Patent Publication Number: US-11021140-B2

Title: Vehicle brake system having plunger power source

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a continuation in part of International Application PCT/US17/44547 filed Jul. 28, 2017 which designated the U.S. and that International Application was published in English under PCT Article 21(2) on Feb. 1, 2018 as International Publication Number WO 2018/023091, the full disclosure of which is incorporated herein by reference. PCT/US17/44547 claims priority to U.S. patent application Ser. No. 15/221,648, filed Jul. 28, 2016, the full disclosure of which is incorporated herein by reference. Thus, the subject non provisional application claims priority to U.S. patent application Ser. No. 15/221,648, filed Jul. 28, 2016. This application claims priority to, and the benefit of, U.S. Provisional Application Ser. No. 62/611,926, filed Dec. 29, 2017, the full 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. 
     SUMMARY OF THE INVENTION 
     The invention concerns an improved plunger assembly for a vehicle brake system. The plunger assembly is operable as a pressure source to control brake fluid pressure supplied to one or more wheel brakes. The plunger assembly comprises a housing defining a cylinder having a first port; a reversible motor supported by the housing and having a rotor; and a linear actuator driven by the motor. Preferably, the linear actuator includes a ball screw mechanism having a screw and a nut, with one of the screw and the nut defining a rotatable component connected to the motor rotor, and the other one of the screw and the nut defining a translatable component. The plunger assembly also includes an anti-rotation member coupled to the translatable component to allow translation and resist rotation of the translatable component within the housing. A plunger head is mounted in the cylinder and driven by the translatable component in first and second opposite directions. The plunger head cooperates with the cylinder to define a first chamber containing brake fluid received from a fluid reservoir, and the first chamber is hydraulically connected to the wheel brakes via the first port. In at least one operating mode, fluid pressure in the first chamber is increased when the plunger head is moved in the first direction and is decreased when the plunger head is moved in the second direction. 
     According to another aspect of the invention, a clamped connection is provided at an inner race of the bearing for securing the rotor to the rotatable component. The clamped connection may include a washer having a tapered aperture and a threaded fastener have tapered head and a threaded portion extending from the tapered head, with the mating tapered aperture and tapered head defining a taper interface, and the taper interface establishing an initial contact point proximate the threaded portion and a gap at a distal end of the tapered head. Also, the rotor defines a tapered aperture and the rotatable component defines a complementary tapered extension and wherein the clamped connection engages the extension within the aperture to create a frictional torque transmitting connection. The rotor further defines a second aperture that is concentric with the tapered aperture and defines a torque transmitting profile, and the rotatable component has a mating torque transmitting profile configured to provide a secondary torque transmission path between the rotor and the rotatable component. Generally, the clamped connection includes a washer engaging one side of the inner race, the rotor engages the opposite side of the inner race, and a threaded faster extends through the washer and is tightened into a threaded bore in the rotatable component to clamp the rotor to the inner race. 
     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 first embodiment of a brake system. 
         FIG. 2  is an enlarged schematic illustration of the plunger assembly of the brake system of  FIG. 1 . 
         FIG. 3  is a schematic illustration of a first embodiment of a plunger assembly in accordance with the invention. 
         FIG. 4  is a schematic illustration, in a partially exploded view, of a portion of the plunger assembly of  FIG. 3 . 
         FIG. 5  is a perspective view of a second embodiment of a plunger assembly. 
         FIG. 6  is a perspective view of an opposite side of the plunger assembly of  FIG. 5 . 
         FIG. 7  is an exploded perspective view of the plunger assembly of  FIGS. 4 and 5 . 
         FIG. 8  is an elevational view, in cross section, of the plunger assembly of  FIG. 4 . 
         FIG. 9  is an enlarged elevational view, in cross section, of a connection of the plunger assembly of  FIG. 8 . 
         FIG. 10  is an enlarged view, in cross section, of the connection of the plunger assembly of  FIG. 9 , taken at  18 - 18 . 
         FIG. 11  is a perspective view, in cross section, of the motor and ball screw sub assembly of the plunger assembly of  FIG. 8 . 
         FIG. 12  is an exploded, perspective view of the motor and ball screw sub assembly of  FIG. 11 . 
         FIG. 13  is a partially exploded, perspective view of the motor and ball screw sub assembly of  FIG. 11 . 
         FIG. 14  is an enlarged, cross sectional view of a ball screw assembly of  FIG. 9 . 
         FIG. 15  is an exploded, perspective view of a ball screw assembly and support structure. 
         FIG. 16  is a partially exploded, perspective view of the ball screw and support structure assembly of  FIG. 15  and the plunger housing. 
         FIG. 17  is an assembled view of the ball screw and support structure assembly and the plunger housing of  FIG. 16 . 
         FIG. 18  is an enlarged view, in cross section, of a plunger head and cylinder. 
         FIG. 19  is an exploded view of a plunger head and ball screw assembly. 
         FIG. 20  is a perspective view, in cross section, of the plunger and housing assembly. 
         FIG. 21  is an exploded view of an anti-rotation tube, plunger cylinder sleeve, and end cap sub assembly. 
         FIG. 22  is an enlarged, cross sectional view of the anti-rotation tube, plunger cylinder sleeve, and end cap sub assembly of  FIG. 21 . 
         FIG. 23  is a perspective view, in cross section, of the plunger assembly of  FIG. 6 . 
         FIG. 24  is a perspective view, in cross section, of the plunger assembly of  FIG. 5 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the drawings, there is schematically illustrated in  FIG. 1  a first embodiment of a vehicle brake system, indicated generally at  10 . The brake system  10  is a hydraulic braking system in which fluid pressure from a source is operated to apply braking forces for the brake system  10 . The brake system  10  may suitably be used on a ground vehicle such as an automotive vehicle having four wheels. Furthermore, the brake system  10  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. In the illustrated embodiment of the brake system  10 , there are four wheel brakes  12   a ,  12   b ,  12   c , and  12   d . The wheel brakes  12   a ,  12   b ,  12   c , and  12   d  can have any suitable wheel brake structure operated by the application of pressurized brake fluid. The wheel brakes  12   a ,  12   b ,  12   c , and  12   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  12   a ,  12   b ,  12   c , and  12   d  can be associated with any combination of front and rear wheels of the vehicle in which the brake system  10  is installed. A diagonally split brake system is illustrated such that the wheel brake  12   a  is associated with the left rear wheel, the wheel brake  12   b  is associated with the right front wheel, the wheel brake  12   c  is associated with the left front wheel, and the wheel brake  12   d  is associated with the right rear wheel. Alternatively for a vertically split system, the wheel brakes  12   a  and  12   b  may be associated with the front wheels, and the wheel brakes  12   c  and  12   d  may be associated with the rear wheels. 
     The brake system  10  includes a brake pedal unit, indicated generally at  14 , a pedal simulator  16 , a plunger assembly, indicated generally at  18 , and a reservoir  20 . The reservoir  20  stores and holds hydraulic fluid for the brake system  10 . The fluid within the reservoir  20  is preferably held at or about atmospheric pressure but may store the fluid at other pressures if so desired. The brake system  10  may include a fluid level sensor (not shown) for detecting the fluid level of the reservoir  20 . Note that in the schematic illustration of  FIG. 1 , conduit lines may not be specifically drawn leading to the reservoir  20  but may be represented by conduits ending and labelled with T 1 , T 2 , or T 3  indicating that these various conduits are connected to one or more tanks or sections of the reservoir  20 . Alternatively, the reservoir  20  may include multiple separate housings. As will be discussed in detail below, the plunger assembly  18  of the brake system  10  functions as a source of pressure to provide a desired pressure level to the wheel brakes  12   a ,  12   b ,  12   c , and  12   d  during a typical or normal brake apply. Fluid from the wheel brakes  12   a ,  12   b ,  12   c , and  12   d  may be returned to the plunger assembly  18  and/or diverted to the reservoir  20 . 
     The brake system  10  includes an electronic control unit (ECU)  22 . The ECU  22  may include microprocessors. The ECU  22  receives various signals, processes signals, and controls the operation of various electrical components of the brake system  10  in response to the received signals. The ECU  22  can be connected to various sensors such as pressure sensors, travel sensors, switches, wheel speed sensors, and steering angle sensors. The ECU  22  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  10  during vehicle stability operation. Additionally, the ECU  22  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  10  further includes first and second isolation valves  30  and  32 . The isolation valves  30  and  32  may be solenoid actuated three way valves. The isolation valves  30  and  32  are generally operable to two positions, as schematically shown in  FIG. 1 . The first and second isolation valves  30  and  32  each have a port in selective fluid communication with an output conduit  34  generally in communication with an output of the plunger assembly  18 , as will be discussed below. The first and second isolation valves  30  and  32  also includes ports that are selectively in fluid communication with conduits  36  and  38 , respectively, when the first and second isolation valves  30  and  32  are non-energized, as shown in  FIG. 1 . The first and second isolation valves  30  and  32  further include ports that are in fluid communication with conduits  40  and  42 , respectively, which provide fluid to and from the wheel brakes  12   a ,  12   b ,  12   c , and  12   d.    
     The system  10  further includes various solenoid actuated valves (slip control valve arrangement) for permitting controlled braking operations, such as ABS, traction control, vehicle stability control, and regenerative braking blending. A first set of valves includes a first apply valve  50  and a first dump valve  52  in fluid communication with the conduit  40  for cooperatively supplying fluid received from the first isolation valve  30  to the wheel brake  12   a , and for cooperatively relieving pressurized fluid from the wheel brake  12   a  to a reservoir conduit  53  in fluid communication with the reservoir  20 . A second set of valves includes a second apply valve  54  and a second dump valve  56  in fluid communication with the conduit  40  for cooperatively supplying fluid received from the first isolation valve  30  to the wheel brake  12   b , and for cooperatively relieving pressurized fluid from the wheel brake  12   b  to the reservoir conduit  53 . A third set of valves includes a third apply valve  58  and a third dump valve  60  in fluid communication with the conduit  42  for cooperatively supplying fluid received from the second isolation valve  32  to the wheel brake  12   c , and for cooperatively relieving pressurized fluid from the wheel brake  12   c  to the reservoir conduit  53 . A fourth set of valves includes a fourth apply valve  62  and a fourth dump valve  64  in fluid communication with the conduit  42  for cooperatively supplying fluid received from the second isolation valve  32  to the wheel brake  12   d , and for cooperatively relieving pressurized fluid from the wheel brake  12   d  to the reservoir conduit  53 . Note that in a normal braking event, fluid flows through the non-energized open apply valves  50 ,  54 ,  58 , and  62 . Additionally, the dump valves  52 ,  56 ,  60 , and  64  are preferably in their non-energized closed positions to prevent the flow of fluid to the reservoir  20 . 
     The brake pedal unit  14  is connected to a brake pedal  70  and is actuated by the driver of the vehicle as the driver presses on the brake pedal  70 . A brake sensor or switch  72  may be connected to the ECU  22  to provide a signal indicating a depression of the brake pedal  70 . As will be discussed below, the brake pedal unit  14  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  18  under certain failed conditions of the brake system  10 . The brake pedal unit  14  can supply pressurized fluid in the conduits  36  and  38  (that are normally closed off at the first and second isolation valves  30  and  32  during a normal brake apply) to the wheel brake  12   a ,  12   b ,  12   c , and  12   d  as required. 
     The brake pedal unit  14  includes a housing having a multi-stepped bore  80  formed therein for slidably receiving various cylindrical pistons and other components therein. The housing may be formed as a single unit or include two or more separately formed portions coupled together. An input piston  82 , a primary piston  84 , and a secondary piston  86  are slidably disposed within the bore  80 . The input piston  82  is connected with the brake pedal  70  via a linkage arm  76 . Leftward movement of the input piston  82 , the primary piston  84 , and the secondary piston  86  may cause, under certain conditions, a pressure increase within an input chamber  92 , a primary chamber  94 , and a secondary chamber  96 , respectively. Various seals of the brake pedal unit  14  as well as the structure of the housing and the pistons  82 ,  84 , and  86  define the chambers  92 ,  94 , and  96 . For example, the input chamber  92  is generally defined between the input piston  82  and the primary piston  84 . The primary chamber  94  is generally defined between the primary piston  84  and the secondary piston  86 . The secondary chamber  96  is generally defined between the secondary piston  86  and an end wall of the housing formed by the bore  80 . 
     The input chamber  92  is in fluid communication with the pedal simulator  16  via a conduit  100 , the reason for which will be explained below. The input piston  92  is slidably disposed in the bore  80  of the housing of the brake pedal unit  14 . An outer wall of the input piston  82  is engaged with a lip seal  102  and a seal  104  mounted in grooves formed in the housing. A passageway  106  (or multiple passageways) is formed through a wall of the piston  82 . As shown in  FIG. 1 , when the brake pedal unit  14  is in its rest position (the driver is not depressing the brake pedal  70 ), the passageway  106  is located between the lip seal  102  and the seal  104 . In the rest position, the passageway  106  permits fluid communication between the input chamber  92  and the reservoir  20  via a conduit  108 . Sufficient leftward movement of the input piston  82 , as viewing  FIG. 1 , will cause the passageway  106  to move past the lip seal  102 , thereby preventing the flow of fluid from the input chamber  92  into the conduit  108  and the reservoir  20 . Further leftward movement of the input piston  82  will pressurize the input chamber  92  causing fluid to flow into the pedal simulator  16  via the conduit  100 . As fluid is diverted into the pedal simulator  16 , a simulation chamber  110  within the pedal simulator  16  will expand causing movement of a piston  112  within the pedal simulator  16 . Movement of the piston  112  compresses a spring assembly, schematically represented as a spring  114 . The compression of the spring  114  provides a feedback force to the driver of the vehicle which simulates the forces a driver feels at the brake pedal  70  in a conventional vacuum assist hydraulic brake system, for example. The spring  114  of the pedal simulator  16  can include any number and types of spring members as desired. For example, the spring  114  may include a combination of low rate and high rate spring elements to provide a non-linear force feedback. The simulation chamber  110  is in fluid communication with the conduit  100  which is in fluid communication with the input chamber  92 . A solenoid actuated simulator valve  116  is positioned within the conduit  100  to selectively prevent the flow of fluid from the input chamber  92  to the simulation chamber, such as during a failed condition in which the brake pedal unit  14  is utilized to provide a source of pressurized fluid to the wheel brakes. A check valve  118  in parallel with a restricted orifice  120  may be positioned with the conduit  100 . The spring  114  of the pedal simulator  16  may be housed within a non-pressurized chamber  122  in fluid communication with the reservoir  20  (T 1 ). 
     As discussed above, the input chamber  92  of the brake pedal unit  14  is selectively in fluid communication with the reservoir  20  via a conduit  108  and the passageway  106  formed in the input piston  82 . The brake system  10  may include an optional simulator test valve  130  located within the conduit  108 . The simulator test valve  130  may be electronically controlled between an open position, as shown in  FIG. 1 , and a powered closed position. The simulator test valve  130  is not necessarily needed during a normal boosted brake apply or for a manual push through mode. The simulator test valve  130  can be energized to a closed position during various testing modes to determine the correct operation of other components of the brake system  10 . For example, the simulator test valve  130  may be energized to a closed position to prevent venting to the reservoir  20  via the conduit  108  such that a pressure build up in the brake pedal unit  14  can be used to monitor fluid flow to determine whether leaks may be occurring through seals of various components of the brake system  10 . 
     The primary chamber  94  of the brake pedal unit  14  is in fluid communication with the second isolation valve  32  via the conduit  38 . The primary piston  84  is slidably disposed in the bore  80  of the housing of the brake pedal unit  14 . An outer wall of the primary piston  84  is engaged with a lip seal  132  and a seal  134  mounted in grooves formed in the housing. One or more passageways  136  are formed through a wall of the primary piston  84 . The passageway  136  is located between the lip seal  132  and the seal  134  when the primary piston  84  is in its rest position, as shown in  FIG. 1 . Note that in the rest position the lip seal  132  is just slightly to the left of the passageway  136 , thereby permitting fluid communication between the primary chamber  94  and the reservoir  20 . 
     The secondary chamber  96  of the brake pedal unit  14  is in fluid communication with the first isolation valve  30  via the conduit  36 . The secondary piston  86  is slidably disposed in the bore  80  of the housing of the brake pedal unit  14 . An outer wall of the secondary piston  86  is engaged with a lip seal  140  and a seal  142  mounted in grooves formed in the housing. One or more passageways  144  are formed through a wall of the secondary piston  86 . As shown in  FIG. 1 , the passageway  144  is located between the lip seal  140  and the seal  142  when the secondary piston  86  is in its rest position. Note that in the rest position the lip seal  140  is just slightly to the left of the passageway  144 , thereby permitting fluid communication between the secondary chamber  96  and the reservoir  20  (T 2 ). 
     If desired, the primary and secondary pistons  84  and  86  may be mechanically connected with limited movement therebetween. The mechanical connection of the primary and secondary pistons  84  and  86  prevents a large gap or distance between the primary and secondary pistons  84  and  86  and prevents having to advance the primary and secondary pistons  84  and  86  over a relatively large distance without any increase in pressure in the non-failed circuit. For example, if the brake system  10  is under a manual push through mode and fluid pressure is lost in the output circuit relative to the secondary piston  86 , such as for example in the conduit  36 , the secondary piston  86  will be forced or biased in the leftward direction due to the pressure within the primary chamber  94 . If the primary and secondary pistons  84  and  86  were not connected together, the secondary piston  86  would freely travel to its further most left-hand position, as viewing  FIG. 1 , and the driver would have to depress the pedal  70  a distance to compensate for this loss in travel. However, because the primary and secondary pistons  84  and  86  are connected together, the secondary piston  86  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  84  and  86  may be used. For example, as schematically shown in  FIG. 1 , the right-hand end of the secondary piston  86  may include an outwardly extending flange that extends into a groove formed in an inner wall of the primary piston  84 . The groove has a width which is greater than the width of the flange, thereby providing a relatively small amount of travel between the first and secondary pistons  84  and  86  relative to one another. 
     The brake pedal unit  14  may include an input spring  150  generally disposed between the input piston  82  and the primary piston  84 . Additionally, the brake pedal unit  14  may include a primary spring (not shown) disposed between the primary piston  84  and the secondary piston  86 . A secondary spring  152  may be included and disposed between the secondary piston  86  and a bottom wall of the bore  80 . The input, primary and secondary springs may have any suitable configuration, such as a caged spring assembly, for biasing the pistons in a direction away from each other and also to properly position the pistons within the housing of the brake pedal unit  14 . 
     The brake system  10  may further include a pressure sensor  156  in fluid communication with the conduit  36  to detect the pressure within the secondary pressure chamber  96  and for transmitting the signal indicative of the pressure to the ECU  22 . Additionally, the brake system  10  may further include a pressure sensor  158  in fluid communication with the conduit  34  for transmitting a signal indicative of the pressure at the output of the plunger assembly  18 . 
     As shown schematically in  FIG. 2 , the plunger assembly  18  includes a housing having a multi-stepped bore  200  formed therein. The bore  200  includes a first portion  202  and a second portion  204 . A piston  206  is slidably disposed within the bore  200 . The piston  206  includes an enlarged end portion  208  connected to a smaller diameter central portion  210 . The piston  206  has a second end  211  connected to a ball screw mechanism, indicated generally at  212 . The ball screw mechanism  212  is provided to impart translational or linear motion of the piston  206  along an axis defined by the bore  200  in both a forward direction (leftward as viewing  FIGS. 1 and 2 ), and a rearward direction (rightward as viewing  FIGS. 1 and 2 ) within the bore  200  of the housing. In the embodiment shown, the ball screw mechanism  212  includes a motor  214  rotatably driving a screw shaft  216 . The second end  211  of the piston  206  includes a threaded bore  220  and functions as a driven nut of the ball screw mechanism  212 . The ball screw mechanism  212  includes a plurality of balls  222  that are retained within helical raceways  223  formed in the screw shaft  216  and the threaded bore  220  of the piston  206  to reduce friction. Although a ball screw mechanism  212  is shown and described with respect to the plunger assembly  18 , it should be understood that other suitable mechanical linear actuators may be used for imparting movement of the piston  206 . It should also be understood that although the piston  206  functions as the nut of the ball screw mechanism  212 , the piston  206  could be configured to function as a screw shaft of the ball screw mechanism  212 . Of course, under this circumstance, the screw shaft  216  would be configured to function as a nut having internal helical raceways formed therein. The piston  206  may include structures (not shown) engaged with cooperating structures formed in the housing of the plunger assembly  18  to prevent rotation of the piston  206  as the screw shaft  216  rotates around the piston  206 . For example, the piston  206  may include outwardly extending splines or tabs (not shown) that are disposed within longitudinally extending grooves (not shown) formed in the housing of the plunger assembly  18  such that the tabs slide along within the grooves as the piston  206  travels in the bore  200 . 
     As will be discussed below, the plunger assembly  18  is preferably configured to provide pressure to the conduit  34  when the piston  206  is moved in both the forward and rearward directions. The plunger assembly  18  includes a seal  230  mounted on the enlarged end portion  208  of the piston  206 . The seal  230  slidably engages with the inner cylindrical surface of the first portion  202  of the bore  200  as the piston  206  moves within the bore  200 . A seal  234  and a seal  236  are mounted in grooves formed in the second portion  204  of the bore  200 . The seals  234  and  236  slidably engage with the outer cylindrical surface of the central portion  210  of the piston  206 . A first pressure chamber  240  is generally defined by the first portion  202  of the bore  200 , the enlarged end portion  208  of the piston  206 , and the seal  230 . A second pressure chamber  242 , located generally behind the enlarged end portion  208  of the piston  206 , is generally defined by the first and second portions  202  and  204  of the bore  200 , the seals  230  and  234 , and the central portion  210  of the piston  206 . The seals  230 ,  234 , and  236  can have any suitable seal structure. 
     The plunger assembly  18  preferably includes a sensor, schematically shown as  218 , for detecting the position of the piston  206  within the bore  200 . The sensor  218  is in communication with the ECU  22 . In one embodiment, the sensor  218  may detect the position of the piston  206 , or alternatively, metallic or magnetic elements embedded with the piston  206 . In an alternate embodiment, the sensor  218  may detect the rotational position of the motor  214  and/or other portions of the ball screw mechanism  212  which is indicative of the position of the piston  206 . The sensor  218  can be located at any desired position. 
     For reasons which will be explained below, the piston  206  of the plunger assembly  18  includes a passageway  244  formed therein. The passageway  244  defines a first port  246  extending through the outer cylindrical wall of the piston  206  and is in fluid communication with the secondary chamber  242 . The passageway  244  also defines a second port  248  extending through the outer cylindrical wall of the piston  206  and is in fluid communication with a portion of the bore  200  located between the seals  234  and  236 . The second port  248  is in fluid communication with a conduit  249  which is in fluid communication with the reservoir  20  (T 3 ). 
     Referring back to  FIG. 1 , the brake system  10  further includes a first plunger valve  250 , and a second plunger valve  252 . The first plunger valve  250  is preferably a solenoid actuated normally closed valve. Thus, in the non-energized state, the first plunger valve  250  is in a closed position, as shown in  FIG. 1 . The second plunger valve  252  is preferably a solenoid actuated normally open valve. Thus, in the non-energized state, the second plunger valve  252  is in an open position, as shown in  FIG. 1 . A check valve may be arranged within the second plunger valve  252  so that when the second plunger valve  252  is in its closed position, fluid may still flow through the second plunger valve  252  in the direction from a first output conduit  254  (from the first pressure chamber  240  of the plunger assembly  18 ) to the conduit  34  leading to the isolation valves  30  and  32 . Note that during a rearward stroke of the piston  206  of the plunger assembly  18 , pressure may be generated in the second pressure chamber  242  for output into the conduit  34 . 
     Generally, the first and second plunger valves  250  and  252  are controlled to permit fluid flow at the outputs of the plunger assembly  18  and to permit venting to the reservoir  20  (T 3 ) through the plunger assembly  18  when so desired. For example, the first plunger valve  250  may be energized to its open position during a normal braking event so that both of the first and second plunger valves  250  and  252  are open (which may reduce noise during operation). Preferably, the first plunger valve  250  is almost always energized during an ignition cycle when the engine is running. Of course, the first plunger valve  250  may be purposely moved to its closed position such as during a pressure generating rearward stroke of the plunger assembly  18 . The first and second plunger valves  250  and  252  are preferably in their open positions when the piston  206  of the plunger assembly  18  is operated in its forward stroke to maximize flow. When the driver releases the brake pedal  70 , the first and second plunger valves  250  and  252  preferably remain in their open positions. Note that fluid can flow through the check valve within the closed second plunger valve  252 , as well as through a check valve  258  from the reservoir  20  depending on the travel direction of the piston  206  of the plunger assembly  18 . 
     It may be desirable to configure the first plunger valve  250  with a relatively large orifice therethrough when in its open position. A relatively large orifice of the first plunger assembly  250  helps to provide an easy flow path therethrough. The second plunger valve  252  may be provided with a much smaller orifice in its open position as compared to the first plunger valve  250 . One reason for this is to help prevent the piston  206  of the plunger assembly  18  from rapidly being back driven upon a failed event due to the rushing of fluid through the first output conduit  254  into the first pressure chamber  240  of the plunger assembly  18 , thereby preventing damage to the plunger assembly  18 . 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  206  of the plunger assembly  18  upon failure of the brake system  10 , such as for example, when power is lost to the motor  214  and the pressure within the conduit  34  is relatively high. As shown in  FIG. 2 , the plunger assembly  18  may include an optional spring member, such as a spring washer  277 , to assist in cushioning such a rapid rearward back drive of the piston  206 . The spring washer  277  may also assist in cushioning the piston  206  moving at any such speed as it approaches a rest position near its most retracted position within the bore  200 . Schematically shown in  FIG. 2 , the spring washer  277  is located between the enlarged end portion  208  and a shoulder  279  formed in the bore  200  between the first and second portions  202  and  204 . The spring washer  277  can have any suitable configuration which deflects or compresses upon contact with the piston  206  as the piston  206  moves rearwardly. For example, the spring washer  277  may be in the form of a metal conical spring washer. Alternatively, the spring washer  277  may be in the form of a wave spring. Although the spring washer  277  is shown mounted within the bore  200  of the plunger assembly  18 , the spring washer  277  may alternatively be mounted on the piston  206  such that the spring washer  277  moves with the piston  206 . In this configuration, the spring washer  277  would engage with the shoulder  279  and compress upon sufficient rightward movement of the piston  206 . 
     The first and second plunger valves  250  and  252  provide for an open parallel path between the pressure chambers  240  and  242  of the plunger assembly  18  during a normal braking operation. Although a single open path may be sufficient, the advantage of having both the first and second plunger valves  250  and  252  is that the first plunger valve  250  may provide for an easy flow path through the relatively large orifice thereof, while the second plunger valve  252  may provide for a restricted orifice path during certain failed conditions (when the first plunger valve  250  is de-energized to its closed position. 
     During a typical or normal braking operation, the brake pedal  70  is depressed by the driver of the vehicle. In a preferred embodiment of the brake system  10 , the brake pedal unit  14  includes one or more travel sensors  270  (for redundancy) for producing signals transmitted to the ECU  22  that are indicative of the length of travel of the input piston  82  of the brake pedal unit  14 . 
     During normal braking operations, the plunger assembly  18  is operated to provide pressure to the conduit  34  for actuation of the wheel brakes  12   a ,  12   b ,  12   c , and  12   d . Under certain driving conditions, the ECU  22  communicates with a powertrain control module (not shown) and other additional braking controllers of the vehicle to provide coordinated braking during advanced braking control schemes (e.g., anti-lock braking (AB), traction control (TC), vehicle stability control (VSC), and regenerative brake blending). During a normal brake apply, the flow of pressurized fluid from the brake pedal unit  14 , generated by depression of the brake pedal  70 , is diverted into the pedal simulator  16 . The simulator valve  116  is actuated to divert fluid through the simulator valve  116  from the input chamber  92 . Note that the simulator valve  116  is shown in its energized state in  FIG. 1 . Thus, the simulator valve  116  is a normally closed solenoid valve. Also note that fluid flow from the input chamber  92  to the reservoir  20  is closed off once the passageway  106  in the input piston  82  moves past the seal  104 . 
     During the duration of a normal braking event, the simulator valve  116  remains open, preferably. Also during the normal braking operation, the isolation valves  30  and  32  are energized to secondary positions to prevent the flow of fluid from the conduits  36  and  38  through the isolation valves  30  and  32 , respectively. Preferably, the isolation valves  30  and  32  are energized throughout the duration of an ignition cycle such as when the engine is running instead of being energized on and off to help minimize noise. Note that the primary and secondary pistons  84  and  86  are not in fluid communication with the reservoir  20  due to their passageways  136  and  144 , respectively, being positioned past the lip seals  132  and  140 , respectively. Prevention of fluid flow through the isolation valves  30  and  32  hydraulically locks the primary and secondary chambers  94  and  96  of the brake pedal unit  14  preventing further movement of the primary and secondary pistons  84  and  86 . 
     It is generally desirable to maintain the isolation valves  30  and  32  energized during the normal braking mode to ensure venting of fluid to the reservoir  20  through the plunger assembly  18  such as during a release of the brake pedal  70  by the driver. As best shown in  FIG. 1 , the passageway  244  formed in the piston  206  of the plunger assembly  18  permits this ventilation. 
     During normal braking operations, while the pedal simulator  16  is being actuated by depression of the brake pedal  70 , the plunger assembly  18  can be actuated by the ECU  22  to provide actuation of the wheel brakes  12   a ,  12   b ,  12   c , and  12   d . The plunger assembly  18  is operated to provide desired pressure levels to the wheel brakes  12   a ,  12   b ,  12   c , and  12   d  compared to the pressure generated by the brake pedal unit  14  by the driver depressing the brake pedal  70 . The electronic control unit  22  actuates the motor  214  to rotate the screw shaft  216  in the first rotational direction. Rotation of the screw shaft  216  in the first rotational direction causes the piston  206  to advance in the forward direction (leftward as viewing  FIGS. 1 and 2 ). Movement of the piston  206  causes a pressure increase in the first pressure chamber  240  and fluid to flow out of the first pressure chamber  240  and into the conduit  254 . Fluid can flow into the conduit  34  via the open first and second plunger valves  250  and  252 . Note that fluid is permitted to flow into the second pressure chamber  242  via a conduit  243  as the piston  206  advances in the forward direction. Pressurized fluid from the conduit  34  is directed into the conduits  40  and  42  through the isolation valves  320  and  322 . The pressurized fluid from the conduits  40  and  42  can be directed to the wheel brakes  12   a ,  12   b ,  12   c , and  12   d  through open apply valves  50 ,  54 ,  58 , and  62  while the dump valves  52 ,  56 ,  60 , and  64  remain closed. When the driver lifts off or releases the brake pedal  70 , the ECU  22  can operate the motor  214  to rotate the screw shaft  216  in the second rotational direction causing the piston  206  to retract withdrawing the fluid from the wheel brakes  12   a ,  12   b ,  12   c , and  12   d . The speed and distance of the retraction of the piston  206  is based on the demands of the driver releasing the brake pedal  70  as sensed by the sensor  218 . Under certain conditions, the pressurized fluid from the wheel brakes  12   a ,  12   b ,  12   c , and  12   d  may assist in back-driving the ball screw mechanism  212  moving the piston  206  back towards its rest position. 
     In some situations, the piston  206  of the plunger assembly  18  may reach its full stroke length within the bore  200  of the housing and additional boosted pressure is still desired to be delivered to the wheel brakes  12   a ,  12   b ,  12   c , and  12   d . The plunger assembly  18  is a dual acting plunger assembly such that it is configured to also provide boosted pressure to the conduit  34  when the piston  206  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  206  has reached its full stroke, for example, and additional boosted pressure is still desired, the second plunger valve  252  is energized to its closed check valve position. The first plunger valve  250  is de-energized to its closed position. The electronic control unit  22  actuates the motor  214  in a second rotational direction opposite the first rotational direction to rotate the screw shaft  216  in the second rotational direction. Rotation of the screw shaft  216  in the second rotational direction causes the piston  206  to retract or move in the rearward direction (rightward as viewing  FIGS. 1 and 2 ). Movement of the piston  206  causes a pressure increase in the second pressure chamber  242  and fluid to flow out of the second pressure chamber  242  and into the conduit  243  and the conduit  34 . Pressurized fluid from the conduit  34  is directed into the conduits  40  and  42  through the isolation valves  30  and  32 . The pressurized fluid from the conduits  40  and  42  can be directed to the wheel brakes  12   a ,  12   b ,  12   c , and  12   d  through the opened apply valves  50 ,  54 ,  58 , and  62  while dump valves  52 ,  56 ,  60 , and  64  remain closed. In a similar manner as during a forward stroke of the piston  206 , the ECU  22  can also selectively actuate the apply valves  50 ,  54 ,  58 , and  62  and the dump valves  52 ,  56 ,  60 , and  64  to provide a desired pressure level to the wheel brakes  12   a ,  12   b ,  12   c , and  12   d , respectively. When the driver lifts off or releases the brake pedal  70  during a pressurized rearward stroke of the plunger assembly  18 , the first and second plunger valves  250  and  252  are preferably operated to their open positions, although having only one of the valves  250  and  252  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  206  and the displaced volume within the plunger assembly  18  correlate exactly with the given pressures and fluid volumes within the wheel brakes  12   a ,  12   b ,  12   c , and  12   d . However, when the correlation is not exact, fluid can be drawn from the reservoir  20  via the check valve  258  into the chamber  240  of the plunger assembly  18 . 
     During a braking event, the ECU  22  can selectively actuate the apply valves  50 ,  54 ,  58 , and  62  and the dump valves  52 ,  56 ,  60 , and  64  to provide a desired pressure level to the wheel brakes, respectively. The ECU  22  can also control the brake system  10  during ABS, DRP, TC, VSC, regenerative braking, and autonomous braking events by general operation of the plunger assembly  18  in conjunction with the apply valves and the dump valves. Even if the driver of the vehicle is not depressing the brake pedal  70 , the ECU  22  can operate the plunger assembly  18  to provide a source of pressurized fluid directed to the wheel brakes, such as during an autonomous vehicle braking event. 
     In the event of a loss of electrical power to portions of the brake system  10 , the brake system  10  provides for manual push through or manual apply such that the brake pedal unit  14  can supply relatively high pressure fluid to the conduits  36  and  38 . During an electrical failure, the motor  214  of the plunger assembly  18  might cease to operate, thereby failing to produce pressurized hydraulic brake fluid from the plunger assembly  18 . The isolation valves  30  and  32  will shuttle (or remain) in their positions to permit fluid flow from the conduits  36  and  38  to the wheel brakes  12   a ,  12   b ,  12   c , and  12   d . The simulator valve  116  is shuttled to its closed position to prevent fluid from flowing out of the input chamber  92  to the pedal simulator  16 . During the manual push-through apply, the input piston  82 , the primary piston  84 , and the secondary piston  86  will advance leftwardly such that the passageways  106 ,  136 ,  144  will move past the seals  102 ,  132 , and  140 , respectively, to prevent fluid flow from their respective fluid chambers  92 ,  94 , and  96  to the reservoir  20 , thereby pressurizing the chambers  92 ,  94 , and  96 . Fluid flows from the chambers  94  and  96  into the conduits  38  and  36 , respectively, to actuate the wheel brakes  12   a ,  12   b ,  12   c , and  12   d.    
     Referring now to  FIG. 3 , the plunger assembly includes an output housing  2128 , an electric motor  2130 , and an input housing  2132 . Supported on the housing  2128  is a plunger, indicated generally at  2134 . The plunger  2134  may be dual acting and includes a plunger head  2136  attached to a rod  2138 . The rod  2138  has a threaded portion  2140 . A ball screw assembly, indicated generally at  2142 , operates as known to those skilled in the art. The ball screw assembly  2142  includes a screw shaft  2144 , ball bearings (not illustrated) between the threaded portion  2140  and screw shaft  2144  such that the threaded portion  2140  is supported on the screw shaft  2144 . An anti-rotation tube  2146  is supported on the housing  2128  by a torque coupler, indicated generally at  2148 . The torque coupler  148  is attached or secured between the housing  2128  and anti-rotation tube  2146 . For example, the torque coupler  2148  may be press fit to each of the housing  2128  and anti-rotation tube  2146 . 
     The anti-rotation tube  2146  restrains the threaded portion  2140  from rotation. The anti-rotation tube  2146  has internal ridges  2150  corresponding to slots  2152  on the threaded portion  2140 . When the ridges  2150  are inserted in the slots  2152 , the rod  2138  is restrained from rotating. As such, when the screw shaft  2144  is driven or rotated by the motor  2130 , the head  2136  and rod  2138  move or translate in a first direction X. As illustrated, the plunger  2134  is in an unactuated, rightward position. As the motor  2130  drives the screw shaft  2144 , the head  2136  moves between the rightward position and a leftward position (not illustrated). 
     The movement of the plunger  2134  pressurizes brake fluid in first and second annular chambers  2154 A and  2154 B, respectively, such that brake pressure is generated for the brake system  2102 . The first chamber  2154 A is defined by the head  2136 , a sleeve  2156 , and an end cap  2158  and the second chamber  2154 B is defined between the head  2136 , rod  2138 , and sleeve  2156 . Typically, the first and second chambers  2154 A and  2154 B, respectively, are hydraulically linked. Pressure in the first and second chambers  2154 A and  2154 B, respectively, rises as the plunger  2134  moves away from the motor  2130  and falls as the plunger  2134  moves toward the motor  2130 . During events such as slip control, the first and second chambers  2154 A and  2154 B, respectively, may be hydraulically isolated when the plunger  2134  is moving towards the motor  2130 . When the first and second chambers  2154 A and  2154 B, respectively, are isolated, pressure in the second chamber  2154 B rises and fluid from a reservoir (not shown) flows into the first chamber  2154 A. 
     As shown in  FIG. 3 , the embodied electric motor  2130  comprises a motor housing  2710  for supporting a stator (not shown) that rotates a rotor  2740 . The motor housing  2710  comprises a generally cylindrical enclosure  2711  with a bore  2712  extending from a front side  2713  to a back side  2714  of the cylindrical enclosure  2711 . The bore  2712  opening on the back side of the cylindrical enclosure  2711  has stepped down diameter to define a front bearing stop  2718 . A bearing support surface  2719  is adjacent the front bearing stop  2718 , with a generally horizontal surface for supporting the outer race of a bearing  2720 . The stator (not shown) is mounted within the cylindrical enclosure  2711 , between the front and back sides with the outer circumferential surface of the stator adjacent the bore  2712 . The bearing  2720  is positioned within the cylindrical enclosure  2711  such that the front side of the outer race of the bearing  2720  is proximate to the front bearing stop  2718  and the outermost portion of the outer race is proximate the bearing support surface  2719 . In the illustrated embodiment, one end of the rotor  2740  is supported in the motor housing  2710  by engaging the inner race of the bearing  2720 . The rotor  2740  rotor further defines a tapered bore  2741  for receiving a corresponding tapered end  2621  of the screw shaft  2144 . In the illustrated embodiment, the screw shaft  2144  has a threaded bore  2622  for receiving a threaded fastener  2760 . When threaded into the screw shaft  2144 , the threaded fastener  2760  passes through a washer  2770  and then through the opening in the bearing  2720 . The threaded fastener  2760  is then threaded into the threaded bore  2622  of the screw shaft  2144  and thus is also passes through the center of the rotor  2740 . The clamping force provided by the threaded fastener  2760  compresses the washer  2770  against one side of the inner race of the bearing  2720  and urges the screw shaft  2144  toward the second side of the bearing  2720 . The tapered end  2621  of the screw shaft  2144  frictionally engages the tapered bore  2741  of the rotor  2740 , thus transferring a force that causes the rotor  2740  to frictionally engage the other side of the inner race of the bearing  2720 . 
     As shown in  FIG. 4 , one or more sleeve tabs  2472  project from the sleeve  2156  to form a loose interlocking connection with anti-rotation tube tabs  2434  on the anti-rotation tube  2146 . The tolerances between the anti-rotation tube tabs  2434  and the sleeve tabs  2472  will allow for some rotational movement of the anti-rotation tube  2146 , due to the elastic properties of the torque coupler  2148 , without coming into contact with each other. It is also within the scope of the invention that the tolerances between the anti-rotation tube tabs  2434  and the sleeve tabs  2472  are such that rotation of the anti-rotation tube  2430  is limited by the respective tabs coming into contact with one another. 
     There is illustrated in  FIGS. 5 and 6  an alternate embodiment of a braking system assembly  1500 . The braking assembly  1500  includes a housing  1502  generally comprised of six sides: top  1503 , bottom  1504 , first  1505 , second  1506 , front  1507 , and back  1508 . The housing  1502  may be formed as a single unit, as shown, or include two or more separately formed portions coupled together. The housing  1502  generally includes a first bore  1509  located in the first side of the housing  1502 , and a second bore  1510  extending between the front  1507  and back sides  1508 . A brake pedal unit similar to embodiments described above, but not shown in  FIGS. 5 and 14 , includes an input piston. The input piston is slidably disposed in the first bore  1509  to actuate the master cylinder grouping. A plunger assembly, indicated generally at  1400 , is slidably disposed in the second bore  1510 , the purpose and function of which will be described in further detail below. In addition, the illustrated embodiment of the housing  1502  further comprises a threaded raised end cap mount  1511  extending from the second bore for securing an end cap  1490  to the housing  1502 . In another embodiment, the raised end cap mount  1511  may be generally flush with the front surface  1507  permitting the end cap  1490  to be directly connected to the surface of the housing  1502 . The second bore  1510  may comprise multiple stepped diameters for housing various components described below. In addition, the housing  1502  may also define additional openings or bores for valves, ECU connections, reservoir connections, conduits, and brake line attachments. 
     As shown in an exploded view in  FIG. 7  and a cross sectional view in  FIG. 8 , the braking system  1500  comprises a power transmission unit grouping  1300 . The illustrated power transmission unit  1300  comprises the plunger assembly, generally indicated as  1400 , a ball nut assembly  1600 , and a motor assembly  1700 . In operation, a rotational movement of the motor assembly  1700  will drive a portion of the ball nut assembly  1600  in a rotational manner. The operation of the ball nut assembly  1600  will then convert the rotational movement of the motor assembly  1700  into a linear movement along the axis of rotation. The linear movement of a portion of the ball nut assembly  1600  is then transferred to a portion of the plunger assembly  1400  where the linear movement drives a pressurizing structure, such as a piston/cylinder arrangement, to generate a fluid pressure force that can be used for actuation of the wheel brakes. 
     As shown in  FIGS. 9-13 , the embodied motor assembly  1700  comprises an electric motor, known to those skilled in the art as having a motor housing  1710  for supporting a stator  1730  that rotates a rotor  1740 . The motor housing  1710  comprises a generally cylindrical enclosure  1711  with a bore  1712  extending from a front side  1713  to a back side  1714  of the cylindrical enclosure  1711 . A mounting surface  1715  and front lip  1716  define a front seal groove  1717  adjacent the bore  1712  opening on the front side. The bore  1712  opening on the back side of the cylindrical enclosure has stepped down diameter to define a front bearing stop  1718 . A bearing support surface  1719  is adjacent the front bearing stop  1718 , with a generally horizontal surface for supporting the outer race of a bearing  1720 . The bearing support surface  1719  is adjacent to a larger diameter bore with a horizontally extending surface forming a nut support surface  1721 . The nut support surface  1721  may further include threads or other fastening means for securing a nut  1722 . Further, the back side of the cylindrical enclosure  1711  may also comprise a back lip  1723  forming a portion of the nut support surface  1721  on the interior, and also define a back seal groove  1724  on the exterior. 
     The stator  1730  is mounted within the cylindrical enclosure, between the front and back sides with the outer circumferential surface of the stator  1730  adjacent the bore  1712 . The bearing  1720  is positioned within the cylindrical enclosure  1711  such that the front side of the outer race of the bearing  1720  is proximate to the front bearing stop  1718  and the outer diameter of the outer race is proximate the bearing support surface  1719 . The nut  1722  comprises a threaded outer circumferential surface that threads into the nut support surface  1721 . A portion of the nut  1722  will then be proximate to or abutting the outer race of the bearing  1720 , thus securing the position of the bearing  1720  relative to the cylindrical enclosure  1711 . In order to enclose the back side of the cylindrical enclosure  1711 , a back side seal  1725  is set in the back seal groove  1724 . A cover plate  1726  is mounted to the cylindrical enclosure  1711  such that the back side seal is pressed against a portion of the cover plate  1726  to prevent the ingress or egress of contaminants. In a similar regard, the front side of the cylindrical enclosure  1711  is mounted to the housing  1502  with the mounting surface  1715  proximate the back side  1508  of the housing  1502 . In order to prevent contaminant ingress between the housing  1502  and the cylindrical enclosure  1711 , a front side seal  1727  is set in the front side seal groove  1717 . The front lip  1716  is inserted into the second bore  1510  such that the front side seal  1727  engages the interior surface of the second bore  1510 . In addition, a plurality of fasteners further secure the cylindrical enclosure  1711  to the housing  1502 . 
     In the illustrated embodiment, one end of the rotor  1740  is supported in the motor housing  1710  by engaging the inner race of the bearing  1720 . As shown in  FIGS. 9 and 10 , the rotor  1740  further defines a tapered bore  1741  for receiving a corresponding tarped end  1621  of a ball screw  1620 . In the illustrated embodiment, the ball screw  1620  has a threaded bore  1622  for receiving a threaded fastener  1760 . When threaded into the ball screw  1620 , the threaded fastener  1760  passes through a washer  1770  and then through the opening in the bearing  1720 . The threaded fastener  1760  is then threaded into the threaded bore  1622  of the ball screw  1620  and thus is also passes through the center of the rotor  1740 . As shown in  FIGS. 9 and 10 , the fastener  1760  includes a tapered head  1762  that engages a mating tapered surface  1772  on the washer  1770 . The fit between these mating tapers is such that contact between the surfaces initiates at a point near the base of the head toward the threaded section of the fastener  1760 . As the fastener is drawn into contact with the washer, the tapered surfaces come together though the stress distribution along the taper interface is higher toward the threaded end. This “oil can” effect creates a non-uniform load and stress distribution along the taper interface. This non-uniform stress distribution at the head end of the fastener causes the resultant preload forces acting on the connection to be applied toward the threads of the fastener, rather than concentrated at the fastener/washer interface. By shifting the preload forces into the threads, the integrity of the connection is maintained in response to repeated torsional impact loads from inertial forces during the many cyclic start/stop events as the plunger system is operated. 
     The clamping force provided by the threaded fastener  1760  compresses the washer  1770  against one side of the inner race of the bearing  1720  and urges the ball screw  1620  toward the second side of the bearing  1720 . The tapered end  1621  of the ball screw  1620  frictionally engages the tapered bore  1741  of the rotor  1740 , thus transferring a force that causes the rotor  1740  to frictionally engage the other side of the inner race of the bearing  1720 . In one embodiment, this frictional fit is the primary torque driving mechanism between the rotor  1740  and the ball screw  1620 . In this particular embodiment, the degree of frictional fit between the ball screw taper  1621  and mating taper bore  1741  of the rotor  1740  does not cause an expansion of the rotor hub that engages the inner race of the bearing. By preventing a radial preload of the inner race of the bearing, the fit of the rolling elements between the inner and outer races remains generally unchanged, thus reducing parasitic losses from increased frictional forces. 
     In an alternate embodiment, however, a bearing assembly having clearances sufficient to permit radial expansion of the inner race may be provided. In this embodiment, as the tapered end  1621  is drawn within the tapered bore  1741 , a portion of the rotor  1740  that extends into the inner diameter of bearing  1720  may be displaced radially against the inner race of the bearing  1729 . The added clearance in the bearing is taken up by radial expansion to provide a desired rolling element fit. Thus, the connection of the ball screw to rotor is further compressed to provide additional compressive stresses to resist torsional impact loads, experienced during operation, that resist loosening of the connection. 
     In addition, it is within the scope of the invention that the ball screw  1620  may have one or more flats  1624  adjacent or as part of the tapered end  1621 , as shown in  FIG. 13 . The one or more flats  1624  correspond to one or more respective flats  1744  on the rotor  1740  surface. When assembled, the ball screw flats  1624  and rotor flats  1744  will be in an abutting or nearly abutting relationship. Thus, it is within the scope of the invention that a redundant torque transfer mechanism is provided between the rotor  1740  and the ball screw  1620 , in the event of a loss of friction between the tapered surfaces ( 1621  and  1741 ). It is further within the scope of this invention that one of the above described torque transmission mechanisms can be provided without the other mechanism and retain the intended purpose and operation of the braking system  1500 . 
     As shown in  FIG. 14 , the ball nut assembly  1600  further includes a ball nut  1650  with ball bearings that engage the ball screw  1620  in a manner known to those skilled in the art. For example, complementary grooves are formed on the inner surface of the ball nut  1650  bore which correspond to groove on the ball screw  1620 . The ball bearings are located between the nut grooves and ball screw grooves, and as such engage both the screw  1620  and the ball nut  1650  and transfer torque there between. As best shown in  FIG. 15 , one end the ball nut  1650  includes a threaded end for receiving a portion of the plunger assembly  1400 . Further, the outer circumferential surface of the ball nut  1650  can include one or grooves  1652  that correspond to one or more splines  1432  on an anti-rotation tube  1430 . 
     As shown in  FIG. 15 , the illustrated plunger assembly  1400  is connected to the ball nut assembly  1600  by a rod  1410  with a threaded portion that is threaded into the threaded end of the ball nut  1650 . In operation of the braking system  1500 , rotational movement of the motor assembly  1700  is transferred to the ball screw  1620 . The anti-rotation tube  1430  is provided between the housing  1502  and the ball nut  1650  to prevent rotational movement of the ball nut  1650  as a result of the rotational forces being transferred by the ball screw  1620 . As shown in  FIGS. 14 and 15 , the one or more splines  1432  on anti-rotation tube  1430  will engage the grooves  1652  on the ball nut  1650  to minimize relative rotation between the ball nut  1650  and portions of the plunger assembly connected to the housing to ensure the transfer of rotary to linear motion of the ball screw assembly. When the splines  1432  are inserted into the slots  2152 , rotation of the ball screw  1620  causes the ball nut  1650  to translate axially along the ball screw  1620 . In the illustrated embodiment, the anti-rotation tube  1430  defines the full length of travel of the ball nut  1650  and supports the ball nut  1650  over the full length of travel. As such, the anti-rotation tube  1430  provides substantial support the ball nut  1650  during actuation of the motor assembly  1700 , allowing the braking system to be sufficiently radially supported with the use of only one bearing, such as bearing  1720 . In one embodiment, bearing  1720  may be a four-point ball bearing. Alternatively, bearing  1720  may be a barrel or roller thrust bearing. 
     As shown in  FIG. 16  and  FIG. 17 , the anti-rotation tube  1430  is supported on the housing  1502  by a torque coupler  1450 . In the illustrated embodiment, the torque coupler  1450  is press fit or otherwise fixed into the housing  1502  and onto the outer surface of the anti-rotation tube  1430 . Frictional engagement between the torque coupler  1450 , the housing  1502 , and the anti-rotation tube  1430  is sufficient to prevent rotational slipping between the components. As shown in  FIG. 22 , the torque coupler  1450  includes a center elastomer sleeve  1451  having metal rings  1452  and  1453  crimped to its ends. The ring  1452  is press fit onto the anti-rotation tube  1430 , and the ring  1453  is press fit in the housing  1502 . Additional fasteners or fastening mechanisms can also be utilized in the aforementioned friction connections without deviating from the scope of the invention. For example, the surface of the anti-rotation tube  1430  can be knurled to increase the frictional engagement with the torque coupler  1450 . The torque coupler  1450  is formed from a material with elastic properties such that the torque coupler  1450  can deflect torsionally about the axis of rotation of the ball screw  1620 . In one embodiment, the torque coupler material may be a synthetic rubber and, in particular, may be an EPDM rubber material. In addition, the torque coupler  1450  accommodates torsional movement, and thus provides torsional isolation, of the anti-rotation tube  1430  relative to the housing  1502  in both the clockwise and counter-clockwise directions. The elastic deflection of the torque coupler  1450  in the rotational direction provides isolation and damping of torsional load spikes when the motor assembly  1700  begins rotation from a stopped position or reverses the direction of rotation, as the moment of inertia of the braking system  1500  components will result in higher torsional strains on the system. During such situations, the elastic deflection of the torque coupler  1450  will absorb spikes in shear strain, while also deflecting back to its normal operating position after the initial acceleration of the of the motor assembly  1700 . In a similar regard, the torque coupler  1450  can absorb shock forces that move the anti-rotation tube  1430  out of axial alignment with the ball nut  1650 , but then return the anti-rotation tube  1430  to the correct position due to the elastic properties of the torque coupler  1450 . 
     The rod  1410  is further provided with a threaded bore at one end that corresponds with a threaded end on a plunger head  1480  as shown in  FIG. 18  and  FIG. 19 . While the illustrated embodiment describes a threaded connection between the plunger head  1480  and the rod  1410 , it is within the scope of the invention that other fastening mechanisms can be employed without deviating from the scope of the invention. It is further within the scope of the invention that the rod  1410  and plunger head  1480  can alternately be formed as one continuous part. As illustrated in  FIG. 18 , the plunger head  1480  also defines a widened portion with a plunger head groove  1481  extending circumferentially around. A plunger head seal  1485 , such as an energized Teflon seal  1485  shown in the illustrated embodiment, is seated within the plunger head groove  1481  and abuts the interior surface of a sleeve  1470 . While described as a Teflon seal, the plunger head seal  1485  may be any material or configuration that provides a generally low coefficient of friction contact during relative motion of the plunger head  1480  relative to the bore it is received within. The illustrated embodiment further includes a plunger head O-ring  1486  between the energized Teflon seal  1485  and the bottom of the plunger head groove  1481 . The plunger head O-ring  1486  acts as a resilient support member beneath the energized Teflon seal  1485 , such that the energized Teflon seal  1485  is urged in a radially outward direction. The energized Teflon seal  1485  and O-ring  1486  combination acts as a resilient member that radially positions and supports plunger head  1480  within a bore in sleeve  1470 . However, it is within the scope of the invention that the O-ring  1486  can be incorporated into the energized Teflon seal  1485  while providing the same positioning characteristics described above. 
     The implementation of the energized Teflon seal  1485  provides support to the plunger head  1480 , particularly in the extended position, by acting as a bearing surface. Thus, the embodied braking system  1100  requires only one bearing  1720  located generally opposite the plunger head  1480 . However, it is within in the scope of the invention that two or more bearings can be used within the plunger assembly  1400 . In addition, the circumferential surfaces located on the front  1482  and back  1483  of the plunger head groove  1481  may be stepped to such that the front surface  1482  is of a greater or smaller diameter relative to the back surface  1483 . The stepped design allows for controlled deformation of the energized Teflon seal  1485  while preventing extrusion of the seal in either the forward or reward direction. In addition, the energized Teflon seal  1485  can be readily installed onto the plunger head  1480  by slipping the energized Teflon seal  1485  over the small diameter surface. 
     The plunger assembly  1400  can further comprise a crash washer  1487 , for example a spring washer, Bellville washer, or other resilient member, seated between the plunger head  1480  and the rod  1410 . In an actuated position, for example the position shown in  FIG. 18 , the crash washer  1487  is in a non-deflected state. In the event of a brake system failure or power loss scenario during retraction of the plunger assembly  1400 , the inertia of the plunger assembly  1400  may continue in the retracting direction. While the viscosity of the fluid being forced through the system can reduce or stop this inertial movement, additional measures may be needed to prevent the plunger assembly from damaging components if the plunger head contacts a hard-stop surface. Thus, the arraignment of crash washer  1487  allows it to engage a sleeve park position surface  1471  before the plunger head  1480 , causing the crash washer  1487  to deflect and absorb the inertial forces, as shown in  FIG. 22 . 
     In an embodiment, the crash washer  1487  can be used to determine the park position of the plunger. For example, when the crash washer  1487  abuts a surface of the housing  1502  or a surface connected to the housing  1502 , the motor assembly  1700  will experience increased current or power draw due to the spring forces of the crash washer. Various electrical, software, and or electromechanical means can be used to detect the increased current or power draw to signal the braking system that the plunger assembly  1400  is in the parked position. In  FIG. 22 , the plunger assembly  1400  is illustrated in a position wherein the crash washer  1487  is in a fully deflected state between the plunger head  1480  and the sleeve  1470 . The position of the plunger assembly  1400  in  FIG. 22  can also illustrate a park position of the plunger assembly  1400  when no pressure is applied by the braking system  1500 . 
     As shown in  FIG. 20 , the sleeve  1470  is located within the second bore  1510  of the housing  1502  but extends beyond the length of the second bore  1510 . As further illustrated in  FIG. 21 , the one or more tabs  1472  of the sleeve form an interlocking connection with the tabs  1434  of the anti-rotation tube. The interlocking connection may accommodate various degrees of relative motion between the sleeve  1470  and the anti-rotation tube  1430 , from a loose fit, where no contact is made when the torque coupler  1450  is operable, to a contacting fit that permits relative torsional movement and provides limits to the torsional deflections of the torque coupler  1450  during operation. The tolerances between the anti-rotation tube tabs  1434  and the sleeve tabs  1472  will allow for some rotational movement of the anti-rotation tube  1430 , due to the elastic properties of the torque coupler  1450 , without coming into contact with each other. It is also within the scope of the invention that the tolerances between the anti-rotation tube tabs  1434  and the sleeve tabs  1472  are such that rotation of the anti-rotation tube  1430  is limited by the respective tabs coming into contact with one another. In another embodiment, one set or both sets of the interconnecting tabs  1434  and  1472  may be coated with a resilient material in order to provide a secondary cushioning effect when contact of the interconnecting tabs  1434  and  1472  may occur. 
     In addition, a pin  1475  is secured to the housing  1502  such that one end is inserted into an aperture in the housing and the other end of the pin is disposed in a detent  1473  in at least one of the sleeve tabs  1472  or in the sleeve  1470 . During assembly, the pin  1475  aligns a series of sleeve apertures  1476 , shown in  FIG. 21 , the purpose of which is further discussed below, to the upright position. After assembly, the pin  1475  functions as a fail-safe mechanism in the event that the sleeve  1470 , anti-rotation tube  1430 , or torque coupler  1450  decouple from the housing  1502 . In such an event, the pin  1475  would maintain the position of the sleeve  1470  relative to the housing  1502  and the interlocking connection between the anti-rotation tube tabs  1434  and the sleeve tabs  1472  will resist rotational movement of the anti-rotation tube. 
     As best shown in  FIG. 18 , grooves  1477 , illustrated as three grooves though more or fewer may be used, are defined on the outer circumferential surface of the sleeve  1470 , each holding seal  1478 , such as an O-ring seal  1478 . The sleeve  1470  is inserted into the housing  1502  such that the three or more O-ring seals  1478  engage an inner surface of the housing second bore  1510 . As illustrated in  FIG. 21 , the sleeve can further comprise the series of apertures  1476  located between the three or more O-ring seals  1478 . When the sleeve  1470  is located within the housing bore, the O-rings  1478  will establish hydraulically separated volumes  1514  as illustrated in  FIG. 18 . Further, the apertures located between the O-rings  1478  allow fluid to flow between an inner diameter of the sleeve  1470  and an outer diameter of the sleeve  1470 . Thus, selective fluid communication can occur between a source of fluid at or near the center of the plunger assembly and a passageway extending from the housing second bore  1510  of the housing  1502  by a movable aperture or passageway, as will be described below. 
     As illustrated in  FIG. 18 , the sleeve  1470  comprises a bore with a diameter sized to correspond to the diameter of the rod  1410 . The sleeve bore also defines two circumferential grooves in which an “L” shaped seal  1422  and “E” shaped seal  1424  are seated within. When the rod  1410  and plunger head  1480  are positioned in the sleeve  1470  bore, as illustrated in  FIG. 19 , the “L” shaped seal  1422  and “E” shaped seal  1424  abut the outer diameter of the rod  1410 . The abutting connection is such that the rod  1410  and plunger head  1480  can freely move forward and backward relative to the sleeve  1470 , but also establishes hydraulically separate spaces. The space located between the energized Teflon seal and the “E” shaped seal  1424  establishes the second pressure chamber. The passageway  1421  between the “E” shaped seal  1424  and the “L” shaped seal  1422  establish a flow path for fluid leaving rod flow metering cross holes  1412  in the rod  1410 . The area behind the “L” shaped seal  1422  is generally intended to be free of fluid, but the illustrated embodiment further comprises a drip chamber or drip path  1423 , shown in  FIG. 24 , for the collection of any fluid due to weepage around the “L” shaped seal  1422 . 
     As shown in  FIG. 22 , the illustrated rod flow metering cross holes  1412  allow venting of the brakes to the reservoir in boost mode. In slip control, it allows the release of pressure by pulling the plunger back to vent to the reservoir without the need for dump valve actuation. In addition, the rod flow metering cross holes  1412  allow fluid to flow between the park position relief conduit  1515  and the second pressure chamber. When the plunger assembly  1400  is advanced and retracted to supply pressurized fluid to the system, differences between the actual plunger assembly position and the computed plunger assembly position can arise. Thus, it is important to reestablish the park position to prevent over extension or retraction of the plunger assembly. In reestablishing the park position, the braking system  1500  can retract the plunger assembly  1400  back towards the park position. When the correct positioning is arrived at, the rod flow metering cross holes  1412  will align with the park position relief conduit  1515 . As a result, the pressure in the second chamber will decrease, which can be detected by the braking system  1500  by either fluid flow or pressure change, and the system will stop the retraction of the plunger assembly  1400  and set the current position as the park position. Another advantage of the rod flow metering cross holes  1412  aligning with the park position relief conduit  1515  arises when a boosted braking event has completed and it is desired for the wheel brakes to be unactuated. While the built up pressure in the braking system  1500  could be relieved by actuation of the dump valves. 
     In order to enclose the opening provided by the second bore  1510  in the housing  1502 , an end cap  1490  is secured to the end cap mount  1511 . In the illustrated embodiment, the end cap  1490  is a generally hollow cylindrical device with an open threaded end. When secured to the housing, a threaded end of the end cap  1490  engages the end cap mount  1511 . Further, an end cap seal  1492  can be placed between the end cap  1490  and the housing  1502  to prevent the ingress of contaminants and egress of fluid. As shown in  FIGS. 18 and 22 , the inner bore diameter of the end cap  1490  is greater than the outer diameter of the sleeve  1470 . In operation, fluid is allowed to pass between the sleeve  1470  and the end cap  1490 , such that the end cap  1490  acts as a pressure barrel chamber for the braking system. Thus, the co-axial positioning of the end cap  1490  and the sleeve  1470  cooperates to create a fluid pathway to the first pressure chamber without the need for additional conduits or passageways. 
     As shown in  FIG. 24 , flow slots  1479  in the sleeve  1470  allow fluid to flow from the first pressure chamber to a conduit or from a conduit to the first pressure chamber. In the illustrated embodiment, the flow slots  1479  are evenly spaced about the circumference of sleeve  1470  to improve flow in the first pressure chamber. However, it is also within the scope of invention that the flow slots  1479  can be located on only a portion of the sleeve  1470 . For example, locating the flow slots  1479  or a single larger opening at the top of the sleeve  1470  would aid in the bleeding the braking system  1500  of fluid during a maintenance procedure. 
     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.