Patent Publication Number: US-2023154661-A1

Title: Permanent electromagnetic holder and conveyance device

Description:
TECHNICAL FIELD 
     The present invention relates to a technique of a permanent electromagnetic holder and a conveyance device including the permanent electromagnetic holder. 
     BACKGROUND ART 
     Conventionally, a technique related to a permanent electromagnetic holder including a permanent magnet and a coil is known (see Patent Literature 1). The permanent electromagnetic holder is configured to be capable of attracting an object to be attracted by a magnetic force of the permanent magnet, and is configured to be in an attraction OFF state of the object to be attracted when the coil is energized. 
     In addition, there is known a technique related to a permanent electromagnetic attraction device that includes a first permanent magnet, a second permanent magnet, and a coil, and switches between an attraction ON state and an attraction OFF state of an object to be attracted by switching the magnetizing direction of the first permanent magnet through energization of the coil with the energizing direction switched (see Patent Literature 2). 
     The permanent electromagnetic attraction device has a high attraction force and does not require continuous energization of the coil in the attraction OFF state. Therefore, power consumption can be reduced. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: JP-A 2019-102682 Gazette 
     Patent Literature 2: JP-A 2017-75020 Gazette 
     SUMMARY OF INVENTION 
     Technical Problems 
     However, in the permanent electromagnetic holder, the permanent magnet and an electromagnet are arranged side by side in the height direction (direction orthogonal to the attracting surface). Therefore, the permanent electromagnetic holder has a problem of increasing in size in the height direction. 
     In addition, in the permanent electromagnetic attraction device, the first permanent magnet and the second permanent magnet are arranged side by side in the height direction (direction orthogonal to the attracting surface). For this reason, even though the permanent electromagnetic attraction device has a high attraction force and consumes less power, there is a problem that the size thereof increases in the height direction. 
     The present invention has been made in view of the above circumstances, and an object thereof is to provide a permanent electromagnetic holder that can be relatively thin while having a high attraction force and consuming less power. 
     Solutions to Problems 
     The problems to be solved by the present invention are as described above, and means for solving the problems will be described below. 
     That is, according to a first aspect, there is provided a permanent electromagnetic holder including an attracting surface for an object to be attracted, configured to be capable of attracting the object to be attracted in an attraction ON state, and configured to be capable of detaching the object to be attracted that has been attracted in an attraction OFF state, the permanent electromagnetic holder including: a first magnet that is a magnet having a relatively small coercive force; a second magnet that is a rare-earth magnet having a relatively large coercive force and is configured in a ring shape; and a coil configured to magnetize the first magnet through energization of the coil, the first magnet being configured such that magnetic pole surfaces of different magnetic poles are oriented in a thrust direction of an axial center orthogonal to the attracting surface and passing through a center of the attracting surface, the second magnet being configured such that magnetic pole surfaces of different magnetic poles are oriented in the thrust direction of the axial center and being disposed outside the first magnet in a radial direction of the axial center, the coil being disposed between the first magnet and the second magnet, the first magnet, the second magnet, and the coil being arranged so as to overlap in the radial direction of the axial center, and an operation of switching an attraction ON/OFF state being carried out by switching a magnetizing direction of the first magnet through energization of the coil when the attraction ON/OFF state is switched. 
     A second aspect further includes a front yoke disposed close to the first magnet, the second magnet, and the coil on an attracting surface side in the thrust direction of the axial center of the first magnet, the second magnet, and the coil, and a part of the front yoke protrudes to a side opposite to an attracting surface side in the thrust direction of the axial center with respect to an attracting surface side end part of the coil on a first magnet side of the coil. 
     A third aspect further includes a back yoke in which a bottom part is disposed on a side opposite to an attracting surface side in the thrust direction of the axial center and an opening is disposed on the attracting surface side in the thrust direction of the axial center, and the first magnet, the second magnet, and the coil are disposed inside the back yoke, the front yoke is disposed so as to cover the opening of the back yoke, and a part of the back yoke protrudes to the attracting surface side in the thrust direction of the axial center with respect to an end part on a side opposite to an attracting surface side of the coil on a first magnet side of the coil. 
     According to a fourth aspect, there is provided a conveyance device including the permanent electromagnetic holder. 
     Advantageous Effects of Invention 
     The effects of the present invention are as follows. 
     That is, according to the present invention, it is possible to provide a thin structure while having a high attraction force and consuming less power. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a cross-sectional view illustrating a permanent electromagnetic holder according to an embodiment of the present invention. 
         FIG.  2 A  is a schematic plan view illustrating a first magnet, a second magnet, and a coil in an attraction ON state of the permanent electromagnetic holder, and  FIG.  2 B  is a schematic cross-sectional view illustrating the attraction ON state of the permanent electromagnetic holder. 
         FIG.  3 A  is a schematic plan view illustrating the first magnet, the second magnet, and the coil in an attraction OFF state of the permanent electromagnetic holder, and  FIG.  3 B  is a schematic cross-sectional view illustrating the attraction OFF state of the permanent electromagnetic holder. 
         FIG.  4    is a cross-sectional view illustrating a permanent electromagnetic holder. 
         FIG.  5    is a cross-sectional view illustrating a permanent electromagnetic holder. 
         FIG.  6    is a cross-sectional view illustrating a permanent electromagnetic holder. 
         FIG.  7    is a schematic cross-sectional view illustrating an attraction ON state of a permanent electromagnetic holder. 
         FIG.  8    is a schematic cross-sectional view illustrating the attraction ON state of the permanent electromagnetic holder. 
         FIG.  9    is a schematic cross-sectional view illustrating the attraction ON state of the permanent electromagnetic holder. 
         FIG.  10    is a schematic cross-sectional view illustrating the attraction OFF state of the permanent electromagnetic holder. 
         FIG.  11    is a cross-sectional view illustrating a permanent electromagnetic holder. 
     
    
    
     DESCRIPTION OF EMBODIMENT 
     Next, a permanent electromagnetic holder  1  will be described with reference to  FIGS.  1  to  6   . Note that in the following, a description will be given assuming that a broken line in the drawings indicates a loop of a magnetic field line. 
     The permanent electromagnetic holder  1  is configured to be capable of attracting an iron product or the like (object  2  to be attracted) by using a magnetic force. The permanent electromagnetic holder  1  is configured to be capable of switching an attraction ON/OFF state. The permanent electromagnetic holder  1  is configured to be capable of attracting the object  2  to be attracted in the attraction ON state, and is configured to be capable of detaching the object  2  to be attracted that has been attracted in the OFF state. 
     The permanent electromagnetic holder  1  can be used for, for example, a conveyance device that conveys or transports an iron product or the like, or a holding device that holds a state in which an iron product is attracted to a wall surface, a top surface, or the like. Specific examples of usage of the permanent electromagnetic holder  1  include an industrial robot or a crane device that holds and conveys a workpiece by a magnetic force, an aircraft such as a helicopter or a drone that hangs and conveys a steel frame or the like by a magnetic force, and a wall climbing robot that moves while being attracted to a wall surface by a magnetic force. 
     As illustrated in  FIGS.  1  to  3 B , the permanent electromagnetic holder  1  is formed in a substantially columnar shape, and includes an attracting surface  1   a  for the object  2  to be attracted on one end side in the thrust direction of the axial center α. The axial center α is orthogonal to the attracting surface  1   a  and passes through the center of the attracting surface  1   a.  The thrust direction of the axial center α indicates a direction in which the axial center α extends (height direction of the permanent electromagnetic holder  1 ), and the radial direction of the axial center a indicates a direction orthogonal to the direction of the axial center α. The permanent electromagnetic holder  1  includes a first magnet  3 , a second magnet  4 , a coil  5 , a front yoke  6 , a back yoke  7 , and a spacer  8 . 
     Hereinafter, for convenience, a direction in which the attracting surface  1   a  of the object  2  to be attracted in the permanent electromagnetic holder  1  is oriented will be described as a downward direction; however, the direction of the permanent electromagnetic holder  1  is not limited thereto. 
     The first magnet  3  is a magnet (for example, an Alnico magnet, an iron-chromium-cobalt magnet, or the like) having a relatively small coercive force, and is formed in a columnar shape. The first magnet  3  is configured such that magnetic pole (S pole and N pole) surfaces of different magnetic poles are oriented in the thrust direction of the axial center α. The first magnet  3  is configured such that the magnetic pole surfaces of the different magnetic poles are orthogonal to the thrust direction of the axial center α. 
     The second magnet  4  is a rare-earth magnet (for example, a neodymium magnet) having a relatively large coercive force, and is formed in a tubular shape (ring shape). The height of the second magnet  4  is substantially the same as the height of the first magnet  3 . The second magnet  4  is configured such that magnetic pole (S pole and N pole) surfaces of different magnetic poles are oriented in the thrust direction of the axial center α. The second magnet  4  is configured such that the magnetic pole surfaces of the different magnetic poles are orthogonal to the thrust direction of the axial center α. 
     The second magnet  4  is disposed outside the first magnet  3  in the radial direction (outside the first magnet  4  in the radial direction of the axial center α). The second magnet  4  is concentric with the first magnet  3 . 
     The first magnet  3  and the second magnet  4  are disposed at the same position in the thrust direction of the axial center α. The first magnet  3  and the second magnet  4  are arranged so as to overlap in the radial direction of the axial center α. The upper surface position of the first magnet  3  and the upper surface position of the second magnet  4  match in the thrust direction of the axial center α, and the lower surface position of the first magnet  3  and the lower surface position of the second magnet  4  match in the thrust direction of the axial center α. 
     The coil  5  is a solenoid coil, and generates strong magnetic flux on the axial center α side (first magnet  3  side) to magnetize the first magnet  3  through energization thereof. The coil  5  switches the magnetizing direction of the first magnet  3  through energization thereof with the energizing direction switched. 
     The coil  5  is annularly arranged between the first magnet  3  and the second magnet  4 . The coil  5  is disposed outside the first magnet  3  in the radial direction. The coil  5  is disposed inside the second magnet  4  in the radial direction. The coil  5  is configured to be close to the first magnet  3  and the second magnet  4 . 
     The first magnet  3 , the second magnet  4 , and the coil  5  are arranged so as to overlap in the radial direction of the axial center α. 
     The front yoke  6  is a yoke made of a material such as iron, and is formed in a substantially flat disk shape. The front yoke  6  is disposed outside the first magnet  3 , the second magnet  4 , and the coil  5 . The front yoke  6  is disposed below (on the attracting surface  1   a  side in the thrust direction of the axial center α of) the first magnet  3 , the second magnet  4 , and the coil  5  so as to be close to or in contact with them. The outer diameter of the front yoke  6  is substantially the same as the outer diameter of the second magnet  4 . 
     The back yoke  7  is a yoke made of a material such as iron, and is formed in a substantially tubular shape having a bottom part and an opening. The bottom part of the back yoke  7  is disposed on the upper side, and the opening is disposed on the lower side. The inner diameter of the back yoke  7  is greater than the outer diameter of the front yoke  6 . 
     The back yoke  7  is disposed outside the first magnet  3 , the second magnet  4 , and the coil  5 . The first magnet  3 , the second magnet  4 , and the coil  5  are disposed inside the back yoke  7  (in the back yoke  7 ). The back yoke  7  is configured to be close to or in contact with the first magnet  3 , the second magnet  4 , and the coil  5 . The bottom part of the back yoke  7  is located above the first magnet  3 , the second magnet  4 , and the coil  5 , and the side wall of the back yoke  7  is located outside the first magnet  3 , the second magnet  4 , and the coil  5 . The lower end part (lower opening) of the back yoke  7  is located below the first magnet  3 , the second magnet  4 , and the coil  5 . The lower end surface of the back yoke  7  is substantially flush with the lower surface of the front yoke  6 . 
     The front yoke  6  is disposed so as to cover the opening of the back yoke  7 . The lower surface of the front yoke  6  and the lower end surface of the back yoke  7  constitute the attracting surface  1   a.    
     The spacer  8  is made of a nonmagnetic material such as aluminum, and shields the magnetic flux of the first magnet  3  and the second magnet  4 . The spacer  8  is fitted between the outer surfaces of the second magnet  4  and the front yoke  6  and the inner surface of the back yoke  7 . 
     By energizing the coil  5  with the energizing direction switched, the magnetizing direction of the first magnet  3  is switched to switch the attraction ON state and the attraction OFF state of the object  2  to be attracted. 
     An operation of switching the attraction ON/OFF state is carried out by switching the magnetizing direction of the first magnet  3  through instantaneous energization of the coil  5  (energization OFF of the coil  5  immediately after energization ON of the coil  5 ) when the attraction ON/OFF state is switched. In the permanent electromagnetic holder  1 , for example, the magnetizing direction (attraction ON/OFF state) of the first magnet  3  is switched in 0.01 seconds to 0.2 seconds after the start of energization to the coil  5 . 
     In the attraction ON state, for example, in a case where the magnetic pole of the second magnet  4  on the attracting surface  1   a  side is an N pole, the magnetic pole of the first magnet  3  on the attracting surface  1   a  side becomes an N pole. As described above, the first magnet  3  is magnetized in the same direction as the magnetic poles of the second magnet  4 , and is set to the attraction ON state (see  FIGS.  2 A and  2 B ). 
     In the attraction OFF state, for example, in a case where the magnetic pole of the second magnet  4  on the attracting surface  1   a  side is an N pole, the magnetic pole of the first magnet  3  on the attracting surface  1   a  side becomes an S pole. In this way, the magnetic flux of the first magnet  3  and the second magnet  4  is looped inside the front yoke  6  and the back yoke  7  so that the magnetic flux of the first magnet  3  and the second magnet  4  does not leak to the outside of the attracting surface  1   a,  and the attraction OFF state is established (see  FIGS.  3 A and  3 B ). 
     As described above, the first magnet  3  is configured such that the magnetic pole surfaces of the different magnetic poles are oriented in the thrust direction of the axial center α, the second magnet  4  is configured such that the magnetic pole surfaces of the different magnetic poles are oriented in the thrust direction of the axial center α, the first magnet  3 , the second magnet  4 , and the coil  5  are arranged so as to overlap in the radial direction of the axial center α, and the operation of switching the attraction ON/OFF state is carried out by switching the magnetizing direction of the first magnet  3  through energization (instantaneous energization) of the coil  5  when the attraction ON/OFF state is switched. Therefore, the permanent electromagnetic holder  1  can be configured to be thin while having a high attraction force and reducing power consumption as compared with a conventional permanent electromagnetic holder or the like. 
     In addition, in the conventional permanent electromagnetic holder, in a case where the magnetic force of the permanent magnet is relatively large, it is conceivable that the coil is configured to be relatively large so that the magnetic force of the permanent magnet can be canceled. If the coil is configured to be relatively large in this manner, the size of the permanent electromagnetic holder is increased on the radial direction (radial direction of the axial center) side of the attracting surface. However, in the permanent electromagnetic holder  1  configured as described above, it is possible to suppress an increase in size on the radial direction side of the axial center α while providing a high attraction force as compared with a conventional permanent electromagnetic holder in which the permanent magnet has a relatively strong magnetic force and the coil is relatively large. 
     After switching the attraction ON/OFF state, the permanent electromagnetic holder  1  maintains the attraction ON/OFF state after the switching even when the coil  5  is not energized until the next operation of switching the attraction ON/OFF state is carried out. 
     Therefore, in the permanent electromagnetic holder  1 , for example, it is possible to prevent a magnetic body or the like from being erroneously attracted when the power supply is turned off so that energization is unintentionally stopped and the attraction ON state is established as in the conventional permanent electromagnetic holder. In addition, for example, as in the conventional permanent electromagnetic holder, it is possible to prevent an operation of attracting a magnetic body or the like from being erroneously started when energization is unintentionally stopped before positioning of the hanging position. 
     As illustrated in  FIG.  1   , the front yoke  6  is configured such that a groove  6   a  is formed in an upper surface (surface on a side where the first magnet  3 , the second magnet  4 , or the coil  5  is disposed in thrust direction of the axial center α) and a step is formed on the upper surface. 
     The groove  6   a  of the front yoke  6  is a square groove, is formed in an annular shape in plan view, and is formed concentric with the first magnet  3  or the second magnet  4 . The outer diameter of the groove  6   a  of the front yoke  6  is substantially the same as the inner diameter of the second magnet  4 . The inner diameter of the groove  6   a  of the front yoke  6  is substantially the same as the outer diameter of the first magnet  3 . 
     A part (lower part) of the coil  5  is disposed in the groove  6   a  of the front yoke  6 . A lower end part (an end part on the attracting surface  1   a  side) of the coil  5  is disposed so as to bite into an upper part of the front yoke  6 . Inside the coil  5  (on the first magnet  3  side), a part of the front yoke  6  protrudes upward (side on which the first magnet  3 , the second magnet  4 , or the coil  5  is disposed in the thrust direction of axial center α (side opposite to the attracting surface  1   a  side in the thrust direction of the axial center α)) with respect to the lower end part (end part on the attracting surface  1   a  side) of the coil  5 . Outside the coil  5  (on the second magnet  4  side), a part of the front yoke  6  protrudes above the lower end part of the coil  5 . 
     As described above, the end part on the attracting surface  1   a  side of the coil  5  is arranged so as to bite into the upper part of the front yoke  6 , and on the first magnet  3  side of the coil  5 , a part of the front yoke  6  protrudes to the side opposite to the attracting surface  1   a  side in the thrust direction of the axial center α with respect to the end part on the attracting surface  1   a  side of the coil  5 . Therefore, in the permanent electromagnetic holder  1 , the magnetic flux to the axial center α side (first magnet  3  side) can be increased, and magnetization of the first magnet  3  can be performed with relatively low power consumption. 
     The back yoke  7  is configured such that a groove  7   a  is formed in a lower surface (surface on a side where the first magnet  3 , the second magnet  4 , or the coil  5  is disposed in thrust direction of the axial center α) of the bottom part and a step is formed on the lower surface. 
     The groove  7   a  of the back yoke  7  is a square groove, is formed in an annular shape in bottom view, and is formed concentric with the first magnet  3  or the second magnet  4 . The outer diameter of the groove  7   a  of the back yoke  7  is substantially the same as the inner diameter of the second magnet  4 . The inner diameter of the groove  7   a  of the back yoke  7  is substantially the same as the outer diameter of the first magnet  3 . 
     A part (upper part) of the coil  5  is disposed in the groove  7   a  of the back yoke  7 . An upper end part (end part on the side opposite to the attracting surface  1   a  side) of the coil  5  is disposed so as to bite into the bottom part of the back yoke  7 . Inside the coil  5  (on the first magnet  3  side), a part of the back yoke  7  protrudes downward (side on which the first magnet  3 , the second magnet  4 , or the coil  5  is disposed in the thrust direction of the axial center α (attracting surface  1   a  side in the thrust direction of the axial center α)) with respect to the upper end part (end part on the side opposite to the attracting surface  1   a  side) of the coil  5 . Outside the coil  5  (on the second magnet  4  side), a part of the back yoke  7  protrudes below the upper end part of the coil  5 . 
     As described above, the end part on the side opposite to the attracting surface  1   a  side of the coil  5  is arranged so as to bite into the bottom part of the back yoke  7 , and the back yoke  7  is configured such that on the first magnet  3  side of the coil  5 , a part of the back yoke  7  protrudes to the side opposite to the attracting surface  1   a  side in the thrust direction of the axial center α with respect to the end part on the side opposite to the attracting surface  1   a  side of the coil  5 . Therefore, in the permanent electromagnetic holder  1 , the magnetic flux to the axial center α side (first magnet  3  side) can be increased, and magnetization of the first magnet  3  can be performed with relatively low power consumption. 
     The permanent electromagnetic holder  1  may not be formed in a columnar shape, and may be formed in a polygonal shape (for example, a quadrangular prism). The first magnet  3  may not be formed in a columnar shape, and the second magnet  4  may not be formed in a tubular shape. That is, the first magnet  3  can be formed in a polygonal pole shape (for example, a quadrangular prism), and the second magnet  4  can be formed in a polygonal tube shape for example, a rectangular tube shape). 
     The second magnet  4  may be formed in a ring shape by arranging a plurality of rare-earth magnets having a predetermined shape (for example, a fan shape). 
     The groove  6   a  of the front yoke  6  or the groove  7   a  of the back yoke  7  may be a groove having a circular shape, a polygonal shape, or the like. 
     Note that in the permanent electromagnetic holder  1 , as illustrated in  FIG.  4   , the front yoke  6  may have a flat upper surface without the groove  6   a  in the upper surface, and the back yoke  7  may have a flat lower surface of the bottom part without the groove  7   a  in the lower surface of the bottom part. With this configuration, the permanent electromagnetic holder  1  can be manufactured relatively easily. 
     The permanent electromagnetic holder  1  can also be configured such that the height of the second magnet  4  is different from the height of the first magnet  3 . At this time, for example, as illustrated in  FIG.  5   , the length (height) in the thrust direction of the axial center α of the second magnet  4  is shorter than the length (height) in the thrust direction of the axial center α of the first magnet  3 . A recess is formed in the upper surface of the front yoke  6 , and a recess is formed in the lower surface of the bottom part of the back yoke  7 . The outer diameter of the recess of the front yoke  6  and the outer diameter of the recess of the back yoke are formed to be substantially the same as the inner diameter of the second magnet  4 . The first magnet  3  and the coil  5  are disposed in the recess of the front yoke  6  and the recess of the back yoke. 
     The permanent electromagnetic holder  1  can also be configured such that the outer diameter of the front yoke  6  is different from the outer diameter of the second magnet  4 . At this time, for example, as illustrated in  FIG.  6   , the outer diameter of the front yoke  6  is shorter than the outer diameter of the second magnet  4 . 
     Next, a permanent electromagnetic holder  1  illustrated in  FIGS.  7  to  10    will be described. 
     Note that in the description of the permanent electromagnetic holder  1  illustrated in  FIGS.  7  to  10   , the description of parts similar to those of the permanent electromagnetic holder  1  illustrated in  FIGS.  1  to  6    will be omitted as appropriate, and parts different from those of the permanent electromagnetic holder  1  illustrated in  FIGS.  1  to  6    will be mainly described. 
     As illustrated in  FIGS.  7  to  10   , the permanent electromagnetic holder  1  includes a first magnet part  10  and a second magnet part  20 , and is configured such that the first magnet part  10  and the second magnet part  20  are arranged side by side in the thrust direction of the axial center α. The first magnet part  10  is disposed closer to the attracting surface  1   a  side than the second magnet part  20  is in the thrust direction of the axial center α. 
     The first magnet part  10  includes a first magnet  13 , a second magnet  14 , a first coil  15 , a front yoke  16 , a first back yoke  17 , and a spacer  18 . 
     The first magnet  13  is a magnet (for example, an Alnico magnet, an iron-chromium-cobalt magnet, or the like) having a relatively small coercive force, and is formed in a columnar shape. The first magnet  13  is configured such that magnetic pole (S pole and N pole) surfaces of different magnetic poles are oriented in the thrust direction of the axial center α. The first magnet  13  is configured such that the magnetic pole surfaces of the different magnetic poles are orthogonal to the thrust direction of the axial center α. 
     The second magnet  14  is a rare-earth magnet (for example, a neodymium magnet) having a relatively large coercive force, and is formed in a tubular shape (ring shape). The height of the second magnet  14  is substantially the same as the height of the first magnet  13 . The second magnet  14  is configured such that magnetic pole (S pole and N pole) surfaces of different magnetic poles are oriented in the thrust direction of the axial center α. The second magnet  14  is configured such that the magnetic pole surfaces of the different magnetic poles are orthogonal to the thrust direction of the axial center α. 
     The first magnet  13  and the second magnet  14  are disposed at the same position in the thrust direction of the axial center α. 
     The first coil  15  is a solenoid coil, and generates strong magnetic flux on the axial center a side (first magnet  13  side) through energization thereof to magnetize the first magnet  13 . The coil  15  switches the magnetizing direction of the first magnet  13  through energization thereof with the energizing direction switched. 
     The first coil  15  is disposed between the first magnet  13  and the second magnet  14 . The first coil  15  is disposed outside the first magnet  13  in the radial direction. The first coil  15  is disposed inside the second magnet  14  in the radial direction. The first coil  15  is configured to be close to the first magnet  13  and the second magnet  14 . 
     The first magnet  13 , the second magnet  14 , and the first coil  15  are arranged so as to overlap in the radial direction of the axial center α. 
     The front yoke  16  is a yoke made of a material such as iron, and is formed in a substantially flat disk shape. The front yoke  16  is disposed below the first magnet  13 , the second magnet  14 , and the first coil  15  so as to be close to or in contact with them. 
     The first back yoke  17  is a yoke made of a material such as iron, and is formed in a substantially tubular shape having a bottom part and an opening. The bottom part of the first back yoke  17  is disposed on the upper side, and the opening is disposed on the lower side. The inner diameter of the first back yoke  17  is greater than the outer diameter of the front yoke  16 . 
     The first back yoke  17  is disposed outside the first magnet  13 , the second magnet  14 , and the first coil  15 . The first magnet  13 , the second magnet  14 , and the first coil  15  are disposed inside the first back yoke  17 . The first back yoke  17  is configured to be close to or in contact with the first magnet  13 , the second magnet  14 , and the first coil  15 . The bottom part of the first back yoke  17  is located above the first magnet  13 , the second magnet  14 , and the first coil  15 , and the side wall of the first back yoke  17  is located outside the first magnet  13 , the second magnet  14 , and the first coil  15 . The lower end part (lower opening) of the first back yoke  17  is located lower than the first magnet  13 , the second magnet  14 , and the first coil  15 . The lower end surface of the first back yoke  17  is substantially flush with the lower surface of the front yoke  16 . 
     The front yoke  16  is disposed so as to cover the opening of the first back yoke  17 . 
     The spacer  18  is made of a nonmagnetic material such as aluminum, and shields the magnetic flux of the first magnet  13 , the second magnet  14 , a third magnet  23 , and a fourth magnet  24 . The spacer  18  is fitted between the outer surfaces of the second magnet  14  and the front yoke  16  and the inner surface of the first back yoke  17 . 
     The second magnet part  20  includes the third magnet  23 , the fourth magnet  24 , a second coil  25 , a second back yoke  27 , and a spacer  28 . 
     The third magnet  23  is a magnet (for example, an Alnico magnet, an iron-chromium-cobalt magnet, or the like) having a relatively small coercive force, and is formed in a columnar shape. The outer diameter of the third magnet  23  is greater than the inner diameter of the second magnet  14  and smaller than the outer diameter of the second magnet  14 . The third magnet  23  is configured such that its magnetic pole surfaces are oriented in the thrust direction of the axial center α. The third magnet  23  is configured such that the magnetic pole (S pole and N pole) surfaces of different magnetic poles are orthogonal to the thrust direction of the axial center α. 
     The third magnet  23  is disposed above the first back yoke  17  so as to be close to or in contact with the upper surface of the first back yoke  17 . The third magnet  23  is concentric with the axial center of the first magnet  13  and the second magnet  14 . 
     The fourth magnet  24  is a rare-earth magnet (for example, a neodymium magnet) having a relatively large coercive force, and is formed in a tubular shape (ring shape). The height of the fourth magnet  24  is substantially the same as the height of the third magnet  23 . The outer diameter of the fourth magnet  24  is greater than the outer diameter of the second magnet  14 . The fourth magnet  24  is configured such that magnetic pole (S pole and N pole) surfaces of different magnetic poles are oriented in the thrust direction of the axial center α. The fourth magnet  24  is configured such that the magnetic pole surfaces of the different magnetic poles are orthogonal to the thrust direction of the axial center α. 
     The fourth magnet  24  is disposed above the first back yoke  17  so as to be close to or in contact with the upper surface of the first back yoke  17 . The fourth magnet  24  is disposed outside the third magnet  23  in the radial direction (outside the third magnet  23  in the radial direction of the axial center α). The fourth magnet  24  is concentric with the first magnet  13 , the second magnet  14 , and the third magnet  23 . 
     The third magnet  23  and the fourth magnet  24  are disposed at the same position in the thrust direction of the axial center α. The third magnet  23  and the fourth magnet  24  are arranged so as to overlap in the radial direction of the axial center α. The upper surface position of the third magnet  23  and the upper surface position of the fourth magnet  24  match in the thrust direction of the axial center α, and the lower surface position of the third magnet  23  and the lower surface position of the fourth magnet  24  match in the thrust direction of the axial center α. 
     The second coil  25  is a solenoid coil, and generates strong magnetic flux on the axial center α side (third magnet  23  side) to magnetize the third magnet  23  through energization thereof. The second coil  25  switches the magnetizing direction of the third magnet  23  through energization thereof with the energizing direction switched. 
     The second coil  25  is disposed between the third magnet  23  and the fourth magnet  24 . The second coil  25  is disposed outside the third magnet  23  in the radial direction. The second coil  25  is disposed inside the fourth magnet  24  in the radial direction. The second coil  25  is configured to be close to the third magnet  23  and the fourth magnet  24 . 
     The third magnet  23 , the fourth magnet  24 , and the second coil  25  are arranged so as to overlap in the radial direction of the axial center α. 
     The second back yoke  27  is a yoke made of a material such as iron, and is formed in a substantially tubular shape having a bottom part and an opening. The bottom part of the second back yoke  27  is disposed on the upper side, and the opening is disposed on the lower side. The inner diameter of the second back yoke  27  is greater than the outer diameter of the first back yoke  17 . 
     The second back yoke  27  is disposed outside the first magnet part  10  (the first magnet  13 , the second magnet  14 , and the first coil  15 ), the third magnet  23 , the fourth magnet  24 , and the second coil  25 . The first magnet part  10  (the first magnet  13 , the second magnet  14 , and the first coil  15 ), the third magnet  23 , the fourth magnet  24 , and the second coil  25  are disposed inside the second back yoke  27 . The second back yoke  27  is configured to be close to or in contact with the third magnet  23 , the fourth magnet  24 , and the second coil  25 . The bottom part of the second back yoke  27  is located above the third magnet  23 , the fourth magnet  24 , and the second coil  25 , and the side wall of the second back yoke  27  is located outside the first magnet part  10  (the first magnet  13 , the second magnet  14 , and the first coil  15 ), the third magnet  23 , the fourth magnet  24 , and the second coil  25 . The lower end part (lower opening) of the second back yoke  27  is located lower than the third magnet  23 , the fourth magnet  24 , and the second coil  25 . The lower end surface of the second back yoke  27  is substantially flush with the lower surface of the front yoke  16  and the lower end surface of the first back yoke  17 . 
     The spacer  28  is made of a nonmagnetic material such as aluminum, and shields the magnetic flux of the first magnet  13 , the second magnet  14 , the third magnet  23 , and the fourth magnet  24 . The spacer  28  is fitted between the outer surfaces of the fourth magnet  24  and the first back yoke  17  and the inner surface of the second back yoke  27 . 
     The lower surface of the front yoke  16 , the lower end surface of the first back yoke  17 , and the lower end surface of the second back yoke  27  constitute the attracting surface  1   a.    
     By switching the energizing directions and energizing the first coil  15  and/or the second coil  25 , the magnetizing directions of the first magnet  13  and/or the third magnet  23  are switched, and the attraction ON state and the attraction OFF state of an object  2  to be attracted are switched. 
     An operation of switching the attraction ON/OFF state is carried out by switching the magnetizing direction of the first magnet  13  and/or the magnetizing direction of the third magnet  23  through instantaneous energization of the first coil  15  and/or the second coil  25  (energization OFF of the first coil  15  and/or the second coil  25  immediately after energization ON of the first coil  15  and/or the second coil  25 ) when the attraction ON/OFF state is switched. For example, in the permanent electromagnetic holder  1 , the magnetizing direction of the first magnet  13  and/or the magnetizing direction of the third magnet  23  (attraction ON/OFF state) are switched in 0.01 seconds to 0.2 seconds after the start of energization to the first coil  15  and/or the second coil  25 . 
     For example, in the attraction ON state, in a case where the magnetic pole of the second magnet  14  on the attracting surface  1   a  side is a N pole and the magnetic pole of the fourth magnet  24  on the attracting surface  1   a  side is a S pole, by energizing only the first coil  15 , the magnetic pole of the first magnet  13  on the attracting surface  1   a  side becomes a N pole. In this manner, the first magnet  13  is magnetized in the same direction as the magnetic poles of the second magnet  14 , and the attraction ON state is established (see  FIG.  7   ). 
     In addition, for example, in the attraction ON state, in a case where the magnetic pole of the second magnet  14  on the attracting surface  1   a  side is a N pole and the magnetic pole of the fourth magnet  24  on the attracting surface  1   a  side is a S pole, by energizing only the second coil  25 , the magnetic pole of the third magnet  23  on the attracting surface  1   a  side becomes a S pole. In this manner, the third magnet  23  is magnetized in the same direction as the magnetic poles of the fourth magnet  24 , and the attraction ON state is established (see  FIG.  8   ). At this time, as compared with a case where only the first coil  15  is energized and the attraction ON state is established, a stronger attraction force for attracting the object  2  to be attracted is generated. 
     Furthermore, for example, in the attraction ON state, in a case where the magnetic pole of the second magnet  14  on the attracting surface  1   a  side is a N pole and the magnetic pole of the fourth magnet  24  on the attracting surface  1   a  side is a S pole, by energizing the first coil  15  and the second coil  25 , the magnetic pole of the first magnet  13  on the attracting surface  1   a  side becomes a N pole and the magnetic pole of the third magnet  23  on the attracting surface  1   a  side becomes a S pole. In this manner, the first magnet  13  is magnetized in the same direction as the magnetic poles of the second magnet  14 , the third magnet  23  is magnetized in the same direction as the magnetic poles of the fourth magnet  24 , and the attraction ON state is established (see FIG.  9 ). At this time, as compared with the case where only the first coil  15  is energized or only the second coil  25  is energized to be in the attraction ON state, a stronger attraction force for attracting the object  2  to be attracted is generated. 
     In the attraction OFF state, for example, in a case where the magnetic pole of the second magnet  14  on the attracting surface  1   a  side is a N pole and the magnetic pole of the fourth magnet  24  on the attracting surface  1   a  side is a S pole, the magnetic pole of the first magnet  13  on the attracting surface  1   a  side becomes a S pole and the magnetic pole of the third magnet  23  on the attracting surface  1   a  side becomes a N pole. In this way, the magnetic flux of the first magnet  13 , the second magnet  14 , the third magnet  23 , and the fourth magnet  24  is looped inside the front yoke  16 , the first back yoke  17 , and the second back yoke  27  so that the magnetic flux of the first magnet  13 , the second magnet  14 , the third magnet  23 , and the fourth magnet  24  does not leak to the outside of the attracting surface  1   a,  and the attraction OFF state is established (see  FIG.  10   ). 
     As described above, the first magnet part  10  is disposed closer to the attracting surface  1   a  than the second magnet part  20  is in the thrust direction of the axial center α, the first magnet  13  is configured such that the magnetic pole surfaces of the different magnetic poles are oriented in the thrust direction of the axial center α, the second magnet  14  is configured such that the magnetic pole surfaces of the different magnetic poles are oriented in the thrust direction of the axial center α, the first magnet  13 , the second magnet  14 , and the first coil  15  are disposed so as to overlap in the radial direction of the axial center α, the third magnet  23  is configured such that the magnetic pole surfaces of the different magnetic poles are oriented in the thrust direction of the axial center α, the fourth magnet  24  is configured such that the magnetic pole surfaces of the different magnetic poles are oriented in the thrust direction of the axial center α, and the third magnet  23 , the fourth magnet  24 , and the second coil  25  are arranged so as to overlap in the radial direction of the axial center α. The operation of switching the attraction ON/OFF state is carried out by switching the magnetizing direction of the first magnet  13  and the magnetizing direction of the third magnet  23  through energization (instantaneous energization) of the first coil  15  and/or the second coil  25  when the attraction ON/OFF state is switched. Therefore, the permanent electromagnetic holder  1  can be configured to be relatively thin in the configuration in which the first magnet part  10  and the second magnet part  20  are arranged side by side in the thrust direction of the axial center α while having a higher attraction force and reducing power consumption as compared with a conventional permanent electromagnetic holder and the like. 
     When the attraction ON/OFF state is switched, a mode of energizing the coil (the first coil  15  and/or the second coil  25 ) can be selected from among energizing the first coil  15 , energizing the second coil  25 , and energizing the first coil  15  and/or the second coil  25 . The mode of energizing the coil is selected by, for example, a worker operating an operation switch or the like. 
     For example, in a case where the attraction force for attracting the object  2  to be attracted is desired to be relatively weak, only the first coil  15  is selected to be energized when the attraction ON/OFF state is switched. 
     In addition, for example, in a case where the attraction force for attracting the object  2  to be attracted is desired to be stronger than that when only the first coil  15  is energized, only the second coil  25  is selected to be energized when the attraction ON/OFF state is switched. 
     Furthermore, for example, in a case where the attraction force for attracting the object  2  to be attracted is desired to be stronger than that when only the first coil  15  is energized or only the second coil  25  is energized, the first coil  15  and the second coil  25  are selected to be energized when the attraction ON/OFF state is switched. 
     As described above, when the attraction ON/OFF state is switched, it is possible to select the mode of energizing the coil among energizing the first coil  15 , energizing the second coil  25 , and energizing the first coil  15  and the second coil  25 . Therefore, it is possible to configure settings to change the attraction force for attracting the object  2  to be attracted according to the specifications of the object  2  to be attracted, usage of the permanent electromagnetic holder  1 , and the like. 
     Note that the permanent electromagnetic holder  1  can also be configured such that only the first coil  15  and the second coil  25  can be energized when the attraction ON/OFF state is switched. 
     The front yoke  16  is configured such that a groove (not illustrated) is formed in an upper surface thereof and a step is formed on the upper surface. 
     A part (lower part) of the first coil  15  is disposed in the groove of the front yoke  16 . A lower end part (an end part on the attracting surface  1   a  side) of the first coil  15  is disposed so as to bite into an upper part of the front yoke  16 . Inside (first magnet  13  side) the first coil  15 , a part of the front yoke  16  protrudes above a lower end part (end part on the attracting surface  1   a  side) of the first coil  15 . Outside the first coil  15  (on the second magnet  14  side), a part of the front yoke  16  protrudes above the lower end part of the first coil  15 . 
     As described above, the end part on the attracting surface  1   a  side of the first coil  15  is arranged so as to bite into the upper part of the front yoke  16 , and on the first magnet  13  side of the first coil  15 , a part of the front yoke  16  protrudes to the side opposite to the attracting surface  1   a  side in the thrust direction of the axial center α with respect to the end part on the attracting surface  1   a  side of the first coil  15 . Therefore, in the permanent electromagnetic holder  1 , the magnetic flux to the axial center α side (first magnet  13  side) can be increased, and magnetization of the first magnet  13  can be performed with relatively low power consumption. 
     The first back yoke  17  is configured such that a groove (not illustrated) is formed in a lower surface of the bottom part and a step is formed on the lower surface. 
     A part (upper part) of the first coil  15  is disposed in the groove of the first back yoke  17 . An upper end part (end part on the side opposite to the attracting surface  1   a  side) of the first coil  15  is disposed so as to bite into the bottom part of the first back yoke  17 . Inside the first coil  15  (on the first magnet  13  side), a part of the first back yoke  17  protrudes downward (side on which the first magnet  13 , the second magnet  14 , or the first coil  15  is disposed in the thrust direction of the axial center α (attracting surface  1   a  side in the thrust direction of the axial center α)) with respect to the upper end part (end part on the side opposite to the attracting surface  1   a  side) of the first coil  15 . Outside the first coil  15  (on the second magnet  14  side), part of the first back yoke  17  protrudes below the upper end part of the first coil  15 . 
     As described above, the end part on the side opposite to the attracting surface  1   a  side of the first coil  15  is arranged so as to bite into the bottom part of the first back yoke  17 , and the first back yoke  17  is configured such that on the first magnet  13  side of the first coil  15 , a part of the first back yoke  17  protrudes to the attracting surface  1   a  side in the thrust direction of the axial center α with respect to the end part on the side opposite to the attracting surface  1   a  side of the first coil  15 . Therefore, in the permanent electromagnetic holder  1 , the magnetic flux to the axial center α side (first magnet  13  side) can be increased, and magnetization of the first magnet  13  can be performed with relatively low power consumption. 
     Similarly to the front yoke  16 , the second back yoke  27  can also be configured such that a groove (not illustrated) is formed in an upper surface of the bottom part and a step is formed on the upper surface. 
     A part (lower part) of the second coil  25  is disposed in the groove of the second back yoke  27 . A lower end part (end part on the attracting surface  1   a  side) of the second coil  25  is disposed so as to bite into an upper part of the second back yoke  27 . Inside (third magnet  23  side of) the second coil  25 , a part of the second back yoke  27  protrudes above a lower end part (end part on the attracting surface  1   a  side) of the second coil  25 . Outside the second coil  25  (on the fourth magnet  24  side), a part of the second back yoke  27  protrudes above the lower end part of the second coil  25 . 
     As described above, the end part on the attracting surface  1   a  side of the second coil  25  is arranged so as to bite into the upper part of the second back yoke  27 , and on the third magnet  23  side of the second coil  25 , a part of the second back yoke  27  protrudes to the side opposite to the attracting surface  1   a  side in the thrust direction of the axial center α with respect to the end part on the attracting surface  1   a  side of the second coil  25 . Therefore, in the permanent electromagnetic holder  1 , the magnetic flux to the axial center α side (first magnet  13  side) can be increased, and magnetization of the first magnet  13  can be performed with relatively low power consumption. 
     Similarly to the first back yoke  17 , the second back yoke  27  can also be configured such that a groove (not illustrated) is formed in a lower surface of the bottom part and a step is formed on the lower surface. 
     A part (upper part) of the second coil  25  is disposed in the groove of the second back yoke  27 . An upper end part (end part on the side opposite to the attracting surface  1   a  side) of the second coil  25  is disposed so as to bite into the bottom part of the second back yoke  27 . Inside the second coil  25  (on the third magnet  23  side), a part of the second back yoke  27  protrudes downward (side on which the third magnet  23 , the fourth magnet  24 , or the second coil  25  is disposed in the thrust direction of the axial center α (attracting surface  1   a  side in the thrust direction of the axial center α)) with respect to the upper end part (end part on the side opposite to the attracting surface  1   a  side) of the second coil  25 . Outside the second coil  25  (on the fourth magnet  24  side), a part of the second back yoke  27  protrudes below the upper end part of the second coil  25 . 
     As described above, the end part on the side opposite to the attracting surface  1   a  side of the second coil  25  is arranged so as to bite into the bottom part of the second back yoke  27 , and the second back yoke  27  is configured such that on the third magnet  23  side of the second coil  25 , a part of the second back yoke  27  protrudes to the attracting surface  1   a  side in the thrust direction of the axial center α with respect to the end part on the side opposite to the attracting surface  1   a  side of the second coil  25 . Therefore, in the permanent electromagnetic holder  1 , the magnetic flux to the axial center α side (third magnet  23  side) can be increased, and magnetization of the third magnet  23  can be performed with relatively low power consumption. 
     Note that the permanent electromagnetic holder  1  can also be configured such that three or more magnet parts are arranged side by side in the thrust direction of the axial center α in addition to the first magnet part  10  and the second magnet part  20 . 
     The second magnet  14  or the fourth magnet  24  may be formed in a ring shape by arranging a plurality of rare-earth magnets having a predetermined shape (for example, a fan shape). 
     The groove of the front yoke  16 , the second back yoke  17 , or the second back yoke  27  may be a groove having a circular shape, a polygonal shape, or the like. 
     In the permanent electromagnetic holder  1 , as illustrated in  FIG.  11   , the coil  5  may be disposed above the second magnet  4  (on the side opposite to the attracting surface  1   a  side in the thrust direction of the axial center α) so as to bite into the back yoke  7 . At this time, the first magnet  3  and the second magnet  4  are disposed so as to overlap in the radial direction of the axial center α, the coil  5  is disposed outside the first magnet  3  in the radial direction and above the second magnet  4 , and the coil  5  and the second magnet  4  are disposed so as to overlap in the thrust direction of the axial center α. 
     With this configuration, in the permanent electromagnetic holder  1 , the magnetic flux to the axial center α side (first magnet  3  side) can be increased, and magnetization of the first magnet  3  can be performed with relatively low power consumption. 
     INDUSTRIAL APPLICABILITY 
     The present invention is applied to a permanent electromagnetic holder configured to be capable of attracting an object to be attracted using a magnetic force, and a conveyance device including the permanent electromagnetic holder. 
     REFERENCE SIGNS LIST 
     
         
           1  Permanent electromagnetic holder 
           1   a  Attracting surface 
           2  Object to be attracted 
           3  First magnet 
           4  Second magnet 
           5  Coil 
           6  Front yoke 
           6   a  Groove 
           7  Back yoke 
           7   a  Groove 
           8  Spacer 
           10  First magnet part 
           13  First magnet 
           14  Second magnet 
           15  First coil 
           16  Front yoke 
           17  First back yoke 
           18  Spacer 
           20  Second magnet part 
           23  Third magnet 
           24  Fourth magnet 
           25  Second coil 
           27  Second back yoke 
           28  Spacer 
         α Axial center