Abstract:
A permanent magnet motor for position sensorless drive operation provides a stator design that exhibits a saliency (machine asymmetric) functionally dependent on rotor position as caused by periodic magnetic saturation of stator structure. This saturation property is caused by rotor zigzag leakage flux from surface permanent magnets. The stator structure may be designed to further saturate from zigzag leakage flux to provide greatest spatial saliency in the quadrature phase for motor position sensorless position estimation. The position, velocity, and shaft torque can be extracted by measuring the phase current from the stator coil of permanent magnet motor.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Not applicable. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to position sensorless permanent magnet motor drives and in particular to a method of sensing rotor position in such motors using stator-based saliency (sometimes referred to as motor asymmetry). 
     Permanent magnet synchronous motors provide a torque through the interaction of a magnetic field generated by one or more electrical coils and one or more permanent magnet elements. The magnetic field generated by the current supplied to the coils must be synchronized with the position of the magnetic field of the permanent magnets and consequently such motors are normally termed “synchronous motors”. 
     Permanent magnet synchronous motors place the permanent magnets on the rotor to provide multiple, angularly spaced magnetic poles (eliminating the need for brushes to carry power to the rotating rotor) and sense rotor position to properly synchronize the drive current to coils in the stator. 
     In a “surface permanent magnet synchronous motor” (SPMSM), the permanent magnets of the rotor are on the surface of the rotor. In an “interior permanent magnet synchronous motor” (IPMSM), the permanent magnets are embedded in a metallic core of the rotor. 
     The sensing of rotor position, in order to properly coordinate the drive signals to the coils of the stator, may be accomplished by a variety of means. For example, one or more Hall effect sensors may be placed on the stator to sense changes in the magnetic field generated by the rotor as it spins. Alternatively, position resolution may be obtained by mounting an encoder or a resolver to the rotor shaft. Absolute rotor position is not necessary but only relative position of the motor poles with respect to the stator coils. 
     Discrete rotor position sensors may substantially decrease the reliability of the motor by introducing additional electrical conductors and connectors that are substantially less robust than the motor windings and their terminations. Accordingly there is considerable interest in producing “position sensorless” motor drives, for example, by analyzing slight perturbations in the drive currents caused by motor saliency. Saliency, as used herein, refers to periodic fluctuations in the electrical properties of the motor as a function of the position of the rotor. These saliency-based sensorless control implementations are described in U.S. Patent Publication Nos. US 1996/5565752, US 2000/6069467, US 2006/7045988 B2 and US 2006/7098623 B2 all hereby incorporated by reference. 
     One type of motor saliency that may be exploited for this purpose is the change in reluctance between stator coils caused by the permanent magnet inclusions in the rotor in an IPMSM design. As the rotor moves and the permanent magnet inclusions move into and out of an inter-coil path, this change in reluctance may be detected by a carrier current superimposed on the drive signal of the stator. IPMSM sensorless drive is reported in “Ogasawara et al., Implementation and position control performance of a position sensorless IPM motor drive system based on magnetic saliency,”  Industry Applications, IEEE Transactions on , vol. 34, pp. 806-812, 1998 also hereby incorporated by reference. 
     In a SPMSM design, small amounts of exposed steel in between the magnets may saturate at particular rotor positions. Again, a high frequency injection signal may be used to detect this saturation which will result in a modulation of a measured induction of the stator coils. SPMSM sensorless drive is described in “Jang et al., Sensorless drive of surface-mounted permanent-magnet motor by high-frequency signal injection based on magnetic saliency,”  Industry Applications, IEEE Transactions on , vol. 39, pp. 1031-1039, 2003 also hereby incorporated by reference. 
     Current position sensorless detection systems are relatively ineffective in SPMSM designs which provide a substantially continuous outer magnetic surface. Many position sensorless systems are susceptible to interference by the drive signal which may obscure sensitive measurements of inductance or reluctance. 
     SUMMARY OF THE INVENTION 
     The present invention provides a motor design in which a saliency is implemented through stator design and, in particular, through design of stator teeth to produce measurable saturation as a function of rotor position. In one embodiment, the stator is designed to be saturated by “zigzag” leakage flux conducted between rotor magnetic poles and thus may be out of phase with the drive current of the stator. This phasing permits improved extraction of the sensing spatial signal with reduced interference from the drive signal. Various embodiments of the invention also provide for preferred winding arrangements and numbers of rotor poles and stator slots. 
     Specifically, the present invention provides a permanent magnet synchronous motor system using a motor having a rotor supported to rotate about an axis and holding at least one permanent magnet providing multiple angularly spaced magnetic poles, and having a stator fitting about the rotor so that the rotor may rotate within and with respect to the stator. The stator provides a plurality of angularly spaced teeth extending inward toward the rotor and electrically conductive coils coiling around at least some of the teeth to apply a magnetic field thereto. A position detection unit provides an injection signal generator applying an electrical signal to the coils at a frequency substantially higher than a drive frequency applied to the coils, the latter to cause rotation of the rotor. An injection signal analyzer analyzes the injection signal to detect variations in the injection signal functionally related to the rotor position and caused by magnetic saturation of the teeth by the permanent magnets of the rotor to deduce rotor position. 
     It is thus a feature of a least one embodiment of the invention to permit position sensorless operation for motor types having zero or very low saliency in the rotor. By introducing the saliency in the stator, SPMSM sensorless drives may be used effectively. 
     The permanent magnet may be positioned on an outer surface of the rotor. 
     It is thus a feature of a least one embodiment of the invention to provide a position sensorless system suitable for surface permanent magnet synchronous motors that are not readily adaptable to systems that measure variation in reluctance between coils. 
     The outer surface of the rotor may provide a substantially angularly unbroken permanent magnet material. 
     It is thus a feature of a least one embodiment of the invention to provide a position sensorless system suitable for rotors having a uniform outer surface tending to have zero or very low saliencies. 
     The teeth of the stator may provide circumferentially extending bridges having a predetermined thickness selected so that the rotor causes magnetic saturation in the bridges at periodic rotor positions. 
     It is thus a feature of a least one embodiment of the invention to provide a system adaptable to a wide range of stator designs. 
     The stator may be formed to produce a maximum saturation under the influence of rotor zigzag leakage flux between rotor magnetic poles in a SPMSM. 
     It is thus a feature of a least one embodiment of the invention to provide a saturation effect that is largely indifferent to the distal structure of the stator and its coil but instead determined primarily by the stator tip. It is another feature of a least one embodiment of the invention to provide for a saturation that affects the spatial saliency of the motor to be more robust against load changes and drive induced effects that more strongly affect the quadrature phase inductance L q . 
     The number of teeth may differ by less than three from two times a number of pole pairs. 
     It is thus a feature of a least one embodiment of the invention to provide a motor harmonic structure that improves the ability to detect the stator-based saliency of the present invention. 
     Every tooth of the stator may be surrounded by a coil. 
     It is thus a feature of a least one embodiment of the invention to adopt a double-layer winding that accentuates the stator-based saliency being detected. 
     The permanent magnet synchronous motor system may further include a drive circuit for applying electrical power to the coils as a function of the deduced rotor position. 
     It is thus a feature of a least one embodiment of the invention to provide a position sensorless drive system for a permanent magnet synchronous motor. 
     The drive circuit may further apply electrical power to the coil as a function of detected back-EMF in the coil. 
     It is thus a feature of a least one embodiment of the invention to provide a drive that may use stator-based saliency for low-speed operation and conventional back-EMF (electromotive force) detection for high-speed operation. 
     These particular features and advantages may apply to only some embodiments falling within the claims, and thus do not define the scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  is a simplified block diagram of the motor system of the present invention providing a motor and control electronics; 
         FIG. 2  is a cross-sectional view through a simplified motor per the present invention showing stator teeth approaching a central rotor, the latter having surface-mounted magnets; 
         FIG. 3   a  is a simplified representation of the motor of  FIG. 2  “unrolled” for clarity and showing a given stator tooth during the direct phase, when relatively large drive power is applied to the stator tooth, showing a simplified flux conduction path for drive flux concentrated along a central axis of the tooth; 
         FIG. 3   b  is a figure similar to that of  FIG. 3   a  showing the given stator tooth during the quadrature phase, when relatively low drive power is applied to the stator tooth, showing a simplified flux conduction path for “zigzag” leakage flux directed through the bridges on either side of the stator tooth; 
         FIG. 4  is a detailed fragmentary view of the tip of the stator tooth showing different areas of saturation in the quadrature and direct phases; 
         FIG. 5  is a simplified representation of measured inductance in a particular stator coil associated with a given stator tooth at various angular positions of the rotor; 
         FIG. 6  is a fragmentary schematic representation of the stator tooth and rotor magnets showing particular dimensions that may be optimized to promote saliency; and 
         FIG. 7  is a flow chart of the principal operations executed by the program of  FIG. 1  in low-speed and high-speed operation. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to  FIG. 1 , a position sensorless permanent magnet synchronous motor system  10  may include a permanent magnet motor  12  receiving electrical drive power over leads  14  from a drive system  16 . 
     The hardware of the drive system  16  may be of conventional design providing drive electronics  18  receiving electrical power  20  and synthesizing, for example using pulse width modulation, arbitrary waveforms on leads  14 . The drive electronics  18  may be controlled by a microprocessor  22  or the like communicating with a memory  24  holding a stored program  26  providing for standard control algorithms together with the rotor position-sensing system of the present invention, as will be described in more detail below. The microprocessor  22  may monitor the current through the leads  14  using current sensors  28  and the voltage on the leads  14  via signals provided by the drive electronics  18  for the rotor position-sensing system of the present invention as well as the standard control of the motor  12 . As will be understood in the art, the microprocessor  22  further may receive a command signal  30 , for example, torque, current, velocity or the like, for the control of the motor  12 . 
     As will be described in more detail below, the program  26 , when executed, will control the drive electronics  18  to provide over each drive line  14 , both drive power  32  providing a fundamental component energizing the motor  12  and an injection signal  34  used for position sensing per the present invention. Generally the injection signal  34  will be of lower amplitude and higher frequency than the drive power  32  and in one embodiment of the present invention will be on the order of one kilohertz. 
     Referring now to  FIG. 2 , motor  12  may be a surface mount permanent magnet synchronous motor having a central rotor  36  supported on bearings to rotate about a rotational axis  38 . In one embodiment of the invention, the rotor  36  has on its outer surface a continuous cylindrical magnetic ring  40  magnetized to provide multiple magnetic poles  41  of alternating polarity. Optionally, multiple magnets, each forming a single pole, may be mounted around the periphery of the rotor  36  with alternating polarities. The poles  41  are separated at regular intervals, angularly, about the axis  38 . In the simplified example of  FIG. 2 , four poles are shown of alternating north and south polarity. Because the surface of the rotor  36  is continuous and of a uniform material, saliency in the rotor  36  is low. 
     The rotor  36  fits within a stator  42  being formed, for example, of a set of ferromagnetic laminations providing a set of radially oriented teeth  44  extending inward from outer stator ring  47  to angularly spaced points proximate to the outer surface of the rotor  36 . In a preferred embodiment, each tooth  44  is surrounded with a multi-turn electrical coil  46 , for example, of copper wire whose ends provide leads  14  to receive the drive power  32  and the injection signal  34 . This configuration is termed “double-layer” winding and differs from a single layer winding in which only every other tooth has a coil  46  whose windings fully fill the gaps between the teeth  44 . Generally the leads  14  from the coils  46  will be combined in commonly driven phases of signals produced by the drive electronics  18 . 
     The end of each tooth  44  proximate to the rotor  36  may have tips  48  extending circumferentially on either side of the tip of the tooth  44  by equal distances. The bridges  48  may also curve to follow the surface of the rotor  36 . These bridges  48  may exist in a conventional design to retain the coils  46 , but in the present invention are enlisted to provide for saliency as will be described below. 
     Referring now to  FIG. 3   a , when a given stator tooth  44  is aligned with the rotor in the direct phase (meaning that a pole  41  of the rotor  36  is substantially centered along a radial central axis  50  of the stator tooth  44 ) the coil  46  will receive drive power  32  with a superimposed injection signal  34 , where the drive power  32  has a polarity such as to promote an attraction with the aligned pole  41  of the rotor  36 . The action of the coil  46  receiving drive power  32  is such as to draw flux lines  52  from the pole  41  substantially along the central axis  50  of the stator tooth  44  and thus somewhat removed from the bridges  48 . It will be noted generally that this concentration will be enhanced by a magnetic pole  41  that has smaller angular extent than the width of the bridges  48  leading to a general design principle as will be discussed below. 
     Referring to  FIG. 3   b , in contrast, when the stator tooth  44  is aligned with the rotor  36  in the quadrature phase with a central axis  50  substantially between centers of two poles  41  of the rotor  36 , the coil  46  will be receiving substantially only the injection signal  34  and thus will not contribute significantly to the flux path through the tip of the stator tooth  44 . At this time, a rotor zigzag leakage flux represented by flux lines  52  passes laterally through the stator tooth  44  along the bridges  48  between the poles  41  flanking the central axis  50 . This flux path provides substantially greater flux through the bridges  48  increasing the saturation of the material of the bridges  48 . 
     Referring to  FIG. 4 , the different flux patterns described above will result in two differing saturation modes of the stator tooth  44  providing a substantially greater saturation region  54  during the quadrature phase, the saturation region extending substantially into the bridges  48 , and a substantially lesser saturation region  54 ′ occurring during the direct phase because of the substantially lower flux density in the material of the bridges  48  during that time. The thinness of the bridges  48  promotes a high flux density promoting saturation as will be described below. 
     Referring now to  FIG. 5 , the saturation of the material of the stator tooth  44  will reduce the effective inductance of the coil  46  as may be measured by the injection signal  34  (as affecting the impedance experienced by that signal) per conventional techniques. This measurement may be synchronized with the phase of the rotor  36  (and the drive power  32 ) so as to distinguish between inductance during the quadrature phase L q  and inductance during the direct phase L d  to better isolate the measured saliency using understood phase decomposition techniques. For a given stator tooth  44 , the inductance function  56  (L) of that tooth  44  will have a minimum  58  occurring at quadrature phases of the rotor  36 . This minimum  58  and the shape of the curve about the minimum  58  may be used to accurately determine the position of the rotor  36  with respect to the stator  42  within the range of stator tooth angular periodicity. 
     This position information may be used to synchronize the drive power  32  with rotor position for conventional motor operation. 
     Because the measured inductance is based on the injection signal excitation and is not principally a function of rotor speed, this sensing technique is well suited to low rotor speeds. 
     Referring now to  FIG. 6 , the degree of the saliency provided by the saturation of the rotor tip may be enhanced by adjusting the dimensions of various components of the motor  12  according to some general design rules. A first rule is that the separation between the poles of the magnets on the rotor should be small compared to the spacing between the stator teeth. That is: 
     
       
         
           
             
               
                 
                   
                     w 
                     r 
                   
                   ≤ 
                   
                     
                       
                         w 
                         o 
                       
                       + 
                       
                         w 
                         b 
                       
                     
                     2 
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     where w r  is the circumferential separation between magnetic poles on the rotor; 
     w o  is a circumferential separation between adjacent ends of bridges on adjacent stator teeth; and 
     w b  is the circumferential width of the stator tooth from one bridge to the other. 
     This rule generally expresses the fact that the stator tooth should form a conduit for the rotor zigzag leakage flux between adjacent poles. 
     A second design rule is that the bridge height should generally be relatively thin as expressed by the following equation: 
     
       
         
           
             
               
                 
                   
                     d 
                     ≤ 
                     
                       
                         0.66 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         
                           
                             
                               ( 
                               
                                 
                                   w 
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                                   w 
                                   o 
                                 
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                                   w 
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                             2 
                           
                           
                             4 
                             ⁢ 
                             
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                                   w 
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                   = 
                   
                     
                       0.66 
                       ⁢ 
                       
                         
                           
                               
                           
                           ⁢ 
                           
                             
                               ( 
                               
                                 
                                   τ 
                                   s 
                                 
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                               ⁢ 
                               
                                   
                               
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                           4 
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     where τ s  is the circumferential width of the stator slot width 
     d is the radial height of the bridge. 
     Generally it will be noted that the rotor zigzag leakage flux has greater saturation in the bridges  48  (shown with respect to  FIG. 4 ) by decreasing the slot pitch TS. 
     Desirably, too, the stator teeth  44  will be designed to reduce the harmonic content of the inductance function  56  (shown with respect to  FIG. 5 ) as may be fine-tuned by modeling or the like. 
     The present inventors have also determined that particular combinations of numbers of the poles and stator teeth (slots) provide improved saliency detectability for stator-based saliency and, in particular, that those combinations described by the following formulas are preferred:
 
 N   s =2 p± 2  (3)
 
 N   s =2 p± 1  (4)
 
     where N s  is the number of slots; and 
     p is the number of magnetic pole pairs respectively. 
     Referring now to  FIG. 7 , the present invention contemplates that the sensed position may be used for control of the drive electronics  18  in two distinct operating modes. During low-speed operation, under the control of the program  26  as indicated by process block  60 , the drive electronics  18  will be controlled to inject the injection signal  34 . At process block  62 , the injection signal  34  may be used to detect the saliency using the inductance measuring techniques described above. Because the measured inductance is largely indifferent to rotor speed, this technique is particularly suited for low-speed operation. 
     During high-speed operation, as indicated by process block  64 , the injection signal  34  may be halted, and a conventional back-EMF detection process may be used, per process block  66 , measuring the voltage reflected back into the coils  46  by passage of the magnetic poles  41  for position detection. Sensorless operation based on back-EMF methods are reported in U.S. Patent Publication Nos. US 2005/6924617 B2, US 2006/7026772 B2, US 2007/7245104 B2 and US 2003/6552509 B2 also hereby incorporated by reference. In both cases the detected position may be used to synchronize the drive power  32  appropriately as is understood in the art. 
     Certain terminology is used herein for purposes of reference only, and thus is not intended to be limiting. For example, terms such as “upper”, “lower”, “above”, and “below” refer to directions in the drawings to which reference is made. Terms such as “front”, “back”, “rear”, “bottom” and “side”, describe the orientation of portions of the component within a consistent but arbitrary frame of reference which is made clear by reference to the text and the associated drawings describing the component under discussion. Such terminology may include the words specifically mentioned above, derivatives thereof, and words of similar import. Similarly, the terms “first”, “second” and other such numerical terms referring to structures do not imply a sequence or order unless clearly indicated by the context. 
     When introducing elements or features of the present disclosure and the exemplary embodiments, the articles “a”, “an”, “the” and “said” are intended to mean that there are one or more of such elements or features. The terms “comprising”, “including” and “having” are intended to be inclusive and mean that there may be additional elements or features other than those specifically noted. It is further to be understood that the method steps, processes, and operations described herein are not to be construed as necessarily requiring their performance in the particular order discussed or illustrated, unless specifically identified as an order of performance. It is also to be understood that additional or alternative steps may be employed. 
     References to “a microprocessor” and “a processor” or “the microprocessor” and “the processor,” can be understood to include one or more microprocessors that can communicate in a stand-alone and/or a distributed environment(s), and can thus be configured to communicate via wired or wireless communications with other processors, where such one or more processor can be configured to operate on one or more processor-controlled devices that can be similar or different devices. Furthermore, references to memory, unless otherwise specified, can include one or more processor-readable and accessible memory elements and/or components that can be internal to the processor-controlled device, external to the processor-controlled device, and can be accessed via a wired or wireless network. In the present invention processors should be considered to embrace any functionally capable circuitry including, for example, field programmable gate arrays and the like 
     It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein and the claims should be understood to include modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments as come within the scope of the following claims. All of the publications described herein, including patents and non-patent publications, are hereby incorporated herein by reference in their entireties.