JP2003505959A - Magnet assembly - Google Patents

Abstract

(57) [Abstract] The transducer comprises a magnet (1), a magnetically permeable inner pole tip (3) and an outer pole tip (5), and a voice coil (7) mounted in a gap between the pole tips. And a magnet assembly having: Either or both of the pole tips are conductive in a plane substantially orthogonal to the voice coil axis in a region extending from the end of the pole tip adjacent the gap to the pole tip body.

Description

Detailed Description of the Invention

TECHNICAL FIELD The present invention relates to magnet assemblies, and more particularly to magnet assemblies for actuators such as transducers in loudspeakers.

BACKGROUND OF THE INVENTION Conventional axisymmetric permanent magnet assemblies used to drive tubular coils of actuators require the magnetic flux generated by a permanent magnet or electromagnet to be concentrated within the coil area. This produces a high magnetic flux density and improves overall efficiency.

In existing designs, the concentration of magnetic flux is usually done using a magnetic pole tip made of steel, which increases the effective permeability of the space near the winding coil and, as a result, the effective inductance of the coil. Will increase. Secondary effects occur as the frequency of the alternating current increases. This is because eddy currents are generated in the pole tips to substantially shield the coil from the pole tips, thereby substantially reducing permeability.

The effect will be described below with reference to the apparatus shown in FIG. FIG. 1 shows a permanent magnet 3 sandwiched between a first pole piece 5 and a second pole piece 7. First
The magnetic pole piece 5 has a disk shape having an outer peripheral edge portion forming the inner magnetic pole tip 9. The second pole piece 7 is cup-shaped with an outer pole tip 11. A gap 13 for moving the coil 15 is formed between the inner magnetic pole tip and the outer magnetic pole tip. FIG. 1 shows a cross-sectional view, where the entire magnet assembly has an annular shape symmetrical about an axis 17. That is, the coil is a ring provided in the annular gap 13.

FIG. 2 shows lines of magnetic force generated by a magnet. When an electric current flows through the coil, a force that moves the coil in the axial direction acts on the coil. The force acting on the coil is given by the product of the magnetic flux density B and the current density J. But,
The current density J, and hence the force, decreases at higher frequencies where the coil inductance increases.

To understand the mechanism by which a coil's inductance increases, the magnetic flux associated with the current through this coil can be modeled and plotted. When plotting the magnetic field around the coil in air, the magnetic field is as shown in FIG. The axis of symmetry is shown by the vertical line on the left side of the figure. The magnetic field lines are smooth and the magnetic flux lines are not distorted. The curve that bounds the model forms the boundary of the mathematical model and represents the absorption boundary intended to guarantee a finite model size.

When this coil is attached to the magnet assembly, the magnetic flux distribution is distorted as shown in FIG. There are two effects that limit the inductance of the coil, both found in conventional magnet assemblies. The first effect is that the higher the permeability to air, the higher the magnetic flux density as the flux lines pass through the steel. At higher frequencies, the second effect of eddy currents becomes dominant. With respect to this effect, the magnetic flux lines passing through the steel cause a current (although not so good) to flow in the steel which is a conductor. However, when the frequency of the current flowing through the coil becomes high, it becomes difficult for the AC magnetic flux to penetrate the steel due to the eddy current. The penetration depth is called the epidermal depth.

The skin depth (δ) of the conductor is given by the following equation. Here, μ is relative magnetic permeability, σ is electrical conductivity, and ω is angular frequency (2π × frequency). As the frequency becomes higher, the skin depth becomes shallower and the magnetic flux penetration range becomes narrower. For example, at high frequencies of 10,000 Hz, the penetration rate is very small (10 kHz
Then about 0.6 mm). The skin depth versus frequency for steel is shown in FIG.

In existing methods of reducing inductance, the effect of this skin depth is reduced by covering the pole pieces with a copper foil sheet. This method is effective only at the frequency when the skin depth of copper is about the thickness of the copper foil. However, the addition of this copper foil reduces the available voids and reduces the DC flux density. This method has a contradiction that reducing the inductance also reduces the magnetic flux density.

For the above reasons, the copper foil shroud is only partially effective. Nevertheless, at 10 kHz, the magnetic flux lines do not penetrate as deeply into the steel (
(See FIG. 7). Japanese Patent Laid-Open No. 3-201797 describes an example of this method.

A similar arrangement using two copper rings for the outer pole pieces is DE-19637847.
Explained. However, the inductance reduction due to this configuration is not entirely satisfactory.

DISCLOSURE OF THE INVENTION According to the present invention, a magnet assembly is provided which is magnetically permeable and comprises inner and outer pole tips forming a gap for mounting a voice coil in a predetermined axial direction, comprising: Provided is a magnet assembly, at least one of which is electrically conductive in a plane substantially orthogonal to said predetermined axial direction in a region extending from the end of the pole tip adjacent to the gap to the pole tip. The gap is preferably annular in shape.

Inductance can form an important and also dominant part of the impedance of conventional loudspeaker coils at frequencies around 1-10 kHz and higher. Since the inductance gives jωL to the impedance, the contribution to the impedance increases with frequency. Therefore, when driven at a constant voltage, the coil current will eventually decrease at a higher frequency, which will also reduce the coil force. Therefore, the inductance causes a reduction in the acoustic output at high frequencies. This effect is particularly severe for a coil intended to produce a large force, since the desired large number of turns increases the inductance.

The magnet assembly according to the present invention reduces inductance at high frequencies. The conductivity of the pole tips shields the pole tips from the coils located in the annular gap along the eddy currents. As a result, the inductance of the coil at that frequency is reduced and the acoustic output by a loudspeaker using such a magnet assembly can be more uniform over frequency. Both pole tips are preferably conductive in the plane.

The pole tip may be near magnetic saturation and may help reduce the coil inductance slightly, especially at low frequencies. Therefore, the pole tip may be at least 75%, preferably 90% of the saturation magnetization.

The pole tips may be formed of a stack that is stacked perpendicular to the plane. The stack may be a ferromagnetic material and a highly conductive material, such as steel and copper, respectively. This does not unduly obstruct the magnetic flux flow through the magnet assembly,
The conductivity in the plane can be increased to promote eddy currents.

In another embodiment, the pole tip may be constructed of steel with a slot extending from the end adjacent the annular gap to the pole tip. At that time, the copper shroud is swaged by the pole tip to block the slot.

It should be understood that the eddy currents caused by the magnetic field generated by the coil are substantially perpendicular to the coil axis in the coil plane. Therefore, the conductivity need not be high in the axial direction but cannot be ignored. However, it should be very difficult to configure the magnetic permeability to be high and the conductivity to be high in all directions at the same time. 1
In one configuration, the pole tip can be comprised of a sintered mixture of steel and copper.

The magnet assembly according to the present invention can minimize the increase in permeability encountered by the coil while at the same time improving the conductivity of the pole tips to promote eddy currents. In conventional magnetic circuits such as transformers and inductors, the purpose is to couple the magnetic flux lines to the core to reduce the effects of eddy currents and prevent the magnetic flux lines from penetrating into the core. In this case, the opposite effect is intended: coupling to the core unnecessarily increases the inductance.

According to another aspect of the invention, there is provided a transducer having the aforementioned magnet assembly and a coil disposed in the gap. The coil may include an annular molded body having a conductive wire wound on the outside.

The present invention also provides a loudspeaker having the aforementioned magnet assembly, a diaphragm, and a voice coil that is firmly fixed to the diaphragm and fixed in the annular gap of the magnet assembly. it can. The loudspeaker is WO9
It may be a distributed mode loudspeaker as described in 7/09842. Specific embodiments, which are merely for illustrative purposes, are described below with reference to the accompanying drawings.

BEST MODE FOR CARRYING OUT THE INVENTION A magnet assembly according to a particular embodiment of the present invention shown in FIG. 8 has two pole pieces 5, 7.
And one is an annular steel cup 7 which surrounds the other magnetic pole piece, which is a steel disc 5. The permanent magnet 3 separates the pole pieces. It should be noted that the drawing shows a cross section extending outwards from the center of the magnet assembly, and when viewed from above, the magnet assembly is actually annular.

The outer rim of the pole piece and the inner rim of the pole piece comprise an inner pole tip 9 and an outer pole tip 11. Each pole tip is a stack of alternating steel 19 and copper 21 that allows DC flux to pass from the inner pole piece 5 to the cup 7.

In the annular gap 13 between the pole pieces, the metal wire 2 wound around the forming body 25 is provided.
A coil 15 having 3 is provided. The coil is fixed to the diaphragm 27 of the loudspeaker so as to excite the loudspeaker. The magnet assembly is mounted on the diaphragm by elastic support 28. The diaphragm may be a distributed mode panel and a distributed mode loudspeaker as described in WO / 09842.

The magnetic circuit generated by the permanent magnet 3 is shown by the magnetic flux lines in FIG. As can be seen by comparing FIG. 9 with FIG. 2 which does not have a laminated pole tip, the laminate is composed of permanent magnets 6
Zero has a minimal effect on the magnetic flux density generated.

The thickness of the steel laminate 21 is determined so that it operates in a substantially magnetically saturated state and reduces its permeability while preventing eddy current circulation in the steel. Since the conductivity of copper is much higher than that of steel (about 10 times), the copper plate can be made thin (calculated √10 = about 3 times as thin), and in the present embodiment, the thickness is ½. Is.
For clarity, this difference in thickness is not shown in FIG.
, And 13 are shown. Similar to the desired inductance reduction, this ratio can be modified to fit the available geometric dimensions. The nearly saturated steel pole tip 19 reduces the inductance of the coil 15 slightly at low frequencies.

The magnetic flux from the coil itself is shown in FIG. The magnetic flux lines pass through the thin steel plates 19 and the interleaved copper plates 21 with very high conductivity short out eddy currents.

The steel plate is thin enough at the high frequencies to the depth of the skin so that the frequencies are high, no eddy currents are formed and the alternating arrangement of copper shorts them out. In this embodiment, the steel plate has a thickness of 0.2 mm and the copper plate has a thickness of 0.1 mm. As shown in FIG. 12, even at 10 kHz, the magnetic flux pattern is distorted to keep the inductance low.

In order to contrast possible improvements, a finite element (FE) model of the structure was analyzed and the estimated inductance analysis results are shown in Table 1.

Table 1

The effect that the magnet assembly has on the inductance focuses on the increased inductance at low frequencies caused by the steel core of the standard magnet assembly, which decreases at higher frequencies as a result of eddy currents in the steel. Can be understood by

In the case of stacked pole tips, a slight reduction is obtained at 100 Hz due to the reduced pole tip transmission, leading to a significant improvement at 10 kHz where the inductance is substantially lower than for coils in air. By choosing the right steel plate / copper plate thickness ratio, the inductance can be reduced by 50% -70%. The stack can be applied to either or both of the poles and the cup.

FIG. 13 shows another embodiment in which the pole tips 9, 11 are not a laminate but a sintered mixture of steel and copper.

FIG. 14 shows a pole tip used in another embodiment, which pole tip is constructed of steel with slots 31 extending perpendicular to the coil axis. At this time, the copper piece 3 is swaged on the facing surface of the magnetic pole tip to close the slot.

Although a particular embodiment has been described, several other embodiments are possible. For example, any suitable magnetically transparent material can be used for the pole tip without departing from the scope of the invention. The conductive strip need not be copper, but other conductive materials such as silver or aluminum can be used.

The pole tips and pole pieces may be of any suitable shape. For example, one pole piece may be U-shaped with two ends, with one pole tip at each end. An inner pole piece may be provided having a pole tip opposite the pole tip of each of the U-shaped pole pieces across the two gaps, forming two gaps. The coil may surround the inner pole piece and pass through both gaps.

[Brief description of drawings]

[Figure 1]   1 shows a cross section of a conventional magnet assembly.

[Fig. 2]   2 shows the magnetic flux of the magnet assembly of FIG.

[Figure 3]   The magnetic flux around a coil in air is shown.

[Figure 4]   4 shows the magnetic flux around a coil at 100 Hz for a conventional magnet assembly.

[Figure 5]   Figure 4 shows the magnetic flux around the coil at 10,000 Hz for a conventional magnet assembly.

[Figure 6]   Figure 3 shows the skin depth of steel as a function of frequency.

[Figure 7]   Figure 4 shows the magnetic flux lines of a magnet assembly having a copper shroud around the pole tip.

[Figure 8]   3 shows a magnet assembly according to the present invention.

[Figure 9]   3 shows the magnetic flux lines of a magnet assembly according to the invention.

[Figure 10]   3 shows the magnetic flux lines from the coil at 100 Hz of the magnet assembly according to the invention.

FIG. 11 shows the magnetic flux lines from the coil at 1,000 Hz of the magnet assembly according to the invention.

FIG. 12 shows the magnetic flux lines from the coil at 10,000 Hz of the magnet assembly according to the invention.

[Fig. 13]   3 illustrates another embodiment of the present invention.

FIG. 14   3 illustrates another embodiment of the present invention.

[Procedure for Amendment] Submission for translation of Article 34 Amendment of Patent Cooperation Treaty

[Submission date] August 6, 2001 (2001.8.6)

[Procedure Amendment 1]

[Document name to be amended] Statement

[Name of item to be amended] Claims

[Correction method] Change

[Contents of correction]

[Claims]

[Procedure Amendment 2]

[Document name to be amended] Statement

[Correction target item name] 0011

[Correction method] Change

[Contents of correction]

A similar arrangement using two copper rings for the outer pole pieces is DE-19637847.
Explained. However, the inductance reduction due to this configuration is not entirely satisfactory. According to U.S. Pat. No. 3,830,986 of Pioneer Electronic, an air gap formed by magnetic materials laminated with conductive layers having a relatively high magnetic reluctance in order to increase the magnetic reluctance of the voice coil with respect to magnetic flux. It is known to provide an electro-acoustic moving coil converter with a magnetic circuit.

[Procedure 3]

[Document name to be amended] Statement

[Correction target item name] 0012

[Correction method] Change

[Contents of correction]

DISCLOSURE OF THE INVENTION According to the present invention, a magnet and magnetically permeable inner and outer magnetic pole tips that are magnetically coupled to the magnet and that form a gap to accommodate a voice coil in a predetermined axial direction. And at least one pole tip having a relatively high magnetic resistance in a magnetic circuit in a plane substantially orthogonal to the predetermined axial direction in a region extending from an end of the pole tip adjacent to the gap to the pole tip body. A magnetic assembly having a magnetization of at least 75% of magnetic saturation, the magnetic pole tip having a magnetic resistance of at least 75% of magnetic saturation. The gap is preferably annular in shape.

[Procedure amendment 4]

[Document name to be amended] Statement

[Correction target item name] 0013

[Correction method] Change

[Contents of correction]

Inductance can form an important and also dominant part of the impedance of conventional loudspeaker coils at frequencies around 1-10 kHz and higher. Since the inductance gives jωL to the impedance, the contribution to the impedance increases with frequency. Therefore, when driven at a constant voltage, the coil current will eventually decrease at a higher frequency, which will also reduce the coil force. Therefore, the inductance causes a reduction in the acoustic output at high frequencies. This effect is particularly severe for a coil intended to produce a large force, since the desired large number of turns increases the inductance.

[Procedure Amendment 5]

[Document name to be amended] Statement

[Correction target item name] 0014

[Correction method] Change

[Contents of correction]

The magnet assembly according to the present invention reduces inductance at high frequencies. The conductivity of the pole tips shields the pole tips from the coils located in the annular gap along the eddy currents. As a result, the inductance of the coil at that frequency is reduced and the acoustic output by a loudspeaker using such a magnet assembly can be more uniform over frequency. Both pole tips are preferably conductive in the plane.

[Procedure correction 6]

[Document name to be amended] Statement

[Name of item to be corrected] 0015

[Correction method] Change

[Contents of correction]

The magnetic pole tip is configured to be nearly magnetically saturated, especially at low frequencies.
It may help reduce the inductance of the coil slightly.
Therefore, the pole tip has a magnetization of at least 75%, preferably 90% of its saturation magnetization.

[Procedure Amendment 7]

[Document name to be amended] Statement

[Correction target item name] 0016

[Correction method] Change

[Contents of correction]

The pole tips may be formed of a stack that is stacked perpendicular to the plane. The laminate has a high magnetic resistance to magnetic flux, such as steel and copper,
It may be a ferromagnetic material and a highly conductive material. This can increase conductivity in the plane and promote eddy currents without unduly hindering the magnetic flux flow through the magnet assembly.

[Procedure Amendment 8]

[Document name to be amended] Statement

[Correction target item name] 0017

[Correction method] Change

[Contents of correction]

In another embodiment, the pole tip may be constructed of steel with a slot extending from the end adjacent the annular gap to the pole tip. At that time, the copper shroud is swaged by the pole tip to block the slot.

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI theme code (reference) H04R 9/04 105 H04R 9/04 105A (81) Designated countries EP (AT, BE, CH, CY, DE, DK, ES, FI, FR, GB, GR, IE, IT, LU, MC, NL, PT, SE), OA (BF, BJ, CF, CG, CI, CM, GA, GN, GW, ML, MR, NE, SN, TD, TG), AP (GH, GM, KE, LS, MW, MZ, SD, SL, SZ, TZ, UG, ZW), EA (AM, AZ, BY, KG, KZ) , MD, RU, TJ, TM), AE, AG, AL, AM, AT, AU, AZ, BA, BB, BG, BR, BY, BZ, CA, CH, CN, CR, CU, CZ, DE, DK, DM, DZ, EE, ES, FI, GB, GD, GE, GH, GM, HR, HU, ID, IL, IN, IS, JP, KE, KG, KP , KR, KZ, LC, LK, LR, LS, LT, LU, LV, MA, MD, MG, MK, MN, MW, MX, MZ, NO, NZ, PL, PT, RO, RU, SD, SE, SG, SI, SK, SL, TJ, TM, TR, TT, TZ, UA, UG, US, UZ, VN, YU, ZA, ZW

Claims (19)

[Claims] 1. A magnet assembly comprising: a magnet; and magnetically permeable inner and outer magnetic pole tips magnetically coupled to the magnet to form a gap for mounting a voice coil in a predetermined axial direction, At least one of the pole tips is electrically conductive in a plane substantially orthogonal to the predetermined axial direction in a region extending from the pole tip end adjacent the gap to the pole tip body. Magnet assembly. 2. The magnet assembly according to claim 1, wherein the gap has an annular shape extending around a peripheral portion of the inner magnetic pole tip. 3. A magnet assembly according to any one of the preceding claims, characterized in that both of said pole tips are electrically conductive in said plane. 4. At least one of the pole tips comprises a laminated sheet of ferromagnetic material and highly conductive material, the sheet being in a plane substantially orthogonal to the predetermined axial direction. The magnet assembly according to any one of the preceding claims. 5. The magnet assembly of claim 4, wherein both of the pole tips comprise a stack of ferromagnetic and highly conductive materials. 6. The magnet assembly according to claim 1, wherein the magnetic pole tip is formed of a sintered mixture of steel and copper. 7. A magnetic pole tip formed to have a slot extending from an end adjacent to the annular gap to the pole tip and a conductive shroud provided on the pole tip to close the slot. The magnet assembly according to any one of claims 1 to 3, wherein the magnet assembly is a transparent material. 8. A magnet assembly according to any one of the preceding claims, wherein the pole tips are substantially in magnetic saturation. 9. A magnet, magnetically permeable inner and outer magnetic pole tips magnetically coupled to the magnet to form a gap for mounting a voice coil in a predetermined axial direction, and the gap in the predetermined axial direction. A voice coil attached thereto, wherein at least one of the magnetic pole tips is substantially in the predetermined axial direction in a region extending from the magnetic pole tip end portion adjacent to the gap to the magnetic pole tip body. A transducer characterized by having conductivity in a plane orthogonal to the. 10. The transducer according to claim 9, wherein the gap has an annular shape extending around a peripheral portion of the inner magnetic pole tip. 11. A transducer according to claim 9 or 10, characterized in that both of said pole tips are electrically conductive in said plane. 12. At least one of the pole tips comprises a laminated sheet of ferromagnetic material and highly conductive material, the sheet lying in a plane substantially orthogonal to the predetermined axial direction. The converter according to any one of claims 9 to 11. 13. The transducer of claim 12 wherein both of said pole tips comprise a stack of ferromagnetic and highly conductive materials. 14. A transducer as claimed in any one of claims 9 to 11, characterized in that the pole tips are made of a sintered mixture of steel and copper. 15. A magnetic pole tip formed to have a slot extending from the end adjacent the annular gap to the pole tip, and a conductive shroud provided on the pole tip and closing the slot. 12. Transducer according to any one of claims 9 to 11, characterized in that it is a transparent material. 16. A transducer as claimed in any one of claims 8 to 15 in which the pole tip is substantially in magnetic saturation. 17. The transducer according to claim 9, wherein the coil has an annular molded body on which a conductive wire is wound. 18. The loudspeaker according to claim 9, further comprising a diaphragm and a transducer, wherein the voice coil is firmly attached to the diaphragm and an electric signal is applied to the voice coil. A loudspeaker which excites the diaphragm. 19. The diaphragm is a plate capable of supporting a plurality of bending wave modes of the plate surface, and the transducer excites the resonant bending wave to generate an acoustic output. The loudspeaker according to item 18.