Abstract:
A servovalve pilot stage assembly is provided having a first fluid conduit ( 152 ) having a first orifice ( 156 ), a second fluid conduit ( 154 ) having a second orifice ( 158 ), a flapper ( 44 ) having a deformable first region ( 176 ) disposed between the first orifice and the second orifice, an actuator ( 24 ) arranged to drive the flapper ( 44 ) from a first condition in which the first region of the flapper has a first width between the first and second orifice to a second condition in which the first region of the flapper has a second width between the first and second orifice, the second width being less than the first width.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application is the U.S. national phase of International Application No. PCT/GB2014/052038 filed Jul. 4, 2014 which claims priority of British Application No. 1313612.2 filed Jul. 30, 2013, the entirety of which is incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention is concerned with hydraulic servovalves. More particularly, the present invention is concerned with single stage and multiple stage nozzle-flapper type hydraulic servovalves for use in a variety of industries, including but not limited to aerospace, motorsport and industrial process control. 
       BACKGROUND OF THE INVENTION 
       [0003]    Servovalves are used to magnify a relatively low power input signal (usually an electrical control signal in the order of a fraction of a Watt) to a high power hydraulic output (in the order of many thousands of Watts). Several types of hydraulic servovalves are known in the art—for example deflector jet, jet pipe and nozzle flapper. Each operates by using a pilot stage to create a differential pressure at either end of a spool (the “main stage”). The spool controls the flow of the high pressure working fluid. Servovalves typically comprise some kind of mechanical or electronic feedback system from the main stage to the pilot stage. 
         [0004]    The present invention concerns nozzle-flapper type hydraulic servovalves. Nozzle-flapper type hydraulic servovalves are well known in the art. A prior art nozzle-flapper servovalve is shown in  FIGS. 1 and 2  of the appended drawings. 
         [0005]    Referring to  FIG. 1 , a nozzle-flapper type electro-hydraulic servovalve (EHSV)  10  is shown schematically and in cross-section. The servovalve  10  comprises a pilot stage subassembly  12  and a main stage subassembly  14  as will be described in more detail below. 
         [0006]    The pilot stage subassembly  12  defines a main central axis A and comprises a housing cover  16  and a cylindrical base  18  which co-operate to define an enclosed volume  20 . The base  18  comprises an annular flange  88  which seals against the housing cover  16 . The base  18  further defines a central coaxial bore  22  extending along the main central axis A, and two diametrically opposed bores  52 ,  54  extending radially from the main central axis A. Each of the bores  52 ,  54  is in fluid communication with the bore  22 . Within each bore  52 ,  54  there is provided a respective conduit  53 ,  55  (only shown in  FIG. 2 ) defining a respective fluid nozzle  56 ,  58 . The conduits  53 ,  55  are adjustable along a common nozzle axis Z within the bores  52 ,  54 . 
         [0007]    Contained within the volume  20  there is provided an electro-magnetic actuator  24  comprising a first set of windings  26  and a second set of windings  28 . An armature  30  is provided comprising a tubular, cylindrical body  38  with a first leg  34  and a second leg  36  extending radially outwardly therefrom. The first leg  34  is disposed within the first set of windings  26  and the second leg  36  is disposed within the second set of windings  28 . 
         [0008]    The legs  34 ,  36  are ferromagnetic and as such the armature is arranged for rotation about an armature axis R, intersecting and perpendicular to the main central axis A when the respective windings  26 ,  28  are energised by a control signal. 
         [0009]    A flapper  44  is provided and is generally tubular and cylindrical in structure. The flapper  44  has a bore  84  concentric therewith. Turning to  FIG. 2 , the flapper has a free end  75  and a fixed end  46 . The flapper  44  comprises a body defining, starting from the free end  75 , a first region  76 , a second region  78 , a third region  80  (with a higher wall thickness than the first and second regions) and a fourth region  82  which terminates in a shoulder  74 . The first region  76  and the second region  78  are identical in inner and outlet diameter with the exception that the first region  76  has diametrically opposed flats  77 ,  79 . The shoulder  74  is connected to a collar  72  at the fixed end  46  having a diameter dimensioned for an interference fit with the body  38  of the armature  30 . 
         [0010]    A flexure sleeve  40  is provided, which is generally tubular and cyclindrical in shape having an internal bore  41 . The flexure sleeve has a first end  90  and a second end  92  where it is provided with a surface mounting formation. 
         [0011]    A feedback wire  50  is provided which is solid, cylindrical and extends from a first end  51  to a second end  53 . The first end  51  comprises a solid collar. 
         [0012]    The pilot stage assembly  12  is assembled as follows. 
         [0013]    The collar  72  of the flapper  44  is fitted into the body  38  of the armature  30  such that the fixed end  46  is secured to the armature and as such the flapper  44  is cantilevered thereto. The flapper  44  extends from the fixed end  46 , past the axis R to the free end  75 . With the exception of the collar  72 , an annular gap is provided between the flapper  44  and the body  38  of the armature  30 . 
         [0014]    The flexure sleeve  40  is fitted around the part of the third region  80  and the fourth region  82  of the flapper  44 , and is dimensioned such that the second end  92  terminates partway down the flapper where it is mounted to the base  18  such that its internal bore  41  is in communication with the bore  22  of the base  18 . As such, the flapper sits in the annular gap between the flapper  44  and the body  38  of the armature  30 . The flexure sleeve  40  is closely fitted to the flapper  44  providing an annular gap between the flexure sleeve  40  and the body  38  of the armature  30 . 
         [0015]    The first end  51  of the feedback wire  50  is fitted into the fixed end  46  of the flapper  44 . The feedback wire is therefore fixed within the armature at the same position as the flapper  44 . The feedback wire  50  extends beyond the free end  75  of the flapper  44  to protrude from the base  18 . 
         [0016]    The flapper  44  extends into the bore  22  in the base  18  such that the first region  76  is disposed between the nozzles  56 ,  58 , creating a “hydraulic bridge”—i.e. an arrangement of the nozzles  56 ,  58 , the gaps between the flapper  44  and the nozzles  56 ,  58  and the inlet orifices. The nozzles  56 ,  58  are thereby directed onto the flats  77 ,  79  of the flapper  44 . A clearance gap is provided between each of the nozzles  56 ,  58  and the flapper  44 . 
         [0017]    Turning to the main stage  14 , there is provided a valve  60  comprising a spool  62 . The spool has end pressure faces  64 ,  66 . The spool is arranged to move along a spool axis B to control a flow through the valve  60  in a known manner. In various applications, the movement of the spool  62  directs fluid flow so as to control external apparatus such as actuators, pumps, etc. 
         [0018]    Movement of the spool  62  along the axis B is achieved by the application of differential pressure to the pressure faces  64 ,  66  respectively. Each of the pressure faces  64 ,  66  is open to a respective pressure chamber  68 ,  70  respectively. Each chamber is in fluid communication via supply lines  6 ,  8  to a high pressure source (not shown). Each chamber  68 ,  70  is also in fluid communication with a respective one of the first and second channels  52 ,  54  of the base  18  of the pilot stage (and therefore is in fluid communication with the conduits  53 ,  55 ). Each chamber  68 ,  70  is also in communication with an external pressure source (not shown). 
         [0019]    In operation, the known electro-hydraulic servovalve operates as follows. 
         [0020]    In the null position as shown in  FIG. 1 , without the coils  26 ,  28  being energised, the flapper  44  sits equidistantly between the nozzle outlets  56 ,  58 . As such, the pressure on either end of the spool  62  is equal. 
         [0021]    Should it be desired to move the spool to the left to control the flow through the valve  60 , then the first and second windings  26 ,  28  are energised in order to rotate the armature  30  in an anti-clockwise direction about the armature axis R. This has the effect of rotating the flapper  44  such that the first region  76  moves towards the nozzle  58  and away from the nozzle  56 . During this movement the flexure sleeve  40  elastically deforms by virtue of its attachment to the base  18 . 
         [0022]    The reduction in the flow gap between the nozzle  58  and the flapper  44  results in a rise in pressure upstream of the conduit  55 . This creates a higher pressure in the chamber  70  and consequently at the second pressure face  66  on the spool  62 . The opening of the gap between the nozzle  56  and the flapper  44  causes a reduction in pressure upstream of the conduit  53  and therefore lowers the pressure in the chamber  68  and reduces the pressure on the face  64 . As a result, the spool travels to the left. 
         [0023]    As can be seen in  FIG. 1 , the feedback wire  50  is connected to the centre of the spool  62 . As the spool  62  moves towards its desired position the feedback wire  50  is deformed and a torque, opposing the electrically generated torque, is generated on the armature  30 . When the desired spool position is reached, the mechanical and electrical torques balance and the flapper has returned to the null position between the nozzles (albeit with a bend in the feedback wire  50 ). In this condition the differential pressure across the spool  62  is now zero and the spool stops moving. In other words, this is negative position feedback control of the spool  62 . 
         [0024]    When the coils are deenergised the electrical torque on the armature  30  is removed but because the spool is still displaced from the mid position the mechanical torque from the feedback wire  50  remains. The net effect is to rotate the armature  30  in a clockwise direction which moves the flapper  44  towards the nozzle  56  and away from the nozzle  58 . This generates a differential pressure across the spool  62  that positively drives the spool back towards the null position. When the spool reaches the mid position the feedback wire is no longer bent, the net torque is zero and the differential pressure is zero so the spool stops in the mid position. 
         [0025]    As mentioned, the electro-hydraulic servovave  10  is connected to a constant pressure source into the chambers  68 ,  70  (via lines  6 ,  8 ). In the null position, because of the gaps between the nozzles  56 ,  48  there is a quiescent leakage into the bore  22 , which then flows to a drain. This quiescent leakage flow is undesirable—it is wasted energy which makes operation of the valve inefficient and expensive. 
       SUMMARY OF THE INVENTION 
       [0026]    It is an aim of the present invention to reduce quiescent flow in nozzle-flapper type hydraulic servovalves. 
         [0027]    According to a first aspect of the invention there is provided a servovalve pilot stage assembly comprising: 
         [0028]    a first fluid conduit having a first orifice; 
         [0029]    a second fluid conduit having a second orifice; 
         [0030]    a flapper having a deformable first region disposed between the first orifice and the second orifice; 
         [0031]    an actuator arranged to drive the flapper from a first condition in which the first region of the flapper has a first width between the first and second orifice to a second condition in which the first region of the flapper has a second width between the first and second orifice, the second width being less than the first width so as to separate, or further separate, the flapper and the first orifice. 
         [0032]    By “deformable”, we mean the first region can be elastically compressed to reduce its width. The first region is elastically, or resiliently, compressible. 
         [0033]    Advantageously, by providing a flapper which is deformable, the flow orifices can be placed much closer to the flapper in the null position reducing quiescent flow. During actuation, the required gap between the flapper and the orifices is created by elastic deformation of the flapper. In the present invention, the orifices can even be placed in contact with the flapper in the null position to reduce flow significantly, or almost eliminate it all together (dependent upon the sealing effect between the flapper and the outlet). In some circumstances, the flapper can be pre-compressed by having the gap between the nozzles less than the uncompressed width of the flapper in the first region. 
         [0034]    Preferably the first region is hollow having a wall and a central cavity. This facilitates deformation and allows passage of a feedback wire therethrough. 
         [0035]    Preferably, the first region of the flapper is locally, structurally weakened to elastically deform. The flapper defines: a main longitudinal axis; a width extending between the orifices; and, a depth extending normal to the main longitudinal axis and the width; in which an opening/openings is/are formed through the depth of the first region of the flapper. Advantageously, such openings allow elastic deformation to take place by locally reducing the stiffness of the flapper. 
         [0036]    Preferably the flapper comprises a free end proximate the first region, and the opening is a/are blind slot/slots generally extending in direction of the main longitudinal axis from the free end, through the first region to form a first leg and a second leg of the flapper in the first region. Such slots are relatively simple to manufacture. 
         [0037]    Preferably the blind slots are diametrically opposed. 
         [0038]    Preferably the slot or slots terminate in a curved end region which may be partially circular, and preferably has a diameter greater than the width of the slot proximate the circular curved end region. This acts to eliminate the stress raiser at the end of the slot. 
         [0039]    The slot or slots may be of constant width along substantially their entire length, alternatively they may taper to alter the characteristics of the flapper. 
         [0040]    Preferably at least one of the first and second conduits defining the first and second respective orifices are in contact with the first region of the flapper in the first condition. Preferably both the first and second conduits defining the first and second respective orifices are in contact with the first region of the flapper in the first condition. The first region of the flapper may have an undeformed width greater than the distance between the first and second orifices such that in the first condition the first region of the flapper is pre-compressed. This reduces quiescent flow to an absolute minimum. 
         [0041]    Preferably the first region of the flapper defines flats facing the first and second orifices. This improves sealing contact with the flat orifices. 
         [0042]    Preferably the first and second orifices are defined in nozzles directed towards the flapper. 
         [0043]    According to a second aspect of the invention there is provided a servovalve comprising: 
         [0044]    a servovalve pilot stage assembly according to the first aspect; and, 
         [0045]    a main stage controlled by the pilot stage 
         [0046]    Preferably the servovalve comprises a spool valve having a spool defining a first end face in fluid communication with the first conduit. 
         [0047]    The spool preferably defines a second, opposite, end face in fluid communication with the second conduit. 
         [0048]    Preferably the first conduit is in fluid communication with: 
         [0049]    a pressure source such that the first orifice is an outlet; and, 
         [0050]    a first part of the main stage, 
         [0051]    in which the fluid pressure at the first part of the main stage is controlled by the distance between the flapper and the first orifice. 
         [0052]    The first part is preferably in fluid communication with one end of a spool valve to move it in a first axial direction. 
         [0053]    Similarly, the second conduit is preferably in fluid communication with: 
         [0054]    a pressure source such that the second orifice is an outlet; and, 
         [0055]    a second part of the main stage, 
         [0056]    in which the fluid pressure at the second part of the main stage is controlled by the distance between the flapper and the second orifice. 
         [0057]    The second part can be placed in fluid communication with the opposite end of the spool valve to move it in the opposite direction. 
         [0058]    Preferably there is provided a drain port between the first and second orifices. 
         [0059]    As an alternative to a traditional nozzle/nozzle valve, the servovalve may be a nozzle/elzzon valve in which: 
         [0060]    the first conduit is in fluid communication with a pressure source such that the first orifice is an outlet; 
         [0061]    the second conduit is a connected to a fluid drain such that the second orifice is an outlet; 
         [0062]    a third fluid conduit is provided between the first and second fluid orifices in fluid communication with a first part of the main stage; 
         [0063]    in which the fluid pressure at the first part of the main stage is controlled by the position of the flapper between the first and second orifices. 
         [0064]    Advantageously this type of valve is single inlet and as such mitigates and potential “hard over” failure mode. The main stage will likely require a return mechanism. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING VIEWS 
         [0065]    An example electro-hydraulic servovalve pilot stage in accordance with the present invention will now be described with reference to the accompanying figures in which: 
           [0066]      FIG. 1  is a schematic section view of a known electro-hydraulic servovalve; 
           [0067]      FIG. 2  is a detail view of a part of the valve of  FIG. 1 ; 
           [0068]      FIG. 3  is a detail view of a part of first electro-hydraulic servovalve in accordance with the present invention, similar to the view of  FIG. 2 ; 
           [0069]      FIG. 4 a    is a detail view of a part of the servovalve  FIG. 3 ; 
           [0070]      FIG. 4 b    is a section view along line BB of  FIG. 4   a;    
           [0071]      FIG. 5  is a view of the valve of  FIG. 4 a    in a deformed state; 
           [0072]      FIG. 6  is detail view of a part of a second electro-hydraulic servovalve in accordance with the present invention; and 
           [0073]      FIG. 7  is a detail view of a part of a third electro-hydraulic servovalve in accordance with the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0074]    With reference to  FIG. 3 , the components shown therein are suitable for use in the servovalve of  FIG. 1 , and as such the description of  FIG. 1  is equally applicable the embodiments of the present invention discussed below. 
         [0075]    The view shown in  FIG. 3  is similar to that of  FIG. 2 , and an electro-hydraulic servovalve  100  according to the invention as shown therein comprises a flapper  144  which is similar to the flapper  44  as shown in  FIG. 2 . The flapper  144  is generally tubular and cylindrical in structure. The flapper  144  has a bore  184  concentric therewith. The flapper has a body defining a first region  176 , a second region  178 , a third region  180  with a higher wall thickness than the first and second regions, and a fourth region  182  which terminates in a shoulder  174 . The shoulder  174  defines a collar  172  having a diameter dimensioned for an interference fit with the body  38  of the armature  30  as shown in  FIG. 1 . As such, the flapper  144  is cantilevered from the armature  30  having a fixed end  146  and a free end  175 . 
         [0076]    A more detailed view of the flapper  144  can be seen in  FIG. 4 a   .  FIG. 4 b    shows a cross section through the first region  176 . 
         [0077]    As with the flapper  44 , a pair of diametrically opposed flats  177 ,  179  are provided in the first region  176  (see  FIG. 4 b   ). The distance between the flats  177 ,  179  defines a flapper undeformed width N. 
         [0078]    Part of a base  118  is also shown in  FIG. 3  comprising a central coaxial bore  122  extending along a main central axis A, and two diametrically opposed bores  152 ,  154  extending radially from the main central axis A. Each of the bores  152 ,  154  is in fluid communication with the bore  122 . Within each bore  152 ,  154  there is provided a respective nozzle insert  153 ,  155  defining a respective fluid nozzle  156 ,  158 . The nozzle inserts  153 ,  155  are movable along a common nozzle axis Z within the bores  152 ,  154 . 
         [0079]    The main difference between the flapper  144  and the flapper  44  is the provision of a pair of identical diametrically opposed slots  200 ,  210 . The slot  200  has width W and extends parallel to the main central axis A from the free end  175  of the flapper  144 , through the first region  176 , through the second region  178  and into the third region  180 , where the slot  200  terminates in a circular region  202  having diameter D. The width of the slot  200  is constant from the free end  148  to the circular region  202  and has a width W less than D. The slots  200 ,  210  are identical in shape. The slots  200 ,  210  result in the provision of a first leg  201  and a second leg  203  at the free end  175  of the flapper  144 . The first leg  201  comprises the flat  177  and the second leg  203  comprises the flat  179 . 
         [0080]    As can be seen in  FIG. 4 a   , the nozzles  156 ,  158  are in direct contact with the flats  177 ,  179  of the first region  176  of the flapper  144 . This can also be seen in  FIG. 4   b.    
         [0081]    In operation, the electro-hydraulic servovalve  100  is operated in much the same way as the valve  10 . Taking the same example as described above with respect to the prior art, an anti-clockwise rotation of the armature  30  will result in an anti-clockwise rotation of the flapper  144  about the armature axis R as shown in  FIG. 3 . Because the flapper  144  is in contact with the nozzles  156 ,  158 , the first region  176  of the flapper  144  cannot move any further to the right in  FIG. 5 . As such it deforms, compressing the flapper  144  and closing the slot  200 . The width of the flapper  144  between the nozzles (and between the flats  177 ,  179 ) reduces from the undeformed width N to a deformed width D, where D&lt;N. 
         [0082]    The second leg  203  of the flapper  144  deforms by virtue of the reaction between the flat  179  and the nozzle  158 . The first leg  201  of the flapper  144  remains straight, but moves away from the nozzle  156  thus opening the gap between the nozzle  156  and the flat  177  and reducing the pressure in the chamber  68  in  FIG. 1 . 
         [0083]    As such, although contact between the nozzle  158  and the flat  179  is maintained (and as such so is the pressure in the chamber  70 ) the gap opened between the flat  177  and the nozzle  156  lowers the pressure in the chamber  68 , and as a consequence, moves the spool to the left. 
         [0084]    When returning to the null position, the flapper resiles to its undeformed width N. Deformation of the flapper  144  is kept elastic to avoid permanent deformation. 
         [0085]    It will be noted that in the present invention, in the null position there is very little quiescent flow because the flats  177 ,  179  of the flapper  144  are in contact with the nozzles  156 ,  158 . 
         [0086]    In a further embodiment, in order to further reduce the quiescent flow, the flapper  144  may be slightly compressed by contact with the nozzles  156 ,  158 . In other words, a pre-stress may be applied to the flapper compressing the flats to a pre-stress width P, where N&gt;P&gt;D. This provides even better sealing to reduce quiescent flow. 
         [0087]    In a still further embodiment, a gap between the nozzles  156 ,  158  and the flapper  144  may still be present, although made smaller than the prior art. Under these circumstances, the quiescent flow is reduced (although not eliminated). The advantage of this technique is that a pressure rise would be seen in the chamber connected to the nozzle which the flapper moves towards. As such, a higher differential pressure can be applied to the spool. 
         [0088]    Turning to  FIGS. 6 and 7 , alternative embodiments are shown in which the slots  200  both converge towards the free end of the flapper such that the slot width narrows from w 1  to w 2  ( FIG. 6 ), and in which the slots  200  both diverge towards the free end of the flapper such that the slot width broadens from w 1  to w 2  ( FIG. 7 ). This alters the deformation and spring characteristics of the flapper allowing for its behaviour over the course of it deformation to be tailored to the desired application. 
         [0089]      FIG. 8  is a representation of the hydraulic configuration of the present invention, showing the flapper  144  between the nozzles  156 ,  158 . The nozzles  156 ,  158  and the chambers  68 ,  70  are fed from a common pressure source  300  via pressure lines  304 ,  306  passing through restrictors  308 ,  310  respectively. An inter-nozzle gap  312  feeds to a drain  302 .  FIG. 8  is a traditional nozzle-flapper configuration with two pressure inlet lines  304 ,  306 . 
         [0090]    Turing to  FIG. 9 , an alternative configuration of a servovalve (a Nozzle/Elzzon configuration) is shown. It is sometimes advantageous to have a hydraulic bridge fed by a single pressure conduit. This is known as “single inlet” the traditional nozzle/flapper bridge described with reference to  FIG. 1  is “double inlet” because it has two inlet orifices. A disadvantage with double inlet valves is that in applications where contamination is possible, a piece of fluid borne contamination can block (or partially block) one of the inlet orifices and cause a significant pressure imbalance that can cause the valve to move to one end of its stroke (“hard-over” failure). Such a failure mode does not occur with a single inlet device. If the single inlet starts to block the general performance of the valve will deteriorate (usually the spool will not respond as quickly) but a large offset will not result, leading to more benign failure modes. 
         [0091]    Turning to  FIG. 9 , a single pressure source  400  feeds a pressure line  404  to the nozzle  156  and thence to an inter-nozzle/elzzon gap  412 . An “elzzon”  158  (i.e. the opposite to a nozzle—an inlet as opposed to an outlet) opposite the nozzle  156  provides a drain line  402  on the other side of the gap  412 . A control outlet  406  is configured to control movement of a spool valve via a control line. 
         [0092]    The pressure downstream of the control outlet  406  is determined by the condition of the hydraulic bridge. Therefore the more the flapper  144  moves towards the elzzon  158  the higher the pressure becomes in the outlet  406 . Evidently the use of a deformable flapper  144  is advantageous, as the amount of fluid passing from the nozzle  156  to the elzzon  158  can be minimised in the null position. As with the above embodiments, the nozzle  156  and elzzon  158  may be configured to be in contact with the flapper  144 . 
         [0093]    Unlike the above described embodiments, the embodiment of  FIG. 9  has a single control outlet  406 . Therefore the spool must be provided with a mechanism for applying an opposite force, such as a spring. 
         [0094]    Variations fall within the scope of the present invention. 
         [0095]    The servo valve does not need to be an electromagnetic-hydraulic servo valve, and may be actuated by other means, for example a piezoelectric element, a linear force motor or a limited angle torque motor. 
         [0096]    Instead of the mechanical feedback wire  50 , the main stage may be provided with a movement transducer to provide an electrical feedback signal to a controller which controls the movement of the armature  30  via the provision of power to the windings. As such, electrical feedback is envisaged as a viable alternative to mechanical feedback. 
         [0097]    Electrical position feedback may also be added to the pilot element driver, and this can be advantageous in certain applications.