Patent Publication Number: US-2023141291-A1

Title: Container Storage Facility

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to Japanese Patent Application No. 2021-183244 filed Nov. 10, 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 a container storage facility including a container storage rack including a plurality of container placement sections on each of which a container is placeable, and an inactive gas supply apparatus configured to supply an inactive gas to each of the containers placed on the container placement sections. 
     2. Description of the Related Art 
     An example of such a container storage facility is disclosed in WO 2015/045582 (Patent Document 1). In the following, the reference numerals and the names of the components disclosed in Patent Document 1 are cited in the description of the related art. 
     A purging device described in Patent Document 1 is provided in a stocker ( 2 ) installed in a clean room. An internal space ( 6 ) of the stocker ( 2 ) is divided into a work area ( 12 ) and a non-work area ( 14 ). A partition ( 30 ) for restricting the entry of a purge gas from the non-work area ( 14 ) into the work area ( 12 ) is placed at the boundary between the work area ( 12 ) and the non-work area ( 14 ). The purging device stops purging in the work area ( 12 ) when a worker enters the internal space ( 6 ). The purging device monitors the oxygen concentration in the work area ( 12 ), and, if a detection result of the oxygen concentration in the work area ( 12 ), which is a result obtained from an oxygen concentration sensor ( 54 ), is less than or equal to a predetermined value, stops the supply of the purge gas also in the non-work area ( 14 ). 
     SUMMARY OF THE INVENTION 
     According to the technique described in Patent Document 1, when a worker enters the internal space, purging in the work area is stopped to ensure the safety of the worker. Furthermore, if the oxygen concentration in the work area is less than or equal to the predetermined value, purging in the non-work area is stopped, thus restoring the oxygen concentration in the work area. Incidentally, for example, some container storage racks are suspended from and supported by a ceiling, and include a plurality of container placement sections next to each other. In such a container storage rack, there may be an area in which the oxygen concentration is locally reduced according to the extent of leakage of an inactive gas from the containers. In such a case, a worker cannot enter the area in which the oxygen concentration is reduced. For this reason, in order for the worker to perform work on such a container storage rack, it is necessary to appropriately detect the presence of an area in which the oxygen concentration is locally reduced. However, Patent Document  1  does not disclose detecting a local reduction in the oxygen concentration around such a container storage rack suspended from the ceiling. 
     Therefore, it is desirable to realize a container storage facility capable of appropriately detecting the oxygen concentration around a container storage rack that is suspended from and supported by a ceiling and that includes a plurality of container placement sections next to each other. 
     In view of the foregoing, a characteristic feature of a container storage facility lies in 
     a container storage facility including:   a container storage rack including a plurality of container placement sections on each of which a container is placeable; and   an inactive gas supply apparatus configured to supply an inactive gas to each of the containers placed on the container placement sections,   wherein the container storage rack is configured to be suspended from and supported by a ceiling, and the plurality of container placement sections are next to each other in a first direction extending in a horizontal direction, and,   the container storage facility further includes:   a first sensor group including a plurality of oxygen concentration sensors at an overlapping height and next to each other in the first direction on a first side of the containers in a second direction orthogonal to the first direction as viewed in a vertical direction, the overlapping height being a position overlapping the containers placed on the container placement sections;   a second sensor group including a plurality of oxygen concentration sensors at the overlapping height and next to each other in the first direction on a second side of the containers in the second direction;   a third sensor group including a plurality of oxygen concentration sensors at a first predetermined height and next to each other in the first direction at positions further toward the first side in the second direction than the first sensor group, the first predetermined height being lower than the overlapping height; and   a fourth sensor group including a plurality of oxygen concentration sensors at a second predetermined height and next to each other in the first direction at positions further toward the second side in the second direction than the second sensor group, the second predetermined height being lower than the overlapping height.   

     In a container storage facility including a container storage rack including a plurality of container placement sections on each of which a container is placeable, and an inactive gas supply apparatus configured to supply an inactive gas to each of the containers placed on the container placement sections, the oxygen concentration may be locally reduced according to the extent of leakage of the inactive gas from the containers. In this case, a worker cannot enter the area in which the oxygen concentration is reduced. In particular, with a container storage rack that is suspended from and supported by a ceiling, the oxygen concentration may be locally reduced in the vicinity of the head of a worker approaching the container storage rack. In such a case, it is necessary to appropriately detect the reduction, and alert the worker or the like. With this configuration, it is possible to appropriately detect oxygen concentrations around a container storage rack that is suspended from and supported by a ceiling, wherein the head of a worker is assumed to approach the container storage rack from below. 
     Further features and advantages of the technique according to the present disclosure will become apparent from the following description of illustrative and non-limiting embodiments with reference to the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a perspective view of a container storage facility according to an embodiment of the present disclosure. 
         FIG.  2    shows a container storage rack as viewed from a first direction-second side. 
         FIG.  3    shows the container storage rack as viewed from a second direction-first side. 
         FIG.  4    shows the container storage rack as viewed from a second direction-second side. 
         FIG.  5    is a block diagram showing functional units relating to alarm output performed by a control device. 
         FIG.  6    is a diagram showing a layout of human detection sensors. 
         FIG.  7    shows an example of monitoring area settings. 
         FIG.  8    is a diagram showing an example of detection ranges detected by the human detection sensor. 
         FIG.  9    shows an example of a map displayed on a display device. 
         FIG.  10    is a diagram showing an article transport device that transports containers. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     An embodiment of the container storage facility will be described with reference to the drawings. The present embodiment will be described, taking, as an example, a case where the container storage facility is installed in a down flow-type clean room in which clean air flows downward from the ceiling side toward the floor side. 
     As shown in  FIG.  1   , a container storage facility  1  includes a container storage rack  10  and an inactive gas supply apparatus  45 . The container storage rack  10  is a rack that stores containers  4 . In the present embodiment, the container storage rack  10  is illustrated as an open rack having no peripheral wall portion disposed therearound. The container storage rack  10  includes a plurality of container placement sections  11  on each of which a container  4  is placeable. Also, an inactive gas is supplied from the inactive gas supply apparatus  45  to each of the containers  4  placed on the container placement sections  11 . 
     Here, in the present embodiment, a first direction X is a specific direction (in the present embodiment, the longitudinal direction of the container storage rack  10 , which is a direction that extends in a horizontal direction) extending in the horizontal direction, a second direction Y is a direction extending orthogonal to the first direction X as viewed in a vertical direction, and a third direction Z is a direction extending in the vertical direction, which extends orthogonal to both the first direction X and the second direction Y. Also, the present embodiment is described with a first direction-first side X 1  being one side in the first direction X, and a first direction-second side X 2  being the other side in the first direction X. Also, the present embodiment is described with a second direction-first side Y 1  being one side in the second direction Y, and a second direction-second side Y 2  being the other side in the second direction Y. 
     The container storage rack  10  is configured to be suspended from and supported by a ceiling. The container storage rack  10  includes a pair of suspension parts  10 A spaced apart in the first direction X. The container storage rack  10  is suspended from the ceiling by coupling the suspension parts  10 A to supporters (not shown) provided on the ceiling of a clean room, for example. The pair of suspension parts  10 A are disposed above the container placement sections  11  of the container storage rack  10 . 
     In the present embodiment, as shown in  FIG.  6   , positioning pins  11   a  for positioning a container  4  by engaging with a bottom of portion of the container  4  are disposed on each of the container placement sections  11 . Each of the container placement sections  11  is also provided with a gas supply part  11   b  for supplying an inactive gas to the container  4  that is connected to a connection port provided at the bottom portion of the container  4 . A plurality of container placement sections  11  are next to each other in the first direction X. The container storage rack  10  according to the present embodiment includes four container placement sections  11 . Thus, one container storage rack  10  is capable of storing a maximum of four containers  4 . 
     In the present embodiment, each container  4  can be sealed in such a manner that its internal space is air-tightly sealed. For example, a semiconductor substrate, a reticle substrate, or the like can be housed in the container  4 . In the present embodiment, the container  4  includes a body part, and a cover part configured to be detachable from the body part, and the container  4  is configured such that the internal space of the container  4  is in an air-tight state while the cover part is attached to the body part. 
     The inactive gas supply apparatus  45  is configured to supply an inactive gas via a supply part (not shown) into the containers  4  placed on the plurality of container placement sections  11 . The inactive gas is a gas that has low reactivity with an object housed in the container  4  (a gas that produces substantially no problematic chemical reaction). In the present embodiment, nitrogen gas is used as the inactive gas. As the inactive gas, carbon dioxide may be used in place of nitrogen gas, or a noble gas such as helium, neon, argon, krypton, xenon, or radon may be used. 
     The inactive gas supply apparatus  45  includes a first pipe  45   a  connected to a supply source of the inactive gas, and second pipes  45   b  that connect the first pipe  45   a  and the containers  4  to one another. In the present embodiment, the first pipe  45   a  extends in the third direction Z, and the second pipes  45   b  branch from the first pipe  45   a  and extend in the first direction X and the second direction Y. Also, each of the second pipes  45   b  is connected to the gas supply part  11   b  provided on the corresponding placement part  11 . 
     The inactive gas supply apparatus  45  also includes a flow rate adjustment device  45   c  capable of adjusting the flow rate of the inactive gas in the first pipe  45   a  and the second pipes  45   b . The supply flow rate of the inactive gas supplied to each container  4  can be adjusted by the flow rate adjustment device  45   c  adjusting the flow rate of the inactive gas in the first pipe  45   a  and the second pipes  45   b . In  FIG.  1   , the flow rate adjustment device  45   c  is provided on the first pipe  45   a . However, the flow rate adjustment device  45   c  may be provided on the second pipes  45   b . The inactive gas from the inactive gas supply apparatus  45  is supplied to each of the containers  4 , thus making the pressure in the container  4  positive. When the internal pressure of the container  4  becomes greater than or equal to a certain pressure, part of the inactive gas inside the container  4  is discharged to the outside of the container  4  (a storage space  90  of the container storage facility  1  in the clean room). The air in the storage space  90 , which is air containing the inactive gas discharged from the container  4 , flows downward from the ceiling side toward the floor side in response to the circulation of clean air in the clean room. 
     The container storage facility  1  includes a plurality of oxygen concentration sensors  20 . The oxygen concentration sensors  20  detect the oxygen concentration at their respective positions. The oxygen concentration sensors  20  may be any type of oxygen concentration sensor, including, but not limited to, zirconia, magnetic, laser spectroscopic, and electrode type oxygen concentration sensors. In the present embodiment, the container storage facility  1  includes a first sensor group  31 , a second sensor group  32 , a third sensor group  33 , and a fourth sensor group  34 , each of which includes a plurality of oxygen concentration sensors  20 . 
     The first sensor group  31  includes a plurality of oxygen concentration sensors  20  at an overlapping height S and next to each other in the first direction X on the second direction-first side Y 1  of the containers  4 .  FIG.  2    shows the container storage rack  10  as viewed from the first direction-second side X 2 . The overlapping height S is a position overlapping the containers  4  placed on the container placement sections  11  when the container storage rack  10  is viewed in the first direction X, as shown in  FIG.  2   . That is, the overlapping height S is the height, in the third direction Z, from a lower end  4 B of the containers  4  placed on the container placement sections  11  to an upper end  4 U thereof. In the present embodiment, the second direction-first side Y 1  corresponds to the right side of left and right sides in the second direction Y when the container storage rack  10  is viewed toward the first direction-first side X 1  as shown in  FIG.  2   , for example. The expression “next to each other in the first direction X” means that the plurality of oxygen concentration sensors  20  constituting the first sensor group  31  are aligned in the first direction X. Accordingly, the first sensor group  31  is disposed on the right side in the second direction Y relative to the container storage rack  10  within a range of the height in the third direction Z from the lower end  4 B to the upper end  4 U of the containers  4  placed on the container placement sections  11  when the container storage rack  10  is viewed in the first direction X, and when the container storage rack  10  is viewed toward the first direction-first side X 1 . 
     Here,  FIG.  3    shows the container storage rack  10  as viewed from the second direction-first side Y 1 . In the present embodiment, as shown in  FIG.  3   , the first sensor group  31  includes two oxygen concentration sensors  20  next to each other in the first direction X. In the present example, as shown in  FIG.  3   , the oxygen concentration sensors  20  constituting the first sensor group  31  are respectively disposed at an end portion of the container storage rack  10  on the first direction-first side X 1  and an end portion thereof on the first direction-second side X 2 . Note that the first sensor group  31  may include three or more oxygen concentration sensors  20  next to each other in the first direction X. In this case, the distance in the first direction X between two adjacent oxygen concentration sensors  20  of the first sensor group  31  is shorter than that in the example shown in  FIG.  3   . 
     The second sensor group  32  includes a plurality of oxygen concentration sensors  20  at the overlapping height S and next to each other in the first direction X on the second direction-second side Y 2  of the containers  4 . In the present embodiment, the second direction-second side Y 2  corresponds to the left side of left and right sides in the second direction Y when the container storage rack  10  is viewed toward the first direction-first side X 1  as shown in  FIG.  2   , for example. Accordingly, the second sensor group  32  is disposed on the left side in the second direction Y relative to the container storage rack  10  within a range of the height in the third direction Z from the lower end  4 B to the upper end  4 U of the containers  4  placed on the container placement sections  11  when the container storage rack  10  is viewed in the first direction X, and when the container storage rack  10  is viewed toward the first direction-first side X 1 . In this manner, the first sensor group  31  and the second sensor group  32  are disposed on opposite sides in the second direction Y with the container storage rack  10  interposed therebetween. 
     Here,  FIG.  4    shows the container storage rack  10  as viewed from the second direction-second side Y 2 . In the present embodiment, as shown in  FIG.  4   , the second sensor group  32  includes two oxygen concentration sensors  20  next to each other in the first direction X. In the present example, as shown in  FIG.  4   , the oxygen concentration sensors  20  constituting the second sensor group  32  are respectively disposed at an end portion of the container storage rack  10  on the first direction-first side X 1  side and an end portion thereof on the first direction-second side X 2 . Note that the second sensor group  32  may include three or more oxygen concentration sensors  20  next to each other in the first direction X. In this case, the distance in the first direction X between two adjacent oxygen concentration sensors  20  of the second sensor group  32  is shorter than that in the example shown in  FIG.  4   . 
     The third sensor group  33  includes a plurality of oxygen concentration sensors  20  at a first predetermined height T 1  and next to each other in the first direction X at positions further toward the second direction-first side Y 1  than the first sensor group  31 , the first predetermined height T 1  being lower than the overlapping height S. As shown in  FIG.  2   , the first predetermined height T 1  is a height in the third direction Z that is lower than the lower end  4 B of the containers  4  placed on the container placement sections  11 . In the present embodiment, the first predetermined height T 1  is set to a height such that the distance in the third direction Z from the lower end  4 B of the containers  4  is less than or equal to a length corresponding to the overlapping height S. In the illustrated example, the first predetermined height T 1  is set as the height from a floor surface U. The expression “positions further toward the second direction-first side Y 1  than the first sensor group 31” refers to a region further distanced from a central portion of the container storage rack  10  in the second direction Y than the first sensor group  31 , as shown in  FIG.  2   . Accordingly, the third sensor group  33  is disposed at the first predetermined height T 1  that is lower than the overlapping height S in the third direction Z, at a position further spaced apart from the central portion of the container storage rack  10  in the second direction Y than the first sensor group  31  when the container storage rack  10  is viewed in the first direction X. 
     Here, in the present embodiment, as shown in  FIG.  3   , the third sensor group  33  includes two oxygen concentration sensors  20  next to each other in the first direction X. In the present example, as shown in  FIG.  3   , the oxygen concentration sensors  20  constituting the third sensor group  33  are respectively disposed at an end portion of the container storage rack  10  on the first direction-first side X 1  and an end portion thereof on the first direction-second side X 2 . In the present example, support members  12  are provided on the second direction-first side Y 1  of the container storage rack  10 , respectively at an end portion of the container storage rack  10  on the first direction-first side X 1 , and at an end portion thereof on the first direction-second side X 2 . Also, each of the oxygen concentration sensors  20  constituting the third sensor group  33  is supported on the container storage rack  10  using the corresponding support member  12 . In the present example, each of the support members  12  includes a first extension part  12 A extending downward in the third direction Z on the second direction-first side Y 1  of the container storage rack  10 , from an end portion on the first direction-first side X 1  of the container storage rack  10  or an end portion on the first direction-second side X 2  thereof, and a second extension part  12 B extending toward the second direction-first side Y 1  from a lower end portion of the first extension part  12 A. In the present example, each of the oxygen concentration sensors  20  constituting the third sensor group  33  is disposed at a distal end portion of the corresponding second extension part  12 B on the second direction-first side Y 1 . 
     The fourth sensor group  34  includes a plurality of oxygen concentration sensors  20  at a second predetermined height T 2  and next to each other in the first direction X at positions further toward the second direction-second side Y 2  than the second sensor group  32 , the second predetermined height T 2  being lower than the overlapping height S. As shown in  FIG.  2   , the second predetermined height T 2  is a height in the third direction Z that is lower than the lower end  4 B of the containers  4  placed on the container placement sections  11 . In the present embodiment, the second predetermined height T 2  is set to a height such that the distance in the third direction Z from the lower end  4 B of the containers  4  is less than or equal to a length corresponding to the overlapping height S. In the illustrated example, the second predetermined height T 2  is set as the height from the floor surface U. The expression “further toward the second direction-second side Y 2  than the second sensor group 32” refers to a region further spaced apart from the central portion of the container storage rack  10  in the second direction Y than the second sensor group  32 , as shown in  FIG.  2   . Accordingly, the fourth sensor group  34  is disposed at the second predetermined height T 2  that is lower than the overlapping height S in the third direction Z, at a position further spaced apart from the central portion of the container storage rack  10  in the second direction Y than the second sensor group  32  when the container storage rack  10  is viewed in the first direction X. 
     Here, in the present embodiment, as shown in  FIG.  4   , the fourth sensor group  34  includes two oxygen concentration sensors  20  next to each other in the first direction X. In the present example, as shown in  FIG.  4   , the oxygen concentration sensors  20  constituting the fourth sensor group  34  are respectively disposed at an end portion of the container storage rack  10  on the first direction-first side X 1  and an end portion thereof on the first direction-second side X 2 . In the present example, support members  13  are provided on the second direction-second side Y 2  of the container storage rack  10 , respectively at an end portion of the container storage rack  10  on the first direction-first side X 1 , and at an end portion thereof on the first direction-second side X 2 . Also, each of the oxygen concentration sensors  20  constituting the fourth sensor group  34  is supported on the container storage rack  10  using the corresponding support members  13 . In the present example, each of the support members  13  includes a first extension part  13 A extending downward in the third direction Z on the second direction-second side Y 2  of the container storage rack  10 , from an end portion on the first direction-first side X 1  of the container storage rack  10  or an end portion on the first direction-second side X 2  thereof, and a second extension part  13 B extending toward the second direction-second side Y 2  from a lower end portion of the first extension part  13 A. In the present example, each of the oxygen concentration sensors  20  constituting the fourth sensor group  34  is disposed at a distal end portion of the corresponding second extension part  13 B on the second direction-second side Y 2 . 
     In the present embodiment, the first predetermined height T 1  and the second predetermined height T 2  are set to the same height. With this configuration, the first sensor group  31 , the second sensor group  32 , the third sensor group  33 , and the fourth sensor group  34  are disposed in a trapezoidal shape when the container storage rack  10  is viewed in the first direction X. This facilitates appropriate detection of a reduction in oxygen concentration caused by an inactive gas that is discharged from the containers  4 , carried by an air flow moving from the ceiling portion of the clean room to the floor portion thereof, and diffused in a direction toward the floor surface U. 
     Note that the first predetermined height T 1  and the second predetermined height T 2  may be different from each other. In this case, the first predetermined height T 1  may be higher than the second predetermined height T 2 , or the second predetermined height T 2  may be higher than the first predetermined height T 1 . 
     Here, in the present embodiment, the container storage facility  1  includes a control device  40  and at least one human detection sensor  50 , and the control device  40  is configured to output an alarm according to detection results of the oxygen concentration sensors  20  and a detection result of the human detection sensor  50 . 
       FIG.  5    is a block diagram showing functional units relating to alarm output performed by the control device  40 . 
     The control device  40  includes a monitoring area setting unit  41 , an oxygen concentration estimation unit  42 , an alarm output unit  43 , and a map generation unit  44 . Each of these functional units is constructed by hardware or software, or both hardware and software, with a CPU serving as the core member. 
     The human detection sensor  50  detects the presence of a person in a detection range. In the present example, the human detection sensor  50  detects infrared radiation within a preset detection range, and detects the presence of a person in the detection range based on a change in the infrared radiation. Naturally, a human detection sensor configured to emit ultrasonic waves to the surrounding region thereof, and detect the presence of a person based on reflected ultrasonic waves can be used as the human detection sensor  50 , for example. In the present embodiment, a plurality of human detection sensors  50  are distributed around the container storage rack  10 . 
       FIG.  6    shows an example of the layout of the oxygen concentration sensors  20  and the human detection sensors  50  according to the present embodiment. In the present example, as shown in  FIG.  6   , eight oxygen concentration sensors  20  are provided on the container storage rack  10 . In the present example, four human detection sensors  50  are provided at lower portions of the container storage rack  10 . In the present embodiment, each of the human detection sensors  50  is disposed in such a manner as to detect a range below the container placement section  11  as a detection range. In the present example, a human detection sensor  50  is provided facing the floor surface U side, at a distal end portion of the second extension part  12 B of each of the two support members  12  on which the oxygen concentration sensors  20  of the third sensor group  33  are provided, and a distal end portion of the second extension part  13 B of each of the two support members  13  on which the oxygen concentration sensors  20  of the fourth sensor group  34  are provided. Accordingly, each of the human detection sensors  50  in the present example is disposed in such a manner as to detect, as a detection range, a range below the container placement sections  11  and overlapping the container storage rack  10  as viewed in the third direction Z (as viewed in the vertical direction). 
     The monitoring area setting unit  41  sets monitoring areas A obtained by dividing a space including the container storage rack  10 , the first sensor group  31 , the second sensor group  32 , the third sensor group  33 , and the fourth sensor group  34  into a plurality of segments. The expression “obtained by dividing a space ... into a plurality of segments” means dividing the space into a plurality of segments of a predetermined size. In the present embodiment, the monitoring area setting unit  41  divides a space (hereinafter referred to as a “storage rack-peripheral space”) including all of the container storage rack  10 , the first sensor group  31 , the second sensor group  32 , the third sensor group  33 , and the fourth sensor group  34  into a plurality of segments of a predetermined size. Each of the divided areas is regarded as a monitoring area A. In the present example, as shown in  FIG.  7   , the monitoring area setting unit  41  divides the storage rack-peripheral space into two segments in the first direction X, and two segments in the second direction Y. In the present example, there is one segment in the third direction Z. Accordingly, four monitoring areas A are set in the example shown in  FIG.  7   . 
     The human detection sensors  50  are configured to detect the presence of a person attempting to enter their respective monitoring areas A. As shown in  FIG.  8   , in the present embodiment, each of the human detection sensors  50  is disposed in such a manner as to detect a range below the container placement sections  11  of the container storage rack  10  as a detection range. This allows the human detection sensor  50  to detect a person approaching the container placement section  11  from below. Here, the container storage rack  10  is suspended from a ceiling. Accordingly, in most cases, a person attempting to enter a monitoring area A approaches the container placement section  11  from below. Therefore, it can be determined that a person detected by the human detection sensor  50  is a person attempting to enter the monitoring area A. In the present embodiment, a plurality of human detection sensors  50  are disposed in one-to-one correspondence with the plurality of monitoring areas A. That is, each of the human detection sensors  50  is disposed in such a manner as to detect a range below the corresponding one of the plurality of monitoring areas A as a detection range. In the present example, one human detection sensor  50  is disposed in each of the four monitoring areas A. Also, each of the human detection sensors  50  is configured to detect a person approaching the corresponding monitoring area A from below. 
     The oxygen concentration estimation unit  42  estimates the oxygen concentration of each of the plurality of monitoring areas A based on the detection value of each of the plurality of oxygen concentration sensors  20 . The plurality of oxygen concentration sensors  20  are the two oxygen concentration sensors  20  constituting the first sensor group  31 , the two oxygen concentration sensors  20  constituting the second sensor group  32 , the two oxygen concentration sensors  20  constituting the third sensor group  33 , and the two oxygen concentration sensors  20   constituting the fourth sensor group  34 . Detection values are transmitted to the oxygen concentration estimation unit  42  from the oxygen concentration sensors  20 . In the present example, the detection values of the oxygen concentration sensors  20  are transmitted in real time, or at a certain time interval, to the oxygen concentration estimation unit  42 . Based on the transmitted detection values of the oxygen concentration sensors  20 , the oxygen concentration estimation unit  42  estimates the oxygen concentrations in the plurality of monitoring areas A. In the present example, each of the plurality of monitoring areas A is set so as to include two oxygen concentration sensors  20 . A minimum value or an average value of the detection values of the two oxygen concentrations may be used for each of the monitoring areas A. For example, detection values of two oxygen concentration sensors  20  that are adjacent to each other may be used to calculate a gradient (concentration gradient) of the oxygen concentration, and the oxygen concentration may be estimated for each of the plurality of monitoring areas based on the calculated result. 
     The alarm output unit  43  outputs an alarm in response to at least one of the monitoring areas being a low-oxygen concentration area A 1  (see  FIG.  9   ) in which the estimated oxygen concentration is less than or equal to a predetermined determination threshold, and the corresponding human detection sensor  50  detecting a person attempting to enter the low-oxygen concentration area A 1 . The estimated oxygen concentration is an oxygen concentration estimated by the oxygen concentration estimation unit  42  for each of the monitoring areas. The determination threshold is, for example, a lower limit value defining the oxygen concentration at which a worker can perform work in the container storage facility  1  without issue. Detection results are transmitted to the alarm output unit  43  from the human detection sensors  50 . The alarm output unit  43  determines, in real time, whether or not the oxygen concentration estimated by the oxygen concentration estimation unit  42  for each of the monitoring areas is less than or equal to the determination threshold. Then, in this determination, if there is at least one monitoring area A in which the oxygen concentration is less than or equal to the determination threshold, the alarm output unit  43  determines the monitoring area A as being the low-oxygen concentration area A 1 , and, if a detection result of the corresponding human detection sensor  50  indicates that a person is attempting to enter the low-oxygen concentration area A 1  in which the oxygen concentration is less than or equal to the determination threshold, an alarm is output. Here, the alarm may be an alarm indicating, to a person attempting to enter the low-oxygen concentration area A 1 , that the oxygen concentration in the monitoring area A is reduced, or may be an alarm prompting the person to move away from the monitoring area A. For example, the container storage rack  10  or the clean room may be provided with a speaker, and the alarm may be output from the speaker. The alarm may be output from a terminal carried by a manager of the clean room. Such an alarm may be output using audio, image display, character display, or the like. Alternatively, the alarm may be a simple buzzing sound. 
     The oxygen concentration estimation unit  42  may be configured to, based on detection values of all the oxygen concentration sensors  20  constituting the first sensor group  31 , the second sensor group  32 , the third sensor group  33 , and the fourth sensor group  34 , estimate a minimum value of the oxygen concentration of each of the plurality of monitoring areas A through spatial interpolation. That is, for example, rather than estimating the oxygen concentration for each of the monitoring areas A, detection values of a plurality of oxygen concentration sensors  20  that are adjacent to each other are used to estimate a gradient (concentration gradient) of the oxygen concentration in each location, and the oxygen concentration distribution in each of the plurality of monitoring areas A is estimated based on the estimated value. Also, the oxygen concentration estimation unit  42  estimates the value of the lowest oxygen concentration in the thus estimated oxygen concentration distribution in each of the monitoring areas A as the minimum value of the oxygen concentration of the monitoring area A. With such an oxygen concentration estimation method using spatial interpolation, it is possible to appropriately estimate oxygen concentrations, not only in the above-described configuration in which two oxygen concentration sensors  20  are included in each of the plurality of monitoring areas A, but also in a configuration in which a monitoring area A including the oxygen concentration sensors  20  and a monitoring area A not including the oxygen concentration sensors  20  are present, and a configuration in which one, or three or more oxygen concentration sensors  20  are included in one monitoring area A. 
     The alarm output unit  43  may be configured to determine a monitoring area A in which the minimum value of the oxygen concentration estimated by the oxygen concentration estimation unit  42  is less than or equal to the determination threshold as being a low-oxygen concentration area A 1 , and output an alarm in response to the corresponding human detection sensor  50  detecting a person attempting to enter such a low-oxygen concentration area A 1 . 
     In the case where the container storage facility  1  includes a display device  60 , the map generation unit  44  may display, on the display device  60 , the oxygen concentration estimated for each of the plurality of monitoring areas A as an oxygen concentration map associated with the location of the container storage rack  10 . The oxygen concentration estimated for each of the plurality of monitoring areas A is an oxygen concentration estimated by the oxygen concentration estimation unit  42 . Accordingly, the map generation unit  44  may obtain an estimation result obtained as a result of the oxygen concentration estimation unit  42  estimating the oxygen concentrations. The oxygen concentration map associated with the location of the container storage rack  10  is a map indicating oxygen concentrations in which each of the monitoring areas A for which the oxygen concentration has been estimated is associated with an image obtained by imaging the container storage rack  10 . Specifically, a model of the clean room in which the container storage rack  10  is disposed is created, and the map indicates a plurality of oxygen concentrations that are divided in such a manner as to correspond to the monitoring areas A in the model. The map generation unit  44  may generate such an oxygen concentration map, and display the oxygen concentration map on the display device  60 . Note that a monitor of the control device  40  can be used as the display device  60 . 
       FIG.  9    shows an example of the oxygen concentration map displayed on the display device  60 . Such an oxygen concentration map is displayed on the display device  60 , which is checked by a worker, and thus the oxygen concentrations in the storage space  90  of the container storage facility  1  can be easily visually understood. Although the scale of the oxygen concentration is divided into six levels in the example shown in  FIG.  9   , the scale may be divided into smaller units. To facilitate understanding by a worker who has checked the oxygen concentration map shown in  FIG.  9   , it is preferable that an image representing the container storage rack  10  is superposed on the oxygen concentration map. In  FIG.  9   , the above-described monitoring areas A in which the oxygen concentration is less than or equal to the predetermined determination threshold are shown as the low-oxygen concentration areas A 1 . 
     For example, as shown in  FIG.  10   , there are cases where a clean room is provided with a plurality of container storage facilities  1 , and a transport device  91  that transports containers  4  by moving along a movement path R and is provided in such a manner as to connect the container storage facilities  1  to one another. For example, the transport device  91  is a transport vehicle that travels along a movement path R connecting container transfer locations serving as a transport source of the containers  4  and a transport destination of the containers  4 . The container storage facilities  1  are disposed along the movement path R. In such a configuration, it is preferable that, if at least one of the plurality of monitoring areas A that are set around each of the container storage racks  10  is a low-oxygen concentration area A 1 , and if the corresponding human detection sensor  50  detects a person attempting to enter the low-oxygen concentration area A 1 , the transport device  91  is configured to not transport the containers  4  to the container storage facility  1  in which the monitoring area A that has been determined as being the low-oxygen concentration area A 1  is set. In  FIG.  10   , “O” is assigned to a movement path to a container storage facility  1  to which the transport device  91  transports the containers  4 , and “X” is assigned to a movement path to a container storage facility  1  to which the transport device  91  does not transport the containers  4 . Accordingly, it is possible to prevent a person (worker) located close to the container storage facility  1  and the transport device  91  from coming into contact with each other. Furthermore, it is possible to avoid such a situation where, after a container  4  has been placed in a container storage facility  1  where a monitoring area A that is a low-oxygen concentration area A 1  is set, the worker cannot approach the container  4  despite needing to. 
     Other Embodiments 
     Next, other embodiments of the container storage facility  1  will be described. 
     ( 1 ) In the above embodiment, the container storage rack  10  is described, taking, as an example, an open rack having no peripheral wall portion disposed therearound. However, the present invention is not limited to such a configuration. For example, the container storage rack  10  may be a sealed rack surrounded by a peripheral wall portion. In addition, although the container placement sections  11  are illustrated as being disposed in a single row in the present example, the container placement sections  11  may be disposed in a plurality of rows. Furthermore, the present example illustrates a configuration in which four container placement sections  11  are provided in one container storage rack  10  in such a manner as to extend in the first direction X. However, the number of container placement sections  11  may be one, or may be a plural number other than four. In such a case as well, it is possible to appropriately detect the oxygen concentrations around the container storage rack  10  by providing the container storage rack  10  with the oxygen concentration sensors  20 . 
     ( 2 ) In the above embodiment, the control device  40  is described as being configured to output an alarm in response to at least one of the monitoring areas A being a low-oxygen concentration area A 1  in which the estimated oxygen concentration is less than or equal to a predetermined determination threshold, and the human detection sensor  50  detecting a person attempting to enter the low-oxygen concentration area A 1 . However embodiments of the container storage facility  1  are not limited to such a configuration. For example, the control device  40  can perform control in such a manner as to cause the flow rate adjustment device  45   c  of the inactive gas supply apparatus  45  to reduce the flow rate of the inactive gas supplied to the container storage rack  10 , or stop the supply of the inactive gas, if at least one of the monitoring areas A is a low-oxygen concentration area A 1  in which the estimated oxygen concentration is less than or equal to a predetermined determination threshold. In such a case, the container storage facility  1  need not include any human detection sensor  50 . 
     ( 3 ) The above embodiment illustrates a case where one human detection sensor  50  is provided in one monitoring area A. However embodiments of the container storage facility  1  are not limited to such a configuration. For example, one human detection sensor  50  may be provided for a plurality of monitoring areas A. In this case, one human detection sensor  50  may be configured to include detection ranges corresponding to a plurality of monitoring areas A, and detect a person attempting to enter any of the plurality of monitoring areas A. For example, one human detection sensor  50  may be provided at a central portion of the container storage rack  10  in the second direction Y on the first direction-first side X 1 , in place of the two human detection sensors  50  on the first direction-first side X 1 , and one human detection sensor  50  may be provided at a central portion of the container storage rack  10  in the second direction Y on the first direction-second side X 2 , in place of the two human detection sensors  50  on the first direction-second side X 2 . Alternatively, the container storage facility  1  need not include any human detection sensor  50 . 
     ( 4 ) In the above embodiment, the display device  60  is described as being a monitor of the control device  40 . However embodiments of the container storage facility  1  are not limited to such a configuration. The display device  60  may be a monitor of a portable terminal carried by a worker, or may be a pair of smart glasses (a display device integrated in one piece with glasses) if worn by a worker. 
     ( 5 ) In the above embodiment, the control device  40  is described as including the monitoring area setting unit  41 , the oxygen concentration estimation unit  42 , the alarm output unit  43 , and the map generation unit  44 . However embodiments of the container storage facility  1  are not limited to such a configuration. The functional units constituting the control device  40  are merely examples, and the grouping of the functional units can be changed as appropriate. The control device  40  can be configured to include other functional units. 
     ( 6 ) Note that the configurations disclosed in the embodiments described above are applicable in combination with configurations disclosed in other embodiments as long as no inconsistency arises. With regard to the other configurations as well, the embodiments disclosed herein are illustrative in all respects. Therefore, various modifications and alterations may be made as appropriate without departing from the gist of the present disclosure. 
     Outline of the Embodiment 
     In the following, an outline of the container storage facility described above will be described. 
     A container storage facility including: 
     a container storage rack including a plurality of container placement sections on each of which a container is placeable; and   an inactive gas supply apparatus configured to supply an inactive gas to each of the containers placed on the container placement sections,   wherein the container storage rack is configured to be suspended from and supported by a ceiling, and the plurality of container placement sections are next to each other in a first direction extending in a horizontal direction, and,   the container storage facility further includes:   a first sensor group including a plurality of oxygen concentration sensors at an overlapping height and next to each other in the first direction on a first side of the containers in a second direction orthogonal to the first direction as viewed in a vertical direction, the overlapping height being a position overlapping the containers placed on the container placement sections;   a second sensor group including a plurality of oxygen concentration sensors at the overlapping height and next to each other in the first direction on a second side of the containers in the second direction;   a third sensor group including a plurality of oxygen concentration sensors at a first predetermined height and next to each other in the first direction at positions further toward the first side in the second direction than the first sensor group, the first predetermined height being lower than the overlapping height; and   a fourth sensor group including a plurality of oxygen concentration sensors at a second predetermined height and next to each other in the first direction at positions further toward the second side in the second direction than the second sensor group, the second predetermined height being lower than the overlapping height.   

     In a container storage facility including a container storage rack including a plurality of container placement sections on each of which a container is placeable, and an inactive gas supply apparatus configured to supply an inactive gas to each of the containers placed on the container placement sections, the oxygen concentration may be locally reduced according to the extent of leakage of the inactive gas from the containers. In this case, the worker cannot enter the area in which the oxygen concentration is reduced. In particular, around a container storage rack that is suspended from and supported by a ceiling, the oxygen concentration may be locally reduced in the vicinity of the head of a worker approaching the container storage rack. In such a case, it is necessary to appropriately detect the reduction, and to alert the worker or the like. With this configuration, it is possible to appropriately detect oxygen concentrations around a container storage rack that is suspended from and supported by a ceiling, wherein the head of a worker is assumed to approach the container storage rack from below. 
     Here, it is preferable that the container storage facility further includes: 
     a control device; and   at least one human detection sensor configured to detect a person,   wherein the container storage rack, the first sensor group, the second sensor group, the third sensor group, and the fourth sensor group are located in a space divided into a plurality of monitoring areas, and the at least one human detection sensor is configured to detect presence of a person attempting to enter the monitoring areas, and   the control device is configured to (i) estimate an oxygen concentration of each of the plurality of monitoring areas based on a detection value of each of the plurality of oxygen concentration sensors constituting the first sensor group, the second sensor group, the third sensor group, and the fourth sensor group, and (ii) output an alarm in response to at least one of the monitoring areas being a low-oxygen concentration area in which the estimated oxygen concentration is less than or equal to a predetermined determination threshold, and the at least one human detection sensor detecting a person attempting to enter the low-oxygen concentration area.   

     With this configuration, if a worker approaches a container storage rack that is suspended from and supported by a ceiling, and the oxygen concentration in the vicinity of the head of the worker is locally reduced, it is possible to appropriately detect the reduction and output an alarm. 
     It is preferable that the at least one human detection sensor includes a plurality of the human detection sensors disposed in one-to-one correspondence with the plurality of monitoring areas. 
     With this configuration, it is possible to appropriately detect the presence of a person entering each of the plurality of monitoring areas. 
     It is preferable that the control device is configured to, based on detection values of all the oxygen concentration sensors constituting the first sensor group, the second sensor group, the third sensor group, and the fourth sensor group, estimate a minimum value of the oxygen concentration of each of the plurality of monitoring areas through spatial interpolation, and determine each of the monitoring areas for which the minimum value is less than or equal to the determination threshold to be the low-oxygen concentration area. 
     With this configuration, even if the number of oxygen concentration sensors disposed in the periphery of the container storage rack is relatively small, it is possible to accurately determine whether or not there is a low-oxygen concentration area. 
     It is preferable that the container storage facility further includes a display device, wherein the control device is configured to display, on the display device, an oxygen concentration estimated for each of the plurality of monitoring areas as an oxygen concentration map associated with a location of the container storage rack. 
     With this configuration, the oxygen concentration is estimated for each of the plurality of monitoring areas obtained by dividing the area surrounding the container storage rack, and the result of estimation is displayed as an oxygen concentration map on the display device. Accordingly, it is possible to inform a worker of the oxygen concentration at each location around the container storage rack in an easily understandable manner. 
     INDUSTRIAL APPLICABILITY 
     The technique according to the present disclosure is applicable to a container storage facility including a container storage rack including a plurality of container placement sections on each of which a container is placeable, and an inactive gas supply apparatus configured to supply an inactive gas to each of the containers placed on the container placement sections.