GB2104343A

GB2104343A - Moving iron transducer

Info

Publication number: GB2104343A
Application number: GB08125481A
Authority: GB
Inventors: David Alan Burgess
Original assignee: Standard Telephone and Cables PLC
Current assignee: STC PLC
Priority date: 1981-08-20
Filing date: 1981-08-20
Publication date: 1983-03-02
Also published as: GB2104343B

Abstract

In moving iron transducers used, for example, in telephones, whether of the two pole or of the rocking armature type, the armatures are short enough to move inside the pole-pieces with a narrow clearance, which gives good stability and sensitivity. In the simple case with a U- shaped magnetic circuit, the magnet (22) is in the base of the U and the armature (20) moves into and out of the region between the pole- piece (23, 24) ends. In a rocking armature device, the magnetic circuit is E-shaped with the magnet in the middle leg of the E and the armature rocks about the end of that middle leg. Each of its ends extends almost to the adjacent outer leg so that the ends move in and out of the region within the pole-piece ends. <IMAGE>

Description

SPECIFICATION Moving iron transducer This invention relates to electro-acoustic or acoustic-electric transducers of the moving iron type.

Moving iron transducers as used in telephony as receivers are essentially similar in many respects to the original designs from much earlier times. Thus Fig. 1 shows a basic form of transducer which has a diaphragm 1 made from a magnetic material. This performs three functions; it provides magnetic coupling, balances the static magnetic attraction with a mechanical restoring force, and couples the mechanical forces on the diaphragm to the surrounding air. A magnet 2 provides a static magnetic bias, and a coil 3 wound on the magnet applies the variation in the magnetic field corresponding to the electrical signal applied.

With such a device, mechanical sensitivity is proportional to the static magnetic force.

The force F- displacement x curve for such a device is shown in Fig. 5, and it will be appreciated that sensitivity can be improved by reducing the length of the air gap between the pole ends and the diaphragm. However, this gap cannot be reduced below the point at which "freeze off' occurs. Superimposed on Fig. 5 are two lines A and B representing the opposing force exerted by the diaphragm for two different diaphragms. If the diaphragm exerts a force less than F0 at x = 0, and is displaced in contact with the pole tips, e.g. by mechanical shock or overpressure, the dia phragm "freezes off" and a loss of function occurs. Thus the air gap must be at least GA for a stiff diaphragm A and at least GB for a stiff diaphragm B.In practice it must be greater than this to allow for the effects of thermal expansion, other thermal effects and relaxation of the diaphragm. Thus high me chanical sensitivity requires a stiff diaphragm, which generally causes poor acoustic coupling to the airload, especially if the transducer is used as a microphone.

Two modifications of the device of Fig. 1 are in use, in the first of which the functions of the diaphragm 1 are performed by an armature of high performance. The Western Electric ring armature, and central armature transducers are examples of this. In the sec ond of these modifications, the armature is in a balanced magnetic circuit and provides the restoring force, being coupled by a rod to a light unstressed diaphragm. An example of this is the rocking armature transducer used in the British Telecom earpieces. In these modifi cations the original "horseshoe" magnet is replaced by a small magnet of modern high coercivity material with two or more pole pieces. Such a device of the second type is shown in Fig. 2, where we have a magnet 5 mounted centrally over a U shaped yoke having pole-pieces 6, 7 wound with a coil 8.The armature 9 is mounted to rock about the end of the magnet 5 (or a pole-piece mounted thereon) and drives a diaphragm (not shown) by a rod 10.

The force-displacement curve for a conventional rocking armature transducer is shown in Fig. 7, where the magnetic attractions due to the left and right-hand poles are indicated at 12, 1 3 respectively. The straight line 14 represents the armature restoring force, and the curve 1 5 is the sum of the applied magnetic forces. The lines 16, 1 7 show the limits due to an armature end stroking a magnetic pole-piece. Note that there exists a high value of force, F,, at the limits, and the mechanical restoring force must exceed this as in the case of the central armature type.

It is an object of the invention to provide a moving iron transducer which is an improvement on known devices such as described above, in which the force-displacement curve is improved and "freeze off" is eliminated.

According to the invention there is provided an electro-acoustic, or acoustic-electric, transducer of the moving iron type, in which the magnetic circuit includes a source of polarising magnetization having pole-pieces, there being a coil or coils wound on said polepieces, in which an armature is provided which is coupled to a diaphragm, the armature having a length or diameter slightly less than the distance between the inner edges of the pole-pieces so that a clearance exists between each end of the armature and the polepieces adjacent thereto, whereby movement of the armature, either in response to electrical signals applied to the coil or to sound waves incident on the diaphragm, may take either end thereof inside the pole-piece to which it is adjacent, in which region in its reciprocatory movement a said armature end experiences a reduced axial magnetic force.

Embodiments of the invention will now be described with reference to Figs. 3, 4, 6 and 8 of the accompanying drawings, in which: Figure 3 is a simplified view of a central aramature transducer embodying the invention.

Figure 4 is a simplified view of a rocking armature embodying the invention.

Figure 6 is a graph explanatory of the action of the device of Fig. 3.

Figure 8 is a graph explanatory of the action of the device of Fig. 4.

The transducer of Fig. 3 includes the general improvements already mentioned, i.e. an armature 20 secured to a non-magnetic dia phragm 21, a magnet 22, which may be a high coercivity magnet, and pole-pieces 23, 24. The additional feature is that, considered in the vertical plane in Fig. 3, there is a clearance indicated at 25 between each outer end of the armature and the adjacent pole piece. If the armature 20 is displaced so as to be within the pole-pieces 23, 24, the axial forces are much smaller since the magnetic lines of force are radial. Dependent on the shape of the pole tips, which are bevelled in Fig. 3, the magnetic characteristic has the shape shown in Fig. 6. The working point W is no longer dependent on diaphragm stiff ness, so that the stiffness can be indepen dently chosen to be, for example, that of diaphragm A, B or C.However, stiffness should exceed that of diaphragm D, which has a working point at W, but can also take an alternative stable position WD. Thus ''freeze off" is eliminated since the disphragm always returns to W, no matter where it is displaced.

Further, the curve shows a maximum, so that in the region of the maximum, variation of the airgap does not greatly affect sensitivity.

Hence variation of sensitivity with temperature, diaphragm relaxation, etc., and poor adjustment at assembly time will be reduced.

Note that in its movement, the armature is not restricted by striking a pole tip, but can move into a region where the axial magnetic force is decreasing.

The principle is also applicable, see Fig. 4, to rocking armature transducers. In view of the explanation given above it is felt that no detailed description is needed. However, it will be seen that in this case the balanced nature of the magnetic circuit allows the armature to be close to a neutral point which considerably increases sensitivity and allows very compliant armatures to be used. The inner facing tips of the outer pole-pieces can be bevelled, as in the case of the transducer of Fig. 3.

Fig. 8 shows the characteristics for the transducer of Fig. 4, the armature being shown "inside" the pole-pieces, i.e. beyond the position of maximum attraction, resulting in a "self-centring" action, i.e. the armature is stable if stiffness = 0. This arrangement is not, however, necessary to the function of this invention. Here the line 30 is the armature restoring face, the curves 31, 32 are the magnetic attractions due to the left and righthand poles respectively, and the curve 33 is the resultant magnetic force.

It should be noted that the improved transducers described herein can be used either as transmitters (microphones) or as receivers (earpieces in telephones).

Claims

1. An electro-acoustic, or acoustic-electric, transducer of the moving iron type, in which the magnetic circuit includes a source of polarising magnetization having pole-pieces, there being a coil or coils wound on said polepieces, in which an armature is provided which is coupled to a diaphragm, the arma- ture having a length or diameter slightly less than the distance between the inner edges of the pole-pieces so that a clearance exists between each end of the armature and the pole piece adjacent thereto, whereby movement of the armature, either in response to electrical signals applied to the coil or to sound waves incident on the diaphragm, may take either ends thereof inside the pole-piece to which it is adjacent, in which a region a said armature experiences a reduced axial magnetic force.

2. A transducer as claimed in claim 1, in which the magnetic circuit is U-shaped and the souce of magnetization is in the base of the U, and in which the two pole-pieces are each secured to one end of the magnet.

3. A transducer as claimed in claim 1, in which the magnetic circuit is E shaped with the source of magnetization in the central leg of the E, in which the armature is of the rocking armature type and is rockable about the end of the central leg of the E, the armature length being less than the distance between the inner faces of the two outer legs of the E, in which the diaphragm is driven by a rod secured to the armature at one of its ends, and in which the coil is wound on the two outer legs of the E.

4. A transducer as claimed in claim 2 or claim 3, and in which the inward facing tips of the pole-pieces are bevelled or otherwise shaped.

5. An electro-acoustic, or acoustic-electric transducer of the moving iron type, substantially as described with reference to Figs. 3 and 6, or Figs. 4 and 8, of the accompanying drawings.

CLAIMS (19Jan 1982)

6. An electro-acoustic, or acousto-electric, transducer of the moving iron type, in which the magnetic circuit includes a source of polarising magnetization having pole-pieces, there being a coil or coils wound on said polepieces, in which an armature is provided which is coupled to a disphragm, the armature having a length or diameter slightly less than the distance between the inner edges of the pole-pieces so that a clearance exists between each end of the armature and the polepiece adjacent thereto, whereby movement of the armature, either in response to electrical signals applied to the coil or to sound waves incident on the diaphragm, may take the ends thereof each inside the pole-piece to which it is adjacent, and whereby the armature in its movement does not strike the pole-pieces, and in which when a said armature is in the said region its ends each experience a reduced axial magnetic force.

7. A transducer as claimed in claim 6, in which the magnetic circuit is U-shaped and the source of magnetization is a small magnet of a high coercivity material located in the base of the U, and in which the two polepieces are each secured to one end of the magnet.

8. A transducer as claimed in claim 6, in which the magnetic circuit is E shaped with the source of magnetization formed by a small magnet of a high coercivity material located in the central leg of the E, in which the armature is of the rocking armature type and is rockable about the end of the central leg of the E, the armature length being less than the distance between the inner faces of the two outer legs of the E, in which the diaphragm is driven by a rod secured to the armature at one of its ends, and in which the coil is wound on the two outer legs of the E.

9. A transducer as claimed in claim 6, 7 or 8, in which the armature is flat, is at right agnles to the pole-pieces, and is shorter than the inter-pole-piece gap, and in which the armature is coupled to a diaphragm by a drive rod at one of its ends.

10. A transducer as claimed in claim 6, 7, 8 or 9, and in which the inward facing tips of the pole-pieces are levelled or angled or otherwise shaped to modify the force-displacement characteristics in the working region.