Anti-skid control system and method for automotive vehicle

(1) Field of the Invention

The present invention relates generally to anti-skid control system and method for an automotive vehicle which can prevent each of road wheels from being locked by performing cyclical pressure increase and decrease controls of a brake liquid pressure during the braking of a wheel cylinder equipped on each of the road wheels of the automotive vehicle and, more particularly, techniques adopted in the anti-skid control system and method such as a small sizing of a liquid pressure pump equipped in the anti-skid control system for a circulation of a working liquid and a suppression of vibration due to an excessive drive of the liquid pressure pump.

(2) Description of the Related Art

A Japanese Patent Application First Publication No. Heisei 7-117653 published on May 9, 1995 exemplifies a previously proposed brake liquid pressure controlling apparatus.

In the previously proposed brake liquid pressure controlling apparatus, in accordance with a correlation between a pseudo vehicle body velocity and each road wheel velocity, namely, in accordance with a road wheel slip rate of each road wheel, a pressure increase or decrease control for the road wheel corresponding wheel cylinder is carried out so that such a slip rate suppression control that each or any one of the road wheels does not exceed a predetermined value (so-called, a control target velocity) is carried out to prevent the occurrence of the corresponding road wheel's lock.

Since a maximum road wheel velocity from among the respective wheel velocities of the road wheels is approximated to an actual vehicle body velocity, this maximum road wheel velocity is used for a control purpose as a pseudo vehicle body velocity. However, the maximum wheel velocity may probably be calculated at a value higher than an actual vehicle body velocity due to the slips of some road wheels, an unnecessary pressure reduction control of the brake liquid pressure is started so that a brake insufficient state of the vehicle may occur if the pseudo vehicle body velocity based on the maximum road wheel velocity is directly used. Hence, an offset value toward a lower vehicle velocity side is set to the vehicle body velocity based on the maximum road wheel velocity. It is common practice that the pseudo vehicle body velocity set toward a lower value by the velocity reduction side offset value than the pseudo vehicle body velocity based on the maximum road wheel velocity is used for an anti-skid control purpose.

However, since, in the previously proposed brake liquid pressure controlling apparatus described above, the pseudo vehicle body velocity which is set to the lower value by the offset quantity toward the lower velocity side than the pseudo vehicle body velocity based on the maximum road wheel velocity, the control target velocity (so-called, a threshold value determining a start of the pressure decrease control) of the road wheel velocities generated on the basis of the pseudo vehicle body velocity is also set to be the lower value. Thus, since the pressure increase control is acted upon even when the road wheel velocity becomes deeper (or lower) than an ideal road wheel velocity with respect to the actual vehicle body velocity, the pressure increase becomes excessive. Hence, during the subsequent pressure decrease control, the increase in the pressure increase quantity, i.e., a quantity (consumed liquid quantity) of the brake liquid circulated toward a master cylinder side from a reservoir along with a discharge of a relatively excessive quantity of brake liquid from the wheel cylinder to the reservoir. A liquid pressure pump having a larger capacity is required and a cost of installing the anti-skid control system becomes expensive and a large sizing of the whole system is brought out. In addition, due to a frequent operation of the liquid pressure pump, a performance of a sound suppression in the liquid pressure pump and a brake piping system can be worsened.

It is, therefore, an object of the present invention to provide anti-skid control system and method for an automotive vehicle which enable a whole reduction in a consumed liquid quantity of a working liquid.

According to one aspect of the present invention, there is provided an anti-skid control system for an automotive vehicle, comprising: a brake unit to adjust a brake liquid pressure of a wheel cylinder installed on each road wheel of the vehicle, the brake unit being enabled to form any one of a pressure increase state, a pressure decrease state, and a pressure hold state in each wheel cylinder; a wheel velocity detecting section that detects wheel velocities of the respective road wheels; a pseudo vehicle body velocity calculating section that calculates a pseudo vehicle body velocity on the basis of the detected road wheel velocities of the respective road wheels; a road wheel deceleration calculating section that calculates a deceleration of each of the road wheels; a vehicle body deceleration calculating section that calculates a pseudo vehicle body deceleration from the calculated pseudo vehicle body velocity; and a brake control unit that, when any one of the wheel velocities of the respective road wheels has reached to a target velocity derived on the basis of the pseudo vehicle body velocity, forms the pressure decrease state of the brake unit to execute such a pressure decrease control as to carry out a decrease in pressure of the brake liquid of any one of the wheel cylinders which corresponds to one of the road wheels whose detected wheel velocity has reached to the target velocity and, thereafter, when the calculated wheel deceleration of the corresponding one of the road wheels becomes equal to or below zero, forms the pressure increase state of the brake unit to execute such a pressure increase control as to carry out an increase in pressure of the brake liquid of the corresponding one of the wheel cylinders, the brake control unit suspending the pressure increase control and executing such a pressure hold control as to hold the brake liquid pressure of the road wheel corresponding one of the wheel cylinders when the calculated wheel deceleration of the corresponding one of the road wheels has reached to a predetermined deceleration set on the basis of the vehicle body deceleration.

This summary of the invention does not necessarily describe all necessary features so that the invention may also be a sub-combination of these described features.

FIG. 1 is a system configuration of an anti-skid control system in a first preferred embodiment according to the present invention.

FIG. 2 is a schematic block diagram of a brake liquid pressure circuit in the anti-skid control system shown in FIG. 1 .

FIG. 3 is a basic flowchart of an anti-skid control procedure executed by an ECU in the anti-skid control system shown in FIG. 1 .

FIG. 4 is an operational flowchart representing the contents of a pseudo vehicle body velocity calculation in the ECU of the anti-skid control system shown in FIG.

FIG. 5 is an operational flowchart representing the contents of a vehicle body deceleration calculation executed in the ECU shown in FIG. 1 .

FIG. 6 is an operational flowchart representing the contents of a vehicle body deceleration calculation executed in the ECU shown in FIG. 1 .

FIG. 7 is an operational flowchart representing the contents of a PI control arithmetic and logic operations executed in the ECU shown in FIG. 1 .

FIG. 8 is an operational flowchart representing the contents of the pressure decrease control executed in the ECU shown in FIG. 1 .

FIG. 9 is an operational flowchart representing the contents of pressure hold and increase controls executed in the ECU shown in FIG. 1 .

FIG. 10 is a timing chart representing an anti-skid control operation in a case where the offset value is fixed to a constant value.

FIG. 11 is a timing chart representing the control contents of the ECU executed in the anti-skid control system in the first embodiment.

FIG. 12 is an operational flowchart representing the contents of the PI control arithmetic operation from among the control contents of the ECU of the anti-skid control system in a second preferred embodiment according to the present invention.

FIG. 13 is an operational flowchart representing the PI control arithmetic and logic operation in the ECU in the anti-skid control system in the second embodiment according to the present invention.

FIG. 14 is a timing chart representing the control contents in the ECU of the anti-skid control system in the second preferred embodiment according to the present invention.

Reference will hereinafter be made to the drawings in order to facilitate a better understanding of the present invention.

(First Embodiment)

FIG. 1 shows a system configuration of an anti-skid control system in a first preferred embodiment according to the present invention.

In FIG. 1, an automotive vehicle includes a front tight road wheel 10 , a front left road wheel 14 , a rear right road wheel 20 , and a rear left road wheel 22 . A plurality of wheel velocity sensors 12 , 16 , 24 , and 26 are disposed on the respectively corresponding road wheels 10 , 14 , 20 , and 22 to output pulse train signals representing revolutions of the corresponding road wheels. These pulse train signals are supplied to an electronic control unit (hereinafter, ECU) including a CPU (Central Processing Unit). The front right and left road wheels 10 and 14 are non-driven wheels and the rear right and left road wheels 20 and 22 are driven wheels, in this embodiment.

FIG. 2 shows a system configuration view of a brake liquid pressure circuit applicable to the anti-skid control system in the first embodiment shown in FIG. 1 .

The brake liquid pressure circuit shown in FIG. 2 includes: wheel cylinders (or, so-called, braking cylinders) 50 disposed on the respective road wheels; a master cylinder 52 which develops the brake liquid pressure in accordance with a depression by a vehicle driver; and a main liquid passage (liquid pressure tubular passage) 54 which communicates each of the wheel cylinders 50 with master cylinder 52 . An actuator unit 60 is intervened in a midway through the main liquid passage 54 to control the brake liquid pressure of each of the wheel cylinders 50 . The actuator unit 60 includes: a switch control valve 62 to switch a control state in either the control increase or decrease in the liquid pressures of the corresponding one of the respective wheel cylinder 50 ; a reservoir 64 in which the brake liquid of the corresponding one of the wheel cylinders 50 is reserved during the pressure decrease; and a liquid pressure pump 66 which returns the brake liquid reserved in the reservoir 64 to the main liquid passage 54 .

Next, a basic control procedure of the anti-skid control system executed by ECU 40 will be described below with reference to a control flowchart shown in FIG. 3 (FIG. 3 corresponds to brake controlling means including pressure increase control inhibiting means).

At a step S 1 , ECU 40 calculates respective road wheel velocities VW of the front right and left road wheels 10 and 16 and the rear right and left road wheels 24 and 26 in accordance with the outputted pulse train signals from the wheel velocity sensors 12 , 16 , 24 , and 26 and calculates respective wheel accelerations VWD by differentiating the respectively corresponding wheel velocities VW. At a step S 2 , ECU 40 calculates a pseudo vehicle velocity, namely, a pseudo vehicle body velocity VI from each wheel velocity VW calculated at step S 1 . The detailed content of step S 2 will be described later in flowcharts shown in FIGS. 4 and 5.

At a step S 3 , ECU 40 calculates a control target velocity VWS (pressure decrease determination threshold value) from pseudo vehicle body velocity VI calculated at step S 2 . It is noted that the contents of the calculation on the control target velocity VWS will be described later with reference to a flowchart of FIG. 6 .

At the next step S 4 , ECU 40 calculates a PI (Proportional-Integration) control arithmetic and logic operation process. In details, ECU 40 calculates a target pressure increase·decrease pulse time duration PB representing a control time duration of a target brake liquid and a deceleration offset value OFF_VIK which is an offset value of a threshold value to inhibit the brake liquid increase pressure control. It is noted that the contents of the PI arithmetic operation process at step S 4 will be described in details later with reference to a flowchart in FIG. 7 .

At the next step S 5 , ECU 40 determines if any one of the wheel velocities VW of the respective road wheels calculated at step S 1 is lower than the control target velocity VWS calculated at step S 3 and a flag ZFLAG indicating that the pressure increase for the corresponding one of the wheel cylinders 50 is carried out (the flag indicating that the pressure increase control is being executed) is set to “1”. If Yes (VW<VWS and ZFLAG=1) at step S 5 , the routine goes to a step S 7 for preparing the pressure decrease control.

At step S 7 , EGU 40 performs such a process as listed below and, thereafter, the routine goes to a step S 8 . At step S 8 , ECU 40 carries out the brake liquid pressure decrease control procedure (brake liquid pressure decrement control). The processes to be executed at step S 7 : 1) Set a pressure decrease control performance time duration AS to a predetermined time A; 2) reset a hold control time duration TH 0 JI to “0”; and 3) set a pressure decrease execution flag GFLAG to “1”.

At step S 8 , ECU 40 carries out the brake pressure decrease control procedure. That is to say, FCU 40 outputs a switch signal to switch control valve 62 of actuator unit 60 so that master cylinder 52 , corresponding wheel cylinder 50 , and reservoir 64 are communicated with one another.

On the other hand, if the determination at step S 5 is No (viz., VW≧VWS, or ZFLAG=0), the routine goes to a step S 6 . At step S 6 , ECU 40 determines whether a necessity of the brake liquid pressure decrease control is present, specifically, whether a hold control time duration THOJI is in excess of a predetermined time B msec. and target pressure increase·decrease pulse time duration PB− pressure decrease time duration timer value DECT is in excess of a predetermined time of T1 msec. (T1<B), the hold control time duration THOJI is in excess of a predetermined time C msec. (B<C), or a time duration of the pressure decrease time timer value DECT subtracted from the target pressure increase·decrease pulse duration PB is in excess of a predetermined time of T2 msec. (T2<T1). If either one of the two conditions is established (Yes) at step S 6 , the routine goes to step S 7 .

In addition, if the determination at step S 6 is No (neither one nor the other condition is established) at step S 6 , the routine goes to a step S 9 in order to determine whether the pressure increase or hold control of the brake liquid pressure is needed. Specifically, EGU 40 determines whether the time of the value of the pressure increase time timer INCT subtracted from the target pressure increase·decrease pulse time PB is shorter than a predetermined time of −T2 msec. and the hold control time duration THOJI is in excess of C msec. If the determination at step S 9 is Yes (both conditions are established), ECU 40 determines that both wheels are not yet slipped and the routine goes to a step S 10 .

At step S 10 , ECU 40 determines whether a pressure decrease execution flag GFLAG (a flag indicating that the pressure decrease control is being executed) is set to “1” and the wheel acceleration VWD is in excess of 0 g. If No (at least one of the two conditions is not established) at step S 10 , the routine goes to a step S 11 since the liquid pressure in the corresponding wheel cylinder 50 tends to be insufficient. At step S 11 , hold control time duration THOJI is reset to “0”. Thereafter, the routine goes to a step S 12 at which the pressure decrease control (brake liquid pressure increment control) is executed.

At step S 10 , ECU 40 determines if pressure decrease execution flag GFLAG is set to “1” and determines whether any one of the wheel accelerations is in excess of 0 g.

If No (PB+INCT≦−T2 msec. or THOJI≦C msec.) at step S 9 or Yes (GFLAG=0 or VWD≦VIK+OFF_VIK) at step S 10 , the routine goes to a step S 14 .

As described above, at step S 12 , ECU 40 carries out the brake liquid pressure increase control. That is to say, in this case, the switch control valve 62 of the actuator unit 60 is driven so that master cylinder 52 and the related wheel cylinder 50 are linked to one another. At the next step S 13 , pressure increase control execution flag ZFLAG is set to “1”.

At step S 14 , ECU 40 increments the hold control time duration THOJI and, thereafter, the routine goes to a step S 15 at which the brake liquid pressure hold control for the corresponding wheel cylinder 50 is carried out.

At step S 15 , ECU 40 carries out the brake liquid pressure hold control. That is to say, in this case, the switch control valve 62 is driven at a position at which the corresponding wheel cylinder 50 interrupts the communication to the master cylinder 52 and reservoir 64 .

If any one of steps S 8 , S 13 , and S 15 is executed, the routine goes to a step S 16 . At step S 16 , ECU 40 determines whether 10 milliseconds have passed. If not longer than 10 milliseconds (No), the determination at step S 16 is repeated until indicating Yes. If 10 milliseconds have passed (Yes), the routine goes to a step S 17 . At step S 17 , the pressure decrease control execution time AS is decremented and the routine returns to step S 1 . As described above, the above-described routine of FIG. 3 is executed for 10 millisecond.

Next, the detailed contents of step S 2 shown in FIG. 3, viz., the pseudo vehicle body velocity calculation process will be described with reference to a flowchart of FIG. 4 .

At a step S 21 , ECU 40 sets a maximum value of the four wheel velocities VW as a wheel velocity select value VFS and, thereafter, the routine goes to a step S 22 .

At a step S 22 , ECU 40 determines whether the non-pressure decrease control is being carried out depending on whether pressure decrease control execution time duration AS is zero. If Yes (under the non-pressure decrease control) at step S 22 , the routine goes to a step S 23 . At step S 23 , after the maximum value of the wheel velocities of VW of the non-driven wheels as wheel velocity select value VFS, the routine goes to a step S 24 . If NO (the pressure decrease control is being executed with AS≠0), the routine goes directly to a step S 24 .

At step S 24 , ECU 40 determines whether pseudo vehicle body velocity VI is equal to or higher than the wheel velocity select value VFS. If Yes (VI≧VFS) at step S 24 , the routine goes to a step S 25 . At step S 25 , ECU 40 determines pseudo vehicle body velocity VI during the vehicle deceleration from the following equation and the present flow of FIG. 4 is ended. That is to say, VI=VI−VIK x k. In this equation, VIK denotes a vehicle body deceleration. The contents of calculation of the vehicle body deceleration VIK will be described later with reference to a flowchart of FIG. 5 .

If VI<VFS (No) at step S 24 , ECU 40 determines that the vehicle is being accelerated and the routine goes to a step S 26 . At step S 26 , ECU 40 sets a deceleration limiter constant x to 2 Km/h and the routine goes to a step S 27 . At step S 27 , ECU 40 determines whether the present time is under the non-pressure decrease control depending on whether the pressure decrease control execution time AS indicates zero. If Yes (AS=0) at step S 27 , the routine goes to a step S 28 . At step S 28 , the deceleration limiter constant is set to 0.1 Km/h (x←0.1 (Km/h)) and, thereafter, the routine goes to a step S 29 . If No (AS≠0 and the pressure decrease control is being executed) at step S 27 , the routine jumps from step S 27 to step S 29 . At step S 29 , ECU 40 determines the pseudo vehicle body velocity VI from the following equation and the present once flow is ended: VI=VI+x.

Next, a specific content of the vehicle body deceleration calculation used at step S 25 of FIG. 4 will be described with reference to a flowchart shown in FIG. 5 .

At a step S 251 , ECU 40 determines whether the control state is switched from the non-pressure decrease control state (AS=0) to the pressure decrease control state (AS≠0). If Yes at step S 251 , the routine goes to a step S 252 . At step S 252 , EGU 400 sets the vehicle velocity when the pressure decrease control is first carried out, viz., a pressure decrease control start vehicle velocity VO to pseudo vehicle body velocity VI and resets a timer TO for generating the vehicular deceleration to “0”. Then, the routine goes to a step S 253 . In addition, if No (non-pressure decrease control is being executed) (AS=0)) at step S 251 , the routine jumps to step S 253 . After the vehicle deceleration generation timer TO is incremented, the routine goes to a step S 254 .

At a step S 254 (spin up determination), ECU 40 determines whether the wheel velocity select value VFS is returned to pseudo vehicle body velocity VI. If Yes (VI<VFS→VU≧VFS) at step S 254 , the routine goes from step S 254 to step S 255 . At step S 255 , ECU 40 determines vehicle body deceleration VIK using the following equation and the routine goes to a step S 256 . If No at step S 254 (VI<VFS), the routine jumps to step S 256 . That is to say, VIK=(VO−VI)/TO.

At step S 256 , ECU 40 determines whether the vehicle is running on a road surface having a low frictional coefficient, viz., the pressure decrease time DECT is equal to or longer than D msec. If Yes (DECT≧D msec.=low μ road surface) at step S 256 , the routine goes to a step S 257 at which a low μ flag LouF is set to “1”. Then, the present once flow is ended. If No (DECT<D msec.=a high μ road surface at step S 256 , the present once flow is directly ended.

Next, a specific detailed content of the control target velocity calculation at step S 3 shown in FIG. 3 will be described with reference to a flowchart shown in FIG. 6 .

At a step S 31 , ECU 40 sets an offset value XX of control target velocity VWS to, first of all, 8 Km/h and, thereafter, the routine goes to a step S 32 .

At step S 32 , ECU 40 determines whether the vehicle body deceleration is lower than a predetermined value E and the low μ flag LouF is set to “1” so as to determine whether the vehicle is running on the road surface having the low fictional coefficient (low μ). If Yes at step S 32 (the vehicle is running on the low μ road surface), the routine goes to a step S 33 . At step S 33 , the offset value XX is modified to 4 Km/h and the routine goes to a step S 34 . On the other hand, if No (the vehicle is running on the high μ road surface, the routine goes directly to step S 34 (the offset value of XX is left unchanged as XX=8 Km/h).

At step S 34 , ECU 40 calculates the control target velocity VWS on the basis of the following equation from the pseudo vehicle body velocity VI calculated at the flowchart of FIG. 4 and the offset value of XX and, thereafter, the routine goes to a step S 35 . That is to say, VWS=0.95x VI−XX.

At step S 35 , ECU 40 determines whether the pressure decrease flag GFLAG is set to “1”, the wheel acceleration VWD is in excess of the predetermined value of F, and, furthermore, the wheel velocity VW is in excess of the control target velocity VWS. If Yes at step S 35 , the routine goes to a step S 36 . At step S 36 , ECU 40 sets a target slip vehicle velocity VWM to the pseudo vehicle body velocity VI. If No at step S 35 , the routine goes to a step@ S 37 . At step S 37 , ECU 40 sets the target slip vehicle velocity VWM to the control target velocity VWS. Then, the present once flow is ended.

Next, a specific detailed content of the PI control arithmetic and logic operation process at step S 4 shown in FIG. 3 will be described below with reference to a flowchart of FIG. 7 .

First, at a step S 41 , ECU 40 calculates a deviation ΔVW on the basis of the following equation: ΔVW=VWM−VW.

Next, at a step S 42 , ECU 40 determines a proportional coefficient PP from the following equation: PP=KP xΔVW.

At the next step S 43 , ECU 40 derives an integration coefficient IP of the PI control from the following equation: IP=IP 100 millisecond before+KI xΔVW, wherein KI denotes a coefficient.

At the next step S 44 , ECU 40 derives a target pressure increase·decrease pulse time PB from the following equation: PB=PP+IP.

At the next step S 45 , ECU 40 determines whether an added value of OFF_VIK of the offset value of the deceleration to the vehicle body deceleration VIK is lower than the wheel acceleration −VWD. If Yes (VIK+OFF_VIK<−VWD) at step S 45 , the routine goes to a step S 46 . At step S 46 , ECU 40 resets the target pressure increase·decrease pulse time PB to “0”. Thus, a process of inhibiting the pressure increase control is carried out and, thereafter, the routine goes to a step S 47 . In addition, if No (VIK+OFF_VIK≧−VWD) at step S 46 , the routine goes directly to step S 47 . At step S 47 , ECU 40 sets the deceleration offset value OFF_VIK to a value of the deceleration offset value OFF_VIK before 100 milliseconds added to 0.1 g and, thereafter, the present once flow is ended.

Next, a specific detailed content of the pressure decrease control carried out at step S 12 of FIG. 3 will be described below with reference to a flowchart of FIG. 8 .

At a step S 121 , the ECU 40 resets a pressure increase time counter INCT to “0” and, at a step S 122 , the ECU 40 sets a pressure decrease pulse time GAW to target pressure increase·decrease pulse time PB and the routine goes to a step S 123 .

At a step S 123 , ECU 40 determines whether pressure decrease execution flag ZFLAG is set to “1”.

If Yes (ZFLAG=1) at a step S 123 , the routine goes to a step S 124 . At step S 124 , ECU 40 derives pressure decrease pressure pulse time GAW from following equation: GAW=VWD xα/VIK, wherein a denotes a coefficient. After pressure increase execution flag ZFLAG is reset to “0” at step S 124 , the routine goes to a step S 125 . In addition, if No (ZFLAG=0) at step S 123 , the routine jumps to step S 125 . At step S 125 , ECU 40 carries out a port pressure decrease output process. After pressure decrease time timer DECT is incremented, the routine goes to a step S 126 .

At step S 126 , ECU 40 determines whether pressure decrease time timer DECT is equal to or longer than pressure decrease pulse time GAW or whether wheel acceleration VWD is in excess of the predetermined value of F. If Yes (DECT≧GAW, or VWD>F) at step S 126 , the routine goes to a step S 127 . At step S 127 , ECU 40 carries out the hold control output and decrements pressure decrease time timer DECT. Then, the present once flow is ended. If No (DECT<GAW and VWD≦F) at step S 126 , the present once flow is directly ended.

Next, a specific detailed content of step S 15 (pressure hold control) shown in FIG. 3 will be described below with reference to a flowchart of FIG. 9 .

At a step S 151 , ECU 40 resets pressure decrease time counter DECT to “0” and, at the next step S 152 , ECU 40 sets pressure increase pulse time duration ZAW to target pressure increase·decrease pulse time PB and the routine goes to a step S 153 . That is to say, at step S 45 of the PI control calculation procedure shown in FIG. 7, if ECU 40 determines that the added value between vehicle body deceleration VIK and deceleration offset value OFF_VIK is smaller than wheel acceleration −VWD, the routine shown in FIG. 7 goes to step S 46 at which target pressure increase·decrease pulse time PB is reset to zero. Therefore, under this state, since pressure increase pulse time ZAW at step S 152 is set to PB (=0), a pressure increase control is inhibited.

At the next step S 153 , ECU 40 determines whether pressure decrease execution flag GFLAG is set to “1”. If Yes (GFLAG=1) at step S 153 , the routine goes to a step S 154 . At step S 154 , ECU 40 sets the deceleration offset value OFF_VIK to 0.1 g. Then, the routine goes to a step S 155 . At step S 155 , ECU 40 derives pressure increase pulse time duration ZAW from the following equation: ZAW=VWD xβ/VIK, wherein β denotes a coefficient. After pressure decrease execution flag GFLAG is reset to zero, the routine goes to a step S 156 . If No (GFLAG=0), the routine goes directly to step S 156 .

At step S 156 , ECU 40 carries out the port pressure increase output process and the increment of pressure increase time counter INCT. Then, the routine goes to a step S 157 .

At step S 157 , ECU 40 determines whether pressure increase time timer INCT is equal to or longer than pressure increase time ZAW. If Yes (INCT≧ZAW) at step S 157 , the routine goes to a step S 158 . At step S 158 , ECU 40 carries out the port hold output process and decrements pressure increase time counter INCT and the present once flow is ended. If No (INCT<ZAW) at step S 157 , the present once flow is ended.

Next, operation and advantage of the anti-skid control system in the first embodiment will be described below with reference to FIGS. 10 and 11.

The structure of the anti-skid control system in the first embodiment according to the present invention has been described with reference to FIGS. 1 through 9. ECU 40 executes the pressure decrease control such that switch control valve 62 is switched to the pressure decrease control state so as to decrease the pressure of the liquid pressure of wheel cylinder 50 , thus weakening the brake pressure since there is an increased tendency of the wheel lock when each of wheel velocities VW of respective wheels 10 , 14 , 20 , and 21 detected by wheel velocity sensors 12 , 16 , 24 , and 26 is lower than control target velocity VWS derived from pseudo vehicle body velocity VI. The execution of the pressure decrease control causes each of wheel velocities VW to be changed from the deceleration direction to the acceleration direction and to prevent the wheel lock from being occurred.

Thereafter, the execution of the above-described pressure decrease control causes the wheel acceleration to be decreased and to be equal to or lower than zero. At this time, switch control valve 62 is switched into the pressure increase control state so that such the pressure increase control that the liquid pressure of wheel cylinder 50 is increased. Hence, the braking force is strengthened and a development of the insufficient state of the deceleration of the vehicle body is prevented.

In addition, if wheel acceleration VWD becomes equal to or larger than the deceleration which is the addition of vehicle body deceleration VIK to deceleration offset value OFF_VIK, the pressure increase control is inhibited and the hold control such that the liquid pressure of wheel cylinder 50 is held at its output pressure is executed.

That is to say, if wheel acceleration VWD is in the deceleration direction with respect to vehicle body deceleration VIK, there is no ill effect on the anti-skid control even if the pressure increase control is inhibited. Hence, if the extra pressure increase control is positively inhibited in such a situation as described above, the consumed liquid quantity of the whole working liquid can be reduced. Hence, since the capacity of liquid pressure pump 66 can be reduced, it becomes possible to reduce the cost and the small sizing of the whole system. In addition, since the working time of liquid pressure control valve 66 , the vibration present of low pressure pump 66 and the brake piping system can be improved.

As described above, since the wheel acceleration value which serves as a criterion of determining a start of the pressure increase is set to the added value of the vehicle body deceleration velocity VIK to deceleration offset value OFF_VIK, the pressure increase control is inhibited at an earlier timing so as to enable the entrance of control into the hold control even if, for example, due to an influence of a disturbance input of noises, the vehicle body deceleration such as VIK is calculated at a relatively lower value. Then, a drop in the deceleration and a development in the insufficient acceleration can be generated.

In addition, while the addition value of vehicle body deceleration VIK and deceleration offset value OFF_VIK is lower than wheel acceleration −VWD, deceleration offset value OFF_VIK to be added to the vehicle body deceleration VIK is set to be increased by 0.1 g for each 100 millisecond. Then, action and advantages can be achieved in the following: FIG. 10 shows a timing chart representing an operation of the anti-skid control system in the first embodiment in a case where deceleration offset value OFF_VIK is fixed to a constant value and FIG. 11 shows a timing chart representing the contents of operation in the anti-skid control system in the first embodiment according to the present invention in which deceleration offset value OFF_VIK is gradually increased.

That is to say, for example, when the running road surface is transferred from low μ road surface to high μ road surface and deceleration offset value OFF_VIK is fixed to the constant value, as shown in FIG. 10, the wheel acceleration VWD is changed to the decrease direction as compared with vehicle body deceleration VIK. Hence, if is difficult to start the pressure increase control so that a delay in the rise of the deceleration occurs and a braking distance becomes longer.

On the other hand, as shown in FIG. 11, while the addition value between vehicle body deceleration VIK and the added value of deceleration offset value OFF_VIK is lower than wheel acceleration −VWD, deceleration offset value OFF_VIK to be added to vehicle body deceleration VIK is set to be gradually increased. Hence, even if the running road surface is transferred from low μ road surface to high μ road surface and wheel acceleration VWD is changed to the decrease direction as compared with vehicle body deceleration VIK. Hence, a smooth rise in the deceleration can be achieved since a region in which a gradual pressure increase is possible becomes large. Thus, the braking distance can be shortened in a case where the frictional state of the road surface on which the vehicle is running is changed from low μ road surface into high μ road surface.

(Second Embodiment)

Next, a second preferred embodiment of the anti-skid control system according to the present invention will be described below. If is noted that the same reference numerals used in the second embodiment as those described in the first embodiment designate the like elements and the detailed description of these elements will herein be omitted.

In the anti-skid control system in the second embodiment, when any one of the respective wheel velocities VW derived from corresponding road wheels 10 , 14 , 20 , and 22 is lower than control target velocity VWS derived from pseudo vehicular body velocity VI, switch control valve 62 is switched into the pressure decrease control state and the pressure decrease control is executed such that the liquid pressure of the wheel cylinder is decreased. Thereafter, when wheel acceleration VWD is equal to or lower than zero, switching control valve 62 is switched to the pressure increase state to execute the pressure increase control such as to increase the liquid pressure of wheel cylinder 50 . Furthermore, if wheel velocity VW is equal to a hold control determination velocity λB obtained by adding pseudo vehicle body velocity VI to predetermined offset value LBPFS, switching control valve 62 is switched to the hold control state with the pressure increase control suspended so that the hold control such as to hold the liquid pressure of wheel cylinder 50 is executed. Then, in the setting of hold control determination velocity λB, offset value LBOFS to be added to pseudo vehicle body velocity VI is gradually increased. That is to say, parts of the contents of the flowcharts of the anti-skid control system in the first embodiment are different from those in the case of the second embodiment.

FIG. 12 shows a flowchart of the detailed contents of the PI control arithmetic and logic operations shown in FIG. 7 described in the first embodiment. The contents of steps of S 45 and S 47 are different from those in the case of the first embodiment.

That is to say, at a step S 45 - 1 shown in FIG. 12, ECU 40 determines if wheel velocity VW is lower than hold control determination velocity λB and wheel velocity VW is equal to or higher than control target velocity VWS. If Yes (VW<λB and VW≧VWS), the routine goes to step S 46 . At step S 46 , ECU 40 sets target pressure increase·decrease pulse time PB to zero and the routine goes to a step S 47 - 1 . If No at step S 45 - 1 , the routine goes directly goes to step S 47 - 1 .

At step S 47 - 1 , ECU 40 sets offset value LBOFS to a value of offset value LBOFS before 100 milliseconds added to 0.1 Km/h and the routine goes to a step S 47 - 2 . At step S 47 - 2 , ECU 40 derives the hold control determination velocity λB from the following equation and the present once flow is ended. λB=0.95x VI−LBOFS.

Next, FIG. 13 shows the operational flowchart representing the contents of the pressure increase control corresponding to FIG. 9 described in the first embodiment. The difference point is that the contents of a step S 154 shown in FIG. 13 .

That is to say, if the determination at step S 153 indicates Yes (GFLAG=1), the routine goes to a step S 154 - 1 at which the offset value of LBOFS is set to “0” and, thereafter, advances to step S 155 .

Next, operation and advantage in the case of the second embodiment of the anti-skid control system according to the present invention will be described below with reference to FIG. 14 .

When each of wheel velocities VW becomes lower than hold control determination velocity λB derived by adding pseudo vehicle body velocity VI to predetermined offset value LBOFS, switch control valve 62 is switched into the hold control state with the pressure increase control suspended so that such the hold control that the liquid pressure of wheel cylinder 50 is held. Hence, an extra pressure increase control is actively inhibited so that the whole consumed liquid quantity of the working liquid can be decreased. Hence, since the capacity of the liquid pressure pump 66 can be reduced, the reduction in cost and the small sizing of the system become possible and the drive time of liquid pressure pump 66 becomes short. Hence, the sound vibration performance of liquid pressure pump 66 and the brake piping system can be improved.

In addition, in the setting of hold control determination velocity ?B, offset value LBOFS to be added to pseudo vehicle body velocity VI is set to be gradually increased at a gradient of 0.1 Km/h for each 100 millisecond. Hence, the following advantages can be obtained.

In details, for example, suppose that, in a case where the running road surface is transferred from low μ road surface to high μ road surface. At this time, when the offset value LBOFS is fixedly set at the constant value, wheel acceleration VWD is first changed in the decrease direction as compared with vehicle body deceleration VIK. Hence, the pressure increase control is made difficult to be started, thus the delay in the rise in the deceleration being resulted to occur and the braking distance being elongated. Whereas, as shown in FIG. 14, offset value LBOFS is set so as to be gradually increased. At this time, even if the running road surface is transferred from the low μ road surface to the high μ road surface and wheel acceleration VWD is changed in the direction at which the value of VWD is decreased as compared with vehicle body deceleration VIK, the region in which the gradual pressure increase is possible is enlarged. Hence, the quick rise in the deceleration can be achieved. Thus, the braking distance in a case where the road surface change of the frictional coefficient from the low μ road surface to the high μ road surface occurs. In addition, since the brake efficiency during the execution of the pressure increase inhibit control becomes relatively high, the braking performance can become favorable.

As described above, the embodiments of the anti-skid control system according to the present invention have been described with reference to the accompanying drawings. Various types of design modifications can be made without departing from the scope and sprit of the present invention.

The entire contents of a Japanese Patent Application No. 2001-080263 (filed in Japan on Mar. 21, 2001) are herein incorporated by reference. The scope of the invention is defined with reference to the following claims.