Patent Publication Number: US-10760836-B2

Title: Expansion valve

Description:
BACKGROUND OF THE INVENTION 
     This invention relates in general to valves for controlling fluid flow. In particular, this invention relates to an improved structure for two-stage proportional control valve for use in a fluid system, such as a heating, ventilating, air conditioning, and refrigeration (HVAC-R) system. 
     One known two-stage proportional control valve is an expansion valve, such as a Modular Silicon Expansion Valve (MSEV). MSEVs are electronically controlled, normally closed, and single flow directional valves. MSEVs may be used for refrigerant mass flow control in conventional HVAC-R applications. 
     The first stage of the MSEV is a microvalve that acts as a pilot valve to control a second stage spool valve. When the microvalve receives a Pulse Width Modulation (PWM) signal, the microvalve modulates to change a pressure differential across the second stage spool valve. The spool valve will move to balance the pressure differential, effectively changing an orifice opening of the MSEV to control the flow of refrigerant. 
     There are however, undesirable manufacturing processes associated with known MSEVs. For example, the final machining steps necessary to ensure a required spool bore diameter in a valve body of the MSEV may only be accomplished after fluid inlet and fluid outlet connector tubes and capillary tubes have been brazed to the valve body. This sequence is required because bores machined into the valve body may become distorted by as much as about 30 μm by the heat used in the brazing operation. A typical machined spool bore in an MSEV valve body has a diameter tolerance of about +/−5 μm, and the brazing operation may cause the machined spool bore to become out of tolerance if the brazing operation is performed after the spool bore has been machined. Therefore, components such as the fluid inlet and fluid outlet connector tubes and the capillary tubes are commonly brazed to the valve body prior to the final machining steps. Because components such as the fluid inlet and fluid outlet connector tubes and the capillary tubes are brazed to the valve body prior to the final machining steps, fixtures and tools used to assemble the MSEV may be complex and costly, and manufacturing time may be undesirably lengthy. 
     MEMS (Micro Electro Mechanical Systems) are a class of systems that are physically small, having features with sizes in the micrometer range; i.e., about 10 μm or smaller. 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 “micromachined device” as used in this application means a device having some features with sizes of about 10 μm or smaller, and thus by definition is at least partially formed by micromachining. More particularly, the term “microvalve” as used in this application means a valve having features with sizes of about 10 μm or smaller, and thus by definition is at least partially formed by micromachining. The term “microvalve device” as used in this application means a micromachined 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. Similarly, a micromachined device may include both micromachined components and standard sized (larger) 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 component movably supported by a body for movement between a closed position and a fully open position. When placed in the closed position, the valve component substantially blocks or closes a first fluid port that is otherwise in fluid communication with a second fluid port, thereby substantially preventing fluid from flowing between the fluid ports. Known microvalves thus allow some fluid to leak through a closed valve port, thus substantially preventing, but not completely preventing, fluid flow therethrough. When the valve component moves from the closed position to the fully open position, fluid is increasingly allowed to flow between the fluid ports. 
     U.S. Pat. Nos. 6,523,560; 6,540,203; and 6,845,962, the disclosures of which are incorporated herein by reference, describe microvalves made of multiple layers of material. The multiple layers are micromachined and bonded together to form a microvalve body and the various microvalve components contained therein, including an intermediate mechanical layer containing the movable parts of the microvalve. The movable parts are formed by removing material from an intermediate mechanical layer (by known micromachined device fabrication techniques, such as, but not limited to, Deep Reactive Ion Etching) to create a movable valve element that remains attached to the rest of the part by a spring-like member. Typically, the material is removed by creating a pattern of slots through the material to achieve the desired shape. The movable valve element will then be able to move in one or more directions an amount roughly equal to the slot width. 
     U.S. Pat. No. 7,156,365, the disclosure of which is also incorporated herein by reference, describes a method of controlling the actuator of a microvalve. In the disclosed method, a controller supplies an initial voltage to the actuator which is effective to actuate the microvalve. Then, the controller provides a pulsed voltage to the actuator which is effective to continue the actuation of the microvalve. 
     Because of the undesirable processes associated with manufacturing known two-stage proportional control valves, it would be desirable to provide an improved structure for a two-stage proportional control valve that is easier to manufacture, and in which the final machining steps necessary to manufacture the valve body may be accomplished before components such as the fluid inlet and fluid outlet connector tubes and the capillary tubes must been brazed thereto. 
     SUMMARY OF THE INVENTION 
     This invention relates to an improved structure for a two-stage proportional control valve for use in a fluid system, such as an HVAC-R system. In one embodiment, the two-stage proportional control valve configured for use in a fluid system includes a valve body having a longitudinally extending valve body bore formed therethrough. A first stage microvalve is mounted within the valve body bore, and a second stage spool assembly is mounted within the valve body bore downstream of the microvalve. The second stage spool assembly includes a sleeve and a spool slidably mounted within the sleeve. 
     In a second embodiment, a spool assembly configured for use in a two-stage proportional control valve in a fluid system includes a sleeve. The sleeve is substantially cylindrical and includes an axially extending sleeve bore formed therein and extending from an open first end to an open second end of the sleeve. A spool includes a spool bore extending axially from an open first end to a closed second end and slidably mounted within the sleeve bore. 
     In a third embodiment, a method of assembling a two-stage proportional control valve configured for use in a fluid system includes slidably mounting a spool within a sleeve to define a spool valve assembly. The spool valve assembly is mounted in a longitudinally extending valve body bore formed through a valve body of the two-stage proportional control valve. A first stage microvalve is also mounted within the valve body bore. The spool valve assembly defines a second stage spool assembly of the two-stage proportional control valve and is mounted within the valve body bore downstream of the microvalve. 
     Various aspects of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiments, when read in light of the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a representative embodiment of a refrigeration system including an HVAC-R expansion valve in accordance with this invention. 
         FIG. 2  is a side elevational view of a conventional HVAC-R expansion valve. 
         FIG. 3  is front elevational view of the conventional HVAC-R expansion valve illustrated in  FIG. 2 . 
         FIG. 4  is a cross sectional view taken along the line  4 - 4  of  FIG. 3  and shown with the plugs and spool removed. 
         FIG. 5  is a side elevational view of an improved HVAC-R expansion valve according to this invention. 
         FIG. 6  is a front elevational view of the improved HVAC-R expansion valve illustrated in  FIG. 5 . 
         FIG. 7  is a cross sectional view taken along the line  7 - 7  of  FIG. 5 . 
         FIG. 8  is an enlarged cross sectional view of the valve body shown in  FIG. 7 . 
         FIG. 9  is an end view of the improved spool assembly shown in  FIG. 7 . 
         FIG. 10  is a cross sectional view of the improved spool assembly taken along the line  10 - 10  of  FIG. 9 . 
         FIG. 11  is a cross sectional view of the improved spool assembly taken along the line  11 - 11  of  FIG. 10 . 
         FIG. 12  is an alternate cross sectional view of the of the improved spool assembly shown in  FIG. 11  showing the improved spool assembly in a fully actuated position. 
         FIG. 13  is a cross sectional view of the improved spool assembly taken along the line  13 - 13  of  FIG. 12 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings, there is illustrated in  FIG. 1  a block diagram of a representative embodiment of a refrigeration system, indicated generally at  10 , in accordance with this invention. The illustrated refrigeration system  10  is, in large measure, conventional in the art and is intended merely to illustrate one environment in which this invention may be used. Thus, the scope of this invention is not intended to be limited for use with the specific structure for the refrigeration system  10  illustrated in  FIG. 1  or with refrigeration systems in general. On the contrary, as will become apparent below, this invention may be used in any desired environment for the purposes described below. 
     As is well known in the art, the refrigeration system  10  circulates a refrigerant through a closed circuit, where it is sequentially subjected to compression, condensation, expansion, and evaporation. The circulating refrigerant removes heat from one area (thereby cooling that area) and expels the heat in another area. 
     To accomplish this, the illustrated refrigeration system  10  includes an evaporator  11 , such as an evaporator coil. The evaporator  11  is conventional in the art and is adapted to receive a relatively low pressure liquid refrigerant at an inlet thereof. A relatively warm fluid, such as air, may be caused to flow over the evaporator  11 , causing the relatively low pressure liquid refrigerant flowing in the evaporator  11  to expand, absorb heat from the fluid flowing over the evaporator  11 , and evaporate within the evaporator  11 . The relatively low pressure liquid refrigerant entering into the inlet of the evaporator  11  is thus changed to a relatively low pressure refrigerant gas exiting from an outlet of the evaporator  11 . 
     The outlet of the evaporator  11  communicates with an inlet of a compressor  12 . The compressor  12  is conventional in the art and is adapted to compress the relatively low pressure refrigerant gas exiting from the evaporator  12  and to move such relatively low pressure refrigerant gas through the refrigeration system  10  at a relatively high pressure. The relatively high pressure refrigerant gas is discharged from an outlet of the compressor  12  that communicates with an inlet of a condenser  13 . The condenser  13  is conventional in the art and is configured to remove heat from the relatively high pressure refrigerant gas as it passes therethrough. As a result, the relatively high pressure refrigerant gas condenses and becomes a relatively high pressure refrigerant liquid. 
     The relatively high pressure refrigerant liquid then moves from an outlet of the condenser  13  to an inlet of an expansion device  14 . In the illustrated embodiment, the expansion device  14  is a hybrid spool valve that is configured to restrict the flow of fluid therethrough. As a result, the relatively high pressure refrigerant liquid is changed to a relatively low pressure refrigerant liquid as it leaves the expansion device. The relatively low pressure refrigerant liquid is then returned to the inlet of the evaporator  11 , and the refrigeration cycle is repeated. 
     The illustrated refrigeration system  10  additionally may include a conventional external sensor  15  that communicates with the fluid line that provides fluid communication from the evaporator  11  to the compressor  12 . The external sensor  15  is responsive to one or more properties of the fluid (such as, for example, pressure, temperature, and the like) in such fluid line for generating a signal that is representative of that or those properties to a controller  16 . In response to the signal from the external sensor  15  (and, if desired, other non-illustrated sensors or other inputs), the controller  16  generates a signal to control the operation of the expansion device  14 . If desired, the external sensor  15  and the controller  16  may be embodied together as a conventional universal superheat sensor-controller, such as is commercially available from DunAn Microstaq, Inc. of Austin, Tex. U.S. Pat. No. 9,140,613 to Arunasalam et al. describes superheat sensors, controllers, and processors, and their operation. The disclosure of U.S. Pat. No. 9,140,613 is incorporated herein by reference. 
       FIGS. 2 through 4  illustrate a conventional hybrid spool valve. The illustrated conventional hybrid spool valve is a two-stage proportional control valve configured as a Modular Silicon Expansion Valve (MSEV)  14 . In  FIG. 4 , the MSEV  14  is shown with a conventional first plug and attached conventional first stage microvalve, a conventional second plug, and a conventional second stage spool removed for clarity. 
     Referring now to  FIGS. 5 through 13 , an improved two-stage proportional control valve configured as an MSEV is shown at  50 . The MSEV  50  includes a valve body  52  defining a longitudinally extending bore  54  formed therein and extending between a first end  52   a  and a second end  52   b  of the valve body  52 . The bore  54  includes a first portion or plug bore  60  configured to receive a first plug defining a microvalve assembly  64  (see  FIG. 7 ), and a second portion or spool assembly bore  62  configured to receive a spool assembly  66  (see  FIG. 7 ). An axial end surface  53  of the first end  52   a  of the valve body  52  (the upwardly facing surface when viewing  FIG. 7 ) includes an annular sealing groove  53   a  formed therein. 
     An opening  56  (see  FIG. 8 ) of the bore  54  at the first end  52   a  of the valve body  52  may be closed by the microvalve assembly  64 . Similarly, an opening  58  (see  FIG. 8 ) of the bore  54  at the second end  52   b  of the valve body  52  may be closed by a closure member or second plug  68 . The second plug  68  includes external threads and is configured for threaded attachment within the spool assembly bore  62 . The plugs  64  and  68  may be sealingly fixed in the respective openings  56  and  58  by any suitable means, such as by welding, press fitting, rolling, or as illustrated, held in place by a threaded connection. As shown in  FIG. 7 , the microvalve assembly  64  includes a radially outwardly extending flange  67  at a first end thereof. A sealing surface  67   a  of the flange  67  (the downwardly facing surface when viewing  FIG. 7 ) includes an annular sealing ridge  69  extending outwardly therefrom. 
     The microvalve assembly  64  may be made leak-tight by a metal to metal interference seal S 1  defined between the annular sealing ridge  69  and the annular sealing groove  53   a , and one or more annular seals, such as O-rings  70  and  72 . Similarly, the second plug  68  may be made leak tight by a metal to metal interference seal S 2  defined between an outside surface of the second plug  68  and a shoulder  63  formed in the spool assembly bore  62 . The second plug  68  may be further made leak tight by an O-ring  73 . It will be understood however, that the metal interference seal S 2  may be sufficient to seal the second plug  68  within the spool assembly bore  62 , and the O-ring  73  may not be required. An electrical connector  74  extends outwardly from an outside axial end of the microvalve assembly  64 . A microvalve  76  may be mounted to an inboard axial end of the microvalve assembly  64  (the lower end of the microvalve assembly  64  when viewing  FIG. 7 ) by any suitable method, such as with solder. 
     Electrical connectors, such as posts or pins  78 , extend between a cavity  65  formed in the first end  64   a  of the microvalve assembly  64  and a second end  64   b  of the microvalve assembly  64 . First electrical connectors, such as wires  83 , electrically connect the pins  78  to a source of electrical power (not shown) via the electrical connector  74 . Second electrical connectors, such as wires  84  electrically connect the microvalve  76  to the pins  78  at the second end  64   b  of the microvalve assembly  64 . 
     A substantially cup-shaped cap  80  is attached to an outside surface of the microvalve assembly  64  at a second end  64   b  thereof. The cap  80  has a substantially cylindrical outer surface and includes an opening  81  in an end wall thereof that defines a flow path for fluid between the microvalve  76  and the spool assembly bore  62 . An interior of the cap  80  defines a cavity  82  within which the microvalve  76  is mounted. The illustrated cap  80  is preferably formed from glass filled nylon. Alternatively, the cap  80  may be formed from any desired polymer or other material. 
     Referring to  FIG. 8 , the spool assembly bore  62  includes a first diameter portion  62   a  adjacent the plug bore  60 , a second diameter portion  62   b , and a third diameter portion  62   c  at the second end  52   b  of the valve body  52 . The second diameter portion  62   b  is larger than the first diameter portion  62   a , and smaller than the third diameter portion  62   c . A first circumferentially extending fluid flow groove  85  is formed in an inside surface of the second diameter portion  62   b  of the spool assembly bore  62 , and a second circumferentially extending fluid flow groove  86  is formed in an inside surface of the third diameter portion  62   c  of the spool assembly bore  62 . 
     The valve body  52  further includes a transversely extending fluid inlet port  88  and a transversely extending fluid outlet port  90  in fluid communication with the spool assembly bore  62  via the fluid flow grooves  85  and  86 , respectively. As shown in  FIG. 7 , the fluid inlet port  88  is in fluid communication with the condenser  13  via the inlet connector conduit  36 , and the fluid outlet port  90  is in fluid communication with the evaporator  11  via the outlet connector conduit  38 . Thus, as shown in  FIGS. 6 and 7 , fluid may flow through the MSEV  50  in the direction of the arrows A. 
     As shown in  FIG. 5 , transversely extending capillary bores  92   a  and  92   b  are formed in the valve body  52  and extend outwardly from the fluid flow grooves  86  and  85 , respectively. Transversely extending capillary bores  92   c  and  92   d  are also formed in the valve body  52  and extend outwardly from the plug bore  60  of the bore  54  and are in fluid communication with fluid flow conduits (not shown) formed in the microvalve assembly  64 . These fluid flow conduits (not shown) supply fluid to the microvalve  76 . 
     Referring to  FIGS. 5 and 6 , a first capillary tube  94   a  extends between the capillary bore  92   a  and the capillary bore  92   d . A second capillary tube  94   b  extends between the capillary bore  92   b  and the capillary bore  92   c . The joints between the capillary tubes  94   a  and  94   b  and the valve body  52  may be brazed joints and are shown at B 1  in  FIGS. 5 and 6 . Similarly, the joints between the fluid inlet and fluid outlet ports  88  and  90  and the inlet and outlet connector conduits  36  and  38 , respectively, may also be brazed joints and are shown at B 2  in  FIGS. 6 and 7 . 
     The conventional MSEV  14  illustrated in  FIGS. 2 through 4  includes a valve body  20  defining a longitudinally extending bore  22  having a first portion  24  configured to receive the microvalve assembly  64  (shown removed for clarity), and a second portion or spool bore  26  configured to receive the spool assembly  66  (shown removed for clarity). The spool bore  26  includes three sections  26   a ,  26   b , and  26   c , the inside diameters of each require a dimensional manufacturing tolerance of about +/−5 μm. 
     The spool bore  26  also includes a circumferentially extending first groove defining a fluid inlet chamber  28 , and a circumferentially extending second groove defining a fluid outlet chamber  30 . 
     The valve body  20  further includes a transversely extending inlet port  32  and a transversely extending outlet port  34 . The inlet port  32  is in fluid communication with the condenser  13  via an inlet connector conduit  36 . The outlet port  34  is in fluid communication with the evaporator  11  via an outlet connector conduit  38 . 
     Capillary tubes  40  extend between the inlet and outlet ports  32  and  34  and fluid flow conduits (not shown) formed in the microvalve assembly  64 . These fluid flow conduits supply fluid to the first stage microvalve (not shown). The joints between the capillary tubes  40  and the valve body  20  are typically brazed joints and are shown at B 1  in  FIGS. 2 and 3 . Similarly, the joints between the inlet and outlet ports  32  and  34  and the inlet and outlet connector conduits  36  and  38  are also typically brazed joints and are shown at B 2  in  FIGS. 3 and 4 . 
     When manufacturing the conventional MSEV  14 , the valve body  20 , the capillary tubes  40 , and the inlet and outlet connector conduits  36  and  38  are first assembled and brazed as shown in  FIGS. 2 through 4 . After the brazing step in the manufacturing process, the final machining steps necessary to finish the spool bore  26  of the bore  22  may be completed. This sequence is required because the machined spool bore  26  in the valve body  20  may become distorted by as much as 30 μm by the heat used in the brazing operation. Such distortion is undesirable because spool bores, such as the spool bore  20 , typically require a dimensional manufacturing tolerance of about +/−5 μm, and the brazing operation may cause the spool bore  20  to become out of tolerance if the brazing operation is performed after the spool bore  20  has been machined. 
     Referring to  FIGS. 9 through 13 , a first embodiment of an improved spool assembly  66  in accordance with this invention is shown. The spool assembly includes a substantially cylindrical spool  110  within a sleeve  112 . The spool  110  includes an axially extending bore  114  formed therein and extending from an open first end  110   a  to a closed second end  110   b  of the spool  110 . The first end  110   a  of the spool  110  includes a reduced diameter portion  116  defining a shoulder  118 . A substantially cup-shaped insert  115  is attached within the bore  114  at the open first end  110   a  of the spool  110 . A feedback pressure chamber  117  may be defined in an interior of the insert  115 . The insert  115  has a substantially cylindrical outer surface and includes an opening  119  in an end wall thereof that defines a flow path for fluid between the feedback pressure chamber  117  and the spool bore  114 . 
     A first circumferentially extending groove  120  is formed on an outside surface of the spool  110  intermediate the first and second ends  110   a  and  110   b . The circumferentially extending groove  120  defines a fluid flow path. A second circumferentially extending groove  122  is formed on an outside surface of the spool  110  near the first end  110   a  thereof, and a third circumferentially extending groove  124  is formed on an outside surface of the spool  110  near the second end  110   b  thereof. A circumferentially extending pressure groove  126  is also formed on an outside surface of the spool  110  between the second axial end  110   b  and the third circumferentially extending groove  124 . 
     A first transverse fluid passageway  128  is formed through a side wall of the spool  110  between the bore  114  and the second circumferentially extending groove  122 , and a second transverse fluid passageway  130  is formed through a side wall of the spool  110  between the bore  114  and the third circumferentially extending groove  124 . A third transverse fluid passageway  132  is formed through a side wall of the spool  110  between the bore  114  and the circumferentially extending pressure groove  126 . 
     The sleeve  112  is substantially cylindrical and includes an axially extending spool bore  134  formed therein and extending from an open first end  112   a  to an open second end  112   b  of the sleeve  112 . 
     A first circumferentially extending sealing portion  136  is formed on an outside surface of the sleeve  112  and defines a first circumferentially extending sealing groove  136   a . A second circumferentially extending sealing portion  138  is also formed on an outside surface of the sleeve  112  and defines a second circumferentially extending sealing groove  138   a . Additionally, a third circumferentially extending sealing portion  140  is formed on an outside surface of the sleeve  112  and defines a third circumferentially extending sealing groove  140   a.    
     A first annular seal  142   a , such as an O-ring, may be disposed within the first circumferentially extending sealing groove  136   a . Similarly, second and third annular seals  142   b  and  142   c , such as O-rings, may be disposed within the second and third circumferentially extending sealing groove  138   a  and  140   a , respectively. 
     A circumferentially extending inlet fluid flow groove  144  is defined in the outside surface of the sleeve  112  between the second and third sealing portions  138  and  140 . Similarly, a circumferentially extending outlet fluid flow groove  146  is defined in the outside surface of the sleeve  112  between the first and second sealing portions  136  and  138 . 
     At least one main fluid flow inlet passageway  148  is formed through a side wall of the sleeve  112  between the bore  134  and the inlet fluid flow groove  144 , and at least one main fluid flow outlet passageway  150  is formed through the side wall of the sleeve  112  between the bore  134  and the outlet fluid flow groove  146 . Additionally, at least one feedback flow inlet passageway  152  is formed through the side wall of the sleeve  112  between the bore  134  and the inlet fluid flow groove  144 , and at least one feedback flow outlet passageway  154  is formed through the side wall of the sleeve  112  between the bore  134  and the outlet fluid flow groove  146 . 
     A first cap cavity  156  is formed in the first end  112   a  of the sleeve  112  and a second cap cavity  158  is formed in the second end  112   b  of the sleeve  112 . A closure member or cap  160  is mounted within each of the first and second cap cavities  156  and  158 , and may be attached therein by any desired means, such as by threaded attachment, staking, or by welding. The cap  160  may include one or more fluid passageways  162  (see  FIGS. 9 and 11 ) formed therethrough. A spring  164  extends between the cap  160  at the first end  112   a  of the sleeve  112  and the shoulder  118  of the spool  110 . The spring  164  urges the second end  110   b  of the spool  110  toward the second end  112   b  of the sleeve  112  and thus urges the spool  110  into an un-actuated or closed position, as shown in  FIGS. 10 and 11 . In the closed position, the main fluid flow outlet passageway  150  is closed by the spool  110 , thus preventing fluid flow through the spool assembly  66 . In the closed position, the feedback flow inlet passageway  152  is also closed by the spool  110 , but the feedback flow outlet passageway  154  is open and in fluid communication with the outlet fluid flow groove  146 , the second circumferentially extending fluid flow groove  86  (see  FIG. 8 ), and the fluid outlet port  90  (see  FIG. 8 ). A command chamber  166  may be defined between an axial end face of the second end  110   b  of the spool  110  and the adjacent cap  160 . 
     In operation, when it is desired to operate the spool assembly  66  and move fluid therethrough, the microvalve  76  may be actuated. The fluid discharged from the microvalve  76  controls a command pressure on the second end  110   b  of the spool  110 . The command pressure acting on the second end  110   b  of the spool  110  urges the spool  110  against the force of the spring  164  (downward when viewing  FIG. 7  and to the right when viewing  FIGS. 10 and 11 ). 
     Thus, when actuated, the microvalve  76  causes the spool  110  to move from the closed position to a fully actuated or fully open position as shown in  FIGS. 12 and 13 , and a plurality of partially open positions (not shown) between the closed and fully open positions. In the fully open position, the main fluid flow inlet passageway  148  and the main fluid flow outlet passageway  150  are open, thus permitting a main flow of fluid through the spool assembly  66 , i.e., through the main fluid flow inlet passageway  148 , the first circumferentially extending groove  120  of the spool  110 , and the main fluid flow outlet passageway  150 . In the fully open position, the feedback flow outlet passageway  154  is closed by the spool  110 , but the feedback flow inlet passageway  152  is open and in fluid communication with the inlet fluid flow groove  144 , the first circumferentially extending fluid flow groove  85  (see  FIG. 8 ), and the fluid inlet port  88  (see  FIG. 8 ). 
     The circumferentially extending pressure groove  126  and the fluid passageway  132  are in fluid communication with the bore  114  and are configured to isolate the command chamber  166  from fluid that may leak around the spool  110  (i.e., from the right of the pressure groove  126  when viewing  FIGS. 10 through 13 ), and that may overwhelm the fluid pressure introduced by the microvalve  76 . Any fluid that may leak into the command chamber  166  is thus tied to the feedback pressure within the bore  114  and the feedback pressure chamber  117 . 
     During manufacture and assembly of the MSEV  50 , the spool assembly bore  62  may be machined in the valve body  52  prior to the capillary tubes  40  and the inlet and outlet connector conduits  36  and  38  being brazed to the valve body  52 . 
     The spool  110 , the sleeve  112 , and the caps  160  may be formed and assembled to define the spool assembly  66  independently of the valve body  52 . The piston bore  134  may thus be machined having a diameter tolerance of about +/−5 μm, without being negatively affected by heat from the brazing operation on the valve body  52 . Once assembled, the spool assembly  66  may then be mounted within the spool assembly bore  62 . 
     The spool assembly bore  62  in the valve body  52  is configured to receive, and have fixedly mounted therein, the spool sleeve  112  rather than the slidable spool  110 , as in the conventional MSEV  14 . Because the spool assembly  66  may be sealed within the spool assembly bore  62  by the metal to metal interference seal S 1 , and by the O-rings  142   a ,  142   b , and  142   c , the diameter tolerance for the spool assembly bore  62  may relatively larger than the tolerance for the spool bore  26  in the conventional valve body  20 , such as about +/−50 μm. 
     Thus, the spool assembly bore  62  may be machined prior to brazing, and therefore the capillary tubes  40  and the inlet and outlet connector conduits  36  and  38  may be thereafter brazed without causing the spool assembly bore  62  to become out of tolerance. The relatively small tolerance of about +/−5 μm between the spool  110  and the sleeve  112  in the spool assembly  66  may also be achieved and maintained in a manufacturing process independent of, and at a location separate from, the machining and brazing steps required to manufacture and assemble the valve body  52 . 
     Because the spool  110  is enclosed within the sleeve  112  by the caps  160 , the spool assembly  66  may be easily and safely moved, and may be easily tested independently and separately from the valve body  52  of the MSEV  50 , thus saving time and reducing cost. 
     The principle and mode of operation of this invention have been explained and illustrated in its preferred embodiments. 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.