ANTI-SKID BRAKE CONTROL SYSTEM FOR AUTOMOTIVE BRAKE SYSTEM WITH QUICK TAKE-UP FEATURE

Abstract: An anti-skid brake control system controls an automotive brake system for optimizing braking characteristics by adjust­ ing braking pressure nearby a lock pressure. The control sys­ tem is detective of relatively quick wheel speed recovery while a braking pressure is held at a increased level to disable holding control to continuously increase braking pressure until wheel slippage reaches a given value., This shorten take-up time to increase braking pressure approximately the lock pressure.

Description: BACKGROUND OF THE INVENTION The present invention relates generally to an anti-skid brake control system for an automotive brake system, which holds brake pressure to be applied to each wheel at approximately lock pressure at which the vehicle wheel locks to cause skidding. More specifically, the invention relates to a quick take-up function for building up brake pressure close to the lock pressure at earlier timing. As is well known, optimum braking characteristics are obtained when braking pressure or force can be so adjusted that the peripheral speed of the wheels during braking is held to a given ratio, e.g. about 80% to 85%, of the vehicle speed. This practice is believed to be particularly effective when road conditions and other factors are taken into consideration. On the other hand, if wheel speed is held to a ratio to the vehicle speed higher than the above-mentioned optimal ratio, e.g., 80% to 85%, braking distance may be prolong due to lack of braking pressure. On the other hand, if the braking pressure is so adjusted as to hold the wheel speed to a ratio with respect the vehicle speed less than the aforementioned optimal ratio, the vehicle wheels may lock and skid, resulting in an unnecessarily long braking distance due to reduced traction. In practice, it is very difficult to precisely adjust the braking pressure so that the wheel speed is held to the given optimal ratio to the vehicle speed. In the practical anti-skid brake control operation, braking pressure in one or more wheel cylinders is adjusted by cyclically increasing and decreasing the braking pressure in the wheel cylinder. The anti-skid control system generally decreases braking pressure when the wheel deceleration value becomes less than a given deceleration threshold, which is so chosen as to prevent the wheel from skidding, and increases braking pressure when the wheel acceleration value is greater than a given acceleration threshold. In this conventional anti-skid brake control procedure, wheel speed does not remain in an optimal relationship to the vehicle speed for a satisfactorily long period of time. In order to improve performance, US-A-3637264 discloses an Antilocking Control for Pressure Actuated Brakes. The pressure of the brake-actuating fluid in an antilocking brake control system is varied by pulsing the control valve or valves for durations which are varied to be greater or lesser than the period of that limiting frequency above which the brake system cannot respond. In the former case, a rapid increase in fluid pressure or a rapid decrease in fluid pressure occurs, whereas in the latter case, a less rapid average or net increase or decrease occurs in the fluid pressure to which the brake system responds. These conditions are controlled in dependence on the rotational behavior of the vehicle wheel or wheels and more especially in dependence or predetermined changes in angular velocity of the wheel. Moreover, either variation in pulse duration at a fixed frequency or variation in frequency at a fixed pulse duration may be effected during high-frequency pulsing so as further to alter the net increase or decrease in fluid pressure. This further alternation is effected as a function of time from the beginning of the high-frequency pulsing. In addition; JP-A-51-89096 discloses a system similar to the above. The fluid pressure in the wheel cylinder is increased in a stepwise manner. Duration of increase of the fluid pressure is adjusted in accordance with the rate of increase of the fluid pressure in one or more preceding steps. US-A-4 395 761 discloses an anti-skid brake control apparatus in which the inlet valve and outlet valve of the pressure control valve are opened and closed intermittently in a vibratory manner during applying and releasing mode of the control valve in order to avoid any deterioration of the braking sensation due to an increase of the cross-sectional areas of the passages in the hydraulic control system. Such an increase of the cross-sectional areas of the passages is provided in order to avoid clogging in the fluid passages. In this prior art apparatus the above-mentioned on-off operation of the valves is limited only to the braking period of the apparatus in order to avoid a decrease to durability of the valves and its corresponding solenoids when at least one of the valves is activated also during normal running conditions of the vehicle. In such conventional anti-skid brake control system, hunting of switching control mode between application mode in which brake pressure increases and subsequent hold mode in which brake pressure is held constant at increased level tend to occur when friction on the road surface is relatively high. This occurs by relatively quick recovery of wheel speed during hold mode. Hunting of switching between application mode and subsequent hold mode may degrade riding comfort and, as well prolongate braking distance due to rather long period of time for reaching a lock pressure where braking efficiency becomes approximately optimum. Therefore, it is the object of the invention to avoid hunting of switching between application and hold modes and to build-up a brake pressure close to a lock pressure as quickly as possible. Said object is solved by the features as claimed in the characterizing parts of claims 1, 15 and 16. In order to accomplish the above-mentioned object, an anti-skid brake control system, according to the present invention, is detective of accelerating state of wheel during hold mode following application mode in which brake pressure is increased. The system inhibits control entering into hold mode and continue application mode even when switching criteria for switching operation mode from application mode to hold mode is detected, when acceleration state on the wheel in hold mode is detected. For instance, in the anti-skid brake control system according to the present invention, when road friction is so high as to cause recovery of wheel speed at high rate to put the wheel into accelerating state even when brake pressure is held constant at increased pressure level, increasing of brake pressure in application mode is continued until brake pressure reaches approximately a lock pressure. In accordance one aspect of the invention, an anti-skid brake control system for an automotive brake system comprises a hydraulic brake system including a wheel cylinder for applying braking pressure to a vehicle wheel, and a pressure control valve associated with the wheel cylinder for adjusting fluid pressure in the wheel cylinder, the pressure control valve being operative to increase fluid pressure in the wheel cylinder in its first position, to subsequently hold increased fluid pressure constant at its second position, to decrease fluid pressure in the wheel cylinder in its third position, and to subsequently hold decreased fluid pressure constant in its fourth position, wheel speed sensor detecting rotation speed of the wheel and producing a wheel speed indicative signal having a value indicative of the detected wheel speed, a controller deriving a wheel acceleration on the basis of variation of the wheel speed indicative signal values and selecting operation mode of the pressure control valve on the basis of detected wheel speed and derived wheel acceleration to produce a control signal to control the pressure control valve to one of first, second, third and fourth positions corresponding to the selected operation mode, the controller being detecting of hunting in switching of operation modes between the first and second positions to modify selection selecting the second position to select the first position and whereby maintain the first position until the third position is selected. According to another aspect of the invention, an anti-skid brake control system for an automotive brake system comprises a hydraulic brake system including a wheel cylinder for applying braking pressure to a vehicle wheel, and a pressure control valve associated with the wheel cylinder for adjusting fluid pressure in the wheel cylinder, the pressure control valve being operative to increase fluid pressure in the wheel cylinder in its first position, to subsequently hold increased fluid pressure constant at its second position, to decrease fluid pressure in the wheel cylinder in its third position, and to subsequently hold decreased fluid pressure constant in its fourth position, a wheel speed sensor detecting rotation speed of the wheel and producing a wheel speed indicative signal having a value representative of detected wheel speed, first means for deriving brake control parameters including wheel acceleration, second means for determining operation mode of the pressure control valve in the hydraulic brake system among the first to fourth positions and producing a control signal to operate the pressure control valve to the selected one of valve positions, third means, associated with the second means and responsive to selection selecting the first mode subsequently of selection of the second mode, to produce a hunting indicative signal, and fourth means, responsive to the hunting indicative signal, for modifying selection selecting the second position to the first position. According to a further aspect of the invention, a method for anti-skid control in an automotive brake system comprises the steps of: detecting wheel speed to produce a wheel speed indicative signal value of which is variable proportional to detected wheel speed; deriving brake control parameters including wheel acceleration on the basis of the wheel speed indicative signal values; operating a pressure control valve disposed within a hydraulic brake circuit including a wheel cylinder to its first position to increase braking pressure; subsequently operating the control valve to its second position to hold braking pressure at increased constant level, when wheel acceleration decreases across a first given value; operating the control valve to its third position to decrease the braking pressure, when wheel speed decreases across a second given value; subsequently operating the control valve to its fourth position to hold braking pressure at decreased constant level, when wheel acceleration increases across a third given value; operating the control valve to the first position when wheel speed increases across a given fourth value; and detecting increasing of wheel acceleration across the first given value during operation in which the control value is in the second position, to operate the control valve to the first position and hold the control valve at the first position until the wheel speed decreases across the second value. According to a still further apect of the invention, a method for anti-skid control in an automotive brake system comprises the steps of: detecting wheel speed to produce a wheel speed indicative signal value of which is variable proportional to detected wheel speed; processing the wheel speed indicative signal to derive brake control parameters including wheel acceleration and slip rate relative to an estimated vehicle speed, which estimated vehicle speed is derived on the basis of wheel speed; operating a pressure control valve disposed within a hydraulic brake circuit including a wheel cylinder to its first position to increase braking pressure; subsequently operating the control valve to its second position to hold braking pressure at increased constant level, when wheel acceleration decreases across a first given value; operating the control valve to its third position to decrease braking pressure when the slip rate increases across a second given value; subsequently operating the control valve to its fourth position to hold braking pressure at decreased constant level, when wheel acceleration increases across a third given value; operating the control valve to the first position when the wheel speed increases across a given fourth value which corresponds to the estimated vehicle speed; and detecting increasing of wheel acceleration across the first given value during operation in which the control value is in the second position, to operate the control valve to the first position and hold the control valve at the first position until the wheel speed decreases across the second value. BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be understood more fully from the detailed description given below and from the accompanying drawings of the preferred embodiments of the present invention, which, however, should not be taken to limit the invention to the specific embodiments, but are for explanation and understanding only. Fig. 1 is a schematic block diagram of the general design of the preferred embodiment of an anti-skid brake control system according to the present invention; Fig. 2 is a perspective illustration of the hydraulic circuits of the anti-skid brake system according to the present invention; Fig. 3 is a circuit diagram of the hydraulic circuits performing the anti-skid control according to the present invention; Fig. 4 is an illustration of the operation of an electromagnetic flow control valve employed in the hydraulic circuit, which valve has been shown in an application mode for increasing the fluid pressure in a wheel cylinder; Fig. 5 is a view similar to Fig. 4 but of the valve in a hold mode in which the fluid pressure in the wheel cylinder is held at a substantially constant value; Fig. 6 is a view similar to Fig. 4 but of the valve in a release mode in which the fluid pressure in the wheel cylinder is reduced; Fig. 7 is a perspective view of a wheel speed sensor adapted to detect the speed of a front wheel; Fig. 8 is a side elevation of a wheel speed sensor adapted to detect the speed of a rear wheel; Fig. 9 is an explanatory illustration of the wheel speed sensors of Figs. 7 and 8; Fig. 10 shows the waveform of an alternating current sensor signal produced by the wheel speed sensor; Fig. 11 is a timing chart for the anti-skid control system; Fig. 12 is a block diagram of the preferred embodiment of a controller unit in the anti-skid brake control system according to the present invention; Fig. 13 is a flowchart of a main program of a microcomputer constituting the controller unit of Fig. 12; Fig. 14 is a flowchart of an interrupt program executed by the controller unit; Fig. 15 is a flowchart of a main routine in the main program of Fig. 13; Fig. 16 is an explanatory diagram of the input timing sampling modes and variations thereof; Fig. 17 is another explanatory diagram of the sampling timing of the sensor pulse input timing; Fig. 18 is a diagram of the period of time during which sensor pulses are sampled in accordance with the present invention, which period of time is compared with that used in the typical prior art; Fig. 19 is a flowchart of a sample control program executed by the controller unit; Fig. 20 is a flowchart of a timer overflow program executed periodically as an interrupt program of the main program; Fig. 21 is a graph of the variation of a counter value of a clock counter in the preferred embodiment of controller unit; Fig. 22 is a timing chart of the timer overflow which is shown in relation to the value of the timer overflow interrupt flag; Fig. 23 is a flowchart of an output calculation program for deriving EV and V signals for controlling operation mode of the electromagnetic valve according to the valve conditions of Figs. 4, 5 and 6; Figs. 24 and 25 are diagrams of execution timing of the output calculation program in relation to the main program; Fig. 26 is a table determining the operation mode of the actuator 16, which table is accessed in terms of the wheel acceleration and deceleration and the slip rate; Fig. 27 is a block diagram of another embodiment of the anti-skid brake control system according to the present invention; Fig. 28 is a timing chart showing operation of the anti-skid control system of Fig. 27, under normal condition; and Fig. 29 is a timing chart showing operation of the anti-skid brake control system of Fig. 27, which operation is taken place when road surface friction is relatively high. DESCRIPTION OF THE PREFERRED EMBODIMENTS This application is one of eighteen mutually related co-pending Patent Applications in the United States, filed on the same day. All of the eighteen applications have been filed by the common applicant to this application and commonly assigned to the assignee of this application. The eighteen applications are identified below: Disclosures of other seventeen applications as identified above are hereby incorporated by reference for the sake of disclosure. Referring now to the drawings, particularly to Fig. 1, the preferred embodiment of an anti-skid control system according to the present invention includes a control module 200 including a front-left controller unit (FL) 202, a front-right controller unit (FR) 204 and a rear controller unit (R) 206. The controller unit 202 comprises a microprocessor and is adapted to control brake pressure applied to a front left wheel cylinder 30a of a front left hydraulic brake system 302 of an automotive hydraulic brake system 300. Similarly, the controller unit 204 is adapted to control brake pressure applied to the wheel cylinder 34a of a front right wheel (not shown) in the front right hydraulic brake system 304 and the controller unit 206 is adapted to control brake pressure applied to the rear wheel cylinders 38a of the hydraulic rear brake system 306. Respective brake systems 302, 304 and 306 have electromagnetically operated actuators 16, 18 and 20, each of which controls the pressure of working fluid in the corresponding wheel cylinders. By means of the controlled pressure, the wheel cylinders 30a, 34a and 38a apply braking force to brake disc rotors 28, 32 and 36 mounted on the corresponding wheel axles for rotation with the corresponding vehicle wheels via brake shoe assemblies 30, 34 and 38. Though the shown brake system comprises disc brakes, the anti-skid control system according to the present invention can also be applied to drum-type brake systems. The controller units 202, 204 and 206 are respectively associated with actuator drive circuits 214, 216 and 218 to control operations of corresponding actuators 16, 18 and 20. In addition, each of the controller units 202, 204 and 206 is connected to a corresponding wheel speed sensor 10, 12 and 14 via shaping circuits 208, 210 and 212 incorporated in the controller 200. Each of the wheel speed sensors 10, 12 and 14 is adapted to produce an alternating-current sensor signal having a frequency related to or proportional to the rotation speed of the corresponding vehicle wheel. Each of the A-C sensor signals is converted by the corresponding shaping circuit 208, 210 and 212 into a rectangular pulse signal which will be hereafter referred to as "sensor pulse signal". As can be appreciated, since the frequency of the A-C sensor signals is proportional to the wheel speed, the frequency of the sensor pulse signal should correspond to the wheel rotation speed and the pulse intervals thereof will be inversely proportional to the wheel rotation speed. The controller units 202, 204 and 206 operate independently and continuously process the sensor pulse signal to derive control signals for controlling the fluid pressure in each of the wheel cylinders 30a, 34a and 38a in such a way that the slip rate R at each of the vehicle wheels is optimized to shorten the distance required to stop the vehicle, which distance will be hereafter referred to as "braking distance". In general, each controller unit 202, 204 and 206 monitors receipt of the corresponding sensor pulses so that it can derive the pulse interval between the times of receipt of successive sensor pulses. Based on the derived pulse interval, the controller units 202, 204 and 206 calculate instantaneous wheel speed V w and instantaneous wheel acceleration or deceleration a w . From these measured and derived values, a target wheel speed V i is derived, which is an assumed value derived from the wheel speed at which a slip is assumed to zero or approximately zero. The target wheel speed V i varies at a constant decelerating rate derived from variation of the wheel speed. The target wheel speed is thus correspond to a vehicle speed assumed based on variation of the wheel speed. Based on the difference between the instantaneous wheel speed V w and the target wheel speed V i , a slip rate R is derived. The controller units 202, 204 and 206 determine the appropriate operational mode for increasing, decreasing or holding the hydraulic brake pressure applied to the wheel cylinders 30a, 34a and 38a. The control mode in which the brake pressure is increased will be hereafter referred to as "application mode". The control mode in which the brake pressure is decreased will be hereafter referred to as "release mode". The mode in which the brake pressure is held essentially constant will be hereafter referred to as "hold mode". The anti-skid control operation consists of a loop of application mode, hold mode, release mode and hold mode. This loop is repeated throughout the anti-skid brake control operation cyclically. One cycle of the loop of the control variation will be hereafter referred to as "skid cycle". Fig. 2 shows portions of the hydraulic brake system of an automotive vehicle to which the preferred embodiment of the anti-skid control system is applied. The wheel speed sensors 10 and 12 are respectively provided adjacent the brake disc rotor 28 and 32 for rotation therewith so as to produce sensor signals having frequencies proportional to the wheel rotation speed and variable in accordance with variation of the wheel speed. On the other hand, the wheel speed sensor 14 is provided adjacent a propeller shaft near the differential gear box or drive pinion shaft 116 for rotation therewith. Since the rotation speeds of the left and right rear wheels are free to vary independently depending upon driving conditions due to the effect of the differential gear box 40, the rear wheel speed detected by the rear wheel speed sensor 14 is the average of the speeds of the left and right wheels. Throughout the specification, "rear wheel speed" will mean the average rotation speed of the left and right rear wheels. As shown in Fig. 2, the actuator unit 300 is connected to a master wheel cylinder 24 via primary and secondary outlet ports 41 and 43 thereof and via pressure lines 44 and 42. The master wheel cylinder 24 is, in turn, associated with a brake pedal 22 via a power booster 26 which is adapted to boost the braking force applied to the brake pedal 22 before applying same to the master cylinder. The actuator unit 300 is also connected to wheel cylinders 30a, 34a and 38a via brake pressure lines 46, 48 and 50. The circuit lay-out of the hydraulic brake system circuit will be described in detail below with reference to Fig. 3 which is only an example of the hydraulic brake system to which the preferred embodiment of the anti-skid control system according to the present invention can be applied, and so it should be appreciated that it is not intended to limit the hydraulic system to the shown embodiment. In Fig. 3, the secondary outlet port 43 is connected to the inlet ports 16b and 18b of electromagnetic flow control valves 16a and 18a, the respective outlet ports 16c and 18c of which are connected to corresponding left and right wheel cylinders 30a and 34a, via the secondary pressure line 42. The primary outlet port 41 is connected to the inlet port 20b of the electromagnetic valve 20a, the outlet port 20c of which is connected to the rear wheel cylinders 38a, via the primary pressure line 44. The electromagnetic valves 16a, 18a and 20a also have drain ports 16d, 18d and 20d. The drain ports 16d and 18d are connected to the inlet port 72a of a fluid pump 90 via drain passages 80, 82 and 78. The fluid pump 90 is associated with an electric motor 88 to be driven by the latter which is, in turn, connected to a motor relay 92, the duty cycle of which is controlled by means of a control signal from the control module 200. While the motor relay 92 is energized to be turned ON, the motor 88 is in operation to drive the fluid pump 90. The drain port 20d of the electromagnetic flow control valve 20a is connected to the inlet port 58a of the fluid pump 90 via a drain passage 64. The outlet ports 72b and 58b are respectively connected to the pressure lines 42 and 44 via a return passages 72c and 58c. The outlet ports 16c, 18c and 20c of respective electromagnetic flow control valves 16a, 18a and 20a are connected to corresponding wheel cylinders 30a, 34a and 38a via braking lines 46, 48 and 50. Bypass passages 96 and 98 are provided to connect the braking pressure lines 46 and 48, and 50 respectively to the pressure lines 42 and 44, bypassing the electromagnetic flow control valves. Pump pressure check valves 52 and 66 are installed in the pressure lines 42 and 44. Each of the pump pressure cut valves 66 and 52 is adapted to prevent the working fluid pressurized by the fluid pump 90 from transmitting pressure surges to the master cylinder 24. Since the fluid pump 90 is designed for quick release of the braking pressure in the braking pressure lines 46, 48 and 50 and thus releasing the wheel cylinders 30a, 34a and 38a from the braking pressure, it is driven upon release of the brake pedal. This would result in pressure surges in the working fluid from the fluid pump 90 to the master cylinder 24 if the pump pressure check valves 66 and 52 were not provided. The pump pressure check valves 66 and 52 serve as one-way check valves allowing fluid flow from the master cylinder 24 to the inlet ports 16b, 18b and 20b of the electromagnetic valves 16a, 18a and 20a. Pressure accumulators 70 and 56 are installed in the pressure lines 42 and 44, which pressure accumulators serve to accumulate fluid pressure generated at the outlet ports 72b and 58b of the fluid pump 90 while the inlet ports 16b, 18b and 20b are closed. Toward this end, the pressure accumulators 70 and 56 are connected to the outlet ports 72b and 58b of the fluid pump 90 via the return passages 72c and 58c. Outlet valves 68 and 54 are one-way check valves allowing one-way fluid communication from the fluid pump to the pressure accumulators. These outlet valves 68 and 54 are effective for preventing the pressure accumulated in the pressure accumulators 70 and 56 from surging to the fluid pump when the pump is deactivated. In addition, the outlet valves 68 and 54 are also effective to prevent the pressurized fluid flowing through the pressure lines 42 and 44 from flowing into the fluid pump 90 through the return passages 72c and 58c. Inlet valves 74 and 60 are inserted in the drain passages 78 and 64 for preventing surge flow of the pressurized fluid in the fluid pump 90 to the electromagnetic flow control valves 16a, 18a and 20a after the braking pressure in the wheel cylinders is released. The fluid flowing through the drain passages 78 and 64 is temporarily retained in fluid reservoirs 76 and 62 connected to the former. Bypass check valves 85, 86 and 84 are inserted in the bypass passages 98 and 96 for preventing the fluid in the pressure lines 42 and 44 from flowing to the braking pressure lines 46, 48 and 50 without first passing through the electromagnetic flow control valves 16a, 18a and 20a. On the other hand, the bypass valves 85, 86 and 84 are adapted to permit fluid flow from the braking pressure line 46, 48 and 50 to the pressure lines 42 and 44 when the master cylinder 24 is released and thus the line pressure in the pressure lines 42 and 44 becomes lower than the pressure in the braking pressure lines 46, 48 and 50. The electromagnetic flow control valves 16a, 18a and 20a are respectively associated with the actuators 16, 18 and 20 to be controlled by means of the control signals from the control module 200. The actuators 16, 18 and 20 are all connected to the control module 200 via an actuator relay 94, which thus controls the energization and deenergization of them all. Operation of the electromagnetic valve 16a in cooperation with the actuator 16 will be illustrated with reference to Figs. 4, 5 and 6, in particular illustrating application mode, hold mode and release mode, respectively. It should be appreciated that the operation of the electromagnetic valves 18a and 20a are substantially the same as that of the valve 16a. Therefore, disclosure of the valve operations of the electromagnetic valves 18a and 20a is neglected in order to avoid unnecessary repetition and for simplification of the disclosure. APPLICATION MODE In this position, the actuator 16 remains deenergized. An anchor of the electromagnetic valve 16a thus remains in its initial position allowing fluid flow between the inlet port 16b and the outlet port 16c so that the pressurized fluid supplied from the master cylinder 24 via the pressure line 42 may flow to the left front wheel cylinder 30a via the braking pressure line 46. In this valve position, the drain port 16d is closed to block fluid flow from the pressure line 42 to the drain passage 78. As a result, the line pressure in the braking pressure line 46 is increased in proportion to the magnitude of depression of the brake pedal 22 and thereby the fluid pressure in the left front wheel cylinder 30a is increased correspondingly. In this case, when the braking force applied to the brake pedal is released, the line pressure in the pressure line 42 drops due to return of the master cylinder 24 to its initial position. As a result, the line pressure in the braking pressure line 46 becomes higher than that in the pressure line 42 and so opens the bypass valve 85 to permit fluid flow through the bypass passage 98 to return the working fluid to the fluid reservoir 24a of the master cylinder 24. In the preferring construction, the pump pressure check valve 66, normally serving as a one-way check valve for preventing fluid flow from the electromagnetic valve 16a to the master cylinder 24, becomes wide-open in response to drop of the line pressure in the pressure line below a given pressure. This allows the fluid in the braking pressure line 46 to flow backwards through the electromagnetic valve 16a and the pump pressure check valve 66 to the master cylinder 24 via the pressure line 42. This function of the pump pressure check valve 66 facilitates full release of the braking pressure in the wheel cylinder 30a. For instance, the bypass valve 85 is rated at a given set pressure, e.g. 2 kg/cm² and closes when the pressure difference between the pressure line 42 and the braking pressure line 46 drops below the set pressure. As a result, fluid pressure approximating the bypass valve set pressure tends to remain in the braking pressure line 46, preventing the wheel cylinder 30a from returning to the fully released position. In order to avoid this, in the shown embodiment, the one-way check valve function of the pump pressure check valve 66 is disabled when the line pressure in the pressure line 42 drops below a predetermined pressure, e.g. 10 kg/cm². When the line pressure in the pressure line 42 drops below the predetermined pressure, a bias force normally applied to the pump pressure check valve 66 is released, freeing the valve to allow fluid flow from the braking pressure line 46 to the master cylinder 24 via the pressure line 42. HOLD MODE In this control mode, a limited first value, e.g. 2A of electric current serving as the control signal is applied to the actuator 16 to position the anchor closer to the actuator 16 than in the previous case. As a result, the inlet port 16b and the drain port 16d are closed to block fluid communication between the pressure line 42 and the braking pressure line 46 and between the braking pressure line and the drain passage 78. Therefore, the fluid pressure in the braking pressure line 46 is held at the level extant at the moment the actuator is operated by the control signal. In this case, the fluid pressure applied through the master cylinder flows through the pressure check valve 66 to the pressure accumulator 70. RELEASE MODE In this control mode, a maximum value, e.g. 5A of electric current serving as the control signal is applied to the actuator 16 to shift the anchor all the way toward the actuator 16. As a result, the drain port 16d is opened to establish fluid communication between the drain port 16d and the outlet port 16c. At this time, the fluid pump 90 serves to facilitate fluid flow from the braking pressure line 46 to the drain passage 78. The fluid flowing through the drain passage is partly accumulated in the fluid reservoir 76 and the remainder flows to the pressure accumulator 70 via the check valves 60 and 54 and the fluid pump 90. It will be appreciated that, even in this release mode, the fluid pressure in the pressure line 42 remains at a level higher or equal to that in the braking pressure line 46, so that fluid flow from the braking pressure line 46 to the pressure line 42 via the bypass passage 98 and via the bypass check valve 85 will never occur. In order to resume the braking pressure in the wheel cylinder (FL) 30a after once the braking pressure is reduced by positioning the electromagnetic valve 16a in the release position, the actuator 16 is again denergized. The electromagnetic valve 16a is thus returns to its initial position to allow the fluid flow between the inlet port 16b and the outlet port 16c so that the pressurized fluid may flow to the left front wheel cylinder 30a via the braking pressure line 46. As set forth the drain port 16d is closed to flock fluid flow from the pressure line 42 to the drain passage 78. As a result, the pressure accumulator 70 is connected to the left front wheel cylinder 30a via the electromagnetic valve 16a and the braking pressure line 46. The pressurized fluid in the pressure accumulator 70 is thus supplied to the wheel cylinder 30a so as to resume the fluid pressure in the wheel cylinder 30a. At this time, as the pressure accumulator 70 is connected to the fluid reservoir 76 via the check valves 60 and 54 which allow fluid flow from the fluid reservoir to the pressure accumulator, the extra amount of pressurized fluid may be supplied from the fluid reservoir. The design of the wheel speed sensors 10, 12 and 14 employed in the preferred embodiment of the anti-skid control system will be described in detail with reference to Figs. 7 to 9. Fig. 7 shows the structure of the wheel speed sensor 10 for detecting the rate of rotation of the left front wheel. The wheel speed sensor 10 generally comprises a sensor rotor 104 adapted to rotate with the vehicle wheel, and a sensor assembly 102 fixedly secured to the shim portion 106 of the knuckle spindle 108. The sensor rotor 104 is fixedly secured to a wheel hub 109 for rotation with the vehicle wheel. As shown in Fig. 9, the sensor rotor 104 is formed with a plurality of sensor teeth 120 at regular angular intervals. The width of the teeth 120 and the grooves 122 therebetween are equal in the shown embodiment and define to a unit angle of wheel rotation. The sensor assembly 102 comprises a magnetic core 124 aligned with its north pole (N) near the sensor rotor 104 and its south pole (S) distal from the sensor rotor. A metal element 125 with a smaller diameter section 125a is attached to the end of the magnetic core 124 nearer the sensor rotor. The free end of the metal element 125 faces the sensor teeth 120. An electromagnetic coil 126 encircles the smaller diameter section 125a of the metal element. The electromagnetic coil 126 is adapted to detect variations in the magnetic field generated by the magnetic core 124 to produce an alternating-current sensor signal as shown in Fig. 10. That is, the metal element and the magnetic core 124 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 125 and the sensor rotor surface. Thus, the intensity of the magnetic field fluctuates in relation to the passage of the sensor teeth 120 and accordingly in relation to the angular velocity of the wheel. It should be appreciated that the wheel speed sensor 12 for the right front wheel has the substantially the same structure as the set forth above. Therefore, also explanation of the structure of the right wheel speed sensor 12 will be omitted in order to avoid unnecessary repetition of disclosure and in order to simplify the description. Fig. 8 shows the structure of the rear wheel speed sensor 14. As with the aforementioned left front wheel speed sensor 10, the rear wheel speed sensor 14 comprises a sensor rotor 112 and a sensor assembly 102. The sensor rotor 112 is associated with a companion flange 114 which is, in turn, rigidly secured to a drive shaft 116 for rotation therewith. Thus, the sensor rotor 112 rotates with the drive shaft 116. The sensor assembly 102 is fixed to a final drive housing or a differential gear box (not shown). Each of the sensor assemblies applied to the left and right front wheel speed sensors and the rear wheel sensor is adapted to output an alternating-current sensor signal having a frequency proportional to or corresponding to the rotational speed of the corresponding vehicle wheel. The electromagnetic coil 126 of each of the sensor assemblies 102 is connected to the control module 200 to supply the sensor signals thereto. As set forth above, the control module 200 comprises the controller unit (FL) 202, the controller unit (FR) 204 and the controller unit (R) 206, each of which comprises a microcomputer. Therefore, the wheel speed sensors 10, 12 and 14 are connected to corresponding controller units 202, 204 and 206 and send their sensor signals thereto. Since the structure and operation of each of the controller units is substantially the same as that of the others, the structure and operation of only the controller unit 202 for performing the anti-skid brake control for the front left wheel cylinder will be explained in detail. Fig. 11 is a timing chart of the anti-skid control performed by the controller unit 202. As set forth above, the alternating-current sensor signal output from the wheel speed sensor 10 is converted into a rectangular pulse train, i.e. as the sensor pulse signal mentioned above. The controller unit 202 monitors occurrences of sensor pulses and measures the intervals between individual pulses or between the first pulses of groups of relatively-high-frequency pulses. Pulses are so grouped that the measured intervals will exceed a predetermined value, which value will be hereafter referred to as "pulse interval threshold". The wheel rotation speed V w is calculated in response to each sensor pulse. As is well known, the wheel speed is generally inversely proportional to the intervals between the sensor pulses, and accordingly, the wheel speed V w is derived from the interval between the last sensor pulse input time and the current sensor pulse input time. A target wheel speed V i the resultant wheel speed V w . In addition, the slip rate is derived from the rate of change of the wheel speed and an projected speed V v which is estimated from the wheel speed at the moment the brakes are applied based on the assumption of a continuous, linear deceleration without slippage. In general, the target wheel speed V i is derived from the wheel speed of the last skid cycle during which the wheel deceleration rate was equal to or less than a given value which will be hereafter referred to as "deceleration threshold a ref " , and the wheel speed of the current skid cycle, and by estimating the rate of change of the wheel speed between wheel speeds at which the rate of deceleration is equal to or less than the deceleration threshold. In practice, the first target wheel speed V i is derived based on the projected speed V v which corresponds to a wheel speed at the initial stage of braking operation and at which wheel deceleration exceeds a predetermined value, e.g. -1.2G, and a predetermined deceleration rate, for example 0.4G. The subsequent target wheel speed V i is derived based on the projected speeds V v in last two skid cycles. For instance, the deceleration rate of the target wheel speed V i is derived from a difference of the projected speeds V v in the last two skid cycle and a period of time in which wheel speed varies from the first projected speed to the next projected speed. Based on the last projected speed and the deceleration rate, the target wheel speed in the current skid cycle is derived. The acceleration and deceleration of the wheel is derived based on input times of the successive three sensor pulses. Since the interval of the adjacent sensor signal pulses corresponds wheel speed and the wheel speed is a function of the reciprocal of the interval, by comparing adjacent pulse-to-pulse intervals, a value corresponding to variation or difference of the wheel speed. The resultant may be divided by the period of time in order to obtain the wheel acceleration and deceleration at the unit time. Therefore, the acceleration or deceleration of the wheel is derived from the following equation: where A, B and C are the input times of the sensor pulses in the order given. On the other hand, the slip rate R is a rate of difference of wheel speed relative to the vehicle speed which is assumed as substantially corresponding to the target wheel speed. Therefore, in the shown embodiment, the target wheel speed V i is taken as variable or parameter indicative of the assumed or projected vehicle speed. The slip rate R can be obtained by dividing a difference between the target wheel speed V i and the instantaneous wheel speed V w by the target wheel speed. Therefore, in addition, the slip rate R is derived by solving the following equation: Finally, the controller unit 202 determines the control mode, i.e., release mode, hold mode and application mode from the slip rate R and the wheel acceleration or deceleration a w . General operation of the controller unit 202 will be briefly explained herebelow with reference to Fig. 11. Assuming the brake is applied at t 0 and the wheel deceleration a w exceeds the predetermined value, e.g. 1.2G at a time t₁, the controller unit 202 starts to operate at a time t₁. The first sensor pulse input time (t₁) is held in the controller unit 202. Upon receipt of the subsequent sensor pulse at a time t₂, the wheel speed V w is calculated by deriving the current sensor, pulse period (dt = t₂ - t₁). In response to the subsequently received sensor pulses at times t₃, t₄..., the wheel speed values V w2 , V w3 ... are calculated. On the other hand, at the time t₁, the instantaneous wheel speed is taken as the projected speed V v . Based on the projected speed V v and the predetermined fixed value, e.g. 0.4G, the target wheel speed V i decelerating at the predetermined deceleration rate 0.4G is derived. In anti-skid brake control, the braking force applied to the wheel cylinder is to be so adjusted that the peripheral speed of the wheel, i.e. the wheel speed, during braking is held to a given ratio, e.g. 85% to 80% of the vehicle speed. Therefore, the slip rate R has to be maintained below a given ratio, i.e., 15% to 10%. In the preferred embodiment, the control system controls the braking force so as to maintain the slip rate at about 15%. Therefore, a reference value R ref to be compared with the slip rate R is determined at a value at 85% of the projected speed V v . As will be appreciated, the reference value is thus indicative of a slip rate threshold, which will be hereafter referred to "slip rate threshold R ref " throughout the specification and claims, and varies according to variation of the target wheel speed. In practical brake control operation performed by the preferred embodiment of the anti-skid control system according to the present invention, the electric current applied to the actuator becomes a limited value, e.g., 2A to place the electromagnetic valve 30a in the hold mode as shown in Fig. 5 when the wheel speed remains inbetween the target wheel speed V i and the slip rate threshold R ref . When the slip rate derived from the target wheel speed V i and the wheel speed V w becomes equal to or larger than the slip rate threshold R ref , then the supply current to the actuator 16 is increased to a maximum value, e.g. 5A to place the electromagnetic valve in the release mode as shown in Fig. 6. By maintaining the release mode, the wheel speed V w is recovered to the target wheel speed. When the wheel speed is thus recovered or resumed so that the slip rate R at that wheel speed becomes equal to or less than the slip rate threshold R ref , then the supply current to the actuator 16 is dropped to the limited value, e.g. 2A to return the electromagnetic valve 30a to the hold mode. By holding the reduced fluid pressure in the wheel cylinder, the wheel speed V w is further resumed to the target wheel speed V i . When the wheel speed V w is resumed equal to or higher than the target wheel speed V i , the supply current is further dropped to zero for placing the electromagnetic valve in the application mode as shown in Fig. 4. The electromagnetic valve 30a is maintained in the application mode until the wheel speed is decelerated at a wheel speed at which the wheel deceleration becomes equal to or slightly more than the deceleration threshold R ref -1.2G. At the same time, the projected speed V v is again derived with respect to the wheel speed at which the wheel deceleration a w becomes equal to or slightly larger than the deceleration threshold a ref . From a difference of speed of the last projected speed and the instant projected speed and the period of time from a time obtaining the last projected speed to a time obtaining the instant projected speed, a deceleration rate of the target wheel speed V i is derived. Therefore, assuming the last projected speed is V v1 , the instant projected speed is V v2 , and the period of time is T v , the target wheel speed V i can be obtained from the following equation: V i = V v ₂ - (V v ₁ - V v ₂)/T v x t e where t e is an elapsed time from the time at which the instant projected speed V v2 is obtained. Based on the input timing to t₁, t₂, t₃, t₄..., deceleration rate a w is derived from the foregoing equation (1). In addition, the projected speed V v is estimated as a function of the wheel speed V w and rate of change thereof. Based on the instantaneous wheel speeds V w1 at which the wheel deceleration is equal to or less than the deceleration threshold a ref and the predetermined fixed value, e.g. 0.4G for the first skid cycle of control operation, the target wheel speed V i is calculated. According to equation (2), the slip rate R is calculated, using successive wheel speed values V w1 , V w2 , V w3 ... as parameters. The derived slip rate R is compared with the slip rate threshold R ref . As the wheel speed V w drops below the projected speed V v at the time t₁, the controller unit 202 switches the control mode from the application mode to the hold mode. Assuming also that the slip rate R exceeds the slip rate threshold at the time t₄, then the controller unit 202 switches the control mode to the release mode to release the fluid pressure at the wheel cylinder. Upon release of the brake pressure in the wheel cylinder, the wheel speed V w recovers, i.e. the slip rate R drops until it is smaller than the slip rate threshold at time t₇. The controller unit 202 detects when the slip rate R is smaller than the slip rate threshold R ref and switches the control mode from release mode to the hold mode. By maintaining the brake system in the hold mode in which reduced brake pressure is applied to the wheel cylinder, the wheel speed increases until it reaches the projected speed as indicated by the intersection of the dashed line (V v ) and the solid trace in the graph of V w in Fig. 11. When the wheel speed V w becomes equal to the target wheel speed V w (at a time t₈), the controller unit 202 switches the control mode from the hold mode to the application mode. As can be appreciated from the foregoing description, the control mode will tend to cycle through the control modes in the order application mode, hold mode, release mode and hold mode, as exemplified in the period of time from t₁ to t₈. This cycle of variation of the control modes will be referred to hereafter as "skid cycle". Practically speaking, there will of course be some hunting and other minor deviations from the standard skid cycle. The projected speed V v , which is meant to represent ideal vehicle speed behavior, at time t₁ can be obtained directly from the wheel speed V w at that time since zero slip is assumed. At the same time, the deceleration rate of the vehicle will be assumed to be a predetermined fixed value or the appropriate one of a family thereof, in order to enable calculation of the target wheel speed for the first skid cycle operation. Specifically, in the shown example, the projected speed V v at the time t₁ will be derived from the wheel speed V w1 at that time. Using the predetermined deceleration rate. the projected speed will be calculated at each time the wheel deceleration a w in the application mode reaches the deceleration threshold a ref . At time t₉, the wheel deceleration a w becomes equal to or slightly larger than the deceleration threshold a ref , then the second projected speed V v2 is obtained at a value equal to the instantaneous wheel speed V w at the time t₉. According to the above-mentioned equation, the deceleration rate da can be obtained da = (V v1 - V v2 )/(t₉ - t₁) Based on the derived deceleration rate da, the target wheel speed V i ' for the second skid cycle of control operation is derived by: V i ' = V v2 - da x t e Based on the derived target wheel speed, the slip rate threshold R ref for the second cycle of control operation is also derived. As will be appreciated from Fig. 11, the control mode will be varied during the second cycle of skid control operation, to hold mode at time t₉ at which the wheel deceleration reaches the deceleration threshold a ref as set forth above, to release mode at time t₁₀ at which the slip rate R reaches the slip rate threshold R ref , to hold mode at time t₁₁ at which the slip rate R is recovered to the slip rate threshold R ref , and to application mode at time t₁₂ at which the wheel speed V w recovered or resumed to the target wheel speed V i '. Further, it should be appreciated that in the subsequent cycles of the skid control operations, the control of the operational mode of the electromagnetic valve as set forth with respect to the second cycle of control operation, will be repeated. Relating the above control operations to the structure of Figs. 3 through 6, when application mode is used, no electrical current is applied to the actuator of the electromagnetic valve 16a so that the inlet port 16b communicates with the outlet port 16c, allowing fluid flow between the pressure passage 42 and the brake pressure line 46. A limited amount of electrical current (e.g. 2A) is applied at times t₁, t₇, t₉ and t₁₁, so as to actuate the electromagnetic valve 16a to its limited stroke position by means of the actuator 16, and the maximum current is applied to the actuator as long as the wheel speed V w is not less than the projected speed and the slip rate is greater than the slip rate threshold R ref . Therefore, in the shown example, the control mode is switched from the application mode to the hold mode at time t₁ and then to the release mode at time t₄. At time t₇, the slip rate increases back up to the slip rate threshold R ref , so that the control mode returns to the hold mode, the actuator driving the electromagnetic valve 16a to its central holding position with the limited amount of electrical current as the control signal. When the wheel speed V w finally returns to the level of the target wheel speed V i at time t₈, the actuator 16 supply current is cut off so that the electromagnetic valve 16a returns to its rest position in order to establish fluid communication between the pressure line 42 and the braking pressure line 46 via inlet and outlet ports 16b and 16c. Referring to Fig. 12, the controller unit 202 includes an input interface 230, CPU 232, an output interface 234, RAM 236 and ROM 238. The input interface 230 includes an interrupt command generator 229 which produces an interrupt command in response to every sensor pulse. In ROM, a plurality of programs including a main program (Fig. 13), an interrupt program (Fig. 15), an sample control program (Fig. 19), a timer overflow program (Fig. 20) and an output calculation program (Fig. 23) are stored in respectively corresponding address blocks 244, 246, 250, 252 and 254. The input interface also has a temporary register for temporarily holding input timing for the sensor pulses. RAM 236 similarly has a memory block holding input timing for the sensor pulses. The contents of the memory block 240 of RAM may be shifted whenever calculations of the pulse interval, wheel speed, wheel acceleration or deceleration, target wheel speed, slip rate and so forth are completed. One method of shifting the contents is known from the corresponding disclosure of the United States Patent No. 4,408,290. The disclosure of the U.S. Patent 4,408,290 is hereby incorporated by reference. RAM also has a memory block 242 for holding pulse intervals of the input sensor pulses. The memory block 242 is also adapted to shift the contents thereof according to the manner similar to set forth in the U.S. Patent 4,408,290. An interrupt flag 256 is provided in the controller unit 202 for signalling interrupt requests to the CPU. The interrupt flag 256 is set in response to the interrupt command from the interrupt command generator 229. A timer overflow interrupt flag 258 is adapted to set an overflow flag when the measured interval between any pair of monitored sensor pulses exceeds the a capacity of a clock counter. In order to time the arrival of the sensor pulses, a clock is connected to the controller unit 202 to feed time signals indicative of elapsed real time. The timer signal value is latched whenever a sensor pulse is received and stored in either or both of the temporary register 231 in the input interface 230 and the memory block 240 of RAM 236. The operation of the controller unit 202 and the function of each elements mentioned above will be described with reference to Figs. 13 to 30. Fig. 13 illustrates the main program for the anti-skid control system. Practically speaking, this program will generally be executed as a background job, i.e. it will have a lower priority than most other programs under the control of the same processor. Its first step 1002 is to wait until at least one sample period, covering a single sensor pulse or a group thereof, as described in more detail below, is completed as indicated when a sample flag FL has a non-zero value. In subsequent step 1004, the sample flag FL is checked for a value greater than one, which would indicate the sample period is too short. If this is the case, control passes to a sample control program labelled "1006" in Fig. 13 but shown in more detail in Fig. 19. If FL=1, then the control process is according to plan, and control passes to a main routine explained later with reference to Fig. 15. Finally, after completion of the main routine, a time overflow flag OFL is reset to signify successful completion of another sample processing cycle, and the main program ends. Fig. 14 shows the interrupt program stored in the memory block 246 of ROM 238 and executed in response to the interrupt command generated by the interrupt command generator 229 whenever a sensor pulse is received. It should be noted that a counter value NC of an auxiliary counter 233 is initially set to 1, a register N representing the frequency divider ratio is set at 1, and a counter value M of an auxiliary counter 235 is set at -1. After starting execution of the interrupt program, the counter value NC of the auxiliary counter 233 is decremented by 1 at a block 3002. The auxiliary counter value NC is then checked at a block 3004 for a value greater than zero. For the first sensor pulse, since the counter value NC is decremented by 1 (1 - 1 = 0) at the block 3002 and thus is zero, the answer of the block 3004 is NO. In this case, the clock counter value t is latched in a temporary register 231 in the input interface 230 at a block 3006. The counter value NC of the auxiliary counter 233 is thereafter assigned the value N in a register 235, which register value N is representative of frequency dividing ratio determined during execution of the main routine explained later, at a block 3008. The value M of an auxiliary counter 235 is then incremented by 1. The counter value M of the auxiliary counter 235 labels each of a sequence of sample periods covering an increasing number of sensor pulses. After this, the sample flag FL is incremented by 1 at a block 3012. After the block 3012, interrupt program ends, returning control to the main program or back to block 3002, whichever comes first. On the other hand, when the counter value NC is non-zero when checked at the block 3004, this indicates that not all of the pulses for this sample period have been received, and so the interrupt program ends immediately. This interrupt routine thus serves to monitor the input timing t of each pulse sampling period, 1.e. the time t required to receive NC pulses, and signals completion of each sample period (M=0 through M=10, for example) for the information of the main program. Before describing the operation in the main routine, the general method of grouping the sensor pulses into sample periods will be explained to facilitate understanding of the description of the operation in the main routine. In order to enable the controller unit 202 to accurately calculate the wheel acceleration and deceleration a w , it is necessary that the difference between the pulse intervals of the single or grouped sensor pulses exceeding a given period of time, e.g. 4 ms. In order to obtain the pulse interval difference exceeding the given period of time, 4 ms, which given period of time will be hereafter referred to as "pulse interval threshold S", some sensor pulses are ignored so that the recorded input timing t of the sensor pulse groups can satisfy the following formula: where A, B and C are the input times of three successive sensor pulse groups. The controller unit 202 has different sample modes, i.e. MODE 1, MODE 2, MODE 3 and MODE 4 determining the number of sensor pulses in each sample period group. As shown in Fig. 16, in MODE 1 every sensor pulse input time is recorded and therefore the register value N is 1. In MODE 2, every other sensor pulse is ignored and the register value N is 2. In MODE 3, every fourth sensor pulse is monitored, i.e. its input time is recorded, and the register value N is 4. In MODE 4, every eighth sensor pulse is sampled and the register value N is then 8. The controller unit 202 thus samples the input timing of three successive sensor pulses to calculate the pulse interval difference dT while operating in MODE 1. If the derived pulse interval difference is equal to or greater than the pulse interval threshold S, then sensor pulses will continue to be sampled in MODE 1. Otherwise, the input timing of every other sensor pulse is sampled in MODE 2 and from the sampled input timing of the next three sensor pulses sampled, the pulse interval difference dT is calculated to again be compared with the pulse interval threshold S. If the derived pulse interval difference is equal to or greater than the pulse interval threshold S, we remain in MODE 2. Otherwise, every four sensor pulses are sampled in MODE 3. The input timings of the next three sampled sensor pulses are processed to derive the pulse interval difference dT. The derived pulse interval difference dT is again compared with the pulse interval threshold S. If the derived pulse interval difference is equal to or greater than the pulse interval threshold S, the MODE remains at 3 and the value N is set to 4. On the other hand, if the derived pulse interval difference dT is less than the pulse interval threshold S, the mode is shifted to MODE 4 to sample the input timing of every eighth sensor pulse. In this MODE 4, the value N is set at 8. For instance, in Fig. 16, the sensor pulses A₁, B₁ and C₁ are sampled under MODE 1. In MODE 2, the sensor pulses a₁ and c₁ are ignored and the sensor pulses A₁ (= A₂), B₂ (= b₁) and C₂ (= b₂ = a₃) are sampled. In MODE 3, the three sensor pulses c₂ (= b₃ = a₄), c₃ (= b₄) and c₄ following B₃ (= c₂) are ignored and the sensor pulses A₃ (= A₁ = A₂), B₃ (= b₂ = a₃) and C₃ (= b₅ = a₆) are sampled. In MODE 4, the seven sensor pulses c₅ (= b₆ = a₇), c₆ (= b₇ = a₈), c₇ (= b₈ = a₉), c₈ (= b₉ = a₁₀), c₉ (= b₁₀ = a₁₁), c₁₀ (= b₁₁) and c₁₁ following B₄ (= c₃) are ignored and the sensor pulses A₄ (= A₁ = A₂ = A₃), B₄ (= C₃ = b₅ = a₆) and C₄ are sampled. Referring to Fig. 15, the main routine serves to periodically derive an updated wheel acceleration rate value a w . In general, this is done by sampling larger and larger groups of pulses until the difference between the durations of the groups is large enough to yield an accurate value. In the main routine, the sample flag FL is reset to zero at a block 2001. Then the counter value M of the auxiliary counter 233, indicating the current sample period of the current a w calculation cycle, is read out at a block 2002 to dictate the subsequent program steps. Specifically, after the first sample period (M=), the input timing t temporarily stored in the temporary register 231 corresponding to the sensor pulse number (M = 0) is read out and transferred to a memory block 240 of RAM at a block 2004, which memory block 240 will be hereafter referred to as "input timing memory". Then control passes to the block 1008 of the main program. When M = 2, the corresponding input timing t is read out from the temporary register 231 and transferred to the input timing memory 240 at a block 2006. Then, at a block 2008, a pulse interval Ts between the sensor pulses of M = 1 is derived from the two input timing values in the input timing memory 240. That is, the pulse interval of the sensor pulse (M = 1) is derived by: Ts = t₁ - t₀ where t₁ is input time of the sensor pulse M1; and t₀ is input time of the sensor pulse M0. The derived pulse interval T₅ of the sensor pulse M1 is then compared with a reference value, e.g. 4 ms., at a block 2010. If the pulse interval T s is shorter than the reference value, 4 ms., control passes to a block 2012 wherein the value N and the pulse interval T s are multiplied by 2. The doubled timing value (2T s ) is again compared with the reference value by returning to the block 2010. The blocks 2010 and 2012 constitute a loop which is repeated until the pulse interval (2nT s ) exceeds the reference value. When the pulse interval (2nT s ) exceeds the reference value at the block 2010, a corresponding value of N (2N) is automatically selected. This N value represents the number of pulses to be treated as a single pulse with regard to timing. After setting the value of N and thus deriving the sensor pulse group size then the auxiliary counter value NC is set to 1, at a block 2016. The register value N is then checked for a value of 1, at a block 2018. If N=1, then the auxiliary counter value M is set to 3 at a block 2020 and otherwise control returns to the main program. When the register value N equals 1, the next sensor pulse, which would normally be ignored, will instead be treated as the sensor pulse having the sample period number M = 3. In the processing path for the sample period number M = 3, the corresponding input timing is read from the corresponding address of the temporary register 231 and transferred to the input timing memory 240, at a block 2024. The pulse interval T₂ between the sensor pulses at M = 1 and M = 3 is then calculated at a block 2026. The derived pulse interval T₂ is written in a storage section of a memory block 242 of RAM 236 for a current pulse interval data, which storage section will be hereafter referred at as "first pulse interval storage" and which memory block 242 will be hereafter referred to as "pulse interval memory". After the block 2026, control returns to the main program to await the next sensor pulse, i.e. the sensor pulse for sample period number M = 4. When the sensor pulse for M = 4 is received, the value t of the temporary register 231 is read out and transferred to the input timing memory 240 at block 2028. Baked on the input timing of the sensor pulses for M = 3 and M = 4, the pulse interval T₃ is calculated at a block 2030. The pulse interval T₃ derived at the block 2030 is then written into the first pulse interval storage of the pulse interval memory. At the same time, the pulse interval data T₂ previously stored in the first pulse interval storage is transferred to another storage section of the pulse interval memory adapted to store previous pulse interval data. This other storage section will be hereafter referred to as "second pulse interval storage". Subsequently, at a block 2032 the contents of the first and second storages, i.e. the pulse interval data T₂ and T₃ are read out. Based on the read out pulse interval data T₂ and T₃, a pulse interval difference dT is calculated at block 2032 and compared with the pulse interval threshold S to determine whether or not the pulse interval difference dT is large enough for accurate calculation of wheel acceleration or deceleration a w . If so, process goes to the block 2040 to calculate the wheel acceleration or deceleration according to the equation (1). The register value N is then set to 1 at the block 2044 and thus MODE 1 is selected. In addition sample period number M is reset to -1, and the a w derivation cycle starts again. On the other hand, if at the block 2032 the pulse interval difference dT is too small to calculate the wheel acceleration or deceleration a w , then the value N is multiplied by 2 at a block 2034. Due the updating of the value N, the sample mode of the sensor pulses is shifted to the next mode. When the block 2034 is performed and thus the sample mode is shifted to MODE 2 with respect to the sensor pulse of M = 4', the sensor pulse c₂ input following to the sensor pulse of M = 4' is ignored. The sensor pulse c₃ following to the ignored sensor pulse c₂ is then taken as the sensor pulse to be sampled as M = 3". At this time, the sensor pulse of M = 4' is treated as the sensor pulse of M = 2" and the sensor pulse of M = 2 is treated as the sensor pulse of M = 1". Therefore, calculation of the interval difference dT and discrimination if the derived interval difference dT is greater than the pulse interval threshold S in the block 2032 will be carried out with respect to the sensor pulse c₃ which will be treated as the sensor pulse of M = 4". The blocks 2032 and 2034 are repeated until the interval difference greater than the pulse interval threshold S is obtained. The procedure taken in each cycle of repetition of the blocks 2032 and 2034 is substantially same as that set forth above. As set forth above, by setting the counter value NC of the auxiliary counter 233 to 1 at the block 2016, the input timing of the sensor pulse received immediately after initially deriving the sample mode at the blocks 2010 and 2012 will be sampled as the first input timing to be used for calculation of the wheel acceleration and deceleration. This may be contrasted with the procedure taken in the known art. Fig. 18 shows timing of calculation of the wheel acceleration and deceleration in comparison with the calculation timing of the wheel acceleration and deceleration in the prior art. As will be appreciated from Fig. 18, in the prior art, after deriving the sample mode so that the pulse interval T s is longer than the reference value, e.g. 4 ms, the first sensor pulse A' is sampled after thinning the corresponding number of sensor pulses e.g. 3 sensor pulses in the shown case. On the other hand, the first sensor pulse A, according to the present invention, can be sampled with respect to the sensor pulse input immediately after deriving the sample mode. As will be appreciated herefrom, sample timing according to the present invention is faster than that in the prior art so that calculation of the wheel acceleration and deceleration can be performed at an earlier timing than that in the conventional art. In other words, the time lag of wheel acceleration/ deceleration calculation due to sensor pulse grouping can be shortened. Fig. 17 shows a modified procedure can be taken for obtaining the interval difference dT larger than the pulse interval threshold S. In this modification, SUB-MODE as illustrated is used instead of performing the block 2034 to shift the sample mode of Fig. 16. In this modification, when MODE 3 is selected during execution of the main routine of Fig. 15, at the blocks 2010, 2012 and 2016, the controller unit 202 is operating under MODE 3, first the sensor pulses A₁,, B₁, and C₁' are sampled as shown in Fig. 17. The pulse interval difference between (C₁, - B₁,) and (B₁, - A₁,) is calculated in response to the sensor pulse C₁, (M = 4). This operation for detecting the interval difference dT larger than the pulse interval threshold S is substantially corresponding to the operation at the blocks 2032, in the main routine of Fig. 15. If the determined pulse interval difference dT is equal to or greater than the pulse interval threshold S, the wheel acceleration or deceleration will be calculated using the derived pulse interval difference dT (SUB-MODE 1) at the block 2040 of the main routine of Fig. 15. On the other hand, if the derived pulse interval difference dT is less than the pulse interval threshold S, the sensor pulses A₂, (= A₁, : M = 2), B₂, (= C₁, : M = 4) and C₂, (M = 6) are sampled in SUB-MODE 2. If the pulse interval difference derived from the input timing of A₂,, B₂, and C₂' is less than the pulse interval threshold S, then the controller unit 202 shifts the operation mode into SUB-MODE 3 in which the sensor pulses A₃' (= A₁' = A₂, : M = 2), B₃' (= C₂, : M = 6) and C₃' (M = 10) are sampled. After the step 2040, the obtained wheel acceleration and deceleration a w is checked at a step 2042 if the wheel is in accelerating state or decelerating state. This can be done by comparing the obtained value a w with 0. When the obtained value a w is greater than 0, accelerating state of the wheel is recognized. When accelerating state of the wheel is detected at the step 2042, an accelerating state indicative flag F A is set at a step 2043 in an accelerating state indicative flag register 257 in the controller 202 of Fig. 12. This accelerating state indicative flag F A is checked when operation mode of the control system is determined in a procedure described in detail later. In both of SUB-MODEs 2 and 3, calculation for deriving the wheel acceleration or deceleration a w relative to the sensor pulses M5, M6, M7, M8 and M9 are performed with taking the input timing of two proceeding sensor pulses similarly to the procedure performed at SUB-MODE 1 when the interval difference dT larger than the pulse interval threshold S can be detected with respect to M7, M8 or M9, for example. As will be appreciated herefrom, SUB-MODE referred to hereabove mean further variations of the sensor pulse sample mode in order to obtain the interval difference dT greater than the pulse interval threshold S for enabling calculation of the wheel acceleration and deceleration at the block 2040 of the main routine of Fig. 15. With the foregoing modification of Fig. 17, even when the interval difference dT greater than the pulse interval threshold S is obtained with respect to the sensor pulse which has to be thinned under the procedure of Fig. 16, the wheel acceleration and deceleration can be derived for reducing loss time. Further, according to this modified procedure, the calculation timing of the wheel acceleration and deceleration can follow relatively abrupt change of the wheel speed. Fig. 19 shows the sample control program stored in the memory block 250 of ROM 238. This sample control program is executed when the sample flag FL reaches a predetermined value. In the shown embodiment, the sample control program is executed when the sample flag value FL equals 2. When the sample flag value FL = 2 at the block 1004 in Fig. 13, then the sample control program is executed to multiply the auxiliary counter value N by 2, at a block 4002 of Fig. 19. At the same time, the auxiliary counter value NC is set to 1. Thereafter, the sample flag is reset to zero at a block 4004. The sample control program of Fig. 19 provides a quick and simple adjustment of the sampling mode for both initial start-up and cases where the wheel accelerates so quickly that two sampling periods are completed within a single acceleration rate a w derivation cycle. Setting N equal to 2N in block 4002 doubles the sample size and so effectively doubles the sample period and setting NC to 1 ensures that the sampling will restart immediately with the next sensor pulse. Fig. 20 shows the timer overflow program stored in the memory block 252 of ROM. As set forth above, the clock counter 259 used in the shown embodiment has the capacity to count the clock pulses from the clock generator 11 for 64 ms. Therefore, the timer overflow program is executed as an interrupt program whenever the counter value of the clock counter 259 reaches its maximum value (counter is full counted), i.e. every 64 ms. Upon starting execution of the timer overflow program, the timer overflow value OFL is incremented by 1, at a block 4010. The overflow value OFL is then checked at a block 4012. If the overflow value OFL is less than a given value, e.g. 2, then control returns to the main routine of the main program. Since the timer overflow value OFL is cleared at the end of the main program at the block 1008, if the timer overflow program is executed twice during one cycle of execution of main program, the overflow value OFL would become 2. In this case, the answer at the block 4012 would be YES and the wheel speed value V w would be set to zero and the wheel acceleration and deceleration value a w would also be set to zero. For instance, if three successive sensor pulses are produced within the period of time for which the clock counter 259 counts the clock pulses from the clock signal generator, as shown in Fig. 21, the input timing of respective sensor pulses may be as shown at A, B and C, corresponding to the counter values C A , C B and C C . The overflow value OFL remains at zero in response to each of the sensor pulses A, B and C, since the sensor pulses are received before the counter time elapses. Therefore, the first time the timer overflow program is executed after receiving the sensor pulse C, the timer overflow value is incremented by 1 during execution of the timer overflow program at the block 4010. In this case, the timer overflow value OFL is still only 1 which is smaller than the limit checked for at the block 4012. On the other hand, if the sensor pulses are produced at intervals relatively long so that the timer overflow program can be executed twice before three successive sensor pulses are sampled, as shown in Fig. 23, then the wheel is movely so slowly that wheel acceleration a w can not be reliably calculated. Therefore, in the timer overflow program, as shown in Fig. 20, the wheel speed V w and the wheel acceleration or deceleration a w are, set to zero at the block 4014. By setting both the wheel speed V w and the wheel acceleration and deceleration a w to zero, serious errors will be avoided. Fig. 23 shows the output program for deriving the wheel speed V w , wheel acceleration and deceleration a w and slip rate R, selecting the operational mode, i.e. application mode, hold mode and release mode and outputting an inlet signal EV and/or an outlet signal AV depending upon the selected operation mode of the actuator 16. When the application mode is selected the inlet signal EV goes HIGH and the outlet signal EV goes HIGH. When the release mode is selected, the inlet signal EV goes LOW and the outlet signal AV also goes LOW. When the selected mode is hold mode, the inlet signal EV remains HIGH while the outlet signal AV goes LOW. These combinations of the inlet signal EV and the outlet signal AV correspond to the actuator supply current levels shown in Fig. 11 and thus actuate the electromagnetic valve to the corresponding positions illustrated in Figs. 4, 5 and 6. The output program is stored in the memory block 254 and adapted to be read out periodically, e.g. every 10 ms, to be executed as an interrupt program. The output calculation program is executed in the time regions shown in hatching in Figs. 24 and 25. During execution of the output calculation program, the pulse interval T is read out from a memory block 241 of RAM which stores the pulse interval, at a block 5002. As set forth above, since the pulse interval T is inversely proportional to the wheel rotation speed V w , the wheel speed can be derived by calculating the reciprocal (1/T) of the pulse interval T. This calculation of the wheel speed V w is performed at a block 5004 in the output program. After the block 5004, the target wheel speed V i is calculated at a block 5006. Manner of deriving the target wheel speed V i has been illustrated in the United States Patents 4,392,202 to Toshiro MATSUDA, issued on July 5, 1983, 4,384,330 also to Toshiro MATSUDA, issued May 17, 1983 and 4,430,714 also to Toshiro MATSUDA, issued on February 7, 1984, which are all assigned to the assignee of the present invention. The disclosure of the above-identified three United States Patents are hereby incorporated by reference for the sake of disclosure. In addition, the method of deriving the wheel speed V w may be appreciated from Fig. 31. As is obvious herefrom, the target wheel speed V i is derived as a function of wheel speed deceleration as actually detected. For instance, the wheel speed V w at (a) of Fig. 30 at which the wheel deceleration a w exceeds the deceleration threshold a ref , e.g. -1.2G is taken as one reference point for deriving the target wheel speed V i . The wheel speed at (b) of the current skid cycle, at which the wheel deceleration a w also exceeds the deceleration threshold a ref , is taken as the other reference point. In addition, the period of time between the points a and b is measured. Based on the wheel speed V w1 and V w2 and the measured period P, the deceleration rate dV i is derived from: This target wheel speed V i is used for skid control in the next skid cycle. It should be appreciated that in the first skid cycle, the target wheel speed V i cannot be obtained. Therefore, for the first skid cycle, a predetermined fixed value will be used as the target wheel speed V i . At a block 5008, the slip rate R is calculated according to the foregoing formula (2). After deriving the slip rate R at the block 5008, the derived slip rate R is compared with the reference value R ref which is representative of the slip rate threshold to switch operational mode from hold mode H H following the application mode to release mode in usual control, at a block 5010. When the slip rate R as compared with the reference value R ref at the block 5010, is equal to or greater than the reference value, then the acceleration state indicative flag F A as set at the block 2043 of the main routine is reset at a block 5012. After process at the block 5012 or when the slip rate as compared with the reference value at the block 5010 is smaller than the reference value R ref process goes to a block 5014. At the block 5014, the operational mode is determined on the basis of the wheel acceleration and deceleration a w and the slip rate R. Fig. 26 shows a table used in determining or selecting the operational mode of the actuator 16 and which is accessed according to the wheel acceleration and deceleration a w and the slip rate R. As can be seen, when the wheel slip rate R is in the range of 0 to 15%, the hold mode is selected when the wheel acceleration and deceleration a w is lower than -1.0G and the application mode is selected when the wheel acceleration and deceleration a w is in the range of -1.0G to 0.6G. On the other hand, when the slip rate R remains above 15%, the release mode is selected when the wheel acceleration and deceleration a w is equal to or less than 0.6G, and the hold mode is selected when the wheel acceleration and deceleration is in a range of 0.6G to 1.5G. When the wheel acceleration and deceleration a w is equal to or greater than 1.5G, the application mode is selected regardless of the slip rate. After deriving the operational mode at the step 5014, the determined operational mode is checked if it is hold mode H H to hold the brake pressure at the increased level, at a block 5016. When the hold mode H H is detected at the block 5016, then, the acceleration state indicative flag F A is checked at a block 5018. When set state of the acceleration state indicative flag F A is detected at the block 5018, the determined operational mode is modified to application mode at a block 5020. On the other hand, when the determined operational mode is other than hold mode H H as checked at the block 5016 or when the acceleration state indicative flag F A is reset as checked at the block 5018, then process of the output program goes to a block 5022. At the block 5022, according to the operational mode selected at the block 5014 or the modified operational mode modified at the block 5020, the signal levels of the inlet signal EV and the outlet signal AV are determined so that the combination of the signal levels corresponds to the selected operation mode of the actuator 16. The determined combination of the inlet signal EV and the outlet signal AV are output to the actuator 16 to control the electromagnetic valve. As set forth above, according to the shown embodiment, the operational modes cycle is modified when the wheel speed recovery rate is higher than an usual rate due to high friction of the road surface. This avoids prolongation of period of time to increase the brake pressure at required pressure level to hold the brake pressure nearby the lock pressure. As a result, the desired quick take-up feature in the anti-skid brake control system can be obtained. It should be appreciated that, although the execution timing of the output calculation program has been specified to be about 10 ms in the foregoing disclosure, the timing is not necessarily fixed to the mentioned timing and may be selectable from the approximate range of 1 ms to 20 ms. The execution timing of the output program is fundamentaly to be determined in accordance with the response characteristics of the actuator. Fig. 27 shows a circuit for embodying another embodiment of the anti-skid brake control system with quick take-up feature, in accordance with the present invention. In this embodiment, the controller 202, 204 and 206 of the overall system as illustrated in Fig. 1 have respectively identical circuit construction to each other. Therefore, in order to simplify the disclosure, the detailed description given herebelow may be concentrated to the controller unit 202. With respect to the controller units 204 and 206, it should be noted that the circuit construction with respect to the controller unit 202 is applicable thereto. In Fig. 27, a wheel speed deriving circuit 410 is connected to the wheel speed sensor 10 to receive wheel speed indicative signals. The wheel speed deriving circuit 410 is adapted to output a wheel speed indicative signal having a value proportional to the pulse frequency of wheel speed sensor signal from the wheel speed sensor. The wheel speed indicative signal is supplied to an acceleration deriving circuit 412. The wheel acceleration deriving circuit 412 differentiates the wheel speed indicative signal value to derive wheel acceleration value a w and outputs a wheel acceleration indicative signal. The wheel acceleration indicative signal is input to the negative input terminal of a differential amplifier 414. The positive input terminal of the differential amplifier 414 is connected to a reference signal generator 416 to receive a reference signal. The reference signal value is representative of a preset deceleration value, e.g. -1G. Therefore, as long as the wheel acceleration indicative signal value is greater than the preset deceleration value, the output level of the differential amplifier remains LOW. On the other hand, when the wheel acceleration indicative signal value becomes less than the preset deceleration value, output level of the differential amplifier 414 goes HIGH. The output of the differential amplifier 414 is supplied to one of the three input terminals of an OR gate 436. The wheel speed deriving circuit 410 is also connected to a projected speed deriving circuit 420. The projected speed deriving circuit is also connected to the wheel acceleration deriving circuit 412 to receive the wheel acceleration indicative signal. The projected speed deriving circuit 420 is adapted to latch the wheel speed indicative signal value when wheel acceleration indicative signal value becomes equal to or greater than the preset deceleration value. The projected speed deriving circuit 420 includes memories for storing latched wheel speed indicative signal values of the current skid cycle and the immediately preceding skid cycle. In addition, the projected speed deriving circuit measures the interval between occurrences of latching of the wheel speed indicative signal values and from the measured period of time, the projected speed deriving circuit derives an average angular deceleration value. This deceleration value may used to derive a projected speed value for the next cycle of anti-skid control. For instance, each instantaneous projected speed may be derived by the following equation: V c = V wnew + dV w x t where V c is the projected speed; V wnew is the wheel speed at which the wheel acceleration indicative signal value equal to or less than the preset deceleration value is obtained; dV w is the derived deceleration value; and t is elapsed time since deviation of the value V wnew . The projected speed V c represents an estimated vehicle speed based on the measured wheel speed. The vehicle speed can be obtained directly from the wheel speed whenever zero slip can be assumed. Therefore, in the shown embodiment, it is assumed that, when the preset deceleration value, e.g. -1G, is obtained, wheel slip relative to the vehicle ground speed will be zero or negligible and so can ignored. The timing at which the wheel acceleration value becomes equal to or less than the preset deceleration value is thus regarded as cripping point for increasing wheel slippage relative to vehicle from zero by further decelerating operation. In addition, it should be appreciated that, in the first cycle of anti-skid control, a fixed value, e.g. -0.4G will be used as the deceleration value. Procedures for deriving the projected speed can also be seen in the United States Patents 4,392,202, issued July 5, 1983; 4,384,330, issued May 17, 1983; and 4,430,714 issued February 7, 1984, respectively to the inventor of this invention and commonly assigned to the assignee of this invention. Disclosure of the above-identified U.S. Patents are herewith incorporated by reference for the sake of disclosure. Returning to Fig. 11, the projected speed deriving circuit 420 is connected to a target wheel speed deriving circuit 422. The target wheel speed deriving circuit 422 is adapted to derive a target wheel speed which is optimally related to the vehicle speed. The target wheel speed means a wheel speed to which the wheel speed is to be adjusted in order to obtain optimal braking characteristics. In general, as is well known, braking characteristics are optimized when wheel slippage relative to the vehicle speed is in the range of 15% to 20%. Therefore, according to the preferred embodiment of the invention, the target wheel speed is chosen to be 85% of the projected vehicle speed. The target wheel speed deriving circuit 422 thus outputs a target wheel speed indicative signal having a value corresponding to 85% of the projected speed. The target wheel speed deriving circuit 422 is connected to the positive input terminal of a differential amplifier 424. The negative input terminal of the differential amplifier is connected to the wheel speed deriving circuit 410. The differential amplifier 424 compares the wheel speed indicative signal value with the target wheel speed indicative signal value and outputs a HIGH-level comparator signal as long as the wheel speed indicative signal value is less than the target wheel indicative signal value. On the other hand, the output level of the differential amplifier goes LOW when the wheel speed indicative signal value becomes greater than the target wheel speed indicative signal value. The output terminal of the differential amplifier 424 is connected to an AND gate 428 to supply the comparator output thereto. The wheel acceleration deriving circuit 412 is also connected to the positive terminal of a differential amplifier 430. The negative input terminal of the differential amplifier 430 is connected to a reference signal generator 426. The reference signal generator 426 outputs a reference signal having a value representative of a preset acceleration value, e.g. 0.6G. The differential amplifier 430 outputs a HIGH-level signal when the wheel acceleration indicative signal value is greater than the reference signal value, and, conversely, a LOW-level signal when the wheel acceleration indicative signal value is less than the reference signal value. The OR gate 436 is connected to the output terminal of the differential amplifiers 424. Also, the OR gate 436 is connected to the output terminal Q of a set/reset gate circuit 437 which is, in turn, connected to the differential amplifier 414. The gate circuit 437 is also connected to an one-shot multivibrator (M/M) 415 which is adapted to be triggered by the trailing edge of HIGH level output of the differential amplifier 414, at the reset terminal thereof. The set terminal of the gate circuit 437 is connected to the output terminal of the differential amplifier 424 through an one-shot multivibrator (M/M) 431. The one-shot multivibrator 431 is adapted to be triggered by the trailing edge of the HIGH-level comparator signal from the differential amplifier 424. The gate circuit 437 allows to pass the output of the differential amplifier 414 to the OR gate 436 at the set state thereof and blocks communication between the differential amplifier 414 and the OR gate 436 at its reset state. The differential amplifier 430 is connected to an AND gate 439 which is, in turn, connected to a retriggerable timer 424 of a motor control circuit 500 described later. The output terminal of the AND gate 439 is connected to the OR gate 436. The differential amplifier 414 outputs a HIGI-level comparator signal when the wheel acceleration indicative signal value is less than the preset deceleration value. The preset deceleration value represents a pressure release threshold. Therefore, a HIGH-level output from comparator 414 indicates deceleration of the wheel beyond the pressure release threshold. The differential amplifier 430 outputs a HIGH-level comparator signal when the wheel acceleration indicative signal value is greater than the preset acceleration value. The preset acceleration value is representative of a pressure apply threshold. Therefore, a HIGH-level signal from comparator 430 indicates acceleration of the wheel beyond the pressure apply threshold. On the other hand, the differential amplifier 424 outputs a HIGH-level comparator signal when the wheel speed value is less than the target wheel speed value. Therefore, under the normal state, the output of the OR gate 436 is HIGH when wheel acceleration is less than the preset deceleration value or greater than the preset acceleration value, or when the wheel speed is less than the target wheel speed. The output of the OR gate 436 is supplied to one input terminal of an AND gate 438. The differential amplifier 424 is also connected for output to an AND gate 428. In addition, the differential amplifier 430 is connected to an inverting input terminal of the AND gate 428. The differential amplifier 424 outputs a HIGH-level comparator signal when wheel speed is less than the target wheel speed, the differential amplifier 430 outputs a LOW-level comparator signal when the wheel acceleration value is smaller the preset acceleration value, and the AND gate 428 outputs a HIGH-level gate signal only when both of these conditions are satisfied. Otherwise, the output level of the AND gate 428 remains LOW. The AND gate is connected for output to an amplifier 440 which in turn sends an amplifier signal to outlet valve as outlet signal AV. The AND gate 428 is also connected to the retriggable timer 442. The retriggable timer 442 is responsive to a HIGH-level output from the AND gate 428 to be triggered for a period of time longer than maximum duration of one cycle of skid control. The retriggable timer 442 is connected for output to the base electrode of a switching transistor 502 in a motor control circuit 500 which controls operation of the motor 88 of the fluid pump 90. The transistor 502 is rendered conductive by the HIGH-level trigger signal from the retriggable timer 442 to energize a relay coil 504 connected to collecter electrode thereof. A relay switch 506 is turned ON by energization of the relay coil 504 to close a power supply circuit for the motor 88. Therefore, the motor 88 is driven for the period of time for which the retriggable timer 442 is triggered. The usual operation of the first embodiment of the anti-skid brake control system as set out above, will be described with reference to Fig. 28. After starting application of the brakes, the wheels decelerate, i.e. the deceleration value increases. At a time t₁, the wheel deceleration value a w becomes less than the preset deceleration value b₁. Therefore, the output level of the differential amplifier 414 goes from LOW level to HIGH level. The HIGH level comparator signal of the differential amplifier 414 is outputted to the OR gate 436. At this time, as the wheel acceleration value a w is less than the preset acceleration value a₁, the output level of the differential amplifier 430 remains LOW. Also, as the wheel speed V w is higher than the target wheel speed V i , the output level of the differential amplifier 424 remains LOW. Therefore, the output of the OR gate 436 goes HIGH and the output of the AND gate 428 goes LOW. Thus, the output of the OR gate 436 goes HIGH, thus transmitting a HIGH-level inlet signal EV to the inlet valve 16b through the amplifier 438. On the other hand, since the output level of the AND gate 428 remains LOW, a LOW-level outlet signal AV is transmitted to the outlet valve 16c via the amplifier 440. In the shown embodiment, the pressure control valve 16a including the inlet valve 16b and the outlet valve 16c operates in different operation modes as set out below: Therefore, at the time t₁, the pressure control valve 16a is actuated to HOLD mode in which the fluid pressure to be applied to the wheel cylinder is held at the pressure level at time t₁. As a result, the wheel continues to decelerate and drops below the target wheel speed V i at a time t₂. As a result, the output of the differential amplifier 424 goes HIGH at the time t₂. Since the output level of the differential amplifier 430 still remains LOW, the input level of the AND gate 428 from the differential amplifier 430 is HIGH. Thus, the AND gate 428 is opened to transmit a HIGH-level outlet signal AV to the outlet valve 16c via the amplifier 440. At this time, the inlet signal EV remains HIGH. Therefore, at the time t₂, the operation mode of the pressure control valve 16a changes to RELEASE mode. In response to the rising edge of the outlet signal AV, the retriggable timer 442 is triggered to energize the relay coil 504 to turn ON the relay switch 506 to drive the motor 88 of the pressure reduction fluid pump 90. The fluid pump 90 assists reduction of the fluid pressure in the wheel cylinder. By actuating the pressure control valve 16a to the RELEASE mode, the wheel speed is allowed to increase again. As a result, the wheel acceleration a w gradually increases and exceeds the preset deceleration value b₁ at a time t₃. Whereupon the output level of the differential amplifier 414 goes LOW. However, in this case, since the output level of the differential amplifier 424 still remains HIGH, the output of the OR gate 424 remains HIGH. Therefore, RELEASE mode is maintained, allowing the wheel speed to increase further towards the vehicle speed. Wheel acceleration a w thus increases until it exceeds the preset acceleration value a₁ at a time t₄. As a result, the output level of the differential amplifier 430 goes HIGH at the time t₄. The OR gate 418 thus outputs a HIGH-level gate signal. On the other hand, as the input level of the AND gate 428 from the differential amplifier 430 goes LOW, the output level of the AND GATE 428 goes LOW. Therefore, operation returns to HOLD mode in which the fluid pressure in the wheel cylinder is held at the current pressure level at the time t₄. Since the fluid pressure at the time t₄ is relatively low, wheel speed continues to increase toward the vehicle speed. The wheel speed thus exceeds the target wheel speed V i at a time t₅. Therefore, output level of the differential amplifier 424 turns LOW level at the time t₅. At this time, since the output level of the differential amplifier 430 is HIGH, the OR gate 418 still outputs a HIGH-level gate signal. When the road surface friction is relatively high and thus wheel speed recovery during HOLD mode holding the brake pressure occurs as shown in Fig. 29, the wheel acceleration a w which once drops below the reference value b₁ at a time t₁, after starting application of brake at atime t₀', is increased across the reference value b₁ at a time t₂'. By occurrence of the increasing of wheel acceleration a w and thus occurring of wheel speed recovery, slip rate R cannot become equal to or greater the slip rate reference R ref . This, in turn, means that due to recovery of wheel speed, the wheel speed varies away from the target wheel speed V i during pressure holding at the HOLD mode during the period t₁, to t₂,. At the time t₂', the one-shot multivibrator 415 is triggered by the trailing edge of the HIGH level comparator signal from the differential amplifier 414. The gate circuit 437 is thus reset at the time t₂'. By this, the differential amplifier 414 is blocked communication to the OR gate 436. This blocking state of the gate circuit 437 maintains until the wheel speed V w once dropped below the target wheel speed V i is recovered to across the target wheel speed. In Fig. 29, the wheel acceleration a w increases across the reference value a₁ at a time __, to cause HIGH-level comparator signal is not active at this time, AND condition in the AND gate 439 is not established to hold the output of AND gate 430 LOW. By this, HOLD mode to maintain the brake pressure at the increased level and following to APPLICATION mode is inhibitted. As a result, even when the wheel acceleration a w drops below the reference value b₁ at this condition, operational mode is held at APPLICATION mode. Thus, the invention fulfills all objects and sought therefor.

Claims: An anti-skid brake control system for an automotive brake system comprising: a hydraulic brake system including a wheel cylinder (30a, 34a, 38a) for applying braking pressure to a vehicle wheel, and a pressure control valve (16a, 18a, 20a) associated with said wheel cylinder for adjusting fluid pressure in said wheel cylinder, said pressure control valve being operative to increase fluid pressure in the wheel cylinder in its first position, to subsequently hold increased fluid pressure constant at its second position, to decrease fluid pressure in said wheel cylinder in its third position, and to subsequently hold decreased fluid pressure constant in its fourth position; wheel speed sensor (10, 12, 14) detecting rotation speed of the wheel and producing a wheel speed indicative signal having a value indicative of the detected wheel speed; a controller (200) deriving a wheel acceleration on the basis of variation of said wheel speed indicative signal values and selecting operation mode of said pressure control valve (16a, 18a, 20a) on the basis of detected wheel speed and derived wheel acceleration to produce a control signal to control said pressure control valve to one of first, second , third and fourth positions corresponding to the selected operation mode, CHARACTERIZED IN THAT said controller (200) being detecting hunting in switching operation modes between said first and second positions to select said first position instead of selecting said second position and to maintain said first position until said third position is selected. The anti-skid brake control system as set forth in claim 1, wherein said controller (200) further estimates a vehicle speed on the basis of said wheel speed indicative signal value and projects a target wheel speed. The anti-skid brake control system as set forth in claim 2, wherein said controller (200) selects said second position when derived wheel acceleration decreases below a first acceleration threshold, said third position when detected wheel speed drops below said target wheel speed, said fourth position when derived wheel acceleration increases across a second acceleration threshold, and said first position when detected wheel speed increases across a third value corresponding to estimated vehicle speed. The anti-skid brake control system as set forth in claim 3, wherein said controller (200) detects wheel acceleration once decreases said first acceleration threshold and then increases across said first acceleration threshold before detecting wheel speed decreasing across said target wheel speed, to detect hunting of switching of mode selection between said first and second positions. The anti-skid brake control system as set forth in claim 1 , wherein said pressure control valve (16a, 18a, 20a) comprises a first valve (16b) for feeding a pressurized fluid into said wheel cylinder (30a) and a second valve (16c) for draining the pressurized fluid in said wheel cylinder (30a). The anti-skid brake control system as set forth in claim 5, wherein said first valve (16b) opens and said second valve (16c) closes in said first position, said first valve (16b) closes and second valve (16c) opens in said third position and both of first and second valves (16b, 16c) close in said second and fourth positions. The anti-skid brake control system as set forth in claim 6, wherein said first and second control valve (16b, 16c) open in response to high-level control signal from said controller (200) and close in response to low-level control signal. The anti-skid brake control system as set forth in claim 7, wherein said controller (200) holds the control signal to be applied to said first valve (16b) at a high-level when hunting of switching between said first and second positions is detected. An anti-skid brake control system as set forth in any of claims 1 to 8, wherein said controller (200) comprises: first means (410, 412) for deriving brake control parameters including wheel acceleration; second means (402) for determining operation mode of said pressure control valve (16a) in said hydraulilc brake system among said first to fourth positions and producing a control signal to operate said pressure control valve (16a) to the selected one of valve positions; third means (439), associated with said second means (402) and responsive to selection selecting said first mode subsequently of selection of said second mode, to produce a hunting indicative signal; and fourth means (437, 436,438), responsive to said hunting indicative signal, for modifying selection selecting said second position to said first position. The anti-skid brake control system as set forth in claim 9, wherein said second means (402) selects one of said valve position in such a manner that: when wheel acceleration derived by said first means (410, 412) as one of brake control parameters decreases across a given first value, it selects said second mode; when a slip rate derived by said first means (410, 412) as one of brake control parameters increases across a given second value, it selects said third position; when the wheel acceleration increases across a given third value, it selects said fourth position; and when the wheel speed increases across a given fourth value, it selects said first position. The anti-skid brake control system as set forth in claim 10, wherein said wheel speed sensor (10, 12, 14) produces a train of pulses separated by intervals representative of the detected wheel rotation speed. The anti-skid brake control system as set forth in claim 11, wherein said first means (410, 412) is associated with a timer means (11) for producing a timer signal having a value indicative of elapsed time, and samples said timer signal value in response to sensor signal pulses, stores the sampled timer signal value as input timing data and processes said input timing data for deriving wheel speed. The anti-skid brake control system as set forth in claim 12, wherein said second means (402) produces first and second control signals respectively applied to said first and second valves (16b, 16c) to open said valve by low-level signal. The anti-skid brake control system as set forth in claim 13, wherein said fourth means (437) is responsive to said hunting indicative signal to hold said first control signal high. A method for anti-skid control in an automotive brake system comprising the steps of: detecting wheel speed to produce a wheel speed indicative signal value of which is variable proportional to detected wheel speed; deriving brake control parameters including wheel acceleration on the basis of said wheel speed indicative signal values; operating a pressure control valve disposed within a hydraulic brake circuit including a wheel cylinder to its first position to increase braking pressure; subsequently operating said control valve to its second position to hold braking pressure at increased constant level, when wheel acceleration decreases across a first given value; operating said control valve to its third position to decrease said braking pressure, when wheel speed decreases across a second given value; subsequently operating said control valve to its fourth position to hold braking pressure at decreased constant level, when wheel acceleration increases across a third given value; and operating said control valve to said first position when wheel speed increases across a given fourth value; CHARACTERIZED BY detecting increasing of wheel acceleration across said first given value during operation in which said control valve is in said second position, to operate said control valve to said first position and hold the control valve at said first position until the wheel speed decreases across said second value. A method for anti-skid control in an automotive brake system comprising the steps of: detecting wheel speed to produce a wheel speed indicative signal value fo which is variable proportional to detected wheel speed; processing said wheel speed indicative signal to derive brake control parameters including wheel acceleration and slip rate relative to an estimated vehicle speed, which esstimated vehicle speed is derived on the basis of wheel speed; operating a pressure control valve disposed within a hydrauliic brake circuit including a wheel cylinder to its first position to increase braking pressure; subsequently operating said control valve to its second position to hold braking pressure at increased constant level, when wheel acceleration decreases across a first given value; operating said control valve to its third position to decrease braking pressure when said slip rate increases across a second given value; subsequently operating said control valve to its fourth position to hold braking pressure at decreased constant level, when wheel acceleration increases across a third given value; and operating said control valve to said first position when said wheel speed increases across a given fourth value which corresponds to said estimated vehicle speed; CHARACTERIZED BY detecting increasing of wheel acceleration across said first given value during operation in which said control valve is in said second position, to operate said control valve to said first position and hold the control valve at said first position until the wheel speed decreases across said second value.

Inventors: MATSUDA, TOSHIRO
Applicants: NISSAN MOTOR CO., LTD.
Application year: 1984
Main classification: B60T8
IPC codes: B60T8