Patent Publication Number: US-2020288631-A1

Title: Riding type vehicle

Description:
CROSS REFERENCE TO RELATED APPLICATION(S) 
     The entire disclosures of Japanese Patent Application No. 2016-070103, Japanese Patent Application No. 2016-071091, Japanese Patent Application No. 2016-071132, and Japanese Patent Application No. 2016-071163, filed on Mar. 31, 2016 including the specification, claims, drawings, and abstract are incorporated herein by reference in their entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a riding type vehicle that includes a driving source, a left wheel and a right wheel capable of being independently driven with regard to a rotation direction and a rotation speed, and caster wheels separately provided in a front-rear direction with respect to the left wheel and the right wheel. 
     2. Description of the Related Art(s) 
     Lawnmower vehicles that include a lawnmower driven for performing lawn mowing work are conventionally known. Moreover, lawnmower vehicles that include a left wheel and a right wheel, which are main driving wheels independently travel-driven by respective motors such as electric motors or hydraulic motors, and caster wheels, can also be considered in such lawnmower vehicles. 
     Moreover, as lawnmower vehicles, there are lawnmower vehicles capable of self-travel that perform travelling and control of lawn mowing on the vehicle ridden by a driver, and these are called riding lawnmower vehicles. For example, as lawnmowers there are propeller-type rotation blade type and rotation winding blade type lawnmower rotating tools or the like. 
     A riding lawnmower vehicle is used entirely in a so-called off-road situation such as in a garden, and moves on the ground surface for lawn mowing work. 
     For example, Japanese Patent Application Publication No. 2006-507789 (corresponding to US publication No. 2004-0134175) describes a hybrid power device equipped with an engine-dynamo combined unit that connects a rotor to an engine shaft of an internal combustion engine. A lawnmower vehicle illustrated as a power device is described as having independent electric motors respectively connected to multiple driving wheels, the respective driving wheels can be independently controlled at variable speeds, and smooth starting, stopping, speed variation, and direction switching can be performed for such a lawnmower vehicle. FIG. 4 and the description of FIG. 4 in Japanese Patent Application Publication No. 2006-507789 (corresponding to US publication No. 2004-0134175) describe a riding lawnmower vehicle capable of turning with a zero rotation radius. 
     In the case of the vehicle described in FIG. 4 and the description of FIG. 4 in Japanese Patent Application Publication No. 2006-507789 (corresponding to US publication No. 2004-0134175), a turn is made possible by having the speeds of the left and right rear wheels differ. In such a vehicle, a rapid turn can be performed with a small rotation radius. In this case, a driver performs a turn to the rear while looking back to the rear. However, in the case where there is an obstacle target in a region that becomes a blind spot with respect to the driver&#39;s visual field, there is a possibility that this obstacle target will not be able to be confirmed. In particular, at the time when there is an obstacle target, which is a person or object on the outer side in the left-right direction, there is a possibility that this obstacle target will not be able to be confirmed, more to the rear than the driver&#39;s seat of the vehicle and more to the front than the rear end of the vehicle. As a result, it is desirable to implement a configuration where it is easy to automatically detect an obstacle target, at the time of turning travel to the rear. Moreover, in a configuration of a vehicle capable of a rapid turn, where only one of the left wheel and the right wheel rotates around a turn center position, or the left wheel and the right wheel rotate in opposite directions, it will become easy for the vehicle to approach an obstacle target, which is a person or object positioned in a difficult-to-confirm position in a surrounding part, at the time of a rapid turn to the rear. Also, considerable attention is required by the driver in order to avoid colliding with an obstacle target. As a result, it is desirable to implement a configuration where it is easy to automatically avoid a collision with an obstacle target at the time of a rapid turn to the rear. 
     If an obstacle target can be automatically detected at the time of turning travel to the rear, for example, it will be easy to avoid colliding with the obstacle target. Moreover, in a lawnmower vehicle, there is the possibility that an obstacle target will be wound around the lawnmower, by having the obstacle target be near the lawnmower at the time of turning travel to the rear. If an obstacle target can be automatically detected at the time of turning travel to the rear, it will be easy to avoid the obstacle target being wound around the lawnmower at the time of turning travel to the rear. 
     Moreover, in the vehicle described in FIG. 4 and the description of FIG. 4 in Japanese Patent Publication Application No. 2006-507789 (corresponding to US publication No. 2004-0134175), a rapid turn with a small rotation radius is possible by causing only one of the left wheel and the right wheel to rotate, or causing the left wheel and the right wheel to rotate in opposite directions. However, in the case where rapidly turning, it is possible for unstable turning to be performed, where the behavior of the vehicle becomes unstable due to a high turning speed or the like, by the operation of a driver. While such an unstable turn is based on the operation by the driver, it is not desirable from the viewpoint of safe travelling of the vehicle. As a result, a structure is desirable that can automatically suppress an unstable turn of the vehicle. 
     SUMMARY OF THE INVENTION 
     At least one advantage of the present invention, in a configuration of a riding type vehicle where left and right wheels are capable of being independently driven with regard to a rotation direction and a rotation speed, is that it is possible to implement a configuration where it is easy to automatically detect an obstacle target that approaches the vehicle at the time of turning travel to the rear. 
     At least one advantage of the present invention, in a configuration of a riding type vehicle where left and right wheels are capable of being independently driven with regard to a rotation direction and a rotation speed, is that it is possible to implement a configuration where it is easy to automatically avoid colliding with an obstacle target at the time of a rapid turn to the rear. 
     At least one advantage of the present invention, in a configuration of a riding type vehicle where left and right wheels are capable of being independently driven with regard to a rotation direction and a rotation speed, is it is possible to implement a configuration where an unstable turn may be automatically suppressed. 
     A first riding type vehicle according to the present invention has a driving source, a left wheel and a right wheel, a transmission configured to receive power from the driving source to independently operate and drive the left wheel and the right wheel with regard to a rotation direction and a rotation speed, and caster wheels separately provided in a front-rear direction with respect to the left wheel and the right wheel, the first riding type vehicle including two first sensors arranged on both left and right sides more to a front side than a rear end of the vehicle, the two first sensors configured to detect an obstacle target located on a rear side, the obstacle target being a target becoming an obstacle at the time of reversing or turning. 
     A second riding type vehicle according to the present invention has a driving source, a left wheel and a right wheel, a transmission configured to receive power from the driving source to independently operate and drive the left wheel and the right wheel with regard to a rotation direction and a rotation speed, and caster wheels separately provided in a front-rear direction with respect to the left wheel and the right wheel, wherein the second riding type vehicle is capable of a rapid turn where only one of the left wheel and the right wheel rotates around a turn center position, or the left wheel and the right wheel rotate in opposite directions, the second riding type vehicle including a sensor, arranged on the vehicle, capable of detecting an obstacle target on a rear side, the obstacle target being a target becoming an obstacle at the time of reversing or turning, and a control device for causing a rapid turn to the rear to stop, or causing a stop of a rapid turn to be maintained, at the time when the obstacle target has been detected by the sensor. 
     A third riding type vehicle according to the present invention has a driving source, a left wheel and a right wheel, a transmission configured to receive power from the driving source to independently operate and drive the left wheel and the right wheel with regard to a rotation direction and a rotation speed, and caster wheels separately provided in a front-rear direction with respect to the left wheel and the right wheel, wherein the third riding type vehicle is capable of a rapid turn where only one of the left wheel and the right wheel rotates around a turn center position, or the left wheel and the right wheel rotate in opposite directions, the third riding type vehicle including a sensor, arranged on the vehicle, capable of detecting an obstacle target on a rear side, the obstacle target being a target becoming an obstacle at the time of reversing or turning, and a control device for causing a turn to decelerate, at the time when the obstacle target has been detected by the sensor and a rapid turn is performed to the rear, and causing a turn to stop prior to the vehicle colliding with the obstacle target. 
     A fourth riding type vehicle according to the present invention has a driving source, a left wheel and a right wheel, a transmission configured to receive power from the driving source to independently operate and drive the left wheel and the right wheel with regard to a rotation direction and a rotation speed, and caster wheels separately provided in a front-rear direction with respect to the left wheel and the right wheel, wherein the fourth riding type vehicle is capable of a rapid turn where only one of the left wheel and the right wheel rotates, or the left wheel and the right wheel rotate in opposite directions, the fourth riding type vehicle including a rapid turn detection section for detecting the vehicle performing a rapid turn, and a turning speed suppression section for suppressing a turning speed when a rapid turn is being performed and a turning stability relationship amount, which is a physical quantity related to turning stability, is equal to or higher than a threshold. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING(S) 
       Embodiment(s) of the present disclosure will be described based on the following figures, wherein: 
         FIG. 1  is a perspective illustration of a riding type vehicle in an embodiment according to the present invention; 
         FIG. 2  is a view, when the vehicle is seen from above, showing detection ranges of first sensors in an embodiment; 
         FIG. 3  is a block diagram showing the characteristic configuration of the vehicle in an embodiment; 
         FIG. 4A  is a view, seen from the upper side of the vehicle, of a power transmission structure between power generation units for the left wheel and the right wheel, and an engine, in an embodiment; 
         FIG. 4B  is a view of the vehicle showing hydraulic circuits of the power generation units for the left wheel and the right wheel, in an embodiment; 
         FIG. 5  is a schematic illustration showing a state of straight travel of the vehicle in an embodiment; 
         FIG. 6A  is a schematic illustration showing a state of turning travel of the vehicle to the front, in an embodiment; 
         FIG. 6B  is a schematic illustration showing a state of turning of the vehicle, centered on one wheel of the left and right wheels, in an embodiment; 
         FIG. 6C  is a schematic illustration showing a state of turning of the vehicle, centered on the center between the left and right wheels, in an embodiment; 
         FIG. 7  is a schematic illustration showing the inconvenience for the vehicle, when turning to the rear, in an embodiment; 
         FIG. 8  is a view showing a state when the vehicle, from the state of  FIG. 2 , performs an ultra-pivot turn in an a direction, and the sensor detects an obstacle; 
         FIG. 9  is a view showing a state when the vehicle, from the state of  FIG. 2 , performs an ultra-pivot turn in a β direction, and the sensor detects an obstacle; 
         FIG. 10  is a block diagram showing the characteristic configuration of a riding type vehicle in another example of an embodiment according to the present invention; 
         FIG. 11  is a view corresponding to  FIG. 2 , in the configuration shown in  FIG. 10 ; 
         FIG. 12  is a block diagram showing the characteristic configuration of a riding type vehicle in another example of an embodiment according to the present invention; 
         FIG. 13  is a block diagram showing the characteristic configuration of a riding type vehicle in another example of an embodiment of the present invention; 
         FIG. 14  is a schematic illustration showing the inconvenience for the vehicle, when rapidly turning to the rear, centered on one wheel of the left and right wheels, in an embodiment; 
         FIG. 15  is a block diagram showing the characteristic configuration of a riding type vehicle in another example of an embodiment of the present invention; 
         FIG. 16A  is a view corresponding to  FIG. 4A , in the configuration shown in  FIG. 15 , and  FIG. 16B  is a view seen from an arrow A direction of  FIG. 16A ; 
         FIG. 17  is a block diagram showing the characteristic configuration of a riding type vehicle in another example of an embodiment of the present invention; 
         FIG. 18  is a block diagram showing the characteristic configuration of a riding type vehicle in another example of an embodiment of the present invention; 
         FIG. 19A  is a view corresponding to  FIG. 4A , in the configuration shown in  FIG. 18 , and  FIG. 19B  is a view seen from an arrow B direction of  FIG. 19A ; 
         FIG. 20  is a sectional view showing bypass valves operated by bypass actuators mounted in a riding type vehicle in another example of an embodiment of the present invention; 
         FIG. 21  is a block diagram showing the characteristic configuration of a riding type vehicle in another example of an embodiment of the present invention; 
         FIG. 22A  is a view, seen from the upper side of the vehicle, of a power transmission structure between power generation units for the left wheel and the right wheel, and an engine, in another example of an embodiment, and  FIG. 22B  is a view seen from an arrow C direction of  FIG. 22B ; 
         FIG. 23  is a block diagram showing the characteristic configuration of a riding type vehicle in another example of an embodiment of the present invention; 
         FIG. 24  is a block diagram showing the characteristic configuration of a riding type vehicle in another example of an embodiment of the present invention; 
         FIG. 25A  is a view corresponding to  FIG. 4A , in the configuration shown in  FIG. 24 , and  FIG. 25B  is a view seen from an arrow D direction of  FIG. 25A ; 
         FIG. 26  is a block diagram showing the characteristic configuration of a riding type vehicle in another example of an embodiment of the present invention; 
         FIG. 27  is a view corresponding to  FIG. 4A , in the configuration shown in  FIG. 26 ; 
         FIG. 28  is a perspective illustration of a riding type vehicle in another example of an embodiment of the present invention; 
         FIG. 29  is a view, when the vehicle is seen from above, showing a circumscribed circle of the caster wheels, in the case where rapidly turning centered on the center between the left and right wheels, in an embodiment; 
         FIG. 30  is a block diagram showing the characteristic configuration of a vehicle in another example of an embodiment; 
         FIG. 31A  is a view showing the principles of an acceleration sensor, and is a view at the time when a weight of the acceleration sensor is at a neutral position; 
         FIG. 31B  is a view showing the principles of an acceleration sensor, and is a view at the time when a weight of the acceleration sensor is displaced in an X-direction; 
         FIG. 31C  is a view showing the principles of an acceleration sensor, and is a view at the time when a weight of the acceleration sensor is displaced in a Y-direction; 
         FIG. 31D  is a view showing the principles of an acceleration sensor, and is a view at the time when a weight of the acceleration sensor is displaced in a Z-direction; 
         FIG. 32  is a block diagram showing the characteristic configuration of a riding type vehicle in another example of an embodiment of the present invention; 
         FIG. 33  is a view corresponding to  FIG. 4B , in a riding type vehicle in another example of an embodiment of the present invention; 
         FIG. 34  is a view corresponding to  FIG. 4B , in a riding type vehicle in another example of an embodiment of the present invention; 
         FIG. 35A  is a view showing two of the conditions for suppressing the turning speed by using a relationship between an operation amount of a left operation lever and pressure detection values of a first oil path and a second oil path, in the configuration shown in  FIG. 34 , and  FIG. 35B  is a view showing two of the conditions for suppressing the turning speed by using a relationship between an operation amount of a right operation lever and a pressure detection values of a third oil path and a fourth oil path; and 
         FIG. 36A  is a view corresponding to  FIG. 4B , in a riding type vehicle in another example of an embodiment of the present invention, and  FIG. 36B  is a view seen from an arrow H direction of  FIG. 36A . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S) 
     Hereinafter, embodiments according to the present invention will be described in detail using the figures. Note that hereinafter, while a configuration will be mainly described where left and right wheels of a riding lawnmower vehicle are driven by hydraulic motors as motors for travelling, the motors for travelling may be other motors, such as electric motors. Hereinafter, while a case will be described where the wheels are arranged on the rear side as left and right main driving wheels, and caster wheels are arranged on the front side, the wheels may be on the front side and the caster wheels may be on the rear side. 
     The shape, number, and the arrangement relationships of parts or the like stated hereinafter are illustrations for the description, and arbitrary changes are possible, in accordance with the specifications or the like of the riding lawnmower vehicle. Moreover, hereinafter, the same reference numerals will be attached to similar elements in all of the figures, and overlapping descriptions will be omitted or simplified. 
       FIG. 1  to  FIG. 9  show a riding lawnmower vehicle that is a riding type vehicle according to an embodiment. Hereinafter, a riding lawnmower vehicle  10  will be described as a vehicle  10 .  FIG. 1  is a perspective illustration of the vehicle  10 .  FIG. 2  is a view, when the vehicle  10  is seen from above, showing detection ranges of first sensors  50   a ,  50   b .  FIG. 3  is a block diagram showing the characteristic configuration of the vehicle  10 .  FIG. 4A  is a view, seen from the upper side of the vehicle  10 , of a power transmission structure between power generation units  26 ,  27  for a left wheel  12  and a right wheel  13 , and an engine  14 .  FIG. 4B  is a view of the vehicle  10  showing hydraulic circuits  28 ,  29  of the power generation unit  26 ,  27  for the left wheel  12  and the right wheel  13 . 
     The vehicle  10  is a self-propelled type off-road vehicle suitable for lawnmowing. The vehicle  10  includes a left wheel  12  and a right wheel  13 , caster wheels  15 ,  16 , a lawnmower  18 , two first sensors  50   a ,  50   b , a tension switching actuator  43  ( FIG. 3 ,  FIG. 4A ), and a controller  60 , which is a control device ( FIG. 3 ). 
     The left wheel  12  and the right wheel  13  are rear wheels supported on both the left and right sides of the rear side of a main frame  20 , which is a vehicle body, and are main driving wheels. The main frame  20  is formed in a beam structure or the like, by a metal such as steel. The main frame  20  includes side plate parts  20   a ,  20   b  extending in an approximately front-rear direction at both the left and right ends, and a connection part  20   c  that connects the side plate parts  20   a ,  20   b  of both the left and right sides. A driver&#39;s seat  21 , on which a driver sits, is fixed on the upper side between the rear end parts of the left and right side plate parts  20   a ,  20   b.    
     Left and right operation levers  22 ,  23  are supported, on the main frame  20 , so as to project from the front side floor of the driver&#39;s seat  21 . The tip part of each of the operation levers  22 ,  23  is gripped by the driver, and is used for indicating the rotation direction and rotation speed of the left wheel  12  and the right wheel  13 . Each of the operation levers  22 ,  23  is approximately L-shaped, and has a gripping part  24  formed extending in the left-right direction on the upper end part. The gripping part  24  is gripped and operated by the driver. Each of the operation levers  22 ,  23  is capable of swinging, centered on an axis in position of the lower end part along the left-right direction. 
     The left wheel  12  and the right wheel  13  protrude more on the outer side than the left-right direction outer end of the side plate parts  20   a ,  20   b  of the main frame  20 . The upper side of each of the wheels  12 ,  13  has at least one part covered by a wheel cover  25 , and the left-right direction inner side end parts of the wheel covers  25  are fixed to the side plate parts  20   a ,  20   b.    
     The two left and right caster wheels  15 ,  16  are steering control wheels supported on the front end part of the main frame  20 , and are front wheels. The left wheel  12  and the right wheel  13  are independently travel-driven by a left hydraulic motor  30  ( FIG. 4B ) and a right hydraulic motor  31  ( FIG. 4B ), described below, which are two motors for travelling. As a result, each of the caster wheels  15 ,  16  is separately provided in the front-rear direction with respect to the left wheel  12  and the right wheel  13 , in the front-rear direction of the vehicle  10 . Each of the caster wheels  15 ,  16  is capable of freely rotating through 360 degrees or more centered on a vertical axis (the up-down direction of  FIG. 1 ). Note that the caster wheels are not limited to the configuration where two are arranged on the vehicle, and one, or three or more, may be arranged on the vehicle. Hereinafter, the left wheel  12  and the right wheel  13  will sometimes be described as the left and right wheels  12 ,  13 . 
     As shown in  FIG. 4B , the left wheel  12  and the right wheel  13  are capable of being independently driven, with regard to a rotation direction and a rotation speed, by a transmission  11 . For example, the transmission  11  is constituted to be capable of independently operating and driving the left wheel  12  and the right wheel  13  receiving power from a driving source, with regard to a rotation direction and a rotation speed. The transmission  11  includes left and right power generation units  26 ,  27 . Power of the engine  14 , as a driving source, is input to the transmission  11 , and outputs of the left and right hydraulic pumps  32 ,  33  are output, via deceleration gear mechanisms  26   b ,  27   b , to drive shafts of the left and right wheels  12 ,  13 . As a result, the transmission  11  is constituted to be capable of independently driving the left wheel  12  and the right wheel  13 , which receive power from the engine  14 , with regard to a rotation direction and a rotation speed. Left and right fixed capacity type hydraulic motors  30 ,  31  respectively constitute the left and right power generation units  26 ,  27 . The left and right hydraulic motors  30 ,  31  are respectively connected to the drive shafts of the left wheel  12  and the right wheel  13 . The power generation units  26 ,  27  generate power for a wheel, and include cases  26   a ,  27   a , and hydraulic circuits  28 ,  29  on the inner side, respectively. The hydraulic circuits  28 ,  29  respectively have variable-capacity swash-plate type hydraulic pumps  32 ,  33 , and hydraulic motors  30 ,  31  driven by having pressurized oil supplied from the hydraulic pumps  32 ,  33 , and oil paths  34  connecting the hydraulic pumps  32 ,  33  and the hydraulic motors  30 ,  31 . The hydraulic motors  30 ,  31  are, for example, fixed capacity type motors. Driven pulleys  35  are respectively fixed to drive shafts  32   a ,  33   a  of the hydraulic pumps  32 ,  33 , and are driven, via a belt  36 , by the engine  14  as a power source, described below. The hydraulic pumps  32 ,  33  function as input sections of the transmission  11 . 
     Each of the hydraulic pumps  32 ,  33  includes a left swash plate operation shaft  32   b , which is a left adjustment shaft, and a right swash plate operation shaft  33   b , which is a right adjustment shaft, for changing a tilting angle and orientation of a movable swash plate by rotation, and a swash plate operation lever  32   c ,  33   c  connected to the swash plate operation shaft  32   b ,  33   b . The left swash plate operation shaft  32   b  adjusts a pressurized oil discharge amount of the left hydraulic pump  32 . The right swash plate operation shaft  33   b  adjusts a pressurized oil discharge amount of the right hydraulic pump  33 . The lower end parts of the operation levers  22 ,  23  of left and right corresponding sides are respectively connected, via a link  37 , to the swash plate operation levers  32   c ,  33   c . As a result, by having the operation levers  22 ,  23  swing in the front-rear direction, the swash plate operation shafts  32   b ,  33   b  will rotate. Also, the tilting angles and orientations of the movable swash plates of the hydraulic pumps  32 ,  33  will change. The discharge amounts of the hydraulic pumps  32 ,  33  change, in accordance with the change of tilting angles of the movable swash plates. By lowering the operation levers  22 ,  23  significantly to the front or the rear, the discharge amounts of the hydraulic pumps  32 ,  33  will increase. The left hydraulic motor  30  is driven by a pressurized oil supply from the left hydraulic pump  32 . The right hydraulic motor  31  is driven by a pressurized oil supply from the right hydraulic pump  33 . By lowering the operation levers  22 ,  23  more to the front than a neutral state, discharge directions will be prescribed so that the hydraulic pumps  32 ,  33  cause the hydraulic motors  30 ,  31  to rotate to one side. By having the operation levers  22 ,  23  fall more to the rear than a neutral state, discharge directions will be prescribed so that the hydraulic pumps  32 ,  33  cause the hydraulic motors  30 ,  31  to rotate to the other side. A neutral state is a state where there is no discharge of oil at a position the operation levers  22 ,  23  automatically return to in a state not gripped by the driver. For the rotation directions of the hydraulic motors  30 ,  31 , one side corresponds to the rotation of a forward direction of the wheels  12 ,  13  and the other side corresponds to the rotation of a backward direction of the wheels  12 ,  13 . Moreover, swing angle positions of the operation levers  22 ,  23  are detected by lever potentiometers  38 ,  39 , which are swing angle detection sections. Detection signals of the lever potentiometers  38 ,  39  are transmitted to a controller  60  ( FIG. 3 ), described below. 
     Moreover, in the hydraulic circuits  28 ,  29  of  FIG. 4B , a charge oil path C 1  is connected to two main oil paths S 1 , S 2 , which connect the hydraulic pumps  32 ,  33  and the hydraulic motors  30 ,  31 . The charge oil path C 1  connects each of the main oil paths S 1 , S 2 , and an oil reservoir E, via check valves F 1 , F 2 . The charge oil path C 1  is for replenishing oil from the oil reservoir E to the main oil path of a low pressure side, from among the main oil paths S 1 , S 2 . Moreover, bypass valves  28   a ,  29   a  are connected between each of the main oil paths S 1  S 2 , and the oil reservoir E. The bypass valves  28   a ,  29   a  are configured to be capable of switching the connection and disconnection between the main oil paths S 1 , S 2 , and the oil reservoir E, manually. In the example of  FIG. 4B , in the case where each of the bypass valves  28   a ,  29   a  has been switched to an opened side, the main oil paths S 1 , S 2  will be connected to the oil reservoir E, via throttles, and the throttles may be omitted. 
     The left and right wheels  12 ,  13  are respectively connected to the output shafts of the left and right hydraulic motors  30 ,  31 , to be capable of transmitting power via the deceleration gear mechanisms  26   b ,  27   b , which constitute the power generation units  26 ,  27 . As will be described below, the vehicle  10  is capable of straight travel and turning travel by independent control of the left and right wheels  12 ,  13 . 
     The engine  14  is arranged, in the vehicle  10 , on the rear side of the driver&#39;s seat  21  ( FIG. 1 ). As shown in  FIG. 4A , the engine  14  has a drive shaft  14   a , along the vertical direction (the front-back direction of the paper surface of  FIG. 4A ), that rotates centered on the vertical direction. A drive pulley  40  is fixed to the drive shaft  14   a , and a belt  36  is suspended between the drive pulley  40  and the two driven pulleys  35  provided in the power generation units  26 ,  27 . The drive pulley  40  and the two driven pulleys  35  correspond to shaft pulleys. As a result, by driving the engine  14 , the hydraulic pumps  32 ,  33  will be driven, via the drive pulley  40 , the belt  36 , and the driven pulleys  35 . Pressurized oil is discharged from the hydraulic pumps  32 ,  33 , and the hydraulic motors  30 ,  31  rotate, by operations of the operation levers  22 ,  23 . Moreover, the vehicle  10  is controlled so that the engine  14  is driven at a constant rotation speed determined beforehand, by having a start switch (not illustrated) set to ON by a user. Electric motors may be provided as the driving sources of the hydraulic pumps  32 ,  33 . 
     By rotating the left and right wheels  12 ,  13  in mutually opposite directions at the same speed, such as described below, it will be possible for the vehicle  10  to rapidly turn around a turn center position  70  ( FIG. 2 ), which is a position in the middle of the left wheel  12  and the right wheel  13 . 
     Moreover, as shown in  FIG. 4A , the belt  36  can include a belt tension switching mechanism  41 , to function as a clutch arranged between an output section of the driving source and an input section of the transmission  11 , and as a result, the presence or absence of tension can be switched. The belt tension switching mechanism  41  includes a pressing force pulley  42 , which presses the belt  36  from an outer circumference side, and a tension switching actuator  43 , which switches the presence or absence of the pressing force applied to the belt  36  from the pressing force pulley  42 . The pressing force pulley  42  is supported on one end (the left end of  FIG. 4A ) of a swinging plate part  44 . The swinging plate part  44  is supported to be capable of swinging centered on a vertical axis, which is positioned at the middle part of the swinging plate part  44 , in the main frame  20  ( FIG. 1 ). The tension switching actuator  43  includes a cylinder member  45 , a rod  46  supported on the cylinder member  45  to be capable of being displaced in an axial direction, and a linear-type solenoid (not illustrated) for changing the projection length of the rod  46  from the cylinder member  45 . 
     The solenoid is arranged surrounding the rod  46  on the inner side of the cylinder member  45 , and operates so as to cause the rod  46  to project from the cylinder member  45  as a result of energizing the solenoid. The tip part of the rod  46  is joined to the other end part (the right end part of  FIG. 4A ) of the swinging plate part  44 . A spring  47  is attached to the swinging plate part  44 , and the spring  47  applies an elastic force in a direction that presses the pressing force pulley  42  against an outer circumference surface of the belt  36 . As a result, the swinging plate part  44  will swing in a direction that separates the pressing force pulley  42  from the belt  36 , as the projection length of the rod  46  increases by having the solenoid energized. Accordingly, since the tension of the belt  36  becomes zero, and the power transmission from the engine  14  to the hydraulic pumps  32 ,  33  ( FIG. 4B ) is blocked, the discharge amounts of the hydraulic pumps  32 ,  33  will become zero or extremely small, and the rotation of the hydraulic motors  30 ,  31  ( FIG. 4B ) will stop. At this time, power transmission in the clutch, between the engine  14  and the transmission  11 , will be cut. Therefore, the rotation of the left and right wheels  12 ,  13 , connected to be capable of transmitting the power of the hydraulic motors  30 ,  31 , will also stop. As a result of this, travelling of the vehicle  10  is stopped, and turning, in the case where the vehicle  10  is turning, is also stopped. The tension switching actuator  43  is controlled by the controller  60  ( FIG. 3 ), described below, and engages/disengages the clutch. At the time when at least one of obstacle targets T 1 , T 2 , and T 3  ( FIG. 2 ) has been detected by the first sensors  50   a ,  50   b , described below, the controller  60  causes turning of the vehicle  10  to stop, by operating the tension switching actuator  43 . As a result, it will become difficult for the vehicle  10  to collide with the obstacle target at the time of a turn. In the case where the solenoid is not energized, tension is generated in the belt  36 , and power is transmitted from the engine  14  to the hydraulic pumps  32 ,  33 , and therefore power transmission in the clutch, between the engine  14  and the transmission  11 , will be in a connected state. 
     Returning to  FIG. 1 , the lawnmower  18  is supported on the lower side of the longitudinal direction middle part of the main frame  20 . The lawnmower  18  is arranged between the caster wheels  15 ,  16  and the left and right wheels  12 ,  13 , in the front-rear direction. The lawnmower  18  includes a lawnmowing blade (not illustrated), which is a lawnmower rotating tool arranged on the inner side of a mower deck  19 , which is a cover. The lawnmowing blade is covered on the upper side by the mower deck  19 . The lawnmowing blade has multiple blade components (not illustrated) that rotate around a shaft facing in a vertical direction (the up-down direction of  FIG. 1 ). As a result, mowing is possible by having the blade elements rotate and cut grass. The lawnmowing blade is rotatably driven by a lawnmower drive motor  48  ( FIG. 3 ) controlled by the controller  60  ( FIG. 3 ), described below. Note that the lawnmower may be configured to be capable of being driven by receiving power from the engine  14 , by suspending a belt between the drive pulley fixed to the drive shaft of the engine  14  and a driven pulley fixed to the drive shaft of the lawnmowing blade or the like. The mowed grass is discharged to one side in the left-right direction of the vehicle  10  through a discharge port, which is not illustrated, provided on one side (the left side of  FIG. 1 ) in the left-right direction of the mower deck  19 . A grass collection duct can be connected to the mower deck  19 , and mowed grass can be collected in a grass collection tank connected to the grass collection duct. 
     Moreover, both end parts in the left-right direction of the mower deck  19  respectively project to the outer side from both the left and right ends, in a front-rear direction middle part of the side plate parts  20   a ,  20   b  on both the left and right sides, which constitute the main frame  20 . Moreover, the left and right wheels  12 ,  13  are respectively arranged more to the rear than the portion where the mower deck  19  projects to the outer side, and more on the outer side than the outer end in the left-right direction, on the side plate parts  20   a ,  20   b  of the main frame  20 . 
     As shown in  FIG. 2 , the two first sensors  50   a ,  50   b  are arranged separated on both the left and right sides of the vehicle  10 . Specifically, the two first sensors  50   a ,  50   b  are arranged separately fixed on both the left and right end parts projecting to the outer side from the side plate parts  20   a ,  20   b  of the main frame  20 , on the upper side portion of the upper surface or the like of the mower deck  19 . As a result, the two first sensors  50   a ,  50   b  are arranged on both the left and right sides more to the front than the rear end of the vehicle  10 . Each of the first sensors  50   a ,  50   b  is constituted so as to detect the presence or absence of an obstacle or person, which is an obstacle target positioned on the rear side. An obstacle target is a target, positioned on the rear side of the vehicle, for example, that will be an obstacle at the time of reversing or at the time of turning. A millimeter wave radar is used, for example, as such first sensors  50   a ,  50   b . At this time, the millimeter wave radar can detect the presence of an obstacle target in a detection region determined beforehand, by having electrical waves transmitted from a transmission section reflected by the obstacle target, and receiving this reflection using a reception section. Moreover, it is preferable for the first sensors  50   a ,  50   b  to have directivity to one direction, in order to prevent the vehicle  10  itself being detected. In addition, it is preferable for the first sensors  50   a ,  50   b  to be capable of measuring a distance up to an obstacle target. For example, the first sensors  50   a ,  50   b  are capable of measuring the distance up to an obstacle target, by receiving electrical waves transmitted from one transmission section, in the millimeter wave radar, with two reception sections provided at different positions. In  FIG. 2 , the detection regions of each of the first sensors  50   a ,  50   b  are shown by shaded regions. The detection regions extend to the rear, and do not cross the vehicle  10 . Detection signals of the first sensors  50   a ,  50   b  are transmitted to the controller  60  ( FIG. 3 ). Laser radars, ultrasonic wave sensors, infrared sensors or the like may be used as the first sensors  50   a ,  50   b.    
     As shown in  FIG. 3 , the controller  60  includes an operation section such as a CPU and a storage section such as a memory, and is constituted, for example, by a microcomputer. The controller  60  has a rear turn determination section  61 , a turn stop section  62 , and a lawnmower drive stop section  63 . The rear turn determination section  61  determines whether or not the vehicle  10  is turning to the rear from detection signals of the left and right lever potentiometers  38 ,  39 . For example, the rotation directions and rotation angles of the left and right wheels  12 ,  13  are calculated from these detection signals. In the case where the left and right wheels  12 ,  13  rotate to the rear, and the rotation speeds of the left and right wheels  12 ,  13  differ, it is determined that the vehicle  10  is turning to the rear. Moreover, in the case where one wheel of the left and right wheels rotates to the front, and the other wheel rotates to the rear, and in the case where an absolute value of the rotation speed of the other wheel is larger than an absolute value of the rotation speed of the one wheel, it is determined that the vehicle  10  is rapidly turning to the rear. Moreover, in the case where only one wheel of the left and right wheels  12 ,  13  rotates in a backward direction, it is also determined that the vehicle  10  is rapidly turning to the rear. Such a rapid turn will be described afterwards using  FIG. 8  and  FIG. 9 . At the time when the respective absolute values of ground movement speeds of a rear rotating wheel, which is a wheel rotating to the rear, and a front rotating wheel, which is a wheel rotating to the front, are larger than zero, and a difference between both absolute values is zero, the rapid turn will be a zero-turn. 
     At the time when an obstacle target has been detected by at least one of the first sensors  50   a ,  50   b , in the case where it is determined that the vehicle  10  is turning to the rear by the rear turn determination section  61 , the turn stop section  62  causes the rear turn of the vehicle  10  to stop. At this time, by having the turn stop section  62  control the driving of the tension switching actuator  43 , the driving of the left and right hydraulic motors  30 ,  31  is stopped by making the tension of the belt  36  zero, namely, by cutting the clutch. As a result, the left and right wheels  12 ,  13  stop, and therefore the turn to the rear stops. Moreover, at the time when an obstacle target has been detected by at least one of the first sensors  50   a ,  50   b , in the case where it is determined that the vehicle  10  is stopped, the turn stop section  62  causes the stop of the turn to the rear of the vehicle  10  to be maintained. At this time, the stopping of straight travel to the rear may be maintained, along with maintaining a stop of a turn to the rear of the vehicle  10 . 
     In addition, at the time when the lawnmower drive motor  48  is driving, at the time when an obstacle target has been detected by any one of the first sensors  50   a ,  50   b , the lawnmower drive stop section  63  causes the driving of the lawnmower drive motor  48  to stop. At this time, the lawnmower drive stop section  63  causes rotation to stop by controlling the driving of the lawnmower drive motor  48 . Moreover, at the time when the lawnmower drive motor  48  is drive-stopped, at the time when an obstacle target has been detected by any one of the first sensors  50   a ,  50   b , the lawnmower drive stop section  63  causes the drive stop of the lawnmower drive motor  48  to be maintained. 
       FIG. 5  is a schematic illustration showing a state of straight travel of the vehicle  10 . In  FIG. 5 , a positional relationship of the left and right wheels  12 ,  13  and the caster wheels  15 ,  16  is shown. As shown in  FIG. 5 , straight travel of the vehicle  10  is possible by causing the rotation speeds of the left and right wheels  12 ,  13  to match, using the left and right hydraulic motors  30 ,  31  ( FIG. 4B ). At this time, ground movement speeds V 1 , V 2 , which are the movement speeds of the ground position with respect to the ground surface of the left and right wheels  12 ,  13 , will match. The power source is not connected to the left and right caster wheels  15 ,  16 , and the caster wheels  15 ,  16  are rotated in a following manner from the ground surface in accordance with the travelling of the vehicle  10  by the driving of the left and right wheels  12 ,  13 . On the other hand, turning travel of the vehicle  10  is possible by generating a rotation speed difference between the left and right wheels  12 ,  13 . 
       FIG. 6A ,  FIG. 6B , and  FIG. 6C  show three examples of turning travel of the vehicle. Also in  FIG. 6A ,  FIG. 6B , and  FIG. 6C , similar to  FIG. 5 , a positional relationship of the left and right wheels  12 ,  13  and the caster wheels  15 ,  16  is shown.  FIG. 6A  is a schematic illustration showing a state of turning travel of the vehicle  10  to the front. In  FIG. 6A , a turn center position  70 , at the time when seen from above, is the outer side of the left wheel  12  on an extension line of a wheel axis direction of the left and right wheels  12 ,  13 . At this time, the vehicle  10  turns in a comparatively gradual manner. 
       FIG. 6B  is a schematic illustration showing a state of turning of the vehicle  10 , centered on the one wheel  12  of the left and right wheels  12 ,  13 . In  FIG. 6B , the turn center position  70  is the ground position of the one wheel  12 . Such a turn is called a pivot turn, and the vehicle  10  turns more rapidly than in the case of  FIG. 6A . 
       FIG. 6C  is a schematic illustration showing a state of turning of the vehicle  10 , centered on the center between the left and right wheels  12 ,  13 . In  FIG. 6C , the turn center position  70 , when seen from above, is a center position between the left and right wheels  12 ,  13  on an extension line in a wheel axis direction of the left and right wheels  12 ,  13 . Moreover, while the absolute values of the speeds V 1 , V 2  of the left and right wheels  12 ,  13  are the same, the direction of the speed V 1  of the one wheel  12  is opposite to the direction of the speed V 2  of the other wheel  13 . In this case, the vehicle  10  additionally turns more rapidly than in the case of  FIG. 6B . Such a turn is called an ultra-pivot turn, a spin turn, or a zero turn radius (ZTR) turn, where a turn radius is zero. 
       FIG. 7  is a schematic illustration showing the inconvenience for the vehicle  10 , when turning to the rear. In  FIG. 7 , for ease of understanding, the vehicle  10  is shown by a dash-dotted line G 1  rectangle and a dotted line G 2  rectangle. From a state where the vehicle  10  is the dash-dotted line G 1 , the ground movement speeds of the left and right wheels  12 ,  13  are V 1 , V 2  in the backward direction, and there will be cases where an absolute value of the ground movement speed V 1  of the left wheel  12  is larger than an absolute value of the ground movement speed V 2  of the right wheel  13 . In this case, as shown by the dotted line G 2 , the vehicle  10  turns to the rear as shown by the arrow a direction. Also, in the state of the dash-dotted line G 1 , there will be cases where there is an obstacle target, shown by T, more to the front than the rear end of the vehicle  10 , and nearer to the outer side than the left side surface. At this time, there will be times where the obstacle target T cannot be seen by the driver in the driver&#39;s seat  21 , or the driver fails to notice the obstacle target T. At this time, since the vehicle  10  continues to turn while extending in the left-right direction outer side at the front side, the vehicle  10  will collide with the obstacle target T, in the state of the dotted line G 2 . In the vehicle  10  of an embodiment shown from  FIG. 1  to  FIG. 9 , even in the case where obstacle targets, such as shown by T 1  and T 2  in  FIG. 2 , are positioned on the left-right direction outer sides of the vehicle  10 , the obstacle targets can be detected at an early stage by at least one of the first sensors  50   a ,  50   b , from among the two first sensors  50   a ,  50   b . In this state, a turn to the rear of the vehicle  10  is stopped, or a stop of a turn is maintained. Moreover, driving of the lawnmower  18  is stopped, or a drive stop is maintained. 
     Moreover, in order to release these stop and stop maintenance states, after a turn to the rear and the driving of the lawnmower  18  are stopped, or a stop maintenance is performed, for example, the driver causes the vehicle  10  to travel to the front or the like, and the obstacle targets will fall outside the detection regions of the first sensors  50   a ,  50   b . Also, in this state, the controller  60  may be configured to perform a reset, for example, by returning the left and right operation levers  22 ,  23  to a neutral state. This reset causes the controller  60  to permit a turn to the rear of the vehicle, or driving of the lawnmower. 
     According to the above described vehicle  10 , in a configuration where the left and right wheels  12 ,  13  are independently travel-driven by the hydraulic motors  30 ,  31 , it will be easy to automatically detect an obstacle target that approaches the vehicle  10  in a relative manner at the time of turning travel to the rear. For example, the visual field of the driver riding in the driver&#39;s seat is the range shown by arrow Q in  FIG. 2 . At the time when there is an obstacle target at a position falling outside this range, in particular, when being obstructed by an engine bonnet  141 , it may be necessary for the driver to confirm by changing the orientation of his or her body to the rear or alighting from the vehicle  10 . In particular, the two first sensors  50   a ,  50   b  are respectively arranged on both the left and right sides, more to the front than the rear end of the vehicle  10 , and are constituted so as to detect an obstacle target positioned comparatively near the ground surface of the rear side. In this way, different to the case where sensors capable of detecting the rear are arranged only on the rear end of the vehicle  10 , it will be easy to detect an obstacle target positioned more on an outer side than both the left and right ends of the vehicle  10  and more to the front than the rear end of the vehicle  10 . Also, a turn of the vehicle  10  can be stopped at an early stage, by the detection of an obstacle target. 
     Moreover, as shown by T 3  in  FIG. 2 , there will be cases where an obstacle target is not in either of the detection ranges of the left and right first sensors  50   a ,  50   b . However, when the vehicle  10  turns, using a zero-turn or the like, to the rear in the arrow a direction, such as shown in  FIG. 2  and  FIG. 8 , the obstacle target T 3  will be detected by the first sensor  50   b  of the left side, in the state of  FIG. 8 . As a result, the turn of the vehicle  10  is stopped, and the vehicle  10  is prevented from colliding with the obstacle target T 3 . Moreover, since the driving of the lawnmower  18  is stopped when the obstacle target T 3  has been detected, the obstacle target T 3  is prevented from being wound around the lawnmower  18 . Note that in  FIG. 8 , for the ground movement speeds V 1 , V 2  of the left and right wheels  12 ,  13 , the speed corresponding to the forward direction is shown as positive, and the speed corresponding to the backward direction is shown as negative. 
     Moreover, as shown in  FIG. 8  or the like, at the time when the left and right wheels  12 ,  13  rotate in opposite directions, and an absolute value of the ground movement speed of the wheel rotating to the rear is equal to or higher than an absolute value of the ground movement speed of the wheel rotating to the front, a rapid turn to the rear of the vehicle  10  will occur. Moreover, at the time of a pivot turn to the rear, in the case where only one wheel of the left and right wheels  12 ,  13  rotates in the backward direction, and the other wheel is stopped, a rapid turn to the rear of the vehicle  10  will also occur. In this way, at the time when the vehicle  10  turns rapidly to the rear, it will become easy to approach an obstacle target positioned in a difficult-to-confirm position. Also, considerable attention is required by the driver in order to prevent the vehicle colliding with the obstacle target. In an embodiment, when performing such a rapid turn, the effect achieved with a configuration that includes the first sensors  50   a ,  50   b  will be remarkable. 
     While a case has been described, in  FIG. 8 , where the vehicle  10  turns rapidly in the arrow α direction, there will be cases where the vehicle  10  turns rapidly to the rear using a zero-turn in an arrow β direction opposite to the arrow α, such as shown in  FIG. 9 . At this time, in the state shown in  FIG. 9 , the obstacle target T 3  is detected by the first sensor  50   b  of the right side. As a result, the turn of the vehicle  10  is stopped, and the driving of the lawnmower  18  is also stopped. 
       FIG. 10  is a block diagram showing the characteristic configuration of the vehicle  10  in another example of an embodiment.  FIG. 11  is a view corresponding to  FIG. 2 , in the configuration shown in  FIG. 10 . In the configurations shown in  FIG. 10  and  FIG. 11 , second sensors  51   a ,  51   b  are arranged more to the rear than the first sensors  50   a ,  50   b  of the vehicle  10 , in the configurations from  FIG. 1  to  FIG. 9 . Specifically, the two second sensors  51   a ,  51   b  are respectively fixed to the upper sides of the left and right wheel covers  25 , which respectively cover the upper sides of the left and right wheels  12 ,  13 . Similar to the first sensors  50   a ,  50   b , each of the second sensors  51   a ,  51   b  is constituted so as to detect an obstacle target positioned on the rear side. In  FIG. 11 , the detection regions of each of the second sensors  51   a ,  51   b  are shown by dispersed dot-shaped regions. The detection regions extend to the rear, and do not cross the vehicle  10 . Detection signals of the second sensors  51   a ,  51   b  are transmitted to the controller  60 . In order to reduce undetectable regions, it is preferable for the detection regions of the two second sensors  51   a ,  51   b  to partially overlap, such as in  FIG. 11 . Moreover, in order to reduce undetectable regions, it is preferable for the detection regions of each of the second sensors  51   a ,  51   b  to partially overlap with the detection regions of the first sensors  50   a ,  50   b.    
     The controller  60  has the turn stop section  62  ( FIG. 3 ) and the lawnmower drive stop section  63  ( FIG. 3 ). At the time when an obstacle target has been detected by at least one of the first sensors  50   a ,  50   b  and the second sensors  51   a ,  51   b , the turn stop section  62  causes a turn to the rear of the vehicle  10  to stop, or causes a stop of a turn to be maintained. 
     In addition, at the time when an obstacle target has been detected by at least one of the first sensors  50   a ,  50   b  and the second sensors  51   a ,  51   b , the lawnmower drive stop section  63  causes the driving of the lawnmower drive motor  48  to stop, or causes a drive stop to be maintained. 
     According to the above described configuration, since the range in which it is possible to detect an obstacle target is extended, it will be easier to automatically detect an obstacle target that approaches the vehicle  10  at the time of turning travel to the rear. For example, even when obstacle targets T 3  and T 4  are positioned near the vehicle  10  at the rear of the vehicle  10 , and these obstacle targets cannot be detected by the first sensors  50   a ,  50   b , it will be easy to detect the obstacle targets T 3  and T 4  with the second sensors  51   a ,  51   b . Configurations and actions other than these will be the same as the configurations of  FIG. 1  to  FIG. 9 . 
     Moreover, in the configurations of  FIGS. 10 and 11 , it will be easy to detect obstacle targets T 3  and T 4  more to the rear than the rear end of the vehicle  10 . Accordingly, the controller  60  may configured as when the obstacle targets T 3  and T 4  are detected, and it is determined to be going straight to the rear, the controller  60  causes straight travel to the rear of the vehicle  10  to stop. 
       FIG. 12  is a block diagram showing the characteristic configuration of the vehicle  10  in another example of an embodiment according to the present invention. In the configuration of  FIG. 12 , the tension switching actuator, in the configurations of  FIG. 1  to  FIG. 9 , is not provided. In the configuration of  FIG. 12 , the vehicle  10  includes a warning buzzer  72  arranged near the driver&#39;s seat. The warning buzzer  72  corresponds to a warning section. The operation of the warning buzzer  72  is controlled by the controller  60 , and warns of an approach to an obstacle target using a sound. At the time when an obstacle target has been detected by one or both of the left and right first sensors  50   a ,  50   b , the controller  60  causes the warning buzzer  72  to operate. Since the driver can recognize an approach to an obstacle target, in response to the operation of the warning buzzer  72  the driver will cause a rear turn, or straight travel to the rear, to stop by returning the left and right operation levers  22 ,  23  to a neutral state, or will turn a switch (not illustrated) for the operation of the lawnmower  18  to OFF. Accordingly, safe securement of the vehicle will be carried out. 
     Moreover, in the configuration of  FIG. 12 , a warning light  73  can also be arranged near the driver&#39;s seat  21  of the vehicle, instead of the warning buzzer  72 , or together with the warning buzzer  72 . The warning light  73  also corresponds to a warning section. For example, the warning light  73  may be fixed near the feet of the driver&#39;s seat  21 , in the vehicle. Moreover, the warning light  73  may be set above a supporting part of the caster wheels  15 ,  16  where it is easy to come into the visual field when turning to the front. The operation of the warning light  73  is controlled by the controller  60 , and warns of an approach to an obstacle target by turning on or flashing a light. At the time when an obstacle target has been detected by one or both of the left and right first sensors  50   a ,  50   b , the controller  60  causes the warning light  73  to operate. The driver can recognize an approach to an obstacle target by the operation of the warning light  73 . Configurations and actions other than these will be the same as the configurations of  FIG. 1  to  FIG. 9 . 
     Note that in the configuration of  FIG. 12 , the two second sensors  51   a ,  51   b  can be provided such as in the configurations of  FIG. 10  and  FIG. 11 . Moreover, the configurations of  FIG. 1  to  FIG. 9 , or the configurations of  FIG. 10  and  FIG. 11 , may be configurations that include the tension switching actuator  43 , and configurations where a warning section such as the warning buzzer  72  is provided. At this time, when an obstacle target has been detected by any one of the first sensors  50   a ,  50   b  and the second sensors  51   a ,  51   b , the warning section is operated along with stopping of a turn and the driving of the lawnmower  18 , or performing a stop maintenance. As a result, a warning can be issued to the driver, and the interruption of a rapid turn operation can be prompted. 
     While a case has been described, heretofore, where the controller  60  has both the turn stop section  62  and the lawnmower drive stop section  63 , the controller may be configured to have only one of the turn stop section  62  and the lawnmower drive stop section  63 . Moreover, reverse switches, which detect that the left and right operation levers  22 ,  23  are in a region indicating reversing, may be respectively provided, in the vehicle  10 , near the left and right operation levers  22 ,  23 . Detection signals of the reverse switches are transmitted to the controller  60 . At this time, by using not only the lever potentiometers  38 ,  39 , but also the detection signals of the reverse switches in an auxiliary manner, the controller  60  can more stably determine whether or not the vehicle  10  is turning to the rear. 
     Moreover, two left and right direction indication lights can be fixed at positions separated in the left-right direction on the front end part of the vehicle  10 , such as at positions near a supporting part of the caster wheels  15 ,  16 , for example, along with attaching direction indication switches to the left and right operation levers  22 ,  23 . Each of the direction indication lights is constituted to be capable of flashing a light in the case were the direction indication switch of the side corresponding to the left or the right has been pressed. In such a configuration, since the vehicle  10  turning to the front or the rear side can be notified to a person nearby by the flashing of the direction indication lights, it becomes possible to perform safer travelling. Moreover, the direction indication lights may be used as the above described warning section. Specifically, when an obstacle target has been detected by one or both of the left and right first sensors  50   a ,  50   b , the controller causes the left and right direction indication lights to turn on or flash at the same time. 
     Moreover, while a case has been described, heretofore, where a turn of the vehicle  10  is stopped by controlling the tension switching actuator  43 , a turn of the vehicle  10  may be stopped by various methods other than this. For example, a throttle actuator that mechanically or electrically adjusts the opening of a throttle valve of the engine may be included, and a turn may be stopped by closing the throttle valve by having the controller  60  control the driving of the throttle actuator. Moreover, there may be a configuration that includes bypass valves  28   a ,  29   a  ( FIG. 4B ) arranged between the hydraulic circuits of the power generation units  26 ,  27  and an oil reservoir, and bypass actuators that open-close the bypass valves  28   a ,  29   a  at the same time. Also, a turn may be stopped by stopping the supply of oil to the hydraulic motors, by setting the bypass valves  28   a ,  29   a  to an opened state, namely, a state where the main oil paths S 1 , S 2  and the oil reservoir are connected, by having the controller  60  control the driving of the bypass actuators. Moreover, there may be a configuration that includes actuators that drive the swash plate operation levers  32   c ,  33   c  connected to the swash plate operation shafts  32   b ,  33   b  of the hydraulic pumps  32 ,  33 , and these actuators are controlled by the controller  60  by converting the operation amounts of each of the operation levers  22 ,  23  into electrical signals. Also, the supply oil to the hydraulic motors may be stopped, by setting the tilting angles of the movable swash plates to a neutral state by causing the swash plate operation shafts to rotate, and stopping the discharge of oil from the hydraulic pumps  32 ,  33 . As a result, it becomes possible to stop a turn. 
       FIG. 13  is a block diagram showing the characteristic configuration of a riding type vehicle in another example of an embodiment according to the present invention. In the configuration of this example, the vehicle  10 , in the configurations of  FIG. 1  to  FIG. 9 , includes the warning light  73   a  ( FIG. 13 ) arranged near the driver&#39;s seat  21  of the vehicle. The warning light  73   a  corresponds to a warning section. For example, the warning light  73   a  may be fixed near the feet of the driver&#39;s seat  21 , in the vehicle. Moreover, the warning light  73   a  may be set above a supporting part of the caster wheels  15 ,  16  where it is easy to come into the visual field when turning to the front. Moreover, the operation of the warning light  73   a  is controlled by the controller  60 , described later, and warns of an approach to an obstacle target by turning on or flashing a light. 
     As shown in  FIG. 13 , the controller  60  includes an operation section such as a CPU and a storage section such as a memory. The controller  60  has a rear rapid turn determination section  61   a , and a turn stop section  62   a . The rear rapid turn determination section  61   a  determines whether or not the vehicle  10  is rapidly turning to the rear from detection signals of the left and right lever potentiometers  38 ,  39 . For example, the rotation directions and rotation angles of the left and right wheels  12 ,  13  are calculated from these detection signals. In the case where the left and right wheels  12 ,  13  rotate to the rear, and the rotation speeds of the left and right wheels  12 ,  13  differ, it is determined that the vehicle  10  is turning to the rear. 
     In addition, in the case where only one wheel of the left and right wheels  12 ,  13  rotates to the rear, it is determined that the vehicle  10  is rapidly turning to the rear. Moreover, in the case where the left and right wheels  12 ,  13  rotate in opposite directions, and an absolute value of a ground movement speed of a rear rotating wheel, which is the wheel rotating to the rear, is larger than an absolute value of a ground movement speed of a front rotating wheel, which is the wheel rotating to the front, it is also determined that the vehicle  10  is rapidly turning to the rear. Such a rapid turn will be described afterwards using  FIG. 14 . When the respective absolute values of the ground movement speeds of the rear rotating wheel and the front rotating wheel are larger than zero, and a difference of both absolute values is zero, the rapid turn will be a zero-turn. 
     When an obstacle target has been detected by at least one of the first sensors  50   a ,  50   b , in the case where it is determined that the vehicle  10  is rapidly turning to the rear by the rear rapid turn determination section  61   a , the turn stop section  62   a  causes the rapid turn to the rear of the vehicle  10  to stop. At this time, by having the turn stop section  62   a  control the driving of the tension switching actuator  43 , the driving of the left and right hydraulic motors  30 ,  31  is stopped by making the tension of the belt  36  zero, namely, by cutting the clutch. As a result, the left and right wheels  12 ,  13  stop, and therefore the rapid turn to the rear stops. Moreover, when an obstacle target has been detected by at least one of the first sensors  50   a ,  50   b , in the case where it is determined that the vehicle  10  is stopped, the turn stop section  62  causes the stop of a rapid turn to the rear of the vehicle  10  to be maintained. At this time, a straight travel stop to the rear may be maintained, along with maintaining a rapid turn stop to the rear of the vehicle  10 . 
     Straight travel, turning travel, a pivot turn, and an ultra-pivot turn of the vehicle  10  are the same as the travelling or turning described using  FIG. 5  and  FIG. 6A  to  FIG. 6C . 
       FIG. 14  is a schematic illustration showing the inconvenience for the vehicle  10 , at the time when rapidly turning to the rear. In  FIG. 14 , for ease of understanding, the vehicle  10  is shown by a dash-dotted line G 1  rectangle and a dotted line G 2  rectangle. From a state where the vehicle  10  is the dash-dotted line G 1 , there will be cases where only the left wheel  12 , which is one wheel of the left and right wheels  12 ,  13 , rotates in the backward direction, and the stopping of the right wheel  13 , which is the other wheel, is maintained. In this case, as shown by the dotted line G 2 , the vehicle  10  turns rapidly to the rear as shown by the arrow a direction, by setting the ground position of the right wheel  13  to the turn center position  70 . Also, in the state of the dash-dotted line G 1 , there will be cases where there is an obstacle target, shown by T, more to the front than the rear end of the vehicle  10 , and nearer to the outer side than the left side surface. At this time, there will be times where the obstacle target T cannot be seen by the driver in the driver&#39;s seat  21 , or the driver fails to notice the obstacle target T. At this time, since the vehicle  10  continues to turn while extending in the left-right direction outer side at the front side, there is the possibility that the vehicle  10  will collide with the obstacle target T, in the state of the dotted line G 2 . In the vehicle  10  of an embodiment shown in  FIG. 13 , even in the case where obstacle targets, such as shown by T 1  and T 2  by referring to  FIG. 2 , are positioned on the left-right direction outer sides of the vehicle  10 , the obstacle targets can be detected at an early stage by at least one of the first sensors  50   a ,  50   b , from among the two first sensors  50   a ,  50   b . In this state, a rapid turn to the rear of the vehicle  10  is stopped, or a stop of a turn is maintained. 
     Moreover, in order to release these stop and stop maintenance states, after a turn to the rear is stopped, or a stop maintenance is performed, for example, the driver causes the vehicle  10  to travel to the front or the like, and the obstacle targets will then fall outside the detection regions of the first sensors  50   a ,  50   b . Also, in this state, the controller  60  may be configured to perform a reset, for example, by returning the left and right operation levers  22 ,  23  to a neutral state. This reset causes the controller  60  to permit a turn to the rear of the vehicle. 
     At the time when an obstacle target has been detected by at least one of the first sensors  50   a ,  50   b , the controller  60  causes the warning light  73   a  to operate, along with causing a rapid turn to the rear of the vehicle  10  to stop, or causing a stop of a rapid turn to be maintained. As a result, the driver can recognize an approach to an obstacle target. Note that the warning buzzer  72   a  can be arranged near the driver&#39;s seat  21  of the vehicle, instead of the warning light  73   a , or together with the warning light  73   a . The warning buzzer  72   a  corresponds to a warning section. The operation of the warning buzzer  72   a  is controlled by the controller  60 , and warns of an approach to an obstacle target using a sound. At the time when an obstacle target has been detected by one or both of the left and right first sensors  50   a ,  50   b , the controller  60  causes the warning buzzer  72   a  to operate. The driver can recognize an approach to an obstacle target, by the operation of the warning buzzer  72   a.    
     According to the above described vehicle  10 , in a configuration where the left and right wheels  12 ,  13  are independently travel-driven by the hydraulic motors  30 ,  31 , it will be easy to automatically avoid a collision with an obstacle target by the vehicle  10  at the time of rapid turning travel to the rear. In this case, different to the case where sensors capable of detecting the rear are arranged only on the rear end of the vehicle  10 , similar to the configurations of  FIG. 1  to  FIG. 9 , it will be easy to detect an obstacle target positioned more on the outer side than both the left and right ends of the vehicle  10  and more to the front than the rear end of the vehicle  10 . Also, a turn of the vehicle  10  can be stopped at an early stage, by the detection of an obstacle target. 
     Moreover, when the vehicle  10  rapidly turns, using a zero-turn or the like, to the rear in the arrow a direction, as shown in  FIG. 2  and  FIG. 8 , the obstacle target T 3  will be detected by the first sensor  50   b  of the left side in the state of  FIG. 8 . As a result, the rapid turn of the vehicle  10  is stopped, and the vehicle  10  is prevented from colliding with the obstacle target T 3 . 
     Moreover, in the case where the vehicle  10  turns rapidly using a pivot turn to the rear, as in  FIG. 14 , there will be cases where an obstacle target is not in either of the detection ranges of the left and right first sensors  50   a ,  50   b , in a stop state. Also in this case, it will be easy for an obstacle target to be detected by any one of the first sensors  50   a ,  50   b , in a state where a rapid turn is continued. As a result, the rapid turn of the vehicle  10  is stopped, and the vehicle  10  is prevented from colliding with the obstacle target T. In this way, when the left and right wheels  12 ,  13  rotate in opposite directions, and an absolute value of the ground movement speed of the wheel rotating to the rear is equal to or higher than an absolute value of the ground movement speed of the wheel rotating to the front, a rapid turn to the rear of the vehicle  10  will occur. Moreover, in the case where only one wheel of the left and right wheels  12 ,  13  rotates in the backward direction, and the other wheel is stopped, a rapid turn to the rear will occur. In this way, when the vehicle  10  turns rapidly to the rear, it will become easy for the vehicle to approach an obstacle target positioned in a difficult-to-confirm position. Moreover, in the case where the vehicle turns rapidly using a zero-turn, the vehicle will significantly deflect in the left-right direction at this location, and therefore it will become easy for the vehicle to approach an obstacle target positioned in a difficult-to-confirm position. Also, considerable attention will be required by the driver in order to prevent the vehicle colliding with the obstacle target. In an embodiment, when the obstacle target has been detected by the first sensors  50   a ,  50   b , the effect achieved with a configuration where the controller  60  causes a rapid turn to the rear to be stopped, or causes a stop of a rapid turn to be maintained, will be remarkable. Configurations and actions other than these will be the same as the configurations of  FIG. 1  to  FIG. 9 . 
     By referring to  FIG. 10  and  FIG. 11 , a vehicle in another example of an embodiment will be described. In the configuration of this example, the second sensors  51   a ,  51   b  are arranged more to the rear than the first sensors  50   a ,  50   b  of the vehicle, in the configuration of  FIG. 13 . Moreover, the controller  60  has the turn stop section  62   a  ( FIG. 13 ). When an obstacle target has been detected by at least one of the first sensors  50   a ,  50   b  and the second sensors Ma,  51   b , the turn stop section  62   a  causes a rapid turn to the rear of the vehicle  10  to stop, or causes a stop of a rapid turn to be maintained. 
     According to the above described configuration, since the range capable of detecting an obstacle target is extended, it will be easier to automatically detect an obstacle target that approaches the vehicle  10  at the time of turning travel to the rear. For example, even when obstacle targets T 3  and T 4  ( FIG. 11 ) are positioned near the vehicle  10  at the rear of the vehicle  10 , and these obstacle targets can not be detected by the first sensors  50   a ,  50   b , it will be easy to detect the obstacle targets T 3  and T 4  with the second sensors  51   a ,  51   b . As a result, it will be easy to automatically and effectively avoid a collision of the vehicle  10  with an obstacle target at the time of rapid turning travel to the rear. Configurations and actions other than these will be the same as the configurations of  FIG. 1  to  FIG. 9 , the configurations of  FIG. 10  and  FIG. 11 , or the configuration of  FIG. 13 . 
       FIG. 15  is a block diagram showing the characteristic configuration of the vehicle  10  in another example of an embodiment.  FIG. 16A  is a view corresponding to  FIG. 4A , in the configuration shown in  FIG. 15 , and  FIG. 16B  is a view seen from an arrow A direction of  FIG. 16A . By referring to  FIG. 1  to  FIG. 9 , the vehicle  10  of  FIG. 15 ,  FIG. 16A  and  FIG. 16B  includes a left reverse switch  75  arranged in the surrounding part of the lower end part of the left operation lever  22 , and a right reverse switch  76  arranged in the surrounding part of the lower end part of the right operation lever  23 , in the configuration of  FIG. 13 . The left and right reverse switches  75 ,  76  detect whether or not the left and right operation levers  22 ,  23  have been swung to regions (the R regions of  FIG. 16A  and FIG.  16 B) indicating reversing, centered on shafts S in the left-right direction of the lower end part. Also, in the case where it is detected that the left and right operation levers  22 ,  23  have been swung to the regions for indicate reversing, the left and right reverse switches  75 ,  76  transmit these detection signals to the controller  60 . For example, in the case where the front end parts of the reverse switches  75 ,  76  have been pressed downward, by the front end of the lower end parts of the operation levers  22 ,  23 , it is detected that reversing has been instructed by the operation levers  22 ,  23  lowering from a neutral state to the rear. By using not only the left and right lever potentiometers  38 ,  39 , but also the detection signals of the reverse switches  75 ,  76  as an assistance, the controller  60  will more stably determine whether or not the vehicle is rapidly turning to the rear. The left and right operation levers  22 ,  23  are capable of being displaced, from a maximum displacement position Fmax of a region indicating advancing to a maximum displacement position Rmax of a region indicating reversing, centered on the neutral position of  FIG. 16A  and  FIG. 16B . The left and right lever potentiometers  38 ,  39  are arranged near shafts S of the left and right operation levers  22 ,  23 . An illustration of the left and right lever potentiometers  38 ,  39  is omitted in  FIG. 16A . 
     Moreover, the vehicle  10  includes a throttle actuator  78  that mechanically or electrically adjusts the opening of a throttle valve of the engine  14 . The throttle actuator  78  includes a motor (not illustrated) fixed to a rotating shaft (not illustrated) of the throttle valve. The controller  60  controls the motor of the throttle actuator  78 , so that the engine  14  is driven at a constant rotation speed determined beforehand, by having a start switch (not illustrated) set to ON by a user. The controller  60  controls the throttle actuator  78  so as to cause the throttle valve to close, by having the start switch set to OFF by the user. 
     Moreover, when an obstacle target has been detected by at least one of the first sensors  50   a ,  50   b , in the case where it is determined that the vehicle  10  is rapidly turning to the rear by the rear rapid turn determination section  61   a  ( FIG. 13 ), the controller  60  causes the rapid turn to the rear of the vehicle  10  to stop. When an obstacle target has been detected by at least one of the first sensors  50   a ,  50   b , in the case where it is determined that the vehicle is stopped, the controller  60  causes a stop of a rapid turn to the rear of the vehicle  10  to be maintained. At this time, by having the turn stop section  62   a  ( FIG. 13 ) control the driving of the throttle actuator  78 , the rapid turn to the rear of the vehicle is stopped, or the stop of the rapid turn is maintained, by closing the throttle value. Configurations and actions other than these will be the same as the configurations of  FIG. 1  to  FIG. 9 , or the configuration of  FIG. 13 . 
       FIG. 17  is a block diagram showing the characteristic configuration of the vehicle  10  in another example of an embodiment. In the configuration of  FIG. 13 , the vehicle  10  of  FIG. 17  includes left and right bypass actuators  79 ,  80 . The left and right bypass actuators  79 ,  80  respectively drive the bypass valves  28   a ,  29   a  connected between the main oil paths S 1 , S 2  of the hydraulic circuits  28 ,  29  of the left and right power generation units  26 ,  29 , and the oil reservoir E, by referring to  FIG. 4B . The bypass valves  28   a ,  29   a  cause oil of the main oil paths  51 , S 2  to be discharged to the oil reservoir E, in an opened state, namely, in a connection state between the main oil paths  51 , S 2  and the oil reservoir E. On the other hand, the bypass valves  28   a ,  29   a  cause oil to circulate to the main oil paths S 1 , S 2  in a closed state, namely, in a disconnection state between the main oil paths S 1 , S 2  and the oil reservoir E. For example, the left bypass actuator  79  includes a left solenoid that electrically switches the opening and closing of the left bypass valve  28   a , and the right bypass actuator  80  includes a right solenoid that electrically switches the opening and closing of the right bypass valve  29   a . Each of the bypass actuators  79 ,  80  is controlled by the controller  60 . Also, the controller  60  stops the supply of oil to the hydraulic motors  30 ,  31 , even during the driving of the hydraulic pumps  32 ,  33 , by setting the left and right bypass valves  28   a ,  29   a  to an opened state at the same time by controlling the driving of the left and right bypass actuators  79 ,  80 . As a result, since the hydraulic motors  30 ,  31  are in an idling state, a rapid turn to the rear of the vehicle  10  will be stopped, or a stop of a rapid turn will be maintained. Note that at this time, the vehicle  10  will be stopped by inertia, and therefore there is the possibility of making contact with an obstacle target during travel. However, driving power is not provided to the hydraulic motors  30 ,  31  in this case, and therefore an applied force at the time of contact can be significantly reduced. Configurations and actions other than these will be the same as the configurations of  FIG. 1  to  FIG. 9 , or the configuration of  FIG. 13 . Even in the configuration of  FIG. 17 , similar to the configurations of  FIG. 15 ,  FIG. 16A  and  FIG. 16B , reverse switches can be included as an assistance. Reverse switches can be similarly included in each of the configurations shown in  FIG. 1  to  FIG. 13 . 
       FIG. 18  is a block diagram showing the characteristic configuration of the vehicle  10  in another example of an embodiment.  FIG. 19A  is a view corresponding to  FIG. 4A , in the configuration shown in  FIG. 18 , and  FIG. 19B  is a view seen from an arrow B direction of  FIG. 19A . The vehicle  10  of  FIG. 18  and  FIG. 19A  and  FIG. 19B  does not have the swash plate operation levers  32   c ,  33   c  of the left and right power generation units  26 ,  27 , in the configuration of  FIG. 13 , connected via the operation levers  22 ,  23  and a link. Instead of this, the vehicle  10  includes left and right swash plate actuators  81 ,  82 . The swash plate actuators  81 ,  82  drive the swash plate operation levers  32   c ,  33   c  connected to the swash plate operation shafts  32   b ,  33   b  ( FIG. 4B ) of the hydraulic pumps  32 ,  33  ( FIG. 4B ) of the corresponding sides, and are controlled by the controller  60 . The swash plate actuators  81 ,  82  include a piston cylinder mechanism, or a motor, for example, which rotates the swash plate operation levers  32   c ,  33   c  of the corresponding sides. 
     The controller  60  drives the left and right swash plate operation levers  32   c ,  33   c  by controlling the driving of the left and right swash plate actuators  81 ,  82 , in accordance with detection signals from the left and right lever potentiometers  38 ,  39 . For example, in the case where the left and right operation levers  22 ,  23  have been lowered to the front, the controller  60  drives the swash plate operation levers  32   c ,  33   c  in one direction by controlling the swash plate actuators  81 ,  82 , in accordance with detection signals from the lever potentiometers  38 ,  39 . As a result, the discharge amounts of the left and right hydraulic pumps  32 ,  33  will change, and the movable swash plates of each of the hydraulic pumps  32 ,  33  will tilt, so that the discharge amounts of each of the hydraulic pumps  32 ,  33  increase at the advancing side. In the case where the left and right operation levers  22 ,  23  have been lowered to the rear, the controller  60  drives the swash plate operation levers  32   c ,  33   c  in the other direction by controlling the swash plate actuators  81 ,  82 . As a result, the discharge amounts of the left and right hydraulic pumps  32 ,  33  will change, and the movable swash plates of each of the hydraulic pumps  32  and  33  will tilt, so that the discharge amounts of each of the hydraulic pumps  32 ,  33  increase in a direction of a rotation of the reversing side. Accordingly, the controller  60  causes the movable swash plates of the left and right hydraulic pumps  32 ,  33  to tilt, and causes the discharge amounts of the hydraulic pumps  32  and  33  to change, by controlling the driving of the left and right swash plate actuators  81 ,  82  in accordance with detection signals of the left and right lever potentiometers  38 ,  39 . 
     In addition, the controller  60  sets the discharge amounts of each of the hydraulic pumps  32 ,  33  to substantially zero, by setting the tilting angles of the movable swash plates of the left and right hydraulic pumps  32 ,  33  to approximately a neutral state, by controlling the driving of the left and right swash plate actuators  81 ,  82 . As a result, a rapid turn to the rear of the vehicle is stopped, or a stop of a rapid turn is maintained. Configurations and actions other than these will be the same as the configurations of  FIG. 1  to  FIG. 9 , or the configuration of  FIG. 13 . Even in the configurations of  FIG. 18  and  FIG. 19 , similar to the configurations of  FIG. 15 ,  FIG. 16A  and  FIG. 16B , reverse switches can be included as assistance. 
       FIG. 20  is a sectional view showing the bypass valves  28   a ,  29   a  operated by the bypass actuators  79 ,  80  mounted in a vehicle in another example of an embodiment. The bypass valves  28   a ,  29   a  of  FIG. 20  are used as opening-closing valves for switching the connection and disconnection between the main oil paths S 1 , S 2  of the hydraulic circuits  28 ,  29 , and the oil reservoir E, of  FIG. 4B . Specifically, the bypass valves  28   a ,  29   a  include a housing  83 , and a piston  84  and check valve  85  arranged inside this housing. At the time of closing the bypass valves  28   a ,  29   a , the connection between the main oil paths  51 , S 2  and the oil reservoir E is disconnected, by having a ball  86  of the check valve  85  energized at a valve seat  88  by a spring  87 . The spring side of the ball  86  passes through the oil reservoir E via a hole  89   a  of a spring pressing part  89 , and the valve seat side of the ball passes through the main oil paths  51 , S 2 . Also, one end (the lower end in  FIG. 20 ) of the piston  84  opposes the ball  86 , and the piston  84  is energized by a second spring  90  so as to separate from the ball  86 . The other end (the upper end in  FIG. 20 ) of the piston  84  protrudes from the housing  83 . The bypass valves  28   a ,  29   a  are opened by causing the bypass actuators  79 ,  80 , which are advancing-and-backing controlled by the controller  60 , to operate by protruding in the lower orientation of  FIG. 20 . Specifically, at the time of opening the bypass valves  28   a ,  29   a , namely, at the time of connecting the main oil paths and the oil reservoir, the bypass valves are opened, by having the portions protruding at the other end of the piston  84  pressed to the ball  86  side by the bypass actuators. Configurations and actions other than these will be the same as the configurations of  FIG. 1  to  FIG. 9 , the configuration of  FIG. 13 , or the configuration of  FIG. 17 . 
       FIG. 21  is a block diagram showing the characteristic configuration of a riding type vehicle in another example of an embodiment according to the present invention.  FIG. 22A  is a view, seen from the upper side of a vehicle, of a power transmission structure between power generation units for the left wheel and the right wheel, and an engine, in another example of an embodiment, and  FIG. 22B  is a view seen from an arrow C direction of  FIG. 22A . In the configuration of this example, the vehicle  10 , in the configurations of  FIG. 1  to  FIG. 9 , includes the warning light  73   a  ( FIG. 21 ) corresponding to a warning section, arranged near the driver&#39;s seat  21  of the vehicle. The operation of the warning light  73   a  is controlled by the controller  60 , and warns if an approach to an obstacle target by turning on or flashing a light.
         Moreover, as shown in  FIG. 22A  and  FIG. 22B , the vehicle  10  includes the left reverse
 
switch  75  arranged in the surrounding part of the lower end part of the left operation lever  22 , and the right reverse switch  76  arranged in the surrounding part of the lower end part of the right operation lever  23 . The left and right reverse switches  75 ,  76  detect whether or not the left and right operation levers  22 ,  23  have been swung to regions (the R regions of  FIG. 22A  and  FIG. 22B ) indicating reversing, centered on shafts S in the left-right direction of the lower end part. Also, in the case where it is detected that the left and right operation levers  22 ,  23  have been swung to the regions for indicating reversing, the left and right reverse switches  75 ,  76  transmit these detection signals to the controller  60 . For example, in the case where the front end parts of the reverse switches  75 ,  76  have been pressed downward, by the front end of the lower end parts of the operation levers  22 ,  23 , it is detected that reversing has been instructing by the operation levers  22 ,  23  lowering from a neutral state to the rear. By using detection signals of the left and right lever potentiometers  38 ,  39 , the controller  60  will determine whether or not the vehicle is rapidly turning to the rear. At this time, by having detection signals of the reverse switches  75 ,  76  used as assistance, it can be determined with stability whether or not the vehicle is rapidly turning to the rear.
       

     The left and right operation levers  22 ,  23  are capable of being displaced, from a maximum displacement position Fmax of a region indicating advancing to a maximum displacement position Rmax of a region indicating reversing, centered on the neutral position of  FIG. 22A  and  FIG. 22B . The left and right lever potentiometers  38 ,  39  are arranged near shafts S of the left and right operation levers  22 ,  23 . An illustration of the left and right lever potentiometers  38 ,  39  is omitted in  FIG. 22A .
         Moreover, the vehicle  10  of  FIG. 21 ,  FIG. 22A  and  FIG. 22B  includes a throttle
 
actuator  78 . The configuration of the throttle actuator  78  is the same as the configuration described using  FIG. 15 ,  FIG. 16A  and  FIG. 16B . The throttle actuator  78  is controlled by the controller  60  ( FIG. 21 ). When at least one of obstacle targets T 1 , T 2 , and T 3  ( FIG. 2 ) has been detected by the first sensors  50   a ,  50   b , the controller  60  controls the driving of the throttle actuator  78 . As a result, the controller  60  causes a turn of the vehicle  10  to decelerate by gradually bringing the throttle valve near to a closed state, and causes a turn to the rear to stop by closing the valve. Accordingly, it will become difficult for the vehicle  10  to collide with the obstacle target at the time of a turn.
       

     Similar to the configuration shown in  FIG. 2 , the two first sensors  50   a ,  50   b  are arranged, in the vehicle  10 , separated on both the left and right sides of the vehicle  10 . The first sensors  50   a ,  50   b  are capable of measuring a distance up to an obstacle target. 
     Moreover, the vehicle  10  includes a left wheel rotation number sensor  142 , which detects the number of rotations per unit time (for example, per minute) of the left wheel  12  in electromagnetism, and a right wheel rotation number sensor  143 , which detects the number of rotations per unit time (for example, per minute) of the right wheel  13  in electromagnetism. The left and right wheel rotation number sensors  142 ,  143  correspond to left and right wheel rotation number detection sections. The controller  60  calculates the orientation of the vehicle in accordance with detection signals from the left and right wheel rotation number sensors  142 ,  143 . For example, the controller  60  calculates a change in orientation of the vehicle with respect to a standard state, by calculating a change in the number of rotations of the left and right wheels  12 ,  13  from a standard state determined beforehand. 
     As shown in  FIG. 21 , the controller  60  has a rear rapid turn determination section  61   a , and a turn deceleration stop section  62   b . The rear rapid turn determination section  61   a  determines whether or not the vehicle  10  is rapidly turning to the rear from detection signals of the left and right lever potentiometers  38 ,  39 . For example, the rotation directions and rotation angles of the left and right wheels  12 ,  13  are calculated from these detection signals. In the case where the left and right wheels  12 ,  13  rotate to the rear, and the rotation speeds of the left and right wheels  12 ,  13  differ, it is determined that the vehicle  10  is turning to the rear. 
     In addition, in the case where only one wheel of the left and right wheels  12 ,  13  rotates to the rear, it is determined that the vehicle  10  is rapidly turning to the rear. Moreover, in the case where the left and right wheels  12 ,  13  rotate in opposite directions, and an absolute value of a ground movement speed of a rear rotating wheel, which is the wheel rotating to the rear, is larger than an absolute value of a ground movement speed of a front rotating wheel, which is the wheel rotating to the front, it is also determined that the vehicle  10  is rapidly turning to the rear. Such a rapid turn is as described using  FIG. 17 . At the time when the respective absolute values of the ground movement speeds of the rear rotating wheel and the front rotating wheel are larger than zero, and a difference of both absolute values is zero, the rapid turn will be a zero-turn. 
     When an obstacle target has been detected by at least one of the first sensors  50   a ,  50   b , in the case where it is determined that the vehicle  10  is rapidly turning to the rear by the rear rapid turn determination section  61   a , the turn deceleration stop section  62   b  causes the turn to the rear of the vehicle  10  to decelerate. Also, the turn deceleration stop section  62   b  causes the turn to stop prior to the vehicle colliding with the obstacle target. At this time, the turn deceleration stop section  62   b  causes the turn of the vehicle to decelerate by gradually bringing the throttle valve near to a closed state and causes the turn of the vehicle to the rear to stop by closing the valve, by controlling the driving of the throttle actuator  78 . 
     It will be necessary for a turn stop of the vehicle to be performed prior to the vehicle colliding with the obstacle target. Accordingly, the controller  60  calculates a first orientation of the vehicle at the point in time when the obstacle target has been detected by the first sensors  50   a ,  50   b , and a second orientation of the vehicle when the vehicle collides with the obstacle target. The first sensors  50   a ,  50   b  are capable of measuring a distance up to the obstacle target. Accordingly, the controller  60  can calculate the number of rotations of the left and right wheels  12 ,  13  until the obstacle target collides at the left-right direction side end of the vehicle, from a distance from the first sensor to the obstacle target at the time when the obstacle target begins to fall into the detection regions of the first sensors  50   a ,  50   b  at the time of a turn of the vehicle. At this time, for example, a rectangular parallelepiped vehicle simulation model including the vehicle  10  and simulating the vehicle  10  may be set, and a second orientation may be calculated at the time when the vehicle simulation model collides with an obstacle target. For example, the vehicle simulation model may be in contact with the outside of the vehicle, or may have a shape slightly larger than the vehicle. By setting such a vehicle simulation model, it will be easy to prevent an obstacle target colliding with the vehicle. 
     The second orientation may be calculated by assuming that the swing positions of the left and right operation levers  22 ,  23  are constant, from a point in time when an obstacle target has been detected by the first sensors  50   a ,  50   b . Also, in the case where the actual swing positions of the left and right operation levers  22 ,  23  change after the first orientation, the second orientation may be corrected in accordance with this change. 
     Also, the controller  60  sets a third orientation prior to changing from the first orientation to the second orientation, and controls the rotation state of the left and right wheels  12 ,  13 , so as to cause a turn to decelerate until the vehicle changes to the third orientation, and cause the turn to stop at the third orientation. At this time, the controller  60  can control the throttle actuator  78 , and can cause the left and right wheels  12 ,  13  to stop in the third orientation, by closing the throttle valve. 
     Straight travel, turning travel, a pivot turn, and an ultra-pivot turn of the vehicle  10  are the same as the travelling or turning described using  FIG. 5  and  FIG. 6A  to  FIG. 6C . 
     By referring to  FIG. 14 , a state will be described, in the vehicle  10 , where a collision of the vehicle to an obstacle target P is avoided, at the time of rapidly turning to the rear, centered on the ground position of the right wheel  13 , which is one wheel of the left and right wheels  12 ,  13 . At this time, from a state where the vehicle  10  is the dash-dotted line G 1 , there will be cases where only the left wheel  12 , which is the other wheel of the left and right wheels  12 ,  13 , rotates in the backward direction, and the stopping of the right wheel  13 , which is the one wheel, is maintained. In this case, as shown by the dotted line G 2 , the vehicle  10  turns rapidly to the rear as shown by the arrow a direction, by setting the ground position of the right wheel  13  to the turn center position  70 . Also, in the state of the dash-dotted line G 1 , there will be cases where there is an obstacle target, shown by P, more to front than the rear end of the vehicle  10 , and nearer to the outer side than the left side surface. At this time, there will be times where the obstacle target P can not be seen by the driver of the driver&#39;s seat  21 , or the driver fails to notice the obstacle target P. At this time, since the vehicle  10  continues to turn while extending in the left-right direction outer side at the front side, there is the possibility that the vehicle  10  will collide with the obstacle target P, in the state of the dotted line G 2 . In the vehicle  10  of an embodiment shown in  FIG. 21 ,  FIG. 22A  and  FIG. 22B , it is calculated by the controller  60  that the vehicle  10  will change from the first orientation of the dash-dotted line G 1  to the second orientation of the dotted line G 2 . Also, the controller  60  controls the driving of the left and right wheels  12 ,  13 , so that the vehicle is stopped at the third orientation prior to the second orientation, after the first orientation of the vehicle. As a result, at the time of a rapid turn to the rear, it will become easy to detect an obstacle target at an early stage by at least one of the first sensors  50   a ,  50   b , from among the two first sensors  50   a ,  50   b , and it will be easy to cause a turn to stop prior to the vehicle colliding with the obstacle target. Moreover, since the turn decelerates from the point in time when an obstacle target has been detected by one of the first sensors  50   a ,  50   b , additional deceleration to the driver can be reduced compared to the case where a turn is stopped suddenly, and therefore the burden on the driver can be reduced. 
     Moreover, in order to release this stop, after a turn to the rear is stopped, for example, the driver causes the vehicle  10  to travel to the front or the like, and the obstacle target will fall outside the detection regions of the first sensors  50   a ,  50   b . Also, in this state, the controller  60  may be configured to perform a reset, for example, by returning the left and right operation levers  22 ,  23  to a neutral state. This reset is to make the controller  60  permit a turn to the rear of the vehicle. 
     Moreover, when an obstacle target has been detected by at least one of the first sensors  50   a ,  50   b , the controller  60  causes the warning light  73   a  to operate, along with causing a rapid turn to the rear of the vehicle  10  to decelerate. As a result, the driver can recognize an approach to an obstacle target. Note that the warning buzzer  72   a , which corresponds to a warning section, can be arranged near the driver&#39;s seat  21  of the vehicle, instead of the warning light  73   a , or together with the warning light  73   a . The operation of the warning buzzer  72   a  is controlled by the controller  60 , and warns of an approach to an obstacle target using a sound. When an obstacle target has been detected by one or both of the left and right first sensors  50   a ,  50   b , the controller  60  causes the warning buzzer  72   a  to operate. The driver can recognize an approach to an obstacle target, as a result of operation of the warning buzzer  72   a.    
     According to the above described vehicle  10 , in a configuration where the left and right wheels  12 ,  13  are independently travel-driven by the hydraulic motors  30 ,  31 , it will be easy to automatically avoid a collision of the vehicle  10  with an obstacle target at the time of rapid turning travel to the rear. In this case, different to the case where sensors capable of detecting the rear are arranged only on the rear end of the vehicle  10 , similar to the configurations of  FIG. 1  to  FIG. 9 , it will be easy to detect an obstacle target positioned more on the outer side than both left and right ends of the vehicle  10  and more to the front than the rear end of the vehicle  10 . Also, a turn of the vehicle  10  can be automatically stopped by the detection of an obstacle target. 
     Further, as shown by T 3 , referring to  FIG. 2 , there will be cases where an obstacle target is not in either of the detection ranges of the left and right first sensors  50   a ,  50   b , in a stop state of the vehicle. However, when the vehicle  10  turns rapidly, by means of a zero-turn or the like, to the rear in the arrow a direction, such as shown in  FIG. 2  and  FIG. 8 , the obstacle target T 3  will be detected by the first sensor  50   b  of the left side in the state of  FIG. 8 . As a result, the vehicle  10  is prevented from colliding with the obstacle target T 3 , by having the rapid turn of the vehicle  10  decelerate, and thereafter stopped. 
     In this way, when the left and right wheels  12 ,  13  rotate in opposite directions, and an absolute value of the ground movement speed of the wheel rotating to the rear is equal to or higher than an absolute value of the ground movement speed of the wheel rotating to the front, a rapid turn to the rear of the vehicle  10  will occur. Moreover, as shown in  FIG. 14 , in the case where only one wheel of the left and right wheels  12 ,  13  rotates in the backward direction, and the other wheel is stopped, a rapid turn to the rear will occur. In this way, at the time when the vehicle  10  turns rapidly to the rear, it will become easy for the vehicle to approach an obstacle target positioned in a difficult-to-confirm position. Moreover, in the case where the vehicle rapidly turns in a zero-turn, the vehicle will significantly deflect in the left-right direction at this location, and therefore it will become easy for the vehicle to approach an obstacle target positioned in a difficult-to-confirm position. Also, considerable attention will be required by the driver in order to prevent the vehicle colliding with the obstacle target. In an embodiment, when the obstacle target has been detected by the first sensors  50   a ,  50   b , the effect achieved by a configuration where the controller  60  causes a rapid turn to the rear to decelerate, and thereafter stop, will be remarkable. Configurations and actions other than these will be the same as the configurations of  FIG. 1  to  FIG. 9 , or the configuration of  FIG. 13 . 
       FIG. 23  is a block diagram showing the characteristic configuration of the vehicle  10  in another example of an embodiment. In the configuration of this example, the two second sensors  51   a ,  51   b  are arranged more to the rear of the vehicle  10  than the first sensors  50   a ,  50   b , in the configuration of  FIG. 21 ,  FIG. 22A  and  FIG. 22B . The arrangement positions of each of the sensors  50   a ,  50   b , Ma, and Mb are the same as the configurations of  FIG. 10  and  FIG. 11 . Moreover, the controller  60  has the turn deceleration stop section  62   b  ( FIG. 21 ). When an obstacle target has been detected by at least one of the first sensors  50   a ,  50   b  and the second sensors Ma, Mb, the turn deceleration stop section  62   b  causes a rapid turn to the rear of the vehicle  10  to decelerate, and causes the vehicle  10  to stop prior to colliding with the obstacle target. 
     According to the above described configuration, since the range in which detection of an obstacle target is possible is extended, it will be easier to automatically detect an obstacle target that approaches the vehicle  10  at the time of turning travel to the rear. For example, even at the time when obstacle targets T 3  and T 4  ( FIG. 11 ) are positioned near the vehicle  10  at the rear of the vehicle  10 , and these obstacle targets can not be detected by the first sensors  50   a ,  50   b , it will be easy to detect the obstacle targets T 3  and T 4  with the second sensors Ma, Mb. As a result, it will be easy to automatically and effectively avoid a collision of the vehicle  10  with an obstacle target at the time of rapid turning travel to the rear. Configurations and actions other than these will be the same as the configurations of  FIG. 1  to  FIG. 9 , the configurations of  FIG. 10  and  FIG. 11 , or the configurations of  FIG. 21 ,  FIG. 22A  and  FIG. 22B . 
       FIG. 24  is a block diagram showing the characteristic configuration of the vehicle  10  in another example of an embodiment.  FIG. 25A  is a view corresponding to  FIG. 4A , in the configuration shown in  FIG. 24 , and  FIG. 25B  is a view seen from an arrow D direction of  FIG. 25A . The vehicle  10  of  FIG. 24  and  FIG. 25A  and  FIG. 25B  has the swash plate operation levers  32   c ,  33   c  of each of the left and right power generation units  26 ,  27 , in the configurations of  FIG. 21 ,  FIG. 22A  and  FIG. 22B , not connected via the operation levers  22 ,  23  and a link. Instead of this, the vehicle  10  includes the left and right swash plate actuators  81 ,  82 . The swash plate actuators  81 ,  82  drive the swash plate operation levers  32   c ,  33   c  connected to the swash plate operation shafts  32   b ,  33   b  ( FIG. 4B ) of the hydraulic pumps  32 ,  33  ( FIG. 4B ) of the corresponding sides, and are controlled by the controller  60 . The swash plate actuators  81 ,  82  include a piston cylinder mechanism, or a motor, for example, which rotates the swash plate operation levers  32   c ,  33   c  of the corresponding sides. 
     The controller  60  drives the left and right swash plate operation levers  32   c ,  33   c  by controlling the driving of the left and right swash plate actuators  81 ,  82 , in accordance with detection signals from the left and right lever potentiometers  38 ,  39 . For example, in the case where the left and right operation levers  22 ,  23  have lowered to the front, the controller  60  drives the swash plate operation levers  32   c ,  33   c  in one direction by the control of the swash plate actuators  81 ,  82 , in accordance with detection signals from the lever potentiometers  38 ,  39 . As a result, the discharge amounts of the left and right hydraulic pumps  32 ,  33  will change, and the movable swash plates of each of the hydraulic pumps  32 ,  33  will tilt, so that the discharge amounts of each of the hydraulic pumps  32 ,  33  increase at the advancing side. In the case where the left and right operation levers  22 ,  23  have lowered to the rear, the controller  60  drives the swash plate operation levers  32   c ,  33   c  in the other direction by control of the swash plate actuators  81 ,  82 . As a result, the discharge amounts of the left and right hydraulic pumps  32 ,  33  will change, and the movable swash plates of each of the hydraulic pumps  32 ,  33  will tilt, so that the discharge amounts of each of the hydraulic pumps  32 ,  33  increase in a direction of a rotation of the reversing side. Accordingly, the controller  60  causes the movable swash plates of the left and right hydraulic pumps  32 ,  33  to tilt, and causes the discharge amounts of the hydraulic pumps  32 ,  33  to change, by controlling the driving of the left and right swash plate actuators  81 ,  82  in accordance with detection signals of the left and right lever potentiometers  38 ,  39 . 
     In addition, the controller  60  brings the discharge amounts of each of the hydraulic pumps  32 ,  33  near to zero, by setting the tilting angles of the movable swash plates of the left and right hydraulic pumps  32 ,  33  to a neutral state, by controlling the driving of the left and right swash plate actuators  81 ,  82 . As a result, a rapid turn to the rear of the vehicle decelerates. Also, the controller  60  sets the discharge amounts of each of the hydraulic pumps  32 ,  33  to substantially zero, by setting the tilting angles of the movable swash plates of the left and right hydraulic pumps  32 ,  33  to approximately a neutral state. As a result, a turn to the rear of the vehicle is stopped. Configurations and actions other than these will be the same as the configurations of  FIG. 21 ,  FIG. 22A  and  FIG. 22B . Even in the configurations of  FIG. 24  and  FIG. 25A  and  FIG. 25B , similar to the configurations of  FIG. 21 ,  FIG. 22A  and  FIG. 22B , reverse switches can be included as assistance. 
       FIG. 26  is a block diagram showing the characteristic configuration of the vehicle  10  in another example of an embodiment.  FIG. 27  is a view corresponding to  FIG. 4A , in the configuration shown in  FIG. 26 . The vehicle  10  of  FIG. 26  and  FIG. 27  has the belt  36  ( FIG. 27 ) including the belt tension switching mechanism  41 , in the configurations of  FIG. 21 ,  FIG. 22A  and  FIG. 22B , to function as a clutch arranged between an output section of the driving source and an input section of the transmission  11 . As a result, the presence or absence of tension can be switched. The belt tension switching mechanism  41  includes a pressing force pulley  42 , and a tension switching actuator  43 . The configuration of the belt tension switching mechanism  41  is the same as the configurations of  FIG. 1  to  FIG. 9 . Such a tension switching actuator  43  is controlled by the controller  60 , and engages/disengages the above described clutch. 
     In addition, the turn deceleration stop section  62   b  ( FIG. 21 ) of the controller  60  sets the power transmission in the clutch to a half-transmission state by causing the tension of the belt  36  to gradually reduce by controlling the driving of the tension switching actuator  43 . As a result, a turn of the vehicle decelerates. Also, by setting the tension of the belt  36  to zero, the drive transmission in the clutch is set to zero, namely, is cut. As a result, a turn to the rear of the vehicle is stopped. Configurations and actions other than these will be the same as the configurations of  FIG. 21 ,  FIG. 22A  and  FIG. 22B , or the configurations of  FIG. 1  to  FIG. 9 . Even in the configurations of  FIG. 26  and  FIG. 27 , similar to the configurations of  FIG. 21 ,  FIG. 22A  and  FIG. 22B , reverse switches can be included as assistance. 
     Note that while illustration is omitted, the configurations in each example shown in  FIG. 13  to  FIG. 27  may be configurations that include hydraulic sensors for respectively detecting pressurized oil in the two main oil paths S 1 , S 2  of the left and right hydraulic circuits, instead of the lever potentiometers. Also, the controller may be configured to have either of the main oil paths of each of the hydraulic circuits as a high pressure side, and to calculate absolute values of the speed directions and speeds of each of the wheels  12 ,  13 , in accordance with detection signals of each of the hydraulic sensors. Also, the controller may determine whether or not the vehicle is rapidly turning to the rear, in accordance with this calculation result. 
     Moreover, the configurations in each of the above described examples can have two left and right direction indication lights fixed at positions separated in the left-right direction of the front end part of the vehicle  10 , such as at positions near a supporting part of the caster wheels  15 ,  16 , for example, along with attaching direction indication switches to the left and right operation levers  22 ,  23 . Each of the direction indication lights is constituted to be capable of flashing a light in the case where the direction indication switch of the corresponding left or right side has been pressed. In such a configuration, since the vehicle  10  turning to the front or the rear side can be notified to a person in the surroundings by the flashing of the direction indication lights, it becomes possible to perform safer travelling. Moreover, the direction indication light may be used as the above described warning section. Specifically, when an obstacle target has been detected by one or both of the left and right first sensors  50   a ,  50   b , the controller causes the left and right direction indication lights to turn on or flash at the same time. 
       FIG. 28  to  FIG. 31D  show a riding type vehicle according to another example of an embodiment.  FIG. 28  is a perspective illustration of the vehicle  10 .  FIG. 29  is a view, when the vehicle  10  is seen from above, showing a circumscribed circle of the caster wheels, in the case of rapidly turning centered on the center between the left and right wheels.  FIG. 30  is a block diagram showing the characteristic configuration of the vehicle  10 . 
     The configuration of this example has the left and right operation levers  22 ,  23  of both left and right sides, in the configurations of  FIG. 1  to  FIG. 9 , supported on surrounding parts of the driver&#39;s seat  21  in the main frame  20 . 
     Moreover, the view, seen from the upper side of the vehicle, of a power transmission structure between power generation units for the left wheel and the right wheel, and an engine, is the same as that of  FIG. 19A  and  FIG. 19B . As shown in  FIG. 30 , the vehicle includes two left and right swash plate actuators  81 ,  82 .
         By referring to  FIG. 19A  and  FIG. 19B , the two left and right swash plate actuators  81 ,
 
 82  drive the swash plate operation levers  32   c ,  33   c  of the left and right corresponding sides, and the driving is controlled by the controller  60 , which is a control device. The swash plate actuators  81 ,  82  include a piston cylinder mechanism, or a motor, for example, which rotates the swash plate operation levers  32   c ,  33   c  of the corresponding sides.
       

     Moreover, the vehicle  10  includes the two left and right lever potentiometers  38 ,  39 , which are swing angle detection sections. The left lever potentiometer  38  detects a swing angle position of the left operation lever  22 , and the right lever potentiometer  39  detects a swing angle position of the right operation lever  23 . Detection signals of each of the lever potentiometers  38 ,  39  are transmitted to the controller  60  ( FIG. 30 ). 
     The left and right operation levers  22 ,  23  are capable of being displaced, from a maximum displacement position Fmax of a region indicating advancing to a maximum displacement position Rmax of a region indicating reversing, centered on the neutral position of  FIG. 19A  and  FIG. 19B . The left and right lever potentiometers  38 ,  39  are arranged near shafts S of the left and right operation levers  22 ,  23 . 
     Referring to  FIG. 4B , the controller  60  causes the swash plate operation shafts  32   b ,  33   b  to rotate, in accordance with the swing of the front-rear direction of the operation levers  22 ,  23 . Specifically, as described below, the controller  60  drives the left and right swash plate operation levers  32   c ,  33   c  by controlling the driving of the left and right swash plate actuators  81 ,  82 , in accordance with detection signals from the left and right lever potentiometers  38 ,  39 . The swash plate operation shafts  32   b ,  33   b  will rotate due to the rotation of the swash plate operation levers  32   c ,  33   c . Also, the tilting angles and orientations of the movable swash plates of the hydraulic pumps  32 ,  33  will change. The discharge amounts of the hydraulic pumps  32 ,  33  change, in accordance with the change of tilting angles of the movable swash plates. By lowering the operation levers  22 ,  23  significantly to the front or the rear, the discharge amounts of the hydraulic pumps  32 ,  33  will increase. The left hydraulic motor  30  is driven by a pressurized oil supply from the left hydraulic pump  32 . The right hydraulic motor  31  is driven by a pressurized oil supply from the right hydraulic pump  33 . By lowering the operation levers  22 ,  23  more to the front than a neutral state, discharge directions will be prescribed so that the hydraulic pumps  32 ,  33  cause the hydraulic motors  30 ,  31  to rotate to one side. By lowering the operation levers  22 ,  23  more to the rear than a neutral state, discharge directions will be prescribed so that the hydraulic pumps  32 ,  33  cause the hydraulic motors  30 ,  31  to rotate to the other side. A neutral state is a state where there is no discharge of oil at a position the operation levers  22 ,  23  automatically return to in a state of not being gripped by the driver. For the rotation directions of the hydraulic motors  30 ,  31 , one side corresponds to the rotation of a forward direction of the wheels  12 ,  13 , and the other side corresponds to the rotation of a backward direction of the wheels  12 ,  13 . 
     In addition, at the time a rapid turn has been performed, and it is determined to be an unstable turn, the controller  60  controls the driving of the left and right swash plate actuators  81 ,  82 , as described below. As a result, the discharge amounts of each of the hydraulic pumps  32 ,  33  approach zero as a result of bringing the tilting angles of the movable swash plates of the left and right hydraulic pumps  32 ,  33  close to an approximately neutral state, and as a result this, the turning speed of the vehicle is reduced. Alternatively, the discharge amounts of each of the hydraulic pumps  32 ,  33  are substantially set to zero by setting the tilting angles of the movable swash plates of the left and right hydraulic pumps  32 ,  33  to an approximately neutral state, and as a result of this, the turning speed of the vehicle is suppressed. Specifically, the turning speed is reduced, or the turning speed is set to zero. In the case where the tilting angles of the movable swash plates are in an approximately neutral state, the movable swash plates will be in an approximately neutral position. At this time, the hydraulic motors  30 ,  31  will not be driven. 
     Moreover, by referring to  FIG. 4B , in the hydraulic circuits  28 ,  29  where each of the power generation units  26 ,  27  are included, a charge oil path C 1  is connected, respectively at the left and right, to the two main oil paths  51 , S 2  connecting the hydraulic pumps  32 ,  33  and the hydraulic motors  30 ,  31 . The charge oil path C 1  connects each of the main oil paths  51 , S 2 , and the oil reservoir E, via check valves F 1 , F 2 . Specifically, the hydraulic circuit  28  of the left power generation unit  26  includes the first main oil path  51 , which is a first oil path, and the second main oil path S 2 , which is a second oil path. The first main oil path  51  connects one port P 1  from among the two ports P 1 , P 2  of the left hydraulic pump  32 , and one port Q 1  from among the two ports Q 1 , Q 2  of the left hydraulic motor  30 . The second main oil path S 2  connects the other port P 2  from among the two ports P 1 , P 2  of the left hydraulic pump  32 , and the other port Q 2  from among the two ports Q 1 , Q 2  of the left hydraulic motor  30 . On the other hand, the hydraulic circuit  29  of the right power generation unit  27  includes a third main oil path S 3 , which is a third oil path, and a fourth main oil path S 4 , which is a fourth oil path. The third main oil path S 3  connects one port P 3  from among the two ports P 3 , P 4  of the right hydraulic pump  33 , and one port Q 3  from among the two ports Q 3 , Q 4  of the right hydraulic motor  31 . The fourth main oil path S 4  connects the other port P 4  from among the two ports P 3 , P 4  of the right hydraulic pump  33 , and the other port Q 4  from among the two ports Q 3 , Q 4  of the right hydraulic motor  31 . 
     The charge oil path C 1  is for replenishing oil from the oil reservoir E to the main oil path of a low pressure side, from among each of the main oil paths S 1 , S 2 , S 3 , and S 4 . Moreover, a bypass valve  28   a  is connected between both the first main oil path S 1  and the second main oil path S 2 , and the oil reservoir E. A bypass valve  29   a  is connected between both the third main oil path S 3  and the fourth main oil path S 4 , and the oil reservoir E. The bypass valves  28   a ,  29   a  are configured to be capable of switching between opening, which is a connection between the main oil paths S 1 , S 2 , S 3 , and S 4  and the oil reservoir E, and closing, which is a disconnection, manually. 
     As shown in  FIG. 28  and  FIG. 29 , an acceleration sensor  150  is arranged near the center between the two caster wheels  15 ,  16 , on the main frame  20 , or on a member with high rigidity fixed to the main frame  20 , at the front end part of the vehicle  10 . The acceleration sensor  50  is called a G sensor, and detects the acceleration in the left-right direction of the vehicle  10 , as a turn stability relationship amount, which is a physical quantity related to turn stability. The acceleration sensor  150  corresponds to a turn stability relationship sensor. 
       FIG. 31A ,  FIG. 31B ,  FIG. 31C , and  FIG. 31D  are views showing the principles of the acceleration sensor  150 .  FIG. 31A  is a view at the time when a weight  150   b  of the acceleration sensor  150  is at a neutral position.  FIG. 31B  is a view at the time when the weight  150   b  is displaced in an X-direction.  FIG. 31C  is a view at the time when the weight  150   b  is displaced in a Y-direction.  FIG. 31D  is a view at the time when the weight  150   b  is displaced in a Z-direction. For example, as shown in  FIG. 31A , the acceleration sensor  150  includes the weight  150   b  arranged on the inner side of a case  150   a , and springs  150   c  joined to multiple positions of the surroundings of the weight  150   b , and is supported within the case  150   a  in a state where the weight  150   b  is balanced and steady at a neutral position. At this time, the acceleration can be measured by detecting a displacement of the weight  150   b . For example, in the case where X, Y, and Z-directions are considered as orthogonal 3-axis directions such as shown in  FIG. 31B , a displacement amount will be obtained at the time when the weight  150   b  is displaced from the neutral position in the X-direction. As a result, an acceleration in the X-direction can be detected. Moreover, as shown in  FIG. 31C  and  FIG. 31D , a displacement amount will be obtained at the time when the weight  150   b  is displaced from the neutral position in the Y-direction or the Z-direction. Based on this displacement amount, an acceleration in the Y-direction or the Z-direction can be detected. The acceleration sensor  150  has a configuration based on such principles, and the acceleration sensor  150  is arranged in the vehicle  10  so that the X-direction of the acceleration sensor  150 , for example, is along the left-right direction of the vehicle  10 . A semiconductor system such as an electrostatic capacitance type, piezo resistance type, distortion gauge detection type, or gas temperature distribution type semiconductor system may be included in the acceleration sensor  150 . 
     In the case where such a detection value of the acceleration sensor  150  is excessively high, it can be determined to be an unstable turn where the front end part of the vehicle  10  is drastically turning to the left or right. Detection signals of the acceleration sensor  150  are transmitted to the controller  60  ( FIG. 30 ). 
     As shown in  FIG. 30 , the controller  60  has a rapid turn determination section  60   a , an unstable turn determination section  60   b , a turning speed suppression section  60   c , and a swash plate actuator control section  60   d . The rapid turn determination section  60   a  determines whether or not the vehicle  10  is rapidly turning from detection signals from the left and right lever potentiometers  38 ,  39 . For example, the rotation directions and rotation angles of the left and right wheels  12 ,  13  are calculated from these detection signals. In the case where the left and right wheels  12 ,  13  rotate in the same direction, and the rotation speeds of the left and right wheels  12 ,  13  differ, it is determined that the vehicle  10  is turning moderately to the front or the rear. A rapid turn detection section  137  is constituted by the left and right lever potentiometers  38 ,  39  and the rapid turn determination section  60   a . The rapid turn detection section  137  detects that the vehicle  10  is turning rapidly. Performing a rapid turn, which is any one of a pivot turn, a zero-turn, and a turn between a pivot turn and a zero-turn, which are described next, is detected by the rapid turn detection section  137 . Also, the turning speed will increase as the values of each of the left and right detection signals from the lever potentiometers  38 ,  39  increase. 
     For example, in the case where only one wheel of the left and right wheels  12 ,  13  rotates, it is determined that the vehicle  10  is rapidly turning, which is called a pivot turn, which is a turn centered on the ground position of the one wheel of the left and right wheels  12 ,  13 . Moreover, in the case where the left and right wheels  12 ,  13  rotate in opposite directions, it is determined that the vehicle  10  is rapidly turning with a reduced turning radius. In particular, in the case where the left and right wheels  12 ,  13  rotate in opposite directions, and the absolute values of the rotation speeds of the left and right wheels  12 ,  13  are the same, it is determined that the vehicle  10  is turning rapidly, which is called a zero-turn or an ultra-pivot turn, which is a turn centered on the center between the left and right wheels  12 ,  13 . As a result, it can be detected that the vehicle is turning rapidly. Such a rapid turn is as described using  FIG. 6B  and  FIG. 6C . 
     When the detection value of the acceleration sensor  150  is equal to or higher than a threshold determined beforehand for the acceleration, the unstable turn determination section  60   b  determines that it is an unstable turn. The swash plate actuator control section  60   d  causes the movable swash plate of the left hydraulic pump  32  to tilt by controlling the driving of the left swash plate actuator  81  in accordance with a detection signal of the left lever potentiometer  38 . Moreover, the swash plate actuator control section  60   d  causes the movable swash plate of the right hydraulic pump  33  to tilt by controlling the driving of the right swash plate actuator  82  in accordance with a detection signal of the right lever potentiometer  39 . 
     In addition, when a rapid turn has been performed, and it is determined to be an unstable turn, the turning speed suppression section  60   c  reduces the turning speed, in the vehicle  10 , in accordance with a determination result of the rapid turn determination section  60   a  and the unstable turn determination section  60   b . Specifically, the turning speed suppression section  60   c  brings the tilting angles of the swash plates of the left hydraulic pump  32  and the right hydraulic pump  33  to a neutral state by controlling the driving of the left swash plate actuator  81  and the right swash plate actuator  82 . As a result, the turning speed of the vehicle  10  is reduced, by bringing the discharge amounts of the left hydraulic pump  32  and the right hydraulic pump  33  close to zero. Moreover, the turning speed suppression section  60   c  may set the turning speed to zero by setting the discharge amounts of the left hydraulic pump  32  and the right hydraulic pump  33  to substantially zero, by setting the tilting angles of the swash plates of the left hydraulic pump  32  and the right hydraulic pump  33  to an approximately neutral state. As a result, the vehicle is stopped. While the portions that execute each of the functions of the rapid turn determination section  60   a , the unstable turn determination section  60   b  or the like of the controller  60  may be integrated into one controller, multiple controllers may be connected by cables, and multiple functions may be separately executed by the multiple controllers. 
     Straight travel, turning travel, a pivot turn, and an ultra-pivot turn of the vehicle  10  are the same as the travelling or turning described by using  FIG. 5  and  FIG. 6A  to  FIG. 6C . 
     According to the above described vehicle  10 , in a configuration where the left and right wheels  12 ,  13  are independently travel-driven by the left and right hydraulic motors  30 ,  31 , an unstable turn can be automatically suppressed. For example, at the time when the turning speed is low, in the case where the vehicle turns rapidly using a zero-turn such as in  FIG. 29 , the acceleration in the left-right direction detected by the acceleration sensor  150  will be less than a threshold, and therefore the rapid turn will not be suppressed. On the other hand, when the turning speed is excessively high, in the case where the vehicle turns rapidly using a zero-turn, the acceleration in the left-right direction detected by the acceleration sensor  150  will be equal to or higher than a threshold. In this case, it is determined to be an unstable turn, and therefore the vehicle decelerates or stops as a result of the operation of the swash plate actuators  81 ,  82 . As a result, the turning speed is suppressed, and therefore an unstable turn is automatically suppressed. In addition, in a configuration where the turning speed of the vehicle is gradually lowered by the turning speed suppression section  60   c , different to the configuration where the vehicle rapidly stops, an additional impact to the driver can be eased by deceleration. Moreover, by having the turning speed suppressed, it will become easy for the driver to recognize a person or object present in the surroundings, and therefore safety can be improved. 
     Moreover, the front end part of the vehicle  10  will move significantly in the left-right direction, in a rapid turn where the turning center is positioned between the ground positions of the left and right wheels  12 ,  13 , or the ground position of one wheel of the left and right wheels  12 ,  13 . As a result, in the case where the acceleration sensor  150  is arranged on the front end part of the vehicle  10 , such as in an embodiment, the detection accuracy of turning stability will improve. Note that the arrangement position of the acceleration sensor  150  is not limited to the configuration of being arranged on the front end part of the vehicle  10 , and can be arranged at various positions, as long as it is arranged on a member with high rigidity in the vehicle  10 . For example, the acceleration sensor  150  may be arranged near the center of gravity of the vehicle  10 . Moreover, in the case where the front wheels are driving wheels, and the rear wheels are caster wheels, the rear end part of the vehicle will move significantly in the left-right direction, and therefore the detection accuracy of turning stability can be improved, by arranging the acceleration sensor on the rear end part of the vehicle. 
     Moreover, a case has been described, heretofore, where the acceleration sensor  150  is used as a turning stability relationship sensor that detects a turning stability amount related to turning stability. On the other hand, an angular velocity sensor  151  ( FIG. 30 ) can be used instead of an acceleration sensor, as a turning stability relationship sensor. The angular velocity sensor  151  can be arranged at a position similar to the arrangement position of the acceleration sensor  150  of  FIG. 28 , on the front end part of the vehicle, or can be arranged on a member with high rigidity such as the rear end part of the vehicle near the center position between the left and right wheels  12 ,  13 , in the vehicle. The angular velocity sensor  151  detects an angular velocity of the vehicle around an axis in a vertical direction. A gyro sensor can be used, for example, in the angular velocity sensor  151 . 
     When a rapid turn has been performed, and a detection value of the angular velocity sensor  151  is equal to or higher than a threshold, the turning speed suppression section  60   c  of the controller  60  reduces the turning speed, by decelerating or stopping the vehicle by the operation of the swash plate actuators  81 ,  82 . In the case of such a configuration, an unstable turn can also be automatically suppressed similar to the case of using the acceleration sensor  150 . For example, when the turning speed is low, in the case where the vehicle turns rapidly, the angular velocity detected by the angular velocity sensor  151  will be less than a threshold, and therefore the rapid turn will not be suppressed. On the other hand, at the time when the turning speed is excessively high, in the case where the vehicle turns rapidly, the angular velocity detected by the angular velocity sensor  151  will be equal to or higher than a threshold, and therefore the rapid turn will be suppressed. As a result, unstable turning can be automatically suppressed. Configurations and actions other than these will be the same as the configurations of  FIG. 1  to  FIG. 9 . 
       FIG. 32  is a block diagram showing the characteristic configuration of the vehicle  10  in another example of an embodiment. In the configuration of  FIG. 32 , the view, seen from the upper side of the vehicle, of a power transmission structure between power generation units for the left wheel and the right wheel, and an engine, is the same as that of  FIG. 4A . In the configuration shown in  FIG. 32 , the vehicle  10 , in the configurations of  FIG. 28  to  FIG. 31D , includes the tension switching actuator  43 , instead of the swash plate actuators. Specifically, by referring to  FIG. 4A , the lower end parts of the operation levers  22 ,  23  of the left and right corresponding sides are respectively connected, via a link  37 , to the left and right swash plate operation levers  32   c ,  33   c . As a result, by having the operation levers  22 ,  23  swing in the front-rear direction, the swash plate operation shafts  32   b ,  33   b  will rotate. Also, the tilting angles and orientations of the movable swash plates of the hydraulic pumps  32 ,  33  (refer to  FIG. 4B ) will change. 
     Moreover, referring to  FIG. 4A , a belt  36  is suspended between a drive pulley  40  fixed to a drive shaft  14   a  of the engine  14 , and driven pulleys  35  fixed to driven shafts of the hydraulic pumps  32 ,  33 . This belt  36  can include a belt tension switching mechanism  41 , to function as a clutch arranged between an output section of the driving source and an input section of the transmission  11 , and as a result, the presence or absence of tension can be switched. The belt tension switching mechanism  41  includes a pressing force pulley  42 , and a tension switching actuator  43 . The configuration of the belt tension switching mechanism  41  is the same as that of the configurations of  FIG. 1  to  FIG. 9 . The driving of the tension switching actuator  43  is controlled by the controller  60  ( FIG. 30 ), and engages/disengages the above described clutch. 
     In addition, when a rapid turn has been performed, and a detection value of the acceleration sensor  150  or the angular velocity sensor  151  is equal to or higher than a threshold, the turning speed suppression section  60   c  (refer to  FIG. 30 ) of the controller  60  controls the driving of the tension switching actuator  43 . Also, the turning speed is reduced by bringing the tension of the belt  36  close to zero, or setting to zero. In the case where there is no energization of a solenoid, tension is generated in the belt  36 , and power is transmitted from the engine  14  to the hydraulic pumps  32 ,  33 , and therefore power transmission in the clutch, between the engine  14  and the transmission  11 , will be in a connection state. Configurations and actions other than these will be the same as the configurations of  FIG. 1  to  FIG. 9 , or the configurations of  FIG. 28  to  FIG. 31D . 
     Another example of an embodiment will be described, by referring to  FIG. 16A - FIG. 16B . The vehicle  10  in another example of an embodiment, in the configurations of  FIG. 28  to  FIG. 31D , includes left and right reverse switches  75 ,  76  respectively arranged in the surrounding parts of the lower end parts of the left and right operation levers  22 ,  23 . In the case where it is detected that the left and right operation levers  22 ,  23  have been swung to regions where reversing will be instructed, the left and right back switches  75 ,  76  transmit these detection signals to the controller  60 . By using not only the left and right lever potentiometers  38 ,  39 , but also the detection signals of the reverse switches  75 ,  76  as assistance, the controller  60  will more stably determine whether or not the vehicle is turning rapidly. 
     Moreover, the vehicle  10  includes a throttle actuator  78  that mechanically or electrically adjusts the opening of the throttle valve of the engine  14 . The controller  60  (refer to  FIG. 30 ) controls the motor of the throttle actuator  78 , so that the engine  14  is driven at a constant rotation speed determined beforehand, by having a start switch (not illustrated) set to ON by a user. The controller  60  controls the throttle actuator  78  so as cause the throttle valve to close, by having the start switch set to OFF by the user. 
     Moreover, when a rapid turn has been performed, and a detection value of the acceleration sensor  150  ( FIG. 30 ) or the angular velocity sensor  151  ( FIG. 30 ) is equal to or higher than a threshold, the turning speed suppression section  60   c  ( FIG. 30 ) of the controller  60  controls the driving of the throttle actuator  78 . Also, a turn of the vehicle decelerates or is stopped, or the turning speed is reduced, by bringing the throttle valve close to a closed state, or closing. Configurations and actions other than these will be the same as the configurations of  FIG. 15  to  FIG. 16A - FIG. 16B , or the configurations of  FIG. 28  to  FIG. 31D . The reverse switches  75 ,  76  can be used in the configurations of  FIG. 28  to  FIG. 31D , or the configuration of  FIG. 32 . 
       FIG. 33  is a view corresponding to  FIG. 4B , in the vehicle  10  in another example of an embodiment. The vehicle  10  of  FIG. 33  includes left-right bypass actuators  179 ,  180 , in the configurations of  FIG. 28  to  FIG. 31D . The left and right bypass actuators  179 ,  180  open-close drive the bypass valves  28   a ,  29   a  connected between the main oil paths S 1 , S 2 , S 3 , S 4  of the hydraulic circuits  28 ,  29  of the left and right power generation units  26 ,  27 , and the oil reservoir E, at the same time. The bypass valves  28   a ,  29   a  discharge oil of the main oil paths  51 , S 2 , S 3 , S 4  to the oil reservoir E, in an opened state, namely, in a connection state between the main oil paths  51 , S 2 , S 3 , S 4  and the oil reservoir E. On the other hand, the bypass valves  28   a ,  29   a  circulate oil to the main oil paths  51 , S 2 , S 3 , S 4  in a closed state, namely, in a disconnection state between the main oil paths  51 , S 2 , S 3 , S 4  and the oil reservoir E. For example, the left bypass actuator  179  includes a left solenoid that electrically switches the closing and opening of the left bypass valve  28   a , and the right bypass actuator  180  includes a right solenoid that electrically switches the closing and opening of the right bypass valve  29   a . Each of the bypass actuators  179 ,  180  is controlled by the controller  60  (refer to  FIG. 3 ). Also, the controller  60  stops the supply of oil to the hydraulic motors  30 ,  31 , even during the driving of the hydraulic pumps  32 ,  33 , by setting the left and right bypass valves  28   a ,  29   a  to an opened state at the same time by controlling the driving of the left and right bypass actuators  179 ,  180 . As a result, since the hydraulic motors  30 ,  31  are in an idling state, a turn of the vehicle  10  will be stopped. Note that at this time, the hydraulic pumps  32 ,  33  will not stop rapidly, and will be stopped by inertia, and therefore the vehicle  10  will also be stopped by inertia. Configurations and actions other than these will be the same as the configurations of  FIG. 28  to  FIG. 31D . In the configuration of  FIG. 33  also, reverse switches can be included as assistance. 
       FIG. 34  is a view corresponding to  FIG. 4B , in the vehicle  10  in another example of an embodiment. In the vehicle  10 , a first pressure sensor  181 , a second pressure sensor  182 , a third pressure sensor  183 , and a fourth pressure sensor  184  are included, as turning stability relationship sensors. The first pressure sensor  181  detects the pressure of the first main oil path S 1  connecting one port P 1  of the left hydraulic pump  32  and one port Q 1  of the left hydraulic motor  30 . The second pressure sensor  182  detects the pressure of the second main oil path S 2  connecting the other port P 2  of the left hydraulic pump  32  and the other port Q 2  of the left hydraulic motor  30 . 
     Moreover, the third pressure sensor  183  detects the pressure of the third main oil path S 3  connecting one port P 3  of the right hydraulic pump  33  and one port Q 3  of the right hydraulic motor  31 . In addition, the fourth pressure sensor  184  detects the pressure of the fourth main oil path S 4  connecting the other port P 4  of the right hydraulic pump  33  and the other port Q 4  of the right hydraulic motor  31 . Detection signals of each of the pressure sensors  181 ,  182 ,  183 ,  184  are transmitted to the controller  60  ( FIG. 30 ). Also, when a rapid turn has been performed, and an absolute value of a detection value of at least one of the pressure sensors of each of the pressure sensors  181 ,  182 ,  183 ,  184  is equal to or higher than a threshold, the turning speed suppression section  60   c  ( FIG. 30 ) of the controller  60  suppresses the turning speed. Specifically, the turning speed is reduced, or is set to zero. 
       FIG. 35A  is a view showing two of the conditions for suppressing the turning speed by using a relationship between an operation amount of the left operation lever  22  ( FIG. 29 ) and pressure detection values of the first main oil path S 1  and the second main oil path S 2 , in the configuration shown in  FIG. 34 .  FIG. 35B  is a view showing two of the conditions for suppressing the turning speed using a relationship between an operation amount of the right operation lever  23  and pressure detection values of the third main oil path S 3  and the fourth main oil path S 4 , in the configuration shown in  FIG. 34 . 
     For example, the region of the shaded part U 1  of  FIG. 35A  shows a condition where the operation amount of the left operation lever  22 , that has been operated to a front side F corresponding to advancing rotation, is equal to or more than a prescribed amount close to a maximum value Max, and the pressure of the first main oil path S 1  of the left hydraulic pump  32  side is equal to or higher than a prescribed value close to a maximum value Max. The region of the shaded part U 2  of  FIG. 35A  shows a condition where the operation amount of the left operation lever  22 , that has been operated to a rear side R corresponding to reversing rotation, is equal to or higher than a prescribed amount close to a maximum value Max, and the pressure of the second main oil path S 2  of the left hydraulic pump  32  side is equal to or higher than a prescribed value close to a maximum value Max. 
     Moreover, the region of the shaded part U 3  of  FIG. 35B  shows a condition where the operation amount of the right operation lever  23 , that has been operated to a front side F corresponding to advancing rotation, is equal to or higher than a prescribed amount close to a maximum value Max, and the pressure of the third main oil path S 3  of the right hydraulic pump  33  side is equal to or higher than a prescribed value close to a maximum value Max. The region of the shaded part U 4  of  FIG. 35B  shows a condition where the operation amount of the right operation lever  23 , that has been operated to a rear side R corresponding to backing rotation, is equal to or higher than a prescribed amount close to a maximum value Max, and the pressure of the fourth main oil path S 4  of the right hydraulic pump  33  side is equal to or higher than a prescribed value close to a maximum value Max. 
     For example, combining the two conditions shown by arrow J 1  means pressure of the first main oil path S 1  increasing and the left operation lever  22  being operated significantly to the front, and additionally the pressure of the fourth main oil path S 4  increasing and the right operation lever  23  being operated significantly to the rear. In the case where this combination is established, the vehicle  10  will turn rapidly in the state of a zero-turn or near to a zero-turn to the left side, and will have a high speed. At this time, by determining that a rapid turn has been performed, and that it is an unstable turn, the controller  60  ( FIG. 30 ) will reduce the turning speed using the swash plate actuators  81 ,  82  ( FIG. 30 ). 
     On the other hand, combining the two conditions shown by arrow J 2  means the pressure of the second main oil path S 2  increasing and the left operation lever  22  being operated significantly to the rear, and additionally the pressure of the third main oil path S 3  increasing and the right operation lever  23  being operated significantly to the front. In the case where this combination is established, the vehicle will turn rapidly in the state of a zero-turn or near to a zero-turn to the right side, and will have a high speed. At this time, by determining that a rapid turn has been performed, and that it is an unstable turn, the controller will reduce the turning speed by the swash plate actuators. Configurations and actions other than these will be the same as the configurations of  FIG. 28  to  FIG. 31D . Note that in the configurations of each of the examples of  FIG. 28  to  FIG. 33 , and the configurations in another example described by referring to  FIG. 16A - FIG. 16B , the pressure sensors  181 ,  182 ,  183 ,  184  such as in the configurations of  FIG. 34  and  FIG. 35A  and  FIG. 35B  can be used as sensors that detect an unstable turning amount. Even in the configuration of  FIG. 34 , the reverse switches  75 ,  76  (refer to  FIG. 16A - FIG. 16B ) can be included as an assistance. Note that in  FIG. 35A  and  FIG. 35B , a combination of conditions is shown in the case of turning rapidly in the state of a zero-turn or near to a zero-turn, and having a high speed. On the other hand, even in the case where the combination of conditions of  FIG. 35A  and  FIG. 35B  are not established, the turning speed suppression section  60   c  ( FIG. 30 ) may be configured to reduce the turning speed at the time when a rapid turn has been performed, and an absolute value of a detection value of at least one pressure sensor of each of the pressure sensors  181 ,  182 ,  183 ,  184  is equal to or higher than a threshold. The configuration of  FIG. 34  is not limited to a configuration that reduces the turning speed using the swash plate actuators  81 ,  82  ( FIG. 30 ), and can be combined with a configuration that reduces the turning speed in the configurations of each of the examples shown in  FIG. 32  to  FIG. 33 , and the configuration in another example described by referring to  FIG. 16 . 
       FIG. 36A  is a view corresponding to  FIG. 4A , in the vehicle  10  in another example of an embodiment, and  FIG. 36B  is a view seen from an arrow H direction of  FIG. 36A . In the vehicle  10  shown in  FIG. 36A , one of the acceleration sensor  150  ( FIG. 30 ) and the angular velocity sensor  151  ( FIG. 30 ), in the configurations of  FIG. 28  to  FIG. 31D , may not be included. Instead of this, the vehicle  10  uses a left lever swing angle centered on a left lever neutral position of the left operation lever  22 , as a first turning stability relationship amount, and uses a right lever swing angle centered on a right lever neutral position of the right operation lever  23 , as a second turning stability relationship amount. Also, at the time when it is determined that at least one of the left lever swing angle and the right lever swing angle is equal to or higher than a threshold, and it is determined that a rapid turn has been performed from a determination result of the rapid turn determination section  60   a , the turning speed suppression section  60   c  ( FIG. 3 ) of the controller  60  suppresses the turning speed. Specifically, the turning speed suppression section  60   c  suppresses the turning speed by controlling the driving of the left and right swash plate actuators  81 ,  82 . For example, in the case where both the left lever swing angle and the right lever swing angle are within the range of arrow A 1  or arrow A 2  near a lever neutral position, such as shown in  FIG. 36B , from the detection values of each of the lever potentiometers  38 ,  39 , a rapid turn, such as a pivot turn or a zero-turn, will be permitted. On the other hand, in the case where at least one of the left lever swing angle and the right lever swing angle is within the range of arrow A 3  or arrow A 4 , which is a range far from a lever neutral position, a rapid turn will be restricted. At this time, the driving of the swash plate actuators  81 ,  82  is controlled, and the turning speed is reduced, by bringing the tilting angles of the swash plates of the left hydraulic pump  32  and the right hydraulic pump  33  close to a neutral state, or setting to a neutral state. The ranges of the arrows A 1  and A 2  shown in FIG.  36 A and  FIG. 36B  are rapid turn allowable ranges, and the ranges of the arrows A 3  and A 4  are rapid turn restriction ranges. According to the above described configuration, it is not necessary to include either of the acceleration sensor  150  and the angular velocity sensor  151 , and therefore a cost reduction is achieved. Configurations and actions other than these will be the same as the configurations of  FIG. 28  to  FIG. 31D . The configuration of  FIG. 36A  and  FIG. 36B  are not limited to a configuration that suppresses the turning speed using the swash plate actuators  81 ,  82 , and can be combined with a configuration that suppresses the turning speed in the configurations of each of the examples shown in  FIG. 32  to  FIG. 33 , and the configuration in another example described by referring to  FIG. 16 . 
     Note that while a case has been described, in each of the above described embodiments, where the riding type vehicle is a riding lawnmower vehicle that includes a lawnmower, the present invention is not limited to such a configuration, and the riding type vehicle may be a vehicle that does not include a lawnmower. For example, each of the above described embodiments may be a vehicle capable of travelling on uneven ground or a road, by omitting the lawnmower. A forklift, wheelchair or the like driven by left and right electric motors can be included, as specific examples. 
     At least one riding type vehicle of each of the above described embodiments has the configuration of the above described first riding type vehicle. Accordingly, in a configuration where the left and right wheels are capable of being independently driven, with regard to a rotation direction and a rotation speed, it will be easy to automatically detect an obstacle target that approaches the vehicle at the time of turning travel to the rear. In particular, since each of the two first sensors are arranged on both the left and right sides, more to the front than the rear end of the vehicle, and are configured to detect an obstacle target positioned on the rear side, different to the case where sensors capable of detecting the rear are arranged on the rear end of the vehicle, it will be easy to detect an obstacle target positioned more on the outer side than both left and right ends of the vehicle and more to the front than the rear end of the vehicle. 
     At least one riding type vehicle of each of the above described embodiments has the configuration of the above described second riding type vehicle or third riding type vehicle. Accordingly in a configuration where the left and right wheels are capable of being independently driven, with regard to a rotation direction and a rotation speed, it will be easy to automatically avoid a collision with an obstacle target at the time of a rapid turn to the rear. 
     At least one riding type vehicle of each of the above described embodiments has the configuration of the above described fourth riding type vehicle. Accordingly, in a configuration where the left and right wheels are capable of being independently driven, with regard to a rotation direction and a rotation speed, an unstable turn can be automatically suppressed.