Abstract:
A permanent magnet structure for use in brushless motors is disclosed. In an exemplary embodiment of the invention, the magnet structure includes a parallelogram shaped body. The body has an outer surface and an inner surface, with the outer surface and the inner surface being arcuate in shape.

Description:
BACKGROUND  
         [0001]    Polyphase permanent magnet, brushless motors driven by a sinusoidal current offer the capability of providing low torque ripple, noise and vibration in comparison with those driven by a square wave current. Theoretically, if a motor controller can produce polyphase sinusoidal currents with the same frequency as that of the sinusoidal back EMFs, the torque output of the motor will be a constant, and zero torque ripple can be achieved. However, due to practical limitations of motor design and controller implementation, there are deviations from pure sinusoidal back EMF and current waveforms. The deviations will typically result in parasitic torque ripple components at various frequencies and magnitudes.  
           [0002]    Another component of torque ripple in a conventional permanent magnet, brushless motor is cogging torque. Cogging torque is a result of the magnetic interaction between the permanent magnets of the rotor and the slotted structure of the armature. As the leading edge of a magnet approaches an individual stator tooth, a positive torque is produced by the magnetic attraction force exerted therebetween. However, as the magnet leading edge passes and the trailing edge approaches, a negative torque is produced. The instantaneous value of the cogging torque varies with rotor position and alternates at a frequency that is proportional to the motor speed and the number of slots. The amplitude of the cogging torque is affected by certain design parameters such as slot opening/slot pitch ratio, magnet strength and air gap length.  
           [0003]    One approach to reducing torque ripple is to employ a slotless armature, which allows for precise winding patterns in order to achieve a pure sinusoidal back EMF. In addition, the absence of slots in the armature eliminates the cogging torque resulting therefrom. However, the manufacturing process for slotless motors is not well defined and thus the manufacturing costs thereof may be prohibitive.  
         SUMMARY  
         [0004]    The problems and disadvantages of the prior art are overcome and alleviated by a permanent magnet structure for use in brushless motors. In an exemplary embodiment of the invention, the magnet structure includes a parallelogram shaped body. The body has an outer surface and an inner surface, with the outer surface and the inner surface being arcuate in shape.  
           [0005]    In a preferred embodiment, the outer surface and the inner surface are generally concentric with one another. The body is preferably comprised of neodymium-iron-boron material and is epoxy coated. In an alternative embodiment, the body is nickel-plated. In still another alternative embodiment, the body is aluminum deposition coated. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]    The present invention will now be described, by way of example only, with reference to the accompanying drawings which are meant to be exemplary, not limiting, and wherein like elements are numbered alike in several Figures, in which:  
         [0007]    [0007]FIG. 1 is a schematic diagram of an electric power steering system using a polyphase brushless motor having rotor magnets in accordance with an embodiment of the invention;  
         [0008]    [0008]FIG. 2 is a perspective view of a partial rotor assembly of a motor, partially illustrating a plurality of rotor magnets mounted thereon, in accordance with an embodiment of the invention;  
         [0009]    [0009]FIG. 3 is a perspective view of one of the rotor magnets shown in FIG. 2;  
         [0010]    [0010]FIG. 4 is a side view of the magnet shown in FIG. 3;  
         [0011]    [0011]FIG. 5 is a top view of the magnet shown in FIG. 3; and  
         [0012]    [0012]FIG. 6 is an end view of the magnet shown in FIG. 3.  
     
    
     DETAILED DESCRIPTION  
       [0013]    Referring initially to FIG. 1, a motor vehicle  10  is provided with an electric power steering system  12 . Electric power steering system  12  may include a conventional rack and pinion steering mechanism  14  having a toothed rack  15  and a pinion gear (not shown) under a gear housing  16 . As steering wheel  18  is turned, an upper steering shaft  20  turns a lower shaft  22  through a universal joint  24 . Lower steering shaft  22  turns the pinion gear. The rotation of the pinion gear moves the pinion rack  15 , which then moves tie rods  28  (only one shown). In turn, tie rods  28  move steering knuckles  30  (only one shown) to turn wheels  32 .  
         [0014]    An electric power assist is provided through a controller  34  and a power assist actuator comprising a motor  36 . Controller  34  receives electric power from a vehicle electric power source  38  through a line  40 . The controller  34  also receives a signal representative of the vehicle velocity on line  41 , as well as steering pinion gear angle from a rotational position sensor  42  on line  44 . As steering wheel  18  is turned, a torque sensor  46  senses the torque applied to steering wheel  18  by the vehicle operator and provides an operator torque signal to controller  34  on line  48 . In addition, as the rotor of motor  36  turns, rotor position signals for each phase are generated within motor  36  and provided over bus  50  to controller  34 . In response to vehicle velocity, operator torque, steering pinion gear angle and rotor position signals received, the controller  34  derives desired motor phase currents. The motor phase currents are provided to motor  36  through a bus  52  to motor  36 , which thereby provides torque assist to steering shaft  20  through worm  54  and worm gear  56 .  
         [0015]    Referring now to FIG. 2, motor  36  features a rotor assembly  60 , which has a plurality of rotor magnets  62  circumferentially mounted upon a core  64 . Core  64  is preferably circular in shape and may comprise a plurality of lamina of iron, steel or other magnetic material. A central hole  66  is centrally formed within the core  64  for receiving a rotor shaft (not shown) therewithin. A pair of positioning holes  68  is disposed on opposite sides of central hole  66  for properly positioning the magnets  62  on the periphery of core  64 . A retention sleeve  70 , shown partially cut away in FIG. 2, surrounds the rotor magnets  62  for retaining the magnets therewithin during rotation of the rotor assembly  60  and is made of a non-magnetic material such as stainless steel.  
         [0016]    [0016]FIGS. 3 through 6 illustrate a permanent magnet structure for an individual rotor magnet  62 , in accordance with an embodiment of the invention. Each magnet  62  comprises a parallelogram shaped body  72  (as best seen in FIG. 5), having an arcuate outer surface  74  and an arcuate inner surface  76 . FIG. 6 particularly illustrates outer and inner surfaces  74 ,  76 , which are also generally concentric with one another.  
         [0017]    Referring to FIGS. 5 and 6, it will be seen that the parallelogram shape of each of the magnets  62  provides skewed magnetic poles on rotor assembly  60 . Body  72  has a pair of opposing side edges  78  that are parallel with one another, but which also form a skew angle with respect to the rotational axis  80  of rotor assembly  60 . The skew angle is generally defined as 360°/n, where n represents the number of slots in the stator assembly (not shown). In the embodiment shown, each magnet  62  is designed for a motor  36  having 27 slots. Thus, the skew angle of each magnet  62  shown is approximately 360°/27, or 13.33°. Naturally, if magnets  62  were to be used in conjunction with a motor having a different number of slots, the skew angle would vary accordingly.  
         [0018]    Referring again to FIG. 5, opposing end edges  82  are parallel to one another and are perpendicular to the rotational axis  80  of rotor assembly  60 . Thus configured, magnet  62  has a leading corner  84  and a trailing corner  86  with respect to the direction of rotation of the rotor assembly  60  (or leading corner  86  and trailing corner  84  if the direction is reversed).  
         [0019]    In a preferred embodiment, magnets  62  are comprised of a rare earth-based permanent magnet material such as neodymium-iron-boron, with each individual magnet  62  is also preferably epoxy coated, nickel-plated or aluminum deposition coated for corrosion protection.  
         [0020]    By using the permanent magnet structure for an individual rotor magnet as described above, it has been found that the magnitude of the 5 th  &amp; 7 th  harmonic components are reduced to less than 0.3% and 0.1% of the fundamental frequency component, respectively. Furthermore, it has also been found that the resulting cogging torque has been significantly reduced, having a value of approximately 0.002 Newton-meters (N·m).  
         [0021]    While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.