Patent Publication Number: US-6667677-B2

Title: Magnet movable electromagnetic actuator

Description:
TECHNICAL FIELD 
     The present invention relates to a magnet movable electromagnetic actuator for moving and positioning an object with satisfactory responsivity. 
     PRIOR ART 
     Conventionally, an electromagnetic solenoid (actuator) in which voltage is applied to an exciting coil to apply a linear motion to a movable core by a magnetic force is well known as a reciprocation apparatus for magnetically moving an object. Although a structure of this electromagnetic solenoid is simple, the electromagnetic solenoid includes a core inside the coil. Therefore, it is difficult to improve electrical responsivity. Moreover, because thrust cannot be generated when a current is not passed, uses of the electromagnetic solenoid are limited. 
     To cope with these problems, large voltage is applied on startup or positioning in non-energization is carried out by using a spring. Therefore, complication of the structure and increase in the number of parts are inevitable. 
     DISCLOSURE OF THE INVENTION 
     It is an object of the present invention to provide a magnet movable electromagnetic actuator for generating steady-state thrust in a short time with satisfactory responsivity without applying large voltage on startup unlike the prior-art electromagnetic solenoid. 
     It is another object of the invention to provide a magnet movable electromagnetic actuator in which a movable member can be easily retained in non-energization. 
     It is yet another object of the invention to provide a small-sized and inexpensive magnet movable electromagnetic actuator including the small number of parts, the electromagnetic actuator showing the above-described features by a simple structure in which a cylindrical permanent magnet polarized in a radial direction is used. 
     To achieve the above object, a first electromagnetic actuator of the invention comprises: an annular exciting coil; a main yoke surrounding a periphery of the exciting coil and having at a portion of the main yoke a pair of polar teeth positioned to face each other at axial opposite end portions of a central hole of the exciting coil; and a cylindrical permanent magnet disposed in the central hole of the exciting coil to be movable in an axial direction of the central hole and polarized into a north pole and a south pole in a radial direction. 
     A second magnet movable electromagnetic actuator of the invention comprises: an annular exciting coil; a main yoke surrounding a periphery of the exciting coil and having at a portion of the main yoke a pair of polar teeth positioned to face each other at axial opposite end portions of an outer periphery of the exciting coil; and a cylindrical permanent magnet disposed on an outer peripheral side of the exciting coil to be movable in an axial direction of the coil and polarized into a north pole and a south pole in a radial direction. 
     In the first and second magnetic movable electromagnetic actuators having the above structures, if the exciting coil is energized, the one polar tooth of the main yoke becomes the north pole while the other polar tooth becomes the south pole according to a direction of the current. If the magnetic poles generated in these polar teeth and a magnetic pole of the permanent magnet on a side facing the polar teeth are different from each other, an attracting force acts between them. If they are the same as each other, repulsion acts between them. Therefore, these forces become axial thrust acting on the permanent magnet and the permanent magnet moves in the axial direction in the central hole of the coil or outside the coil. If the exciting coil is energized in a reverse direction, the magnetic poles, i.e., the north pole and the south pole generated in both the polar teeth of the main yoke are reverse to the above-described case. As a result, the thrust acting on the permanent magnet is also in a reversed direction and the permanent magnet moves in a reverse direction. 
     As described above, according to the invention, it is advantageously possible to generate steady-state thrust in a short time with satisfactory responsivity without applying large voltage on startup unlike the prior-art electromagnetic solenoid. 
     In the invention, a cylindrical back yoke positioned coaxially with the cylindrical permanent magnet may be provided on an opposite side to the exciting coil through the permanent magnet, i.e., inside the permanent magnet in the first electromagnetic actuator and outside the permanent magnet in the second electromagnetic actuator. With this structure, because a magnetic path extending from the one polar tooth through the permanent magnet and the back yoke to reach the other polar tooth can be formed, it is possible to reduce a magnetic reluctance and to further increase thrust and the magnetic adsorbing force of the permanent magnet. 
     If the back yoke is formed to have such a thickness as to be magnetically saturated by a magnetomotive force of the permanent magnet, the permanent magnet can be retained in a neutral position by a magnetic force when the exciting coil is not energized. If the back yoke is formed to have such a thickness as not to be magnetically saturated by a magnetomotive force of the permanent magnet, the permanent magnet can be retained in two positions, i.e., a forward movement end or a rearward movement end by a magnetic force when the exciting coil is not energized. 
     According to the invention, as a third electromagnetic actuator, there is provided a magnet movable electromagnetic actuator comprising: an annular exciting coil; an annular main yoke surrounding a periphery of the exciting coil and having at a portion of the main yoke a pair of polar teeth positioned to face each other at axial opposite end portions of a central hole of the exciting coil; a cover and a cap respectively mounted to axial opposite end portions of the main yoke to form a casing with the main yoke; a magnet chamber formed inside the casing and having an outer periphery surrounded by the exciting coil and the pair of polar teeth; a permanent magnet formed in a cylindrical shape, polarized into a north pole and a south pole in a radial direction, and disposed in the magnet chamber inside the exciting coil and the polar teeth to be movable in an axial direction of the casing; a magnet holder for holding the per manent magnet and movable with the permanent magnet; and an output shaft passing through a central portion of the magnet chamber to slide in the axial direction of the casing and connected to the magnet holder. 
     The cylindrical back yoke may be mounted in a fixed manner to the casing to be positioned concentrically with the permanent magnet inside the permanent magnet. 
     The magnet holder may be repulsed by a spring in a returning direction. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a sectional view of a structure of a first magnet movable electromagnetic actuator according to the present invention in terms of a principle. 
     FIG. 2 is a sectional view of a structure of a second magnet movable electromagnetic actuator according to the invention in terms of a principle. 
     FIG. 3 is a sectional view for explaining a switching operation with regard to an example of the first electromagnetic actuator. 
     FIG. 4 is a sectional view for explaining a switching operation with regard to another example of the first electromagnetic actuator. 
     FIG. 5 is a diagram showing an operating property in non-energization according to presence or absence of the back yoke. 
     FIG. 6 is a diagram showing a relationship between a space between polar teeth and thrust in non-energization. 
     FIG. 7 is a diagram showing an operating property when the thrust in non-energization is minimized throughout a stroke. 
     FIG. 8 is a sectional view showing an embodiment in which the electromagnetic actuator in FIG. 1 is embodied and showing different operating states in upper and lower half portions. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows a structure of a first magnet movable electromagnetic actuator according to the present invention in terms of a principle. The first electromagnetic actuator  1 A includes an annular exciting coil  10 , an annular main yoke  12  surrounding a periphery of the exciting coil  10  and having at a portion of the main yoke  12  cylindrical polar teeth  12   a  and  12   b  positioned to face each other at opposite end portions of a central hole  11  of the exciting coil  10 , a cylindrical permanent magnet  13  disposed in the central hole  11  of the exciting coil to be movable in an axial direction of the hole and polarized into the north pole and the south pole in a radial direction, and a cylindrical back yoke  14  inside the permanent magnet  13 . The main yoke  12  and the back yoke  14  are respectively made of magnetic material. 
     A preferable length of the cylindrical permanent magnet  13  is a length with which a gap between both the polar teeth  12   a  and  12   b  is covered and especially such a length that one end of the permanent magnet  13  reaches one movement end in the central hole  11  of the exciting coil when the other end of the permanent magnet  13  partially overlaps the opposite polar tooth or is positioned close to the polar tooth. The back yoke  14  is not necessarily provided. If the back yoke  14  is provided, the back yoke  14  preferably has a length with which most of the permanent magnet  13  is covered wherever the permanent magnet  13  is in movement. 
     On the other hand, a second magnet movable electromagnetic actuator  1 B of the invention shown in FIG. 2 includes an annular exciting coil  20 , an annular main yoke  22  surrounding a periphery of the exciting coil  20  and having at a portion of the main yoke  22  cylindrical polar teeth  22   a  and  22   b  positioned to face each other at axial opposite end portions of an outer periphery of the exciting coil  20 , a cylindrical permanent magnet  23  disposed outside the exciting coil  20  to be movable in an axial direction of the coil and polarized into the north pole and the south pole in a radial direction, and a cylindrical back yoke  24  disposed outside the permanent magnet  23 . Lengths of the permanent magnet  23  and the back yoke  24  and the like are similar to those in the above-described first electromagnetic actuator  1 A. 
     Because the second electromagnetic actuator  1 B is different from the first electromagnetic actuator  1 A shown in FIG. 1 only in disposition of the exciting coil, the permanent magnet, and the back yoke and there is substantially no difference between the actuators  1 A and  1 B in terms of functions, only operation of the first electromagnetic actuator  1 A in FIG. 1 will be described below and description of operation of the second electromagnetic actuator  1 B will be omitted. 
     In the first electromagnetic actuator  1 A having the above structure, as shown in FIG. 1, the permanent magnet  13  is polarized in the radial direction such that an outer side of the permanent magnet  13  is the south pole and an inner side is the north pole. If the exciting coil  10  is energized in a direction shown with symbols in FIG. 1 in this state, the one polar tooth  12   a  of the main yoke  12  becomes the north pole and the other polar tooth  12   b  becomes the south pole due to this direction of a current. Therefore, an attracting force acts between the north pole generated in the polar tooth  12   a  and the south pole on an outer face side of the permanent magnet  13  facing the north pole and repulsion acts between the south pole generated in the polar tooth  12   b  and the south pole of the permanent magnet. Therefore, these forces generate axial thrust in the permanent magnet  13  and the permanent magnet  13  moves axially (rightward in FIG. 1) in the central hole  11  of the coil by the thrust. 
     If the exciting coil  10  is energized in a reverse direction, magnetic poles of the north pole and the south pole generated in both the polar teeth  12   a  and  12   b  of the main yoke  12  are reverse to the above-described case. As a result, the direction of the thrust generated in the permanent magnet  13  is also reversed (leftward in FIG. 1) and the permanent magnet  13  moves in a direction reverse to the above direction. 
     Here, if the back yoke  14  is provided, because a magnetic path extending from the polar tooth on the north polar side of the main yoke  12  through the permanent magnet  13  to the back yoke  14  and passing through an outside space to reach the other polar tooth is formed, a magnetic reluctance and the like of the magnetic path are adjusted by a magnetic property, a form of disposition, and the like of the back yoke  14  to thereby adjust the thrust and the magnetic adsorbing force of the permanent magnet  13 . 
     On the other hand, a stop position of the permanent magnet  13  when the exiting coil  10  is not energized changes depending on presence or absence of the back yoke  14 , a magnetic saturation property of the back yoke  14 , and the like. This will be described below. 
     First, if the back yoke  14  is not disposed or if the back yoke  14  is disposed but is thin-walled to such a degree that the back yoke  14  is magnetically saturated by a magnetomotive force of the permanent magnet  13 , the permanent magnet  13  is retained in a neutral position when the exciting coil  10  is not energized. In other words, if energization of the exciting coil  10  is interrupted in a state in which the exciting coil  10  has been energized and the permanent magnet  13  has been moved forward to a stroke end on the polar tooth  12   a  side, because the magnetic reluctance of a magnetic path Sa on the polar tooth  12   a  side is smaller than the magnetic reluctance of a magnetic path Sb on the polar tooth  12   b  side at this forward movement end as shown in FIG. 3, magnetic flux Φ b passing through the magnetic path Sb is more than magnetic flux Φ a passing through the magnetic path Sa in magnetic flux generated by the magnetomotive force of the permanent magnet  13 . As a result, the permanent magnet  13  is attracted and moves toward the polar tooth  12   b . Then, when the permanent magnet  13  moves to the neutral position, because the magnetic reluctances in the magnetic paths Sa and Sb become equal to each other and a balance is achieved between the magnetic fluxes Φ a and Φ b, the permanent magnet  13  stops in this neutral position. On the other hand, if energization of the exciting coil  10  is interrupted in a state in which the permanent magnet  13  has been moved to a rearward movement stroke end on the polar tooth  12   b  side, the permanent magnet  3  is attracted and moves toward the polar tooth  12   a  in a way reverse to the above case. When the permanent magnet  13  moves to the neutral position, the permanent magnet  13  stops and is retained in the position. 
     Therefore, if an object to be driven is connected to the permanent magnet  13  and the exciting coil  10  is energized in a normal or reverse direction to move the permanent magnet  13  forward or rearward and then the energization is canceled, the object can be positioned in the neutral position of the permanent magnet  13 . This structure is equivalent to provision of mechanical return springs on opposite sides of the permanent magnet  13 . Therefore, the structure is efficient when it is used to continuously drive the permanent magnet  13  for reciprocation because switching of the permanent magnet  13  is promoted by a resonant phenomenon. 
     Next, if the back yoke  14  is thick to such a degree that the back yoke  14  is not magnetically saturated by the magnetomotive force of the permanent magnet  13 , the permanent magnet  13  is retained in two positions, i.e., the forward movement end or the rearward movement end when the exciting coil  10  is not energized. In other words, if energization of the exciting coil  10  is interrupted in a state in which the exciting coil  10  has been energized and the permanent magnet  13  has been moved forward to a stroke end on the polar tooth  12   a  side, a magnetic flux generated from the permanent magnet  13  is divided into a magnetic flux Φ a extending from the north pole through the back yoke  14  and the polar tooth  12   a  to the south pole, a magnetic flux Φ b extending from the north pole through the back yoke  14  and the polar tooth  12   b  to the south pole, and a magnetic flux Φ c extending from the north pole through the back yoke  14 , the polar tooth  12   b , the main yoke  12 , and the polar tooth  12   a  to the south pole as shown in FIG.  4 . Therefore, the magnetic flux passing through the polar tooth  12   a  and entering the south polar is Φ a+Φ c which is more than Φ b passing through the polar tooth  12   b  and entering the south pole. As a result, the permanent magnet  13  is retained at the forward movement end while being attracted toward the polar tooth  12   a . This is also true for a case of interrupting energization of the exciting coil  10  in a state in which the permanent magnet  13  has been moved to the stroke end on the polar tooth  12   b  side. In this case, the permanent magnet  13  is retained at the rearward movement end while being attracted toward the polar tooth  12   b.    
     Therefore, if an object to be driven is connected to the permanent magnet  13  and the exciting coil  10  is energized in a normal or reverse direction to move the permanent magnet  13  forward or rearward and then the energization is canceled, the object can be reliably positioned in two positions, i.e., the forward movement end or the rearward movement end. 
     FIG. 5 shows a relationship between an operating position of the permanent magnet  13  and magnitude and a direction of the thrust generated by the magnetomotive force of the permanent magnet  13  itself. In FIG. 5, a graphm is a case in which the back yoke  14  is not provided or the back yoke  14  which is thin-walled to such a degree as to be magnetically saturated by the magnetomotive force of the permanent magnet  13  is provided and a graph n is a case in which the back yoke  14  which is thick to such a degree as not to be magnetically saturated by the magnetomotive force of the permanent magnet  13  is provided. 
     The graph m shows a fact that thrust in a minus direction (rearward direction) acts on the permanent magnet  13  when the permanent magnet  13  is at the forward movement end as shown in FIG. 3 while thrust in a plus direction (forward direction) acts on the permanent magnet  13  when the permanent magnet  13  is at the rearward movement end. Therefore, it is found that the permanent magnet  13  moves to the neutral position and is retained in the neutral position whichever of the forward movement end and the rearward movement end the permanent magnet  13  is at 
     The graph n shows a fact that thrust in the plus direction (forward direction) acts on the permanent magnet  13  when the permanent magnet  13  is at the forward movement end as shown in FIG. 4 while thrust in the minus direction (rearward direction) acts on the permanent magnet  13  when the permanent magnet  13  is at the rearward movement end. Therefore, it is found that the permanent magnet  13  is retained in the respective positions. In this case, the thrust does not similarly act on the permanent magnet when the permanent magnet is in the neutral position. 
     As described above, the magnitude of the thrust acting on the permanent magnet  13  when the exciting coil  10  is not energized can be adjusted freely by changing material and a thickness of the back yoke  14 , a space between the pair of polar teeth  12   a  and  12   b , the length of the permanent magnet  13 , and the like. As an example of this, FIG. 6 shows an influence of the space between the pair of polar teeth on the thrust property. From FIG. 6, it is found that the thrust reduces as the space between the polar teeth reduces. It is also possible to minimize the thrust acting on the permanent magnet throughout the stroke of the permanent magnet as shown in FIG.  7 . In this case, it is possible to stop and retain the permanent magnet and the object and the like retained on the permanent magnet in an arbitrary position. Because the electromagnetic actuator having such a feature has good controllability, the actuator can be applied to a motor for controlling and the like. 
     FIG. 8 shows an embodiment in which the first electromagnetic actuator  1 A shown in FIG. 1 is embodied. 
     This electromagnetic actuator  1 C includes an annular exciting coil  30  formed by providing winding  32  to a bobbin  31  and an annular main yoke  33  surrounding a periphery of the exciting coil  30 . This main yoke  33  is formed of an outer yoke  34  in which an outer tube portion  34   a  also functioning as an outer wall of a casing and one polar tooth  34   b  are integrated with each other and a bottom yoke  35  in a L-shaped sectional shape having the other polar tooth  35   a . The outer yoke  34  and the bottom yoke  35  are mounted to each other such that the polar teeth  35   a  and  34   b  in the pair are positioned at opposite end portions of a central hole of the exciting coil  30  to face each other and the outer yoke  34  and the bottom yoke  35  are connected to each other by means such as screwing. 
     A cover  37  is fixed to axial one end side of the main yoke  33  through a screw  38  and a cap  39  is fixed to the other end side of the main yoke  33  through a C-type snap ring  40 . The casing  41  is formed of the main yoke  33 , the cover  37 , and the cap  39 . In this casing  41 , a magnet chamber  42  an outer periphery of which is surrounded by the exciting coil  30  and the pair of polar teeth  35   a  and  34   b  is formed. In this magnet chamber  42 , a hollow output shaft  45  which passes through a center of the magnet chamber  42  and can slide in an axial direction is provided, a cylindrical magnet holder  46  is mounted around the shaft  45  to move with the shaft  45 , and a cylindrical permanent magnet  47  is mounted to an outer peripheral face of the magnet holder  46  to face the exciting coil  30  and the pair of polar teeth  35   a  and  34   b  inside the coil  30  and the polar teeth  35   a  and  34   b.    
     The permanent magnet  47  is polarized into the north pole and the south pole in a radial direction and has such a length that a gap between both the polar teeth  35   a  and  34   b  of the main yoke  33  is covered with the permanent magnet  47  and that one end of the permanent magnet  47  reaches a movement end in the central hole of the exciting coil  30  when the other end of the permanent magnet  47  partially overlaps the opposite polar tooth or is positioned close to the polar tooth. 
     In the permanent magnet  47 , as shown by a chain line in FIG. 8, a cylindrical back yoke  48  can be disposed coaxially with the permanent magnet  47  in a fixed manner by mounting the back yoke  48  to the cap  39 . If the back yoke  48  is provided, the back yoke  48  preferably has such a length as to face the permanent magnet  47  wherever the permanent magnet  47  is in movement. As described above, the back yoke  48  is not necessarily provided. 
     In FIG. 8, a reference numeral  50  designates a bearing provided to the cover  37  to support the shaft  45  for sliding,  51  and  52  designate dampers provided to the cover  37  and the cap  39  to stop the magnet holder  46  at stroke ends in a cushioned manner,  53  designates a screw hole for mounting the electromagnetic actuator to a predetermined place, and  55  designates a return spring for returning the shaft  45  to a return position in a non-energized state. 
     The electromagnetic actuator  1 C having the above structure is used for carrying the object and the like by connecting the object to the shaft  45 . In an operating state in which the shaft  45  is positioned at the left end as shown in a lower half of FIG. 8, if the exciting coil  30  is energized and such a current that the one polar tooth  35   a  becomes the north pole and that the other polar tooth  34   b  becomes the south pole is passed, an attracting force acts between the north pole generated in the polar tooth  35   a  and the south pole on the outer face side of the permanent magnet  47  and repulsion acts between the south pole generated in the polar tooth  34   b  and the south pole of the permanent magnet. Therefore, these forces act on the permanent magnet  47  as axial thrust and the permanent magnet  47  moves forward with the shaft  45  to the right end shown in an upper half of FIG.  8 . 
     If a current in a reverse direction is passed through the exciting coil  30  when the permanent magnet  47  is positioned at the forward movement end, magnetic poles reverse to the above-described case are generated in both the polar teeth  35   a  and  34   b . Therefore, the permanent magnet  47  and the shaft  45  quickly move rearward to the return ends by the resultant of the thrust due to the magnetic force and a repulsing force of the return spring  55 . Even if energization of the exciting coil  30  is cancelled at the forward movement end, the permanent magnet  47  and the shaft  45  move to the rearward movement end shown in the lower half portion of FIG. 8 due to the repulsing force of the spring  55 . 
     As described above, if the return spring  55  is provided, the permanent magnet  47  can be switched to two positions, i.e., the forward movement end and the rearward movement end. If the spring  55  is not provided, different switching operations, i.e., passing a current in a reverse direction through the exciting coil  30  or interrupting energization at each the stroke end are carried out according to conditions such as presence or absence of the back yoke  48  and if the back yoke  48  is magnetically saturated by the magnetomotive force of the permanent magnet  47 . Because these switching operations are substantially similar to the case described in regard to the first electromagnetic actuator  1 A, descriptions of them are omitted here. 
     Because the radially polarized permanent magnet  47  is used in the electromagnetic actuator  1 C, a lateral load acting on a movable portion including the shaft  45 , the magnet holder  46 , and the movable magnet  47  is small. Therefore, the bearing  50  for supporting the shaft  45  may be a simple one and reduction of cost and improvement of durability due to the small lateral load are expected. 
     Because the number of members made of iron and provided in the exciting coil  30  can be reduced in the electromagnetic actuator  1 C, an inductance of the exciting coil can be reduced. Therefore, rising of a current is satisfactory when step voltage is applied to the coil, electrical responsivity can be improved, and as a result, steady-state thrust can be generated in a short time (about a few ms). 
     According to the electromagnetic actuator of the invention described above in detail, by simple means in which the cylindrical permanent magnet polarized in the radial direction is used, it is possible to generate steady-state thrust in a short time with satisfactory responsivity without applying large voltage on startup unlike the prior-art electromagnetic solenoid. Furthermore, by the above structure in which the permanent magnet is used, it is possible to reliably retain the object in the desired operating position in non-energization, the number of parts can be reduced to thereby reduce cost, and durability can be improved. 
     According to the electromagnetic actuator of the invention, based on the above-described structure, it is possible to generate greater thrust than the prior-art electromagnetic solenoid of the same outer dimensions. With the same outer dimensions, it is possible to generate greater thrust. Furthermore, it is possible to reduce the outer dimensions to generate the same degree of thrust.