Abstract:
In a shift by wire transmission, a control unit limits the shift position (P, R, N, D, L) to be changed beyond a prescribed shift position depending on an initial shift position and an amount of the movement of the shift member in a single shifting operation. Thereby, even when the shift member is excessively advanced or otherwise improperly advanced, the actually selected shift position can be suitably selected so that an undesired change of the shift position can be avoided. The control unit may also limit the selected shift position to be changed beyond a prescribed shift position depending on a traveling speed of the vehicle so that an improper change in the shift position due to vehicle speed constraints can be avoided by using a simple structure.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a shift device for a transmission of a motor vehicle, and in particular to a shift device using a shift member that can be advanced in either direction in an endless manner. 
     BACKGROUND ART 
     The shift device for the transmission of a motor vehicle is provided with a shift lever which is typically located in the center console. When there is no center console or the center console does not provide a space for installing the shift lever, the shift lever may also be provided adjacent to the steering column or on the instrument panel. 
     In a typical automatic transmission, the shift positions are arranged in the order of the P (parking) position, the R (reverse) position, the N (neutral) position and the D (drive position) in that other. Oftentimes, a special forward travel position is provided in addition to the D position, and is typically located next to the drive position on the other side of the N position. Such an additional forward travel position is called as the S (sport mode) position, the 2 (second) position and the L (low speed) position depending on the purpose thereof. In the case of an electric vehicle or a hybrid vehicle, the additional forward travel position may consist of the B (regenerative braking) position. As the special forward travel position is designed for a special purpose such as increasing the engine brake and improving the response of the vehicle, it is selected only under special circumstances. As the special forward travel position is located next to the D position, it is possible that the vehicle operator inadvertently selects the special forward travel position, and it may irritate the vehicle operator. 
     The conventional shift device for an automatic transmission is provided with an interlock that prevents an improper shifting to be made under certain conditions. For instance, the shift position cannot be moved from the P position unless the brake pedal is depressed. The shift position cannot be shifted from the N position unless a button fitted to the shift level is depressed. 
     The shift lever was traditionally connected to the automatic transmission via a mechanical link. In recent years, various proposals have been made to detect the displacement of the shift lever with an electric sensor, and transmit the detection signal of the electric sensor to the automatic transmission via a control unit. Such an arrangement is known as the shift by wire (SBW) system. In the shift device illustrated in FIGS. 9 and 10 of U.S. Pat. No. 7,971,498, a shift knob for rotational movement is surrounded by a C-shaped stopper member that restricts the rotation of the shift knob to a range defined by the gap in the stopper member. This range can be varied by turning the C-shaped stopper member around the shift knob. 
     JP 2010-090925A discloses a SBW shift device using a shift lever of a momentary switch type. According to this shift device, the shift lever is configured to return to the N position upon release of the shift lever. The desired shift change is effected by moving the shift lever away from the N position in the corresponding direction, and releasing the shift lever. In this shift device, to avoid the inadvertent shifting from the D position to the R position even though the vehicle operator intends to change the shift position to the N position, the time period required for recognizing the completion of the shift change is varied depending on the kind of the shift change. 
     In the invention disclosed in U.S. Pat. No. 7,971,498, the selected shift position can be determined independently from the angular position of the shift knob while the rotational range of the shift knob is variably restricted by the stopper member. According to this invention, even when the shift knob is placed at a position other than the P position when deactivating the vehicle, the P position may be automatically restored when restarting the engine. However, no arrangement is made for preventing an inadvertent selection of a shift position or a shift interlock under special conditions. 
     The invention disclosed in JP 2010-090925A is applicable only to a momentary switch type shift device, and not to a shift device using an endlessly rotatable shift knob. 
     BRIEF SUMMARY OF THE INVENTION 
     In view of such problems of the prior art, a primary object of the present invention is to provide a shift device for a transmission of a motor vehicle which is provided with a shift knob that can be rotated or otherwise advanced in either direction in an endless manner. 
     A second object of the present invention is to provide a shift device for a transmission of a motor vehicle that can be shifted by a shift knob configured to be advanced in either direction in an endless manner and provided with an arrangement for preventing an inadvertent selection of a shift position not intended by the vehicle operator. 
     A third object of the present invention is to provide a shift device for a transmission of a motor vehicle that can be shifted by a shift knob configured to be advanced in either direction in an endless manner and provided with an arrangement for preventing an undesired selection of a shift position under a prescribed operating condition of the vehicle in a reliable manner by using a highly simple structure. 
     To achieve such objects, the present invention provides a speed shift device for a transmission of a motor vehicle, comprising: a shift member that can be manually advanced selectively in a first direction and a second direction in an endless manner; a movement detector that detects a movement of the shift member; a control unit for selecting a shift position of the transmission from a plurality of shift positions in a prescribed order according to a detection signal of the movement detector; and an indicator that indicates the selected shift position; wherein the control unit is configured to cause the selected shift position to be changed in the prescribed order in correspondence to a displacement of the shift member in each single shifting operation under normal condition, and to limit the selected shift position to be changed beyond a prescribed shift position depending on an initial shift position and an amount of the movement of the shift member in a single shifting operation. 
     According to this arrangement, even when the shift member is excessively advanced or otherwise improperly advanced, the actually selected shift position can be suitably selected depending on the initial shift position and the amount of the movement of the shift member so that an undesired change of the shift position can be avoided. Furthermore, such an arrangement can be realized by an electric circuit or without using any complex mechanical arrangements. 
     Typically, the shift member comprises a shift knob that can be rotated in either direction in an endless manner. In such a case, the shift member may further comprise a detent mechanism that retains the shift knob resiliently at each of the shift positions. Thereby, the user is allowed to manipulate the shift member both accurately and comfortably. The single shifting operation can be determined as a continuous movement of the shift member that does not include a pause of more than a prescribed time period. 
     According to an alternate embodiment of the present invention, the shift member consists of a momentary switch type shift member that can be moved in either direction from a neutral position and configured to be returned to the neutral position when released, and the movement detector is configured to detect the movement of the shift member according to a time duration of the displacement of the shift member from the neutral direction in each given direction. Alternately, the movement detector may detect the movement of the shift member according to a number of prescribed displacements of the shift member from the neutral direction in each given direction. 
     According to a basic aspect of the present invention, the shift positions include a non-forward travel position, a D (drive) position for a normal forward travel and an additional forward travel position for a special purpose, such as an L position, arranged in that order in the first direction, and the control unit is configured to limit the change in the shift position from the non-forward travel position in a single shifting operation in the first direction to the D position without regard to the displacement of the shift member. 
     Therefore, when the vehicle operator starts off the vehicle, even if the shift member is advanced from the non-forward travel position such as a P position excessively beyond the D position, the shift position is limited to the D position, instead of selecting the L position. Therefore, the vehicle operator is not required to advance the shift member exactly to the D position, and is not required to turn back the shift member if the additional forward travel position such as the L position even when the shift member is excessively advanced. Thereby, the vehicle operator is allowed to operate the shift member in a comfortable manner. The non-forward travel position may include at least one of a P (park) position, a R (reverse) position and a N (neutral) position. 
     According to another aspect of the present invention, the non-forward travel position comprises a P (park) position, and the control unit is configured to prevent the change in the shift position from the P position in a single shifting operation in the second direction without regard to the displacement of the shift member in the second direction. 
     According to yet another aspect of the present invention, the control unit is configured to limit the selected shift positions to be changed beyond a prescribed shift position additionally depending on a state of a brake of the vehicle. For instance, the shift device prevents the changing of the shift position from the P position unless the brake pedal is depressed. 
     According to a particularly preferred embodiment of the present invention, the control unit is configured to limit the selected shift position to be changed beyond a prescribed shift position additionally depending on a traveling speed of the vehicle. Typically, the non-forward travel position includes a P (park) position, a R (reverse) position and a N (neutral) position arranged in that order in the first direction. 
     For instance, the control unit may be configured to prevent the change in the shift position from the R position in a single shifting operation by an advancing of the shift member by one or more stages in the second direction when the vehicle is traveling at more than a prescribed speed in either direction (Vth 1 ). The changing the shift position to the P position when the vehicle is in motion is damaging to the transmission. Therefore, it is advantageous that the changing of the shift position from the R position to the P position is prevented when the vehicle is not substantially stationary. 
     In another example, the control unit is configured to limit the change in the shift position from the R position to the N position in a single shifting operation by an advancing of the shift member by two or more stages in the first direction when the vehicle is traveling rearward at more than a prescribed speed (Vth 3 ). Changing the shift position from the R position to the D position while the vehicle is traveling rearward is damaging to the transmission. Therefore, it is advantageous to limit the change of the shift position from the R position to the N position or short of the D position. 
     In yet another example, the control unit is configured to limit the change in the shift position from the N position to the R position in a single shifting operation by an advancing of the shift member by two or more stages in the second direction when the vehicle is traveling at more than a prescribed speed (Vth 1 ) in either direction but not more than a second prescribed speed (Vth 2 ) greater in value than the first prescribed speed. The changing the shift position to the P position when the vehicle is in motion is damaging to the transmission. However, if the vehicle is traveling rearward or not traveling forward at high speed, the changing the shift position to the P position may not be permissible but the changing the shift position to the R position is permissible. Therefore, it is advantageous that the changing of the shift position from the N position to the P position is prevented but the changing of the shift position from the N position to the R position is permitted when the traveling speed of the vehicle is appropriate. 
     In yet another example, the control unit is configured to prevent the change in the shift position from the N position in a single shifting operation by an advancing of the shift member by one stage in the second direction when the vehicle is traveling forward at more than a prescribed speed (Vth 2 ). When the vehicle is traveling forward at a relatively high speed, the changing of the shift position to either the R position or the P position should not be permitted, and it is advantageous to prevent the shift position from the N position in the second direction. 
     In yet another example, the control unit is configured to prevent the change in the shift position from the N position in a single shifting operation by an advancing of one or more stages in the first direction when the vehicle is traveling rearward at more than a prescribed speed (Vth 3 ). When vehicle is traveling rearward at some speed, it is damaging to the transmission to change the shift position to the D position. It is therefore advantageous to prevent the change of the shift position from the N position in the first direction. 
     In yet another example, the control unit is configured to limit the change in the shift position from the D position to the N position in a single shifting operation by an advancing of the shift member by two stages in the second direction when the vehicle is traveling forward at more than a prescribed speed (Vth 2 ). When the vehicle is traveling forward at some speed, it may be damaging to the transmission to change the shift position to the R position. It is therefore advantageous to limit the change of the shift position from the D position in the second direction to the N position. 
     In yet another example, the control unit is configured to limit the change in the shift position from the D position to the N position in a single shifting operation by an advancing of the shift member by three or more stages in the second direction when the vehicle is traveling at more than a prescribed speed (Vth 1 ) in either direction. When the vehicle is traveling in either direction at some speed, it may be damaging to the transmission to change the shift position to the PR position. It is therefore advantageous to limit the change of the shift position from the D position in the second direction to the N position. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF DRAWINGS 
       Now the present invention is described in the following with reference to the appended drawings, in which: 
         FIG. 1  is a block diagram showing an overall functional structure of a motor vehicle to which the present invention is applied; 
         FIG. 2  is a perspective view of a part of a passenger compartment of the vehicle surrounding a vehicle operator&#39;s seat; 
         FIG. 3  is a diagram showing a shift knob and a display panel provided around the shift knob; 
         FIG. 4  is a functional block diagram of a speed shift device embodying the present invention; 
         FIGS. 5 to 9  show a flowchart of the control flow in changing shift positions given as a first embodiment of the present invention; 
         FIG. 10  is a diagram showing the mode of operation of a part of the first embodiment; 
         FIGS. 11 to 15  show a flowchart of the control flow in changing shift positions given as a second embodiment of the present invention; 
         FIG. 16  is a diagram showing the mode of operation of a part of the second embodiment; 
         FIG. 17  is a diagram showing the operation of another part of the second embodiment; and 
         FIG. 18  is a simplified view showing an alternate embodiment of the shift member. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIG. 1 , a motor vehicle  1  embodying the present invention is provided with an internal combustion engine  2  as a power source, and the output of the engine  2  is transmitted to driven road wheels via a front axle  5  via an automatic transmission  4 . The driven road wheels of the motor vehicle  1  of the illustrated embodiment consist of front wheels  6   f , but the present invention is equally applicable to rear wheel drive vehicles where rear wheels  6   r  consist of driven road wheels and four wheel drive vehicles where both the front and rear wheels  6  consist of driven road wheels. The automatic transmission  4  of the illustrated embodiment consists of a multi gear stage automatic transmission system having four forward drive stages and one reverse drive stage, but may also consist of a continuously variable transmission (CVT). 
     The motor vehicle  1  may also consist of an electric vehicle fitted with an electric motor instead of an internal combustion engine or a hybrid vehicle fitted with an electric motor in combination with an internal combustion engine. In such a case, the electric motor may consist of a motor generator  3  that receives a supply of electric power from a battery  7  and charges the battery  7  depending on the operating condition of the vehicle under the control of an inverter  8 . For instance, when the vehicle is decelerating, the motor generator  3  provides a braking force to the vehicle, and recovers the energy from the deceleration of the vehicle. 
     The vehicle  1  includes a control unit  11  incorporated with a microcomputer, ROM, RAM, peripheral circuits, input/output interfaces and various drivers, a shift knob  14  for manually selecting the gear range of the transmission, a rotary encoder  15  for detecting the displacement (angular displacement θs) of the shift knob  14 , a vehicle speed sensor  16  for detecting the traveling speed of the vehicle  1 , an accelerator pedal sensor  18  for detecting the displacement θa of the accelerator pedal  17  and a brake pedal sensor  23  for detecting the depression of the brake pedal  22  or the engagement of the brake device. The shift device  10  of the present invention is essentially constituted by the control unit  11 , the shift knob  14 , the rotary encoder  15  and the vehicle speed sensor  16 . 
     The control unit  11  performs a shift control whereby the gear range of the transmission  4  is selected according to the operation of the shift knob  14  and the gears of the transmission  4  are changed according to the selected gear range and the operating condition of the engine, in addition to controlling the operation of the engine  2 . The shifting of the gears is performed via the control of the shift solenoid valves provided in a hydraulic circuit of the transmission. When the vehicle consists of an electric vehicle or a hybrid vehicle, the control unit  11  controls the driving and regenerating action of the motor generator  3  as well. 
     As shown in  FIGS. 2 and 3 , the shift knob  14  is disk-shaped, and is located in a lower left part of an instrument panel  19  of the vehicle  1  with respect to a steering wheel  20 . This part of the instrument panel  19  may be more protruded than the remaining part of the instrument panel  19  so that the vehicle operator can comfortably reach the shift knob  14 . The provision of the shift knob  14  on the instrument panel  19  eliminates the need for a center console so that the inner space of the passenger compartment can be used in an efficient manner. The positioning of the shift knob  14  is not essential for the present invention, and may also be placed in a center console or any other convenient place without departing from the spirit of the present invention. 
     The shift knob  14  is configured to be turned endlessly in either direction, and provides detent action at every 30 degrees of rotation. Therefore, the vehicle operator is enabled to sense each 30 degrees of rotation by tactile sensation, and the shift knob  14  is held at any of the angular positions separated by the 30 degree angle while prevented from being held at any intermediate position. Such a detent action can be achieved by using any known detent mechanism such as the one disclosed in U.S. Pat. No. 7,971,498. 
     As shown in  FIG. 3 , a display panel  21  is provided in a part of the instrument panel  19  surrounding the shift knob  14 , and includes markings for the shift positions P, R, N, D and L in that order in counter-clockwise direction. The display panel  21  is provided with an internal lighting arrangement such that any selected one of the shift positions is illuminated. The arrangement of the shift positions is not limited by the illustrated embodiment, but may be selected as desired without departing from the spirit of the present invention. For instance, these shift positions may be arranged in clockwise direction, a S position and/or a 2 position may be provided in succession to the L position or instead of the L position. The B position may also be provided instead of the L position. 
     The rotary encoder  15  is provided on the back side of the instrument panel  19 , and is configured to detect the angular displacement θs of the shift knob  14 . The detection signal of the rotary encoder  15  is forwarded to the control unit  11 , and is used for the shift control of the transmission  4 . 
     As shown in  FIG. 4 , the control unit  11  includes a shift position selecting unit  12  and a gear ratio selecting unit  13 . The shift position selecting unit  12  changes the current shift position to a new shift position depending on the angular displacement θs and the direction of the angular movement. More specifically, normally, every time the shift knob  14  is turned by a prescribed angle (30 degrees in the illustrated embodiment), the control unit  11  changes the shift position by one stage. For instance, suppose that the current shift position is the P position, and the shift knob  14  is tuned in counter clockwise direction by 90 degrees. The control unit  11  changes the shift position by three stages, and sets the shift position to the D position. When turning the shift knob  14 , the shift knob  14  may slightly overshoot the intended shift position, and is turned back to the D position. The detent mechanism of the shift knob  14  then finally settles the shift position at D. In such a case, it can be said that the shift knob  14  was turned from the initial position to the D position. At any event, the angular displacement of any shift operation is determined as that between the initial angular position and the final angular position as a result of a single shifting operation. 
     However, such an arrangement may create a certain inconvenience. The vehicle operator when starting the vehicle turns the shift knob in counter clockwise direction, and it is highly possible that the shift knob is turned by more than 90 degrees. In such a case, the L position is inadvertently selected, and the vehicle operator is required to turn back the shift knob in clockwise direction by 30 degrees. Such an occurrence may cause an irritation to the vehicle operator. 
     Based on such a consideration, according to the illustrated embodiment, a certain arrangement is made such that the shift knob is turned in counter-clockwise direction by 90 degrees in effect even when the shift knob is turned in counter-clockwise direction by well more than 90 degrees without a pause. According to the particular arrangement of the illustrated embodiment, when an angular displacement of 90 degrees is made from the P position without a pause, the D position is selected. Also when an angular displacement of 120 degrees or more is made without a pause, the D position is still selected. Thereby, the vehicle is enabled to operate the shift knob  14  without experiencing the irritation mentioned above. 
     “Without a pause” in this case may mean that the shift knob was turned by a certain angle without being held stationary for more than a prescribed time period such as 0.2 seconds. Turning of the shift knob without a pause can be regarded as a single shifting operation. If there is a pause of more than 0.2 seconds, the process of turning the shift knob is considered as being completed, and any subsequent rotation of the shift knob is considered as a second shifting operation which is separated from the first shifting operation by the pause. For instance, if the shift knob is turned in one direction and then in the opposite direction without a pause of more than 0.2 seconds during the whole process, it is no different from turning the shift knob directly from the initial position to the final position without reversing the direction of turning the knob. 
     The shift position selecting unit  12  is provided with a counter that is incremented every time the shift knob passes a shift position according to the detection signal from the rotary encoder  15  so that the angular displacement of the shift knob  14  in each shifting operation can be determined from the count value C of the counter provided in the shift position selecting unit  12 . More specifically, the initial count value C is zero when the shift position is at P, and the count value C is incremented by +1 when the shift knob  14  is turned in counter-clockwise direction by 30 degrees. When the shift knob  14  is turned in counter-clockwise direction by 90 degrees in a single shifting operation, the count value C is incremented by +3. When the shift knob  14  is turned in clockwise direction by 30 degrees in a single shifting operation, the count value C is incremented by −1, or decremented by +1. 
     In this manner, the shift position selecting unit  12  keeps track of the shift actions according to the count value C. For instance, when the shift knob is turned from the P position in counter-clockwise direction by 30 degrees, and the count value C=+1 is registered, the shift position selecting unit  12  sets the R range. Likewise, when the shift knob is turned from the P position in counter-clockwise direction by 60 degrees and 90 degrees, and the count value C=+2 and +3 are registered, shift position selecting unit  12  sets the N and D ranges, respectively. The shift device is configured such that the P position is selected at the time of vehicle start up. 
     The gear ratio selecting unit  13  selects the gear ratio of the transmission  4  according to the shift position selected by the shift position selecting unit  12  by selectively activating corresponding solenoid valves of the hydraulic circuit not shown in the drawings. More specifically, when the P position or the N position is selected, the gear ratio selecting unit  13  disconnects the path of the power transmission in the transmission  4  in a per se known manner. When the R position is selected, the reverse gear is engaged. When the D position is selected, the gears are shifted according to the depression θa of the accelerator  17  detected by the accelerator pedal sensor  18  and the traveling speed V of the vehicle by using a suitable map. When the L position is selected, the range of the gear ratios is restricted such that an increased engine brake may be obtained. 
       FIGS. 5 to 9  show the control flow of a first embodiment of the present invention.  FIG. 5  shows the control flow when the current shift position is at the P position. When the vehicle is initially started, the shift position is at the P position. It can be accomplished by using any known arrangement. During the operation of the vehicle, the vehicle operator may select the P position when the vehicle is stationary. When the P position is selected (ST 1 : Yes), a completion of a single shift operation is detected in step ST 2 . As discussed above, a single shift operation may be detected as a continuous angular movement of the shift knob  14  without a pause of more than a prescribed time period such as 0.2 seconds. During a single shift operation, the rotational direction of the shift knob  14  may be reversed. In such a case, the resulting angular movement is measured as the angular displacement between the initial shift position and the final shift position. 
     Upon detecting the completion of a single shift operation from the P position (ST 2 : Yes), it is determined if the count value C is equal to or smaller than zero (ST 3 ). If not, it is determined if the count value C is equal to 1 (ST 4 ). If not, it is determined if the count value C is equal to 2 (ST 5 ). If not, it means that the count value C is equal to or greater 3. 
     If the count value C is equal to or smaller than zero in step ST 3 , the P position is maintained (ST 7 ), and the program flow returns to the main flow. Therefore, when the shift knob  14  is not turned at all in effect or turned in clockwise direction by any angular displacement, the shift position remains at the P position. 
     If the count value C is equal to 1 in step ST 4 , the shift position is changed by one stage to the R position (ST 8 ). If the count value C is equal to 2 in step ST 5 , the shift position is changed by two stages to the N position (ST 9 ). If the count value C is equal to 3 (ST 5 : No), the shift position is changed by three stages to the D position, and the vehicle is brought ready to start off. In any of the cases, the program flow returns to the main flow. 
     If the count value C is greater than 3 (ST 5 : No), the change of the shift position from the P position is still limited to the D position. This is advantageous because the shift position is normally desired to be at the D position when starting off the vehicle, and it is annoying to the vehicle operator if the shift position is brought to the L position owing to the inadvertent excessive turning of the shift knob. Owing to this arrangement, when changing the shift position from the P position to the D position to start off the vehicle, the vehicle operator is allowed to turn the shift knob in counter-clockwise direction by large enough an angle without being required to stop the movement of the shift knob exactly at the D position. 
       FIG. 10  illustrates this arrangement. When the shift knob  14  is turned from the P position, the shift position is changed according to the number of 30 degree increments of the rotation of the shift knob  14  up to the D position. However, when the shift knob  14  is turned beyond the three 30 degree increments of the rotation of the shift knob  14 , the shift position is limited to the D position, and does not advance to the L position. When the shift operation is terminated at the D position, and a subsequent shift operation in counter-clockwise direction is performed following a pause of more than 0.2 seconds, the shift position is allowed to be changed to the L position, without regard to the number of the 30 degree increments beyond the L position. 
     Referring to  FIG. 6 , upon detecting that the P position is not selected in step ST 1  ( FIG. 5 ), it is determined if the R position is selected in step ST  12 . If the R position is detected (ST 12 : Yes), a completion of a single shift operation is detected in step ST 13 . Upon detecting the completion of a single shift operation from the R position, it is determined if the count value C is equal to or smaller than −1 (step ST 14 ). If not, it is determined if the count value C is equal to 0 (step ST 15 ). If not, it is determined if the count value C is equal to 1 (step ST 16 ). If not (ST 16 : No), it means that the count value C is equal to or greater 2. 
     If the count value C is equal to or smaller than −1 in step ST 14 , the shift position is changed to the P position (ST 18 ). A separate arrangement may be made to prohibit this change of the shift position when the vehicle is traveling at more than a prescribed speed. If the count value C is equal to 0 in step ST 15 , the R position is maintained (ST 19 ). If the count value C is equal to 1 in step ST 16 , the shift position is changed to the N position (ST 20 ). If the count value C is equal to 2 (ST 16 : No), the shift position is changed to the D position. If the count value C is greater than 2 (ST 16 : No), the change of the shift position from the R position in a single shift operation is still limited to the D position. In any of the cases, the program flow returns to the main flow. 
     Referring to  FIG. 7 , upon detecting that the R position is not selected in step ST 12  ( FIG. 6 ), it is determined if the N position is selected in step ST  23 . If the N position is detected (ST 23 : Yes), a completion of a single shift operation is detected in step ST 24 . Upon detecting the completion of a single shift operation from the N position, it is determined if the count value C is equal to or smaller than −2 (ST 25 ). If not, it is determined if the count value C is equal to −1 (ST 26 ). If not, it is determined if the count value C is equal to 0 (ST 27 ). If not, it means that the count value C is equal to or greater than 1. 
     If the count value C is equal to or smaller than −2 in step ST 25 , the shift position is changed to the P position. A separate arrangement may be made to prohibit this change of the shift position if the vehicle is traveling at more than a prescribed speed. If the count value C is equal to −1 in step ST 26 , the shift position is changed to the R position (ST 30 ). If the count value C is equal to 0 in step ST 27 , the N position is maintained (ST 31 ). If the count value C is equal to 1 (ST 27 : No), the shift position is changed to the D position (ST 32 ). Even if the count value C is greater than 1, the change of the shift position from the N position in a single shift operation is still limited to the D position. In any of the cases, the program flow returns to the main flow. 
     Referring to  FIG. 8 , upon detecting that the N position is not selected in step ST  23  ( FIG. 7 ), it is determined if the D position is selected in step ST  34 . Upon detecting the D position (ST 34 : Yes), a completion of a single shift operation is detected in step ST 35 . 
     Upon detecting the completion of a single shift operation from the D position (ST 35 : Yes), it is determined if the count value C is equal to or smaller than −3 in step ST 36 . If not, it is determined if the count value C is equal to −2 in step ST 37 . If not, it is determined if the count value C is equal to −1 in step ST 38 . If not, it is determined if the count value C is equal to 0 in step ST 39 . If not, it means that C is equal to or greater 1. 
     If the count value C is equal to or smaller than −3 in step ST 36 , the shift position is changed to the P position (ST 40 ). A separate arrangement may be made to prohibit this change of the shift position if the vehicle is traveling at more than a prescribed speed. If the count value C is equal to −2 in step ST 37 , the shift position is changed to the R position (ST 41 ). If the count value C is equal to −1 in step ST 38 , the shift position is changed to the N position (ST 42 ). If the count value C is equal to 0 in step ST 38 , the D position is maintained (ST 43 ). If the count value C is equal to or greater than 1 (ST 39 : No), the shift position is changed to the L position (ST 44 ). In any of the cases, the program flow returns to the main flow. 
     Referring to  FIG. 9 , upon failing to detect the D position in ST 34  ( FIG. 8 ), a completion of a single shift operation is detected in step ST 45 . Upon detecting the completion of a single shift operation from the L position in step ST 45 , it is determined if the count value C is equal to or smaller than −4 in step ST 46 . If not, it is determined if the count value C is equal to −3 in step ST 47 . If not, it is determined if the count value C is equal to −2 in step ST 48 . If not, it is determined if the count value C is equal to −1 in step ST 49 . If not, it means that the count value C is equal to or greater than 0. 
     If the count value C is equal to or smaller than −4 in step ST 46 , the shift position is changed to the P position (ST 50 ). A separate arrangement may be made to prohibit this change of the shift position if the vehicle is traveling at more than a prescribed speed. If the count value C is equal to −3 in step  47 , the shift position is changed to the R position (ST 51 ). If the count value C is equal to −2 in step  48 , the shift position is changed to the N position (ST 52 ). If the count value C is equal to −1 in step  49 , the shift position is changed to the D position (ST 53 ). If the count value C is equal to or greater than 0 (ST 49 : No), the L position is maintained (ST 54 ). In any of the cases, the program flow returns to the main flow. 
       FIGS. 11 to 15  show the control flow of a second embodiment of the present invention. This embodiment differs from the first embodiment mainly in that the change in the shift position is limited also depending on the vehicle speed. More specifically, an improper change in the shift position due to vehicle speed constraints can be avoided by using a simple structure. When the vehicle is initially started, the shift position is at the P position on account of a per se known interlock system provided in the vehicle. Also, during the operation of the vehicle, the vehicle operator may intentionally select the P position when the vehicle is stationary. Referring to  FIG. 11 , upon detecting the P position (ST 61 : Yes), it is determined if a prescribed condition for allowing the shift position to be moved from the P position is met in step ST 62 . The prescribed condition may include that the brake pedal  22  is depressed, the accelerator pedal  17  is released and/the engine is in operation. If this condition is not met (ST 62 : No), the shift knob  14  may be turned in either direction, but it does not cause the shift position to be moved from the P position. 
     If this condition is met (ST 62 : Yes), a completion of a single shift operation is detected in step ST 63 . As discussed above, a single shift operation may be detected as a continuous angular movement of the shift knob  14  without a pause of more than a prescribed time period such as 0.2 seconds. During a single shift operation, the rotational direction of the shift knob  14  may be reversed. In such a case, the resulting angular movement is measured as the angular displacement between the initial shift position and the final shift position. 
     Upon detecting the completion of a single shift operation from the P position in step ST 63 , it is determined if the count value C is equal to or smaller than zero in step ST 64 . If not, it is determined if the count value C is equal to 1 in step ST 65 . If not, it is determined if the count value C is equal to 2 in step ST 66 . If not, it means that the count value C is equal to or greater 3. 
     If the count value C is equal to or smaller than zero in step  64 , the P position is maintained (ST 68 ), and the program flow returns to the main flow. Therefore, when the shift knob  14  is turned in clockwise direction by any angular displacement, the shift position remains at the P position. 
     If the count value C is equal to 1 in step ST 65 , the shift position is changed by one stage to the R position (ST 69 ). If the count value C is equal to 2 in step ST 66 , the shift position is changed by two stages to the N position (ST 70 ). If the count value C is equal to or greater than 3 (ST 66 : No), the shift position is changed by three stages to the D position, and the vehicle is brought ready to start off. Again, no matter how far more the shift knob  14  is turned beyond this position, the D position is still selected as illustrated in  FIG. 10 . This is advantageous because the shift position is desired to be at the D position when starting off the vehicle, and it is annoying to the vehicle operator if the shift position is brought to the L position owing to the inadvertent excessive turning of the shift knob. Owing to this arrangement, when changing the shift position from the P position to the D position to start off the vehicle, the vehicle operator is allowed to turn the shift knob in counter-clockwise direction by large enough an angle without being required to stop the movement of the shift knob exactly at the D position. In any of the cases, the program flow returns to the main flow. 
     Referring to  FIG. 12 , if it is determined that the P position is not selected in step ST 61  ( FIG. 11 ), it is determined if the R position is selected in step ST 72 . Upon detecting the R position (ST 72 : Yes), a completion of a single shift operation is detected in step ST 73 . Upon detecting the completion of a single shift operation from the R position in step ST 73 , it is determined if the count value C is equal to or smaller than −1 in step ST 74 . If not, it is determined if the count value C is equal to 0 in step ST 75 . If not, it is determined if the count value C is equal to 1 in step ST 76 . If not, it means that the count value C is equal to or greater 2. 
     If the count value C is equal to or smaller than −1 in step ST 74 , it is determined if the absolute value of the vehicle speed V is equal to or lower than a first threshold value Vth 1  (5 km/h, for instance) in step ST 78 . If the absolute value of the vehicle speed V is equal to or lower than the first threshold value Vth 1  (ST 78 : Yes), the shift position is allowed to be changed from the R position to the P position (ST 80 ). If the absolute value of the vehicle speed V is greater than the first threshold value Vth 1  (ST 78 : No), the shift position is prevented from being changed from the R position or maintained at the R position (ST 81 ). Therefore, the change of the shift position from the R position to the P position is permitted only when the vehicle is substantially stationary. In other words, if the vehicle is traveling at some speed in the R position, the clockwise rotation of the shift knob is disregarded, and the R position is maintained. However, when the vehicle is stationary, the clockwise rotation of the shift knob causes the shift position to be changed from the R position to the P position. This is illustrated on the left hand side of  FIG. 16 . 
     If the count value C is equal to 0 in step ST 75 , the R position is maintained (ST 81 ). If the count value C is equal to 1 in step ST 76 , the shift position is changed to the N position (ST 82 ). If the count value C is equal to or greater than 2 (ST 76 : No), it is determined if the vehicle speed is higher than a third threshold value Vth 3  (which is negative in value, and greater in absolute value than the first threshold value Vth 1 ; for instance −10 km/h). If the vehicle is not traveling rearward at a relatively high speed (if the vehicle is traveling rearward at a low speed or traveling forward) (ST 79 : Yes), the shift position is allowed to be changed from the R to the D position (ST 83 ). If the vehicle is traveling rearward at a high speed (ST 79 : No), the change of the shift position is limited to the N position (ST 82 ). Therefore, when the vehicle is traveling rearward at a high speed, the change of the shift position from the R position to the D position is prohibited. 
     This is illustrated on the right hand side of  FIG. 16 . If the vehicle is traveling at some speed in the R position, the counter-clockwise rotation of the shift knob allows the shift position to be changed only to the N position. If the vehicle is stationary, the counter-clockwise rotation of the shift knob allows the shift position to be changed only to the D position. In any of the cases, upon finalizing the shift change, the program flow returns to the main flow. 
     Referring to  FIG. 13 , if it is determined that the R position is not selected in step ST 72  ( FIG. 12 ), it is determined if the N position is selected in step ST 84 . Upon detecting the N position (ST 84 : Yes), a completion of a single shift operation is detected in step ST 85 . Upon detecting the completion of a single shift operation from the N position in step ST 85 , it is determined if the count value C is equal to or smaller than −2 in step ST 86 . If not, it is determined if the count value C is equal to −1 in step ST 87 . If not, it is determined if the count value C is equal to 0 in step DST 88 . If not, it means that the count value C is equal to or greater 1. 
     If the count value C is equal to or smaller than −2 in step ST 86 , it is determined if the absolute value of the vehicle speed V is equal to or lower than a first threshold value Vth 1  (5 km/h, for instance) in step ST 90 . If the absolute value of the vehicle speed V is equal to or lower than the first threshold value Vth 1  (ST 90 : Yes), the shift position is allowed to be changed from the N position to the P position (ST 93 ). If the absolute value of the vehicle speed V is greater than the first threshold value Vth 1  (ST 90 : No), the program flow advances to step ST 91  where the vehicle speed V is compared with a second threshold value Vth 2  which is positive in value and greater in absolute value than the first threshold value Vth 1 , and may be 10 km/h, for instance. 
     If the vehicle speed V is equal to or lower than the second threshold value Vth 2  in step ST 91 , the shift position is changed to the R position (ST 94 ). If the vehicle speed V is higher than the second threshold value Vth 2  in step ST 91 , the shift position is maintained at the N position (ST 95 ). 
     In other words, when the vehicle speed is very small (Vth 1 ≧|V|), be it forward or rearward, the shift position can be changed from the N position to the P position (by two stages). If the vehicle speed is relatively small (Vth 2 ≧|V|&gt;Vth 1 ) or the vehicle is traveling rearward (−Vth 1 &gt;V), the shift position can be changed from the N position only to the R position no matter how far more the shift knob  14  is turned in the clockwise direction. When the vehicle speed V is relatively high (V&gt;Vth 2 ), the shift position is maintained at the N position how far more the shift knob  14  is turned in the clockwise direction. 
     If the count value C is equal to −1 in step ST 87 , the vehicle speed V is compared with the second threshold value Vth 2  in step ST 91 . Again, if the vehicle speed V is equal to or lower than the second threshold value Vth 2  in step ST 91 , the shift position is changed to the R position (ST 94 ). If the vehicle speed V is higher than the second threshold value Vth 2  in step ST 91 , the shift position is maintained at the N position (ST 95 ). 
     If the count value C is equal to 0 in step ST 88 , the shift position is maintained at the N position. If the count value C is equal to greater than 1 (ST 88 : No), the vehicle speed is compared with the third threshold value Vth 3 . If the vehicle is not traveling rearward at a relatively high speed (if the vehicle is traveling rearward at a low speed or traveling forward) (ST 92 : Yes), the shift position is allowed to be changed from the N position to the D position (ST 96 ). If the vehicle is traveling rearward at a high speed (ST 92 : No), the shift position is maintained at the N position (ST 95 ). Therefore, when the vehicle is traveling rearward at a high speed, the change of the shift position from the N position to the D position is prohibited. 
     In any of the cases, upon finalizing the shift change, the program flow returns to the main flow. 
     Referring to  FIG. 14 , if it is determined that the N position is not selected in step ST 84  ( FIG. 13 ), it is determined if the D position is selected in step ST 97 . Upon detecting the D position (ST 97 : Yes), a completion of a single shift operation is detected in step ST 98 . 
     Upon detecting the completion of a single shift operation from the D position (ST 98 : Yes), it is determined if the count value C is equal to or smaller than −3 in step ST 99 . If not, it is determined if the count value C is equal to −2 in step ST 100 . If not, it is determined if the count value C is equal to −1 in step ST 101 . If not, it is determined if the count value C is equal to 0 in step ST 102 . If not, it means that the count value is equal to or greater than 1. 
     If the count value C is equal to or smaller than −3 in step ST 99 , the absolute value of the vehicle speed V is compared with the first threshold value Vth 1  in step ST 103 . If the absolute value of the vehicle speed V is equal to or lower than the first threshold value Vth 1  (ST 103 : Yes), the shift position is allowed to be changed from the D position to the P position (ST 105 ). If the absolute value of the vehicle speed V is greater than the first threshold value Vth 1  (ST 103 : No), the change of the shift position from the D position is limited to the N position (ST 107 ). 
     If the count value C is equal to −2 in step ST 100 , the vehicle speed V is compared with the second threshold value Vth 2 . If the vehicle speed is equal to or lower than the second threshold value Vth 2  in step ST 104 , the shift position is allowed to be changed from the D position to the R position (ST 106 ). If the vehicle speed V is higher than the second threshold value Vth 2  in step ST 104 , the change of the shift position from the D position is limited to the N position (ST 107 ). On the other hand, when the vehicle is stationary, the shift position can be changed from the D position to either the P position or the R position at will. This is illustrated on the left hand side of  FIG. 17 . 
     If the count value C is equal to −1 in step ST 101 , the shift position is changed from the D position to the N position (ST 108 ). If the count value C is equal to 0, the D position is maintained. If the count value C is equal to or greater than 1 (ST 102 : No), the shift position is changed from the D position to the L position. In any of the cases, upon finalizing the change in the shift position, the program flow returns to the main flow. 
     Referring to  FIG. 15 , if it is determined that the D position is not selected in step ST 97  ( FIG. 14 ), it means that the L position is selected. Upon detecting the L position (ST 97 : No), a completion of a single shift operation is detected in step ST 110 . 
     Upon detecting the completion of a single shift operation from the L position (ST 110 : Yes), it is determined if the count value C is equal to or smaller than −4 in step ST 111 . If not, it is determined if the count value C is equal to −3 in step ST 112 . If not, it is determined if the count value C is equal to −2 in step ST 113 . If not, it is determined if the count value C is equal to −1 in step ST 114 . If not, it means that the count value is equal to or greater than 0. 
     If the count value C is equal to or smaller than −4 in step ST 111 , the absolute value of the vehicle speed V is compared with the first threshold value Vth 1  in step ST 115 . If the absolute value of the vehicle speed V is equal to or lower than the first threshold value Vth 1  (ST 115 : Yes), the shift position is allowed to be changed from the L position to the P position (ST 117 ). If the absolute value of the vehicle speed V is greater than the first threshold value Vth 1  (ST 115 : No), the change of the shift position from the L position is limited to the N position (ST 119 ). 
     If the count value C is equal to −3 in step ST 112 , the vehicle speed V is compared with the second threshold value Vth 2 . If the vehicle speed is equal to or lower than the Vth 2  in step ST 116 , the shift position is allowed to be changed from the L position to the R position (ST 118 ). If the vehicle speed V is higher than the Vth 2  in step ST 116 , the change of the shift position from the L position is limited to the N position (ST 119 ). 
     If the count value C is equal to −2 in step ST 113 , the shift position is changed from the L position to the N position (ST 119 ). If the count value C is equal to −1 in step ST 114 , the shift position is changed from the L position to the D position (ST 120 ). If the count value C is equal to or greater than 0 (ST 114 : No), the L position is maintained (ST 121 ). In any of the cases, upon finalizing the change in the shift position, the program flow returns to the main flow. 
     In the foregoing embodiments, the shift member consisted of a shift knob that can be rotated in either direction in an endless manner, and the shift position is normally advanced in a stepwise manner for each prescribed angular displacement of the shift knob  14  such as 30 degrees. However, as shown in  FIG. 18 , the shift member may also consist of a joystick or a momentary switch type shift member  27  that can be moved or tilted in either direction from a neutral (N) position in both forward (F) and rearward (R) directions and configured to be returned to the neutral position when released by using a spring member or the like, and a movement detector associated with the shift member  27  is configured to detect the movement of the shift member according to a time duration of the displacement of the shift member from the neutral direction in each given direction. In such a case, a parking position may be achieved by pressing a switch  25  provided next to the shift member  24 . Alternatively, the movement detector may detect the movement of the shift member according to a number of prescribed displacements of the shift member from the neutral direction in each given direction. 
     The control processes of the first and second embodiments are applicable to these modified embodiments possibly with minor modifications. 
     Although the present invention has been described in terms of preferred embodiments thereof, it is obvious to a person skilled in the art that various alterations and modifications are possible without departing from the scope of the present invention which is set forth in the appended claims. The contents of the original Japanese patent application on which the Paris Convention priority claim is made for the present application as well as the contents of the prior art references mentioned in this application are incorporated in this application by reference.