Abstract:
A braking system includes an emulator cylinder, an emulator actuator piston within the emulator cylinder, an emulator actuator chamber within the emulator cylinder defined in part by the emulator actuator piston, a pedal feel simulator including an input port in fluid communication with the emulator actuator chamber and an output port, a selector piston movable within the emulator cylinder between a first position and a second position, and a poppet valve operably connected to the emulator actuator piston, the poppet valve movable by the emulator actuator piston between a deactivated position and an actuated position and configured such that (i) when the selector piston is in the first position and the poppet valve is in the actuated position, the poppet valve is closed and the emulator actuator chamber is not in fluid communication with the reservoir through the poppet valve, and (ii) when the selector piston is in the second position and the poppet valve is in the actuated position, the poppet valve is open and the emulator actuator chamber is in fluid communication with the reservoir through the poppet valve.

Description:
FIELD 
       [0001]    The invention relates to braking systems, and in particular to a braking system with a pedal feel simulator. 
       BACKGROUND 
       [0002]    Significant progress has been made in vehicular braking systems in recent years. Among these developments are anti-lock braking systems (ABS) and regenerative braking systems. The latter is used in electric and hybrid-electric vehicles. In regenerative braking systems, a vehicle&#39;s brake pedal is mechanically decoupled from downstream braking circuits. A sensor associated with an input rod coupled to a brake pedal provides an electrical signal to an electronic control unit (ECU). The signal is 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 a force feedback to the vehicle operator at the brake pedal. At the same time, 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 braking system provide the necessary braking force. 
         [0003]    In the event of a failure of the hydraulic system and/or the electrical regenerative system, it is necessary for the braking system to switch modes of operation so that the brake pedal is mechanically coupled to the downstream brake circuits. In such a failure mode, the force applied to the brake pedal is transferred to the downstream brake circuits to generate the necessary braking force to halt a vehicle. 
         [0004]    There is a need to provide an improved braking system that is operable in a normal 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 
       [0005]    According to one embodiment of the present disclosure, there is provided a braking system. The braking system includes an emulator cylinder, an emulator actuator piston within the emulator cylinder, an emulator actuator chamber within the emulator cylinder defined in part by the emulator actuator piston, a pedal feel simulator including an input port in fluid communication with the emulator actuator chamber and an output port, a selector piston movable within the emulator cylinder between a first position and a second position, and a poppet valve operably connected to the emulator actuator piston, the poppet valve movable by the emulator actuator piston between a deactivated position and an actuated position and configured such that (i) when the selector piston is in the first position and the poppet valve is in the actuated position, the poppet valve is closed and the emulator actuator chamber is not in fluid communication with the reservoir through the poppet valve, and (ii) when the selector piston is in the second position and the poppet valve is in the actuated position, the poppet valve is open and the emulator actuator chamber is in fluid communication with the reservoir through the poppet valve. 
         [0006]    According to one embodiment of the present disclosure, there is provided a braking system. The braking system includes an emulator cylinder, an emulator actuator piston within the emulator cylinder, an emulator actuator chamber located forwardly of the emulator actuator piston within the emulator cylinder, a pedal feel simulator including an input port in fluid communication with the emulator actuator chamber and an output port, a selector piston located forwardly of the emulator actuator chamber and movable within the emulator cylinder between a first position and a second position, and a poppet valve operably connected to the emulator actuator piston, the poppet valve movable by the emulator actuator piston between a rearward position and a forward position and configured such that (i) when the selector piston is in the first position the poppet valve can be seated against the selector piston with the emulator actuator chamber not in fluid communication with a reservoir through the poppet valve, and (ii) when the selector piston is in the second position the poppet valve cannot be seated against the selector piston and the emulator actuator chamber is in fluid communication with the reservoir through the poppet valve. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  depicts a fragmentary cross sectional view of a braking system including a high pressure regulator with a high pressure accumulator, a reservoir, a master cylinder assembly, and an emulator assembly; 
           [0008]      FIG. 2  depicts a fragmentary cross sectional view of the emulator assembly of  FIG. 1 , the emulator includes a cylinder, an actuator piston assembly, an actuator shaft, a poppet valve assembly, a selector piston assembly, and a pin assembly; 
           [0009]      FIG. 3A  depicts a cross sectional view of the actuator piston assembly of  FIG. 2 ; 
           [0010]      FIG. 3B  depicts a cross section view of the actuator shaft of  FIG. 2 ; 
           [0011]      FIG. 4  depicts a cross sectional view of the poppet valve assembly of  FIG. 2 ; 
           [0012]      FIG. 5A  depicts a front view of the selector valve assembly of  FIG. 2 ; 
           [0013]      FIG. 5B  depicts a cross sectional view of the selector valve assembly of  FIG. 2 ; 
           [0014]      FIG. 6  depicts a front view of the pin assembly of  FIG. 2 ; 
           [0015]      FIG. 7  depicts a fragmentary cross sectional view of the emulator assembly of  FIG. 1  in one operational position in a normal mode, wherein the poppet assembly is firmly engaged with the poppet valve assembly in an actuated position; 
           [0016]      FIG. 8  depicts a fragmentary cross sectional view of the emulator assembly of  FIG. 1  in another operation position in the normal mode, wherein the actuator piston assembly is further moved leftward within the cylinder as compared to the actuator piston of  FIG. 7 ; 
           [0017]      FIG. 9  depicts a fragmentary cross sectional view of the emulator assembly of  FIG. 1  in one operation position in a fallback mode, wherein the poppet assembly is in a deactivated position; and 
           [0018]      FIG. 10  depicts an enlarged fragmentary cross sectional view of a portion of the emulator assembly of  FIG. 9 . 
       
    
    
     DESCRIPTION 
       [0019]    For the purposes 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 specification. It is understood that no limitation to the scope of the invention is thereby intended. It is further 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 of ordinary skill in the art to which this invention pertains. 
         [0020]    Referring to  FIG. 1 , a fragmentary cross sectional view of a braking system  100  is depicted. The braking system  100  includes a high pressure regulating system  102 , a reservoir  104 , a master cylinder assembly  106 , a booster assembly  108 , an emulator assembly  110 , and an input rod assembly  130 . The high pressure regulating system  102  is fluidly coupled to the booster assembly  108 . The booster assembly  108  is mechanically coupled to the master cylinder assembly  106 . The master cylinder assembly  106  is fluidly coupled to the reservoir  104  and to downstream braking circuits (not shown). 
         [0021]    The high pressure regulating system  102  includes a high pressure accumulator  112  for storing high pressure fluid, and a regulating valve assembly  114 . The regulating valve assembly  114  is electrically coupled to and controlled by an electronic control unit (ECU) (not shown). The regulating valve assembly  114  is configured to regulate pressure from the high pressure accumulator  112 , which is supplied from a high pressure fluid generator, e.g., a pump (not shown), and provide high pressure fluid (with the pressure regulated) to a boost chamber  116  of the booster assembly  108 . 
         [0022]    The reservoir  104  provides fluid to a primary chamber  118  and a secondary chamber  120  of the master cylinder assembly through inlets  146  and  148 , respectively. The primary chamber  118  is defined by a primary piston  122  and a secondary piston  124 , while the secondary chamber  120  is defined by the secondary piston  124  and a cylinder wall portion  126  of the master cylinder assembly  106 . The primary chamber  118  is coupled to a first downstream braking circuit (not shown) through a first outlet (not shown) while the secondary chamber  120  is coupled to a second downstream circuit chamber (not shown) through a second outlet (not shown). 
         [0023]    The booster assembly  108  is coupled to the high pressure regulating system  102 . The booster assembly includes a booster piston  128  which is in fluid contact with the boost chamber  116 . The boost chamber  116  is thus defined by the booster piston  128  and a housing  134 . The booster piston  128  includes a rear portion  136  which extends through the housing  134 . 
         [0024]    The input rod assembly  130  includes a rod  140 , a clevis  142 , and a ball portion  144 . The clevis  142  is mechanically coupled to a brake pedal (not shown). The ball portion  144  allows pivoting of the entire input rod assembly  130  by coupling with the emulator assembly  110 . The input rod assembly  130  is configured to transfer movement of the brake pedal (not shown) to the emulator assembly  110 . 
         [0025]    The input rod assembly  130  is also coupled to an apply link  132  and is configured to transfer movement of the brake pedal (not shown) to the rear portion  136  of the booster piston  128  in one operational mode, as described further below. 
         [0026]    Referring to  FIG. 2 , a detailed cross sectional view of the emulator assembly  110  is depicted. The emulator assembly  110  includes a cylinder  202 , an actuator piston assembly  204 , a poppet valve assembly  206 , a selector piston assembly  208 , and a pin assembly  210 . The emulator assembly  110  also includes a pedal feel simulator  212 , a normally open solenoid valve  214 , a selector piston spring  216 , and an actuator piston spring  218 . A travel sensor assembly  220  is coupled to the cylinder  202 . 
         [0027]    The actuator piston assembly  204  is slidably disposed within the cylinder  202 . The actuator piston assembly  204  and the cylinder  202  define an actuator chamber  222 . The actuator chamber  222  is in fluid communication with the reservoir  104  (see  FIG. 1 ) through the poppet valve assembly  206  when the poppet valve assembly  206  is not in a sealed position against the selector piston assembly  208 . The actuator piston assembly  204  is coupled to the poppet valve assembly  206  by the actuator piston spring  218  and an actuator shaft  224 . 
         [0028]    The poppet valve assembly  206  is moveably disposed within the cylinder  202 . The poppet valve assembly  206  is coupled to the selector piston assembly  208  and is configured to be supported by the selector piston assembly  208 , as described further below. 
         [0029]    The selector piston assembly  208  is slidably disposed within the cylinder  202  and is biased rightward (with reference to  FIG. 2 ) by the selector piston spring  216  which is disposed between the selector piston assembly  208  and the cylinder  202 . The selector piston assembly  208 , the cylinder  202 , and the normally open solenoid valve  214  define a selector chamber  226 . The solenoid valve  214  is a normally open bidirectional solenoid valve. The solenoid valve  214  is thus configured to fluidly isolate the selector chamber  226  from the reservoir  104  when energized. When the normally open solenoid valve  214  is not energized, it provides unrestricted fluid communication between the selector chamber  226  and the reservoir  104 . 
         [0030]    The pin assembly  210  is positioned about the selector piston assembly  208 . The pin assembly  210  is fixedly coupled to the cylinder  202 . The selector piston assembly  208  is configured to slide with respect to the pin assembly  210 . 
         [0031]    The pedal feel simulator  212  is also coupled to the cylinder  202  via an inlet  228  and an outlet  230 . The inlet  228  is fluidly coupled to the actuator chamber  222 , while the outlet  230  is coupled to the reservoir  104  by a fluid path about the selector piston assembly  208  and the pin assembly  210 . 
         [0032]    Fluid passageways  232  and  234  fluidly coupled the emulator assembly  110  to the reservoir  104  (see  FIG. 1 ) through the master cylinder assembly  106 . Specifically, the fluid passageway  232  is fluidly coupled to the actuator chamber  222  when the poppet valve assembly  206  is not in a sealed position against the selector piston assembly  208 . Further, the fluid passageway  234  is fluidly coupled to the selector chamber  226  when the normally open solenoid valve  214  is not energized. 
         [0033]    Referring to  FIG. 3A , the actuator piston assembly  204  is depicted. The actuator piston assembly  204  includes a body portion  252 , an actuator bracket  254 , and a ball-and-socket interface  256 . The ball portion  144  of the input rod assembly  130  (see  FIG. 1 ) is coupled to the ball-and-socket interface  256  and is thereby configured to rotate with respect to the emulator assembly  110 . The actuator bracket  254  includes a rearward facing cavity  258  which is aligned with a cavity  260  formed in the body portion  252 . Attached to the body portion  252  are also a magnet  262  and an apply link extension  264 . The apply link extension  264  is fixedly coupled to the apply link  132  (see  FIG. 1 ). 
         [0034]    The actuator bracket  254  includes a central opening  268  through which the actuator shaft  224  is slidably received. The central opening  268  is smaller than the rearward facing cavity  258 , thereby defining a step  270  at the transition between the central opening  268  and the rearward facing cavity  258 . 
         [0035]    Referring to  FIG. 3B , the actuator shaft  224  is depicted. The actuator shaft  224  includes a key portion  280  and an end portion  282 . The key portion  280  interfaces with the step  270  of the actuator piston assembly  204  in order to limit leftward movement, with reference to  FIG. 2 , of the actuator shaft  224 . The end portion  282  interfaces with the poppet valve assembly  206 , as discussed in further detail with respect to  FIG. 4 . 
         [0036]    The poppet valve assembly  206 , depicted in  FIG. 4 , includes a poppet housing  302 , a poppet body  304 , a poppet spring  306 , and a seal  308 . The poppet spring  306  is located between the poppet housing  302  and the poppet body  304  and is configured to bias the poppet body  304  leftward, with reference to  FIG. 4 . The poppet body  304  is sealingly coupled to the poppet housing  302  by the seal  308 , and is configured to slide within the poppet housing  302 . The poppet body  304  is fixedly coupled to the actuator shaft  224 . In particular, the poppet body  304  interfaces with the end portion  282  of the actuator shaft  224 . 
         [0037]    The poppet housing  302  includes a step  310  which interfaces with the actuator piston spring  218  on the right side of the step  310 , with reference to  FIG. 4 . On the left side of the step  310 , the poppet valve assembly  206  interfaces with the selector piston assembly  208  (see  FIG. 2 ). 
         [0038]    Referring to  FIGS. 5A and 5B , a front view and a cross sectional view of the selector piston assembly  208  are depicted, respectively. In general the selector piston assembly  208  is divided into a front portion  352  and a rear portion  354 . The front portion  352  includes a front face  356 , a seal  358 , and four wedges  368 . The rear portion  354  includes a sealing face  360 , a cavity  362 , a seal  364 , and a step  366 . While the selector piston assembly  208  is depicted as a single piece (i.e., the front portion  352  and the rear portions  354  are depicted as a unitary component), the reader should understand that the front portion  352  and the rear portion  354  may be two separate pieces that are assembled about the pin assembly  210 . 
         [0039]    The front portion  352  interfaces with the selector spring  216 . In particular, the front face  356  provides a seating surface for the selector spring  216 . As described above, the front portion  352 , the cylinder  202 , and the normally open valve  214  define the selector chamber  226 . The rear portion  354  interfaces with the poppet valve assembly  206 . In particular, the step  366  interfaces with the step  310  of the poppet housing  302  of the poppet valve assembly  206 . The sealing face  360  provides a sealing surface for the seal  308  of the poppet valve assembly  206 . The cavity  362  provides a space for slidably receiving elongated members of the pin assembly  210 . 
         [0040]    Referring to  FIG. 6 , the pin assembly  210  is depicted. The pin assembly  210  includes a disk portion  402  and elongated members  404  and  406  projecting rearwardly from the disk portion  402 . The elongated members  404  and  406  form a cross-shaped pin. As described above, the selector pin assembly  208  is formed or assembled about the pin assembly  210 . As a result, openings  408  are formed on the disk portion  402  to allow passage of the front portion  352  of the selector piston assembly  208  and in particular the four wedges  368  (see  FIG. 5B ) through the opening  408 . The four wedges  368  engage the rear portion  354  of the selector piston assembly  208 , e.g. by threads. Other interfaces between the selector piston assembly  208  and the pin assembly  210  are also possible. In any of these interfaces, the selector piston assembly  208  is configured to slide with respect to the pin assembly  210  which has a fixed position with respect to the cylinder  202 . 
         [0041]    Operation of the brake assembly  100  is described with respect to two different modes. In the first mode, hereinafter referred to as the normal mode, the normally open valve  214  is energized by the ECU (not shown). The normal mode corresponds to an operational mode in which the high pressure regulating system  102  is working normally and producing appropriate fluid pressures. In the second mode, hereinafter referred to as the fallback mode, the normally open valve  214  is not energized. The fallback mode corresponds to an operational mode in which the high pressure regulating system  102  is not working properly and thereby not producing appropriate fluid pressures in the boost chamber  116 . 
         [0042]    With reference to  FIG. 2 , the input rod assembly  130  is coupled to the brake pedal (not shown) and to the actuator piston assembly  204  which is depicted in an unapplied condition (i.e., with the brake pedal (not shown) in an unapplied position). The actuator chamber  222  is in fluid communication with the pedal feel simulator  212  through the inlet  228 . Also, the actuator chamber  222  is selectively in fluid communication with the reservoir  104  via the poppet valve assembly  206  through the openings  408  of the pin assembly and through the fluid passageway  232 . 
         [0043]    In the unapplied condition, the poppet valve spring  306  is in a state of compression (i.e., the poppet body  304  is biased leftward, with reference to  FIG. 4 ). The interface between the step  270  of the actuator bracket  254  of the actuator piston assembly  204  and the key portion  280  of the actuator shaft  224 , limits the leftward movement of the actuator shaft  224 . Therefore, while the poppet body  304  is biased leftward, the leftward travel of poppet body  304 , being fixedly coupled to the actuator shaft  224 , is limited. The rest position hereinafter is also referred to as the deactivated position or the rearward position of the poppet valve assembly  206 . 
         [0044]    In the normal mode, as described above, the normally open valve  214  is energized. As an operator of a vehicle applies force to the brake pedal (not shown) the input rod assembly  130  moves leftward. 
         [0045]    The actuator piston assembly  204  moves leftward because of the leftward movement of the input rod assembly  130 . The actuator spring  218  compresses because of the leftward movement of the actuator piston assembly  204 , which applies a leftward force to the housing  302  of the poppet valve assembly  206 . The poppet valve assembly  206  transfers the leftward force to the selector piston assembly  208 . Since the normally open solenoid valve  214  is energized, the selector piston chamber  226  is isolated from fluid communication with the reservoir  104 . Therefore, since the fluid is contained in the selector chamber  226 , the above mentioned leftward force results in no movement of the selector piston assembly  208 . The depicted position of the selector piston assembly  208  with the normally open solenoid valve  214  being energized may hereinafter be called the first position of the selector piston assembly  208 . 
         [0046]    With the leftward movement of the actuator piston assembly  204  the step  270  of the actuator bracket  254  and the key portion  280  are no longer limiting the leftward movement of the actuator shaft  224  which is fixedly coupled to poppet body  304 . Since the poppet spring  306  in a compressed state, the poppet body  304  moves leftward. Continuous leftward movement of the actuator piston assembly  204  allows further leftward movement of the poppet body  304  until the poppet body  304  makes contact with the sealing surface  360 . 
         [0047]    Since the selector piston assembly  208  is hydraulically locked with respect to the cylinder  202 , the poppet assembly  206  is in contact with the selector piston assembly  208 , and the actuator shaft  224  is fixedly coupled to the poppet assembly  206 , the actuator shaft  224  is unable to move leftward once the poppet valve assembly  206  makes contact with the selector piston assembly  208 . As a result, the key portion  280  separates from the step  270  of the actuator piston assembly (see  FIGS. 3A and 3B ). Continued leftward movement of the actuator piston assembly  204 , with the key the portion  280  remaining stationary, allows the actuator shaft to move within the rearward facing cavity  258  of the actuator bracket  254  or even into the cavity  260  of the body portion  252  of the actuator piston assembly  204 , as depicted in  FIGS. 7 and 8 . 
         [0048]    Once the poppet body  304  makes contact with the sealing face  360  of the selector piston assembly  208 , the actuator chamber  222  is no longer in fluid communication with the reservoir  104  via pin assembly  210  (i.e., through the opening  408 , see  FIG. 6 , and through the passageway  232 , see  FIG. 2 ). Since the actuator chamber  222  is isolated from the reservoir, fluid within the actuator chamber  222  enters the pedal feel simulator  212  through the inlet  228 . The pedal feel simulator  212  is of the type that is known in the art. Generally, this type of pedal feel simulator includes a high pressure side and a low pressure side. The high pressure side communicates with the actuator chamber  222  via the inlet  228 . The low pressure side vents to the reservoir  104  via the outlet  230 , and as in the case of the braking system  100 , through the master cylinder assembly  106  (see  FIG. 1 ). The high pressure side of this type of pedal simulator typically includes a piston which is biased away from a bottom portion of a cylinder by one or more simulator springs. The piston is fluidly coupled to the actuator chamber  222  via the inlet  228 . Pressure build up in the actuator chamber  222  causes the piston to move against the biasing force of the simulator springs which provides a pedal feel of a conventional braking system to the operator. The low pressure side is vented to the reservoir  104  in order to prevent a hydraulic lock situation. In alternative embodiments, the low pressure side is of a pneumatic type and is coupled to the atmosphere to prevent a pneumatic lock situation. 
         [0049]    As the operator moves the brake pedal (not shown), as described above, leftward movement of the actuator piston assembly  204  also causes leftward movement of the magnet  262  (see  FIG. 3A ). The travel sensor assembly  220  is configured to sense position of the magnet  262 , and thereby determines the position of the brake pedal (not shown), and to generate a corresponding signal which it communicates with the ECU (not shown). The ECU (not shown) correspondingly controls the regulating valve assembly  114  which regulates pressure in the boost chamber  116 . In response to fluid pressure in the boost chamber  116 , the booster piston  128  moves leftward which moves the primary piston  122  and the secondary piston  124  in order to pressurize fluid in the downstream braking circuits (not shown). As a result, the booster piston  128  applies a boosted force to the primary and secondary pistons  122  and  124  to effectuate the braking function. 
         [0050]    While the apply link  132  which is positioned near the booster piston  128  is coupled to the input rod assembly  130  and moves leftward with the leftward movement of the input rod assembly  130 , the booster piston  128  is mechanically decoupled from the input rod assembly  130 , and thus from the brake pedal (not shown). Specifically, the apply link  132  does not make contact with the booster piston  128 . Therefore, the only feedback the operator receives from the brake pedal (not shown) is from the actuator spring  218  and the pedal feel simulator  212 . 
         [0051]    When the operator releases the brake pedal (not shown), the actuator piston assembly  204  returns to the unapplied position (see  FIG. 2 ). Prior to reaching the unapplied position, the poppet valve assembly  206  unseals from the selector piston assembly  208  and the actuator chamber  222  returns to fluid communication with the reservoir through the poppet valve assembly  206 . 
         [0052]    In the fallback mode, the braking system  100  is initially in condition of  FIG. 1  and operates similar to the normal mode except that the normally open solenoid valve  214  is not energized. As a result, the selector chamber  226  remains in fluid communication with the reservoir  104  through the normally open solenoid valve  214  and through the fluid passageway  234  (see  FIG. 2 ). Therefore, the abovementioned leftward force applied by the poppet valve assembly  206  to the selector piston assembly  208  causes the selector piston assembly  208  to move leftward, thereby collapsing the selector chamber  226  by compressing the selector spring  216 , as depicted in  FIG. 9 . The depicted position in  FIG. 9  is hereinafter referred to as the second position of the selector valve assembly  208 . 
         [0053]    Referring to  FIG. 10 , an enlarged fragmentary cross sectional view of the pin assembly  210  and the poppet valve assembly  206  is depicted. Leftward movement of the selector piston assembly  208  exposes the elongated members  404  and  406  of the pin assembly  210 , which is fixedly coupled to the cylinder  202 . With the elongated members  404  and  406  exposed, the poppet valve assembly  206  (specifically, the poppet body  304 ) cannot seal against the sealing face  360  of the selector piston assembly  208 . The depicted position of the poppet valve assembly  206  is hereinafter referred to as the forward position or the actuated position. Therefore, the actuator chamber  222  remains in fluid communication with the reservoir  104  through the poppet valve assembly  206 . As a result, fluid does not enter the pedal feel simulator  112  through the inlet  228  (see  FIG. 2 ). 
         [0054]    In the fallback mode, the ECU (not shown) does not control the regulating valve assembly  114  which regulates pressure in the boost chamber  116 . As a result, the booster piston  128  is not moved by boost pressure in the boost chamber  116  as the operator is moving the brake pedal (not shown), thereby effectuating no boosted braking function. 
         [0055]    Because the booster piston  128  is not moving, movement of the input rod assembly  130  moves the apply link  132  into contact with rear portion  136  of the booster piston  128  (see  FIG. 1 ). The apply link  132  is coupled to the input rod  130  through the actuator piston assembly  204  (see  FIG. 2 ) and is configured to move leftward along with leftward movement of the input rod assembly  130 . Once the apply link  132  makes contact with the rear portion  136  of the booster piston  128 , the operator is able to move the primary and secondary pistons  122  and  124  to provide the desired braking function without a booster function or with a degraded booster function. In addition, with the brake pedal (not shown), now mechanically coupled to the primary and secondary pistons  122  and  124 , the operator receives the feedback at the brake pedal (not shown) corresponding to springs in the primary chamber  118  and the secondary chamber  120 , the hydraulic force from the fluid in the downstream braking circuits (not shown), and the actuator spring  218 . 
         [0056]    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.