Patent Publication Number: US-11641169-B2

Title: Method and apparatus for commutation of drive coils in a linear drive system with independent movers

Description:
This application is a continuation of and claims priority to U.S. application Ser. No. 16/449,835, filed Jun. 24, 2019, which is, in turn, a continuation of and claims priority to U.S. application Ser. No. 15/719,153, which was filed Sep. 28, 2017 and issued Aug. 13, 2019 as U.S. Pat. No. 10,381,958, the entire contents of each is incorporated herein by reference. 
    
    
     BACKGROUND INFORMATION 
     The subject matter disclosed herein relates to motion control systems and, more specifically, to a method and apparatus for determining coil current references for drive coils used to control operation of multiple independent movers traveling along a track in 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. 
     Typically, each mover includes one or more permanent magnets mounted to the mover which, in combination with the drive coils spaced along the track, form a linear drive system. A motor controller generates a voltage having a variable amplitude and variable frequency which, in turn, results in a desired current flowing through each drive coil. The current flowing through the drive coil generates an electromagnetic field which interacts with the magnetic field produced by the permanent magnets to cause the movers to travel along the track. 
     As each mover travels past a coil, the permanent magnets create a back-emf voltage in each coil that is counter to the applied voltage. The back-emf voltage interacts with the applied voltage and current in each coil. If the back-emf voltage and the current in the coil are both sinusoidal waveforms, the interaction between the back-emf and the current in the coil is smooth, meaning there are no force pulsations. If, however, one or both of the waveforms are non-sinusoidal, then an undesirable force pulsation may be present on the mover. In addition, the permanent magnets mounted on the mover attempt to align themselves with the maximum amount of ferromagentic material present on the track, creating a cogging force. The force pulsations due to non-sinusoidal waveforms combines with the cogging force to generate an undesirable force on the mover as it travels in the linear drive system. 
     Several factors impact the shape of the waveforms in the linear drive system. Some of the factors include the design, shape, and placement of the magnets on the mover as well as the design, pitch, and placement of the coils along the track. These factors make it difficult to design an ideal linear drive system with purely sinusoidal waveforms and no cogging force. Still additional factors include the size of the mover and the number of coils with which the mover will interact at one time. These factors impact not only the force pulsations on the mover but also whether the currents are balanced between coils and the amount of copper losses in the coils. 
     A designer for the linear drive system must balance the competing effects of the different factors when designing the linear drive system. The designer must further balance manufacturing and material costs associated with the various design factors. As a result, a linear drive system typically has current and back-emf waveforms that are not purely sinusoidal as well as some amount of cogging force between the magnets in the movers and the laminations of the track. In addition, there may be variations in the linear drive system due, for example, to variations in placement of coils on the track along a straight segment and along a curved segment. Therefore, movers of different sizes and of different construction will interact differently with the coils along a track and may further interact differently along different sections of a single track. 
     Thus, it would be desirable to provide an improved method and system for providing current to the coils in a linear drive system. It is also desirable to provide different methods of regulating the current to the coils at different segments of the track according to the application requirements. 
     BRIEF DESCRIPTION 
     The subject matter disclosed herein describes an improved method and system for providing current to the drive coils in a linear drive system, where the linear drive system includes multiple, independent movers traveling along a track. A motor controller is provided that utilizes different criterion for regulating the current to the coils at different segments of the track according to the application requirements. A motor controller is configured to execute a commutation routine in one of a plurality of operating modes, where each operating mode utilizes one of the different criterion for regulating the current. The motor controller identifies a set of drive coils that will be energized to control operation of a mover as a function of the position of the mover on the track. The motor controller then generates the current for each of the drive coils in the set to control operation of the mover. In a first operating mode, the motor controller generates currents for each of the drive coils in order to minimize the copper losses in the drive coils. In a second operating mode, the motor controller generates currents for each of the drive coils to maximize the force applied to the mover. In a third operating mode, the motor controller generates current for each of the drive coils that are balanced between the drive coils. In a fourth operating mode, the motor controller generates current for each of the drive coils according to a selected operating point that combines characteristics of the first three operating modes. It is another aspect of the invention, that the motor controller monitors each of the drive coils for saturation and redistributes at least a portion of the current required to control operation of the mover to the other drive coils when one of the drive coils is saturated. 
     According to one embodiment of the invention, a method for controlling commutation of drive coils to control operation of a mover along a track in a linear drive system is disclosed. A position of the mover along the track is obtained with a motor controller, and a plurality of drive coils proximate the position of the mover are identified with the motor controller. An electromagnetic field generated by a current flowing in each of the plurality of drive coils is operative to engage at least one drive magnet on the mover. A reference signal is received at the motor controller corresponding to a desired operation of the mover, and the motor controller selects one of a plurality of commutation modes to determine a current reference for each of the plurality of drive coils. The current reference for each of the plurality of drive coils is determined with the motor controller according to the selected commutation mode. 
     According to another embodiment of the invention, a motor controller for controlling commutation of drive coils to control operation of a mover along a track in a linear drive system is disclosed. The motor controller includes at least one first input, at least one second input, a power segment, and a processor. The first input is operative to receive a position feedback signal corresponding to a position of the mover along the track, and the second input is operative to receive a reference signal corresponding to a desired operation of the mover along the track. The power segment is operative to provide a current to each of a plurality of drive coils operatively connected to the power segment. The processor is operative to identify a portion of the drive coils proximate the position of the mover along the track, where an electromagnetic field generated by the current flowing in each coil is operative to engage at least one drive magnet on the mover. The processor is also operative to generate a current reference signal for each coil in the portion of the drive coils proximate the position of the mover along the track according to one of a plurality of commutation modes and to transmit the current reference signal for each drive coil in the portion of the drive coils proximate the position of the mover to the power segment. The power segment provides the current to each coil according to the corresponding current reference signal. 
     According to still another embodiment of the invention, a method for controlling commutation of drive coils to control operation of a mover along a track in a linear drive system is disclosed. A position of the mover along the track is identified with a motor controller, and the motor controller identifies multiple drive coils proximate the position of the mover, where an electromagnetic field generated by a current flowing in each of the drive coils is operative to engage at least one drive magnet on the mover. A reference signal is received at the motor controller corresponding to a desired operation of the mover, and the motor controller determines a desired force to be applied to the mover as a function of the reference signal and of the position of the mover. One of a plurality of commutation modes is selected in the motor controller to determine a current reference for each of the drive coils. The motor controller determines the current reference for each of the plurality of drive coils as a function of: the selected commutation mode, the position of the mover, the desired force to be applied to the mover as a function of the reference signal, and a back-emf value for each of the drive coils. 
     According to yet another embodiment of the invention, a method for selecting commutation of drive coils to control operation of a mover along a track in a linear drive system receives a reference signal corresponding to a desired operation of the mover at a controller for at least one track segment of the track. At the controller, a first desired commutation mode for generating a current reference for a plurality of drive coils positioned along the track is determined. The mover is propelled along the track responsive to the reference signal using the first desired commutation mode, and as the mover is propelled along the track, a selection signal is received at the controller corresponding to a second desired commutation mode for generating the current reference for the drive coils positioned along the track. At the controller, the current reference is generated for drive coils positioned along the track with the second desired commutation mode responsive to receiving the selection signal. 
     According to still another embodiment of the invention, a motor controller for controlling commutation of drive coils to control operation of a mover along a track in a linear drive system includes at least one input, a power segment, and a processor. The at least one input is operative to receive a reference signal corresponding to a desired operation of the mover along the track, and the power segment is operative to provide a current to drive coils operatively connected to the power segment. The processor is operative to determine a first desired commutation mode for generating a current reference corresponding to the current provided to each of the drive coils, generate the current reference with the first desired commutation mode to propel the mover along the track, receive a selection signal corresponding to a second desired commutation mode for generating the current reference as the mover travels along the track, and generate the current reference with the second desired commutation mode to propel the mover along the track responsive to receiving the selection signal. 
     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 one embodiment of the transport system of  FIG.  1    illustrating activation coils distributed along one surface of the track segment; 
         FIG.  3    is an isometric view of a mover from the transport system of  FIG.  1   ; 
         FIG.  4    is a partial sectional view of the transport system of  FIG.  1   ; 
         FIG.  5    is a partial sectional view of a mover illustrating an exemplary magnet configuration for a mover having a first width; 
         FIG.  6    is a partial sectional view of a mover illustrating an exemplary magnet configuration for a mover having a second width; 
         FIG.  7    is a partial sectional view of a mover illustrating an exemplary magnet configuration for a mover having a third width; 
         FIG.  8    is a block diagram representation of an exemplary power and control system for the transport system  FIG.  1   ; 
         FIG.  9    is an exemplary schematic for a portion of the power and control system of  FIG.  8   ; 
         FIG.  10    is a block diagram representation of an exemplary control module executing in one of the segment controllers of  FIG.  8   ; 
         FIG.  11    is a graphical representation of an exemplary back-emf waveform stored in the motor controller for the mover of  FIG.  5   ; 
         FIG.  12    is a graphical representation of an exemplary back-emf waveform stored in the motor controller for the mover of  FIG.  6   ; 
         FIG.  13    is a graphical representation of an exemplary back-emf waveform stored in the motor controller for the mover of  FIG.  7   ; 
         FIG.  14    is a graphical representation of a force constant plotted with respect to an amplitude of the sum of coil currents for an exemplary mover according to one embodiment of the invention; and 
         FIG.  15    is a graphical representation of an rms-value of current plotted with respect to the amplitude of the sum of coil currents for the exemplary mover of  FIG.  14   . 
     
    
    
     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. 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. According to the illustrated embodiment, each rail  20  includes a base portion  22  mounted to the track segment and a track portion  24  along which the mover  100  runs. 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  FIG.  3   , 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 within 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. According to one embodiment of the invention shown in  FIG.  2   , the linear drive system includes drive magnets  140  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 alternately having a north pole, N, and south pole, S, pole facing the track segment  12 . The drive magnets  140  are typically permanent magnets, and two adjacent magnet segments including a north pole and a south pole 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.  4   , 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  141  and interacts with the drive magnets  140  mounted to the mover  100  to control operation of the mover  100 . 
     It is contemplated that a track  10  may be configured to have movers  100  of different sizes and/or movers  100  having different magnet configurations traveling along the same track. With reference next to  FIGS.  5 - 7   , three movers  100 , each having a different width and a different magnet configuration are illustrated. Turning first to  FIG.  5   , a mover  100  having a first width, W 1 , is illustrated. The mover  100  includes a first half of a drive magnet  140  mounted proximate one side of the mover  100  and a second half of a drive magnet mounted proximate the other side of the mover  100 . Between the two halves, a whole drive magnet  140  is mounted. Each of the two halves are arranged such that one polarity of the drive magnet  140  faces the drive coils and the whole drive magnet  140  is arranged such that the other polarity of the drive magnet  140  faces the drive coils. As illustrated, each of the half drive magnets  140  has a north pole, N, facing the drive coils and the whole drive magnet  140  has a south pole, S, facing the drive coils. Turning then to  FIG.  6   , a mover  100  having a second width, W 2 , is illustrated. The mover  100  includes four whole drive magnets positioned adjacent to each other. In a manner similar to that illustrated in  FIG.  3   , adjacent drive magnets  140  alternately having a north pole, N, and south pole, S, pole facing the drive coils. Turning next to  FIG.  7   , a mover  100  having a third width, W 3 , is illustrated. The mover  100  includes a first half of a drive magnet  140  mounted proximate one side of the mover  100  and a second half of a drive magnet mounted proximate the other side of the mover  100 . Between the two halves, a series of whole drive magnets  140  are mounted. Each of the two halves are arranged such that one polarity of the drive magnet  140  faces the drive coils and the whole drive magnets  140  are arranged such that the polarity of the drive magnets  140  alternate between the two half magnets. As illustrated, each of the half drive magnets  140  has a north pole, N, facing the drive coils and the whole drive magnets  140  have alternating south and north poles facing the drive coils. The illustrated magnet configurations are exemplary only and not intended to be limiting. It is contemplated that magnets having, for example, different widths or different arrangements of north and south poles may be utilized without deviating from the scope of the invention. 
     Turning next to  FIG.  8   , an exemplary power and control system for the track  10  and linear drive system is illustrated. A segment controller  200  is mounted within each track segment  12 . The segment controller  200  receives command signals from a system controller  30  and generates switching signals for power segments  210  ( FIG.  9   ) 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  32  may be 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. The system controller  30  could be a Programmable Logic Controller (PLC). 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  via a control network  201 , such as an EtherNet/IP network. 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  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 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  controlled by that system controller  30 . 
     With additional reference to  FIG.  9   , each segment controller  200  generates switching signals  207  to control operation of switching devices within one or more power segments  210  mounted within the track segment  12 . 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 . The switching signals  207  control operation of switching devices  220  in communication with the drive coils  50 , including upper switch devices  220   a  and lower switching devices  220   b . The switching devices  220  may be solid-state devices that are activated by the switching signals  207 , including, but not limited to, transistors, such as insulated-gate bipolar transistors, thyristors, or silicon-controlled rectifiers. 
     According to the illustrated embodiment, an AC converter  222  ( FIG.  8   ) can receive a single or multi-phase AC voltage  224  from a power grid. The AC converter  222 , in turn, can provide a DC voltage  226  using, for example, a rectifier front end, at input terminals of a DC supply  228 , which could be a DC-to-DC buck converter. The DC supply  228 , in turn, can provide a distributed DC bus  230  at the input terminals of the segments  12 , including: a DC reference voltage rail  232 , configured to provide a DC reference voltage (“DC-”) such as ground (0 V); a mid-bus DC voltage rail  234 , configured to provide half DC power at a mid-bus voltage (“DC 1”) such as 200 V; and a full-bus DC voltage rail  236 , configured to provide DC power at a full-bus voltage (“DC 2”), such as 400 V. Although illustrated external to the track segment  12 , it is contemplated that the DC bus  230  would extend within the segments  12 . Each segment  12  includes connectors to which either the DC supply or another track segment may be connected such that the DC bus  230  may extend for the length of the track  10 . Optionally, each track segment  12  may be configured to include a rectifier section (not shown) and receive an AC voltage input. The rectifier section in each track segment  12  may convert the AC voltage to the DC bus  230  utilized by the corresponding track segment. It is contemplated that the polarities and magnitudes of the various rails of the DC bus  230  may vary within the scope of the invention. 
     The processor  204  also receives a feedback signal  209  from the position sensors  150  along the track segment  12  to provide an indication of the presence of one or more movers  100 . In each power segment  210 , the processor  204  can generate the switching signals  207  to control the various switching devices  220  to provide power to respective coils  50  for propelling a mover  100  while continuously receiving feedback signals for determining positions of the mover  100 . For example, in a first leg “A,” the processor  204  can drive the upper and lower switching devices  220   a  and  220   b , respectively, to control a corresponding coil  50  in the first leg A to propel the mover  100 . The processor  204  can detect movement of the mover  100  from the first leg A toward an area corresponding to the second leg “B” via the feedback signals from the position sensors  150 . The processor  204  can then drive the upper and lower switching devices  220   a  and  220   b , respectively, to control a corresponding coil  50  in the second leg B to continue propelling the mover  100 , according to a predetermined motion profile. In each leg, the lower switching devices  220   b  can be coupled to the DC-voltage rail  232 , the upper switching device  220   a  can be coupled to the full-bus DC voltage rail  236 , and the coil  50  can be coupled between the upper and lower switching devices  220   a  and  220   b , respectively, on a first side and the mid-bus DC-voltage rail  234  on a second side. Accordingly, the switching devices  220  in each leg can be configured to connect a coil  50  in the leg between rails of the DC bus  230  in various states, such as the upper switching devices  220   a  connecting or disconnecting full-bus DC voltage rail  236  to a coil  50  causing positive current flow in coil  50 , and/or the lower switching device  220   b  connecting or disconnecting DC-voltage rail  232  to a coil  50  causing negative current flow in coil  50 . 
     The processor  204  receives feedback signals from voltage and/or current sensors mounted at an input or output of the power segment  210  providing an indication of the current operating conditions of the DC bus  230  within the power segment  210  or the current operating conditions of a coil  50  connected to the power segment  210 , respectively. According to the illustrated embodiment, sensing resistors  260  are shown between lower switching devices  220   b  and the DC-reference voltage rail  232  to detect current through the lower switching devices. Signals from either side of the sensing resistors are provided to the signal conditioning circuitry  244 . Similarly, a bus sensing resistor  240  is shown in series with the mid-bus DC-voltage rail  234 . Signals from either side of the bus sensing resistor  240  are provided to the signal condition circuitry  244  through an isolation circuit  246 . The signals are provided via an amplifier  248  and an Analog-to-Digital Converter (ADC)  250  to the processor  204  to provide a measurement of the current flowing through each of the sensing resistors  260  and the bus sensing resistor  240 . It is contemplated that still other sensing resistors or other current transducers and voltage transducers may be located at various locations within the power segment  210  to provide current and/or voltage feedback signals to the processor  204  corresponding to current and/or voltage levels present on any leg of the DC bus  230  or at the output to any of the coils  50  connected to the power segment  210 . 
     In operation, the processor  204  for each segment controller  200  is configured to execute a control module  300  which utilizes position and current feedback information to regulate the current output to each coil  50  to achieve desired operation of each mover  100 . Referring next to  FIG.  10   , a control module  300  according to one embodiment of the invention is illustrated. The control module  300  receives a position reference signal (x*)  301  as an input. The position reference signal (x*)  301  is compared to a position feedback signal (x)  303  at a first summing junction  302 . A position error signal is output from the first summing junction  302  and input to a position loop controller  304 . According to the illustrated embodiment, the position loop controller  304  is a proportional-integral (PI) controller. Optionally, the position loop controller  304  may be just a proportional (P) controller or further include a derivative (D) component. Each of the proportional (P), integral (I), and/or derivative (D) components of the position loop controller  304  includes a controller gain. The controller gains are commonly referred to as a proportional gain (Kpp), integral gain (Kpi), and a derivative gain (Kpd). The output of the position loop controller  304  is a velocity reference signal (v*). The velocity reference signal (v*) is compared to a velocity feedback signal (v) at a second summing junction  306 . The velocity feedback signal (v) is generated by taking a derivative, as shown in the derivative block  311 , of the position feedback signal (x). The velocity feedback signal (v) may also be filtered by a velocity filter block  310 . A velocity error signal is output from the second summing junction  306  and input to a velocity loop controller  308 . According to the illustrated embodiment, the velocity loop controller  308  is a proportional-integral (PI) controller. Optionally, the velocity loop controller  308  may be just a proportional (P) controller or further include a derivative (D) component. Each of the proportional (P), integral (I), and/or derivative (D) components of the velocity loop controller  308  includes a controller gain. The controller gains are commonly referred to as a proportional gain (Kvp), integral gain (Kvi), and a derivative gain (Kvd). The output of the velocity loop controller  308  is an acceleration reference signal. 
     The control module  300  may also include feed forward branches. According to the illustrated embodiment, the control module  300  includes feed forward branches for both the velocity and the acceleration elements. The position reference signal (x*) is passed through a first derivative element  312  to obtain a velocity feed forward signal. The velocity feed forward signal is multiplied by a velocity feed forward gain (Kvff)  314  and combined with the velocity reference signal (v*) and the velocity feedback signal (v) at the second summing junction  306 . The velocity feed forward signal is passed through a second derivative element  316  to obtain an acceleration feed forward signal. The acceleration feed forward signal is multiplied by an acceleration feed forward gain (Kaff)  318  and combined with the acceleration reference signal at a third summing junction  320 . 
     As indicated above, the derivative of velocity feedforward is an acceleration feedforward signal and the output of the velocity loop controller  308 , absent other gains, is an acceleration reference signal. Each of these signals is combined at the third summing junction  320 . It is understood that the acceleration is proportional to a force required to achieve the acceleration according to Newton&#39;s second law of motion, which states that a force is equal to a mass multiplied by acceleration. Thus, in order to convert the acceleration terms at the summing junction  320  to force terms, each acceleration term would be multiplied by the mass to be accelerated. 
     In some embodiments of the control module  300 , calculations may be performed in a per unit system. Depending on the per unit system, a range of zero to one hundred percent acceleration may be equivalent to a range of zero to one hundred percent torque. As a result, a per unit value of acceleration would be equivalent to a per unit value of torque and no further gain needs to be applied at the summing junction  320  to convert the acceleration to force. In other embodiments, the mass of the mover  100  or of the mover and load may be incorporated into the controller gains of the velocity loop controller  308  and in the acceleration feedforward path to output a force reference from the third summing junction  320 . In still other embodiments, the mass of the mover  100  or of the mover and load may be included as an additional gain before or after the summing junction  320 . According to the illustrated embodiment, it is contemplated that the mass is incorporated into the controller gains for the velocity loop controller  308  and in the acceleration feed forward gain (Kaff)  318 . Incorporating the mass of the mover into the controller gains reduces the computational requirements during run-time by eliminating an additional gain calculation. The output of the third summing junction  320 , therefore, is a force reference, or a desired force, F d , to be applied to a mover  100 . 
     The control module  300  may also be used to reduce or minimize cogging forces present in the linear drive system. The memory device  206  in the segment controller may include a table  321  storing a magnitude of a cogging force that results from a mover  100  traveling along the track segment  12 . The table  321  includes a plurality of values corresponding to the position of the mover with respect to the track segment. As shown in  FIG.  10   , the position feedback signal, x, may be used by the control module  300  to access the cogging table  321  and to identify a magnitude of cogging force at the position. The magnitude of cogging force may be supplied to the third summing junction such that the desired force, F d , compensates for the cogging force. The table  321  may include a separate list of cogging forces for each mover  100 . Optionally, movers  100  of similar construction may use a single list of cogging forces and the table may include a separate list of cogging forces for movers of differing construction. The control module  300  may select the stored value for a position that most closely corresponds to the mover&#39;s position. Optionally, the control module  300  may two stored values for the cogging force, where the two stored values are located on either side of the present position of the mover, and the control module  300  may interpolate between the two stored values to obtain a more accurate value of the cogging force at the present location. 
     The commutator  322  in the control module  300  receives the desired force, F d , as an input and determines a current reference for each of the coils  50  that need to be energized to control operation of a mover  100 . A track segment  12  may include, for example, twelve coils  50  spaced along the side of the segment. When a single mover  100  is present on the track segment  12 , only those coils  50  located under the mover and proximate the drive magnets  140  need to be energized to control operation of the mover  100 . With reference, for example, to  FIGS.  5 - 7   , it is contemplated that two or three coils  50  may be energized to control operation of the mover  100  illustrated in  FIG.  5    depending on the location of the mover along the track, four coils may be energized to control operation of the mover illustrated in  FIG.  6   , and five or six coils may be energized to control operation of the mover illustrated in  FIG.  7    depending on the location of the mover along the track. However, the number of coils required to control operation of a mover is dependent on the construction of the coils, the mover and the drive magnets  140  located on the mover. If multiple movers  100  are present on the track segment  12 , the coils  50  located under each of the movers and proximate the respective sets of drive magnets  140  need to be energized to control operation of the respective mover  100 . The processor  204  receives position feedback information for each mover  100  and, therefore, identifies the coils  50  located behind each mover(s) that require energization to control operation of the mover  100 . 
     The processor  204  receives a reference signal corresponding to a desired operation of each mover  100  located on the track segment  12 . As discussed above, the reference signal may be a motion profile or portion thereof, defining operation of the mover  100  along the track segment  12 . The processor  204  may convert the motion profile to the position reference signal (x*)  301  and provide the position reference signal to the control module  300 . Based on the current position of each mover  100 , the coils present under each mover, the desired force, F d , to be applied to each mover  100  and the desired operating mode, the commutator  322  generates a current reference signal, I*, for each coil  50  proximate a mover. 
     According to one aspect of the invention, the commutator  322  is operative to determine a desired current reference signal, I*, for each drive coil  50  in one of multiple commutation modes. The commutator  322  may operate in a first operating mode to generate current reference signals for each of the drive coils  50  which minimize the copper losses in the drive coils. In a second operating mode, the commutator  322  generates current reference signals for each of the drive coils  50  to maximize the force applied to the mover  100 . In a third operating mode, the commutator  322  generates current reference signals for each of the drive coils  50  that result in balanced currents between the drive coils, where balanced currents indicates that a sum of the currents in each of the drive coils  50  activated for one mover  100  is zero or near zero. In a fourth operating mode, the commutator  322  generates current reference signals for each of the drive coils  50  according to a selected operating point that combines characteristics of the first three operating modes. 
     The current reference signal, I*, for each drive coil  50  is determined for each of the different operating modes using a single closed form equation as shown below in Equation 1. The commutator  322  iteratively utilizes Equation 1 to determine a current reference signal for each of the “n” coils under a mover  100 . 
                         i   *     (   x   )     k     =             F   d     (   x   )     ⁢     (         ne   ⁡   (   x   )     k     -     β   ⁢       ∑     j   =   1     n             e   ⁡   (   x   )     j           )       +       α   ⁡   (   x   )     ⁢     (         ∑     j   =   1     n             e   ⁡   (   x   )     j   2       -         e   ⁡   (   x   )     k     ⁢       ∑     j   =   1     n             e   ⁡   (   x   )     j           )             n   ⁢       ∑     j   =   1     n         e   ⁡   (   x   )     j   2         -       β   ⁡   (       ∑     j   =   1     n             e   ⁡   (   x   )     j       )     2                 (   1   )               
where: x is the present position of the mover;
 
     n is the number of coils energized to control the mover; 
     k is an individual coil number between 1 and n; 
     i*(x) k  is the current for the k th  coil at the present position of the mover; 
     F d (x) is the desired force output from the control module; 
     e(x) k  is the back-emf for the k th  coil at the present position of the mover; and 
                     α   ⁡   (   x   )     =     a   ⁢     sin   ⁡   (       2   ⁢   π   ⁢   x     τ     )               (   2   )               
where: τ is the coil pitch; and
 
     a is the desired sum of active coil currents. 
     The desired operating mode is selected, at least in part, by the selection of values for α and β in equations 1 and 2 above. When α and β are both set equal to zero, the commutator  322  will operate in the first operating mode. When α is set equal to zero and β is set equal to one, the commutator  322  will operate in the third operating mode. When β is set equal to one and α is equal to a non-zero value, the commutator  322  will operate in the desired operating mode as a function of the desired sum of active coil currents indicated in “a” of equation 2. 
     As indicated in Equation 1, the back-emf induced in each coil by the drive magnets  140  on the mover is used to determine the current reference signals. It is contemplated that the back-emf for each coil  50  may be stored in a table in the memory device  206  on the segment controller  200 . With reference to  FIGS.  11 - 13   , three exemplary back-emf waveforms are illustrated. Each waveform corresponds to the back-emf induced in a single coil  50  by a mover  100  passing by the coil. Further, the three waveforms correspond to the same coil  50  generating a different back-emf waveform as a result of a mover  100  having a different construction passing the coil. The exemplary back-emf waveforms in  FIGS.  11 - 13    correspond to the movers shown in  FIGS.  5 - 7    each passing the same coil  50 . According to the illustrated embodiment, a first back-emf waveform  270  shown in  FIG.  11    is generated by the mover in  FIG.  5    passing a coil, a second back-emf waveform  280  shown in  FIG.  12    is generated by the mover in  FIG.  6    passing the coil, and a third back-emf waveform  290  shown in  FIG.  13    is generated by the mover in  FIG.  7    passing a coil. 
     Each of the plots  270 ,  280 ,  290  in  FIGS.  11 - 13    are illustrated with both a continuous waveform and a number of points. The continuous waveform  271 ,  281 ,  291  corresponds to the back-emf generated by the mover  100  as it passes the coil. The points  272 ,  282 ,  292  correspond to values stored in the memory device  206  for each waveform. A finite number of points may be stored for each waveform and each point corresponds to a particular location along the track  10 . When the commutator  322  determines the current reference for a coil, the mover&#39;s position along the track is obtained from the position sensors  150  located along the track. The value for the back-emf waveform is then read from the memory device, where the particular stored back-emf waveform is selected based on the coil and the mover. In one embodiment, the commutator  322  may select the stored value for a position that most closely corresponds to the mover&#39;s position. Optionally, the commutator  322  may read the stored values for two positions of the back-emf waveform, where the two stored values are located on either side of the present position of the mover, and the commutator  322  may interpolate between the two stored values to obtain a more accurate value of the back-emf waveform for use in Equation 1. 
     As previously indicated, the commutator  322  may be configured to operate in different modes. In each mode, the commutator  322  determines current references for each coil  50  according to different criterion. With reference to Equation 2 above, the term “a” is used to select in which operating mode the commutator operates. 
     Selection of a desired operating mode is dependent on a number of factors. For discussion, an exemplary mover receives an object for delivery at a first location and delivers the object to a second location. At a loading station, the mover is stopped and the object is loaded on the mover. The mover then accelerates from zero speed up to a transport speed during which the object is loaded on the mover. As the mover approaches the delivery station, the mover decelerates from the transport speed to zero speed. At the delivery station, the object is removed from the mover. The mover again accelerates from zero speed up to a return speed during which the mover is unloaded. The mover decelerates from the return speed back to zero speed as it approaches the loading station. 
     In the example, periods of acceleration and deceleration typically require the greatest amounts of current. During these periods, it may be desirable to operate under the first operating mode to minimize copper losses and, therefore, reduce heating in the coils when the current is high. When the mover is traveling between the loading and unloading station, it may require operating mode two such that the maximum force is provided to the mover, allowing the mover to transport the object loaded on the mover. When the mover is returning from the unloading station to the loading station, it may be desirable to operate in the third operating mode, such that there is a zero sum of current present in the coils used to control the mover. Having a zero sum current lowers the demand on the power supply providing current to the coils and, therefore, allows either a greater number of movers to be controlled by one power supply or allows for a power supply having a lower rating, which typically results in a lower cost, to be utilized. The above example is not intended to be limiting and it is understood that various other factors and application requirements may influence the selection of zones without deviating from the scope of the invention. Each zone may span an entire track segment  12  or there may be multiple zones on one track segment. The segment controller is configured to operate in the desired operating mode or modes when a mover  100  is travelling along the segment and is located in the corresponding zone or zones. 
     With reference next to  FIGS.  14  and  15   , operation of a mover  100  may be initially characterized at a number of different operating points. In the illustrated embodiment, a mover  100  is characterized at fourteen operating points  355 ; however, it is understood that varying numbers of operating points at different current intervals may be selected without deviating from the scope of the invention. Each operating point corresponds to a different sum of the currents present in the coils  50  controlling operation of the mover  100 .  FIG.  14    illustrates a force constant, Kf, corresponding to the amount of force applied to the mover  100  as a result of the sum of the currents present in the coils  50  controlling operation of the mover  100 . As the force constant increases, the force applied to the mover  100  increases.  FIG.  15    illustrates the rms value of current present in the coils controlling operation of the mover  100 . The rms value of current includes both the components of current that are producing force on the mover and the components of current that are causing copper losses in the coils. Thus, for the same applied force on the mover, as the rms value of the current is reduced, the magnitude of copper losses present in the coils is reduced. 
     A set of curves, corresponding to the plots in  FIGS.  14  and  15   , may be stored in the memory  206  on the segment controller for each mover  100 . Optionally, one set of curves for multiple movers  100  of similar construction may be stored. The set of curves define each of the operating modes for the commutator  322 . In the first operating mode, the commutator  322  generates current reference signals for each of the drive coils  50  which minimize the copper losses in the drive coils. The processor may determine a minimum value in the plot in  FIG.  15    (or in the values for the plot stored in memory) corresponding to a minimum value of current present that causes the copper losses. In  FIG.  15   , the operating point is identified by reference numeral  350 . According to the illustrated embodiment, the sum of the coil currents at this point is eight amps. The operating point  350  at eight amps is similarly marked in  FIG.  14    for reference. In the second operating mode, the commutator  322  generates current reference signals for each of the drive coils  50  to maximize the force applied to the mover  100 . The processor may determine a maximum value in the plot in  FIG.  14    (or in the values for the plot stored in memory) corresponding to the maximum force constant determined during the characterization of the mover  100 . In  FIG.  14   , the operating point is identified by reference numeral  360 . According to the illustrated embodiment, the sum of the coil currents at this point is four and one-half amps. The operating point  360  at four and one-half amps is similarly marked in  FIG.  15    for reference. In the third operating mode, the commutator  322  generates current reference signals for each of the drive coils  50  that result in balanced currents between the drive coils. As previously indicated, when the commutator  322  generates current reference signals for each of the drive coils  50  that result in balanced currents, the sum of the currents in each of the drive coils is zero or near zero. In both  FIGS.  14  and  15   , the operating point identified by the reference numeral  370  is located at zero amps. For each operating point identified above, the corresponding current at the operating point is the value of “a” for Equation 2 above. Setting “a” to the corresponding current will cause the closed form determination of current reference signals performed in Equation 1 to determine the current reference for the desired operating mode. 
     In addition to the three operating modes discussed above, the commutator  322  may be configured to operate in a fourth operating mode. A value of current for desired operating point may be stored in the memory  206  of the segment controller which corresponds to the desired operating point. In one aspect of the invention, the value of current for the desired operating point may be determined automatically by the processor  204  as discussed above. Optionally, a user may adjust the value of the current via a user interface connected to the segment controller  200  or via the user interface  36  connected to the system controller  30 , where the system controller  30  transmits the new value of the current to the segment controller  200 . The new value of current may be for any sum of coil currents shown in  FIGS.  14  and  15   . Consequently, an operating point between any of the first three operating points may be selected. 
     In addition to selection based on the location of the mover, the segment controller may include an input signal by which an operating mode may be selected. In one aspect of the invention, the input signal may be, for example, a data word or multiple bits by which any of the different operating modes may be selected. In another aspect of the invention, the input may be a logical input receiving either a logical zero or a logical one as an input. The segment controller may be configured to execute in one operating mode when the input is a logical zero and in another operating mode when the input is a logical one. In this manner, each the segment controller  20  may be operative to select different commutation modes for the same mover  100  based on other factors in the controlled system. 
     According to another aspect of the invention, the commutator  322  may be further configured to detect when one of the coils  50  will be saturated and distribute the excess current from the saturated coil to the other coils controlling operation of a mover  100 . During the calculation of current reference signals, I*, the back-emf for each of the coils is determined, as described above. The commutator  322  may initially sort the values for each of the back-emf values in descending order. By sorting the back-emf values in descending order, the coil with the greatest back-emf is identified. Further, it is the coil with the greatest back-emf that will be the first coil to become saturated. Therefore, sorting the back-emf values according to descending order identifies the coil or coils most likely to saturate. 
     After sorting the back-emf values, the commutator  322  determines an initial set of current reference signals, I*, using Equation 1 as described above. The current reference signal, I*, for the coil with the greatest back-emf value is first compared to a maximum current value. If the current reference signal is less than the maximum current value, the commutator  322  outputs the current reference signals to the current regulator  324 . If, however, the current reference signal for the coil with the greatest back-emf value is greater than the maximum current value, then the commutator  322  sets the current reference signal for the coil with the greatest back-emf value to the maximum current value and redistributes the excess current to the other coils. 
     In order to redistribute the excess current, the commutator  322  first determines the force that will be generated as a result of the current in the coil that has been limited to the maximum current. The force applied to the mover by the coil is equal to the magnitude of the current multiplied by the back-emf value for the coil. This force is subtracted from the desired force that was previously determined and which was used to determine the initial set of current reference signals. The new desired force is utilized by Equation 1 to determine a new set of current reference signals for the remaining drive coils which were not set to the maximum current value. After obtaining a new set of current reference signals, the commutator  322  can again verify that there are no current reference signals greater than the maximum current value using the steps described above. If there are still any current reference signals greater than the maximum current value, another current reference signal may be set to the maximum current value and the remaining current redistributed to the coils not limited. The process repeats until either all of the coils are set to output the maximum current value or there are no current reference signals greater than the maximum current value. 
     With reference again to  FIG.  10   , the current reference signal, I*, is then output from the commutator  322  and provided as an input to the current regulator  324 . According to the illustrated embodiment, the current regulator  324  is a proportional-integral (PI) controller. Optionally, the current regulator  324  may be just a proportional (P) controller or further include a derivative (D) component. Each of the proportional (P), integral (I), and/or derivative (D) components of the current regulator  324  includes a controller gain. The controller gains are commonly referred to as a proportional gain (Kip), integral gain (Kii), and a derivative gain (Kid). The output of the current regulator  324  is used to generate the switching signals  207  which, in turn, connect the DC bus  230  and provide the desired current to the coils  50 . It is further contemplated that the current reference signal, I*, output from the commutator  322  may be a vector value. In other words, the current reference signal, I*, may include multiple reference values, where each reference value corresponds to the current to be provided to one of the coils  50 . The current regulator  324  may be configured to operate on the vector or, optionally, separate current regulators  324  may be provided for each coil  50 . 
     The current is output from the power segment  210  to each of the coils  50 . The plant  326  in  FIG.  10    corresponds to the linear drive system, including the coils and the movers traveling along the track. Current feedback signals, I, are fed back from each coil to the control module  300  for the current regulators  324  to control the current. The position feedback signals from each sensor  150  are provided to the control module  300  as a mover travels along the corresponding track segment  12 . 
     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.