GB2223812A - Servo assisted brake system - Google Patents

Abstract

A servo assisted brake system includes a master cylinder (10), said master cylinder (10) being connected to a brake actuator via a servo actuator (14) and valve means (15) is provided to control the servo actuator (14) so that upon brake actuation it will apply a boost pressure to the brake actuator. The master cylinder (10) (Fig 3, not shown) has a piston (30), slidable within a stepped cylindrical bore (21), said piston (30) having a first portion (31) slidably sealed within the smaller diameter portion of the bore (21) and a second portion (33) slidably sealed within the larger diameter portion of the bore (21). The smaller diameter portion of the bore (21) is closed to define a first working chamber (50) between the closed end (22) and the first portion (31) of the piston (30), a second working chamber (52) being defined between the first and second portions (31, 33) of the piston (30). The first working chamber (50) is connected to the brake actuator via the servo actuator (14) and the second working chamber (52) is connected to a cylinder (115) to control said valve means (15). <IMAGE>

Description

SERVO ASSISTED BRAKE SYSTEMS The present invention relates to servo assisted brake systems and in particular brake systems in which the servo actuator is positioned remote from the master cylinder and is actuated in response to actuation of the brake.

Conventionally, in servo assisted brake systems of this type, a vacuum servo is actuated by a valve means which responds to pressure of fluid in the brake system. However, before sufficient pressure is established in the brake system to operate the valve means, clearances, for example between friction elements and drums or discs, have to be closed and the piston controlling the valve moved.

Consequently, there is a significant delay between application of the brakes and the servo actuator providing a boost to the brake pressure.

According to one aspect of the present invention a servo assisted brake system comprises a master cylinder, said master cylinder being connected to a brake actuator via a servo actuator and valve means being provided to control the servo actuator so that upon brake actuation it will apply a boost pressure to the brake actuator, said master cylinder having a piston slidable within a stepped cylindrical bore, said piston having a first portion slidably sealed within the smaller diameter portion of the bore and the second portion slidably sealed within the larger diameter portion of the bore, the smaller diameter portion of the bore being closed thereby defining a first working chamber between the closed end of the bore and the first portion of the piston and a second working chamber between the first and second portions of the piston, a first outlet being provided from said first working chamber and a second . outlet being provided from said second working chamber, said first outlet being connected via the servo actuator to the brake actuator and the second outlet being connected to a cylinder to control said valve means.

In the brake curcuit described above, as the master cylinder piston moves, the second portion thereof will expel fluid from the larger diameter portion of the cylinder and actuate the valve means, thus avoiding the delay as fluid from the smaller diameter portion of the cylinder is displaced to take up clearances in the braking system. Preferably, the working area of the second portion of the piston is greater than the working area of the first portion, so that volume displacement from the second outlet is relatively greater than that from the first outlet. In this manner, the response time of the servo actuator to brake actuation, is significantly reduced and brake boost can be applied almost instantaneously.

An embodiment of the invention is now described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 illustrates diagramatically a brake system in accordance with the present invention Figure 2 shows a cross-sectional elevation of the valve means used in the system illustrated in Figure 1; Figure 3 is a cross-sectional partial view of the master cylinder used in the system illustrated in Figure 1; and Figure 4 is a sectional elevation of the servo actuator used in the system illustrated in Figure 1.

Figure 1 illustrates a dual braking system having two brake circuits, one controlling the brakes associated with the front wheels of the vehicle and the other controlling the brakes associated with the rear wheels of the vehicle.

The system is controlled by a master cylinder 10 which in response to depression of the brake pedal 11 provides a source of fluid under pressure for each of the brake circuits. In each circuit, a separate outlet 12, 13 of the master cylinder 10 is connected via separate servo actuators 14, 14' to actuators (not shown) adapted to control the brakes associated with the front and with the rear wheels of the vehicle respectively. The servo actuators 14, 14' are simultaneously controlled to provide a boost pressure to both sets of brake actuators, by valve means 15 which in turn is controlled by fluid pressure from a further outlet 16 of the master cylinder 10.

As illustrated in greater detail in Figure 3, the master cylinder 10 is a dual master cylinder of the "quick fill" type and is of generally conventional design. The master cylinder 10 comprises a housing 20 defining a stepped bore 21, the stepped bore 21 being closed at the smaller diameter end 22. A reservoir 23 for hydraulic fluid is mounted on the housing 20 on a pair of bosses 24 and 25.

A first piston 30 is slidingly sealed in the bore 21 adjacent the stepped end thereof. The piston 30 has a first portion 31 which is sealed with respect to the smaller diameter portion of bore 21 by means of a cup seal 32 and an axially spaced larger diameter portion 33 which is sealed with respect to the larger diameter portion of the bore 21.

The difference in cross-sectional areas of portions 31 and 32 of piston 30 is in excess of the cross-sectional area of portion 31. The piston 30 is trapped in the bore 21 by means of a circlip 34 at the open end 35 of the larger diameter portion of bore 21.

A second piston 40 is slidingly sealed within the smaller diameter portion of bore 21. A compression spring 41 acts between the smaller diameter portion 31 of piston 30 and a cage 42 which is slidingly located on a rod 43 extending axially from portion 31 of piston 30. The rod 43 has an enlarged end 44 which acts as stop for the cage 42 and restricts the maximum extension of spring 41. A second compression spring 45 acts between the end 22 of bore 21 and piston 40 to maintain piston 40 in abutment with the cage 42/spring 41 assembly on piston 30.

The pistons 30 and 40 thus divide bore 21 into three working chambers, chamber 50 between the end 22 of bore 21 and piston 40, chamber 51 between pistons 30 and 40 and chamber 52 between portions 31 and 33 of piston 30. The outlet 12 communicates with chamber 50, outlet 13 with chamber 51 and outlet 16 with chamber 52, the latter being the only significant departure from the conventional master cylinders of this type. Chamber 50 is connected to the reservoir 23 via boss 25 and a port 55 which is positioned just forward of a seal 56 between the piston 40 and bore 21, when the master cylinder is not actuated. Chamber 51 is connected to reservoir 23 via port 58 which is forward of the seal 32 and chamber 52 via port 59.The ports 58 and 59 are connected to reservoir 23 via a valve assembly 60 located in the boss 24, said valve assembly 60 comprising a float valve 61 which permits fluid to flow from the reservoir to the chambers 51 and 52 when the pressure therein is below the pressure in the reservoir 23 and a pressure release valve 62 which permits flow of fluid from chamber 52 to the reservoir 23, when pressure in chamber 52 reaches a pre-determined value.

The master cylinder 10 is actuated by a brake pedal controlled push rod 65 which engages in a closed bore 66 in the end of piston 30.

IJhen master cylinder 10 is actuated by depression of the brake pedal 11, the push rod 65 will move piston 30 to the left (as illustrated). Movement of the piston 30 is also transmitted to piston 40 by spring 41. Initial movement of pistons 30 and 40 will first interrupt communication of chambers 51 and 50 witri the reservoir 23 via ports 58 and 55 respectively, and continued movement will then expel fluid via outlets 13 and 12. Movement of the piston 30 will also cause the pressure of fluid in chamber 52 to increase, causing fluid to be expelled through outlet 16.

Because of the difference in cross-sectional areas of portions 31 and 33 of the piston 30, the pressure in chamber 52 will increase more rapidly than in chambers 51 and 50.

The cup seal 32 is arranged such that when pressure in chamber 52 in excess of that in chamber 51, fluid will flow past the seal 32 from chamber 52 to 51. In this manner, excessive movement of pistons 30 and 40 in order to take up clearances in the brake system is avoided.

The servo actuators 14 and 14' are of identical construction and are as illustrated is Figure 4. As illustrated in Figure 4, the servo actuators 14, 14' comprises a cylinder 70 having a closed bore 71 and a coaxial casing 72 mounted on the open end. The casing 72 is divided into two compartments 73 and 74 by a diaphram 75 which is mounted between the wall of the casing 72 and a plunger 76 located within the casing 72. The plunger 76 has a stem portion 77 which extends into the bore 71 through a gland seal 78 at the open end of the bore 71. A helical compression spring 79 urges the plunger 76 away from the cylinder 70.

An annular piston 80 is slidingly located in the bore 71 of cylinder 70 and is sealed with respect to the walls thereof. The annular piston 80 is mounted on the end of stem portion 77 in a manner which will permit limited axial movement therebetween. A fluid path 81 is provided between the stem portion 77 and annular piston 80 to permit access of hydraulic fluid from an inlet 82 opening into the bore 71 adjacent the casing 72 past the piston 80 to the portion of bore 71 adjacent its closed end. A valve seat 84 is provided on the piston 80 and may be engaged by the end of stem 77 to close the fluid path 81. An outlet 83 opens into the bore 71 of cylinder 70 adjacent the closed end thereof.

Chambers 73 and 74 defined by casing 72 are provided with outlets 85 and 86 respectively. The inlets 82 of servo actuators 14 and 14' are connected to the outlets 13 and 12 of master cylinder 10 respectively while the outlet 83 of servo 14 is connected to the actuators of the brakes associated with the front wheels of the vehicle and outlet 83 of servo actuator 14' is connected to the actuators of the brakes associated with the rear wheels of the vehicle.

Outlets 85 from compartment 73 of servo actuators 14 and 14 are connected to a vacuum reservoir (not shown) while outlets 86 from compartment 74 are selectively connected via valve means 15 to the vacuum reservoir or to atmosphere.

As illustrated in greater detail in Figure 2, valve means 15 includes a casing 90 which is formed in two parts 91 and 92. The casing 90 is divided into two compartments 93 and 94 by means of an annular flexible diaphragm 95, the outer periphery of the diaphragm 95 being trapped between the parts 91 and 92 of casing 90 and the inner periphery engaging a valve member 96 which is slidingly located within the casing 90, axially thereof, on a part cylindrical formation 97. The valve member 96 defines a port 98 which is normally open and interconnects compartments 93 and 94. A further port 100 which is coaxial with port 98 is provided in part 92 of casing 90, this port 100 when open connecting compartment 94 to atmosphere through an inlet 101. A valve spool 102 is mounted coaxially of ports 98 and 100 and has axially separated sealing means 103 and 104.A helical compression spring 105 acts on the valve spool 102 to urge the spool 102 towards valve member 96, so that normally sealing member 104 will engage a seat formation 106 to close port 100 while sealing means 103 remains spaced axially from port 98. The compartment 93 is provided with a connection 110 by means of which it may be connected to a vacuum reservoir and compartment 94 is provided with connection 111 by means of which it may be connected to outlets 86 of the servo actuators 14 and 14'.

The casing 90 is mounted coaxially of a cylinder body 115 which defines a stepped bore 116 coaxial with ports 98 and 100. The bore 116 is closed at the smaller diameter end 117 which is remote from the casing 90. A first piston 120 is slidingly sealed in the larger diameter portion 121 of bore 116 and engages a stem 122 which extends axially from the valve member 96, the valve member 96 being urged into engagement with piston 120 by a light return spring 107.

Two further piston 123 and 124 are slidingly sealed in the smaller diameter portion 125 of bore 116, the pistons 123 and 124 being sealed to the bore 116 by centrally disposed enlarged diameter portions 126, thereby defining; working chamber 127 between the closed end 117 of bore 116 and piston 124, working chamber 128 between piston 124 and piston 123 and working chamber 129 between piston 123 and piston 120.

Inlets 130, 131 and 132 are provided to the chambers 127, 128 and 129 respectively. Inlet 130 is connected to outlet 12 of master cylinder 10, inlet 131 is connected to outlet 13 of master cylinder 10 and inlet 132 is connected to outlet 16 of master cylinder 10.

When the master cylinder 10 is actuated by depression of the brake pedal 11, fluid from chamber 52 will be forced into chamber 129 of valve 15 thus forcing piston 120 towards the open end of bore 116. Movement of piston 120 will move the valve member 96 so that sealing means 103 will first close port 98. Continued movement of piston 120 will then move the spool valve 102 so that the port 100 is opened and compartment 94 connected to atmosphere. Air will then flow through the compartment 94 to compartments 74 of servo actuators 14 and 14', thus establishing pressure differentials across the diaphragms 75. These pressure differentials will cause the plungers 76 to move towards cylinders 70 against the springs 79.Movement of the stems 77 to engage with the seats 80 will close the fluid paths 81 and further movement of the plungers 76 will move the pistons 80 to provide a boost to the pressure applied to the brake actuators.

As the pressure in chamber 94 increases the pressure differential across diaphragm 95 will begin the balance the pressure applied to piston 120 until the force on diaphragm 95 is sufficient to move the valve member 96 and close port 100. Should the effort applied to the brake pedal 11 be reduced, excess force on the diaphragm 95 will then further move the valve member 96 to open port 98, so that chamber 94 is reconnected to vacuum and the boost pressure is reduced.

The boost pressure applied by the servo actuators 14 and 14' is consequently proportional to the pressure applied by the master cylinder 10 from chamber 52. This pressure is however limited by pressure release valve 62.

When pressure in chambers 50 and 51 of the master cylinder exceeds that in chamber 52, the excess pressure in chambers 127 and 128 will cause the pistons 123 and 124 to act on piston 120, so that the valve member 96 will then be moved in relation to the pressure in chambers 50 and 51 to further increase the boost pressure.

Various modifications may be made without departing from the invention. For example, while in the above embodiment vacuum servo actuators are used, air or hydraulic pressure servo actuators may alternatively be used. Also rather than using independent servo actuators, a back-to-back servo actuator with a common working chamber 74 may be used. The present invention also applies to single as well as dual brake circuits and rather than controlling the front brakes and rear brakes separately, dual circuits may be arranged to control other combinations of brakes, for example, diagonally opposite brakes or one circuit controlling the two front brakes and one rear and the other circuit controlling the two front brakes and the other rear.

Claims (9)

1. A servo assisted brake system comprising a master cylinder, said master cylinder being connected to a brake actuator via a servo actuator and valve means being provided to control the servo actuator so that upon brake actuation it will apply a boost pressure to the brake actuator, said master cylinder having a piston slidable within a stepped cylindrical bore, said piston having a first portion slidably sealed within the smaller diameter portion of the bore and the second portion slidably sealed within the larger diameter portion of the bore, the smaller diameter portion of the bore being closed thereby defining a first working chamber between the closed end of the bore and the first portion of the piston and a second working chamber between the first and second portions of the piston, a first outlet being provided from said first working chamber and a second outlset being provided from said second working chamber, said first outlet being connected via the servo actuator to the brake actuator and the second outlet being connected to a cylinder to control said valve means.

2. A servo assisted brake system according to Claim 1 in which the working area of the second portion of the piston is greater than that of the first.

3. A servo assisted brake system according to Claim 1 or 2 in which a release valve is provided to limit the pressure of fluid in the second working chamber to a predetermined value.

4. A servo assisted brake system according to Claim 3 in which the master cylinder is of the "quick fill" type (as herein before defined), an outlet being provided from the quick fill chamber of the master cylinder to control the valve means.

5. A servo assisted brake system according to Claims 1 to 4 in which said valve means controls the servo actuator to provide a boost to the pressure of fluid applied to the brake actuator, said boost being proportional to the pressure of fluid applied by the master cylinder to control the valve means.

6. A servo assisted brake system according to Claim 5 in which the valve means comprises a piston located in a closed bore, said piston engaging a valve member to control movement of the valve member between one extremity in which said outlet from the valve is connected to a source of low pressure fluid and a second extremity in which the outlet is connected to a source of high pressure fluid.

7. A servo assisted brake system according to Claim 6 in which the pressure of fluid applied to the servo actuator acts upon the valve member to oppose movement of the valve member under the load applied thereto by the piston.

8. A servo assisted brake system according to Claim 6 or 7 in which a further piston is provided within the closed bore of the valve means, fluid from the first outlet of the master cylinder being applied to said piston to urge it towards said first piston of the valve means so that when pressure in the first chamber of the master cylinder is in excess of that in the second chamber, the pressure in the first chamber of the master cylinder will act upon the valve member.

9. A servo assisted brake system substantially as described herein with reference to and as shown in Figures 1 to 4 of the accompanying drawings.