CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority from Japanese Patent Application No. 2006-313229, filed on Nov. 20, 2006. 
       TECHNICAL FIELD OF THE INVENTION 
       [0002]    The present invention relates to a motor control apparatus that supplies power to a high-output motor by using a plurality of power supply processing units. 
       BACKGROUND OF THE ART 
       [0003]    When generating power for driving a high-output motor by means of a power supply processing unit such as an inverter unit, it is necessary to provide a large-capacity power supply processing unit. However, such a large-capacity power supply processing unit has the following problems: 1) increased housing size 2) since the unit is often a custom-designed product, development cost is higher than when using a general-purpose power supply processing unit, and 3) increased maintenance cost. 
         [0004]    In view of the above problems, it has been proposed, as shown in  FIG. 1 , to construct the windings of a high-output motor  1  from a plurality of electrically independent three-phase windings and connect a plurality of general-purpose power supply processing units  2  ( 2 A,  2 B,  2 C, and  2 D) to the respective three-phase windings to drive high-output motor  1 . Since this configuration not only reduces the volume of each individual power supply processing unit, but also permits the use of general-purpose power supply processing units, development and maintenance costs can be reduced. However, the configuration of  FIG. 1 , has the disadvantage that the number of motors that can be connected to control processing unit  3  is limited. This is due a limit of the number of power supply processing units  2  that can be connected to control processing unit (CNC)  3 . 
         [0005]      FIG. 2  shows a motor control apparatus as proposed in Japanese Unexamined Patent Publication No. 2005-86918 to overcome the above disadvantage. In this apparatus, one signal relay processing unit (PDM)  4  is provided between control processing unit  3  and the plurality of power supply processing units  2  ( 2 A,  2 B,  2 C, and  2 D), and a motor drive command from control processing unit  3  is supplied to the plurality of power supply processing units  2  in a parallel fashion via signal relay processing unit  4 . With this configuration, since signal relay processing unit (PDM)  4  can be handled as though it were one power supply processing unit, it is possible to connect a plurality of signal relay processing units  4  to control processing unit  3  and drive a large number of motors under the control of control processing unit  3 . This solves the problem associated with the apparatus of  FIG. 1 , i.e., a limited number of motors that can be connected to it. 
         [0006]      FIG. 3  is a diagram showing a signal control system in a motor control apparatus of the type shown in  FIG. 1  in which motor control processing unit  3  is directly connected to power supply processing units  2 . As shown, control processing unit  3  includes a receiving part  3   a , a transmitting part  3   b , and a preparation request flag  3   c , and each power supply processing unit  2  includes a receiving part  2   a , a transmitting part  2   b , and a preparation complete flag  2   c . When power supply processing unit  2  starts preparation for driving the motor, control processing unit  3  enables the preparation request flag. When the preparation for supplying power to the motor is completed, power supply processing unit  2  enables preparation complete flag  2   c . The enabled or disabled state of preparation request flag  3   c  is transmitted via its transmitting part  3   c  to receiving part  2   a  of power supply processing unit  2 . The enabled or disabled state of preparation complete flag  2   c  in supply processing unit  2  is transmitted via its transmitting part  2   c  to receiving part  3   a  of control processing unit  3 . 
         [0007]      FIGS. 4 and 5  are time charts illustrating the operation of the apparatus shown in  FIG. 3 . As shown in  FIG. 4 , control processing unit  3  at time T 1  changes the state of preparation request flag  3   c  from disabled to enabled and transmits the preparation request flag enabled state to power supply processing unit  2 , whereupon power supply processing unit  2  starts preparation for supplying power to the motor, and transmits the preparation complete flag enabled state to control processing unit  3  upon completion of the preparation (time T 2 ). When control processing unit  3  receives the preparation complete flag enabled state from power supply processing unit  2  within a predefined time interval (T 1  to T 3 ), for example, within several seconds, control processing unit  3  outputs a motor drive command by determining that power supply processing unit  2  has completed the preparation for driving the motor. In order to effect the supply of power from power supply processing unit  2  to the motor, not only the condition that the motor drive command is received, but also other conditions, including the condition that an emergency stop signal (alarm) for stopping power supply processing unit  2  is disabled, must be satisfied. 
         [0008]    On the other hand, if power supply processing unit  2  fails, in order to complete the preparation for supplying power to the motor in response to the preparation request flag enabled state transmitted from control processing unit  3 , preparation complete flag  2   c  in power supply processing unit  2  continues to be held in the disabled state, as shown in  FIG. 5 . As a result, control processing unit  3  cannot receive the preparation complete flag enabled state within the predefined time interval, and therefore, at time T 3  control processing unit  3  transmits the preparation request flag disabled state to power supply processing unit  2 , causing power supply processing unit  2  to stop the preparation for driving the motor, and subsequently carries out alarm processing. 
         [0009]    However, when the flag control shown in  FIGS. 3 to 5  is applied to the motor control apparatus of the type shown in  FIG. 2 , the following problem occurs. 
         [0010]      FIG. 6  is a block diagram showing the configuration when the flag control described above with reference to  FIGS. 3 to 5  is applied to the motor control apparatus of the type shown in  FIG. 2 . As shown, signal relay processing unit  4  includes: a preparation request flag  4   c  which is enabled or disabled depending on the state of preparation request flag  3   c  received from control processing unit  3 ; a preparation complete flag register  4   f  which stores preparation complete flags  2   c  ( 2 A to  2 D) received from respective power supply processing units  2 A to  2 D; and a preparation complete flag  4   d  which is enabled or disabled according to the contents of preparation complete flag register  4   f , that is, according to whether all the preparation complete flags from the respective power supply processing units  2 A to  2 D are enabled or not. Preparation complete flag register  4   f  is, for example, a four-bit register when four power supply processing units  2 A to  2 D are connected to signal relay processing unit  4 , and each bit is set to a 1 or a 0 according to whether preparation complete flag  2   c  from a corresponding one of the power supply processing units is enabled or disabled. 
         [0011]    Preparation complete flag  4   d  in signal relay processing unit  4  is enabled when the value of preparation complete flag register  4   f  is “1111”, i.e., when all the preparation complete flags received from the respective power supply processing units are in enabled state. On the other hand, when the value of preparation complete flag register  4   f  is not “1111”, i.e., when the preparation complete flag received from any one of the power supply processing unit is not in enabled state, preparation complete flag  4   d  is disabled. In signal relay processing unit  4 , reference numeral  4   a  indicates a receiving part which receives the preparation request flag from control processing unit  3  and the preparation complete flags from the respective power supply processing units  2 A to  2 D, and  4   b  designates a transmitting part which transmits the state of preparation request flag  4   c  to power supply processing units  2 A to  2 D and the state of preparation complete flag  4   d  to control processing unit  3 . 
         [0012]    The operation of the apparatus shown in  FIG. 6  and the problem associated with it will be described with reference to the time charts of  FIGS. 7 and 8 . As shown in  FIG. 7 , when preparation request flag  3   c  in control processing unit  3  changes from disabled to enabled at time T 1 , the preparation request flag enabled state is transmitted to signal relay processing unit  4 , and preparation request flag  4   c  changes from disabled to enabled. This change of state is transmitted to the respective power supply processing units  2 A to  2 D in a parallel fashion, causing the power supply processing units to start preparation for driving the motor. Power supply processing units  2 A,  2 B, and  2 C that have completed the preparation for supplying power to the motor change their preparation complete flags  2   c  ( 2 A),  2   c  ( 2 B), and  2   c  ( 2 C) from disabled to enabled upon completion of the preparation (T 4  and T 5 ). 
         [0013]    On the other hand, power supply processing unit  2 D has failed to complete the power supply preparation due to some kind of trouble, and therefore, its preparation complete flag  2   c  ( 2 D) continues to be held in the disabled state. As a result, the value of preparation complete flag register  4   f  in signal relay processing unit  4  does not change to “1111”, and preparation complete flag  4   d  cannot change to the enabled state. Since control processing unit  3  is not able to receive the enable signal of preparation complete flag  4   d  from signal relay processing unit  4  within the predefined time after changing the preparation request flag from disabled to enabled, preparation request flag  3   c  is automatically changed from enabled to disabled at time T 3 . Since this change of state is transmitted to the respective power supply processing units  2 A to  2 D in a parallel fashion via preparation request flag  4   c  in signal relay processing unit  4 , the preparation complete flags in the respective power supply processing units  2 A to  2 D are forcefully disabled at time T 6 . This information is transmitted to signal relay processing unit  4  at time T 7 , updating the contents of the preparation complete flag register to “0000”. When preparation request flag  3   c  in control processing unit  3  is changed from disabled to enabled, preparation complete flag register  4   f  in signal relay processing unit  4  may be reset as shown at time T 8 . 
         [0014]    As described above, in the apparatus shown in  FIG. 6 , when control processing unit  3  does not receive the enabled state of preparation complete flag  4   d  from signal relay processing unit  4  within the predefined time after enabling preparation request flag  3   c , preparation request flag  3   c  is automatically changed from enabled to disabled, and the preparation complete flags in the respective power supply processing units  2 A to  2 D are forcefully disabled, as a result of which the contents of preparation complete flag register  4   f  in signal relay processing unit  4  are updated to “0000”. In this case, control processing unit  3  cannot identify the failed power supply processing unit from the value of preparation complete flag register  4   f.    
         [0015]      FIG. 8  is a time chart showing the flag states in the respective processing units when the states of preparation complete flags  2   c  ( 2 A to  2 D) in the respective power supply processing units connected to the signal relay processing unit  4  are all enabled but a certain power supply processing unit, for example, power supply processing unit  2 D, fails when power is being normally supplied to the motor. In the case where control processing unit  3  has transmitted the enabled state of preparation request flag  3   c , and in response, power supply processing units  2 A to  2 D have started operating normally and preparation complete flags  2   c  ( 2 A) to  2   c  ( 2 D) are transmitting enabled states. In this case, if power supply processing unit  2 D fails at time T 10 , causing its preparation complete flag  2   c  ( 2 D) to change from enabled to disabled, information notifying this change of state is received by signal relay processing unit  4  which, at time T 11 , updates the contents of the preparation complete flag register from “1111” to “0111” and changes the preparation complete flag  4   d  from enabled to disabled. 
         [0016]    This enabled to disabled state change of preparation complete flag  4   d  is transmitted to control processing unit  3 , whereupon the preparation request flag  3   c  in control processing unit  3  changes from enabled to disabled. Then, at time T 12 , preparation request flag  4   c  in signal relay processing unit  4  is caused to change from enabled to disabled. This change of state is transmitted to power supply processing units  2 A to  2 D in a parallel fashion (time T 13 ), and as a result, all of preparation complete flags  2   c  ( 2 A) to  2   c  ( 2 D) are disabled, in response to which preparation complete flag register  4   f  in signal relay processing unit  4  is cleared to “0000” at time T 14 . As a result, as in the case of  FIG. 7 , control processing unit  3  cannot identify the failed power supply processing unit from the value of preparation complete flag register  4   f.    
       SUMMARY OF THE INVENTION 
       [0017]    The present invention has been devised in order to solve the above problem associated with the motor control apparatus in which the signal relay processing unit is provided between the control processing unit and the plurality of power supply processing units, and an object of the invention is to provide a motor control apparatus that can identify any power supply processing unit that has caused a failure from among the plurality of power supply processing units. 
         [0018]    To achieve the above object, according to a first invention, there is provided a motor control apparatus comprising a control processing unit for creating a motor drive command based on an input signal, a plurality of power supply processing units for supplying a driving voltage to a motor based on the motor drive command, and a signal relay processing unit for supplying the motor drive command received from the control processing unit to the plurality of power supply processing units in a parallel fashion, wherein the control processing unit transmits a preparation request flag enabled state to the plurality of power supply processing units via the signal relay processing unit in order to instruct the plurality of power supply processing units to start preparation for driving the motor, the signal relay processing unit includes a preparation complete flag register for storing preparation complete flags each transmitted from each of the plurality of power supply processing units to indicate the completion of preparation for supplying power to the motor, and transmits a preparation complete flag enabled state to the control processing unit if it can be determined from the contents of the preparation complete flag register that the preparation is completed in all of the plurality of power supply processing units, and the control processing unit transmits a preparation request flag disabled state if the control processing unit does not receive the preparation complete flag enabled state from the signal relay processing unit within a predefined time after transmitting the preparation request flag enabled state, and wherein upon receiving the preparation request flag disabled state from the control processing unit, the signal relay processing unit stops updating the contents of the preparation complete flag register. 
         [0019]    In the first invention, the signal relay processing unit may be configured to stop updating the contents of the preparation complete flag register until the signal relay processing unit receives a next preparation request flag enabled state from the control processing unit. 
         [0020]    Further, the contents of the preparation complete flag register the updating of which has been stopped may be reset upon the signal relay processing unit receiving a next preparation request flag from the control processing unit. 
         [0021]    To achieve the above object, according to a second invention, there is provided a motor control apparatus comprising a control processing unit for creating a motor drive command based on an input signal, a plurality of power supply processing units for supplying a driving voltage to a motor based on the motor drive command, and a signal relay processing unit for supplying the motor drive command received from the control processing unit to the plurality of power supply processing units in a parallel fashion, wherein the control processing unit transmits a preparation request flag enabled state to the plurality of power supply processing units via the signal relay processing unit in order to instruct the plurality of power supply processing units to start preparation for driving the motor, the signal relay processing unit includes a preparation complete flag register for storing preparation complete flags each transmitted from each of the plurality of power supply processing units to indicate the completion of preparation for supplying power to the motor, and transmits a preparation complete flag enabled state to the control processing unit if it can be determined from the contents of the preparation complete flag register that the preparation is completed in all of the plurality of power supply processing units, and the control processing unit transmits a preparation request flag disabled state if the control processing unit does not receive the preparation complete flag enabled state from the signal relay processing unit within a predefined time after transmitting the preparation request flag enabled state, and wherein upon receiving the preparation request flag disabled state from the control processing unit, the signal relay processing unit stops transmission of the preparation request flag disabled state to the plurality of power supply processing units, and thereby holds the contents of the preparation complete flag register in a state that existed just before the reception of the preparation request flag disabled state. 
         [0022]    In the second invention, the contents of the preparation complete flag register may be held in that state until the signal relay processing unit receives a next preparation request flag enabled state from the control processing unit. Further, the contents of the preparation complete flag register held in that state may be reset upon the signal relay processing unit receiving a next preparation request flag from the control processing unit. 
         [0023]    Further, in the first and second inventions, the contents of the preparation complete flag register the updating of which has been stopped may be transmitted from the signal relay processing unit to the control processing unit, thereby enabling the control processing unit to identify any power supply processing unit that failed to complete the preparation for supplying power to the motor. In this case, upon identifying any power supply processing unit that failed to complete the preparation for supplying power to the motor, the control processing unit may transmit an alarm along with information of the identification to outside of the apparatus. 
         [0024]    In the first invention, after the control processing unit has transmitted the preparation request flag enabled state, if any power supply processing unit fails to operate normally due to some kind of trouble or fault, the signal relay processing unit stops updating the contents of the preparation complete flag register upon detecting the enabled to disabled state change of the preparation request flag. Therefore, the control processing unit can identify the failed power supply processing unit from the contents of the preparation complete flag register the updating of which has been stopped. 
         [0025]    In the second invention, after the control processing unit has transmitted the preparation request flag enabled state, if any power supply processing unit fails to operate normally due to some kind of trouble or fault, the signal relay processing unit may detect the enabled to disabled state change of the preparation request flag but does not transmit this change of state to the power supply processing units. As a result, since each power supply processing unit holds its preparation complete flag in the state effected at the time of the occurrence of the fault, the contents of the preparation complete flag register are also held accordingly. Therefore, the control processing unit can identify the failed power supply processing unit by checking the contents of the preparation complete flag register. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]      FIG. 1  is a diagram showing the configuration of a prior art motor control apparatus. 
           [0027]      FIG. 2  is a diagram showing the configuration of a motor control apparatus constructed by improving the apparatus of  FIG. 1 . 
           [0028]      FIG. 3  is a diagram showing a flag control mechanism for the apparatus shown in  FIG. 1 . 
           [0029]      FIG. 4  is a time chart explaining the operation of the apparatus shown in  FIG. 1 . 
           [0030]      FIG. 5  is another time chart for explaining the operation of the apparatus shown in  FIG. 1 . 
           [0031]      FIG. 6  is a diagram showing a flag control mechanism for the apparatus shown in  FIG. 2 . 
           [0032]      FIG. 7  is a time chart explaining the operation of the apparatus shown in  FIG. 2 . 
           [0033]      FIG. 8  is another time chart for explaining the operation of the apparatus shown in  FIG. 2 . 
           [0034]      FIG. 9  is a diagram showing, among others, a flag control mechanism for a motor control apparatus according to first and second embodiments of the present invention. 
           [0035]      FIG. 10  is a time chart explaining the operation of the motor control apparatus according to the first embodiment of the present invention. 
           [0036]      FIG. 11  is another time chart explaining the operation of the motor control apparatus according to the first embodiment of the present invention. 
           [0037]      FIG. 12A  is a diagram showing the first half of a flow chart illustrating the procedure for carrying out the operation shown in  FIGS. 10 and 11 . 
           [0038]      FIG. 12B  is a diagram showing the second half of the flow chart illustrating the procedure for carrying out the operation shown in  FIGS. 10 and 11 . 
           [0039]      FIG. 13  is still another time chart for explaining the operation of the motor control apparatus according to the first embodiment of the present invention. 
           [0040]      FIG. 14  is a flow chart illustrating the procedure for carrying out the operation shown in  FIG. 13 . 
           [0041]      FIG. 15  is a time chart explaining the operation of the motor control apparatus according to the second embodiment of the present invention. 
           [0042]      FIG. 16A  is a diagram showing the first half of a flow chart illustrating the procedure for carrying out the operation shown in  FIG. 15 . 
           [0043]      FIG. 16B  is a diagram showing the second half of the flow chart illustrating the procedure for carrying out the operation shown in  FIG. 15 . 
           [0044]      FIG. 17  is another time chart explaining the operation of the motor control apparatus according to the second embodiment of the present invention. 
           [0045]      FIG. 18  is a flow chart illustrating the procedure for carrying out the operation shown in  FIG. 17 . 
       
    
    
     DETAILED DESCRIPTION 
       [0046]    The embodiments of the present invention will be described with reference to the drawings. In the drawings given hereinafter, the same reference numerals as those in  FIGS. 1 to 8  designate the same or similar component elements, and the description of such elements will not be repeated herein. 
         [0047]      FIG. 9  is a block diagram showing the configuration of a motor control apparatus according to first and second embodiments of the present invention; as in  FIGS. 3 and 6 , the configuration of the major portion for implementing the transmission and reception of the preparation request and preparation complete flags is shown here. In the motor control apparatus according to the first embodiment, the signal relay processing unit (PDM)  4  includes a preparation request flag change detector  4   e  in order to identify a power supply processing unit (for example, an inverter unit) that has caused a failure, and provisions are made to stop updating the contents of preparation complete flag register  4   f , i.e., the values of preparation complete flags  2   c  ( 2 A) to  2   c  ( 2 D) in the respective power supply processing units  2 A to  2 D connected to signal relay processing unit  4 , when the value of preparation request flag  4   c  has changed from enabled to disabled. 
         [0048]    On the other hand, in the motor control apparatus according to the second embodiment, in order to identify a power supply processing unit that has caused a failure, provisions are made not to transmit the disable signal of the preparation request flag to power supply processing units  2 A to  2 D by controlling transmitting part  4   b  when preparation request flag change detector  4   e  provided in signal relay processing unit  4  has detected that the value of preparation request flag  4   c  has changed from enabled to disabled. With this arrangement, since the disabled state of the preparation request flag is not transmitted to power supply processing units  2 A to  2 D, preparation complete flags  2   c  ( 2 A) to  2   c  ( 2 D) are not forcefully disabled, and therefore, preparation complete flag register  4   f  can hold the contents at the time of the occurrence of the failure. In the above first and second embodiments, the contents of preparation complete flag register  4   f  the updating of which has been stopped are transmitted to control processing unit  3  which can thus identify the failed power supply processing unit. 
         [0049]    The operation of the apparatus according to the first embodiment will be described below with reference to the time charts and flow charts shown in  FIGS. 10 to 14 . 
         [0050]      FIG. 10  is a time chart illustrating the operation for the case where, in the apparatus shown in  FIG. 9 , control processing unit (CNC)  3  has been able to receive the enabled state of the preparation complete flag within the predefined time interval (T 1  to T 3 ) from signal relay processing unit  4  and, as a result, control processing unit  3  has been able to normally output the motor drive command to signal relay processing unit  4 . On the other hand,  FIG. 11  is a time chart illustrating the operation for the case where, despite the output of the preparation complete flag enabled state from control processing unit  3 , power supply processing unit  2 D has been unable to change preparation complete flag  2   c  ( 2 D) from disabled to enabled due to some kind of trouble, and as a result, control processing unit  3  has failed to receive the enabled state of the preparation complete flag from signal relay processing unit  4  within the predefined time interval (T 1  to T 3 ). 
         [0051]      FIGS. 12A and 12B  are flowcharts illustrating the flow of the operation shown in  FIGS. 10 and 11 . The operation of the motor control apparatus shown in  FIGS. 10 and 11  will be described below with reference to the flowchart of  FIGS. 12A and 12B . As shown in  FIGS. 12A and 12B , when control processing unit  3  changes the state of preparation request flag  3   c  from disabled to enabled (step S 1 ), the signal indicating the change of state is transmitted via its transmitting part  3   b  to receiving part  4   a  of signal relay processing unit  4  (step S 2 ). When the preparation request flag enabled state is received (step P 1 ), the signal relay processing unit  4  resets preparation complete flag register  4   f  (step P 2 ) and transmits the preparation request flag enabled state to the respective power supply processing units  2 A to  2 D in a parallel fashion (step P 3 ). In  FIGS. 10 and 11 , until time t 1 , preparation complete flag register  4   f  retains the contents updated when preparation complete flag  4   c  was last changed from enabled to disabled, as will be described later, but when preparation complete flag  4   c  changes from disabled to enabled at time t 1 , the contents are reset (to “0000”) and the updating is resumed. 
         [0052]    When power supply processing units  2 A to  2 D receive the preparation request flag enabled state (step R 1 ), each power supply processing unit changes its preparation complete flag state from disabled to enabled as it completes preparation for supplying power to the motor, and transmits information indicating the change of state to signal relay processing unit  4  (step R 2 ). As a result, as shown in  FIGS. 10 and 11 , the contents of preparation complete flag register  4   f  in signal relay processing unit  4  are updated to “0011” at time t 2 , then to “1011” at time t 3 . Here, the value “1011”, for example, means that preparation complete flags  2   c  ( 2 A),  2   c  ( 2 B), and  2   c  ( 2 D) in power supply processing units  2 A,  2 B, and  2 D are enabled but preparation complete flag  2   c  ( 2 C) in power supply processing unit  2 C remains disabled. 
         [0053]    As signal relay processing unit  4  receives the preparation complete flag from each power supply processing unit  2 , signal relay processing unit  4  updates preparation complete flag register  4   f  (step P 401 ) and checks preparation complete flag register  4   f  to see if all the preparation complete flags have been enabled or not (step P 5 ). If the answer is YES, preparation complete flag  4   d  in signal relay processing unit  4  is enabled. In the example shown in  FIG. 10 , since all the preparation complete flags from power supply processing unit  2  have been enabled by time t 4 , preparation complete flag  4   d  in signal relay processing unit  4  is enabled at time t 4 . If the answer is NO in step P 5 , preparation complete flag  4   d  is disabled (step P 7 ). 
         [0054]    Signal relay processing unit  4  transmits preparation complete flag  4   d  to control processing unit  3  (step P 8 ) by repeating the process from step P 4  onward until the predefined time (T 1  to T 3 ) elapses (YES in step P 9 ). Control processing unit  3  continues to receive preparation complete flag  4   d  from signal relay processing unit  4  over the predefined time interval (T 1  to T 3 ) (the period during which the answer in step S 4  is NO) and, after the predefined time has elapsed (YES in step S 4 ), the control processing unit  3  checks to see whether received preparation complete flag  4   d  is enabled or not (step S 5 ). If YES in step S 5 , that is, if received preparation complete flag  4   d  is enabled, control processing unit  3  outputs the motor drive command by determining that power supply processing units  2 A to  2 D have started operating normally (step S 6 ). 
         [0055]    In the example of  FIG. 10 , since preparation complete flag  4   d  is enabled at time t 4 , and information indicating the enabled state is transmitted to control processing unit  3 , control processing unit  3  carries out step S 6  at time T 3  and outputs the motor drive command. On the other hand, as in the example of  FIG. 11 , if all the preparation complete flags from power supply processing unit  2  are not enabled even after the predefined time has elapsed (NO in step S 5 ), control processing unit  3  transmits the disabled state of preparation request flag  3   c  to signal relay processing unit  4  (step S 7 ). When signal relay processing unit  4  receives the disabled state of preparation request flag  3   c  from control processing unit  3  (step P 10 ), signal relay processing unit  4  stops updating value  20  of preparation complete flag register  4   f  and transmits the last updated value to control processing unit  3  (step P 11 ). Control processing unit  3  checks the thus received value of the preparation complete flag register to identify the power supply processing unit whose preparation complete flag did not change to the enabled state because of the occurrence of failure (step S 8 ), and carries out fault processing such as an alarm indication (step S 9 ). 
         [0056]    Signal relay processing unit  4  transmits the disabled state of preparation request flag  4   c  to all the power supply processing units (step P 12 ) after completing step P 11  or at the same time that step P 11  is carried out, and when the power supply processing units receive the disabled state of preparation request flag  4   c  (step R 3 ), the power supply processing units disable their preparation complete flags  2   c  ( 2 A) to  2   c  ( 2 D) (step R 4 ). 
         [0057]      FIG. 13  is a time chart showing the operation of the motor control apparatus for the case where the preparation complete flag in the power supply processing unit  2 D changes from enabled to disabled during the operation, and  FIG. 14  is a flowchart showing the flow of processing performed between the respective processing units. As shown in  FIG. 13 , when the motor is operating normally with all the preparation complete flags  2   c  ( 2 A) to  2   c  ( 2 D) enabled in power supply processing units  2 A to  2 D, if preparation complete flag  2   c  ( 2 D) in power supply processing unit  2 D changes from enabled to disabled, for example, at time t 10  due to some kind of trouble (step R 20  in  FIG. 14 ), information indicating this change of state is transmitted to signal relay processing unit  4  (step R 21 ). When this information is received (step P 20 ), signal relay processing unit  4  updates preparation complete flag register  4   f  (step P 201 ) and checks the contents of the preparation complete flag register to see whether all the preparation complete flags are enabled or not (step P 202 ). If YES, preparation complete flag  4   d  in signal relay processing unit  4  is enabled (step P 203 ). If NO in step P 202 , preparation complete flag  4   d  is disabled (step P 204 ). 
         [0058]    In the example shown in  FIG. 13 , since the value of preparation complete flag register  4   f  changes from “1111” to “0111”, the state of preparation complete flag  4   d  in signal relay processing unit  4  changes from enabled to disabled, and information indicating this change of state is transmitted to control processing unit  3  (step P 21 ). When the disabled state of preparation complete flag  4   d  is received (step S 20 ), control processing unit  3  changes preparation request flag  3   c  from enabled to disabled, and transmits the disabled state of preparation request flag  3   c  to signal relay processing unit  4  (step S 21 ). When the disabled state of preparation request flag  3   c  is received (step P 22 ), signal relay processing unit  4  stops updating preparation complete flag register  4   f , and transmits the last updated value to control processing unit  3  (step P 23 ). Then, control processing unit  3  identifies the disabled power supply processing unit from the value of preparation complete flag register  4   f  thus received (step S 22 ), and carries out fault processing such as an alarm indication (step S 24 ). 
         [0059]    After stopping the updating of preparation complete flag register  4   f  and transmitting the last updated value to control processing unit  3  (step P 23 ), signal relay processing unit  4  transmits the disabled state of preparation request flag  3   c  to power supply processing units  2 A to  2 D (step P 24 ), upon receiving the disabled state (step R 22 ), power supply processing units  2 A to  2 D change their preparation complete flags  2   c  ( 2 A) to  2   c  ( 2 D) from enabled to disabled (step R 23 ) and make preparation to stop the motor. 
         [0060]      FIGS. 15 to 18  are time charts and flow charts illustrating the operation of the motor control apparatus according to the second embodiment of the present invention. As shown in  FIG. 9 , in the motor control apparatus according to the second embodiment of the present invention, signal relay processing unit  4  includes preparation request flag change detector  4   e , and when detector  4   e  detects that preparation request flag  4   c  has changed from enabled to disabled, transmitting part  4   b  is controlled so as not to transmit the disabled state of preparation request flag  4   c  to power supply processing units  2 A to  2 D. With this arrangement, power supply processing units  2 A to  2 D continue to hold the enabled or disabled states effected before preparation request flag  4   c  was changed from enabled to disabled and, as a result, the contents of preparation complete flag register  4   f  in signal relay processing unit  4  remain unchanged. Since this information is transmitted to control processing unit  3 , control processing unit  3  can identify the failed power supply processing unit. 
         [0061]      FIG. 15  is a time chart illustrating the operation for the case where, in the motor control apparatus according to the second embodiment of the present invention, control processing unit  3  does not receive the preparation complete flag enabled state within the predefined time after transmitting the preparation request flag enabled state. The time chart of  FIG. 15  and the flow chart of  FIGS. 16A and 16B  are substantially the same as the time chart of  FIG. 11  and the flow chart of  FIG. 12 , respectively, but the difference is that even when control processing unit  3  transmits the disabled state of preparation request flag  3   c  to signal relay processing unit  4  (step S 7  in  FIG. 16B ) after failing to receive the enabled state of preparation complete flag  4   d  from signal relay processing unit  4  by time T 3  shown in  FIG. 15 , signal relay processing unit  4  that received the disabled state (step P 10  in  FIG. 16B ) does not transmit the disabled state of preparation request flag  4   c  to power supply processing units  2  (step P 16 ). Since power supply processing units  2  cannot receive the disabled state of the preparation request flag (step R 10 ), the power supply processing units continue to hold the preparation complete flag states effected before time T 3  (step R 11 ). 
         [0062]    Accordingly, when control processing unit  3  changes the state of preparation request flag  3   c  from enabled to disabled, the contents of preparation complete flag register  4   f  before the change are retained (indicated at  24  in  FIG. 15 ), and the thus retained value is transmitted to control processing unit  3  (step P 15 ). In this way, control processing unit  3  can identify the failed power supply processing unit (step S 8 ) and can carry out fault processing (step S 9 ). 
         [0063]      FIG. 17  shows the time chart for the case where, in the second embodiment, all the power supply processing units are enabled and power is supplied normally to the motor, and  FIG. 18  is a flow chart illustrating the sequence of operations for that case. The time chart and the flow chart shown here are substantially the same as those of the first embodiment shown in  FIGS. 13 and 14 , but the difference from the first embodiment lies in the process to be performed after control processing unit  3  receives the disabled state of preparation complete flag  4   d  from signal relay processing unit  4  (step S 20  in  FIG. 18 ) and disables preparation request flag  3   c  and transmits this disabled state to signal relay processing unit  4  (step S 21  in  FIG. 18 ). 
         [0064]    More specifically, when the disabled state of preparation request flag  3   c  is received (step P 22 ), signal relay processing unit  4  disables its own preparation request flag  4   c , but does not transmit this disabled state to power supply processing units  2 A to  2 D (step P 31 ). Since power supply processing units  2 A to  2 D cannot receive the disabled state of the preparation request flag (step R 30 ), preparation complete flags  2   c  ( 2 A) to  2   c  ( 2 D) in the respective power supply processing units retain the states effected before preparation request flags  3   c  and  4   c  were disabled (indicated at  26  in  FIG. 17 , step R 31  in  FIG. 18 ). The thus retained contents of preparation complete flag register  4   f  are transmitted to control processing unit  3  (step P 30 ) which thus identifies the disabled power supply processing unit (step S 22 ). After that, control processing unit  3  carries out fault processing in step S 23 , and outputs a motor drive stop command causing power supply processing units  2 A to  2 D to stop supplying power to the motor.
A motor control apparatus comprises a control processing unit ( 3) creating a motor drive command, power supply processing units ( 2 A to 2 D) supplying a driving voltage to a motor, and a signal relay processing unit ( 4) supplying the motor drive command received from unit ( 3) to power supply processing units ( 2 A to 2 D). Control processing unit ( 3) transmits a preparation request flag to power supply processing units ( 2 A to 2 D) via signal relay processing unit ( 4), which includes a preparation complete flag register storing preparation complete flags each transmitted from each of power supply processing units ( 2 A to 2 D) to indicate the completion of preparation for supplying power to the motor, and transmits a preparation complete flag enabled state to control processing unit ( 3) if contents of the preparation complete flag register indicate that the preparation is completed in all units ( 2 A to 2 D).