Patent Publication Number: US-2011060490-A1

Title: Moving vehicle system and method of controlling moving vehicles

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a system in which moving vehicles travel under control on the ground. 
     2. Description of the Related Art 
     In the system of moving a plurality of moving vehicles using a linear motor having a primary side on the ground, the inventor studied a system of monitoring positions of moving vehicles continuously, and driving the moving vehicles continuously using the linear motor. If such a system is obtained, it becomes possible to implement travel control in a manner that positions of moving vehicles such as overhead traveling vehicles are monitored substantially all the time. JP62-152303A proposed to provide primary coils of a linear induction motor on the ground and a secondary side conductor on a moving vehicle, and to arrange the primary coils at a pitch not more than the length of the secondary side conductor. 
     SUMMARY OF THE INVENTION 
     Preferred embodiments of the present invention provide a method and apparatus to control a primary side of a linear motor continuously to drive a moving vehicle while continuously monitoring a position of the moving vehicle. 
     Another preferred embodiment of the present invention provides a method and apparatus to monitor a position of a moving vehicle continuously over the entire travel route for providing feedback in travel control. 
     Still another preferred embodiment of the present invention provides a method and apparatus to control a plurality of moving vehicles to travel synchronously. 
     According to a preferred embodiment of the present invention, a moving vehicle system includes a plurality of moving vehicles each including a secondary element of a linear motor, primary elements of the linear motor arranged along a travel route of the moving vehicles at a pitch not more than the length of the secondary element, a plurality of sensors arranged at the pitch not more than the length of the secondary element to detect positions of the moving vehicles, and a controller arranged and programmed to control the primary elements based on position signals from the sensors. 
     In a preferred embodiment of the present invention, in a segment where the primary elements of the linear motor and the sensors are arranged at the pitch not more than the length of the secondary conductor, feedback control of the primary side of the linear motor can be implemented all the time. Therefore, it becomes possible to monitor the positions of the plurality of moving vehicles continuously, and to control the moving vehicles continuously. Further, the travel route includes a diverge segment, a merge segment, and a curve segment in addition to a straight segment, and the secondary element of the linear motor is bendable in the diverge segment, the merge segment, and the curve segment along the travel route. In the system, feedback control of the primary side of the linear motor can be implemented all the time. 
     Preferably, in one preferred embodiment of the present invention, the linear motor is a linear synchronous motor, the secondary element is an array of magnets, each of the sensors is a sensor arranged to detect an absolute position, and the sensors and the primary elements are arranged at a pitch not more than the length of the secondary element, for example. In this manner, the positions of the moving vehicles can be monitored over the entire length of the travel route, and feedback control for the travel can be implemented. 
     Preferably, each of the sensors is a sensor arranged to detect an array of magnets of the secondary elements, for example. Thus, the positions of the moving vehicles can be detected using the magnet array of the secondary elements of the linear motor. 
     Further, preferably, the controller includes a system controller and a plurality of zone controllers below the system controller, for example. The zone controllers control the primary elements based on a position instruction of each moving vehicle from the system controller and position signals from the sensors. 
     Each of the zone controllers reports positions of the moving vehicles in each of predetermined control cycles. 
     The system controller sends position instructions to each of the zone controllers in each of the control cycles. 
     In this manner, centralized monitoring of the positions of the moving vehicles is carried out by the system controller, and the moving vehicles are controlled to travel synchronously in accordance with the position instructions from the system controller. 
     In particular, preferably, the controller controls the moving vehicles to travel synchronously at the same speed along the travel route. In this manner, a large number of traveling vehicles can be controlled to move efficiently without any interference. 
     Further, a method of controlling moving vehicles according to another preferred embodiment of the present invention is carried out in a system including a plurality of the moving vehicles each including a secondary element of a linear motor, and primary elements of the linear motor arranged along a travel route of the moving vehicles at a pitch not more than the length of the secondary element. The method includes the steps of a) detecting positions of the moving vehicles continuously using a plurality of sensors arranged at the pitch not more than the length of the secondary element, and b) controlling the primary elements based on the determined positions. 
     In this specification, the description regarding the moving vehicle system is directly applicable to the method of controlling moving vehicles, and conversely, the description regarding the method of controlling moving vehicles is directly applicable to the description regarding the moving vehicle system. 
     The above and other elements, features, steps, characteristics and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a plan view showing a layout of a moving vehicle system according to a preferred embodiment of the present invention. 
         FIG. 2  is a view showing a moving vehicle having a secondary side of a linear motor, a primary side of the linear motor provided on the ground, and the linear sensor. 
         FIG. 3  is a view showing a model of a bendable magnet array. 
         FIG. 4  is a diagram showing a layout of the secondary side of the linear motor and linear sensors. 
         FIG. 5  is a diagram showing a positional relationship between the secondary side of the linear motor and the linear sensor. 
         FIG. 6  is a diagram showing another positional relationship between the secondary side of the linear motor and the linear sensor. 
         FIG. 7  is a block diagram showing a linear sensor shown in  FIG. 2 . 
         FIG. 8  is a block diagram showing a sensor including Hall elements. 
         FIG. 9  is a curve diagram showing signal processing for the output of the Hall elements shown in  FIG. 8 . 
         FIG. 10  is a block diagram showing the relationship between a zone controller and a drive unit of a linear motor in a preferred embodiment of the present invention. 
         FIG. 11  is a diagram showing a model of synchronization control between apparatuses in a preferred embodiment of the present invention. 
         FIG. 12  is a graph showing timings between packets and a model of delay time in a preferred embodiment of the present invention. 
         FIG. 13  is a curve diagram including a curve ( 1 ) showing reports from the zone controller to a system controller, a curve ( 2 ) showing position instructions sent from the system controller to the zone controller, and a curve ( 3 ) showing velocity instructions sent from the zone controller to the drive unit of the liner motor. 
         FIG. 14  is a view schematically showing communication data transmitted between the zone controller and the system controller. 
         FIG. 15  is a block diagram showing the system controller in a preferred embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, preferred embodiments for carrying out the present invention in the most preferred form will be described. The preferred embodiments can be modified suitably with reference to techniques known in this technical field, and do not limit the scope of the present invention. 
       FIGS. 1 to 15  show a moving vehicle system  2  according to a preferred embodiment and its modification. As shown in  FIG. 1 , in a travel route for moving vehicles  20 , for example, a plurality of intra-bay routes  4  are connected each other through an inter-bay route  6 . A plurality of stations  8  are provided along the routes  4 ,  6 . Reference numerals  10  denote straight segments in the routes  4 ,  6 . Reference numerals  11  denote curve segments, and reference numerals  12  denote diverge and merge segments. Diverge segments and merge segments are collectively referred to as the diverge and merge segments. Dotted lines  13  in  FIG. 1  show zone boundaries. For example, overhead traveling vehicles, rail vehicles that travel along rails on the ground, stacker cranes or the like may be preferably used as the moving vehicles  20 . In the present preferred embodiment, the moving vehicle  20  preferably does not have any transfer apparatus. Therefore, a transfer apparatus that travels synchronously with the moving vehicle  20  is provided in each station  8 , for example. By controlling the moving vehicle  20  and the transfer apparatus to travel at the same speed, transfer of an article transported by the moving vehicle  20  is performed. 
     Reference numerals  22  denote zone controllers. Each of the intra-bay routes  4  and the inter-bay route  6  is considered as a unit of the zone. The zone may include more than one intra-bay route  4  or the inter-bay route  6 . Alternatively, one intra-bay route  4  may be divided into a plurality of zones, and the inter-bay route  6  may be divided into a plurality of zones. A reference numeral  24  denotes a system controller provided to control the entire moving vehicle system  2  through the zone controllers  22 . In particular, the system controller  24  is arranged to control travel of the moving vehicles  20  and the transfer apparatus provided at each of the stations  8 . The zone controllers  22  and the system controller  24  are connected by a LAN  25 . The zone controller  22  implements control of the moving vehicles  20  and the transfer apparatus at each station  8  in the zone in accordance with instructions from the system controller  24 . 
       FIG. 2  shows structure of the moving vehicle  20 . A load receiver is provided at an upper position of the moving vehicle  20 . Further, for example, a pair of front and rear bogies  26  are provided at the bottom of the moving vehicle  20 . Reference numerals  27  denote bogie shafts for rotation of the bogies  26 . Reference numerals  28  denote wheels. For example, magnet arrays  30  are provided at the bottoms of the bogies  26 . Clearance is provided between the front and rear magnet arrays  30  to prevent interference during rotation of the bogies  26 . Alternatively, instead of providing such clearance, the front and rear magnet arrays  30  may have different heights to prevent interference even if these magnet arrays  30  are overlapped with each other. Further, the magnet arrays  30  may be provided at the fixed bottom of the moving vehicle  20 , instead of the bottoms of the bogies  26 . The moving vehicle  20  may not have the bogie  26 . The moving vehicle  20  may be equipped with a non-contact power feeding apparatus, a power generator or the like, and components such as sensors, communication devices, and the transfer apparatus may be mounted in the moving vehicle  20 . 
     Reference numerals  32  denote primary coils of a linear synchronous motor. The primary coils  32  are arranged at a pitch not more than the total length L of the front and rear magnet arrays  30 . It is assumed that the length of the primary coil  32  is d. Reference numerals  34  denote drive units arranged to control the primary coils  32  to operate as a linear synchronous motor. Reference numerals  36  denote linear sensors arranged to detect a position of the moving vehicle  20  by detection of the magnet arrays  30 . One linear sensor  36  is provided in each space between the primary coils  32 . The linear sensors  36  are provided at a pitch not more than the total length of the magnet arrays  30 . It should be noted that the expression “not more than” includes “equal to”. An alphabet “g” denotes a gap length between coils  32 . The linear sensor  36  is provided at each gap. 
       FIG. 3  shows a model for allowing the magnet arrays  30  to be bendable. A plurality of magnet arrays  30  are mounted in each of the bogies to form joints  41  between the respective magnet arrays  30 . Therefore, the moving vehicle can be accelerated or decelerated also in the curve segment or the like. 
       FIG. 4  shows a layout of the primary coils (motors)  32  and the linear sensors  36 .  FIG. 5  shows an example where the primary coils  32  and the linear sensors  36  are arranged in the same straight line.  FIG. 6  shows an example where the primary coils  32  and the linear sensors  36  are arranged in two straight lines in parallel. Reference numerals  38  denote signal processing units. For example, the signal processing units  38  may preferably be microcomputers or digital signal processors. Each of the signal processing units  38  processes a signal from the linear sensor  36 , and outputs a position of an absolute coordinate of the moving vehicle  20  to the zone controller  22 . A reference numeral  40  denotes a zone LAN arranged to connect the zone controllers  22  and the drive units  34  to the signal processing units  38 . Preferably, the zone controller  22  is provided at intervals of every predetermined number of coils  32  and every predetermined number of linear sensors  36 . 
     The linear sensor  36  directly detects the position of the magnet pole essential to control the synchronous motor, and process information of the magnet pole position to determine the absolute coordinate of the moving vehicle.  FIG. 7  shows the linear sensor  36  and the corresponding signal processing unit  38 . In the magnet array  30 , the magnets  31  are arranged in the moving direction of the moving vehicle at a predetermined pitch. In the linear sensor  36 , a plurality of coils  42  are arranged e.g., in one line in the same direction as the magnet array  30 . In the present preferred embodiment, the pitch of the coils  42  is the same as the pitch of the magnets  31 . The pitch of the coils  42  may be an integral multiple of the pitch of the magnets  31 , or the pitch of the magnets  31  may be an integral multiple of the pitch of the coils  42 . Further, reference numerals  44  denote Hall elements provided on both sides of the array of the coil  42 . Alternatively, the Hall element  44  may be provided in each gap between the coils  42 . The Hall element  44  detects appearance of the magnet array  30  and appearance of the boundary between the magnets  31 . Thus, the positions of the magnet arrays  30  at a pitch longer than the length of the magnet  31  can be determined by the Hall elements  44 . 
     The signal processing unit  38  generates a phase signal wt by a counter  50 , and a sine curve power supply  49  outputs sine curves V 1 , V 2  having the phase of ωt. The phase of the sine curve V 1  is positive, and the phase of the sine curve V 2  is negative. For example, these voltages are applied to a pair of coils  42 . That is, an even number of coils  42  are provided as pairs of left and right coils, from the side close to the center to the side remote from the center. The voltage V 1  is applied to one ends of the pairs of the coils, and the voltage V 2  is applied to the other ends of the pairs of coils. A processing circuit  46  processes a signal from the pairs of the coils  42 , e.g., processes the voltage at a point between one pair of coils, and determines the position of the magnet array  30  for each unit of distance which is smaller than the length of the coil  42 . For example, assuming that the pitch of the magnets  31  is equal to the pitch of the coils  42 , the signal from the coil  42  is repeated periodically, each time the magnet array  30  is shifted by a distance corresponding to two magnets  31 . Therefore, the processing circuit  46  determines a position based on the unit having the distance not more than the length of one magnet  31 . Then, the number of magnets  31  detected by the Hall elements  44  is counted by the counter  47 . The counted value is multiplied by the length of one magnet  31 , and the resulting value is added to the position determined from the coils  42  to determine the position of the magnet array  30 . Further, an offset memory  48  stores an absolute coordinate at the origin point (e.g., central position) of the linear sensor  36 . By adding the determined position to the absolute coordinate at the origin point, the absolute coordinate of the magnet array  30  is outputted. 
     The linear sensor  36  can distinguish the state where all the coils  42  face the magnet array  30  from the state where only some of the coils  42  face the magnet array  30 , and the magnet array  30  can distinguish the coils that face the magnet array  30  from the coils that do not face the magnet array  30 . Therefore, the position of the magnet array  30  can be detected without counting signals from the Hall elements  44 . 
     In the case where a high response type Hall element  52  having response time of 1 msec, preferably, 0.1 msec is used, for example, the position of the magnet array  30  can be detected without using the linear sensor  36 .  FIGS. 8 and 9  show a modified example of such a case. For example, a pair of Hall elements  52  are arranged at an interval of two pitches of the magnets  31 . Signals from the Hall elements  52  are processed by an input interface  54  to monitor which of the left and right Hall elements  52  has detected the magnet first. In a curve memory  55 , the signal from the Hall element that has detected the magnet first is stored for one cycle, i.e., two pitches of the magnets  31 . A comparator  56  compares an output of the Hall element that has detected the magnet  31  with delay and the stored curve, and a phase calculator  57  determines a phase θ. Further, each time the phase calculator  57  reaches a predetermined phase, the value of the counter  58  is incremented, e.g., by 1. The position of the magnet  31  in the magnet array  30  is determined by counting pitches. In  FIG. 8 , this variable is denoted by “n”. 
     If the left and right Hall elements  52  have the same characteristics, the Hall elements  52  output the same curve to the magnet array  30 . Therefore, by storing the curve detected on the upstream side, and detecting the phase based on the comparison with this curve, even if the interval between the Hall element  52  and the magnet  31  is changed, or even if the magnetic field applied to the Hall elements  42  does not have a sine waveform, the phase can be determined correctly. In this example, the pair of left and right Hall elements are preferably used. Alternatively, a pair of Hall elements for comparison may be provided additionally on both sides of the Hall elements for detection with spaces of two pitches of the magnets  31 . 
       FIG. 10  shows the relationship between the zone controller  22  and components such as the primary coil  32 , and the linear sensor  36 . The zone controller  22  reports the position, velocity and other states of the moving vehicle in each control cycle through the LAN  25 . The system controller  24  sends a position instruction or the like to the zone controller  22  in each control cycle. For example, the control cycle preferably is in a range of 1 msec to 100 msec, and more preferably, in a range of 1 msec to 10 msec. In the present preferred embodiment, it is assumed that the next control cycle starts at the time when the system controller  24  receives the report of the position and velocity. 
     A communication unit  60  of the zone controller  22  communicates with the system controller  24 , and a multiplexer  61  passes the signal of the position and velocity from a sensor unit to a position instruction generator  62 . The position instruction generator  62  generates position instructions to a plurality of coil units  68 . Therefore, the signal of the position and velocity is passed to the position instruction generator after information indicating which sensor unit  66  supplied the data is added, or together with the absolute coordinate of the moving vehicle. The position instruction generator  62  generates a target position and a target velocity for each moving vehicle, and controls the corresponding coil unit  68 . An alarm unit  63  generates an alarm signal when the moving vehicle is significantly deviated from the target position or the target velocity, or if any incident such as overvoltage, overcurrent, or voltage drop greater than a predetermined value occurs. Further, the alarm unit  63  uses information regarding positions of vehicles on the front and back sides stored by the system controller to stop the vehicles, e.g., safely without any collision. 
     The sensor unit  66  preferably includes the linear sensor  36  and the signal processing unit  38  described above. The position and velocity are reported at a cycle shorter than one control cycle, e.g., about 10 to 100 times per one control cycle. The coil unit  68  preferably includes the primary coil  32  and its drive unit  34  described above. The drive unit  34  receives the position instructions in each control cycle, and controls the phase and frequency of the electrical current applied to the primary coils  32  for movement to the position designated by the position instruction. 
       FIGS. 11 and 12  show models of synchronous travel of a plurality of moving vehicles  20  according to structure in  FIG. 10 . The system controller  24  has a clock as a reference in the entire system. Each controller in the system synchronizes its clock with the clock of the system controller  24 . The system controller  24  sends a synchronization packet (a) to the zone controller  22  for synchronization. After the zone controller  22  receives the synchronization packet (a), the zone controller  22  changes its internal settings in accordance with the synchronization packet (a), and as a result, the system controller  24  and the zone controller  22  are synchronized with each other. At the same time, the zone controller  22  sends a synchronization packet (b) to the servo amplifier (the drive unit  34  and the signal processing unit  38  for the primary coil). When the servo amplifier receives this packet, reading processing (c) of reading the sensor signal, velocity control (d) of the moving vehicle, and position control (e) of the moving vehicle are performed. Thus, the zone controller  22  is synchronized with the servo amplifier, and the entire system is operated synchronously within the delay time Δt shown in  FIG. 12 . By the synchronization control, the position and velocity of a plurality of moving vehicles can be controlled together, and transition between the primary coils of the linear motor is smoothly performed. 
     For synchronization control in the system, the LAN  25  requires to have a high data rate and a high capacity. In this system, since the apparatuses (controller and sevo amplifier) are concentrated on the ground. Synchronization between the apparatuses can be obtained even in a large scale system. 
       FIG. 13  shows 1) reports of positions or the like sent from the system controller to the zone controller, 2) position instructions sent from the system controller to the zone controller, and 3) velocity instructions sent from the zone controller to the coil unit. For example, the reports are preferably sent to the system controller  24  in each control cycle in a range of 1 msec to 100 msec, and one control cycle corresponds to a period from a position report to the next report. In each control cycle, preferably, at the time of starting the control cycle, the system controller  24  sends a position instruction to the zone controller. In one control cycle, the zone controller receives a report of the position and velocity multiple times from the linear sensor, and generates velocity instructions for implementing feedback control of the coil units  68 . 
       FIG. 14  shows a report packet  80  sent from the zone controller to the system controller, and a packet  82  of a position instruction sent from the system controller to the zone controller. In the report packet  80  from the zone controller, information showing that this packet is a report from the zone controller, and an ID of the zone controller are written. Next, for each of one or a plurality of moving vehicles under the control of the zone controller, the position, velocity, ID, and other information are notified. The packet  80  may be sent for each of the moving vehicles, and one packet may not include information of a plurality of moving vehicles. 
     In the packet  82  from the system controller, information to the effect that this packet is transmitted from the system controller, and an ID of the zone controller at the destination are written. For each of the moving vehicles, a target position and other information in the next control cycle are added. One or more dedicated packets  82  may preferably be used for each of the moving vehicles. Assuming that the velocity of the moving vehicle is, e.g., 10 m per second at the maximum, since the control cycle is in a range of 1 to 100 msec, the target position of the moving vehicle is 1 m to 10 mm ahead of the current position, for example. Even if the moving vehicles are provided as densely as possible, the distances between the centers of bodies of the moving vehicles is preferably not less than 1 m, for example. Therefore, the zone controller can determine the correspondence between the target positions and the moving vehicles based on the packet  82 . Assuming that the control cycle is in a range of 1 to 10 msec, the target position is 100 to 10 mm ahead of the current position, for example. 
       FIG. 15  shows structure of the system controller  24 . A communication unit  91  communicates with a communication unit  60  of the zone controller. An allocation controller  93  communicates with, e.g., a production controller or a host controller of the production controller and the system controller  24 , receives a transportation request, and reports a transportation result. A position instruction generator  90  generates position instructions for respective moving vehicles in each control cycle. The state table  92  stores positions of the moving vehicles along the travel route, velocity, destination, travel route to the destination, travel priority, and states, e.g., indicating whether any article is being transported or not, or the transportation vehicle is out of order. The state table  92  stores states of the moving vehicles along the travel route, and update the states in each control cycle. 
     A retraction controller  94  determines the necessity of retraction based on the travel route, priority, and position or the like of the moving vehicle in the state table  92 , and changes the travel route, destination or the like of the moving vehicle based on the determination. Then, the retraction controller  94  changes the travel route written in the state table  92 . A merge controller  95  determines combinations of moving vehicles that may cause interference in the diverge and merge segment, and notifies such combinations to the position instruction generator  90 . An interference search unit  96  reads the position and velocity of each moving vehicle from data in the state table  92  to prevent interference between moving vehicles outside the diverge and merge segments, and notifies such combinations to the position instruction generator  90 . The position instruction generator  90  implements velocity control to avoid the interference. 
     In the present preferred embodiment, at least the following advantages are obtained. 
     At least in straight segments, it is possible to continuously monitor positions of the moving vehicles  20  to continuously implement feedback control of the coil units  68 . 
     By the system controller  24 , absolute positions of a plurality of the moving vehicle  20  can be controlled over the entire travel route. 
     Since centralized control is implemented by the system controller  24 , in comparison with the case of distribution control where position instructions are generated by each of zone controllers, processing at the time of passing the boundary between the zone controllers is simplified. 
     Since the system controller implements position control over the entire area, positions of all the moving vehicles  20  can be controlled accurately at every time point. Further, control of movement of moving vehicles and control of inter-vehicle distance between front and rear moving vehicles can be implemented over the entire area. Accordingly, instructions are allocated to the moving vehicles optimally, optimum retraction of the moving vehicles can be carried out to avoid jams, and interference can be prevented reliably. 
     The moving vehicles  20  can be controlled to travel densely, i.e., with a small inter-vehicle distance. As in the case of the present preferred embodiment, by arranging the primary coils  32  at a pitch more than the length of the magnet array  30 , at the maximum, one moving vehicle can be provided at interval of two primary coils  32 . In particular, under the control of the system controller  24  and the zone controllers  22 , a plurality of moving vehicle can travel synchronously at the same speed. 
     The moving vehicles  20  can be controlled to travel synchronously with the transfer apparatus at the station  8  easily. When the moving vehicle  20  is decelerated for transfer of the article, the subsequent moving vehicles can be decelerated synchronously under control. 
     Although the linear synchronization motor is preferably used in the present preferred embodiment, alternatively, a linear induction motor may be used, and a secondary conductor of aluminum or the like may be provided in the moving vehicle  20 . In this case, a magnetic mark of aluminum or the like may be provided in addition to the secondary conductor, and the magnetic mark is detected by the linear sensor  36 . The diverge and merge of the moving vehicle may be controlled mechanically by a guide roller and guide rail (not shown) or the like, or may be controlled electromagnetically by attraction or reaction between the magnet on a side of the bogie vehicle and the coils on the ground. Further, diverge and merge control is implemented by the system controller  24  through the zone controller  22 . 
     DESCRIPTION OF THE NUMERALS 
     
         
           2 : moving vehicle system 
           4 : intra-bay route 
           6 : inter-bay route 
           8 : station 
           10 : straight segment 
           11 : curve segment 
           12 : diverge and merge segment 
           13 : zone boundary 
           20 : moving vehicle 
           22 : zone controller 
           24 : system controller 
           25 : LAN 
           26 : bogie vehicle 
           27 : bogie shaft 
           28 : wheel 
           30 : magnet array 
           32 : primary coil 
           34 : drive unit 
           36 : linear sensor 
           38 : signal processor 
           40 : zone LAN 
           41 : joint 
           42 : coil 
           44 : hall element 
           46 : processing circuit 
           47 : counter 
           48 : offset memory 
           49 : sine curve power supply 
           50 : counter 
           52 : hall element 
           54 : input interface 
           55 : curve memory 
           56 : comparator 
           57 : phase calculator 
           58 : counter 
           60 ,  91 : communication unit 
           61 : multiplexer 
           62 : position instruction generator 
           63 : alarm unit 
           66 : sensor unit 
           68 : coil unit 
           80 ,  82 : packet 
           90 : position instruction generator 
           92 : state table 
           93 : allocation controller 
           94 : retraction controller 
           95 : merge controller 
           96 : interference search unit 
           98 : error detection unit 
       
    
     While preferred embodiments of the present invention have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing the scope and spirit of the present invention. The scope of the present invention, therefore, is to be determined solely by the following claims.