Patent Document

BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a linear actuator.  
         [0003]     2. Description of the Related Art  
         [0004]      FIG. 3  is a schematic cross sectional view of a conventional linear actuator. The linear actuator of  FIG. 3  comprises a stator assembly  10 , a rotor assembly  20 , a rear end cap  30 , an output shaft  40 , and a front end cap  50 .  
         [0005]     The stator assembly  10  is composed of two stator units, one of which is structured such that two stator yokes  13   a ,  13   b  shaped into a ring oppose each other so as to sandwich therebetween a bobbin  12  having a winding  11  provided therearound, and the other of which is structured such that two stator yokes  16   a ,  16   b  shaped into a ring oppose each other so as to sandwich therebetween a bobbin  15  having a winding  14  provided therearound. The two stator units structured as above are coaxially stacked on each other forming a hollow-cylinder looking like a doughnut. The stator yokes  13   a ,  13   b  each have an array of pole teeth and are coupled to each other with their respective pole teeth intermeshing with each other. In the same way, the stator yokes  16   a ,  16   b  each have an array of pole teeth and are coupled to each other with their respective pole teeth intermeshing with each other. The pole teeth constitute the inner circumference of the stator assembly  10 . The windings  11 ,  14  are responsible for exciting the pole teeth. The stator yokes  13   a ,  13   b , and  16   a ,  16   b , and the bobbins  12 ,  15  are integrally fixed together by resin injection-molding.  
         [0006]     The rotor assembly  20  is housed in the stator assembly  10 . The rotor assembly  20  is composed of a rotor magnet  21 , and a resin segment  22 , and is shaped into a hollow-cylinder. The rotor magnet  21  has a plurality of magnetic poles, and constitutes the outer circumference of the rotor assembly  20  thus opposing the pole teeth of the stator assembly  10  with a  5  predetermined gap therebetween. The resin segment  22  is shaped tube-like, and disposed inside the rotor magnet  21 , and a female screw  23  is fixedly attached inside the resin segment  22 .  
         [0007]     The rear end cap  30  is disposed at the rear end face of the stator assembly  10  so as to cover the hollow of the stator assembly  10 . The rear end  10  cap  30  has a cavity  31  at its inner side facing the rotor assembly  20  and has a rear ball bearing  32  fitted into a circular recess formed coaxially with the cavity  31 . The ball bearing  32  supports rotatably the rear end portion of the rotor assembly  20 .  
         [0008]     The output shaft  40  is shaped round in its cross section, has a male  15  screw  41  formed at its rear end portion, has a stopper pin  24  disposed at its frontward portion, and has its rearward portion inserted through the rotor assembly  20 . The male screw  41  engages threadedly with the female screw  23  of the rotor assembly  20 , whereby the output shaft  40  travels in the axial direction linearly without turning or with less than one turn when the rotor  20  assembly  20  rotates. In this connection, the stopper pin  24  prohibits or restricts rotation of the output shaft  40  within one turn.  
         [0009]     The front end cap  50  is attached to the front end of the stator assembly  10  so as to cover the hollow of the stator assembly  10  housing the rotor assembly  20 . The front end cap  50  has a round center hole  51 , and the output  25  shaft  40  is inserted through the center hole  51  so as to have its front end portion sticking out from the front end cap  50 . The front end cap  50  has a circular recess  52  coaxial with the center hole  51 , and has a groove  53  extending parallel to the length of the output shaft  40 . The recess  52  receives a front ball bearing  54  fitted thereinto, which rotatably supports the front end portion of the rotor assembly  20 . The aforementioned stopper pin  24  is lodged in and guided by the groove  53  so as to prohibit or restrict rotation of the output shaft  40 , and to restrict the frontward travel amount of the output shaft  40 , and, in some cases, the rearward travel amount thereof as well.  
         [0010]     In the above described linear actuator of  FIG. 3 , when current is caused to flow in the windings  11 ,  14 , the pole teeth of the stator assembly  10  are excited thereby rotating the rotor assembly  20  due to magnetism of the rotor magnet  21  having magnetic poles. When the rotor assembly  20  rotates, the rotational movement of the rotor assembly  20  is converted into linear movement of the output shaft  40  by means of the female screw  23  of the rotor assembly  20  threadedly engaging with the male screw  41  of the output shaft  40 . The output shaft  40  travels in a reciprocating manner within the length of the groove  53  in response to the rotational direction of the rotor assembly  20 . The output shaft  40  stops and reverses its movement when the rotor assembly  20  stops and reverses its rotation.  
         [0011]     The linear actuator above described may encounter troubles incurred when the output shaft  40  stops its movement, that is, when the rotor assembly  20  is caused to stop its rotation. The male screw  41  of the output shaft  40  has a predetermined length defined by its proximal end portion  41   a , and in some linear actuators in which the rearward travel amount of the output shaft  40  is restricted by the proximal end portion  41   a , the rotor assembly  20  is caused to stop its rotation when the proximal end portions  41   a  of the male screw  41  touches the female screw  23 . In this case, the thread of the female screw  23  may possibly bite into the proximal end portion  41   a  depending on the magnitude of inertial force of the rotation of the rotor assembly  20  at the time of touching, which, depending on the degree of the biting, can cause a critical problem that the rotor assembly  20  will not start off its rotation in the reversed direction therefore failing to move the output shaft  40 . On the other hand, if the length of the male screw  41  is increased to keep off the screw biting problem, the output shaft  40  is caused to stop its rearward movement when the stopper pin  24  touches the front end of the rotor assembly  20  or the inner ring of the front ball bearing  54 . In this case, since the touching area of the stopper pin  24  therewith is positioned outside the pitch diameter of the female and male screws  23 ,  41 , an extra torque is required for the rotor assembly  20  to duly start its rotation in the reversed direction thus, in the worst case, making it possibly happen that the rotor assembly  20  will not start off its rotation.  
         [0012]     A linear actuator to address the above problems is disclosed in Unexamined Japanese Patent Application KOKAI Publication No. H06-335228 and will be explained below based on reference numbers in  FIG. 3 . The linear actuator disclosed therein has a pointed stopper disposed at the center of the cavity  31  of the rear end cap  30 . In the linear actuator, the output shaft  40  is caused to stop its rearward movement when the rear end surface of the output shaft  40  touches the pointed stopper. This structure eliminates the two problems described above, specifically one is that the thread of the female screw  23  bites into the proximal end portion  41   a  of the male screw  41 , and the other is that an increased torque is required for the rotor assembly  20  to duly start off its rotation in the reversed direction. However, since the stopper has a pointed head, the head of the stopper can possibly be readily worn away or damaged due to the rear end surface of the output shaft  40  repeatedly touching the head. If the head of the stopper is worn away or damaged, the output shaft  40  cannot be stopped precisely at a place originally determined thus failing to perform an accurate control.  
       SUMMARY OF THE INVENTION  
       [0013]     The present invention has been made in light of the above problems, and it is an object of the present invention to provide a linear actuator, in which the rearward movement of an output shaft can be surely and accurately stopped without undergoing malfunction.  
         [0014]     In order to achieve the object, according to one aspect of the present invention, a linear actuator comprises a stator assembly, a rotor assembly, an output shaft, a stopper pin, and a stopper member, wherein: the stator assembly is shaped into a cylinder defining a hollow, and includes a plurality of windings, and a plurality of stator yokes each having an array of pole teeth which constitute an inner circumference of the stator assembly, and which are excited by the windings; the rotor assembly is shaped into a hollow-cylinder, is rotatably housed in the hollow of the stator assembly, and includes a ring-shaped magnet which has a plurality of magnetic poles, constitutes an outer circumference of the rotor assembly, and which opposes the pole teeth of the stator assembly with a predetermined distance therebetween, and a female screw disposed at an inner circumference of the rotor assembly; the output shaft is inserted through the rotor assembly, and has a male screw which is formed at a rearward portion thereof, and which engages threadedly with the female screw of the rotor assembly; the stopper pin is fixedly disposed at a frontward portion of the output shaft, and controls axially the mode and amount of movement of the output shaft initiated by rotation of the rotor assembly; and the stopper member is disposed fixedly with respect to the stator assembly, and stops the axial movement of the output shaft without making it happen that the stopper pin which moves with the output shaft touches the rotor assembly.  
         [0015]     Consequently, the female screw does not touch the proximal end portion of the male screw therefore eliminating the aforementioned screw biting problem, and the stopper pin does not touch any portion of the rotor assembly therefore not requiring any extra torque for restarting the rotation of the rotor assembly. Also, the stopper member does not have any pointed portion therefore exhibiting little wear and keeping from damage, eventually resulting in a stable and accurate positioning control of the output shaft. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]     The above object and other advantages of the present invention will become more apparent by describing in detail the preferred embodiments of the present invention with reference to the attached drawings in which:  
         [0017]      FIG. 1  is a schematic cross sectional view of a linear actuator according to a first embodiment of the present invention;  
         [0018]      FIG. 2  is a schematic cross sectional view of a linear actuator according to a second embodiment of the present invention; and  
         [0019]      FIG. 3  is a schematic cross sectional view of a conventional linear actuator. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     First Embodiment  
       [0020]     A linear actuator according to a first embodiment of the present invention will hereinafter be described with reference to  FIG. 1 . A linear actuator of  FIG. 1  generally comprises a stator assembly  60 , a rotor assembly  70 , an output shaft  80 , a rear end cap  90 ; a front end protrusion  100 ; and a front end cap  110 .  
         [0021]     The stator assembly  60  is composed of two stator units  63 ,  66 , one  63  of which is structured such that two stator yokes  63   a ,  63   b  shaped into a ring oppose each other so as to sandwich therebetween a bobbin  62  having a winding  61  provided therearound, and the other  66  of which is structured such that two stator yokes  66   a ,  66   b  shaped into a ring oppose each other so as to sandwich therebetween a bobbin  65  having a winding  64  provided therearound, and the two stator units  63 ,  66  structured as above are coaxially stacked on each other forming a hollow-cylinder looking like a doughnut. The two stator yokes  63   a ,  63   b  of the stator unit  63  each have an array of pole teeth and are coupled to each other with their respective pole teeth intermeshing with each other. In the same way, the two stator yokes  66   a ,  66   b  of the stator unit  66  each have an array of pole teeth and are coupled to each other with their respective pole teeth intermeshing with each other. And, respective pole teeth of the two stator units  63 ,  66  are appropriately shifted from each other for two-phase driving. The pole teeth constitute the inner circumference of the stator assembly  60 . The windings  61 ,  64  are responsible for exciting the respective pole teeth of the stator units  63 ,  66 . The stator yokes  63   a ,  63   b  and  66   a ,  66   b , and bobbins  62 ,  65  with the windings  61 ,  64  are integrally fixed together by means of a yoke support member  67  which is formed of resin by injection-molding.  
         [0022]     The rotor assembly  70  is housed in the stator assembly  60 . The rotor assembly  70  is composed of a rotor magnet  71 , a resin segment  72 , and a female screw  73 , and is shaped into a hollow-cylinder. The rotor magnet  71  is shaped in a ring, has a plurality of magnetic poles, and constitutes the outer circumference of the rotor assembly thus opposing the pole teeth of the stator assembly  60  with a predetermined gap therebetween. The resin segment  72  is shaped tube-like, and disposed inside the rotor magnet  71 , and the female screw  73  is attached inside the resin segment  72  by means of resin injection-molding.  
         [0023]     The rear end cap  90  is positioned at a rear end face  60   b  of the stator assembly  60  and covers the hollow of the stator assembly  60 . The rear end cap  90  is formed of resin simultaneously and integrally with the yoke support member  67  by resin injection-molding. The rear end cap  90  has a cavity  91  at its inner side facing the rotor assembly  70 . The cavity  91  constitutes a sleeve bearing and supports rotatably a rear end portion  70   b  of the rotor assembly  70 . The cavity  91  is not necessarily configured as shown in  FIG. 1  but may alternatively be configured so as to receive a ball bearing fitted thereinto for rotatably supporting the rotor assembly  70 .  
         [0024]     The output shaft  80  is shaped round in its cross section, has a male screw  81  formed at a portion toward a rear end  80   b  thereof, has a stopper pin  82  disposed at a portion toward a front end  80   a , and has its rearward portion inserted through the rotor assembly  70 . The male screw  81  engages threadedly with the female screw  73  of the rotor assembly  70 , whereby the output shaft  80  travels in the axial direction linearly without turning or with less than one turn when the rotor assembly  70  rotates. In this connection, the stopper pin  82  prohibits or restricts rotation of the output shaft  80  within one turn.  
         [0025]     The front end protrusion  100  is shaped into a ring and positioned at a front end face  60   a  of the stator assembly  60 . The front end protrusion  100  is formed of resin simultaneously and integrally with the yoke support member  67  and also with the rear end cap  90  by resin injection-molding. The front end protrusion  100  has an inner diameter larger than the inner diameter of the stator assembly  60 , and has a front ball bearing  101  fitted thereinto. A portion of the resin segment  72  of the rotor assembly  70  is fixedly fitted into the inner ring of the front ball bearing  101 , whereby the front end portion of the rotor assembly  70  is rotatably supported. The front end protrusion  100  has a stopper member  103  attached to its frontward portion. The stopper member  103  is shaped into a disk, has a center hole for inserting the output shaft  80 , and has a circular recess formed at its inner side facing the front ball bearing  101 . The recess of the stopper member  103  defines a diameter larger than an outer diameter of the inner ring of the front ball bearing  101  thereby forming a clearance from the inner ring so as not to block the rotation of the rotor assembly  70 . In the structure described above, the output shaft  80  is caused to stop its rearward movement when the stopper pin  82  of the output shaft  80  touches the stopper member  103 .  
         [0026]     The front end cap  110  is attached by means of a metal fitting  114  to the front end face  60   a  of the stator assembly  60  so as to cover the hollow of the stator assembly  60  housing the rotor assembly  70 . The front end cap  110  has a round center hole  111 , and the output shaft  80  is inserted through the center hole  111  so as to have its front end portion (toward the front end  80   a ) sticking out from the front end cap  110 . The front end cap  110  has a groove  112  extending parallel to the length of the output shaft  80 . The aforementioned stopper pin  82  is lodged in and guided by the groove  112  so as to prohibit or restrict rotation of the output shaft  80  and to restrict the travel distance of the output shaft  80 .  
         [0027]     The actuation of the linear actuator of  FIG. 1  will be discussed. Current is caused to flow in the windings  61 ,  64  so as to excite the respective pole teeth of the stator units  63 ,  66 , whereby the rotor assembly  70  is caused to rotate due to magnetism from the rotor magnet  71 . When the rotor assembly  70  rotates, the output shaft  80  is caused to move in the axial direction by means of the female screw  73  of the rotor assembly  70  threadedly engaging with the male screw  81  of the output shaft  80 . In this connection, the stopper pin  82  moves along the groove  112  while prohibiting or restricting the rotation of the output shaft  80 . The stopper pin  82  moves rearward together with the output shaft  80  moving rearward, and the output shaft  80  stops its movement when the stopper pin  82  touches the stopper member  103 .  
         [0028]     As described above, the linear actuator according to the first embodiment of the present invention includes the stopper member  103 , and the output shaft  80  is caused to stop its rearward movement when the stopper pin  82  touches the stopper member  103 . Thus, the female screw  73  does not touch the proximal end portion of the male screw  81  therefore eliminating the aforementioned screw biting problem, and the stopper pin  82  does not touch any portion of the rotor assembly  70  or the inner ring of the front ball bearing  101  therefore not requiring any extra torque for restarting the rotation of the rotor assembly  70 . Also, the stopper member  103  does not have any pointed portion therefore exhibiting little wear and keeping off damage, eventually resulting in a stable and accurate positioning control of the output shaft  80 .  
       Second Embodiment  
       [0029]     A linear actuator according to a second embodiment of the present invention will be described with reference to  FIG. 2 . In  FIG. 2 , like reference numerals refer to like elements in  FIG. 1 .  
         [0030]     The linear actuator according to the second embodiment differs from the first embodiment principally in bearing type, and in stopper member structure. Specifically, in the first embodiment, the front and rear end portions of the rotor assembly  70  are rotatably supported respectively by the ball bearing  101  and the cavity  91  constituting a sleeve bearing, and the stopper member  103  is attached to the front end protrusion  100 . On the other hand, in the second embodiment, the front and rear end portions of a rotor assembly are rotatably supported respectively by a sleeve bearing and a ball bearing (reversed compared with the first embodiment), and a stopper member is constituted by a portion of the sleeve bearing which rotatably supports the front end portion of the rotor assembly.  
         [0031]     The linear actuator according to the second embodiment shown in  FIG. 2  basically comprises a stator assembly  60 , a rotor assembly  70 , and an output shaft  80 , which are of the same structure as the first embodiment shown in  FIG. 1 .  
         [0032]     A rear end cap  120  is disposed at a rear end face  60   b  of the stator assembly  60  so as to cover the hollow of the stator assembly  60 . The rear end cap  120  is formed of resin integrally with a yoke support member  67  by injection-molding, and has a cavity  121  at its inner side facing the rotor assembly  70 . The cavity  121  has a circular recess formed coaxially therewith, and a rear ball bearing  122  to rotatably support a rear end  70   b  of the rotor assembly  70  is fitted into the recess. A front end cap  130  is attached at a front end face  60   a  of the stator assembly  60  so as to cover the hollow of the stator assembly  60  housing the rotor assembly  70 . The front end cap  130  defines a cavity at its inner side facing the rotor assembly  70 , and has a round center hole  131 . A sleeve bearing  140  is fitted into the cavity of the front end cap  130  and supports rotatably a front end portion  70   a  of the rotor assembly  70 , and the output shaft  80  is inserted through the center hole  131  so as to have its front end portion (toward a front end  80   a ) sticking out from the front end cap  130 . The output shaft  80  is movably inserted through the sleeve bearing  140 . The sleeve bearing  140  has a groove  141  formed at its inner circumference so as to extend parallel to the length of the output shaft  80 . A frontward end of the groove  141  is open, and the other end is blind so as to constitute a stopper member  142 . A stopper pin  82  is lodged in and guided by the groove  141  thereby controlling the movement of the output shaft  80 . A front plate  150  may be attached as required.  
         [0033]     The linear actuator structured above actuates basically in the same way as the linear actuator of the first embodiment. The output shaft  80  moves linearly when the rotor assembly  70  rotates. The stopper pin  82  fixedly disposed at the output shaft  80  also moves linearly along the groove  141  while prohibiting or restricting the rotation of the output shaft  80 . When the output shaft  80  moves rearward, the stopper pin  82  moves also rearward, and the output shaft  80  stops its movement upon the stopper pin  82  touching the stopper member  142 .  
         [0034]     As described above, the linear actuator according to the second embodiment of the present invention includes the stopper member  142 , and the rotor assembly  70  is caused to stop its linear movement when the stopper pin  82  touches the stopper member  142 . Thus, the female screw  73  does not touch the proximal end portion of the male screw  81  therefore eliminating the aforementioned screw biting problem, and the stopper pin  82  does not touch any portion of the rotor assembly  70  therefore not requiring any extra torque for duly restarting the rotation of the rotor assembly  70 . Also, the stopper member  142  does not have any pointed portion therefore exhibiting little wear and keeping from damage, eventually resulting in a stable and accurate positioning control of the output shaft  80 .  
         [0035]     The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes that come within the meaning and range of equivalency of the claims are to be embraced within their scope.  
         [0036]     This application is based on Japanese Patent Application No. 2003-50097 filed on Feb. 26, 2003 and including specification, claims, drawings and summary. The disclosure of the above Japanese Patent Application is incorporated herein by reference in its entirety.

Technology Category: 5