GB2293635A

GB2293635A - An anti-skid braking system

Info

Publication number: GB2293635A
Application number: GB9519703A
Authority: GB
Inventors: Naohiko Tsuru; Hiroyuki Shinkai; Katsuji Shioda; Toshiya Morikawa
Original assignee: NipponDenso Co Ltd
Current assignee: Denso Corp
Priority date: 1994-09-27
Filing date: 1995-09-27
Publication date: 1996-04-03
Also published as: DE19535591A1; GB9519703D0; CN1139057A; JPH0891193A

Description

ANTI-SKID APPARATUS 2293635 This invention relates to a brake system for

automotive vehicles, and more specifically to an anti-skid control system that prevents driveability impairment due to wheel locking in a braking operation.

In a usual brake system of a vehicle, a master cylinder connected to a brake pedal, a wheel cylinder provided in a brake mechanism of each one of the wheels of the vehicles and a brake fluid reservoir are connected by respective conduits. An antiskid system used in this type of braking system is described, for example, in the specification of Japanese Patent Examined No. Sho. 49-32494. In this example, a flow-in valve is installed in a conduit connecting a master cylinder and a wheel cylinder while a flow-out valve is installed in a conduit connecting the wheel cylinder and a reservoir. A brake fluid pressure in the wheel cylinder can be. increased or decreased by controlling the switching operation of the flowin and the flow-out valves. The brake fluid released from the wheel cylinder to the reservoir is returned to a conduit between the master cylinder and the flow-in valve by operation of a pump.

In this kind of system, there is a drawback in that the system gives a driver an unnatural feeling during the braking operation because the brake pedal is pushed back by the brake fluid when the pressure in the wheel cylinder is decreased (a socalled kickback phenomenon), or the brake pedal is unnaturally depressed when the brake fluid pressure in the wheel cylinder is increased.

Japanese Patent Laid-Open No. Sho. 61-202965 shows an anti-skid control system in which the pump is connected to the wheel cylinder and supplies the brake fluid thereto during antiskid operation. Thus, the feeling during a braking operation is improved.

As seen from Figure 1 in the aforementioned publication, three valves to control the on or off state of the brake fluid flow are required for each front wheel in this system, resulting in six valves altogether for the front wheels. This anti-skid system requiring many valves is, of course, costly and bulky in size.

This invention aims to solve these problems and supply an anti-skid control system that is small in siz.e and low in cost. The invention offers an anti-skid apparatus that can be manufactured at low cost and in a small size by reducing the number of valves for controlling the flow of the brake fluid.

TO solve the abc--e-mentioned problems, the present invention provides an anti-skid control system which includes a master cylinder connected to wheel cylinders and a reservoir via a first conduit, a switching valve in the first conduit for controlling flow of brake fluid into and out of the master cylinder into the first conduit, a flow-out valve in the first conduit for controlling flow of brake fluid into and out of the reservoir, flow-in valves in the first conduit for controlling flow of brake fluid into and out of respective wheel cylinders, and a pump connected in parallel with the flow-out valve for sending the brake fluid from the reservoir to the wheel cylinders and a second conduit as a fluid path between the wheel cylinders and the reservoir. During normal braking operations, the flowout valve is closed, the first and second flow-in valves and the switching valve are open and brake fluid is supplied to the wheel cylinders through the switching valve.

During anti-skid control, the switching valve is closed and brake fluid from the master cylinder is shut off. When pressure in the wheel cylinders must be maintained, the first and the second flow-in valves are closed and the flow-out valve is open. In this stage, the brake fluid pumped out from the pump is returned to the reservoir through the flow- out valve because the flow-out valve is connected to the pump in parallel. When brake fluid pressure increase in one of the wheel cylinders is required, brake fluid is supplied from the pump, the flow-out valve is closed and one of the flow-in valves is open. When pressure decrease in one of the wheel cylinders is required, the flow-out valve and the corresponding flow-in valve are open and brake fluid in the wheel cylinder is released therefrom.

Additionally, the system may include a one-way valve in parallel with the switching valve to provide a relief path from the pump to the master cylinder when all of the valves are closed. A relief valve may also be provided in parallel with the pump to provide a relief path when the pressure at the pump outlet is too high.

The system may also include one-way valves providing a brake fluid path between each wheel cylinder and the master cylinder to discharge fluid in the wheel cylinders to the master cylinder when a driver releases the brake pedal during an anti-skid control operation.

A second preferred embodiment of the invention uses separate flow-out valves between the reservoir and each wheel cylinder for greater flexibility and control at the expense of a slight increase in parts count. A third preferred embodiment of the invention uses a single flowin valve between the switching valve and the wheel cylinders to reduce the number of parts, cost and complexity of the system even more at the expense of a slight reduction in flexibility.

Other objects and features of the invention will appear in the course of the description thereof, which follows.

Additional objects and advantages of the present invention will be more readily apparent from the following detailed description of preferred embodiments thereof when taken together with the accompanying drawings in which:

FIG. 1 shows a brake system used in a first embodiment of the present FIG.

invention; 2 shows a brake system used in a second embodiment of the present invention; FIG. 3 shows a brake system used in a third embodiment of the present invention; and FIG. 4 shows a brake system used in a fourth embodiment of the present invention.

The presently preferred embodiments of the present invention are hereinafter described with reference to the accompanying drawings.

The first embodiment according to this invention will be explained in reference to FIG. 1. The brake system of the first embodiment includes control systems for front and rear wheels. However, FIG. 1 shows the system only for front wheels, i.e., front-right and front-left wheels. The same system as shown here for the front wheels may be used for the rear wheels.

In FIG. 1, a brake pedal 1 is connected to a master cylinder 2 having its own reservoir. When the brake fluid pressure generated in the master cylinder 2 by pressing down the brake pedal 1 is supplied to the first wheel cylinder 3 for the front-right wheel through passages described hereafter, a braking operation will be performed. In the same manner, when the brake fluid pressure generated in the master cylinder 2 is fed to the second wheel cylinder 4 for the front-left wheel through the passages described hereafter, a braking operation will be performed..

The outlet port of the master cylinder 2 and a switching valve 5 are connected by a pipe 20. A passage extending from the switching valve 5 is connected through pipes 21 - 24 to the first wheel cylinder 3 and the second wheel cylinder 4, thereby supplying the brake fluid from the master cylinder 2 to the first wheel cylinder 3 and the second wheel cylinder 4.

A first conduit is formed by pipes 20 - 24. The first conduit includes a first branch conduit 23 and a second branch conduit 24 which branch out at one end of the pipe 22. The first branch conduit 23 and the second branch conduit 24 supply the brake fluid to the first wheel cylinder 3 and the second wheel cylinder 4, respectively. In the first branch conduit 23, a first flow-in valve 9 is installed for controlling the flow of the brake fluid from the master cylinder 2 to the first wheel cylinder 3. In the same manner, a second flow-in valve 10 is installed in the second branch conduit 24 for controlling the flow of the brake fluid from the master cylinder 2 to the second wheel cylinder 4.

A reservoir 7 is connected to the junction point of pipes 21 and 22 through a pipe 25 which serves as a second conduit for supplying the brake fluid from the reservoir 7 to the wheel cylinders 3 and 4. In the pipe 25, a flow-out valve 6 is installed for controlling the flow of the brake fluid to and from the reservoir 7. A pump 8 is connected in parallel with the flow-out valve 6 by a pipe 26. The pump 8 pumps up the brake fluid from the reservoir 7 and supplies it to the wheel cylinders 3 and 4.

A one-way valve 11 is installed in parallel with the switching valve 5 by a pipe 27. This one-way valve 11 allows flow of the brake fluid only in one direction, that is, from the wheel cylinders 3 and 4 and the pump 8 to the master cylinder 2. This is done because the inlet port of the one- way valve 11 is connected to the pipe 21 and the outlet port of the one- way valve 11 to the pipe 20.

A first one-way valve 12 is connected to one end of a pipe 30 extending from the pipe 23 at a point between the first flow-in valve 9 and the first wheel cylinder 3. Another one-way valve 13 is connected to one end of a pipe 31 extending from the pipe 24 at a point between the second flow-in valve 10 and the second wheel cylinder 4. These one-way valves 12 and 13 are connected to each other at their outlet ports by a pipe 32 which is further connected to the master cylinder 2 by a pipe 33 and the pipe 20. The one-way valves 12 and 13 thus connected allow the brake fluid to flow from the wheel cylinders 3 and 4, respectively. In other words, the first one-way valve 12 allows the brake fluid flow only in one direction from the pipe 30 to the pipe 32. In the same way, the second one-way valve 13 allows - 7 the brake fluid flow only in one direction from the pipe 31 to the pipe 32.

All the aforementioned valves, i.e., the switching valve 5, the flow-out valve 6, and the first and second flow-in valves 9 and 10, are 2-port, 2position valves. The valve body in each respective valve is operated by an electric solenoid, which is not shown in the drawing, and assumes two positions, on or off. When the electric solenoid is not energized, each valve assumes the respective position shown in FIG. 1. In place of the valves operated by the electric solenoids as mentioned above, mechanical valves may also be used in this system.

The first embodiment of this invention as shown in FIG. 1 operates as follows. On or off positions of the valves, i.e., the switching valve 5, the flow-out valve 6, the first and second flow-in valves 9 and 10, are shown in TABLE I for each operating mode.

Mode Cyl. 3 Cyl. 4 Switch. Flow-In Flow-In Flow Mode 1 Mode s 9 10 Out 6 A Normal Braking 0 0 0 X B Maintain Maintain X X X 0 c Maintain Decrease X X 0 0 D Decrease Maintain X 0 X 0 E Decrease Decrease X 0 0 0 F Increase Maintain X 0 X X Maintain Increase X X 0 X Reservoir Full X / 0 X X (0 = Open valve, X = Closed valve) TABLE I

8 - In a normal braking stage, all these valves 5, 6, 9, and 10 take the positions as shown in FIG. 1. That is, as shown in Mode A in FIG. 2, the switching valve 5 makes a path between the pipes 20 and 21, the flow-out valve 6 cuts off a path in the pipe 25, and the first and second flow-in valves 9 and 10 make paths in the pipes 23 and 24, respectively. Therefore, the brake fluid pressure generated in the master cylinder 2 by pressing down the brake pedal 1 is transferred to the first and second wheel cylinders 3 and 4 through a passage formed by pipes 20 24, the first and the second flow-in valves 9 and 10.

When the wheels are about to be locked, which is detected by sensors (not shown in the Figure) based on certain relations between wheel speeds and vehicle speeds which are wellknown in the art, anti-skid operation will be performed to avoid wheel locking and to maintain a slip ratio at an optimum value. During anti-skid operation, the brake fluid pressure to be imposed on the first and the second wheel cylinders 3 and 4 is controlled as shown in Modes B to G in TABLE I in accordance with respective antiskid control modes, i.e., pressure maintenance, increase and decrease. Operations of respective valves 5, 6, 9 and 10 being controlled in these anti-skid control modes will be explained below. In all of the following modes, the pump 8 starts working at the same time as the anti-skid control operation begins so as to draw in or pump out the brake fluid to or from the reservoir 7.

In the pressure maintenance mode for keeping the brake fluid pressure in the wheel cylinders 3 and 4 constant, as shown in Mode B in TABLE I, the switching valve 5 and the first and the second flow-in valves 9 and 10 are closed while the flow-out valve 6 is open. The switching valve 5 is always closed during the anti-skid operation in order to shut off the brake fluid flow from the master cylinder 2. In this mode, the brake fluid pumped out from the pump 8 can be recirculated to the reservoir 7 through the flow-out valve 6 which is open. This recirculation passage is formed by the parallel connection of the pump 8 with the flow-out valve 6 through pipes 25 and 26, thereby avoiding pressure accumulation due to the pumped out fluid from the pump 8 in the passages or in the master cylinder 2 during the pressure maintenance mode.

In the pressure decrease mode for decreasing the brake f luid pressure in both wheel cylinders 3 and 4, as shown in Mode E in FIG. 2, the brake fluid pressure in the first and the second wheel cylinders 3 and 4 is returned to the reservoir 7 through the passage formed by the first and second flow-in valves 9 and 10 and the flow-out valve 6, which are all open, and the pipes 22 - 25. Though the pump 8 is operated to pump out the brake fluid from reservoir 7 in this mode, the pressure in the wheel cylinders 3 and 4 is invariably released because the releasing speed is much higher than the pumping out speed. This pressure decrease in the wheel cylinders is performed when the slip ratio of the wheels becomes high such as in a locking state, thereby effecting recovery of the wheel speed.

Next, the anti-skid control operation in first alternate modes shown by Modes C and D of TABLE I will be - explained. In these modes, the brake fluid pressure in one of the wheel cylinders 3 and 4 is maintained while the pressure in the other is decreased. As seen from Modes C and D in TABLE I, the flow-out valve 6 is open in either case. As for the first and the second flow-in valves 9 and 10, the valve corresponding to the wheel cylinder in which the pressure is to be maintained is closed and the valve corresponding to the wheel cylinder in which the pressure is to be decreased is open. When the pressure in the first wheel cylinder 3 is maintained and the pressure in the second wheel cylinder 4 is decreased, the first flow-in valve 9 is closed and the second flow-in valve 10 is open. In the same manner, when the pressure in the first wheel cylinder 3 is decreased and the pressure in the second wheel cylinder 4 is maintained, then the first flow-in valve 9 is open and the second flow-in valve 10 is closed. The anti-skid control operation in these alternate modes is necessary when road conditions under the right and left wheels are different and the slip ratios or braking conditions of the two wheels are not uniform. In this case, the pressure in each wheel cylinder is controlled differently so that the braking condition for each of the right and left wheels is kept optimum.

Further, the anti-skid control operation in second alternate modes will be discussed. When road conditions under the right and left wheels are not uniform and friction between the wheels and the road is large, the anti-skid control operations described in Modes F and G in TABLE I may be required. In these second alternate modes, the brake fluid pressure in one 11 - of the wheel cylinders 3 and 4 is maintained while the pressure in the other is increased. As seen in Modes F and G in TABLE I, the flow-out valve 6 is closed in either case, and the one of the first and second flow-in valves 9 and 10 corresponding to the wheel cylinder in which the brake fluid pressure is maintained is closed and the other flow-in valve corresponding to the wheel cylinder in which the pressure is required to be increased is open. When the pressure in the f irst wheel cylinder 3 is increased and the pressure in the second wheel cylinder 4 is maintained, the first flow-in valve 9 is open and the second flow-in valve 10 is closed. When the pressure in the first wheel cylinder 3 is maintained and the pressure in the second wheel cylinder 4 is increased, the first flowin valve is closed and the second flow-in valve is open.

As explained above, the first and the second wheel cylinders 3 and 4 can be controlled independently in different modes because each of the wheel cylinders has its own flow-in valve, i.e., valve 9 or 10.

As shown in TABLE I, all the valves 5, 6, 9 and 10 are not closed at the same time during the normal anti-skid control operation. However, there may be a case where the all valves 5, 6, 9 and 10 are closed simultaneously for a transient period due to some mechanical delays or signal errors from a control computer (not shown) In this case, if the one-way valve 11 were not installed, the brake fluid pressure in the pipes 21 - 26 might exceed a maximum permissible value due to the fluid pumped out from the pump 8 and cause damage in the pressurized passages 12 - in the system. To prevent such possible damage, the one-way valve 11 is installed in parallel with the switching valve 5. The brake fluid can therefore bypass the switching valve 5 through the one-way valve 11 from the pipe 21 to the pipe 22, thereby reaching the master cylinder 2.

However, this one-way valve 11 may not be always necessary. If the mechanical strength of the fluid pressure system is high enough or the computer control system is designed not to cause such moments where all the valves are closed simultaneously, then the one-way valve 11 may be eliminated from the system.

Next, the operation of the first and the second one-way valves 12 and 13 will be explained. As mentioned above, the first and the second one-way valves 12 and 13 are connected to the rest of the system through the pipes 31 - 33. The anti-skid operation begins when the brake pedal 1 is depressed by a driver and the wheels tend to be locked. When the driver releases the brake pedal 1 during the anti-skid operation, the pressure in the wheel cylinders 3 and 4 has to be decreased corresponding to the driver's intent. If the pressure in the wheel cylinders 3 and 4 continues to be increased under the anti-skid control operation even the driver releases the brake pedal 1, the driver may feel awkward. In order to eliminate this awkward feeling, the pressure in the wheel cylinder 3 and 4 must be decreased when the pressure in the master cylinder 2 is decreased as a result of releasing the brake pedal 1 by the driver. For this purpose, the one-way valves 12 and 13 are installed in the passage between the 13 pipes 23 and 24 and the pipe 22 through the pipes 30 - 33. The brake fluid pressure in the wheel cylinders 3 and 4 is transferred to the master cylinder 2 through the one-way valves 12 and 13. As a result, the driver can feel a returning motion of the brake pedal 1. The brake fluid pressure in the master cylinder 2 is not transferred to the wheel cylinders 3 and 4 when the driver presses down the brake pedal 1 during the anti-skid operation, because the one-way valves 12 and 13 permit the fluid flow only in the direction from the pipes 30 and 31 to the pipes 32 and 33. In addition, the one-way valves 12 and 13 are respectively installed for each wheel cylinder 3 and 4. Therefore, fluid communication between pipes 23 and 24, and between pipes 32 and 33 is not permitted. If the fluid communication were permitted therebetween, independent control for each of the wheel cylinders 3 and 4 would not be attained.

In case the system is designed to sense the brake pedal 1 release by a driver and to open the valves 5, 9 and 10 to bring the system to the normal braking operation, the one-way valves 12 and 13 above mentioned can be eliminated.

Advantages of the anti-skid apparatus according to the above-mentioned embodiment will be explained. In this apparatus, the brake fluid return passage for returning the fluid pumped out from the pump 8 to the reservoir 7 is installed and brake fluid flow control can be performed in the same pipe 22 both for increasing and decreasing the pressure in the wheel cylinders 3 and 4. Therefore, the number of solenoid valves required for controlling the front-right and front-left wheels is four, compared with six in the conventional apparatus. This means two solenoid valves can be eliminated and, thereby, the apparatus can be made low in cost and small in size. Further, in the apparatus, the switching valve 5 is closed whenever the anti-skid control operation begins and cuts out the flow of the brake fluid from the master cylinder 2 during the anti- skid operation. Therefore, it is not necessary for the pump 8 to generate a higher fluid pressure to overcome the pressure generated in the master cylinder 2. This means a smaller pump with less output can be used in the apparatus, thus achieving low cost.

Now, a second embodiment as shown in FIG. 2 will be explained. Components or parts which perform the same function as in the first embodiment are given the same numbers as in FIG. 1 and detailed explanation thereof will not be made here. FIG. 2 shows the braking system only for front wheels as in the FIG. 1 for the first embodiment.

In FIG. 2, the switching valve 5 is connected to the master cylinder 2 through the pipe 20 and the one-way valve 11 is connected in parallel with the switching valve 5. The outlet port of the pump 8 is connected to the switching valve 5 through a pipe 60 and the pipe 21. The outlet port of the pump 8 is further connected to the first and second flow-in valves 9 and 10 through the pipes 60, 23 and 24. The inlet port of the pump 8 is connected to the reservoir 7 through a pipe 61 and the brake fluid is supplied to the pump 8 therethrough. The branch conduits 23 and 24 extend from the pipe 21 and are connected to the first and the second flow-in valves 9 and 10, respectively.

- 15 The flow-in valves 9, 10 control the flow of the brake fluid between the wheel cylinders 3 and 4 and the master cylinder 2. In the same manner as in the first embodiment, the one-way valves 12 and 13 are installed between the flow-in valves 9 and 10 and the master cylinder 2 via the pipes 20 and 30 - 32.

A first flow-out valve 50 is installed between the first wheel cylinder 3 and the reservoir 7 through a pipe 62 and the pipe 61. The first flow-out valve 50 performs switching of the fluid flow from the first wheel cylinder 3 and the reservoir 7. A second flow-out valve 51 is installed between the second wheel cylinder 4 and the reservoir 7 through a pipe 63 and the pipe 61. The second flow-out valve 51 performs switching of the fluid flow from the second wheel cylinder 4 and the reservoir 7.

Valve positions to effect the various pressure control modes are shown below in TABLE II.

Mode Cyl. 3 Cyl. 4 Switch F-I F-I F-0 F-0 Mode [ Mode 5 9 10 50 51 A Normal Braking 0 0 0 X X B Maintain Maintain X X X X X c Maintain Decrease X X X X 0 D Decrease Maintain X X X 0 X E Decrease Decrease X X X 0 0 F Increase Maintain X 0 X X X G Maintain Increase X X 0 X X r -E Reservoir Full X / 0 X X X X (0 = open valve, X = Close37vaI-v-e) TABLE II

The brake fluid pressure in the first and the second wheel cylinders 3 and 4 are independently controlled by the first - 16 - and the second flow-in valves 9 and 10 and the first and the second flow- out valves 50 and 51. In other words, the pressure in the first wheel cylinder 3 is controlled by the first flowin valve 9 and the first flow- out valve 50. In the same way, the pressure in the second wheel cylinder 4 is controlled by the second flow-in valve 10 and the second flow-out valve 51. In this embodiment, the first and second flow-out valves 50 and 51 are closed when the corresponding wheel cylinder is in the pressure maintenance mode or pressure increase mode, while the flow-out valves 50 and 51 are open when the corresponding wheel cylinder is in the pressure decrease mode. In the pressure maintenance mode, all the valves 5, 9, 10, 50 and 51 are closed and the brake fluid pumped out by the pump 8 is returned to the master cylinder 2 through the one-way valve 11. In the second embodiment, the number of the solenoid valves required is five for the control of the front wheels as opposed to six in the conventional system. If the same control system is applied both for front and rear wheels, the number of the solenoid valves to be saved in this embodiment is two. Therefore, the apparatus can be made small in size and low in cost.

This invention is not limited to the embodiments described above. other forms of the invention are possible as described below. For example, the one-way valve 11 in the first embodiment shown in FIG. 1 can be replaced by a relief valve lla as shown in FIG. 3. The relief valve lla performs a similar Junction as the one-way valve 11 in the first embodiment. The relief valve lla is connected in parallel with the pump 8 through 17 - a pipe 27a and allows the flow of the brake fluid only in the direction from the outlet port to the inlet port of the pump 8 when the fluid pressure at the outlet port of the pump 8 exceeds a predetermined value. If the brake fluid pressure in passages becomes excessively high due to the fluid pumped out from the pump 8 when all the valves, i.e., the switching valve 5, the first and second flow-in valves 9 and 10 and the flow-out valve 6, are closed, then the fluid flows from the outlet port to the inlet port of the pump 8 through the relief valve lla. The pipe 27a is not necessarily connected as shown in FIG. 3. For example, one end of the pipe 27a connected to the outlet port of the pump may be instead connected to any one of the pipes 21, 22 or 25 or the pump-side portions of pipes 23 and 24. The other end of the pipe 2-7a connected to the inlet port of the pump 8 may be connected to the pipe 25 between the flow-out valve 6 and the reservoir 7.

In the above mentioned embodiments, one control system controls a set of wheels, a front-right wheel and a front-left wheel, in which each of the wheels is independently controlled. This type of the system is called a two-channel system. In this case, another two-channel system controls a set of rear wheels. In other words, the system as a whole including a front system and a rear system is called a four-channel system. However, the set of wheels such as a front-right wheel and a front-left wheel can be changed to a different combination. That is, a set of wheels consisting of a front-right and a rear-left wheel and another set of wheels consisting of a front-left and a rear-right 18 - wheel can be made. This type of arrangement is called an Xarrangement. This invention can be used for the X-arrangement, too.

Further, this invention can be applied to a threechannel system which includes a two channel system for the front wheels and a one-channel system for rear wheels (the one-channel system will be explained later using FIG. 4). In this application, the first or second aforementioned embodiment may be used as the two channel system for the front wheels and the embodiment shown in FIG. 4 as the one-channel system for the rear wheels.

An anti-skid control apparatus shown in FIG. 4 controls a set of wheels, e.g., a right wheel and a left wheel, in the same manner and simultaneously as opposed to the independent control mentioned before in the first and second embodiments.

In this embodiment, a flow-in valve 9a controls the brake fluid pressure both in the right and left wheel cylinders 3 and 4. A one-way valve 12a is installed between the wheel cylinders 3 and 4 and the master cylinder 2 through a pipe 100 so that the pressure in both wheel cylinders 3 and 4 can be released to the master cylinder 2. The number of solenoid valves used in the apparatus shown in FIG. 4 is three as is apparent from theFigure and the number of the one-way valves is two. This means that one solenoid valve and one one-way valve are saved compared with the first embodiment shown in FIG. 1. AS another application, separate one-channel systems as shown in FIG. 4 may be used for each of the front wheels and rear wheels, - 19 thus constituting a two channel system as a whole.

Further, the first and second embodiments may be modified as follows. Each of valves 5, 6, 9 and 10 may be controlled as shown in Mode H in TABLE I, so that the brake fluid returns from the reservoir 7, which has been filled with the brake fluid released from the wheel cylinders 3 and 4 during the pressure decrease mode in the anti-skid control operation, to the master cylinder 2. As seen in Mode H in TABLE I, the switching valve 5 is brought to the on- or off-position intermittently while the other three valves 6, 9, 10 are all closed (offposition). In this modification, however, a sensor for sensing the fluid level in the reservoir 7 has to be provided.

Although the present invention has been fully described in connection with the preferred embodiment thereof with reference to the accompanying drawings, it is to be noted that various changes and modifications will become apparent to those skilled in the art. Such changes and modifications are to be understood as being included within the scope of the present invention as defined by the appended claims.

Claims

1. An anti-skid apparatus comprising: a master cylinder for generating brake fluid pressure responsive to depression of a brake pedal; a first conduit having first and second branch conduits for supplying brake fluid pressure from said master cylinder to first and second wheel cylinders, respectively; a switching valve, connected between said master cylinder and a junction point of said first and second branch conduits, for selectively supplying and cutting-off brake fluid from said master cylinder; a first flow-in valve, in said first branch conduit, for selectively supplying and cutting-off brake fluid to said first wheel cylinder; a second flow-in valve, in said second branch conduit, for selectively supplying and cutting-off brake fluid to said second wheel cylinder; a second conduit, extending from said junction point of said first and second branch conduits and connected to a brake fluid reservoir; a flow-out valve, in said second conduit, for selectively supplying and cutting-off brake fluid in said second conduit; and a pump, in parallel with said flow-out valve, for sending brake fluid to said first and second wheel cylinders.

- 21

2.

An anti-skid apparatus according to claim 1, further comprising a one-way valve in parallel with said switching valve, so that brake fluid flows therethrough only from the MP side of said switching valve to said master cylinder.

3. An anti-skid apparatus according to claim 1 or claim 2, further comprising a relief valve, in parallel with said pump, so that brake fluid flows therethrough only from an outlet side of said pump to an inlet side of said pump when brake fluid pressure at said outlet side of said pump exceeds a predetermined value.

4. An anti-skid apparatus according to claim 1, 2 or 3, further comprising a relief valve in a passage connecting an area encircled by said switching valve, said flow-out valve, said first flow-in valve and said second flow-in valve at an outlet side of said pump and another area encircled by said reservoir and said flow-out valve at an inlet side of said pump, so that brake fluid flows therethrough only from said outlet side of said pump to said inlet side of said pump when brake fluid pressure at said outlet side of said pump exceeds a predetermined value.

5. An anti-skid apparatus according to any ing claim, further comprising:

a third conduit connecting a point between said first flow-in valve and said first wheel cylinder in said first branch conduit and another point between said master cylinder and said switching valve in said first conduit; a fourth conduit connecting a point between said second flow-in valve and said second wheel cylinder in said second branch conduit and another point between said master cylinder and said switching valve in said first conduit; a first one-way valve, in said third conduit, so that brake fluid flows therethrough only from said first wheel cylinder to said master cylinder; and a second one-way valve, in said fourth conduit, so that brake fluid flows only from said second wheel cylinder to said master cylinder.

6. An anti-skid apparatus according to claim 5, further comprising a common conduit, in a part of said third and fourth conduits, connecting one of said first and second one-way valves and said another point between said switching valve and said master cylinder.

An anti-skid apparatus accDrdino to any precedinq claim, further comprising a recirculation circuit connecting said pump, said flow-out valve and said reservoir so that brake fluid pumped out from said pump can be returned to said reservoir through said flow-out valve in an on-position when said switching valve and said first and second flow-in valves are all in an off-position simultaneously.

8. An anti-skid apparatus accOrding tO anY preceding claim,.1n:

- 23 pressurized brake fluid is supplied via a fluid supply path to one of said first and second wheel cylinders through one of said first and second flow-in valves, respectively, from one of said master cylinder and said pump when pressure increase in said one of said first and second wheel cylinders is required; and brake fluid is returned via a fluid return path to said reservoir from said one of said first and said second wheel cylinders through said one of said first and second flow-in valves, respectively, and said flow-out valve when pressure decrease in said one of said first and second wheel cylinders is required; and said first conduit additionally serves as one of said fluid supply path and said fluid return path.

9. An anti-skid apparatus comprising:

master cylinder for generating brake fluid pressure responsive to depression of a brake pedal; first conduit having first and second branch conduits for supplying brake fluid pressure from said master cylinder to first and second wheel cylinders, respectively; a two-port switching valve, connected between said master cylinder and a junction point of said first and second branch conduits, for selectively supplying and cutting-off brake fluid from said master cylinder; a first flow-in valve, in said first branch conduit, for selectively supplying and cutting-off brake fluid to said - 24 first wheel cylinder; a seco nd flow-in valve, in said second branch conduit, for selectively supplying and cutting-off brake fluid to said second wheel cylinder; a second conduit extending from said junction point of said first and second branch conduits to a brake fluid reservoir; a pump, in said second conduit, for sending brake fluid to said first and second wheel cylinders; a first flow-out valve for selectively opening and closing a brake fluid path between said first wheel cylinder and said reservoir; and a second flow-out valve for selectively opening and closing a path between said second wheel cylinder and said reservoir.

10. An anti-skid apparatus according to claim 9, further comprising a oneway valve in parallel with said switching valve so that brake fluid flows only from a pump side of said switching valve to said master cylinder.

11. An anti-skid apparatus according to claim 9 or claim 10, further comprising passages permitting brake fluid in said first and second wheel cylinders to be returned to said master cylinder therethrough, each of said passages having a respective one-way valve so that brake fluid flows only from said wheel cylinders to said master cylinder.

12. An anti-skid brake apparatus comprising:

master cylinder; plurality of wheel cylinders; conduit providing a brake fluid path between said master cylinder and each of said plurality of wheel cylinders; a two-port switching valve disposed in said brake fluid path; a flow-in valve disposed in said brake fluid path between said switching valve and at least one of said wheel cylinders; reservoi pump having an intake port connected to said reservoir and an outlet port connected to said conduit; and a flow-out valve having a first port connected to said reservoir and a second port connected to said conduit.

13. The anti-skid brake apparatus of claim 12, wherein:

said flow-in valve is disposed in said brake fluid path between said switching valve and a first one of said wheel cylinders; and said apparatus further comprises an additional flow-in valve disposed in said brake fluid path between said switching valve and a second one of said wheel cylinders.

14. The anti-skid brake apparatus of claim 12 or claim 13, f comprising a one-way valve having a first port connected to said conduit between said flowin valve and said at least one of said wheel cylinders and a second port connected to said conduit between said master cylinder and said switching valve.

15. A method of performing anti-skid brake control, said method comprising the steps of: performing a normal braking operation by establishing masterwheel brake fluid flow path from a master cylinder to wheel cylinder using a two-port switching valve and a flow-in valve, each in an open state; performing an anti-skid braking operation in which pressure in said wheel cylinder is maintained by terminating said master- wheel brake fluid flow path using said switching valve and said flow-in valve, each in a closed state; performing an anti-skid braking operation in which pressure in said wheel cylinder is decreased by terminating said master-wheel brake fluid flow path using said switching valve in a closed state and establishing a wheel-reservoir brake fluid flow Path from said wheel cylinder to a reservoir using a flowout valve in an open state.

16. The method of claim 15, wherein said pressure maintaining step includes a step of establishing a pump-pump brake fluid flow path, from an outlet of a pump, to an inlet of said pump and a reservoir using said flow-out valve in said open state.

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17. The method of claim 15 or claim 16, in sa-id pressure maintaining step includes a step of establishing a reservoirmaster brake fluid flow path from a reservoir to said master cylinder using said flow-out valve in a closed state.

18. The method af claim 15, 16 or 17, in said pressure decreasing step comprises a step of using said flow-in valve in an open state to establish said wheel-reservoir brake fluid flow path.

19. The ff of &W -af clains 15 to 18, Aerein said pressure decreasing step comprises a step of using said flow-in valve in a closed state to establish said wheel-reservoir brake fluid flow path.

20. The ',. of aT af clairis 15 to 19, ftrther sing the step of performing an anti-skid braking operation in which pressure in said wheel cylinder is increased by terminating said masterwheel brake fluid flow path using said switching valve in a closed state and establishing a pump- wheel brake fluid flow path from said pump to said wheel cylinder using said flow-in valve in an open state and said flow-out valve in a closed state.

21. The of any of clairtis 15 to 20, further comprising the step of establishing a reservoir-master brake fluid flow path using said switching valve in an open state, said flow-out valve in a closed state and said flow-in valve in a closed state to perform a master cylinder fill operation in which brake fluid in said reservoir is supplied to said master cylinder.

22. Anti-skid apparatus substantially as described herein with reference to Fig. 1, 2, 3 or 4 of the accompanying drawings.

23. A method of performing anti-skid brake control substantially as described herein with reference to Fig. 1, 2, 3 or 4 of the accompanying drawings.

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