Anti-Skid Brake Control System for Automotive Vehicle With Feature of Assuring Prevention of Wheel-Locking at Any Driving Condition

1. Field of the Invention

The present invention relates generally to a method and system for performing anti-skid brake control for an automotive vehicle. More specifically, the invention relates to an anti-skid brake control assuring effect of prevention of locking of wheels even at low friction of the road surface condition.

2. Description of the Prior Art

As is well known, wheel slip control, such as anti-skid brake control and traction control which controls driving torque on vehicular wheels for eliminating wheel-spin, is performed generally based on a wheel slippage. The wheel slippage is generally derived on the basis of a difference between a wheel rotation speed and a vehicle speed. Namely, anti-skid nnbrake control is carried out to maintain wheel slippage around 10 to 20% for optimizing vehicular braking characteristics. On the other hand, in the traction control, a wheel-spinning condition is detected by detecting wheel speed exceeding a vehicle speed representative value.

Therefore, in order to perform precise wheel slip control, it is essential to detect vehicle speed accurately. Substantially high accuracy of vehicle speed detection can be made by means of a known lader systems, such as a dopper lader system. However, such lader systems are expensive for facilitating wheel slip control. Therefore, in practice, the vehicle speed representing value has to be derived based on other parameters which can be obtained an inexpensive sensor or sensors.

Conventionally, the vehicle speed representing values have been derived on the basis of rotation speeds of vehicular wheels. For example, the Japanese Patent Second (examined) Publication (Tokko) Showa No. 41-17082 discloses an anti-skid brake control system with derivation of the vehicle speed representing value. In the disclosed procedure, one of the wheels which is rotatating at the highest speed is selected to take the rotation speed thereof thus deriving the vehicle speed representing value. However, during abrupt deceleration of the vehicle, even the wheel speed of the highest speed wheel will not represent the vehicle speed. Therefore, in the conventional system, the vehicle speed representing value may be arithmetically derived by latching a highest wheel speed upon initiation of one cycle of wheel slip control operation and decreasing a constant value at every given interval for deriving assumed vehicle speed representing values to be used during abrupt deceleration.

Difficulty in performing accurate or precise anti-ski brake control arises when all of wheels effect wheel-locking in order to abruptly drop the wheel speed. When all of the wheels are locked and skidding, the vehicle speed representing value, derived from the highest wheel speed value, becomes completely useless, since that value does not represent the true vehicle speed. In order to avoid this, there is one approach which can be taken in order to provide a back-up value which is to be used as vehicle speed representing value while all the wheels are skidding. In such a system, locking of all the wheels has to be detected in a precise manner so as to assure switching of the use of the vehicle speed representing value derived based on the highest wheel speed and the back-up value. However, difficulty is encountered in detecting when all of the wheels are locking. Particularly, when the anti-skid control is utilized in a four wheel drive vehicle and all of the wheels are connected to the engine, the intertia moment exerted onto each wheel becomes relatively high to slow-down dropping of the wheel speed.

Therefore, it is an object of the present invention to provide an anti-skid brake control which assures the prevention of wheel locking at any vehicle driving condition.

Another object of the invention is to provide an anti-skid brake control which enables derivation of a vehicle speed representing value at any vehicle condition, wherein the vehicle speed representing value is accurately corresponding to the actual vehicle speed.

In order to accomplish the aforementioned and other objects, a method for anti-skid controlling an automotive brake system, according to the present invention, allows control for braking pressure at each wheel independently of other wheels. The anti-skid brake control is initiated in response to a drop of wheel acceleration across a predetermined wheel deceleration threshold. In the anti-skid brake control operation, braking pressure for at least two wheels are released in synchronism with each other. On the other hand, increasing rate of the braking pressure at the synchroneously released wheels are varied to each other so that when one of the wheels causes wheel-locking, the other wheel is held in a non-locking condition.

This assures derivation of the vehicle speed representing value based on the wheel speed of the other wheel which is not locking and assures the prevention of locking at any of the wheels.

On the other hand, according to the present invention, there is provided an anti-skid brake control system which implements the aforementioned anti-skid controlling process according to the present invention.

According to one aspect of the invention, an anti-skid brake control system for an automotive vehicle, comprises a hydraulic brake circuit including a plurality of wheel cylinders for establishing braking pressure for decelerating vehicular wheels, first and second anti-skid brake control valves, each associated at least one of first and second wheel cylinders associated with first and second wheels for adjusting braking pressure therein, each of the anti-skid brake control values being operable to increase braking pressure in the associated wheel cylinder in a first mode position, and to decrease the braking pressure in the associated wheel cylinder in a second mode position, first sensor for monitoring rotation speed of the first wheel for producing a first wheel speed indicative signal, second sensor for monitoring rotation speed of the second wheel for producing a second wheel speed indicative signal, first means for monitoring wheels slippage at the first and second wheels based on the first and second wheel speed indicative signals and detecting wheel slippage at one of the first and second wheels increasing across a given slippage threshold for operating the first and second anti-skid brake control valves into the second position, and second means for detecting the first and second wheel speed indicative values for operating the first and second anti-skid brake control valves into the first mode position when a given brake resuming condition is satisfied while the anti-skid brake control value is in the second position, the second means adjusting increasing rate of braking pressure in the first and second wheel cylinders so that the braking pressure in the third wheel cylinder is increased at a greater increasing rate than that in the fourth wheel cylinder.

According to one embodiment, the anti-skid brake control valves are further operable to hold the braking pressure in the first and second wheel cylinders constant in a third mode position. The second means alternatively switchs the mode positions of the first and second anti-skid brake control valves between the first and third mode positions so as to increase the braking pressure in the first and second wheel cylinders in stepwise fashion. In the embodiment, the second means adjusts the period for holding the second anti-skid brake control valve in the third mode position for a longer period than that for the first anti-skid brake control valves.

In the alternative embodiment, the second means provides a given delay time to operate for the increasing braking pressure of the second anti-skid brake control valve in stepwise fashion after the brake resuming condition is satisfied. On the other hand, in the further alternative embodiment, the second means alternatively switches the mode positions of the first and second anti-skid brake control valves between the first and second mode positions so as to increase the braking pressure in the first and second wheel cylinders in stepwise fashion.

According to another aspect of the invention, a method for anti-skid controlling an automotive hydraulic brake system including first and second anti-skid brake control valves, each associated at least one of first and second wheel cylinders associated with first and second wheels for adjusting braking pressure therein, each of the anti-skid brake control valves being operable to increase braking pressure in the associated wheel cylinder in a first mode position, and to decrease the braking pressure in the associated wheel cylinder in a second mode position, comprising the steps of:

monitoring rotation speed of the first wheel for producing a first wheel speed indicative signal;

monitoring rotation speed of the second wheel for producing a second wheel speed indicative signal;

detecting one of the first and second wheel condition satisfying a predetermined brake release condition to operate the first and second anti-skid brake control valves into the second mode position for decreasing brake pressure in the first and second wheel cylinders;

detecting one of the first and second wheel condition satisfying a predetermined brake resume condition for increasing braking pressure in the first and second wheel cylinders; and

controlling the first and second anti-skid brake control valves for increasing braking pressures in the first and second wheel cylinders at mutually different rates.

In the drawings:

FIGS. 1(A) and 1(B) are block diagram of an overall wheel slip control system, to which the preferred embodiment of a vehicle speed representing value derivation system according to the invention, is applied;

FIG. 2 is a circuit diagram of a peak detecting circuit in the wheel speed representing value derivation circuit of FIG. 1;

FIG. 3 is a chart showing manner of detection of the wheel speed peak to be carried out by the peak detecting circuit of FIG. 2;

FIG. 4 is a circuit diagram of a variable timer circuit employed in the wheel slip control system of FIG. 1;

FIG. 5 is a chart showing operation of the variable timer circuit of FIG. 4;

FIG. 6 is a block diagram of the preferred embodiment of the vehicle speed representing value derivation circuit in the wheel slip control system of FIG. 1;

FIG. 7 is a timing chart showing operation of the vehicle speed representing value derivation circuit of FIG. 6;

FIG. 8 is a timing chart showing operation of the anti-skid brake control to be performed by the wheel slip control system of FIG. 1;

FIG. 9 is a block diagram of a modified embodiment of the vehicle speed representing value derivation circuit in the wheel slip control system of FIG. 1;

FIGS. 10(A) and 10(B) are block diagrams of another embodiment of the preferred embodiment of the vehicle speed representing value derivation circuit of the invention;

FIG. 11 is a timing chart showing operation of the another embodiment of an anti-skid brake control system of FIG. 10;

FIGS. 12(A) and 12(B) are block diagrams of a further embodiment of an anti-skid brake control system according to the invention;

FIG. 13 is a timing chart showing operation of the anti-skid brake control system of FIG. 12; and

FIG. 14 is a diagramatical illustration of a modified embodiment of the anti-skid conmtrol valve assembly employed in the further embodiment of the anti-skid brake control system of FIG. 12.

Referring now to the drawings, particularly to FIG. 1, a wheel slip control system is designed for controlling wheel slip at respective front and rear wheels 1, 2, 3 and 4. Each front and rear wheels 1, 2, 3 and 4 are co-operated with wheel cylinders 1a, 2a, 3a and 4a so as to apply braking force therethrough. On the other hand, the rear wheels 3 and 4 are connected to an automotive engine 5 through a power train constituted by a power transmission 6, a propeller shaft 7, front and rear differential gear units 8a and 8b and drive shafts 9a, 9b, 9c and 9d. In the shown embodiment, the preferred embodiment of the anti-skid brake control system is employed in a four-wheel drive vehicle as being appreciated from hereinabove. Therefore, the output of the power transmission 6 is transmitted through a center differential unit 10 which controls distribution of the engine driving torque to the front and rear wheels 1, 2, 3 and 4 through the propeller shaft 7 and the front and rear differential gear units 8a and 8b.

In the shown embodiment, a hydraulic brake system for applying braking fluid pressure to respective wheel cylinders 1a, 2a, 3a and 4a includes two separate hydraulic circuits 12 and 14. One of the circuits 12 connects one of the outlet ports of a master cylinder 11 to front-right wheel cylinder 1a which is associated with the front-right wheel 1. The circuit 12 includes a branch circuit 13 connecting one of the outlet ports to the front-left wheel cylinder 2a of the front-left wheel 2. The other outlet port of the master cylinder 11 is connected to the rear-right wheel cylinder 3a and, through a branch circuit 15, to the rear-left wheel cylinder 4a. As is well known, the master cylinder 11 is mechanically coupled with a brake pedal 16 for building-up braking fluid pressure depending upon the magnitude of depression of the brake pedal. The braking fluid pressure built up in the master cylinder 11 is distributed to respective wheel cylinders 1a, 2a, 3a and 4a through the circuits 12, 13, 14 and 15 for exerting braking force to respectively associated wheels 1, 2, 3 and 4. As will be seen from FIGS. 1(A) and 1(B), the shown embodiment takes so-called XX circuit connection in establishing the hydraulic brake circuit. Therefore, one pressure line 11a extending from one port of the master cylinder 11 is connected to the right-front wheel cylinder 1a and left-rear wheel cylinder 4a and the other pressure line extending from the other port of the master cylinder is connected to the left-front wheel cylinder 2a and the right-rear wheel cylinder 3a.

Anti-skid brake control valves assemblies 17a and 17b, 17c and 17d are disposed in the circuits 12, 13, 14 and 15. As will be seen from FIGS. 1(A) and 1(B), the anti-skid brake control valve assembly 17a is respectively disposed in the circuit 12 for controlling braking pressure in the right-front wheel cylinder 1a. Similarly, the anti-skid brake control valve assembly 17b is disposed in the branch circuit 15 for controlling braking pressure in the left-front wheel cylinder 2a. The anti-skid brake control valve assembly 17c is disposed in the branch circuit 14 for controlling brake pressure in the right-rear wheel cylinder 3a. The anti-skid brake control valve assembly 17d is disposed in the branch circuit 13 for controlling braking pressure in the left-rear wheel cylinder 4a. As will be appreciated herefrom, each of the anti-skid brake control valve assemblies 17a, 17b, 17c and 17d control the braking pressure in each associated wheel cylinder 1a, 2a, 3a and 4a independently to the others.

The anti-skid brake control valve assembies 17a, 17b, 17c and 17d have mutually identical constructions to each other. Therefore, it would not be necessary to give a detailed description of the constructions for the valve assemblies 17a, 17b, 17c and 17d, respectively. In order to avoid redundant recitation and to avoid confusion in understanding the invention, the construction of the anti-skid brake control valve assembly 17a will only be described hereinbelow. The constructions of the valve assemblies 17b, 17c and 17d should be understood as indentical to that of the valve assembly 17a set out below.

The anti-skid brake control valve assembly 17a includes an inlet (EV) valve 19a, outlet (AV) valve 20a, a fluid pump 21a, accumulator 22a and one-way check valve 23a. The inlet valve 19a has an inlet port connected to the associated outlet port of the master cylinder 11 via the hydraulic circuit 12 and an outlet port connected to the front-right wheel cylinder 1a. On the other hand, the AV valve 20a has an inlet port connected to the wheel cylinder 1a and an outlet port connected to the accumulator 22a. The accumulator 22a is further connected to the hydraulic circuit 12 via the fluid pump 21a and the one-way check valve 23a for feeding back excessive fluid pressure therethrough. The fluid pump 23a is designed to draw the pressurized fluid in the associated wheel cylinder 1a to quickly reduce the braking pressure in the wheel cylinder while the anti-skid brake control is performed.

The EV valve 19a valve position is controlled

by an inlet control signal EV.sub.1 from a control unit 18 between closed position and open position. Similarly, the AV valve 20a valve position is controlled

by an outlet control signal AV.sub.1 from the control unit 18 between a closed position and an open position. Furthermore, the fluid pump 21a is connected to the control unit 18 to receive a pump drive signal MR,

its operation is controlled between driving condition and resting condition.

In the preferred embodiment, the EV valve 19a is in the open position while the inlet control signal EV.sub.1 is LOW level. On the other hand, the AV valve 20a is in the closed position while the outlet control signal AV.sub.1 is LOW level. The anti-skid brake control valve assembly 17a is operable in APPLICATION mode, RELEASE mode and HOLD mode. In the APPLICATION mode, the inlet control signal EV.sub.1 is held LOW to maintain the EV valve 19a in open position. At the same time, the outlet control valve AV.sub.1 is also held LOW to maintain the AV valve 20a in the closed position. Therefore, the outlet port of the master cylinder 11 is connected to the wheel cylinder 1a through the hydraulic circuit 12 and the EV valve 19a. Therefore, the braking fluid pressure in the wheel cylinder 1a is increased proportionally to that built-up in the master cylinder. On the other hand, in the RELEASE mode, the inlet control signal EV.sub.1 is switched into HIGH level to operate the EV valve 19a at the closed position. This shuts off the fluid communication between the outlet port of the maaster cylinder 11 and the wheel cylinder 1a therethrough. On the other hand, at this time, the outlet control signal AV.sub.1 also becomes HIGH level to open the AV valve 20a. As a result, fluid communication between the wheel cylinder 1a and the accumulator 22a is established. At the same time, the pump drive signal MR turns into HIGH to drive the fluid pump 21. Therefore, the fluid pressure in the accumulator 22a becomes lower than that in the wheel clinder 1a. Therefore, the braking fluid in the wheel cylinder 1a is drawn into the pressure accumulator.

On the other hand, in the HOLD mode, the inlet control signal EV.sub.1 is set HIGH level to close the EV valve 19a and the outlet control signal AV.sub.1 is set LOW level to maintain the AV valve 20a at closed position. Therefore, the wheel cylinder 1a is blocked from fluid communication with the master cylinder 11 and the accumulator 22a. Therefore, the fluid pressure in the wheel cylinder 1a is held constant. The relationship of the inlet and outlet control signals EV.sub.1 and AV.sub.1 and the pump drive signal MR will be clearly seen from the following table:

The control unit 18 is connected to a wheel speed sensors 26a, 26b, 26c and 26d for receiving pulse signals having frequency proportional to the rotation speed of the associated wheel 1. In practice, each of the wheel speed sensors 26a, 26b, 26c and 26d comprises a sensor rotor adapted to rotate with the vehicle wheel and a sensor assembly fixedly secured to the shim portion of the knucle spindle. The sensor rotor is fixedly secured to a wheel hub for rotation with the vehicle wheel. The sensor rotor may be formed with a plurality of sensor teeth at regular angular intervals. The width of the teeth and the grooves therebetween are preferably equal to each other and define a unit angle of wheel rotation. The sensor assembly comprises a magnetic core aligned with a north pole near the sensor rotor and a south pole distance from the sensor rotor. A metal element with a smaller diameter section is attached to the end of the magnetic core near the sensor rotor. The free end of the metal element faces the sensor teeth. An electromagnetic coil encircles the smaller diameter section of of the metal element. The electromagnetic coil is adapted to detect a variation in the magnetic field generated by the magnetic core to produce an alternating-current sensor signal. Namely, the metal element and the magnetic core form a kind of proximity switch which adjusts the magnitude of the magnetic field depending upon the distance between the free end of the metal element and the sensor rotor surface. Thus, the intensity of the magnetic field fluctuates in relation to the passage of the sensor teeth and accordingly in relation to the annular velocity of the wheel.

Such a wheel speed sensor has been disclosed in U.S. Pat. No. 4,597,052, issued on June 24, 1986, for example. The disclosure of the U.S. Pat. No. 4,597,052 is herein incorporated by reference for the sake of disclosure.

The control unit 18 has a controller circuit section 18a for performing wheel slip control for the right-front wheel 1. Similarly, the control unit 18 has controller circuit sections 18b and 18c respectively adapted to perform wheel slip control for respectively associated left-front wheel 2 and rear wheels 3 and 4. The controller circuit section 18a receives the wheel speed indicative pulse signal from the wheel speed sensor 26a. Similarly, the controller circuit section 18b receives a wheel speed indicative pulse signal from a wheel speed sensor 26b which monitors rotation speed of the left-front wheel 2. A wheel speed sensors 26c and 26d are connected to the controller circuit section 18c for inputting wheel speed indicative pulse signals, which monitor rotation speeds of respectively associated right-rear wheel 3 and left-rear wheel 4. As the anti-skid brake control valves 17a, 17b, 17c and 17d, the controller circuit sections 18a, 18b and 18c are essentially the identical circuit constructions to each other. Therefore, the following discussion will be given only for the controller circuit section 18a. The corresponding circuit elements in the controller circuit sections 18b and 18c are represented by the same reference numerals with different suffixes which identifies the section to include the circuit elements.

As will be seen from FIG. 1, the controller circuit section 18a includes a wheel speed derivation circuit 31a and a wheel acceleration derivation circuit 32a. The wheel speed derivation circuit 31a receives the wheel speed indicative pulse signal from the wheel speed sensor 26a. Based on the frequency or pulse period of the wheel speed indicative pulse signal of the wheel speed sensor 26a and rotation radius of the right-front wheel, the wheel speed derivation circuit 31a derives an angular velocity of the right-front wheel and derives an instantaneous wheel speed Vw.sub.1 of the right-front wheel 1. The wheel speed derivation circuit 31a produces a wheel speed signal indicative of the derived wheel speed Vw.sub.1. The wheel acceleration derivation circuit 32a receives the wheel speed signal from the wheel speed derivation circuit 31a. The wheel acceleration derivation circuit 32a derives wheel acceleration aw.sub.1 and produces a wheel acceleration indicative signal. Derivation of the wheel acceleration aw.sub.1 based on the variation of wheel speed Vw.sub.1 indicated in the wheel speed signal, can be performed by differentiating the differences of the wheel speeds input at different and consecutive timing. Otherwise, the wheel acceleration may be derived directly from the wheel speed indicative pulse signal from the wheel speed sensor 26a in a manner disclosed in the aforementioned U.S. Pat. No. 4,597,052.

The wheel acceleration indicative signal of the wheel acceleration derivation circuit 32a is transmitted to a comparators 33a and 34a. In order to receive the wheel acceleration indicative signal, the comparator 33a is connected to the wheel acceleration derivation circuit 32a at an inverting input terminal. On the other hand, the comparator 34a is connected to the wheel acceleration derivation circuit 32a at a non-inverting input terminal thereof for receiving the wheel acceleration indicative signal therethrough. The wheel acceleration derivation circuit 32a is further connected to a peak detector circuit 44a which is designed for detecting a peak value of the wheel acceleration aw.sub.1 and holds the peak value.

The non-inverting input terminal of the comparator 33a is connected to a reference signal generator (not shown) for receiving therefrom a deceleration threshold indicative reference signal -b. The deceleration threshold indicative reference signal has a value representative of a predetermined deceleration threshold to be compared with the wheel acceleration value aw.sub.1. The comparator 33a normally outputs a LOW level comparator signal as long as the wheel acceleration .alpha.w.sub.1 is maintained higher than the deceleration threshold -b. The comparator 33a is responsive to the wheel acceleration aw.sub.1 dropping across the deceleration threshold -b to output a HIGH level comparator signal.

The inverting input terminal of the comparator 34a is connected to a reference signal generator (not shown) which generates an acceleration threshold indicative reference signal having a value indicative of a predetermined wheel acceleration threshold +a. The comparator 34a thus compares the wheel acceleration aw.sub.1 as indicated in the wheel acceleration indicative signal of the wheel acceleration derivation circuit 32a with the wheel acceleration threhshold +a. The comparator 34a generally outputs a LOW level comparator signal while the wheel acceleration .alpha.w.sub.1 is held lower than the wheel acceleration threshold +a. The comparator 34a produces a HIGH level comparator signal when the wheel acceleration .alpha.w.sub.1 rises across the wheel acceleration threshold.

Another comparator 35a is provided in the controller circuit section 18a. The comparator 35a has an inverting input terminal connected the wheel speed derivation circuit 31a for receiving therefrom the wheel speed signal. The comparator 35a also has a non-inverting input terminal connected to a target wheel speed derivation circuit 28a. The target wheel speed derivation circuit 28a generally derives a target wheel speed V.lambda. on the basis of a vehicle speed representing value Vi and a desired optimum wheel slippage for optimizing vehicular braking efficiency. As is well known, the vehicular braking efficiency becomes maximum in a wheel slippage range of 10% to 20%. In the practical embodiment, the target wheel slippage .alpha. is set at 15%. On the other hand, the vehicle speed representing value Vi is derived on the basis of an instantaneous wheel speed Vw at the begining of each cycle of anti-skid brake control. The procedure of derivation of the vehicule speed representing value Vi discussed later. The target wheel speed V.lambda. is derived at a value of 85% of the vehicle speed representing value Vi to indicate 15% of wheel slippage. The comparator 35a maintains a LOW level comparator signal while the wheel speed Vw is maintained higher than the target wheel speed V.lambda.. On the other hand, the comparator signal level of the comparator 35a turns HIGH when the wheel speed Vw drops across the target wheel speed V.lambda..

The comparator signals of the comparators 33a and 34a and 35a are fed to input terminals of an OR gate 36. The comparator 34a and 35a are also connected to an AND gate 38 to feed the comparator signals. The AND gate 38a has an inverting input terminal connected to the comparator 35a to receive therefrom the comparator signal. The comparator 35a is further connected to a variable timer circuit 42a which will be described later.

The output terminal of the OR gate 36a is connected to one input terminal of an OR gate 40a. The other input terminal of the OR gate 40a is connected to an AND gate 41a. The AND gate 41a has one input terminal connected to the variable timer 42a. Another input terminal of the AND gate 41a is connected to an oscillator 43a which serves as a clock generator. The other input terminal of the AND gate 41a is connected to a retriggerable timer circuit 30 which is desiged for producing a pump drive signal MR for driving the fluid pump 21a, 21b and 21c by applying the pump drive signal MR to pump drive motor 24 through a switching transistor 25.

The output terminal of the OR gate 40a is connected to the EV valve 19a through an amplifier 37a to feed the output thereof as the inlet control signal EV.sub.1. On the other hand, the output terminal of the AND gate 38a is connected to the AV valve 20a via an amplifier 39a to feed the output as the outlet control signal AV.sub.1.

A vehicle speed representing value derivation circuit 27a is connected to the wheel speed derivation circuit 31a to receive therefrom the wheel speed signal from time-to-time. The vehicle speed representing value derivation circuit 27a is also connected to the retriggerable timer circuit 30. The vehicle speed representing value derivation circuit 27a is designed to latch an instantaneous wheel speed Vw as an initial vehicle speed representing value Vi.sub.1 in response to the leading edge of a HIGH level timer signal serving as the pump drive signal MR. The vehicle speed representing value derivation circuit 27a derives the vehicle speed representing value Vi.sub.1 on the basis of the initial vehicle speed representing value corresponding to the latched wheel speed value Vw.sub.1 as set forth above.

The vehicle speed representing value derivation circuit 27a is associated with other vehicle speed representing value derivation circuits 27b, 27c and 27d which derive the vehicle speed representing values Vi.sub.2, Vi.sub.3 and Vi.sub.4 with respect to respectively associated left-front wheel 2, right-rear wheel 3 and left-rear wheel 4, in order to form the preferred embodiment of a vehicle speed representing value derivation system 27. The vehicle speed representing value derivation system 27 also includes a select-LOW switch 58 having three terminals respectively connected to the vehicle speed representinv value derivation circuits 27a, 27b, 27c and 27d. The select-LOW switch 58 selects the smallest value among three vehicle speed representing values Vi.sub.1, Vi.sub.2, Vi.sub.3 and Vi.sub.4 from respective vehicle speed representing value derivation circuits 27a, 27b and 27c and outputs the selected value as a common vehicle speed representing value Vi. The comon vehicle speed representing value Vi is transferred to respective target wheel speed derivation circuits 28a, 28b and 28c in the controller circuit sections 18a, 18b and 18c so that the target wheel derivation circuits will be able to derive the target wheel speeds V.lambda..sub.1, V.lambda..sub.2 V.lambda..sub.3 and V.lambda..sub.4 based thereon.

As will be appreciated herefrom, the select-HIGH switch 58 selects one of the vehicle speed representing values Vi.sub.1, Vi.sub.2, Vi.sub.3 and Vi.sub.4 as select-HIGH value Vi.sub.H. The select-HIGH switch 58 outputs a select-HIGH value indicative signal and distributes the same to respectively corresponding target wheel speed derivation circuits 28a, 28b and 28c.

FIG. 2 shows detailed construction of the peak detector circuit 44a in the controller circuit section 18a set forth above. As will be seen from FIG. 2, the peak hold circuit 44a generally comprises a peak hold circuit 49a and an analog switch 49b. The peak hold circuit 48 is consisted of buffer amplifiers 45 and 46, a diode 47 and a capacitor 48. The buffer amplifier 45 of the peak hold circuit 49a is connected to the wheel acceleration derivation circuit 32a to receive therefrom the wheel acceleration indicative signal having a value representative of the wheel acceleration .alpha.w.sub.1, at a non-inverted input terminal. An inverting input terminal of the buffer amplifier 45 is connected to an output terminal thereof for receiving the amplifier output as feedback input. The buffer amplifier outputs through its output terminal the amplifier output indicative of one of the inputs greater than the other. The amplifier output of the buffer amplifier 45 is fed to a charge/discharge circuit consisting of the diode 47 and the capacitor 48 and to a non-inverted input terminal of the other buffer amplifier 46. Similarly to the aforementioned buffer amplifier 45, an inverting input terminal of the buffer amplifier 46 is connected to an output terminal thereof to receive the amplifier output as feedback input.

The inmput side of the capacitor 48 is connected to the ground through the analog switch 49b. The analog switch 49b is connected to the comparator 33a. Therefore, the analog switch 49a becomes conductive at every leading edge of the HIGH level comparator signal from the comparator 33a for connecting the capacitor 48 to the ground. As a result, the potential in the capacitor 48 is discharged to the ground. Since the analog switch 49a is held conductive while the comparator signal from the comparator 33a is held at HIGH level, the potential in the capacitor 48 is maintained substantially zero during this period. The analog switch 49a becomes non-conductive in response to the trailing edge of the HIGH level comparator signal from the comparator 33a to block communication between the capacitor and the ground. As a result, the capacitor 48 starts to be charged by the amplifier output from the buffer amplifier 45. The potential in the capacitor 48 is increased as increasing of the wheel acceleration .alpha.w.sub.1 and held at the value corresponding to the peak value of the wheel acceleration, as shown in FIG. 3. Therefore, the amplifier output from the buffer amplifier 46 indicates peak value .alpha.w.sub.max of the wheel acceleration. As set forth above, since the potential in the capacitor 48 is discharged everytime the comparator signal rises to HIGh, the peak value .alpha.w.sub.max output from the peak detector circuit 44a represents the peak value of the wheel acceleration .alpha.w.sub.1 in each skid cycle, while the anti-skid brake control is performed.

FIG. 4 shows a detailed construction of the variable timer 42a. The variable timer 42a generally comprises a first timer 51 and a second timer 52. The first timer 50 has an input port B connected to the output terminal of the comparator 34a through an inverter 52 to receive therefrom the inverted comparator signal. The first timer 50 is designed to be triggered in response to the trailing edge of the HIGH level input at the input port B to output timer signal Q.sub.A for a given period of time T.sub.1. The period T.sub.1 to maintain the timer signal Q.sub.A is determined by a time constant derived from a capacity of a capacitor 54 and a resistance of a variable resistor 55 connected to the terminals T.sub.1 and T.sub.2 of the first timer 50. The variable resistor 55 is connected to the aforementioned peak detector circuit 44a to receive therefrom the amplifier output as a wheel acceleration peak indicative input. The resistance of the variable resistor 55 is adjusted depnding upon the peak value .alpha.w.sub.max as indicated in the wheel acceleration peak indicative signal. Therefore, the timer period of the first timer 50 is variable in proportion to the magnitude of the peak value .alpha.w.sub.max.

The timer signal Q.sub.A of the first timer 50 is fed to a B input port of the second timer 51. The second timer 51 has a time constant circuit consisted of a capacitor 56 and a variable resistor 57 for providing a timer period T.sub.2. The time constant of the time constant circuit of the capacitor 56 and the variable resistor 57 is set constant to set the timer period T.sub.2 constant. Therefore, the second timer 51 is triggered by the trailing edge of the HIGH level input at the B input terminal. The second timer 51 as triggerd outputs HIGH level timer signal for the timer period determined by the time constant of the time constant circuit of the capacitor 56 and the variable resistor 57, through Q.sub.B output terminal for the given period of time T.sub.2, as shown in FIG. 5.

FIG. 6 shows the detailed construction of the vehicle speed representing value derivation circuit 27a. As set forth above, the vehicle speed representing value derivation circuit 27a derives a vehicle speed representing value Vi.sub.1 based on the wheel speed Vw.sub.1 as indicated in the wheel speed signal from the wheel speed derivation circuit 31a. The vehicle speed representing value derivation circuit 27a includes comparators 59 and 60. The comparator 59 has a non-inverting input terminal connected to the wheel speed derivation circuit 31a. On the other hand, the comparator 60 is connected to the wheel speed derivation circuit 31a at an inverting input terminal. An inverting input terminal of the comparator 59 is connected to output terminal of the vehicle speed representing value derivation circuit 27a through which the vehicle speed representing value Vi.sub.1 is output, through an adder 61. On the other hand, the non-inverting input terminal of the comparator 60 is connected to the output terminal of the vehicle speed representing value derivation circuit 27a through a subtractor 62. The adder 61 is designed to add a given value corresponding to 1 km/h of vehicle speed to the vehicle speed representing value Vi.sub.1 for providing dead band of +1 km/h. The value as the sum of the vehicle speed representing value Vi.sub.1 and the dead band value 1 km/h will be hereafter referred to as higher vehicle speed reference value . Similarly, the subtractor 672 subtracts a given value corresponding to 1 km/h of the vehicle speed from the vehicle speed representing value Vi.sub.1 for providing dead band of -1 km/h. The value as the difference of the vehicle speed representing value Vi.sub.1 and the dead band value -1 km/h will be hereafter referred to as lower vehicle speed reference value. The comparator 59 outputs HIGH level comparator signal when the wheel speed vw.sub.1 is higher than or equal to the higher vehicle speed reference value (Vi.sub.1 +1 km/h). In other words, the comparator signal level of the comparator 59 is held LOW as long as the wheel speed Vw.sub.1 is maintained lower than the lower vehicle speed reference value (Vi.sub.1 +1 km/h). The comparator 60 outputs HIGH level comparator signal when the wheel speed vw.sub.1 is lower than to the lower vehicle speed reference value (Vi.sub.1 -1 km/h). In other words, the comparator signal level of the comparator 59 is held LOW as long as the wheel speed Vw.sub.1 is maintained higher than or equal to the lower vehicle speed reference value (Vi.sub.1 -1 km/h).

The output terminals of the comparators 59 and 60 are connected to input terminals of NOR gate 63 to feed the comparator signals c.sub.1 and c.sub.2 thereto. The NOR gate 63 outputs HIGH level gate signal while signal levels of both of the comparator signals c.sub.1 and c.sub.2 are maintained LOW. Namely, the gate signal output from the NOR gate 63 is held LOW while the wheel speed Vw.sub.1 is maintained higher than or equal to the vehicle speed representing value Vi.sub.1 -1 km/h and lower than the higher vehicle speed reference value (Vi.sub.1 +1 km/h). The gate signal of the NOR gate 63 is fed to a timer 64, an OR gate 65 and a shot-pulse generator 66, respectively. The timer 64 is responsive to the trailing edge of the HIGH level NOR gate signal to output a timer signal for a given period of time T.sub.3, e.g. 0.1 sec. The timer signal is fed to the OR gate 65.

The OR gate 65 thus receives the NOR gate signal at one input terminal and the timer signal from the timer 64 at the other input terminal. An OR gate signal of the OR gate 65 is transmitted to a gate of an analog switch 67 as a selector signal S.sub.3. The output terminal of the OR gate 65 is also connected to one input terminal of an AND gates 69 and 70 via an inverter 68. The other input terminal of the AND gate 69 is connected to the output terminal of the comparator 59 to receive therefrom the comparator signal c.sub.1. Similarly, the other input terminal of the AND gate 70 is connected to the output terminal of the comparator 60 to receive the comparator signal c.sub.2 therefrom. Therefore, the gate signal S.sub.2 of the AND gate 69 becomes HIGH while the comparator signal c.sub.1 is maintained at HIGH level and the NOR gate signal is held LOW. The gate signal S.sub.2 serves as a selector signal. On the other hand, the gate signal S.sub.4 of the AND gate 70 becomes HIGH level while the comparator signal c.sub.2 is maintained HIGH and the NOR gate signal is held LOW. This gate signal S.sub.4 also serves as selector signal. The AND gates 69 and 70 are connected to gates of analog switches 71 and 72.

The analog switch 67 is turned ON in response to HIGH level selector signal S.sub.3 to drop the supply voltage to an integrator circuit 73 to zero. On the other hand, the analog switch 71 is turned ON in response to the HIGH level selector signal S.sub.2 to supply a voltage E corresponding to a possible maximum wheel acceleration, e.g. 0.4G, to the integrator circuit 73. The analog switch 72 is likewise turned ON in response to the HIGH level selector signal S.sub.4 to supply a voltage corresponding to possible minimum wheel acceleration value, e.g. -1.2G, to the integrator circuit 73.

The integrator circuit 73 has a per se well known construction and is consisted of an amplifier 74, a capacitor 75 and an analog switch 76. The gate of the analog switch 76 is connected to the shot-pulse generator 66 to receive therefrom a shot-pulse which serves as a reset signal S.sub.1. The integrator 73 is reset by the HIGH level reset signal S.sub.1 and is responsive to the trailing edge of the HIGH level reset signal to reset the integrated value. The integrator circuit 73 integrates the supply voltage E after termination of the HIGH level reset signal S.sub.1 to output the integrator signal. The shot-pulse generator 66 is responsive to an ON-set signal IG of an ignition switch to generate a first shot pulse as the first reset signal for resetting the integrator circuit 73. The shot-pulse generator 66 subsequently generates the shot-pulses serving as the reset signal S.sub.1 at every leading edge of the HIGH level NOR gate signal. As set forth above, since the NOR gate signal becomes HIGH when the wheel speed Vw.sub.1 satisfies (Vi.sub.1 -1 km/h).ltoreq.Vw.sub.1 <(Vi.sub.1 +1 km/h), the integrated value of the integrator 73 is reset every occurence of the wheel speed Vw.sub.1 in the aforementioned range. The reset signal S.sub.1 of the shot-pulse generator 66 is also supplied to a sample hold circuit 77. The sample hold circuit 77 comprises buffer amplifiers 78 and 79, a capacitor 80 and an analog switch 81. The analog switch 81 is connected to the shot-pulse generator 66 to receive the reset signal S.sub.1 at the gate thereof to be turned ON. The sample hold circuit 77 is rsponsive to turning ON of the analog switch 81 to reset the held wheel speed value. The sample hold circuit 77 in absence of the reset signal S.sub.1 from the shot-pulse generator 66, samples and holds the instantaneous wheel speed value Vw.sub.1 at the occurence of the reset signal as a sample value Vs. The sample hold circuit 77 outputs a sample/hold signal having a value indicative of the sample value Vs to an adder 82. The adder receives the sample/hold signal from the sample hold circuit 77 and integrator signal from the integrator 73. As will be appreciated, the integrator signal has a value indicative of an integrated value ##EQU1##

Therefore, the adder 82 adds the integrated value Ve to the sample value Vs to derive the vehicle speed representing value Vi.sub.1. The output terminal of the adder 82 is connected to a switching circuit 83. The switching circuit 83 is also directly connected to the wheel spee derivation circuit 31a to be input the wheel speed signal. On the other hand, the switching circuit 83 also connected to an AND gate 84. The AND gate 84 has one input terminal connected to a retriggerable timer 30 to receive therefrom the pump drive signal MR. The other input terminal of the AND gate 84 is connected to the output terminal of the comparator 59. The AND gate 84 controls the switch position of the switching circuit 83 to selectively connect the wheel speed derivation circuit 31a or the adder 82 to the output terminal of the vehuicle speed representing value derivation circuit 27a.

Namely, the gate signal of the AND gate is normally maintained LOW level due to absence of the HIGH level pump drive signal MR. The gate signal of the AND gate is also held LOW level while the wheel acceleration is negative or that the wheel speed Vw.sub.1 is lower than the Vi.sub.1 +1 km/h value as compared in the comparator 59. While the gate signal is held LOW, the switching circuit 82 is held at a first switch position where the adder 82 is connected to the output terminal of the vehicle speed representing value derivation circuit therethrough. On the other hand, when the HIGH level pump drive signal MR and the HIGH level comparator signal of the comparator 59 are both input to the AND gate, the gate signal of the AND gate 84 turns HIGH to switch the switching circuit 83 to a second switch position where the wheel speed derivation circuit 31a is directly connected to the output terminal of the vehicle speed representing value derivation circuit 27a.

Operation of the vehicle speed representing derivation circuit 27a will be described herebelow with reference to FIG. 7. In FIG. 7, the operation of the vehicle speed representing value derivation circuit 27a is described in a condition where the gate signal level of the AND gate 84 is maintained LOW level due to absence of the HIGH level pump signal MR or the comparator signal c.sub.1 from the comparator 59 is held LOW. At this condition, by the LOW level gate signal of the AND gate 84, the switching circuit 83 is switched at a switch position where the adder 82 is connected to the output terminal of the vehicle speed representing value derivation circuit.

In the process of FIG. 7, the engine is started up at a time t.sub.0. In response thereto, the On-set signal IG is input to the shot-pulse generator 66. Therefore, the shot pulse s.sub.1 is output at the time t.sub.0 from the shot-pulse generator 66. With the shot pulse at the time t.sub.0, the sample hold circuit 77 is reset. The sample/hold circuit 77 as reset by the shot pulse s.sub.1 at the time t.sub.0, samples and holds the wheel speed signal value Vw.sub.1 as sample value Vs. Therefore, after the time t.sub.0, the held sample value Vs is output from the sample hold circuit 77 as an initial vehicle speed representing value. At the same time, i.e. at the time t.sub.0, the integrator circuit 73 is reset by the reset signal s.sub.1. Therefore, the value Ve of the integrator signal of the integrator circuit 73 drops to zero. As a result, the output value Vi.sub.1 from the adder 82 becomes that equal to the held initial vehicle speed representing value Vs, as shown by broken line in FIG. 7.

At this time, the comparator signals c.sub.1 and c.sub.2 of the comparators 59 and 60 are maintained LOW level. Therefore, the NOR gate signal of the NOR gate 63 is maintained HIGH level. Therefore, the gate signal level of the OR gate 65 is maintained HIGH level and the gate signal is fed to the analog switch 67 as the select signal S.sub.3. Therefore, the analog switch 67 is turned ON by the HIGH level gate signal from the OR gate 65. On the other hand, the HIGH level gate signal of the OR gate 65 is fed to the AND gates 69 and 70 through the inverter 68. Therefore, the select signals S.sub.2 and S.sub.4 of the AND gates 69 and 70 are held LOW to maintain the analog switches 71 and 72 at OFF position. Since the analog switch 67 serves to maintain the input level at the inverting input terminal of the comparator 74 in the intergrator circuit 73 zero, the integrated value of the integrator circuit 73 is maintained zero. As a consequence, the output value of the adder 82 as the vehicle speed representing value Vi is maintained at the equal value to the initial vehicle speed representing value as the sample value Vs.

After the vehicle starts running, the wheel speed Vw becomes greater than or equal to Vi.sub.1 +1 km/h, at a time t.sub.1. In response to this, the comparator signal of the comparator 59 turns HIGH level. By the HIGH level comparator signal c.sub.1 from the comparator 59, the gate signal of the NOR gate 63 turns LOW. At this time, since the timer 64 becomes active to output HIGH level timer signal for a period of time T.sub.3, the gate signal level of the OR gate 65 is maintained HIGH level for the corresponding T.sub.3 period. Therefore, the select signal S.sub.3 is maintained HIGH level and the selector signals S.sub.2 and S.sub.4 are held at LOW level. Therefore, even after the vehicle starts running the vehicle speed representing value Vi.sub.1 is held at the equal value to the sample value Vs for the T.sub.3 period.

After expiration of the T.sub.3 period, at a time t.sub.2, the gate signal of the OR gate 65 turned into LOW level due to termination of the HIGH level timer signal from the timer 64. Since the comparator signal c.sub.1 and the inverted gate signal from the OR gate 65 through the inverter 68 are both becomes HIGH level, the selector signal S.sub.2 of the AND gate 69 turns HIGH. At the same time, because of LOW level gate signal of the OR gate 65 is applied to the gate of the analog switch 67 as the select signal S.sub.3 to turn the latter OFF. At this time, since the comparator signal from the comparator 60 is maintained LOW level, the AND gate 70 is maintained non-conductive to feed the lOW level select signal S.sub.4. Therefore, only analog switch 71 is turned ON to input a value corresponding to wheel acceleration magnitude of 0.4G. This value, e.g. 0.4G serves for defining inclination of the vehicle speed representing valuie Vi.sub.1. The integrator circuit 73 thus receives the value (0.4G) through the analog switch 71 to output the integrator signal having a value Ve as set forth above. Therefore, the output value of the adder 82 increases from time-to-time as increasing of the integrator signal value Ve.

At a time t.sub.3, the vehicle speed representing value Vi.sub.1 (=Vs+Ve) reaches a value to establish Vw.sub.1 <Vi.sub.1 +1 km/h. Then, the comparator signal c.sub.1 turns into LOW level. Therefore, the gate signal level of the NOR gate 63 again turns into HIGH level. The shot-pulse generator 66 is triggered by the leading edge of the HIGH level gate signal of the NOR gate to output the shot pulse serving as the reset pulse S.sub.1. Therefore, the sample hold circuit 77 and the integrator circuit 73 are reset. At the same time, the instantaneous wheel speed Vw.sub.1 at the time t.sub.3 is sampled and held in the sample hold circuit 77 as the renewed sample value Vs. By renewing the sample value Vs, the vehicle speed representing value Vi.sub.1 becomes equal to the instantaneous wheel speed value Vw.sub.1 and thus establishes Vw.sub.1 .gtoreq.Vi+1 km/h. Therefore, the signal c.sub.1 again turns ON at the time t.sub.3. Similarly to the control behavior at the period from t.sub.1 to t.sub.3, the gate signal of the OR gate 65 is held HIGH for the T.sub.3 period by HIGH level timer signal of the timer 64. Similarly, at the time t.sub.4, the shot-pulse generator 66 is triggered to output the reset signal S.sub.1 to renew the sample value Vs by the instantaneous wheel speed Vi.sub.1 at the time t.sub.4. After the time t.sub.4, the sampled value Vs is maintained constant for the given period T.sub.3 by the HIGH level timer signal of the timer 64. Before the HIGH level timer signal terminates, the shot pulse as the reset signal is generated by the shot pulse generator 66, as shown in FIG. 7. While the interval of the shot pulse of the shot pulse generator 66 is shorter than the timer period T.sub.3, the integrated value of the integrator 73 is maintained zero. Therefore, the adder outputs the sampled value Vs as the vehicle speed representing value Vi.sub.1. After a time t.sub.5, at which the reset signal s.sub.1 is produced for resetting the sample hold circuit 77 and the intergrator 73, the gate signal of the OR gate turns LOW at a time t.sub.6 after expeiration of the timer period T.sub.3. During the period between the times t.sub.5 and t.sub.6, the wheel speed Vw.sub.1 drops to be lower than (Vi.sub.1 -1 km/h). Since the wheel speed Vw.sub.1 is maintained lower than the value (Vi.sub.1 -1 km/h) at the time t.sub.6, the comparator signal c.sub.1 of the comparator 59 is maintained LOW and the comparator signal c.sub.2 of the comparator 60 is maintained HIGH. Therefore, the analog switch 71 is held OFF and the analog switch 72 is turned ON. Therefore, a value corresponding to a predetermined deceleration magnitude of -1.2G is input through the analog switch 72. Consenquently, the integrated value in the integrator 73 becomes negative. The negative integrated value is summed with the sample value Vs which corresponds to the instantaneous wheel speed Vw.sub.1 at a time t.sub.5 to gradually reduce the value of the vehicle speed representing value Vi.sub.1. At a time t.sub.7, the wheel speed Vw.sub.1 is increaseed across the value (Vi.sub.1 +1 km/h). As a result, the reset signal s.sub.1 is generated by the shot pulse generator 66. Then sample hold circuit 77 and the integrator 73 are thus reset.

At a time t.sub.8, braking operation is initiated to abruptly decelerate the vehicle. As a result, the wheel speed Vw.sub.1 drops across the value of (Vi.sub.1 -1 km/h). By this, the comparator signal c.sub.2 of the comparator rises to HIGH level to destroy the NOR condition at the NOR gate 63. Therefore, the NOR gate signal of the NOR gate turns into LOW level. The timer 64 is thus triggered by the trailing edge of the HIGh level NOR gate signal to output HIGH level timer signal for the timer period T.sub.3. After expiration of the timer period T.sub.3, the value corresponding to the deceleration magnitude of -1.2G is input to the integrator. Therefore, the vehicle speed representing value Vi.sub.1 is gradually decreased.

FIG. 8 shows a timing chart of the operation of the controller circuit section 18c for showing the principle of the present invention.

It is assumed that vehicular brake is applied for deceleration of the vehicle at a time t.sub.10, hydraulic braking pressure is built up and distributed to all of the wheel cylinders 1a, 2a, 3a and 4a in common. According to the increasing of the braking pressure in the wheel cylinders 3a and 4a, the rear wheels 3 and 4 are decelerated. As shown in FIG. 8, during this deceleration period, the right-rear wheel speed Vw.sub.3 is higher than the left-rear wheel speed Vw.sub.4. Therefore, the select-LOW switch 90 connects the wheel speed derivation circuit 31d which outputs the left-rear wheel speed indicative signal to the wheel acceleration derivation circuit 32c and to the comparator 35c. This wheel speed indicative signal selected by the select-LOW switch 90 will be hereafter referred to as "selected rear wheel speed indicative signal" and the value thereof will be referred to as "lower rear wheel speed Vw.sub.R "". The wheel acceleration aw.sub.3 in the rear wheels 3 and 4 are derived on the basis of the lower rear wheel speed Vw.sub.R. In FIG. 8, a one-dotted line identified by "Vc" represents the actual vehicle speed for a reference.

At a time t.sub.11, the wheel acceleration aw.sub.3 decreases across the wheel deceleration threshold -b. This results in initiation of anti-skid brake control. Namely, since the wheel acceleration aw.sub.3 becomes smaller than the wheel deceleration threshold -b, the comparator signal of the comparator 33c turns into HIGH level. This turns the gate signals of the OR gates 36c, 40c and 40d into HIGh level. Therefore, the inlet control signals EV.sub.3 and EV.sub.4 output through the amplifiers 37c and 37d turns HIGH level. Therefore, both of the EV valves 19c and 19d are shut at the time .sub.1. At this time, since the output of the AND gate 38c is held LOW level, the outlet control signal AV.sub.3 which is commonly used as the outlet signal AV.sub.4 for the left-rear wheel cylinder 4a, is held LOW. Therefore, the AV valves 20c and 20d are also maintained at closed position. Therefore, the anti-skid brake control valve assemblies 17c and 17d are placed in HOLD mode position. The braking pressure in the wheel cylinders 3a and 4a are thus held constant at the pressure level at the time t.sub.1.

For the initial cycle of the anti-skid brake control, the vehicle speed representing value Vi is derived by the vehicle speed representing value derivation system 27. In the shown example, since the right-rear wheel speed Vw.sub.3 is higher than the left-rear wheel speed Vw.sub.4 as set forth above, the vehicle speed representing value Vi.sub.3 derived by the vehicle speed representing value derivation circuit 27c is necessarily higher than the vehicle speed representing value Vi.sub.4 as derived by the vehicle speed representing value derivation circuit 27d. Therefore, the select-HIGH switch 58 selects the Vi.sub.3 as the common vehicle speed representing value Vi. Based on the common vehicle speed representing value Vi, the target wheel speed VL.sub.3 is derived at a value 85% of Vi. As described above, since the vehicle speed representing value decreases according to the integrated value of the integrator 73, the target wheel speed VL.sub.3 decreases continuously. At a time t.sub.12, the lower rear wheel speed Vw.sub.R decelerated across the target wheel speed VL.sub.3. Then, the comparator signal of the comparator 35c turns into HIGh level. At this time, since the comparator signal of the comparator 34c is held LOW to apply HIGH level input to the AND gate 38c through the inverting input terminal, AND condition is established at the AND gate 38c. Therefore, the gate signal of the AND gate 38c turns into HIGH level to cause HIGH level outlet control signals AV.sub.3 and AV.sub.4. This operates the anti-skid brake control valve assemblies 17c and 17d into the RELEASE mode position. Therefore, the braking pressure in the wheel cylinders 3a and 4a are released by feeding back the pressurized braking fluid to the pressure accumulators 22c and 22d. At the same time, by the HIGH level outlet control signal AV.sub.3 and AV.sub.4, the retriggerable timer 30 is triggered to start outputting of the pump drive signal MR. Therefore, the pump motor 24 is start to drive the fluid pumps 21c and 21d.

By decreasing the braking pressure in the wheel cylinders 3a and 4a, the wheel speeds Vw.sub.3 and Vw.sub.4 are resumed and therefore wheel acceleration .alpha.w.sub.3 is increased. The wheel acceleration .alpha.w.sub.3 increases across the wheel deceleration threshold -b at a time t.sub.13. Therefore, the comparator signal of the comparator 33c turns into LOW level at the time .sub.13. However, at this time, since the HIGH level comparator signal is input to the OR gate 36 is maintained HIGH level. Therefore, the gate signal of the OR gates 40c and 40d are held HIGH to maintain the inlet control signals EV.sub.3 and EV.sub.4 HIGH level. Therefore, the anti-skid control valve assemblies 17c and 17d are maintained at the RELEASE mode, at the time t.sub.13. thus, the wheel speeds Vw.sub.3 and Vw.sub.4 are continued to increase. Accordingly, the wheel acceleration .alpha.w.sub.3 increases. At a time .sub.14, the wheel acceleration .alpha.w.sub.3 increases across the wheel acceleration threshold +a. This results in HIGH level comparator signal of the comparator 34c. This HIGH level comparator signal of the comparator 34c turns the input level at the inverting input terminal of the AND gate 38c into LOW level. Therefore, the AND gate signal of the AND gate 38c turns into LOW level. Therefore, the outlet control signals AV.sub.3 and AV.sub.4 turns into LOW level to close the AV valves 20c and 20d in the anti-skid brake control valve assemblies 17c and 17d . As a result, the anti-skid brake control valve assemblies 17c and 17d are again placed into the HOLD mode to maintain the braking pressure level constant at the pressure level at the time t.sub.14. By holding the braking pressure at decreased level, the wheel speed Vw.sub.3 and Vw.sub.4 still increases. Also, according to increasing of the wheel speed, wheel acceleration .alpha.w.sub.3 increases toward the peak .alpha.w.sub.max. As seen from FIG. 8, the lower rear wheel speed Vw.sub.R increases across the target wheel speed VL.sub.3 at a time t.sub.15. This results in LOW level comparator signal of the comparator 35c. After this, at a time .sub.16, the wheel acceleration .alpha.w.sub.3 drops across the wheel acceleration threshold +a. In response to drop of the wheel acceleration .alpha.w.sub.3 across the wheel acceleration threshold +a, the comparator signal of the comparator 34c turns into LOW level. Therefore, all of the inputs to the OR gate 36c becomes LOW level. Therefore, the OR gate signal of the OR gate 36c turns into LOW level to cause LOW level inlet control signals EV.sub.3 and EV.sub.4 at the time t.sub.16. At the same time, the variable timer 42c is activated by the trailing edge of the HIGH level comparator signal of the comparator 34c to output HIGH level timer signal for a period T.sub.2 after a delay time T.sub.1 which is variable depending upon the wheel acceleration peak value as latched by the peak hold circuit 44c. During the period T.sub.2, the oscillators 43c and 43d are triggered to output constant pulse signals. As will be seen from FIG. 8, the duty cycle of the pulse signal generated by the oscillator 43d is greater than that of the oscillator 43c to have longer ON period than that of the pulse signal of the oscillator 43c. As will be appreciated, while the pulse signal is maintained ON (HIGH) level, ANd conditions are established in the AND ate 41c and 41d to maintain the inlet control signal EV.sub.3 and EV.sub.4. Therefore, during the delay time T.sub.1, the anti-skid brake control valves 17c and 17d are held in APPLICATION mode to increase the braking pressure. On the other hand, during the period T.sub.2, the operation mode of the anti-skid brake control valves 17c and 17d are alternated between APPLICATION mode and HOLD mode repeatedly.

At this time, since the oscillator 43d is provided greater duty cycle in comparison with that of the oscillator 43c, each period to maintain the anti-skid brake control valve assembly 17d in HOLD mode position is longer than that of the anti-skid brake control valve assembly 17c . As a result, braking pressure level at the wheel cylinders 3a and 4a becomes different. Namely, due to longer period to hold the anti-skid brake control valve assembly 17d in the HOLD mode, the braking pressure in the wheel cylinder 4a becomes lower than that in the wheel cylinder 3a. Therefore, the right-rear wheel 3 is decelerated at higher magnitude than that of the left-rear wheel 4.

During the period after the time t.sub.16, the right-rear wheel speed Vw.sub.3 becomes lower than the left-rear wheel speed Vw.sub.4 due to higher braking pressure to be built up in the right-rear wheel cylinder 3a than that in the left-rear wheel cylinder. Therefore, the select-LOW switch 90 is switched to pass the right-rear wheel speed Vw.sub.3 as the lower rear wheel speed Vw.sub.R. Therefore, wheel acceleration .alpha.w.sub.3 is derived on the basis of variation of the right-rear wheel speed Vw.sub.3.

At a time t.sub.17, the wheel acceleration .alpha.w.sub.3 drops across the wheel deceleration threshold -b. Therefore, another cycle of anti-skid brake control is initiated. At the same time, the peak hold circuit 44c is reset by the leading edge of the HIGH level comparator signal of the comparator 33c. Thereafter, one skid cycle of anti-skid brake control operation is performed during the period t.sub.17 to t.sub.22.

As will be appreciated herefrom, the anti-skid brake control to be taken place for releasing the braking pressure in both of the rear wheel cylinders in response to decreasing of the lower rear wheel speed Vw.sub.R across the target wheel speed V.lambda..sub.3 in synchronism with each other and at the same rate. On the other hand, in the APPLICATION mode, the braking pressures in the rear wheel cylinders are built up at different rate to each other. As a result, when one of the wheel is decelerated across the target wheel speed, the other wheel is rotating at higher speed than the target wheel speed. Therefore, it is successfully avoided to cause locking of both wheels at the same time. This assures derivation of the vehicle speed representing value Vi.sub.3 at the value precisely reflecting the vehicle speed to allow precise control for the brake.

It should be appreciated that though the aforementioned embodiment has been disclosed for simultaneously releasing the braking pressure at same rate and building up the braking pressure at the different rate only in the rear wheel cylinders, it should be possible to apply the equivalent control procedure for all of the wheels. Furthermore, it should be appreciated that the invention should not be appreciated to be specified to the foregoing specific embodiment to achieve the desired result. Various embodiments and modifications can be derived without departing from the principle of the invention as set out in the appended claims.

For instance, FIG. 9 shows a modification of the aforementioned vehicle speed representing value derivation circuit 27a , 27b, 27c and 27d. In this modified embodiment, the wheel acceleration indicative value to be introduced to the integrator 73 is variable between +1.0G and +0.4G through the analog switch 71. For selectively introducing the +0.4G value and +10G value, a selector switch 85 is provided. The selector switch 85 is normally in a position to pass the +0.4G value to the integrator 73 and is responsive to HIGH level pump drive signal MR to turn the switch position to pass the +1.0 G value. In the modification, the switch 83 is neglected.

FIG. 10 show another embodiment of the anti-skid brake control system according to the invention. In this embodiment, the AND gate 41d is connected to an OR gate 92 at one input terminal. Other inputs are connected to the variable timer 42c and the oscillator 43c similarly to that in the former embodiment. The OR gate 82 is connected to a timer 91 which is connected to the variable timer to be triggered by the leaidng edge of the timer signal of the variable timer to produce a HIGH level timer signal for a given period of time T.sub.4. As will be seen from FIG. 10, the oscillator 43c are commonly applied for increasing the braking pressures in the right-rear and left-rear wheel cylinders 3a and 4a in stepwise fashion. Therefore, the oscillator 43d employed in the former embodiment can be neglected.

As will be seen from FIG. 11, in the anti-skid brake control system of FIG. 10, the HIGH level timer signal of the timer 91 is applied to the OR gate for the given period of time T.sub.4 during the T.sub.2 period defined by the variable timer 42c. As a result, the output of the OR gate 92 is constantly maintained HIGH during this T.sub.4 period. As a result, the inlet control signal EV.sub.4 is maintained HIGH to maintain the EV valve 19d in the closed position. This provides T.sub.4 period of delay in building up of the braking pressure in the left-rear wheel cylinder 4a in the application mode. Therefore, the braking pressure in the wheel cylinder 4a is maintained lower than that in the wheel cylinder 3a throughout the APPLICATION mode operation. This provides the same effect as that achieved by the former embodiment.

FIG. 12 shows a further embodiment of the anti-skid brake control system according to the present invention. In this embodiment, an OR gate 93 and an amplifier 39d are added to the circuit of FIG. 1. The OR gate 93 is connected to the AND gate 38c at one input terminal. The other input terminal of the OR gate 93 is connected to an AND gate 94 which is a replacement for AND gate 41d in the embodiment of FIG. 1. The AND gate 94 has an inverting input terminal connected to the output terminal of the OR gate 36c. Similarly to the embodiment of FIG. 1, the oscillator 43d is provided greater duty cycle than that of the oscillator 43c.

In this arrangement, while the gate signal of the OR gate 36c is LOW and the timer signal from the variable timer 42c is held HIGH level, the input at the OR gate 93 from the AND gate 94 becomes HIGH in response to HIGH level pulse signal from the oscillator 43d. Therefore, ON period of the pulse signal of the oscillator 43d, HIGH level outlet control signal AV.sub.4 is output to open the AV valve.

On the other hand, the OR gate 40d is connected to the AND gate 94 at one input terminal and to the output terminal of the OR gate 40. Therefore OR gate signal of the OR gate 40 turns HIGH any time HIGH level OR gate signal is out put from the OR gate 40c and HIGH level gats signal.

Therefore, though the increasing cycle of the braking pressure in the left-rear wheel cylinder 4a is same as that of the right-rear wheel cylinder 3a. The braking pressure in the wheel cylinder 4a is maintained lower than that in the wheel cylinder 3a due to opening of the AV valve 20d cyclically as controlled by the pulse signal of the oscillator 43d. This provides essentially the same effect as that provided by the former embodiments of FIG. 1 and FIG. 10.

In analternative, delay of increasing of the braking pressure in one of the wheel cylinder may be provided by providing orifaces 95 in the in the circuit for restricting fluid flow, as shown in FIG. 14.

Therefore, the present invention fulfills all of the objects and advantages sought therefor.