Patent Publication Number: US-11643281-B2

Title: Method and apparatus for identifying a mover on a track

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of and claims priority to U.S. application Ser. No. 15/672,788, filed Aug. 9, 2017, the entire contents of which is incorporated herein by reference. 
    
    
     BACKGROUND INFORMATION 
     The present invention relates to motion control systems and, more specifically, to identification of a mover in a motion control system incorporating multiple movers propelled along a track using a linear drive system. 
     Motion control systems utilizing movers and linear motors can be used in a wide variety of processes (e.g. packaging, manufacturing, and machining) and can provide an advantage over conventional conveyor belt systems with enhanced flexibility, extremely high speed movement, and mechanical simplicity. The motion control system includes a set of independently controlled “movers” each supported on a track for motion along the track. The track is made up of a number of track segments that, in turn, hold individually controllable electric coils. Successive activation of the coils establishes a moving electromagnetic field that interacts with the movers and causes the mover to travel along the track. Sensors may be spaced at fixed positions along the track and/or on the movers to provide information about the position and speed of the movers. 
     Each of the movers may be independently moved and positioned along the track in response to the moving electromagnetic field generated by the coils. In a typical system, the track forms a closed path over which each mover repeatedly travels. At certain positions along the track other actuators may interact with each mover. For example, the mover may be stopped at a loading station at which a first actuator places a product on the mover. The mover may then be moved along a process segment of the track where various other actuators may fill, machine, position, or otherwise interact with the product on the mover. The mover may be programmed to stop at various locations or to move at a controlled speed past each of the other actuators. After the various processes are performed, the mover may pass or stop at an unloading station at which the product is removed from the mover. The mover then completes a cycle along the closed path by returning to the loading station to receive another unit of the product. 
     On some tracks, each of the movers are of identical construction. Although each mover is typically of “identical” construction, it is understood that there is some variation between movers due, for example, to manufacturing tolerances, wear, and the like. The variations between movers may result in variations, for example, in acceleration, deceleration, positioning, and the like as the mover is positioned along the track. In some applications, precise activation and positioning of the movers is required requiring, for example, sub-millimeter accuracy. As a result, each mover may be characterized, for example, during a commissioning process to detect any variations in individual movers from a design standard. A set of compensating variables may be stored in a controller corresponding to each mover to allow for the precise activation and positioning desired for each mover. However, in order to apply the correct compensation to each mover, it is necessary to know the identity of each mover. 
     On other tracks, movers of different construction may be utilized. Different movers may include, for example, different tooling to perform different operations. Optionally, movers may be of different size, for example, of alternating size to accommodate two different products on the same line. In still other systems, movers may operate independently or in combination with another mover. Adjacent movers may be configured to alternately operate with another mover but only in a specific direction from the mover. In order for the controller to properly control each mover, it must know the identity of each mover. 
     As is known to those skilled in the art, one mode of identifying movers is the use of radio frequency identification (RFID) in which RFID tags with unique identifiers are mounted on each mover. The unique identifier may be, for example, a multi-bit serial number associated with each mover. An RFID reader is positioned along the track which detects RFID tags on the movers and reads the unique identifier as the movers pass the RFID reader positioned. Upon power-up, an application program will typically execute a system start-up routine which drives the movers along the track so that each mover passes near the RFID reader. The RFID reader will obtain the unique identifier from each tag as it passes so that the identity of the mover can be determined. Use of RFID in product delivery systems is described in U.S. Pat. No. 7,931,197, entitled “RFID-based product manufacturing and lifecycle management,” assigned to the present applicant, and hereby incorporated by reference. 
     The use of RFID tags and readers requires unique identifying information to be installed on each mover and a database of identifying information to be stored in a system database. Also, each mover must pass in close proximity to a RFID reader so that the RFID tags can be read. This process can be time consuming and intrusive, especially when resuming mover operations already in process. 
     Thus, it would be desirable to provide an improved system for determining the identification of movers in a motion control system. 
     BRIEF DESCRIPTION 
     The subject matter disclosed herein describes an improved system for determining the identification of movers in a motion control system. The motion control system includes multiple movers traveling on a track. The physical construction of at least one element of one of the movers is different on one mover than the physical construction of the corresponding element on each of the other movers. The control system for the movers is configured to detect the difference in construction and identify the unique mover as a first mover. Each of the other movers along the track are assigned an identifier based on their relative position to the first mover. According to one embodiment, a position sensing system is utilized to identify the first mover. The position sensing system is provided to determine a location of each mover on the track. The position sensing system includes an array of sensors positioned along the track and a position magnet located on each mover. The position magnet on the first mover has a different construction than the position magnet on the other movers, resulting in a different magnetic field being generated by the position magnet on the first mover than by the position magnets on the other movers. The array of sensors can detect the different magnetic field and identify the first mover. According to another embodiment, the drive system for the movers is utilized to identify the first mover. The drive system includes a plurality of drive magnets mounted on each mover. On the first mover, the polarity of the drive magnets is reversed from the polarity of the drive magnets on the other movers. As the coils along the track are energized, the response from each mover is detected and the first mover is identified. In still other embodiments, a combination of the position sensing system and the drive system is utilized to identify the first mover. In some instances when the position magnet on the first mover has a different construction than the position magnets on the other movers, some movement of the movers may be required to enable the sensors to detect the different construction. Optionally, a high frequency signal may be output by the coils to identify the location of the drive magnets and the location of the drive magnets may be compared to the location of the position magnets. 
     In one embodiment of the invention, a system for identifying a first mover is disclosed. The first mover is selected from multiple movers, and each of the movers travels along a track. The system includes multiple position magnets and multiple sensors. Each position magnet is mounted to one of the movers and generates a magnetic field. The sensors are spaced apart along the track, and each of the sensors generates a signal corresponding to the magnetic field generated by one of the position magnets. A first position magnet, selected from the multiple position magnets, is mounted to the first mover, and each of the plurality of sensors generates a first signal corresponding to the magnetic field generated by the first position magnet. Each of the sensors generates a second signal corresponding to the magnetic field generated by each of the other position magnets, and the first signal is different than the second signal. 
     According to another embodiment of the invention, a method for identifying a first mover is disclosed. The first mover is selected from multiple movers, and each of the movers travels along a track. Position signals are received at a controller, where each position signal is generated by one of multiple sensors spaced apart along the track. Each position signal corresponds to at least one position magnet mounted to one mover, and the controller is configured to determine a relative location of each mover along the track as a function of each position signal and of an identity of the sensor which detected the corresponding position signal. Each of the plurality of position signals is compared to each other in the controller and a magnetic field generated by the position magnets on one mover that is different than a magnetic field generated by the position magnets mounted on each of the other movers is identified in the controller. The first mover is identified with the controller as the one mover with the magnetic field different than the other magnetic fields. 
     According to still another embodiment of the invention, a system for identifying a first mover is disclosed. The first mover is selected from multiple movers, where each of the movers travels along a track. The system includes a position sensing system and a drive system. The position sensing system includes multiple position magnets and multiple sensors. Each position magnet is mounted to one of the movers and generates a magnetic field. Each sensor is spaced apart along the track and generates a signal corresponding to the magnetic field generated by one of the plurality of position magnets. The drive system includes multiple coils and multiple drive magnets. The coils are mounted along the track, and the drive magnets are mounted to each mover. A controlled current supplied to the plurality of coils generates an electromagnetic field that interacts with the plurality of drive magnets to control motion of each of the plurality of movers. The first mover includes a first position magnet and a first set of drive magnets, and each of the other movers includes a second position magnet and a second set of drive magnets. At least one of the first position magnet or the first set of drive magnets is mounted differently or of a different construction than the second position magnet and the second set of drive magnets, respectively, and the first mover is identified as a function of the different mounting or of the different construction between the first and second position or drive magnets. 
     These and other advantages and features of the invention will become apparent to those skilled in the art from the detailed description and the accompanying drawings. It should be understood, however, that the detailed description and accompanying drawings, while indicating preferred embodiments of the present invention, are given by way of illustration and not of limitation. Many changes and modifications may be made within the scope of the present invention without departing from the spirit thereof, and the invention includes all such modifications. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various exemplary embodiments of the subject matter disclosed herein are illustrated in the accompanying drawings in which like reference numerals represent like parts throughout, and in which: 
         FIG.  1    is an isometric view of an exemplary transport system incorporating multiple movers travelling along a closed curvilinear track according to one embodiment of the present invention; 
         FIG.  2    is a partial side elevation view of one segment of the transport system of  FIG.  1    illustrating activation coils distributed along one surface of the track segment; 
         FIG.  3    is a partial sectional view of the transport system of  FIG.  1   ; 
         FIG.  4    is an isometric view of a mover from the transport system of  FIG.  1   ; 
         FIG.  5    is a side elevation view of the mover of  FIG.  4   ; 
         FIG.  6    is a front elevation view of the mover of  FIG.  4   ; 
         FIG.  7    is a block diagram representation of a motion control system incorporating an embodiment of the mover identification system disclosed herein as applied to the transport system of  FIG.  1   ; 
         FIG.  8   a    is a partial side elevation view of the mover of  FIG.  1    and a sensor for detecting a position magnet mounted on the mover with a first air gap defined between the position magnet and the sensor; 
         FIG.  8   b    is a partial side elevation view of the mover of  FIG.  1    and the sensor for detecting the position magnet mounted on the mover with a second air gap defined between the position magnet and the sensor; 
         FIG.  9   a    is a partial front elevation view of the mover of  FIG.  1    with a position magnet mounted on the mover in a first position with respect to a central axis of the mover; 
         FIG.  9   b    is a partial front elevation view of the mover of  FIG.  1    with a position magnet mounted on the mover in a second position with respect to a central axis of the mover; 
         FIG.  10   a    is a partial front elevation view of the mover of  FIG.  1    illustrating a set of drive magnets mounted in a first configuration on the mover; 
         FIG.  10   b    is a partial front elevation view of the mover of  FIG.  1    illustrating a set of drive magnets mounted in a second configuration on the mover; 
         FIG.  11   a    is a partial front elevation view of the mover of  FIG.  1    with a single position magnet mounted on the mover; 
         FIG.  11   b    is a partial front elevation view of the mover of  FIG.  1    with multiple position magnets mounted on the mover; 
         FIG.  12   a    is a partial front elevation view of the mover of  FIG.  1    with a position magnet having a first shape mounted on the mover; 
         FIG.  12   b    is a partial front elevation view of the mover of  FIG.  1    with a position magnet having a second shape mounted on the mover; 
         FIG.  13    is a graphical representation of a signal generated by a magnetic sensor incorporated into one embodiment of the position sensing system to identify movers along the track; 
         FIG.  14   a    is a partial top sectional view illustrating a mover at a first orientation with respect to the magnetic sensor of  FIG.  3   ; and 
         FIG.  14   b    is a partial top sectional view illustrating a mover at a second orientation with respect to the magnetic sensor of  FIG.  3   . 
     
    
    
     In describing the various embodiments of the invention which are illustrated in the drawings, specific terminology will be resorted to for the sake of clarity. However, it is not intended that the invention be limited to the specific terms so selected and it is understood that each specific term includes all technical equivalents which operate in a similar manner to accomplish a similar purpose. For example, the word “connected,” “attached,” or terms similar thereto are often used. They are not limited to direct connection but include connection through other elements where such connection is recognized as being equivalent by those skilled in the art. 
     DETAILED DESCRIPTION 
     The various features and advantageous details of the subject matter disclosed herein are explained more fully with reference to the non-limiting embodiments described in detail in the following description. 
     Turning initially to  FIG.  1   , an exemplary transport system for moving articles or products includes a track  10  made up of multiple segments  12 ,  14 . According to the illustrated embodiment, the segments define a generally closed loop supporting a set of movers  100  movable along the track  10 . The track  10  is oriented in a horizontal plane and supported above the ground by a base  15  extending vertically downward from the track  10 . According to the illustrated embodiment, the base  15  includes a pair of generally planar support plates  17 , located on opposite sides of the track  10 , with mounting feet  19  on each support plate  17  to secure the track  10  to a surface. The illustrated track  10  includes four straight segments  12 , with two straight segments  12  located along each side of the track and spaced apart from the other pair. The track  10  also includes four curved segments  14  where a pair of curved segments  14  is located at each end of the track  10  to connect the pairs of straight segments  12 . The four straight segments  12  and the four curved segments  14  form a generally oval track and define a closed surface over which each of the movers  100  may travel. It is understood that track segments of various sizes, lengths, and shapes may be connected together to form a track  10  without deviating from the scope of the invention. 
     For convenience, the horizontal orientation of the track  10  shown in  FIG.  1    will be discussed herein. Terms such as upper, lower, inner, and outer will be used with respect to the illustrated track orientation. These terms are relational with respect to the illustrated track and are not intended to be limiting. It is understood that the track may be installed in different orientations, such as sloped or vertical, and include different shaped segments including, but not limited to, straight segments, inward bends, outward bends, up slopes, down slopes and various combinations thereof. Further, each track segment  12 ,  14  is shown in a generally horizontal orientation. The track segments  12 ,  14  may also be oriented in a generally vertical orientation and the width of the track  10  may be greater in either the horizontal or vertical direction according to application requirements. The movers  100  will travel along the track and take various orientations according to the configuration of the track  10  and the relationships discussed herein may vary accordingly. 
     Each track segment  12 ,  14  includes a number of independently attached rails  20  on which each mover  100  runs. According to the illustrated embodiment, rails  20  extend generally along the outer periphery of the track  10 . A first rail  20  extends along an upper surface  11  of each segment and a second rail  20  extends along a lower surface  13  of each segment. With reference also to  FIG.  3   , the illustrated embodiment of each rail  20  includes a base  22  and a track portion  24 . The base  22  is secured to the upper surface  11  or lower surface  13  of each segment  12 ,  14  and the track portion  24  is mounted to the base  22 . It is contemplated that each rail  20  may be a singular, molded or extruded member or formed from multiple members. It is also contemplated that the cross section of the rails  20  may be circular, square, rectangular, or any other desired cross-sectional shape without deviating from the scope of the invention. The rails  20  generally conform to the curvature of the track  10  thus extending in a straight path along the straight track segments  12  and in a curved path along the curved track segments  14 . The rails  20  may be thin with respect to the width of the track  10  and span only a partial width of the surface of the track  10  on which it is attached. Each mover  100  includes complementary rollers  110  to engage the track portion  24  of the rail  20  for movement along the track  10 . 
     One or more movers  100  are mounted to and movable along the rails  20  on the track  10 . With reference next to  FIGS.  4 - 6   , an exemplary mover  100  is illustrated. Each mover  100  includes a side member  102 , a top member  104 , and a bottom member  106 . The side member  102  extends for a height at least spanning a distance between the rail  20  on the top surface  11  of the track  10  and the rail  20  on the bottom surface  13  of the track  10  and is oriented generally parallel to a side surface  21  when mounted to the track  10 . The top member  104  extends generally orthogonal to the side member  102  at a top end of the side member  102  and extends across the rail  20  on the top surface  11  of the track  10 . The top member  104  includes a first segment  103 , extending orthogonally from the side member  102  for the width of the rail  20 , which is generally the same width as the side member  102 . A set of rollers  110  are mounted on the lower side of the first segment  103  and are configured to engage the track portion  24  of the rail  20  mounted to the upper surface  11  of the track segment. According to the illustrated embodiment two pairs of rollers  110  are mounted to the lower side of the first segment  103  with a first pair located along a first edge of the track portion  24  of the rail and a second pair located along a second edge of the track portion  24  of the rail  20 . The first and second edges and, therefore, the first and second pairs of rollers  110  are on opposite sides of the rail  20  and positively retain the mover  100  to the rail  20 . The bottom member  106  extends generally orthogonal to the side member  102  at a bottom end of the side member  102  and extends for a distance sufficient to receive a third pair of rollers  110  along the bottom of the mover  100 . The third pair of rollers  110  engage an outer edge of the track portion  24  of the rail  20  mounted to the lower surface  13  of the track segment. Thus, the mover  100  rides along the rails  20  on the rollers  110  mounted to both the top member  104  and the bottom member  106  of each mover  100 . The top member  104  also includes a second segment  120  which protrudes from the first segment  103  an additional distance beyond the rail  20  and is configured to hold a position magnet  130 . According to the illustrated embodiment, the second segment  120  of the top member  104  includes a first portion  122  extending generally parallel to the rail  20  and tapering to a smaller width than the first segment  103  of the top member  104 . The second segment  120  also includes a second portion  124  extending downward from and generally orthogonal to the first portion  122 . The second portion  124  extends downward a distance less than the distance to the upper surface  11  of the track segment but of sufficient distance to have the position magnet  130  mounted thereto. According to the illustrated embodiment, a position magnet  130  is mounted within a recess  126  on the second portion  124  and is configured to align with a sensor  150  mounted to the top surface  11  of the track segment. 
     A linear drive system is incorporated in part on each mover  100  and in part within each track segment  12 ,  14  to control motion of each mover  100  along the segment. On each mover  100 , the linear drive system includes multiple drive magnets mounted to the side member  102 . According to the illustrated embodiment, the drive magnets  140  are arranged in a block along an inner surface of the side member  102  with separate magnet segments  142 ,  144  alternately having a north pole  142 , N, and south pole  144 , S, pole facing the track segment  12  (see also  FIG.  10   a ,  10   b   ). The drive magnets  140  are typically permanent magnets, and two adjacent magnet segments including a north pole  142  and a south pole  144  may be considered a pole-pair. The drive magnets  140  are mounted on the inner surface of the side member  102  and when mounted to the track  10  are spaced apart from a series of coils  50  extending along the track  10 . As shown in  FIG.  3   , an air gap  141  is provided between each set of drive magnets  140  and the coils  50  along the track  10 . On the track  10 , the linear drive system includes a series of parallel coils  50  spaced along each track segment  12  as shown in  FIG.  2   . According to the illustrated embodiment, each coil  50  is placed in a channel  23  extending longitudinally along one surface of the track segment  12 . The electromagnetic field generated by each coil  50  spans the air gap and interacts with the drive magnets  140  mounted to the mover  100  to control operation of the mover  100 . 
     Turning next to  FIG.  7   , an exemplary control system for the track  10  and linear drive system is illustrated. A segment controller  200  is mounted within each track segment  12 ,  14 . The segment controller  200  receives command signals from a system controller  30  and generates switching signals for power segments  210  which, in turn, control activation of each coil  50 . Activation of the coils  50  are controlled to drive and position each of the movers  100  along the track segment  12  according to the command signals received from the system controller  30 . 
     The illustrated motion control system includes a system controller  30  having a processor  32  and a memory device  34 . It is contemplated that the processor  32  and memory device  34  may each be a single electronic device or formed from multiple devices. The processor may be  32  a microprocessor. Optionally, the processor  32  and/or the memory device  34  may be integrated on a field programmable array (FPGA) or an application specific integrated circuit (ASIC). The memory device  34  may include volatile memory, non-volatile memory, or a combination thereof. A user interface  36  is provided for an operator to configure the system controller  30  and to load or configure desired motion profiles for the movers  100  on the system controller  30 . It is contemplated that the system controller  30  and user interface  36  may be a single device, such as a laptop, notebook, tablet or other mobile computing device. Optionally, the user interface  36  may include one or more separate devices such as a keyboard, mouse, display, touchscreen, interface port, removable storage medium or medium reader and the like for receiving information from and displaying information to a user. Optionally, the system controller  30  and user interface  36  may be integrated into an industrial computer mounted within a control cabinet and configured to withstand harsh operating environments. It is contemplated that still other combinations of computing devices and peripherals as would be understood in the art may be utilized or incorporated into the system controller  30  and user interface  36  without deviating from the scope of the invention. 
     One or more programs may be stored in the memory device  34  for execution by the processor  32 . The system controller  30  receives one or more motion profiles for the movers  100  to follow along the track  10 . A program executing on the processor  32  is in communication with a segment controller  200  on each track segment  12 ,  14 . The system controller  30  may transfer a desired motion profile to each segment controller  200  or, optionally, the system controller  30  may perform some initial processing based on the motion profile to transmit a segment of the motion profile to each segment controller  200  according to the portion of the motion profile to be executed along that segment. Optionally, the system controller  30  may perform still further processing on the motion profile and generate a desired switching sequence for each segment  12 ,  14  that may be transmitted to the segment controller  200 . 
     A gateway  202  in each segment controller  200  receives the communications from the system controller  30  and passes the communication to a processor  204  executing in the segment controller  200 . The processor may be a microprocessor. Optionally, the processor  204  and/or a memory device  206  within the segment controller  200  may be integrated on a field programmable array (FPGA) or an application specific integrated circuit (ASIC). It is contemplated that the processor  204  and memory device  206  may each be a single electronic device or formed from multiple devices. The memory device  206  may include volatile memory, non-volatile memory, or a combination thereof. The segment controller  200  receives the motion profile, or portion thereof, or the switching sequence transmitted from the system controller  30  and utilizes the motion profile or switching sequence to control movers  100  present along the track segment  12 ,  14  controlled by that system controller  30 . 
     Each segment controller  200  generates switching signals to control operation of switching devices within one or more power segments  210  mounted within the track segment  12 ,  14 . The processor  204  receives feedback signals from sensors providing an indication of the current operating conditions within the power segment  210  or the current operating conditions of a coil  50  connected to the power segment  210 . The switching devices within each power segment  210  are connected between a power source and the coils  50 . The switching signals are generated to sequentially energize coils  50  along a track segment, where the energized coils  50  create an electromagnetic field that interacts with the drive magnets  140  on each mover  100  to control motion of the movers  100  along the corresponding track segment  12 ,  14 . 
     In operation, the system controller  30  executes to control each of the movers  100  on the track  10 . As previously discussed, each mover  100  may have some variation in construction and, therefore, the controller  30  may compensate control of the coils  50  in the drive system according to the mover  100  to be controlled to accurately position each mover. In order to provide the varying compensation to each mover  100 , the system controller  30  must know the identification of each mover  100  along the track  10 . 
     When power is cycled, the potential exists that movers  100  are manually repositioned, added, or removed for maintenance. As a result, the system controller  30  determines the identification of each mover along the track  10  during each power up cycle. According to the illustrated embodiment, the track  10  is a closed track. In other words, after power-up and during normal operation, movers  100  repeatedly travel over the same segments  12 ,  14  and no movers  100  are introduced to or removed from the track  10 . Rather than providing unique identifiers for every mover  100 , the present inventors have determined a method of identifying a single mover  100  along the track. The identified mover  100  is referred to herein as the first mover. After identifying the first mover, the system controller  30  may then incrementally assign numbers to each subsequent mover  100  in either a positive or a negative direction along the track  10  to identify each of the movers  100  along the track. It is understood that various other numbering methods, such as decrementing, incrementing by intervals greater than one, and the like may be utilized without deviating from the scope of the invention. 
     A position sensor system is provided to detect the location of each mover  100  along the track. The position sensor system may be as described in U.S. Pat. No. 9,511,681, entitled “Controlled motion system having an improved track configuration,” and US Patent Publication No. 2014/0265645, entitled “Controlled motion system having a magnetic flux bridge joining linear motor sections,” both assigned to the present applicant, and both of which are hereby incorporated by reference. The position sensor system includes a first member and a second member where the first member is mounted to each mover  100  and the second member is mounted to the track  10 . One member is to be sensed while the other member senses. As a mover  100  travels along the track  10 , the first and second members interact to detect the position of each mover  100 . 
     Referring again to  FIGS.  1  and  3   , an array of sensors  150  is provided along the top surface  11  of the track  10 . A position magnet  130  is mounted in the top member  104  of each mover at a location that is proximate to each sensor  150  as the mover  100  passes the sensor  150 . The sensors  150  are a suitable magnetic field detector including, for example, a Hall Effect sensor, a magneto-diode, an anisotropic magnetoresistive (AMR) device, a giant magnetoresistive (GMR) device, a tunnel magnetoresistance (TMR) device, fluxgate sensor, or other microelectromechanical (MEMS) device configured to generate an electrical signal corresponding to the presence of a magnetic field. The magnetic field sensor  150  outputs a feedback signal  151  to the segment controller  200  for the corresponding track segment  12 ,  14  on which it is mounted. The feedback signal  151  may be an analog signal  155  of a type illustrated, for example, in  FIG.  13   . The sensor  150  includes a support member  152  and a transducer  154 , where the support member  152  is affixed to the top surface  11  of the track and positions the transducer  154  at a desired location. The transducer  154  includes a central axis  157  extending horizontally, as illustrated, across the gap, G 1 , (see  FIG.  14   a ,  14   b   ) between the sensor and position magnet  130  in a mover  100  passing by the transducer  154 . As the center of the position magnet  130  approaches the central axis  157 , the angle, α, changes and the magnitude of the signal generated by the transducer  154  similarly changes as a function of the angle α. 
     With reference again to  FIG.  7   , each magnetic field sensor  150  transmits the feedback signal  151  to the segment controller  200 . An array of sensors  150  are spaced along the segment, such that the mover generates a signal on at least one sensor  150  at all times as it travels along the track segment. According to an exemplary embodiment, the sensors  150  are positioned at 20 mm intervals. The segment controller  200  identifies which sensor  150  is generating a signal. The segment controller  200  may convert the feedback signal  151  to a digital signal corresponding to the analog feedback signal  151  and transmit the signal to the system controller  30 . Optionally, the segment controller  200  may transmit the feedback signal  151  directly or perform further processing on the feedback signal  151  prior to transmitting the signal to the system controller  30 . For example, each segment controller  200  may have knowledge of the location of each sensor  150  along its segment and generate a value corresponding to a position along the length of the segment at which each mover  100  is located. According to still another option, the segment controller  200  may transmit the identity of the sensor  150  along with the corresponding feedback signal  151  to the system controller  30  which, in turn, determines the position of each mover  100  along the track  10 . 
     In order to identify the first mover, the position magnet  130  for one of the movers  100  has a different construction than the position magnet  130  for each of the other movers  100 . The feedback signal  151  generated by the sensor varies, for example, as a function of the location of the position magnet  130  with respect to the sensor  150  and as a function of the strength of the magnetic field generated by the position magnet  130 . Therefore, a first position magnet is mounted to the first mover and each of the remaining movers receive a second position magnet, where the first position magnet is different than second position magnet. As a result, the sensor  150  generates a first feedback signal corresponding to the first position magnet and a second feedback signal corresponding to the second position magnet, where the first feedback signal is different than the second feedback signal. The inventors have identified a number of embodiments of the invention by which the first and second position magnets may vary resulting in a different feedback signal being generated by the sensor  150  to identify the first mover  100 . 
     According to a first embodiment of the invention, the gap between the position magnet  130  and the sensor  150  is set differently for the first mover than for each of the other movers. Referring next to  FIGS.  8   a  and  8   b   , a first air gap, G 1 , and a second air gap, G 2 , are illustrated. In  FIG.  8   a   , it is contemplated that the position magnet  130  will typically be mounted flush with the surface of the second portion  124  of the second segment  120  of the top member  104  of the mover  100 . A first air gap, G 1 , is defined between the surface of the position magnet  130  and the transducer  154  in the magnetic field sensor  150 . As each mover  100   a  passes a sensor  150 , the feedback signal  151  will have a first peak voltage (i.e., V min  and V max  in  FIG.  13   ). In  FIG.  8   b   , the recess  126  has a greater depth such that the surface of the position magnet  130  is recessed by a width, W, from the surface of the second portion  124  of the second segment  120  of the top member  104  of the mover  100 . As a result, a second air gap, G 2 , is defined between the surface of the position magnet  130  and the transducer  154  in the magnetic field sensor  150 . As the second mover  100   b  passes each sensor  150  the feedback signal  151  will have a second peak voltage, where the second peak voltage is different than the first peak voltage. It is contemplated that the first mover could be constructed with either the first air gap, G 1 , or second air gap, G 2 , and the remaining movers would then be constructed with the second air gap or the first air gap, respectively. 
     It is further contemplated that the sensor  150  may be configured to generate both a sine waveform  155   a  and a cosine waveform  155   b  corresponding to the magnetic field of each position magnet  130  passing in front of the sensor  150 , and both signals may be provided as feedback signals  151  to the segment controller  200 . Either the segment controller  200  or the system controller  30  may determine a sum of the squared values of both the sine and the cosine feedback signals. The resulting sum allows the controller to determine the width of the air gap. The system controller  30  may receive or may determine the width of the air gap for each mover  100  and identify the first mover according to the position magnet  130  that has an air gap that differs from the air gap of the other position magnets  130 . Optionally, a preset value of the width of the air gap for the first mover or of the width for each of the other movers may be stored in the memory  34  or  206  for either the segment controller  200  or the system controller  30 , and the controller may compare the widths measured from the feedback signals  151  to the stored preset values and identify which of the movers  100  is the first mover. In this manner, the first mover may be identified quickly upon power up without requiring motion of any of the movers  100 . 
     According to another aspect of the invention, the position sensors  150  may be spaced along the track at a distance that permits multiple sensors  150  to detect the magnetic field generated by one position magnet  130 . Thus, the strength of the magnetic field detected at two or more sensors and, therefore, at two or more locations along the track may be compared to determine the location of each position magnet  130  with respect to the locations of each sensor  150  sensing the magnetic field generated by the magnet  130 . If the width of the air gap varies, the strength of the magnetic field detected by the position sensor  150  will vary for two movers at the same location. Similarly, if a mover  100  is at a first location and a second location with respect to a position sensor  150 , the strength of the magnetic field detected by the sensor  150  will vary as a function of the distance along the rail  20  that each position magnet  130  is displaced from the sensor  150 . In order to distinguish between a different width of the air gap or displacement along the rails  20 , the controller uses the signals from multiple sensors  150  and the relative strength of the signal present at each of the multiple sensors to determine the location of each position magnet  130 . The memory  34 ,  306  in the system controller  30  or the segment controller  200 , respectively, may include a look-up table which includes the relationship between the strength of the magnetic field detected at each sensor  150  and the correspondence to the width of the air gap between the position magnet  130  and the sensor  150 . The controller may utilize the look-up table to identify the width of the air gap on each mover and, thereby identify on which mover the air gap is different than the air gap on the other movers. 
     According to another embodiment of the invention, the location of the position magnet  130  on the mover  100  is set differently for the first mover than for each of the other movers. Referring next to  FIGS.  9   a  and  9   b   , each mover  100   a ,  100   b  includes a central axis  101  extending generally orthogonal to the direction of travel along the track. According to the illustrated embodiment, each mover  100  has a greater height than width and the central axis  101  extends longitudinally from the bottom to the top of the mover  100 . The position magnet  130  also includes a central axis  131  for the magnet, extending generally parallel to the central axis  101  of the mover  100 . In a first construction, shown in  FIG.  9   a   , the position magnet  130  is mounted on the mover  100  such that the central axis  101  of the mover  100  and the central axis  131  of the position magnet  130  are aligned with each other when viewed from the front of the mover  100  and both central axes are generally orthogonal to the direction of travel of the mover  100 . As illustrated, both axis  101 ,  131  are located a first width, W 1 , from one side of the mover  100  and a second width, W 2 , from the other side of the mover  100 , where the first width, W 1 , and the second width, W 2 , are identical. In a second construction, shown in  FIG.  9   b   , the position magnet  130  is mounted on the mover  100  such that the central axis  131  of the position magnet  130  is offset in a lateral direction, or along the direction of travel, from the central axis  101  of the mover  100 . As illustrated, the central axis  101  of the mover  100  is still located a first width, W 1 , from a first side of the mover  100  and a second width, W 2 , from a second side of the mover  100 , where the first width, W 1 , and the second width, W 2 , are identical. However, the central axis  131  of the position magnet  130  is located a third width, W 3 , from the first side of the mover  100  and a fourth width, W 4 , from the second side of the mover  100 , where the third width, W 3 , and the fourth width, W 4 , are different. It is contemplated that the first mover could be constructed either with the central axes  101 ,  131  having the same width from each side or with the central axes  101 ,  131  having different widths from each side and the remaining movers would then have the opposite construction. 
     When the location of the position magnet  130  is offset from the sides of the mover  100  differently for the first mover than for each of the other movers, the controller utilizes both the position sensing system and the drive system to detect the first mover. The segment controller  200  generates switching signals to control operation of the switching devices in each power segment  210  at a high frequency, where the high frequency may be, for example, an order of magnitude or more greater than a rated excitation frequency used to drive each mover  100 . The power segments  210  are further controlled, such that an amplitude of voltage and/or current output to the coils  50 , in combination with the higher frequency of the output current, generates little or no movement of the movers  100  along the track. When generating the high frequency output voltage to each coil, the voltage and/or current in each coil is sensed. The presence of a mover  100  adjacent to a coil will generate a saliency in the feedback signal. The saliency is a ripple, spike, or other disturbance in the feedback signal that is repeatable and detectable and is function of the location of the drive magnets  140  on the mover with respect to the coil. The controller uses the detected saliencies to determine the location of the drive magnets  140  on the mover along the track. Based on the location of the drive magnets  140  for each mover, the controller, in turn, determines the location of the central axis  101  for each mover  100 . The position sensing system detects the location of the central axis  131  for each position magnet  130  along the length of the track. Each segment controller  200  may then compare the locations of the central axes  101  for each mover  100  with the locations of the central axes  131  for each position magnet  130  along its respective section of track  10  to identify whether each mover has aligned or offset central axes. Optionally, the system controller  30  may determine whether each mover  100  has aligned or offset central axes for each of the movers  100  along the entire track  10 . The first mover is identified as the mover  100  that has central axes  101 ,  131  aligned differently than the central axes of each of the other movers  100 . In this embodiment, the first mover may again be identified quickly upon power up without requiring motion of any of the movers  100 . 
     According to still another embodiment of the invention, the configuration of the drive magnets  140  is set differently for the first mover than for each of the other movers. Referring next to  FIGS.  10   a  and  10   b   , a first direction of travel  146  and a second direction of travel  148  are identified with respect to a set of drive magnets  140  for one of the movers  100 , where the first direction of travel  146  is illustrated as a positive direction and the second direction of travel  148  is illustrated as a negative direction. In  FIG.  10   a   , an arrangement for a first set of drive magnets  140   a  is shown, and in  FIG.  10   b   , and arrangement for a second set of drive magnets  140   b  is shown. Both sets of drive magnets  140   a ,  140   b  include magnet segments alternately having a north pole, N,  142  and a south pole, S,  144  facing the track segment. However, the first set of drive magnets  140   a  begins with a magnet segment having a north pole, N,  142  facing toward the track and adjacent the side of the mover toward the negative direction of travel  148  and the second set of drive magnets  140   b  begins with a magnet segment having a south pole, S,  144  facing toward the track and adjacent the side of the mover toward the negative direction of travel  148 . 
     At power up, a current is supplied to the coils  50  along the drive which generates an electromagnetic field in the coil  50  and, as a result, applies a small positive driving force to each mover  100 . The resulting motion of the movers is used to identify the first mover. Each mover  100  having the first set of drive magnets  140   a  will move in one direction, and each mover  100  having the second set of drive magnets  140   b  will move in the opposite direction. The first mover is constructed to have either the first set  140   a  or second set  140   b  of drive magnets and each of the other movers  100  are constructed to have the other set of drive magnets. Although a small amount of motion is required to identify the first mover, this embodiment allows the uniform construction of the position sensing system. 
     According to yet another embodiment of the invention as illustrated in  FIGS.  11   a  and  11   b   , the first mover includes a second position magnet  130  mounted to the mover  100  while each of the other movers have a single position magnet  130 . With reference first to  FIG.  11   a   , a mover identical to those illustrated and discussed above with respect to  FIGS.  8   a  and  9   a    is shown. Each of the movers  100  other than the first mover in the transport system have the single position magnet  130  shown in  FIG.  11   a   . With reference then to  FIG.  11   b   , the second portion  124  of the second segment  120  of the top member  104  of the mover  100  has a first position magnet  130   a  and a second position magnet  130   b  mounted therein. The first position magnet  130   a  has a first central axis  131   a , and the second position magnet  130   b  has a second central axis  131   b . The first central axis  131   a  is offset in a first direction from the central axis  101  of the mover, and the second central axis  131   b  is offset in a second direction from the central axis  101  of the mover. As a result, the first central axis  131   a  is positioned at a fifth width, W 5 , from one edge of the mover  100 , the second central axis  131   b  is positioned at a sixth width, W 6 , from the other edge of the mover  100 , and the first and second central axes  131   a ,  13   b  are spaced apart for a seventh width, W 7 . 
     The position sensing system senses the location of each position magnet  130   a ,  130   b  present along the length of the track. For each of the other movers, the position sensing system detects a single position magnet  130 . For the first mover, the position sensing system detects both the first position magnet  130   a  and the second position magnet  130   b . Either the segment controller  200  or the system controller  35  may be configured to compare the distances between each position magnet located. The seventh width, W 7 , as illustrated in  FIG.  11   b   , is a known distance and may be stored in memory of one of the controllers. The distance detected between position magnets  130  may be compared to the seventh width and the mover  100  present at the location with the two position magnets  130  spaced apart by the seventh width is determined to be the first mover. According to another embodiment, the distance between each adjacent position magnet  130  may be compared. If the distance between any two position magnets  130  is less than the width of the mover  100 , the mover located by these two position magnets  130  is identified as the first mover. If, however, two movers  100  are adjacent to each other, it is possible, the distance between the position magnet  130  for one of the other movers and the distance to one of the position magnets  130   a ,  130   b  on the first mover is also less than the width of a mover  100 . Preferably, the seventh width, W 7 , is less than half the width of the mover  100  and, therefore, if two movers  100  are adjacent such that multiple distances between position magnets  130  are less than the width of a mover  100 , the smaller distance between position magnets identifies the first mover. As a result, this embodiment may also identify the first mover quickly upon power up without requiring motion of any of the movers  100 . 
     According to yet another embodiment of the invention, the shape of the position magnet  130  is set different for the first mover than for each of the other movers. The magnetic field generated by the position magnet  130  is a function of the construction of the position magnet  130 , including, but not limited to, the material from which the magnet is constructed, the shape of the magnet, and the orientation of the magnet. Referring next to  FIGS.  12   a  and  12   b   , a position magnet  130  mounted on the mover  100   a  in  FIG.  12   a    has a different shape and size than a position magnet  130  mounted on the mover  100   b  in  FIG.  12   b   . Consequently, the magnetic field generated by the position magnet  130  mounted on the mover  100   a  in  FIG.  12   a    may have a different shape, a different orientation of the magnetic flux lines, or a different strength than the magnetic field generated by the position magnet  130  mounted on the mover  100   b  in  FIG.  12   b   . It is contemplated that the first mover could be constructed with the position magnet as shown in either  FIG.  12   a    or  FIG.  12   b    and the other movers would then be constructed with the other position magnet. Further, the illustrated magnets represent one embodiment of the invention and it is contemplated that other shapes of magnets, magnets having identical shapes but different field strengths, or magnets having different physical orientations may be utilized without deviating from the scope of the invention. 
     As previously discussed, the sensors  150  in the position sensing system may be spaced along the track at a distance that permits multiple sensors  150  to detect the magnetic field generated by one position magnet  130 . Thus, if the strength of the magnetic field varies due to the size, shape, or physical material of the position magnet  130  being different, the relative strength of the magnetic field detected at two or more sensors and, therefore, at two or more locations along the track will vary and may be compared to determine the location of each position magnet  130  with respect to the locations of each sensor  150  sensing the magnetic field generated by the magnet  130 . The memory  34 ,  306  in the system controller  30  or the segment controller  200 , respectively, may include a look-up table which includes the relationship between the strength of the magnetic field detected at each sensor  150  and the correspondence to the size, shape, or physical material of each position magnet  130 . The controller may utilize the look-up table to identify each position magnet  130  and, more specifically, to identify which position magnet  130  is different than the other position magnets and, thereby, identify the first mover. 
     According to still another embodiment of the invention, the position sensing system may include a first set of sensors  150  and a second set of sensors. The first set of sensors may generate an analog signal, or signals,  155  as illustrated in  FIG.  13    that varies as a function of the strength or angle of the magnetic field detected by each sensor  150 . The second set of sensors may be, for example, a Hall Effect sensor that generates a signal corresponding to a polarity of the magnetic field detected by each sensor  150 . The position magnet  130  on a first mover  100  may be mounted with either the north or the south polarity facing the sensors  150 , and the position magnets  130  on each of the other movers  100  may be mounted with the opposite polarity facing the sensors  150 . In this manner, the strength or angle of the magnetic field for each position magnet  130  corresponds to the same displacement along the rail  12  from a sensor  150  for each mover. The controller detects the first mover  100  by identifying the mover in which the polarity of the position magnet  130  is reversed. 
     It should be understood that the invention is not limited in its application to the details of construction and arrangements of the components set forth herein. The invention is capable of other embodiments and of being practiced or carried out in various ways. Variations and modifications of the foregoing are within the scope of the present invention. It also being understood that the invention disclosed and defined herein extends to all alternative combinations of two or more of the individual features mentioned or evident from the text and/or drawings. All of these different combinations constitute various alternative aspects of the present invention. The embodiments described herein explain the best modes known for practicing the invention and will enable others skilled in the art to utilize the invention.