Patent Publication Number: US-9404513-B2

Title: Servo valve

Description:
BACKGROUND 
     This specification generally relates to a servo valve, and more particularly to a hydraulic servo valve for regulating fluid flow. 
     Servo valves can be used to control fluid flow, for example, in hydraulic systems and continuous fluid flow systems. In some implementations, servo valves include a movable piston in a housing actuated by a movable flapper. 
     SUMMARY 
     The description below relates to servo valves. 
     In some aspects, a servo valve includes a valve housing, a piston cylinder disposed in the housing, a piston disposed within the piston cylinder, and a flapper assembly. The piston is fluidly connected on a first end to a first fluid pressure pathway, and fluidly connected on a second end to a second fluid pressure pathway. The piston is configured to translate axially within the piston cylinder in response to a pressure differential between a first fluid in the first fluid pressure pathway and a second fluid in the second fluid pressure pathway. The flapper assembly includes an activation portion and closure portion. The closure portion extends from the activation portion, and the flapper assembly is configured to move the closure portion to engage a first fluid flow control element on the first fluid pressure pathway when the closure portion is in a first position, and configured to move the closure portion to engage a second fluid flow control element on the second fluid pressure pathway when the closure portion is in a second position. The servo valve also includes a third fluid flow control element disposed in the piston cylinder in a portion of the first fluid pressure pathway. The third fluid flow control element is configured to stop a flow of fluid through the first fluid pressure pathway when the piston engages the third fluid control element. 
     In some aspects, a method of operating a servo valve includes providing a servo valve that includes a valve housing, a piston cylinder disposed in the housing, a piston disposed within the piston cylinder, and a flapper assembly. The piston is fluidly connected on a first end to a first fluid pressure pathway, and fluidly connected on a second end to a second fluid pressure pathway. The piston is configured to translate axially within the piston cylinder in response to a pressure differential between a first fluid in the first fluid pressure pathway and a second fluid in the second fluid pressure pathway. The flapper assembly includes an activation portion and closure portion. The closure portion extends from the activation portion, and the flapper assembly is configured to pivotably move the closure portion to engage a first fluid flow control element on the first fluid pressure pathway when the closure portion is in a first position, and configured to move the closure portion to engage a second fluid flow control element on the second fluid pressure pathway when the closure portion is in a second position. The servo valve also includes a third fluid flow control element disposed in the piston cylinder in a portion of the first fluid pressure pathway. The third fluid flow control element is configured to stop a flow of fluid through the first fluid pressure pathway when the piston engages the third fluid control element. The method further includes moving the closure portion of the flapper assembly to a first position, where the closure portion of the flapper assembly engages with the second flow control element, resulting in a pressure differential between the first fluid pressure pathway and second fluid pressure pathway that translates the piston within the piston cylinder to a first position, where the piston engages the third flow control element to seal the first fluid pressure pathway. 
     Some implementations may include one or more of the following features. The flapper assembly further includes one or more electrical coils disposed proximal to the activation portion of the flapper assembly. The first fluid control element includes a first nozzle in the first fluid pressure pathway configured to seal against the closure portion of the flapper assembly when the closure portion engages the first nozzle, and the second fluid control element includes a second nozzle in the second fluid pressure pathway configured to seal against the closure portion of the flapper assembly when the closure portion engages the second nozzle. The servo valve includes a fourth fluid control element disposed in the piston cylinder in a portion of the second fluid pressure pathway, the fourth fluid control element configured to stop a flow of fluid through the second fluid pressure pathway when the piston engages the fourth fluid control element. An outer periphery portion of the piston pressure-seals against an inner surface of the piston cylinder. The first fluid pressure pathway is connected on one end to a high pressure fluid pathway via a first pressure change element and on another end to a low pressure fluid pathway via the first fluid flow control element in the first fluid pathway. The second fluid pressure pathway is connected on one end to the high pressure fluid pathway via a second pressure change element and on another end to the low pressure fluid pathway via the second fluid flow control element in the second fluid pathway. The piston includes an outer groove disposed circumferentially in a substantially cylindrical outer surface of the piston. The piston cylinder includes an opening in a sidewall of the piston cylinder fluidly connected to a high pressure fluid pathway, an opening in a sidewall of the piston cylinder fluidly connected to a low pressure fluid pathway, and an opening in a sidewall of the piston cylinder fluidly connected to an output fluid pathway. The opening to the output fluid pathway is positioned in the piston cylinder such that when the groove in the piston translates as the piston moves axially, fluid in the groove remains in fluid communication with the opening to the output fluid pathway. The opening to the high pressure fluid pathway is spaced apart from and positioned in the sidewall to a first side of the opening to the output fluid pathway, and the opening to the low pressure fluid pathway is spaced apart from and positioned in the sidewall to a second side of the opening to the output fluid pathway in an opposite axial direction from the opening to the high pressure fluid pathway. The opening to the high pressure fluid pathway is positioned in the piston cylinder such that when the groove in the piston translates as the piston moves axially in a first direction, fluid in the groove remains in fluid communication with the opening to the high pressure fluid pathway and an outer surface of the piston closes the opening to the low pressure fluid pathway. The opening to the low pressure fluid pathway is positioned in the piston cylinder such that when the groove in the piston translates as the piston moves axially in a second direction opposite the first direction, fluid in the groove remains in fluid communication with the opening to the low pressure fluid pathway and an outer surface of the piston closes the opening to the high pressure fluid pathway. The piston includes a second outer groove disposed circumferentially in the substantially cylindrical outer surface of the piston. The piston cylinder includes a second opening in the sidewall of the piston cylinder fluidly connected to the high pressure fluid pathway, a second opening in the sidewall of the piston cylinder fluidly connected to the low pressure fluid pathway, and an opening in the sidewall of the piston cylinder fluidly connected to a second output fluid pathway. The opening to the second output fluid pathway is positioned in the piston cylinder such that when the groove in the piston translates as the piston moves axially, fluid in the second groove remains in fluid communication with the opening to the second output fluid pathway. The second opening to the high pressure fluid pathway is spaced apart from and positioned in the sidewall to a first side of the opening to the second output fluid pathway, and the second opening to the low pressure fluid pathway is spaced apart from and positioned in the sidewall to a second side of the opening to the second output fluid pathway in an opposite axial direction from the second opening to the high pressure fluid pathway. The second opening to the low pressure fluid pathway is positioned in the piston cylinder such that when the second groove of the piston translates as the piston moves axially in the first direction, fluid in the second groove remains in fluid communication with the second opening to the low pressure fluid pathway and an outer surface of the piston closes the second opening to the high pressure fluid pathway. The second opening to the high pressure fluid pathway is positioned in the piston cylinder such that when the second groove of the piston translates as the piston moves axially in the second direction, fluid in the second groove remains in fluid communication with the second opening to the high pressure fluid pathway and an outer surface of the piston closes the second opening to the low pressure fluid pathway. The first mentioned output fluid pathway and the second output fluid pathway are operably connected to a hydraulic drive system. The servo valve includes a feedback spring connected to the closure portion of the flapper assembly on one end and the piston on another end. The closure portion of the flapper assembly is movably attached to the housing. The closure portion of the flapper assembly is rotatably attached to the housing by a pivot, wherein the pivot comprises a pivot spring. The method includes moving the closure portion of the flapper assembly to a second position, where the closure portion engages with the second flow control element, resulting in a pressure differential between the first fluid pressure pathway and second fluid pressure pathway that translates the piston within the piston cylinder to a second position, where the piston engages a fourth flow control element to seal the second fluid pressure pathway. The fourth flow control element is disposed in the piston cylinder in a portion of the second fluid pressure pathway, and the fourth flow control element is configured to stop a flow of fluid through the second fluid pressure pathway when the piston engages the fourth fluid control element. Moving the closure portion of the flapper assembly to a first position includes providing an electrical input to one or more coils disposed proximal to the activation portion of the flapper assembly and thereby moving the closure portion of the flapper assembly to a first position. The method includes connecting the output fluid pathway to a hydraulic drive system. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic, partial cross-sectional front view of an example electrohydraulic servo valve. 
         FIGS. 2A and 2B  are schematic front views of an example electrohydraulic servo valve in a center position and a first position, respectively. 
         FIGS. 3A through 3C  are schematic front views of an example servo valve in a center position, a first position, and a second position, respectively. 
         FIG. 4  is a schematic front view of an example servo valve in a second position. 
     
    
    
     Like reference symbols in the various drawings indicate like elements. 
     DETAILED DESCRIPTION 
       FIG. 1  shows an example electrohydraulic servo valve (“EHSV”)  100  in a schematic, partial cross-sectional front view. The EHSV  100  includes a valve housing  102 , a piston cylinder  104  with a sleeve  106  disposed in the housing  102 , a piston  108  disposed in the sleeve  106 , and a flapper assembly  110  with an activation portion  112  and a closure portion  114 . It will be understood that the sleeve  106  is not a required element for implementation of this disclosure. In alternate embodiments, the piston  108  may be disposed directly in a bore of the piston cylinder  104 . The piston  108  is fluidly connected on a first end to a first fluid pressure pathway  116 , and is fluidly connected on a second end to a second fluid pressure pathway  118 . The piston  108  is configured to translate axially within the sleeve  106  in response to a pressure differential between a first fluid in the first fluid pressure pathway  116  and a second fluid in the second fluid pressure pathway  118 . The closure portion  114  of the flapper assembly  110  extends from the activation portion  112 , and the flapper assembly  110  is configured to move the closure portion  114 . In some instances, the flapper assembly  110  is configured to move the closure portion  114  to engage a first fluid flow control element  120  on the first fluid pressure pathway  116  when the closure portion  114  is in a first position, and is configured to move the closure portion  114  to engage a second fluid flow control element  122  on the second fluid pressure pathway  118  when the closure portion  114  is in a second position. 
     In certain instances, the first fluid flow control element  120  includes a first nozzle in the first fluid pressure pathway  116 , and the second fluid flow control element  122  includes a second nozzle in the second fluid pressure pathway  118 . The first nozzle is configured to seal against the closure portion  114  of the flapper assembly  110  when the closure portion  114  engages with the first nozzle in the first position. Similarly, the second nozzle is configured to seal against the closure portion  114  of the flapper assembly  110  when the closure portion  114  engages with the second nozzle in the second position. In other instances, the fluid flow control elements  120  and  122  include other, different flow control features. 
     The activation portion  112  of the flapper assembly  110  can be implemented in various manners. For example, the activation portion  112  can include a pressure activated diaphragm, a linear actuator, a pneumatic actuator, a servo motor, an armature with electrified coils about ends of the armature, and/or a different activation component. In the example shown in  FIG. 1 , the example EHSV  100  includes two electrical coils  124  disposed proximal to the activation portion  112  of the flapper assembly  110 . The flapper assembly  110  is movably attached to the housing  102 , for example, by a pivot spring  126  configured to resist rotation of the flapper assembly  110 . In the example shown in  FIG. 1 , the two electrical coils  124  coil about opposite ends of the activation portion  112 . In some instances, an electrical input, such as an input voltage or current, to the electrical coils  124  produces an electromagnetic force that results in a torque acting on the activation portion  112  to rotate the closure portion  114  to a specific position. In certain instances, the pivot spring  126  is configured to resist rotation of the flapper assembly  110  while the electrical coils  124  promote rotation of the flapper assembly  110 , such that the rotation of the flapper assembly  110  is proportional to the electrical input to the electrical coils  124 . The example EHSV  100  can include a different number of coils  124 , for example, one coil, or three or more coils. In some instances, the coils  124  can include a solenoid, coiled copper wire, and/or other electrical components. 
     In some instances, the EHSV  100  includes a feedback spring  128  connected to the closure portion  114  of the flapper assembly  110  on one end and the piston  108  on another end. The feedback spring  128  is configured to provide a force balance between the piston  108  and the flapper assembly  110 . For example, the piston  108  translates until torque on the flapper assembly  110  from the feedback spring  128  balances torque on the flapper assembly  110  applied by the electrical input of the electrical coils  124 . 
     In some instances, an outer periphery portion of the piston  108  pressure-seals against an inner surface of the sleeve  106  such that the first fluid in the first fluid pressure pathway  116  is separated from the second fluid in the second fluid pressure pathway  118 . For example, peripheries of the opposite ends of the piston  108  can seal against the sleeve  106  such that the first fluid is retained on one end of the sleeve  106  against a first end of the piston  108 , and the second fluid is retained on an opposite end of the sleeve  106  against a second, opposite end of the piston  108 . Pressure differentials between the first fluid and the second fluid can actuate the piston  108  to translate within the sleeve  106 . 
     The cross-sectional shape of the piston  108  and sleeve  106  can vary. For example, the piston  108  and sleeve  106  can each have a rectangular, square, circular, or different cross-sectional shape. The piston  108  has the same cross sectional shape as the sleeve  106  such that a pressure seal can exist between the piston and the sleeve while allowing translative movement of the piston  108  within the sleeve  106 . In an alternative embodiment without a sleeve  106 , the piston cylinder  104  will be configured with a cross-section to slidably receive the piston  108  of a non-cylindrical cross-section. In the example shown in  FIG. 1 , the piston  108  is substantially cylindrical with a circular cross-sectional shape that matches (substantially or wholly) a substantially cylindrical inner sidewall of the sleeve  106 . The piston  108  includes an outer groove  130  disposed circumferentially in a substantially cylindrical outer surface of the piston  108 . The sleeve  106  includes an opening  132  in the sidewall of the sleeve  106  fluidically connected to a high pressure fluid pathway  134 , an opening  136  in the sidewall of the sleeve  106  fluidically connected to a low pressure fluid pathway  138 , and an opening  140  in the sidewall of the sleeve  106  fluidically connected to an output fluid pathway  142 . The opening  140  to the output fluid pathway  142  is positioned in the sleeve  106  such that when the groove  130  in the piston  108  translates as the piston  108  moves axially, fluid in the groove  130  remains in fluid communication with the opening  140  to the output fluid pathway  142 . The opening  132  to the high pressure fluid pathway  134  is spaced apart from and positioned in the sidewall to a first side of the opening  140  to the output fluid pathway  142 , and the opening  136  to the low pressure fluid pathway  138  is spaced apart from and positioned in the sidewall to a second side of the opening  140  to the output fluid pathway  142  in an opposite axial direction from the opening  132  to the high pressure fluid pathway  134 . The opening  132  to the high pressure fluid pathway  134  is positioned in the sleeve  106  such that when the groove  130  in the piston  108  translates as the piston  108  moves axially in a first direction, fluid in the groove  130  remains in fluid communication with the opening  132  to the high pressure fluid pathway  134  and an outer surface of the piston  108  closes the opening  136  to the low pressure fluid pathway  138  (See  FIG. 3B ). The opening  136  to the low pressure fluid pathway  138  is positioned in the sleeve  106  such that when the groove  130  in the piston  108  translates as the piston  108  moves axially in a second direction opposite the first direction, fluid in the groove  130  remains in fluid communication with the opening  136  to the low pressure fluid pathway  138  and an outer surface of the piston  108  closes the opening  132  to the high pressure fluid pathway  134  (See  FIG. 3C ). 
     In some instances, such as the example EHSV  100  of  FIG. 1 , the piston  108  includes a second outer groove  144  disposed circumferentially in the substantially cylindrical outer surface of the piston  108 . The sleeve  106  includes a second opening  146  in the sidewall of the sleeve  106  fluidly connected to the high pressure fluid pathway  134 , a second opening  148  in the sidewall of the sleeve  106  fluidly connected to the low pressure fluid pathway  138 , and an opening  150  in the sidewall of the sleeve  106  fluidly connected to a second output fluid pathway  152 . The opening  150  to the second output fluid pathway  152  is positioned in the sleeve  106  such that when the groove in the piston  108  translates as the piston  108  moves axially, fluid in the second groove remains in fluid communication with the opening  150  to the second output fluid pathway  152 . The second opening  146  to the high pressure fluid pathway  134  is spaced apart from and positioned in the sidewall to a first side of the opening  150  to the second output fluid pathway  152 , and the second opening  148  to the low pressure fluid pathway  138  is spaced apart from and positioned in the sidewall to a second side of the opening  150  to the second output fluid pathway  152  in an opposite axial direction from the second opening  146  to the high pressure fluid pathway  134 . The second opening  148  to the low pressure fluid pathway  138  is positioned in the sleeve  106  such that when the second groove  144  of the piston  108  translates as the piston  108  moves axially in the first direction, fluid in the second groove  144  remains in fluid communication with the second opening  148  to the low pressure fluid pathway  138  and an outer surface of the piston  108  closes the second opening  146  to the high pressure fluid pathway  134 . The second opening  146  to the high pressure fluid pathway  134  is positioned in the sleeve  106  such that when the second groove  144  of the piston  108  translates as the piston  108  moves axially in the second direction, fluid in the second groove  144  remains in fluid communication with the second opening  146  to the high pressure fluid pathway  134  and an outer surface of the piston  108  closes the second opening  148  to the low pressure fluid pathway  138 . In some instances, the openings  136  and  148  to the low pressure fluid pathway  138  are a single opening in the sidewall of the sleeve  106 . In other instances, the openings  132  and  146  to the high pressure fluid pathway  134  are a single opening in the sidewall of the sleeve  106 . 
     In some instances, the first mentioned output fluid pathway  142 , the second output fluid pathway  152 , or both are operably connected to a hydraulic drive system, for example, a hydraulic actuator. The hydraulic actuator may be used to mechanically move an element of a device from a first position to a second position. By way of example and not limitation, the hydraulic output may be used to move an object (e.g. piston, actuator, fuel nozzle, etc.) on an aircraft from a first position to a second position and to intermediate positions there between. 
     In the example EHSV  100  shown in  FIG. 1 , the first fluid pressure pathway  116  is connected on one end to the high pressure fluid pathway  134  via a first pressure change element  154 , and connected on another end to the low pressure fluid pathway  138  via the first fluid flow control element  120 . The second fluid pressure pathway  118  is connected on one end to the high pressure fluid pathway  134  via a second pressure change element  156  and on another end to the low pressure fluid pathway  138  via the second fluid flow control element  122 , with an intermediate section extending into the sleeve  106  proximate the second end of the piston  108 . The first pressure change element  154  regulates pressure between fluid in the high pressure fluid pathway  134  and fluid in the first fluid pressure pathway  116  based on fluid flow through the first pressure change element  154 . Similarly, the first fluid flow control element  120  regulates pressure between fluid in the low pressure fluid pathway  138  and fluid in the first fluid pressure pathway  116 . For example, the first pressure change element  154  creates a pressure drop between the high pressure fluid pathway  134  and first fluid pressure pathway  116 , and the first fluid flow control element  120  creates a pressure drop between the first fluid pressure pathway  116  and the low pressure fluid pathway  138 , such that fluid in the first fluid pressure pathway  116  is at an intermediate pressure between the higher pressure in the high pressure fluid pathway  134  and the lower pressure in the low pressure fluid pathway  138 . The second pressure change element  156  regulates pressure between fluid in the high pressure fluid pathway  134  and fluid in the second fluid pressure pathway  118  based on fluid flow through the second pressure change element  156 . Similarly, the second fluid flow control element  122  regulates pressure between fluid in the low pressure fluid pathway  138  and fluid in the second fluid pressure pathway  118 . For example, the second pressure change element  156  creates a pressure drop between the high pressure fluid pathway  134  and second fluid pressure pathway  118 , and the second fluid flow control element  122  creates a pressure drop between the second fluid pressure pathway  118  and the low pressure fluid pathway  138 , such that fluid in the second fluid pressure pathway  118  is at an intermediate pressure between the higher pressure in the high pressure fluid pathway  134  and the lower pressure in the low pressure fluid pathway  138 . The first pressure change element  154  and second pressure change element  156  can each include a hydraulic bridge with an orifice, where the orifice is adapted to regulate pressure based on fluid flow through the orifice, for example, fluid flow from the high pressure fluid pathway  134  through the orifice and to the first fluid pressure pathway  116 , or fluid flow from the high pressure fluid pathway  134  through the orifice and to the second fluid pressure pathway  118 . 
     A third fluid flow control element  158  is disposed in the piston cylinder  104  in a portion of the first fluid pressure pathway  116 . The third fluid flow control element  158  is configured to stop a flow of fluid through the first fluid pressure pathway  116  when the piston  108  engages the third fluid flow control element  158 . The third fluid flow control element  158  can allow the example EHSV  100  to achieve a leakage shutoff condition for either a high pressure output or low pressure output in the output fluid pathway  142 . 
     The third fluid flow control element  158  can take many forms. In the example implementation shown in  FIG. 1 , the third fluid flow control element  158  comprises an inlet opening of the first fluid pressure pathway  116  into the piston cylinder  104 , where the piston  108  is configured to engage and block the inlet opening to stop a flow of fluid through the first fluid pressure pathway  116 . In some instances, the third fluid flow control element  158  includes a seat in the opening of first fluid pressure pathway  116  into the piston cylinder  104 , where the seat is configured to seal against the piston  108  when the piston  108  translates in the piston cylinder  104  and engages the seat. Fluid flow in the first fluid pressure pathway  116  is restricted (wholly or substantially) at the engagement of the piston  108  and the inlet opening and/or seat. In some instances (not shown), the third fluid flow control element  158  includes an extension or protrusion of the sleeve  106  or piston cylinder  104  into a portion of the first fluid pressure pathway  116 , with the protrusion or extension configured to abut the piston  108  when the piston  108  translates in the piston cylinder  104  and engages the protrusion or extension. In other instances (not shown), the third fluid flow control element  158  includes an extension or protrusion of the piston  108  into the first fluid pressure pathway  116 . The extension or protrusion of the piston  108  can be configured to seal against and engage a portion of the first fluid pressure pathway  116  such that fluid flow in the first fluid pressure pathway  116  is restricted (wholly or substantially) where the protrusion or extension of the piston  108  engages the portion of the first fluid pressure pathway  116 . For example, the piston  108  can include a cylindrical protrusion at a longitudinal end of the piston  108  adjacent the first fluid pressure pathway  116 , with the cylindrical protrusion configured to surround an opening of the first fluid pressure pathway  116  into a piston chamber portion of the first fluid pressure pathway  116 . In another example (not shown), a cylindrical protrusion of the piston  108  is configured to be received in and substantially seal the opening of the first fluid pressure pathway  116  to the piston chamber portion of the first fluid pressure pathway  116 . In other instances, the third fluid flow control element includes a fixed protrusion from the housing  102  into the first fluid pressure pathway  116  (See element  158 ′ in  FIGS. 3A, 3B, and 3C ). In further instances (not shown), the third fluid flow control element  158  includes another, different component configured to stop a flow of fluid through the first fluid pressure pathway  116  when engaged with the piston  108 . 
     In certain instances, the example EHSV  100  includes a fourth fluid flow control element (see  FIG. 4 ) disposed in the piston cylinder  104  in a portion of the second fluid pressure pathway  118 . For example, the second fluid pressure pathway  118  can mirror the first fluid pressure pathway  116  on an opposite side of the piston  108  from the first fluid pressure pathway  116 . The fourth fluid flow control element is configured to stop a flow of fluid through the second fluid pressure pathway  118  when the piston  108  engages the fourth fluid control element. In certain instances, the fourth fluid flow control element includes elements and components of the third fluid flow control element  158 . For example, the example servo valve  400  in  FIG. 4  shows a fourth fluid flow control element  160  including a fixed protrusion from the housing  102  into the second fluid pressure pathway  118 . An example servo valve with the third fluid flow control element  158  and the fourth fluid flow control element can achieve multiple leakage shutoff conditions. For example, a first leakage shutoff condition can correspond to a high pressure output for the output fluid pathway  142  when the third fluid flow control element  158  engages the piston  108 , and a second leakage shutoff condition can correspond to a low pressure output for the output fluid pathway  142  when the fourth fluid flow control element engages the piston  108 . 
       FIGS. 2A and 2B  show an example EHSV  200  in schematic front views. The example EHSV  200  is like the example EHSV  100  of  FIG. 1 , except the example EHSV  200  does not include a second opening in the sidewall of the sleeve  106  fluidly connected to the high pressure fluid pathway  134 , a second opening in the sidewall of the sleeve  106  fluidly connected to the low pressure fluid pathway  138 , and an opening in the sidewall of the sleeve  106  fluidly connected to a second output fluid pathway. In some instances, the example EHSV  200  includes the second opening to the high pressure fluid pathway  134 , the second opening to the low pressure fluid pathway  138 , and the opening to the second output fluid pathway. 
       FIG. 2A  illustrates the example EHSV  200  in a center position, where the closure portion  114  of the flapper assembly  110  is not engaged with the first fluid flow control element  120  or the second fluid flow control element  122 , and the piston  108  is generally centered in the sleeve  106 .  FIG. 2B  shows the example EHSV  200  in a first position, where the closure portion  114  is engaged with the second fluid flow control element  122  and the piston  108  is engaged with the third fluid flow control element  158 . In some instances, an electrical input to the coils  124  moves the flapper assembly  110  such that the closure portion  114  engages the second fluid flow control element  122 , thereby blocking fluid flow from the second fluid pressure pathway  118  from leaking into the low pressure fluid pathway  138  and allowing fluid flow from the high pressure fluid pathway  134  to enter the second fluid pressure pathway  118 . A higher pressure in the second fluid pressure pathway  118  relative to the pressure in the first fluid pressure pathway  116  creates a pressure differential between the first fluid pressure pathway  116  and second fluid pressure pathway  118 . The pressure differential effects translation of the piston  108  in a first direction (e.g. toward the first fluid pressure pathway  116 ) to engage the third fluid flow control element  158 , thereby blocking fluid leakage from the high pressure fluid pathway  134  into the first fluid pressure pathway  116 . In certain instances, translation of the piston  108  in the first direction effects a high pressure fluid through the output fluid pathway  142 . In other instances, translation of the piston  108  in a second, opposite direction from the first direction effects a low pressure fluid through the output fluid pathway  142 . 
       FIGS. 3A through 3C  show an example servo valve  300  in schematic front views. The example servo valve  300  includes components of the example EHSV  200  of  FIGS. 2A and 2B , except the third fluid flow control element is different. The servo valve  300  includes a third fluid flow control element  158 ′ disposed in the piston cylinder  104  in a portion of the first fluid pressure pathway  116 . The third fluid flow control element  158 ′ is configured to stop a flow of fluid through the first fluid pressure pathway  116  when the piston  108  engages the third fluid flow control element  158 ′. In the example servo valve  300  of  FIGS. 3A, 3B , and  3 C, the third fluid flow control element  158 ′ includes a fixed protrusion from the housing  102  into the first fluid pressure pathway  116 .  FIG. 3A  illustrates the servo valve  400  in the center position, and  FIG. 3B  illustrates the servo valve  300  in the first position.  FIG. 3C  illustrates the servo valve  300  in a second position, where the closure portion  114  is engaged with the first fluid flow control element  120  and the piston  108  is engaged with an end of the sleeve  106 . In some instances, the flapper assembly  110  is activated such that the closure portion  114  engages the first fluid flow control element  120 , thereby blocking fluid flow from the first fluid pressure pathway  116  from leaking into the low pressure fluid pathway  138  and allowing fluid flow from the high pressure fluid pathway  134  to enter the first fluid pressure pathway  116 . A higher pressure in the first fluid pressure pathway  116  relative to the pressure in the second fluid pressure pathway  118  creates a pressure differential between the first fluid pressure pathway  116  and second fluid pressure pathway  118 . The pressure differential effects translation of the piston  108  in a second direction (e.g. toward the second fluid pressure pathway  118 ) to engage the end of the sleeve  106 . 
       FIG. 4  shows an example servo valve  400  in a schematic front view, where the servo valve  400  is in the second position like the servo valve  300  in  FIG. 3C . The example servo valve  400  is like the example servo valve  300  of  FIGS. 3A, 3B, and 3C , except the example servo valve  400  includes a fourth fluid flow control element  160  disposed in the piston cylinder  104  in a portion of the second fluid pressure pathway  118 . The fourth fluid control element  160  is configured to stop a flow of fluid through the second fluid pressure pathway  118  when the piston  108  engages the fourth fluid flow control element  160 . In the example servo valve  400  of  FIG. 4 , the fourth fluid flow control element  160  includes a fixed protrusion from the housing  102  into the second fluid pressure pathway  118 . In other instances, the fourth fluid control element  160  includes elements and components of the third fluid flow control element  158  of  FIG. 1 . 
     In some instances, the flapper assembly  110  is activated such that the closure portion  114  engages the first fluid flow control element  120 , thereby blocking fluid flow from the first fluid pressure pathway  116  from leaking into the low pressure fluid pathway  138  and allowing fluid flow from the high pressure fluid pathway  134  to enter the first fluid pressure pathway  116 . A higher pressure in the first fluid pressure pathway  116  relative to the pressure in the second fluid pressure pathway  118  creates a pressure differential between the first fluid pressure pathway  116  and second fluid pressure pathway  118 . The pressure differential effects translation of the piston  108  in a second direction (e.g. toward the second fluid pressure pathway  118 ) to engage the fourth fluid flow control element  160 , thereby blocking fluid leakage from the high pressure fluid pathway  134  into the second fluid pressure pathway  118 . 
     One or more of the following advantages may be achieved by the apparatus, systems, and methods described below: reduced fluid leakage; reduced fluid input pump size; heat load, size, weight, and cost reductions; and/or ability to shut off leakage while controlling hydraulic output. 
     In the foregoing description of the example servo valves  100 ,  200 ,  300 , and  400 , various components, such as seals, bearings, fasteners, fittings, cables, channels, piping, etc., may have been omitted to simply the description. However, those skilled in the art will realize that such conventional equipment can be employed as desired. Those skilled in the art will further appreciate that various components described are recited as illustrative for contextual purposes and do not limit the scope of this disclosure. 
     Further, the use of a reference axes throughout the specification and/or claims is for describing the relative positions of various components of the system, apparatus, and other elements described herein. Unless otherwise stated explicitly, the use of such terminology does not imply a particular position or orientation of any components during operation, manufacturing, and/or transportation. 
     A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the inventions.