Abstract:
A linear electric machine includes a plurality of teeth spaced apart from each other, a yoke magnetically connected to the teeth, a plurality of coils mounted on the teeth and a movable core disposed opposite the teeth to reciprocate relative to the teeth along an axis of the movable core. The movable core has a plurality of permanent magnets at axially opposite ends thereof for supplying magnetic flux to the teeth, and a member for suppressing generation of a magnetic field disposed at an axial center of the movable core to magnetically separate the permanent magnets.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
         [0001]    The present application is based on and claims priority from Japanese Patent Application 2002-245180, filed Aug. 26, 2002, the contents of which are incorporated herein by reference.  
         BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to a linear electric machine that moves back and forth along a straight line, such as a linear motor, a vibrator or a linear generator.  
           [0004]    2. Description of the Related Art  
           [0005]    U.S. Pat. No. 6,499,972 B2 discloses an actuator of a linear compressor. The actuator is constituted of a movable core with a plurality of circumferentially disposed permanent magnets on it, a plurality of teeth disposed around the movable core and a plurality of exciting coils respectively mounted on the teeth.  
           [0006]    In the linear electric machine, the polarity of the permanent magnets that are disposed opposite ends in the direction of the linear motion has to be made different. The actuator of the linear compressor disclosed in the US patent includes permanents magnets disposed on the opposite ends of the movable core, which are in contact with each other. Therefore, a considerable amount of magnetic flux is confined in the movable core, and magnetic force between the teeth and the movable core can not be sufficiently generated. This lowers the efficiency of the linear electric machine.  
           [0007]    That is, a linear actuator or motor having the above structure can not generate sufficient driving force, and a linear generator having the above structure can not generate sufficient electric power.  
         SUMMARY OF THE INVENTION  
         [0008]    In view of the above described problem, a main object of the invention is to provide an improved linear electric machine that can provide sufficient driving force or electric power.  
           [0009]    According to a main feature of the invention, a linear electric machine includes a movable core that has a plurality of first permanent magnets at axially opposite ends for supplying magnetic flux to the teeth and means for suppressing generation of a magnetic field disposed at an axial center thereof to magnetically separate the permanent magnets.  
           [0010]    According to another aspect of the invention, a linear electric machine includes a movable core that includes magnetically shielding means for suppressing generation of a magnetic field and a pair of first permanent magnets respectively disposed on the opposite sides of the shielding means in the axial direction.  
           [0011]    Therefore, the magnetic flux of the permanent magnet can be effectively utilized.  
           [0012]    In the above linear electric machine, the magnetically shielding means may be made of a non-magnetic material. The magnetically shielding means may also be made of a second permanent magnet that has opposite polarity to the first permanent magnets.  
           [0013]    The movable core may further include inductors made of magnetic material disposed in magnetic paths between the first permanent magnets and the teeth. 6. In this case, the first permanent magnets are disposed around a center of a plane that is perpendicular to the reciprocating direction of the movable core and polarized in directions perpendicular to the reciprocating direction. The magnetic inductors are disposed between the first permanent magnets in the direction perpendicular to the reciprocating direction. Preferably, a center line (L 1 ) of each first permanent magnet in a radial direction inclines to a center line (L 2 ) of the teeth. In the linear electric machine having the above features, the first permanent magnets may project from the inductors to be located between the adjacent teeth. The movable core may have a magnet shielding member at the center of the cross-section perpendicular to the reciprocating direction of said movable core.  
           [0014]    The coils of the linear electric machine may be connected to an ac power source to reciprocate the movable core as a linear motor. On the other hand, the movable core may be connected to means for reciprocating the movable core to generate electric power at the coils. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    Other objects, features and characteristics of the present invention as well as the functions of related parts of the present invention will become clear from a study of the following detailed description, the appended claims and the drawings. In the drawings:  
         [0016]    [0016]FIG. 1 is a schematic diagram illustrating a pulse tube type cooling machine to which a linear electric machine according to the invention is applied;  
         [0017]    [0017]FIG. 2A is a cross-sectional view of the linear electric machine cut along line II-II in FIG. 1 and FIG. 2B is a cross sectional side view of the linear electric machine cut along line IIB-IIB in FIG. 2A;  
         [0018]    [0018]FIG. 3 is a schematic diagram illustrating a main feature of the linear electric machine according to the first embodiment of the invention;  
         [0019]    [0019]FIG. 4A is a perspective view of a movable core of the linear electric machine according to the first embodiment of the invention, and FIG. 4B is a cross sectional view of the movable core;  
         [0020]    [0020]FIGS. 5A and 5B are schematic perspective views of a spacer of the linear electric machine according to the second embodiment of the invention;  
         [0021]    [0021]FIG. 6A is a schematic cross-sectional plan view of a linear electric machine according to the third embodiment of the invention, and FIG. 6B is a perspective view thereof;  
         [0022]    [0022]FIG. 7A is a schematic cross-sectional plan view of a linear electric machine according to the fourth embodiment of the invention, and FIG. 7B is a cross-sectional side view thereof;  
         [0023]    [0023]FIG. 8 is a schematic cross-sectional plan view of a modification of the linear electric machine according to invention;  
         [0024]    [0024]FIG. 9 is a schematic cross-sectional plan view of a modification of the linear electric machine according to invention;  
         [0025]    [0025]FIG. 10 is a schematic cross-sectional plan view of a modification of the linear electric machine according to invention; and  
         [0026]    [0026]FIG. 11 is a schematic diagram illustrating a generator driven by a thermoacoustic engine. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0027]    A linear electric machine according to the first embodiment of the invention, which is applied to an actuator of a linear compressor for a pulse tube type cooling machine, will be described with reference to FIGS.  1 - 4 . As disclosed in U.S. Pat. No. 6,499,972 B2, the pulse tube type cooling machine includes a radiator  1 , a heat accumulator  2 , a heat extractor  3 , a pulse tube  4 , a capillary tube  5 , a buffer tank  6 , a linear compressor  10 , etc. The linear compressor  10  includes a cylinder  11 , a piston  12  disposed in the cylinder  11  and an actuator  13  that reciprocates the piston  12  along the inside surface of the cylinder  11 .  
         [0028]    The actuator  13  includes a movable core  14  that is disposed to move back and forth on a line of reciprocation, a plurality of magnetic teeth  15  that extends in the radial direction perpendicular to the line of reciprocation, a plurality of solenoid coils  16  respectively mounted on the teeth  15 , a leaf spring  17  that supports the movable core  14  and a casing  18  that accommodates the above described parts or components. The casing  18  is a pressure vessel made of stainless steel. The piston  12  and the movable core  14  are connected by a non-magnetic shaft  19  that penetrates the movable core  14  in the axial direction thereof or in the direction of motion of the movable core  14 . The solenoid coil  16  is constituted of a resinous non-magnetic bobbin and a conductive coil made of copper or aluminum.  
         [0029]    A plurality (e.g. four) of the teeth  15  are disposed around the generally columnar movable core  14  to protrude radially inward from an annular yoke  20 , as shown in FIG. 2A. The teeth  15  and the yoke  20  are constituted of laminated magnetic plates such as magnetic iron sheets or permendur sheets to form magnetic paths of the magnetic flux induced by the solenoid coil  16 . The laminated magnetic plates are disposed so that the lamination direction thereof can be parallel to the direction of motion of the movable core  14 . In other words, the movable core  14  and the teeth  15  are disposed so that the lamination directions thereof can be parallel to each other.  
         [0030]    As shown in FIG. 2B, the movable core  14  includes a magnet shielding plate  14   a , a plurality (e.g. 8) of permanent magnets  14   b  and a plurality (e.g. 8) of inductors  14   c . The permanent magnets  14   b  are equally disposed on the opposite sides of the shield plate  14   a . The magnet shielding plate  14   a  is made of non-magnetic material, such as stainless steel, copper and aluminum to prevent the magnetic flux of the permanent magnets from being confined in the movable core  14 . In other words, the magnet shielding plate  14   a  increases effective magnetic flux supplied from the permanent magnets  14   b  to the teeth  15 . The permanent magnets  14   b  are made from neodymium-iron alloy, samarium-cobalt alloy or ferrite. The permanent magnets  14   b  are disposed so that the center lines L 1  of the permanent magnets  14   b  in the radial direction can incline to the center lines L 2  of the teeth  15 , as shown in FIG. 3, and magnetized so that the lines of the magnetic force can be perpendicular to the direction of the motion of the movable core  14 . The inclination angle between the center line L 1  and the center line L 2  is 360 degrees/the number (4) of the teeth/2, or 45 degrees. One of the permanent magnets  14   b  disposed on one side of the magnet shielding plate  14   a  is magnetized to have the polarity opposite to the polarity of an adjacent permanent magnet  14   b  disposed on the other side.  
         [0031]    As shown in FIGS. 4A and 4B, each one of the inductors  14   c  is constituted of radially arranged magnetic plates disposed in a quarter section of the columnar movable core  14  that are respectively divided by the permanent magnets  14   b . Each plate lies so that the rolled surface thereof can be approximately in parallel to the direction of motion of the movable core  14 . The magnetic plates are made of the same material as the teeth  15 , such as magnetic iron sheets or permendur sheets. Therefore, the magnetic flux mainly flows from the permanent magnets  14   b  through the inductors  14   c  to the tooth  14 .  
         [0032]    The movable core  14  has a magnet shielding center hole  14   d  at the center of the cross-section thereof perpendicular to the direction of motion thereof. The shaft  19  is made of non-magnetic material, such as stainless steel, copper or aluminum, and is force-fitted to the center hole  14   d . Each of the magnetic plates disposed around the center hole  14   d  are arranged to be perpendicular to the direction of motion of the movable core  14 . The magnet shielding center hole  14   d  increases the effective magnetic flux supplied to the teeth  15  from the permanent magnets  14   b.    
         [0033]    The solenoid coil  16  is applied an output voltage of a electronically control unit, which cyclically changes the magnetic polarity of the teeth  15  at a frequency corresponding to the natural vibration frequency caused by the elasticity of the movable core  14 , the leaf spring  17  and working fluid. Accordingly, the attractive and repulsive forces, which are exerted between the teeth  15  and the movable core  14 , are cyclically reversed to reciprocate the movable core  14 .  
         [0034]    The movable core  14  and the teeth  15  are disposed so that the permeance between the movable core  14  and the teeth  15  becomes a maximum when the piston  12  is located at the middle (the center of vibration) between the top dead center and the bottom dead center. In other words, the thrust of the movable core  14  becomes the maximum at the middle or the center of vibration.  
         [0035]    A linear electric machine according to the second embodiment of the invention will be described with reference to FIGS. 5A and 5B.  
         [0036]    As shown in FIG. 5A, the magnet shielding plate  14   a  is made of a permanent magnet instead of a non-magnetic material. The permanent magnet shielding plate  14   a  is magnetized to have the polarities opposite to the polarities of the permanent magnets  14   b  that is shown in FIG. 4A.  
         [0037]    The magnet shielding plate  14   a  may be constituted of a non-magnetic plate and a plurality of permanent magnet bars  14   a ′ that are embedded into the non-magnetic plate and magnetized to have the polarities opposite to the polarities of the permanent magnets  14   b , as shown in FIG. 5B.  
         [0038]    The magnet shielding plate  14   a , or  14   a ′ that is constituted of a permanent magnet or a plurality of permanent magnets increases the effective magnetic flux of the movable core  14 , so that the thrust of the movable core  14  can be significantly increased.  
         [0039]    A linear electric machine according to the third embodiment of the invention will be described with reference to FIGS. 6A and 6B. Incidentally, the same reference numeral that indicates a part or component of the following embodiments as the previous embodiment will indicate the same or substantially the same part or component, hereafter.  
         [0040]    As shown in FIGS. 6A and 6B, the radially outer portion of each permanent magnets  14   b  projects from the inductors  14   c  of the movable core  14  to be located between the adjacent teeth  15 .  
         [0041]    Therefore, the size of the permanent magnets  14   b  can be increased so that the effective magnetic flux of the permanent magnets  14   b  or the thrust of the movable core  14  can be increased without increasing the total volume of the actuator  13 .  
         [0042]    A linear electric machine according to the fourth embodiment of the invention will be described with reference to FIGS. 7A and 7B.  
         [0043]    The movable core  14  is constituted of four arc-shaped permanent magnets  14   b  that are disposed at the peripheral portion of the movable core  14  and four arc-shaped inductors  14   c  that cover the outer surfaces of the permanent magnets  14   b . Numeral  14   e  indicates a longitudinal magnet shielding groove.  
         [0044]    Various modifications of the linear electric machine according to the invention will be described with reference to FIGS.  8 - 11 .  
         [0045]    The above described linear electric machines can be utilized as a generator, as shown in FIGS.  8 - 10 . FIG. 11 shows a thermoacoustic engine for reciprocating the movable core  14  of the generators shown in FIGS.  8 - 10 .  
         [0046]    In the foregoing description of the present invention, the invention has been disclosed with reference to specific embodiments thereof. It will, however, be evident that various modifications and changes may be made to the specific embodiments of the present invention without departing from the scope of the invention as set forth in the appended claims. Accordingly, the description of the present invention is to be regarded in an illustrative, rather than a restrictive, sense.