Abstract:
A method for cyclically controlling the braking of a vehicle includes the following steps: (a) detecting a road surface frictional force of said vehicle, (b) detecting a brake fluid pressure, and (c) controlling the brake fluid pressure in response to detected values including the brake fluid pressure and the road surface frictional force, the controlling including decreasing the brake fluid pressure in response to the road surface frictional force declining during an increase of the brake fluid pressure and increasing the brake fluid pressure in response to the road surface frictional force declining during fall-off of the brake fluid pressure.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Technical Field  
         [0002]     This invention relates to an antilock braking device for precluding the locking of the wheels of a vehicle on sudden application of the brake and a road surface friction sensor and a road surface friction coefficient detector which can be used as components of said antilock braking device.  
         [0003]     2. Technical Background  
         [0004]     The conventional antilock braking device for cars or other vehicles generally employs a system such that the braking action is automatically controlled according to the chassis speed and wheel speed in such a manner that the slip ratio will fall within a definite range (See, for example, Japanese Patent Publication No. 30585/1984 and Japanese laid-open Patent Application KOKAI No. 61354/1985). The relationship between road surface friction coefficient and slip ratio is variable according to the texture of the road surface and, for this reason, the above system does not always provide the maximum braking force depending on the condition of the road surface and, in such cases, does not insure the minimum braking distance. Furthermore, because the chassis speed value used is an approximate value estimated from the wheel speed, the precision of slip ratio control is not sufficiently high. In order to ascertain the exact chassis speed, one has to rely on complicated devices such as the ground relative speed sensor (for example, Japanese laid-open Patent Application No. 64861/1988) or chassis deceleration sensor (for example, Japanese laid-open Patent Application No. 170157/1988).  
         [0005]     In the conventional antilock braking device described in Japanese laid-open Patent Application No. 25169/1988, the road surface friction torque acting on the wheel (tire torque) is calculated from the wheel angular acceleration and brake fluid pressure values and the beginning of a fall in tire torque during the elevation of brake fluid pressure is utilized as one of the criteria for ascertaining the condition immediately preceding a wheel lock. However, since the tire torque is indirectly calculated from the wheel angular acceleration and brake fluid pressure, the above system does not take care of indefinite constants such as the moment of inertia of the wheels, the braking efficiency of the brake and so on, thus presenting problems in terms of the accuracy of data. There also is the problem that since the distance between the wheel to the road surface varies according to the deceleration of the chassis depending on the pneumatic pressure of the tires and the weight of the chassis, the road surface friction force and the tire torque are not necessarily maintained in a fixed ratio.  
         [0006]     It is an object of this invention to provide an antilock braking device free from the above-mentioned disadvantages of the conventional device.  
         [0007]     It is another object to provide a road surface frictional force sensor and a road surface friction coefficient detector which can be used as components of said antilock braking device.  
       SUMMARY OF THE INVENTION  
       [0008]     A first antilock braking device according to this invention includes a brake control means adapted to cyclically perform an operational series which comprises sensing the road surface frictional force, increasing the brake fluid pressure while the road surface frictional force is increasing in response to the elevation of brake fluid pressure, decreasing the brake fluid pressure when the road surface frictional force declines despite elevation of the brake fluid pressure, and increasing the brake fluid pressure again when the road surface frictional force decreases in response to a fall-off of brake fluid pressure. The road surface frictional force can be known from measured values of the tire strain or the strain around the wheel of the vehicle.  
         [0009]     A second antilock braking device according to this invention includes a brake control means adapted to cyclically perform an operational series which comprises detecting the coefficient of road surface friction, increasing the brake fluid pressure while the road surface friction coefficient is increasing in response to the elevation of brake fluid pressure, relieving or releasing the brake fluid pressure as the velocity of gain in road surface friction coefficient falls below a set value and increasing the brake fluid pressure again after the road surface friction coefficient has declined below said set value. The road surface friction coefficient value used in this second antilock braking device can be calculated from the road surface frictional force value and the vertical load value obtainable from measured values of the tire strain or the strain around the wheel. The relationship between wheel-road surface slip ratio and road surface friction coefficient can be represented by curves such as shown in  FIG. 1 . On the ordinary road surface, this relation can be expressed by a curve having a peak as shown at C 1 . On an extraordinary road surface, such as a snow-clad road surface, the relation may be represented by a curve without a peak as shown at C 2 . Not only the presence or absence of a peak but also the height of the peak and the magnitude of the slip ratio corresponding to the peak vary with the condition of the road surface and the chassis speed. On the other hand, as represented by curve C 3 , the cornering force (lateral drag) decreases monotonously in response to an increase in slip ratio. Therefore, as far as trackless vehicles such as automobiles are concerned, in order to obtain the maximum braking force without sacrificing the cornering force, it is ideal to apply the brake in the neighborhood of P 1  or P 2  on curve C 1  or C 2  as the case may be.  
         [0010]     Let it be supposed that the vehicle is running on a road surface such that the relation between road surface friction coefficient and slip ratio can be represented by the curve C 1  shown in  FIG. 1 . It should be understood that the road surface friction force is approximately proportional to the road surface friction coefficient. Under these conditions, the first anti-lock braking device according to this invention functions as follows. First, as a sudden brake is applied by depressing the brake pedal or manipulating the brake lever, the brake fluid pressure increases. While the detected road surface frictional force value continues to rise, the brake fluid pressure is increased consistently to apply the brake with an increasing force. This phase corresponds to the segment to the left of P 1  on the curve C 1  shown in  FIG. 1 . As the brake fluid pressure is increased to apply the brake more forcefully, the slip ratio increases to approach to the point P 1  of maximum road surface friction coefficient. As the brake fluid pressure is further increased, the point P 1  is passed over in due course. Beyond P 1 , the locking of the wheels begins to occur as the road surface frictional force turns to decline against the elevation of brake fluid pressure. When the road surface friction sensor output decreases in this manner, the brake fluid pressure is decreased to releave the brake action. Therefore, the locking of the wheels is prevented. As the road surface frictional force decreases in response to a decline in brake fluid pressure, the brake fluid pressure is increased again. As the result of this action, as long as the vehicle runs on the road surface which can be represented by a curve with a peak in regard to the road surface friction coefficient-slip ratio relation, the locking of the wheels can be prevented irrespective of road condition and, moreover, the braking action making the most of road surface frictional force can be realized.  
         [0011]     The frictional force which acts between each wheel of the vehicle and the road surface is dynamically equivalent to the braking force applied by the wheel on the chassis. Therefore, strains and stresses proportional to the road surface frictional force are generated in all given positions of the structure between the point of contact of the wheel with the road surface and the chassis. Therefore, it is possible for one to measure the structural strain at an appropriate point of the structure and detect the road surface frictional force through the strain value. The member of the structure in which the maximum strain is generated is the tire in case the vehicle has tires in its wheels. Therefore, the road surface frictional force can be detected from measured values of the tire strain. It is also possible to affix strain gauges to the bearing shaft supporting the wheel, for instance, and measure the strain around the wheel. This strain is smaller than the tire strain but since said shaft is not a rotary element, the construction of the road surface friction sensor can be simplified.  
         [0012]     The vertical drag exerted by the road surface on each wheel, or the vertical load which the wheel applies to the road surface as a reaction thereto, can, for the same reason as above, also be detected from a measured value of the tire strain or the strain around the wheel.  
         [0013]     The second antilock braking device according to this invention functions as follows. As the motorist depresses the brake pedal or manipulates the brake lever with a great force, the antilock braking device is started. In the segment to the left of P 1  or P 2  on curve C 1  or C 2 , the road surface friction coefficient p increases in response to an elevation of brake fluid pressure. However, when the velocity of gain in μ falls off below a predetermined reference level (slightly to the left of the point P 1  or at the point P 2 ), the brake fluid pressure is releaved or released, whereupon the value of μ turns to diminish. After a decline corresponding to a given proportion of the maximum value immediately preceding the beginning of decrease of the road surface friction coefficient μ, the brake fluid pressure begins to rise again. Thereafter, the above sequence of events is repeated. In this manner, not only when the vehicle is running on a road surface such that the relation between road surface friction coefficient and slip ratio traces the aforementioned curve C 1  but also when the road surface can be represented by curve C 2  without a peak, the road surface friction coefficient at application of the brake is maintained in the neighborhood of P 1  and P 2 , thus insuring a more or less ideal braking action. For vehicles (rolling stock, etc,) which run on tracks, in which no cornering force is required, said predetermined reference value for the velocity of gain in μ is set at zero or an appropriate negative value. Then, the braking action utilizing the maximum road surface friction force can be insured. The road surface friction coefficient value to be used in this second antilock braking device can be found by computation from the above-mentioned road surface frictional force value and the vertical load value obtainable through tire strain data or the data of strain around the wheel.  
         [0014]     Thus, according to the device of this invention, the braking distance can be minimized irrespective of the condition of the road surface and, at the same time, the object of an antilock braking effect can be accomplished. Furthermore, the device does not require a complicated setup for measuring the chassis speed. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]      FIG. 1  is a diagrammatic representation of the relationship among slip ratio, road surface friction coefficient and cornering force;  
         [0016]      FIG. 2  is a block diagram of the antilock braking device according to an embodiment of this invention;  
         [0017]      FIG. 3  is a flow chart showing the execution of the program routine in the control means built into the antiblock braking device illustrated in  FIG. 2 ;  
         [0018]      FIG. 4  is a block diagram of the antilock braking device according to another embodiment of this invention;  
         [0019]      FIG. 5  is a flow chart showing the execution of the main routine in the control means built into the antilock braking device illustrated in  FIG. 4 ;  
         [0020]      FIG. 6  is a flow chart showing the brake fluid depression routine of  FIG. 5  in detail;  
         [0021]      FIG. 7  is a flow chart showing the brake fluid recompression routine of  FIG. 5  in detail;  
         [0022]      FIG. 8  is a flow chart showing an interruption of the main routine shown in  FIG. 5 ;  
         [0023]      FIG. 9  is a front view showing the position of installation of strain gauges constituting the road surface friction coefficient detector embodying the principle of this invention for one rear wheel in the vicinity of the rear wheel;  
         [0024]      FIG. 10  is a front view, on exaggerated scale, of the position of installation of the strain gauges illustrated in  FIG. 9 ;  
         [0025]      FIG. 11  is a plan view, on exaggerated scale, of the,position of installation of the strain gauges illustrated in  FIG. 10 ;  
         [0026]      FIG. 12  is a block diagram of the road surface friction coefficient detector embodying the principle of this invention as applicable to one rear wheel;  
         [0027]      FIG. 13  is a front view showing another example of the installation of strain gauges in the vicinity of the rear wheel of a vehicle;  
         [0028]      FIG. 14  is a front view showing the position of installation of strain gauges constituting the road surface friction coefficient detector embodying the principle of this invention for one front wheel in the vicinity of the front wheel;  
         [0029]      FIG. 15  is a perspective view showing, on exaggerated scale, the position of installation of the strain gauges illustrated in  FIG. 14 ;  
         [0030]      FIG. 16  is a similar perspective view showing, on exaggerated scale, the position of installation of the strain gauges illustrated in  FIG. 14 ;  
         [0031]      FIG. 17  is a block diagram of the road surface friction coefficient detector embodying the principle of this invention as applicable to one front wheel;  
         [0032]      FIG. 18  is a front view showing a further example of the installation of strain gauges in the vicinity of the front wheel of a vehicle;  
         [0033]      FIG. 19  is a perspective view showing, on exaggerated scale, the position of installation of the strain gauges illustrated in  FIG. 18 ; and  
         [0034]      FIG. 20  is a schematic side-elevation view showing the displacement of the tire on application of the brake. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0035]     In the following description of embodiments, an automobile will be taken as an example. However, it should be understood that the present invention is applicable to other types of vehicles as well.  
         [0036]      FIG. 2  and  3  illustrate an antilock braking device embodying the principle of this invention.  
         [0037]     The control means  3  controls the pressure of brake fluid with reference to measured values of brake pedal depressing force, road surface frictional force and brake fluid pressure. A brake pedal depressing force sensor  2 , brake fluid pressure generator  4 , brake means  5  and brake fluid pressure sensor  6  may each be implemented by utilizing the known technologies. The control means  3  comprises an electronic circuit including a microprocessor, a memory and an input-output interface and functions according to program written into the memory. An example of the action of this control means  3  is illustrated in the flow chart of  FIG. 3 . When the brake pedal-depressing force exceeds a set value, the antilock braking device of this invention is started to make a transition from the ordinary braking action to the antilock braking action. Referring to the flow chart of  FIG. 3 , step  10  represents the beginning of the antilock braking action. Subsequently at step  12 , the road surface frictional force F is detected, and at step  14 , the above value is stored in a variable labeled with F t-1 . Then, at step  16 , the brake fluid pressure is increased. Further at step  18  the road surface frictional force F is detected, and at step  20  this road surface frictional force value is stored in a variable F t . Then, at step  22 , it is judged whether the difference between the two variable F t  and F t-1 , viz. F t −F t-1 , is positive or negative. If it is positive, the stored value of variable F t-1  is updated to the value of variable F t  at step  24 . Then, the sequence returns to step  16 . If the judgement at step  22  is not positive, the sequence proceeds to step  26 . At step  26 , just as at step  24 , the stored value of variable F t-1  is updated to the value of variable F t . Then, at step  28 , the brake fluid pressure is decreased. Thereafter, the road surface frictional force F is detected at step  30  and this value is stored in variable F t  at step  32 . Subsequently at step  34 , as at step  22 , the value of difference F t −F t-1  is compared with zero. If the difference is negative, the sequence proceeds to step  24 . If the judgement at step  34  is not negative, the sequence returns to step  26 .  
         [0038]     As the control means  3  executes the above program, the brake fluid pressure is controlled in such a manner that the brake will always be applied at or near the maximum road surface friction coefficient (P 1  on curve C 1  in  FIG. 1 ) irrespective of the condition of the road surface.  
         [0039]     However, the above embodiment has the following drawbacks (1) through (4).  
         [0040]     (1) The antilock braking device according to the above embodiment functions in pursuit of the maximum value of road surface frictional force. Therefore, when the road surface has no peak of road surface friction coefficient relative to slip ratio as represented by curve C 2  in  FIG. 1 , it may happen that ultimately a state of complete lock (slip ratio S=1) occurs to frustrate the object of an antilock brake.  
         [0041]     (2) The road surface friction coefficient is the relative value of road surface frictional force to the vertical load acting on the wheel. However, since the vertical load which acts on the wheels during the running of the vehicle is not necessarily constant, the relation between road surface friction coefficient and road surface frictional force is not exactly proportional but merely approximately so. Therefore, it does not necessarily hold true that the antilock braking device of the above embodiment which pursues the maximum value of road surface frictional force pursues the peak point P 1  on the curve Cl shown in  FIG. 1 . The antilock braking device described in Japanese laid-open Patent Application No. 25169/1988, which controls the braking action with reference to the brake torque of the wheel, also has a similar disadvantage.  
         [0042]     (3) The above surface frictional force increases as the brake fluid pressure is increased. However, as moments of inertia around the axle exist in the wheel, there is a delay in the increases in slip ratio and road surface frictional force that follow the increasing brake fluid pressure. Therefore, when the point of braking action makes an ingress beyond and into the segment to the right of P 1  on the curve C 1  of  FIG. 1  and the road surface friction coefficient (and the road surface frictional force, too, if the vertical load is constant) begins to decline, the brake fluid pressure may have reached an excessively high level. Therefore, even if a decline in road surface frictional force is detected at this time-point and the brake fluid pressure is accordingly relieved, it is not certain that the slip ratio immediately begins to decrease and the point of braking action reapproaches to P 1  from the right-hand side with a consequent upward turn of road surface frictional force. In other words, in the antilock braking device of the above embodiment in which the forthcoming decompression or compression of brake fluid is predicated on the increase or decrease in road surface frictional force in response to a fall-off of brake fluid pressure, it may happen that a complete lock occurs without a reapproach of the point of braking action, to P 1  from its right-hand side on the curve C 1 .  
         [0043]     (4) Because of the absence of a means for sensing the stationary state of the vehicle, the unnecessary antilock braking action may continue to occur even after the vehicle has stopped to run.  
         [0044]     An antilock braking device according to another embodiment of this invention, which has overcome the above-mentioned drawbacks is described hereinafter with reference to  FIGS. 4 through 8 .  
         [0045]     A control means  103  controls the brake fluid pressure with reference to measured values of brake pedal-depressing force, road surface friction coefficient μ, chassis speed detection signal and brake fluid pressure. A brake pedal-depressing force sensor  102 , brake fluid pressure generator  104 , brake fluid pressure sensor  107  and brake means  105  may all be implemented by utilizing the known technologies. The road surface friction coefficient and chassis speed detection signal are obtained from a road surface friction coefficient detector  101  and a chassis speed sensor  106 , both of which are described hereinafter in detail.  
         [0046]     Like the above-mentioned control means  3  shown in  FIG. 2 , the control means  103  comprises an electronic circuitry including a microprocessor, a memory and an input-output interface, and functions according to a program written into the memory. An example of the action of this control means  103  is shown in the flow charts of  FIGS. 5 through 8 .  
         [0047]     As the brake pedal-depressing force reaches a set value, the antilock braking device of this embodiment starts functioning to make a transition from the ordinary braking action to the antilock braking action. Referring to the main routine shown in  FIG. 5 , step  110  represents the beginning of this antilock braking action. Subsequently at step  111 , the road surface friction coefficient μ is detected and at step  112 , this value of μ is stored in the variable labeled with μ t-1 . At step  113 , this value is stored in the variable labeled with μ p . Then, the brake fluid pressure is increased at step  115  and the value of μ is detected at step  116 . At step  117 , the value of μ detected at step  116  is stored in the variable labeled with μ t . The sequence proceeds to step  118 , where the difference μ t −μ t-1  between the two stored values μ t  and μ t-1  is compared with a predetermined reference value μ c . If the difference μ t −μ t-1  is larger than μ c , the sequence proceeds to step  119 . If said difference is either equal to or smaller than μ c , the sequence proceeds to the brake fluid decompression routine at step  123 . At step  119 , the value stored in the variable μ t  is stored in the variable μ t-1  and the stored value μ t-1  is updated. The sequence then returns to step  113 .  
         [0048]     In the brake fluid decompression routine  123 , as illustrated in  FIG. 6  the brake fluid pressure is either released or decreased to a given low level at step  142 . Then, μ is detected at step  143  and this detected value is stored in the variable μ t-1  at step  144 . The sequence then proceeds to step  146  where μ t-1  is compared with α·μ p . The coefficient α is a preset appropriate constant within the range of 0 to 1. If the variable μ t-1  is smaller, the sequence proceeds to step  149  to terminate the brake fluid decompression routine  123  and, then, to the brake fluid recompression routine at step  124 . If the variable μ t-1  is larger or equal, the sequence returns to step  142 .  
         [0049]     In the brake fluid recompression routine  124  following the brake fluid decompression routine  123 , the operation illustrated in  FIG. 7  is executed. First, at step  162 , the brake fluid pressure is increased. Then, μ is detected at step  163  and this value is stored in the variable μ t  at step  164 . Thereafter, at step  165 , the variable μ t  is compared with the variable μ t-1 . If the variable μ t  is larger, the sequence proceeds to step  166  where the value of variable μ t  is stored in the variable μ t-1  to update the stored value of variable μ t-1 . Then, the sequence proceeds to step  171  to terminate the brake fluid recompression routine and returns to the main routine at step  113 . If the variable μ t  is found to be either smaller or equal at step  165 , the sequence proceeds to step  167  where, as at step  166 , the value of variable μ t-1  is updated to the value of μ t . The sequence then returns to step  162 .  
         [0050]     As the control means  103  executes the above program, the antilock braking device according to this embodiment functions as follows. After the beginning of operation of the antilock braking device, the brake fluid pressure is increased while the rising velocity of road surface friction coefficient continues to exceed a predetermined reference value. As the rising velocity of road surface friction coefficient μ drops below said reference value, the brake fluid pressure is relieved or released. At this stage, the value of road surface friction coefficient μ immediately prior to the beginning of decline is memorized. This memorized value is referred to as μ p . When the road surface friction coefficient μ has declined to a predetermined percentage, for example 50%, which is dependent on α, the brake fluid pressure is caused to increase again. Thereafter, the above sequence of operation is repeated.  
         [0051]     The chassis speed sensor  106  illustrated in  FIG. 4  may be the conventional device adapted for installation at the front of the driver&#39;s seat. This chassis speed can be found by electrical processing of, for example, a sensor output of the rotational speed of the change-speed drive shaft. There is usually a time lag between the rotational speed of the drive shaft and the chassis speed indication value. The detected chassis speed value carrying this time lag may be utilized as it is. As an alternative, the sensor of the change-speed drive shaft speed may be provided with a signal processing system adapted to output an appropriate chassis speed delay signal to cause a delay in the chassis speed signal output and this value be fed to the control means  103  as chassis speed data. When this chassis speed falls below a predetermined value (for example, several kilometers per hour), the control means  103  is not caused to make a transition from ordinary braking action to antilbck braking action even if the brake pedal is depressed with a force over a predetermined value. When the chassis speed drops below said predetermined value during an antilock braking action, irrespective of the stage which the control means  103  is executing in the flow charts illustrated in  FIGS. 5 through 7 , it immediately executes the interruption routine shown in  FIG. 8  to terminate the antilock braking action and controls the brake fluid system so that the ordinary braking action resumes. If the chassis speed is very low, the need for antilock braking action is not great and this action is not required at all when the vehicle is standing still. In consideration of the above, this embodiment is so designed that the antilock braking action will not take place when the chassis speed is below a predetermined value. Since the chassis speed data referred to has a delay from the rotational speed of the drive shaft, there is no response to a sudden decrease in drive shaft rotational speed due to locking of the drive wheels upon application of the brake. Therefore, the risk of a failure to enter into an antilock braking action due to locking prior to the transition from ordinary braking to antilock braking on depression of the brake pedal is reduced. Furthermore, the risk of release of the antilock braking action and return to the ordinary braking action in the event of locking during antilock braking is eliminated.  
         [0052]     Irrespective of the stage of the flow chart which the control means  103  is executing, the interruption routine shown in  FIG. 8  is executed immediately upon generation of an antilock brake release instruction signal to terminate the antilock braking action and the ordinary braking action resumes. This action can be incorporated in the first-mentioned embodiment illustrated in  FIGS. 2 and 3 . The generation timing of said antilock braking release instruction may for example be (1) when the brake pedal-depressing force has decreased below a predetermined reference value, (2) when the brake pedal is re-depressed after releasing of the depressing force, or (3) when the engine key is turned off. Of these alternatives, (2) and (3) are advantageous in that even if an accident disables the driver to manipulate the controls, the vehicle can be automatically brought to standstill with the ideal braking characteristic.  
         [0053]     A more effective antilock brake control can be implemented by replacing the comparison at step  165  in the brake fluid recompression routine  124  with the comparison of whether the relation of μ t −μ t-1 &gt;μ c2′  where μ c2  is a predetermined appropriate positive reference value, holds true. This reference value μ c2  preferably satisfies the condition μ C2 /Δt 2 &gt;μ c /Δt 1 , wherein Δt 2  is the larger of the time period in which the sequence proceeds from step  143  to step  146  and to step  149  in the brake fluid decompression routine  123  and further to step  160  to step  163  in the brake fluid recompression routine  124  and the time period of the loop in which the sequence proceeds from step  163  in the brake fluid recompression routine  124  and returns through steps  165 ,  167  and  162  back to step  163 , and Δt 1  is the time period of the loop in which the sequence proceeds from step  116  in the main routine through steps  118 ,  119  and  113  and back to step  116 . It is also preferable that the above-mentioned brake recompression routine be interposed downstream of step  112  in the above-mentioned main routine shown in  FIG. 5 . The course of return from step  119  to step  113  remains unchanged. In the above case, the reference value μ c2  in the brake fluid recompression routine  124  may be an appropriate value satisfying the relation μ c2 /Δt 2 &gt;μ c /Δ t , wherein Δt 2  is the larger of the values of the time period in which the sequence proceeds from step  111  to step  163  in the interposed brake fluid recompression routine  124  and the time period of the loop in which the sequence proceeds from step  163  in the inserted brake fluid recompression routine through steps  165 ,  167  and  162  back to step  163 .  
         [0054]     Now, the road surface friction coefficient detector  101  embodying the principle of this invention is described below with reference to  FIG. 9  through  19 .  
         [0055]     In this embodiment, the strain in the axle or in the vicinity of the axle is measured by means of strain gauges  41  to  44  and  51  to  54  for the rear wheels and strain gauges  71  to  74 ,  75  to  78 ,  81  to  84  and  85  to  88  or strain gauges  41  to  44  and  51  to  54 , for the front wheels, whereby the road surface frictional force and the load in the vertical direction are detected. The strain gauge itself is a known technology utilizing the fact that the electric resistance of a resistance wire changes in proportion with strain. Typically, it comprises a rectangular film in which a resistance wire has been embedded and detects the tensile strain and compressive strain in its longitudinal direction.  
         [0056]     An example of the position of attachment of strain gauges in the vicinity of a rear wheel  64  is shown in  FIGS. 9 through 11 . It should be understood that the arrowmarks  61 ,  62  and  63  indicate the vertical direction, direction of advance, and axle direction, respectively, of the wheels  64 . A total of 8 strain gauges  41  to  44  and  51  to  54  are affixed to the surface of a rear axle housing  66  between the wheel  64  and a rear spring  65  secured rigidly to a car body  58 . A set of 4 strain gauges  41 ,  42 ,  43  and  44  for detection of the vertical load that acts on the rear wheel  64  and another set of  4  strain gauges  51 ,  52 ,  53  and  54  for detection of the road surface frictional force which acts on the same wheel  64  are assembled into the respective bridge circuits as shown in  FIG. 12  and the outputs of the respective bridge circuits are fed to amplifiers  45  and  55 .  
         [0057]     As shown in  FIG. 10 , the set of strain gauges  41 ,  42 ,  43  and  44  are affixed on the line of intersection of a horizontal plane including the axis or centerline of the rear axle housing  66  with the surface of the housing  66  for measuring the compressive and tensile strains in the direction at an angle of 45 degrees from said line of intersection. However, strain gauges  41  and  42  and strain gauges  43  and  44  are respectively disposed close to each other and the strain gauges  41  and  44  and the strain gauges  42  and  43  are respectively disposed in symmetric relation with respect to the centerline. The other set of strain gauges  51 ,  52 ,  53  and  54  are disposed as illustrated in  FIG. 11 . Thus, they are affixed on the line of intersection of a vertical plane including the centerline of rear axle housing  66  with the surface of said housing  66  for measuring the compressive and tensile strains in the direction at an angle of 45 degrees from said line of intersection. Furthermore, the strain gauges  51  and  52  and the strain gauges  53  and  54  are respectively disposed close to each other and the strain gauges  51  and  54  and the strain gauges  52  and  53  are respectively disposed in symmetric relation with respect to the centerline.  
         [0058]     By the vertical load acting on the wheel  64 , the rear axle housing  66  is subject to a bending deformation such that the centerline or axis of the housing  66  is bent on a vertical plane including the centerline. At the same time, a shearing force equivalent to the vertical load is applied vertically to the cross-sectional area perpendicular to the centerline of the rear axle housing  66 . In proportion with this shearing force, a shear strain is generated in the rear axle housing  66 . The bridge circuit consisting of the set of strain gauges  41 ,  42 ,  43  and  44  detects this shear strain. Thus, even if the above-mentioned bending deformation caused the respective strain gauges to undergo compression or elongation, the effects of the bending deformation on strain gauges  41 ,  42 ,  43  and  44  are mutually offset in this bridge circuit. Thus, the voltage output of the amplifier  45  is only proportional to the vertical load acting on the wheel and not subject to the effect of the moments around a rear spring  65 .  
         [0059]     The road surface frictional force acting on the wheel  64  causes a bending deformation such that the centerline of the rear axle housing  66  is bent on a horizontal plane including the centerline. At the same time, a shearing force equivalent to the road surface frictional force is applied horizontally to the cross-sectional area perpendicular to the centerline of rear axle housing  66 . In proportion with this shearing force, a shear strain is generated in the rear axle housing  66 . The bridge circuit consisting of strain gauges  51 ,  52 ,  53  and  54  detects this shear strain. Just as mentioned above, the effects of the bending deformation on the respective strain gauges are mutually offset in this bridge circuit. Therefore, the voltage output of the amplifier  55  is only proportional to the road surface frictional force applied to the wheel  64  and is not subject to the influence of the moments around the rear spring  65 .  
         [0060]     Furthermore, the bending deformation and shear strain due to the vertical load do not interfere with the output voltage of the bridge circuit consisting of strain gauges  51  through  54  constituting said one set, and the bending deformation and shear strain due to the road surface frictional force do not influence the output voltage of the bridge circuit consisting of strain gauges  41  through  44  constituting the other set. Though the cornering force (lateral drag) acting on the wheel  64  adds a compressive or tensile strain to the rear axle housing  66  in the direction of its center-line, these strains do not affect the output voltage of the bridge circuit consisting of strain gauges  41  through  44 , nor do they interfere with the voltage output of the bridge circuit consisting of strain gauges  51  through  54 .  
         [0061]     Furthermore, as the brake is applied to the wheel  64 , the brake torque (the moment about the centerline of the axle) generates a torsional deformation in the rear axle housing  66  about its axis. However, the voltage outputs of the bridge circuits consisting of said sets of strain gauges  41  to  44  and  51  to  54  are not affected by the torsional deformation.  
         [0062]     Furthermore, in view of the fact that the rear axle housing  66  is made of a steel material with a high thermal conductivity, the temperature difference among said strain gauges  41  through  44  or among said strain gauges  51  through  54  is so small that a change in atmospheric temperature exerts little influence on the output voltages of the respective amplifiers  45  and  55 . Thus, the influence of atmospheric temperature on detected vertical load and road surface frictional force values is almost negligible. If the peripheral surface of the rear axle housing  60  is locally treated with copper and the eight strain gauges are affixed to the treated area, the inter-gauge temperature gradient and, hence, the influence of atmospheric temperature will be effectively minimized.  
         [0063]     The moments around the rear spring  65  which act on the rear axle housing  66  owing to the vertical load and road surface frictional force applied to the rear wheel  64  vary with the shift of the point of contact on the tire surface with the ground in the axle direction, even if the vertical load or road surface frictional force remains constant. Therefore, it is necessary to detect the very vertical load and road surface frictional force without picking up such moments. This embodiment meets the above demand.  
         [0064]     As shown in  FIG. 12 , the road surface friction coefficient detector  101  according to this embodiment feeds to an operational circuit  56  a voltage signal proportional to the vertical load as obtainable as the output of an amplifier  45  and a voltage signal proportional to the road surface frictional force as obtainable as the output of an amplifier  55 . The operational circuit  56  calculates the quotient of road surface frictional force and vertical load and outputs a voltage signal corresponding to the road surface friction coefficient μ.  
         [0065]     It should be noted that a strain gauge (tentatively called “cross gauge”) which consists of two strain gauges disposed as intersecting each other at right angles and is capable of measuring the tensile or compressive strains in biaxial directions is commercially available. Therefore, as shown in  FIG. 13 , each of the couples of strain gauges  41  and  42 ;  43  and  44 ;  51  and  52 ; and  53  and  54  may be replaced with one cross gauge to constitute bridge circuits as illustrated in  FIG. 12  to accomplish the desired effect with greater efficiency. When cross gauges are used, the interval between strain gauges in each pair (for example,  41  and  42 ) become zero and the tensile or compressive strains in two perpendicular directions can be measured in one and the same position, with the result that the measurement of the vertical load and road surface frictional force can be performed with improved accuracy.  
         [0066]      FIGS. 14 through 16  show exemplary positions of attachment of strain gauges in the vicinity of the axle of the front wheel  57 , taking as an example a front wheel suspension structure of the so-called wishbone type. In the views, the arrowmarks  61 ,  47  and  48  indicate the vertical direction, direction of advance and axle direction, respectively, of the wheel  57 . The arrowmark  46  is parallel to the arrowmark  48  and points to the side on which the wheel is positioned. It is recommended that strain gauges  71  through  74  and strain gages  81  through  84  be affixed respectively on the lateral sides of two upper and lower wheel supporting members  60  and  69  for transmitting the force acting on the wheel  57  to a suspension mechanism  59  connected to the vehicle body  58  and strain gauges  75  through  78  and strain gauges  85  through  88  be affixed on the top and bottom sides, respectively. In these views, the use of aforesaid cross gauges is represented.  
         [0067]     As shown in  FIG. 15 , a set of four strain gauges  71 ,  72 ,  73  and  74  are affixed on both lateral sides of the lower wheel supporting member  60  in such a manner that the strain gauges  71  and  72  and the strain gauges  73  and  74  are respectively disposed in symmetric relation with each other on the respective lateral sides. These gauges are affixed at an angle of 45 degrees from the vertical direction  61  and the axle direction  48 . Quite similarly, the strain gauges  81 ,  82 ,  83  and  84  constituting another set are affixed on both lateral sides of the upper wheel supporting member  69 . The positions and direction of attachment are similar to those shown in  FIG. 15  except that the supporting member  60  should read  69  and the strain gauges  71 ,  72 ,  73  and  74  read  81 ,  82 ,  83  and  84 , respectively.  
         [0068]     As illustrated in  FIG. 16 , strain gauges  75 ,  76 ,  77  and  78  forming another set are affixed on the top and bottom sides of the lower wheel supporting member  60  in such a manner that the strain gauges  75  and  76  and the strain gauges  77  and  78  are respectively disposed in symmetric relation on the respective sides. These gauges are affixed at an angle of 45 degrees from the direction of advance  47  and axle direction  48 . Quite similarly, strain gauges  85 ,  86 ,  87  and  88  forming another set are affixed on the top and bottom sides of the upper wheel supporting member  69 . The positions and direction of attachment are similar to those indicated in  FIG. 16  except that the supporting member  60  should read  69  and the strain gauges  75 ,  76 ,  77  and  78  read  85 ,  86 ,  87  and  88 , respectively.  
         [0069]     As it was the case with the rear wheel  64 , each set of these strain gauges  71  through  74 ,  81  through  84 ,  75  through  78  and  85  through  88  constitutes a bridge circuit and is connected to the corresponding one of amplifiers  91 ,  92 ,  93  and  94  as shown in  FIG. 17 .  
         [0070]     The vertical load acting on the wheel  57  generates a shearing force equivalent to the vertical load, as a sum of shearing forces for the upper and lower wheel supporting members  60  and  69 , in vertical direction  61  in the cross-sectional area perpendicular to the axle direction  48  of each of the supporting members  60  and  69 . As a result, shear strains proportional to the shearing forces acting on the supporting members  60  and  69 , respectively, are generated in the respective wheel supporting members  60  and  69 . The bridge circuits consisting of strain gauges  71 ,  72 ,  73  and  74  and strain gauges  81 ,  82 ,  83  and  84 , respectively, detect these respective shear strains. The outputs of amplifiers  91  and  92  representing detected values of these two shear strains are added in a predetermined suitable ratio in an operational circuit  95  shown in  FIG. 17  and the result is outputted. In this manner, a voltage signal proportional to the vertical load applied to the wheel  57  is obtained as the output of said operational circuit  95 .  
         [0071]     Similarly, the road surface frictional force applied to the wheel  57  generates a shearing force equivalent to the road surface frictional force, as the sum of forces for the upper and lower wheel supporting members  60  and  69 , in travel direction  47  in the cross-sectional area perpendicular to the axle direction  48  of each of the wheel supporting members  60  and  69 . As a result, shear strains proportional to the shearing forces acting on the supporting members  60  and  69 , respectively, are generated in the wheel supporting members  60  and  69 , respectively. The bridge circuits consisting of strain gauges  75 ,  76 ,  77  and  78  and strain gauges  85 ,  86 ,  87  and  88 , respectively, detect these shear strains, respectively. The outputs of amplifiers  93  and  94  which represent detected values of these two shear strains are added in a predetermined suitable ratio in an operational circuit  96  shown in  FIG. 17  and the result is outputted. In this manner, a voltage signal proportional to the road surface frictional force acting on the wheel  57  is obtained as the output of the operational circuit  96 . The outputs of the two operational circuits  95  and  96  are fed to an operational circuit  97  which, like the operational circuit  56  shown in  FIG. 12 , calculates the quotient of road surface frictional force and vertical load and outputs a voltage signal corresponding to the road surface friction coefficient μ.  
         [0072]     As it is the case with the rear wheel  64 , the cornering force applied to the wheel does not affect detected values of vertical load and road surface frictional force. Furthermore, the vertical load does not interfere with the detected road surface frictional force value and the reverse is also true. Moreover, for the same reason as mentioned in connection with the rear wheel  64 , the influence of atmospheric temperature is also small. Similarly, too, the influence of atmospheric temperature can be further minimized by treating the surfaces of the wheel supporting members  60  and  69  with copper and affixing the strain gauges thereon. By the vertical load acting on the wheel  57 , the wheel supporting members  60  and  69  are subjected not only to the above-mentioned shearing forces but also to compressive and tensile forces in the axle direction  48 . However, as it is the case with the effect of cornering force, neither the detected value of road surface frictional force nor that of vertical load is influenced. Furthermore, by the braking torque acting as the brake for wheel  57  is applied, a shearing force is generated in the direction of advance  47  in the cross-sectional area perpendicular to the axle direction  48  of each of the wheel supporting members  60  and  69 . As a result, shear strains, proportional to the shearing forces acting on the wheel supporting members  60  and  69  are generated in the supporting members  60  and  69 . The bridge circuits consisting of strain gauges  75 ,  76 ,  77  and  78  and strain gauges  85 ,  86 ,  87  and  88 , respectively, detect these shear strains, respectively. However, as mentioned hereinbefore, the outputs of amplifiers  93  and  94  are added in a predetermined suitable ratio in the operational circuit  96 . Therefore, the effects of shearing forces due to said torque are offset and consequently a voltage signal proportional to the road surface frictional force acting on the wheel  57  is obtained as the output of said operational circuit  96 .  
         [0073]     As will be apparent from the construction illustrated in  FIG. 14 , the upper wheel supporting member does not substantially bear the vertical load on the wheel. Therefore, even if the set of strain gauges  81  through  84 , amplifier circuit  92  and operational circuit  95  are omitted and the output of the amplifier  91  is fed as the detected value of vertical load directly to the operational circuit  97 , the error will be almost negligible.  
         [0074]      FIGS. 18 and 19  show the positions of installation of strain gauges in the front wheel  57  axle region where the suspension mechanism is the so-called “strat” type. Preferably, strain gauges  41  through  44  are affixed on the lateral sides and strain gauges  51  through  54  on the top and bottom sides of a support member  99  adapted to transmit the force acting on the wheel  57  to a strat  98 . In these views, the cross gauges mentioned above are shown by way of example. Like the above-mentioned strain gauges  71  through  74  or  81  through  84 , the strain gauges  41  through  44  are positioned in the direction at an angle of  45  degrees with respect to vertical direction  61  and in such a manner that the couple of  41  and  42  and the couple of  43  and  44  are symmetrically positioned on the lateral sides of the support member  99 . Similarly, the strain gauges  51  through  54  are affixed in the direction at an angle of 45 degrees with respect to the axle direction  48  and in such a manner that the couple of  51  and  52  and the couple of  53  and  54  are symmetrically disposed on the top and bottom sides of the support member  99 . These sets of strain gauges  41  to  44  and strain gauges  51  to  54  each constitutes a bridge as illustrated in  FIG. 12  and are connected to amplifiers  45  and  55 , respectively. The respective amplifiers  45  and  55  output voltage signals proportional to the vertical load and road surface friction force acting on the wheel  57 , respectively. These voltage signals are fed to an operational circuit  56 . Just as in the case of the rear wheel  64 , the operational circuit  56  outputs a voltage signal corresponding to the road surface friction coefficient μ.  
         [0075]     In this embodiment, as it is the case with the rear wheel  64  and the front wheel  57  having the “wishbone” suspension mechanism, the cornering force acting on the wheel does not interfere with the detected values of vertical load and road surface friction force. Moreover, the vertical load does not influence the detected value of road surface friction force and the reverse also holds true. Furthermore, the influence of variation in atmospheric temperature is also negligible. This influence of atmospheric temperature may be further diminished by treating the surface of the support member  99  locally with copper and affixing the strain gauges to the treated areas. In addition, just as it was the case with the rear wheel  64 , the vertical load acting on the wheel  57  generates not only a shear strain but a bending deformation in the support member  99 . Furthermore, when the brake is applied to the wheel  57 , the brake torque superimposes a torsional deformation in the support member  99 . However, neither the bending deformation nor the torsional deformation affects the voltage outputs of the bridge circuits consisting of said sets of strain gauges  41  to  44  and  51  to  54 . Therefore, the amplifiers  45  and  55  each outputs a voltage signal which is exclusively proportional to the vertical load and road surface friction force acting on the wheel  57 .  
         [0076]     The road surface friction sensor  1  included in the illustration of  FIG. 2  can be constituted, for the rear wheel and the front wheel connected to the strat type suspension mechanism, by the bridge circuit consisting of strain gauges  51  through  54  and amplifier  55  as shown in  FIG. 12  and a voltage signal proportional to the road surface frictional force as obtainable as the output of the amplifier  55  can be directly fed to the control means  3 . For the front wheel which is connected to the “wishbone” suspension mechanism, the sensor  1  can be constituted by bridge circuits comprising two sets of strain gauges  75  through  77  and  85  through  87 , amplifiers  93  and  94  and an operational circuit  96  as shown in  FIG. 17  and, then, a voltage signal proportional to the road surface frictional force as obtainable as the output of the amplifier  96  can be directly fed to the control means  3 .  
         [0077]     When the brake is applied to the wheel  57  during the running of the vehicle in the direction of arrow-mark  67  as illustrated in  FIG. 20 , the centerline AB along the lateral side of the tire of this wheel  57  is displaced, for example to AB′, according to the magnitude of the frictional force from a road surface  68 . Moreover, the tire is deformed by the vertical load so that the distance between A and B is altered. Therefore, when the lateral side of the tire is locally marked in a suitable pattern and the deformation of the marking is measured by optical means such as image pickup elements disposed in the vicinity of the wheel  57 , the strain of the tire itself can be known through the image data. Then, based on the result, the road surface frictional force and vertical load values are calculated. Using the road surface friction coefficient μ obtained by computation from the two calculated values, the antilock braking action shown in  FIGS. 5 through 8  is effected. It is also possible to calculate the road surface frictional force only and perform the antilock braking action of  FIG. 3  according to the result of the calculation.