ANTI-SKID BRAKE CONTROL SYSTEM FOR AN AUTOMOTIVE BRAKE SYSTEM HAVING VARIABLE DECELERATION THRESHOLD

Abstract: An anti-skid brake control system detects wheel acceler­ ation and deceleration. The operational mode, i.e. whether to increase or decrease fluid pressure in a wheel cylinder or to hold the fluid pressure in the wheel cylinder constant, is selected on the basis of brake control parameters including the wheel acceleration and deceleration. In an initial stage of each cycle of anti-skid control, the fluid pressure is held at a level at which wheel deceleration exceeds a predetermined deceleration threshold. The predetermined deceleration threshold is reduced to a predetermined modified value after wheel deceleration has dropped below the predetermined deceleration of the wheel and anti-skid brake control is thus started.

Description: BACKGROUND OF THE INVENTION The invention relates to an anti-skid brake control system comprising the features as indicated in the precharacterizing part of claim 1. 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 abovementioned 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, U.S. Patent 3,637,264, issued on January 25, 1972 to Leiber et al 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, Published Japanese Patent Application (Tokkai) Showa 51-89096, published on August 4, 1976 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. Anti-skid control systems such as those set forth above in which braking pressure is increased in a stepwise manner have a drawback in that the wheel will decelerate to a speed lower than that obtained by increasing braking pressure linearly and continuously. In general, the anti-skid brake control system becomes active when wheel deceleration exceeds a given deceleration threshold. Therefore, it is possible for the braking pressure to increase while the deceleration remains below the deceleration threshold until it excees a lock pressure, resulting in skidding of the wheel. This is due to the effective disabling of the anti-skid brake control system due to relatively low wheel deceleration. Such an anti-skid brake control system 1 as indicated in the precharacterizing part of claim 1 and as disclosed in DE-B-1 755 578 comprises wheel cylinders and pressure control valves which operate in an application mode, release mode and hold mode. Wheel speed sensors are provided in which signals are used to detect control parameters such as acceleration, deceleration and slip rate. In the case that the wheel tends to skid, i.e. a predetermined slip threshold is obtained, the control valve is switched to release mode. It is the object of the invention to provide an anti-skid brake control system which increases braking pressure in a stepwise manner and is able to perform skid control even when wheel deceleration is relatively low. In accordance with the invention, this object is solved by the features as claimed in the characterizing parts of claims 1 and 6. In the inventive anti-skid brake control system means are provided for detecting wheel acceleration and deceleration. In an initial stage of each cycle of anti-skid control, control of the fluid pressure is performed when wheel deceleration exceeds a predetermind deceleration threshold. The predetermined deceleration threshold is reduced to a predetermined modified value after wheel deceleration has dropped below the predetermined deceleration of the wheel. The modified deceleration threshold is smaller than the first deceleration threshold. Thus the inventive system is intended to quickly take-up the braking pressure close to the lock pressure at the initial stage of the braking operation and to maintain the braking pressure close to the lock pressure in the vicinity of the lock pressure as long as possible so as to obtain the optimum braking performance. 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 block diagram of the first embodiment of a controller unit in the anti-skid brake control system according to the present invention; Fig. 12 is a timing chart showing operations of the controller unit of Fig. 13; Fig. 13 is a block diagram of the second embodiment of the controller unit in the anti-skid brake control system of Fig. 1; Fig. 14 is a flowchart of a main program to be executed by the controller unit of Fig. 13; Fig. 15 is an flowchart of an interrupt program to be executed by the controller unit of Fig. 13; Fig. 16 is a flowchart of a main routine inthe main program of Fig. 13; Fig. 17 is an explanatory diagram of the input timing sampling mode and variation thereof; Fig. 18 is a flowchart of an output calculation program to be executed by the controller unit of Fig. 13; Figs. 19 and 20 are diagram of execution timing of the output calculation program in relation to the main program of Fig. 14; Fig. 21 is a table for determining the operation mode of the actuator 16, which table is accessed in terms of the wheel acceleration and deceleration and the slip rate; and Fig. 22 is a flowchart of an inlet signal and an outlet signal deriving routine in the output calculation. DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the drawings, particularly to Figs. 1 to 10, an anti-skid brake control system, according to the present invention, includes independently operative three anti-skid control circuits 402, 404 and 406 respectively controlling front-left (FL) wheel, front-right (FR) wheel and rear (R) wheels. The anti-skid control circuit 402, 404 and 406 respectively includes digital controller units 202, 204 and 206 which are housed in a common controller housing to form a control module 200. The controller unit 202 provided in the front-left anti-skid control circuit 402, is connected to a wheel speed sensor 10 for producing an alternative current form sensor signal having a frequency proportional to rotation speed of a front-left wheel (not shown). On the other hand, the controller unit 202 is also connected to an electromagnetic actuator 16 in a front-left brake circuit 302. The brake circuit 302 includes a front-left wheel cylinder 30a for operating a brake shoe assembly 30 for applying braking force to a brake disc rotor 28, and an electromagnetic pressure control valve 16a operated by the actuator 16 for controlling fluid pressure to be applied to the wheel cylinder 30a and whereby controlling the braking force. Similarly, the controller unit 204 of the front-right anti-skid control circuit 404 is connected to a wheel speed sensor 204 to receive alternative current form sensor signal with a frequency representative of rotation speed of front-right wheel. The controller unit 204 is, in turn, connected to an actuator 18 in a front-right brake circuit 304. The actuator 18 is adapted to operate an electromagnetic pressure control valve 18a for controlling hydraulic pressure to be applied to a front-right wheel cylinder 34a. With the controlled hydraulic pressure, the wheel cylinder 34a operates a front-right brake shoe assembly 34 for applying braking force to a brake disc rotor 32 rotating with the front-right wheel. In addition, the controller unit 206 is connected to a wheel speed sensor 14 to receive therefrom an alternative current sensor signal having a frequency indicative of the average rotation speed of rear wheels. In order to detect average rotation speed of the rear wheels, the wheel speed sensor 14 may be adapted to detect rotation speed of propeller shaft or the equivalent rotating at the approximately average speed of the rear wheels. The controller unit 206 is also connected to an electromagnetic actuator 20 of an electromagnetic pressure control valve 20a. The electromagnetic valve 20a is associated with rear wheel cylinders 38a for controlling fluid pressure to be applied to the rear wheel cylinders and whereby controlling braking pressure to be applied to rear brake disc rotors 36 through rear brake shoe assemblies 38a. It should be appreciated that through the controller units 202, 204 and 206 are adapted to control respectively the front-left, front-right and rear brake circuits 302, 304 and 306, since the shown embodiment is directed to an anti-skid brake control system for a vehicle having driving arrangement of front-engine, rear wheel drive, the invention can be modified to apply any driving arrangement of vehicle, such as front-engine, front wheel drive or four wheel drive arrangements. In addition, though the shown brake system comprises disc brakes, the anti-skid brake control system according to the invention can also be applied to drum-type brake system. 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 less 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 shows the first embodiment of controller unit 202 of the present invention. The controller units 204 and 206 are designed in substantially the same way as the controller unit described herebelow. Therefore, in order to simplify the disclosure, the detailed explanation of the controller units 204 and 206 will be omitted. In Fig. 11, a wheel speed deriving circuit 210 is connected to the wheel speed sensor 10 to receive wheel speed indicative signals. The wheel speed deriving circuit 210 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 fed to an acceleration and deceleration deriving circuit 212. The wheel acceleration and deceleration deriving circuit 212 differentiates the wheel speed indicative signal value to derive wheel acceleration and deceleration value a w to output a wheel acceleration and deceleration indicative signal. The wheel acceleration and deceleration indicative signal is input to a negative input terminal of a differential amplifier 214. A positive input terminal of the differential amplifier 214 is connected to first and second reference signal generators 216 and 217 to selectively receive a first or a second reference signal via a switching circuit 219. The reference signal value of the first reference signal generator 216 is representative of a preset deceleration value a dec , e.g. -1G. On the other hand, the reference signal value of the second reference signal generator 217 is representative of a modified preset deceleration value a mod , which value is greater, i.e. less negative, than the preset deceleration value a dec of the first reference signal. Therefore, as long as the wheel acceleration and deceleration indicative signal value is larger than either the preset deceleration value or the modified preset deceleration value, output level of the differential amplifier remains LOW. On the other than, when the wheel acceleration and deceleration indicative signal value becomes smaller than the preset deceleration value, output level of the differential amplifier 214 turns to HIGH. The output of the differential amplifier 214 is fed to one of three input terminals of an OR gate 218. The wheel speed deriving circuit 210 is also connected to a projected speed deriving circuit 220. The projected speed deriving circuit is also connected to the wheel acceleration deriving circuit 212 to receive the wheel acceleration indicative signal. The projected speed deriving circuit 220 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 220 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 220 is connected to a target wheel speed deriving circuit 222. The target wheel speed deriving circuit 222 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 222 thus outputs a target wheel speed indicative signal having a value corresponding to 85% of the projected speed. The target wheel speed deriving circuit 222 is connected to the positive input terminal of a differential amplifier 224. The negative input terminal of the differential amplifier is connected to the wheel speed deriving circuit 210. The differential amplifier 224 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 224 is connected to an AND gate 228 to supply the comparator output thereto. The wheel acceleration deriving circuit 212 is also connected to the positive terminal of a differential amplifier 230. The negative input terminal of the differential amplifier 230 is connected to a reference signal generator 226. The reference signal generator 226 outputs a reference signal having a value representative of a preset acceleration value, e.g. 0.6G. The differential amplifier 230 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 differential amplifiers 224 and 230 are also connected to the OR gate 218 to supply their comparator signals. Therefore, OR gate 218 outputs a HIGH-level gate signal in response to a HIGH-level input from any of the differential amplifiers 214, 224 and 230. The differential amplifier 214 outputs a HIGH-level comparator signal when the wheel acceleration and deceleration indicative signal value is smaller than the preset deceleration value. The preset deceleration value represents a pressure release threshold. Therefore, a HIGH-level comparator signal is indicative of a decelerating state of the wheel beyond the pressure release threshold. The differential amplifier 230 outputs HIGH level comparator signal when the wheel acceleration and deceleration indicative signal value is greater than the preset acceleration value. The preset acceleration value is representative of a pressure apply threshold. Therefore, HIGH level comparator signal is indicative of acceleration state of the wheel across the pressure apply threshold. On the other hand, the differential amplifier 224 outputs HIGH level comparator signal when the wheel speed value is smaller than the target wheel speed value. Therefore, output of the OR gate 218 become HIGH when whell acceleration and deceleration is smaller than the preset deceleration value or greater than the preset acceleration value, or when the wheel speed remains lower than the target wheel speed. The output of the OR gate 218 is supplied to an amplifier 238 which in turn sends and amplified output to the inlet valve as inlet signal EV. The differential amplifiers 224 is also connected to an AND gate 228. Also, the differential amplifier 230 is connected to the AND gate 228 via an inverting input terminal. As the differential amplifier 224 outputs HIGH level comparator signal when wheel speed is lower than the target wheel speed and the differential amplifier 230 outputs LOW level comparator signal when the wheel acceleration and deceleration is smaller the preset acceleration value, the AND gate 228 outputs HIGH level gate signal only wheel the aforementioned and-condition is established. Otherwise, the gate signal of the AND gate 228 remains LOW level. The AND gate is connected to an amplifier 240 to feed amplifier signal to outlet valve as outlet signal AV. On the other hand, the differential amplifier 214 is also connected to a retriggable timer 242. The retriggable timer 242 is responsive to a HIGH-level gate signal from the AND gate 228 to be triggered for a period of time longer than the maximum possible duration of one cycle of skid control. The retriggable timer 242 is connected to the switching circuit 219. The switching circuit normally connects the first reference signal generator 216 to the differential amplifier 214, but is responsive to a HIGH-level signal from the retriggable timer 228 to connect the second reference signal generator 217 to the differential amplifier 214. The operation of the first embodiment of the anti-skid brake control system as set out above, will be described with reference to Fig. 12. After starting application of the brakes, the wheel decelerates, i.e. the deceleration value increases. At a time t₁, the wheel deceleration a w becomes smaller than the preset deceleration value a dec . Therefore, the output level of the differential amplifier 214 changes from LOW to HIGH. The HIGH level comparator signal of the differential amplifier 214 is fed to the OR gate 218. At the same time, the HIGH level comparator signal of the differential amplifier 214 is fed to the retriggable timer 242 to trigger the latter. In response to the HIGH level signal from the retriggable timer 242 and with a given time delay T₁, the switching circuit 219 changes position so as to connect the second reference signal generator 217 to the differential amplifier 214. At this time, since the wheel acceleration and deceleration value a w is less than the preset acceleration value a₁, the output level of the differential amplifier 230 remains LOW. Also, since the wheel speed V w is higher than the target wheel speed V i , the output level of the differential amplifier 224 remains LOW. Therefore, the gate signal output from the OR gate 218 turns HIGH level and the gate signal output from the AND gate 228 remains LOW level. Thus, the gate signal from the OR gate 236 turns HIGH for feeding HIGH level inlet signal EV to the inlet valve 16b through the amplifier 238. On the other hand, since the output level of the AND gate 228 remains LOW, LOW level outlet signal AV is fed to the outlet valve 16c via the amplifier 240. Here, 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: Operation Mode EV AV APPLICATION MODE LOW LOW HOLD MODE HIGH LOW RELEASE MODE HIGH HIGH Therefore, at the time t₁, the pressure control valve 16a is operated to HOLD mode to hold the fluid pressure to be applied to the wheel cylinder at a pressure level at the time t₁. By holding the fluid pressure at the level of the time t₁, the wheel is decelerated across the target wheel speed V i at a time t₂. As a result, the output of the differential amplifier 224 turns HIGH level, at the time t₂. Since the output level of the differential amplifier 230 still remains LOW, the input level of the AND gate 228 from the differential amplifier 230 is HIGH. Thus, the AND gate 228 is opened to feed HIGH level outlet signal AV to the outlet valve 16c via the amplifier 240. At this time, the inlet signal EV is maintained HIGH level. Therefore, at the time t₂, the operation mode of the pressure control valve 16a turns to RELEASE mode. Due to operation of the pressure control valve 16a in the RELEASE mode, the wheel speed increases toward the vehicle speed and the wheel acceleration a w gradually increases to beyond the modified preset deceleration value a mod at a time t₃. Due to the increase of the wheel acceleration past the modified preset deceleration value a mod , the output level of the differential amplifier 214 turns LOW at the time t₃. However, at this time, since the output level of the differential amplifier 224 still remains HIGH level, the gate signal of the OR gate 224 remains HIGH level. Therefore, RELEASE mode is maintained to further resume the wheel speed towards the vehicle speed. Wheel acceleration a w is thus increased across the preset acceleration value a acc , at a time t₄. As a result, the output level of the differential amplifier 230 turns HIGH level at the time t₄. At this time, the output level of the differential amplifier 224 is still maintained at HIGH level. The OR gate 218 thus outputs HIGH level gate signal. On the other hand, as the input level of the AND gate 228 from the differential amplifier 230 turns LOW, the signal level of the AND gate 228 becomes LOW. Therefore, operation mode returns to HOLD mode to hold the fluid pressure in the wheel cylinder at a pressure level at the time t₄. As the fluid pressure at the time t₄ is reduced level, wheel speed is still resumed toward the vehicle speed. The wheel speed is thus exceeds the target wheel speed V i at a time t₅. Therefore, output level of the differential amplifier 224 turns LOW level at the time t₅. At this time, since the output level of the differential amplifier 230 is HIGH level, the OR gate 218 still outputs HIGH level gate signal. Wheel acceleration gradually decreases until it is again below the preset acceleration value a acc at a time t₆. At this time, the output level of the differential amplifier 230 turns LOW level. As the wheel speed V w is still higher than the target wheel speed, the output level of the differential amplifier 224 is also LOW level. Further, since the wheel acceleration a w is greater than the modified preset deceleration value a mod , the output level of the differential amplifier 214 is also LOW. Thus, the signal level of the OR gate 218 turns LOW level. The wheel deceleration drops below the modified preset deceleration value a mod at a time t₇. The differential amplifier 214 thus outputs HIGH level comparator signal to turn the operation mode into HOLD mode, as the status of the wheel is very similar to that at the time t₁. Therefore, the second cycle of skid control operation is performed from the time t₇. Procedure to be performed in the second skid cycle is very similar to that set forth above with respect to the first cycle of skid control operation. As will be appreciated, the fluid pressure in the wheel cylinder is increased in the application mode in a stepwise manner in order to gradually and moderately increase the braking pressure. Figs. 13 to 22 show another embodiment of the anti-skid brake control system according to the present invention. In this embodiment, the present invention is applied for a digital control system in use with a microprocessor. In order to make the alternating current form wheel sensor signal applicable for the digital control system, the sensor signal is converted into pulse signals having intervals corresponding to or representative of the peripheral speed of the wheel as detected. Before entry of disclosure with respect to the shown embodiment, procedure for performing anti-skid brake control by means of the digital control system will be briefly described herebelow for the sake of better understanding of the invention. 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 . 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 20%. 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 a ref e.g. -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 v2 - (V v1 - V v2 )/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, 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. 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. When the wheel speed V w becomes equal to the target wheel speed V w , 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. 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 . 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 . 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. The control mode will be varied during the second cycle of skid control operation, to hold mode, when the wheel deceleration reaches the deceleration threshold a ref as set forth above, to release mode, when the slip rate R reaches the slip rate threshold R ref , to hold mode when the slip rate R is recovered to the slip rate threshold R ref , and to application mode when 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 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, the control mode is switched from the application mode to the hold mode and then to the release mode. 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 , 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. 13, the controller unit 202 includes an input interface 1230, CPU 1232, an output interface 1234, RAM 1236 and ROM 1238. The input interface 1230 includes an interrupt command generator 1229 which produces an interrupt command in response to every sensor pulse. In ROM, a plurality of programs including a main program (Fig. 18), an interrupt program (Fig. 19), an sample control program, a timer overflow program and an output calculation program (Fig. 22) are stored in respectively corresponding address blocks 1244, 1246, 1250, 1252 and 1254. The input interface also has a temporary register for temporarily holding input timing for the sensor pulses. RAM 1236 similarly has a memory block holding input timing for the sensor pulses. The contents of the memory block 1240 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 1242 for holding pulse intervals of the input sensor pulses. The memory black 1242 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 1256 is provided in the controller unit 202 for signalling interrupt requests to the CPU. The interrupt flag 1256 is set in response to the interrupt command from the interrupt command generator 1229. A timer overflow interrupt flag 1258 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 1231 in the input interface 1230 and the memory block 1240 of RAM 1236. The operation of the controller unit 202 and the function of each elements mentioned above will be described with reference to Figs. 14 to 22. Fig. 14 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. 16. 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. 15 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, i.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: dT = (C - B) - (B - A) ≧ S (4 ms.) (3) 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. 17, 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. 17, 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. 16, 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 1233, 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 1240 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 1240 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 1240. 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 T s 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 1240, 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 1242 of RAM 1236 for a current pulse interval data, which storage section will be hereafter referred at as "first pulse interval storage" and which memory block 1242 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 1231 is read out and transferred to the input timing memory 240 at block 2028. Based 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). Detail of the wheel acceleration or deceleration deriving routine at the block 2040 will be described later with reference to Fig. 17. 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 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 AV 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. 19 and 20. 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. 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 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 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: dV i = (V w1 - V w2 )/P (4) 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). Subsequently, the operational mode is determined on the basis of the wheel acceleration and deceleration a w and the slip rate R, at a block 5010. Fig. 22 is a flowchart of an EV/AV deriving routine to be executed at the step 5010 of Fig. 18. The schedule of selection of the operation mode of the actuator 16 is shown in the form of a table in Fig. 21. The table 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. According to the operational mode selected at the block 5010, 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. 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. In Fig. 22, selection of the operation mode is performed according to the table of Fig. 21. Therefore, wheel acceleration and deceleration and slip rate are taken as parameters for selecting the operation mode. At first, wheel acceleration and deceleration a w and slip rate R are read out at steps 5010-1 and 5010-2. Wheel acceleration and deceleration a w is first compared with the given acceleration threshold a acc at a step 5010-3. If wheel acceleration a w is equal to or less than the acceleration threshold a acc , the slip rate R is compared with the given slip rate threshold R th at a step 5010-4. If the slip rate R is equal to or greater than the slip rate threshold R th , both the inlet and outlet signals EV and AV are set to 1 (HIGH) at a step 5010-5. On the other hand, if the slip rate R is less than the slip rate threshold R th , the wheel deceleration a w is compared with the given deceleration threshold a dec at a step 5010-6. If the wheel deceleration is equal to or greater than the deceleration threshold a dec , in other words, if the wheel acceleration is equal to or less than the deceleration threshold, the contents of the flag register 255 are checked at a step 5010-8. If a flag FL MOD , which indicates whether the modified deceleration threshold a mod is currently being used in the step 5010-6, is not set in the flag register 255, the deceleration threshold a dec is replaced with the larger the modified deceleration threshold a mod at a step 5010-9. The modified deceleration threshold a mod is thereafter used as the deceleration threshold to be compared with the wheel acceleration and deceleration a w at the step 5010-6. Then, the flag FL MOD is set at a step 5010-10. The inlet signal is set to 1 (HIGH) and the outlet signal AV is set to 0 (LOW) at a step 5010-11. On the other hand, if the wheel acceleration is greater than the deceleration threshold a dec , both of the inlet and outlet signals EV and AV are set to 0 (LOW) at a step 5010-7. If the wheel acceleration a w when checked at the step 5010-3 is greater than the acceleration threshold a acc , then the wheel accelertion a w is compared with another acceleration threshold a ref which is greater than the acceleration threshold a cc , at a step 5010-12. If the wheel acceleration a w is greater than the acceleration threshold a ref , both the inlet and outlet signals EV and AV are set to 0(LOW) at a step 5010-14. When the wheel acceleration a w is equal to or less than the acceleration threshold a ref , then the inlet signal is set to 1 (HIGH) and the outlet signal AV is set to 0 (LOW), at a step 5010-13. Although not clearly shown in the drawings, the controller unit 202 will be re-initialized at a given timing after completion of anti-skid brake control. During initialization of the controller unit 202, the flag register 255 will be reset and the deceleration threshold modified at the step 5010-9 will be returned to its original value a dec . Completion of the anti-skid brake control can be recognized in various ways. According to the preferred embodiment, completion of the anti-skid control operation is determined when wheel acceleration exceeds the deceleration threshold for a given period of time, e.g. 2 sec. It will be appreciated that, as in the first embodiment, the fluid pressure will be increased in a stepwise manner as shown in Fig. 12 in order to gradually increase the braking force. According to the present invention, since the deceleration threshold for initiating anti-skid brake control is increased after the system has entered into operation, anti-skid brake control can be repeated even when wheel deceleration is relatively low, which would otherwise disable the anti-skid control system. This allows the control system to increase the brake pressure in a stepwise manner so as to prolong the period of time for which the brake pressure is held to near a lock pressure in order to obtain optimal braking characteristics. Thus, the invention fulfills all of the objects and advantages sought therefor.

Claims: An anti-skid brake control system for an automotive vehicle comprising: a hydraulic brake system including a wheel cylinder (30a, 34a, 38a) and a pressure control valve (16a, 18a, 20a) associated with said wheel cylinder (30a, 34a, 38a) for increasing braking pressure in said wheel cylinder (30a, 34a, 38a) in its first position, decreasing braking pressure in said wheel cylinder (30a, 34a, 38a) in its second position and holding braking pressure in the wheel cylinder (30a, 34a, 38a) constant in its third position; detector for detecting control parameters for performing anti-skid brake control based thereon, said control parameters including wheel acceleration (a w ) and deceleration (a w ); and a controller (202, 204, 206) associated with said pressure control valve (16a, 18a, 20a) for controlling the position of the latter depending upon detected control parameters for optimizing braking characteristics, the improvement CHARACTERIZED BY said controller (202, 204, 206) detecting wheel deceleration (a w ) in excess of a given first deceleration threshold (a dec ) for initiation of the first cycle of skid control and replacing said given first deceleration threshold (a dec ) with a given second deceleration threshold (a mod ) which is smaller than said first deceleration threshold (a dec ) for subsequent skid control cycles. The anti-skid brake control systems set forth in claim 1, wherein said detector includes a wheel speed sensor (10, 12, 14) for detecting rotation speed (v w ) of the wheel, and said detector derives wheel acceleration (a w ) and deceleration (a w ) on the basis of the detected wheel rotation speed (v w ). The anti-skid brake control system as set forth in claim 1 or 2, wherein, in said first position, said pressure control valve (16a, 18a, 20a) increases the braking pressure in said wheel cylinder (30a, 34a, 38a) in a stepwise manner. The anti-skid brake control system as set forht in one of claims 1 to 3, wherein said controller (202, 204, 206) cyclically actuates said pressure control valve (16a, 18a, 20a) in said first and third positions alternatingly when the braking pressure in the wheel cylinder (30a, 34a, 38a) is to be increased. The anti-skid brake control system as set forth in one of claims 1 to 4, wherein said controller (202, 204, 206) uses said second deceleration threshold (a mod ) throughout all control operations after anti-skid control has been initiated, and returns to the first deceleration threshold (a dec ) after anti-skid control has been completed. A method for anti-skid controlling an automotive brake control system including a hydraulic circuit including a wheel cylinder for applying braking force to a wheel and a pressure control valve for increasing fluid pressure in said wheel cylinder in its first position, decreasing fluid pressure in said wheel cylinder in its second position and holding fluid pressure in said wheel cylinder constant in its third position, the method comprising the step of: detecting wheel acceleration and deceleration; CHARACTERIZED BY the steps of: detecting when wheel deceleration exceeds a given first deceleration threshold during application of the brakes; operating said pressure control valve when wheel deceleration exceeding said first deceleration threshold; adjusting said deceleration threshold to a given second value when wheel deceleration exceeds said given first deceleration threshold; and detecting completion of anti skid control and initializing the anti-skid control system including returning said deceleration threshold to said given first deleceration threshold. The method as set forth in claim 6, which further comprises the steps of: operating said pressure control valve in said second position when wheel speed is lower than a first projected speed derived as a function of an instantaneous wheel speed at which wheel deceleration first exceeds said deceleration threshold; operating said pressure control valve in said third position when wheel acceleration exceeds a predetermined acceleration threshold; and operating said pressure control valve in said first position when wheel speed exceeds a second projected speed derived as a function of said instantaneous wheel speed, which second projected speed is higher than said first projected speed. The method as set forth in claim 7, wherein said first projected speed is about 80 to 85% of said second projected speed. The method as set forth in claim 8, which further comprises a step of detecting wheel speed and wheel acceleration and deceleration is derived on the basis of the detected wheel speed.

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