Abstract:
A robot cleaner includes a body; a dust box to store dust; and a dust sensing unit to detect dust stored in the dust box, the dust sensing unit including a light emitting unit to transmit a signal to an interior of the dust box and a light receiving sensor to sense the signal transmitted by the light emitting unit. The light emitting unit and the light receiving sensor are positioned between the dust box and the body, and face each other at the same height.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of Korean Patent Application Nos. P2010-68670 and P2010-108235, respectively filed on Jul. 15, 2010 and Nov. 2, 2010 in the Korean Intellectual Property Office, the disclosures of which are incorporated herein by reference. 
     BACKGROUND 
     1. Field 
     Embodiments of the present disclosure relate to a system for performing a cleaning operation using an autonomous robot. 
     2. Description of the Related Art 
     An autonomous robot is a device for performing a desired task while traveling about a certain region without being operated by a user. Such a robot may substantially operate autonomously. Autonomous operation may be achieved in various manners. In particular, a robot cleaner is a device for removing dust from a floor while traveling about a region to be cleaned without being operated by a user. In detail, such a robot cleaner may perform a vacuum cleaning operation and a wiping operation in a home. Here, dust may mean (soil) dust, mote, powder, debris, and other dust particles. 
     SUMMARY 
     Therefore, it is an aspect of the present disclosure to provide a cleaning system capable of preventing the cleaning performance of a robot cleaner from being degraded. 
     Additional aspects of the disclosure will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the disclosure. 
     In accordance with one aspect of the present disclosure, a robot cleaner includes a body having an opening, a dust box provided at the body, to store dust, and a brush unit provided at the opening of the body, to sweep dust on a floor into the dust box, wherein the dust swept into the dust box is suspended in air introduced into the dust box through the opening of the body, and is then discharged through the opening of the body. 
     The air may be introduced into the dust body through a side region of the opening of the body, and may then be outwardly discharged through a central region of the opening of the body. 
     The robot cleaner may further include a brush unit provided at the body such that the brush unit is rotatable. The brush unit may be controlled to allow dust to be more effectively discharged. 
     The brush unit may include a roller, and the roller of the brush unit changes a rotation direction at least one time during the dust discharge. 
     During the dust discharge, the roller of the brush unit may rotate slowly in an initial period of time when light dust is discharged, and may then rotate rapidly. 
     The robot cleaner may further include a maintenance station to generate a flow to discharge air toward the body, and a flow to suck air from the body. The opening of the body may communicate with an opening provided at the maintenance station. 
     In accordance with another aspect of the present disclosure, a maintenance station blows air into a dust box included in a robot cleaner through an opening of the robot cleaner where a brush unit is installed, and sucks dust stored in the dust box while being suspended in the air blown into the dust box. 
     The air sucked from the dust box of the robot cleaner may be re-blown into the dust box through the opening of the robot cleaner. 
     The maintenance station may further include an opening to communicate with the opening of the robot cleaner. The dust stored in the dust box of the robot cleaner may be discharged to the opening of the robot cleaner, so as to be introduced into the opening of the maintenance station. 
     The maintenance station may further include a pump unit, a suction duct provided at a suction side of the pump unit, and a discharge duct provided at a discharge side of the pump unit. The suction duct may have a suction port arranged at the opening of the maintenance station, and the discharge duct may have a discharge port arranged at the opening of the maintenance station. 
     The maintenance station may further include a pump unit, a suction duct provided at a suction side of the pump unit, and a discharge duct provided at a discharge side of the pump unit. The suction duct may have a suction port arranged at the opening of the maintenance station. The discharge duct may have a discharge port. The suction port and the discharge port may form the opening of the maintenance station. 
     The suction port of the suction duct may be formed at a large region of the opening in the maintenance station in a longitudinal direction of the opening, and the discharge port of the discharge duct may be formed at an end region of the opening as viewed in the longitudinal direction of the opening. 
     The suction port of the suction duct may have a larger cross-sectional area than the discharge port of the discharge duct. 
     The maintenance station may further include a dust box arranged between the suction duct and the pump unit. Air discharged from the pump unit may be circulated to the pump unit after sequentially passing through the discharge duct, the opening of the robot cleaner, the dust box of the robot cleaner, the opening of the robot cleaner, the suction duct, and the dust box of the maintenance station. 
     The discharge duct may include a first discharge duct having a first discharge port to allow air to be blown into a larger dust box included in the dust box of the robot cleaner, and a second discharge port to allow air to be blown into a smaller dust box included in the dust box of the robot cleaner. 
     The first and second discharge ports of the first discharge duct may be arranged at opposite ends of the second opening in a width direction in one side region of the second opening, respectively. 
     The discharge duct may include a second discharge duct having a third discharge port to allow air to be blown into a larger dust box included in the dust box of the robot cleaner, and a fourth discharge port to allow air to be blown into a smaller dust box included in the dust box of the robot cleaner. 
     The third and fourth discharge ports of the first discharge duct may be arranged at opposite ends of the second opening in a width direction in the other side region of the second opening, respectively. 
     The maintenance station may further include a suction/discharge dual tube to guide air to be blown to a sensor provided at the robot cleaner and to be again sucked from the sensor. 
     The maintenance station may further include a pump unit, a suction duct provided at a suction side of the pump unit, and a discharge duct provided at a discharge side of the pump unit. The suction duct may communicate with a suction tube of the suction/discharge dual tube, and the discharge duct may communicate with a discharge tube of the suction/discharge dual tube. 
     The maintenance station may further include a pump unit, a suction duct provided at a suction side of the pump unit, and a port assembly to divide the suction duct into two portions respectively having first and second suction ports. 
     The port assembly may include a suction port forming member to form the first and second suction ports. 
     The second suction port may surround at least a portion of the first suction port. 
     The first suction port may be provided at a position substantially corresponding to the opening of the robot cleaner. At least a portion of the second suction port is arranged outside the opening of the robot cleaner. 
     A cover having a plurality of through holes may be provided at the second suction port. 
     The maintenance station may further include a pump unit, first and second discharge ducts provided at a discharge side of the pump unit, and a port assembly to divide the first discharge duct into two portions respectively having first and second discharge ports, and to divide the second discharge duct into two portions respectively having third and fourth discharge ports. 
     The port assembly may include a first discharge port forming member to form the first discharge port, a second discharge port forming member to form the second discharge port, a third discharge port forming member to form the third discharge port, and a fourth discharge port forming member to form the fourth discharge port. 
     The second suction port may surround at least a portion of each of the first, second, third and fourth discharge ports. 
     The port assembly may further include a plurality of brush cleaning members to clean the brush unit of the robot cleaner. 
     Each of the plural brush cleaning members may include a guide extending inclinedly with respect to a rotation direction of the brush unit, and at least one hook protruded from a side surface of the guide. 
     The port assembly may be detachably mounted to the opening of the maintenance station. 
     The port assembly may further include a first spacer provided at a bottom of the port assembly, and second spacers provided at opposite sides of the first spacer. 
     The opening of the maintenance station may be larger than the opening of the robot cleaner. 
     The maintenance station may further include a pump unit, and a suction duct provided at a suction side of the pump unit. The suction duct may have a suction port, which is larger than the opening of the robot cleaner. 
     In accordance with another aspect of the present disclosure, a cleaning system includes a robot cleaner including a first opening, and a first dust box communicating with the first opening, and a maintenance station including a second opening, and a second dust box communicating with the second opening, wherein dust stored in the first dust box of the robot cleaner is discharged to the second opening of the maintenance station through the first opening of the robot cleaner after being suspended in air introduced into the first dust box of the robot cleaner. 
     The air introduced into the first dust box of the robot cleaner may pass through the first opening of the robot cleaner. 
     The cleaning system may further include a dust removal unit to suck air from the first dust box of the robot cleaner through the first opening of the robot cleaner, and to again blow air to the first opening of the robot cleaner. 
     The dust removal unit may suck air such that the air blown to the first opening of the robot cleaner emerges from the first opening of the robot cleaner after circulating through the first dust box of the robot cleaner. 
     The dust removal unit may blow air in a side region of the first opening of the robot cleaner as viewed in a longitudinal direction of the first opening, and may suck air in a large region of the first opening as viewed in the longitudinal direction of the first opening. 
     The dust removal unit may include a pump unit, and a first discharge duct provided at a discharge side of the pump unit. The first discharge duct may have a first discharge port to allow air to be blown into a larger dust box included in the first dust box, and a second discharge port to allow air to be blown into a smaller dust box included in the first dust box. 
     The dust removal unit may further include a second discharge duct provided at the discharge side of the pump unit. The second discharge duct may have a third discharge port to allow air to be blown into the larger dust box of the first dust box, and a fourth discharge port to allow air to be blown into a smaller dust box included in the first dust box. 
     The dust removal unit may include a pump unit, and a suction duct provided at a suction side of the pump unit. The suction duct may have a suction port, which is larger than the opening of the robot cleaner. 
     The dust removal unit may include a pump unit, a suction duct provided at a suction side of the pump unit, first and second discharge ducts provided at a discharge side of the pump unit, and a port assembly to divide the suction duct into two portions respectively having first and second suction ports, to divide the first discharge duct into two portions respectively having first and second discharge ports, and to divide the second discharge duct into two portions respectively having third and fourth discharge ports. 
     The port assembly may include a suction port forming member to form the first and second suction ports, a first discharge port forming member to form the first discharge port, a second discharge port forming member to form the second discharge port, a third discharge port forming member to form the third discharge port, and a fourth discharge port forming member to form the fourth discharge port. 
     The second suction port may surround the first suction port, the first discharge port, the second discharge port, the third discharge port, and the fourth discharge port. 
     The dust removal unit may include a pump unit, a suction duct provided at a suction side of the pump unit, and a discharge duct provided at a discharge side of the pump unit. The suction duct may have a suction port arranged in a large region of the first opening of the robot cleaner in a longitudinal direction of the first opening, and the discharge duct may have a discharge port arranged at a side region of the first opening as viewed in the longitudinal direction of the first opening. 
     The suction port of the suction duct may have a larger cross-sectional area than the discharge port of the discharge duct. 
     A cross-sectional area ratio between the suction port of the suction duct and the discharge port of the discharge duct may be 7.5:1. 
     The suction port of the suction duct and the discharge port of the discharge duct may form the second opening of the maintenance system. 
     The maintenance station may further include a cover to open or close the second opening of the maintenance station. 
     The maintenance station may further include a bridge extending along a central portion of the second opening of the maintenance station. 
     The robot cleaner may further include a brush unit provided at the first opening of the robot cleaner. The brush unit may be controlled to allow dust stored in the first dust box of the robot cleaner to be more effectively discharged to the second opening of the maintenance station. 
     The brush unit may include a roller, and the roller of the brush unit changes a rotation direction at least one time during the dust discharge. 
     The roller may rotate slowly in an initial period of time when light dust is discharged, and may then rotate rapidly. 
     The maintenance station may further include a brush cleaning member to clean the brush unit. 
     The brush cleaning member may be arranged adjacent to the second opening of the maintenance station. 
     The brush cleaning member may include a guide extending inclinedly with respect to a rotation direction of the brush unit, and at least one hook protruded from a side surface of the guide. 
     The robot cleaner may further include a dust sensing unit to sense an amount of dust stored in the first dust box. The dust sensing unit may include a light emitting sensor and a light receiving sensor, which are installed at regions other than the first dust box, and a reflecting member installed in the first dust box, to reflect a signal transmitted from the light emitting sensor to the light receiving sensor. 
     The robot cleaner may further include a dust sensing unit to sense an amount of dust stored in the first dust box. The robot cleaner may be moved to the maintenance station when the dust amount sensed by the dust sensing unit corresponds to a predetermined amount or more. 
     In accordance with another aspect of the present disclosure, a cleaning system includes docking a robot cleaner at a maintenance station, determining whether or not docking is completed, discharging dust stored in the robot cleaner into the maintenance station through an opening where a brush unit included in the robot cleaner is installed, upon completion of docking, and operating a brush unit of the robot cleaner during dust discharge. 
     The brush unit may change a rotation direction at least one time. 
     The brush unit may rotate slowly in an initial period of time when light dust is discharged, and may then rotate rapidly. 
     The cleaning system may further include determining whether or not dust is completely filled in a dust box of the robot cleaner. 
     In accordance with another aspect of the present disclosure, a robot cleaner includes a body, a dust box provided at the body, to store dust, and a dust sensing unit to measure an amount of dust stored in the dust box, wherein the dust sensing unit includes a light emitting sensor installed at a region other than the dust box, to transmit a signal to an interior of the dust box, and a light receiving sensor installed at a region other than the dust box, to sense a signal emerging from the interior of the dust box. 
     The dust sensing unit may further include a reflecting member installed within the dust box, to reflect the signal transmitted from the light emitting sensor to the light receiving sensor. 
     The dust box may include at least one inlet, through which dust is introduced into the dust box. The light emitting sensor and the light receiving sensor may be provided at a portion of the body corresponding to the inlet of the dust box, to perform signal transmission and signal reception through the inlet of the dust box, respectively. 
     The robot cleaner may further include a display provided at the body, to display various information. The display unit may display dust sensing information from the dust sensing unit. 
     There may be no connecting terminal connected to the dust box. 
     In accordance with another aspect of the present disclosure, a robot cleaner may include a body, a dust box provided at the body, to store dust, and a dust sensing unit to measure an amount of dust stored in the dust box. The dust sensing unit may include a light emitting sensor installed at a region other than the dust box. A signal transmitted from the light emitting sensor may reach the light receiving sensor after passing through the dust box. 
     The dust box may be made of a transparent material to allow a signal to pass through the dust box. 
     The light emitting sensor and the light receiving sensor may be installed so as to face each other. 
     The dust box may include a transmitted-signal passing portion arranged at a position corresponding to the light emitting sensor, to allow a signal to enter the dust box, and a received-signal passing portion arranged at a position corresponding to the light receiving sensor, to allow a signal to emerge from the dust box. 
     The transmitted-signal passing portion and the received-signal passing portion may be made of a transparent material. 
     There may be no connecting terminal connected to the dust box. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and/or other aspects of the disclosure will become apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings of which: 
         FIG. 1  is a view illustrating a cleaning system according to an exemplary embodiment of the present disclosure; 
         FIG. 2  is a sectional view illustrating a configuration of the robot cleaner according to an exemplary embodiment of the present disclosure; 
         FIG. 3  is a perspective view illustrating a bottom of the robot cleaner according to the illustrated embodiment of the present disclosure; 
         FIG. 4A  is a plan view illustrating a dust sensing unit according to an exemplary embodiment of the present disclosure; 
         FIG. 4B  is a plan view illustrating a dust sensing unit according to another exemplary embodiment of the present disclosure; 
         FIG. 4C  is a plan view illustrating a dust sensing unit according to another exemplary embodiment of the present disclosure; 
         FIG. 5A  is a top perspective view illustrating a configuration of a maintenance station according to an exemplary embodiment of the present disclosure; 
         FIG. 5B  is a top perspective view illustrating a configuration of the maintenance station according to another exemplary embodiment of the present disclosure; 
         FIG. 5C  is a top perspective view illustrating a configuration of the maintenance station according to another exemplary embodiment of the present disclosure; 
         FIG. 5D  is a top perspective view illustrating a configuration of the maintenance station according to another exemplary embodiment of the present disclosure; 
         FIG. 5E  is a sectional view illustrating a configuration of the maintenance station according to another exemplary embodiment of the present disclosure; 
         FIG. 6  is a plan view illustrating a duct included in the maintenance station according to the embodiment of  FIG. 5A ; 
         FIG. 7  is a plan view illustrating the maintenance station according to the embodiment of  FIG. 5A ; 
         FIG. 8  is a sectional view illustrating a docking state of the robot cleaner and maintenance station; 
         FIG. 9A  is a view illustrating a configuration of a brush cleaning member according to an exemplary embodiment of the present disclosure; 
         FIG. 9B  is a view illustrating a configuration of the brush cleaning member according to another exemplary embodiment of the present disclosure; 
         FIG. 9C  is a view illustrating a configuration of the brush cleaning member according to another exemplary embodiment of the present disclosure; 
         FIG. 10  is a view schematically illustrating a cleaning system according to another exemplary embodiment of the present disclosure; 
         FIG. 11  is a perspective view illustrating a suction/discharge dual tube; 
         FIG. 12  is a view illustrating flow of air in the cleaning system according to the embodiment shown in  FIG. 10 ; 
         FIG. 13  is a view schematically illustrating a cleaning system according to another embodiment of the present disclosure; 
         FIG. 14  is a view schematically illustrating a cleaning system according to another embodiment of the present disclosure. 
         FIG. 15  is a top perspective view illustrating a configuration of the maintenance station according to another exemplary embodiment of the present disclosure; 
         FIG. 16  is an exploded perspective view illustrating a configuration of the maintenance station according to the illustrated embodiment of the present disclosure; 
         FIG. 17  is a plan view illustrating a duct included in the maintenance station according to the illustrated embodiment of the present disclosure; 
         FIG. 18  is a sectional view illustrating a flow of air discharged through a second opening during a docking operation; 
         FIG. 19  is a sectional view illustrating a flow of air sucked through the second opening during the docking operation; 
         FIG. 20  is a top perspective view illustrating a port assembly according to another exemplary embodiment of the present disclosure; and 
         FIG. 21  is a bottom perspective view illustrating the port assembly according to the illustrated embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, a robot cleaner, a maintenance station, and a cleaning system according to embodiments of the present disclosure will be described with reference to the accompanying drawings. 
       FIG. 1  is a view illustrating a cleaning system according to an exemplary embodiment of the present disclosure. 
     As shown in  FIG. 1 , the cleaning system  10  may include a robot cleaner  20  and a maintenance station  60 . The robot cleaner  20  is a device for autonomously performing various cleaning tasks. The maintenance station  60  is a device for repair and maintenance. The maintenance station  60  may charge a battery of the robot cleaner  20 , and empties a dust box of the robot cleaner  20 . 
       FIG. 2  is a sectional view illustrating a configuration of the robot cleaner according to an exemplary embodiment of the present disclosure.  FIG. 3  is a perspective view illustrating a bottom of the robot cleaner according to the illustrated embodiment of the present disclosure. 
     As shown in  FIGS. 1 to 3 , the robot cleaner  20  includes a body  21 , a driving unit  30 , a cleaning unit  40 , various sensors  50 , and a controller (not shown). 
     The body  21  may have various shapes. For example, the body  21  may have a circular shape. Where the body  21  has a circular shape, it may be prevented from coming into contact with surrounding obstacles and may easily achieve direction change, even during rotation thereof, because it has a constant radius of rotation. Also, it may be possible to prevent the body  21  from being obstructed by a surrounding obstacle during travel thereof. Thus, the body  21  cannot be trapped by an obstacle during travel thereof. 
     Various constituent elements to perform cleaning tasks, that is, the driving unit  30 , cleaning unit  40 , various sensors  50 , controller (not shown), and a display  23 , may be installed on the body  21 . 
     The driving unit  30  may enable the body  21  to travel about a region to be cleaned. The driving unit  30  may include left and right driving wheels  31   a  and  31   b , and a caster  32 . The left and right driving wheels  31   a  and  31   b  are mounted to a central portion of a bottom of the body  21 . The caster  32  is mounted to a front portion of the bottom of the body  21 , to maintain stability of the robot cleaner  20 . 
     The left and right driving wheels  31   a  and  31   b  may be controlled to move the robot cleaner  20  forward or backward, or to change the running direction of the robot cleaner  20 . For example, it may be possible to move the robot cleaner  20  forward or backward by uniformly controlling the left and right driving wheels  31   a  and  31   b . Also, it may be possible to change the running direction of the robot cleaner  20  by differently controlling the left and right driving wheels  31   a  and  31   b.    
     Meanwhile, each of the left and right driving wheels  31   a  and  31   b , and the caster  32  may be configured into a single assembly detachably mounted to the body  21 . 
     The cleaning unit  40  may clean the region underneath the body  21  and surrounding portions thereof. The cleaning unit  40  may include a brush unit  41 , a side brush  42 , and a first dust box  43 . 
     The brush unit  41  may be mounted to a first opening  21   a  formed through the bottom of the body  21 . The brush unit  41  may be arranged at a position other than the central portion of the body  21 . That is, the brush unit  41  may be arranged at a position adjacent to the driving wheels  31   a  and  31   b  while being spaced apart from the driving wheels  31   a  and  31   b  in a rearward direction R of the body  21 . 
     The brush unit  41  may sweep dust accumulated on a floor beneath the body  21  into the first dust box  43 . The brush unit  41  may include a roller  41   a  rotatably mounted to the first opening  21   a , and a brush  41   b  fixed to an outer peripheral surface of the roller  41   a . When the roller  41   a  rotates, the brush  41   b , which is made of an elastic material, may sweep up dust accumulated on the floor. In accordance with this sweeping operation, the dust accumulated on the floor may be collected in the first dust box  43  through the first opening  21   a.    
     The brush unit  41  may be controlled to rotate at a constant speed, in order to exhibit a uniform cleaning performance. When the brush unit  41  cleans a rough floor surface, the rotating speed thereof may be lowered, as compared to the case in which the brush unit  41  cleans a smooth floor surface. In this case, an increased amount of current may be supplied to keep the speed of the brush unit  41  constant. 
     The side brush  42  may be rotatably mounted to a peripheral portion of the bottom of the body  21  at one side of the body  21 . The side brush  42  may be mounted at a position spaced apart from the central portion of the body  21  in a forward direction F while being biased toward one side of the body  21 . 
     The side brush  42  may move dust accumulated around the body  21  to the brush unit  41 . The side brush  42  may expand the cleaning zone of the robot cleaner  20  to the bottom of the body  21  and surroundings thereof. The dust moved to the brush unit  41  may be collected in the first dust box  43  through the first opening  21   a , as described above. 
     The first dust box  43  may be mounted to a rear portion of the body  21 . The first dust box  43  includes an inlet  43 ′ communicating with the first opening  21   a , to allow dust to be introduced into the first dust box  43 . 
     The first dust box  43  may be divided into a larger dust box  43   a  and a smaller dust box  43   b  by a partition  43   c . The brush unit  41  may sweep dust having a relatively-large size into the larger dust box  43   a  via the first inlet  43   a ′. A fan unit  22  may be provided to suck small-size dust such as hairs via a second inlet  43   b ′, and thus to collect the dust in the smaller dust box  43   b . In particular, a brush cleaning member  41   c  is arranged at a position adjacent to the second inlet  43   b ′. The brush cleaning member  41   c  removes hairs wound around the brush unit  41 , and then collects the removed hairs in the smaller dust box  43   b  via the second inlet  43   b ′, using a suction force of the fan unit  22 . 
     Meanwhile, each of the brush unit  41 , side brush  42 , and first dust box  43  may be configured into a single assembly detachably mountable to the body  21 . 
       FIG. 4A  is a plan view illustrating a dust sensing unit according to an exemplary embodiment of the present disclosure.  FIG. 4B  is a plan view illustrating a dust sensing unit according to another exemplary embodiment of the present disclosure.  FIG. 4C  is a plan view illustrating a dust sensing unit according to another exemplary embodiment of the present disclosure. 
     As shown in  FIG. 4A , the dust sensing unit may be installed within the first dust box  43 , in order to sense the amount of dust in the first dust box  43 . 
     In this case, the dust sensing unit  44  may include a light emitting sensor  44   a  and a light receiving sensor  44   b . A signal transmitted from the light emitting sensor  44   a  within the first dust box  43  may be directly received by the light receiving sensor  44   b.    
     Each of the light emitting sensor  44   a  and light receiving sensor  44   b  may include a photodiode or a phototransistor. In this case, it may be possible to determine whether or not the first dust box  43  is completely filled with dust, based on the amount of energy sensed by the photodiode or phototransistor. That is, as dust is accumulated in the first dust box  43 , the amount of energy sensed by the photodiode or phototransistor may be considerably reduced. Through comparison of the sensed energy amount with a predetermined reference value, the controller may determine that the first dust box  43  is completely filled with dust, when the sensed energy amount is less than the reference value. Since the light emitting sensor  44   a  and light receiving sensor  44   b , which are configured by photodiodes or phototransistors, are considerably influenced by disturbance, it may be possible to more accurately sense the amount of dust where a structure such as a slit or a light guide is installed to guide a signal transmitted from the light emitting sensor  44   a  or a signal received by the light receiving sensor  44   b.    
     Each of the light emitting sensor  44   a  and light receiving sensor  44   b  may also be configured by a remote-controller receiving module. In this case, it may be possible to determine whether or not the first dust box  43  is completely filled with dust, based on whether or not a signal has been received by the light receiving sensor  44   b . That is, when dust is accumulated, the light receiving sensor  44   b  may not receive a signal transmitted from the light emitting sensor  44   a . In this case, the controller may determine that the amount of dust in the first dust box  43  corresponds to a predetermined amount or more. The light emitting sensors  44   a  and light receiving sensor  44   b , which are remote-controller receiving modules, may not require a slit or light guide structure because they filter low-frequency waves while exhibiting high intensity and sensitivity. 
     For the signal transmitted from the light emitting sensor  44   a  and received by the light receiving sensor  44   b , visible light, infrared light, sound waves, ultrasonic waves, etc. may be used. 
     Meanwhile, as shown in  FIG. 4B , the dust sensing unit  44  may include a light emitting sensor  44   a , a light receiving sensor  44   b , and a reflecting member  44   c.    
     In this case, the light emitting sensor  44   a  and light receiving sensor  44   b  are not installed within the first dust box  43 , but are instead installed in an area other than the first dust box  43 . That is, the light emitting sensor  44   a  and light receiving sensor  44   b  may be installed at a portion of the body  21  facing the first dust box  43 . In detail, the light emitting sensor  44   a  and light receiving sensor  44   b  may be installed adjacent to the inlet  43 ′ of the first dust box  43 . In this case, accordingly, the light emitting sensor  44   a  may transmit a signal into the first dust box  43  through the inlet  43 ′. The light receiving sensor  44   b  may receive the signal, which emerges from the first dust box  43  through the inlet  43 ′ of the first dust box  43 . 
     The reflecting member  44   c  may be installed within the first dust box  43 . The reflecting member  44   c  may reflect a signal emitted from the light emitting sensor  44   a  toward the light receiving sensor  44   b.    
     When the first dust box  43  is completely filled with dust in this case, the reflecting member  44   c  is shielded by the dust, so that the signal emitted from the light emitting sensor  44   a  cannot be received by the light receiving sensor  44   b , or the amount of energy received by the light receiving sensor  44   b  is considerably reduced. In this state, accordingly, the controller may determine that the first dust box  43  is filled with a predetermined amount of dust or more. 
     Meanwhile, where the light emitting sensors  44   a  and light receiving sensors  44   b  are configured by remote-controller modules, it may be unnecessary to use a slit or light guide structure because the light emitting sensors  44   a  and light receiving sensors  44   b  filter low-frequency waves while exhibiting high intensity and sensitivity, as described above. That is, the light emitting sensors  44   a  and light receiving sensors  44   b , which are configured by remote-controller modules, may determine whether or not the first dust box  43  is completely filled with dust, even though there is no structure such as the reflecting member  44   c  within the first dust box  43 . 
     Since the light emitting sensor  44   a  and light receiving sensor  44   b  may not be installed within the first dust box  43 , as described above, it may be unnecessary to install an electrical connecting terminal within the first dust box  43 . Accordingly, the user may clean the first dust box  43 , using water. 
     The dust sensing unit  44  may also include a light emitting sensor  44   a  and a light receiving sensor  44   b , which are configured as shown in  FIG. 4C . 
     In this case, the light emitting sensor  44   a  and light receiving sensor  44   b  need not be installed within the first dust box  43 , and may instead be installed at regions other than the first dust box  43 . That is, the light emitting sensors  44   a  and light receiving sensors  44   b  may be installed on the body  21 , to face each other. In detail, the light emitting sensor  44   a  may be installed at a portion of the body  21  facing one side of the first dust box  43 , whereas the light receiving sensor  44   b  may be installed at another portion of the body  21  facing the other side of the first dust box  43 . In this case, the first dust box  43  is arranged between the light emitting sensor  44   a  and the light receiving sensor  44   b , so that a signal transmitted from the light emitting sensor  44   a  may be received by the light receiving sensor  44   b  through the first dust box  43 . The first dust box  43  may be formed to be completely transparent, so as to allow a signal to pass therethrough. The first dust box  43  may include a transparent transmitted-signal passing portion  43   a ″ at a position corresponding to the light emitting sensor  44   a , in order to allow a signal to pass therethrough, and a transparent received-signal passing portion  43   b ″ at a position corresponding to the light receiving sensor  44   b , in order to allow a signal to pass therethrough. 
     The signal transmitted from the light emitting sensor  44   a  may be directly received by the light receiving sensor  44   b . When the first dust box  43  is completely filled with dust, the light receiving sensor  44   b  does not sense any signal, or the amount of energy sensed by the light receiving sensor  44   b  may be considerably reduced. In this case, the controller may determine that the first dust box  43  is completely filled with dust. 
     Since an electrical connecting structure is not installed within the first dust box  43 , it may be possible to clean the first dust box  43 , using water. 
     When the dust sensing unit  44  senses a predetermined amount of dust or more, the robot cleaner  20  may display information about the sensed result on the display  23 . The user may directly clean the first dust box  43 . Meanwhile, the robot cleaner  20  may automatically dock with the maintenance station  60 , to automatically discharge dust collected in the first dust box  43 . 
     The various sensors  50 , which are mounted to the body  21 , may be used to sense obstacles. As these sensors  50 , a contact sensor, a proximity sensor, etc. may be used. For example, a bumper  51 , which is arranged at a front portion of the body  21 , to be directed to a front direction F of the body  21 , may be used to sense a front obstacle such as a wall. It may also be possible to sense a front obstacle, using an infrared sensor (or an ultrasonic sensor). 
     An infrared sensor  52  (or an ultrasonic sensor), which is arranged on the bottom of the body  21 , may be used to sense a condition of the floor, for example, condition of steps. A plurality of infrared sensors  52  may be installed on the bottom of the body  21  along an arc-shaped peripheral portion of the body  21 . 
     Various sensors other than the above-described sensors may also be installed on the body  21 , to transfer various conditions of the robot cleaner  20  to the controller. 
     The controller receives signals from the various sensors  50 , and controls the driving unit  30  and cleaning unit  40 , based on the received signals, thereby more efficiently controlling the robot cleaner  20 . 
       FIG. 5A  is a perspective view illustrating a top perspective view illustrating a configuration of the maintenance station according to an exemplary embodiment of the present disclosure.  FIG. 5B  is a top perspective view illustrating a configuration of the maintenance station according to another exemplary embodiment of the present disclosure.  FIG. 5C  is a top perspective view illustrating a configuration of the maintenance station according to another exemplary embodiment of the present disclosure.  FIG. 5D  is a top perspective view illustrating a configuration of the maintenance station according to another exemplary embodiment of the present disclosure.  FIG. 5E  is a sectional view illustrating a configuration of the maintenance station according to another exemplary embodiment of the present disclosure.  FIG. 6  is a plan view illustrating a duct included in the maintenance station according to the embodiment of  FIG. 5A .  FIG. 7  is a plan view illustrating the maintenance station according to the embodiment of  FIG. 5A . 
     As shown in  FIGS. 1 to 7 , the robot cleaner  20  may dock with the maintenance station  60  in various situations. For example, there may be various situations such as a situation in which the battery (not shown) of the robot cleaner  20  needs to be charged, a situation in which the robot cleaner  20  has performed a cleaning task for a predetermined time, a situation in which the robot cleaner  20  has completed a cleaning task, and a situation in which the first dust box  43  of the robot cleaner  20  is completely filled with dust. 
     The maintenance station  60  may include a housing  61 , a docking guide unit  70 , a charging unit  80 , a dust removal unit  90 , and a controller (not shown). 
     A platform  62  may be provided at the housing  61 . The platform  62  may support the robot cleaner  20  while the robot cleaner  20  docks with the maintenance station  60 . 
     The platform  62  has an inclined structure to allow the robot cleaner  20  to easily ascend along or descend from the platform  62 . A caster guide  63   a  may be formed at the platform  62 , to guide the caster  32  of the robot cleaner  20 . Driving wheel guides  63   b  and  63   c  may also be formed at the platform  62 , to guide the left and right driving wheels  31   a  and  31   b  of the robot cleaner  20 . The caster guide  63   a  and driving wheel guides  63   b  and  63   c  may be formed to be recessed, as compared to portions of the platform  62  therearound. 
     A second opening  62   a  may be formed through the platform  62 . The second opening  62   a  of the platform  62  may be arranged at a position where the second opening  62   a  may communicate with the first opening  21   a  of the robot cleaner  20 . In accordance with this arrangement, dust discharged through the first opening  21   a  of the robot cleaner  20  may be introduced into the second opening  62   a  of the platform  62 . The dust introduced into the second opening  62   a  of the platform  62  may be collected in a second dust box  94  included in the maintenance station  60 . 
     The second dust box  94  of the maintenance station  60  is different from the first dust box  43  of the robot cleaner  20 . The first dust box  43  of the robot cleaner  20  stores dust collected by the robot cleaner  20  during movement of the robot cleaner  20 . The second dust box  94  of the maintenance station  60  collects and stores dust discharged from the first dust box  43 . In this regard, the second dust box  94  of the maintenance station  60   n  may have a greater capacity than the first dust box  43  of the robot cleaner  20 . 
     The dust sensing unit  44  may also be installed within the second dust box  94 , in order to sense the amount of dust in the second dust box  94 . 
     In this case, the dust sensing unit  44  may include a light emitting sensor  44   a  and a light receiving sensor  44   b . When the light receiving sensor  44   b  cannot receive a signal transmitted from the light emitting sensor  44   a , the controller may determine that the amount of dust in the second dust box  94  corresponds to a predetermined amount or more. 
     The second opening  62   a  of the platform  62  may have an open structure, as shown in  FIG. 5A . That is, the second opening  62   a  of the platform  62  may always be open without being covered by a separate cover. 
     The platform  62  may be formed to be inclined at a predetermined angle θ or more ( FIG. 7 ). When the robot cleaner  20  moves on the platform  62  inclined at the predetermined angle θ or more, the front portion of the robot cleaner  20  may be slightly lifted because the weight of the robot cleaner  20  is rearwardly biased. As a result, the caster  32  of the robot cleaner  20  may pass the second opening  62   a  of the platform  62  without falling into the second opening  62   a.    
     Meanwhile, a cover  64  may be installed at the second opening  62   a  of the platform  62 , to slidably move along the second opening  62   a , as shown in  FIG. 5B . When the robot cleaner  20  is completely docked, the cover  64  may be opened, to allow the robot cleaner  20  to discharge dust through the second opening  62   a  of the platform  62 . On the other hand, when the docked state of the robot cleaner  20  is released, the cover  64  may be closed to close the second opening  62   a  of the platform  62 . 
     The cover  64  may also function as a bridge upon which the caster  32  of the robot cleaner  20  will move. The cover  64  may be opened or closed in linkage with docking of the robot cleaner  20 . That is, the cover  64  may be opened while or before the caster  32  passes the cover  64  during docking of the robot cleaner  20 . The cover  64  may be closed while or after the caster  32  passes the cover  64  during docking release of the robot cleaner  20 . It may also be possible to open or close the cover  64 , using a separate device. 
     On the other hand, as shown in  FIG. 5C , a cover  65  may be installed at the second opening  62   a  of the platform  62 , to slidably move along the second opening  62   a . Of course, the cover  65  may be installed only at a central portion of the second opening  62   a  of the platform  62  in the case of  FIG. 5C , different from the case of  FIG. 5B . This structure is adapted to allow the caster  32  of the robot cleaner  20  to pass the second opening  62   a  of the platform  62 . The opening/closing operation of the cover  65  may be achieved in the same manner as described above. 
     On the other hand, as shown in  FIG. 5D , a bridge  66  may be installed at the second opening  62   a  of the platform  62 . The bridge  66  may be installed only at a central portion of the second opening  62   a  of the platform  62 , to achieve a bridge function allowing the caster  32  of the robot cleaner  20  to pass the bridge  66 . 
     As shown in  FIG. 5E , the bridge  66  may be installed at the second opening  62   a  of the platform  62  to move upward and downward. That is, when the robot cleaner  20  enters the platform  62 , the bridge  67   a  moves upward to allow the caster  32  of the robot cleaner  20  to move thereon. When the docking of the robot cleaner  20  is completed, the bridge  67   b  moves downward to allow the second opening  62   a  of the platform  62  to secure an increased opening area. 
     The docking guide unit  70  may be installed at an upper portion of the housing  61 . The docking guide unit  70  may include a plurality of sensors  71 . The sensors  71  may define a docking guide region and a docking region, to accurately guide the robot cleaner  20  to dock with the maintenance station  60 . 
     The charging unit  80  may be installed at the platform  62 . The charging unit  80  may include a plurality of connecting terminals  81   a  and  81   b . The connecting terminals  81   a  and  81   b  may correspond to a plurality of connecting terminals  23   a  and  23   b  provided at the robot cleaner  20 . When docking of the robot cleaner  20  is completed, current may be supplied to the plural connecting terminals  23   a  and  23   b  of the robot cleaner  20  via the plural connecting terminals  81   a  and  81   b  of the maintenance station  60 . 
     The charging unit  80  may supply current after determining whether or not the plural connecting terminals  23   a  and  23   b  of the robot cleaner  20  are connected to the charging unit  80 . That is, when the charging unit  80  is connected to an element other than the plural connecting terminals  23   a  and  23   b , the charging unit  80  interrupts supply of current, to avoid occurrence of an accident. 
     The dust removal unit  90  may be installed at the housing  61 . The dust removal unit  90  may discharge dust stored in the first dust box  43  of the robot cleaner  20  into the second dust box  94  of the maintenance station  60 , to empty the first dust box  43 . Thus, the dust removal unit  90  may maintain desired cleaning performance of the robot cleaner  20 . 
     The dust removal unit  90  may include a pump unit  91 , a suction duct  92 , and a discharge duct  93 , in addition to the second dust box  94 . The dust removal unit  90  functions to force a flow of air discharged from the discharge duct  93  to be sucked back into the suction duct  92 . Using such a circulating air flow, the dust removal unit  90  removes dust stored in the first dust box  43  of the robot cleaner  20 . 
     The pump unit  91  is a device to suck/discharge air. The pump unit  91  may include a fan and a motor. 
     The suction duct  92  may be installed at a suction side of the pump unit  91 . The suction duct  92  may include a suction port  92   a , which may form a portion of the second opening  62   a . Alternatively, the suction port  92   a  may be separate from the second opening  62   a . In this case, the suction duct  92   a  may be arranged at a position adjacent to the second opening  62   a.    
     The suction port  92   a  may extend in a longitudinal direction of the second opening  62   a , to occupy a portion of the second opening  62   a , except for a portion of the second opening  62   a  occupied by discharge ports  93   a  and  93   b  of the discharge duct  93 . 
     The discharge duct  93  may be installed at a discharge side of the pump unit  91 . The discharge duct  93  may be divided into two portions, which form the two discharge ports  93   a  and  93   b . The discharge ports  93   a  and  93   b  may form portions of the second opening  62   a . Alternatively, the discharge ports  93   a  and  93   b  may be separate from the second opening  62   a . In this case, the discharge ports  93   a  and  93   b  may be arranged at positions adjacent to the second opening  62   a.    
     The discharge ports  93   a  and  93   b  may be formed at longitudinal ends of the second opening  62   a , namely, opposite side regions of the second opening  62   a , respectively. 
     The suction port  92   a  of the suction duct  92  may have a larger cross-sectional area than the sum of the cross-sectional areas of the discharge ports  93   a  and  93   b  of the discharge duct  93 . Hereinafter, the sum of the cross-sectional areas of the discharge ports  93   a  and  93   b  of the discharge duct  93  will be simply referred to as “the cross-sectional area of the discharge ports  93   a  and  93   b ”. The cross-sectional area ratio between the suction port  92   a  of the suction duct  92  and the discharge ports  93   a  and  93   b  of the discharge duct may be 7.5:1. Of course, the cross-sectional area ratio of the suction port  92   a  of the suction duct  92  to the discharge ports  93   a  and  93   b  of the discharge duct may be smaller than the above-described ratio, for example, may be 7:1, 6.5:1, or 6:1. Even when the cross-sectional area ratio is slightly reduced, as described above, it falls within the technical scope of the present disclosure. 
     Accordingly, the air flow velocity at the discharge ports  93   a  and  93   b  of the discharge duct  93  may be higher than the air flow velocity at the suction port  92   a  of the suction duct  92  because there is a cross-sectional area difference between the suction port  92   a  and the discharge ports  93   a  and  93   b  under the condition that the suction flow rate and discharge flow rate of the pump unit  91  are substantially equal. By virtue of this flow velocity difference, it may be possible to prevent air emerging from the discharge ports  93   a  and  93   b  from being sucked into the suction port  92   a . That is, air emerging from the discharge ports  93   a  and  93   b  may be injected into the first dust box  34  without being directly sucked into the suction port  92   a  by a suction force at the suction port  92   a , because the air flow velocity of the discharged air is very high. Thus, air injected into the first dust box  43  may emerge from the first dust box  43  after circulating through the first dust box  34 , and may then enter the suction port  92   a.    
       FIG. 8  is a sectional view illustrating a docking state of the robot cleaner and maintenance station. 
     As shown in  FIGS. 1 to 8 , when the robot cleaner  20  docks with the maintenance station  60 , the first opening  21   a  of the robot cleaner  20  may communicate with the second opening  62   a  of the maintenance station  60 . 
     When docking is achieved, the suction port  92   a  of the suction duct  92  may be arranged adjacent to the first opening  21   a  of the robot cleaner  20  while extending in the longitudinal direction of the first opening  21   a . Also, the discharge ports  93   a  and  93   b  of the discharge duct  93  may be arranged adjacent to the first opening  21   a  of the robot cleaner  20  at the longitudinal ends of the first opening  21   a  of the robot cleaner  20 , namely, the opposite side regions of the first opening  21   a , respectively. 
     In accordance with the above-described configuration, air circulated (returned) by the dust removing device  90  during the docking operation may form a closed loop. That is, air discharged from the pump unit  91  rapidly emerges from the discharge ports  93   a  and  93   b  of the discharge duct  93 , and then enters the first dust box  43  of the robot cleaner  20  after passing through the opposite side regions of the first opening  21   a . The air introduced into the first dust box  43  of the robot cleaner  20  is discharged through the central region of the first opening  21   a , to be introduced into the second dust box  94  of the maintenance station  60  through the suction port  92   a  of the suction duct  92 . Thereafter, the air is again sucked into the pump unit  91 . 
       FIG. 9A  is a view illustrating a configuration of the brush cleaning member according to an exemplary embodiment of the present disclosure.  FIG. 9B  is a view illustrating a configuration of the brush cleaning member according to another exemplary embodiment of the present disclosure.  FIG. 9C  is a view illustrating a configuration of the brush cleaning member according to another exemplary embodiment of the present disclosure. 
     As shown in  FIG. 9A , the maintenance station  60  may include a brush cleaning member  95   a  to clean the brush unit  41  of the robot cleaner  20 . The brush cleaning member  95   a  of the maintenance station  60  is different from the brush cleaning member  41   c  of the robot cleaner  20 . 
     The brush cleaning member  95   a  of the maintenance station  60  may be arranged adjacent to the second opening  62   a . The brush cleaning member  95   a  of the maintenance station  60  may be protruded from the bottom of the housing  61  toward the second opening  62   a . The brush cleaning member  95   a  may include a plurality of brush cleaning members arranged in a longitudinal direction of the second opening  62   a.    
     In a docking state, the brush cleaning member  95   a  of the maintenance station  60  may be in contact with the brush unit  41  of the robot cleaner  20 . The brush cleaning member  95   a  of the maintenance station  60  may remove foreign matter such as hairs wound around the brush unit  41  of the robot cleaner  20 . In particular, the foreign matter removed by the brush cleaning member  95   a  of the maintenance station  60  may be introduced into the second dust box  94  by the suction force of the pump unit  91  because the brush cleaning member  95   a  of the maintenance station  60  may be arranged at the suction duct  92 . 
     In accordance with another embodiment of the present disclosure, the brush cleaning member  95   b  of the maintenance station  60  may be arranged to be slidably movable in the longitudinal direction of the second opening  62   a , as shown in  FIG. 9B . The brush cleaning member  95   b  of the maintenance station  60  may remove foreign matter wound around the brush unit  41  of the robot cleaner  20  while sliding. 
     In accordance with another embodiment of the present disclosure, the brush cleaning member  95   c  of the maintenance station  60  may be installed to be upwardly and downwardly movable, as shown in  FIG. 9C . The brush cleaning member  95   c  may move upward when the docking of the robot cleaner is completed, so that the brush cleaning member  95   c  comes into contact with the brush unit  41  of the robot cleaner  20 . On the other hand, when the docking of the robot cleaner is released, the brush cleaning member  95   c  may move downward. Meanwhile, the upward and downward movement of the brush cleaning member  95   c  may be carried out in linkage with docking of the robot cleaner  20 . 
     The brush unit  41  of the robot cleaner  20  may more effectively move dust in cooperation with the dust removal unit  90 . When the dust removal unit  90  circulates air, the brush unit  41  of the robot cleaner  20  may rotate in a clockwise direction in  FIG. 8 . In this case, the brush unit  41  of the robot cleaner  20  may assist introduction of air into the first dust box  43  of the robot cleaner  20 . Furthermore, the brush unit  41  may assist introduction of air emerging from the first dust box  43  of the robot cleaner  20  into the suction port  92   a  of the suction duct  92 . 
     The brush unit  41  of the robot cleaner may rotate at various speeds, to more effectively move dust. For example, when the dust removal unit  90  circulates air, the brush unit  41  of the robot cleaner  20  may slowly rotate in an early stage, and may then rapidly rotate. Here, the “early stage” means a certain period of time. This period may be set to be a sufficient time to allow light dust such as hairs to be discharged. As the brush unit  41  of the robot cleaner  20  rotates slowly in the early stage, foreign matter such as relatively-light hairs may be easily moved to the suction port  92   a  of the suction duct  92  by the suction force of the dust removal unit  90 . As the brush unit  41  of the robot cleaner  20  then rotates rapidly, relatively-heavy dust may be easily moved to the suction port  92   a  of the suction duct  92  by virtue of the rotating force of the brush unit  41 . 
     The brush unit  41  of the robot cleaner  20  may remove foreign matter wound around the brush unit  41  while changing the rotation direction thereof at least one time. Dust stored in the first dust box  43  of the robot cleaner  20  may be wound around the brush unit  41  of the robot cleaner  20  because it is discharged through the first opening  21   a  of the robot cleaner  20  after passing the brush unit  41  of the robot cleaner  20 . At this time, it may be possible to unwind the foreign matter wound around the brush unit  41  of the robot cleaner  20  by changing the rotation direction of the brush unit  41  of the robot cleaner  20 . The unwound foreign matter is moved to the suction port  92   a  of the suction duct  92 , and is then stored in the second dust box  94  of the maintenance station  60 . Subsequently, the brush unit  41  of the robot cleaner  20  may again change the rotation direction, so as to rotate in the original direction. The brush unit  41  of the robot cleaner  20  may repeat the change of the rotation direction several times. 
     Hereinafter, operation of the cleaning system according to an exemplary embodiment of the present disclosure will be described. 
     As shown in  FIGS. 1 to 9C , the robot cleaner  20  may sense a signal from the docking guide unit  70 , to accurately dock with the maintenance station  60  in accordance with the sensed signal. Docking is initiated as the body  21  enters the platform  62 , starting from the front portion of the body  21 . Docking is completed at a position where the first opening  231   a  of the robot cleaner  20  communicates with the second opening  62   a  of the maintenance station  60 . 
     Upon completion of docking, the dust removal unit  90  may discharge dust stored in the robot cleaner  20  to the maintenance station  60 . In detail, the pump unit  91  may discharge air at a high flow velocity through the discharge ports  93   a  and  93   b  of the discharge duct  93 . The air emerging from the discharge ports  93   a  and  93   b  may be introduced into the first dust box  43  after passing through the first opening  21   a  of the robot cleaner  20 . The air introduced into the first dust box  43  of the robot cleaner  20  may completely circulate the entire space of the first dust box  43  without forming a dead space in the first dust box  43 . In particular, air emerging from the discharge ports  93   a  and  93   b  may completely stir dust, starting from the side portion of the first dust box  43 , because the discharge ports  93   a  and  93   b  are arranged at the opposite side regions of the first opening  20   a  of the robot cleaner  20  as viewed in the longitudinal direction of the first opening  20   a . Subsequently, the dust stored in the first dust box  43  may be suspended in the air introduced into the first dust box  43 , and may then be discharged through the first opening  21   a , along with the air introduced into the first dust box  43 . The suction port  92   a  of the suction duct  92  applies a suction force to the first opening  21   a  of the robot cleaner  20 , thereby causing dust emerging from the first dust box  43  of the robot cleaner  20  to be sucked. The dust introduced into the suction port  92   a  of the suction duct  92  may be stored in the second dust box  94  of the maintenance station  60 . Air is again sucked into the pump unit  91  via a filter  94   a.    
     Thus, the air discharged from the pump unit  91  may be reintroduced into the pump unit  91  after sequentially passing through the discharge duct  93 , the first opening  21   a  of the robot cleaner  20 , the first dust box  43  of the robot cleaner  20 , the first opening  21   a  of the robot cleaner  20 , the suction duct  92 , and the second dust box  94  of the maintenance station  60 . As air circulates (returns) as described above, it may be possible to maximally prevent outward discharge of air. Accordingly, it may be possible to reduce the performance of the filter  94   a . Furthermore, it may be possible to achieve suction/discharge of air, using a single pump unit as the pump unit  91 . 
     Dust emerging from the first dust box  43  of the robot cleaner  20  may be moved to a large central region of the first opening  21   a  of the robot cleaner  20  and a large central region of the second opening  62   a  of the maintenance station  60  because the air emerging from the discharge ports  93   a  and  93   b  of the discharge duct  93  may be discharged through the opposite side regions of the first opening  21   a  of the robot cleaner  20  and second opening  62   a  of the maintenance station  60  as viewed in the longitudinal direction of the first and second openings  21   a  and  62   a , and the air sucked at the suction port  92   a  of the suction duct  92  may be sucked through the large regions of the first opening  21   a  of the robot cleaner  20  and second opening  62   a  of the maintenance station  60  as viewed in the longitudinal direction of the first and second openings  21   a  and  62   a . The arrangements of the suction port  92   a  and discharge ports  93   a  and  93   b  may prevent dust emerging from the first dust box  43  of the robot cleaner  20  from moving through the opposite side regions, and thus may prevent the dust from being outwardly discharged. The positions of the suction port  92   a  and discharge ports  93   a  and  93   b  with regard to the first opening  21   a  of the robot cleaner  20  and the second opening  62   a  of the maintenance station  60  may provide a certain sealing effect between the robot cleaner  20  and the maintenance station  60 . 
     Meanwhile, the brush unit  41  may be controlled to rotate slowly in an early stage, and then to rotate rapidly while the dust removal unit  90  circulates air, in order to assist the dust removal unit  90 . In detail, the brush unit  41  assists, in the early stage, the dust removal unit  90  to rapidly suck light dust such as hairs while rotating slowly. Subsequently, the brush unit  41  assists the dust removal unit  90  to suck relatively-heavy dust while rotating rapidly. 
     Furthermore, the brush unit  41  may be controlled to change the rotation direction thereof at least one time while the dust removal unit  90  circulates air, in order to assist the dust removal unit  90 . In detail, foreign matter such as hairs may be wound around the brush unit  41 . The wound foreign matter such as hairs may be unwound as the rotation direction of the brush unit  41  is changed. In this case, the dust removal unit  90  may suck the foreign matter such as hairs off of the brush unit  41 . 
     Meanwhile, the brush cleaning member  95  of the maintenance station  60  may remove foreign mater such as hairs wound around the brush unit  41  of the robot cleaner  20 . Foreign matter wound around the brush unit  41  of the robot cleaner  20  during rotation of the brush unit  41  comes into contact with the brush cleaning member  95  of the maintenance station  60 , so that the foreign matter may be removed from the brush unit  41  by the brush cleaning member  95  of the maintenance station  60 . The removed foreign matter may be collected in the second dust box  94  by the suction force of the dust removal unit  90 . 
       FIG. 10  is a view schematically illustrating a cleaning system according to another exemplary embodiment of the present disclosure.  FIG. 11  is a perspective view illustrating a suction/discharge dual tube.  FIG. 12  is a view illustrating flow of air in the cleaning system according to the embodiment shown in  FIG. 10 . 
     As shown in  FIGS. 10 to 12 , the cleaning system  100  may discharge dust stored in a first dust box  143  included in a robot cleaner  120  to a second dust box  194  included in a maintenance station  160 . The following description will be given only in conjunction with matters different from those of the previous embodiments. 
     The maintenance station  160  may include a suction/discharge dual tube  200 , to which a suction air flow and a discharge air flow are applied. Here, the “suction air flow” is an air flow emerging from the first dust box  143  of the robot cleaner  120 , whereas the “discharge air flow” is an air flow introduced into the first dust box  143  of the robot cleaner  120 . When docking is carried out, the first dust box  143  of the robot cleaner  120  may be coupled with the suction/discharge dual tube  200  of the maintenance station  160  via a communicating member  145 . 
     The suction/discharge dual tube  200  may have a concentric dual tube structure. For example, the suction/discharge dual tube  200  may include a discharge tube  293  arranged at a central portion of the suction/discharge dual tube  200 , and a suction tube  292  surrounding an outer peripheral surface of the discharge tube  293 . 
     On the other hand, the suction/discharge dual, tube may have a parallel dual tube structure in accordance with another embodiment. For example, the suction/discharge dual tube may include suction and discharge tubes arranged in parallel in a longitudinal direction or in a lateral direction. 
     The maintenance station  160  may include a dust removal unit  190 . The dust removal unit  190  may include a pump unit  191 , a suction duct  192  installed at a suction side of the pump unit  191 , and connected to the suction tube  292  of the suction/discharge dual tube  200 , a discharge duct  193  installed at a discharge side of the pump unit  191 , and connected to the discharge tube  293  of the suction/discharge dual tube  200 , and a second dust box  194 . 
     When the robot cleaner  20  docks with the maintenance station  160 , air discharged from the pump unit  191  may be introduced into the first dust box  143  of the robot cleaner  120  after entering the discharge tube  293  of the suction/discharge dual tube  200  via the discharge duct  193 . Thereafter, the air introduced into the first dust box  143  may pass through the suction duct  192  after being sucked into the suction tube  292  of the suction/discharge dual tube  200 , along with dust stored in the first dust box  143 . The dust passing through the suction duct  192  may be stored in the second dust box  194 , and may then be sucked into the pump unit  191  again. 
     Thus, the air discharged from the pump unit  191  may be reintroduced into the pump unit  191  after sequentially passing through the discharge duct  193 , the discharge tube  293  of the suction/discharge dual tube  200 , the first dust box  143  of the robot cleaner  120 , the suction tube  292  of the suction/discharge dual tube  200 , the suction duct  192 , and the second dust box  194  of the maintenance station  160 . 
       FIG. 13  is a view schematically illustrating a cleaning system according to another embodiment of the present disclosure. 
     As shown in  FIG. 13 , the cleaning system  300  may discharge dust stored in a first dust box  343  included in a robot cleaner  320  to a second dust box  394  included in a maintenance station  360 . The following description will be given only in conjunction with matters different from those of the previous embodiments. 
     The first dust box  343  of the robot cleaner  320  may include an inlet communicating with a first opening  321   a  included in the robot cleaner  320 , and a communicating member  345  to directly communicate with the maintenance station  360 . 
     The maintenance station  360  may include a dust removal unit  390 . The dust removal unit  390  may include a pump unit  391 , a suction duct  392  installed at a suction side of the pump unit  391 , and a discharge duct  393  installed at a discharge side of the pump unit  391 . 
     When the robot cleaner  320  docks with the maintenance station  360 , the first opening  321   a  of the robot cleaner  320  may be connected to the suction duct  392  of the maintenance station  360 , and the communicating member  345  of the first dust box  343  in the robot cleaner  320  may be connected to the discharge duct  393  of the maintenance station  360 . 
     Air discharged from the pump unit  391  may be introduced into the first dust box  343  of the robot cleaner  320  via the discharge duct  393 . The air introduced into the first dust box  343  of the robot cleaner  320  may be moved to the suction duct  392  after passing through the inlet  343 ′ of the first dust box  343  and the first opening  321   a  of the robot cleaner  320 , along with dust stored in the first dust box  343 . The dust moved to the suction duct  392  is stored in the second dust box  394  of the maintenance station  360 , whereas the air may be sucked into the pump unit  391  again. 
     Thus, the air discharged from the pump unit  391  may be reintroduced into the pump unit  391  after sequentially passing through the discharge duct  393 , communicating member  345  of the first dust box  343 , the first dust box  343  of the robot cleaner  320 , the inlet  343 ′ of the first dust box  343 , the suction duct  392 , and the second dust box  394  of the maintenance station  360 . 
       FIG. 14  is a view schematically illustrating a cleaning system according to another embodiment of the present disclosure. 
     As shown in  FIG. 14 , the cleaning system  400  may discharge dust stored in a first dust box  443  included in a robot cleaner  420  to a second dust box  494  included in a maintenance station  460 . The following description will be given only in conjunction with matters different from those of the previous embodiments. 
     When the robot cleaner  420  docks with the maintenance station  460 , a first opening  421   a  of the robot cleaner  420  may be connected to a discharge duct  493  of the maintenance station  460 , and a communicating member  445  included in the first dust box  443  of the robot cleaner  420  may be connected to a suction duct  492  of the maintenance station  460 . 
     Air discharged from the pump unit  491  may be introduced into the first dust box  443  of the robot cleaner  320  via the discharge duct  493 , the first opening  421   a  of the robot cleaner  420 , and an inlet  443 ′ of the first dust box  443 . The air introduced into the first dust box  443  of the robot cleaner  420  may be moved to the suction duct  492  after passing through the communicating member  445  of the first dust box  443 , along with dust stored in the first dust box  443 . The dust moved to the suction duct  492  is stored in the second dust box  494  of the maintenance station  460 , whereas the air may be sucked into the pump unit  491  again. 
     Thus, the air discharged from the pump unit  491  may be reintroduced into the pump unit  491  after sequentially passing through the discharge duct  493 , the inlet  443 ′ of the first dust box  443 , the first dust box  443  of the robot cleaner  420 , the communicating member  445  of the first dust box  443 , the suction duct  492 , and the second dust box  494  of the maintenance station  460 . 
       FIG. 15  is a top perspective view illustrating a configuration of the maintenance station according to another exemplary embodiment of the present disclosure.  FIG. 16  is an exploded perspective view illustrating a configuration of the maintenance station according to the illustrated embodiment of the present disclosure.  FIG. 17  is a plan view illustrating a duct included in the maintenance station according to the illustrated embodiment of the present disclosure.  FIG. 18  is a sectional view illustrating a flow of air discharged through a second opening during a docking operation.  FIG. 19  is a sectional view illustrating a flow of air sucked through the second opening during the docking operation.  FIG. 20  is a top perspective view illustrating a port assembly according to another exemplary embodiment of the present disclosure.  FIG. 21  is a bottom perspective view illustrating the port assembly according to the illustrated embodiment of the present disclosure. 
     Referring to  FIGS. 15 to 21 , a cleaning system  510  is illustrated. The cleaning system  510  has the same basic structure as the above-described cleaning system  10 . Accordingly, the following description will be given mainly in conjunction with portions of the cleaning system  510  different from the cleaning system  10 , and no description will be given of the same portions of the cleaning system  510  as the cleaning system  10 , if possible. 
     The maintenance station  560  may include a housing  561 , a docking guide unit  570 , a charging unit  580 , a dust removal unit  590 , and a controller (not shown). 
     A platform  562  may be provided at the housing  561 . A second opening  562   a  may be formed at the platform  562 . The second opening  562   a  of the platform  562  is arranged at a position where the second opening  562   a  may communicate with a first opening  521   a  of the robot cleaner  520 . Dust discharged through the first opening  521   a  of the robot cleaner  520  may be introduced into the second opening  562   a  of the platform  562 , and is then stored in a second dust box  594  of the maintenance station  560 . In this case, the second opening  562   a  of the platform  562  may be larger than the first opening  521   a  of the robot cleaner  520 . 
     The dust removal unit  590  may be installed at the housing  561 . The dust removal unit  590  may discharge dust stored in the first dust box  543  of the robot cleaner  520  into the second dust box  594  of the maintenance station  560 , to empty the first dust box  543 . Thus, the dust removal unit  590  may maintain desired cleaning performance of the robot cleaner  520 . 
     The dust removal unit  590  may include a pump unit  591 , a suction duct  592 , a first discharge duct  593   a , a second discharge duct  593   b , a port assembly  600 , and a suction/discharge dual tube  200 , in addition to the second dust box  594 . The dust removal unit  590  functions to force air discharged from the first and second discharge ducts  593   a  and  593   b  to be sucked back into the suction duct  592 . Using such a circulating air flow, the dust removal unit  590  removes dust stored in the first dust box  543  of the robot cleaner  520 . 
     The suction duct  592  may be installed at a suction side of the pump unit  591 . The first and second discharge ducts  593   a  and  593   b  may be installed at a discharge side of the pump unit  591 . The port assembly  600  may be separably mounted to the second opening  562   a . The port assembly  600  communicates with the suction duct  592 , first discharge duct  593   a , and second discharge duct  593   b.    
     The port assembly  600  may include a suction port forming member  610 , a first discharge port forming member  621 , a second discharge port forming member  622 , a third discharge port forming member  623 , a fourth discharge port forming member  624 , and a brush cleaning member  630 . 
     The suction port forming member  610  divides the suction duct  592  into two portions, which form first and second suction ports  592   a  and  592   b , respectively. First spacers  610   a  and  610   b  are formed at a lower surface of the suction port forming member  610 . The first spacers  610   a  and  610   b  function to space the suction port forming member  610  from the bottom of the housing  561 . 
     Air or dust introduced into the first suction port  592   a  flows toward the suction duct  592  along an upper surface of the suction port forming member  610 . Air or dust introduced into the second suction port  592   b  flows toward the suction duct  592  along a lower surface of the suction port forming member  610 . The dust is subsequently stored in the second dust box  594  of the maintenance station  560 . 
     The first discharge port forming member  621  and second discharge port forming member  622  divide the first discharge duct  593   a , into two portions, which form first and second discharge ports  593   a ′ and  593   a ″, respectively. On the other hand, the third discharge port forming member  623  and fourth discharge port forming member  624  divide the second discharge duct  593   b , into two portions, which form third and fourth discharge ports  593   b ′ and  593   b ″, respectively. 
     Air discharged through the first discharge port  593   a ′ and third discharge port  593   b ′ is fed to a large dust box  543   a  of the robot cleaner  520 , whereas air discharged through the second discharge port  593   a ″ and fourth discharge port  593   b ″ is fed to a small dust box  543   b  of the robot cleaner  520 . The first discharge port  593   a ′ and third discharge port  593   b ′ directly face the large dust box  543   a . Accordingly, air discharged through the first discharge port  593   a ′ and third discharge port  593   b ′ is fed to the large dust box  543   a  while passing through the brush unit  541  at high flow rate. 
     However, the second discharge port  593   a ″ and fourth discharge port  593   b ″ do not directly fact the small dust box  543   b . For this reason, air discharged through the second discharge port  593   a ″ and fourth discharge port  593   b ″ is guided by a brush drum  540   a , to be fed to the small dust box  543   b . When the brush unit  541  rotates in a counterclockwise direction in  FIG. 18 , air discharged through the second discharge port  593   a ″ and fourth discharge port  593   b ″ may be more smoothly fed to the small dust box  543   b.    
     The first discharge port  593   a ′ and third discharge port  593   b ′ are arranged at opposite longitudinal (or lateral) ends of the second opening  562   a , namely, opposite side regions of the second opening  562   a , respectively. Also, the second discharge port  593   a ″ and fourth discharge port  593   b ″ are arranged at opposite longitudinal (or lateral) ends of the second opening  562   a , namely, opposite side regions of the second opening  562   a , respectively. On the other hand, the first discharge port  593   a ′ and second discharge port  593   a ″ are arranged at opposite ends of the second opening  562   a  in a width (forward or backward) direction in one side region of the second opening  562   a , respectively. Also, the third discharge port  593   b ′ and fourth discharge port  593   b ″ are arranged at opposite ends of the second opening  562   a  in the width (forward or backward) direction in the other side region of the second opening  562   a , respectively. Thus, the first discharge port  593   a ′ to fourth discharge port  593   b ″ are arranged at respective corner regions of the second opening  562   a.    
     Meanwhile, second spacers  622   a  and  624   a  are formed at side walls of the second discharge port forming member  622  and fourth discharge port forming member  624 , respectively. The second spacers  622   a  and  624   a  function to prevent the port assembly  600  from being biased toward one side of the second opening  562   a.    
     Thus, the second suction port  592   b  may be formed to have a structure surrounding the first suction port  592   a , first discharge port  593   a ′, second discharge port  593   a ″, third discharge port  593   b ′, and fourth discharge port  593   b ″. The area occupied by the first suction port  592   a  and the first to fourth discharge ports  593   a ′,  593   a ″,  593   b ′, and  593   b ″ corresponds to the area of the first opening  521   a  of the robot cleaner  520 . The second suction port  592   b  may suck dust dispersed outside the first opening  521   a  of the robot cleaner  520  because it is arranged outside the first opening  521   a  of the robot cleaner  520 . 
     A cover  640  formed with a plurality of through holes  640   a  may be mounted to the second suction port  592   a . In this case, dust dispersed outside the first opening  521   a  of the robot cleaner  520  may be sucked into the second suction port  592   b  through the through holes  640   a . Normally, the cover  640  prevents foreign matter having a large size from entering the second suction port  592   a , thereby preventing the suction passage from becoming clogged. 
     The brush cleaning member  630  is formed at the suction port forming member  610 , to be protruded from the suction port forming member  610 , and thus to come into contact with brushes  541   b  of the brush unit  541 . A plurality of brush cleaning members  630  may be installed to be arranged in a longitudinal direction of the suction port forming member  610 , as in the illustrated case. In the illustrated case, the brush cleaning members  630  are arranged in two rows in the longitudinal direction of the suction port forming member  610 . In another embodiment, a plurality of brush cleaning members  630  may be arranged in one row, two rows, or more. 
     The brush cleaning member  630  may include a guide  631  and a hook  632 . 
     The guide  631  extends inclinedly with respect to a rotation direction of the brush unit  541 . The hook  632  is protruded from a side surface of an end of the guide  631 . When the brush unit  541  rotates, the brushes  541   b , which are made of an elastic material, are inclined in the inclined direction of the guide  631  while coming into contact with the guide  631 . Accordingly, foreign matter, which may be hairs wound around the brushes  541   b , may be caught by the hook  632  which, in turn, separates the foreign matter from the brushes  541   b.    
     Meanwhile, in another embodiment, a plurality of guides  631  may be arranged in a longitudinal direction of the suction port forming member  610 , and a plurality of hooks  632  may be protruded from side surfaces of guides  631 , respectively. The guides  631 , which are arranged in the longitudinal direction of the suction port forming member  610 , may be laterally symmetrically arranged. 
     A plurality of suction/discharge dual tubes  200  may be provided at the platform  562 . The plural suction/discharge dual tubes  200  are arranged at positions corresponding to a plurality of infrared sensors  552  installed on a bottom of the robot cleaner  520 . The concrete shape of each suction/discharge dual tubes  200  may be referred to the description given with reference to  FIG. 11 . 
     Each suction/discharge dual tube  200  generates a suction air flow and a discharge air flow. Here, the suction air flow is an air flow introduced into the housing  561  through a suction tube  292  communicating with the suction duct  592 , whereas the discharge air flow is an air flow outwardly discharged from the housing  561  through a discharge tube  293  communicating with the first discharge duct  593   a  or second discharge duct  593   b.    
     The infrared sensors  552  of the robot cleaner  520  may be cleaned by air flowing through the corresponding suction/discharge dual tubes  200 , respectively. That is, air is blown to each infrared sensor  552  of the robot cleaner  520  through the discharge tube  293  of the corresponding suction/discharge dual tube  200 , to remove dust from the infrared sensor  552 , and the removed dust is then sucked through the suction tube  292  of the corresponding suction/discharge dual tube  200 . The dust introduced into the suction tube  292  is collected in the second dust box  594  of the maintenance station  560 . 
     Thus, dust attached to each infrared sensor  552  is removed, so that desired sensing performance may be maintained. Since the dust removed from the infrared sensor  552  is sucked back without being dispersed, the surroundings of the station  560  may be kept clean. 
     As apparent from the above description, the cleaning system according to each of the illustrated embodiments may prevent the cleaning performance of the robot cleaner from being degraded. 
     The cleaning system may also achieve a reduction in energy and material costs by circulating air between the robot cleaner and the maintenance station. 
     The cleaning system may also easily achieve automatic dust discharge by discharging dust through the opening of the robot cleaner. 
     The cleaning system may cut off dust dispersed during automatic dust discharge, thereby keeping clean the surroundings of the maintenance station. 
     The cleaning system also may clean the sensors using circulating discharge air, thereby preventing dust from dispersed around the surroundings of the cleaning system. 
     Also, the cleaning system may effectively remove foreign matter wound on the brush unit during automatic dust discharge. 
     Although a few embodiments of the present disclosure have been shown and described, 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.