Abstract:
An inverted vehicle in which the angle of tilt of a vehicle body due to acceleration can be reduced by ground engagement members. The vehicle causes assist wheels to engage the ground when acceleration of the vehicle exceeds a predetermined threshold value to accelerate/decelerate the vehicle. The acceleration may be either requested acceleration or actual acceleration. The ground contact points of the assist wheels are set so that higher the acceleration, the farther away the ground contact points are from the ground contact points of the drive wheels, in the direction opposite to the direction of the acceleration. When the acceleration is within a predetermined range, the vehicle body tilts to a corresponding angle, and when the acceleration exceeds the predetermined threshold value, the assist wheels engage the ground to prevent the vehicle body from tilting beyond a predetermined value.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a vehicle, and in particular relates to a vehicle operating as an inverted pendulum for posture control, for example. 
       BACKGROUND ART 
       [0002]    Vehicles operating as an inverted pendulum for posture control (hereafter simply termed “inverted pendulum vehicles”) have attracted attention. A sensor unit provided in an inverted pendulum vehicle detects the state of balance of a housing and a transportation device is placed in a stationary or moving state by controlling the operation of a rotating body by a control unit. 
         [0003]    JP-A-2004-74814 and JP-A-2004-217170 disclose inverted pendulum vehicles which employ retractable auxiliary wheels as a ground contact member for limiting inclination by placing a section of the vehicle body in contact with the ground. 
         [0004]    JP-A-2004-74814 discusses facilitating the mounting and dismounting of the vehicle by a rider with ground contact of the auxiliary wheels stabilizing the vehicle posture. Furthermore the extension of the auxiliary wheels maintains the vehicle posture when the posture control encounters difficult conditions. 
         [0005]    JP-A-2004-217170 discloses extension of the auxiliary wheels responsive to abnormal operating conditions to maintain vehicle body stability. 
       DISCLOSURE OF THE INVENTION 
     Problem to be Solved by the Invention 
       [0006]    In an inverted pendulum vehicle, the vehicle body must undergo large forward and rearward inclination responsive to a request for rapid acceleration and responsive to sharp braking, in order to maintain balance in the vehicle body by tilting the vehicle body during acceleration or deceleration of the vehicle. Since the field of vision of an occupant is moved through a large vertical range, riding comfort tends to be adversely affected. However the ground contact member (auxiliary wheels) in both of the above patent documents only makes ground contact when the vehicle is stationary or during abnormal operation. 
         [0007]    Thus, it is an object of the present invention to provide a vehicle enabling reduction of an angle of vehicle inclination in response to acceleration and deceleration by using a ground contact member. 
       Means for Solving the Problem 
       [0008]    In order to achieve the above object, the invention provides a vehicle including coaxially disposed drive wheels, a vehicle body having a riding section, acceleration request acquisition means for receiving a requested acceleration of the vehicle, running control means for maintaining the vehicle body, including the riding section, upright by controlling torque to the drive wheels and running in response to the requested acceleration, a ground contact member disposed to be switchable between a ground contact state and a non-ground contact state at a position forward or forward of the drive wheels, and ground contact member control means for extending the ground contact member into ground contact in a direction opposite to the direction of acceleration of the drive wheels when the absolute value of the requested acceleration acquired is greater than or equal to a predetermined threshold. 
         [0009]    Preferably, the ground contact member control means places the ground contact member in ground contact at a position further away from the drive wheels as the absolute value of the requested acceleration increases. 
         [0010]    Preferably, for the non-ground contact state, the ground contact member control means places the ground contact member in a standby position by lifting the ground contact member by a predetermined distance at a position on a vertical line passing through the rotational axis of the drive wheels, or at a ground contact position when the absolute value of the requested acceleration is a predetermined threshold. 
         [0011]    The vehicle may further include selection means for selecting a maximum value for vehicle body angle of inclination. The ground contact member control means makes the acceleration corresponding to a maximum value of the selected vehicle body angle of inclination coincide with the predetermined threshold. 
         [0012]    The vehicle may further include slip detecting means for detecting slip of the drive wheels when the ground contact member is in ground contact. When slip in the drive wheels is detected, the ground contact member control means displaces the ground contact member in a direction away from the drive wheels. 
         [0013]    In another aspect, the invention provides a vehicle including coaxially disposed drive wheels, a vehicle body having a riding section, acceleration request acquisition means for receiving a requested acceleration for the vehicle, running control means for maintaining the vehicle body including the riding section in an upright state by controlling torque to the drive wheels and running in response to the requested acceleration, a ground contact member selectively movable between a ground contact state and a non-ground contact state in a position forward, or forward of the drive wheels, ground contact member control means for placing the ground contact member in ground contact in a direction opposite to the direction of acceleration of the drive wheels when the absolute value of the requested acceleration is greater than or equal to a predetermined threshold, position determination means for determining the position of the ground contact member when in the ground contact state, and correction means for correcting the requested acceleration used by the running control means to a value equal to or less than a limiting acceleration when the absolute value of the limiting acceleration corresponding to the determined position of the ground contact member is smaller than an absolute value of the requested acceleration. 
         [0014]    In yet another aspect, the invention provides a vehicle including coaxially disposed drive wheels, a vehicle body having a riding section, acceleration request acquisition means for acquiring a requested acceleration for the vehicle, running control means for maintaining the vehicle body including the riding section in an upright state by controlling torque of the drive wheels and running in response to the requested acceleration, a ground contact member switchable between a ground contact state and a non-ground contact state in a position forward, or forward of the drive wheels, and ground contact member control means for extending the ground contact member into ground contact in a direction opposite to the direction of acceleration of the drive wheels when the absolute value of the requested acceleration acquired is greater than or equal to a predetermined threshold and when emergency braking is requested. 
         [0015]    According to the present invention, when the absolute value of the requested acceleration is greater than or equal to a predetermined threshold, since the ground contact member is placed in ground contact in a direction opposite to the direction acceleration of the drive wheels, it is possible to reduce the angle of inclination of the vehicle body resulting from acceleration. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]      FIG. 1A  and  FIG. 1B  are external views with occupant mounted on a vehicle according to the present embodiment. 
           [0017]      FIG. 2  shows the constitution of the control unit. 
           [0018]      FIGS. 3A and 3B  show dynamic modes of a vehicle posture control system according to the present embodiment. 
           [0019]      FIG. 4  is a flowchart showing a deceleration running control process according to a first embodiment. 
           [0020]      FIG. 5  shows the relationship between a target value θ 1 * for vehicle body inclination angle and a target value b* for auxiliary wheel position relative to the target value α* for deceleration. 
           [0021]      FIG. 6  shows the constitution of the control unit according to a second embodiment. 
           [0022]      FIG. 7  shows the relationship of an inclination angle command and a maximum vehicle body inclination angle θ 1,Max . 
           [0023]      FIG. 8  is a flowchart of a deceleration running control process according to the second embodiment. 
           [0024]      FIG. 9  is a flowchart of a deceleration running control process according to a third embodiment. 
           [0025]      FIG. 10  is a flowchart of a deceleration running control process according to a fourth embodiment. 
           [0026]      FIG. 11  is a flowchart of a deceleration running control process according to a fifth embodiment. 
       
    
    
     DESCRIPTION OF THE REFERENCE NUMERALS 
       [0000]    
       
         
           
               11  DRIVE WHEELS 
               12  DRIVE MOTOR 
               13  RIDING SECTION 
               14  SUPPORT MEMBER 
               15  ASSIST WHEELS 
               131  SEAT CUSHION 
               132  SEAT BACK 
               133  HEAD RESTRAINT 
               16  CONTROL UNIT 
               20  CONTROL ECU 
               21  MAIN CONTROL ECU 
               22  DRIVE WHEEL CONTROL ECU 
               23  ROD CONTROL ECU 
               30  INPUT DEVICE 
               31  ACCELERATION/DECELERATION COMMAND DEVICE 
               40  VEHICLE BODY CONTROL SYSTEM 
               41  ANGLE METER 
               50  DRIVE WHEEL CONTROL SYSTEM 
               51  DRIVE WHEEL ROTATION ANGLE METER 
               52  DRIVE WHEEL ACTUATOR 
               60  ROD CONTROL SYSTEM 
               61  ROD DRIVE MOTOR ROTATION ANGLE METER 
               62  ROD ACTUATOR F 
               63  ROD ACTUATOR R 
           
         
       
     
       DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0051]    Preferred embodiments of a vehicle according to the present invention will be described with reference to  FIGS. 1 to 11 . 
       (1) Overview of the Embodiments 
       [0052]    A vehicle according to the present embodiment is an inverted pendulum vehicle having a structure in which the shaft of coaxially disposed drive wheels is connected with a riding section. The vehicle maintains the riding section in an upright state by using a sensor to measure the rotation of the vehicle wheels and the inclination of the vehicle body and uses the measurement value by controlling with the drive wheels. 
         [0053]    Furthermore a moveable auxiliary wheel mechanism is provided to function as a ground contact member. The moveable auxiliary wheel mechanism is formed by auxiliary wheels, a rod actuator and a rod control ECU. 
         [0054]    When the absolute value for acceleration exceeds a predetermined threshold during sharp acceleration or deceleration, the auxiliary wheels are placed in ground contact. Acceleration may be either requested acceleration or actual acceleration. The ground contact position of the auxiliary wheels is set to be spaced from the drive wheels (from a reference position) during acceleration, in a direction forward of the ground contact point of the drive wheels, and during deceleration, in a rearward direction, as acceleration increases. 
         [0055]    In the vehicle according to this embodiment, both superior characteristics as an inverted pendulum vehicle and superior characteristics for auxiliary wheels are realized by placing the auxiliary wheels in ground contract at the required position at the required time. 
         [0056]    More precisely, when the absolute value of acceleration is smaller than a predetermined threshold, vehicle body posture is maintained by displacing the center of gravity resulting from vehicle body inclination. Conversely when the absolute value of acceleration is larger than a predetermined threshold, the vehicle body is inclined to an angle of inclination corresponding to the predetermined threshold (the maximum value of the vehicle inclination) and the auxiliary wheels is displaced in contact with the ground in a direction opposite to that of acceleration in order to maintain vehicle body posture by displacement of the center of gravity resulting from vehicle body inclination and a reactive force (normal force) to the auxiliary wheels. 
         [0057]    Although the vehicle body is inclined by an angle corresponding to a predetermined acceleration, since inclination of the vehicle body greater than or equal to the predetermined threshold is limited by the ground contact of the auxiliary wheels relative to the acceleration exceeding the predetermined threshold, an occupant experiences comfortable acceleration and deceleration. 
         [0058]    When the auxiliary wheels make ground contact, the position of the auxiliary wheels (wheel base between the drive wheels and the auxiliary wheels) is varied to a position which maintains the minimum required ground contact load of the drive wheels in response to the dimension of the acceleration. In this manner, accurate variation in speed is realized since the ground contact load of the drive wheels is ensured. 
         [0059]    In the present embodiment, since the auxiliary wheels do not make ground contact during acceleration and deceleration at speeds less than or equal to the predetermined threshold, energy loss resulting from shaft friction and rotational inertia caused by unnecessary ground contact by the auxiliary wheels can be reduced. 
         [0060]    It is not always necessary to provide auxiliary wheels as a ground contact member and a curved member having a predetermined curvature on a distal portion may be placed into ground contact as a ground contact member. 
         [0061]    Another embodiment enables variation of the degree of vehicle body inclination in response to the preference of an occupant. 
         [0062]    Furthermore a target deceleration can be limited with respect to an actual auxiliary wheel position. 
         [0063]    When the drive wheels slip, slip may be avoided by increasing the ground contact load of the drive wheels by increasing the separation of the auxiliary wheels. 
         [0064]    Limiting the target deceleration with respect to an actual auxiliary wheel position eliminates the production of a large acceleration which cannot be dealt with prior to displacement of the auxiliary wheels. 
         [0065]    Furthermore during emergency braking, running control can take priority to vehicle body posture control. In other words, during emergency braking, braking delays resulting from a rearward tilting posture due to braking can be eliminated by maintaining an upright vehicle body or by placing the auxiliary wheels in ground contact immediately while tilting the vehicle body in a direction opposite to the direction of acceleration. 
         [0066]      FIGS. 1A and 1B  show examples of an external appearance with an occupant riding a vehicle according to a first embodiment. As shown in  FIG. 1A , the vehicle includes two co-axially disposed drive wheels  11   a  ( 11   b ). Drive wheels  11   a ,  11   b  are driven respectively by drive motors  12   a ,  12   b . A riding section  13  (seat) for mounting an occupant or cargo (weight bodies) is disposed on an upper section of the drive wheels  11   a ,  11   b  (hereafter, the drive wheels  11   a ,  11   b  will be collectively referred to as drive wheels  11 . Other components will be treated the same below) and the drive motor  12 . The riding section  13  is formed from a seat cushion  131  on which a driver sits, a seat back  132  and a head restraint  133 . The riding section  13  is supported by a support member  14  fixed to a drive motor housing that houses the drive motor  12 . 
         [0067]    An input device  30  is disposed on the side of the riding section  13 . The input device  30  is operated by a driver to perform vehicle commands such as acceleration, deceleration, turning, stationary turning, stopping and braking. Although the input device  30  in the present embodiment is fixed to the seat cushion  131 , the input device  30  may be formed by either a hard-wired or wireless remote controller. Furthermore an armrest may be provided and the input device  30  may be provided on an upper section thereof. Although the input device  30  is provided in a vehicle according to the present embodiment, when the vehicle operates automatically using pre-set running command data, a running command data readout section may be provided in substitution for the input device  30 . The running command data readout section may include, for example, reading means for reading running command data from various types of memory media such as semiconductor memories and/or transmission control means for reading out running command data from an external section by wireless transmission. 
         [0068]      FIGS. 1A and 1B  show a person mounted on the riding section  13 . However, the vehicle is not limited to always transporting a person and may carry only cargo and run and stop by remote control operation, for example, from an external section, carry only cargo and run and stop according to running command data, or run and stop without carrying anything. In the present embodiment, control such as acceleration or deceleration is performed by an operation signal output by operating the input device  30 . 
         [0069]    A control unit (not shown) is disposed between the riding section  13  and the drive wheels  11 . The control unit in the present invention is mounted on a lower face of the seat cushion  131 . 
         [0070]    A moveable auxiliary wheel mechanism is disposed in the seat cushion  131 . The moveable auxiliary wheel mechanism includes auxiliary wheels  15 , rod actuator F 62  and rod actuator R 63 . 
         [0071]    As shown in  FIG. 1A , an end  62   a  of one end of the rod actuator F 62  is disposed in front of the seat cushion  131 . The other end  62   b  is disposed coaxially with the rotation shaft of the auxiliary wheels  15 . An end  63   a  of one end of the rod actuator F 62  is disposed in the back of the seat cushion  131 . The other end  63   b  is disposed coaxially with the rotation shaft of the auxiliary wheels  15 . Both ends of the rod actuator F 62  and rod actuator R 63  are mounted rotatably with respect to the seat cushion  131  and the auxiliary wheels  15 . One end  62   a ,  63   a  of both rod actuators F 62 , R 63  may be attached rotatably with respect to another section of the vehicle body rather than the seat cushion  131 , the support member  14 , for example. 
         [0072]    Both rod actuators F 62 , R 63  have a structure in which the overall length can be varied by compression and expansion. 
         [0073]      FIG. 1A  shows a reference state, that is to say, the state in which the auxiliary wheels  15  are directly below the drive shaft of the drive wheels  11  when the vehicle body posture is upright. In contrast,  FIG. 1B  shows the state of both rod actuators F 62 , R 63  and the inclination of the vehicle body during deceleration. As shown in  FIG. 1B , when the absolute value of acceleration is greater than or equal to a predetermined threshold, as shown in  FIG. 1B , the vehicle body is inclined rearward to an angle of inclination corresponding to the predetermined threshold (a maximum inclination angle) and the auxiliary wheels are placed in ground contact having a separation corresponding to the acceleration forward (opposite direction to that of acceleration) of the drive wheels. In this manner, the effect of the anti-torque of the drive wheels and the inertial force due to the acceleration or deceleration is cancelled out and balance of the vehicle body is maintained by the normal force at the point of ground contact of the auxiliary wheels and the gravitational torque resulting from vehicle body inclination. In this state, the auxiliary wheels are placed in ground contact at a position which is separated forward by a fixed amount by expanding the rear rod actuator R 63  more than the forward rod actuator R 62 . In this manner, the auxiliary wheels can be placed in ground contact at an arbitrary position by adjusting the amount of extension or compression of both rod actuators F 62 , R 63 . 
         [0074]    When the auxiliary wheels  15  make ground contact at an arbitrary position, the auxiliary wheels can be slightly raised at that position by slightly compressing both rod actuators F 62 , R 63  and placed in a standby non-ground contact state. 
         [0075]      FIG. 2  shows the constitution of the control unit. The control unit includes a control electronic control unit (ECU)  20 , an acceleration/deceleration command device  31 , an angle meter (angular velocity meter)  41 , a drive wheel rotation angle meter  51 , a drive wheel actuator  52  (drive motor  12 ), a rod drive motor rotation angle meter (expansion/compression sensor)  61 , rod actuators F 62 , R 63  and other devices. 
         [0076]    The control unit is provided with other devices such as batteries (not shown). The batteries can supply power for driving operations and calculation operations to the drive motor  12 , the drive actuator  52 , both rod actuators F 62 , R 63  and the control ECU  20 . 
         [0077]    The control ECU  20  is provided with a main control ECU  21 , a drive wheel control ECU  22  and a rod control ECU  23 . Each type of control including vehicle running and posture control is performed by the drive wheel control or vehicle body control (inverted pendulum control). The control ECU  20  performs posture control using the auxiliary wheels  15  during acceleration and deceleration in this embodiment. The control ECU  20  is formed from a computer system including a ROM storing data and various programs, a RAM used as an operational section, an external memory device and an interface section. 
         [0078]    The drive wheel rotation angle meter  51 , the angle meter (angular velocity meter)  41 , the rod drive motor rotation angle meter  61  and the acceleration/deceleration command device  31  as the input device  30  are connected to the main control ECU  21 . 
         [0079]    The acceleration/deceleration command device  31  is formed by a joystick for example and running commands based on an operation of an occupant are supplied to the main control ECU  21 . An upright joystick position is a neutral position and commands acceleration by tilting in a longitudinal direction and commands a turning curve by tilting to the right or left. When the angle of inclination increases, the requested acceleration/deceleration or turning curve increases. 
         [0080]    The main control ECU  21  functions as a vehicle body control system  40  together with the angle meter  41  and posture control of the inverted pendulum vehicle is performed by controlling vehicle body posture with the anti-torque of the drive motor  12  based on vehicle body inclination. 
         [0081]    The main control ECU  21  functions as a drive wheel control system  50  together with the drive wheel control ECU  22 , the drive wheel rotation angle meter  51 , and the drive wheel actuator  52 . 
         [0082]    The drive wheel rotation angle meter  51  supplies a rotation angle of the drive wheel  11  to the main control ECU  21 . The main control ECU  21  supplies a drive torque command value to the drive wheel control ECU  22  and the drive wheel control ECU  22  supplies a drive voltage corresponding to the drive command value to the drive wheel actuator  52 . The drive wheel actuator  52  controls both drive wheels  11   a ,  11   b  independently according to the command value. 
         [0083]    The main control ECU  21  functions as drive wheel torque determination means. The main control ECU  21  also functions as a rod control system  60  (ground contact member control means) together with the rod control ECU  23 , the rod drive motor rotation angle meter (expansion/compression sensor)  61  and rod actuators F 62 , R 63 . 
         [0084]    The rod drive motor rotation angle meter  61  supplies a rotation angle of a rod drive motor, that is to say, the compression/expansion amount λ F , λ R  of both rod actuators to the main control ECU  21 . The main control ECU  21  supplies a drive thrust command value to the rod control ECU  23 . The rod control ECU  23  supplies a drive voltage corresponding to the drive thrust command value respectively to both rod actuators F 62 , R 63 . Both rod actuators F 62 , R 63  undergo compression and expansion in response to the command value and in this manner enables switching of the ground contact or non-ground contact and movement of the auxiliary wheels  15  to the predetermined position. 
         [0085]    Assist wheel control will be described with respect to acceleration in the vehicle according to the present embodiment constituted as described above. Although there are cases in which the acceleration requested by the input device will be positive (acceleration) and negative (deceleration), since both are subject to the same control with the direction reversed, the description below will describe negative acceleration, that is to say, an example of deceleration will be described. 
         [0086]      FIGS. 3A and 3B  show dynamic models for a vehicle posture control system according to the present embodiment. The reference numerals in  FIGS. 3A  and  3 B are as follows and the reference numeral in each embodiment below are those reference numerals corresponding to the dynamic model. 
       (a) State Quantities 
       [0000]    
       
         
           
             θ w : rotational angle of drive wheels [rad] 
             θ 1 : angle of inclination of main body (vertical axis standard) [rad] 
             b: distance between the ground contact points of the auxiliary wheels and drive wheels (wheel base) [m] 
             λ F : expansion/compression amount of rod actuator F 
             λ R : expansion/compression amount of rod actuator R 
           
         
       
     
       (b) Input Force 
       [0000]    
       
         
           
             τ W : drive motor torque (2-wheel total) [Nm] 
             T F : thrust of rod actuator F [N] 
             T R : thrust of rod actuator R [N] 
           
         
       
     
       (c) Physical Constants 
       [0000]    
       
         
           
             g: gravitational acceleration [m/s 2 ] 
           
         
       
     
       (d) Parameter 
       [0000]    
       
         
           
             m W : mass of drive wheels [kg] 
             R W : radius of drive wheels [m] 
             I W : inertial moment of drive wheels (about wheel shaft) [kgm 2 ] 
             r W : radius of auxiliary wheels [m] 
             m 1 : mass of main body (including occupant) [ ] 
             I 1 : distance of center of gravity of main body (from wheel shaft) [m] 
             I 1  inertial moment of main body (about center of gravity) [kgm 2 ] 
           
         
       
     
         [0103]      FIG. 4  is a flowchart showing a deceleration running control process according to a first embodiment. 
         [0104]    The main control ECU  21  acquires the respective state quantities from a sensor (step  1 ). In other words, the main control ECU  21  acquires a drive wheel rotation angle θ W  from the drive wheel rotation angle meter  51 , a vehicle body inclination angle θ 1  (angular velocity) from the angle meter (angular velocity meter)  41  and a rotation angle (expansion/compression amount λ F , λ R ) from the rod drive motor rotation angle meter (expansion/compression sensor)  61 . 
         [0105]    The main control ECU  21  acquires an operational amount of an occupant inputted from an acceleration/deceleration command device  31  (for example a joystick operational amount) (step  2 ) and determines a target value α* for deceleration based on the operational amount (step  3 ). The target value α* for deceleration is determined to be a value proportional to the acquired operational amount, for example. 
         [0106]    Next, the main control ECU  21  determines a target value {θ w *} for drive wheel angular velocity from the target value α* for deceleration determined in step  3  (step  4 ). The target value {θ w *} for drive wheel angular velocity is a value obtained by converting an acceleration to a velocity by time integration of the target value α* for deceleration and then dividing that value by a predetermined drive wheel ground contact radius R W . The symbol {X} expresses a time differential for X. 
         [0107]    Next the main control ECU  21  determines a target value θ 1 * for a vehicle inclination angle from Formula 1 and determines a target value b* for auxiliary wheel position from Formula 2 (step  5 ). In other words, the main control ECU  21  determines a required target value θ 1 * for a vehicle inclination angle and a target value b* for auxiliary wheel position to realize the deceleration at the target value α* for deceleration determined in step  3 . 
         [0000]      When α*&lt;α Max , θ 1 *=φ*+sin −1 (tan γ sin φ*) 
         [0000]      When α*≧α Max , θ 1 *=θ 1,Max   Formula 1 
         [0000]      When α*&lt;α Max , b*=0 
         [0000]      When α*≧α Max , b*=C safe b 0 *  Formula 2 
         [0108]    In Formula 1, φ* is an equilibrium axis inclination angle, and is given as φ*=tan −1 α*. When the value for the deceleration target α* increases, φ* increases. 
         [0109]    In Formula 2, b 0 * is a slip limit auxiliary wheel position and is a function of the deceleration target α*. When the deceleration target α* increases, the value for b 0 * increases (refer to Formula 2-2 hereafter). 
         [0110]      FIG. 5  shows the relationship between the target value θ 1 * for a vehicle inclination angle and the target value b* for auxiliary wheel position relative to the target value α* for deceleration determined in step  3  (Formula 1, Formula 2). As shown in  FIG. 5  and Formula 1, when the target value α* for deceleration is less than a threshold α Max , the target value θ 1 * for a vehicle inclination angle increases up to a maximum vehicle body inclination angle α Max  (set value) as the value for the target value α* increases. On the other hand, when the target value α* for deceleration is greater than or equal to the threshold α Max , the target value θ 1 * takes the maximum vehicle body inclination angle θ 1Max  and the vehicle body does not incline greater than that value. 
         [0111]    As shown in  FIG. 5  and Formula 2, when the target value α* for deceleration is less than a threshold α Max , the target value b* for auxiliary wheel position is determined to be zero. When the target value α* is greater than or equal to the threshold α Max , the target value b* for auxiliary wheel position increases together with the target value α*. 
         [0112]    In this manner, when the target value α* for deceleration is less than a threshold α Max , the vehicle body inclines within the range defined by the maximum vehicle body inclination angle θ 1,Max  and the balance of the vehicle body is maintained during deceleration by displacement of the center of gravity due to the vehicle inclination. Within this range, the auxiliary wheels  15  are not in ground contact and elimination of unnecessary ground contact of the auxiliary wheels  15  enables a reduction in energy loss resulting from ground contact of the auxiliary wheels  15 . On the other hand, when the target value α* for deceleration is greater than or equal to the threshold α Max , the target value θ 1 * for the angle of inclination of the vehicle body is maintained to the maximum vehicle body inclination angle θ 1,Max . Since vehicle balance during deceleration is not maintained by only displacement of the center of gravity due to the vehicle inclination, the vehicle body inclines forward and thereafter the shortfall in the inclining torque is compensated for by the normal force at the point of ground contact of the auxiliary wheels  15 . 
         [0113]    A wheelbase b corresponding to the target value α* is set by increasing the target value b* for the auxiliary wheel position together with the target value α* for deceleration. In this manner, forward inclination of the vehicle body when the target value α* for deceleration is large, or slip resulting from a decrease in the ground contact load of the drive wheels can be prevented. 
         [0114]    In Formula 1 and Formula 2, the threshold α Max  is determined from Formula 1-2 below using the predetermined maximum vehicle body inclination angle θ 1Max . Tan γ in Formula 2 is determined from Formula 1-3 and the value M in Formula 1-3 is determined from Formula 1-4. 
         [0000]      α Max =(sin θ 1,Max )/(cos θ 1,Max +tan γ)  Formula 1-2 
         [0000]      tan γ=( MR   W )/( m   1   l   t )  Formula 1-3 
         [0000]        M=m   1   +m   w   +I   w   /R   w   2   Formula 1-4 
         [0115]    In Formula 2, b 0 * is the auxiliary wheel position at the slip limit and is expressed in Formula 2-2, and M b * is expressed in Formula 2-3. 
         [0116]    Idle rotation of the drive wheels can be suppressed by applying a safety coefficient C safe  with respect to the auxiliary wheel position at the slip limit b 0 * whereby safety is ensured. The slip limit is determined with respect to the static friction coefficient μ between the drive wheels and the road surface (predetermined measurement value). The safety coefficient C safe  is a predetermined set value. 
         [0000]        b   0   *=l   1 ( m   1   /M   b *)(tan γ sin φ*+sin(φ*−θ 1,Max ))/cos φ*  Formula 2-2 
         [0000]        M   b *=(1−(α*/μ)) M   Formula 2-3 
         [0117]    The main control ECU  21  displaces the auxiliary wheels  15  towards the target value b* for auxiliary wheel position determined by Formula 2 and therefore that value is used to determine the target values λ F *, λ R * for the rod expansion/contraction amount relative to the rod actuator F 62 , R 63  are determined from Formula 3 below (step  6 ). 
         [0118]    In Formula 3, ε is determined from Formula 3-2 and λ 0  is determined from Formula 3-3. Formula 3 
         [0000]      λ F *=√(( d  cos θ 1,Max   −h  sin θ 1,Max   −b *) 2 +( h  cos θ 1,Max   +d  sin θ 1,Max   +R   W   −r   W +ε) 2 )− l   0    
         [0000]      λ R *=√(( d  cos θ 1,Max   −h  sin θ 1,Max   −b *) 2 +( h  cos θ 1,Max   +d  sin θ 1,Max   +R   W   −r   W +ε) 2 )− l   0    
         [0000]      When b*=0, ε=−δ, 
         [0000]      When b*&gt;0, ε=0,  Formula 3-2 
         [0000]        l   0 =√( d   2 ( h+R   W   r   W ) 2 )  Formula 3-3 
         [0119]    In Formula 3-2, δ is a minute contraction amount for lifting the auxiliary wheels  15  from the ground contact surface. In other words, when target value α*&lt;threshold α Max  (b*=0), from Formula 3, the auxiliary wheels  15  are placed into a standby position not making ground contact at the position δ directly above the ground contact position nearest to the drive wheel  11 . 
         [0120]    Although the contraction amount δ in the present embodiment is arbitrary, it can be set to 5 mm or 1 cm for example. The contraction amount δ may be a value which differs in response to the state of the road surface such as a paved road or an unpaved road. In this case, the state of the road is determined from a vibration state detected by a vibration sensor. The amplitude of the vibration may be detected, and a variation can be performed to increase the contraction amount δ in response to the amplitude. Furthermore a corresponding contraction amount δ may be employed by the occupant inputting paved road or unpaved road. 
         [0121]    In Formula 3-3, l 0  is the reference length of both rod actuators F 62 , R 63 . When the posture of the vehicle is upright, a state in which the auxiliary wheels  15  make ground contact directly below the drive shaft of the drive wheel  11  is taken to be a reference state and the length of the rod at the reference state is taken to be l 0 . The difference from the reference length l 0  is taken to be the rod expansion/contraction amount λ. The symbol d denotes the value when the distance between the ends (fixed points)  62   a ,  63   a  on the riding section  13  side of the rod actuators F 62 , R 63  takes a value of 2d. h is the distance from the median point of both fixed points  62   a ,  63   a  to the rotational center of the drive wheels  11 . 
         [0122]    The structure of the rod actuators F 62 , R 63  in the present embodiment is an example of an auxiliary wheel position control structure and another structure may be employed. For example, one end of the rod actuator may be mounted on the vehicle body such as the seat cushion  131  and the ground contact of the auxiliary wheels  15 , the non-ground contact and the ground contact position may be varied by using the drive motor to adjust the expansion/contraction amount and the angle of the rod. In this case, a target value corresponding to that structure is set in place of Formula 3. 
         [0123]    The main control ECU  21  determines the output command value for each actuator (step  7 ). In other words, the main control ECU  21  determines a torque command value τ W  for the drive wheels  11  from Formula 4 and determines a drive thrust command value T F , T R  for both rod actuators F 62 , R 63  from Formula 5. 
         [0124]    In Formula 4, the target value {θ W *} for the drive wheel angular velocity determined in step  4  and the target value θ 1 * for the vehicle body angle of inclination determined in step  5  are used. 
         [0125]    In the Formula 5, the target values λ F *, λ R * for the rod expansion/contraction amount determined in step  6  are used. 
         [0000]      τ W   =−K   W2 ([θ W ]−[θ W   *]−K   W3 (θ 1 −θ 1 *)− K   W4 ([θ 1 ]−[θ 1 *])  Formula 4 
         [0000]        T   F   =−K   L1 (λ F −λ F *)− K   L2 ([λ F ]−[λ F *])− K   L3 ∫(λ F −λ F *) dt    
         [0000]        T   R   =−K   L1 (λ R −λ R *)− K   L2 ([λ R ]−[λ R *])− K   L3 ∫(λ R −λ R *) dt   Formula 5 
         [0126]    In Formulas 4 and 5, the feedback gain values K W2 , K W3 , K W4  and K L1 , K L2 , K L3  are set in advance using a pole assignment method, for example. In Formula 4, when the auxiliary wheels  15  are in ground contact, the feedback gain may have a value of K W3 =K W4 =0 such that inverted pendulum posture control is not performed. 
         [0127]    In Formula 5, the effect of gravity or dry friction is compensated for by applying an integral gain K L3 . However in a feedforward sense, provision may be made for input application. 
         [0128]    The main control ECU  21  applies the respective command values to each control system and returns to the main routine (step  8 ). In other words, the main control ECU  21  supplies a torque command value τ W  for the drive wheels  11  to the drive wheel control ECU  22  and supplies the drive thrust command value T F , T R  for both rod actuators F 62 , R 63  to the rod control ECU  23 . In this manner, the drive wheel control ECU  22  supplies a drive voltage corresponding to the command value τ W  to the drive wheel actuator  52 , applies the drive torque τ W  to the drive wheels  11  and performs feedback control to coincide with the target value {θ W *} for drive wheel angular velocity and the target value θ 1 * for vehicle body angle of inclination determined in step  4 . 
         [0129]    The rod control ECU  23  supplies the drive voltage corresponding to the drive thrust command value T F , T R  to both rod actuators F 62 , R 63  and performs feedback control to coincide with the target value λ F *, λ R * for the rod extension/contraction amount determined in step  6 . In this manner, the position of the auxiliary wheels  15  coincides with the target value b* for the auxiliary wheel position determined in the step  5 . 
         [0130]    A second embodiment will be described below. 
         [0131]    In the first embodiment, the maximum vehicle body angle of inclination θ 1,Max  determining the threshold α Max  are fixed values determined in advance by a designer. In contrast, in the second embodiment, the differences in a permissible range of vehicle body inclination for occupants are taken into account and the occupant can select the maximum vehicle body angle of inclination θ 1,Max . For example, control is performed to ensure balance to the greatest degree possible when the vehicle body is inclined by increasing the maximum vehicle body angle of inclination θ 1,Max  (threshold α Max ). Conversely, for an occupant requiring running without inclination of the vehicle body, control is performed to maintain the posture to the greatest degree possible with the auxiliary wheels  15  by reducing the maximum vehicle body angle of inclination θ 1,Max  (threshold α Max ). Thus an inverted pendulum vehicle is realized by limiting the vehicle body angle of inclination in accordance with the preferences of an occupant. 
         [0132]      FIG. 6  shows the structure of a control unit according to the second embodiment. The control unit according to the second embodiment includes a vehicle body inclination command device  32  in the input device  30 . The vehicle body inclination command device  32  is an input device for indication of occupant preferences with respect to inclination of the vehicle body. The operational amount is supplied to the main control ECU  21  as an inclination command. 
         [0133]      FIG. 7  shows the relationship between the inclination command supplied from the vehicle body inclination command device  32  and the maximum vehicle body inclination angle θ 1,Max . The main control ECU  21  has a corresponding conversion table or a corresponding conversion formula to  FIG. 7  in a predetermined storage section which is used to determine the maximum vehicle body inclination angle θ 1,Max . In the present embodiment, as shown by the solid line in  FIG. 7 , although the occupant can select an arbitrary value from 0 to a maximum value as a vehicle body inclination angle, a system may be provided enabling selection of a discrete vehicle body inclination angle. For example, a selection may be enabled with respect to two modes being a smooth mode having a small value for the maximum vehicle body inclination angle θ 1,Max  or an active mode with a large value for the maximum vehicle body inclination angle θ 1,Max . Furthermore selection of more stages may be enabled. When selection of a discrete maximum vehicle body inclination angle θ 1,Max  is enabled, the value for the maximum vehicle body inclination angle θ 1,Max  corresponding to the selectable mode or the vehicle body inclination is pre-stored. Other constituent sections of the control unit according to the second embodiment are the same as those of the first embodiment as shown in FIG.  2 . 
         [0134]    Next the operation of the second embodiment will be described. 
         [0135]    In addition to a deceleration running process performed in this embodiment, the main control ECU  21  monitors whether or not an inclination command value has been supplied from the vehicle body inclination command device  32  in accordance with an input of a vehicle body inclination angle by an occupant. When the inclination command is supplied, the vehicle body inclination value is stored in a storage section such as a RAM. The vehicle body inclination command may be stored in a non-volatile storage section rather than a RAM, and once inputted, the vehicle body inclination command may be used continuously with respect to subsequent running operations. Of course, when the occupant changes, a new vehicle body inclination may be inputted by the new occupant and in this case, the data in the storage section can be updated. Vehicle body inclination commands may be stored for respective occupants by discriminating between the occupants. In this case, a load meter is disposed on the seat cushion  131  to infer an occupant from a measured load or the occupant may input their own discrimination data. 
         [0136]      FIG. 8  is a flowchart showing the details of a deceleration running control process according to the second embodiment. Sections which are the same as those processes described in the first embodiment with reference to the flowchart in  FIG. 4  including the embodiment below are designated by the same step numbers, additional description will be omitted and the description will concentrate on points of difference. 
         [0137]    In the same manner as the first embodiment, the main control ECU  21  acquires respective state quantities θ W , θ 1 , λ F , λ R  from a sensor (step  1 ), acquires an operational amount from an occupant (step  2 ), determines a target value α* for deceleration (step  3 ) and determines a target value {θ W *} for drive wheel angular velocity (step  4 ). The main control ECU  21  determines whether or not an inclination command value inputted from the vehicle body inclination command device  32  is present in the input device storage section (step  41 ). 
         [0138]    When a vehicle body inclination command value is present in the input device storage section (step  41 : Y), the main control ECU  21  determines a maximum vehicle body inclination angle θ 1,Max  corresponding to the vehicle body inclination command value from the relationship shown in  FIG. 8  and updates the value of the maximum vehicle body inclination angle used in Formula 1 and Formula 2 (step  42 ). On the other hand, when a vehicle body inclination command value is not present in the input device storage section (step  41 : N), in other words, when the occupant has not set a vehicle body inclination by operation of the vehicle body inclination command device  32 , the main control ECU  21  omits step  42  and proceeds to step  5 . The value of the maximum vehicle body inclination angle θ 1,Max  in this case is determined in the same manner as the first embodiment and is used as a default value. 
         [0139]    Thereafter in the same manner as the first embodiment, the main control ECU  21  determines both the target values θ 1 *, b* for the vehicle inclination angle and auxiliary wheel position (step  5 ), determines the target values λ F *, λ R * for the rod expansion/contraction amount (step  6 ), determines the output command values τ W , T F , T R  for each actuator (step  7 ) and supplies the respective command values τ W , T F , T R  to each control system (step  8 ) and then returns to the main routine. 
         [0140]    Next a third embodiment will be described. 
         [0141]    As described with reference to the first embodiment, the auxiliary wheels  15  are displaced to a target position b* by feedback control. As a result, when a displacement lag is produced, since braking is applied before the auxiliary wheels  15  reach the target position b* for the auxiliary wheels, there is the possibility of temporary forward inclination of the vehicle body or drive wheel slip. Thus in the third embodiment, the target deceleration α* is limited until the auxiliary wheels  15  reach the target position b* with respect to “lags” in the displacement of the auxiliary wheels  15 . More precisely, the target deceleration α* is limited by the actual auxiliary wheel position b. In this manner, loss of balance during deceleration due to vehicle body inclination and auxiliary wheel ground contact can be prevented and forward vehicle inclination and slip can be prevented. This serves as a failsafe if a system of displacing the auxiliary wheels  15  malfunctions. 
         [0142]    The structure of the control unit according to the third embodiment is the same as that of the first embodiment shown in  FIG. 2 . 
         [0143]      FIG. 9  is a flowchart showing the details of deceleration running control process according to the third embodiment. 
         [0144]    In the same manner as the first embodiment, the main control ECU  21  acquires respective state quantities θ W , θ 1 , λ F , λ R  from a sensor (step  1 ), acquires an operational amount from an occupant (step  2 ) and determines a target value f for deceleration (step  3 ). 
         [0145]    The main control ECU  21  uses Formula 6 below to determine the current auxiliary wheel position b (step  31 ). Formula 6 corresponds to the auxiliary wheel position control mechanism (rod actuator F 62 , R 63 ) shown in  FIGS. 1A and 1B . As described in the first embodiment, when using another mechanism the auxiliary wheel position b is determined using an equation corresponding to the structure employed in substitution of Formula 6. 
         [0000]        b =((λ R   −l   0 ) 2 −(λ F   −l   0 ) 2 )/(4 d  cos θ 1 )+( R   W   −r   W )tan θ 1   Formula 6 
         [0146]    Next the main control ECU  21  determines a limiting value α lim  for deceleration corresponding to the current auxiliary wheel position b (step  32 ). Tan η in Formula 7 is determined from Formula 7-2 using the current auxiliary wheel position b and M b  is determined using Formula 7-3. 
         [0000]      α lim =(sin θ 1 −tan η)/(cos θ 1 +tan γ)  Formula 7 
         [0000]      tan η=( M   b   b )/( m   1   l   t )  Formula 7-2 
         [0000]        M   b =(1−(α*/μ)) M   Formula 7-3 
         [0147]    Although the deceleration limit α lim  is defined with reference to the slip limit in the third embodiment, the deceleration limit may be defined with reference to the overturning limit. In this case, M b =M in Formula 7-3. Control stability may be increased by dividing the deceleration limit obtained by Formula 7 by a safety coefficient. 
         [0148]    When the deceleration limiting value determined in response to the current auxiliary wheel position b is smaller than the target value α* for deceleration determined in step  3 , the main control ECU  21  performs a reduction operation to the limiting value α lim  which determined the target value α* for deceleration in step  4  (step  33 ). For example, when the limiting value α lim  for deceleration corresponding to the current auxiliary wheel position b takes a value of 0.3 G at a deceleration determined in step  3  of 0.4 G, the target value α* for deceleration used in step  4  is reduced to 0.3 G. 
         [0149]    The main control ECU  21  determines a target value {θ W *} for the drive wheel angular velocity from the target value α* for deceleration corrected in step  33  (step  4 ). In the same manner as the first embodiment, the main control ECU  21  thereafter determines both the target values θ 1 *, b* for the vehicle inclination angle and auxiliary wheel position (however in this case, the value α* uses the deceleration target value before limiting determined in step  3 ) (step  5 ), determines the target values λ F *, λ R * for the rod expansion/contraction amount (step  6 ), determines the output command values τ W , T F , T R  for each actuator (step  7 ) and supplies the respective command values τ W , T F , T R  to each control system (step  8 ) and then returns to the main routine. 
         [0150]    Next a fourth embodiment will be described. 
         [0151]    In the fourth embodiment, when slip in the drive wheels  11  is detected by slip detection means, the ground contact position of the auxiliary wheels  15  displaces forward while maintaining the vehicle body angle of inclination. In this manner, the vehicle can emerge from a slip condition since the vehicle body center of gravity undergoes relative displacement from the auxiliary wheels  15  towards the drive wheel  11  and the ground contact load of the drive wheels  11  is increased. The structure of the control unit in the fourth embodiment is the same as that of the first embodiment shown in  FIG. 2 . 
         [0152]      FIG. 10  is a flowchart showing the details of a deceleration running control process according to the fourth embodiment. 
         [0153]    In the same manner as the first embodiment, the main control ECU  21  acquires respective state quantities θ W , θ 1 , λ F , λ R  from a sensor (step  1 ), acquires an operational amount from an occupant (step  2 ), determines a target value α* for deceleration (step  3 ) and determines a target value {θ W *} for drive wheel angular velocity (step  4 ). 
         [0154]    The main control ECU  21  determines whether or not the drive wheels  11  are in a state of slip (step  43 ). The method of determining whether or the drive wheels  11  are in a state of slip includes determination using a method employing an observer based on a dynamic model with respect to rotational motion of the drive wheels and a method of comparing the rotation speed of the drive wheels  11  with a value from an acceleration sensor mounted on the vehicle. 
         [0155]    Next the main control ECU  21  determines the current auxiliary wheel position b (step  44 ). The process is the same as step  31  in the third embodiment. The main control ECU  21  determines the current auxiliary wheel position b from Formula 6. The main control ECU  21  corrects the value for coefficient of static frictional μ used in Formulas 2 and 3 to a value obtained from Formula 8 (step  45 ). 
         [0000]      μ=α/(1−( m   1   l   t   /Mb )((cos θ 1 +tan γ)α−sin θ 1 ))  Formula 8 
         [0156]    Acceleration a in Formula 8 is obtained from a history of drive wheel rotation speed immediately prior to slip or from a value from an acceleration sensor mounted on the vehicle. The estimation method (estimation value) may be stabilized by applying the calculation result from Formula 8 to a low pass filter. 
         [0157]    In the same manner as the first embodiment, the main control ECU  21  thereafter determines both the target values θ 1 *, b* for the vehicle inclination angle and auxiliary wheel position (step  5 ), determines the target values λ F *, λ R * for the rod expansion/contraction amount (step  6 ), determines the output command values τ W , T F , T R  for each actuator (step  7 ) and supplies the respective output command values τ W , T F , T R  to each control system (step  8 ) and then returns to the main routine. 
         [0158]    In the fourth embodiment, once the vehicle has slipped, when the estimated value for the coefficient of static friction is decreased, as long as a single cycle of control is not completed the value is not recovered. A reset signal transmission device may be provided to the input device and a value may be initialized using the input signal. Otherwise the value may be gradually recovered over the course of time. 
         [0159]    Next a fifth embodiment will be described. 
         [0160]    The fifth embodiment is a process for dealing with emergency braking operations. During emergency braking, a large braking force is required in addition to deceleration in as short a time as possible. During deceleration, although balance of the drive wheels  11  is maintained by tilting the vehicle rearward, the vehicle is temporarily accelerated by the reactive force of the drive wheel torque tilting the vehicle body rearward and the period of rearward tilting of the vehicle body results in a time loss until commencement of emergency braking. In the fifth embodiment, during emergency braking, running control (deceleration control) is prioritized over vehicle body posture control. More precisely, when a request for emergency braking is detected, the vehicle is deceleration while maintaining vehicle posture by placing the auxiliary wheels  15  in immediately ground contact with the target position without inclining the vehicle body to the rear. 
         [0161]    The structure of the control unit according to the fifth embodiment is the same as that of the first embodiment shown in  FIG. 2 . 
         [0162]      FIG. 11  is a flowchart showing the details of deceleration running control process according to the third embodiment. 
         [0163]    In the same manner as the first embodiment, the main control ECU  21  acquires respective state quantities θ W , θ 1 , λ F , λ R  from a sensor (step  1 ), acquires an operational amount from an occupant (step  2 ), determines a target value a for deceleration (step  3 ) and determines a target value {θ W *} for drive wheel angular velocity (step  4 ). 
         [0164]    The main control ECU  21  determines whether or not an occupant has requested emergency braking (step  46 ). Although the determination of whether an occupant has requested emergency braking is determined from the acceleration/deceleration command value supplied from the input device  30  or the variation rate of that value, it may be determined from a signal from an emergency braking command input device in the input device  30 . 
         [0165]    When emergency braking is not requested (step  46 : N), the main control ECU  21  proceeds the routine to step  5  and in this case, the same processing as the first embodiment is applied. On the other hand, when emergency braking is requested (step  46 : Y), the main control ECU  21  corrects the maximum vehicle body inclination angle θ 1,Max  used in Formula 1 and Formula 2 to θ 1,Max =0. In this manner, the target value θ 1 * for vehicle body inclination angle takes a value of 0, thus time loss and temporary acceleration resulting from vehicle body inclination can be eliminated. 
         [0166]    In the same manner as the first embodiment, the main control ECU  21  determines both the target values θ 1 *, b* for the vehicle inclination angle and auxiliary wheel position from Formula 1 and Formula 2 (the value θ 1,Max  for is varied with respect to the present or absence of a correction) (step  5 ), determines the target values λ F *, λ R * for the rod expansion/contraction amount (step  6 ), determines the output command values τ W , T F , T R  for each actuator (step  7 ) and supplies the respective output command values τ W , T F , T R  to each control system (step  8 ) and then returns to the main routine. 
         [0167]    In the fifth embodiment described above, during emergency braking, the target value θ 1 * for vehicle body inclination angle is placed to a value of 0 and deceleration is enabled while maintaining an upright posture with the auxiliary wheels  15 . However deceleration may be enabled while tilting forward by placing the target value θ 1 * for vehicle body inclination angle to a negative value. In this manner, the reactive force with respect to the vehicle body forward tilting torque can be compensated for as deceleration torque. Posture control may not be performed (waived) by placing the feedback gain KW 3 , KW 4  in Formula 4 to a value of zero. 
         [0168]    In each of the embodiments above, as shown in  FIG. 5  and expressed in Formula 2, when the target value α* for deceleration is greater than or equal to the threshold α Max , although the target value b* for auxiliary wheel position can be increased together with the target value α* for deceleration, the target value b* may be a fixed value. In other words, in Formula 2, if b* is placed to a value of b 0  when α*≧α Max , in the event that the target value α* for deceleration is greater than or equal to the threshold α Max , the auxiliary wheels  15  make ground contact with a position b 0  at a predetermined distance from the drive wheel  11  in a forward position during deceleration or a rear position during acceleration. A value for a position corresponding to the value of maximum envisaged acceleration or deceleration, for example, can be applied in advance as the predetermined value b 0 . There is no necessity for the auxiliary wheels  15  to displace forward or to the rear by an arbitrary amount in response to the acceleration or deceleration. For example, retractable auxiliary wheels  15 F,  15 R may be disposed respectively at a front or back position corresponding to the maximum envisaged acceleration or deceleration. 
         [0169]    In each of the embodiments described above, the target position for the auxiliary wheels when not in ground contact is given by b*=0 (Formula 2). The auxiliary wheels  15  are usually in a standby position δ directly above a ground contact position nearest to the drive wheels  11  with respect to an arbitrary vehicle body angle of inclination θ 1  of target value α* for deceleration&lt;threshold value α Max  (Formula 3). Thus when required, ground contact at a suitable position may be rapidly performed for early enablement of the effect of ground contact by the auxiliary wheels  15 . In this regard, the standby position of the auxiliary wheels may be varied in response to the running velocity of the vehicle in order to adapt rapidly to variation in acceleration or deceleration. For example, when the vehicle is stationary, sharp acceleration may be provided for by moving the auxiliary wheels in a rear direction in advance or when running at near to maximum velocity, sharp braking may be provided for by moving the auxiliary wheels forward in advance. 
         [0170]    Furthermore energy saving may be realized by not performing any control of the auxiliary wheels standby position when the vehicle is not in use.