Field of the Invention [0001] This invention relates to an actuator, i.e. a means of transforming electrical energy into mechanical energy, producing relatively low linear speed, high torque movements from relatively swiftly changing electromagnetic fields. The invention is particularly suited to portable applications requiring a high degree of efficient control, e.g. applications in which human-like movement needs to be simulated or interacted with. Such applications include actuators for prosthetic limbs, prostheses and orthoses, and for service and remotely operated robots.

Background of the Invention [0002] Electrical rotary actuators typically take the form of rotary stepping motors and brushless motors mechanically connected to harmonic drives, ball-screws, and planetary gear transmissions.

[0003] There is a need for a rotary actuator that has certain advantageous characteristics. For certain applications, an actuator should be compact, lightweight, powerful and efficient. Ideally it should also be highly integrated and provide a highly controllable transformation of electrical to mechanical energy. This technical agenda is driven by the attempt to make a practical and competitively performing actuator that can be portable, and may be used in prosthetics, robotics and automation.

[0004] In the prior art, such as EP 1306963, Outrunner' type rotary actuators are known. However, it is a general problem in the field that Outrunner' actuators such as these do not provide any form of gearing, thereby making them inappropriate for certain fields of application. The present invention aims to solve this problem by providing a rotary actuator with integral gearing. Summary of the Invention

[0005] Aspects of the present invention are defined by the accompanying claims. In a preferred embodiment of the invention, there is a stator and a first, second and third cylindrical elements, wherein the stator and the first and the second and the third cylindrical elements being arranged concentrically around a central axis. The third cylindrical element is mounted around the second cylindrical element, the second cylindrical element is mounted around the first cylindrical element, and the first cylindrical element is mounted around the stator. The stator comprises electromagnetic sectors, the first cylindrical element comprises permanent magnetic elements magnetised radially, the second cylindrical element comprises alternating sectors of high and low magnetic permeability, and the third cylindrical element comprises permanent magnetic elements magnetised radially. Phased magnetisation of the stator causes the first cylindrical element to rotate around the central axis, thereby causing the second cylindrical element to exert a rotational force around the central axis.

[0006] Efficient transformation of energy is achieved by providing a motor with an integral gearing mechanism that uses changing electromagnetic fields of a stator to propel a rotor element within this gearing mechanism. Integrating the use of magnetic and electromagnetic fields in both the primary torque generating components and also the transmission components, means that friction is absent. By using a rotor element that aligns with a helical path, the work done by the rotor is better matched, by the mechanical advantage of the pitch of the helix and the diameter of the rotor, to the forces and torques required, e.g. as exerted by human limbs of a comparable scale. The absence of conventional mechanical gears and screws, and the associated friction generated by these components, makes for an efficient actuator that is also very quiet in use, making it highly appropriate for prosthetic devices in which undue operational noise can draw unwanted attention to the wearer. [0007] The first cylindrical element is preferably mounted around the stator, with the second cylindrical element mounted around the first cylindrical element, and the third cylindrical element mounted around the second cylindrical element. Preferably, the first cylindrical element and the second cylindrical element are free to rotate relative to an axis, with the stator and the third cylindrical element rotationally fixed relative to that axis. The second rotational cylindrical element is attached to a drive arm and is used to exert a rotational force relative to the stator. [0008] The first cylindrical element may comprise permanent magnetic elements magnetized radially and arranged in first and second axially separated helices on its external surface, the magnetic elements of the first helix being of opposite polarity to the magnetic elements of the second helix. The first cylindrical element may also comprises different permanent magnetic elements magnetized radially and arranged in first and second axially separated helices on its internal surface, the magnetic elements of the first helix and the second helix creating a pattern of axial stripes of alternate magnetic polarity on the first cylindrical element's internal surface.

[0009] The second cylindrical element may comprise radially alternating sectors of high and low magnetic permeability.

[0010] The third cylindrical element may comprise permanent magnetic elements magnetized radially and arranged in first and second axially separated helices, the magnetic elements of the first helix being of opposite polarity to the magnetic elements of the second helix.

[0011] Preferably the stator has N sectors and the first cylindrical element comprising permanent magnetic elements arranged has at least one helix with at least N+l sectors. More preferably the first cylindrical element comprising permanent magnetic elements arranged has at least one helix with at least 4N/3 sectors.

[0012] A method for producing rotational force is provided, comprising providing a stator comprising electromagnetic sectors for generating phased electromagnetic fields around the stator, providing a first cylindrical element, comprising permanent magnetic elements magnetised radially, providing a second cylindrical element having alternating sectors of high and low magnetic permeability, and providing a third cylindrical element, comprising permanent magnetic elements magnetised radially. The stator and the first, second and third cylindrical elements are arranged concentrically around a central axis with the first and second cylindrical elements being free to rotate relative to the stator. Magnetising the sectors of the stator in angular phases to causes the first cylindrical element to rotate around the central axis, thereby causing the second cylindrical element to exert a rotational force around the central axis.

[0013] The absence of mechanical interconnection between the relatively moving rotor and stator parts means that when the stator is un-powered, there are no electromagnetic fields to constrain the rotor and it can rotate freely along the helical path as long as radial magnetic cogging torque between the stator and rotor parts are designed to be minimal or non-existent. This feature is useful if the actuator is used to propel the hip joint of a prosthetic lower-limb, as it enables the limb to be extended using the momentum generated by the movement of the persons intact body sections (hip, glutteal and core muscles), permitting the person to walk naturally with the device for long periods without using portable electrical battery supplies. This feature is also useful in service robotics as it presents a failsafe feature for active joints that come into close proximity to people, permitting the robot's joints to be entirely limp and back-drivable once electrical power is removed from the robot. In contrast, if the off-power magnetic cogging torque between the rotor and stator are designed to produce a certain torque value then this will be amplified by the mechanical advantage of the magnetic helix.

[0014] The stator is preferably made from a stack of a number of relatively thin ferromagnetic electrical steel laminates stacked and bonded together and electrically insulated from one another. Using this method produces a stator 'core' that can provide a good path for magnetic flux and relatively poor electrically conductive path, in this way reducing unwanted eddy currents and so promoting efficiency. In such cases, cogging torque is tailored by (i) increasing the circular air gap between the stator and rotor (but this approach significantly reduces wanted on-power torque), (ii) reducing the differential reluctance to the magnetic flux as the rotor turns, i.e. making the air gap slot openings as small as possible (wide air gap = high magnetic flux reluctance), (iii) skewing (partially radially offsetting) the laminates in the build of the stack to partially (or fully) span the air gap relative to the depth of the stator. [0015] A further benefit of the absence of mechanical interconnection between rotor and stator sections is that the actuator is more durable, as contaminant ingress such as; dust, dirt and sand are not as damaging to its function, as they might be to a precisely toleranced mechanical transmission. This is particularly useful to a portable device that may be used outdoors. The absence of mechanical parts in the transmission also adds to the longevity of the actuator and the operational duration between service intervals.

[0016] The electromagnetic engagement between the rotor and stator sections means that there is a high degree of control that can be achieved in the movement and force generated by the actuator; additionally, the intrinsic electrical control of this engagement means that this can be changed very rapidly and subtly. This is particularly appropriate for an actuator that is used as a replacement prosthetic hip, as it permits the actuator to simulate the action of a highly controllable variable stiffness damper enabling the person in one instance high-stiffness stable low-speed movement, whilst in another instance low-stiffness efficient high rate mobility. This feature is also appropriate to orthoses that support intact but poorly functioning human limbs, as in the case of limbs exhibiting tremors, where through the swift control of the actuator mechanically linked in parallel to the person's own limb it might be used to damp unwanted tremors whilst supporting force and movement of their desired movement. The subtly of control is also appropriate for using the actuator for haptic devices that need to simulate the forces exerted on remote or virtual objects to the operator, such devices include fly-by-wire joysticks and haptic styluses and other tools in the control of teleoperated surgery. [0017] The electrical control and electromagnetic engagement of the relatively moving parts means that positioning can be achieved with high repeatability and with zero backlash. This is especially appropriate to positioning systems such as computer- controlled optical devices, and computer-controlled machine tools. [0018] The invention integrates torque generating and transmission stages into a single combined stage, aiding a minimal bulk and mass unit to be created. High power and small mass actuator units are appropriate to many portable devices and especially appropriate to actuators that need to be supported from a person's body for a sustained duration, as is the case with prosthetic limbs.

[0019] Preferred embodiments of the invention are now described, by way of example only, with reference to the accompanying drawings.

Brief Description of the Drawings

[0020] There now follows, by way of example only, a detailed description of preferred embodiments of the present invention, with reference to the figures identified below:

Fig. 1 is a perspective view of a complete actuator unit in accordance with an embodiment of the invention.

Figs. 2a to 2d show an exploded view of concentric parts of an actuator motor contained within the actuator unit of Fig. 1.

Figs. 3a and 3b show a cross-sectional view and an end-view of the first rotational cylindrical element of Fig. 2c.

Fig. 4 is a perspective view of the constructed actuator motor of Fig. 2

Fig. 5 shows an end-view of the stator inside the first rotational cylindrical element.

Figs. 6a and 6b show a diagrammatic representation of the magnetised stator component of Fig. 5 separated into three phases.

Figs. 7a to 7f show the preferred stator components in cross-section with different phase states.

Figs. 8a and 8b show the preferred first rotational cylindrical element and stator components in cross-section with different phase states.

Fig. 9 shows an exploded view of the second rotational cylindrical element, the first rotational cylindrical element and the outer cylinder when diagrammatically represented as flat plates.

Fig. 10 shows a magnified view of a portion of Fig. 9.

Figs. 11a and lib show a schematic view of the outer cylinder overlaying the first rotational cylindrical element in a flat diagrammatic representation for the purpose of demonstrating the determination of high -permeability bar locations. Figs. 12a to 12m show a schematic view of the elements of Fig. 11a in a flat diagrammatic representation for the purpose of demonstrating an example gearing.

Detailed Description of the Embodiments

[0021] In the following description, functionally similar parts may carry the same reference numerals between figures. Preferred embodiments of the invention are now described, by way of example only, with reference to the accompanying drawings. [0022] Referring to Fig. 1, an actuator unit is shown, including a first drive arm 101 and a second drive arm 103, a protective outer cover 105, and cabling 107 routed through the centre of the actuator unit. The first drive arm 101 and/or the second drive arm 103 is configured to be attached to a load. The internal structure of the actuator unit and its mechanism of operation will be described below from Fig.2 onwards.

[0023] In operation, the first drive arm 101, upon application of electric current to the actuator motor in manners described below and provided through cabling 107, rotates relative to the actuator unit whilst the second drive arm 103 remains static relative to the actuator unit. The first drive arm 101 and/or the second drive arm 103 is connected to a load to exert a rotational force on the load. It can apply a rotational force in either rotational direction. Significantly, it can also resist a force in both of these directions in the power-off state. Its ability to resist a force can, in some circumstances, be greater than its ability to rotate a load against an applied force. [0024] Referring to Fig. 2, an exploded view of concentric parts of an actuator motor contained within the actuator unit of Fig. 1 is shown. The central axis 202 represents the common axis between all of the concentric parts when they are constructed into the single actuator motor. Referring to Fig. 2a, a detailed view of an outer cylinder 201 of the actuator motor is shown. The outer cylinder 201 is attached to a back-plate 203. The outer cylinder 201 is made from a non-ferromagnetic material and includes a helically drilled arrangement of holes assembled with magnets with a similar pole facing radially outward and other magnets with a similar pole facing radially inward. These magnets are powerful, and are preferably rare-earth magnets such as neodymium iron boron magnets, but may also be ceramic, alnico, samarium cobalt, injection molded or flexible magnets. The magnets are configured to have poles orientated along radial projections in a pattern described below. These permanent magnetic elements of the outer cylinder 201 may be formed as magnetized plugs mounted in radial holes formed in a cylinder of non-ferromagnetic material. Means may be provided to retain each magnetic plug in its hole. E.g. each plug may have an interference fit with its respective hole, or each plug and hole may be screw-treaded, or the plugs may be glued in place, or the holes may not pass completely through the outer cylinder 201, to retain the plugs at their inner end or outer end, with (optionally) a thin tight-fitting sleeve to retain them at the opposite (outer or inner) end. The permanent magnetic elements may have, for instance, circular or square faces in the radial direction, or any other appropriate shape. Alternatively, the permanent magnetic elements of the outer cylinder 201 may be formed to have a cross- section being a sector of a circle. [0025] An alternating helical array is arranged in the outer cylinder 201 with separate threads 205 and 207 containing a different magnetic pole, for example magnetic thread 205 may contain outwardly north facing magnets and magnetic thread 207 may contain outwardly south facing magnets. As shown, the alternating magnetic pattern formed by the helical threads 205 and 207 is repeated around the external surface of the outer cylinder 201 to form an alternating helical magnetic pattern.

[0026] Referring to Fig. 2b, a detailed view of a second rotational cylindrical element 209 is shown. When the actuator motor of Fig. 2 is constructed, the second rotational cylindrical element 209 is internally concentric to the outer cylinder 201, and is separated from the outer cylinder 201 by a small radial air-gap. The second rotational cylindrical element 209 is free to rotate about its axis relative to the outer cylinder 201, and is coupled to the stator by means of appropriate ball-bearing race elements. These ball-bearings also serve to prevent axial translation and to maintain the small radial air- gap between second rotational cylindrical element 209 and the outer cylinder 201. The second rotational cylindrical element 209 is provided with a circumferentially alternating arrangement of bars 211 and windows 213, wherein the bars 211 are made of a material with a high magnetic permeability and the windows 213 are made of a material with a low magnetic permeability. For example, the magnetically permeable bars 211 may be made of materials such as Metglas, Mu-metal, iron, nickel, electrical steel and cobalt alloys materials, and the windows 213 may be made of materials such as aluminium, ceramic and plastic. Alternatively, the windows 213 may be air gaps. In a preferred embodiment, the windows are webs of the same material as the bars, but of reduced thickness. This arrangement lends itself to manufacture in material such as nickel that is easily machined, e.g. electrochemically. Alternatively, other strong low- permeability material such as nylon may be used to improve structural integrity and withstand torque and mechanical stress on this component. Nylon or other polymer can be co-molded by first forming the high permeability part and then injection molding the polymer.

[0027] The second rotational cylindrical element 209 further includes an end-cap 215 which comprises various means for attaching the first drive arm 101 of Fig. 1 to the second rotational cylindrical element 209.

[0028] Referring to Fig. 2c, a detailed view of a first rotational cylindrical element 217 is shown. When the actuator motor of Fig. 2 is constructed, the first rotational cylindrical element 217 is internally concentric to the second rotational cylindrical element 209, and is separated from the second rotational cylindrical element 209 by a small radial air-gap. The first rotational cylindrical element 217 is free to rotate about its axis relative to the outer cylinder 201 in a similar manner to the second rotational cylindrical element 209, which is similarly facilitated by means of appropriate ballbearing race elements These ball-bearings also serve to prevent axial translation and to maintain the small radial air-gap between the first rotational cylindrical element 217 and both the second rotational cylindrical element 209 and the stator 223 (described below). The first rotational cylindrical element 217 is made from a non-ferromagnetic material and includes a helically drilled arrangement of holes assembled with magnets with a similar pole facing radially outward and other magnets with a similar pole facing radially inward. These magnets are powerful, and are preferably rare-earth magnets such as neodymium iron boron magnets, but may also be ceramic, alnico, samarium cobalt, injection molded or flexible magnets. The permanent magnetic elements are configured to have poles orientated along radial projections in a pattern described below. These permanent magnetic elements of the first rotational cylindrical element 217 may be formed as magnetized plugs mounted in radial holes formed in a cylinder of non-ferromagnetic material. Means may be provided to retain each magnetic plug in its hole. E.g. each plug may have an interference fit with its respective hole, or each plug and hole may be screw-treaded, or the plugs may be glued in place, or the holes may not pass completely through the first rotational cylindrical element 217, to retain the plugs at their inner end or outer end, with (optionally) a thin tight -fitting sleeve to retain them at the opposite (outer or inner) end. The permanent magnetic elements may have, for instance, circular or square faces in the radial direction, or any other appropriate shape. Alternatively, the permanent magnetic elements of the outer cylinder 201 may be formed to have a cross-section being a sector of a circle.

[0029] An alternating helical array is arranged with separate threads 219 and 221 containing a different magnetic pole, for example magnetic thread 219 may contain outwardly north facing magnets and magnetic thread 221 may contain outwardly south facing magnets. For diagrammatic purposes, the first rotational cylindrical element 219 is shown with only a single outwardly north facing magnetic helical row 219 and a single outwardly south facing magnetic helical row 221. Further, the magnetic pattern of the threads as formed on the external surface of the first rotational cylindrical element 217 is different and not merely opposite the magnetic pattern of the threads as formed on the internal surfaces of the first rotational cylindrical element 217, this is described in more detail below with reference to Fig. 3a. In a preferred embodiment, the alternating magnetic patterns formed by threads 219 and 221 on the internal and external surfaces of the first rotational cylindrical element 217 is repeated around the internal and external surfaces of the first rotational cylindrical element 217.

[0030] Referring to Fig. 2d, a detailed view of a stator 223 is shown. When the actuator motor of Fig. 2 is constructed, the stator 223 is internally concentric to the first rotational cylindrical element 217, and is separated from the first rotational cylindrical element 217 by a small radial air-gap. Further, the stator 223 is fixed to the back -plate 203 of the outer cylinder 201, making the outer cylinder 201 and the stator 223 fixed with respect to each other, unable to rotate. The stator 223 is made from a highly ferromagnetic material for most high torque applications and greatest axial restraint, but it may be constructed from non-ferromagnetic material if zero off -power cogging torque is priority. The stator 223 is provided with ferromagnetic radial projections, each being wound with electrically conductive laminated wire. These form an array of radial electromagnets 225. Preferably, each electromagnet pole (e.g. outward north pole) runs the entire axial length of the stator. This interacts with the permanent magnetic threads 219 and 221 on the internal surface of the first rotational cylindrical element 217. Stator 223 further comprises a circuitry and sensory portion 227, including for instance optical sensors and magnetic sensors, for instance Hall effect sensors, for closed loop control of phase currents to the stator 223 and precise rotary position encoders used in closed loop positioning control.

[0031] The air gaps and the non-ferromagnetic material present in the constructed actuated motor create reluctance to the flux created by the electromagnet. This reluctance reduces the flux greatly with distance (approx = k * l/distance ^A 3). For instance, the flux is negligible at the external diameter of the outer cylinder 201 and presents minimal deleterious influence against the second rotational cylindrical element 209.

[0032] Referring to Fig. 3a, a cross-sectional view of the first rotational cylindrical element 217 is shown. The first rotational cylindrical element 217 is provided with the external magnetic threads 219 and 221 as described above. The internal magnetic arrangement of the threads 219 and 221, as seen from inside the first rotational cylindrical element 217, is different and not merely opposite the external magnetic arrangement of the threads 219 and 221 as seen from the outside of first rotational cylindrical element 217. This difference in magnetic arrangement is achieved by using different lengths of permanent magnet inserted into the drilled threads 219 and 221, allowing for a single drilled hole either of the threads 219 or 221 of the first rotational cylindrical element 217 to contain either single or multiple permanent magnets. This difference is demonstrated with reference to the possibility of inserting two permanent magnets inserted into a single hole 301, and/or the possibility of inserting a single permanent magnet into a single hole 303.

[0033] On the internal surface of the first rotational cylindrical element 217, the helical threads 219 and 221 are magnetically arranged such that an axial pattern of magnetic stripes 305 and 307 is created, each corresponding to a stripe of a single magnetic polarity, for example magnetic stripe 305 is internally north facing and magnetic stripe 307 is internally south facing. This results in a radially alternating magnetic pattern on the internal surface of the first rotational cylindrical element 217, forming stripes 305 and 307 of a single polarity running axially down the length of the first rotational cylindrical element 217. On the external surface of the first rotational cylindrical element 217, the helical threads 219 and 221 are magnetically arranged such that each magnetic helical array 319 and 321 contain only a single magnetic polarity. This results in an axially and radially alternating magnetic pattern on the external surface of the first rotational cylindrical element 217, but wherein each helix 319 and 321 contains only a single outward facing magnetic polarity. Magnetic stripes 305 and 307, and external alternating polarity magnetic helices 319 and 321 may extend to the full length of the first rotational cylindrical element 217, as when physically implemented there would be a greater number of helical threads 219 and 221 on the first rotational cylindrical element 217 which are currently omitted for diagrammatic purposes. The first rotational cylindrical element 217 further includes a code-wheel 309 fixed into an end of the first rotational cylindrical element 217.

[0034] Referring to Fig. 3b, an end-view of the first rotational cylindrical element 217 is shown, with detail of the code-wheel 309 clearly visible. The code-wheel 309 includes radial groups of north facing magnets 311 and radial groups of south facing magnets 313, arranged in a circle on the face of the code-wheel 309. The radial groups of magnets 311 and 313 are arranged to correspond to the internal axial magnetic stripes 305 and 307, in order to be aligned magnetic stripes 305 and 307 of identical polarity to themselves. During construction of the first rotational cylindrical element 217, the radial magnet groups 311 and 313 are orientated to register with axial magnetic stripes 305 and 307 of identical polarity, with the code-wheel 309 being fixed into location in the end of the first rotational cylindrical element 217 in this orientation by means of locating pins 315. In this way, the code-wheel 309 is fixed to the first rotational cylindrical element 217 and rotates in equal measure with it during actuator operation. Code-wheel 309 further includes code-pattern 317 preferably comprising multiple equally spaced bars of alternating optically reflective and non -reflective material. However, code-pattern 317 may instead be replaced by a magnetic code-pattern of finely spaced alternating polarity magnets, or by optical elements comprising multiple equally space bars of alternating transmissive and non-transmissive materials. Further, the code-pattern 317 may also be changed from equally spaced elements to any of Gray Scale, Nonius or Vernier patterns to provide absolute angular encoding. The code- pattern 317, when compared to the radial magnets 311 and 313, provides a finer graduation of rotary orientation and movement. As such, the detection of the orientation of the radial magnets 311 and 313 by sensors in the circuitry and sensory portion 227 is used primarily to determine the next electromagnetic phase to apply to electromagnets 225, whereas the detection of the code-pattern 317 may be used for more subtle rotary positioning and speed regulation purposes. In a preferred embodiment, a conceptually similar code-wheel to code-wheel 309 is also provided between the second rotational cylindrical element 209 and the end-cap 203 of the outer cylinder 201. In a preferred embodiment, the rotation of the code-pattern 317 is also measured by sensors in the sensory and control circuit 227, wherein the output from the sensors measuring the rotation of the code-pattern 317 is input into motor control circuitry in circuitry portion 227, thereby providing a more accurate, or subtle, rotary positioning and speed regulation functionality.

[0035] In operation, upon application of a controlled, phased electric current, the radial electromagnets 225 become magnets of phased polarity, as described in detail below with reference to Figs. 6, 7, and 8. The arrangement of electromagnetic poles created on the electromagnets 225 of the stator 223 interact with the stripes of alternating polarity 305 and 307 on the internal surface of the first rotational cylindrical element 217. As the first rotational cylindrical element 217 is free to rotate relative to the stator 223, the magnetic force interactions created between the first rotational cylindrical element 217 and the stator 223 cause the first rotational cylindrical element 217 to rotate in its attempt to find a stable magnetic orientation, commonly referred to as 'alignment torque'. Through the configuration of alternating magnetic stripes 305 and 307 on the internal surface of the first rotational cylindrical element 217, the configuration of magnetic poles created on the electromagnetic array 225 urges the first rotational cylindrical element 217 to a position whereby there is alignment between north poles on the inner surface of the first rotational cylindrical element 217 with south poles on the stator 223 and vice-versa. At any given phase position, the design may be such that the alignment is complete. In the preferred embodiment, alignment is incomplete to increase torque, but clockwise offsets and anti -clockwise offsets are balanced. The electrical current through the electromagnets 225 will then be altered as described below, causing further rotation as the first rotational cylindrical element 217 is forced to seek a new stable configuration. In this manner the first rotational cylindrical element 217 is rotated relative to both the stator 223 and the outer cylinder 201.

[0036] As the first rotational cylindrical element 217 rotates, the orientation of the second rotational cylindrical element 209 is constrained by the magnetic interaction between the helical magnetic arrangement 319 and 321 on the external surface of the first rotational cylindrical element 217 and the magnetic threads 205 and 207 of the outer cylinder 201 that encase it. The magnetic flux created between magnetic threads 205 and 207 of the outer cylinder 201 and the magnetic arrangement 319 and 321 of the external surface of the first rotational cylindrical element 217 causes the second rotational cylindrical element 209 to settle in a particular orientation as the high- permeability bars 211 align in accordance with the magnetic flux (this is described in further detail below with reference to Figs. 9 and 10). The attractive and repulsive magnetic forces between the helical threads 205 and 207 of the outer cylinder 201 and the helical magnetic arrangements 319 and 321 on the outer surface of the first rotational cylindrical element 217 interact with the low reluctance flux path represented by the high-permeability bars 211 of the second rotational cylindrical element 209 to constrain the second rotational cylindrical element 209 to particular orientations. The interleaved magnetic threads of opposite polarity 319 and 321 on the outer surface of the first rotational cylindrical element 217 constrain the interacting high-permeability bars 211 to movement only in the manner dictated by the magnetic flux paths created between the outer cylinder 201 and the first rotational cylindrical element 217. In this way, the first rotational cylindrical element 217 achieves rotational axial movement resulting in the rotation of the first drive arm 101. Rotational axial movement is constrained to the gearing as defined by the relative pitch of the magnetic threads 205 and 207 in comparison to magnetic threads 219 and 221. Rotational axial movement that in any way does not conform to this gearing would entail the high-permeability bars 211 of the second rotational cylindrical element 209 moving towards an orientation that is not the state of lowest magnetic equilibrium, i.e. an orientation that is not aligned with the path of magnetic flux, or the path of least magnetic reluctance, created between the magnetic arrangement 319 and 321 of the first rotational cylindrical element 217 and the magnetic threads 205 and 207 of the external cylinder 201 that encase it. This movement is hence doubly opposed by the helical magnetic arrangement 319 and 321 on the external surface of the first rotational cylindrical element 217 being attracted to magnetic poles of its facing helical magnetic threads 205 and 207 of opposite polarity on outer cylinder 201, and also by the helical magnetic arrangements 319 and 321 on the outer surface of the first rotational cylindrical element 217 being repulsed by the adjacent facing helical magnetic threads 205 and 207 of similar polarity on the outer cylinder 201. The specific gearing defined by the relative pitch of the helical threads 203 and 205 on the outer cylinder 201 when compared to the helical threads 219 and 221 on the first rotational cylindrical element 217 is described in detail below.

[0037] The above process describes the rotation of the second rotational cylindrical element 209 that is entailed by application of single controlled phased electric current to the radial electromagnets 225, causing the electromagnets 225 to become magnets of single polarity. In full operation of the actuator unit, the above process is repeated multiply through repeated application of numerous controlled, phased electric currents, causing the radial electromagnets 225 to become magnets of differing phased polarities as time progresses and the phased electric current pattern applied changes - this process is described in further detail below with reference to Figs. 6, 7 and 8. The determination of which phase of the controlled electrical current is to be applied at any given time is based on a determination of the current orientation of at least the second cylindrical element 217. The process of determination is described below.

[0038] In operation, any rotation of the first rotational cylindrical element 217 causes the code-wheel 309 to rotate in equal measure. With reference to Fig. 2, when the first rotational cylindrical element 217 is caused to rotate by the above described electromagnetic mechanisms, the code-wheel 309 rotates equally. The rotation of the code-wheel 309 is detected by the sensors located in the sensory and circuitry portion 227 of the stator 223, for instance Hall Effect sensors, through measuring the changing magnetic flux caused by the rotation of the radial magnet groups 311 and 313. As these magnetic groups 311 and 313 are arranged to correspond with the polarity of the magnetic stripes 305 and 307 of the first rotational cylindrical element 217, in this way, the orientation of the first rotational cylindrical element 217 is measured. The electrical signals arising from this measurement may be stored and processed by circuitry in the sensory and circuitry portion 227, and also provided as input to motor control circuitry also within the circuitry portion 227, wherein the motor control circuitry calculates which phase is to be next applied to the radial electromagnets 225. In a preferred embodiment, there is also a conceptually similar code-wheel to code-wheel 309 provided between the second rotational cylindrical element 209 and the end-cap 203 of the outer cylinder 201. In this configuration, it would be possible via similar mechanisms as described above in relation to the code-wheel 309 to measure the actual output position of the second rotational cylindrical element 209, from which computations arising from the difference between calculated and actual output position of the second rotational cylindrical element 209 could be used to deduce the output torque.

[0039] In this way, electric current creating a magnetic field in the electromagnets 225 is converted to a rotational axial movement of the second rotational cylindrical element 209. This rotational axial movement of the second rotational cylindrical element 209 results in a rotational axial movement of the first drive arm 101 attached to the second rotational cylindrical element 209, as the first drive arm 101 is coupled to the second rotational cylindrical element 209 by the end-cap 215. In this way, the actuator creates a rotational force, which it can apply in either rotational direction around the axis 202. In a preferred embodiment, the rotation of both the second rotational cylindrical element 209 and the first rotational cylindrical element 217 is measured by sensors in the sensory and circuitry portion 227, wherein the output from these sensors is input into a motor control circuitry which may then calculate which phase is next to be applied to the electromagnets 225 based on the current orientation of the second rotational cylindrical element 209 and/or the first rotational cylindrical element 217. [0040] Referring to Fig. 4, the complete actuator motor is shown, comprising the concentric elements of Fig. 2. The end-cap 215 of the second rotational cylindrical element 209 is shown inside the outer cylinder 201. [0041] Referring to Fig. 5, an end-view of the stator 223 is shown inside the first rotational cylindrical element 217. The stator 223 is made from a highly ferromagnetic material for most high torque applications and greatest axial restraint, but it may be constructed from non-ferromagnetic material if zero off-power cogging torque is priority. The stator 223 is provided with ferromagnetic radial projections, each being wound with electrically conductive laminated wire. These form an array of radial electromagnets 225. Preferably, each electromagnet pole (e.g. outward north pole) runs the entire axial length of the stator. This interacts with the permanent magnetic threads 219 and 221 of the first rotational cylindrical element 217. The radial array of electromagnets 225 is comprised of multiple 'teeth' 501, each of which can be individually addressed with an independent electrical current in order to facilitate a pattern of electromagnetic phases across the 'teeth' 501, as described in further detail below. [0042] Referring to Fig. 6, a diagrammatic representation of the stator of Fig. 5 is shown separated into three electrical phases. The electrical operation of the radial electromagnets 225 is displayed using truth-tables 601, in which current flowing in each of the winding circuits A, B and C is shown in its relative state of positive '+', negative '-' or no current 'Ο'. This shows a simple method of electrical energising, referred to as 'block commutation'; however, this pattern may be represented by sinusoids offset by 120 degrees; referred to 'sinusoidal commutation' or trapezoidal wave forms or custom waveforms, but all following the basic block commutation pattern.

[0043] Referring to Fig. 6a and Fig. 6b, the electrical current configuration of the electromagnetic arrays 225 of the stator 223 is expanded upon. Fig. 6a shows a complete diagram of the circuit arrangement of the electromagnetic arrays 225 as they occur on the stator 223. Fig. 6b shows the winding circuits A, B and C separated for clarity. [0044] Referring to Figs. 7a to 7f, a detailed cross-sectional view of the subsequent stages of rotation between the first rotational cylindrical element 217 and the stator 223 is shown. The views 7a to 7f show the magnetic arrangement across a full electrical cycle that constitutes the relative rotary motion between the first rotational cylindrical element 217 and the stator 223. An indicative dot 701, that is fixed with respect to the first rotational cylindrical element 217, is added to the diagram to aid comprehension. A group of four permanent magnets 707 is shown grouped together to form a single band, i.e. axial magnetic stripe 305 or 307 of the first rotational cylindrical element 217 as shown in Fig. 3a. As such, the individual magnetic pole pairs that comprise each magnetic group 707 in Fig. 7 represent only the magnets comprising the internal magnetic arrangement of threads 219 and 221 as seen from the inside of the first rotational cylindrical element 217, as described with reference to Fig. 3a above. For simplicity, the magnets that comprise the outer magnetic arrangement of threads 219 and 221 as seen from the outside of the first rotational cylindrical element 217 are not shown, but if were diagrammatically represented they would form a circular arrangement around each of the magnet groups 707.

[0045] In this figure, N can represent a north pole and S can represent a south pole. Neutral poles have no polarity indications. Electromagnet 501 is a radial electromagnet as shown in the electromagnetic array 225 of Fig. 5. A diametric pole-pair is shown on the stator 223, formed by two diametrically opposed sectors of permanent magnets 703a and 703b. A corresponding pole-pair 705 of the stator 223 is shown. The pole-pair 703a and 703b is magnetized with south radially outwards and north radially inwards. The pole-pair 705 is magnetized (in Fig. 7b) with south radially outwards and north radially inwards.

[0046] In operation, the electrical current is directed through each winding circuit A, B and C in the order shown in the truth -tables 601, Figs. 7a to 7f. The varying states of magnetization of electromagnets 501 interact with the alternating magnetic bands 707, i.e. magnetic stripes 305 and 307 of the first rotational cylindrical element 217, resulting in relative rotary motion of the first rotational cylindrical element 217. At any stage of the electrical cycle the electric current is applied as illustrated. In response to this change in magnetic field, there is a rotation in the first rotational cylindrical element 217 towards a position of registration between the radial electromagnets 501 and the alternating bands 707, i.e. 305 and 307, that constitute the internal surface magnetic pattern of first rotational cylindrical element 217. Each subsequent stage of the electrical cycle applies the electric current as illustrated, and the same process of alignment occurs. In this way, a continued rotation of the first rotational cylindrical element 217 is achieved.

[0047] By comparing the start location of dot 701 in Fig. 7a, to the final location of dot 701 in Fig. 7f, it can be seen that a full electrical cycle results in only a partial rotation of the first rotational cylindrical element 217.

[0048] By energising the radial electromagnets 501 in the manner shown in Fig. 7, a high on-power radial registration between the first rotational cylindrical element 217 and the stator 223 is created, producing the required torque to controllably rotate the first rotational cylindrical element 217. There is a high electromechanical advantage. Every three arms of the stator 223 (spanning 90 degrees of arc) are energized in three phases, in two magnetic orientations, giving six phase combinations. A full cycle of six electrical phase combinations causes rotation of the rotor (the first rotational cylindrical element 217) to rotate 37.5 degrees. Forty eight phase combinations are required to give a full rotation of the rotor. Each step is 360/48 = 7.5 degrees. More generally, each step angle is given by 360 / (no. of stator pole pairs * no. of rotor pole pairs). If the rotor had only a single pole pair, it would rotate 360 degrees in 6 steps, i.e. each step would be 360/6 = 60 degrees.

[0049] For the given cross-sectional arrangement there are, in the preferred embodiment, 48 sequence steps required to complete a full 360 degree mechanical rotation of the rotor. [0050] In the preferred embodiment, there is a mismatch in the number of alternating band pole-pairs 703 to radial electromagnet pole-pairs 705. The first rotational cylindrical element 217 has more (preferably eight) alternating band pole-pairs 703, and the stator 223 has fewer (preferably six) radial electromagnet pole-pairs 705. This is more clearly illustrated in Figs. 8a and 8b, which shows the stator 223 in first and second phases of magnetization, with the first rotational cylindrical element 217 in the same position (indicated by the dot 701) to illustrate the change in magnetic forces as the phase changes, but before the rotor rotates. [0051] The stator is shown in the first (Fig. 8a) and second (Fig. 8b) phases of magnetization. The varying states of magnetization of electromagnets 501a to 501h interact with the alternating magnetic bands 707 of the first rotational cylindrical element 217, resulting in relative rotary motion of the first rotational cylindrical element 217.

[0052] Each single electromagnet 501 faces a combination of two separate magnetic bands 707, partially facing each of the two bands. As each magnetic band 707 is formed from four (of 64) similarly orientated permanent magnets (or is an arcuate magnet spanning 22.5 degrees of arc), a single electromagnet 501 (spanning 30 degrees of arc) will never experience full registration with a single magnetic band 707. Instead the electromagnet 501 will either be facing: three north poles and one south pole, two north poles and two south poles, or three south poles and one north pole (or their equivalents in terms of degrees of arc).

[0053] The system begins in a state of stability (Fig. 7a and Fig. 8a), with total repulsive magnetic force as experienced by the entire system minimized, and total attractive forces maximised. A rotation only occurs after a change in phase of the electromagnets 501 of the stator 223. The phase change causes a new state (Fig. 8b), which is orientated so that band/sector of permanent magnets is incrementally offset from an electromagnet of opposite polarity. This maximizes the tangential attractive and repulsive magnetic forces, and such forces are all in the same rotational direction (forcing the rotor clockwise). Thus, electromagnet 501a is fully registered; 501b is one permanent magnet (about 5.6 degrees) displaced from full registration; 501c is fully registered; 50 Id is one magnet displaced from full registration; 50 le is fully registered; 501f is one magnet displaced from full registration; 501g is fully registered; 501h is one magnet displaced from full registration. Each magnetized arm of the stator is applying force in the same direction upon the facing magnetic band 707. This cumulative force acts to turn the first rotational cylindrical element 217 in the preferred direction, which in this embodiment is clockwise.

[0054] The numerical mismatch ensures that at least some of the electromagnets 501 are always partially facing two magnetic bands 707, and are never fully registering with a single magnetic band 707 and the first rotational cylindrical element 217 will always have a preferred direction of motion in which it can move. This preferred direction of rotation occurs in part because the electromagnet 501 is always facing at least one element (or a partial segment) of a magnetic band 707 that is of the same polarity as itself and is repulsive.

[0055] After each further change in phase, the mismatch configuration of the magnets results in the first rotational cylindrical element 217 rotating in the desired direction. The magnetic mismatch configuration always creates an unambiguous direction of preferred rotation for the first rotational cylindrical element 217 to move to a stable position.

[0056] The configuration of the magnetic mismatch, formed by the larger number of alternating magnetic bands 707 facing the fewer number of radial electromagnets 501, combined with the three phase (six phase/polarity combination) electrical setup, ensures that the phase change experienced by each of the electromagnets 501 around the entire stator 223 creates the same preferred direction of movement at every point around the first rotational cylindrical element 217. [0057] It is possible to have a system with an equal number of radial electromagnets and alternating band poles. For example, a system could be envisaged with a complete registration of twelve radial electromagnets to twelve alternating band poles, while giving good torque and unambiguous direction. [0058] More generally, preferred configurations comply with the following equation:

K ₀ = — where K ₀ ≠S (Equation 1)

3N

K0 = Integer Slot Offset (offset between permanent magnet poles and stator slots) NS = Number of slots between the radial electromagnets NM = Number of magnets on the second cylindrical element (i.e. fractions of a full circle)

q = any positive integer

S = Coil span (= the number of stator projections, i.e. radial electromagnets, that a coil winding spans)

[0059] The preferred configurations have the following constraints:

(i) there is an even number of magnets (NM) on the second cylindrical element, orientated in such a way that there is an equal number of north and south poles facing the electromagnets.

(ii) for S = 1, as depicted in Figs. 6 and 7, the defining variable is the number of slots (NS) between the radial electromagnets, where NS is required to be a factor of six i.e. NS = n/6 where n is a positive integer. [0060] With reference to (i), Fig. 7 has each alternating band pole 707, formed from four identically orientated bar magnets, representing one value of NM. Hence in Fig. 7 NM = 16.

[0061] With reference to (ii), Figs. 6 and 7 have NS = 12, but it is possible to use any valid value of NS as defined above.

[0062] Referring to Fig. 9, an exploded view of the outer cylinder 201, the second rotational cylindrical element 209 and the first rotational cylindrical element 217 are diagrammatically represented as flat plates. The outer cylinder 201, the second rotational cylindrical element 209 and the first rotational cylindrical element 217 are shown in an aligned position, as would be the case when a particular and non-changing phase current is being applied to the electromagnets 225 of the stator 223 (not shown here for simplicity). As such, a position of equilibrium is shown, wherein the line of magnetic flux 901 created by the interaction of the energized electromagnets 225 of the stator 223, the first rotational cylindrical element 217, the second rotational cylindrical element 209 and the outer cylinder 201 has aligned each of these elements in the orientation of least reluctance. The orientation of least reluctance is that where the first rotational cylindrical element 217 is constrained to rotate until it is aligned in an orientation in accordance with the polarity of the electromagnets 225 of the stator 203, and where the second rotational cylindrical element 209 is subsequently constrained to rotate until it is aligned in an orientation constrained by both the polarity of the first rotational cylindrical element 217 internal to it and similarly by the polarity of the outer cylinder 201 external to it. This is demonstrated at the scale of a single magnet pair in Fig. 10

[0063] Referring to Fig. 10, a magnified view of a single magnet pair of Fig. 9 is shown. A single magnet 1001 of the outer cylinder 201 is shown, as well as a single magnet 1003 of the first rotational cylindrical element 217. The magnetic flux 901 between the outer cylinder 201 and the first rotational cylindrical element 217 serves to align the high -permeability bar 211. The intersection -marking 1005 indicates the location where magnets 1001 and 1003 intersect with high-permeability bar 211; this symbolism is also used in subsequent figures.

[0064] Referring to Fig. 11, a schematic view of the outer cylinder 201 overlaying the first rotational cylindrical element 217 is shown in a flat diagrammatic representation for the purpose of demonstrating how the locations where high-permeability bars 211 are required to be constructed on the second rotational cylindrical element 209 are determined. Referring to Fig. 11a, the dimension labeled H is the circumference of the outer cylinder 201, and the dimension labeled J ⁷ is the length of the outer cylinder 201. Preferably, there must always be an integer number of helical threads arranged on both the outer cylinder 201 and the first rotational cylindrical element 217. The helical threads 1101 represent the helical magnetic threads 205 and 207 of the outer cylinder 201, and the helical thread 1103 represents a single helical magnetic thread 319 or 321 of the first rotational cylindrical element 217. The locations at which the helical threads 1101 of the outer cylinder 201 intersect with the helical thread 1103 of the first rotational cylindrical element 217 are indicated by intersection-markings 1005. These intersection -markings 1005 indicate a location where a high-permeability bar 211 should be constructed to correspond to in order for the high-permeability bars 211 to be able to orientate into positions of least magnetic reluctance. As can be seen, for the relative helical pitches of the outer cylinder 201 and the first rotational cylindrical element 217 of Fig. 11a, the second rotational cylindrical element 209 should be constructed with five high-permeability bars 211. By comparison, for the relative helical pitches of the outer cylinder 201 and the first rotational cylindrical element 217 of Fig. lib, the second rotational cylindrical element 209 should be constructed with six high- permeability bars 211.

[0065] The calculation of the required number of high -permeability bars 211 that should be constructed on the second rotational cylindrical element 209 can be generalised as: I ₁₁ = h ₁₁ + v„ (Equation 2) where

I _n = number of high -permeability bars 211 required;

h„ = number of times the helical threads 1101 of the outer cylinder 201 intersect the H dimension;

v _n = number of times the helical threads 1101 of the outer cylinder 201 intersect the V dimension.

[0066] This is demonstrated in Fig. 11a, where the pitch of the helical threads 1101 of the outer cylinder 201 is such that the helical threads 1101 intersect the H dimension four times, and intersect the V dimension only a single time. Using equation 2, it can be shown that in this instance five high-permeability bars 211 will be required. In Fig. lib, the pitch of the helical threads 1101 of the outer cylinder 201 is such that the helical threads 1101 intersect the H dimension four times, and intersect the J ⁷ dimension two times. Using equation 2, it can be shown that in this instance six high-permeability bars 211 will be required. Highlighted section 1109 shows the location where a high- permeability bar 211 would subsequently be constructed on the second rotational cylindrical element 209 based on the above calculations.

[0067] The general equation for the gearing, defined as the relative rotational movement of the first rotational cylindrical element 217 compared to the movement of the second rotational cylindrical element 209, is given by the below equation, with reference to the symbols defined above for equation 2: Gearing = (I„ / v„) : 1 (Equation 3)

As such, using the attributes of Fig. 11a, it can be shown that the resultant gearing defined by this particular relative pitch of the threads 219 and 221 on the first rotational cylindrical element 217 compared with the threads 205 and 207 on the outer cylinder 201 is 5:1. That is to say, for each single revolution of the second rotational cylindrical element 209, the first rotational cylindrical element 217 has rotated five times. It is a step-down gearing in which the mechanical advantage is greater than one. Speed of rotation is reduced and torque is increased. For Fig. 1 lb, the gearing is 3 : 1.

[0068] Preferably the gearing is at least 2:1. In other words:

I _n / V _n >2,

so (h _n + v _n )/ v _n >2,

so h _n / v _n >l

so, preferably, h _n > v _n

The arrangement works better at higher ratios. Preferably h _n > v _n + 1. More preferably

[0069] In a preferred embodiment, the desired rotary speed of first drive arm 101 is approximately 1 radian per second. This is in order to conform to the speed at which human limb sections typically move. However, as efficient conversion of electrical to mechanical energy at voltages appropriate to portable battery supplies occurs at rotational rates quicker than 1 radian per second, the gearing should be adjusted to appropriately match this requirement.

[0070] Adjustments to the gearing are achieved by changing the relative angle of pitch between the helix threads 205 and 207 on outer cylinder 201, and the helix threads 221 and 219 on the first rotational cylindrical element 217. Looking to Fig. 11a, for instance, the relative angle of pitch 1107 that results in the 5: 1 gearing mentioned above is indicated by angle 1107, wherein angle 1107 is approximately 75 degrees. Looking to Fig. lib, the relative angle of pitch 1110 is approximately 90 degrees. In preferred embodiments, the angular range of the relative angle of pitch is between 60 degrees and 100 degrees. [0071] Different values for the relative angle of pitch can be achieved by varying the angle of offset of helical threads 205 and 207 from the axial direction on outer cylinder 201, and/or the angle of offset of helical threads 219 and 221 from the axial direction on the first rotational cylindrical element 217. In preferred embodiments, the angle of offset from the axial direction of the helical threads 219 and 221 of the first rotational cylindrical element is greater than the angle of offset from the axial direction of the helical threads 205 and 207 on the outer cylinder 201. As such, as can be seen for instance in Fig. 2, the helical threads 219 and 221 on the first rotational cylindrical element 217 may be 'wound tighter' than the helical threads 205 and 207 on the outer cylinder 201.

[0072] Alternatively, instead of varying the pitch of the helical threads 205 and 207 on the outer cylinder 201, the pitch of the helical threads of the second cylindrical element 217 could be varied, or the pitch of the helical threads on both the outer cylinder 201 and the second cylindrical element 217 could be varied. In either of these cases, the equations and description above in relation to gearing apply equally to these scenarios.

[0073] Referring to Fig. 12, a schematic view of the outer cylinder 201 overlaying the second rotational cylindrical element 209 and the first rotational cylindrical element 217 of Fig. 11a is shown in a flat diagrammatic representation for the purpose of demonstrating an example gearing of the relative motion between the first rotational cylindrical element 217 and the second rotational cylindrical element 209 during operation of the actuator. The circumference H of the outer cylinder 201 is indicated on Fig. 12a only, but is common to each of the figures in Fig. 12.

[0074] In particular, Figs. 12a to 12m demonstrate the relative motion of the first rotational cylindrical element 217 and the second rotational cylindrical element 209 with respect to the outer cylinder 201, during a single full revolution of the first rotational cylindrical element 217. This relative motion is indicated by the progress bars 1201, wherein the full length of the progress bars represents a single full revolution of the each of the first rotational cylindrical element 217, indicated by a square, the second rotational cylindrical element 209, indicated by a circle, and the outer cylinder 201, indicated by a triangle. Each progress bar 1201 is graduated into twelve equal increments, whereby each increment therefore represents 30 degrees of rotation. As such, each figure beginning with Fig. 12a, represents a progression of 30 degrees of rotation of the second rotational cylinder 217 from the figure that precedes it. In this way, Fig. 12m shows the relative rotary orientations of each of the first rotational cylindrical element 217, the second rotational cylindrical element 209, and the outer cylinder 201 at the point where the first rotational cylindrical element 217 has completed a single full revolution. Using equation 3, it can be shown that for a single revolution of the first rotational cylindrical element 217, the second rotational cylindrical element 209 has rotated 72 degrees from its starting position. In terms of the progress bars 1201, this corresponds to 2.4 sections of the progress bar. In accordance with this, Fig. 12m demonstrates that for the particular relative pitches of the first rotational cylindrical element 217 compared to the outer cylinder 201 as shown in Fig. 11a, a single revolution of the first rotational cylindrical element 217 does indeed result in 72 degrees of rotation of the second rotational cylindrical element 209, which is equivalent to 2.4 sections of the progress bar. In the preferred embodiment, the outer cylinder 201 is fixed and therefore has not rotated at all.

[0075] Many aspects of the enclosed invention present significant advantages over the prior art. The dynamic properties of the actuator result from interacting magnetic and electromagnetic fields without the need for a separate mechanical transmission and this leads to the following benefits:

[0076] The actuator does not have the added mass and bulk of a mechanical transmission stage. This promotes its use as an actuator for a portable device, whilst also enabling the actuator to fit within a cosmetic envelope such as the form of an absent body part for a prosthetic limb replacement. Similarly, it facilitates the actuator to be used in designs that need to be worn closely to the user's body, such as for a powered orthosis or exoskeleton. Further, the lower mass per unit power developed allows an increase in efficiency, and where applicable an increase in battery life of the actuator. [0077] The actuator has an advantageously increased efficiency in converting electrical energy to mechanical energy. In the first instance, the actuator does not invoke the inefficiencies due to friction that occur with a mechanical transmission stage. In the second instance, there is a dramatic reduction in the inertial loading when compared to relatively heavy mechanical transmission components.

[0078] The actuator does not require high tolerance mechanical parts within a mechanical transmission stage. This feature promotes cost-effective manufacture, and also facilitates easier and less skilled assembly, hence also requiring less skilled personnel in maintenance and repair. The transmission does not require lubricant for close fitting mechanical parts in order to work efficiently. This enables the actuator to be worn close to the user's body without the potential hazard of lubricant contamination, and also enables reliable use without the need for regular maintenance and re-lubrication. In terms of functionality, this feature promotes the use of the actuator within portable devices where gravitational effects are not constantly from the same direction. The absence of lubrication enables the actuator to be used in dirty, dusty or sandy environments that present seizing hazards to lubricated mechanical transmissions, and also enables the actuator to be used in low pressure environments where common lubricants evaporate and lose their effectiveness. The absence of lubricant is also beneficial to applications intolerant to material contamination such as clean room operations and automated food preparation.

[0079] The transmission generates less noise in operation than a comparable mechanical transmission. This enables the actuator to be used within devices that covertly aim to replace the function of an absent limb where undue noise would betray the limb as artificial, hence enabling the actuator to be used to support the action of inadequately functioning intact joints without drawing attention to the wearer through undue operational noise. Similarly, it enables the actuator to be used in multi-degree or multi-axis systems where cumulative noise would be distracting for operation.

[0080] The actuator has rotating sections that when un-powered can be designed to be free running. This promotes the use of the actuator for an active prosthetic hip, for instance, where the body generated 'swing' and momentum of the wearer's natural leg, thigh and hip can be used to swing the hip joint into a natural position during walking without the need for external power so conserving portable power supplies and increasing aesthetically acceptable movement. Free-running and absence of mechanical connection can also increase the longevity of the actuator as shock loadings are not transmitted to mechanical parts within the actuator. This feature also promotes the use of the actuator for service robotics where compliance to human interaction is necessary for safety.

[0081] Further, the ability of the actuator to be back-driven is a key failsafe requirement, for instance in aerospace industry. In flight control actuation systems, the actuators are often mounted in parallel and a power failure of one actuator needs to be overcome by the remaining functional actuators, thus necessitating that the failed actuator can be passively back-driven. [0082] The actuator has a rotating mechanism that allows it to be electronically positioned extremely accurately and repeatedly. This enables the actuator to be used for applications that require extremely accurate and repeatable positioning, such as robot- assisted surgery, without the need to constantly adjust mechanical parts as they wear. [0083] The particular design of the actuator provides a high-torque and relatively low- speed movement that simultaneously allows back-drive. This is highly advantageous when compared to conventional and commercially available electrical motors, where only high speed and low torque movement is possible without the addition of a separate mechanical transmission which itself often prevents efficient back-drive. Further, the high torque and low speed movement of the actuator makes it highly comparable to the movement of natural limbs as controlled by skeletal muscle. As such, the actuator is highly suited to being used in the applications related to prostheses and orthoses.

[0084] The rotating sections of the actuator can be controlled with intermediate force engagement under electrical control. As a result, the actuator can be used as a damper by adjusting the electromagnetic engagement between the rotating and static elements and used to actuate a hip prosthesis to better simulate natural walking gait. With an appropriate control system the actuator may also be used as a mechanical 'active-filter' orthosis, such as at the hip for a person suffering with tremors, where their unwanted limb movement might be damped and their desired movements assisted using suitable control strategies and sensors. The actuator may be used in part of a remote control or teleoperation system, with this feature used to accurately reflect and mimic the force and position exerted upon the remote or virtual object.

Alternative Embodiments

[0085] The embodiments described above are illustrative of, rather than limiting to, the present invention. Alternative embodiments apparent on reading the above description may nevertheless fall within the scope of the invention.

[0086] In another aspect, the outer cylinder 201 may instead have permanent magnetic arrangements parallel to the central axis, and the second cylindrical element 209 may have bars of high magnetic permeability in a helical arrangement around the central axis. The minimum requirement is that the high-permeability bars of the second rotational cylindrical element are helical relative to the rows of permanent magnets arranged on the outer cylinder 201.

[0087] In another aspect, whilst the above embodiments have considered the first rotational cylindrical element 217 as the driver of the actuator when operating in a speed-reducer mode, embodiments of the invention could equally include driving the actuator mechanism in the opposite way as a speed-increaser in what would be a generator-like mode. Operating the actuator in this free-running way presents the opportunity to use the actuator as a sensor or generator as in reverse mode, where imparted rotational forces to the second rotational cylindrical element 209 of the actuator will cause rapid rotation of the second cylindrical element 217, thereby inducing electrical currents to be formed into the phase windings of the stator 233.

[0088] In another aspect, whilst the embodiments above have considered the first rotational cylindrical element 217 to have a relative helical pitch when compared with the outer cylinder 201 that results in the actuator operation having a speed-reduction gearing, embodiments may also be envisaged where the relative pitches between the helical threads of the first rotational cylindrical element 217 and the outer cylinder 201 result in a speed-increasing gearing in the manner of a generator.

[0089] The theoretical workings of each of the above described alternative embodiments is equivalent to those of the main embodiments described above, as such, the described operation of the main embodiments is equally applicable to the alternative embodiments.