This invention relates to vehicular braking systems having a power assist feature therefor and more particularly to an electrohydraulic brake booster system. The system may have a common boost and input chamber or a separate input chamber within the boost piston. 
     BACKGROUND OF THE INVENTION 
     Early power assisted braking systems were vacuum actuated utilizing the engine intake manifold as a source of power. More recently, hydraulic assist systems have become popular. The hydraulic systems usually either rely on a power steering pump as a source of pressurized fluid or include a separate dedicated fluid pump. The hydraulic systems typically include a power assist unit or booster having a driver input piston disposed within an input cylinder and a power output piston disposed in a separate output cylinder for powering a master brake cylinder. Typically, the pressure on the working face of the input piston is monitored and the pressure applied to the working face of the output piston set accordingly. The distances traveled by the input and output pistons are largely ignored. 
     Brake booster systems are sometimes commanded to apply the maximum available fluid pressure for braking. Any additional brake pedal pressure fails to raise the force applied to the master cylinder. If no precautions are taken, the driver may continue to depress the brake pedal, but feel no increase in resistance to pedal motion. Finally, vehicle braking should still be possible despite catastrophic failure of the boost fluid pressure as by engine stoppage, or a belt slipping or breaking. Desirably, the system reverts to a conventional (no boost) braking mode. 
     SUMMARY OF THE INVENTION 
     It is desirable to take into account the travel of the input piston in setting output piston face pressure. Moreover, also monitoring the output piston travel allows the output pressure to be in part determined by that travel allowing variations in the ratio of input to output piston travel to be incorporated in the braking system. 
     The present invention provides solutions to the above concerns by providing a boost pressure system which may include linear measures of both driver input and boost output travel, may utilize a single boost chamber to supply both force to a vehicle master cylinder and opposition force to the driver input, or may employ a separate driver input force opposition from a pressure chamber enclosed within a boost piston. 
     The invention comprises, in one form thereof, a vehicle brake booster having a source of pressure fluid, a booster housing with first and second generally cylindrical bores therein, a driver actuable brake input piston reciprocally disposed in the first cylindrical bore, and a hydraulically powered brake master cylinder actuating output piston reciprocally disposed in the second cylindrical bore. The second cylindrical bore together with the output piston define a boost chamber with the input piston extending from the second bore into the boost chamber. A pressure fluid conduit couples the source of pressure fluid to the boost chamber, and the boost chamber is completely defined by the second cylindrical bore, the input piston, the output piston and the fluid conduit. 
     In another form, the invention includes a vehicle brake booster having a housing with a generally cylindrical bore and a hydraulically powered brake master cylinder actuating boost piston reciprocally disposed in the cylindrical bore. The cylindrical bore and a working face of the boost piston define a boost chamber. There is a generally cylindrical bore in the boost piston extending from the piston working face part way through the piston. A driver actuable brake input piston passes through the boost chamber and is reciprocally disposed within the boost piston bore. The boost piston bore and a working face of the input piston define an input chamber. There is a source of pressure fluid and an arrangement for selectively supplying fluid pressure from the source to the boost chamber and to the input chamber. 
     The invention also comprises a method of amplifying a hydraulic brake force applied by a vehicle operator in which the driver input brake force is sensed by monitoring linear motion of a driver actuable input piston. The amplified hydraulic braking force is sensed by monitoring linear motion of an output piston or pressure. Fluid from a fluid pressure source is conveyed to a boost chamber to move the output piston a distance proportional to the distance moved by the input piston to actuate a vehicle brake master cylinder in proportion to the sensed travel associated with the driver applied input brake force. The ratio of input piston travel to output piston travel may be modified. 
     An advantage of the present invention is that the ratio of input piston travel to that of output piston travel may be selectively modified as desired to facilitate driver pedal feel. 
     Another advantage of the present invention is that the system is fail-safe reverting to a completely manual mode in the event of hydraulic or electrical failure of the boost system. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a partially cross-sectional, partially schematic view of a brake booster system according to a first embodiment of the invention; and 
     FIG. 2 is a partially cross-sectional, partially schematic view of a brake booster system according to a second embodiment of the invention. 
     Corresponding reference characters indicate corresponding parts throughout the several views. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the drawings and particularly to FIG. 1, there is shown a hydraulic brake booster  11  in cross-section with associated hydraulic and electrical circuitry shown in block schematic form. The booster includes a housing  13  with a power output piston  15  reciprocally mounted within a generally cylindrical bore  17  within the housing. The output piston is mechanically coupled to a conventional vehicle braking system master cylinder (not shown) by output rod  33 . One piston face  19  is exposed to hydraulic fluid pressure within a boost chamber  21  within the housing. The force of hydraulic pressure in chamber  21  acting on boost piston face  19  is opposed by a coil biasing spring  37  retained in housing  13  by spider  45 . A driver actuable pedal input piston  23  is also reciprocally mounted within the housing  13  in a second generally cylindrical bore  25 . The input piston is mechanically coupled to a conventional driver actuable vehicle brake pedal (not shown) by way of socket  35 . The input piston  23  has a piston face  27  exposed to hydraulic fluid pressure within the boost chamber  21 . The force of hydraulic pressure in chamber  21  acting on input piston face  27  is supplemented or reinforced by a return spring  39  that is coupled thereto by way of connecting rod  41 . The return spring  39  being utilized to retain a brake pedal in a rest position. Outward motion of the input piston  23  is limited by flange  43  engaging the housing  13  while outward motion of output piston  15  is limited by the piston engaging stop pin  24  located in housing  13 . The bias or return springs  37 , 39  urge their respective pistons toward the rest or non-actuated positions shown in FIG. 1 with the separation between the pistons (and the rightward motion of piston  15 ) maintained by connecting rod  41 . Linear motion of the input piston  23  within bore  25  is sensed by a travel sensor  29  and the linear motion of the output piston  15  is similarly monitored by travel sensor  31 . Fluid leakage from chamber  21  is controlled by conventional seals such as at  47  and  49 . The sole fluid connection to the chamber  21  is by way of conduit  51 . 
     A supply of hydraulic fluid resides in the reservoir  53  and is selectively pumped to a pressure fluid accumulator  55  by pump  57 . A solenoid actuable valve  59  is normally closed blocking fluid passage from the accumulator to conduit  51 . A second solenoid actuable valve  61  that is normally open provides a fluid drain path from chamber  21  by way of conduit  51  to the reservoir  53 . 
     The travel sensors  29  and  31  provide two inputs to an electronic control unit (ECU)  63 . The ECU  63  also receives an enabling input on line  65  when the vehicle ignition is enabled and a hydraulic fluid level indicative input on line  67 . The ECU  63  provides output signals to selectively enable the solid state switches  71  and  73 . A malfunction such as inadequate fluid level may be indicated to the vehicle driver by causing switch  73  to conduct enabling a lamp by way of line  69 . When the ECU  63  causes switch  71  to conduct, a circuit is established between vehicle battery voltage on line  75  and vehicle ground enabling the motor  77  to drive fluid pump  57 . The ECU  63  also selectively provides solenoid enabling signals to actuate valves  59  and  61  on lines  79  and  81  respectively. 
     The driver initiates a braking command by moving the input piston  23  toward the left as viewed in FIG. 1, movement of which is sensed by travel sensor  29 . The ECU  63  then closes the valve  61  isolating the boost chamber and opens the valve  59  allowing high pressure fluid from accumulator  55  into the chamber  21 . This moves the output piston  15 , which pushes on a conventional type piston of master cylinder, toward the left until it reaches a predetermined position as monitored by travel sensor  31 . Travel of the output piston  15  can be set to exceed that of the input rod thereby providing a ratio change function provides an improving pedal feel to the driver. Pressurized fluid in the boost chamber  21  reacts against the piston face  27  and input rod to provide feedback to the driver as appropriate. When the driver reduces the input force, the input piston  23  correspondingly moves back toward the positon of rest. The ECU  63  also responds by reducing the fluid pressure in boost chamber  21  by alternately opening and closing the valves  59  and  61  to effect a reduction in pressure. Typical types of pulse width modulation (PWM) of these valves can be utilized to control the pressure. In the event of loss of boost by hydraulic or electrical means, manual push through is maintained by a direct link between the input  23 , rod  41  and output piston  15 . Additionally, independent control of the valves  59  and  61  permit self actuation of the booster thereby enhancing the function of typical advanced modulation functions including ESP and ROM. 
     A hydraulic brake booster  83  in cross-section with associated hydraulic and electrical circuitry in block schematic form is shown in FIG. 2 that defines a second brake system according to the invention. The booster  83  includes a housing  85  with a power output piston  87  reciprocally mounted within a generally cylindrical bore  89  within the housing. The output piston  87  is mechanically coupled to a conventional vehicle braking system master cylinder (not shown) by output rod  91 . One piston face  93  is exposed to hydraulic fluid pressure presented to a boost chamber  95  within housing  85 . The force of hydraulic pressure in chamber  95  acts on the face  93  of output piston  87  and is opposed by a coil biasing spring  97 . A driver actuable pedal input piston  99  is also reciprocally mounted within the housing  85  in a second generally cylindrical bore  101 . The input piston is mechanically coupled to a conventional driver actuable vehicle brake pedal (not shown) by way of rod  103 . The input piston  99  extends through chamber  95  and into a generally cylindrical bore  105  within output piston  87 . The cylindrical bores  89 ,  101  and  105 , and the input and output pistons all share a common axis  107 . Input piston  99  has a piston face  109  that is exposed to hydraulic fluid pressure within a separate chamber  111  and moves with the output piston  87 . The force of hydraulic pressure in chamber  111  acts on input piston face  109  is supplemented or reinforced by return spring  113  that biases a brake pedal toward a rest position and define a reation force. Linear motion of the input piston  99  is sensed by a travel sensor  115 . Linear motion of the output piston may be monitored by a travel sensor  98  that function in a similar manner as sensor  31  in FIG.  1 . Fluid leakage from chamber  95  is controlled by conventional seals such as  117  mounted within the housing  85  and  119  located on the outer cylindrical surface of output piston  87 . There is a fluid connection to the chamber  95  by way of conduit  121  and a separate fluid connection to chamber  111  by way of housing conduit  123  and output piston conduit  125 . In addition, chamber  95  is connected to chamber  111  as a lip seal  98  carried by piston  99  provides a one way flow path such that a fluid pressure in chamber  95  is in equilibrium with a fluid pressure in chamber  111 . The conduits  123  and  125  are in fluid transmitting communication by way of annular piston groove  127  in all operational positions of the piston  87  within the housing. 
     Hydraulic fluid is supplied to a pressure fluid accumulator  129  from a fluid source line  131 . Line  131  may connect to a dedicated pump as in FIG. 1., to a power steering pump, or any other suitable fluid pressure source. A solenoid actuable valve  133  is normally closed blocking fluid passage from the accumulator. A second solenoid actuable valve  135  is normally open providing a fluid drain path from chamber  95  by way of conduit  121  to sump or other reservoir  137 . A third solenoid actuable valve  139  is also normally open providing a drain path from chamber  111  by way of conduits  123  and  125  and the valve  135  to the sump  137 . A pressure sensor  141  provides an indication of fluid pressure within chamber  95  and supplies an indication of that chamber pressure to the ECU  143  by way of line  145 . The ECU  143  may also receives travel sensor information regarding the input piston from sensor  115  by way of line  147  and, optionally, regarding the output piston from sensor  98  by way of line  149  from a travel sensor  98 . The ECU  143  provides output signals to selectively enable the solenoids of valves  133 ,  135  and  139  as indicated by the dotted lines. 
     The FIG. 2 embodiment is similar to that of FIG. 1 in that the hydraulically powered brake master cylinder actuating boost piston  87  is reciprocally disposed in the cylindrical bore  105  with the cylindrical bore and a working face  93  of the boost piston defining a boost chamber, but differs somewhat from the FIG. 1 version in that there is a generally cylindrical bore in the boost piston extending from the piston working face  93  part way through the piston for receiving the driver actuable brake input piston  87  which passes through the boost chamber and is reciprocally disposed within the boost piston bore. The boost piston bore, and a working face  109  of the input piston define an input chamber  111 . Fluid pressure from the source of pressure fluid on line  131  is selectively supplied to the boost chamber through conduit  121  and to the input chamber by way of conduits  123  and  125 . As before, there is a travel sensor  115  for monitoring the motion of the input piston and fluid is selectively supplied to the boost chamber in accordance with the monitored input piston motion. The electronic control unit  143  monitors boost chamber pressure and input piston travel, and controls the normally closed solenoid actuable valve  133  to selectively couple the source  131  and  129  to the boost chamber. A normally open solenoid actuable valve  139  is controlled by the electronic control unit to selectively trap pressured fluid from the normally closed valve  133  to the input chamber  111  and to selectively vent pressure fluid from the input chamber. A pressure sensor determines fluid pressure within the boost chamber as a function of signals from input travel sensors  115 , 147  and fluid is selectively supplied to the boost chamber in accordance with the determined fluid pressure. The electronic control unit  143  is also operable upon sensing input piston travel unaccompanied by boost chamber pressure change to close the normally open valve  139  trapping a fixed volume of fluid in the input chamber  111 , whereby the ratio of output piston travel to input piston travel is fixed at 1:1. A pressure fluid conduit  121  in the housing couples the source of pressure fluid to the boost chamber  95  when the normally closed valve  133  opens, and a fluid path including the normally closed valve  133 , when open, the normally open valve  139 , a housing aperture  123 , an output piston aperture  125 , and an annular space  127  intermediate the housing bore and output piston which couples the housing aperture and output piston aperture regardless of output piston position cooperate to supply pressure fluid from the source to the input chamber. The conduit  121  provides the only path for fluid entering or exiting the boost chamber. In the event of boost system failure, valves  135  and  139  revert to the normally open position, input piston  99  is allowed to engage the end of bore  105  and brake pressure reverts to unassisted manual operation. A feature of this embodiment resides in an ability for the ECU  145  to also evaluate the movement of piston  87  through either the travel sensor  116  or the pressure sensor  141  as a function of the movement of piston  99  as measured by travel sensor  115  is effecting a desired brake application. 
     In an event that the ECU  143  determines that the fulid pressure supplied to chamber  95  does not correspond to a desired braking application as derived from the travel sensed by travel sensors  115  and the travel of power piston  87  as derived from the travel sensed by travel sensor  98  or the pressure sensed by sensor  145 , a signal is sent to close solenoid valve  139  and trap fluid in chamber  111 . Thereafter, the input force applied to rod  103  moves input piston  99  into engagement with power piston  87  to provides a 1:1 manual force that supplements the output force derived from the pressuried fluid supplied to chamber  95 . On termination of the input force to rod  103 , the return springs  113  and  97  move the input piston  99  and poer popoun ibabemnnvalve balsignal derived from sensor piston  97  to the rest positon as valves  139  and  135  are opened to the reservoir  137 .
A brake booster amplifies driver brake pedal input into an output force and travel for operating a master cylinder. A power unit builds and stores high pressure fluid to provide boost. Inlet and outlet solenoid valves regulate pressurized fluid to the amplifying mechanism. In one embodiment, a single boost chamber provides fluid pressure to operate the master cylinder and to provide a brake pressure indicative opposing force to driver input. One travel sensor monitors the position and movement of an input rod and piston, and a second travel sensor monitors the position and movement of an output piston. An ECU monitors system parameters and controls a motor pump, inlet and outlet valves and peripherals. In another embodiment, the opposing force to the brake pedal input is provided by a separate pressure fluid chamber located within and movable with the output piston. Boost chamber pressure and, optionally, output piston travel are monitored to provide a braking force indication. An ECU monitors system parameters including input travel and boost chamber pressure, and controls the inlet and outlet valves and peripherals.