Abstract:
A lock device that restricts movement of a movable body includes: a lock member that is engageable with an engagement portion formed at the movable body; an urging member that urges the lock member in a direction away from the engagement portion; and an actuator that moves the lock member toward the engagement portion to engage the lock member with the engagement portion against an urging force generated by the urging member.

Description:
INCORPORATION BY REFERENCE 
     The disclosure of Japanese Patent Applications No. 2010-255737 filed on Nov. 16, 2010 and No. 2011-097123 filed on Apr. 25, 2011 including the specification, drawings and abstract is incorporated herein by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a lock device and an electric power steering system. 
     2. Description of Related Art 
     Conventional lock devices are described in, for example, Japanese Patent Application Publication No. 2001-219816 (JP 2001-219816) and Japanese Patent Application Publication No. 2002-308049 (JP 2002-308049). 
     JP 2001-219816 describes a column-type electric power steering system provided with a steering lock device. An engagement portion is formed at a worm shaft side (input side), the worm shaft transmitting the rotation of an electric motor to a speed reducer, or at an electric motor output shaft side. A locked state is achieved by inserting a lock member into the engagement portion, and an unlocked state is achieved by removing the lock member from the engagement portion. 
     The lock member is advanced or retracted with the use of an elastic member that urges (advances) the lock member toward the engagement portion and an actuator that attracts the lock member to remove (retract) the lock member from engagement portion. The lock member is advanced or retracted in the following manner. When an ignition key is turned on, electric current application to a solenoid that serves as the actuator and that is fixed to a casing is controlled. Thus, an attraction force that counteracts an urging force generated by the elastic member is generated to retract the lock member provided with a moving core so that the lock member is removed from the engagement portion. As a result, the unlocked state is achieved. When the ignition key is turned off, attraction of the moving core by the solenoid is stopped, and the lock member is advanced toward the engagement portion by an urging force generated by the elastic member. As a result, the lock member is engaged with the engagement portion, whereby the locked state is achieved. 
     As described above, the locked state and the unlocked state are achieved at the electric motor output shaft side or at the worm shaft side that is the input side of the speed reducer (i.e., at a stage prior to output of assist torque based on the torque applied to a steering wheel). Thus, in the locked state where the lock member is engaged with the engagement portion, a large force based on the torque applied to the steering wheel is no longer applied directly to the lock member, which enables downsizing of the lock member. 
     However, when the ignition switch is on, electric current is applied to a coil to retract the lock member from the engagement portion. Therefore, if, for example, breakage of a harness, disconnection of a connector, or an instantaneous reduction in battery voltage occurs, electric current application to the coil is stopped. As a result, the lock member is advanced and engaged with the engagement portion due to an urging force generated by the elastic member. In some cases, the steering wheel may be locked while a vehicle is traveling. With this regards, there is still room for improvement. 
     JP 2002-308049 describes a structure in which a key portion and a lock mechanism portion of a steering shaft are unitized so as to be mechanically linked to each other. In the structure, a cam member that rotates together with a key rotor is provided between the key rotor and the lock mechanism portion of the steering shaft and extends to the lock mechanism portion coaxially with the key rotor. A locking lever that is linked to insertion and removal of the key is provided. When the key is turned from ACC position to LOCK position, the cam member is operated. In accordance with the operation of the cam member, the lock member that is provided at the lock mechanism on the steering shaft side is operated and is brought to a state where the lock member can be locked with the steering shaft. When the key is removed from LOCK position, the locking lever is operated. In accordance with the operation of the locking lever, the lock member is operated to be inserted in a groove of the steering shaft. Thus, the locked state is achieved. 
     JP 2002-308049 A describes the structure in which the key portion and the lock mechanism portion of the steering shaft are unitized so as to be mechanically liked to each other. Therefore, if the key portion is provided at an instrument panel at a driver&#39;s seat, the lock mechanism portion is located in front of the knee of a driver, which may impose restrictions on the strength and installation position of the lock mechanism portion. 
     In order to address this problem, the following configuration may be employed. An operation portion such as a key device and an actuator portion such as a lock mechanism are separated from each other. A lock member at the lock mechanism is moved to the lock position by a spring member. When the key is inserted and turned to ACC position (when locking is cancelled), the fact that the key is turned to ACC position is detected by, for example, detection unit, and drive unit such as a motor is driven based on a detection signal to move the lock member to the locking cancellation position. 
     A device is required which maintains the locking cancellation state so that the locking operation is not erroneously performed in the locking cancellation state where locking by the lock member is cancelled. Conventionally, the key portion and the lock mechanism portion of the steering shaft are mechanically linked to each other. Therefore, as long as the key rotor is at a predetermined rotation position, the locking cancellation state where locking by the lock member is cancelled is maintained by the cam member. 
     In the above-described structure where the operation portion such as the key device and the actuator portion such as the lock mechanism are separated from each other, there is no cam member. Accordingly, it is necessary to provide a device that maintains the locking cancellation state, at the actuator portion. For example, a locking cancellation maintaining member is attached to a plunger of a solenoid, which is an electric drive unit. An electric signal is generated based on the operation of the operation portion, the solenoid is driven according to the electric signal, and the locking cancellation state in which locking by the lock member is cancelled is maintained by the locking cancellation maintaining member. However, in the structure in which the locking cancellation maintaining member is operated by electric drive unit such as a solenoid, malfunction due to an electrical problem (e.g., breakage of a harness, disconnection of a connector, or an instantaneous reduction in battery voltage) may occur. In this regard, there is still room for improvement. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a lock device that is able to reliably maintain the locked state with low power consumption while operating a lock cancellation maintaining member using an electric drive unit without being affected by an electrical trouble. 
     An aspect of the invention relates to a lock device that restricts movement of a movable body. The lock device includes: a lock member that is engageable with an engagement portion formed at the movable body; an urging member that urges the lock member in a direction away from the engagement portion; and an actuator that moves the lock member toward the engagement portion to engage the lock member with the engagement portion against an urging force generated by the urging member. 
     With the configuration described above, even if electric current application to a coil or electric current application to the actuator is stopped due to an electrical trouble, for example, breakage of a harness, disconnection of a connector or an instantaneous drop in battery voltage, it is possible to maintain the disengaged state by moving the lock member using the urging member. 
     As a result, even if electric current application is stopped, the lock member is maintained in the disengaged state. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Features, advantages, and technical and industrial significance of exemplary embodiments of the invention will be described below with reference to the accompanying drawings, in which like numerals denote like elements, and wherein: 
         FIG. 1  is an overall view of an electric power steering system according to an embodiment of the invention; 
         FIG. 2  is a partial sectional view which is taken along the line II-II in  FIG. 1 , and from which a steering wheel and a universal joint are omitted; 
         FIG. 3  is a sectional view taken along the line III-III in  FIG. 2 ; 
         FIG. 4  is a view showing the structure of a steering lock device; 
         FIG. 5  is an electrical diagram for the steering lock device; 
         FIG. 6  is a graph showing the relationship between a stroke of a lock pin of the steering lock device and forces that act on the lock pin while electric current is not applied; 
         FIG. 7  is a graph showing the relationship between a stroke of the lock pin of the steering lock device and forces that act on the lock pin while electric current is applied to achieve the locked state; 
         FIG. 8  is a graph showing the relationship between a stroke of the lock pin of the steering lock device and forces that act on the lock pin while electric current is applied to achieve the unlocked state; 
         FIG. 9  is a view showing the state where the lock pin of the steering lock device is about to start advancing from the unlock end; 
         FIG. 10  is a view showing the state where the lock pin has advanced from the unlock end in  FIG. 9  and a lock detection switch is turned on; 
         FIG. 11  is a view showing the state where the lock pin has further advanced and come close to the lock end; 
         FIG. 12  is a view showing the state where the lock pin is about to start retracting from the lock end; 
         FIG. 13  is a state where the lock pin has retracted from the lock end in  FIG. 12  and the lock detection switch is turned off; 
         FIG. 14  is a state where the lock pin has further retracted and come close to the unlock end; and 
         FIG. 15  is a flowchart showing an operation of the steering lock device. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Hereafter, an embodiment of the invention will be described with reference to the accompanying drawings. As shown in  FIG. 1  and  FIG. 2 , an input shaft  1  of an electric power steering system is rotatably supported by a steering column  2 . The input shaft  1  includes an upper shaft  4  and a lower shaft  6 . A steering wheel  3  is attached to the upper shaft  4 . The lower shaft  6  is fitted in a tubular portion  5  formed at a lower end portion of the upper shaft  4  such that relative rotation between the lower shaft  6  and the tubular portion  5  is restricted and such that relative displacement between the tubular portion  5  and the lower shaft  6  in the axial direction is allowed if an axial force equal to or larger than a predetermined value is applied. Accordingly, if a driver hits the steering wheel  3  upon a vehicle collision and an axial force equal to or larger than the predetermined value is applied to the input shaft  1 , the upper shaft  4  is displaced relative to the lower shaft  6  in the axial direction. Thus, impact energy is absorbed. 
     The steering column  2  includes a tubular upper column  8  and a tubular lower column  9 . The upper column  8  rotatably supports the upper shaft  4  via a bearing  7 . The lower column  9  is fitted at its upper end portion to the inner periphery of a lower end portion of the upper column  8 . An upper bracket  10  is used to fit the upper column  8  to a vehicle body. If a vehicle collision occurs and the upper column  8  is moved forward due to an impact, the upper bracket  10  is removed from the vehicle body, thus allowing the upper column  8  and the upper shaft  4  to move forward. 
     A housing  11  is fixed to a lower end of the lower column  9 , and fitted to the vehicle body via a lower bracket  12 . An output shaft  13  is an output member rotatably supported by the housing  11 , and is connected to the lower shaft  6  via a torsion bar  14 . The output shaft  13  is connected to steered wheels  18  via, for example, a universal joint  15 , an intermediate shaft  16 , and a rack and pinion mechanism  17 . A torque detector  19  detects a steering torque that is applied to the input shaft  1  via the steering wheel  3 . The steering torque is detected by electrically measuring a minute relative rotational displacement between the input shaft  1  and the output shaft  13 , which is proportional to torsion of the torsion bar  14  due to the steering torque. 
     As shown in  FIG. 3 , a wheel gear  21  of a speed reducer  20  is fixed to the output shaft  13 . A worm shaft (input member of the speed reducer  20 )  22  is rotatably supported by the housing  11  at both ends via bearings  23 , and is in mesh with the wheel gear  21 . An electric motor  24  is fixed to the housing  11 . An output shaft  25  that serves as a rotational output member of the electric motor  24  is spline-connected to the worm shaft  22 . A ring  31  having a plurality of lock holes (engagement portions)  29  in its periphery is fitted to the output shaft  25 . A steering lock device (lock device)  35  is fixed to the housing  11 . The steering lock device  35  places the output shaft  25  in the locked state by inserting a lock pin (lock member)  26  into the lock hole  29  formed in the ring  31  fitted to the output shaft  25 , and places the output shaft  25  in the unlocked state by removing the lock pin  26  from the lock hole  29 . 
     Next, the structure of the steering lock device  35  will be described with reference to  FIG. 4 . The steering lock device  35  includes an actuator  30  and the ring  31 . The actuator  30  causes the lock pin  26  to advance toward the lock hole  29  or to retract from the lock hole  29 . The ring  31  is fitted to the output shaft  25  of the motor  24 , and has the multiple lock holes  29  in its periphery. 
     The actuator  30  has a plunger  51  that is secured to the lock pin  26  so as to move together with the lock pin  26 . The plunger  51  is formed of a magnet with one pole pair. For example, as shown in  FIG. 4 , the left side portion of the plunger  51  is the south pole, and the right side portion of the plunger  51  is the north pole. 
     A coil  52  is wound around the plunger  51  to generate an electromagnetic force for advancing or retracting the lock pin  26 . The coil  54  is surrounded by a yoke  53  that serves as a magnetic path for an electromagnetic force generated by the coil  54 . 
     Two bushes  55  are provided between the plunger  51  and the coil  54 . Thus, the plunger  51  is smoothly advanced or retracted by an electromagnetic force generated by the coil  54 . 
     A front portion of the actuator  30  is covered with a front cover  56 . A compression spring (urging member)  28 , which is used to remove the lock pin  26  from the lock hole  29  formed in the ring  31  fitted to the output shaft  25  of the electric motor  24 , is provided between the front cover  56  and the left end of the plunger  51 . 
     A rear portion of the actuator  30  is covered with a rear cover  57 . A lock detection switch (lock detection unit)  45 , which detects the state of engagement of the lock pin  26  with the lock hole  29  based on the position of the plunger  51 , is provided between the rear rover  57  and the yoke  53 . 
     Next, the electrical configuration of the steering lock device  35  will be described with reference to  FIG. 5 . A control unit for the steering lock device  35  includes an ECL  40  that is a main control portion, a battery  46 , an ignition switch  44 , the lock detection switch  45 , and the actuator  30 . 
     The ECU  40  includes a CPU  41  that executes control processes, an input interface (I/F)  42 , and an output interface (I/F)  43 . The input interface  42  receives signals from the ignition switch  44  and the lock detection switch  45 . The output interface  43  outputs electric current to the actuator  30 . 
     Next, the operations of the steering lock device  35  and the ECU  40  for the steering lock device  35  will be described with reference to  FIGS. 6 ,  7  and  8 . 
     With regard to the ordinate axis in  FIG. 6 , the upward arrow represents a retraction force Fr for retracting the lock pin  26  from the lock hole  29  (placing the lock pin  26  in the disengaged state), and the downward arrow represents an advance force for advancing the lock pin  26  toward the lock hole  29 . 
     The abscissa axis represents a stroke of the lock pin  26 . When the lock pin  26  is in the state shown in  FIG. 4 , the stroke is zero. The stroke in this state is indicated by an unlock end (retraction end) P 1 . As the lock pin  26  moves toward a lock end (advance end), the value of stroke shifts rightward on the abscissa axis. 
     L 1  indicates the relationship between the stroke of the lock pin  26  and a retraction force Fr, generated by the compression spring  28 , for retracting the lock pin  26  from the lock hole  29 . L 2  indicates an attraction force that acts between the magnet with one pole pair, which constitutes the plunger  51 , and the yoke  53 . The attraction force acts as an advance force Fa for advancing the lock pin  26  toward the lock hole  29 . 
     L 3  indicates a resultant of L 1  and L 2  while electric current is not applied to the coil  54 , that is, a resultant of the retraction force Fr generated by the compression spring  28  and the attraction force (advance force) Fa that acts between the plunger  51  and the yoke  53 . P 3  indicates a balance point at which the retraction force Fr generated by the compression spring  28  and the advance force Fa that acts between the plunger  51  and the yoke  53  cancel out each other. P 10  indicates a position to which the lock pin  26  is allowed to be advanced maximally by the resultant of the retraction force Fr generated by the compression spring  28  and the attraction force Fa that acts between the magnet of the plunger  51  and the yoke  53 . Note that, the lock pin  26  is configured to mechanically stop at the lock end (advance end) P 2 , therefore, the lock pin  26  never reaches P 10 . 
     Between P 1  and P 3 , the retraction force Fr is larger than the advance force Fa, and therefore a force for retracting the lock pin  26  acts on the lock pin  26 . Between P 3  and P 10 , the advance force Fa is larger than the retraction force Fr, and therefore a force for advancing the lock pin  26  acts on the lock pin  26 . 
     As a result, when the value of stroke is on the left side of P 3 , the lock pin  26  is pushed by the retraction force Fr generated by the compression spring  28  such that the lock pin  26  is directed toward the unlock end (retraction end) P 1 . That is, the unlocked state is achieved. 
     Next, as indicated by L 4  in  FIG. 7 , a lock pin advancing current Ia is applied to the coil  54  of the actuator  30  by the ECU  40  to apply the advance force Fa that overcomes the retraction force Fr generated by the compression spring  28  to the lock pin  26 . Then, the lock pin  26  is advanced by a resultant (indicated by L 5 ) of the advance force Fa indicated by L 4  and the retraction force Fr indicated by L 3 . 
     The lock pin advancing current Ta is shut off at a stroke (e.g., P 4 ) at which the lock pin  26  is able to be advanced even when electric current is not applied to the coil  54  of the actuator  30  by the ECU  40  as shown by L 3 . Then, the advance force Fa is shifted from the advance force Fa indicated by L 5  to the advance force Fa indicated by L 3  at P 4 . However, the advance force Fa continuously acts on the lock pin  26  to bring the lock pin  26  to the lock end (advance end) P 2  at which the lock pin  26  mechanically stops, and the lock pin  26  is maintained at the lock end P 2  (engaged state). That is, the locked state is achieved. As described later in detail (see  FIG. 15 ), whether the lock pin  26  has reached P 4  is determined based on a signal from the lock detection switch  45  and a value indicated by a lock pin advance checking timer Tr 1 . More specifically, when the lock pin  26  reaches P 4 , the lock pin  26  is in an immediately-before engaged state that is a state achieved immediately before the engaged state where the tip of the lock pin  26  reaches the lock end of the lock hole  29  formed in the ring  31 . As a result, the lock pin advancing current Ia is shut off at P 4 . Therefore, steering lock is achieved in the electric power steering system with lower power consumption. 
     As indicated by L 6  in  FIG. 8 , a lock pin retracting current Ib is applied to the coil  54  of the actuator  30  by the ECU  40  to apply the retraction force Fr that overcomes the resultant of the compressing spring force and the attraction force that acts between the magnet that constitutes the plunger  51  and yoke  53 . With the resultant, steering lock has been maintained. Then, the lock pin  26  is retracted by a resultant (indicated by L 7 ) of the retraction force Fr indicated by L 6  and the advance force Fa indicated by L 3 . 
     The lock pin retracting current Ib is shut off at a stroke (e.g., P 5 ) at which the lock pin  26  is able to be retracted even when electric current is not applied to the coil  54  of the actuator  30  by the ECU  40  as shown by L 3 . Then, the retraction force Fr is shifted from the retraction force Fr indicated by L 7  to the retraction force Fr indicated by L 3  at P 5 . However, the retraction force Fr continuously acts on the lock pin  26  to bring the lock pin  26  to the unlock end (retraction end) P 1  at which the lock pin  26  mechanically stops, and the lock pin  26  is maintained at the unlock end P 1 . As described later in detail (see  FIG. 15 ), whether the lock pin  26  has reached P 5  is determined based on a signal from the lock detection switch  45  and a value indicated by a lock pin retraction checking timer Tr 2 . 
     Next, transition of the steering lock device  35  from the unlocked state to the locked state and transition of the steering lock device  35  from the locked state to the unlocked state will be described with reference to  FIG. 9  to  FIG. 14 . 
     As shown in  FIG. 9 , in the state where the plunger  51  secured to the lock pin  26  so as to move together with the lock pin  26  is standstill at the unlock end P 1 , electric current is applied to the coil  54  of the actuator  30  by the ECU  40 . Electric current is applied to the coil  54  of the actuator  30  in such a direction that the north pole is formed in the left side portion of the yoke  53  and the south pole is formed in the right side portion of the yoke  53  (see Ia in  FIG. 5 ). Thus, the south pole of the plunger  51  is attracted to the north pole formed in the yoke  53 , and the plunger  51  is advanced toward the lock hole  29 . 
     When the plunger  51  is advanced to a predetermined position (P 4  in  FIG. 7 ), the lock detection switch  45  is turned on as shown in  FIG. 10 . Therefore, based on a signal from the lock detection switch  45  and a value indicated by the lock pin advance checking timer Tr 1 , electric current application to the coil  54  of the actuator  30  is stopped by the ECU  40 . The south pole of the plunger  51  has been attracted to the north pole formed in the yoke  53 , and thus the plunger  51  has been advanced. However, the north pole that has been formed in the yoke  53  disappears when electric current application to the coil  54  is stopped. 
     However, even if electric current application to the coil  54  of the actuator  30  is stopped by the ECU  40  when the lock pin  26  reaches the predetermined position (P 4  in  FIG. 7 ), a magnetic force acts between the plunger  51  formed of the magnet with one pole pair and the yoke  53  made of magnetic material. Thus, the lock pin  26  is advanced to the lock end P 2  against a spring force of the compression spring  28 , and the lock pin  26  is maintained at the lock end P 2  (see  FIG. 11 ). The lock pin advancing current Ia is shut off at P 4 . Therefore, it is possible to achieve steering lock in the electric power steering system with lower power consumption. 
     Next, if the ignition switch  44  is turned on when the plunger  51  is stopped at the lock end P 2  as shown in  FIG. 12 , electric current is applied to the coil  54  of the actuator  30  by the ECU  40 . Electric current is applied to the coil  54  of the actuator  30  in such a direction that the south pole is formed in the left side portion of the yoke  53  and the north pole is formed in the right side portion of the yoke  53  (see Ib in  FIG. 8 ). Then, the south pole of the plunger  51  repels the south pole formed in the yoke  53 , and the spring force of the compression spring  28  is added to the repelling force. With this force, the lock pin  21  secured to the plunger  51  is retracted from the lock hole  29 . 
     When the plunger  51  is retracted to a predetermined position (P 5  in  FIG. 8 ), the lock detection switch  45  is turned off as shown in  FIG. 13 . Therefore, based on a signal from the lock detection switch  45  and a value indicated by the lock pin retraction checking timer Tr 2 , electric current application to the coil  54  of the actuator  30  is stopped by the ECU  40 . However, at this time, the plunger  51  is retracted to the unlock end P 1  by a resultant of a spring force of the compression spring  28  and a magnetic force that acts between the plunger  51  and the yoke  53  (see  FIG. 14 ). 
     Next, the operations of the steering lock device  35  and the ECU  40  will be described in detail with reference to a flowchart shown in  FIG. 15 . First, it is determined whether the ignition switch  44  is off (step  101 ). If it is determined in step  101  that the ignition switch  44  is off (YES in step  101 ), the lock pin advance checking time Tr 1  is reset (step  102 : Tr 1 =0). 
     Next, electric current is applied to the coil  54  of the solenoid  30  in such a direction that the lock pin  26  is advanced (step  103 : apply current Ia). Further, the lock pin advance checking timer Tr 1  is incremented (step  104 : Tr 1 =Tr 1 +T 1 ). 
     Next, it is determined whether the value indicated by the lock pin advance checking time Tr 1  is equal to or larger than a predetermined value (step  105 : Tr 1 ≧Tr 01 ). If the value indicated by the lock pin advance checking timer Tr 1  is equal to or larger than the predetermined value (YES in step  105 : Tr 1 ≧Tr 01 ), it is determined whether the lock detection switch  45  is on (step  106 ). 
     If the lock detection switch  54  is on (YES in step  106 ), electric current application to the coil  54  of the actuator  30  is stopped (step  107 : stop application of current Ia), after which the process ends. Thus, the lock pin  26  is engaged with the lock hole  29 , whereby the locked state is achieved. If the value indicated by the lock pin advance checking time Tr 1  is smaller than the predetermined value (NO in step  105 : Tr 1 &lt;Tr 01 ), or if the lock detection switch  45  is off (NO in step  106 ), step  103  is executed again to apply electric current to the coil  54  of the actuator  30  in such a direction that the lock pin  26  is advanced (step  103 :apply current Ia). 
     If it is determined in step  101  that the ignition switch  44  is on (NO in step  101 ), the lock pin retraction checking time Tr 2  is reset (step  108 : Tr 2 =0). 
     Then, electric current is applied to the coil  54  of the actuator  30  in such a direction that the lock pin  26  is retracted (step  109 : apply current Ib). In addition, the lock pin retraction checking time Tr 2  is incremented (step  110 : Tr 2 =Tr 2 +T 2 ). 
     Next, it is determined whether the lock detection switch  45  is off (step  111 ). If the lock detection switch  45  is off (YES in step  111 ), it is determined whether the value indicated by the lock pin retraction checking time Tr 2  is equal to or larger than a predetermined value (step  112 : Tr 2 ≧Tr 02 ). If the value indicated by the lock pin retraction checking time Tr 2  is equal to or larger than the predetermined value (YES in step  112 : Tr 2 ≧Tr 02 ), electric current application to the coil  54  of the actuator  30  is stopped (step  113 : stop application of current Ib), after which the process ends. Thus, the lock pin  26  is disengaged from the lock hole  29 , whereby the unlocked state is achieved. 
     If the lock detection switch  45  is on (NO in step  111 ), or if the value indicated by the lock pin retraction checking timer Tr 2  is smaller than the predetermined value (NO in step  112 : Tr 2 &lt;Tr 02 ), step  109  is executed again to apply electric current to the coil  54  of the actuator  30  in such a direction that the lock pin  26  is retracted (step  109 : apply current Ib). 
     According to the present embodiment, the following operations and effects are obtained. The steering lock device  35  is configured such that, when the ignition switch is turned on, electric current is applied to the coil  54  of the actuator  30  in such a direction that the lock pin  26  secured to the plunger  51  moves away from the ring  31  that has a plurality of lock holes  29  in its periphery and that is fitted to the output shaft  25  of the electric motor  24 . In addition, the urging member for increasing a force for moving the lock pin  26  away from the ring  31  is provided. 
     With the configuration described above, when the ignition switch  44  is on, in other words, when the steering wheel  3  is being operated, the unlocked state is maintained by a spring force of the urging member. 
     With the configuration described above, even if electric current application to the coil is stopped due to an electrical problem (for example, breakage of a harness, disconnection of a connector, or an instantaneous drop of battery voltage), it is possible to maintain the disengaged state by retracting the lock member using the urging member. 
     As a result, even if electric current application is stopped, the lock member is maintained in the disengaged state. 
     In addition, the steering lock device  35  is configured such that, when the ignition switch is turned off, electric current is applied to the coil  54  of the actuator  30  by the ECU  40  in such a direction that the lock pin  26  secured to the plunger  51  is engaged with the ring  31  that has a plurality of lock holes  29  in its periphery and that is fitted to the output shaft  25  of the electric motor  24 . 
     When the lock detection switch  45  that detects the position of the plunger  51  secured to the lock pin  26  is turned on, electric current application is stopped based on a signal from the lock detection switch  45  and a value indicated by the lock pin advance checking timer Tr 1 . After that, the lock pin  26  is advanced by a magnetic force acting between the yoke  53  of the actuator  30  and the magnet with one pole pair, which is fitted to the plunger  51 , and then engaged with the engagement portion formed in the rotational output member of the electric motor  24 . 
     With the configuration described above, when the lock pin  26  is advanced beyond P 4  and approaches P 2 , even if electric current application to the coil  54  of the actuator  30  is stopped, the lock pin  26  is engaged with the engagement portion formed in the rotational output member of the electric motor  24  by a magnetic force between the plunger  51  formed of the magnet with one pole pair and the yoke  53  made of magnetic material. 
     As a result, steering lock in the electric power steering system is maintained with lower power consumption. 
     In the present embodiment, the ring  31  is provided at the output shaft  25  of the motor  24 . Accordingly, the engagement force of the lock device  35  is increased by the speed reducer  20 . As a result, steering lock is achieved with lower power consumption. 
     The present embodiment may be modified as follows. 
     In the present embodiment, the invention is applied to a column assist-type EPS. Alternatively, the invention may be applied to a rack assist-type EPS or a pinion assist-type EPS. 
     In the present embodiment, the steering lock device is actuated based on the on/off state of the ignition switch. However, how the steering lock device is actuated is not limited to this. The steering lock device may be actuated by a remote controller that uses radio waves or infrared rays. 
     In the present embodiment, the left side portion of the plunger  51  is the south pole, and the right side portion of the plunger  51  is the north pole. However, as a matter of course, the right side portion of the plunger  51  may be the south pole and the left side portion of the plunger  51  may be the north pole. 
     In the present embodiment, a magnet with one pole pair is used as the plunger  51 . Alternatively, a magnet with two or more pole pairs may be used as the plunger  51 . 
     If the steering lock device according to the present embodiment is applied to a hybrid vehicle, a plug hybrid vehicle or a electric vehicle having a large-capacity battery, it is possible to maintain steering lock state for a long period of time. 
     According to the invention, it is possible to provide a lock device that is able to maintain a lock member in the disengaged state and to reliably maintain the locked state with low power consumption while operating a lock cancellation maintaining member using an electric drive unit without being affected by an electrical trouble.