Patent Publication Number: US-10310551-B2

Title: Foldable device

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority from Korean Patent Application No. 10-2015-0088718, filed on Jun. 22, 2015 in the Korean Intellectual Property Office, and U.S. Provisional Application No. 62/145,135, filed on Apr. 9, 2015 in the U.S. Patent and Trademark Office, the disclosures of which are incorporated herein in their entirety by reference. 
    
    
     BACKGROUND 
     1. Field 
     Apparatuses and methods consistent with exemplary embodiments relate to a foldable device including a flexible display device. 
     2. Description of the Related Art 
     A portable foldable device (hereinafter, referred to as a mobile device) such as a communication terminal, a game console, a multimedia device, a portable computer, or a photographing apparatus generally includes a display device that displays image information and an input unit such as a keypad. Many mobile devices have a foldable structure that may be folded into a smaller size in order to improve the portability thereof. In such mobile devices, two bodies are connected to each other via the foldable structure. Since a related art display device is not foldable, the display device may be arranged on one of two bodies. Hence, it may difficult to implement a large display device to a mobile device having a foldable structure. 
     Along with the development of a flexible and bendable display device, there have been attempts to implement the display device to a mobile device having a foldable structure. In this case, since the flexible display device may be arranged over two bodies to cross over the foldable structure, a large screen may be provided. However, although the flexible display device may be bent, if the flexible display device is sharply bent, the flexible display device may be damaged. Hence, a curved portion having a predetermined curvature is formed at the center of the flexible display device when the flexible display device folds. 
     SUMMARY 
     Exemplary embodiments address at least the above problems and/or disadvantages and other disadvantages not described above. Also, the exemplary embodiments are not required to overcome the disadvantages described above, and may not overcome any of the problems described above. 
     One or more exemplary embodiments include a foldable device that has a small size and is portable. 
     One or more exemplary embodiments include a foldable device on which a flexible display device is supported flat when the foldable device unfolds. 
     One or more exemplary embodiments include a foldable device that may reduce distortion of a screen when the foldable device unfolds at an obtuse angle. 
     According to an aspect of an exemplary embodiment, there is provided a foldable device including: a hinge shaft that includes first and second hinge shafts; a flexible display device that includes a first part, a second part, and a third part between the first part and the second part; a first body that includes a first fixed portion to which the first part is fixed, rotates around the first hinge shaft, and moves between a folding position and an unfolding position, and is movable to or from the first hinge shaft; a first support portion that is provided on the first body and moves between a support position and a receiving position, the support position enabling the first support portion to partially support the third part, the receiving position enabling the third part to be bent to form a curved receiving space in the first body; and a restriction unit configured to maintain the first support portion at the support position when the first body is in a position between the unfolding position and a start point of the receiving position and allow the first support portion to move to the receiving position when the first body folds beyond the receiving start position. 
     The foldable device may further include: a second body that includes a second fixed portion to which the second part is fixed, rotates around the second hinge shaft, moves between a folding position and an unfolding position, and is movable to or from the second hinge shaft; and a second support portion that is provided on the second body and moves between a support position and a receiving position, the support position enabling the second support portion to partially support the third part, the receiving position enabling the third part to be bent to form a curved receiving space in the second body. The restriction unit may be configured to maintain the second support portion at the support position when the second body is in a position between the unfolding position and a start point of the receiving position and allow the second support portion to move to the receiving position when the second body folds beyond the receiving start position. The support position enabling the second support portion to partially support the third part may correspond to the support position enabling the first support portion to partially support the third part. The receiving position enabling the third part to be bent to form a curved receiving space in the first body may correspond to the receiving position enabling the third part to be bent to form a curved receiving space in the second body. 
     At the folding position, the first part and the second part may be located closer to each other than at the unfolding position, and face each other. The third part may be bent from the first and second parts into the first and second bodies and received in the receiving space in a droplet shape. 
     The foldable device may further include a first gear portion and a second gear portion that are respectively provided on the first and second hinge shafts and are engaged with each other so that the first and second bodies synchronously rotate together. 
     The foldable device may further include a locking unit configured to lock the first body and the second body at at least one inclined position between the unfolding position and the folding position. An opening angle between the first body and the second body at the receiving start position may be equal to or less than an opening angle between the first body and the second body at the at least one inclined position. 
     The restriction unit may include: a first support arm and a second support arm that respectively support the first support portion and the second support portion and respectively pivot about fixed positions with respect to the first hinge shaft and the second hinge shaft; a first guide pin and a second guide pin that are respectively provided on the first support arm and the second support arm; and a first guide slot and a second guide slot that respectively rotate around the first hinge shaft and the second hinge shaft along with the first body and the second body, respectively guide the first guide pin and the second guide pin, and include support parts that maintain the first support arm and the second support arm at a position at which the first support arm and the second support arm support the first support portion and the second support portion and receiving parts that allow the first support arm and the second support arm to move away from the first support portion and the second support portion so that the first support portion and second support portion are allowed to move to the receiving position. 
     Each of the first support arm and the second support arm may include: a first link arm that pivots about the fixed position; and a second link arm having one end portion that is pivotably connected to the first link arm by the first guide pin or the second guide pin and another end portion that is pivotably connected to the first support portion or the second support portion and supports the first support portion or the second support portion. 
     The restriction unit may include: a rotating cam that supports the first support portion and the second support portion and includes a first cam path corresponding to the support position and a second cam path corresponding to the receiving position; a pinion gear that is connected to the rotating cam; a rack gear that is engaged with the pinion gear; a pulley; and a connection member having one end portion that is connected to a fixed position with respect to the hinge shaft and another end portion that is connected through the pulley to the rack gear and moves the rack gear when the first body and the second body fold or unfold. 
     The connection member may have a strip shape having bending elasticity. 
     The connection member may include a wire, and the foldable device may further include a return spring that is connected to the rack gear and applies a tensile force to the wire. 
     The foldable device may further include a path difference compensating unit configured to restrict an amount of movement of at least one of the first body and the second body to or from the hinge shaft, and compensate for a path difference between the flexible display device and the first body and a path difference between the flexible device and the second body while the foldable device folds or unfolds. 
     The path difference compensating unit may be further configured to enable at least one of the first body and the second body to move away from the hinge shaft during a period when the foldable device moves from the unfolding position to a changing position and enable at least one of the first body and the second body to move toward the hinge shaft during a period when the foldable device moves from the changing position to the folding position. 
     An opening angle between the first and second bodies at the changing position may be equal to an opening angle between the first and second bodies at the receiving start position. 
     The foldable device may further include: a first hinge unit that is pivotably connected to the first hinge shaft and is connected to the first body; and a second hinge unit that is pivotably connected to the second hinge shaft and is connected to the second body. The first body and the second body may be respectively connected to the first hinge unit and the second hinge unit so that the first body and the second body respectively move to or from the first hinge shaft and the second hinge shaft. 
     The first hinge unit and the second hinge unit may respectively include: a first hinge frame and a second hinge frame that are respectively rotatably connected to the first hinge shaft and the second hinge shaft; and a first movable frame and a second movable frame that are respectively supported on the first hinge frame and the second hinge frame so that the first movable frame and the second movable frame respectively move to or from the first hinge shaft and the second hinge shaft. The first body and the second body are respectively coupled to the first movable frame and the second movable frame. 
     The path difference compensating unit may include: a first restriction pin and a second restriction pin that have fixed positions with respect to the first hinge shaft and the second hinge shaft; and restriction slots into which the first restriction pin and the second restriction pin are respectively inserted and that respectively restrict amounts of movement of the first body and the second body according to rotation angles of the first body and the second body while the restriction slots pivot along with the first body and the second body. 
     According to an aspect of another exemplary embodiment, there is provided a foldable device including: a flexible display device that includes a first part, a second part, and a third part between the first part and the second part; a first body and a second body to which the first part and the second part are respectively fixed, and that move between an unfolding position and a folding position at which the first part and the second part are located closer to each other that at the unfolding position and face each other and the third part is bent from the first part and the second part into the first body and the second body and is received in a receiving space in a droplet shape; and a first support portion and a second support portion that are respectively provided on the first body and the second body to support the third part and are maintained at a support position at which the first support portion and the second support portion support the third part when the first body and the second body are in a position between the unfolding position and a starting point of a receiving position and move to the receiving position at which the first support portion and the second support portion move away from the third part to form the receiving space when the first body and the second body fold beyond the receiving start position. 
     The first body and the second body may respectively rotate about a first hinge shaft and a second hinge shaft and move between the unfolding position and the folding position. The foldable device may further include a first gear portion and a second gear portion that are respectively provided on the first hinge shaft and the second hinge shaft and are engaged with each other so that the first body and the second body synchronously rotate together. 
     The foldable device may further include a locking unit configured to lock the first body and the second body at at least one inclined position between the unfolding position and the folding position. An opening angle between the first body and the second body at the starting point of the receiving position may be equal to or less than an opening angle between the first body and the second body at the at least one inclined position. 
     The first body and the second body may be respectively movable to or from the first hinge shaft and the second hinge shaft. 
     The foldable device may further include a path difference compensating unit configured to restrict an amount of movement of at least one of the first body and the second body to or from the first hinge shaft and the second hinge shaft, and compensate for a path difference between the flexible display device and the first body and a path difference between the flexible display device and the second body while the foldable device folds or unfolds. 
     According to an aspect of another exemplary embodiment, there is provided a foldable device including: a hinge unit including a hinge shaft, a restriction slot, and a restriction pin inserted into the restriction slot; and a body configured to be move between a folding position and a unfolding position and connected to the hinge unit through the restriction pin; wherein a position of the restriction pin changes along the restriction slot while the body moves between the folding position and the unfolding position, wherein the body moves away from the hinge shaft when the body moves from the unfolding position to the folding position. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and/or other aspects will be more apparent by describing certain exemplary embodiments, taken with reference to the accompanying drawings, in which: 
         FIG. 1  is a perspective view illustrating an outer appearance of a foldable device according to an exemplary embodiment; 
         FIG. 2  is a side view illustrating an unfolding state of the foldable device of  FIG. 1  according to an exemplary embodiment; 
         FIG. 3  is a side view illustrating a folding state of the foldable device of  FIG. 1  according to an exemplary embodiment; 
         FIG. 4  is a cross-sectional view of a flexible display device according to an exemplary embodiment; 
         FIGS. 5A and 5B  are schematic views illustrating a folding state of the foldable device at an angle of, for example, 90°; 
         FIG. 6  is a schematic exploded perspective view of the foldable device according to an exemplary embodiment; 
         FIG. 7  is an exploded perspective view of the foldable device of  FIG. 6  according to an exemplary embodiment; 
         FIG. 8  is an exploded perspective view of a hinge unit according to an exemplary embodiment; 
         FIG. 9  is a side view for explaining an operation of a locking unit; 
         FIGS. 10A, 10B, and 10C  are schematic views of a structure in which first and second bodies move to or from first and second hinge shafts according to an exemplary embodiment, respectively illustrating a state wherein the first and second bodies are unfolded, a state wherein the first and second bodies are spaced apart by a maximum distance from the first and second hinge shafts, and a state wherein the first and second bodies are folded; 
         FIG. 10D  is a graph showing a movement distance according to a folding angle between the first and second bodies in the structure of  FIG. 10A ; 
         FIGS. 11A, 11B, and 11C  are schematic views of a structure in which the first and second bodies move to or from the first and second hinge shafts according to an exemplary embodiment, respectively illustrating a state wherein the first and second bodies are unfolded, a state wherein the first and second bodies are spaced apart by a maximum distance from the first and second hinge shafts, and a state wherein the first and second bodies are folded; 
         FIG. 11D  is a graph showing a movement distance according to a folding angle between the first and second bodies in the structure of  FIG. 11A ; 
         FIG. 12A  is an exploded perspective view for explaining a connection relationship between a first restriction pin and a first restriction slot according to an exemplary embodiment; 
         FIG. 12B  is a cross-sectional view taken along line E-E′ of  FIG. 12A ; 
         FIG. 13  is a schematic view of a restriction unit according to an exemplary embodiment; 
         FIG. 14  is an exploded perspective view of a structure in which first and second support arms are pivotably supported according to an exemplary embodiment; 
         FIG. 15  is a schematic view of the restriction unit including two link arms according to an exemplary embodiment; 
         FIGS. 16A, 16B, and 16C  are schematic views of the restriction unit according to another exemplary embodiment; 
         FIG. 17  is an exploded perspective view of a structure that moves a movable support member between a first position and a second position according to an exemplary embodiment; and 
         FIGS. 18A and 18B  are cross-sectional views taken along line F-F′ of  FIG. 17 , respectively illustrating a state wherein the movable support member is located at the second position and a state wherein the movable support member is located at the first position. 
     
    
    
     DETAILED DESCRIPTION 
     Exemplary embodiments are described in greater detail below with reference to the accompanying drawings. 
     In the following description, like drawing reference numerals are used for like elements, even in different drawings. The matters defined in the description, such as detailed construction and elements, are provided to assist in a comprehensive understanding of the exemplary embodiments. However, it is apparent that the exemplary embodiments can be practiced without those specifically defined matters. Also, well-known functions or constructions are not described in detail since they would obscure the description with unnecessary detail. 
     As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. Expressions such as “at least one of” when preceding a list of elements, modify the entire list of elements and do not modify the individual elements of the list. 
       FIG. 1  is a perspective view illustrating an outer appearance of a foldable device according to an exemplary embodiment.  FIG. 2  is a side view illustrating an unfolding state of the foldable device of  FIG. 1  according to an exemplary embodiment.  FIG. 3  is a side view illustrating a folding state of the foldable device of  FIG. 1  according to an exemplary embodiment. 
     Referring to  FIGS. 1 through 3 , the foldable device may include first and second bodies  1  and  2  and a flexible display device  4 . The first and second bodies  1  and  2  are connected to each other and may move between an unfolding position of  FIG. 2  and a folding position of  FIG. 3 . 
       FIG. 4  is a cross-sectional view of the flexible display device  4  according to an exemplary embodiment. Referring to  FIG. 4 , the flexible display device  4  may include a flexible display panel  41  that displays an image and a transparent protective panel  43  that is disposed on the flexible display panel  41 . An additional panel or substrate may be disposed between the transparent protective panel  43  and the flexible display panel  41 . The flexible display panel  41  may be, for example, an organic light-emitting diode (OLED) panel. When the flexible display panel  41  is an OLED panel, an organic emission layer may be disposed between an upper substrate and a lower substrate. A polarization plate may be disposed on the upper substrate from which light is emitted. Also, the flexible display device  4  may further include a touch panel  42  as an input unit. For example, the touch panel  42  may be disposed between the transparent protective panel  43  and the flexible display panel  41 . The flexible display panel  41 , the touch panel  42 , and the transparent protective panel  43  may be adhered to one another by using an optically-clear adhesive (OCA) layer. However, the present exemplary embodiment is not limited thereto, and the flexible display device  4  may further include any of various other optical panels or optical films. 
     The flexible display device  4  is partially fixed to the first body  1  and the second body  2 . The flexible display device  4  may be divided into a first part  4   a  that is fixed to the first body  1 , a second part  4   b  that is fixed to the second body  2 , and a third part  4   c  that is formed between the first body  1  and the second body  2 . The first body  1  includes a first fixed portion  1   a  to which the first part  4   a  is fixed, and the second body  2  includes a second fixed portion  2   a  to which the second part  4   b  is fixed. For example, the first part  4   a  and the second part  4   b  of the flexible display device  4  may be respectively fixed to the first fixed portion  1   a  and the second fixed portion  2   a  by using an adhesive unit such as an adhesive or a double-sided tape. Alternatively or additionally, the first part  4   a  and the second part  4   b  be may respectively fixed to the first fixed portion  1   a  and the second fixed portion  2   b  through an interference fit or a press fit. In other words, fastening between the first part  4   a  and the first fixed portion  1   a  may be achieved by friction after the first part  4   a  and the first fixed portion  1   a  are pushed together. Similarly, fastening between the second part  4   b  and the second fixed portion  2   b  may be achieved by friction after the second part  4   b  and the second fixed portion  2   b  are pushed together. The third part  4   c  of the flexible display device  4  may not be fixed to the first body  1  or the second body  2 . That is, the third part  4   c  may not be adhered to the first and second bodies  1  and  2 . When the first and second bodies  1  and  2  are located at the folding position of  FIG. 3 , the third part  4   c  forms a curved portion  4   d  having a predetermined radius of curvature. Accordingly, since the flexible display device  4  may not be sharply bent and the third part  4   c  is bent by forming the curved portion  4   d  or is spread, the first and second bodies  1  and  2  may change between the folding position and the unfolding position. Also, when the foldable device folds, since the flexible display device  4  may be received in a receiving space  3  in the first and second bodies  1  and  2 , the foldable device may have a compact size and may be easily carried. That is, when the foldable device of the present exemplary embodiment folds, the first part  4   a  and the second part  4   b  of the flexible display device  4  are located close to each other to face each other. At the same time, the third part  4   c  may be bent from the first and second parts  4   a  and  4   b  into the first and second bodies  1  and  2  and is received in the receiving space  3  to have a droplet shape. Hence, the foldable device may have a compact size when the foldable device folds. 
     The first and second bodies  1  and  2  may be connected to each other. The position of the first and second bodies  1  and  2  may change between the unfolding position and the folding position due to a hinge unit  5 . For example, the hinge unit  5  may include hinge shafts. The hinge shafts of the present exemplary embodiment may include first and second hinge shafts  5   a - 1  and  5   a - 2  that are spaced apart from each other in a longitudinal direction L. The first and second bodies  1  and  2  may respectively rotate about the first and second hinge shafts  5   a - 1  and  5   a - 2  to change between the unfolding position and the folding position. 
     The foldable device may be a portable mobile device such as a communication terminal, a game console, a multimedia device, a portable computer, or a photographing apparatus. The present exemplary embodiment is not limited thereto, and the foldable device may be any device as long as it includes the first body  1  to which the first part  4   a  of the flexible display device  4  is fixed and the second body  2  to which the second part  4   b  of the flexible display device  4  is fixed and that is foldably connected to the first body  1 . 
     A processing unit and input/output units for performing functions according to the use of the foldable device may be provided on the first and second bodies  1  and  2 . When the foldable device is a multimedia terminal that provides images and music, the processing unit may include an image information processing unit and an audio information processing unit. When the foldable device is a communication terminal, the processing unit may include a communication module. The input/output units may include an image input/output unit, an audio input/output unit, and a manipulation unit for user manipulation. The manipulation unit may be realized by using the touch panel  42  of the flexible display device  4 . 
       FIGS. 5A and 5B  are schematic views illustrating a state wherein the foldable device is folded at an angle of, for example, 90°. Referring to  FIGS. 5A and 5B , there is a difference (hereinafter, referred to as a folding/unfolding path difference) between a folding/unfolding path of the first and second bodies  1  and  2  and a folding/unfolding path of the flexible display device  4  when the first and second bodies  1  and  2  are folded/unfolded. For example, assuming that the flexible display device  4  is entirely fixed to the first and second bodies  1  and  2 , when the first and second bodies  1  and  2  fold at an angle of 90°, the third part  4   c  is bent at a right angle as marked by a dashed line  4   e  in  FIG. 5A . In this case, the folding/unfolding path difference is “0”. 
     However, when the flexible display device  4  is sharply bent, the flexible display device  4  may be permanently deformed or may lose its function. Accordingly, in order for the flexible display device  4  to be smoothly bent during a folding/unfolding process, the third part  4   c  of the flexible display device  4  is not fixed to the first and second bodies  1  and  2 . In this case, the third part  4   c  of the flexible display device  4  may have an arc shape having a radius of curvature R as marked by a solid line  4   f  in  FIG. 5A  due to its elasticity. A length difference  4   g  between a portion corresponding to the dashed line  4   e  and a portion corresponding to the solid line  4   f  is the folding/unfolding path difference. The folding/unfolding path difference may be presented as 2R−2πR/4. Hereinafter, the folding/unfolding path difference is denoted by reference numeral  4   g . If the second part  4   b  of the flexible display device  4  is fixed to the second body  2  and the first part  4   a  of the flexible display device  4  is not fixed to the first body  1 , the flexible display device  4  is pushed toward an end portion of the first body  1  by as much as the folding/unfolding path difference  4   g . Accordingly, the third part  4   c  of the flexible display device  4  may be smoothly bent inward as marked by the solid line  4   f.    
     If the first and second parts  4   a  and  4   b  of the flexible display device  4  are respectively fixed to the first and second bodies  1  and  2 , the third part  4   c  of the flexible display device  4  is bent both inward and outward and thus an uneven portion  4   h  is formed as shown in  FIG. 5B . The uneven portion  4   h  may cause the flexible display device  4  to be permanently deformed. Also, when the first and second bodies  1  and  2  are used at an opening angle ranging from, for example, about 90° to about 180°, distortion of a screen may occur at the uneven portion  4   h . Also, when the third part  4   c  is repeatedly bent inward and outward, the flexible display device  4  may be permanently deformed or damaged at the uneven portion  4   h.    
     A length of the first body  1  may be extended by as much as the folding/unfolding path difference  4   g  so that the first body  1  has an extended portion and the first part  4   a  of the flexible display device  4  may slide along the first body  1 . However, in this case, when the first and second  1  and  2  completely unfold, the extended portion of the first body  1  may not be covered by the flexible display device  4 . 
     According to an exemplary embodiment, at least one of the first and second bodies  1  and  2  may move to or from the first and second hinge shafts  5   a - 1  and  5   a - 2  to compensate for the folding/unfolding path difference  4   g  during a folding/unfolding process. For example, when the first and second bodies  1  and  2  fold, the first and second bodies  1  and  2  may move away from the first and second hinge shafts  5   a - 1  and  5   a - 2 . As the third part  4   c  of the flexible display device  4  forms the curved portion  4   d  and is received in the receiving space  3  in the first and second bodies  1  and  2 , the first and second bodies  1  and  2  may move toward the first and second hinge shafts  5   a - 1  and  5   a - 2 . When the first and second bodies  1  and  2  unfold, the first and second bodies  1  and  2  may move away from the first and second hinge shafts  5   a - 1  and  5   a - 2 . As the curved portion  4   d  of the third part  4   c  of the flexible display device  4  unfolds and is separated from the receiving space  3  in the first and second bodies  1  and  2 , the first and second bodies  1  and  2  may move again toward the first and second hinge shafts  5   a - 1  and  5   a - 2 . Accordingly, in order to compensate for the folding/unfolding path difference  4   g  during the folding/unfolding process, the foldable device may include a path difference compensating unit that restricts an amount of movement of the first and second bodies  1  and  2  to or from the first and second hinge shafts  5   a - 1  and  5   a - 2 . The path difference compensating unit may enable at least one of the first and second bodies  1  and  2  to move away from the first and second hinge shafts  5   a - 1  and  5   a - 2  during a period when the foldable device moves from the unfolding position to a changing position. The path difference compensating unit may enable at least one of the first and second bodies  1  and  2  to move toward the first and second hinge shafts  5   a - 1  and  5   a - 2  during a period when the foldable device moves from the changing position to the folding position. The changing position may be the same as a receiving start position of first and second support portions  1   b  and  2   b  which will be explained below. In other words, an opening angle between the first and second bodies  1  and  2  at the changing position may be equal to or less than an opening angle between the first and second bodies  1  and  2  at the receiving start position. The path difference compensating unit will be explained below in detail. 
     Referring to  FIGS. 2 and 3 , the first and second support portions  1   b  and  2   b  that partially support the third part  4   c  of the flexible display device  4  at the unfolding position may be respectively provided on the first and second bodies  1  and  2 . When the first and second bodies  1  and  2  are located at the folding position, the first and second support portions  1   b  and  2   b  pivot to a position at which the receiving space  3  in which the curved portion  4   d  is received is formed in the first and second bodies  1  and  2 . In this configuration, the flexible display device  4  may be supported flat at the unfolding position, and may be received with a compact size in the first and second bodies  1  and  2  at the folding position. 
     The first and second support portions  1   b  and  2   b  may be maintained at a support position (see  FIG. 2 ) at which the first and second support portions  1   b  and  2   b  partially support the third part  4   c  of the flexible display device  4  when an angle between the first and second bodies  1  and  2  ranges from 180° to a receiving start angle (of the receiving start position), and may pivot to a receiving position (see  FIG. 3 ) at which the receiving space  3  is formed when the angle between the first and second bodies  1  and  2  is less than the receiving start angle. Since the foldable device is rarely used when the angle between the first and second bodies  1  and  2  is less than 90°, the receiving start angle may be, for example, about 90°. However, the present exemplary embodiment is not limited thereto, and the receiving start angle may be slightly greater or less than 90°. In order to stably support the flexible display device  4  at an inclined position which will be explained below, the receiving start angle may be equal to or less than an opening angle at the inclined position. 
       FIG. 6  is a schematic exploded perspective view of the foldable device according to an exemplary embodiment.  FIG. 7  is an exploded perspective view of the foldable device of  FIG. 6  according to an exemplary embodiment.  FIG. 8  is an exploded perspective view of the hinge unit  5  according to an exemplary embodiment. Since the hinge unit  5  is symmetric in a width direction W of the foldable device, only one side of the hinge unit  5  in the width direction W is illustrated in  FIG. 8 . 
     Referring to  FIGS. 6, 7, and 8 , the hinge unit  5  includes a fixed hinge  5 - 3  including the first and second hinge shafts  5   a - 1  and  5   a - 2 . The fixed hinge  5 - 3  is located at a fixed position without rotating or moving, The first and second bodies  1  and  2  may move to or from the fixed hinge  5 - 3  and may rotate around the fixed hinge  5 - 3 . The hinge unit  5  may further include a first hinge unit  5 - 1  that pivots about the first hinge shaft  5   a - 1  and a second hinge unit  5 - 2  that pivots about the second hinge shaft  5   a - 2 . The fixed hinge  5 - 3  includes the first and second hinge shafts  5   a - 1  and  5   a - 2 . The fixed hinge  5 - 3  may further include a shielding plate  5   a - 3 . The first and second hinge shafts  5   a - 1  and  5   a - 2  may be supported on the shielding plate  5   a - 3 . The first and second hinge shafts  5   a - 1  and  5   a - 2  may be integrally formed with the shielding plate  5   a - 3 . The shielding plate  5   a - 3  is located on a side portion of the flexible display device  4  in the width direction W and covers a gap between an end portion of the flexible display device  4  in the width direction W and the first and second bodies  1  and  2 . Accordingly, the inside of the foldable device may be prevented from being exposed through the gap and foreign materials may be prevented from being introduced into the foldable device through the gap. 
     The first body  1  is connected to the first hinge unit  5 - 1  and the second body  2  is connected to the second hinge unit  5 - 2 . The first body  1  includes a first lower case  1 - 1  and a first upper case  1 - 2 . The first upper case  1 - 2  includes the first fixed portion  1   a  and the first support portion  1   b . The first support portion  1   b  is coupled to the first upper case  1 - 2  so that the first support portion  1   b  pivots between the support position and the receiving position. The second body  2  includes a second lower case  2 - 1  and a second upper case  2 - 2 . The second upper case  2 - 2  includes the second fixed portion  2   a  and the second support portion  2   b . The second support portion  2   b  is coupled to the second upper case  2 - 2  so that the second support portion  2   b  pivots between the support position and the receiving position. 
     Referring to  FIG. 8 , the first hinge unit  5 - 1  includes a first hinge frame  51 - 1 . The first hinge shaft  5   a - 1  is inserted into first through-holes  51 - 1   a  that are formed in the first hinge frame  51 - 1 . A diameter of each of the first through-holes  51 - 1   a  is determined so that the first hinge frame  51 - 1  pivots about the first hinge shaft  5   a - 1 . Accordingly, the first hinge frame  51 - 1  may pivot about the first hinge shaft  5   a - 1 . 
     A first movable frame  52 - 1  is supported on the first hinge frame  51 - 1  so that the first movable frame  52 - 1  moves to or from the first hinge shaft  5   a - 1 . For example, a first guide shaft  53 - 1  that extends in the longitudinal direction L is provided on the first hinge frame  51 - 1 , and the first movable frame  52 - 1  is slidably supported on the first guide shaft  53 - 1 . The first lower case  1 - 1  and the first upper case  1 - 2  may be coupled to the first movable frame  52 - 1  to be respectively disposed under and over the first movable frame  52 - 1 . According to another exemplary embodiment, one of the first lower case  1 - 1  and the first upper case  1 - 2  may be coupled to the first movable frame  52 - 1  and the other case. The first body  1  is connected to the first hinge unit  5 - 1  so that the first body  1  moves to or from the first hinge shaft  5   a - 1 . 
     The second hinge unit  5 - 2  includes a second hinge frame  51 - 2 . The second hinge shaft  5   a - 2  is inserted into second through-holes  51 - 2   a  that are formed in the second hinge frame  51 - 2 . A diameter of each of the second through-holes  51 - 2   a  is determined so that the second hinge frame  51 - 2  pivots about the second hinge shat  5   a - 2 . Accordingly, the second hinge frame  51 - 2  may pivot about the second hinge shaft  5   a - 2 . 
     A second movable frame  52 - 2  is supported on the second hinge frame  51 - 2  so that the second movable frame  52 - 2  moves to or from the second hinge shaft  5   a - 2 . For example, a second guide shaft  53 - 2  that extends in the longitudinal direction L is provided on the second hinge frame  51 - 2 , and the second movable frame  52 - 2  is slidably supported on the second guide shaft  53 - 2 . The second lower case  2 - 1  and the second upper case  2 - 2  may be coupled to the second movable frame  52 - 2  to be respectively disposed under and over the second movable frame  52 - 2 . Accordingly to another exemplary embodiment, one of the second lower case  2 - 1  and the second upper case  2 - 2  may be coupled to the second movable frame  52 - 2  and the other case. The second body  2  is connected to the second hinge unit  5 - 2  so that the second body  2  moves to or from the second hinge shaft  5   a - 2 . 
     First and second gear portions  51 - 1   b  and  51 - 2   b  having at the center the first and second through-holes  51 - 1   a  and  51 - 2   a  are respectively provided on the first and second hinge frames  51 - 1  and  51 - 2 . The first and second gear portions  51 - 1   b  and  51 - 2   b  are engaged with each other. In this configuration, the first and second bodies  1  and  2  may synchronously rotate about the first and second hinge shafts  5   a - 1  and  5   a - 2 . 
     The foldable device may include a locking unit that locks the first and second bodies  1  and  2  at any of a plurality of positions. For example, the locking unit may lock the first and second bodies  1  and  2  at the unfolding position (see  FIG. 2 ) and at the folding position (see  FIG. 3 ). The locking unit may further lock the first and second bodies  1  and  2  at a position (inclined position) having a predetermined opening angle between the unfolding position (see  FIG. 2 ) and the folding position (see  FIG. 3 ). The opening angle at the inclined position may be greater than 90°. The locking unit may lock the first and second bodies  1  and  2  at any of a plurality of inclined positions having an opening angle between 180° and 90°. An opening angle of an inclined position may be, for example, 120°. 
       FIG. 8  illustrates the locking unit according to an exemplary embodiment.  FIG. 9  is a side view for explaining an operation of the locking unit. Referring to  FIGS. 8 and 9 , the locking unit may include a locking member  5   c  that is located at a fixed position, and a protrusion member that is fixed by the locking member  5   c  while rotating along with the first and second bodies  1  and  2 . The locking member  5   c  may include locking portions  5   c - 1 ,  5   c - 2 , and  5   c - 3  that respectively lock the first and second bodies  1  and  2  at the unfolding position, the inclined position, and the folding position. The protrusion member may include first and second protrusion members  54 - 1  and  54 - 2  that are selectively fixed by the locking portions  5   c - 1 ,  5   c - 2 , and  5   c - 3  according to an opening angle between the first and second bodies  1  and  2 . 
     The locking member  5   c  is located at the fixed position. The locking member  5   c  is provided on the fixed hinge  5 - 3 . The locking member  5   c  may be fixed to the first and second hinge shafts  5   a - 1  and  5   a - 2 . In the present exemplary embodiment, the first and second hinge shafts  5   a - 1  and  5   a - 2  pass through through-holes  5   c - 4  that are formed in the locking member  5   c . Accordingly, the locking member  5   c  has the fixed position with respect to the first and second hinge shafts  5   a - 1  and  5   a - 2 . 
     The first and second protrusion members  54 - 1  and  54 - 2  rotate about the first and second hinge shafts  5   a - 1  and  5   a - 2  along with the first and second bodies  1  and  2 . The first and second protrusion members  54 - 1  and  54 - 2  are supported on the first and second hinge frames  51 - 1  and  51 - 2  so that the first and second protrusion members  54 - 1  and  54 - 2  move toward or away from the locking member  5   c . First and second elastic members  55 - 1  and  55 - 2  apply elastic forces to the first and second protrusion members  54 - 1  and  54 - 2  to hold the movement of the first and second protrusion members  54 - 1  and  54 - 2  at the locking portions  5   c - 1 ,  5   c - 2 , and  5   c - 3 . In the present exemplary embodiment, the first and second protrusion members  54 - 1  and  54 - 2  are fixed to the first and second guide shafts  53 - 1  and  53 - 2 . The first and second guide shafts  53 - 1  and  53 - 2  are supported on the first and second hinge frames  51 - 1  and  51 - 2  so that the first and second guide shafts  53 - 1  and  53 - 2  slide toward or away from the locking member  5   c.    
     Since the first and second bodies  1  and  2  synchronously rotate due to the first and second gear portions  51 - 2   a  and  51 - 2   b  in the present exemplary embodiment, only one of the first and second bodies  1  and  2  may need to be locked. Accordingly, once one of the first and second protrusion members  54 - 1  and  54 - 2  is fixed by the locking portions  5   c - 1 ,  5   c - 2 , and  5   c - 3 , the first and second bodies  1  and  2  may be locked not to rotate. 
     Referring to  FIG. 9 , when the first and second bodies  1  and  2  are located at the unfolding position, the second protrusion member  54 - 2  is fixed or locked by the locking portion  5   c - 1 . Since the first and second bodies  1  and  2  synchronously rotate due to the first and second gear portions  51 - 2   a  and  51 - 2   b  as described above, once the second body  2  is locked at the unfolding position, the first body  1  is also locked at the unfolding position. 
     When the first and second bodies  1  and  2  begin to fold, the second protrusion member  54 - 2  is pushed in a direction that is opposite to a direction in which an elastic force of the second elastic member  55 - 2  is applied and thus is separated from the locking portion  5   c - 1 . When the first and second bodies  1  and  2  reach the inclined position, for example, a position having an opening angle of 120°, the first protrusion member  54 - 1  is fixed by the locking portion  5   c - 2 . Accordingly, the first and second bodies  1  and  2  are locked at the inclined position. 
     When the first and second bodies  1  and  2  continuously move to be folded, the first protrusion member  54 - 1  is pushed in a direction that is opposite to a direction in which an elastic force of the first elastic member  55 - 1  is applied and thus is separated from the locking portion  5   c - 2 . When the first and second bodies  1  and  2  reach the folding position, the movement of the first and second protrusion members  54 - 1  and  54 - 2  are stopped and locked by one pair of locking portions  5   c - 3 . Accordingly, the first and second bodies  1  and  2  are locked at the folding position. 
     As such, since the foldable device may be locked at the unfolding position and the folding position, convenience in use and portability may be improved. Also, since the foldable device may be located at a predetermined inclination angle, convenience in use may be further improved. Although the locking member  5   c  including one locking portion  5   c - 2  corresponding to an inclination angle is provided in the present exemplary embodiment, two or more locking portions  5   c - 2  corresponding to two or more inclination angles may be provided, and thus the foldable device may be located at positions having the two or more inclination angles. 
     As described above, amounts of movement of the first and second bodies  1  and  2  to or from a hinge shaft, for example, the first and second hinge shafts  5   a - 1  and  5   a - 2  are restricted during a folding/unfolding process in order to compensate for the folding/unfolding path difference  4   g . To this end, the foldable device includes the path difference compensating unit. The path difference compensating unit enables at least one of the first and second bodies  1  and  2  to move away from the hinge shaft during a period when the foldable device moves from the unfolding position to the changing position. The path difference compensating unit enables at least one of the first and second bodies  1  and  2  to move toward the hinge shaft during a period when the foldable device moves from the changing position to the folding position. 
     For example, the path difference compensating unit may include a restriction pin that has a fixed position with respect to the first and second hinge shafts  5   a - 1  and  5   a - 2  and a restriction slot that rotates about and moves to or from the first and second hinge shafts  5   a - 1  and  5   a - 2  along with the first and second bodies  1  and  2 . The restriction pin stays fixed and does not rotate around the first and second hinge shafts  5   a - 1  and  5   a - 2 . The restriction slot is shaped to restrict amounts of movement of the first and second bodies  1  according to a rotation angle of the first and second bodies  1  and  2 . 
       FIGS. 10A, 10B, and 10C  are schematic views of a structure of a hinge shaft  5   a  including the first and second bodies  1  and  2  that move to or from the first and second hinge shafts  5   a - 1  and  5   a - 2  according to an exemplary embodiment. FIG.  10 A illustrates a state wherein the first and second bodies  1  and  2  are unfolded,  FIG. 10B  illustrates a state wherein the first and second bodies  1  and  2  are spaced apart by a maximum distance from the first and second hinge shafts  5   a - 1  and  5   a - 2 , and  FIG. 10C  illustrates a state wherein the first and second bodies  1  and  2  are folded. In the foldable device according to the present exemplary embodiment, the first and second bodies  1  and  2  move to or from the first and second hinge shafts  5   a - 1  and  5   a - 2 . In  FIGS. 10A, 10B, and 10C , the flexible display device  4  is not shown.  FIG. 10D  is a graph showing a movement distance according to a folding angle between the first and second bodies  1  and  2 . 
     Referring to  FIG. 10A , the first and second bodies  1  and  2  may move along the first and second guide shafts  53 - 1  and  53 - 2  as described above. Amounts of movement of the first and second bodies  1  and  2  are restricted by the path difference compensating unit. The path difference compensating unit may include first and second restriction pins  5   b - 1  and  5   b - 2  having fixed positions, and first and second restriction slots  1   c  and  2   c  that rotate and move along with the first and second bodies  1  and  2 . For example, the first and second restriction slots  1   c  and  2   c  may be formed in the first and second bodies  1  and  2  or the first and second movable frames  52 - 1  and  52 - 2  (see  FIG. 8 ). The first and second restriction pins  5   b - 1  and  5   b - 2  are respectively inserted into the first and second restriction slots  1   c  and  2   c . The first and second restriction slots  1   c  and  2   c  respectively include first end portions  1   c - 1  and  2   c - 1  and second end portions  1   c - 2  and  2   c - 2 . At the unfolding position, the first and second restriction pins  5   b - 1  and  5   b - 2  are respectively located in the first end portions  1   c - 1  and  2   c - 1 , and at the folding position, the first and second restriction pins  5   b - 1  and  5   b - 2  are respectively located in the second end portions  1   c - 2  and  2   c - 2 . As shown in  FIG. 10A , each of the first and second restriction slots  1   c  and  2   c  linearly extends in a direction that is perpendicular to the longitudinal direction L of the first and second bodies  1  and  2 , and is symmetric with respect to a line X 1  that connects the first and second hinge shafts  5   a - 1  and  5   a - 2 . In this configuration, a movement distance of the first and second bodies  1  and  2  to or from the first and second hinge shafts  5   a - 1  and  5   a - 2  when the first and second bodies  1  and  2  fold/unfold is symmetric about an angle of 90° as shown in the graph of  FIG. 10D . 
     When the first and second bodies  1  and  2  are completely unfolded as shown in  FIG. 10A , the folding/unfolding path difference  4   g  between the flexible display device  4  and the first and second bodies  1  and  2  is “0” and the first and second restriction pins  5   b - 1  and  5   b - 2  are located in the first end portions  1   c - 2  and  2   c - 2  of the first and second restriction slots  1   c  and  2   c . In this state, when the first and second bodies  1  and  2  begin to pivot about the first and second hinge shafts  5   a - 1  and  5   a - 2  to fold, the first and second bodies  1  and  2  are guided by the first and second restriction pins  5   b - 1  and  5   b - 2  and the first and second restriction slots  1   c  and  2   c  to gradually move away from the first and second hinge shafts  5   a - 1  and  5   a - 2 . When the first and second bodies  1  and  2  form an angle of, for example, 90°, therebetween as shown in  FIG. 10B , the first and second restriction pins  5   b - 1  and  5   b - 2  are located in the middle of the first and second restriction slots  1   c  and  2   c  and the first and second bodies  1  and  2  reach a farthest position from the first and second hinge shafts  5   a - 1  and  5   a - 2 . When the first and second bodies  1  and  2  move continuously to be folded, the first and second bodies  1  and  2  gradually move toward the first and second hinge shafts  5   a - 1  and  5   a - 2 . When the first and second bodies  1  and  2  are completely folded as shown in  FIG. 10C , the first and second restriction pins  5   b - 1  and  5   b - 2  reach the second end portions  1   c - 2  and  2   c - 2  of the first and second restriction slots  1   c  and  2   c.    
     As such, since the first and second bodies  1  and  2  move away from the first and second hinge shafts  5   a - 1  and  5   a - 2  and then move toward the first and second hinge shafts  5   a - 1  and  5   a - 2  during a folding/unfolding process to compensate for the folding/unfolding path difference  4   g , the third part  4   c  of the flexible display device  4  may be bent into a smooth curved shape. Also, when the foldable device is used in a state wherein the first and second bodies  1  and  2  form a predetermined unfolding angle therebetween, the third part  4   c  has a smooth curved shape, thereby reducing distortion of a screen. 
     The first and second restriction slots  1   c  and  2   c  are not limited to linear shapes of  FIG. 10A , and may have any of various shapes as long as the folding/unfolding path difference  4   g  according to a folding/unfolding structure of the first and second bodies  1  and  2  may be compensated for. 
       FIGS. 11A, 11B, and 11C  are schematic views of a structure of a hinge shaft  5   a  including the first and second bodies  1  and  2  that move to or from the first and second hinge shafts  5   a - 1  and  5   a - 2  according to an exemplary embodiment.  FIG. 11A  illustrates a state wherein the first and second bodies  1  and  2  are unfolded,  FIG. 11B  illustrates a state wherein the first and second bodies  1  and  2  are spaced apart by a maximum distance from the first and second hinge shafts  5   a - 1  and  5   a - 2 , and  FIG. 11C  illustrates a state wherein the first and second bodies  1  and  2  are folded. In  FIGS. 11A, 11B, and 11C , the flexible display device  4  is not shown.  FIG. 11D  is a graph showing a movement distance according to a folding angle between the first and second bodies  1  and  2 . 
     Referring to  FIG. 11A , the first and second restriction slots  1   c  and  2   c  include first parts  1   c - a  and  2   c - a  corresponding to a folding/unfolding process from an unfolding state to a state having an angle of, for example, 90°, between the first and second bodies  1  and  2  and second parts  1   c - b  and  2   c - b  corresponding to a folding/unfolding process from the state having the angle of 90° between the first and second bodies  1  and  2  and a folding state. The first and second bodies  1  and  2  are folded or unfolded by rotating about the first and second hinge shafts  5   a - 1  and  5   a - 2 . When the first and second bodies  1  and  2  are folded, the first and second bodies  1  and  2  form an obtuse angle during a period when the foldable device moves from the unfolding state to a state having the receiving start angle of, for example, 90°, and the first and second support portions  1   b  and  2   b  are maintained at the support position. As a result, the folding/unfolding path difference  4   g  may be sharply increased. When the angle between the first and second bodies  1  and  2  is less than the receiving start angle, the first and second support portions  1   b  and  2   b  may pivot to the receiving position at which the receiving space  3  is formed, thereby reducing the folding/unfolding path difference  4   g  to some extent due to the receiving space  3 . 
     In this regard, the first parts  1   c - a  and  2   c - a  have profiles corresponding to a sharp change in the folding/unfolding path difference  4   g  during a folding/unfolding process. When the angle between the first and second bodies  1  and  2  is less than 90°, since the third part  4   c  of the flexible display device  4  is received in the receiving space  3  and thus the folding/unfolding path difference  4   g  is reduced to some extent due to the receiving space  3 , the second parts  1   c - b  and  2   c - b  do not need to have strict profiles for compensating for the folding/unfolding path difference  4   g  when compared to the first parts  1   c - a  and  2   c - a . The second parts  1   c - b  and  2   c - b  may just have profiles for moving the first and second bodies  1  and  2  so that the third part  4   c  of the flexible display device  4  smoothly forms the curved portion  4   d  and is received in the receiving space  3 . 
     According to the first and second restriction slots  1   c  and  2   c  of  FIG. 11A , a movement distance of the first and second bodies  1  and  2  to or from the first and second hinge shafts  5   a - 1  and  5   a - 2  when the first and second bodies  1  and  2  fold or unfold is asymmetric about an angle of 90° as shown in the graph of  FIG. 11D . That is, a change in a movement distance when the first and second bodies  1  and  2  are guided by the first parts  1   c - a  and  2   c - a , that is, a gradient of a curve indicating the movement distance in  FIG. 11D , is sharply increased, and a movement distance when the first and second bodies  1  and  2  are guided by the second parts  1   c - b  and  2   c - b  is relatively moderately changed. 
     When the first and second bodies  1  and  2  are completely unfolded as shown in  FIG. 11A , the folding/unfolding path difference  4   g  between the flexible display device  4  and the first and second bodies  1  and  2  is “0” and the first and second restriction pins  5   b - 1  and  5   b - 2  are located in the first end portions  1   c - 2  and  2   c - 1  of the first and second restriction slots  1   c  and  2   c . In this state, when the first and second bodies  1  and  2  begin to pivot about the first and second hinge shafts  5   a - 1  and  5   a - 2  to fold, due to operations of the first and second restriction pins  5   b - 1  and  5   b - 2  and the first parts  1   c - a  and  2   c - a , the first and second bodies  1  and  2  rapidly move away from the first and second hinge shafts  5   a - 1  and  5   a - 2 . When the first and second bodies  1  and  2  form an angle of 90° therebetween as shown in  FIG. 11B , the first and second restriction pins  5   b - 1  and  5   b - 2  are located in the middle of the first and second restriction slots  1   c  and  2   c , that is, a boundary between the first parts  1   c - a  and  2   c - a  and the second parts  1   c - b  and  2   c - b , and the first and second bodies  1  and  2  reach a farthest position from the first and second hinge shafts  5   a - 1  and  5   a - 2 . When the first and second bodies  1  and  2  move continuously to folded, the first and second restriction pins  5   b - 1  and  5   b - 2  are guided by the second parts  1   c - b  and  2   c - b , and the first and second bodies  1  and  2  gradually move toward the first and second hinge shafts  5   a - 1  and  5   a - 2 . When the first and second bodies  1  and  2  are completely folded as shown in  FIG. 11C , the first and second restriction pins  5   b - 1  and  5   b - 2  reach the second end portions  1   c - 2  and  2   c - 2  of the first and second restriction slots  1   c  and  2   c.    
     In other exemplary embodiments, the first and second restriction slots  1   c  and  2   c  may have any of various shapes for compensating for the folding/unfolding path difference  4   g  between the flexible display device  4  and the first and second bodies  1  and  2  according to a folding/unfolding angle. 
       FIG. 12A  is an exploded perspective view illustrating a connection relationship between the first restriction pin  5   b - 1  and the first restriction slot  1   c  according to an exemplary embodiment.  FIG. 12B  is a cross-sectional view taken along line E-E′ of  FIG. 12A . The path difference compensating unit will now be explained with reference to  FIGS. 8, 12A, and 12B . 
     Referring to  FIGS. 8, 12A, and 12B , first and second pin members  5   d - 1  and  5   d - 2  are illustrated. The first and second restriction pins  5   b - 1  and  5   b - 2  are respectively provided on the first and second pin members  5   d - 1  and  5   d - 2 . The first and second pin members  5   d - 1  and  5   d - 2  are provided on the fixed hinge  5 - 3 . For example, the first and second pin members  5   d - 1  and  5   d - 2  are inserted into through-holes  5   c - 4  of the locking member  5   c  and are respectively fixed to the first and second hinge shafts  5   a - 1  and  5   a - 2  due to a fastening member  5   e . Accordingly, the first and second restriction pins  5   b - 1  and  5   b - 2  may have fixed positions with respect to the first and second hinge shafts  5   a - 1  and  5   a - 2 . 
     The first restriction slot  1   c  including the first part  1   c - a  and the second part  1   c - b  shown in  FIG. 11A  is formed in an end portion of the first movable frame  52 - 1  that is close to the first hinge shaft  5   a - 1 . The first restriction pin  5   b - 1  is inserted into the first restriction slot  1   c . As marked by brackets in  FIGS. 12A and 12B , the second restriction slot  2   c  including the first part  2   c - a  and the second part  2   c - b  shown in  FIG. 11A  is also formed in the second movable frame  52 - 2 . The second restriction pin  5   b - 2  is inserted into the second restriction slot  2   c.    
     The path difference compensating unit according to an exemplary embodiment may be realized by using the above configuration. Shapes of the first and second restriction slots  1   c  and  2   c  are not limited to those in the above, and may be determined to compensate for the folding/unfolding path difference  4   g  in consideration of the folding/unfolding path difference  4   g  during a folding/unfolding process of the first and second bodies  1  and  2 . 
     The first and second support portions  1   b  and  2   b  may move between the support position and the receiving position as described above. The first and second support portions  1   b  and  2   b  are located at the support position when the first and second bodies  1  and  2  are located at the unfolding position, and move to the receiving position when the first and second bodies  1  and  2  are located at the folding position. In order for the first and second bodies  1  and  2  to be used at the inclined position between the unfolding position and the folding position, the first and second support portions  1   b  and  2   b  may be located at the support position when the first and second bodies  1  and  2  are in a position from the unfolding position to the inclined position, and may move to the receiving position when the first and second bodies  1  and  2  fold beyond the inclined position. Accordingly, the receiving start angle may be equal to or greater than an angle corresponding to the inclined position. Since the foldable device is rarely used at an opening angle that is less than 90°, the first and second support portions  1   b  and  2   b  may be maintained at the support position at the opening angle ranging from 180° to 90°. 
     The foldable device may include a restriction unit that maintains the first and second support portions  1   b  and  2   b  at the support position when the first and second bodies  1  and  2  are in a position from the unfolding position to the receiving start position corresponding to the receiving start angle, and allows the first and second support portions  1   b  and  2   b  to move to the receiving position when the first and second bodies  1  and  2  folds beyond the receiving start position. 
       FIG. 13  is a schematic view of the restriction unit according to an exemplary embodiment. Although only a mechanism that restricts the movement of the first support portion  1   b  is shown in  FIG. 13 , the same mechanism applies to a structure that restricts the movement of the second support portion  2   b . Hence, reference numerals without parentheses in  FIG. 13  denotes an element of the structure that restricts the movement of the first support portion  1   b , and reference numerals in parentheses denotes an element of the structure that restricts the movement of the second support portion  2   b.    
     Referring to  FIG. 13 , first and second support arms  56 - 1  and  56 - 2  that respectively support the first and second support portions  1   b  and  2   b  are illustrated. The first and second support arms  56 - 1  and  56 - 2  may respectively pivot about fixed positions with respect to the first and second hinge shafts  5   a - 1  and  5   a - 2 . For example, the first and second support arms  56 - 1  and  56 - 2  may be pivotably supported on the fixed hinge  5 - 3 . 
       FIG. 14  is an exploded perspective view of a structure in which the first and second support arms  56 - 1  and  56 - 2  are pivotably supported according to an exemplary embodiment. Referring to  FIGS. 8 and 14 , first and second pivot pins  56 - 1   a  and  56 - 2   a  are respectively provided on end portions of the first and second support arms  56 - 1  and  56 - 2 , and are respectively pivotably inserted into first and second pivot holes  5   c - 4   a  and  5   c - 4   b  that are formed in the locking member  5   c.    
     First and second guide pins  56 - 1   b  and  56 - 2   b  are respectively provided on the first and second support arms  56 - 1  and  56 - 2 . The first and second guide pins  56 - 1   b  and  56 - 2   b  are respectively inserted into first and second guide slots  57 - 1  and  57 - 2 . Although the first and second guide pins  56 - 1   b  and  56 - 2   b  pass through the first and second guide slots  57 - 1  and  57 - 2 , extend under the first and second support portions  1   b  and  2   b , and support the first and second support portions  1   b  and  2   b  in the present exemplary embodiment, for convenience of explanation,  FIG. 13  illustrates that support protrusions  56 - 1   c  and  56 - 2   c  that support the first and second support portions  1   b  and  2   b  are respectively provided on the first and second support arms  56 - 1  and  56 - 2 . 
     The first and second guide slots  57 - 1  and  57 - 2  are respectively formed in the first and second hinge frames  51 - 1  and  51 - 2 . The first and second guide slots  57 - 1  and  57 - 2  include support parts  57 - 1   a  and  57 - 2   a  and receiving parts  57 - 1   b  and  57 - 2   b . The support parts  57 - 1   a  and  57 - 2   a  correspond to a period from the unfolding position to the receiving start position, and the receiving parts  57 - 1   b  and  57 - 2   b  correspond to a period from the receiving start position to the folding position. The support parts  57 - 1   a  and  57 - 2   a  are formed so that the first and second support arms  56 - 1  and  56 - 2  are maintained at a position at which the first and second support arms  56 - 1  and  56 - 2  support the first and second support portions  1   b  and  2   b  while the first and second bodies  1  and  2  pivot from the unfolding position to the receiving start position. The receiving parts  57 - 1   b  and  57 - 2   b  are formed so that the first and second support arms  56 - 1  and  56 - 2  are allowed to pivot away from the first and second support portions  1   b  and  2   b  and thus the first and second support portions  1   b  and  2   b  are allowed to pivot to the receiving position while the first and second bodies  1  and  2  pivot from the receiving start position to the folding position. Although the first and second support portions  1   b  and  2   b  pivot due to their weights from the support position to the receiving position in the present exemplary embodiment, an elastic member that applies an elastic force in a direction in which the first and second support portions  1   b  and  2   b  pivot to the receiving position may be provided, and thus the first and second support portions  1   b  and  2   b  may pivot from the support position to the receiving position due to the elastic force of the elastic member. 
       FIG. 13  illustrates a state (a) wherein the first and second bodies  1  and  2  are located at the unfolding position. The first and second guide pins  56 - 1   b  and  56 - 2   b  are located in the support parts  57 - 1   a  and  57 - 2   a , and the first and second support portions  1   b  and  2   b  are supported by the first and second support arms  56 - 1  and  56 - 2  and are maintained at the support position. Accordingly, the flexible display device  4  is supported by the first and second fixed portions  1   a  and  2   a , the first and second support portions  1   b  and  2   b , and a movable guide member  7  which will be explained below and is maintained flat. 
     In the state (a) of  FIG. 13 , when the first and second bodies  1  and  2  begin to fold, the first and second bodies  1  and  2  move away from the first and second hinge shafts  5   a - 1  and  5   a - 2  due to the path difference compensating unit. Until the first and second bodies  1  and  2  reach the receiving start position as in a state (b) of  FIG. 13 , since the first and second guide pins  56 - 1   b  and  56 - 2   b  are guided by the support parts  57 - 1   a  and  57 - 2   a , the first and second support arms  56 - 1  and  56 - 2  are maintained at a position at which the first and second support arms  56 - 1  and  56 - 2  support the first and second support portions  1   b  and  2   b.    
     When the first and second bodies  1  and  2  fold beyond the receiving start position, the first and second guide pins  56 - 1   b  and  56 - 2   b  are guided by the receiving parts  57 - 1   b  and  57 - 2   b . The first and second support arms  56 - 1  and  56 - 2  pivot about the first and second pivot pins  56 - 1   a  and  56 - 2   a  away from the first and second support portions  1   b  and  2   b . The first and second support portions  1   b  and  2   b  pivot away from a rear surface of the flexible display device  4 , and the receiving space  3  (see  FIG. 3 ) begins to be formed in the first and second bodies  1  and  2 . The first and second bodies  1  and  2  slide toward the first and second hinge shafts  5   a - 1  and  5   a - 2  due to the path difference compensating unit, and the third part  4   c  of the flexible display device  4  is moderately bent and is received in the receiving space  3 . 
     When the first and second bodies  1  and  2  reach the folding position as in a state (c) of  FIG. 13 , the first and second support portions  1   b  and  2   b  reach the receiving position, and the receiving space  3  is formed in the first and second bodies  1  and  2  as shown in  FIG. 3 . The third part  4   c  of the flexible display device  4  forms the curved portion  4   d  and is received in the receiving space  3 . 
     The first and second guide slots  57 - 1  and  57 - 2  are not limited to shapes in  FIG. 13 . Shapes of the first and second guide slots  57 - 1  and  57 - 2  may vary according to positions of the first and second pivot holes  5   c - 4   a  and  5   c - 4   b  relative to the first and second hinge shafts  5   a - 1  and  5   a - 2 . 
     A structure of the restriction unit is not limited to that in  FIGS. 13 and 14 . The restriction unit according to an exemplary embodiment is shown in  FIG. 15 . The restriction unit of the present exemplary embodiment includes two link arms. Although only a mechanism that restricts the movement of the first support portion  1   b  is shown in  FIG. 15 , the same mechanism applies to a structure that restricts the movement of the second support portion  2   b.    
     Referring to  FIG. 15 , first and second link arms  61  and  62  are illustrated. The first link arm  61  may pivot about a fixed position with respect to the first hinge shaft  5   a - 1 . For example, the first link arm  61  may be pivotably supported on the fixed hinge  5 - 3 . For example, a pivot pin  61   a  that is provided on one end portion of the first link arm  61  may be pivotably inserted into the first pivot hole  5   c - 4   a  of  FIG. 14 . One end portion of the second link arm  62  and the other end portion of the first link arm  61  are pivotably connected to each other by a guide pin  63 . The other end portion of the second link arm  62  is pivotably connected to the first support portion  1   b . The guide pin  63  is guided by a guide slot  64 . The guide slot  64  may be formed in, for example, the first hinge frame  51 - 1 . The guide slot  64  includes a support part  64   a  and a receiving part  64   b . The support part  64   a  accommodates the guide pin  63  during a period when the foldable device moves from the unfolding position to the receiving start position. The receiving part  64   b  accommodates the guide pin  63  during a period when the foldable device moves from the receiving start position to the folding position. 
     In this configuration, the first and second link arms  61  and  62  and the first support portion  1   b  constitute a three-link (3-link) structure. In other words, in  FIG. 13 , the first and second support arms  56 - 1  and  56 - 2  respectively include the first and second link arms  61  and  62 , and the first and second guide pins  56 - 1   b  and  56 - 2   b  pivotably connect the first and second link arms  61  and  62 . The guide slot  64  is a modification of the first and second guide slots  57 - 1  and  57 - 2  of  FIG. 13  in order to suit the 3-link structure. 
       FIG. 15  illustrates a state (a) wherein the first and second bodies  1  and  2  are located at the unfolding position. The guide pin  63  is located in the support part  64   a  and the first support portion  1   b  is maintained at the support position. In the state (a) of  FIG. 15 , when the first and second bodies  1  and  2  begin to fold, the first body  1  moves away from the first hinge shaft  5   a - 1  due to the path difference compensating unit. Until the first body  1  reaches the receiving start position as in a state (b) of  FIG. 15 , since the guide pin  63  is guided by the support part  64   a , the first support portion  1   b  is maintained at the support position. 
     When the first and second bodies  1  and  2  fold beyond the receiving start position, the guide pin  63  is guided by the receiving part  64   b . The first and second link arms  61  and  62  pull the first support portion  1   b  toward the receiving position, and thus the first support portion  1   b  pivots away from the rear surface of the flexible display device  4  and the receiving space  3  (see  FIG. 3 ) begins to be formed in the first and second bodies  1  and  2 . The first body  1  slides toward the first hinge shaft  5   a - 1  due to the path difference compensating unit, and the third part  4   c  of the flexible display device  4  is moderately bent and is received in the receiving space  3 . 
     When the first and second bodies  1  and  2  reach the folding position as shown in a state (c) of  FIG. 15 , the first support portions  1   b  and  2   b  reach the receiving position and the receiving space  3  is formed in the first and second bodies  1  and  2  as shown in  FIG. 3 . The third part  4   c  of the flexible display device  4  forms the curved portion  4   d  and is received in the receiving space  3 . 
       FIGS. 16A, 16B, and 16C  are schematic views of the restriction unit according to another exemplary embodiment. Although a mechanism that moves the first support portion  1   b  between the support position and a receiving position is illustrated in  FIGS. 16A, 16B, and 16C , the same mechanism applies to a structure that moves the second support portion  2   b  between the support position and the receiving position. 
     Referring to  FIGS. 16A, 16B, and 16C , a connection member  5   j  is illustrated. The connection member  5   j  may be, for example, a wire. One end portion  5   j - 1  of the connection member  5   j  may be fixed to a fixed position with respect to the first hinge shaft  5   a - 1 . The other end portion  5   j - 2  of the connection member  5   j  is connected to a rack gear  5   g . The rack gear  5   g  may be slidably provided on, for example, the first hinge frame  51 - 1 . 
     For example, the one end portion  5   j - 1  of the connection member  5   j  may be connected to the fixed hinge  5 - 3 . A pulley  5   f - 1  is provided on the first hinge shaft  5   a - 1 . The connection member  5   j  is in contact with the pulley  5   f - 1  and is wound around the pulley  5   f - 1  during a folding/unfolding process of the first and second bodies  1  and  2 . When the connection member  5   j  is wound around the pulley  5   f - 1 , a distance between a contact point between the connection member  5  and the pulley  5   f - 1  and the other end portion  5   j - 2  of the connection member  5  is changed, and the rack gear  5   g  slides. 
     When the one end portion  5   j - 1  of the connection member  5   j  is connected to the second hinge frame  51 - 2  and a pulley  5   f - 2  is provided on the second hinge shaft  5   a - 2 , a sliding amount of the rack gear  5   g  may be increased by causing the connection member  5   j  to be wound around the pulleys  5   f - 1  and  5   f - 2  during a folding/unfolding process. 
     When the first and second bodies  1  and  2  unfold, a return spring  5   k  applies an elastic force to the rack gear  5   g  so that the rack gear  5   g  moves away from the first hinge shaft  5   a - 1 . 
     The first support portion  1   b  is supported by a rotating cam  5   h . A pinion gear  5   h - 1  that is engaged with the rack gear  5   g  is provided on the rotating cam  5   h . Accordingly, as the rack gear  5   g  slides, the rotating cam  5   h  rotates. The rotating cam  5   h  includes a first cam path  5   h - 2  that has a constant distance from the center of rotation and a second cam path  5   h - 3  that has a reduced distance from the center of rotation. While the first support portion  1   b  is supported by the first cam path  5   h - 2 , the first support portion  1   b  is maintained at the support position. When contact between the first support portion  1   b  and the first cam path  5   h - 2  ends, the first support portion  1   b  is supported by the second cam path  5   h - 3  and pivots to the receiving position. 
       FIG. 16A  illustrates a state wherein the first and second bodies  1  and  2  are located at the unfolding position. The first support portion  1   b  is supported by the first cam path  5   h - 2  and is maintained at the support position. 
     In the state of  FIG. 16A , when the first and second bodies  1  and  2  begin to fold, the connection member  5   j  begins to be wound around the first and second pulleys  5   f - 1  and  5   f - 2 . Accordingly, the rack gear  5  slides toward the first hinge shaft  5   a - 1 , and the rotating cam  5   h  rotates. Until the first and second bodies  1  and  2  reach the receiving start position as shown in  FIG. 16B , since the first support portion  1   b  is guided by the first cam path  5   h - 2 , the first support portion  1   b  is maintained at the support position. 
     When the first and second bodies  1  and  2  fold beyond the receiving start position, contact between the first cam path  5   h - 2  and the first support portion  1   b  ends and the first support portion  1   b  is supported by the second cam path  5   h - 3 . Accordingly, the first support portion  1   b  pivots away from the rear surface of the flexible display device  4  and the receiving space  3  (see  FIG. 3 ) begins to be formed in the first and second bodies  1  and  2 . The third part  4   c  of the flexible display device  4  is moderately bent and begins to be received in the receiving space  3 . 
     When the first and second bodies  1  and  2  reach the folding position as shown in  FIG. 16C , the first support portion  1   b  reaches the receiving position and the receiving space  3  is formed in the first and second bodies  1  and  2  as shown in  FIG. 3 . The third part  4   c  of the flexible display device  4  forms the curved portion  4   d  and is received in the receiving space  3 . 
     The return spring  5   k  is connected to the rack gear  5   g  and applies a tensile force to the connection member  5   j  having a wire shape. Accordingly, when the first and second bodies  1  and  2  unfold, due to an elastic force of the return spring  5 , the rack gear  5   g  slides in a direction that is opposite to a direction in which the rack gear  5   g  slides when the first and second bodies  1  and  2  fold, and the rotating cam  5   h  also rotates in an opposite direction. Accordingly, the first support portion  1   b  returns from the receiving position to the support position. 
     Assuming that a strip having bending elasticity is used as the connection member  5   j , when the first and second bodies  1  and  2  unfold, the strip is spread due to elasticity and pushes the rack gear  5   g  in a direction that is opposite to a direction in which the rack gear  5   g  slides when the first and second bodies  1  and  2  fold. Accordingly, the return spring  5   k  may be omitted. 
     Referring back to  FIG. 6 , a cover member  8  surrounds an area between the first and second hinge units  5 - 1  and  5 - 2  so that a receiving space in the foldable device is prevented from being exposed to the outside. For example, the cover member  8  may include an extending portion  81  that extends along facing edges of the first and second hinge units  5 - 1  and  5 - 2  and side wall portions  82  that are located on both end portions of the extending portion  81 . The cover member  8  is coupled to the hinge unit  5  by slightly opening outward the side wall portions  82  to surround the first and second gear portions  51 - 1   b  and  51 - 2   b.    
     Referring back to  FIGS. 6 and 7 , the movable support member  7  including a support surface  71  on which the third part  4   c  of the flexible display device  4  is supported is illustrated. The movable support member  7  is located at a first position at which the movable support member  7  supports the third part  4   c  of the flexible display device  4  when the first and second bodies  1  and  2  are located at the unfolding position. The movable support member  7  supports the third part  4   c  of the flexible display device  4  between the first and second support portions  1   b  and  2   b . When the first and second bodies  1  and  2  change to the folding position, the movable support member  7  is spaced apart from the third part  4   c  of the flexible display device  4  and is located at a second position at which the receiving space  3  for receiving the curved portion  4   d  is formed. That is, the movable support member  7  moves between the first position and the second position as the first and second bodies  1  and  2  change between the unfolding position and the folding position. As such, when the first and second bodies  1  and  2  are located at the unfolding position, the third part  4   c  of the flexible display device  4  may be supported by the movable support member  7  to be spread flat. As the foldable device changes from the folding position to the unfolding position and the movable support member  7  is raised from the second position to the first position, the movable support member  7  may adjacently support the third part  4   c  of the flexible display device  4 . Accordingly, even when the foldable device is used at the inclined position having a predetermined angle, the movable support member  7  may stably support the third part  4   c  of the flexible display device  4  along with the first and second support portions  1   b  and  2   b , thereby reducing distortion of a screen at the third part  4   c.    
       FIG. 17  is an exploded perspective view of a structure that moves the movable support member  7  between the first position and the second position according to an exemplary embodiment. Referring to  FIG. 17 , slots  72  that extend in a folding/unfolding direction of the first and second bodies  1  and  2 , that is, the longitudinal direction L, are formed in the movable support member  7 . One pair of guide portions  73 - 1  and  73 - 2  that are inserted into the slots  72  are provided around facing edges of the first and second hinge units  5 - 1  and  5 - 2 . For example, the guide portions  73 - 1  and  73 - 2  may be provided on upper ends of ribs  74 - 1  and  74 - 2  that respectively upwardly extend from the first and second hinge frames  51 - 1  and  51 - 2 . Insertion grooves  75  are formed in the movable support member  7 , and the guide portions  73 - 1  and  73 - 2  are inserted into the slots  72  through the insertion grooves  75 . The insertion grooves  75  are formed at central portions of the slots  72  in a longitudinal direction  72 L of the slots  72 . While the first and second bodies  1  and  2  fold or unfold, the guide portions  73 - 1  and  73 - 1  move in the longitudinal direction  72 L of the slots  72  and a movement path of the guide portions  73 - 1  and  73 - 2  does not overlap the insertion grooves  75 . Accordingly, while the guide portions  73 - 1  and  73 - 2  are inserted into the slots  72  and then the movable support member  7  is raised and lowered, the guide portions  73 - 1  and  73 - 2  are not separated from the slots  72  through the insertion grooves  75 . 
       FIGS. 18A and 18B  are cross-sectional views taken along line F-F′ of  FIG. 17 , respectively illustrating a state wherein the movable support member  7  is located at the second position and a state wherein the movable support member  7  is located at the first position. Referring to  FIG. 18A , the first and second bodies  1  and  2  are located at the folding position. The guide portions  73 - 1  and  73 - 2  are located at a farthest position from each other, and the movable support member  7  is located at the second position. The curved portion  4   d  of the flexible display device  4  is received in the receiving space  3  that is formed due to the first and second support portions  1   b  and  2   b  and the movable support member  7 . 
     When the first and second bodies  1  and  2  unfold in the state of  FIG. 18A , the guide portions  73 - 1  and  73 - 2  slide in the slots  72  to move toward each other, and the movable support member  7  is raised toward the flexible display device  4  and continuously adjacently supports the third part  4   c  of the flexible display device  4 . Accordingly, even when the foldable device is used at the inclined position, the third part  4   c  of the flexible display device  4  may be stably supported by the first and second fixed portions  1   a  and  2   a , the first and second support portions  1   b  and  2   b , and the support surface  71  of the movable support member  7 . 
     When the first and second bodies  1  and  2  are completely unfolded as shown in  FIG. 18B , the movable support member  7  is located at the first position and the third part  4   c  of the flexible display device  4  is stably supported by the support surface  71 . 
     The guide portions  73 - 1  and  73 - 2  and the slots  72  may be provided at two or more places in the width direction W of the foldable device in order for the movable support member  7  not to be inclined when being raised and lowered between the first and second positions. 
     The movement of the guide portions  73 - 1  and  73 - 2  in a width direction  72 W of the slots  72  is restricted and the movement of the guide portions  73 - 1  and  73 - 2  in the longitudinal direction  72 L of the slots  72  is allowed. That is, the vertical movement of the movable support member  7  is restricted by the guide portions  73 - 1  and  73 - 2 . Accordingly, the movable support member  7  may be stably vertically raised and lowered without being shaken. Also, even when the first and second bodies  1  and  2  are located at the unfolding position, the support surface  71  of the movable support member  7  may not be inclined and may stably support flat the third part  4   c  of the flexible display device  4 . 
     The foregoing exemplary embodiments and advantages are merely exemplary and are not to be construed as limiting. The present teaching can be readily applied to other types of apparatuses. Also, the description of the exemplary embodiments is intended to be illustrative, and not to limit the scope of the claims, and many alternatives, modifications, and variations will be apparent to those skilled in the art.