Abstract:
A robot cleaner capable of moving in diverse directions and enhancing cleaning efficiency by increasing frictional force between a pad and a floor includes two or more driving units. Each of the driving units includes plural motors, a first subframe connected to at least any one of the motors and configured to rotate by receiving rotational force from the motor, a rotating plate assembly mounted to the first subframe and configured to be slanted with respect to a floor by rotation of the first subframe and to rotate clockwise or counterclockwise by receiving rotational force from another motor, and a pad provided at the rotating plate assembly and configured to contact the floor. When the rotating plate assembly is slanted with respect to the floor, nonuniform frictional force is generated between the pad and the floor, through which the robot cleaner travels.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of Korean Patent Application No. 10-2013-0004841, filed on Jan. 16, 2013 in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference. 
     BACKGROUND 
     1. Field 
     One or more embodiments relate to a robot cleaner capable of moving in diverse directions and enhancing cleaning efficiency by increasing frictional force between a pad and a floor. 
     2. Description of the Related Art 
     A robot cleaner is an appliance utilizing an automatic travel function to clean a room or the like by vacuuming foreign materials, such as dust, from a floor of a room without user intervention. The robot cleaner detects a distance to an obstacle, such as furniture, office supplies, walls or the like, present in a zone to be cleaned using a distance sensor, and changes traveling directions by selectively driving a left-wheel motor and a right-wheel motor to perform cleaning of the zone to be cleaned. 
     Besides robot cleaners capable of vacuuming foreign materials, such as dust, from a floor, robot cleaners capable of wiping floors have been developed recently. A conventional robot cleaner includes pads provided at a bottom thereof and wipes a floor by traveling with the pads closely contacting the floor. The conventional robot cleaner travels using a moving unit which is provided separately from a cleaning unit. 
     SUMMARY 
     One or more embodiments relate to a robot cleaner capable of smoothly moving in diverse directions and enhancing cleaning efficiency by increasing frictional force between a pad and a floor. 
     Additional aspects and/or advantages of one or more embodiments will be set forth in part in the description which follows and, in part, will be apparent from the description, or may be learned by practice of one or more embodiments of disclosure. One or more embodiments are inclusive of such additional aspects. 
     In accordance with one or more embodiments, a robot cleaner may include a main body that may include two or more driving units and a case disposed outside the main body to define an external appearance thereof, wherein each of the driving units may include plural motors to generate rotational force, a first subframe connected to at least one of the motors and configured to rotate by receiving the rotational force from the motor, a rotating plate assembly mounted to the first subframe and configured to be slanted with respect to a floor by rotation of the first subframe and to rotate clockwise or counterclockwise by receiving the rotational force from another one of the plural motors, and a pad provided at a bottom surface of the rotating plate assembly and configured to contact the floor. When the bottom surface of the rotating plate assembly is slanted with respect to the floor, nonuniform frictional force may be generated between a bottom surface of the pad and the floor. The robot cleaner may travel in a specific direction through the nonuniform frictional force. 
     The plural motors may include a first motor and a second motor. The first subframe may rotate about an X-axis by the first motor and may rotate about a Y-axis by the second motor, and the bottom surface of the rotating plate assembly may be slanted with respect to the floor by rotation of the first subframe. 
     A slanted direction and a slanted angle of the bottom surface of the rotating plate assembly with respect to the floor may be changed according to a degree of rotation of the first subframe about the X-axis or the Y-axis. If the slanted direction of the bottom surface of the rotating plate assembly is changed, a traveling direction of the robot cleaner may be changed. If the slanted angle of the bottom surface of the rotating plate assembly is changed, the frictional force between the pad and floor may be changed. 
     The pad may be made of a compressible material so that even when the bottom surface of the rotating plate assembly is slanted with respect to the floor, a majority of the bottom surface of the pad comes into contact with the floor. 
     The first motor may be positioned above the first subframe and the first subframe may be configured to rotate through a rotating shaft connecting the first motor to the first subframe. 
     The second motor may be positioned next to the first subframe and the first subframe may be configured to rotate through a link connecting the second motor to the first subframe. 
     The rotating plate assembly may include a rotating shaft mounted to the first subframe and a rotating plate mounted to the rotating shaft. 
     The plural motors may further include a third motor and the rotating plate may be configured to rotate about the rotating shaft by the third motor. 
     The robot cleaner may travel through the two or more driving units, in each of which the rotating plate rotates about the rotating shaft while the first subframe is slanted with respect to the floor. 
     The robot cleaner may travel by reaction to the frictional force between the pad and the floor, and a traveling direction of the robot cleaner may be determined by a slanted direction with respect to the floor and a rotating direction of the rotating plate provided at each of the two or more driving units. 
     The two or more driving units may include a first driving unit and a second driving unit, and the first driving unit and the second driving unit may be mounted to a main frame. 
     If a right portion of the bottom surface of the rotating plate provided at the first driving unit and a right portion of the bottom surface of the rotating plate provided at the second driving unit contact the floor and if the rotating plate provided at the first driving unit rotates counterclockwise and the rotating plate provided at the second driving unit rotates clockwise, the robot cleaner may perform cleaning of the floor at a stationary location. 
     The robot cleaner may further include a third driving unit and a fourth driving unit. The second driving unit may be positioned next to the first driving unit, the third driving unit may be positioned to the rear of the first driving unit, and the fourth driving unit may be positioned to the rear of the second driving unit. 
     A rotating plate of the third driving unit may move in the same rotating direction and slanted angle as the rotating plate of the first driving unit, and a rotating plate of the fourth driving unit may move in the same rotating direction and slanted angle as the rotating plate of the second driving unit. 
     In accordance with one or more embodiments, a robot cleaner may include two or more driving units, each of which may include a rotating plate assembly provided with a pad at a bottom surface thereof, a frame to which the rotating plate assembly is mounted, a first motor connected with the frame through a first link and configured to enable the frame to be slanted with respect to a floor, a second motor connected with the frame through a second link arranged to cross the first link and configured to enable the frame to be slanted with respect to the floor, and a third motor mounted to the frame and configured to rotate the rotating plate assembly. When the frame is slanted with respect to the floor, nonuniform frictional force may be generated between the pad and the floor, and the robot cleaner may travel through the nonuniform frictional force and rotation of the rotating plate assembly. 
     The first link and the second link may be arranged perpendicular to each other. 
     The rotating plate assembly may include a rotating shaft provided with a gear at an outer circumferential surface thereof and a rotating plate connected with the rotating shaft. 
     The third motor may be mounted with a gear unit, which may be tooth-engaged with the gear of the rotating shaft. 
     The frame and the rotating plate may be fixed to each other through a locking unit inserted through the frame and the rotating plate. 
     The rotating plate may be formed with a hole at a side surface thereof, into which one end portion of the locking unit may be inserted. The frame may be formed with a hole into which the other end portion of the locking unit may be inserted. 
     Balls may be mounted to both end portions of the first link and both end portions of the second link. 
     The frame may be provided with ball receiving parts in which the balls are received, and rotational force of the first motor and the second motor may be transmitted to the frame. 
     As described above, the robot cleaner according to one or more embodiments may travel in all directions on the floor and may enhance cleaning efficiency by increasing frictional force between the pad and the floor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and/or other aspects will become apparent and more readily appreciated from the following description of embodiments, taken in conjunction with the accompanying drawings of which: 
         FIG. 1  is a perspective view illustrating a robot cleaner according to one or more embodiments; 
         FIG. 2  is a perspective view illustrating a main body of the robot cleaner according to one or more embodiments; 
         FIG. 3  is a partially exploded perspective view of the main body of the robot cleaner according to one or more embodiments; 
         FIG. 4  is a side view of the main body of the robot cleaner according to one or more embodiments; 
         FIG. 5  is a view illustrating a rotating state of a first driving unit about an X-axis in the robot cleaner according to one or more embodiments; 
         FIG. 6  is a view illustrating a rotating state of the first driving unit about a Y-axis in the robot cleaner according to one or more embodiments; 
         FIG. 7 a    is a view illustrating an operating state at a stationary location of the robot cleaner according to one or more embodiments; 
         FIG. 7 b    is a view illustrating bottom surfaces of pads when the robot cleaner according to one or more embodiments operates at a stationary location; 
         FIG. 8 a    is a view illustrating a backward moving state of the robot cleaner according to one or more embodiments; 
         FIG. 8 b    is a view illustrating the bottom surfaces of the pads when the robot cleaner according to one embodiment of the present invention moves backward; 
         FIG. 9 a    is a view illustrating a sideward moving state of the robot cleaner according to one or more embodiments; 
         FIG. 9 b    is a view illustrating the bottom surfaces of the pads when the robot cleaner according to one or more embodiments moves sideways; 
         FIG. 10 a    is a view illustrating a diagonally moving state of the robot cleaner according to one or more embodiments; 
         FIG. 10 b    is a view illustrating the bottom surfaces of the pads when the robot cleaner according to one or more embodiments moves diagonally; 
         FIG. 11  is a view illustrating a main body of a robot cleaner according to one or more embodiments; 
         FIG. 12  is an exploded perspective view of a first driving unit of the robot cleaner according to one or more embodiments; 
         FIG. 13  is a sectional view of the first driving unit of the robot cleaner according to one or more embodiments; 
         FIG. 14  is a view illustrating a rotating state of a rotating plate of the first driving unit about the X-axis in the robot cleaner according to one or more embodiments; and 
         FIG. 15  is a view illustrating a rotating state of the rotating plate of the first driving unit about the Y-axis in the robot cleaner according to one or more embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to one or more embodiments, illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout. In this regard, embodiments of the present invention may be embodied in many different forms and should not be construed as being limited to embodiments set forth herein, as various changes, modifications, and equivalents of the systems, apparatuses and/or methods described herein will be understood to be included in the invention by those of ordinary skill in the art after embodiments discussed herein are understood. Accordingly, embodiments are merely described below, by referring to the figures, to explain aspects of the present invention. 
       FIG. 1  is a perspective view illustrating a robot cleaner according to one or more embodiments, and  FIG. 2  is a perspective view illustrating a main body of the robot cleaner according to one or more embodiments. 
     Referring to  FIGS. 1 and 2 , a robot cleaner  1  according to one or more embodiments may include a main body  2  and a housing  3 . The housing  3  may envelop the main body  2  and may form an external appearance of the robot cleaner  1 . 
     The main body  2  may include a first driving unit  10  and a second driving unit  20 . The second driving unit  20  may be provided corresponding to the first driving unit  10 . Hereinafter, the first driving unit  10  will be explained. Operational structure and method of the first driving unit  10  may be similarly applied to the second driving unit  20 . 
     The first driving unit  10  may include a frame  30 , a motor  40  and a rotating plate assembly  50 . The motor  40  and the rotating plate assembly  50  may be mounted to the frame  30 . The frame  30  may include a main frame  31 , and the first driving unit  10  and the second driving unit  20  may be mounted to the main frame  31 , thereby forming the main body  2 . The motor  40  may rotate the frame  30  mounted with the rotating plate assembly  50 , to thereby vary a contact region of the rotating plate assembly  50  with the floor. The motor  40  also may rotate the rotating plate assembly  50  to clean the floor. 
     Hereinafter, a structure of the first driving unit  10  will be explained in detail with reference to the drawings. 
       FIG. 3  is a partially exploded perspective view of the main body of the robot cleaner according to one or more embodiments, and  FIG. 4  is a side view of the main body of the robot cleaner according to one or more embodiments. 
     Referring to  FIGS. 3 and 4 , the robot cleaner  1  according to one or more embodiments may include the frame  30 , the motor  40  and the rotating plate assembly  50 . The frame  30  may include a main frame  31 , a fixing frame  32  and a first subframe  33 . The first driving unit  10  and the second driving unit  20  may be mounted to the main frame  31 , thereby forming the main body  2 . 
     The fixing frame  32  may be coupled to the main frame  31  to cover an upper portion of the first driving unit  10 . The fixing frame  32  may be fixed to an upper surface of the main frame  31  using a fastening member. 
     The first subframe  33  may be disposed below the main frame  31 . The first subframe  33  may be formed with a receiving part  330 , in which a driving motor  43  (which will be described later) and the rotating plate assembly  50  may be inserted. The rotating plate assembly  50  may include a rotating shaft  51 , which may be connected to the driving motor  43  in the receiving part  330 . The rotating shaft  51  may rotate clockwise or counterclockwise by the driving motor  43 . A hole  331  may be formed at a portion of the first subframe  33 . 
     A case  34  may also be inserted in the receiving part  330 . The case  34  may be formed with a through-hole  340  through which the rotating shaft  51  may pass. The case  34  may further provided with a link coupling part  341 . The rotating shaft  51  may pass through the through-hole  340  of the case  34  and may be connected to the driving motor  43  that may be disposed in the receiving part  330 . The link coupling part  341  may be disposed at an outer surface of the case  34 . The link coupling part  341  may be exposed to the outside of the first subframe  33  through the hole  331  formed at the first subframe  33 . 
     The main frame  31  may be provided with a support frame  36  at a portion thereof to support the main frame  31 . The support frame  36  may extend from a bottom surface of the main frame  31  and may contact the floor. Accordingly, in spite of rotation of the first subframe  33  and rotation of the rotating plate assembly  50 , the main body  2  may stably move on the floor without shaking by virtue of the support frame  36 . 
     The motor  40  may include a first control motor  41 , a second control motor  42  and a driving motor  43 . The first control motor  41  and the second control motor  42  may be servomotors, and the driving motor  43  may be a DC motor. The first control motor  41  may be positioned above the main frame  31 . The second control motor  42  and the driving motor  43  may be positioned below the main frame  31 . However, the positions of the first control motor  41  and the second control motor  42  are not limited to the above-described positions. The first control motor  41  and the second control motor  42  may be controlled to enable the first driving unit  10  to be slanted by a designated angle. In detail, the first control motor  41  may be controlled to rotate the first driving unit  10  about an X-axis, and the second control motor  42  may be controlled to rotate the first driving unit  10  about a Y-axis. The driving motor  43  may be controlled to rotate the rotating plate assembly  50  to rub the floor. 
     The first control motor  41  may be connected with a rotating shaft  360 . The rotating shaft  360  may rotate clockwise or counterclockwise by the first control motor  41 . When the rotating shaft  360  rotates, the first subframe  33  may rotate together with the rotating shaft  360 . 
     In detail, the rotating shaft  360  may be connected with a second subframe  361 . The second subframe  361  may be fixed to or formed integrally with the rotating shaft  360  in order to rotate together with the rotating shaft  360 . The second subframe  361  may be connected to the first subframe  33 . Accordingly, the first subframe  33  may also rotate when the second subframe  361  rotates. As a result, the first subframe  33  may rotate about the X-axis by the rotating shaft  360  which may rotate by the first control motor  41 . 
     The second control motor  42  may be connected with the first subframe  33  to rotate the first subframe  33  about the Y-axis. In detail, the link coupling part  341  may be provided at a side surface of the case  34  that may be inserted in the receiving part  330  of the first subframe  33  and may be connected to the second control motor  42  through a link assembly  35 . 
     The link assembly  35  may include a first link  351 , a second link  352 , a third link  353  and a fourth link  354 . The first link  351  may be mounted to a drive shaft  420  that may be provided at the second control motor  42 . The second link  352  may be mounted to the link coupling part  341  that may be provided at the side surface of the case  34 . The third link  353  and the fourth link  354  may connect the first link  351  and the second link  352 . The third link  353  and the fourth link  354  may be arranged in parallel with each other. Accordingly, if the drive shaft  420  of the second control motor  42  rotates clockwise or counterclockwise, the case  34  may rotate about the Y-axis by the link assembly  35 . If the case  34  rotates about the Y-axis, the first subframe  33 , in which the case  34  may be inserted, may also rotate about the Y-axis. 
     The rotating plate assembly  50  may include a rotating shaft  51 , a rotating plate  52  and a pad  53 . The rotating shaft  51  may be inserted into the case  34 . One end of the rotating shaft  51  may be mounted to the driving motor  43  in the first subframe  33 , and the other end of the rotating shaft  51  may be mounted with the rotating plate  52 . The rotating plate  52  may rotate together with the rotating shaft  51  when the rotating shaft  51  rotates. The rotating plate  52  may be formed integrally with the rotating shaft  51 . The pad  53 , which may be made of a textile material to accomplish wet cleaning, may be attached to a bottom surface of the rotating plate  52 . The rotating plate  52  may be provided with an adhesive part (not shown) at the bottom surface thereof, to which the pad  53  may be adhered. In the above-structured rotating plate assembly  50 , if the rotating plate  52  rotates about a Z-axis by the driving motor  43 , the pad  53  attached to the bottom surface of the rotating plate  52  may wipe the floor. 
       FIG. 5  is a view illustrating a rotating state of the first driving unit about the X-axis in the robot cleaner according to one or more embodiments, and  FIG. 6  is a view illustrating a rotating state of the first driving unit about the Y-axis in the robot cleaner according to one or more embodiments. 
     Referring to  FIGS. 5 and 6 , the rotating plate  52  may be slanted by a designated angle with respect to the floor by the first driving unit  10  rotating about the X-axis or Y-axis. 
     If the rotating shaft  360  rotates a designated angle clockwise or counterclockwise about the X-axis by the first control motor  41 , the second subframe  361  coupled to the rotating shaft  360 , the first subframe  33  mounted to the second subframe  361 , and the rotating plate assembly  50  mounted to the first subframe  33  may all be simultaneously pivoted. As exemplarily shown in  FIG. 5 , an angle T 1  may be formed between an imaginary line A extending from the rotating shaft  51  of the rotating plate assembly  50  before rotation of the rotating shaft  360  and an imaginary line B extending from the rotating shaft  51  of the rotating plate assembly  50  after rotation of the rotating shaft  360  by a designated angle. At this time, an angle T 2  may be formed between the bottom surface of the rotating plate  52  and the floor. The rotation angle T 1  of the rotating shaft  360  may be the same as the angle T 2  between the bottom surface of the rotating plate  52  and the floor. As described above, the rotating plate  52  may pivot about the X-axis. 
     If the drive shaft  420  of the second control motor  42  rotates, the first link  351  connected to the drive shaft  420  may also be rotated thereby. If the first link  351  rotates, the second link  352  connected to the first link  351  by the third and fourth links  353  and  354  may rotate in the same direction as the first link  351 . For instance, if an angle T 3  is formed between the imaginary lines C and D connecting a contact point of the first and third links  351  and  353  and a contact point of the first and fourth links  351  and  354  before and after rotation of the drive shaft  420 , an angle T 3  may be identically formed between imaginary lines connecting a contact point of the second and third links  352  and  353  and a contact point of the second and fourth links  352  and  354  before and after rotation of the drive shaft  420 . As the second link  352  rotates, the case  34  connected with the second link  352  and the rotating plate assembly  50  provided at the case  34  may also pivot. Accordingly, an angle T 4  may be formed between the bottom surface of the rotating plate  52  and the floor. The angle T 3  and the angle T 4  may be the same. As described above, the rotating plate  52  may pivot about the Y-axis. 
     Because the rotating plate  52  may be slanted with respect to the floor, nonuniform frictional force may be generated between the pad  53  provided at the bottom surface of the rotating plate  52  and the floor. 
     The rotating shaft  51  of the rotating plate assembly  50  may rotate by the driving motor  43  that may be positioned in the first subframe  33 . Accordingly, the rotating plate  52  may rotate about the Z-axis. 
     The rotating plate  52  may rotate while being slanted by a designated angle with respect to the floor by the first control motor  41  and the second control motor  42 . The rotating plate  52  may vary in terms of slanted angle and slanted direction with respect to the floor by the first control motor  41  and the second control motor  42 . In detail, the rotating plate  52  may be slanted by pivoting about the X-axis by the first control motor  41  and may also be slanted by pivoting about the Y-axis by the second control motor  42 . The slanted angle and the slanted direction of the rotating plate  52  with respect to the floor may be determined by a degree of rotation about the X-axis or Y-axis. 
     The pad  53  provided at the bottom surface of the rotating plate  52  may be made of a compressible material. Accordingly, the entire bottom surface of the pad  53  may come into contact with the floor even when the rotating plate  52  is slanted. Since force needed to wipe the floor is relatively large at a region having large frictional force between the pad  53  and the floor, cleaning efficiency (e.g., removal of stains from the floor) may be enhanced. 
     Difference in partial frictional force of the pad  53  may be generated according to the slanted angle of the rotating plate  52 . Frictional force between the floor and a portion of the pad  53  corresponding to a region at which the rotating plate  52  and the floor are relatively close to each other may exceed frictional force between the floor and a portion of the pad  53  corresponding to a region at which the rotating plate  52  and the floor are relatively far from each other. Differences in frictional force between the pad  53  and the floor may cause the traveling direction and speed of the robot cleaner  1  to be varied. 
     The robot cleaner  1  may travel toward a region of large frictional force between the pad  53  and the floor. As frictional force between the pad  53  and the floor increases, magnitude of frictional force required to drive the robot cleaner  1  may be increased accordingly. Therefore, the traveling speed of the robot cleaner  1  may also be increased. 
     The first subframe  33  may be slanted by a designated angle in the X-axis direction by the first control motor  41  and may be slanted by a designated angle in the Y-axis direction by the second control motor  42 . A control unit (not shown) controls the first control motor  41  and the second control motor  42  so that each of the slanted angles of the first subframe  33  in the X-axis direction and the Y-axis direction may be adequately adjusted. Accordingly, a specific portion of the pad  53  provided at the bottom surface of the rotating plate  52  may contact the floor at a specific contact area. 
     The robot cleaner  1  may perform cleaning while traveling in a specific direction according to a contact position of the pad  53  with the floor and a rotating direction of the rotating plate  52  or the pad  53  about the Z-axis. In detail, the traveling direction of the robot cleaner  1  may be changed according to the relative contact positions of the pad  53  provided at the first driving unit  10  and the pad  53  provided at the second driving unit  20  with the floor, or according to the rotating directions of the pad  53  provided at the first driving unit  10  and the pad  53  provided at the second driving unit  20  about the Z-axis. For instance, although the contact portions of the pads  53  of the first and second driving units  10  and  20  with the floor may be identical, when one of the pads  53  rotates clockwise and the other rotates counterclockwise, the traveling direction of the robot cleaner  1  may be changed. 
     As described above, because the robot cleaner  1  may travel by frictional force, between the pad  53  and the floor, functioning as driving force, traveling of the robot cleaner  1  may be free from restraint by a material of the floor or obstacles in comparison with a robot cleaner configured to travel using wheels. Further, the robot cleaner  1  may travel in all directions by adjusting the slanted angle and slanted direction of the rotating plate  52  and the rotating direction of the pad  53 . 
       FIG. 7 a    is a view illustrating an operating state at a stationary location of the robot cleaner according to one or more embodiments, and  FIG. 7 b    is a view illustrating the bottom surfaces of the pads when the robot cleaner according to one or more embodiments operates at a stationary location. 
     Referring to  FIGS. 7 a  and 7 b   , the rotating plate  52  provided at the first driving unit  10  and the rotating plate  52  provided at the second driving unit  20  may rotate while being slanted such that a right portion of the bottom surface of each rotating plate  52  is closest to the floor. At this time, frictional force between a right portion P 1  of the bottom surface of the pad  53  provided at the first driving unit  10  and the floor may exceed frictional force between any other portion of the pad  53  and the floor. Similarly, frictional force between a right portion P 2  of the bottom surface of the pad  53  provided at the second driving unit  20  and the floor may exceed frictional force between any other portion of the pad  53  and the floor. That is, P 1  and P 2  indicated in  FIG. 7 b    are portions of the bottom surfaces of the respective pads  53 , at which frictional force with the floor may be increased. At this time, the center of gravity of the robot cleaner may be between the pad  53  provided at the first driving unit  10  and the pad  53  provided at the second driving unit  20 . 
     The rotating plate  52  of the first driving unit  10  may rotate counterclockwise R 1 . The rotating plate  52  of the second driving unit  20  may rotate clockwise R 2 . Because the rotating plate  52  of the first driving unit  10  and the rotating plate  52  of the second driving unit  20  may rotate in opposite directions with increased frictional force being applied between the right portions of the bottom surfaces of the pads  53  provided at the first and second driving units  10  and  20  and the floor, the robot cleaner  1  may operate at a stationary location, that is, without movement. Further, because the rotating plate  52  may rotate about the Z-axis, the pad  53  may clean the floor by rotating at a stationary location. 
       FIG. 8 a    is a view illustrating a backward moving state of the robot cleaner according to one or more embodiments, and  FIG. 8 b    is a view illustrating the bottom surfaces of the pads when the robot cleaner according to one or more embodiments moves backward. 
     Referring to  FIGS. 8 a  and 8 b   , the rotating plate  52  provided at the first driving unit  10  and the rotating plate  52  provided at the second driving unit  20  may rotate while being slanted such that a right portion of the bottom surface of the rotating plate  52  of the first driving unit  10  and a left portion of the bottom surface of the rotating plate  52  of the second driving unit  20  are closest to the floor. At this time, frictional force between a right portion P 1  of the bottom surface of the pad  53  provided at the first driving unit  10  and the floor may exceed frictional force between any other portion of the pad  53  and the floor. Similarly, frictional force between a left portion P 3  of the bottom surface of the pad  53  provided at the second driving unit  20  and the floor may exceed frictional force between any other portion of the pad  53  and the floor. That is, P 1  and P 3  indicated in  FIG. 8 b    are portions of the bottom surfaces of the respective pads  53 , at which frictional force with the floor may be increased. 
     At this time, the rotating plate  52  of the first driving unit  10  may rotate counterclockwise R 1 . The rotating plate  52  of the second driving unit  20  may rotate clockwise R 2 . Because the rotating plate  52  of the first driving unit  10  and the rotating plate  52  of the second driving unit  20  may rotate in opposite directions (i.e., counterclockwise R 1  and clockwise R 2 , respectively) with increased frictional force being applied between the right portion P 1  of the bottom surface of the pad  53  of the first driving unit  10  and the floor and between the left portion P 3  of the bottom surface of the pad  53  of the second driving unit  20  and the floor, the robot cleaner  1  may move backward (in a direction F indicated in  FIG. 8 b   ) by reaction to frictional force applied between the pads  53  of the first and second driving units  10  and  20  and the floor. 
       FIG. 9 a    is a view illustrating a sideward moving state of the robot cleaner according to one or more embodiments, and  FIG. 9 b    is a view illustrating the bottom surfaces of the pads when the robot cleaner according to one or more embodiments moves sideways. 
     Referring to  FIGS. 9 a  and 9 b   , the rotating plate  52  provided at the first driving unit  10  and the rotating plate  52  provided at the second driving unit  20  may rotate while being slanted such that a front portion of the bottom surface of the rotating plate  52  of the first driving unit  10  and a rear portion of the bottom surface of the rotating plate  52  of the second driving unit  20  are closest to the floor. At this time, frictional force between a front portion P 4  of the bottom surface of the pad  53  provided at the first driving unit  10  and the floor may exceed frictional force between any other portion of the pad  53  and the floor. Similarly, frictional force between a rear portion P 5  of the bottom surface of the pad  53  provided at the second driving unit  20  and the floor may exceed frictional force between any other portion of the pad  53  and the floor. That is, P 4  and P 5  indicated in  FIG. 9 b    are portions of the bottom surfaces of the respective pads  53 , at which frictional force with the floor may be increased. 
     At this time, the rotating plate  52  of the first driving unit  10  may rotate counterclockwise R 1 . The rotating plate  52  of the second driving unit  20  may rotate clockwise R 2 . Because the rotating plate  52  of the first driving unit  10  and the rotating plate  52  of the second driving unit  20  may rotate in opposite directions (i.e., counterclockwise R 1  and clockwise R 2 , respectively) with increased frictional force being applied between the front portion P 4  of the bottom surface of the pad  53  of the first driving unit  10  and the floor and between the rear portion P 5  of the bottom surface of the pad  53  of the second driving unit  20  and the floor, the robot cleaner  1  may move to the right (in a direction S indicated in  FIG. 9 b   ) by reaction to frictional force applied between the pads  53  of the first and second driving units  10  and  20  and the floor. 
       FIG. 10 a    is a view illustrating a diagonally moving state of the robot cleaner according to one or more embodiments, and  FIG. 10 b    is a view illustrating the bottom surfaces of the pads when the robot cleaner according to one or more embodiments moves diagonally. 
     Referring to  FIGS. 10 a  and 10 b   , the rotating plate  52  provided at the first driving unit  10  and the rotating plate  52  provided at the second driving unit  20  may rotate while being slanted such that a right-front portion of the bottom surface of the rotating plate  52  of the first driving unit  10  and a left-rear portion of the bottom surface of the rotating plate  52  of the second driving unit  20  are closest to the floor. At this time, frictional force between a right-front portion P 6  of the bottom surface of the pad  53  provided at the first driving unit  10  and the floor may exceed frictional force between any other portion of the pad  53  and the floor. Similarly, frictional force between a left-rear portion P 7  of the bottom surface of the pad  53  provided at the second driving unit  20  and the floor may exceed frictional force between any other portion of the pad  53  and the floor. That is, P 6  and P 7  indicated in  FIG. 10 b    are portions of the bottom surfaces of the respective pads  53 , at which frictional force with the floor may be increased. 
     At this time, the rotating plate  52  of the first driving unit  10  may rotate counterclockwise R 1 . The rotating plate  52  of the second driving unit  20  may rotate clockwise R 2 . Because the rotating plate  52  of the first driving unit  10  and the rotating plate  52  of the second driving unit  20  may rotate in opposite directions (i.e., counterclockwise R 1  and clockwise R 2 , respectively) with increased frictional force being applied between the right-front portion P 6  of the bottom surface of the pad  53  of the first driving unit  10  and the floor and between the left-rear portion P 7  of the bottom surface of the pad  53  of the second driving unit  20  and the floor, the robot cleaner  1  may move diagonally backward to the right (in a direction R indicated in  FIG. 10 b   ) by reaction to frictional force applied between the pads  53  of the first and second driving units  10  and  20  and the floor. 
       FIG. 11  is a view illustrating a main body of a robot cleaner according to one or more embodiments. 
     Referring to  FIG. 11 , a main body  4  of a robot cleaner according to one or more embodiments may include a first driving unit  60 , a second driving unit  70 , a third driving unit  80  and a fourth driving unit  90 . The first driving unit  60 , the second driving unit  70 , the third driving unit  80  and the fourth driving unit  90  may be mounted to a frame  600 . Since the second driving unit  70 , the third driving unit  80  and the fourth driving unit  90  have the same constitution as the first driving unit  60 , constitution of the first driving unit  60  will be representatively explained hereinafter. 
       FIG. 12  is an exploded perspective view of the first driving unit of the robot cleaner according to one or more embodiments, and  FIG. 13  is a sectional view of the first driving unit of the robot cleaner according to one or more embodiments. 
     Referring to  FIGS. 12 and 13 , the first driving unit  60  may include a fixing frame  61 , a first subframe  62 , a second subframe  63 , a rotating plate assembly  64 , and a pad  65 . A first control motor  610  and a second control motor  611  may be disposed above the fixing frame  61 . A driving motor  612  may be mounted to the first subframe  62 . 
     The first control motor  610  may be connected to the first link  614  and the second control motor  611  may be connected to the second link  613 . The first link  614  may be connected to a drive shaft of the first control motor  610 . The first link  614  may have a pair of end portions which may be bent to be seated on the first subframe  62 . A ball  615  may be mounted to each of the end portions of the first link  614 . Similarly, the second link  613  may be connected to a drive shaft of the second control motor  611  and may have a pair of end portions which may be bent to be seated on the first subframe  62 . A ball  615  may be mounted to each of the end portions of the second link  613 . The first link  614  and the second link  613  may be arranged to cross each other. For instance, the first link  614  connected to the first control motor  610  and the second link  613  connected to the second control motor  611  may be arranged perpendicular to each other. However, the crossing angle between the first link  614  and the second link  613  is not limited to 90 degrees. The fixing frame  61  may be formed with a hole  616  into which the driving motor  612  may be inserted so as to be connected to an external power source or the like. 
     The first subframe  62  may be disposed below the fixing frame  61 . The first subframe  62  may be provided with s  623  defining spaces in which the balls  615  respectively provided at both end portions of the first link  614  and the second link  613  may be inserted. The first subframe  62  may be formed with a hole  625  in which the driving motor  612  may be inserted and a rotating shaft insertion hole  622  in which a rotating shaft  641  provided at the rotating plate assembly  64  may be inserted. The hole  625  and the rotating shaft insertion hole  622  may be positioned adjacently. The hole  625  of the first subframe  62  may communicate with the hole  616  of the fixing frame  61 . 
     The second subframe  63  may be formed with a rotating shaft insertion hole  632  in which the rotating shaft  641  provided at the rotating plate assembly  64  may be inserted. The rotating shaft insertion hole  632  of the second subframe  63  may communicate with the rotating shaft insertion hole  622  of the first subframe  62 . The second subframe  63  may also be formed with a gear receiving part  631  in which a gear unit  651  may be received. 
     The second subframe  63  may be provided with second ball receiving parts  633  defining spaces in which the balls  615  respectively provided at both end portions of the first link  614  and the second link  613  may be inserted. When the first subframe  62  and the second subframe  63  are coupled, each of the first ball receiving parts  623  of the first subframe  62  and each of the second ball receiving parts  633  of the second subframe  63  may define a unitary ball receiving space together, in which each of the balls  615  provided at the first and second links  614  and  613  may be received. Through such engagement using the balls  615  and the first and second ball receiving parts  623  and  633 , the first subframe  62  and the second subframe  63  may be tilted according to operation of the first link  614  and the second link  613 . 
     The gear receiving part  631  may receive one end portion of the driving motor  612  inserted through the hole  616  of the fixing frame  61  and the hole  621  of the first subframe  62 . That is, the gear unit  651  may be coupled to one end portion of the driving motor  612  and the gear unit  651  may be received in the gear receiving part  631  of the second subframe  63 . A portion of the gear receiving part  631  and a portion of the rotating shaft insertion hole  632  may communicate with each other, so that the rotating shaft  641  of the rotating plate assembly  64  may be tooth-engaged with the gear unit  651 . 
     The rotating plate assembly  64  provided with the rotating shaft  641  may further include a rotating plate  640 . The rotating shaft  641  may be positioned at a center portion of an upper surface of the rotating plate  640 . The rotating shaft  641  may be formed with a through-hole  643 . A chamber (not shown) may be mounted in the through-hole  643  to supply water to the pad  65  mounted to a bottom surface of the rotating plate  640 . The rotating shaft  641  may be provided with a gear at an outer circumferential surface thereof, which may be configured to be tooth-engaged with the gear unit  651 . The rotating plate  640  may be formed with a locking hole  642  at a side surface thereof, into which a locking unit  66  may be inserted. 
     In order to mount the pad  65  to the bottom surface of the rotating plate assembly  64 , the rotating plate  640  may be provided with an adhesive part  650 , to which the pad  65  may be adhered, at the bottom surface thereof. 
     If the gear unit  651  coupled to the driving motor  612  rotates, the rotating shaft  641  tooth-engaged with the gear unit  651  may rotate and thus the rotating plate  640  may rotate about the Z-axis. Accordingly, the pad  65  may wipe the floor while rotating at a stationary location. 
     The first driving unit  60  further may include a locking unit  66 . The locking unit  66  may pass through the rotating shaft insertion hole  622  of the first subframe  62 , the rotating shaft insertion hole  632  of the second subframe  63 , and the through-hole  643  of the rotating shaft  641 . The locking unit  66  may include a bent part  662  formed at a top thereof. The bent part  662  may pass through the rotating shaft insertion holes  622  and  632  and may be hung on an upper surface of the first subframe  62 . A lower end of the locking unit  66  may be positioned at a bottom surface of the rotating plate  640 . The lower end of the locking unit  66  may be inserted into the locking hole  642  formed at the side surface of the rotating plate  640 . Accordingly, the first subframe  62 , the second subframe  63  and the rotating plate  640  may be fixedly locked by the locking unit  66 . 
       FIG. 14  is a view illustrating a rotating state of the rotating plate of the first driving unit about the X-axis in the robot cleaner according to one or more embodiments, and  FIG. 15  is a view illustrating a rotating state of the rotating plate of the first driving unit about the Y-axis in the robot cleaner according to one or more embodiments. 
     Referring to  FIGS. 14 and 15 , the rotating plate  640  of the first driving unit  60  may pivot about the X-axis or Y-axis in order to be slanted with respect to the floor by the first control motor  610  and the second control motor  611 . For instance, if the first link  614  rotates clockwise or counterclockwise by rotation of the first control motor  610 , the rotating plate  640  may pivot about the Y-axis according to the rotating direction of the first link  614  and may be slanted with respect to the floor. As the first link  614  rotates, the first subframe  62  and the second subframe  63  may pivot by the balls  615  provided at both end portions of the first link  614  and received in the ball receiving parts, and the rotating plate assembly  64  coupled to the first and second subframes  62  and  63  may also be pivoted thereby. Accordingly, the rotating plate  640  may pivot about the Y-axis to be slanted with respect to the floor. 
     If the second link  613  rotates clockwise or counterclockwise by rotation of the second control motor  611 , the rotating plate  640  may pivot about the X-axis according to the rotating direction of the second link  613  and may be slanted with respect to the floor. As the second link  613  rotates, the first subframe  62  and the second subframe  63  may pivot by the balls  615  provided at both end portions of the second link  613  and received in the ball receiving parts, and the rotating plate assembly  64  coupled to the first and second subframes  62  and  63  may also be pivoted thereby. Accordingly, the rotating plate  640  may pivot about the X-axis to be slanted with respect to the floor. 
     As described above, if the rotating plate  640  may pivot about the X-axis or Y-axis and may be slanted with respect to the floor, even when the whole bottom surface of the pad  65  provided at the bottom surface of the rotating plate  640  comes into contact with the floor, frictional force between a specific portion of the bottom surface of the pad  65  and the floor may exceed frictional force between any other portion of the bottom surface of the pad  65  and the floor. 
     A control unit (not shown) may adjust the slanted angle and the slanted direction of the rotating plate  640  by controlling the first control motor  610  and the second control motor  611 . The control method of the rotating plate  640  of the first driving unit  60  may be similarly applied to control of the rotating plate of the second driving unit  70 . Further, the main body  4  may move in a specific direction according to the slanted angles and the slanted directions of the first and second driving units  60  and  70  and the rotating directions of the rotating plates  640  of the first and second driving units  60  and  70 . The control method of movement of the main body in the previous embodiment may be similarly applied to control of movement of the main body  4  in this embodiment. 
     In addition to the first driving unit  60  and the second driving unit  70 , the main body  4  may further include the third driving unit  80  and the fourth driving unit  90 . If the second driving unit  70  is positioned next to the first driving unit  60 , the third driving unit  80  is positioned to the rear of the first driving unit  60  and the fourth driving unit  90  is positioned to the rear of the second driving unit  70 , the rotating plate  640  of the third driving unit  80  may move in the same rotating direction and slanted direction as the rotating plate  640  of the first driving unit  60 , and the rotating plate  640  of the fourth driving unit  90  may move in the same rotating direction and slanted direction as the rotating plate  640  of the second driving unit  70 . The number of driving units provided at the main body  4  is not limited to four. The first control motor, the second control motor, and the driving motor may be named the first motor, the second motor, and the third motor, respectively. 
     As is apparent from the above description, the robot cleaner according to the embodiments of the present invention may travel in all directions possibly without interference with obstacles. Further, since frictional force may be sufficiently generated between the pad and the floor, cleaning efficiency may be enhanced. 
     While aspects of the present invention have been particularly shown and described with reference to differing embodiments thereof, it should be understood that these embodiments should be considered in a descriptive sense only and not for purposes of limitation. Descriptions of features or aspects within each embodiment should typically be considered as available for other similar features or aspects in the remaining embodiments. Suitable results may equally be achieved if the described techniques are performed in a different order and/or if components in a described system, architecture, device, or circuit are combined in a different manner and/or replaced or supplemented by other components or their equivalents. 
     Thus, although a few embodiments have been shown and described, with additional embodiments being equally available, it would be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the claims and their equivalents.