The present invention discloses a refrigerator and relates to the technical field of refrigerators. The refrigerator is invented to settle problems, such as increased external space occupied by a refrigerator when its secondary door is opened, increased difficulty in fetching goods in the refrigerator, and heavy loss of cooling capacity resulted from the full opening of an opening. The inventive refrigerator comprises a main door; an opening is formed on a door body of the main door; a guide rail is provided at an edge of the opening, and a secondary door is fitted on the guide rail; and the secondary door is connected with a driving mechanism which can drive the secondary door to slide along the guide rail, and as the secondary door slides along the guide rail, the secondary door can enclose or open the opening. The refrigerator of the present invention is used for preserving and freezing foods.

[0001]    The application claims both the priority of Chinese Patent Application No. 201510009262.9, submitted to Chinese Patent Office on Jan. 7, 2015, titled “REFRIGERATOR”, and the priority of Chinese Patent Application No. 201510107398.3, submitted to Chinese Patent Office on Mar. 11, 2015, titled “REFRIAGERATOR”, the entirety of which is incorporated herein by reference. 
       FIELD OF TECHNOLOGY 
       [0002]    The present invention relates to the technical field of refrigerators, and in particular to a refrigerator. 
       BACKGROUND 
       [0003]    With the improvement of people's life quality, a gradually increasing storage requirement for a refrigerator makes refrigerator products with a large capacity become popular. However, an enlarged capacity will lead to a correspondingly increased size of the refrigerator, and the door body of the refrigerator will also become taller and wider. Because users will frequently open the large-sized door body when they fetch goods, cooling capacity in the refrigerator will leak heavily which causes a compressor to frequently start and thus leads to increased energy consumption of the refrigerator. In addition, due to the deeper depth of the refrigerator, it will be more difficult for users to fetch goods if the goods are not placed in good classification, when a large number of goods are stored in the refrigerator. 
         [0004]      FIG. 1  is a refrigerator in the prior art, including a refrigerator door  01 . A storage space (not shown) is provided on the refrigerator door  01 . A secondary door  03  is provided outside of an opening  02  of the storage space. A revolving shaft  04  of the secondary door  03  is arranged horizontally on the bottom of the opening  02 . The secondary door  03  is rotated about the revolving shaft  04  to open or enclose the opening  02 . Because commonly used goods are arranged inside the storage space on the refrigerator door  01 , it is just needed to open the secondary door  03  when users fetch them. This avoids opening and closing the large-sized refrigerator door  01  frequently, thereby reducing the leakage of the cooling capacity in the refrigerator and decreasing the energy consumption of the refrigerator. 
         [0005]    When the secondary door  03  is opened, an angle between the secondary door  03  and the refrigerator door  01  will become larger as the secondary door  03  rotates about the revolving shaft  04 , leading to increased space occupied by the refrigerator. The secondary door  03  will occupy a certain external space when it is fully opened, causing unnecessary limitations. And, the secondary door  03  sometimes blocks in front of the human body and thus increases the difficulty in fetching goods. In addition, opening the door each time will cause the full opening of the opening  02  on the refrigerator door  01 . As a result, loss of cooling capacity remains heavy. 
       SUMMARY OF THE INVENTION 
       [0006]    Embodiments of the present invention provide a refrigerator which may solves problems such as increased external space occupied by a refrigerator when its secondary door is opened, increased difficulty in fetching goods in the refrigerator, and heavy loss of cooling capacity resulted from the full opening of the opening. 
         [0007]    In order to achieve this objective, the embodiments of the present invention adopt the following technical solution. 
         [0008]    A refrigerator is provided, including a main door; an opening is formed on a door body of the main door; a guide rail is provided at an edge of the opening, and a secondary door is fitted on the guide rail; and the secondary door is connected with a driving mechanism which can drive the secondary door to slide along the guide rail, and as the secondary door slides along the guide rail, the secondary door can enclose or open the opening. 
         [0009]    The embodiments of the present invention provide a refrigerator. The secondary door is provided on the main door. When users fetch goods, the opening on the main door can be opened by driving the secondary door to slide along the guide rail by the driving mechanism, so that it is possible to fetch the goods in the refrigerator; and the opening on the main door can be enclosed by driving the secondary door by the driving mechanism to slide along the guide rail in the opposite direction. With regard to the refrigerator provided by the embodiments of the present invention, a small-sized secondary door can be opened partially or fully when users fetch the commonly used goods, so as to reduce the loss of cooling capacity, fetch and place goods conveniently, and improve the user experience. Furthermore, the secondary door, when opened, is located in the main door and in a same plane as the main door, so that it will not block in front of the human body and will not exert an influence on the external space occupied by the refrigerator and the difficulty in fetching goods. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    In order to describe technical solutions in the embodiments of the present invention or in the prior art more clearly, the accompanying drawings to be used for describing the embodiments or the prior art will be introduced briefly. Obviously, the accompanying drawings to be described below are merely some embodiments of the present invention, and a person of ordinary skill in the art can obtain other drawings according to those drawings without paying any creative effort. 
           [0011]      FIG. 1  is a side view of a refrigerator door in the prior art; 
           [0012]      FIG. 2  is a schematic structure diagram of a main door according to one embodiment of the present invention; 
           [0013]      FIG. 3  is a schematic structure diagram of a second gear provided in a transmission assembly according to one embodiment of the present invention; 
           [0014]      FIG. 4  is a schematic structure diagram of a transmission assembly, which is a gear rack, according to one embodiment of the present invention; 
           [0015]      FIG. 5  is a schematic overall structure diagram in which a secondary door is driven by a transmission rope according to one embodiment of the present invention; 
           [0016]      FIG. 6  is a partially schematic structure diagram of the transmission assembly of  FIG. 5 ; 
           [0017]      FIG. 7  is a schematic structure diagram of a driving wheel in the transmission assembly of  FIG. 5 ; 
           [0018]      FIG. 8  is a partially top view of the transmission assembly of  FIG. 5 ; 
           [0019]      FIG. 9  is a schematic structure diagram in which a guide rail of the refrigerator is arranged in a horizontal direction, according to one embodiment of the present invention. 
           [0020]      FIG. 10  is a side view of the main door when a thickening layer is provided, according to one embodiment of the present invention; 
           [0021]      FIG. 11  is a side view of the main door when a vacuum insulation panel is provided, according to one embodiment of the present invention; 
           [0022]      FIG. 12  is a schematic diagram when a sealing strip is not in contact with the secondary door, according to one embodiment of the present invention; 
           [0023]      FIG. 13  is a schematic diagram when the sealing strip is in contact with the secondary door, according to one embodiment of the present invention; and 
           [0024]      FIG. 14  is a schematic diagram when a guide wheel is provided in a guide rail, according to one embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0025]    The technical solutions in the embodiments of the present invention will be described clearly and completely with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the embodiments to be described are merely some but not all of embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by a person of ordinary skill in the art without paying any creative effort are included the protection scope of the present invention. 
         [0026]    In the description of the present invention, it should be understood that orientation or location relationships indicated by terms “center”, “up”, “down”, “front”, “behind”, “left”, “right”, “vertical”, “horizontal”, “top”, “bottom”, “inside”, “outside” and the like are the orientation or location relationships based on the accompanying drawings, provided just for ease of describing the present invention and simplifying the description. They are not intended to indicate or imply that the stated devices or elements must have the specific orientation and be constructed and operated in the specific orientation. Hence, they shall not be understood as any limitation to the present invention. 
         [0027]    Terms “first” and “second” are simply used for description, and shall not be understood to indicate or imply relative importance or to imply the amount of the stated technical features. Therefore, features defined with “first” and “second” can explicitly or impliedly include one or more such features. In the description of the present invention, “more” means “two or more than two”, unless otherwise specifically stated. 
         [0028]    In the description of the present invention, it should be noted that, unless otherwise clearly specified and defined, terms “mount”, “connected with” and “connected to” should be understood in a broad sense, for example, it can be fixed connection, and can also be detachable connection or integral connection; and, it can be direct connection, can also be connection by intermediate members, and can be internal connection between two elements. For a person of ordinary skill in the art, the specific meaning of those terms in the present invention can be understood in specific circumstances. 
         [0029]    The refrigerator mainly comprises a cabinet, a main door, a refrigerating system and a controlling system. A storage space is provided in the cabinet, and a storage space may be provided on one side of the main door close to the inside of the cabinet. Users can fetch goods in the above storage spaces by opening the main door. The refrigerating system comprises a compressor and so on, and is configured to lower temperature in the cabinet in order to refrigerate goods. The controlling system comprises a temperature controller and so on, and is configured to control the temperature in the cabinet to be within a range. 
         [0030]      FIG. 2  is one specific embodiment of the refrigerator according to the embodiments of the present invention. The refrigerator in this embodiment includes a main door  1 ; an opening  2  is formed on the door body of the main door  1 ; a guide rail  3  is provided at an edge of the opening  2 , and a secondary door  4  is fitted on the guide rail  3 ; and the secondary door  4  is connected to a driving mechanism  5  which can drive the secondary door  4  to slide along the guide rail  3 , and as the secondary door  4  slides along the guide rail  3 , the secondary door  4  can enclose or open the opening  2 . 
         [0031]    With regard to the refrigerator provided in this embodiment of the present invention, the secondary door  4  is provided on the main door  1 , and the opening  2  on the main door  1  can be opened by driving the secondary door  4  to slide along the guide rail  3  by the driving mechanism  5  so that it is possible to fetch the goods in the refrigerator; and the opening  2  on the main door  1  can be enclosed by driving the secondary door  4  by the driving mechanism  5  to slide along the guide rail  3  in the opposite direction. With regard to the refrigerator provided in this embodiment of the present invention, the small-sized secondary door  4  can be opened partially or fully when users fetch the commonly used goods, so as to reduce the loss of cooling capacity in the refrigerator, fetch and place goods conveniently for users, and improve the user experience. Furthermore, the secondary door  4 , when opened, is located in the main door  1  and in a same plane as the main door  1 , so that it will not block in front of the human body and will not exert an influence on the external space occupied by the refrigerator and the difficulty in fetching goods. By opening the secondary door by sliding the secondary door  4  along the guide rail  3 , the door hinge parts, which are easy to wear, are omitted, and the durability of the secondary door  4  is enhanced. In addition, the opening or enclosing of the secondary door  4  can be implemented by the driving mechanism  5 , which is beneficial to the automation development of refrigerators. 
         [0032]    The arrangement of the guide rail  3  at an edge of the opening  2  can be implemented in the following two ways. The first implementation way is to arrange the guide rail  3  at an edge of only one side of the opening  2 . In this case, the sliding of the secondary door  4  can be implemented by limiting the edge of the one side of the secondary door  4  by the guide rail  3  on the corresponding one side, and as a result, the sliding stability of the secondary door  4  is relatively low. The second implementation way is to arrange parallel guide rails  3  at two opposite edges of the opening  2 , respectively. Such implementation, in which edges of two sides of the secondary door  4  are limited by guide rails  3  on the two sides, makes the secondary door  4  slide more smoothly and stably. Therefore, it is preferable to arrange guide rails  3  at the two opposite edges of the opening  2 , respectively. 
         [0033]    In this embodiment, the driving mechanism  5  includes a motor  6  and a transmission assembly  7 ; an output shaft of the motor  6  is connected to the transmission assembly  7  in a transmission way, and the transmission assembly  7  is connected to the secondary door  4  in a transmission way; and the transmission assembly  7  can transform a rotary motion of the output shaft of the motor  6  to a linear motion to drive the secondary door  4  to slide along the guide rail  3 . Power of the motor  6  is transmitted to the secondary door  4  by the transmission assembly  7 , to drive the secondary door  4  to slide along the guide rail  3 , so as to realize the opening and enclosing of the opening  2  on the main door  1 . 
         [0034]      FIG. 2  is one implementation of the transmission assembly  7 , including a first gear  71 , a first connecting rod  72  and a second connecting rod  73 ; the first connecting rod  72  is connected to the door body of the main door  1  by a first shaft  75 , and the first connecting rod  72  can rotate about the first shaft  75 ; the second connecting rod  73  is hinged with the first connecting rod  72  by a second shaft  76 ; the first gear  71  is fixed onto the output shaft of the motor  6 ; one end of the first connecting rod  72  is a gear structure  721 , and the other end thereof is connected with a first guide pin  722 ; the gear structure  721  is engaged with the first gear  71 ; a first chute  41  perpendicular to the guide rail  3  is provided on one side of the secondary door  4  close to the driving mechanism  5 , and the first guide pin  722  is fitted inside the first chute  41 ; one end of the second connecting rod  73  is connected with a second guide pin  731  and the other end thereof is connected with a third guide pin  732 , and the second guide pin  731  is fitted inside the first chute  41 ; and a second chute  74  perpendicular to the guide rail  3  is provided on the door body of the main door  1 , and the third guide pin  732  is fitted inside the second chute  74 . When the secondary door  4  changes to the opened position from the closed position, the motor  6  starts, and drives the first gear  71  to rotate about the motor shaft counterclockwise. Because the first gear  71  is engaged with the gear structure  721 , the first gear  71  drives the first connecting rod  72  to rotate about the first shaft  75  clockwise, and drives the second shaft  76  to rotate about the first shaft  75  clockwise. In this case, the first guide pin  722  slides along the first chute  41  in a direction away from the first gear  71 ; the second connecting rod  73  rotates about the second shaft  76  counterclockwise, so that the second guide pin  731  slides along the first chute  41  in a direction opposite to the sliding direction of the first guide pin  722 ; and the third guide pin  732  slides along the second chute  74  in a direction the same as the sliding direction of the first guide pin  722 , so as to drive the secondary door  4  to slide along the guide rail  3  in a direction close to the first gear  71  until the opening  2  is fully opened. When the secondary door  4  changes to the closed state from the opened state, the motor  6  starts, and drives the first gear  71  to rotate about the motor shaft clockwise. Because the first gear  71  is engaged with the gear structure  721 , the first gear  71  drives the first connecting rod  72  to rotate about the first shaft  75  counterclockwise, and drives the second shaft  76  to rotate about the first shaft  75  counterclockwise. In this case, the first guide pin  722  slides along the first chute  41  in a direction close to the first gear  71 ; the second connecting rod  73  rotates about the second shaft  76  clockwise, so that the second guide pin  731  slides along the first chute  41  in a direction opposite to the sliding direction of the first guide pin  722 ; and the third guide pin  732  slides along the second chute  74  in a direction the same as the sliding direction of the first guide pin  722 , so as to drive the secondary door  4  to slide along the guide rail  3  in a direction away from the first gear  71  until the opening  2  is fully enclosed. Power of the motor  6  is transmitted to the first connecting rod  72  and the second connecting rod  73  by engaging the first gear  71  and the gear structure  721  on the first connecting rod  72 , so that the first connecting rod  72  and the second connecting rod  73  drive the secondary door  4  to slide along the guide rail  3  smoothly and stably, so as to realize the opening and enclosing of the opening  2 . The bearing capacity and impact resistance of the transmission assembly  7  are so high that there is a relatively small abrasion during the transmission. Furthermore, the manufacturing is convenient and it is easy to obtain a relatively high precision when producing a refrigerator. 
         [0035]    In the above embodiment, because the second chute  74  is formed on the door body of the main door  1 , strength at the corresponding position of the main door  1  will be reduced. If the second chute  74  is damaged, the main door  1  will be scraped entirely. In order to avoid this case, in another embodiment of the present invention, a support rod (not shown) perpendicular to the guide rail  3  is preferably arranged on the door body of the main door  1 , and the second chute  74  is formed on the support rod. The third guide pin  732  is fitted inside the second chute  74 . Therefore, when the second chute  74  is damaged, it is just needed to replace the support rod. This prevents the main door  1  from entirely scraping and also guarantees the strength of the main door  1  not being impacted. 
         [0036]    In the above embodiment, because only some of teeth in the gear structure  721  are used during the swing process of the first connecting rod  72 , in order to save material of the gear structure  721  and simplify its processing process, the gear structure  721  is preferably a sector gear structure as shown in  FIG. 2 . 
         [0037]    With reference to  FIG. 3 , the transmission assembly  7  in one embodiment of the invention also includes a second gear  77 , and the gear structure  721  is engaged with the first gear  71  by the second gear  77 . 
         [0038]    Another implementation of the transmission assembly  7  can adopt a gear rack for transmission, including a transmission gear, a rack and a rack guide rail; the rack guide rail is fixed onto the door body of the main door  1 , the gear guide rail is in parallel to the guide rail  3 , and the rack can slide along the rack guide rail and one end of the rack is connected to the secondary door  4 ; and the transmission gear is connected with an output shaft of the motor  6  in a transmission way, and engaged with the rack. When the secondary door  4  changes to the opened state from the closed state, the motor  6  starts, and drives the transmission gear to rotate about the motor shaft counterclockwise. In this case, the rack engaged with the transmission gear slides along the rack guide rail so as to drive the secondary door  4  to slide along the guide rail  3  in a direction close to the transmission gear until the opening  2  is fully opened. When the secondary door  4  changes to the closed state from the opened state, the motor  6  starts, and drives the transmission gear to rotate about the motor shaft clockwise. In this case, the rack engaged with the transmission gear slides along the rack guide rail so as to drive the secondary door  4  to slide along the guide rail  3  in a direction away from the transmission gear until the opening  2  is fully closed. The rotary motion of the output shaft of the motor  6  is transformed to a linear motion by the gear rack. The secondary door  4  is driven to slide along the guide rail  3 , so as to realize the opening and enclosing of the opening  2 . Such transmission assembly  7  can ensure a constant transmission ratio, high transmission efficiency, and smooth and stable transmission, so that the secondary door  4  slides more smoothly and stably along the guide rail  3 . In this way, the service life becomes longer. 
         [0039]    In the above embodiment, there can be one transmission gear, one rack and one rack guide rail. The motor  6  drives this transmission gear to rotate when it starts. The rotation of the transmission gear drives the rack engaged with the transmission gear to slide along the rack guide rail, so as to drive the secondary door  4  to slide along the guide rail and thus to realize the opening and closing of the opening  2 . 
         [0040]    In another embodiment of the present invention, there can be two transmission gears, two racks and two rack guide rails. With reference to  FIG. 4 , specifically, the transmission gear includes two gears, i.e., a third gear  78  and a fourth gear  79 . There are two racks  710  and two rack guide rails  711 . The two racks  710  are spaced apart from each other and in parallel to the guide rail  3 , and one end of each of the two racks  710  is connected with the secondary door  4 . The two rack guide rails  711  are spaced apart from each other and in parallel to the guide rail  3 . The two racks  710  are fitted on the two rack guide rails  711 , respectively. The third gear  78  is fixed on the output shaft of the motor  6 , and is engaged with one rack  710  and the fourth gear  79 , respectively. The fourth gear  79  is engaged with the other rack  710 . When the secondary door  4  changes to the opened state from the closed state, the motor  6  starts and drives the third gear  78  to rotate about the motor shaft counterclockwise. In this case, the fourth gear  79  engaged with the third gear  78  rotates about its rotary center clockwise; the third gear  78  and the fourth gear  79  drive the secondary door  4  to slide along the guide rail  3  in a direction close to the third gear  78  until the opening  2  is fully opened, by driving two racks  710  engaged with them to slide along the rack guide rail  711 . When the secondary door  4  changes to the closed state from the opened state, the motor  6  starts, and drives the third gear  78  to rotate about the motor shaft clockwise. In this case, the fourth gear  79  engaged with the third gear  78  rotates its rotary center counterclockwise. The third gear  78  and the fourth gear  79  drive the secondary door  4  to slide along the guide rail  3  in a direction away from the third gear  78  until the opening  2  is fully closed, by driving two racks  710  engaged with them to slide along the rack guide rail  711 . Because there are two transmission gears, two racks and two rack guide rails, the secondary door  4  is driven from two sides. Compared with driving the secondary door  4  from one side, this implementation enables the secondary door  4  to be stressed more evenly, preventing the secondary door  4  from being jammed during the sliding. Therefore, it is preferred that there are two transmission gears, two racks and two rack guide rails. 
         [0041]    As shown in  FIG. 5  and  FIG. 6 , in another embodiment of the present invention, the transmission assembly  7  includes a guide block  701  which is arranged along a direction in parallel to the guide rail  3 ; at both ends of the guide block  701 , a first guide wheel  703  and a second guide wheel  704  are provided, respectively; a driving wheel  705  is sleeved on the output shaft of the motor  6 ; the secondary door  4  includes a sliding bottom plate  702  which is sleeved on the guide block  701  and can slide along the guide block  701 ; the sliding bottom plate  702  is located between the first guide wheel  703  and the second guide wheel  704  and connected with a transmission rope  706 ; the transmission rope  706  includes a first transmission rope segment  7061  and a second transmission rope segment  7062  which are located on both sides of the sliding bottom plate  702 , respectively; and the first transmission rope segment  7061  is passed over the first guide wheel  703  and wound onto the driving wheel  705  in a first direction, and the second transmission rope segment  7062  is passed beneath the second guide wheel  704  and wound onto the driving wheel  705  in a direction opposite to the first direction. 
         [0042]    The operating process of the above embodiment is as follows. When the secondary door  4  moves from the opened position to the closed position, the output shaft of the motor  6  drives the driving wheel  705  to rotate counterclockwise, so as to pull the first transmission rope segment  7061  and release the second transmission rope segment  7062 , so that the transmission rope located between the first guide wheel  703  and the second wheel  704  moves upward because of being partially stressed and then, guided by the guide block  701  and the guide rail  3  in terms of direction, pulls the sliding bottom plate  702  in order to drive the secondary door  4  to move upward to reach the closed position. When the secondary door  4  moves from the closed position to the opened position, the output shaft of the motor  6  drives the driving wheel  705  to rotate clockwise, so as to pull the second transmission rope segment  7062  and release the first transmission rope segment  7061 , so that the transmission rope located between the first guide wheel  703  and the second wheel  704  moves downward and then, guided by the guide block  701  and the guide rail  3  in terms of direction, pulls the sliding bottom plate  702  in order to drive the secondary door  4  to move downward drive to reach the opened state. In a transmission structure shown in  FIG. 5 , compared with a common transmission structure, the number of components and parts assembled is reduced, and the production efficiency is increased. Furthermore, because the transmission rope is continuously coordinated with the driving wheel and the guide wheels, and the transmission rope is a flexible member, compared with gear transmission, the friction and collision between the teeth generated when engaged with each other are avoided, and as a result, noise generated during the transmission is reduced. 
         [0043]    In order to make the stressed direction of the secondary door consistent with the arrangement direction of the guide rail  3 , a portion of the transmission rope  706  located between the first guide wheel  703  and the second wheel  704  can be made in parallel to the guide rail  3  by arranging the positions and sizes of the first guide wheel  703  and the second guide wheel  704 . For example, the first guide wheel  703  and the second wheel  704  can be set to have an equal radius and their centers can be located in a same straight line. Therefore, the stressed direction of the secondary door is made consistent with the arrangement direction of the guide rail  3 . This avoids jamming during the movement. 
         [0044]    In order to prevent the first transmission rope segment  7061  and the second transmission rope segment  7062  wound onto the driving wheel  705  from interfering with each other, as shown in  FIG. 7 , a first guide groove  7051  and a second guide groove  7052 , which are in parallel to each other, are preferably provided along an outer circumference of the driving wheel  705 ; and the first transmission rope segment  7061  is passed over the first guide wheel  703  and wounded into the first guide groove  7051  in a first direction, and is fixedly connected to the first guide groove  7051 ; and the second diving rope segment  7062  is passed beneath the second guide wheel  704  and wounded into the second guide groove  7052  in a direction opposite to the first direction, and is fixedly connected to the second guide groove  7052 . Therefore, when the secondary door  4  moves from the opened position to the closed position, the output shaft of the motor  6  drives the driving wheel  705  to rotate counterclockwise so as to: pull the first transmission rope segment  7061  so that the first transmission rope segment  7061  is gradually wound into the guide groove  7051 ; and to release the second transmission rope segment  7062  so that the second transmission rope segment  7062  is gradually separated from the second guide groove  7052 , and as a result, transmission rope located between the first guide wheel  703  and the second guide wheel  704  moves upward because of being partially stressed, and then, guided by the guide block  701  and the guide rail  3  in terms of direction, pulls the sliding bottom plate  702  in order to drive the secondary door  4  to move upward to reach the closed position. Therefore, by designing the driving wheel  705  in a structure having two guide grooves can separate the first transmission rope segment  7061  and the second transmission rope segment  7062  wound onto the driving wheel  705  from each other, thereby preventing increasing the resistance of the movement of the secondary door due to the contact and friction between the first transmission rope segment  7061  and the second transmission rope segment  7062  during the transmission. 
         [0045]    Wherein, a length of the transmission rope  706  wound onto the driving wheel  705  should be enough to allow for a stroke traveled by the secondary door between the fully enclosed position and the fully opened position. As such, the whole movement of the secondary door between the fully enclosed position and the fully opened position can be ensured. Specifically, when the secondary door is in the fully enclosed position (that is, the position shown in  FIG. 5 ), the length of the transmission rope  706  wound into the first guide groove  7051  should be equal to or greater than the stroke. 
         [0046]    It should be noted that, when the secondary door moves to a limiting position, the portion of the transmission rope  706  wound onto the driving wheel  705  may have been fully released. Such limiting cases are also in the explanation scope of the “wind” in the embodiment of the present invention. 
         [0047]    In the above embodiment, the transmission rope  706  can be a whole rope, and can also be separated into two segments. When the transmission rope  706  is a whole rope, the middle portion of the transmission rope  706  penetrates through and is connected with the sliding bottom plate  702 . When the transmission rope  706  is separated into two segments, as shown in  FIG. 6 , the two transmission ropes are connected with an upper end and a lower end, of the sliding bottom plate  702 , respectively. 
         [0048]    As shown in  FIG. 8 , in order to reduce the friction force when the sliding bottom plate  702  slides along the guide block  701 , pulleys  7022  can be provided between the sliding bottom plate  702  and the guide block  701 . Specifically, rotary pulleys  7022  can be provided on an inner wall of the sliding bottom plate  702  in contact with the guide block  701 , and then the sliding bottom plate  702  is sleeved onto the guide block  702 . Therefore, the sliding between the sliding bottom plate  702  and the guide block  701  is supported by the pulleys  7022 , so that the friction force when the sliding bottom plate  702  slides along the guide block  701  becomes a rolling fiction force which significantly reduces the movement resistance of the secondary door. 
         [0049]    Wherein, the transmission rope  706  is preferably made of a steel rope which is more resistant to loss. 
         [0050]    As shown in  FIG. 6 , in order to realize a more stable connection between the sliding bottom plate  702  and the secondary door  4 , it is preferable to form a mounting groove  7021  on a surface of the sliding bottom plate  702  facing the secondary door. An edge of the secondary door is clamped into the mounting groove  7021 . Therefore, it may prevent the secondary door and the sliding bottom plate  702  from being separated during the transmission. 
         [0051]    Because the driving mechanism  5  of the secondary door  4  is arranged inside the door body of the main door  1 , a thickness of a foam layer on the main door  1  at a corresponding position is reduced, and as a result, the thermal insulation performance of the main door  1  is decreased. In order to keep the thermal insulation performance of the main door  1 , as shown in  FIG. 10 , a thickening layer  11  which is protruded from a surface of an inner wall is provided on the inner wall of the main door  1  at a position corresponding to the driving mechanism  5 . By the arrangement of the thickening layer  11 , the thermal insulation performance of the main door  1  may be enhanced. 
         [0052]    In addition, with reference to  FIG. 11 , a vacuum insulation panel  8  is provided on an external surface of the driving mechanism  5 . By the arrangement of the vacuum insulation panel  8 , the thermal insulation performance of the main door  1  is enhanced. 
         [0053]    With reference to  FIG. 10  to  FIG. 13 , in order to ensure the sealing performance of the opening  2  when the secondary door  4  is closed, a sealing strip  9  is provided around an inner wall of the opening  2 , a groove  91  is provided on one side of the sealing strip  9  facing the secondary door  4 , and elastic projections  92  are provided inside the groove  91 ; and when the secondary door  4  is closed, an edge of the secondary door  4  can be extended into the groove  91  to press against the elastic projections  92 . At this time, the elastic projections  92  are in close contact with the secondary door  4 , which ensures the sealing performance of the opening  2  when the secondary door is closed. 
         [0054]    To prolong the service life of the elastic projections  92  and improve the sealing performance and easy sliding at the junction of the secondary door  4  and the sealing strip  9 , external surfaces of the elastic projections  92  are planted with fluff by flocking. Therefore, the friction resistance of the elastic projections  92  is increased, thereby prolonging the service time of the elastic projections  92  and improving the sealing performance and easy sliding at the junction of the secondary door  4  and the sealing strip  9 . 
         [0055]    Because the sealing strip  9  is arranged on the inner wall of the opening  2  and a temperature on the inner wall of the opening  2  is relatively low, the sealing strip  9  is required to keep a good elasticity at low temperature, so as to ensure the sealing performance of the opening  2 . The material of the sealing strip  9  is preferably EPDM (Ethylene-Propylene-Diene Monomer), TPE (Thermoplastic Elastomer) or TPR (Thermoplastic Rubber). The three materials mentioned above have a good elasticity at low temperature, so that the sealing performance of the opening  2  is ensured. 
         [0056]    In order to enhance the thermal insulation performance of the refrigerator, the secondary door  4  is made of heat insulating glass. The heat insulating glass may prevent cooling capacity inside the refrigerator from leaking, so that the thermal insulation performance of the refrigerator is enhanced. In addition, users may also check the storage condition of goods from the secondary door  4 , when the main door  1  and the secondary door  4  are both closed. It is helpful to fetch goods. 
         [0057]    The guide rail  3  in this embodiment can be arranged in a vertical direction, and in this case, the secondary door  4  can slide up and down along the guide rail  3 . In addition, as shown in  FIG. 9 , the guide rail  3  can also be arranged in a horizontal direction, and in this case, the secondary door  4  can slide left and right along the guide rail  3 . If a storage rack is horizontally arranged inside the main door  1 , when the guide rail  3  is arranged in a horizontal direction, users may fetch only goods on half of the storage rack even if the secondary door  4  is fully opened; and when the guide rail  3  is arranged in a vertical direction, users may fetch goods on the whole storage rack even if the secondary door  4  is partially opened. For example, users only need to open the upper part of the secondary door  4  to fetch goods on the upper layer of the storage rack, and this thus reduces the loss of cooling capacity. Therefore, it is preferable to arrange the guide rail  3  in a vertical direction. 
         [0058]    With reference to  FIG. 14 , in order to make the secondary door  4  slide more stably along the guide rail  3 , a rotary guide wheel  31  is provided on inner walls of two sides of the guide rail  3 , when the secondary door  4  slides along the guide rail  3 , the surfaces of the two sides of the secondary door  4  are fitted to the guide wheel  31 , respectively. The position of the secondary door  4  is limited by the guide wheel  31 , thereby preventing the secondary door  4  from swinging toward the two sides during the sliding and allowing the secondary door  4  to slide more smoothly and stably along the guide rail  3 . In addition, the material of the guide wheel  31  is preferably rubber or nylon, and this may prevent the guide wheel  31  from scratching the glass secondary door  4  to influence the appearance. 
         [0059]    The above description is merely specific implementation of the present invention, and the protection scope of the present invention is not limited thereto. Changes or replacements readily obtained by any technical person who is familiar with the technical field within the disclosed technical scope of the present invention should be included in the protection scope of the present invention. Therefore, the protection scope of the present invention should be subject to the protection scope of the claims.