Abstract:
A control unit for a rotary electric machine includes a first current command module, a second current command module, a change module, and a return module. The first module performs a first current command on a maximum efficiency characteristic line on a d-q plane thereby to drive the machine at a maximum efficiency. The second module performs a second current command on a switching line set at a retard angle side relative to the maximum efficiency characteristic line. The change module changes a control mode from a rectangular wave voltage phase control mode to an overmodulation current control mode when an operation point of the machine reaches the switching line. The return module returns the current command from the second command to the first command after performance of the second command for a predetermined period.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a Continuation of application Ser. No. 12/574,855, filed Oct. 7, 2009, which is based on and incorporates herein by reference Japanese Patent Applications No. 2008-261333 filed on Oct. 8, 2008 and No 2008-286696 filed on Nov. 7, 2008. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to an electric power source circuit and an abnormality diagnosis system for the electric power source circuit. The electric power source circuit includes an power supply relay for opening and closing an electric power conversion circuit and an power supply part, a capacitor connected between an electric current path, which is formed between the power conversion circuit and the power supply relay, and a reference potential source, a pre-charge path bypassing the power supply relay for pre-charging the capacitor, and an on/off switch for connecting and disconnecting the pre-charge path and the power supply part. 
       BACKGROUND OF THE INVENTION 
       [0003]    In a conventional electric power source circuit, an power supply relay is provided in an electric current path between an electric power conversion circuit and a battery to turn on and off the electric current path. In a case that the length of the current path between the power conversion circuit and the battery is long, a capacitor is conventionally provided near the power conversion circuit so that the capacitor is pre-charged by the battery to stably supply the power conversion circuit with electric power. 
         [0004]    If the power supply relay is used, it is desired to diagnose whether the power supply relay normally operates. The power supply relay is diagnosed with respect to its abnormality (malfunction) of fixed-closure, in which the power supply relay is persistently closed due to fixation of a movable contact. This fixed-closure can be determined by detecting a charge voltage of the capacitor in response to a trigger that is caused when an on/off switch part (ignition switch) for connecting and disconnecting the battery and a part operating the power conversion circuit is closed. At the time immediately after the ignition switch is turned on, the capacitor is supposed to have not been charged yet because the power supply relay has been turned off. If the capacitor has been charged, the power supply relay is diagnosed as being in the abnormal condition of fixed-closure. 
         [0005]    Fusion abnormality is considered as one of the causes of the fixed-closure of the power supply relay. This abnormality arises, when a large electric current flows from the battery to the capacitor at the time of turning on the power supply relay to the closed state and melts the movable contact to a fixed contact during closure of the power supply relay. It is therefore desired to avoid the fusion abnormality. A power supply relay, which can supply a large electric current, may be used to avoid the fusion abnormality. This power supply relay however is large in size and costly. 
         [0006]    JP 11-245829 proposes to connect a capacitor connected between a H-bridge circuit of an electric motor and a power supply relay to a battery through an on/off switch part (ignition switch) and a pre-charge resistor. When the ignition switch is turned on, the charge of the battery is supplied to charge the capacitor through the ignition switch and the resistor. As a result, even when the power supply relay is turned on subsequently, a large current is restricted from flowing from the battery to the capacitor through the power supply relay. 
         [0007]    The fixed-closure of the power supply relay may arise for causes other than the large current, which flows to the capacitor. It is therefore also desired in the proposed power source circuit to diagnose whether the power supply relay is in the fixed-closure state. In the case of an arrangement, in which the capacitor is charged by turning on the ignition switch, the capacitor may not produce a large voltage difference, by which the fixed-closure of the power supply relay is detectable, between the capacitor voltages at the time of turning on the ignition switch. It is therefore difficult to diagnose the power supply relay with respect to the fixed-closure abnormality. 
         [0008]    It is also possibly difficult to diagnose fixed-open abnormality, in which the power supply relay is persistently held in the fixed-open state, because the capacitor is charged by turning on the ignition switch. 
       SUMMARY OF THE INVENTION 
       [0009]    It is therefore an object of the present invention to improve an electric power source circuit and an abnormality diagnosis system, so that an abnormality of an power supply relay may be diagnosed even in an arrangement that the power source circuit has a pre-charge path for pre-charging a capacitor connected between a current path of an electric power conversion circuit and the power supply relay and the ground. 
         [0010]    According to the present invention, an abnormality diagnosis system is provided for a power source circuit, which includes an electric power source, a power supply relay, a capacitor, a pre-charge path and a switch. The power supply relay is provided to open and close a current path formed between the electric power source and a power conversion circuit. The capacitor is connected between a reference potential and the current path formed between the power conversion circuit and the power supply relay. The pre-charge path is formed between the power source and the capacitor to pre-charge the capacitor by the power source. The pre-charge path bypasses the power supply relay. The switch is provided to open and close the pre-charge path. 
         [0011]    In one aspect, a diagnosis circuit is provided for diagnosing the power supply relay with respect to fixed-closure of the power supply relay based on a drop of a charge voltage of the capacitor produced after both the switch and the power supply relay are operated to open. The fixed-closure abnormality indicates that the power supply relay maintains a closed state even if operated to open. 
         [0012]    In another aspect, a motor relay is provided between the power conversion circuit and a motor to open and close a current path from the power conversion circuit to the motor. A diagnosis circuit is provided to diagnose the power supply relay with respect to fixed-open abnormality based on a charge voltage of the capacitor produced when all of the switch, the power supply relay and the motor relay are operated to close, the fixed-open abnormality indicating that the power relay maintains an open state even if operated to close. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description made with reference to the accompanying drawings. In the drawings: 
           [0014]      FIG. 1  is a circuit diagram showing an abnormality diagnosis system according to a first embodiment of the present invention; 
           [0015]      FIG. 2  is a flowchart showing processing of diagnosing fixed-closure abnormality of a power supply relay in the first embodiment; 
           [0016]      FIG. 3  is a time chart showing an example of the processing of diagnosing fixed-closure abnormality in the first embodiment; 
           [0017]      FIGS. 4A and 4B  are time charts showing a principle of processing of diagnosing fixed-open abnormality of a power supply relay in a second embodiment of the present invention; 
           [0018]      FIG. 5  is a flowchart showing the processing of diagnosing the fixed-open abnormality of the power supply relay in the second embodiment; 
           [0019]      FIG. 6  is a flowchart showing processing of diagnosing fixed-open abnormality of a power supply relay in a third embodiment of the present invention; 
           [0020]      FIG. 7  is a circuit diagram showing an abnormality diagnosis system according a fourth embodiment of the present invention; and 
           [0021]      FIG. 8  is a flowchart showing processing of diagnosing fixed-open abnormality of a power supply relay in a fifth embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     First Embodiment 
       [0022]    According to a first embodiment, an abnormality diagnosis system for an electric power source circuit of the present invention is applied to an abnormality diagnosis system for a power source circuit of an electric power steering apparatus mounted on a vehicle. 
         [0023]    Referring to  FIG. 1 , an electric motor  10  is a DC motor with brushes. The motor  10  is an actuator provided in a steering apparatus, which power-assists steering operation of a driver. The motor  10  is supplied with electric power of a battery  12  through a H-bridge circuit  20 . 
         [0024]    The H-bridge circuit  20  is an electric power conversion circuit for supplying the electric power of the battery  12  to the motor  10 . Specifically, the H-bridge circuit  20  has parallel-connected series arms. In one series arm, switching elements  21  and  22  are connected in series. In the other series arm, switching elements  23  and  24  are connected in series. The switching elements  21  to  24  may be transistors. A pair of terminals of the motor  10  is connected to junctions of two switching elements of each arm, respectively. Resistors  25  and  26  are connected to the switching elements  21  and  24  for fail-safe operation, respectively. 
         [0025]    A microcomputer  30  is provided to control a drive amount (torque, etc.) of the motor  10  as a subject of control by controlling the H-bridge circuit  20 . The microcomputer  30  includes a CPU, a ROM, a RAM, etc. in the known manner. The microcomputer  30  further includes a non-volatile memory  32 . The memory  32  may be an EEPROM, etc., which maintains its storage even when the power supply is interrupted. 
         [0026]    As an electric power source circuit for supplying the electric power of the battery  12  to the motor  10  through the H-bridge circuit  20 , a power supply relay  40  is connected to connect and disconnect the battery  12  and the H-bridge circuit  20 , that is, to open and close a current path between the battery  12  and the H-bridge circuit  20 . The power supply relay  40  is an analog normally-open type relay, which is an electromagnetic relay such as a movable core-type relay. The power supply relay  40  closes a current path when excited to turn on. The power source circuit further includes a capacitor  42  and a discharge resistor  44 , which connects a current path between the power supply relay  40  and the H-bridge circuit  20  and the reference potential source (for example, ground). The capacitor  42  is connected at a position closest to the H-bridge circuit  20  so that the length of a wire (current path) between the capacitor  42  and the H-bridge circuit  20  may be shortened to a minimum. The capacitor  42  has a large capacitance so that the electric power is supplied to the H-bridge circuit  20  stably. The discharge resistor  44  is provided to discharge the capacitor  42 . 
         [0027]    The power source circuit further includes an electric motor relay (motor relay)  46 , which connects and disconnects the terminals of the motor  10  and the H-bridge circuit  20 . The motor relay  46  also may be an analog normally-open type relay, which is an electromagnetic relay such as a movable core-type relay. The motor relay  46  closes a current path when excited to turn on. 
         [0028]    The power source circuit further includes an on/off activation switch (ignition switch  50 ) operable for activating a vehicle-mounted control system by a user, and a diode  52  for allowing supply of the electric power of the battery  12  to the microcomputer  30  through the ignition switch  50 . A diode  54  is connected between a junction of the power supply relay  40  and the H-bridge circuit  20  and a junction of the cathode of the diode  52  and the microcomputer  30 . The diode is forward-biased in a direction from the junction between the junction of the power supply relay  40  to the microcomputer  30 . A resistor  56  is connected in parallel to the diode  54  between the junction of the power supply relay  40  and the H-bridge circuit  20  and the junction between the cathode of the diode  52  and the microcomputer  30 . 
         [0029]    The microcomputer  30  is activated when the electric power of the battery  12  is supplied through at least one of the diode  52  and the diode  54 . The microcomputer  30  controls the drive amount of the motor  10 . The microcomputer  30  drives the motor  10  in the forward rotation direction or in the reverse rotation direction by selectively turning on and off the switching elements  21  and  24  periodically or turning on and off the switching elements  22  and  23  periodically. The amount of current supplied to the motor  10  is controlled based on a duty ratio of an on-time of the switching elements relative to one cycle time (on-and-off time) of the same in the periodic on-off control. 
         [0030]    The microcomputer  30  executes processing to turn on (close) the power supply relay  40  and the motor relay  46  in advance of controlling the drive amount of the motor  10 . 
         [0031]    One terminal of an excitation coil of the power relay  40  is connected to one terminal (cathode of the diode  52 ) of the ignition switch  50 , which is not connected to the positive-side of the battery  12 . The other terminal of the excitation coil is grounded through the switching element  60 . When the microcomputer  30  turns on the switching element  60  under a condition that the ignition switch  50  is in the closed state, a current flows from the battery  12  to the ground through the excitation coil of the power supply relay  40  from the battery  12 . The excitation coil of the power supply relay  40  responsively generates magnetic force thereby to move a movable contact of the power supply relay  40 . As a result, an input terminal, which is connected to the battery  12 , and an output terminal, which is connected the H-bridge circuit  20 , are connected (turned on to closed state). 
         [0032]    The motor relay  46  has an excitation coil, one terminal of which is connected to the capacitor  42  and the H-bridge circuit  20  and the other terminal of which is grounded through a switching element  62 . When the microcomputer  30  turns on the switching element  62  to the closed state, a current flows to the ground from the battery  12  through the excitation coil of the motor relay  46 . The excitation coil of the motor relay  46  responsively generates magnetic force thereby to move a movable contact of the motor relay  46 . As a result, an input terminal, which is connected to the H-bridge circuit  20 , and an output terminal, which is connected to the motor  10 , are connected (turned on to closed state). 
         [0033]    When the ignition switch  50  is turned on, the microcomputer turns on the switching element  60  to turn on the power supply relay  40  to the closed state, and then turns on the switching element  62  to turn on the motor relay  46  to the closed state. With the relays  40  and  46  being thus set to the closed state, the drive amount of the motor  10  is controlled by controlling the H-bridge circuit  20  so that the motor  10  assists the steering operation of a driver. 
         [0034]    When the ignition switch  50  is turned on, the capacitor  42  is pre-charged by the current supplied from the battery  12  through the pre-charge resistor  56 , before the power supply relay  40  is turned on to the closed state by the microcomputer  30 . As a result, the capacitor  42  is charged to the charge voltage, which corresponds to the voltage of the battery  12 , before a time point the power supply relay  40  is turned on to close. Specifically, the charge voltage of the capacitor  42  is a voltage VPC, which corresponds to a resistance division ratio R 44 /(R 44 +R 56 ) of the voltage VIG of the battery  12  (specifically less a voltage V 52  of the diode  52 ). The resistance R 56  of the pre-charge resistor  56  is set to be far smaller than the resistance R 44  of the discharge resistor  44 . Therefore, at the time point the power supply relay  40  is turned on, the charging of the capacitor  42  to be close to the voltage of the battery  12  is completed readily. The current, which flows from the battery  12  to the capacitor  42  at the time of turning on the power supply relay  40 , is reduced to be sufficiently small. 
         [0035]    Since the capacitor  42  is pre-charged by the turn-on of the ignition switch  50 , it is difficult to diagnose whether the power supply relay  40  has fixed-closure abnormality with respect to some charge voltages of the capacitor  42  produced before the power supply relay  40  is turned on. Therefore, the fixed-closure abnormality of the power supply relay  40  is detected by monitoring or checking whether the charge voltage of the capacitor  42  drops by turning off the power supply relay  40  to the open state after the ignition switch  50  is turned off to the open state by a driver. 
         [0036]    This fixed-closure malfunction diagnosis processing is executed by the microcomputer  30  as shown in  FIG. 2  in response to the turn-on of the ignition switch  50  as a trigger. The microcomputer  30  thus operates as a diagnosis circuit as well as a motor control circuit. 
         [0037]    First at step S 10 , a keyword stored in a predetermined address of the non-volatile memory  32  is read out. The keyword is provided to indicate a result of diagnosis (presence or absence) of the fixed-closure abnormality. The keyword is initially set to “A,” which is predetermined to indicate “normal (no abnormality).” At step S 12 , it is checked whether the keyword is “A” indicating no abnormality. If the keyword is “A,” the switching elements  60  and  62  are turned on at step S 14  to thereby turn on the power supply relay  40  and the motor relay  46  to the closed state. The drive amount of the motor  10  is also controlled so that the steering operation by a driver is assisted by the motor  10 . 
         [0038]    This step S 14  is repeated until it is determined at step S 16  that the ignition switch  50  is turned off to the open state. After the ignition switch  50  is turned off (S 16 : YES), the keyword “A” is written in the predetermined address of the memory  32 . At step S 20 , the power supply relay  40  is turned off to the open state. Specifically, by turning off the switching element  60 , the current flowing in the excitation coil of the power supply relay  40  is interrupted. Since the magnetic flux generated by the current flowing in the excitation coil is reduce to zero, the power supply relay  40  is turned off to the open state, in which the movable contact is disconnected from the fixed contact in the power relay  40 . 
         [0039]    After the power supply relay  40  is turned off to the open state, a charge path from the battery  12  to the capacitor  42  is interrupted, the charge voltage of the capacitor  42  decreases by discharging through the discharge resistor  44  and power consumption of the microcomputer  30 . It is noted that the microcomputer  30  is maintained operative with the charged power of the capacitor  42  even after the ignition switch  50  and the power supply relay  40  are turned off. The capacitor  42  is discharged to a lowest limit voltage (for example, 5V) required for the microcomputer  30  to operate in about a certain time after the power supply relay  40  is turned off. At step S 22 , it is checked whether a predetermined time T 1  has elapsed after the power supply relay  40  is turned off. This time T 1  is set to correspond to a possible minimum of the certain time or more to check whether the charge voltage of the capacitor  42  drops in the predetermined time T 1 . If the time T 1  has elapsed ( 522 : YES), the keyword is changed at S 24  to “B” thereby indicating that the power supply relay  40  has fixed-closure abnormality, in which the movable contact is continuously connected to and not disconnected from the fixed contact in the power supply relay  40 . In the case that the power supply relay  40  has no fixed-closure abnormality, the microcomputer  30  becomes inoperative before step S 24  is executed, that is, before the elapse of time T 1 , and the keyword is not changed to “B.” 
         [0040]    If it is determined that the keyword is not “A” (S 12 : NO), it indicates that step S 24  has been executed and the keyword has changed to “B,” which indicates that the power supply relay  40  has the fixed-closure abnormality. At step S 26 , the fixed-closure abnormality is indicated to an outside. For example, this abnormality is indicated by turning on a malfunction indicator light (MIL)  34 . In addition, the assist control is prohibited. As a result, the motor relay  46  is not turned on to the closed state and hence the motor  10  is not driven because of no supply of power from the battery  46 . 
         [0041]    After step S 26 , it is checked at step S 28  whether the ignition switch  50  is turned off by a driver. If the ignition switch  50  is turned off (S 28 : YES), the keyword “A” is written in the non-volatile memory  32  in place of the keyword “B” at step S 30 . Steps S 22  and S 24  follow step S 30 . 
         [0042]    After step S 24 , the sequence of processing is ended. 
         [0043]    The operation of diagnosing the fixed-closure is shown in  FIG. 3 . In  FIG. 3 , (a) indicates a change of the ignition switch  50 , (b) indicates a change of the output voltage VIG corresponding to the battery voltage (anode-side voltage of the diode  52 ) of the ignition switch, (c) indicates a change of the charge voltage VPC of the capacitor  42 , (d) indicates change of the keyword, (e) indicates a change of the power supply relay  40 , (f) indicates a change of a check result of the fixed-closure abnormality, and (g) indicates a change of the malfunction indicator light  34 . 
         [0044]    As shown in  FIG. 3 , when the ignition switch  50  is turned on by a user at time t 1 , the output voltage VIG of the ignition switch  50  and the capacitor  42  starts to be charged to provide the voltage VPC. Then, when the keyword is referred to and confirmed to be “A,” the microcomputer  30  turns on the switching element  60  and maintains the turn-on of the power supply relay  40  at time t 2 . As a result, the current path between the battery  12  and the capacitor  42  is closed. Even after the ignition switch  50  is turned off at time t 3 , the microcomputer  30  maintains the switching element  60  in the turned-on condition to maintain the power supply relay  40  in the turned-on condition, so that the capacitor  42  maintains its charge voltage VPC. The microcomputer  30  maintains this condition until it completes the required various post-processing, for example, writing the keyword “A.” At time t 4 , at which the post-processing is completed, the microcomputer  30  turns off the switching element  60 . As long as the power supply relay  40  has no abnormality, it is turned off to open the current path between the battery and the capacitor  42 . 
         [0045]    However, if the power supply relay  40  has the fixed-closure abnormality, the power supply relay  40  cannot turn off to open the current path. As a result, the capacitor  42  is persistently connected to the battery  12 . The charge voltage VPC therefore does not drop after time t 4 . If this condition continues to be more than the predetermined time T 1 , the microcomputer  30  changes the keyword and writes “B” in place of “A” at time t 5 . 
         [0046]    When the ignition switch  50  is turned on again at time t 6 , the microcomputer  30  refers to the keyword stored in the non-volatile memory  32 . Since the stored keyword is “B,” the microcomputer  30  determines that the power supply relay  40  has the fixed-closure abnormality and turns on the malfunction indicator light  54  at time t 7 . When the ignition switch  50  is turned off at time t 8  again, the microcomputer  30  changes the keyword to “A.” 
         [0047]    If the power supply relay  40  does not have the fixed-closure abnormality any more, the capacitor  42  is allowed to discharge through the resistor  44 . The charge voltage VPC falls and becomes less than the predetermined threshold level Vth. As a result, when the ignition switch  50  is turned on next time, the power supply relay  40  is determined to be normal and the assist control is performed. 
         [0048]    The first embodiment provides the following advantages. 
         [0049]    (1) The power supply relay  40  is diagnosed whether it has the fixed-closure abnormality, based on the charge voltage of the capacitor  42  after both of the ignition switch  50  and the power supply relay  40  are turned off to the open state. Thus, the fixed-closure abnormality can be detected. 
         [0050]    (2) The operability of the microcomputer  30  is stored as data of the keyword, if the microcomputer  30  remains operable after the elapse of the predetermined time T 1  from the turn-off of both of the ignition switch  50  and the power supply relay  40  to the open state. By referring to the value of the keyword when the ignition switch  50  is turned on, the power supply relay  40  is diagnosed whether it has the fixed-closure abnormality. If the fixed-closure abnormality of the power supply relay  40  is confirmed when the ignition switch  50  is turned on, a driver can be notified of the abnormality appropriately. 
         [0051]    (3) The capacitor  42  is charged by the battery  12  through the pre-charge resistor  56 , the resistance of which is smaller than that of the discharge resistor  44 . The capacitor  42  can therefore be charged to increase its charge voltage closely to the voltage of the battery  12  before the power supply relay  40  is turned on to connect the capacitor  42  and the battery  12  therethrough. 
         [0052]    (4) The discharge resistor  44  is connected in parallel to the capacitor  42 . The speed of drop of the charge voltage of the capacitor  42  caused when the ignition switch  50  and the power supply relay  40  are turned off can be increased. As a result, the predetermined time T 1  can be set to be short. Even if the ignition switch  50  is turned on again in a short time, the fixed-closure of the power supply relay  40  can be diagnosed readily. 
       Second Embodiment 
       [0053]    A second embodiment is described next with respect to parts different from the first embodiment. 
         [0054]    In the second embodiment, the power supply relay  40  is further diagnosed with respect to its fixed-open abnormality, in which the movable contact cannot be moved to close the input terminal and the output terminal in the power supply relay  40 . 
         [0055]    The principle of diagnosing the power supply relay  40  with respect to the fixed-open abnormality is shown in  FIGS. 4A and 4B , in which the power supply relay  40  is assumed to be normal and abnormal, respectively. In  FIGS. 4A and 4B , (a 1 ) and (a 2 ) indicate changes of the ignition switch  50 , (b 1 ) and (b 2 ) indicate changes of the power supply relay  40 , (c 1 ) and (c 2 ) indicate changes of the motor relay  46 , (d 1 ) and (d 2 ) indicate changes of the output voltage VIG of the ignition switch  50 , and (e 1 ) and (e 2 ) indicate changes of the charge voltage VPC of the capacitor  42 . 
         [0056]    If the power supply relay  40  is normal, as shown in  FIG. 4A , when the ignition switch  50  is turned on, the output voltage VIG of the ignition switch  50  rises and the capacitor  42  starts to be charged. The capacitor  42  is charged to the voltage VPC, which is close to the output voltage VIG. This charge voltage VPC corresponds to a division of the voltage VIG of the battery  12  by the pre-charge resistor  56  and the discharge register  44 . When the power supply relay  40  is turned on to the closed state by turning on the switching element  60 , the charge voltage VPC of the capacitor  42  further rises to the output voltage of the power supply relay  40 , which is substantially the same as the battery voltage. Even if the motor relay  46  is turned on to the closed state by turning on the switching element  62 , the charge voltage VPC of the capacitor  42  does not change. 
         [0057]    If the power supply relay  40  has the fixed-open abnormality, the power supply relay  40  is not turned on to the closed state because of its abnormality even when the switching element  60  is turned on. That is, the current path between the capacitor  42  and the battery  12  through the power supply relay  40  remains disconnected. The charge voltage VPC of the capacitor  42  remains to be slightly lower than the ignition output voltage VIG. Since this voltage difference is small, it is not easy to detect the fixed-open abnormality of the power supply relay  40  accurately based on this small difference. If the motor relay  46  is turned on to the closed state by turning on the switching element  62  under this condition, the charge voltage VPC of the capacitor  42  falls. This is because the resistance of the discharge path of the capacitor  42  is decreased. Specifically, the capacitor  42  is connected in parallel to not only the discharge resistor  44  but also the excitation coil of the motor relay  46  as well as the resistors  25 ,  26  and the motor  10 . Thus the resistance of the discharge path of the capacitor  42  is reduced to be less than the resistance of the discharge resistor  44  itself. The ratio of division of the voltage VIG by the pre-charge resistor  56  and the discharge path parallel to the capacitor  42  becomes smaller than that provided by the pre-charge resistor  56  and the discharge resistor  44 . Thus, the charge voltage VPC of the capacitor  42  is decreased considerably from the ignition output voltage VIG. Based on this large charge voltage drop, the fixed-open abnormality is detected. 
         [0058]    The fixed-open abnormality diagnosis processing is executed in the second embodiment as shown in  FIG. 5 . This processing is executed by the microcomputer  30  in response to the turn-on of the ignition switch  50  as a trigger. 
         [0059]    First, at step S 40 , the fixed-closure abnormality of the power supply relay  40  is diagnosed by referring to the keyword stored in the non-volatile memory  32 . At step S 42 , it is checked whether the keyword is “B,” indicating the fixed-closure abnormality. If the keyword is “B” (S 42 : YES), the abnormality is indicated by the malfunction indicator light  34  and the assist control is prohibited. 
         [0060]    If the keyword is not “B” (S 42 : NO), the motor relay  46  is turned on to the closed state by turning on the switching element  62 . It is checked at step S 48  whether a predetermined time T 2  has elapsed after the turn-on of the motor relay  46 . The predetermined time T 2  is set to a shortest possible time, in which the charge voltage VPC of the capacitor  42  is supposed to fall by the amount α. After an elapse of the predetermined time T 2  (S 48 : YES), it is checked at step S 50  whether the charge voltage VPC of the capacitor  42  is less than a predetermined threshold voltage, which is less than the ignition output voltage VIG by the amount α. This step is for diagnosing the power supply relay  40  with respect to the fixed-open abnormality. The amount of drop a of the capacitor voltage is determined in accordance with the voltage VIG of the battery  12 , the resistance of the pre-charge resistor  56  and the resistance of the discharge path, which includes the discharge resistor  44  and is connected in parallel to the capacitor  42 . If the charge voltage VPC is less than the predetermined threshold voltage (S 50 : YES), it is determined at step S 52  that the power supply relay  40  has the fixed-open abnormality. At step S 54 , the fixed-open abnormality is indicated by the malfunction indicator light  34  and the assist control is prohibited. 
         [0061]    This processing is terminated when steps S 44  or S 54  has been completed or when it is determined at step S 50  that the charge voltage VPC remain higher than the predetermined threshold voltage. 
         [0062]    The second embodiment provides the following advantages in addition to the foregoing advantages of the first embodiment. 
         [0063]    (5) The power supply relay  40  is diagnosed with respect to the fixed-open abnormality based on the charge voltage VPC of the capacitor  42  present when the ignition switch  50 , the power supply relay  40  and the motor relay  46  are turned on. Even if the capacitor  42  is pre-charged, the power supply relay  40  can be diagnosed accurately with respect to the fixed-open abnormality. 
         [0064]    (6) The power supply relay  40  is diagnosed with respect to the fixed-open abnormality based on the charge voltage VPC of the capacitor  42  and the voltage of the battery  12  (ignition output voltage VIG), when the ignition switch  50 , the power supply relay  40  and the motor relay  46  are turned on. Thus, the predetermined threshold voltage for checking the fixed-open abnormality of the power supply relay  40  by comparison with the charge voltage VPC can be set by using the ignition output voltage VIG. 
       Third Embodiment 
       [0065]    A third embodiment is described next with respect to parts different from the second embodiment. 
         [0066]    In the third embodiment, the abnormality diagnosis processing is executed as shown in  FIG. 6  in response to the turn-on of the ignition switch  50  as a trigger. In  FIG. 6 , steps S 60 , S 62 , S 64  and S 66  are executed in addition to the steps executed in the second embodiment ( FIG. 5 ). 
         [0067]    If it is determined at step S 42  that the fixed-closure abnormality is not present (NO), it is then checked at step S 60  whether a predetermined time T 3  has elapsed. The predetermined time T 3  is set to correspond to a period required to charge the capacitor  42  to a predetermined voltage β by the ignition voltage VIG through the pre-charge resistor  56 . If the predetermined time elapses (S 62 : YES), it is checked at step S 62  whether the charge voltage VPC is greater than the predetermined voltage β. This step S 62  is for checking whether the pre-charge path including the pre-charge resistor  56  has any abnormality. Specifically, the pre-charge path is determined to be abnormal, if the charge voltage VPC does not rise sufficiently even after an elapse of time, in which the capacitor  42  is supposed to have been pre-charged sufficiently. The predetermined voltage β is therefore set to correspond to the charge voltage, which the capacitor  42  normally attains in the predetermined time T 3 . If the charge voltage VPC is less than the predetermined voltage  3  (S 62 : NO), this abnormality of low pre-charge voltage is indicated by the malfunction indicator light  34  at step S 64  and the motor relay  46  is turned on to the closed state at step S 66 . 
         [0068]    If the charge voltage VPC is greater than the predetermined voltage β (S 62 : YES), steps S 46  to S 54  are executed to diagnose the power supply relay  40  with respect to the fixed-open abnormality in the similar manner as in the second embodiment. 
         [0069]    The third embodiment provides the following advantage in addition to the advantages of the first embodiment and the advantages of the second embodiment. 
         [0070]    (7) The pre-charge path is diagnosed with respect to its abnormality based on the charge voltage VPC of the capacitor  42  produced after the ignition switch  50  is turned on. It can be therefore detected in advance, by turning on the power supply relay  40  to the closed state, that a large current will flow in the motor relay  46 . 
       Fourth Embodiment 
       [0071]    A fourth embodiment is described next with reference to parts different from the first embodiment. 
         [0072]    In the fourth embodiment, as shown in  FIG. 7 , a Zener diode  80  is connected to the pre-charge resistor  56  between two current paths, one of which is between the diode  52  and the microcomputer  30  and the other of which is between the power supply relay  40  and the H-bridge circuit  20 . The breakdown voltage Vz of the Zener diode  80  is set such that the current, which flows in the power supply relay  40  when the power supply relay is turned on, does not exceed an upper limit value of an allowable current. The breakdown voltage Vz is also set such that the charge voltage VPC produced by the capacitor  42  before and after the turn-on of the power supply relay  40  may be distinguished accurately. The charge voltage VPC of the capacitor  42  is preferably greater than one half (½) of the voltage of the battery  12  and less than nine-tenth ( 9/10) of the same. 
         [0073]    According to the fourth embodiment, the power supply relay can be diagnosed with respect to its fixed-closure abnormality based on the charge voltage VPC of the capacitor  42  after the ignition switch  50  is turned on and before the power supply relay  40  is turned on to the closed state. 
         [0074]    It is possible to perform this operation by increasing the resistance of the pre-charge resistor  56  without the Zener diode  80 . The increase of the resistance of the pre-charge resistor  56  will necessarily reduce the current, which flows from the battery  12  to the capacitor  42 . As a result, time required to pre-charge the capacitor  42  becomes long and the start of control of the motor  10  is delayed more. 
         [0075]    The fourth embodiment provides the following additional advantage. 
         [0076]    (8) Since the Zener diode  80  is connected in series to the pre-charge resistor  56 , the capacitor  42  can be quickly charge to a voltage, which is sufficiently lower than the charge voltage VPC attained when the power supply relay  40  is turned on to the closed state. As a result, the pre-charge can be completed quickly and the fixed-closure abnormality of the power supply relay  40  can be detected in advance of the turn-on of the power supply relay  40 . 
       Fifth Embodiment 
       [0077]    A fifth embodiment is described next with reference to parts different from the second embodiment ( FIG. 5 ). 
         [0078]    In this embodiment, abnormality diagnosis processing is executed as shown in  FIG. 8  by the microcomputer  30  in response to the turn-on of the ignition switch  50  as a trigger. In the fifth embodiment shown in  FIG. 8 , steps S 46   a  and S 70  are additionally executed relative to the processing of the second embodiment shown in  FIG. 5 . Step S 46   a  is a replacement of step S 46 . 
         [0079]    Specifically, if the power supply relay  40  has no fixed-closure abnormality (S 42 : NO), the motor relay  46  is turned on to the closed state and the duty ratio Duty of the H-bridge circuit  20  is controlled to be less than a minimum duty ratio Dmin at step S 46   a . The minimum duty ratio Dmin is a minimum value of the duty control for the motor  10  by the H-bridge circuit  20  at the time the motor  10  is driven. When the switching elements  21  and  24  are turned on, the current path formed of the switching elements  21 ,  24  and the motor  10  is added in parallel to the discharge resistor  44 . When the switching elements  22  and  23  are turned on, two current paths are connected in parallel to the discharge resistor  44 . One current path is formed of the switching elements  22 ,  23  and the motor  10 , and the other current path is formed of the resistors  25 ,  26  and the motor  10 . As a result, the resistance of the discharge paths including the discharge resistor  44  and connected in parallel to the capacitor  42  is decreased, and the charge voltage VPC of the capacitor  42  is decreased more quickly. Thus, the fixed-open abnormality can be detected at step S 50  more accurately. The duty ratio is controlled within a range, in which the rotor of the motor  10  is not displaced. Thus, it is prevented that the steering wheel is moved by the motor unintentionally. 
         [0080]    In the duty ratio control, the switching elements  21  and  24  may be turned on and off periodically at the same time, or the switching elements  22  and  23  may be turned on and off periodically at the same time. The switching elements  21 ,  24  and the switching elements  22 ,  23  may be turned on alternately. In either of the cases, the period in which the switching elements  21 ,  24  are turned on continuously and the period in which the switching elements  22 ,  23  are turned on continuously should be limited to a period not to move the rotor of the motor  10 . 
         [0081]    The duty ratio control is stopped at S 70 , if the main relay  40  has no fixed-open abnormality (S 50 : NO) or the assist control is prohibited (S 54 ). 
         [0082]    The fifth embodiment provides the following advantage in addition to the advantages of the second embodiment. 
         [0083]    (9) Since the H-bridge circuit  20  is duty-controlled in diagnosing the power supply relay  40  with respect to the fixed-open abnormality, the diagnosis operation can be performed more accurately. 
       Other Embodiments 
       [0084]    The foregoing embodiments may be modified as follows. 
         [0085]    The fourth embodiment may be modified similarly as the second embodiment is modified by the third embodiment. 
         [0086]    The third embodiment may be modified similarly as the second embodiment is modified by the fifth embodiment. 
         [0087]    The fixed-closure diagnosis performed in the first embodiment need not be based on the drop of the charge voltage VPC produced after both of the ignition switch  50  and the power supply relay  40  are turned on to the closed state. For example, this diagnosis may be performed based on the drop condition of the voltage during a period in which a voltage greater than the low limit value of the operation voltage of the microcomputer  30  after both of the ignition switch  50  and the power supply relay  40  are turned on to the closed state. 
         [0088]    In the second and the third embodiments, the microcomputer need not have input terminals provided exclusively to monitor the ignition output voltage VIG and the charge voltage VPC to detect the fixed-open abnormality. The fixed-open abnormality may be detected based on only the charge voltage VPC. In this case, when the motor relay  46  is turned on after the ignition switch  50  and the power supply relay  40  are turned on, the charge voltage VPC does not fall if the power supply relay  40  has no fixed-open abnormality. However, the charge voltage falls if the power supply relay  40  has the fixed-open abnormality. Therefore it is possible to diagnose the power supply relay  40  with respect to the fixed-open abnormality based on whether the charge voltage VPC falls more than a predetermined amount after the motor relay  46  is turned on to the closed state. 
         [0089]    In each embodiment, the keyword need not be written and stored in the non-volatile memory  32  for the fixed-closure diagnosis. For example, the keyword may be written and stored in a back-up RAM, which is maintained operable irrespective of a condition of the ignition switch  50 , that is, a condition of connection between the microcomputer  30  and the battery  12 . The keyword may be written and stored in a volatile memory. Even in this case, the keyword “B” indicating the fixed-closure abnormality can be maintained, because the power supply to the microcomputer  30  is maintained by the power supply relay  40 . It is thus possible to perform the fixed-closure abnormality based on checking whether the keyword B is stored in the volatile memory, when the ignition switch  50  is turned on next time. The processing of writing the keyword “A” at the time the ignition switch  50  is turned off may be obviated. 
         [0090]    In the foregoing embodiments, the power supply relay  40  may be diagnosed with respect to the fixed-closure abnormality in the conventional manner by setting the resistances of the pre-charge resistor  56  and the discharge resistor  44 . 
         [0091]    In the fourth embodiment, the abnormality diagnosis performed in the other embodiments may be performed. 
         [0092]    In the foregoing embodiments, the discharge resistor  44  may be obviated. Even in this case, the current path for charging the capacitor  42  is interrupted by turning off the power supply relay  40  to the open state under the condition that the ignition switch  50  is in the turned-off condition. The voltage of the capacitor  42  thus falls as the power of the capacitor  42  is consumed by the microcomputer  30 . As a result, the power supply relay  40  is diagnosed with respect to the fixed-closure abnormality based on the charge voltage VPC of the capacitor  42  in the similar manner as in the foregoing embodiments. 
         [0093]    The motor  10  is not limited to the brush-type DC motor but may be a brushless-type DC motor. In this case, a three-phase inverter may be used as the power conversion circuit. 
         [0094]    The motor  10  is not limited to be used in the electric power-assisting apparatus but may be used in a gear ratio varying apparatus, which is provided between an input shaft mechanically coupled to a steering wheel and an output shaft driven to rotate by an electric motor and varies a ratio of rotation amount of the output shaft relative to a rotation amount of the input shaft. The motor  10  may be used in a steering system of a steer-by-wire system. In these cases, it becomes possible in the fifth embodiment that the duty ratio may be set to a value, which will cause a small amount of movement of the rotor of the motor  10  without any turning the steering wheel. The motor  10  may also be used to drive an air compressor or a motive power generator mounted in a vehicle.