GB2119855A

GB2119855A - Fluid-pressure rotary actuator

Info

Publication number: GB2119855A
Application number: GB08309418A
Authority: GB
Inventors: John T Caruso
Original assignee: Moog Inc
Current assignee: Moog Inc
Priority date: 1982-04-12
Filing date: 1983-04-07
Publication date: 1983-11-23
Also published as: FR2524944A1; JPH0235163B2; GB2119855B; JPS58178001A

Abstract

In an actuator that may be one of a set of three connected fluidically in parallel and mechanically in series so as to produce a wrist-like action, Figs. 5 and 6, a passage 16 is provided in the output shaft 12 to facilitate conduction of working fluid from an inlet passage 66 to e.g. the adjacent actuator. The shaft may also be provided with a passage 20 through which working fluid can be discharged from e.g. the adjacent actuator, into an outlet passage 19. The actuator is controlled by a valve e.g. a servo- valve, 13 and may be furnished with a transducer 100 to indicate the angular position of the shaft. <IMAGE>

Description

SPECIFICATION Fluid-powered actuator This invention relates to fluid-powered actuators, and more particularly to a rotary actuator which may be usefully employed in a wrist actuator capable of movement about multiple axes.

A fluid-powered rotary actuator is basically a device for converting pressurized fluid flow into rotational movement of a shaft relative to a body.

In recent years, there has been increasing interest in the development of robots for the performance of mechanical acts, such as on automobile assembly lines and the like. Such robots have been designed with degrees of freedom comparable to human joints. The human wrist, for example, is capable of rotational movement about three mutually perpendicular axes, and may be selectively positioned by extension or retraction of the person's arm. The desire to develop satisfactory robots has lead to a need for actuators simulating human joints.

According to the present invention there is provided a rotary actuator comprising a body, a shaft rotatably mounted on said body and having one portion arranged within said body and another portion arranged outside said body, a valve having a supply port which, when provided with a supply pressure, is selectively operable to provide a control pressure, and a motor arranged to receive said control pressure and operative to urge said shaft to rotate relative to said body, and also comprising a body supply passage provided in said body and extending between an inlet port for receiving supply pressure, said valve supply port, and a supply commutating port exposed to a surface of said one portion of said shaft, and a shaft supply passage provided within said shaft and extending from said supply commutating port to an outlet port opening onto a surface of said other portion of said shaft, whereby supply pressure provided to said body inlet port will also be provided to said other portion of said shaft at a location outside said body.

The present invention also provides a rotary actuator comprising a first actuator having a first valve, a first body, a first shaft rotatably mounted on said first body and having one portion arranged within said first body and having another portion arranged outside said first body, a first motor associated with said first valve and operative to urge said first shaft to rotate relative to said first body, and a supply passage communicating an inlet port on said first body with said first valve and with an outlet port on said first shaft other portion, and a first bracket mounted fast to said first shaft other portion for rotation therewith, said first bracket being provided with a supply passage extending between an inlet port in communication with said first shaft outlet port, and an outlet port, whereby pressurized fluid supplied to said first body inlet port will be provided at said first bracket outlet port.

The invention will now be particularly described with reference to the accompanying drawings in which: Figure 1 is a longitudinal vertical sectional view of a rotary actuator according to the invention showing a servovalve in elevation: Figure 2 is a horizontal sectional view of the actuator taken on line 2-2 of Figure 1; Figure 3 is a transverse vertical sectional view of the actuator taken on line 3-3 of Figure 1; Figure 4 is an exploded view of the actuator of Figure 1; Figure 5 is a perspective view of a wrist actuator employing three of the rotary actuators according to the invention and two brackets; and Figure 6 is a view of the wrist actuator shown in Figure 5, but showing the wrist actuator as having been articulated to a "flattened" condition.

As used herein, expressions such as "left", right", "up", "down", and the like, are oriented to the indicated drawing figure as it faces the reader.

As shown in Figures 1 to 4, the rotary actuator 10 comprises a body 11, a shaft 12 rotatably mounted on the body, a servovalve 13, and a vane-type motor 1 4 (Fig. 3) operatively arranged to urge the shaft to rotate relatively to the body.

Interconnected supply passages 1 5, 1 6 are provided in the body 11, and the shaft 12, respectively, so that supply pressure provided to a body supply passage inlet port 18 from some external source (not shown) is provided to the valve, and is also made available at the distal end of the shaft, for a purpose hereinafter discussed.

Preferably, these supply passages are accompanied by communicating body and shaft return passages 1 9, 20 which are available to convey a flow of return fluid from the valve and/or the distal end of the shaft to a body return passage outlet port 21.

Body 11 As best shown in Figures 1 and 4, the body is formed by suitably assembling a left section 22, an intermediate section 23, and a right section 24.

Body left section 22 is shown as including, inter alia, a planar vertical right face 25, from which a large diameter annular recess extends leftwardly into the left section to accommodate an O-Ring 26; an opposite annular vertical left face 28, from which a circular recess extends righwardly into the left section to accommodate an O-Ring 29; and a specially-configured throughbore communicating the end faces and including, in pertinent part (from left to right in Fig. 4), an internally-threaded portion 30 extending rightwardly from left face 28, an inwardlyextending annular boss 31, a leftwardly-facing annular vertical shoulder 32, and a cyjindrical surface 33 continuing rightwardly therefrom to join right face 25.

A bearing assembly 34 is mounted on the left section with its right face arranged adjacent shoulder 32, and has its outer race 35 engaging the inwardly-facing cylindrical surface of boss 31.

The bearing's inner race 36 engages a proximate portion of shaft 12.

An annular left end cap 38 is mounted on the left section. End cap 38 has an annular vertical right face 39 engaging the left face of bearing assembly 34, has an externally-threaded portion 40 extending leftwardly therefrom and adapted to engage left section internally-threaded portion 30, and has a rightwardly-facing annular vertical surface 41 arranged to compressively engage 0 Ring 29 and abut section left face 28. An annular corner recess extends into the end cap from its inwardly-facing cylindrical surface 42 and its right face 39 to accommodate a resilient annular seal 43, having a somewhat C-shaped cross-section.

The inwardly facing cylindrical surface of seal 43 wipingly engages a proximate portion of shaft 12.

The body intermediate section 23 is shown as having a planar vertical left face 44 arranged to compress O-Ring 26 and abut left section right face 25, a planar vertical right face 45, and an uppermost planar horizontal surface 46, against which the servovalve is mounted. The intermediate section is also provided with a specially-configured through-bore having a somewhat C-shaped appearance when viewed in transverse cross-section (Fig. 3). Specifically, this through-bore is bounded by a relatively-large diameter cylindrical surface segment 48 occupying an arc of about 280 degrees, a smaller diameter cylindrical surface 49 occupying an arc of about 60 degrees, and planar surfaces 50, 51 symmetrically joining these surfaces.

The intermediate section is further provided with a pluraiity of drilled holes which form parts of the body supply and return passages. Specifically, a horizontal hole 52 extends through the intermediate section above its through-bore, and is intersected by a hole 53 extending downwardly from section upper surface 46. Holes 52 and 53 constitute a part of the body return passage 1 9. If desired, body hole 52 may communicate with an aligned hole 54 in the left body section to relieve pressure in the vicinity of the bearing. A horizontal hole 55 extends leftwardly into the intermediate section from its right face, and is intersected by a hole 56 extending downwardly from upper surface 46. Holes 55 and 56 constitute part of the body supply passage 1 5.

The right body section 24 has a planar vertical left face 58, from which a relatively large diameter annular recess extends rightwardly into the section to accommodate an O-Ring 59; an opposite planar vertical right face 60, and pianar horizontal upper and lower surfaces 61,62, respectively. The right face 45 of the intermediate section is arranged to compressively engage 0 Ring 59 and abut the left face 58 of the right section. The right section is also provided with a through-bore bounded by an inwardly-facing cylindrical surface 63, and has an annular recess extending outwardly into the section to accommodate an annular resilient seal 64, the inwardly-facing cylindrical surface of which wipingly engage a proximate portion of the shaft 12.

The right section is provided with a plurality of holes and recesses which form other portions of the body supply and return passages 1 5, 19.

Specifically, a horizontal hole 65 extends into the section from inlet port 1 8, and is intersected by a hole 66 extending upwardly from lower surface 62 to intersect an annular recess 68 encircling the bore, and continuing therebeyond. A horizontal hole 69, aligned with intermediate section hole 55, extends into the right section from its left face to intersect the extension hole 66. Hole 66 is tapped between body lower surface 62 and hole 65 to receive a suitable closure plug 70. A horizontal hole 71 extends leftwardly into the right section from outlet port 21, and is intersected by a hole 72 extending downwardly from the body upper surface 61 to intersect an annular recess 73 surrounding the bore at a location spaced axially from recess 68.

Hole 72 is tapped between body upper surface 61 and hole 71 to receive a closure plug 74. A horizontal hole 75, aligned with intermediate section hole 52, extends into the right section from its left face 58 to intersect hole 72.

The three body sections are assembled together, as shown in Figure 1, and are held in this position by means of four bolts, severally indicated at 76 (Figs. 2-3). The assembled body is thus provided with a supply passage 15, which includes inlet port 18, holes 65 and 66, annular recess 68, and the extension of hole 66, and holes 69, 55 and 56. The body return passage 1 9 includes outlet port 21, holes 71 and 72, recess 73, and holes 75, 52, 53 (and hole 54, if provided).Holes 56 and 53 open onto the intermediate section upper surface 46 to form ports aligned with the supply and return ports (not shown), respectively, of servovalve 1 3. Thus, the body supply passage 1 5 is arranged to convey supply pressure provided to inlet port 1 8 from an external source (not shown) to one location on the shaft, and to the supply port of valve 13. The body return passage 1 9 is arranged to convey fluid from the valve's return port, the chamber housing bearing 34 (if hole 54 is provided), and from another location on the shaft, to outlet port 21.

Shaft 12 The shaft 12 is a horizionally-elongated member having a stepped outer surface, and is operatively arranged in the body bore. Shaft 12 has an annular vertical left end face 78 arranged without or outside the body; an annular vertical right end face 79; and an outer surface including (from left to right in Fig. 4) a cylindrical surface 80 extending rightwardly from left face 78, an annular recess 81, an annular boss having a leftwardly-facing annular vertical shoulder 82, an annular notch 83, a leftwardly-facing annular vertical shoulder 84 rotatably engaging the left section right face 25, a cylindrical surface 85 rotatably embraced by intermediate section arcuate surface 49 (Fig. 3), a rightwardly-facing annular vertical shoulder 86 rotatably engaging the right section left face 58, an annular notch 88, and a cylindrical surface 89 continuing rightwardly therefrom to join right face 79.

The shaft is shown provided with supply and return passages 1 6, 20. The shaft supply passage 1 6 includes a horizontal hole 90 extending into the shaft from its left end face 78. The left marginal end portion of hole 90 may be tapped to receive a closure plug 91. Hole 90 is intersected within the body by a radial hole 92 in registry with annular recess 68, and without the body by a radial hole 93 adjacent left end face 78. The shaft return passage 20 includes a horizontal hole 94 extending rightwardly into the shaft from its left end face 78. The left marginal end portion of hole 94 is tapped to receive closure plug 95. Hole 94 is intersected within the body by a hole 96 in registry with annular recess 73, and without the body by a hole 98 arranged adjacent left end face 78.Thus, the shaft supply passage 1 6 has an inlet port communicating with the body supply passage outlet port, and is arranged to convey fluid to a location (i.e., hole 98) on the shaft beyond the body.

Adjacent its left end, shaft 12 is provided with a tapped hole 99 to accommodate a set screw (not shown) by which some additional structure (not shown in Figs. 1 ) may be coupled to the shaft for rotation therewith. In the preferred embodiment, a rotary position transducer, generally indicated at 100, is mounted on the body right section and engages the shaft via splined connection 101. This transducer is arranged to sense the angular position of the shaft relative to the body and to provide an electrical feedback signal reflective of such relative position.

Together, the combination of a rotary actuator having a shaft rotatably mounted on the body and a motor operative to urge the shaft to rotate relative to the body, a servovalve arranged to control fluid power to the motor, and a feedback transducer to sense angular position of the shaft relative to the body, constitute a rotary servoactuator.

Valve 13 Valve 13 is preferably an electrohydraulic servovalve which has a supply port (not shown) communicating with hole 56, and has a return port (not shown) communicating with hole 53.

Valve 13 also has two control ports C1 and C2, which communicate with the intermediate section bore through holes 102, 103 respectively (Figs. 2-3). Since valve 13 is individually "old" in this art, it will not be explicitly described.

Suffice it to say here that valve 13 is supplied with pressurized fluid via the body and shaft supply passages, communicates with return via the body and shaft return passages, and is selectively operable in response to an electrical command signal to produce differential fluid control of either polarity between ports C1 and C2.

Motor 14 As best shown in Figure 3, a vane 104 extends radially beyond shaft surface 85 and sealingly engages body surface. The vane, thus divides the intermediate section bore into a first chamber 105 arranged to be supplied with control pressure from valve port C, via hole 102, and a second chamber 106 supplied with control pressure from valve port C2 via hole 102. The differential between control pressure C, and C2, acting on the opposing faces of the vane, selectively urges the shaft to rotate in either angular direction relative to the body, according to the polarity of the differential.

Operation of rotary actuator 18 In the rotary actuator, the labyrinth of communicating supply passages function to connect body inlet port 1 8 to the valve supply port (not shown) and to a commutating port consisting of annular cavity 68 and shaft hole 92, thence to shaft outlet port 93. The commutating port serves to maintain the supply pressure connection to the shaft outlet port throughout the range of shaft rotational displacement.Similarly, the communicating return passage connects body return port 98 to the valve return port (not shown) and to the shaft return inlet port 98 by way of commutating ports 73, 96. Thus, fluid power available between body supply and return ports 1 8, 21, will be always provided to the servovalve and to the distal end of the shaft beyond the body, independently of the angular position of the shaft relative to the body. This latter feature is deemed particularly advantageous because the fluid supply is made available for use in powering some other mechanism (not shown in Figs. 4) which may be attached to the end of the shaft. At the same time, the supply and return passages are provided within the body and shaft, and obviate the need for external flexible hoses.

Modifications of rotary actuator 10 The present invention contemplates that many possible changes and modifications may be made to rotary actuator 1 0. For example, the body need not be of a three-piece sectional construction, nor need the various parts, always have the same shape or function as in the preferred embodiment.

While it is preferred that the valve be an electrohydraulic servovalve, other types of valves may be necessarily mounted directly on the body, and the shaft may have different shapes. Also, the commutating ports between the body and the shaft may take various forms, such as annular cavities formed on the shaft communicating with holes formed in the body, or similar cavities and/or holes formed at the end surface of the shaft which would communicate across an interface surface provided between the shaft and the body. The method of construction or assembly is not deemed critical and may be readily changed. For example, the various holes may be cast or otherwise formed, instead of drilled. Other types of motors and/or mechanisms may be substituted for the single vane-type motor of the preferred embodiment.The improved actuator may be serviced by a variety of pressurized fluids (i.e., liquids or gases). In some applications, it may not be necessary to provide the return passages, such as when the valve or other fluid-powered mechanism is vented directly.

Wrist actuator 110 (Figs. 56) The rotary actuator 10 heretofore described may be advantageously employed in an improved wrist actuator, generally indicated at 110 in Figures 5 and 6.

Wrist actuator 110 is shown as including a lowermost first rotary actuator 1 or, an intermediate second rotary actuator 1 OB, and an uppermost third rotary actuator 1 0C. The output shaft 12A (Fig. 6) of the first actuator is arranged vertically in Figure 5. A base 111 of a U-shaped first bracket 112, is mounted fast to the first actuator's output shaft, with its left and right arms 11 3, 114 embracing the body of second actuator 1 OB. The base 115 (Fig. 6) of another U-shaped second bracket 11 6, is mounted on the body of the third actuator 1 OC, and has its right arm 11 7 mounted fast to the second actuator's output shaft 1 2B.The left arm 11 8 of second bracket has its distal marginal end portion journalled in a bearing 11 9 provided on the body of the second actuator 1 OB. The first actuator 1 OA may be mounted on a suitable object (not shown), such as the end of a movable robot arm. As best shown in Figure 6, seiective operation of the first actuator 1 OA causes the first bracket member 112 to pivot about the axis of the first actuator's shaft. The third actuator 10C may be selectively operated to cause desired rotation of its shaft. The third actuator's output shaft 1 2C is adapted to have mounted fast thereon, a suitable tool (not shown) or workpiece (not shown).The first and second actuators 1 OA, 1 OB function to position the third actuator's output shaft relative to an object.

One feature of wrist actuator 110 is the manner by which the supply and return ports of the three servovalves are connected hydraulically in parallel, so as to obviate the need for external hoses which might otherwise interfere with or be unduly fatigued by articulation of the wrist actuator.

As has been previously described, the supply and return passages communicate the body inlet and return ports (18,21 in in Fig. 1-4) with the shaft outlet and inlet ports (93, 98 in Figs.1 4), respectively. Thus, the fluid power supply between the body ports is available at the end of each shaft.

The first bracket 112 is mounted fast to the first shaft 1 2A by means of a set screw 1 20 received in shaft hole 99, and the second bracket is mounted fast to the second actuator output shaft by means of a like set screw. The first bracket includes a pair of horizontal supply and return passageways 121, 122, respectively, which extend rightwardly into the base 111 to communicate with the shaft's supply and return ports 93, 98, respectively. The left marginal end of these first bracket base passageways 1 21, 1 22 are closed by suitable plugs 123, 123. A pair of supply and return passages 124, 125 extend downwardly within the first bracket left arm 11 3 to intersect the base supply and return passages.

The ends of these passageways may be closed by suitable closure plugs, again severally indicated at 123. These arm passageways 124,125 communicate with the supply and return ports 1 8B, 21 B of the second rotary actuator.

In like manner, the second bracket 116 has internal supply and return passages 126, 128 in its base 11 5 and arm 117, which communicate with the second rotary actuator shaft supply and return ports 93B, 98B with the third rotary actuator body supply and return ports 1 8C, 21 C respectively.

Thus, the supply and return ports of the three servovalves are connected hydraulically in parallel, by a labyrinth of communicating internal passageways. Supply pressure provided to the body supply port 1 8A of the first rotary actuator is provided through the first bracket to the body supply port 1 8B of the second rotary actuator, and is further provided through the second bracket to the body supply port 1 8C of the third rotary actuator. Similarly, return fluid from the third actuator's return port 21 C is provided through the second bracket to the second actuator's return port 21 B, and is further provided through the first bracket to the first actuator's return port 21 A. Each servovalve may be operated independently of one another to articulate the wrist actuator to the extent desired.In this manner, the wrist actuator obviates the need for external hoses providing supply and return connections to each servovalve. It should also be noted that each rotary actuator of the three-axis wrist assembly of Figure 5 can be equipped with a rotary position transducer as indicated at 1 0OA, B, C so that wrist motion is accomplished by multi-axis, rotary position servoactuators.

Modifications of wrist actuator 110 As with the rotary actuator, it is contemplated that many changes and modifications may be made. While the preferred embodiment is shown as including three rotary actuators and two intermediate bracket members, a greater or lesser number may be provided. Rotary actuators other than that specifically disclosed, might readily be substituted. The structure of the brackets might also be changed, as, for example, to communicate with the alternative ports on the end faces of the shafts.

Claims

1. A rotary actuator comprising a body, a shaft rotatably mounted on said body and having one portion arranged within said body and another portion arranged outside said body, a valve having a supply port which, when provided with a supply pressure, is selectively operable to provide a control pressure, and a motor arranged to receive said control pressure and operative to urge said shaft to rotate relative to said body, and also comprising a body supply passage provided in said body and extending between an inlet port for receiving supply pressure, said valve supply port, and a supply commutating port exposed to a surface of said one portion of said shaft, and a shaft supply passage provided within said shaft and extending from said supply commutating port to an outlet port opening onto a surface of said other portion of said shaft, whereby supply pressure provided to said body inlet port will also be provided to said other portion of said shaft at a location outside said body.

2. A rotary actuator according to claim 1, wherein said commutating port is provided by an annular recess encircling said shaft to maintain communication between said body inlet port and said shaft outlet port independently of the angular position of said shaft relative to said body.

3. A rotary actuator according to claim 1 or claim 2, wherein said shaft other portion surface is an end face of said shaft.

4 A rotary actuator according to claim 1 or claim 2, wherein said shaft other portion surface is the circumferential surface of said shaft.

5. A rotary actuator according to any preceding claim wherein said valve is selectively operable to provide differential fluid control between said control pressure and a second control pressure, said differential fluid control being provided to said motor such that said shaft can be urged to rotate bidirectionally relative to said body in response to the polarity of said differential control pressure.

6. A rotary actuator according to claim 5, wherein said motor is a vane-type motor.

7. A rotary actuator according to any preceding claim, wherein said valve has a return port, and further comprising a shaft return passage provided within said shaft and extending between an inlet port opening onto a surface of said shaft other portion and a return commutating port opening onto a surface of said shaft one portion at a location spaced from said pressure commutating port, and a body return passage provided in said body and extending between said return commutating port, said valve return port, and a body output port, whereby the fluid power available between said body inlet port and said body outlet port will be provided to said valve and will also be provided to said shaft other portion at a location outside said body.

8. A rotary actuator according to claim 7, wherein said valve is an electrohydraulic servovalve.

9. A rotary actuator comprising a first actuator having a first valve, a first body, a first shaft rotatably mounted on said first body and having one portion arranged within said first body and having another portion arranged outside said first body, a first motor associated with said first valve and operative to urge said first shaft to rotate relative to said first body, and a supply passage communicating an inlet port on said first body with said first valve and with an outlet port on said first shaft other portion, and a first bracket mounted fast to said first shaft other portion for rotation therewith, said first bracket being provided with a supply passage extending between an inlet port in communication with said first shaft outlet port, and an outlet port, whereby pressurized fluid supplied to said first body inlet port will be provided at said first bracket outlet port.

10. A rotary actuator according to claim 9 and further comprising a second actuator having a second valve, a second body, a second shaft rotatably mounted on said second body and having one portion arranged within said second body and having another portion arranged outside to said second body, a second motor associated with said second valve and operative to urge said second shaft to rotate relative to said second body, and a second supply passage communicating an inlet port on said second body with said second valve and with an outlet port on said second shaft other portion, said second body being mounted on said first bracket with said second body inlet port in communication with said first bracket outlet port, whereby pressurized fluid supplied to said first body inlet port will be provided to said second valve, and to said second shaft other portion.

11. A rotary actuator according to claim 10 and further comprising a second bracket mounted fast to said second shaft other portion for rotation therewith, said second bracket being provided with an internal supply passage between an inlet port in communication with said second shaft outlet port and an outlet port, whereby pressurized fluid supplied to said first body inlet.

port will be provided at said second bracket outlet port.

12. A rotary actuator according to claim 10 or claim 11, and further comprising a third actuator having a third valve, a third body, a third shaft rotatably mounted on said third body and having one portion arranged within said third body and another portion arranged outside said third body, a third motor associated with said third valve and operable to urge said third shaft to rotate relative to said third body, and a third supply passage communicating an inlet port on said third body with said third valve, said third body being mounted on said second bracket with said third body inlet port in communication with said second bracket outlet port, whereby pressurized fluid supplied to said first body inlet port will be provided to said third valve.

1 3. A rotary actuator according to claim 10, wherein the axis of said second shaft is substantially perpendicular to the axis of said first shaft.

14. A rotary actuator according to claim 12, wherein the axis of said third shaft is substantially perpendicular to the axis of said second shaft.

1 5. A rotary actuator according to claim 12, wherein the axis of said second shaft is substantially perpendicular to the axis of said first shaft, and wherein the axis of said third shaft is substantially perpendicular to the axis of said second shaft.

1 6. A rotary actuator according to claim 12, wherein said second bracket is provided with an internal return passage between an inlet port in communication with said third body outlet port and an outlet port, whereby return fluid from said third valve will be provided to said second bracket outlet port.

1 7. A rotary actuator according to claim 16, wherein said second actuator is provided with a return passage having an inlet port on said second shaft other portion in communication with said second bracket outlet port, and communicating with said second valve and with an outlet port on said second body, whereby return fluid from said second and third valves will be provided to said second body outlet port.

18. A rotary actuator according to claim 17, wherein said first bracket is provided with an internal return passage between an inlet port in communication with said second body outlet port, whereby return fluid from said second and third valves will be provided to said first bracket outlet port.

19. A rotary actuator according to claim 18, wherein said first actuator is provided with a return passage having an inlet on said first shaft other portion in communication with said first bracket outlet port and communicating with said first valve and with an outlet port on said first body, whereby return fluid from said first, second and third valves will be provided to said first body outlet port.

20. A rotary actuator according to claim 9 and further comprising a first position transducer operatively arranged between said first body and said first shaft.

21. A rotary actuator according to claim 10 and further comprising a second position transducer operatively arranged between said second body and said second shaft.

22. A rotary actuator according to claim 12 and further comprising a third position transducer operatively arranged between said third body and said third shaft.

23. A rotary actuator substantially as herein described with reference to the accompanying drawings.