MAGNET ASSEMBLY

Abstract: A transducer comprises a magnet assembly comprising a magnet (1), magnetically permeable inner (3) and outer pole tips (5) and a voice coil (7) mounted in a gap between the pole tips. Either or both pole tips is electrically conductive in a plane substantially orthogonal to the axis of the voice coil in a region extending into the body of the pole tips from the edge of the pole tips adjoining the gap.

Description: TITLE: MAGNET ASSEMBLY DESCRIPTION The invention relates to a magnet assembly, in particular a magnet assembly for an actuator such as a loudspeaker transducer. In traditional axisymmetric permanent magnet assemblies which are used to drive tubular coils for actuators, there is a requirement for the flux produced by a permanent or electromagnet to be focussed in the region of the coil. This produces a high flux density, and helps to improve overall efficiency. In existing designs, focussing is usually done with steel pole tips, which increase the effective permeability of the space as seen by the coil of wire, and thus increase the effective inductance of the coil. A secondary effect occurs as the frequency of the alternating current is increased. This is the generation of eddy currents in the pole tips, which effectively screen the coil from the pole tips, whereby the permeability is effectively reduced. The effect will now be explained with reference to a device as shown in Figure 1. This shows a permanent magnet 3 sandwiched between first 5 and second 7 pole pieces. The first pole piece 5 is in the form of a disc having an outer periphery forming an inner pole tip 9. The second pole piece 7 is in the form of a cup having an outer pole tip 11. Inner and outer pole tips define a gap 13 therebetween in which a coil 15 moves. The view shown is in section; the whole magnet assembly is circularly symmetrical about the axis 17. Thus, the coil is a ring that is provided in the gap 13 which has the form of an annulus . The magnetic field lines caused by the magnet are shown in Fig.2. When an electrical current is passed through the coil a force is exerted on the coil that tends to cause the coil to move in an axial direction. The force exerted on the coil is given by the product of the flux density B, and current density J. However, the current density J, and hence the force, will fall at high frequencies, due to the rise in inductance of the coil. In order to understand the mechanism whereby the coil inductance rises, it is possible to model and plot the flux associated with this current flowing in the coil. If you plot the field around the coil in air, then the field looks like that shown in Figure 3. The axis of symmetry is shown as a vertical line to the left of the figure. The field lines are smooth and there is no distortion of the flux lines. The curved line, which bounds the model, represents an absorbing boundary whose purpose is to terminate the mathematical model, ensuring a finite model size . When this coil is immersed in the magnet assembly, this flux distribution is distorted as shown in Figure 4. There are two effects that control the coil inductance, which are both present in traditional magnet assemblies. The first is the increase in flux density when the flux lines pass through the steel, because of its increased permeability over air. As the frequency rises, the second effect, which is caused by eddy currents, dominates. In this effect, the flux lines passing through the steel cause a current to flow in the steel, since it is an electrical conductor (although not a very good one) . When the frequency of current through the coil is increased, however, the AC flux has increasing difficulty penetrating the steel because of the eddy current effect. The depth of penetration called the skin depth. The skin depth (S) for a conductor is given by; where μ is relat permeability σ is the electrical conductivity o is the angular frequency ( . π- frequency) . As the frequency is increased, the skin depth 5 decreases, and the flux penetrates to a lesser extent. At high frequencies, for example 10,000 Hz, the penetration is quite small (about 0.6 mm at 10 kHz) . For steel, the variation of skin depth with frequency is shown in Figure 6. 10 An existing technique for reducing inductance employs this skin depth effect by shrouding the pole piece with a sheet of copper foil. This is only effective at frequencies when the skin depth for copper is of the order of the foil thickness. However, adding this foil reduces the available 15 air gap and the DC flux density falls. In this way, there will be a significant trade-off between reducing inductance, and reduced flux density. For this reason the copper foil shroud is only partially successful. Nevertheless it can be seen that at 20 10 kHz (see Figure 7) the flux lines do not penetrate very far into the steel. JP-3201797 describes an example of this technique . A similar arrangement is described in DE19637847 which describes the use of two copper rings on the outer pole 25 piece. However, the inductance reduction in this arrangement also is not wholly satisfactory. According to the invention there is provided a magnet assembly comprising inner and outer pole tips defining a gap therebetween for mounting a voice coil in a predetermined axial direction, the pole tips being magnetically permeable, wherein at least one of the pole tips is electrically conductive in a plane substantially orthogonal to the predetermined axial direction in a region extending into the body of the pole tips from the edge of the pole tips adjoining the gap. Preferably, the gap is in the form of an annulus . The inductance can make up a significant, even dominant part of the impedance of a conventional loudspeaker coil at frequencies around 1-10 KHz or higher. Since the inductance contributes jωL to the impedance, the contribution to the impedance rises with frequency. When driven at a constant voltage, the current in the coil accordingly falls at higher frequencies which likewise reduces the force on the coil. The inductance therefore causes a reduction in sound output at high frequencies. This effect is particularly severe for coils intended to produce a large force - the large number of turns required increases the inductance. The magnet assembly according to the invention reduces the inductance at higher frequencies. The electrical conductivity of the pole tips causes the tips to be shielded from a coil placed in the annular gap by eddy currents. Accordingly, the coil inductance at these frequencies is reduced, and the sound pressure level output by a loudspeaker using such a magnet assembly can accordingly be more uniform with frequency. Preferably, both pole tips are electrically conductive in the said plane. The pole tips may be near to magnetic saturation which also helps reduce the inductance of the coil slightly, especially at low frequencies. The pole tips may accordingly have a magnetisation that is at lest 75%, preferably 90% of the magnetisation of the saturation. The pole tips can be formed by a laminate stacked perpendicularly to the plane. The laminate can be a laminate of a ferromagnetic material and a high conductivity material, such as steel and copper respectively. This achieves the increase in conductivity in the said plane to encourage eddy currents, without impeding excessively the flow of magnetic flux through the magnet assembly. In an alternative embodiment the pole tips may be formed from steel with slots extending into the pole tips from the edge adjoining the annular gap. A copper shroud can then be swaged onto the tips to fill the slots. It will be understood that eddy currents caused by a magnetic field produced by a coil are substantially perpendicular to the axis of the coil in the plane of the coil. Accordingly, it is not necessary that the electrical conductivity be high in the axial direction, although it is not excluded. It can however be more difficult to arrange for the magnetic permeability to be high as well as to arrange for the electrical conductivity to be high in all directions. In one arrangement, the pole tips may be formed from a sintered mixture of steel and copper. The magnet assembly according to the invention may minimise the increase in permeability that the coil sees, and at the same time improve the conductivity of the pole tips, to encourage the eddy currents. In normal magnetic circuits, such as transformers and inductors, the object is to couple the flux lines into the core, and to reduce the effect of eddy currents, which prevent these flux lines penetrating into the core. In this case the opposite effect is intended; coupling to the core might undesirably increase the inductance . According to another aspect of the invention there is provided a transducer having a magnet assembly as discussed above and a coil positioned in the gap. The coil may comprise an annular former with electrically conductive wire wound outside it. The invention may also provide a loudspeaker having a magnet assembly as discussed above, a diaphragm and a voice coil rigidly attached to the diaphragm and positioned in the annular gap in the magnet assembly. The loudspeaker may be a distributed mode loudspeaker as described in WO 97/09842. A specific embodiment of the invention will now be described, purely by way of example, with reference to the accompanying drawings, in which Figure 1 shows a cross section through a traditional magnet assembly, Figure 2 shows the flux in the magnet assembly of Figure 1, Figure 3 shows the flux around a coil in air, Figure 4 shows the flux around a coil in a conventional magnet assembly at 100Hz, Figure 5 shows the flux around a coil in a conventional magnet assembly at lOOOOHz, Figure 6 shows the skin depth of steel as a function of frequency, Figure 7 shows the flux lines in a magnet assembly having a copper shroud around the pole tips, Figure 8 shows the magnet assembly according to the invention, Figure 9 shows the flux lines in a magnet assembly according to the invention, Figure 10 shows the flux lines from the coil at 100Hz in a magnet assembly according to the invention, Figure 11 shows the flux lines from the coil at 1000Hz in a magnet assembly according to the invention, Figure 12 shows the flux lines from the coil at lOOOOHz in a magnet assembly according to the invention, Figure 13 shows another embodiment of the invention, and Figure 14 shows a further embodiment of the invention. The magnet assembly according to a specific embodiment of the invention shown in Figure 8 comprises two pole pieces 5,7, one being a circular steel cup 7 which surrounds the other pole piece, a steel disk 5. A permanent magnet 3 separates the pole pieces. It should be noted that the Figures show a cross section extending outwards from the centre of the assembly, the assembly is in fact circular when viewed from above . The outer rim of the pole and the inner rim of the pole have inner 9 and outer 11 pole tips. Each pole tip is a lamination of alternate layers of steel 19 and copper 21 to allow the DC flux to pass through, from inner pole piece 5 to cup 7. A coil 15 having a metallic wire 23 wound on a former 25 is provided in the annular gap 13 between the pole pieces. The coil is fixed to a diaphragm 27 of a loudspeaker to drive the loudspeaker. The magnet assembly is mounted on the diaphragm by resilient supports 28. The diaphragm may be a distributed mode panel and the loudspeaker a distributed mode loudspeaker, such as described in WO98/09842. The magnetic circuit caused by the permanent magnet 3 is shown by flux lines in Figure 9. As can be seen by comparing this Figure with Figure 2 which shows the flux without laminated pole tips, the lamination has only a minimal effect on the flux density generated by the permanent magnet 60. The thickness of the steel laminations 21 is determined so that they operate close to magnetic saturation, and thus reduce their permeability, whilst preventing eddy currents circulating in the steel. Because the copper has a much higher conductivity (around 10 times greater) than steel, the copper plates can be thinner, (nominally VlO = approximately 3 times thinner) and in this example are 2 times thinner. For increased clarity this variation in thickness is not shown in Fig.8, although it may be seen in Figures 10, 11 and 12. This ratio can be changed to suit the geometry available, as well as the reduction of inductance required. The near saturation of the steel pole tips 19 helps to reduce the inductance of the coil 15 slightly, at low frequencies. The flux from the coil itself is shown in Figure 10. The flux lines pass through the thin steel plates 19, the interleaved copper plates 21, having much higher conductivity, short the eddy currents. The steel plates are thin enough to be the order of the skin depth at high frequencies, so that the frequency rises, eddy currents cannot form and the copper interleave shorts them out anyway. In this example, the steel plates are 0.2mm in thickness, and the copper plates are 0.1 mm in thickness . Even at 10 kHz the flux pattern is distorted in such a way as to keep the inductance low as shown in Figure 12. To compare the improvement possible, a finite element (FE) model of the structure was analysed and the results for the estimates inductance are given in Table 1. Table 1 Here the effect that a magnet assembly has on the inductance can be seen by noting the increase in inductance at low frequencies caused by the steel core of a standard assembly which reduces as the frequency rises as a result of eddy currents in the steel . For the case of the laminated pole tips, a small reduction is gained at 100 Hz, because the pole tip permeability is reduced, and a significant improvement is made at 10 kHz, where the inductance is actually less than the case of the coil in air. Reductions of up to 50% - 70% in the inductance are possible by choosing the correct ratio of steel plate/copper plate thicknesses. The lamination can be applied to either pole and cup or both. Fig.13 illustrates an alternative embodiment in which the pole tips 9,11 are not a laminate, but a sintered mixture of steel and copper. Fig.14 illustrates a pole tip as used in a further alternative embodiment in which the pole tips are steel formed with slots 31 extending in a direction perpendicular to the axis of the coil. A copper piece 33 is then swaged onto the opposed face of the pole pieces to fill the slots. Although a specific embodiment has been discussed a number of alternatives are possible. For example, without departing from the scope of the invention any suitable magnetically permeable material may be used for the pole tips. The conductive pieces need not be copper, but other conductive materials may also be used, such as silver or aluminium. Any suitable shape of the pole tips and pieces may also be used. For example, one pole piece may be a U- shaped piece having two ends, one pole tip being on each end. An inner pole piece having a pole tip facing each pole tip of the U-shaped pole piece across two gaps may be provided; defining two gaps. The coil may surround the inner pole piece and pass through both gaps.

Claims: CLAIMS 1. A magnet assembly comprising a magnet, and magnetically permeable inner and outer pole tips magnetically coupled to the magnet and defining a gap between the inner and outer pole tips for mounting a voice coil in a predetermined axial direction, wherein at least one of the pole tips is electrically conductive in a plane substantially orthogonal to the predetermined axial direction in a region extending into the body of the pole tips from the edge of the pole tips adjoining the gap. 2. A magnet assembly according to claim 1 wherein the gap is in the form of an annulus extending around the perimeter of the inner pole tip. 3. A magnet assembly according to any preceding claim wherein both pole tips are electrically conductive in the said plane. 4. A magnet assembly according to any preceding claim wherein at least one of the pole tips comprises a laminate of sheets of a ferromagnetic material and a high conductivity material, the sheets being substantially in the plane orthogonal to the predetermined axial direction. 5. A magnet assembly according to claim 4 wherein both pole tips comprise a laminate of a ferromagnetic material and a high conductivity material . 6. A magnet assembly according to any of claims 1 to 3 wherein the pole tips are formed from a sintered mixture of steel and copper. 7. A magnet assembly according to any of claims 1 to 3 wherein the pole tips are of a magnetically permeable material formed to have slots extending into the pole tips from the edge adjoining the annular gap and a conductive shroud provided on the tips and filling the slots. 8. A magnet assembly according to any preceding claim wherein the pole tips are substantially magnetically saturated. 9. A transducer comprising a magnet , magnetically permeable inner and outer pole tips magnetically coupled to the magnet and defining a gap between the inner and outer pole tips for mounting a voice coil in a predetermined axial direction, and a voice coil mounted in the gap in the predetermined axial direction, wherein at least one of the pole tips is electrically conductive in a plane substantially orthogonal to the predetermined axial direction in a region extending into the body of the pole tips from the edge of the pole tips adjoining the gap. 10. A transducer according to claim 9 wherein the gap is in the form of an annulus extending around the perimeter of the inner pole tip. 11. A transducer according to claim 9 or 10 wherein both pole tips are electrically conductive in the said plane. 12. A transducer according to any of claims 9 to 11 wherein at least one of the pole tips comprises a laminate of sheets of a ferromagnetic material and a high conductivity material, the sheets being substantially in the plane orthogonal to the predetermined axial direction. 13. A transducer according to claim 12 wherein both pole tips comprise a laminate of a ferromagnetic material and a high conductivity material . 14. A transducer according to any of claims 9 to 11 wherein the pole tips are formed from a sintered mixture of steel and copper. 15. A transducer according to any of claims 9 to 11 wherein the pole tips are of a magnetically permeable material formed to have slots extending into the pole tips from the edge adjoining the annular gap and a conductive shroud provided on the tips and filling the slots. 16. A transducer according to any of claims 8 to 15 wherein the pole tips are substantially magnetically saturated. 17. A transducer according to any of claims 9 to 16 wherein the coil comprises an annular former with electrically conductive wire wound outside it . 18. A loudspeaker comprising a diaphragm and a transducer according to any of claims 9 to 17, wherein the voice coil is rigidly attached to the diaphragm to excite the diaphragm when an electric signal is applied to the voice coil . 19. A loudspeaker according to claim 18 wherein the diaphragm is a plate capable of supporting a plurality of resonant bending wave modes in the plane of the plate and wherein the transducer excites the resonant bending waves to produce an acoustic output.

Inventors: BANK GRAHAM, BANK, GRAHAM
Applicants: BANK GRAHAM, NEW TRANSDUCERS LTD, BANK, GRAHAM, NEW TRANSDUCERS LIMITED
Application year: 2000
Main classification: H01F
IPC codes: H04R7, H04R9