Wheel braking control system

Claim 2

Vehicle wheel braking control system comprising means (20, 25b, 25a, 20a) supplying a brake fluid medium under pressure; controllable brake pressure valve means (21) having a control winding (26) and applying pressurized brake fluid to a wheel brake cylinder (23); controllable bleeder valve means (22) having a control winding (27) bleeding brake fluid from the wheel brake cylinder (23); means (93) providing a signal (r omega ) representative of wheel speed; differentiator means (30, 230) providing a differentiated signal (r omega ) representative of change of wheel speed; a group of at least three threshold circuits (41-44) connected to the differentiator means (30), the first threshold circuit (41) responding to wheel deceleration above a predetermined first threshold level (b1) corresponding to maximum wheel deceleration and being connected to open the bleeder valve means (22) and to close the pressure valve means (21) to effect a steep drop in brake fluid pressure applied to the wheel; the second threshold circuit (42) responding to about 0 change in wheel speed (b2) upon transition of wheel speed change from deceleration to acceleration and being connected to close the bleeder valve means (22) to effect termination of the steep drop in brake fluid pressure; the third threshold circuit (43) responding to a first value of wheel acceleration (b3) corresponding to wheel reacceleration and being connected to open the pressure valve means (21) to effect increase in brake fluid pressure applied to the wheel; controlled connecting means (65) interlocked with a control connection (92) and interlocking the pressure brake valve winding (26) and the bleeder valve winding (27) to permit bleeding of brake fluid only if the brake pressure valve means (21) interrupts supply of pressurized brake fluid to the wheel brake cylinder (23); and a low-pass filter (31; 233, 234) connected between the output of the differentiator means (30, 230) and the input of the first (41, 241) and the third (43, 243) threshold circuit, and filtering stray and noise signals from being applied to said first and third threshold circuits effecting steep drop in brake fluid pressure, or increasE in brake fluid pressure, respectively.

Claim 3

System according to claim 1, wherein the controlled connection means comprises an AND-gate (65) controlling energization of the bleeder valve winding (27) to permit brake fluid flow through the bleeder valve means (22) only if pressurized fluid flow to the brake cylinder (23) has been interrupted.

Claim 4

System according to claim 1, further comprising a memory circuit (51) connected to respond to the first threshold circuit (41) and storing information representative of wheel deceleration, the first memory circuit being connected to one of the other threshold circuits of the group and terminating storage of wheel decelerating information upon response of one of said other threshold circuits.

Claim 5

System according to claim 3, comprising conjunctive gate means (64, 164, 272) connected to control energization of the brake valve winding (26); a second memory (52), the output of the second memory being connected to the conjunctive gate; means controlling setting of the second memory when said threshold level of the first threshold switch (41) is exceeded, and means (95, 195, 295) re-setting the second memory (52) upon release of pressure from the pressurized brake fluid.

Claim 6

System according to claim 4, wherein the second memory is a bistable flip-flop.

Claim 7

System according to claim 3, further comprising an AND-gate (61) having its output connected to the memory circuit (51), the input of said AND-gate (61) being connected to and derived from the first and second threshold switches (41, 42); a speed comparison circuit (47) is provided having an output signal representative of difference in circumferential wheel speed and vehicle speed; a second AND-gate (62) being provided, and AND-gate being connected to the output of the second threshold switch (42) and the speed comparator circuit (47), the output of said second AND-gates, and the output of the third threshold switch (43) controlling re-setting and termination of storage of the memory circuit (51).

Claim 8

System according to claim 6, further comprising a fourth threshold switch (44) connected to and controlled by the differentiator means (30, 230) and responding to drop of wheel acceleration below a second value of wheel acceleration, and corresponding to decrease in the rate of change of wheel acceleration, and being connected to operate the pressure valve means (21) to close said pressure valve means and inhibit rapid rise of application of brake fluid pressure applied to the wheel; a third memory (53); a third AND-gate (63), the third and fourth threshold switches (43, 44) being connected to the input of the third AND-gate, the output of said third AND-gate (63) being connected to set the third memory (53); the fourth threshold switch (44) and the output ot the memory circuit (51) being connected to control selectively (OR-gate 71) re-setting of the third memory; a second memory being provided (52) controlled by the first threshold switch (41) and re-setting upon termination of brake pedal pressure (95); the output of the speed comparison circuit and the third memory being connected disjunctively (OR-gate 73) to conjunctively (AND-gate 64) control, with the output of the second memory, energization of the brake valve winding (26).

Claim 9

System according to claim 7, wherein the third memory comprises a monstable flip-flop.

Claim 10

System according to claim 7, wherein the first and the third memories are monostable flip-flops having an unstable pulse period of about 200 msec.

Claim 11

System according to claim 3, wherein the first memory comprises a monostable flip-flop (51).

Claim 12

System according to claim 10, wherein the monostable flip-flop (51) has an unstable time longer than the re-set period normally expected under vehicle operating conditions, the monostable flip-flop re-Setting automatically even in the absence of a re-set signal to prevent malfunction of the system and to provide for faiL-safe operation.

Claim 13

System according to claim 3, comprising a fourth threshold switch (44) connected to and controlled by the differentiator means (30, 230) and responding to drop of wheel acceleration below a second value of wheel acceleration, and corresponding to decrease in the rate of change of wheel acceleration, and being connected to operate the pressure valve means (21) to close said pressure valve means and inhibit rapid rise of application of brake fluid pressure applied to the wheel; wherein the first threshold switch (241) is connected to the set input (S) of the memory circuit (251) and the fourth threshold switch (244) is connected to the re-set input (R) of the first memory means.

Claim 14

System according to claim 12, further comprising a second memory (252) and a third memory (253); the output of the first memory (251) controlling the set input of the second and third memories (252, 253); and a disjunctive circuit connected to the output of the fourth threshold circuit (244) and the output of the first memory circuit (251), the disjunctive circuit (271) controlling the brake valve winding (26) to control application of pressurized brake fluid to the wheel brake cylinder.

Claim 15

System according to claim 13, wherein the first memory circuit (51) and the second memory (252) are bistable flip-flops; and the third memory (253) is a monostable flip-flop having a pulse duration in the order of about 80 msec.

Claim 16

System according to claim 13, comprising a conjunctive circuit (261) having its inputs controlled by the outputs of the third memory (253) and the third threshold switch (243), the output of the conjunctive circuit forming a control input for the brake valve winding (26) controlling application of pressurized brake fluid to the wheel cylinder.

Claim 17

System according to claim 13, further comprising an astable multivibrator (255) having a blocking input terminal (E); and means (262; 271) logically interconnecting the blocking input (E) of the astable multivibrator with the third threshold circuit (243), the fourth threshold circuit (244) and the memory circuit (251), the output of the astable multivibrator controlling pulsed opening and closing of the brake valve (21) to permit gradual rise, only, of pressure due to gradual admission of pressurized brake fluid to the wheel brake cylinder.

Claim 18

System according to claim 16, including a NOR-gate (272) having its output connected to the brake valve winding (26), the NOR-gate (272) having its input connected to the output of the astable multivibrator (255), and the second and third memories (252, 253).

Claim 19

System according to claim 17, further comprising an OR-gate (273) connected in advance of the brake valve winding (26) for selective application of signals thereto; and a speed comparison circuit (247) providing an output signal representative of difference between wheel speed and vehicle speed in excess of a predetermined value; the output from said speed comparison circuit (247), the NOR-gate (272 and the fifth threshold circuit (240) being connected to the input of the OR-gate (273).

Claim 20

System according to claim 17, wherein the controlled connection means comprises a three-input AND-gate (265), one input being derived from the memory circuit (251), a second input being derived from the output of the NOR-gate (272) and a third input being derived from the second threshold switch (242).

Claim 21

System according to claim 3, further comprising a second memory circuit (52) having its set input connected to the output of the first memory circuit (51), and having a reset input (R); the wheel braking control system further comprising brake pedal means (20''), and electrical connection means (95) operative upon release of the brake pedal means connected to the re-set terminal of the second memory circuit (52) and operative to re-set the second memory circuit upon release of the brake pedal means.

Claim 22

System according to claim 3, further comprising a second memory circuit (52) having its set input terminal (S) connected to be controlled by the first threshold switch (41), the second memory circuit having a re-set terminal (R); the wheel braking control system further comprising brake pedal means (20'') and electrical connection means (95) operative upon release of the brake pedal means connected to the re-set terminal (R) of the second memory (52) to re-set the second memory circuit upon release of the brake pedal means.

Claim 23

System according to claim 21, wherein the vehicle is provided with a brake light switch, and the electrical connection means are interconnected with the brake light switch.

Claim 24

System according to claim 1, further comprising a fifth threshold switch (140) connected to the output of the differentiator (130) and responding to a small negative value of wheel rate of change of speed.

Claim 25

System according to claim 1 comprising a NOR-gate (272) forming a conjunctive gate and controlling energization of the brake pressure valve winding (26) of the brake pressure valve (21), one output from the NOR-gate (272) forming a control input to the controlled connection means (265; FIG. 5).

Claim 26

System according to claim 24, comprising an additional low-pass filter (233), connected between the output of the differentiator means (230) and the low-pass filter (234); a fourth threshold switch (244), a fifth threshold switch (240), the fourth (244) and fifth (240) threshold switches being connected to the output of the first series connected low-pass filter (233).

Claim 27

System according to claim 25, wherein both low-pass filters (233, 234) have the same limiting frequency.

Claim 28

System according to claim 26, wherein the limiting frequency is about 18 Hz.

Claim 29

System according to claim 26, wherein the transfer characteristics with respect to time of the low-pass filter (234) and the additional low-pass filter (233) are different, to provide for different time delay periods of signals passing through the low-pass filter and the additional filter.

Claim 30

System according to claim 28, wherein the time delay of the low-pass filter (234) is greater than that of the additional low-pass filter (233).

Claim 31

System according to claim 1, wherein at least one of the threshold switches has a variable threshold level characteristic; the threshold switch being provided with a control input (V) connected to a speed comparator circuit (47), having, at one input (94) a signal representative of vehicle speed applied thereto and having, at a second input (93) a signal representative of wheel speed applied thereto, the output from said speed comparator circuit controlling energization of the brake valve winding (26) and hence flow of pressurized brake fluid to the wheel cylinder (23) in a direction to inhibit application of pressurized brake fluid to the wheel cylinder when the comparator circuit indicates an excessive speed difference between circumferential wheel speed and vehicle speed, to change the threshold response level of the respective threshold switch upon application of a signal to the threshold switch from the speed comparator circuit.

Claim 32

System according to claim 30, wherein the first and second threshold switches are provided with auxiliary control inputs connected to the speed comparator circuit (47), the threshold levels (b1, b2) of the first and second threshold switches (41, 42) changing, in positive direction, upon application of a signal to the control input.

Claim 33

System according to claim 1, comprising a pressure switch (28, 29) responsive to brake fluid pressure in the wheel brake cylinder (23), and providing a pressure signal when the pressure reaches a predetermined level; at least one of the threshold switches (41, 42) having an additional input changing the threshold level of the threshold switches; the pressure switch being connected to the level-change input of the threshold switch, or switches to Change the threshold level in a positive direction when the pressure level in the brake cylinder drops below said predetermined level.

Claim 34

System according to claim 32, wherein the pressure level is about 30 at - gauge.

Claim 35

System according to claim 1, wherein at least one of the threshold switches (40-45) comprises an operational amplifier (420) with a positive feedback circuit (FIG. 10), the differentiator means (30) providing the differentiated wheel speed signal (r omega ) representative of rate of change of wheel speed, and connected to the inverting input of the operational amplifier (420); and means (424, 425, 426) providing a reference voltage to the non-inverting input of the operational amplifier.

Claim 36

System according to claim 34, wherein the means providing a reference voltage comprises a voltage divider (424, 425, 426), and controllable resistance means (427, 428) connected to the voltage divider to change the voltage division ratio thereof, so that the threshold level of the threshold switch is changed by applying a different reference voltage to the non-inverting input of the operational amplifier (420) upon control of the resistance value of the controllable resistance path connected to the voltage divider.

Claim 37

System according to claim 35, wherein the controllable resistance path comprises a transistor (427) having its emitter-collector path connected in parallel with a portion (426) of the voltage divider, the base of the transistor (427) forming a control input (429) for a control voltage.

Claim 38

System according to claim 36, including a delay circuit (FIG. 11: 417, 418) connected between the means providing the signal representative of wheel speed and the base of the transistor (419).

Claim 39

System according to claim 36, wherein the voltage division ratio of the voltage divider (FIG. 10: 424, 425, 426; FIG. 11: 414, 415, 416) and the resistance of the emitter-collector path (419b, 419, 419a) and the signal representative of wheel speed are adjusted to provide a threshold value (b1) of the threshold switch corresponding to about -3.5 g when the wheel has a speed corresponding to rotating speed of about 40 km/h; the threshold level is about -2 g when the wheel has a speed corresponding to rotating speed of 20 km/h or less; and the threshold level changes approximately linearly proportionately from -2 g to -3.5 g for speed variations between 20 km/h to 40 km/h (FIG. 12).

Claim 40

System according to claim 36, wherein the means (93) providing the signal (r omega ) representative of wheel speed is applied to the base of the transistor.

Claim 41

System according to claim 3, wherein the second threshold switch (42) includes a timed holding circuit (54) having a time delay which is short with respect to the holding time of the memory circuit (51), the holding circuit removing the signal from the second threshold circuit (42) after the threshold level (b2) has been passed in a negative direction during the delay time of the holding circuit.

Claim 42

System according to claim 40, wherein the holding circuit has a delay time of about 20 msec., and connects the input of the second threshold circuit to ground.

Claim 43

System according to claim 1, further comprising a circuit simulating vehicle speed (VF) comprising a storage capacitor (FIG. 13: 452); a constant current circuit (450, 451, 457, 458) connected to discharge the storage condenser (452); and parallel-connected diodes (453, 454, 455, 456), one each connected to the means providing a signal (r omega ) representative of the speed of one of the respective wheels of the vehicle to charge the storage capacitor to the maximum voltage derived from any of said wheel speed signal generating means (93) associated with any of the wheels, and to provide for uniform discharge of the capacitor at a constant rate.

Claim 44

System according to claim 42, including Pressure switches (29, 29a) connected to the brake cylinders (23) of the brakes associated with non-driven wheels; and means interconnecting the pressure switches to modify the discharge current of the constant current circuit to decrease the discharge current when low brake pressure is sensed.

Claim 45

System according to claim 43, wherein the constant current circuit includes a transistor having a controlled base voltage applied thereto; the pressure switches (29, 29a) being connected in the base circuit of the transistor to modify the base voltage applied to the transistor; and a delay circuit between the pressure switches and the base circuit of the transistor to delay change in the discharge rate of the storage capacitor upon closing of one of said pressure switches.

Claim 46

System according to claim 43, wherein at least two pressure switches are provided, one each being associated with a non-driven wheel of the vehicle, each pressure switch separately influencing the discharge rate of said capacitor so that, if only one pressure switch responds, the discharge rate will be changed by a first value, and when both pressure switches respond, the discharge rate will be changed by a second value.

Claim 47

System according to claim 1, further comprising timing means (253) energizable to measure a predetermined time interval and then provide an output signal; said timing means (253) being connected to be energized upon sensing of a first level (b1) of wheel deceleration upon response of the corresponding threshold switch (41); a common response circuit (261) connected to the output of one of the other threshold switches having a threshold level different from said first threshold level, and the output of said timing means, thus determining the time interval during which a predetermined rate of change of wheel speed occurred, the common response circuit providing an output signal representative of the second derivative of wheel speed (r omega ).

Claim 48

System according to claim 1, further comprising threshold circuit means (44) connected to and controlled by the differentiator means (30, 230) and responding to drop of wheel acceleration below a second value corresponding to decrease in the rate of wheel acceleration and being connected to operate the pressure valve means (21) to close said pressure valve means and inhibit rapid rise in brake fluid pressure being applied to the wheel.

Claim 49

System according to claim 47, wherein the threshold circuit means comprises a fourth threshold switch, said value of acceleration being between the threshold level of said second and said third threshold switch.

Claim 50

System according to claim 48, comprising a first low-pass filter (132) having a limiting frequency of about 40 Hz and connected between the output of the differentiator (130), and the inputs of the second and fourth threshold switches (142, 144, 140).

Claim 51

System according to claim 47, further comprising an astable multivibrator (155) having a blocking input (E); and means (163) controlling blocking and unblocking of the astable multivibrator as a function of response of the third threshold circuit (143) and the threshold circuit means (144), the output of the astable multivibrator (155) controlling the brake valve winding (26) to repetitively, in pulsed manner, open and close the brake valve (21) to provide for gradual, slowly rising pressure increase of the pressurized brake fluid being applied to the brake cylinder (23).

Claim 52

System according to claim 50, comprising conjunctive connection means (164) having its output connected to the brake valve winding (26), the input to said conjunctive connection means being controlled by the output of the astable multivibrator (155) and the controlled connection means (51).

Claim 53

Vehicle wheel braking control system comprising means (25, 25b, 25a, 20a) supplying a brake fluid medium under pressure; means (21, 26) controlling increase of brake Fluid pressure in a wheel brake cylinder (23); means (22, 27) controlling decrease of brake fluid pressure in the wheel brake cylinder (23); means (93) providing a signal (r omega ) representative of wheel speed; means (30) providing a signal (r omega ) representative of wheel acceleration; a group of at least four acceleration threshold circuits, the first circuit (41, 214) responding to negative acceleration in excess of a first level (b1); the second circuit (42, 242) responding to about 0 acceleration (b2); the third circuit (43, 243) responding to positive wheel acceleration above a third level (b3); and the fourth threshold circuit (44, 244) responding to positive wheel acceleration above a fourth level, the fourth level lying between 0 and the third level; a low-pass filter (3, 233, 234) connected between the output of said means (30) providing a signal representative of wheel acceleration and the input to said first (41, 241) and third (43, 243) threshold circuits, and filtering stray and noise signals; a memory circuit (51) providing an output signal, the memory circuit responding and being connected to the first threshold circuit when the first level (b1) is passed in a negative direction, said output signal being connected to control said means (22, 27) controlling decrease of brake fluid pressure; logic interconnection means (62, 72) connected to another one of said threshold circuits of the group to re-set the memory circuit (51) and remove control of decrease of brake fluid pressure from said memory circuit (51); and further logic control circuit means interconnected from said second, third and fourth threshold circuits to the brake fluid pressure increase control means (21, 26) and the brake fluid pressure decrease control means (22, 26) selectively inhibiting flow of pressurized brake fluid to, or from, or to and from the wheel brake cylinder (23) in accordance with sensed wheel acceleration, and as wheel acceleration changes and causes response of the respective threshold circuits, said logic control circuit means being connected to effect response by the fourth threshold circuit to smoothe swings in brake fluid pressure and control decrease of brake fluid pressure when wheel acceleration has not reached the third threshold level.

Claim 54

Vehicle wheel braking control system comprising means (25, 25b, 25a, 20a) supplying a brake fluid medium under pressure; means (21, 26) controlling increase of brake Fluid pressure in a wheel brake cylinder (23); means (22, 27) controlling decrease of brake fluid pressure in the wheel brake cylinder (23); means (93) providing a signal (r omega ) representative of wheel speed; means (30) providing a signal (r omega ) representative of wheel acceleration; a group of at least four acceleration threshold circuits, the first circuit (41, 214) responding to negative acceleration in excess of a first level (b1); the second circuit (42, 242) responding to about 0 acceleration (b2); the third circuit (43, 243) responding to positive wheel acceleration above a third level (b3); and the fourth threshold circuit (44, 244) responding to positive wheel acceleration above a fourth level, the fourth level lying between 0 and the third level; a low-pass filter (3, 233, 234) connected between the output of said means (30) providing a signal representative of wheel acceleration and the input to said first (41, 241) and third (43, 243) threshold circuits, and filtering stray and noise signals; a memory circuit (51) providing an output signal, the memory circuit responding and being connected to the first threshold circuit when the first level (b1) is passed in a negative direction, said output signal being connected to control said means (22, 27) controlling decrease of brake fluid pressure; logic interconnection means (62, 72) connected to another one of said threshold circuits of the group to re-set the memory circuit (51) and remove control of decrease of brake fluid pressure from said memory circuit (51); and further logic control circuit means interconnected from said second, third and fourth threshold circuits to the brake fluid pressure increase control means (21, 26) and the brake fluid pressure decrease control means (22, 26) selectively inhibiting flow of pressurized brake fluid to, or from, or to and from the wheel brake cylinder (23) in accordance with sensed wheel acceleration, and as wheel acceleration changes and causes response of the respective threshold circuits, said logic control circuit means being connected to effect response by the fourth threshold circuit to smoothe swings in brake fluid pressure and control decrease of brake fluid pressure when wheel acceleration has not reached the third threshold level.

Claim 55

System according o claim 52, further comprising a fifth threshold circuit responding to negative acceleration at a fifth threshold level (b0), the fifth level being between 0 and the first threshold level (b1), the fifth threshold circuit having its output connected in the logical control circuit to further additionally control flow of pressurized brake fluid to the wheel brake cylinder (23) and to provide an intermediate pressure response level to additionally smoothe swings in brake fluid pressure and control increase in brake fluid pressure when negative wheel acceleration has not reached the first threshold level.

Claim 56

System according to claim 52, further comprising an astable multivibrator (155) having a blocking input; means (163) controlling unblocking of the astable multivibrator as a function of response of at least one of the third and fourth threshold circuits, the output of the astable multivibrator (155) controlling the brake fluid pressure increase control means (21, 26) to apply rapidly recurring pulses of pressurized brake fluid to the wheel brake cylinder and thus provide for gradual increase of brake fluid pressure in the wheel brake cylinder.

Claim 57

System according to claim 52, further comprising means providing a signal (r omega ) indicative of predetermined limiting values or rate of change of the acceleration signal and thus providing an output signal when the second derivative of wheel speed changes beyond a predetermined value, comprising timing means (253) energizable to measure a predetermined time interval and then provide an output signAl, said timing means being connected to be energized when one of said acceleration threshold circuits (41) responds upon the acceleration signal passing the respective level; a common response circuit (261) connected to the output of said timing means and further to the output of another one of the threshold switches to determine the time interval during which a predetermined acceleration of the wheel occurred, the output signal from the common response circuit being indicative of the second derivative of wheel speed being above or below the predetermined value.

Claim 58

Vehicle wheel braking control system comprising means (20, 25b, 25a, 20a) supplying a brake fluid medium under pressure; controllable brake valve means (21) having a control winding (26) and applying pressurized brake fluid to a wheel brake cylinder (23); controllable bleeder valve means (22) having a control winding (27) bleeding brake fluid from the wheel brake cylinder (23); means (93) providing a signal (r omega ) representative of wheel speed; differentiator means (30) providing a differentiated signal (r omega ) representative of change of wheel speed; a time delay means including a low-pass filter (31); a first threshold switch (41) responding to wheel deceleration exceeding a first threshold level (b1) connected through said time delay means (31) to the differentiator means (30) and being connected to and controlling said bleeder valve means (22) to bleed pressurized brake fluid, and hence reduce the pressure level of brake fluid in the wheel brake cylinder (23) when said first threshold level (b1), after said time delay, is exceeded; a second threshold switch (42) responding to about zero change in wheel speed and having a second threshold level (b2), said second threshold switch being connected to the differentiator means (30) without time delay and being connected to and controlling said bleeder valve means (22) to terminate bleeding of pressurized brake fluid and hence terminate reduction of the pressure level of the brake fluid in the wheel brake cylinder (23) upon failure to sense a change in speed of the wheel.

Claim 59

Vehicle wheel braking control system comprising means (20, 25b, 25a, 20a) supplying a brake fluid medium under pressure; controllable brake valve means (21) having a control winding (26) and applying pressurized brake fluid to a wheel brake cylinder (23); controllable bleeder valve means (22) having a control winding (27) bleeding brake fluid from the wheel brake cylinder (23); means (93) providing a signal (r omega ) representative of wheel speed; differentiator means (30) providing a differentiated signal (r omega ) representative of change of wheel speed; a time delay means including a low-pass filter (31); a first threshold switch (41) responding to wheel deceleration exceeding a first threshold level (b1) connected through said time delay means (31) to the differentiator means (30) and being connected to and controlling said bleeder valve means (22) to bleed pressurized brake fluid, and hence reduce the pressure level of brake fluid in the wheel brake cylinder (23) when said first threshold level (b1), after said time delay, is exceeded; a second threshold switch (42) responding to about zero change in wheel speed and having a second threshold level (b2), said second threshold switch being connected to the differentiator means (30) without time delay and being connected to and controlling said bleeder valve means (22) to terminate bleeding of pressurized brake fluid and hence terminate reduction of the pressure level of the brake fluid in the wheel brake cylinder (23) upon failure to sense a change in speed of the wheel.

Claim 60

System according to claim 56 further comprising a third threshold switch (43) responding to a first value of wheel acceleration exceeding a third threshold level (b3), and connected through said low-pass filter (31) to the differentiator means (30), and being connected to and controlling said brake valve means (21) to introduce pressurized brake fluid into the wheel brake cylinder (23) and hence increase the pressure level of brake fluid in the wheel brake cylinder (23) when said third threshold level, after said time delay, is exceeded; means (253) determining the time interval between response of said first threshold switch (41) and response of the third threshold switch (43) to determine the time interval of re-acceleration of the wheel, and thereby provide an electrical representation of at least two differential conditions of friction between the wheel and its support surface; and means (255) controlled by said timing means (253) controlling the rate of rise in pressure in the wheel cylinder (23) in such direction that said pressure rise will occur only gradually and slowly if a high rate of wheel reacceleration is sensed, indicative of a slippery support surface.

Claim 61

System according to claim 57 wherein the timing means comprises a monostable multivibrator (253), the rate of pressure increase control means being responsive to the comparison of the time interval between response of said first and third threshold switches (41, 43) and the unstable time of said monostable multivibrator (253).

Claim 62

System according to claim 56 further comprising a third threshold switch (43) responding to a first value of wheel acceleration exceeding a third threshold level (b3) connected through said low-pass filter (31) to the differentiator means (30) and beiNg connected to and controlling said brake valve means (21) to introduce pressurized brake fluid into the wheel brake cylinder (23) and hence increase the pressure level of brake fluid in the wheel brake cylinder (23) when said threshold level, after said time delay, is exceeded; a fourth threshold switch (44) responding to a second value wheel acceleration (b4) exceeding a fourth threshold level, the fourth value of acceleration being less than said third level and greater than zero, said fourth threshold switch (44) being connected to and controlling said brake valve means (21) and said bleeder valve means (22) to maintain the pressure level of the brake fluid in the wheel brake cylinder (23) constant, response of said third threshold switch upon further wheel acceleration overriding control by said fourth threshold switch and causing increase of the pressure level in the wheel brake cylinder.

Claim 63

System according to claim 59 further comprising a fifth threshold switch (240) responding to a low level of wheel deceleration (b0) being between zero and said first threshold level (b1), said fifth threshold switch being connected to and controlling said brake valve means (21) and said bleeder valve means (22) to maintain the pressure level of the brake fluid in the wheel brake cylinder (23) constant, response of another of said threshold switches, upon further rate of change of wheel speed overriding the control of said fifth threshold switch, the constant pressure level commanded by said fifth threshold switch affecting a temporal extension of the control cycle and thus a reduction of frequency of control swings.

Claim 64

System according to claim 56 further comprising a speed comparator circuit (47) responsive to the difference in speed between the circumferential speed of the wheel and the vehicle speed, said speed comparator being connected to and controlling said bleeder valve means (22) to bleed pressurized brake fluid, and hence reduce the pressure level of the brake fluid in said wheel cylinder (23) when said speed difference exceeds a predetermined value.