ANTI-SKID BRAKE CONTROL SYSTEM WITH OPERATIONAL MODE CONTROL AND METHOD THEREFOR

Abstract: An anti-skid brake control system controls the operation mode of a pressure control valve in a hydraulic brake circuit for increasing the fluid pressure in its application mode, for de­ creasing the fluid pressure in its releasing mode and for hold­ ing the fluid pressure at a constant value in its holding mode. The control system has a predetermined operation pattern in which the operation modes of the pressure control valve cycle through a predetermined schedule. Measured wheel accelera­ tion and slip rate values dictate which operation modes are to be selected next and when the change from mode to mode should be performed. When the operation mode is to be changed, the next operation mode selected is checked to see if it is consistent with that scheduled. When the selected oper­ ation mode is different from the scheduled one, the selected operation mode is modified to perform the operation accord­ ing to the scheduled operation mode which would otherwise be skipped.

Description: The invention relates to an anti-skid brake control system and a method for anti-skid controlling comprising the features as indicated in the precharacterizing parts of Claims 1 and 12. Such an anti-skid brake control system as disclosed in DE-A-3 103 967 comprises a pressure control valve for applying, holding and releasing pressure in the wheel cylinder of a vehicle. The system further includes a wheel speed sensor, a timer means, a wheel acceleration detecting means and a slip rate detecting means. Basing on the corresponding input data, a control signal is provided in order to actuate the pressure control valve in an applying, releasing or holding position. It is the object of the invention to provide a reliable operating anti-skid brake control system. In accordance with the invention, this object is solved by the features as claimed in the characterizing parts of Claims 1 and 12. In the inventive anti-skid brake control system there is provided a predetermined sequence of application mode, hold mode and release mode. A means for checking said predetermined sequence is provided. In the case that predetermined sequence is irregular, the control signal will be modified in order to agree with said predetermined sequence. Thus, a skipping of any one of scheduled operation modes causing a skid is avoided. the foregoing operation mode is selected depending upon the braking condition derived based on a wheel speed, a wheel acceleration, a slip rate and so forth. In U.S. Patent 4,408,290, issued on October 4, 1983 to the common inventor and commonly assigned to a common assignee, discloses a method and system for sampling input time data for use in calculation of acceleration and deceleration. In the disclosure of the applicant's prior invention, an acceleration sensor acts on the variable-frequency pulses of a speed sensor signal to recognize any variation of the pulse period thereof and to produce an output indicative of the magnitude of the detected variation to within a fixed degree of accuracy. The durations of groups of pulses are held to within a fixed range by adjusting the number of pulses in each group. The duration of groups of pulses are measured with reference to a fixed-frequency clock pulse signal and the measurement periods of successive groups of equal numbers of pulses are compared. If the difference between pulse group periods is zero or less than a predetermined value, the number of pulses in each group is increased in order to increase the total number of clock pulses during the measurement interval. The number of pulses per group is increased until the difference between measured periods exceeds the predetermined value or until the number or pulses per group reaches a predetermined maximum. Acceleration data calculation and memory control procedure are designed to take into account the variation of the number of pulse per group. The wheel speed is calculated periodically after the pulse interval or duration of the grouped sensor pulses becomes greater than a predetermined value. A slip rate is periodically derived from the derived wheel speed. On the other hand, the wheel acceleration is calculated only when the difference between the sensor pulse intervals or the durations of successive sensor pulse groups becomes larger than a given value. Therefore, wheel acceleration may be derived later than the wheel speed and the slip rate. Since the operation mode is derived periodically corresponding to calculation timing of the wheel speed and slip rate, the delay in deriving the wheel acceleration value can cause errors in selecting the operation mode. In particularly significant cases, one of the operation modes in a predetermined skid control pattern may be skipped, i.e. not performed during one cycle of the skid control operation. For example, it is possible for the holding mode to be selected immediately after having selected the holding mode, i.e. to skip either a release mode or application mode. In such case, if it is an application mode which is skipped, and thus the fluid pressure in the wheel cylinder is held at a reduced value, wheel speed will increase non-optimally, resulting in an unnecessarily prolonged braking distance. On the other hand, if a release mode period is skipped, the fluid pressure in the wheel cylinder will be held at an excessively high value, increasing the slip rate non-optimally, which may result in a skid. Such danger is caused when one period of a scheduled operation mode cycle is skipped. SUMMARY OF THE INVENTION Therefore, it is an object of the present invention to provide an anti-skid brake control system in which none of the scheduled operation modes in a predetermined operation cycle can be skipped. Another and more specific object of the invention is to provide an anti-skid brake control system, in which skipping of any of the scheduled operation modes in a predetermined operation pattern is detected and operation is modified to perform the scheduled operation mode which otherwise would be skipped. In order to achieve the above-mentioned and other objects, an anti-skid brake control system according to the present invention controls the operation mode of a pressure control valve in a hydraulic brake circuit for increasing the fluid pressure in its application mode, for decreasing the fluid pressure in its releasing mode and for holding the fluid pressure at a constant value in its holding mode. The control system has a predetermined operation pattern in which the operation modes of the pressure control valve cycle through a predetermined schedule. Measured wheel acceleration and slip rate values dictate which operation modes are to be selected next and when the change from mode to mode should be performed. When the operation mode is to be changed, the next operation mode selected is checked to see if it is consistent with that scheduled. When the selected operation mode is different from the scheduled one, the selected operation mode is modified to perform the operation according to the scheduled operation mode which would otherwise be skipped. In accordance with one aspect of the invention, an anti-skid brake control system for an automotive hydraulic brake system comprises a hydraulic brake circuit including a wheel cylinder for applying braking force to a vehicle wheel, a pressure control valve disposed within the hydraulic circuit and operative to increase fluid pressure in the wheel cylinder in its first position, to decrease the fluid pressure in the wheel cylinder in its second position and to hold the fluid pressure in the wheel cylinder constant in its third position, a wheel speed sensor means for detecting the rotational speed of the vehicle wheel and producing sensor signal pulses separated by intervals representative of the detected wheel speed, a timer means 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. 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 an elevation of a wheel speed sensor adapted to detect the speed of a front wheel; Fig. 8 is an 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 is a timing chart for the anti-skid control system; Fig. 11 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. 12 is a flowchart of a main program of a microprocessor constituting the controller unit of Fig. 11; Fig. 13 is a flowchart of an interrupt program executed by the controller unit; Fig. 14 is a flowchart of a main routine in the main program of Fig. 12; Fig. 15 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; Fig. 16 is a table determining the operation mode of the actuator 16, which table is accessed in terms of the wheel acceleration and the slip rate; Fig. 17 shows variation of derived wheel speed; Fig. 18 shows variation of derived wheel acceleration when MODE 4 of the sample mode is selected; Fig. 19 is a timing chart of calculation of the wheel speed and wheel acceleration performed by the anti-skid control system; Fig. 20 shows a Lissajous figure illustrated in terms of the wheel acceleration and the slip rate in a normal skid control cycle; Fig. 21 is a similar graph to Fig. 20 but showing deformation of the Lissajous' figure due to delay of calculation of the wheel acceleration and deceleration; Fig. 22 shows the variation of wheel speed and wheel acceleration as anti-skid brake control is performed, in which the solid line shows a normal state and the broken line shows a state in which one scheduled operation mode is skipped; Fig. 23 is a flowchart of a control or operation mode determining routine in the output calculation program of Fig. 15; Fig. 24 is a block diagram of another embodiment of the controller unit in the preferred embodiment of the anti-skid brake control system according to the present invention; Fig. 25 is a block diagram of an output unit in the controller unit of Fig. 24; Fig. 26 is a block diagram of a modification of the output unit of Fig. 25; and Fig. 27 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. DESCRIPTION OF THE PREFERRED EMBODIMENTS 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. 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. 10 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 speedV 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 t₀ 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 Vv. 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 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 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 s 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: Vi' = 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. 11, 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. 12), an interrupt program (Fig. 14), an sample control program, a timer overflow program and an output calculation program (Fig. 15) 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. 12 to 27. Fig. 12 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. 12. If FL=1, then the control process is according to plan, and control passes to a main routine explained later with reference to Fig. 14. 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. 13 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=O 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 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. Referring to Fig. 14, 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 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=l, 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. 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). 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. 15 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. 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 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: 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. 16 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. 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. The output program and the main program are executed on a timing-sharing basis. As shown in Fig. 17, the output program and the main program are executed within a fixed period of time, e.g. every 10 ms. Therefore, the wheel speed V w can be derived every 10 ms, as shown in Fig. 17, during execution of the output program. During execution of the output program, the slip rate R can be derived every 10 ms. On the other hand, as set forth with respect to the main routine of Fig. 14 and illustrated in Fig. 18, the wheel acceleration a w is not always derived every 10 ms. Specifically, the wheel acceleration a w is calculated only after the difference between the pulse intervals of the signals or grouped sensor pulses becomes larger than a given period of time. For example, when sample mode MODE 4 is selected, the acceleration and deceleration of the wheel will be derived every eight cycles of main program execution, as shown in Fig. 19. As set forth with respect to Fig. 16, the operational mode of the actuator 16 is determined relative to the wheel acceleration and deceleration a w and the slip rate R, at the block 5010 of the output program. As this block for determining the operational mode of the actuator is part of the output program, the operational mode may change every 10 ms. Since the wheel speed V w and the slip rate R is derived as part of the main program, a fresh slip rate value is obtained each time the block 5010 is executed. However, with regard to the wheel acceleration data, new values are not obtained as frequently and old data must necessarily be used until wheel acceleration is newly calculated. As the operational mode is determined in terms of the wheel acceleration and deceleration a w and the slip rate R, it is possible that one or more operation modes which constitute part of each cycle of anti-skid control may be skipped due to this delay in the calculation of fresh wheel acceleration values. For instance, in the normal state in which the calculation of the wheel acceleration data is not so significantly delayed as to affect selection of the operational mode, the Lissajous figure relating the wheel acceleration and the slip rate is almost a perfect circle, as shown in Fig. 20. On the other hand, when the delay is significant, the corresponding Lissajous figure may become sufficiently oblate to skip one of the modes, as shown in Fig. 21. In Figs. 20 and 21, a₁ and b₁ represent the hold mode, a₀ represents application mode and b₂ represents release mode. Fig. 22 charts selection of the operational mode relative to variations in the wheel speed and the wheel acceleration. Fig. 22(c) corresponds to the normal cycle of skid control illustrated in Fig. 20 and Fig. 22(d) corresponds to a cycle in which one or more operational modes are skipped. In the example shown in Fig. 22, b₁ (holding mode) is skipped so that application mode a₀ follows release mode b₂ directly. In this case, since the brake system enters application mode immediately after the wheel speed V w reaches the optimal wheel speed (shown in dashed-dotted lines in Fig. 22a) derived from an assumed optimal slip rate and assumed optimal vehicle speed during braking, the wheel speed will very quickly drop agin below the optimal leve, triggering a subsequent hold mode which only serves to drive the wheel speed lower. Accordingly, the slip rate R increases beyond an allowable limit, which may result in a skid despite the anti-skid control operation. In addition, it can be expected that due to the increase in the slip rate beyond the allowable limit, the braking distance will be dangerously prolonged. Therefore, in order to prevent the anti-skid control system from skipping one or more operational modes at the block 5010 of the output program, the control mode select routine of the block 5010 includes steps for checking the sequence of operational modes and correcting the determined mode so that all of the necessary operational modes are performed in order in each skid cycle. Fig. 23 is a flowchart of one example of the control mode select routine of the block 5010 of Fig. 15. This example is designed to prevent the circumstances shown in Fig. 22(d), i.e. release mode being followed directly by application mode, this case being particularly liable to result in noticeable and possibly dangerous malfunction of the brake system. It is, of course, also possible for the cntrol mode select routine to check all possible mode changes, or only selected other modes. In the control mode select routine, the wheel acceleration a w and the slip rate R are read out at a first block 5010-1. The table is referred to according to these values at a block 5010-2 in order to determine the operational mode of the actuator 16. In subsequent block 5010-3, a release mode-indicative flag FLb₂ is checked to see if it is set, indicating that the system is currently in release mode. If the flag FLb₂ is not set, then control passes to a block 5010-5, described below. Therefore, in this case, the operational mode of the actuator 16 is output as derived from the look-up table without change. On the other hand, when the flag FLb₂ is set when checked at th block 5010-3, then the determined operational mode is checked to see if it is the application mode, at a block 5010-4. When the determined operational mode is not the application mode as checked at the block 5010-4, then the determined operational mode is again checked to see if it is the release mode at a block 5010-5. If the release mode is detected at the block 5010-5, then control returns to the output program after the flag FLb₂ is set in block 5010-6. Thus, the determined release mode is actuated as dictated by the look-up table. On the other hand, if the determined operational mode is not the release mode when checked at the block 5010-5, then the flag FLb₂ is reset at a block 5010-7. Thereafter control returns to the output program. When the determined operational mode when checked at the block 5010-6 is the application mode (a₀), then the operational mode is changed to the holding mode (a₁) at a block 5010-8. Thereafter, the flag FLb₂ is reset at the block 5010-7. Therefore, the operational mode becomes holding mode. As set forth above, according to the present invention, the holding mode is always carried out subsequent to the release mode and thus skipping of the holding mode between therelease mode and the application mode is satisfactorily avoided. However, the shown embodiment uses a flag to identify the preceding operational mode, this may be done in various ways. In addition, although the shown embodiment prevents the control system from skipping the hold mode between the release mode and the application mode, it is possible as stated previously to provide a more comprehensive program for avoiding skipping of any mode in each cycle of skid control operatin. Fig. 24 shows another embodiment of the controller unit 202 in the preferred embodiment of the anti-skid control system according to the present invention. In practice, the circuit shown in Fig. 24 performs the same procedure in controlling the actuator 16 and each block of the circuit performs operation substantially corresponding to that performed by the foregoing computer flowchart. In Fig. 24, the wheel speed sensor 10 is connected to a shaping circuit 260 provided in the controller unit 202. The shaping circuit 260 produces the rectangular sensor pulses having a pulse interval inversely proportional to the wheel speed V w . The sensor pulse output from the shaping circuit 260 is fed to a pulse pre-scaler 262 which counts the sensor pulses to produce a sample command for sampling input timing when the counter value reaches a predetermined value. The predetermined value to be compared with the counter value in the pulse pre-scaler 262 is determined such that the intervals between the pairs of three successive sample commands will be sufficiently different to allow calculation of the wheel acceleration and deceleration rate. The sample command is fed to a flag generator 264. The flag generator 264 is responsive to the sample command to produce a flag signal. The flag signal of the flag generator 264 is fed to a flag counter 266 which is adapted to count the flag signals and output a counter signal having a value representative of its counter value. At the same time, the sample command of the pulse pre-scaler 262 is fed to a latching circuit 268 which is adapted to latch the signal value of a clock counter signal from a clock counter 267 counting the clock pulse output by a clock generator 11. The latched value of the clock counter signal is representative of the input timing of the sensor pulse which activates the pulse pre-scaler 262 to produce the sample command. The latching circuit 268 sends the input timing indicative signal having a value corresponding to the latched clock counter signal value, to a memory controller 274. The memory controller 274 is responsive to a memory command input from an interrupt processing circuit 272 which in turn is responsive to the flag counter signal to issue a memory command which activates the memory controller 274 to transfer the input timing indicative signal from the latching circuit 268 to a memory area 276. The memory 276 sends the stored input timing indicative signal to a sample controller 270 whenever the input timing signal value corresponding to the latched value of the latching circuit 268 is written therein. The sample controller 270 performs operations substantially corresponding to that performed in the blocks 2008, 2010, 2012, 2032 and 2034 in Fig. 15, i.e. it determines number of sensor pulses in each group to be ignored. The sample controller 270 outputs a pulse number indicative signal to the pulse pre-scaler 262, which pulse number indicative signal has a value approximating the predetermined value to be compared with the counter value in the pulse pre-scaler 262. The memory 276 also feeds the stored input timing indicative signal to a wheel acceleration and deceleration calculation circuit 278 and a pulse interval calculation circuit 280. The wheel acceleration and deceleration calculation circuit 278 first calculates a pulse interval difference between pairs of three successively sampled sensor pulses. The obtained pulse interval difference is compared with a reference value so as to distinguish if the pulse interval difference is great enough to allow calculation of the wheel acceleration and deceleration a w . If the obtained pulse interval difference is greater than the reference value, then the wheel acceleration and deceleration calculation circuit 278 performs calculation of the wheel acceleration and deceleration according to the foregoing formula (1) . If the obtained pulse interval difference is smaller than the reference value, the wheel acceleration and deceleration calculation circuit 278 shifts the operational mode thereof in a manner substantially corresponding to the procedure disclosed set forth above, so as to achieve a pulse interval difference large enough to permit the wheel acceleration and deceleration calculation. On the other hand, the pulse interval calculation circuit 280 performs calculations to obtain the pulse interval between the current pulse and the immediate preceding pulse and sends a pulse interval indicative signal to a memory 282. The memory 282 sends a stored pulse interval indicative signal to a wheel speed calculation circuit 284 which is associated with a 10 ms timer 292. The 10 ms timer 292 produces a timer signal every 10 ms to activate the wheel speed calculation circuit 284. The wheel speed calculation circuit 284 is responsive to the timer signal to perform calculation of the wheel speed V w by calculating the reciprocal value of the pulse interval indicative signal from the memory 282. The wheel speed calculation circuit 284 thus produces a wheel speed indicative signal to be fed to a target wheel speed calculation circuit 288 and to a slip rate calculation circuit 290 which is also associated with the 10 ms timer to be activated by the timer signal every 10 ms. The target wheel speed calculation circuit 288 is adapted to detect the wheel speed V w at which the wheel acceleration and deceleration a w calculated by the wheel acceleration and deceleration calculating circuit 278 exceeds than a predetermined deceleration rate -b. The target wheel speed calculation circuit 288 measures the interval between times at which the wheel deceleration exceeds the predetermined deceleration value. Based on the wheel speed at the foregoing times and the measured period of time, the target wheel speed calculation circuit 288 derives a decelerating ratio of the wheel speed to produce a target wheel speed indicative signal. The target wheel indicative signal of the target wheel speed calculation circuit 288 and the wheel speed indicative signal from the wheel speed calculation circuit 284 are fed to a slip rate calculation cicuit 290. The slip rate calculation circuit 290 is also responsive to the timer signal from the 10 ms timer 292 to perform calculation of the slip rate R based on the wheel speed indicative signal from the wheel speed calculation circuit 284 and the target wheel speed calculation circuit 288, in accordance with the formula (2). The slip rate calculation circuit 290 and the wheel acceleration and deceleration calculation circuit 278 are connected to an output unit 294 to feed the acceleration and deceleration indicative signal and the slip rate control signal thereto. The output unit 294 determines the operation mode of the actuator 16 based on the wheel acceleration and deceleration indicative signal value and the slip rate indicative signal value according to the table of Fig. 16. The output unit 294 thus produces the inlet and outlet signals EV and AV with a combination of signal levels corresponding to the selected operation mode of the actuator. On the other hand, the wheel speed calculation circuit 284 is also connected to the flag counter 266 to feed a decrementing signal whenever the calculation of the wheel speed is completed and so decrement the counter value of the flag counter by 1. The flag counter 266 is also connected to a comparator 295 which is adapted to compare the counter value of the flag counter with a reference value, e.g. 2. When the counter value of the flag counter 266 is greater than or equal to the reference value, the comparator 295 outputs a comparator signal to an overflow detector 296. The overflow detector 296 is responsive to the comparator signal to feed a sample mode shifting command to be fed to the pulse pre-scaler 262 to shift the sample mode to increase the number of the sensor pulses in each sample group. On the other hand, the clock counter 267 is connected to an overflow flag generator 297 which detects when the counter value reaches the full count of the clock counter to produce an overflow flag signal. The overflow flag signal of the overflow flag generator 297 is fed to an overflow flag counter 298 which is adapted to count the overflow flag signals and send an overflow counter value indicative signal to a judgment circuit 299. The judgment circuit 299 compares the overflow counter indicative signal value with a reference value e.g. 2. The judgment circuit 299 produces a reset signal when the overflow counter indicative signal value is equal to or greater than the reference value. The reset signal resets the wheel acceleration and deceleration calculation circuit 278 and the wheel speed calculation circuit 284 to zero. On the other hand, the overflow flag counter is connected to the wheel speed calculation circuit 284 and is responsive to the decrementing signal output from the wheel speed calculation circuit as set forth above to be reset in response to the decrementing signal. Fig. 25 shows the output unit 294 of the controller unit 202 of Fig. 24. The output unit 294 includes a mode selector circuit 294-1 including a memory 294-2 storing the table of the control modes accessible in terms of the wheel acceleration and the slip rate, and an error signal generator 294-3 associated with the mode selector circuit 294-1 for monitoring the control mode determined by the mode selector circuit. In order to determine the control mode, the mode selector circuit 294-1 is connected to the slip rate calculation circuit 290 to receive a signal indicative of slip rate. Also, the mode selector circuit 294-1 is connected to the wheel acceleration calculation circuit 278 to receive the wheel acceleration indicative signal. Based on the slip rate indicative signal value and the wheel acceleration indicative signal values, the mode selector circuit 294-1 derives a control signal indicative of the control mode determined by table loop-up. The control signal derived by the mode selector 294-1 compreises the inlet signal EV and the outlet signal AV. The combination of signal levels of the inlet signal EV and the outlet signal AV vary according to the selected operation mode of the actuator, which combinations are summarized in the following table: Therefore, the mode selector circuit 294-1 has an inlet signal output terminal 294-4 which will hereafter be referred to as "EV terminal", and an outlet signal output terminal 294-5 which hereafter be referred to as "AV terminal". The AV termina 294-5 is connected to one input terminal of an AND gate 294-6. The other input terminal of the AND gate 294-6 is connected to the error signal generator 294-3. The error signal generator 294-3 is designed to detect when either the application mode or the release mode is selected by the mode selector circuit 294-1. The error signal generator 294-3 normally sends a HIGH-level output to the AND gate 294-6. In this case when the outlet signal AV is HIGH, the AND gate output goes HIGH, and when the outlet signal AV is low, the output of the AND gate 294-6 goes LOW. The error signal generator 294-3 detects when the application mode is selected immediately after the releasing mode and in such cases, switches its output level to LOW. In this case, the otput level at the AV terminal 294-5 is HIGH as the application mode is selected. Due to the LOW-level input from the error signal generator 294-3, the output level of the AND gate 294-6 goes LOW. Therefore, the hold mode is performed. Thereafter, the erro signal generator 294-3 returns to normal state, sending a HIGH-level signal to the AND gate 294-6. Fig. 26 shows a modification of the foregoing embodiment of Fig. 25. In this modification, in the table in which the operational modes are stored for later reference in terms of the wheel acceleration a w and the slip rate R, the operational modes are represented in the form of 3-bit binary numbers, as shown in Fig. 27. The mode selector circuit 294-1' therefore includes a digital memory 294-2' storing the operational mode-indicative binary numbers. The mode selector circuit 294-1' is also provided with another memory 294-11 identifying which mode was selected in the previous cycle, and a calculation circuit 294-12 which compares the binary number stored in the memory 294-11 with the binary number selected from memory 294-2'. The binary number indicative of the previous operation mode will be hereafter referred to as "old mode data N n-1 " and the binary number indicative of the current operational mode will be referred hereafter as "new mode data N n ". The calculation circuit 294-12 produces a signal indicative of the difference between the old operational mode and the new operation mode, i.e.: dN = |N n - N n-1 | The output signal of the calculation circuit 294-12 is fed to an error signal generator 294-3'. The error signal generator 294-3' has two output terminals 294-13 and 294-14 respectively connected to an AND gate 294-15 and an OR gate 294-16. The AND gate 294-15 is also connected to the EV terminal 294-4'. The OR gate 294-16 is also connected to the AV terminal 294-5 of the mode selector circuit 294-1'. In general, the error signal generator 294-3' is adapted to detect abnormal switching of the operational mode which would otherwise skip one of the application mode and release mode. For instance, the error signal generator 294-3' detects switching of operational mode from holde mode (001) to the holde mode (011), skipping the application mode (000). In the normal state, the error signal generator feeds a HIGH-level output to the AND gate 294-15 and a LOW-level output to the OR gate 294-16. Upon detecting skipping of either the application mode or the release mode, the error signal generator adjusts its combination of outputs according to the following pattern: Therefore, in case where the application mode would be skipped by selecting the holde mode (001) immediately after the holde mode (011), the output of the error signal generator 294-3' to the OR gate 294-16 goes HIGH. At this time, although the AV output in the hold mode is LOW, the OR gate output goes HIGH. As a result, the AND gate output, i.e. the EV signal and the OR gate output, i.e. the AV signal both go HIGH, actuating the application mode. On the other hand, when the release mode is to be skipped due to change of the operational mode from holde mode (011) to the holde mode (001), the output of the error signal generator 294-3' to the AND gate 294-15 goes LOW. Although the EV output from the EV terminal 294-4' is HIGH in the hold mode, the output of the AND gate goes LOW. Therefore, both of the outputs of the AND gate 294-15 and the OR gate 294-16 i.e. the EV and AV signals go LOW to perform the fluid control operation under the release mode. As can be appreciated from the table of Fig. 27, when the operational modes are selected in the normal state, the difference between the selected mode-indicative signals will be 001 or 011 and on the other hand, when the mode selection is abnormal in that it would skip either the application mode or the release mode, the difference would be 010. Therefore, by detecting when the difference is 010, the error is detected. In addition, by checking the binary number of either the old or the new operational mode, which of the application mode or the release mode should be performed can be detected. Therefore, according to the invention, skipping of operational modes can be effectively prevented by modifying the selected operational mode to the mode skipped. This fulfills and satisfies all of the object and advantages sought for the invention.

Claims: An anti-skid brake control system for an automotive hydraulic brake system comprising: a hydraulic brake circuit including a wheel cylinder (30a, 34a, 38a) for applying braking force to a vehicle wheel; a pressure control valve (16a, 18a, 20a) disposed within said hydraulic circuit and operative to increase fluid pressure in said wheel cylinder (30a, 34a, 38a) in its first position, to decrease the fluid pressure in said wheel cylinder (30a, 34a, 38a) in its second position and to hold the fluid pressure in third position; a wheel speed sensor means (10, 12, 14) for detecting the rotational speed (V w ) of said vehicle wheel and producing sensor signal pulses separated by intervals representative of the detected wheel speed (v w ); a timer means (11) for producing a timer signal having a value indicative of elapsed time; a first menas (2040;278) for detecting wheel acceleration (a w ) and producing a first signal indicative of the detected wheel acceleration (a w ); a second means (5008;290) for detecting the slip rate (R) of the vehicle wheel with respect to the ground and producing a second signal indicative of the detected slip rate (R); third means (5010;294) for producing a control signal (EV,AV) which actuates said pressure control valve (16a, 18a, 20a) to one of said first, second and third positions so as to adjust the wheel rotational speed (v w ) toward an optimum relationship with vehicle speed, wherein the improvement CHARACTERIZED BY said third means (5010;294) selecting said control signal (EV,AV) according to predetermined relationship between first and second signal values and according to a predetermined sequence of actuation of said first, second and third positions in a predetermined timing; and by a fourth means (5010-1 to 5010-8;294-3;294-3') associated with said third means (5010;294) for checking the control signal (EV,AV) and when said control signal (EV,AV) is not in accordance with said predetermined sequence, modifying said control signal (EV,AV) so as to accord with said predetermined sequence. The anti-skid brake control system as set forth in claim 1, wherein said third means (5010;294) selects one of first, second, third and fourth modes (a₀,b₁,b₂,a₁> according to said fixed order in the predetermined pattern, in said first mode (a₀) said pressure control valve (16a,18a,20a) is operated to said first position, in said second mode (b₁), said pressure control valve (16a,18a,20a) is operated to said third position, in said third mode (b₂) said pressure control valve (16a,18a,20a) is operated to said second position and in said fourth mode (a₁ ), said pressure control valve (16a,18a,20a) is operated to said third position. The anti-skid brake control system as set forth in claim 2, wherein said fourth means (5010-1 to 5010-8;294-3;294-3') is adapted to detect one of said first and third mode (a₀,b₂) irregularly selected by skipping one of said second and fourth mode (b₁,a₁) so as to modify the control signal (EV,AV) to represent one of said second and fourth mode (b₁,a₁) skipped. The anti-skid brake control system as set forth in claim 2, wherein said fourth means (5010-1 to 5010-8;294-3;294-3') is adapted to detect one of said second and fourth modes (b₁ ,a₁) irregularly selected by skipping one of said first and third modes (a₀,b₂) as to modify the control signal (EV,AV) to represent one of said first and third modes (a₀,b₂) which is otherwise skipped. The anti-skid brake control system as set forth in one of claims 2 to 4 wherein said third means (5010;294) deriving one of said control modes (a₀,b₁,b₂,a₁) by means of a table look-up in terms of said first and second signal values. The anti-skid brake control system as set forth in one of claims 2, 1 to 5, which further comprises a fifth means (2000;262) for sampling said timer signal value to gain a measure of the sensor signal pulse input timing (t). The anti-skid brake control system as set forth in claim 6, wherein said fifth means (2000;262) is adapted to sample said sensor signal pulse input timing (t) every given number of pulses grouped, said number of pulses in each group being adjusted so that a difference of the durations of each group is large enough to calculate said wheel acceleration and deceleration (a w ). The anti-skid brake control system as set forth in claim 7, wherein said third means (294) derives said wheel acceleration and deceleration value (a w ) from the difference between the durations of each three successive groups of sensor signal pulses. The anti-skid brake control system as set forth in claim 8, wherein said slip rate (R) is derived based on a wheel speed (v w ) derived from the interval between successive sensor signal pulses and an assumed vehicle speed assumed based on a wheel speed (v x ) at starting of said first mode (a₀). The anti-skid brake control system as set forth in one of claims 2 to 9, wherein said third means (5010;294) selects one of said control modes (a₀,b₁,b₂,a₁) by means of table look-up, which table stores binary numbers respectively representative of one of said control modes (a₀,b₁,b₂,a₁). The anti-skid brake control system as set forth in claim 10, wherein said binary numbers are given to respective control modes (a₀,b₁ ,b₂,a₁) so that a difference of the previously selected binary number and the currently selected binary number becomes a specific value when the currently selected control mode (a₀;b₁;b₂,a₁) is irregular in view of said fixed order in said predetermined pattern. A method for anti-skid controlling an automotive brake system including a hydraulic brake circuit with a wheel cylinder and a pressure control valve associated with said wheel cylinder for controlling fluid pressure to be applied to the wheel cylinder, said pressure control valve operative to increase the fluid pressure in said wheel cylinder in its first position, to decrease the fluid pressure in said wheel cylinder in its second position and to hold the fluid pressure at a constant value in its third position, the method comprising the step of: detecting a braking condition including a wheel acceleration and deceleration to produce a braking condition indicative signal, CHARACTERIZED BY the steps of: selecting one of a plurality of control mode each corresponding to one of said first, second and third valve positions according to a predetermined operation pattern and in a fixed order based on said braking condition indicative signal, to produce a control signal representative of selected control mode for operating said pressure control valve to the corresponding valve position; and detecting the selected control mode and modifying the control signal when the detected control mode is irregular with respect to said predetermined operation pattern and to said fixed order, so that the modified control signal is representative of the control mode which is otherwise skipped. The method as set forth in claim 12, which further comprising a step of storing binary numbers respectively representative of said control modes, one of said binary number being identified by said first signal value. The method as set forth in claim 12 or 13 in which said wheel acceleration and deceleration as a first braking condition indicative factor and a slip rate as a second braking condition indicative factor. The method as set forth in one of claims 12 to 14, wherein in selecting one of the control modes, one of first, second, third and fourth modes is selected according to the predetermined pattern and the fixed order depending upon said first signal value, in said first mode, said pressure control valve being operated to said first position, in said second mode, said pressure control valve being operated to said third position, in said third mode, said pressure control valve being operated to said second position and in said fourth mode, said pressure control valve being operated at said third position. The method as set forth in one of claims 12 to 15 wherein, at said step for detecting control mode, one of said second and fourth modes irregularly selected by skipping one of said first and third modes is detected so as to modify the control signal to represent one of said first and third modes which is otherwise skipped. The method as set forth in one of claims 12 to 15 wherein, at said step of detecting control mode, one of said first and third modes irregularly selected by skipping one of said second and fourth modes is detected so as to modify the control signal to represent one of said second and fourth modes which is otherwise skipped.

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