Abstract:
A transmission ratio of a rotational angle of an output shaft to a rotational angle of an input shaft is adjustable by a transmission ratio changing mechanism when a coupling member is in a unlocked position. A resilient member urges the coupling member toward the locked position. A solenoid holds the coupling member at the unlocked position against a resilient force of the resilient member. A solenoid drive control apparatus applies a drive voltage to the solenoid to hold the coupling member positioned at the unlocked position and reduces the drive voltage through a voltage attenuation process before the coupling member finally reaches the locked position. During the voltage attenuation process, the solenoid produces an electromagnetic force yielding to the resilient force of the resilient member and decreases a shifting speed of the coupling member on the way to the locked position.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates to a steering apparatus equipped with a transmission ratio adjusting mechanism (hereinafter, referred to as a transmission ratio adjustable steering apparatus) that is preferably employed in automotive vehicles and is capable of changing a transmission ratio of a steered angle of a steerable wheel or tire to a steering angle of a steering wheel controlled or manipulated by a driver or an operator.  
         [0002]     Some of advanced automotive steering control apparatuses employ a transmission ratio changing mechanism, such as Variable Gear Ratio Steering (VGRS), that can change a steering angle conversion ratio (i.e. a transmission ratio of a steered angle of a steerable wheel to a steering angle of a steering wheel). In general, it is desirable to change the steering angle conversion ratio with reference to vehicle driving conditions including a vehicle traveling speed. For example, when a vehicle is traveling at a higher speed, the steering angle conversion ratio should be decreased to avoid rapid change in the steered angle of the wheel relative to a steering angle of the steering wheel controlled or manipulated by a driver because the traveling performance of the vehicle is stabilized at higher speeds.  
         [0003]     On the other hand, when the vehicle is traveling at a lower speed, it is desirable to increase the steering angle conversion ratio in order to reduce a required minimum steering angle of the steering wheel when a driver must manage to put his/her vehicle into a garage or into a limited parking space and accordingly in order to reduce the burden of a driver who must control or manipulate the steering wheel.  
         [0004]     Conventionally, many motor type steering mechanisms are widely used for adjusting the steering angle conversion ratio because they are excellent in independently rotating and driving a steered wheel shaft. More specifically, an angle detecting section is provided to detect a steering angle of a steering wheel controlled or manipulated by a driver. The steering angle conversion ratio is determined based on the detected steering angle of the steering wheel and actual vehicle driving conditions. Through computer processing, a finally required wheel steering angle (i.e. target wheel angle) is calculated based on the determined steering angle conversion ratio. Then, the steered wheel shaft, mechanically separated from the steering shaft of the steering wheel, is rotated or driven by the motor so as to adjust the angle of the steerable wheel to the target wheel angle.  
         [0005]     According to this kind of steering control system, the steered wheel shaft is follow-up controlled so as to accurately agree with a rotation of the steering shaft. More specifically, the rotational speed of a steered wheel shaft driving motor is adjusted in accordance with a difference between an actual rotational angle of the steered wheel shaft (i.e. actual wheel steering angle) and a target wheel steering angle. The follow-up control should be performed speedily so that the actual steering shaft angle agrees with a target steering angle as quick as possible. However, to avoid undesirable overshoot phenomenon occurring during a final stage of this kind of follow-up control, it is necessary to sufficiently decelerate and accurately control a rotational speed of the motor. On the other hand, a driver may suddenly turn the steering wheel to avoid immediate danger. In such a case, the motor must rotate at a very high speed.  
         [0006]     Furthermore, a transmission ratio changing mechanism may include a locking apparatus, according to which the transmission ratio is mechanically fixed to a predetermined value in case of failure occurring in the transmission ratio changing mechanism. For example, a locking apparatus includes an arch-shaped locking arm swingably supported to a motor housing of the transmission ratio changing mechanism. And, a disc-shaped lock holder is fixed around a rotor shaft. The locking arm has a projection selectively engageable with a recess of this lock holder. When the projection of the locking arm engages with the rotor shaft, the relative rotation between the motor housing and the rotor shaft is locked. Accordingly, in a case that the engine is stopped or in an event that an excessive force is applied to the transmission ratio changing mechanism, the locking apparatus is brought into a locked condition to surely maintain the accurate and regulated relationship between the steering angle of the steering wheel and a steered angle of the steerable wheel. (Refer to Japanese Patent Application Laid-open No. 11-34894).  
         [0007]     However, according to the transmission ratio adjustable steering apparatus disclosed in the above-described prior art, there is a drawback such that the locking arm and the lock holder make uncomfortable noises when they engage to bring the locking apparatus into the lock condition.  
       SUMMARY OF THE INVENTION  
       [0008]     In view of the foregoing problems, the present invention has an object to provide a transmission ratio adjustable steering apparatus that is capable of suppressing noise sounds generating from a transmission ratio changing mechanism, for example, when a locking arm engages with a lock holder in the locking operation of a locking mechanism.  
         [0009]     In order to accomplish the above and other related objects, the present invention provides a transmission ratio adjustable steering apparatus, including an input shaft, an output shaft, a transmission ratio changing mechanism, a coupling member, a resilient member, a solenoid, and a solenoid drive control apparatus. The input shaft is connected to a steering wheel. An output shaft is connected to a steerable wheel. A transmission ratio changing mechanism changes a ratio of a rotational angle of the output shaft to a rotational angle of the input shaft. A coupling member (e.g. locking arm) is shiftable between a locked position (i.e. the position of a lock holder) and an unlocked position. The input shaft and the output shaft integrally rotate when the coupling member is in the locked position, while the input shaft and the output shaft are released from a coupling condition and brought into an unlocked condition. The transmission ratio of the rotational angle of the output shaft to the rotational angle of the input shaft is adjustable by the transmission ratio changing mechanism when the coupling member is in the unlocked position. The resilient member resiliently urges the coupling member toward the locked position. The solenoid holds the coupling member at the unlocked position against a resilient force of the resilient member. And, the solenoid drive control apparatus applies a drive voltage to the solenoid to hold the coupling member positioned at the unlocked position and reduces the drive voltage through a voltage attenuation process before the coupling member finally reaches the locked position. During the voltage attenuation process, the solenoid produces an electromagnetic force yielding to the resilient force of the resilient member and decreases a shifting speed of the coupling member on the way to the locked position.  
         [0010]     According to a preferred embodiment of the present invention, the solenoid drive control apparatus, in the process of shifting the coupling member from the unlocked position to the locked position, gradually reduces the drive voltage applied to the solenoid to allow the coupling member to finally reach the locked position.  
         [0011]     In general, noise sounds generating from the coupling member will be roughly proportional to an impact force generated by the coupling member. Furthermore, an impact force of the coupling member is proportional to the square of its shifting speed. Therefore, noise sounds can be suppressed by decreasing the shifting speed of this coupling member.  
         [0012]     In a practical control, even if the voltage applied to the solenoid is reduced to zero to shift the coupling member from the unlocked position to the locked position, the drive voltage of the solenoid does not decrease to zero immediately due to its inductance component and resistance component. However, it is effective to control a reduction rate of the voltage applied to the solenoid to be smaller than a reduction rate of the voltage caused due to its inductance component and resistance component in realizing a slower shifting speed of the coupling member and accordingly in suppressing noise sounds, compared with a case that a voltage applied to the solenoid is suddenly dropped to zero (refer to  FIG. 5A ).  
         [0013]     Furthermore, it is preferable that the solenoid drive control apparatus, in the process of shifting the coupling member from the unlocked position to the locked position, reduces the drive voltage applied to the solenoid so that an electromagnetic force of the solenoid balances with a resilient force of the resilient member at a near side of the locked position to momentarily stop the coupling member at the near side before the coupling member reaches the locked position, and then finally decreases a value of the drive voltage to zero.  
         [0014]     Even when the voltage applied to the solenoid is controlled so as to shift the coupling member from the unlocked position to the locked position, an actual drive voltage of the solenoid cannot be immediately adjusted to an applied voltage value due to inductance component and resistance component of the solenoid. However, momentarily stopping the coupling member makes it possible to reduce a delay in the change of the drive voltage applied to the solenoid. Furthermore, when the voltage finally applied to the solenoid reduces zero at a time the coupling member has not yet reached the locked position, an impact force of the coupling member is substantially proportional to a product of an elastic modulus of the resilient member and a shift distance of the coupling member. Thus, noise sounds generating from the coupling member in its locking operation become small.  
         [0015]     If the shifting speed of the coupling member is slow in the process of shifting the coupling member from the unlocked position to the locked position, noise sounds generating from the coupling member in its locking operation are small but a long time is required for accomplishing the locking operation. In this respect, employing the above-described arrangement is effective in not only suppressing noise sounds but also in shortening a required operation time, compared with a case that the voltage applied to the solenoid is simply reduced.  
         [0016]     More specifically, according to a preferred embodiment of the present invention, a housing integrally rotates with the input shaft. The transmission ratio changing mechanism is a transmission ratio changing motor that is fixed in the housing and has a rotary shaft for transmitting motor rotation to the output shaft via a speed reduction gear unit. A rotary member is formed coaxially and integrally with the rotary shaft of the transmission ratio changing motor and has at least one engaging recess formed on an outer circumferential surface thereof. The coupling member has an engaging hook that is attached to the housing so as to oppose to the outer circumferential surface of the rotary member and is shiftable between the locked position and the unlocked position. The engaging hook engages with the engaging recess in the locked position. The engaging hook disengages from the engaging recess in the unlocked position so that a predetermined distance is kept between the engaging hook and the outer circumferential surface of the rotary member. The resilient member resiliently urges the coupling member toward the locked position where the engaging hook of the coupling member engages with the engaging recess of the rotary member. The solenoid, when activated, shifts the coupling member against the resilient force of the resilient member and holds the coupling member at the unlocked position where the engaging hook of the coupling member disengages from the engaging recess of the rotary member and keeps a predetermined distance from the outer circumferential surface of the rotary member. And, the solenoid drive control apparatus, in the voltage attenuation process, causes the coupling member held at the unlocked position to shift toward the locked position based on the resilient force of the resilient member, thereby integrating the input shaft with the rotary shaft of the transmission ratio changing motor via the housing, and causing the input shaft and the output shaft to rotate integrally via the speed reduction gear unit.  
         [0017]     The above-described arrangement is effective in realizing the transmission ratio adjustable steering apparatus that is capable of suppressing noise sounds generating when the coupling member is brought into the locked position and also capable of firmly locking the input shaft (i.e. the steering shaft) with the output shaft (i.e. the steered wheel shaft) so that they rotate integrally. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]     The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description which is to be read in conjunction with the accompanying drawings, in which:  
         [0019]      FIG. 1  is a schematic diagram showing an example of a transmission ratio adjustable steering apparatus in accordance with the present invention;  
         [0020]      FIG. 2  is a cross-sectional view showing a transmission ratio changing mechanism of the transmission ratio adjustable steering apparatus in accordance with the present invention;  
         [0021]      FIGS. 3A and 3B  are cross-sectional views explaining the operation of a locking mechanism incorporated in the transmission ratio changing mechanism in accordance with a first embodiment of the present invention;  
         [0022]      FIG. 4  is a cross-sectional view explaining the locked condition of the locking mechanism incorporated in the transmission ratio changing mechanism in accordance with the first embodiment of the present invention;  
         [0023]      FIGS. 5A  to  5 D are graphs explaining the change of a drive voltage applied to an electromagnetic coil or to a solenoid in accordance with the present invention;  
         [0024]      FIG. 6  is a flowchart showing a first method for applying the drive voltage to the electromagnetic coil or to the solenoid in accordance with the present invention;  
         [0025]      FIG. 7  is a flowchart showing a second method for applying the drive voltage to the electromagnetic coil or to the solenoid in accordance with the present invention;  
         [0026]      FIG. 8  is a flowchart showing a third method for applying the drive voltage to the electromagnetic coil or to the solenoid in accordance with the present invention;  
         [0027]      FIGS. 9A and 9B  are cross-sectional views explaining the operation of another locking mechanism incorporated in the transmission ratio changing mechanism in accordance with a second embodiment of the present invention;  
         [0028]      FIG. 10  is a cross-sectional view explaining the locked condition of the locking mechanism incorporated in the transmission ratio changing mechanism in accordance with the second embodiment of the present invention;  
         [0029]      FIGS. 11A and 11B  are cross-sectional views explaining the operation of a modified locking mechanism incorporated in the transmission ratio changing mechanism in accordance with the second embodiment of the present invention; and  
         [0030]      FIG. 12  is a cross-sectional view explaining the operation of a modified locking mechanism incorporated in the transmission ratio changing mechanism in accordance with the second embodiment of the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0031]     Hereinafter, preferred embodiments of the present invention will be explained with reference to attached drawings.  
       First Embodiment  
       [0032]     First, a transmission ratio adjustable steering apparatus will be explained in accordance with a first embodiment of the present invention.  FIG. 1  shows a schematic arrangement for a transmission ratio adjustable steering apparatus  1  according to the first embodiment of the present invention. A steering wheel  10  is fixed to an upper end of a steering shaft  12   a  (corresponding to an input shaft of the present invention). The steering shaft  12   a  has a lower end connected to a transmission ratio changing mechanism  14 . An upper end of a pinion shaft  12   b  (corresponding to an output shaft of the present invention) is connected to the transmission ratio changing mechanism  14 . Furthermore, a pinion (not shown) is provided at a lower end of the pinion shaft  12   b . This pinion meshes or engages with a rack bar  18  disposed in a steering wheel gearbox  16 . Furthermore, each end of the rack bar  18  is linked to an inner end of a corresponding tie rod  20 . The outer end of each tie rod  20  is connected to a steerable wheel  24  via knuckle arm  22 . This embodiment employs a power steering device including a power assist mechanism (not shown) that is capable of assisting the rack bar  18  when it moves. For example, a hydraulic type, a motor driven type, or an electro-hydraulic type will be used.  
         [0033]     Furthermore, a steering angle sensor  25  is attached to the steering shaft  12   a  to detect a steering angle of the steering wheel  10 . The steering angle sensor  25  is, for example, a conventionally known rotary encoder or a comparable angle detecting device. Similarly, an output angle sensor  26  is attached to the pinion shaft  12   b  to detect a steered angle of the steerable wheel  24 . The output angle sensor  26  is, for example, a conventionally known rotary encoder or a comparable angle detecting device. In this case, it is possible to incorporate the output angle sensor  26  into the transmission ratio changing mechanism  14 . A steering control section  30  inputs a steering angle of the steering wheel  10  detected by the steering angle sensor  25  and a steered angle of the steerable wheel  24  detected by the output angle sensor  26 . Furthermore, the steering control section  30  inputs a vehicle traveling speed detected by a vehicle speed sensor  27 . Moreover, the steering control section  30  can function as a solenoid drive control apparatus of the present invention that produces a control signal for controlling the transmission ratio changing mechanism  14 .  
         [0034]     Furthermore, the steering control section  30  is constituted by a well-known microcomputer including CPU  31 , RAM  32 , ROM  33 , and an input/output interface  34  that are mutually connected via a bus line  35  and are capable of communicating with each other. Furthermore, ROM  33  includes a program storing region  33   a  and a data memorizing region  33   b . The program storing region  33   a  stores a steering control program  33   p . The data memorizing region  33   b  stores the data used in the steering control.  
         [0035]     As shown in  FIG. 2 , the transmission ratio changing mechanism  14  includes a motor  40  and a speed reduction gear unit  42 . The motor  40  includes a stator  46  and a rotor  48 . The stator  46  is fixed to a motor housing  44 . The speed reduction gear unit  42  is, for example, constituted by a planetary gear mechanism or a harmonic drive gear mechanism. According to the planetary gear mechanism, a rotary shaft  50  rotates together with the rotor  48  and is fixed to a sun gear  52 . A predetermined number of planetary gears  54  are disposed around the sun gear  52  and angularly spaced at equal intervals. Each planetary gear  54  meshes with the sun gear  52  at the radial inner side and also meshes with a ring gear  56  formed on the inner cylindrical surface of the motor housing  44 . Furthermore, each planetary gear  54  is rotatably supported by a carrier  58 .  
         [0036]     Furthermore, a locking mechanism (refer to  FIGS. 3A and 3B ) is provided in the motor housing  44 . The locking mechanism is located at the height corresponding to an upper part of the rotor  48  (i.e. the level of a line A-A shown in  FIG. 2 ). More specifically, the locking mechanism includes an arch-shaped locking arm  60  (corresponding to a coupling member of the present invention) disposed in the motor housing  44 . The locking arm  60  corresponds in altitude to the upper part of the rotor  48 . The locking arm  60  has an engaging projection  60   a  (corresponding to an engaging hook of the present invention) formed at the inner side of its arch-shaped body. The locking arm  60  has one end (i.e. a pivot end or a proximal end) swingably supported by the motor housing  44  by means of a pin  44   a . An electromagnetic coil  62  is provided at the other end (i.e. a free end or a distal end) of the locking arm  60 . Furthermore, as shown in  FIG. 2 , a plate magnet  64  is fixed to a ceiling of the motor housing  44 . The plate magnet  64  opposes closely, from above, to the electromagnetic coil  62 . On the other hand, a metallic plate  66  is fixed to the stator  46  and is positioned under the electromagnetic coil  62  in the opposed relationship. The electromagnetic coil  62 , the plate magnet  64 , and the metallic plate  66  cooperatively constitute a solenoid of the present invention.  
         [0037]     Furthermore, a spring  67  (corresponding to the resilient member of the present invention) is provided in the vicinity of the free end of the locking arm  60  where the electromagnetic coil  62  is provided. One end of the spring  67  is connected to the locking arm  60  and the other end of the spring  67  is anchored to the inner cylindrical surface of the motor housing  44 . The spring  67  resiliently urges or pulls the locking arm  60  toward the rotary shaft  50 .  
         [0038]     On the other hand, a rotary lock holder  68  (corresponding to a rotary member of the present invention) is provided on the upper surface of the rotor  48  of the motor  40 . The lock holder  68  is fixed to the rotary shaft  50  and rotates together with the rotor  48 . The lock holder  68  is provided with at least one engaging recess  68   a  (corresponding to an engaging recess of the present invention) that is engageable with the engaging projection  60   a  of the locking arm  60 . According to the embodiment disclosed in  FIGS. 3A and 3B , a total of four engaging recess  68   a  are provided.  
         [0039]     The motor housing  44  of the motor  40  is connected to the upper end of the pinion shaft  12   b . The carrier  58  is connected to the lower end of the steering shaft  12   a  of a universal joint (not shown).  
         [0040]     According to the transmission ratio adjustable steering apparatus  1  of this embodiment, the steering control section  30  inputs a vehicle traveling speed detected by the vehicle speed sensor  27  and a steering angle detected by the steering angle sensor  25 . The steering control section  30  calculates a target steering angle based on the entered vehicle traveling speed and the steering angle according to the steering control program  33   p  executed by CPU  31 . The steering control section  30  outputs a control signal corresponding to the obtained target steering angle to the transmission ratio changing mechanism  14 . The motor  40  of the transmission ratio changing mechanism  14  is driven based on the control signal so as to equalize the actual steered angle of the steerable wheel  24  with the target steering angle.  
         [0041]     The above-described locking mechanism operates in the following manner.  
         [0042]     The steering control section  30 , when the engine is operating (i.e. when the ignition switch is in ON state) and when the motor  40  is not in a failed condition, supplies electric power to the electromagnetic coil  62 . The electromagnetic coil  62  generates electromagnetic force acting in the direction parallel to the metallic plate  66  that is positioned beneath the locking arm  60 . The locking arm  60  is pulled toward the inner cylindrical wall of the motor housing  44  against the spring force of the spring  67 . Namely, the locking arm  60  shifts in the direction departing from the lock holder  68  and accordingly the engaging projection  60   a  of the locking arm  60  disengages from the engaging recess  68   a  of the lock holder  68  as shown in  FIG. 3A  (corresponding to unlocked position of the present invention). When no excessive input is entered from the steerable wheel  24  and when the motor  40  is not in the failed condition, the transmission ratio changing mechanism adjusts a steered angle of the steerable wheel  24  based on the detected vehicle traveling speed.  
         [0043]     On the other hand, when the engine is stopped (i.e. when the ignition switch is in OFF state) or when the motor  40  is in the failed condition, the steering control section  30  stops electric power supply to the electromagnetic coil  62 . The spring  67  resiliently forces the engaging projection  60   a  of the locking arm  60  to move toward the lock holder  68  and forces the engaging projection  60   a  to engage with the engaging recess  68   a  of the lock holder  68 , as shown in  FIG. 4  (corresponding to the locked position of the present invention).  
         [0044]     An engagement of the engaging projection  60   a  and the engaging recess  68   a  is carried out in the following manner.  
         [0045]     In a case that the engaging projection  60   a  of the locking arm  60  agrees with the engaging recess  68   a  of the lock holder  68  in their angular positions in the circumferential direction, the engaging projection  60   a  directly and immediately engages with the engaging recess  68   a . When the engaging projection  60   a  of the locking arm  60  disagrees with the engaging recess  68   a  of the lock holder  68 , the locking operation cannot be accomplished immediately and accordingly the lock holder  68  can rotate relative to the locking arm  60  for a while until the engaging projection  60   a  agrees with the engaging recess  68   a  in their angular positions. Thus, the engaging projection  60   a  immediately engages with the engaging recess  68   a  upon the engaging projection  60   a  agreeing with the engaging recess  68   a  in their angular positions. Accordingly, even when the motor  40  is failed in operation, the transmission ratio can be fixed to a predetermined value and accordingly the driver can safely steer the wheels  24 .  
         [0046]     Furthermore, this embodiment uses the PWM (=Pulse Width Modulation) control to adjust a drive voltage applied to the electromagnetic coil  62 . The steering control section  30  determines the drive voltage applied to the electromagnetic coil  62  according to the steering control program  33   p , and produces a PWM signal based on a duty ratio corresponding to the applied voltage.  
         [0047]     The steering control section  30  (more specifically, CPU 31 ) executes the operational processing for the locking mechanism with the steering control program  33   p . Hereinafter, a first example of the steering control program  33   p  according to the first embodiment of the present invention will be explained with reference to the flowchart of  FIG. 6  together with  FIGS. 3A, 3B , and  4  and the graph of  FIG. 5B . First, it is checked whether or not the ignition switch (not shown in the drawing) is changed from the ON state to the OFF state (refer to step S 1 ). When the condition of the ignition switch is changed to the OFF state (i.e. YES in step S 1 ), the steering control section  30  changes the PWM duty ratio so that the voltage applied to the electromagnetic coil  62  decreases stepwise from V 0  to Vb as shown in  FIG. 5B  (refer to step S 2 ). Then, the steering control section  30  sets a locking flag indicating that the locking mechanism is in operation (refer to step S 3 ). In this condition, the electromagnetic force of the electromagnetic coil  62  decreases by an amount corresponding to the reduction in the applied voltage. In other words, the spring force of the spring  67  becomes larger than the electromagnetic force of the electromagnetic coil  62  and accordingly the engaging projection  60   a  of the locking arm  60  shifts toward the lock holder  68  and is held at a predetermined balancing point. More specifically, when the voltage V 0  is applied to the electromagnetic coil  62 , there is a gap of distance d 11 , between the engaging projection  60   a  and the outer peripheral portion of the lock holder  68  as shown in  FIG. 3A  (corresponding to the unlocked position of the present invention). On the other hand, when the voltage applied to the electromagnetic coil  62  is reduced to Vb, the gap between the engaging projection  60   a  and the outer peripheral portion of the lock holder  68  decreases to distance d 12  as shown in  FIG. 3B  (corresponding to the near side position of the present invention). At this moment, the engaging projection  60   a  is not yet brought into contact with the outer peripheral portion of the lock holder  68  and accordingly the lock holder  68  can rotate continuously.  
         [0048]     When the ignition switch is not changed from the ON state to the OFF state (i.e. NO in step S 1 ), it is checked whether or not the ignition switch is in the OFF state (refer to step S 4 ). When the ignition switch is in the ON state (i.e. NO in step S 4 ), the locking operation is cancelled (refer to step S 10 ). The steering control section  30  releases the locking arm  60  as shown in  FIG. 3A , and executes the processing for the transmission ratio adjustable steering apparatus  1  being in an ordinary condition. When the ignition switch is in the OFF state (i.e. YES in step S 4 ), it is checked whether or not the locking mechanism is in operation (refer to step S 5 ). When the locking mechanism is not in operation (i.e. NO in step S 5 ), the steering control section  30  terminates this processing immediately.  
         [0049]     When the ignition switch is in the OFF state and the locking mechanism is in operation (i.e. YES in step S 5 ), it is checked whether or not a predetermined time Tb (e.g. 5 sec) has passed since the ignition switch has turned from the ON state to the OFF state (refer to step S 6 ). When the predetermined time Tb has not yet passed (i.e. NO in step S 6 ), the steering control section  30  terminates this processing immediately. When the predetermined time Tb has already passed (i.e. YES in step S 6 ), the steering control section  30  reduces the voltage V applied to the electromagnetic coil  62  to  0 V (refer to step S 7 ). Then, the steering control section  30  clears the locking flag that indicates the locking mechanism being in operation (refer to step S 8 ). In this case, the electromagnetic coil  62  generates no electromagnetic force. Thus, the contraction force of the spring  67  causes the engaging projection  60   a  to shift from the condition of  FIG. 3B  to the condition of  FIG. 4 . The engaging projection  60   a  completely engages with the engaging recess  68   a . In other words, the locking arm  60  is locked with the lock holder  68 .  
         [0050]     Tb is a sufficiently long time compared with a time required for the locking arm  60  to accomplish a shifting movement from the condition shown in  FIG. 3A  to the condition shown in  FIG. 3B . In the condition of  FIG. 3A , there is a gap of distance d 11  between the engaging projection  60   a  and an outer peripheral portion of the lock holder  68 . In the condition of  FIG. 3B , the gap between the engaging projection  60   a  and the outer peripheral portion of the lock holder  68  reduces to a distance of d 12 . Furthermore, Tb should be determined considering a time constant determined by an inductance of the electromagnetic coil  62  and a resistance component contained in the electromagnetic coil  62  (i.e. a delay time of the voltage applied to the electromagnetic coil  62  that changes from V 0  to Vb).  
         [0051]     Hereinafter, a second example of the steering control program  33   p  according to the first embodiment of the present invention will be explained with reference to the flowchart of  FIG. 7  together with  FIGS. 3A, 3B , and  4  and the graph of  FIG. 5C . The steering control section  30  (more specifically, CPU 31 ) executes the operational processing for the locking mechanism with this steering control program  33   p . First, it is checked whether or not the ignition switch (not shown in the drawing) is changed from the ON state to the OFF state (refer to step S 11 ). When the condition of the ignition switch is changed to the OFF state (i.e. YES in step S 11 ), the steering control section  30  changes the PWM duty ratio so that the voltage applied to the electromagnetic coil  62  decreases stepwise from V 0  to Vc as shown in  FIG. 5C  (refer to step S 12 ). Then, the steering control section  30  sets a locking flag indicating that the locking mechanism is in operation (refer to step S 13 ). In this condition, the electromagnetic force of the electromagnetic coil  62  decreases by an amount corresponding to a reduction in the applied voltage. In other words, the spring force of the spring  67  becomes larger than the electromagnetic force of the electromagnetic coil  62  and accordingly the engaging projection  60   a  of the locking arm  60  shifts toward the lock holder  68 . More specifically, when the voltage V 0  is applied to the electromagnetic coil  62 , there is a gap of distance d 11  between the engaging projection  60   a  and an outer peripheral portion of the lock holder  68  as shown in  FIG. 3A  (corresponding to the unlocked position of the present invention). On the other hand, when the voltage applied to the electromagnetic coil  62  is reduced to Vc, the gap between the engaging projection  60   a  and the outer peripheral portion of the lock holder  68  decreases to distance d 12  as shown in  FIG. 3B  (corresponding to the near side position of the present invention). At this moment, the engaging projection  60   a  is not yet brought into contact with the outer peripheral portion of the lock holder  68  and accordingly the lock holder  68  can rotate continuously.  
         [0052]     When the ignition switch is not changed from the ON state to the OFF state (i.e. NO in step S 11 ), it is checked whether or not the ignition switch is in the OFF state (refer to step S 14 ). When the ignition switch is in the ON state (i.e. NO in step S 14 ), the locking operation is cancelled (refer to step S 21 ). The steering control section  30  releases the locking arm  60  as shown in  FIG. 3A , and executes the processing for the transmission ratio adjustable steering apparatus  1  being in an ordinary condition. When the ignition switch is in the OFF state (i.e. YES in step S 14 ), it is then checked whether or not the locking mechanism is in operation (refer to step S 15 ). When the locking mechanism is not in operation (i.e. NO in step S 15 ), the steering control section  30  terminates this processing immediately.  
         [0053]     When the ignition switch is in the OFF state and the locking mechanism is in operation (i.e. YES in step S 15 ), the steering control section  30  then checks in step S 16  whether or not it is the time to change the voltage V (i.e. PWM duty ratio) applied to the electromagnetic coil  62 . When it is the time to change the voltage (i.e. YES in step S 16 ), the steering control section  30  changes the PWM duty ratio so that the voltage applied to the electromagnetic coil  62  is reduced by an amount of Vcof (e.g. 0.1V) (refer to step S 17 ). The electromagnetic force of the electromagnetic coil  62  decreases by an amount corresponding to a reduction in the applied voltage. In other words, the spring force of the spring  67  becomes larger than the electromagnetic force of the electromagnetic coil  62  and accordingly the engaging projection  60   a  of the locking arm  60  shifts toward the lock holder  68  and is held at a new balancing point (closer to the lock holder  68 ). When the voltage change timing has not come yet (i.e. NO in step S 16 ), the steering control section  30  skips the step S 17  and proceeds to the next step S 18 .  
         [0054]     According to the voltage characteristics shown in  FIG. 5C , the contraction force of the spring  67  exceeds the electromagnetic force of the electromagnetic coil  62  when the voltage V applied to the electromagnetic coil  62  approaches to V 1 . The spring  67  and the electromagnetic coil  62  cannot maintain a balanced condition. Accordingly, the engaging projection  60   a  engages with the engaging recess  68   a  (refer to  FIG. 4 ).  
         [0055]     After the PWM duty ratio is changed, it is checked whether or not the voltage V applied to the electromagnetic coil  62  has reduced to V 2  (refer to step S 18 ). When the voltage V applied to the electromagnetic coil  62  is larger than V 2  (NO in step S 18 ), the steering control section  30  terminates this processing. When the voltage V applied to the electromagnetic coil  62  is equal to or smaller than V 2  (YES in step S 18 ), the steering control section  30  reduces the voltage V applied to the electromagnetic coil  62  to 0V (refer to step S 19 ). Then, the steering control section  30  clears the locking flag that indicates the locking mechanism being in operation (refer to step S 20 ).  
         [0056]     In this case, voltage V 1  is an averaged voltage necessary for the electromagnetic coil  62  to maintain the engaged condition of the engaging projection  60   a  and the engaging recess  68   a . Furthermore, voltage V 2  is a minimum (or lowest) voltage necessary for the electromagnetic coil  62  to maintain the above engaged condition when various differences of constituent components need to be taken into consideration.  
         [0057]     Hereinafter, a third example of the steering control program  33   p  according to the first embodiment of the present invention will be explained with reference to the flowchart of  FIG. 8  together with  FIGS. 3A, 3B , and  4  and the graph of  FIG. 5D . The steering control section  30  (more specifically, CPU 31 ) executes the operational processing for the locking mechanism with this steering control program  33   p . First, it is checked whether or not the ignition switch (not shown in the drawing) is changed from the ON state to the OFF state (refer to step S 31 ). When the condition of the ignition switch is changed to the OFF state (i.e. YES in step S 31 ), the steering control section  30  changes the PWM duty ratio so that the voltage applied to the electromagnetic coil  62  decreases linearly at a constant rate from V 0  to 0 as shown in  FIG. 5D  (refer to step S 32 ). More specifically, in response to turning-off action of the ignition switch, the steering control section  30  changes the PWM duty ratio to decrease the voltage applied to the electromagnetic coil  62  by an amount of Vdof. Then, the steering control section  30  sets a locking flag indicating that the locking mechanism is in operation (refer to step S 33 ).  
         [0058]     When the ignition switch is not changed from the ON state to the OFF state (i.e. NO in step S 31 ), it is checked whether or not the ignition switch is in the OFF state (refer to step S 34 ). When the ignition switch is in the ON state (i.e. NO in step S 34 ), the locking operation is cancelled (refer to step S 41 ). The steering control section  30  releases the locking arm  60  as shown in  FIG. 3A , and executes the processing for the transmission ratio adjustable steering apparatus  1  being in an ordinary condition. When the ignition switch is in the OFF state (i.e. YES in step S 34 ), it is checked whether or not the locking mechanism is in operation (refer to step S 35 ). When the locking mechanism is not in operation (i.e. NO in step S 55 ), the steering control section  30  terminates this processing immediately.  
         [0059]     When the ignition switch is in the OFF state and the locking mechanism is in operation (i.e. YES in step S 35 ), the steering control section  30  checks whether or not it is the time to change the voltage V (i.e. PWM duty ratio) applied to the electromagnetic coil  62 . When it is the time to change the voltage (i.e. YES in step S 36 ), the steering control section  30  changes the PWM duty ratio so that the voltage applied to the electromagnetic coil  62  decreases by an amount of Vdof (refer to step S 37 ). When the voltage change timing has not come yet (i.e. NO in step S 36 ), the steering control section  30  skips the step S 37  and proceeds to the next step S 38 . Accordingly, an electromagnetic force of the electromagnetic coil  62  decreases by an amount corresponding to a reduction in the applied voltage. In other words, the spring force of the spring  67  becomes larger than the electromagnetic force of the electromagnetic coil  62  and accordingly the engaging projection  60   a  of the locking arm  60  shifts toward the lock holder  68  and is held, as a result, at a balancing point (closer to the lock holder  68 ).  
         [0060]     According to the voltage characteristics shown in  FIG. 5D , the contraction force of the spring  67  exceeds the electromagnetic force of the electromagnetic coil  62  when the voltage V applied to the electromagnetic coil  62  approaches to V 1 . The spring  67  and the electromagnetic coil  62  cannot maintain a balanced condition. Accordingly, the engaging projection  60   a  engages with the engaging recess  68   a  (refer to  FIG. 4 ).  
         [0061]     After the PWM duty ratio is changed, it is checked whether or not the voltage V applied to the electromagnetic coil  62  has reduced to 0 (refer to step S 38 ). When the voltage V applied to the electromagnetic coil  62  is larger than 0 (NO in step S 38 ), the steering control section  30  terminates this processing. When the voltage V applied to the electromagnetic coil  62  is equal to or smaller than 0 (YES in step S 38 ), it is regarded that voltage V applied to the electromagnetic coil  62  is 0V (refer to step S 39 ). Then, the steering control section  30  clears the locking flag that indicates the locking mechanism being in operation (refer to step S 40 ).  
         [0062]     In this third example, Vdof represents a decreasing rate (V 0 /Td) of the voltage V applied to the electromagnetic coil  62 . Although the Vdof is set to be constant in this example, it is possible to change Vdof at the final stage of the shifting movement of the engaging projection  60   a  approaching to the lock holder  68  (e.g. when the voltage V decreases below the voltage value V 1 ).  
         [0063]     Besides the above-described arrangement, as shown in  FIG. 11A , it is possible to attach a rubber or resilient member  60   b  to the engaging projection  60   a  or attach a rubber or resilient member  68   b  to the engaging recess  68   a . In other words, it is desirable to provide a rubber or comparable resilient member to suppress the noise sounds generating when the engaging projection  60   a  engages with the engaging recess  68   a . For example, this arrangement allows the steering control section  30  to use the voltage characteristics shown in  FIG. 5A , instead of employing the operational processing for the locking mechanism described above. According to  FIG. 5A , when the ignition switch changes from the ON state to the OFF state, the voltage V applied to the electromagnetic coil  62  immediately drops to 0V. Even if the voltage V decreases abruptly, the resilient member attached to the engaging projection  60   a  and/or to the engaging recess  68   a  can effectively reduce the noise sounds generating when the locking arm  60  engages with the lock holder  68 .  
         [0064]     Furthermore, as shown in  FIG. 11B , it is possible to constitute the spring  67  by a combination of two springs  67   a  and  67   b  which are connected in series and different in their elastic modulus. Using the combination of two springs  67   a  and  67   b  makes it possible to further reduce the noise sounds generating when the engaging projection  60   a  engages with the engaging recess  68   a . According to this arrangement, one spring  67   a  with a larger elastic modulus contracts first and then the other spring  67   b  with a smaller elastic modulus contracts next. In other words, the shifting speed of the locking arm  60  becomes small at the final stage of the contracting process of the composite spring  67 . This is effective in absorbing the shock (and accordingly in suppressing noise sounds) occurring in the engagement of the engaging projection  60   a  with the engaging recess  68   a . For example, this arrangement allows the steering control section  30  to use the voltage characteristics shown in  FIG. 5A , instead of employing the operational processing for the locking mechanism described above. As described above, according to  FIG. 5A , the voltage V applied to the electromagnetic coil  62  immediately drops to 0V in response to turning-off action of the ignition switch. Even if the voltage V decreases abruptly, the composite spring  67  consisting of two springs  67   a  and  67   b  that are connected in series and different in their elastic modulus can effectively reduce noise sounds generating when the locking arm  60  engages with the lock holder  68 .  
         [0065]     Furthermore, it is possible to employ both of the above-proposed arrangements shown in  FIGS. 11A and 11B . In this case, a rubber or resilient member is attached to at least one of the engaging projection  60   a  of the locking arm  60  and the engaging recess  68   a  or the lock holder  68 . The spring  67  is constituted by a combination of two springs  67   a  and  67   b  which are connected in series and different in their elastic modulus. This is effective in further reducing the noise sounds generating when the engaging projection  60   a  engages with the engaging recess  68   a.    
         [0066]     For example, this arrangement allows the steering control section  30  to use the voltage characteristics shown in  FIG. 5A , instead of employing the operational processing for the locking mechanism described above. As described above, according to  FIG. 5A , the voltage V applied to the electromagnetic coil  62  immediately drops to 0V in response to turning-off action of the ignition switch. Even if the voltage V decreases abruptly, the resilient member attached to the engaging projection  60   a  and/or to the engaging recess  68   a  and the composite spring  67  consisting of two springs  67   a  and  67   b  that are connected in series and different in their elastic modulus can effectively reduce noise sounds generating when the locking arm  60  engages with the lock holder  68 .  
       Second Embodiment  
       [0067]     Next, with reference to  FIGS. 9A and 9B  and  10 , a transmission ratio adjustable steering apparatus will be explained with a second embodiment of the present invention. The transmission ratio adjustable steering apparatus according to the second embodiment is different from the first embodiment shown in  FIG. 1  in that the locking mechanism (refer to  FIG. 2  or  FIGS. 3A and 3B ) of the transmission ratio adjustable steering apparatus  1  is replaced with an arrangement shown in  FIGS. 9A and 9B . Accordingly, except for the locking mechanism, the transmission ratio adjustable steering apparatus according to the second embodiment is structurally identical with the transmission ratio adjustable steering apparatus of the first embodiment. Thus, the same reference numerals are attached to the components identical with those of the transmission ratio adjustable steering apparatus  1  of the first embodiment already explained with reference to FIGS.  1  to  8 .  
         [0068]     In  FIGS. 9A and 9B , a locking pin  70  (corresponding to a coupling member of the present invention) is rotatably attached to a rotary base  72 . The rotary base  72  is fixed to the motor housing. A rear end portion  73  is connected to a solenoid  75  (corresponding to a solenoid of the present invention). Furthermore, a coil spring  71  (corresponding to a resilient member of the present invention) is provided around the rotary base  72  to resiliently return the locking pin  70  to its original position when the solenoid  75  is deactivated. According to this arrangement, the solenoid  75  generates an electromagnetic force in accordance with an applied voltage. An electromagnetic force produced from the solenoid  75  causes the locking pin  70  to rotate about the rotary base  72  against a resilient force of the coil spring  71 . The engaging projection  70   a  (corresponding to an engaging hook of the present invention) of the locking pin  70  separates or disengages from the engaging recess  68   a  (corresponding to an engaging recess of the present invention) of the lock holder  68 . This arrangement enables the locking pin  70  to rotate about the rotary base  72  so that the engaging projection  70   a  selectively engages with or disengages from the engaging recess  68   a . The solenoid  75  is duty driven by the PWM control.  
         [0069]     The steering control section  30  (more specifically, CPU 31 ) executes the operational processing for the locking mechanism with the steering control program  33   p . Hereinafter, a first example of the steering control program  33   p  according to the second embodiment of the present invention will be explained with reference to the flowchart of  FIG. 6  together with  FIGS. 9A, 9B , and  10  and the graph of  FIG. 5B . First, it is checked whether or not the ignition switch (not shown in the drawing) is changed from the ON state to the OFF state (refer to step S 1 ). When the condition of the ignition switch is changed to the OFF state (i.e. YES in step S 1 ), the steering control section  30  changes the PWM duty ratio so that the voltage V applied to the solenoid  75  decreases stepwise from V 0  to Vb as shown in  FIG. 5B  (refer to step S 2 ). Then, the steering control section  30  sets a locking flag indicating that the locking mechanism is in operation (refer to step S 3 ). In this condition, a pulling force of the solenoid  75  decreases by an amount corresponding to the reduction in the applied voltage. In other words, the spring force of the coil spring  71  becomes larger than the pulling force of the solenoid  75  and accordingly the engaging projection  70   a  of the locking pin  70  shifts toward the lock holder  68  and is held at a predetermined balancing point. More specifically, when the voltage V 0  is applied to the solenoid  75 , there is a gap of distance d 21  between the engaging projection  70   a  and an outer peripheral portion of the lock holder  68  as shown in  FIG. 9A  (corresponding to the unlocked position of the present invention). On the other hand, when the voltage applied to the solenoid  75  is reduced to Vb, the gap between the engaging projection  70   a  and the outer peripheral portion of the lock holder  68  decreases to distance d 22  as shown in  FIG. 9B  (corresponding to the near side position of the present invention). At this moment, the engaging projection  70   a  is not yet brought into contact with the outer peripheral portion of the lock holder  68  and accordingly the lock holder  68  can rotate continuously.  
         [0070]     When the ignition switch is not changed from the ON state to the OFF state (i.e. NO in step S 1 ), it is checked whether or not the ignition switch is in the OFF state (refer to step S 4 ). When the ignition switch is in the ON state (i.e. NO in step S 4 ), the locking operation is cancelled (refer to step S 10 ). The steering control section  30  releases the locking pin  70  as shown in  FIG. 9A , and executes the processing for the transmission ratio adjustable steering apparatus  1  being in an ordinary condition. When the ignition switch is in the OFF state (i.e. YES in step S 4 ), it is checked whether or not the locking mechanism is in operation (refer to step S 5 ). When the locking mechanism is not in operation (i.e. NO in step S 5 ), the steering control section  30  terminates this processing immediately.  
         [0071]     When the ignition switch is in the OFF state and the locking mechanism is in operation (i.e. YES in step S 5 ), it is checked whether or not a predetermined time Tb (e.g. 5 sec) has passed since the ignition switch has turned from the ON state to the OFF state (refer to step S 6 ). When the predetermined time Tb has not yet passed (i.e. NO in step S 6 ), the steering control section  30  terminates this processing immediately. When the predetermined time Tb has already passed (i.e. YES in step S 6 ), the steering control section  30  reduces the voltage V applied to the solenoid  75  to 0V (refer to step S 7 ). Then, the steering control section  30  clears the locking flag that indicates the locking mechanism being in operation (refer to step S 8 ). In this case, the solenoid  75  generates no pulling force. Thus, a rotational force of the coil spring  71  causes the engaging projection  70   a  to shift from the condition of  FIG. 9B  to the condition of  FIG. 10 . The engaging projection  70   a  completely engages with the engaging recess  68   a . In other words, the locking pin  70  is locked with the lock holder  68 .  
         [0072]     Tb is a sufficiently long time compared with a time required for the locking pin  70  to accomplish a shifting movement from the condition shown in  FIG. 9A  to the condition shown in  FIG. 9B . In the condition of  FIG. 9A , there is a gap of distance d 21  between the engaging projection  70   a  and an outer peripheral portion of the lock holder  68 . In the condition of  FIG. 9B , the gap between the engaging projection  70   a  and the outer peripheral portion of the lock holder  68  reduces to a distance of d 22 . Furthermore, Tb should be determined considering a time constant determined by an inductance of the solenoid  75  and a resistance component contained in the solenoid  75  (i.e. a delay time of the drive voltage applied to the solenoid  75  that changes from V 0  to Vb).  
         [0073]     Hereinafter, a second example of the steering control program  33   p  according to the second embodiment of the present invention will be explained with reference to the flowchart of  FIG. 7  together with  FIGS. 9A, 9B , and  10  and the graph of  FIG. 5C . The steering control section  30  (more specifically, CPU 31 ) executes the operational processing for the locking mechanism with this steering control program  33   p . First, it is checked whether or not the ignition switch (not shown in the drawing) is changed from the ON state to the OFF state (refer to step S 11 ). When the condition of the ignition switch is changed to the OFF state (i.e. YES in step S 11 ), the steering control section  30  changes the PWM duty ratio so that the voltage V applied to the solenoid  75  decreases stepwise from V 0  to Vc as shown in  FIG. 5C  (refer to step S 12 ). Then, the steering control section  30  sets a locking flag indicating that the locking mechanism is in operation (refer to step S 13 ). In this condition, the pulling force of the solenoid  75  decreases by an amount corresponding to a reduction in the applied voltage. In other words, a spring force of the coil spring  71  becomes larger than the pulling force of the solenoid  75  and accordingly the engaging projection  70   a  of the locking pin  70  shifts toward the lock holder  68 . More specifically, when the voltage V 0  is applied to the solenoid  75 , there is a gap of distance d 21  between the engaging projection  70   a  and an outer peripheral portion of the lock holder  68  as shown in  FIG. 9A  (corresponding to the unlocked position of the present invention). On the other hand, when the voltage applied to the solenoid  75  is reduced to Vc, the gap between the engaging projection  70   a  and the outer peripheral portion of the lock holder  68  decreases to a distance d 22  as shown in  FIG. 9B  (corresponding to the near side position of the present invention). At this moment, the engaging projection  70   a  is not yet brought into contact with the outer peripheral portion of the lock holder  68  and accordingly the lock holder  68  can rotate continuously.  
         [0074]     When the ignition switch is not changed from the ON state to the OFF state (i.e. NO in step S 11 ), it is checked whether or not the ignition switch is in the OFF state (refer to step S 14 ). When the ignition switch is in the ON state (i.e. NO in step S 14 ), the locking operation is cancelled (refer to step S 21 ). The steering control section  30  releases the locking pin  70  as shown in  FIG. 9A , and executes the processing for the transmission ratio adjustable steering apparatus  1  being in an ordinary condition. When the ignition switch is in the OFF state (i.e. YES in step S 14 ), it is then checked whether or not the locking mechanism is in operation (refer to step S 15 ). When the locking mechanism is not in operation (i.e. NO in step S 15 ), the steering control section  30  terminates this processing immediately.  
         [0075]     When the ignition switch is in the OFF state and the locking mechanism is in operation (i.e. YES in step S 15 ), the steering control section  30  then checks in step S 16  whether or not it is the time to change the voltage V (i.e. PWM duty ratio) applied to the solenoid  75 . When it is the time to change the voltage (i.e. YES in step S 16 ), the steering control section  30  changes the PWM duty ratio so that the voltage applied to the solenoid  75  is reduced by an amount of Vcof (e.g. 0.1V) (refer to step S 17 ). A pulling force of the solenoid  75  decreases by an amount corresponding to a reduction in the applied voltage. In other words, the spring force of the coil spring  71  becomes larger than the pulling force of the solenoid  75  and accordingly the engaging projection  70   a  of the locking pin  70  shifts toward the lock holder  68  and is held, as a result, at a new balancing point (closer to the lock holder  68 ). When the voltage change timing has not come yet (i.e. NO in step S 16 ), the steering control section  30  skips the step S 17  and proceeds to the next step S 18 .  
         [0076]     According to the voltage characteristics shown in  FIG. 5C , the rotational force of the coil spring  71  exceeds the pulling force of the solenoid  75  when the voltage V applied to the solenoid  75  approaches to V 1 . The coil spring  71  and the solenoid  75  cannot maintain a balanced condition. Accordingly, the engaging projection  70   a  engages with the engaging recess  68   a  (refer to  FIG. 10 ).  
         [0077]     After the PWM duty ratio is changed, it is checked whether or not the voltage V applied to the solenoid  75  has reduced to V 2  (refer to step S 18 ). When the voltage V applied to the solenoid  75  is larger than V 2  (NO in step S 18 ), the steering control section  30  terminates this processing. When the voltage V applied to the solenoid  75  is equal to or smaller than V 2  (YES in step S 18 ), the steering control section  30  reduces the voltage V applied to the solenoid  75  to 0V (refer to step S 19 ). Then, the steering control section  30  clears the locking flag that indicates the locking mechanism being in operation (refer to step S 20 ).  
         [0078]     In this case, voltage V 1  is an averaged voltage necessary for the solenoid  75  to maintain the engaged condition of the engaging projection  70   a  and the engaging recess  68   a . Furthermore, voltage V 2  is a minimum (or lowest) voltage necessary for the solenoid  75  to maintain the above engaged condition when various differences of constituent components need to be taken into consideration.  
         [0079]     Hereinafter, a third example of the steering control program  33   p  according to the second embodiment of the present invention will be explained with reference to the flowchart of  FIG. 8  together with  FIGS. 9A, 9B , and  10  and the graph of  FIG. 5D . The steering control section  30  (more specifically, CPU 31 ) executes the operational processing for the locking mechanism with this steering control program  33   p . First, it is checked whether or not the ignition switch (not shown in the drawing) is changed from the ON state to the OFF state (refer to step S 31 ). When the condition of the ignition switch is changed to the OFF state (i.e. YES in step S 31 ), the steering control section  30  changes the PWM duty ratio so that the voltage V applied to the solenoid  75  decreases linearly at a constant rate from V 0  to 0 as shown in  FIG. 5D  (refer to step S 32 ). More specifically, in response to turning-off action of the ignition switch, the steering control section  30  changes the PWM duty ratio to decrease the voltage V applied to the solenoid  75  by an amount of Vdof. Then, the steering control section  30  sets a locking flag indicating that the locking mechanism is in operation (refer to step S 33 ).  
         [0080]     When the ignition switch is not changed from the ON state to the OFF state (i.e. NO in step S 31 ), it is then checked whether or not the ignition switch is in the OFF state (refer to step S 34 ). When the ignition switch is in the ON state (i.e. NO in step S 34 ), the locking operation is cancelled (refer to step S 41 ). The steering control section  30  releases the locking pin  70  as shown in  FIG. 9A , and executes the processing for the transmission ratio adjustable steering apparatus  1  being in an ordinary condition. When the ignition switch is in the OFF state (i.e. YES in step S 34 ), it is checked whether or not the locking mechanism is in operation (refer to step S 35 ). When the locking mechanism is not in operation (i.e. NO in step S 55 ), the steering control section  30  terminates this processing immediately.  
         [0081]     When the ignition switch is in the OFF state and the locking mechanism is in operation (i.e. YES in step S 35 ), the steering control section  30  checks whether or not it is the time to change the voltage V (i.e. PWM duty ratio) applied to the solenoid  75 . When it is the time to change the voltage (i.e. YES in step S 36 ), the steering control section  30  changes the PWM duty ratio so that the voltage V applied to the solenoid  75  decreases by an amount of Vdof (refer to step S 37 ). On the other hand, when the voltage change timing has not come yet (i.e. NO in step S 36 ), the steering control section  30  skips the step S 37  and proceeds to the next step S 38 . Accordingly, a pulling force of the solenoid  75  decreases by an amount corresponding to a reduction in the applied voltage. In other words, the spring force of the coil spring  71  becomes larger than the pulling force of the solenoid  75  and accordingly the engaging projection  70   a  of the locking pin  70  shifts toward the lock holder  68  and is held, as a result, at a balancing point (closer to the lock holder  68 ).  
         [0082]     According to the voltage characteristics shown in  FIG. 5D , the rotational force of the coil spring  71  exceeds the pulling force of the solenoid  75  when the voltage V applied to the solenoid  75  approaches to V 1 . The coil spring  71  and the solenoid  75  cannot maintain a balanced condition. Accordingly, the engaging projection  70   a  engages with the engaging recess  68   a  (refer to  FIG. 10 ).  
         [0083]     After the PWM duty ratio is changed, it is checked whether or not the voltage V applied to the solenoid  75  has reduced to 0 (refer to step S 38 ). When the voltage V applied to the solenoid  75  is larger than 0 (NO in step S 38 ), the steering control section  30  terminates this processing. When the voltage V applied to the solenoid  75  is equal to or smaller than 0 (YES in step S 38 ), it is regarded that the voltage V applied to the solenoid  75  is 0V (refer to step S 39 ). Then, the steering control section  30  clears the locking flag that indicates the locking mechanism being in operation (refer to step S 40 ).  
         [0084]     In this third example, Vdof represents a decreasing rate (V 0 /Td) of the voltage V applied to the solenoid  75 . Although the Vdof is set to be constant in this example, it is possible to change Vdof at the final stage of the shifting movement of the engaging projection  70   a  approaching to the lock holder  68  (e.g. when the voltage V decreases below the voltage value V 1 ).  
         [0085]     Besides the above-described arrangement, as shown in  FIG. 12 , it is possible to attach a rubber or resilient member  70   b  to the engaging projection  70   a  or attach a rubber or resilient member  68   b  to the engaging recess  68   a . In other words, it is desirable to provide a rubber or comparable resilient member to suppress the noise sounds generating when the engaging projection  70   a  engages with the engaging recess  68   a . For example, this arrangement allows the steering control section  30  to use the voltage characteristics shown in  FIG. 5A , instead of employing the operational processing for the locking mechanism described above. According to  FIG. 5A , when the ignition switch changes from the ON state to the OFF state, the voltage V applied to the solenoid  75  immediately drops to 0V. Even if the voltage V decreases abruptly, the resilient member attached to the engaging projection  70   a  and/or to the engaging recess  68   a  can effectively reduce the noise sounds generating when the locking pin  70  engages with the lock holder  68 .  
         [0086]     Furthermore, although not shown in the drawing, it is possible to constitute the coil spring  71  by a combination of two coil springs which are connected in series and different in their elastic modulus. Using the combination of two coil springs makes it possible to further reduce the noise sounds generating when the engaging projection  70   a  engages with the engaging recess  68   a . According to this arrangement, the coil spring having a larger elastic modulus rotates first and then the coil spring having a smaller elastic modulus rotates next. In other words, the shifting speed of the locking pin  70  becomes small at the final stage of the rotating process of the composite coil spring  71 . This is effective in absorbing the shock (and accordingly in suppressing the noise sounds) occurring in the engagement of the engaging projection  70   a  and the engaging recess  68   a . For example, this arrangement allows the steering control section  30  to use the voltage characteristics shown in  FIG. 5A , instead of employing the operational processing for the locking mechanism described above. As described above, according to  FIG. 5A , the voltage V applied to the solenoid  75  immediately drops to 0V in response to turning-off action of the ignition switch. Even if the voltage V decreases abruptly, the composite coil spring  71  consisting of two coil springs that are connected in series and different in their elastic modulus can effectively reduce the noise sounds generating when the locking pin  70  engages with the lock holder  68 .  
         [0087]     Furthermore, it is possible to employ both of the above-proposed arrangements. In this case, a rubber or resilient member is attached to at least one of the engaging projection  70   a  of the locking pin  70  and the engaging recess  68   a  or the lock holder  68 . The coil spring  71  is constituted by a combination of two coil springs which are connected in series and different in their elastic modulus. This is effective in further reducing the noise sounds generating when the engaging projection  70   a  engages with the engaging recess  68   a . For example, this arrangement allows the steering control section  30  to use the voltage characteristics shown in  FIG. 5A , instead of employing the operational processing for the locking mechanism described above. As described above, according to  FIG. 5A , the voltage V applied to the solenoid  75  immediately drops to 0V in response to turning-off action of the ignition switch. Even if the voltage V decreases abruptly, the resilient member attached to the engaging projection  70   a  and/or to the engaging recess  68   a  and the composite coil spring  71  consisting of two coil springs that are connected in series and different in their elastic modulus can effectively reduce noise sounds generating when the locking pin  70  engages with the lock holder  68 .  
         [0088]     The above-described embodiments of the present invention are mere practical examples, and accordingly the present invention is not limited to these embodiments and can be modified in various ways without departing from the scope of the present invention.