A VOICE COIL ACTUATOR HAVING A FLUX GUIDE AT BOTH

ENDS

BACKGROUND OF THE INVENTION 1. FIELD OF THE INVENTION

The invention is related to the field of voice coils, and in particular, to a voice coil with a flux guide at both ends.

2. DESCRIPTION OF THE PRIOR ART

Voice coils come in many different varieties. Some advanced voice coils use flux focused interleaved magnetic circuits. One example of such a voice coil is disclosed in US patent 5,345,206 entitled "Moving coil actuator utilizing flux-focused interleaved magnetic circuit" issued 9/6/1994 and hereby incorporated by reference.

Figure 1 is a cross sectional view of voice coil 100 as disclosed by the above cited patent. Voice coil 100 has magnetic core 102 made from a plurality of permanent magnets 106 separated by a plurality of pole pieces 108. The polarity of the permanent magnets is alternated such that the north and south poles of magnets adjacent to the pole pieces face each other. Magnetic core 102 is encased in a cylindrical shell 104 that is open on one end and closed on the other end. Magnetic core 102 is attached to the shell 104 at the closed end. A cylindrical moving coil 110 is inserted in an air gap between the magnetic coil 102 and the case 104. The cylindrical moving coil 110 has at least two sets of windings. The sets of windings have alternating polarities. There is a winding in the cylindrical moving coil 110 corresponding to each one of the plurality of pole pieces in the magnetic core 102.

Figure 2 is a plot of the magnetic flux in an example embodiment of a coil disclosed in the above cited patent. Flux lines going through the closed end of the shell (at F and G) are more concentrated than the flux lines going from the magnetic pole piece 208 back into the shell near the open end of the shell. This is because the flux at the closed end of the shell is being concentrated by the closed end of the ferro-magnetic shell. The flux traveling through the ferro-magnetic pole piece 208 have to jump an air gap to re-enter the open end of the shell. This lack of flux concentration may cause a loss of power for the magnetic coil. Therefore there is a need for an improved voice coil.

SUMMARY OF THE INVENTION

A voice coil actuator having a magnetic core comprising a plurality of permanent magnets separated by a plurality of pole pieces. The polarity of the permanent magnets alternate such that the north and south poles of the magnets adjacent to the pole pieces face each other. The magnetic core is encased in a cylindrical shell that has a magnetic flux guide at both ends. A cylindrical moving coil is installed in an air gap between the magnetic core and the shell and is captured between the two flux guides. Members are attached to the coil and extend through holes in one of the flux guides.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional view of voice coil 100 as disclosed by US patent 5,345,206 (Prior art).

FIG. 2 is a plot of the magnetic flux in an example embodiment of a coil disclosed by US patent 5,345,206 (Prior art). FIG. 3 is a cross sectional view of a voice coil actuator in an example embodiment of the current invention.

FIG. 4 is a cross sectional view of a voice coil actuator having the first flux guide integrated into the outer shell in an example embodiment of the current invention. FIG. 5 is a plot of the magnetic flux in a voice coil actuator in an example embodiment of the current invention.

FIG. 6a is an isometric view of voice coil 600 in an example embodiment of the invention.

FIG 6b is a front view of voice coil 600 in an example embodiment of the invention. FIG. 7a is a front view of a voice coil with three members in an example embodiment of the invention.

FIG. 7b is a front view of a voice coil with four members in an example embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT FIGS. 1 - 7 and the following description depict specific examples to teach those skilled in the art how to make and use the best mode of the invention. For the purpose of teaching inventive principles, some conventional aspects have been simplified or omitted.

Those skilled in the art will appreciate variations from these examples that fall within the scope of the invention. Those skilled in the art will appreciate that the features described below can be combined in various ways to form multiple variations of the invention. As a result, the invention is not limited to the specific examples described below, but only by the claims and their equivalents.

Figure 3 a is a cross sectional view of voice coil 300 in an example embodiment of the invention. Voice coil 300 comprises a cylindrical shell 302, a flux guide 304, a cylindrical moving coil 306 with two members 308 attached to one end, and a cylindrical core comprising a plurality of permanent magnets 312 and two pole pieces 314. The cylindrical shell 302 has a closed end and an open end. The flux guide 304 is attached to the open end of the shell 302, essentially closing the open end. The cylindrical core is centered inside cylindrical shell 302 and attached to the closed end of the shell and held in place by the flux guide 304. The cylindrical core is comprised of the two pole pieces captured between the plurality of permanent magnets, with the magnets oriented such that the polarity of the magnets adjacent to each pole piece align (i.e. one pole piece surrounded by two north poles with the next pole piece surrounded by two south poles). An air gap is formed between the core and the shell 302. The cylindrical moving coil 306 is located in the air gap between the core and the shell 302. The two members 308 attached to movable coil 306 extend out through two openings 310 in flux guide 304. The cylindrical moving coil 306 has two sets of windings. The two sets of windings have opposite polarities. In operation, an electrical current running through the windings in the cylindrical moving coil 306 located inside the magnetic field generated by the core, cause a force to be exerted against the cylindrical moving coil 306. The force is transferred from the cylindrical moving coil 306 through the two members 308, attached to the coil, to the outside of the flux guide 304. Figure 3b is a cross sectional view of voice coil 300 in another example embodiment of the invention. Voice coil 300 comprises a cylindrical shell 302, two flux guides 304, a cylindrical moving coil 306 with two members 308 attached to one end, and a cylindrical core comprising a plurality of permanent magnets 312 and two pole pieces 314. The cylindrical shell 302 has both ends open. A flux guide 304 is attached to each end of the shell 302, essentially enclosing the shell. The cylindrical core is centered inside cylindrical shell 302 and attached to the two flux guides 304. The cylindrical core is comprised of the two pole pieces captured between the plurality of permanent magnets, with the magnets

oriented such that the polarity of the magnets adjacent to each pole piece align (i.e. one pole piece surrounded by two north poles with the next pole piece surrounded by two south poles). An air gap is formed between the core and the shell 302. The cylindrical moving coil 306 is located in the air gap between the core and the shell 302. The two members 308 attached to movable coil 306 extend out through two openings 310 in one of the flux guides 304. The cylindrical moving coil 306 has two sets of windings. The two sets of windings have opposite polarities. In operation, an electrical current running through the windings in the cylindrical moving coil 306 located inside the magnetic field generated by the core, cause a force to be exerted against the cylindrical moving coil 306. The force is transferred from the cylindrical moving coil 306 through the two members 308, attached to the coil, to the outside of the flux guide 304.

Figure 4 is a cross sectional view of voice coil 400 in another example embodiment of the invention. Voice coil 400 comprises a cylindrical shell 402, two flux guides 404, a cylindrical moving coil 406 with two rods 408 attached to one end, and a cylindrical core comprising a plurality of ring shaped permanent magnets 412 and two ring shaped pole pieces 414. The cylindrical shell 402 has both ends open. One flux guide 404 is located at each end of the shell 402, essentially enclosing the shell. The cylindrical core is centered inside cylindrical shell 402. The cylindrical core is comprised of the two ring shaped pole pieces captured between the plurality of ring shaped permanent magnets, with the magnets oriented such that the polarity of the magnets adjacent to each pole piece align (i.e. one pole piece surrounded by two north poles with the next pole piece surrounded by two south poles). The core, the two flux guides, and the shell are held together by bolt 416 and nut 420. An air gap is formed between the core and the shell 402. The cylindrical moving coil 406 is located in the air gap between the core and the shell 402. The two rods 408 attached to movable coil 406 extend out through two openings 410 in the lower flux guides 404. A face plate 422 is attached to the two rods 408. The cylindrical moving coil 406 has two sets of windings. The two sets of windings have opposite polarities. The cylindrical moving coil has a travel range of approximately distance d, which is the distance between the end of cylindrical moving coil and the lower flux guide, when the cylindrical moving coil is in the up most position. In operation, an electrical current running through the windings in the cylindrical moving coil 406 located inside the magnetic field generated by the core, cause a force to be exerted against the cylindrical moving coil 406. The force is transferred from the

cylindrical moving coil 406 through the two rods 408, attached to the coil, to the face plate 422 on the outside of the flux guide 404. Permanent magnet 424 is attached to face plate 422. Permanent magnet 424 is used to hold or move cylindrical moving coil 406 into the up most position when no current is flowing through the windings on cylindrical moving coil 406. In one example embodiment of the invention, a spring may be used instead of permanent magnet 424.

Figure 5 is a plot of the flux lines from one side of voice coil 400 in one example embodiment of the invention. In figure 5 you can see one side of the plurality of permanent magnets 512 surrounding the plurality of pole pieces 514 with the two flux guides 504 on either end with bolt 516 holding the assembly together. Shell 502 is held between the two flux guides 504 and the cylindrical moving coil 506 is located in the air gap between the core and the shell 502. The flux lines on either end of the voice coil are concentrated by the two flux guides 504 attached to either ends of the voice coil. Having the flux lines concentrated at both ends, instead of only one end, can increase the actuating power of the voice coil by as much as 20 percent.

The examples shown above have only two pole pieces in the core with two corresponding windings in the moving cylindrical coil. In other example embodiments of the invention, additional pole pieces may be used with a corresponding increase in the number of windings. For example, a core that contains 5 pole pieces would have a cylindrical moving coil having 5 windings where the windings have alternating polarities.

Figure 6a is an isometric view of a voice coil in an example embodiment of the invention. Voice coil 600 has a cylindrical shell 602 with a flux guide 604 attached to one end of the cylindrical shell 602. Two circular rods 608 extend out from the inside of cylindrical shell 602 through two holes in flux guide 604. Figure 6b is a front view of voice coil 600. The two circular rods 608 are placed symmetrically about the center of voice coil 600. Voice coil 600 uses two circular rods to transfer the force from the inner moving coil to the outside of the voice coil. Shapes other than circular rods may be used, for example squares, rectangles, circular segments, or the like. The number of rods or member used is also unimportant. Figure 7a is a front view of a voice coil with three members in an example embodiment of the invention. Figure 7b is a front view of a voice coil with four members in an example embodiment of the invention. Having the rods or members spaced symmetrically around the center of the voice coil minimizes the torque induced into the

moving coil inside the voice coil. A voice coil with only one rod attached to the inner moving coil is possible, but may need a stronger suspension for the inner moving coil.