CN112421925A - Electromagnetic linear actuator - Google Patents

Abstract

A linear actuator comprising: a first stationary part comprising a linear array of stator teeth, each tooth being surrounded by an electrical coil; a controller that generates a set of currents that are applied to the phase windings of the first stationary portion to generate a pattern of magnetic poles along the teeth of the array; a second stationary portion 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 stroke length of the actuator and longer than the length of the fixed part, 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 magnetic flux acting between the poles of the first and second fixed parts, the controller being arranged in use to produce linear movement of the output part by moving the poles of the first fixed part along the array.

Description

Electromagnetic linear actuator

Technical Field

The present invention relates to linear actuators, and in particular to electromagnetic linear actuators suitable for applications requiring high stroke lengths along a line.

Background

A linear actuator is a device that produces movement of an output member (typically an elongated 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 to move the threaded rod along a linear path. Devices incorporating these types of linear actuators are widely used in automotive jacks because rotation of the fixed part can be provided by manual force. In another type of linear actuator, hydraulic pressure in a cylinder may be used to drive a piston along a linear path.

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 the rotor, arranged along a line rather than wound about an axis. Thus, a translational motion is output rather than a rotational motion. A simple linear motor has a set of windings on a fixed part that produce magnetic flux that interacts with a flux pattern (flux pattern) produced by a set of magnets mounted on a movable elongate part. By moving the magnetic flux pattern along the stator, the lorentz forces will move the linear member along a linear path when the magnetic flux patterns from the permanent magnets attempt to align themselves with the magnetic flux pattern generated by the stator.

The applicant has realised that known linear motors using permanent magnets on the moving part, although capable of producing high precision linear motion, can be extremely expensive in the case where it is desired to produce a linear actuator having a large stroke length and which can generate large linear forces. In order to generate a large force, the magnets arranged along the moving member must be able to generate a high level of magnetic flux. Considering that a rare earth element is contained in the composition of the magnet, the magnet capable of generating a high level of magnetic flux is relatively expensive. Also, because the magnets must be spaced along the entire length of the member, this would require the use of a large number of magnets on the moving member for a large stroke length. This is quite wasteful in that in practice only the magnets aligned with the stator are active at any given time, the remaining magnets being carried only as the output section moves.

Disclosure of Invention

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

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

a first stationary part comprising a linear array of stator teeth, each stator tooth being surrounded by one or more turns of an electrical coil,

a controller that generates a set of currents that are applied to the phase windings of the first stationary 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 stationary portion,

a second stationary portion comprising a set of alternating magnetic poles, the spacing between adjacent poles being less than the spacing of the magnetic poles of the first array produced by the controller, an

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 stroke length of the actuator and longer than the length of the fixed part, whereby in use the length of the movable output part is always between the first and second fixed parts, wherein the pole pieces between the fixed parts shape the magnetic flux acting between the poles of the first and second fixed parts, and

wherein, in use, the controller is arranged to cause linear movement of the output portion by moving the poles of the first fixed portion along the array.

By means of the 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 stationary part, which has been shaped by the pole piece of the output part. The speed of movement of the poles with the shaped flux pattern will be lower than the speed of movement of the poles of the first fixed part but in the same direction, which provides a degree of drive for the actuator, giving a high force density. Because the movable array includes only relatively low cost pole pieces, rather than fixed magnets as known in the art, the use of only pole pieces spaced along the output, rather than magnets, can produce a cost effective linear actuator.

The first stationary portion may define two poles, the second stationary portion may define more than two poles, and the pole piece may shape magnetic flux from a pole of the second pole to generate the two poles in a region where the magnetic flux from the first stationary portion intersects the magnetic flux from the second stationary portion.

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

Thus, the output section in a viable example would need to have more than 12 pole pieces along its length.

Other numbers of poles and pole pieces may be used in order for the pole pieces to couple the strong, ideal first harmonic of the magnetic flux from the second stationary part to the matching pattern of the poles generated by the windings and teeth of the first stationary part.

The pole pieces of the output section may comprise ferrous metal pole pieces, most preferably steel pole pieces. The pole pieces may be supported by a carrier that is non-ferrous. For example, the pole pieces may be fixed to the carrier or embedded within the carrier. Unlike prior art linear actuators, the pole pieces need not be magnets, as their function is simply to shape the magnetic flux from the second stationary part in the air gap between the two stationary parts.

The fixed portion may be fixed in a reference frame, wherein the movable array translates relative to the frame along the stroke length. The fixed frame of reference may be fixed relative to the ground or relative to the body of the vehicle carrying the linear actuator. For example, the fixed reference frame may be fixed relative to the reference frame of the platform on which the linear actuator is fixed. A fixing device, such as a bracket or support, may be provided for fixing the fixing portion in position, for example for fixing the fixing portion to the body of a vehicle or other fixed component of the vehicle. The securing apparatus may include an opening for receiving a fastener such as a bolt. The fixing device may comprise a substrate fixed to or integral with the first and/or second fixing portions.

The output portion may comprise an elongate member having a constant cross-section along a major part of its length, which cross-section may pass between the fixed portions. The cross-section may be a square, rectangular or circular cross-section, but any cross-section is envisaged.

The output member may have a length of at least 30cm, or at least 1m or more. In theory, there is no limit to the stroke of the linear actuator, but in practice the limit will depend on the friction and if the output part moves vertically, the limit will depend on the weight of the output part. Since only low cost pole pieces are required compared to magnets, a linear actuator with a large stroke length can be realized in a cost-effective manner.

The second stationary part may comprise a linear array of permanent magnets, each magnet defining one pole of the second stationary part. The magnets may 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 stationary part and the output member.

In the alternative, the second stationary part may comprise a linear array of electromagnets. Each electromagnet may include a tooth around which a length of wire is wound to form a coil. The coil may be supplied with current from a controller that generates a fixed, non-moving flux pattern defining poles of the second fixed portion.

A single controller may drive the windings of the first and second fixed parts, but of course different modes of current may be applied to the coils of each fixed part when one requires a moving DC field and the other requires a fixed DC field.

Using an electromagnet to generate the poles of the second fixed part may be preferred if costs are to be minimized, but results in a compromise of the electrical efficiency of the linear actuator due to ohmic losses and potentially an increase in weight.

The applicant has realised that the above arrangement with first and second fixed portions and an output portion therebetween will generate some lateral force on the output portion.

Thus, the output section may be supported at spaced locations along its length by one or more bearing assemblies that resist lateral forces.

In an alternative arrangement (which may be combined with the use of bearings if desired), the linear actuator may further comprise:

a third stationary part comprising a linear array of stator teeth, each stator tooth being surrounded by one or more turns of an electrical coil,

the controller generates a set of currents that are applied to the phase windings of the third fixed portion to generate the same alternating pattern of magnetic poles along the teeth of the array as the first fixed portion, an

A second movable output portion comprising a linear array of pole pieces, at least a portion of the movable output portion being located between the second fixed portion and the third fixed portion.

The number and position of the pole pieces are matched to those of the first movable output part and the magnetic flux acting between the poles of the second fixed part and the third fixed part is shaped.

This arrangement places the second fixing part in the middle of the sandwich construction, where the sandwich element has one moving part on each side, and the first and third fixing parts form the outermost layers of the sandwich.

The two movable output portions may be mechanically connected or may be formed from a single unitary member. For example, there may be an elongate member having an elongate slot along its length which receives the second securing member, the two sets of pole pieces extending along opposite sides of the slot.

All of the fixed and moving parts may lie in a single plane, with each array being rectilinear.

Alternatively, each of the fixed and moving parts may be axisymmetric and have a cylindrical or partially cylindrical cross-section along their length.

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

a first stationary part comprising a linear array of stator teeth, each stator tooth being surrounded by one or more turns of an electrical coil,

a second stationary part comprising a set of alternating magnetic poles, an

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

Drawings

Four embodiments of the invention will now be described, by way of example only, in which:

FIG. 1 is a plan view of a first embodiment of a linear actuator according to the present invention with the output portion toward one end of the actuator stroke and in a retracted position;

FIG. 2 is a plan view corresponding to FIG. 1 showing the linear actuator with the output portion toward the opposite end of actuator travel and in a fully extended position;

FIG. 3 is a schematic diagram of the major components of an actuator body forming part of the linear actuator of FIG. 1;

FIG. 4 is a schematic illustration of the major components of an actuator body similar to FIG. 3, which forms part of a second embodiment of a linear actuator in accordance with an aspect of the present invention;

FIG. 5 is a schematic view of the major components of an actuator body similar to FIG. 3, which forms part of a third embodiment of a linear actuator in accordance with an aspect of the present invention;

fig. 6 (a) shows the teeth and windings of the first or third fixed part (where the third fixed part is present) of the actuator of fig. 1 to 5, and fig. 6 (b) shows the magnetic flux pattern generated corresponding to the two poles when current is applied to the windings using the first set of current waveforms; and (c) in fig. 6 shows the movement of the pattern of the two poles along the teeth of the array when a different set of current waveforms is applied;

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

fig. 8 is an isometric view showing a possible, alternative fourth embodiment of the linear actuator shown in fig. 2.

Detailed Description

A linear actuator according to an aspect of the present invention includes an actuator having a fixed body 30 and a moving output portion 40. The fixed body portion is fixed to the 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 movement is caused by the lorentz force acting between the fixed body 30 and the moving output portion 40 and is controlled by the 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 hereinafter.

The linear actuator may 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 elongated fixed parts 1, 2 arranged in parallel and facing each other across an air gap. The moving output part 3 is located in the air gap and comprises an elongated rod. The rod has a length along its long axis that is longer than the length of the air gap, such that for any position along its stroke, a portion of the rod, but not the entire rod, is located in the air gap. A bearing assembly (not shown) may be provided to laterally support the rod to ensure that the rod is held centrally in the air gap between the two elongate fixing portions.

A first one of the elongate stationary portions 1 comprises a linear array of stator teeth 6 extending from a continuous back iron (back iron)7 extending from one end of the stationary 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 an electrical coil forming one winding 8 of a set of coil windings. The windings are configured in phase and the windings are connected to an output of the controller.

The controller generates a set of phase currents which are applied to the windings 8 of the first stationary part 1 so as to generate a pattern of magnetic poles along the teeth of the array. In this example, a current waveform is applied that: this current waveform 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 smaller 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. Fig. 6 (a) to (c) show how the pattern of poles is moved along the teeth of the array simply by varying the current applied to the windings. As can be seen, the spacing between the two poles does not change, but only the pattern moves along the first stationary part 1.

The elongate second fixing part 2 comprises an elongate array of permanent magnet poles 5 alternating between north and south poles along the length of the fixing part 2. The permanent magnet poles are supported by an elongated tailgate. The spacing between adjacent poles is less than the spacing of the poles of the first array produced by the controller and in this example there are 21 magnets equally spaced along the elongate second fixed portion. In this example, the first and second fixing portions have the same length.

The movable output section 3 comprises an elongate, non-ferrous carrier 9 which supports a linear array of pole pieces 4 extending along the entire length of the output section. For any position of the output portion along its stroke, 12 of the pole pieces are located in the air gap between the two fixed portions. The pole pieces 4 are evenly spaced along the carrier. Each pole piece 4 comprises a portion of ferrous material.

The presence of the pole piece 4 in the air gap shapes or distorts the flux pattern from the magnet 5 of the second stationary part. This can be seen in fig. 7. Without the pole pieces, the flux in the air gap from the magnet 5 of the second stationary part would be as shown in fig. 7 (a). With the output part in place, the flux is shaped to form a flux pattern: this flux pattern is similar to that produced with only two pole pairs on the second stationary portion, as shown in fig. 7 (b). In practice, a small distance of movement of the output portion relative to the second fixed portion will result in the same magnetic flux pattern, but offset in the direction of movement of the output portion, as shown in fig. 7 (c). The reason for this shaping is well understood in the context of a pseudo direct drive motor and a specific discussion is given in international patent application WO 2007/125284 in the name of University of sheffield, where the concept is used in the design of a rotating electrical machine.

The reader will understand that the modified magnetic flux pattern from the permanent magnet 5 will interact with a similar magnetic flux pattern from the electromagnet of the first fixed part and by moving the pole pattern of the first fixed part, a lorentz force will act on the pole piece 4 of the output part, thereby moving the output part so as to realign the magnetic flux pattern to the position of the output part where no force is acting. Since this causes movement of the output portion 3, the magnetic flux from the second fixed portion is changed, and this causes the output portion 3 to move at a slower speed than the moving speed of the pole pattern of the first fixed member. The result is a form of magnetic transmission which is advantageous in providing a high force density for the linear actuator.

Fig. 4 is a view of a second embodiment of a linear actuator 200 according to an aspect of the present invention. In this arrangement, a first stationary part 201 and a moving output part 202 are provided, which are identical in shape and function to those of the first embodiment 100 of fig. 3. This embodiment differs, among other things, in the arrangement of the second stationary part 203, which comprises a set of electromagnets 204 instead of 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 windings of each electromagnet which produces a fixed DC field which is functionally the same as that generated by the permanent magnets of figure 3. The electromagnet, when driven, presents alternating north and south poles for the moving output section 202. In a simple arrangement every other tooth is wound in series to form a group that will provide a north pole, and every intervening tooth is wound in series to form a second group that will form a south pole.

Fig. 5 is a view of a third embodiment 300 of a linear actuator in accordance with an aspect of the present invention. In this embodiment, the first fixing part 301 and the second fixing part 302 are identical in shape and function to those of the first embodiment, but as shown in the figure, there is an additional third fixing portion 303 located on the opposite side of the second fixing portion 302 so as to define a second air gap. The output portion is bifurcated along its length to define a central slot 304 extending around the second fixing element. The second stationary part comprises an elongated support for groups 309 of alternating north and south pole permanent magnets. Thus, the output section can be considered to form two parallel, elongated output sections 305, 306. Each of the two elongate output portions carries a respective row of pole pieces 307, 308 arranged on each side of the slot 304 such that one set of pole pieces is located in the air gap between the first and second fixed portions and the other set of pole pieces is located in the same air gap between the second and third fixed portions.

By making the first and third fixed portions 301, 303 identical and applying the same current waveform so that the air gap is the same on each side of the second fixed portion, and by making the two rows of pole pieces identical, the lateral forces acting on the output portion of the embodiment of figures 3 and 4 can be eliminated. This may allow the bearing assembly to be omitted as the balanced forces will hold the output shaft securely, preventing lateral movement when in use.

Of course, the same effect can be achieved with different components by controlling the current applied to the windings of each of the fixed components to compensate for any variations. Furthermore, the skilled person will understand that the permanent magnets of the second stationary part may be replaced by electromagnets as shown in the second embodiment without substantially changing the way the actuator operates.

The skilled person will understand 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 generally all lie in one plane, the carrier for the magnets may be arranged so that the magnets do not all lie in the same plane. An example of such a design of a linear actuator 400 is shown in cross-section in fig. 8, where the outer stationary part 410 comprises a tube with annular teeth 420 spaced along its bore. The moving part 403 comprises a smaller tube fitting within the outer fixed part. The smaller tube supports annular ferrous pole pieces 404 spaced along its length. These pole pieces are shown in dashed lines. The second stationary part 405 comprises a rod which is located in the bore of the moving part and which carries a stationary magnet 406, also shown in dashed lines. The two fixed portions are fixed at one end to the bottom plate 407.

The skilled person will also understand that the term "linear" as used throughout this document is intended to cover translational movements along the line of the output portion as opposed to rotational movements, and therefore linear movements along a straight line as well as curvilinear movements along a curved line are encompassed within the scope of the present invention.

Claims (10)

1. A linear actuator, the linear actuator comprising:

a first stationary part comprising a linear array of stator teeth, each stator tooth being surrounded by one or more turns of an electrical coil,

a controller that generates a set of currents that are applied to the phase windings of the first stationary portion so as to generate a pattern of magnetic poles along the teeth of the array, a spacing between the poles being greater than a spacing between adjacent teeth of the first stationary portion,

a second stationary part comprising a set of alternating magnetic poles, the spacing between adjacent poles of the second stationary part being less than the spacing of the magnetic poles of the first array produced by the controller, an

A movable output part comprising a linear array of pole pieces extending along a length of the output part which is greater than the stroke length of the actuator and longer than the length of the fixed part, whereby in use the length of the movable output part is always between the first and second fixed parts, wherein the pole pieces between the fixed parts shape the magnetic flux acting between the poles of the first and second fixed parts, and

wherein, in use, the controller is arranged to cause linear movement of the output portion by moving the poles of the first fixed portion along the array.

2. The linear actuator of claim 1 wherein the first fixed portion defines two poles, the second fixed portion defines more than two poles, and the pole piece shapes magnetic flux from a pole of the second pole to generate two poles in an area where the magnetic flux from the first fixed portion intersects the magnetic flux from the second fixed portion.

3. A linear actuator according to claim 1 or 2, wherein the pole pieces of the movable output part comprise ferrous metal pole pieces supported by a non-ferrous carrier.

4. A linear actuator according to any preceding claim, wherein the movable output section comprises an elongate member having a constant cross-section along a major part of its length.

5. The linear actuator according to any one of the preceding claims, wherein the second stationary part comprises: a linear array of permanent magnets, each of said magnets defining a pole of said second stationary portion; or a linear array of electromagnets.

6. The linear actuator according to any one of the preceding claims, further comprising:

a third stationary part comprising a linear array of stator teeth, each stator tooth being surrounded by one or more turns of an electrical coil,

the controller generates a set of currents to be applied to the phase windings of the third fixed portion to generate the same alternating pattern of magnetic poles along the teeth of the array as the first fixed portion, an

A second movable output portion comprising a linear array of pole pieces, at least a portion of the movable output portion being located between the second fixed portion and the third fixed portion.

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

8. A linear actuator according to claim 6 or 7, wherein the two movable output parts are mechanically connected or formed from a single integral member.

9. A linear actuator according to any preceding claim, wherein all fixed and moving parts lie in a single plane, with each array being rectilinear.

10. A linear actuator for use with a controller so as to form a linear actuator assembly according to any one of claims 1 to 9, the linear actuator comprising:

a first stationary part comprising a linear array of stator teeth, each stator tooth being surrounded by one or more turns of an electrical coil,

a second stationary part comprising a set of alternating magnetic poles, an

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

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