Patent Publication Number: US-8540207-B2

Title: Fluid flow control assembly

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 61/120,412 filed Dec. 6, 2008. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates in general to valves for controlling fluid flow and more particularly, to a fluid flow control assembly for controlling fluid flow in two directions of flow. 
     Valves are widely used for controlling the flow of a fluid from a source of pressurized fluid to a load device or from a load device to a low pressure reservoir. Frequently, a pump, or other device, is provided as the source of pressured fluid. The flow of the fluid is selectively controlled by a valve to control the operation of the load device. 
     One type of valve is a microvalve. A microvalve system is a MicroElectroMechanical System (MEMS) relating in general to semiconductor electromechanical devices. 
     MEMS is a class of systems that are physically small, having features with sizes in the micrometer range or smaller. A MEMS device is a device that at least in part forms part of such a system. These systems have both electrical and mechanical components. The term “micromachining” is commonly understood to mean the production of three-dimensional structures and moving parts of MEMS devices. 
     MEMS originally used modified integrated circuit (computer chip) fabrication techniques (such as chemical etching) and materials (such as silicon semiconductor material) to micromachine these very small mechanical devices. Today there are many more micromachining techniques and materials available. 
     The term “microvalve,” as used in this application, means a valve having features with sizes in the micrometer range or smaller, and thus by definition is at least partially formed by micromachining. The term “microvalve device,” as used in this application, means a device that includes a microvalve, and that may include other components. It should be noted that if components other than a microvalve are included in the microvalve device, these other components may be micromachined components or standard sized (larger) components, also known as macro sized components. 
     Various microvalve devices have been proposed for controlling fluid flow within a fluid circuit. A typical microvalve device includes a displaceable member or valve movably supported by a body and operatively coupled to an actuator for movement between a closed position and a fully open position. When placed in the closed position, the valve blocks or closes a first fluid port that is placed in fluid communication with a second fluid port, thereby preventing fluid from flowing between the fluid ports. When the valve moves from the closed position to the fully open position, fluid is increasingly allowed to flow between the fluid ports. 
     One type of microvalve is the micro spool valve. The micro spool valve typically consists of a micromachined spool disposed in a chamber formed in an intermediate layer of multilayer valve housing. A variety of ports through the layers of the housing provide fluid communication with the chamber. The micromachined spool is moveable in the chamber to selectively allow fluid communication though the chamber by blocking particular ports depending on the desired result. In operation, a differential pressure is exerted across the micromachined spool to move the micromachined spool into a desired position. Typically, the differential pressure is controlled by a pilot valve. 
     Another type of microvalve, often used as a pilot valve, consists of a beam resiliently supported by the body at one end. In operation, an actuator forces the beam to bend about the supported end of the beam. In order to bend the beam, the actuator must generate a force sufficient to overcome the spring force associated with the beam. As a general rule, the output force required by the actuator to bend or displace the beam increases as the displacement requirement of the beam increases. 
     In addition to generating a force sufficient to overcome the spring force associated with the beam, the actuator must generate a force capable of overcoming the fluid flow forces acting on the beam that oppose the intended displacement of the beam. These fluid flow forces generally increase as the flow rate through the fluid ports increases. 
     As such, the output force requirement of the actuator and in turn the size of the actuator and the power required to drive the actuator generally must increase as the displacement requirement of the beam increases and/or as the flow rate requirement through the fluid ports increases. 
     One specific type of microvalve system is the pilot operated microvalve. Typically, such a microvalve device includes a micro spool valve that is pilot operated by a microvalve of the type as described above. For example, U.S. Pat. Nos. 6,494,804; 6,540,203; 6,637,722; 6,694,998; 6,755,761; 6,845,962; and 6,994,115, the disclosures of which are herein incorporated by reference, disclose pilot operated microvalves and microvalves acting as pilot valves. 
     Microvalve devices have application in many fields for controlling the flow of fluids in systems such as hydraulic, pneumatic, and refrigerant systems, including the Heating, Ventilation, and Air Conditioning (HVAC) field. HVAC systems may include, without limitation, such systems as refrigeration systems, air conditioning systems, air handling systems, chilled water systems, etc. Many HVAC systems, including air conditioning and refrigeration systems operate by circulating a refrigerant fluid between a first heat exchanger (an evaporator), where the refrigerant fluid gains heat energy, and a second heat exchanger (a condenser), where heat energy in the refrigerant fluid is rejected from the HVAC system. One type of HVAC system is the heat pump system, which provides the ability to reverse flow of refrigerant through portions of the HVAC system. This allows the heat pump system to act as an air conditioning system in the summer, cooling air that flows through a first heat exchanger by absorbing the heat from the air into a refrigerant pumped through the first heat exchanger. The refrigerant then flows to a second heat exchanger, where the heat gained by the refrigerant in the first heat exchanger is rejected. However, during the winter, the flow of refrigerant between the first and second heat exchangers is reversed. Heat is absorbed into the refrigerant in the second heat exchanger, and the refrigerant flows to the first heat exchanger, where the heat is rejected from the refrigerant into the air flowing through the first heat exchanger, warming the air passing through the first heat exchanger. 
     SUMMARY OF THE INVENTION 
     This invention relates to an improved device for controlling fluid flow in a system, such as, but not limited to, a hydraulic, pneumatic, or HVAC system, and in particular to a reversible fluid flow control assembly. 
     The assembly may include a pilot valve responsive to a command signal for supplying a fluid at a command pressure to a pilot valve control port; and a pilot operated spool valve. The pilot operated spool valve may have a body having a first connector and a second connector, each of the first connector and the second connector being adapted for fluid communication with an external circuit. A spool may be disposed for sliding movement in the body. The spool may have a first end portion and a second end portion opposite the first end portion. The first end portion of the spool may be in fluid communication with the pilot valve control port such that the spool is urged to move in a first direction by the fluid at the command pressure. The spool may be movable to control a fluid flow between the first connector and the second connector through the body proportionally to the command pressure when the fluid flow is a forward flow from the first connector to the second connector and when the fluid flow is a reverse flow from the second connector to the first connector. The spool valve may use negative feedback in the form of fluid at a feedback pressure acting on the spool in a second direction, opposite the first direction, to position the spool in conjunction with the fluid at the command pressure. The spool valve may utilize unstable equilibrium of fluid forces to switch between controlling the forward flow and the reverse flow. 
     According to another aspect, the reversible fluid flow control assembly may include a spool valve with a body having a first connector and a second connector and a spool movable relative to the body for controlling flow between the first connector and the second connector. The reversible flow control assembly further may include a pilot valve device developing a single pressure command. The spool valve may be responsive to the single pressure command developed in said pilot valve device to control flow between the first connector and the second connector without regard to the direction of flow. The majority of forces acting on the spool to position the spool relative to the body when fluid is flowing through the valve may be fluid forces. 
     Various aspects of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiment, when read in light of the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a partial cross-section and partial schematic representation of a reversible fluid flow control device. 
         FIG. 2  is an exploded perspective view of the reversible fluid flow control device. 
         FIG. 3  is a sectional view of a spool of the reversible fluid flow control device. 
         FIG. 4  is a sectional view of a spool valve of the reversible fluid flow control device, showing the spool thereof in a first position. 
         FIG. 5  is an enlarged view of a portion, indicated by the circle  5 , of  FIG. 4 . 
         FIG. 6  is a view similar to  FIG. 4 , except showing the spool in a second position. 
         FIG. 7  is an enlarged view of a portion, indicated by the circle  7 , of  FIG. 6 . 
         FIG. 8  is a view similar to  FIG. 4 , except showing the spool in a shutoff position. 
         FIG. 9  is an enlarged view of a portion, indicated by the circle  9 , of  FIG. 8 . 
         FIG. 10  is a graph of operating regions of the reversible fluid flow control device for forward flow. 
         FIG. 11  is a graph similar to  FIG. 10 , except for reverse flow. 
         FIG. 12  is a sectional view similar to  FIG. 4 , but showing an alternate embodiment of a spool 
         FIG. 13  is a view similar to  FIG. 12 , except showing the spool in a second position. 
         FIG. 14  is a partial cross-section and partial schematic representation of an alternate embodiment of a reversible fluid flow control device with a spool thereof in a position within a first range of positions. 
         FIG. 15  is an enlarged cross-sectional view of the spool illustrated in  FIG. 14 . 
         FIG. 16  is a partial cross-section and partial schematic representation of the reversible fluid flow control device illustrated in  FIG. 14 , with the spool shown in a forward flow position with the first range of positions. 
         FIG. 17  is a partial cross-section and partial schematic representation of the reversible fluid flow control device illustrated in  FIG. 14 , with the spool shown in an unpowered or failed-power mode within a second range of positions. 
         FIG. 18  is a partial cross-section and partial schematic representation of the reversible fluid flow control device illustrated in  FIG. 17 , with the spool shown in a reverse flow position within the second range of positions. 
         FIG. 19  is a partial cross-section and partial schematic representation of the reversible fluid flow control device illustrated in  FIG. 14 , with the spool thereof shown in a shut off position intermediate the first range of positions and the second range of positions. 
         FIG. 20  is a partial cross-section and partial schematic representation of an alternate embodiment of a reversible fluid flow control device having a spool providing for unequal forward and reverse flow cross-sectional areas. 
         FIG. 21  is a first perspective view of the control device body illustrated in  FIG. 14 . 
         FIG. 22  is a second perspective view of the control device body illustrated in  FIG. 14 . 
         FIG. 23  is a partial cross-section and partial schematic representation of an alternate embodiment of a reversible fluid flow control device body. 
         FIG. 24  is a perspective cross-sectional view of the control device body illustrated in  FIG. 23 . 
         FIG. 25  is an alternate perspective view of the control device body illustrated in  FIGS. 23 and 24 , illustrating fluid filled spaces thereof. 
         FIG. 26  is a view similar to  FIG. 25 , except from a generally opposite perspective. 
         FIG. 27A  is an enlarged cross-sectional view of a portion of the spool valve illustrated in  FIG. 17 , showing the spool in a first metered position. 
         FIG. 27B  is an enlarged cross-sectional view of a portion of the spool valve illustrated in  FIG. 17 , showing the spool in a second metered position. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Preliminarily, it should be understood that in this description and in the claims, the use of the singular word “port”, “aperture”, “fluid conduit”, “passageway”, or words of similar import, should be considered to include the possibility of multiple ports (apertures, fluid conduits, passageways, etc.) with the same functionality attributed to the single port (apertures, fluid conduits, passageways, etc.) unless explicitly and definitely limited to the singular. Furthermore, the use of directional terms such as “left” and “right”, and words of similar import, should be interpreted in the context of the figure(s) under discussion, and should not be interpreted as limitations on orientation during use or the scope of the claims. 
     Referring now to the drawings, wherein like reference numbers and characters may represent like elements through out all of the figures, there is illustrated in  FIGS. 1 and 2  a reversible fluid flow control assembly, generally indicated at  10 . The flow control assembly  10  may include a spool valve, indicated generally at  12 , and a pilot valve device, indicated generally at  14 . The spool valve  12  and the pilot valve device  14  each may be in fluid communication with a first connector  16 , by means of which the flow control assembly  10  may be connected in fluid communication with a first portion of a system (not shown) in which the flow control assembly  10  may be installed, as will be described in detail below. As will also be described in detail below, the spool valve  12  and the pilot valve device  14  each may be in fluid communication with a second connector  18 , by means of which the flow control assembly  10  may be connected in fluid communication with a second portion of the system which the flow control assembly  10  may be installed. The first connector  16  and the second connector  18  each may be any suitable structure by means of which the flow control assembly  10  may be connected for installation in the system, including without limitation, threaded connections, welded connections, brazed connections, press-fit connections, rolled connections, permanently deformable connections, adhesive connections, compression fitting connections, etc. 
     The spool valve  12  may include a body  20 . Preferably the first connector  16  and the second connector  18  are at least partially formed in the body  20 , as is the case in the embodiment illustrated in  FIGS. 1 and 2 , where each of the first connector  16  and the second connector  18  is shown as a threaded connection port which can threadably accept standard hydraulic tube fittings  19 . The body  20  may be made of any material suitable for the application, such as aluminum or other metal. 
     The body  20  may have an interior wall surface  21  defining a bore  22  therethrough. The bore  22  may have a first end portion, indicated generally at  24 , a second end portion, indicated generally at  26 , and a central portion, indicated generally at  27 . The first end portion  24  of the bore  22  may be enlarged and threaded as shown to accept a plug  28 . Similarly, the second end portion  26  of the bore  22  may be enlarged and threaded as shown to accept another plug  28 . 
     The spool valve  12  may further include a spool  29  disposed for sliding movement in the bore  22 . The spool  29  may have a first end portion  29   a  and a second end portion  29   b . As illustrated in  FIGS. 1 and 2 , the spool  29  may be oriented in the bore  22  with the first end portion  29   a  of the spool  29  near the first end portion  24  of the bore  22 , and the second end portion  29   b  of the spool  29  near the second end portion  26  of the bore  22 . The structure of the spool  29  will be discussed in further detail below. 
     The spool  29  and the plug  28  in the first end portion  24  of the bore  22  cooperate with the body  20  to define a command chamber  30  in the first end portion  24  of the bore  22 . The purpose of the command chamber  30  will be discussed below. A fluid conduit  31  is formed in the body  20  which may be in fluid communication with the command chamber  30  and, as will be discussed further below, in fluid communication with the pilot valve device  14 . The spool  29  and the plug  28  in the second end portion  26  of the bore  22  cooperate with the body  20  to define a feedback chamber  32  in the second end portion  26  of the bore  22 . The purpose of the feedback chamber  32  will be discussed below. 
     As illustrated in  FIG. 1 , a plurality of cavities may be formed in the body  20  in fluid communication with the central portion  27  of the bore  22 , at axially spaced locations along the bore  22 . A first one of this plurality of cavities may take the form of a circumferentially-extending first groove  34  formed in the surface  21  of the body  20  defining the bore  22  at a first axial location along the bore  22 , which, compared to the locations of the rest of the plurality of cavities, may be seen to be relatively close to the first end portion  24  of the bore  22 , and thus closest to the command chamber  30 . A second one of this plurality of cavities may take the form of a circumferentially-extending second groove  36  formed in the surface  21  of the body  20  defining the bore  22  at a second axial location along the bore  22  which may be closer to the second end portion  26  of the bore  22  (and thus closer to the feedback chamber  32 ) than the first axial location where the first groove  32  may be located. A third one of this plurality of cavities may take the form of a circumferentially-extending third groove  38  formed in the surface  21  defining the bore  22  at a third axial location along the bore  22  which is intermediate, preferably midway between the first axial location at which the first groove  34  may be located and the second axial location at which the second groove  36  may be located. 
     The body  20  may define a fluid conduit  40  providing fluid communication between the second connector  18  and the third groove  38 . The body  20  may also define a fluid conduit  42  providing fluid communication between the first connector  16  and both of the first groove  34  and the second groove  36 . In the example illustrated in  FIGS. 1 and 2 , the fluid conduit  42  is comprised in part by intersecting bores  42   a  and  42   b , which may be formed, for example, by drilling through the body  20  from the surface of the body  20 , and then closing the outer ends of the bores  42   a  and  42   b , in some manner, such as by pressing in balls  44 , which may be followed by deformation of the body  20  by rolling, staking, etc., to capture the balls  44  in their respective bores. The bore  42   a  intersects with the first groove  34 , while the bore  42   b  intersects with the first connector  16 . The body  20  also defines a third bore  42   c , which also comprises a portion of the fluid conduit  42 , and may also, for example, be drilled from the surface of the body  20  to intersect and communicate with the bore  42   b . However, the outer end of the bore  42   c  is not closed, but rather may be open to provide fluid communication with the pilot valve  14  in a manner which will be discussed below. The body  20  also defines a fourth bore  42   d  which provides fluid communication between the bore  42   b  and the second groove  36 . The bore  42   d  may be formed, for example, by drilling axially from an inner end of the first connector  16  to the second groove  36 . 
     The body  20  may also define a fluid conduit  46  providing fluid communication between the second connector  18  and the pilot valve  14  in a manner which will be discussed below. In the example illustrated in  FIG. 1 , the fluid conduit  46  is comprised of intersecting bores  46   a  and  46   b , which may be formed, for example, by drilling through the body  20  from the surface of the body  20 . The outer end of the bore  46   a  is closed in some manner, such as by pressing in a ball  44 , which may be followed by deformation of the body  20  by rolling, staking, etc., to capture the ball  44  in the bore  46   a . The bore  46   b  remains open to provide communication with the pilot valve  14  in a manner which will be discussed below. The bore  46   a  intersects with the second connector  18 . 
     Now referring additionally to  FIG. 3 , the spool  29  may have a central portion, indicated generally at  50 , between the first end portion  29   a  and the second end portion  29   b . The spool  29  may have a first axial end face, indicated generally at  52  on the first end portion  29   a  which is in fluid communication with the command chamber  30 . The first axial end face  52  may have a central boss  53  formed thereon, the purpose of which will be discussed below. The spool  29  may have a second axial end face, indicated generally at  54 , on the second end portion  29   b  which is in fluid communication with the feedback chamber  32 . The second axial end face  54  may have an opening  56  defined therein. 
     The spool  29  may have an internal axial passageway  58  defined therein. The axial passageway  58  may communicate with the opening  56  in the second axial end face  54 . The axial passageway  58  may extend from the opening  56  into the central portion of the spool  29 . The second end portion  29   a  may include a damping orifice  59  that restricts communication between the portion of the axial passageway in the central portion  50  of the spool  29  and the feedback chamber  32 , in order to dampen movement of the spool  29  during operation. In the illustrated embodiment, the orifice  59  is formed as a threaded insert which is threadably fixed in the second end  29   b  of the spool  29 . A slot  60  may be formed in the threaded orifice  59  to allow the use of a screwdriver or other tool to turn the threaded orifice  59  during installation. Of course, the orifice  59  may be affixed to the spool  29  in any suitable manner, or may be integrally formed with the spool  29 , if a reduced diameter (compared to the diameter of the rest of the axial passageway  58 ) orifice is provided for damping at all. 
     The spool  29  may have an exterior surface  62 . The spool  29  may have a first port  64  at a first axial location in the central portion  50  of the spool  29  providing fluid communication between the exterior surface  62  and the axial passageway  58 . The spool  29  may have a second port  66  in the central portion  50  of the spool  29  at a second axial location between the first axial location and the second end portion  29   b  of the spool  29  providing fluid communication between the axial passageway and the exterior surface  62  of the spool  29 . In the illustrated embodiment, the first port  64  may be one of a plurality of ports spaced apart circumferentially about the spool  29  at the first axial location, and the second port  66  may be one of a plurality of ports spaced apart circumferentially about the spool  29  at the second axial location. 
     The spool  29  may have a circumferential groove  67  formed in the exterior surface  62  at an axial location between the first axial location and the first end portion  29   a  of the spool  29 . The spool  29  may further have an aperture  68  providing fluid communication between the circumferential groove  67  and the axial passageway  58  formed in the spool  29 . The aperture  68  allows fluid at feedback pressure existing in the axial passageway  58  during equilibrium conditions to be distributed about the spool  29  in the groove  67 , which, as will become clearer during the discussion of operation below, minimizes the differential pressure between the command chamber  30  and the groove  67 , and thus minimizes leakage out of the command chamber  30  between the surface  21  defining the bore  22  and the surface  62  of the spool  29 . 
     The spool  29  may further be provided with a plurality of circumferentially extending grooves  69 , which may be relatively shallow compared to the groove  67 . The grooves  69  may be formed in the surface  62 , for example, on either side of the groove  67 , between the first axial location and the second axial location (i.e., between the first port  64  and the second port  66 , and in the second end portion  29   b ). The grooves  69  are believed to help distribute any leakage that may occur between the outer surface  62  of the spool  29  and the surface  21  defining the bore  22  about the circumference of the spool  29 , equalizing pressures and minimizing unequal radial loading on the spool  29  which might occur from circumferentially unequal leakage along the spool  29 , thereby minimizing friction between the surface  21  and the surface  62 . 
     Referring again to  FIGS. 1 and 2 , a coil spring  70  may be disposed in the command chamber  30 , acting between the plug  28  in the first end portion  24  of the bore  22  and the spool  29  to urge the spool  29  toward the second end portion  26  of the bore  22 . The boss  53  on the spool  29  may help to radially center the spring  70 . Similarly, a coil spring  72  may be disposed in the feedback chamber  32 , acting between the plug  28  in the second end portion  26  of the bore  22  and the spool  29  to urge the spool  29  toward the first end portion  24  of the bore  22 . As illustrated, the orifice  59  may extend out of the second end portion  29   b  of the spool  29  to help to radially center the spring  72 . 
     A stop structure  74  may be provided that will limit motion of the spool  29  in a first direction toward the second end portion  26  of the bore  22 . In particular, the stop structure  74  may be provided to prevent the spool  29  from traveling past a desired maximum travel position, shown in  FIGS. 4 and 5 . The stop structure  74  may, for example, be provided on the plug  28  disposed in the second end portion  24 . The stop structure  74  may be adjustable to allow adjustment of the maximum travel position. For example, the stop structure  74  in the illustrated embodiment may be a threaded member  75  threadably engaging a threaded bore formed in the associated plug  28 . As most clearly seen in  FIG. 5 , a suitable maximum travel position may be a first position of the spool  29 , which is defined as the first position of the spool  29  which is reached during movement in the first direction in which the port  66  is fully uncovered in communication with the second groove  36  and the port  64  is fully uncovered in communication with the third groove  38 . If the spool  29  is moved in a second direction toward the first end portion  24  of the bore  22  from the first position illustrated in  FIG. 5 , the portion of the body  20  forming a land between the second groove  36  and the third groove  38  will progressively cover the port  66 , decreasing the cross-sectional area through which fluid can flow between the second groove  36  and the axial passageway  58  in the spool  29 . As will be further described below, the spool  29  can be positioned in any of a first range of positions, including the first position, each position in the first range of positions having a different cross-sectional area for fluid communication between the second groove  36  and the axial passageway  58  via the port  66 . 
     Similarly, a stop structure  76  may be provided that will engage the spool  29 , limiting motion of the spool  29  in a second direction toward the first end portion  24  of the bore  22 , preventing the spool  29  from traveling past a desired maximum travel position, shown in  FIGS. 6 and 7 . The stop structure  76  may, for example, be provided on the plug  28  disposed in the first end portion  24 . The stop structure  76  may be adjustable to allow adjustment of the maximum travel position. For example, the stop structure  76  in the illustrated embodiment may be a threaded member  77  threadably engaging a threaded bore formed in the associated plug  28 . As most clearly seen in  FIG. 7 , a suitable maximum travel position may be a second position of the spool  29 , which is defined as the first position of the spool  29  which is reached during travel in the second position in which the port  64  is fully uncovered in communication with the first groove  34  and the port  66  is fully uncovered in communication with the third groove  38 . If the spool  29  is moved in the second direction toward the first end portion  24  of the bore  22  from the first position illustrated in  FIGS. 4 and 5 , the portion of the body  20  forming a land between the first groove  34  and the third groove  38  will progressively cover the port  64 , decreasing the cross-sectional area through which fluid can flow between the second groove  36  and the axial passageway  58  in the spool  29 . As will be further described below, the spool  29  can be positioned in any of a second range of positions, including the second position, each position in the second range of positions having a different cross-sectional area for fluid communication between the first groove  34  and the axial passageway  58  via the port  64 . 
     The springs  70  and  72  may urge the spool  29  to a shutoff position, between the first range of positions and the second range of positions of the spool  29 , which is illustrated in  FIGS. 8 and 9 . More specifically, the spring  70  may urge the spool  29  to move from the second range of positions toward the shutoff position, and the spring  72  may urge the spool  29  to move from the first range of positions toward the shutoff position. 
     In the shutoff position, both the port  64  and the port  66  may be completely uncovered to communicate with the third groove  38 ; however, neither the port  64  nor the port  66  is in substantial direct fluid communication with either the first groove  34  or the second groove  36 , and thus substantially no fluid communication exists between the axial passageway  58  in the spool  29  and either the first groove  34  or the second groove  36 . 
     Referring to  FIGS. 1 and 2 , the pilot valve device  14  may include a valve or valves  80  and a manifold  82  provided with fluid passageways interconnecting the valve  80  and the spool valve  12 , as will be described below. 
     The valve  80  may include a fluid conduit  84  extending between a first pilot connection port  86  and a second pilot connection port  88 . The flow through the fluid conduit  84  may be regulated by two variable orifices in series arrangement in the fluid conduit  84 . A variable first orifice  90  may be a normally closed orifice; that is the orifice may be closed in the absence of a command signal to the valve  80 . A variable second orifice  92  may be a normally open orifice. A pilot valve control port  94  may be connected in fluid communication with the fluid conduit  84  between the first orifice  90  and the second orifice  92 . The valve  80  may be a single valve or microvalve containing moving components acting as the first orifice  90  and the second orifice  92 . Alternatively, the valve  80  may be embodied as a plurality of valves or microvalves acting as the first orifice  90  and the second orifice  92 . 
     One and only one pressure command used for control of the spool valve  12  is developed in the pilot valve device  14 . In the illustrated embodiment, for example, the pressure command is developed in the fluid conduit  84  between the first orifice  90  and the second orifice  92  when pressurized fluid is supplied to the valve  80 . The pressure developed there is the command pressure, and fluid at the command pressure is conveyed from the pilot valve device  14  to the command chamber  30  of the spool valve  12 . As illustrated herein, the pressure command may be conveyed to the command chamber  30  via a single fluid conduit via a single pilot valve control port  94  and a single fluid conduit  31 . However, it is contemplated that multiple fluid paths may be used, perhaps even simultaneously, to convey the single pressure command between the point at which the pressure command is developed to the point at which the pressure command is utilized to control the operation of operation of the spool valve  12 , and such should be considered within the scope of the claims. 
     If the valve  80  is a microvalve, the manifold  82  may be advantageously used to adapt the small package size of a microvalve to the large package size of the body  20 . The valve  80  may be mounted by any suitable method (such as brazing, soldering, adhesively bonding, mechanically connection, etc.) on the manifold  82 , or on the body  20  if the manifold  82  is omitted. The first pilot connection port  86  is connected in fluid communication with the fluid conduit  42 , via the bore  42   c , providing uninterrupted fluid communication between the normally closed orifice  90  and the first connection  16 . The second pilot connection port  88  is connected in fluid communication with the fluid conduit  46 , via the bore  46   b , thus providing uninterrupted fluid communication between the normally open orifice  92  and the second connection  18 . The pilot valve control port  94  is connected in fluid communication with the fluid conduit  31 , and the pilot valve control port  94  is thus in uninterrupted fluid communication with the command chamber  30 . 
     As seen in  FIG. 2 , O-rings  96  may be utilized between the manifold  82  and the body  20  to prevent leakage at the interface between the manifold and the body  20  from the fluid conduit  42 , the fluid conduit  46 , or the fluid conduit  31 . 
     Operation of the illustrated embodiment will now be discussed. 
     During operation, the reversible fluid flow control assembly  10  is installed in a system (not shown) via the first connection  16  and the second connection  18 . During operation of the system, normally one of the first connection  16  and the second connection  18  will be supplied with a higher pressure (hereinafter “supply pressure”) and the other of the first connection  16  and the second connection  18  will be supplied with a lower pressure (hereinafter “return pressure”). During operation, when there are differences between supply pressure and return pressure, the components of the reversible fluid control assembly  10  operate to develop two separate fluid pressures acting in opposition across the spool  29 . On one side, the left as drawn in  FIGS. 1 and 4 , a command pressure developed in the pilot valve device  14  and supplied to the command chamber  30  pushes on the first axial end face  52  of the spool  29  to urge the spool  29  in the first direction (rightward as seen in  FIGS. 1 ,  4  and  5 ), moving the spool  29  into the first range of positions of the spool  29 . A pressure proportional to the position of the spool  29 , referred to as feedback pressure, is developed in the axial passageway of the spool  29  as will be described below. The feedback pressure is communicated via the opening  56  from the axial passageway of the spool  29  to the feedback chamber  32  on the right side (as seen in  FIGS. 1 and 4 ) of the spool  29 . Feedback pressure in the feedback chamber  32  acting on the second axial end face  54  of the spool  29 , urges the spool  29  in the second direction (leftward as seen in  FIGS. 1 and 4 ). The spool  29  is free to move until the forces acting on either end face  52 ,  54  of the spool  29  balance. Note that in this discussion the forces exerted by the springs  70 ,  72  will not be discussed, as the springs  70 ,  72  would normally be chosen to have a very low spring rate, so as to not exert significant force on the spool compared to the fluid forces acting on the axial end faces  52 ,  54  of the spool  29 ; if the spring forces are significant, calculation of their effect is relatively simple and predictable balance of forces calculation for one of ordinary skill in the art. Indeed, in some applications, the springs  70 ,  72  may be omitted entirely. In any case, it will be appreciated that it is contemplated that in at least some embodiments, a majority of axial forces acting on the spool  29  to position the spool  29  relative to the body  20  when fluid is flowing through the spool valve  12  will be fluid forces. 
     Both the command pressure and the feedback pressures will fall between supply pressure and return pressure in normal operation.  FIG. 10  is a graph of feedback pressure versus position of the spool  29  during forward flow through the spool valve  12 .  FIG. 11  is a graph of feedback pressure versus position of the spool  29  during reverse flow of fluid through the spool valve  12 . 
     The feedback pressure is a pressure developed between the first port  64  and the second port  66  in the axial passageway  58 . During forward flow, with the spool  29  in the first range of positions, flow of fluid through the spool valve  12  travels from the first connection  16 , through the second port  66 , through the axial passageway  58  of the spool  29 , through the first port  64  and then out through the second connection  18 , as illustrated in  FIGS. 4 and 5 . As the spool  29  moves from the shutoff position toward the first position, the land formed by the body  20  between the second groove  36  and the third groove  38  progressively uncovers the second port  66 , and the first port  64  remains uncovered and in full communication with the third groove  38 . In forward flow, the third groove  38  will be at return pressure, while the second groove  36  will be at supply pressure. As the second port  66  is progressively uncovered, pressure in the axial passageway  58  will rise, as indicated on the right half of the graph in  FIG. 10 , where spool position “S” is the shutoff position illustrated in  FIG. 1 , and spool position “ 1 ” is the first position, illustrated in  FIGS. 4 and 5 . However, feedback pressure will not rise to the magnitude of supply pressure, since the first port  64  is continually venting fluid from the axial passageway  58  to the third groove  38 , which is at return pressure. The feedback pressure in the feedback chamber  32  will equal the pressure in the axial passageway  58  once steady state operating conditions exist. During transient conditions, the pressure in the feedback chamber  32  may lag the pressure in the axial passageway due to the damping effect of the orifice  59 . However, this lag may be ignored for the purpose of analyzing the steady-state to steady-state operation of the reversible fluid control assembly  10 . 
     The current concept is best explained by describing functionality around three points, the first position of the spool  29 , which is illustrated in  FIGS. 4 and 5 , the second position of the spool  29 , which is illustrated in  FIGS. 6 and 7 , and the shutoff position of the spool  29 , which is illustrated in  FIGS. 1 ,  8 , and  9 . 
     In the first position with forward flow, the spool valve  12  is considered stable. Stability is defined herein as any state of operation of the spool valve  12  where a small deviation in command pressure results in movement of the spool  29  that generates a proportionate change in feedback pressure that tends to return operation of the spool valve  12  to an equilibrium condition with the spool  29  continuing to operate on the same side of the shutoff position as before the deviation in command pressure. Conversely, instability (or unstable condition) is defined as any state of operation of the spool valve where a small deviation in command pressure results in movement of the spool  29  that generates a feedback pressure that does not tend to return operation of the spool valve  12  to an equilibrium condition with the spool  29  continuing to operate on the same side of the shutoff position as before the deviation in command pressure. 
     Assume the spool valve  12  is operating in equilibrium with forward flow, and the spool  29  is at a position within the first range of positions, and more particularly, in a position intermediate the shutoff position and the first position (which, it will be recalled, are indicated as “S” and “ 1 ” on the graph of  FIG. 10 ). The command signal supplied to the pilot valve device  14  is at an intermediate value. The normally closed orifice  90  of the pilot valve device  14  is partially opened, and the normally open orifice  92  of the pilot valve device  14  is also partially opened, and the pressure in the passageway  84  between the orifice  90  and the orifice  92  (the command pressure supplied via the pilot valve control port  94  to the command chamber  30  of the spool valve  12 ) is a steady percentage of the difference between the supply pressure supplied to the first pilot connection port  86  and the return pressure at the second pilot connection port  88 . Now assume that the command signal to the pilot valve device  14  is increased. This causes the normally closed orifice  90  to open further and the normally open orifice  92  to close further. This causes the command pressure to rise. An increase in command pressure causes the spool  29  to move in the first direction, away from the command chamber  30  (rightward as seen in  FIGS. 4 ,  5 , and  10 ). As the spool  29  moves away from the command chamber  30 , feedback pressure increases. Feedback pressure increases due to the increasing ratio of cross-sectional flow area through the second port  66  (which is the port opening to supply pressure) to the cross-sectional flow area of the first port  64  (which is the opening to return pressure), raising the pressure in the axial passageway  58 . As feedback pressure increases, the spool  29  will come to rest in a new equilibrium position where feedback pressure substantially equals command pressure. The converse is true with decreasing command pressure (the spool  29  will move in the second direction, the second port  66  gets increasingly covered, lowering the ratio of the cross-sectional flow area of the through the second port  66  (which is the port opening to supply pressure) to the cross-sectional flow area of the first port  64  (which is the opening to return pressure), lowering the pressure in the axial passageway  58 , and the spool  29  will come to rest in a new equilibrium position within the first range of positions when the feedback pressure falls to approximate the lowered command pressure. 
     Similarly, with the spool  29  positioned within the second range of positions, and the reversible fluid flow control assembly  10  operating with reverse flow (supply pressure supplied to the second connection  18 , with return pressure at the first connection), the spool  29  will also be operating in a stable manner, as illustrated in  FIGS. 6 ,  7  and the left half of the graph in  FIG. 11 . 
     Assume spool  29  is at a position within the second range of positions, and more particularly, in a position intermediate the shutoff position and the second position (which are indicated as “S” and “ 2 ” on the graph of  FIG. 11 ). The command signal supplied to the pilot valve device  14  is at an intermediate value. The normally closed orifice  90  of the pilot valve device  14  is partially opened, and the normally open orifice  92  of the pilot valve device  14  is also partially opened, and the pressure in the passageway  84  between the orifice  90  and the orifice  92  (the command pressure supplied via the pilot valve control port  94  to the command chamber  30  of the spool valve  12 ) is a steady percentage of the difference between the supply pressure at the second pilot connection port  88  and the return pressure at the first pilot connection port  86 . 
     Now assume it is desired to open the spool  29  more, that is, move the spool toward the second position to increase fluid flow through the spool valve  12 . The command signal supplied to the pilot valve device  14  is increased. This causes the normally closed orifice  90  to open further, opening up the release path to return pressure at the first connection  16 , and the normally open orifice  92  to close further, throttling the supply pressure supplied from the second connection  18 . This causes the command pressure supplied to the command chamber to decrease. A decrease in command pressure causes the spool  29  to move in the second direction, toward the command chamber  30  (leftward as seen in  FIGS. 6 ,  7 , and  11 ). As the spool  29  moves toward the command chamber  30 , feedback pressure will decrease. Feedback pressure decreases due to the increasing the ratio of cross-sectional flow area through the first port  66  (which is the port opening to return pressure in the first groove  34 ) to the cross-sectional flow area of the second port  64  (which is the opening to supply pressure). With the release path to return through the port  64  opened up, and the cross-sectional area of the flow path from supply fixed, the pressure in the axial passageway  58  will also fall. As feedback pressure decreases, the spool  29  will come to rest in a new equilibrium position where feedback pressure substantially equals command pressure, with the increased flow through the spool valve  12  that was desired. The converse is true with a decreasing command signal, which will generate an increase command pressure in the pilot valve device  14 . This will cause the spool  29  to move in the first direction, so that the first port  64  will get increasingly covered, lowering the ratio of the cross-sectional flow area of the first port  64  (which is the port opening to return pressure) to the cross-sectional flow area of the second port  66  (which is the opening to supply pressure), raising the pressure in the axial passageway  58 , and the spool  29  will come to rest in a new equilibrium position within the second range of positions when the feedback pressure rises to approximate the increased command pressure. The flow rate through the spool valve  12  will be lower than the original flow rate. 
     Now consider the possible scenarios in which the spool valve  12  is operating in an unstable operating region. As seen in  FIGS. 10 and 11 , there are two unstable regions: The first unstable operating region is operation in the second range of positions during forward flow, and the second unstable operating region is operation in the first range of positions during reverse flow. For each of the two unstable operating regions, the command pressure can be changed in two directions: command pressure can be increased or command pressure can be decreased. Thus, there are four scenarios to consider. 
     For the first scenario, consider the case in which command pressure is increased while the spool  29  is in the second range of positions during forward flow operation (for example, when forward flow is first initiated), as illustrated in  FIGS. 6 and 7  and the left half of the graph of  FIG. 10 . The spool valve  12  will be in an unstable mode of operation, and will respond according to the principles of unstable equilibrium to changes in command pressure. While unstable, a small increase or decrease in command pressure does not result in a proportional movement in spool position, nor is there a return to equilibrium operation in a spool position on the same said of the shutoff position in which the spool  29  was before the change in command pressure. 
     Assuming that the spool  29  is at equilibrium at the second position (that is, with feedback and command pressures exerting equal and opposite forces on the spool  29 ) with forward flow existing, increased command pressure causes the spool  29  to move in the first direction, away from the command chamber  30 . As the spool  29  moves away from the command chamber  30 , the first port  64  will become increasingly covered, throttling the flow path from the groove  34  (which is at supply pressure during forward flow) to the axial passageway  58 . The release path through the second port  66  remains wide open, and feedback pressure will decrease as the pressure in the axial passageway  58  decreases. As feedback pressure decreases, the net force pushing the spool  29  in the first direction (the right as viewed in  FIGS. 6 ,  7  and  10 ) increases, accelerating movement in the first direction (away from the command chamber  30 ). Movement of the spool  29  will not stop in the second range of positions, but instead the spool  29  will continue past the shutoff position into the first range of positions. Once past the shutoff position, the spool valve  12  returns to stable operation for forward flow, since further movement in the first direction will result in increased communication between the groove  36  (which is at supply pressure) and the axial passageway  58 , raising feedback pressure until feedback pressure counterbalances command pressure, as discussed above. At this point, the spool  29  comes to rest pending further changes to command pressure. 
       FIG. 10  illustrates this transition. The spool valve  12  is initially at condition O 1 , which corresponds to spool position S 1  in the second range of positions, with command and feedback pressures at P 1 . If the command pressure is raised to P 2 , the spool  29  is urged in the first direction by the imbalance of command pressure and feedback pressure. There is no position on the operating curve between position S 1  and the shutoff position S in which feedback pressure will equal P 2 , so the spool  29  moves over into the first range of positions, and moves from S to S 2 , at which point the feedback pressure (pressure in the axial passageway  58 ) rises to P 2 . Position S 2  is in the stable operating region of the graph of  FIG. 10 . Once in the stable region, the spool valve  12  will remain stable while forward flow continues. 
     For the second scenario, consider what would happen if all the conditions were the same as in the preceding scenario, but command pressure were reduced while the system was operating with forward flow and the spool  29  was in the second range of positions. Again, assume the spool valve  12  is initially at condition O 1 , which corresponds to spool position S 1  in the second range of positions, with command and feedback pressures at P 1 . If the command pressure is lowered, the spool  29  is urged in the second direction by the imbalance of command pressure and feedback pressure. This causes the first port  62  to become more uncovered, increasing the cross-sectional flow area between the first groove  34 , which is at supply pressure, and the axial passageway  58 . This causes an increase in feedback pressure, further increasing the imbalance of command pressure and feedback pressure. There is no position on the operating curve between position S 1  and the second position (indicated as “ 2 ” in  FIG. 10 ) in which feedback pressure will drop to equal a command pressure less than P 1 , so the spool  29  moves in the second direction until the stop  74  is encountered, at which point the spool  29  is in the second position. Although the spool  29  is no longer moving, the spool valve  12  is still considered to be operating in an unstable manner, since the spool  29  has not returned to equilibrium, because the command pressure and the feedback pressure are not substantially equal. To return to stable operation, a command pressure greater than maximum feedback pressure must be generated to initiate movement of the spool in the first direction. Once the command pressure exceeds the feedback pressure, the spool valve  12  will return to stable operation by moving the spool  29  to the first range of positions, in the same manner as discussed in the first scenario. 
     Command pressure may be raised above maximum feedback pressure in all operating modes, because, when the spool  29  is moved to the second position, the axial passageway  58  will be connected to return pressure either through the wide-open first port  64  or through the wide open second port  66  (see  FIGS. 6 and 7 ). Thus, feedback pressure cannot reach supply pressure, and may have a magnitude only about half that of supply pressure. In contrast, by manipulating the normally open orifice  90  and the normally closed orifice  92 , the pilot valve control port  94  may be substantially isolated from return pressure, and fully connected to supply pressure so that command pressure can substantially equal supply pressure. 
     For the third scenario, consider case where the spool  29  is in the first range of positions, and reverse flow exists, which describes the unstable region of the graph in the right half of  FIG. 11 . From an initial position between the shutoff position “S” and the first position “ 1 ” illustrated in  FIGS. 4 and 5 , any decrease in command pressure causes an imbalance with feedback pressure which urges the spool to move in the second direction (to the left in  FIGS. 4 ,  5 , and  11 ), past the shutoff position, and into the second range of positions (the stable region). Movement of the spool  29  in the second direction while in the second range of positions (refer to  FIGS. 6 and 7 ) increasingly uncovers the bore  64  to open a release path through the axial passageway  58 , lowering the pressure in the axial passageway  58 , and thus feedback pressure. This continues until feedback pressure drops to command pressure, at which point the spool  29  comes to rest pending further changes to command pressure as the spool valve  12  returns to stable operation. Thus, the third scenario is similar to the first scenario. 
     For the fourth scenario, consider the case the same initial unstable conditions as the third scenario and consider the response to an increase in command pressure. Any increase in command pressure causes an imbalance with feedback pressure which urges the spool to move in the first direction (to the right in  FIGS. 4 ,  5 , and  11 ), past the shutoff position, and into the second range of positions (the stable region). Movement of the spool  29  in the second direction while in the second range of positions (refer to  FIGS. 6 and 7 ) increasingly uncovers the bore  64  to open a release path through the axial passageway  58 , lowering the pressure in the axial passageway  58 , and thus feedback pressure. The spool  29  moves disproportionately in the first direction and will move until the spool  29  engages the stop  74 , with the spool  29  in the first position. At this point, feedback pressure will be about half of the difference between supply and return pressure, since both the first port  64  and the second port  66  are both fully uncovered. The spool  29  will remain in the first position until the command pressure is dropped below feedback pressure. When this occurs, the spool  29  will begin to move disproportionately in the second direction, and will continue past the shutoff position into the second range of position, until a position is reached in which the feedback pressure decreases to the newly lowered command pressure. At this point, the spool valve  12  returns to stable operation, and the spool  29  comes to rest pending further changes to command pressure. 
     The shutoff position, illustrated in  FIGS. 1 ,  8  and  9 , represents the transition point for both forward and reverse flow. It will be appreciated from  FIG. 9 , that there may actually be a range of positions in which the spool  29  is positioned such that flow through the spool valve  12  is not possible because neither the first port  64  nor the second port  66  is aligned even partially with a groove other than the third groove  38 , and thus no flow path through the spool valve  12  to or from the third groove  38  (and the second connection  18 ) is established. The extent of this range of positions in which flow is shut off of course depends on the spacing between the first port  64  and the second port  66  relative to the spacing between the first groove  34  and the second groove  36 , and the width of the lands between the first, second, and third grooves  34 ,  36 , and  38 . For the purposes of this discussion, this entire range of positions in which flow is shut off, which are physically located between the first range of positions and the second range of positions, will be referred to the shutoff position. Relative to the shutoff position, any spool position in the second direction from the shutoff position (to the left of the shutoff position illustrated in  FIGS. 1 ,  8 , and  9 ) is stable with reverse flow and unstable in forward flow. Conversely, any spool position in the first direction from the shutoff position (to the right of the shutoff position illustrated in  FIGS. 1 ,  8  and  9 ) is stable with forward flow and unstable with reverse flow. 
     The illustrated arrangement for the reversible flow control assembly  10  is particularly well suited for use of a microvalve in the pilot valve device  14 , because the arrangement allows flow area through the spool valve  12  to be a function of the command pressure supplied by the pilot valve device  15 , regardless of supply and return pressure, assuming stable operation of the spool valve  12 . As described above, flow opening (the effective cross-sectional area of the flow path through the spool valve) is a function of feedback pressure. Since feedback pressure is developed in the spool valve  12  by throttling fluid flowing between supply and return pressure, feedback pressure is function of the relative pressure difference between supply and return pressure. A microvalve or series of microvalves that develop a “working pressure” between a series of orifices responsive to an electrical command supplied to the microvalve and arranged in a fluid conduit between a supply pressure and a return pressure also outputs a command pressure relative to the difference between supply and return pressure. 
     For the pilot valve  14 , this may be expressed 
                     (         P   C     -     P   T           P   S     -     P   T         )     =     f   ⁡     (     C   e     )               Equation   ⁢           ⁢   1               
where
         P C  is Command Pressure;   P T  is Return Pressure;   P S  is Supply Pressure;   C e  is Electrical signal supplied to the microvalve, and   ƒ( ) means “is a function of” the term within the parenthesis.       

     For the spool valve  12 , this may be expressed, 
                     A   F     =     f   ⁡     (         P   F     -     P   T           P   S     -     P   T         )               Equation   ⁢           ⁢   2               
where
         P F  is Feedback Pressure; and   A F  is Flow Area       

     When the spool valve  12  is in equilibrium, then the following is true regardless of supply and return pressure, assuming the spool is in a stable position.
 
 P   F   =P   C   Equation 3
 
and
 
 A   F   =f ( C   e )  Equation 4
 
     Command pressure P C  is a percentage of the difference between supply pressure P S  and return pressure P T . At full power (i.e., when maximum flow through the reversible flow control assembly  10  is demanded), the normally closed (NC) orifice  90  is full open and the normally open (NO) orifice  92  is closed, irrespective of whether flow through the reversible flow control assembly  10  is forward or reverse. 
     As illustrated in  FIGS. 4 and 6 , travel of the spool  29  may be limited to the second position (on the left in  FIG. 6 ) by the stop  74  and to the first position (on the right in  FIG. 4 ) by the stop  74 . This may be done for two reasons. First, peak flow through the spool valve  12  occurs when the first port  64  and the second port  66  are both completely uncovered, which occurs at the first position and the second position by definition. Limiting the stroke of the spool  29  only to those positions from the first position to the second position helps ensure a linear response, aiding in control of the reversible flow control assembly  10 . Second, this is done to ensure transition between operation between the first range of positions and the second range of positions is always possible: Assuming the spool valve  12  is positioned at an unstable point for the direction of flow (i.e., one of the unstable operating regions shown in  FIGS. 10 and 11 ), transition to stability requires a higher command pressure than feedback pressure with forward flow, and lower pressure command pressure than feedback with reverse flow. In this case, limiting travel to the ranges between the first position and the second position means that feedback pressure P F  will always be less than or equal to the sum of return pressure P T  and the average of return pressure P T  and supply pressure P S  during forward flow (Equation 5), and P F  will always be greater than or equal to the sum of return pressure P T  and the average of return pressure P T  and supply pressure P S  during reverse flow (Equation 6) 
     
       
         
           
             
               
                 
                   
                     P 
                     F 
                   
                   ≤ 
                   
                     
                       ( 
                       
                         
                           
                             P 
                             S 
                           
                           - 
                           
                             P 
                             T 
                           
                         
                         2 
                       
                       ) 
                     
                     + 
                     
                       
                         P 
                         T 
                       
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       in 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       forward 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       flow 
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   5 
                 
               
             
             
               
                 
                   
                     P 
                     F 
                   
                   ≥ 
                   
                     
                       ( 
                       
                         
                           
                             P 
                             S 
                           
                           - 
                           
                             P 
                             T 
                           
                         
                         2 
                       
                       ) 
                     
                     + 
                     
                       
                         P 
                         T 
                       
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       in 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       reverse 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       flow 
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   6 
                 
               
             
           
         
       
     
     Assuming command pressure P C  is capable of any pressure between supply pressure P S  and return pressure P T , the difference between maximum feedback pressure (Maximum P F ) and maximum command pressure (Maximum P C ) will be sufficiently large to overcome any factors negatively affecting operation of the spool valve  12  to enable transition, such as any leakage from the fluid conduit providing communication from the pilot valve control port  94  to the command chamber  30  (which would in essence reduce the command pressure), hysteresis due to friction, or other force. 
     The current discussion assumes equal size first port  64  and second port  66  for ease of explanation; they may be different sizes. Further, as will be further described below with respect to an alternate embodiment of a spool for the spool valve  12 , it may be possible to utilize different sizes of ports with forward flow versus reverse flow. 
     As indicated above, the spring  70  and the spring  72  may be installed in the spool valve  12  to ensure the spool  29  stays centered in the shutoff position when the spool valve  12  is “off” (electrical command is zero), thereby minimizing leakage through the spool valve  12  between the first connection  16  and the second connection  18 . As indicated above, the springs  70 ,  72  may provide minimal force in comparison to the axial forces due to fluid pressure acting on the axial end faces  53 ,  54  of the spool  29  when fluid is flowing through the spool valve  12 . 
     With the implementation pictured in  FIG. 1 , if the pilot valve device  14  is powered off (i.e., the electrical command signal is zero), or with the spool  29  is positioned in a stable region (as labeled in  FIGS. 10 and 11 , the spool  29  will travel to the shutoff position in both forward and in reverse flow conditions. In this shutoff position, the flow path through the spool valve  12  is closed. Effectively this means the spool valve  12  (and the reversible fluid flow control assembly  10 ) is a normally closed flow control valve. 
     Regardless of the flow direction through the spool valve  12 , the following may be true: 1) Flow through the spool valve  12  may increase proportionally full scale, from zero flow to full flow, as the electrical command signal to the pilot valve device  14  changes by only half scale (0 to 50%, or 100% to 50%), assuming the spool  29  is operating in a stable region. 2) 100% pilot command (maximum electrical signal causing the normally closed (NC) orifice  90  to fully open and the normally open (NO) orifice  92  to fully close) generates a 100% pressure command (i.e., substantially equal to supply pressure) that forces the spool  29  into a stable region, regardless of initial position (again, without regard to the direction of flow through the spool valve  12 ). 
     It should be realized that in an alternate embodiment (not shown) where the normally open and normally closed states of the orifices  90  and  92  are reversed (i.e., if the orifice  90  were normally open, and the orifice  92  were normally closed), and all other components of the reversible fluid flow control assembly  10  were as shown and discussed above, then the control signal to the pilot valve device  14  could be inverted to achieve control of the system. In other words, in such case, a 0% pilot command (minimum or zero electrical signal causing such a normally open (NO) orifice  90  to fully open and such normally closed (NC) orifice  92  to fully close) generates a pressure command that forces the spool  29  into a stable region, regardless of initial position (again, without regard to the direction of flow through the spool valve  12 ). 
     Because the spool  29  may start in any position, especially if the springs  70 ,  72  are omitted, it would normally be expected that, at startup of the system in which the reversible fluid flow control assembly  10  is installed, a 100% pilot command would be momentarily initially applied to ensure the spool  29  is correctly moved into a stable region prior to resuming normal proportional control. Of course, it is expected that in most systems in which flow is reversed, such as a heat pump system, the system would be shut down in one direction, and then restarted in the opposite direction However, if the reversible fluid flow control assembly  10  were installed in a system in which fluid flow through the spool valve  12  and pilot valve device  10  could be reversed without shutting down the system first, provision could be made such that, upon reversing the flow in the system in which the reversible fluid flow control assembly  10  is installed, a 100% pilot command would be momentarily initially applied to ensure the spool  29  is correctly moved into a stable region prior to resuming normal proportional control. 
     An alternate embodiment of a spool, indicated generally at  129 , is illustrated in  FIGS. 10 and 11 . The spool  129  may be utilized in the reversible flow control assembly  10 . The spool  129  may be similar to the spool  29  except that the spool  129  is provided with control ports at more axial locations than the spool  29 ; accordingly, the same reference numbers will be utilized for similar features in the following description of the structure and operation of the spool  129 . More specifically, compared to the spool  29 , the spool  129  may define ports in the central portion  50  of the spool  129  at four axial locations. The spool  129  may have a first port  164  at a first axial location in the central portion  50  of the spool  29  providing fluid communication between the exterior surface  62  and the axial passageway  58 . The spool  129  may have a second port  166  in the central portion  50  of the spool  129  at a second axial location between the first axial location and the second end portion  29   b  of the spool  129  providing fluid communication between the axial passageway and the exterior surface  62  of the spool  129 . The spool  129  may further have a third port  264  in the central portion  50  of the spool  129  at a third axial location spaced a first axial distance X from the first axial location toward the first end portion  29   a  of the spool  129 , the third port  264  providing communication between the exterior surface and the axial passageway  50 . Finally, the spool  129  may have a fourth port  266  in the central portion  50  of the spool at a fourth axial location spaced the first axial distance X from the second axial location toward the first axial location and, the fourth port  266  providing communication between the exterior surface and the axial passageway and the axial passageway  50 . Suitably, the ports  164 ,  166 ,  264 , and  266  may have different cross-sectional areas. More specifically, in the embodiment illustrated in  FIGS. 10 and 11 , the ports  164  and  166  each may have a first cross-sectional area, while the ports  264  and  266  each may have a second cross-sectional area which is different from the first cross-sectional area. Even more specifically, the first cross-sectional area is larger than the second cross sectional area. 
     Thus, when the spool  129  is in the first range of positions, as illustrated in  FIG. 10 , a flow path for forward flow of fluid through the spool valve  12  is established from the first connector  16  through the fluid conduit  42 , the second groove  36  through the relatively larger first cross-sectional flow area of the second port  166 , through the axial passageway  58 , through the relatively larger diameter first cross-sectional flow area of the first port  164 , through the third groove  38 , and thence to the second connector  18 . In contrast, when the spool  129  is in the second range of positions, as illustrated in  FIG. 11 , a flow path for reverse flow of fluid through the spool valve  12  is established from the second connector  18 , to the third groove  38 , through the relatively smaller first cross-sectional flow area of the fourth port  266 , through the axial passageway  58 , through the relatively smaller first cross-sectional flow area of the third port  264  to the first groove  34 , to the fluid conduit  42 , and thence to the first connector  16 . Thus, all other factors being equal, the reversible flow control assembly  10  utilizing the spool  129  permits a greater volumetric flow rate for forward flow in the first position illustrated in  FIG. 10  than permitted for reverse flow in the second position illustrated in  FIG. 11 . This feature could be useful, for example, in a heat pump HVAC system, when reversing flow and switching between cooling (higher desired volumetric flow rate) and heating (lower desired volumetric flow rate) functions. The operation of the reversible flow control assembly  10  utilizing the spool  129  is otherwise similar to the operation of the reversible flow control assembly  10  utilizing the spool  29 . 
     Referring now to  FIGS. 14 through 27B , wherein like reference numbers and characters may represent like elements through out all of the figures, there is illustrated an additional alternate embodiment of a reversible fluid flow control assembly, generally indicated at  300 . The flow control assembly  300  may include a spool valve, indicated generally at  312 , and a pilot valve device, indicated schematically at  314 . The spool valve  312  and the pilot valve device  314  each may be in fluid communication with a first connector  316 , by means of which the flow control assembly  300  may be connected in fluid communication with a first portion of a system (not shown) in which the flow control assembly  300  may be installed, as described above. 
     The spool valve  312  and the pilot valve device  314  each may be in fluid communication with a second connector  318 , by means of which the flow control assembly  300  may be connected in fluid communication with a second portion of the system which the flow control assembly  300  may be installed. The first connector  316  and the second connector  318  each may be any suitable structure by means of which the flow control assembly  300  may be connected for installation in the system, including without limitation, threaded connections, welded connections, brazed connections, press-fit connections, rolled connections, permanently deformable connections, adhesive connections, compression fitting connections, etc. 
     The spool valve  312  may include a body  320 . Preferably the first connector  316  and the second connector  318  are at least partially formed in the body  320 , as is the case in the embodiment illustrated in  FIGS. 14 through 20 , where each of the first connector  316  and the second connector  318  is shown as a threaded connection port which can threadably accept standard hydraulic tube fittings, such as the tube fitting  19  shown above. The body  320  may be made of any material suitable for the application, such as a polymeric material, or a metal such as brass or aluminum, for example. 
     The body  320  may have an interior wall surface  321  defining a bore  322  therethrough. The bore  322  may have a first end portion, indicated generally at  324 , a second end portion, indicated generally at  326 , and a central portion, indicated generally at  327 . The first end portion  324  of the bore  322  may be provided with a plate  325  fixed to the body  320  to close the first end portion  324  of the bore  322  in a fluid tight manner. Similarly, the second end portion  326  of the bore  322  may be closed such as by a ball  328  disposed therein. The ball  328  may be pressed in the bore  322  to close the second end portion  326  of the bore  322  in a pressure tight manner. 
     The spool valve  312  may further include a spool  329  disposed for sliding movement in the bore  322 . The spool  329  may have a first end portion  329   a  and a second end portion  329   b . As illustrated in  FIGS. 14 through 20 , the spool  329  may be oriented in the bore  322  with the first end portion  329   a  of the spool  329  near the first end portion  324  of the bore  322 , and the second end portion  329   b  of the spool  329  near the second end portion  326  of the bore  322 . The structure of the spool  329  will be discussed in further detail below. 
     The spool  329  and the plate  325  closing the first end portion  324  of the bore  322  cooperate with the body  320  to define a command chamber  330  in the first end portion  324  of the bore  322 . The purpose of the command chamber  330  will be discussed below. A fluid conduit  331  is formed in the body  320  which may be in fluid communication with the command chamber  330  and, as will be discussed further below, in fluid communication with the pilot valve device  314 . The spool  329  and the ball  328  in the second end portion  326  of the bore  322  cooperate with the body  320  to define a feedback chamber  332  in the second end portion  326  of the bore  322 . 
     As illustrated in  FIG. 14 , a pair of cavities may be formed in the body  320  in fluid communication with the central portion  327  of the bore  322 , at axially spaced locations along the bore  322 . A first one of this pair of cavities may take the form of a circumferentially-extending first groove  334  formed in the surface  321  of the body  320  defining the bore  322  at a first axial location along the bore  322 , which, compared to the locations of the other of the pair of cavities, may be seen to be relatively close to the first end portion  324  of the bore  322 , and thus closest to the command chamber  330 . A second one of this pair of cavities may take the form of a circumferentially-extending second groove  336  formed in the surface  321  of the body  320  defining the bore  322  at a second axial location along the bore  322  which may be closer to the second end portion  326  of the bore  322  (and thus closer to the feedback chamber  332 ) than the first axial location where the first groove  334  may be located. 
     The body  320  may define a fluid conduit  340  providing fluid communication between the second connector  318  and the second groove  336 . The body  320  may also define a fluid conduit  342  providing fluid communication between the first connector  316  and the first groove  334 . 
     A bore  344  is provided in fluid communication between the first connector  316  and the pilot valve device  314 . The bore  344  may be formed, for example, by drilling through the body  320  from the surface of the body  320 . A bore  346  is provided in fluid communication between the first connector  318  and the pilot valve device  314 . The bore  346  may be formed, for example, by drilling through the body  320  from the surface of the body  320 . 
     Referring now to  FIGS. 14 and 15 , the spool  329  may have a central portion, indicated generally at  350 , between the first end portion  329   a  and the second end portion  329   b . The spool  329  may have a first axial end face, indicated generally at  352  on the first end portion  329   a  which is in fluid communication with the command chamber  330 . In the illustrated embodiment, the first end portion  329   a  is frusto-conically shaped, for reasons will be discussed below. The spool  329  may have a second axial end face, indicated generally at  354 , on the second end portion  329   b  which is in fluid communication with the feedback chamber  332 . In the illustrated embodiment, the second end portion  329   b  is frusto-conically shaped, for reasons will be discussed below. The second axial end face  354  may have an opening  356  defined therein. 
     The spool  329  may have an internal axial passageway  358  defined therein. The axial passageway  358  may provide fluid communication from the opening  356  in the second axial end face  354  to a blind end in an interior portion of the first end portion  329   a  of the spool  329 . In the illustrated embodiment, an insert  360  is fixed in the opening  356  in the second end  329   b  of the spool  329  by a suitable mechanism such as threaded engagement. The insert  360  may include a first bore  361  extending axially inwardly from the second axial face  354  and a damping orifice  359  that restricts communication between the axial passageway  358  of the spool  329  and the feedback chamber  332 , in order to dampen movement of the spool  329  during operation. In the illustrated embodiment, the orifice  359  forms a reduced diameter bore between the bore  361  and the axial passageway  358 . 
     The insert  360  may be affixed to the spool  329  in any suitable manner, or may be integrally formed with the spool  329 , if a reduced diameter (relative to the diameter of the rest of the axial passageway  358 ) orifice is provided. It is anticipated that in some applications, no orifice providing damping will be needed at all, and the insert  360  may be omitted. 
     The spool  329  may have an exterior surface  362 . The spool  329  may have a plurality of ports formed in the spool  329 . In the illustrated embodiment, a first port  363  is formed at a first axial location in the spool  329  providing fluid communication between the exterior surface  362  and the axial passageway  358 . Similarly, a second port  364  is formed at a second axial location, a third port  365  is formed at a third axial location, and a fourth port  366  is formed at a fourth axial location in the spool  329 . Each of the ports  363 ,  364 ,  365 , and  366  may be one of a plurality of ports spaced apart circumferentially about the spool  329  at the respective axial location of the ports  363 ,  364 ,  365 , and  366 . 
     The spool  329  may have a circumferential groove  367  formed in the exterior surface  362  at an axial location between the first port  363  and the first end portion  329   a  of the spool  329 . The spool  329  may further have an aperture  368  providing fluid communication between the circumferential groove  367  and the axial passageway  358  formed in the spool  329 . The aperture  368  allows fluid at feedback pressure existing in the axial passageway  358  during equilibrium conditions to be distributed about the spool  329  in the groove  367 , which, as discussed above, minimizes the differential pressure between the command chamber  330  and the groove  367 , and thus minimizes leakage into or out of the command chamber  330  between the surface  321  defining the bore  322  and the surface  362  of the spool  329 . 
     Referring again to  FIGS. 15 and 19 , a coil spring  370  may be disposed in the command chamber  330 , acting between the plate  325  at the first end portion  324  of the bore  322  and the spool  329  to urge the spool  329  toward the second end portion  326  of the bore  322 . The frusto-conically shaped first end portion  329   a  of the spool  329  may help to radially center the spring  370 . Similarly, a coil spring  372  may be disposed in the feedback chamber  332 , acting between the ball  328  in the second end portion  326  of the bore  322  and the spool  329  to urge the spool  329  toward the first end portion  324  of the bore  322 . As illustrated, the frusto-conically shaped second end portion  329   b  of the spool  329  may help to radially center the spring  372 . 
     The ball  328  defines a stop structure that will limit motion of the spool  329  in a first direction toward the second end portion  326  of the bore  322 . In particular, the stop structure may prevent the spool  329  from travelling past a desired first maximum travel position, shown in  FIG. 16 . Similarly, the plate  325  defines a stop structure that will engage the spool  329 , limiting motion of the spool  329  in a second direction toward the first end portion  324  of the bore  322 . The stop structure may prevent the spool  329  from travelling past a desired second maximum travel position, shown in  FIG. 20 . 
     A first position of the spool  329  is shown in  FIG. 16 . The first position in the illustrated embodiment may be the desired first maximum travel position, which may be the first position of the spool  329  which is reached during movement in the first direction in which the port  363  is fully uncovered in communication with the first groove  334  and the port  365  is fully uncovered in communication with the second groove  336 . If the spool  329  is moved in the second direction toward the first end portion  324  of the bore  322  from the first position illustrated in  FIG. 16 , the portion of the body  320  between the first groove  334  and the first end portion  324  will progressively cover the port  363 . Likewise, the portion of the body  320  forming a land between the first groove  334  and the second groove  336  will progressively cover the port  365 . The spool  329  may be positioned in any of a first range of positions as the spool  329  moves in the second direction from the first maximum travel position to a shut off position describe below. 
     A second position of the spool  329  is seen in  FIG. 18 . The second position in this embodiment may be the desired second maximum travel position, which may be the first position of the spool  329  which is reached during travel in the second direction in which the port  364  is fully uncovered in communication with the first groove  334  and the port  366  is fully uncovered in communication with the second groove  336 . The spool  329  may be positioned in any of a second range of positions as the spool  329  moves in the first direction from the second maximum travel position to a shut off position described below. 
     The springs  370  and  372  may urge the spool  329  to a centered or shut off position, between the first range of positions and the second range of positions of the spool  329 . This centered position is illustrated in  FIG. 19 . More specifically, the spring  370  may urge the spool  329  to move in the first direction (leftward as viewed in  FIG. 19 ) from the second range of positions toward the centered position; the spring  372  may urge the spool  329  to move in the second direction from the first range of positions (rightward as viewed in  FIG. 19 ) toward the centered position. The first range of positions is to the left of the centered position illustrated in  FIG. 19 , and the second range of positions is to the right of the centered position illustrated in  FIG. 19 . 
     In the centered position, both the port  365  and the port  366  may be partially uncovered to communicate with the second groove  336 ; however, neither the port  363  nor the port  364  is in substantial direct fluid communication with the first groove  334 . There will be no fluid communication between the axial passageway  358  in the spool  329  and the first groove  334 , and thus no fluid communication between the first connector  316  and the second connector  318 . 
     Referring again to  FIG. 14 , the pilot valve device  314  may include a valve or valves  380  and a manifold, such as the manifold  82  described above, provided with fluid passageways interconnecting the valve  380  and the spool valve  312 . 
     The valve  380  may include a fluid conduit  384 . The flow through the fluid conduit  384  may be regulated by two variable orifices in series arrangement in the fluid conduit  384 . A variable first orifice  390  may be a normally closed orifice, that is, the first orifice  390  may be closed in the absence of a command signal to the valve  380 . A variable second orifice  392  may be a normally open orifice. The fluid conduit  331  may be connected in fluid communication with the fluid conduit  384  between the first orifice  390  and the second orifice  392 . The valve  380  may be a single valve or microvalve containing one or more moving components acting as the first orifice  390  and the second orifice  392 . Alternatively, the valve  380  may be embodied as a plurality of valves or microvalves acting as the first orifice  390  and the second orifice  392 . The first orifice  390  and the second orifice  392  may move inversely proportionally—that is, when one is open, the other is closed. As one opens, the other simultaneously closes, and when one is half open, the other is also half open (and half closed). 
     Referring now to  FIGS. 14 ,  15 , and  16 , the operation of the spool valve  312  will be described. A pressure command used for control of the spool valve  312  is developed in the pilot valve device  314 , as described above. In the illustrated embodiment, for example, the pressure command is developed in the fluid conduit  384  between the first orifice  390  and the second orifice  392  when pressurized fluid is supplied to the valve  380 . The pressure developed there is the command pressure, and fluid at the command pressure is conveyed from the pilot valve device  314  to the command chamber  330  of the spool valve  312 . The command pressure may be conveyed to the command chamber  330  via a single fluid conduit via a pilot valve control port (not illustrated) and the single fluid conduit  331 . 
     During operation, the reversible fluid flow control assembly  300  is installed in a system (not shown) via the first connection  316  and the second connection  318 . During operation of the system, normally one of the first connection  316  and the second connection  318  will be supplied with a higher pressure (hereinafter “supply pressure”) and the other of the first connection  316  and the second connection  318  will be supplied with a lower pressure (hereinafter “return pressure”). During operation, when there are differences between supply pressure and return pressure, the components of the reversible fluid control assembly  300  operate to develop two separate fluid pressures acting in opposition across the spool  329 . 
     On one side, to the right as drawn in  FIGS. 14  though  19 , the command pressure is developed in the pilot valve device  314  by positioning the first orifice  390  and the second orifice  392  to achieve a desired pressure. The command pressure may be supplied to the command chamber  330  to push on the first axial end face  352  of the spool  329  to urge the spool  329  in the first direction (leftward to the first range of positions as seen in  FIGS. 14 through 19 ), moving the spool  329  into the first range of positions of the spool  329 . A pressure proportional to the position of the spool  329 , referred to as feedback pressure, is developed in the axial passageway of the spool  329  as will be described below. The feedback pressure is communicated via the bore  361  from the axial passageway  358  of the spool  329  to the feedback chamber  332  on the left side (as seen in  FIGS. 14 through 19 ) of the spool  329 . 
     Feedback pressure in the feedback chamber  332  acting on the second axial end face  354  of the spool  329 , urges the spool  329  in the second direction (rightward as seen in  FIGS. 14 through 19 ). The spool  329  is free to move until the forces acting on the end faces  352 ,  354  of the spool  329  balance. Note that in this discussion the forces exerted by the springs  370 ,  372  will not be discussed, as the springs  370 ,  372  would normally be chosen to have a very low spring rate, as discussed above. It will be appreciated that in at least some embodiments, a majority of axial forces acting on the spool  329  to position the spool  329  relative to the body  320  when fluid is flowing through the spool valve  312  will be fluid forces. 
     Both the command pressure and the feedback pressures may fall between supply pressure and return pressure in normal operation, as described above. 
     The feedback pressure is a pressure developed between the first port  363  and the third port  365  in the axial passageway  358 . During forward flow (illustrated in  FIG. 16  by the arrow R 1 ), with the spool  329  in the first range of positions, flow of fluid through the spool valve  312  travels from the first connection  316 , through the first port  363 , through the axial passageway  358  of the spool  329 , through the second port  365  and then out through the second connection  318 , as illustrated in  FIG. 16 . 
     In forward flow, the second groove  336  will be at return pressure, while the first groove  334  may be at supply pressure. As the first port  363  is progressively uncovered while moving from the shut off position illustrated in  FIG. 19 , through the position illustrated in  FIG. 14  (in which fluid flows through the spool valve  312 ), to the first position illustrated in  FIG. 16 , pressure in the axial passageway  358  may rise. However, feedback pressure may not rise to the magnitude of supply pressure, since the third port  365  is continually venting fluid from the axial passageway  358  to the second groove  336 , which may be at return pressure. 
     Maximum flow through the valve  380 , which occurs when both the orifice  390  and the orifice  392  are half open, as shown in  FIG. 16 , since any further opening of one of the orifices  390 ,  392  will also mean that the other of the orifices  390 ,  392  goes closed (since, as described above, in this embodiment the orifices  390 ,  392  operate equally and oppositely), limiting flow to a net lower valve. With forward flow, and both the orifices  390 ,  392  half open, the command pressure may be about half of the supply pressure P 1 . If the spool  329  moves to the right from the position illustrated in  FIG. 16 , the port  363  will start to be covered by the body  320 , decreasing the cross-sectional flow area between the groove  334  (supply pressure) and the axial passageway (feedback pressure). Therefore, the feedback pressure is lowered. With the command pressure unchanged, the net pressure imbalance between command and feedback pressure may urge the spool  329  back to the left until the stop (ball  328 ) is encountered or the pressure imbalance is eliminated by the resultant rise in feedback pressure. 
     This feedback mechanism causes the feedback pressure P′ 2 , in the passageway  358 , to be equal to the command pressure P 2 . The command pressure P 2  may be represented by the following equation: 
     
       
         
           
             
               
                 
                   
                     P 
                     2 
                   
                   = 
                   
                     ( 
                     
                       
                         
                           P 
                           1 
                         
                         · 
                         
                           A 
                           1 
                           2 
                         
                       
                       
                         
                           A 
                           1 
                           2 
                         
                         + 
                         
                           A 
                           2 
                           2 
                         
                       
                     
                     ) 
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   7 
                 
               
             
           
         
       
     
     Feedback pressure P 2 ′ may be represented by the following equation: 
                     P   2   ′     =     (         P   1   ′     ·       (     A   1   ′     )     2             (     A   1   ′     )     2     +       (     A   2   ′     )     2         )             Equation   ⁢           ⁢   8               
where:
         P 1  is Supply Pressure to the pilot valve device  314 ;   P′ 1  is Supply Pressure to the spool valve  312  (from the first connection  316  during forward flow; from the second connection  318  during reverse flow) (in the illustrated embodiment P 1 =P′ 1 );   P 2  is Command Pressure;   P′ 2  is Feedback Pressure;   A 1  is the cross-sectional flow ( 390  during forward flow,  392  during reverse flow) area of the upstream pilot orifice where fluid flows from the fluid conduit at supply pressure into the command chamber;   A′ 1  is the inlet cross-sectional flow area of the spool valve  312  where fluid flows into the feedback chamber from either the groove  334  (during forward flow) or the groove  336  (during reverse flow);   A 2  is the cross-sectional flow area of the downstream pilot orifice where fluid flows out of the command chamber into the fluid conduit at return pressure; and   A′ 2  is the outlet cross-sectional flow area of the spool valve  312  where fluid flows out of the feedback chamber into either the groove  336  (during forward flow) or the groove  334  (during reverse flow).       

     Equation 7 can be rearranged as: 
     
       
         
           
             
               
                 
                   
                     
                       A 
                       2 
                     
                     
                       A 
                       1 
                     
                   
                   = 
                   
                     
                       
                         
                           P 
                           1 
                         
                         - 
                         
                           P 
                           2 
                         
                       
                       
                         P 
                         2 
                       
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   9 
                 
               
             
           
         
       
     
     Similarly, Equation 8 can be rearranged as: 
     
       
         
           
             
               
                 
                   
                     
                       A 
                       2 
                       ′ 
                     
                     
                       A 
                       1 
                       ′ 
                     
                   
                   = 
                   
                     
                       
                         
                           P 
                           1 
                           ′ 
                         
                         - 
                         
                           P 
                           2 
                           ′ 
                         
                       
                       
                         P 
                         2 
                         ′ 
                       
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   10 
                 
               
             
           
         
       
     
     Since the pressure forces acting on the spool valve  312  balance when the spool valve  312  is at equilibrium, the following is true when the spool valve  312  is at equilibrium:
 
 P   2   =P′   2   Equation 11
 
     As indicated above, P 1 =P′ 1  since, in the illustrated embodiment, the pilot valve device  314  and the spool valve  314  are both fed fluid from a common source. Therefore, equation 10 can be rewritten as: 
     
       
         
           
             
               
                 
                   
                     
                       A 
                       2 
                       ′ 
                     
                     
                       A 
                       1 
                       ′ 
                     
                   
                   = 
                   
                     
                       
                         
                           P 
                           1 
                         
                         - 
                         
                           P 
                           2 
                         
                       
                       
                         P 
                         2 
                       
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   12 
                 
               
             
           
         
       
     
     From Equation 9 and 12, therefore: 
     
       
         
           
             
               
                 
                   
                     
                       A 
                       2 
                       ′ 
                     
                     
                       A 
                       1 
                       ′ 
                     
                   
                   = 
                   
                     
                       A 
                       2 
                     
                     
                       A 
                       1 
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   13 
                 
               
             
           
         
       
     
     Equation 13 shows that the ratio of the cross-sectional flow area of the pilot downstream orifice to the cross-sectional flow area of the pilot upstream orifice is equal to the ratio of the outlet cross-sectional flow area out of the spool valve  312  to the inlet cross-sectional flow area into the spool valve  312 . Equation 13 can be rewritten thusly: 
     
       
         
           
             
               
                 
                   
                     
                       A 
                       1 
                       ′ 
                     
                     
                       A 
                       2 
                       ′ 
                     
                   
                   = 
                   
                     
                       A 
                       1 
                     
                     
                       A 
                       2 
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   14 
                 
               
             
           
         
       
     
     Equation 14 shows that the ratio of the cross-sectional flow area of the pilot upstream orifice to the cross-sectional flow area of the pilot downstream orifice is equal to the ratio of the inlet cross-sectional flow area into the spool valve  312  to the outlet cross-sectional flow area out of the spool valve  312 . 
     Thus it is clear from Equations 13 and 14 that the ratio of inlet and outlet cross-sectional flow areas of the spool valve  312  can be set by controlling the ratio of cross-sectional flow areas of the upstream and downstream orifices of the pilot valve device  314 . This relationship can be used for developing control algorithms for the reversible flow control assembly  300 . Trying to control the spool valve  312  using downstream pressure or flow as a direct feedback signal can be difficult in some applications, such as when a two phase fluid (a mixture of fluid and gas) is flowing through the spool valve. A specific example could be a refrigerant such as 1,1,1,2-tetrafluoroethane (R134a), which, at an appropriate temperature, could have some portion of liquid entering the spool valve  312  change to a gas due to the pressure drop experienced flowing through the spool valve  312 . Slight movements of the spool  329 , changing the pressure drop slightly, could result in significant changes in flow volume and pressure downstream of the spool valve  312  by changing the percentages of gas and liquid in the fluid stream downstream of the spool valve  312 . Therefore, the reversible fluid flow control assembly  300  can advantageously set a desired cross-sectional flow area through the spool valve  312  utilizing the pilot valve device  314 . 
     Other suitable parameters, which are not as unstable as downstream pressure and flow, and which are application specific, but which would be readily apparent to those of ordinary skill in the applicable art, may be used as part of the control algorithm controlling the pilot valve device  314 . As an example, in a refrigeration system, where the reversible fluid flow control assembly  300  is used to supply an evaporator coil, the temperature of the refrigerant tubing at the outlet of the evaporator coil could be used as a parameter considered in controlling the operation of the reversible fluid flow control assembly  314 . Other parameters which might alternatively, or additionally be utilized, and which would be apparent to those of ordinary skill in the art of refrigeration systems, might include the temperature change of the refrigerant tubing between the inlet and the outlet of the evaporator coil, degrees of superheat or subcooling of the refrigerant at the outlet of the evaporator coil, and energy content or temperature change of the fluid being cooled by the evaporator coil after passing through the evaporator coil. 
     In the embodiment illustrated in  FIGS. 17 and 18 , the second connection  318  is at the supply pressure P 1 . Command pressure P 2  is high (command pressure P 2  may equal supply pressure P 1 ) since the orifice  392  is open and the downstream orifice  390  is shut (i.e., in their normal positions). Supply pressure is communicated from the second connection  318  through the port  366  to the axial passageway  358 , however no connection between the axial passageway  358  and the first connection  316  exists, therefore the feedback pressure P′ 2  is high (feedback pressure P′ 2  may equal supply pressure P′ 1 ). If the spool  329  moves to the right for any reason, the feedback pressure P′ 2  will drop and the command pressure P 2  will urge the spool  329  back to the left. If the spool  329  moves to the left for any reason, the feedback pressure P′ 2  remains constant and equal to the command pressure P 2 . Therefore, the springs  370  and  372  move back to the shut off position. 
     If flow through the system is reversed, for example if a heat pump is switched from a cooling to a heating function, then the second connection  318  will be supplied with a higher pressure (supply pressure) and the first connection  316  will be supplied with a lower pressure (return pressure). The spool  329  will then operate in the second range of positions, i.e. the range of positions bounded by the position shown in  FIG. 18  and the centered (or shut off) position shown in  FIG. 19 . 
     The spool valve  312  may operate in one or more metering positions. Assume, for example, that the spool  329  is positioned within the second range of positions at a first metering position as shown in  FIG. 27A , and the reversible fluid flow control assembly  300  is operating with reverse flow (supply pressure supplied to the second connection  318 , with return pressure at the first connection  316 ), and that the spool  329  is in equilibrium. The command signal supplied to the pilot valve device  314  is at an intermediate value. The normally closed orifice  390  of the pilot valve device  314  (the down stream orifice in this direction of flow) is also partially opened, and the normally open orifice  392  of the pilot valve device  314  (the upstream orifice) is also partially opened, and the pressure in the passageway  384  between the orifice  390  and the orifice  392  (the command pressure supplied via the pilot valve control device  314  to the command chamber  330  of the spool valve  312 ) is a steady percentage of the difference between the supply pressure at the second pilot connection port  388  and the return pressure at the first pilot connection port  386 . 
     Now assume it is desired to open the spool  329  more, that is, to increase the cross-section flow area through the spool valve  312  by moving the spool  329  toward a second metering position illustrated in  FIG. 27B  in order to increase fluid flow through the spool valve  312 . The command signal supplied to the pilot valve device  314  is increased. This causes the normally closed orifice  390  to open further, opening up the release path to return pressure at the first connection  316 , and the normally open orifice  392  to close further, throttling or metering the supply pressure supplied from the second connection  318 . This causes the command pressure P 2  supplied to the command chamber  330  to decrease. A decrease in command pressure P 2  causes the spool  329  to move in the second direction, toward the command chamber  330  (rightward as seen in  FIGS. 16 ,  27 A, and  27 B). As the spool  329  moves in the second direction toward the command chamber  330 , the spool  329  moves through a plurality of metering positions, from the first metering position shown in  FIG. 27A , the second metering position shown in  FIG. 27B  as the spool  329  moves in the second direction; the feedback pressure will decrease. Feedback pressure P 2   1  decreases due to increasing the ratio of the outlet cross-sectional flow area A 2   1  through the second port  364  (which is the port opening to return pressure in the first groove  334 ) to the inlet cross-sectional flow area A 1   1  of the fourth port  366  (which is the opening to supply pressure). 
     More specifically, with the release path to return pressure through the port  366  opened up, and the cross-sectional flow area of the flow path from supply unchanged when moving from the first metering position illustrated in  FIG. 27A  to the second metering position illustrated in  FIG. 27B , the feedback pressure P 2   1  in the axial passageway  358  will also fall. As feedback pressure P 2  decreases, the spool  329  will come to rest in a new equilibrium position, such as shown in  FIG. 27B , where feedback pressure P 2   1  substantially equals command pressure P 2 , with the increased cross-sectional flow area and associated increased flow through the spool valve  312  that was desired. 
     The converse is also true with a decreasing command signal, which will generate an increased command pressure in the pilot valve device  314 . This will cause the spool  329  to move in the first direction, so that the second port  364  will get increasingly covered, lowering the ratio of the cross-sectional flow area of the second port  364  (which is the port opening to return pressure) to the cross-sectional flow area of the fourth port  366  (which is the opening to supply pressure), raising the pressure in the axial passageway  358 , and the spool  329  will come to rest in a new equilibrium position within the second range of positions, such as the first metering position shown in  FIG. 27A , when the feedback pressure rises to equal the increased command pressure. The mass flow rate through the spool valve  312  will be lower than the original mass flow rate, since the cross-sectional flow area through the spool valve is described. 
     It should be emphasized that the first metering position shown in  FIG. 27A  and the second metering position shown in  FIG. 27B  represent only two of an infinite number of metering (or “throttling”) positions within the second range of positions; there are similarly an infinite number of metering positions within the first range of positions. 
     The spool valve  312 , shown in  FIGS. 14 through 19 ,  27 A, and  27 B, is an example of a symmetric valve with a spool  329  for a symmetric valve. As used herein, a symmetric valve is defined as a valve in which the maximum cross-sectional flow area in forward and reverse flow configurations are substantially the same. An asymmetric valve, conversely, is defined as a valve in which the maximum cross-sectional flow area through the valve in a forward flow direction is substantially different from the maximum cross-sectional flow area in a reverse flow direction. 
     The spool valve  412 , shown in  FIG. 20  is an example of an asymmetric valve. In the asymmetric valve  412 , a spool  429  for an asymmetric valve is disposed for sliding movement in the bore  322  of the valve body  320 . It should be noted that the body  320 , advantageously, may be used with either the spool  329  (described previously) to form a symmetric valve, or the spool  429  to form an asymmetric valve. The illustrated spool  429  has a plurality of ports spaced apart circumferentially about the spool  429 . In the illustrated embodiment, one or more first ports  463  are formed at a first axial location in the spool  429  providing fluid communication between the exterior surface  462  and the axial passageway  458 . Similarly, one or more second ports  464  are formed at a second axial location, one or more third ports  465  are formed at a third axial location, and one or more fourth port  466  is formed at a fourth axial location in the spool  429 . The cross-sectional flow areas at the second and fourth axial locations, used during forward flow, are smaller than the cross-sectional flow areas in the first and third axial locations used during reverse flow. This spool structure allows a heat pump, for example, to have different refrigerant flow rates when heating a building than when cooling a building. 
     In the illustrated embodiment, all the ports  463 ,  464 ,  465 , and  466  are of the same diameter; a greater cross-sectional flow area is achieved in the first and third axial locations by providing more ports  463  and  465  at the first and third axial locations, respectively, than the number of ports  464  and  466  at the second and fourth axial locations, respectively. However, a difference in cross-sectional flow areas may be achieved by any suitable arrangement. For example, a greater cross-sectional flow area could also be achieved in the first and third axial locations than at the second and fourth axial locations by providing the same number of ports  463 ,  464 ,  465 , and  466  at the first, second, third, and fourth axial locations in the same number, but having the individual ports  463  and  465  be formed with greater diameter (greater individual cross-sectional flow area), than the ports  464  and  466 . Such an arrangement is illustrated in the alternate embodiment illustrated in  FIGS. 23 and 24 , which will be described below. 
     Regarding the housing or body  320 ,  FIGS. 21 and 22  illustrate that the first connection  316  and the second connection  318  are connected to the grooves  334  and  336  by bores or conduits  342  and  340 , respectively. The conduit  340  may be formed by one or more bores  340   a  drilled between the first connection  316  and the groove  336 , as shown in  FIG. 21 , similarly, the conduit  342  may be formed by one or more bores  342   a  drilled between the second connection  318  and the grooves  334 , as shown in  FIG. 22 . 
     An alternate body  320 ′ may be provided, as best shown in  FIGS. 23 ,  24 ,  25 , and  26 . The conduits  340 ′ and  342 ′ may be formed as slots formed between the grooves  334 ′ and  336 ′ and the first and second connections  316  and  318 , respectively. The slots defining the conduits  340 ′ and  342 ′ may be formed by any desired method, such as by milling.  FIGS. 25 and 26  are illustrations of the shapes of the fluid volumes about the spool  329  and elsewhere, with the body  320 ′ in phantom. The illustrations of  FIGS. 25 and 26  are included to provide a better sense of the configuration of the fluid passageway in the body  320 ′. 
     The spool shown in  FIGS. 23 and 24  is another example of a spool for an asymmetric valve, and more specifically, an alternate embodiment of the spool  429 , which is disposed for sliding movement in the bore of the valve body  320 ′. The embodiment of the spool illustrated in  FIGS. 23 and 24  utilizes a different method of achieving asymmetric flow than that utilized by the spool  429  illustrated in  FIG. 20 . The spool illustrated in  FIGS. 23 and 24  has a plurality of ports axially spaced apart along the spool in groups at each of four axial locations. Unlike the spool  429  illustrated in  FIG. 20 , the number of ports at an axial location is the same as the number at each of the other axial locations. In other words, in the illustrated embodiment, at least a first port  463  is formed at the first axial location in the spool providing fluid communication between the exterior surface of the spool and the axial passageway through the spool&#39;s longitudinal axis. Similarly, ports  464  in the same number of ports as at the first axial location are formed at the second axial location, ports  465  in the same number of ports as at the first axial location are formed at the third axial location, and ports  466  in the same number of ports as at the first axial location are formed at the fourth axial location in the spool  429 . The second and fourth ports,  464  and  466  respectively, used during forward flow, have a smaller diameter, and therefore a smaller cross-sectional flow area, than the ports  463  and  465 , used during reverse flow. 
     In partial summary, among the advantages of the illustrated reversible fluid flow control assembly is the ability control flow in either direction proportional to a single pressure command from a pilot valve device, without using a spring as the primary spool closing force, and utilizing unstable equilibrium forces to switch between forward and reverse flow functionality. 
     In further partial summary, a device has been disclosed, including a pilot valve responsive to a command signal for supplying a fluid at a command pressure to a pilot valve control port; and a pilot operated spool valve. The pilot operated spool valve may have a body having a first connector and a second connector, each of the first connector and second connector being adapted for fluid communication with an external circuit; and a spool disposed for sliding movement in the body, the spool having a first end portion and a second end portion opposite the first end portion. The first end portion of the spool may be in fluid communication with the pilot valve control port such that the spool is urged to move in a first direction by the fluid at the command pressure. The spool may be movable to control a fluid flow between the first connector and the second connector through the body proportionally to the command pressure when the fluid flow is a forward flow from the first connector to the second connector and when the fluid flow is a reverse flow from the second connector to the first connector. The spool valve may use negative feedback in the form of fluid at a feedback pressure acting on the spool in a second direction, opposite the first direction, to position the spool in conjunction with the fluid at the command pressure. The spool valve may utilize unstable equilibrium of fluid forces to switch between controlling the forward flow and the reverse flow of fluid through the spool valve. 
     In further partial summary, a device has been disclosed that includes a pilot valve device responsive to a command signal for supplying a fluid at a command pressure to a pilot valve control port; and a pilot operated spool valve. The pilot operated spool valve may have a body having a first connector and a second connector, each of the first connector and second connector being adapted for fluid communication with an external circuit; and a spool disposed for sliding movement in the body. The spool may have a first end portion and a second end portion opposite the first end portion, the first end portion of the spool being in fluid communication with the pilot valve control port such that the spool is urged to move in a first direction by the fluid at the command pressure. The spool may be movable through a first range of positions to control, proportionally to the command pressure, the flow of a fluid when the fluid is flowing through the body in a forward direction from the first connector to the second connector. The spool may be movable through a second range of positions, offset from the first range of positions, to control, proportionally to the command pressure, the flow of the fluid when the fluid is flowing through the body in a reverse direction from the second connector to the first connector. A portion of the fluid flowing through the body may have a feedback pressure and acting on the spool in a second direction, opposite the first direction, to position the spool in conjunction with the fluid at the command pressure, the magnitude of the feedback pressure being generated at least in part as a function of the position of the spool. A portion of the fluid flowing through the body may develop the feedback pressure when flowing from the body into a passageway within the spool and be directed out of the spool into a feedback chamber to act on the spool in the second direction. 
     In further partial summary, a device has been disclosed that has a command chamber in fluid communication with the pilot valve control port to receive the fluid at the command pressure, a feedback chamber receiving the fluid having the feedback pressure; and a bore communicating at a first end portion with the command chamber and at a second end portion with the feedback chamber, the spool being disposed for sliding movement in the bore. 
     In further partial summary, a device has been disclosed in which the spool may further define an exterior surface, a first end portion, a second end portion, and a central portion between the first end portion and the second end portion. A first axial end face may be defined on the first end portion which is in fluid communication with the command chamber. A second axial end face may be defined on the second end portion which is in fluid communication with the feedback chamber and having an opening defined therein. An axial passageway may be defined communicating with the opening in the second axial end face, the axial passageway extending into the central portion of the spool. A first port at a first axial location in the central portion of the spool may provide communication between the exterior surface and the axial passageway. Finally, a second port in the central portion of the spool at a second axial location between the first axial location and the second end portion of the spool may provide communication between the exterior surface and the axial passageway. 
     In further partial summary, a device has been disclosed in which the body may define a first cavity communicating with the bore in the body at a first axial location along the bore. The body may also define a second cavity communicating with the bore in the body at a second axial location along the bore which is closer to the feedback chamber than the first axial location. The body may also define a third cavity communicating with the bore in the body at a third axial location along the bore. The third location may be located between the first axial location and the second axial location. The first connector may be in fluid communication with the first cavity and with the second cavity. The second connector may be in fluid communication with the third cavity. When the spool is in the first range of positions, a flow path for forward flow of fluid through the spool valve is established from the first connector, to the second cavity, through the spool via, sequentially the second port, the axial passageway, the first port, to the third cavity, and thence to the second connector, and such that when the spool is in the second range of positions, a flow path for reverse flow of fluid through the spool valve is established from the second connector, to the third cavity, through the spool via, sequentially the second port, the axial passageway, and the first port, to the first cavity, and thence to the second connector. Each of the first, second, and third cavities may be in the form of a circumferentially extending groove formed in the surface of the wall defining the bore in the body. 
     In further partial summary, a device has been disclosed the spool is movable to a shutoff position between the first range of positions and the second range of positions, where substantially no fluid communication exists between the axial passageway in the spool and either the first cavity or the second cavity. 
     In further partial summary, a device has been disclosed wherein the spool valve may further have a first spring urging the spool to move from the second range of positions toward the shutoff position, and may have a second spring urging the spool to move from the first range of positions toward the shutoff position. 
     In further partial summary, a device has been disclosed wherein the a circumferential groove may be formed in the exterior surface of the spool at a third axial location between the first axial location and the first end portion of the spool; and an aperture may be formed in the spool providing fluid communication between the circumferential groove in the exterior surface of the spool and the axial passageway formed in the spool. 
     In further partial summary, a device has been disclosed wherein the first port may be one of a plurality of ports spaced apart circumferentially about the spool at the first axial location. Furthermore the second port may be one of a plurality of ports spaced apart circumferentially about the spool at the second axial location. 
     In further partial summary, a device has been disclosed wherein the spool may further define a third port in the central portion of the spool at a third axial location spaced a first axial distance from the first axial location toward the first end portion of the spool. The third port may provide communication between the exterior surface and the axial passageway. Furthermore, the spool may also define a fourth port in the central portion of the spool at a fourth axial location spaced the first axial distance from the second axial location toward the first axial location. The fourth port may also provide communication between the exterior surface and the axial passageway. The body may further define a first cavity communicating with the bore in the body at a first axial location along the bore, a second cavity communicating with the bore in the body at a second axial location along the bore which is closer to the feedback chamber than the first axial location, a third cavity communicating with the bore in the body at a third axial location along the bore, the third location being between the first axial location and the second axial location. The first connector may be in fluid communication with the first cavity and with the second cavity. The second connector may be in fluid communication with the third cavity, such that, when the spool is in the first range of positions, a flow path for forward flow of fluid through the spool valve is established from the first connector, to the second cavity, through the spool via, sequentially the second port, the axial passageway, and the first port, to the third cavity, and thence to the second connector, and such that when the spool is in the second range of positions, a flow path for reverse flow of fluid through the spool valve is established from the second connector, to the third cavity, through the spool via, sequentially the fourth port, the axial passageway, and the third port, to the first cavity, and thence to the second connector. 
     In further partial summary, a device has been disclosed wherein the first and the second ports each have a first cross-sectional flow area, and wherein the third and the fourth ports each have a second cross-sectional flow area different than the first cross-sectional flow area. 
     In further partial summary, a device has been disclosed that wherein, when the spool is in the first range of positions, and fluid communication is established between the first connector and the second connector, through the second cavity, through the spool via the second port, the axial passageway, and the first port, and through the third cavity, the presence of fluid in the second connector at a pressure higher than that existing in the first connector results in an instability in flow such that any decrease in command pressure would cause the spool to move in the second direction toward the command chamber, resulting in decreased communication between the second cavity and the second port, resulting in an increase in pressure in the axial passageway and thus pressure in the feedback chamber, further urging the spool to move in the second direction toward the command chamber, resulting in the spool moving disproportionately to the change in command pressure, the spool moving out of the first range of positions toward the second range of positions. 
     In further partial summary, a device has been disclosed that may include a first stop structure limiting movement of the spool in the first direction at a position providing substantially the least resistance to flow through the body of any of the first range of positions, and a second stop structure limiting movement of the spool in the second direction at a position providing substantially the least resistance to flow through the body of any of the second range of positions. 
     In further partial summary, a device has been disclosed that may utilize a microvalve as a pilot valve device. 
     In further partial summary, a device has been disclosed wherein the pilot valve device may comprise a fluid conduit extending between a first pilot connection port and a second pilot connection port, the flow through which fluid conduit is regulated by two variable orifices in series, one of which is normally open and one of which is normally closed, the pilot valve control port being connected in fluid communication with the fluid conduit between the variable orifices. 
     In further partial summary, a device has been disclosed wherein the normally closed orifice may be connected in fluid communication with the first connector via the first pilot connection port and the normally open orifice is in fluid communication with the second connector via the second pilot connection port. 
     In further partial summary, a device has been disclosed with a spool having first, second, third, and fourth ports formed in the spool at first, second, third, and fourth axial locations along the spool, respectively, each of the ports communicating with an axial passageway in the spool, each of the ports having the same cross-sectional flow area, and wherein there are more of one of the first, second, third, and fourth ports at the associated one of the first, second, third, and fourth axial locations than at another of the first, second, third, and fourth axial locations, whereby the device forms an asymmetric valve. 
     In further partial summary, a device has been disclosed with a spool having first, second, third, and fourth ports formed in the spool at first, second, third, and fourth axial locations along the spool, respectively, each of the ports communicating with an axial passageway in the spool, wherein at least one the first, second, third, and fourth ports has a different cross-sectional flow area from another of the first, second, third, and fourth ports at a different one of the first, second, third, and fourth axial locations, whereby the device forms an asymmetric valve. 
     In further partial summary, a device has been disclosed including a body having a first connector and a second connector, each of the first connector and second connector being adapted for fluid communication with an external circuit; and a spool disposed for sliding movement in the body. The spool has a first end portion and a second end portion opposite the first end portion, the first end portion of the spool being in fluid communication with a pilot valve producing a command pressure such that the spool is urged to move in a first direction by the command pressure, the spool being movable through a first range of positions to control, proportionally to the command pressure, the flow of a fluid when the fluid is flowing through the body in a forward direction from the first connector to the second connector, the spool being movable through a second range of positions, offset from the first range of positions, to control, proportionally to the command pressure, the flow of the fluid when the fluid is flowing through the body in a reverse direction from the second connector to the first connector, a portion of the fluid flowing through the body having a feedback pressure and acting on the spool in a second direction, opposite the first direction, to position the spool in conjunction with the fluid at the command pressure, the magnitude of the feedback pressure being generated at least in part as a function of the position of the spool. The body may further define a command chamber in fluid communication with the pilot valve control port to receive the fluid at the command pressure; a feedback chamber receiving the fluid having the feedback pressure; and a bore communicating at a first end portion with the command chamber and at a second end portion with the feedback chamber, the spool being disposed for sliding movement in the bore. The spool may further define an exterior surface; a central portion between the first end portion and the second end portion; a first axial end face on the first end portion which is in fluid communication with the command chamber; a second axial end face on the second end portion which is in fluid communication with the feedback chamber and having an opening defined therein; an axial passageway communicating with the opening in the second axial end face, the axial passageway extending into the central portion of the spool; a first port at a first axial location in the central portion of the spool providing communication between the exterior surface and the axial passageway; a second port in the central portion of the spool at a second axial location between the first axial location and the second end portion of the spool providing communication between the exterior surface and the axial passageway; a third port in the central portion of the spool at a third axial location spaced a first axial distance from the first axial location toward the first end portion of the spool, the third port providing communication between the exterior surface and the axial passageway; and a fourth port in the central portion of the spool at a fourth axial location spaced the first axial distance from the second axial location toward the first axial location and, the fourth port providing communication between the exterior surface and the axial passageway. The body may further define a first cavity communicating with the bore in the body at a first axial location along the bore, the first connector being in fluid communication with the first cavity; and a second cavity communicating with the bore in the body at a second axial location along the bore which is closer to the feedback chamber than the first axial location the second connector being in fluid communication with the second cavity, such that when the spool is in the first range of positions, a flow path for forward flow of fluid through the spool valve is established from the first connector, to the first cavity, through the spool via, sequentially the first port, the axial passageway, and the third port, to the second cavity, and thence to the second connector, and such that when the spool is in the second range of positions, a flow path for reverse flow of fluid through the spool valve is established from the second connector, to the second cavity, through the spool via, sequentially the fourth port, the axial passageway, and the second port, to the first cavity, and thence to the first connector. The device may have a greater maximum cross-sectional flow area when controlling one of forward flow and reverse flow, than when controlling the other one of forward flow and reverse flow. In further partial summary, this difference in maximum cross-sectional flow area might be achieved in a device in which, the first, second, third, and fourth ports each has the same cross-sectional flow area, and wherein there more of one of the first, second, third, and fourth ports at the associated one of the first, second, third, and fourth axial locations than at another of the first, second, third, and fourth axial locations, whereby the device forms an asymmetric valve. In further partial summary, another way in which this difference in maximum cross-sectional flow area might be achieved is in a device in which at least one the first, second, third, and fourth ports has a different cross-sectional flow area from another of the first, second, third, and fourth ports at a different one of the first, second, third, and fourth axial locations, whereby the device forms an asymmetric valve. 
     In further partial summary, a device has been disclosed that may include a spool valve including a body having a first connector and a second connector and a spool movable relative to the body for controlling flow between the first connector and the second connector. The reversible flow control assembly further may include a pilot valve device developing a single pressure command. The spool valve may be responsive to the single pressure command developed in the pilot valve device to control flow between the first connector and the second connector without regard to the direction of flow. The majority of forces acting on the spool in opposition to the pressure command to position the spool relative to the body when fluid is flowing through the valve may be fluid forces. 
     The principle and mode of operation of this invention have been explained and illustrated in its preferred embodiment. However, it must be understood that this invention may be practiced otherwise than as specifically explained and illustrated without departing from its spirit or scope.