Abstract:
An electric machine includes a stator and a rotor interacting with the stator, wherein the stator and/or the rotor has/have a conducting device for guiding a magnetic flux. The conducting device includes a gap which is provided in a path of the magnetic flux, and a sensor which is received in the gap for measuring the magnetic flux.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS  
       [0001]     This application claims the priority of German Patent Application, Serial No. 103 29 150.4, filed June  27 ,  2003 , pursuant to 35 U.S.C. 119(a)-(d), the disclosure of which is incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     The present invention relates, in general, to an electric machine.  
         [0003]     Electric machines of a type involved here typically include a primary (normally the stator) and a secondary (normally the rotor or armature). Examples of electric machines include a linear motor or a torque motor. Oftentimes the position of the stator or the rotor has to be determined. For example, when determining the position of the primary in relation to a permanent-magnet-excited secondary, the provision of an encoder or a sensor is required. U.S. Pat. No. 5,091,665 describes the use of a Hall sensor for determining the position of the armature relative to the stator. This type of sensor is hereby a separate element of the primary.  
         [0004]     It would be desirable and advantageous to provide an improved electric machine which is compact in structure and yet allows reliable determination of a position of a stator or a rotor.  
       SUMMARY OF THE INVENTION  
       [0005]     According to one aspect of the present invention, an electric machine includes a stator and a rotor interacting with the stator, wherein the stator and/or the rotor has a conducting device for guiding a magnetic flux, with the conducting device including a gap provided in a path of the magnetic flux, and a sensor received in the gap for measuring the magnetic flux.  
         [0006]     The present invention resolves prior art problems by placing the sensor, e.g. a Hall sensor, for measuring the magnetic flux in a gap, thereby realizing a compact configuration of the electric machine. While the gap can be configured in different shapes, it is bounded by at least two elements in opposite disposition and can be integrated in many different ways. As a consequence, the sensor can be integrated in an optimum manner within the conducting device. Despite the provision of the gap, the geometric size of the electric machine remains unchanged and, in particular, is not enlarged.  
         [0007]     According to another feature of the present invention, the conducting device may be designed as a lamination stack and include at least one flux conductor constructed to form the gap. By providing one or more flux conductors, a partial magnetic flux, also called magnetic sensor flux, may be branched off a main magnetic flux and conducted across the gap with the incorporated sensor. The gap may be an air gap, or a gap filled with a filler such as, e.g., resin or plastic.  
         [0008]     According to another feature of the present invention, the conducting device may be provided in the area of the flux conductor(s) with a recess. By providing the recess, splitting of the magnetic main flux and resultant formation of a magnetic sensor flux can be controlled. As the flux conductor affects the magnetic flux, the size of the recess can be suited to so adjust the magnetic flux as to remain unaffected by a flux conductor in comparison to an area without flux conductor.  
         [0009]     The conducting device may be constructed as a lamination stack or so constructed as to contain at least one ferrous material such as, e.g., an injection molding compound containing a filler material with e.g. iron particles, e.g. in the form of ferrous spheres. The stator as well as the rotor of the electric machine may also be constructed in the form of a lamination stack.  
         [0010]     According to another feature of the present invention, the stator as well as the rotor of the electric machine may be constructed to have windings. As an alternative, it is also conceivable to provide the stator with windings whereas the rotor may have permanent magnets. As a consequence of the arrangement of permanent magnets, a magnetic field is constantly present which can continuously be ascertained by the sensor so that through measurement of the magnetic field the physical position of the stator or the rotor of the electric machine can be determined when operatively connecting the sensor to a suitable device. Such a device may, e.g. realized in the controller of the electric machine or in an associated power converter.  
         [0011]     The electric machine may be constructed as linear motor with either the primary or the secondary being intended for executing a linear motion. Another example of an electric machine may involve a rotary machine, whereby the rotor may be constructed as inside rotor or outside rotor. An example of a rotary motor includes a torque motor.  
         [0012]     According to another feature of the present invention, the sensor may be constructed to determine a position of the stator or the rotor. The sensor may hereby be operatively connected with a suitable evaluation device for position determination.  
         [0013]     An electric machine according to the present invention is advantageous in particular for application with permanent-magnet-excited machines. Startup of permanent-magnet-excited machines requires recognition of the absolute rotor position. The rotor position recognition relates to the commutation position, i.e. 360 electrical degrees. This normally requires the provision of an absolute encoder. The provision of a relative encoder may be sufficient for the operation of the electric machine.  
         [0014]     According to another aspect of the present invention, a method of operating an electric machine includes the steps of measuring a magnetic flux between a stator and a rotor by a sensor which provides a signal which is indicative of the magnetic flux, determining an actual value and a target value of one or more or all stator currents, and determining a commutation position of the electric machine on the basis of the signal and the actual and target values.  
         [0015]     According to still another aspect of the present invention, a method of operating an electric machine includes the steps of determining a position of a stator or a rotor of the electric machine, and initializing an incremental encoder of the electric machine.  
         [0016]     Heretofore, the absolute rotor position recognition, i.e. recognition of the commutation position of the rotor, has been realized by separate sensors which are arranged outside of the magnetic circuit of the machine or by software-based processes. An example includes a linear motor having Hall sensors in a separate box outside the housing for the primary. Evaluation of the stray flux of the permanent magnets in air enables hereby the position determination. Position recognition in a torque motor is possible, for example, by a separate transducer track. Through integration of the rotor position recognition according to the invention, the beginning commutation position and initialization of a relative encoder can be realized, without provision of additional devices and processes for recognition of the rotor position. An advantage of the method according to the invention is their applicability in situations, when other methods are unsuitable.  
         [0017]     The integrated rotor position recognition according to the present invention replaces the Hall sensor box in linear motors. As a result, the electric machine according to the present invention is more compact and no additional space is required within the installations space (e.g. carriage of a machine tool). There is no need for a precise adjustment of the assembly position between the Hall senor box and the primary. Installation of a Hall sensor box in torque motors in not possible in view of the absence of free-standing magnets. The integrated rotor position recognition according to the present invention expands the function of the torque motor by assuring a safe startup under load or in situations that do not permit a motion-based process for rotor position recognition. This is, for example, the case when limitations make it impossible or do not allow an initialization motion of the machine. Integrated rotor position recognition according to the present invention thus provides a simple and cost-efficient solution in connection with torque motors.  
         [0018]     According to another feature of the present invention, the feedback signal of the sensor for measuring the magnetic flux is evaluated in operation by compensation of the signal influences, when current is applied to the stator. As a consequence, it becomes possible to ascertain the commutation position also during operation of an electric machine. The term “stator currents” relates hereby to actual values or target values of one or more or all electric stator currents. For example, drives without transducer, can thus be improved through determination of the additional variable “magnetic flux” and diagnostic systems can be provided. Examples include observers and motor models.  
         [0019]     One or more sensors such as, for example, Hall sensors, are thus integrated in accordance with the present invention at a suitable location in the magnetic circuit of the electric machine. Thus, the signal can be generated, for example, as a result of the magnetic main flux, e.g. by the excitation flux of the permanent magnets in the primary (the stator) of the electric machine. Evaluation of the signal is indicative for rotor position required for commutation and an incremental encoder can, for example, be initialized. As a consequence, recognition of the commutation position can be realized as function of a defined startup of the electric machine.  
         [0020]     According to another feature of the present invention, in a permanent-magnet-excited machine, the part of the electric machine which carries the permanent magnets is provided with the gap for accommodation of the sensor which provides feedback of the present position of the moving motor part, with the magnitude of the magnetic field or its change being indicative through data connection of the position or speed of the moving motor part. In permanent-magnet-excited electric machines, having juxtaposed permanent magnets, the magnetic field of the permanent magnets is indicative for position determination.  
         [0021]     An electric machine according to the present invention can be constructed in such a manner that the magnetic field strength can be ascertained locally at several, at least two, points. These points are spaced from one another in movement direction of the motor part, whereby, according to another feature of the present invention, the distances between the single points may differ from the distances of the permanent magnets. In this way, varying phases of the measuring signals can be established at the single measuring points, thereby increasing the accuracy of the position determination. Thus, in accordance with another feature of the present invention, several sensors may be provided which are, preferably, associated to the coils of the primary.  
         [0022]     According to another feature of the present invention, the sensor may be cooled down by an active cooling device of the primary. This not only enhances the accuracy of the sensors but requires no particular requirement of the ambient temperature.  
     
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0023]     Other features and advantages of the present invention will be more readily apparent upon reading the following description of currently preferred exemplified embodiments of the invention with reference to the accompanying drawing, in which:  
         [0024]      FIG. 1  is a principal illustration of one embodiment of an electric machine according to the present invention, constructed as a linear motor;  
         [0025]      FIG. 2  is a detailed view of a lamination of a laminated core of a primary of a linear motor according to the present invention;  
         [0026]      FIG. 3  is a schematic view of a variation of a laminated core for a primary of the linear motor;  
         [0027]      FIG. 4  is a fundamental illustration of another embodiment of an electric machine according to the present invention, constructed as a linear motor;  
         [0028]      FIG. 5  is a schematic illustration of a lamination of a laminated core for a stator of a rotary machine;  
         [0029]      FIG. 6  is a schematic illustration of still another embodiment of an electric machine according to the present invention, constructed as a linear motor; and  
         [0030]      FIG. 6   a  is an enlarged detailed view of the area encircled in  FIG. 6  and marked X. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0031]     Throughout all the Figures, same or corresponding elements are generally indicated by same reference numerals. These depicted embodiments are to be understood as illustrative of the invention and not as limiting in any way. It should also be understood that the drawings are not necessarily to scale and that the embodiments are sometimes illustrated by graphic symbols, phantom lines, diagrammatic representations and fragmentary views. In certain instances, details which are not necessary for an understanding of the present invention or which render other details difficult to perceive may have been omitted.  
         [0032]     Turning now to the drawing, and in particular to  FIG. 1 , there is shown a schematic principal illustration of one embodiment of an electric machine, according to the present invention, generally designated by reference numeral  1  and constructed as a linear motor having a fixed stator  3  (here the secondary) which extends over the length to be traveled, and a rotor  5  (here the primary) which interacts with the stator  3 , thereby producing thrust for travel of the rotor  5  along the stator  3 . It is to be understood by persons skilled in the art that the term “rotor” is used here in a generic sense to indicate the moving member of an electric machine. An equivalent term for rotor is “armature”. The stator  3  is constructed in the form of a laminated core  8  and includes permanent magnets  33  having a magnetic flux orientation, as indicated by arrows  35 , to produce a magnetic flux  11 , shown in  FIG. 1  as field lines.  
         [0033]     Also the rotor  5  is constructed in the form of a laminated core  18  having teeth  10  for arrangement of windings  23  between the teeth  10 . The rotor  5  is provided with a conducting device for guiding the magnetic flux  11 , whereby the conducting device is realized by a laminated structure having flux conductors  25 ,  27  disposed in opposite relationship to thereby bound a gap  13 . Situated in the gap  13  is a position sensor  17  which provides feedback of the present rotor position, with the magnitude of the feedback signal being indicative of the rotor position. In the area of the flux conductors  25 ,  27 , the magnetic flux  11  splits into a magnetic main flux  12  which flows across a cross section  21  of the laminated core  18  and a magnetic sensor flux  14 , which flows across the sensor  17 .  
         [0034]     The laminated core  18  has a length dimension  39  and a transverse dimension  37 . The length dimension  39  also designates the travel direction of the rotor  5  in relation to the stator  3 . The flux conductors  25 ,  27  may extend over the entire transverse dimension  37  of the laminated core  18 , or also over a portion thereof. According to a presently preferred embodiment, the flux conductors  25 ,  27  are situated in midsection of the transverse dimension  37 .  
         [0035]      FIG. 2  is a detailed view of a lamination  40  of a modified laminated core  18  of the rotor  5 . Parts corresponding with those in  FIG. 1  are denoted by identical reference numerals and not explained again. The description below will center on the differences between the embodiments. In this embodiment, the laminations  40  of the rotor  5  have a recess  31  adjacent to the gap  13 , thereby enabling to suit the signal amplitude of the sensor  17 . The recess  31  reduces the cross section  21  of the laminated core  18  for passage of the magnetic main flux  12 . Although not shown in the drawing, it is, of course, also possible to enlarge the cross section  21  in relation to the region without flux conductors  25 ,  27 .  
         [0036]     Referring now to  FIG. 3 , there is shown a schematic view of a variation of a laminated core  18  for the rotor  5  of the linear motor. Again, parts corresponding with those in  FIG. 1  are denoted by identical reference numerals and not explained again. In this embodiment, the laminated core  18  has laminations  16  formed with a gap  15  for accommodating the sensor  17 . In this way, the laminated core  18  has a compact construction. As a consequence of this configuration, while the gap  15  reduces the cross section  21 , the outer dimension of the lamination  16  remains unchanged throughout, irrespective of the provision of the gap  15 .  
         [0037]      FIG. 4  is a principal illustration of another embodiment of an electric machine  1  according to the present invention, constructed as a linear motor. Again, parts corresponding with those in  FIG. 1  are denoted by identical reference numerals and not explained again. In this embodiment, the laminated core  18  of the rotor  5  is provided in addition to the gap  13  with a further gap  14  which is demarcated between a flux conductor  29  and a confronting side of the laminations  40  of the laminated core  18 , and accommodates a further sensor  17 . In addition, the laminated core  8  of the stator  3  of the electric machine  1  of  FIG. 4  is formed with a gap  15  for accommodation of still another sensor  17 . In this way, the magnetic flux can be measured also in the area of the stator  3 .  
         [0038]     Turning now to  FIG. 5 , there is shown a schematic illustration of a lamination  41  of a laminated core  30  for a stator of a rotary machine. The lamination  41  has teeth  9  and is formed with opposite flux conductors  25 ,  27  in spaced apart relationship to define a gap  13  for placement of a position sensor  17 . Thus, a conducting device according to the present invention for determination of a rotor position is applicable for linear motors as well as rotary motors.  
         [0039]      FIG. 6  is a principal illustration of still another embodiment of an electric machine  1  according to the present invention, constructed as a linear motor. Parts corresponding with those in  FIG. 1  will be identified by corresponding reference numerals followed by an “a”. The description below will center only on the differences between the embodiments. In this embodiment, the stator  3   a  has a laminated core  8   a  having teeth  10   a  for arrangement of windings  23   a  between the teeth  10   a,  whereas the rotor  5   a  is constructed in the form of a laminated core  18   a  and includes permanent magnets  33   a.    
         [0040]     Suitably, the sensor  17  is cooled down by an active cooling device of the stator  3  or the rotor  5 . By way of example,  FIG. 6  shows also the provision of cooling pipes  32  as part of a cooling device.  
         [0041]     Referring now to  FIG. 6   a,  there is shown an enlarged detailed view of the area encircled in  FIG. 6  and marked X to illustrate, by way of example, the provision of a filler  34 , such as, e.g., resin or plastic, to fill the gap  13  in which the sensor  17  is placed. The provision of a filler  34  is, of course, also applicable, if desired, for filling the gaps  15  and  44  as described in conjunction with the embodiments of  FIGS. 1-5 . As an alternative, the gaps  13 ,  15 ,  44  may also be air gaps.  
         [0042]     Although not shown in the drawing, the electric machine may include an incremental encoder which typically includes a wheel with tally marks arranged in a circular array about the wheel, with a sensor detecting the passage of tally marks as the orientation of the wheel is changed. The incremental encoder has to be initialized relative to an origin to provide information as to absolute position. In accordance with the present invention, the initialization of the incremental encoder can be realized after the position of the stator or rotor has been determined.  
         [0043]     While the invention has been illustrated and described in connection with currently preferred embodiments shown and described in detail, it is not intended to be limited to the details shown since various modifications and structural changes may be made without departing in any way from the spirit of the present invention. The embodiments were chosen and described in order to best explain the principles of the invention and practical application to thereby enable a person skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated.  
         [0044]     What is claimed as new and desired to be protected by Letters Patent is set forth in the appended claims and includes equivalents of the elements recited therein: