Patent Publication Number: US-2022219551-A1

Title: Article transport system

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to Japanese Patent Application No. 2021-002843 filed Jan. 12, 2021, the disclosure of which is hereby incorporated by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an article transport system provided with a plurality of transport bodies on each of which an article is placeable or each of which is configured to house the article, a plurality of unmanned transport vehicles for transporting the transport bodies, and a control unit for controlling the transport bodies and the unmanned transport vehicles. 
     2. Description of the Related Art 
     An example of such an article transport system is described in JP 2014-117067 (Patent Document 1). In the following description of the related art, the reference signs and names used in Patent Document 1 are put in parentheses. 
     The article transport system of Patent Document 1 is provided with an unmanned transport vehicle (10) including a first electrical energy storage device (vehicle side battery 14) and a transport body (pallet 20) including a second electrical energy storage device (pallet side battery 24). Also, the transport body (20) is provided with a power supply unit capable of supplying electric power of the second electrical energy storage device (24) to an external component, and the unmanned transport vehicle (10) is provided with a power receiving unit capable of receiving electric power from the power supply unit of the transport body (20). Also, when the transport body (20) is placed on the unmanned transport vehicle (10), the unmanned transport vehicle (10) runs on the electric power from the second electrical energy storage device (24) and charges the first electrical energy storage device (14), and when the transport body (20) is not placed on the unmanned transport vehicle (10), the unmanned transport vehicle (10) runs on the electric power from the first electrical energy storage device (14). The first electrical energy storage device (14) and the second electrical energy storage device (24) are charged by a charging device (charger 40) installed at a station (ST) where the unmanned transport vehicle (10) picks up or puts down the transport body (20). 
     SUMMARY OF THE INVENTION 
     However, in the article transport system of Patent Document 1, depending on the overall layout of the system, for example, there may be too many transport bodies in actual operation relative to the number of charging devices. In such cases, the charge amount of the second electrical energy storage device of a transport body may be insufficient, resulting in a sufficient amount of electric power being unable to be supplied to the unmanned transport vehicle. If such a situation occurs, the operation efficiency of the unmanned transport vehicles is decreased. Also, such a situation is likely to occur in a busy period when there is a large number of transport bodies to be transported by the unmanned transport vehicles. On the other hand, by installing many charging devices to accommodate for the busy period, a charging device excess is created in the slack periods. This increases the cost of the article transport system and constricts the space of the facility where the article transport system is installed due to the space taken up by the charging devices. 
     Thus, there is a need for an article transport system capable of minimizing or preventing a decrease in the operation efficiency of an unmanned transport vehicle in a busy period without having an excessive number of charging devices. 
     An article transport system according to the present disclosure includes: a plurality of transport bodies on each of which an article is placeable or each of which is configured to house the article; a plurality of unmanned transport vehicles each configured to transport the transport body; and a control unit configured to control the transport bodies and the unmanned transport vehicles, wherein each of the unmanned transport vehicles: includes a first electrical energy storage device; a travel section configured to travel on electric power from the first electrical energy storage device, a holding section configured to hold the transport bodies; a first power receiving unit configured to receive electric power from an external power source and at least either charge the first electrical energy storage device or supply electric power to the travel section; and a first power transmitting unit configured to transmit electric power of the first electrical energy storage device to an external component; each of the transport bodies includes: a second electrical energy storage device; a second power receiving unit configured to receive electric power from an external power source and charge the second electrical energy storage device; and a second power transmitting unit configured to transmit electric power of the second electrical energy storage device to an external component; and the control unit is configured to selectively execute a first control of, while the unmanned transport vehicle is holding the transport body on the holding section, receiving electric power from the second electrical energy storage device via the second power transmitting unit and the first power receiving unit and at least either charging the first electrical energy storage device or supplying electric power to the travel section and a second control of, while the unmanned transport vehicle is holding the transport body on the holding section, receiving electric power from the first electrical energy storage device via the first power transmitting unit and the second power receiving unit and charging the second electrical energy storage device. 
     According to the present configuration, the first control, in which electric power from the second electrical energy storage device of the transport body is supplied to at least one of the first electrical energy storage device of the unmanned transport vehicle and the travel section, and the second control, in which electric power from the first electrical energy storage device of the unmanned transport vehicle is supplied to the second electrical energy storage device of the transport body, can be selectively executed. Thus, the balance in the charge amount between the first electrical energy storage devices of the plurality of unmanned transport vehicles and the second electrical energy storage devices of the plurality of transport bodies in the article transport system overall can be constantly appropriately adjusted. Accordingly, for example, in the slack period, supply of electric power from the first electrical energy storage device of the unmanned transport vehicle not assigned with a transportation task to the second electrical energy storage device of the transport body can be actively performed. Thus, the number of times the transport bodies are transported to the charging devices can be kept to a minimum, and the charge amount of the second electrical energy storage devices of the transport bodies can be quickly increased. Also, in the busy period, supply of electric power from the second electrical energy storage device of the transport body with a large charge amount to the first electrical energy storage device of the unmanned transport vehicle can be actively performed. Thus, the number of times the unmanned transport vehicles are moved to the charging devices can be kept to a minimum, and many articles and transport bodies can be transported. In this manner, a decrease in the operation efficiency of an unmanned transport vehicle in a busy period can be minimized or prevented without having an excessive number of charging devices. 
     Advantages and effects of the article transport system will be made clear from the embodiments described below with reference to the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic plan view of an article transport facility using an article transport system. 
         FIG. 2  is a control block diagram. 
         FIG. 3  is an explanatory diagram of the transportation operation of transport bodies by unmanned transport vehicles. 
         FIG. 4  is an explanatory diagram illustrating a transportation operation of transport bodies by unmanned transport vehicles in a time series. 
         FIG. 5  is a side view of an unmanned transport vehicle and a transport body in a non-held state according to a first embodiment. 
         FIG. 6  is a side view of the unmanned transport vehicle and the transport body in a held state according to the first embodiment. 
         FIG. 7  is a perspective view illustrating a portion of the unmanned transport vehicle and the transport body according to the first embodiment. 
         FIG. 8  is a side view of the unmanned transport vehicle, the transport body, and a charging device according to the first embodiment. 
         FIG. 9  is a side view of an unmanned transport vehicle and a transport body in a non-held state according to a second embodiment. 
         FIG. 10  is a side view of the unmanned transport vehicle and the transport body in a held state according to the second embodiment. 
         FIG. 11  is a perspective view illustrating the unmanned transport vehicle and the transport body according to the second embodiment. 
         FIG. 12  is a side view of the unmanned transport vehicle, the transport body, and a charging device according to the second embodiment. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     First Embodiment 
     An article transport system according to the first embodiment will be described with reference to the drawings ( FIGS. 1 to 8 ). 
     An article transport system  1  is a system for transporting articles  2  (see  FIG. 5 ). The article transport system  1  is used in an article transport facility such as the example illustrated in  FIG. 1 . As illustrated in  FIG. 1 , the article transport system  1  is provided with a plurality of transport bodies  20  on each of which an article  2  (see  FIG. 5 ) is placeable or each of which is configured to house the article  2 , and a plurality of unmanned transport vehicles  10  each configured to transport the transport body  20 . The article transport system  1  is further provided with charging devices  60  for charging the unmanned transport vehicles  10  (specifically, a first electrical energy storage device  11  described below) and the transport bodies  20  (specifically, a second electrical energy storage device  21  described below). In the present embodiment, the articles  2  are articles housing a load (not illustrated) such as a product. The articles  2  may be containers, corrugated cardboard boxes, or the like, for example. 
     As illustrated in  FIG. 5 , the unmanned transport vehicle  10  is provided with a travel section  14  and a holding section  15  for holding the transport body  20 . The unmanned transport vehicle  10  is a transport vehicle capable of autonomous travel. In the present embodiment, the travel section  14  is provided with wheels that roll on a floor  4 , and the unmanned transport vehicle  10  autonomously travels along the floor  4 . The unmanned transport vehicle  10  drives the travel section  14  by rotating the wheels via drive power from a drive power source such as an electric motor. The movement operation of the unmanned transport vehicles  10  is caused by the travel operation of the travel section  14 . 
     In the present embodiment, the holding section  15  holds the transport body  20  by supporting the transport body  20  from below. Specifically, the unmanned transport vehicle  10  (specifically, the holding section  15 ) is provided with a placement section  16  on which the transport body  20  is placeable. A placed section  24 , described below, of the transport body  20 , is placed on the placement section  16 . The transport body  20  is placed on the placement section  16  while being held by the holding section  15 . In the present embodiment, the holding section  15  is supported by the travel section  14  in a manner allowing it to move vertically. The unmanned transport vehicle  10  raises and lowers the holding section  15  via drive power from a drive power source such as an electric motor. 
     The unmanned transport vehicles  10  holds the transport body  20  via the holding section  15  by, after moving under the transport body  20  (see  FIG. 5 ) with the holding section  15  in a lowered state, raising the holding section  15  and raising the transport body  20  off the floor  4  (see  FIG. 6 ). The unmanned transport vehicle  10  transports the transport body  20  by moving with the transport body  20  held by the holding section  15  (in other words, with the transport body  20  placed on the placement section  16 ). Then, the unmanned transport vehicle  10 , after moving to a position of a transportation destination D, lowers the holding section  15  and lowers the transport body  20  to the floor  4  (see  FIG. 5 ). In this manner, in the present embodiment, the holding operation of the transport body  20  by the holding section  15  is performed via a raising operation of the holding section  15 , and the hold releasing operation of the transport body  20  by the holding section  15  is performed via a lowering operation of the holding section  15 . 
     As illustrated in  FIGS. 2 and 5 , the unmanned transport vehicle  10  is provided with the first electrical energy storage device  11 , a first power receiving unit  12 , and a first power transmitting unit  13 . In the present embodiment, the first power receiving unit  12  and the first power transmitting unit  13  are disposed (see  FIG. 5 ) on the holding section  15  (specifically, the placement section  16 ). The first electrical energy storage device  11  is electrically connected to the travel section  14 . Note that “electrically connected” includes in its meaning both being connected via a wired connection or a wireless (i.e., non-contact) connection. In the present embodiment, the first electrical energy storage device  11  is disposed on the travel section  14  (see  FIG. 5 ). The travel section  14  receives a supply of electric power from the first electrical energy storage device  11  to travel. The drive power source (for example, an electric motor) of the travel section  14  generates the travel drive power of the travel section  14  via electric power supplied from the first electrical energy storage device  11 . The travel section  14  may also receive a supply of electric power from the first power receiving unit  12  to travel. The first electrical energy storage device  11  is an electrical energy storage device capable of charging and discharging and is a battery or capacitor, for example. A sensor for detecting the charge amount is provided on the first electrical energy storage device  11 . 
     The first power receiving unit  12  receives electric power from an external (external to the unmanned transport vehicle  10 ) power source and at least either charge the first electrical energy storage device  11  or supply electric power to the travel section  14 . In the present embodiment, the electric power received by the first power receiving unit  12  can be directly supplied to the travel section  14  bypassing the first electrical energy storage device  11 . The first power receiving unit  12  (specifically, a power receiving circuit of the first power receiving unit  12 ) is electrically connected to the first electrical energy storage device  11  and electrically connected to the travel section  14 . The first power receiving unit  12  is provided with a first power receiving connection unit  12   a  (see  FIG. 7 ) for receiving a supply of electric power from an external power source and supplies the electric power received at the first power receiving connection unit  12   a  to the supply destination (the first electrical energy storage device  11  or the travel section  14 ). In the present embodiment, the first power receiving connection unit  12   a  is disposed in the holding section  15  (specifically, the placement section  16 ). The power receiving operation by the first power receiving unit  12  (specifically the operation of the power receiving circuit electrically connected to the first power receiving connection unit  12   a ) is controlled by a controller (power receiving controller) provided in the unmanned transport vehicle  10 . The power receiving operation by the first power receiving unit  12  includes an operation of charging the first electrical energy storage device  11  via electric power received by the first power receiving unit  12  and an operation of supplying the travel section  14  with electric power received by the first power receiving unit  12 , and the charging operation and the supplying operation can be performed in parallel. 
     Supplying electric power from the external power source (for example, the transport body  20  or the charging device  60 ) to the first power receiving unit  12  may be performed via an electromagnetic induction method or a magnetic field resonance method via a non-contact method (wireless method) or a contact method (wired method), for example. In a case where the electric power is supplied from the external power source to the first power receiving unit  12  using a non-contact method, the first power receiving connection unit  12   a  is provided with a non-contact connection unit (for example, a power receiving coil), and electric power is supplied with the non-contact connection unit and a non-contact connection unit (for example, a power transmitting coil) of the external power source being electrically connected to one another via a non-contact method. In a case where the electric power is supplied from the external power source to the first power receiving unit  12  using a contact method, the first power receiving connection unit  12   a  is provided with a contact connection unit (for example, a power receiving terminal), and electric power is supplied with the contact connection unit and a contact connection unit (for example, a power transmitting terminal) of the external power source being electrically connected to one another via contact. In another possible configuration, the first power receiving connection unit  12   a  is provided with both a non-contact connection unit and a contact connection unit, and electric power can be supplied from the external power source to the first power receiving unit  12  via either a non-contact method or a contact method. A possible configuration includes electric power being supplied from the transport body  20  (specifically, the second electrical energy storage device  21  described below) to the first power receiving unit  12  via a non-contact method and electric power being supplied from the charging device  60  to the first power receiving unit  12  via a contact method. 
     The first power transmitting unit  13  transmits the electric power of the first electrical energy storage device  11  to an external component. The first power transmitting unit  13  (specifically, a power transmitting circuit of the first power transmitting unit  13 ) is electrically connected to the first electrical energy storage device  11 . The first power transmitting unit  13  is provided with a first power transmitting connection unit  13   a  (see  FIG. 7 ) for supplying electric power to an external component and supplies the electric power of the first electrical energy storage device  11  from the first power transmitting connection unit  13   a  to an external component. In the present embodiment, the first power transmitting connection unit  13   a  is disposed in the holding section  15  (specifically, the placement section  16 ). The power transmitting operation by the first power transmitting unit  13  (specifically the operation of the power transmitting circuit electrically connected to the first power transmitting connection unit  13   a ) is controlled by a controller (power transmitting controller) provided in the unmanned transport vehicle  10 . 
     Supplying the electric power from the first power transmitting unit  13  to the external supply target (for example, the transport body  20 ) may be performed by a non-contact method or a contact method. In a case where the electric power is supplied from the first power transmitting unit  13  to the external supply target using a non-contact method, the first power transmitting connection unit  13   a  is provided with a non-contact connection unit (for example, a power transmitting coil), and electric power is supplied with the non-contact connection unit and a non-contact connection unit (for example, a power receiving coil) of the external supply target being electrically connected to one another via a non-contact method. In a case where the electric power is supplied from the first power transmitting unit  13  to the external supply target using a contact method, the first power transmitting connection unit  13   a  is provided with a contact connection unit (for example, a power transmitting terminal), and electric power is supplied with the contact connection unit and a contact connection unit (for example, a power receiving terminal) of the external supply target being electrically connected to one another via contact. In another possible configuration, the first power transmitting connection unit  13   a  is provided with both a non-contact connection unit and a contact connection unit, and electric power can be supplied from the first power transmitting unit  13  to the external supply target via either a non-contact method or a contact method. 
     In the present embodiment, as illustrated in  FIG. 7 , the unmanned transport vehicle  10  is provided with a first power transmitting and receiving unit  19  that functions as both the first power receiving unit  12  and the first power transmitting unit  13 , and the first power transmitting and receiving unit  19  is provided with a first power transmitting and receiving connection unit  19   a  that functions as both the first power receiving connection unit  12   a  and the first power transmitting connection unit  13   a . The first power transmitting and receiving connection unit  19   a  is provided with at least one of a non-contact connection unit (for example, a power transmitting and receiving coil) and a contact connection unit (for example, a power transmitting and receiving terminal). The first power transmitting and receiving unit  19  is provided with a power transmitting and receiving circuit capable of operating as either a power receiving circuit or a power transmitting circuit; and when the power transmitting and receiving circuit operates as a power receiving circuit, the first power transmitting and receiving unit  19  functions as the first power receiving unit  12 , and when the power transmitting and receiving circuit operates as a power transmitting circuit, the first power transmitting and receiving unit  19  functions as the first power transmitting unit  13 . The power transmitting and receiving operation by the first power transmitting and receiving unit  19  (specifically the operation of the power transmitting and receiving circuit electrically connected to the first power transmitting and receiving connection unit  19   a ) is controlled by a controller (power transmitting and receiving controller) provided in the unmanned transport vehicle  10 . The power transmitting and receiving operation by the first power transmitting and receiving unit  19  includes the power receiving operation by the first power receiving unit  12  and the power transmitting operation by the first power transmitting unit  13 . 
     As illustrated in  FIG. 6 , in the present embodiment, the transport body  20  is provided with the placed section  24  which is placed on the placement section  16  of the unmanned transport vehicle  10 . In the present embodiment, the transport body  20  is a housing shelf  30  for housing the articles  2 . The housing shelf  30  is provided with a housing section  31  where the articles  2  are housed. The housing shelf  30  is provided with a shelf  32  where the articles  2  are placed, with the shelf  32  forming the housing section  31 . In this example, the housing shelf  30  is provided with the shelves  32  at a plurality of positions (specifically, two positions) in a vertical direction V (an up-down direction), and a plurality of the housing sections  31  (specifically, two housing sections  31 ) are formed side by side in the vertical direction V. In this manner, in the present embodiment, the transport body  20  (housing shelf  30 ) is provided with the housing sections  31  arranged next to one another in the vertical direction V, allowing the articles  2  to be housed in the housing sections  31  at a plurality of levels. 
     As illustrated in  FIG. 5 , in the present embodiment, the housing shelf  30  is provided with leg portions  33  extending downward from the placed section  24 , and the housing shelf  30  is placed on the floor  4  by the leg portions  33  coming into contact with the floor  4 . The length of the leg portions  33  in the vertical direction V is set so as to form a space between the placed section  24  and the floor  4  into which the unmanned transport vehicle  10  can enter when the housing shelf  30  is placed on the floor  4 . 
     As illustrated in  FIGS. 2 and 5 , the transport body  20  is provided with the second electrical energy storage device  21 , a second power receiving unit  22 , and a second power transmitting unit  23 . The second electrical energy storage device  21  is an electrical energy storage device capable of charging and discharging and is a battery or capacitor, for example. A sensor for detecting the charge amount is provided on the second electrical energy storage device  21 . The second electrical energy storage device  21 , for example, is an electrical energy storage device compatible with the capacity of the first electrical energy storage device  11 . 
     As illustrated in  FIG. 5 , in the present embodiment, the second power receiving unit  22  and the second power transmitting unit  23  are below a lowermost one of the housing sections  31  of the transport body  20 . In other words, the second power receiving unit  22  and the second power transmitting unit  23  are arranged in a position which is lower than the lowest housing section  31  of the transport body  20  and where the second power receiving unit  22  and the second power transmitting unit  23  overlap the housing sections  31  as seen in a plan view (as seen in a direction aligned with the vertical direction V). In this example, the transport body  20  is provided with a power storage unit  25  including the second electrical energy storage device  21 , the second power receiving unit  22 , and the second power transmitting unit  23  integrating into one unit. Also, the power storage unit  25  is disposed below the lowest housing section  31  in the transport body  20 . 
     The second power receiving unit  22  receives electric power from an external (external to the transport body  20 ) power source and charges the second electrical energy storage device  21 . The second power receiving unit  22  (specifically, a power receiving circuit of the second power receiving unit  22 ) is electrically connected to the second electrical energy storage device  21 . The second power receiving unit  22  is provided with a second power receiving connection unit  22   a  (see  FIG. 7 ) for receiving a supply of electric power from an external power source and supplies the electric power received at the second power receiving connection unit  22   a  to the second electrical energy storage device  21 . In the present embodiment, the second power receiving connection unit  22   a  is disposed in the placed section  24 . In the example illustrated in  FIG. 5 , the placed section  24  and the power storage unit  25  are distinct components, but the placed section  24  may be a part of the power storage unit  25  or, as in the example illustrated in  FIG. 7 , the lower portion of the power storage unit  25  may correspond to the placed section  24 . The power receiving operation by the second power receiving unit  22  (specifically the operation of the power receiving circuit electrically connected to the second power receiving connection unit  22   a ) is controlled by a controller (power receiving controller) provided in the transport body  20 . 
     Supplying the electric power from the external power source (for example, the unmanned transport vehicles  10  or the charging device  60 ) to the second power receiving unit  22  may be performed via a non-contact method or a contact method. In a case where the electric power is supplied from the external power source to the second power receiving unit  22  using a non-contact method, the second power receiving connection unit  22   a  is provided with a non-contact connection unit (for example, a power receiving coil), and electric power is supplied with the non-contact connection unit and a non-contact connection unit (for example, a power transmitting coil) of the external power source being electrically connected to one another via a non-contact method. In a case where the electric power is supplied from the external power source to the second power receiving unit  22  using a contact method, the second power receiving connection unit  22   a  is provided with a contact connection unit (for example, a power receiving terminal), and electric power is supplied with the contact connection unit and a contact connection unit (for example, a power transmitting terminal) of the external power source being electrically connected to one another via contact. In another possible configuration, the second power receiving connection unit  22   a  is provided with both a non-contact connection unit and a contact connection unit, and electric power can be supplied from the external power source to the second power receiving unit  22  via either a non-contact method or a contact method. A possible configuration includes electric power being supplied from the unmanned transport vehicles  10  (specifically, the first electrical energy storage device  11 ) to the second power receiving unit  22  via a non-contact method and electric power being supplied from the charging device  60  to the second power receiving unit  22  via a contact method. 
     The second power transmitting unit  23  transmits the electric power of the second electrical energy storage device  21  to an external component. The second power transmitting unit  23  (specifically, the power transmitting circuit of the second power transmitting unit  23 ) is electrically connected to the second electrical energy storage device  21 . The second power transmitting unit  23  is provided with a second power transmitting connection unit  23   a  (see  FIG. 7 ) for supplying electric power to an external component and supplies the electric power of the second electrical energy storage device  21  from the second power transmitting connection unit  23   a  to an external component. In the present embodiment, the second power transmitting connection unit  23   a  is disposed in the placed section  24 . The power transmitting operation by the second power transmitting unit  23  (specifically the operation of the power transmitting circuit electrically connected to the second power transmitting connection unit  23   a ) is controlled by a controller (power transmitting controller) provided in the transport body  20 . 
     Supplying the electric power from the second power transmitting unit  23  to the external supply target (for example, the unmanned transport vehicle  10 ) may be performed by a non-contact method or a contact method. In a case where the electric power is supplied from the second power transmitting unit  23  to the external supply target using a non-contact method, the second power transmitting connection unit  23   a  is provided with a non-contact connection unit (for example, a power transmitting coil), and electric power is supplied with the non-contact connection unit and a non-contact connection unit (for example, a power receiving coil) of the external supply target being electrically connected to one another via a non-contact method. In a case where the electric power is supplied from the second power transmitting unit  23  to the external supply target using a contact method, the second power transmitting connection unit  23   a  is provided with a contact connection unit (for example, a power transmitting terminal), and electric power is supplied with the contact connection unit and a contact connection unit (for example, a power receiving terminal) of the external supply target being electrically connected to one another via contact. In another possible configuration, the second power transmitting connection unit  23   a  is provided with both a non-contact connection unit and a contact connection unit, and electric power can be supplied from the second power transmitting unit  23  to the external supply target via either a non-contact method or a contact method. 
     In the present embodiment, as illustrated in  FIG. 7 , the transport body  20  is provided with a second power transmitting and receiving unit  29  that functions as both the second power receiving unit  22  and the second power transmitting unit  23 , and the second power transmitting and receiving unit  29  is provided with a second power transmitting and receiving connection unit  29   a  that functions as both the second power receiving connection unit  22   a  and the second power transmitting connection unit  23   a . The second power transmitting and receiving connection unit  29   a  is provided with at least one of a non-contact connection unit (for example, a power transmitting and receiving coil) and a contact connection unit (for example, a power transmitting and receiving terminal). The second power transmitting and receiving unit  29  is provided with a power transmitting and receiving circuit capable of operating as either a power receiving circuit or a power transmitting circuit; and when the power transmitting and receiving circuit operates as a power receiving circuit, the second power transmitting and receiving unit  29  functions as the second power receiving unit  22 , and when the power transmitting and receiving circuit operates as a power transmitting circuit, the second power transmitting and receiving unit  29  functions as the second power transmitting unit  23 . The power transmitting and receiving operation by the second power transmitting and receiving unit  29  (specifically the operation of the power transmitting and receiving circuit electrically connected to the second power transmitting and receiving connection unit  29   a ) is controlled by a controller (power transmitting and receiving controller) provided in the transport body  20 . The power transmitting and receiving operation by the second power transmitting and receiving unit  29  includes the power receiving operation by the second power receiving unit  22  and the power transmitting operation by the second power transmitting unit  23 . 
     In the article transport system  1 , electric power can be supplied in both directions between the unmanned transport vehicle  10  and the transport body  20 . As described above, the first power receiving unit  12  is provided with the first power receiving connection unit  12   a , the first power transmitting unit  13  is provided with the first power transmitting connection unit  13   a , the second power receiving unit  22  is provided with the second power receiving connection unit  22   a , and the second power transmitting unit  23  is provided with the second power transmitting connection unit  23   a . Also, when electric power is supplied from the transport body  20  to the unmanned transport vehicle  10 , the first power receiving connection unit  12   a  and the second power transmitting connection unit  23   a  are electrically connected, and when electric power is supplied from the unmanned transport vehicle  10  to the transport body  20 , the first power transmitting connection unit  13   a  is electrically connected to the second power receiving connection unit  22   a . In this manner, the first power receiving connection unit  12   a  is a connection unit that is electrically connected to the second power transmitting unit  23 , the first power transmitting connection unit  13   a  is a connection unit that is electrically connected to the second power receiving unit  22 , the second power receiving connection unit  22   a  is a connection unit that is electrically connected to the first power transmitting unit  13 , and the second power transmitting connection unit  23   a  is a connection unit that is electrically connected to the first power receiving unit  12 . Herein, “connected” means being connected when electric power is transmitted between the unmanned transport vehicle  10  and the transport body  20 . 
     Here, the state (see  FIG. 6 ) where the transport body  20  is supported on the holding section  15  of the unmanned transport vehicle  10  is defined as the held state. The held state includes in its meaning both a state when the unmanned transport vehicle  10  is traveling and a state when the unmanned transport vehicle  10  is stopped. In the held state, the first power receiving connection unit  12   a  and the second power transmitting connection unit  23   a  are electrically connected and the first power transmitting connection unit  13   a  and the second power receiving connection unit  22   a  are electrically connected. Thus, in the held state, electric power can be transmitted in both directions between the unmanned transport vehicle  10  and the transport body  20 . In the present embodiment, electric power is transmitted between the unmanned transport vehicle  10  and the transport body  20  via a non-contact method. Thus, even in a state (see  FIG. 5 ) where the unmanned transport vehicle  10  is not holding the transport body  20  on the holding section  15 , specifically, a state where the placement section  16  is disposed opposite the placed section  24  with a gap therebetween, the first power receiving connection unit  12   a  and the second power transmitting connection unit  23   a  are electrically connected, the first power transmitting connection unit  13   a  and the second power receiving connection unit  22   a  are electrically connected, and electric power can be transmitted in both directions between the unmanned transport vehicle  10  and the transport body  20 . 
     In the present embodiment, as schematically illustrated in  FIG. 7 , the first power transmitting and receiving connection unit  19   a  and the second power transmitting and receiving connection unit  29   a  are each provided with a power transmitting and receiving coil as a non-contact connection unit, and electric power is transmitted between the unmanned transport vehicle  10  and the transport body  20  via a non-contact method with the power transmitting and receiving coil of the first power transmitting and receiving connection unit  19   a  and the power transmitting and receiving coil of the second power transmitting and receiving connection unit  29   a  being disposed opposite one another (specifically, opposite one another in the vertical direction V). In the example illustrated in  FIG. 7 , the first power transmitting and receiving connection unit  19   a  and the second power transmitting and receiving connection unit  29   a  are each provided with a plurality (specifically, four) of power transmitting and receiving coils, and the plurality of power transmitting and receiving coils of the first power transmitting and receiving connection unit  19   a  are each disposed opposite to the corresponding power transmitting and receiving coils of the plurality of power transmitting and receiving coils of the second power transmitting and receiving connection unit  29   a.    
     In the example illustrated in  FIG. 7 , an identification information carrier  34  for holding identification information of the transport body  20  is provided in a central portion of the placed section  24  as seen in a plan view, and a reading device  17  for reading the identification information held by the identification information carrier  34  is provided in a central portion of the placement section  16  as seen in a plan view. The identification information carrier  34 , for example, is a one-dimensional code such as a barcode, a two-dimensional code such as a QR code (registered trademark), or a storage medium such as a radio frequency (RF) tag or the like. Also, the power transmitting and receiving coil of the first power transmitting and receiving connection unit  19   a  is provided outside of the region where the reading device  17  is disposed so that the reading device  17  is not disposed within the power transmitting and receiving coil as seen in a plan view, and the power transmitting and receiving coil of the second power transmitting and receiving connection unit  29   a  is provided outside of the region where the identification information carrier  34  is disposed so that the identification information carrier  34  is not disposed within the power transmitting and receiving coil as seen in a plan view. Accordingly, effects of the magnetic field from the power transmitting and receiving coil on the reading device  17  can be kept to a minimum. 
     Note that in cases where the effects of the magnetic field from the power transmitting and receiving coil on the reading device  17  can be minimized or prevented, such as a case where the reading device  17  does not move up and down together with the placement section  16  and is disposed lower than the power transmitting and receiving coil of the first power transmitting and receiving connection unit  19   a  when the placement section  16  is raised, the power transmitting and receiving coil of the first power transmitting and receiving connection unit  19   a  may be disposed so that the reading device  17  is not disposed within the power transmitting and receiving coil as seen in a plan view, and the power transmitting and receiving coil of the second power transmitting and receiving connection unit  29   a  may be disposed so that the identification information carrier  34  is not disposed within the power transmitting and receiving coil as seen in a plan view. 
     As illustrated in  FIG. 8 , the charging device  60  is provided with a power supply unit  61  for supplying electric power to the first power receiving unit  12  and the second power receiving unit  22 . Supplying the electric power from the power supply unit  61  to the first power receiving unit  12  and the second power receiving unit  22  may be performed via a non-contact method or a contact method. In a case where the electric power is supplied from the power supply unit  61  to the first power receiving unit  12  via a non-contact method, the first power receiving connection unit  12   a  is provided with a non-contact connection unit (for example, a power receiving coil), and in a case where the electric power is supplied from the power supply unit  61  to the first power receiving unit  12  via a contact method, the first power receiving connection unit  12   a  is provided with a contact connection unit (for example, a power receiving terminal). Also, in a case where electric power is supplied from the power supply unit  61  to the second power receiving unit  22  via a non-contact method, the second power receiving connection unit  22   a  is provided with a non-contact connection unit (for example, a power receiving coil), and in a case where electric power is supplied from the power supply unit  61  to the second power receiving unit  22  via a contact method, the second power receiving connection unit  22   a  is provided with a contact connection unit (for example, a power receiving terminal). 
     In the present embodiment, the charging device  60  is capable of simultaneously charging both the first electrical energy storage device  11  of the unmanned transport vehicle  10  and the second electrical energy storage device  21  of the transport body  20 . Specifically, as illustrated in  FIG. 8 , in the charging device  60 , the power supply unit  61  is able to be disposed between the placement section  16  and the placed section  24  in the vertical direction V. The power supply unit  61  is disposed between the placement section  16  and the placed section  24  in the vertical direction V when the PROFFERED power supply unit  61  is supported by a support portion  62  of the charging device  60 . Also, when the power supply unit  61  is disposed in this manner, the power supply unit  61  is electrically connected to both the first power receiving connection unit  12   a  and the second power receiving connection unit  22   a  and capable of simultaneously supplying electric power to the first power receiving unit  12  and the second power receiving unit  22 . 
       FIG. 8  is a diagram illustrating an example in which electric power is intended to be supplied from the power supply unit  61  to the first power receiving unit  12  and the second power receiving unit  22  via a non-contact method. Thus, though the details are omitted herein, in the example illustrated in  FIG. 8 , the power supply unit  61  is provided with a power transmitting coil disposed opposite the power receiving coil (in the present embodiment, a power transmitting and receiving coil) of the first power receiving connection unit  12   a  and the second power receiving connection unit  22   a . The power supply unit  61  may be separately provided with a power transmitting coil disposed opposite the power receiving coil (in the present embodiment, a power transmitting and receiving coil) of the first power receiving connection unit  12   a  and a power transmitting coil disposed opposite the power receiving coil (in the present embodiment, a power transmitting and receiving coil) of the second power receiving connection unit  22   a.    
     As illustrated in  FIG. 2 , the article transport system  1  is provided with a control unit  50  configured to control the unmanned transport vehicles  10  and the transport bodies  20 . The control unit  50  is communicatively connected to the unmanned transport vehicles  10  and the transport bodies  20 . The control unit  50  is provided with an arithmetic processing device such as a CPU and a peripheral circuit such as a memory, and the functions of the control unit  50  are implemented by cooperation of these pieces of hardware and a program executed on the hardware such as the arithmetic processing device.  FIG. 2  is a diagram illustrating an example in which the control unit  50  is provided separate from the unmanned transport vehicle  10  and the transport body  20 . However, at least a portion (for example, at least the portion of the control unit  50  with the function of controlling the unmanned transport vehicle  10 ) of the control unit  50  may be provided in the unmanned transport vehicle  10 , or at least a portion (for example, at least the portion of the control unit  50  with the function of controlling the transport body  20 ) of the control unit  50  may be provided in the transport body  20 . Note that the technology of the control unit  50  disclosed in the present specification can be applied to a method for controlling the unmanned transport vehicles  10  and the transport bodies  20  in the article transport system  1 , and a method for controlling the unmanned transport vehicles  10  and the transport bodies  20  is also disclosed in the present specification. 
     The unmanned transport vehicle  10  is controlled by the control unit  50  and performs the travel operation of the travel section  14 , the holding and hold releasing operation of the transport body  20  by the holding section  15  (in the present embodiment, the raising and lowering operation of the holding section  15 ), the power receiving operation by the first power receiving unit  12  and the power transmitting operation by the first power transmitting unit  13 . A controller (device controller) provided in the unmanned transport vehicle  10  controls the operations described above in response to commands from the control unit  50 . Also, the transport body  20  is controlled by the control unit  50  and performs the power receiving operation by the second power receiving unit  22  and the power transmitting operation by the second power transmitting unit  23 . A controller provided in the transport body  20  controls the operations described above in response to commands from the control unit  50 . 
     The control unit  50  manages the information of the current position of the unmanned transport vehicle  10  and the information of the charge amount of the first electrical energy storage device  11  for each of the plurality of unmanned transport vehicles  10 . Also, the control unit  50  manages the information of the current position of the transport body  20 , the information of the article  2  placed or housed in the transport body  20  (for example, information of the type or quantity of the load housed in the article  2 ), and the information of the charge amount of the second electrical energy storage device  21  for each of the plurality of transport bodies  20 . The charge amount of the first electrical energy storage device  11  and the second electrical energy storage device  21  is represented by a proportion (for example, a percentage) relative to the charge amount when fully charged, for example. Also, the control unit  50  controls the operations (specifically, the travel operation of the travel section  14  and the holding and hold releasing operation of the transport body  20  by the holding section  15 ) of the unmanned transport vehicle  10  in response to a transportation task so that the transport body  20  which is the target for transportation is transported to the transportation destination D. In this example, the transportation task is a command to be assigned to each of the plurality of unmanned transport vehicles  10  and specifying the transport body  20  to be transported and the transportation destination D of the transport body  20 . The transportation task is generated by the control unit  50  or generated by another control unit (for example, a higher level control unit in the control unit  50 ) capable of communicating with the control unit  50 . 
     In the present embodiment, in the article transport system to which the article transport system  1  is applied, picking is performed. Thus, in a transportation task, the transport body  20  housing the article  2  which is the picking target is specified as the transport body  20  which is the transportation target, and a work area A 2  (see  FIG. 1 ) where picking is performed is specified as the transportation destination D of the transport body  20 . Accordingly, the transport body  20  housing the article  2  which is the picking target is transported to the work area A 2 , and, in the work area A 2 , picking is performed to retrieve the type and quantity of the load specified by an order from the article  2 . The order is generated by the control unit  50  or another control unit on the basis of an order from shipment destination.  FIG. 1  is a diagram illustrating an example in which picking is performed by workers P. However, picking may be performed by robots or the like instead of workers P. 
     In the example illustrated in  FIG. 1 , in addition to the work area A 2 , there is also a storage area A 1  and a charging area A 3 . The transport bodies  20  that are not required to be transported to the work area A 2  are primarily placed in the storage area A 1 . The charging devices  60  are provided in the charging area A 3 . By transporting the transport body  20  to the charging area A 3 , the second electrical energy storage device  21  of the transport body  20  can be charged at the charging devices  60 , and by moving the unmanned transport vehicle  10  to the charging area A 3 , the first electrical energy storage device  11  of the unmanned transport vehicle  10  can be charged at the charging devices  60 . In the example illustrated in  FIG. 1 , the work area A 2  is also provided with the charging devices  60 , allowing for the second electrical energy storage device  21  of the transport body  20  housing the article  2  which is the picking target to be charged while picking is being performed. 
     In the article transport system  1 , at least in the held state, electric power can be transmitted in both directions between the unmanned transport vehicle  10  and the transport body  20 . Also, the control unit  50  selectively executes a first control and a second control. Herein, the first control is control of, in the held state, receiving electric power from the second electrical energy storage device  21  via the second power transmitting unit  23  and the first power receiving unit  12  and at least either charging the first electrical energy storage device  11  or supplying electric power to the travel section  14 . In the first control, the control unit  50  controls the power transmitting operation by the second power transmitting unit  23  and the power receiving operation by the first power receiving unit  12  so that, by receiving a supply of electric power from the second electrical energy storage device  21 , at least one of charging the first electrical energy storage device  11  and supplying electric power to the travel section  14  is performed. In the first control, in a case where both the first electrical energy storage device  11  is charged by electric power received from the second electrical energy storage device  21  and electric power received from the second electrical energy storage device  21  is supplied to the travel section  14 , the travel section  14  can be driven while the first electrical energy storage device  11  is being charged. 
     Herein, the second control is control of, in the held state, receiving a supply of electric power from the first electrical energy storage device  11  via the first power transmitting unit  13  and the second power receiving unit  22  and charging the second electrical energy storage device  21 . In the second control, the control unit  50  controls the power transmitting operation by the first power transmitting unit  13  and the power receiving operation by the second power receiving unit  22  so that a supply of electric power is received from the first electrical energy storage device  11  and charging of the second electrical energy storage device  21  is performed. For example, when the control unit  50  executes the second control, in accordance with a transportation task already assigned to the unmanned transport vehicle  10 , the electric power of the first electrical energy storage device  11  can be supplied from the first power transmitting unit  13  to the second power receiving unit  22  in such a manner that the first electrical energy storage device  11  keeps electric power necessary to execute the transportation task. 
     Next, specific examples of the control executed by the control unit  50  according to the present embodiment will be described below. However, the control unit  50  may have a configuration in which it cannot execute one or more of the controls described below. 
     In the present embodiment, the control unit  50  executes busy/slack determination process to determine based on at least the number of transportation tasks whether a current period is a busy period or a slack period. Herein, the “transportation task” may include in its meaning a transportation task already assigned to any one of the unmanned transport vehicles  10  or a transportation task is the determination phase for future allocation to any one of the unmanned transport vehicles  10 . For example, using the ratio of unmanned transport vehicles  10  with a transportation task assigned to the total number of unmanned transport vehicles  10  as a task allocation rate, a configuration can be used in which the control unit  50  determines that a current period is a slack period in a case where the task allocation rate is less than a predetermined busy/slack determination threshold and determines that the current period is a busy period in a case where the task allocation rate is equal to or greater than the busy/slack determination threshold. 
     Also, in a case where the control unit  50  determines through the busy/slack determination process that a current period is a slack period, the control unit  50  executes the second control more frequently than the first control. Accordingly, in the slack period, using the unmanned transport vehicle  10  not assigned with a transportation task, supply of electric power from the first electrical energy storage device  11  of the unmanned transport vehicle  10  to the second electrical energy storage device  21  of the transport body  20  can be actively performed. As a result, the number of times the transport bodies  20  are transported to the charging devices  60  can be kept to a minimum, and the charge amount of the second electrical energy storage devices  21  of the transport bodies  20  can be quickly increased. Also, in a case where the control unit  50  determines through the busy/slack determination process that the current period is a busy period, the control unit  50  executes the first control more frequently than the second control. Accordingly, in the busy period, using the transport body  20  with a large charge amount, supply of electric power from the second electrical energy storage device  21  of the transport body  20  to the first electrical energy storage device  11  of the unmanned transport vehicle  10  can be actively performed. As a result, the number of times the unmanned transport vehicles  10  are moved to the charging devices  60  can be kept to a minimum, and many articles  2  and transport bodies  20  can be transported. Accordingly, the article transport system  1  according to the present embodiment is particularly suited to cases in which the number of transport bodies  20  is larger than the number of unmanned transport vehicles  10 . 
     Herein, the transport body  20  which is specified as the transportation target by the transportation task is defined as the target transport body, and the unmanned transport vehicle  10  which transports the target transport body is defined as the target transport vehicle. In the present embodiment, in a case where the charge amount of the second electrical energy storage device  21  of the target transport body is equal to or smaller than a predetermined first charge threshold (in this example, in a case where the charge amount is equal to or smaller than the first charge threshold and larger than a second charge threshold) and the transportation destination D of the target transport body transported by the target transport vehicle is not the charging device  60 , the control unit  50  executes the second control to charge the second electrical energy storage device  21  of the target transport body with use of the electric power of the first electrical energy storage device  11  of the target transport vehicle. Also, in the present embodiment, in a case where there is a charge-requiring transport body, which is the transport body  20  with a charge amount of the second electrical energy storage device  21  that is equal to or smaller than the second charge threshold lower than the first charge threshold, the control unit  50  assigns, to any one of the plurality of unmanned transport vehicles  10 , a transportation task for which the transportation target is the charge-requiring transport body and the transportation destination D is the charging device  60 , and causes the charging device  60  to charge the second electrical energy storage device  21  of the charge-requiring transport body. 
       FIG. 3  is a diagram illustrating a state in which a first unmanned transport vehicle  10 A has been assigned with a transportation task in which a first transport body  20 A with a charge amount of the second electrical energy storage device  21  that is equal to or smaller than the first charge threshold is set as the target transport body and the storage area A 1  is set as the transportation destination D and there is a second transport body  20 B which is a charge-requiring transport body present. In  FIG. 3 , the power storage amount of the second electrical energy storage device  21  of the transport body  20  is schematically illustrated within the border indicating the transport body  20 . In the state illustrated in  FIG. 3 , the first unmanned transport vehicle  10 A is being controlled to transport the first transport body  20 A to the storage area A 1 , and the first unmanned transport vehicle  10 A and the first transport body  20 A are being controlled to execute the second control. Also, a second unmanned transport vehicle  10 B is assigned with a transportation task in which the second transport body  20 B is the transportation target and the charging device  60  is the transportation destination D, and the second unmanned transport vehicle  10 B is controlled to transport the second transport body  20 B to the charging device  60 . 
     Also, in the present embodiment, in a case where the charge amount of the second electrical energy storage device  21  of the target transport body is equal to or smaller than a predetermined third charge threshold (in this example, in a case where the charge amount is equal to or smaller than the third charge threshold and larger than the second charge threshold) and the charge amount of the first electrical energy storage device  11  of the target transport vehicle is equal to or smaller than a fourth charge threshold, the control unit  50  executes the second control to charge the second electrical energy storage device  21  of the target transport body with use of the electric power of the first electrical energy storage device  11  of the unmanned transport vehicle  10  (hereinafter, referred to as the “specified transport vehicle”) other than the target transport vehicle before start of the transport of the target transport body by the target transport vehicle. For example, in a possible configuration, the unmanned transport vehicle  10  with a charge amount of the first electrical energy storage device  11  which is equal to or larger than a fifth charge threshold greater than the fourth charge threshold can be selected as the specified transport vehicle. Note that, preferably, the unmanned transport vehicle  10  located at a position closer to the target transport body than the target transport vehicle (the unmanned transport vehicle  10  with a distance to the target transport body that is less than the distance from the target transport vehicle to the target transport body) is selected as the specified transport vehicle. Also, preferably, the second control in which the second electrical energy storage device  21  of the target transport body is charged via the electric power of the first electrical energy storage device  11  of the specified transport vehicle is started before the target transport vehicle arrives at the place where the target transport body is located. Also, in the present embodiment, even in a state where the unmanned transport vehicle  10  is not holding the transport body  20  on the holding section  15 , electric power can be transmitted between the unmanned transport vehicle  10  and the transport body  20 . Thus, the second electrical energy storage device  21  of the target transport body may be charged via the electric power of the first electrical energy storage device  11  of the specified transport vehicle when the specified transport vehicle is not holding the target transport body on the holding section  15 . 
     The control unit  50  ends the second control on the condition of, for example, the charge amount of the second electrical energy storage device  21  of the target transport body reaching a sixth charge threshold greater than the third charge threshold. Alternatively, the control unit  50  ends the second control on the condition of, for example, the charge amount of the second electrical energy storage device  21  of the target transport body being larger than the third charge threshold and the target transport vehicle having arrived at the place where the target transport body is located. After the second control ends, the control unit  50  executes the first control which includes starting the transportation of the target transport body to the transportation destination D and at least one of charging the first electrical energy storage device  11  of the target transport vehicle via the electric power of the second electrical energy storage device  21  of the target transport body and supplying the electric power to the travel section  14 . The third charge threshold may be a value equal to or different from the first charge threshold. 
     On the other hand, in a case where the charge amount of the second electrical energy storage device  21  of the target transport body is equal to or smaller than the third charge threshold and the charge amount of the first electrical energy storage device  11  of the target transport vehicle is larger than the fourth charge threshold, the control unit  50  does not execute second control, in which the second electrical energy storage device  21  of the target transport body is charged via the electric power of the first electrical energy storage device  11  of the unmanned transport vehicle  10  other than the target transport vehicle, and executes the second control, in which transportation of the target transport body to the transportation destination D is started and the second electrical energy storage device  21  of the target transport body is charged via the electric power of the first electrical energy storage device  11  of the target transport vehicle. 
       FIG. 4A  is a diagram illustrating a state in which the charge amount of the second electrical energy storage device  21  of the transport body  20  which is the target transport body is equal to or smaller than the third charge threshold and the charge amount of the first electrical energy storage device  11  of the first unmanned transport vehicle  10 A which is the target transport vehicle is equal to or smaller than the fourth charge threshold. In  FIGS. 4A to 4C , the power storage amount of the first electrical energy storage device  11  of the unmanned transport vehicle  10  is schematically illustrated within the border indicating the unmanned transport vehicle  10 , and the power storage amount of the second electrical energy storage device  21  of the transport body  20  is schematically illustrated within the border indicating the transport body  20 . In the state illustrated in  FIGS. 4A to 4C , the second unmanned transport vehicle  10 B and the transport body  20  are controlled to execute the second control (see  FIG. 4A ) after the second unmanned transport vehicle  10 B, which is the unmanned transport vehicle  10  other than the first unmanned transport vehicle  10 A, is controlled to move to the place where the transport body  20  is located. When the second control ends, the second unmanned transport vehicle  10 B is controlled to move away from the place where the transport body  20  is located (see  FIG. 4B ). Thereafter, the first unmanned transport vehicle  10 A is controlled to transport the transport body  20  to the transportation destination D, and the first unmanned transport vehicle  10 A and the transport body  20  are controlled to execute the first control (see  FIG. 4C ). 
     SECOND EMBODIMENT 
     An article transport system according to the second embodiment will be described with reference to the drawings ( FIGS. 9 to 12 ). The article transport system according to the present embodiment will be described below, focusing on the points that differ from the first embodiment. Points not explicitly described are as in the first embodiment, with the same reference sign being given and a detailed description being omitted. 
     As illustrated in  FIGS. 9 and 11 , in the present embodiment, the transport body  20  is a pallet  40  in a shape of a rectangular board with a prescribed thickness. Note that rectangular includes in its meaning both a rectangle and a square. The pallet  40  is the transport body  20  where the article  2  is placed and is provided with a placement surface  41  where the article  2  is placed. The placement surface  41  is formed on the upper surface of the pallet  40 . As illustrated in  FIG. 11 , the transport body  20  (pallet  40 ) is provided with a pair of insertion holes  43  for fork insertion in at least one of four side surfaces  42  of the outer edge of the pallet  40  as seen in a board thickness direction T, the board thickness direction T being orthogonal to the placement surface  41  of the pallet  40 . When a forklift supports the pallet  40 , the fork of the forklift is inserted into the pallet  40  from the insertion holes  43 . The pair of insertion holes  43  are formed on opposite side of the central portion of the side surface  42  (the central portion in a direction along the side surface  42  in the direction orthogonal to the board thickness direction T). The pallet  40  is held by the holding section  15  with the board thickness direction T being aligned with the vertical direction V. Thus, in a state where the pallet  40  is held by the holding section  15 , a board thickness direction view and a plan view are synonymous. 
     As illustrated in  FIG. 11 , with the side surface  42  where the pair of insertion holes  43  are formed being defined as the target side surface, a pair of hole portions extending in the direction orthogonal to the target side surface are formed in the pallet  40 , and the portion of each of the pair of hole portions that opens to the side surface  42  is formed as the insertion hole  43 . Because the pair of hole portions are formed extending through the pallet  40 , the pair of insertion holes  43  are formed in the pair of side surfaces  42  on opposite sides from one another. Also, in the present embodiment, in the pallet  40 , two sets of a pair of hole portions are formed with the extension direction of the two sets being orthogonal to one another. Thus, in the present embodiment, the pair of insertion holes  43  are formed in all four of the side surfaces  42 . 
     As illustrated in  FIG. 11 , in the central portion of the transport body  20  as seen in the board thickness direction, the hole portions described above that form the insertion holes  43  are not formed. Also, in the present embodiment, the second power receiving unit  22  and the second power transmitting unit  23  are disposed in the central portion of the transport body  20  as seen in the board thickness direction. In addition, in the present embodiment, the first power receiving unit  12  and the first power transmitting unit  13  are disposed in the central portion of the holding section  15  (placement section  16 ) as seen in a plan view. Also, the second electrical energy storage device  21  is arranged outward of the second power receiving unit  22  and the second power transmitting unit  23  so as not to overlap with the pair of insertion holes  43  (and also the pair of hole portions that form the pair of insertion holes  43 ) as seen in the board thickness direction. Note that, in the present embodiment, as in the first embodiment described above, the transport body  20  (pallet  40 ) is provided with a second power transmitting and receiving unit  29  that functions as both the second power receiving unit  22  and the second power transmitting unit  23 , and the second power transmitting and receiving unit  29  is provided with a second power transmitting and receiving connection unit  29   a  that functions as both the second power receiving connection unit  22   a  and the second power transmitting connection unit  23   a.    
     As illustrated in  FIG. 9 , in the present embodiment, the transport body  20  is placed on a support body  3  provided extending upward from the floor  4 . Also, the unmanned transport vehicles  10  holds the transport body  20  via the holding section  15  by, after moving under the transport body  20  (see  FIG. 9 ) with the holding section  15  in a lowered state, raising the holding section  15  and raising the transport body  20  off the support body  3  (see  FIG. 10 ). Also, the unmanned transport vehicle  10 , after moving to the position of the transportation destination D, lowers the transport body  20  at the position (see  FIG. 9 ) by lowering the holding section  15  and placing the transport body  20  on the support body  3  (the support body  3  provided at the transportation destination D). Note that the support surface of the support body  3  for the transport body  20  may be capable of moving vertically, and the holding operation and the hold releasing operation of the transport body  20  by the holding section  15  may be performed via the raising and lowering operation of the support surface. In this possible configuration, the holding section  15  is not supported by the travel section  14  in a manner allowing for vertical movement, and the height of the holding section  15  is fixed. 
     In the present embodiment, as in the first embodiment described above, the charging device  60  is capable of simultaneously charging both the first electrical energy storage device  11  of the unmanned transport vehicle  10  and the second electrical energy storage device  21  of the transport body  20 . Also, in the present embodiment, as illustrated in  FIG. 12 , at the charging device  60 , with a plurality of the transport body  20  stacked on top of one another, the second electrical energy storage devices  21  of the transport bodies  20  can be charged. By each of the transport bodies  20  disposed between the lowest transport body  20  and the highest transport body  20  supplying at least a portion of the electric power supplied from the transport body  20  below to the transport body  20  above, the electric power supplied from the power supply unit  61  of the charging device  60  can be supplied to each of the plurality of transport bodies  20  stacked on top of one another. Note that the electric power can be transmitted between the stacked transport bodies  20  via a non-contact method or a contact method. 
     OTHER EMBODIMENTS 
     Next, article transport systems according to other embodiments will be described. 
     1. The embodiments described above includes a configuration in which the electric power is transmitted between the unmanned transport vehicle  10  and the transport body  20  via a non-contact method. However, the present disclosure is not limited to such a configuration, and the electric power may be transmitted between the unmanned transport vehicle  10  and the transport body  20  via a contact method. 
     2. The embodiments described above includes a configuration in which the electric power is supplied from the power supply unit  61  to the first power receiving unit  12  and the second power receiving unit  22  via a non-contact method. However, the present disclosure is not limited to such a configuration, and the electric power from the power supply unit  61  to the first power receiving unit  12  and/or the electric power from the power supply unit  61  to the second power receiving unit  22  may be supplied via a contact method. 
     3. The embodiments described above includes a configuration in which the charging device  60  is capable of simultaneously charging both the first electrical energy storage device  11  of the unmanned transport vehicle  10  and the second electrical energy storage device  21  of the transport body  20 . However, the present disclosure is not limited to such a configuration, and the charging device  60  for charging the first electrical energy storage device  11  and the charging device  60  for charging the second electrical energy storage device  21  may be separately provided. 
     4. The embodiments described above includes a configuration in which the unmanned transport vehicle  10  is provided with the first power transmitting and receiving unit  19  that functions as both the first power receiving unit  12  and the first power transmitting unit  13 , and the transport body  20  is provided with a second power transmitting and receiving unit  29  that functions as both the second power receiving unit  22  and the second power transmitting unit  23 . However, the present disclosure is not limited to such a configuration, and the unmanned transport vehicle  10  may be provided with the first power receiving unit  12  and the first power transmitting unit  13  separated from one another, and the transport body  20  may be provided with the second power receiving unit  22  and the second power transmitting unit  23  separated from one another. In this case, the first power receiving connection unit  12   a  and the first power transmitting connection unit  13   a  are provided separated from one another, and the second power receiving connection unit  22   a  and the second power transmitting connection unit  23   a  are provided separated from one another. 
     5. The embodiments described above includes a configuration in which the housing shelf  30  is provided with housing sections  31  arranged next to one another in the vertical direction V. However, the present disclosure is not limited to such a configuration, and the housing shelf  30  may be provided with only one housing section  31 . Also, the first embodiment described above included a configuration in which the housing shelf  30  is directly placed on the floor  4 . However, the present disclosure is not limited to such a configuration, and the housing shelf  30  may be placed on a support body (for example, a support body similar to the support body  3  illustrated in  FIG. 9 ) provided extending upward from the floor  4 . 
     6. The second embodiment described above includes a configuration in which the pair of insertion holes  43  are formed in all four of the side surfaces  42  of the pallet  40 . However, the present disclosure is not limited to such a configuration, and, for example, the pair of insertion holes  43  may be formed in only two of the four side surfaces  42  of the pallet  40  on opposite sides from one another. 
     7. The first embodiment described above includes a configuration in which the transport body  20  is the housing shelf  30 , and the second embodiment described above includes a configuration in which the transport body  20  is the pallet  40 . However, the present disclosure is not limited to such a configuration, and the transport body  20  may be something (for example, a container) other than the housing shelf  30  and the pallet  40 . Note that the arrangement position of the second electrical energy storage device  21 , the second power receiving unit  22  (specifically, the second power receiving connection unit  22   a ), and the second power transmitting unit  23  (specifically, the second power transmitting connection unit  23   a ) in the transport body  20  is not limited to the positions described in the embodiments described above and may be changed as appropriate, such as being changed depending on the configuration of the transport body  20 . 
     8. The embodiments described above includes a configuration in which the transport body  20  is supported by the holding section  15  supporting the transport body  20  from below (specifically, a configuration in which the holding section  15  is provided with the placement section  16  where the transport body  20  is placed, and the transport body  20  is provided with the placed section  24  placed on the placement section  16 ). However, the present disclosure is not limited to such a configuration, and how the transport body  20  is held by the holding section  15  can be changed as appropriate. Also, the embodiments described above included a configuration in which the unmanned transport vehicle  10  autonomously travels along the floor  4 . However, the configuration of the unmanned transport vehicle  10  may be changed as appropriate. Note that the arrangement position of the first electrical energy storage device  11 , the first power receiving unit  12  (specifically, the first power receiving connection unit  12   a ), and the first power transmitting unit  13  (specifically, the first power transmitting connection unit  13   a ) in the unmanned transport vehicle  10  is not limited to the positions described in the embodiments described above and may be changed as appropriate, such as being changed depending on the configuration of the unmanned transport vehicle  10 . 
     9. Note that the configurations described in the embodiments described above can be used in combination with configurations described in other embodiments (including combinations embodiments described as other embodiments) as long as inconsistencies are avoided. Also, regarding the other configurations, the embodiments described in the present specification are merely examples in all aspects. Accordingly, various modifications within the range of the present disclosure can be made as appropriate. 
     Supplement 
     A summary of the article transport system described above will be given below. 
     An article transport system includes: a plurality of transport bodies on each of which an article is placeable or each of which is configured to house the article; a plurality of unmanned transport vehicles each configured to transport the transport body; and a control unit configured to control the transport bodies and the unmanned transport vehicles, wherein each of the unmanned transport vehicles includes: a first electrical energy storage device; a travel section configured to travel on electric power from the first electrical energy storage device, a holding section configured to hold the transport body; a first power receiving unit configured to receive electric power from an external power source and at least either charge the first electrical energy storage device or supply electric power to the travel section; and a first power transmitting unit configured to transmit electric power of the first electrical energy storage device to an external component; each of the transport bodies includes: a second electrical energy storage device; a second power receiving unit configured to receive electric power from an external power source and charge the second electrical energy storage device; and a second power transmitting unit configured to transmit electric power of the second electrical energy storage device to an external component, and the control unit is configured to selectively execute a first control of, while the unmanned transport vehicle is holding the transport body on the holding section, receiving electric power from the second electrical energy storage device via the second power transmitting unit and the first power receiving unit and at least either charging the first electrical energy storage device or supplying electric power to the travel section and a second control of, while the unmanned transport vehicle is holding the transport body on the holding section, receiving electric power from the first electrical energy storage device via the first power transmitting unit and the second power receiving unit and charging the second electrical energy storage device. 
     According to the present configuration, the first control, in which electric power from the second electrical energy storage device of the transport body is supplied to at least one of the firs electrical energy storage device of the unmanned transport vehicle and the travel section, and the second control, in which electric power from the first electrical energy storage device of the unmanned transport vehicle is supplied to the second electrical energy storage device of the transport body, can be selectively executed. Thus, the balance in the charge amount between the first electrical energy storage devices of the plurality of unmanned transport vehicles and the second electrical energy storage devices of the plurality of transport bodies in the article transport system overall can be constantly appropriately adjusted. Accordingly, for example, in the slack period, supply of electric power from the first electrical energy storage device of the unmanned transport vehicle not assigned with a transportation task to the second electrical energy storage device of the transport body can be actively performed. Thus, the number of times the transport bodies are transported to the charging devices can be kept to a minimum, and the charge amount of the second electrical energy storage devices of the transport bodies can be quickly increased. Also, in the busy period, supply of electric power from the second electrical energy storage device of the transport body with a large charge amount to the first electrical energy storage device of the unmanned transport vehicle can be actively performed. Thus, the number of times the unmanned transport vehicles are moved to the charging devices can be kept to a minimum, and many articles and transport bodies can be transported. In this manner, a decrease in the operation efficiency of an unmanned transport vehicle in a busy period can be minimized or prevented without having an excessive number of charging devices. 
     Here, preferably, the control unit, in executing the second control, supplies electric power of the first electrical energy storage device via the first power transmitting unit to the second power receiving unit in such a manner that the first electrical energy storage device keeps electric power necessary to execute a transportation task already assigned to the unmanned transport vehicle, the transportation task being a command to be assigned to each of the plurality of unmanned transport vehicles and specifying a target transport body and a transportation destination of the target transport body. 
     According to the present configuration, even in a case where electric power has been supplied from the first electrical energy storage device of the unmanned transport vehicle to the second electrical energy storage device of the transport body, a situation in which the unmanned transport vehicle cannot complete the transportation task assigned to it can be avoided. 
     Also, preferably, the control unit executes a busy/slack determination process to determine based on at least the number of transportation tasks whether a current period is a busy period or a slack period, the transportation task being a command to be assigned to each of the plurality of unmanned transport vehicles and specifying a target transport body and a transportation destination of the target transport body; in a case where the control unit has determined through the busy/slack determination process determines that the current period is the slack period, the control unit executes the second control more frequently than the first control; and in a case where the control unit has determined through the busy/slack determination process determines that the current period is the busy period, the control unit executes the first control more frequently than the second control. 
     According to the present configuration, in the slack period, using the unmanned transport vehicle not assigned with a transportation task, supply of electric power from the first electrical energy storage device of the unmanned transport vehicle to the second electrical energy storage device of the transport body can be actively performed. Thus, the number of times the transport bodies are transported to the charging device can be kept to a minimum, and the charge amount of the second electrical energy storage devices of the transport bodies can be quickly increased. Also, in the busy period, using the unmanned transport vehicle with a large charge amount, supply of electric power from the second electrical energy storage device of the transport body to the first electrical energy storage device of the unmanned transport vehicle can be actively performed. Thus, the number of times the unmanned transport vehicles are moved to the charging device can be kept to a minimum, and many articles and transport bodies can be transported. In this manner, a decrease in the operation efficiency of an unmanned transport vehicle in a busy period can be minimized or prevented without having an excessive number of charging devices. 
     Also, preferably, a charging device configured to charge the first electrical energy storage device of the unmanned transport vehicle and the second electrical energy storage device of the transport body is further provided, wherein with (i) a transportation task being a command to be assigned to each of the plurality of unmanned transport vehicles and specifying a target transport body and a transportation destination of the target transport body and (ii) a target transport vehicle being an unmanned transport vehicle that transports the target transport body, the control unit executes the second control to charge the second electrical energy storage device of the target transport body with use of electric power of the first electrical energy storage device of the target transport vehicle in a case where a charge amount of the second electrical energy storage device of the target transport body is equal to or smaller than a predetermined first charge threshold and a transportation destination of the target transport body transported by the target transport vehicle is not the charging device, and in a case where there is a charge-requiring transport body, which is the transport body with a charge amount of the second electrical energy storage device that is equal to or smaller than a second charge threshold lower than the first charge threshold, the control unit assigns, to any one of the plurality of unmanned transport vehicles, a transportation task for which the transportation target is the charge-requiring transport body and the transportation destination is the charging device, and causes the charging device to charge the second electrical energy storage device of the charge-requiring transport body. 
     According to the present configuration, in a case where the charge amount of the second electrical energy storage device of the target transport body is relatively small and the transportation destination of the target transport body is not the charging device, the second control is executed to charge the second electrical energy storage device of the target transport body via the electric power of the first electrical energy storage device of the target transport vehicle, allowing the charge amount of the second electrical energy storage device to be increased. Also, in a case where there is a charge-requiring transport body with a charge amount of the second electrical energy storage device that is even lower, the charge-requiring transport body can be transported to the charging device by any one of the unmanned transport vehicles and the second electrical energy storage device of the charge-requiring transport body can be charged. Thus, the number of times the transport bodies are transported to the charging device can be kept to a minimum, and the second electrical energy storage devices of the transport body can be appropriately charged as necessary. 
     Also, preferably, with (i) a transportation task being a command to be assigned to each of the plurality of unmanned transport vehicles and specifying a target transport body and a transportation destination of the target transport body and (ii) a target transport vehicle being an unmanned transport vehicle that transports the target transport body, the control unit executes the second control to charge the second electrical energy storage device of the target transport body with use of electric power of the first electrical energy storage device of an unmanned transport vehicle other than the target transport vehicle before start of the transport of the target transport body by the target transport vehicle in a case where a charge amount of the second electrical energy storage device of the target transport body is equal to or smaller than a predetermined third charge threshold, and a charge amount of the first electrical energy storage device of the target transport vehicle is equal to or smaller than a predetermined fourth charge threshold. 
     According to the present configuration, in a case where the charge amount of the second electrical energy storage device of the target transport body as per the transportation task is small and the charge amount of the first electrical energy storage device of the target transport vehicle as per the transportation task is small, the second electrical energy storage device of the target transport body can be charged via the electric power of the first electrical energy storage device of another unmanned transport vehicle before transportation as per the transportation task begins. In this manner, even in a case where the charge amount of the first electrical energy storage device of the target transport vehicle is low, the transportation as per the transportation task can be promptly begun without sending the target transport vehicle to the charging device, and the first electrical energy storage device of the target transport vehicle can be charged via the electric power of the second electrical energy storage device of the target transport body. This allows the transportation task to be promptly completed. 
     Also, preferably, the transport body includes a plurality of housing sections arranged in a vertical direction; each of the plurality of housing sections is capable of housing the article; and the second power receiving unit and the second power transmitting unit are below a lowermost one of the housing sections of the transport body. 
     According to the present configuration, the transport body is formed in the shape of a shelf provided with a plurality of housing sections arranged next to one another in the vertical direction, and, in a case where the transport body is transported while supported from below by the unmanned transport vehicle, electric power can be easily and appropriately transmitted between the first power receiving unit and the first power transmitting unit of the unmanned transport vehicle and the second power receiving unit and the second power transmitting unit of the transport body. 
     Also, preferably, the transport body is a pallet in a shape of a rectangular board with a prescribed thickness and includes a pair of insertion holes for fork insertion in at least one of four side surfaces of an outer edge of the pallet as seen in a board thickness direction, the board thickness direction being orthogonal to a placement surface of the pallet on which the article is placeable; the second power receiving unit and the second power transmitting unit are disposed in a central portion of the transport body as seen in the board thickness direction; and the second electrical energy storage device is arranged outward of the second power receiving unit and the second power transmitting unit so as not to overlap with the pair of insertion holes as seen in the board thickness direction. 
     According to the present configuration, in a case where the transport body is a pallet, and the transport body is transported while supported from below by the unmanned transport vehicle, electric power can be easily and appropriately transmitted between the first power receiving unit and the first power transmitting unit of the unmanned transport vehicle and the second power receiving unit and the second power transmitting unit of the transport body. Also, this configuration is preferable because the second electrical energy storage device is disposed at an appropriate position that does not impair the function of the transport body. 
     Also, preferably, a charging device configured to charge the first electrical energy storage device of the unmanned transport vehicle and the second electrical energy storage device of the transport body is further provided, wherein the unmanned transport vehicle includes a placement section on which the transport body is placeable; the first power receiving unit includes a first power receiving connection unit electrically connected to the second power transmitting unit; the first power transmitting unit includes a first power transmitting connection unit electrically connected to the second power receiving unit; the second power receiving unit includes a second power receiving connection unit electrically connected to the first power transmitting unit; the second power transmitting unit includes a second power transmitting connection unit electrically connected to the first power receiving unit; the first power receiving connection unit and the first power transmitting connection unit are on the placement section; the second power receiving connection unit and the second power transmitting connection unit are on a placed section placed on the placement section of the transport body; the charging device includes a power supply unit configured to supply electric power to the first power receiving unit and the second power receiving unit; and the power supply unit is configured to able to be disposed between the placement section and the placed section in a vertical direction and to be able to be electrically connected to both the first power receiving connection unit and the second power receiving connection unit when disposed in this manner and simultaneously supply electric power to both the first power receiving unit and the second power receiving unit. 
     According to the present configuration, at the charging device, the first power receiving unit and the second power receiving unit can be simultaneously supplied with electric power, and the first electrical energy storage device of the unmanned transport vehicle and the second electrical energy storage device of the transport body can be simultaneously charged. Thus, the number of charging devices can be kept to a minimum, and charging can be performed efficiently by only charging when necessary. 
     The article transport system according to the present disclosure is only required to achieve at least one of the effects described above.