ELECTROMAGNETIC ACTUATOR

Abstract: An electromagnetic actuator (6) has a substantially linear force displacement characteristic. This is achieved by two coils (16, 17) which produce magnetic fields which interact with respective armature members (19, 20). The force displacement characteristic of the armatures (19, 20) individually is a square law. However, in the actuator (6) they are arranged to operate in a push-pull fashion resulting in a linearised force displacement characteristic for the armature (10) as a whole. Biasing magnets (32, 33) are associated with respective coils (16, 17) in order to optimise the linearisation of the armature force displacement characteristic.

Description: ELECTRQMA£3iETIC ACTUATOR DESCRIPTION The present invention relates to an improved electromagnetic actuator for use in vibration cancellation systems. It has been proposed to cancel vibrations in a mechanical system by sensing the vibrations and utilising an electronic control circuit to drive a mechanical actuator in order to apply vibrations to the system which tend to cancel out the original source of vibration. For example, it has been proposed to use a mechanical actuator in association with an engine mount for an internal combustion engine to apply cancelling vibrations to the engine for producing smooth running. ( Known actuators have the disadvantage that they are able to produce only a limited force for a unit of a given size. It is therefore an object of the present invention to provide such an actuator producing a greater force for a given unit size than has heretofore been reliably achieved. A further object of the present invention is to provide an actuator having improved linearity. According to a first aspect of the present invention there is provided an electromagnetic actuator for use in a vibration cancellation system, comprising a stator, an armature including first and second armature members, and first and second conducting coil means for producing magnetic fields which interact with the first and second armature members respectively, the arrangement being such that when the coils are energised the resultant force produced by the actuator as a result of interaction of the magnetic fields produced by the coil members with the respective armature members exhibits a more linear relationship with the energising current than the individual forces produced by the magnetic field of either said coil means and its respective armature member. Advantageously, a permenant magnet may be incorpoated into the stator, in order to produce a biasing magnetic field. Alternatively, the biasing field can be produced by a coil, e.g. at least one of the coil members. Preferably, the actuator is provided with a straight passageway therethrough and the armature includes a linear portion which passes through said passageway. Bearing means may be provided whereby the armature is constrained such that it may only move parallel to the axis of said passageway. The bearing means may comprise a PTFE bushing located between the wall of the passageway and said linear portion, or a pair of substantially planar spring members each having an aperture through which respective extensions of said linear portions are affixed, and fixed at their peripheries to the stator. Preferably, the planar spring members are disc-shaped. According to a second aspect of the present invention there is provided an electromagnetic actuator for use in a vibration cancellation system, comprising a stator, an armature, and conducting coil means for driving the armature, wherein a magnetic circuit is partially defined by said coil and armature and includes a portion through non-ferromagnetic material, and said magnetic circuit has a substantially constant reluctance as a function of armature displacement. Preferably, said constant reluctance is achieved by ensuring that the cross sectional area of each said magnetic circuit through non-ferromagnetic material is varied as a function of armature displacement together with the path length of the circuit in such a manner as to achieve a substantially constant total reluctance during movement of the armature. Alternatively, said constant reluctance may be achieved by providing each magnetic circuit with two portions through non-ferromagnetic material, wherein in operation a widening of one such portion is accompanied by a corresponding increase in the effective area of the other so as to achieve a substantially constant total reluctance upon movement of the armature. According to a third aspect of the present invention there is provided a vibration cancelling system; including an actuator comprising a stator, an armature, and coil means for driving the armature wherein the relationship between force and displacement of the armature is linearised. In one arrangement according to the invention means are provided for energising the coil means such that the energising current is varied in dependence on the armature displacement, so as to tend to linearise the force generated by the actuator as a function of armature displacement. In another arrangement, spring means are provided including first and second spring devices which have respective non-linear force/displacement characteristics, which operate in opposition such that the combined effect thereof has a linearised force/displacement characteristic. Preferably, said vibration cancellation system further includes an electrically controlled amplifier and displacement sensing means wherein a signal is produced by said displacement sensing means in dependence on the displacement of the armature and said signal is used to control the gain of said electrically controlled amplifier. It is intended that under certain circumstances the electrically controlled amplifier will be replaced by an electrically controlled attenuator. Advantageously, the vibration cancelling system further comprises a vibration sensor, electronic control means, a source of vibrations and a fluid-type coupler, wherein the coupler is operative such that only vibrations above a predetermined frequency are transmitted from the source of vibrations to the actuator and vice versa. According to a fourth aspect of the present invention there is provided a vibration cancellation system comprising an actuator according to the first or second aspects of the present invention, a vibration sensor, electronic control means, a source of vibrations and a fluid-type damping element, wherein said damping element is located such that only vibrations above a predetermined frequency are transmitted from the source of vibrations to the actuator. Preferably, said source of vibrations comprises an internal combustion engine and said actuator functions as an engine mounting. A number of embodiments of the present invention will now be described by way of example, with reference to the accompanying drawings, in which: Figure 1 is a diagrammatic representation of the principle of operation of an electromagnetic actuator; Figure 2 is a sectional view of an actuator according to the present invention; Figure 3 is a graph showing the force-current relationship for an actuator according to the present invention; Figure 4 is a sectional view illustrating an alternative actuator structure; Figure 5 is a partial sectional view of a further actuator according to the present invention; Figure 6 is a sectional view of a further actuator according to the present invention; Figure 7 is a sectional view of a further actuator according to the present invention; Figure 8 shows a schematic of a coil current control loop according to the present invention; Figure 9 shows an example of an actuator in combination with a damper according to the present invention; and Figure 10 shows a schematic of a vibration cancellation system according to the present invention. Referring to Figure 1, two portions of a magnetic circuit 1, 2 are separated by an air-gap 3. A magnetic field is generated in the magnetic circuit by the passage of a current I through a coil 4. When the coil 4 is energised, a force is generated which tries to close the air-gap 3. The force generated is predicted by the following relationship: F « uAN*I a (1) 2d 2 where A is the area of the end 5 of the magnetic circuit portion 1; N is the number of turns in the coil 4; I is the current flowing in the coil 4; d is the length of the air-gap 3; and μ is the permeability of the air-gap. It can be seen from the above relationship that the force F generated is non-linear as a function of both energising current I and the length d of the air-gap 3. It has been found that actuators operating on this principle have been unsatisfactory in feedback control systems because of their non-linearities. However, in accordance with the invention it has been found possible to improve their performance, and that of control systems incorporating them, by linearising their force-current and force-displacement relationships. Referring to Figure 2, an actuator 6 according to the invention comprises a substantially cylindrical stator 7, first and second dish-shaped end caps 8, 9, and a bobbin-shaped armature 10. The end caps 8, 9 threadably engage the stator 7 about opposite end faces 7a, 7b thereof respectively, such that first and second chambers 11, 12 are formed. Each of the chambers 11, 12 is defined by a stator end face 7a, 7b and the inner surfaces of an end cap 8, 9. The stator 7 is further provided with an axially extending straight passageway 13. First and second annular slots 14, 15, extend axially into the stator 7 from respective end faces 7a, 7b thereof and coaxially with the passageway. First and second coils 16, 17 are located within respective slots 14, 15. The bobbin-shaped armature 10 comprises a central spindle 18, separating first and second disc-shaped armature members 19, 20 which lie in planes normal to the axis of said spindle 18. The armature 10 is located such that the spindle 18 passes through the passageway 13 in the stator 7 and the armature members 19, 20 lie parallel to respective stator end faces 7a, 7b. First and second annular channels 21 and 22 are provided in respective stator end faces 7a, 7b whilst third and fourth annular channels 23, 24 are provided in the faces of respective armature members 19, 20 facing the stator end faces 7a, 7b, such that when the armature 10 is mounted on the stator said first and second channels 21, 22 are aligned with said third and fourth channels 23, 24 respectively. First and second resiliently deformable rings 25, 26 are held between respective aligned pairs of channels 21, 23, 22, 24. The spindle 18 is maintained parallel to the axis of the passageway 13 by means of a pair of annular PTFE bearings 27, 28 located in said passageway 13 adjacent opposite ends thereof. It will be appreciated that other forms of bearing could be utilised e.g. brass, roller and the like. The outer curved wall of each of the bearings 27, 28 contacts the wall of the passageway 13 and the armature spindle 18 passes though the central aperture in each bearing 27, 28. A central aperture 29 is provided in the first end cap 8 through which projects a threaded extension 30 of the armature spindle 18. The second end cap 9 is provided with a centrally located threaded stud 31 which extends away from the stator 7 and coaxial with the armature spindle 18. The extension 30 and the stud 31 enable the actuator to be mounted between a source of vibrations and a structure to be isolated therefrom. A pair of annular magnets 32, 33 are incorporated in the stator 7 in respective annular openings provided between the coil slof 14, 15 and the passageway 13. The magnets 32, 33 provide a biasing magnetic field. In operation the coils are variably energised with the same current. This produces lines of flux in what are effectively two isolated magnetic circuits. Each magnetic circuit comprises half of the stator 7 and an associated armature member 19, 20. The interaction of the magnetic fields on the armature members 19, 20 results in opposed forces tending to cause the armature members 19, 20 to move towards the stator 7. The force/current relationship for the individual magnetic circuits has a square law i.e. non-linear relationship as per equation (1) hereinbefore. However, these forces when summed with the force on the armature produced by the biasing magnetic field from the magnets 32, 33 produce a resultant force/current relationship which is substantially linear, i.e. the force on the armature is linearly related to the applied current. Referring to Figure 3, the force-current relationships for the individual armature members are shown as curves A and B respectively and are significantly non-linear. However, the curves are shifted apart from one another on the graph as a result of the bias field, so that their resultant, the dashed line C, is a substantially straight line, this providing the desired linear relationship. Whilst the bias fields are preferably produced by the magnets 32, 33 they can be generated by passing a bias field producing current through one or both of the coils, 16, 17 or by means of an additional coil. Referring to Figure 4, the PTFE bearings 27, 28 and the rings 25, 16 of Figure 1 have been replaced by an alternative form of bearing comprising first and second disc-shaped spring elements 34, 35, eg of metal. The first end cap 8 has been modified such that its planar portion is now formed by a first disc-shaped spring element 34. The first spring element 34 is affixed about the spindle extension 30, passing therethrough by means of a circular welded plate 34a for stiffening the central portion of the spring. A second disc-shaped spring element 35 is located within the second end cap 9 and is similarly connected to an additional extension 36 from the other end of the spindle 18 by means of circular welded plate 35a of same diameter to plate 34a. This arrangement allows axial movement of the armature but greatly restricts all other modes of armature movement. Unfortunately, the use of the disc-shaped springs introduced non-linearities due to the characteristics of the springs. However, this may be overcome by designing each of the spring elements 34, 35 to have substantially similar square law relationships between force F and displacement d. Characteristics of the spring elements 34, 35 can be controlled by careful choice of the material used therefor . and the geometry and material of the welded plates 34a, 35a. If, the springs are given opposite initial offsets, resulting in opposing initial biases, the resultant sum of their spring characteristics results in a substantially linear relationship, between force and displacement much in the manner explained with reference to Figure 3. It will be appreciated that the springs 34, 35 could be stiffened by means other that the welded plates 34a, 35a to achieve the square law characteristics. Appropriate design of the spring elements 34, 35 can also produce a degree of linearisation of the armature displacement force relationship. The armature displacement force relationship can also be substantially linearised by careful design of the magnetic circuit geometry. Referring to Figure 5, an additional annular slot 37 is formed in the first end face 7a of the stator 7. The additional annular slot 37 is coaxial with the first coil slot 14. The armature member 19 has been extended into the additional annular slot 37, by means of an annular extension 38. The extention 38 is provided with e.g. three axially spaced ridges 39a, 39b, 39c which extend radially inwardly towards complementary ridge structures 40a, 40b, 40c on the radially outward facing wall of said additional annular slot 37. A similar arrangement is provided for the second armature member. The ridges 39a, 39b, 39c, 40a, 40b, 40c are arranged such that as the armature 10 moves axially within the stator 7. The reduction in air-gap length between the stator end face 7a and the adjacent face of the armature member 19 is compensated for by a reduction in the area of the overlap between the ridges 39a, 39b, 39c on the armature member 19 and those on the stator 40a, 40b, 40c. The aim of this is to keep a substantially constant reluctance in the magnetic circuit thus producing a substantially linear relationship between armature displacement and resultant force. Referring to Figure 6, a practical form of the actuator is shown which comprises a bobbin shaped armature 60 and a substantially cylindrical stator 61. The bobbin shaped armature 60 comprises a central spindle 62, separating first and second disc-shaped armature members 63 and 64 which lie in planes normal to the axis of the spindle 62. Additionally, the armature 60 is provided with an annular central flange 65 located midway between the armature members 63, 64 and extending radially outwardly. Biasing magnets 66, 67 are incorporated in the spindle 62 where the spindle 62 meets the armature members 63, 64. Each armature member 63, 64 is provided with three concentric axially extending annular flanges 68, the flanges 68 being arranged such that the flanges 68 on respective armature members 63, 64 extend towards each other. Two coils 69, 70 are wound around the spindle 62 with one located on each side of the flange 65. The stator 61 comprises first and second stator rings 71, 72, aligned coaxially, and coupled by a coupling ring 73 of non-magnetic material. The coupling ring 73 has a radially outwardly extending flange located midway along its axial length. The coupling ring 73 is joined to the margins of the outer surfaces of the first and second stator rings 71, 72, such that a groove is formed between the adjacent end faces of the first and second stator ring 71, 72. In its assembled form, the stator 61 is disposed about the spindle 62 with the outer margin of the central flange 65 received in the groove 74 thereby forming an air gap between each side of the central flange 65 and respective stator rings 71, 72. A pair of spiders 75, 76 are respectively located towards each end of the spindle 62 for holding the spindle 62 relative to the stator 61. The free end of the first stator ring 71 is provided with a first axially extending stator flange 100 running around its outer margin and a second axially extending stator flange 101 running around its inner margin. A channel 77 is formed between the stator flanges 100, 101. A second channel 78 is similarly formed by a second pair of stator flanges 102, 103 on the second stator ring 72. The channels 77, 78 allow the armature 60 to move axially with respect to the stator 61 by receiving one of the annular flanges 68 on the adjacent armature member 63, 64, the other two flanges 68 passing outside the stator flanges 100, 101, 102, 103. Air gaps are defined between the overlapping portions of the stator flanges 100, 101, 102, 103 and the flanges 68 on the armature members 63, 64. Threaded holes 79, 80 are centrally located in the outer axial faces of the armature members 63, 64 for coupling the actuator to other apparatus. In operation, currents are fed to the coils 69, 70 which produce respective magnetic fields. The lines of magnetic flux from the first coil 69 passes around a circuit comprising the spindle 62, the first armature member 63, the air gap between the armature flanges 68 the first stator ring 71, the first stator ring 71, the air gap between the first stator ring 71 and the central flange 65, and finally back to the spindle 62 through the central flange 65. A similar circuit passes through the second stator ring 72 for the magnetic field produced by the second coil 70. The presence of the magnetic field produces an electromotive force acting to move the armature 60 axially with respect to the stator 61. Considering the upper half of the actuator, using the orientation of Figure 6, as the armature 60 moves downwards, the middle flange 68 on the first armature member 63 penetrates further into the channel 77. This increases the effective area of the air gap between the first stator ring 71 and the first armature member 63. However, at the same time the air gap between the first stator ring 71 and the central flange 65 widens. The increase in the width of the air gap between the first stator ring 71 and the flange 65 compensates for the increase in the effective area of the air gap between the first stator ring 71 and the first armature member 63, thereby ensuring a substantially constant reluctance in the magnetic circuit as the armature 60 moves. An alternative embodiment is shown in Figure 7. A hollow cylindrical stator 81 is provided with first and second annular end flanges 82, 83 extending radially inwardly from the axial extremities of the stator 81 and a central annular flange extending radially inwardly midway between the end flanges 82, 83. A further annular flange 85 extends radially outwardly from the stator 81 midway along its length. Three concentric annular flanges 88 are located about the inner margin of each end flange 82, 83 such that they extend axially into the stator 81. An armature 91 extends axially through the length of the stator 81. The armature 91 comprises a non-magnetic spindle 92 which extends the full length of the stator 81 and a pair of ferromagnetic bodies 93, 94 mounted coaxially on the spindle 92, such that one is either side of the central flange 84. Considered moving away from the central flange 84, the ferromagnetic bodies 93, 94 are initially cylindrical but as the end flanges 82, 83 are approached they taper outwardly. Two concentric annular flanges 95 extend from each of the ferromagnetic bodies 93, 94 towards the end flanges 82, 83 of the stator 81. The concentric flanges 95 are arranged such that they are in interdigital relation with the concentric flanges 88 on the stator 81. The armature is maintained in its correct relation to the stator 81 by means of a pair of spiders 96 and 97 extending between the spindle 92 and further annular flanges 89, 90 extending axially from the axial extremities of the stator 81. In operation, changes in the overlap between the concentric flanges 95 on the armature 91 and the concentric flanges 88 on the stator 81 are compensated for by variations in the width of the air gaps between the ferromagnetic bodies 93, 94 and the central flange 84. While the structures in Figures 5, 6 and 7 are double ended, it will be appreciated that the techniques employed are equally applicable to single ended devices. Referring to Figure 8, an alternative method of achieving a linear displacement-force relationship is to provided a feedback loop which controls the energising current in order to overcome the effects of varying magnetic circuit reluctance. This arrangement can be used with the embodiment of Figure 2. The actuator 6 is energised with current supplied from an electrically controlled current amplifier 41. The current amplifier has a signal input 42, an energising current output 43 and a control signal output 44. A displacement sensor 45, which may be an optical device, a linear variable differential transformer, capacitance sensor or other convenient device, senses the armature displacement. A signal derived form the displacement sensor 45 is fed to the control signal input 44 of the amplifier 41 and controls the gain of the amplifier 41 such that the signal applied to the input 42 is amplified by a greater amount for greater armature displacement, in either direction, and consequently the energising current is increased to overcome the increased magnetic circuit reluctance. Referring to Figure 9, an electromagnetic actuator 6 suitable for use in a vibration control system in particular for an internal combustion engine is provided with a coupler 46. The coupler 46 is located mounted between the threaded extension 30 and the armature 10. The coupler 46 isolates the actuator 6 and internal combustion engine from load variations below a predetermined frequency applied to the theaded end of the extension 30 but permits the transmission of higher frequency loads such as those caused by engine vibration. The coupler 46 comprises a dashpot comprising a dish shaped receptacle 47 formed on the armature member 19 and a paddle device 48 connected by shaft 50 to the threaded extension 30. The receptacle 47 is filled with a viscous fluid e.g. hydraulic fluid or a viscous polymer, which is relatively rigid in respect of high frequency vibrations but compliant for static and low frequency loads. A tapered flexible rubber boot 47 shrouds the coupler 46. Referring to Figure 10, an electromagnetic actuator 6 is employed as an engine mounting between an i.e. engine 51 and the body 52 of a vehicle. In order to isolate the vehicle body 52 from engine vibrations, the actuator forms part of a control system. The control system further comprises a vibration sensor 53 and an electronic control circuit 54, for example including a microcomputer running a suitable program. In operation the control cicuit 54 processes signals derived from the vibration sensor 53 and produces an energising signal for the actuator which then produces a force on the engine 51 which tends to cancel the engine vibration. The coupler 46 associated with the actuator 6 isolates the actuator from low frequency load variations such as produced due to the inertia of the engine 51 during cornering, braking etc., together with static loads. While the actuator has been described with reference to a vibration cancellation system for a vehicle engine it is not intended to limit the scope of the invention thereto.

Claims: CLAIMS 1. An electromagnetic actuator for use in a vibration cancellation system, comprising a stator, an armature, coil means for driving the armature and means for linearising the relationship between generated force and displacement of the armature. 2. An electromagnetic actuator according to claim 1, wherein the armature includes first and second armature members and the coil means includes first and second conducting coils for producing magnetic fields which interact with the first and second armature members respectively, the arrangement being such that when the coils are energised, the resultant force produced by the actuator as a result of interaction of the magnetic fields, produced by the coils, with respective armature members exhibits a more linear relationship with the energising current than the individual forces produced by the magnetic field of either said coil and its respective armature member. 3. An electromagnetic actuator according to claim 2, further comprising means to produce a bias magnetic field. 4. An electromagnetic actuator according to claim 3, wherein the means to produce a bias magnetic field comprises a permanent magnet. 5. An electromagnetic actuator according to claim 3 or 4, wherein the means to produce a bias magnetic field is an auxilliary coil. 6. An electromagnetic actuator according to any one of claims 2 to 5, wherein the stator is provided with a straight passageway therethrough, the armature includes a linear portion which passes through said passageway and bearing means to ensure that the armature is constrained to move only substantially parallel to the longitudinal axis of the passageway. 7. An electromagnetic actuator according to claim 6, wherein the bearing means comprises a PTFE bushing, located between the wall of the passageway and the linear portion of the armature. 8. An electromagnetic actuator according to claim 6, wherein the bearing means comprises first and second spring members coupling the armature to the stator and which have respective non-linear force/displacement characteristics that operate in opposition such that the combined effect thereof has a linearisied force/displacement charateristic. 9. An electromagnetic actuator according to claim 8, wherein said spring members are substantially planar and are coupled to the stator at their peripheries and in central regions thereof to respective portions of the armature. 10. An electromagnetic actuator according to claim 1, including a magnetic circuit, partially defined by the coil means, the stator and the armature and having a portion through non-ferromagnetic material between the armature and the stator, wherein the magnetic circuit has a substantially constant reluctance as a function of armature displacement. 11. An electromagnetic actuator according to claim 10, wherein the cross-sectional area of said non-ferromagnetic portion is varied as a function of the length of said non-fezrromagnetic portion, said length being a function of armature displacement, so as to maintain a substantially constant reluctance for said non-ferromagnetic portion as a function of armature displacement. 12. An electromagnetic actuator according to claim 10, wherein the magnetic circuit includes two non-ferromagnetic portions arranged such that in operation a widening of one such portion is accompanied by a corresponding increase in the effective area of the other so as to achieve a substatnially constant total reluctance upon movement of the armature. 13. An electromagnetic actuator according to any one of claims 10 to 12, wherein the actuator is a double ended device. 14, An electromagnetic actuator according to any one of claims 10 to 13, wherein a non-ferromagnetic portion is an air gap. 15. An electromagnetic actuator according to claim 1, further comprising means to energise the coil means such that the energising current is varied as a function of armature displacement, so as to tend to linearise the force generated by the actuator as a function of armature displacement. 16. An electromagnetic actuator according to claim 15, further comprising sensor means for sensing armature displacement. 17. An electromagnetic actuator according to claim 16, wherein the sensor means is an optical sensor. 18. An electromagnetic actuator according to claim 17, wherein the sensor means is a linear variable differential transformer. 19. An electromagnetic actuator according to claim 16, wherein the sensor means is a capacitance sensor. 20. An electromagnetic actuator according to any preceding claim, further comprising a fluid-type coupler, wherein the coupler is operative such that only vibrations above a predetermined frequency are transmitted through said coupler. 21. A vibration cancelling system, including an electromagnetic actuator according to any one of claims 1 to 19, a vibration sensor for outputting signals representative of sensed vibrations, electronic control means, a source of vibrations, wherein the electronic control means controls the actuator in reponse to said signals from the vibration sensor so as to cancel vibrations from the source of vibrations. 22. A vibration cancelling system, including an electromagnetic actuator according to claim 21, further comprising a fluid-type damping element, wherein the damping element is located such that only vibrations above a predetermined frequency are transmitted from the source of vibrations to the actuator. 23. A vibration cancelling system, including an electromagnetic actuator according to claim 21 or 22, wherein the source of vibrations is an internal combustion engine.- 24. An engine mounting, comprising an electro- magnetic actuator according to any one of claims 1 to 20.

Inventors: JONES OWEN, TRINDER MICHAEL CHARLES JOHN, JONES, OWEN, TRINDER, MICHAEL, CHARLES, JOHN
Applicants: ACTIVE NOISE & VIBRATION TECH, ACTIVE NOISE AND VIBRATION TECHNOLOGIES INC.
Application year: 1991
Main classification: H02K33
IPC codes: F16F15, H01F7, H02K33