Riding work vehicle

A riding work vehicle includes right and left wheels, at least one caster wheel, and a working machine. The lawnmower vehicle further includes traveling motor, steering motor, a traveling system clutch, and a steering system clutch. The traveling system clutch is configured to disable transmission of the rotational force from an axle of the caster wheel to the traveling motor in a state where the traveling motor is deactivated. The steering system clutch is configured to disable transmission of the rotational force from a steering shaft for the caster wheel to the steering motor in a state where the steering motor is deactivated.

PRIORITY INFORMATION

The present invention claims priority from Japanese Patent Application No. 2009-30949 filed on Feb. 13, 2009, which is hereby incorporated herein in its entirety.

BACKGROUND

1. Technical Field

The present invention relates to a riding work vehicle, which includes right and left wheels that are operable as main driving wheels and respectively driven independently for traveling, at least one caster wheel that is operable as a steering wheel whose operational state is selectable between free traveling and forced traveling and also between free steering and forced steering, and a working machine that is operable for work on the ground.

2. Related Art

A work vehicle equipped with a working machine is available for work on the ground, such as a conventionally known lawn mower, farm tractor, or the like. The work vehicle can include right and left wheels that are operable as main driving wheels and respectively driven independently for traveling by a power source (e.g., an electric motor) and a caster wheel that is a freely steerable steering control wheel.

For example, a riding lawnmower vehicle is an automotive work vehicle that is equipped with a lawnmower as a working machine and enables a worker to ride thereon and perform operations for traveling and for ground work. The lawnmower mounted on the riding lawnmower vehicle is, for example, a lawnmower rotary tool.

The riding lawnmower vehicle is an off-road vehicle that usually travels on the ground surface of a garden or the like to operate for lawn mowing work, although the riding lawnmower vehicle is a type of a vehicle. To this end, the riding lawnmower vehicle includes a power source (e.g., an electric motor) mounted thereon for wheel driving and lawnmower driving. The electric motor can receive electric power from a power supply source (e.g., a battery). If the riding lawnmower vehicle is a hybrid-type riding lawnmower vehicle, the battery can receive electric power supplied from an electric generator that is driven by an internal combustion engine for electric power generation.

For example, as discussed in Japanese Patent Application Laid-Open No. 2008-168869, a hybrid type riding lawnmower vehicle includes an engine and an electric generator that can supply electric power to a power source unit. The power source unit can supply electrical energy to an electric rotary machine. The riding lawnmower vehicle further includes a main frame that supports right and left wheels operable as main driving wheels and right and left caster wheels operable as steering control wheels. The right and left wheels are connected to an axle driving electric rotary machine. The caster wheels are connected to a steering wheel driving electric rotary machine. Further, a steering actuator that can function as a motor is connected to respective steering shafts of the right and left caster wheels, to cause the right and left caster wheels to rotate around their steering shafts, respectively. Further, a controller is provided to set arbitrary steering angles for respective caster wheels in a state where the steering actuator is connected to the steering shafts.

Furthermore, as discussed in Japanese Patent Application Laid-Open No. 2008-168871, another riding lawnmower vehicle includes right and left main driving wheels, right and left caster wheels operable as two steering control wheels, and the main driving wheels are driven by an electric motor and the caster wheels are steered by an electric steering motor. The controller includes a switching module that switches the operational state between a forced steering mode and a free steering mode. When the switching module selects the forced steering mode, the electric steering motor forcibly steers the caster wheels. On the other hand, in the free steering mode, the switching module brings the caster wheels into a freely steerable state by stopping power generation for the electric traveling motor or stopping power transmission for steering from the electric steering motor to the caster wheels.

According to the riding lawnmower vehicles discussed in Japanese Patent Application Laid-Open No. 2008-168869 and Japanese Patent Application Laid-Open No. 2008-168871, the main driving wheels are driven by the electric motor serving as the axle driving electric rotary machine and the caster wheels are driven by the electric motor serving as the steering control wheel driving electric rotary machine. Further, the electric motor serving as the steering actuator is provided to rotate respective caster wheels around their steering shafts.

However, according to the vehicle discussed in Japanese Patent Application Laid-Open No. 2008-168869, if the steering control wheel driving electric rotary machine and the steering actuator are in a deactivated state, the steering control wheel driving electric rotary machine and the steering actuator apply a significant amount of load to the caster wheels when the caster wheels are forcibly rotated for traveling or steering by the driving force of the main driving wheels. In this respect, there is some room for improvement in reduction of the torque required to drive the main driving wheels.

For example, permanent magnet equipped motors having permanent magnet equipped rotors may be used for the motors functioning as the steering control wheel driving electric rotary machine and the steering actuator. In this case, a cogging torque may be generated as resistance by the magnetic reaction force of the stator positioned in a confronting relationship with the permanent magnet in a situation where the caster wheels are driven by the force acting from the ground when the main driving wheels are driven in a non-energized state of the motors.

The cogging torque is a component that resists the vehicle when the vehicle is traveling. The cogging torque also resists the caster wheels when the caster wheels are in the freely steerable state. Therefore, the cogging torque is not preferable. Therefore, a relatively large amount of electric power is supplied to the axle driving electric rotary machine dedicated to the main driving wheels to cause the vehicle to travel at an intended speed and in an intended direction. In this respect, there is some room for improvement in energy saving.

Further, brush equipped motors or the like (i.e., motors other than the permanent magnet equipped motors) may be used for the motors functioning as the steering control wheel driving electric rotary machine and the steering actuator. In this case, a significant amount of sliding resistance occurs between the brush and a commutator and therefore the vehicle is subjected to a relatively large amount of resistance in the non-energized state of the motors when the vehicle is in a traveling or freely steering state. Thus, there is some room for improvement in energy saving.

Further, according to the vehicle discussed in Japanese Patent Application Laid-Open No. 2008-168871, as understood from a structure illustrated in FIG. 17 of Japanese Patent Application Laid-Open No. 2008-168871, a spur gear that constitutes a spur gear mechanism is supported, via a one-way clutch, around one end portion of a lower side rotary shaft fixed to the caster wheel. The spur gear is operatively connected to a rotary shaft of an electric motor.

According to this arrangement, if there is the tendency for the rotational speed of the spur gear to become slower than the rotational speed of the caster wheel, the power transmission from the electric motor to the lower side rotary shaft is blocked, thereby preventing the electric motor from obstructing smooth rotation of the caster wheels. Hence, there is a possibility that energy saving can be attained to a certain extent by disconnecting the electric motors (i.e., the steering control wheel driving electric rotary machines) when the electric motors are deactivated.

Further, as discussed in Japanese Patent Application Laid-Open No. 2008-168871, when the free steering mode is selected, electric power supply to the electric steering motor can be stopped or power transmission from the electric steering motor to the caster wheel can be blocked. To this end, it is useful to provide a clutch mechanism in a power transmission path between the electric steering motor and a driving portion of the caster wheel.

However, the above described Japanese Patent Application Laid-Open No. 2008-168871 has no mention of any mechanism for preventing the steering motor from obstructing the steering operation of the caster wheel that is driven by the force acting from the ground when the vehicle travels by the driving force of the main driving wheels, in a situation where the caster wheel is freely steerable in the deactivated state of the steering motor. Namely, there is no discussion that suggests the clutch that is capable of blocking transmission of the rotational force in the deactivated state of the electric steering motor.

Therefore, in the configuration discussed in Japanese Patent Application Laid-Open No. 2008-168871, electric power is wastefully consumed if the clutch is disengaged to block the power transmission for the steering operation in the power-supply state of the electric steering motor. There is some room for improvement in energy saving.

SUMMARY

The present invention is intended to provide a riding work vehicle that can effectively save energy in a configuration that includes at least one caster wheel configured to switch its operational state between free traveling and forced traveling and also switch the operational state between free steering and forced steering.

A riding work vehicle according to the present invention includes right and left wheels configured to as main driving wheels that are respectively driven independently for traveling, at least one caster wheel as a steering control wheel whose operational state is selectable between free traveling and forced traveling and also between free steering and forced steering, a working machine that is operable for a work on the ground, a traveling motor configured to drive the caster wheel for traveling, a steering motor configured to steer the caster wheel, a traveling system clutch that is provided in a traveling system power transmission path for transmitting the power from the traveling motor to an axle of the caster wheel and is configured to disable transmission of the rotational force from the axle to the traveling motor in a state where the traveling motor is deactivated, and a steering system clutch that is provided in a steering system power transmission path for transmitting the power between the steering motor and a steering shaft for the caster wheel and is configured to disable transmission of the rotational force from the steering shaft to the steering motor in a state where the steering motor is deactivated.

According to the above described configuration, the operational state of the caster wheel can be selected between the free traveling and the forced traveling by switching the traveling system clutch between an engaged state and a disengaged state. The operational state of the caster wheel can be further selected between the free steering and the forced steering by switching the steering system clutch between an engaged state and a disengaged state. Further, the traveling system clutch can disable transmission of the rotational force from the axle to the traveling motor in a state where the traveling motor is deactivated. The steering system clutch can disable the transmission of the rotational force from the steering shaft to the steering motor in a state where the steering motor is deactivated. Therefore, in a state where each motor is deactivated, the power transmission via a corresponding clutch can be prevented. Therefore, even in a case where there is a tendency for the force to be transmitted from the ground to the motor via the axle of the caster wheel or via the steering shaft when the vehicle is traveling, the present invention can prevent the force from being transmitted to the motor. More specifically, when the caster wheel is in the free traveling or free steering state, the present invention can effectively prevent the motor from acting as resistance for rotation. Accordingly, the present invention can realize a configuration capable of effectively saving energy.

Further, it is preferable that the riding work vehicle according to the present invention includes a steering angle sensor that is provided on a downstream side of the steering system clutch in a power transmission direction of the steering system power transmission path, and is configured to detect the steering angle of the steering shaft or a portion connected to the steering shaft.

According to the above described configuration, in the configuration in which the steering system clutch is provided in the steering system power transmission path, the steering angle of the steering shaft or the portion connected to the steering shaft can be detected regardless of engagement/disengagement of the steering system clutch.

Further, in the riding work vehicle according to the present invention, it is preferable that the steering angle sensor includes a sensor shaft disposed in parallel with the steering shaft, and a gear mechanism provided between the sensor shaft and the steering shaft to transmit the rotation of the steering shaft to the sensor shaft at a rotational speed identical to that of the steering shaft or a rotational speed slower than that of the steering shaft.

According to the above described configuration, the configuration of the portion including the steering shaft and the steering angle sensor can be prevented from being excessively enlarged. Therefore, a portion to be modified to detect the steering angle of the steering shaft can be realized by a relatively simple, compared to the basic configuration. Further, the number of the modified portion can be decreased.

Further, in the riding work vehicle according to the present invention, it is preferable that the traveling system clutch is configured to switch its operational state between an engaged state and a disengaged state according to a control signal input from a control unit.

According to the above described configuration, the traveling system clutch can disable or enable power transmission between the caster wheel and the traveling motor according to the signal supplied from the control unit that reflects a driver's operation or a traveling state of the vehicle. Therefore, in a case where the traveling motor is an electric rotary machine, the traveling system clutch is brought into the engaged state when the vehicle is decelerating, thereby causing the caster wheel to realize the regenerative braking. The rotational force of the caster wheel can be transmitted to the traveling motor. The traveling motor can generate electric power, and thus the riding work vehicle can further save energy.

Further, in the riding work vehicle according to the present invention, it is preferable that the steering system clutch is configured to switch its operational state between an engaged state and a disengaged state according to a control signal input from a control unit.

Further, in the riding work vehicle according to the present invention, in which the steering system clutch is configured to switch its operational state between the engaged state and the disengaged state according to the control signal input from the control unit, it is preferable that the traveling system clutch is configured to switch its operational state between an engaged state and a disengaged state according to a control signal input from the control unit, and the control unit is configured to constantly engage the steering system clutch when the traveling system clutch is engaged.

The above described configuration can effectively prevent the planted lawn from being damaged (or scuffed) by the caster wheel that is driven forcibly when being unexpectedly steered, when the vehicle is turning.

Further, in the riding work vehicle according to the present invention, it is preferable that the traveling system clutch is a one-way clutch.

According to the above described configuration, a control mechanism for the traveling clutch (e.g. a cable connecting the traveling clutch to the control unit) can be omitted.

Further, in the riding work vehicle according to the present invention, it is preferable that the traveling system clutch is a two-way clutch.

According to the above described configuration, the control mechanism for the traveling system clutch can be simplified or can be omitted. For example, a cable connecting the traveling system clutch to the control unit can be omitted.

Further, it is preferable that the riding work vehicle according to the present invention further includes a control unit configured to supply electric power generated by the traveling motor to an electric power storage unit to charge the electric power storage unit if it is determined that a regenerative braking request is present, wherein the control unit is configured to supply electric power to the traveling motor to cause the traveling motor to rotate in a direction opposed to that in a normal traveling state if it is determined that the regenerative braking request is present, thereby causing the traveling motor to generate electric power by the regenerative braking after the traveling system clutch is engaged.

According to the above described configuration, even when the traveling system clutch is constituted by a one-way clutch or a two-way clutch, the rotational force of the caster wheel can be transmitted via the traveling system clutch to the traveling motor. The traveling motor can generate electric power by regenerative braking.

Further, it is preferable that the riding work vehicle according to the present invention further includes a hill-climbing detection sensor configured to detect whether the vehicle is in a hill-climbing state, wherein if it is determined that the vehicle is in the hill-climbing state, the speed of the caster wheel relative to the ground is set to be higher than the speed of the right and left wheels relative to the ground. It is preferable that if it is determined that the vehicle is in the hill-climbing state, the speed of the caster wheel relative to the ground is set to be higher, by the amount equivalent to 20% (more preferably 15%) or less, than the speed of the right and left wheels relative to the ground.

According to the above described configuration, the vehicle can constantly maintain four-wheel drive traveling to prevent frequent switching between a two-wheel driving state and a four-wheel driving state that may occur when the vehicle is in a hill-climbing state because the wheels tend to slip. This configuration can improve ride comfort of the vehicle and realize stable traveling.

Further, in the riding work vehicle according to the present invention, it is preferable that the steering system clutch is a two-way clutch.

According to the above described configuration, the control mechanism for the steering system clutch can be simplified or can be omitted. For example, the cable connecting the steering system clutch to the control unit) can be omitted.

Further, in the riding work vehicle according to the present invention, it is preferable that the steering angle sensor includes a unit to be sensed that is provided on the steering shaft and disposed on the downstream side of the steering system clutch in the power transmission direction of the steering system power transmission path, or provided on another shaft disposed coaxially with the steering shaft and connected to the steering shaft, and a detection unit provided at a position confronting with the unit to be sensed.

As described above, the riding work vehicle according to the present invention is configured to switch the operational state of the caster wheel between the free traveling state and the forced traveling state and also between the free steering state and the forced steering state, and is capable of realizing a configuration that can effectively save energy.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

[First Embodiment of the Present Invention]

A first embodiment of the present invention is described below in detail with reference to attached drawings.FIG. 1is a perspective view illustrating a configuration of a lawnmower vehicle, which is a riding work vehicle according to the first embodiment of the present invention.FIG. 2is a plan view illustrating a swing arm and caster wheel support units attached to each end of the swing arm as illustrated inFIG. 1.FIG. 3is a front view illustrating the swing arm and the caster wheel support unit illustrated inFIG. 2.FIG. 4is a side view illustrating a swing arm and one caster wheel support unit, seen from the vehicle illustrated inFIG. 1.FIG. 5is an enlarged cross-sectional view taken along a line A-A illustrated inFIG. 2.FIG. 6is a side view illustrating one caster wheel support unit, seen from the vehicle illustrated inFIG. 1.FIG. 7is a cross-sectional view taken along a line B-B illustrated inFIG. 6.FIG. 8is a cross-sectional view taken along a line C-C illustrated inFIG. 6.FIG. 9is a cross-sectional view taken along a line D-D illustrated inFIG. 8, which illustrates a caster wheel attached portion of a rotatable housing in a state reversed in the right-and-left direction with respect to a traveling system power transmission shaft.FIG. 10illustrates a fundamental configuration relating to constituent components of an electric system, which includes a controller, provided in the lawnmower vehicle according to the first embodiment of the present invention.FIG. 11is a block diagram illustrating a portion relating to a turning function of the lawnmower vehicle illustrated inFIG. 1.FIG. 12illustrates an example of linear traveling according to the first embodiment of the present invention.FIG. 13Aillustrates an example of turn traveling according to the first embodiment of the present invention.FIG. 13Billustrates another example of turn traveling according to the first embodiment of the present invention.FIG. 13Cillustrates another example of turn traveling according to the first embodiment of the present invention.FIG. 14Aillustrates an example of determination of a turning center position in response to a given turning instruction according to the first embodiment of the present invention.FIG. 14Billustrates an example of calculation for determining the turning center position in response to the given turning instruction according to the first embodiment of the present invention.FIG. 15Aillustrates an example of determination of steering angles of caster wheels and the like based on the turning center position according to the first embodiment of the present invention.FIG. 15Billustrates an example of calculation for determining the steering angles of the caster wheels and the like based on the turning center position according to the first embodiment of the present invention.FIG. 16illustrates an example of calculation for determining a caster wheel speed and the like based on the turning center position according to the first embodiment of the present invention.

In the following description, a lawnmower vehicle includes a plurality of electric motors, which are electric rotary machines (i.e., driving sources). More specifically, the lawnmower vehicle includes a driving source dedicated to right and left wheels (i.e., rear wheels that serve as main driving wheels). The lawnmower vehicle further includes a driving source for travelling and a driving source for steering dedicated to right and left caster wheels operable as front wheels. Additionally, the lawnmower vehicle further includes a driving source for a lawnmower blade or a lawnmower reel, which is a lawnmower rotary tool constituting a lawnmower. The driving sources are not limited to the electric motors and are replaceable with hydraulic motors or any other driving sources. In the following description of the present invention, the “left and right” direction and the “back and forth” direction represent corresponding directions of the lawnmower vehicle, unless specifically stated otherwise.

Further, in the following description, a battery (i.e., an electric power storage unit) is a power source unit capable of supplying electric power to each electric motor. If the lawnmower vehicle is configured as a hybrid-type riding lawnmower vehicle, an internal combustion engine can be used and an electric generator can be driven by the engine to supply generated electric power to the battery. Further, an appropriate power transmission mechanism can be employed to transmit driving power from an output shaft of the engine to the lawnmower blade, so that the engine can be used as a driving source for the lawnmower blade when the hybrid-type riding lawnmower vehicle is employed.

Further, each of the electric motor serving as the driving source dedicated to the right and left wheels and the electric motor serving as the driving source dedicated to the caster wheels can generate driving force to rotate respective wheels or respective caster wheels by supplying electric power to each of the electric motor. On the other hand, when the wheels or the caster wheels are in a decelerating state, these motors can function as electric generators capable of recovering regenerative energy.

Further, the power source unit is not limited to the electric power storage unit configured to receive charging power from an external device. The power source unit can be replaced by, or can be used together with, a fuel cell unit or a solar battery or the like that has a power generation function.

Moreover, the layout of various components constituting the lawnmower vehicle can be appropriately changed according to practical specifications of the lawnmower vehicle. An example of the lawnmower vehicle is described below in detail with reference to the attached drawings.

As illustrated inFIG. 1, a lawnmower vehicle10is a self-propelled off-road vehicle, which includes a main frame12that supports, at the rear end portion (i.e., the right side portion inFIG. 1) thereof, left and right wheels14and15(although only the left wheel14is illustrated inFIG. 1andFIG. 11can be referred to for the right wheel15) that are two main driving wheels respectively driven independently for traveling. The main frame12has a front end portion (i.e., a left end portion inFIG. 1) that supports a swing arm16that can swing. The lawnmower vehicle10includes left and right caster wheels18and20(i.e., two steering control wheels) that are supported at both end portions of the swing arm16in the right-and-left direction.

The lawnmower vehicle10further includes a lawnmower (or a mower)22, which is illustrated inFIG. 10although not illustrated inFIG. 1. The lawnmower22is provided between the caster wheels18and20and the left and right wheels14and15in the back-and-forth direction of the main frame12(i.e., the direction indicated by an arrow α inFIG. 1). The lawnmower22is a working machine usable for a job on the ground.

The lawnmower22is operatively connected to an electric mower motor (or a hydraulic motor), which serves as a power source for driving the lawnmower22, although it is not illustrated. For example, the electric mower motor (or the hydraulic motor) is operatively connected to the lawnmower22via a gear mechanism (not illustrated), or a universal joint and a transmission shaft, so that the power of the motor can be transmitted to the lawnmower22.

The lawnmower rotary tool, which constitutes the lawnmower22, is disposed in a mower deck (not illustrated). The lawn grass having been cut by the lawnmower rotary tool can be directly discharged to the side of the lawnmower. In a case where a discharge duct (not illustrated) is provided, the grass cut by the lawnmower22can be discharged rearward via the discharge duct to a grass collection tank (not illustrated) that is connected to the rear end of the discharge duct and mounted on a vehicle body.

Further, the lawnmower rotary tool of the lawnmower22can be a lawnmower blade type that includes a rotary shaft extending vertically relative to the ground surface and one or a plurality of blades disposed around the rotary shaft and rotatable to cut the lawn. Alternatively, the lawnmower rotary tool of the lawnmower22can be a lawnmower reel type that includes a cylinder having a rotary shaft extending in parallel with the ground surface and a helical blade disposed in the cylinder to sandwich and cut the lawn.

A driver's seat24is provided on the upper surface side of the main frame12. A steering operator26is provided on the front side of the driver's seat24. Further, a forward movement accelerator pedal28and a rearward movement accelerator pedal30, which can be operated to accelerate the lawnmower vehicle10, are provided on the front side of the driver's seat24. Similarly, a brake pedal32, which can be operated to decelerate the lawnmower vehicle10, is provided on the front side of the driver's seat24.

The steering operator26is, for example, a circular or C-shaped steering wheel, which can be operated to instruct turning. The steering operator26, which is configured to be rotatable or swingable, has a function of adjusting the turning direction of the vehicle. For example, in a case where the steering operator26is a steering wheel, the steering operator26can rotate an arbitrary angle around its rotary shaft (i.e., the rotational center) in the clockwise direction or in the counterclockwise direction. The positional relationship between the accelerator pedals28and30in the right-and-left direction can be reversed if desired. Although the illustrated example of the accelerator pedal is a combination of the forward movement accelerator pedal28and the rearward movement accelerator pedal30, the accelerator pedal can be constituted by a single pedal that is swingable around and supported by a horizontal shaft fixed to the main frame12. In this case, if the front side of the accelerator pedal is depressed, the accelerator pedal can function as a forward movement accelerator pedal. If the rear side of the accelerator pedal is depressed, the accelerator pedal can function as a rearward movement accelerator pedal.

Further, the left and right wheels14and15(i.e., rear wheels) can be respectively driven by two wheel driving motors34and36(seeFIG. 11), which are electric motors capable of driving respective wheels14and15for traveling. More specifically, the wheel driving motors34and36have rotary shafts that can independently drive the left and right wheels14and15, respectively. Each of the wheel driving motors34and36can be a permanent magnet equipped DC brushless motor, a brush equipped motor, or the like, which can rotate in both forward and reverse directions. A controller38, which is a below-described control unit, can control the rotational speed (i.e., the number of revolutions per unit time) of respective wheel driving motors34and36.

Further, the left and right caster wheels18and20(i.e., front wheels) can be respectively driven by two traveling motors42and44, which are electric motors capable of driving respective caster wheels18and20for traveling. Further, the left and right caster wheels18and20can be respectively driven by two steering motors46and48, which are electric motors capable of steering respective caster wheels18and20. Each of the traveling motors42and44and the steering motors46and48is, for example, a permanent magnet equipped DC brushless motor, a brush equipped motor, or the like, which can rotate in both forward and reverse directions to drive or steer the load (i.e., the wheel). At least one of two batteries50and51, which are power source units disposed at left and right sides of the driver's seat24and the upper side of the main frame12, can supply electric power to the above described motors34,36,42,44,46, and48.

Further, although not illustrated in the drawings, a starter switch that serves as an operation unit configured to operate the lawnmower22, and a parking brake lever or a similar manipulator that serves as a parking brake mechanism configured to hold a stopped state of the vehicle, can also be provided in the vicinity of the driver's seat24. The steering operator26is not limited to the above described steering wheel and can be a two-lever type operator that includes two operation levers separately provided on the right and left sides to enable a worker sitting on the driver's seat24to turn, accelerate, and decelerate the lawnmower vehicle10. In this case, the accelerator pedals28and30can be omitted.

Further, the controller38is disposed at an appropriate position on the upper surface side or the bottom surface side of the main frame12. The controller38integrally controls various operations to be performed by constituent components of the vehicle including the batteries50and51and the motors including the wheel driving motors34and36. The controller38is an electric circuit including electric components that can be separately provided at different portions, that is different from other mechanism components. According to the example illustrated inFIG. 1, only one controller38is disposed on the rear side of the driver's seat24and on the upper surface side of the main frame12.

Alternatively, the controller38can be constituted by a plurality of control units that are separately disposed at different portions and mutually connected via appropriate signal cables. The controller38includes a driver circuit, such as an inverter circuit used for the wheel driving motors34and36and other motors, and also includes a control logic circuit, such as a central processing unit (CPU). Although the illustrated example includes two batteries50and51mounted on the vehicle, only one battery can be provided.

Further, the front end portion of the main frame12supports, via the swing arm16, the left and right caster wheels18and20. More specifically, an upright frame52is fixed at the center of the main frame12in the right-and-left direction. The upright frame52has an upper side that supports a rotary supporting portion of the steering operator26. The upright frame52has a front end portion that supports the swing arm16so as to be swingable around a back-and-forth shaft extending in the back-and-forth direction and in the horizontal direction. Left and right caster wheel support units54and56, which support the caster wheels18and20that are rotatable, are fixed to both end portions of the swing arm16.

More specifically, a protruding member58extending in the back-and-forth direction is fixed to the front end portion of the upright frame52provided on the front side of the main frame12. As illustrated inFIG. 2toFIG. 5, the protruding member58includes an intermediate bracket60having a U-shaped configuration in cross section with an opened lower side, and a mounting bracket62fixed to the rear end portion (i.e., the upper end portion illustrated inFIG. 2) of the intermediate bracket60so as to close the rear end opening of the intermediate bracket60. The protruding member58further includes a plate portion64fixed to the front end portion (i.e., the lower end portion illustrated inFIG. 2) of the intermediate bracket60so as to close the front end opening of the intermediate bracket60, and a shaft66(i.e., back-and-forth shaft) having both end portions fixed to the mounting bracket62and the plate portion64so as to extend in the intermediate bracket60in the back-and-forth direction. The mounting bracket62is fixed to the upright frame52by means of bolts. Further, as illustrated inFIG. 5, a shaft supporting bracket68is supported by and rotatable around the shaft66. The shaft supporting bracket68has a lower end portion fixed to the swing arm16that extends in the right-and-left direction. The above described configuration enables the swing arm16to swing around the shaft66that extends in the back-and-forth direction and in the horizontal direction at the front end portion of the main frame12(seeFIG. 1).

Further, as illustrated inFIG. 5, in a state where the swing arm16is swingably supported by the protruding member58via the shaft66, a clearance72is provided between an upper end surface of the swing arm16and the lower end periphery of a pair of wall portions70provided at the right and left ends of the intermediate bracket60. The lower end periphery of each wall portion70can function as a swing stopper that restricts a swingable range of the swing arm16. More specifically, the swing stopper capable of regulating the swingable range of the swing arm16is provided between the swing arm16and a portion fixed to the main frame12.

Further, as illustrated inFIG. 2toFIG. 4, the left and right caster wheel support units54and56are fixed to the left and right end portions of the swing arm16by means of appropriate fastening members (e.g., bolts). As illustrated in detail inFIG. 6toFIG. 9, each of the caster wheel support units54and56includes a stationary housing74, the traveling motor42(or44) and the steering motor46(or48) that are fixed on the upper side of the stationary housing74, a rotatable housing76, the steering shaft78, a steering system power transmission shaft80, a traveling system power transmission shaft82(i.e., driving shaft), a steering system gear mechanism84, a traveling system gear mechanism86, a steering angle sensor88, the caster wheels18and20, an axle90, a traveling system clutch92, and a steering system clutch94. Two caster wheel support units54and56are components that are identical to each other. The caster wheel support units54and56are mutually symmetrical with respect to the center line of the vehicle body extending in the back-and-forth direction, and are fixedly attached to the front and upper surfaces of the swing arm16(seeFIG. 1) by fastening members (e.g., bolts).

As illustrated inFIG. 8, the stationary housing74includes right and left cylindrical motor connection members96, a planar upper side member98, an intermediate member100having upper and lower recessed portions, and a lower side member104having an upper recessed portion and a tube-like portion102that protrudes downward at one end in the right-and-left direction, which are assembled in the up-and-down direction and fastened together by means of bolts. Two motor casings constituting the steering motor46and the traveling motor42are fixed via respective motor connection members96to left and right separated portions on the upper side of the upper side member98. In this state, the motors46and42have rotary shafts106and108that extend in the vertical direction. The rotary shafts106and108are rotatably supported by respective motor casings. Further, the lower end portions of the rotary shafts106and108of the corresponding motors46and42are inserted into the upper portions of the motor connection members96.

In the stationary housing74, the steering system power transmission shaft80is disposed coaxially with the rotary shaft108of the steering motor46. The steering system power transmission shaft80is rotatably supported by the stationary housing74and is positioned beneath the rotary shaft108. The lower end of the rotary shaft108is opposed via a clearance to the upper end of the steering system power transmission shaft80. The steering system power transmission shaft80, as illustrated inFIG. 8, can be constituted by two coaxial shafts that are mutually fixed with a key and are synchronously rotatable. Further, the steering system clutch94can mechanically, i.e., operatively, connect the rotary shaft108to the steering system power transmission shaft80.

More specifically, the steering system clutch94is an electromagnetic clutch that includes an exciting member110(e.g., a solenoid), a displacement member112, and a shaft-rotation synchronizing member113. When the steering system clutch94is connected, i.e., when electric power is supplied to the exciting member110, the displacement member112moves downward to the upper side of the exciting member110due to the magnetic force generated by the exciting member110. The power of a rotary member114that rotates synchronously with the rotary shaft108is transmitted to the shaft-rotation synchronizing member113. The shaft-rotation synchronizing member113can engage, via a key, with the upper cylindrical surface of the steering system power transmission shaft80and can rotate synchronously with the steering system power transmission shaft80. The displacement member112is supported by the steering system power transmission shaft80and can move in the up-and-down direction relative to the steering system power transmission shaft80. The displacement member112and the rotary member114are connected via a plate spring (not illustrated), which is an elastic member having a tensile spring function.

When no electric power is supplied to the exciting member110(i.e., in non-energized state), the displacement member112moves upward due to the elastic force of the plate spring. As a result, a clearance is formed between the displacement member112and the shaft-rotation synchronizing member113. On the other hand, when electric power is supplied to the exciting member110(i.e., in energized state), the displacement member112moves downward and, as a result, the lower surface of the displacement member112is firmly brought into contact with the upper surface of the shaft-rotation synchronizing member113. Thus, the displacement member112and the shaft-rotation synchronizing member113can rotate synchronously. Accordingly, if the displacement member112rotates in accordance with the rotation of the steering motor46, the steering system power transmission shaft80and the shaft-rotation synchronizing member113rotate together in synchronization with the displacement member112.

The steering system clutch94can switch its operational state between an engaged state (i.e., ON) and a disengaged state (i.e., OFF) according to a control signal supplied from the controller38(seeFIG. 1). In a case where the steering system clutch94is in the engaged state, the power of the rotary shaft108can be transmitted to the steering system power transmission shaft80. In a case where the steering system clutch94is in the disengaged state, the power of the rotary shaft108cannot be transmitted to the steering system power transmission shaft80. To this end, the steering system clutch94and the controller38are connected via a cable116.

Further, the steering shaft78having a cylindrical body extending in the vertical direction is rotatably supported by an inside surface of the tube-like portion102provided on the lower side of the lower side member104. The steering shaft78has a lower end portion that protrudes from the lower end of the tube-like portion102and is fixed to an upper end portion of the rotatable housing76. Further, the steering system gear mechanism84is provided between the steering system power transmission shaft80and the steering shaft78. The steering system gear mechanism84can transmit the rotation of the steering system power transmission shaft80to the steering shaft78. According to the illustrated example, the steering system gear mechanism84includes a plurality of spur gears. Further, as illustrated inFIG. 7, the axle90extending in the horizontal direction is rotatably supported at the lower side of the rotatable housing76. The caster wheel18(or20) attached to the axle90can rotate integrally with the axle90.

Further, as illustrated inFIG. 8, the rotary shaft108of the steering motor46, the rotary member114, the steering system clutch94, the steering system power transmission shaft80, the steering system gear mechanism84, and the steering shaft78cooperatively constitute a steering system power transmission path. According to this configuration, the steering system clutch94provided in the steering system power transmission path can selectively transmit the power between the steering motor46and the steering shaft78. Further, the steering system clutch94can prevent the rotational force of the steering shaft78from being transmitted to the steering motor46in a state where no electric power is supplied to the steering motor46, namely in a state where the driving operation of the steering motor46is stopped.

Further, in the stationary housing74, the traveling system power transmission shaft82is disposed coaxially with the rotary shaft106of the traveling motor42. The traveling system power transmission shaft82is rotatably supported by stationary housing74and is positioned beneath the rotary shaft106. The lower end of the rotary shaft106is opposed via a clearance to the upper end of the traveling system power transmission shaft82. The traveling system power transmission shaft82, as illustrated inFIG. 8, can be constituted by two coaxial shafts that are mutually fixed with a key and are synchronously rotatable. Further, the traveling system clutch92can mechanically, i.e., operatively, connect the rotary shaft106to the traveling system power transmission shaft82. More specifically, the traveling system clutch92is an electromagnetic clutch that includes an exciting member110, a displacement member112, and a shaft-rotation synchronizing member113, which are similar to those of the above described steering system clutch94.

The shaft-rotation synchronizing member113can engage with the upper cylindrical surface of the traveling system power transmission shaft82and can rotate synchronously with the traveling system power transmission shaft82. The displacement member112is supported by the traveling system power transmission shaft82and can move in the up-and-down direction relative to the traveling system power transmission shaft82. The displacement member112and a rotary member120that can rotate synchronously with the rotary shaft106are connected via a plate spring (not illustrated), which is an elastic member having a tensile spring function.

When no electric power is supplied to the exciting member110(i.e., in non-energized state), the displacement member112moves upward due to the elastic force of the plate spring. As a result, a clearance is formed between the displacement member112and the shaft-rotation synchronizing member113. On the other hand, when electric power is supplied to the exciting member110(i.e., in energized state), the displacement member112moves downward and, as a result, the lower surface of the displacement member112is firmly brought into contact with the upper surface of the shaft-rotation synchronizing member113. Thus, the displacement member112and the shaft-rotation synchronizing member113can rotate synchronously. Accordingly, if the displacement member112rotates in accordance with the rotation of the traveling motor42when the traveling system clutch92is in the engaged state, the traveling system power transmission shaft82and the shaft-rotation synchronizing member113rotate together in synchronization with the displacement member112.

Similar to the steering system clutch94, the traveling system clutch92can switch its operational state between an engaged state (i.e., ON) and a disengaged state (i.e., OFF) according to a control signal input from the controller38(seeFIG. 1). In a case where the traveling system clutch92is in the engaged state, the power of the rotary shaft106can be transmitted to the traveling system power transmission shaft82. In a case where the traveling system clutch92is in the disengaged state, the power of the rotary shaft106cannot be transmitted to the traveling system power transmission shaft82. To this end, the traveling system clutch92and the controller38are connected via the cable116.

As illustrated inFIG. 7, an upper side rotary shaft118is rotatable around a horizontally extending axis and is supported at the inside of the upper portion of the rotatable housing76. The traveling system power transmission shaft82has a lower end portion connected via a bevel gear mechanism to one end portion (i.e., the left end portion illustrated inFIG. 7) of the upper side rotary shaft118, so that the power can be transmitted from the traveling system power transmission shaft82to the upper side rotary shaft118. The traveling system gear mechanism86, which is provided inside the rotatable housing76, can transmit the rotation of the upper side rotary shaft118to the axle90. According to the illustrated example, the traveling system gear mechanism86includes a plurality of spur gears.

The traveling system power transmission shaft82extends thoroughly inside the steering shaft78and is rotatably supported by the steering shaft78. Further, the rotary shaft106of the traveling motor42, the rotary member120that can rotate synchronously with the rotary shaft106, the traveling system clutch92, the traveling system power transmission shaft82, the traveling system gear mechanism86, and the axle90cooperatively constitute a traveling system power transmission path. According to this configuration, the traveling system clutch92provided in the traveling system power transmission path can selectively transmit the power between the traveling motor42and the axle90. Further, the traveling system clutch92can prevent the rotational force of the axle90from being transmitted to the traveling motor42in a state where no electric power is supplied to the traveling motor42, namely in a state where the driving operation of the traveling motor42is stopped.

Further, the steering angle sensor88capable of detecting the steering angle of the steering shaft78is provided on the caster wheel support units54and56. The steering angle sensor88includes a sensor shaft122that is elongated in the vertical direction (i.e., in parallel with the steering shaft78) and is rotatably supported by the lower side member104, a gear mechanism124for the sensor that is provided between the sensor shaft122and the steering shaft78, and a detection unit126that is provided on the lower end side of the sensor shaft122. The gear mechanism124for the sensor includes a shaft side gear128and a sensor side gear130meshing with each other. The shaft side gear128is a spur gear fixed to the upper portion of the steering shaft78, and the sensor side gear130is a spur gear fixed to the upper portion of the sensor shaft122.

The sensor shaft122has a lower end portion inserted in the detection unit126, which is configured to detect a steering angle of the sensor shaft122that represents an angular deviation from a reference position and also detect a rotational direction of the sensor shaft122. The steering angle sensor88can generate a steering angle signal (e.g., +α or −α, in which plus and minus indicate rotational directions) that represents the angular deviation and the rotational direction of the steering shaft78relative to the reference position of the steering shaft78(that corresponds to the straight forward direction). The detected steering angle signal is sent via the cable116to the controller38(seeFIG. 1). The steering angle sensor88is disposed on the downstream side of the steering system clutch94in the power transmission direction of the steering system power transmission path.

Alternatively, the steering angle sensor88can be configured to include a unit to be sensed that is provided on the steering shaft78and disposed on the downstream side of the steering system clutch94in the power transmission direction of the steering system power transmission path, or provided on another shaft disposed coaxially with the steering shaft78and connected to the steering shaft78, and a detection unit provided at a position confronting with the unit to be sensed. Further, the steering angle sensor88can be configured to detect a steering angle of the portion connected to the steering shaft78.

In the present embodiment, the shaft side gear128and the sensor side gear130are the same in the number of teeth. Therefore, the gear mechanism124for the sensor transmits the rotation of the steering shaft78to the sensor shaft122at a rotational speed identical to that of the steering shaft78. Alternatively, the number of teeth of the sensor side gear130can be set to be greater than that of the shaft side gear128. In this case, the gear mechanism124for the sensor transmits the rotation of the steering shaft78to the sensor shaft122at a rotational speed slower than that of the steering shaft78. In this case, the rotational angle of the steering shaft78is different from the rotational angle of the sensor shaft122. The steering angle of the steering shaft78can be calculated based on a one-to-one relationship when the rotational angle of the sensor shaft122is detected. On the other hand, if the number of teeth of the sensor side gear130is set to be smaller than that of the shaft side gear128, the steering shaft78may cause a phase change of 2 when the sensor shaft122keeps 1 phase. Therefore, it may be difficult to accurately detect the steering angle of the steering shaft78.

The caster wheel support units54and56, each having the above described constituent components, are the same parts that are supported by the left and right end portions of the swing arm16(seeFIG. 1). Further, as illustrated inFIG. 4andFIG. 6, the caster wheel support units54and56include a pair of first flat portions134and a pair of second flat portions138. The first flat portions134are positioned on parallel virtual planes or the same virtual plane and are directed in the downward direction with respect to the lower side member104, so as to serve as reference planes that can abut against a flat portion132(seeFIG. 4) on the upper surface of the right and left end portions of the swing arm16. The second flat portions138are parallel to each other and are directed in the back-and-forth direction (i.e., the right-and-left direction inFIG. 4andFIG. 6) with respect to the lower side member104, so as to serve as reference planes that can abut against a front flat portion136(seeFIG. 4) of the right and left end portions of the swing arm16. The length of the lower side member104in the back-and-forth direction, at its upper end portion, is longer than that of the tube-like portion102. The above described first flat portions134are provided on the lower surface of the upper end portion.

Further, the second flat portions138are provided on an outer circumferential surface, which is directed to the back-and-forth direction, of the tube-like portion102that is located at the lower end portion of the lower side member104. Two caster wheel support units54and56have mutually different orientations in the back-and-forth direction, and the traveling motors42and44are disposed on the outer side of the steering motors46and48. The stationary housing74is firmly fixed to the swing arm16by means of bolts or the like in a state where the first flat portions134and the second flat portions138are brought into contact with the flat portions132and136on the upper surface and the front surface of the swing arm16.

The caster wheels18and20are freely rotatable (i.e., freely steerable) more than 360 degrees around the steering shaft78in a state where the steering system clutch94is in the disengaged state. The caster wheels18and20can be forcibly steered by the steering motors46and48in a state where the steering system clutch94is in the engaged state. Further, the caster wheels18and20are freely rotatable (i.e., freely travelable) around the axle90(seeFIG. 7) in a state where the traveling system clutch92is in the disengaged state. The caster wheels18and20can be forcibly steered by the traveling motors42and44in a state where the traveling system clutch92is in the engaged state. In other words, the steering system clutch94can select free steering or forced steering for the caster wheels18and20. The traveling system clutch92can select free traveling or forced traveling for the caster wheels18and20.

According to the above described configuration, the swing arm16is swingable around the horizontal shaft extending in the back-and-forth direction and the caster wheels18and20are supported by the swing arm16. Therefore, in a state where the ground surface is in bad condition (for example, when the ground surface is an undulated or inclined surface), the degree of being parallel, i.e., the degree of inclination, relative to the horizontal direction can be changed for each of the axle of the left and right wheels14and15and the axle90(seeFIG. 7) of the caster wheels18and20. Therefore, the caster wheels18and20can firmly grip the ground while they rotate. As a result, the driving power of the caster wheels18and20and the wheels14and15can be effectively transmitted to the ground.

FIG. 10illustrates a fundamental configuration of the lawnmower vehicle10that includes the controller38. The controller38, for example, includes an electronic control unit (ECU)140that is a control circuit unit including a CPU, first driving circuits (i.e., two motor drivers although not illustrated) that can drive the wheel driving motors34and36dedicated to the left and right wheels14and15illustrated inFIG. 11, second driving circuits (i.e., two motor drivers)142and144that can drive the traveling motors42and44dedicated to the left and right caster wheels18and20, a third driving circuit (i.e., two motor drivers)146and148that can drive the steering motors46and48dedicated to the left and right caster wheels18and20, and a plurality of power regeneration units (not illustrated) that correspond to the wheel driving motors34and36and the traveling motors42and44.

For example, the first driving circuits drive the wheel driving motors34and36according to a control signal supplied from the ECU140. The wheel driving motors34and36return, to the controller38, feedback signals representing rotational speed, rotational direction, and current values. Further, electrically operable braking units (not illustrated) are provided to decelerate the wheels14and15(seeFIG. 11) associated with the wheel driving motors34and36. When the brake pedal32(seeFIG. 1) is operated, the braking units receive a control signal from the controller38and generate the braking force in accordance with the received control signal.

Further, the ECU140supplies control signals to the traveling system clutch92and the steering system clutch94to control connection/disconnection of respective clutches. The ECU140further supplies control signals to the lawnmower22to control operations to be performed by the lawnmower22. The batteries50and51supply electric power to the lawnmower22, the clutches92and94, and the motors34,36,42,44,46, and48.

A forward movement pedal sensor150can detect a depression amount (i.e., an operation amount) of the forward movement accelerator pedal28(seeFIG. 1). A rearward movement pedal sensor152can detect a depression amount (i.e., an operation amount) of the rearward movement accelerator pedal30(seeFIG. 1). The ECU140acquirers detection signals sent from respective pedal sensors150and152. The ECU140further acquirers detection signals from left and right steering angle sensors88associated with the left and right caster wheels18and20as well as a detection signal representing an operation amount, i.e., a steering position detection value, from the steering operator26(seeFIG. 1).

InFIG. 10, the solid line is a communication line of the CANbus network (i.e., one of the on-vehicle networks) that connects the ECU140to various components, the bold dotted line is a signal line that transmits a detection signal from each sensor to the ECU140, and the thin dotted line is a power line that supplies electric power to each component from the batteries50and51. More specifically, the driving circuits142,144,146, and148of respective motors34,36,42,44,46, and48, the clutches92and94, the lawnmower22, and the ECU140are mutually connected via a single wiring that enables multiplex communication of signals among a plurality of components. If desirable, the communication line of the CANbus network can be used to connect only the driving circuits142,144,146, and148of respective motors34,36,42,44,46, and48and the ECU140. Further, respective sensors88,150,152, and154can be connected via the communication line of the CANbus network. Moreover, the communication line of another on-vehicle network (e.g., FlexRay network) can also be used to connect the above-described components.

Further, the wheel driving motors34and36can function as electric generators in a phase where the left and right wheels14and15(seeFIG. 11) are decelerated. The generated electric power can be supplied via the power regeneration unit to the batteries50and51(i.e., the power source units) for charging. The first driving circuits capable of driving the wheel driving motors34and36and the power regeneration units can be combined as an integrated inverter circuit. Further, the traveling motors42and44capable of driving the caster wheels18and20(seeFIG. 11) are similar to the wheel driving motors34and36in the above described functions. The power regeneration units corresponding to the traveling motors42and44are similar to the power regeneration units corresponding to the wheel driving motors34and36.

The batteries50and51are secondary batteries that can store electric energy and can supply electric power, if necessary, to respective electric loads such as the wheel driving motors34and36(seeFIG. 11) and the traveling motors42and44. The secondary batteries50and51are, for example, lead storage batteries, lithium-ion rechargeable assembled cells, or nickel metal hydride assembled cells.

The batteries50and51can be configured to receive charging power from an external power source according to a plug-in method or other method. In this case, the batteries50and51can be sufficiently charged by the external power source when the lawnmower vehicle10is in a non-operative state. Further, in a case where the lawnmower vehicle10is a hybrid lawnmower vehicle that includes an engine in addition to the batteries50and51, the batteries50and51can be charged by the external power source.

A mower motor (not illustrated) is an electric motor capable of driving the lawnmower22and is, for example, connected to the batteries50and51to rotate a lawnmower blade (or lawnmower blades) of the lawnmower22. A lawnmower starter switch (not illustrated) is provided in the vicinity of the driver's seat24(seeFIG. 1) to enable a worker to ON/OFF control the mower motor. More specifically, the ECU140detects an ON/OFF state of the lawnmower starter switch and controls a driver of the mower motor according to the detected state to cause the mower motor to start or stop its operation, thereby activating or deactivating the lawnmower22.

In the present embodiment, the ECU140controls operations to be performed by the wheel driving motors34and36connected to the left and right wheels14and15(seeFIG. 11) according to the operation amount of the steering operator26and the depression amount of respective accelerator pedals28and30. Further, in a case where the steering system clutch94is in the engaged state, the ECU140controls operations to be performed by the steering motors46and48in addition to the control for the wheel driving motors34and36. In a case where the traveling system clutch92is in the engaged state, the ECU140controls operations to be performed by the traveling motors42and44in addition to the control for the wheel driving motors34and36.

Further, in a case where the clutches92and94are in the disengaged state, the ECU140controls operations to be performed by the wheel driving motors34and36connected to the left and right wheels14and15according to the operation amount of the steering operator26and the depression amount of respective accelerator pedals28and30. The ECU140sets an average speed for the rotational speeds of the left and right wheel driving motors34and36according to the depression amount of respective accelerator pedals28and30. The ECU140further sets a speed difference between the left and right wheel driving motors34and36according to the operation amount of the steering operator26. The ECU140can further set a speed ratio between the left and right wheel driving motors34and36according to the operation amount of the steering operator26.

For example, in a state where the steering operator26is held in a neutral position indicating the straight forward direction, if the forward movement accelerator pedal28is depressed, the ECU140performs control for rotating the wheels14and15(seeFIG. 11) in the forward direction. As the depression amount increases, the rotational speeds of respective wheels14and15increase and the forward movement speed shifts to the high-speed side. On the other hand, if the rearward movement accelerator pedal30is depressed, the ECU140performs control for rotating the wheels14and15in the rearward direction.

As the depression amount increases, the rotational speeds of respective wheels14and15increase and the rearward movement speed shifts to the high-speed side. In this manner, the ECU140can control the lawnmower vehicle10to move at an arbitrary speed in the forward or rearward direction.

Further, in a state where the forward movement accelerator pedal28is held at an appropriate depression amount, if the steering operator26is rotated in the clockwise direction, the ECU140sets the rotational speed of the left wheel14(seeFIG. 11) to be faster than the rotational speed of the right wheel15(seeFIG. 11) so that the lawnmower vehicle10can turn to the right while traveling. If the rotational amount of the steering operator26becomes larger, the ECU140increases the difference between the rotational speed of the left wheel14and the rotational speed of the right wheel15correspondingly. On the other hand, if the rotational amount of the steering operator26becomes smaller, the ECU140decreases the difference between the rotational speed of the left wheel14and the rotational speed of the right wheel15correspondingly. In this manner, the ECU140can adjust the turning radius of the vehicle. If the steering operator26is rotated in the counterclockwise direction, the ECU140sets the rotational speed of the right wheel15to be greater than the rotational speed of the left wheel14so that the lawnmower vehicle10can turn to the left while traveling.

Further, if the depression amount of the forward movement accelerator pedal28is changed while the vehicle is traveling, the vehicle can turn while changing the traveling speed. If the steering operator26is operated in a state where the rearward movement accelerator pedal30is depressed, the vehicle can turn while traveling in the rearward direction.

As described above, the ECU140can independently adjust the rotational speeds of the left and right wheel driving motors34and36according to a rotational operation of the steering operator26and depressing operations of the accelerator pedals28and30, so as to control the traveling and turning of the vehicle. Further, if the steering system clutch94is in the disengaged state, the caster wheels18and20are freely rotatable around the steering shaft78. In this case, the steering angle is determined according to the traveling of the left and right wheels14and15that reflect the driving operations of respective wheel driving motors34and36.

Further, if the right and left steering system clutches94are in the engaged state, the controller38can forcibly adjust the steering angles of respective caster wheels18and20to arbitrary angles. For example, in a state where the steering system clutch94is disconnected, the steering angles of the caster wheels18and20may become inappropriate if the ground surface is inclined or undulated. In such a case, an appropriate steering angle detection unit can be used to monitor the steering angle. If the steering angle is greatly deviated from an appropriate value, the ECU140can bring each steering system clutch94into the engaged state and supply control signals to the third driving circuits146and148that drive the steering motors46and48to accurately control the steering angles of respective caster wheels18and20to appropriate angles.

In this case, each steering system clutch94can be disconnected again after the steering angles are adjusted to the appropriate angles. According to the above described configuration, it is required to monitor the steering angles of the caster wheels18and20. Therefore, the ECU140performs control for engaging or disengaging the steering system clutch94according to a detection value of each steering angle sensor88.

The ECU140includes a control logic circuit, such as a CPU, which is capable of processing an input detection signal that represents an operational state of the lawnmower vehicle10and also generating a control signal to be supplied to each constituent component. The ECU140further includes a memory.

The control to be performed for the wheel driving motors34and36and the steering motors46and48is basically the rotational speed control for adjusting the traveling speed to a target value. In particular, when the vehicle is turning, the speed difference between the left and right wheels14and15corresponding to the turning center position can be instructed based on an operation amount signal representing a steering amount of the steering operator26. The mean traveling speed, i.e., the speed relative to the ground at exactly the midpoint between the left and right wheels14and15on the axle of the left and right wheels14and15, can be instructed based on an operation amount signal representing the depression amount of respective accelerator pedals28and30. Therefore, the ECU140controls a target rotational speed or a target rotational angle for each of respective motors34,36,46, and48so that these motors can be integrally controlled in association with each other.

It may be useful to obtain, beforehand, a relationship between the operation amount signal representing the depression amount of respective accelerator pedals28and30and the mean traveling speed, and a relationship between the operation amount signal representing the steering position and the speed difference or the speed ratio between the left and right wheels14and15. The obtained relationships data can be stored, for example, in a memory unit158(seeFIG. 11) in the controller38. When the vehicle is traveling straight, the ECU140can perform control for adjusting the output torque to a target value. For example, the vector control is available to control the torque.

Next, a turning function of the lawnmower vehicle10is described below. In the following description, components similar to those illustrated inFIG. 1toFIG. 10are denoted by the same reference numerals and detailed descriptions for these components are not repeated. InFIG. 11, a portion that corresponds to the controller38illustrated inFIG. 10is a control unit156and the memory unit158. In the control unit156, a turning driver module168corresponds to a portion of the controller38that includes the driving circuits for respective motors34,36,42,44,46, and48(seeFIG. 10regarding46and48) that drive the wheels14and15and the caster wheels18and20. The remaining portion of the control unit156and the memory unit158connected to the control unit156correspond to another portion of the controller38that includes the control logic circuit.

As illustrated inFIG. 11, the wheel driving motors34and36(MDR, MDL) are connected to the wheels14and15and the traveling motors42and44(MSR, MSL) are connected to the caster wheels18and20. The control unit156receives an operation amount signal from the steering operator26that represents a steering amount of the steering operator26. The control unit156further receives an operation amount signal from respective accelerator pedals28and30that represents the depression amount of respective accelerator pedals28and30. The control unit156supplies driving signals to the wheel driving motors34and36, the traveling motors42and44, and the steering motors46and48, respectively.

The control unit156can generate the driving signals to be supplied to the wheel driving motors34and36, the traveling motors42and44, and the steering motors46and48, respectively, based on the operation amount signals received from the steering operator26and the accelerator pedals28and30. The control unit156is capable of causing the wheels14and15and the caster wheels18and20to turn around their turning center positions corresponding to a turning instruction.

The control unit156includes a mean traveling speed acquisition module160, a turning center position acquisition module162, a left and right wheel speed acquisition module164, a caster wheel speed acquisition module166, and the turning driver module168. The mean traveling speed acquisition module160can acquire a mean traveling speed corresponding to the operation amount signal of respective accelerator pedals28and30. The turning center position acquisition module162can determine and acquire a turning center position based on the mean traveling speed and an acquired turning instruction input, which is a speed difference between the left and right wheels14and15according the operation amount of the steering operator26.

The left and right wheel speed acquisition module164can determine and acquire traveling speeds of the left and right wheels14and15based on the turning center position. The caster wheel speed acquisition module166can determine and acquire traveling speeds of the caster wheels18and20based on the turning center position and the traveling of the left and right wheels14and15. The turning driver module168can generate control signals to be supplied to the motors34and36and the like dedicated to the wheels14and15and the caster wheels18and20based on the traveling speeds of the left and right wheels14and15and the traveling speeds of the caster wheels18and20. The wheels14and15and the caster wheels18and20can turn around the turning center position based on the control signals supplied from the turning driver module168.

A computer mounted on the vehicle can implement processing for realizing each of the above described various mechanisms, except for a driver portion of the turning driver module168. Software programs, such as a lawnmower control program, installed on the computer can execute each of the above described functions. As a modified embodiment, an appropriate hardware configuration may be additionally employed to partly realize the above described functions.

The lawnmower control program is, for example, stored in the memory unit158connected to the control unit156. Various functions to be realized by the control unit156are described below in detail. Prior to the description for these functions, linear traveling and turn traveling are described below with reference toFIG. 12,FIG. 13A,FIG. 13B, andFIG. 13C. The reference numerals illustrated inFIG. 1toFIG. 11are similarly used in the following description. These drawings include plan views schematically illustrating various traveling states of the lawnmower vehicle10that includes two main wheels14and15and two caster wheels18and20. In the present embodiment, all of the wheels14and15and the caster wheels18and20are independently driven for traveling.

FIG. 12illustrates an example of linear traveling in which all of the wheels14and15and the caster wheels18and20travel straight. Therefore, in this case, the wheels14and15and the caster wheels18and20have the same traveling speed. In the context of the present disclosure, the term “traveling speed” refers to a horizontal moving speed of each wheel relative to the ground. If there is a difference between the diameter of the wheels14and15and the diameter of the caster wheels18and20, the rotational speed of the wheels14and15differs from the rotational speed of the caster wheels18and20even when the wheels14and15and the caster wheels18and20have the same traveling speed.

FIG. 13A,FIG. 13B, andFIG. 13Cillustrate examples of turn traveling.FIG. 13Aillustrates an example of turn traveling in which a turning center position170is on an extension of the axle of the wheels14and15and is offset radially inward relative to the inside wheel14.FIG. 13Billustrates another example of turn traveling, which is generally referred to as a “pivot turn”, according to which the turning center position170is at a ground-contact position of either one of the wheels14and15(i.e., the inside wheel14according to the illustrated example).FIG. 13Cillustrates yet another example of turn traveling, which is generally referred to as a “zero turn” or a “spin turn”, according to which the turning center position170is exactly at the midpoint between the wheels14and15on the axle of wheels14and15. In this case, the traveling speeds of the wheels14and15are the same in absolute value. However, the direction of the rotation of the wheel14positioned on one side and the direction of the rotation of the wheel15positioned on the other side are opposed to each other. Therefore, the lawnmower vehicle10rotates around the turning center position170while the caster wheels18and20move along a circular line.

FIG. 13A,FIG. 13B, andFIG. 13Cillustrate typical examples of turn traveling. Although not illustrated, there are other examples of turn traveling whose patterns are between these typical examples. For example, the turning center position may be somewhere inside between the wheels14and15on the axle of the wheels14and15, although the turning center position is not at exactly the midpoint between the wheels14and15. The turning center position may be closer to one of the wheels14and15. In any of these cases, the wheels14and15and the caster wheels18and20turn around the turning center position without changing the planar layout relationship in the lawnmower vehicle10.

Next, various functions to be realized by the system configuration illustrated inFIG. 11are described below. The control unit156starts executing the lawnmower control program upon startup of the lawnmower vehicle10that a worker can manipulate with the steering operator26. The control unit156receives a turning instruction input from the steering operator26if it is operated. More specifically, the control unit156receives, as the turning instruction input signals, an operation amount signal from the steering operator26that represents the steering amount and an operation amount signal from the accelerator pedals28and30that represents the depression amount.

When the steering position deviates from the neutral position in the clockwise direction, the control unit156generates an instruction that sets the rotational speed of the left wheel14to be higher than the rotational speed of the right wheel15. Further, when the steering position deviates from the neutral position in the counterclockwise direction, the control unit156generates an instruction that sets the rotational speed of the right wheel15to be higher than the rotational speed of the left wheel14. If the deviation of the steering position relative to the neutral position is increased, the control unit156generates an instruction that increases the rotational speed difference between the left wheel14and the right wheel15.

Further, if the depression amount of the accelerator pedals28and30is increased, the control unit156generates an instruction that increases the traveling speed. If the depression amount of the accelerator pedals28and30is decreased, the control unit156generates an instruction that decreases the traveling speed. Accordingly, the control unit156can instruct a mean traveling speed based on the operation amount signal representing the depression amount, which is received from the accelerator pedals28and30. Further, the control unit156can instruct a speed difference between the left and right wheels14and15, which corresponds to a turning center position, based on the operation amount signal representing the steering position.

As described above, in the lawnmower vehicle10equipped with the steering operator26, the mean traveling speed and the speed difference between the left and right wheels14and15are acquired as the turning instruction input. In the present embodiment, the steering operator26is not limited to the above described steering wheel and can be two-lever type swingable operators that are disposed on the right and left sides of the driver's seat24. In this case, the traveling speeds of the left and right wheels14and15can be increased or decreased by changing the inclination angle of a corresponding lever. For example, a worker can push either one of the two levers forward to instruct the forward movement of the wheel positioned on the same side and can pull either one of the two levers rearward to instruct the rearward movement of the wheel positioned on the same side. When the lawnmower vehicle10is equipped with the two-lever type operator, the control unit156can acquire the traveling speeds of the left and right wheels14and15, as turning instruction input signals, based on the operation amounts of respective levers that constitute the two-lever type operator.

Next, the control unit156determines and acquires the turning center position based on the acquired turning instruction input. Then, the control unit156determines and acquires the traveling speeds of the left and right wheels14and15. The turning center position acquisition module162and the left and right wheel speed acquisition module164can implement the above described function to be realized by the control unit156.

More specifically, turning center position acquisition module162acquires the turning center position using the method illustrated inFIG. 14AandFIG. 14B.FIG. 14Ais a view that corresponds toFIG. 13A.FIG. 14Aillustrates the layout of the wheels14and15and the turning center position170to be determined. InFIG. 14A, Vo represents the speed of the outside wheel15(i.e., the wheel positioned on the outside when the vehicle is turning) relative to the ground, and Vi represents the speed of the inside wheel14(i.e., the wheel positioned on the inside when the vehicle is turning) relative to the ground. Further, VMrepresents the speed of the vehicle relative to the ground at exactly the midpoint between the left and right wheels14and15on the axle of the left and right wheels14and15. The speed VMcorresponds to the mean traveling speed, which can be defined using the formula VM=(Vo+Vi)/2. The turning center position acquisition module162can execute the processing for determining and acquiring the mean traveling speed. However, there may be a case in which only this portion in particular is executed and utilized. Therefore, inFIG. 11, the mean traveling speed acquisition module160is illustrated as an independent function of the control unit156.

Further, 2T represents a main driving wheel tread, which is equal to the clearance between the wheels14and15, and rrrepresents the radius of respective wheels14and15. Accordingly, a rotational speed No of the outside wheel15around the axle thereof can be defined using the formula (60 Vo)/(2π rr), and a rotational speed Ni of the inside wheel14around the axle thereof can be defined using the formula (60 Vi)/(2π rr).

FIG. 14Billustrates example processes for calculations in determining the turning center position170using the above described symbols. In this case, it is assumed that the turning center position170is represented by a distance R from exactly the midpoint between the wheels14and15on the axle of the wheels14and15. As illustrated inFIG. 14B, the turning center position can be defined using the formula R=T×{(No+Ni)/(No−Ni)}. Accordingly, if T is given according to the configuration of the lawnmower vehicle10, the turning center position R can be determined based on a mean rotational speed NM=(No+Ni)/2 that corresponds to the mean traveling speed VMbased on the depression amount of respective accelerator pedals28and30and also based on a rotational speed difference Δ=(No−Ni) that corresponds to a speed difference based on the steering position.

Further, rotational speeds No and Ni that correspond to the traveling speeds of the outside wheel15and the inside wheel14can be obtained based on the turning center position R, the mean rotational speed NM=(No+Ni)/2, and the rotational speed difference Δ=(No−Ni) that corresponds to the speed difference. Further, the corresponding traveling speeds Vo and Vi of the wheels15and14can be determined based on the obtained rotational speeds No and Ni.

Next, the caster wheel speed acquisition module166of the control unit156determines and acquires the traveling speeds of the caster wheels18and20based on the traveling speeds of the left and right wheels14and15and the turning center position.

FIG. 15A,FIG. 15B, andFIG. 16are views describing example processes for determining the traveling speeds of the caster wheels18and20using the turning center position R obtained inFIG. 14AandFIG. 14B. The reference numerals illustrated inFIG. 11,FIG. 14A, andFIG. 14Bare similarly used in the following description.FIG. 15Ais a view that corresponds toFIG. 13A,FIG. 14A, andFIG. 14B.FIG. 15Aillustrates the layout of the wheels14and15, the layout of the caster wheels18and20, and the turning center position170. InFIG. 15A, VFiand VFoare traveling speeds of the caster wheels18and20to be obtained. More specifically, VForepresents the speed of the caster wheel20(i.e., the caster wheel positioned on the outside when seen from the turning center position170) relative to the ground. VFirepresents the speed of the caster wheel18(i.e., the caster wheel positioned on the inside when seen from the turning center position170) relative to the ground.

Further, 2t represents a caster wheel tread that is the clearance between the caster wheels18and20, W represents a wheel base length that is the distance between the midpoint between the wheels14and15and the midpoint between the caster wheels18and20, and rfrepresents the radius of the caster wheels18and20. Accordingly, when the vehicle is turning, a rotational speed NFoof the outside caster wheel20around the axle can be defined using the formula (60 VFo)/(2π rf) and a rotational speed NFiof the inside caster wheel18around the axle can be defined using the formula (60 VFi)/(2π rf).

The caster wheels18and20can be brought, for example, into a freely rotatable state around the steering shaft78, so that the steering angle of respective caster wheels18and20can follow the traveling direction of the wheels14and15. Alternatively, the caster wheels18and20can be driven by the steering motors46and48so that the steering angle of respective caster wheels18and20can be forcibly adjusted to the following predetermined angles. The controller38disengages the steering system clutch94to bring the caster wheels18and20into a freely rotatable state around the steering shaft78only when the caster wheels18and20are not driven by the traveling motors42and44, i.e., in a case where the traveling system clutch92is in the disengaged state. In other words, in a case where the traveling system clutch92is in the engaged state, the controller38controls the steering system clutch94to be constantly engaged so that the caster wheels18and20can be forcibly steered.

FIG. 15Aillustrates a steering angle Gi of the inside caster wheel18and a steering angle Go of the outside caster wheel20. Further, inFIG. 15A, Ri represents the distance between a ground-contact position of the inside caster wheel18and the turning center position170, and Ro represents the distance between a ground-contact position of the outside caster wheel20and the turning center position170.

FIG. 15Billustrates example processes for calculations in determining the steering angles θiand θoof respective caster wheels18and20using the above described symbols. In this case, the control unit156determines the distances Riand Rothat represent turning radii of respective caster wheels18and20based on R that is determined as described inFIG. 14AandFIG. 14B, the wheel base length W, and the distance t that is a half of the caster wheel tread. Then, the control unit156determines the steering angles θoand θibased on the relationship of the obtained values of Roand Riand the determined R. The radii Roand Riare the distance between the turning center position170and the ground-contact positions of respective caster wheels18and20.

FIG. 16illustrates example processes for calculations in determining the traveling speeds VFiand VFoof the caster wheels18and20that correspond to the mean traveling speed VMof the wheels14and15. The control unit156can determine the traveling speed VFoof the outside caster wheel20and its rotational speed NFoas illustrated inFIG. 16, as R is already obtained as illustrated inFIG. 14AandFIGS. 14Band Rois already obtained as illustrated inFIG. 15B. Similarly, the control unit156can determine the traveling speed VFiof the inside caster wheel18and its rotational speed NFi. Accordingly, the memory unit158can be used to store formulae and maps to be required in calculations beforehand in addition to detailed specifications of the vehicle. The control unit156can use the data stored beforehand in the memory unit158to facilitate the processes for determining and acquiring the turning center position and the traveling speeds of the wheels14and15and the caster wheels18and20in response to the turning instruction input.

Next, to cause the lawnmower vehicle10to turn, the turning driver module168of the control unit156performs driving control for the wheel driving motors34and36and the traveling motors42and44based on the wheel traveling speeds and the caster wheel traveling speeds, or based on the rotational speeds Niand Noof the wheels14and15and the rotational speeds NFiand NFoof the caster wheels18and20. More specifically, the turning driver module168independently supplies the acquired wheel traveling speeds or the wheel rotational speeds Niand Noto the wheel driving motors34and36. Further, the turning driver module168independently supplies the acquired caster wheel traveling speeds or the caster wheel rotational speeds NFiand NFoto the traveling motors42and44. Thus, the turning driver module168integrally controls the wheels14and15and the caster wheels18and20while associating them with each other so that these wheels can independently rotate around their axles. As a result, the lawnmower vehicle10can turn around the turning center position170while traveling.

In the present embodiment, the traveling motors42and44can supply driving force to the caster wheels18and20while satisfying the conditions for the rotational speeds of the caster wheels18and20in a state where the traveling system clutch92is engaged. Accordingly, the lawnmower vehicle10can increase the entire torque and can turn smoothly without increasing the rotation of each wheel excessively, and accordingly, without damaging the planted lawn excessively. As described above, the caster wheels18and20can be supplied with the driving power so that the lawnmower vehicle10can increase the overall torque while causing the wheels to turn appropriately.

In the foregoing description, the control unit156determines the rotational speeds of the caster wheels18and20assuming that the turning center position170is on the outside of the wheels14and15as illustrated inFIG. 13A. Similarly, in the case of the pivot turn illustrated inFIG. 13Band in the case of the spin turn illustrated inFIG. 13C, the control unit156can determine the rotational speeds of the caster wheels18and20based on the vehicle traveling speed and the turning center position referring to the geometrical dimensions of the lawnmower vehicle10, as described with reference toFIG. 14AandFIG. 14BtoFIG. 16.

Further, the lawnmower vehicle10described in the foregoing description is a four-wheel drive vehicle having two wheels14and15and two caster wheels18and20that can be driven by the motors. However, even in a case where the lawnmower vehicle10is a three-wheel drive lawnmower vehicle having only one caster wheel, the control unit156can determine the rotational speed of the single caster wheel based on the vehicle traveling speed and the turning center position referring to the geometrical dimensions of the lawnmower vehicle10, as described with reference toFIG. 14AandFIG. 14BtoFIG. 16. Similarly, in a case where the number of the wheels14and15is other than 2, or in a case where the number of the caster wheels18and20is three or more, the control unit156can determine the rotational speeds of respective wheels14and15and respective caster wheels18and20based on the vehicle traveling speed and the turning center position referring to the geometrical dimensions of the lawnmower vehicle10.

Further, in the foregoing description, the controller38engages the traveling system clutch92dedicated to the caster wheels18and20and causes the traveling motors42and44to drive the caster wheels18and20. However, if the controller38determines that the driving power of the wheels14and15is sufficient for the lawnmower vehicle10to travel, the controller38can select a two-wheel driving mode. In this case, only when it is determined that the torque is insufficient, the driving power of the caster wheels18and20can be added. To detect the shortage of torque, it is useful to provide a gradient sensor (not illustrated), which can detect the gradient of the vehicle body, on the lawnmower vehicle10. The controller38receives a detection signal from the gradient sensor. If the controller38determines that the gradient of the vehicle relative to the ground exceeds a predetermined threshold, the controller38can engage the traveling system clutch92and otherwise disengage the traveling system clutch92.

In a case where a mechanism for constantly transmitting the driving force to the caster wheels18and20is employed, the driving force required for the wheels14and15can be reduced correspondingly. In this case, the motors to be mounted on the lawnmower vehicle10can be downsized as a whole. In a case where a mechanism for transmitting the driving force to the caster wheels18and20only when it is required, is employed, the electric power consumption of the lawnmower vehicle10can be reduced when the vehicle is traveling on a flat or similar ground that does not particularly require the torque.

Further, the controller38can engage the steering system clutch94dedicated to the caster wheels18and20and cause the steering motors46and48to forcibly rotate respective caster wheels18and20around the steering shaft78to set a desired steering angle. For example, the orientation of each caster wheel may be unexpectedly changed due to bad ground surface conditions. If such an unstable state is not controlled, desired traveling and turning of the lawnmower vehicle10cannot be attained. Therefore, the controller38monitors the steering angle based on a detection signal of the steering angle sensor88provided for respective caster wheels18and20. For example, if the actual steering angle deviates greatly more than given range from a target steering angle (i.e., calculated steering angle) that can be determined as illustrated inFIG. 15A,FIG. 15B, andFIG. 16, the controller38can perform control for adjusting the actual steering angle to the target steering angle. Thus, the traveling and turning of the lawnmower vehicle10can be stabilized, regardless of actual ground surface conditions.

As one aspect, the controller38can automatically control engagement/disengagement of the traveling system clutch92and the steering system clutch94dedicated to the caster wheels18and20in accordance with an input signal that represents a vehicle state, as described above. As another aspect, an operation unit (e.g., a switch unit) can be provided in the vicinity of the driver's seat24to enable a worker to manually instruct the switching of engagement/disengagement for the traveling system clutch92and the steering system clutch94. In this case, the controller38receives an instruction signal from the operation unit and supplies a control signal to a corresponding one of the traveling system clutch92and the steering system clutch94. The designated clutch switches the engagement/disengagement state according to the control signal received from the controller38.

Even when the steering system is configured to manually switch the engagement/disengagement of respective clutches92and94, if in a situation where the traveling system clutch92is engaged to forcibly drive the caster wheels18and20for traveling, the controller38can constantly engage the steering system clutch94and control the engagement/disengagement of the steering system clutch94so that the steering angles of respective caster wheels18and20can be forcibly set to the target angles (i.e., calculated angles). The above described system configuration can effectively prevent the planted lawn from being damaged (or scuffed) by the caster wheels18and20that are driven forcibly when being unexpectedly steered when the vehicle is turning.

Furthermore, as a case other than the above described manual switching of clutch engagement/disengagement, the steering system may be configured to automatically switch the engagement/disengagement of respective clutches92and94if the actual steering angle deviates greatly more than given range from the target steering angle (i.e., calculated steering angle). Even in this case, in a situation where the traveling system clutch92is engaged to forcibly drive the caster wheels18and20for traveling, the controller38can constantly engage the steering system clutch94and control the engagement/disengagement of the steering system clutch94so that the steering angles of respective caster wheels18and20can be forcibly set to the target angles (i.e., calculated angles). Therefore, as described above, the planted lawn can be effectively prevented from being damaged by the caster wheels18and20that are driven forcibly when being unexpectedly steered when the vehicle is turning.

According to the above described riding work vehicle, the operational state of respective caster wheels18and20can be selected between free traveling and forced traveling by changing the engagement/disengagement of the traveling system clutch92. Further, the operational state of respective caster wheels18and20can be selected between free steering and forced steering by changing the engagement/disengagement of the steering system clutch94. Furthermore, the traveling system clutch92can prevent the rotational force of the axle90from being transmitted to the traveling motors42and44in a state where the traveling motors42and44are deactivated. The steering system clutch94can prevent the rotational force of the steering shaft78from being transmitted to the steering motors46and48in a state where the steering motors46and48are deactivated.

Therefore, in a state where the motors42,44,46, and48dedicated to the caster wheels18and20are all deactivated, the corresponding clutches92and94are disengaged. Therefore, even in a case where there is a tendency for the force to be transmitted from the ground to the motors42,44,46, and48via the axle90of the caster wheels18and20or via the steering shaft78when the vehicle is traveling, the above described system configuration can prevent the force from being transmitted to the motors42,44,46, and48. More specifically, when the caster wheels18and20are in the free traveling state or in the free steering state, the present embodiment can prevent the motors42,44,46, and48from acting as resistance. Therefore, the axle90or the steering shaft78can rotate smoothly. In other words, the present embodiment can realize the configuration capable of effectively saving energy.

Further, the steering angle sensor88that can detect the steering angle of the steering shaft78is disposed on the downstream side of the steering system clutch94in the power transmission direction of the steering system power transmission path. Therefore, in the configuration in which the steering system clutch94is provided in the steering system power transmission path, the steering angle of the steering shaft78can be detected regardless of the engagement/disengagement of the steering system clutch94.

Further, the sensor shaft122is disposed in parallel with the steering shaft78. The gear mechanism124for the sensor is provided between the sensor shaft122and the steering shaft78. The gear mechanism124can transmit the rotation of the steering shaft78to the sensor shaft122at a rotational speed identical to that of the steering shaft78or at a rotational speed slower than that of the steering shaft78. Therefore, the present embodiment can prevent the mechanical configuration including the steering shaft78and the steering angle sensor88from being excessively enlarged. Therefore, a portion to be modified to detect the steering angle of the steering shaft78can be realized by a relatively simple, compared to the basic configuration. The number of the modified portion can be decreased. Further, a general conventional steering angle sensor can be used as the detection unit126.

Further, the traveling system clutch92can switch its operation between the engaged state and the disengaged state according to a control signal supplied from the controller38. Therefore, the controller38supplies a control signal to the traveling system clutch92to enable or disable power transmission between the traveling motors42and44and the caster wheels18and20according to an operation of a driver or according to a traveling state of the vehicle.

Accordingly, in a case where the traveling motors42and44are electric motors as described in the present embodiment, the traveling system clutch92is brought into the engaged state when the vehicle is in a decelerating state to realize regenerative braking by the caster wheels18and20. In this case, the rotational force of respective caster wheels18and20can be transmitted to the traveling motors42and44. The electric power generated by the traveling motors42and44can be used to charge the batteries50and51. The lawnmower vehicle10can further save energy. The layout of the traveling system clutch92is not limited to the example illustrated inFIG. 8. For example, the traveling system clutch92can be disposed between an outer circumferential surface of the axle90of the caster wheels18and20and an inner circumferential surface of a lowermost spur gear that constitutes the traveling system gear mechanism86provided around the axle90.

[Second Embodiment of the Present Invention]

Next, a second embodiment of the present invention is described below. A lawnmower vehicle that can serve as a riding work vehicle according to the present embodiment has a fundamental configuration similar to that of the lawnmower vehicle10according to the first embodiment described with reference toFIG. 1toFIG. 16. In the following description, components similar to or corresponding to those illustrated inFIG. 1toFIG. 16are denoted by the same reference numerals. In the present embodiment, the traveling system clutch92is a one-way clutch or a two-way clutch, although it was described as an electromagnetic clutch in the first embodiment with reference toFIG. 8. Similarly, the steering system clutch94is a two-way clutch (i.e., not the electromagnetic clutch). These clutches92and94can be disposed at appropriate positions that are similar to or may be different from the positions described in the first embodiment.

However, it is desired that the steering system clutch94is provided on the upstream side of the steering angle sensor88in the power transmission direction of the steering system power transmission path, so that the steering angle sensor88can constantly detect the steering angle regardless of the engagement/disengagement of the steering system clutch94. Further, the traveling system clutch92can be provided, for example, between the outer circumferential surface of the axle90of the caster wheels18and20and the inner circumferential surface of the lowermost spur gear that constitutes the traveling system gear mechanism86provided around the axle90, as can be understood with reference toFIG. 7.

Further, as can be understood with reference toFIG. 7andFIG. 8, in a case where the traveling system clutch92is a one-way clutch, the traveling system clutch92can disable power transmission from the axle90of the caster wheels18and20to the traveling motors42and44if the rotational speed of the traveling motors42and44becomes slower by a predetermined ratio with respect to the vehicle traveling speed, i.e., the rotational speed of the caster wheels18and20, for example, when there is a tendency for the rotary shaft106of the traveling motors42and44to be slower than the traveling system power transmission shaft82. Even in this case, the traveling system clutch92can prevent the rotational force of the axle90from being transmitted to the traveling motors42and44in a state where the traveling motors42and44are deactivated, in the same manner as in the above described first embodiment in which the electromagnetic clutch is used. Even in this configuration, the present embodiment can prevent the traveling motors42and44from acting as resistance for smooth traveling in a state where the traveling motors42and44are deactivated. Therefore, the vehicle can save energy.

The positional relationship between the steering shaft78and the axle90of the caster wheels18and20in the back-and-forth direction can be reversed according to the traveling direction of the vehicle (i.e., the forward movement or the rearward movement) when the caster wheels18and20are in the free steering state. For example, as understood from the example illustrated inFIG. 6, a caster trail176(i.e., an offset) represents the distance between a steering shaft center172and the axle center (i.e., a wheel center174) in the back-and-forth direction (i.e., in the right-and-left direction inFIG. 6).

In this case, the steering angle tends to be determined according to the traveling state of the left and right wheels14and15when the steering shaft is freely rotatable. For example, when the vehicle is traveling forward, the wheel center174of respective caster wheels18and20is positioned on the rear side of the steering shaft center172. When the vehicle is traveling rearward, the wheel center174is positioned on the front side of the steering shaft center172. Accordingly, even when the traveling system clutch92is a one-way clutch, the traveling system clutch92can prevent the rotation of the caster wheels18and20from being transmitted to the traveling motors42and44in a state where the traveling motors42and44are deactivated regardless of the forward movement or the rearward movement of the vehicle and the wheels14and15are driven by the wheel driving motors34and36. Therefore, this configuration can prevent the traveling motors42and44from acting as resistance for smooth traveling. However, in this case, compared to the above described first embodiment, the regenerative braking by the caster wheels18and20is not available and the power regeneration using the traveling motors42and44cannot be implemented.

On the other hand, as understood fromFIG. 7andFIG. 8, when the traveling system clutch92is a two-way clutch, the traveling system clutch92enables power transmission between the traveling motors42and44and the axle90only when there is a tendency for the power to be transmitted from the traveling motors42and44to the axle90. Further, the traveling system clutch92disables the power transmission when there is the tendency for the power to be transmitted from the axle90to the traveling motors42and44. Further, the two-way clutch can transmit the power from the traveling motors42and44to the axle90regardless of rotational direction of respective traveling motors42and44.

Further, in a case where the traveling system clutch92is constituted by the two-way clutch, the controller38(i.e., the control unit) can supply electric power generated by the traveling motors42and44to the batteries50and51(which serve as the power source units and the electric power storage units) to charge the batteries50and51, if it is determined that the regenerative braking request is present. For example, the controller38can determine the presence of the regenerative braking request if a sensor detects that the accelerator pedals28and30are off, i.e., the depression amount is 0, when the vehicle is traveling, or if a sensor detects that the brake pedal32is depressed when the vehicle is traveling. In this case, the controller38controls the driving circuits142and144(seeFIG. 10) for the wheel driving motors34and36and the traveling motors42and44to charge the batteries50and51with the electric power generated by respective motors34,36,42, and44.

Further, in this case, simply configuring the traveling system clutch92as a two-way clutch is not sufficient to transmit the rotation of the caster wheels18and20to the traveling motors42and44when the vehicle is decelerating. Accordingly, the regenerative braking by the caster wheels18and20cannot be realized. Therefore, to realize the regenerative braking by the caster wheels18and20, the controller38executes the following regenerative braking control processing. More specifically, in the regenerative braking control processing, the controller38controls the traveling motors42and44to rotate in a direction opposed to that in the normal traveling state. If it is determined that the regenerative braking request is present, the controller38supplies electric power to the traveling motors42and44to cause the traveling motors42and44to rotate in a direction opposed to that in the normal traveling state (i.e., motor acceleration direction in the traveling state). As a result, the traveling system clutch92is brought into the engaged state. Then, the controller38holds the engaged state of the traveling system clutch92so as to realize the regenerative braking by the traveling motors42and44that can regenerate electric power.

Further, as can be understood fromFIG. 8, the steering system clutch94is a two-way clutch that enables power transmission between the steering motors46and48and the steering shaft78only when there is a tendency for the power to be transmitted from the steering motors46and48to the steering shaft78. Further, the steering system clutch94disables the power transmission when there is a tendency for the power to be transmitted from the steering shaft78to the steering motors46and48. Even in this case, the steering system clutch94can prevent the rotational force of the steering shaft78from being transmitted to the steering motors46and48in a state where the steering motors46and48are deactivated.

According to the above described configuration, the vehicle can prevent the steering motors46and48from acting as resistance for free steering in a state where the steering motors46and48are deactivated. Therefore, the vehicle can save energy. The caster wheels18and20are required to be directed in each of the right and left directions. Therefore, the steering motors46and48are required to rotate in both forward and reverse directions to flexibly change the direction of the power to be transmitted to the steering shaft78. This is the reason why the steering system clutch94cannot be simply constituted by a one-way clutch.

In the present embodiment, the traveling system clutch92is constituted by a one-way clutch or a two-way clutch and the steering system clutch94is configured by a two-way clutch. Therefore, compared to the above described first embodiment, the present embodiment can simplify the control mechanism for the clutches92and94or can omit the mechanism. For example, the cable116connecting the clutches92and94to the controller38can be omitted.

Further, in the present embodiment, as understood fromFIG. 10, the lawnmower vehicle10includes a hill-climbing detection sensor178that can detect whether the vehicle is in a hill-climbing state, namely whether the front end of the lawnmower vehicle10is positioned higher than the rear end thereof, or whether the hill-climbing angle of the vehicle is equal to or greater than a predetermined value. The ECU140, which is provided in the controller38, receives a detection signal from the hill-climbing detection sensor178. If it is determined that the detection signal of the hill-climbing detection sensor178indicates that the vehicle is in the hill-climbing state, the ECU140controls the traveling motors42and44to set the speed VFof the left and right caster wheels18and20relative to the ground to be higher than the speed VRof the left and right wheels14and15relative to the ground (i.e., VF>VR). For example, in a case where the diameter of the caster wheels18and20is identical to the diameter of the wheels14and15, the ECU140sets the rotational speed of the caster wheels18and20to be higher than the rotational speed of the wheels14and15.

On the other hand, if it is determined that the detection signal of the hill-climbing detection sensor178indicates that the vehicle is not in the hill-climbing state, the ECU140controls the traveling motors42and44to set the speed VFof the left and right caster wheels18and20relative to the ground to be lower than the speed VRof the left and right wheels14and15relative to the ground (i.e., VF<VR). For example, the ECU140performs this control when the front end of the lawnmower vehicle10is positioned at the same height as the rear end of the lawnmower vehicle10, namely, when the ground is substantially flat, or when the hill-climbing angle of the vehicle is less than the predetermined value. For example, in a case where the diameter of the caster wheels18and20is identical to the diameter of the wheels14and15, the ECU140sets the rotational speed of the caster wheels18and20to be lower than the rotational speed of the wheels14and15.

To realize the above described control, the controller38can use detection values of the rotational speeds of the wheel driving motors34and36that correspond to the speeds of the left and right wheels14and15relative to the ground and detection values of the rotational speeds of the traveling motors42and44that correspond to the speeds of the left and right caster wheels18and20relative to the ground. It is preferable that, if it is determined that the vehicle is in the hill-climbing state, the controller38controls the traveling motors42and44so as to set the speed VFof the left and right caster wheels18and20relative to the ground to be higher, by the amount equivalent to 20% or less, than the speed VRof the left and right wheels14and15relative to the ground.

More preferably, the controller38controls the traveling motors42and44so as to set the speed VFto be higher, by the amount equivalent to 15% or less, than the speed VR. According to the above described configuration, in a case where the traveling system clutch92is a one-way clutch or a two-way clutch, these clutches can be used to realize four-wheel driving only when the driving power of the left and right wheels14and15is not sufficiently transmitted to the ground, if the vehicle is traveling on a flat ground. For example, the left and right wheels14and15may slip when the vehicle is traveling on a road having a low surface resistance, e.g., when the left and right wheels14and15are trapped in mud. On the other hand, these clutches can be used to realize two-wheel driving only when the driving power of the left and right wheels14and15is sufficiently transmitted to the ground.

Further, in a case where the traveling system clutch92is a one-way clutch or a two-way clutch, the vehicle can constantly maintain four-wheel drive traveling to prevent frequent switching between the two-wheel driving state and the four-wheel driving state that may occur when the vehicle is in a hill-climbing traveling state because the left and right wheels14and15tend to slip. Thus, the present embodiment can improve ride comfort of the vehicle and can realize stable traveling. The rest of the configuration is similar to that described in the above described first embodiment.

In the present embodiment, one of the traveling system clutch92and the steering system clutch94can be constituted by a one-way clutch or a two-way clutch (i.e., a non-electromagnetic clutch) and the other can be constituted by an electromagnetic clutch similar to that described in the first embodiment, which can be engaged or disengaged according to a control signal input from the controller38.

Although not related to the present invention, in the above described embodiments, the traveling motors42and44dedicated to the caster wheels18and20for traveling and the traveling system clutch92can be omitted if desired to constantly realize the two-wheel driving by the left and right wheels14and15.