Patent Publication Number: US-2020282972-A1

Title: Work Vehicle

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to Japanese Patent Application No. 2019-040293 filed Mar. 6, 2019, the disclosure of which is hereby incorporated by reference in its entirety. 
     TECHNICAL FIELD 
     The present invention relates to a work vehicle which travels in a field while carrying out a utility work. 
     BACKGROUND ART 
     Conventionally, a work vehicle has been used when a utility work is carried out in a field. Such work vehicle is configured to travel while carrying out a utility work with using an output of at least one of an engine and a motor (to be referred to as a “power source” hereinafter). In case such power source is an engine, fuel is supplied from a fuel tank to the power source. Whereas, in case the power source is a motor, in many cases, the power source is configured to be supplied with electric power from a battery (fuel and electric power will be referred to as “energy source” hereinafter). If such energy source is exhausted during traveling or a utility work of the work vehicle, this will present a trouble in the work in the field. For this reason, the art has contemplated a technique of monitoring a remaining amount in a reservoir section (a fuel tank or a battery) which stores such energy source and reporting or alarming when the remaining amount becomes below a predetermined value (e.g. Patent Document 1). 
     Patent Document 1 discloses a driving assistance device that utilizes at least one of an internal combustion engine and an electric power generator as a power source for traveling and that is mounted on a traveling vehicle. This driving assistance device obtains information relating to at least one of a road condition of a predetermined area which is present in the traveling destination of the vehicle, a current vehicle situation, and a past operational situation. And, this driving assistance device, based on such information, will make a prediction whether deterioration of fuel consumption efficiency of the vehicle will occur or not in the traveling course of the vehicle. If it is predicted that deterioration of fuel consumption efficiency of the vehicle will be invited, this driving assistance device will report (inform) in advance a riding person of the vehicle an operation for improving fuel consumption efficiency of the vehicle. 
     PRIOR ART DOCUMENT 
     
         
         Patent Document 1: JP2012-13021A 
       
    
     DISCLOSURE OF THE INVENTION 
     Problem to be Solved by Invention 
     The technique disclosed in Patent Document 1 above is configured such that in case it is predicted that fuel consumption efficiency deterioration will be invited, an operation for improving fuel consumption efficiency of the vehicle is reported in advance to the riding person of the vehicle. However, for a user using the traveling work vehicle while being engaged in a utility work in a field, even if such situation is reported, as the user concentrates himself/herself in the work or traveling operation, this user may fail to take notice of the reporting. In such case, it may be anticipated that the work vehicle may make an unexpected stop due to exhaust of the energy source at a site distant from a site where energy replenishment is possible (this site will be referred to as “replenishment site” hereinafter). If this happens, the user will need to transport energy source from the replenishment site to the site of the stopped vehicle or to carry the reservoir to the replenishment site, fill it with the energy source, and then carry the replenished reservoir to the site of the stopped vehicle. These causes relatively much trouble. 
     In view of the above, there is a need for a work vehicle allowing a user to grasp diminishing of the energy source. 
     Means for Solving Problem 
     According to a characterizing feature of a work vehicle relating to the present invention, there is provided a work vehicle that travels in a field while carrying out a utility work, the work vehicle comprising: 
     a driving wheel provided in a machine body; 
     a power source for inputting a rotational force to the driving wheel; 
     an output instruction acquisition section for acquiring an output instruction for the power source according to an operation of an operation input unit; 
     a traveling control unit for controlling driving of the power source in accordance with the output instruction; 
     a reservoir section for reserving energy source for driving the power source; 
     a remaining amount detection section for detecting a remaining amount of the energy source in the reservoir section; and 
     a restriction section for restricting a maximum speed of the machine body to a preset speed irrespectively of the output instruction if the remaining amount of the energy source is equal to or smaller than a preset first threshold value. 
     With the above-described characterizing arrangement, in correspondence with reduction in the remaining amount of the energy source, it is possible to dare to forcibly reduce a vehicle speed at which the work vehicle can travel. As a result, by providing the user with “inconvenience”, the user can be made to become aware of the diminishing of the remaining amount of the energy source. Therefore, it becomes possible to avoid the situation of energy shortage occurring in the middle of the field for instance, and to urge the user to replenish the energy source spontaneously. 
     Further, preferably, the restriction section is configured to make restriction such that when the remaining amount of the energy source is equal to or smaller than the first threshold value, the maximum speed is progressively decreased in accordance with diminishing of the remaining amount of the energy source. 
     With the above-described arrangement, the restriction of the maximum speed can be carried out easily in accordance with the remaining amount of the energy source. 
     Further, preferably, the restriction section restricts the maximum speed to a preset fixed value if the remaining amount of the energy source is equal to or smaller than a second threshold value which is smaller than the first threshold value. 
     With the above-described arrangement, it becomes possible to secure a speed needed when moving to the energy sourced replenishment site due to diminishing of the remaining amount of the energy source. Therefore, it is possible to avoid prolongation of a time needed for movement to the energy source replenishment site. 
     Further, preferably, the work vehicle further comprises: 
     a work unit driven with using the energy source for carrying out the utility work; and 
     a stopping section for stopping driving of the work unit if the remaining amount of the energy source is equal to or smaller than a preset third threshold value. 
     With the above-described arrangement, in association with reduction in the energy source, the work unit that consumes a large electric power can be stopped for instance. As a result, by giving a feeling of “wrongness” to the user by intentionally interfering with progress of the utility work, it is possible to secure energy source needed at least for traveling of the work vehicle to the energy source replenishment site. 
     Still further, preferably, the power source comprises a motor and the reservoir section comprises a battery. 
     With the above-described arrangement, if the state of the battery (e.g. charged state of the battery, an output voltage state of the battery, etc.) becomes equal to or smaller than the first threshold value, the speed of the machine body by the operation input unit can be reduced in accordance with this battery state. As a result, it is possible to prompt the user to effect charging by providing the user with feeling of wrongness. 
     Further, preferably, the restriction section restricts an electric current running in the motor according to stored power amount of the battery. 
     With the above-described arrangement, the restriction section effects the restriction of the maximum speed easily. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a work vehicle according to an instant embodiment, 
         FIG. 2  is a system diagram showing an electric system and a power system of the work vehicle, and 
         FIG. 3  is a view illustrating restriction of a maximum speed. 
     
    
    
     BEST MODE OF CARRYING OUT THE INVENTION 
     A work vehicle relating to the present invention is configured to allow a user to grasp diminishing of an energy source used for driving the work vehicle. Next, a work vehicle  1  according to the instant embodiment will be explained. 
       FIG. 1  shows a perspective view of a riding electric grass cutting machine as one example of the work vehicle  1  of this embodiment. Also,  FIG. 2  shows a system diagram showing an electric system and a power system of the work vehicle  1 . The work vehicle  1  according to the instant embodiment is configured to restrict a maximum speed of the work vehicle  1  when a predetermined condition (to be detailed later) is satisfied. 
     In the instant embodiment, as an example of the work vehicle  1 , a riding electric grass cutting machine will be cited and explained. In this case, a grass cutting work corresponds to a work (“a utility work”) relating to the invention. The work vehicle  1  travels in a field while carrying out a grass cutting work. 
     An electric power control device  9  controls electric power used by the work vehicle  1  which travels while carrying out a utility work. The electric power used by the work vehicle  1  includes electric power used for traveling of the work vehicle  1 , electric power used for the utility work carried out by the work vehicle  1 , and electric power for a user who uses the work vehicle  1 . A specific example of the electric power used for traveling of the work vehicle  1  is electric power supplied to motors  20  (an example of “power source”) which input rotational forces to driving wheels  3  included in a machine body  4  of the work vehicle  1 . A specific example of the electric power used for a utility work carried out by the work vehicle  1  is electric power to be supplied to motors (“mower motors  130   a ,  130   b ,  130   c ” to be described later) which rotate cutter blades used for a grass cutting work. A specific example of the electric power used for a user who uses the work vehicle  1  is electric power used for displaying by an instrument panel  99 . The electric power control device  9  is mounted on the work vehicle  1  that carries out such work. The electric power control device  9  appropriately distributes the respective electric powers to functional sections for carrying out the respective functions. 
     As shown in  FIG. 1  and  FIG. 2 , the riding electric grass cutting machine includes the machine body  4 , a battery (an example of a “reservoir section”)  23 , a driver&#39;s seat  11 , a ROPS frame  12  and a mower unit (an example of a “work unit”)  13 . The machine body  4  is supported by caster wheels  5  constituting front wheels and the driving wheels  3  constituting rear wheels. The battery  23  is disposed at a rear portion of the machine body  4 . The driver&#39;s seat  11  is disposed forwardly of the battery  23 . The ROPS frame  12  is mounted erect from the rear side of the driver&#39;s seat  11 . The mower unit  13  is suspended from the machine body  4  to be lifted up/down via a lift (elevating/lowering) mechanism in a space beneath the machine body  4  and between the caster wheels  5  and the driving wheels  3 . 
     The driving wheels  3  are driven by a traveling control unit  6 . Operations of this traveling control unit  6  are controlled by a traveling control device  2  (an example of an “output instruction acquisition section”). Operations of the mower unit  13  are controlled by a mower control device  7 . Here, it is noted that the caster wheels  5  consist of a left caster wheel  5   a  and a right caster wheel  5   b  and the driving wheels  3  consist of a left driving wheel  3   a  and a right driving wheel  3   b.    
     Forwardly of the driver&#39;s seat  11 , there is provided a floor plate acting as a “footrest” for the driver. From this floor plate, a brake pedal  14  protrudes. On the left and right opposed sides of the driver&#39;s seat  11 , there are disposed a left maneuvering lever  8   a  and a right maneuvering lever  8   b . Further, on the side of the driver&#39;s seat  11 , there is provided an electric operational panel  18 . This electric operational panel  18  includes switch buttons, switch levers or the like of the electric control system. On this electric operational panel  18 , a mower switch for activating the mower unit  13  and the instrument panel  99  described above are disposed. Incidentally, in the following explanation, the above-described left maneuvering lever  8   a  and the right maneuvering lever  8   b  will be generically referred to as “maneuvering lever  8 ” (an example of “operation input unit”) when no distinction therebetween is particularly needed. 
     In the instant embodiment, power sources for the left driving wheel  3   a  and the right driving wheel  3   b  are rotational forces from a left motor  21  and a right motor  22 , respectively. Both the left motor  21  and the right motor  22  are in-wheel motors. Via a left power supply section  41 , electric power is supplied to the left motor  21 . The left power supply section  41  is included in an inverter  10 . Via a right power supply section  42 , electric power is supplied to the right motor  22 . The right power supply section  42  also is included in the inverter  10 . By changing the supplied electric powers respectively, it is possible to change at least one of a rotational speed and a torque. With this, respective rotational speeds (circumferential speeds) of the left driving wheel  3   a  and the right driving wheel  3   b  can be made different from each other. As a result, due to a rotational speed difference between the left driving wheel  3   a  and the right driving wheel  3   b , the riding electric grass cutting machine makes a turn. 
     The traveling control unit  6  is a functional section for controlling traveling and turning of the riding electric grass cutting machine. In the instant embodiment, the traveling control unit  6  includes the left motor  21 , the right motor  22  and the inverter  10  (in particular, the left power supply section  41  and the right power supply section  42 ) which were mentioned above. The inverter  10  supplies electric power to the left motor  21  and the right motor  22  respectively. Electric power outputted from the inverter  10  corresponds to a speed instruction value (a target value) calculated by the traveling control device  2 . When the actual rotational speed (the actual speed) becomes smaller than a target value due to a traveling load, the electric power outputted from the inverter  10  will be corrected to increase the motor output torque. On the other hand, in the case of e.g. traveling on a downslope, if the actual rotational speed (the actual speed) becomes greater than the target value, the electric power outputted from the inverter  10  will be corrected to decrease the motor output torque. 
     The mower unit  13  includes three rotary blades  131   a ,  131   b ,  131   c . These rotary blades  131   a ,  131   b ,  131   c  respectively use the mower motors  130   a ,  130   b ,  130   c  as their driving power sources. Electric power supplies to the mower motors  130   a ,  130   b ,  130   c  are carried out via a mower power supply section  43 . This mower power supply section  43  too is included in the inverter  10 . The mower power supply section  43  is controlled by the mower control device  7 . This mower control device  7  constitutes a control device  100 , together with the traveling control device  2  and the electric power control device  9  which were described above. 
     An operational amount (a pivot angle) of the left maneuvering lever  8   a  is detected by a left maneuvering angle detection sensor  80   a . An operational amount (a pivot angle) of the right maneuvering lever  8   b  is detected by a right maneuvering angle detection sensor  80   b . Further, an operational angle of the brake pedal  14  is detected by a brake detection sensor  80   c . An operation of the mower switch is detected by a mower sensor  80   d . Further, a rotational speed of the left driving wheel  3   a  is detected by a left rear wheel rotation detection sensor  70   a . A rotational speed of the right driving wheel  3   b  is detected by a right rear wheel rotation detection sensor  70   b . Further, rotational speeds of the mower motors  130   a ,  130   b ,  130   c  are detected respectively by rotation sensors  100   a ,  100   b ,  100   c . And, detection results from the respective sensors are transmitted to the control device  100  to be used by this control device  100  when necessary or appropriate. 
     In the traveling control device  2 , based on operational amounts of the respective maneuvering levers  8 , electric powers to be supplied respectively to the left motor  21  and the right motor  22  are calculated. Further, in this traveling control device  2 , by a known feedback control technique, the above-describe electric powers are corrected. Namely, the traveling control device  2  calculates required driving torques (to be referred to simply as “necessary torques” hereinafter) required by the left motor  21  and the right motor  22 . Here, the necessary torque means a torque amount required by the left motor  21  or the right motor  22  for the actual speed to reach the target speed in case an actual rotational speed has not arrived at a target rotational speed with a control amount calculated by a target rotational speed set in correspondence with an operational amount by the left maneuvering lever  8   a  or the right maneuvering lever  8   b . The traveling control device  2  derives such necessary torques from the target rotational speeds of the left driving wheel  3   a  and the right driving wheel  3   b  and the actual rotational speeds of the left driving wheel  3   a  and the right driving wheel  3   b . Such target rotational speeds of the left driving wheel  3   a  and the right driving wheel  3   b  are obtained, based on detection results of the left maneuvering angle detection sensor  80   a  and the right maneuvering angle detection sensor  80   b . The actual rotational speeds of the left driving wheel  3   a  and the right driving wheel  3   b  are obtained by the left rear wheel rotation detection sensor  70   a  and the right rear wheel rotation detection sensor  70   b  respectively. Then, based on such calculated necessary torques, the traveling control device  2  corrects the electric power amounts. 
     Next, there will be explained a function for allowing a user to grasp diminishing of energy source for driving the work vehicle  1  (to be referred to as “present inventive function” hereinafter) relating to the instant embodiment. The present inventive function is realized by the respective functional sections of the driving wheels  3 , the motor  20 , the traveling control device  2 , the traveling control unit  6 , the battery  23 , the remaining amount detection section  52 , the restriction section  53  and a stopping section  54 . In particular, the traveling control device  2 , the remaining amount detection section  52 , the restriction section  53 , and the stopping section  54  are constituted of hardware and/or software including a CPU as the core component thereof, in order to carry out processes relating to the above-described present inventive function. 
     The driving wheels  3  are provided in the machine body  4  as described above and consist of the left driving wheel  3   a  and the right driving wheel  3   b , in the instant embodiment. The motor  20  inputs rotational forces to the driving wheels  3 . And, this motor  20  consists of the left motor  21  and the right motor  22  in this embodiment, as described above. The left motor  21  inputs a rotational force to the left driving wheel  3   a . The right motor  22  inputs a rotational force to the right driving wheel  3   b.    
     The traveling control device  2  acquires output instructions for the motor  20  according to an operation of the maneuvering lever  8 . The maneuvering lever  8  consists of a left maneuvering lever  8   a  and a right maneuvering lever  8   b . The output instructions for the motor  20  according to an operation of the maneuvering lever  8  refer to electric power amounts to be supplied to the left motor  21  and the right motor  22  calculated based on operations of the left maneuvering lever  8   a  and the right maneuvering lever  8   b  by the user. Such electric power amounts are calculated by the traveling control device  2  in the instant embodiment. 
     The traveling control unit  6  controls driving of the motor  20  according to output instructions. In this embodiment, as described above, the traveling control unit  6  includes the left motor  21 , the right motor  22 , the left power supply section  41  and the right power supply section  42 . Here, in particular, the left power supply section  41  and the right power supply section  42  correspond to the traveling control unit  6 . The output instructions refer to the electric power amounts calculated by the traveling control device  2 . Therefore, the left power supply section  41  and the right power supply section  42  control driving of the left motor  21  and the right motor  22  according to electric power amounts calculated by the traveling control device  2 . 
     The battery  23  stores energy source for driving the motor  20 . The motor  20  is driven by electric energy. Therefore, the electric energy in this embodiment corresponds to the “energy source” relating to the present invention. The battery  23  stores such electric energy. 
     The remaining amount detection section  52  detects a remaining amount of the energy source in the battery  23 . The energy source in the battery  23  means electric energy charged and stored in the battery  23 . Therefore, the remaining amount detection section  52  detects a remaining amount of electric energy stored in the battery  23 . Here, the remaining amount may be a value of the electric energy stored in the battery  23  or may be an output voltage of the battery  23  for instance or it may also be a state of charge (SOC: State of Charge). Incidentally, in case the remaining amount detection section  52  detects an output voltage of the battery  23  or a state of charge, a predetermined filtering process (a lowpass filter process) may be effected to a detected value in order to facilitate the detection. 
     The restriction section  53  restricts (limits) the maximum speed of the machine body  4  to a preset speed irrespectively of an output instruction, in case the remaining amount of the energy source is equal to or smaller than a preset first threshold value. The remaining amount of the energy source, in the instant embodiment, is the remaining amount of the electric energy stored in the battery  23  detected by the remaining amount detection section  52 . Output instructions refer to electric power amounts to be supplied to the left motor  21  and the right motor  22  calculated based on operations on the left maneuvering lever  8   a  and the right maneuvering lever  8   b  by the user. Namely, if the electric power amounts to be supplied to the left motor  21  and the right motor  22  calculated based on operations on the left maneuvering lever  8   a  and the right maneuvering lever  8   b  by the user are large, output torques of the left motor  21  and the right motor  22  are large. As a result, the speed will increase if the road (ground) surface of the field is horizontal or down-sloped, whereby the speed of the work vehicle  1  will increase also. 
     However, with the present inventive work vehicle  1 , if the remaining amount of the electric energy stored in the battery  23  is equal to or smaller than the preset first threshold value, the restriction section  53  restricts (limits) the maximum speed at the time of traveling of the work vehicle  1  to a preset speed. One preferred example of such restriction of maximum speed is restriction of electric power amounts to be supplied to the left motor  21  and the right motor  22  to electric power amounts according to such preset speed. With this arrangement, irrespectively of user&#39;s operations on the left maneuvering lever  8   a  and the right maneuvering lever  8   b , the electric power amounts to be supplied to the left motor  21  and the right motor  22  will be reduced. As a result, the traveling speed of the machine body  4  does not become a speed according to the operations on the left maneuvering lever  8   a  and the right maneuvering lever  8   b . With this, feeling of “wrongness” can be given to the user. Thus, it is possible to cause the user to grasp (take notice of) the remaining amount of electric energy stored in the battery  23  having become equal to or smaller than the preset first threshold value. Incidentally, such first threshold value can be set to about 50% of the maximum electric energy amount that can be stored in the battery  23 , for example. Needless to say, it may be set to any other amount. 
       FIG. 3  is a view showing one example of maximum speed restriction of the work vehicle  1 . In  FIG. 3  (A), the horizontal axis is the time axis and the vertical axis represents the stored electric power amount of the battery  23 . In particular, in the case of this example of  FIG. 3  (A), there is shown a manner of the stored electric power amount progressively decreasing from the 100% state with lapse of time. Further,  FIG. 3  (B) shows maximum speeds at which the work vehicle  1  (machine body  4 ) can travel in the case of transition of the stored electric power amount that occurs as shown in  FIG. 3  (A). 
     The restriction section  53  can effect restriction to progressively decrease the maximum speed in accordance with diminishing of the remaining amount of the energy source in case the remaining amount of the energy source is equal to or smaller than the first threshold value. Here, the remaining amount of the energy source means the stored electric power amount of the battery  23 . The first threshold value is not particularly limited. But, here, it is defined as a stored electric power amount corresponding to 50% of the full charge of the battery  23 . This threshold value is denoted by a sign T 1  in  FIG. 3  (A). In case the stored electric power amount is greater than the first threshold value T 1 , as shown in  FIG. 3  (B), the maximum speed at which the work vehicle  1  can travel is a preset value V 1 . 
     When the stored electric power amount becomes equal to or smaller than the first threshold value T 1  in association with continued traveling of the work vehicle  1  (timing t 1 ), the restriction section  53 , as shown in  FIG. 3  (B), restricts the maximum speed travelable by the work vehicle  1  so that it may gradually decrease in accordance with the stored electric power amount (after timing: t 1 ). 
     The restriction section  53  is configured also to restrict the maximum speed to a preset fixed value, in case the remaining amount of the energy source is equal to or smaller than a second threshold value which is smaller than the first threshold value T 1 . This second threshold value is not particularly limited. But, here, it is defined as a stored electric power amount corresponding to 20% of the full charge of the battery  23 . This threshold value is denoted by a sign T 2  in  FIG. 3  (A). If the stored electric power amount becomes equal or smaller than the second threshold value T 2 , then, as shown in  FIG. 3  (B), the maximum speed travelable by the work vehicle  1  will be restricted to a preset fixed value V 2  by the restriction section  53  (after timing: t 2 ). 
     In such case as above, the restriction section  53  may alternatively be configured to restrict electric current running in the motor  20  according the stored electric power amount of the battery  23 . The motor  20  inputs rotational forces to the driving wheels  3 . Thus, the electric currents will be restricted. As output torques of the motor  20  are restricted, the maximum speed of the work vehicle  1  can be restricted also. 
     As described above, as the restriction section  53  restricts the maximum speed of the work vehicle  1  at the time of its traveling irrespectively of user&#39;s operation of the maneuvering lever  8 , it is possible to give feeling of wrongness to the user, thus making the user grasp diminishing of the remaining amount of the battery  23  in a reliable manner. 
     Here, as described above, the work vehicle  1  mounts the mower unit  13  for carrying out a grass (lawn) cutting work. This mower unit  13  is driven by using the electric energy stored in the battery  23  (an example of the “energy source”). 
     Preferably, the stopping section  54  is configured to stop driving of the mower unit  13 , if the remaining amount of the electric energy is equal to or smaller than a preset third threshold value. Though not particularly limited, in the case of the example shown in  FIG. 3 , such third threshold value is equal to the first threshold value T 1 . In  FIG. 3  (A), the third threshold value is denoted by a sign T 3 . If the stored electric power amount becomes equal to or lower than this third threshold value T 3 , as shown in  FIG. 3  (C), the stopping section  54  stops operation of the mower unit  13  (after timing t 1 ). 
     With the above-described configuration, in association with diminishing of the stored electric power amount of the battery  23 , the mower unit  13  is stopped. Thus, it becomes possible to give feeling of wrongness to the user, thereby to make this user grasp diminishing of the remaining amount of the battery  23 . 
     Other Embodiments 
     In the foregoing embodiment, the explanation was made with citing an electric grass cutting machine as an example of the work vehicle  1 . However, the work vehicle  1  may be a tractor for carrying out a plowing work in a field for example, or may be a rice planting machine also. Still further, it may be a combine for carrying out a culm harvesting work in a field or may be a direct seed sowing machine for sowing seeds. Still further, it may be a riding management machine for carrying out e.g. a chemical agent spraying work, etc. Namely, specific examples of the work vehicle  1  include a grass cutting machine, a tractor, a rice planting machine, a combine, a direct seed sowing machine, a riding management machine, etc. Needless to say, the present invention is applicable also to a work vehicle  1  used for carrying out works different from the above in a field. Moreover, the present invention is applicable not only to a riding work vehicle  1 , but also to a work vehicle  1  that can be remotely controlled. 
     In the foregoing embodiment, it was explained that the restriction section  53  restricts the maximum speed such that the maximum speed is progressively decreased in accordance with diminishing of the remaining amount of the energy source in case the remaining amount of the energy source is equal to or less than the first threshold value T 1 . However, the restriction section  53  may alternatively be configured such that the maximum speed is restricted to a preset fixed value in case the remaining amount of the energy source is equal to or less than the first threshold value T 1 . 
     In the foregoing embodiment, it was explained that the restriction section  53  restricts the maximum speed to a preset fixed value in case the remaining amount of the energy source is equal to or smaller than the second threshold value T 2  which is smaller than the first threshold value T 1 . However, the restriction section  53  may be configured to restrict the maximum speed in such a manner that the maximum speed becomes progressively smaller even in case the remaining amount of the energy source is equal to or smaller than the second threshold value T 2  which is smaller than the first threshold value T 1 . 
     In the foregoing embodiment, it was explained that the stopping section  54  stops the driving of the mower unit  13  in case the remaining amount of the energy source is equal to or smaller than the preset third threshold value T 3 . However, the stopping section  54  may be configured not to stop driving of the mower unit  13  even in case the remaining amount of the energy source is equal to or smaller than the preset third threshold value T 3 . Further, the third threshold value T 3  may be a different (may be greater or smaller) value from the first threshold value T 1 . 
     In the foregoing embodiment, it was explained that the power source is the motor  20  and the reservoir section is the battery  23 . However, the power source may be an engine, not a motor. In such case, a fuel tank will correspond to the “reservoir section” relating to the present invention. In this case, preferably, the remaining amount detection section  52  will be configured to detect a remaining amount of fuel. Further, preferably, the restriction section  53  may be configured to restrict the output of the engine. 
     In the foregoing embodiment, it was explained that the restriction section  53 , when restricting the maximum speed, restricts electric current running in the motor  20  in accordance with a stored electric power amount of the battery  23 . However, the restriction section  53  may alternatively be configured to restrict the maximum speed by some other method. 
     In the foregoing embodiment, it was explained that the restriction section  53  restricts the maximum speed of the work vehicle  1  in accordance with a remaining amount of the energy source and the stopping section  54  stops driving of the mower unit  13  in accordance with the remaining amount of the energy source. However, it may alternatively be possible to arrange such that reporting indicative of restriction of the maximum speed being carried out is made during restriction by the restriction section  53 . Further, it may be arranged that reporting indicative of stopping being carried out by the stopping section  54  is made during stopping of the driving of the mower unit  13 . 
     In the foregoing embodiment, it was explained that the restriction section  53  restricts the maximum speed travelable by the work vehicle  1  as illustrated in  FIG. 3 . However, it may alternatively be arranged such that a change amount of the maximum speed is calculated by a following calculation formula in case e.g. the remaining amount detection section  52  detects SOC and the restriction section  53  restricts the maximum speed based on the calculated change amount of the maximum speed. 
       vehicle speed upper limit change amount= V 1−( V 1− V 2)/( T 1− T 2)×( T 1−current SOC).
 
     where, V 1 , V 2 , T 1  and T 2  have values shown in  FIG. 3 . 
     Further, the restriction section  53  may alternatively be configured such that in place of the above-described calculation formula, it stores in advance a map which defines relationship between remaining amounts of energy source and the maximum speeds and it restricts the maximum speed based on this map. 
     INDUSTRIAL APPLICABILITY 
     The present invention is applicable to a work vehicle which travels in a field while carrying out a utility work. 
     DESCRIPTION OF REFERENCE SIGNS 
     
         
         
           
               1 : work vehicle 
               2 : traveling control device (output instruction acquisition section) 
               3 : driving wheel 
               4 : machine body 
               6 : traveling control unit 
               8 : maneuvering lever (operation input unit) 
               13 : mower unit (utility work unit) 
               20 : motor (power source) 
               23 : battery (reservoir section) 
               52 : remaining amount detection section 
               53 : restriction section 
               54 : stopping section 
             T 1 : first threshold value 
             T 2 : second threshold value 
             T 3 : third threshold value