Abstract:
A mobile robot configured to travel across a residential floor or other surface while cleaning the surface with a cleaning pad and cleaning solvent is disclosed. The robot includes a controller for managing the movement of the robot as well as the treatment of the surface with a cleaning solvent. The movement of the robot can be characterized by a class of trajectories that achieve effective cleaning. The trajectories include sequences of steps that are repeated, the sequences including forward and backward motion and optional left and right motion along arcuate paths.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application claims the benefit of U.S. Provisional Patent Application Ser. No. 61/292,753 filed Jan. 6, 2010, entitled “A method for effective autonomous mopping of a floor surface,” which is hereby incorporated by reference herein for all purposes. 
    
    
     TECHNICAL FIELD 
     The invention pertains to a mobile robot for cleaning a floor or other surface. In particular, the invention pertains to a robot configured to implement a class of trajectories designed to efficiently scrub or otherwise clean the floor. 
     BACKGROUND 
     From their inception, robots have been designed to perform tasks that people prefer not to do or cannot do safely. Cleaning and vacuuming, for example, are just the type of jobs that people would like to delegate to such mechanical helpers. The challenge, however, has been to design robots that can clean the floor of a home well enough to satisfy the exacting standards of the people the live in it. Although robots have been designed to vacuum floors, robots designed to perform mopping present unique challenges. In particular, such a robot should be able to dispense cleaning solution, scrub the floor with the solution, and then effectively remove the spent cleaning solution. Robots tend to perform unsatisfactorily, however, because hard deposits on the floor may require time for the cleaning solution to penetrate and removal of the dirty solution may leave streak marks on the floor. There is therefore a need for a cleaning robot able to implement a cleaning plan that enables the robot to apply cleaning solution, repeatedly scrub the floor with the solution, and leave the floor free of streak marks. 
     SUMMARY 
     The present invention pertains to a mobile robot configured to travel across a residential floor or other surface while cleaning the surface with a cleaning pad and cleaning solvent. The robot includes a controller for managing the movement of the robot as well as the treatment of the surface with a cleaning solvent. The movement of the robot can be characterized by a class of trajectories that achieve effective cleaning. These trajectories seek to: maximize usage of the cleaning solvent, reduce streaking, utilize absorption properties of the pad, and use as much of the surface of the pad as possible. In an exemplary embodiment, the trajectory may include an oscillatory motion with a bias in a forward direction by repeatedly moving forward a greater distance than backward. In the same exemplary embodiment, the cleaning pad is a disposable sheet impregnated with solvent that is then applied to and recovered from the surface by means of the trajectory. 
     In one embodiment, the cleaning robot includes a cleaning assembly; a path planner for generating a cleaning trajectory; and a drive system for moving the robot in accordance with the cleaning trajectory. The cleaning trajectory is a sequence of steps or motions that are repeated a plurality times in a prescribed order to effectively scrub the floor. Repetition of the sequence, in combination with the forward and back motion, causes the cleaning assembly to pass of areas of the floor a plurality of times while allowing time for the cleaning solution to penetrate dirt deposits. 
     The sequence repeated by the cleaning trajectory preferably comprises: (i) a first path for guiding the robot forward and to the left; (ii) a second path for guiding the robot backward and to the right; (iii) a third path for guiding the robot forward and to the right; and (iv) a fourth path for guiding the robot backward and to the left. The first path and third path result in a longitudinal displacement of the robot (movement parallel to the direction of progression) referred to as a first distance forward, and the second path and fourth path result in a longitudinal displacement referred to as a second distance backward. The first distance is greater than the second distance, preferably twice as large. In addition, the second path results in a lateral displacement (movement perpendicular to the direction of progression) which is referred to as the third distance, and the fourth path moves the robot laterally by a fourth distance that is equal in magnitude but opposite in direction from the third distance. In the preferred embodiment, the first through fourth paths are arcuate paths. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention is illustrated by way of example and not limitation in the figures of the accompanying drawings, and in which: 
         FIG. 1A  is an autonomous mobile robot, in accordance with a preferred embodiment; 
         FIG. 1B  is the autonomous mobile robot moving in the forward direction, in accordance with a preferred embodiment; 
         FIG. 1C  is the autonomous mobile robot moving in the backward direction, in accordance with a preferred embodiment; 
         FIG. 2  is a schematic diagram of a navigation system, in accordance with a preferred embodiment; 
         FIG. 3  is a cleaning trajectory for scrubbing a floor, in accordance with an exemplary embodiment; 
         FIGS. 4A-4D  depict a sequence of steps that produce the trajectory of  FIG. 3 ; 
         FIG. 5  is a cleaning trajectory for scrubbing a floor, in accordance with another exemplary embodiment; 
         FIGS. 6A-6D  depict a sequence of steps that produce the trajectory of  FIG. 5 , 
         FIG. 7  is a cleaning trajectory for scrubbing a floor, in accordance with still another exemplary embodiment; and 
         FIGS. 8A-8B  depict a sequence of steps that produce the trajectory of  FIG. 7 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Illustrated in  FIG. 1A  is an autonomous mobile robot  100  configured to clean or otherwise treat a floor or other surface using a trajectory designed to repeatedly scrub the floor. In the preferred embodiment, the mobile robot  100  includes a housing with a controller and navigation system (not shown) for generating a path to clean the entire floor, a drive system  110  configured to move the robot around a room or other space in a determined trajectory, and a cleaning assembly  120 A pivotably attached to the robot housing by means of a hinge  130 . The cleaning assembly  120 A preferably includes a curved bottom surface  122  configured to press a cleaning sheet to the floor. The mobile robot in the preferred embodiment is based on the cleaning robot taught in pending U.S. patent application Ser. No. 12/429,963 filed on Apr. 24, 2009, which is hereby incorporated by reference herein. 
     The cleaning component in the preferred embodiment is configured to scrub the floor with a disposable cleaning sheet, preferably a wet cleaning sheet impregnated with cleaning solution. In other embodiments, the cleaning assembly is configured to dispense cleaning solution directly on the floor and then scrub the floor with a dry cleaning sheet. In still other embodiments, the cleaning assembly is configured to employ cleaning components for brushing, dusting, polishing, mopping, or vacuuming the floor, which may be a wood, tile, or carpet, for example. 
     Illustrated in  FIG. 2  is a schematic diagram of the navigation system in the preferred embodiment. The navigation system  200  includes a navigation module  210  configured to maintain an estimate of the robot&#39;s current pose while it navigates around its environment, preferably an interior room bounded by walls. The pose  212 , which includes both the position and orientation, is based on multiple sensor readings including wheel odometry  214  provided by encoders integrated into the wheels  110 , orientation readings  216  from a gyroscope (not shown) on board the robot, and position coordinates from an optical sensor configured to sense light reflected from the room&#39;s ceiling. The optical sensor may include one or more photo diodes, one or more position sensitive detectors (PSDs), or one or more laser range finders, for example. The optical sensor employed in the preferred embodiment is taught in U.S. Pat. No. 7,720,554, which is hereby incorporated by reference herein. 
     The navigation system further includes a path planner  220  for generating or executing logic to traverse a desired trajectory or path  222  to scrub the entire floor with no gaps. In path  222  designed by the path planner  220  is a combination of a first trajectory from a room coverage planner  222  and a second trajectory from a local scrub planner  226 , which are discussed in more detail below. Based on the current pose  212  and the desired path  222 , the motion controller generates motion commands  232  for the robot drive  240 . The commands in the preferred embodiment include the angular velocity for each of a pair of wheels  110 , which are sufficient to control the speed and direction of the mobile robot. As the robot navigates through its environment, the navigation module  210  continually generates a current robot pose estimate while the path planner  220  updates the desired robot path. 
     The first trajectory is designed to guide the robot throughout the entire room until each section of the floor has been traversed. The second trajectory is a pattern including a plurality of incremental steps that drive the cleaning assembly both forward and backward, and optionally left and right. The first trajectory ensures every section of the floor is traversed with the cleaning assembly while the second trajectory ensures each section of floor traversed is effectively treated with cleaning solution and scrubbed with multiple passes of the cleaning assembly. 
     The first trajectory may take the form of any of a number of space-filling patterns intended to efficiently traverse each part of the room. For example, the first trajectory may be a rectilinear pattern in which the robot traverses the entire width of the room multiple times, each traversal of the room covering a unique swath or row adjacent to the prior row traversed. The pattern in repeated until the entire room is covered. In another embodiment, the robot follows a path around the contour of the room to complete a loop, then advances to an interior path just inside the path traversed in the preceding loop. Successively smaller looping patterns are traversed until the center of the room is reached. In still another embodiment, the robot traverses the room in one or more spiral patterns, each spiral including a series of substantially concentric circular or substantially square paths of different diameter. These and other cleaning contours are taught in U.S. patent application Ser. No. 12/429,963 filed on Apr. 24, 2009. 
     The second trajectory scrubs the floor using a combination of forward and backward motion. The step in the forward direction is generally larger than the step in the backward direction to produce a net forward movement. If the second trajectory includes lateral movement, the steps to the left and right are generally equal. The repeated forward/backward motion, in combination with hinge  130 , causes the orientation of the cleaning assembly to oscillate between a small angle forward or a small angle backward as shown in  FIGS. 1B and 1C , respectively. When driven in the forward direction, the cleaning assembly  120 B pivots forward which presses the front half of the cleaning pad against the floor while lifting the back half away from the floor. When driven in the reverse direction, the cleaning assembly  120 C pivots backward to press the back half of the cleaning pad against the floor while lifting the front half away from the floor. As a result, the front half of the cleaning sheet (1) scrubs the floor with the cleaning sheet impregnated with cleaning solution and (2) captures/collects dirt and debris as the robot advances in the forward direction (see  FIG. 1B ). On the other hand, the back half of the cleaning sheet, which remains generally free of debris, scrubs the floor with cleaning solution released from the cleaning sheet. This serves to: (i) evenly apply cleaning solution, (ii) recover the cleaning solution mixed with dissolved dirt, (iii) produce an elapse time between application and recovery of the cleaning solution, (iv) mechanically agitate the cleaning solution on the floor, and (v) produce little or no visible streak marks on the floor. In the exemplary embodiment, the cleaning sheet is impregnated with a cleaning solution that is transferred to the floor by contact. In another embodiment the mobile robot includes a reservoir with at least one nozzle configured to either spray cleaning solution on the floor surface in front of the cleaning assembly or diffuse the cleaning solution directly into the upper surface of the sheet through capillary action. 
     Illustrated in  FIG. 3  is an example of a second trajectory for scrubbing a floor. The trajectory is produced by repetition of the sequence of steps shown in  FIGS. 4A-4D . The solid line  310  represents the path traced by the midpoint of the leading edge of the cleaning assembly  120  while the shaded region  320  represents the region of floor scrubbed by the cleaning assembly. When this sequence is employed, the robotic cleaner oscillates in the forward direction of motion as well as laterally. For each oscillation, the forward displacement (defined as the “fwd_height”) exceeds the backward displacement (defined as the “back_height”) so the robot advances in a generally forward direction (defined as the “direction of progression”). The robot also moves left and right equal amounts (defined as the fwd_width) which causes the robot to travel in a generally straight line. 
     The trajectory shown in trajectory in  FIG. 3  is produced by repetition of the sequence of steps illustrated in  FIG. 4A-4D  in the prescribed order. Each step or leg comprises a motion with an arcuate path. The first leg  410  of the sequence shown in  FIG. 4A  advances the robot forward by fwd_height and to the left. In the second leg  420  shown in  FIG. 4B , the robot moves backward by back_height and to the right. In the third leg  430  shown in  FIG. 4C , the robot moves forward by fwd_height and to the right. In the fourth leg  440  shown in  FIG. 4D , the robot moves backward by back_height and to the left. The forward arcing motions have a larger radius than the back motions such that the robot is oriented parallel to the direction of progression upon completion of the backward motion. 
     Trajectories that include arced or arcuate paths can provide several benefits over trajectories having only straight paths. For example, the trajectory shown in  FIG. 3 , which consists of arcuate paths, causes the robot to continually turn or rotate while in motion. This rotation, in turn, is detected by the on-board gyroscope and monitored by the navigation system for purposes of detecting slippage of the wheels  110 . When the detected rotation is different than the rotation associated with the curvature of the path, the robot can confirm slippage due to loss of traction, for example, and correct the robots course accordingly. In contrast, trajectories with straight paths make it difficult to detect slippage when, for example, both wheels slip at the same rate which cannot be detect with the gyro. 
     For the trajectory shown in FIGS.  3  and  4 A- 4 D, the parameters are as follows:
         (a) fwd_height: the distance traveled in the direction of progression on the forward legs or strokes has a value of approximately 1.5 times with width of the cleaning assembly  120 , the width being measured in the direction perpendicular to the direction of progression;   (b) back_height: the distance traveled in the direction opposite the direction of progression on the backward legs or strokes has a value of approximately 0.75 times the width of the cleaning assembly  120 ; and   (c) fwd_width: the distance traveled orthogonal to the direction of progression on the forward legs or strokes has a value of approximately 0.3 times the width of the cleaning assembly  120 .       

     In general, however, fwd_height may range between one and five times the width of the cleaning assembly  120 , the back_height may range between one third and four times the width of the cleaning assembly  120 , and the elapse time of a cleaning single sequence may range between five second and sixty seconds. 
     Where the cleaning sheet is a Swiffer® Wet Cleaning Pad, for example, each sequence of the trajectory is completed in a time between 15 to 30 seconds, which enables the cleaning solution to remain on the floor long enough to dissolve dirt but not so long that it first evaporates. 
     Illustrated in  FIG. 5  is another example of a second trajectory for scrubbing a floor. The trajectory is produced by repetition of the sequence of steps shown in  FIGS. 6A-6D . The solid line  510  represents the path traced by the midpoint of the leading edge of the cleaning assembly  120  while the shaded region  520  represents the region of floor scrubbed by the cleaning assembly. When this sequence is employed, the robotic cleaner oscillates in the forward direction of motion as well as laterally. For each oscillation, the forward displacement (“fwd_height”) exceeds the backward displacement (“back_height”) so the robot advances in a generally forward direction (“direction of progression”). The robot also moves left and right equal amounts (fwd_width) which causes the robot to travel in a generally straight line. 
     The trajectory shown ion trajectory in  FIG. 5  is produced by repetition of the sequence of steps illustrated in  FIG. 6A-6D . Each step or leg comprises a motion with an arcuate path. The first leg  610  of the sequence shown in  FIG. 6A  advances the robot forward by fwd_height and to the left with a predetermined radius. In the second leg  620  shown in  FIG. 4B , the robot moves backward by back_height and to the right along the same radius as leg  610 . In the third leg  630  shown in  FIG. 6C , the robot moves forward by fwd_height and to the right with the same predetermined radius. In the fourth leg  640  shown in  FIG. 6D , the robot moves backward by back_height and to the left with the same radius as above. The forward arcing motions progress a greater distance than the back motions so that the robot generally progresses in the forward direction. 
     For the trajectory shown in FIGS.  5  and  6 A- 6 D, the parameters are as follows:
         (a) fwd_height: the distance traveled in the direction of progression on the forward legs or strokes has a value of approximately 1.5 times with width of the cleaning assembly  120 , namely the direction perpendicular to the direction of progression;   (b) back_height: the distance traveled in the direction opposite the direction of progression on the backward legs or strokes has a value of approximately 0.75 times the width of the cleaning assembly  120 ; and   (c) radius: the radius of each arc is approximately equal to the diameter of the mobile robot, although the radius may range between 0.5 and 3 times the width of the cleaning assembly.       

     Illustrated in  FIG. 7  is another example of a second trajectory for scrubbing a floor. The trajectory is produced by repetition of two legs that are both straight and parallel, as shown in  FIGS. 7A-7B . The solid line  710  represents the path traced by the midpoint of the leading edge of the cleaning assembly  120  while the shaded region  720  represents the region of floor scrubbed by the cleaning assembly. When this sequence is employed, the robotic cleaner oscillates in the forward direction but not laterally. For each oscillation, the forward displacement (“fwd_height”) of the forward leg  810  exceeds the backward displacement (“back_height”) of the back leg  820 , so the robot advances in a generally forward direction. 
     In some embodiments, the robot further includes a bump sensor for detecting walls and other obstacles. When a wall is detected, the robot is configured to make a U-turn by completing a 180 degree rotation while moving the robot to one side, the distance moved being approximately equal to the width of the cleaning assembly. After completing the turn, the robot is then driven across the room along a row parallel with and adjacent to the preceding row traversed. By repeating this maneuver each time a wall is encountered, the robot is made to traverse a trajectory that takes the robot across each portion of the room. 
     The trajectory is preferably based, in part, on the pose of the robot which is tracked over time to ensure that the robot traverses a different section of the floor with each pass, thereby avoiding areas of the floor that have already been cleaned while there are areas still left to be cleaned. 
     One or more of the components of the mobile robot, including the navigation system, may be implemented in hardware, software, firmware, or any combination thereof. Software may be stored in memory as machine-readable instructions or code, or used to configure one or more processors, chips, or computers for purposes of executing the steps of the present invention. Memory includes hard drives, solid state memory, optical storage means including compact discs, and all other forms of volatile and non-volatile memory. 
     Although the description above contains many specifications, these should not be construed as limiting the scope of the invention but as merely providing illustrations of some of the presently preferred embodiments of this invention. 
     Therefore, the invention has been disclosed by way of example and not limitation, and reference should be made to the following claims to determine the scope of the present invention.