GB2098293A - Anti-skid brake system - Google Patents

Abstract

An anti-skid hydro-pneumatic brake system comprises a common three-port, two-position valve (6) and two two-port, two-position valves (7, 8) for monitoring two brake cylinders (43, 44). The three-port, two-position valve (6) is disposed in an air line from a brake valve 1 to a hydraulic fluid displacer (39) and the two-port, two- position valves are disposed in a hydraulic brake line (41). All three valves (6, 7, 8) are pilot-controlled and actuated by pressure medium, and the actuating pressure medium for the two two-port, two-position valves and preferably also for the third valve is tapped from an air line (control line 3) which is subjected to pressure during braking. In this manner, the same hydro- pneumatic master cylinders can be used for the front and rear axles, or the same electronic system can be used in a vehicle having a hydraulically braked front axle and a pneumatically braked rear axle, since the solenoid pilot control valves (17, 18, 19) required for control are identical to one another. <IMAGE>

Description

SPECIFICATION Anti-skid brake system The invention relates to an anti-skid brake system having a three-port, two-position valve common to several brake cylinders and two-port, two-position valves individual to the brake cylinders.

An anti-skid hydro-pneumatic brake system of this kind is known (German patent specification (Offenlegungsschrift) No. 26 56 983). However, this known system requires a master cylinder of special construction. In addition to this, only relatively small valve cross sections are to be monitored by the directly controlled hydraulic twoport, two-position solenoid valves. Small valve cross sections of this kind are inadequate when used in commercial vehicle brake systems.

The present invention provides an anti-skid hydro-pneumatic brake system for the wheels of a motor vehicle with common regulation of the brake pressure in at least two brake cylinders, comprising a three-port, two-position valve, common to the said brake cylinders an individual two-port, two-position valve for each brake cylinder, and the three-port, two-position valve being incorporated in an air line leading from a brake valve to a hydro-pneumatic master cylinder and the two-port, two-position valves being incorporated in a hydraulic brake line leading from the master cylinder to the brake cylinders, the two-port, two-position valves being pressuremedium-operated valves, and an actuating pressure medium therefor being tapped from a pressure-medium line which is pressurized during a braking operation.

This has the advantage that conventional, mass-produced biassing cylinders can be used.

Since the two-port, two-position valves are pilot controlled, they can be of larger construction and can monitor larger valve cross sections. In a vehicle having hydraulic brakes on the front axle and pneumatic brakes on the rear axle, the electrical control for the two axles can be the same, since the solenoid pilot control valve required for control are identical to one another.

The invention is further described by way of example, with reference to the accompanying drawings, in which: Figure 1 is a diagrammatic sectional illustration of an anti-skid system having a valve unit, Figure 2 shows a modification of a detail of the valve unit, and Figure 3 shows a different detail of the valve unit.

A hydro-pneumatic brake system for the wheels of a motor vehicle has a brake valve 1 which introduces air into a control line 3 from a reservoir 2, or vents the control line 3 to the atmospheric air. The control line 3 leads to a housing 4 of a valve unit 5 in which a three-port, two-position valve 6 and two two-port, twoposition valves 7 and 8 are disposed side by side.

The three-port, two-position valve 6 has two valve seats 9 and 10, that is to say, an inlet valve seat 9 and an outlet valve seat 10, between which is disposed a closure body 11 which can alternately cover one or other of the two valve seats 9 and 10. The valve 6 has an inlet connection 12 from which a control passage 1 3 branches to lead to a respective inlet valve seat 14, 1 5, 1 6 of three electro-magnetically operable pilot control valves 17, 18 and 19. Each pilot control valve 1 7, 18, 19 has an outlet seat designated 20, 21 and 22 respectively.

Each of the three valves 6, 7 and 8 has a respective control chamber 23, 24, 25 to which a respective diaphragm 26, 27, 28 is exposed and which is connected to the respective, corresponding inlet valve seat 14, 1 5, 1 6. The diaphragm 26 carries a push rod 29 for actuating the closure body 11, and the two diaphragms 27 and 28 of the two-port, two-position valves 7 and 8 also carry push rods 30 and 31 for the actuation of balls serving as closure bodies 32 and 33. A respective spring-loaded push rod 34 and 35 is mounted on the other side of each ball-type closure body 32 or 33 and seeks to keep the balltype closure body 32 or 33 away from a respective valve seat 36 or 37. It will be seen that the two valves 7 and 8 are poppet type valves and have relatively large valve cross sections.

Alternatively, however, in a modification to the type of construction illustrated, the two valves 7 and 8 can be in the form of slide valves.

Downstream of the three-port, two-position valve 6, the control line 3 continues in the form of a line 3' which leads to an air side 38 of a brake master cylinder 39. The brake master cylinder 39 is of conventional construction for hydropneumatic brake systems, that is to say, special manufacture is not required. A hydraulic side 40 of the master cylinder 39 supplies a hydraulic brake line 41 with brake fluid which originates from a topping-up reservoir 42. The brake line 41 leads to the two valves 7 and 8 in the valve unit 5 where it splits into two brake line branches 41' and 41" which lead to wheel brake cylinders 43 and 44 disposed on the vehicle wheels.

Rotation of the vehicle wheels is monitored by sensors 45 and 46 which are connected to an electronic control device 47. Electrical connection leads for the solenoids 48, 49 and 50 of the pilot control valves 17, 1 8 and 1 9 are also connected to the control device 47.

The system which has been described operates in the following manner: As will be seen in Fig. 1 of the drawings, brake pressure modulation in the anti-skid system in.

accordance with invention is effected on the compressed air side by the three-port, twoposition valve 6, and on the hydraulic side by means of the two two-port, two-position valves 7 and 8. The two two-port, two-position hydraulic valves 7 and 8 are pilot-controlled, in the same manner as the air valve 6, by way of the pilotcontrol valves 17, 18 and 19 by the compressed air introduced into the control chambers 23, 24 and 25. This has the advantage that the two hydraulic valves 7 and 8 can be in the form of simple, uncomplicated ball valves, and that the cross sections in the valves 7 and 8 can be relatively large, since the valves 7 and 8 (each provided with pilot-control) reliably change over even under large forces. In this manner, valve cross sections having a diameter of up to 4 mm can be used.

In addition to this, the brake valve 1 , the valve unit 5 and the master cylinder 39 are separate components, the brake valve 1 and the master cylinder 39 being conventional mass-produced products. All the parts 1,5 and 39 are interconnected by lines. In this type of construction, it is a simple matter to install a conventional load-dependent brake force regulator by fitting it selectively in the air control line 3, 3' or in the hydraulic brake line 41,41 41".

Finally, it will be seen that, by virtue of the special type of air control of the hydraulic valves 7 and 8, it is possible to conceive a brake system for a vehicle whose front axle is braked hydraulically and whose rear axle is braked with compressed air. The front axle brakes would then have to be monitored by a valve unit 5 in accordance with the present invention, and the rear axle by a valve unit as described for example, in German patent specification (Offenlegungsschrift) No. 27 53 992 (pressure-control valve 22). The electronic control for the two valve units could be the same and could be performed by the same electronic control device.

The following may also be said with respect to the mode of operation of the valve unit 5: In the event of anti-skid operation, the brake pressure is reduced by the central three-port, twoposition valve 6, and a pressure-holding function is performed by the two hydraulic two-port, twoposition valves 7 and 8 connected downstream. A reduction in the brake pressure is effected in the control line 3' by way of a drop in the air pressure, and in the master cylinder 39 by virtue of an increase in volume at the hydraulic side by virtue of the yielding master cylinder input piston. Brake pressure is held solely in the hydraulic circuit.

Two-channel control is thereby provided by the possibility of shutting off the prevailing pressure in one wheel brake cylinder relative to that in the other brake cylinder. However, since, in principle, a hydraulic fluid is incompressible, the two valves 7 and 8 must be prevented from having the same opening times in order to avoid equalisation of the pressure in the two brake cylinders 43 and 44.

This is rendered possible by actuating the two valves in anti-phase during the pressure build-up phases. This does not result in any functional disadvantage, since the ratio of the pressure-build up phase (holding valve open) to the pressureholding phase (holding valve closed) is approximately 1:6 and therefore asynchronization can be readily achieved. Thus, as a result of this simple measure, different brake pressures in the two brake cylinders 43 and 44 can be obtained even when using hydraulic fluid as the pressure medium.

On the other hand, however, vehicle states occur in which only a predetermined pressure difference may be permitted in the two brake cylinders in order to limit the yawing moment. This can be achieved in this case, by permitting synchronous valve control, limited with respect to time.

Thus, it is possible to eliminate the pressure difference in the two brake cylinders by synchronous control of the valves 7 and 8, to modulate a maximum pressure difference by asynchronous control of the valves 7 and 8 or, alternatively, to modulate only a predetermined differential pressure according to the ratio of the synchronous control with respect to time to the asynchronous control with respect to time.

By way of example, it is possible to choose a ratio of synchronous control to asynchronous control to 4:6 in the case of a pulsed, flat build-up of pressure with an open period of the valves 7 and 8 of, for example,10 msec, so that the brake cylinder having the higher pressure level can only assume a predetermined pressure difference compared with the lower pressure level of the other brake cylinder.

It is also conceivable to change the valve control from synchronous to asynchronous modulation during the regulated braking operation, in dependence upon time or in dependence upon the vehicle velocity. The greatest limitation of the yawing moment is thereby established by the synchronous valve control upon commencement of braking or at a high vehicle velocity, and a greater yawing moment is then permitted by a more asynchronous control towards the termination of the braking operation.

As a result of this measure, the driver is clearly relieved of stress in the generally critical commencement of braking phase, although, as a result of the greater differential pressure, greater deceleration is achieved towards the end of the braking operation.

As may be seen in Figure 2, additional throttle adaptation by means of a throttle 51 in the brake line 41' can be undertaken in order to optimize the time behaviour of the pressure rise and also of the pressure drop.

For the purpose of these technical measures, it may be necessary to provide additional measures when using simple master brake cylinders for antiskid regulation. In these simple and thus inexpensive master brake cylinders, a breather port control serves, in a known manner, to connect the piston chamber to the topping-up reservoir in the non-braked state.

Breather-port-controlled master brake cylinders of this kind involve the risk that, upon rapid braking, the piston seal may be forced into the breather port owing to the dynamic pressure and may be damaged. For this reason, master cylinders having a tilting valve are more suitable and can achieve higher brake pressures even with very small piston strokes. In order, nevertheless, to be able to use breather-port-controlled master brake cylinders for anti-skid regulation, the following additional measures are conceivable.

1. As will be seen in Figure 2, a pressure accumulator 52 having a small volume (approximately 2 to 5 cm3), and whose accumulator pressure lies below the pressure for applying the brake shoes, is incorporated in the brake line 41 between the master cylinder 39 and the valve unit 5.

The pressure accumulator 52 prevents the piston seal from being pressed into the breather port upon rapid actuation of the master brake cylinder and thereby possibly causing an operational failure.

It will be appreciated that the pressure accumulator 52 may be integrated in the valve unit 5.

2. A further possibility of protecting the piston sleeve against damage resides in preventing the piston seal from moving across the breather port upon the return stroke of the piston during antiskid regulation.

This is rendered possible, as is shown in Figure 3, by disposing a hold-back valve 53, whose closing pressure lies below the pressure for applying the brake shoes, in the brake pressure control unit in the line connecting the three-port, two-position valve 6 to the atmospheric air. Thus, a residual pressure remains in the master cylinder 39 during regulation and enables the full release of the wheel brake, although it does not permit the piston seal to move across the breather port.

On the other hand, the master cylinder is fully vented directly by way of the brake valve for normal braking.

Claims (13)

1. An anti-skid hydro-pneumatic brake system for the wheels of a motor vehicle with common regulation of the brake pressure in at least two brake cylinders, comprising a three-port, twoposition valve, common to the said brake cylinders an individual two-port, two-position valve for each brake cylinder, and the three-port, two-position valve being incorporated in an air line leading from a brake valve to a hydro-pneumatic master cylinder and the two-port, two-position valves being incorporated in a hydraulic brake line leading from the master cylinder to the brake cylinders, the two-port, two-position valves being pressure-medium-operated valves, and an actuating pressure medium therefor being tapped from a pressure-medium line which is pressurized during a braking operation.

2. An anti-skid brake system as claimed in claim 1, in which the three-port, two-position valve and also the two two-port, two-position valves are main valves which are pilot-controlled by pilot control valves.

3. An anti-skid brake system as claimed in claim 2, in which the pilot-controlled pressure medium is the same for all three valves.

4. An anti-skid brake system as claimed in any of claims 1 to 3, in which each of the two-port, two-position valves has a ball serving as a closure member, each ball being actuatable at opposite sides thereof by a respective push rod.

5. An anti-skid brake system as claimed in any of claims 1 to 3, in which the two-port, twoposition valves are slide valves.

6. An anti-skid brake system as claimed in any preceding claim, in which a pressure accumulator is connected to the hydraulic brake line between the master cylinder and the two-port, two-position valves.

7. An anti-skid brake system as claimed in any of claims 1 to 6, in which the pressure-medium line from which the pressure medium is tapped for actuating the valves is a compressed air line.

8. An anti-skid brake system as claimed in claim 7, in which in operation during an anti-skid regulating operation, a reduction of the brake pressure is primarily effected by a pressure drop in the compressed air circuit and a holding function in the hydraulic circuit.

9. An anti-skid brake system as claimed in claim 8, in which, in anti-skid operation, the pressure build-up phases are effected in antiphase by the two two-port, two-position valves in order to effect different brake pressure control in the two brake cylinders.

10. An anti-skid brake system as claimed in claim 9, in which the phase shift of the pressure build-up phase during regulated braking is variable in dependence upon time or in dependence upon velocity.

11. An anti-skid brake system as claimed in any of claims 8 to 10, in which a throttle is incorporated in the hydraulic brake line to optimize the time behaviour during a pressure rise or pressure drop.

12. An anti-skid brake system as claimed in any of claims 8 to 11, in which a hold-back valve is incorporated in the line connection between the three-port, two-position valve and the atmospheric air.

13. An anti-skid brake system, constructed and adapted to operate substantially as herein described with reference to and as illustrated in the accompanying drawings.