hydraulic pressure smaller than the front wheel applied hydraulic pressure PF is set in the αPF term, and feedback is added to this to determine the rear wheel applied hydraulic pressure PR. Here, if a large slip occurs in the front wheel WF, NF>
NR, the second term of equation (25) takes a positive value, and the hydraulic pressure PR applied to the rear wheels increases in accordance with the rotational difference. That is, the torque capacity of the rear wheel side clutch 53 increases. Therefore, the torque distribution rate on the rear wheel side increases, the torque distribution rate on the front wheel side relatively decreases, and the slip on the front wheel side becomes small. Conversely, if a larger slip occurs in the rear wheel WR, the NF
<NR, so the second term of equation (25) becomes a negative value, and the hydraulic pressure PR applied to the rear wheels decreases in accordance with the rotational difference. That is, the torque capacity of the rear wheel side clutch 53 decreases. Therefore, the torque distribution rate on the rear wheel side decreases, the torque distribution rate on the front wheel side relatively increases, and the slip on the rear wheel side becomes small. On the other hand, if it is determined in step S21 that PF<PR, the process proceeds to step S24. In step S24, the rear wheel side applied hydraulic pressure PR is changed to It is determined based on the transmission output shaft torque Tt according to the formula. PR= f(Tt) ・・・・・・・・・(
26) In the next step S25, the hydraulic pressure PF applied to the front wheel clutch 52 is determined by the following equation. PP=αPR+K (NR-NF)/Nt...
...(27) The function of this step S24.325 is also the function of step S22.
, is the same as S23, and if a larger slip occurs in the rear wheel WR, the torque distribution ratio of the front wheel WF is increased, and the front wheel WF is
If a larger slip occurs, the torque distribution ratio of the front VaWF is lowered and feedback is applied to eliminate the rotational difference between the two wheels. Here, in step S21, it is determined which torque distribution ratio is larger, and in the case where the torque distribution ratio is larger, the clutch is fully engaged, and on the other hand, the clutch torque capacity is adjusted only by one clutch. This is because the aim is to minimize powertrain loss by causing slippage. After passing through step 323 or S25, step S2
In this step S26, proceeding to 6, the control value VF is determined by the following equation, so that the pressure regulation value of the electromagnetic pressure control valve (not shown) that regulates the pressure applied to the clutch 52 is the applied pressure PF. VF= f(PF) ・・・・・・・・・(2
8) In the next step S27, an electromagnetic pressure ml (also not shown) is applied to adjust the hydraulic pressure applied to the rear wheel clutch 3.
In order to set the pressure regulation value of the control valve to PR, the control value VR is determined by the following equation. VR= f(PR) ・・・・・・・・・(2
9) Here, both equations (28) and (two) express the relationship measured in advance between the control value and the pressure regulation value of the electromagnetic pressure control valve. After step S27, in step S28, the control value VF,
VR is output, and then the process returns to step S21 again. In this way, in the sixth embodiment, although feedback is not directly applied to the drive torque distribution ratio, the clutch 52.
By using the hydraulic pressure applied to 53 as the amount of operation and controlling the transmitted torque so as to eliminate the difference in rotation between the front and rear wheels, it is possible to obtain the same optimum drive torque distribution ratio as in the first embodiment. Furthermore, if it is known that the torque distribution between the front and rear wheels will always be large due to the structure of the vehicle, it is possible to suffice with only one clutch that can vary the torque capacity. If the shared load on the side is sufficiently large and the torque distribution ratio of the rear wheels is always greater than the torque distribution ratio of the front wheels in normal driving, even if the drive torque distribution ratio is controlled to the optimum drive torque distribution ratio, then the clutch shown in Fig. 5 53 can be omitted and directly connected. The control at this time can be expressed only by equation (27), and in FIG.
22, S23, S24, and S27 become unnecessary. Although various modifications have been described above, the present invention is basically applicable to any 4WD vehicle that can change the drive torque distribution ratio between the front and rear wheels. [Effects of the Invention] As explained above, according to the present invention, unnecessary slip of the wheels is reduced and the traction of the vehicle is increased. Therefore, the road condition and 8ti! ! This has the excellent effect of always realizing an optimum drive torque distribution ratio according to the driving state of the vehicle, regardless of the conditions.

[Brief explanation of the drawing]

FIG. 1 is a schematic plan view showing a power train system of a four-wheel drive vehicle in which a first embodiment of the control method for a four-wheel drive vehicle according to the present invention is adopted, and FIG. Skeleton diagram showing the control system of two continuously variable 41.1s, Figure 3 (A
), (B), and (C) are flowcharts showing the control flow used in the first embodiment, and FIG. 4 is a power train of a four-wheel drive vehicle in which the second embodiment of the present invention is adopted. FIG. 5 is a schematic plan view showing the power train system of a four-wheel drive vehicle in which the sixth embodiment is also adopted.
The figure is a flowchart showing the control flow used in the sixth embodiment. 1... Engine, 4... Center differential, 5.6... Continuously variable transmission mechanism, 9.10... Output shaft (drive shaft), 13.56... Propeller shaft, WF... Front wheel, WR... Rear wheel, 28... Controller, 32.33.54.55... Rotation speed sensor, Nout
R, Nout F-... Output shaft rotation speed, 34... Acceleration sensor, G... Vehicle acceleration, γP, γC... Target drive torque distribution ratio, ε... Target rotation difference rate, 52.53 ...Wet type multi-plate clutch.

Claims (3)

[Claims] (1) A four-wheel drive vehicle having a drive torque distribution ratio control means that allows the drive torque distribution ratio between the front and rear wheels to be arbitrarily set by changing either or both of the front wheel drive torque and the rear wheel drive torque. In the control method, a rotation difference between a front wheel drive shaft and a rear wheel drive shaft is detected, and the drive torque distribution ratio control means is feedback-controlled so that the rotation difference becomes a target value, so that the drive torque between the front wheels and the rear wheels is adjusted. A control method for a four-wheel drive vehicle characterized by setting the distribution ratio to the fastest value. (2) A method for controlling a four-wheel drive vehicle according to claim 1, wherein the target value of the rotational difference is zero. (3) When performing feedback control of the drive torque distribution ratio control means, the target drive torque distribution ratio determined in advance from the running state of the vehicle is corrected by a feedback term including a rotation difference. A method for controlling a four-wheel drive vehicle as described in Section 1.