Anti-Skid Brake Control System for Automotive Brake System With Extrapolation of Wheel Acceleration

The present invention relates generally to an anti-skid brake control system for an automotive hydraulic brake system. More particularly, the invention relates to a technique for measuring wheel acceleration including interporation over the interval required for deriving a wheel acceleration value on the basis of wheel speed variations.

As is well known, optimum braking characteristics are obtained when braking pressure or force can be so adjusted that the peripheral speed of the wheels during braking is held to a given ratio, e.g. about 80% to 85%, of the vehicle speed. This practice is believed to be particularly effective when road conditions and other factors are taken into consideration. On the other hand, if wheel speed/vehicle speed ratio is maintained higher than the above-mentioned optimal ratio, e.g., 80% to 85%, braking distance may be prolong due to a lack of braking pressure. On the other hand, if the braking pressure is so adjusted as to maintain the wheel speed/vehicle speed to a ratio less than the aforementioned optimal ratio, the vehicle wheels may lock and skid, resulting in an unnecessarily long braking distance due to reduced traction. In practice, it is very difficult to precisely adjust the braking pressure so that the wheel speed is held to the given optimal ratio to the vehicle speed.

In the practical anti-skid brake control operation, braking pressure in one or more wheel cylinders is adjusted by cyclically increasing and decreasing the braking pressure in the wheel cylinder. The anti-skid control system generally decreases braking pressure when the wheel deceleration value becomes less than a given deceleration threshold, which is so chosen as to prevent the wheel from skidding, and increases braking pressure when the wheel acceleration value is greater than a given acceleration threshold. In this conventional anti-skid brake control procedure, wheel speed does not remain at an optimal relationship to the vehicle speed for a satisfactorily long period of time.

U.S. Pat. No. 3,637,264, issued on Jan. 25, 1972 to Leiber et al discloses an Antilocking Control for Pressure Actuated Brakes. The pressure of the brake-actuating fluid in an antilocking brake control system is varied by pulsing the control valve or valves for durations which are varied to be greater or lesser than the period of that limiting frequency above which the brake system cannot respond. In the former case, a rapid increase in fluid pressure or a rapid decrease in fluid pressure occurs, whereas in the latter case, a less rapid average or net increase or decrease occurs in the fluid pressure to which the brake system responds. These conditions are controlled in dependence on the rotational behavior of the vehicle wheel or wheels and more especially in dependence or predetermined changes in angular velocity of the wheel. Moreover, either variation in pulse duration at a fixed frequency or variation in frequency at a fixed pulse duration may be effected during high-frequency pulsing so as further to alter the net increase or decrease in fluid pressure. This further alternation is effected as a function of time from the beginning of the high-frequency pulsing.

In addition, Published Japanese Patent Application (Tokkai) Showa 51-89096, published on Aug. 4, 1976 discloses a system similar to the above. The fluid pressure in the wheel cylinder is increased in a stepwise manner. Duration of the increase of the fluid pressure is adjusted in accordance with the rate of increase of the fluid pressure in one or more preceding steps.

European Patent First Publications Nos. 01 23 286, 01 23 287, 01 24 035, 01 23 283, 01 23 281, 01 23 285, 01 23 280, 01 23 279, 01 23 282, 01 26 377 and 01 25 613 and German Patent First Publications Nos. 34 17 587, 34 18 217, 34 17 389, 34 17 144 and 34 18 235 disclose anti-skid brake control systems including derivation of wheel speed and wheel acceleration. In the disclosed systems, the anti-skid control system holds braking pressure at a constant level after wheel acceleration drops below a predetermined threshold.

In the recent anti-skid brake control systems disclosed in the publications listed above, an attempt has been made to maintain the braking pressure near a lock pressure at which the wheels tend to lock. A typical attempt at maintaining the braking pressure near the lock pressure has been disclosed in the European Patent First Publication No. 01 34 512. In this approach, the possibility of skidding is increased since the braking pressure can easily exceed the lock pressure briefly. In order to prevent the wheels from locking, brake pressure must be controlled more precisely. For more precise control, it is essential to detect when wheel acceleration drops below the deceleration threshold with greater accuracy.

However, in the anti-skid control systems proposed in the aforementioned publications, wheel acceleration derivation routines require much more time than in wheel speed derivation. This may cause delays in detection of when the wheel acceleration drops below the deceleration threshold which may cause the wheels to lock.

Therefore, it is a principle object of the present invention to minimize the time required for deriving wheel acceleraion by deriving interporating values of wheel acceleration during the derivation intervals.

Another object of the present invention is to provide an anti-skid brake control system with interporating technique for deriving wheel acceleration interporating value according to braking condition.

In order to accomplish the aforementiond and other objects, an anti-skid brake control system according to the invention sequentially or cyclically performs interporating operation for increasing and decreasing the wheel acceleration value depending upon increasing and decreasing of the braking pressure.

In practical operation, wheel acceleration is incremented by a given value as the braking pressure decreased and is decremented by a given value as the braking pressure increases.

In accordance with one aspect of the invention, an anti-skid brake control system for an automotive brake system comprises a hydraulic brake system including a pressure control valve for adjusing braking pressure on a vehicle wheel, the pressure control valve performing braking pressure adjustment through at least one skid cycle which includes a first mode for increasing braking pressure and a second mode for decreasing braking pressure, a wheel speed sensor monitoring rotation speed of the vehicle wheel and producing a sensor signal, a controller for controlling the pressure control valve so as to prevent the wheel from skidding, the controller periodically calculating wheel acceleration over a given interval on the basis of the sensor signal, and extrapolating changes in the wheel acceleration value in accordance with a predetermined extrapolation coefficient which varies with the operation mode of the pressure control valve, the controller further deriving a wheel slippage value on the basis of the sensor signal, and a control signal for ordering the pressure control valve to an operation mode selected according to the wheel acceleration value and the wheel slippage value.

In accordance with another aspect of the invention, an anti-skid brake control system for an automotive brake system comprises a hydraulic brake system including a wheel cylinder for applying braking pressure to a vehicle wheel, a pressure control valve associated with the wheel cylinder for adjusting fluid pressure in the wheel cylinder, the pressure control valve being operative to increase fluid pressure in the wheel cylinder in its first mode of operation, to hold the fluid pressure in the wheel cylinder at an increased constant value in its second mode of operation, to decrease fluid pressure in the wheel cylinder in its third mode of operation and to hold the fluid pressure in the wheel cylinder at a decreased constant value in its fourth mode of operation, a wheel speed sensor detecting rotation speed of the wheel and producing a wheel speed indicative signal having a value indicative of the detected wheel speed, and a controller periodically deriving a wheel acceleration value on the basis of wheel speed indicative signal variation, comparing the wheel acceleration value with a predetermined deceleration threshold and switching the pressure control valve from the first mode to the second mode whenever the wheel acceleration drops below the deceleration threshold, the controller adjusting the wheel acceleration value with a predetermined extrapolation coefficient which varies with the operation mode of the pressure control valve.

In accordance with a further aspect of the invention, a method for anti-skid controlling an automotive brake system comprises the steps of:

producing a wheel speed indicative signal having a value indicative of the wheel speed;

deriving brake control parameters on the basis of the wheel speed indicative signal value;

periodically deriving a wheel acceleration value from the wheel speed indicative signal;

extrapolating the wheel acceleration value in accordance with a time dependent extrapolation value which varies with the operation mode of the brake system, which include a first mode in which braking pressure increases and a second mode in which braking pressure decreases;

comparing the wheel speed indicative signal value to a first reference value and decreasing braking pressure on a vehicle wheel when the wheel speed indicative signal value achieves a predetermined specific relationship with the first reference value; and

comparing the wheel speed indicative signal value to a second reference value and increasing braking pressure on the vehicle wheel when the wheel speed indicative signal value achieves a predetermined specific relationship with the second reference value.

The present invention will be understood more fully from the detailed description given below and from the accompanying drawings of the preferred embodiments of the present invention, which, however, should not be taken to limit the invention to the specific embodiments, but are for explanation and understanding only.

FIG. 1 is a schematic block diagram of the overall design of the preferred embodiment of an anti-skid brake control system according to the present invention;

FIG. 2 is a perspective view of the hydraulic circuits of the anti-skid brake system according to the present invention;

FIG. 3 is a circuit diagram of the hydraulic circuits performing the anti-skid control according to the present invention;

FIG. 4 is an illustration of the operation of an electromagnetic flow control valve employed in the hydraulic circuit, which valve has been shown in an application mode for increasing the fluid pressure in a wheel cylinder;

FIG. 5 is a view similar to FIG. 4 but of the valve in a hold mode in which the fluid pressure in the wheel cylinder is held at a substantially constant value;

FIG. 6 is a view similar to FIG. 4 but of the valve in a release mode in which the fluid pressure in the wheel cylinder is reduced;

FIG. 7 is a perspective view of a wheel speed sensor adapted to detect the speed of a front wheel;

FIG. 8 is a side elevation of a wheel speed sensor adapted to detect the speed of a rear wheel;

FIG. 9 is an explanatory illustration of the wheel speed sensors of FIGS. 7 and 8;

FIG. 10 shows the waveform of an alternating current sensor signal produced by the wheel speed sensor;

FIG. 11 is a block diagram of the first embodiment of a controller unit in the anti-skid brake control system according to the present invention;

FIG. 12 is a circuit diagram of a wheel acceleration calculating circuit in a wheel acceleration deriving circuit in the first embodiment of the controller unit of FIG. 11;

FIG. 13 is a circuit diagram of a wheel acceleration adjustment circuit in the wheel acceleration deriving circuit of FIG. 11;

FIG. 14 is a timing chart for the controller unit of FIG. 11;

FIG. 15 shows a characteristics of increase of brake pressure and a relation of the brake pressure and lock pressure;

FIG. 16 is a block diagram of a modification to the first embodiment of the controller unit in FIG. 11;

FIG. 17 is a block diagram of a projected speed deriving circuit in the controller unit of FIG. 16;

FIG. 18 is a timing chart for the control unit of FIG. 16;

FIG. 19 is a circuit diagram of the wheel acceleration adjustment circuit in the wheel acceleration deriving circuit employed in the controller unit of FIG. 16;

FIG. 20 is a block diagram of another embodiment of a controller unit in the anti-skid brake control system according to the present invention;

FIG. 21 is a flowchart of a main program of a microcomputer constituting the controller unit of FIG. 20;

FIG. 22 is a flowchart of an interrupt program executed by the controller unit;

FIG. 23 is a flowchart of a main routine in the main program of FIG. 21;

FIG. 24 is an explanatory diagram of the input timing sampling modes and variations thereof;

FIG. 25 is a flowchart of an output calculation program for deriving EV and AV signals for controlling operation mode of the electromagnetic valve according to the valve conditions of FIGS. 4, 5 and 6;

FIG. 26 is a flowchart of a wheel acceleration deriving routine in the main routine of FIG. 23;

FIGS. 27 and 28 are diagrams of execution timing of the output calculation program in relation to the main program of FIG. 21;

FIG. 29 is a table for determining the operation mode of the actuator 16, which table is accessed in terms of the wheel acceleration and the slip rate; and

FIG. 30 is a flowchart of the projected speed deriving in the output calculation program of FIG. 25;

FIG. 31 is a block diagram of the third embodiment of the controller unit in the anti-skid brake control system according to the invention;

FIG. 32 is a circuit diagram of the wheel acceleration adjustment circuit in the wheel acceleration deriving circuit in the controller unit of FIG. 31; and

FIG. 33 is a timing chart of the signals produced in the controller unit of FIG. 31.

Referring to the drawings, particularly to FIGS. 1 to 10, an anti-skid brake control system, according to the present invention, includes independently operative three anti-skid control circuits 402, 404 and 406 respectively controlling front-left (FL) wheel, front-right (FR) wheel and rear (R) wheels. The anti-skid control circuit 402, 404 and 406 respectively includes digital controller units 202, 204 and 206 which are housed in a common controller housing to form a control module 200.

The controller unit 202 provided in the front-left anti-skid control circuit 402, is connected to a wheel speed sensor 10 for producing an alternative current form sensor signal having a frequency proportional to the rotation speed of a front-left wheel (not shown). On the other hand, the controller unit 202 is also connected to an electromagnetic actuator 16 in a front-left brake circuit 302. The brake circuit 302 includes a front-left wheel cylinder 30a for operating a brake shoe assembly 30 for applying braking force to a brake disc rotor 28, and an electromagnetic pressure control valve 16a operated by the actuator 16 for controlling fluid pressure to be applied to the wheel cylinder 30a and whereby controlling the braking force.

Similarly, the controller unit 204 of the front-right anti-skid control circuit 404 is connected to a wheel speed sensor 204 to receive alternative current form sensor signal with a frequency representative of rotation speed of the front-right wheel. The controller unit 204 is, in turn, connected to an actuator 18 in a front-right brake circuit 304. The actuator 18 is adapted to operate an electromagnetic pressure control valve 18a for controlling hydraulic pressure to be applied to a front-right wheel cylinder 34a. With the controlled hydraulic pressure, the wheel cylinder 34a operates a front-right brake shoe assembly 34 for applying braking force to a brake disc rotor 32 rotating with the front-right wheel.

In addition, the controller unit 206 is connected to a wheel speed sensor 14 to receive therefrom an alternative current sensor signal having a frequency indicative of the average rotation speed of rear wheels. In order to detect average rotation speed of the rear wheels, the wheel speed sensor 14 may be adapted to detect rotation speed of propeller shaft or the equivalent rotating at the approximately average speed of the rear wheels. The controller unit 206 is also connected to an electromagnetic actuator 20 of an electromagnetic pressure control valve 20a. The electromagnetic valve 20a is associated with rear wheel cylinders 38a for controlling fluid pressure to be applied to the rear wheel cylinders and whereby controlling braking pressure to be applied to rear brake disc rotors 36 through rear brake shoe assemblies 38a.

It should be appreciated that through the controller units 202, 204 and 206 are adapted to control respectively the front-left, front-right and rear brake circuits 302, 304 and 306, since the embodiment shown is directed to an anti-skid brake control system for a vehicle having a driving arrangement of a front-engine, rear wheel drive vehicle, the invention can be modified to apply any driving arrangement of vehicle, such as front-engine, front wheel drive or four wheel drive arrangements. In addition, though the disclosed brake system comprises disc brakes, the anti-skid brake control system according to the invention can also be applied to drum-type brake system.

The controller units 202, 204 and 206 are respectively associated with actuator drive circuits 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 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 signal is converted by the corresponding shaping circuit 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.sub.w and instantaneous wheel acceleration or deceleration a.sub.w. From these measured and derived values, a target wheel speed V.sub.i is derived, which is an assumed value derived from the wheel speed at which a slip is assumed to be zero or approximately zero. The target wheel speed V.sub.i varies at a constant decelerating rate derived from the variation of the wheel speed. The target wheel speed thus corresponds to a vehicle speed which itself is based on variations of the wheel speed. Based on the difference between the instantaneous wheel speed V.sub.w and the target wheel speed V.sub.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 the 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 (see FIG. 8). 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 embodiment shown. 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 lines 46 and 48. 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 a primary pressure line 50. 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 passage 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 check 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 check 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 check valves 85, 86 and 84 are adapted to permit fluid flow from the braking pressure line 46, 48 and 50 to the pressure lines 42 and 44 when the master cylinder 24 is released and thus the line pressure in the pressure lines 42 and 44 becomes less than the pressure in the braking pressure lines 46, 48 and 50.

The electromagnetic flow control valves 16a, 18a and 20a are respectively associated with the actuators 16, 18 and 20 to be controlled by means of the control signals from the control module 200. The actuators 16, 18 and 20 are all connected to the control module 200 via an actuator relay 94, which thus controls the energization and deenergization of them all. Operation of the electromagnetic valve 16a in cooperation with the actuator 16 will be illustrated with reference to FIGS. 4, 5 and 6, in particular illustrating the 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 omitted in order to avoid unnecessary repetition and for simplification of the disclosure.

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/cm.sup.2 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/cm.sup.2. 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.

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.

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 deenergized. 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 block 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 a unit angle of wheel rotation. The sensor assembly 102 comprises a magnetic core 124 aligned with its north pole (N) near the sensor rotor 104 and its south pole (S) distal from the sensor rotor. A metal element 125 with a smaller diameter section 125a is attached to the end of the magnetic core 124 nearer the sensor rotor. The free end of the metal element 125 faces the sensor teeth 120. An electromagnetic coil 126 encircles the smaller diameter section 125a of the metal element. The electromagnetic coil 126 is adapted to detect variations in the magnetic field generated by the magnetic core 124 to produce an alternating-current sensor signal as shown in FIG. 10. That is, the metal element and the magnetic core 124 form a kind of proximity switch which adjusts the magnitude of the magnetic field depending upon the distance between the free end of the metal element 125 and the sensor rotor surface. Thus, the intensity of the magnetic field fluctuates in relation to the passage of the sensor teeth 120 and accordingly in relation to the angular velocity of the wheel.

It should be appreciated that the wheel speed sensor 12 for the right front wheel has the substantially the same structure as the set forth above. Therefore, explanation of the structure of the right wheel speed sensor 12 will be omitted in order to avoid unnecessary repetition in the disclosure and in order to simplify the description.

FIG. 8 shows the structure of the rear wheel speed sensor 14. As with the aforementioned left front wheel speed sensor 10, the rear wheel speed sensor 14 comprises a sensor rotor 112 and a sensor assembly 102. The sensor rotor 112 is associated with a companion flange 114 which is, in turn, rigidly secured to a drive shaft 116 for rotation therewith. Thus, the sensor rotor 112 rotates with the drive shaft 116. The sensor assembly 102 is fixed to a final drive housing or a differential gear box (not shown).

Each of the sensor assemblies applied to the left and right front wheel speed sensors and the rear wheel sensor is adapted to output an alternating-current sensor signal having a frequency proportional to or corresponding to the rotational speed of the corresponding vehicle wheel. The electromagnetic coil 126 of each of the sensor assemblies 102 is connected to the control module 200 to supply the sensor signals thereto.

As set forth above, the control module 200 comprises the controller unit (FL) 202, the controller unit (FR) 204 and the controller unit (R) 206, each of which comprises a microcomputer. Therefore, the wheel speed sensors 10, 12 and 14 are connected to corresponding controller units 202, 204 and 206 and send their sensor signals thereto. Since the structure and operation of each of the controller units is substantially the same as that of the others, the structure and operation of only the controller unit 202 for performing the anti-skid brake control for the front left wheel cylinder will be explained in detail.

FIG. 11 shows the first embodiment of controller unit 202 of the present invention. The controller units 204 and 206 are designed in substantially the same way as the controller unit described herebelow. Therefore, in order to simplify the disclosure, the detailed explanation of the controller units 204 and 206 will be omitted.

In FIG. 11, a wheel speed deriving circuit 210 is connected to the wheel speed sensor 10 to receive wheel speed indicative signals. The wheel speed driving circuit 210 is adapted to output a wheel speed indicative signal having a value proportional to the pulse frequency of the wheel speed sensor signal from the wheel speed sensor. A wheel acceleration deriving circuit 212 derives a wheel acceleration value a.sub.w and outputs a wheel acceleration indicative signal. The wheel acceleration indicative signal is input to the negative input terminal of a differential amplifier 24. The positive input terminal of the differential amplifier 214 is connected to a reference signal generator 216 to receive a reference signal. The reference signal value is representative of a preset deceleration value, e.g. -1 G. Therefore, as long as the wheel acceleration indicative signal value is greater than the preset deceleration value, the output level of the differential amplifier remains LOW. On the other hand, when the wheel acceleration indicative signal value becomes less than the preset deceleration value, the output level of the differential amplifier 214 goes HIGH. The output of the differential amplifier 214 is supplied to one of the three input terminals of an OR gate 218.

The wheel speed deriving circuit 210 is also connected to a projected speed deriving circuit 220. The projected speed deriving circuit 220 is also connected to the wheel acceleration deriving circuit 212 to receive the wheel acceleration indicative signal. The projected speed deriving circuit 220 is adapted to latch the wheel speed indicative signal value when wheel acceleration indicative signal value becomes equal to or greater than the preset deceleration value. The projected speed deriving circuit 220 includes memories for storing latched wheel speed indicative signal values of the current skid cycle and the immediately preceding skid cycle. In addition, the projected speed deriving circuit measures the interval between occurrences of latching of the wheel speed indicative signal values and from the measured period of time, the projected speed deriving circuit derives an average angular deceleration value. This deceleration value may used to derive a projected speed value for the next cycle of anti-skid control. For instance, each instantaneous projected speed may be derived by the following equation:

where

V.sub.c is the projected speed;

V.sub.wnew is the wheel speed at which the wheel acceleration indicative signal value equal to or less than the preset deceleration value is obtained;

dV.sub.c is the derived deceleration value; and

t is elapsed time since deviation of the value V.sub.wnew.

The projected speed V.sub.c represents an estimated vehicle speed based on the measured wheel speed. The vehicle speed can be obtained directly from the wheel speed whenever zero slip can be assumed. Therefore, in the embodiment shown, it is assumed that, when the preset deceleration value, e.g. -1 G, is obtained, wheel slip relative to the vehicle ground speed will be zero or negligible and so can ignored. The timing at which the wheel acceleration value becomes equal to or less than the preset deceleration value is thus regarded as cripping point for increasing wheel slippage relative to vehicle from zero by further decelerating operation.

In addition, it should be appreciated that, in the first cycle of anti-skid control, a fixed value, e.g. --0.4 G will be used as the deceleration value.

Procedures for deriving the projected speed can also be seen in the U.S. Pat. Nos. 4,392,202, issued July 5, 1983; 4,384,330, issued May 17, 1983; and 4,430,714 issued Feb. 7, 1984, respectively to the inventor of this invention and commonly assigned to the assignee of this invention. Disclosure of the above-identified U.S. Patents are herewith incorporated by reference for the sake of disclosure.

Returning to FIG. 11, the projected speed deriving circuit 220 is connected to a target wheel speed deriving circuit 222. The target wheel speed deriving circuit 222 is adapted to derive a target wheel speed which is optimally related to the vehicle speed. The target wheel speed means a wheel speed to which the wheel speed is to be adjusted in order to obtain optimal braking characteristics. In general, as is well known, braking characteristics are optimized when wheel slippage relative to the vehicle speed is in the range of 15% to 20%. Therefore, according to the preferred embodiment of the invention, the target wheel speed is chosen to be 85% of the projected vehicle speed. The target wheel speed deriving circuit 222 thus outputs a target wheel speed indicative signal having a value corresponding to 85% of the projected speed.

The target wheel speed deriving circuit 222 is connected to the positive input terminal of a differential amplifier 224. The negative input terminal of the differential amplifier is connected to the wheel speed deriving circuit 210. The differential amplifier 224 compares the wheel speed indicative signal value with the target wheel speed indicative signal value and outputs a HIGH-level comparator signal as long as the wheel speed indicative signal value is less than the target wheel indicative signal value. On the other hand, the output level of the differential amplifier goes LOW when the wheel speed indicative signal value becomes greater than the target wheel speed indicative signal value. The output terminal of the differential amplifier 224 is connected to an AND gate 228 to supply the comparator output thereto.

The wheel acceleration deriving circuit 212 is also connected to the positive terminal of a differential amplifier 230. The negative input terminal of the differential amplifier 230 is connected to a reference signal generator 232. The reference signal generator 232 outputs a reference signal having a value representative of a preset acceleration value, e.g. 0.6 G. The differential amplifier 230 outputs a HIGH-level signal when the wheel acceleration indicative signal value is greater than the reference signal value, and, conversely, a LOW-level signal when the wheel acceleration indicative signal value is less than the reference signal value.

The OR gate 218 is connected to the output terminals of the three differential amplifiers 214, 224 and 230. The differential amplifier 214 outputs a HIGH-level comparator signal when the wheel acceleration indicative signal value is less than the preset deceleration value. The preset deceleration value represents a pressure release threshold. Therefore, a HIGH-level output from comparator 214 indicates deceleration of the wheel beyond the pressure release threshold. The differential amplifier 230 outputs a HIGH-level comparator signal when the wheel acceleration indicative signal value is greater than the preset acceleration value. The preset acceleration value is representative of a pressure apply threshold. Therefore, a HIGH-level signal from comparator 230 indicates acceleration of the wheel beyond the pressure apply threshold. On the other hand, the differential amplifier 224 outputs a HIGH-level comparator signal when the wheel speed value is less than the target wheel speed value. Therefore, the output of the OR gate 218 is HIGH when wheel acceleration is less than the preset deceleration value or greater than the preset acceleration value, or when the wheel speed is less than the target wheel speed.

The output of the OR gate 218 is supplied to an amplifier 238 which in turn supplies an amplifier output to the inlet valve as inlet signal EV.

The differential amplifier 224 is also connected for output to an AND gate 228. In addition, the differential amplifier 230 is connected to an inverting input terminal of the AND gate 228. The differential amplifier 224 outputs a HIGH-level comparator signal when wheel speed is less than the target wheel speed, the differential amplifier 230 outputs a LOW-level comparator signal when the wheel acceleration value is smaller the preset acceleration value, and the AND gate 228 outputs a HIGH-level gate signal only when both of these conditions are satisfied. Otherwise, the output level of the AND gate 228 remains LOW. The AND gate is connected for output to an amplifier 240 which in turn sends an amplifier signal to outlet valve as outlet signal AV.

The AND gate 228 is also connected to a retriggerable timer 242. The retriggerable timer 242 is responsive to a HIGH-level output from the AND gate 228 to be triggered for a period of time longer than the maximum duration of one cycle of skid control. The retriggerable timer 242 generates a signal MR and is connected for output to the base electrode of a switching transistor 502 in a motor control circuit 500 which controls operation of the motor 88 of the fluid pump 90. The transistor 502 is rendered conductive by the HIGH-level trigger signal from the retriggerable timer 242 is energize a relay coil 504 connected to collector electrode thereof. A relay switch 506 is turned ON by energization of the relay coil 504 to close a power supply circuit for the motor 88. Therefore, the motor 88 is driven for the period of time for which the retriggerable timer 242 is triggered.

As shown in FIG. 11, in the first embodiment, the wheel acceleration deriving circuit 212 comprises a wheel acceleration calculating circuit 212a and a wheel acceleration adjustment circuit 212b.

As shown in FIG. 12, the wheel acceleration calculating section 212a comprises a differential amplifier 212a-1, an arithmetic circuit 212a-2 and a memory 212a-3. The differential amplifier 212a-1 has a positive input terminal connected to the wheel speed sensor 10 to receive the AC wheel speed indicative signal with a frequency proportional to the wheel speed. The differential amplifier 212a-1 outputs a pulse train having a frequency matching the frequency of the wheel speed sensor signal from the wheel speed sensor 10 to the arithmetic circuit 212a-2. The arithmetic circuit 212a-2 performs arithmetic operations in order to derive the wheel acceleration value a.sub.w on the basis of pulse period of the output pulse train from the differential amplifier. The resultant wheel acceleration value a.sub.w is fed to the memory 212a-3 to be temporarily stored. The arithmetic circuity 212a-2 also outputs an updating indicative signal S.sub.a every time a new wheel acceleration value a.sub.w has been derived.

At the same time, the updating indicative signal S.sub.a from the arithmetic circuit 212a-2 is supplied to the wheel acceleration adjustment section 212b. As shown in FIG. 13, the wheel acceleration adjustment circuit 212b has an AND gate 212b-1 connected for input from the arithmetic circuit 212a-2 of the wheel acceleration calculating section 212a. The AND gate has an input terminal connected to the retriggerable timer 242 to receive the output signal of the latter. The output terminal of the AND gate 212b-1 is connected to a switch 212b-2 which comprises an FET in the shown embodiment and which is closed by a LOW-level output from the AND gate. The switch 212b-2 enables the disables an integrator 212b-3 according to its switch position.

The integrator 212b-3 comprises a differential amplifier 212b-4, a resistor R and a capacitor C. The input terminal of the integrator is connected parallelly to a positive potential source 212b-5, a negative potential source 212b-6 and ground via respective switches 212b-7, 212b-8 and 212b-9. The positive potential source 212b-5 supplies a given potential +B which corresponds to a given rate of increase of the wheel acceleration value a.sub.w. Similarly, the negative potential source 212b-6 supplies a given potential -B which corresponds to a given rate of decrease of the wheel acceleration value a.sub.w.

The switch 212b-7 is opened and closed depending upon the output level of a NOR gate 212b-10. The NOR gate 212b-10 has input terminals respectively connected to the OR gate 218 and the AND gate 228. The switch 212b-7 is closed in response to a HIGH-level NOR gate output i.e. when the outputs of both the OR gate 218 and the AND gate 228 are LOW, which in turn means that the system is operating in APPLICATION mode. In this case, the switch 212b-7 transmits the potential +B from the positive potential source 212b-5 to the negative input terminal of the differential amplifier 212b-4 via the resistor R.

The switch 212b-8 is opened and closed depending upon the output level of an AND gate 212b-11. As with the NOR gate 212b-10, the AND gate 212b-11 is connected to the OR gate 218 and the AND gate 228. The AND gate 212b-11 outputs a HIGH-level signal when both of the outputs of the OR gate 218 and the AND gate 228 are HIGH, which in turn, means that the system is operating in the RELEASE mode. The switch 212b-8 closes in response to HIGH-level outputs from the AND gate 212b-11 so as to transmit the potential -B to the negative input terminal of the differential amplifier 212b-4 of the integrator 212b-3.

The switch 212b-9 is likewise connected to an AND gate 212b-12 which has an input terminal connected to the OR gate 218 and an inverting input terminal connected to the AND gate 228. Therefore, the output of the AND gate 212b-12 goes HIGH when the output of the OR gate is HIGH and the output of the AND gate 228 is LOW. The Switch 212b-9 closes in response to the HIGH-level input from the AND gate 212b-12. When closed, the switch 212b-12 connects the negative input terminal of the differential amplifier to ground. Since the positive input terminal of the differential amplifier 212b-4 is connected to ground, the input levels at the positive and negative input terminals in this case would be equal and the output of the differential amplifier would be zero.

The integrator 212b-3 integrates the input potential and outputs the integrated value to an adder 212b-13. The adder 212b-13 is also connected to the memory 212a-3 of the wheel acceleration calculating section 212a to receive the wheel acceleration value a.sub.w. The adder 212b-13 outputs an extrapolated value signal a.sub.w ' representing the sum of the integrated value and the wheel acceleration value a.sub.w.

The operation of the first embodiment of the anti-skid brake control system will be described herebelow. Upon application of the brakes, the wheels start to decelerate, i.e. the deceleration value increases. When the wheel deceleration value a.sub.w drops below the preset deceleration value b.sub.1, the output level of the differential amplifier 214 goes from LOW level to HIGH level. The HIGH level comparator signal of the differential amplifier 214 is outputted to the OR gate 218. PG,31

At this time, as the wheel acceleration value a.sub.w is less than the preset acceleration value a.sub.1, the output level of the differential amplifier 230 remains LOW. Also, as the wheel speed V.sub.w is higher than the target wheel speed V.sub.i, the output level of the differential amplifier 224 remains LOW. Therefore, the output of the OR gate 218 goes HIGH, thus transmitting a HIGH-level inlet signal EV to the inlet valve 16b through the amplifier 238. On the other hand, since the output level of the AND gate 228 remains LOW, a LOW-level outlet signal AV is transmitted to the outlet valve 16c via the amplifier 240.

In the shown embodiment, the pressure control valve 16a including the inlet valve 16b and the outlet valve 16c operates in different operation modes as set out below:

Therefore, the pressure control valve 16a is actuated to the HOLD mode in which the fluid pressure to be applied to the wheel cylinder is held at the pressure level. As a result, the wheel continues to decelerate and drops below the target wheel speed V.sub.i. As a result, the output of the differential amplifier 224 goes HIGH. Since the output level of the differential amplifier 230 still remains LOW, the input level of the AND gate 228 from the differential amplifier 230 is HIGH. Thus, the AND gate 228 is opened to transmit a HIGH-level outlet signal AV to the outlet valve 16c via the amplifier 240. At this time, the inlet signal EV remains HIGH. Therefore, the operation mode of the pressure control valve 16a changes to RELEASE mode.

In response to the rising edge of the outlet signal AV, the retriggerable timer 242 is triggered to energize the relay coil 504 to turn ON the relay switch 506 to drive the motor 88 of the pressure reduction fluid pump 90. The fluid pump 90 assists reduction of the fluid pressure in the wheel cylinder.

By actuating the pressure control valve 16a to the RELEASE mode, the wheel speed is allowed to increase again. As a result, the wheel acceleration a.sub.w gradually increases and exceeds the preset deceleration value b.sub.1. Whereupon the output level of the differential amplifier 214 goes LOW. However, in this case, since the output level of the differential amplifier 224 still remains HIGH, the output of the OR gate 224 remains HIGH. Therefore, RELEASE mode is maintained, allowing the wheel speed to increase further towards the vehicle speed. Wheel acceleration a.sub.w thus increases until it exceeds the preset acceleration value a.sub.1. As a result, the output level of the differential amplifier 230 goes HIGH. The OR gate 218 thus outputs a HIGH-level gate signal. On the other hand, as the input level of the AND gate 228 from the differential amplifier 230 goes LOW, the output level of the AND gate 228 goes LOW. Therefore, operation returns to the HOLD mode in which the fluid pressure in the wheel cylinder is held at the current pressure level. Since the fluid pressure is relatively low, wheel speed continues to increase toward the vehicle speed. The wheel speed thus exceeds the target wheel speed V.sub.i. Therefore, the output level of the differential amplifier 224 turns LOW level. At this time, since the output level of the differential amplifier 230 is HIGH, the OR gate 218 still outputs a HIGH-level gate signal.

During the aforementioned skid control operation the wheel acceleration deriving circuit 212 periodically derives the wheel acceleration value a.sub.w and output the wheel acceleration indicative signal. Between derivations of the wheel acceleration values, the system extrapolates the wheel acceleration value a.sub.w, using one of given values +B or -B depending upon the operating mode of the system.

The operation of the wheel acceleration deriving circuit will be explained with reference to FIG. 14. As long as the wheel acceleration value a.sub.w remains higher than the deceleration threshold (-b), the retriggerable timer 242 remains inoperative due to the LOW-level output of the AND gate 228. On the other hand, due to application of braking pressure, the wheel acceleration value a.sub.w derived by the wheel acceleration calculating section 212a will be negative. Therefore, the output level of the AND gate 212b-1 is held LOW to keep the switch 212b-2 closed. As a result, the integrator circuit 212b-3 is disabled and so outputs a zero-value signal. The adder 212b-13 thus outputs the wheel acceleration value a.sub.w as input from the memory 212a-3.

During this initial state, the wheel acceleration value a.sub.w varies stepwise with each wheel acceleration value derivation cycle.

When the wheel acceleration value a.sub.w drops equal to or less than the deceleration threshold (-b), at time t.sub.1, the operation mode of the anti-skid brake control system is switched to HOLD mode so as to hold the braking pressure at an increased constant value. At this time, since the output level of the AND gate 228 is still LOW, the retriggerable timer 242 is still inoperative. Therefore, the integrator circuit 212b-3 remains disabled and thus outputs a zero value.

During the HOLD mode operation starting from the timer t.sub.1, the wheel speed V.sub.w continues to decrease due to the constant enhanced braking presure. When the wheel speed value V.sub.w decreases below the target wheel speed value V.sub.i at time t.sub.2, the output level of the AND gate 228 goes HIGH. The retriggerable timer 242 is thus triggered by the leading edge of the HIGH-level AND gate signal to output a HIGH-level signal to the AND gate 212b-1. As a result, the output of the AND gate 212b-1 goes HIGH and so opens the switch 212b-2. The integrator 212b-3 then become active to integrate the given value +B or -B depending upon the operation mode of the anti-skid brake control system. Since the trigger period of the retriggerable timer 242 is longer than the maximum length of the HIGH-level output of the AND gate 242, the integrator 212b-3 remains active throughout the current anti-skid brake control episode.

At the time t.sub.2, since the output of the OR gate 218 and the AND gate 228 are both HIGH, the anti-skid brake control system operates in RELEASE mode so that the braking pressure decreases. Therefore, the output level of the AND gate 212b-11 goes HIGH and closes the switch 212b-8. The integrator 212b-3 is thus connected to the negative potential source 212b-6 to receive the negative potential -B.

Since, the rate of change of the integrated value is determined by the capacitance C of the capacitor C and the resistance R of the resistor, the integrated value can be expressed by the following equation: ##EQU1##

As shown in FIG. 13, since the negative potential source 212b-6 is connected to the negative input terminal of the differential amplifier 212b-4, the positive value increases with time in accordance with the above equation. Therefore, the wheel acceleration value a.sub.w is adjusted by extrapolation at a linear rate. The adder 212b-13 outputs the adjusted value as a replacement for the initially calculated wheel acceleration value.

At a time t.sub.3, a new wheel acceleration value a.sub.w is calculated. At the same time, the updating indicative signal S.sub.a is also produced to temporarily turn the output level of the OR gate 212b-1 LOW. The LOW-level OR gate signal closes the switch 212b-2 momentarily to clear the integrated value in the integrator 212b-3 to zero. At the same time, the memory 212a-3 is also updated with the new wheel acceleration value a.sub.w, which is supplied to the adder 212b-13. After the time t.sub.3, the adjustment by extrapolation described abobve is repeated for the wheel acceleration value.

At the time t.sub.4, the wheel acceleration indicative value, which is equal either to the initial wheel acceleration value a.sub.w or the adjusted value, reaches the acceleration threshold (+a), whereupon the system switches operation mode for RELEASE mode to HOLD mode so that the braking pressure is held at a decreased constant value. At this time, the output of the AND gate 228 goes LOW and the output level of the OR gate 218 remains HIGH. As a result, the AND condition of the AND gate 212b-12 is satisfied. Therefore, the HIGH-level output from the AND gate 212b-12 closes the switch 212b-7 to connect the negative input terminal of the differential amplifier 212b-4 to ground. Therefore, the integrated value of the integrator 212b-3 is held at zero. This results in stepwise variation of the wheel acceleration value a.sub.w as long as the system remains in HOLD mode.

Therefore, at times t.sub.5, t.sub.6, t.sub.7 and t.sub.8, the wheel acceleration value a.sub.w is updated with new wheel acceleration values a.sub.w derived by the arithmetic circuit 212a-2. During the intervals between t.sub.5 and t.sub.6, t.sub.6 and t.sub.7, and t.sub.7 and t.sub.8, the output value of the adder 212b-13 remains constant.

During this HOLD mode operation starting from the time t.sub.4, the wheel speed V.sub.w increases beyond the projected speed V.sub.c and then quickly drops below the projected speed V.sub.c again at time t.sub.9. This induces LOW level outputs from the OR gate 218 and the AND gate 228 whereupon the anti-skid brake control system switches APPLICATION mode. The LOW output levels of both of the OR gate 218 and the AND gate 228 satisfy the NOR condition of the NOR gate 212b-10. Therefore, the NOR gate 212b-10 outputs a HIGH-level gate signal to the switch 212b-5 and so closes it. The switch 212b-5 then connects the positive potential source 212b-5 to the negative input terminal of the differential amplifier. The integrator 212b-3 thus outputs a linearly decreasing integrated value. The wheel acceleration value a.sub.w derived at the time t.sub.8 thus decreases linearly in accordance with the adjustment value.

APPLICATION mode operation causes the wheel acceleration to drop below the deceleration threshold (-b) again, at a time t.sub.9. This initiates the second cycle of skid control operation through the period t.sub.10, t.sub.11, t.sub.12 . . . .

Thus, the preferred embodiment can promptly detect when the wheel acceleration a.sub.w drops below the deceleration threshold (-b) with the help of the extrapolation adjustment to the wheel acceleration value. This helps prevent the vehicle wheel from skidding due to over-pressure.

Although the first embodiment of the anti-skid brake control system uses a constant rate +B and -B of extrapolated change of the wheel acceleration value, it might be preferable to adjust the value used for extrapolation. One approach that may be adopted would be to adjust the extrapolation value according to road surface conditions.

As shown in FIG. 15, the fluid pressure in the wheel cylinder builds up non-linearly. Specifically, in the low pressure range the fluid pressure increases relatively slowly. On the other hand, at higher pressures the fluid pressure increases at a relatively high rate of increase.

If the road surface affords relatively good traction, the lock pressure at which the wheels will tend to skid will be relatively high. As a result, the braking pressure can vary within a relatively high pressure range E.sub.1. On the other hand, if the coefficient of friction of the road surface is relatively low, the lock pressure P.sub.lock will be relatively low. In this case, the fluid pressure varies within a pressure range E.sub.2 which is substantially lower than the pressure range E.sub.1.

As should be obvious, the behavoir of wheel acceleration depends strongly on the braking pressure applied. This means that when the braking pressure varies within the pressure range E.sub.1, the rate of change of wheel acceleration will be much higher than in the pressure range E.sub.2.

FIGS. 16 to 19 show a modification to the first embodiment of the anti-skid brake control system set forth above, with the added feature of making the extrapolation value dependent upon the rate of deceleration of the projected speed. As shown in FIG. 16, the wheel acceleration adjustment circuit 212b of this embodiment is additionally connected to the projected speed deriving circuit 220 to receive a projected speed deceleration indicative signal which has a value indicative of the rate of deceleration of the projected speed. The value A indicative of the deceleration rate of the projected speed V.sub.c is used as the extrapolation value and which therefore will be referred to hereafter as "extrapolation coefficient".

FIG. 17 shows the projected speed V.sub.c deriving circuit 220 employed in the shown modification to the first embodiment of the anti-skid brake control system of FIG. 11. The projected speed deriving circuit F has four sample/hold circuits 220-1, 220-2, 220-3 and 220-4. The sample/hold circuit 220-1 is connected for input from the wheel speed deriving circuit 210 and from the differential amplifier 219 through an AND gate 220-5. The AND gate 220-5 is connected to the differential amplifier 219 at one input terminal and to the retriggerable timer 242 through an inverter 220-6 at its other input terminal.

The sample/hold circuit 220-1 is triggered by HIGH-level signals from the AND gate 220-5 produced the output of the differential amplifier 219 is HIGH and the retriggerable timer output remains LOW. Since the retriggerable timer 242 outputs a HIGH-level signal in response to a HIGH-level output from the differential amplifier 224 and remains HIGH for a given period which is long enough to be retriggered by a HIGH-level output from the differential amplifier 224 in the next skid cycle, the sample/hold circuit 220-1 is able to sample the wheel speed V.sub.w only during HOLD mode at the initial stage of skid control operation. Therefore, the sample hold circuit holds the initial wheel speed V.sub.w0 throughout the anti-skid brake control operation.

The sample/hold circuit 220-2 is also connected for input from the wheel speed deriving circuit 210 and the differential amplifier 219 directly. The sample/hold circuit 220-2 is triggered by the HIGH level output of the differential amplifier 219 to latch the wheel indicative signal value V.sub.w. Since the sample/hold circuit is triggered every time the wheel acceleration drops below the wheel deceleration threshold (-b.sub.1) i.e. in response to such HIGH-level differential amplifier output, its value is updated each time it is triggered.

The sample/hold circuit 220-3 is connected for input from a timer 220-7 which is, in turn, connected for input from a clock generator 220-8, for counting the clock pulses therefrom. The sample/hold circuit 220-3 is also connected for input from the AND gate 220-5 and is triggered by HIGH-level signals therefrom to latch the timer value t as an initial timer value t.sub.0.

Although the shown embodiment employs the sample/hold circuit 220-3 for holding the initial timer value, it is not always necessary. As an alternative to the sample/hold circuit 220-3, the timer 220-7 may be designed to be reset by the HIGH-level gate signal from the AND gate 220-5. In this case, when the initial wheel speed V.sub.w0 is latched, the timer value t will be 0 by force and thus need not be held for the target wheel deriving operation set out below.

The sample/hold circuit 220-4 is connected for input from the timer and is triggered by HIGH-level outputs from the differential amplifier. The sample/hold circuit 220-4, when triggered, latches the timer value t. The contents t of the sample/hold circuit 220-4 is updated every time the output level of the differential amplifier 219 goes HIGH.

The sample/hold circuits 220-1 and 220-2 are connected for output to a subtractor 220-9. The subtractor 220-9 is adapted to receive the holding values V.sub.w0 and V.sub.w from the sample/hold circuits 220-1 and 220-2. The subtractor 220-9 calculates the difference between the values held (V.sub.w0 -V.sub.w) and outputs a wheel speed difference indicative signal dV.sub.w. On the other hand, the sample/hold circuits 220-3 and 220-4 are connected for output to a subtractor 220-10. The subtractor 220-10 receives the holding values t.sub.0 and t of the sample/hold circuits 220-3 and 220-4. The subtractor 220-10 calculates the difference between their values (t.sub.0 -t) and outputs an elapsed time indicative signal d.sub.t.

The subtractors 220-9 and 220-10 are connected for output to a divider 220-11. The divider 220-11 calculates the rate of wheel speed change (d.sub.vw /d.sub.t) and produces a deceleration rate indicative signal dV.sub.c which is representative of decrement to the projected speed in the next skid cycle.

The divider 220-11 is connected for output to a multiplier 220-12 through a switching circuit 220-13. The switching circuit 220-13 is also connected to an initial deceleration value generator 220-14. The initial deceleration value generator 220-14 outputs a constant signal representative of a fixed deceleration rate for use in the first skid cycle. On the other hand, the switching circuit 220-13 is connected for control input from a flip-flop 220-15. The flip-flop 220-15 has a set input terminal connected to the output terminal of an AND gate 220-16. One input terminal of the AND gate 220-16 is connected to the differential amplifier 219, and the other input terminal of the AND gate 220-16, which is an inverting input terminal, is connected to the retriggerable timer 242. Therefore, similarly to the AND gate 220-5, the AND gate 220-16 outputs a HIGH-level signal only at the beginning of the anti-skid brake control operation. The flip-flop 220-15 also has a reset input terminal connected to the retriggerable timer 242. The flip-flop 220-15 is thus set by HIGH-level signals from the AND gate 220-16 to output a HIGH-level switching signal to the switching circuit 220-13. The switching circuit 220-13 is responsive to HIGH-level switching signals to connect the initial deceleration value generator 220-14 to the multiplier 220-12. On the other hand, the flip-flop 220-15 is reset by HIGH-level signals from the retriggerable timer 242 to output a LOW-level switching signal to the switching circuit. While the switching signal from the flip-flop 220-15 remains LOW, the switching circuit 220-13 connects the divider 220-11 to the multiplier 220-12.

The multiplier 220-12 is also connected for input from to a subtractor 220-17. The subtractor 220-17 is connected for input from the timer 220-7 and the sample/hold circuit 220-4. The subtractor 220-17 outputs an elapsed period indicative signal d.sub.p representative of the time elapsed since the beginning of the current skid cycle. The multiplier 220-12 multiplies the deceleration rate indicative signal value dV.sub.c either from the divider 220-11 or the initial deceleration generator 220-14 by the elapsed period indicative signal d.sub.p to derive a wheel speed deceleration value. The multiplier 220-12 outputs a projected deceleration indicative signal V.sub.c ' to a subtractor 220-18 which is also connected to the sample/hold circuit 220-2 to receive the current vehicle speed value. The subtractor 220-18 subtracts the two input values to derive the instantaneous projected speed V.sub.c (=V.sub.w -V.sub.c ').

The subtractor 220-18 outputs the projected speed indicative signal to the target wheel speed V.sub.c deriving circuit 222.

A retriggerable timer 220-19 is also connected for input from the differential amplifier 219 and is triggered by the leading edge of its HIGH-level output. The retriggerable timer 220-19 remains at its triggered state for a given period of timer, e.g. 2 sec. The retriggerable timer 220-19 is connected for output to a switching circuit 220-20. The switching circuit 220-20 is normally connected to the wheel speed deriving circuit 210 to the target wheel speed deriving circuit 222 while it remains inactivated. The switching circuit 220-20 responds to HIGH-level signals from the retriggerable timer 220-19 by connecting the subtractor 220-18 to the target wheel speed deriving circuit 222.

The operation of the projected speed deriving circuit is shown in FIG. 18 in the form of a timing chart.

As shown in FIG. 18, at the time, t.sub.20, T.sub.21 . . . t.sub.26 . . . , the wheel acceleration a.sub.w drops below the deceleration threshold (-b.sub.1) to trigger HIGH-level differential amplifier outputs from the differential amplifiers 219 and 214. This induces HOLD mode operation by which the braking pressure at the wheel cylinders is held at an increased constant value. At the time t.sub.20, the sample/hold circuits 220-1 and 220-2 latch the wheel speed indicative signal V.sub.w from the wheel speed deriving circuit 202. When the wheel speed V.sub.w drops below the target wheel speed V.sub.i and so triggers RELEASE mode operation, the retriggerable timer 242 becomes active to output a HIGH-level signal. This disables the sample/hold circuit 220-1. Therefore, from this time on the sample/hold circuit 220-1 holds the initial wheel speed V.sub.w sampled at the time t.sub.20. On the other hand, the value of the sample/hold circuit 220-2 is updated every time the output of the differential amplifier 219 goes HIGH, i.e. at times t.sub.21, t.sub.22, t.sub.23, t.sub.24, t.sub.25, t.sub.26 . . . , at which the instantaneous wheel speed V.sub.w1, V.sub.w2, V.sub.w3, V.sub.w4, V.sub.w5, V.sub.w6 . . . respectively prevail. At the same time, the sample-hold circuits 220-3 and 220-4 sample and hold the timer values. The sample/hold circuit 220-3 holds the timer value at the time t.sub.20 throughout the skid control operation as set forth above. The sample/hold circuit 220-4 updates the held value in response to the leading edge of the HIGH-level output of the differential amplifier 219 at times t.sub.21, t.sub.22, t.sub.23, t.sub.24, t.sub.25, t.sub.26 . . . . Based on the values of the sample hold circuits 220-1, 220-2, 220-3 and 220-4, projecting speed deceleration rates dV.sub.c1, dV.sub.c2, dV.sub.c3, dV.sub.c4, dV.sub.c5, dV.sub.c6 . . . are derived. The projected speed values V.sub.c1, V.sub.c2, V.sub.c3, V.sub.c4, V.sub.c5, V.sub.c6 . . . are derived by subtracting the deceleration values dV.sub.c1, dV.sub.c2, dV.sub.c3, dV.sub.c4, dV.sub.c5, dV.sub.c6 . . . from the sampled wheel speed values V.sub.w1, V.sub.w2, V.sub.w3, V.sub.w4, V.sub.w5, V.sub.w6 . . . .

In the first skid cycle between times t.sub.20 and t.sub.21, the projected speed deceleration rate dV.sub.cset preset by the initial deceleration generator 220-14 is used and the initial wheel speed V.sub.w0 is taken as the starting value.

On the other hand, the output of the switching circuit 220-13 will be either the initial deceleration rate indicative value from the initial deceleration generator 220-14 or the deceleration rate indicative signal from the divider 220-11 and is supplied to the wheel acceleration adjustment circuit 212b of the wheel speed deriving circuit 212.

As shown in FIG. 19, the output of the switching circuit 220-13 is connected to the negative input terminal of the differential amplifier 212b-4 of the intetrator 212b-3 through the switch 212b-7. Also, the output of the switching circuit 220-13 is connected to an inverter 212b-14. The inverter is, in turn, connected for output to the negative input terminal of the differential amplifier 212b-4 through the switch 212b-8. Therefore, either the deceleration rate indicative signal value A or its invert (-A) will serve as the extrapolating coefficient.

FIGS. 20 to 29 show another embodiment of the anti-skid brake control system according to the present invention. In this embodiment, the present invention is applied to a microprocessor-based digital control system. In order to make the alternating-current wheel sensor signal applicable to the digital control system, the sensor signal is converted into a train of pulses separated by intervals corresponding to or representative of the detected peripheral speed of the wheel. Before explaining the embodiment shown, the theory of anti-skid brake control by means of the digital control system will be briefly described hereinbelow for the sake of better understanding of the invention.

The wheel rotation speed V.sub.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.sub.w is derived from the interval between the last sensor pulse input time and the current sensor pulse input time. A target wheel speed is designated V.sub.i and the resultant wheel speed is designated V.sub.w. In addition, the slip rate is derived from the rate of change of the wheel speed and an projected speed V.sub.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.sub.i is derived from the wheel speed of the last skid cycle during which the wheel deceleration value was equal to or less than a given value which will be hereafter referred to as "deceleration threshold a.sub.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.sub.i is derived based on the projected speed V.sub.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. -1.2 G, and a predetermined deceleration value, for example 0.4 G. The subsequent target wheel speed V.sub.i is derived based on the projected speeds V.sub.v in last two skid cycles. For instance, the deceleration value of the target wheel speed V.sub.i is derived from a difference of the projected speeds V.sub.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 value, the target wheel speed in the current skid cycle is derived.

The acceleration of the wheel is derived based on the input times of three successive sensor pulses. Since the interval of the adjacent sensor signal pulses corresponds to the 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 the or difference of the wheel speed may be obtained. The resultant may be divided by the period of time in order to obtain the wheel acceleration at the unit time. Therefore, the acceleration or deceleration of the wheel is derived from the following equation: ##EQU2## 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.sub.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.sub.i and the instantaneous wheel speed V.sub.w by the target wheel speed. Therefore, in addition, the slip rate R is derived by solving the following equation: ##EQU3##

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.sub.w.

In anti-skid brake control, the braking force applied to the wheel cylinder is to be so adjusted that the peripheral speed of the wheel, i.e. the wheel speed, during braking is held to a given ratio, e.g. 85% to 80% of the vehicle speed. Therefore, the slip rate R has to be maintained below a given ratio, i.e., 15% to 20%. In the preferred embodiment, the control system controls the braking force so as to maintain the slip rate at about 15%. Therefore, a reference value R.sub.ref to be compared with the slip rate R is determined at a value at 85% of the projected speed V.sub.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.sub.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 attains a limited value, e.g., 2 A to place the electromagnetic value 30a in the hold mode as shown in FIG. 5 when the wheel speed remains inbetween the target wheel speed V.sub.i and the slip rate threshold R.sub.ref. When the slip rate derived from the target wheel speed V.sub.i and the wheel speed V.sub.w becomes equal to or greater than the slip rate threshold R.sub.ref, then the supply current to the actuator 16 is increased to a maximum value, e.g. 5 A to place the electromagnetic valve in the release mode as shown in FIG. 6. By maintaining the release mode, the wheel speed V.sub.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.sub.ref, then the supply current to the actuator 16 is dropped to the limited value, e.g. 2 A to return the electromagnetic valve 30a to the hold mode. By holding the reduced fluid pressure in the wheel cylinder, the wheel speed V.sub.w is further resumed to the target wheel speed V.sub.i. When the wheel speed V.sub.w is resumed equal to or higher than the target wheel speed V.sub.i, the supply current is further dropped to zero for placing the electromagnetic valve in the application mode as shown in FIG. 4. The electromagnetic valve 30a is maintained in the application mode until the wheel speed is decelerated at a wheel speed at which the wheel deceleration becomes equal to or slightly more than the deceleration threshold a.sub.ref e.g. -1.2 G. At the same time, the projected speed V.sub.v is again derived with respect to the wheel speed at which the wheel deceleration a.sub.w becomes equal to or slightly greater than the deceleration threshold a.sub.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.sub.i is derived. Therefore, assuming the last projected speed is V.sub.v1, the instant projected speed is V.sub.v2, and the period of time is T.sub.v, the target wheel speed V.sub.i can be obtained from the following equation:

where t.sub.e is an elapsed time from the time at which the instant projected speed V.sub.v2 is obtained.

Based on the input timing, deceleration value a.sub.w is derived from the foregoing equation (1). In addition, the projected speed V.sub.v is estimated as a function of the wheel speed V.sub.w and rate of change thereof. Based on the instantaneous wheel speeds V.sub.w1 at which the wheel deceleration is equal to or less than the deceleration threshold a.sub.ref and the predetermined fixed value, e.g. 0.4 G for the first skid cycle of control operation, the target wheel speed V.sub.i is calculated. According to equation (2), the slip rate R is calculated, using successive wheel speed values V.sub.w1, V.sub.w2, V.sub.w3 . . . as parameters. The derived slip rate R is compared with the slip rate threshold R.sub.ref. As the wheel speed V.sub.w drops below the projected speed V.sub.v at the time t.sub.1, 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 t.sub.4, 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.sub.w recovers, i.e. the slip rate R drops until it is less than the slip rate threshold. The controller unit 202 detects when the slip rate R is less than the slip rate threshold R.sub.ref and switches the control mode from release mode to the hold mode.

By maintaining the brake system in the hold mode in which reduced brake pressure is applied to the wheel cylinder, the wheel speed increases until it reaches the projected speed. When the wheel speed V.sub.w becomes equal to the target wheel speed V.sub.i, the controller unit 202 switches the control mode from the hold mode to the application mode.

As can be appreciated from the foregoing description, the control mode will tend to cycle through the control modes in the order application mode, hold mode, release mode and hold mode. This cycle of variation of the control modes will be referred to hereafter as "skid cycle". Practically speaking, there will of course be some hunting and other minor deviations from the standard skid cycle.

The projected speed V.sub.v, which is meant to represent ideal vehicle speed behavior, at time t.sub.1 can be obtained directly from the wheel speed V.sub.w at that time since zero slip is assumed. At the same time, the deceleration vlaue 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.sub.v at the time t.sub.1 will be derived from the wheel speed V.sub.w1 at that time. Using the predetermined deceleration value, the projected speed will be calculated at each time the wheel deceleration a.sub.w in the application mode reaches the deceleration threshold a.sub.ref.

The wheel deceleration a.sub.w becomes equal to or slightly greater than the deceleration threshold a.sub.ref, then the second projected speed V.sub.v2 is obtained at a value equal to the instantaneous wheel speed V.sub.w. According to the above-mentioned equation, the deceleration value da can be obtained

Based on the derived deceleration value da, the target wheel speed V.sub.i ' for the second skid cycle of control operation is derived by:

Based on the derived target wheel speed, the slip rate threshold R.sub.ref for the second cycle of control operation is also derived. The control mode will be varied during the second cycle of skid control operation, to hold mode, when the wheel deceleration reaches the deceleration threshold a.sub.ref as set forth above, to release mode, when the slip rate R reaches the slip rate threshold R.sub.ref, to hold mode when the slip rate R is recovered to the slip rate threshold R.sub.ref, and to application mode when the wheel speed V.sub.w recovered or resumed to the target wheel speed V.sub.i '. In addition, 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. 2 A) is applied 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.sub.w is not less than the projected speed and the slip rate is greater than the slip rate threshold R.sub.ref. Therefore, the control mode is switched from the application mode to the hold mode and then to the release mode. The slip rate increases back up to the slip rate threshold R.sub.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.sub.w finally returns to the level of the target wheel speed V.sub.i, the actuator 16 supply current is cut off so that the electromagnetic valve 16a returns to its rest position in order to establish fluid communication between the pressure line 42 and the braking pressure line 46 via inlet and outlet ports 16b and 16c.

Referring to FIG. 20, the controller unit 202 includes an input interface 1230, CPU 1232, an output interface 1234, RAM 1236 and ROM 1238. The input interface 1230 includes an interrupt command generator 1229 which produces an interrupt command in response to every sensor pulse. In ROM, a plurality of programs including a main program (FIG. 21), an interrupt program (FIG. 22), an sample control program, a timer overflow program and an output calculation program (FIG. 25) are stored in respectively corresponding address blocks 1244, 1246, 1250, 1252 and 1254.

The input interface also has a temporary register for temporarily holding input timing for the sensor pulses. RAM 1236 similarly has a memory block holding input timing for the sensor pulses. The contents of the memory block 1240 of RAM may be shifted whenever calculations of the pulse interval, wheel speed, wheel acceleration or deceleration, target wheel speed, slip rate and so forth are completed. One method of shifting the contents is known from the corresponding disclosure of the U.S. Pat. No. 4,408,290. The disclosure of the U.S. Pat. No. 4,408,290 is hereby incorporated by reference. RAM also has a memory block 1242 for holding pulse intervals of the input sensor pulses. The memory block 1242 is also adapted to shift the contents thereof according to the manner similar to set forth in the U.S. Pat. No. 4,408,290.

An interrupt flag 1256 is provided in the controller unit 202 for signalling interrupt requests to the CPU. The interrupt flag 1256 is set in response to the interrupt command from the interrupt command generator 1229. A timer overflow interrupt flag 1258 is adapted to set an overflow flag when the measured interval between any pair of monitored sensor pulses exceeds the 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 time signals indicative of elapsed real time. The timer signal value is latched whenever a sensor pulse is received and stored in either or both of the temporary register 1231 in the input interface 1230 and the memory block 1240 of RAM 1236.

The RAM of the controller 202 has a memory block 1255 allocated for a flag FLRL which is set during RELEASE mode operation. The flag FLRL will be referred to hereafter "RELEASE mode indicative flag".

The controller unit 202 also includes memory blocks 1235 and 1237 in RAM for storing the last two wheel acceleration values. The wheel speed indicative signal value V.sub.w latched at the first and skid cycle will be hereafter referred to as "initial wheel speed value V.sub.w0 " and the wheel speed indicative signal value latched at the beginning of the current skid cycle will be hereafter referred to as "latched wheel speed value". The memory block 1235 is allocated for storage of the initial wheel speed value V.sub.w0. The memory block 1237 stores the latched wheel speed value V.sub.w and updates the stored value every time a new wheel speed indicative signal value is latched. The controller unit 202 also has a timers 1260 and 1261 and flag registers 1257 and 1258 respectively allocated for flags FLCT and FLHD. FLCT is indicative of the operating status of the anti-skid control system. The flag FLHD is indicative of operation in HOLD mode. The timer 1255 measures elapsed time since the initial wheel speed indicative signal value was latched. The timer 1256 measures the interval over which the hold-indicative flag FLHD is set.

The operation of the controller unit 202 and the function of each elements mentioned above will be described with reference to FIGS. 21 to 29.

FIG. 21 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. 21 but shown in more detail in FIG. 22. If FL=1, then the control process is according to plan, and control passes to a main routine explained later with reference to FIG. 23. 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. 22 shows the interrupt program stored in the memory block 1246 of ROM 1238 and executed in response to the interrupt command generated by the interrupt command generator 1229 whenever a sensor pulse is received. It should be noted that a counter value NC of an auxiliary counter 1233 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 1235 is set at -1. After starting execution of the interrupt program, the counter value NC of the auxiliary counter 1233 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 1231 in the input interface 1230 at a block 3006. The counter value NC of the auxiliary counter 1233 is thereafter assigned the value N in a register 1235, 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 1235 is then incremented by 1. The counter value M of the auxiliary counter 1235 labels each of a sequence of sample periods covering an increasing number of sensor pulses. After this, the sample flag FL is incremented by 1 at a block 3012. After the block 3012, interrupt program ends, returning control to the main program or back to block 3002, whichever comes first.

On the other hand, when the counter value NC is non-zero when checked at the block 3004, this indicates that not all of the pulses for this sample period have been received, and so the interrupt program ends immediately.

This interrupt routine thus serves to monitor the input timing t of each pulse sampling period, i.e. the time t required to receive NC pulses, and signals completion of each sample period (M=0 through M=10, for example) for the information of the main program.

Before describing the operation in the main routine, the general method of grouping the sensor pulses into sample periods will be explained to facilitate understanding of the description of the operation in the main routine.

In order to enable the controller unit 202 to accurately calculate the wheel acceleration a.sub.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:

wherein A, B and C are the input times of three successive sensor pulse groups.

The controller unit 202 has different sample modes, i.e. MODE 1, MODE 2, MODE 3 and MODE 4 determining the number of sensor pulses in each sample period group. As shown in FIG. 24, in MODE 1 every sensor pulse input time is recorded and therefore the register value N is 1. In MODE 2, every other sensor pulse is ignored and the register value N is 2. In MODE 3, every fourth sensor pulse is monitored, i.e. its input time is recorded, and the register value N is 4. In MODE 4, every eighth sensor pulse is sampled and the register value N is then 8.

The controller unit 202 thus samples the input timing of three successive sensor pulses to calculate the pulse interval difference dT while operating in MODE 1. If the derived pulse interval difference is equal to or greater than the pulse interval threshold S, then sensor pulses will continue to be sampled in MODE 1. Otherwise, the input timing of every other sensor pulse is sampled in MODE 2 and from the sampled input timing of the next three sensor pulses sampled, the pulse interval difference dT is calculated to again be compared with the pulse interval threshold S. If the derived pulse interval difference is equal to or greater than the pulse interval threshold S, we remain in MODE 2. Otherwise, every four sensor pulses are sampled in MODE 3. The input timings of the next three sampled sensor pulses are processed to derive the pulse interval difference dT. The derived pulse interval difference dT is again compared with the pulse interval threshold S. If the derived pulse interval difference is equal to or greater than the pulse interval threshold S, the MODE remains at 3 and the value N is set to 4. On the other hand, if the derived pulse interval difference dT is less than the pulse interval threshold S, the mode is shifted to MODE 4 to sample the input timing of every eighth sensor pulse. In this MODE 4, the value N is set at 8.

For instance, in FIG. 24, the sensor pulses A.sub.1, B.sub.1 and C.sub.1 are sampled under MODE 1. In MODE 2, the sensor pulses a.sub.1 and c.sub.1 are ignored and the sensor pulses A.sub.1 (=A.sub.2), B.sub.2 (=b.sub.1) and C.sub.2 (=b.sub.2 =a.sub.3) are sampled. In MODE 3, the three sensor pulses c.sub.2 (=b.sub.3 =a.sub.4), c.sub.3 (=b.sub.4) and c.sub.4 following B.sub.3 (=c.sub.2) are ignored and the sensor pulses A.sub.3 (=A.sub.1 =A.sub.2), B.sub.3 (=b.sub.2 =a.sub.3) and C.sub.3 (=b.sub.5 =a.sub.6) are sampled. In MODE 4, the seven sensor pulses c.sub.5 (=b.sub.6 =a.sub.7), c.sub.6 (=b.sub.7 =a.sub.8), c.sub.7 (=b.sub.8 =a.sub.9), c.sub.8 (=b.sub.9 =a.sub.10), c.sub.9 (=b.sub.10 =a.sub.11), c.sub.10 (=b.sub.11) and c.sub.11 following B.sub.4 (=c.sub.3) are ignored and the sensor pulses A.sub.4 (=A.sub.1 =A.sub.2 =A.sub.3), B.sub.4 (=C.sub.3 =b.sub.5 =a.sub.6) and C.sub.4 are sampled.

Referring to FIG. 23, the main routine serves to periodically derive an updated wheel acceleration value a.sub.w. In general, this is done by sampling greater and larger groups of pulses until the difference between the durations of the groups is large enough to yield an accurate value. In the main routine, the sample flag FL is reset to zero at a block 2001. Then the counter value M of the auxiliary counter 1233, indicating the current sample period of the current a.sub.w calculation cycle, is read out at a block 2002 to dictate the subsequent program steps.

Specifically, after the first sample period (M=.0.), the input timing t temporarily stored in the temporary register 1231 corresponding to the sensor pulse number (M=0) is read out and transferred to a memory block 240 of RAM at a block 2004, which memory block 1240 will be hereafter referred to as "input timing memory". Then control passes to the block 1008 of the main program. When M=2, the corresponding input timing t is read out from the temporary register 1231 and transferred to the input timing memory 1240 at a block 2006. Then, at a block 2008, a pulse interval Ts between the sensor pulses of M=1 is derived from the two input timing values in the input timing memory 1240. That is, the pulse interval of the sensor pulse (M=1) is derived by:

where

t.sub.1 is input time of the sensor pulse M1; and

t.sub.0 is input time of the sensor pulse M0.

The derived pulse interval T.sub.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.sub.s is shorter than the reference value, 4 ms., control passes to a block 2012 wherein the value N and the pulse interval T.sub.s are multiplied by 2. The doubled timing value (2T.sub.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.sub.s) exceeds the reference value. When the pulse interval (2nT.sub.s) exceeds the reference value at the block 2010, a corresponding value of N (2N) is automatically selected. This N value represents the number of pulses to be treated as a single pulse with regard to timing.

After setting the value of N and thus deriving the sensor pulse group size then the auxiliary counter value NC is set to 1, at a block 2016. The register value N is then checked for a value of 1, at a block 2018. If N=1, then the auxiliary counter value M is set to 3 at a block 2020 and otherwise control returns to the main program. When the register value N equals 1, the next sensor pulse, which would normally be ignored, will instead be treated as the sensor pulse having the sample period number M=3.

In the processing path for the sample period number M=3, the corresponding input timing is read from the corresponding address of the temporary register 231 and transferred to the input timing memory 1240, at a block 2024. The pulse interval T.sub.2 between the sensor pulses at M=1 and M=3 is then calculated at a block 2026. The derived pulse interval T.sub.2 is written in a storage section of a memory block 1242 of RAM 1236 for a current pulse interval data, which storage section will be hereafter referred at as "first pulse interval storage" and which memory block 1242 will be hereafter referred to as "pulse interval memory". After the block 2026, control returns to the main program to await the next sensor pulse, i.e. the sensor pulse for sample period number M=4.

When the sensor pulse for M=4 is received, the value t of the temporary register 1231 is read out and transferred to the input timing memory 240 at block 2028. Based on the input timing of the sensor pulses for M=3 and M=4, the pulse interval T.sub.3 is calculated at a block 2030. The pulse interval T.sub.3 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 T.sub.2 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 storage, i.e. the pulse interval data T.sub.2 and T.sub.3 are read out. Based on the read out pulse interval data T.sub.2 and T.sub.3, 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.sub.w. If so, process goes to the block 2040 to calculate the wheel acceleration or deceleration according to the equation (1). Detail of the wheel acceleration or deceleration deriving routine at the block 2040 will be described later with reference to FIG. 24. 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.sub.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.sub.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 c.sub.2 input following to the sensor pulse of M=4' is ignored. The sensor pulse c.sub.3 following to the ignored sensor pulse c.sub.2 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 c.sub.3 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. This may be contrasted with the procedure taken in the known art.

FIG. 25 shows the output program for deriving the wheel speed V.sub.w, wheel acceleration a.sub.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 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 the 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 1254 and adapted to be read out periodically, e.g. every 10 ms, to be executed as an interrupt program. The output calculation program is executed in the time regions shown in hatching in FIGS. 27 and 28.

During execution of the output calculation program, the pulse interval T is read out from a memory block 1241 of RAM which stores the pulse interval, at a block 5002. Then, the adjustment of the wheel acceleration value a.sub.w is performed at block 5003 for use in deriving the EV, AV signals. The derivation of the extrapolated adjustment will be described later with reference to FIG. 23. As set forth above, since the pulse interval T is inversely proportional to the wheel rotation speed V.sub.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.sub.w is performed at a block 5004 in the output program. After the block 5004, the target wheel speed V.sub.i is calculated at a block 5006. The manner of deriving the target wheel speed V.sub.i has been illustrated in the U.S. Pat. Nos. 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 Feb. 7, 1984, which are all assigned to the assignee of the present invention. The disclosure of the above-identified three United States Patents are hereby incorporated by reference for the sake of disclosure. As is obvious herefrom, the target wheel speed V.sub.i is derived as a function of wheel speed deceleration as actually detected. For instance, the wheel speed V.sub.w at which the wheel deceleration a.sub.w exceeds the deceleration threshold a.sub.ref, e.g. -1.2 G is taken as one reference point for deriving the target wheel speed V.sub.i. The wheel speed at which the wheel deceleraion a.sub.w also exceeds the deceleration threshold a.sub.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.sub.w1 and V.sub.w2 and the measured period P, the deceleration value dV.sub.i is derived from:

This target wheel speed V.sub.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.sub.i cannot be obtained. Therefore, for the first skid cycle, a predetermined fixed value will be used as the target wheel speed V.sub.i.

At a block 5008 (FIG. 25), 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 a.sub.w and the slip rate R, at a block 5010. The schedule of operation mode selection of the actuator 16 is illustrated in the form of a table in FIG. 29. The table is accessed according to the wheel acceleration a.sub.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 a.sub.w is less than -1.0 G and the application mode is selected when the wheel acceleration a.sub.w is in the range of -1.0 G to 0.6 G. On the other hand, when the slip rate R remains above 15%, the release mode is selected when the wheel acceleration a.sub.w is equal to or less than 0.6 G, and the hold mode is selected when the wheel acceleration is in a range of 0.6 G to 1.5 G. When the wheel acceleration a.sub.w is equal to or greater than 1.5 G, 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.

FIG. 26 shows the wheel acceleration adjustment routine executed at the block 5003 in the output calculation program of FIG. 22. At a block 5003-1, the HOLD mode indicative flag FLHD is checked to see whether or not it is set. If the HOLD mode indicative flag FLHD is set, then control returns to the output calculation program without adjusting the acceleration value a.sub.w. Therefore, while the system is in HOLD mode, the wheel acceleration value a.sub.w will not be adjusted.

On the other hand, if the HOLD mode indicative flag is not set when checked at the block 5003-1, then the RELEASE mode indicative flag FLRL is checked to see whether or not it is set, at a block 5003-2. If the RELEASE state indicative flag FLRL is set, then a timer value t representing the elapsed time since the last derivation of the wheel acceleration at the block 2040 of FIG. 23 is read out at a block 5003-3. The adjusted value a.sub.w ' is then calculated using the following equation:

at a block 5003-4. The wheel acceleration value a.sub.w is thereafter updated by the adjusted value a.sub.w ' derived at the block 5003-4, at a block 5003-5.

On the other hand, if the RELEASE mode indicative flag FLRL is not set when checked at the block 5003-2, then, the timer value t is read out at a block 5003-6. The adjusted value a.sub.w ' is calculated at a block 5003-7 according to the following equation:

The wheel acceleration value a.sub.w is then replaced with the adjusted value a.sub.w ' derived at the block 5003-7, at a block 5003-8.

FIG. 30 shows a routine for deriving the projected speed V.sub.c, used in turn to derive the target wheel speed V.sub.i in the output calculation program of FIG. 22. The routine illustrated performs essentially as explained with reference to the circuit of FIG. 17.

In FIG. 30, the routine is triggered during output calculation program prior to execution of the step 5006. Immediately after starting execution, first, the measured wheel acceleration a.sub.w is compared to the deceleration threshold (-b.sub.1) at a block 5005-1. If the wheel acceleration a.sub.w is equal to or less than the deceleration threshold (-b.sub.1), the control state indicative flag FLCT in the memory block 1257 is checked at a block 5005-2. If the control state indicative flag FLCT is not set when checked at the block 5005-2, and thus the initial stage of anti-skid control is recognized, the HOLD mode indicative flag FLHD in the memory block 1258 is checked at a block 5005-3. If the HOLD mode indicative flag FLHD is not set when checked at the block 5005-3, the wheel speed indicative signal value V.sub.w is latched at a block 5005-4 and is used to preset an initial projected speed value V.sub.c0 and an initial wheel speed value V.sub.w0 at a block 5005-5. Thereafter, the first timer 1255 is started at a block 5005-6. At a block 5005-7, the HOLD mode indicative flag FLHD is set to 1. Thereafter, control returns to the output deriving program of FIG. 25.

On the other hand, if the HOLD mode indicative flag FLHD is set when checked at the block 5005-3, a preset initial deceleration rate indicative value dV.sub.ci, the initial projected speed value V.sub.c0 and the timer value t are read from memory at a block 5005-8. It should be noted that the initial deceleration rate indicative value dV.sub.cset corresponds to the value indicated by the signal produced by the initial deceleration value generator in the first embodiment. In the next block 5005-9, a projected deceleration value (dV.sub.c) is calculated by multiplying the preset value dV.sub.ci by the timer value t. The projected deceleration value dV.sub.c is then used in block 5005-10 to derive the current projected speed value V.sub.c, using the formula . . . V.sub.c =V.sub.c0 -dV.sub.c. Control then returns to the main program.

If the control state indicative flag FLCT is set when checked at the block 5005-2, then the HOLD mode indicative flag FLHD is checked at a block 5005-11. If the HOLD mode indicative flag FLHD is not set as checked in the block 5005-11, then the wheel speed indicative signal value V.sub.w is latched at a block 5005-12. The newly latched speed V.sub.w is used to update the initial projected speed value V.sub.c0 in subsequent block 5005-13. The timer signal value t and initial vehicle speed value V.sub.w0 are then read from memory at a block 5005-14. The timer value t is used to update an initial timer value register t.sub.0 at a block 5005-15. In subsequent block 5005-16, an initial wheel speed deceleration rate value dV.sub.c0 is calculated from the formula dV.sub.co =(V.sub.w0 -V.sub.w)/t. Finally, the HOLD mode indicative flag FLHD is set at a block 5005-17, before control returns to the main program.

If the HOLD mode indicative flag FLHD is set when checked at the block 5005-11, then the initial wheel speed deceleration rate dV.sub.c0, the timer value t and the initial timer value t.sub.o are read memory in a block 5005-18. These values are then used in block 5005-19 to find the projected speed deceleration value V.sub.c from the formula dV.sub.c0 .times.(t-t.sub.o). Then the projected speed V.sub.c is calculated using the expression V.sub.c0 -dV.sub.c, at a block 5005-20. Thereafter, control returns to the main program.

On the other hand, if the measured wheel acceleration a.sub.w checked at the block 5005-1 is greater than the deceleration threshold (-b.sub.1), control goes to a block 5005-21, in which the HOLD mode indicative flag FLHD is checked. If the HOLD mode indicative flag FLHD is set, the HOLD mode indicative flag FLHD is reset at a block 5005-22. Another timer t.sub.2 is then started at a block 5005-23. Thereafter, the control state indicative flag FLCT is checked at a block 5005-24. If the control state indicative flag FLCT is not set when checked at the block 5005-24, then FLCT is set at a block 5005-25, and control returns to the main program.

If the HOLD mode indicative flag FLHD is not set when checked at the block 5005-21, then the control state indicative flag FLCT is checked at a block 5005-26. If the control state indicative flag is set when checked at either block 5005-24 or block 5005-26, the timer value t.sub.2 of the timer t.sub.2 is read at a block 5005-27. The timer value t.sub.2 is then compared with a control termination threshold t.sub.cT at a block 5005-28. If the timer value t.sub.2 is equal to or greater than the control termination threshold t.sub.cT, the control state indicative flag FLCT is reset at a block 5005-29. If either case, control returns to the main program.

During execution of this routine, the deceleration rate dV.sub.c derived at either of the blocks 5005-11, 5005-28 is stored in a deceleration rate register 1262 of the controller. In the wheel acceleration adjustment routine of FIG. 29, the projected speed deceleration value in the deceleration rate register 1262 is used as a coefficient for deriving the wheel acceleration adjustment value.

FIGS. 31 to 33 show the third embodiment of the controller used in the anti-skid brake control system according to the invention. In this third embodiment the wheel acceleration value is adjusted in accordance with the fluid pressure in the wheel cylinder.

The basic concept behind this third embodiment is that the extrapolated rate of change of the wheel acceleration da.sub.w is essentially proportional to the rate of change .DELTA.P of the braking pressure, i.e. fluid pressure in the wheel cylinder, and can be illustrated as below:

where

I is constant;

dw is the change in angular velocity of the wheel and is proportional to the wheel acceleration; and

-b is constant.

Thus, the rate of change of the wheel acceleration a.sub.w can be approximated by finding the rate of change of the fluid pressure. The rate of change of the wheel acceleration a.sub.w can be expressed:

Therefore, by monitoring the fluid pressure, the rate of change of the wheel acceleration can be estimated.

In the embodiment shown, as shown in FIG. 31, the wheel acceleration adjustment circuit 212b of the wheel acceleration deriving circuit 212 receives only the wheel acceleration value a.sub.w from the wheel acceleration calculating circuit 212a.

As shown in FIG. 32, the wheel acceleration adjustment circuit includes a retriggerable timer 212b-15 which has three input terminals respectively connected to the outputs of the NOR gate 212b-10, AND gate 212b-11 and AND gate 212b-12. The output terminal of the retriggerable timer 212b-15 is connected to the inverting input terminal of an OR gate 212b-16. The OR gate has another inverting input terminal connected to the retriggerable timer 242 of FIG. 31 which provides output signal MR.

The retriggerable timer 212b-15 is triggered by the leading edge of any one of the gate outputs of the NOR gate 212b-10, AND gate 212b-11 and AND gate 212b-12. The retriggerable timer 212b-15 outputs a HIGH-level signal when triggered. The OR gate 212b-16 is connected for output to the switch 212b-2. Therefore, while the input from the retriggerable timer 212b-15 is LOW, the switch 212b-2 remains open to enable operation of the integrator.

As will be appreciated herefrom, the integrated value of the integrator 212b-3 in this embodiment is thus reset when the control system switches from one operation mode to another.

The trigger period of the retriggerable timer is set to be longer than the maximum duration of either APPLICATION or RELEASE mode.

It should be appreciated that although variations in the fluid pressure are monitored by monitoring the outputs of the NOR gate and AND gates in the shown third embodiment, they can also be monitored by monitoring the fluid pressure directly. In this case, a fluid pressure sensor will be utilized to monitor the fluid pressure variation.

Although the specific embodiments have been of the present invention have been disclosed for clarity, the invention should be understood to include all possible modifications of the shown embodiments and other embodiments which do not depart from the principle of the invention, which is set out in the appended claims.