Abstract:
A two, three or four-way valve includes two stages each with working valve components linked by a feedback spring so that a first stage servo unit is auto-nulling and the stroke of the valve member in a second stage valve unit is proportional to the input current to the first stage. An inherently balanced clevis member in the first stage controls flow of pressurized control media, which in combination with direct pressure flow, controls the position of a spool member in the second stage. The spool in turn controls pressure flow to a either one or two metering or input/output flow ports. Transient movement of the clevis due to change in input current to a magnetic servo drive assembly is opposed by bending of the feedback spring arising from the associated movement of the spool, which returns the clevis to its centered null position and stops the spool.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]    This application claims benefit to U.S. provisional application Ser. No. 60/366,761 filed Mar. 21, 2002. 
     
    
     
       STATEMENT OF FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
         [0002]    Not applicable.  
         BACKGROUND OF THE INVENTION  
         [0003]    1. Technical Field  
           [0004]    The present invention relates to valves, and in particular, to multistage electrohydraulic servo valves.  
           [0005]    2. Description of the Related Art  
           [0006]    Electrohydraulic servo valves are well-known, particularly for use in pilot stages of directional control valves. One such application is for an actuator operating the compressor bleed valve of an aircraft turbofan engine. Electrohydraulic servo valves can have a first stage with an electrical or electromagnetic force motor controlling flow of a hydraulic fluid driving a valve member, such as a spool valve, of a second stage, which in turn can control flow of hydraulic fluid to an actuator driving the load. The force motor can operate to position a movable member, such as a flapper, in response to an input drive signal to drive the second stage valve member. Electrical or mechanical feedback can be provided to return the force motor to the original or null position after the valve member has been moved to its desired position, thereby stopping its movement.  
           [0007]    U.S. Pat. No. 4,456,031 discloses one example of an electrohydraulic servo valve. In this case, the valve first stage has a torque motor driving an armature to pivot a flapper member toward and away from two nozzles through which hydraulic fluid can be directed at either of opposing ends of a spool so as to move the spool and thereby control flow to an actuator. Redundant mechanical spring and electrical transducer feedback systems are employed to prevent shut-down of the valve in the event of failure of malfunction of one of the feedback systems. See also U.S. Pat. No. 5,249,603. However, these and other existing systems are disadvantageous in that they do not exhibit both high response and low null leakage, competing attributes that are highly advantageous in hydraulic systems.  
           [0008]    Accordingly, an improved multi-stage valve is desired.  
         SUMMARY OF THE INVENTION  
         [0009]    The present invention provide a two stage electrohydraulic servo valve in which the first stage has an inherently balanced variable flow valve member that is auto-nulled by a feedback force from associated movement of the valve member in the second stage. The stroke of the hydraulically drive second stage valve member is proportional to a drive signal input to the first stage.  
           [0010]    Specifically, the invention provides an electrohydraulic servo valve having first and second stage units. The first stage servo valve unit has a drive assembly adapted to move a forked clevis member from a null position to alternatively open and close first and second nozzle orifices. When the first nozzle orifice is open flow is permitted between a pressure port and a control port and when the second nozzle orifice is open flow is permitted between the control port and a return port. The second stage valve unit has a sliding valve member as well as an inlet port in communication with the first stage control port, a flow port and a pressure port. Flow from the second stage pressure port to the flow port is controlled by the sliding valve member. The clevis member and the sliding valve member are linked by a feedback spring such that movement of the sliding valve member imparts a feedback force to the feedback spring to return the clevis member to the null position.  
           [0011]    In one preferred form, the sliding valve member is a spool cooperating with a half-area piston held stationary by fluid pressure and disposed within a fixed guide sleeve. The spool member moves under the force of flow from the first stage control pressure to close off flow from the second stage pressure or return ports.  
           [0012]    In other forms, the valve can have a two-way second stage in which the flow is controlled from the second stage pressure port (with no return port) to a single metering flow port. Or, the valve can have a three-way second stage having a pressure port, a return port and a single input/output flow port. Or, the valve can have a four-way second stage in having pressure, return and two input/output flow ports, in which case flow is discharged from one flow port and taken in through the other flow port.  
           [0013]    In another preferred form, wherein the drive assembly is a permanent magnet motor having a wire coil and a movable actuator member connected to the clevis member and disposed along a main axis. The first stage valve unit further includes a flexure pivot allowing the clevis member to pivot with respect to the main axis to control flow through the nozzle orifices. The flexure pivot has a movable part and a non-moving part in a plane spaced from the movable part and joined thereto by a flexible spoke.  
           [0014]    In still other preferred forms, the feedback spring has a ball end that is pivotally engaged with the socket or groove in the spool to alleviate binding. The first stage valve unit can include a separate valve body defining the nozzle orifices, which are preferably two pairs of slots through opposite flat sides of the nozzle body. The nozzle body can be partitioned and have a bore through the partition through which the feedback spring extends. In this case, one nozzle orifice is on each side of the partition. Further, the forked end of the clevis member has two prongs, one disposed on opposite sides of the nozzle body. Preferably, each prong has tapered lateral leading edges.  
           [0015]    The present invention thus provides an improved electrohydraulic servo valve that is both highly responsive and exhibits low null leakage. These and other benefits are derived in large part to the use of the clevis valve member in the first stage. The clevis arrangement is inherently pressure balanced such that it is highly insensitive to the affects of pressure loading and transient flow forces as well as to pump pressure ripple or noise common in hydraulic or fuel systems, which works to maximize the net drive force during operation. The valve is also highly efficient, empirically exhibiting very high first stage pressure recovery (approximately 97%) and very low hysteresis. The valve is also highly reliable and suitable for use in highly particle contaminated environments, such as jet fuel applications because of a high first stage pressure gain working to clear the spool in the event of sticking. In addition, the valve arrangement is closed centered in that the clevis prongs close the nozzle orifices at the null position such that the valve provides very low null leakage with variable transient flow.  
           [0016]    These and still other advantages of the invention will be apparent from the detailed description and drawings. What follows is a preferred embodiment of the present invention. To assess the full scope of the invention the claims should be looked to as the preferred embodiment is not intended as the only embodiment within the scope of the invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    [0017]FIG. 1 is bottom plan view of the electrohydraulic valve according to the present invention;  
         [0018]    [0018]FIG. 2 is a cross-sectional view thereof taken along line  2 - 2  of FIG. 1 showing the valve in its null position;  
         [0019]    [0019]FIG. 3 is an enlarged partial sectional view as in FIG. 2 showing a servo first stage of the valve;  
         [0020]    [0020]FIG. 4 is an enlarged partial sectional view as in FIG. 2 showing a valve second stage with a spool member in an extreme left position in which a first cylinder port is open to pressure;  
         [0021]    [0021]FIG. 5 is a partial sectional view taken along line  5 - 5  of FIG. 4 showing a clevis member opening a set of nozzle orifices allowing communication between a first stage pressure and control port;  
         [0022]    [0022]FIG. 6 is a view similar to FIG. 4 albeit with the spool valve in an extreme right position in which a second cylinder port is open to pressure;  
         [0023]    [0023]FIG. 7 is a sectional view similar to FIG. 5 albeit taken along line  77  of FIG. 6 showing the clevis member opening another set of nozzle orifices allowing communication between the first stage control port and a first stage return port;  
         [0024]    [0024]FIG. 8 is an exploded perspective view of a clevis pivot and feedback assembly of the valve; and  
         [0025]    [0025]FIG. 9 is a partial sectional view taken along line  9 - 9  of FIG. 3 showing a permanent magnet arrangement of the first stage. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0026]    The present invention provides a two stage electrohydraulic servo valve  10 , as shown in FIGS. 1 and 2. The valve  10  includes a first stage servo unit  12  and a second stage valve unit  14 . With reference to FIGS. 2 and 3, the first stage servo unit  12  has a housing  16  which bolts onto a housing  18  of the second stage valve unit  14 . The first stage housing  16  has pressure  20 , control  22  and return  24  ports. The pressure  20  and return  24  ports in communication with associated internal routing passageways  26  and  28  in the second stage housing  18 , which acts as a manifold block to which connect fuel lines (not shown) leading to and from a fuel supply (not shown). O-rings  32  seal the pressure  20  and return  24  ports.  
         [0027]    Referring to FIG. 3, the first stage housing  16  is enclosed by a cylindrical cover  34  and defines a nozzle chamber  36  concentric with a nozzle axis  38  and a main chamber  40  concentric with a main axis  42  such that the two chambers intersect each other at a right angle. A nozzle body  44  is slid into the nozzle chamber  36  from one end until stopped by abutment of a flared end  45  with the first stage housing  16 . The open ends of the nozzle chamber  36  are sealed by plugs  46  (with o-rings  48 ) bolted to the first stage housing  16 . One plug  46  also retains the nozzle body  44  in place. The nozzle body  44  is sealed circumferentially by three spaced apart o-rings  50 , one located on one side and two on the other side of an intermediate section  52 . This section  52  has a square outer cross-section and defines a partition wall  54  separating the nozzle body  44  into two passageways  56  and  58 . A bore  60  concentric with the main axis  42  passes through the wall  54 . Four spaced apart slots parallel to the main axis  42  are formed in two opposite sides of the intermediate section  52  of the nozzle body  44  forming two pair of nozzle orifices  62  and  64  on respective left and right sides of the partition wall  54  (see FIGS. 3, 5 and  7 ). The nozzle orifices  62  open to passageway  56  and nozzle orifices  64  open to passageway  58 . Thus, hydraulic fluid, jet fuel in one preferred case, can flow from the pressure port  20  into the nozzle chamber  36  and passageway  56  of the nozzle body  44 , and when the nozzle orifices  62  are open (as shown in FIG. 5), out through the control port  22  to the second stage. Alternatively, when nozzle orifices  64  are open (as shown in FIG. 7), fluid can flow from the control port  22  into the nozzle body passageway  58  and out through the return port  24  through four openings  66 .  
         [0028]    The nozzle orifices  62  and  64  are controlled by a clevis member  68  (shown best in FIGS. 3 and 11) disposed along the main axis  42 . The clevis member  68  has a cylindrical stepped diameter stem  70  disposed in the main chamber  40  and an opposite forked end  72  within the nozzle chamber  36  straddling the square intermediate section  52  of the nozzle body  44 . The forked end  72  has two spaced apart prongs  74  with tapered leading edges  76  (along their lateral sides) to lower shear forces during operation. The prongs  74  are sized so that the leading edges  76  cover all four of the nozzle orifices  62  and  64  when in a null position in which the clevis member  68  is symmetric about the main axis  42 . The symmetric configuration of the clevis member  68  makes it inherently balanced since the same pressure forces will act on each one of the prongs  74 .  
         [0029]    The clevis member  68  is supported at its stem  70 , which fits through a central opening  78  (sized smaller than shoulder  80 ) of a fixed part  82  of a flexure pivot  84  bolted to the first stage housing  16  concentric with the main axis  42 . The flexure pivot  84  has a movable part  86  connected to the fixed part  82  by two spokes  88 . The spokes  88  are strong but slightly deflectable to allow relative movement of part  86  with respect to part  82 . The movable part  86  has a central opening  90  fit over the smaller diameter section of the clevis stem  70 . The movable part  86  and the stem  70  are brazed together with an armature  92  of a magnetic drive assembly  94 . The armature  92  is supported by the flexure pivot  84  in a magnetically inert guide sleeve  96  having a flanged end  98  which seals off the main chamber  40 , via o-ring  100 , by seating against the first stage housing  16 . The flanged end  98  of the guide sleeve  96  is held in place by an end plate  102  mounted to the first stage housing  16  by bolts  104  also mounting the drive assembly  94 .  
         [0030]    In addition to the armature  92 , the drive assembly  94  includes a wire coil  106  disposed about the guide sleeve  96  between the end plate  102  and a permanent magnet assembly  108 . As shown in FIG. 9, the permanent magnet assembly  108  includes two arch shaped permanent magnets  110  as well as two identical ferromagnetic pole pieces  112  arranged in a circle about the main axis  42 . The pole pieces  112  extend in a direction parallel to the main axis  42  to fit around an outer diameter of the coil  106 . Non-magnetic spacers  114  take up the gap between the ends of the pole pieces  112  and the end plate  102 .  
         [0031]    The drive assembly  94  thus provides a permanent magnet motor for driving the clevis member  68 . Specifically, the pole pieces  112  become magnetized by the permanent magnets  110  and establish north and south poles providing a unidirectional magnetic flux force acting on the armature  92  in the direction from the north pole to the south pole. When current is applied to the coil  106  it acts as an electromagnet providing magnetic flux lines acting on the armature  92  that vary depending on the input current to the coil  106 , tending to add or subtract from the force of the permanent magnet flux. The guide sleeve  96  and spacers  114  do not effect the flux path because they are made of magnetically inert materials. The negative spring rate acting on the armature  92  from the magnetic flux lines is coupled with the positive spring rate of the flexure pivot  84  such that the combined force effect on the armature  92  is proportional to the input current to the coil  106 . Thus, the net effect on the armature  92  is a force proportional to input current tending to move the armature  92  toward one of opposite sides of the main axis  42  where either of the pole pieces  112  reside.  
         [0032]    Because the clevis member  68  (and thereby the armature  92 ) are supported by the flexure pivot  84 , driving the armature  92  side to side will cause the armature  92 /clevis member  68 /movable part  86  assembly to pivot generally about the center of the fixed part  82  of the flexure pivot  84 . Movement of the armature  92  toward the one side of the main axis  42  (e.g., to the right in FIG. 3) moves the forked end  72  of the clevis member  68  in the opposite direction (left in FIG. 3). Note that in the preferred embodiment described herein, the amplitude of travel of the forked end  72  of the clevis member  68  is approximately 0.01 to 0.001 inches in either direction from its resting or null position in which both sets of nozzle orifices  62  and  64  are closed (as shown in FIGS. 2 and 3).  
         [0033]    The clevis arrangement thus provides variable fluid flow during transient operation thereby increasing its responsiveness. This arrangement also makes the valve better suited for use in particle contaminated environments since there is no fixed area open orifices as is conventional (which are necessarily very small in diameter due to the low amount of torque provided by the drive assembly) that are susceptible to clogging. Moreover, this arrangement provides high pressure gain at the first stage which assists the valve in the second stage to clear in the event of binding or sticking, thus making it selfclearing.  
         [0034]    Referring to FIGS. 3, 4 and  6 , the clevis member  68  includes an enlarged body section  116  with a threaded bore  118  concentric with the main axis  42 . A feedback spring  120  has a threaded end  121  that threads into the bore  118  to secure it to the clevis member  68 . The feedback spring  120  provides a mechanical link between the first and second stages and provides an auto-nulling function for the first stage, as described below.  
         [0035]    Referring now to FIGS. 4 and 6, the second stage valve unit  14  will be described in detail in a four-way valve construction. Note that it is within the scope of the invention to incorporate a two or three port second stage, especially in the event the valve is to be used for metering applications. The second stage housing  18  defines a valve chamber  122  and the passageways  26  and  28  mentioned above. The second stage housing  18  also defines a pressure port  124 , a return port  126  and two flow ports, preferably input/output actuator cylinder ports  128  and  130 , as shown in FIG. 1. The pressure  124  and return  126  ports couple the valve to a supply (and possibly a separate return) tank (not shown). In one application, the cylinder ports  128  and  130  can be coupled to separate cylinders of a piston actuating unit, such as for operating a compressor bleed valve in an aircraft turbofan engine. The second stage valve housing  18  has internal porting that leads from each of the pressure  124  and return  126  ports to open at two locations in the valve chamber  122 . The second stage housing  18  also defines a central control inlet  134  in communication with the control port  22  of the first stage.  
         [0036]    A guide sleeve  142  (inserted from an open end of the valve chamber  122  until a flange  144  abuts a ledge  146 ) is fixed at the interior of the valve chamber  122 . The outer diameter of the guide sleeve  142  has a plurality of circumferential grooves holding o-rings  150  that seal against inwardly projecting circular lands  151  of the valve chamber  122  dividing it into seven separate annular channels  152 - 164  in communication with ports  124 - 130  and  134 . Specifically, outer channels  152  and  164  communicate with the return port  126 , intermediate channels  154  and  162  communicate separately with respective cylinder ports  128  and  130 , inner channels  156  and  160  communicate with the pressure port  124  and the central channel  158  communicates with the control inlet port  134 .  
         [0037]    Within the center of the guide sleeve  142  is a spool member  166  having a cup end  168  defining a cavity in which is disposed a half-area piston  170  (as known in the art) having a circular cross-section of an area essentially one half that of the cup end  168 . The piston  170  and the spool member  166  are disposed along a stroke axis  172  with the spool member  166  being slidable along the stroke axis  172  and the piston be fixed. Note that the slide surfaces of the spool member  166  and the piston  170  have a series of so called cleaning grooves to equalized pressure therebetween and reduce side loading on the spool member  166 . An end cap  174  with an o-ring  176  seals the valve chamber  122  and presses against the flange  144  of the guide sleeve  142  to fix its position.  
         [0038]    The spool member  166  defines an outer circumferential groove  178  at its center which receives a ball end  180  of the feedback spring  120  in a pivotal or swivel connection and defines five annular channels  182 - 190  spaced from the groove  178 . The spool member  166  also defines two unconnected passageways  192  and  194  concentric with the stroke axis  172  and opening to opposite ends of the spool member  166 . Passageway  192  communicates with annular channel  184  via bore  196  and passageway  194  communicates with annular channels  186  and  188  via respective bores  198  and  200 .  
         [0039]    As mentioned above, FIG. 2 illustrates the valve when both of the first and second stages are at the centered null position. In this position, both sets of nozzle orifices  62  and  64  are closed by the clevis member  68  and the spool member  166  is positioned so that the pressure port  124  is closed off from the cylinder ports  128  and  130 . FIGS. 4 and 5 show the valve in a transient state in which the spool member  166  is being moved away from the piston  170 . FIGS. 6 and 7 show the valve in another transient state in which the spool member  166  is being moved toward the piston  170 .  
         [0040]    With reference to FIGS.  3 - 5 , the flow path of the fluid will now be described when positioning the spool member  166  from null to allow flow to pass out cylinder port  130  to one cylinder of the piston actuator. In this case, a current signal is supplied to the electromagnet to pull the armature  92  to the left side of the main axis  42  (as shown in the figures). This causes the clevis member  68  to pivot about the flexure pivot  84  so that its forked end  72  moves to the right, which opens nozzle orifices  62 . This in turn allows flow from the open pressure port  20  to the control port  22  and into the second stage.  
         [0041]    Referring to FIGS. 4 and 5, flow passes from the control inlet port  124  and into channel  188  then through bore  200  into passageway  194 . The accumulation of pressurized fluid at space  202  bears against the right end of the spool member  166  to slide it toward the piston  170 . This shift opens an orifice  204  to permit flow from the pressure port  124  (see FIG. 1) to pass into channel  190  and out through orifice  206  into channel  162  and eventually out through cylinder port  130  to one cylinder of the piston actuator. Flow from the opposite actuator cylinder flows through cylinder port  128  into channel  154  and then to the return port  126  (see FIG. 1) via orifice  208 , channel  182 , orifice  210  and channel  152 . At the same time, the fluid displaced by movement of the spool member  166  passes through two orifices  212  in the flange  144  and into channel  152  to the return port. Equalizing flow from the displaced fluid in the cup end  168  of the spool member  166  passes through passageway  192 , bore  196 , channel  186 , orifice  214  and channel  156  as needed to maintain the fixed position of the piston  170 .  
         [0042]    As can be seen, moving the spool member  166  in this way moves the ball end  180  of the feedback spring  120  to the left which bends the feedback spring  120  and imparts a spring force biasing the forked end  72  of the clevis member  68  to the left, back to its null position, thereby closing nozzle orifices  62  and stopping movement of the spool member  166 . Since the pivoting of the clevis member  68  is proportional to the input current to the electromagnet, the stroke of the spool member  166  is also proportional to the input current, thus allowing the valve to be controlled very accurately.  
         [0043]    Referring now to FIGS. 6 and 7, the flow path of the fluid will now be described when positioning the spool member  166  from null to allow flow to pass out cylinder port  128  to the other cylinder of the piston actuator. Here, an opposite polarity current signal is supplied to the electromagnet to pull the armature  92  to the right side of the main axis  42 , which causes the clevis member  68  to pivot about the flexure pivot  84  so that its forked end  72  moves to the left to open nozzle orifices  64 . This in turn allows flow from the control port  22  to the now open return port  24 .  
         [0044]    Flow passes from the control inlet port  124  after passing from channel  156 , channel  186 , bore  198 , and passageway  194  by virtue of the spool member  166  being driven away from the piston  170  under pressure from flow passing from the pressure port  124  (see FIG. 1), through orifice  214 , channel  184 , bore  196  and passageway  192 . This movement opens orifice  216  to allow flow from the pressure port to pass through channel  182 , orifice  208 , channel  154  and exit through the cylinder port  128 . Fluid from the opposite actuator cylinder passes through port  130  into channel  162  through orifice  206  into channel  190  through orifice  218  to channel  164  and to the return port. Make up fluid is provided to the evacuated space between the end cap  174  and the cup end  168  of the spool member  166  through orifices  212  in the flange  144 . Moving the spool member  166  in this way moves the ball end  180  of the feedback spring  120  to the right which imparts a spring force biasing the forked end  72  of the clevis member  68  to the right, back to its null position, thereby closing nozzle orifices  64  and stopping movement of the spool member  166 .  
         [0045]    The present invention thus provides an improved 4-way electrohydraulic servo valve that is both highly responsive and exhibits low null leakage. These and other benefits are derived in large part to the use of the clevis valve member in the first stage. The clevis arrangement is inherently pressure balanced such that it is highly insensitive to the affects of pressure loading and transient flow forces as well as to pump pressure noise common in hydraulic systems, which works to maximize the net drive force during operation. The valve is also highly efficient, empirically exhibiting very high first stage pressure recovery (approximately 97%) and very low hysteresis. The valve is also highly reliable and suitable for use in highly particle contaminated environments, such as jet fuel applications because of a high first stage pressure gain working to clear the spool in the event of sticking. In addition, the valve arrangement is closed centered in that the clevis prongs close the nozzle orifices at the null position such that the valve provides very low null leakage with variable transient flow.  
         [0046]    It should be appreciated that merely a preferred embodiment of the invention has been described above. However, many modifications and variations to the preferred embodiment will be apparent to those skilled in the art, which will be within the spirit and scope of the invention. For example, the drawings and the above description describe a 4-way electrohydraulic servo valve, however, it is within the scope of the invention for the valve to have a three-way second stage in which case the second stage valve housing has only one cylinder port (along with the pressure and return ports) or a two-way second stage in which case it has only a pressure port and one unidirectional output flow port. As such, the valve is capable of operating as a metering valve in which fluid passes into the second stage from pressure and exits through the metering flow port. Therefore, the invention should not be limited to the described embodiment. To ascertain the full scope of the invention, the following claims should be referenced.