Patent Publication Number: US-2023148766-A1

Title: Refrigerator having panel assembly covering opening of outer plate of door

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a Continuation application of Ser. No. 17/557,536 filed on Dec. 21, 2021, which is a continuation of U.S. patent application Ser. No. 16/781,143 filed Feb. 4, 2020, now U.S. Pat. No. 11,300,350, which is a continuation of prior U.S. patent application Ser. No. 16/416,498 filed May 20, 2019, now U.S. Pat. No. 10,588,428, which is a continuation of U.S. patent application Ser. No. 15/340,056 filed Nov. 1, 2016, now U.S. Pat. No. 10,342,363, which claims priority under 35 U.S.C. § 119 to Korean Application No. 10-2016-0001298 filed on Jan. 5, 2016, whose entire disclosures are hereby incorporated by reference. 
    
    
     BACKGROUND 
     Generally, a refrigerator is a home appliance which stores food at a low temperature in a storage space. The refrigerator is formed to cool an inside of the storage space using cooling air generated through heat exchange with a refrigerant circulated in a refrigeration cycle, and keeps the stored food in an optimum state. 
     SUMMARY 
     The present invention is directed to a refrigerator which enables at least a part of a refrigerator door to be selectively transparent by an user&#39;s operation, such that the user sees through an inside of the refrigerator while the refrigerator door is closed. 
     Also, the present invention is directed to a refrigerator in which at least a part of a front surface of a refrigerator door is formed of half glass, and a lighting unit in the refrigerator is turned on/off by a user&#39;s operation, and thus the user sees through an inside of the refrigerator while the refrigerator door is closed. 
     Also, the present invention is directed to a refrigerator which uniformly illuminates an entire opening part when a lighting unit is turned on. 
     Also, the present invention is directed to a refrigerator which is selectively transparent by a knocking operation on a refrigerator door, and enables an inside of the refrigerator to be visible. 
     Also, the present invention is directed to a refrigerator which enhances recognition performance and reliability of an operation for enabling an inside of the refrigerator to be visible through a panel assembly while a refrigerator door is closed. 
     Also, the present invention is directed to a refrigerator which prevents exposure of a detection device detecting an operation for enabling at least a part of a refrigerator door to be selectively transparent. 
     Also, the present invention is directed to a refrigerator in which a detection device for detecting a user&#39;s operation is easily installed and assembled, and maintenance thereof is easily performed. 
     Also, the present invention is directed to a refrigerator which is able to prevent dew condensation on a surface of a refrigerator door formed of a glass material and enabling an inside of the refrigerator to be seen through. 
     Also, the present invention is directed to a refrigerator which enables a panel plate formed of a glass material to be firmly installed and fixed to a door, and also enables a step difference or a gap in an installed state to be minimized. 
     Also, the present invention is directed to a refrigerator which enables an insulation to be completely filled in an inside of a door. 
     Also, the present invention is directed to a refrigerator which easily guides an electric wire inside a door, and prevents interference when the door is rotated. 
     Also, the present invention is directed to a refrigerator in which an internal space of a door basket is able to be utilized variously. 
     According to an aspect of the present invention, there is provided a refrigerator including a cabinet configured to form a storage space; a main door configured to open and store the storage space, and having an opening part which is in communication with the storage space; a sub-door rotatably installed at the main door and configured to open and close the opening part; a panel assembly provided at the sub-door, and having a half-mirror structure which allows to selectively see through an inside of the opening part; a knock detection device disposed at a rear surface of the panel assembly, and configured to detect a user&#39; knocking operation on the panel assembly; and a door lighting unit provided above the panel assembly, turned on or off by the knock detection device to selectively illuminate an internal space of the opening part, such that the panel assembly selectively becomes transparent. 
     The sub-door may include an outer plate which is formed of a metal plate material, forms an exterior of the sub-door and has a panel installation opening at which the panel assembly is installed; a door liner which is spaced apart from the outer plate, forms a perimeter of a rear surface of the sub-door, and forms a space, in which an insulation is filled, at an outside of the panel assembly; an upper cap decoration which is coupled to upper ends of the outer plate and the door liner, and forms an upper surface of the sub-door; and a lower cap decoration which is coupled to lower ends of the outer plate and the door liner, and forms a lower surface of the sub-door. 
     A support frame which supports the outer plate and the panel assembly may be formed at a perimeter of the panel installation opening. 
     A plate insertion part in which a plate bent part bent along the perimeter of the penal installation opening is accommodated may be formed at the support frame. 
     A plurality of protrusions may be formed at an inside of the plate insertion part, and a plate hole in which the plurality of protrusions are inserted may be formed at the plate bent part. 
     A plurality of holes through which a foaming solution is introduced may be formed along the plate insertion part. 
     The holes may be formed at positions corresponding to the protrusions. 
     The plate bent part may be in close contact with a side end of the panel assembly. 
     The support frame may be bonded to the rear surface of the panel assembly. 
     A heater groove in which a heater disposed along an edge of the panel assembly is accommodated may be formed at the support frame. 
     An aluminum sheet may be interposed between the support frame and the panel assembly. 
     The support frame may include an upper frame which accommodates an upper end of the panel installation opening, side frames which accommodate both side ends of the panel installation opening; and a lower frame which accommodates a lower end of the panel installation opening, and the upper frame, the side frames and the lower frame may be coupled to each other. 
     A detection device hole through which a front end of the knock detection device passes may be formed at the lower frame. 
     An upper hinge installation part to which an upper hinge connecting the main door with the cabinet is shaft-coupled may be formed at an upper end of the main door, and a sub-upper hinge installation part to which a sub-upper hinge connecting the sub-door with the cabinet is shaft-coupled may be formed at an upper end of the sub-door, and a hinge shaft of the sub-upper hinge may be formed in a cylindrical shape which guides an access of an electric wire to an inside of the sub-door. 
     The hinge shaft of the sub-upper hinge may be formed to have a circular cross section of which one side is opened. 
     A connector may be provided at an end of the electric wire guided to an outside of the sub-door through the hinge shaft of the sub-upper hinge, and may be coupled to an electric wire guided toward the cabinet. 
     The sub-upper hinge installation part may include a sub-hinge cover which accommodates the sub-upper hinge and further accommodates the electric wire coupled to the connector. 
     A main hinge cover which accommodates the upper hinge may be provided, and a sub-hinge cover which accommodates the sub-upper hinge may be provided, and the main hinge cover and the sub-hinge cover may have a connection part which rotatably connects each other, and the electric wire may be guided from the inside of the sub-door to the cabinet through the connection part. 
     The connection part may include a main connection part which is located at an upper side of a rotating center of the upper hinge and of which at least a part of an upper surface is opened; and a sub-connection part which is seated on the main connection part and is in communication with the main connection part, and centers of the upper hinge, the main connection part and the sub-connection part may be concentrically located. 
     The sub-hinge cover may be formed in a shape corresponding to a space between the sub-upper hinge installation part and the main hinge cover. 
     A plurality of door baskets may be installed at the opening part. 
     A front end of each of the door baskets which face the sub-door may be spaced part from a rear surface of the sub-door. 
     A front surface of each of the door baskets may be formed to be inclined from one side thereof toward the other side thereof and thus to be gradually distant from the rear surface of the sub-door. 
     The door lighting unit may be disposed at an upper portion of the opening part, and may also be disposed between the front surface of the door basket and the rear surface of the sub-door. 
     The refrigerator may further include a dividing member which is rotatably provided at each of both side surfaces of the door basket, extends so as to cross an inside of the door basket, and is rotated at a preset angle to divide the inside of the door basket. 
     The refrigerator may further include a stopper which is installed at each of both side surfaces of the door basket so that the dividing member is inserted therein, and in which a fixing groove in which the dividing member is inserted is formed at each set angle of a circumference thereof so as to fix the dividing member at the set angle. 
     The dividing member may include a dividing part which crosses the inside of the door basket, a first bent part which is bent from each of both ends of the dividing part, a second bent part which is bent again from the first bent part and inserted into the fixing groove, and an insertion part which is bent from an end of the second bent part and rotatably inserted into a center of the stopper. 
     The first bent part and the second bent part may form to have different lengths from each other. 
     A sensor opening in which the knock detection device is inserted may be formed at a rear surface of the door liner. 
     The refrigerator may further include an accommodation case which is installed at the door liner to be in communication with the sensor opening, extends to the rear surface of the panel assembly, and forms a space for accommodating the knock detection device. 
     The knock detection device and a detection device PCB which determines a signal of the knock detection device may be installed at an inside of the accommodation case. 
     An insert cover may be installed at the sensor opening, and the detection device PCB which determines a signal of the knock detection device may be installed at the insert cover. 
     An insert cover which shields the sensor opening may be installed at the sensor opening, and the insert cover may include a cover part which is formed in a shape corresponding to the sensor opening, and an insertion part which extends from the cover part, is inserted into the sub-door, and at which the detection device PCB for determining a signal of the knock detection device is installed. 
     An insert cover which shields the sensor opening may be installed at the sensor opening, and a cover groove which forms a part of a liner groove in which a sub-gasket is installed may be formed at the insert cover. 
     The knock detection device may include a microphone module which is in contact with the rear surface of the panel assembly and receives a sound generated by vibration due to a knock transmitted through the panel assembly; an elastic member which presses the microphone module toward the panel assembly; and a support member in which the elastic member is accommodated and which is installed and fixed to the inside of the sub-door. 
     The microphone module may include a microphone accommodation part which is formed of an elastic material to accommodate a microphone, and an opening which faces the panel assembly and a protrusion which protrudes along a circumference of the opening and is in close contact with the panel assembly may be formed at the microphone accommodation part. 
     The panel assembly may include a front panel which forms a front surface of the sub-door, and is formed in a half mirror which is selectively transparent by reflecting a part of light and transmitting another part of the light; a plurality of insulation panels which are spaced apart from the front panel, and formed of transparent tempered glass; and a spacer bar which is provided between the front panel and the insulation panel and between the insulation panels, and spaces and seals between the front panel and the insulation panel and between the insulation panels. 
     The front panel may extend outward further than the insulation panel, and a bezel which blocks the light may be formed along an extending edge of the front panel, and the knock detection device may be disposed at a rear surface of the bezel. 
     The details of one or more implementations are set forth in the accompanying drawings and the description below. Other features will be apparent from the description and drawings, and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a perspective view of an example refrigerator; 
         FIG.  2    is a perspective view illustrating a state in which a sub-door of the refrigerator is opened; 
         FIG.  3    is a perspective view illustrating a state in which a main door of the refrigerator is opened; 
         FIG.  4    is an exploded perspective view illustrating a coupling structure between the main door and the sub-door; 
         FIG.  5    is a partial perspective view illustrating a hinge coupling structure between the main door and an upper end of the sub-door; 
         FIG.  6    is a partial perspective view illustrating a state in which a hinge cover is installed at the main door and the sub-door; 
         FIG.  7    is a partial perspective view illustrating the hinge coupling structure of a lower end of the main door; 
         FIG.  8    is a perspective view of a refrigerator door of the refrigerator when being seen from a rear; 
         FIG.  9    is a perspective view of an example of a door basket 
         FIG.  10    is an exploded perspective view of the door basket; 
         FIG.  11    is a perspective view illustrating a use state of a dividing member of the door basket; 
         FIG.  12    is a front view of the sub-door; 
         FIG.  13    is an exploded perspective view of the sub-door; 
         FIG.  14    is an exploded perspective view of an example of a door lighting unit; 
         FIG.  15    is a cross-sectional view taken along line  15 - 15 ′ of  FIG.  1   ; 
         FIG.  16    is a cross-sectional view taken along line  16 - 16 ′ of  FIG.  12   ; 
         FIG.  17    is a perspective view of an example of a support frame  FIG.  18    is an exploded perspective view illustrating a coupling structure of an upper portion of the support frame; 
         FIG.  19    is a perspective view of an upper frame forming the support frame; 
         FIG.  20    is a perspective view of a side frame forming the support frame when being from a front; 
         FIG.  21    is a perspective view of the side frame forming the support frame when being from a rear; 
         FIG.  22    is an exploded perspective view illustrating a coupling structure of a lower portion of the support frame; 
         FIG.  23    is a perspective view of a lower frame forming the support frame; 
         FIG.  24    is a cross-sectional view taken along line  24 - 24 ′ of  FIG.  12   ; 
         FIG.  25    is a cross-sectional view taken along line  25 - 25 ′ of  FIG.  12   ; 
         FIG.  26    is a cross-sectional view taken along line  26 - 26 ′ of  FIG.  12   ; 
         FIG.  27    is a cross-sectional view taken along line  27 - 27 ′ of  FIG.  12   ; 
         FIG.  28    is an exploded perspective view illustrating an example of a coupling structure of a knock detection device when being seeing from a front; 
         FIG.  29    is an exploded perspective view illustrating the coupling structure of the knock detection device when being seeing from a rear; 
         FIG.  30    is an exploded perspective view of the knock detection device; 
         FIG.  31    is a cross-sectional view illustrating a microphone structure of the knock detection device; 
         FIG.  32    is a cross-sectional view taken along line  32 - 32 ′ of  FIG.  12   ; 
         FIG.  33    is a cross-sectional view taken along line  33 - 33 ′ of  FIG.  12   ; 
         FIG.  34    is a block diagram illustrating a flow of a control signal of the refrigerator; 
         FIG.  35    is a flowchart sequentially illustrating an operation of the sub-door of the refrigerator; 
         FIG.  36    is a perspective view illustrating a state before an knocking operation in the refrigerator; and 
         FIG.  37    is a perspective view illustrating a state after the knocking operation in the refrigerator. 
     
    
    
     DETAILED DESCRIPTION 
     As illustrated in  FIG.  1   , an external appearance of a refrigerator  1  may be formed by a cabinet  10  which forms a storage space and a door which opens and closes the storage space. 
     An inside of the cabinet  10  may be divided by a barrier  11 , and a refrigerator compartment  12  may be formed at an upper portion of the cabinet  10 , and a freezer compartment  13  may be formed at a lower portion of the cabinet  10 . 
     The door may include a refrigerator compartment door  20  and a freezer compartment door  30 . The refrigerator compartment door  20  may be formed to open and close an opened front surface of the refrigerator compartment  12 , and the freezer compartment door  30  may be formed to open and close an opened front surface of the freezer compartment  13 . 
     One pair of refrigerator compartment doors  20  may be provided left and right, and the refrigerator compartment  12  may be formed to be shielded by the pair of refrigerator compartment doors  20 . The freezer compartment door  30  may be withdrawn in a drawer manner, and thus may be opened and closed by inserting and withdrawing motions. 
     Various accommodation members such as a shelf, a drawer and a basket may be provided inside the refrigerator compartment  12  and the freezer compartment  13 . The accommodation members may be inserted or withdrawn while the refrigerator compartment door  20  and the freezer compartment door  30  are opened, and food may be accommodated and stored in the refrigerator and freezer compartments by the inserting and withdrawing motions of the accommodation members. 
     The exterior of the refrigerator  1  may be formed of a metal material t. A dispenser  22  which dispenses water or ice may be provided at the refrigerator compartment door  20 . 
     A right one of the pair of refrigerator compartment doors  20  may be formed to be doubly opened and closed. Specifically, the right refrigerator compartment door  20  may include a main door  40  which opens and closes the refrigerator compartment  12 , and a sub-door  50  which is rotatably disposed at the main door  40  to open and close an opening part  403  formed at the main door  40 . 
     The main door  40  may be formed to have the same size as that of a left one of the pair of refrigerator compartment doors  20 , may be rotatably installed at the cabinet  10  by an upper hinge  401  and a lower hinge  402 , and may open and close at least a part of the refrigerator compartment  12 . 
     The opening part  403  which is opened in a predetermined size is formed at the main door  40 . A door basket  43  is installed at a rear surface of the main door  40  including an inside of the opening part  403 . A user may have access to the door basket  43  through the opening part  403  without opening of the main door  40 . At this point, a size of the opening part  403  may correspond to most of a front surface of the main door  40  except a part of an edge of the main door  40 . 
     The sub-door  50  is rotatably installed at the front surface of the main door  40 , and opens and closes the opening part  403 . Therefore, access to the opening part  403  may be allowed by opening the sub-door  50 . 
     A size of the sub-door  50  is formed to be same as that of the main door  40  and thus to shield an entire front surface of the main door  40 . The main door  40  and the sub-door  50  may be formed to be coupled to each other while the sub-door  50  is closed, and to have the same size and shape as those of the left refrigerator compartment door  20 . A sub-gasket  591  may be provided at a rear surface of the sub-door  50  to seal between the main door  40  and the sub-door  50 . 
     A panel assembly  54  formed of a transparent material such as glass is provided at a center of the sub-door  50 . Therefore, even while the sub-door  50  is closed, the user may see through an inside of the opening part  403 . The sub-door  50  may be referred to as a see-through door. 
     The panel assembly  54  may be formed to have a controllable light transmittance and reflectivity, and thus may be selectively changed into a transparent or opaque state according to a user&#39;s operation. The panel assembly  54  may be changed into the transparent state only upon a user&#39;s request to allow the user to see through the refrigerator, otherwise may be maintained in the opaque state. 
     A sub-upper hinge  51  and a sub-lower hinge  52  may be provided at an upper end and a lower end of the sub-door  50 , respectively, and the sub-door  50  may be rotatably installed at the front surface of the main door  40 . The sub-door  50  may be formed to be independently rotated while the main door  40  is closed, and thus to open and close the opening part  403 . 
     A handle  23  may be provided at a front surface of the refrigerator compartment door  20 . The handle  23  may be formed at each of adjacent ends of the pair of refrigerator compartment doors  20 . The handle  23  of the right refrigerator compartment door  20  may be provided at a front surface of the sub-door  50 . 
     An operating button  231  is provided at the handle  23 , and a locking unit  232  which is operated by the operating button  231  is provided at the sub-door  50 . The locking unit  232  protrudes to a rear of the sub-door  50 , and may be selectively separated from a restricting member  404  of the main door  40  by an operation of the operating button  231 . 
     Therefore, while the sub-door  50  is closed, the locking unit  232  is maintained in a coupled state to the restricting member  404 . In this state, when the handle  23  is pulled, the main door  40  is rotated, and thus the refrigerator compartment  12  may be opened. 
     When the user pushes the operating button  231  while the sub-door  50  is closed, the locking unit  232  is separated from the restricting member  404 , and only the sub-door  50  may be rotated. The opening part  403  may be opened by rotation of the sub-door  50 . 
     An accommodation case  41  is formed at the rear surface of the main door  40 . The accommodation case  41  serves to define an accommodation space located at the rear surface of the main door  40 , and a plurality of door baskets  43  are disposed therein. 
     As illustrated in  FIGS.  4 - 6   , an upper hinge installation part  405  at which the upper hinge  401  is installed is formed at an upper end of the main door  40  to be recessed. The upper hinge  401  is installed and fixed to the cabinet  10 , and a hinge shaft  4011  is inserted into a hinge hole  4051  of the upper hinge installation part  405 , and thus the main door  40  is rotatably installed at the cabinet  10 . 
     A sub-upper hinge installation part  501  at which the sub-upper hinge  51  is installed is formed at an upper end of the sub-door  50  to be recessed. The sub-upper hinge  51  is installed and fixed to the upper hinge installation part  405 , and a hinge shaft  511  is inserted into a hinge hole  5011  of the sub-upper hinge installation part  501 , and the sub-door  50  is rotatably installed at the main door  40 . 
     The hinge shaft  511  of the sub-upper hinge  51  may be formed in a tube shape having a cross section of which one side is opened. Such a cross-sectional structure provides an internal space of the hinge shaft  511  which further expands. Therefore, electric wires L which are connected with electronic components (a heater, a knock detection device and a door lighting unit which will be described below) provided at the sub-door  50  may be guided outside the sub-door  50  through the hinge shaft  511  of the sub-upper hinge  51 , and may be connected to a main control part  2  located on the cabinet  10 . 
     The upper hinge  401  and the sub-upper hinge  51  are shielded by a main hinge cover  45  and a sub-hinge cover  53 . The main hinge cover  45  and the sub-hinge cover  53  are connected to each other, and the electric wires L which are guided to an outside through the sub-upper hinge  51  may pass through the sub-hinge cover  53 , and may be guided toward the main control part  2  through an inside of the main hinge cover  45 . 
     Specifically, the main hinge cover  45  is formed so that a lower portion of the hinge is opened, and a recessed space is formed in the hinge to accommodate the entire upper hinge  401 . A separate PCB for controlling the electronic components provided at a side of the sub-door  50  may be accommodated inside the main hinge cover  45 . 
     The main hinge cover  45  may have a structure which is matched with one side of the cabinet  10 , or a coupling structure which is installed and fixed by a separate coupling member. A part of the main hinge cover  45  is formed to extend toward the upper hinge installation part  405  of the main door  40 . 
     An extension part  451  which extends toward a rotating axis of the upper hinge  401  may be further formed at one side of the main hinge cover  45 . A main connection part  452  may be formed at an end of the extension part  451 . The main connection part  452  may be formed to be stepped downward from the end of the extension part  451 . The main connection part  452  may be formed in a circular shape, and a center of the main connection part  452  may be located on an extension line of the hinge shaft  4011  of the upper hinge  401 . A connection part hole  453  is opened at an upper surface of the main connection part  452  so that the electric wires L pass through the connection part hole. 
     The sub-hinge cover  53  is formed so that a lower portion thereof is opened, and a recessed space is formed therein to accommodate the entire sub-upper hinge  51 . The sub-hinge cover  53  may be installed and fixed to the upper hinge installation part  405 . And a part of the sub-hinge cover  53  may extend toward the sub-upper hinge installation part  501 , and may shield the sub-upper hinge  51 . 
     A cover fixing part  531  which is recessed downward is formed at one side of the sub-upper hinge  51 , and a coupling member like a screw may pass through the cover fixing part  531 , may be fastened to the sub-upper hinge installation part  501 , and thus may fix the sub-hinge cover  53 . 
     A shape of the sub-hinge cover  53  is formed corresponding to a space between the sub-upper hinge installation part  501  and the main hinge cover  45  while the sub-door  50  is closed, and does not interfere with the sub-door  50  and the main door  40  when the sub-door  50  and the main door  40  are opened and closed. 
     A sub-connection part  532  is formed at one side of the sub-hinge cover  53 . The sub-connection part  532  is formed to extend toward the main door  40 , and seated on the upper surface of the main connection part  452 . At this time, a lower surface of the sub-connection part  532  is opened, and thus an inside of the sub-connection part  532  may be in communication with an inside of the main connection part  452 . Like the main connection part  452 , the sub-connection part  532  is formed to have a circular cross section, and located on an extension line of a center of the rotating axis of the upper hinge  401 . 
     That is, the center of the rotating axis of the upper hinge  401  and centers of the main connection part  452  and the sub-connection part  532  are concentrically disposed. Therefore, while the main door  40  and the sub-door  50  are rotated, the rotating axis of the upper hinge  401 , the main connection part  452  and the sub-connection part  532  may be stably rotated without being shaken. When the main door  40  or the sub-door  50  is rotated, the electric wires L guided through the sub-hinge cover  53  may pass, in turn, through the sub-connection part  532  and the main connection part  452 , and may be guided inside the main hinge cover  45 . 
     The electric wires L may be connected by connectors  502  and  406 , and the connectors  502  and  406  may be located inside the sub-hinge cover  53 . Therefore, after the sub-connector  502  which is connected to ends of the electric wires L exposed outward through a rotating axis of the sub-upper hinge  51  and the main connector  406  of the electric wires L which are connected to the main control part  2  and guided toward the sub-upper hinge installation part  501  through the main hinge cover  45  are connected to each other, the sub-hinge cover  53  may be covered thereon, and thus the electric wires L, the connectors  502  and  406  and the sub-upper hinge  51  may be shielded. 
     The lower hinge  402  which supports a lower end of the main door  40  is installed at one side of a front surface of the cabinet  10 . The sub-lower hinge  52  which supports the sub-door  50  is installed at the lower end of the main door  40 . 
     As illustrated in  FIG.  7   , the lower hinge  402  is installed at the front surface of the cabinet  10 , and serves to enable the main door  40  to be rotatably supported. 
     The lower hinge  402  may include a main fixing part  4021  which is fixed to the front surface of the cabinet  10 , a main support part  4022  which is vertically bent from the main fixing part  4021  and supports the lower end of the main door  40 , and a hinge shaft  4023  which extends upward from the main support part  4022  and is inserted into the main door  40 . The lower end of the main door  40  is supported by the lower hinge  402 , and the main door  40  may be rotated about the hinge shaft  4023 . 
     The sub-lower hinge  52  is located at a front of the lower hinge  402 , and provided at the lower end of the main door  40 . The sub-lower hinge  52  may be installed at a sub-lower hinge installation part  407  which is recessed from the lower end of the front surface of the main door  40 . 
     The sub-lower hinge  52  may include a sub-fixing part  521  which is fixed to the sub-lower hinge installation part  407 , and a sub-support part  522  which is bent from a lower end of the sub-fixing part  521  and extends forward, and a hinge shaft  523  which extends upward from the sub-support part  522  and is inserted into the sub-door  50 . Therefore, the lower end of the sub-door  50  is supported by the sub-lower hinge  52 , and the sub-door  50  may be rotated about the hinge shaft  523 . 
       FIG.  8    is a perspective view of the refrigerator door of the refrigerator when being seen from a rear. 
     As illustrated in  FIG.  8   , a front surface of the accommodation case  41  is opened, and the opened front surface of the accommodation case  41  may be formed corresponding to the size of the opening part  403 . Therefore, when the sub-door  50  is opened, the opening part  403  is exposed, and the access to the door basket  43  inside the accommodation case  41  through the opening part  403  may be allowed. 
     A case door  42  which is opened and closed by rotation may be provided at a rear surface of the accommodation case  41 . While the main door  40  is opened, the access to the door basket  43  may be allowed by opening the case door  42 . 
     A case opening  410  and a door opening  421  which allows the cooling air in the refrigerator to be introduced into the accommodation case  41  may be formed at the accommodation case  41  and the case door  42 . A temperature inside the accommodation case  41  may be maintained to be the same as an internal temperature of the refrigerator compartment  12 . 
     A molded end at which the door basket  43  is installed may be formed at the rear surface of the main door  40 , more specifically, both side surfaces of the opening part  403 . The molded end may be formed in an uneven structure, and may also be formed to enable the door basket  43  to be separated. 
     As illustrated in  FIGS.  9  and  10   , the door basket  43  is formed so that an upper surface of the door basket is opened, and has an installation part  431  which is disposed inside the opening part  403 . The installation part  431  has a width corresponding to that of the opening part  403 , and an installation groove  4311  which is coupled to the molded end of the opening part  403  may be formed at both side surfaces thereof. 
     A rear surface of the installation part  431  which faces the rear surface of the sub-door  50  may be formed to be inclined. Therefore, a predetermined distance may be formed between the rear surface of the sub-door  50  and a rear surface of the installation part  431 , and light emitted from a door lighting unit  49  is not blocked, and the light illuminates the entire opening part  403 . 
     The door basket  43  may include a dividing member  432 , a stopper  433  which enables the dividing member  432  to be fixed at a set angle, and a stopper cap  434  for installation of the stopper  433 . 
     The dividing member  432  may be formed of a wire or rod material, and may have a circular cross section having a predetermined diameter. The dividing member  432  may include a dividing part  4321  which extends to cross the installation part  431 , a first bent part  4322  which is bent vertically from each of both ends of the dividing part  4321 , a second bent part  4323  which is bent vertically from the first bent part  4322 , and an insertion part  4324  which is bent outward from each of both ends of the second bent part  4323  and inserted into the stopper  433 . 
     The stopper  433  is installed at an inner side surface of the installation part  431 , and disposed at each of left and right side surfaces to fix the dividing member  432 . The stopper  433  may be formed in a cylindrical shape, and a cap insertion hole  4331  in which the stopper cap  434  is installed may be formed at one side surface thereof which is contact with the installation part  431 . A stopper hook  4332  which fixes the stopper  433  to the installation part  431  may be formed at an outside of the cap insertion hole  4331 . 
     A center hole  4333  in which the insertion part  4324  of the dividing member  432  is inserted may be formed at a center of opposite surfaces of the stopper  433  which are disposed at both sides of the stopper  433 . A fixing groove which is recessed inward centering on the center hole  4333  may be formed at a circumference of the stopper  433 . The fixing groove is formed in a shape corresponding to that of an outer surface of the dividing member  432 , and accommodates and fixes the second bent part  4323  of the dividing member  432 . 
     The fixing groove may include a first fixing groove  4334  which enables the dividing member  432  to be located at a first position, a second fixing groove  4335  which enables the dividing member  432  to be located at a second position, and a third fixing groove  4336  which enables the dividing member  432  to be located at a third position. The first fixing groove  4334  and the third fixing groove  4336  may be formed at positions which vertically face each other in upward and downward directions of the stopper  433 , and the second fixing groove  4335  may be formed at a middle portion between the first fixing groove  4334  and the third fixing groove  4336 . Each of the fixing grooves may be disposed at an angular interval of 90°, and may fix the dividing member  432  at each position which is rotated by an angle of 90°. 
     The first position corresponds to a state in which the second bent part  4323  is inserted into the first fixing groove  4334 . In this state, the dividing part  4321  of the dividing member  432  is in close contact with an inner wall surface of the door basket  43 , as illustrated in  FIG.  9   , and an entire internal space of the door basket  43  may be used as one accommodation space. 
     The second position corresponds to a state in which the second bent part  4323  is inserted into the second fixing groove  4335 . In this state, the dividing part  4321  of the dividing member  432  is located at a position which is spaced apart from the inner wall surface of the door basket  43 , and thus the internal space of the door basket  43  is divided into two spaces. At this time, the position of the dividing part  4321  is located at a position which is spaced apart from a position of the stopper  433  by a length of the second bent part  4323 , and divides the space. 
     The third position corresponds to a state in which the second bent part  4323  is inserted into the third fixing groove  4336 . In this state, like the second position, the dividing part  4321  of the dividing member  432  is located at a position which is spaced apart from the inner wall surface of the door basket  43 , and the internal space of the door basket  43  is divided into two spaces. 
     At this time, the position of the dividing part  4321  is located at a position which is spaced apart from the position of the stopper  433  by a length of the first bent part  4322 , and divides the space. Since the first bent part  4322  and the second bent part  4323  are formed to have different lengths from each other, the user may selectively use the space according to a divided degree of the space occurring by a length difference between the first bent part  4322  and the second bent part  4323 . 
     In particular, as illustrated in  FIG.  11   , in the case in which the dividing member  432  is located at the second position and the third position, and thus divides the internal space of the door basket  43  into the two spaces, each of the spaces has a narrow width and a high height, and thus may effectively store sauce bottles or tube containers which easily fall down. 
     A stopper fixing hole  435  is formed at both side surfaces of the installation part  431 . A fixing groove  4351  is further formed at the stopper fixing hole  435  so that the stopper hook  4332  is hooked and restricted thereto. Therefore, the stopper  433  may be fixed to the inner side surface of the installation part  431  so as not to be rotated. The stopper cap  434  may be provided outside the installation part  431 . The stopper cap  434  is installed in the stopper fixing hole  435 , and a cap hook  4341  provided at a center of the stopper cap  434  may pass through the stopper fixing hole  435 , and may be hooked and restricted into the cap insertion hole  4331 . Therefore, the stopper  433  may be further firmly fixed by the stopper cap  434 . 
     As illustrated in  FIGS.  12  and  13   , the sub-door  50  may include an outer plate  55  which forms an exterior of the sub-door  50 , the panel assembly  54  which is installed at an opening of the outer plate  55 , a door liner  58  which is installed to be spaced apart from the outer plate  55 , and an upper cap decoration  56  and a lower cap decoration  57  which form an upper surface and a lower surface of the sub-door  50 . 
     The outer plate  55  forms the exterior of the front surface and a part of a perimeter surface of the sub-door  50 , and may be formed of a stainless material. A panel installation opening  551  in which the panel assembly  54  is formed is formed at a center of the outer plate  55 . The panel installation opening  551  is a space which sees through the inside of the opening part  403  of the main door  40 , and may be formed to be the same as or similar to the size of the opening part  403 . A plate bent part  552  which is vertically bent inward along a perimeter of the panel installation opening  551  is formed. The plate bent part  552  is bent so as to be inserted into a support frame  60 , and plate holes  5521  which are opened at regular intervals may be continuously formed at the plate bent part  552 . 
     The panel assembly  54  is formed to shield the panel installation opening  551 , and while the panel assembly  54  is installed, a front surface of the panel assembly  54  may form the same plane as a front surface of the outer plate  55 . The panel assembly  54  may be selectively transparent according to On/Off of the door lighting unit  49 , and may be formed to enable the user to selectively see through the inside of the opening part  403 . The upper cap decoration  56  forms the upper surface of the sub-door  50 , and is coupled to upper ends of the outer plate  55  and the door liner  58 . The sub-upper hinge installation part  501  is formed at one end of the upper cap decoration  56 , and an upper boss  5012  in which the hinge shaft  523  of the upper hinge  401  is inserted may be installed at the sub-upper hinge installation part  501 . 
     The lower cap decoration  57  forms the lower surface of the sub-door  50 , and is coupled to lower ends of the outer plate  55  and the door liner  58 . A hinge plate  571  is installed at a position of the lower cap decoration  57  at which the sub-lower hinge  52  is installed, and a lower boss  572  in which the hinge shaft  523  of the sub-lower hinge  52  is inserted may be installed at the hinge plate  571 . 
     Two foaming solution injecting holes  573  may be formed at the lower cap decoration  57 . The foaming solution injecting holes  573  are formed to inject a foaming solution which forms an insulation at a perimeter of the panel assembly  54  in an internal space of the sub-door  50 . At this point, to smoothly inject the foaming solution into the sub-door  50 , a vertical lower side corresponding to a space between an outside of the panel assembly  54  and an inner side surface of the sub-door  50  is formed to be opened. When the foaming solution is injected, the foaming solution may be prevented from flowing back or being incompletely filled, and may be effectively injected. The foaming solution injecting holes  573  may be shielded by injecting hole caps  574  which are installed at the lower cap decoration  57 . 
     The door liner  58  forms the rear surface of the sub-door  50 , and a liner opening  581  is formed at an area at which the panel assembly  54  is disposed. A liner groove  582  which maintains a shape of the door liner  58  may be formed along the liner opening  581  at a perimeter of the door liner  58 . The liner groove  582  is recessed from the rear surface of the sub-door  50 , and the sub-gasket  591  which seals between the sub-door  50  and the main door  40  may be installed thereat. 
     The locking unit  232  may be installed at one side of the door liner  58 , and a knock detection device  82  and a door opening assist device  588  which are described below in detail may be installed. 
     A heater  59  may be provided inside the sub-door  50 . The heater  59  is disposed along a perimeter of a rear surface of the panel assembly  54 . Specifically, the heater  59  may be disposed at a rear surface of a front panel  541  which forms the front surface of the panel assembly  54 , and may be installed at an area of a bezel  5411 , which is formed at a perimeter of the front panel  541 , so as not to be exposed outside. 
     The support frame  60  which fixes the outer plate  55  and the panel assembly  54  along the perimeter of the panel installation opening  551  is provided at a rear surface of the outer plate  55 . The support frame  60  may be formed by coupling an upper frame  61 , a lower frame  63  and one pair of side frames  62 , may also be formed along an edge of the panel installation opening  551  to accommodate the plate bent part  552 , and may be in contact with the rear surface of the panel assembly  54 . The support frame  60  may fix the heater  59  which is disposed at the rear surface of the panel assembly  54 . The door lighting unit  49  may be installed at an upper end of the liner opening  581 . The door lighting unit  49  may illuminate a portion at the plurality of door baskets  43  disposed at the main door  40  are arranged, may enable the inside of the refrigerator to be brighter than an outside thereof, and thus may enable the panel assembly  54  to look transparent. 
     The door lighting unit  49  may include a lamp case  491  which is installed at the door liner  58 , a lamp PCB  492  which is accommodated in the lamp case  491  and at which a plurality of LEDs  4921  are disposed, and a lamp cover  493  which shields an opened lower surface of the lamp case  491  and is exposed through the opening. 
     As illustrated in  FIGS.  14 ,  15  and  16    the door lighting unit  49  may be installed at a lighting unit installation part  583  which is formed at an upper end of the liner opening  581  of the door liner  58 . 
     A case fixing part  5831  is formed at both ends of the lighting unit installation part  583 , and a case rib  4911  which protrudes from both ends of the lamp case  491  is hooked and restricted to the case fixing part  5831 , and thus the door lighting unit  49  may be installed and fixed. 
     The lamp cover  493  is formed to extend long along the door liner  58 , and includes a recessed part  4912  which is formed in the lamp cover to have a recessed space for accommodating the lamp PCB  492 . Specifically, a surface of the recessed part  4912  which faces the lamp PCB  492  may be formed to be rounded, and light emitted from the lamp PCB  492  is reflected by a round surface  4913  having a predetermined curvature, and directed to the lamp case  491 . A film which increases reflectivity of the light may be attached or coated on an inner surface of the recessed part  4912 , in particular, the round surface  4913 . 
     A lamp PCB installation part  4914  at which the lamp PCB  492  is installed is formed at one surface which faces the round surface  4913 . The lamp PCB installation part  4914  is formed so that the lamp PCB  492  is installed and fixed in a direction crossing the lamp cover  493  at an inside of the lamp case. 
     An electric wire entrance hole  4915  through which access of the electric wires L connected to the lamp PCB  492  is allowed may be formed at one side of the lamp PCB installation part  4914 . The electric wire entrance hole  4915  may be guided to the internal space of the sub-door  50 , and may pass through the hinge shaft  511  of the sub-upper hinge  52 . 
     The lamp cover  493  is formed so that the light reflected by the round surface  4913  of the recessed part  4912  is transmitted through the lamp cover, and also formed to shield an opening of the recessed part  4912 . The lamp cover  493  is formed to be transparent or semitransparent, such that the light reflected by the round surface  4913  and uniformly spread is transmitted. The light passing through the lamp cover  493  illuminates the inside of the refrigerator in an indirect lighting method, and has a surface emitting effect. 
     The film may be attached to or coated on the lamp cover  493  to more effectively diffuse the light. When the lamp cover  493  is injection-molded, particles or a material which diffuses the light may be added. 
     The lighting unit installation part  583  at which the door lighting unit  49  is installed extends toward the inside of the opening part  403 , and the lamp cover  493  radiates downward the light from an upper end of the opening part  403  when the door lighting unit  49  is installed. 
     Rear ends of the plurality of door baskets  43  installed at the opening part  403  are spaced apart from the rear surface of the sub-door  50 , and located at the inside of the refrigerator further than the door lighting unit  49 . As illustrated in  FIG.  15   , the light emitted downward from the door lighting unit  49  is prevented from being blocked by the door baskets  43  or the food accommodated in the door baskets  43 . 
     A front surface of each of the door baskets  43  close to a rear end of the panel assembly  54  is formed to be inclined, and the light emitted downward from the door lighting unit  49  may be prevented from being interfered, and may also illuminate the entire opening part  403 . 
     The door lighting unit  49  may be selectively turned on/off according to a user&#39;s operation. When the door lighting unit  49  is turned on, the accommodation space formed at the main door  40  is illuminated. When the door lighting unit  49  is turned on, and the accommodation space of the main door  40 , i.e., the inside of the refrigerator becomes brighter than the outside, the light emitted from the door lighting unit  49  penetrates the sub-door  50 . The user may recognize the transparent sub-door  50 , and may see the accommodation space inside the main door  40  through the sub-door  50  from the outside. 
     The panel assembly  54  may be installed and fixed to the sub-door  50  by the support frame  60 . One side of the support frame  60  may be fixed to the outer plate  55 , and the other side of the support frame  60  may be bonded to the panel assembly  54  so that the panel assembly  54  is fixed to the panel installation opening  551 . 
     The panel assembly  54  may include the front panel  541 , at least one or more insulation panels  542  disposed at a rear of the front panel  541 , and a spacer bar  543  which supports the front panel  541  and the insulation panel  542  and also supports between a plurality of insulation panels  542 . 
     The front panel  541  may be formed of a glass material which is selectively see through according to the light transmittance and reflectivity, and may be referred to as a half mirror. 
     That is, in a state in which the door lighting unit  49  is turned on, the light inside the refrigerator penetrates the front panel  541 , and the front panel  541  looks transparent. Therefore, the space inside the refrigerator located at a rear of the sub-door  50  or the accommodation space formed at the main door  40  may be seen from the outside while the sub-door  50  is closed. 
     When the door lighting unit  49  is turned off, the light does not penetrate the front panel  541 , but is reflected, and the front panel  541  serves as a mirror surface. In this state, the space inside the refrigerator located at the rear of the sub-door  50  or the accommodation space formed at the main door  40  may not be seen from the outside. 
     The front panel  541  may be surface-treated in various methods so that a half mirror effect is provided at the front panel  541 . For example, a titan compound may be vacuum-deposited on a front surface of a glass layer forming the front panel  541 . 
     A silk-screen printing may be performed on a rear surface of the glass layer forming the front panel  541  using a titan compound (TiO 2 )-based printing ink which includes a resin for adjusting viscosity, an organic solvent and an additive, and thus a ceramic printed layer may be formed. 
     In some examples, a hard coating layer configured with triple layers of iron, cobalt and chrome may be formed on the rear surface of the glass layer forming the front panel  541  using an atmospheric pressure chemical vapor deposition (APCAVD) method or a spraying method in which a liquid coating material is sprayed. 
     When the ceramic printed layer or the hard coating layer is formed on the rear surface of the glass layer, the front panel  541  may be formed to have a light transmittance of 20% to 30% from the outside. When the transmittance is 20% or less, it is not easy to see through the inside from the outside even when the door lighting unit  49  is turned on, and when the transmittance is 30% or more, the front surface of the panel assembly  54  does not look like the mirror surface, and a part of the inside of the refrigerator may be seen through. 
     The bezel  5411  which is formed along the perimeter of the rear surface of the front panel  541  may be formed so that the light is not transmitted through it, and an edge of the front panel  541  at which the bezel  5411  is provided is formed to extend outward further than the insulation panel  542 . Therefore, at the rear surface of the front panel  541  at which the bezel  5411  is formed, the support frame  60 , the heater  59  and the spacer bar  543  are provided so as not to be exposed forward through the front panel  541 . 
     The knock detection device  82  may be disposed at the bezel  5411  which is formed at a lower end of the front panel  541 . Therefore, the knock detection device  82  may also be covered by the bezel  5411  so as not to be exposed to the outside. 
     Specifically, the support frame  60  is disposed at an area of the bezel  5411  to fix the panel assembly  54 . The heater  59  fixed by the support frame  60  may also be located at the area of the bezel  5411 . Therefore, all of the heater  59  and the support frame  60  which are disposed along an edge of the panel assembly  54  are covered by the bezel  5411  and thus not exposed to the outside. 
     The spacer bar  543  is formed at the perimeter of the rear surface of the front panel  541 . The spacer bar  543  serves to enable the front panel  541  and the insulation panel  542  to be spaced apart from each other, and also to seal the space between the front panel  541  and the insulation part  542 . 
     The spacer bar  543  may also be disposed between the plurality of insulation panels  542 . The front panel  541 , the insulation panel  542  and the plurality of spacer bars  543  may be bonded to each other by an adhesive, and a sealant may be coated to seal an outer surface of the panel assembly  54  and among the front panel  541 , the insulation panel  542  and the spacer bar  543 . 
     The insulation panel  542  may be formed to have a size smaller than that of the front panel  541 , and may be located within an internal area of the front panel  541 . The insulation panel  542  may be chemical strengthening glass in which glass is soaked in an electrolyte solution at a glass transition temperature or more, and chemically strengthened. 
     A low-radiation coating layer for reducing heat transfer into the storage compartment due to radiation may be formed at a rear surface of the insulation panel  542 . Glass on which the low-radiation coating layer is formed is referred to as low-c glass. The low-radiation coating layer may be formed by sputtering silver or the like on a surface of the glass. 
     A sealed space between the front panel  541  and the insulation panel  542  and a sealed space between the plurality of insulation panels  542  which are formed by the spacer bar  543  may create a vacuum state so as to be insulated. 
     In some examples, an inert gas for the insulation, such as argon, may be filled in the sealed space between the front panel  541  and the insulation panel  542  and the sealed space between the plurality of insulation panels  542 . Insulation performance may be ensured by forming a predetermined space between the front panel  541  and the insulation panel  542  and between the plurality of insulation panels  542  in which the inert gas is filled since inert gases have a better insulation property than air. 
     The insulation panel  542  may be formed in a single panel, and may be installed to be spaced apart from the front panel  541 . Two or more insulation panels  542  may be provided to be spaced apart from each other. 
     As illustrated in in  FIG.  17   ., the support frame  60  may be formed so that the upper frame  61  and the lower frame  63  are disposed up and down, and the pair of side frames  62  connect between the upper frame  61  and the lower frame  63 , and thus may be formed to have a quadrangular shape. 
     The support frame  60  may be injection molded with a plastic material. The support frame  60  may be formed so that the plate bent part  552  of the outer plate  55  is inserted into the support frame  60  while the support frame  60  is assembled, and may also be formed so that the heater  59  is fixed to the frame  60 . 
     Specifically, plate insertion parts  611 ,  621  and  631  in which the plate bent part  552  is inserted may be formed at the assembled support frame  60 . The plate insertion parts  611 ,  621  and  631  may be formed along an entire perimeter of the support frame  60 . A plurality of restricting protrusions  612 ,  622  and  632  may be formed at the plate insertion parts  611 ,  621  and  631  to be spaced apart from each other at regular intervals. A plurality of holes  614 ,  624  and  634  corresponding to the restricting protrusions  612 ,  622  and  632  may be formed at the plate insertion parts  611 ,  621  and  631  so that the foaming solution is easily introduced into the plate insertion parts  611 ,  621  and  631 . The restricting protrusions  612 ,  622  and  632  may be formed at positions corresponding to the plurality of holes  614 ,  624  and  634 , and thus the restricting protrusions  612 ,  622  and  632  may be easily molded during an injection molding. 
     Heater grooves  613 ,  623  and  633  may be formed at the support frame  60  to correspond to an arrangement of the heater  59 , and the heater  59  may be installed. 
     An entire width of the support frame  60  is formed to be covered by the bezel  5411  provided at the panel assembly  54 , and may be formed to extend to an outer end of the insulation panel  542 . 
     As illustrated in  FIGS.  18  and  19   , the upper frame  61  forms an upper portion of the support frame  60 . Both ends of the upper frame  61  are bent downward, and may be coupled to upper ends of the side frames  62 . To this end, both ends of the upper frame  61  may be bent downward, and then may be opened. An upper hook groove  615  in which a side hook  625  formed at the upper end of the side frame  62  is inserted may be formed at each of both ends of the upper frame  61 . 
     An upper insertion part  611  may be formed to be recessed from one surface of the upper frame  61  which is in contact with the rear surface of the outer plate  55 . The upper insertion part  611  is formed so that a portion of the plate bent part  552  which forms an upper end of the panel installation opening  551  is inserted therein. 
     The upper insertion part  611  may be formed to have a thickness greater than that of the outer plate  55  and a protruding height of an upper restricting protrusion  612  formed at the upper insertion part  611 . Therefore, the plate bent part  552  may be inserted into the upper insertion part  611 , and then the upper restricting protrusion  612  may be inserted into the plate hole  5521 , and thus the outer plate  55  and the upper frame  61  may be coupled to each other. 
     An upper heater groove  613  may be formed at an inner side further than the upper insertion part  611 , i.e., at an opening side of the support frame  60 , and thus may accommodate an upper portion of the heater  59  which is disposed along the edge of the front panel  541 . An electric wire entrance part  616  is formed at one side of the upper heater groove  613 , and thus the electric wire L connected to the heater  59  may enter therethrough. 
     An upper hole  614  may be further formed at the upper insertion part  611  corresponding to the upper restricting protrusion  612 , and the foaming solution for forming the insulation may be injected through the upper hole  614 , and may be filled inside the upper insertion part  611  of the upper frame  61 . 
     As illustrated in  FIGS.  20  and  21   , the side frames  62  form both side surfaces of the support frame  60 . Each of the side frames  62  is formed to have a length which connects between the upper frame  61  and the lower frame  63 . To this end, the side hook  625  is formed at an upper end and a lower end of each of the side frames  62 . The side hook  625  may be coupled to the upper hook groove  615  of the upper frame  61  and a lower hook groove  635  of the lower frame  63 . 
     Side ribs  626  which are inserted into the upper insertion part  611  of the upper frame  61  and a lower insertion part  631  of the lower frame  63  may be formed at the upper end and the lower end of each of the side frames  62 , respectively. Therefore, the side frames  62 , the upper frame  61  and the lower frame  63  may be maintained in a stably coupled state to each other. 
     A side insertion part  621  may be formed to be recessed from one surface of each of the side frames  62  in contact with the rear surface of the outer plate  55 . The side insertion part  621  may be formed to be inserted into a portion of the plate bent part  552  which forms both side ends of the panel installation opening  551 . 
     At this time, the side insertion part  621  may be formed to have a thickness greater than that of the outer plate  55  and a protruding height of a side restricting protrusion  622  formed at the side insertion part  621 . Therefore, the plate bent part  552  may be inserted into the side insertion part  621 , and then the side restricting protrusion  622  may be inserted into the plate hole  5521 , and the outer plate  55  and the side frames  62  may be coupled to each other. 
     A side heater groove  623  may be formed at an inner side further than the side insertion part  621 , and may accommodate a side portion of the heater  59  which is disposed along the edge of the front panel  541 . 
     Meanwhile, a side hole  624  may be further formed at the side insertion part  621  corresponding to the side restricting protrusion  622 , and the foaming solution may be injected through the side hole  624 , and may be fully filled inside the side insertion part  621  of each of the side frames  62 . 
     As illustrated in  FIGS.  22  and  23   , the lower frame  63  forms a lower portion of the support frame  60 . Both ends of the lower frame  63  are bent upward, and may be coupled to lower ends of the side frames  62 . To this end, both ends of the lower frame  63  may be bent upward, and then may be opened. The lower hook groove  635  in which the side hook  625  formed at the lower end of the side frame  62  is inserted may be formed at each of both ends of the lower frame  63 . 
     The lower insertion part  631  may be formed to be recessed from one surface of the lower frame  63  which is in contact with the rear surface of the outer plate  55 . The lower insertion part  631  is formed so that a portion of the plate bent part  552  which forms a lower end of the panel installation opening  551  is inserted into the lower insertion part  631 . 
     The lower insertion part  631  may be formed to have a thickness greater than that of the outer plate  55  and a protruding height of a lower restricting protrusion  632  formed at the lower insertion part  631 . Therefore, the plate bent part  552  may be inserted into the lower insertion part  631 , and then the lower restricting protrusion  632  may be inserted into the plate hole  5521 , and thus the outer plate  55  and the lower frame  63  may be coupled to each other. 
     A lower heater groove  633  may be formed at an inner side further than the lower insertion part  631 , and may accommodate a lower end of the heater  59  which is disposed along the edge of the front panel  541 . 
     Meanwhile, a lower hole  634  may be further formed at the lower insertion part  631  corresponding to the lower restricting protrusion  632 , and the foaming solution may be injected through the lower hole  634 , and thus may be fully filled inside the lower insertion part  631  of the lower frame  63 , and the insulation may be formed. 
     A detection device hole  636  through which the knock detection device  82  which will be described below in detail is exposed is formed at one side of the lower frame  63 . Therefore, the knock detection device  82  may be provided at a rear of the lower frame  63 , and may be in close contact with the rear surface of the front panel  541  through the detection device hole  636 . 
     Referring to  FIGS.  24 - 27   , all of the upper frame  61 , the side frames  62  and the lower frame  63  are attached to a protruding edge of the front panel  541 . The upper frame  61 , the side frames  62  and the lower frame  63  may be bonded to the rear surface of the front panel  541 , at which the bezel  5411  is formed, by an adhesive sheet  5412  or an adhesive. 
     Ends of the upper frame  61 , the side frames  62  and the lower frame  63  may extend to an outer end of the insulation panel  542 . The upper heater groove  613 , the side heater groove  623  and the lower heater groove  633  are formed at the ends of the upper frame  61 , the side frames  62  and the lower frame  63 , and thus the edge of the front panel  541  of which a temperature of the rear surface thereof is relatively lower than that of the front surface thereof may be heated. And an aluminum sheet  5413  having a predetermined width may be disposed between the heater  59  and the front panel  541 . Therefore, when the heater  59  generates heat, an area at which the bezel  5411  of the front panel  541  is formed may be equally heated. 
     The plate bent part  552  of the outer plate  55  is inserted into the upper insertion part  611 , the side insertion part  621  and the lower insertion part  631  of the upper frame  61 , the side frame  62  and the lower frame  63  which are attached to the front panel  541 . 
     As illustrated in  FIG.  24   , the plate bent part  552  is in close contact with the front panel  541  while being inserted into the side insertion part  621 . The plate hole  5521  of the plate bent part  552  is hooked and restricted to the side restricting protrusion  622 , and thus the outer plate  55  and the side frames  62  may be coupled and fixed to each other. 
     The foaming solution may be introduced into the side insertion part  621  through the side hole  624 , and the introduced foaming solution may enable the plate bent part  552  to be in close contact with the front panel  541 . 
     Due to such a structure, the plate bent part  552  formed at the outer plate  55  may be in close contact with an outer end of the front panel  541 . Therefore, when seen from a front, a gap between an end of the outer plate  55  forming the panel installation opening  551  and the outer end of the front panel  541  may be minimized. 
     As illustrated in  FIGS.  28  and  29   , a sensor opening  584  in which the knock detection device  82  is inserted is formed at a lower portion of the door liner  58 . The sensor opening  584  is formed so that the knock detection device  82  is inserted from an outside. A detection device PCB  83  for processing a knock detecting signal of the knock detection device  82  may be formed to have a shape corresponding to an insert cover  585  which will be installed, and may be shielded by the insert cover  585 . 
     In a state in which the insert cover  585  is installed, an outer surface of the insert cover  585  has the same shape as that of an outer surface of the door liner  58 . Therefore, the insert cover  585  is installed at a position through which the liner groove  582  formed at the door liner  58  passes, and a cover groove  5854  which is connected with the liner groove  582  is formed at the insert cover  585 . Therefore, the sub-gasket  591  may be installed to pass through the liner groove  582  and the cover groove  5854 . 
     A detection device accommodating part  586  may be formed along a perimeter of the sensor opening  584 . The detection device accommodating part  586  forms a part of a space which accommodates the knock detection device  82 , is formed to extend toward the outer plate  55 , and coupled to an accommodating part case  587  so as to form a space for accommodating the knock detection device  82 . 
     The accommodating part case  587  is coupled to a front end of the detection device accommodating part  586 , and forms a remaining space for accommodating the knock detection device  82 . A case coupling part  5871  to which a screw is fastened is formed at both ends of the accommodating part case  587 , and thus the accommodating part case  587  may be coupled and fixed to the detection device accommodating part  586 . 
     A detection device inserting part  5872  in which a front end of the knock detection device  82  is inserted is formed at the accommodating part case  587 . An insertion part hole  5873  is formed at a front surface of the detection device inserting part  5872 , and the detection device inserting part  5872  is inserted into the detection device hole  636 . Therefore, a front surface of the knock detection device  82  which is inserted into the detection device inserting part  5872  may pass through the insertion part hole  5873  and the detection device hole  636 , and may be in close contact with the rear surface of the front panel  541 . 
     An end of the detection device accommodating part  586  and an end of the accommodating part case  587  which are in contact with each other may be formed to have slopes corresponding to each other, and thus may be coupled to each other with orientation. Due to coupling between the detection device accommodating part  586  and the accommodating part case  587 , a space in which the knock detection device  82  and the detection device PCB  83  are installed is formed inside the sub-door  50 . 
     While the detection device accommodating part  586  and the accommodating part case  587  are coupled to each other, the detection device accommodating part  586  and the accommodating part case  587  form a space independent from the space inside the sub-door  50  in which the foaming solution is injected. Therefore, the user may insert and install the knock detection device  82  through the sensor opening  584  after a foaming process is performed at the sub-door  50 . 
     Meanwhile, the insert cover  585  may include a cover part  5851  which shields the sensor opening  584 , and a PCB insertion part  5852  in which the detection device PCB  83  is installed. The cover part  5851  is formed to have a shape corresponding to the sensor opening  584  and to shield the sensor opening  584 , and forms a part of an exterior of the door liner  58 . And a screw hole  5853  may be formed at one side of the cover part  5851 , and the cover part  5851  may be fixed to the door liner  58  by a screw. 
     The PCB insertion part  5852  is formed to extend from the cover part  5851 , and also formed to be inserted into the detection device accommodating part  586  when the insert cover  585  is inserted. The cover part  5851  may be formed corresponding to a size of the detection device PCB  83  so that the insert cover  585  is inserted and installed into the sensor opening  584  while the detection device PCB  83  is installed. The electric wire L which is connected to the knock detection device  82  may be connected to the adjacent detection device PCB  83 . Since the detection device PCB  83  may be disposed at a lateral side of the knock detection device  82 , the detection device PCB  83  may be connected to the knock detection device  82  in a short distance. 
     In the case in which the detection device PCB  83  for processing the signal is located at a distance, there may be a problem that noise generated when the signal to be processed is transferred through a signal line may be increased. However, since the detection device PCB  83  is located at a position at which the knock detection device  82  is installed, the main control part  2  receives only a valid knock-on signal. Accordingly, the noise due to the signal line between the main control part  2  and the detection device PCB  83  may be minimized. That is, the main control part  2  may receive the signal of which the noise is minimized through the detection device PCB  83 . Therefore, it is possible to ensure an accurate recognition rate. 
     In particular, in the case of the knock detection device  82 , a signal output through a microphone  8211  is indicated by mV unit, but the main control part  2  which controls an entire operation of the refrigerator generally receives a signal which is basically indicated by V unit. Therefore, due to a scale difference in a physical signal, it is not preferable that the main control part  2  determines whether the knock-on signal is normal. 
     The refrigerator is an electronic appliance using a high voltage/a high current. Therefore, an electrical noise generation amount is relatively great. This means that the signal of mV unit output from the microphone  8211  may be further vulnerable to the electrical noise. 
     Therefore, since the detection device PCB  83  is located close to the knock detection device  82 , the noise may be remarkably reduced, and thus the recognition rate may be enhanced. 
     Referring to  FIGS.  30 - 33   , a structure of the knock detection device  82  will be described in detail. The knock detection device  82  is a device which detects a user&#39;s knocking operation on the sub-door  50 . 
     The knock detection device  82  may include a microphone module  821  which detects the knock-on signal, a holder  823  which accommodates the microphone module  821 , an elastic member  824  which presses the holder  823  and the microphone module  821  toward the front panel  541  so that the holder  823  and the microphone module  821  are in close contact with the front panel  541 , and a support member  825  which supports the elastic member  824  and the holder  823 . 
     The microphone module  821  includes the microphone  8211  which directly senses a sound wave, and a microphone accommodation part  8212  which accommodates the microphone  8211 . The microphone  8211  serves to directly sense the sound wave, is formed in a circular shape having a predetermined thickness, and installed and fixed into the microphone module  821 . One surface of the microphone  8211  may be referred to as a sound wave receiving part  8213  which receives the sound wave, and the sound wave receiving part  8213  is disposed toward an opening  8214  of the microphone accommodation part  8212 . The other side of the microphone  8211  may be connected to a signal line  8216 , and the signal line  8216  may also be connected to the detection device PCB  83 . 
     The microphone accommodation part  8212  is formed of an elastic material such as rubber, and also formed to be in close contact with the front panel  541 . To this end, the opening  8214  may be formed at one side of the microphone accommodation part  8212  close to the microphone  8211  installed in the microphone accommodation part  8212 , and a circular protrusion  8215  may be formed at a circumference of the opening  8214 . The protrusion  8215  serves to enable the microphone accommodation part  8212  not to be inclined in one direction when the microphone accommodation part  8212  is in close contact with the front panel  541 , and also to enable an entire opened front surface of the opening part  403  to be maintained in a closely contacting state with the front panel  541 . 
     A predetermined sealed space may be formed between the opening  8214  and the sound wave receiving part  8213  which are in close contact with each other by the protrusion  8215 . Therefore, a front of the closely contacting space is sealed by a medium, i.e., the front panel  541 . Accordingly, vibration transmitted through an inside of the medium vibrates air in the predetermined space, and the sound wave due to the vibration may be received by the microphone  8211 . 
     Due to such a sealing process, introduction of external noise or vibration into the predetermined space may be minimized. Thus, an error in determining a knock-on operation or a malfunction due to the external noise may be considerably reduced, and very accurate recognition rate may be ensured. That is, accuracy in determining the knock-on operation when a knock-on input is applied may be remarkably increased. 
     A module seating part  8231  in which the microphone module  821  is accommodated and which is opened toward the front panel  541  may be formed at the holder  823 . The microphone module  821  may be formed so that at least the protrusion  8215  protrudes further than a front surface of the holder  823  while the microphone module  821  is seated on the module seating part  8231 . 
     A holder slot  8232  through which the signal line connected to the microphone  8211  passes is formed at the holder  823 . The holder slot  8232  is formed to be opened at one side of the module seating part  8231 . 
     Also, a first elastic member fixing part  8233  which protrudes so that the elastic member  824  is installed and fixed thereto is formed at a rear surface of the holder  823 . The first elastic member fixing part  8233  may be formed to extend and to pass through one end of the elastic member  824  having a coil shape. 
     A holder coupling part  8234  which is formed in a hook shape and coupled to the support member  825  is formed at both sides of the holder  823 . Due to the holder coupling part  8234 , the holder  823  is coupled so as not to be separated by the support member  825 . And also, due to the hook shape of the holder coupling part  8234 , movement of the holder  823  in a direction which is inserted into the support member  825  is not restricted. 
     A front surface of the support member  825  is formed to be opened, and also formed so that the holder  823  is inserted through the opened front surface thereof. And a second elastic member fixing part  8251  which protrudes so that the elastic member  824  is installed and fixed thereto may be formed at an inside of the holder  823 . The second elastic member fixing part  8251  may be located on an extension line of the first elastic member fixing part  8233 , and may be inserted so as to pass through one end of the elastic member  824 . 
     Therefore, even though the elastic member  824  is compressed to press the holder  823 , the elastic member  824  may stably press the holder  823  toward the front panel  541  without being buckled. 
     By the elastic member  824 , the microphone module  821  may be maintained in a closely contacting state with the front panel  541 , and particularly, may be always maintained in the closely contacting state with the front panel  541  without a position change of the microphone module  821  due to a shock generated when the main door  40  and the sub-door  50  are closed and opened or an inertial force generated when the main door  40  and the sub-door  50  are rotated. 
     A support member slot  8252  may be formed at one side of the support member  825 . The support member slot  8252  may be formed on an extension line of the holder slot  8232 . Therefore, the signal line passing through the holder slot  8232  may pass through the support member slot  8252 , and may be connected to the detection device PCB  83 . 
     A support member fixing part  8254  is formed at the other side of the support member  825 . The support member fixing part  8254  may be formed to extend outward, and may be installed and fixed to a detecting device coupling boss  5874  formed on the accommodating part case  587  by a screw. The detecting device coupling boss  5874  is formed to be exposed through the sensor opening  584 , and enables the knock detection device  82  to be installed and fixed through the sensor opening  584  after the knock detection device  82  is inserted. 
     The knock detection device  82  is located at the edge of the front panel  541 , but an effective input part for the user&#39;s knocking operation is not limited to this configuration. In a state in which the knock detection device  82  is in close contact with the medium, even though the knocking operation is applied to any positions, the sound wave may be transmitted through the continuous same medium due to a property of the microphone  8211  which detects the sound wave generated by the vibration, instead of the vibration itself, and thus may be effectively detected. Therefore, a position of the knock detection device  82  may be disposed at one end at which the electric wires may be arranged and a visible area of the sub-door  50  may also be maximized. At the same time, even though the user knocks on any point of the front panel  541 , the sound wave may be detected through the microphone  8211  which is in close contact with the same medium. 
     Specifically, an area to which a user&#39;s knocking input is applied may be an entire area which is defined by the front surface of the front panel  541 . Most of the front panel  541  except a boundary portion thereof is a see-through area which selectively becomes transparent, and the knock detection device  82  may not be disposed thereat. 
     Therefore, it is preferable that the knock detection device  82  is located at the area of the bezel  5411  in the front panel  541 . In particular, the bezel  5411  located at the upper end and left and right sides of the front panel  541  may be minimized by locating the knock detection device  82  at the lower end of the front panel  541  rather than both of the left and right sides thereof. By such a shape of the bezel  5411 , the see-through area may be expanded. Since the knock detection device  82  is located at the lower end of the front panel  541  on which a user&#39;s eyes are relatively less focused, the wider see-through area may be provided to the user. 
     Since the knock detection device  82  is located at the area of the bezel  5411 , is not exposed to an outside, and has a structure which is in close contact with the front panel  541 , the user&#39;s knocking operation may be detected even through the user knocks on any position of the front panel  541 . 
     The front surface of the panel assembly  54  may be vibrated by the shock generated when the main door  40  and the sub-door  50  are opened and closed, an external loud noise or the like, and such an input due to the external environments may be recognized as a knock signal. 
     The detection device PCB  83  may be set so that a user&#39;s operation which knocks several times the front surface of the panel assembly  54  may be recognized as a normal knock input. More specifically, the user&#39;s operation which knocks several times the front surface of the panel assembly  54  at predetermined time intervals may be recognized as the normal knock input. 
     For example, when the user knocks twice the front surface of the panel assembly  54  within a predetermined time, it may be recognized as the normal knock input. 
     The door opening assist device  588  is provided at the lower portion of the door liner  58 . The door opening assist device  588  may include an elastic member accommodation part  5881  which is recessed inside the door liner  58 , an elastic member  5882  which is accommodated in the elastic member accommodation part  5881 , and a pressing member  5883  which is supported by the elastic member  5882 . 
     While an external force is not applied, the pressing member  5883  is supported by the elastic member  5882 , and protrudes from the door liner  58 , as illustrated in  FIG.  33   . Therefore, when the sub-door  50  is closed, the pressing member  5883  becomes in contact with the main door  40 , and compresses the elastic member  5882 . When the user operates the operating button  231  of the handle  23  to open the sub-door  50  while the sub-door  50  is closed, the coupling between the locking unit  232  and the restricting member  404  is released, and the sub-door  50  is rotated and opened by an elastic force of the elastic member  5882 . 
     As illustrated in  FIGS.  34 - 37   , the refrigerator  1  includes the main control part  2  which controls the operation of the refrigerator  1 , and the main control part  2  may be connected to a door switch  21 . The door switch  21  may be provided at the cabinet  10 , and may detect opening of the refrigerator compartment door  20  or the main door  40 , and may also be provided at the main door  40 , and may detect opening of the sub-door  50 . 
     The main control part  2  may be connected to the door lighting unit  49 , and may enable the door lighting unit  49  to be turned on when the sub-door  50  is opened or the knock-on signal is input. The main control part  2  may be connected to the detection device PCB  83  which is connected to the knock detection device  82 . 
     In a general state in which a separate operation is not applied to the refrigerator  1 , the panel assembly  54  is in the opaque state like the mirror surface, as illustrated in  FIG.  36   . In this state, it is impossible to see through the inside of the refrigerator. 
     In this state, the knock detection device  82  is maintained in an activated state in which the user may input the operation anytime [S 110 ]. 
     In this state, when the user performs a knock-on operation which knocks on the front surface of the sub-door  50 , i.e., the front panel  541  to check a food storing state in the refrigerator, the knock detection device  82  may detect the knock-on operation, and the detection device PCB  83  determines whether the knock-on operation is valid. 
     Specifically, when the user knocks on the front panel  541 , the sound wave due to the vibration generated at this point is transmitted along the front panel  541  formed of the same medium, and the microphone  8211  which is in close contact with the front panel  541  receives the sound wave. 
     The received sound wave is filtered and amplified while passing through a filter and an amplifier, and transmitted to the detection device PCB  83 . The detection device PCB  83  determines the knock with the signal which is collected and analyzed to detect the knock signal. 
     That is, in the case of the sound wave which is generated by the noise or the shock inside or outside the refrigerator  1 , there is a difference from the sound wave generated by the knocking operation in a property thereof, and thus the detection device PCB  83  determines whether the user performs the knocking operation through the signal corresponding to the property of the knock signal. 
     In a specific situation, a signal similar to the knock signal may be generated, or a shock similar to the knock may be applied to the front panel  541  due to the user&#39;s carelessness or inexperienced operation, or the external noise may be recognized as a signal similar to a wavelength of the knock signal. 
     When the main door  40  or the sub-door  50  is opened, the knock detection device  82  may be deactivated, or may ignore the input signal, and thus the malfunction may be prevented [S 120 ]. 
     Meanwhile, when the valid knock-on signal is detected, and the detection device PCB  83  transmits the valid signal to the main control part  2 , the main control part  2  turns on the door lighting unit  49 . 
     When the door lighting unit  49  is turned on, the inside of the opening part  403  becomes bright, and the light inside the refrigerator passes through the panel assembly  54 . In particular, when the light passes through the front panel  541 , the front panel  541  becomes transparent, and thus the inside thereof may be seen through, as illustrated in  FIG.  37   . 
     When the sub-door  50  becomes transparent, the user may confirm the accommodation space inside the main door  40  or the space inside the refrigerator, and thus may open the sub-door  50  to store the food, or may perform a necessary operation [S 130 ]. 
     The turned-on door lighting unit  49  may be maintained in a turned-on state for a preset time, e.g., 10 seconds, and thus may allow the user to sufficiently confirm an internal state of the refrigerator  1 . 
     When the present time passes, the door lighting unit  49  is turned off [S 140 ]. 
     While the door lighting unit  49  is turned on, a valid knock-on operating signal may be input by the user before the preset time passes. 
     That is, when the user performs the knocking operation to confirm the inside the refrigerator  1 , but a separate operation is not needed, the door lighting unit  49  may be turned off before the preset time passes. 
     For example, in a state in which the user confirms an accommodation state inside the refrigerator  1  within 5 seconds after the door lighting unit  49  is turned on, when it is intended that the sub-door  50  becomes opaque, the knocking operation may be performed again on the front surface of the sub-door  50 , i.e., the front panel  541 . 
     At this point, when it is determined that the knocking operation is valid, the door lighting unit  49  may be turned off before the preset time passes. Of course, validity determination of the knocking operation may be set to be the same as the operation S 120 , and if necessary, may be set to another knock input pattern [S 150 ]. 
     When the preset time passes after the door lighting unit  49  is turned on, or the valid knock-on signal is input, the door lighting unit  49  may be turned off. 
     When the door lighting unit  49  is turned off, the inside of the refrigerator  1  becomes dark, and the outside thereof is in a bright state. In this state, the light outside the refrigerator  1  is reflected by the front panel  541 , and thus the front surface of the sub-door  50  is in a mirror-like state, and the user may not see through the inside thereof. Therefore, the sub-door  50  is maintained in the opaque state until a new operation is input [S 160 ].