Abstract:
A fluid compensator is provided in a hydraulic control unit of vehicular brake system. The fluid compensator permits additional displacement of a master cylinder piston during anti-lock braking events only by storing fluid in the hydraulic control unit. The additional displacement moves a seal mounted on the master cylinder piston away from a compensator port orifice, thereby preventing repeated “nibbling” of the seal which can cause premature seal failure.

Description:
This application is a continuation of PCT/U.S. 99/02256 filed Feb. 2, 1999. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates in general to vehicular brake systems, and in particular is concerned with a fluid compensator provided in a hydraulic control unit to accommodate travel of a master cylinder piston. 
     Hydraulic braking systems for vehicles are well known- A typical hydraulic brake system includes a master cylinder, fluid conduit arranged into a desired circuit and wheel brakes. The master cylinder generates hydraulic forces in the fluid circuit by pressurizing brake fluid when the driver steps on the brake pedal. A piston in the master cylinder travels in response to the brake pedal. The pressurized fluid travels through the fluid conduit in the circuit to actuate brake cylinders at the wheel brakes and slow the vehicle. 
     Anti-lock braking systems (ABS) for vehicles are also well known hydraulic systems. A hydraulic control unit (HCU) or housing, containing control valves and other components such as control valves and pumps, is located between the master cylinder and the wheel brake assemblies. Through an electronic controller, the control valves and other components selectively control pressure to the wheel brake assemblies to provide a desired braking response of the vehicle. 
     A seal is provided about the master cylinder piston. Preferably, this seal is formed as a one-way seal commonly known as a lip seal. During a braking event, this seal can be positioned nearby a compensation port orifice of the master cylinder. Repeated passes of a seal holding pressurized fluid across the compensation port orifice as the master cylinder piston is stroked can result in deterioration of the seal and shortened effective life of the seal. Known braking systems can provide additional displacement of the master cylinder piston to move the seal away from the compensation port orifice, but such systems add extra pedal stroke to every braking event, i.e., normal (base) braking as well as anti-lock braking. Additional pedal stroke is generally not desired in many-brake systems. 
     SUMMARY OF THE INVENTION 
     This invention includes a fluid compensator provided in a hydraulic control unit of vehicular brake systems. The fluid compensator permits additional displacement of a master cylinder piston during anti-lock braking events only by storing fluid in the hydraulic control unit. The additional displacement moves a seal mounted on the master cylinder piston away from a orifice, thereby preventing repeated “nibbling” of the seal compensator port which can cause premature seal failure. One advantage provided by this fluid compensator is that additional pedal stroke is not present during normal base braking, since this fluid compensator has no effect during normal braking. The fluid compensator is provided in a hydraulic control unit and can be used with many variations of anti-lock brake systems. 
     In a first preferred embodiment, a fluid compensator to accommodate piston displacement of a master cylinder in a vehicular brake system includes a body formed from a resilient material having a gas trapped in pores. The body includes a fluid impermeable outer surface. The body can be positioned in a bore of an attenuator downstream of a pump outlet. As fluid enters the bore, the body compresses to permit additional displacement of a master cylinder piston. 
     In a second preferred embodiment, a fluid compensator to accommodate piston displacement of a master cylinder in a vehicular braking system includes a fluid chamber formed between an end wall of a bore mounting a slidable piston in a low pressure accumulator and a seal mounted about the piston. A fluid passageway intersects a side wall of the bore at a predetermined distance away from the end wall. At predetermined positions of the piston, fluid flow out of the fluid chamber is restricted as a suction force from a pump inlet draws the piston against the fluid passageway. In a third preferred embodiment, a fluid compensator is formed in a manner similar to the second embodiment with the inclusion of an undercut provided about an outer surface of the piston. The undercut increases the annular restriction between the piston and the bore at the area where the fluid passageway intersects the side wall. 
     Various objects and advantages of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiment, when read in light of the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic circuit diagram of a vehicular brake system according to this invention illustrating a first embodiment of a fluid compensator provided on a high pressure side of a pump in a hydraulic control unit. 
     FIG. 2 is a sectional view through the fluid compensator of FIG.  1 . 
     FIG. 3 is a schematic circuit diagram of a vehicular brake system according to this invention illustrating a second embodiment of a fluid compensator provided on a low pressure side of a pump in a hydraulic control unit. 
     FIG. 4 is a sectional view through a third embodiment of a fluid compensator according to this invention provided on a low pressure side of a pump in a hydraulic control unit. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A vehicular brake system according to this invention is indicated generally at  10  in FIG.  1 . System  10  includes valves and other components described below to provide anti-lock braking functions. In other embodiments, system  10  can also include components to provide traction control functions and/or vehicle stability control functions. 
     In system  10 , a brake pedal  12  is connected to a master cylinder  14  to provide pressurized brake fluid to a wheel brake  16 . In the embodiment prove illustrated in FIG. 1, the wheel brake  16  is illustrated as a disc assembly; however, wheel brake  16  may be any type found on vehicles. 
     As is well known in the art, the master cylinder  14  includes a piston (not illustrated) which moves in response to application of the brake pedal  12 . The master cylinder piston (not illustrated) includes seals (not illustrated) which separate fluid into various chambers in the master cylinder  14 . It is known to use lip seals about the master cylinder piston. 
     A hydraulic control unit (HCU)  18  is a housing having bores for receiving control valves and other components described below. Fluid passageways or conduits are provided between the bores to provide fluid communication between the valves and other components. For purposes of clarity of illustration, only one set of components is illustrated in the schematic of FIG.  1 . However, it is understood that the HCU  18  can also house corresponding components for other circuits and/or wheels of the vehicle. 
     The HCU  18  includes a normally open control valve  20 , commonly referred to as an isolation valve, and a normally closed control valve  22 , commonly known as a dump valve, disposed between the master cylinder  14  and the wheel brake  16 . A low pressure accumulator  24  is disposed between the dump valve  22  and a reciprocating hydraulic pump  26 . The pump  26  is driven by an electric motor (not illustrated) in a well known manner. A fluid compensator  100 , described in detail below, is provided in the HCU  18  between an outlet of the pump  26  and an inlet to the isolation valve  20 . In this position, the fluid compensator  100  can be said to be mounted on a high pressure side of the pump  26 . 
     The isolation valve  20  is preferably formed as a solenoid valve switchable between two positions. The dump valve  22  is preferably formed as a solenoid valve switchable between two positions. Valves  20  and  22 , as well as pump  26 , are electrically connected to an electronic control module (not illustrated) and operated to provide desired braking in a well known manner. 
     A schematic sectional view of a portion of the HCU  18  is presented in FIG.  2 . As stated above, the HCU  18  is formed as a housing containing valves and other components described below. The HCU  18  includes bores for receiving the isolation valve  20 , the dump valve  22 , the low pressure accumulator  24 , and the fluid compensator  100 . The fluid compensator  100  accommodates additional displacement of a master cylinder piston (not illustrated) during anti-lock braking only, thereby moving seals of the master cylinder piston away from a compensator port orifice (not illustrated) of the master cylinder  14 . 
     The fluid compensator  100  is mounted in the HCU  18  that can be used with various anti-lock brake systems. A bore  102  is provided in the HCU  18 . The bore  102  and its interior volume can be referred to as an attenuator to damp fluid pulses entering the bore  102 . A resilient body  104  is fitted within the bore  102 . The body  104  is retained within the bore  102  by an end plug  106 . The end plug  106  is secured to the HCU  18  by any desired means. 
     The body  104  can be formed as a molded member having an interior portion  108  having at least one pore or cavity  110 . A membrane  112  is provided about the outer surface of the interior portion  108 . The membrane  112  is formed from a material that prevents brake fluid from penetrating the porous interior portion  108 . The pores  110  are filled with a suitable trapped gas. The interior portion  108  and the membrane  112  are formed from suitable resilient materials. 
     The body  104  can be formed by a method that utilizes a controlled porosity in a thermal plastic injection molding process. In such a process, closed-cell foaming agents are utilized during an injection process to produce controlled porosity (resulting in pores  110 ) in the interior portion  108 . The pores  110  can be limited in size to provide controlled fluid displacement during ABS mode (to provide the master cylinder seal saver function). Also, porosity effectively reduces the bulk modulus of the body  104 , resulting in improved noise damping properties. The body  104  can also be formed by other means that produce a resilient interior portion having trapped gas. For example, a fluid impermeable outer surface of the body  104  can be integrally formed with the interior portion  108  having a lower modulus. 
     The fluid compensator  100  is preferably located adjacent an outlet  113  of the pump  26  and adjacent an inlet  114  of a passageway  116  which leads to the isolation valve  20  (not illustrated in FIG.  2 ). Preferably, an end of the body  104  adjacent the outlet  113  is reduced. A restricted orifice  118  is preferably provided between bore  102  and the inlet  114 . Preferably, a one-way check valve  120  is provided at the inlet  114 . Valve  120  permits fluid flow from the fluid compensator  100  to the isolation valve  20  but not in the opposite direction. The check valve  120  can be formed by a ball  122  retained by cold stakes  123  at one end of the inlet  114 . A valve seat  124  is provided in the inlet  114  for seating the ball  122  to prevent fluid from flowing into the restricted orifice  118  from passageway  116 . 
     When system  10  is operating in normal braking (i.e., not an ABS mode), the fluid compensator  100  has no effect on the system  10  as fluid from the master cylinder  14  is checked by the check valve  120 . Thus, the fluid compensator  100  does not affect pedal stroke. 
     When the system  10  is operating in an ABS mode, i.e., when pump  26  is pumping brake fluid, the fluid compensator  100  permits the master cylinder  14  to continue to travel, thereby moving a seal of the master cylinder piston away from a compensator port orifice. As the pump  26  forces fluid into bore  102 , body  104  compresses as its pores  110  collapse upon themselves. Thus, additional fluid is stored within the bore  102 , and not routed to the master cylinder  14 . When the pump  26  stops operating (i.e., when an ABS event is complete), the resilient body  104  expands to its approximate original configuration. The additional fluid that had been stored in the bore  102  is returned to the master cylinder  14 . 
     As stated above, the resilient body  104  also improves noise damping in system  10  as fluid pulses exit the pump  26 . The relatively soft interior portion  108  collapses as fluid enters bore  102  from the pump outlet  113 . 
     A second embodiment of a vehicular brake system according to this invention is indicated generally at  200  and illustrated in FIG.  3 . System  200  includes valves and other components described below to provide an anti-lock braking function. In other embodiments, system  200  can also include components to provide traction control and vehicle stability control functions. 
     In system  200 , a brake pedal  212  is connected to a master cylinder  214  to provide pressurized brake fluid to a wheel brake  216 . In the embodiment illustrated in FIG. 3, the wheel brake  216  is illustrated as a disc assembly; however, the wheel brake  216  may be any type found on vehicles. 
     As is well known in the art, the master cylinder  214  includes a piston (not illustrated) which moves in response to application of the brake pedal  212 . The master cylinder piston includes seals that separate fluid into various chambers in the master cylinder  214 . It is known to use lip seals about the master cylinder piston. 
     A hydraulic control unit (HCU)  218  is a housing having bores for receiving control valves and other components described below. Fluid conduits are provided between the bores to provide fluid communication between the valves and other components. For purposes of clarity of illustration, only one set of components is illustrated in the schematic of FIG.  3 . However, it is understood that the HCU  218  can also house corresponding components for other circuits and/or wheels of the vehicle. 
     The HCU  218  includes a normally open control valve  220 , commonly referred to as an isolation valve, and a normally closed control valve  222 , commonly known as a dump valve, disposed between the master cylinder  214  and the wheel brake  216 . A low pressure accumulator (LPA)  224  is disposed between the dump valve  222  and a reciprocating hydraulic pump  226 . The pump  226  is driven by an electric motor in a well known manner. As described below, the LPA  224  includes a fluid compensator  225  that improves the operating life of seats mounted on a piston in the master cylinder  214 . 
     The isolation valve  220  is preferably formed as a solenoid valve switchable between two positions. The dump valve  222  is preferably formed as a solenoid valve switchable between two positions. Valves  220  and  222 , as well as pump  226 , are electrically connected to an electronic control module (not illustrated) and operated to provide desired system braking in a well known manner. 
     The LPA  224  includes a bore  232  formed in the HCU  218 . A cup-shaped piston  234  is slidably mounted in the bore  232 . A spring  236  biases the piston  234  away from an end cap  238  that closes the bore  232 . The end cap  238  can be retained in any desired manner. A seal  240  is received in a groove formed in an outer surface of the end cap  238  to seal the bore  232 . A seal  242  is received in a groove formed in an outer surface of the piston  234 . 
     The fluid compensator  225  includes a fluid chamber  244  formed between an end wall  246  of the bore  232  and seal  242  placed about the piston  234 . Fluid is routed to the chamber  244  via a passageway  248  intersecting a side wall  250  of the bore  232 . The passageway  248  is spaced a predetermined distance away from the end wall  246 . At predetermined positions of the piston  234 , the piston  234  restricts fluid flow into and out of the chamber  244 . Another predetermined positions of the piston  234 , fluid travels freely into and out of the chamber  244  via passageway  248 . 
     Some fluid from the first few dump pulses remains in the chamber  244  during an ABS event. During an ABS event, i.e., when the pump  226  is operating, a suction force to the pump inlet is generated in passageway  248 . The suction force is applied against a side of the piston  234  and pulls the piston  234  against the side wall  250 , thereby restricting fluid flow through passageway  248 , and reducing fluid flow to the pump  226 . This feature insures that a predetermined amount of fluid will be delayed (at least until the next dump sequence) in chamber  244  prior to returning to the master cylinder  14  during the remainder of an ABS event. This storage of fluid in the HCU  18  provides a seal saver function for a seal on a master cylinder piston, since the stored fluid does not displace the master cylinder piston. 
     Once the pump  226  is deactivated, i.e., after an ABS event is completed, piston  234  will recenter in the bore  232 , allowing fluid trapped in chamber  244 .to drain out over lip seals (not illustrated) in the dump valve  222 . 
     When the system is operating in normal braking (i.e., not an ABS event), the fluid compensator  225  has no effect on the system  200 . When the system  200  is operating in an ABS mode, i.e., when pump  226  is operating, the fluid compensator  225  permits a master cylinder piston to travel so that seals will be moved away from compensator ports in the master cylinder  14 . 
     A third embodiment of a fluid compensator according to this invention is indicated generally at  325  in FIG. 4. A low pressure accumulator (LPA)  324  is disposed between a dump valve (not illustrated) and a reciprocating hydraulic pump (not illustrated) in a hydraulic control unit (HCU)  318  in a manner similar to corresponding elements of system  200  illustrated in FIG.  3 . The LPA  324  includes a bore  332  formed in the HCU  318 . A cup-shaped piston  334  is slidably mounted in the bore  332 . A spring  336  biases the piston  334  away from an end cap  338  that closes the bore  332 . The end cap  338  can be retained onto the HCU  318  in any desired manner. A seal  340  is received in a groove formed in an outer surface of the end cap  338  to seal the bore  332 . A seal  342  is received in a groove formed in an outer surface of the piston  334 . 
     The fluid compensator  325  includes a fluid chamber  344  formed between an end wall  346  of the bore  332  and the seal  342  placed about the piston  334 . Fluid is routed to the chamber  344  via a passageway  348  intersecting a side wall  350  of the bore  332 . The passageway  348  is spaced a predetermined distance away from the end wall  346 . At predetermined positions of the piston  334 , fluid is restricted from entering and exiting in the chamber  344 . At other predetermined positions of the piston  334 , fluid travels freely into and out of the chamber  344  via passageway  348 . 
     An undercut or groove  352  is provided in an outer surface of the piston  334 . Preferably, the undercut  352  is formed along a portion of the length of the piston  334  spanning the distance indicated at B in FIG.  4 . The undercut  352  is formed between a portion of the length of the piston  334  indicated at A and between a portion of the piston  334  mounting seal  342 . 
     The undercut  352  improves the first dump performance by reducing the annular restriction between the piston  324  and the bore  332 . The time to drain fluid trapped in chamber  344  at the end of an ABS event is reduced by minimizing the length of surface A and by maximizing the length of surface B. 
     When a brake system is operating in normal braking (i.e., not an ABS event), the fluid compensator  325  has no effect on the system. When the system is operating in an ABS mode, i.e., when a pump operating, the fluid compensator  325  permits a master cylinder piston to travel so that seals will be moved away from compensator port orifices in the master cylinder. 
     In accordance with the provisions of the patent statutes, the principle and mode of operation of this invention have been explained and illustrated in its preferred embodiments. However, it must be understood that this invention may be practiced otherwise than as specifically explained and illustrated without departing from its spirit or scope.