Abstract:
A multi-phase electric motor including a housing, a stator mounted to the housing, a rotor rotatably mounted relative to the stator, and a position sensing system configured and disposed to output a signal representing a position of the rotor relative to the stator. The position sensing system includes a rotating member mounted relative to the rotor and a plurality of digital sensors mounted relative to the rotating member. At least two of the plurality of digital sensors are configured and disposed to generate a quadrature output signal. The plurality of digital sensors being configured and disposed to sense discrete portions of the rotating member to detect a position of the rotor relative to the stator.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    Exemplary embodiments pertain to the art of electric motors and, more particularly, to a position sensing system for a three-phase electric motor. 
         [0002]    It is desirable to know rotor position relative to a stator of a multi-phase electric machine before activation. Knowledge of the rotor position enables the multi-phase electric machine to be activated in such away so as to achieve a desired direction of rotation. In addition, it is desirable to monitor rotor position to allow for proper timing of current. That is, applying current when the rotor is in a particular position range relative to the stator results in maximum output torque. Without proper timing, the multi-phase motor will perform poorly, operate in a reverse direction, or not operate at all. 
         [0003]    Current systems for monitoring motor position include rotary encoders and resolvers. Rotary encoders are electro-mechanical devices that convert angular position of a rotor shaft to an analog or digital code. Generally, rotary encoders include an encoder housing that is mounted externally to the multi-phase electric motor. Rotary encoders include both mechanical encoders and optical encoders. Mechanical encoders include a metal disc containing a concentric rings of openings fixed to an insulating disk that is rigidly mounted to the rotor shaft. A row of sliding contacts is fixed to a stationary object, such as the encoder housing, such that each contact wipes against the metal disc at a different distance from the shaft. The contacts signal a presence or absence or material on the metal disc to provide electric signals that are representative of shaft position. Optical encoders employ discs made from glass or plastic having transparent and opaque areas. A light source directs light at the disc and a photo-detector reads optical patterns passing through the disc to provide signals representative of shaft position. 
         [0004]    Resolvers are rotary transformers that are mounted to a multi-phase electric motor. A brushless transmitter resolver includes a stator and a rotor. The stator includes three windings, an exciter winding and two-phase windings. The exciter winding forms part of a transformer. The two phase windings are arranged 90 degrees offset from the exciter winding. A sinusoidal electric current is induced into the exciter winding. The current flows to the two-phase windings producing a sinusoidal and cosine feedback current each having an associated voltage. The relative magnitude of the two voltages is measured to determine an angle of the rotor relative to the stator. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0005]    Disclosed is a multi-phase electric motor including a housing, a stator mounted to the housing, a rotor rotatably mounted relative to the stator, and a position sensing system configured and disposed to output a signal representing a position of the rotor relative to the stator. The position sensing system includes a rotating member mounted relative to the rotor and a plurality of digital sensors mounted to the housing relative to the rotating member. At least two of the plurality of digital sensors are configured and disposed to generate a quadrature output signal. The plurality of digital sensors being configured and disposed to sense discrete portions of the rotating member to detect a position of the rotor relative to the stator. 
         [0006]    Also disclosed is a method of sensing a position of a rotor relative to a stator of a multi-phase electric motor. The method includes digitally sensing with a first digital sensor a first trigger element arranged on a first sensing portion of a rotating member mounted to the rotor of the multi-phase electric motor. The first trigger element includes a first sensing portion and a first transition portion. The method also includes sensing with a second digital sensor a second trigger element arranged on the first sensing portion. The second trigger element is ninety degrees out of phase relative to the first trigger element. A quadrature output signal having a quadrature signal period is generated from the first and second digital sensors. The method further includes digitally sensing with a third digital sensor a trigger member arranged on a second sensing portion of the rotary member, generating an output signal from the third digital sensor, and determining a position of the rotor relative to the stator based on the quadrature signal and the output signal from the third digital sensor. 
         [0007]    Further disclosed is a position sensing system for sensing a position of a rotor relative to a stator. The position sensing system includes a rotating member, and a plurality of digital sensors mounted relative to the rotating member. At least two of the plurality of digital sensors being configured and disposed to generate a quadrature output signal. The plurality of digital sensors are configured and disposed to sense discrete portions of the rotating member to detect a position of the rotor relative to the stator. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    The following descriptions should not be considered limiting in any way. With reference to the accompanying drawings, like elements are numbered alike: 
           [0009]      FIG. 1  depicts a cross-sectional view of an three phase electric motor including a position sensing system in accordance with an exemplary embodiment; 
           [0010]      FIG. 2  depicts a plan view of the position sensing system of  FIG. 1 ; 
           [0011]      FIG. 3  depicts a detail view of a portion of the position sensing system of  FIG. 2 ; and 
           [0012]      FIG. 4  depicts output signals from the position sensing system in accordance with an exemplary embodiment. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0013]    A detailed description of one or more embodiments of the disclosed apparatus and method are presented herein by way of exemplification and not limitation with reference to the Figures. 
         [0014]      FIG. 1  illustrates an electric machine, shown in the form of a multi-phase electric motor  2 , in accordance with an exemplary embodiment. As will be detailed more fully below, multi-phase electric motor  2  takes the form of a three-phase electric motor. Multi-phase electric motor  2  includes a housing  4  having an outer surface  6  and an inner surface  7  that defines an interior portion  10 . A connector housing  12  is mounted to outer surface  6 . A plurality of electric supply cables  14 - 16  pass into connector housing  12 . Multi-phase electric motor  2  is also shown to include a stator  20  operatively connected to inner surface  8  of housing  4  and electrically connected to the plurality of electric supply cables  14 - 16 . More specifically, stator  20  includes a plurality of stator phase windings (not separately labeled) electrically connected to electric supply cables  14 - 16 . That is, stator  20  includes a first phase stator phase winding connected to electric supply cable  14 , a second stator phase winding connected to electric supply cable  15 , and a third stator phase winding connected to electric supply cable  16 . A rotor  28  is rotatably mounted within interior portion  10 . Rotor  28  includes a rotor hub  29  that supports a plurality of rotor laminations  30  that are rotated relative to stator  20 . Rotor laminations  30  include a plurality of rotor phase windings (not separately labeled). More specifically, rotor laminations  30  include a first rotor phase winding, a second rotor phase winding and a third rotor phase winding. An output shaft  38  is mounted to rotor hub  29 . Output shaft  38  is supported by first and second bearings  39  and  40  and provides a mechanical interface to a driven member (not shown). 
         [0015]    Multi-phase electric motor  2  is electrical connected to a controller  41  that establishes a desired rotational speed, and rotational direction for rotor  28 . However, prior to any application of current to multi-phase electric motor  2 , it is desirable to sense a position of rotor  28  relative to stator  20 . Sensing a relative position of rotor  28  to stator  20  allows controller  41  to initially apply current to a desired one of the stator phase windings. In addition to initial current application, controller  41  establishes a desired current timing. That is, controller  41  applies current when rotor  28  is in a particular position range relative to stator  20  in order to produce a desired output torque from multiphase electric motor  2 . In order to sense the position of rotor  28  relative to stator  22 , multi-phase electric motor  2  includes a position sensing system  50 . 
         [0016]    In accordance with an exemplary embodiment, position sensing system  50  includes a rotary member  54  and a plurality of digital sensors  60 - 62  fixedly mounted relative to housing  4  and electrically connected to controller  41 . As best shown in  FIGS. 2 and 3 , rotary member  54  takes the form of a tone wheel  70  having a first sensing portion  74  and a second sensing portion  75 . First sensing portion  74  includes a plurality of trigger elements, one of which is indicated at  77  that are positioned at the first and second rotor phases. Each trigger element  77  includes a sensing portion  90  having a first transition portion  92 , and a second transition portion  93 . Trigger elements  77  collectively establish a 50% duty cycle that indicates a position of the first rotor phase and the second rotor phase to establish a quadrature sensing period  95 . More specifically, digital sensors  60  and  61  take the form of, for example, Hall Effect sensors that are positioned to detect trigger elements  77 . Digital sensor  61  is arranged about 90° out of phase relative to digital sensor  60 . With this arrangement, as tone wheel  70  rotates digital sensor  60  detects sensing portions  90  to produce a first quadrature output signal  140  having a first quadrature period  142 , and digital sensor  61  detects sensing portions  90  to produce a second quadrature output signal  144  having a second quadrature period  146  such as shown in  FIG. 4  and in the Table 1 below. In accordance with one aspect of the exemplary embodiment, trigger elements  77  are arranged to produce first and second quadrature sensing signals  140  and  144  having eight (8) cycles per phase. With digital sensor  61  being 90° out of phase relative to digital sensor  60  quadrature output signals provide position accuracy of about 11.25° for the first and second rotor phases 
         [0000]    
       
         
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Sensor 60 
                 Sensor 61 
               
               
                   
                   
               
             
             
               
                   
                 1 
                 1 
               
               
                   
                 0 
                 1 
               
               
                   
                 0 
                 0 
               
               
                   
                 1 
                 0 
               
               
                   
                   
               
             
          
         
       
     
         [0017]    In further accordance with the exemplary embodiment, second sensing portion  75  extends concentrically about first sensing portion  74  and includes a plurality of trigger members  107  that are closely aligned with the third rotor phase. Each trigger members  107  includes a sensing section  120  having a first transition section  122  and a second transition section  123  that collectively establish a sensing period  125 . In accordance with the exemplary embodiment, first transition section  122  is closely aligned with one of the first transition portions  92  of trigger elements  77 . Digital sensor  62  is positioned to detect sensing sections  120  to produce an output signal  150  having an output signal period  152  that in accordance with one aspect of the exemplary embodiment is less than each of quadrature output periods  142  and  144 . In accordance with another aspect of the exemplary embodiment, output signal period  152  is an integer multiple of quadrature output periods  142  and  146 . In this manner, a positive output from digital sensor  62  aligns with applied force of the third rotor phase in one direction (e.g., clockwise) and a negative output from digital sensor  62  aligns with applied force of the third rotor phase in an opposite direction (e.g., counter-clockwise) to increase position detection accuracy of position sensing system  50 . 
         [0018]    In the above described arrangement, output from digital sensor  60  aligns with output from digital sensor  62  to provide controller  41  with position indication that allows for proper current application to achieve desired torque output. While position accuracy may be lower during a first portion of rotor movement leading to a slight reduction of torque at initial start-up, after a first transition of signal  150 , position sensing system signals precise rotor position to controller  41 . Thus following only a small rotation of rotor  28 , full motor capability is available. If multi-phase electric motor is used as power for a vehicle, full motor capability would be available after only a few centimeters of movement. 
         [0019]    At this point, it should be understood that position sensing system  50  provides a low cost system for detecting rotor position of a multi-phase motor. In addition to low cost, the position sensing system in accordance with the exemplary embodiment has a small form factor. That is, in contrast to existing resolvers and encoders that increase a size of a motor assembly, the position sensing system in accordance with the exemplary embodiment allows for the design and construction of smaller multi-phase electric motors having without sacrificing operating characteristics. It should also be understood that while described as Hall Effect sensors, the digital sensors can take on a variety of forms. Furthermore, while described as including three digital sensors with one sensor closely aligned with a rotor phase winding, additional accuracy could be realized with the addition of a fourth sensor closely aligned with another of the rotor phase windings. 
         [0020]    While the invention has been described with reference to an exemplary embodiment or embodiments, 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 claims.