Electromagnetic actuator and portable device having the same

An electromagnetic actuator includes a cylindrical stator made of a permanent magnet and having annular end faces magnetized to plural magnetic poles, a coil securely disposed in a hollow portion of the stator via a bobbin on which the coil is wound and which is fixed to the stator's hollow portion, a rotary shaft put through the coil in the direction of the center axis thereof in such a way as to be rotatably supported at the bobbin, and a rotor of a soft magnetic material provided in such a way as to rotate together with the rotary shaft and having plural rotor arms radially extending from both end portions of the rotary shaft and facing the respective annular end faces of the stator.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application claims priority to Japanese Patent Application No. 2003-368600 filed on Oct. 29, 2003, which is incorporated herein by reference for all purposes.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an electromagnetic actuator which rotates with a 1-phase excitation driving method, and a portable device having the electromagnetic actuator.

2. Description of the Related Art

There has been proposed a brushless motor as a DC motor, which has a rotor made of a permanent magnet and a stator made by a coil provided in the circumferential direction about the rotational axis of the rotor or the permanent magnet.

As the conventional brushless motor has the stator coil arranged in the circumferential direction about the rotational axis of the rotor in such a way as to surround the rotor, it is restricted on its mount space in the radial direction.

BRIEF SUMMARY OF THE INVENTION

Accordingly, it is an object of the invention to provide an electromagnetic actuator which is less restricted on its mount space in the radial direction, and a portable device having the electromagnetic actuator.

To achieve the object, according to one aspect of the invention, there is provided an electromagnetic actuator which comprises a cylindrical stator permanent magnet having annular end faces magnetized to a plurality of magnetic poles; a coil securely disposed inside a hollow interior of the stator; a rotary shaft put through the coil in a direction of a center axis thereof in such a way as to be rotatably supported; and a rotor of a soft magnetic material having a plurality of rotor arms facing the annular end faces of the stator and provided in such a way as to rotate together with the rotary shaft.

The rotor is disposed at at least one of the annular end faces of the stator.

More practically, the rotor arms include at least two main rotor arms and at least one rotational-direction determining rotor arm which extend in a direction of the annular end faces of the stator from both end portions of the rotary shaft and face the annular end faces, the at least one rotational-direction determining rotor arm is less in quantity than the main rotor arms rotor, those rotor arms which are disposed at one of the annular end faces are arranged in phase with magnetic poles of the facing annular end face, those rotor arms which are disposed at the other annular end face are arranged in an opposite phase to the phase with respect to magnetic poles of the facing annular end face, and the rotational-direction determining rotor arm is so arranged as to be shifted by a given angle with respect to the magnetic poles of the facing annular end face.

Alternatively, the rotor arms include at least two main rotor arms and at least one rotational-direction determining rotor arm which extend in a direction of the annular end faces of the stator from both end portions of the rotary shaft and face the annular end faces, the at least one rotational-direction determining rotor arm is less in quantity than the main rotor arms rotor, those rotor arms which are disposed at each one of the annular end faces are arranged in phase with magnetic poles of the facing annular end face, and the rotational-direction determining rotor arm is so arranged as to be shifted by a given angle from the phase with respect to the magnetic poles of the facing annular end face.

The rotational-direction determining rotor arm may be formed as a separate member from the main rotor arms.

The rotor arms which are disposed at the one of the annular end faces may include four rotor arms arranged at angles of 90 from one another, the rotor arms which are disposed at the other annular end face may include three rotor arms in such a way that angles defined by a center one of the three rotor arms and the other two rotor arms are 90 degrees, and the rotational-direction determining rotor arm may be arranged at the other annular end face between the other two rotor arms.

The electromagnetic actuator further comprises a coil bobbin on which the coil is wound and which is secured to the hollow interior of the stator, and the coil bobbin constitutes a bearing which supports the rotary shaft in a rotatable manner.

In the electromagnetic actuator, when an alternate current or a current whose positive and negative polarities are switched in arbitrary periods is supplied to the coil, the rotor rotates stepwise in a same direction in units of the magnetic poles every time a direction of the current is changed.

According to another aspect of the invention, there is provided a portable device having an electromagnetic actuator which comprises a cylindrical stator permanent magnet having annular end faces magnetized to a plurality of magnetic poles; a coil securely disposed inside a hollow interior of the stator; a rotary shaft put through the coil in a direction of a center axis thereof in such a way as to be rotatably supported; and a rotor of a soft magnetic material having a plurality of rotor arms facing the annular end faces of the stator and provided in such a way as to rotate together with the rotary shaft.

In the portable device, the rotor arms include at least two main rotor arms and at least one rotational-direction determining rotor arm which extend in a direction of the annular end faces of the stator from both end portions of the rotary shaft and face the annular end faces, the at least one rotational-direction determining rotor arm is less in quantity than the main rotor arms rotor, those rotor arms which are disposed at one of the annular end faces are arranged in phase with magnetic poles of the facing annular end face, those rotor arms which are disposed at the other annular end face are arranged in an opposite phase to the phase with respect to magnetic poles of the facing annular end face, and the rotational-direction determining rotor arm is so arranged as to be shifted by a given angle with respect to the magnetic poles of the facing annular end face.

The electromagnetic actuator according to the invention is less restricted on its mount space in the radial direction and generates a sufficient rotational torque with 1-phase excitation.

As the moment of inertia of the rotor is small, the power consumed when the electromagnetic actuator is driven can be reduced.

The portable device according to the invention can be made more compact and lighter.

DETAILED DESCRIPTION OF THE INVENTION

Electromagnetic actuators according to preferred embodiments of the invention will be described below with reference to the accompanying drawings.

FIG. 1shows an example of the structure of an electromagnetic actuator according to one embodiment of the invention.

As shown inFIG. 1, the electromagnetic actuator includes a cylindrical stator11made of a permanent magnet and having annular end faces11aand11bmagnetized to a plurality of magnetic poles, a coil13securely disposed in a hollow portion11cof the stator11via a bobbin15on which the coil13is wound and which is fixed to the hollow portion11cof the stator11, a rotary shaft17which is put through the coil13in the direction of the center axis thereof in such a way as to be rotatably supported at the bobbin15, and a rotor20of a soft magnetic material which is provided in such a way as to rotate together with the rotary shaft17and has a plurality of rotor arms19(19ato19d) and21(21ato21d) radially extending from both end portions of the rotary shaft17and facing the respective annular end faces11aand11bof the stator11. The coil13is wound on a cylindrical bobbin15made of an insulator, which serves as a bearing to support the rotary shaft17in a rotatable manner. The rotary shaft17is put through a hollow portion15aof the bobbin15so as to be rotatably supported. The bearing and the bobbin may be formed as separate members.

The cylindrical stator11is formed of a ferromagnetic material, and is magnetized in the perpendicular direction to both annular end faces11aand11b.The stator11is constituted by a plurality of permanent magnet portions lying at equal intervals in the circumferential direction in such a way as to surround the hollow portion11c.That is, magnetic poles of plural permanent magnets extending in the radial direction are present alternately on both annular end faces11aand11bof the stator11.

The number of the magnets or the number of segments, n, is an even number, and is “8” in this embodiment. In other words, eight S/N poles are alternately arranged at equal intervals of 45° on the annular end faces11aand11bof the stator11.

The rotary shaft17has a shaft portion17c,which is so supported as to rotate smoothly and without shaking, and drive shafts17aand17bof small diameters coaxially protruding from both end portions of the shaft portion17c.The first and second rotor arms19and21are fixed to near the boundaries between the shaft portion17cand the drive shafts17aand17bwhich protrude from the first and second rotor arms19and21in the axial direction. InFIGS. 1 and 8, the vicinity of the center portion of the shaft portion17cis omitted for the sake of easier illustration.

The first rotor arm19has 4-pole main rotor arms19a,19b,19cand19dformed in a cross shape and arranged at equal intervals about the rotary shaft17. The angle that is defined by each of the main rotor arms19a,19b,19cand19dand an adjoining one of the remaining rotor arms is 90°. That is, the main rotor arms19ato19dare formed in such a way as to be in phase with respect to the magnetic poles of the facing or associated annular end face11aof the stator11. A boss portion19eis formed at the first rotor arm19, which is secured to the rotary shaft17as the boss portion19eis press-fitted over the drive shaft17a.

The second rotor arm21has 4-pole rotor arms, namely 3-pole main rotor arms21a,21band21cformed in a T shape about the rotary shaft17, and a 1-pole rotational-direction determining rotor arm21dextending to the opposite side to the main rotor arm21bfrom between the main rotor arms21aand21cextending in the diametrical direction. The 3-pole main rotor arms21ato21care formed at equal intervals in such a way that the main rotor arms21aand21cform 90° with respect to the center main rotor arm21b.The rotational-direction determining rotor arm21dis formed at an angle different from 90° with respect to the main rotor arm21aor21c.That is, the main rotor arms21ato21care arranged in such a way as to be in phase with the magnetic poles of the associated annular end face11bof the stator11, and the rotational-direction determining rotor arm21dis arranged in such a way as to be shifted by a predetermined angle (offset angle α) with respect to that phase.

FIGS. 2 and 3show in perspective views as seen from the front side and the rear side respectively, the assembled electromagnetic actuator according to the embodiment in which the bobbin15having the coil13wound thereon is securely inserted into the hollow portion11cof the stator11, the rotary shaft17is inserted into the hollow portion15aof the bobbin15, and the first rotor arm19is fixed to the end portion of the rotary shaft17protruding from the bobbin15.

The second rotor arm21is formed integrally with the rotary shaft17, while the first rotor arm19is formed as a separate member from the rotary shaft17. The first rotor arm19formed as a separate member is fixed after the rotary shaft17is put through the bobbin15. At this time, the first rotor arm19and the second rotor arm21are secured and arranged in such a way that the phase of the main rotor arms21ato21cbecome opposite to the phase of the main rotor arms19ato19c(in such a way that the main rotor arms19ato19coverlap the respective main rotor arms21ato21cin projection in the axial direction of the rotary shaft17in the embodiment). The second rotor arm21can be formed as a separate member from the rotary shaft17, the first rotor arm19can be formed integrally with the rotary shaft17, or the first and second rotor arms19and21can both be formed as separate members from the rotary shaft17.

The rotor arms19ato19dand21ato21d,and the annular end faces11aand11bof the stator11are formed in parallel to each other and to have smooth flat surfaces. For example, with spacers intervened between the first and second rotor arms19and21and the bobbin15, the rotor arms19ato19dand21ato21dare separated from the annular end faces11aand11bof the stator11by a predetermined distance and in a contactless manner.

The electromagnetic actuator according to the embodiment is constructed in such a way that when an alternate current or a current whose positive and negative polarities are switched in arbitrary periods is supplied to the coil13from a power supply23, the first and second rotor arms19and21and the rotary shaft17rotate stepwise in the same direction in units of magnetic poles every time the direction of the current is changed. The torque is transmitted to outside via the drive shafts17aand17bprotruding from both ends of the electromagnetic actuator.

The rotational-direction determining rotor arm21din the embodiment is formed in such a way as to be shifted by a predetermined offset angle α from the angle of 90° that is defined by the adjoining main rotor arms21ato21c.The offset angle α determines whether to rotate the electromagnetic actuator rightward or leftward. The offset angle α in the embodiment is set to α=22.5° so that the angle defined between the rotational-direction determining rotor arm21dand the main rotor arm19cbecomes smaller than the pitch angle of 90°. This angle setting allows the first and second rotor arms19and21and the rotary shaft17(17ato17c) to rotate rightward.

While the integral formation of the rotor arms19ato19dof the first rotor arm19and the rotor arms21ato21dof the second rotor arm21can improve the assembling efficiency, such as making adjusting among arms unnecessary, the rotor arms may be formed as separate members. The main rotor arms21ato21cof the second rotor arm21may be formed integrally, while only the rotational-direction determining rotor arm21dis formed separately. This structure can ensure adjustment of the maximum rotational speed or so and switching of the rotational direction by adjusting the phase at the time the rotational-direction determining rotor arm21dis secured.

The widths of the rotor arms19ato19dand21ato21dare set in such a way as to be narrower than the circumferential width of the magnetic poles on the annular end faces11aand11bof the stator11(the innermost width). While the rotor arms19ato19dand21ato21dmay have equal widths, they may be so formed in a fan shape as to become wider outward.

The rotational operation of the electromagnetic actuator10will be described referring to operational charts shown inFIGS. 4A to 4Dand5A to5E. Those diagramsFIGS. 4A to 4Dshow the stator11and the first and second rotor arms19and21developed linearly and show the rotor arms19ato19dand21ato21dof the first and second rotor arms19and21at positions corresponding to the annular end faces11aand11bof the developed stator11. InFIGS. 4A to 4Dand5A to5E, the rightward direction corresponds to the rightward rotational direction of the first and second rotor arms19and21, and the angles given in the diagrams indicate the rotational angles of the first and second rotor arms19and21.

It is assumed that the first and second rotor arms19and21are initially held at a stable (stationary or balanced) position as shown inFIG. 4A. At this stable position the rotational moment to rotate the 4-pole main rotor arms19ato19dof the first rotor arm19and the 3-pole main rotor arms21ato21cof the second rotor arm21to the stable state by the repulsive/attracting force acting between those rotor arms and the magnetic poles of the associated annular end faces11aand11bis balanced with the rotational moment to rotate the rotational-direction determining rotor arm21dto the stable state by the repulsive/attracting force acting between the rotational-direction determining rotor arm21dand the magnetic poles of the associated annular end face11b.The rotor arms19ato19dand21ato21din the illustrated embodiment are rotated rightward or moved rightward in the diagram by a minute angle β from the stable position in case where the rotational-direction determining rotor arm21dis not present, and are stopped there. In this way, the action of the rotational-direction determining rotor arm21dholds the first and second rotor arms19and21to a state slightly turned rightward from the stable position that is achieved by only the main rotor arms19ato19dand the main rotor arms21ato21c.

When the forward current flows across the coil13at the stable position of the first and second rotor arms19and21shown inFIG. 4A, the rotor arms19ato19dare magnetized to the N pole and the rotor arms21ato21dare magnetized to the S pole as shown inFIG. 4B. Then, the main rotor arms19ato19d,and the rotor arms21ato21dare rotated rightward by attraction of the closest S pole on the annular end face11aand the closest N pole on the annular end face11bby one step (45°) and are stopped at the stable position (FIG. 4B). If the current to the coil13is cut off then, the rotor arms19ato19dand21ato21dare held at the stable position.

When the reverse current flows across the coil13at the stable position of the first and second rotor arms19and21shown inFIG. 4B, the magnetic poles of the main rotor arms19ato19dand the rotor arms21ato21dare inverted, so that the main rotor arms19ato19dare magnetized to the S pole and the rotor arms21ato21dare magnetized to the N pole as shown inFIG. 4C. Then, the main rotor arms19ato19dare attracted to the closest N pole on the associated annular end face11ain the rotational direction, and the rotor arms21ato21dare attracted to the closest S pole on the associated annular end face11bin the rotational direction. As a result, the main rotor arms19ato19d,the rotor arms21ato21dand the rotary shaft17are rotated rightward by one step and stopped at the stable position (FIG. 4C).

Thereafter, every time the direction of the current to the coil13is changed, the main rotor arms19ato19d,the rotor arms21ato21dand the rotary shaft17repeat the stepwise rotation of turning rightward by one step and stopping at the stable position (FIG. 4DtoFIG. 5E). That is, the drive shafts17aand17bof the electromagnetic actuator10are rotated rightward step by step of 45° every time the direction of the current to the coil13is changed.

Although the offset angle α of the rotational-direction determining rotor arm21dis set to 22.5° in the embodiment, it is possible to achieve leftward rotation by changing the offset angle α to a predetermined range. In the illustrated embodiment, the offset angle α of the rotational-direction determining rotor arm21dand the rotational direction has the following relationship. Note that the offset angle α is positive for the angle measured leftward from the pitch angle (phase) of the main rotor arms and is negative for the angle measured rightward from the pitch angle.

When the offset angle α is 0°, 45° (=90°/2) and 90°, whether to rotate rightward or leftward is not settled.

The number of segmented magnetic poles of the stator11, the number of poles of the main rotor arms and the number of poles of the rotational-direction determining rotor arm21dare not limited to those of the embodiment. Although the rotational-direction determining rotor arm21dis provided only at the second rotor arm21in the illustrated embodiment, the first rotor arm19may be constructed in the same way as the second rotor arm21. Further, the number of poles of the main rotor arms of the second rotor arm21may be set equal to the number of poles of the main rotor arms of the first rotor arm19, and the rotational-direction determining rotor arm may be provided between the adjacent two of the main rotor arms of either rotor arm.

The rotational-direction determining rotor arm should not necessarily be one, but plural rotational-direction determining rotor arms may be provided. The number and the shape or so of the rotational-direction determining rotor arms have only to be set so that the main rotor arms do not stop at the center of each magnetic pole of the stator or at the boundary between the magnetic poles of the stator.

Although the number of segmented magnetic poles of the stator11is set to “8” and the number of poles of the first and second rotor arms (the rotor arms19ato19dand21ato21d) is set to “8” in the illustrated embodiment, the invention is not limited to this case. Given that the number of segmented magnetic poles of the stator11is m, the numbers of poles of the first and second rotor arms, X1and X2become equal to or less than m/2. The rotational step angle of the rotor can be made smaller by increasing the number of segmented magnetic poles of the stator.

The number of magnetic poles, Y, of the rotational-direction determining rotor arm which determines the rotational direction is not limited because they should only shift the magnetic poles of the first and second rotor arms in a given direction from the center of each magnetic pole of the stator. The number of magnetic poles Y is however normally 1≦Y<(X1+X2). The optimal number of magnetic poles Y of the rotational-direction determining rotor arm is set based on the material, shape and so forth of the rotational-direction determining rotor arm.

FIG. 6presents a block diagram of a control circuit in case where the electromagnetic actuator10is adapted to an aperture device for a camera as a portable device. A control unit50has a CPU (Central Processing Unit)51, a memory52and a driver53. The CPU51performs the general control of the electromagnetic actuator and arithmetic operations. A program and control information for controlling the electromagnetic actuator10are stored in the memory52. The driver53supplies a positive or negative drive current to the coil13and excites the coil13according to a control signal from the CPU51.

When a release switch62is depressed, the CPU51reads luminance information of a shooting target, which is detected by a light quantity detector61, and discriminates whether the electromagnetic actuator10should be driven (the amount of exposure by the aperture should be restricted) or not. When the aperture action is needed according to the discrimination result, therefore, the CPU51instructs the driver53to output the positive or negative current to drive the electromagnetic actuator10. In response to the instruction, the driver53supplies the positive or negative current to the coil13of the electromagnetic actuator10.

The control of the energization of the coil13of the electromagnetic actuator10this way allows the rotor20and the rotary shaft17to take a rotational motion in a same direction.

The aperture device for a camera which uses the electromagnetic actuator10will be described referring toFIG. 7. The aperture device has the control unit50, the light quantity detector61and the release switch62inFIG. 6. The aperture device controls the light quantity by restricting a lens aperture71as the lens aperture71is placed over an aperture72smaller in diameter than the lens aperture71by turning an aperture blade70.

The aperture72smaller in diameter than the lens aperture71and a fitting hole73are formed in the aperture blade70. The drive shaft17aof the electromagnetic actuator10is fitted in the fitting hole73of the aperture blade70, so that the aperture blade70is secured to the drive shaft17a.As the rotary shaft17rotates, therefore, the aperture blade70rotates too. Further as a plurality of various apertures72smaller in diameter than the lens aperture71are formed in the aperture blade70around the drive shaft17a,an aperture device which restricts the lens aperture71in various steps can be provided.

FIGS. 8,9and10illustrate an electromagnetic actuator40according to another embodiment which is comprised of the minimum number of rotor arms and can achieve the invention. The embodiment has a rotor30which has a single rotor arm25(a main rotor arm25a) at one end portion of the rotary shaft17, and two rotor arms27(a main rotor arm27aand a rotational-direction determining rotor arm27b) at the other end portion. The other structure of the electromagnetic actuator according to this embodiment of the invention is the same as that of the embodiment shown inFIGS. 1,2and3. The main rotor arms25aand27aare formed in phase with each other. The rotational-direction determining rotor arm27bis formed in such a way that the phase is shifted by a predetermined offset angle α2from the phase of the main rotor arm27aso that the rotational-direction determining rotor arm27bis not in phase with the main rotor arms25aand27a.The offset angle α2is at least α2≠0° and α2≠180° and is set so that at the stable position, the main rotor arms are not positioned at the middle of each magnetic pole and at the boundary between the magnetic poles.

In this embodiment too, when an AC current or the direction of the current that changes its positive/negative polarity every predetermined time is let to flow across the coil, the rotor arms and the rotary shaft17rotate in a given direction step by step of 45° every time the direction of the current to the coil is changed.

Because the coil13is wound in such a way as to surround the rotary shaft17, thereby making it possible to reduce the overall diameter of the electromagnetic actuator, according to the embodiments of the invention, the radial-directional space can be made wider.

The number of turns can be increased easily without changing the diameter by making the coil13longer in the axial direction, and the permanent magnet constituting the stator11is formed cylindrical and extending in the axial direction. This makes the magnetic force stronger, so that the rotational torque and the detent torque can be made greater.

As the rotor20is constructed by rotor arms, the moment of inertia of the rotor20becomes smaller, thus making it possible to reduce the power consumed when the electromagnetic actuator is activated.

A portable device according to any embodiment of the invention can be made smaller in size and lighter by reducing the size of the electromagnetic actuator10in the radial direction. As the portable device according to the embodiment has the electromagnetic actuator10which generates a sufficient rotational torque and has smaller power consumption, the performance can be enhanced.

Although the portable device according to the embodiment has been described as an aperture device for a camera, the invention can be adapted to any portable device with an electromagnetic actuator, such as a shutter device for a camera, a lens driving device, a wristwatch or a portable telephone.

Although the rotor arms are rotated in the above-described embodiments, an electromagnetic actuator according to a further embodiment of the invention may be designed in such a way that the rotary shaft and the rotor arms are fixed while a cylindrical permanent magnet separated to segments which are magnetized to a plurality of poles is rotated as a rotor. In this case, as the coil rotates together with the permanent magnet, a current is supplied to the coil by means of a brush structure or so.

This application is based on Japanese Patent Application No. 2003-368600 filed on Oct. 29, 2003, and including the specification, claims, drawings and summary. The disclosure of the above Japanese Patent Application is incorporated herein by reference in its entirety.