Improvements relating to a vacuum servo unit for use in traction control

Abstract: The present specification discloses a servo unit for use in a vehicle braking system, the servo unit comprising a housing (1) within which a flexible diaphragm (7) separates a vacuum pressure chamber (3) from an atmospheric pressure chamber (5). An actuator rod assembly (11) extends through the housing (1) and includes a valve assembly (17) having a valve control piston (23) which has a laterally extending key (21), the key (21) being engageable with a fixed stop in the housing (1) to limit the movement of the valve control piston (23) and cause a balance in the valve assembly (17) between the vacuum and atmospheric pressure. A solenoid valve (25) is also provided, the solenoid valve (25) being controllable to either connect a flexible passage member (29) extending around the actuator rod assembly (11) across the vacuum pressure chamber (3) between the housing (1) and the diaphragm (7) in the region of the valve assembly (17), to said vacuum pressure chamber (5), or to connect both said flexible passage member (29) and a bypass (31; 37,39; 35; 41,45; 47) leading from the solenoid valve to said atmospheric pressure chamber (5), to the atmosphere. The bypass incorporates a one-way valve (33;49).

Description: The present invention relates to a vacuum servo unit for use in a vehicle braking system. In particular the present invention relates to a servo unit of the type comprising a housing, the interior of which housing is divided into two chambers by a flexible diaphragm, one chamber being connectible to the exhaust manifold of an engine and thus then being subjected to a vacuum pressure, while the other chamber is connectible via a valve assembly to atmospheric pressure, so that the pressure difference across the diaphragm aids axial movement of a brake actuator rod assembly which extends through the housing and is connected to the diaphragm and to a master cylinder and brake pedal. More particularly, the present invention relates to a servo unit of the above-described type wherein a key arrangement limits the movement of a control piston of the valve assembly relative to a power piston which is connected to the diaphragm. Such a construction is disclosed in our Spanish Patent Application No. 551919. By virtue of this arrangement a balance is achieved in the rest position of the servo unit, i.e. with the brakes released, such that any movement of the actuator rod assembly in the brake applying direction will immediately cause the valve assembly to introduce atmospheric pressure and actuate the servo unit. Should such a servo unit be required to be used in a traction control system such as disclosed in our European Patent Application No.88307406.4, bearing in mind its immediate response advantage, this would not be possible as the introduction of atmospheric pressure via a solenoid valve and the valve assembly would not actuate the servo unit in view of the balanced condition of the valve assembly, the balanced condition precluding the provision of an air flow path through the valve assembly. The present invention thus aims to modify the above balanced type of servo unit for advantageous immediate response use in a traction control system. According to the present invention there is provided a servo unit for use in a vehicle braking system, the servo unit comprising a housing within which a flexible diaphragm separates a vacuum pressure chamber from an atmospheric pressure chamber, an actuator rod assembly extending through the housing and including a valve assembly having a valve control piston which has a laterally extending key, the key being engageable with a fixed stop in the housing to limit the movement of the valve control piston and cause a balance in the valve assembly between the vacuum and atmospheric pressure, a solenoid valve being controllable to either connect a flexible passage member extending around the actuator rod assembly across the vacuum pressure chamber between the housing and the diaphragm in the region of the valve assembly, to said vacuum pressure chamber, or to connect both said flexible passage member and a bypass leading from the solenoid valve to said atmospheric pressure chamber, to the atmosphere, said bypass incorporating a one-way valve. The provision of the bypass thus enables the atmospheric pressure chamber to be immediately connected to atmospheric pressure as and when traction control is required, whilst allowing for the advantageous immediate response of the balanced valve facility produced by the lateral key design of the valve control piston. In one embodiment of the present invention the bypass is a pipe which extends externally of the housing. Aternatively the bypass may pass, at least partially, through the wall of the housing, or through the diaphragm within the flexible passage member. In a further embodiment an axially flexible tube extends between the diaphragm and a wall of the housing, across the vacuum pressure chamber, the tube connecting the solenoid valve with the atmospheric chamber of the housing. In a lightweight housing design which is strengthened by a number of axially extending tie bars, at least one tie bar extends through the axially flexible tube which is connected across the vacuum pressure chamber between the diaphragm and the wall of the housing. One end region of this flexible tube opens through the diaphragm into the atmospheric pressure chamber, whilst the other end region connects with the solenoid valve via a further pipe. The present invention will now be further described, by way of example, with reference to the accompanying drawings, in which:- Fig.1 is a cross-sectional view through one embodiment of the present invention; Fig.2 is a cross-sectional view through another embodiment of the present invention; Fig.3 is a cross-sectional view through a further embodiment of the present invention; Fig.4 is a cross-sectional view through a still further embodiment of the present invention; and Fig.5 is a cross-sectional view through a still further embodiment of the present invention. The servo units illustrated in Figs. 1 to 5 of the accompanying drawings comprise a housing 1 which is divided internally into a vacuum pressure chamber 3 and an atmospheric pressure chamber 5 by a flexible diaphragm 7. The diaphragm 7 includes a power piston 9 which is connected in an actuator rod assembly 11. The actuator rod assembly 11 extends through the housing 1 and projects from either side, one end region 13 being connected to a master cylinder (not shown) and the other end region being connected to a brake pedal 15. A valve assembly 17 controls the flow of air to the atmospheric pressure chamber 5, air normally entering the unit via inlet 19. This valve assembly 17 incorporates a key arrangement 21 as disclosed in our Spanish Patent Application No. 551919, the key arrrangement 21 limiting the available movement of a valve control piston 23 relative to the servo housing 1 and resulting in a balanced condition when the brakes are released and the servo unit is at rest, this balanced condition providing for the immediate response of the servo unit as soon as the brake pedal 15 is moved to apply the brakes. To provide for traction control independent of normal braking via the brake pedal 15, a solenoid valve 25 is provided in the wall of the housing 1, the solenoid valve 25 being controlled by an electronic control unit 27 which monitors speed and torque at the vehicle wheels. For normal braking the solenoid valve opens a passage 26 between the vacuum pressure chamber 3 and an axially flexible passage member 29 which extends from the power piston 9 to the wall of the housing, across the vacuum pressure chamber 3 and around the actuator rod assembly 11. When traction control is required the solenoid valve 25 closes the passage 26 to the vacuum pressure chamber 3, and opens the passage 26 to the atmosphere. Thus atmospheric pressure is fed to the axially flexible passage member 29. However, due to the balanced condition of the valve assembly 17 this atmospheric pressure cannot pass to the atmospheric pressure chamber 5 as can occur in the servo unit of our European Patent Application No.88307406.4. To solve this situation a bypass 35, incorporating a one-way valve 33, extends from the solenoid valve 25 to the atmospheric pressure chamber 5 so that when traction control is required, the bypass 31 is opened to allow atmospheric pressure to be introduced into the atmospheric pressure chamber 5. The resulting initial movement of the diaphragm 7 and thus the power piston 9, upsets the balanced condition of valve assembly 17 so that the valve assembly 17 opens so as to allow air under atmospheric pressure to also flow from the axially flexible passage member 29 to the atmospheric pressure chamber 5 via the valve assembly 17, resulting in the brakes being actuated. The one-way valve 33 retains the pressure in the atmospheric pressure chamber 5 during normal braking when valve assembly 17 opens to allow air under atmospheric pressure to enter the atmospheric pressure chamber 5 via inlet 19. In Fig.1, the bypass 31 is a pipe 35 which extends externally of the housing. In Fig.2, the bypass 31 has a section 37 which extends through the wall of the housing 1, this section 37 being connected by a pipe 39 in the vacuum pressure chamber 3 to the one-way valve 33. In Fig.3, the bypass 31 is formed by an axially flexible tube 41 which extends across the vacuum pressure chamber 3 from the diaphragm 7 to the wall of the housing 1 adjacent to the solenoid valve 25, the tube 41 flexing as the diaphragm 7 moves. The tube 41 connects, through the diaphragm 7, with the atmospheric pressure chamber 5. A modified form of the embodiment of Fig.3 is shown in Fig.4, the housing 1 of the embodiment of Fig.4 being of a lightweight design and requiring axial tie bars 43 to strengthen its construction. One of these tie bars 43 extends through an axially flexible tube 41, the tube 41 connecting, through the diaphragm 7, with the atmospheric pressure chamber 5, and by a further pipe 45 with the one-way valve 33 and solenoid valve 25. A still further embodiment of the present invention is shown in Fig.5 of the accompanying drawings, the bypass 35 being provided by passages 47 which extend through the power piston 9 connecting the flexible passage member 29 to the atmospheric pressure chamber 5. Thus, when traction control is required, the solenoid valve 25 opens the flexible passage member 29 to the atmosphere so that atmospheric pressure immediately passes to the atmospheric pressure chamber 5 to actuate the servo unit and apply the brakes. For normal braking the flexible passage member 29 is connected to the vacuum pressure chamber 3 via the solenoid valve 25, flap valves 49, i.e. the said one-way valve, being closed to prevent the chambers 3,5 from interconnecting. The present invention thus provides a servo unit with immediate response for normal braking, and with a traction control facility.

Claims: A servo unit for use in a vehicle braking system, the servo unit comprising a housing (1) within which a flexible diaphragm (7) separates a vacuum pressure chamber (3) from an atmospheric pressure chamber (5), an actuator rod assembly (11) extending through the housing (1) and including a valve assembly (17) having a valve control piston (23) which has a laterally extending key (21), the key (21) being engageable with a fixed stop in the housing to limit the movement of the valve control piston (23) and cause a balance in the valve assembly (17) between the vacuum and atmospheric pressure, characterised in that a solenoid valve (25) is controllable to either connect a flexible passage member (29) extending around the actuator rod assembly (11) across the vacuum pressure chamber (3) between the housing and the diaphragm (7) in the region of the valve assembly (17), to said vacuum pressure chamber (3), or to connect both said flexible passage member (29) and a bypass (31; 37,39; 35, 41,45; 47) leading from the solenoid valve (25) to said atmospheric pressure chamber (5), to the atmosphere, said bypass incorporating a one-way valve (33;49). A servo unit as claimed in claim 1, wherein the bypass (31) is formed by a pipe (35) which extends at least in part externally of the housing (1). A servo unit as claimed in claim 1, wherein the bypass (37,39) extends, at least partially (37) through a wall of the housing (1). A servo unit as claimed in claim 1, wherein the bypass (47) extends through the diaphragm (7) in the region within the flexible passage member (29). A servo unit as claimed in claim 1, wherein the bypass (35) is formed by an axially flexible tube (41) which extends between the diaphragm (7) and a wall of the housing (1) across the vacuum pressure chamber (3), the tube (41) connecting the solenoid valve (25) with the atmospheric chamber (5) of the housing (1). A servo unit as claimed in claim 1, wherein at least one tie bar (43) extends across the housing (1) through the diaphragm (7), the bypass (41,45) being formed by an axially flexible hollow tube (41) through which a tie bar extends, the tube (41) being connected across the vacuum pressure chamber (3) between the diaphragm (7) and a wall of the housing (1), one end region of the tube (41) communicating with the atmospheric pressure chamber (5) through the diaphragm (7), whilst the other end region of the tube (41) communicates with the solenoid valve (25) via a further pipe (45).

Inventors: BAGLEY FRANK JOHN, BYRNES SEAN, MCDONALD COLIN, MORTIMER IVAN, BAGLEY, FRANK JOHN, BYRNES, SEAN, MCDONALD, COLIN, MORTIMER, IVAN
Applicants: LUCAS IND PLC, LUCAS INDUSTRIES PUBLIC LIMITED COMPANY
Application year: 1990
Main classification: B60T13
IPC codes: B60T13, B60T8