Abstract:
A motorized vehicle has a vehicle body, at least a pair of wheels mounted on the vehicle body for undergoing rotation to cause the motorized vehicle to undergo travelling, and a pair of electric motors each mounted on the vehicle body to selectively undergo forward and reverse rotation to rotationally drive a respective one of the wheels. A first switch is connected to the electric motors so that operation of the first switch causes the electric motors to undergo rotation simultaneously in opposite directions to turn the motorized vehicle in a first direction while the motorized vehicle does not undergo travelling. A second switch is connected to the electric motors so that operation of the second switch causes the electric motors to undergo rotation simultaneously in opposite directions to turn the motorized vehicle in a second direction opposite to the first direction while the motorized vehicle does not undergo travelling.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to a motorized vehicle having left and right driving wheels independently driven by left and right electric motors, respectively.  
         BACKGROUND OF THE INVENTION  
         [0002]    The term “working machine” is used herein in a comprehensive sense, i.e., to broadly refer to a load-carrying vehicle, a tiller, a tractor, a lawn mower, a snowplow and so on. In case of the tiller, uncultivated areas are formed at ends of an arable land where the tiller makes a 180° turn. The uncultivated areas should preferably be as small as possible. To meet this condition, the tiller is designed to have a smaller turning radius and, ideally, the tiller can make a turn while staying at the same position. Such a turn is referred to as “spot turn”. The spot turn is very useful not only for the tiller but also for other sorts of working machines because they are required to make sharp or abrupt turns frequently to avoid interference with obstacles.  
           [0003]    Conventional techniques proposed to improve turning performance characteristics of working vehicles are disclosed in Japanese Patent Laid-open Publications Nos. 10-95360 and 6-87340.  
           [0004]    The working vehicle disclosed in Japanese Patent Laid-open Publications Nos. 10-95360 includes a travel HST continuously variable shift mechanism and a turning HST continuously variable shift mechanism disposed in juxtaposition. The travel HST continuously variable shift mechanism is operated by a speed change lever while the turning HST continuously variable shift mechanism is operated a round-type steering handle. The disclosed working vehicle is complicated in construction because a number of links are disposed in a complicated manner below the steering handle and speed change lever. Furthermore, the side-by-side arrangement of two shift mechanisms increases the number of components of the working vehicle and makes the working vehicle expensive to manufacture.  
           [0005]    The working machine disclosed in Japanese Patent Laid-open Publications No. 6-87340 includes a hydraulic continuous variable transmission mechanism equipped with left and right neutral valves adapted to be operated by left and right side clutch control levers provided on left and right handlebars, respectively, of the working vehicle. When the left side clutch control lever is gripped together with the left handlebar, the left neutral valve is activated to realize a clutch-off state of the continuous variable transmission mechanism. Similarly, when the right side clutch control lever is gripped together with the right handlebar, the right neutral valve is activated to realize the clutch-off state of the continuous variable transmission mechanism. With this construction, when a spot turn is to be made, the operator is required to manipulate left and right side clutch control levers with high dexterity. A similar attempt by a non-skilled operator would result in a turn of the working vehicle achieved with an increased turning radius much larger than that attained by the spot turn.  
         SUMMARY OF THE INVENTION  
         [0006]    It is accordingly an object of the present invention to provide a motorized vehicle which is simple in construction but can achieve a spot turn easily and reliably.  
           [0007]    To achieve the foregoing object, according to the present invention, there is provided a motorized vehicle comprising: a vehicle body; a left driving wheel and a right driving wheel that are rotatably mounted on the vehicle body; a left electric motor and a right electric motor that are mounted on the vehicle body for independently rotating the left and right driving wheels, respectively, at variable speeds; and an actuator for causing one of the left and right electric motors to rotate in one direction and, at the same time, causing the other of the left and right electric motors to rotate in the opposite direction, thereby ensuring that the vehicle making a turn while staying at the same position.  
           [0008]    In one preferred form, the motorized vehicle further includes a pair of left and right handlebars extending from the vehicle body in a rearward direction of the motorized vehicle, each of the handlebars having a handgrip adapted to be gripped by the operator. The actuator comprises a left brake and a right brake that are mounted on the vehicle body for independently applying brake forces to the left and right driving wheels, respectively, and a pair of left and right turn control levers pivotally mounted to the left and right handlebars, respectively, so as to extend along the corresponding handgrips. The left and right turn control levers are operatively connected to both the left and right brakes and the left and right electric motors, respectively, such that the left and right electric motors are caused to rotate simultaneously in opposite directions based on the angular positions of the left and right turn control levers. The left and right brakes are associated with the left and right electric motors, respectively, and separately apply the brake forces to the left and right driving wheels via the left and right electric motors.  
           [0009]    It is preferable that the left and right turn control levers are angularly movable between an initial zero-brake position and a stroke end position opposite to the zero-brake position across a full-brake position. The left and right turn control levers are operatively linked with the left and right brakes and the left and right electric motors such that when the left turn control lever moves within a first range defined between the zero-brake position and the full-brake position, the brake force applied from the left brake varies linearly with the amount of displacement of the left turn control lever, when the left turn control lever moves within a second range defined between the full-brake position and the stroke end position, the left electric motor is rotated in the reverse direction, and the right electric motor is rotated in the forward direction, when the right turn control lever moves within the first range, the brake force applied from the right brake varies linearly with the amount of displacement of the right turn control lever, and when the right turn control lever moves within the second range, the right electric motor is rotated in the reverse direction, and the left electric motor is rotated in the forward direction.  
           [0010]    In another preferred form, the actuator comprises a left spot turn switch operatively connected to the left and right electric motors and manually operable to cause the left electric motor to rotate in the reverse direction and the right electric motor to rotate in the forward direction, and a right spot turn switch operatively connected to the left and right electric motors and manually operable to cause the right electric motor to rotate in the reverse direction and the left electric motor to rotate in the forward direction. The motorized vehicle may further include an operator control panel mounted to the vehicle body in which instance, the left and right spot turn switches are provided on the operator control panel.  
           [0011]    The motorized vehicle may further include a pair of left and right crawler belts driven by the left and right driving wheels.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    Certain preferred embodiments of the present invention will hereinafter be described in detail, by way of example only, with reference to the accompanying drawings, in which:  
         [0013]    [0013]FIG. 1 is a plan view of a motorized vehicle according to a first embodiment of the present invention;  
         [0014]    [0014]FIG. 2A is a diagrammatical view showing the operation of an accelerator lever of the motorized vehicle;  
         [0015]    [0015]FIG. 2B is a graph showing the relationship between the output from an accelerator potentiometer and the position of the accelerator lever;  
         [0016]    [0016]FIG. 3 is a side view showing a brake control lever serving also as a turn control lever of the motorized vehicle;  
         [0017]    [0017]FIG. 4A is a diagrammatical view showing the operation of a brake potentiometer taken in conjunction with the position of the turn control lever;  
         [0018]    [0018]FIG. 4B is a graph showing the relationship between the output from the brake potentiometer and position of the turn control lever;  
         [0019]    [0019]FIG. 5 is a pictorial block diagram showing a control system of the motorized vehicle;  
         [0020]    [0020]FIG. 6 is a flowchart showing a series of operations achieved by the control system when the vehicle makes a spot turn;  
         [0021]    [0021]FIGS. 7A to  7 C are diagrammatical views illustrative of the manner in which the vehicle makes a sport turn;  
         [0022]    [0022]FIGS. 8A and 8B are diagrammatical views illustrative of the manner in which the vehicle makes a normal pivot turn;  
         [0023]    [0023]FIG. 9 is a plan view of a motorized vehicle according to a second embodiment of the present invention;  
         [0024]    [0024]FIG. 10A is a diagrammatical view showing the operation of a brake potentiometer taken in conjunction with the position of a brake control lever;  
         [0025]    [0025]FIG. 10B is a graph showing the relationship between the output from the brake potentiometer and position of the brake control lever;  
         [0026]    [0026]FIG. 11 is a pictorial block diagram showing a control system of the motorized vehicle shown in FIG. 9;  
         [0027]    [0027]FIG. 12 is a flowchart showing a series of operations achieved by the control system when the vehicle of FIG. 9 makes a spot turn;  
         [0028]    [0028]FIGS. 13A to  13 C are diagrammatical views illustrative of the manner in which the vehicle shown in FIG. 9 makes a sport turn;  
         [0029]    [0029]FIG. 14 is a side view of a snowplow embodying the present invention;  
         [0030]    [0030]FIG. 15 is a plan view of the snowplow; and  
         [0031]    [0031]FIG. 16 is a diagrammatical, partly perspective view showing a control system of the snowplow. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0032]    [0032]FIG. 1 shows in plan view a motorized vehicle  10  according to a first embodiment of the present invention, the vehicle  10  taking the form of a walk-behind motorized crawler cart. The motorized crawler cart  10  generally comprises a vehicle frame or body  11 , batteries  12  mounted on the vehicle body  11 , left and right electric motors  13 L,  13 R powered with the batteries  12 , left and right driving axles  14 L,  14 R rotatably mounted on the vehicle frame  11  and independently driven by the left and right electric motors  13 L,  13 R, respectively, left and right driving wheels  15 L,  15 R attached to an end of the left and right driving axles  14 L,  14 R, respectively, left and right crawler belts  16 L,  16 R each stretched between the driving wheel  15 L,  15 R and a driven wheel  15 ′L,  15 ′R and driven by the driving wheel  15 L,  15 R, and left and right brakes  17 L,  17 R for independently applying a braking force to the left and right driving wheels  15 L,  15 R, respectively. In the illustrated embodiment, the left and right brakes  17 L,  17 R are associated with the left and right electric motors  13 L,  13 R, respectively, for independently braking the motors  13 L,  13 R to vary the speeds of the left and right driving wheels  15 L,  15 R. The driven wheels  15 ′L,  15 ′R are rotatably mounted on opposite ends of a front axle  14 ′ rotatably mounted on the vehicle body  11 .  
         [0033]    The vehicle  10  further has a load-carrying platform  20  mounted on the vehicle body  11 , an operator control panel  21  mounted to a read end of the load-carrying platform  20 , and left and right operation handlebars  30 L,  30 R extending from a rear portion of the operator control panel  21  obliquely upward in a rearward direction of the snowplow  10 . The handlebars  30 L,  30 R may be so arranged to extend form the vehicle body  11  or the platform  20 . The operator control panel  21  is provided with an accelerator lever  22 .  
         [0034]    The operation handlebars  30 L,  30 R have handgrips  25 L,  25 R at free ends thereof for being gripped with hands of the operator. Left and right turn control levers  23 L,  23 R attached to the left and left handlebars  30 L,  30 R so as to extend along the left and right handgrips  25 L,  25 R, respectively. The turn control levers  23 L,  23 R are manually operated to control operation of the corresponding electric motors  13 L,  13 R and the brakes  17 L,  17 R in a manner as described below.  
         [0035]    The operator manipulates levers and buttons including the accelerator lever  22  on the operator control panel  21  and the turn control levers  23 L,  23 R while walking behind the vehicle  10  so as to move the vehicle forward or backward, turn the vehicle leftward or rightward, and stop the vehicle.  
         [0036]    A control unit  24  is disposed inside the operator control panel  21  and controls operation of the electric motors  13 L,  13 R and the left and right brakes  17 L,  17 R based on the positions of the accelerator lever  22  and turn control levers  23 L,  23 R. The brakes  17 L,  17 R may be an electromagnetic brake, a hydraulic brake, a mechanical brake, regenerative brake and so on.  
         [0037]    The accelerator lever  22  is manually actuated to control the direction and speed of movement of the vehicle  10 . The accelerator lever  22  is normally disposed in a neutral position where the vehicle is stopped. The position of the acceleration lever  22  is monitored by an accelerator potentiometer  26  shown in FIG. 2A. The output from the accelerator potentiometer  28  varies linearly with the amount of angular displacement of the accelerator lever  22 , as indicated by a graph shown in FIG. 2B. In the illustrated embodiment, the output from the accelerator potentiometer  26  is set to vary within a range from 0 to 5.0 volts (V). A maximum forward speed of the vehicle is achieved when the output from the accelerator potentiometer  28  is +5.0 V. A maximum backward vehicle speed is achieved when the accelerator potentiometer output is 0 volt. The vehicle is stopped when the accelerator potentiometer output is 2.5 V.  
         [0038]    [0038]FIG. 3 shows a free end portion of the operation handlebar  30 L,  30 R including the handgrip  25 L,  25 R. The turn control lever  23 L,  23 R is pivotally connected by a hinge pin  31 L,  31 R to the handlebar  30 L,  30 R so as to extend along the handgrip  25 L,  25 R. The turn control lever  23 L,  23 R is firmly connected to one end of an actuator arm  32 L,  32 R of a brake potentiometer  27  so that the actuator arm  32 L,  32 R angularly moves or turns in unison with the turn control lever  25 L,  25 R. The brake potentiometer  27 L,  27 R is designed such that the output from the brake potentiometer  27  varies linearly with the amount of angular displacement of the actuator arm  32 L,  32 R and turn control lever  23 L,  23 R. As shown in FIG. 3, the turn control lever  23 L,  23 R is angularly movable between an initial zero-brake position (first position) P 1  indicated by the solid line and a stroke end position (second position) P 2  indicated by two-dot chain line through a full-brake position (third position) P 3  indicated by the dashed line. The turn control lever  23 L,  23 R is normally disposed in the solid-lined zero-brake position P 1  by the force of a return spring  33 L,  33 R.  
         [0039]    [0039]FIG. 4A shows a range of angular movement of the actuator arm  32 L,  32 R of the brake potentiometer  27 L,  27 R, which corresponds to the range of movement of the turn control lever  23 L,  23 R shown in FIG. 3. As shown in FIG. 4, the actuator arm  32 L,  32 R is angularly movable between the first position (zero-brake position) P 1  and the second position (stroke end position) P 2  through the third position (full-brake position) P 3 . The output from the brake potentiometer  27 L,  27 R varies linearly with the position of the actuator arm  32 L,  32 R and turn control lever  23 L,  23 R, as indicated by a graph shown in FIG. 4B. In the illustrated embodiment, the output from the brake potentiometer  27 L,  27 R is set to vary within a range from 0 to 5.0 volts (V). When the turn control lever  23 L,  23 R is in the initial zero-brake position P 1 , the output from the brake potentiometer is nil. When the turn control lever  23 L,  23 R is in the stoke end position P 3 , the output from the brake potentiometer is 5.0 V. And when the turn control lever  23 L,  23 R is in the intermediate full-brake position P 2 , the output from the brake potentiometer is Vm volts, where Vm is greater than 0 and smaller than 5.0. The output voltage Vm may be 1.5, 2.0 or 2.5 volts.  
         [0040]    As shown in FIGS. 4A and 4B, when the turn control lever  23 L,  23 R (i.e., the actuator arm  32 L,  32 R) moves within a range defined between the zero-brake position P 1  and the full-brake position P 3 , brake control operation is achieved. On the other hand, when the turn control lever  23 L,  23 R (actuator arm  32 L,  32 R) moves within a range defined between the full-brake position P 3  and the stroke end position P 2 , turn control operation is achieved.  
         [0041]    [0041]FIG. 5 shows a control system of the motorized vehicle  10 . As shown in this figure, the accelerator potentiometer  26  and the left and right brake potentiometers  27 L,  27 R are electrically connected to the control unit  24 . Also connected to the control unit  24  is a vehicle speed sensor  34  for detecting the speed of the vehicle  10 . The control unit  24  is electrically connected to the left and right brakes  17 L,  17 R via left and right brake drivers  28 L,  28 R, respectively, for controlling operation of the brakes  17 L,  17 R based on the position of the corresponding turn control levers  23 L,  23 R in a manner described below. Similarly, the control unit  24  is electrically connected to the left and right electric motors  13 L,  13 R via left and right motor drivers  29 L,  29 R, respectively, for controlling operation of the motors  13 L,  13 R based on the position of the accelerator lever  22  in a manner described below. In a practical sense, the brake drivers  28 L,  28 R and the motor drivers  29 L,  29 R are formed as a part of the control unit  24 .  
         [0042]    When the left turn control lever  23 L is manipulated or otherwise pulled by the operator, the left brake potentiometer  27 L generates an output signal BKLV corresponding in magnitude to the amount of angular displacement of the turn control lever  23 L. Upon receipt of the output signal BKLV from the brake potentiometer  27 L, the controller  24  sends a command signal to the left brake driver  28 L so that the left brake  17 L is driven to apply to the left driving wheel  15 L a brake force corresponding to the position of the left turn control lever  23 L. When the left turn control lever  23 L (i.e., the actuator arm  32 L of the left brake potentiometer  27 L) is in the brake control range defined between the zero-brake position P 1  and the full-brake position P 3  (FIGS. 4A and 4B), brake control operation is achieved, in which the brake force applied from the left brake  17 L to the left driving wheel  15 L varies linearly with the amount of angular displacement of the left turn control lever  23 L.  
         [0043]    Similarly, when the right turn control lever  23 R is manipulated or otherwise pulled by the operator, the right brake potentiometer  27 R generates an output signal BKRV corresponding in magnitude to the amount of angular displacement of the turn control lever  23 R. Upon receipt of the output signal BKRV from the brake potentiometer  27 R, the controller  24  sends a command signal to the right brake driver  28 R so that the right brake  17 L is driven to apply to the right driving wheel  15 R a brake force corresponding to the position of the right turn control lever  23 R. When the right turn control lever  23 R (i.e., the actuator arm  32 R of the right brake potentiometer  27 R) is in the brake control range defined between the zero-brake position P 1  and the full-brake position P 3  (FIGS. 4A and 4B), brake control operation is achieved, in which the brake force applied from the right brake  17 R to the right driving wheel  15 R varies linearly with the amount of angular displacement of the right turn control lever  23 R.  
         [0044]    When the accelerator lever  22  is actuated or otherwise tilted by the operator, the accelerator potentiometer  26  generates an output signal ACCV corresponding in magnitude to the amount of angular displacement of the accelerator lever  22 . Upon receipt of the output signal ACCV from the accelerator potentiometer  26 , the controller  24  sends a command signal to the left and right motor drivers  29 L,  29 R so that the left and right electric motors  13 L,  13 R rotate the corresponding driving wheels  15 L,  15 R in the forward or backward direction at a speed corresponding to the position of the accelerator lever  22 . Thus, the vehicle (crawler cart) with crawler belts  16 L,  16 R independently driven by the driving wheels  15 L,  15 R moves in the forward or backward direction at the desired speed.  
         [0045]    When the left or right turn control lever  23 L,  23 R is pulled to approach the handgrip  25 L,  25  across the full-brake position P 2  (FIGS. 4A and 4B), turn control operation is achieved under the control of the control unit  24  so as to ensure that the vehicle makes a turn while staying at the same direction (spot turn). The turn control operation will be described with reference to a flowchart shown in FIG. 6.  
         [0046]    At a first step ST 01 , a judgment is made to determine as to whether or not the output signal BKLV from the left brake potentiometer  27 L (FIG. 5) is greater than Vm (FIG. 4B). When the result of judgment is “YES” (BKLV&gt;Vm), this means that the left turn control lever  23 L is dispose in the turn control range defined between the full-brake position P 3  and the stroke end position P 2  (FIGS. 3 and 4A). The control then goes on to a step ST 02 . Alternately, when the result of judgment is “NO”(BKLV≦Vm), the control moves to a step ST 07 .  
         [0047]    At the step ST 02 , the output signal V from the vehicle speed sensor  34  (FIG. 5) is monitored so as to determine whether or not the vehicle speed V is not more than V 0  where V 0  represents the vehicle being at halt or moving at a slow speed which allows the vehicle to make an abrupt turn. When the result of judgment is “YES” (V&lt;V 0 ), the control advances to a step ST 04 . Alternately when the judgment result is “NO” (V≧V 0 ), the control moves to a step ST 03 .  
         [0048]    At the step ST 03 , slowdown control is achieved in which the control unit  24  (FIG. 5) controls the electric motors  13 L,  13 R via the motor drivers  29 L,  29 R so as to slow down the rotational speed of the driving wheels  15 L,  15 R. This operation continues until the vehicle speed V is below V 0 .  
         [0049]    At the step ST 04 , the left and right brakes  17 L,  17 R (FIG. 5) are released or de-activated to allow rotation of the left and right driving wheels  15 L,  15 R. After the step ST 04 , the control goes on to a step ST 05 .  
         [0050]    The step ST 05  is achieved on condition that VKLV&gt;Vm and V&lt;V 0  (that is, the left turn control lever  23 L is in the turn control range defined between the full-brake position P 3  and the stroke end position P 2 , and the vehicle is stopped or moving at a slow speed which allow the vehicle to make an abrupt turn). At the step ST 05 , the left electric motor  13 L (FIG. 5) is rotated in the reverse direction and, at the same time, the right electric motor  13 R is rotated in the forward direction. The term “forward direction” is used to refer to a direction to move the vehicle forward, and the term “reverse direction” is used to refer to a direction to move the vehicle backward. By thus driving the left and right electric motors  13 L,  13 R simultaneously in opposite directions, the vehicle starts to make an abrupt turn in the leftward direction while staying at the same position (spot turn).  
         [0051]    When the vehicle has turned leftward through a desired angle (180 degrees, for example), the operator releases the left turn control lever  23 L, allowing the lever  23 L to return to its initial zero-brake position P 1  (FIGS. 3 and 4B). This causes the output BKLV from the left brake potentiometer  27 L to go down to or below Vm (BKLV≦Vm). This condition is detected at a step ST 06  whereupon the control comes to an end and operation of the vehicle returns to a regular operation mode.  
         [0052]    At the step ST 07 , which follows the “NO” state at the preceding step ST 01 , a judgment is made to determine as to whether or not the output signal BKRV from the right brake potentiometer  27 R (FIG. 5) is greater than Vm (FIG. 4B). When the result of judgment is “YES” (BKRV&gt;Vm), the control advances to a step ST 08 . Alternately, when the judgment result is “NO”(BKRV≦Vm), this means that either lever  23 L,  23 R (actuator arm  32 L,  32 R of the brake potentiometer  27 L,  27 R) is not in the turn control range defined between the full-brake position P 3  and the stroke end position P 2 . Accordingly, the control is terminated.  
         [0053]    At the step ST 08 , following the “YES” state in the preceding step ST 07 , the output signal V from the vehicle speed sensor  34  (FIG. 5) is compared with V 0  so as to determine whether or not V&lt;V 0 . When the comparison result is “YES” (V&lt;V 0 ), the control advances to a step ST 10 . Alternately when the comparison result is “NO” (V≧V 0 ), the control moves to a step ST 09 .  
         [0054]    At the step ST 09 , slowdown control is achieved in which the control unit  24  (FIG. 5) controls the electric motors  13 L,  13 R via the motor drivers  29 L,  29 R so as to slow down the rotational speed of the driving wheels  15 L,  15 R. This operation continues until the vehicle speed V is below V 0 .  
         [0055]    At the step ST 10 , the left and right brakes  17 L,  17 R (FIG. 5) are released or de-activated to allow rotation of the left and right driving wheels  15 L,  15 R. After the step ST 10 , the control goes on to a step ST 11 .  
         [0056]    The step ST 11  is achieved on condition that VKRV&gt;Vm and V&lt;V 0  (that is, the right turn control lever  23 R is in the turn control range defined between the full-brake position P 3  and the stroke end position P 2 , and the vehicle is stopped or moving at a slow speed which allows the vehicle to make an abrupt turn). At the step ST 11 , the right electric motor  13 R (FIG. 5) is rotated in the reverse direction and, at the same time, the left electric motor  13 L is rotated in the forward direction. As a result of simultaneous driving of the left and right electric motors  13 L,  13 R in opposite directions, the vehicle starts to make an abrupt turn in the rightward direction while staying at the same position (spot turn).  
         [0057]    When the vehicle has turned rightward through a desired angle (180 degrees, for example), the operator releases the right turn control lever  23 R, allowing the lever  23 R to return to its initial zero-brake position P 1  (FIGS. 3 and 4B). This causes the output BKRV from the right brake potentiometer  27 R to go down to or below Vm (BKRV≦Vm). This condition is detected at a step ST 12  where-upon the control is terminated and operation of the vehicle returns to the regular operation mode.  
         [0058]    The speed of the electric motors  13 L,  13 R achieved at the steps ST 05  and ST 11  may be either fixed at a predetermined value, or alternately variable. In the latter case, the motor speed is set to be proportional to the output ACCV from the accelerator potentiometer  26  (corresponding to the position of the accelerator lever  22 ). By thus setting the motor speed, the vehicle can make a spot turn at the same speed as a preceding working operation which the vehicle has done.  
         [0059]    [0059]FIGS. 7A to  7 C are illustrative of the manner in which the vehicle makes a spot turn in the rightward direction through an angle of 180 degrees. In these figures, the left turn control lever is not shown for the purpose of illustration. When the right turn control lever  23 R is manipulated or otherwise pulled so as to approach the handgrip  25 R across the full-brake position P 2  (FIG. 3), the left electric motor  13 L is driven to rotate in the forward direction and, at the same time, the right electric motor  13 R is driven to rotate in the reverse direction. This means that the left crawler belt  16 L is driven to run or travel in the forward direction, while the right crawler belt  16 R is driven to run or travel in the backward direction. As a result of simultaneous running of the left and right crawler belts  16 L,  16 R in the forward and backward directions, respectively, the vehicle  10  starts to turn rightward about a center G 1  common to the left and right crawler belts  16 L,  16 R, with a turning radius R 1  equal to the distance from the turning center G 1  to a front left corner of the load-carrying platform  20 , as shown in FIG. 7A.  
         [0060]    Continuing operation of the left and right motors  13 L,  13 R will place the vehicle  10  to a position shown in FIG. 7B where the vehicle  10  has turned about the center G 1  in the rightward direction through an angle of 90 degrees. As the turning operation further continues, the vehicle  10  completes a 180° turn while staying at the same position, as shown in FIG. 7C. Then the operator releases the right turn control lever  23  to thereby terminate the spot turn operation. A spot turn in the leftward direction can be achieved in the same manner as described above by pulling the left turn control lever  23 L until it assumes a position located within the turn control range defined between the full-brake position P 3  and the stroke end position P 2  shown in FIGS. 3 and 4B.  
         [0061]    For comparative purposes, description will be made to a normal pivot turn operation of the vehicle  10  with reference to FIGS. 8A and 8B. When a right turn of the vehicle  10  is desired, the right turn control lever  23 R is pulled to assume the full-brake position P 3  (FIGS. 3 and 4B) or a position immediately before the full-brake position P 3 , whereupon by the effect of a maximum brake force applied from the right brake  17 R to the right driving wheel  15 R, the right crawler belt  16 R is stopped. In this instance, since the left crawler belt  16 L continues its running in the forward direction, the vehicle  10  starts to turn rightward about a turning center G 2  located at a longitudinal center of the right crawler belt  16 R, with a turning radius R 2  equal to the distance from the turning center G 2  to the front left corner of the platform  20 , as shown in FIG. 8B.  
         [0062]    As the turning operation further continues, the vehicle  10  completes a 180° turn about the turning center G 2 . A comparative review of FIGS. 7C and 8B indicates that a turning area in a circle drawn with the turning radius R 1  achieved by the spot turn operation (FIG. 7C) is much smaller than that in a circle drawn with the turning radius R 2  achieved by the normal pivot turn operation (FIG. 8B). This proves that the spot turn is optimum to minimize the turning area of the vehicle  10 .  
         [0063]    When the direction of travel of the vehicle  10  is to be adjusted, the left or the right turn control lever  23 L,  23 R is lightly pulled to create a speed difference between the left and right crawler belts  16 L,  16 R due to a brake force applied from the left or right brake  17 L,  17 R to the corresponding driving wheel  15 L,  15 R. Thus, the vehicle starts to make a gradual turn in a desired direction. When a rapid direction change is needed, the left or right turn control lever  23 L,  23 R is pulled to an increased extent. In this instance, when the turn control lever  23 L,  23 R is in the brake full-brake position P 3 , the normal pivot turn will be achieved in the same manner as described above with reference to FIGS. 8A and 8B. Alternatively, when the turn lever  23 L,  23 R is in the turn control region defined between the full-brake position P 3  and the stroke end position P 2 , the spot turn will be achieved in the same manner as described above with reference to FIGS. 7A to  7 C.  
         [0064]    It will readily be understood that by merely manipulating the turn control levers  23 L,  23 R in an appropriate manner, the vehicle can make a gradual turn, a normal pivot turn or a spot turn. The turn control levers  23 L,  23 R double in function as brake control levers to achieve gradual turns and a normal pivot turn, and also as a spot-turn initiating levers to achieve a spot turn. This obviates the need for the provision of a separate lever used exclusively for achieving different sorts of turn. The motorized vehicle is relatively simple in construction and can easily be operated even by an un-skilled operator.  
         [0065]    [0065]FIG. 9 shows a motorized vehicle  10   a  taking the form of a walk-behind motorized crawler cart according to a second embodiment of the present invention. The vehicle  10   a  is structurally and operationally the same as the vehicle  10  of the first embodiment shown in FIG. 1, with the exception that the left and right turn control levers  23 L,  23 R serve only as brake control levers, and left and right spot turn switches  35 L,  35 R are provided separately to achieve a spot turn. Due to this similarly, these parts which are identical to those shown in FIG. 1 are designated by the same reference characters and further description thereof can, therefore, be omitted to avoid duplicate description.  
         [0066]    As shown in FIG. 9, the left and right spot turn switches  35 L,  35 R are provided on an operator control panel  21  and electrically connected to a control unit  24  disposed inside the operator control panel  21 . The left and right turn control levers  23 L,  23 R (hereinafter referred to as brake control levers) are electrically connected to the control unit  24  via left and right brake potentiometers  27 L,  27 R (FIGS. 10A and 11). The potentiometers  27 L,  29 L each have an actuator arm  32 L,  32 R (FIG. 10A) directly connected to the corresponding brake control lever  23 L,  23 R.  
         [0067]    As understood from FIG. 10A, the brake control levers  23 L,  23 R (i.e., the actuator arms  32 L,  32 R of the brake potentiometers  27 L,  27 R) are angularly movable between an initial zero-brake position (first position) P 1  and a full-brake position (second position) P 2 . The output from the brake potentiometer  27 L,  27 R varies linearly with the position of the actuator arm  32 L,  32 R (i.e., the position of the brake control lever  23 L,  23 R), as indicated by a graph shown in FIG. 10B. In the illustrated embodiment, the output from the brake potentiometer  27 L,  27 R is set to vary within a range from 0 to 5.0 volts (V). When the brake control lever  23 L,  23 R is in the initial zero-brake position P 1 , the output from the brake potentiometer is nil. When the turn control lever  23 L,  23 R is in the full-brake position P 2 , the output from the brake potentiometer is 5.0 V. In terms of the output, the full-brake position P 2  in this position corresponds to the stroke end position P 2  of the first embodiment shown in FIG. 4B.  
         [0068]    [0068]FIG. 11 shows a control system of the motorized vehicle  10   a . The control system structurally differs from the control system of the first embodiment shown in FIG. 5 in that the spot turn switches  35 L,  35 R are provided separately from the brake control levers (turn control levers)  23 L,  23 R. Due to this similarity, these parts which are identical to those shown in FIG. 5 are designated by the same reference characters, and no further description thereof is needed.  
         [0069]    With the control system arranged as shown in FIG. 11, when the left brake control lever  23 L is manipulated or otherwise pulled by the operator, the left brake potentiometer  27 L generates an output signal BKLV corresponding in magnitude to the amount of angular displacement of the brake control lever  23 L. Upon receipt of the output signal BKLV from the brake potentiometer  27 L, the controller  24  sends a command signal to the left brake driver  28 L so that the left brake  17 L is driven to apply to the left electric motor  13 L a brake force corresponding to the position of the left brake control lever  23 L. By thus braking the electric motor  13 L, the rotating speed of the left driving wheel  15 L decreases linearly with the amount of displacement of the left brake control lever  23 L. When the brake control lever  23 L is pulled so as to assume the full-brake position P 2  (FIG. 10A), a maximum brake force is applied from the left brake  17 L to the left motor  13 L, thereby stopping rotation of the left motor  13 L. Thus, the left driving wheel  15 K is stopped. Similarly, when the right brake control lever  23 R is manipulated or otherwise pulled by the operator, the control unit  24  controls operation of the right brake  17 R via the right brake driver  28 R so that the right motor  13 R is braked with a brake force variable linearly with the output BKRV from the right brake potentiometer  27 R. When the right brake control lever  23 R is in the full-brake position P 2  (FIG. 10A), the output BKRV from the right brake potentiometer  27 R has a maximum value. This makes the right motor  13 R to stop rotation by the effect of a maximum brake force applied from the right brake  17 R.  
         [0070]    When the accelerator lever  22  is actuated or otherwise tilted by the operator, the accelerator potentiometer  26  generates an output signal ACCV corresponding in magnitude to the amount of angular displacement of the accelerator lever  22 . Upon receipt of the output signal ACCV from the accelerator potentiometer  26 , the controller  24  sends a command signal to the left and right motor drivers  29 L,  29 R so that the left and right electric motors  13 L,  13 R rotate the corresponding driving wheels  15 L,  15 R in the forward or backward direction at a speed corresponding to the position of the accelerator lever  22 . Thus, the vehicle (crawler cart) with crawler belts  16 L,  16 R independently driven by the driving wheels  15 L,  15 R moves in the forward or backward direction at the desired speed.  
         [0071]    When the left or right spot turn switch  35 L,  35 R is activated, turn control operation is achieved under the control of the control unit  24  so as to ensure that the vehicle makes a turn while staying at the same direction (spot). The turn control operation will be described with reference to a flowchart shown in FIG. 12.  
         [0072]    At a first step ST 01 , a judgment is made to determine as to whether or not the left spot turn switch  35 L is in the “ON” state. When the result of judgment is “YES”, the control then goes on to a step ST 02 . Alternately, when the judgment result is “NO”, the control moves to a step ST 06 .  
         [0073]    At the step ST 02 , the output signal V from the vehicle speed sensor  34  (FIG. 11) is monitored so as to determine whether or not the vehicle speed V is not more than V 0  where V 0  represents the vehicle being at halt or moving at a slow speed which allows the vehicle to make an abrupt turn. When the judgment result is “YES” (V&lt;V 0 ), the control advances to a step ST 04 . Alternately when the judgment result is “NO” (V≧V 0 ), the control moves to a step ST 03 .  
         [0074]    At the step ST 03 , slowdown control is achieved in which the control unit  24  (FIG. 11) controls the electric motors  13 L,  13 R via the motor drivers  29 L,  29 R so as to slow down the rotational speed of the driving wheels  15 L,  15 R. This operation continues until the vehicle speed V is below V 0 .  
         [0075]    The step ST 04  is achieved on condition that VKLV&gt;Vm and V&lt;V 0  (that is, the left spot turn switch  35 L is in the “ON” state, and the vehicle is stopped or moving at a slow speed which allows the vehicle to make an abrupt turn). At the step ST 04 , the left electric motor  13 L (FIG. 11) is rotated in the reverse direction and, at the same time, the right electric motor  13 R is rotated in the forward direction. By thus driving the left and right electric-motors  13 L,  13 R simultaneously in opposite directions, the vehicle starts to make an abrupt turn in the leftward direction while staying at the same position (spot turn).  
         [0076]    When the vehicle has turned leftward through a desired angle ( 180  degrees, for example), the operator deactivates the left spot turn switch  35 L, causing the output BKLV from the left brake potentiometer  27 L to go down to or below Vm (BKLV≦Vm). This condition is detected at a step ST 05 , and upon detention of this condition, the control comes to an end and operation of the vehicle returns to a regular operation mode.  
         [0077]    At the step ST 06 , which follows the “NO” state at the preceding step ST 01 , a judgment is made to determine as to whether or not the right spot turn switch  35 R is in the “ON” state. When the result of judgment is “YES”, the control advances to a step ST 07 . Alternately, when the judgment result is “NO”, this means that either switch  35 L,  35 R is not activated. Accordingly, the control is terminated.  
         [0078]    At the step ST 07 , following the “YES” state in the preceding step ST 06 , the output signal V from the vehicle speed sensor  34  (FIG. 11) is compared with V 0  so as to determine whether or not V&lt;V 0 . When the comparison result is “YES” (V&lt;V 0 ), the control advances to a step ST 09 . Alternately when the comparison result is “NO” (V≧V 0 ), the control moves to a step ST 08 .  
         [0079]    At the step ST 08 , slowdown control is achieved in which the control unit  24  (FIG. 11) controls the electric motors  13 L,  13 R via the motor drivers  29 L,  29 R so as to slow down the rotational speed of the driving wheels  15 L,  15 R. This operation continues until the vehicle speed V is below V 0 .  
         [0080]    The step ST 09  is achieved on condition that VKRV&gt;Vm and V&lt;V 0  (that is, the right spot turn switch  35 R is in the “ON” state, and the vehicle is stopped or moving at a slow speed which allows the vehicle to make an abrupt turn). At the step ST 09 , the right electric motor  13 R (FIG. 11) is rotated in the reverse direction and, at the same time, the left electric motor  13 L is rotated in the forward direction. As a result of simultaneous driving of the left and right electric motors  13 L,  13 R in opposite directions, the vehicle starts to make an abrupt turn in the rightward direction while staying at the same position (spot turn).  
         [0081]    When the vehicle has turned rightward through a desired angle (180 degrees, for example), the operator deactivates the right spot turn switch  35 R, causing the output BKRV from the right brake potentiometer  27 R to go down to or below Vm (BKRV≦Vm). This condition is detected at a step ST 010 , and upon detention of this condition, the control is terminated operation of the vehicle returns to a regular operation mode.  
         [0082]    The speed of the electric motors  13 L,  13 R achieved at the steps ST 04  and ST 09  may be either fixed at a predetermined value, or alternately variable. In the latter case, the motor speed is set to be proportional to the output ACCV from the accelerator potentiometer  26  (FIG. 11) By thus setting the motor speed, the vehicle can make a spot turn at the same speed as a preceding working operation which the vehicle has done.  
         [0083]    [0083]FIGS. 13A to  13 C are illustrative of the manner in which the vehicle  10   a  makes a spot turn in the rightward direction through an angle of  180  degrees. In these figures, the brake control levers are not shown for the purpose of illustration. When the right spot turn switch  35 R is activated, the left electric motor  13 L is driven to rotate in the forward direction and, at the same time, the right electric motor  13 R is driven to rotate in the reverse direction. This means that the left crawler belt  16 L is driven to run or travel in the forward direction, while the right crawler belt  16 R is driven to run or travel in the backward direction. As a result of simultaneous running of the left and right crawler belts  16 L,  16 R in the forward and backward directions, respectively, the vehicle  10   a  starts to turn rightward about a center G common to the left and right crawler belts  16 L,  16 R, with a turning radius R equal to the distance from the turning center G to a front left corner of the load-carrying platform  20 , as shown in FIG. 13A.  
         [0084]    Continuing operation of the left and right motors  13 L,  13 R will place the vehicle  10   a  to a position shown in FIG. 13B where the vehicle  10  has turned about the turning center G in the rightward direction through an angle of 90 degrees. As the turning operation further continues, the vehicle  10   a  completes a 180° turn while staying at the same position, as shown in FIG. 13C. Then the operator deactivates the right spot turn switch  35 R to thereby terminate the spot turn operation. A spot turn in the leftward direction can be achieved in the same manner as described above by activating the left spot turn switch  35 L.  
         [0085]    The spot turn switches  35 L,  35 R may be comprised of a push button switch, a self-hold push-push switch, a self-hold toggle switch, or a self-hold dial switch. Though not shown, these switches  35 L,  35 R may be mounted to the left and right handlebars  30 L,  30 R adjacent to the handgrips  25 ,  25 R.  
         [0086]    [0086]FIGS. 14 and 15 show a walk-behind self-propelled crawler snowplow  40  embodying the present invention. The snowplow  40  generally comprises a propelling frame  42  carrying thereon left and right crawler belts  41 L, a vehicle frame  45  carrying thereon a snowplow mechanism  43  and an engine (prime motor)  44  for driving the snowplow mechanism  43 , a frame lift mechanism  46  operable to lift a front end portion of the vehicle frame  45  up and down relative to the propelling frame  42 , and a pair of left and right operation handlebars  47 L and  47 R extending from a rear portion of the propelling frame  42  obliquely upward in a rearward direction of the snowplow  40 . The propelling frame  42  and the vehicle frame  45  jointly form a vehicle body  49 .  
         [0087]    The left and right crawler belts  41 L,  41 R are driven by left and right electric motors  71 L,  71 R, respectively. The crawler belts  41 L,  41 R are each trained around a driving wheel  72 L,  72 R and an idler wheel  73 L,  73 R. The driving wheel  72 L,  72 R is disposed on a rear side of the crawler belt  41 L,  41 R, and the idler wheel  73 L,  73 R is disposed on a front side of the crawler belt  41 L,  41 R.  
         [0088]    The snowplow mechanism  43  has an auger  43   a , a blower  43   b  and a discharge duct  43   c  that are mounted to a front portion of the vehicle frame  45 . In operation, the auger  43   a  rotates to cut snow away from a road, for example, and feed the cut mass of snow to the blower  43   b  which blows out the snow through the discharge duct  43   c  to a position far distant from the snowplow  40 .  
         [0089]    The operation handlebars  47 L,  47 R are adapted to be gripped by a human operator (not shown) walking behind the snowplow  40  in order to maneuver the snowplow  40 . An operator control panel  51 , a control unit  52  and batteries  53  are arranged in a vertical space defined between the handlebars  47 L,  47 R and they are mounted to the handlebars  47 L,  47 R in the order named when viewed from the top to the bottom of FIG. 14.  
         [0090]    The operation handlebars  47 L,  47 R each have a handgrip  48 L,  48 R at the distal end (free end) thereof. The left handlebar  47 L has a parking brake lever  54  disposed in close proximity to the handgrip  48 L. The parking brake lever  54  is of the deadman lever type and is adapted to be gripped by the operator together with the left handgrip  48 L. When gripped, the parking brake lever  54  turns about a pivot pin  54   a  in a direction toward the handgrip  48 L. With this movement of the parking brake lever  54 , a brake switch  55  (FIG. 16) is turned on, thereby releasing a brake on the driving wheels  72 L,  72 R. The left and right handlebars  14 L,  47 R further have turn control levers  56 L,  56 R associated with the respective handgrips  18 ,  18 .  
         [0091]    The crawler snowplow  40  of the foregoing construction is self-propelled by the crawler belts  41 L,  41 R driven by the electric motors  71 L,  71 R and is also maneuvered by the human operator walking behind the snowplow  40  while gripping the handlebars  47 L,  47 R.  
         [0092]    In the crawler snowplow  40 , a generator driving pulley  75  is attached to an output shaft  65  of the engine  44 . The diving pulley  75  is connected by an endless belt  77  to a generator driven pulley  76  mounted to the shaft of a generator  69 . Thus, rotation of the engine output shaft  65  is transmitted via the belt  77  to the generator  69 . That is, when the engine  44  is running, the generator  69  is driven via the belt drive  75 - 77  so that the batteries  53  (FIG. 14) are charged with electric current supplied from the generator  69 .  
         [0093]    A second driving pulley  67   a  is coupled via an electromagnetic clutch  66  to the output shaft  65  of the engine  44 , and a second driven pulley  68   b  is connected to one end of a rotating shaft  68   a . The second driving and driven pulleys  67   a ,  68   b  are connected by a second endless belt  67   b . The rotating shaft  68   a  is connected to a central shaft of the auger  43   a  via a worm gear speed reducing mechanism (not designated). The rotating shaft  168   a  is also connected to the blower  43   b . While the engine  44  is running, the auger  43   a  and blower  43   b  are drivable through the second belt drive  67   a ,  67   b ,  68   b  when the electromagnetic clutch  66  is in the engaged state.  
         [0094]    The operator control panel  51  has a lift control lever  60   a  for controlling operation of the frame lift mechanism  46  (FIG. 14), a duct control lever  60   b  for changing direction of the discharge duct  43   c , an accelerator lever  22  for controlling the direction and speed of travel of the snowplow  40 , and a throttle lever  64  for controlling the speed of the engine  44 . The operator control panel  51  further has a clutch switch  59  disposed adjacent to the right operation handlebar  47 R. The clutch switch  59  is a normally open contact switch and adapted to be turned on and off to achieve on-off control of the electromagnetic clutch  66 .  
         [0095]    As shown in FIG. 16, the left and right turn control levers  56 L,  56 R each have an integral pivot pin  56   a  by means of which the lever  56 L,  56 R is pivotally mounted to the corresponding handlebar  47 L,  47 R. The pivot pin  56   a  serves also as a rotating shaft of a rotary type brake potentiometer  57 L,  57 R which is associated with the turn control lever  56 L,  56 R to monitor the position of the turn control lever  56 L,  56 R. The brake potentiometer  57 L,  57 R are electrically connected to the control unit  52 . Left and right brakes  74 L,  74 R are associated with the left and right motors  71 L,  71 R, respectively, for independently applying a brake force to the corresponding motors  71 L,  71 R. The Left and right brakes  74 L,  74 R are driven by left and right brake drivers  58 L,  58 R under the control of the control unit  52  based on the amount of angular displacement of the turn control levers  56 L,  56 R detected by the brake potentiometers  57 L,  57 R. The accelerator lever  22  is electrically connected to the control unit  52  via an accelerator potentiometer  26 . The left and right motors  711 ,  71 r are driven by left and right motor drivers  29 L,  29 R under the control of the control unit  52  based on the amount of angular displacement of the accelerator lever  22  detected by the accelerator potentiometer  26 . The operation of the accelerator lever  22  and turn control levers  56 L,  56 R are identical to the operation of those  22 ,  23 L,  23 R described above with reference to the first embodiment shown in FIGS. 1-8, and further description thereof can be omitted.  
         [0096]    It will be appreciated from the foregoing description that by virtue of the left and right turn control levers mounted to the left and right handlebars so as to extend along the left and right handgrips, the operator can manipulate the turn control levers while keeping a grip on the handgrips. This enables the operator to steer the motorized vehicle stably and reliably in a desired direction. Furthermore, the turn control levers can be easily manipulated with operator&#39;s fingers of the operator. This will lessen the load on the operator.  
         [0097]    The present disclosure relates to the subject matter of Japanese Patent Applications Nos. 2000-331554, 2000-331554 and 2001-134689, filed Oct. 30, 2000, Oct. 30, 2000 and May 1, 2001, respectively, the disclosures of which are expressly incorporated herein by reference in their entirety.