Patent Publication Number: US-2020298807-A1

Title: Vehicle brake system having a brake pedal unit

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 62/592,929, filed Nov. 30, 2017, the disclosure of which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates in general to vehicle braking systems. Vehicles are commonly slowed and stopped with hydraulic brake systems. These systems vary in complexity but a base brake system typically includes a brake pedal, a tandem master cylinder, fluid conduits arranged in two similar but separate brake circuits, and wheel brakes in each circuit. The driver of the vehicle operates a brake pedal which is connected to the master cylinder. When the brake pedal is depressed, the master cylinder generates hydraulic forces in both brake circuits by pressurizing brake fluid. The pressurized fluid travels through the fluid conduit in both circuits to actuate brake cylinders at the wheels to slow the vehicle. 
     Base brake systems typically use a brake booster which provides a force to the master cylinder which assists the pedal force created by the driver. The booster can be vacuum or hydraulically operated. A typical hydraulic booster senses the movement of the brake pedal and generates pressurized fluid which is introduced into the master cylinder. The fluid from the booster assists the pedal force acting on the pistons of the master cylinder which generate pressurized fluid in the conduit in fluid communication with the wheel brakes. Thus, the pressures generated by the master cylinder are increased. Hydraulic boosters are commonly located adjacent the master cylinder piston and use a boost valve to control the pressurized fluid applied to the booster. 
     Braking a vehicle in a controlled manner under adverse conditions requires precise application of the brakes by the driver. Under these conditions, a driver can easily apply excessive braking pressure thus causing one or more wheels to lock, resulting in excessive slippage between the wheel and road surface. Such wheel lock-up conditions can lead to greater stopping distances and possible loss of directional control. 
     Advances in braking technology have led to the introduction of Anti-lock Braking Systems (ABS). An ABS system monitors wheel rotational behavior and selectively applies and relieves brake pressure in the corresponding wheel brakes in order to maintain the wheel speed within a selected slip range to achieve maximum braking force. While such systems are typically adapted to control the braking of each braked wheel of the vehicle, some systems have been developed for controlling the braking of only a portion of the plurality of braked wheels. 
     Electronically controlled ABS valves, comprising apply valves and dump valves, are located between the master cylinder and the wheel brakes. The ABS valves regulate the pressure between the master cylinder and the wheel brakes. Typically, when activated, these ABS valves operate in three pressure control modes: pressure apply, pressure dump and pressure hold. The apply valves allow pressurized brake fluid into respective ones of the wheel brakes to increase pressure during the apply mode, and the dump valves relieve brake fluid from their associated wheel brakes during the dump mode. Wheel brake pressure is held constant during the hold mode by closing both the apply valves and the dump valves. 
     To achieve maximum braking forces while maintaining vehicle stability, it is desirable to achieve optimum slip levels at the wheels of both the front and rear axles. During vehicle deceleration different braking forces are required at the front and rear axles to reach the desired slip levels. Therefore, the brake pressures should be proportioned between the front and rear brakes to achieve the highest braking forces at each axle. ABS systems with such ability, known as Dynamic Rear Proportioning (DRP) systems, use the ABS valves to separately control the braking pressures on the front and rear wheels to dynamically achieve optimum braking performance at the front and rear axles under the then current conditions. 
     A further development in braking technology has led to the introduction of Traction Control (TC) systems. Typically, valves have been added to existing ABS systems to provide a brake system which controls wheel speed during acceleration. Excessive wheel speed during vehicle acceleration leads to wheel slippage and a loss of traction. An electronic control system senses this condition and automatically applies braking pressure to the wheel cylinders of the slipping wheel to reduce the slippage and increase the traction available. In order to achieve optimal vehicle acceleration, pressurized brake fluid is made available to the wheel cylinders even if the master cylinder is not actuated by the driver. 
     During vehicle motion such as cornering, dynamic forces are generated which can reduce vehicle stability. A Vehicle Stability Control (VSC) brake system improves the stability of the vehicle by counteracting these forces through selective brake actuation. These forces and other vehicle parameters are detected by sensors which signal an electronic control unit. The electronic control unit automatically operates pressure control devices to regulate the amount of hydraulic pressure applied to specific individual wheel brakes. In order to achieve optimal vehicle stability, braking pressures greater than the master cylinder pressure must quickly be available at all times. 
     Brake systems may also be used for regenerative braking to recapture energy. An electromagnetic force of an electric motor/generator is used in regenerative braking for providing a portion of the braking torque to the vehicle to meet the braking needs of the vehicle. A control module in the brake system communicates with a powertrain control module to provide coordinated braking during regenerative braking as well as braking for wheel lock and skid conditions. For example, as the operator of the vehicle begins to brake during regenerative braking, electromagnet energy of the motor/generator will be used to apply braking torque (i.e., electromagnetic resistance for providing torque to the powertrain) to the vehicle. If it is determined that there is no longer a sufficient amount of storage means to store energy recovered from the regenerative braking or if the regenerative braking cannot meet the demands of the operator, hydraulic braking will be activated to complete all or part of the braking action demanded by the operator. Preferably, the hydraulic braking operates in a regenerative brake blending manner so that the blending is effectively and unnoticeably picked up where the electromagnetic braking left off. It is desired that the vehicle movement should have a smooth transitional change to the hydraulic braking such that the changeover goes unnoticed by the driver of the vehicle. 
     Brake systems may also include autonomous braking capabilities such as adaptive cruise control (ACC). During an autonomous braking event, various sensors and systems monitor the traffic conditions ahead of the vehicle and automatically activate the brake system to decelerate the vehicle as needed. Autonomous braking may be configured to respond rapidly in order to avoid an emergency situation. The brake system may be activated without the driver depressing the brake pedal or even if the driver fails to apply adequate pressure to the brake pedal. Advanced autonomous braking systems are configured to operate the vehicle without any driver input and rely solely on the various sensors and systems that monitor the traffic conditions surrounding the vehicle. 
     There is illustrated in  FIG. 1  a schematic illustration of a prior art master cylinder or a brake pedal unit, indicated generally at  10 . The brake pedal unit  10  is an example of a prior art brake pedal unit used in the brake systems described above. The brake pedal unit  10  includes a housing with a multi-stepped bore  12  formed therein. An input piston  14 , a primary piston  16 , and a secondary piston  18  are slidably disposed within the bore  12 . The input piston  14  is connected with a brake pedal (not shown) via a linkage arm  20 . Leftward movement of the input piston  14 , the primary piston  16 , and the secondary piston  18  may cause, under certain conditions, a pressure increase within an input chamber  24 , a primary chamber  26 , and a secondary chamber  28 , respectively. Various seals of the brake pedal unit  10  as well as the structure of the housing and the pistons  14 ,  16 , and  18  define the chambers  24 ,  26 , and  28 . For example, the input chamber  24  is generally defined between the input piston  14  and the primary piston  16 . The primary chamber  26  is generally defined between the primary piston  16  and the secondary piston  18 . The secondary chamber  28  is generally defined between the secondary piston  18  and an end wall  30  of the housing formed by the bore  12 . 
     The input chamber  24  is selectively in fluid communication with a pedal simulator (not shown) for simulating a force feedback on the brake pedal to the driver of the vehicle. An outer cylindrical wall of the input piston  14  is engaged with a lip seal  32  and a pair of seals  34  and  36 . The seal  36  functions as a secondary sealing structure in conjunction with the seal  34  and provides an extra layer of leak protection so that fluid does not leak from the input chamber  24  out of the brake pedal unit  10 . A fluid passageway  38  (or multiple passageways) is formed through a wall of the input piston  14 . As shown in  FIG. 1 , when the brake pedal unit  10  is in its rest position (the driver is not depressing the brake pedal), the passageway  38  is located between the lip seal  32  and the seal  36 . In the rest position, the passageway  38  permits fluid communication between the input chamber  24  and a fluid reservoir (not shown). Note that in the rest position of the brake pedal unit  10 , the lip seal  32  is to the left of the passageway  38 , thereby permitting fluid communication between the input chamber  24  and the fluid reservoir. During initial operation of the brake pedal unit  10 , sufficient leftward movement of the input piston  14 , as viewing  FIG. 1 , will cause the passageway  38  to move past the lip seal  32 , thereby preventing the flow of fluid from the input chamber  24  into the reservoir. Further leftward movement of the input piston  14  will pressurize the input chamber  24  causing fluid to flow into the pedal simulator. 
     Under certain conditions, the primary and secondary chambers  26  and  28  are each in fluid communication with of a pair of wheel brakes (not shown) which provide pressurized fluid to the wheel brakes when a buildup of pressure occurs within the first and second pressure chambers  26  and  28 . An outer wall of the primary piston  16  is engaged with a lip seal  40  and a seal  42  mounted in grooves formed in the housing. A fluid passageway  44  (or passageways) is formed through a wall of the primary piston  16 . The passageway  44  is located between the lip seal  40  and the seal  42  when the primary piston  16  is in its rest position. Note that in the rest position of the brake pedal unit  10 , the lip seal  40  is to the left of the passageway  44 , thereby permitting fluid communication between the primary chamber  26  and the reservoir. An outer wall of the secondary piston  18  is engaged with a lip seal  46  and a seal  48  mounted in grooves formed in the housing. A fluid passageway  50  (or passageways) is formed through a wall of the secondary piston  18 . The passageway  50  is located between the lip seal  46  and the seal  48  when the secondary piston  18  is in its rest position. Note that in the rest position of the brake pedal unit  10 , the lip seal  50  is to the left of the passageway  50 , thereby permitting fluid communication between the secondary chamber  28  and the fluid reservoir. 
     The brake pedal unit  10  includes an input spring assembly  52  generally disposed between the input piston  14  and the primary piston  16 . A primary spring assembly  54  is disposed between the primary piston  16  and the secondary piston  18 . A secondary spring assembly  56  is disposed between the secondary piston  18  and a bottom wall of the housing. The input, primary and secondary spring assemblies  52 ,  54 , and  56  function as caged spring assemblies for biasing the pistons  14 ,  16 , and  18  away from each other as well as functioning to properly position the pistons  14 ,  16 , and  18  within the housing of the brake pedal unit  10 . A caged spring assembly includes structures which limit and define the length of the caged spring assembly, thereby positioning the pistons at a predefined distance relative to one another. A spring preload may exist in the spring members of the caged spring assembly such that an initial force is required to compress the spring members of the caged spring assembly. The brake pedal unit  10  further includes a return spring  58  biasing the input piston  14  in the rightward direction as viewing  FIG. 1 . Note that in the rest position of the brake pedal unit  10 , the caged secondary spring assembly  56  is provided with a gap  90  between an enlarged head portion  62  of a stem  64  and a retainer  66 . 
     In the rest position of the brake pedal unit  10 , all three chambers  24 ,  26 , and  28  are in fluid communication with the fluid reservoir via the respective passageways  38 ,  44 , and  50 . During operation of the brake pedal unit  10 , the driver depresses the brake pedal causing leftward movement of the input piston  14 . Fluid from the input chamber  24  is vented to the fluid reservoir until the passageway  38  slips past the lip seal  32 . Further leftward movement of the input piston  14  causes an increase in pressure within the input pressure chamber  24  causing actuation of the pedal simulator. Note that upon movement of the input piston  14 , there is a simultaneous cut-off of fluid to the reservoir from all three pressure chambers  24 ,  26 , and  28  due to the respective passageways  38 ,  44 , and  50  slipping past the respective lip seals  32 ,  40 , and  46 . Under normal braking conditions, the pressure from the primary and secondary chambers  26  and  28  is prevented from any fluid flow to the wheel brakes, thereby hydraulically locking the primary and secondary chambers  26  and  28  and preventing further movement of the primary and secondary pistons  16  and  18 . Instead, another source of pressurized fluid is utilized to provide controlled fluid pressure to actuate the wheel brakes. However, under certain failed conditions of the brake system, the brake pedal unit  10  could be used to provide pressurized fluid to the wheel brakes by permitting fluid from the primary and secondary chambers  26  and  28  to be directed to the wheel brakes. 
     Although the design of the brake pedal unit  10  functions adequately, there is a relatively high initial force required by the driver to actuate the brake pedal unit. The forces from the spring arrangements and seal friction much first be overcome. This may impart an undesirable pedal feel characteristic for the driver. Although the brake pedal unit  10  may be suitable for a truck or larger vehicle, it may not be desirable for a smaller passenger vehicle. 
     SUMMARY OF THE INVENTION 
     This invention relates to a master cylinder or brake pedal unit which is connected to a brake pedal and is in selective fluid communication with a fluid reservoir. The pedal unit includes a housing defining a bore formed therein. An input piston is slidably disposed in the bore. The input piston is connected to the brake pedal such that engagement of the brake pedal causes movement of the input piston within the bore of the housing of the brake pedal unit. The brake pedal unit is defined to be in an at rest position when the brake pedal in not engaged causing movement of the input piston. A primary piston is slidably disposed in the bore for pressurizing a primary chamber. A primary passageway permits fluid communication between the primary chamber and the reservoir, wherein fluid flow through the primary passageway is blocked when the brake pedal unit is in the rest position. A secondary piston is slidably disposed in the bore for pressurizing a secondary chamber. A secondary passageway permits fluid communication between the secondary chamber and the reservoir, wherein fluid flow through the secondary passageway is blocked when the brake pedal unit is in the rest position. 
     Various aspects of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiment, when read in light of the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic illustration of a prior art brake pedal unit. 
         FIG. 2  is a schematic illustration of an embodiment of a brake system in accordance with the present invention. 
         FIG. 3  is a schematic cross-sectional illustration of the brake pedal unit of the brake system of  FIG. 2 . 
         FIG. 4  is a schematic cross-sectional illustration of the pedal simulator of the brake system of  FIG. 2 . 
         FIG. 5  is a schematic cross-sectional illustration of the plunger assembly of the brake system of  FIG. 2 . 
         FIG. 6  is a graphical representation of pedal force vs. pedal travel of the brake pedal units of  FIGS. 1 and 5  during operation thereof. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the drawings, there is schematically illustrated in  FIG. 2  an embodiment of a vehicle brake system, indicated generally at  100 . The brake system  100  is a hydraulic boost braking system in which boosted fluid pressure is utilized to apply braking forces for the brake system  100 . The brake system  100  may suitably be used on a ground vehicle such as an automotive vehicle having four wheels with a wheel brake associated with each wheel. Furthermore, the brake system  100  can be provided with other braking functions such as anti-lock braking (ABS) and other slip control features to effectively brake the vehicle, as will be discussed below. Components of the brake system  100  may be housed in one or more blocks or housings. The block or housing may be made from solid material, such as aluminum, that has been drilled, machined, or otherwise formed to house the various components. Fluid conduits may also be formed in the block or housing 
     In the illustrated embodiment of the brake system  100 , there are four wheel brakes  120   a ,  120   b ,  120   c , and  120   d . The wheel brakes  120   a ,  120   b ,  120   c , and  120   d  can have any suitable wheel brake structure operated by the application of pressurized brake fluid. The wheel brakes  120   a ,  120   b ,  120   c , and  120   d  may include, for example, a brake caliper mounted on the vehicle to engage a frictional element (such as a brake disc) that rotates with a vehicle wheel to effect braking of the associated vehicle wheel. The wheel brakes  120   a ,  120   b ,  120   c , and  120   d  can be associated with any combination of front and rear wheels of the vehicle in which the brake system  100  is installed. For example, the brake system  100  may be configured as a vertical split system such that a front pressure circuit is associated with providing fluid to the wheel brakes  120   a  and  120   b , and a rear pressure circuit is associated with providing fluid to the wheel brakes  120   c  and  120   d . In this example, the wheel brake  120   a  may be associated with a left front wheel of the vehicle in which the brake system  100  is installed, and the wheel brake  120   b  may be associated with the right front wheel. The wheel brake  120   c  may be associated with the left rear wheel, and the wheel brake  120   d  may be associated with the right rear wheel. Alternatively, the brake system  10  may be configured as a diagonally split brake system such that the wheel brakes  120   a  and  120   d  are associated with wheels at opposite corners of the vehicle, and the wheel brakes  120   b  and  12   c  are associated with the other opposite corners of the vehicle. 
     The brake system  100  generally includes a brake pedal unit, indicated generally at  130 , a pedal simulator, indicated generally at  132 , a plunger assembly, indicated generally at  134 , and a fluid reservoir  136 . The reservoir  136  stores and holds hydraulic fluid for the brake system  100 . The fluid within the reservoir  136  is preferably held at or about atmospheric pressure but may store the fluid at other pressures if so desired. The reservoir  136  is shown schematically having three tanks or sections with three fluid conduit lines connected thereto. The sections can be separated by several interior walls within the reservoir  136  and are provided to prevent complete drainage of the reservoir  136  in case one of the sections is depleted due to a leakage via one of the three lines connected to the reservoir  136 . Alternatively, the reservoir  136  may include multiple separate housings. The reservoir  136  may include a fluid level sensor  138  for detecting the fluid level of one or more of the sections of the reservoir  136 . 
     As will be discussed in more detail below, the plunger assembly  134  of the brake system  100  functions as a source of pressure to provide a desired pressure level to the wheel brakes  120   a ,  120   b ,  120   c , and  120   d  during a typical or normal brake apply. After a brake apply, fluid from the wheel brakes  120   a ,  120   b ,  120   c , and  120   d  may be returned to the plunger assembly  134  and/or diverted to the reservoir  136 . In a preferred embodiment, the plunger assembly  134  is a dual acting plunger assembly such that it is configured to also provide boosted pressure to the brake system  100  when a piston of the plunger assembly  134  is stroked rearwardly as well as forwardly, as will be described in more detail below. 
     The brake system  100  further includes an electronic control unit or ECU  140 . The ECU  140  may include microprocessors and other electrical circuitry. The ECU  140  receives various signals, processes signals, and controls the operation of various electrical components of the brake system  100  in response to the received signals. The ECU  140  can be connected to various sensors such as the reservoir fluid level sensor  138 , pressure sensors, travel sensors, switches, wheel speed sensors, and steering angle sensors. The ECU  140  may also be connected to an external module (not shown) for receiving information related to yaw rate, lateral acceleration, longitudinal acceleration of the vehicle such as for controlling the brake system  100  during vehicle stability operation. Additionally, the ECU  140  may be connected to the instrument cluster for collecting and supplying information related to warning indicators such as an ABS warning light, a brake fluid level warning light, and a traction control/vehicle stability control indicator light. 
     The brake system  100  further includes first and second isolation valves  150  and  152 . The isolation valves  150  and  152  may be solenoid actuated three way valves. The isolation valves  150  and  152  are generally operable to two positions, as schematically shown in  FIG. 2 . The first and second isolation valves  150  and  152  each have a port in selective fluid communication with an output conduit  154  generally in communication with the output of the plunger assembly  134 , as will be discussed below. The first and second isolation valves  150  and  152  also includes ports that are in fluid communication with conduits  156  and  158 , respectively, which are connected to the brake pedal unit  130  when the first and second isolation valves  150  and  152  are de-energized, as shown in  FIG. 2 . The first and second isolation valves  150  and  152  further include ports that are in fluid communication with conduits  160  and  162 , respectively, which provide fluid to and from the wheel brakes  120   a ,  120   b ,  120   c , and  120   d.    
     In a preferred embodiment, the first and/or second isolation valves  150  and  152  may be mechanically designed such that flow is permitted to flow in the reverse direction (from the output conduit  154  to the conduits  156  and  158 , respectively) when in their de-energized positions and can bypass the normally closed seat of the valves  150  and  152 . Thus, although the 3-way valves  150  and  152  are not shown schematically to indicate this fluid flow position, it is noted that that the valve design may permit such fluid flow. This may be helpful in performing self-diagnostic tests of the brake system  100 . 
     The system  100  further includes various solenoid actuated valves (slip control valve arrangement) for permitting controlled braking operations, such as ABS, traction control, vehicle stability control, dynamic rear proportioning, regenerative braking blending, and autonomous braking. A first set of valves includes a first apply valve  170  and a first dump valve  172  in fluid communication with the conduit  160  for cooperatively supplying fluid received from the first isolation valve  150  to the front wheel brake  120   a , and for cooperatively relieving pressurized fluid from the wheel brake  120   a  to a reservoir conduit  173  in fluid communication with the reservoir  136 . A second set of valves includes a second apply valve  174  and a second dump valve  176  in fluid communication with the conduit  160  for cooperatively supplying fluid received from the first isolation valve  150  to the wheel brake  120   b , and for cooperatively relieving pressurized fluid from the wheel brake  120   b  to the reservoir conduit  173 . A third set of valves includes a third apply valve  178  and a third dump valve  180  in fluid communication with the conduit  162  for cooperatively supplying fluid received from the second isolation valve  152  to the wheel brake  120   c , and for cooperatively relieving pressurized fluid from the wheel brake  120   c  to the reservoir conduit  173 . A fourth set of valves includes a fourth apply valve  182  and a fourth dump valve  184  in fluid communication with the conduit  162  for cooperatively supplying fluid received from the second isolation valve  152  to the wheel brake  120   d , and for cooperatively relieving pressurized fluid from the wheel brake  120   d  to the reservoir conduit  173 . Note that in a normal braking event, fluid flows through the de-energized open apply valves  170 ,  174 ,  178 , and  182 . Additionally, the dump valves  172 ,  176 ,  180 , and  184  are preferably in their de-energized closed positions to prevent the flow of fluid to the reservoir  136 . 
     The brake pedal unit  130  is connected to a brake pedal  190  and is actuated by the driver of the vehicle as the driver presses on the brake pedal  190 . A brake sensor or switch  192  may be connected to the ECU  140  to provide a signal indicating a depression of the brake pedal  190 . As will be discussed below, the brake pedal unit  130  may be used as a back-up source of pressurized fluid to essentially replace the normally supplied source of pressurized fluid from the plunger assembly  134  under certain failed conditions of the brake system  100 . This situation is referred to as a manual push-through event. The brake pedal unit  130  can supply pressurized fluid to the conduits  156  and  158  (that are normally closed off at the first and second isolation valves  150  and  152  during a normal brake apply) to the wheel brake  120   a ,  120   b ,  120   c , and  120   d  as required. 
     As shown schematically in  FIG. 3 , the brake pedal unit  130  includes a housing having a multi-stepped bore  200  formed therein for slidably receiving various cylindrical pistons and other components therein. Note that the housing is not specifically schematically shown in  FIG. 3  but instead the walls of the bore  200  are illustrated. The housing may be formed as a single unit or include two or more separately formed portions coupled together. An input piston  202 , a primary piston  204 , and a secondary piston  206  are slidably disposed within the bore  200 . The input piston  202  is connected with the brake pedal  190  via a linkage arm  208 . Leftward movement of the input piston  202 , the primary piston  204 , and the secondary piston  206  may cause, under certain conditions, a pressure increase within an input chamber  210 , a primary chamber  212 , and a secondary chamber  214 , respectively. Various seals of the brake pedal unit  130  as well as the structure of the housing and the pistons  202 ,  204 , and  206  define the chambers  210 ,  212 , and  214 , respectively. For example, the input chamber  210  is generally defined between the input piston  202  and the primary piston  204 . The primary chamber  212  is generally defined between the primary piston  204  and the secondary piston  206 . The secondary chamber  214  is generally defined between the secondary piston  206  and an end wall  216  of the housing formed by the bore  200 . The primary and secondary pistons  204  and  206  define a pair of output pistons for the brake pedal unit  130 . The primary and secondary chambers  212  and  214  define a pair of outputs of the brake pedal unit  130 . 
     The input chamber  210  is in fluid communication with the pedal simulator  132  via a conduit  218 , the reason for which will be explained below. The input piston  202  is slidably disposed in the bore  200  of the housing of the brake pedal unit  130 . An outer cylindrical wall  219  of the input piston  202  is engaged with a lip seal  220  and a seal  222  mounted in grooves formed in the housing. A fluid passageway  224  (or multiple passageways) is formed through a wall of the input piston  202 . As shown in  FIGS. 2 and 3 , when the brake pedal unit  130  is in its rest position (the driver is not depressing the brake pedal  190 ), the passageway  224  is located between the lip seal  220  and the seal  222 . In the rest position, the passageway  224  permits fluid communication between the input chamber  210  and the reservoir  136  via a conduit  226 . 
     Referring back to  FIG. 1 , the brake system  100  may further include an optional solenoid actuated simulator test valve  227  which may be electronically controlled between an open position, as shown in  FIG. 2 , and a powered closed position. The simulator test valve  227  is not necessarily needed during a normal brake apply or for a manual push through mode. The simulator test valve  227  can be actuated to a closed position during various testing modes to determine the correct operation of other components of the brake system  100 . For example, the simulator test valve  227  may be actuated to a closed position to prevent venting to the reservoir  136  via the conduit  226  such that a pressure build up in the brake pedal unit  130  can be used to monitor fluid flow to determine whether leaks may be occurring through seals of various components of the brake system  100 . 
     During initial operation of the brake pedal unit  130 , sufficient leftward movement of the input piston  202 , as viewing  FIG. 3 , will cause the passageway  224  to move past the lip seal  220 , thereby preventing the flow of fluid from the input chamber  210  into the conduit  226  and into the reservoir  136 . Further leftward movement of the input piston  202  will pressurize the input chamber  210  causing fluid to flow into the pedal simulator  132  via the conduit  218 . As fluid is diverted into the pedal simulator  132 , the pedal simulator  132  is actuated to provide a feedback force to the driver of the vehicle via the brake pedal  190  which simulates the forces a driver feels at the brake pedal  190  in a conventional vacuum assist hydraulic brake system, for example. 
     Referring now to  FIG. 4 , the embodiment of the pedal simulator  132  includes a housing defining a bore  230 . A cup-shaped piston  232  is slidably disposed in the bore  230 . The piston  232  is sealingly engaged with the wall of the bore  230  by a seal  234 . A pressure chamber  235  is defined by the bore  230  and the piston  232 . The pressure chamber  235  is in fluid communication with the input chamber  210  of the brake pedal unit  130  via the conduit  218 . The pedal simulator  132  may include a first spring  236  and a second spring  238  having a relatively low spring rate compared to the first spring  236 . In the example shown, the second spring  238  has a preload value of about  3  N. A tubular retainer  240  is disposed between the first and second springs  236  and  238  such that the first and second springs act against one another via the retainer  240 . The pedal simulator  132  may further include a spring washer assembly  242 . The spring washer assembly  242  may include one or more conical washer springs having a relatively high spring rate. Of course, the spring washer assembly  242  may include any suitable type of spring designs such as wave springs, Belleville washers, or elastomeric pad(s). In the embodiment shown, the spring washer assembly  242  includes a pair of conical washers  243  and  244 . After sufficient compression of the first spring  236  and the retainer  240  has moved a sufficient distance to close the gap, the right hand end of the retainer  240  will start compressing the conical spring washer assembly  242  along with the compression of the spring  236  and  238 . This arrangement assists in causing a non-linear progressive spring rate characteristic for obtaining a desirable force feedback to the driver. 
     As discussed above, the simulation pressure chamber  235  of the pedal simulator  132  is in fluid communication with the conduit  218  which is in fluid communication with the input chamber  210  of the brake pedal unit  130 . As shown in  FIG. 2 , a solenoid actuated simulator valve  246  is positioned within the conduit  218  to selectively prevent the flow of fluid from the input chamber  210  to the simulation pressure chamber  235 , such as during a failed condition in which the brake pedal unit  130  is utilized to provide a source of pressurized fluid to the wheel brakes  120   a ,  120   b ,  120   c , and  120   d.    
     The brake system  100  may further include a check valve  247  which is in a parallel path arrangement with a restricted orifice  248  in the conduit  118 . The check valve  247  and the restricted orifice  248  could be integrally built or formed in the simulator valve  246  or may be formed separately therefrom. The restricted orifice  248  provides damping during a spike apply in which the driver depresses the brake pedal  190  rapidly and forcefully. This damping provides a force feedback making depression of the brake pedal  190  feel more like a traditional vacuum booster, which may be a desirable characteristic of the brake system  100 . The damping may also provide a more accurate relationship between brake pedal travel and vehicle deceleration by generally avoiding too much brake pedal travel for the vehicle deceleration that can be delivered by the brake system  100 . The check valve  247  provides an easy flow path and allows the brake pedal  190  to return quickly, which allows the associated brake pressure to decrease quickly per the driver&#39;s intent. 
     The primary chamber  212  of the brake pedal unit  130  is in fluid communication with the second isolation valve  152  via the conduit  158 . The primary piston  204  is slidably disposed in the bore  200  of the housing of the brake pedal unit  130 . An outer wall  249  of the primary piston  104  is engaged with a lip seal  250  (primary seal) and a seal  252  mounted in grooves formed in the housing. One or more passageway(s)  254  are formed through a wall of the primary piston  204 . Unlike the arrangement of the input piston  202 , when the brake pedal unit  130  is in its rest position (driver is not pressing on the brake pedal  190 ), the passageway  254  is just to the left of the lip seal  250 . As shown in  FIG. 3 , the passageway  254  is spaced from the lip seal  250  by about a relatively small distance Dp. This position prevents the fluid communication between primary chamber  212  and the reservoir  136  via the conduit  265 . Note with respect to the input piston  202 , the passageway  224  is spaced from the lip seal  220  by a slightly larger distance D I . It is noted that the schematic illustrations of  FIG. 3  are not to scale and the dimensions are shown for ease of explanation. 
     The secondary chamber  214  of the brake pedal unit  130  is in fluid communication with the first isolation valve  150  via the conduit  156 . Referring now to  FIG. 3 , the secondary piston  206  is slidably disposed in the bore  200  of the housing of the brake pedal unit  130 . An outer wall  259  of the secondary piston  206  is engaged with a lip seal  260  (secondary seal) and a seal  262  mounted in grooves formed in the housing. One or more passageway(s)  264  are formed through a wall of the secondary piston  206 . Similar to the arrangement of the primary piston  204 , when the brake pedal unit  130  is in its rest position (driver is not pressing on the brake pedal  190 ), the passageway  264  is just to the left of the lip seal  260 . As shown in  FIG. 3 , the passageway  264  is spaced from the lip seal  260  by about a relatively small width D S . This position prevents the fluid communication between secondary chamber  214  and the reservoir  136  via the conduit  266 . 
     The lip seals  220 ,  250  and  260  may have any suitable seal structure. The lip seals  250  and  260  may be designed such that fluid may flow in the direction from the reservoir  136  into the primary and secondary chambers  212  and  214  via the conduits  265  and  266 , respectively, when the pressure within the chambers  212  and  214  falls below atmospheric pressure (the pressure within the reservoir  136 ). This may be true even if the brake pedal unit  130  is in its rest position. This may be caused by minor leakage or fluid volume changes in the wheel brakes. 
     In an alternate embodiment, lip seals similar to the lip seals  220 ,  250  and/or  260  may be mounted on the pistons  202 ,  204 , and/or  206 , respectively. Passageways similar to the passageways  224 ,  254 , and  264  would then be formed in the housing of the brake pedal unit and in fluid communication with the reservoir  136 . 
     If desired, the primary and secondary pistons  204  and  206  may be mechanically connected but with limited movement therebetween. The mechanical connection of the primary and secondary pistons  204  and  206  prevents a large gap or distance between the primary and secondary pistons  204  and  206 . This helps prevent lost pedal travel by preventing having to advance the primary and secondary pistons  204  and  206  over a relatively large distance without any increase in pressure during a failed system event. For example, if the brake system  100  is under a manual push-through mode and fluid pressure is lost in the front circuit relative to the secondary piston  206  (secondary chamber  214 ), such as for example in the conduit  156 , the secondary piston  206  will be forced or biased in the leftward direction due to the greater pressure within the primary chamber  212 . If the primary and secondary pistons  204  and  206  were not connected together, the secondary piston  206  would freely travel to its further most left-hand position, as viewing  FIG. 3 , and the driver would have to depress the pedal  190  a distance to compensate for this loss in travel. However, because the primary and secondary pistons  204  and  206  are connected together, the secondary piston  206  is prevented from this movement and relatively little loss of travel occurs in this type of failure. 
     Any suitable mechanical connection between the primary and secondary pistons  204  and  206  may be used. For example, as schematically shown in  FIG. 3 , the right-hand end of the secondary piston  206  includes an outwardly extending flange  270  that extends into a groove  272  formed in an inner wall  274  of the primary piston  204 . The groove  272  has a width which is greater than the width of the flange  270 , thereby providing a relatively small amount of travel between the primary and secondary pistons  204  and  206  relative to one another. 
     The brake pedal unit  130  further includes a return spring  286  biasing the input piston  202  in the rightward direction as viewing  FIG. 3 . An input spring  288  is disposed about an axial stem  290  formed in the input piston  202  and engages with a washer  292  which is in direct contact with a shoulder  294  formed in the right-hand end of the primary piston  212 . The axial stem  290  extends into a bore  296  formed in the right-hand end of the primary piston  212 . An elastomeric pad  298  is disposed in the bore  296  and will engage with an enlarged head  300  formed at the end of the axial stem  290  when the input piston  210  is moved a sufficient distance towards the primary piston  212 . Compression of the elastomeric pad  298  by the head  300  of the stem  290  provides for a desired spring rate characteristic. The enlarged head  300  is spaced from the washer  292  and the shoulder  294  by a gap  302 . Note that the washer  292  actually engages with and touches the shoulder  294 , and not spaced therefrom as shown in the schematic illustration of  FIG. 3 . The very slight gap schematically shown between the washer  292  and the shoulder  294  is shown for clarity purposes such that the lines are not drawn on top of one another. This schematic illustration is true of other components of the brake pedal unit  130  that are actually in contact with one another yet shown just slightly out of contact with one another. 
     The brake pedal unit  2702  further includes a primary spring  304  generally disposed between the secondary piston  206  and the primary piston  204 . The primary spring  304  is disposed within the bore  274  and engages with a retainer  306  forming a caged spring assembly configuration with an axial stem  308  extending from bottom of the bore  274  of the primary piston  204 . The retainer  306  is restrained by an enlarged head  310  formed on the end of the axial stem  308 . 
     The brake pedal unit  2702  further includes a secondary spring  312  generally disposed between the secondary piston  206  and the bottom wall  216  of the bore  200 . The secondary spring  312  is disposed within a bore  314  formed in the left-hand end of the secondary piston  206  and engages with a retainer  316  forming a caged spring assembly configuration with an axial stem  318  extending from the bottom of the bore  314  of the secondary piston  206 . The retainer  316  is restrained by an enlarged head  320  formed on the end of the axial stem  318 . Note that at the rest position, the enlarged head  320  contacts the end of the retainer  316  such that there is essentially no gap therebetween. 
     As shown in  FIG. 2 , the brake system  100  may further include a pressure sensor  330  in fluid communication with the conduit  156  to detect the pressure within the secondary pressure chamber  214  of the brake pedal unit  130  and for transmitting the signal indicative of the pressure to the ECU  140 . The brake system  100  further includes a pressure sensor  332  in fluid communication with the output conduit  154  for transmitting a signal indicative of the pressure at the output of the plunger assembly  134 . The ECU  140  utilizes the signals from the pressure sensors  330  and  332  to actuate the brake system  100  under various braking events. 
     In a preferred embodiment of the brake system  100 , the brake pedal unit  130  includes a pair of travel sensors  340  (one redundant) for producing signals that are indicative of the length of travel and/or rate of travel of the input piston  202  and providing the signals to the ECU  140 . 
     Referring now to  FIG. 5 , there is schematically illustrated an enlarged view of the plunger assembly  134 . The plunger assembly  134  includes a housing having a multi-stepped bore  400  formed therein. Note that the housing is not specifically schematically shown in  FIG. 5  but instead the walls of the bore  400  are illustrated. The bore  400  includes a first portion  402  and a second portion  404 . A piston  406  is slidably disposed within the bore  400 . The piston  406  includes an enlarged end portion  408  connected to a smaller diameter central portion  410 . The piston  406  has a second end  411  connected to a ball screw mechanism, indicated generally at  412 . The ball screw mechanism  412  is provided to impart translational or linear motion of the piston  406  along an axis defined by the bore  400  in both a forward direction (leftward as viewing  FIG. 5 ), and a rearward direction (rightward as viewing  FIG. 5 ) within the bore  400  of the housing. In the embodiment shown, the ball screw mechanism  412  includes an electric motor, indicated schematically and generally at  414 , which is electrically connected to the ECU  140  for actuation thereof. The motor  414  rotatably drives a screw shaft  416 . The motor  414  generally includes a stator  415  and a rotor  417 . In the schematic embodiment shown in  FIG. 5 , the rotor  417  and the shaft  416  are integrally formed together. The second end  411  of the piston  406  includes a threaded bore  420  and functions as a driven nut of the ball screw mechanism  412 . The ball screw mechanism  412  includes a plurality of balls  422  that are retained within helical raceways  423  formed in the screw shaft  416  and the threaded bore  420  of the piston  406  to reduce friction. 
     Although a ball screw mechanism  412  is shown and described with respect to the plunger assembly  134 , it should be understood that other suitable mechanical linear actuators may be used for imparting movement of the piston  406 . It should also be understood that although the piston  406  functions as the nut of the ball screw mechanism  412 , the piston  406  could be configured to function as a screw shaft of the ball screw mechanism  412 . Of course, under this circumstance, the screw shaft  416  would be configured to function as a nut having internal helical raceways formed therein. 
     The piston  406  may include structures engaged with cooperating structures formed in the housing of the plunger assembly  134  to prevent rotation of the piston  406  as the screw shaft  416  rotates around the piston  406 . For example, the piston  206  may include outwardly extending splines or tabs  426  (See  FIG. 5 ) that are disposed within longitudinally extending grooves  428  formed in the housing of the plunger assembly  134  such that the tabs  426  slide along within the grooves  428  as the piston  406  travels in the bore  400 . 
     As will be discussed below, the plunger assembly  134  is preferably configured to provide pressure to the output conduit  154  when the piston  406  is moved in either the forward or rearward direction. The plunger assembly  134  includes a seal  430  mounted on the enlarged end portion  408  of the piston  406 . The seal  430  slidably engages with the inner cylindrical surface of the first portion  402  of the bore  400  as the piston  406  moves within the bore  400 . A seal  434  and a seal  436  are mounted in grooves formed in the second portion  404  of the bore  400 . The seals  434  and  436  slidably engage with the outer cylindrical surface of the central portion  410  of the piston  406 . A first pressure chamber  440  is generally defined by the first portion  402  of the bore  400 , the enlarged end portion  408  of the piston  406 , and the seal  430 . An annular shaped second pressure chamber  442 , located generally behind the enlarged end portion  408  of the piston  406 , is generally defined by the first and second portions  402  and  404  of the bore  400 , the seals  430  and  434 , and the central portion  410  of the piston  406 . The seals  430 ,  434 , and  436  may have any suitable seal structure. 
     Although the plunger assembly  134  may be configured to any suitable size and arrangement, in one embodiment, the effective hydraulic area of the first pressure chamber  440  is greater than the effective hydraulic area of the annular shaped second pressure chamber  442 . The first pressure chamber  440  generally has an effective hydraulic area corresponding to the diameter of the central portion  410  of the piston  406  (the inner diameter of the seal  434 ) since fluid is diverted through the output conduit  154  and conduits  443  and  454  as the piston  406  is advanced in the forward direction. The second pressure chamber  442  generally has an effective hydraulic area corresponding to the diameter of the first portion  402  of the bore  400  minus the diameter of the central portion  410  of the piston  406 . In general, if the annular area is less than the diameter of the central portion  410 , this configuration provides that on the back stroke in which the piston  406  is moving rearwardly, less torque (or power) is required by the motor  414  to maintain the same pressure as in its forward stroke. Besides using less power, the motor  414  may also generate less heat during the rearward stroke of piston  406 . Under circumstances when high brake pressure is desired, the plunger assembly  134  could be operated from a forward stroke to a rearward stroke. So while a forward stroke is used in most brake applications, a rearward pressure stroke can be utilized. Also, under circumstances in which the driver presses on the pedal  190  for long durations, the brake system  10  could be operated to maintain brake pressure (instead of continuously energizing the plunger assembly  134 ) by controlling the first and second plunger valves  450  and  452  (as will be discussed below) to closed positions and then turn off the motor or the plunger assembly  134 . 
     The plunger assembly  134  preferably includes a sensor, schematically shown as  418 , for indirectly detecting the position of the piston  406  within the bore  400 . The sensor  418  is in communication with the ECU  140 . In one embodiment, the sensor  418  detects the rotational position of the rotor  417  which may have metallic or magnetic elements embedded therein. Since the rotor  417  is integrally formed with the shaft  416 , the rotational position of the shaft  416  corresponds to the linear position of the piston  406 . Thus, the position of the piston  406  can be determined by sensing the rotational position of the rotor  417  via the sensor  418 . 
     The piston  406  of the plunger assembly  134  includes a passageway  444  formed therein. The passageway  444  defines a first port  446  extending through the outer cylindrical wall of the piston  406  and is in fluid communication with the secondary chamber  442 . The passageway  444  also defines a second port  448  extending through the outer cylindrical wall of the piston  406  and is in fluid communication with a portion of the bore  400  located between the seals  434  and  436 . The second port  448  is in fluid communication with a conduit  449  which is in fluid communication with the reservoir  136 . When in the rest position, as shown in  FIGS. 2 and 5 , the pressure chambers  440  and  442  are in fluid communication with the reservoir  136  via the conduits  449 ,  454  and  443 . This helps in ensuring a proper release of pressure at the output of the plunger assembly  34  and within the pressure chambers  440  and  442  themselves. After an initial forward movement of the piston  406  from its rest position, the port  448  will move past the lip seal  434 , thereby closing off fluid communication of the pressure chambers  440  and  442  from the reservoir  136 , thereby permitting the pressure chambers  440  and  442  to build up pressure as the piston  406  moves further. 
     Referring back to  FIG. 2 , the brake system  100  further includes a first plunger valve  450 , and a second plunger valve  452 . The first plunger valve  450  is preferably a solenoid actuated normally closed valve. Thus, in the non-energized state, the first plunger valve  450  is in a closed position, as shown in  FIG. 2 . The second plunger valve  452  is preferably a solenoid actuated normally open valve. Thus, in the non-energized state, the second plunger valve  452  is in an open position, as shown in  FIG. 2 . A check valve may be arranged within the second plunger valve  452  so that when the second plunger valve  452  is in its closed position, fluid may still flow through the second plunger valve  452  in the direction from the first output conduit  454  (from the first pressure chamber  440  of the plunger assembly  134 ) to the output conduit  154  leading to the isolation valves  150  and  152 . Note that during a rearward stroke of the piston  406  of the plunger assembly  134 , pressure may be generated in the second pressure chamber  442  for output into the output conduit  154 . The brake system  100  further includes a check valve  451  permitting fluid to flow in the direction from the conduit  449  (from the reservoir  136 ) to the conduit  454  and into the first pressure chamber  440  of the plunger assembly  134  such as during a pressure generating rearward stroke of the piston  406 . 
     Generally, the first and second plunger valves  450  and  452  are controlled to permit fluid flow at the outputs of the plunger assembly  134  and to permit venting to the reservoir  136  through the plunger assembly  134  when so desired. For example, the first plunger valve  450  is preferably energized to its open position during a normal braking event. Additionally, it is preferred that both the first and second plunger valves  450  and  452  remain open (which may reduce noise during operation). Preferably, the first plunger valve  450  is almost always energized during an ignition cycle when the engine is running Of course, the first and second plunger valves  450  and  452  may be purposely operated to their closed positions such as during a pressure generating rearward stroke of the plunger assembly  134  or during a hill hold brake operation. The first and second plunger valves  450  and  452  are preferably in their open positions when the piston  406  of the plunger assembly  134  is operated in its forward stroke to maximize flow. When the driver releases the brake pedal  190 , the first and second plunger valves  450  and  452  preferably remain in their open positions. However, under certain circumstances, such as during slip control and the driver is pushing hard on the brake pedal  190  during controlled low pressures and then the driver releases half way on the brake pedal  190 , it may be desirable to operate the first and second plunger valves  450  and  452  to their closed positions. Note that fluid can flow through the check valve within the closed second plunger valve  452 , as well as through the check valve  451  from the reservoir  136  depending on the travel direction of the piston  406  of the plunger assembly  134  and the state of the first and second plunger valves  450  and  452 . 
     It may be desirable to configure the first plunger valve  450  with a relatively large orifice therethrough when in its open position. A relatively large orifice of the first plunger valve  450  helps to provide an easy flow path therethrough. The second plunger valve  452  may be provided with a much smaller orifice in its open position as compared to the first plunger valve  450 . One reason for this is to help prevent the piston  406  of the plunger assembly  134  from rapidly being back driven upon a failed event due to the rushing of fluid through the first output conduit  454  into the first pressure chamber  440  of the plunger assembly  134 , thereby preventing damage to the plunger assembly  134 . As fluid is restricted in its flow through the relatively small orifice, dissipation will occur as some of the energy is transferred into heat. Thus, the orifice should be of a sufficiently small size so as to help prevent a sudden catastrophic back drive of the piston  406  of the plunger assembly  134  upon failure of the brake system  100 , such as for example, when power is interrupted or lost to the motor  414  and the pressure within the output conduit  154  is relatively high. The plunger assembly  134  may include an optional spring member, such as a spring washer  419 , to assist in cushioning such a rapid rearward back drive of the piston  406 . The spring washer  419  may also assist in cushioning the piston  406  moving at any such speed as it approaches a rest position near its most retracted position within the bore  400 . It is noted that although the isolation valves  150  and  152  could shuttle to their positions shown in  FIG. 2  during a power failure, the presence of the spring washer  419  enables the isolation valves  150  and  152  to be made cheaply with a smaller solenoid wherein they might hydraulically lock and not shuttle, thereby allowing this rapid rearward back drive of the piston  406 . The spring washer  419  can also function as a parking element such that the piston  406  can lightly hit the spring washer  419  on a return stroke to determine its homing, start or at rest position. When it is detected that the piston  406  has stopped moving by hitting the spring washer  419 , the homing position can be determined. 
     The first and second plunger valves  450  and  452  provide for an open parallel path between the pressure chambers  440  and  442  of the plunger assembly  134  during a normal braking operation (with the first plunger valve  450  energized). Although a single open path may be sufficient, the advantage of having both the first and second plunger valves  450  and  452  is that the first plunger valve  450  may provide for an easy flow path through the relatively large orifice thereof, while the second plunger valve  452  may provide for a restricted orifice path during certain failed conditions (when the first plunger valve  450  is de-energized to its closed position). It is noted that a single normally open valve with a relatively large orifice could be sufficient instead of the two plunger valves  450  and  452 , however, the single valve may need a relatively large solenoid and during power losses the single valve could close causing possible locking of the isolation valves  150  and  152 . 
     The operation of the brake system  100  will now be described. It is noted that the terms “normal braking” or “normal brake apply” generally refers to a braking event in which all of the components of the brake system  100  are functioning normally. Additionally, under a normal braking event, the brake system  100  is not experiencing any detrimental leakage that could hinder proper operation of the brake system  100 .  FIGS. 2 and 3  illustrates the brake system  100  and the brake pedal unit  130  in their rest positions. In this condition, the driver is not depressing the brake pedal  190 . In a non-autonomous braking event, the brake pedal  190  is depressed by the driver of the vehicle indicating their intent in actuating the brake system  100  to decelerate the vehicle. The ECU  140  detects this braking event by signals from the travel sensors  340  and also by the pressure sensor  330 . 
     During a normal brake apply braking operation, the flow of pressurized fluid from the brake pedal unit  130  generated by depression of the brake pedal  190  is diverted into the pedal simulator  132 . The simulation valve  246  is actuated or energized to divert fluid through the simulation valve  246  from the input chamber  210  of the brake pedal unit  130  as the input piston  202  is moved via the brake peda 1190 . Note that fluid flow from the input chamber  210  to the reservoir  136  is closed off once the passageway  224  in the input piston  202  moves past the lip seal  220 . As the input piston  202  generates fluid pressure within the input chamber  210 , the pressurized fluid is diverted into the pressure chamber  235  of the pedal simulator  132 . The build-up of pressure within the pressure chamber  235  of the pedal simulator  132  moves the piston  232  against the bias of the springs  236  and  238 . Compression of the springs  236  and  238  provides a force feedback to the driver of vehicle as the driver feels the resistance on the driver&#39;s foot via the brake pedal  190 . 
     During the duration of the normal braking apply, the simulation valve  246  remains open, preferably, in its energized state. Preferably, the simulation valve  246  is energized throughout the duration of an ignition cycle. Also during the normal boost apply braking operation, the isolation valves  150  and  152  are energized to secondary positions to prevent the flow of fluid from the conduits  156  and  158  through the isolation valves  150  and  152 , respectively. In one embodiment, the isolation valves  150  and  152  are energized throughout the duration of an ignition cycle such as when the engine is running instead of being energized on and off. This constant energizing helps to minimize noise. 
     Note that the primary and secondary pistons  204  and  206  are not in fluid communication with the reservoir  136  due to their passageways  254  and  264 , respectively, being positioned past the lip seals  250  and  260  (unlike the prior art brake pedal unit  10  shown in  FIG. 1 ). Prevention of fluid flow through the isolation valves  152  and  150  hydraulically locks the primary and secondary chambers  212  and  214  preventing further movement of the primary and secondary pistons  204  and  206 . Thus, required further initial movement of the brake pedal  190  is not necessary to close of these chambers, as is required in the prior art brake pedal  10 . 
     It is generally desirable to maintain the isolation valves  150  and  152  energized during the normal braking mode to ensure any necessary venting of fluid to the reservoir  136  through the plunger assembly  134 . As shown in  FIG. 5 , the piston  406  of the plunger assembly  134  includes the passageway  444  formed therein to permit this ventilation. However, during a failed condition in which the isolation valves  150  and  152  are not able to be energized (such as an electrical failure or failure of the ECU  140 ), fluid from the wheel brakes  120   a ,  120   b ,  120   c , and  120   d  can still be vented via the conduits  156  and  158  and through the brake pedal unit  134  if the pressure within the secondary and primary chambers  214  and  212  exceeds a predetermined pressure level. In one embodiment, the predetermined pressure level is about  0 . 65  bars, which is sufficient to move the primary and secondary pistons  204  and  206  in the right-hand direction such that the passageways  254  and  264  are to the right of the lip seals  250  and  260 , respectively. In this position, fluid can flow from the primary and secondary chambers  212  and  214  into the reservoir  136  via the conduits  265  and  266 , respectively. The requirement of exceeding the predetermined pressure level, e.g. 0.65 bar, is generally only required during certain failed conditions. During a failed condition wherein the driver is not applying pressure to the brake pedal  190 , this slight pressure (about 0.65 bar for example) may cause a slight braking at one or more of the wheel brakes  120   a ,  120   b ,  120   c , and  120   d  until a volume increase in the brake fluid due to heating thereof may offset the pressure increase, thereby moving the primary and secondary pistons  204  and  206  to a position to permit venting through the brake pedal unit  130 . It has been found that this minor brake application and heating will not cause undue brake fade problems at the wheel brakes. 
     During a normal brake apply while the pedal simulator  132  is being actuated by depression of the brake pedal  190 , the ECU  140  operates the brake system  100  to provide actuation of the wheel brakes  120   a ,  120   b ,  120   c , and  120   d . The ECU  140  actuates and regulates the plunger assembly  134  to provide pressure for the wheel brakes  120   a ,  120   b ,  120   c , and  120   d  based on the information from the travel sensors  340  which corresponds to the driver&#39;s intent. The plunger assembly  134  is operated to provide desired pressure levels to the wheel brakes  120   a ,  120   b ,  120   c , and  120   d  in relation to the driver&#39;s intent. The ECU  140  may also use information from the pressure sensor  332  generally located at the output of the plunger assembly  134  to regulate the motor  414  of the plunger assembly  134  to obtain a desired pressure level within the output conduit  54 . 
     To operate the plunger assembly  134 , the ECU  140  actuates the motor  414  to rotate the screw shaft  416  in a first rotational direction. Rotation of the screw shaft  416  in the first rotational direction causes the piston  406  to advance in the forward direction (leftward as viewing  FIGS. 2 and 5 ). Movement of the piston  406  causes a pressure increase in the first pressure chamber  440  and fluid to flow out of the first pressure chamber  440  and into the conduit  454 . Fluid can flow into the output conduit  154  via the open first and second plunger valves  450  and  452 . Note that fluid is permitted to flow into the second pressure chamber  442  via the conduit  443  as the piston  406  advances in the forward direction. Pressurized fluid from the output conduit  154  is directed into the conduits  160  and  162  through the isolation valves  150  and  152 , respectively. The pressurized fluid from the conduits  160  and  162  can be directed to the wheel brakes  120   a ,  120   b ,  120   c , and  120   d  through open apply valves  170 ,  174 ,  178 , and  182  while the dump valves  172 ,  176 ,  180 , and  184  remain closed. It is noted that the first plunger valve  450  is energized to its open position during a normal braking event so that both of the first and second plunger valves  450  and  452  are open. The plunger valve  450  may be energized throughout the duration of an ignition cycle. 
     When the driver lifts off or releases the brake pedal  190 , the ECU  140  can operate the motor  414  to rotate the screw shaft  416  in the second rotational direction causing the piston  406  to retract withdrawing the fluid from the wheel brakes  120   a ,  120   b ,  120   c , and  120   d . The speed and distance of the retraction of the piston  406  is based on the demands of the driver releasing the brake pedal  190  with the cooperation from the sensor  418 . Of course, if the driver rapidly releases the brake pedal  190 , the plunger assembly  134  may be operated to avoid such an instant drop in pressure. Under certain conditions, such as in a non-boosted slip control event, the pressurized fluid from the wheel brakes  120   a ,  120   b ,  120   c , and  120   d  may assist in back-driving the ball screw mechanism  412  moving the piston  406  back towards its rest position. Note that when the driver releases the brake pedal  190 , the first and second plunger valves  450  and  452  preferably remain in their open positions during a non-slip control event. 
     During a braking event, the ECU  140  can also selectively actuate the apply valves  170 ,  174 ,  178 , and  182  and the dump valves  172 ,  176 ,  180 , and  184 , respectively, to provide a desired pressure level to the wheel brakes  120   a ,  120   b ,  120   c , and  120   d . This selective actuation of the wheel brakes  120   a ,  120   b ,  120   c , and  120   d  may even be in conflict of the driver&#39;s intent as sensed by the travel sensors  340 . For example, the ECU  140  can control the brake system  100  during ABS, DRP, TC, VSC, regenerative braking, and/or autonomous braking events by general operation of the plunger assembly  134  in conjunction with selective actuation the apply valves  170 ,  174 ,  178 , and  182  and the dump valves  172 ,  176 ,  180 , and  184 . Under certain driving conditions, the ECU  140  communicates with a powertrain control module (not shown) and other additional braking controllers of the vehicle to provide coordinated braking during these advanced braking control schemes (e.g., anti-lock braking (AB), traction control (TC), vehicle stability control (VSC), and regenerative brake blending, amongst others). 
     In some situations such as a relatively large brake pressure demand or extended slip control event, the piston  406  of the plunger assembly  134  may reach its full stroke length within the bore  400  of the housing and additional boosted pressure is still desired to be delivered to the wheel brakes  120   a ,  120   b ,  120   c , and  120   d . The plunger assembly  134  is a dual acting plunger assembly such that it is configured to also provide boosted pressure to the output conduit  154  when the piston  406  is stroked rearwardly (rightward) or in a reverse direction. This has the advantage over a conventional plunger assembly that first requires its piston to be brought back to its rest or retracted position before it can again advance the piston to create pressure within a single pressure chamber. If the piston  406  has reached its full stroke, for example, and additional boosted pressure is still desired, the second plunger valve  452  is energized to its closed check valve position. The first plunger valve  450  is de-energized to its closed position. The ECU  140  actuates the motor  414  in a second rotational direction opposite the first rotational direction to rotate the screw shaft  416  in the second rotational direction. Rotation of the screw shaft  416  in the second rotational direction causes the piston  406  to retract or move in the rearward direction (rightward as viewing  FIGS. 2 and 5 ). Movement of the piston  406  causes a pressure increase in the second pressure chamber  442  and fluid to flow out of the second pressure chamber  442  and into the conduit  443  and the output conduit  154 . Pressurized fluid from the output conduit  154  is directed into the conduits  160  and  162  through the isolation valves  150  and  152 . The pressurized fluid from the conduits  160  and  162  can be directed to the wheel brakes  120   a ,  120   b ,  120   c , and  120   d  through the opened apply valves  170 ,  174 ,  178 , and  182  while dump valves  172 ,  176 ,  180 , andl  84  remain closed. 
     In a similar manner as during a forward stroke of the piston  406 , the ECU  140  can also selectively actuate the apply valves  170 ,  174 ,  178 , and  182  and the dump valves  172 ,  176 ,  180 , and  184  to provide a desired pressure level to the wheel brakes  120   a ,  120   b , 1   20   c , and  120   d , respectively. When the driver completely lifts off or releases the brake pedal  190  during a pressurized rearward stroke of the plunger assembly  134 , the first and second plunger valves  450  and  452  are preferably operated to their open positions, as discussed above, although having only one of the valves  450  and  452  open would generally still be sufficient. Note that when transitioning out of a slip control event, the ideal situation would be to have the position of the piston  406  and the displaced volume within the plunger assembly  134  correlate just about exactly with the given pressures and fluid volumes within the wheel brakes  120   a ,  120   b ,  120   c , and  120   d . However, when the correlation is not exact, fluid can be drawn from the reservoir  136  via the check valve  451  into the chamber  440  of the plunger assembly  134 . To vent the plunger assembly  134 , fluid may flow through the port  448  to the reservoir  136 . 
     In the event of a loss of electrical power to portions of the brake system  100  provides for a manual push-through or manual apply such that the brake pedal unit  130 , which is operated by the driver via the brake pedal  190 , can supply relatively high pressure fluid to the primary output conduit  158  and the secondary output conduit  156 . Thus, the terms “manual push-through” or “a manual push-through event or mode” refers to the situation in which the brake pedal  190  is being depressed for actuation of the brake pedal unit  130 , and at least a portion of the brake system  100  is not operating properly or a detrimental leak has occurred within the brake system  100 . Note that during a manual push-through event, the pedal simulator  132  is not being utilized as normally intended. 
     During an electrical failure, the motor  2414  of the plunger assembly  134  might cease to operate, thereby failing to produce pressurized hydraulic brake fluid from the plunger assembly  134  to the output conduit  154 . During the electrical failure, the ECU  140  may also be inoperable and may be unable to actuate the plunger assembly  134  or the solenoid valves of the brake system  100 . In this situation, the isolation valves  150  and  152  will shuttle (or remain) in their de-energized positions to permit fluid flow from the conduits  156  and  158  to the wheel brakes  120   a ,  120   b ,  120   c , and  120   d . The pedal simulator valve  246  will shuttle (or remain) to its de-energized closed position to prevent fluid from flowing out of the input chamber  210  to the pedal simulator  132 . The closing of the conduit  218  to the pedal simulator  132  will assist in causing the input chamber  210  to be hydraulically locked. During the manual push-through apply, when the driver continues to push on the brake pedal  190 , the input piston  202 , the primary piston  204 , and the secondary piston  206  will advance leftwardly, as viewing  FIGS. 2 and 3 . This advancement will cause the passageway  224  to move past the seal  220  to prevent fluid flow from the fluid chambers  210  to the reservoir  136 . Fluid flows from the primary and the secondary chambers  212  and  214  into the conduits  158  and  156 , respectively, to actuate the wheel brakes  120   a ,  120   b ,  120   c , and  120   d.    
     Referring now back to  FIG. 3 , the brake pedal unit  130  includes various features which have advantages over the prior art brake pedal unit  10  of  FIG. 1 . One advantage is the lower force requirement to actuate the brake pedal unit  130  as compared to actuation of the brake pedal unit  10 . The brake pedal unit  10  includes springs therein having a higher preload force values compared to the brake pedal unit  130 . The higher spring preload values are necessary for proper operation of the brake pedal unit  10 . It is known that the return spring  58  of the brake pedal unit  10  has a preload force value of about 40 N (newton). The input, primary, and secondary spring assemblies  52 ,  54 , and  56  have preload force values of 100 N, 75 N, and 50 N, respectively. However, for a preferred embodiment of the brake pedal unit  130 , the return spring  286  has a preload force value of about 45 N, and the input, primary, and secondary springs  288 ,  304 , and  312  have a preload force value of about 5 N, 80 N, and 55 N, respectively. The preload values generally indicate the required force acting on the spring to initiate the start of deflection of the spring. It should be understood that these values are indicative of just some examples of desired embodiments of the brake pedal unit  130  and are not to be held to limit the invention as disclosed herein. It is also noted that generally each of the seals of the brake pedal units  10  and  130  provide about 7 N of nominal frictional forces that are required to overcome. 
     Since the primary and secondary pistons  204  and  206  of the brake pedal unit  130  do not need to be moved to a vented position with respect to a cut-off with the reservoir  136 , as compared to the brake pedal unit  10 , this design enables a lowering in force required by the driver acting on the brake pedal  190  to initiate movement. 
     Due to stack up tolerances in manufacturing and assembly of the various components of the brake pedal units  10  and  130 , it is desirable to design gaps between some components while providing contact or no gaps between other components for proper alignment of the pistons within the bore of the brake pedal units  10  and  130 . Providing design aspects for gaps and contacts may also help in determining the preload spring forces within the brake pedal units  10  and  130 . For example, eliminating a gap  90  (shown in the brake pedal unit  10  in  FIG. 1 ) from the design of the brake pedal unit  130  assists in assuring alignment of the passageways  254  and  264  of the primary and secondary pistons  204  and  206 . Since a gap  90  is eliminated in the brake pedal unit  130 , it may be desirable to include the gap  302  (described above) to compensate for any manufacturing tolerances when assembling the components and springs of the brake pedal unit  130  within its housing. In designing the brake pedal unit  130 , the desired spring forces and seal frictions are typically first determined, and then the location and spacing of desired gaps may be appropriately calculated. Since the gap  302  is associated with the spring  288  having the lowest preload force (5 N), compression of this spring  288  during initial actuation of the brake pedal unit  130  is advantageously relatively low. 
     All three pistons (the input piston  14 , the primary piston  16 , and the secondary piston  18 ) of the brake pedal unit  10  are generally simultaneously cut off from the reservoir as all three pistons move leftward as viewing  FIG. 1 . Contrary, only the input piston  202  of the brake pedal unit  130  requires initial movement for cut off from the reservoir  136  o occur. The elimination of movement of the primary and secondary pistons  204  and  206  of the brake pedal unit  130  reduces the force required by the driver during initial operation of the brake pedal unit  130  as compared to the brake pedal unit  10 . 
     Another difference between the brake pedal units  130  and  10  is that the brake pedal unit  10  includes a redundant seal  36  adjacent a seal  34 . The redundant seal  36  helps to prevent minor fluid leakage occurring out of the brake pedal unit  10  adjacent the input piston  14  and the opening of the bore of the housing. Due to the elimination of a redundant seal from the brake pedal unit  130 , a seal interface (not shown) may be utilized at the end of the bore  200  of the housing of the brake pedal unit  130  to trap any fluid that bypasses the seal  222 . The seal interface may include an optional sponge (not shown) and covered by a retainer or cover. Although the addition of a redundant seal  36  may assist in helping prevent fluid leakage, the addition of the redundant seal  36  adds seal friction resistance increasing the force necessary to initiate movement of the input piston  14 . The addition of the redundant seal  36  also necessitates the addition of additional spring forces to overcome this friction resistance which further increases the force necessary to initiate movement of the input piston  14 . 
     The operation of a generally low braking event comparing the differences between the prior art brake pedal unit  10  and the brake pedal unit  130  as felt by the driver of the vehicle of the brake pedal will now be described. As will be seen from the graph of  FIG. 6 , the structural and operational differences between the brake pedal units  10  and  130  (as discussed above) generally provide a reduced pedal force for the brake pedal unit  130  compared to the prior art brake pedal unit  10 . 
       FIG. 6  illustrates graphical representations of a low pedal travel apply and release occurrence of a normal braking event for the brake pedal units  10  and  130 . More specifically, the graphical representation is of a pedal force vs a pedal travel. The pedal travel corresponds directly with travel of the input pistons  14  and  202  within the brake pedal units  10  and  130 , respectively. As described above, movement of the brake pedal  190  causes movement of the respective input piston ( 14  or  202 ) via the linkage arm  208  of the brake pedal  190 . It is noted that the numerical data for the pedal travel apply and release occurrence represented in  FIG. 6  is used in conjunction with a brake pedal having a pedal ratio of about  3 . 8 . The reactionary forces from the input piston provide feedback as experienced by the foot of the driver as the driver operates the brake pedal  190 . A dashed line  500  is associated with the operation of the brake pedal unit  130  of  FIG. 3  and the pedal simulator  132  of  FIG. 4 . A solid line  510  is associated with the prior art brake pedal unit  10  of  FIG. 1  and a respective pedal simulator, such as one that is similar to the pedal simulator  132 .  FIG. 6  provides an appropriate graphical representation highlighting the differences between the brake pedal units  10  and  130 . However, it should be understood that these values are indicative of just one example of a braking event of the brake pedal units  10  and  130  for descriptive purposes and are not to be held to limit the invention as disclosed herein. 
     Referring now to the broken or dashed line  500  associated with the brake pedal unit  130 , an initial preloaded force input, indicated by a nearly vertical path  500   a , generally corresponds to the force required to overcome the various preload requirements of the springs within the brake pedal unit  130  as well as overcome various frictional forces imparted by the various seals of the brake pedal unit  130 . For the example illustrated in  FIG. 6 , the force at which these forces are overcome is at about 16.8 N. This is derived by adding the preload spring force of 45 N of the return spring  286 , plus the preload spring force of 5 N of the input spring  288 , plus the nominal frictional forces of about 7 N each for the seals  220  and  22   s  of the input piston  202 , and then dividing by the pedal ratio of about 3.8 to obtain about 16.8 N 
     Once the preload force of about 16.8 N is overcome, the input piston  202  starts to move towards a position such that the passageway  224  moves just past the lip seal  220 , as indicated by the sloped path  500   b . Note that during the path  500   b , the primary and secondary pistons  204  and  206  are not moving (unlike the brake pedal unit  10 ). The gradual rise along the path  500   b  generally accounts for the spring rate of the compressing return spring  286  and input spring  288 . An assumption can be made with respect to the graphical representations of  FIG. 6  that all springs have the same spring rate, such as for example, of about 2 N/m. 
     Once the passageway  224  of the input piston  202  moves just past the lip seal  220  cutting off fluid communication of the input chamber  210  with the reservoir  136 , as indicated at the end of the sloped path  500   b , a further force is required to overcome the preload forces of the first and second springs  236  and  238  of the pedal simulator  132  and overcome the seal friction of the seals in the pedal simulator  132 . This is represented by the nearly vertical path  500   c . Upon a sufficient further force, generally at about  20  N, the input piston  202  continues to move (compressing the input spring  288  and actuating the pedal simulator  132 ), as represented by a sloped path  500   d  until the driver halts applying force to the brake pedal  190 , as represented by this example in  FIG. 6 . Note that there is generally no significance to the travel length (about 15 mm) when the driver starts release of the brake pedal  190  and this graphical representation is used only for demonstrative purposes of a relatively low pedal travel apply. During the path  500   d , the pedal simulator  132  is being actuated and its piston  232  is moving therein compressing the first and second springs  236  and  238 . Paths  500   e ,  500   f , and  500   g  represent the release of the brake pedal  190  by the driver back to an at rest state. The nearly vertical drop of path  500   e  represents the change of direction and associated seal friction change of the input piston  202  and the pedal simulator  132 . 
     Referring now to the solid line  510  associated with the prior art brake pedal unit  10 , an initial preloaded force input, indicated by a nearly vertical path  510   a  generally corresponds to the force required to overcome the various preload requirements of the springs within the brake pedal unit  10  as well as overcome various friction forces imparted by the various seals of the brake pedal unit  10 . A larger force is required compared to the brake pedal unit  130  due to all three pistons (the input piston  14 , the primary piston  16 , and the secondary piston  18 ) now having to be moved until their respective passageways  38 ,  44 , and  50 , respectively, are blocked. For the example illustrated in  FIG. 6 , the force at which these forces are overcome is at about 36.6 N. This is derived by adding the preload spring force of 40 N of the return spring  58 , plus the preload spring force of 50 N of the secondary spring  56 , plus the nominal frictional forces of about 7 N each for all of the seals  32 ,  24 ,  26 ,  40 ,  42 ,  46 , and  48 , and then dividing by the pedal ratio of about 3.8 to obtain about 36.6 N. 
     Once the preload force of about 36.6 N is overcome, all three pistons (input piston  14 , primary piston  16 , and secondary piston  18 ) start to move towards their respective positions closing off the fluid communication with the reservoir. More specifically, the passageways  38 ,  44 , and  50  move past the lip seals  32 ,  40 , and  46 , as indicated by the sloped path  510   b . The gradual rise along the path  510   b  generally accounts for the spring rate of the compressing return spring  58  and the secondary spring  56 . The gradual rise along the path  510   b  is similar to the path  500   b.    
     Once the passageways  38 ,  44 , and  50  move past the lip seals  32 ,  40 , and  46 , as indicated at the end of the sloped path  510   b , a further force is required to overcome the preload forces of the springs of the pedal simulator  132  and the input spring  288  of the brake pedal unit  130 , which is represented by the nearly vertical path  510   c . Upon a sufficient further force, generally at about 44 N, the input piston  14  continues to move (compressing the input spring  52  and actuating the pedal simulator), as represented by a sloped path  510   d  until the driver halts applying force to the brake pedal. During the path  510   d , the pedal simulator is being actuated. 
     Upon release, paths  510   e  and  510   f  are similar to paths  500   e  and  500   f  until about 5.7 mm when the pedal simulator empties of fluid and all three pistons (the input piston  14 , the primary piston  16 , and the secondary piston  18 ) start their movement back to their rest positions, as represented by path  510   g . Movement of the three pistons back to their rest positions is indicated by path  510   h , and return to a rest state represented by path  510   i.    
     As can be seen from the graph of  FIG. 6 , the differences between the brake pedal units  10  and  130  generally provide a reduced pedal force requirement for the brake pedal unit  130  compared to the brake pedal unit  10 . The input piston spring  52  (100 N preload) of the brake pedal unit  10  has a greater preload value than the input piston spring  288  (5 N preload) of the brake pedal unit  130  since the spring  52  is utilized during the initial movement of the primary and secondary pistons  16  and  18  during simultaneous cut off. With respect to the brake pedal unit  130 , the input spring  288  is not utilized to move the primary and secondary pistons  204  and  206 . The input spring  288  may provide a much lesser force acting on the primary and secondary pistons  204  and  206  to prevent movements caused by bumps or motion of the vehicle that would add cut off travel in the next brake apply after inadvertent movement of the pistons in the brake pedal unit  130 . 
     With respect to the various valves of the brake system  10 , the terms “operate” or “operating” (or “actuate”, “moving”, “positioning”) used herein (including the claims) may not necessarily refer to energizing the solenoid of the valve, but rather refers to placing or permitting the valve to be in a desired position or valve state. For example, a solenoid actuated normally open valve can be operated into an open position by simply permitting the valve to remain in its non-energized normally open state. Operating the normally open valve to a closed position may include energizing the solenoid to move internal structures of the valve to block or prevent the flow of fluid therethrough. Thus, the term “operating” should not be construed as meaning moving the valve to a different position nor should it mean to always energizing an associated solenoid of the valve. 
     The principle and mode of operation of this invention have been explained and illustrated in its preferred embodiment. However, it must be understood that this invention may be practiced otherwise than as specifically explained and illustrated without departing from its spirit or scope.