Abstract:
A method of making stepper motor includes: providing a permanent magnet rotor carried by an output shaft; forming a bobbin having an alignment feature, a bearing opening for supporting the shaft, and a passage for receiving the permanent magnet rotor; forming a magnetically soft stator-yoke located around the inside of the passage; winding a coil of electrically conducting wire around the bobbin; and providing a motor housing for receiving the bobbin and where the motor housing has a corresponding alignment feature to align the bobbin with the housing.

Description:
CROSS REFERENCE TO REFERENCE TO RELATED APPLICATIONS 
     This is a divisional of application U.S. Ser. No. 08/613,730, filed Feb. 9, 1996, now abandoned. 
    
    
     FIELD OF THE INVENTION 
     The invention relates generally to the field of DC motors, and in particular to permanent magnet stepper motors. 
     BACKGROUND OF THE INVENTION 
     Permanent magnet stepper motors are currently used in a wide variety of apparatus including cameras, printers and scanners. Their ability to effect discrete and precise movement makes them the preferred choice for driving mechanical elements in this type of equipment. 
     Referring to FIG. 1, an exploded perspective view of a permanent magnet stepper motor of the type known in the prior art is shown. The stepper motor 10 includes a multi-pole permanent magnet rotor 11 carried by a shaft 12. The shaft 12 is mounted for rotation in front and back bearings 14 and 16 respectively. The bearings are mounted in front and back end plates 18 and 20. A pair of plastic bobbins 22 and 24 carry a pair of coils 26 and 28. A pair of soft magnetic metal stator yokes 30 and 32 each having a plurality of stator fingers 34, and a pair of soft magnetic stator shells 36 and 38, each having a corresponding plurality of stator fingers 40 are arranged such that the stator fingers 34 and 40 extend into the plastic bobbins in an interdigitated fashion to form stator poles when current is applied to the coils 26 and 28. The front and back end plates 18 and 20 are mounted on the outside ends of the stator shells 36 and 38, and the rotor 11 is supported by shaft 12 for rotation within the bobbins 22 and 24. The stator fingers in bobbin 22 are angularly offset from the fingers in bobbin 24 by one half the finger spacing so that current can be alternately applied to coils 26 and 28 to turn the rotor 10 in a stepwise fashion, thereby applying torque to shaft 12. 
     The manufacture and assembly of such stepper motors is presently a complex process requiring many steps and critical alignments, thereby making the motors relatively expensive. There is a need for an improved motor design and assembly method that is adapted to robotic automated assembly to reduce the cost and improve the reliability of the motors. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to overcoming one or more of the problems set forth above. Briefly summarized, according to one aspect of the present invention, a stepper motor includes an output shaft; a permanent magnet rotor carried by the output shaft; a bobbin having an alignment feature and defining a bearing for supporting the shaft and a passage for receiving the permanent magnet rotor; a magnetically soft stator-yoke located around the inside of the passage; a coil wound around the bobbin; and a motor housing for receiving the bobbin and having a complementary alignment feature to align the bobbin within the housing. 
     According to another aspect of the invention, the bobbin with the stator yoke is formed by insertion molding. According to another aspect of the invention, the bobbin with the stator yoke is formed by electroforming and insertion molding. According to yet another aspect of the invention, the bobbin with the stator yoke is formed by two step injection molding and plating. 
     These and other aspects, objects, features and advantages of the present invention will be more clearly understood and appreciated from a review of the following detailed description of the preferred embodiments and appended claims, and by reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an exploded perspective view of a prior art stepper motor; 
     FIG. 2 is an exploded perspective view of a stepper motor according to the present invention; 
     FIG. 3 is a perspective view of an assembled stepper motor according to the present invention; 
     FIG. 4 is a schematic diagram illustrating the process of forming a bobbin having an integral bearing and stator by insertion molding; 
     FIG. 5 is a schematic diagram illustrating the process of forming a bobbin having an integral bearing and stator by an electroforming and insertion molding process; and 
     FIG. 6 is schematic diagram illustrating the process of forming a bobbin having an integral bearing and stator by a two step molding and bulk plating process. 
     To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 2 shows an exploded perspective view of a stepper motor according to the present invention. The stepper motor, generally designated 10, includes a multi-pole permanent magnet rotor 11 carried by a shaft 12. The shaft 12 is mounted for rotation in front and back bearings that are formed as integral parts of plastic bobbins 22 and 24. Only back bearing 16 is shown in FIG. 2. The bobbins carry a pair of coils 26 and 28 each having leads 29 and 31. A pair of soft magnetic metal stator yokes 30 and 32 each having a plurality of stator fingers 34, and a second pair of soft magnetic stator yokes 42 and 44, each having a corresponding plurality of stator fingers 40 are arranged such that the stator fingers 34 and 40 extend into the plastic bobbins 22 and 24 in an interdigitated fashion to form stator poles when current is applied to the coils 26 and 28. The bobbin, coil and stator yokes form a bobbin assembly. Both bobbin assemblies are enclosed in a soft magnetic housing 46 that is twice the length of one of the bobbin assemblies. The housing 46 includes tabs 47 for securing the bobbin assemblies in the motor housing. 
     The rotor 11 is supported by shaft 12 for rotation within the bobbins. The stator fingers in the bobbin 22 are angularly offset from the fingers in bobbin 24 by one half the finger spacing so that current can be alternately applied to coils 26 and 28 to turn the rotor 11 in a stepwise fashion thereby applying torque to shaft 12. This angular offset is achieved by alignment features comprising tabs 48 and 50 on bobbins 22 and 24 respectively that cooperate with complementary alignment features comprising cutouts 52 and 54 on the housing 46. Tabs 48 and 50 each support electrical termination pins 56 and 58. 
     In the stepper motor manufacturing process, the bobbin assemblies are fabricated by forming the bobbins with the stator yokes, winding the coils 26 and 28 on the bobbins 22 and 24 and connecting the leads 29 and 31 to the termination pins 56 and 58 respectively. The rotor 11 is inserted between the two bobbin assemblies in the housing 46 using the alignment features to align the bobbin assemblies in the housing. Finally the tabs 47 on the housing 46 are crimped over the outside edges of the bobbin assemblies to retain the bobbins and the rotor in the housing. The bearing ends of the bobbin assemblies define the ends of the motor, thereby eliminating the need to provide separate end plates, and eliminating the steps of mounting bearings in the end plates and mounting the end plates on the motor. 
     Since the motor of the present invention incorporates fewer parts than the prior art motors, it is more easily assembled than the prior art motors and thereby more readily lends itself to robotic assembly, can be produced more economically and be made more reliable than prior art stepper motors. 
     FIG. 3 shows a perspective view of the completed stepper motor 10. Typical stepper motors according to the present invention measures 8-20 mm in outside diameter, with a 4-10 mm diameter rotors. 
     Referring to FIG. 4, one method of manufacturing a bobbin with stator yokes by insertion molding according to the present invention will be described. In step 1, a pair of stator yokes 32 and 44 formed from soft iron sheet by a compound punch and die are placed on a mandrel 60. The mandrel 60 has a first portion 62 that is slightly larger in diameter than the rotor 11 for forming the passage in the bobbin that receives the rotor and a second portion 64 that is slightly larger in diameter than the rotor shaft 12 for forming the bearing in the end of the bobbin. The mandrel 60 includes an alignment feature 66 for aligning the mandrel in an injection mold. The stator yokes 32 and 34 are placed on the mandrel 60 so that their fingers 34 and 40 are interdigitated and aligned in a preferred orientation with respect to the alignment feature 66 on the mandrel 60. The mandrel 60 bearing the stator yokes 32 and 44 is placed in the cavity 68 of an injection mold 70. The injection mold cavity 68 is configured for forming the bobbin, having bearing and alignment features, around the stator yokes. 
     In step 2, the mold 70 is closed and plastic is injected into the cavity 68. In step 3, after the plastic for forming the bobbin has been injected and allowed to cool, the mold 70 is opened. In step 4, the mandrel 60 and bobbin 22 are removed from the mold and the completed bobbin 22 is removed from the mandrel 60. 
     Referring to FIG. 5, an alternative method for forming the bobbin with the stator yokes by electroforming and insertion molding will be described. In step 1, the mandrel 60 is coated with a release agent and a conductive pattern 72 representing the stator fingers is formed on the mandrel 60 by well known photolithographic techniques. The conductive pattern 72 is formed in a preferred orientation with respect to the alignment feature 66 on the mandrel 60. In step 2, platable plastic end caps 74 are placed on the mandrel at the ends of the conductive pattern 72. In step 3, a layer of soft magnetic material is electroformed onto the conductive pattern 72 and platable end caps 74 to form the electroformed stator yokes 76. In step 4, the mandrel bearing the electroformed stator yokes is placed in the cavity of a mold 70, similar to the mold described with respect to FIG. 4, and plastic is injected into the mold. 
     In step 5, mold 70 is opened and the mandrel and bobbin are removed. In step 6, the mandrel 60 is removed from the completed bobbin 22. The electroforming method enables the formation of more uniform stator fingers that are more accurately located on the inside of the bobbin. This method is particularly useful for making very small motors, e.g. less than 6 mm in diameter. 
     Referring to FIG. 6, a further alternative method for forming the bobbin with stator yokes by two step injection molding and plating will be described. In the first step, a mold 77, has a cavity 78 that is configured to produce a portion of the bobbin that will not be plated when forming the stator yokes. A mandrel 60, is placed in the mold cavity to provide the inside diameters of the bearing and rotor passage as described above. A first non-platable plastic is injected into mold 70. In step 2, the mandrel bearing the first molded plastic features is removed from the mold 77 and placed in a second mold 80 having a cavity 82, slightly larger than cavity 78 in areas that will be plated to form the stator yokes. The mold 80 is closed and a second platable plastic is injected to form a plastic bobbin having platable areas where the stator yokes will be formed. 
     In step 3, mandrel 60 is removed from the plastic bobbin 22. In step 4, a number of plastic bobbins 22 are placed in a bulk plating tank 84, and soft magnetic material is plated onto the platable plastic portions of the bobbins to form the stator yokes. This process is less expensive than the electroforming technique described above, however the mold is more complex. It will be understood that the two step molding process may be performed in a single complex mold by moving mold parts to provide the additional volume for the platable plastic after the non-platable plastic has been injected. 
     The invention has been described with reference to a preferred embodiment. However, it will be appreciated that variations and modifications can be effected by a person of ordinary skill in the art without departing from the scope of the invention. 
     PARTS LIST 
     10 stepper motor 
     11 magnet rotor 
     12 shaft 
     14 front bearing 
     16 back bearing 
     18 front end plate 
     20 back end plate 
     22 plastic bobbin 
     24 plastic bobbin 
     26 coil 
     28 coil 
     29 coil lead 
     30 stator yoke 
     31 coil lead 
     32 stator yoke 
     34 stator finger 
     36 stator shell 
     38 stator shell 
     40 stator finger 
     42 stator yoke 
     44 stator yoke 
     46 motor housing 
     47 housing tabs 
     48 alignment tabs 
     50 alignment tabs 
     52 alignment cutout 
     54 alignment cutout 
     56 electrical termination pin 
     58 electrical termination pin 
     60 mandrel 
     62 first mandrel portion 
     64 second mandrel portion 
     66 alignment feature 
     68 mold cavity 
     70 injection mold 
     72 conductive pattern 
     74 platable plastic end caps 
     76 electroformed stator yokes 
     77 injection mold 
     78 mold cavity 
     80 injection mold 
     84 bulk plating tank