Brake fluid pressure control system

In a brake pressure control system for ABS control, traction control or vehicle motion control, a control unit controls each of pressure control solenoid valves in a PWM (pulse width modulation) control mode to prevent pulsation in the wheel cylinder pressure during the presence of a predetermined temperature or pressure condition, and a non-PWM control mode in the absence of the predetermined condition. The control unit includes a process section such as a computer and a PWM switch section including subsections each for a unique one of the solenoid valves. The process section has a single PWM port connected through a PWM signal line to each subsection of the PWM switch section.

BACKGROUND OF THE INVENTION

The present invention relates to a vehicle ABS control for controlling a brake fluid pressure to prevent wheel locking on braking, a vehicle motion control for controlling the brake fluid pressure to prevent unwanted vehicle behavior due to an excessive over-steer condition or an excessive under-steer condition during steering operation, and a vehicle traction control for controlling the brake fluid pressure to optimize the driving forces of the drive wheels during starting operation.

An ABS brake fluid pressure control system in general compares a wheel speed with a pseudo vehicle body speed (approximating the vehicle speed) to determine a slip rate of each wheel. If the slip rate exceeds a predetermined level, the control system starts the ABS control, and controls the fluid pressure applied to the associated wheel cylinder to increase, hold and decrease the brake fluid pressure in a manner to prevent wheel locking.

A brake fluid pressure control system shown in a Japanese Patent Provisional Publication No. 9(1997)-95229 is arranged to prevent pulsation of a wheel cylinder pressure and reduce operating noises by performing, in an ABS control operation, a pulse width (or pulse duration) modulation control (PWM control) based on a duty ratio control to one or more solenoid valves in a brake actuator for varying the brake fluid pressure.

This system uses the PWM control in opening and closing a solenoid valve in the ABS control and varies the duty ratio to effectively reduce pulsation of the wheel cylinder pressure.

Moreover, in consideration of deviation, from a desired correspondence, of an actual correspondence between a PWM-controlled duty ratio and a coil current due to changes in the resistance of the coil or solenoid and the source voltage caused by variation in the temperature, the control system determines a PWM signal D 1 by the following equation.

In this equation, D 1 is an original PWM signal before correction, I 0 is a coil current at a duty ratio of 100% in a theoretical relationship between the duty ratio and the coil current, and I 0 is a coil current at the duty ratio of 100% in the coil current versus duty ratio characteristic curve, determined by the original PWM signal.

SUMMARY OF THE INVENTION

In the present invention, attention is directed to a relation between an operating condition, such as a coil temperature of a solenoid valve, and reduction of pulsation.

FIG. 17 shows control waveform according to a PWM control. The PWM control includes a T 1 pressure increase interval and a T 2 pressure increase interval. The T 1 pressure increase interval is a short period immediately after the solenoid valve is turned OFF. During this, the PWM control is still inoperative. The T 2 pressure increase interval is a period during which the PWM control is operative (the PWM duty ratio is effective). The T 1 pressure increase time interval, the T 2 pressure increase time interval and the duty ratio are preliminarily determined to achieve a pressure increase quantity equaling a moderate pressure increase quantity in the normal ABS control. If the coil temperature or the ambient temperature varies beyond limits A 1 and A 2 shown in FIG. 15 , then the target current and the amount of the valve lift vary for the same duty ratio, so that the system becomes unable to achieve a desired moderate pressure increase, as shown at A in FIG. 17 .

As shown in FIG. 15 , a region of good PWM control performance is only part of a region of good normal ABS performance. The normal ABS control mode is a mode in which the PWM control based on the duty ratio control is not performed, and the controller outputs the drive signal simply made up of constant on signal and off signal. In the PWM control mode, by contrast, the drive signal is produced according to the PWM control. In the PWM mode, the control system controls the current supplied to the coil according to the PWM control, and this current is affected by variation in the temperature of the coil, so that the control system cannot achieve a desired moderate pressure increase. Therefore, the performance of the PWM control is satisfactory only in the narrow region shown in FIG. 15 , and the performance becomes worse outside the region defined between A 1 and A 2 . The control system may be able to achieve a desired moderate pressure increase by varying the PWM control appropriately when the temperature becomes closer to A 1 or A 2 . This arrangement, however, requires costly microcomputer and complicated control program to continuously vary PWM.

It is therefore an object of the present invention to provide a brake fluid pressure control for reducing cost and preventing undesired pulsation.

According to the present invention, a brake fluid pressure control system comprises a brake actuator, a sensor and a control unit.

The brake actuator comprises at least one solenoid valve for regulating a brake fluid pressure for at least one wheel of a vehicle. The sensor senses a vehicle operating condition. The control unit is for controlling the brake fluid pressure by producing a drive signal to drive the solenoid valve of the actuator in accordance with the vehicle operating condition sensed by the sensor, and for producing, as the drive signal, a PWM control signal according to a pulse width modulation in the case of existence of a predetermined PWM control condition, and a normal control signal comprising an on signal and an off signal in the case of nonexistence of the predetermined PWM control condition.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a hydraulic circuit of a brake fluid pressure control system according to a first embodiment of the present invention. Between a master cylinder 3 and wheel cylinders 9 , 10 , 11 and 12 for road wheels of a vehicle, there is provided a brake unit 60 . The brake unit 60 includes an arrangement of pressure increase side solenoid valves 33 , 34 , 35 and 36 and pressure decrease side solenoid valves 43 , 44 , 45 and 46 for increasing, holding and decreasing the oil pressure for each wheel cylinder 9 , 10 , 11 or 12 . The brake unit 60 further includes pumps 37 and 38 , driven by a motor 30 , for returning, to the master cylinder 3 , the oil drained to reservoirs 55 and 56 from the wheel cylinders 9 12 during a pressure decreasing operation. In this example, each of the pressure increase side solenoid valves 33 36 and the pressure decrease side solenoid valves 43 46 is an on-off type solenoid valve, as shown in FIG. 11 , having a plunger valve element 30 a and a valve seat 30 b for allowing and blocking the flow of the brake fluid.

Wheel speed sensors 51 , 52 , 53 and 54 sense wheel speeds of the road wheels of the vehicle and produce pulse signals representing the sensed wheel speeds, respectively. The wheel speed sensors 51 54 are connected to a control unit 50 . In FIG. 1 , signal lines for the control unit 50 are partly omitted.

FIG. 2 illustrates variation of the master cylinder fluid pressure and wheel cylinder fluid pressure when pressure increasing, holding and decreasing operations are performed. In a section shown as an example by a circle A from a pressure increasing operation to a pressure holding operation, a change from the OFF (deenergized) state increasing the pressure to the ON (energized) state in one or more of the pressure increase solenoid valves 33 36 tends to cause a fluid pressure surge due to violent collision of the plunger valve element against the valve seat. This pressure surge produces pulsation in the fluid passages and fluctuates the wheel cylinder pressure, resulting in variation in the braking force and vibration in the suspension system. The driver can feel this as unpleasant operating sound of the ABS system.

FIG. 3 shows the control system according to the first embodiment of the present invention.

The control unit 50 includes a wheel speed input circuit 4 for processing the signals from the wheel speed sensors 51 54 . Signals having processed waveforms are sent from the input circuit 4 to a microcomputer 1 .

A PMW signal line 21 is a line through which a PMW signal of a PWM duty ratio with a predetermined frequency is sent from the microcomputer 1 to a PWM circuit 5 . Furthermore, there extend, from the microcomputer 1 to the PCM circuit 5 , a pair of PWM switch signal line 33 a and solenoid signal line 33 b for operating the pressure increasing solenoid valve 33 , a pair of PWM switch signal line 34 a and solenoid signal line 34 b for operating the pressure increasing solenoid valve 34 , a pair of PWM switch signal line 35 a and solenoid signal line 35 b for operating the pressure increasing solenoid valve 35 , and a pair of PWM switch signal line 36 a and solenoid signal line 36 b for operating the pressure increasing solenoid valve 36 .

The PWM circuit 5 determines the mode of the control signal (the PWM control signal or the normal ABS control signal) to be inputted to each of the pressure increasing solenoid valves 33 36 , and sends the signals to an pressure increase side solenoid valve drive circuit 7 . The pressure increase side solenoid drive circuit 7 is connected to the pressure increase side solenoid valves 33 36 , respectively, and further connected with a resistor circuit having at least one fixed resistor 22 . This fixed resistor 22 is connected with an amplifier circuit 6 having a circuit voltage source 6 a (of 5 V). The amplifier circuit 6 sends a detection signal to the microcomputer 1 . The resistance of the fixed resistor 22 is preliminarily examined at an ordinary temperature at a predetermined voltage.

The circuit shown in FIG. 3 further includes a battery 14 , a key switch 15 , an actuator relay 16 , a motor relay 17 , an actuator relay drive circuit 18 , a motor relay drive circuit 19 , a fail-safe circuit 20 , a fault diagnostic circuit 23 and an external diagnosis connector 24 . It is optional to provide at least the fail-safe circuit 20 and the diagnostic circuit 23 in the microcomputer 1 .

FIG. 4 shows the PWM circuit 5 more in detail. The PWM circuit 5 has four circuit sections for delivering controls signals, respectively, through four output lines 33 c, 34 c, 35 c and 36 c to the pressure increase side solenoid valves 33 36 . The four circuit sections are substantially identical to one another. The PWM signal is inputted to each section through the PWM signal line 21 . The first circuit section, as an example, further receives the solenoid signal through the solenoid signal line 33 b and the PWM switch signal through the PWM switch signal line 33 a, and delivers the output signal through the output signal line 33 c. When the first solenoid signal line 33 b is High, the first PWM circuit section delivers, through the first output signal line 33 c to the first pressure increase side solenoid valve 33 , the output signal in the normal ABS ON state (the hold state without the PWM control). When the solenoid signal line 33 b is Low and the PWM switch signal line 33 a is High, the first PWM circuit section delivers the output signal in the form of the PWM control signal to the first increase side solenoid valve 33 . When the solenoid signal line 33 b is Low and the PWM switch signal line 33 a is Low, then the first PWM circuit section delivers the output signal in the OFF state to the first pressure increase side solenoid valve 33 . Input and output conditions in these three operating states are listed in FIG. 5 . The second, third and fourth PWM circuit sections are operated in the same manner. In the example shown in FIG. 4 , each PWM circuit section includes an AND subsection having a first input receiving the PWM signal through the line 21 , a second input receiving the switch signal through the line 33 a 36 a, and an output, and an OR subsection having a first input connected with the output of the AND subsection, a second input receiving the solenoid signal through the line 33 b 36 b and an output for delivering the output signal through the line 33 c 36 c.

FIG. 6 shows the pressure increase side solenoid valve drive circuit 7 in detail. The output signal lines 33 c 36 c from the PCW circuit 5 are connected, respectively, to drive devices (or drive elements) 33 d, 34 d, 35 d and 36 d for driving the pressure increase side solenoid valves 33 36 . In this example, the fixed resistor 22 is connected between the drive device 33 d and a ground. In the example depicted in FIG. 6 , each drive device is a transistor having a control terminal (such as a base electrode) connected with the corresponding output signal line 33 c 36 c, a first terminal (such as a collector electrode) connected to the corresponding solenoid valve, and a second terminal (such as an emitter electrode) grounded (through the fixed resistor 22 in the case of the first drive device 33 d ).

FIG. 7 is a flowchart showing a basic flow of the ABS control process performed by a cycle of 10 msec. After a an initialization following a start of this control process, a step S 1 is reached. At the step S 1 , the control unit 50 (or the microcomputer 1 of the control unit 50 ) calculates the wheel speed Vw of each wheel by determining the sensor signal frequency from the number of pulses and period of the sensor signal from the corresponding one of the wheel speed sensors 51 54 , and further calculates a wheel acceleration/deceleration Vw of each wheel. The wheel acceleration/deceleration Vw is a time rate of change of the wheel speed. In this example, the wheel acceleration/deceleration Vw is an amount of change of the wheel speed Vw during a predetermined time interval.

At a step S 2 following the step S 1 , a pseudo vehicle body speed VI is determined by a select-high operation of changes in the wheel speeds Vw, or from a longitudinal acceleration sensed by a longitudinal acceleration sensor. A next step S 3 is for determining a vehicle deceleration VI from current and previous values of the pseudo body speed VI. A step S 4 is for determining a pressure decrease threshold B for use in the ABS control.

Then, the wheel speed of each wheel is checked and the brake fluid pressure for each wheel is controlled in the following manner. At a step S 5 following the step SS 4 , the control unit 50 checks whether the wheel speed Vw of each wheel is lower than the threshold B. If Vw< B, the control unit 50 proceeds to a step S 7 for a pressure decrease operation. If Vw B, the control unit 50 proceeds to a step S 6 to further check whether the wheel acceleration/deceleration Vw is lower than a predetermined hold level. If Vw<the hold level, the control unit 50 proceeds to a step S 9 for a pressure hold operation. If Vw the hold level, the control unit 50 proceeds to a step S 8 for a pressure increase operation. Thereafter, the control unit 50 checks at a step S 10 whether 10 msec has elapsed. After the elapse of 10 msec, the control unit 10 returns to the step S 1 .

FIG. 8 shows the brake fluid pressure hold control of the step S 9 in detail. First at a step S 91 , the control unit 50 checks whether this operation is the first hold operation after the start of the ABS control. The control unit 50 proceeds to a step S 92 in the case of the first hold operation, and to a step S 96 in the case of a second or subsequent hold operation.

At the step S 92 , the control unit 50 reads the resistance value of the fixed resistor 22 and the battery voltage. Then, at a step S 93 , the control unit 50 determines the value I of a current flowing through the fixed resistor 22 , and checks the current I. From the step S 93 , the control unit 50 proceeds to a step S 94 if I 1 I I 2 , and to a step S 97 if I<I 1 or I 2 <I. The step S 93 is for deciding whether to perform the PWM control by checking the current I through the fixed resistor 22 . The current values I 1 and I 2 are limit values for defining a region in which the PWM control is carried out.

At the step S 94 , the control unit 50 starts the PWM control, sets a PWM flag to one, and sets the PWM switch signal to Low (the low state). At a next step S 95 , a hold signal is inputted, and the solenoid signal is set to High (the high state).

At the step S 96 reached from the step S 91 in the case of a second or subsequent hold operation, the control unit 50 checks the PWM flag, and proceeds to the step S 97 if the PWM flag is zero and to the step S 95 if the PWM flag 1.

At the step S 97 , the control unit 50 performs the normal ABS control, resets the PWM flag to zero, and sets the PWM switch signal to Low.

In this brake fluid pressure hold control of the step S 9 , either of the flow from S 94 to S 95 and the flow from S 97 to S 95 sets the PWM signal to Low and the solenoid signal to High, and the corresponding one of the pressure increase side solenoid valves 33 36 operates in the hold state.

FIG. 9 shows another example of the pressure hold control of the step S 9 . Steps S 93 , S 94 , S 95 and S 97 are substantially identical to the corresponding steps of FIG. 8 .

In the example of FIG. 9 unlike FIG. 8 , the resistance value of the fixed resistor 22 and the battery voltage are read at a step S 101 irrespective of whether the first hold operation or not. This configuration can switch precisely the control mode during the ABS control between the mode with the PWM control and the mode without the PWM control, and thereby achieve both reduction of operating noise and satisfactory ABS performance.

FIG. 10 shows the brake fluid pressure increase control of the step S 8 shown in FIG. 7 .

At a step S 81 , the control unit 50 examines whether the PWM flag is equal to zero, and proceeds to a step S 82 to perform a PWM control mode pressure increase operation if the PWM flag is not equal to zero. At the step S 82 , the control unit 50 first sets the solenoid signal to Low for the T 1 pressure increase, and then sets the PWM signal to High for the T 2 pressure increase. If the PWM flag is equal to zero, the control unit 50 proceeds from the step S 81 to a step S 83 . At the step S 83 , the control unit 50 performs a normal ABS pressure increase operation and sets the PWM switch signal to Low. The PWM flag is reset to zero in the initializing section shown in FIG. 7 or at the end of the ABS control.

In the first embodiment, as shown in FIG. 3 , there is provided only one of the PWM signal port, connected with the PWM signal line 21 , for outputting the PWM signal based on the duty ratio. The microcomputer 1 is configured to deliver the PMW signal of a single duty ratio from this single port. There is no need for the control for continuously varying the PWM, so that an uncostly computer can be used as the microcomputer 1 .

Despite the simplified arrangement for producing only one PWM signal, the control system according to this embodiment carries out the PWM control only in the region as shown in FIG. 15 where the PWM control is sufficiently effective in preventing pulsation and reducing noises and vibrations. The PWM control is not performed outside this effective region. Therefore, the control system can reduce pulsation, noises and vibrations sufficiently without performing the continuous control for continuously controlling the PWM in accordance with the temperature.

FIG. 12 shows a brake fluid pressure control system according to a second embodiment of the present invention. In this control system, there are provided flywheel circuits 73 , 74 , 75 and 76 each of which is connected in parallel to the coil of a unique one of the pressure increase side solenoid valves 33 36 . The flywheel circuits 73 76 function to reduce the occurrence of counter electromotive force in an ON to Off transition in the solenoid valves 33 36 . As a result, the control system according to the second embodiment can improve the accuracy in the PWM control (improve the ability of controlling the current value accurately to the desired value, that is). In other respects, the control system of FIG. 12 is substantially identical to the system of FIG. 3 . Each flywheel circuit includes a diode as shown in FIG. 12 .

FIGS. 13 and 14 show a control process performed by a brake pressure control system according to a third embodiment of the present invention. In the third embodiment, the control system is arranged to monitor a differential pressure (or pressure difference) between the fluid pressure of the master cylinder 3 and the fluid pressure in each of the wheel cylinders 9 12 , instead of the temperature (or the current I of the fixed resistor 22 ), to switch the control mode in the brake pressure hold control in the step S 9 between the modes with and without PWM.

As the results of investigation, the inventors of the present application has gained knowledge that the PWM control is influenced by the differential pressure between the master cylinder pressure and wheel cylinder pressure as well as the temperature of the solenoid. When the differential pressure is greater than a predetermined pressure, solenoid valves tend to be inaccurate and insufficient for intended control target and prevention of pulsation. When the differential pressure is smaller than a predetermined pressure, solenoid valves of some types could be inaccurate and deficient for the PWM control.

FIG. 15 shows a PWM control region in which satisfactory PWM control performance can be expected. In this example, the PWM control region is bounded between a lower limit pressure P 1 and an upper limit pressure P 2 and between an upper limit temperature A 1 and a lower limit temperature A 2 . When the differential pressure becomes greater than the upper limit pressure P 2 in hard braking, it is difficult to accurately control the valve opening degree to the desired opening degree by controlling the control current with the PWM control, so that the suppression of undesired pulsation is insufficient. FIG. 16 is a table showing numerical examples of the pressure range of P 1 P 2 , obtained by experiments of the inventors. FIG. 16 shows values of the master cylinder pressure and wheel cylinder pressure in braking on a road covered with ice and a road covered with pressed snow. As known from these figures, the reduction of noises and vibrations is demanded only in a limited region.

As shown in FIG. 13 , the control unit 50 calculates an estimated wheel cylinder pressure Pwc at a step S 304 , and then calculates the differential pressure Ps ( Pmc Pwc) between the master cylinder pressure Pmc and the wheel cylinder pressure Pwc at a step S 305 . At a step S 306 following the step S 305 , the control unit 50 checks the differential pressure Ps to determine whether Ps is within a range between a predetermined first pressure Ps 1 and a predetermined second pressure Ps 2 . From the step S 306 , the control unit 50 proceeds to a step S 307 if Ps 1 Ps Ps 2 , and to a step S 309 if Ps<Ps 1 or Ps>Ps 2 . The pressures Ps 1 and Ps 2 serve as lower and upper limits, such as P 1 and P 2 , for defining the region of satisfactory PWM performance.

At the step S 307 , the control unit 50 starts the PWM control, sets the PWM flag to one, and sets the PWM switch signal to Low. At a next step S 308 , the solenoid signal is set to High. At the step S 309 , the control unit 50 resets the PWM flag to zero, sets the PWM switch signal to Low, and then proceeds to the step S 308 .

FIG. 14 shows the estimation of the wheel cylinder pressure Pwc in the step S 304 .

At a step S 310 , the control unit 50 reads the master cylinder pressure Pmc sensed by a master cylinder pressure sensor. At a step S 311 , the control unit 50 checks the ABS control by examining whether AS 0. The control unit 50 proceeds to a step S 315 if AS 0, and to a step S 312 if As 1.

At the step S 312 , the control unit 50 examines whether the previous control operation is a pressure decreasing operation or a pressure increasing operation, and proceeds to a step S 313 in the case of the pressure increasing operation, and to a step S 314 in the case of the pressure decreasing operation.

At the step S 313 , the control unit 50 determines the pressure increase time (time length) T 1 of the previous operation from stored data on the time T 1 . (The pressure increase time T 1 is equal to T 1 T 2 when the previous increase operation is an increase in the PWM control mode.) Furthermore, the control unit 50 sets the current master cylinder pressure Pmc as P 1 , and the estimated wheel cylinder pressure Pwc as P 2 . Then, at a step S 316 , the control unit 50 calculates a per-unit-time inflow quantity (rate) Q according to the following equation (1). The per-unit-time inflow quantity Q is an amount of fluid flowing into the corresponding wheel cylinder ( 9 12 ) per unit time (10 msec) by the current differential pressure. In the pressure increase operation, the brake fluid flows into the wheel cylinder ( 9 12 ) by the differential pressure between the pressure Pmc of the master cylinder 3 as the pressure source and the pressure of the wheel cylinder. Therefore, the control unit 50 can calculate the amount of flow during a predetermined unit time interval from the current differential pressure between the master cylinder and the (estimated) wheel cylinder. Q = C ( d 2 / 4 ) [ ( 2 / ) ( P1 - P2 ) 1 / 2 ] = C ( P1 - P2 ) 1 / 2 ( 1 )

In this equation, C is a flow coefficient, d is an orifice diameter and is the density of the brake fluid. The orifice diameter is synonymous with the opening size or opening area of the pressure increase side solenoid valve ( 33 36 ). When the opening areas are different among the solenoid valves 33 36 , the orifice diameter d differ among the solenoid valves 33 36 . The quantity of the fluid flow from the master cylinder 3 to each wheel cylinder WC through the corresponding one of the solenoid valves 33 36 is dependent on the pressure difference between the master cylinder pressure Pmc and the wheel cylinder pressure Pwc, and the quantity of the flow increases as the pressure difference increases. Therefore, the flow quantity Q in this example is a function of the differential pressure (P 1 P 2 ), as expressed by the equation (1).

At a next step S 318 , the control unit 50 calculates a per-unit-time pressure increase quantity P 1 O per unit time, from the per-unit-time flow quantity Q and the previous estimated wheel cylinder pressure Pwc(old) by using a characteristic between the wheel cylinder pressure and the fluid flow quantity which is determined experimentally and stored as data. According to this characteristic, the control unit 50 calculates a wheel cylinder pressure Paft resulting from the inflow of the quantity Q into the wheel cylinder having the estimated wheel cylinder pressure Pwc(old). Then, the pressure increase quantity P 10 is determined from the difference between Paft and Pwc(old) as shown in the step S 318 . If the driver decreases the brake pedal depressing force during the ABS control, then the master cylinder pressure Pmc becomes lower than the wheel cylinder pressure Pwc, so that the inflow quantity Q becomes negative, and hence the pressure increase quantity P 10 becomes negative too. Then, the control unit 50 determines a pressure increase quantity P by multiplying P 10 by a quotient obtained by dividing the pressure increase time T 1 by the control cycle (10 msec).

Then, at a next step S 320 , the current estimated wheel cylinder pressure Pwc is determined from the previous estimated wheel cylinder pressure Pwc(old) and P. Pwc Pwc(previous) P.

The step S 314 is reached from the step S 312 when the previous control operation is a pressure decrease operation. At the step S 314 , the control unit 50 determines the pressure decrease time (length) T 2 of the previous operation from stored data on the time T 2 . Furthermore, the control unit 50 sets the estimated wheel cylinder pressure Pwc as P 1 , and a reservoir fluid pressure as P 2 . In this example, the reservoir fluid pressure P 2 is equal to a predetermined value which is equal to 0 Kgf/cm 2 .

Then, at a step S 317 , the control unit 50 calculates the per-unit-time fluid flow quantity Q according to the before-mentioned equation (1). The per-unit-time flow quantity Q is an amount of flow into or out of the wheel cylinder per unit time (10 msec) by the current differential pressure. In the pressure decrease operation, the brake fluid flows out of the wheel cylinder ( 9 12 ) by the differential pressure between the wheel cylinder pressure Pwc and the reservoir pressure Pr of the reservoir 55 or 56 . Therefore, the control unit 50 can calculate the amount of flow during a predetermined unit time interval from the current differential pressure.

The orifice diameter d is synonymous with the opening size or opening area of the outflow control solenoid valve ( 43 46 ). When the opening areas are different among the solenoid valves 43 46 , the orifice diameter d differ among the solenoid valves 43 46 . The quantity of the fluid flow in the pressure decrease operation from each of the wheel cylinders 9 12 to the reservoir 55 or 56 through the corresponding one of the solenoid valves 43 46 is dependent on the pressure difference between the wheel cylinder pressure Pwc and the reservoir pressure Pr. Therefore, the per-unit-time outflow quantity Q in this example is the function of the differential pressure (P 1 P 2 ), as expressed by the equation (1).

At a next step S 319 , the control unit 50 calculates a per-unit-time pressure decrease quantity P 10 per unit time, from the per-unit-time outflow quantity Q determined at the step 317 and the previous estimated wheel cylinder pressure Pwc(old) by using a characteristic between the wheel cylinder pressure and the flow quantity which is determined experimentally and stored as data. In this example, the same characteristic between the wheel cylinder pressure and flow quantity is used both in the steps S 318 and S 319 . According to this characteristic, the control unit 50 calculates a wheel cylinder pressure Paft resulting from the outflow of the quantity Q from the wheel cylinder having the estimated wheel cylinder pressure Pwc(old). Then, the per-unit-time pressure decrease quantity P 10 is determined from the difference between Paft and Pwc(old) as shown in the step S 319 . Then, the control unit 50 determines a pressure decrease quantity P by multiplying P 10 by T 2 over the control cycle.

Then, at the step S 320 , the current estimated wheel cylinder pressure Pwc is determined from the previous estimated wheel cylinder pressure Pwc(old) and P. Pwc Pwc(old) P.

By performing the PWM control only in the region as shown in FIG. 15 where the PWM control is sufficiently effective in preventing pulsation and reducing noises and vibrations, the control system according to the third embodiment can achieve the intended effects by using the computer 1 of a simple construction. In the present invention, it is possible to employ the first and third embodiments independently and individually, or employ both of the first and third embodiments in combination.

It is possible to use a fluid pressure sensor for sensing the wheel cylinder pressure of each wheel cylinder. The above-mentioned estimation of the wheel cylinder pressure can eliminate the need for such fluid pressure sensors.

The present invention is not limited to the illustrated embodiments. Various variations and modifications are possible in the present invention.

The present invention is applicable to a brake fluid pressure control system for a vehicle traction control for preventing drive wheel slip or acceleration slip or a vehicle motion control system for producing a yaw moment of the vehicle by controlling braking forces applied on wheels to restrain excessive oversteering or understeering during cornering operation in accordance with a vehicle running condition sensed by vehicle motion sensors such as wheel speed sensors, longitudinal acceleration sensor, lateral acceleration sensor, steering angle sensor, a yaw sensor and a yaw rate sensor.

FIG. 18 schematically shows one possible structure of a control system according to the present invention. This control system includes an output section 100 b having a brake actuator or modulator unit such as the item 60 shown in FIG. 1 , an input section for collecting input information on vehicle operating conditions needed for the control, and a control unit 100 e. The brake actuator unit of the output section 100 b includes a plurality of solenoid valves 100 a such as the solenoid valves 33 36 and 43 45 for regulating brake fluid pressure. The input section includes at least one vehicle operating condition sensor 100 c (such as the wheel speed sensors 51 54 ) for sensing a vehicle operating condition needed for the ABS control, or traction control or vehicle behavior (or motion) control. The input section may further includes a sensor or element 100 p for sensing the temperature of a solenoid of at least one solenoid valve, and/or a pressure sensor 100 q for sensing the master cylinder pressure and/or wheel cylinder pressures. The control unit may includes a drive section 100 d, a switch or switching section 100 f, and a control section 100 g. The control section g includes two or more normal signal ports (or terminals) 100 h which is equal in number to the solenoid valves 100 a, and only one PWM signal port 100 j. The switch section 100 f includes two or more switch signal ports 100 k which are equal in number to the normal signal ports 100 h. The drive section 100 d includes two or more output ports or terminal 100 m which are equal in number to the solenoid valves 100 a, and input ports for receiving signals from the ports of the control section 100 g and the switch section 100 f. In the example of FIG. 18 , the number of the solenoid valves 100 a is four, and the number of the input ports of the drive section 100 d is nine.

In the third embodiment, the characteristic between the wheel cylinder pressure and the fluid flow quantity used in the step S 319 is the same as the characteristic used in the step S 318 . In the preceding embodiments, PWM signal transmitted through the PWM signal line has a single duty ratio which is fixed at a determined constant value.