The present invention relates to a balanced electrodynamic motor assembly for bone conduction vibrators. In particular, the present invention relates to a balanced

electrodynamic motor assembly which is mechanically simple, cost effective and suitable for being manufactured in relatively large numbers in a reproducible manner.

BACKGROUND OF THE INVENTION

WO 01/67813 discloses the so-called Balanced Electromagnetic Separation Transducer (BEST). A BEST involves an electromagnetic vibrator of variable reluctance type which provides higher efficiency, smaller dimensions, and a higher reliability compared to known technology. These advantages have been obtained by providing a magnetic signal flux around a coil which is closed through a bobbin body and a yoke and through air gaps formed between bobbin body and yoke(s) where a static flux from one or more permanent magnets and the signal flux cooperates so that static forces are outbalanced and so that axial signal forces are generated.

Despite its superior performance a BEST involves a relatively complicated mechanical design with small tolerances which may be difficult to cope with if a large number of transducers are to be manufactured in a reproducible manner.

It may be seen as an object of embodiments of the present invention to provide a balanced electrodynamic motor assembly which is mechanically simple, cost effective and suitable for being manufactured in relatively large numbers in a reproducible manner.

DESCRIPTION OF THE INVENTION

The above-mentioned object is complied with by providing, in a first aspect, a magnetically balanced electrodynamic motor assembly for bone conductor vibrators comprising 1) a first magnetic circuit adapted to generate an essential constant magnetic flux,

2) a second magnetic circuit adapted to generate an alternating electromagnetic flux, the second magnetic circuit having a through-going opening arranged therein wherein the first and second magnetic circuits are mechanically connected via at least one resilient member.

The present invention provides an optimized design of a balanced electrodynamic motor assembly. The design is mechanically simple, cost effective and in particular suitable for being manufactured in relatively large numbers in a reproducible manner. Preferably, the motor assembly of the present invention is a variable reluctance type motor assembly.

The through-going opening may be a centre through-going opening via which the motor assembly may be mounted using for example a bolt.

The first magnetic circuit of the motor assembly may comprise an inner yoke having a plurality of permanent magnets thereon.

In one embodiment a total of four permanent magnets are arranged on two oppositely arranged surfaces of the inner yoke. It should be noted however, that the number of permanent magnets arranged on the two oppositely arranged surfaces of the inner yoke may differ from four. Thus, a total of 2, 6, 8 or even more permanent magnets may be arranged on the two oppositely arranged surfaces of the inner yoke.

The first magnetic circuit may further comprise first and second outer yokes, wherein the first outer yoke is arranged on at least four permanent magnets on one surface of the inner yoke, and wherein the second outer yoke is arranged on at least four permanent magnets on an oppositely arranged surface of the inner yoke. The first and second outer yokes may each comprise a recess or indentation in a surface facing away from the inner yoke. This recess or indentation facilitates that the second magnetic circuit may move relative to the first magnetic circuit.

The inner and outer yokes may be manufactured using a soft-magnetic material or soft- magnetic alloy. The first magnetic circuit may further comprise a substantially plane first resilient member being arranged on the first outer yoke surface facing away from the inner yoke, the first resilient members expanding across the recess or indentation of the first outer yoke whereby the first resilient member is bendable due to the presence of said recess, and further comprising a substantially plane second resilient member being arranged on the second outer yoke surface facing away from the inner yoke, the second resilient members expanding across the recess or indentation of the second outer yoke whereby the second resilient member is bendable due to the presence of said recess. The second magnetic circuit of the motor assembly may comprise an electromagnetic arrangement suspended in the first and second resilient members, the electromagnetic arrangement comprising a coil element and a coil core element being concentrically arranged around the through-going opening. The coil core element may be manufactured using a soft- magnetic material or soft-magnetic alloy.

The second magnetic circuit further comprising a pair of plates being secured to oppositely arranged surfaces of the coil core element. These plates may be manufactured using a soft- magnetic material or soft-magnetic alloy.

The soft-magnetic material or soft-magnetic alloy plates are secured to the second magnetic circuit and extend at least partly into air gaps provided by the first magnetic circuit. In this way the essential constant magnetic flux of the first magnetic circuit is mixed with the alternating electromagnetic flux of the second magnetic circuit.

In a second aspect the present invention relates to a bone conductor vibrator comprising a motor assembly according to the first aspect. BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described in further details with reference to the accompanying figures, wherein

Fig. 1 shows a motor assembly of the present invention,

Fig. 2 illustrates the assembling process, Fig. 3 shows a cross-sectional view of a bare motor assembly, and

Fig. 4 shows a cross-sectional view of a motor assembly including a bolt positioned in the centred through-going opening.

While the invention is susceptible to various modifications and alternative forms specific embodiments have been shown by way of examples in the drawings and will be described in details herein. It should be understood, however, that the invention is not intended to be limited to the particular forms disclosed. Rather, the invention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims. DETAILED DESCRIPTION OF THE INVENTION

In its broadest aspect the present invention relates to a magnetically balanced

electrodynamic motor assembly for bone conductor vibrators. In order to easy manufacturing and subsequent mounting of the motor assembly a through-going opening is provided in the assembly. The through-going opening is adapted to receive suitable fixation means in the form of for example a bolt. Preferably, the motor assembly is of the reluctance type having the through-going opening arranged in a centre thereof.

Referring now to Fig. 1 a motor assembly 100 according to a preferred embodiment of the present invention is shown. As seen in Fig. 1 the motor assembly is a sandwiched type structure having permanent magnets 107, 108, 109, 110 arranged between inner yoke 102 and outer yokes 101, 103. The outer yoke 101 has a recess 113 formed therein so that the centre portion of the resilient element 104 is allowed to move up and down when a drive signal is applied to the coil (not shown) via the lead-outs 105, 106. The recess 113 defines an edge 114 to the peripheral region of the outer yoke 101. It should be noted that the outer yoke 103 also processes a recess (not shown) so that the centre portion of the resilient element 1112 is allowed to move up and down when a drive signal is applied to the lead-outs 105, 106.

The motor assembly 100 has a through-going opening 111 in its centre region. This through- going opening may be utilized during manufacturing in order to ensure proper aligning of the various components. Also, the through-going opening is advantageous in that it may receive suitable fixation means which may include a bolt or similar means. A recess 115 is provided to at least partly hide a bolt being positioned in the through-going opening 111.

Referring now to Fig. 2 the assembling process 200 is illustrated. Starting in the upper left corner a circular flank 201 having a through-going opening is provided. The flank 201 is secured to a soft magnetic material 202 which is at least partly being adapted to be positioned in the air gaps of an assembled motor. The combining of the flank 201 and the soft magnetic material 202 leads to sub-assembly 203.

The outer yoke 205 has a recess 206 so that upon fixation of the resilient member 204 to the outer yoke 205 this resilient member 204 is allowed to move up and down. The outer yoke 205 and the resilient member 204 combines into sub-assembly 207 which is assembled with sub-assembly 203 to form sub-assembly 208. A total for four permanent magnets 209 are then arranged on the outer yoke 205 whereby sub-assembly 210 is provided. It should be noted that sub-assemblies 210 and 215 are identical. By adding the inner yoke 217 to the sub-assembly 210 sub-assembly 213 is provided. A positioning of the coil 212 and the coil core 211 within the opening of the inner yoke 217 leads to sub-assembly 214. The final motor assembly 216 is provided by combining subassemblies 214 and 215. The assembling process depicted in Fig. 2 may be simplified and streamlined by aligning the various components relative to each other by positioning a pole like structure in the through- going openings of the components.

Referring now to Fig. 3 a cross-sectional view of a motor assembly 300 having a centre through-going opening 301 is depicted. As seen in Fig. 3 the inner yoke 316, 317 (same yoke) is sandwiched between the four permanent magnets 304, 305, 306, 307. The outer yokes 303, 319 and 302, 318 are attached to the permanent magnets 304, 305, 306, 307. The coil assembly of the motor assembly is suspended in the oppositely arranged resilient members 314 and 315. Properly designed recesses 320, 321 formed in the outer yokes 319, 318 allow that the coil assembly may move up and down upon applying a drive signal to the coil 309 due to the spacing 322, 323.

The coil assembly of the motor assembly comprises a coil 309 and a coil core 308 arranged concentrically therewith. The coil 309 and the coil core 308 are sandwiched between two plates 310, 311 of a soft-magnetic material or a soft-magnetic alloy. These soft-magnetic material/alloy plates 310, 311 are secured to the resilient members 314, 315 via the flanks 312, 313. As seen in Fig. 3 the soft-magnetic/alloy plates 310, 311 extend into the four regions between the inner yoke 316, 317 and the outer yokes 303, 319 and 302, 318. In each of these four regions a DC magnetic flux originating from each of the permanent magnets 304, 305, 306, 307 is present. If an AC drive signal is applied to the coil 309 an AC magnetic flux is induced in the flux path involving the coil core 308, soft-magnetic/alloy plates 310, 311 and the inner yoke 316, 317. As a result the coil assembly of the motor assembly will move or vibrate relative to the fixed portions of the motor assembly. The flanks 312, 313, which mechanically connects the resilient members 314, 315 to the respective soft-magnetic/alloy plates 310, 311, each comprises a recess having a depth which is suitable for hiding at least a portion of a bolt head, cf. Fig. 4. Fig. 4 shows a motor assembly 400 similar to that of Fig. 3 except that a bolt 401 has been inserted into the through-going opening. The bolt 401 and the nut 402 press the coil assembly together. Also, the thread of the bolt 401 facilitates that easy mounting of the motor assembly may be provided. Thus, the coil assembly including the coil 309, the coil core 308 and two soft-magnetic/alloy plates 310, 311 is adapted to be bolted to an external housing I