Electromagnetic linear actuator

Abstract

A linear actuator comprising: a first fixed part comprising a linear array of stator teeth, each tooth surrounded by an electrical coil; a controller generating a set of currents applied to phase windings of the first fixed part so as to generate a pattern of magnetic poles along the teeth of the array; a second fixed part comprising a set of alternating magnetic poles; and a movable output part comprising a linear array of pole pieces extending along a length of the output part, the length being greater than the length of the stroke of the actuator and longer than the length of the fixed parts, whereby in use the length of the movable output part is always located between the first and second fixed parts, wherein the pole pieces located between the fixed parts shape the magnetic flux acting between the poles of the first fixed part and the poles of the second fixed part, the controller being arranged in use to generate linear movement of the output part by moving the magnetic poles of the first fixed part along the array.

Description

TECHNICAL FIELD

[0001] The present invention relates to linear actuators, and in particular to electromagnetic linear actuators suitable for applications requiring high travel length along a line. BACKGROUND

[0002] A linear actuator is a device that produces motion of an output member, typically an elongate rod, along a straight or curved line. Various types of linear actuators are known. One of the earliest types of linear actuators is a screw drive, in which a threaded rod is engaged with a threaded body that can be rotated so as to cause the threaded rod to move along a linear path. Devices incorporating these types of linear actuators are widely used in automotive jacks, since rotation of the fixed component can be provided by human power. In another class of linear actuators, hydraulic pressure in a cylinder can be used to drive a piston along a linear path.

[0003] Another type of linear actuator is a linear motor. These linear motors operate in a similar manner to conventional motors, but they have a stator and a component functionally equivalent to a rotor that is arranged along a line rather than wound around an axis. Thus, an output is a translational motion rather than a rotational motion. A simple linear motor has a set of windings on a fixed component that produces a magnetic flux that interacts with a magnetic flux pattern produced by a set of magnets mounted on a movable elongate component. By moving the magnetic flux pattern along the stator, the Lorentz force will cause the linear member to move along a linear path as the magnetic flux pattern from the permanent magnets attempts to align themselves with the magnetic flux pattern generated by the stator.

[0004] The Applicant has recognised that known linear motors using permanent magnets on the moving component, while capable of producing high precision linear motion, can be extremely expensive in situations where it is desirable to produce a linear actuator with a large travel length and that can generate a large linear force. In order to produce a large force, the magnets arranged along the moving member must be capable of producing a high level of magnetic flux. Magnets capable of producing a high level of magnetic flux are relatively expensive, given the inclusion of rare earth elements in their composition. Moreover, since the magnets must be spaced along the entire length of the member, this will require a large number of magnets to be used on the moving member for a large travel length. This is quite wasteful, since in fact only the magnets that are aligned with the stator are working at any given time, the rest of the magnets are merely being carried along as the output part moves. SUMMARY

[0005] It is an object of the present invention to provide a linear actuator that ameliorates some of the limitations of prior art linear actuator designs.

[0006] According to a first aspect, the present invention provides a linear actuator assembly comprising:

[0007] a first fixed portion comprising a linear array of stator teeth, each stator tooth being surrounded by one or more turns of electrical coil,

[0008] a controller generating a set of currents applied to phase windings of the first fixed portion so as to generate a pattern of magnetic poles along the teeth of the array, the spacing between poles being greater than the spacing between adjacent teeth of the first fixed portion,

[0009] a second fixed portion comprising a set of alternating magnetic poles, the spacing between adjacent poles being less than the spacing of the poles of the first array generated by the controller, and

[0010] a moveable output portion comprising a linear array of pole pieces extending along a length of the output portion, the length being greater than the length of the stroke of the actuator and longer than the length of the fixed portions, whereby in use the length of the moveable output portion is always located between the first fixed portion and the second fixed portion, wherein the pole pieces located between the fixed portions shape the magnetic flux acting between the poles of the first fixed portion and the poles of the second fixed portion, and

[0011] wherein in use the controller is arranged to cause linear movement of the output portion by causing movement of the poles of the first fixed portion along the array.

[0012] With the present invention, the controller generates a moving magnetic flux pattern at the first stator which interacts with a similar moving magnetic flux pattern from the second fixed portion which has been shaped by the pole pieces of the output portion. The speed of movement of the poles with the shaped magnetic flux pattern will be lower than the speed of movement of the poles of the first fixed portion but in the same direction, which provides a degree of gearing to the actuator giving a high force density. Because the moveable array comprises only relatively low cost pole pieces, rather than fixed magnets as known in the prior art, a cost effective linear actuator can be produced using only pole pieces spaced along the output rather than magnets.

[0013] The first fixed portion can define two magnetic poles, the second fixed portion can define more than two magnetic poles, and the pole pieces can shape the magnetic flux from the poles of the second magnetic poles so as to generate two magnetic poles in the region where the magnetic flux from the first fixed portion intersects the magnetic flux from the second fixed portion.

[0014] In one possible arrangement, the second fixed part can comprise 21 magnetic poles generated by 21 magnets, and for all positions of the output part along its travel, the output part can carry 12 pole pieces along the part of the output part between the two fixed parts, with the first fixed part generating two pole pairs, each pole pair comprising a pair of north poles or a pair of south poles.

[0015] Thus, in possible examples, the output part will need to have more than 12 pole pieces along its length.

[0016] Other numbers of magnetic poles and pole pieces can be used, with the aim of matching the pole pieces to couple the strong, desired first harmonic of the magnetic flux from the second fixed part to the pattern of poles generated by the windings and teeth of the first fixed part.

[0017] The pole pieces of the output part can comprise pole pieces of ferrous metal, most preferably steel pole pieces. The pole pieces can be supported by a carrier that does not comprise ferrous metal. For example, the pole pieces can be fixed to or embedded in the carrier. Unlike linear actuators of the prior art, the pole pieces do not need to be magnets, as their function is merely to shape the magnetic flux from the second fixed part in the air gap between the two fixed parts.

[0018] The fixed parts can be fixed in a reference frame, with the movable array translating relative to the frame along the length of the travel. The fixed reference frame can be fixed relative to the ground, or relative to the body of a vehicle that carries the linear actuator. For example, the fixed reference frame can be fixed relative to a reference frame of a platform on which the linear actuator is fixed. Fixing apparatus, such as brackets or supports, can be provided for fixing the fixed parts in place, for example for fixing the fixed parts to the body of a vehicle or other fixed parts of a vehicle. The fixing apparatus can comprise openings for receiving fasteners, such as bolts. The fixing apparatus can comprise a base plate that is fixed to or integral with the first fixed part and / or the second fixed part.

[0019] The output part can comprise an elongate member having a constant cross-section along a major part of its length, which cross-section can pass between the fixed parts. The cross-section can be a square, rectangular or circular cross-section, but the cross-section can envisage any cross-section.

[0020] The output member can have a length of at least 30 cm, or at least 1 m or more. In theory, there is no limit to the stroke of the linear actuator, but in practice the limit will depend on frictional forces, and if the output portion is moved vertically, the limit will depend on the weight of the output portion. Because only low cost pole pieces are required compared to magnets, a linear actuator with a large stroke length can be implemented in a cost effective manner.

[0021] The second fixed portion can comprise a linear array of permanent magnets, each magnet defining a pole of the second fixed portion. The magnets can be arranged in an alternating north-south pattern so as to produce alternating north and south magnetic poles in the air gap between the second fixed portion and the output member.

[0022] In an alternative, the second fixed portion can comprise a linear array of electromagnets. Each electromagnet can comprise a tooth around which a length of wire is wound to form a coil. The coil can be supplied with current from a controller which generates a fixed, non-moving magnetic flux pattern defining a pole of the second fixed portion.

[0023] A single controller can drive the windings of the first and second fixed portions, but of course different patterns of current can be applied to the coils of each fixed portion when one requires a moving DC field and the other requires a fixed DC field.

[0024] The use of electromagnets to generate the poles of the second fixed portion can be preferable where cost minimisation is to be achieved, but at the expense of the electrical efficiency of the linear actuator due to Ohmic losses and potentially an increase in weight.

[0025] The Applicant has recognised that the above described arrangement with first and second fixed portions and an output portion therebetween will generate some sideways force on the output portion.

[0026] Accordingly, the output portion can be supported at spaced apart locations along its length by one or more bearing assemblies which resist the sideways force.

[0027] In an alternative arrangement, which can be combined with the use of bearings if required, the linear actuator can further comprise:

[0028] a third fixed portion comprising a linear array of stator teeth, each stator tooth being surrounded by one or more turns of electrical coil,

[0029] the controller generating a set of currents which are applied to the phase windings of the third fixed portion so as to generate the same alternating pattern of magnetic poles along the teeth of the array as the first fixed portion, and

[0030] a second moveable output section comprising a linear array of pole pieces, at least a portion of the moveable output section being located between the second fixed section and the third fixed section.

[0031] The number and location of the pole pieces matches that of the first moveable output section, and shapes the magnetic flux acting between the poles of the second fixed section and the third fixed section.

[0032] This arrangement places the second fixed component in the middle of a sandwich construction, with the sandwich component having one moving section on each side, and the first and third fixed sections forming the outermost layers of the sandwich.

[0033] The two moveable output sections can be mechanically connected or can be formed from a single unitary component. For example, there can be an elongate component having an elongate slot along its length that receives the second fixed component, with the two sets of pole pieces extending along opposite sides of the slot.

[0034] All of the fixed sections and moving sections can lie in a single plane, with each array being rectilinear.

[0035] Alternatively, each of the fixed sections and moving sections can be axisymmetric, and have a cylindrical or partially cylindrical cross-section along their length.

[0036] According to a second aspect, the present application provides a linear actuator for use with a controller to form a linear actuator assembly according to the first aspect of the present application, the actuator body comprising:

[0037] a first fixed section comprising a linear array of stator teeth, each stator tooth being surrounded by one or more turns of electrical coil,

[0038] a second fixed section comprising sets of alternating magnetic poles, and

[0039] a moveable output section comprising a linear array of pole pieces extending along a length of the output section, the length being greater than the length of the stroke of the actuator and longer than the length of the fixed sections, whereby in use the length of the moveable output section is always located between the first fixed section and the second fixed section, wherein the pole pieces located between the fixed sections shape the magnetic flux acting between the poles of the first fixed section and the second fixed section. BRIEF DESCRIPTION OF DRAWINGS

[0040] Four embodiments of the present application will now be described by way of example only, in which:

[0041] Figure 1This is a plan view of a first embodiment of a linear actuator according to the present invention, wherein the output portion is facing one end of the actuator travel and is in the retracted position;

[0042] Figure 2 To illustrate the linear actuator, corresponding to Figure 1 The plan view shows the output section facing the opposite end of the actuator stroke and in the fully extended position;

[0043] Figure 3 To form Figure 1 A schematic diagram of the main components of the actuator body, which is a part of the linear actuator;

[0044] Figure 4 To and Figure 3 A schematic diagram of the main components of a similar actuator body, which forms part of a second embodiment of a linear actuator according to one aspect of the invention;

[0045] Figure 5 To and Figure 3 A schematic diagram of the main components of a similar actuator body, which forms part of a third embodiment of a linear actuator according to one aspect of the present invention;

[0046] Figure 6 (a) in the middle shows Figures 1 to 5 The teeth and windings of the first or third fixed part (in the case of the third fixed part) of the actuator. Figure 6 (b) shows the flux patterns generated corresponding to the two poles when current is applied to the winding using the first set of current waveforms; and Figure 6 (c) shows the movement of the patterns of the two poles along the teeth of the array when different sets of current waveforms are applied;

[0047] Figure 7 (a) in the middle shows Figure 1 The arrangement of permanent magnets in the second fixed section of a linear actuator, wherein there is no output section and no first fixed section. Figure 7 (b) shows the effect of the output section modifying the magnetic flux, where the first fixed section is also absent, and Figure 7 (c) shows the same modified flux pattern moving along the array due to the different alignment of the output section with the second fixed section, wherein the output section has moved to the right by a distance equal to the spacing between adjacent pole pieces; and

[0048] Figure 8 This is an isometric view, which shows Figure 2 A possible, alternative fourth embodiment of the linear actuator shown. Detailed Implementation

[0049] A linear actuator according to an aspect of the present application comprises an actuator having a fixed body 30 and a moving output portion 40. The fixed body portion is fixed to a platform 50 or other support and is not intended to move in use. The moving output portion 40 performs work and in use moves linearly, reciprocally along a path from a retracted position as shown in Figure 1 to an extended position as shown in Figure 2 The actuator comprises an arrangement of ferrous poles and electromagnets. The motion is generated by Lorentz forces acting between the fixed body 30 and the moving output portion 40 and is controlled by a controller 20 which controls the flow of current through the windings of the electromagnets in the fixed body portion 30 in a manner to be described below.

[0050] The linear actuator can be arranged in various ways and a first embodiment 100 is shown in Figure 3 of the drawings. In this embodiment the actuator 100 comprises two elongate fixed portions 1, 2 arranged in parallel and facing each other across an air gap. A moving output portion 3 is located in the air gap and comprises an elongate rod. The rod has a length along its long axis which is longer than the length of the air gap so that for any position along its travel a part of the rod rather than the whole rod is located in the air gap. A bearing assembly (not shown) can be provided to laterally support the rod so as to ensure that the rod is held central in the air gap between the two elongate fixed portions.

[0051] The first of the elongate fixed portions 1 comprises a linear array of stator teeth 6 extending from a continuous back iron 7 which extends from one end of the fixed portion to the other. In this example there are 12 teeth equally spaced along the back iron. Each tooth 6 is surrounded by one or more turns of electrical coil forming one winding 8 of a set of coil windings. The windings are phased and the windings are connected to the outputs of the controller.

[0052] The controller generates a set of phase currents which are applied to the windings 8 of the first fixed portion 1 so as to generate a pattern of magnetic poles along the teeth of the array. In this example a current waveform is applied which generates a pattern with two pairs of poles (two north poles and two south poles) with equal spacing between the poles. Thus the number of poles is less than the number of teeth. Importantly, by varying the current applied to the windings the pattern of poles can be moved along the first fixed portion. Figure 6(a) through (c) illustrate how the pattern of the poles can be moved along the teeth of the array simply by changing the current applied to the winding. As can be seen, the spacing between the two poles does not change; only the pattern moves along the first fixed portion 1.

[0053] The elongated second fixed portion 2 includes an elongated array of permanent magnet poles 5, which alternate between north and south poles along the length of the fixed portion 2. These permanent magnet poles are supported by an elongated backplate. The spacing between adjacent poles is smaller than the spacing between the poles of the first array generated by the controller, and in this example, there are 21 magnets evenly spaced along the elongated second fixed portion. In this example, the first and second fixed portions have the same length.

[0054] The movable output section 3 includes an elongated, iron-free carrier 9 that supports a linear array of electrodes 4 extending along the entire length of the output section. At any point along the travel of the output section, 12 electrodes are located in an air gap between two fixed sections. The electrodes 4 are evenly spaced along the carrier. Each electrode 4 includes a portion of iron-containing material.

[0055] The presence of the pole piece 4 in the air gap shapes or distorts the magnetic flux pattern from the magnet 5 in the second fixed section. This can... Figure 7 As seen in the image. Without pole pieces, the magnetic flux from magnet 5 in the air gap, originating from the second fixed section, will be as follows: Figure 7 As shown in (a) above. With the output section in the proper position, the magnetic flux is shaped to form a flux pattern similar to that produced using only two magnetic pole pairs on the second fixed section, such as... Figure 7 As shown in (b) above. In fact, a small movement of the output section relative to the second fixed section will result in the same flux pattern, but offset along the direction of movement of the output section, as... Figure 7 As shown in (c). The reason for this formation can be well understood in the context of pseudo-direct drive motors, and is specifically discussed in international patent application WO 2007 / 125284 under the name of the University of Sheffield, in which the concept was used in the design of rotary motors.

[0056] The reader will appreciate that the modified flux pattern from the permanent magnets 5 will interact with the similar flux pattern from the electromagnets of the first fixed part and by moving the pole pattern of the first fixed part, Lorentz forces will act on the pole pieces 4 of the output part, moving it so as to realign the flux pattern to a position of the output part where no forces act. As a result of this, the flux from the second fixed part is changed and this causes the output part 3 to move at a slower speed than the movement of the pole pattern of the first fixed part. The result is a form of magnetic transmission which is advantageous in terms of providing a high force density for a linear actuator.

[0057] Figure 4 A view of a second embodiment of a linear actuator 200 according to an aspect of the application. In this arrangement, a first fixed part 201 and a moving output part 202 are provided which are identical in shape and function to those of the first embodiment 100. Figure 3 The difference in this embodiment is in the arrangement of a second fixed part 203 which comprises a set of electromagnets 204 rather than a set of permanent magnets. Each electromagnet comprises a tooth around which a coil of wire is wound in a conventional manner. The electromagnets 204 are not modulated but are simply driven by applying a current to the winding of each electromagnet which produces a fixed DC field which is functionally identical to that produced by the permanent magnets of the first embodiment 100. Figure 3 When the electromagnets are driven, they present an alternating north and south pole to the moving output part 202. In a simple arrangement, every other tooth is wound in series to form a set which will provide a north pole and every other tooth interleaved is wound in series to form a second set which will form a south pole.

[0058] Figure 5 A view of a third embodiment 300 of a linear actuator according to an aspect of the application. In this embodiment, a first fixed part 301 and a second fixed part 302 are identical in shape and function to those of the first embodiment but as shown there is a further third fixed part 303 located on the opposite side to the second fixed part 302 so as to define a second air gap. The output part is divided into two along its length so as to define a central slot 304 which extends around the second fixed part. This second fixed part comprises an elongate support for a set 309 of alternating north south pole permanent magnets. Thus the output part can be considered to form two parallel, elongate output parts 305, 306. Each of the two elongate output parts carries a respective row of pole pieces 307, 308 which are arranged on each side of the slot 304 so that one set of pole pieces is located in the air gap between the first fixed part and the second fixed part and the other set of pole pieces is located in the same air gap between the second fixed part and the third fixed part.

[0059] By making the first and third fixed parts 301, 303 the same and applying the same current waveform, the air gap is made the same on each side of the second fixed part, and by making the two rows of pole pieces the same, the side forces acting on the output part of the embodiment of Figure 3 and Figure 4 may be eliminated. This can allow the omission of a bearing assembly, as the balanced forces will hold the output shaft firmly, preventing lateral movement in use.

[0060] Of course, the same effect can be achieved with different components, by controlling the current applied to the windings in each of the fixed parts to compensate for any variations. Also, the skilled person will appreciate that the permanent magnets of the second fixed part can be replaced by electromagnets as shown in the second embodiment, without materially changing the way the actuator operates.

[0061] The skilled person will appreciate that many modifications are possible within the scope of the invention. In particular, although the three embodiments described above include magnets and pole pieces that are generally all located in one plane, the carriers for the magnets can be arranged so that the magnets are not all located in the same plane. An example of such a design for a linear actuator 400 is shown in cross-section in Figure 8 where the outer fixed part 410 comprises a tube having annular teeth 420 spaced along its bore. The moving part 403 comprises a smaller tube that fits within the outer fixed part. This smaller tube supports annular ferrous pole pieces 404 spaced along its length. These pole pieces are shown in broken line. The second fixed part 405 comprises a rod that sits within the bore of the moving part and carries a fixed magnet 406, also shown in broken line. The two fixed parts are fixed to a base plate 407 at one end.

[0062] The skilled person will also appreciate that the term "linear" used throughout this document is intended to cover translational motion along a line, as distinct from rotational motion, and thus covers linear motion along a straight line as well as curvilinear motion along a curve, within the scope of the invention.

Claims

1. A linear actuator assembly, the linear actuator assembly comprising: The first fixed section includes a linear array of stator teeth, each stator tooth being surrounded by one or more turns of electrical coil. A controller generates a set of currents applied to the phase windings of the first fixed portion to generate a pattern of magnetic poles along the teeth of the array, the spacing between the poles being greater than the spacing between adjacent teeth of the first fixed portion. The second fixed portion includes alternating groups of magnetic poles, wherein the spacing between adjacent poles of the second fixed portion is smaller than the spacing between the magnetic poles of the first array generated by the controller, and A movable output section includes a linear array of pole pieces extending along a length greater than the stroke length of the linear actuator assembly and longer than the length of the fixed section, such that in use, this length of the movable output section is always located between the first and second fixed sections, wherein the pole pieces located between the fixed sections shape the magnetic flux acting between the poles of the first and second fixed sections. In use, the controller is arranged to cause linear movement of the output section by moving the magnetic poles of the first fixed section along the array.

2. The linear actuator assembly according to claim 1, wherein, The first fixed portion defines two magnetic poles, the second fixed portion defines more than two magnetic poles, and the pole piece shapes the magnetic flux from the second magnetic pole to generate two magnetic poles in the region where the magnetic flux from the first fixed portion intersects with the magnetic flux from the second fixed portion.

3. The linear actuator assembly according to claim 1 or 2, wherein, The electrodes of the movable output section consist of iron-containing metal electrodes supported by an iron-free carrier.

4. The linear actuator assembly according to claim 1 or 2, wherein, The movable output section includes an elongated member having a constant cross-section along the main portion of its length.

5. The linear actuator assembly according to claim 1 or 2, wherein, The second fixed portion includes: a linear array of permanent magnets, each magnet defining a pole of the second fixed portion; or a linear array of electromagnets.

6. The linear actuator assembly of claim 1, wherein the movable output portion is a first movable output portion, and the linear actuator assembly further comprises: The third fixed section includes a linear array of stator teeth, each stator tooth being surrounded by one or more turns of electrical coil. The controller generates a set of currents applied to the phase windings of the third fixed portion to generate magnetic poles along the teeth of the array in the same alternating pattern as the first fixed portion. The second movable output section includes a linear array of pole pieces, and at least a portion of the second movable output section is located between the second fixed section and the third fixed section.

7. The linear actuator assembly of claim 6, wherein, The number and position of the poles of the second movable output section match the number and position of the poles of the first movable output section, and shape the magnetic flux acting between the poles of the second fixed section and the poles of the third fixed section.

8. The linear actuator assembly according to claim 6 or 7, wherein, The first movable output portion and the second movable output portion are mechanically connected or formed from a single integral component.

9. The linear actuator assembly according to any one of claims 1, 2, and 6, wherein, All the fixed parts and all the movable output parts are located in a single plane, where each array is linear.

10. A linear actuator for use with a controller to form a linear actuator assembly according to any one of claims 1 to 9, said linear actuator comprising: The first fixed section includes a linear array of stator teeth, each stator tooth being surrounded by one or more turns of electrical coil. The second fixed part includes a group of alternating magnetic poles, and A movable output section includes a linear array of pole pieces extending along a length of the output section, the length of which is greater than the stroke length of the linear actuator assembly and longer than the length of the fixed section, such that the length of the movable output section is always located between the first fixed section and the second fixed section during use, wherein the pole pieces located between the fixed sections shape the magnetic flux acting between the poles of the first fixed section and the poles of the second fixed section.

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