GB2104249A - Servovalves - Google Patents

Abstract

A single-stage servovalve includes a valve spool rotatably mounted in a body bore. A torque motor has a stator mounted on the body and has a rotor mounted fast to the valve spool. The angular position of the spool relative to the body is proportional to the magnitude of the input current supplied to the torque motor. Cooperative ports on the spool and body are cooperatively shaped such that the size of the metering orifice will be proportional to angular position of the spool relative to the body. The valve may be easily nulled at a desired angular position. <IMAGE>

Description

SPECIFICATION Servovalves The present invention relates to servovalves.

Most high performance servovalves today are two-stage configurations having a substantially friction less pilot stage and a sliding spool output stage. In such valves, the pilot stage provides power amplification such that an electro-mechanical torque motor, typically with low power input, can be used to control a high power sliding spool valve. Full servovalve output is typically obtained with a maximum input power of from 50 to 100 milliwatts. One representative example of such a two-stage servovalve is shown in U.S. Patent No.

3,023,782.

The pilot stage of such a two-stage servovalve is believed to possess certain inherent limitations. For example, frictionless pilot stages are inherently open centre and have a continuous leakage flow. This represents a power loss which can become significant in a complex control system, particularly where redundancy is provided. A redundant control system for a piloted aircraft may typically have from 15 to 40 servovalves. The pilot stage of each servovalve may consume 1/4 horsepower. Hence, the continuous power drain associated with such pilot stage leakage flow may be significant.

Contamination of the serviced fluid poses yet another problem. A valve according to the aforesaid U.S.

Patent No. 3,023,782 includes small size orifices in the pilot stage. Hence, servovalve performance may be substantially impaired by flow-restricting contaminants. Other problems subsisting in such conventional two-stage servovalve designs include the relative delicateness of the components parts, sensitivity to temperature variations, stresses due to pressure, shock and acceleration, and other changes due to variations in environmental and operational conditions. These factors are the primary cause of servovalve null shift. Moreover, the dynamic response of two-stage servovalves may be affected by fluid viscosity, and open-centre pilot stages are susceptible to instability at high temperatures and high pressures.

In light of the foregoing difficulties, it would be desirable to eliminate the pilot stage from a high performance servovalve. Previous attempts to create such a high performance single-stage servovalve with a power control capability comparable to two-stage servovalves, have been deficient in one or more of the following respects: lack of adequate spool driving force (so-called "chip shearing" force), excessive size and weight, and a dependence on motion reduction and/or conversion mechanism (e.g., levers, ball/screws, etc.).

According to one aspect of the present invention, there is provided a single-stage servovalve which comprises, a body, a rotatable valve element mounted in the body, and an electromechanical actuator having a rotor, the rotor being mounted fast with said valve element for rotation therewith, and the valve element providing bearing support for the rotor.

In a preferred embodiment to be described, the rotor and valve element are directly and rigidly coupled, so that there is no backlash or lost motion between these elements. In addition, this servovalve possesses inherently high torque amplification related to the ratio of the diameter of the air gaps of the rotary torque motor to the diameter of the rotary valve element.

According to another aspect of the invention, there is provided a method of nulling a rotatable valve element relative to a body, which method includes the steps of: machining a hole in either the valve element or the body to provide a fluid flow passageway terminating in a port at the interface between the valve element and the body, providing in the other of the valve element and body, another passageway terminating in a complementarily-shaped port at the same interface, the shape of this other port being such that, when the ports are uncovered by rotation of the valve element, the cross-sectional area of the flow metering orifice defined thereby will be proportional to the angular position of the valve element relative to the body, and selectively increasing the diameter of the hole such that the valve will be nulled when the valve element is in a desired angular position relative to the body.

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying diagrammatic drawings, in which: Figure 1 is a fragmentary vertical sectional view of a single-stage servovalve in accordance with the present invention; Figure 2 is a bottom view thereof; Figure 3 is a transverse vertical sectional view thereof, taken generally on line 3-3 of Figure 1; Figure 4 is a simplified schematic vertical sectional view of a torque motor; Figure 5 is a transverse vertical sectional view, taken generally on line 5-5 of Figure 4, and showing the motor stator in elevation; Figure 6 is a transverse vertical sectional view, taken generally on line 6-6 of Figure 4, and showing one rotor flux plate in elevation; Figure 7 is a perspective view of a spool shown in Figure 1;; Figure 8 is a fragmentary vertical sectional view of a portion of a modified single-stage servovalve, incorporating a position sensing transducer; Figure 9 is a block diagram of the modified servovalve shown in Figure 8; Figure 10 is an enlarged fragmentary view of the valve spool and sleeve shown in Figure 1; Figure 7 1 is a fragmentary transverse vertical sectional view thereof, taken generally on line 11-11 of Figure 10; Figure 12 is a fragmentary transverse vertical sectional view thereof, taken generally on line 12-12 of Figure 10; Figure 13 is a fragmentary transverse vertical sectional view thereof, taken generally on line 13-13 of Figure 10; Figure 14 is a fragmentary transverse vertical sectional view thereof, taken generally on line 14-14 of Figure 10;; Figure 15 is a fragmentary transverse vertical sectional view thereof, taken generally on line 15-15 of Figure 10; and Figure 16 is a fragmentary transverse vertical sectional view thereof, taken generally on line 16-16 of Figure 10.

Referring now to the drawings, and more particularly to Figure 1 thereof, there is shown a single-stage servovalve generally indicated at 10. Valve 10 is shown as broadly including a valve body 11 provided with a horizontally, elongated through-bore 12, a sleeve 13 arranged in bore 10, a valve spool 14, a rotary torque motor 15 including a rotor 16 and a stator 18, and a cover 19 mounted on body 11 and protectively enclosing the torque motor 15. Persons skilled in this art will appreciate that, because of its many passageways, the sleeve 13 is normally formed separately for convenience in manufacturing, and subsequently assembled into the body. After such insertion and assembly, the sleeve becomes a practical part of the body.Hence, as used herein, the term "body" is intended to mean either a body having the various ports, passages and surfaces of the sleeve, or as incorporating a separately-formed sleeve, unless otherwise specified. Also, servovalve 10 is adapted to handle a wide range of serviced fluids, either liquids or gases.

Referring now to Figures 1 and 2, the body 11 is shown as having a somewhat L-shaped vertical cross-section. The upper portion of the body is provided with a stepped horizontal through-opening 20 to accommodate mounting of an electrical connecton 21 via fasteners 22. Thus, connector 21 is adapted to be supplied with suitable electrical signals from an external source (not shown), and to supply such signals via conductors 23 to the torque motor 15. As previously mentioned, the lower portion of body 11 is provided with a horizontally-elongated through-bore 12. A plurality of shallow annular recesses are shown as extending into the body from the cylindrical wall defining body bore 12 at various axially-spaced locations, to mate with various passageways provided in sleeve 13.Four separate passageways through the body communicate five axially-spaced discrete places along bore 12 with four ports on the bottom of the body (Figure 2). For convenience, these four bottom parts are denominated pressure (P), return (R), control 1 (C1) and control 2 (C2). The five discrete places of bore 12 represent, from left to right in Figure 1, return (R), control (C1), pressure (P), control (C1), pressure (P), control 2 (C2), and return (R). As best shown in Figure 1, a hole 24 communicates bottom port C1 with the second bore position (C1), and a hole 25 communicates bottom port C2 with the fourth bore position (C2). The two bore end return positions, representing bore positions one and five, communicate with bottom return port R through suitable passageways (not shown).

Similarly, the bore middle position (P) communicates with bottom pressure port P via a suitable passageway (not shown). Thus, pressurized fluid from a suitable source (not shown) is supplied through bottom pressure port P to the middle bore position, the two end ports, are connected to a suitable drain or sump (not shown) via return ports R, and control ports C1 and C2 are used to control the flow of fluid with respect to a suitable hydraulic or pneumatic device. For example, control ports C1 and C2 may communicate with the opposite chambers of a hydraulic actuator having a piston arranged in a cylinder.

A suitable end cap 26, having a somewhat U-shaped cross-section, sealingly closes the right end of body bore 12, and is secured to the body via fasteners 28.

Referring now to Figure 10, the sleeve 13 is shown as being a horizontally-elongated tubular member operatively arranged in body bore 12. Sleeve 13 has an annular vertical left end face 29 arranged to abut end plate 44, an annular vertical right end face 30 abutting a portion of end cap 26, a cylindrical outer surface 31, and a cylindrical inner surface 32. A pair of annular recesses 33, 34 extend radially outwardly from inner surface 32 and rightwardly from the left and right end faces, respectively, to severally accommodate a U-shaped resilient or elastomeric seal member 35 acting between the sleeve and the spool. The sleeve is provided with a plurality of holes, severally machined diametrically therethrough, which holes are axially-spaced from one another so as to be aligned with the five bore positions.From left to right in Figure 1, the sleeve includes a first hole 37 communicating with the body return position R, a second hole 38 communicating with control port position C1, third and fourth holes 39,40 severally communicating with body pressure position P, fifth hole 41 communicating with body control port C2, and a sixth hole 42 communicating with body return port R. The sleeve is shown as being further provided with a plurality of annular recesses intermediate holes 37-42, each of which accommodates a suitable O-Ring to sealingly separate one fluid passage from another. The sleeve is shown as being non-rotatively mounted on the body by means of a pin 43 (Figure 1) interconnecting the sleeve with the end cap 26. If desired, the sleeve may be further provided with one or more pressure relief passages (not shown) to alleviate pressure build-up at appropriate places between the sleeve and spool. The sleeve is appropriately held in this operative position by means of a left vertical end plate 44 (Figure 1) suitable secured to the body by means of fasteners 45.

Still referring principally to Figure 10, the valve element is specifically shown as being in the form of a valve spool 14 mounted within sleeve 13 for rotative movement relative thereto. Spool 14 has an annular vertical left end face 46, an annular vertical right end face 48, and a stepped outer surface including a cylindrical surface 49 extending rightwardly from left face 46, a leftwardly-facing annular vertical shoulder 50 rotatably engaging end plate 44, a cylindrical surface 51 continuing rightwardlytherefrom and arranged within the sleeve, a rightwardly-facing annular vertical shoulder 52 rotatably engaging end cap 26, and a substantially cylindrical portion 53 continuing rightwardly therefrom and arranged within the recess of end cap 26.The spool is provided with six openings 37'-42', severally machined diametrically therethrough, which openings are spaced from one another so as to be axially aligned with corresponding sleeve holes 37-42, respectively. An axial hole 54 extends rightwardly into the spool from its left end face 46, and communicates diametrical spool openings 37', 38' and 39'. The left end of hole is closed by a suitable plug 55. Another axial hole 56 extends leftwardly into the spool from its right end face 48, and communicates diametrical spool openings 40', 41', and 42'. The right end of hole 56 is closed by a suitable plug 58.

As best shown in Figures 11-16, each of the six diametrical holes 37-42 is machined (or otherwise formed) through the sleeve from the same angular position, (i.e. as shown, from top to bottom). However, the corresponding diametrical openings 37'-42' are machined through the spool from different angular positions.Thus, when the spool is in the angular position, shown in Figures 11-16, spool port 37' is rotatably nulled on the edge of sleeve port 37 (Figure 11), spool control port 38' communicated fully with sleeve control port 38' (Figure 12), spool pressure port 39' is rotatably nulled on sleeve pressure port 39 (Figure 13), spool pressure port 40' is rotatably nulled on sleeve pressure port 40 (Figure 14), spool control port 41' communicated fully with sleeve control port 41 (Figure 15), and spool return port 42' is rotatably nulled on sleeve return port 42 (Figure 16). Thus, in the position shown in Figures 11-16, there is no flow (other than annular leakage flow) from pressure to return because all pressure and return ports are in their nulled conditions.If the spool were to move from this initial position in a clockwise direction relative to the sleeve, control port 38 would communicate with now-open return port 37, and control port 41 would communicate with now-open pressure port 40. Alternatively, if the spool were to be moved from the position shown in Figures 11-16 in a counter-clockwise direction relative to the sleeve, control port 38 would communicate with now-open pressure port 39, and control port 41 would communicate with now-open return port 42. In this manner, the angular position of the spool relative to the sleeve may be used to meter the flow of fluid via control ports C1 and C2 to a pneumatically- or hydraulically-operated device (now shown).Another desirable feature of this construction is that the spool is pressure-balanced in that pressure of the serviced fluid is applied to diametrically-opposite positions of the spool, as Figures 11, 13, 14 and 16 indicate.

Another feature lies in the cooperative shapes of the spool and sleeve ports, and in the method by which these ports are formed. Persons skilled in this art will appreciate that it is highly desirable to have the fluid output of the valve be a linear function of spool position relative to the sleeve and body. At the same time, the valve must be nulled, as shown in Figures 11, 13, 14 and 16, when the spool is in a particular angular position. To achieve such proportionality, the cross-sectional area of the flow-metering orifices defined by uncovering each overlapping spool and sleeve port should vary linearly with angular spool position. In the preferred embodiment, various radial round holes 37-42 are provided in the sleeve. Hence, the operative mating edges of the mating spool ports must be complementarily shaped.Thus, to the extent that the operative edge of a sleeve port is concave along the axis of rotation of the spool, the mating edge of its cooperative spool port are convex, as severally indicated at 37', 39', 40' and 42' in Figure 7, so that when these pairs of ports are nulled, the two mating edges will be superimposed in substantially line contact.

Hence, when the spool is thereafter moved relatively in the appropriate angular direction so as to uncover the underlying port, the cross-sectional area of the flow-metering orifice so defined will increase substantially proportionally with continued spool rotation. These convex openings in the spool may be formed by an appropriate electrical discharge machining (EDM) technique through access holes 36 (Figure 7). Having thus formed flow metering ports in the spool having convex edges, the concave edges of the mating sleeve ports may be "nulled" by selectively and individually enlarging the diameters of the holes 37, 39,40 and 42 through the sleeve so that when the spool is in a particular angular position relative to the sleeve, the metering orifice between each desired cooperative spool and sleeve port will be just closed.Any of a variety of known techniques, such as honing, jig boring, or electric discharge machining, may be used to selectively enlarge the sleeve holes.

Referring now to Figures 1 and 3, four rectangularly-spaced lugs 59, 60, 61 extend leftwardly from plate 44, and are arranged in cooperative pairs. The lugs 59, 60 and 61,62 of each cooperative pair are severally provided with a tapped hole, aligned with its cooperative mate. A yoke assembly 63, including a split clamp 64, is secured to an extended cylindrical portion 49 of valve spool 14 via bolts 66,66 so as to be positioned between the four lugs. Each clamp section has a rightwardly-extending arm 68, and a leftwardly-extending arm 69. The left arms 69, 69 are provided with spring-receiver recesses 70 arranged to face one another. A stop pin 71 is threaded into each tapped hole of right lugs 59,60 so as to provide adjustable stops for limiting the extent of rotational movement of the spool relative to the body.Threaded pins 72,72 are threaded into the tapped holes of left lugs 61,62. A helical compression spring 73 is arranged to act between each left arm spring receiver recess 70 and a pin 72. Hence, these springs exert opposing forces on the left arm to bias the spool toward a centered angular position (Figures 11-16) relative to the body. The individual magnitudes of these spring forces may be varied by selectively threading or unthreading pins 72,72 relative to left lugs 61, 62 so as to achieve a desired position of zero net force. Clockwise rotation of the spool relative to the body (Figure 3) will increase the force from the upper bias spring and reduce that from the lower, thereby creating a counter-clockwise torque which acts to recenter the spool, and vice versa.

As best shown in Figure 1, the torque motor is mounted on the body within the protective casing 19. A structurally-simplified embodiment of the torque motor is depected in Figures 4-6 for ease of understanding.

There, torque motor 15 is shown as including a rotor 16 and a stator 18. The rotor is mounted fast to the left end portion of the valve spool via keyed connection 77 for rigid coupled movement therewith. Moreover, in the presently preferred embodiment, the valve spool provides the sole bearing supportforthe rotor.

As best shown in Figures 4-6, the stator 18 includes a segmented core 75 suitably secured to the body by fasteners 65 and encircling the valve spool. The core also supports four rectangularly-shaped coils, severally indicated at 76. Each individual coil 76 provides on either side of the core, a sheet of current carrying conductors, severally indicated 78,78', for a purpose hereinafter explained. These four coils may be connected in series or in parallel, but are oppositely wound and/or selectively energized, so as to induce series-bucking flux within the core as indicated by magnetic polarities "N" and "S" in Figure 5.

The rotor is shown as indicating a pair of axially-spaced left and right magnet carriers 79,79', each of which is securely connected to the spool. These two magnet carriers are arranged as mirror images of one another. As best shown in Figure 6, four permanent magnets, severally indicated at 81, and preferably formed of samarium cobalt, are mounted in two cooperative pairs on each magnet carrier. Stationary flux plates 80, 80' (Figure 1) are secured to the body in close proximity to the outer faces of each set of magnets by fasteners 65, 65. As best shown in Figure 4, the sets of magnets on the left and right sides of the torque motor are arranged to face one another with their magnetic poles "N" and "S" as indicated, with a coil 76 positioned therebetween.The space between each sheet of coil conductors 78, 78' and each facing magnet is an active air gap, severally indicated at 82,82'. The adjacent magnets of each cooperative pair, 81 in Figure 6, have their opposite poles, "N" and "S", facing into the air gaps 82. Likewise, the poles of the oppositely-facing magnets, 81", have opposing polarities facing air gaps 82'.

Thus, the torque motor has a first magnetic flux path including the magnets 81 of one cooperative pair, stationary flux plate 80, active air gaps 82, the facing sheets of current-carrying conductors 78, and core 75. In like manner, the torque motor has a second magnetic flux path including the magnets 81 of the other cooperative pair located on magnet carrier 80, stationary flux plate 80, active air gaps 82, the facing sheets of current-carrying conductors 78, and core 75. Similarly, a third and fourth magnetic flux path are formed by pairs of cooperative magnets 81', stationary flux plate 80', air gaps 82', and core 75.

The force developned by current in a conductor located transversely in a magnetic field of flux is expressed by the equation: F = ies where F = lateral force; = = currentin conductor; e = length of conductor; B = flux in air gap.

The direction of this force will be mutually perpendicular to the directions of the current and the magnetic flux. The conductors run radially with respect to the axis of rotation of the rotor, so the torque developed by a single conductor in one current sheet is: Tc = J =Ir2idrB)r r1 r1 where Tc = torque due to current in one conductor; r = radius of conductor; r, r2 = inner and outer radius of sheet of; current carrying conductors dr = de = incremental length of conductor Since the air gap flux, B, is constant, for every ready-state condition of current, the torque per conductor will be:: Tc = iB r22-r21 = B R L 2 where R = r2 + r1 = average radius of sheet of current carrying conductors, and L = r2 - r1 = length of conductor The direction of flux in air gap 82' is opposite to that in air gap 82, and the direction of current in sheet 78' is opposite to that in sheet 78, so that the torque developed by forces on each side of each coil will be additive. Also, the adjacent coils of a cooperative pair are energized and/or wound so as to give additive torques. Likewise, the magnet, coil winding and current polarities associated with the second set of active air gaps are arranged to produce additive torques. Therefore, the total torque developed will be: T = i (8NBRL) where T = total torque; N number of conductors in each coil.

Since all parameters inside the brackets are constant, it is seen that the magnitude of the motor torque will be proportional to the magnitude of the current, and the direction of torque (i.e. clockwise or counterclockwise) will be determined by the polarity of the current.

The steady-state torque created by electrical current in the four coils 76 is balanced by appropriate deflection of bias springs 73. In this manner, the supplied current will produce a desired rotation of the spool relative to the sleeve.

A modified single-stage servovalve is shown in Figure 8, in which the same numeral primed indicates the corresponding structure heretobefore described. In this embodiment, however, end cap 26 has been replaced by a plate 84 mounted on the body via fasteners 28, and provided with an axial hole 85 through which an extended portion 86 of spool 14' extends. This extended portion 86 is splined and is appropriately coupled to a position sensing transducer, generally indicated at 88, which is mounted on a suitable frame 89 and encased within a protective cover 90. An electrical connector 91 penetrates cover 90. Functionally, the position sensing transducer 88 is arranged to produce an electrical signal reflective of the actual angular position of the spool relative to the body.

Afunctional block diagram of this modification is shown in Figure 9. An electrical command signal, reflective of the desired angular spool position, is supplied to a summing point 92. A feedback signal developed by spool position transducer 88 is also supplied to summing point 92. An error signal is supplied from the summing point to an amplifier 93 which supplies current to the coils of single stage servovalve 10 to produce a rotational displacement of the spool. The error signal is the algebraic sum of the command signal and the negative feedback signal. Hence, if the spool is actually in the commanded position, the error signal will be zero. If such actual position differs from the commanded position, an appropriate error signal will be supplied to the torque motor coils to produce corrective movement.

Modifications Many modifications and changes may be made. The various materials of construction and design shapes may be readily varied. The use of a position sensing transducer is clearly optional. The provision of centering springs 73 is optional, particularly if a position sensing transducer is employed. The dynamic seals 35 at the ends of the spool may be omitted and laminar leakage that would then pass these sealing spaces would be ported to return. Also, a torsional flexure tube as shown in U.S. Patent No. 2,835,265 could be utilized between the body and the spool to provide zero leakage between the torque motor cavity under cover 19 and the fluid cavities contained in body 11. The torsional spring restraint of such a torsion tube could replace the centering force of spring 73. The particular engagement of the spool sleeve and body may be changed.If desired, the sleeve can be made to rotate relative to the spool and body. The method of forming the cooperatively shaped ports may likewise be varied. The shapes of the cooperative ports need not invariably by concave and convex, although this is preferred. The specific torque motor shown is but one species of a iarger class of electro-mechanical actuators. Such an actuator, it its motion is rotary, may have separate rotational support for the rotor, in which case the rotor could be attached to the rotary valve member by a suitable anti-backlash coupling or a flexure coupling, such as illustrated at 94 in Figure 8, for attaching the position transducer 88. While the disclosed embodiment employs eight permanent magnets in the torque motor, a lesser or greater number may be used, as desired. If desired, the coil or coils may be mounted on the rotor, and the magnets mounted on the same element, if so desired. The four coils need not necessarily be connected in series or parallel circuit energized by a single command source. Instead, these four coils may be energized individually from four separate command sources, such as in a quad-redundant flight control system. The particular material of which the permanent magnets are formed is not critical, although samarium cobalt is presently preferred.

The single-stage servovalve particularly described duplicates many of the performance characteristics of a two-stage servovalve.

Claims (41)

1. A single-stage servovalve arranged to control a fluid output in response to an electrical command signal, comprising a body provided with a bore and having a passageway terminating in a port arranged to face into said bore, a valve spool arranged in said bore for rotary movement relative to said body, said spool being provided with a passageway terminating in a port arranged to face the wall of said bore, said body and spool ports defining a variable area flow-metering orifice therebetween, and an electromechanical actuator having a rotor, said rotor being directly coupled to said valve element so as to move rotatably therewith.

2. A single-stage servovalve as claimed in claim 1, wherein said valve element provides the sole bearing support for said rotor.

3. A single-stage servovalve as claimed in claim 1 or claim 2, wherein said electromechanical actuator is a torque motor.

4. A single-stage servovalve as claimed in any one of claims 1 to 3, wherein one of said ports has a circular transverse cross-section.

5. A single-stage servovalve as claimed in claim 4, wherein said one port is the body port.

6. A single-stage servovalve as claimed in any one of claims 1 to 3, wherein said body includes a sleeve and wherein said body port is provided on said sleeve.

7. A single-stage servovalve as claimed in claim 6, wherein the port on said sleeve has a circular cross-section.

8. A single-stage servovalve as claimed in claim 4, wherein the other of said ports is bounded by an arcuate edge.

9. A single-stage servovalve as claimed in claim 8, wherein said edge is convex.

10. A single-stage servovalve as claimed in any one of claims 1 to 9, further comprising spring means operatively arranged to urge said valve element to a particular angular position relative to said body.

11. A single-stage servovalve as claimed in any one of claims 1 to 10, further comprising a position sensing transducer arranged to sense the angular position of said valve element and operatively arranged to supply such sensed position as a negative feedback signal.

12. A single-stage servovalve as claimed in claim 11, further comprising means for supplying the algebraic sum of a command signal and said negative feedback signal is an error signal to said electromechanical actuator.

13. A single-stage servovalve as claimed in any one of claims 1 to 12, further comprising a fluid seal separating said body from said electromechanical actuator.

14. A single-stage servovalve comprising a body, a valve element rotatably mounted on said body, and an electromechanical actuator having a rotor, said rotor being mounted fast with said valve element for rotation therewith, said valve element providing bearing support for said rotor.

15. A single-stage servovalve as claimed in claim 14, wherein said valve element is operatively arranged to meter a flow of fluid through said body.

16. A single-stage servovalve as claimed in claim 15, further comprising a spool and sleeve, the relative positions of which meter the flow of fluid through said body, and wherein said valve element is one of said spool and sleeve.

17. A single-stage servovalve as claimed in claim 16, wherein said sleeve is stationary with said body.

18. A single-stage servovalve as claimed in claim 14, wherein said valve element is a spool.

19. A single-stage servovalve as claimed in claim 18, wherein each of said body and spool is provided with a fluid flow passageway terminating in a port, the extent to which said body and spool ports overlap one another determining a variable orifice through which fluid may flow, and wherein said body and spool ports are co-operatively shaped such that the flow control area of said orifice is substantially proportional to the relative angular positions between said spool and body.

20. A single-stage servovalve, comprising a body, a valve element rotatably mounted on said body, a torque motor having a rotor, said rotor being directly coupled to said valve element for rotation therewith, a stator, at least one permanent magnet mounted eccentrically on one of said rotor and stator, a first flux path through said rotor and stator, said first flux path including a first air gap, and at least one coil mounted on one of said rotor and stator and arranged in said first air gap such that current supplied to said coil will produce a force on said rotor whereby said force will produce a torque on said rotor.

21. A single-stage servovalve as claimed in claim 20, including a second air gap, and wherein a second permanent magnet is mounted on said one of said rotor and stator and arranged to provide flux in said second air gap, said one coil being mounted such that a portion of the coil conductors are located in said first air gap and another portion located in said second air gap, said magnets and air gaps being cooperatively arranged such that forces produced in both air gaps by a coil current will produce additive individual torques on said rotor.

22. A single-stage servovalve as claimed in claim 21, wherein two coils are mounted on the other of said rotor and stator, each of said coils being appropriately wound such that, when a current is supplied to said coils, the individual torques exerted on said rotor will be additive.

23. A single-stage servovalve as claimed in claim 20, wherein four magnets are mounted on said one of said rotor and stator, and wherein said magnets are arranged in two cooperative pairs such that their magnetic potentials will be additive.

24. A single-stage servovalve as claimed in claim 23, wherein four coils are mounted on the other of said rotor and stator, and wherein each of said coils is arranged in said air gap and appropriately wound such that, when a current is supplied to said coils, the torques exerted on said rotor will be additive.

25. A single-stage servovalve as claimed in claim 20, further comprising a second flux path through said rotor and stator, at least one other permanent magnet arranged in said second flux path and eccentrically mounted on said one of said rotor and stator, said second flux path including a second air gap, and wherein said coil provides a first sheet of current-carrying conductors in said second air gap, such that when current is supplied to said coil, the torques exerted on said rotor will be additive.

26. A single-stage servovalve as claimed in claim 25, wherein said first and second flux paths share a common path in one of said rotor and stator.

27. A single-stage servovalve as claimed in claim 20, wherein each of said magnets is mounted on said rotor, and each of said coils is mounted on said stator.

28. A single-stage servovalve as claimed in claim 20, wherein each magnet is mounted at the same radial distance from the axis of said rotor.

29. A single-stage servovalve as claimed in claim 25, wherein each coil is arranged between magnets of said first and second flux paths.

30. A single-stage servovalve as claimed in claim 20, and further comprising a resilient member acting on said rotor in a direction to oppose such electrically-created torque.

31. A single-stage servovalve as claimed in claim 30, wherein said rotor is resiliently biased to a particular angular position relative to said body.

32. A single-stage servovalve as claimed in claim 30, wherein the magnitude of the angular displacement of said rotor is proportional to the magnitude of the current supplied to said coil.

33. A single-stage servovalve as claimed in claim 20, further comprising a position sensing transducer arranged to sense the angular position of said rotor and to provide a negative feedback signal, and wherein the current supplied to said coil is related to the algebraic sum of a command signal and said negative feedback signal.

34. A single-stage servovalve as claimed in claim 20, wherein each of said coils is positioned at the same radial arm distance from the axis of said rotor.

35. A servovalve comprising a body, a valve element movably mounted on said body and operatively arranged to meter a flow of fluid through said body, and a torque motor mounted on the body and having an electrically-movable member, said valve element being arranged so as to provide the sole bearing support for said torque motor movable member.

36. A servovalve comprising a body, a rotatable valve element mounted on said body, the angular position of said valve element relative to said body defining a variable-area orifice adapted to meter a flow of fluid, and a torque motor mounted on the body and having a rotor, said rotor being mounted on said valve element for rotational movement about the rotational axis of said valve element.

37. A servovalve as claimed in claim 36, wherein said valve element provides the sole bearing support for said rotor.

38. A method of nulling a rotatable valve element relative to a body, which comprises the steps of forming a hole in one of said valve element and body to provide a fluid flow passageway terminating in one port arranged to face the other of said valve element and body, providing in the other of said valve element and body a fluid flow passageway terminating in another port arranged to face said one of said valve element and body, shaping such other port such that when said ports lap one another the size of the orifice defined thereby will be substantially proportional to the angular position of said valve element relative to said body, and selectively enlarging the diameter of said one port such that said valve will be nulled when said valve element is in a desired angular position relative to said body.

39. A method as claimed in claim 38, wherein said other port is shaped by an electric discharge machining technique.

40. A method as claimed in claim 38, wherein said one port is honed to enlarge its diameter.

41. A servovalve substantially as hereinbefore described with reference to the accompanying drawings.