Patent Publication Number: US-8523294-B2

Title: Vehicular brake system operable in dual modes

Description:
FIELD 
     The present invention relates generally to a brake system for a vehicle, and particularly to a brake system that is operable in an active mode and a conventional mode. 
     BACKGROUND 
     Major developments have taken place in vehicular braking systems in recent years. Among these developments are anti-lock braking systems (ABS) and regenerative braking systems used in electric and hybrid-electric systems. In regenerative braking systems, a vehicle&#39;s brake pedal is mechanically decoupled from the downstream braking circuits. Sensors associated with the brake pedal provide electrical signals to an electronic control unit (ECU). These signals are representative of the brake pedal position. Since the brake pedal is mechanically decoupled from the downstream braking circuits, a brake pedal feel simulator is often used to simulate the feel of a conventional braking system by providing pressure feedback to the vehicle operator at the brake pedal. The force of the brake pedal is transferred to the brake pedal feel simulator. Meanwhile, the ECU controls the braking system to apply a braking force consistent with the brake pedal position. An electrical regenerative system and/or a hydraulic system provide the necessary braking force. 
     In the event of a failure of the hydraulic system or the electrical regenerative system, it will become necessary for the braking system to switch its mode of operation so that the brake pedal is mechanically coupled to the downstream brake circuits. In its changed mode, the force applied to the brake pedal would be transferred to the downstream brake circuits to generate the necessary braking force to halt a vehicle. 
     Therefore, there is a need to provide an improved braking system that is operable in a conventional mode in which a brake pedal is mechanically decoupled from the downstream braking circuits and a fallback mode in which the brake pedal is mechanically coupled to the downstream braking circuits. 
     SUMMARY 
     According to one embodiment of the present disclosure, there is provided a system for use in a vehicle with a brake pedal and a brake circuit. The system includes a master cylinder assembly configured to pressurize fluid therein in response to movement of the brake pedal. The system also includes a sensor assembly configured to generate a pedal position signal indicative of position of the brake pedal. Further the system includes an electronic control unit configured to (i) generate a brake request signal in response to generation of the pedal position signal, and (ii) generate a selector control signal. The system also includes a selector valve assembly operable in a first mode and a second mode, the selector valve assembly being moved from the first mode to the second mode in response to generation of the selector control signal. Furthermore the system includes a pedal feel simulator (i) in fluid communication with the master cylinder when the selector valve assembly is positioned in the first mode, and (ii) isolated from fluid communication with the master cylinder when the selector valve assembly is positioned in the second mode. The system also includes a booster actuator configured to generate force in response to generation of the brake request signal, and a booster assembly configured to pressurize fluid therein in response to generation of force by the booster actuator. The master cylinder assembly is (i) isolated from fluid communication with the brake circuit when the selector valve assembly is positioned in the first mode, and (ii) in fluid communication with the brake circuit when the selector valve assembly is positioned in the second mode. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above described features and advantages, as well as others, will become more readily apparent to those of ordinary skill in the art by reference to the following detailed description and accompanying drawings. 
         FIG. 1  depicts a block diagram schematic of a braking system of the present disclosure; 
         FIG. 2  depicts a more detailed schematic of the braking system of  FIG. 1  including a master cylinder, a selector valve, a booster actuator, and a booster assembly; 
         FIG. 2A  is a schematic of the downstream braking circuits of  FIG. 1 ; 
         FIG. 2B  is a schematic of the booster actuator of  FIG. 2 ; 
         FIG. 2C  is a schematic of an alternative embodiment of the booster actuator of  FIG. 2  including a valve assembly; 
         FIG. 2D  is a schematic of the valve assembly of  FIG. 2C ; 
         FIG. 3  depicts a cross sectional view of the master cylinder of  FIG. 2 ; 
         FIG. 4  depicts a cross sectional view of the booster assembly of  FIG. 2 ; 
         FIG. 5  depicts a cross sectional view of the selector valve of  FIG. 2  including a solenoid shown in an actuated position; 
         FIG. 6  depicts the selector valve of  FIG. 5  with the solenoid shown in a de-actuated position; 
         FIG. 7  depicts a schematic of the selector valve of  FIG. 2  shown in a first mode; 
         FIG. 8  depicts a schematic of the selector valve of  FIG. 2  shown in a second mode; 
         FIG. 9  depicts a schematic of an alternative embodiment of the braking system of  FIG. 1  including a master cylinder, a selector valve, and a booster assembly; 
         FIG. 10  depicts a cross sectional view of the booster assembly of  FIG. 9 ; 
         FIG. 11  depicts a cross sectional view of the selector valve of  FIG. 9  including a solenoid shown in an actuated position; 
         FIG. 12  depicts a cross sectional view of the selector valve of  FIG. 9  with the solenoid shown in a de-actuated position; 
         FIG. 13  depicts a schematic of the selector valve of  FIG. 9  shown in a first mode; 
         FIG. 14  depicts a schematic of the selector valve of  FIG. 9  shown in a second mode; 
         FIG. 15  depicts a cross sectional view of an alternative embodiment of the selector valve of  FIG. 9  configured to be actuated hydraulically, with the selector valve shown in an actuated position; and 
         FIG. 16  depicts a schematic of another alternative embodiment of the braking system of  FIG. 1  which includes two selector valves and two brake pedal feel simulators. 
     
    
    
     DETAILED DESCRIPTION 
     For the purpose of promoting an understanding of the principles of the invention, reference will now be made to the embodiments illustrated in the drawings and described in the following written description. It is to be understood that no limitation to the scope of the invention is thereby intended. It is further to be understood that the present invention includes any alterations and modifications to the illustrated embodiments and includes further applications of the principles of the invention as would normally occur to one skilled in the art to which this invention pertains. 
     Referring to  FIG. 1 , there is depicted a block diagram schematic of a braking system  100 . The braking system  100  includes a master cylinder assembly  102 , a brake pedal feel simulator  104 , a reservoir  106 , a booster assembly  108 , a booster actuator  110 , a selector valve  112 , an electronic control unit (ECU)  114 . The booster actuator  110  includes an electric motor and a screw gear (not shown) for actuating the booster assembly  108 . In an alternative embodiment, the booster actuator  110  may include a fluid pump station for providing a source of high pressure fluid which includes a motor, a pump, and a high pressure reservoir (not shown) for actuating the booster actuator. In  FIG. 1 , thin lines indicate electrical lines, while the thicker lines indicate fluid lines. The master cylinder assembly  102  contains a sensor ( FIG. 3 ) which is used to relay information about position of a brake pedal ( FIG. 2 ) to the ECU  114 . The master cylinder assembly  102  is fluidly coupled to the selector valve  112  and is also fluidly coupled to the reservoir  106 . The selector valve  112  is electrically coupled to the ECU  114  so that the ECU  114  can energize a solenoid ( FIG. 4 ) of the selector valve for switching the selector valve from a first mode to a second mode. Alternatively the selector valve  112  is fluidly coupled to the fluid pumping station of the booster actuator  110  or another high pressure fluid source for switching the selector valve  112  from the first mode to the second mode. The selector valve  112  is also fluidly coupled to the brake pedal feel simulator  104 . In addition, the selector valve  112  is fluidly coupled to the booster assembly  108 , which in turn is coupled to the booster actuator  110 . The booster assembly  108  is fluidly coupled to the downstream brake circuits  500  (see  FIG. 2A ) by fluid lines  122  and  124 . In an alternative embodiment, the selector valve  112  is coupled to the downstream brake circuits  500  (See  FIG. 9 ). 
     Referring to  FIG. 2 , there is shown a more detailed schematic of the braking system  100  of  FIG. 1 . The braking system  100  further includes the brake pedal  126 . In  FIG. 2 , the thin lines indicate electrical lines, while the thicker lines indicate hydraulic lines. Hydraulic lines  122  and  124  provide pressurized hydraulic fluid to downstream brake circuits  500  as shown in  FIG. 2A .  FIG. 2A  shows a schematic of the downstream braking circuits  500  which includes a first downstream braking circuit  502  and a second downstream braking circuit  504 . The fluid line  122  provides a high pressure fluid coupling to the first downstream braking circuit  502 . The fluid line  124  provides a high pressure fluid coupling to the second downstream braking circuit  504 . 
     The reservoir  106  is fluidly coupled to the master cylinder assembly  102 . The master cylinder  102  is also fluidly coupled to the selector valve  112 . The selector valve  112  is fluidly coupled to the brake pedal feel simulator  104 . The selector valve  112  is also fluidly coupled to the booster assembly  108 . The booster assembly  108  is fluidly coupled to the reservoir  106 . The coupling of the booster assembly  108  to the reservoir  106  is via a valve assembly  128  which provides selective coupling between the booster assembly  108  and the reservoir  106 . The booster assembly is also fluidly coupled to the downstream brake circuits  500  via the fluid lines  122  and  124 . The ECU  114  is electrically coupled to the master cylinder assembly  102 , the booster actuator  110 , the selector valve  112 , and the valve assembly  128 . 
     The booster actuator  110  is configured to provide a force to the booster assembly  108 . The booster actuator  110  is coupled to a piston  202  of the booster assembly  108 , as shown in  FIG. 2B . Referring to  FIG. 2B , the booster actuator  110  is shown which includes an electric motor  552  in communication with a drive circuit  554  which is coupled to the ECU  114  by an electrical line  556 . Preferably, the electric motor  552  is a stepper motor. A coupling  130  is depicted in  FIG. 2B  by a dotted line. The coupling  130  is a screw-type member coupling the electric motor  552  with the piston  202  of the booster assembly  108  via a ball screw gear mechanism. 
     In an alternative embodiment shown in  FIG. 2C , the booster actuator  110 ′ includes a fluid pumping station  107  and a valve assembly  133 . The fluid pumping station  107  includes a motor, a pump, a check valve, and a high pressure reservoir  143  for pressurizing hydraulic fluid in the fluid pumping station  107 . The valve assembly  133  is fluidly coupled to the piston  202  of the booster assembly  108  to provide a force to the piston  202 . The valve assembly  133  is also coupled to the reservoir  106  via the hydraulic line  137 . Further, the valve assembly  133  is fluidly coupled to the high pressure reservoir  143  via a fluid line  145 . The hydraulic line  137  is also coupled to the fluid pumping station  107 . The valve assembly  133  can selectively couple the fluid pumping station  107  with the piston  202  of the booster assembly  108 . The valve assembly  133  is controlled by the ECU  114  via an electric line  139  which is connected to a solenoid valve  141 . The coupling  130 ′ in  FIG. 2C  is a fluid coupling. Since the booster actuator  110  can be electromechanically coupled to the booster assembly  108 , as shown in  FIG. 2B , or fluidly coupled to the booster assembly  108 , as shown in  FIG. 2C , the electric lines  556  and  139  between the booster actuator  110  and  110 ′ and the ECU  114  are depicted as dotted lines. Also, the coupling  130 / 130 ′ between the booster actuator  110 / 110 ′ and the booster assembly  108 , shown in  FIGS. 2B and 2C , are depicted as dotted lines. Also, since the coupling  130  can be a mechanical coupling (screw) or a fluid coupling it is depicted as a dotted line. The coupling  130 / 130 ′ in  FIGS. 2B and 2C  are shown in double dotted lines to signify the high mechanical force or high pressure fluid coupling between the booster actuator  110 / 110 ′ and the booster assembly  108 . 
     Referring to  FIG. 2D , an example of a schematic of the valve assembly  133  is depicted. The valve assembly  133  has inlets  137 ′ and  145 ′ which are fluidly coupled to fluid lines  137  and  145  which are fluidly coupled to the reservoir  106  and to the high pressure fluid reservoir  143 , respectively. The valve assembly  133  also has a valve mechanism which includes a ball  147 , a spring  149 , and a valve seat  153 . 
     Referring to  FIG. 3 , the master cylinder assembly  102  is depicted. The master cylinder assembly  102  includes a sensor assembly  150  configured to generate an electrical signal on line  151  corresponding to the position of the brake pedal  126 . The line  151  is connected to the ECU  115  so that the ECU  114  receives the electrical signal. Also depicted in  FIG. 3  are pistons  152  and  154 , a chamber  156  formed between the pistons  152  and  154 , a chamber  158  formed between the piston  154  and a housing  160 , a fluid channel  162  in fluid communication between the reservoir  106  and the chamber  156 , a fluid channel  164  in fluid communication between the reservoir  106  and the chamber  158 , an outlet  166  in fluid communication with the chamber  156 , an outlet  168  for fluid communication with the chamber  158 , a biasing element  170  located between the pistons  152  and  154  within the chamber  156 , a biasing element  172  located between the piston  154  and the housing  160  within the chamber  158 , and a brake pedal connecting rod  174  coupling the piston  152  to the brake pedal  126 . 
     Referring to  FIG. 4 , the booster assembly  108  is depicted. The booster assembly  108  includes pistons  202  and  204 , a chamber  206  located between the pistons  202  and  204 , a chamber  208  located between the piston  204  and a housing  210 , an inlet  212  in fluid communication with the chamber  208 , an inlet  214  in fluid communication with the chamber  208 , an inlet  216  in fluid communication with the chamber  206 , an inlet  218  in fluid communication with the chamber  206 , a biasing element  220  located between the pistons  202  and  204  within the chamber  206 , a biasing element  222  located between the piston  204  and the housing  210  within the chamber  208 , an outlet  224  in fluid communication between the chamber  208  and the first brake circuit  502 , and an outlet  226  in fluid communication between the chamber  206  and the second brake circuit  504 . The outlets  224  and  226  are fluidly coupled to the fluid lines  122  and  124 , respectively. 
     Referring to  FIG. 5 , the selector valve  112  is depicted in further detail. The selector valve  112  includes a solenoid  250 , a housing  252 , inlets  254 ,  256 ,  260 , and  264 , outlets  266 ,  268 ,  270 , a shaft  272 , a biasing member  274  in the form of a spring disposed between the shaft  272  and the housing  252 , seals  276 , and a coil  278  coupled to the shaft  272 . The selector valve  112  is depicted in  FIG. 5  as being in an actuated position, i.e., electrical current is applied to the solenoid  250  causing the coil  278  and the shaft  272  to move to the right in the direction of an arrow  280 . In the actuated position, the selector valve  112  places the outlets  166  and  168  of the master cylinder assembly  102  in fluid communications with the brake pedal feel simulator  104 . This position constitutes a first mode of operation, which is hereinafter referred to as the active mode. The shaft  272  has a hollow section  282  which allows hydraulic fluid to travel through the center of the shaft  272 . In this position, fluid can travel from inlets  256  and  260 , through the hollow section  282 , and through the outlet  268 . The dots inside the aforementioned path in  FIG. 5  indicate presence of pressurized hydraulic fluid. The inlets  256  and  260  are connected to the outlets  166  and  168  of the master cylinder assembly  102 , respectively as shown in  FIG. 2 . Fluid that is ejected through the outlets  166  and  168  of the master cylinder assembly  102  travel through the inlets  256  and  260 , through the hollow section  282  of the shaft  272  and unite in the outlet  268 . The outlet  268  is fluidly connected to the brake pedal feel simulator  104 , as shown in  FIG. 2 . The inlets  254  and  264  are coupled to the reservoir  106  via one way valves to provide pressure relief when the solenoid  250  is de-energized. 
       FIG. 6  shows the selector valve  112  positioned in a de-actuated position, i.e., electrical current is not applied to the solenoid  250 . The biasing member  274  causes the coil  278  and the shaft  272  to move to the left in the direction of an arrow  282 , relative to its position in  FIG. 5 . In the de-actuated position, the selector valve  112  couples the outlets  166  and  168  of the master cylinder assembly  102  to the inlets  212  and  216  of the booster assembly  108 , respectively. This position constitutes a second mode of operation, which is hereinafter referred to as the conventional mode. In this position, pressurized fluid can travel from inlets  256  and  260  and through the outlets  266  and  270 . The dots inside the aforementioned path in  FIG. 6  indicate presence of pressurized hydraulic fluid. The inlets  256  and  260  are fluidly connected to outlets  166  and  168  of the master cylinder assembly  102 , respectively. Fluid that is ejected through the outlets  166  and  168  of the master cylinder assembly  102  travel through the inlets  256  and  260 , and through outlets  266  and  270 . The outlets  266  and  270  are connected to the inlets  212  and  216  of the booster assembly  108 . In the conventional mode, the hollow section  282  of the shaft  272  is isolated from fluid communication with the master cylinder assembly  102 . 
     Referring to  FIG. 7 , a schematic of the selector valve  112  and surrounding components is depicted with the selector valve  112  positioned in the active mode. The selector valve  112  is depicted with fluid connections showing operations in the active mode in which the master cylinder assembly  102  is in fluid communication with the brake pedal feel simulator  104 , but is fluidly isolated from the brake circuits  500 . Further, in the active mode, the booster assembly  108  is in fluid communication with the downstream brake circuits  500 , but is fluidly isolated from the master cylinder assembly  102 . In the active mode, the selector valve  112  is also fluidly coupled to the reservoir  106 . 
     Referring to  FIG. 8 , a schematic of the selector valve  112  and surrounding components is depicted with the selector valve positioned in the conventional mode. The selector valve  112  is depicted with fluid connections showing operations in the conventional mode in which the master cylinder assembly  102  is in fluid communication with the downstream brake circuits  500  through the selector valve  112  and the booster assembly  108  via fluid lines  122  and  124 . In the conventional mode, the selector valve is also fluidly coupled to the reservoir  106 . 
     In operation, the ECU  114  determines the mode of the selector valve  112 , i.e., active or conventional. The ECU is configured to make this decision based on the fluid pressure and other diagnostic information, as is well known to one skilled in the art. The ECU generates a selector signal that is used to place the selector valve  112  in one of the two modes. In the active mode, the brake pedal  126  is mechanically decoupled from the downstream brake circuits  500 . When the brake pedal  126  is in a rest position, i.e., no pressure is being applied to the brake pedal  126  by the vehicle operator, the chambers  156  and  158  of the master cylinder assembly  102  are fluidly coupled to the reservoir  106  through inlets  162  and  164 , respectively. In this position, no appreciable fluid pressure exists in the chambers  156  and  158 . As the brake pedal  126  is pressed by the vehicle operator, the pistons  152  and  154  of the master cylinder assembly  102  travel and thereby seal the inlets  162  and  164 . Once the inlets  162  and  164  are completely cut off from fluid communication with the chambers  156  and  158 , these chambers are now only fluidly coupled to the selector valve inlets  256  and  260 , the hollow section  282  of the shaft  272 , the outlet  268 , the brake pedal feel simulator  104  and the fluid lines there between. Therefore, continued depression of the brake pedal  126  raises the pressure inside the aforementioned path. The brake pedal feel simulator  104  provides a resistance to the brake pedal  126  similar to a conventional braking system (e.g., a resistance similar to that experienced by the vehicle operator when the selector valve is positioned in the conventional mode). 
     Furthermore, in the active mode the combination of seals  276 , the shaft  272 , and the housing  252  of the selector valve  112  prevents any appreciable fluid communication between the master cylinder assembly  102  and the booster assembly  108 . Therefore, the master cylinder assembly  102  is isolated from fluid communication with the booster assembly  108  when the selector valve is positioned in the active mode. 
     While the brake pedal  126  is in a rest position, i.e., no pressure being applied by the vehicle operator, the chambers  206  and  208  are in fluid communication with the reservoir  106  via the valve assembly  128 . The valve assembly  128  can be a normally closed valve assembly, thereby requiring energization of its solenoid in order to establish fluid coupling between its inlets and outlets. Therefore, while the brake pedal  126  is in the rest position, the valve assembly  128  can be intermittently energized in order to maintain fluid coupling between the chambers  206  and  208  and the reservoir  106 . Once the brake pedal  126  is pressed by the vehicle operator, the connection between the chambers  206  and  208  and the reservoir  106  is terminated by de-energizing the valve assembly  128 . 
     When the booster actuator  110  is actuated, fluid pressure inside the chambers  206  and  208  begin to rise. The outlets  224  and  226  are fluidly coupled to the downstream brake circuits  500 . The level of actuation of the booster actuator  110  is commensurate with the degree of depression of the brake pedal  126 . This level of actuation is determined by the electrical signal that is generated by the sensor  150  and received by the ECU  114 . One sensor that may be used as sensor  150  is a potentiometer which requires connection to a rail voltage. Depression of the brake pedal  126  results in a proportional change in the received signal. As one example, the signal generated by the sensor  150  is at a maximum level when the brake pedal  126  is in a rest position. Conversely, the signal generated by the sensor  150  is at a minimum value when the brake pedal is completely depressed. Therefore, depending on the strength of the signal generated by the sensor  150 , the ECU  114  determines the level of actuation of the booster actuator  110 . 
     As described above, the booster actuator  110  is an electromechanical actuator that includes an electric motor  552  and a screw-type coupling  130  (i.e., ballscrew gear mechanism) disposed adjacent to the piston  202  of the booster assembly  108 . By sensing the level of the signal generated by the sensor  150 , the ECU generates corresponding signals that cause the electric motor of the booster actuator  110  to rotate a precise number of turns to effect the required amount of braking by the downstream braking circuits  500 . The braking is accomplished by moving the piston  202  of the booster assembly  108  to the right in  FIG. 4 . The biasing element  220  applies pressure to the piston  204  which causes the piston  204  to move to the right in  FIG. 4 . Movement of the piston  204  compresses the biasing element  222 . The more the piston  202  travels to the right, the more fluid pressure builds up in the chambers  206  and  208 . Higher pressures in the chambers  206  and  208  result in higher braking in the downstream braking circuits  500 . 
     Upon a complete or partial release of the brake pedal  126 , the ECU  114  senses the change in the signal generated by the sensor  150  and in response thereto causes the motor of the booster actuator  110  to rotate in a reverse direction. The biasing elements  220  and  222 , and the fluid pressure inside the chambers  206  and  208  cause the pistons  202  and  204  to move to the left in  FIG. 4 , thereby reducing the pressures inside the chamber  206  and  208 . 
     Considering now the booster actuator  110 ′ of  FIG. 2C , the booster actuator  110 ′ is operable to provide a hydraulically generated force to move the piston  202  of the booster assembly  108 . Different methods can be used to govern the pressure behind the piston  202  to effect the precise amount of piston  202  travel to correspond to the degree of depression of the brake pedal  126 . In this embodiment, the ECU  114  modulates a valve assembly  133  of the booster actuator  110 ′ to allow a precise amount of pressure build up behind the piston  202 . The ECU accesses a look-up table that correlates pedal travel to pressure. By modulating the valve assembly  133  between the fluid pumping station  107  and the booster assembly  108 , and by monitoring the pressure, the ECU  114  controls the pressure buildup behind the piston  202 . The valve assembly  133 , therefore, requires a control signal from the ECU  114  on line  139 , a fluid connection to the fluid pumping station  107 , a fluid connection  137  to the reservoir  106 , and a fluid connection  130 ′ to the booster assembly  108 . 
     To reduce the fluid pressures inside the chambers  206  and  208 , the ECU  114  deactivates the valve assembly  133  of the booster actuator  110 ′ and activates the valve assembly  128  to fluidly couple the chambers  206  and  208  to the reservoir  106 . 
     In the conventional mode, the brake pedal  126  is mechanically coupled to the downstream braking circuit  500 . In particular, when the brake pedal  126  is in the rest position, i.e., no pressure is being applied to the brake pedal  126  by the vehicle operator, the chambers  156  and  158  of the master cylinder assembly  102  are fluidly coupled to the reservoir  106  through inlets  162  and  164 , respectively. In this position, no appreciable fluid pressure exists in the chambers  156  and  158 . As the brake pedal  126  is pressed by the vehicle operator, the pistons  152  and  154  of the master cylinder assembly  102  travel and thereby seal the inlets  162  and  164 . Once the inlets  162  and  164  are completely cut off from fluid communication with the chambers  156  and  158 , these chambers are now only fluidly coupled to the selector valve inlets  256  and  260 , the selector valve outlets  266  and  270 , inlets  212  and  216 , outlets  224  and  226 , fluid lines  122  and  124 , and the fluid connections there between. Therefore, continued depression of the brake pedal  126  raises the pressure inside the aforementioned path. It should be appreciated that in the conventional mode, the brake pedal feel simulator  104  is not fluidly coupled to the brake pedal  126 . 
     While the brake pedal  126  is in the rest position (i.e., no pressure being applied by the vehicle operator) the chambers  206  and  208  are in fluid communication with the reservoir  106  via the valve assembly  128 . Once the brake pedal  126  is pressed by the vehicle operator, the connection between the chambers  206  and  208  and the reservoir  106  is terminated by de-energizing the valve assembly  128 . In the conventional mode, the booster actuator  110 / 110 ′ is not actuated. Instead the required fluid pressure to effect proper braking is produced in the master cylinder assembly  102  by depression of the brake pedal  126 , and is transferred to the downstream brake circuits  500  through the selector valve  112  and the booster assembly  108 . 
     Referring to  FIG. 9 , an alternative embodiment of a braking system  300  is depicted. The braking system  300  includes a master cylinder assembly  102 ′, a brake pedal feel simulator  104 ′, a reservoir  306 , a booster assembly  308 , a booster actuator  310 , a selector valve  312 , an electronic control unit (ECU)  114 ′, and a brake pedal  126 . The booster actuator  310  includes a fluid pumping station  307 , a motor assembly  318  for pressurizing hydraulic fluid, a high pressure reservoir  316 , and a valve assembly  333 . In  FIG. 9 , the thin lines indicate electrical lines, while the thicker lines indicate hydraulic lines. Hydraulic lines  322  and  324  provide pressurized hydraulic fluid to downstream brake circuits  500 ′. 
     The reservoir  306  is fluidly coupled to the master cylinder assembly  102 ′. The master cylinder  102 ′ is also fluidly coupled to the selector valve  312 . The selector valve  312  is fluidly coupled to the brake pedal feel simulator  104 ′. The selector valve  312  is also fluidly coupled to the booster assembly  308 . The booster assembly  308  is fluidly coupled to the reservoir  306 . The coupling of the booster assembly  308  to the reservoir  306  is via a valve assembly  128 ′ which provides selective fluid coupling between the booster assembly  308  and the reservoir  306 . The booster assembly  308  is also fluidly coupled to the downstream brake circuits  500 ′ via fluid lines  322  and  324 . The ECU  114 ′ is electrically coupled to the master cylinder assembly  102 , the booster actuator  310 , the selector valve  312 , and the valve assembly  128 ′. 
     The booster actuator  310  is configured to provide a force to the booster assembly  308 . The booster actuator  310  is coupled to a piston ( 402 ,  FIG. 10 ) of the booster assembly  308 . The force generated by the booster actuator  310  causes the piston  402  of the boost assembly  308  to move to the right in  FIG. 10 . Alternatively, the booster actuator  310  may be configured and operate identical to the booster actuator  110  so as to generate a force with a motor and a ball screw gear arrangement. 
     In the embodiment of  FIG. 9 , the valve assembly  333  is controlled by the ECU  114 ′ and the coupling  330  is a fluid coupling. The booster actuator  310  is configured and operates in the same manner as the booster actuator  110 ′, discussed above. 
     Referring to  FIG. 10 , the booster assembly  308  is depicted. The booster assembly  308  includes pistons  402  and  404 , a chamber  406  located between the pistons  402  and  404 , a chamber  408  located between the piston  404  and a housing  410 , an outlet  412  in fluid communication with the chamber  408 , an inlet  414  in fluid communication with the chamber  408 , an outlet  416  in fluid communication with the chamber  406 , an inlet  418  in fluid communication with the chamber  406 , a biasing element  420  disposed between the pistons  402  and  404  within the chamber  406 , a biasing element  422  disposed between the piston  404  and the housing  410  within the chamber  408 . 
     Referring to  FIG. 11 , the selector valve  312  is depicted in further detail. The selector valve  312  includes a solenoid  450 , a housing  452 , inlets  454 ,  456 ,  458 ,  460 ,  462 , and  464 , outlets  466 ,  468 ,  470 , a shaft  472 , a biasing member  474  in the form of a spring disposed between the shaft  472  and the housing  452 , seals  476 , and a coil  478  coupled to the shaft  472 . The selector valve  312  is depicted in  FIG. 11  as being in an actuated position, i.e., electrical current is applied to the solenoid  450  causing the coil  478  and the shaft  472  to move to the right in the direction of an arrow  480 . The activated position depicted in  FIG. 11  is hereinafter referred to as the active mode. 
     In the active mode the selector valve  312  places the outlets of the master cylinder assembly  102  in fluid communication with the brake pedal feel simulator  104 ′. The shaft  472  has a hollow section  482  which allows hydraulic fluid to travel through the center of the shaft. In this position, fluid can travel from inlets  456  and  460 , through the hollow section  482 , and through the outlet  468 . The dots inside the aforementioned path in  FIG. 11  indicate presence of pressurized hydraulic fluid. The inlets  456  and  460  are connected to outlets  166 ′ and  168 ′ of the master cylinder assembly  102 ′, respectively as shown in  FIG. 9 . Fluid that is ejected through the outlets  166 ′ and  168 ′ of the master cylinder assembly  102 ′ travel through the inlets  456  and  460 , through the hollow section  482  of the shaft  472  and unite in the outlet  468 . The outlet  468  is fluidly connected to the brake pedal feel simulator  104  as shown in  FIG. 9 . The inlets  454  and  464  are fluidly coupled to the reservoir  306 . The inlets  458  and  462  are connected to the outlets  412  and  416  of the booster assembly  308 . 
       FIG. 12  shows the selector valve  312  positioned in a de-actuated position, i.e., electrical current is not applied to the solenoid  450 . The biasing member  474  causes the coil  478  and the shaft  472  to move to the left in the direction of an arrow  482  relative to its position in  FIG. 11 . In the de-actuated position, the selector valve  312  couples the outlets  166 ′ and  168 ′ of the master cylinder assembly  102 ′ to the downstream brake circuits  500  via fluid lines  322  and  324  through the selector valve  312 . This position constitutes a second mode of operation, which is hereinafter referred to as the conventional mode. In this position fluid can travel from inlets  456  and  460  and through the outlets  466  and  470 . The dots inside the aforementioned path indicate presence of pressurized hydraulic fluid. The inlets  456  and  460  are connected to outlets  166 ′ and  168 ′ of the master cylinder assembly  102 ′, respectively as shown in  FIG. 9 . Fluid that is ejected through the outlets  166 ′ and  168 ′ of the master cylinder assembly  102 ′ travel through the inlets  456  and  460 , and through outlets  466  and  470 . The outlets  466  and  470  are fluidly connected to the downstream brake circuits  500 . In the conventional mode, the hollow section  482  of the shaft  472  is isolated from fluid communication of the master cylinder assembly  102 ′. 
     Referring to  FIG. 13 , a schematic of the selector valve  312  and surrounding components is depicted with selector valve positioned in the active mode. The selector valve  312  is depicted with fluid connections showing operations in the active mode, in which the master cylinder assembly  102 ′ is in fluid communication with the brake pedal feel simulator  104 , but is isolated from the downstream brake circuits  500 ′. Further, in the active mode, the booster assembly  308  is in fluid communication with the downstream brake circuits  500 ′, but is fluidly isolated from the master cylinder assembly  102 ′. In the active mode, the selector valve  312  is also fluidly coupled to the reservoir  306 . 
     Referring to  FIG. 14 , a schematic of the selector valve  312  and surrounding components is depicted with the selector valve positioned in the conventional mode. The selector valve  312  is depicted with fluid connections showing operations in the conventional mode in which the master cylinder assembly  102 ′ is in fluid communication with the downstream brake circuits  500 ′ through the selector valve  312  and the booster assembly  308  via fluid lines  322  and  324 . In the conventional mode, the selector valve  312  is also fluidly coupled to the reservoir  306 . 
     In operation, the ECU  114 ′ determines the mode of the selector valve  312 , i.e., active or conventional. The ECU  114 ′ is configured to make this decision based on the fluid pressure and other diagnostic information, as is well known to one skilled in the art. The ECU  114 ′ generates a selector signal that is used to place the selector valve  312  in one of the two modes. In the active mode, the brake pedal  126 ′ is mechanically decoupled from the downstream brake circuits  500 ′. When the brake pedal  126 ′ is in a rest position, i.e., no pressure is being applied to the brake pedal  126 ′ by the vehicle operator, chambers  156 ′ and  158 ′ of the master cylinder assembly  102 ′ are fluidly coupled to the reservoir  306  through inlets  162 ′ and  164 ′, respectively. In this position, no appreciable fluid pressure exists in the chambers  156 ′ and  158 ′. As the brake pedal  126 ′ is pressed by the vehicle operator, pistons of the master cylinder assembly  102 ′ travel and thereby seal the inlets  162 ′ and  164 ′. Once the inlets  162 ′ and  164 ′ are completely cut off from fluid communication with the chambers  156 ′ and  158 ′, these chambers are now only fluidly coupled to the selector valve inlets  456  and  460 , the hollow section  482  of the shaft  472 , the outlet  468 , the brake pedal feel simulator  104 ′ and the fluid lines there between. Therefore, continued depression of the brake pedal  126 ′ raises the pressure inside the aforementioned path. The brake pedal feel simulator  104 ′ provides a resistance to the brake pedal  126 ′ similar to a conventional braking system (e.g., a resistance similar to that experienced by the vehicle operator when the selector valve is positioned in the conventional mode). 
     Furthermore, in the active mode the combination of seals  476 , the shaft  472 , and the housing  452  of the selector valve  312  prevents any appreciable fluid communication between the master cylinder assembly  102 ′ and the booster assembly  308 . Therefore, the master cylinder assembly  102 ′ is isolated from fluid communication with the booster assembly  308  when the selector valve is positioned in the active mode. 
     While the brake pedal  126 ′ is in a rest position, i.e., no pressure being applied by the vehicle operator, the chambers  406  and  408  are in fluid communication with the reservoir  306  via the valve assembly  128 ′. The valve assembly  128 ′ can be a normally closed valve assembly, thereby requiring energization of its solenoid in order to establish fluid coupling between its inlets and outlets. Therefore, while the brake pedal  126 ′ is in the rest position, the valve assembly  128 ′ can be intermittently energized in order to maintain fluid coupling between the chambers  406  and  408  and the reservoir  306 . Once the brake pedal  126 ′ is pressed by the vehicle operator, the connection between the chambers  406  and  408  and the reservoir  306  is terminated by de-energizing the valve assembly  128 ′. 
     In the active mode when the booster actuator  310  is actuated, fluid pressure inside the chambers  406  and  408  begins to rise. The outlets  412  and  416  are fluidly coupled to the inlets  458  and  462  of the selector valve  312 . The level of actuation of the booster actuator  310  is commensurate with the degree of depression of the brake pedal  126 ′. This level of actuation is determined by the electrical signal that is generated by a sensor  150 ′ and received by the ECU  114 ′. One sensor that may be used as sensor  150 ′ is a potentiometer. Depression of the brake pedal  126 ′ results in a proportional change in the received signal. As one example, the signal generated by the sensor  150 ′ is at a maximum level when the brake pedal  126 ′ is in a rest position. Conversely, the signal generated by the sensor  150 ′ is at a minimum value when the brake pedal is completely depressed. Therefore, depending on the strength of the signal generated by the sensor  150 ′, the ECU  114 ′ determines the level of actuation of the booster actuator  110 . 
     The booster actuator  310  is operable to provide a hydraulically generated force to move the piston  402  of the booster assembly  308 . Different methods can be used to govern the pressure behind the piston  402  to effect the precise amount of piston travel  402  to correspond to the degree of depression of the brake pedal  126 ′. The ECU  114 ′ modulates a valve assembly  333  of the booster actuator  310  to allow a precise amount of pressure build up behind the piston  402 . The ECU  114 ′ accesses a look-up table that correlates pedal travel to pressure. By modulating the valve assembly  333  between the fluid pumping station  307  and the booster assembly  308 , and by monitoring the pressure, the ECU  114 ′ controls the pressure buildup behind the piston  402 . The valve assembly  333 , therefore, requires a control signal from the ECU  114 ′ on line  339  which is connected to a solenoid valve  341 , a fluid connection  345  to the high pressure reservoir  316 , a fluid connection  337  to the reservoir  306 , and a fluid connection  330  to the booster assembly  308 . 
     In the conventional mode, the brake pedal  126 ′ is mechanically coupled to the downstream braking circuit  500 ′. In particular, when the brake pedal  126 ′ is in the rest position, i.e., no pressure is being applied to the brake pedal  126 ′ by the vehicle operator, the chambers  156 ′ and  158 ′ of the master cylinder assembly  102 ′ are fluidly coupled to the reservoir  306  through inlets  162 ′ and  164 ′, respectively. In this position, no appreciable fluid pressure exists in the chambers  156 ′ and  158 ′. As the brake pedal  126 ′ is pressed by the vehicle operator, the pistons of the master cylinder assembly  102 ′ travel and thereby seal the inlets  162  and  164 . Once the inlets  162  and  164  are completely cut off from fluid communication with the chambers  156 ′ and  158 ′, these chambers are now only fluidly coupled to the selector valve inlets  456  and  460 , the selector valve outlets  466  and  470 , fluid lines  322  and  324 , and the fluid connections there between. Therefore, continued depression of the brake pedal  126 ′ raises the pressure inside the aforementioned path. It should be appreciated that in the conventional mode, the brake pedal feel simulator  104 ′ is not fluidly coupled to the master cylinder assembly  102 ′. 
     While the brake pedal  126 ′ is in the rest position (i.e., no pressure being applied by the vehicle operator) the chambers  406  and  408  are in fluid communication with the reservoir  306  via the valve assembly  128 ′. Once the brake pedal  126 ′ is pressed by the vehicle operator, the connection between the chambers  406  and  408  and the reservoir  306  is terminated by de-energizing the valve assembly  128 ′. In the conventional mode, the booster actuator  310  is not actuated. Instead the required fluid pressure to effect proper braking is produced in the master cylinder assembly  102 ′ by depression of the brake pedal  126 ′, and is transferred to the downstream brake circuits  500 ′ through the selector valve  312  and the booster assembly  308 . 
     Referring to  FIG. 15 , an alternative embodiment of a selector valve  512  is depicted. The selector valve  512  is similar to the selector valve  312  with the following differences. The coil  478  and the solenoid  450  are replaced with a fluid inlet  578  that is fluidly coupled to a source of pressurized fluid (e.g., the pumping station  307 ) to effect movement of a shaft  572  in the direction of an arrow  580 . A hollow section  582  is also present in the selector valve  512 , similar to the hollow section  482  depicted in the selector valve  312 . An electrically actuated valve (not shown) can be used to selectively place a chamber  581  behind the shaft  572  in fluid communication with the source of the pressurized fluid to cause the shaft to move to the right in the direction of the arrow  580 . This actuated position (shown in  FIG. 15 ) places the selector valve  512  in the active mode. The electrically actuated valve is actuated and deactuated by the ECU  114 ′. 
     The electrically actuated valve is also configured to couple the chamber  581  to the reservoir  306  to relieve the pressure that has built up in the chamber  581 . A biasing member  574  in the form of a spring is configured to return the shaft  572  to a rest position. This position places the selector valve  512  in the conventional mode. It should be appreciated that the selector valve  512  as depicted in  FIG. 15  can be used in place of the selector valve  312  as that depicted in  FIG. 2  and the selector valve  312  in  FIG. 9 . 
     Referring to  FIG. 16 , an alternative to the selector valves  112 ,  312 , and  512  is shown. In particular, a selector valve assembly including two selector valves components is shown. The two selector valve components are coupled to one booster assembly, and each individual selector valve member is coupled to a separate brake pedal feel simulator. In the embodiment that is shown in  FIG. 16 , each selector valve member is coupled to a downstream braking circuit  502  and  504 , respectively. In yet another alternative embodiment, each selector valve member can be coupled to an inlet  212  and  216  of the booster assembly  108 , in a manner similar to that depicted in  FIG. 2 . One advantage of dividing the functionality of the selector valve assembly into two selector valves components is to increase safety. With two selector valve components, if one of the two selector valve components fails, the other selector valve component can continue to provide a braking function to its respective downstream braking circuit (such as braking circuit  502  and  504 ). As described above, each selector valve component can be actuated by electrically actuated solenoid valves or by application of high pressure fluid from a source of high pressure fluid to a chamber behind the shafts of the selector valve components (such as chamber  580  in  FIG. 15 ). 
     While the invention has been illustrated and described in detail in the drawings and foregoing description, the same should be considered as illustrative and not restrictive in character. It is understood that only the preferred embodiments have been presented and that all changes, modifications and further applications that come within the spirit of the invention are desired to be protected.