Vehicle Brake System With Plunger Assembly

A brake system includes a plunger assembly for actuating wheel brakes during a normal brake apply. The plunger assembly includes a motor mounted on the housing for driving an actuator. A first piston is connected to the actuator. The first piston is slidably mounted within the housing for pressurizing a first fluid chamber in the housing. A second piston is slidably mounted within the housing for pressurizing a second fluid chamber in the housing. A pump-less control valve arrangement includes a first control valve regulating the flow of fluid between a first fluid chamber and the first wheel brake. A second control valve regulates the flow of fluid between a second fluid chamber and the second wheel brake. An isolation valve arrangement switches the brake system between the normal braking mode and the manual push-through mode.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawings, there is schematically illustrated inFIG. 1a first embodiment of a vehicle brake system, indicated generally at10. The brake system10is a hydraulic boost braking system in which boosted fluid pressure is utilized to apply braking forces for the brake system10. The brake system10may suitably be used on a ground vehicle such as an automotive vehicle having four wheels with a wheel brake associated with each wheel. Furthermore, the brake system10can be provided with other braking functions such as anti-lock braking (ABS) and other slip control features to effectively brake the vehicle, as will be discussed below.

The brake system10generally includes a first block or brake pedal unit assembly, indicated by broken lines12, and a second block or hydraulic control unit, indicated by broken lines14. The various components of the brake system10are housed in the brake pedal unit assembly12and the hydraulic control unit14. The brake pedal unit assembly12and the hydraulic control unit14may include one or more blocks or housings made from a solid material, such as aluminum, that has been drilled, machined, or otherwise formed to house the various components. Fluid conduits may also be formed in the housings to provide fluid passageways between the various components. The housings of the brake pedal unit assembly12and the hydraulic control unit14may be single structures or may be made of two or more parts assembled together. As schematically shown, the hydraulic control unit14is located remotely from the brake pedal unit assembly12with hydraulic lines hydraulically coupling the brake pedal unit assembly12and the hydraulic control unit14. Alternatively, the brake pedal unit assembly12and the hydraulic control unit14may be housed in a single housing. It should also be understood that the grouping of components as illustrated inFIG. 1is not intended to be limiting and any number of components may be housed in either of the housings.

The brake pedal unit assembly12cooperatively acts with the hydraulic control unit14for actuating wheel brakes16a,16b,16c, and16d. The wheel brakes16a,16b,16c, and16dcan be any suitable wheel brake structure operated by the application of pressurized brake fluid. The wheel brake16a,16b,16c, and16dmay include, for example, a brake caliper mounted on the vehicle to engage a frictional element (such as a brake disc) that rotates with a vehicle wheel to effect braking of the associated vehicle wheel. The wheel brakes16a,16b,16c, and16dcan be associated with any combination of front and rear wheels of the vehicle in which the brake system10is installed. For example, for a vertically split system, the wheel brakes16aand16dmay be associated with the wheels on the same axle. For a diagonally split brake system, the wheel brakes16aand16bmay be associated with the front wheel brakes.

The brake pedal unit assembly12includes a fluid reservoir18for storing and holding hydraulic fluid for the brake system10. The fluid within the reservoir18may be held generally at atmospheric pressure or can store the fluid at other pressures if so desired. The brake system10may include a fluid level sensor19for detecting the fluid level of the reservoir. The fluid level sensor19may be helpful in determining whether a leak has occurred in the system10.

The brake pedal control unit assembly12includes a brake pedal unit (BPU), indicated generally at20. The brake pedal unit20is also schematically shown enlarged inFIG. 2. It should be understood that the structural details of the components of the brake pedal unit20illustrate only one example of a brake pedal unit20. The brake pedal unit20could be configured differently having different components than that shown inFIGS. 1 and 2.

The brake pedal unit20includes a housing24(shown broken away inFIG. 2) having various bores formed in for slidably receiving various cylindrical pistons and other components therein. The housing24may be formed as a single unit or include two or more separately formed portions coupled together. The housing24generally includes a first bore26, an intermediate second bore28, and a third bore30. The second bore28has a larger diameter than the first bore26and the third bore30. The brake pedal unit20further includes an input piston34, a primary piston38, and a secondary piston40. The input piston34is slidably disposed in the first bore26. The primary piston38is slidably disposed in the second bore28. The secondary piston40is slidably disposed in the third bore30.

A brake pedal, indicated schematically at42inFIGS. 1 and 2, is coupled to a first end44of the input piston34via an input rod45. The input rod45can be coupled directly to the input piston34or can be indirectly connected through a coupler (not shown). The input piston34includes an enlarged second end52that defines a shoulder54. In the rest position shown inFIGS. 1 and 2, the shoulder54of the input piston engages with a shoulder56formed between the first and second bores26and28of the housing24. An outer cylindrical surface57of the input piston34is engaged with a seal58and a lip seal60mounted in grooves formed in the housing24. The outer cylindrical surface57may be continuous along its length or it may be stepped having two or more different diameter portions. The input piston34includes a central bore62formed through the second end52. One or more lateral passageways64are formed through the input piston34. The lateral passageways64extend from the outer cylindrical surface57to the central bore62to provide fluid communication therebetween. The brake pedal unit20is in a “rest” position as shown inFIGS. 1 and 2. In the “rest” position, the pedal42has not been depressed by the driver of the vehicle. In the rest position, the passageways64of the input piston34are between the seals58and60. In this position, the passageways64are in fluid communication with a conduit66formed though the housing24. The conduit66is in fluid communication with a conduit68formed in the housing24. The conduit68is in fluid communication with a reservoir port70connected to the reservoir18. A filter69may be disposed in the port70or the conduit68. The conduits66and68can be formed by various bores, grooves and passageways formed in the housing24. In the rest position, the passageways64are also in fluid communication with a conduit72formed in the housing24which leads to a simulation valve74. The simulation valve74may be a cut off valve which may be electrically operated. The simulation valve74may be mounted in the housing24or may be remotely located therefrom

The primary piston38is slidably disposed in the second bore28of the housing24. An outer wall79of the primary piston38is engaged with a lip seal80and a lip seal81mounted in grooves formed in the housing24. The primary piston38includes a first end82having a cavity84formed therein. A second end86of the primary piston38includes a cavity88formed therein. One or more passageways85are formed in the primary piston38which extend from the cavity88to the outer wall of the primary piston38. As shown inFIG. 2, the passageway85is located between the lip seals80and81when the primary piston38is in its rest position. For reasons which will be explained below, the passageway85is in selective fluid communication with a conduit154which is in fluid communication with the reservoir18.

The central bore62of the input piston34and the cavity84of the primary piston38house various components defining a pedal simulator, indicated generally at100. A caged spring assembly, indicated generally at102, is defined by a pin104, a retainer106, and a low rate simulator spring108. The pin104is shown schematically as being part of the input piston34and disposed in the central bore62. The pin104could be configured as a pin having a first end which is press fit or threadably engaged with the input piston34. The pin104extends axially within the central bore62and into the cavity84of the primary piston38. A second end112of the pin104includes a circular flange114extending radially outwardly therefrom. The second end112is spaced from an elastomeric pad118disposed in the cavity84. The elastomeric pad118is axially aligned with the second end112of the pin104, the reason for which will be explained below. The retainer106of the caged spring assembly102includes a stepped through bore122. The stepped through bore122defines a shoulder124. The second end112of the pin104extends through the through bore122. The flange114of the pin104engages with the shoulder124of the retainer106to prevent the pin104and the retainer106from separating from each other. One end of the low rate simulator spring108engages with the second end52of the input piston34, and the other end of the low rate simulator spring108engages with the retainer106to bias the retainer106in a direction away from the pin104.

The pedal simulator100further includes a high rate simulator spring130which is disposed about the pin104. The terms low rate and high rate are used for description purposes and are not intended to be limiting. It should be understood that that the various springs of the pedal simulator100may have any suitable spring coefficient or spring rate. In the illustrated embodiment, the high rate simulator spring130preferably has a higher spring rate than the low rate simulator spring108. One end of the high rate simulator spring130engages with the bottom of the central bore62of the input piston34. The other end of the high rate simulator spring130is shown inFIG. 2in a non-engaged position and spaced away from an end of the retainer106. The housing24, the input piston34(and its seals), and the primary piston38(and its seals) generally define a fluid simulation chamber144. The simulation chamber144is in fluid communication with a conduit146which is in fluid communication with the simulation valve74. A filter145may be housed within the conduit146.

As discussed above, the brake pedal unit20includes the primary and secondary pistons38and40that are disposed in the second and third bores28and32, respectively, which are formed in the housing24. The primary and secondary pistons38and40are generally coaxial with one another. A primary output conduit156is formed in the housing24and is in fluid communication with the second bore28. The primary output conduit156may be extended via external piping or a hose connected to the housing24. A secondary output conduit166is formed in the housing24and is in fluid communication with the third bore30. The secondary output conduit166may be extended via external piping or a hose connected to the housing24. As will be discussed in detail below, rightward movement of the primary and secondary pistons38and40, as viewingFIGS. 1 and 2, provides pressurized fluid out through the conduits156and166, respectively. A return spring151is housed in the second bore28and biases the primary piston38in the leftward direction.

The secondary piston40is slidably disposed in the third bore30. An outer wall152of the secondary piston is engaged with a lip seal153and a lip seal154mounted in grooves formed in the housing24. A secondary pressure chamber228is generally defined by the third bore30, the secondary piston40, and the lip seal154. Rightward movement of the secondary piston40, as viewingFIGS. 1 and 2, causes a buildup of pressure in the secondary pressure chamber228. The secondary pressure chamber228is in fluid communication with the secondary output conduit166such that pressurized fluid is selectively provided to the hydraulic control unit14. One or more passageways155are formed in the secondary piston40. The passageway155extends between the outer wall of the primary piston38and a right-hand end of the secondary piston40. As shown inFIG. 2, the passageway155is located between the seal153and the lip seal154when the secondary piston40is in its rest position, the reason for which will be explained below. For reasons which will be explained below, the passageway155is in selective fluid communication with a conduit164which is in fluid communication with the reservoir18.

A primary pressure chamber198is generally defined by the second bore28, the primary piston38, the secondary piston40, the lip seal81, and the seal153. Although the various seals shown in the drawings are schematically represented as O-ring or lip seals, it should be understood that they can have any configuration. Rightward movement of the primary piston38, as viewingFIGS. 1 and 2, causes a buildup of pressure in the primary pressure chamber198. The primary pressure chamber198is in fluid communication with the primary output conduit156such that pressurized fluid is selectively provided to the hydraulic control unit14.

The primary and secondary pistons38and40may be mechanically connected together such that there is limited play or movement between the pistons38and40. This type of connection permits the primary and secondary pistons38and40to move relative to one another by relatively small increments to compensate for pressure and/or volume differences in their respective output circuits. However, under certain failure modes it is desirable that the secondary piston40is connected to the primary piston38. For example, if the brake system10is under a manual push through mode, as will be explained in detail below, and additionally fluid pressure is lost in the output circuit relative to the secondary piston40, such as for example, in the conduit166, the secondary piston40will be forced or biased in the rightward direction due to the pressure within the primary chamber1798. If the primary and secondary pistons38and40were not connected together, the secondary piston40would freely travel to its further most right-hand position, as viewingFIGS. 1 and 2, and the driver would have to depress the pedal42a distance to compensate for this loss in travel. However, because the primary and secondary pistons38and40are connected together, the secondary piston40is prevented from this movement and relatively little loss of travel occurs in this type of failure.

The primary and secondary pistons38and40can be connected together by any suitable manner. For example, as schematically shown inFIGS. 1 and 2, a locking member180is disposed and trapped between the primary and secondary pistons38and40. The locking member180includes a first end182and a second end184. The first end182is trapped within the cavity88of the second end86of the primary piston38. The second end184of the locking member180is trapped within a recess or cavity186formed in the secondary piston40. The first and second ends182and184may include enlarged head portions which are trapped behind narrower openings192and194of the cavities88and186, respectively. A first spring188is housed within the cavity88of the primary piston38and biases the locking member180in a direction towards the primary piston38and away from the secondary piston40. A second spring190is housed within the cavity186of the secondary piston40and biases the locking member180in a direction towards the primary piston38and away from the secondary piston40. The springs188and190and the locking member180maintain the first and second output piston at a spaced apart distance from one another while permitting limited movement towards and away from each other by compression of the springs188or190. This limited play mechanical connection permits the primary and secondary pistons38and40to move relative to one another by small increments to compensate for pressure and/or volume differences in their respective output circuits.

Referring back toFIG. 1, the system10may further include a travel sensor, schematically shown at240inFIG. 1, for producing a signal that is indicative of the length of travel of the input piston34which is indicative of the pedal travel. The system10may also include a switch252for producing a signal for actuation of a brake light and to provide a signal indicative of movement of the input piston34. The brake system10may further include sensors such as pressure transducers257and259for monitoring the pressure in the conduits156and166, respectively.

The system10further includes a source of pressure in the form of a plunger assembly, indicated generally at300. As will be explained in detail below, the system10uses the plunger assembly300to provide a desired pressure level to each of the wheel brakes16a-d. Fluid from the wheel brakes16a-dis returned to the plunger assembly300.

The system10further includes a first isolation valve302and a second isolation valve304(or referred to as switching valves or switching valve arrangement). The isolation valves302and304may be solenoid actuated valves. The isolation valves302and304are generally operable between an open position302a, as schematically shown inFIG. 1, and a closed position302b. The first isolation valve302is in fluid communication with the primary output conduit156such that when the first isolation valve302is in its open position302a, fluid flow is permitted between the first output pressure chamber198and the plunger assembly300via the primary output conduit156and a conduit306. When the first isolation valve302is in its closed position302b, fluid flow is restricted from flowing though the primary output conduit156to the conduit306. The second isolation valve304is in fluid communication with the secondary output conduit166such that when the second isolation valve304is in its open position304a, fluid flow is permitted between the second output pressure chamber228and the plunger assembly300via the secondary output conduit166and a conduit308. When the second isolation valve304is in its closed position304b, fluid flow is restricted from flowing though the secondary output conduit166to the conduit308.

The system10may further include various valves, such as a slip control valve arrangement, for permitting controlled braking operations, such as ABS, traction control, vehicle stability control, and regenerative braking blending. In the embodiment shown inFIG. 1, the system10includes first, second, third, and fourth control valves310,312,314, and316. Similar to the isolation valves302and304, the control valves310,312,314, and316may be solenoid actuated valves movable between open and closed positions and constructed to permit high pressure fluid to flow in both directions through the valve. The control valve310is in fluid communication with the plunger assembly300via conduit320. The control valve310is also in fluid communication with the wheel brake16avia a wheel conduit322. The control valve312is in fluid communication with the plunger assembly300via conduit324. The control valve312is also in fluid communication with the wheel brake16bvia a wheel conduit326. The control valve314is in fluid communication with the plunger assembly300via the conduit324. The control valve314is also in fluid communication with the wheel brake16bvia a wheel conduit328. The control valve316is in fluid communication with the plunger assembly300via the conduit320. The control valve316is also in fluid communication with the wheel brake16bvia a wheel conduit328. A pressure transducer321or other sensor may be included in the system10to monitor the pressure within the conduit320. The system10may also include a pressure transducer or sensor (not shown) to monitor the pressure within the conduit324.

As stated above, the system10includes a source of pressure in the form of the plunger assembly300to provide a desired pressure level to each of the wheel brakes16a-d. As best shown inFIG. 3, the plunger assembly300includes a housing340having a bore342formed therein. Slidably disposed in the bore342are first and second pistons344and346, respectively. The plunger assembly300further includes a ball screw mechanism, indicated generally at350. The ball screw mechanism350is provided to impart translational or linear motion of the first piston344along an axis defined by the bore342in both an actuation direction (leftward as viewingFIGS. 1 and 3), and a retraction direction (rightward as viewingFIGS. 1 and 3) within the bore342of the housing340. In the embodiment shown, the ball screw mechanism350includes a motor352rotatably driving a screw shaft354. A motor352may include a sensor353for detecting the rotational position of the motor352and/or ball screw mechanism350which is indicative of the position of the first piston344. This may be particular useful for a motor352which is capable of very accurate control including controlling the motor to minute movements for providing multiplex control as will be described below. The first piston344includes a threaded bore356and functions as a driven nut of the ball screw mechanism350. The ball screw mechanism350includes a plurality of balls358that are retained within helical raceways formed in the screw shaft354and the threaded bore356of the first piston344to reduce friction. Although a ball screw mechanism350is shown and described with respect to the plunger assembly300, it should be understood that other suitable mechanical linear actuators may be used for imparting movement of the first piston344. It should also be understood that although the first piston344functions as the nut of the ball screw mechanism250, the first piston344could be configured to function as a screw shaft of the ball screw mechanism350. Of course, under this circumstance, the screw shaft354would be configured to function as a nut having internal helical raceways formed therein.

The first piston344includes an outer cylindrical surface360. An O-ring362is mounted within a groove364formed in the bore342. A lip seal366is mounted within a groove368formed in the bore342. The O-ring362and the lip seal366sealingly engage with the outer cylindrical surface360of the first piston344. The first piston344includes a pin or an extension370extending towards the second piston346. The extension370includes an enlarged head372. The enlarged head372is trapped within a cavity374formed in the second piston356by an inwardly extending flange376. The first piston344is mechanically connected to the second piston346by the cooperation of the extension370and the flange376while still permitting a predetermined amount of movement therebetween. The first piston344is biased in a direction away from the second piston346by a spring380. The spring380generally acts on end surfaces of the pistons344and346which face one another. The spring380can be generally housed within a recess382formed in the first piston344.

The second piston346includes an outer cylindrical surface384. An O-ring386is mounted within a groove387formed in the bore342. A lip seal388is mounted within a groove389formed in the bore342. The O-ring386and the lip seal388sealingly engages with the outer cylindrical surface384of the second piston346. It should be understood that any suitable sealing structure may be used for the O-rings362and386and the lip seals366and388. The second piston346includes a pin or an extension390extending towards the end of the bore342. The extension390includes an enlarged head392. The enlarged head392is trapped within a cavity394formed in the end of the bore342of the housing340by an inwardly extending flange396. The second piston346is mechanically connected to the housing340by the cooperation of the extension390and the flange396while still permitting a predetermined amount of movement therebetween. The second piston346is biased in a direction away from the end of the bore340(and towards the first piston344, by a spring400. The spring400can be generally housed within a recess402formed in the second piston346. The springs380and400generally position the second piston346relative to the first piston344within the bore342. The springs380and400also function as return springs by biasing the first and second pistons344and346into their rest positions as shown inFIGS. 1 and 3.

The plunger assembly300includes a first pressure chamber410and a second pressure chamber412. The first pressure chamber410is generally defined by the bore340, the first and second pistons344and346, the lip seal366, and the O-ring386. The first pressure chamber410communicates with the conduit308which is in selective communication with the secondary output conduit166via the second isolation valve304. The first pressure chamber410is also in fluid communication with the conduit324which is in selective fluid communication with the wheel brakes16band16cvia the control valves312and314. The second pressure chamber412is generally defined by the bore340, the second piston346, and the lip seal388. The second pressure chamber412communicates with the conduit306which is in selective communication with the primary output conduit156via the first isolation valve302. The second pressure chamber412is also in fluid communication with the conduit320which is in selective fluid communication with the wheel brakes16aand16dvia the control valves310and316.

The gap between the O-ring362and the lip seal366is vented or in fluid communication with the reservoir18via the conduit296. Similarly, the gap between the O-ring386and the lip seal388is vented or in fluid communication with the reservoir18via the conduit296.

As stated above, the brake pedal unit assembly12includes a simulation valve74which may be mounted in the housing24or remotely from the housing24. As schematically shown inFIGS. 1 and 2, the simulation valve74may be a solenoid actuated valve. The simulation valve74includes a first port75and a second port77. The port75is in fluid communication with the conduit146which is in fluid communication with the simulation chamber144. The port77is in fluid communication with the conduit72which is in fluid communication with the reservoir18via the conduits66and68. The simulation valve74is movable between a first position74arestricting the flow of fluid from the simulation chamber144to the reservoir18, and a second position74bpermitting the flow of fluid between the reservoir18and the simulation chamber144. The simulation valve74is in the first position or normally closed position when not actuated such that fluid is prevented from flowing out of the simulation chamber144through conduit72, as will be explained in detail below.

The following is a description of the operation of the brake system10.FIGS. 1 and 2illustrate the brake system10and the brake pedal unit20in the rest position. In this condition, the driver is not depressing the brake pedal42. Also in the rest condition, the simulation valve74may be energized or not energized. During a typical braking condition, the brake pedal42is depressed by the driver of the vehicle. The brake pedal42is coupled to the travel sensor240for producing a signal that is indicative of the length of travel of the input piston34and providing the signal to an electronic control module (not shown). The control module may include a microprocessor. The control module receives various signals, processes signals, and controls the operation of various electrical components of the brake system10in response to the received signals. The control module can be connected to various sensors such as pressure sensors, travel sensors, switches, wheel speed sensors, and steering angle sensors. The control module may also be connected to an external module (not shown) for receiving information related to yaw rate, lateral acceleration, longitudinal acceleration of the vehicle such as for controlling the brake system10during vehicle stability operation. Additionally, the control module may be connected to the instrument cluster for collecting and supplying information related to warning indicators such as ABS warning light, brake fluid level warning light, and traction control/vehicle stability control indicator light.

During normal braking operations (normal boost apply braking operation) the plunger assembly300is operated to provide boost pressure to the conduit320and324for actuation of the wheel brakes16a-d. Under certain driving conditions, the control module communicates with a powertrain control module (not shown) and other additional braking controllers of the vehicle to provide coordinated braking during advanced braking control schemes (e.g., anti-lock braking (AB), traction control (TC), vehicle stability control (VSC), and regenerative brake blending). During a normal boost apply braking operation, the flow of pressurized fluid from the brake pedal unit20generated by depression of the brake pedal42is diverted into the internal pedal simulator assembly100. The simulation valve74is actuated to divert fluid through the simulation valve74from the simulation chamber144to the reservoir18via the conduits146,72,66, and68. Note that fluid flow from the simulation chamber144to the reservoir18is closed off once the passageways64in the input piston34move past the seal60. Prior to movement of the input piston34, as shown inFIGS. 1 and 2, the simulation chamber144is in fluid communication with the reservoir18via the conduits66and68.

During the duration of the normal braking mode, the simulation valve74remains open permitting the fluid to flow from the simulation chamber144to the reservoir18. The fluid within the simulation chamber144is non-pressurized and is under very low pressures, such as atmospheric or low reservoir pressure. This non-pressurized configuration has an advantage of not subjecting the sealing surfaces of the pedal simulator to large frictional forces from seals acting against surfaces due to high pressure fluid. In conventional pedal simulators, the piston(s) are under increasingly high pressures as the brake pedal is depressed subjecting them large frictional forces from the seals, thereby adversely effecting the pedal feel.

Also during the normal boost apply braking operation, the first and second isolation valves302and304are energized to their closed positions302band304b, respectively, to prevent the flow of fluid from the conduits156and166to the plunger assembly300and wheel brakes16a-d. Thus, the fluid within the first and second output pressure chambers198and228of the brake pressure unit20are fluidly locked which generally prevents the first and second output pistons38and40from moving further. More specifically, during the initial stage of the normal boost apply braking operation, movement of the input rod45causes movement of the input piston34in a rightward direction, as viewingFIG. 2. Initial movement of the input piston34causes movement of the primary piston38via the low rate simulator spring108. Movement of the primary piston38causes initial movement of the secondary piston40due to the mechanical connection therebetween by the locking member180and the springs188and190. Note that during this initial movement of the primary piston38, fluid is free to flow from the primary pressure chamber198to the reservoir18via conduits85,154, and68until the conduit85moves past the seal81. Also, during initial movement of the secondary piston40, fluid is free to flow from the secondary pressure chamber228to the reservoir18via the conduits155and164until the conduit155moves past the seal154.

After the primary and secondary pistons38and40stop moving (by closing of the conduits85and155and closing of the first and second base brake valves320and322), the input piston34continues to move rightward, as viewingFIGS. 1 and 2, upon further movement by the driver depressing the brake pedal42. Further movement of the input piston34compresses the various springs of the pedal simulator assembly100, thereby providing a feedback force to the driver of the vehicle.

During normal braking operations (normal boost apply braking operation) while the pedal simulator assembly100is being actuated by depression of the brake pedal42, the plunger assembly300can be actuated by the electronic control unit to provide actuation of the wheel brakes16a-d. Actuation of the isolation valves302and304to their closed positions302band304bisolated the brake pedal unit12from the wheel brakes16a-d. The plunger assembly300may provide “boosted” or higher pressure levels to the wheel brakes16a-dcompared to the pressure generated by the brake pedal unit12by the driver depressing the brake pedal42. Thus, the system10provides for assisted braking in which boosted pressure is supplied to the wheel brakes16a-dduring a normal boost apply braking operation helping reduce the force required by the driver acting on the brake pedal42.

To actuate the wheel brakes16a-dvia the plunger assembly300, the electronic control unit actuates the motor352in a first rotational direction to rotate the screw shaft354in the first rotational direction. Rotation of the screw shaft354in the first rotational direction causes the first piston344to advance in the actuation direction (leftward as viewingFIGS. 1 and 3). Movement of the first piston344causes the spring380to push against the second piston346, thereby initiating movement of the second piston346. Further movement of the first piston344also causes a pressure increase in the first pressure chamber410and fluid to flow out of the first pressure chamber410and into the conduit324. Note that fluid is prevented from flowing into the conduit308from the first pressure chamber410due to the isolation valve304being in its closed position304b. A pressure increase in the first pressure chamber410may also cause the second piston412to move in the actuation direction, thereby causing a pressure increase in the second pressure chamber412. Fluid flows out of the second pressure chamber412through the conduit320. Note that fluid is prevented from flowing into the conduit306from the second pressure chamber412due to the isolation valve302being in its closed position302b. Pressurized fluid flowing into the conduits320and324and through the open control valves310,312,314, and316causes actuation of the wheel brakes16a-dvia. Braking can be increased by advancing the first and second pistons via the screw shaft354of the ball screw mechanism350.

When the driver releases the brake pedal42, the pressurized fluid from the wheel brakes16a-dmay back drive the ball screw mechanism350moving the first and second pistons344and346back towards their rest position. Under certain circumstances, it may also be desirable to actuate the motor352to a second rotational direction opposite the first rotational direction to cause the first and second pistons344and346to move in a retraction direction (rightward as viewingFIGS. 1 and 3), thereby withdrawing the fluid from the wheel brakes16a-dand replenishing the first and second pressure chambers410and412. The motor352of the plunger assembly300may be actuated in the first and second rotational directions to provide an increase and decrease, respectively, in braking pressure at the wheel brakes16a-d. All of the control valves310,312,314, and316can be controlled (non-energized) to an open position to provide braking to all wheel brakes16a-dsimultaneously. Alternatively, as will be explained below, the control valves310,312,314, and316can be actuated individually between their open and closed positions to provide different braking pressures within the wheel brakes16a-d.

A stated above, the control valves310,312,314, and316can be actuated individually between their open and closed positions to provide different braking pressures within the wheel brakes16a-d. This may be used during various braking functions such as anti-lock braking, traction control, dynamic rear proportioning, vehicle stability control, hill hold, and regenerative braking. The plunger assembly300and the control valves310,312,314, and316are operated by the electronic control unit (not shown). The plunger assembly300is preferably configured and operated by the electronic control unit (not shown) such that relatively small rotational increments of the motor352and/or ball screw mechanism350are obtainable. Thus, small volumes of fluid and relatively minute pressure levels are able to be applied and removed from the conduits320and324. For example, the motor352may be actuated to turn 0.5 of a degree to provide a relatively small amount of fluid and pressure increase. This enables a multiplexing arrangement such that the plunger assembly300can be controlled to provide individual wheel pressure control. For example, if it is determined by the electronic control unit that the wheel brakes16aand16drequire an increase in pressure to stabilize the vehicle, the control valves310and316can be actuated to their open positions. The remaining control valves312and314are actuated to their closed positions. The motor352of the plunger assembly300is then actuated to deliver the required pressure level to the wheel brakes16aand16dvia the pressure chamber412and the conduits320,322and330. To maintain the pressure level within the wheel brakes16aand16d, the control valves310and316can be actuated to their closed positions. To decrease the pressure within the wheel brakes16aand16d, the motor352can be actuated into its opposite rotational direction and the control valves310and316are actuated accordingly. If during the event, the electronic control unit determines that different pressures are required in the wheel brakes16aand16d, the control valves310and316can be controlled individually to permit in increase or decrease in pressure via the conduits310and330, respectively, as required. Thus, the plunger assembly300and the system10can be operated to provide individual control for the wheel brakes16a-dor can be used to control one or more wheel brakes16a-dsimultaneously by opening and closing the appropriate control valves310,312,314, and316.

Although the system10is shown using single control valves310,312,314, and316for each of the wheel brakes16a-d, respectively, the system may be configured to include a pair of solenoid actuated control valves (not shown) for each of the wheel brakes16a-d. Each pair of valves would be arranged in a parallel arrangement with respect to the conduits between the plunger assembly300and the respective wheel brake16a-d. Thus, the illustrated system10would include eight control valves instead of the four control valves310,312,314, and316. The dual valves are controlled simultaneously between their open and closed positions. It may be more cost effective to have two smaller valves actuated simultaneously compared to having a single but larger control valve. To provide the generally same volume and pressure flow, the pair of valves may have smaller springs with lower spring rates compared to the single valve configuration. This may reduce the overall cost as well as being a quieter system since the solenoid required to overcome the bias of the springs may be smaller. For the dual control valve arrangement (not shown), the dual valves can be arranged within the system10such that the fluid flow through one of the dual valves is reversed relative to the other dual valve. The dual valves may include a valve seat arrangement in which flow can flow through the valve seat in either of two directions. In the first direction, the fluid flows first through the valve seat and around the ball or valve member. In the second direction, the fluid flows first around the ball or valve member and then through the valve seat. Although the dual valves may be generally identically structured, they may be situated within the housing of the hydraulic control unit14in a reversed manner. The use of a pair of dual valves having smaller spring rates with a reverse flow arrangement may provide better proportional control than a single larger spring valve. Proportional control is when a pressure increase or decrease is provided to more than wheel brake at a time, wherein the wheel brakes are at different pressures. Proportional control can be accomplished in the brake system10by using the plunger assembly300in cooperation with the respective control valve(s) for a first wheel brake, and then simultaneously using only control of the respective control valve(s) for a second wheel brake. The use of dual valves in a parallel arrangement may also prevent unwanted hydraulic braking in both directions of fluid flow.

In the event of a loss of electrical power to portions of the brake system10, the brake system10provides for manual push through or manual apply such that the brake pedal unit20can supply relatively high pressure fluid to the primary output conduit156and the secondary output conduit166. During an electrical failure, the motor352of the plunger assembly300might cease to operate, thereby failing to produce pressurized hydraulic brake fluid from the plunger assembly300. The isolation valves302and304will shuttle (or remain) in their open positions302aand304aas shown inFIG. 1. In these positions, the isolation valves302and304permit fluid flow from the conduits156and166to the wheel brakes16a-dthrough the plunger assembly300. More specifically, fluid flow is permitted to flow from the primary output conduit156then through the open isolation valve302, the conduit306, the secondary pressure chamber412, the conduit320, the open control valves310and316, the conduits322and330, and to the wheel brakes16aand16d. Similarly, fluid flow is permitted to flow from the secondary output conduit166then through the open isolation valve304, the conduit308, the primary pressure chamber410, the conduit324, the open control valves312and314, the conduits326and328, and to the wheel brakes16band16c. Thus, the brake pedal unit20may now provide a manual apply for energizing the conduits320and324for actuation of the wheel brakes16a-d. The simulation valve74is shuttled to its closed position74a, as shown inFIGS. 1 and 2, to prevent fluid from flowing out of the simulation chamber144to the reservoir18. Thus, moving the simulation valve74to its closed position74ahydraulically locks the simulation chamber144trapping fluid therein. During the manual push-through apply, the primary and secondary output pistons38and40will advance rightward pressurizing the chambers198and228, respectively. Fluid flows from the chambers198and228into the conduits156and166, respectively, to actuate the wheel brakes16a-das described above.

During the manual push-through apply, initial movement of the input piston34forces the spring(s) of the pedal simulator to start moving the pistons38and40. After further movement of the input piston34, in which the fluid within the simulation chamber144is trapped or hydraulically locked, further movement of the input piston34pressurizes the simulation chamber144causing movement of the primary piston38which also causes movement of the secondary piston40due to pressurizing of the primary chamber144. As shown inFIGS. 1 and 2, the input piston34has a smaller diameter (about the seal60) than the diameter of the primary piston38(about the seal80). Since the hydraulic effective area of the input piston34is less than the hydraulic effective area of the primary piston38, the input piston34may travel more axially in the right-hand direction as viewingFIGS. 1 and 2than the primary piston38. An advantage of this configuration is that although a reduced diameter effective area of the input piston34compared to the larger diameter effective area of the primary piston38requires further travel, the force input by the driver's foot is reduced. Thus, less force is required by the driver acting on the brake pedal42to pressurize the wheel brakes compared to a system in which the input piston and the primary piston have equal diameters.

In another example of a failed condition of the brake system10, the hydraulic control unit12may fail as discussed above and furthermore one of the output pressure chambers198and228may be reduced to zero or reservoir pressure, such as failure of a seal or a leak in one of the conduits156or166. The mechanical connection of the primary and secondary pistons38and40prevents a large gap or distance between the pistons38and40and prevents having to advance the pistons38and40over a relatively large distance without any increase in pressure in the non-failed circuit. For example, if the brake system10is under a manual push through mode and additionally fluid pressure is lost in the output circuit relative to the secondary piston40, such as for example in the conduit166, the secondary piston40will be forced or biased in the rightward direction due to the pressure within the primary chamber198. If the primary and secondary pistons38and40were not connected together, the secondary piston40would freely travel to its further most right-hand position, as viewingFIGS. 1 and 2, and the driver would have to depress the pedal42a distance to compensate for this loss in travel. However, because the primary and secondary pistons38and40are connected together through the locking member180, the secondary piston40is prevented from this movement and relatively little loss of travel occurs in this type of failure. Thus, the maximum volume of the primary pressure chamber198is limited had the secondary piston40not be connected to the primary piston38.

In another example, if the brake system10is under a manual push through mode and additionally fluid pressure is lost in the output circuit relative to the primary piston40, such as for example, in the conduit156, the secondary piston40will be forced or biased in the leftward direction due to the pressure within the secondary chamber228. Due to the configuration of the brake pedal unit20, the left-hand end of the secondary piston40is relatively close to the right-hand end of the primary piston38. Thus, movement of the secondary piston40towards the primary piston38during this loss of pressure is reduced compared to a conventional master cylinder in which the primary and secondary pistons have equal diameters and are slidably disposed in the same diameter bore. To accomplish this advantage, the housing24of the brake pedal unit20includes a stepped bore arrangement such that diameter of the second bore28which houses the primary piston38is larger than the third bore30housing the secondary piston40. A portion of the primary chamber198includes an annular region surrounding a left-hand portion of the secondary piston40such that the primary and secondary pistons38and40can remain relatively close to one another during a manual push-through operation. In the configuration shown, the primary and secondary pistons38and40travel together during a manual push-through operation in which both of the circuits corresponding to the conduits156and166are intact. This same travel speed is due to the hydraulic effective areas of the pistons38and40, for their respective output pressure chambers198and228, are approximately equal. In a preferred embodiment, the area of the diameter of the secondary piston40is approximately equal to the area of the diameter of the primary piston38minus the area of the diameter of the secondary piston40. Of course, the brake pedal unit20could be configured differently such that the primary and secondary pistons38and40travel at different speeds and distances during a manual push though operation.

During a manual push-through operation in which both of the circuits corresponding to the conduits156and166are intact, such as during an electrical failure described above, the combined hydraulic effective area of the primary and secondary pistons38and40is the area of the diameter of the primary piston38. However, during a failure of one of the circuits corresponding to the conduits156and166, such as by a leak in the conduit166, the hydraulic effective area is halved such that the driver can now generate double the pressure within the primary chamber198and the non-failed conduit156when advancing the primary piston38during a manual push-through operation via depression of the brake pedal42. Thus, even though the driver is actuating only two of the wheel brakes16aand16dduring this manual push through operation, a greater pressure is obtainable in the non-failed primary chamber198. Of course, the stroke length of the primary piston38will need to be increased to compensate.

The plunger assembly300also includes features to assist during certain failed conditions. The extension370and the enlarged head372of the first piston344and the extension390and enlarged head392of the second piston346restrict the movement of the second piston346relative to the first piston344. Rearward travel of the second piston346is also limited by the connection of the enlarged head392to the housing340. This configuration limits the maximum volume of the first and second pressure chambers410and412. This restriction in movement may be useful during a downstream failed condition in which one of the circuits corresponding to the conduits320and324leaks. For example, in a detected failed condition in which fluid within one or more of the conduits320,322, and330leaks, the electronic control unit may enter into a manual push-through mode such that the isolation valves302and304are actuated to their open positions and the brake pedal unit20is used to provide pressure within the output conduits156and166. In this manual push-through situation, fluid flows through the pressure chambers410and412of the plunger assembly300. In this example of a failed condition, fluid is leaking from the conduit320. The configuration of the plunger assembly300prevents the first pressure chamber410from expanding due to its increase in pressure relative to the second pressure chamber412associated with the leak. If the secondary piston346was not mechanically connected and permitted to move, the first pressure chamber410would expand and the pistons of the brake pedal unit20would need to be advanced to accommodate the expanding first pressure chamber410, thereby causing loss pedal travel experienced by the driver. Similarly, a failed condition in which fluid within one or more of the conduits324,326, and328leaks, a loss in pressure within the first pressure chamber410would occur. The extension390and the enlarged head392prevents appreciable retraction of the second piston346in the retraction direction (rightward as viewingFIGS. 1 and 3) due to the greater pressure within the second pressure chamber412relative to the first pressure chamber410.

During a normal boost apply operation of the system10in which the plunger assembly300is supplying pressure to one of the conduits320and/or324, the first and second pistons344and346will have been advanced in the actuation direction by the ball screw assembly350. If a failure occurs under this condition in which the electronic control unit enters the system10into a manual push-through mode, the isolations valves302and304may be de-energized from a closed position to an open position. When the isolation valves302and304are moved into their open positions, the pressure within the first and second pressure chambers410and412would either cause the ball screw assembly350to back drive such that the first piston344is moved in the retraction direction (rightward as viewingFIGS. 1 and 3) or cause the pressure within the conduits156and166to force the pistons of the brake pedal unit20rearwardly forcing the pedal42rearwardly as well. The driver may compensate for the back driven ball screw mechanism350by further pressing on the pedal42to advance the pistons of the brake pedal unit20, thereby compensating for this lost travel. Although this may be very acceptable during such a failed condition, the valve components of the isolation valves302and304could be configured to hydraulically lock during this situation to prevent rearward movement of the pistons of the brake pedal unit20even though the isolation valves302and304are de-energized. Thus, even in an electrical shut down of the system10during a normal boost apply mode, the isolation valves302and304, which would be de-energized to their normally open positions, would remain in an internally closed condition (hydraulically locked) such that the internal valves of the isolation valves302and304would prevent fluid from flowing from the first and second chambers410and412into the conduits156and166. Thus, this built up in pressure within the plunger assembly300would not initially force the pistons of the brake pedal unit20rearwardly forcing the pedal42rearwardly as well. The isolation valves302and304could also be configured such that when the driver depresses the pedal42to generate pressure within the brake pedal unit20during a manual push-though mode, the increase in pressure within the conduits156and166opens the internal valves of the isolation valves302and304permitting flow into the wheel brakes16a-dsuch as during normal manual push-though operation described above. Preferably, the isolation valves302and304are configured such that they will not hydraulically lock under other conditions such as during a spike or rapid apply. Thus, the isolation valves302and304may be configured such that a low pressure level acting on the valves302and304will unlock the valves from their temporary hydraulic lock condition.

There is schematically illustrated inFIG. 4a second embodiment of a vehicle brake system, indicated generally at500. Similar to the above described brake system10, the brake system500may suitably be used on a ground vehicle such as an automotive vehicle having four wheels and a wheel brake for each wheel. Furthermore, the brake system500can be provided with other braking functions such as anti-lock braking (ABS), other slip control features, and regenerative braking blending to effectively brake the vehicle. The brake system500is similar in function and structure of some aspects of the brake system10and, therefore, like numbers and/or names may be used to reference similar components.

Similar to the brake system10, the brake system500includes a brake pedal unit assembly, indicated by broken lines12, including a brake pedal unit20, reservoir18, brake pedal42, and simulation valve74which are similar in function and structure as describe above with respect to the brake system10. The brake system500also includes wheel brakes16a-d, first and second isolation valves302and304, and control valves310,312,314, and316which are similar in function and structure as described above with respect to the brake system10. The components located out of the brake pedal unit assembly12may be housed in a hydraulic control unit housing or may be located remotely from one another.

The brake assembly500further includes a plunger assembly, indicated generally at502. Although the plunger assembly502is similar to the plunger assembly300described above with respect to the brake system10, there are some differences that will be described below. As best shown inFIG. 5, the plunger assembly502includes a housing540having a bore542formed therein. Slidably disposed in the bore542are first and second pistons544and546, respectively. The plunger assembly502further includes a ball screw mechanism, indicated generally at550. The ball screw mechanism550is provided to impart translational or linear motion of the first piston544along an axis defined by the bore542in both an actuation direction (downward as viewingFIGS. 4 and 5), and a retraction direction (upward as viewingFIGS. 4 and 5) within the bore542of the housing540. In the embodiment shown, the ball screw mechanism550includes a motor552rotatably driving a screw shaft554. A motor552may include a sensor553for detecting the rotational position of the motor552and/or ball screw mechanism550which is indicative of the position of the first piston544. This may be particular useful for a motor552which is capable of very accurate control including controlling the motor to minute movements for providing multiplex control as will be described below. The first piston544includes a threaded bore556and functions as a driven nut of the ball screw mechanism550. The ball screw mechanism550includes a plurality of balls558that are retained within helical raceways formed in the screw shaft554and the threaded bore556of the first piston544to reduce friction.

The first piston544includes an outer cylindrical surface560. A seal, or O-ring562, is mounted within a groove564formed in the bore542. A lip seal566is mounted within a groove568formed in the bore542. The O-ring562and the lip seal566sealingly engage with the outer cylindrical surface560of the first piston544. The first piston544includes a pin or an extension570extending towards the second piston546. The extension570includes an enlarged head572. The enlarged head572is trapped within a cavity574formed in the second piston556by an inwardly extending flange576. The first piston544is mechanically connected to the second piston546by the cooperation of the extension570and the flange576while still permitting a predetermined amount of movement therebetween. The first piston544is biased in a direction away from the second piston546by a spring580. The spring580generally acts on end surfaces of the pistons544and546which face one another. The spring580can be generally housed within a recess582formed in the first piston544.

The second piston546includes an outer cylindrical surface584. An O-ring586is mounted within a groove587formed in outer cylindrical surface584of the second piston546. The O-ring586sealingly engage with the wall of the bore542. Instead of having a pin or an extension, the second piston546is limited in its travel by an outwardly extending flange590positioned and trapped within a recess592formed in the bore542. The recess592defines a pair of shoulders593and594which may engage with the flange590of the second piston546to mechanically connect the second piston546to the housing540while still permitting a predetermined amount of movement therebetween. The second piston546is biased in a direction away from the end of the bore550, and towards the first piston544, by a spring600. The springs580and600generally position the second piston546relative to the first piston544within the bore542. The springs580and600also function as return springs by biasing the first and second pistons544and546into their rest positions as shown inFIGS. 4 and 5.

The plunger assembly502includes a first pressure chamber610and a second pressure chamber612. The first pressure chamber610is generally defined by the bore540, the first and second pistons544and546, the lip seal566, and the O-ring586. The first pressure chamber610communicates with a conduit324awhich is in fluid communication with the conduit324. The second pressure chamber612is generally defined by the bore540, the second piston546, and the O-ring586. The second pressure chamber612communicates with a conduit320awhich is in fluid communication with the conduit320. Unlike the plunger assembly300, adjacent areas of the seals of the plunger assembly502are not vented or in fluid communication with the reservoir18.

The brake system500operates similarly as the system10described above. To actuate the wheel brakes16a-dvia the plunger assembly502, the electronic control unit actuates the motor552in a first rotational direction to rotate the screw shaft554in the first rotational direction. Rotation of the screw shaft554in the first rotational direction causes the first piston544to advance in the actuation direction (downward as viewingFIGS. 4 and 5) causing initial movement of the second piston612by the spring570. Movement of the first piston544causes a pressure increase in the first pressure chamber610and fluid to flow out of the first pressure chamber610and into the conduit324a. Note that fluid is prevented from flowing into the secondary output conduit166from the first pressure chamber610due to the isolation valve304being in its closed position. A pressure increase in the first pressure chamber610will also cause the second piston612to move in the actuation direction, thereby causing a pressure increase in the second pressure chamber612. Fluid flows out of the second pressure chamber612through the conduit320a. Note that fluid is prevented from flowing into the conduit320from the second pressure chamber612due to the isolation valve302being in its closed position. Pressurized fluid flowing into the conduits320and324and through the open control valves310,312,314, and316causes actuation of the wheel brakes16a-dvia. Similar to the brake system10, braking can be increased by advancing the first and second pistons544and546via the screw shaft554of the ball screw mechanism550. To reduce pressure within the wheel brakes16a-d, the motor552is actuated to a second rotational direction opposite the first rotational direction to cause the first and second pistons544and546to move in a retraction direction (upward as viewingFIGS. 1 and 3), thereby withdrawing the fluid from the wheel brakes16a-dand replenishing the first and second pressure chambers610and612. The motor552of the plunger assembly502may be actuated in the first and second rotational directions to provide an increase and decrease, respectively, in braking pressure at the wheel brakes16a-d. All of the control valves310,312,314, and316can be controlled (non-energized) to an open position to provide braking to all wheel brakes16a-dsimultaneously. Alternatively, the control valves310,312,314, and316can be actuated individually between their open and closed positions to provide different braking pressures within the wheel brakes16a-d.

Similar to the plunger assembly300, the plunger assembly502includes features to assist during certain failed conditions such as limiting the maximum volume of the first and second pressure chambers410and412. In a failed condition in which fluid within the conduit324aleaks, a loss in pressure within the first pressure chamber610would occur. The cooperation of the flange590and the shoulder593of the recess592prevents appreciable retraction of the second piston346in the retraction direction (upward as viewingFIGS. 4 and 5) due to the greater pressure within the second pressure chamber612relative to the first pressure chamber610.

One of the advantages of the plunger assembly502compared to the plunger assembly300is the reduced number of seals. The plunger assembly300includes four seals362,366,386, and388compared to the three seals562,566, and586of the plunger assembly502. With a lower number of seals, the overall length of the plunger assembly502may be reduced. Another advantage is that at higher pressures within the plunger assembly, the fewer number of seals may reduce friction. Friction is also reduced because the delta pressure across a seal on the secondary piston is reduced during normal boos operation.

Another difference between the plunger assemblies300and502is that the plunger assembly300includes a conduit296connecting the reservoir18to the plunger assembly300. The conduit296branches into a pair of conduits such that a gap between the O-ring362and the lip seal366and a gap between the O-ring386and the lip seal388are in fluid communication with the reservoir18via the conduit296. With this configuration, it may be easier to detect a failure of one of the seals of the plunger assembly300as compared to the plunger assembly502. The conduit296allows failure detection during normal boos operation by determining abnormal travel which is out of sync with anticipated pressure. During non-braking events, the electronic control unit may perform testing on the system10to detect a failure of the seals by monitoring the abnormal fluid flow into the conduit296past a failed seal within the plunger assembly300.

There is illustrated inFIGS. 6 and 7a third embodiment of a plunger assembly, indicated generally at700. The plunger assembly700is similar in structure and function as the plunger assemblies300and502. One of the differences is that the plunger assembly700includes a hollow outer sleeve702which is mounted within a bore704of a housing706. First and second pistons744and746are slidably mounted in a stepped inner bore of the outer sleeve702, as will be discussed below. The outer sleeve702may be helpful for bleeding and evacuation purposes of the plunger assembly700compared to a plunger assembly having first and second pistons mounted in a bore of the housing. If the housing706were made of aluminum, a separate sleeve702made of a hard coat anodized material may be desirable for housing the first and second pistons744and746. The sleeve702may also assist in assembly due to the stepped bore design. As will be described below, one of the advantages of the plunger assembly700is that the pistons744and746and associated pressure chambers are arranged in an overlapping manner which helps reduce the overall length of plunger assembly700.

The plunger assembly700further includes a ball screw mechanism, indicated schematically at750. The ball screw mechanism750is provided to impart translational or linear motion of the first piston744along an axis defined by the bore704in both an actuation direction (leftward as viewingFIGS. 1 and 3), and a retraction direction (rightward as viewingFIGS. 1 and 3). In the embodiment shown, the ball screw mechanism750includes a motor (not shown) driving an actuator754. The actuator754may be prevented from rotation by an anti-rotation device including a pair of rollers714translating in corresponding tracks716, as shown inFIG. 6. Thus, the actuator754is moved in a liner manner by the ball screw mechanism750. The actuator754engages a retainer720which is threadably connected to an end of the first piston744. The retainer720includes a seal722for sealing off the interior of the first piston744relative to the ball screw mechanism750.

The first piston744includes an outer cylindrical surface760. Pair of seals762and766is mounted on the outer sleeve702and sealingly engage with the surface760of the first piston744. The plunger assembly700also includes a mechanical coupling to the second piston746via a caged spring assembly, indicated generally at747. The caged spring assembly747includes an extension pin770threadably connected to the end of the second piston746and having an enlarged head772engaged and trapped by an inwardly extending flange774of the first piston744. A spring780biases the second piston746in a direction away from the first piston744.

The second piston746includes an outer cylindrical surface784. An O-ring786is mounted on the second piston746. A caged spring assembly, indicated generally at789, includes an extension pin790threadably connected to the second piston746and having an enlarged head792engaged and trapped by a retainer796disposed in the end of the bore704. A spring800biases the retainer796away from the second piston746by a predetermined distance.

The plunger assembly700includes a first pressure chamber810and a second pressure chamber812. The first pressure chamber810includes an expanding and contraction portion generally disposed around the outer surface of the second piston746to help reduce the length of the plunger assembly700. The first pressure chamber810can communicate with the conduit324a, such as for the system500shown inFIG. 4, via passageways813formed through the outer sleeve702. The second chamber812can communicate with the conduit320a, such as for the system500, via slots815formed in the retainer796.

The outer sleeve702includes first bore portion703and a second bore portion705. The first bore portion703is defined by a diameter D1which is slightly smaller than the diameter D2of the second bore portion705. This configuration of the outer sleeve702enables communication between the primary chamber810and the conduit324aeven during long stroke lengths of the first piston744, as shown inFIG. 7, wherein the end of the first piston744moves past the passageways813.

The caged spring assemblies747and789limit the maximum volume of the first and secondary pressure chambers810and812, respectively, thereby reducing travel in certain types of failure conditions such as discussed above with respect to the other plunger assemblies described and shown herein.

There is illustrated inFIG. 8a fourth embodiment of a plunger assembly, indicated generally at900. The plunger assembly90is similar in structure and function as the plunger assemblies described above. The plunger assembly900includes a housing902having a bore904formed therein. First and second pistons910and912are slidably disposed in the bore904for generating pressure within first and second pressure chambers920and922, respectively. The bore904includes a pair of slot portions906to provide fluid communication between the first and second pressure chambers920and922and conduits930and932, respectively. The conduits930and932can be in fluid communication with the wheel brakes of a brake system shown and described above with respect to other plunger assemblies described herein. The slots906also provide flow paths from the reservoir ports to the pressure chambers past the recuperating lip seals.

The plunger assembly900may also included caged spring assemblies936and938, for limiting the maximum volume of the first and secondary pressure chambers920and922. The plunger assembly900may further includes a threaded cap942threadably connected to a retainer944of the caged spring assembly938for sealing off an opening943of the end of the bore904via a seal946. The removable cap942permits installation of various components of the plunger assembly900through the opening943as well as permitting access for threading various components together during installation, such as for example the caged spring assembly938. The plunger assembly900can be pre-assembled and slid into the bore. The opening943provides an access hole to adjust the threaded connection at the end of the bore. The cap942seals the opening943and locks the threaded connection in place.