Abstract:
An antilock brake method and device for a four wheel drive vehicle having front and rear axles, left and right wheels on each axle with brakes associated with each wheel, a power unit drivingly connected to one of the axles, the other axle being coupled for drive together with the driven axle, a hydraulic braking system for controlling the hydraulic pressure applied to the brakes, and an antilock control system coupled to the braking system for controlling the hydraulic braking pressure to reduce the brake pressure when a wheel is about to be locked. The antilock control system includes a front wheel control section for controlling the brakes of the front wheels, a rear wheel control section for controlling the brakes of the rear wheels, and circuit elements included in the front and rear wheel control sections for keeping the hyraulic braking pressure reduced up to the completion of a braking operation, when any of the wheels is about to be locked. In a particular embodiment the rear wheel control section reduces the braking pressure in the rear wheel brakes and maintains the pressure reduced during a braking operation when any of the wheels is about to be locked.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a four wheel drive vehicle with an antilock brake device, in which a power unit is connected to either one of the front and rear axles, with the other axle being connected to said one axle in intercoupled associated relation such that the drive of the axles interferes with each other, and an antilock control unit for controlling the braking hydraulic pressure to reduce the pressure when a wheel is about to be locked, said antilock control unit being associated with a braking hydraulic pressure system for controlling the hydraulic pressure of the brakes respectively mounted on each axle. 
     The invention further relates to a method of operation of the antilock brake device. 
     Incidentally, it should be noted here that throughout the specification when it is described that the axles interfere with each other, this means that the axles are in four-wheel drive state having a substantially rigid interconnection therebetween and the application of the brakes on the wheels of one axle will have effect on the drive of the wheels of the other axle. 
     DESCRIPTION OF THE PRIOR ART 
     In the conventional four wheel drive vehicle improvements are sought in the operating and travel performance on a road surface having a low friction coefficient, and attempts have been made to employ an antilock brake device on the four wheel drive vehicle. 
     However, if an antilock control brake device as employed in the conventional two wheel drive vehicle is used in a four wheel drive vehicle, a disadvantage is produced in that, in the four wheel drive vehicle, the front and rear wheels interfere with each other and consequently, a satisfactory antilock effect cannot be obtained. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to overcome the above disadvantage of providing a four wheel drive vehicle with an antilock brake device in which the antilock control of the front and rear wheels is achieved efficiently and effectively. 
     According to the present invention, an antilock control device is provided which includes a front wheel control section for controlling the brakes for the front wheels and a rear wheel control section for controlling the brakes for the rear wheels, and the front and rear wheel control sections are of such a contruction that the braking hydraulic pressure of the rear brake is maintained at reduced pressure up to the completion of the braking operation, when any of the wheels is about to be locked. 
     During antilock control, the hydraulic braking pressure of the rear brake is reduced and therefore, the antilock control of the rear wheels can be insured by effecting the antilock control of the front wheels, thus providing a satisfactory antilock effect. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES OF THE DRAWING 
     FIG. 1 is a schematic diagram of a four wheel drive system. 
     FIG. 2 is a block diagram of a brake system for the drive system in FIG. 1. 
     FIG. 3 is a schematic circuit diagram of a front wheel control section of the brake system in FIG. 2. 
     FIG. 4 is a schematic circuit diagram of a rear wheel control section of the brake system in FIG. 2 
     FIG. 5 is a schematic circuit diagram of a rear wheel control section according to a second embodiment of the invention. 
     FIG. 5A is a diagram illustrating the characteristic of a delay circuit in the circuit of FIG. 5. 
     FIG. 6 is a schematic circuit diagram of a rear wheel control section according to a third embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The present invention will now be described with reference to the embodiments illustrated in the accompanying drawing. Referring first to FIG. 1 illustrating a first embodiment of the present invention, a pair ofleft and right front wheels Wfl and Wfr and a pair of left and right rear wheels Wrl and Wrr are suspended respectively at the front and rear portions of a body of a vehicle (not shown). 
     A pair of front axles Afl and Afr, which are connected respectively to the left and right front wheels Wfl and Wfr, are interconnected through a front differential Df, and a pair of rear axles Arl and Arr, which are connected respectively to the left and right rear wheels Wrl and Wrr, are interconnected a rear differential Dr. The input of the front differentialDf is connected to the power unit P. The input of the rear differential Dr is connected to a rear propeller shaft Pr which is coaxially connected to a front prepeller shaft Pf through a viscous clutch 1 serving as a torque transmitting mechanism, and the drive force from the power unit P is transmitted to the front propeller shaft Pf. 
     The viscous clutch 1 includes a closed oil chamber 4 containing inner and outer clutch elements 2 and 3 which are rotatable relative to each other. A highly viscous oil with a small amount of air to permit the thermal expansion of the highly viscous oil is sealed in the closed oil chamber 4.The viscous clutch 1 also includes a plurality of outer clutch plates 5 spline-connected to the outer clutch element 2 and a plurality of inner clutch plates 6 spline-connected to the inner clutch element 3, the inner and outer clutch plates being interleaved with each other. Each of the plates 5 and 6 is provided with an opening or groove (not shown) which permits the passage of the oil. The outer clutch element 2 is integral with the front propeller shaft Pf, and the inner clutch element 3 is integral with the rear propeller shaft Pr. 
     In the viscous clutch 1, when relative rotation occurs between the outer clutch element 2 and the inner clutch element 3, both the clutch plates 5 and 6 rotate relative to one another while shearing the highly viscous oil, so that the viscous transmission of torque is obtained between the clutch plates 5 and 6. When the speed of the relative rotation is further increased, a complex temperature gradient is produced in both clutch plates 5 and 6 due to the increase in oil temperature, causing a synergistic effect of a strain attributable to this temperature gradient with an increase in pressure within the closed oil chamber 4 to provide a frictionally engaged portion or a portion having an extremely small gap between the adjacent clutch plates 5 and 6. As a result, the frictional transmission of torque is insured between the outer clutch element 2 and the inner clutch element 3. 
     With such a viscous clutch 1, the front propeller shaft Pf and the rear propeller shaft Pr and thus, the front axles Afl, Afr and the rear axles Arl, Arr are always in a substantially rigidly interconnected state, so that the front wheels Wfl, Wfr and the rear wheels Wrl, Wrr interfere witheach other. 
     Brakes Bfl, Bfr are respectively mounted on the front wheels Wfl, Wfr, and brakes Brl, Brr are respectively mounted on the rear wheels Wfl, Wrr. 
     Referring to FIG. 2, a braking hydraulic pressure system 7 for controlling the hydraulic pressure of each of the brakes Bfl, Bfr, Brl and Brr comprises a tandem type master cylinder 8 having a pair of output ports 8aand 8b, modulators Mfl and Mrr for regulating the hydraulic pressure supplied from the output port 8a to transmit the pressure to the left front wheel brake Bfl and the right rear wheel brake Brr, and modulators Mfr and Mrl for regulating the hydraulic pressure supplied from the outputport Stb to transmit the pressure to the right front wheel brake Bfr and the left rear wheel brake Wrl. The braking hydraulic pressure system 7 is associated with an antilock control device 9 for controlling the operationof the modulators Mfl, Mfr, Mrl and Mrr to prevent the wheels from going into a locked state. 
     The antilock control device 9 includes a front wheel control section 9a forindividually controlling the modulators Mfl and Mfr of the front wheels Wfland Wfr, and a rear wheel control section 9b for simultaneously controllingthe modulators Mrl and Mrr of the rear wheels Wrl and Wrr, so that signals from detectors 10l and 10r for detecting the speeds of the front wheels Wfl and Wfr are supplied to the front control section 9a, and signals fromdetectors 11l, and 11r for detecting the speeds of the rear wheels Wrl and Wrr are supplied to the rear control section 9b. 
     Reference is next made to FIG. 3 for the description of the arrangement of the front wheel control section 9a, wherein because the portion corresponding to one of the modulators Mfl is basically of the same construction as the portion corresponding to the other modulator Mfr, the parts associated with modulator Mfl are designated by addition of reference characters l and they will be explained hereinafter. Meanwhile, the parts associated with the modulator Mfr are designated by addition of reference characters r and their description will be omitted. 
     To determine whether the wheel is ready to go into the locked state, a signal representing wheel speed Vw detected in the detector 10l is fed to the inverse terminal of a first comparator 13l and to an operator circuit 12l in which a signal representing wheel acceleration Vw is produced and fed to the inverse terminal of a second comparator 14l and the non-inverseterminal of a third comparator 15l, respectively. In the first comparator 13l, the wheel speed Vw is compared with a reference wheel speed Vr storedin the non-inverse terminal thereof and when Vr&gt;Vw, a signal λ for moderating the hydraulic braking pressure is delivered from the first comparator 13l. In the second comparator 14l, the wheel acceleration Vw iscompared with a reference wheel deceleration -Vw O  stored in the non-inverse terminal and when -Vw O  &gt;Vw, a signal β for moderating the hydraulic pressure is delivered from the second comparator 14l. Further, in the third comparator 15l, the wheel acceleration Vw is compared with a reference wheel acceleration +Vw O  stored in the inverse terminal and when Vw&gt;+Vw O , a signal α is delivered fromthe third comparator 15l. The signal α serves to check whether the wheel speed Vw is increasing, so that the period of time for which the moderation of the hydraulic braking presure is continued is determined by signal α. 
     The output terminal of the first comparator 13l is connected to the input terminal of an AND gate 16l and to the input terminal of an OR gate 17l. The output terminal of the second comparator 14l is connected to the inputterminals of AND gate 16l and OR gate 17l. Further, the output terminal of the third comparator 15l is connected to the input terminal of OR gate 17l. 
     The output terminal of the AND gate 16l is invertedly connected to the input terminals of AND gates 18l and 19l and to an output terminal 20l. The output terminal of the OR gate 17l is connected to the input terminal of the AND gate 18l whose output terminal is connected to an output terminal 22l and invertendly connected to the input terminal of the AND gate 19l. Further, the output terminal of the AND gate 19l is connected toan output terminal 21l. 
     In the front control section 9a, a signal for reducing the braking pressureis delivered from the output terminals 20l and 20r, and a signal for increasing the braking pressure is delivered from the output terminals 21land 21r, and further, a signal for maintaining the braking pressure constant is delivered from the output terminals 22l and 22r. The modulators Mfl is operated in response to the signals from the output terminals 20l, 21l and 22l, and the modulator Mfr is operated in response to the signals from the output terminals 20r, 21r, and 22r, whereby the antilock control operations are individually conducted for both the brakesBfl and Bfr. 
     Description will next be made of the arrangement of the rear wheel control section 9b with reference to FIG. 4. The arrangement of the rear wheel control section 9b is similar to that of the front wheel control section 9a and hence, the parts corresponding to those of the front wheel control section 9a are designated by the same reference characters with no letter l or r affixed thereto. 
     Attention is directed in the rear wheel control section 9b to the arrangement in which the signals representing the wheel speeds detected inthe detectors 11l and 11r are fed to a low speed selector circuit 23 and the lower wheel speed selected in the low speed selector circuit 23 is fedto the first comparator 13 and the operator circuit 12. More specifically, the antilock control is conducted in coordination to that one of the left and right rear wheels Wrl and Wrr which is more easily locked, i.e., the wheel having the lower speed, and the activation of both modulators Mrl and Mrr are simultaneously controlled through control signals derived fromthe output terminals 20, 21 and 22. 
     Moreover, in the rear wheel control section 9b, a flip-flop 24 is interposed between the AND gate 16 and the AND gates 18 and 19 as well as the output terminal 20. More particularly, the output terminal of the AND gate 16 is connected to a set input terminal S of the flip-flop 24 whose set output terminal Q is connected to the output terminal 20 and invertedly connected to the AND gates 18 and 19. In addition, a braking operation detector 26, which delivers a high level signal upon the detection of the braking operation by a brake pedal 25 (see FIG. 2), is invertedly connected to a reset input terminal R of the flip-flop 24. 
     With such arrangement of the rear wheel control section 9b, when it is detected that one of the rear wheels Wrl and Wrr is about to be locked by generation of high level signals λ and β from the first and second comparators 13 and 14, so that the output of the AND gate 16 is at a high level, the set output of the flip-flop 24 is at a high level until the output of the braking operation detector 26 becomes a low level at thecompletion of the braking operation, i.e., until the set input of the flip-flop 24 becomes a low level. Consequently, when the rear wheels Wrl and Wrr are about to be locked, the output of the output terminal 20 is ata high level until the braking operation is completed, and the hydraulic braking pressure of the rear wheel brakes Brl and Brr is maintained at a decreased level. 
     The operation of this embodiment is as follows. When the rear wheels Wrl and Wrr are about to be locked upon braking during travel of the vehicle, the output of the flip-flop 24 goes into a high level state in accordance with the output of the AND gate 16 going into a high level state, so that the hydraulic braking pressure of both rear wheel brakes Brl and Brr is substantially reduced to the level of atmospheric pressure by the high level signal at the output terminai 20. Moreover, such condition is continued up to the completion of the braking operation. 
     For this duration of time, the rear wheels Wrl and Wrr are responsive to the rotation of the front wheels Wfl and Wfr, and the hydraulic braking pressure of the front wheel brakes Bfl and Bfr is controlled in the front wheel control section 9a, whereby the rotation of the rear wheels Wrl and Wrr can be controlled, thus providing a satisfactory antilock effect. 
     The above embodiment has been described as being applied to a four wheel drive vehicle in which the front axles Afl and Afr are connected with the rear axles Arl and Arr through the viscous clutch 1, but the present invention is also applicable to a four wheel drive vehicle of a part time type having axles Afl, Afr and Arl, Arr interconnected through a clutch adapted to be manually shifted between engaged and disengaged states, whenthe clutch is in the engaged state, and to a four wheel drive vehicle having axles Afl, Afr and Arl, Arr interconnected through a differential having a locking mechanism, when the differential is in a locked state. The following is the description of embodiments applied to such four wheeldrive vehicles. 
     FIG. 5 illustrates a rear wheel control section in accordance with a secondembodiment of the present invention, wherein the parts corresponding to those of the first embodiment are denoted by the same reference characters. 
     The output terminal of AND gate 16 is connected to one of the input terminals of OR gate 27 whose output terminal is connected to output terminal 20 and invertedly connected to the input terminals of AND gates 18 and 19. The output terminal of OR gate 17 is connected to the input terminal of OR gate 28 whose output terminal is connected to the input terminal of the AND gate 18. 
     In addition, the output terminal of tne AND gate 16 is also connected to the input terminal of an OR gate 29 having other input terminals to which are also connected output terminals 20l and 20r in the front wheel controisection 9a. The output terminal of the OR gate 29 is connected to one of the input terminals of an AND gate 30. A four wheel drive state detector 33 is connected to the other input terminal of the AND gate 30. The four wheel drive state detector 33 detects the engagement of a clutch or the locked state of a differential to produce a high level signal. Thus, the output of the AND gate 30 goes into a high level state, when the hydraulicbraking pressure is intended to be reduced upon slipping of either of the front wheels Wfl and Wfr or the rear wheels Wrl and Wrr. 
     The output terminal of the AND gate 30 is connected to a delay circuit 31. When a signal as shown in FIG. 5A (a) is fed to circuit 31 the output of delay circuit 31 increases in coordination with the increase of the input signal and falls after the lapse of a given time T from the fall of the input signal, as shown in FIG. 5A (b). 
     The output terminal of the delay circuit 31 is connected to an OR gate 27 and also to the set input terminal S of a flip-flop 32. A brake operation detector 26 is invertedly connected to the reset input terminal R of the flip-flop 32 whose set output terminal Q is connected to an OR gate 28. 
     In the operation of the second embodiment, when the rear wheels Wrl and Wrrare about to be locked during a braking operation, the output of the outputterminal 20 goes into a high level state, as the output of the AND gate 16 goes into a high level state. Even after the state of the rear wheels being about to be locked is released, the output of the OR gate 27 is at ahigh level state up to the lapse of the given time T and hence, the output of the output terminal 20 maintains a high level state during this time T.This causes the hydraulic braking pressure of the rear wheel brakes Brl andBrr to be significantly reduced. After the lapse of the aforesaid time T, the output of the output terminal 22 becomes a high level state as the output of the AND gate 18 goes into a high level state. Such state will bemaintained up to the completion of the braking operation. 
     Also when either of the front wheels Wfl and Wfr is about to be locked, theoutput of the OR gate 27 goes into a high level state in the same manner asdescribed above, and after the lapse of the given time T, the output of theAND gate 18 becomes a high level state. 
     Thus, when any of the wheels Wfl, Wfr, Wrl and Wrr is about to be locked, the braking hydraulic pressure of the rear wheel brakes Brl and Brr is reduced, and such state will be maintained up to the completion of the braking operation and consequently, a satisfactory antilock effect is provided as in the first embodiment. 
     FIG. 6 illustrates a third embodiment of the present invention, wherein theelements corresponding to those of the second embodiment are designated by the same reference characters as in the second embodiment. 
     In this third embodiment, the OR gate 28 and the delay circuit 31 in the second embodiment are omitted, and the output terminal of OR gate 17 is connected to the input terminal of AND gate 18. The output terminal of ANDgate 30 is connected to the set input terminal S of flip-flop 32 whose set output terminal Q is connected to the input terminal of OR gate 27. 
     In this third embodiment, if any of the wheels Wfl, Wfr, Wrl and Wrr is about to lock, an output from the OR gate 27, i.e., a signal derived from the output terminal 20 is at a high level state up to the completion of the braking operation, and the hydraulic braking pressure of the rear wheel brakes Brl and Brr continues to be reduced. 
     As seen from the above, according to the present invention, the hydraulic braking pressure in the rear wheel brakes remains reduced up to the completion of the braking operation, when any of the wheels is about to belocked. Therefore, travel stability can be maintained and an antilock control for the front wheels can be effected without and interference fromthe rear wheels, leading to an effective antilock control for all the wheels. 
     Although the invention has been described in relation to specific embodiments thereof, it will become apparent to those skilled in the art that numerous modifications and variations can be made without departing from the scope and spirit of the invention as defined in the attached claims.