This application is a continuation of application Ser. No. 09/812,264, filed Mar. 19, 2001, now abandonded, which is a Continuation of Ser. No. 08/825,846 filed Apr. 4, 1997, now Abandoned, which is a Continuation of Ser. No. 08/558,228 filed Nov. 17, 1995, now Abandoned, which is a Continuation of Ser. No. 08/112,495 filed Aug. 26, 1993, now Abandoned. 


   BACKGROUND OF THE INVENTION 
   The present invention relates to a novel system for controlling a vehicle antilock brake system (ABS) by means of stress sensors for detecting road surface friction forces of road surface friction coefficients for respective wheels which insures safe steering without locking of its wheels even during sudden braking. 
   There has been a mounting interest in the so-called antilock brake system which controls the brake fluid pressure on the wheel within a certain range (about 20%) of slip ratio between braking force and cornering force during sudden braking and an increasing number of vehicles are carrying such ABS systems for safe steering even during sudden braking. 
   The antilock brake system generally comprises a wheel speed sensor for detecting the locking tendency of the tire (wheel), a controller for outputting an actuator drive command according to the current wheel speed data and an actuator which, in response to said drive command, adjusts the brake fluid pressure. Among the known modes of ABS control are the three-system control (selectro-control) mode in which the brake fluid pressures to the front right and left wheels are independently controlled and, with one of the two rear wheels which is more liable to be locked as a reference, the brake fluid pressures for both rear wheels are controlled as a unit. A two-system control exists in which the front and rear wheels are respectively controlled as units or the diagonally located wheels are simultaneously controlled. Also, a simultaneous front and rear wheel control mode exists in which one of the rear wheels is controlled by the selectro-control method and with one of the front wheels which is harder to control as a reference, the brake fluid pressures to the two front wheels are simultaneously controlled. 
   Since the conventional antilock brake system described above uses one controller for controlling the brake fluid pressures to the four wheels either through three-system control or through two-system control, a long pipeline is required between the wheel cylinder of each wheel and the actuator. In the case of a large-sized vehicle such as a trailer or a large bus, which has a great overall length, the brake fluid pipeline has to span a great distance, with the result that not only a time lag is inevitable after the actuator receives a drive command and before the wheel cylinder of the wheel is supplied with a brake fluid pressure but also a transmission loss of the brake fluid pressure is liable to occur, so that the system cannot provide for exact brake control and, hence, cannot be said to be a fully safe antilock brake system. 
   The conventional ABS employing wheel speed sensors is a system which automatically controls the brake so as to bring the slip ratio into a certain range based on chassis speed and wheel speed but since the relationship between road surface friction coefficient and slip ratio is a variable dependent on the changing road surface condition, the conventional system may not provide for the maximal braking force depending on the road surface condition, with the result that the minimum braking distance cannot be insured for certain. 
   Furthermore, since the chassis speed is a value estimated from wheel speeds, the accuracy of slip ratio control is not high enough and in order to find the exact chassis speed, a complicated device such as a ground speed sensor or a chassis deceleration sensor is needed. 
   SUMMARY OF THE INVENTION 
   Designed to overcome the above drawbacks of the prior art, the present invention has for its object to provide an ABS control system by which the right and left front and rear wheels of a vehicle are independently subjected to brake fluid pressure control through the utilization of a wheel sensor means adapted to directly detect the road surface friction force or road surface friction coefficient to thereby provide for exact anitlock brake control. 
   The invention provides an ABS control device comprising a plurality of control units each provided for each wheel or each set of wheels and consisting of a stress sensor for detecting a wheel stress such as road surface friction force or road surface friction coefficient, an actuator controller and an actuator for controlling a brake fluid pressure, the stress sensor detecting the stress, such as road surface friction force or road surface friction coefficient, of the corresponding wheel according to road surface condition and the controller responding to the signal output of the stress sensor to cause the actuator to adjust the brake fluid pressure so that the respective wheels are independently controlled. 
   According to a feature of the invention, the stress values such as road surface friction force or road surface friction coefficient values for respective wheels are independently detected automatically on sudden braking, the corresponding controllers of the respective control units output drive signals to the corresponding actuators according to the respective detection signals, and the actuators independently control the brake oil or air brake pressures so that the antilock brake system for each wheel or each set of wheels functions independently of the corresponding systems for the other wheels. 
   As will be understood from the above description, the invention comprises control units each consisting of a stress sensor for detecting the road surface friction force or road surface friction coefficient value, a controller and an actuator, each of the units being installed close to the corresponding wheel, so that it is no longer necessary to provide long hydraulic fluid lines, nor is there a time lag between a drive command from the controller and the application of a brake fluid pressure with the consequent improvement in braking precision. Moreover, each wheel or set of wheels can be provided with an antilock brake system which functions independently of the corresponding systems for the other wheels or sets of wheels, in which cases the antilock brake systems work independently according to detected stress values such as road surface friction force or road surface friction coefficient values for the respective wheels or sets of wheels so that a vehicle equipped with highly safe antilock brake systems can be provided. 
   The invention further provides an ABS control system wherein the respective control units are supplied with a control hydraulic pressure from a foot brake master cylinder. 
   In accordance with a feature of the invention, the control units provided for respective wheels are supplied with hydraulic fluid pressures from a single master cylinder and the respective actuators receiving drive commands from the corresponding controllers function independently to adjust the brake fluid pressures so that the antilock brake systems can be independently actuated for the respective wheels. 
   The invention also provides an ABS control system wherein control hydraulic oil sources for control units are provided for respective wheels or sets of wheels and each of said control oil sources comprises a high pressure control pressure generating means, a fluid reservoir means and a reversing means, so that the respective wheels or sets of wheels may be independently controlled. 
   In accordance with another feature of the invention, the control unit for each wheel or set of wheels is provided with a high-pressure control pressure generating means, a hydraulic oil reservoir means and a reversing means, thus shortening the pipelines connecting the respective controllers to the corresponding actuators and allowing each control unit to function independently and with high precision. 
   The invention further features an ABS control system wherein the control hydraulic pressure source is supplied with an auxiliary oil pressure from a foot brake master cylinder. 
   According to another feature of the invention, any deficiencies in control pressure in the control pressure sure generating means are compensated for by an auxiliary supply of pressure from the foot brake master cylinder so that the actuators of the respective control units can be driven at necessary hydraulic pressures with high precision. 
   The invention further provides an ABS control system wherein the control units are actuated on application of a sudden brake or receipt of the corresponding signal and the operations of the respective control units are coordinated and controlled by a central controller. 
   According to another feature of the invention, the operations of the control units provided for the respective wheels or sets of wheels are coordinated by a central controller so that a good coordination of the respective ABSs can be obtained. 


   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a basic hardware block diagram illustrating a vehicle ABS control system according to the present invention as claimed in claim  1 ; 
       FIG. 2  is a hardware block diagram illustrating an embodiment in which the vehicle antilock system control device of the invention is applied to a large-sized bus; 
       FIG. 3  is a hardware block diagram illustrating an embodiment of the invention in which the vehicle antilock brake system control device of the-invention is applied to a trailer; 
       FIG. 4  is hardware block diagram illustrating another embodiment of a vehicle antilock brake system control device of the present invention; 
       FIG. 5  is a circuit diagram showing-an exemplary stress sensor system; and 
       FIG. 6  is a perspective drawing of an embodiment of a bridge of  FIG. 5 . 
   


   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   The preferred embodiments of the present invention are now described in detail with reference to the accompanying drawings. 
     FIG. 1  is a basic hardware block diagram illustrating the vehicle antilock brake system control according to claim  1  as applied to a passenger car. 
   Referring to  FIG. 1 , this vehicle ABS control system comprises four control units U each consisting of three elements, namely a strain sensor μ adapted to directly detect a wheel stress such as road surface friction force or road surface friction coefficient, a controller C comprising a microcomputer or the like and adapted to output a command signal, and an actuator A, which is adapted to adjust the brake fluid pressure according to the command signal as respectively mounted on the front axle  10  and rear axle  20  of a vehicle B in correspondence with the front right wheel FR, front left wheel FL, rear right wheel RR and rear left wheel RL thereof in such a manner that they may independently detect the stress values, such as road surface friction force or road surface friction coefficient values, for the corresponding wheels to thereby allow the respective wheels to be controlled independently of the others, said actuator A of each control unit U being connected to a master cylinder MP of a foot brake through a brake fluid line  30  so as to constitute antilock brake systems ABS 1 , ABS 2 , ABS 3  and ABS 4  for independent control of the respective wheels. 
   Referring to  FIG. 5 , the stress sensor μ con, for example, be the road surface friction coefficient detector which the present applicant has disclosed in Japanese Patent Application H-3-130840. Thus, the road surface coefficient detector comprises a road surface friction force sensor  40  consisting of 4 strain gauges  41 – 44  affixed on both sides of a plastic, silicon or metal wafer in a perpendicular arrangement to form a bridge  45  and installed in a hole provided in the axle or any structure close to the axle. A potential  46  is applied to the bridge  45  and output terminals of the bridge  45  are connected to an amplifier  47  so as to determine the stress on the axle or the structure adjacent therefor and thereby detect the road surface friction force. A vertical load sensor  50  is constructed in the same manner as above, for detecting the vertical reaction load. Four strain gauges  51 – 54  form a bridge  55 . A potential  56  is applied to the bridge  55  and an output of the bridge  55  is processed by an amplifier  57 . As shown in  FIG. 5 , these two sensors  40  and  50  are connected to an operation circuit  60  to divide the road surface friction force by the vertical reaction load output to output a road surface friction coefficient. The controller C is a microcomputer or an LSI electronic controller. 
   Referring to  FIG. 6 , an embodiment of the bridges  45  and  55  of  FIG. 5  is shown which is used to measure either the road surface frictional force or the vertical drag in application. Thus, each of the strain gauges  4 – 44  ( 51 – 54 ) is preferably installed at an angle of 45° with respect to the y-axis. For the purpose of measuring the road surface frictional force, the x, y and z axes in  FIG. 6  are preferably coincident with the vertical direction, direction of wheel advance and axle direction, respectively. For the purpose of measuring the vertical drag, the x, y and z axes in  FIG. 6  are preferably coincident with the direction of wheel advance, vertical direction and axle direction. The relationship of the bridges  45  and  55  installation position to and axle centerline is such that opposing sides of the bridges  45  and  55  installation position to and axle centerline is such that opposing sides of the bridges  45  and  55  straddle the centerline of the axle. The centerline of the axle is line of zero bending strain and zero shear strain due to torsional stress. The strain gauges  41 – 44  ( 51 – 54 ) are connected to a signal processing circuit shown in  FIG. 5 . Thus, they are formed into bridges  45  and  55  which, in turn, is connected to the amplification circuits  47  and  57 . These amplification circuits  47  and  57  output signals to operation circuit  60 . 
   The strain sensor μ need not be the above-mentioned detector utilizing strain gauges but may for example be a semiconductor sensor, shear stress sensor, acceleration sensor, chassis speed sensor, wheel speed sensor or the like. 
   The control unit U need not be an integral assembly of the stress sensor μ, controller C and actuator A but these components may be disposed near the wheel or axle independently but operatively associated with one another so that they may function in the optimum manner to provide for necessary control according to wheel stress detection signals. 
   As the driver suddenly brakes, the stress sensors μ of the respective control units U independently detect stresses, such as current road surface friction forces or road surface friction coefficients, and independently transmit detection signals to the corresponding controllers C which are adapted to output drive commands to the corresponding actuators A. The respective actuators A receiving the drive commands from the controllers C according to the detection signals for the respective wheels are driven independently so that an antilock brake may be applied independently for each wheel. As indicated by broken lines in  FIG. 1 , a central controller M providing for a failsafe telemetering monitor of the ABS control status of each wheel and a coordination of the respective wheel actions controls the operation of the respective control units U so that the actions of the antilock brakes for the wheels can be efficiently coordinated. The ABS is controlled according to the strain sensor output signal representing the road surface friction force or road surface friction coefficient. As shown, CB represents control panel. 
     FIG. 2  shows an embodiment in which the ABS control system of the invention is mounted on a large-sized bus of the two-front axle/8-wheel and two-rear axle/8-wheel type. The above-mentioned control unit U is provided for each of the front axles  10 , 10  and rear axles  20 , 20  so that the respective control units U may independently detect stress values, such as road surface friction force or road surface friction coefficient values, for the front right wheels FR 1 , FR 2 , front left wheels FL 2 , FL 2 , and rear right wheels RR 2 , RR 2  and rear left wheels RL 1 , RL 2  mounted on the front axles  10 , 10  and rear axles  20 , 20 , and actuators A of these control units U are supplied with a brake fluid from a master cylinder MP. 
   In this manner, the bus is equipped with 8 independently acting an antilock brake systems in a total of 8 positions, namely 4, positions, right and left, for the front wheels and 4 positions, right and left, for the rear wheels. The respective actuators A receive drive commands from the corresponding controllers according to detected stress values, such as road surface friction force or road surface friction coefficient values, for the respective wheels too adjust the brake fluid pressures independently. Thus, the antilock brakes for respective wheels can be independently actuated without requiring extended brake fluid lines. 
     FIG. 3  shows an embodiment in which the ABS control system of the invention is applied to a:trailer of the front 1-axle/2-wheel, rear two-axle/4-wheel diagonal two-wheel simultaneous control type. Here, the independently acting control unit U is provided in a total of 4 positions, namely 2 positions, right and left, for the front wheels and 2 positions, right and left, for the four rear wheels, with the rear right wheel RR 2  being connected to the control unit U for the rear left wheel RL 1  through a connecting pipe  31  and the rear left wheel RL 2  to the control unit U for the rear right wheel RR 1  through a connecting pipe  32 . The actuators A of these four control units U are respectively supplied faith brake fluid from the master cylinder MP. 
   The trailer is, thus., equipped with independently acting antilock brake systems ABC in two positions, right and left, for the front wheels and two positions, right and left, for the four rear wheels. The respective actuators A of said control units receive drive commands from the corresponding controllers C according to detected stress values, such as road surface friction force or road surface friction coefficient values, for each front wheel and for each couple of rear wheels couples to adjust the brake fluid pressures acting on the respective wheels independently. Thus, antilock brakes can be applied for sudden stopping without regard to chassis length or axle-to-axle distance for each front wheel and for each couple of rear wheels independently. 
   In the above embodiment, the actuator A of each control unit is supplied with a brake fluid from the master cylinder MP through a brake fluid line  30  and, therefore, complexity is introduced by the routing of the fluid line  30 . However, when a control hydraulic pressure source S comprising a high-pressure control hydraulic pressure generating means, a fluid reservoir means and a reversing means is disposed for each control unit U as shown in  FIG. 4 . Brake fluid line can be dispensed with and an antilock brake system having its own control hydraulic pressure source can be independently provided for each wheel or each set of wheels. Moreover, when a foot brake master cylinder MP is additionally provided as indicated by broken lines in  FIG. 4  so as to make up for deficiencies in brake fluid pressure in the control, hydraulic pressure sources S, the actuators of the respective control units can be driven at exact fluid pressures for precision control. In  FIG. 4 , the letter G represents a signal transmission system comprising a microcomputer or the like and  1  represents its signal line. 
   While the above embodiments have been described with reference to hydraulic oil brake control, the present invention is not limited to such hydraulic oil control but can be applied to pneumatic brake control with equal success.
A vehicular antilock brake control system does not require a lengthy extension piping from its actuator. The system provides for an accurate controlling method for maximizing a calculated road surface friction coefficient. Each wheel has a control unit consisting of a stress sensor, a controller and an actuator installed at the respective wheel. A stress value is detected, representative of road surface friction value or road surface friction coefficient value, for the corresponding wheel independently of the other wheels. In response to the output signal of the sensor, the controller regulates the actuator which controls brake fluid pressure applied to the respective wheel.