Patent Publication Number: US-8538061-B2

Title: Earphone driver and method of manufacture

Description:
TECHNICAL FIELD 
     The disclosure herein relates to the field of sound reproduction, more specifically to the field of sound reproduction using an earphone. Aspects of the disclosure relate to earphones for in-ear listening devices ranging from hearing aids to high quality audio listening devices to consumer listening devices. 
     BACKGROUND 
     Personal “in-ear” monitoring systems are utilized by musicians, recording studio engineers, and live sound engineers to monitor performances on stage and in the recording studio. In-ear systems deliver a music mix directly to the musician&#39;s or engineer&#39;s ears without competing with other stage or studio sounds. These systems provide the musician or engineer with increased control over the balance and volume of instruments and tracks, and serve to protect the musician&#39;s or engineer&#39;s hearing through better sound quality at a lower volume setting. In-ear monitoring systems offer an improved alternative to conventional floor wedges or speakers, and in turn, have significantly changed the way musicians and sound engineers work on stage and in the studio. 
     Moreover, many consumers desire high quality audio sound, whether they are listening to music, DVD soundtracks, podcasts, or mobile telephone conversations. Users may desire small earphones that effectively block background ambient sounds from the user&#39;s outside environment. 
     Hearing aids, in-ear systems, and consumer listening devices typically utilize earphones that are engaged at least partially inside of the ear of the listener. Typical earphones have one or more drivers or balanced armatures mounted within a housing. Typically, sound is conveyed from the output of the driver(s) through a cylindrical sound port or a nozzle. 
     BRIEF SUMMARY 
     The present disclosure contemplates earphone driver assemblies, specifically balanced armature driver assemblies. The earphone driver assemblies can be used in any hearing aid, high quality listening device, or consumer listening device. For example, the present disclosure could be implemented in or in conjunction with the earphone assemblies, drivers, and methods disclosed Ser. No. 12/833,651, titled “Earphone Assembly” and Ser. No. 12/833,639 titled “Drive Pin Forming Method and Assembly for a Transducer,” which are herein incorporated fully by reference. 
     The following presents a simplified summary of the disclosure in order to provide a basic understanding of some aspects. It is not intended to identify key or critical elements of the invention or to delineate the scope of the invention. The following summary merely presents some concepts of the disclosure in a simplified form as a prelude to the more detailed description provided below. 
     In one exemplary embodiment, a balanced armature motor assembly comprising: an armature having a flexible reed; a pole piece containing a pair of magnets; a bobbin comprising a first cutout, a second cutout, and a center post; a wire coil surrounding the bobbin having a first end and a second end; and a circuit board mounted to the bobbin is disclosed. The circuit board comprises a first terminal and a second terminal. A drive pin is operatively connected between the reed and a paddle. The first end of the wire coil is secured to the first terminal of the circuit board and passes through the first cutout of the bobbin and the second end of the wire coil is secured to the second terminal of the circuit board and passes through the second cutout of the bobbin. The first end of the wire coil is oriented along a first line tangent to the center post of the bobbin and the second end of the wire coil is oriented along a second line tangent to the center post of the bobbin. The circuit board comprises first and second notches, the first end of the wire coil is located in the first notch of the circuit board, and the second end of the wire coil is located in the second notch of the circuit board. The first cutout and the second cutout in the bobbin can be formed L-shaped. 
     In another exemplary embodiment, a method of forming a balanced armature motor assembly comprising an armature having a flexible reed, a pole piece containing a pair of magnets, a bobbin, a wire coil, a drive pin, a paddle, and a circuit board having first and second terminals thereon is disclosed. The method comprises wrapping a first end of a wire around a center post located on the bobbin; placing a portion of the first end of the wire in a first cutout located on the bobbin; wrapping a central portion of the bobbin with the wire to form the wire coil; locating a portion of a second end of the wire in a second cutout located on the bobbin; wrapping the second end of the wire around the center post; and affixing the first end of the wire to the first terminal and the second end of the wire to the second terminal. The method further comprises cutting the first end of the wire between the first terminal and the center post and discarding a first remainder portion of the first end wrapped around the center post and cutting the second end of the wire between the second terminal and the center post and discarding a second remainder portion of the second end wrapped around the center post. The first and second ends of the wire can be attached to the first and second terminals by a thermo-compression or soldering process. 
     In another exemplary embodiment a balanced armature motor assembly comprising: an armature having a flexible reed; a pole piece housing a first magnet and a second magnet; a bobbin having at least one post extending therefrom; a wire coil surrounding the bobbin; a circuit board mounted to the bobbin; a drive pin operatively connected to the reed and to a paddle is disclosed. A compressed polymer material can be interposed between the first magnet and the post and between the second magnet and the post. The polymer material forces the first and second magnets into contact with the pole piece. The polymer material comprises at least one glue dot secured to each of the first magnet and the second magnet or a plurality of glue dots located on each of the first magnet and the second magnet. The at least one post can comprise a pair of T-shaped posts. The at least one glue dot on the first magnet rests on a first side of the T-shaped posts, and the at least one glue dot on the second magnet rests on a second side of the T-shaped posts. The first magnet and the second magnet are further welded to the pole piece. 
     In another exemplary embodiment, a method of forming a balanced armature motor assembly comprising an armature having a flexible reed, a pole piece containing a first magnet and a second magnet, a bobbin, a wire coil, a drive pin, a paddle, and a circuit board is disclosed. The method comprises placing a polymer material on the first magnet and the second magnet; positioning the first magnet and the second magnet such that the polymer material contacts at least one post extending from the bobbin; placing the pole piece over the first magnet and the second magnet and compressing the polymer material to cause the polymer material to force the first magnet and the second magnet into contact with the pole piece; and securing the first magnet and the second magnet to the pole piece. The polymer material comprises an adhesive, and the adhesive can comprise a plurality of glue dots on each of the first magnet and the second magnet. The step of compressing the polymer material can comprise moving the magnets inwardly towards each other. The securing step can comprise welding the first and second magnets to the pole piece. The at least one post can comprise a pair of T-shaped posts extending from the bobbin. Additionally, the reed passes in between the first and second magnets, and is equidistant from the first and second magnets. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present disclosure is illustrated by way of example and not limited in the accompanying figures: 
         FIG. 1  shows a perspective view of prior art fixture for assembling a balanced armature driver assembly; 
         FIG. 2  shows a close up perspective view of the prior art fixture of  FIG. 1 ; 
         FIG. 3A  shows a perspective exploded left front view of an exemplary embodiment of a balanced armature motor assembly disclosed herein; 
         FIG. 3B  shows another perspective exploded left front view of the balanced armature motor assembly in  FIG. 3A ; 
         FIG. 3C  shows a perspective exploded left rear view of the balanced armature motor assembly in  FIG. 3A ; 
         FIG. 3D  shows another perspective left exploded front view of the balanced armature motor assembly in  FIG. 3A ; 
         FIG. 3E  shows another perspective exploded left rear view of the balanced armature motor assembly in  FIG. 3A ; 
         FIG. 3F  shows another perspective exploded left front view of the balanced armature motor assembly in  FIG. 3A ; 
         FIG. 3G  shows another perspective exploded left front view of the balanced armature motor assembly in  FIG. 3A ; 
         FIG. 4A  shows an isometric left front view of the balanced armature motor assembly shown in  FIG. 3A  and a nozzle base; 
         FIG. 4B  shows another isometric left front view of the balanced armature motor assembly in  FIG. 3A ; 
         FIG. 4C  shows an isometric left rear view of the balanced armature motor assembly in  FIG. 3A ; 
         FIG. 5A  shows a bottom view of another exemplary embodiment of a balanced armature motor assembly disclosed herein; 
       FIG.  5 A 1  shows the exemplary embodiment in  FIG. 5A  after an assembly operation; 
         FIG. 5B  shows a left rear perspective top view of the bobbin shown in  FIG. 5A ; 
         FIG. 5C  shows a rear view of the balanced armature motor assembly of  FIG. 5A ; 
         FIG. 6A  shows a front view of another exemplary embodiment of a balanced armature motor assembly prior to a welding operation disclosed herein; 
         FIG. 6B  shows the embodiment of  FIG. 6A  after a welding operation; 
         FIG. 7  shows a bottom view of a pair of magnets and corresponding glue dots used in an embodiment of a balanced armature motor assembly disclosed herein; 
         FIG. 8  shows an end view of the magnets and glue dots of  FIG. 7 ; 
         FIG. 9  shows a top view of another exemplary embodiment of an unassembled balanced armature motor assembly disclosed herein; 
         FIG. 10  shows a representative schematic of an exemplary embodiment disclosed herein; 
         FIGS. 11A-K  show an exemplary assembly method of a balanced armature motor assembly; 
         FIG. 12  shows a graph of comparing glue dot size, compression percentage, and force for an exemplary embodiment disclosed herein. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Exploded views of a balanced armature motor assembly are shown in  FIGS. 3A-3G  and assembled views of a balanced armature motor assembly  150  are shown in  FIGS. 4A ,  4 B, and  4 C. Such a balanced armature motor assembly  150  can be used with any earphone ranging from hearing aids to high quality audio listening devices to consumer listening devices. 
     As shown in  FIGS. 3A and 4A , a balanced armature motor assembly  150  generally consists of an armature  156 , upper and lower magnets  158 A,  158 B, a pole piece  160 , a bobbin  162 , a coil  164 , a drive pin  174 , and a flex board  167  or any suitable type of circuit board. The magnets  158 A,  158 B are secured to the pole piece  160 , and held in contact with the pole piece  160  by a plurality of glue dots  182  which provide a resilient force against a pair of “T” shaped posts  184  extending from the bobbin  162 , as described in greater detail herein. While so held in place, the magnets  158 A,  158 B may be welded to the pole piece  160  as described in greater detail herein. The flex board  167  is a flexible printed circuit board that mounts to the bobbin  162  and free ends of wire forming the coil  164  are secured to the flex board  167  (as discussed in further detail herein). 
     The armature  156  is generally E-shaped from a top view. In other embodiments, the armature  156  may have a U-shape or any other known, suitable shape. The armature  156  has a flexible metal reed  166  which extends through the bobbin  162  and coil  164  between the upper and lower magnets  158 A,  158 B and is located equidistant from the upper and lower magnets  158 A,  158 B. The armature  156  also has two outer legs  168 A,  168 B, lying generally parallel with each other and interconnected at one end by a connecting part  170 . As illustrated in  FIG. 4A , the reed  166  is positioned within an air gap  172  formed by the magnets  158 A,  158 B. The two outer armature legs  168 A and  168 B extend along the outer side along the bobbin  162 , coil  164 , and pole piece  160 . The coil  164  can be formed between two flanges  171 A,  171 B. The two outer armature legs  168 A and  168 B are affixed to the pole piece  160 . The reed  166  can be connected to paddle  152  with the drive pin  174 . The drive pin  174  can be formed of stainless steel wire or any other known suitable material. 
     The electrical input signal is routed to the flex board  167  via a signal cable comprised of two conductors. Each conductor is terminated via a soldered connection or any suitable securing method to one more pads on the flex board  167  which are electrically connected (via the traces of the flex board  167 ) to the respective terminals  178 A,  178 B as shown in FIG.  5 A 1 . In an embodiment, the pads are larger than the terminals  178 A,  178 B and thus serve the purpose of providing a larger surface area for connecting the signal cable conductors, which are relatively larger than the wire forming the coil  164 . In an embodiment, the pads are located on an end of the flex board  167  generally opposite from the terminals  178 A,  178 B, as shown in FIGS.  5 A and  5 A 1 . Each of these terminals  178 A,  178 B is electrically connected to a corresponding lead  165 A or  165 B on each end of the coil  164 . When signal current flows through the signal cable and into the coil&#39;s  164  windings, magnetic flux is induced into the soft magnetic reed  166  around which the coil  164  is wound. The signal current polarity determines the polarity of the magnetic flux induced in the reed  166 . The free end of the reed  166  is suspended between the two permanent magnets  158 A,  158 B. The magnetic axes of these two permanent magnets  158 A,  158 B are both aligned perpendicular to the lengthwise axis of the reed  166 . The lower face of the upper magnet  158 A acts as a magnetic south pole while the upper face of the lower magnet  158 B acts as a magnetic north pole. 
     As the input signal current oscillates between positive and negative polarity, the free end of the reed  166  oscillates its behavior between that of a magnetic north pole and south pole, respectively. When acting as a magnetic north pole, the free end of the reed  166  repels from the north-pole face of the lower magnet and attracts to the south-pole face of the upper magnet. As the free end of the reed oscillates between north and south pole behavior, its physical location in the air gap  172  oscillates in kind, thus mirroring the waveform of the electrical input signal. The motion of the reed  166  by itself functions as an extremely inefficient acoustic radiator due to its minimal surface area and lack of an acoustic seal between its front and rear surfaces. In order to improve the acoustic efficiency of the motor, the drive pin  174  is utilized to couple the mechanical motion of the free end of the reed  166  to an acoustically sealed, lightweight paddle  152  of significantly larger surface area. The resulting acoustic volume velocity is then transmitted through the earphone nozzle  212  and ultimately into the user&#39;s ear canal, thus completing the transduction of the electrical input signal into the acoustical energy detected by the user. 
     As shown in  FIG. 5A , the flex board  167  is formed with first and second terminals  178 A,  178 B. In an embodiment, during assembly, the ends of the wire forming the coil  164  are secured to the flex board  167  at the first and second terminals  178 A,  178 B. Stated differently, a start lead  165 A or a first end of the coil  164  and a finish lead  165 B or a second end of the coil  164  are affixed to the terminals  178 A,  178 B. The flex board  167  may optionally include first and second notches  169 A,  169 B for permitting the start and finish leads  165 A,  165 B of the coil  164  to rest in adjacent notches (or “L-shaped cutouts”  176 A,  176 B as described later herein) in the underlying bobbin  162  without distorting or putting pressure on the flex board  167 . 
     The bobbin  162  has a spool  163 , along with a first post  180 A, a second or center post  180 B, and a third post  180 C. The first, second, and third posts  180 A,  180 B,  180 C are used to locate the flex board  167  onto the bobbin  162 , and the second or center post  180 B is further used for securing the wire during the coiling process. More specifically, the second post  180 B is used in conjunction with the L-shaped cutouts  176 A,  176 B described later herein to locate the start and finish leads  165 A,  165 B at appropriate locations relative to the first and second terminals  178 A,  178 B for affixing thereto. The center post  180 B can also be configured so as to contact an earphone housing once assembled to provide for stability in preventing the motor assembly  150  from moving inside the earphone housing. Additionally, the center post  180 B can aid in leveling the nozzle base  201  to keep the motor assembly  150  parallel to the paddle  152  plane while maintaining needed clearances. As shown in  FIG. 5B , first and second L-shaped cutouts  176 A,  176 B may be provided on the bobbin  162  for locating the start lead  165 A and the finish lead  165 B properly over the first and second terminals  178 A,  178 B. 
     Specifically, the ends of the wires of the coil  164  which form the leads  165 A,  165 B pass through the L-shaped cutouts  176 A,  176 B, through the notches  169 A,  169 B of the flex board  167 , pass diagonally over the terminals  178 A,  178 B of the flex board  167 , and are wrapped around the center post  180 B. It should be understood that the notches  169 A,  169 B are optional, and are present in some embodiments so as to avoid interference between the leads  165 A,  165 B and the flex board  167 . In other embodiments, the flex board  167  may have no notches  169 A,  169 B and may instead be configured in different shapes and arrangements such that the leads  165 A,  165 B pass through the L-shaped cutouts  176 A,  176 B and over the terminals  178 A,  178 B without contacting any edges of the flex board  167 . 
     The center post  180 B and the L-shaped cutouts  176 A,  176 B in the bobbin  164  aid in maintaining the start lead  165 A and the finish lead  165 B properly in place over the terminals while the leads  165 A,  165 B are secured to the terminals  178 A,  178 B. This improves the manufacturability of the motor assembly  150  such that when the coil  164  is formed around the bobbin  162 , the terminal leads  165 A,  165 B of the coil  164  can be properly and consistently located on the flex board  167  and affixed to the terminals  178 A,  178 B. Locating the leads  165 A,  165 B between the fixed structures of the L-shaped cutouts  176 A,  176 B and the center post  180 B ensures that an appropriate and sufficient amount of wire from the leads  165 A,  165 B is in contact with the terminals  165 A,  165 B. 
     In an embodiment, during manufacturing, wire is wrapped around a central portion or spool  163  of the bobbin  162  to form the coil  164 . This winding process may be done manually, may be done using an automated, machine-driven process, or may involve a combination of manual and automated steps. First, the wire is wrapped around the center post  180 B approximately two to four times. Next, the wire is captured in the first L-shaped cutout  176 A located on the bobbin  162 , passing through the first notch  169 A. Next the wire is wrapped around the spool  163  in layers with a specified number of turns per layer. In an embodiment, wire is wrapped around the spool  163  in eight (8) layers, with each layer having thirty-one turns of wire per layer. The wire is then captured in the second L-shaped cutout  176 B located on the bobbin  162 , passing through the second notch  169 B. The wire is then again wrapped around the center post  180 B approximately two to four times. The wire can then be cut to form the finish lead  165 B. This process causes the start and finish leads  165 A,  165 B to be optimally positioned over the terminals  178 A,  178 B for securing the start and finish leads  165 A,  165 B to the terminals  178 A,  178 B, as described herein. 
     Once the start and finish leads  165 A,  165 B are properly positioned over the terminals  178 A,  178 B, they can be secured to the terminals  178 A,  178 B on the flex board  167  by any known appropriate method for connecting wires to metallic terminals, such as by a soldering or by a thermo-compression process. Once the leads  165 A,  165 B are secured to the terminals  178 A,  178 B, the wire of the start and finish leads  165 A,  165 B is cut near the second post  180 B. The excess wire remaining around the center post  180 B is trimmed such that it can be removed and discarded. In one exemplary embodiment the first end  165 A of the wire is cut between the first terminal  178 A and the center post  180 B and a first remainder portion of the first end wrapped around the center post is discarded, and the second end  165 B of the wire is cut between the second terminal  178 B and the center post  180 B and a second remainder portion of the second end wrapped around the center post  180 B is discarded. 
     Thus, the resulting flex board  167  and bobbin  162  with finished leads  165 A,  165 B secured to terminals  178 A,  178 B appear as shown in FIG.  5 A 1 . As shown in the resulting assembly in FIG.  5 A 1 , the first end  165 A of the wire coil  164  is oriented along a first line tangent to the center post  180 B of the bobbin  162  and the second end  165 B of the wire coil  164  is oriented along a second line tangent to the center post  180 B of the bobbin  162 . 
       FIGS. 1 and 2  show a prior art assembly method for installing magnets  58  into a driver assembly. As shown in  FIGS. 1 and 2 , ten pole pieces  60  are loaded into a fixture block  40 , while the magnets  58  are installed and held against the inner walls of each pole piece  60 , using removable compliant spacers  80 . A lateral spacer  10  is also used to center the magnets along the upper and lower pole piece  60  walls. The fixture block  40  is then installed in a laser welder and each magnet is accurately welded to the pole pieces  60  with two spot welds  61 . Next the ten pole pieces  60  are removed and flipped around to perform the same welding operation on the other end in order to completely secure the magnets. A coil and bobbin is then fastened to the pole piece magnet sub-assembly with an adhesive. 
     In exemplary embodiments according to various aspects of the invention, as shown in  FIGS. 3G and 7 , a plurality of glue dots  182  are placed on the magnets  158 , which aid in holding the magnets  158  against the pole piece  160  during welding the magnets to the pole piece  160 . Although  FIG. 3G  depicts four glue dots  182  on magnets  158 A,  158 B and  FIG. 7  depicts two glue dots  182  on magnets  158 A,  158 B, any suitable number of glue dots  182  is contemplated.  FIG. 8  shows a side profile of the glue dots  182  on magnets  158 A,  158 B. As shown in  FIG. 8 , in an embodiment, the glue dots  182  have a generally hemispherical shape. In other embodiment, the glue dots  182  may take on a variety of shapes and configurations. 
     As shown in  FIGS. 5A and 5B , the bobbin  162  incorporates two “T” shaped posts  184  extending from a front flange  171 A on the bobbin  162  to locate and support the magnets  158  and the pole piece  160 . The “T” shaped posts  184  aid in assembling the magnets  158  to the pole piece  160 .  FIG. 9  shows glue dot contact points  187  on opposing surfaces or sides of the “T” shaped posts  184 . As shown in  FIG. 6A , the “T” posts  184  have first sides  185 A and second sides  185 B and the magnets  158 A,  158 B are positioned on each of the first sides  185 A and the second sides  185 B of the “T” shaped posts  184 , with the glue dots  182  in contact with the first and second sides  185 A,  185 B of the T-shaped posts  184 . Although in this embodiment glue “dots” are discussed, the resilient glue or adhesive used can take on other shapes and configurations, such as a strip or line of glue, Additionally, other types of suitable polymers in place of the glue dots are also contemplated. In addition, it is also contemplated that the glue could be placed onto first and second sides  185 A,  185 B or other appropriate locations on the “T” shaped posts  184  instead of the magnets  158 . In addition, other shapes and configurations of the “T” posts are contemplated, for example the posts  184  can be formed as straight posts, legs, or flat narrow strips. 
     The purpose of the glue dots  182  is to aid the assembly of the magnets  158  into the pole piece  160  and provide an improved structure to the balanced armature driver assembly  150  as a whole. It is desirable for the magnets  158  to be held tight against the upper and lower walls of the pole piece  160 . In order to complete the magnetic flux path, it is preferable for performance reasons to minimize or eliminate the existence of any air gaps between the pole piece  160  and magnets  158 . The glue dots  182  provide a resilient, spring-like structure to hold the magnets  158  tightly against the interior of the pole piece  160  while welding the magnets  158  to the pole piece  160 . In one embodiment shown in  FIG. 6B , a plurality of welds  161 A-D are placed between the magnets  158 A,  158 B and the pole piece  160 . Thus, in one respect, the glue dots  182  replace and perform the function of the compliant spacers  80  in the prior art (see  FIGS. 1 and 2 ). In addition to glue, other suitable polymers, such as cured silicon rubber, can be secured to the magnets to provide this resilient function. 
     According to one embodiment of the present invention as shown in  FIGS. 11A-11K , during assembly the magnets  158  are positioned on either side of the “T” shaped posts  184 , compressed and/or “tilted forward” at their front ends, and then captured by the pole piece  160  as it is slid over the magnets  158 . In an embodiment, an assembly fixture  186  can be used to aid the assembly of the magnets  158  to the bobbin  162  and the pole piece  160 . In an embodiment, an assembly fixture  186  can be used to aid the assembly of the magnets  158  to the bobbin  162  and the pole piece  160 . In particular, the assembly fixture  186  holds and manipulates the magnets  158  as the pole piece  160  is added. 
       FIG. 11A  shows the overall assembly fixture  186  and a guide fork  188 .  FIG. 11B  shows the assembly fixture  186  before receiving the bobbin  162 . As shown in  FIG. 11D , the guide fork  188  has a first wider area  191 , a transition area  192 , and a narrower area  193  all of which allow the magnets  158  to be moved closer together as the guide fork  188  is moved inwardly. As shown in  FIG. 11B  the assembly fixture  186  has notches  190  for supporting the bobbin  162  while the magnets  158  and the pole piece  160  are assembled to the bobbin  162 . 
     First as shown in  FIG. 11C  a bobbin  162  is installed in the fixture  186 . Next as shown in  FIG. 11D , the guide fork  188  is moved over the bobbin  162 . Next as shown in  FIG. 11E  the magnets  158  are inserted with the glue dots  182  located on the bobbin “T” shaped posts  184  on the first wider area  191  of the guide fork  188 .  FIGS. 11F and 11G  show the guide fork  188  being moved inwardly (to the left) into position such that the magnets  158  contact the transition area  192  and are compressed as they enter the narrower area  193  of the guide fork  188  in order to bring the magnets  158  closer together for placement of the pole piece  160 . The resilient glue dots  182  are also compressed during the assembly to force the magnets  158  against the pole piece and also counteract the force provided by the guide fork  188 . 
     As shown in  FIG. 11H , the pole piece  160  is next installed over the magnets  158 . At this point the pole piece  160  is resting on top of the guide fork  188  and is located only half way down over the magnets  158 , so as to aid in inserting the magnets  158  into the pole piece  160 . As shown in  FIG. 11I-11K , the guide fork  188  is retracted (moved to the right) and the pole piece  160  is pushed all the way down over the magnets  158 . The glue dots  182  are compressed trapping the magnets  158  between the bobbin “T” shaped posts  184  and the pole piece walls. The entire assembly is then removed from the fixture  186 , and the magnets  158  can then later be welded to the pole piece  160  using any suitable and known welding method, such as laser welding.  FIG. 6B  shows approximate weld locations  161 A-D between the magnets  158 A,  158 B and the pole piece  160 . Thus, the glue dots  182  both secure the magnets  158  into position in the pole piece  160 , and hold them in proper place until a later welding operation is conducted. 
     In an embodiment, the glue can have an elongation property of 150% when fully cured, which provides for adequate compressibility. For consistency in manufacturing and operation, it is preferable that the glue dot  182  be of a consistent height (+/−0.001″) and be accurately located on the magnet  158 . This can be accomplished with proper fixturing and controlled dispensing of the adhesive. The compliance of the glue dot  182  takes up the tolerance in the assembly while providing enough force to keep the magnets  158  against the pole piece  160 . 
     A suitable adhesive that may be used to form the glue dots  182  is Dymax 3013-T, which is a compliant elastomeric adhesive. However, other adhesives and suitable polymers are contemplated. In an embodiment, the glue dots  182  are shaped roughly hemispherically after being dispensed, and are ‘pancaked’ under compression during the assembly process described in  FIGS. 11A-11K . 
     The relative forces provided by each glue dot are based on factors such as material property, amount of compression, and size of each dot. As shown in  FIG. 10 , the glue dot  182  can be modeled as a hemisphere having a radius (R) and the amount of force can be treated like a linear spring with the exception that, as the gap between the bobbin and the magnet (z gap ) is reduced linearly, the volume changes exponentially (3 rd  power) per the below equation. In  FIG. 10 , the glue dot  182  is shown in an uncompressed state, while magnet  158  and portion of the post  184  are shown in a typical compressed spacing illustrating a z gap  less than radius R. The optimal design will match the adhesive dot size capability with the system tolerances that impact the gap. 
     
       
         
           
             
               v 
               comp 
             
             = 
             
               
                 2 
                 3 
               
               ⁢ 
               
                 
                   π 
                   ⁡ 
                   
                     ( 
                     
                       R 
                       - 
                       
                         z 
                         gap 
                       
                     
                     ) 
                   
                 
                 3 
               
             
           
         
       
     
     Estimated forces that will be provided by the glue dots can be calculated by multiplying the displaced volume (ν comp ) by a spring factor (e.g. modulus of elasticity). The exact force may not be easily predictable due to the complex nature of the system behavior and imperfect “hemispheres,” but for design purposes the graph shown in  FIG. 12  shows example system tolerances (bobbin, magnet, pole piece) along with the varying impact of different glue dot heights. 
     The graph shows glue dot compression as a percentage (%) on the x-axis verses force (N) on the y-axis. The top line (dashed) shows the comparison for a dot size of 0.004 in., the middle line (dash-dot) shows the comparison for a dot size of 0.003 in., and the bottom line (solid) shows the comparison for a dot size of 0.002 in. There is a feasible region that works within the least material condition “LMC” (largest gap between the bobbin and the magnet), and maximum material condition “MMC” (smallest gap between the bobbin and the magnet). An LMC/MMC range of the parts to establish a gap is shown as a target design window in  FIG. 12 . The target design window shows an acceptable region for the glue dots  182 . 
     In an alternative embodiment, structures known as “crush ribs” can be molded to the bobbin to arrange the magnets in the pole piece. The ribs can be located half way back along the length of the posts of the bobbin, in an area under the outer edges of the magnets. This also would allow the magnets to be tilted towards each other in the front, as the pole piece is installed over them. As the pole piece is fully installed, the magnets would pivot back around the crush rib to a parallel position, and be forced against the walls of the pole piece by the crush rib. A type of spring or rubber part is also required in this embodiment to keep pressure on the magnets holding them tight against the pole piece. 
     Aspects of the invention have been described in terms of illustrative embodiments thereof. Numerous other embodiments, modifications and variations within the scope and spirit of the disclosed invention will occur to persons of ordinary skill in the art from a review of this entire disclosure. For example, one of ordinary skill in the art will appreciate that the steps illustrated in the illustrative figures may be performed in other than the recited order, and that one or more steps illustrated may be optional in accordance with aspects of the disclosure.