Multiple tow-stage electrohydraulic servovalve apparatus
I United States Patent 1191 Thayer 1 Apr. 1, 1975 1 1 MULTIPLE TOW-STAGE 3,333,139 8/1967 Wood 1. 91/363 A x ELECTR UL] SE v VALVE 3,470,694 10/1969 Bjkzich 91/444 X gg C R 0 3,505,929 4/1970 Coppola et al. 91/363 A X 3,722,543 3/1973 Tennis l37/596.12 [75] Inventor: William J. Thayer, Holland, N.Y. [73] Assignee: Moog- 111e,, East Aurora, NY. Primary E&#34;ami&#39; &#39;er william Cline Assistant Examiner-Robert J. Miller [22] Filed: Aug. 24, 1973 [21] Appl. No.: 391,268 [57] ABSTRACT Two or more independently controllable two-stage [52] U.S. CL... 137/596.16, 137/625.62, 137/625.64 electrohydraulic servovalves are collectively serviced [51] Int. Cl. Fl6k 11/00 by a single fluid supply and a single fluid return al- [58] Field of Search l37/596.l6, 625.64, 625.5, though each servovalve is associated with its own load 137/596.l5, 596.17, 625.63, 625.62, 85, actuator, by providing each servovalve with a first 625.6; 91/48, 336 A, 444, 411 A stage symmetrical hydraulic amplifier and with a second stage valve spool assocaited with metering ports, [56] References Cited the hydraulic amplifiers of the various servovalves UNITED STATES PATENTS being connected in series fluid flow communication, 2 Sig 900 SHQSO Gei or ct a] 9H4 A X so as to maintain quiescent flow essentially the same 1 M1958 Paige 37/85 X as for one such servovalve independently, and the me- 2:835:265 5/1958 51211165156161:1111;211:1113: 137/85 x fling P0rts Oflhe Various Servovalves being in Parallel 2.942.553 6/1960 MOCllCICIal. 91/411 A x fluid flow communication with the fl pp y and 3.019.735 2/1962 M66|1er C1131. 91/411 A x fluid return so as to be capable of developing full dif- 3.()23,782 3/1962 Chaves, Jr. et a1. 137/85 ferential pressure across the load for each servovalve. 3,070,071 12/1962 Cooper 91/363 A X 3,338,138 8/1967 WOUd 91/363 A x 7 Claims, 2 Drawing Flgures T U n f qf Tg i I04 20 .3 20 r 2oL-.. 2o JON A 8 -30 1&#39;22 r16 63 X M10 L j -l6 /4 B5 &#34;5| 1 1 l 9 9o l l V/ s6 H/17&#39; 1 y 4 \ILIIIII&#39;LF. &#39;IOW-STAGF. El.EC&#39;IROHYDRAUl.l( SI-IRYOVALYI&#39;I AIPARATIS BACKUROLND ()F lHl-I INVENTION l. Field of the lmention l&#39;his invention relates to multiple two-stage electrohydraulic ser\o\alve apparatus. and more particularly to such apparatus which comprises two or more servovalves independently controllable and collectively scr- \iceable by a single fluid supply and a single fluid return 2. Description of Prior Art Many applications of electrohydraulic servovalves require the use of two or more servo\alves for multiple function control within the same vehicle or equipment. Examples include missile fin control such as pitch. yaw and roll. radar antenna control such as azimuth and elevation. nozzle gimbaling control. laser beam control. jet tab control. and others.  
  in order to obtain high performance of the control system. two-stage electrohydniulic servovalves having a frictionless hydraulic amplifier are generally preferred. Examples of such servovalves include two-stage double nozzle-flapper servovalves such as disclosed in LKS. Pat. No. 3.023.782. jet pipe servovalves such as disclosed in L. Pat. No. 2.884.907. and deflector jet servovalves such as disclosed in US. Pat. No. 3.6] 2J0 A critical requirement for many applications is that power consumption be minimized. This requirement arises as many such systems are carried by missiles or other vehicles or equipment that include a selfcontained energy sou rce. Needless power consumption results in increased size and weight of the energy source. For airborne vehicles. this sacrifice in size and weight may penalize range. For hand-held equipment. the increased size and weight of the energy source may decrease the effective payload of the equipment.  
  Also. in some applications. if the total energy consumption of the control system can be sufliciently low. then a blow-down system may be utilized. such a system employing. for example. a stored cold gas or hot gas propellant charge. together with a one-shot hydraulic accumulator. Such a blow-down system offers siniplicity. higher reliability. and lower costs than a conventional battery. electric motor and hydraulic pump configuration.  
  In many applications of the type mentioned. the duty cycle of the control system consists of an initial flurry of control activity. such as to correct for launch errors. followed by a relatively long period of coast that requires only small. vernier-type course corrections. The energy consumed in the initial control transients is unavoidable. as well as that required for vernier corrections. Superimposed on this necessary control activity. however. is the quiescent power drain of the servovalve hydraulic amplifiers. Often times this quiescent power drain. which continues throughout use of the control system. accounts for more total energy than does the necessary control activity.  
  A previous approach to solving this problem in some systems heretofore used has been to employ servo- \al\es that have a spool type first-stage hydraulic amplifier. ln systems that require low control flow a singlestage servo\alve in which an electromagnetic positioning means drives directly a sliding spool valve may sut fice. for higher flow systems a two-stage. double spool valve has been used. In either case. the servovalve can he made to have very low quiescent leakage by appropriate overlap of the spool null edges. Howe\ er. these spool type servovalves suffer from the presence of fric tion acting directly upon the electromagnetic drive means. This friction seriously compromises valve performance. and only through use of complex means for adding dither. are such valves made workable. The dither requires quiescent povver loss. and also adds considcrably to the cost of the control system. Also, even with dither. the performance achievable with a servo- \alve having a sliding spool driven directly by the electromagnetic input means is far less than that obtainable with a servovalve having a frictionless first-stage hydraulic amplifier. In particular. the threshold. hystersis and null shift of single-stage spool type servo\ alves. or double spool two-stage servovalves. is generally inferim&#34; to that obtainable with servovalves ha\ing a frictionless first-stage hydraulic amplifier. These performance deficiencies detract from system accuracy and effectiveness.  
  It would he desirable in many instances to replace both spool type single-stage servovalves. and double spool type two-stage servovalves. with the more advanced two stagc servovalve of the frictionless hydraulic amplifier type. in order to reali7e the improved system performance. However. this change has not been practicable heretofore without a corresponding change in the stored energy portion of the vehicle or equipment in order to provide the increased energy necessary to sustain the quiescent power drain associated with the continuous flow through the frictionless hydraulic amplifier. Most often this requirement for increased capacity of the stored energy source necessitated change to a battery. motor and pump apparatus with the attendant increase in complexity. weight and cost.  
 SUMMARY OF THE INVENTION The solution to the problem afforded by the present invention is based upon recognition of the fact that the quiescent flow in the first-stage of a symmetrical hydraulic amplifier of the nozzle-flapper. jet pipe. or deflector jet types. is essentially constant throughout all levels of signal input. i.e. from (1 to either plus or minus 100 percent signal input.  
  Further. the solution to this problem afforded by the present invention is based upon recognition of the fact that in the two-stage servovalve that utilizes feedback of spool position. whether achieved through mechanical feedback to the armature of the torque motor or electrical feedback or otherwise, the differential pressure necessary to move the second-stage valve spool. or to maintain the spool in any displaced position. is but a small portion of normal supply pressure.  
  Still further. the solution afforded to this problem by the present invention is based upon a recognition of the fact that dynamic response of such a two-stage servovalvc incorporating spool position feedback is related primarily to the flow gain of the hydraulic amplifier. rather than the pressure gain ofthe hydraulic amplifier.  
  Accordingly. the primary object of the present invention is to relate two or more independently controllable two-stage electrohydraulic servovalves of the high performance type severally having: (at a first-stage hydraulic amplifier which is frictionless but which requires quicscent flow; and (b) a second-stage power valve. to one another and to a single fluid supply and a single fluid return. such that the quiescent flow for all the first-stage hydraulic amplifiers does not exceed that essentially for one such servovalvc if operated by itself. and the full differential pressure between said supply and return is available for load control by the second stage power valve of each servovalve.  
  This is achieved. in accordance with the inventive concept. by providing two or more servovalves each including first-stage hydraulic amplifier means capable of producing a differential pressure responsive to an independent electrical command signal. and second-stage fluid flow control valve means responsive to such differential pressure. each of the hydraulic amplifier means during operation requiring input operating fluid and producing output operating fluid. each of the fluid flow control valve means including an actuating port alternately communicable with a pressure metering port and a return metering port determined by the position of a movable valve element arranged to be moved by the differential pressure. the operating fluid for the hydraulic amplifier means of the various servovalves being in series fluid flow communication between the fluid supply and fluid return. the pressure and return metering ports of the various servovalves being in par allel fluid flow communication with the fluid supply and fluid return. respectively, and the actuating ports of the various servovalves being operatively associat&#39; able with separate load actuators.  
  Other objects and advantages of the present invention will be apparent from the following detailed description of a preferred embodiment illustrated in the accompanying drawings.  
 BRIEF DESCRIPTION OF THE DRAWINGS Fl(l. I is a vertical. central. longitudinal. sectional view through multiple independently controllable twostage electrohydraulic servovalve apparatus embodying the present invention and illustrating the internal con struction of two servovalves combined in a more or less diagramatic manner, the valve spool of each servovalve being illustrated in elevation and in a null or centered position. and each servovalve being further depicted schematically in operative association with a separate load actuator.  
  FIG. 2 is a block diagram indicating a relationship of factors involved between electrical current input to one servo\ alve of the apparatus and output from such servovalve in the form of flow to the actuator controlled thereby.  
 DESCRIPTION OF PREFERRED EMBODIMENT The multiple two-stage electrohydraulic servovalve apparatus of the present invention is shown generally as comprising two two-stage electrohydraulic servovalves severally of the double nozzle-flapper. mechanical feedback. flow control type illustrated and described in US, Pat. No. 3.023.782 but consolidated into a single body and related to each other so as to be collectively serviced by a single fluid supply port and a single fluid return port and further arranged so that their firststage hydraulic amplifiers are connected in series fluid flow communication and their second-stage valve spools are connected in parallel across the supplyto-return differential pressure available.  
  More specifically and referring to FIG. I. the apparatus is shown as comprising left and right electrohydraulie servovalves 8 and 9. respectively. having a common valve body I0. This body is internally formed to provide a left cylindrical chamber II in which is slidably arranged the second-stage valve spool I2 of servovalvc 5 8 which also includes a torque motor 13 and a hydraulic amplifier I4. Body Ill is also internally formed with a right cylindrical chamber 15 in which is slidably arranged the seeond-stage valve spool 16 of servovalve 9 which also includes a torque motor l7 and a hydraulic amplifier 18.  
  Each of the electromagnetic torque motors I3 and I7 is suitably mounted on body It) and is specifically in the form of a polarized force motor including an upper pole piece 20.11 lower pole piece 2| to provide therebetween a pair of air gaps 20&#39; and 2I&#39; and a pair of coils 22 and 23. Two permanent magnets (not shown). one on each side of the torque motor between these pole pieces. provide polarization of the air gaps. The coils of each torque motor may be connected in any suitable manner so as to provide differential coils. series coils. parallel coils or single coils. As shown. the leads of coils 22 and 23 are connected in a differential coil configu ration. Specifically. coil 22 has two leads Z4 and 2S. and coil 23 has two leads 26 and 27. Leads and 27 are connected together to provide common lead 28. The tips of an armature 30 is shown arranged in air gaps 20&#39;. 2I&#39;. This armature is shown as rigidly supported on the upper end of a flexure tube 31 the lower end of which is suitably secured to body I0.  
  Hydraulic amplifier I4 of left servovalve 8 is shown as including a flapper 32 rigidly mounted at its upper end on the corresponding flexure member 31 and ar mature 30 and having its lower end movably arranged between a pair of oppositely facing, spaced apart. fixed nozzles 33 and 34. In the hydraulic amplifier I8 for the right servovalve 9. the corresponding flapper is designated 35 and the two corresponding nozzles are designated 36 and 37.  
  Valve body I0 is shown as having a pressurized fluid supply port 39 adapted to be connected to any suitable source of pressurized hydraulic fluid (not shown). and also a fluid return port 40 adapted to be connected to any sump or receptacle for collecting return hydraulic fluid (not shown]. Pressure port 39 is shown as connected by passage 41 to a branch passage 42, in turn leading to an enlarged left compartment 43. This compartment houses a filter screen 44. schematically illustrated as being of cylindrical configuration Entrance of the branch passage 42 to compartment 43 is shown as being midway of the length of this compartment. Fluid entering into this compartment flows through the interstices of the filter and thence divides. some of the fluid flowing to the left through a fixed restrictor or orifice 4S and some flowing to the right through a similar fixed restrictor or orifice 46.  
  Restrictor 45 is shown as arranged in the lower end of a passage 48 provided in body I0 communicating with eompartn&#39;ient 43. the opposite end of this passage leading to left nozzle 33. Intermediate its ends. passage 48 intercepts the left end of valve spool compartment I I to provide a left spool end chamber indicated at 49. The other restrictor 46 is shown as arranged in one end of a passage 50 provided in body II) the other end of which leads to right nozzle 34. Intermediate its ends. this passage 50 intercepts the right end of spool compartment l I so as to provide a right spool end chamber indicated at SI.  
  Left valve spool 12 is shown as having a left end lobe 52. a right end lobe 53 and an intermediate lobe 54. these lobes being connected by intermediate stems of reduced diameter so that the valve spool moves as an unit. left lobe 52 is shown as provided with an annular groove 55 which receives in a substantially frictionless rolling contact manner a ball 56 formed on the lower end of a feedback spring member 57. The upper end of this member 57 is cantilever-mounted orl flapper 32.  
  Valve spool I2 is shown in a null or centered position. In this position. center lobe S4 closes a pressure metering port 58 which is connected in fluid flow coinmunication via passage 59 with passage 42. In this null position of valve spool 12. left lobe 52 is shown as closing a left return metering port 60. A similar but right fluid metering port 6l is shown as closed by right spool lobe 53. These return metering ports 60 and 6] are shown as connected by branch passage 62 with main fluid return passage 63 which leads to fluid return port 40.  
  The space between lobes 52 and S4 in compartment II is shown as connnunicating via passage 64 with an actuating port 65. The space between lobes 53 and 54 is shown as communicating via passage 66 with a second actuating port 67. Actuating ports and 67 are shown as opcratively associated with opposite ends of an actuator 68 including a cylinder 69 housing a movable piston 70 mounted on a piston rod 7| which projects from opposite end walls of this cylinder and is suitably connected to a load (not shown) to be serviced by servovalve 8.  
  Nozzles 33 and 34 discharge fluid into an outlet chamber 72 which surrounds left lobe 52 of valve spool 12 and is connected via passage 73 with a right conipartment 74 which is similar to left compartment 43 but separate therefrom. Compartment 74 also houses a cylindrical filter screen 75 and has its ends communicating with passages 76 and 77. Left passage 76 adjacent its lower end has arranged therein a fixed restrictor or orifice 78. The opposite end of this passage 76 leads to left nozzle 36 of right servovalve 9. Intermediate its ends. this passage 76 intercepts the left end of compartment I5 so as to provide left spool end chamber 79 for the right valve spool 16. The other or right passage 77 has therein a similar fixed restrictor or orifice 80 adjacent its lower end. The upper end of this passage 77 communicates with right nozzle 37 of right servovalve 9. Intermediate its ends. this passage 77 intercepts the right end of compartment 15 to provide right spool end chamber 8|.  
  \alve spool I6 is shown as having a left end lobe 82. a right end lobe 83 and a center lobe 84. these lobes being spaced apart and connected together by intermediate stems of reduced diameter so as to provide an unitary valve spool. This spool is also shown in its mill or centered position in which center lobe 84 closes a pressure metering port 85 connected via passage 86 to passage 4l. Left lobe 82 in this position of the spool closes a left return metering port 87. and right lobe 83 closes a right fluid metering port 88. These return metering ports 87 and 88 are shown as connected via passage 89 to main fluid return passage 63.  
  The space between lobes 82 and 84 of right valve spool 16 is shown as communicating with a passage 90 leading from compartment 15 to a left actuating port 9]. The space between lobes 83 and 84 is shown as connected by a passage 92 leading from compartment l5 to a right actuating port 93. Actuating ports 91 and 93 are operatively associated with a separate hydraulic actuator indicated generally at 94 comprising a cylin der 95 housing a piston 96 fast to a piston rod 97 which is shown as extending outwardly through the end walls ofthis cylinder and connected to any suitable load (not shown).  
  Right lobe 83 of right valve spool 16 is shown as pro \ided with an annular groove 98 on the walls of which a ball 99 has a substantially frictionless rolling contact. This ball 99 is mounted on the lower end of a feedback spring wire member 100 the upper end of which is cantilever-mounted on flapper 35 for right servovalve 9.  
  Nozzles 36 and 37 of this right servovalve 9 discharge fluid into an outlet chamber I01 which is shown as having a fluid flow connection with passage 89 via a short passage I02 in which a restrictor or fixed orifice [03 is arranged.  
  Front the foregoing arrangement. it will be seen that ach hydraulic amplifier l4 or [8 during operation requires input operating fluid and produces output operating fluid. In the case of left hydraulic amplifier I4. the operating fluid therefor is derived from fluid supply port 39 flowing through connected passages 4| and 42 into compartment 43. thence dividing to flow through a left course including restrictor 4S. passage 48 and nozzle 33 into chamber 72 and a right course including restrictor 46. passage 50 and nozzle 34 into chamber 72. thence through passage 73 into compartment 74. The output operating fluid from left hydraulic amplifier 14 entering compartment 74 via passage 73 becomes the input operating fluid for right hydraulic amplifier l8. From compartment 74 this fluid divides and flows through left and right courses. The left flow course includes restrictor 78. passage 76 and nozzle 36 into chamber ml. The right flow course includes restrictor 80. passage 77 and nozzle 37 into chamber 101. From this chamber I01 flow passes through restrictor 103 into passage I02 and connected passages 89 and 63 to fluid return port 40. In this manner the operating fluid flow through these hydraulic amplifiers is in series.  
  On the other hand. the pressure and return metering ports 58. 60 and 6] of left spool 12. and the pressure and return metering ports 85. 87 and 88 of right spool 16. are connected in parallel fluid flow communication with fluid supply port 39 and fluid return port 40. respectively. Thus. pressurized fluid flows from pressure supply port 39 through passage 4| and branch passages 59 and 86 to pressure metering ports 58 and 85. respectively. Depending upon the position ofthe valve spools l2 and 16. these pressure metering ports 58 and 85 can be placed in fluid flow communication with one of their respective actuating port passages such as 64. 66 in the case of port 58 or passages 90. 92 in the port 85. while the other of such actuating port passages is connected with the appropriate return metering port such as 60 or 6] for left scrvovalve 8. and 87 or 88 for right servovalve 9. Thus. fluid which flows from supply port 39 through pressure metering port 58 ofleft valve spool 12 (while in a displaced position) to one side of actuator piston 70. in turn displacing fluid from the opposite side of said piston which flows past the appropriate return metering port of left spool 12 to fluid return port 40. is in parallel with fluid that may similarly flow through right spool 16 for displacement of piston 96.  
  Each of the two servovalves operates independently of the other, except that the output operating fluid from the hydraulic amplifier of left senovahe is utilized as the input operating iluid for the right servovalve. If more than two servovalves are employed. as there can be if desired, the output operating lluid from the second servovalve would be utilized as the input operating fluid for the hydraulic amplifier of the third servovalve. and so on. Insofar as command signals for the various servovalves are concerned. these are applied independently to each valve as electrical current inputs through corresponding leads 24. 26. 28. Each servovalve is operatively associated with its own load actuator and the second stage of each servovalvc has access to the full hydraulic power available. i.e. the pressure differential between supply and return across ports 3&#39;) and Ml.  
  The flexure tube 3! ofeach servovalve provides complete isolation of the electromagnetic torque motor l3 or 17 and the associated air gaps from the hydraulic fluid. thereby eliminating problems from magnetic eontamination. Inasmuch as the exterior of such flesure tube is exposed to atmosphere. both torque motors I3 and [7 can be covered by a single removable cover or cap I04 mounted on body member it].  
  As is well known to those skilled in the art. the air gaps 20&#39;. 21&#39; formed by the upper and lower pole pieces 20 and Zl. respectively. of each torque motor are polarized by permanent magnets (not shown) arranged between these pole pieces. These magnets are charged in parallel so that the entire upper pole piece is at the same magnetic potential. Electrical current in the torque motor coils 22 and 23 causes polarization of the armature .30. The resulting unbalance of flux at the air gaps produces a torque on the armature. This torque is then balanced by torque from the spool feedback wire. 57 for the left servovalve 8 and Hill for the right servovalve 9.  
  As previously indicated. the two identical torque motor coils 22 and 23 can be wired with equal ease for series. or parallel. or differential electrical inputs. The differential connection shown is generally used with a push-pull electrical circuit having a nominal quiescent current in each coil.  
  Each electrohydraulic servovalve operates independently of the other. except for the series fluid flow connection between the hydraulic amplifiers of the valves. ln this arrangement. quiescent flow through the hydraulic amplifier l4 of the left servovalve 8. passes also through the hydraulic amplifier I8 of the right servovalve 9. and in any succeeding servovalve. The secondstage spools l2 and I6, however. remain connected in parallel across the total system supply. so are capable of developing full flow and/or full differential pressure across their respective actuators.  
  Should two servovalves be hydraulically connected in the manner described. then in the typical 3.000 psi hydraulic system. the pressure drop across each first-stage hydraulic amplifier will be L500 psi nominally (neglecting the relatively small pressure drop across restrictor 103). Similarly. with three servovalves. each hydraulic amplifier will have one-third of supply or l.()tltl psi. and with four servovalves. 750 psi.  
  In each case. the quiescent hydraulic amplifier How gain can be maintained the same by appropriate selection ofsiving ot&#39;the first-stage hydraulic orifices such as 45. 46 and 78 and H0. Thus. the quiescent flow with two. three. four or more servovalves can be essentially the same as obtainable with a single sertovalve inning the same dynamic performance. liach individual servovalve in such a combination can operate independently and with essentially no sympathetic response from ad joining servovalves connected in the same control system configuration.  
  Fixed restrictor or orifice I03 in return passage I02 minimizes the effects of back surges in the return line and improves the discharge flow characteristics of no?- zles 36 and 37 of right servovalve 9 by maintaining a back pressure in the outlet chamber l0]. The flow through this restrictor Hi3 produces a pressure drop in the range of from 100 to Ztltl psi typically.  
  The action of a given servovalve of the multiple servovalve apparatus ofthe present invention is illustrated schematically in FIG. 2. Referring to that figure. elec tric current is fed to the torque motor which produces torque on the armature and ilapper assembly.  
  The cantilever feedback spring member of each servovalve likewise produces torque on the armature and flapper assembly corresponding to displacement of the servovalve spool. Any difference between the input torque caused by electrical current. and feedback torque caused by spool displacement. acts to move or displace the flapper between the nozzles. This difference in torques is represented in FIG. 2 by the torque summation point. The actual displacement of the flapper in response to a torque difference will be determined by the combined mass. stiffness. and damping of the armature. flapper and llexure tube assembly. It is known to those skilled in the art that the stiffness of this assembly is determined by the centering spring rate of the flesure tube. together with the decentering spring rate associated with permanent magnet flux in the air gaps of the torque motor.  
  Displacement of the flapper between the nozzles causes an unbalance in the orifice relationship between the left and right sides of the hydraulic amplifier. The intermediate chambers on each side of the hydraulic amplifiers connect to their respective spool end chambers as previously described. Thus. the spool will be responsive to unbalance of the hydraulic amplifier.  
  The spool during all normal operations will move whenever very small pressure differentials exist between its spool ends. However. the velocity of spool motion will be determined by the unbalance in hydraulic amplifier flow that results from flapper displacement. Tints. spool displacement will be the integral of the spool velocity.  
  Therefore. it is seen that the spool displacement in a two-stage servovalve of the spool feedback type (where no spool centering springs are necessary) is related to the differential flow characteristics of the hydraulic amplifier. rather than to the differential pressure characteristics of this amplifier. There is a small. but negligible. centering spring effect associated with displacement of the cantilever feedback spring caused by spool displacement.  
  Thus. performance of such a two-stage servovalve of the spool feedback type is influenced primarily by the flow characteristics of the hydraulic amplifier.  
  Further. with the basic symmetry inherent in a double norzlc servovalve of the type described. or the summetry also inherent in a jet pipe or deflector jet servovalve. the total quiescent flow through the hydraulic amplifier will be essentially constant throughout all positions of the first-stage moving valve element. or for all positions of the second-stage moving valve element.  
  From the foregoing, it will be seen that quiescent flow through the series-coupled hydraulic amplifiers is essentially constant at all times whether or not an electrical current input is applied to the torque motor coils. Moreover. the differential pressure developed by a given hydraulic amplifier to move or hold its valve spool is a small portion of the pressurized fluid supply. Further. the dynamic performance of a servovalve having spool position feedback is related to the flow gain of its hydraulic amplifier and flow gain is related to bydraulic amplifier quiescent flow. Thus the multiple twostage electrohydraulic servovalve apparatus of the present invention provides an alternate valve configuration possessing significantly improved performance over the prior art type of apparatus which employed servovalves having a spool type firsbstage hydraulic amplifier.  
 What is claimed is:  
  I. Multiple two-stage clectrohydraulic scrvovalve apparatus collectively serviceable by a single fluid supply and a single fluid return. comprising at least two servovalves each including first-stage hydraulic amplifier means capable of producing a differential pressure responsive to an electrical command signal and secondstage fluid flow control valve means responsive to said differential pressure. each of said hydraulic amplifier means utilizing input operating fluid and producing output operating fluid. each of said fluid flow control valve means including an actuating port alternately communicable with a pressure metering port and a return metering port determined by the position of a movable valve element arranged to be moved by said differential pressure, the operating fluid for the hydraulie amplifier means of the various servovalves being in series fluid flow communication between said fluid stipply and said fluid return. the pressure and return metering ports of the flow control valve means of the various servovalves being operatively connected with said fluid supply and said fluid return. respectively. and the actuating ports ot&#39;the flow control valve means of the servovalves being operatively associatable with separate load actuators.  
  2. Apparatus according to claim I which further comprises position feedback means operatively interposed between said flow control valve means and said hydraulic amplifier means of each of said servovalves.  
  3. Apparatus according to claim 2 wherein each of said hydraulic amplifiers includes a pivotal element and each of said servovalves further comprises a flcxure tube isolating the hydraulic section from the electrical section of the servovalve and also serving as a frictionless pivotal support for said pivotal element.  
  4. Apparatus according to claim 2 wherein each of said hydraulic amplifiers is of the double nozzle-flapper type.  
  5. Apparatus according to claim 4 wherein said position feedback means comprises a spring member operatively interposed between said flow control valve means and the corresponding hydraulic amplifier means.  
  6. Apparatus according to claim 4 wherein said position feedback means comprises a spring member cantilever-mounted at one end on the flapper of the corresponding hydraulic amplifier and having its other end constrained to move with the corresponding valve element.  
  7. Apparatus according to claim 6 wherein each of said servovalves further comprises a flexure tube isolating the hydraulic section from the electrical section of the servovalve and also serving as a frictionless pivotal support for said flapper.