Patent Publication Number: US-7595571-B2

Title: High performance linear motor and magnet assembly therefor

Description:
REFERENCE TO RELATED APPLICATIONS 
   This is a divisional application of U.S. patent Ser. No. 11/553,209, filed Nov. 19, 2007, entitled HIGH PERFORMANCE LINEAR MOTOR AND MAGNET ASSEMBLY THEREFOR, which is a divisional application of U.S. patent application Ser. No. 11/151,152, filed Jun. 13, 2005, entitled HIGH PERFORMANCE LINEAR MOTOR AND MAGNET ASSEMBLY THEREFOR, which is a divisional application of U.S. patent application Ser. No. 10/889,384, filed Jul. 12, 2004, entitled HIGH PERFORMANCE LINEAR MOTOR AND MAGNET ASSEMBLY THEREFOR, which is a divisional application of U.S. patent application Ser. No. 10/369,161, filed Feb. 19, 2003, entitled HIGH PERFORMANCE LINEAR MOTOR AND MAGNET ASSEMBLY THEREFOR, which claims the benefit of U.S. Provisional Patent Application Ser. No. 60/358,654, filed on Feb. 21, 2002, and entitled HIGH PERFORMANCE LINEAR MOTOR MAGNET ASSEMBLY THEREFOR. The entireties of these applications are incorporated by reference herein. 

   BACKGROUND OF THE INVENTION 
   Linear motors are used in various types of systems, such as for positioning and moving applications, including machining and gantry type systems. The high performance systems often require moving elements subjected to high acceleration levels. In order to achieve such high acceleration, the linear motor must exert large forces upon the elements to be moved. 
   There are various configurations of linear motors, including flat motors, U-channel motors and tubular shaped motors. Different types of linear motors are also available, including brush, AC brushless, stepper, and induction motors. Common to most linear motors is a moving assembly, usually called a forcer or stage, which moves relative to a stationary platen (or path) according to magnetic fields generated by application of current through one or more associated windings. The windings can be on the forcer or at the platen depending on the type of motor. For example, in a permanent magnet linear motor, a series of armature windings can be mounted within a forcer that is movable relative a stationary path. The path can include an array of permanent magnets configured to interact with the coils in the stage when energized with an excitation current. 
   Alternatively, in another type of conventional linear motor, permanent magnets can be part of a moveable stage with the coils situated in the platen. Usually, the permanent magnets are attached to a back iron plate above the coils, which are oriented along a path of travel. The magnets usually are rectangular in shape. The magnets are arranged along the back iron so that adjacent pairs of magnets have opposite magnetic pole orientations. The magnets can be oriented generally normal to the direction of travel or inclined at a slight angle from normal to an axis of the direction of travel for the linear motor. The inclined angle creates a flux distribution along the axis of movement which is generally sinusoidal in nature. Such a resulting distribution due to the optimized motor geometry tends to reduce cogging during operation of the linear motor, which would otherwise occur if the magnets were aligned, normal to the axis of movement. 
   Although an inclined angle of the magnets can reduce some cogging, it presents a disadvantage in that a larger area typically must be covered by the rectangular magnets in order to sufficiently cover and interact with the coils of the armature. When the magnets are implemented with a larger area so as to reduce cogging effects, a larger footprint for the back iron also is required. This tends to increase the overall weight and size of the stage. Such increases in size and weight can present additional obstacles, such as in applications were there are size constraints and low mass is desirable. For example, as the mass of the stage increases, the available acceleration experiences a corresponding reduction, and the ability to stop the motor accurately also reduces because of the increased power dissipation needed to stop the motor. 
   As the use of linear motors in manufacturing equipment continues to increase, nominal increases in the speed of operation translate into significant savings in the cost of production. Accordingly, it is desirable to provide a magnet assembly that can be part of a high performance linear motor. 
   SUMMARY 
   The following presents a simplified summary of the invention in order to provide a basic understanding of some aspects of the invention. This summary is not an extensive overview of the invention. It is intended to neither identify key or critical elements of the invention nor delineate the scope of the invention. Its sole purpose is to present some concepts of the invention in a simplified form as a prelude to the more detailed description that is presented later. 
   One aspect of the present invention provides a magnet assembly that can be employed as part of a linear motor stage to form a high performance linear motor. The magnet assembly includes a plurality of magnets operatively associated with magnetically conductive plate, commonly known as a back iron. The magnets extend from a common side of the back iron. The back iron is dimensioned and configured to substantially conform to magnetic flux that travels through the back iron when the magnet assembly is exposed to a magnetic field, such as from windings of a motor path. In one particular aspect of the present invention, a cross-sectional dimension of the back iron varies between opposed ends of the back iron as a function of the position and/or orientation of the magnets. For example, a thickness of the back iron is greater at locations between adjacent pairs of the magnets than at locations generally centered with the respective magnets. As a result of such back iron geometry, force output to moving mass ratio of a motor incorporating the magnet assembly is improved over conventional configurations of magnet assemblies. Also, the back iron geometry reduces leakage flux. 
   Another aspect of the present invention provides a linear motor system that includes a path having a plurality of windings, which can be energized to produce desired magnetic fields. The linear motor system also includes a magnet assembly, such as described above. The linear motor system achieves high performance because the magnet assembly has a reduced mass, which substantially conforms to magnet flux lines that travel through the magnet assembly during energization of path windings. The mass further can be reduced by employing generally elongated octagonal magnets, such as by removing corner portions from rectangular magnets. 
   To the accomplishment of the foregoing and related ends, certain illustrative aspects of the invention are described herein in connection with the following description and the annexed drawings. These aspects are indicative, however, of but a few of the various ways in which the principles of the invention may be employed and the present invention is intended to include all such aspects and their equivalents. Other advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an isometric view of a moving magnet assembly in accordance with an aspect of the present invention. 
       FIG. 2  is a top elevation of the magnet assembly of  FIG. 1 . 
       FIG. 3  is a side sectional view of the magnet assembly taken along line  3 - 3  of  FIG. 2 . 
       FIG. 4  is side sectional view of part of a linear motor in accordance with an aspect of the present invention. 
       FIG. 5  is a side sectional view similar to  FIG. 4 , illustrating magnetic flux lines for an energized linear motor in accordance with an aspect of the present invention. 
       FIG. 6  is a side sectional view of a motor magnet assembly in accordance with another aspect of the present invention. 
       FIG. 7  is a side sectional view of a motor magnet assembly in accordance with another aspect of the present invention. 
       FIG. 8  is a side sectional view of a motor magnet assembly in accordance with another aspect of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The present invention provides a magnet assembly for use in a linear motor. The magnet assembly includes a back iron and an array of magnets. The back iron is in the form of a plate having opposed surfaces of the back iron. The magnets are arranged in a generally linear array along one of the surfaces. The other surface of the back iron plate is dimensioned and configured according to the magnetic field distribution and/or localized regions of saturation associated with the motor geometry/topology. For example, the surface of the back iron plate opposite to which the magnets are attached can be scalloped, such that a dimension between the opposed surfaces at locations generally aligned with the magnet centers is less than a dimension between the opposed surfaces at locations between adjacent magnets. 
     FIGS. 1 ,  2  and  3  illustrate a magnet assembly  10  in accordance with an aspect of the present invention. The magnet assembly  10  includes an array of permanent magnets  12 ,  14 ,  16 ,  18 ,  20 ,  22 ,  24 , and  26  of alternating magnetic polarity (see  FIG. 2 ). The magnets  12 - 26  are arranged in a substantially parallel relationship to each other and mounted to a generally rigid and magnetically conductive plate  30 , commonly referred to (and hereinafter referred to) as a back iron. The alternating polarity facilitates the flow of magnetic flux through the magnets  12 - 26  and the back iron  30 . The assembly  10  also includes an outer encapsulation  32  of a suitable non-conducting material, such as an epoxy or a polymer material. The encapsulation  32  helps hold the magnets  12 - 26  and back iron  30  in a desired relationship. 
     FIG. 2  is a top elevation of the magnet assembly of  FIG. 1  in which the encapsulation  32  has been removed. As shown in  FIG. 2 , the magnets  12 - 26  have a generally rectangular geometry and are spaced apart from each other by a predetermined distance. The magnets  12 - 26  have long axes, which are oriented generally perpendicular to a desired direction of travel for the assembly  10 , indicated at  34 , and which are aligned substantially parallel to each other. To provide desired flux distribution, the corners of each of the magnets  12 - 26  have been chamfered to form magnets having elongated octagonal geometries, such as shown in  FIG. 2 . The precise configuration can vary depending on the size of the magnets  12 - 26 , the size of the motor in which the assembly is to be employed as well as the desired characteristics for the motor. In this example, the illustrated magnet geometry also helps reduce the mass of the magnet assembly  10 . By way of example, the magnets are formed of a NdFeB material or other type of high performance permanent magnetic materials. 
   Referring to the side-sectional view of  FIG. 3 , the  12 - 26  magnets are mounted to and extend from a common side  36  of the back iron  30 . The back iron  30  also includes another side  38  opposite the side  36  to which the magnets  12 - 26  are mounted. In particular, the magnets  12 - 26  are position in slots or receptacles on the side  36 , which are dimensioned and configured to receive a portion of the respective magnets therein. Adjacent pairs of the slots define notches  40 ,  42 ,  44 ,  46 ,  48 ,  50 , and  52  of the back iron material that extend between adjacent pairs of magnets. The notches  40 - 52  operate to separate adjacent pairs of the magnets  12 - 26  by a predetermined distance, indicated at  56 , which corresponds to the width of the respective notches. For example, less than one-half the width of the magnets  12 - 26  are recessed into the back iron  30 , such that more than one-half the width of the magnets extend outwardly from the side  36  of the back iron  30 . The notches  40 - 52  and remaining surface of the side  36  are generally coplanar, although other shapes and configurations could be used in accordance with an aspect of the present invention. Also, the notches  40 - 52  act as retainers locking the magnets in place providing the desired stiffness. 
   In accordance with an aspect of the present invention, a cross-sectional dimension of the back iron  30  varies along its length between spaced apart ends  58  and  60  so as to substantially conform to the magnetic flux generated during operation of a motor that includes the magnet assembly  10 . In the example of  FIG. 3 , the thickness of the back iron  30  between the opposed sides  36  and  38  is greater at locations between adjacent pairs of the magnets  12 - 26  than at locations generally aligned with centers of the respective magnets. The back iron  30  can be formed of substantially any generally rigid material capable of conducting a magnetic field, so as to help form a magnetic circuit formed of the magnets  12 - 26  of different polarities and associated motor windings (not shown). 
   For example, the back iron  30  is formed of a non-linear material having a high magnetic permeability and desired saturation characteristics. In a particular aspect of the present invention, the back iron is formed of vanadium permeadur (e.g., cobalt-48.75%, Vanadium-2%, Carbon-0.004%, Manganese-0.05%, Silicon-0.05%, Iron-balance), which has particularly high saturation characteristics compared to other non-linear materials. While such material is considerably more expensive than steel, its superior magnetic properties are desirable in ultra-high performance motors according to the present invention. It is to be understood and appreciated that a high performance magnet assembly, in accordance with an aspect of the present invention, could employ other types of non-linear materials (e.g., M19 steel) than vanadium permeadur. 
   By way of illustration, the back iron  30  has a maximum thickness, indicated at  62 , at its ends  40  and  42  and at locations  64 ,  66 ,  68 ,  70 ,  72 ,  74 , and  76  between adjacent pairs of magnets  12 - 26 . In the example of  FIG. 3 , the locations  64 - 76  having the maximum thickness  62  are substantially coextensive with the notches  40 - 52 . Additionally, the side  38  of the back iron  30  at the ends  58  and  60  and at the locations  64 - 76  are generally coplanar and substantially parallel to the other side  36  of the back iron. It is to be appreciated, however, that back iron other shapes (e.g., curved in the direction of travel) also could utilized in accordance with an aspect of the present invention. Thus, as shown in  FIGS. 1 and 2 , the side  38  defines generally rectangular and coplanar strips extend between side edges  80  and  82  of the back iron  30  at the ends  58  and  60  and at the locations  64 - 76 . 
   The back iron  30  further has a minimum thickness, indicated at  84 , at locations  86 ,  88 ,  90 ,  92 ,  94 ,  96 ,  98 , and  100  substantially centered with the long axes of the respective magnets  12 - 26 . In the example of  FIG. 3 , the locations  86 - 100  have the minimum thickness  84 , which define generally rectangular planes or strips in the side surface  38  spaced from and substantially parallel to the magnets  12 - 26  over which the respective locations are positioned. The locations generally rectangular strips, which can be coplanar, extend between the side edges  80  and  82  of the back iron  30 . 
   The portions of the side  38  extending between the locations of maximum thickness (e.g., the ends  58  and  60  and the locations  64 - 76 ) and the locations of minimum thickness  86 - 100  slope upwardly and downwardly to provide a desired scalloped or sawtooth cross section, as illustrated in  FIG. 3 . That is, the locations (or strips)  64 - 76  and  86 - 100  respectively provide alternating peaks and valleys along the surface  38  of the back iron. 
   Referring to  FIG. 2 , each of the locations  64 - 76  of maximum back iron thickness has a width  104  in the direction  34 , which width is greater than or equal to zero. Similarly, each of the locations  86 - 100  of minimum back iron thickness has a width  106  in the direction  34 , which width is greater than or equal to zero. Accordingly, while the locations of maximum and minimum thickness are illustrated as generally planar and parallel to the side  36 , those skilled in the art will understand and appreciated that virtually any widths  104  and  106  can be employed to provide different varying cross-sectional configurations for the back iron in accordance with an aspect of the present invention. Additionally or alternatively, while the locations  64 - 76 , the locations  86 - 100  and the portion of the side surface extending therebetween are illustrated as generally planar surfaces, it is to be appreciated that one or more of such surface portions could be curved in accordance with an aspect of the present invention. 
     FIG. 4  illustrates a cross-sectional view of a linear motor system  130  in accordance with an aspect of the present invention. The system  130  includes a moving magnet assembly (or stage)  132  that is moveable in a direction of travel, indicated at  134 , relative to a path  136 . For example, the magnet assembly  132  is supported relative to the path  136  for movement in the direction  134 , such as by low or no friction bearings (e.g., air bearings, not shown) to provide a desired air gap between the magnet assembly and the path. 
   The magnet assembly  132  includes a plurality of magnets  138 ,  140 ,  142 ,  144 ,  146 ,  148 ,  150 , and  152 , which are attached to and extend from a common side  156  of a back iron  158 . An opposite side  160  of the back iron  158  is dimensioned and configured to conform to flux lines of a magnetic circuit formed between the magnet assembly and the path when the path is energized. That is, the thickness of the back iron  158  between the opposed sides  156  and  160  is greater at locations between adjacent pairs of the magnets  12 - 26  than at locations generally aligned with centers of the respective magnets. As a result, the side surface  160  has a generally scalloped or ribbed appearance between its ends; e.g., it is formed of alternating peaks and valleys between spaced apart ends of the back iron. The particular cross-sectional configuration of the back iron can vary, such as described herein. 
   The path  136  includes a plurality of spaced apart teeth  162  that extend from a base portion  164  toward the magnet assembly  132  located above the path  136 . Typically, the teeth  162  are oriented substantially parallel relative to each other and to the magnets  138 - 152 . The path  136  also includes windings  166  disposed around selected teeth. The windings  166  could be pre-wound coil assemblies or wound in-situ around the teeth  162 . 
   Those skilled in the art will understand and appreciate that the linear motor system typically includes a motor controller programmed and/or configured to control operation of the motor system  130 . For example, an encoder or other positioning system provides the controller with position information, based on which the controller controls energization of the associated windings  166  to effect desired movement of the magnet assembly  132  relative to the path. Those skilled in the art further will understand and appreciate various configurations of paths  136  and coil windings that could be utilized in combination with a magnet assembly in accordance with an aspect of the present invention. 
     FIG. 5  depicts a graphical representation of part of linear motor system  200 , similar to that shown in  FIG. 4 , illustrating magnetic flux lines  202  for magnet circuits formed by a magnet assembly  204  and energized windings of a motor path  206  in accordance with an aspect of the present invention. The magnet assembly  204  includes a plurality of permanent magnets  208 ,  210 ,  212 ,  214 ,  216 ,  218 ,  220 , and  222  that are operatively coupled to a back iron  226 . 
   In accordance with an aspect of the present invention, as shown in  FIG. 5 , the back iron  226  is dimensioned and configured to conform to the magnet flux lines that travel through the magnetic circuits formed of the magnet assembly and the path  206 . The back iron  226  has a greater cross-sectional dimension at locations at ends of the magnet assembly  204  and between adjacent pairs of magnets than at the locations generally centered with the respective magnets. Consequently, the overall mass of the moveable magnet assembly  204  is less than if such portions had not been removed. Additionally, because the selected portions have been removed according to the magnetic flux lines during energization of the path windings, the forces generated between the assembly  204  and the path remain substantially unchanged from a back plane that would include a substantially planar surface opposite the magnets. 
   To further reduce the mass of the magnet assembly, the magnets can be configured to have chamfered corners, so as to provide a generally elongated octagonal geometry. The particular dimensions and configuration of magnets and back iron can be further optimized based on magnetic finite element analysis. As a result, under Newton&#39;s law, the acceleration of the magnet assembly  204  relative to the path  206  is increased by an amount proportional to the reduced mass of the magnet assembly. Additionally, the geometry further provides flux distribution that is substantially sinusoidal distribution, which mitigates total harmonic distortion (THD). A lower THD corresponds to a more efficient motor. 
   As mentioned above, it is to be appreciated that various configurations of magnet assemblies can be implemented in accordance with an aspect of the present invention.  FIGS. 6-9  illustrate some examples of other configurations of magnet assemblies that can be utilized. It will be understood and appreciated that such examples are solely for illustrative purposes and that numerous other possible configurations exist, all of which are within the scope of the appended claims. 
     FIG. 6  illustrates a magnet assembly  240  for use in a linear motor in accordance with an aspect of the present invention. The assembly  240  includes a plurality of elongated permanent magnets  242  operatively coupled to a back iron  244 . As shown in  FIG. 6 , a cross-sectional dimension of the back iron  244  varies along its length between spaced apart end portions  246  and  248  of the back iron. In particular, a side surface  250  of the back iron  244  opposite a side  252  to which the magnets  242  are attached has a substantially triangular or sawtooth geometry having alternating peaks  254  and valleys  256 . The other side  252  is generally planar, although it includes slots or receptacles in which a portion of the respective magnets  242  is received. As a result of such back iron  244  configuration, the thickness of the back iron at locations between adjacent pairs of magnets  242  and at the end portions  246  and  248  is greater than its thickness at locations generally centered with the long axes of the respective magnets. 
   The geometry of the back iron  244  substantially conforms to magnetic flux lines that travel through the back iron from the magnets so as to provide extremely high flux densities. The geometry further enables the back iron  244  to have a reduced mass. The magnets also can be configured to have chamfered corners, so as to provide a generally elongated octagonal geometry, such that the mass of the magnet assembly is further reduced. The combination of high flux densities and reduce back iron mass result in a high performance motor capable of achieving rapid acceleration compared to conventional linear motors of similar size. 
     FIG. 7  illustrates a magnet assembly  260  for use in a linear motor in accordance with another aspect of the present invention. The assembly  260  includes a plurality of elongated permanent magnets  262  operatively coupled at a side surface  264  of a back iron  266 . The back iron  266  has a cross-sectional dimension that varies along its length between spaced apart end portions  268  and  270  in accordance with an aspect of the present invention. In particular, a side surface  272  of the back iron  244  opposite the side  264  to which the magnets  262  are attached has a plurality of substantially elongated rectangular peaks (or protrusions)  276 . The peaks  276  extend between side edges of the back iron. That is, the side  272  has alternating rectangular peaks  276  and valleys  278  to provide a generally square wave cross-sectional geometry between the end portions  268  and  270 . The peaks  276  are generally centered over spaces between adjacent pairs of the magnets  262  and the valleys are generally centered over the long axes of the respective magnets. The side  264  is generally planar, although it includes slots or receptacles in which a portion of the respective magnets  262  is received. 
   As a result of such geometry for the back iron  266 , the thickness of the back iron at locations between adjacent pairs of magnets  262  and at the end portions  268  and  270  is greater than its thickness at locations generally centered with the magnets. This geometry substantially conforms to magnetic flux lines that travel through the back iron  266  from the magnets  262  so as to provide extremely high flux densities, such as when associated windings of a motor incorporating the magnet assembly  260  are energized. The geometry further enables the back iron  266  to have a reduced mass. The combination of high flux densities and reduced back iron mass result in a high performance motor capable of achieving rapid acceleration compared to conventional linear motors of similar size. 
     FIG. 8  depicts yet another magnet assembly  280  in accordance with an aspect of the present invention. The magnet assembly includes a plurality of permanent magnets  282  operative connected to a generally planar side surface  284  of a back iron  286 . Specifically, a portion of the magnets  282  can be received in associated slots or receptacles formed in the side  284 , although the magnets could be attached to the back iron in the absence of such slots. 
   In accordance with an aspect of the present invention, a side surface  288  of the back iron  286  opposite the side  284  to which the magnets are connected is dimensioned and configured to substantially conform to magnetic flux lines associated with the magnet assembly when exposed to magnetic fields from energized windings of an associated motor path (not shown). In the example of  FIG. 8 , the side surface  288  has alternating peaks  290  and valleys  292  to provide a generally sinusoidal cross-sectional configuration between spaced apart end portions  294  and  296  of the back iron  286 . The peaks  290  are generally centered over spaces located between adjacent pairs of magnets  282  and the valleys  292  are generally centered over corresponding centers of the respective magnets. 
   As a result of the back iron geometry shown in  FIG. 8 , the magnet assembly  280  is able complete magnetic circuits in an associated linear motor so as to provide extremely high flux densities, such as when associated windings of the motor incorporating the magnet assembly are energized. The geometry further provides the back iron  286  with a reduced mass. To further reduce the mass of the magnet assembly, the magnets  282  can be configured as a generally elongated octagon, such as by removing corner portions of the magnets. The combination of high flux densities and reduced back iron mass result in a high performance motor capable of achieving rapid acceleration compared to conventional linear motors of similar size. 
   What has been described above includes exemplary implementations of the present invention. It is, of course, not possible to describe every conceivable combination of components or methodologies for purposes of describing the present invention, but one of ordinary skill in the art will recognize that many further combinations and permutations of the present invention are possible. For example, a magnet assembly, in accordance with an aspect of the present invention can have different numbers of magnets from that shown and described herein. Additionally, the magnet assembly can have a different contour from the substantially flat configuration shown herein, such as to conform to the contour of the path with which the magnet assembly is to be utilized. Accordingly, the present invention is intended to embrace all such alterations, modifications and variations that fall within the spirit and scope of the appended claims.