Patent Publication Number: US-11661045-B2

Title: Simulator valve

Description:
TECHNICAL FIELD 
     This disclosure relates to an apparatus and method for use of a simulator valve and, more particularly, to a method and apparatus of a brake pedal simulator valve for controlling and/or adjusting brake pedal drop during transition between manual apply and boosted braking. 
     BACKGROUND 
     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 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. 
     During initial movement of the brake pedal unit in boosted mode, the driver pushes on the brake pedal, causing initial movement of an input piston of the master cylinder. Further movement of the input piston will pressurize the input chamber of the master cylinder, causing fluid to flow into a pedal simulator. As fluid is diverted into the pedal simulator, a simulation pressure chamber within the pedal simulator will expand, causing movement of a piston within the pedal simulator. Movement of the piston compresses a spring assembly housed within the pedal simulator and biasing the piston to provide a feedback force to the driver of the vehicle via the brake pedal which simulates the forces a driver feels at the brake pedal in a conventional vacuum assist hydraulic brake system, for example, and therefore is an expected and comforting “brake feel” for the driver. 
     When the vehicle is first started, the brake fluid is under little to no pressure. In certain cases, the driver manually applies the brake in a “push-through” condition, in which the master cylinder directly energizes pressure to at least two, and often four, of the wheel brakes. As the brake system comes online and pressure builds, the system transitions to a “boost” mode wherein the booster is used to supplement or supplant pressurized fluid sent to the wheel brakes from the driver&#39;s push-through force on the brake pedal. However, this is not always a smooth transition and can result in a “pedal drop” condition when the pedal simulator is pressurized that can be discomfiting to the driver. 
     Descriptions of prior art brake systems are in U.S. Pat. No. 10,730,501, issued 4 Aug. 2020 to Blaise Ganzel and titled “Vehicle Brake System with Auxiliary Pressure Source”, and in U.S. Patent Application Publication No. 2020/0307538, published 1 Oct. 2020 by Blaise Ganzel and titled “Brake System with Multiple Pressure Sources”, both of which are incorporated herein by reference in their entirety for all purposes. 
     SUMMARY 
     In an aspect, a simulator valve is described. A housing has a center bore extending longitudinally from a first housing surface. The housing includes a pedal simulator passage extending therethrough to at least partially place the center bore in fluid communication with a pedal simulator. The housing includes a master cylinder passage extending therethrough to at least partially place the center bore in fluid communication with a master cylinder. The master cylinder passage is located longitudinally between the first housing surface and the pedal simulator passage. An armature is located at least partially within the housing for selective longitudinally reciprocating motion with respect thereto between first and second armature positions. A poppet is located within the housing and is at least partially located within an armature bore of the armature for selective longitudinally reciprocating motion with respect thereto between first and second poppet positions. The poppet defines a first valve cooperatively with a first valve seat of at least a portion of the armature bore. The poppet at least partially defines a second valve longitudinally spaced from, and oppositely facing, the first valve seat. The second valve includes a second valve seat located within the center bore and at least partially spaced apart from a bore wall of the center bore. The poppet includes a poppet bore extending longitudinally therethrough and selectively occluded by the first valve. The armature, poppet, and center bore cooperatively define a damped flow fluid path therebetween. The damped flow fluid path selectively permits fluid communication therethrough from the master cylinder passage to the pedal simulator passage. The damped flow fluid path permits fluid communication therethrough when the armature is in the second armature position and the poppet is in the first poppet position. The armature, poppet, and center bore cooperatively define a free fluid path therebetween. The free flow fluid path selectively permits fluid communication therethrough from the pedal simulator passage to the master cylinder passage. The free flow fluid path permits fluid communication therethrough when the armature is in the second armature position and the poppet is in the second poppet position. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a better understanding, reference may be made to the accompanying drawings, in which: 
         FIG.  1    is a schematic cross sectional side view of a simulator valve according to an aspect of the present invention, in a first condition; 
         FIG.  2    is a schematic cross sectional side view of the simulator valve of  FIG.  1    in a second condition; 
         FIG.  3    is a schematic cross sectional side view of the simulator valve of  FIG.  1    in a third condition; 
         FIG.  4    is a schematic hydraulic diagram, including the simulator valve of  FIG.  1   , in a first phase of operation; 
         FIG.  5    is a schematic hydraulic diagram, including the simulator valve of  FIG.  1   , in a second phase of operation; and 
         FIG.  6    is a schematic hydraulic diagram, including the simulator valve of  FIG.  1   , in a third phase of operation. 
     
    
    
     DESCRIPTION OF ASPECTS OF THE DISCLOSURE 
     Unless defined otherwise, all technical and scientific terms used herein have the same meaning as is commonly understood by one of ordinary skill in the art to which the present disclosure pertains. 
     The invention comprises, consists of, or consists essentially of the following features, in any combination. 
       FIG.  1    depicts a simulator valve  100 , comprising a housing  102  having a center bore  104  extending longitudinally downward from a first housing surface  106 . Although the valve  100  is shown in cross-section in  FIGS.  1 - 3   , one of ordinary skill in the art will readily be able to envision the manner in which the component parts of the valve  100  interact three-dimensionally (e.g., in sealing and/or fluid communicating manners) with each other, given the teachings of the present invention. The housing  102  includes a pedal simulator passage  108  extending therethrough to at least partially place the center bore  104  in fluid communication with a pedal simulator (shown schematically at  110 ). The housing  102  includes a master cylinder passage  112  extending therethrough to at least partially place the center bore  104  in fluid communication with a master cylinder (shown schematically at  114 ). The master cylinder passage  112  is imposed longitudinally between the first housing surface  106  and the pedal simulator passage  108 . 
     Any suitable number, configuration, and style of additional structures may be provided to the valve  100  to facilitate assembly and/or use thereof, such as, but not limited to, the stepped inner bore IB shown in the Figures. 
     An armature  116  is located at least partially within the housing  102  for selective longitudinally reciprocating motion with respect to the housing  102 . The armature  116  moves between first and second armature positions in any desired manner, such as the electrically and/or magnetically controlled and exerted forces described below with reference to  FIGS.  1 - 3   . 
     A poppet  118  is located within the housing  102  and is at least partially located within an armature bore  120  of the armature  116 . The poppet  118  is configured for selective longitudinally reciprocating motion with respect to the armature between first and second poppet positions in any desired manner, such as the electrically and/or magnetically controlled and exerted forces described below with reference to  FIGS.  1 - 3   . The poppet  118  at least partially defines a first valve (shown schematically at the area indicated by  122 ) cooperatively with a first valve seat  124  of at least a portion of the armature bore  120 . 
     As shown in the Figures, the first valve  122  may include a valve ball  126  maintained in longitudinal position with respect to the first valve seat  124  of the armature bore  120 . When present, the valve ball  126  may be press-fit into a portion of the armature bore  120 , and may be provided for self-sealing, wear-compensation, and/or any other purpose. As shown here, the valve ball  126  interacts sealingly with a shoulder of the poppet  118  to collectively form the first valve  122 . 
     The poppet  118  also at least partially defines a second valve (shown schematically at the area indicated by  128 ) at a position longitudinally spaced from, and oppositely facing, the first valve seat  124 . The second valve  128  includes a second valve seat  130  located within the center bore  104  and at least partially spaced apart from a bore wall  132  of the center bore  104 . The poppet  118  includes a poppet bore  134  extending longitudinally therethrough. The poppet bore  134  is selectively occluded by the first valve  122  (e.g., and as will be presumed below, by the valve ball  126  of the first valve  122 ). 
       FIG.  1    depicts a situation in which the simulator valve  100  is de-energized, or closed. This situation occurs, for example, when the vehicle and/or a brake system including the simulator valve  100  is in a deactivated state. As can be seen in  FIG.  1   , a magnetic gap  136  is present between the armature  116  and a core  138 , and a core spring  140  is urging the armature  116  downward, in the orientation of  FIG.  1   , to simultaneously close both the first and second valve seats  124  and  130  by pushing the armature  116  downward upon the poppet  118 . 
     The core  138  is provided for selectively magnetically attracting a first end  142  of the armature  116  longitudinally. The armature  116  is itself longitudinally interposed between the core  138  and the poppet  118 . As shown in the Figures, an armature-attracting face  144  of the core  138  may be substantially planar. This is in contrast to the stepped armature-attracting face of known prior art two-stage simulator valves, and may be helpful in attracting the first end  142  of the armature with more efficient and forceful motion than in those known valves. For example, the armature  116  does not need to travel as far to close the magnetic gap  136  with the depicted, substantially planar armature-attracting face  144 , compared to known devices. 
     As mentioned above, the core spring  140  may be interposed longitudinally between the armature  116  and the core  138  to normally bias the armature  116  longitudinally away from the core  138 . Magnetic force from the core  138  must then overcome the spring force of the core spring  140  to move the armature  116  from the first position, shown in  FIG.  1   , to the second position, shown in  FIG.  2   . 
     Also as shown in the Figures, a core sleeve  146  may be received at least partially in the center bore  104  of the housing  102  to maintain the core  138  in a predetermined spaced relationship therewith. The armature  116  is at least partially enclosed within the core sleeve  146 , and is guided thereby for selective longitudinal reciprocating motion with respect to the core  138 , between the first and second armature positions. 
     It is contemplated that the core sleeve  146  may entirely longitudinally enclose the armature  116  therein. The core  138  may be located at a first end  148  of the core sleeve  146 . The core sleeve  146  defines the second valve seat  130  at or adjacent a second end  150  of the core sleeve  146 , the second end  150  being longitudinally spaced from the first end  148 . As shown in the Figures, the second valve seat  130  may be provided by a shoulder of the core sleeve  146 . 
     As previously mentioned,  FIG.  1    depicts the armature  116  in a first (lowermost) armature position. The poppet  118  is maintained in the first (lowermost) poppet position by the armature  116  when the armature  116  is in that first armature position, by virtue of the core spring  140  pushing down on the armature  116  and thereby preventing both the armature  116  and the poppet  118  from traveling upward, in the orientation of  FIG.  1     
     Turning now to  FIG.  2   , the simulator valve  100  is shown in a second, “transition” configuration. (The transition configuration is also the boosted configuration when applying the brake pedal.) The second configuration may occur, for example, when the vehicle is first being started and the brake system of the vehicle is transitioning between a manual apply mode for the brakes and a boosted mode. The armature  116 , poppet  118 , and center bore  104  collectively and cooperatively define a damped flow fluid path therebetween during this initial or “transition” period of operation of the simulator valve  100 ; this damped fluid flow path is indicated by arrows “D” in  FIG.  2   . The damped flow fluid path D selectively permits fluid communication therethrough from the master cylinder passage  112  to the pedal simulator passage  108 . The damped flow fluid path D permits fluid communication therethrough when the armature  116  is in the second, raised armature position (by virtue of being attracted magnetically to the core  138 ) and the poppet  118  is still in the first, lowermost poppet position. As a result, fluid can be sent from the master cylinder  114  (energized at least partially by pressure of the driver&#39;s foot upon the brake pedal) to the pedal simulator  110  through the damped fluid flow path D when the brake system is transitioning from manual apply to boosted apply. 
     During the transition phase, with the simulator valve  100  in the configuration shown in  FIG.  2   , the dual acting plunger of the brake system pushes fluid into the pedal simulator  110 . Active pedal feel control for the driver is then based on the pedal travel sensor, as well as pressure sensor feedback. 
     When a valve ball  126  is present, longitudinal motion of the poppet  118  away from the armature  116  opens the first valve  122  for fluid flow (that is, the damped fluid flow path D) past the valve ball  126  and into the poppet bore  134 . This fluid flow path is created at least partially because the valve ball  126  is maintained (e.g., via a frictional fit with the armature bore  120 ) in engagement with the armature  116 , which moves away from the poppet  118  during the transition from the configuration of  FIG.  1    to that of  FIG.  2   . As a result of this creation of damped fluid flow path D, brake fluid can travel from the dual acting plunger and/or the master cylinder  114  to the pedal simulator  110 . 
     With reference now to  FIG.  3   , backpressure resulting from a buildup of fluid within the pedal simulator  110  has pushed the poppet  118  upward, in the orientation of  FIG.  3   , into the second poppet position. This is the “boosted” configuration when the driver is releasing the brake pedal. Here, the first valve  122  is once more closed, with renewed contact between the valve ball  126  and the poppet  118 . Accordingly, a fluid path through the poppet bore  134  is also no longer available. As a result, the armature  116 , poppet  118 , and center bore  104  cooperatively define a free fluid path therebetween; this relatively undamped fluid flow path is indicated by arrows “F” in  FIG.  3   . The free flow fluid path F selectively permits fluid communication therethrough from the pedal simulator passage  108  to the master cylinder passage  112 . 
     The free flow fluid path F permits fluid communication therethrough when the armature  116  is in the second armature position and the poppet  118  is in the second poppet position. Once the free flow fluid path F is established, pedal simulator  110  can be used to selectively feed pressure back into the master cylinder  114  to assist with achieving a suitable pedal feel for the driver, or for any other desired reason. The dual acting plunger will continue to build higher boosted pressure during this mode, based on the driver&#39;s request, conveyed via the brake pedal. 
       FIGS.  2  and  3    show the “boosted apply and release” modes, respectively, for the simulator valve  100 , indeed, for the brake system as a whole, and it persists until such time as the vehicle is turned off, or some other event occurs which prompts cessation of the boosted apply situation. The simulator valve  100  is “open” anytime boost is active. Which one(s) of the first and second valve seats  124  and  130  is(are) open depends on if the driver is applying or releasing the brake pedal. Both of the first and second valve seats  124  and  130  could be partially open if the driver is not applying or releasing the brake pedal. 
     In the event of a loss of power to the solenoid controlling the core  138 , either intentional or not, the core spring  140  will overcome the force previously provided by the now-de-energized solenoid and applied magnetically via the core  130 , to push the armature  116  back down to the first armature position and thus reestablish the magnetic gap  136  and return the simulator valve  100  to a “manual apply” mode. 
     With reference again back to  FIG.  1   , it is contemplated that fluid flow may be permitted along the free flow fluid path F when the armature  116  is in the first armature position and the poppet  118  is in the first poppet position, as shown in  FIG.  1   . This flow may be permitted when the pressure in the pedal simulator  110  is greater than the pressure in the master cylinder  114  by an amount that can overcome the spring (and other) forces acting on the area of the second valve seat  130 . That is, a predetermined amount of fluid may be permitted to flow through the second valve  128 , between the second valve seat  130  and the poppet  118 , even with the simulator valve  100  in the manual apply, “closed” or de-energized, position. This permissive quality may be helpful in achieving desired efficiencies in the system, as well as brake pedal feel and response characteristics for the driver, and will occur, for example, when fluid pressure in the pedal simulator  110  is higher than fluid pressure in the master cylinder  114 . To emphasize the situation of  FIG.  1   , however, the free and damped flow fluid paths F and D are both substantially blocked from fluid flow when the armature  116  is in the first armature position and the poppet  118  is in the first poppet position—that is, both are in their lowermost position, in the orientation of  FIGS.  1 - 3   . 
     It is also contemplated that at least one of the free and damped fluid flow paths F and D may include at least one filter  152  (two shown in  FIGS.  1 - 3    by way of example) for filtering fluid flow therethrough. When present, the filter(s)  152  may be of any desired type, and may be located in any desired position in the simulator valve  100 . For example, the depicted lower filter  152  is located below the poppet  118  in the center bore  104 . It is contemplated that a cylindrical upper filter  152  could also or instead be located in an area substantially surrounding the poppet  118 ; that is, immediately between the master cylinder passage  112  and the core sleeve  146 . One of ordinary skill in the art will be readily able to provide one or more suitable filters  152 , as desired for a particular use environment of the present invention. 
       FIGS.  4 - 6    schematically depict a brake system  200  utilizing the simulator valve  100  of  FIGS.  1 - 3   . In  FIGS.  4 - 6   , the “bold”, or heavy, lines indicate a component or portion of the brake system  154  which is under fluid pressure. The dotted lines indicate a component or portion of the brake system  200  which is subject, in the modes shown in the Figures, to both pressure and flow of fluid. 
     The simulator valve  100  is not limited to use in the brake system  200  of  FIGS.  4 - 6   , but could be used in any suitable setting in which magnetically-actuated control of hydraulic flow is desirable. For example, the simulator valve  100  could be used in the brake systems shown and depicted in co-pending patent applications U.S. patent application Ser. No. 17/88,227, filed concurrently herewith and titled “Hydraulic Brake Boost”, U.S. patent application Ser. No. 17/188,363, filed concurrently herewith and titled “Apparatus and Method for Control of a Hydraulic Brake System”, and/or U.S. patent application Ser. No. 17/188,288, filed concurrently herewith and titled “Apparatus and Method for Control of a Hydraulic Brake System”, all of which are hereby incorporated by reference in their entirety for all purposes. 
     The brake system  200  is a hydraulic boost braking system in which boosted fluid pressure is utilized to apply braking forces for the brake system  200 . The brake system  200  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  200  can be provided with other braking functions such as anti-lock braking (ABS) and other slip control features to effectively brake the vehicle. Components of the brake system  200  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  200 , there are four wheel brakes  202 A,  202 B,  202 C, and  202 D. The wheel brakes  202 A,  202 B,  202 C, and  202 D can have any suitable wheel brake structure operated by the application of pressurized brake fluid. Each of the wheel brakes  202 A,  202 B,  202 C, and  202 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  202 A,  202 B,  202 C, and  202 D can be associated with any combination of front and rear wheels of the vehicle in which the brake system  200  is installed. For example, the brake system  200  may be configured as a diagonally split system, as shown, such that a master cylinder secondary pressure circuit is associated with providing fluid to the diagonal wheel brakes  202 A and  202 B, and a master cylinder primary pressure circuit is associated with providing fluid to the diagonal wheel brakes  202 C and  202 D. In this example, the wheel brake  202 A may be associated with a right rear wheel of the vehicle in which the brake system  200  is installed, and the wheel brake  202 B may be associated with the left front wheel. The wheel brake  202 C may be associated with the left rear wheel, and the wheel brake  202 D may be associated with the right front wheel. Alternatively, though not depicted here, the brake system  10  may be configured as a vertical split brake system such that the wheel brakes  202 A and  202 B are associated with wheels at the front or rear axle of the vehicle, and the wheel brakes  202 C and  202 D are associated with wheels at the other axle of the vehicle. 
     The brake system  200  generally includes a brake pedal unit, indicated generally at  204 , a pedal simulator, indicated generally at  110 , a plunger assembly (also known as a dual acting plunger), indicated generally at  208 , and a fluid reservoir  210 . The reservoir  210  stores and holds hydraulic fluid for the brake system  200 . The fluid within the reservoir  210  is preferably held at or about atmospheric pressure, but the fluid may be stored at other pressures if desired. The reservoir  210  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  210  and are provided to prevent complete drainage of the reservoir  210  in case one of the sections is depleted due to a leakage via one of the three lines connected to the reservoir  210 . Alternatively, the reservoir  210  may include multiple separate housings. The reservoir  210  may include at least one fluid level sensor  212  for detecting the fluid level of one or more of the sections of the reservoir  210 . 
     The plunger assembly  208  of the brake system  200  functions as a source of pressure to provide a desired pressure level to the wheel brakes  202 A,  202 B,  202 C, and  202 D during a typical or normal brake apply. After a brake apply, fluid from the wheel brakes  202 A,  202 B,  202 C, and  202 D may be returned to the plunger assembly  208  and/or diverted to the reservoir  210 . In the depicted embodiment, the plunger assembly  208  is a dual acting plunger assembly which is configured to also provide boosted pressure to the brake system  200  when a piston of the plunger assembly  208  is stroked rearwardly as well as forwardly. 
     The brake system  200  also includes at least one electronic control unit or ECU  214 . The ECU  214  may include microprocessors and other electrical circuitry. The ECU  214  receives various signals, processes signals, and controls the operation of various electrical components of the brake system  200  in response to the received signals. The ECU  214  can be connected to various sensors such as the reservoir fluid level sensor  212 , pressure sensors, travel sensors, switches, wheel speed sensors, and steering angle sensors. The ECU  214  may also be connected to an external module (not shown) for receiving information related to yaw rate, lateral acceleration, longitudinal acceleration of the vehicle, or other characteristics of vehicle operation for any reason, such as, but not limited to, controlling the brake system  200  during vehicle braking, stability operation, or other modes of operation. Additionally, the ECU  214  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  216  and  218 . The isolation valves  216  and  218  may be, for example, solenoid actuated three way valves. The isolation valves  216  and  218  are generally operable to two positions, as schematically shown in  FIGS.  4 - 6   . The first and second isolation valves  216  and  218  each have a port in selective fluid communication with an output conduit  220  generally in communication with the output of the plunger assembly  208 . The first and second isolation valves  216  and  218  also include ports that are in fluid communication with first and second master cylinder conduits  222  and  224 , respectively, which are each connected to the brake pedal unit  204 , via the master cylinder  114 , as shown in  FIG.  4   . The first and second isolation valves  216  and  218  further include ports that are in fluid communication with first and second wheel brake conduits  226  and  228 , respectively, which provide fluid to and from the wheel brakes  202 A,  202 B,  202 C, and  202 D. 
     In some use environments, the first and/or second isolation valves  216  and  218  may be mechanically designed such that flow is permitted to flow in a direction from the output conduit  220  to the first and second wheel brake conduits  226  and  228  and to the first and second master cylinder conduits  222  and  224 , respectively, when in their de-energized positions and can bypass the normally closed seat of the valves  216  and  218 . Thus, although the 3-way valves  216  and  218  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, for example, in performing self-diagnostic tests of the brake system  100 . 
     The brake system  200  further includes various solenoid-actuated valves (“slip control valve arrangement”) for permitting controlled braking operations, such as, but not limited to, 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  230  and a first dump valve  232  in fluid communication with the first wheel brake conduit  226  for cooperatively supplying fluid received from the first isolation valve  216  to the right rear wheel brake  202 A, and for cooperatively relieving pressurized fluid from the right rear wheel brake  202 A to a reservoir conduit  234  in fluid communication with the reservoir  210 . A second set of valves includes a second apply valve  236  and a second dump valve  238  in fluid communication with the first wheel brake conduit  226  for cooperatively supplying fluid received from the first isolation valve  216  to the left front wheel brake  202 B, and for cooperatively relieving pressurized fluid from the left front wheel brake  202 B to the reservoir conduit  234 . A third set of valves includes a third apply valve  240  and a third dump valve  242  in fluid communication with the second wheel brake conduit  228  for cooperatively supplying fluid received from the second isolation valve  218  to the left rear wheel brake  202 C, and for cooperatively relieving pressurized fluid from the left rear wheel brake  202 C to the reservoir conduit  234 . A fourth set of valves includes a fourth apply valve  244  and a fourth dump valve  246  in fluid communication with the second wheel brake conduit  228  for cooperatively supplying fluid received from the second isolation valve  218  to the right front wheel brake  202 D, and for cooperatively relieving pressurized fluid from the right front wheel brake  202 D to the reservoir conduit  234 . Note that in a normal braking event, fluid flows through the de-energized open apply valves  230 ,  236 ,  240 ,  244 . Additionally, the dump valves  232 ,  238 ,  242 ,  246  are preferably in their de-energized closed positions during normal braking to prevent unwanted flow of fluid to the reservoir  210 . 
     The brake pedal unit  204  is connected to a brake pedal  248  and is actuated by the driver of the vehicle as the driver presses on the brake pedal  248 . A brake sensor or switch  250  may be connected to the ECU  214  to provide a signal indicating a depression of the brake pedal  248 . The brake pedal unit  204  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  208  under certain failed conditions of the brake system  200 , and/or upon initial startup of the brake system  200 . This situation is referred to as a manual push-through event, or a “manual apply”, and is the situation shown in  FIG.  4   . The brake pedal unit  204  can supply pressurized fluid to the first and second master cylinder conduits  222  and  224  (that are normally closed off at the first and second isolation valves  216  and  218  during a normal brake apply) to the wheel brakes  202 A,  202 B,  202 C, and  202 D as desired. This situation is depicted in  FIG.  4    schematically by showing the master cylinder  114  and the wheel brakes  202 A,  202 B,  202 C, and  202 D as being under pressure (depicted as bolded), with both pressure and fluid flow through at least the first and second master cylinder conduits  222  and  224 , the first and second wheel brake conduits  226  and  228 , and from then on to the wheel brakes  202 A,  202 B,  202 C, and  202 D. This flow is pushed through, largely under mechanical pressure upon the brake pedal  248  from the driver&#39;s foot, from the master cylinder  114 . With reference back to the simulator valve  100 , the situation shown in  FIG.  1    is present in the circuit of  FIG.  4   , with both the armature  116  and the poppet  118  in their first positions. 
     As shown schematically in  FIGS.  4 - 6   , the brake pedal unit  204  includes a master cylinder with a housing  252  for slidably receiving various cylindrical pistons and other components therein. Note that the housing is not specifically schematically shown in the Figures, but instead the walls of the bore are illustrated. The housing  252  may be formed as a single unit or include two or more separately formed portions coupled together. An input piston  254  is connected with the brake pedal  248  via a linkage arm  256 . Leftward movement of the input piston  254  may cause, under certain conditions, a pressure increase within the master cylinder  114 . 
     The master cylinder  114  is in fluid communication with the pedal simulator  110  via a master cylinder passage  108 . The input piston  254  is slidably disposed in the bore of the housing  252  of the master cylinder  114 . When the brake pedal unit  204  is in its rest position (the driver is not depressing the brake pedal  248 ), the structures of the master cylinder  114  permit fluid communication between the bore of the housing  252  and the reservoir  210  via a reservoir conduit  258 . 
     The brake system  200  may further include an optional solenoid actuated simulator test valve  260  which may be electronically controlled between an open position and a powered closed position. The simulator test valve  260  is not necessarily needed during a normal brake apply or for a manual push-through mode. The simulator test valve  260  can be actuated to a closed position during various testing modes to determine the correct operation of other components of the brake system  200 . For example, the simulator test valve  260  may be actuated to a closed position to prevent venting to the reservoir  210  via the reservoir conduit  258  such that a pressure build up in the brake pedal unit  204  can be used to monitor fluid flow to determine whether leaks may be occurring through seals of various components of the brake system  200 . 
     The brake system  200  further includes a first plunger valve  262 , and a second plunger valve  264 . The first plunger valve  262  is preferably a solenoid actuated normally closed valve. Thus, in the non-energized state, as shown in  FIG.  4   , the first plunger valve  262  is in a closed position. The second plunger valve  264  is preferably a solenoid actuated normally open valve. Thus, in the non-energized state, the second plunger valve  264  is in an open position, as shown in the Figures. A check valve may be arranged within the second plunger valve  264  so that when the second plunger valve  264  is in its closed position, fluid may still flow through the second plunger valve  264  in the direction from the first plunger output conduit  266  (from the plunger assembly  208 ) to the output conduit  220  leading to the first and second isolation valves  216  and  218 . During a rearward stroke of the piston of the plunger assembly  208 , pressure may be generated within the plunger assembly  208  for output into the output conduit  220 . The brake system  200  further includes a check valve  268  permitting fluid to flow in the direction from the conduit  270  (from the reservoir to  10 ) to the conduit  266  and into the plunger assembly  208 , such as during a pressure generating rearward stroke of the piston of the plunger assembly  208 . 
     During initial manual push through operation of the brake pedal unit  204 , sufficient leftward movement of the input piston  254  will prevent the flow of fluid from the master cylinder  114  into the reservoir conduit  258  and thus into the reservoir  210 , but will put the system into a flow-through state, where the driver&#39;s foot applies pressure to the master cylinder  114 , which then flows fluid (responsive to that pressure from the brake pedal  248 ) through the first and second master cylinder conduits  222  and  224 , the first and second isolation valves  216  and  218 , the first and second wheel brake conduits  226  and  228 , and on to the wheel brakes  202 A,  202 B,  202 C, and  202 D. Further leftward movement of the input piston  254  will pressurize the master cylinder  114 , causing fluid to flow into the pedal simulator  110  via the pedal simulator passage  108 , which is concurrently being opened in the transition portion of operation of the simulator valve  100 , illustrated in  FIG.  2    and described above. As fluid is diverted into the pedal simulator  110 , the pedal simulator  110  provides a feedback force to the driver of the vehicle via the brake pedal  248 . This feedback force simulates the forces a driver feels at the brake pedal  248  in a conventional vacuum assist hydraulic brake system, for example. 
     A simulation pressure chamber  272  of the pedal simulator  110  is in fluid communication with the pedal simulator passage  108 , which is in fluid communication with the master cylinder  114  of the brake pedal unit  204 . As shown in  FIGS.  4 - 6   , the solenoid actuated simulator valve  100  (described above in detail with reference to  FIGS.  1 - 3   ) is positioned between the pedal simulator passage  108  and the master cylinder passage  112  to selectively control the flow of fluid between the master cylinder  114  and the pedal simulator  110  for any desired reason. One example of desired operation of the simulator valve  100  is during a failed and/or initial/startup condition, in which the brake pedal unit  204  is utilized to provide a source of pressurized fluid to the wheel brakes  202 A,  202 B,  202 C, and  202 D in a push-through manner, as described herein. 
     In summary, then, a brake system  200  is provided for actuating a pair of front wheel brakes  202 B,  202 D and a pair of rear wheel brakes  202 A,  202 C. (The front and rear wheel brakes  202 A,  202 B,  202 C, and  202 D could be actuated, as desired, in any order or sequence, any combination(s), or otherwise individually or in groups as desired for a particular use environment. While a diagonal brake system is shown herein as an example, it is contemplated that the present invention could be used with a split brake system or any other configuration, whether or not currently known.) The system includes a reservoir  210  for holding fluid and a master cylinder  114  operable during a manual push-through mode by actuation of a brake pedal  248  connected to the master cylinder  114  to generate brake actuating pressure. The brake actuating pressure is applied at first and second outputs (e.g., first and second master cylinder conduits  222  and  224 ) for actuating the pair of front wheel brakes  202 B,  202 D and the pair of rear wheel brakes  202 A,  202 C. A source of pressurized fluid (here, the plunger assembly  208 ) is provided for actuating the pair of front wheel brakes  202 B,  202 D and the pair of rear wheel brakes  202 A,  202 C during a non-failure normal braking event. An electronic control unit  214  is provided for controlling the source of pressurized fluid. A pedal simulator  110  is in selective fluid communication with the master cylinder. The brake system  200  further includes the simulator valve  100  shown and described herein, which selectively permits fluid communication between the master cylinder  114  and the pedal simulator  110 . 
     In use, the brake system  200  moves from a startup, or “manual apply” mode, through a “transition” mode, into a steady state “boosted” mode, as described above with reference to  FIGS.  1 - 3   , respectively. A manual apply mode is generally only entered if a boosted startup is not available for a particular starting sequence.  FIGS.  4 - 6    schematically depict the condition and operation of the brake system  200  as a whole, respective to those three modes. 
     In  FIG.  4   , the “manual apply” mode, the simulator valve  100  is in the de-energized, or closed, position shown in  FIG.  1   . The driver is just starting to apply foot pressure to the brake pedal  248 , which pressurizes the master cylinder  114  (as indicated by the bold shading of that component), and causes fluid to flow (as indicated by the dotted lines of these components) through first and second master cylinder conduits  222 ,  224  and first and second wheel brake conduits  226 ,  228 , and from there directly to the wheel brakes  202 A,  202 B,  202 C, and  202 D. Again, this is a push-through situation where the driver&#39;s foot pressure upon the brake pedal  248  is directly pushing brake fluid to the wheel brakes  202 A,  202 B,  202 C, and  202 D. 
     During the shift from the configuration of  FIGS.  1  and  4    to the “boosted” configuration of  FIG.  2    and the “transition” configuration of  FIG.  5   , at least the simulator valve  100 , the plunger assembly  208 , the first plunger valve  262 , and the first isolation valve  216  are electrically actuated, which causes the brake system  200  to move into the “transition” mode. (As a side note, though second isolation valve  218  is not yet energized at this point, a predetermined amount of flow through that second isolation valve  218  may be permitted, to help to dampen or soften the transition phase, as desired.) 
     With reference now to  FIG.  5   , at least the master cylinder  114 , the first and second master cylinder conduits  222 ,  224  and first and second wheel brake conduits  226 ,  228 , the check valve  268 , and the hydraulic lines leading directly to the wheel brakes  202 A,  202 B,  202 C, and  202 D, are held under a desired amount of pressure, and the simulator valve  100  has moved into the position shown in  FIG.  2   . Accordingly, the armature  116  has been magnetically pulled by the core  138  upward, in the orientation of  FIG.  2   , into the second armature position, thus pulling the valve ball  126  away from the poppet  118  (which remains in the first poppet position) and opening the damped fluid flow path D. As a result, fluid is permitted to flow from the master cylinder  114 , through the damped fluid flow path D, and into the pedal simulator  110 . 
     The “transition” mode passes rather quickly (on the order of a small fraction of a second to one or two seconds, depending on a number of other factors), and once the pedal simulator  110  has attained sufficient pressurization (represented by the downward arrow within the simulation pressure chamber  272  in  FIG.  5   ), the system enters the “boosted apply” mode, as shown schematically for the entire brake system  200  in  FIG.  6   . 
     With reference to  FIG.  6   , the second isolation valve  218  has been actuated (in addition to the previously actuated components). The “boosted apply” mode shown in  FIG.  6    includes the master cylinder  114 , the plunger assembly  208 , the check valve  268 , the pedal simulator  110 , the first and second master cylinder conduits  222 ,  224 , and the wheel brakes  202 A,  202 B,  202 C, and  202 D (along with portions of the wheel brake hydraulics leading to the dump valves  232 ,  238 ,  242 ,  246 ) either “holding” fluid pressure or helping to prevent fluid flow back to the reservoir  210  while boost pressure is permitted to ebb and flow. Fluid is permitted to flow, under pressure, through the output conduit  220 , the first and second wheel brake conduits  226 ,  228 , and through the apply valves  230 ,  236 ,  240 ,  244  to the wheel brakes  202 A,  202 B,  202 C, and  202 D. The plunger assembly  208  continues to build higher boosted pressure based upon the driver&#39;s request, and the ECU  214  controls the system to carry out braking requests from the driver during further operation of the brake system  200  until there is a system failure or the vehicle is deactivated. 
     It is contemplated that pedal drop (the “feel” of the brake pedal to the driver) during the transition between manual apply and boost braking, using the simulator valve  100  shown and described herein, can be actively controlled and adjusted based on customer preferences. A brake system including the simulator valve  100  according to aspects of the present invention could also allow a user to eliminate pedal drop if that transition pedal feel is preferred. 
     As used herein, the singular forms “a”, “an”, and “the” can include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising”, as used herein, can specify the presence of stated features, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, steps, operations, elements, components, and/or groups thereof. 
     As used herein, the term “and/or” can include any and all combinations of one or more of the associated listed items. 
     It will be understood that when an element is referred to as being “on”, “attached” to, “connected” to, “coupled” with, “contacting”, “adjacent”, etc., another element, it can be directly on, attached to, connected to, coupled with, contacting, or adjacent the other element, or intervening elements may also be present. In contrast, when an element is referred to as being, for example, “directly on”, “directly attached” to, “directly connected” to, “directly coupled” with, “directly contacting”, or “directly adjacent” another element, there are no intervening elements present. It will also be appreciated by those of ordinary skill in the art that references to a structure or feature that is disposed “directly adjacent” another feature may have portions that overlap or underlie the adjacent feature, whereas a structure or feature that is disposed “adjacent” another feature might not have portions that overlap or underlie the adjacent feature. 
     Spatially relative terms, such as “under”, “below”, “lower”, “over”, “upper”, “proximal”, “distal”, and the like, may be used herein for ease of description to describe one element or feature&#39;s relationship to another element(s) or feature(s) as illustrated in the figures. It will be understood that the spatially relative terms can encompass different orientations of a device in use or operation, in addition to the orientation depicted in the figures. For example, if a device in the figures is inverted, elements described as “under” or “beneath” other elements or features would then be oriented “over” the other elements or features. 
     As used herein, the phrase “at least one of X and Y” can be interpreted to include X, Y, or a combination of X and Y. For example, if an element is described as having at least one of X and Y, the element may, at a particular time, include X, Y, or a combination of X and Y, the selection of which could vary from time to time. In contrast, the phrase “at least one of X” can be interpreted to include one or more Xs. 
     It will be understood that, although the terms “first”, “second”, etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. Thus, a “first” element discussed below could also be termed a “second” element without departing from the teachings of the present disclosure. The sequence of operations (or steps) is not limited to the order presented in the claims or figures unless specifically indicated otherwise. 
     While aspects of this disclosure have been particularly shown and described with reference to the example aspects above, it will be understood by those of ordinary skill in the art that various additional aspects may be contemplated. For example, the specific methods described above for using the apparatus are merely illustrative; one of ordinary skill in the art could readily determine any number of tools, sequences of steps, or other means/options for placing the above-described apparatus, or components thereof, into positions substantively similar to those shown and described herein. In an effort to maintain clarity in the Figures, certain ones of duplicative components shown have not been specifically numbered, but one of ordinary skill in the art will realize, based upon the components that were numbered, the element numbers which should be associated with the unnumbered components; no differentiation between similar components is intended or implied solely by the presence or absence of an element number in the Figures. Any of the described structures and components could be integrally formed as a single unitary or monolithic piece or made up of separate sub-components, with either of these formations involving any suitable stock or bespoke components and/or any suitable material or combinations of materials. Any of the described structures and components could be disposable or reusable as desired for a particular use environment. Any component could be provided with a user-perceptible marking to indicate a material, configuration, at least one dimension, or the like pertaining to that component, the user-perceptible marking potentially aiding a user in selecting one component from an array of similar components for a particular use environment. A “predetermined” status may be determined at any time before the structures being manipulated actually reach that status, the “predetermination” being made as late as immediately before the structure achieves the predetermined status. The term “substantially” is used herein to indicate a quality that is largely, but not necessarily wholly, that which is specified—a “substantial” quality admits of the potential for some relatively minor inclusion of a non-quality item. Though certain components described herein are shown as having specific geometric shapes, all structures of this disclosure may have any suitable shapes, sizes, configurations, relative relationships, cross-sectional areas, or any other physical characteristics as desirable for a particular application. Any structures or features described with reference to one aspect or configuration could be provided, singly or in combination with other structures or features, to any other aspect or configuration, as it would be impractical to describe each of the aspects and configurations discussed herein as having all of the options discussed with respect to all of the other aspects and configurations. A device or method incorporating any of these features should be understood to fall under the scope of this disclosure as determined based upon the claims below and any equivalents thereof. 
     Other aspects, objects, and advantages can be obtained from a study of the drawings, the disclosure, and the appended claims.