Abstract:
A permanent magnet micro-rotor comprises a cylinder of magnetic material having a plurality of spaced-apart slots disposed around the periphery of the magnetic material. The magnetic material is magnetized by placing conductors in the spaced apart slots and energizing the conductors so that magnetic poles are formed between the spaced-apart slots.

Description:
FIELD OF THE INVENTION 
     This invention relates to the fabrication of multipole permanent magnet rotors and, more particularly, to the fabrication of very high field magnetized micro-rotors for use in stepper motors, and the method of making same. 
     BACKGROUND OF THE INVENTION 
     Multipole cylindrical permanent magnets are widely used as rotors for conventional stepper motors. Conventional stepper motor rotors are as small as 0.2″ in diameter with 8 alternating magnetic poles around their circumference. These rotors can be fabricated and polarized using standard technology. Specifically, such rotors are fabricated from a cylinder of magnetic material and polarized using fixtures made by threading standard gauge wire through holes in a block of phenolic or other suitable insulating material. The threading of the wire through the holes is done in a serpentine pattern generating the alternating poles of the rotor when a high current pulse is fired through the wires. This method is applicable when the requirements for the number of poles and the pole pitch are modest. However, as the number of poles increase, for example a 20 pole roller having a diameter of only 0.13″, the pitch must be 0.2″; an increase by a factor of 10 in making the magnetized poles closer than that attained by use of the magnetizing fixtures known in the prior art. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to overcoming one or more of the problems set forth above. In summary, one aspect of the present invention is directed to a permanent magnet micro-rotor comprising: a) a cylinder of magnetic material having a plurality of spaced-apart slots disposed around the periphery of said magnetic material; said magnetic material is magnetized by placing conductors in the spaced apart slots and energizing the conductors so that magnetic poles are formed between the spaced-apart slots. 
     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. 1A illustrates the permanent magnet microrotor of the present invention in a perspective view; 
     FIG. 1B is a cross-sectional view taken along line A—A of FIG. 1A; 
     FIG. 2 is a perspective view of a conductive structure of the present invention; 
     FIG. 3 is a perspective view of the magnetizing fixture of the present invention; 
     FIG. 4A is the electrical structure of the fixture of the present invention shown in a perspective view; 
     FIG. 4B is a cross-sectional view taken along line A—A of FIG. 4A; and, 
     FIG. 5 is a cross-sectional view of the magnetizing fixture of the present invention taken along line B—B of FIG.  3 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIGS. 1A and 1B, a permanent magnet cylindrical micro-rotor  10  is shown. The permanent magnet cylindrical micro-rotor  10  is magnetized with a plurality of alternating magnetic poles  12 ,  14 ,  16 ,  18 ,  20 ,  22 ,  24 , and  26 , around its circumference. These poles are respectively separated by a plurality of grooves  32 ,  34 ,  36 ,  38 ,  40 ,  42 ,  44 , and  46 . An axial hole  48  is positioned in the center portion of the micro-motor  10  for accepting a shaft (not shown). The fabrication of micro-rotor  10  is a multi-step process. First, a permanent magnet material is selected. Preferably, the micro-rotor  10  is fabricated from a high energy isotropic material (NdFeB) having a magnetic energy product (BH)max of 7.0 MGOe for injection molded parts, or from 10-12 MGOe for compression molded parts, or from 5-10 MGOe, and surface field at the center of a pole of up to 3000 Oe. Next, the selected magnetic material is formed into the shape of the micro-rotor  10 . Typical dimensions of the micro-rotor  10  are OD from 0.012″ to 0.500″. In the preferred embodiment, the outside diameter is 0.130″, inside diameter is 0.05511″ and the length is 0.059″. It is instructive to note the presence of the surface grooves on the micro-rotor  10 ; these grooves permit a higher degree of magnetization of the micro-rotor  10  thereby enhancing its field strength and performance as will be described hereinbelow. The micro-rotor  10  can be done by injection molding process, compaction (compression) molded process, or extrusion process. For example, for the magnet rotor, the magnet could be made from a block of material that was made from one of these processes and wired by an electric discharge machine (EDM) to the desired specification. 
     The final step in the fabrication of micro-rotor  10  is its magnetization. Prior art magnetization processes were described hereinabove in the background, where the magnetic material is exposed to the high magnetic field, and the magnetizer circuits included capacitors bank, ignitron or thyristor, and pulse transformers. Preferably, in the present invention, approximately 2400 micro Farads and 1800 volts are use. The fabrication of the magnetizing fixture is described below. 
     Referring to FIG. 2, a conductive structure  50  of the present invention for magnetizing the rotor is shown. The constructive structure  50  is an element of the present invention that provides a magnetizing fixture for magnetizing the micro-rotor  10 , as will be described. The conductive structure  50  includes a series of spaced-apart conductive bars  52 ,  54 ,  56 ,  58 ,  60 ,  62 ,  64 , and  66 , conductive connectors  70 ,  72 ,  74 ,  76 ,  78 ,  80 , and  82 , and lead wires  90  and  92 . The pairs of conductive bars  54 - 56 ,  56 - 58 ,  58 - 60 ,  60 - 62 ,  62 - 64 ,  64 - 66 , and  66 - 52  are electrically connected at one end by the conductive connectors  70 ,  72 ,  74 ,  76 ,  78 ,  80 , and  82 , respectively, as shown. Lead wires  90  and  92  are electrically connected to the ends of conductive bars  52  and  54  as shown. 
     Referring to FIG. 3, a magnetizing fixture  100  of the present invention into which magnetizing fixture  100  the conductive structure  50  (as shown in FIG. 2) is mated, as described hereinbelow. The magnetizing fixture  100  includes an insulating support structure  110  that encases the conductive structure  50 . To fabricate the magnetizing fixture  100 , the conductive structure  50  is potted using an insulating epoxy, and the center of the potted structure is axially cored out to provide a cavity  95  into which the micro-rotor  10  fits with a small amount of clearance therebetween. The cavity  95  is of such dimensions that portions of the conductive bars  52 ,  54 ,  56 ,  58 ,  60 ,  62 ,  64 , and  66  protrude into cavity  95  and are exposed as shown (only exposed portions of conductive bars  64  and  66  are shown). 
     Referring to FIGS. 4A and 4B, the electrical structure of the fixture  100  is shown without the insulating support structure  110  (as shown in FIG.  3 ). In FIG. 4A, the rotors  130  and  140  are shown positioned in the fixture  100 , prior to magnetization. It is a feature of the present invention that when the unmagnetized rotors  130 , and  140  are properly seated in the fixture  100 , the grooves  32 ,  34 ,  36 ,  38 ,  40 ,  42 ,  44 , and  46  of unmagnetized rotors  130 , and  140  are aligned with, and partially surround, conductive bars  52 ,  54 ,  56 ,  58 ,  60 ,  62 ,  64 , and  66 , respectively, as shown. This is to be noted that, when a pulse (50 to 100 microseconds) of high current (10,000 to 50,000 amps) flows through the conductive bars  52 ,  54 ,  56 ,  58 ,  60 ,  62 ,  64 , and  66 , the magnetic field that they produce penetrates unmagnetized rotors  130 , and  140  thereby magnetizing them with the magnetization pattern shown in FIGS. 1A and 1B. The present invention is directed to the magnetization of high coercivity micro-rotors (Hci is approximately 10 kOe). To meet impedance of the magnetizer to the fixture, a high magnetic energy is needed. To insure saturation of the magnetic material, it is necessary that the magnetizing field be greater than the coercivity of the material being magnetized. This requires sufficient current flowing through the conductive structure to provide the required field. On the other hand, too high a current can result in electromagnetically induced stresses of sufficient intensity to cause the fixture to explode. To magnetize the above referenced micro roller of NdFeB, intrinsic coercivity of about 10000 Oe, with OD=0.6″, ID=0.5″, and L=0.1″, and having from 36 to 48 poles, a Model 8500 magnetizer, manufactured by Magnetic Instrumentation, Inc., had its capacitor bank tailored to the size of the fixture. The magnetizer&#39;s capacitor bank, set at 1600 micro farads was charged to 1600 volts and discharged by an Ignitron through the fixture. Current pulses on the order of 50,000 amperes, lasting approximately 50-100 microseconds saturate the NdFeB microrotor without damage to the fixture. 
     Referring to FIG. 5, a cross-sectional view of the magnetizing fixture  100  is shown taken along line B—B of FIG.  3 . Before the unmagnetized rotor  130  is magnetized, a soft ferromagnetic element  160  is inserted into the axial hole (cavity)  48  of unmagnetized rotor  130 . The ferromagnetic element  160  is preferably formed from a soft magnetic material including permalloy, supermalloy, sendust, iron, nickel, nickel-iron or alloys thereof. The function of the ferromagnetic element  160  is to enhance the penetration of the magnetizing field created by energized conductive bars  52 ,  54 ,  56 ,  58 ,  60 ,  62 ,  64 , and  66 , thereby enhancing the magnetization of rotor  130 . This, in turn, enhances the performance of the stepper motor. 
     The invention has been described with reference to a preferred embodiment. However, it will be appreciated that variations and modifications an be effected by a person of ordinary skill in the art without departing from the cope of the invention. 
     PARTS LIST 
       10  micro-rotor 
       12  alternating magnetic pole 
       14  alternating magnetic pole 
       16  alternating magnetic pole 
       18  alternating magnetic pole 
       20  alternating magnetic pole 
       22  alternating magnetic pole 
       24  alternating magnetic pole 
       26  alternating magnetic pole 
       32  groove 
       34  groove 
       36  groove 
       38  groove 
       40  groove 
       42  groove 
       44  groove 
       46  groove 
       48  axial hole 
       50  conductive structure 
       52  conductive bar 
       54  conductive bar 
       56  conductive bar 
       58  conductive bar 
       60  conductive bar 
       62  conductive bar 
       64  conductive bar 
       66  conductive bar 
       70  conductive connector 
       72  conductive connector 
       74  conductive connector 
       76  conductive connector 
       78  conductive connector 
       80  conductive connector 
       82  conductive connector 
       90  lead wire 
       92  lead wire 
       95  cavity 
       100  magnetizing fixture 
       110  insulating support structure 
       130  unmagnetized rotor 
       140  unmagnetized rotor 
       160  ferromagnetic element