Patent Publication Number: US-8967179-B2

Title: Servo-valve pilot stage and a two-stage servo-valve including such a stage

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This is a National Stage of International Application No. PCT/EP2011/063153 filed Jul. 29, 2011, claiming priority based on French Patent Application No. 10 56269 filed Jul. 29, 2010, the contents of all of which are incorporated herein by reference in their entirety. 
     The invention relates to a servo-valve pilot stage suitable for acting as a first stage in a two-stage servo-valve. The invention also provides a two-stage servo-valve including a pilot stage of the above-specified type. 
     TECHNOLOGICAL BACKGROUND OF THE INVENTION 
     Jet servo-valves are well known. It is known that they are better at withstanding pollution of the fluid because the distance between the ejector and the deflector is greater than the distance between a nozzle and the flapper. 
     The pilot stage of a jet servo-valve has an ejector for ejecting a jet of fluid towards a receiver, such as deflector or an orifice. The ejector and the receiver are movable relative to each other. The relative movement between the receiver and the jet leaving the ejector enables the receiver to create pressure differences that are used for obtaining fine control over the movement of the spool of the distribution stage of the servo-valve. 
     Nevertheless, a known drawback of servo-valves with a jet pilot stage is the need to channel the fluid to the ejector by passing over the moving assembly of the servo-valve. Global standard SAE ARP490E requires servo-valves to be fastened and fed with hydraulic fluid via their bottom faces. 
     OBJECT OF THE INVENTION 
     An object of the invention is to provide a pilot stage having a movable ejector that is simpler than known stages. 
     BRIEF SUMMARY OF THE INVENTION 
     In order to achieve this object, the invention provides a pilot stage for a jet type servo-valve, the pilot stage comprising an ejector for ejecting a jet of fluid and that is movable facing a deflector suitable for generating a pressure difference that can be used for moving a spool of the servo-valve, and wherein the ejector extends radially projecting from a column, the column has a first end that is embedded in the servo-valve and through which the fluid is introduced into the column, and the column has a second end that is subjected to drive from a torque motor for selectively twisting the column in one direction or the other about a rest position. According to the invention, the column is a single piece and the ejector is fastened at the end of a tube that extends radially from the column while being in fluid-flow communication with a central bore of the column through which the ejector is fed with fluid. 
     The pilot stage of the invention thus makes use of a member that is deformable in twisting in order to move the ejector by acting directly on the deformable member that carries the ejector by means of a torque motor that acts in constant manner on the column regardless of the angle through which the column has twisted, while maintaining a high degree of proportionality between the action of the motor and the movement of the ejector, thereby making it possible to achieve fine control over the angular position of the ejector. Furthermore, the embedded end may be implanted in a low portion of the servo-valve, thereby eliminating the need to cause an ejector feed duct to pass over the distribution assembly. 
     A central location for the column contributes to obtaining a balanced design for the servo-valve that can improve its ability to withstand vibration and that can also improve its dynamic response. Designing the twistable column as a single piece reduces the number of moving parts and the number of seals that need to be made between them. The invention also provides a servo-valve including such a pilot stage. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       The invention can be better understood in the light of the following description of a particular embodiment of the invention, given with reference to the following figures: 
         FIG. 1  is a diagram of the invention as applied to a two-stage servo-valve in a first particular embodiment of the invention, the torque motor being omitted; 
         FIG. 2  is a section view on line II-II of  FIG. 3  showing a servo-valve in a second particular embodiment of the invention; 
         FIG. 3  is a section view on line III-III of  FIG. 2 ; 
         FIG. 4  is a view analogous to the view of  FIG. 3 , the torque motor being shown; 
         FIG. 5  is a section view on line V-V of  FIG. 6 ; 
         FIG. 6  is a fragmentary side view of the servo-valve of  FIGS. 2 to 5 ; 
         FIG. 7  is a diagram showing the respective polarizations of the flapper and of the stator of the servo-valve; and 
         FIG. 8  is a view of the pilot stage of the servo-valve in a third embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE FIGURES 
     With reference to  FIG. 1 , the invention is shown in application to a servo-valve with barometric flowrate-regulation and two stages including a pilot stage. Naturally, the invention is not limited to this application and it may be used with other types of servo-valve. 
     The servo-valve shown comprises a body  1  in which a spool  2  is mounted to slide in leaktight manner in a cylindrical bore  3  by forming the distribution stage. The servo-valve rests on a machined bearing face  1000  having a port P for feeding the servo-valve with fluid, two utilization ports U 1  and U 2 , and a return port R. These ports are in fluid-flow communication with corresponding ports of the support on which the servo-valve is fastened. The spool  2  is movable between two extreme positions and it is shaped to define leaktight chambers C 1 , C 2 , C 3 , and C 4  inside the bore  3  respectively for use, depending on the extreme position of the spool  2  relative to a central position (or neutral position), for putting:
         either the feed port P into communication with a first utilization port U 1 , and a return port R with a second utilization port U 2 ;   or else the feed port P into communication with the second utilization port U 2 , the return port R being in communication with the first utilization port U 1 . The sliding of the spool  2  in the bore  3  is controlled by pilot chambers  4  and  5  that are fed with fluid under pressure by a pressure-sharing member, specifically in this example a deflector  6  engaged in leaktight manner in a housing  7  of the body  1 . The deflector  6  has a central flat  8  in which a sharing orifice  9  is formed. The sharing orifice  9  is put into communication via ducts  10  and  11  with the pilot chambers  4  and  5 . Springs  60  are provided to exert forces reacting against the pilot pressures induced on the spool  2  in order to enable its position to be servo-controlled.       

     Facing the central flat  8  there is an ejector  20  that ejects a jet of fluid towards the sharing orifice  9 . The ejector  20  is movable facing the sharing orifice  9  so as to move the point of impact of the jet on the central flat  8 , thereby having the effect of varying the pressures that exist in the pilot chambers  4  and  5 , thus enabling the spool to be moved in response to the movement of the ejector  20 . The above is well known and is recalled merely to situate the context of the invention. 
     According to an essential aspect of the invention, the ejector  20  is secured to a one-piece column  21  that is twistable and has a tube fastened to its end, which tube extends radially therefrom, and is in fluid-flow communication with a central bore  22  of the column, through which the ejector  20  is fed with fluid. The column  21  has a first end  23  that is fastened in leaktight manner in the body  1  in a direction that is substantially perpendicular to the bearing face  1000  and through which the fluid is introduced into the central bore of the column, the fluid coming from the feed port P (the feed duct is drawn in dashed lines and may be drilled directly in the body  1 ). The first end of the column may be implanted in a low portion of the body  1 , close to the pressure feed, thereby avoiding any need to pass feed ducts for the ejector  20  over the distribution assembly. 
     The column  21  has a second end  24  that is secured to the rotor  25  of a torque motor  26  having its stator  27  fastened on the body  1 . 
     Thus, when the torque motor  26  is powered, it twists the column  21  about its axis Z, thereby causing the ejector  20  to move angularly facing the sharing orifice  9  so that the impact of the jet produced by the ejector  20  moves relative to the sharing orifice  9 . 
     The movement of the point of impact of the jet is small and may be considered to be a movement in translation along the tangent to the trajectory of the ejector  20 . A high degree of proportionality is conserved between this movement and the torque that is imposed by the torque motor  26  on the column, and thus with the electric current fed thereto. 
     When the torque motor  26  is unpowered, the column  21  is at rest, and the jet produced by the ejector  20  impacts the central flat  8  of the deflector at a location for which the pressures in the pilot chambers  4  and  5  are in equilibrium. For this purpose, the deflector  6  is provided with adjustment means enabling its precise positioning in the housing  9  facing the ejector to be adjusted. 
     With reference below to the second particular embodiment shown in  FIGS. 2 and 3 , in which the references for elements that are common with those of  FIG. 1  are the same plus one hundred, the servo-valve comprises, as above, a body  101  in which a spool  102  is slidably mounted. The pilot stage has a deflector  106  and an ejector  120  that is secured to a column  121  by being mounted at the end of a tube  130  that extends radially from the column  121 . The column  121  has a first end that is embedded in leaktight manner in the body  101 , and a second end  124  that is subjected to the action of a torque motor  126 . The column  121  has a central bore  122  enabling the ejector  120  and the feed port P to be put into fluid-flow communication by the first end  123  via the central bore  122  and the tube  130 . It can be seen in this embodiment that the embedded end of the column is likewise implanted close to the pressure feed of the servo-valve. 
     As can be seen more particularly in  FIG. 3 , the column  121  has a twistable section  140  of small thickness, with the remainder of the column being, in comparison, very stiff in twisting. The twisting stiffness of the column  121  thus depends essentially on the thickness, on the diameter, and on the length of this twistable section. This makes it simple to adapt the twisting stiffness of the column  121  by acting on these manufacturing parameters. It should be observed that it is ensured that the twistable section extends over a fraction of the length of the central bore  122 , thus making it possible to achieve stiffness that is small compared with the stiffness of the column  121  (being about 20%), thereby increasing the angle through which the injector can move relative to the angular movement of the flapper  150 . 
     It is advantageous to obtain stiffness that is relatively small, thus making it possible for a required angular stroke of the deflector  120  to make use of a torque motor of smaller power. Thus, the torque to be withstood by the embedded end is made smaller and this may be guaranteed merely by the first end  123  of the column  121  being a tight fit in its housing. Sealing is then guaranteed by a simple static gasket  131 . 
     In this embodiment and according to a particular aspect of the invention, the column  121  is surrounded by a thin-walled tube  127  that extends from a soleplate  128  that is fastened in leaktight manner to the body of the servo-valve to a flange  129  tightly surrounding the end  124  of the column. The flange  129  and said end are fastened together so that during twisting driven by the torque motor  126 , the thin-walled tube  127  and the twistable portion  140  work in parallel and are subjected to the twisting. These two parts serve to seal the chamber  145  into which the ejector  120  ejects the fluid, without having recourse to a sealing gasket rubbing against the end of the column that co-operates with the torque motor, which could give rise to hysteresis. 
     In another particular aspect of the invention, the resilient return force between the spool  102  and the ejector  120  that is secured to the column  121  is provided in this embodiment by a flexible rod  132  connected at one of its ends to the column  121  and extending as far as the spool  102 . The rod  132  extends parallel to the column  121 . 
     In another particular embodiment that is shown in  FIG. 8 , the return-force rod  132  is secured to the column  121 . Ideally it is in the form of a flexible blade  132  that is generally triangular in shape. The base of the triangle is radially connected to the column  121 , with the vertex opposite from that side being in connection with the spool  102 . In this embodiment, the rod  132  is connected to the column  121  by a bushing  160  shrink-fitted on the column  121 . This bushing  160  carries the rod  132  and extends beyond the tube  130 . A longitudinal notch allows the tube  130  to be engaged in the bushing  160 , so as to provide the mechanical connection between the flexible blade  132  and the ejector  120 . 
     The torque motor  126  is described in detail below with reference to  FIGS. 4 to 6 . It comprises a flapper  150  having two opposite arms  150   a  and  150   b  and that is connected to the flange  129  by screw-fastening. The flapper  150  is surrounded by a ferromagnetic structure having two flanks  151  and  152  that are connected together in their top portions by a permanent magnet  153  that is north-south biased as shown in  FIG. 4 . 
     As can be seen in  FIG. 6 , the flanks  151  and  152  present active faces  155  and  156  that are arranged immediately facing the faces of the flapper  150 , leaving only a small airgap, with this being on either side of the twist axis Z. The permanent magnet  153  thus generates magnetic fluxes that pass via the active faces  155 ,  156 , with each of them looping via one of the arms of the flapper  150  on either side of the axis. Since the fluxes are equal, the flapper is not subjected to any torque. 
     Coils  157  and  158 , each arranged to surround one of the arms of the flapper  150 , are powered in opposition, thereby producing torque on the flapper  150  that is proportional to the product of the currents fed to the coils  157  multiplied by the number of turns in the coils so as to generate a magnetic flux within the flapper that produces a north polarization on the portion  150   a  and a south polarization on the portion  150   b  (see  FIG. 7 ). This serves to establish a torque on the flapper  150  that serves to twist the column  121  and the tube  127 . 
     Naturally, this twisting is very small, being of the order of a few tenths of a degree. It suffices to reverse the direction of the current fed to the coils in order to reverse the direction of the twisting. 
     It should be observed that in the variant shown in  FIG. 5 , the base  122  of the column  121  is embedded not by means of a tight fit, but by means of at least one clamping screw, and specifically in this example two clamping screws  160 . 
     Naturally, the invention is not limited to the above description, but covers any variant coming within the ambit defined by the claims. 
     In particular, although the above-described column is mounted parallel with a twistable thin-walled tube, such a configuration could be avoided if sealing can be ensured for the chamber into which the ejector sends fluid. In particular, it is possible to use a bellows, or a gasket that is capable of deforming in twisting without sliding and without friction and that does not present hysteresis. 
     The two stages of the servo-valve may constitute a single module or they may be in the form of separate modules enabling servo-valves to be constructed in modular manner.