Linear actuator

Abstract

A linear actuator having first and second cylindrical elements 122, 116 and a stator 118 arranged concentrically around a central axis. The stator comprises electromagnetic sectors 128 for generating phased electromagnetic fields around the stator. The first cylindrical element has permanent magnetic elements arranged as a first helix. The second cylindrical element has permanent magnetic elements 124 arranged as a second helix in screw engagement with the first helix. Phased magnetization of the stator causes the first cylindrical element to rotate about the axis. Rotation of the first cylindrical element with respect to the second cylindrical element causes a longitudinal force to be exerted along the axis. The actuator comprises a first fitting 104A at a first end and a second fitting 104B at a second end, wherein the stator is connected to the first end and the second cylindrical element is connected to the second end. The actuator comprises a shaft (Fig.2D, 108) extending from the first and / or second ends to the stator. A conduit extends from the first end alongside the shaft to provide cables and / or cooling to the stator. The permanent magnetic elements may be strip magnets (Fig.10A, 173). The helixes of magnets may have pitch p. The first or second helix may be split by a circumferential gap (Fig.14, 1400). The actuator is for applications which require low back drive force, low audible noise, precision, ability to maintain a holding position when current is removed and impulse force tolerance.

Description

Field of the Invention

[0001] The present invention relates to an actuator, and particularly to a linear actuator for transforming electrical energy into mechanical energy. Background of the Invention

[0002] Electromechanical linear actuators are used in a broad range of fields, including in domains from aerospace to limb prosthetics. As a result, there are many simultaneous demands placed on electromechanical linear actuators, such as the need high efficiency, robustness, small size, compliance, low noise and fail-free reliable operation. For example, lower-limb prosthetics require high efficiency to conserve portable battery supply and to limit audible noise, whilst back-drivability in a failed condition is a necessity for aerospace actuators that are often arrayed in a parallel manner for redundancy.

[0003] Traditional electromechanical actuators use an electrical motor in series with a mechanical transmission to create the linear thrusting function. These mechanical transmissions may be Acme (trapezoidal form), roller screws, ‘RohTix’ angled bearings, or ball screws. All of these disadvantageously incur friction and need to be lubricated in some manner to function over long periods, and must have clearance to permit relative surface movement. Further, as state of the art electromechanical actuators become smaller the proportional volume of motor relative to transmission becomes disproportionately small, and thereby limiting power density of the actuator. Additionally, the more miniature transmission components become the more costly to manufacture and vulnerable to impulse loading they are. Hence these products tend to disadvantageously cost more whilst delivering worse performance.

[0004] In response to some of these disadvantageous, the subject matter of the applicant’s own prior application GB 2512074 A was developed, which amongst other numerous advantages provided an integrated electromagnetic transmission actuator having an advantageously large motor volume for high power capacity, a zero-friction screw transmission for high efficiency, and a non-interdigitating configuration to aid shock tolerance, compliance, and fail-free back-drivability. In particular, GB 2512074 B disclosed a particularly advantageous linear actuator arrangement including a stator comprising electromagnetic sectors for generating phased electromagnetic fields around the stator, at least one cylindrical element, the stator and the cylindrical element being arranged concentrically around a central axis, the cylindrical element comprising permanent magnetic elements magnetized radially and arranged as one or more discontinuous helices, whereby phased magnetization of the stator causes the cylindrical element to rotate around the axis along a helical path, thereby causing the cylindrical element to exert a longitudinal force along the axis.

[0005] In aiming to implement electromechanical actuators into physical products for specific applications, further desires in the state of the art arise. Examples of such desires are: the need to securely and robustly provide power and cabling to the different elements, in particular the stator, of the electromechanical actuator that avoids stress and breakages of the cabling due to movement in use; the need to include single spherical fittings at each end of the actuator to allow integration with and connection to aspects of hydraulic and pneumatic systems and mechanical brackets. Further, in the majority of applications the electromechanical actuator is located at or near the centre of moving and articulated applications, leading to difficulties with inadequate heat dissipation. This problem is exacerbated in electromechanical actuator configurations in which the stator is situated within the centre of the rotor as in the subject matter of the applicant’s own prior application GB 2512074 A, as opposed to around the periphery with the rotor central as is typical for most conventional EMA’s. The problem may become particularly apparent at higher power levels, and / or statically commanded to support significant force over long periods.

[0006] More broadly, there is a desire for practical and competitively performing actuators that can be portable, and may be used in prosthetics, robotics, aerospace and automation. There is a desire to provide linear actuators which are advantageously compact, lightweight, powerful and efficient. There is also a desire to provide linear actuators which are advantageously highly integrated and provide a highly controllable transformation of electrical to mechanical energy. Statement of the Invention

[0007] According to a first aspect of the present invention, an actuator is provided comprising: a stator comprising electromagnetic sectors for generating phased electromagnetic fields around the stator; and a first cylindrical element, the stator and the first cylindrical element being arranged concentrically around a central axis, the first cylindrical element having permanent magnetic elements arranged as at least a first helix; a second cylindrical element arranged concentrically with respect to the first cylindrical element, the second cylindrical element having permanent magnetic elements arranged as at least a second helix in screw engagement with the first helix. A set of strip magnets is arranged circumferentially around the first cylindrical element, whereby phased magnetization of the stator causes the first cylindrical element to rotate with respect to the second cylindrical element, thereby causing a longitudinal force to be exerted along the axis.

[0008] The magnets of the first cylindrical element and the second cylindrical element are preferably magnetized in a direction that is generally radial with respect to the axis. They may be magnetized in a parallel manner (i.e. parallel within each element) or radial manner (i.e. each element is arcuate and the lines of magnetization are themselves radial).

[0009] The first cylindrical element is preferably connected to the first end through the stator being fixed to the axis.

[0010] The magnets of the set of strip magnets are preferaby magnetized radially with respect to the axis and are alternately polarized to produce a torsional force.

[0011] In a preferred arrangement, the first cylindrical element surrounds the stator, the second cylindrical element surrounds the first cylindrical element and the strip magnets are located inside the first helix of permanent magnetic elements, preferably longitudinally with respect to the central axis. They may have a skew angle relative to a direction parallel to the central axis.

[0012] According to a second aspect of the present invention, an actuator is provided with a first fitting at a first end and a second fitting at a second end; a stator, connected to the first end, comprising electromagnetic sectors for generating phased electromagnetic fields around the stator; a first cylindrical element, the stator and the first cylindrical element being arranged concentrically around a central axis, the first cylindrical element having permanent magnetic elements magnetized radially and arranged as at least a first helix and further arranged such that phased magnetization of the stator causes the first cylindrical element to rotate about the axis. A second cylindrical element is provided, connected to the second end and arranged concentrically with respect to the first cylindrical element, the second cylindrical element having permanent magnetic elements magnetized radially and arranged as at least a second helix in screw engagement with the first helix, whereby rotation of the first cylindrical element with respect to the second cylindrical element causing a longitudinal force to be exerted along the axis between the first and second fittings; and a shaft extending from the first end to the stator. At least one conduit extends alongside the shaft to provide cables and / or cooling to the stator.

[0013] The shaft preferably has first and second telescopic parts, the first part being connected to the first end and the second part connected to the second end. The first part may be arranged to extend out from and retract into the second part. The conduit(s) extend generally parallel to the first part. At least one cable conduit and at least one coolant conduit may extend alongside the shaft.

[0014] The coolant conduit may extend alongside the shaft to a heat sink at the first end of the actuator, e.g. to a set of external fins.

[0015] The conduit or each conduit is preferably a load bearing strut rigidly connecting the stator to the first fitting.

[0016] The stator may have stator arms extending generally longitudinally with respect to the central axis with a skew angle relative to a direction parallel to the central axis.

[0017] The stator may be provided with circumferential rollers within which the shaft can pass to permit smooth longitudinal movement of the shaft within the stator Other means may be provided for permitting prismatic translation and resisting torque.

[0018] According to a third aspect of the present invention, an actuator is provided having a stator comprising electromagnetic sectors for generating phased electromagnetic fields around the stator; and a first cylindrical element. The stator and the first cylindrical element are arranged concentrically around a central axis, the first cylindrical element having permanent magnetic elements magnetized radially and arranged as at least a first helix of pitch p. A second cylindrical element is arranged concentrically with respect to the first cylindrical element, the second cylindrical element having permanent magnetic elements magnetized radially and arranged as at least a second helix of pitch p in screw engagement with the first helix. One of the first helix and the second helix is split by a circumferential gap. The width of the circumferential gap is equal to Fp + Np, where Fisa fraction less than 1 / 2 and N is an integer. In a preferred arrangement, the circumferential gap is in the first helix and the second helix is continuous.

[0019] Preferred optional features are defined in the dependent claims.

[0020] Various embodiments and aspects of the invention are described without limitation below, with reference to the figures.

[0021] Embodiments of the present invention may provide relatively low linear speed, high thrust movements from relatively swiftly changing electromagnetic fields. Embodiments of the present invention may be particularly suited to portable applications requiring a high degree of efficient control, e.g. applications in which human-like and / or muscle-like movement needs to be simulated or interacted with. Embodiments of the present invention may be particularly suited to applications in active prosthetics, exoskeleton, assistive devices, rehabilitation tele-care and service robotics. Embodiments of the present invention may be particularly suited to applications in aerospace, med-tech and transport.

[0022] In particular, embodiments of the present invention may advantageously be at least one of lightweight, compact, powerful, efficient, quiet, low maintenance and / or able to be backdriven (e.g. providing free leg swinging movement in prosthetic applications). Brief Description of the Drawings

[0023] 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. Figure 1A shows a cross-sectional side view of a prototype actuator in a retracted position according to an embodiment of the invention. Figure IB shows a cross-sectional side view of the prototype actuator of Figure 1A in an extended position according to an embodiment of the invention. Figure 2A shows a cross-sectional view of the actuator of Figures 1A and IB in a retracted position. Figure 2B shows a cross-sectional view of the actuator of Figures 1A and IB in an extended position. Figure 2C shows a cross-sectional perspective view of the actuator of Figure 2A in a retracted position. Figure 2D shows a cross-sectional perspective view of the actuator of Figure 2C at 45 degrees rotation, in mid-extension position. Figure 3 is a cross-sectional perspective view of a shaft portion and aspects of stator element the actuator of Figure 2D. Figure 4 is a cross-sectional perspective view of the outer cylindrical element of the actuator of Figures 2 A and 2B. Figure 5 is a cross-sectional side view of the actuator in a retracted position. Figure 6 is a cross-sectional perspective view of a portion of the actuator of Figure 5. Figure 7 shows a cross-sectional perspective view of a portion of the actuator according to a specific embodiment. Figure 8A shows a cross-sectional view of an outer cylindrical element of the actuator. Figure 8B shows a cross-sectional view of a cylindrical rotor component of the actuator. Figure 8C shows a cross-sectional view of a central stator element of the actuator. Figure 8D shows a cross-sectional view of the combination of the outer cylindrical element of Figure 8A with the cylindrical rotor component of Figure 8B with the central stator element of Figure 8C of the actuator. Figure 9A shows a cross-sectional perspective view of a cylindrical element of the actuator. Figure 9B shows a cross-sectional side view of the outer cylindrical element of Figure 9A. Figure 10A shows a side view of a cylindrical rotor component of the actuator. Figure 10B shows a side perspective view of the cylindrical rotor component of Figure 10A with a quarter of the component visually cut away to allow a perspective inside the cylindrical rotor component. Figure 11A shows a cross-section view through an aspect of a stator of the actuator according to an alternative embodiment. Figure 1 IB shows a side-vide of an aspect of a stator of the actuator according to the alternative embodiment shown in Figure 11 A. Figure 12 shows a side view of an outer cylindrical element of the actuator according to an alternative embodiment. Figures 13 and 14 show arrangements of the outer cylindrical element and the outer magnetic element of the cylindrical rotor component in schematic detail. Detailed Description

[0024] In the following description, functionally similar parts carry the same reference numerals between figures. Embodiments of the invention are now described, by way of example only, with reference to the accompanying drawings.

[0025] Figure 1A shows a side view of a prototype linear actuator 100 in a retracted position. Figure IB shows a side view of the prototype actuator 100 in an extended position. The actuator 100 is for inclusion in a system which requires an actuated movement as part of its functionality. The actuator 100 includes a casing 102 such that the internal mechanisms are protected from ingress of foreign material, such as dirt or sand or water, thereby allowing the actuator 100 to be used in a wide range of different applications. Casing 114 shown as a cylindrical tube overlapping the main actuator body. However, other variants (especially for aerospace) may use bellows, to assure a continuous membrane between the external and interior parts of the actuator. The actuator 100 includes end fittings 104 A and 104B, such as ring end fittings as shown in Figures 1A and IB, which are provided for connection to the aspects of the system which are to be moved by the linear force of the actuator 100 when moving between the retracted and extended positions in use. Other fittings may also be used, such as trunnion fittings, where two spigots project laterally across the actuator 100. A shaft 108 internal to the casing 102 extends from the end fitting 104A, as will be described further below. The shaft may be telescopic, in which case it is also connected to the opposite end fitting 104B.

[0026] The shaft 108 is preferably hollow. It may be provided with a linear encoder target scale along its internal bore. A telescoping linear optical encoder sensor may extend from the end cap 150 inside the hollow shaft. The sensor senses the absolute linear position of the actuator. Alternatively, the sensor and scale can be reversed, with the scale extending in the form of a rod from the end cap 150 to the inside of the shaft in a telescoping manner, with the sensor mounted on or in the vicinity of the internal end cap 133 to read the scale on the rod.

[0027] The casing 102 is formed of a first outer casing 114 and a second outer casing 112 which linearly translates to move telescopically partially into and out of the first outer casing 114 when the actuator 100 moves between the retracted and extended position. The first outer casing 114 end has an actuator end cap 150. (The cap has a cover that is present in some of the figures but removed in others.) One end fitting 104A is fixed to the second outer casing 112 and one end fitting 104B is fixed to the first outer casing 114. The end of the actuator 100 defined by the first outer casing 114 remains static throughout operation of the actuator 100. The end of the actuator 100 defined by the second outer casing 112 linearly translates along the axis 106 during operation of the actuator 100 (or vice-versa). The second outer casing 112 has a helical magnetic element 124 running its entire length, as will be explained.

[0028] In operation, the actuator 100, upon an application of electric current and through interaction of different magnetic elements in manners described in detail below, is caused to move longitudinally along central axis 106. The end fittings 104A and 104B will be connected to a load within the system application to exert a force on the load. In this way, the actuator acts like a piston. It can apply a force in either direction along the axis 106. Significantly, it can also resist a force along that axis. Its ability to resist a force can, in some circumstances, be greater than its ability to move a load against an applied force.

[0029] Figures 2A and 2B show cross-sectional side views of the actuator 100 in a retracted and extended position respectively. Figure 2C shows a cross-sectional perspective view of the actuator of Figure 2A in the retracted position, with the cover of the actuator end cap 150 removed. Figure 2D shows a cross-sectional perspective view of the actuator of Figure 2C in a mid-extension position, at 45 degrees to the view of Figure 2D and with the cover of the actuator end cap 150 in place. Elements of the actuator are shown in greater detail. The shaft 108 can be seen running along the central axis of the actuator 100. The shaft 108 is connected to end fitting 104A at the left-hand end of the actuator 100 as shown firmly connected to the second outer casing 112.

[0030] In an alternative configuration (not shown), the shaft 108 includes two portions: a hollow shaft portion and a telescopic shaft portion which can telescopically retract and extend into and out of the hollow shaft portion. In this arrangement, the telescopic shaft is connected to both ends of the actuator 100. The telescopic shaft portion is connected and fixed to the static outer casing 114, and the hollow shaft portion is fixed to the linearly translating second outer casing 112 (or vice-versa).

[0031] The shaft (whether fixed or telescopic) is preferably generally circular in cross section with flattened faces, as will be described, but it may be square, triangular or fully circular in cross section or have some other cross sectional shape. In the preferred embodiment, shaft 108 allows linear translation of the stator and rotor assembly with the flat side or sides preventing the stator from rotating about shaft 108 when the stator creates torque to the rotor. There are cylindrical and flatted sections to shaft 108 to maximise bearing life. That is, if the shaft were fully flatted, and the stator had roller bearings fixed at either end, any slight misalignment between the bearings at either end would adversely load the bearings and foreshorten their service life.

[0032] The external sides of the second outer casing 112 and the first outer casing 114 are formed of a non-magnetic housing. The second outer casing 112 is attached to an external end cap 134 and an internal end cap 133. (From the viewpoint of the actuator as a whole, the internal end cap 133 may be considered a bulkhead). The second outer casing 112 and the end caps 133, 134 together form a closed cylinder. This closed cylinder will hereafter be referred to as “outer cylindrical element 116” and will be described further below with reference to Figures 3 and 4. Each end of the shaft 108 is fixed to the respective end cap 133, 134 the second outer casing 112 and the shaft 108 move as a single body during liner translation.

[0033] Radially within the outer cylindrical element 116 is a stator 118. The stator is preferably (but not essentially) formed of a central stator element 126 and a slider part 138. The central stator element 126 surrounds the slider part 138. Annular teeth on the slider part engage with corresponding grooves in the central stator element 126 to prevent rotation between the two. Stator 118 has a collar 110 and roller bearings 120 (Figure 2D). The collar and roller bearings interface with the shaft 108 for allowing relative linear movement of the shaft 108.

[0034] The stator 118 includes load bearing struts 132 which pass through the internal end cap 133 and fixedly connect the stator 118 to outer casing 114 at the static end of the actuator 100. There may be a plurality of load bearing struts 132, such as two, three or four, and they may be spaced around the collar 110, and for instance may be evenly spaced around the collar 110. Some or all of these will act as conduits, as will be explained. Each strut passes through a slider bush 139 of the internal end cap 133. Each slider bush 139 may take the form of a low-friction collar or lining that lines hole in the internal end cap 133. The gap between the strut and the internal end cap 133 is minimized to prevent lateral buckling of the strut but not so tight as to prevent low-friction sliding of the one relative to the other. As an option, further bearings (e.g. roller bearings) can be provided here to allow contact but facilitate sliding.

[0035] A cylindrical rotor component 122 surrounds the stator 118 and is free to rotate around the stator 118. The rotor component 122 has a magnetic element 123 comprising inner and outer sets of magnets that are described in greater detail with reference to Figure 8. The magnetic element 123 sits between the stator 118 and the internal face of the outer cylindrical element 116.

[0036] An annular set of ball bearings 130 between the rotor 122 and the collar 110 facilitate rotation of the collar around the rotor. There is another set of such bearings at the opposite end of the rotor 122. These bearings are paired angular contact ball bearings that can take high radial and axial loads. At one end, the inner race of one of the bearings abuts the collar 110 on slider part 138 of stator 118 (shown in Fig. 3). At the other end, the inner race of the other bearing race abuts a distal tightening nut 119. There is a fine screw thread between the distal tightening nut 119 and the slider part 138. This screw thread permits setting a correct angular contact bearing pre-load. Other bearing choices that take radial and axial loads include; four-point bearings, tapered roller, combined radial and thrust bearing packages, and cross-roller bearings. Of these options, angular contact bearings are light in weight, compact and give good force capability.

[0037] Advantageously, the magnetic element 124 of the outer cylindrical element 116 may be lined by a lining 144 on the radially internal surface to protect the magnets of the magnetic element 124 physically and / or in terms of magnetic integrity, and to assist in retaining the magnets in their location. The lining 144 is preferably a continuous non-ferrous, non-electrically conductive and non-eddy current forming membrane, such as a polymer membrane. Advantageously, this lining 144 reduces eddy currents forming and creating undesired magnetic field interactions. However, ferrous membranes may also be used. The eddy current forming lining 144 is useful at the ends of travel, as it aids 'snubbing'. That is, it passively aids the braking of the actuator as it nears its end stops. In the mid-section of travel, where speeds are highest, it is preferrable to have a non-conductive, non-eddy current forming membrane, as this increases efficiency.

[0038] Inside the cylindrical rotor component 122 is the stator 118. The stator 118 is made from a ferromagnetic material, which has conductive wire windings 128 around stator arms (not shown in Figures 2A-2D but described below). The cylindrical rotor component 122 rotates around the central axis 106, between the stator 118 and the outer cylindrical element 116, in a manner determined by the electrical operation of the actuator 100 as will be described further below. The cylindrical rotor component 122 is constrained to rotate about the stator 118 by angular contact ball bearings 130. The outer cylindrical element 116 does not rotate but translates linearly.

[0039] The roller bearings 120 of the collar 110 interface with the shaft 108 to allow relative linear motion along axis 106. The ball bearings 130 of stator 118 interface with the cylindrical rotor component 122 to allow it to rotate independently of all other elements such as shaft 108, stator 118 and the outer cylindrical element 116.

[0040] The cover of the actuator end cap 150 is removed in Figure 2C. Electronic circuitry 152 may be located inside the actuator end cap 150, including for instance electronic control boards, processors and memory, which may provide electronic processing, control and signaling to the various electronic components of the actuator 100, such as motors and sensors, and provided data storage therefor. Alternatively, the electronic circuitry 152 may be provided remote from and outside the actuator 100 with the associated electric wiring passing into and / or through the actuator end cap 150. Both the power inverter circuitry for the phases of the motor and the low-level motor control can fit within this small volume. Advantageously, only DC bus power and signal cabling need go to each actuator. This greatly reduces cabling weight, cost and complexity.

[0041] In operation, controlled electrical operation of the windings 128 of the stator 118 generates magnetic fields which interact with the magnetic element 123 of the cylindrical rotor component 122 and which thereby cause the cylindrical rotor component 122 to rotate. The rotation of the cylindrical rotor component 122 causes a magnetic field interaction between the magnetic element 123 of the cylindrical rotor component 122 and the magnetic element 124 of the outer cylindrical element 116, which causes the dynamic outer cylindrical element 116 to move linearly in the axial direction, either retracting or extending depending on the electric operation applied. The movement of the outer cylindrical element 116 includes the shaft 108 which is fixed to the end cap 134. The roller bearings 120 of the collar 110 allow the shaft 108 to linearly translate relative to the stator 118. Linear force is transferred from the stator 118 by the load bearing struts 132 to the end cap 150 of the static outer casing 114 of the actuator 100.

[0042] In this way, relative separation of the end fittings 104 A and 104B of the second outer casing 112 and the first outer casing 114 respectively is achieved as linear force is generated to move the load of the system application in which the actuator 100 has been included.

[0043] In configurations that include a telescopic shaft portion, the telescopic shaft portion either extends out of or retracts into the hollow shaft portion respectively.

[0044] Figure 3 is a cross-sectional perspective view of the shaft 108 in combination with the collar 110 and stator 118 of the actuator 100 of Figures 2A - 2D, with the other features of the actuator 100 removed for visual clarity. The spaces 135 at either end of the central stator element 126 are the location of the angular contact bearings 130 for allowing the cylindrical rotor component 122 to rotate around the stator 118. When paired, angular contact bearings permit support for both radial and axial loads, as the contact points between the inner race, outer race, and the ball bearings are at an angle. At the end of the stator 118 there is a circuit board (not shown) that terminates the stator windings 128. There may be a plurality of fixing points 136 for fixing load bearing struts. There may, for example, be two, three or four fixing points 136. The fixing points 136 may be spaced around the collar 110, and for instance may be evenly spaced around the collar 110.

[0045] The collar 110 may incorporate sensors 111 to sense the position of the cylindrical rotor component 122 such that this can be reported to a controller (not shown), for instance located in the actuator end cap 150 at the static outer casing 114 end of the actuator 100 such as electronic circuitry 152, as will be described further below. These sensors may include magnetic sensors, such as Hall Effect sensors for closed loop control of phase currents to the stator and precise rotary position encoders used in closed loop positioning control. In specific embodiments, a sensor 111 may detect the position of the cylindrical rotor component 122 using absolute encoding that provides information of the actual angle of the cylindrical rotor component 122 even when returning from a power-off condition. This type of encoding advantageous provides efficient, quiet and safe electrical operation. In certain applications, such as many industrial and domestic applications, commercial-off-the-shelf Hall Effect magnetic absolute encoders may be used. However, other sensors are envisaged. Aerospace applications require a much greater temperature range and have more stringent environmental requirements. For these more challenging applications, instead of using soft bonded “refrigerator” magnet ‘encoder targets’, hard sintered magnets with high coercivity may be used which may be mechanically registered to the position of the cylindrical rotor component 122 using threaded fasteners or pins. In this instance magnet arrays will be sensed by sensors 111 such as Hall Effect sensors, and by virtue of the pattern of the magnetic target array the sensor outputs a sine wave signal in correspondence to the full rotation of the cylindrical rotor component 122.

[0046] The innermost radial component of the stator 118 is a slider part 138 which interfaces the stator 118 with the shaft 108 to allow the relative linear translation of the stator 118 — including the collar 110 - with respect to the shaft 108 Preferably, the slider part 138 is manufactured from a material with high thermal conductivity such that heat can advantageously be conducted out from the centre of the actuator 100.

[0047] The shaft 108 has a circular profile at one end 140 (the end to which fitting 104A is attached) and a flattened part 142 having an array of flat surfaces, such as four flat surfaces forming a square profile at the opposite end. The slider part 138 has a circular bore that is a sliding fit with the outer circumference of the circular profile end 140. Advantageously, this close fit further facilitates thermal conduction and dissipation away from the centre of the stator 118 and slider part 138 to the shaft 108 where it may be transmitted to the end cap 134 of the outer cylindrical element 116 and thereby dissipated.

[0048] The collar 110 includes the roller bearings 120 that roll on the flat surfaces of the flattened part 142 of the shaft 108. The roller bearings 120 constrain the slider part 138 against rotating relative to the shaft 108 whilst simultaneously allowing easy linear translation. This is advantageous in view of high torque that will be generated between the stator and the rotor.

[0049] Whilst different means of achieving this torque-resisting effect are possible, such as using splines, linear bearings, and shaped plain bearings, the use of roller bearings 120 has proven to be advantageously quiet, efficient, accurate, miniature, robust and low-cost. There can be more or fewer roller bearings - e.g. three in the case of a shaft that is triangular in cross section or has three flattened surfaces. Advantageously, to further reduce friction and therefore heat generation in the actuator 100, the shaft 108 may be formed with a polished hardened surface, preferably greater than 58 Rockwell hardness (HRC). [00501 Further advantageously, the shaft 108 has a hollow cavity that may be used to accommodate a probe of a linear displacement sensor such as an linear displacement voltage transducer (LDVT) for detecting the relative displacement of the two ends of the actuator 100.

[0051] Figure 4 is a cross-sectional perspective view of the outer cylindrical element 116 of the actuator 100 of Figures 2A - 2D. The shaft 108 is fixedly attached to the end caps 133 &134 of the outer cylindrical element 116 such that the two elements are constrained to move as one when the magnetic element 124 of the outer cylindrical element 116 is forced to move in a linear direction by the magnetic interaction of the changing magnetic fields caused by the rotation of the magnetic element 123 of the cylindrical rotor component 122 of the stator 118. The shaft 108 may be fixed to the end caps by at a plurality of fixing points 146, which may for instance be screws. The end caps 133 &134 are rigidly fixed to the outer cylindrical element 116, or are fixed by being integrally formed as part of the outer cylindrical element 116, and thereby relative rotation is prevented. The end cap 133 closest to the static end of the actuator 100 with the outer casing 114 includes at least one hole 148 through which the at least one load bearing strut 132 of the stator 118 passes, thereby allowing the load bearing struts 132 to transfer the linear force applied to stator 118 to the static end of the actuator 100. There are as many holes 148 as load bearing struts 132.

[0052] Hence the actuator 100 arrangement has broadly two separate parts which move linearly relative to each other: first, the central shaft 108 being fixed to the outer cylindrical element 116, encased by the second outer casing 112 of the actuator 100 which defines the dynamic moving end fitting 104 A of the actuator 100; second, the stator 118 and collar 110, the stator 118 being fixedly attached to the static end fitting 104B of the actuator 100 which is defined in part by the outer casing 114. The cylindrical rotor component rotates around the stator 118, within the outer cylindrical element 116. It acts and an internal “nut” rotating within a magnetic thread of the outer cylindrical element 116, acting like a hollow “bolt”. (Or the cylindrical rotor component can be considered a bolt rotating within a nut formed by the outer cylindrical element 116.)

[0053] This arrangement of fixing the central shaft 108 to the outer cylindrical element 116 is particularly advantageous as compared to other arrangements. Specifically, in an alternative arrangement the stator 118 may be fixed connected to the shaft 108, or portions thereof, such that relative linear movement of the stator 118 and the shaft 108 is constrained and prevented. In these arrangements the outer cylindrical element 116 is entirely static and it is the combined shaft 108 and stator 118 - including the cylindrical rotor component 122 - which is forced to move linearly within the outer cylindrical element 116 by the magnetic interaction between the magnetic element 124 of the outer cylindrical element 116 and the magnetic element 123 of the cylindrical rotor component 122 of the stator 118. Such an arrangement is shown in certain specific embodiments of the applicant’s own prior application GB 2512074 B. In these arrangements, in contrast to the presently described arrangements, the central shaft 108 is supported by bearings in the end caps 113, 134 of the outer cylindrical element 116 such that the shaft 108 can linearly move therethrough with respect to the outer cylindrical element 116.

[0054] The present arrangement is advantageous over these alternative arrangements in a number of ways. First, because in the alternative arrangements the shaft 108 was the conduit for the electrical wiring which provided the external powering of the stator 118, and hence when the shaft 108 moved it caused the electrical wiring to move which caused stress and strain on the wiring leading to potential failure and breakage. This problem is entirely overcome by the present arrangement, which advantageously allows the stator 118 to be static in operation and for the electrical wiring to be provided through the static load bearing struts 132 connected to the stator 118. Second, the deflection of the central shaft 108 in the alternative arrangement occurred in a ‘simply supported’ manner. However, securing the shaft 108 to the end caps 134 of the outer cylindrical element 116 in the ‘built-in’ or ‘encastre’ manner as provided in the present arrangement advantageously provides for approximately % of the deflection of the central shaft 108 in the ‘simply supported’ manner for a given load. Third, in the present arrangement there is no need to accommodate the displacement of the central shaft 108 into the static end of the actuator 100, which previously limited the number and configuration of different physical system applications the actuator could be included in. Fourth, the present arrangement provides improved heat transfer and dissipation from the central shaft 108 by efficiently transferring the heat to the large surface areas of the end caps 133, 134 and thereby to the outer cylindrical element 116 for dissipation from the second outer casing 112 to the surrounding environment, thereby significantly improving cooling at high power. Hence the present arrangement aids thermal conduction between the central shaft 108 and the end caps 133 &134, so that a heat dissipation path from the stator 118 to the second outer casing 112 of the actuator 100 and hence to atmosphere is improved. Fifth, the present arrangement specifically allows spherical fittings 104 to be fitted at both ends of the actuator 100, which is prevented in the alternative version due to the moving ‘back-shaft’ arrangement. Spherical end fittings are particularly advantageous as they are commonly required for hydraulic and pneumatic system applications.

[0055] Figure 5 is a cross-sectional side view of the actuator 100 in a retracted position. Figure 6 is a cross-sectional perspective view of a portion of the actuator 100 of Figure 5, in particular the static outer casing 114 end. The position of the actuator 100 in Figure 6 is rotated by 90 degrees around the central axis 106 with respect to the actuator 100 of Figure 5, and the hollow shaft 108 and the actuator end cap 150 has been removed for visual clarity.

[0056] As previously described, the actuator 100 includes load bearing struts 132 having the principal function of transferring the axial force generated in the system (by the magnetic interaction between the magnetic element 123 of the of the cylindrical rotor component 122 and the magnetic element 124 of the outer cylindrical element 116) to the static outer casing 114 end of the actuator 100. However, in an advantageous embodiment at least one of the plurality of load bearing structs 132 may include a hollow central cavity such that the load bearing strut 132 in question can additionally house electrical wiring 156 for connecting the electronic circuitry 152 in the actuator end cap 150 to the circuit board and / or sensors located on the collar 110 (as previously described with reference to Figure 3), and thereby provide signaling and power thereto.

[0057] Alternatively or additionally, in a specific embodiment at least one of the load bearing struts 132 may include a heat pipe 154 cavity for providing coolant to and from the stator 118 to provide advantageous cooling by transferring heat out from the centre of the actuator 100 and directly to the actuator end cap 150 at the static outer casing 114 end of the actuator 100, such that the heat may be readily transferred to the atmosphere. Indeed, the heat transferred from the stator 118 may be transferred through the actuator end cap 150 to the large surface area of the outer casing 114 for particularly advantageous cooling efficiency. The heat pipe 154 may include liquid-to-gas coolant typically used in microchip cooling. The heat pipe 154 may be connected to the slider part 138 at the centre of the stator 118 system. The implementation of a heat pipe 154 with coolant is particularly advantageous in larger actuator 100 systems. In smaller actuator systems, the heat pipe 154 coolant may be absent. The heat pipe 154 may additionally aid in cooling the electronic circuitry 152 located in the actuator end cap 150.

[0058] The actuator 100 may have load bearing struts 132 containing electrical wiring 156, or may have load bearing struts 132 containing heat pipes 154, or may have a combination of load bearing struts 132 containing electrical wiring 156 and load bearing struts 132 containing heat pipes 154. For instance, as shown from the perspective of Figure 5 there can be seen one load bearing strut 132 containing electrical wiring 156 and one bearing strut 132 containing a heat pipe 154, whereas when the actuator 100 is rotated 90 degrees as is shown in the perspective of Figure 6 there can be seen two load bearing struts 132 each containing a heat pipe. Accordingly, the actuator may have for instance two, three, or four load bearing struts 132 having a combination of heat pipes 154 and electrical wiring 156 therein. In embodiments in which there are a plurality of load bearing struts 132 containing heat pipes 154, it is advantageous to locate these in diametrically opposite load bearing strut 132 locations around the central axis 106 for maximum heat transfer.

[0059] Figure 7 shows a cross-sectional perspective view of a portion of the actuator 100, in particular the static outer casing 114 end, according to a specific embodiment. As previously described, the actuator 100 is advantageously formed such that it has a protected and sealed internal environment. However, to allow the parts internal to the actuator 100 to move relative to each other in operation, it is necessary that there is at least a small level of gas transfer between the internal environment of the actuator 100 and the external atmosphere. This may be achieved in a number of different manners, with for instance a specific example implementation being machining the relative dimensions of the second outer casing 112 and the outer casing 114 such that there is not a perfect hermetic seal but instead a small gap dimensioned such that gas may pass between the second outer casing 112 and the first outer casing 114, whilst still preventing liquid and solid matter passing between them. Other aspects of the actuator 100 may also allow the passage of gas therethrough to the external atmosphere.

[0060] Accordingly, as previously described, in operation the dynamic second outer casing 112 moves relative to and telescopically partially into and out of the static first outer casing 114. When the actuator 100 is in the fully extended position, air is contained within the cavity inside the outer casing 114 in which the outer cylindrical element 116 is not currently occupying, but which it will occupy once it translates linearly toward the static outer casing 114 end as the actuator 100 moves to the fully retracted position. As the actuator 100 moves from the extended position to the retracted position, the volume of the unoccupied cavity within the second outer casing 112 progressively decreases as the outer cylindrical element 116 progressively occupies the cavity as it translates toward the static first outer casing 114 end. As this happens, the air within the cavity undergoes an increase in pressure and is thereby forced and released through the gap between the second outer casing 112 and the first outer casing 114 to the external atmosphere. Contrastingly, when the actuator 100 moves towards the extended position, a partial vacuum is created in the cavity in the first outer casing 114 left by the outer cylindrical element 116, and air thereby flows into the cavity through the gap between the second outer casing 112 and the first outer casing 114.

[0061] In many actuator system applications this arrangement presents few issues. However, in certain environments, the external atmosphere may include particulate matter such as sand or dust, and / or may include liquid matter such as water in high humidity environments. In these harsher environments, the partial vacuum created as the actuator 100 moves to the extended position may disadvantageously draw in particulate and / or fluid matter which may break, damage and / or impair the function of the actuator 100, for instance by icing or premature wearing and corroding of the actuator 100. To address such conditions and overcome this problem, certain specific embodiments of the actuator 100 may advantageously include a barrier membrane 158, such as a bellows, located in the cavity of the outer casing 114 that maintains a constant membrane separating the internal environment of the cavity from the external atmosphere of the actuator 100. The membrane may for instance allow gas to pass therethrough, but not liquid or solid. Alternatively, the membrane may be non-gas-permeable and made of natural rubber, chloroprene (Neoprene) or hydroformed beryllium copper. The bellows as shown in Figure 7 are not to scale.

[0062] Further advantageous features illustrated in Figure 7 lie in the shape of the heat pipes 154. It can be seen that each load bearing strut has a fixing 155 that firmly attaches the load bearing strut to the end cap 150. The fixing 155 conveys forces between the end cap and the load bearing strut. The fixing has a nut on the external side of the end cap 150. Through the centre of the fixing 155 and the nut, the heat pipe 154 emerges, and the heat pipe 154 has a flange portion 159 within the end cap 150. In the illustrated example, the flange portion takes the form of a 90 degree bend in the heat pipe. The heat pipe undergoes a further 90 degree bend in the plane of the end cap and follows an annular path around the end cap. Other manners of providing a flange portion at the end of the heat pipe can be envisaged. The flange portion serves to convey heat to the end cap and dissipate the heat through and around the end cap. A cover can be provided on the end cap with fins or the like to form a heat sink to further convey heat away from the end cap.

[0063] The magnetic arrangement and operation of the actuator 100 will now be described with reference to Figures 8 to 10. As previously described, the linear actuation function of the actuator 100 depends upon the relative movement of the stator 118 - including the central stator element 126 and the cylindrical rotor component 122 - with respect to the outer cylindrical element 116 and shaft 108 fixed to the outer cylindrical element 116. Figure 8A shows a cross-sectional view of the outer cylindrical element 116 with all other features of the actuator 100 removed. On the radial outside of the outer cylindrical element 116 is the casing 114, and on the radial inside is the magnetic element 124 which will be described in further detail below. As previously described, the radially innermost feature of the outer cylindrical element 116 may include a lining 144 to protect the magnets of the magnetic element 124 physically and / or in terms of magnetic integrity, and to assist in retaining the magnets in their location. The lining 144 is preferably a continuous non-ferrous, non-electrically conductive and non-eddy current forming membrane, such as a polymer membrane. Advantageously, this lining 144 reduces eddy currents forming and creating undesired magnetic field interactions. However, ferrous membranes may also be used. The radially magnetic element 124 is formed of permanent magnets.

[0064] Figure 8B shows a cross-sectional view of the cylindrical rotor component 122 with all other features of the actuator 100 removed. The cylindrical rotor component 122 includes the magnetic element 123. On the radial outside of the cylindrical rotor component 122 is an outer magnetic element 123 A, and on the radial inside of the cylinder rotor component 122 is an inner magnetic element 123B. Advantageously, the outer magnetic element 123A and the inner magnetic element 123B may be separated by a membrane 160 to protect the magnets of the magnetic element 123 physically and / or in terms of magnetic integrity, and to assist in retaining the magnets in their location. The membrane 160 is preferably a continuous nonferrous, non-electrically conductive and non-eddy current forming membrane, such as a polymer membrane. Advantageously, this membrane 160 reduces eddy currents forming and creating undesired magnetic field interactions. However, ferrous membranes may also be used. The magnetic elements 123A and 123B are formed of permanent magnets. Both the non-ferromagnetic lining 144 and membrane 160 present reluctance to the flux created by the central stator element 126. This reluctance reduces the flux greatly with distance (approx = k * l / distance 3). Over the distance of the membrane 160 thickness the flux is negligible at the external diameter of the cylindrical rotor component 122 and presents minimal deleterious influence against the outer cylindrical element 116.

[0065] Figure 8C shows a cross-sectional view of the stator 118 with all other features of the actuator 100 removed. The stator 118 is made from a highly ferromagnetic material for most high torque applications and greatest axial restraint, but it may be constructed from nonferromagnetic material if zero off-power cogging torque is priority.

[0066] The stator 118 has ferromagnetic radial projections in the form of stator arms 129, each being wound with electrically conductive enameled wire 128. In use, each combined stator arms 129 and electrically conductive laminated wire 128 form a radial array of electromagnetic poles 131. Preferably, each pole 131 formed (e.g. outward north pole) runs the entire axial length of the stator 118. In use, these poles 131 interact with the permanent magnetic elements 123 A and 123B on the cylindrical rotor component 122 to provide a highly registered low reluctance flux path for the magnetic flux from the cylindrical rotor component 122. This high axial registration resists axial displacement of the cylindrical rotor component 122. This aids both on and off-power axial force capacity. In contrast to this high axial registration, some embodiments may tune or minimise off-power radial registration, as described below. Further, embodiments may include relatively lower axial registration, as will be described below.

[0067] Because the stator electrical steel material is highly magnetically permeable, it creates a low reluctance path for the magnetic flux coming from the permanent magnets to go around. If there is a clear integer relationship between magnets and stator teeth, then alignment magnet-to-tooth becomes the preferred lowest reluctance path (and so lowest energy) for the magnetic flux, which can be perceived as a mechanical 'cog' as the rotor is rotated. If there is no integer divisible relationship, lower reluctance paths for the magnetism are less pronounced, and this is further aided by adding axial skew. For an application that is principally a positioning application; e.g. opening the boot (trunk) of a car, cogging may be desirable, as, once at the fully extended position, the full weight of the boot is supported by the actuator. If the actuator is very easily back-drivable considerable current is still needed to support the static load. However, if the actuator has significant cogging, this cogging is amplified by the mechanical advantage of the magnetic screw and this significantly reduces the current needed to support the static load. By contrast, for an application that is almost wholly dynamic, such as a prosthetic lower limb, easy back drive is advantageous, as this aids a naturalness of gait, as the person’s body power and momentum can swing the limb straight; rather. This also conserves battery life. Additionally, a lack of cogging reduces torque-ripple, in turn reducing audible noise.

[0068] Figure 8D shows a cross-sectional view of the combined stator 118, cylindrical rotor component 122 and outer cylindrical element 116. As previously described, the stator 118 and the outer cylindrical element 116 are rotationally fixed and do not move rotationally relative to each other. The cylindrical rotor component 122 rotates relative to both the stator 118 and the outer cylindrical element 116. Each of the stator elements 118, cylindrical rotor component 122 and outer cylindrical element 116 are separate by a small air gap, with magnetic interactions occurring across the air gap.

[0069] In Figures 9 to 10 the polarity of various magnetic elements is depicted with the north and the south polarities shown in opposite cross-hatching.

[0070] Figure 9A shows a cross-sectional perspective view of the outer cylindrical element 116 with the shaft 108 removed for visual clarity, and Figure 9B shows a cross-sectional side view. The magnetic arrangement 124 of the radially internal face of the outer cylindrical element 116 is comprised of arc magnets 163 shaped and formed such that they can be arranged to form a pattern of helical threads around the internal face of the outer cylindrical element 116. Preferably, the magnetised direction of flux from these arc magnets 163 is as close to radial as possible.

[0071] In an example, the size of arc of each arc magnet 163 is less than or equal to approximately 40° such that a parallel magnetisation through each arc magnet 163 can produce a satisfactory approximation. This arc angle 40° of the magnetic arrangement 124 is based on minimising the 'cogging' effect between magnets 124 and permanent magnets 123 A, so as to create a transmission with as few losses as possible. Consider, for example, a single pitch: for magnets 124, there are 9 in a pitch and for magnets 123A there are 16 in a pitch. 9 / 16 is not an integer divisible, nor does it have any common factors. Because of this, there is no low energy position for the two threads (124, 123A) to sit. This means an effectively continuous thread can be created from discrete arcs. Other number combinations are possible, such as 5 / 7, etc. The choice of 40° (9 in pitch) is found to be the greatest span that is robust enough not to crack in assembly, given that the magnet must be at an angle to create a helix when, in fact, the magnet form is a simple arc.

[0072] Preferably, the helical pattern is a twin-start helical array, i.e. it has two separate continuous helical paths 161, 162 and each spans the entire radially internal surface of the outer cylindrical element 116 (although other arrangements are possible, including four helices). Each adjacent helical thread 161,162 is of a different magnetic polarity. In the figure, the width of each helical thread 161, 162 is shown as being only a single arc magnet 163 wide, although other arrangements are possible, for instance where each helical thread 161, 162 is two or more arc magnets 163 wide. Hence the helical threads 161, 162 create an alternating pole arrangement between the two threads 161, 162 with a helical pitch labelled £d’ which is the long axis distance between similar polarity directions.

[0073] The arc magnets 163 are powerful magnets, preferably rare-earth magnets such as neodymium iron boron magnets, with poles orientated along radial projections in a pattern described below. Means may be provided to retain each arc magnet 163, e.g. each arc magnet may be glued to the internal surface of the outer cylindrical element 116, or may have an interference fit with a respective retaining fitting, etc.

[0074] Figure 10A shows a side view of the cylindrical rotor component 122. Figure 10B shows a side perspective view of the cylindrical rotor component 122 with a quarter of the component visually cut away to allow a perspective inside the cylindrical rotor component 122. Figure 10B further includes a magnified section to further visually demonstrate the magnetic arrangement 123A of the radially outer face of the cylindrical rotor component 122. As previously described, the magnetic arrangement 123 of the cylindrical rotor component 122 is formed of two different parts: a radially inner magnetic arrangement 123B and a radially outer magnetic arrangement 123A. The radially outer magnetic arrangement 123A is comprised of arc magnets 173 shaped and formed such that they can be arranged to form a pattern of helical threads around the external face of the cylindrical rotor component 122. Preferably, the magnetised direction of flux from these arc magnets 173 is as close to radial as possible. In particular, preferably the size of each arc magnets 173 is less than or equal to approximately 22.5° such that a parallel magnetisation through each arc magnet 173 can produce a satisfactory approximation. Preferably, the helical pattern is a twin-start helical array; having two separate continuous helical paths 171,172 that each spans the entire radially external surface of the cylindrical rotor component 122 (although other arrangements are possible, including four helices). Each adjacent helical thread 171, 172 is of a different magnetic polarity. In the figure, the width of each helical thread 171, 172 is shown as being only a single arc magnet 173 wide, although other arrangements are possible, for instance where each helical thread 171, 172 is two or more arc magnets 173 wide. Hence the helical threads 171, 172 create an alternating pole arrangement between the two threads 171, 172 with a helical pitch labelled ‘d’ which is the long axis distance between similar polarity directions. The arc magnets 173 are powerful magnets, preferably rare-earth magnets such as neodymium iron boron magnets, with poles orientated along radial projections in a pattern described below. Means may be provided to retain each arc magnet 173, e.g. each arc magnet may be glued to the external surface of the cylindrical rotor component 122, or may have an interference fit with a respective retaining fitting, etc. The pitch ‘d’ of the helical threads 171, 172 of the external face of the cylindrical rotor component are similar or the same as the pitch ‘d’ of the helical threads 161, 162 of the internal face of the outer cylindrical element 116, to facilitate magnetic interaction between the magnetic elements 123 of the cylindrical rotor component 122 and the magnetic elements 124 of the outer cylindrical element 116, as will be described further below.

[0075] The choice of the width of the magnet, (and / or spacing between magnets) may be tailored to the application. Smaller width magnets (and / or spacing between magnets) create threads that appear more 'rigid'. This is useful in applications where the absolute position of the load needs to be tightly controlled. E.g. medical dosing applicators, control of mirrors for optics, etc. Large 'leads' (i.e. the amount of axial translation per rotation of the rotor) are still possible with thin width magnets, it just requires more thread 'starts'. Wider magnets (and / or spacing between magnets) create threads that are 'springy'. This is useful in applications where compliance, and / or impulse force tolerance are priorities, e.g. prosthetics, cobots, etc. Also, for applications, like actuating grippers, where the actuator can position to a holding position and then, once current is removed, the holding force is maintained by the magnetic spring force of the rotor magnets 123 A against outer cylindrical element magnets 124.

[0076] The magnetic arrangement 123B on the internal face of the cylindrical rotor component 122 is comprised of alternate polarity strip magnets 174, 175 arranged along the length of the long axis of the cylindrical rotor component 122, and which are magnetized radially (with respect to the central axis) to interact with the electromagnetic poles 131 of the stator 118. Thus, magnets 174 have their south poles facing radially inwards and magnets 175 have their north poles facing radially inwards. Advantageously, this arrangement of alternating polarity strip magnets 174, 175, arranged to correspond to the electromagnetic poles 131 of the central stator 118, allows a high torque interaction between the changing magnetic fields generated by the electromagnetic poles 131 in use and the alternating polarity strip magnets 174, 175 which causes the cylindrical rotor component 122 to rotate in use.

[0077] Each strip magnet may be magnetized in a parallel manner (i.e. parallel lines of magnetization within each element) or radial manner (i.e. each element is arcuate and the lines of magnetization are themselves radial).

[0078] Preferably, for certain applications, the angular size of each strip magnet 174, 175 is similar or the same as the angular size of each of the arc magnets 173 forming the helical threads 171, 172 on the external face of the cylindrical rotor component 122, such that there are the same number of strip magnets 174, 175 as arc magnets 173 in the circumference of one full pitch revolution. For example, as shown in Figure 8D there are sixteen arc magnets 173 and sixteen strip magnets 174, 175 in the circumference of one full pitch revolution. However, different arrangements are possible as outlined below. The alternating polarity strip magnets 174, 175 along the long axis of the cylindrical rotor component 122 create magnetic discontinuities for the changing magnetic poles 131 of the stator 118 to work against to produce the required torque and rotation of the cylindrical rotor component 122.

[0079] As an alternative to providing strip magnets that are magnetized in alternating radial polarities, the strips can be magnetized in alternating polarities tangential to the inner magnetic arrangement 123B.

[0080] As is shown in the magnified cutaway section of Figure 10B, the arc magnets 173 are radially magnetized as previously described, with the north and south polarities shown in opposite cross hatching. The same is true of the arc magnets 163 on the outer cylindrical element 116 Figure 9A, to allow radial magnetization as previously described, however this detail is left out of the majority of the drawings for visual clarity.

[0081] The magnetic operation of the actuator 100 which causes the linear movement of the outer cylindrical element 116 and hence the movement of the actuator 100 from the retracted to the extended position will now be described.

[0082] Upon application of a controlled, phased electric current, the electromagnetic poles 131 become magnets of phased polarity, which can include a pattern of different phased polarities of different electromagnetic poles 131 including positive, neutral and negative. The arrangement of electromagnetic poles 131 created on the stator 118 interact with the of bands of alternating polarity strip magnets 174, 175 of the radially inner surface of the cylindrical rotor component 122. As the cylindrical rotor component 122 is free to rotate relative to the stator 118, the magnetic forces created between the cylindrical rotor component 122 and the stator 118 cause the cylindrical rotor component 122 to rotate in its attempt to find a stable magnetic orientation, commonly referred to as 'alignment torque', and thereby form the shortest magnetic flux path. Through the configuration of alternating magnetic bands of strip magnets 174, 175 of the cylindrical rotor component 122, the configuration of magnetic poles created on the electromagnetic poles 131 urges the cylindrical rotor component 122 to a position whereby there is alignment between north poles 175 on the “bolt” cylindrical rotor component 122 with south poles created on the electromagnetic poles 131 of the stator 118 and vice-versa. At any given phase position, the design may be such that the radial alignment is complete. In a preferred embodiment, radial alignment is incomplete, but clockwise offsets and anticlockwise offsets are balanced, as will be explained in further detail below.

[0083] The electrical current through the electromagnetic 131 arrays will then be altered, causing further rotation as the cylindrical rotor component 122 seeks a new stable configuration. In this manner the cylindrical rotor component 122 is rotated relative to both the stator 118 and the outer cylindrical element 116. Hence the varying states of magnetization of electromagnets 131 at different and subsequent stages of the electrical cycle cause changes in magnetic field leading to a rotation in the cylindrical rotor component 122 ultimately causing the linear movement of the outer cylindrical element 116. A subsequent stage of the electrical cycle causes different electrical fields from the different magnetisations of the electromagnets 131, and the same process of alignment occurs. In this way, a continued rotation of the cylindrical rotor component 122 and subsequent linear movement of the outer cylindrical element 116 is achieved.

[0084] As the cylindrical rotor component 122 rotates, its path is constrained by the interaction between the twin-start helical threads 171, 172 on its radially outer surface interacting with the twin-start helical threads 161, 162 on the surface of the outer cylindrical element 116 that encases it. The attractive and repulsive magnetic forces between the helical threads of the outer cylindrical element 116 and the cylindrical rotor component 122 and the low reluctance flux path created by the tooth form of the stator arms 129 on the stator 118 constrain these components to act as if they are threaded together in a ‘nut and bolt’ manner. The cylindrical rotor component 122 is constrained by collar 110 such that it cannot move linearly with respect to the stator 118, but that it can rotate around the stator 118. (A collar or other means may be provided on either the cylindrical rotor component or the stator to constrain linear movement therebetween.)

[0085] The two interleaved threads of opposite polarity 171, 172 on the cylindrical rotor component 122 (Figure 10B) constrain the interacting threads 161, 162 on the outer cylindrical element 116 (Figure 9B) to movement only in the manner dictated by the path of the helical threads i.e. when the cylindrical rotor component 122 rotates, the outer cylindrical element 116 is forced to resolve this by linear axial movement such that the helical threads are once again aligned and the repulsion between them is minimized. In other words, linear axial movement of the outer cylindrical element 116 without rotation of the cylindrical rotor component 122 would entail a section of helical thread 161, 162 on the outer cylindrical element 116 moving towards a thread 171, 172 with similar polarity on the cylindrical rotor component 122. This movement is hence doubly opposed by the helical thread on the cylindrical rotor component 122 being attracted to its facing helical thread of opposite polarity on the outer cylindrical element 116, and also by the helical thread on the cylindrical rotor component 122 being repulsed by the adjacent facing helical threads of similar polarity on the outer cylindrical element 116.

[0086] In this way, rotation of the cylindrical rotor component 122 is converted to a linear movement of the outer cylindrical element 116 as the outer cylindrical element 116 moves to align the arc magnets 163 in its internal face with the current orientation of the arc magnets 173 on the cylindrical rotor component 122 in the position which forms the shortest magnetic flux path. This mass movement produces the actuator’s 100 shear force and thrust against a load. In particular, as the shaft 108 is fixed to the outer cylindrical element 116 the shaft 108 also moves linearly along axis 106. The end fitting 104A at the dynamic second outer casing 112 end (and / or the end fitting 104B at the static outer casing 114 end) is connected to a load to exert a force on the load relative to the actuator 100. In this way, the actuator 100 acts like a piston. It can apply a force in either direction along the axis 106. Hence the thrust loading borne by the end fittings 104 is ultimately carried by the axial magnetic shear stress between the arc magnets 163 of the outer cylindrical element 116 and the arc magnets 173 of the cylindrical rotor component 122.

[0087] According to embodiments, the magnetic shear stress between cylindrical rotor component 122 and the outer cylindrical element 116 is approximately 1 / 16th that of a physically interdigitating ball screw. Accordingly, to advantageously increase the magnetic shear stress as high as possible, the surface area of the magnetic threads 171, 172 has been maximised to almost the entire length of the external surface of the cylindrical rotor component 122 to thereby advantageously compete in producing equivalent and greater thrust forces to regular electromechanical actuators.

[0088] Further, in existing electromechanical actuators in which the actuating components and their threads physically touch or interdigitate, the coefficient of friction between moving surfaces dictates a large thread angle for efficiency greater than 50%, which thereby disadvantageously necessities a relatively small thread diameter. Accordingly, known electromechanical actuators often have the rotor component and a component equivalent to a ‘nut’ integrated and internal to the stator component. In contrast, embodiments of the invention overcome these disadvantages by providing a close to frictionless mechanism of action - with no thread engagement between the components - as the outer cylindrical element 116, the cylindrical component 122 and the stator 118 are not in physical contact, thereby allowing the advantageous provision of a highly efficient large diameter thread, with a very small lead (linear distance advanced per revolution) which creates high force from relatively low torque.

[0089] With reference to Figure 8D, as previously mentioned, in a preferred embodiment the radial alignment between the electromagnetic poles 131 on the stator 118 to strip magnets 174, 175 on the radial inside surface of the cylindrical rotor component 122 is incomplete, and / or the alignment between the strip magnets 174, 175 on the radial inside surface of the cylindrical rotor component 122 and the arc magnets 173 on the radial outside surface of the cylindrical rotor component 122 is incomplete, and / or the alignment between the arc magnets 173 on the radial outside surface of the cylindrical rotor component 122 to the arc magnets 163 on the radial inside surface of the outer cylindrical element 116 is incomplete. Hence the magnetic elements will not all reach full registration with all adjacent magnetic element at the same time / position.

[0090] To achieve this, in a preferred embodiment, per pitch revolution there is a mismatch in the number of electromagnetic poles 131 on the stator 118 to strip magnets 174, 175 on the radial inside surface of the cylindrical rotor component 122. There are fewer (preferably twelve) electromagnetic poles 131, and more (preferably sixteen) strip magnets 174, 175.

[0091] In a preferred embodiment, per pitch revolution there is the same number of strip magnets 174, 175 (preferably sixteen) on the radial inside surface of the cylindrical rotor component 122 as there are arc magnets 173 (preferably sixteen) on the radial outside surface of the cylindrical rotor component 122.

[0092] In a preferred embodiment, per pitch revolution there is a mismatch between the number of arc magnets 173 (preferably sixteen) on the radial outside surface of the cylindrical rotor component 122 to the number of arc magnets 163 (preferably nine) on the radial inside surface of the outer cylindrical element 116.

[0093] This is most clearly illustrated in Figure 8D. Advantageously, this arrangement - in particular with reference to the nine arc magnets 163 per pitch revolution of the outer cylindrical element 116 versus the sixteen arc magnets 173 of the radial outer surface of the cylindrical rotor component 122 - minimizes the ‘cogging’ between the arc magnets 163 and 173 as they relatively rotate. With a dissimilar non-integer divisible number of arc magnets 163 and 173 facing one another there is no preferential low energy position at which the magnets may align in attempting to minimize repulsive forces. This advantageous configuration is particularly preferred for actuator 100 applications that require very low noise and highly efficient movement.

[0094] Accordingly, in use, the actuator 100 system begins in a state of stability, with total repulsive magnetic force as experienced by the entire system minimized, and total attractive forces maximised. A rotation of the cylindrical rotor component 122 only occurs after a change in phase of the electromagnets 131. The phase change causes a new state, in which the cylindrical rotor component 122 is now orientated so that permanent strip magnets 174, 175 are incrementally offset from an electromagnet 131 of opposite polarity. This maximizes the tangential attractive and repulsive magnetic forces, and such forces are all in the same rotational direction, thereby forcing the cylindrical rotor component 122 to rotate in one direction. Hence the pattern of the controlled electrical operation of the different electromagnets 131 is performed such that each magnetized arm 129 of the stator is applying force in the same direction upon the facing magnetic thread 174, 175. This cumulative force acts to turn the cylindrical rotor component 122 in a preferred direction.

[0095] The numerical mismatch between the magnetic elements as previously described ensures that at least some of the electromagnets 131 are always partially facing two separate strip magnets 174, 175, and are never fully registering with a single strip magnet 174, 175 and hence the cylindrical rotor component 122 will always have a preferred direction of motion in which it can move. This preferred direction of rotation occurs in part because the electromagnet 131 is always facing at least one element (or a partial segment) of a strip magnet 174, 175 that is of the same polarity as itself and is repulsive. Hence this arrangement advantageously reduces ‘cogging’ as previously noted. After each further change in phase, the mismatch configuration of the magnets results in the cylindrical rotor component 122 rotating in the desired direction. The magnetic mismatch configuration always creates an unambiguous direction of preferred rotation for the cylindrical rotor component 122 to move to a stable position.

[0096] Similarly, the configuration of the magnetic mismatch between the arc magnets 173 on the radial outer face of the cylindrical rotor component 122 and the arc magnets 163 on the radially inner face of the outer cylindrical element 116, formed by the fewer number of arc magnets 163 (e.g. nine) per pitch revolution of the outer cylindrical element 116 versus the larger number of arc magnets 173 (e.g. sixteen) of the radial outer surface of the cylindrical rotor component 122 cause the same effect on the relative linear motion of the outer cylindrical element 116 in which there is a preferred direction of movement.

[0097] Alternative embodiments are envisaged for actuator 100 applications that for instance feature a ‘parked’ and loaded position, where for instance the number of opposing arc magnets 163 on the cylindrical rotor component 122 and the number of arc magnets 173 on the outer cylindrical element 116 could be made equal, similar, or integer-divisible. This system would still achieve a good torque and unambiguous direction. Further, a comparatively large radial adjacent airgap could be introduced between adjacent arc magnets 163 and / or between arc magnets 173 to promote cogging, and thereby advantageously creating a magnetic ‘detent’ relieving current demand on the windings 128 of the stator 118.

[0098] As previously noted, the pitch ‘d’ of the helical threads 171, 172 of the external face of the cylindrical rotor component are similar or the same as the pitch ‘d’ of the helical threads 161, 162 of the internal face of the outer cylindrical element 116. Advantageously, for applications which desire maximum shear force between the arc magnets 173 on the outer radial face of the cylindrical rotor component 122 and the arc magnets 163 on the outer cylindrical element 116, this is achieved at ’A pitch ‘d’ distance axial registration between the helical threads 161, 162 of the outer cylindrical element 116 and the helical threads 171, 172 of the cylindrical rotor component 122. Between arrangements in which the axial registration between helical threads 161, 162 of the outer cylindrical element 116 and the helical threads 171, 172 of the cylindrical rotor component 122 is in maximal alignment and arrangements in which they are aligned at A pitch distance ‘d’, the shear force approximates a sine function between 0 and u / 2.

[0099] The fewest number of helices that can function in this manner as a magnetic thread is two: one helical thread entirely magnetized in the north to south direction and the next adjacent helical thread entirely magnetised in the south to north direction (or vice versa). In this preferred embodiment, the lead distance - the axial advance during one complete turn (360°) - of the actuator 100 will equal the pitch distance ‘d’. In alternative embodiments desiring different leads distances, multiples of two helices can be added between the lead distance to produce magnetic threads with multiple starts, which can thereby be used to tailor the compliance or rigidity of the actuator 100 at the design stage for different applications.

[00100] Embodiments may provide certain characteristics and advantages. High off-power axial registration between the outer cylindrical element 116 and the cylindrical rotor component 122 ensures that the actuator 100 can continue to support a load when no power is applied, dependent upon designed cogging torque. The feature of high off-power axial registration presents a failsafe, as the magnets of the cylindrical rotor component 122 will resist linear force applied against the outer cylindrical element 116. For example, in a system including the actuator 100 for use in a prosthetic leg, this feature ensures that the leg does not collapse under the applied weight of the user when the actuator 100 is in the off-power state. High on-power axial registration and high on-power radial registration delivers high torque and therefore high axial force under application of power.

[00101] Embodiments including high off-power magnetic axial registration and low off-power magnetic radial registration, as well as the low friction in the magnetic helix, gives the ability for the cylindrical rotor component 122 to rotate in the off-power state, as the permanent magnets of the twin helices on both the outer cylindrical element 116 and the cylindrical rotor component 122 continue to interact in a thread like manner independent of power.

[00102] The high off-power axial registration created through the magnetic patterns shown in Figures 8 to 10, ensures that a large force is required in the linear axial direction to cause movement solely in this direction, as the magnetic interaction between the helices of both the outer cylindrical element 116 and the cylindrical rotor component 122 provide strong resistance to this movement. Similarly, the low off-power radial registration created through the magnetic patterns shown in Figures 8 to 10 allows free rotary movement of the cylindrical rotor component 122. This combination of highly restricted movement along the linear axial direction, and free movement in the rotary direction, combine to allow the actuator 100 to rotate under application of linear axial force when in the off-power state. This ability for the actuator 100 to linearly translate motion freely is advantageous in applications that aim to utilise body movement to conserve battery power, as well as simulate human-like movement.

[00103] The high off-power axial registration and the low off-power radial registration, are related by tuning the cogging torque produced by the transverse cross sectional profile of the ferromagnetic material of the stator. It is possible to envisage other embodiments based on the same design in which the design is exploited for different advantages e.g. to increase the ability for the component 122 (the bolt) to rotate in the off-power state under application of an external linear force, by increasing the helical pitch ‘d’. This decreases the mechanical advantage and leaves the actuator less able to support a load in the off-power state. Increased pitch delivers greater axial movement for a given rotation but delivers less axial force under application of power. Correspondingly, if the magnets can be made smaller and the pitch decreased, mechanical advantage can be increased but more turns of the component 122 (bolt) are required to give the same axial displacement. Alternative Embodiments

[00104] 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.

[00105] In the embodiments provided above, the stator arms 129 extend in a straight line along the long axis 106 of the actuator 100 and extend parallel to the long axis 106. However, alternative arrangements are envisaged to better suit some applications.

[00106] For instance, in applications which require very low back drive force and very low audible noise, it is important to minimise the cogging torque between the strip magnets 174, 175 of cylindrical rotor component 122 and the ferrous laminate arms 129 of the stator 118. In particular, low back-drive force and very low audible noise are attractive qualities for actuators used in prosthetic applications as easy back-drive facilitates free-swing of the limb without external power, thereby enabling longer usage, and low noise is a desirable quality for use of the actuator 100 in such close proximity to the human user. Additionally, the low back-drive force advantage is also important in aerospace applications, in which actuators 100 are often arranged in parallel to support a load. In these arrangements, in the event of a single failed actuator, the remaining functioning actuators need to be sufficiently able to move the failed actuator. This task is significantly easier if the failed actuator is easily back-driveable with a low back-drive force. Further still, reduced cogging also enables much more precise and accurate ‘jitter-free’ positioning in use, as the controller is not battling to position the cylindrical rotor component 122 in a particular orientation against the inherent preferred cogged positions. Such ‘jitter-free’ movement is useful in many applications, but is particularly advantageous to moving optical systems.

[00107] In an alternative embodiment, one advantageous solution to these problems, and to in general combat cogging between the strip magnets 174, 175 of cylindrical rotor component 122 and the ferrous laminates of the central stator 118, is to choose a non-integer divisible number of stator arms 129 teeth to strip magnetsl74, 175. However, a disadvantage associated with this approach is that it can produce disbalanced orbiting radial forces, and, at some scales, the need for excessively wide stator arms 129 teeth which occupy too large a volume and hence do permit space for sufficient amounts of windings 128 for high power operation.

[00108] In an alternative embodiment, a separate advantageous approach as envisaged is to skew either the strip magnets 174, 175 or to skew the stator arms 129 teeth along the length of the central stator 118, where the skewing is such that the feature in question extends along the axial length of the central stator element 126 but is not parallel to the long axis 106 of the stator 118.

[00109] According to this alternative embodiment, Figure 11A shows a cross-section view through the stator 118 with just the stator arms 129 shown, and Figure 1 IB shows a side-view of the stator 118 with just the stator arms 129 shown. In particular, Figures 11A and 1 IB show the stator arms 129 teeth skewed along the length of the stator 118. In particular, the location of the stator arms 129 teeth on one extremity end of the stator 118 is relatively rotated by ¼ stator tooth pitch ‘x’ with respect to the location of the stator arms 129 teeth on the other extremity end of the stator 118. Advantageously, this arrangement significantly reduces (and in some instances almost eliminates) cogging torque. Additionally, the stator back electromotive force profile becomes much more sinusoidal, rather than trapezoidal, helping reduce torque ripple and consequent operational noise. This also aids electrical to mechanical energy conversion efficiency, as a 'purer' sine wave has fewer higher frequency harmonics. Losses associated with the stator material include eddy current formation and magnetic hysteresis loss. Both of these losses are proportional to frequency raised to an exponent value.

[00110] Optionally, in addition to this embodiment or any preceding embodiment, the internal diameter of the cavity at the centre of the stator arms 129 of stator 118 may additionally include a splined feature 182 to further secure it against a feature at the core of the stator 118, and thereby prevent relative rotation when large torques are developed between the cylindrical rotor component 122 and the stator 118. Alternatively, other fixing methods such as a key and keyway might be used.

[00111] According to an alternative embodiment, Figure 12 shows a side view of the outer cylindrical element 116 of the actuator 100. In particular, the outer cylindrical element 116 is the same as any previously described outer cylindrical element 116, with the exception of the differences described below. In particular, as previously described, the internal radial face of the outer cylindrical element 116 includes a lining 144 covering the entire inner radial surface and extending along the full axial length of the outer cylindrical element 116. The lining 144 is preferably a continuous non-ferrous, non-electrically conductive and non-eddy current forming membrane, such as a polymer membrane. Advantageously, this lining 144 reduces eddy currents forming and creating undesired magnetic field interactions. However, ferrous membranes may also be used. However, in certain applications of the actuator 100 it may be desirable to have a ‘snubbing’ function that passively retards the speed of the actuator 100 as it approaches the ends of its movement in either fully extending or fully retracting. Advantageously, this may be achieved by replacing sections of the lining 144 (preferably a polymer membrane) in the proximity of either end 134 of the outer cylindrical element 116 with an electrically conductive element 184. For instance, these sections of conductive element 184 may extend a short distance, such as approximately 5% to 15%, along the axial length of the outer cylindrical element 116 and around the entire inner radial surface of the outer cylindrical element 116 within this.

[00112] Preferably, the electrically conductive element 184 may be made of aluminum or copper. In particular, in direct contrast to the function of the lining 144, the conductive element 184 allows electrical eddy currents to form and thereby cause a retarding Lorentz force which will oppose the rotational movement of the cylindrical rotor component 122. Accordingly, a desirous snubbing action will be created which passively increases corresponding to the rate at which the arc magnets 173 on the radially external face of the cylindrical component 122 move relative to the conductive element 184. Even further advantageously, this configuration will not adversely affect the advantage of low back-force drivability as the rate of back drive will be proportionately slow, so the eddy current forming effect will be much reduced.

[00113] As an alternative to or in addition to the electrically conductive element 184, the collar 110 may extend along the inside of the stator. The collar may be made of copper or other electrically conductive materials of low friction, providing a good thermal interface to the shaft 108. Thus, the material of the stator can be selected for its magnetic properties and the material of the collar can be selected for its thermal properties.

[00114] Fig. 13 shows the outer cylindrical element 116 and the outer magnetic element 123 A of the cylindrical rotor component 122 in schematic detail (not to scale). In the illustration, these components are shown in cross section on either side of the central axis. Dotted lines represent half-turns of the inner helix. The pitch of each turn is a matter of design choice (smaller pitch - greater mechanical advantage; greater pitch - greater velocity ratio.) There is an air gap 1301 between the components on the upper side and an air gap 1302 between them on the lower side. As illustrated, the magnetic fields of adjacent magnetic elements of each of the outer cylindrical element 116 and the outer magnetic element 123 A are alternately polarised. (There may be regular gaps between adjacent magnets, filled with resin or other binding material, but these are all of equal width and are not shown.) In the illustration, the north poles of the outer cylindrical element 116 are positioned face-to-face with the south poles of the outer magnetic element 123 A and vice versa. This is the position of maximum attraction between the corresponding helices. A force applied along the axis will be resisted by the strong magnetic attraction between the oppositely polarized facing magnetic elements.

[00115] The forces perpendicular to the axis are strong but, in the ideal situation they are evenly balanced. In practice, if the central shaft is not sufficiently rigid, there will be slight bending to one side or the other. If, for example, gap 1301 is slightly less than gap 1302, there will be a net force perpendicular to the axis, in the upwards direction in the figure. Since the magnetic attraction across the gap 1301 is inversely proportional to the square of the gap distance, a small closing of the gap leads to a large increase in the attractive force. The opposite occurs on the opposite side. The gap 1302 will open by a corresponding amount and the force on that side will reduce with the square of the gap. Thus, a slight bending of the central shaft can lead to an unstable situation and lead to mechanical failure. This phenomenon limits the shaft length or mandates a thick, heavy shaft.

[00116] To reduce shaft weight or permit a longer shaft and therefore longer range of operation of the actuator, it is advantageous to introduce a gap between turns of one or other of the outer cylindrical element 116 and the cylindrical rotor component 122. Preferably a gap equivalent to a fraction of a pitch plus N pitches is introduced, where N is an integer. Thus there are at least two sets of helices of a selected pitch (p), with a gap between the two sets equal to F p + N / 2 where Fisa fraction (less than one) of p. In other words, the gap is not an integer multiple of p.

[00117] A single gap in the cylindrical rotor component 122 is sufficient and it is preferably located at a mid-point along the length of the cylindrical rotor component 122. This is illustrated in Figure 14.

[00118] Figure 14 shows a gap of a fraction of one pitch (i.e. N=0) half way along the component 123 A. By virtue of this gap, only half of the face-to-face magnetic elements are fully aligned north-south (those to the right as illustrated). The other half (to the left) are misaligned, thereby reducing the transverse force at that side. The total transverse force is substantially reduced, allowing for a longer shaft or a thinner shaft.

[00119] The gap between adjacent pitches is of width Vp + Np, where F is preferably less than 1 / 2. It is sufficient that F is between about 1 / 8 and 1 / 4.

[00120] The gap 1400 gives rise to a further advantage, which is reduction in longitudinal “give” between the outer cylindrical element 116 and the cylindrical rotor component 122. If, for example, there is a gap between each magnetic element and its adjacent magnetic element, i.e. a gap between the turns of the helix, this gives rise to longitudinal “give” or “play” whereby change of the longitudinal force (load) on the actuator can cause a small unwanted movement. Provision of the gap reduces this.

[00121] Instead of providing the gap 1400 in the cylindrical rotor component 122, one or more gaps can be provided in the outer cylindrical element 116. As this component is longer, it is preferred that several gaps are provided at different positions along its length, preferably spaced apart by about half the length of the cylindrical rotor component 122. At such spacing, there will always be at least one gap along the overlap between the two parts.

[00122] Thus, a means of increasing output rigidity is to ‘split’ the helices of the rotor into two (23, 24) (or more) and space these helices a distance slightly smaller than 1 pitch distance apart (or ‘N’ pitches plus this distance) (25). This can lessen the ultimate shear force available for thrusting against a load in either direction and is particularly advantageous in making very long actuator strokes. This is because as the external rotor magnets move relative to the helical tube magnets to * / 2 pitch alignment, the radial forces transition from attractive to repulsive; consequently, near the ’A pitch registration position the radial concentric constraint demand greatly reduces permitting actuators with much greater stroke length as the beams that concentrically constrain the rotor (26) are under much reduced loading.

[00123] According to another independent aspect of the invention, an actuator is provided comprising: a first fitting at a first end and a second fitting at a second end; a stator, connected to the first end, comprising electromagnetic sectors for generating phased electromagnetic fields around the stator; a first cylindrical element, the stator and the first cylindrical element being arranged concentrically around a central axis, the first cylindrical element having permanent magnetic elements arranged as at least a first helix and further arranged such that phased magnetization of the stator causes the first cylindrical element to rotate about the axis; a second cylindrical element, connected to the second end and arranged concentrically with respect to the first cylindrical element, the second cylindrical element having permanent magnetic elements arranged as at least a second helix in screw engagement with the first helix, whereby rotation of the first cylindrical element with respect to the second cylindrical element causes a longitudinal force to be exerted along the axis between the first and second fittings; and a shaft extending from at least one of the first and second ends to the stator, wherein the first helix comprises first element arc magnets and the second helix comprises second element arc magnets and there is a mismatch between the number of first and second element arc magnets per pitch revolution.

[00124] This aspect has the independent advantage that it reduces cogging.

[00125] The first element arc magnets and the second element arc magnets are preferably magnetized in a direction that is generally radial with respect to the axis. They may be magnetized in a parallel manner (i.e. parallel within each arc magnet) or radial manner (i.e. the lines of magnetization are themselves radial). References 100 - actuator 102 - casing 104 end fittings 104A - dynamic moving end of actuator 104B - static end of the actuator, formed by outer casing 114 106 - axis 107 - hollow part of shaft 108 - shaft 109 - telescopic part of shaft 110 — collar 111- sensor 112- second outer casing 114- first outer casing 116- outer cylindrical element 118- stator 119- distal tightening nut 120 - roller bearings for interface between stator 118 and hollow shaft potion 108 122 - cylindrical rotor component 123 - magnetic element of cylindrical rotor component 122 123A - radially outer magnetic arrangement of cylindrical rotor component 122 123B - radially inner magnetic arrangement of cylindrical rotor component 122 124 - magnetic element of outer casing 114 (i.e. inner face of helical tube) 126 - central stator element 128 - windings of central stator element 129 - stator arms 130 - ball bearings for interface between central stator element and cylindrical rotor component 122 131 - electromagnetic poles formed by central stator element 126 132 - load bearing struts 133, 134 - end caps of outer cylindrical element 116 135 - space for rotor bearings 130 136 - fixing points for load bearing struts 132 138 - slider part 139 - slider bush 140 - circular profile portion of shaft 108 142 - angular portion (flattened end) of shaft 108. 144 - polymer lining 146 - fixing points of shaft 108 to end caps 134 of outer cylindrical element 148 - end cap holes for load bearing struts 132 150 - actuator end cap at static outer casing 114 end 152 - electronic circuitry 154 - heat pipe for coolant 155 - fixing 5 156 - electrical wiring 158 - barrier membrane / bellows 160 - (polymer) membrane of cylindrical rotor component 122 161, 162 helical threads of outer cylindrical element 116 163 - arc magnets of outer cylindrical element 116 10 171, 172 - helical threads of cylindrical rotor component 122 173 - arc magnets of cylindrical rotor component 122 174, 175 - strip magnets of cylindrical rotor component 122 180 - gap between arc magnets 182 - splines 15 184 - conductive element

Claims

1. An actuator comprising:a first fitting at a first end and a second fitting at a second end;a stator, connected to the first end, comprising electromagnetic sectors for generating phased electromagnetic fields around the stator;a first cylindrical element, the stator and the first cylindrical element being arranged concentrically around a central axis, the first cylindrical element having permanent magnetic elements arranged as at least a first helix and further arranged such that phased magnetization of the stator causes the first cylindrical element to rotate about the axis;a second cylindrical element, connected to the second end and arranged concentrically with respect to the first cylindrical element, the second cylindrical element having permanent magnetic elements arranged as at least a second helix in screw engagement with the first helix, whereby rotation of the first cylindrical element with respect to the second cylindrical element causes a longitudinal force to be exerted along the axis between the first and second fittings; anda shaft extending from at least one of the first and second ends to the stator, characterized by:at least one conduit extending from the first end alongside at least part of the shaft to provide cables and / or cooling to the stator.

2. An actuator according to claim 1, wherein the second cylindrical element has an end cap at each of first and second ends thereof and the at least one conduit passes through holes in one of the end caps.

3. An actuator according to claim 1 or 2, wherein the conduit or each conduit is formed within one or more load bearing struts rigidly connecting the stator to the first fitting.

4. An actuator according to claim 3, wherein:the shaft extends from an end cap of the second cylindrical element at the second end of the actuator to an opposite end cap of the second cylindrical element and is fixed to both end caps; andthe stator is free to move longitudinally relative to the shaft.

5. An actuator according to any one of claims 1 to 3, wherein the shaft has first and second telescopic parts, the first part being connected to the first end and the second part connected to the second end.

6. An actuator according to claim 5, wherein the first part is arranged to extend out from and retract into the second part and wherein the at least one conduit extends generally parallel to the first part.

7. An actuator according to any one of claims 1 to 6, comprising at least one cable conduit extending alongside at least part of the shaft and at least one coolant conduit extending alongside at least part of the shaft.

8. An actuator according to any one of claims 1 to 6 comprising at least one coolant conduit extending alongside at least part of the shaft to a heat sink at the first end of the actuator.

9. An actuator according to claim 8 wherein the heat sink comprises a set of external fins.

10. An actuator comprising:a stator comprising electromagnetic sectors for generating phased electromagnetic fields around the stator; anda first cylindrical element, the stator and the first cylindrical element being arranged concentrically around a central axis, the first cylindrical element having permanent magnetic elements magnetized radially and arranged as at least a first helix;a second cylindrical element arranged concentrically with respect to the first cylindrical element, the second cylindrical element having permanent magnetic elements magnetized radially and arranged as at least a second helix in screw engagement with the first helix; characterized by a set of strip magnets arranged circumferentially around the first cylindrical element, whereby phased magnetizationof the stator causes the first cylindrical element to rotate with respect to the second cylindrical element, thereby causing a longitudinal force to be exerted along the axis.

11. An actuator according to claim 10, wherein the magnets of the set of strip magnets are magnetized radially and are alternately polarized.

12. An actuator according to claim 11, wherein the first cylindrical element surrounds the stator, the second cylindrical element surrounds the first cylindrical element and the strip magnets are located inside the first helix of permanent magnetic elements.

13. An actuator according to claim 10, 11 or 12, wherein the strip magnets are arranged generally longitudinally with respect to the central axis.

14. An actuator according to claim 13 wherein the strip magnets are arranged with a skew angle relative to a direction parallel to the central axis.

15. An actuator according to any one of the preceding claims, wherein the stator comprises stator arms extending generally longitudinally with respect to the central axis with a skew angle relative to a direction parallel to the central axis.

16. An actuator according to any one of the preceding claims wherein the stator is provided with circumferential rollers within which the shaft can pass or with or other means for permitting prismatic translation while permitting longitudinal movement of the shaft within the stator and resisting torque.

17. An actuator comprising:a stator comprising electromagnetic sectors for generating phased electromagnetic fields around the stator; anda first cylindrical element;the stator and the first cylindrical element being arranged concentrically around a central axis, the first cylindrical element having permanent magnetic elements magnetized radially and arranged as at least a first helix of pitch p;a second cylindrical element arranged concentrically with respect to the first cylindrical element, the second cylindrical element having permanent magnetic elementsmagnetized radially and arranged as at least a second helix of pitch p in screw engagement with the first helix; characterized in thatone of the first helix and the second helix is split by a circumferential gap.

18. An actuator according to claim 17, wherein the width of the circumferential gap is not an integer multiple ofp.

19. An actuator according to claim 17, wherein the width of the circumferential gap is equal to Yp + Np, where Fisa fraction less than 1 / 2 and N is an integer.

20. An actuator according to claim 19, where F is about 1 / 8 to 1 / 4.

21. An actuator according to any one of claims 17 to 20, wherein the circumferential gap isin the first helix and the second helix is continuous.

22. An actuator according to any one of the preceding claims, wherein the stator includes internal bearings that interface with the shaft for facilitating relative linear movement of the shaft.

23. An actuator according to any one of the preceding claims, wherein the stator includes an internal collar that interfaces with the shaft for facilitating relative linear movement of the shaft, where the collar is of a different material to that of the stator and is selected for greater heat conductivity.

24. An actuator according to any one of the preceding claims, wherein the second cylindrical element is internally lined with a membrane of continuous non-ferrous, nonelectri cally conductive and non-eddy current forming material.

25. An actuator according to any one of the preceding claims wherein the first helix comprises radially magnetized arc magnets and the second helix comprises radially magnetized arc magnets and wherein there is a mismatch between the number of first and second arc magnets per pitch revolution.Application No: GB2319993.8 Examiner: Jonathan MarlowClaims searched: 1-9, 15, 16, 22-25 Date of search: 4 June 2024Patents Act 1977: Search Report under Section 17Documents considered to be relevant:Category Relevant to claims Identity of document and passage or figure of particular relevance X A 1, 2, 5-9, 15, 16, 22, 24 EP 2976828 Bl (WAVEDRIVES) Figures 9, 10 &23-27, and paragraphs [0011 ]-[0012], [0051], [0088] &[0090], GB 2539202 A (ELUMOTION) Figures 1 &2 and claims 1, 3 &4.Categories:v Au Document indicating lack of novelty or inventive step A Document indicating technological background and or state of the art. Y Document indicating lack of inventive step if p Document published on or after the declared priority date but combined with one or more other documents of same category. before the filing date of this invention. & Member of the same patent family E Patent document published on or after, but with priority date earlier than, the filing date of this application.Field of Search:International Classification:Subclass Subgroup Valid From WCiOIZ rrUZix 0041 / 03 01 / 01 / 2006 H02K 0007 / 06 01 / 01 / 2006Application No: GB2319993.8Examiner:Jonathan MarlowClaims searched: 10-16, 22-25Date of search: 27 February 2025Patents Act 1977Further Search Report under Section 17Documents considered to be relevant:Category Relevant to claims Identity of document and passage or figure of particular relevance v A X 10-14, 24, 25 10-16, 22, 24 EP 3484024 Al (SCHUNK ELECTRONIC SOLUTIONS) Figures 1-5 and paragraphs [0018]-[0024], EP 2976828 Bl (WAVEDRIVES) Figures 6-9, 26 &27 and paragraphs [0011], [0012], [0040] &[0090],Categories:X Document indicating lack of novelty or inventive step A Document indicating technological background and / or state of the art. Y Document indicating lack of inventive step if p Document published on or after the declared priority date but combined with one or more other documents of same category. before the filing date of this invention. & Member of the same patent family E Patent document published on or after, but with priority date earlier than, the filing date of this application.Field of Search:International Classification:Subclass Subgroup Valid From H02K 0041 / 03 01 / 01 / 2006 H02K 0007 / 06 01 / 01 / 2006Application No: GB2319993.8Examiner:Jonathan MarlowClaims searched: 17-25Date of search: 27 February 2025Patents Act 1977Further Search Report under Section 17Documents considered to be relevant:Category Relevant to claims Identity of document and passage or figure of particular relevance v A 17-20, 24, 25 EP 3484024 Al (SCHUNK ELECTRONIC SOLUTIONS) Figures 1-5 and paragraphs [0012], [0018], [0021], [0023] &[0024], X 17, 18, 21, 22, 24 EP 2976828 Bl (WAVEDRIVES) Figures 6-9 &27 and paragraphs [0011], [0012], [0036], [0037] &[0090], X 17, 18, 21,24 GB 2539202 A (ELUMOTION) Figures 2a-d and paragraphs [0007]-[0012].Categories:X Document indicating lack of novelty or inventive step A Document indicating technological background and / or state of the art. Y Document indicating lack of inventive step if P Document published on or after the declared priority date but combined with one or more other documents of same category. before the filing date of this invention. & Member of the same patent family E Patent document published on or after, but with priority date earlier than, the filing date of this application.Field of Search:International Classification:Subclass Subgroup Valid From H02K 0041 / 03 01 / 01 / 2006 H02K 0007 / 06 01 / 01 / 2006

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