Abstract:
A SBW control system is disclosed which is powered by an electric power source on a vehicle to electrically control mode shifting of an automatic transmission of the vehicle. The SBW control system includes: 1) means for inputting a shift command to shift a current operating mode of the automatic transmission to a desired operating mode; 2) means for shifting the current operating mode to the desired operating mode according to the shift command; 3) means for locking an output shaft of the automatic transmission when the desired operating mode is P (park) mode and unlocking the same otherwise; 4) means for detecting an abnormal condition of the vehicle; and 5) means for controlling the locking/unlocking means in such a manner that when the abnormal condition is detected, the output shaft of the automatic transmission is unlocked regardless of whether or not the desired operating mode is P mode.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is based on and claims priority from Japanese Patent Application No. 2008-127659, filed on May 14, 2008, the content of which is hereby incorporated by reference in its entirety into this application. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Technical Field of the Invention 
         [0003]    The present invention relates to Shift-By-Wire (SBW) control systems for automatic transmissions of motor vehicles. 
         [0004]    2. Description of the Related Art 
         [0005]    In recent years, there has been a growing tendency of replacing mechanical drive systems with electrical drive systems in motor vehicles, so as to meet the requirements of saving space and improving assembly efficiency and controllability. As an example, there have been developed SBW control systems which electrically control the shifting of operating modes of automatic transmissions in motor vehicles. 
         [0006]    Japanese Patent First Publication No. 2002-243033 discloses a SBW control system for an automatic transmission of a motor vehicle, which is configured to forcibly shift the current operating mode of the automatic transmission to P (park) mode before the engine of the vehicle stops. With this SBW control system, when a shift command to shift the current operating mode of the automatic transmission to P mode is inputted by the driver of the vehicle, it is possible to prevent the current operating mode from being erroneously shifted to any other operating mode than P mode. Generally, in P mode, a parking lock is applied to the automatic transmission to lock the output shaft of the automatic transmission; in other operating modes than P mode, the parking lock is released. Therefore, with the above SBW control system, it is possible to reliably prevent the vehicle from being stolen when parked, thereby ensuring high security of the vehicle. 
         [0007]    However, the above SBW control system can function only when it is powered by an electric power source provided on the vehicle. Therefore, when the power supply from the electric power source to the SBW control system is interrupted with the engine of the vehicle stopped, it is impossible to release the parking lock applied to the automatic transmission. Consequently, when the parked vehicle is in an abnormal condition and it is thus required to move the vehicle to another place (e.g., a repair shop), it is difficult to meet the requirement. Accordingly, the SBW control system may lack in high fail-safe capability. 
       SUMMARY OF THE INVENTION 
       [0008]    The present invention has been made in view of the above-mentioned problems. 
         [0009]    It is, therefore, an object of the present invention to provide a SBW control system for an automatic transmission of a motor vehicle, which has high fail-safe capability as well as ensuring high security of the vehicle. 
         [0010]    According to the present invention, there is provided a SBW control system which is configured to be powered by an electric power source on a vehicle to electrically control mode shifting of an automatic transmission of the vehicle. The SBW control system includes: 1) means for inputting a shift command from a driver of the vehicle to shift a current operating mode of the automatic transmission to a desired operating mode; 2) means for shifting the current operating mode of the automatic transmission to the desired operating mode according to the shift command inputted by the shift command inputting means; 3) means for locking an output shaft of the automatic transmission when the desired operating mode is P (park) mode and unlocking the output shaft when the desired operating mode is not P mode; 4) means for detecting an abnormal condition of the vehicle; and 5) means for controlling the locking/unlocking means in such a manner that when the abnormal condition of the vehicle is detected by the detecting means, the output shaft of the automatic transmission is unlocked regardless of whether or not the desired operating mode is P mode. 
         [0011]    With the above configuration, when the vehicle is in a normal condition and the desired operating mode of the automatic transmission indicated by the shift command is P mode, the shifting means shifts the current operating mode of the automatic transmission to P mode and the locking/unlocking means locks the output shaft of the automatic transmission. Consequently, it is possible to reliably prevent the vehicle from being stolen when parked, ensuring high security of the vehicle. On the other hand, when the abnormal condition of the vehicle is detected by the detecting means, the controlling means controls the locking/unlocking means in such a manner that the output shaft of the automatic transmission is unlocked regardless of whether or not the desired operating mode is P mode. Consequently, even when the power supply from the electric power source to the SBW control system is interrupted due to the abnormal condition of the vehicle, it is still possible to move the vehicle to a desired place (e.g., a repair shop), thus ensuring high fail-safe capability of the SBW control system. 
         [0012]    According to a further implementation of the invention, in the SBW control system, when the abnormal condition of the vehicle is detected by the detecting means with the current operating mode of the automatic transmission being P mode, the controlling means controls the locking/unlocking means to unlock the output shaft of the automatic transmission. 
         [0013]    More specifically, the controlling means controls the shifting means to shift the current operating mode of the automatic transmission from P mode to N (neutral) mode, thereby causing the locking/unlocking means to unlock the output shaft of the automatic transmission. 
         [0014]    Moreover, when the abnormal condition of the vehicle is detected by the detecting means with the current operating mode of the automatic transmission being P mode, the controlling means first determines whether a mechanical brake of the vehicle is activated. When it is determined that the mechanical brake is activated, the controlling means further controls the locking/unlocking means to unlock the output shaft of the automatic transmission. 
         [0015]    The SBW control system further includes means for inputting an ON command to turn on an engine of the vehicle and an OFF command to turn off the engine. The locking/unlocking means unlocks the output shaft of the automatic transmission both when the desired operating mode of the automatic transmission is P mode and when the OFF command is inputted by the ON/OFF commands inputting means. When the abnormal condition of the vehicle is detected by the detecting means, the controlling means controls the locking/unlocking means in such a manner that the output shaft of the automatic transmission is unlocked regardless of whether or not the desired operating mode is P mode and whether or not the OFF command is inputted by the ON/OFF commands inputting means. 
         [0016]    Moreover, when the abnormal condition of the vehicle is detected by the detecting means with the current operating mode of the automatic transmission being not P mode, the controlling means controls the locking/unlocking means to keep the output shaft of the automatic transmission unlocked regardless of whether or not the desired operating mode is P mode and whether or not the OFF command is inputted by the ON/OFF commands inputting means. 
         [0017]    More specifically, the controlling means controls the shifting means to shift or keep the current operating mode of the automatic transmission to or in N (neutral) mode, thereby allowing the locking/unlocking means to keep the output shaft of the automatic transmission unlocked. 
         [0018]    Furthermore, the SBW control system further includes means for outputting a warning. With the output shaft of the automatic transmission unlocked, the controlling means further determines whether a parking brake of the vehicle is activated or deactivated. When it is determined that the parking brake is deactivated, the controlling means controls the warning outputting means to output the warning. 
         [0019]    In the SBW control system, when the abnormal condition of the vehicle is detected by the detecting means with the current operating mode of the automatic transmission being not P mode, the controlling means first determines whether the vehicle is stopped. When it is determined that the vehicle is stopped, the controlling means further controls the shifting means to shift or keep the current operating mode of the automatic transmission to or in N (neutral) mode, thereby allowing the locking/unlocking means to keep the output shaft of the automatic transmission unlocked. 
         [0020]    In the SBW control system, the detecting means may detect, as the abnormal condition of the vehicle, a collision of the vehicle by checking whether an airbag of the vehicle is activated. 
         [0021]    The detecting means may also detect, as the abnormal condition of the vehicle, a power failure in the vehicle by checking whether an output voltage of the electric power source is decreased to below a minimum starting voltage of an engine of the vehicle. 
         [0022]    Further, the detecting means may detect the power failure in the vehicle by checking whether the output voltage of the electric power source is kept below the minimum starting voltage of the engine for longer than a predetermined time. 
         [0023]    It is also possible for the detecting means to detect, as the abnormal condition of the vehicle, a submersion of the vehicle. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]    The present invention will be understood more fully from the detailed description given hereinafter and from the accompanying drawings of one preferred embodiment of the invention, which, however, should not be taken to limit the invention to the specific embodiment but are for the purpose of explanation and understanding only. 
           [0025]    In the accompanying drawings: 
           [0026]      FIG. 1  is a schematic view showing the overall configuration of a SBW control system according to the preferred embodiment of the invention; 
           [0027]      FIG. 2  is a plan view showing the configuration of a shift switch of the SBW control system; 
           [0028]      FIG. 3  is a perspective view showing the configuration of a motion converter of the SBW control system; 
           [0029]      FIG. 4  is a cross-sectional view taken along the line IV-IV in  FIG. 3 ; 
           [0030]      FIG. 5  is a flow chart illustrating a fail-safe control process for responding to a collision of the vehicle according to the preferred embodiment; 
           [0031]      FIG. 6  is a flow chart illustrating a fail-safe control process for responding to a power failure in the vehicle according to the preferred embodiment; and 
           [0032]      FIG. 7  is a flow chart illustrating a fail-safe control process for responding to a submersion of the vehicle according to the preferred embodiment. 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENT 
       [0033]      FIG. 1  shows the overall configuration of a Shift-By-Wire (SBW) control system  2  according to a preferred embodiment of the invention. The SBW control system  2  is installed, along with an automatic transmission  3  and an internal combustion engine  4 , in a motor vehicle to electrically control mode shifting of the automatic transmission  3 . 
         [0034]    As shown in  FIG. 1 , the SBW control system  2  is made up of an automatic transmission control device  10 , a mode selector  20 , a shift control device  30 , an engine control device  40 , and a warning device  50 . 
         [0035]    The automatic transmission control device  10  includes a hydraulic circuit  12  for driving the automatic transmission  3 . The hydraulic circuit  12  includes a spool valve  14  which has a spool that moves linearly. The operating modes of the automatic transmission  3  are shifted from one to another by the output hydraulic pressure of the hydraulic circuit  12  which depends on the position of the spool of the spool valve  14 . 
         [0036]    In the present embodiment, the operating modes of the automatic transmission  3  include non-transmitting modes, in each of which no torque is transmitted by the automatic transmission  3  from the engine  4  to drive wheels of the vehicle, and transmitting modes in each of which the output torque of the engine  4  is transmitted by the automatic transmission  3  to the drive wheels. 
         [0037]    The non-transmitting modes include N (neutral) mode and P (park) mode. In N mode, the automatic transmission  3  is disconnected from the drive wheels of the vehicle so that the vehicle can move freely under its own weight. In P mode, a parking lock is applied to the automatic transmission  3 , restricting the vehicle from moving in any direction. 
         [0038]    The transmitting modes include D (drive) mode, B (brake) mode, and R (reverse) mode. In D mode, the automatic transmission  3  transmits the output torque of the engine  4  to the drive wheels of the vehicle, thereby moving the vehicle forward. In B mode, the automatic transmission  3  transmits torque from the drive wheels of the vehicle to the engine  4 , thereby applying engine brake to the vehicle that is moving forward. In R mode, the automatic transmission  3  transmits the output torque of the engine  4  to the drive wheels of the vehicle, thereby moving the vehicle backward. 
         [0039]    In addition, in the present embodiment, the position of the spool of the spool valve  14  corresponding to D mode of the automatic transmission  3  is set to be the same as that corresponding to B mode of the same. Therefore, D and B modes of the automatic transmission  3  are distinguished from one another by the operating conditions of other components in the engine  4  or in the hydraulic circuit  12  than the spool valve  14 . 
         [0040]    The mode selector  20  includes a parking switch  21  and a shift switch  22 . The parking switch  21  is provided for the vehicle driver to input a shift command that commands the SBW control system  2  to shift the current operating mode of the automatic transmission  3  to P mode. The shift switch  22  is provided for the vehicle driver to input shift commands each commanding the SBW control system  2  to shift the current operating mode of the automatic transmission  3  to a corresponding one of D, B, R, and N modes. The parking switch  21  is located in the vicinity of the driver&#39;s seat in the vehicle. The shift command to shift the current operating mode of the automatic transmission  3  to P mode is inputted by a predetermined manipulation (e.g., a button manipulation or a lever manipulation) to the parking switch  21 . Upon input of the shift command to shift the current operating mode of the automatic transmission  3  to P mode, the parking switch  21  generates a signal that indicates the inputted shift command. The shift switch  22  is also located in the vicinity of the driver&#39;s seat in the vehicle. Each of the shift commands to shift the current operating mode of the automatic transmission  3  to the corresponding ones of D, B, R, and N modes is inputted by a predetermined manipulation of the shift switch  22 . 
         [0041]    More specifically, referring to  FIG. 2 , in the present embodiment, the shift switch  22  includes a shift lever  23  and a shift groove  24  along which the shift lever  23  is moved. The shift lever  23  has, the shift switch  22 , I (initial) position, D (drive) position that corresponds to D mode of the automatic transmission  3 , B (brake) position that corresponds to B mode of the automatic transmission  3 , R (reverse) position that corresponds to R mode of the automatic transmission  3 , and N (neutral) position that corresponds to N mode of the automatic transmission  3 . Each of the shift commands is inputted by moving the shift lever  23  along the shift groove  24  to the corresponding one of D, B, R, and N positions. Upon input of each of the shift commands, the shift switch  22  generates a signal that indicates the inputted shift command. 
         [0042]    Referring back to  FIG. 1 , the shift control device  30  includes a shift actuator  32 , a motion converter  33 , a shift control circuit  34 , and a rotational position sensor  36 . 
         [0043]    The shift actuator  32  is implemented by, for example, an electric actuator which includes an electric motor and a speed reducer. When energized, the shift actuator  32  generates torque and outputs the generated torque via a rotating shaft  32   a  thereof. 
         [0044]    The motion converter  33  converts the rotational motion of the rotating shaft  32   a  of the shift actuator  32  into the linear motion of the spool of the spool valve  14 . Consequently, with the motion converter  33 , the operating modes of the automatic transmission  3  are shifted from one to another according to the rotational position of the rotating shaft  32   a  of the shift actuator  32 . 
         [0045]    In the present embodiment, the rotating shaft  32   a  of the shift actuator  32  has, in its rotational direction, P, R, N, D, and B positions which respectively correspond to P, R, N, D, and B modes of the automatic transmission  3 . 
         [0046]    In addition, as described previously, in the present embodiment, the position of the spool of the spool valve  14  corresponding to D mode of the automatic transmission  3  is set to be the same as that corresponding to B mode of the same. Accordingly, D position of the rotating shaft  32   a  of the shift actuator  32  is set to be the same as B position of the same. 
         [0047]    The rotational position sensor  36  senses the rotational position of the rotating shaft  32   a  of the shift actuator  32  and outputs a signal that indicates the sensed rotational position. Accordingly, based on the signal output from the rotational position sensor  36 , it is possible to determine the current rotational position of the rotating shaft  32   a  and thus the current operating mode of the automatic transmission  3 . In addition, the rotational position sensor  36  may be implemented by, for example, a rotary encoder. 
         [0048]    The shift control circuit  34  is an electric circuit which includes, for example, a microcomputer and various drivers. The shift control circuit  34  is electrically connected to a battery  5  of the vehicle, thereby being powered by the battery  5 . Moreover, the shift control circuit  34  is also electrically connected to the shift actuator  32 , the rotational position sensor  36 , and the parking and shift switches  21  and  22  of the mode selector  20 , thereby powering them using electric power supplied from the battery  5 . When the vehicle is in a normal condition, the shift control circuit  34  controls, based on the signals output from the rotational position sensor  36  and the parking and shift switches  21  and  22  of the mode selector  20 , the rotational position of the rotating shaft  32   a  of the shift actuator  32 , thereby controlling the shifting of the operating modes of the automatic transmission  3 . 
         [0049]    More specifically, when the signal, which indicates the shift command to shift the current operating mode of the automatic transmission  3  to P mode, is output from the parking switch  21 , the shift control circuit  34  controls the shift actuator  32  to bring the rotational position of the rotating shaft  32   a  sensed by the rotational position sensor  36  into agreement with P position of the rotating shaft  32   a . Similarly, when any one of the signals, which respectively indicate the shift commands to shift the current operating mode of the automatic transmission  3  to the corresponding ones of D, B, R, and N modes, is output from the shift switch  22 , the shift control circuit  34  controls the shift actuator  32  to bring the rotational position of the rotating shaft  32   a  sensed by the rotational position sensor  36  into agreement with the one of D, B, R, and N positions of the rotating shaft  32   a  which corresponds to the any one of the signals. 
         [0050]    The shift control device  30  further includes a parking brake sensor  37  and an airbag sensor  38 , both of which are electrically connected to the shift control circuit  34  and are thus powered by the battery  5  via the shift control circuit  34 . The parking brake sensor  37  is mounted to a parking brake  6  of the vehicle which is a mechanical brake. The parking brake sensor  37  senses the operating condition of the parking brake  6  and outputs a signal that indicates the sensed operating condition. The airbag sensor  38  is mounted to an airbag device  8  of the vehicle which is activated when the vehicle has a collision. The airbag sensor  38  senses the operating condition of the airbag device  8  and outputs a signal that indicates the sensed operating condition. 
         [0051]    The engine control device  40  includes an ignition switch  41 , a speed sensor  42 , a foot brake sensor  44 , a submersion sensor  46 , and an engine control circuit  48 . 
         [0052]    The ignition switch  41  is located in the vicinity of the driver&#39;s seat in the vehicle. The ignition switch  41  is provided for the vehicle driver to input an ON command to turn on the engine  4  and an OFF command to turn off the engine  4 . More specifically, each of the ON and OFF commands is inputted by, for example, a predetermined button manipulation of the ignition switch  41 . Upon input of the ON command, the ignition switch  41  generates an ON signal that indicates the inputted ON command. Similarly, upon input of the OFF command, the ignition witch  41  generates an OFF signal that indicates the inputted OFF command. 
         [0053]    The speed sensor  42  is mounted to the automatic transmission  3 . The speed sensor  42  senses the running speed of the vehicle based on the rotating speed of an output shaft of the automatic transmission  3 , and outputs a signal that indicates the sensed running speed. 
         [0054]    The foot brake sensor  44  is mounted to a foot brake  7  of the vehicle. The foot brake  7  is a mechanical brake and is activated by a pedal manipulation. The foot brake sensor  44  senses the operating condition of the foot brake  7  and outputs a signal that indicates the sensed operating condition. 
         [0055]    The submersion sensor  46  is located in the engine compartment of the vehicle along with the engine  4  and the battery  5 . The submersion sensor  46  senses a submersion of the vehicle and outputs a signal that indicates the sensed submersion of the vehicle. 
         [0056]    The engine control circuit  48  is an electric circuit which includes, for example, a microcomputer. The engine control circuit  48  is electrically connected to the battery  5 , thereby being powered by the battery  5 . The engine control circuit  48  is also electrically connected to the ignition switch  41 , the speed sensor  42 , the foot brake sensor  44 , and the submersion sensor  46 , thereby powering them using electric power supplied from the battery  5 . 
         [0057]    The engine control circuit  48  controls operation of the engine  4  based on the signals output from the ignition switch  41 , the speed sensor  42 , the foot brake sensor  44 , and the submersion sensor  46 . Moreover, the engine control circuit  48  is electrically connected to the shift control circuit  34  of the shift control device  30  to send the signals output from the ignition switch  41 , the speed sensor  42 , the foot brake sensor  44 , and the submersion sensor  46  to the shift control circuit  34 . In particular, when the OFF signal, which indicates the OFF command to turn off the engine  4 , is transmitted from the ignition switch  41  to the shift control circuit  34  via the engine control circuit  48  in a normal condition of the vehicle, the shift control circuit  34  shifts the current operating mode of the automatic transmission  3  to P mode. 
         [0058]    The warning device  50  includes an audio output unit  52  and a display unit  54 . 
         [0059]    The audio output unit  52  is configured with, for example, a speaker located in the vicinity of the driver&#39;s seat in the vehicle. The audio output unit  52  is electrically connected to the battery  5 , thereby being powered by the battery  5 . Moreover, the audio output unit  52  is also electrically connected to the shift control circuit  34 , so that it can output an audio warning under control of the shift control circuit  34 . 
         [0060]    The display unit  54  is configured with, for example, a combination meter located in the vicinity of the driver&#39;s seat in the vehicle. The display unit  54  is electrically connected to the battery  5 , thereby being powered by the battery  5 . Moreover, the display unit  54  is also electrically connected to the shift control circuit  34 , so that it can display a warning under control of the shift control circuit  34 . In addition, the display unit  54  can also display the current operating mode of the automatic transmission  3  under control of the shift control circuit  34 . 
         [0061]    After having described the overall configuration of the SBW control system  2 , the detailed configuration of the motion converter  33  of the SBW control system  2  will be described hereinafter with reference to  FIGS. 3 and 4 . 
         [0062]    The motion converter  33  includes, as shown in  FIG. 3 , a detent plate  60 , a detent spring  61 , a park rod  62 , a park pole  63 , and a park gear  64 . 
         [0063]    The detent plate  60  has a drive shaft  66  that is fixed to the rotating shaft  32   a  of the shift actuator  32  (see  FIG. 1 ). The detent plate  60  is also mechanically connected to the spool  16  of the spool valve  14  of the hydraulic circuit  12 . Consequently, as the detent plate  60  is rotated by the torque output from the rotating shaft  32   a  of the shift actuator  32 , the spool  16  of the spool valve  14  is axially moved by the detent plate  60  to shift the operating modes of the automatic transmission  3 . 
         [0064]    Moreover, as shown in  FIG. 4 , the detent plate  60  has four notches  60 P,  60 R,  60 N, and  60 D that are formed in the radially outer periphery of the detent plate  60  and arranged along the rotational direction of the detent plate  60 . The notches  60 P,  60 R,  60 N of the detent plate  60  respectively correspond to P, R, and N modes of the automatic transmission  3 . The notch  60 D of the detent plate  60  corresponds to both D and B modes of the automatic transmission  3 . 
         [0065]    The detent spring  61  is so provided as to be engageable with any one of the notches  60 P,  60 R,  60 N, and  60 D. In the rotational position of the detent plate  60  at which the detent spring  61  engages with the notch  60 P, the current operating mode of the automatic transmission is shifted to P mode. Similarly, in the rotational positions of the detent plate  60  at each of which the detent spring  61  engages with a corresponding one of the notches  60 R,  60 N, and  60 D, the current operating mode of the automatic transmission is shifted to the corresponding ones of R, N, and D modes of the automatic transmission  3 . 
         [0066]    Referring back to  FIG. 3 , the park rod  62  is substantially L-shaped. The park rod  62  has one end fixed the detent plate  60  and the other end on which is fixed a conical member  68  that abuts the park pole  63 . The park pole  63  is so provided as to be swingable and engageable with the park gear  64 . The park gear  64  is fixed on the output shaft  3   a  of the automatic transmission  3  which is linked to the drive wheels of the vehicle. When the park pole  63  is swung to the position at which it engages with park gear  64 , the output shaft  3   a  of the automatic transmission  3  is locked. On the other hand, when the park pole  63  is swung to a position at which it is disengaged from the park gear  64 , the output shaft  3   a  of the automatic transmission  3  is unlocked. 
         [0067]    More specifically, when the detent plate  60  is rotated to the rotational position at which the detent spring  61  engages with the notch  60 P, the park rod  62  is moved toward the park pole  63  and thus the park pole  63  is brought by the conical member  68  into engagement with the park gear  64 , thereby locking the output shaft  3   a  of the automatic transmission  3  together with the park gear  64 . In other words, the parking lock is applied to the automatic transmission  3 . Moreover, when the detent plate  60  is rotated to any one of the rotational positions at which the detent spring  61  engages with the notches  60 R,  60 N, and  60 D respectively, the park rod  62  is moved away from the park pole  63  and thus the park pole  63  is brought by the conical member  68  out of engagement with the park gear  64 , thereby unlocking the output shaft  3   a  of the automatic transmission  3  together with the park gear  64 . In other words, the parking lock to the automatic transmission  3  is released. 
         [0068]    Next, fail-safe control processes of the SBW control system  2  according to the present embodiment will be described. 
         [0069]    In the present embodiment, the fail-safe control processes include a fail-safe control process for responding to a collision of the vehicle, a fail-safe control process for responding to a power failure in the vehicle, and a fail-safe control process for responding to a submersion of the vehicle. In addition, those fail-safe control processes are performed by the shift control circuit  34  by executing predetermined programs. 
         [0070]      FIG. 5  shows the fail-safe control process for responding to a collision of the vehicle according to the present embodiment. 
         [0071]    First, at step S 101 , the shift control circuit  34  determines, based on the signal output from the airbag sensor  38 , whether a collision of the vehicle has been detected. 
         [0072]    In addition, as described previously, the airbag device  8  is activated when the vehicle has a collision. Therefore, it is possible to determine whether a collision of the vehicle is detected based on the signal output from the airbag sensor  38  which indicates the operating condition of the airbag device  8  sensed by the airbag sensor  38 . 
         [0073]    If the determination at step S 101  results in a “NO” answer, then the shift control circuit  34  repeats step S 101 . On the other hand, if the determination at step S 101  results in a “YES” answer, then the process proceeds to step S 102 . 
         [0074]    At step S 102 , the shift control circuit  34  further determines, based on the signal output from the rotational position sensor  36 , whether the current operating mode of the automatic transmission  3  is P mode. 
         [0075]    In addition, as described previously, the signal output from the rotational position sensor  36  indicates the current rotational position of the rotating shaft  32   a  of the shift actuator  32 . Therefore, it is possible to determine whether the current operating mode of the automatic transmission  3  is P mode by checking whether the current rotational position of the rotating shaft  32   a  coincides with the P position of the rotating shaft  32   a.    
         [0076]    If the determination at step S 102  results in a “YES” answer, then the process proceeds to step S 103 . In addition, in this case, the automatic transmission  3  is determined as being currently operating in P mode with the parking lock applied thereto. 
         [0077]    At step S 103 , the shift control circuit  34  determines, based on the signals output from the parking brake sensor  37  and the foot brake sensor  44 , whether at least one of the parking brake  6  and the foot brake  7  is activated. 
         [0078]    If the determination at step S 103  results in a “NO” answer, then the process repeats step S 103 . On the other hand, if the determination at step S 103  results in a “YES” answer, then the process proceeds to step S 104 . 
         [0079]    At step S 104 , the shift control circuit  34  controls the shift actuator  32  to bring the rotational position of the rotating shaft  32   a  sensed by the rotational position sensor  36  into agreement with N position of the rotating shaft  32   a.    
         [0080]    Consequently, with the at least one of the parking brake  6  and the foot brake  7  activated, the current operating mode of the automatic transmission  3  is shifted from P mode to N mode, releasing the parking lock applied to the automatic transmission  3 . 
         [0081]    In addition, at step S 104 , the shift control circuit  34  further controls at least one of the audio output unit  52  and display unit  54  of the warning device  50  to warn the vehicle driver of the release of the parking lock. After that, the process goes to end. 
         [0082]    By performing above steps S 103  and S 104 , it is possible to prevent the vehicle from starting to move by itself upon the release of the parking lock, thereby ensuring high security of the vehicle. Moreover, with the parking lock released, it is possible to move the vehicle by an external force (e.g., by a tow car) to a desired place (e.g., a repair shop) upon deactivating the activated at least one of the parking brake  6  and the foot brake  7  even when the power supply to the SBW control system  2  is interrupted due to the collision. 
         [0083]    On the other hand, if the determination at step S 102  results in a “NO” answer, then the process proceeds to step S 105 . In addition, in this case, the automatic transmission  3  is determined as being currently operating in one of D, B, R, and N modes with the parking lock released. 
         [0084]    At step S 105 , the shift control circuit  34  determines, based on the signals output from the parking switch  21  and the ignition switch  41 , whether there is inputted either of the shift command to shift the current operating mode of the automatic transmission  3  to P mode and the OFF command to turn off the engine  4 . 
         [0085]    If the determination at step S 105  results in a “YES” answer, the process proceeds to step S 106 . 
         [0086]    At step S 106 , the shift control circuit  34  controls the shift actuator  32  to bring the rotational position of the rotating shaft  32   a  sensed by the rotational position sensor  36  into agreement with N position of the rotating shaft  32   a.    
         [0087]    Consequently, though there is inputted the shift command to shift the current operating mode of the automatic transmission  3  to P mode or the OFF command to turn off the engine  4 , the current operating mode of the automatic transmission  3 , which is one of D, B, R, and N modes, is shifted to or kept in N mode, thereby keeping the parking lock released. 
         [0088]    In addition, at step S 106 , the shift control circuit  34  further controls at least one of the audio output unit  52  and display unit  54  of the warning device  50  to warn the vehicle driver of the release of the parking lock. After that, the process proceeds to step S 109 . 
         [0089]    By performing above steps S 105  and S 106 , when the shift command to shift the current operating mode of the automatic transmission  3  to P mode or the OFF command to turn off the engine  4  is erroneously inputted during running of the vehicle, it is possible to keep the parking lock released, thereby preventing the running vehicle from being suddenly stopped. 
         [0090]    On the other hand, if the determination at step S 105  results in a “NO” answer, then the process proceeds to step S 107 . 
         [0091]    At step S 107 , the shift control device  34  determines, based on the signal output from the speed sensor  42 , whether the vehicle is stopped. 
         [0092]    If the determination at step S 107  results in a “NO” answer, then the process returns to step S 105 . On the other hand, if the determination at step S 107  results in a “YES” answer, then the process proceeds to step S 108 . 
         [0093]    At step S 108 , the shift control circuit  34  controls the shift actuator  32  to bring the rotational position of the rotating shaft  32   a  sensed by the rotational position sensor  36  into agreement with N position of the rotating shaft  32   a.    
         [0094]    Consequently, the current operating mode of the automatic transmission  3 , which is one of D, B, R, and N modes, is shifted to or kept in N mode, thereby keeping the parking lock released. 
         [0095]    In addition, at step S 108 , the shift control circuit  34  further controls at least one of the audio output unit  52  and display unit  54  of the warning device  50  to warn the vehicle driver of the release of the parking lock. After that, the process proceeds to step S 109 . 
         [0096]    At step S 109 , the shift control circuit  34  determines, based on the signal output from the parking brake sensor  37 , whether the parking brake  6  is activated. 
         [0097]    If the determination at step S 109  results in a “NO” answer, then the process proceeds to step S 110 , at which the shift control circuit  34  controls at least one of the audio output unit  52  and display unit  54  of the warning device  50  to warn the vehicle driver to activate the parking brake  6 . After that, the process returns to step S 109 . 
         [0098]    On the other hand, if the determination at step S 109  results in a “YES” answer, the process proceeds to step S 111 , at which the shift control circuit  34  stops both the audio output unit  52  and display unit  54  of the warning device  50  from warning the vehicle driver to activate the parking brake  6 . After that, the process goes to the end. 
         [0099]    By performing above steps S 106 , S 106 , and S 109 , it is possible to prevent the vehicle from moving by itself with the parking lock released. Moreover, with the parking lock released, it is possible to move the vehicle by an external force (e.g., by a tow car) to a desired place (e.g., a repair shop) upon deactivating the parking brake  6  even when the power supply to the SBW control system  2  is interrupted due to the collision. 
         [0100]      FIG. 6  shows the fail-safe control process for responding to a power failure in the vehicle according to the present embodiment. 
         [0101]    First, at step S 201 , the shift control circuit  34  determines whether there is a power failure in the vehicle. 
         [0102]    More specifically, in the present embodiment, the shift control circuit  34  determines that there is a power failure in the vehicle when the output voltage of the battery  5  is kept above the minimum operational voltage Vo of the SBW control system  2  but below the minimum starting voltage Vs of the engine  4  for longer than a predetermined time Ts. Therefore, the determination at step S 201  will not produce a “YES” answer when there is a temporary drop in the output voltage of the battery due to a starting operation of the engine  4 . In addition, the minimum starting voltage Vs and the time Ts are predetermined according to the specifications of the battery  5 . The time Ts may be preset to, for example, one day (i.e., 24 hours). 
         [0103]    If the determination at step S 201  results in a “NO” answer, then the process repeats step S 201 . On the other hand, if the determination at step S 202  results in a “YES” answer, then the process proceeds to step S 202 . 
         [0104]    At step S 202 , the shift control circuit  34  further determines, based on the signal output from the rotational position sensor  36 , whether the current operating mode of the automatic transmission  3  is P mode. 
         [0105]    If the determination at step S 202  results in a “NO” answer, then the process directly goes to the end. In addition, in this case, the automatic transmission  3  is determined as being currently operating in one of D, B, R, and N modes with the parking lock released. 
         [0106]    On the other hand, if the determination at step S 202  results in a “YES” answer, then the process proceeds to step S 203 . In addition, in this case, the automatic transmission  3  is determined as being currently operating in P mode with the parking lock applied thereto. 
         [0107]    At step S 203 , the shift control circuit  34  determines, based on the signals output from the parking brake sensor  37  and the foot brake sensor  44 , whether at least one of the parking brake  6  and the foot brake  7  is activated. 
         [0108]    If the determination at step S 203  results in a “NO” answer, then the process repeats step S 203 . On the other hand, if the determination at step S 203  results in a “YES” answer, then the process proceeds to step S 204 . 
         [0109]    At step S 204 , the shift control circuit  34  controls the shift actuator  32  to bring the rotational position of the rotating shaft  32   a  sensed by the rotational position sensor  36  into agreement with N position of the rotating shaft  32   a.    
         [0110]    Consequently, with the at least one of the parking brake  6  and the foot brake  7  activated, the current operating mode of the automatic transmission  3  is shifted from P mode to N mode, thereby releasing the parking lock applied to the automatic transmission  3 . 
         [0111]    In addition, at step S 204 , the shift control circuit  34  further controls at least one of the audio output unit  52  and display unit  54  of the warning device  50  to warn the vehicle driver of the release of the parking lock. After that, the process proceeds to step S 205 . 
         [0112]    At step S 205 , the shift control circuit  34  determines whether the power failure has been resolved. 
         [0113]    More specifically, in the present embodiment, the shift control circuit  34  determines whether the power failure has been resolved by checking whether the output voltage of the battery  5  has been recovered to exceed the minimum starting voltage Vs of the engine  4 . 
         [0114]    If the determination at step S 205  results in a “NO” answer, then the process repeats step S 205 . On the other hand, if the determination at step S 205  results in a “YES” answer, then the process proceeds to step S 206 . 
         [0115]    At step S 206 , the shift control device  34  determines, based on the signal output from the speed sensor  42 , whether the vehicle is stopped. 
         [0116]    If the determination at step S 206  results in a “NO” answer, then the process repeats step S 206 . On the other hand, if the determination at step S 206  results in a “YES” answer, then the process proceeds to step S 207 . 
         [0117]    At step S 207 , the shift control circuit  34  controls the shift actuator  32  to bring the rotational position of the rotating shaft  32   a  sensed by the rotational position sensor  36  into agreement with P position of the rotating shaft  32   a.    
         [0118]    Consequently, the current operating mode of the automatic transmission  3  is shifted from N mode to P mode, applying the parking lock to the automatic transmission  3 . After that, the process goes to the end. 
         [0119]    As above, in the present embodiment, when there is a power failure in the vehicle with at least one of the parking brake  6  and the foot brake  7  activated, the current operating mode of the automatic transmission  3  is shifted from P mode to N mode, thereby releasing the parking lock applied to the automatic transmission  3 . Further, the parking lock is kept released until the power failure is resolved. Consequently, even when the output voltage of the battery  5  is further decreased to below the minimum operational voltage Vo of the SBW control system  2 , it is still possible to move the vehicle by an external force (e.g., by a tow car) to a desired place (e.g., a repair shop) upon deactivating the activated at least one of the parking brake  6  and the foot brake  7 . Moreover, since the parking lock is released with the at least one of the parking brake  6  and the foot brake  7  activated, it is possible to prevent the vehicle from starting to move by itself upon the release of the parking lock, thereby ensuring high security of the vehicle. 
         [0120]      FIG. 7  shows the fail-safe control process for responding to a submersion of the vehicle according to the present embodiment. 
         [0121]    First, at step S 301 , the shift control circuit  34  determines, based on the signal output from the submersion sensor  46 , whether the vehicle is submerged. 
         [0122]    If the determination at step S 301  results in a “NO” answer, then the process repeats step S 301 . On the other hand, if the determination at step S 302  results in a “YES”, answer, then the process proceeds to step S 302 . 
         [0123]    At step S 302 , the shift control circuit  34  further determines, based on the signal output from the rotational position sensor  36 , whether the current operating mode of the automatic transmission  3  is P mode. 
         [0124]    If the determination at step S 302  results in a “YES” answer, then the process proceeds to step S 303 . In addition, in this case, the automatic transmission  3  is determined as being currently operating in P mode with the parking lock applied thereto. 
         [0125]    At step S 303 , the shift control circuit  34  determines, based on the signals output from the parking brake sensor  37  and the foot brake sensor  44 , whether at least one of the parking brake  6  and the foot brake  7  is activated. 
         [0126]    If the determination at step S 303  results in a “NO” answer, then the process repeats step S 303 . On the other hand, if the determination at step S 303  results in a “YES” answer, then the process proceeds to step S 304 . 
         [0127]    At step S 304 , the shift control circuit  34  controls the shift actuator  32  to bring the rotational position of the rotating shaft  32   a  sensed by the rotational position sensor  36  into agreement with N position of the rotating shaft  32   a.    
         [0128]    Consequently, with the at least one of the parking brake  6  and the foot brake  7  activated, the current operating mode of the automatic transmission  3  is shifted from P mode to N mode, thereby releasing the parking lock applied to the automatic transmission  3 . 
         [0129]    In addition, at step S 304 , the shift control circuit  34  further controls at least one of the audio output unit  52  and display unit  54  of the warning device  50  to warn the vehicle driver of the release of the parking lock. After that, the process goes to end. 
         [0130]    By performing above steps S 303  and S 304 , the current operating mode of the automatic transmission  3  is shifted from P mode to N mode, releasing the parking lock applied to the automatic transmission  3 . Further, the parking lock is kept released until the submersion sensor  46  is reset to its initial condition upon repair of the vehicle. Consequently, even when the power supply to the SBW control system  2  is interrupted due to the submersion, it is still possible to move the vehicle by an external force (e.g., by a tow car) to a desired place (e.g., a repair shop) upon deactivating the activated at least one of the parking brake  6  and the foot brake  7 . 
         [0131]    On the other hand, if the determination at step S 302  results in a “NO” answer, then the process proceeds to step S 305 . In addition, in this case, the automatic transmission  3  is determined as being currently operating in one of D, B, R, and N modes with the parking lock released. 
         [0132]    At step S 305 , the shift control circuit  34  determines, based on the signals output from the parking switch  21  and the ignition switch  41 , whether there is inputted either of the shift command to shift the current operating mode of the automatic transmission  3  to P mode and the OFF command to turn off the engine  4 . 
         [0133]    If the determination at step S 305  results in a “NO” answer, then the process repeats step S 305 . On the other hand, if the determination at step S 305  results in a “YES” answer, the process proceeds to step S 306 . 
         [0134]    At step S 306 , the shift control circuit  34  controls the shift actuator  32  to bring the rotational position of the rotating shaft  32   a  sensed by the rotational position sensor  36  into agreement with N position of the rotating shaft  32   a.    
         [0135]    Consequently, though there is inputted the shift command to shift the current operating mode of the automatic transmission  3  to P mode or the OFF command to turn off the engine  4 , the current operating mode of the automatic transmission  3 , which is one of D, B, R, and N modes, is shifted to or kept in N mode, thereby keeping the parking lock released. 
         [0136]    In addition, at step S 306 , the shift control circuit  34  further controls at least one of the audio output unit  52  and display unit  54  of the warning device  50  to warn the vehicle driver of the release of the parking lock. After that, the process goes to the end. 
         [0137]    By performing above steps S 305  and S 306 , when the shift command to shift the current operating mode of the automatic transmission  3  to P mode or the OFF command to turn off the engine  4  is erroneously inputted during running of the vehicle in water, it is possible to keep the parking lock released, thereby preventing the running vehicle from being suddenly stopped to become unable to evacuate from water. 
         [0138]    As described above, in the present embodiment, when the vehicle is in a normal condition, the SBW control system  2  applies the parking lock to the automatic transmission  3  upon input of the shift command to shift the current operating mode of the automatic transmission  3  to P mode or the OFF command to turn off the engine  4 . Consequently, it is possible to reliably prevent the vehicle from being stolen when parked, ensuring high security of the vehicle. On the other hand, when the vehicle is in such a severely abnormal condition as to interrupt the power supply from the battery  5  to the SBW control system  2 , the SBW control system  2  releases the parking lock or keeps the parking lock released. Consequently, when necessary, it is possible to move the vehicle to a desired place (e.g., a repair shop), thus ensuring high fail-safe capability of the SBW control system  2 . 
         [0139]    While the above particular embodiment of the invention has been shown and described, it will be understood by those skilled in the art that various modifications, changes, and improvements may be made without departing from the spirit of the invention. 
         [0140]    For example, in the previous embodiment, the shift control circuit  34  performs the three fail-safe control processes for respectively responding to a collision of the vehicle, a power failure in the vehicle, and a submersion of the vehicle. However, it is also possible for the shift control circuit  34  to perform only one or two of the three fail-safe control processes. 
         [0141]    Moreover, in the previous embodiment, in steps S 103 , S 203 , and S 303  of the three fail-safe control processes, the shift control circuit  34  checks both the operating conditions of the parking brake  6  and the foot brake  7  to determine whether at least one of them is activated. 
         [0142]    However, it is also possible for the shift control circuit  34  to check the operating condition of only one of the parking brake  6  and the foot brake  7  to determine whether it is activated. 
         [0143]    Moreover, it is also possible to omit steps S 103 , S 203 , and S 303  respectively from the three fail-safe control processes. Furthermore, steps S 109  through S 111  can also be omitted from the fail-safe control process for responding to a collision of the vehicle. 
         [0144]    As the airbag sensor  38  and the submersion sensor  46 , it is possible to employ those which have already existed on the vehicle, so as to minimize the manufacturing cost. Alternatively, it is also possible to additionally employ an airbag sensor and a submersion sensor dedicated to the SBW control system  2 . 
         [0145]    Furthermore, in the case of the vehicle being equipped with a communication device (e.g., a telematics device) for informing the occurrence of an abnormal condition of the vehicle, it is possible for the shift control circuit  34  to detect the abnormal condition based on a signal output from the communication device.