GB2149272A - Electromechanical transducer - Google Patents

Abstract

An electromechanical transducer, e.g. a loudspeaker, comprises a movable part in the form of a metallic cylindrical shell or annulus (1) oscillating in the air gap (5) of a closed magnetic circuit including a permanent magnet (3) or a DC-excited electromagnet. The shell (1) has openings (10) accommodating part of the iron core (2) of an electromagnet forming part of the magnetic circuit. When current is passed through the coil 6 of the electromagnet, those parts of the shell (1) around the openings (10) act as short-circuited turns and the resulting induced currents flow through the air gap (5) causing the shell to oscillate. Both the magnet (3) and the electromagnet may be multipolar. <IMAGE>

Description

SPECIFICATION Electrical mechanical transducer The invention relates to an electromechanical transducer for the conversion of electrical oscillations into mechanical or acoustic oscillations, preferably a loudspeaker, having a magnetic circuit with a permanent magnetic field and provided with an air gap, as well as resiliently mounted displaceable part that protrudes into the air gap and a mechanical oscillation-transmitting means connected thereto.

Moving-coil loudspeakers are of this general type of transducer in which the moving or displaceable part ("armature") in the air gap of the magnetic circuit with the permanent magnetic field is a moving coil which is connected to electrical connections fixed to the frame of the loudspeaker through resilient and flexible leads. From the point of view of production and use, moving-coil loudspeakers have several drawbacks and restrictions, as will be detailed below. The moving part is formed by a coil. During operation of the loudspeaker this coil is continuously oscillated. Accordingly, it has to be mechanically resistant to vibration and damage even at the largest oscillation amplitudes.From the point of view of manufacture, this requirement is difficult fully to meet An oscillating part of only restricted thickness may be arranged in the air gap i.e. the size or thickness of the coil is limited. This necessitates partly that a relatively large air gap should be chosen, resulting in reduced induction of the air gap and simultaneously in reduced efficiency, and partly the coil has to be made with a low number of turns thus restricting the impedance of the loudspeakers to a low value. A significant component of this low impedance is the ohmic resistance of the thin wire of the coil, causing also power losses. These losses are due to the unavoidably high current density and thus heating up of the coil, which heat is difficult to dissipate.

A frequency cause of failure of moving coil transducers is due to fatigue in the terminals of the coil of the moving part, which tend to break off after a certain time.

Even a slight mechanical deformation in manufacture may result in friction in the moving-coil, causing not only distortion but also possible short-circuits between the turns of the coil.

An aim of the invention is to develop an electro-mechanical transducer, expediently a loudspeaker, with the aid of which at least some of the above-described disadvantageous features may be eliminated or reduced.

In accordance with the invention, there is provided an electromechanical transducer for the conversion of electrical oscillations into mechanical oscillations, e.g. a loudspeaker, comprising a closed magnetic circuit with a permanent magnetic field and an air gap, a resiliently mounted movable part that protrudes into the air gap and a mechanical oscillation-transmitting member connected to said part, wherein the portion of the movable part protruding into the air gap is cylindrical or annular in shape, is made of metal and is provided with at least two diagonally arranged openings through which the iron core of an electromagnetic forming part of the said closed magnetic circuit is guided.

In a preferred embodiment of the invention, both the magnetic circuit with the permanent magnetic field and the iron core of the electric magnet are multipolar, the poles being uniformly distributed along the periphery of the moving part.

In a further preferred embodiment, the openings are formed in the central part of the moving part. In the direction of the displacement of the moving part, the openings are formed such that they are wider than the iron core of the electromagnet by at least the extent of the largest deflection or displacement.

In a further preferred embodiment according to the invention, the air gap of the magnetic circuit with the permanent magnetic field lies at one edge of the oscillating part. A particularly efficient energy conversion may be achieved in an embodiment in accordance with the invention where the electromechanical transducer has two magnetic circuits each with a permanent magnetic field and into the air gaps of each the two ends of the moving part projects.

The magnetic circuit expediently conains a permanent magnet. However, it is also possible to replace the permanent magnet with a DC-excited electro-magnet (dynamic loudspeaker).

In order to be able to match the electromechanical transducer to different impedances, in accordance with a preferred embodiment of the invention the electromagnet has a pluarlity of coils. In another possible embodiment of the invention, the coil of the electromagnet has one or more taps.

Many of the drawbacks of moving-coil loudspeakers have been eliminated by the electro mechanical transducer according to the invention. So, e.g. the moving part of the loud speaker according to a preferred embodiment of the invention is formed by one single cylindrical annulus or cup, preferably made of aluminium not carrying any coil. The coil is preferably formed by metal parts arranged around the openings in the cylindrical annulus or cup, as short-circuited turns, forming prac tically the secondary coil of a transformer. No electrical power connection is needed for the oscillating part, accordingly, a considerably source of faults could be eliminated.

The moving part is formed as one single metallic cylindrical shell with a relatively thin wall which protrudes into the air gap of the magnetic circuit. By virtue of the small wall thickness a far smaller air gap can be formed than with the known moving-coil loudspeakers, and in such a manner a far higher induction can be produced and simultaneously the efficiency of conversion may be considerably improved.

The large metallic surface enables not only a far higher current intensity but also an increased current density to be permitted, as the large aluminium surface of the said part assures improved cooling.

In case of any possible friction, short-circuiting does not occur.

To a certain extent, the preferred embodiment according to the invention can be considered as "self-correcting", since wear of the frictionally involved part of the operation of the electromechanical transducer remains in practical qualitatively the same.

A further advantage of the electromechanical transducer according to the invention lies in that the coil of the electromagnet can be matched to any desired impedence likely to occur in practice. Lt is also possible to make several taps within one single coil; in such a manner as many impedances may be matched as the number of the branchings. This cannot be achieved with known moving-coil loudspeakers.

Preferred embodiments of an electromechanical transducer according to the invention will be described in detail and purely by way of example with reference to the accompanying drawings wherein: Figure 1 is a schematic section view of a loudspeaker according to the invention; Figure 2 is a section taken along the plane indicated by lines A-A in Fig. 1; Figure 3 is a sectional view of the loudspeaker according to Fig. 1 taken along the plane indicated by lines B-B; Figure 4 is a view similar to Fig. 1 but illustrating a further embodiment; Figure 5 is a section of an arrangement with a six-pole electromagnet; and Figure 6 is a view similar to Fig. 5, but wherein the electromagnet is replaced with a permanent magnet.

Referring to Fig. 1, a preferred embodiment of an electromechanical transducer according to the invention is in the form of a loudspeaker. Regarding its general design, the loudspeaker according to the invention corresponds to that of the traditional moving-coil loudspeaker, insofar as it has a magnetic circuit with a permanent magnetic field with an air gap 5. The circuit is formed in the illustrated embodiment by a permanent magnet 3 and an iron body 4 closing the magnetic circuit thereof. A moving part 1 protrudes into the air gap 5; in accordance with this invention the moving part 1 has the shape of a cylindrical shell or annulus made of metal, preferably of copper or aluminium.

The moving part 1 is resiliently suspended or fixed in a known manner so that it is able to move in the air gap 5 against the spring force. A resilient suspension is assured by means of a suspending member 7 having an inner flange which is connected to the moving part 1, while its outer flange is connected to the base body of the loudspeaker or to a casing 9.

An oscillation-transmitter, i.e. a conical horn 8 is also connected to the moving part 1.

preferably at the suspending member 7. However, the suspension of the moving part 1 and the formation of the oscillation-transmitting member are of no interest from the point of view of the present invention.

In accordance with the invention, two diagonally formed openings 10 are provided on the outer surface of the moving part. The iron core 2 of an electromagnet is passed through the openings 10 in such a manner that the ends of the iron core 2 closing the magnetic circuit of the electromagnetic iie outside the moving part 1. In this way, the iron core 2 passing through the openings 10 is enclosed by the flanges of the two openings 10 of the moving part 1. Preferably, the coil 6 of the electromagnet is wound on the iron core 2 within the moving part 1.

A sectional view of the electromagnet and the moving part 1 may be seen in Fig. 2, showing that the iron core 2 closes the magnetic circuit from two sides.

The shape of the openings 10 formed in the moving part 1 corresponds to the corsssection of the iron core 2 passing through it.

In the lateral direction the openings 10 are larger than the iron core 2, but only to such an extent as is required to ensure that no friction occurs in operation. However, in the direction of the excursion or deviation the opening 10 must be wider in order to avoid contact between the edges of the opening 10 and the iron core 2 of the electromagnet, even at the greatest amplitude of the excursion or deviation. This can be achieved by making the width of the openings 10 in that direction larger than the iron core 2 by at least the maximal deviation peak to peak amplitude of oscillation.

Fig. 3 shows in section the magnetic circuit with the permanent magnetic field. The magnetic field is excited by the permanent magnet 3, at both ends of which there is a respective air gap 5. The magnetic circuit of the permanent magnet 3 is closed by the iron body 4, which encloses - similarly to the iron core 2 the moving part 1 from the outside.

The loudspeaker described above operates as follows.

Passing an alternating current electric signal into the coil 6 of the electromagnet causes an alternating magnetic induction in the iron core 2. The flanges of the two openings 10 on the moving part 1 form a short-circuited turn each around the iron core 2 in which the current is relatively high. The arrangement can be considered also as a transformer, the primary coil of which is formed by the coil 6 of the electromagnet, while the secondary coils are the metal parts of the moving part 1 enclosing the openings 1 0. In said metal parts, as shortcircuited turns, current flows in accordance with the transformer ratio. The parts of the moving part 1 in which current is flowing projects into the air gap 5 such that the force exerted on the moving part 1 depends on the current intensity of the current flowing therein and the induction of the air gap 5.In this way, the moving part 1 is excited to mechanical oscillation in compliance with the electric oscillations fed into the coil 6.

Fig. 4 shows a further preferred embodiment wherein the efficiency of conversion is considerably better, compared with the embodiment according to Fig. 1. The main difference lies in that it is provided with a second magnetic circuit with a permanent magnetic field which lies on the other side of the electromagnet. With this embodiment the second magnetic circuit with the permanent magnetic field consists of a permanent magnet 1 3 and an iron body 14, with an air gap 1 5 between them. While the upper end (as viewed) of the moving part 1 protrudes into the air gap 5, the lower end - which in the previous embodiment was left free is disposd in the air gap 1 5 in such a manner that the power effects arising in the air gaps 5 and 1 5 are in effect added.In order to obtain unidirectional power effects, the direction of current, i.e. of the magnetization in the air gaps 5 and 1 5 must be considered. Accordingly, if the magnetizing direction of the permanent magnet 3 is as shown by the arrow in the Fig.

(from left to right), the direction of magnetization of the lower permanent magnet 1 3 is from right to left, as shown by the arrow in the Figure.

Figs. 5 and 6 illustrate a further embodiment shown in section. In Fig. 5 the iron core 5 of an electromagnet is shown, which has six poles. Accordingly, six openings 10 are formed in the moving part 1. The poles are distributed uniformly around the periphery of the moving part 1.

Fig. 6 shows the magnetic circuits with the permanent magnetic field arranged above and below the electric magnet. Similarly to the electromagnet, the permanent magnet also has six poles.

Compared to the previously described embodiments, the efficiency of conversion is far better with the embodiment shown in Figs. 5 and 6, as practically the whole utilizable surface of the moving part 1 takes part in generating the power effect.

Several variants are possible within the scope of the attached claims. So, e.g. instead of the permanent magnets, electromagnets excited by direct current may be applied. The poles and air gaps of the magnetic circuit with the permanent magnetic field may be arranged differently so long as the mutual relation of the parts in which current flows and the magnetic induction is observed.

The coil of the electromagnet may be dimensioned to an impendence which can be directly matched to the output of a power amplifier. The electromagnet may have a plurality of coils. Where the coil 6 is provided with taps, the loudspeaker can be matched to several different impedances.

Claims (1)

1. An electromechanical transducer for the conversion of electrical oscillations into mechanical oscillations, e.g. a loudspeaker, comprising a closed magnetic circuit with a permanent magnetic field and an air gap, a resiliently mounted movable part that protrudes into the air gap and a mechanical oscillation-transmitting member connected to said part, wherein the portion of the movable part protruding into the air gap is cylindrical or annulus in shape, is made of metal and is provided with at least two diagonally arranged openings through which the iron core of an electromagnet forming part of the said closed magnetic circuit is guided.

2. A transducer as claimed in claim 1, wherein both the magnetic circuit with the permanent magnetic field and the iron core of the electromagnet are multipolar, and the poles are uniformly distributed around the periphery of the cylindrical surface of the said part.

3. A transducer as claimed in claim 1 or claim 2, wherein the openings are formed in the centre of the said part.

4. A transducer as claimed in any preceding claim wherein in the direction of displacement of the said part the openings are wider than the said iron core by at least the magnitude of the maximal displacement of said part.

5. A transducer as claimed in any preceding claim wherein the air gap is at one end of the said part.

6. A transducer as claimed in any preceding claim wherein there are two magnetic circuits with permanent magnetic fields and both ends of the said part protrude into a respective air gap.

7. A transducer as claimed in any preceding claim wherein the magnetic circuit contains a permanent magnet.

8. A transducer as claimed in any of claims 1 to 6, wherein the magnetic circuit contains an electromagnet excited by direct current.

9. A transducer as claimed in any preced ing claim, wherein the electromagnet has a plurality of coils.

10. A transducer as claimed in any preceding claim, wherein the or each coil of the electromagnet is provided with tap(s).

11. A transducer substantially as herein described with reference to and as shown in Figs. 1 to 3 or Fig. 4 or Figs. 5 and 6 of the accompanying drawings.

CLAIMS (25 Sept 1984) Superseded claims 1 New or amended claims:

1. An electromechanical transducer for the conversion of electrical oscillations into mechanical osdillations, e.g. a loudspeaker, comprising at least one closed magnetic circuit with a permanent magnetic field, a resiliently mounted movable part having a cylindrical or annular wall a portion of which protrudes into an air gap across which said permanent magnetic field extends and a mechanical oscillation-transmitting member connected to said part, and wherein said wall is made of metal and is provided with at least two diametrically opposed openings through which the iron core of an electromagnet forming part of a further closed magnetic circuit is guided.