GB2467363A

GB2467363A - A linear actuator

Info

Publication number: GB2467363A
Application number: GB0901611A
Authority: GB
Inventors: Shinichiro Iwasaki; Rajesh Deodhar; Adam Pride
Original assignee: IMRA Europe SAS; IMRA Europe SAS UK Research Center
Current assignee: IMRA Europe SAS; IMRA Europe SAS UK Research Center
Priority date: 2009-01-30
Filing date: 2009-01-30
Publication date: 2010-08-04
Also published as: GB0901611D0

Abstract

A linear actuator comprises an armature 106 and a stator 102 where the armature 106 includes first and second magnets and the stator 102 includes at least one coil arrangement 104. The actuator is arranged such that during use a single magnetic field loop is provided. The armature 106 may have planar magnets secured in a spaced formation, with opposing polarisation, within a planar body of plastic material, which allows a light weight armature to be obtained. If a single sided coil arrangement 104 is employed the armature 106 may include a magnetic-metallic material bridging portion on the side opposite to that of the coil arrangement 104. A number of magnets and coils may be employed according to the force and length of stroke characteristics required of the actuator. Three phase current drive arrangements may also be implemented. Alternatively, the invention may include a linear actuator comprising an armature 106 and a stator 102 where the stator 102 comprises a magnetic core of laminated material arranged such that the laminations lie in a plane which is parallel to the axis of movement of the armature 106.

Description

A Linear Actuator The present invention relates to a linear actuator. Particularly, but not exclusively, the present invention relates to a linear actuator comprising a movable member having a reciprocal linear movement.

A variety of linear actuators are known in the art. A common example is a simple solenoid, in which an armature moves relative to a housing having an electrical coil.

Such simple linear actuators are common in a wide variety of applications and fields of use.

However, there is a need to reduce the weight of linear actuators for various applications, for example, door mechanisms or locks. It is common for these applications to use an electrical motor and a suitable gear coupling to achieve the desired linear motion. However, in order to use a solenoid-type linear actuator for such an application, the actuator would need to be sufficiently lightweight.

It is an object of the present invention to provide an improved linear actuator.

According to one aspect of the invention there is provided a linear actuator comprising a coil body and a reciprocally-movable member, the coil body having at least one coil arrangement and the reciprocally-movable member being linearly movable along an axis of movement relative to the coil body and comprising first and second magnets, wherein the first and second magnets and the or each coil arrangement are arranged such that a single magnetic field loop is formed therebetween.

A single magnetic field loop (as opposed to two or more magnetic loops used in known arrangements) does not require an iron core in order to complete the magnetic field loop, reducing the weight of the arrangement. Further, improved performance can be obtained from a linear actuator so-equipped.

It is desirable that the first and second magnets are spaced by a carrier member comprising a non-magnetic material. By providing such an arrangement, the weight of a linear actuator can be reduced because no iron core is needed in the movable member. The carrier member may be formed from a lightweight material (for example, a plastic) which reduces the weight of the arrangement.

It is an advantage if the first and second magnets are spaced along the axis of movement by the carrier member. This is a useful arrangement for efficient operation of the linear actuator.

Advantageously, the carrier member comprises a plastics material. Plastics materials are cheap to manufacture and can be moulded into a variety of shapes.

It is desirable that the carrier member forms a support structure for the first and second magnets. This is convenient and allows the carrier member to be formed in one piece.

It is desirable for the movable member to have a substantially planar shape. More desirably, each of the first and second magnets is substantially planar. A planar shape is useful for a variety of applications, for example, a door lock.

It is useful for the first and second magnets to have opposing field directions. This arrangement enables the single magnetic field ioop to be realised easily and simply.

In one arrangement, a first coil arrangement is provided on a first side of the movable member and a second coil arrangement is provided on a second side of the movable member, the single magnetic field ioop being formed between the first and second magnets and the first and second coil arrangements.

Preferably, the first and second coil arrangements each comprise separate first and second coils. Whilst additional coils are required, this arrangement enables more compact configurations and layouts to be achieved.

Advantageously, a first coil arrangement is provided on a first side of the movable member and a second coil arrangement is provided on a second side of the movable member, the first and second coil arrangements each comprising separate first, second and third coils and the magnetic field loop being formed between the first magnet, the second magnet and: a) the first and second coils of each of the first and second coil arrangements; or b) the second and third coils of each of the first and second coil arrangements; or c) the first and third coils of each of the first and second coil arrangements.

This arrangement is beneficial in providing a significantly greater stroke length when compared to a two coil arrangement design. Additional coils may be added to increase the stroke length further.

In an alternative arrangement, a single coil arrangement is provided on one side of the movable member and a bridging portion is located on the movable member, the bridging portion extending from a position adjacent the first magnet to a position adjacent the second magnet and comprising a material having magnetic properties.

Both of these alternative arrangements enable the single magnetic field loop to be realised in a simple, lightweight structure. Either two or more coils are used, in which case no iron is required in the movable member, or a single coil is used and a material having magnetic properties is used to complete the magnetic field ioop.

It is desirable for the bridging portion to be located on the opposite side of the movable member to the coil arrangement. Usefully, the bridging portion is formed from a metallic material. This can be in the form of a thin sheet of a metal such as iron. A thin sheet of metal will not add significantly to the weight of the linear actuator as a whole.

It is useful for the coil body to comprise a laminated material arranged such that the laminations lie in a plane which is substantially parallel to the axis of movement of the reciprocally-movable member. This arrangement substantially confines the

magnetic field lines to two dimensions.

Advantageously, at least a part of the or each coil arrangement extends through a part of the coil body. Usefully, said part of the or each coil arrangement extends through a part of the coil body in a direction substantially perpendicular to the plane of the laminations. This arrangement helps to confine the magnetic field lines in substantially two dimensions.

In one arrangement, the laminated material comprises iron. iron is cheap and has suitable magnetic properties.

In an alternative arrangement, the laminated material comprises silicon iron. Silicon iron is more expensive than iron, but has superior magnetic properties.

According to another aspect of the invention, there is provided a linear actuator comprising a coil body and a reciprocally-movable member, the coil body having at least one coil arrangement and the reciprocally-movable member being linearly movable along an axis relative to the coil body, wherein the coil body comprises a laminated material arranged such that the laminations lie in a plane which is parallel to the axis of movement of the reciprocally-movable member.

The above arrangement has low losses because the field lines do not pass through the laminate boundaries. This improves the efficiency of the arrangement because eddy current losses are reduced. Further, the efficiency of movement of the reciprocally-movable member is improved because the magnetic field lines are confined in the correct plane for efficient actuation of the movable member. Further, this arrangement substantially confines the magnetic field lines to two dimensions.

Advantageously, at least a part of the or each coil arrangement extends through a part of the coil body. Usefully, said part of the or each coil arrangement extends through a part of the coil body in a direction substantially perpendicular to the plane of the laminations. This arrangement enables the lines of magnetic flux to be in the same plane as the laminations.

Advantageously, the reciprocally-movable member comprises first and second magnets. Preferably, the first and second magnets are located in a carrier member formed from a non-magnetic material.

By providing such an arrangement, the weight of a linear actuator can be reduced because no iron core is needed in the movable member. The carrier member may be formed from a lightweight material (for example, a plastic) which reduces the weight of the arrangement.

An embodiment of the invention will now be described with reference to the accompanying drawings in which: Figure 1 is a perspective view of a linear actuator according to a first embodiment of the invention; Figure 2 is an end view of the linear actuator of Figure 1; Figure 3 is a side view of a movable member forming part of the linear actuator of Figure 1; Figure 4 is an end view of the movable member of Figure 3; Figure 5 is a side view of the linear actuator of Figure 1; Figure 6 is a cross-section of the linear actuator of Figure 1 taken along the plane Y-Y shown in Figure 5; Figure 7 is a view similar to Figure 6 but showing the movable member in a first stroke position; Figure 8 is a view similar to Figure 6 but showing the movable member in a second stroke position; Figure 9 is a perspective view of a linear actuator according to a second embodiment of the invention; Figure 10 is a cross-section of the linear actuator of Figure 9 taken along the plane Z-Z shown in Figure 11; Figure 11 is an end view of the linear actuator of Figure 9; Figure 12 is a perspective view of a linear actuator according to a third embodiment of the invention; Figure 13 is a plan view of the linear actuator of Figure 12; Figure 14 is a cross-section of the linear actuator of Figure 12; Figure 15 is a view similar to Figure 14 but showing a movable member in a first stroke position; Figure 16 is a view similar to Figure 14 but showing the movable member in a second stroke position; and Figure 17 is a graph showing the flux linkage as a function of stroke position.

Figures 1 and 2 show a linear actuator 100 according to a first embodiment of the invention. The linear actuator 100 comprises a coil body 102, a coil configuration 104 and a movable member 106.

The coil body 102 functions as a housing for the coil configuration 104 and has a first coil housing 108 and a second coil housing 110. The first and second coil housings 108, 110 are each substantially cuboid and are spaced from one another to define a narrow rectangular channel 112 therebetween. The first and second coil housings 108, are connected to one another by means of a first connector 114 located at an upper end thereof and a second connector 116 located at a lower end thereof. The first and second connectors 114, 116 delimit the upper and lower ends of the channel 112 respectively. The channel 112 is dimensioned and arranged to receive the movable member 106.

The coil configuration 104 comprises first and second coil arrangements 118, 120.

The first coil arrangement 118 comprises wire arranged into a flat rectangular pattern and is located partially within the first coil housing 108. The first coil arrangement 118 has first and second portions 11 8a, 11 8b which extend through a respective pair of thorough-holes 122, 124 formed in the first coil housing 108. The first and second portions 118a, 118b comprise a plurality of lengths of wire which are substantially straight, parallel to one another and extend in the direction of an axis A-A.

The second coil arrangement 120 also comprises wire arranged into a rectangular pattern and is located partially within the second coil housing 110. The second coil arrangement 120 has first and second portions 120a, 120b which extend through a corresponding pair of thorough-holes 126, 128 formed in the second coil housing 110.

The first and second portions 120a, 120b comprise a plurality of lengths of wire which are substantially straight, parallel to one another and extend in the direction of an axis B-B. The axis B-B is parallel to the axis A-A. The first and second coil arrangements 118, 120 oppose one another and are located on either side of the channel 112.

The movable member 106 is shown separately from the remainder of the linear actuator 100 in Figures 3 and 4. The movable member 106 functions as an armature in the linear actuator 100 and has a carrier member 130, a first magnet 132 and a second magnet 134. The carrier member 130 carries the first and second magnets 132, 134 and takes the form of a substantially rectangular, planar support member. The carrier member 130 has a projection 136 at one end. The projection 136 acts as a push-rod for the linear actuator 100, i.e. it is arranged to interact with whatever apparatus the linear actuator 100 is integrated into. This may be, for example, a door lock.

The carrier member 130 is not required to have any electromagnetic properties.

Consequently, the carrier member 130 can be formed from a non-magnetic, non-conductive material such as a plastic. Plastics are cheap to manufacture, can be formed into a range of shapes and sizes, and are light in weight. The use of plastic in the movable member 106 enables the weight of the movable member 106 to be reduced, concomitantly reducing the weight of the linear actuator 100 as a whole.

The first and second magnets 132, 134 are each located in the carrier member 130.

and each have an elongate, planar shape. The poles of the first and second magnets 132, 134 are opposed to one another, i.e. the North poles of each magnet 132, 134 face in opposite directions as shown in Figure 3. The first and second magnets 132, 134 are located within the carrier member 130 and spaced apart thereby in the direction of movement of the movable member 106. This will be discussed later.

Figure 5 shows a side view of the linear actuator 100 in more detail. In Figure 5, the movable member 106 is inserted partially into the channel 112. The movable member 106 is arranged to slide within the channel 112 between first and second positions along an axis of movement X-X in response to an electromotive force applied thereto.

The separation between the first and second positions (i.e. the distance of travel of the movable member 106) is known as the stroke of the linear actuator 100.

It can be seen from Figure 5 that the first coil housing 108 and second coil housing which form the coil body 102 comprise a laminate material. The coil body 102 is formed from a plurality of laminate layers 138 which lie in a plane Y-Y. The plane Y-Y is substantially parallel to the axis of movement X-X of the movable member 106.

Further, the plane Y-Y is perpendicular to the axes A-A and B-B of the first and second coil arrangements 118, 120.

As also shown in Figure 5, the wires of the first and second coil arrangements 118, 120 of the coil configuration 104 extend perpendicular to the plane Y-Y of the laminate layers 138. Consequently, the magnetic field lines generated when a current is passed along the first and second coil arrangements 118. 120 flow parallel to the plane Y-Y of the laminate layers 138.

This arrangement has low losses because the magnetic field lines are restricted to the plane of the laminates and so eddy current losses are reduced. This is because the laminate structure of the coil body 102 disrupts the magnetic field lines between the laminate layers 138, preventing the formation of eddy currents. Further, the efficiency of the system is improved because the magnetic field lines are essentially restricted to a two-dimensional arrangement which is beneficial for actuation of the movable member 106.

Figure 6 shows a section taken along the plane Y-Y of the laminate layers 138 shown in Figure 5. The current direction C and the resultant force direction F are shown in Figure 6. The current direction C is the same for the first and the second coil arrangements 118, 120 of the coil configuration 104. Note that, in this view, the movable member 106 is shown in the neutral position, i.e. in the position of zero stroke movement when the magnets 132, 134 are arranged centrally between the two coil arrangements 118, 120. This is the position of the movable member 106 when no current is applied to the coil arrangements 118, 120. The current directions shown in Figure 6 are to illustrate the relative directions of the magnetic field and current loops.

The movement of the movable member 106 will be described in more detail later.

As can be seen, a single magnetic field loop Ml is formed between the first and second coil arrangements 118, 120. The magnetic field ioop Ml extends from the first portion 1 18a of the first coil arrangement 118, through the first magnet 132 to the first portion 120a of the second coil arrangement 120, to the second portion 120b of the second coil arrangement 120, through the second magnet 134, to the second portion 118b of the second coil 118 and back to the first portion 118a of the first coil arrangement 118. In between these parts, the magnetic field passes through the laminated layers 138 of the coil housing 102.

By creating a single magnetic field ioop Ml within the linear actuator 100, there is no requirement for an iron core in the movable member 106 (i.e. in the armature) in order to complete the magnetic field loop. Consequently, the movable member 106 only requires two magnets 134, 136 spaced apart in the direction of movement of the movable member 106 by the non-conductive, non-magnetic carrier member 130.

Therefore, the weight and inertia of the movable member 106 is significantly reduced, improving the efficiency and energy consumption of the linear actuator 100 as a whole. The weight of the linear actuator 100 is also significantly reduced.

Each coil arrangement 118, 120 has a pair of core gaps 140, 142. A core gap 140, 142 is provided for each of the first and second portions 1 18a, 1 18b, 120a, 120b of the first and second coil arrangements 118, 120 respectively. As can be understood from Figure 6, because the magnets 132, 134 are required to complete the magnetic field loop Ml, the movable member 106 is only able to move through a distance corresponding to the length of the magnets 132, 134 whilst maintaining sufficient linearity of response. Further, it is desirable that the distal end of the first and second magnets 132, 134 (i.e. the end of the first magnet 132 furthest from the projection 136 on the front face of the movable member 106) does not enter into a region corresponding to a core gap 140, 142 in order to preserve the linearity of the device.

Figure 6 shows movable member 106 in the neutral position, i.e. in the position of zero stroke movement when the magnets 132, 134 are arranged centrally between the two coil arrangements 118, 120. This is the position of the movable member 106 when no current is applied to the coil arrangements 118, 120.

In use, the coil arrangements 118, 120 are energised by applying a first and second electrical current to the first and second coil arrangements 118, 120 respectively. The coils may be electrically connected in series or in parallel. A control unit (not shown) supplies the coils with electrical current.

Figure 7 shows the position of the movable member 106 in the linear actuator 100 when a current is passed in through the first and second coil arrangements 118, 120 of the linear actuator in the first direction Cl (also shown in Figure 6). This applied current causes the first and second magnets 132, 134 (and, therefore, the whole of the movable member 106) to move from the position shown in Figure 6 to the position shown in Figure 7 as a result of the magnetic coupling between the magnetic field lines of the first and second magnets 132, 134 and the first and second coil arrangements 118, 120.

As shown in Figure 7, the distal edge of the first and second magnets 132, 134 extend up to, but not beyond, the edge of the core gaps 140, 142. This is so that the coupling between the field lines of the first and second magnets 132, 134 and the first and second coil arrangements 118, 120 is kept uniform. It will be appreciated that the movable member 106 may be permitted to move beyond this limit, but that linearity will then be compromised.

Figure 8 shows the position of the movable member 106 in the linear actuator 100 when a current is passed in through the first and second coil arrangements 118, 120 of the linear actuator in a second direction C2 opposite to the first direction Cl. This applied current causes the first and second magnets 132, 134 (and, therefore, the whole of the movable member 106) to move towards the bottom of the page to the position shown in Figure 8 as a result of the magnetic coupling between the magnetic field lines of the first and second magnets 132, 134 and the first and second coil arrangements 118, 120.

Again, similarly to the arrangement shown and described with reference to Figure 7, the proximal edge of the first and second magnets 132, 134 extend up to, but not beyond, the edge of the core gaps 140, 142. This is so that the coupling between the field lines of the first and second magnets 132, 134 and the first and second coil arrangements 1 18, 120 is kept uniform as described above.

The person skilled in the art will recognise that controlling the current in the two coil arrangements 118, 120 will act to move the first and second magnets 132,134 and the movable member 106 to which they are coupled. Precise control of the currents in the coil arrangements 118, 120 allows for precise control of the position of the movable member 106. Furthermore, the currents can be changed and controlled quickly providing a quick response time for the movable member 106 which is further enhanced by the reduced mass of the movable member 106 which provides a corresponding reduction in inertia.

Figures 9, 10 and 11 show a linear actuator 200 according to a second embodiment of the invention. The linear actuator 200 of the second embodiment is shorter in height than the first embodiment and more suitable for applications where space is at a premium, for example, in an automobile. The linear actuator 200 comprises a coil body 202, a coil configuration 204 and a movable member 206.

The coil body 202 functions as a housing for the coil configuration 204 in a similar manner to the coil body 102 of the first embodiment. The coil body 202 has a first coil housing 208 and a second coil housing 210. The first and second coil housings 208, 210 are each substantially cuboid and are spaced from one another to define a narrow rectangular channel 212 therebetween. The first and second coil housings 208, 210 are connected to one another by means of a first connector 214 located at an upper end thereof and a second connector 216 located at a lower end thereof. The first and second connectors 214, 216 delimit the upper and lower ends of the channel 212 respectively. The channel 212 is dimensioned and arranged to receive the movable member 206.

The coil configuration 204 comprises first and second coil arrangements 218, 220.

The first coil arrangement 218 comprises first and second coils 218a, 218b. Each coil 218a, 218b comprises wire wound around, and located partially within, the first coil housing 208. Parts of the first and second coils 218a, 218b extend through a respective pair of thorough-holes 222, 224 formed in the first coil housing 208.

The second coil arrangement 220 also comprises first and second coils 220a, 220b which extend through a corresponding pair of thorough-holes 226, 228 formed in the second coil housing 210. The first and second coil arrangements 218, 220 oppose one another and are located on either side of the channel 212.

The movable member 206 is similar to the movable member 106 of the first embodiment and functions as an armature in the linear actuator 200. The movable member 206 is, however, more elongate than the movable member 106 of the first embodiment, i.e. the length of the movable member 206 is greater in proportion the the height thereof when compared to the movable member 106. The movable member 206 has a carrier member 230, a first magnet 232 and a second magnet 234. The remainder of the structure of the movable member 206 is similar to the movable member 106 and will not be described further here.

Figure 10 shows a section taken along the plane Z-Z shown in Figure 11. The current direction C and the resultant force direction F are shown in Figure 10. The current direction C is opposed for the first and the second coils 218a, 218b, 220a, 220b of each of the first and second coil arrangements 218, 220. Note that, in this view, the movable member 206 is shown in the neutral position, i.e. in the position of zero stroke movement when the magnets 232, 234 are arranged centrally between the two coil arrangements 218, 220. This is the position of the movable member 206 when no current is applied to the coil arrangements 218, 220. The current directions shown in Figure 12 are to illustrate the relative directions of the magnetic field and current loops.

As can be seen, a single magnetic field loop M2 is formed between the first and second coil arrangements 218, 220. The magnetic field ioop M2 extends from the first coil 21 8a of the first coil arrangement 218, through the first magnet 232 to the first coil 220a of the second coil arrangement 220, to the second coil 220b of the second coil arrangement 220, through the second magnet 234, to the second coil 218b of the second coil 218 and back to the first coil 218a of the first coil arrangement 218. In between these parts, the magnetic field passes through the laminated layers of the coil housing 202.

By creating a single magnetic field loop M2 within the linear actuator 200, there is no requirement for an iron core in the movable member 206 (i.e. in the annature) in order to complete the magnetic field loop. Consequently, the movable member 206 only requires two magnets 234, 236 spaced apart in the direction of movement of the movable member 206 by the non-conductive, non-magnetic carrier member 230.

Therefore, the weight and inertia of the movable member 206 is significantly reduced, improving the efficiency and energy consumption of the linear actuator 200 as a whole. The weight of the linear actuator 200 is also significantly reduced.

The operation of the linear actuator 200 is similar to that of the linear actuator 100 of the first embodiment and so will not be described any further here.

Figures 12 to 16 show a linear actuator 300 according to a third embodiment of the invention. The linear actuator 300 of the second embodiment is of a similar configuration to the linear actuator 200 of the second embodiment except that a larger number of coils are provided. The linear actuator 300 comprises a coil body 302, a coil configuration 304 and a movable member 306.

The coil body 302 functions as a housing for the coil configuration 304 in a similar manner to the coil body 202 of the second embodiment. The coil configuration 304 comprises first and second coil arrangements 318, 320. However, in this embodiment, the first coil arrangement 318 comprises first, second and third coils 318a, 318b, 318c.

Each coil 318a, 318b, 318c is similar in arrangement to the coils 218a, 218b of the second embodiment. The second coil arrangement 320 also comprises first, second and third coils 320a, 320b, 320c. The first and second coil arrangements 318, 320 oppose one another.

The movable member 306 is similar to the movable member 206 of the second embodiment and functions as an armature in the linear actuator 300. However, the provision of three sets of coils in each coil arrangement 318, 320 enables the movable member 306 to be longer and to have a longer stroke. As with the movable member 206, the movable member 306 has a carrier member 330, a first magnet 332 and a second magnet 334. The remainder of the structure of the movable member 306 is similar to the movable member 206 and will not be described further here.

The linear actuator 300 operates on a three-phase system. As shown in Figure 13, the respective first coils 318a, 320a, second coils 318b, 320b and third coils 318c, 320c each form a coil group (iph, 2ph, 3ph as shown in Figure 13) and are energised independently to create a three-phase system. However, to create a movable member 306 with only two magnets and which is able to move linearly, only one coil group can be coupled to a single magnet at any one time.

Figure 14 shows a section through the linear actuator 300 similar to that of Figure 10 showing the second embodiment. This is the position of the movable member 306 when no current is applied to the coil arrangements 318, 320. The current directions shown in Figure 14 are to illustrate the relative directions of the magnetic field and current loops. For simplicity, the current direction is shown only for the coils involved in forming the respective magnetic circuit. However, the additional coils may still be energised, but not form part of the magnetic circuit.

As can be seen, a single magnetic field loop M3 is formed between the first and third coils 318a, 320a, 318c, 320c. The magnetic field loop M3 extends from the first coil 318a of the first coil arrangement 318, through the first magnet 332 to the first coil 320a of the second coil arrangement 320, to the third coil 320c of the second coil arrangement 320, through the second magnet 334, to the third coil 318c of the second coil arrangement 318 and back to the first coil 3 18a of the first coil arrangement 318.

In between these parts, the magnetic field passes through the laminated layers of the coil housing 302.

In use, the coil arrangements 318, 320 are energised by applying first, second and third currents to the first, second and third coil groups respectively. The coils may be electrically connected in series or in parallel. A control unit (not shown) supplies the coils with electrical current.

Figure 15 shows the position of the movable member 306 in the linear actuator 300 when a current is passed in through the first and second coil arrangements 318, 320 of the linear actuator 300. This applied current causes the first and second magnets 332, 334 (and, therefore, the whole of the movable member 306) to move from the position shown in Figure 14 to the position shown in Figure 15 as a result of the magnetic coupling between the magnetic field lines of the first and second magnets 332, 334 and the first and second coil arrangements 318, 320.

As shown in Figure 15, the magnetic field loop M4 is formed between the first and second coils 31 8a, 320a, 31 8b, 320b. The magnetic field loop M3 extends from the first coil 318a of the first coil arrangement 318, through the first magnet 332 to the first coil 320a of the second coil arrangement 320, to the second coil 320b of the second coil arrangement 320, through the second magnet 334, to the second coil 318b of the second coil arrangement 318 and back to the first coil 31 8a of the first coil arrangement 318. In between these parts, the magnetic field passes through the laminated layers of the coil housing 302.

Figure 16 shows the position of the movable member 306 in the linear actuator 300 when a current is passed in through the first and second coil arrangements 318, 320 of the linear actuator such that the second and third coil groups are energised. The applied current causes the first and second magnets 332, 334 (and, therefore, the whole of the movable member 306) to move to the position shown in Figure 16 as a result of the magnetic coupling between the magnetic field lines of the first and second magnets 332, 334 and the first and second coil arrangements 318, 320.

As shown in Figure 16, the magnetic field ioop M5 is now formed between the second and third coils 31 8b, 320b, 31 8c, 320c. The magnetic field ioop M4 extends from the second coil 318b of the first coil arrangement 318, through the first magnet 332 to the second coil 320b of the second coil arrangement 320, to the third coil 320c of the second coil arrangement 320, through the second magnet 334, to the third coil 31 8c of the second coil arrangement 318 and back to the second coil 318v of the first coil arrangement 318. In between these parts, the magnetic field passes through the laminated layers of the coil housing 302.

Figure 17 illustrates the flux linkage between each coil phase iph, 2ph, 3ph (see Figure 13). Note that the degree of flux linkage is proportional to the coupling between a respective magnet and a coil group. Thus, as a magnet passes out of the gap for each coil group, the flux linkage will drop rapidly.

The person skilled in the art will recognise that controlling the current in the two coil arrangements 318, 320 will act to move the first and second magnets 332,334 and the movable member 306 to which they are coupled. Precise control of the currents in the coil arrangements 318, 320 allows for precise control of the position of the movable member 306. Furthermore, the currents can be phased to change quickly in a controlled maimer providing a quick response time for the, movable member 306 which is further enhanced by the reduced mass of the movable member 306 which provides a corresponding reduction in inertia. Further, the third embodiment of the invention enables the stroke length to be greater than in the first and second embodiments, leading to improved usability in certain applications.

Additionally, further coil groups could be added to increase the relative stroke length as desired.

Although the invention has been described with reference to the above specific examples, the invention is not limited to the detailed description given above.

Variations will be apparent to the person skilled in the art.

For example, the second coil arrangement could be removed completely so that the linear actuator only has a single coil. In order to complete the magnetic loop, a thin sheet of iron could be provided on the side of the movable member or armature opposed to the first coil. The thin sheet of iron functions as a bridging portion which extends from a position adjacent the first magnet to a position adjacent the second magnet so that the magnetic loop can be completed. This arrangement would provide the desired weight reduction for the linear actuator as a whole, despite the increase in weight of the movable member. In this arrangement, materials other than iron may be used. Whilst a metallic material is preferred, any suitable material having suitable magnetic properties could be used.

The first and second magnets may be arranged in a different spaced configuration to that shown and described, i.e. they need not be spaced in the direction of movement of the movable member.

The coils may take different forms as may be readily envisaged by the person skilled in the art. Further, the coils need not extend through the coil body and instead may be, for example, wrapped around the coil body.

While the invention is susceptible to various modifications and alternative forms, specific embodiments are shown by way of example in the drawings and are herein described in detail. It should be understood, however, that drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, the invention is to cover all modifications, equivalents and alternatives falling within the scope of the appended claims.

Claims

CLAIMS1. A linear actuator comprising a coil body and a reciprocally-movable member, the coil body having at least one coil arrangement and the reciprocally-movable member being linearly movable along an axis of movement relative to the coil body and comprising first and second magnets, wherein the first and second magnets and the or each coil arrangement are arranged such that a single magnetic field loop is formed therebetween.
2. A linear actuator as claimed in claim 1, wherein the first and second magnets are spaced by a carrier member comprising a non-magnetic material.
3. A linear actuator as claimed in claim 2, wherein the first and second magnets are spaced along the axis of movement by the carrier member.
4. A linear actuator as claimed in claim 2 or 3, wherein the carrier member comprises a plastics material.
5. A linear actuator as claimed in any one of claims 1 to 4, wherein the carrier member forms a support stmcture for the first and second magnets.
6. A linear actuator as claimed in any one of the preceding claims, wherein the movable member has a substantially planar shape.
7. A linear actuator as claimed in claim 6, wherein each of the first and second magnets is substantially planar.
8. A linear actuator as claimed in any one of the preceding claims, wherein the first and second magnets have opposing field directions.
9. A linear actuator as claimed in any one of the preceding claims, wherein a first coil arrangement is provided on a first side of the movable member and a second coil arrangement is provided on a second side of the movable member, the single magnetic field loop being formed between the first and second magnets and the first and second coil arrangements.
10. A linear actuator as claimed in claim 9, wherein the first and second coil arrangements each comprise separate first and second coils.
11. A linear actuator as claimed in any one of claims 1 to 8, wherein a first coil arrangement is provided on a first side of the movable member and a second coil arrangement is provided on a second side of the movable member, the first and second coil arrangements each comprising separate first, second and third coils and the magnetic field loop being formed between the first magnet, the second magnet and: a) the first and second coils of each of the first and second coil arrangements; or b) the second and third coils of each of the first and second coil arrangements; or c) the first and third coils of each of the first and second coil arrangements.
12. A linear actuator as claimed in any one of claims ito 8, wherein a single coil arrangement is provided on one side of the movable member and a bridging portion is located on the movable member, the bridging portion extending from a position adjacent the first magnet to a position adjacent the second magnet and comprising a material having magnetic properties.
13. A linear actuator as claimed in claim 12, wherein the bridging portion is located on the opposite side of the movable member to the coil arrangement.
14. A linear actuator as claimed in claim 12 or 13, wherein the bridging portion is formed from a magnetic material, preferably a metallic material.
15. A linear actuator as claimed in any one of the preceding claims, wherein the coil body comprises a laminated material arranged such that the laminations lie in a plane which is substantially parallel to the axis of movement of the reciprocally-movable member.
16. A linear actuator as claimed in any one of the preceding claims, wherein at least a part of the or each coil arrangement extends through a part of the coil body.
17. A linear actuator as claimed in claims 15 and 16, wherein said part of the or each coil arrangement extends through a part of the coil body in a direction substantially perpendicular to the plane of the laminations.
18. A linear actuator as claimed in claim 15, 16, or 17, wherein the laminated material comprises iron.
19. A linear actuator as claimed in claim 15, 16, or 17, wherein the laminated material comprises silicon iron.
20. A linear actuator comprising a coil body and a reciprocally-movable member, the coil body having at least one coil arrangement and the reciprocally-movable member being linearly movable along an axis relative to the coil body, wherein the coil body comprises a laminated material arranged such that the laminations lie in a plane which is parallel to the axis of movement of the reciprocally-movable member.
21. A linear actuator as claimed in claim 20, wherein at least a part of the or each coil arrangement extends through a part of the coil body.
22. A linear actuator as claimed in claim 21, wherein said part of the or each coil arrangement extends through a part of the coil body in a direction substantially perpendicular to the plane of the laminations.
23. A linear actuator as claimed in claim 20, 21 or 22, wherein the laminated material comprises iron.
24. A linear actuator as claimed in claim 20, 21 or 22, wherein the laminated material comprises silicon iron.
25. A linear actuator as claimed in any one of claims 20 to 24, wherein the reciprocally-movable member comprises first and second magnets.
26. A linear actuator as claimed in claim 25, wherein the first and second magnets are located in a carrier member formed from a non-conductive and non-magnetic material.
27. A linear actuator as claimed in claim 26, wherein the first and second magnets are spaced along the axis of movement by the carrier member.
28. A linear actuator as claimed in claim 25 to 27, wherein the first arid second magnets and the or each coil arrangement are arranged such that a single magneticfield loop is formed therebetween.
29. A linear actuator as claimed in any one of claims 26 to 28, wherein the carrier member comprises a plastics material.
30. A linear actuator as claimed in any one of claims 20 to 29, wherein the movable member has a substantially planar shape.
31. A linear actuator as claimed in claim 30, wherein each of the first and second magnets is substantially planar.
32. A linear actuator as claimed in any one of claims 25 to 31, wherein the firstand second magnets have opposing field directions.
33. A linear actuator as claimed in any one of claims 25 to 32, wherein a first coil arrangement is provided on a first side of the movable member and a second coil arrangement is provided on a second side of the movable member, the single magnetic field ioop being formed between the first and second magnets and the first and second coil arrangements.
34. A linear actuator as claimed in claim 33, wherein the first and second coil arrangements each comprise separate first and second coils.
35. A linear actuator as claimed in any one of claims 25 to 32, wherein a first coil arrangement is provided on a first side of the movable member and a second coil arrangement is provided on a second side of the movable member, the first and second coil arrangements each comprising separate first, second and third coils and the magnetic field loop being formed between the first magnet, the second magnet and: a) the first and second coils of each of the first and second coil arrangements; or b) the second and third coils of each of the first and second coil arrangements; or C) the first and third coils of eack of the first and second coil arrangements.
36. A linear actuator as claimed in any one of claims 25 to 32, wherein a single coil arrangement is provided on one side of the movable member and a bridging portion is located on the movable member, the bridging portion extending from a position adjacent the first magnet to a position adjacent the second magnet and comprising a material having magnetic properties.
37. A linear actuator as claimed in claim 36, wherein the bridging portion is located on the opposite side of the movable member to the coil arrangement.
38. A linear actuator as claimed in claim 36 or 37, wherein the bridging portion is formed from a magnetic material, preferably a metallic material.
39. A linear actuator substantially as hereinbefore described with reference to the accompanying drawings.