Patent Publication Number: US-2021164585-A1

Title: Pedal modulating valve assembly including multiple gain states

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to and all advantages of U.S. Application No. 62/942,052, filed on Nov. 29, 2019, the contents of which are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present disclosure generally relates to a pedal modulating valve assembly including a first gain state and a second gain state, and systems including the same, for working units and other applications. 
     BACKGROUND 
     Due to the increasing weights of vehicles, such as off highway vehicles, the brake energy required to stop these vehicles has also increased. To account for these increases, modern off highway vehicles include large and robust wheel brakes designed to be prepared for worst case conditions including an ability to apply the maximum brake pressure needed to bring the vehicle to the shortest possible stop in an emergency situation. While these wheel brakes are effective in worst case conditions, high fidelity control of the wheel brakes during lower pressure braking scenarios is difficult. 
     A wide variety of electrohydraulic proportional pressure control valves are used to provide controlled pressure to working units, such as wheel brakes. Some typical valves are designed for use with an actuator, such as a pedal actuator, in which force is applied to by a user. These pressure control valves provide a linear output characteristic for pressure versus force applied by the user to the actuator. 
     While this linear output characteristic permits braking at both the lower percentage of the brake pressure range and the higher percentage of the braking pressure range, a majority of the braking occurs in the lower percentage of the braking pressure range. Thus, a majority of the braking occurs without high fidelity control of the wheel brakes thereby resulting in abrupt or aggressive braking of the vehicle which affects control of the vehicle and operator comfort. 
     Accordingly, it is desirable to provide an improved valve assembly and a system including the same. Furthermore, other desirable features and characteristics will become apparent from the subsequent summary and detailed description and the appended claims, taken in conjunction with the foregoing technical field and background. 
     BRIEF SUMMARY 
     In one embodiment, a valve assembly is provided. The valve assembly includes a valve body defining a bore. The valve assembly further includes a plunger adjacent the valve body. The valve assembly further includes a modulating biasing member disposed in the bore and abutting the plunger. The valve assembly further includes a spool disposed in the bore and adjacent the plunger with the modulating biasing member disposed therebetween. The plunger is configured to move the spool between a neutral position and an energized position. The spool defines a socket. The valve assembly further includes a piston disposed in the socket and configured to move between a first piston position and a second piston position within the socket. 
     In this and other embodiments, by moving the piston from the first piston position to the second piston position, fluid within the socket is limited to a predefined force and therefore no longer acts to further oppose movement of the spool by the plunger. As a result, force required to move the spool toward the energized position by the plunger when the piston is in the second piston position is decreased relative to the force required when the piston is in the first piston position. 
     In this and other embodiments, the valve assembly has a first gain state and a second gain state. The valve assembly is in the first gain state when the piston is in the first piston position, and the valve assembly is in the second gain state when the piston is in the second piston position. The valve assembly having the first gain state and the second gain state provides the user improved fidelity at lower pressures while still allowing a working unit to reach higher pressures. For working units, such as wheel brakes of a vehicle, lower pressures are typically utilized during a majority of the braking of the vehicle. Thus, improving fidelity of the wheel brakes at lower pressures can improve overall usability of the vehicle. However, higher pressures may be necessary in emergency situations. Therefore, multiple gain states are important to allow the working unit to reach higher pressures while still exhibiting improved fidelity at lower pressures. 
     In another embodiment, a system having a first gain state and a second gain state is also provided. The system includes, but is not limited to, a fluid source configured to provide a fluid force (e.g., hydraulic fluid pressure). The system further includes, but is not limited to, a valve assembly in fluid communication with the fluid source. The valve assembly includes, but is not limited to, a plunger. The valve assembly further includes, but is not limited to, a modulating biasing member abutting the plunger. The valve assembly further includes, but is not limited to, a spool. The spool is adjacent the plunger with the modulating biasing member disposed therebetween. The plunger is configured to move the spool between a neutral position and an energized position. The valve assembly further includes, but is not limited to, a piston. The piston is in fluid communication with the fluid source. The piston is configured to move between a first piston position and a second piston position. The system further includes, but is not limited to, a working unit in fluid communication with the valve assembly and configured to activate in response to the fluid force. The system is in the first gain state when the piston is in the first piston position and the system is in the second gain state when the piston is in the second piston position. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING(S) 
       Other advantages of the disclosed subject matter will be readily appreciated, as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein: 
         FIGS. 1A and 1B  are a cross-sectional plan views illustrating a non-limiting embodiment of a valve assembly; 
         FIG. 2  is another cross-sectional plan view illustrating a non-limiting embodiment of the valve assembly; 
         FIG. 3  is another cross-sectional plan view illustrating a non-limiting embodiment of the valve assembly; and 
         FIG. 4  is a graph illustrating gain states of a non-limiting embodiment of the valve assembly as compared to the prior art. 
     
    
    
     DETAILED DESCRIPTION 
     Except in the examples, or where otherwise expressly indicated, all numerical quantities in this description indicating amounts of material or conditions of reaction and/or use are to be understood as modified by the word “about” in describing the broadest scope of the invention. Practice within the numerical limits stated is generally preferred. Also, unless expressly stated to the contrary: percent, “parts of,” and ratio values are by weight; the description of a group or class of materials as suitable or preferred for a given purpose in connection with the invention implies that mixtures of any two or more of the members of the group or class are equally suitable or preferred; description of constituents in chemical terms refers to the constituents at the time of addition to any combination specified in the description, and does not necessarily preclude chemical interactions among the constituents of a mixture once mixed; the first definition of an acronym or other abbreviation applies to all subsequent uses herein of the same abbreviation and applies mutatis mutandis to normal grammatical variations of the initially defined abbreviation; and, unless expressly stated to the contrary, measurement of a property is determined by the same technique as previously or later referenced for the same property. 
     It must also be noted that, as used in the specification and the appended claims, the singular form “a,” “an,” and “the” comprise plural referents unless the context clearly indicates otherwise. For example, reference to a component in the singular is intended to comprise a plurality of components. 
     The following detailed description is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any theory presented in the preceding background or the following detailed description. 
     A valve assembly is provided herein. In various embodiments, the valve assembly is suitable for controlling a working unit of a vehicle. A system for controlling a working unit of a vehicle is also provided herein. 
       FIGS. 1-3  are cross-sectional plan views illustrating a non-limiting embodiment of a valve assembly  10 . The valve assembly  10  includes a valve body  12  and a plunger  14  adjacent the valve body  12 . In certain embodiments, a user applies a force to the plunger  14  and the plunger  14  is configured to receive the force from the user. In various embodiments, the valve assembly  10  is utilized with a fluid source  16  (e.g., hydraulic pressure unit or hydraulic pump), a tank  18  (e.g., a hydraulic reservoir), and a working unit  20  (e.g., a hydraulic cylinder or wheel brake). In various embodiments, the fluid source  16  is configured to provide fluid force (e.g. hydraulic fluid pressure) to the valve assembly  10 . For purposes of clarification, the valve body  12  will be described as having a first body end  22  and a second body end  24 . 
     The valve body  12  of the valve assembly  10  defines a bore  26 . The bore  26  may be manufactured as a through bore extending through the valve body  12 . It is contemplated that the bore  26  may also be configured as a blind bore. The valve body  12  further defines a pressure port  28 , a work port  30 , and a tank port  32 . The bore  26  typically extends through the valve body  12  between the first body end  22  and the second body end  24 . Each of the ports  28 ,  30 , and  32  may be in fluid communication with the bore  26 . As shown in a non-limiting embodiment of  FIG. 1 , the pressure port  28  is disposed proximate the second body end  24  and the tank port  32  is disposed proximate the first body end  22 . The work port  30  is disposed intermediate the pressure port  28  and the tank port  32 . In certain embodiments, the ports  28 ,  30 , and  32  provide connection locations for establishing fluid communication between the valve body  12  and the hydraulic pump  16 , the working unit  20 , and the tank  18 . Typical port connections include standard SAE straight threads or other configurations for allowing hoses or other conduits to be connected between the components. However, it is to be appreciated that other port configurations are contemplated, for example, the pressure port  28  may be disposed proximate the first body end  22  and the tank port  32  may be disposed proximate the second body end  24 . 
     The bore  26  may include a first annular surface  34  and a second annular surface  36 . These surfaces  34 ,  36  may be utilized to provide fluid communication between the ports  28 ,  30 , and  32 . The bore  26  may also include a countersink region  38 . In various embodiments, the countersink region  38  is proximate the first body end  22 . 
     The valve assembly  10  further includes a spool  40  disposed in the bore  26 . The spool  40  is adjacent and operatively coupled to the plunger  14 . The plunger  14  is configured to move the spool  40  between a neutral position (see  FIG. 1A ) and an energized position (see  FIG. 3 ). In various embodiments, the plunger  14  is configured to move the spool  40  to an intermediate position (see  FIG. 2 ) between the neutral position and the energized position. In certain embodiments, the valve assembly  10  further includes a retaining member  42  operatively coupled to the spool  40  for moving the spool  40  between the neutral position and the energized position. The valve assembly  10  may further includes a ball bearing member  94  as a universal joint between the retaining member  42  and the spool  40 . In various embodiments, the spool  40  includes a first spool end  44  and a second spool end  46  with the retaining member  42  operatively coupled to the first spool end  44 . 
     In certain embodiments, the spool  40  includes a first annular portion  48  and a second annular portion  50 . The first annular portion  48  and the second annular portion  50  may be configured to cooperate with the first annular surface  34  and the second annular surface  36  of the bore  26 , respectively, for manipulating fluid communication between the ports  28 ,  30 , and  32 . The spool  40  may further includes a flow portion  52  having a decreased diameter relative to the first annular portion  48  and the second annular portion  50  for providing fluid communication to the work port  30 . The spool  40  may also include a shoulder  100  proximate the first spool end  44  and configured to cooperate with the countersink region  38  of the valve body  12 . 
     With reference to  FIGS. 1A and 1B , when the spool  40  is in the neutral position, fluid communication may be provided between the work port  30  and the tank port  32 . Further, when the spool  40  is in the neutral position, fluid communication may be prevented between the pressure port  28  and the work port  30 . In particular, when the spool  40  is in the neutral position, the second annular portion  50  may be engaged with the second annular surface  36  thereby preventing fluid to flow between the pressure port  28  and the work port  30 . 
     With reference to  FIGS. 2 and 3 , when the spool  40  is in the intermediate position or the energized position, respectively, fluid communication may be provided between the pressure port  28  and the work port  30 . Further, when the spool  40  is in the intermediate position or the energized position, fluid communication may be prevented between the work port  30  and the tank port  32 . In particular, when the spool  40  is in the intermediate position or the energized position, the first annular portion  48  may be engaged with the first annular surface  34  thereby preventing fluid to flow between the work port  30  and the tank port  32 . 
     It is to be appreciated that the valve assembly  10  may operate in a different manner. For example, the energized position of a spool may provide fluid communication between a work port and a tank port and the neutral position of the spool may provide fluid communication between a pressure port and the work port. 
     With continuing reference to  FIGS. 1-3 , the spool  40  may define a cavity  56  between the first annular portion  48  and the second annular portion  50 . In certain embodiments, the cavity  56  is in fluid communication with the work port  30  and the tank port  32  through an orifice  102  when the spool  40  is in the neutral position. Further, in these embodiments, the cavity  56  is in fluid communication with the pressure port  28  and the work port  30  through the orifice  102  when the spool  40  is in the energized position. 
     In certain embodiments, the spool  40  defines a socket  62  between the second spool end  46  and the cavity  56 . The socket  62  may extend through the second spool end  46 . The spool  40  may have a socket face  65  opposite the second spool end  46  and within the socket  62 . The spool  40  may define a channel  64  extending between the socket  62  and the cavity  56  such that the socket  62  is in fluid communication with the cavity  56 . In various embodiments, fluid provided between the work port  30  and the tank port  32  is also provided to the socket  62  through the orifice  102 , then through the cavity  56 , and then through the channel  64  when the spool  40  is in the neutral position. Likewise, fluid provided between the pressure port  28  and the work port  30  is also provided to the socket  62  through the orifice  102 , then through the cavity  56 , and then through the channel  64  when the spool  40  is in the energized position. 
     The valve assembly  10  further includes a piston  66  disposed in the socket  62 . The piston  66  is configured to move between a first piston position (see  FIGS. 1A and 2 ) and a second piston position (see  FIG. 3 ) in response to the fluid. In certain embodiments, the first piston position and the second piston position are relative to the spool  40 . The piston  66  includes a first piston end  68  and a second piston end  70  spaced from the first piston end  68  with a void  72  defined therebetween. 
     With continuing reference to  FIGS. 1-3 , the valve assembly  10  may further include a dowel  84  extending through the spool  40  and the piston  66 . In various embodiments, the dowel  84  extends through the void  72  of the piston  66 , the socket  62  of the spool  40 , and the channel  64  of the spool  40 . The valve assembly  10  may further include a plug  86  disposed proximate the second body end  24  of the valve body  12  within the bore  26 . The dowel  84  may be adapted to abut the plug  86  to prevent the dowel  84  from moving beyond the plug  86  toward the second body end  24 . The spool  40  may be operatively arranged with the dowel  84  so as to slide relative to the dowel  84 . The piston  66  may also be operatively arranged with the dowel  84  so as to slide relative to the dowel  84 . 
     The spool  40  may have a spool face  58  and the dowel  84  may have a dowel face  60  with the spool face  58  and the dowel face  60  flanking the cavity  56 . As the spool  40  moves from the neutral position to the energized position, fluid provided between the pressure port  28  and the work port  30  acts on the spool face  58  and the dowel face  60 . To this end, the spool face  58  and the dowel face  60  create an imbalanced pressure load on the spool  40  in the presence of the force of the fluid due to the dowel  84  being prevented from moving beyond the plug  86 . In various embodiments, this imbalanced pressure load biases the spool  40  toward the first body end  22  (e.g., toward the neutral position) and the piston  66  may have a piston face  74  with the socket face  65  and piston face  74  flanking the socket  62 . As the spool  40  moves from the neutral position to the energized position, fluid provided between the pressure port  28  and the work port  30  communicates through channel  64  of the piston  66 . To this end, the socket face  65  and the position face  74  create an imbalanced force load on the spool  40  in the presence of the pressure of the fluid. The sum of the pressure force acting on the spool face  58  and the piston face  74  combine to react against the applied force at the plunger  14 . 
     The second piston end  70  includes an extension  76  configured to cooperate with the second spool end  46  to prevent fluid communication between the socket  62  and pressure port  28  when the piston  66  is in the first piston position. As the spool  40  moves from the neutral position to the energized position, fluid provided between the pressure port  28  and the work port  30  through the channel  64  acts on the piston face  74  when the piston  66  is in the first piston position. When the piston  66  is in the second piston position, the piston  66  and the spool  40  cooperate to define a relief passage  78  to limit the force of the fluid acting on the piston face  74  and socket face  65  thereby preventing the piston  66  from absorbing additional force by the fluid. To this end, by moving the piston  66  from the first piston position to the second piston position, force acting on the piston face  74  and socket face  65  due to fluid within the socket  66  is limited to the pressure at which the relief passage  78  begins to meter flow out of socket  66  to oppose movement of the spool  40  by the plunger  14 . As a result, the force required to move the spool  40  toward the energized position by the plunger  14  when the piston  66  is in the second piston position is decreased relative to the force required when the piston  66  is in the first piston position. In various embodiments, the relief passage  78  is in fluid communication with the pressure port  28  due to disengagement of the extension  76  from the second spool end  46 . Thus, when the piston  66  is in the second position, the channel  64  is in fluid communication with the pressure port  28  through the passage  78  such that fluid pressure in channel  64  is limited to the pressure at which the passage  78  started fluid communication to the pressure port  28 . As a result when the piston  66  is in the second position, fluid may flow from the channel  64 , through the socket  62 , around the piston  66 , through the relief passage  78 , and to the pressure port  28 . 
     The valve assembly  10  may further include a first compensating biasing member  80  exhibiting a first force on the spool  40  to bias the spool  40  toward the first body end  22  (e.g., toward the neutral position). The valve assembly  10  may further include a second compensating biasing member  82  exhibiting a second force on the piston  66  to bias the piston  66  to the first piston position. In certain embodiments, the piston  66  is configured to move to the second piston position prior to the spool  40  moving to the energized position due to a ratio of the first force and the second force. The first compensating biasing member  80  and the second compensating biasing member  82  may, independently, include any standard spring commonly used and known by those having skill in the art or any other feed-back device such as pneumatic struts, electromagnets, or elastomeric force feed-back devices. Alternatively, the first compensating biasing member  80  and/or the second compensating biasing member  82  may be omitted in applications where the imbalanced work port pressure alone is used to return the spool to the neutral position and/or return the piston  66  to the first piston position. 
       FIG. 4  is a graph illustrating gain states of a non-limiting embodiment of the valve assembly  10  as compared to the prior art. The valve assembly  10  has a first gain state  88  and a second gain state  90 . The valve assembly  10  may be in the first gain state  88  when the piston  66  is in the first piston position. The valve assembly  10  may be in the second gain state  90  when the piston  66  is in the second piston position. It is to be appreciated that the valve assembly  10  may be configured to have more than two gain states. When the valve assembly  10  is in the first gain state  88 , a greater amount of force by the plunger  14  on the spool  40  is necessary to move the spool  40  toward the second body end  24  as compared to when the valve assembly  10  is in the second gain state  90 . When the valve assembly  10  is in the second gain state  90 , a reduced amount of force by the plunger  14  on the spool  40  is necessary to move the spool  40  toward the second body end  24  as compared to when the valve assembly  10  is in the first gain state  88 . 
     With continuing reference to  FIG. 4 , multiple gain states, such as the first gain state  88  and the second gain state  90  of the valve assembly  10  provide the user improved fidelity at lower pressures while still allowing the working unit  16  to reach higher pressures. For wheel brakes of a vehicle, lower pressures are typically utilized during a majority of the braking of the vehicle. Thus, improving fidelity of the wheel brakes at lower pressures can improve overall usability of the vehicle. However, higher pressures may be necessary in emergency situations. Therefore, multiple gain states are important to allow the working unit  16  to reach higher pressures while still exhibiting improved fidelity at lower pressures. 
     When the valve assembly  10  is in the first gain state  88 , the force of the fluid from the work port  30  acts on the dowel  84  and the piston  66 . This force biases the dowel  84  against the plug  86 . However, this force on the piston  66  is not sufficient to overcome the second force of the second compensating biasing member  82  thereby maintaining the piston  66  in the first piston position. In various embodiments, the piston  66  is adapted to generate a force to oppose movement of the spool  40  by the plunger  14 . In some of these embodiments, the force is in accordance with the second force of the second compensating biasing member  82 . In certain embodiments, the force is equal to the second force of the second compensating biasing member  82 . The pressure force acting on the spool face  58  and the socket face  65  may also act on the spool  40  in a similar amount. Pressure force acting on piston face  74  opposes force from the second compensating member  82 . To this end, the combined pressure force on spool face  58  and the socket face  65 , the first compensating biasing member  80 , and the second compensating biasing member  82  via the piston  66 , are biasing the spool  40  toward the first body end  22  (e.g., toward the neutral position), opposing the force generated by the plunger  14 . Thus, when the valve assembly  10  is in the first gain state  88 , a greater amount of force opposes movement of the spool  40  toward the second body end  24  thereby reducing the force of the fluid acting on the work port  30  relative to the amount of force generated by the plunger  14 . With reference to  FIG. 4 , the first gain state  88  exhibits a lower slope for pressure at the work port  30  relative to the force applied by the user to the plunger  14  as compared to the second gain state  90 . 
     When the valve assembly  10  is in the second gain state  90 , the force of the fluid from the work port  30  continues to act on the spool face  58  and to a reduced amount to the piston face  74  and socket face  65 . In contrast to the first gain state  88 , where fluid from the work port  30  continues to act fully on spool face  58  and the socket face  65  this force on the piston  66  is sufficient to overcome the second force at a second pressure, piston  66  moves to the second piston position and defining the relief passage  78 . With the relief passage  78  defined, via the piston  66  pressure in socket  62  is limited to a maximum pressure. Pressure force acting on the socket face  65  is limited to bias the spool  40  toward the first body end  22 . A pre-defined force in accordance with the force of the biasing member  82  acts on piston  66 . In other words, the force generated by the socket face  65  may be limited to a pre-defined force when the piston  66  is in the second piston position and the pre-defined force may be in accordance with the second force of the second compensating biasing member  82  when the piston  66  is in the second piston position. In certain embodiments, the pre-defined force is equal to the second force of the second compensating biasing member  82  when the piston  66  is in the second piston position. To this end, the force acting on spool face  58  and the limited force acting on the socket face  65 , and the first compensating biasing member  80  are biasing the spool  40  toward the first body end  22  (e.g., toward the neutral position), opposing the force generated by the plunger  14 . Thus, when the valve assembly  10  is in the second gain state  90  as opposed to the first gain state  88 , a reduced amount of force opposes movement of the spool  40  toward the second body end  24  thereby increasing the force of the fluid acting on the work port  30  relative to the amount of force generated by the plunger  14 . With reference to  FIG. 4 , the second gain state  90  exhibits a higher slope for pressure at the work port  30  relative to the force applied by the user to the plunger  14  as compared to the first gain state  88 . 
     The valve assembly  10  may further include a modulating biasing member  92  disposed in the bore  26  and abutting the plunger  14 . In certain embodiments, the modulating biasing member  92  is disposed between the plunger  14  and the retaining member  42  proximate the first body end  22 . The modulating biasing member  92  may be positioned within the countersink region  38  of the bore  26 . The modulating biasing member  92  may include a variety of compression spring configurations. Other spring types that may be used include bevel springs, torsion springs with levers, leaf springs, and the like. 
     The retaining member  42  may be configured with an interior shoulder  54 . The modulating biasing member  92  may be positioned longitudinally between the plunger  14  and the interior shoulder  54  of the retaining member  42 . The retaining member  42  may be configured to transfer force from the modulating biasing member  92  to the spool  40  when the modulating biasing member  92  is compressed by the plunger  14 . In various embodiments, the retaining member  42  includes an extended portion  96  having an inside diameter adapted to guide the modulating biasing member  92 . The extended portion  96  maintains the modulating biasing member  92  in a longitudinal orientation. 
     In various embodiments, a return spring  104  is disposed between the plunger  14  and the valve body  12 . The plunger  14  may be configured to move into the bore  26  against the resistance of the return spring  104 . When pressure is removed from the plunger  14 , the return spring  104  is configured to return the plunger  14  to a normal position. 
     In certain embodiments, a washer  98  is disposed between the shoulder  100  of the spool  40  and the retaining member  42 . The washer  98  may define the neutral position of the spool  40  by providing a mechanical stop to prevent movement of the spool  40  beyond neutral position toward the first body end  22 . As shown in  FIG. 1 , the washer  98  contacts the countersink region  38  due to tension from the retaining member  42  acting on the washer  98 . The washer  98  also contacts the shoulder  100  of the spool  40  when the spool  40  is in the neutral position due tension from the first compensating biasing member  80  acting on the spool  40 . The tension from the first compensating biasing member  80  may also be lower than the tension provided by modulating biasing member  92  when the spool  40  is in the neutral position. 
     It is to be understood that spring compression may be adapted to various applications by modifying the length of the spring retaining member, the thickness of the washer, the stiffness of the spring, or other various structural features as would be obvious to one of ordinary skill in the art. 
     In various embodiments, a check valve passage  106  extends between the tank port  32  and the bore  26 . A check valve  108  may be configured to control flow through the check valve passage  106  between the tank port  32  and the bore  26 . 
     With reference back to  FIGS. 1-3 , non-limiting embodiments of operation of the valve assembly  10  are depicted therein. In certain embodiments, when fluid is desired to operate the working unit  20 , the valve assembly  10  is energized. A user applies a force to the plunger  14  begins developing axial force from the neutral state shown in  FIG. 1 . The plunger  14  moves the spool  40  toward the second body end  24  to the first gain state  88  of the valve assembly  10  shown in  FIG. 2 . In the first gain state  88 , fluid is permitted to flow from the pressure port  28  around the flow portion  52  having a decreased diameter and through cavity  56  of the spool  40 , and to the work port  30  for operation of the working unit  20 . At the same time, fluid flow to the tank port  32  is obstructed by cooperation between the first annular surface  34  of the valve body  12  and the first annular portion  48  of the spool  40 . As described above, when the valve assembly  10  is in the first gain state  88 , a greater amount of force opposes movement of the spool  40  toward the second body end  24  thereby reducing the force of the fluid acting on the work port  30  relative to the amount of force acting on the spool  40  by the plunger  14  due to the force acting on the spool  40  at the spool face  58  and the socket face  65 . In other words, a greater amount of force acting on the spool  40  by the plunger  14  is necessary to move the spool  40  toward the second body end  24  as compared to when the valve assembly  10  is in the second gain state  90 . 
     As the plunger  14  continues providing axial force during the first gain state  88  as shown in  FIG. 2 , the plunger  14  continues to move the spool  40  toward the second body end  24  to the second gain state  90  of the valve assembly  10  as shown in  FIG. 3 . In the second gain state  90 , fluid is still permitted to flow from the pressure port  28  around the flow portion  52  having a decreased diameter and through cavity  56  of the spool  40 , and to the work port  30  for operation of the working unit  20 . At the same time, fluid flow to the tank port  32  is still obstructed by cooperation between the first annular surface  34  of the valve body  12  and the first annular portion  48  of the spool  40 . As described above, when the valve assembly  10  is in the second gain state  90 , a reduced amount of force opposes movement of the spool  40  toward the second body end  24  thereby increasing the force of the fluid acting on the work port  30  relative to the amount of force applied to the spool  40  by the plunger  14  due to the relief passage  78  of the piston  66  being defined when the piston  66  is in the second position. In other words, a reduced amount of force by the plunger  14  on the spool  40  is necessary to move the spool  40  toward the second body end  24  as compared to when the valve assembly  10  is in the first gain state  88 . 
     The force of the fluid acts on the surface areas of the spool face  58  and the socket face  65  of the spool  40 . As the force increases, the force approaches the force applied by the plunger  14  and the spool  40  begins to move toward the first body end  22 . Movement of the spool  40  toward the first body end  22  increases fluid communication with the tank port  32  and decreases fluid communication with the pressure port  28 , thereby causing the force at the work port  30  to stabilize or drop. With force drop, net force of the spool  40  toward the second body end  24  exceeds net force of the spool  40  toward the first body end  22  causing movement of the spool  40  toward the second body end  24 . Movement of the spool  40  toward the second body end  24  decreases fluid communication with the tank port  32  and increases fluid communication with the pressure port  28 . This cycling of movement causes “modulation” (i.e. back and forth movement) of spool  40 . During modulation, the user continues to apply pressure to plunger  14 . The spool  40  modulates until the pressure force and the force of the modulating biasing member  92  is balanced against the force of the plunger  14 . At steady state equilibrium, (when the kinematic energy forces resulting from a changes in the force applied by the plunger  14  or force from the working unit  20  have subsided) the spool  40  will attain a stabilized position where fluid flow from the pressure port  28  to the work port  30  equals the fluid flow from the work port  30  to the tank port  32 . 
     Upon desired release of the fluid, the user reduces pressure to the plunger  14  and no longer generates a force toward the second body end  24 . The spool  40  moves in the toward the first body end  22  to the neutral position by the imbalance of force of the fluid and the force from the first compensating biasing member  80 . In the neutral position, fluid is permitted to flow from the work port  30  around the flow portion  52  of the spool  40  and to the tank port  32 . 
     Flow rate from the work port  30  to the tank port  32  is determined by the amount of flow required in the application, for example, the amount of flow necessary to disengage a hydraulic actuator or hydraulic brake within an acceptable amount of time. For a given spool configuration, the open area or gap providing for fluid communication between ports is a function of spool stroke or spool travel. Greater flow rates require greater cross-sectional flow areas or gaps and therein require the spool  40  to travel farther to increase the area of the gap. Similarly, when the plunger  14  is first energized, the required flow rate from the pressure port  28  to the work port  30  is determined by the amount of flow required in the application, for example, the amount of flow necessary to actuate a hydraulic brake within an acceptable amount of time. 
     As introduced above, a system for controlling the working unit  20  is also provided herein. The system has the first gain state  88  and the second gain state  90 . The system includes a fluid source  16  configured to provide the fluid force. The system further includes the valve assembly  10  with the valve assembly  10  in fluid communication with the fluid source  16 . The valve assembly  10  includes the plunger  14 . The valve assembly further includes the modulating biasing member abutting the plunger  14 . The valve assembly  10  further includes the spool  40 . The spool  40  operatively coupled to the plunger  14 . The plunger  14  is configured to move the spool  40  between the neutral position and the energized position. The valve assembly  10  further includes the piston  66 . The piston  66  is in fluid communication with the fluid source  16 . The piston  66  is configured to move between the first piston position and the second piston position. The system further includes a working unit  20  in fluid communication with the valve assembly  10  and configured to activate in response to the fluid force. The system is in the first gain state  88  when the piston  66  is in the first piston position and the system is in the second gain state  90  when the piston  66  is in the second piston position. 
     Although the invention has been described in connection with specific preferred embodiments, it should be understood that the invention as claimed should not be unduly limited to these specific embodiments. While at least one exemplary embodiment has been presented in the foregoing detailed description of the disclosure, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing an exemplary embodiment of the invention. It being understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope of the disclosure as set forth in the appended claims. 
     Further, any ranges and subranges relied upon in describing various embodiments of the present invention independently and collectively fall within the scope of the appended claims, and are understood to describe and contemplate all ranges including whole and/or fractional values therein, even if such values are not expressly written herein. One of skill in the art readily recognizes that the enumerated ranges and subranges sufficiently describe and enable various embodiments of the present invention, and such ranges and subranges may be further delineated into relevant halves, thirds, quarters, fifths, and so on. As just one example, a range “of from 0.1 to 0.9” may be further delineated into a lower third, i.e., from 0.1 to 0.3, a middle third, i.e., from 0.4 to 0.6, and an upper third, i.e., from 0.7 to 0.9, which individually and collectively are within the scope of the appended claims, and may be relied upon individually and/or collectively and provide adequate support for specific embodiments within the scope of the appended claims. In addition, with respect to the language which defines or modifies a range, such as “at least,” “greater than,” “less than,” “no more than,” and the like, it is to be understood that such language includes subranges and/or an upper or lower limit. As another example, a range of “at least 10” inherently includes a subrange of from at least 10 to 35, a subrange of from at least 10 to 25, a subrange of from 25 to 35, and so on, and each subrange may be relied upon individually and/or collectively and provides adequate support for specific embodiments within the scope of the appended claims. Finally, an individual number within a disclosed range may be relied upon and provides adequate support for specific embodiments within the scope of the appended claims. 
     For example, a range “of from 1 to 9” includes various individual integers, such as 3, as well as individual numbers including a decimal point (or fraction), such as 4.1, which may be relied upon and provide adequate support for specific embodiments within the scope of the appended claims. 
     The present invention has been described herein in an illustrative manner, and it is to be understood that the terminology which has been used is intended to be in the nature of words of description rather than of limitation. Many modifications and variations of the present invention are possible in light of the above teachings. The present invention may be practiced otherwise than as specifically described within the scope of the appended claims. The subject matter of all combinations of independent and dependent claims, both single and multiple dependent, is herein expressly contemplated. 
     INDUSTRIAL APPLICABILITY 
     While the present invention is not limited to a particular end application, use or industry, vehicles often rely on valve assemblies to provide fluid to working units, such as wheel brakes. The valve assembly is configured to move between a first piston position and a second piston position for providing multiple gain states for the valve assembly.