Patent Publication Number: US-10758104-B2

Title: Debris monitoring

Description:
CLAIM OF PRIORITY 
     This U.S. patent application is a continuation of and claims priority to U.S. patent application Ser. No. 15/794,604, filed on Oct. 26, 2017, which is a continuation and claims priority to U.S. patent application Ser. No. 14/985,862, filed on Dec. 31, 2015, which is a continuation of and claims priority to U.S. application Ser. No. 14/258,440, filed Apr. 22, 2014, which is a continuation of and claims priority to U.S. patent application Ser. No. 13/340,784, filed on Dec. 30, 2011 which claims priority under 35 U.S.C. § 1 19(e) to U.S. Provisional Application 61/428,808, filed on Dec. 30, 2010, the disclosure of each of which are considered part of the disclosure of this application and are hereby incorporated by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     This application relates to robots, and more particularly to autonomous coverage robots. 
     BACKGROUND 
     Autonomous robots can perform desired tasks in unstructured environments without continuous human guidance. Many kinds of robots are autonomous to some degree. Different robots can be autonomous in different ways. An autonomous coverage robot can traverse a work surface without continuous human guidance to perform one or more tasks. In the field of home, office and/or consumer-oriented robotics, mobile robots that perform household functions such as removing debris from a surface (e.g., vacuum cleaning and floor washing) have been widely adopted. 
     SUMMARY 
     In one aspect, a debris monitoring system includes a receptacle, a first and a second emitter, and a first receiver. The receptacle defines an opening to receive debris into the receptacle. The first and second emitter are each arranged to emit a signal across at least a portion of the opening. The first receiver is proximate to the first emitter to receive reflections of the signal emitted by the first emitter, and the first receiver is disposed toward the opening to receive an unreflected portion of the signal emitted by the second emitter across at least a portion of the opening. 
     In another aspect, a coverage robot includes a housing, a drive system, a cleaning assembly, a receptacle, a first and a second emitter, and a first receiver. The drive system is coupled to the housing and configured to maneuver the robot across a cleaning surface. The cleaning assembly is coupled to the housing. The receptacle is disposed substantially within the housing, and the receptacle defines an opening to receive debris into the receptacle from the cleaning assembly. The debris monitoring system is disposed substantially within the housing. The debris monitoring system includes a first and a second emitter and a first receiver. The first and second emitter are each arranged to emit a signal across at least a portion of the opening. The first receiver proximate to the first emitter to receive reflections of the signal emitted by the first emitter and the first receiver disposed toward the opening to receive an unreflected portion of the signal emitted by the second emitter across at least a portion of the opening. 
     Implementations of one or more of these aspects of the disclosure may include one or more of the following features. In some implementations, the first receiver and the second emitter are disposed substantially opposite one another across the largest dimension of the opening. The opening can be substantially rectangular. Additionally or alternatively, the first receiver and the second receiver can be substantially diagonally opposed to one another across the opening. In certain implementations, the first and second emitters are arranged relative to one another such that the respective signals emitted by the first and second emitters intersect along at least a portion of the opening. The opening can be defined in a substantially vertical plane as debris is received into the receptacle. 
     In certain implementations, the opening has a top portion and a bottom portion, the top portion above the bottom portion as debris is received into the receptacle, and the first and second emitters and the first receiver each disposed toward a top portion of the opening, with the first receiver disposed above the first and second emitters. 
     In some implementations, the first receiver is arranged about 0.5 inches to about 30 inches from the second emitter. The first receiver can be less than about 5 inches from the first emitter. Additionally or alternatively, the ratio of the distance between the first receiver and the second emitter to the distance between the first receiver and the first emitter is about 0.1 to about 600. 
     In certain implementations, the receptacle is releasably engageable with a housing configured to support the receptacle as debris is received through the opening of the receptacle. The first and second emitters and the first receiver can each be supported on the housing and the receptacle can be movable relative to the first and second emitters and the first receiver. The first and second emitters and the first receiver can each be supported on the receptacle. A controller can be supported on the housing. The first and second emitters and the first receiver can each be in wireless communication (e.g., infrared communication) with the controller. 
     In some implementations, the receptacle is removable from a side portion of the coverage robot when the robot is on the cleaning surface and/or removable from a side portion of the housing. Additionally or alternatively, the receptacle is removable from a top portion of the coverage robot when the robot is on the cleaning surface and/or removable from a top portion of the housing. 
     In another aspect, a debris monitoring system includes a receptacle, a plurality of first emitters and a plurality of second emitters, a first receiver, and a second receiver. The receptacle defines an opening to receive debris into the receptacle. Each emitter of each plurality of emitters is arranged to emit a signal across at least a portion of the opening. The first receiver is proximate to the plurality of first emitters to receive reflections of the signal emitted by each of the plurality of first emitters and the first receiver is disposed toward the opening to receive an unreflected portion of the signal emitted by each of the plurality of second emitters across at least a portion of the opening. The second receiver is proximate to the plurality of second emitters to receive reflections of the signal emitted by each of the plurality of second emitters and the second receiver disposed toward the opening to receive an unreflected portion of the signal emitted by each of the plurality of first emitters across at least a portion of the opening. 
     Implementations of this aspect of the disclosure may include one or more of the following features. In some implementations, a controller is configured to pulse the plurality of first emitters on and off and to pulse the plurality of second emitters on and off. The controller can be configured to sample of each of the first and second receivers synchronously such that a first sample of each receiver is taken when the plurality of first emitters and the plurality of second emitters are off, a second sample of each receiver is taken when the plurality of first emitters is on and the plurality of second emitters is off, and a third sample of each receiver is taken when the plurality of first emitters is off and the plurality of second emitters is on. 
     In certain implementations, the plurality of first emitters and the plurality of second emitters are arranged relative to one another such that the signals emitted by the plurality of first emitters intersect the signals emitted by the plurality of second emitters. The intersection can be along at least a portion of the opening. The plurality of first emitters and the plurality of second emitters can be arranged relative to one another such that the signals emitted by the plurality of first emitters intersect the signals emitted by the plurality of second emitters along a line substantially bisecting the opening. 
     In some implementations, the plurality of first emitters and the plurality of second emitters are spaced relative to one another such that the signals emitted by the plurality of first emitters and the plurality of second emitters span substantially all (e.g., more than 50 percent) of the area of the opening when all of the emitters from each of the plurality of first and second emitters are on. 
     In yet another aspect, a debris monitoring method includes activating and deactivating a first emitter and a second emitter, measuring a first receiver disposed proximate to the first emitter, and detecting movement of debris through the opening. The first emitter and the second emitter are activated to emit respective signals at a substantially constant frequency across an opening defined by a receptacle. The first receiver is disposed proximate to the first emitter to receive a reflected portion of the signal from the first emitter and disposed relative to the second emitter to receive an unreflected portion of the signal from the second emitter. The detection of movement of debris through the opening is based at least in part on a first measurement obtained when the first and second emitters are each deactivated, a second measurement obtained when the first emitter is activated and the second emitter is deactivated, and a third measurement is obtained when the first emitter is deactivated and the second emitter is activated. 
     Implementations of this aspect of the disclosure may include one or more of the following features. In some implementations, detecting movement of debris through the opening includes processing the first, second, and third measurements as a function of time and detecting changes in at least one of the processed second and third measurements. In certain implementations, detecting movement of debris through the opening includes filtering ambient light from the second and third measurements based at least in part on the first measurement. The first, second, and third measurements can be processed as a function of time (e.g., by low pass filtering at least one of the second and third measurements). In some implementations, detecting changes in at least one of the processed second and third measurements includes comparing the instantaneous change to an average value of the respective processed measurement. 
     In certain implementations, the debris monitoring method includes determining the amount of light blocked by debris passing through the opening and periodically assigning a score to the debris based at least in part on the determined amount of light blocked by the debris. Determining the amount of light blocked by debris passing through the opening can be based at least in part the second or third measurement. 
     In some implementations, the debris monitoring method includes summing consecutive debris scores and providing a dirt detection signal if the sum of the debris scores exceeds a threshold value. The sum of the debris score can be decremented over time. The amount of the decrement can be based at least in part on a running average value of the debris scores. 
     In still another aspect, a debris monitoring method including activating and deactivating a first emitter and a second emitter to emit respective signals across an opening defined by a receptacle, measuring a first receiver, and determining whether a receptacle is full of debris. The first receiver is disposed proximate to the first emitter to receive a reflected portion of the signal from the first emitter and disposed relative to the second emitter to receive an unreflected portion of the signal from the second emitter. The determination of whether the receptacle is full of debris is based at least in part on comparing a first reflective signal to a first transmissive signal. The first reflective signal is derived from a measurement by the first receiver when the first emitter is activated and the second emitter is deactivated and first transmissive signal derived from a measurement by the first receiver when the first emitter is deactivated and the second emitter is activated. 
     Implementations of this aspect of the disclosure may include one or more of the following features. In some implementations, determining whether a receptacle is full of debris includes setting a first threshold based at least in part on a comparison of the first reflective signal to the first transmissive signal. The first threshold can be set based in part on the first reflective signal and the first transmissive signal reaching a first crossover point at which the first reflective signal changes from being less than the first transmissive signal to being greater than or equal to the first transmissive signal. The first threshold can be set to a value greater than the value of the first reflective signal at the first crossover point. Additionally or alternatively, the first threshold value can be based at least in part on one or more of the following: the value of the first crossover point and the rate at which the first reflective signal reached the first crossover point. The first threshold can be reset if the first reflective signal falls below the first crossover point after the threshold has been set. 
     In some implementations, the debris monitoring method includes decrementing the threshold over time until the first reflective signal is greater than the first threshold. In certain implementations, the debris monitoring method includes generating a receptacle-full signal if the first reflective signal and the first transmissive signal are each about zero. 
     In some implementations, the debris monitoring method includes measuring a second receiver disposed proximate to the second emitter to receive a reflected portion of the signal from the second emitter and disposed relative to the first emitter to receive an unreflected portion of the signal from the second emitter. Determining whether a receptacle is full of debris can include comparing a second reflective signal, derived from a measurement by the second receiver when the second emitter is activated and the first emitter is deactivated, to a second transmissive signal, derived from a measurement by the second receiver when the second emitter is deactivated and the first emitter is activated. Determining whether a receptacle is full of debris can include setting a second threshold based at least in part on a comparison of the second reflective signal to the second transmissive signal. Additionally or alternatively, the debris monitoring method includes generating a receptacle-full signal if the first and second reflective signals each exceed the respective first and second thresholds. 
     In yet another aspect, a debris monitoring method includes maneuvering an autonomous coverage robot across a cleaning surface, activating and deactivating a first emitter and a second emitter, measuring a first receiver, receiving a signal from the first receiver, detecting movement of the debris through the opening based at least in part on the received signal, and determining whether a receptacle is full of debris based at least in part on the received signal. The robot carries a cleaning assembly and the receptacle arranged relative to the cleaning assembly to receive debris removed from the cleaning surface by the cleaning assembly. The first and second emitter are activated and deactivated to emit respective signals across an opening defined by the receptacle. The first receiver is disposed proximate to the first emitter to receive a reflected portion of the signal from the first emitter and disposed relative to the second emitter to receive an unreflected portion of the signal from the second emitter. Receiving the signal from the first receiver includes receiving a dark signal derived from a measurement by the first receiver when the first emitter is deactivated and the second emitter is deactivated, receiving a reflective signal derived from a measurement by the first receiver when the first emitter is activated and the second emitter is deactivated, and receiving a transmissive signal derived from a measurement by the first receiver when the first emitter is deactivated and the second emitter is activated. Detecting movement of debris through the opening is based at least in part on the dark signal, the reflective signal, and the transmissive signal, and determining whether a receptacle is full of debris is based at least in part on the reflective signal and the transmissive signal. 
     Implementations of this aspect of the disclosure may include one or more of the following features. In some implementations, movement of the robot is altered based at least in part upon detecting movement of debris through the opening. Altering movement of the robot can include initiating a spot coverage cleaning pattern. For example, initiating a spot coverage cleaning pattern can include immediately altering the direction of travel of the robot toward the detected debris. The spot coverage pattern can include one or more of the following: a spiral pattern, a star pattern, and a cornrow pattern. In some implementations, at least one dimension of the spot coverage pattern is at least partly based on a change in detected movement of debris through the opening. Additionally or alternatively, altering movement of the robot includes changing at least one of the following: direction of travel of the robot and speed of travel of the robot. 
     In certain implementations, the debris monitoring method includes altering movement of the robot based at least in part upon determining that the receptacle is full of debris. Altering movement of the robot can include moving the robot toward an evacuation station configured to engage the receptacle. In some implementations, the debris monitoring method includes deactivating the cleaning assembly based at least in part upon determining that the receptacle is full of debris. 
     In still another aspect, an autonomous coverage robot includes a robot body having a forward portion and a rear portion, right and left driven wheels, a debris agitator carried by the robot body, a first and a second cliff sensor, and a controller in communication with the right and left driven wheels and the first and second cliff sensors. The right and left driven wheels define a transverse axis between the forward portion and the rear portion of the robot body and each driven wheel is rotatable about the transverse axis. The debris agitator is configured to remove debris from the cleaning surface. The first cliff sensor is disposed forward of the transverse axis and the second cliff sensor is disposed rear of the transverse axis. The controller is configured to alter the direction of travel of the robot based at least in part on signals received from the first and second cliff sensors. 
     Implementations of this aspect of the disclosure may include one or more of the following features. In some implementations, comprising a waste receptacle carried by the robot body and in fluid communication with the debris agitator to receive the debris removed from the cleaning surface. At least a portion of the waste receptacle can be disposed within the robot body. Additionally or alternatively, the waste receptacle can be carried on the rear portion of the robot body. 
     In certain implementations, the waste receptacle is releasably engageable with the robot body and the second cliff sensor is disposed on the waste receptacle. The controller can be in wireless communication with the second cliff sensor, and this wireless communication can include one or more of the following: optical communication, electromagnetic communication, and radiofrequency communication. 
     In some implementations, a first electrical contact is disposed on the waste receptacle and a second electrical contact is carried on the robot body, wherein the first electrical contact is releasably engageable with the second electrical contact to establish electrical communication between the second cliff sensor and the controller. The controller can be configured to disable the right and left driven wheels if communication with the second cliff sensor is interrupted. 
     In certain implementations, the autonomous coverage robot includes a third cliff sensor disposed rear of the transverse axis. The third cliff sensor can be proximate to the waste receptacle. Additionally or alternatively, the second cliff sensor is proximate to the waste receptacle. 
     In some implementations, the first cliff sensor and the second cliff sensor define a fore-aft axis substantially perpendicular to the transverse axis. In certain implementations, the debris agitator extends substantially parallel to the transverse axis. 
     In another aspect, a waste receptacle for an autonomous coverage robot for removing debris from a cleaning surface includes a housing releasably engageable with a robot body of the autonomous coverage robot and a cliff sensor supported on the housing. The housing defines a volume for containing debris, and the housing defines an opening for receiving debris removed from the cleaning surface. The cliff sensor is arranged to detect a potential cliff while the housing is releasably engaged with the robot body and the robot removes debris from the cleaning surface. 
     Implementations of this aspect of the disclosure may include one or more of the following features. In some implementations, at least a portion of the housing defines at least a portion of a perimeter of the autonomous coverage robot while the housing is releasably engaged with the robot body. Additionally or alternatively, at least a portion of the housing defines at least a portion of a surface of the autonomous coverage robot substantially opposite the cleaning surface when the robot removes debris from the cleaning surface. In some implementations, at least a portion of the housing defines at least a portion of a surface of the autonomous coverage robot substantially perpendicular to the cleaning while the robot removes debris from the cleaning surface. 
     In certain implementations, the cliff sensor is supported on the portion of the housing defining at least a portion of the perimeter of the autonomous coverage robot. The housing can have a substantially arcuate portion and the cliff sensor can be disposed along the substantially arcuate portion. The substantially arcuate portion can be opposite the opening for receiving debris removed from the cleaning surface. 
     In some implementations, the housing has a dimension of less than about ten inches in a direction substantially perpendicular to the cleaning surface when the housing is releasably engaged with the robot body and the robot removes debris from the cleaning surface. 
     In certain implementations, an electrical contact is supported on the housing and in electrical communication with the cliff sensor, the electrical contact configured for releasable engagement with an electrical contact supported on the robot body. In some implementations, an optical emitter supported on the housing and in electrical communication with the cliff sensor, the optical emitter configured for optical communication with an optical receiver supported on the robot body. 
     In another aspect, a method of maneuvering an autonomous coverage robot includes receiving a signal from a first cliff sensor, receiving a signal from a second cliff sensor, and driving the right and left driven wheels to move the robot in a direction substantially opposite a detected potential cliff. The first cliff sensor is arranged to detect a potential cliff forward of a transverse axis defined by right and left driven wheels of the robot. The transverse axis is substantially perpendicular to the fore-aft direction of travel of the robot. The second cliff sensor is arranged to detect a potential cliff aft of the transverse axis. 
     Implementations of this aspect of the disclosure may include one or more of the following features. In some implementations, receiving the signal from the second cliff sensor includes receiving a wireless signal from the second cliff sensor. Additionally or alternatively, receiving the signal from the second cliff sensor includes receiving at least a portion of the signal through a releasably engageable electrical contact. 
     In certain implementations, the first cliff sensor is disposed along the substantially forward-most portion of the robot and the second cliff sensor is disposed along the substantially rear-most portion of the robot. In some implementations, whether the second cliff sensor is present is determined and the right and left driven wheels are disabled if the second cliff detector is not present. In certain implementations, driving the right and left driven wheels to move the robot in a direction substantially opposite a detected potential cliff includes moving the robot a distance greater than the distance between the right and left drive wheels along the transverse axis. 
     In yet another aspect, a method of operating an autonomous cleaning apparatus includes controlling a drive system of the cleaning apparatus to move the cleaning apparatus over a cleaning surface, receiving a signal from a debris sensor of the cleaning apparatus, and moving the cleaning apparatus through a pattern of movement based at least in part on the received debris signal. The signal from the debris sensor indicates that the cleaning apparatus is collecting debris. The pattern of movement includes a plurality of swaths. 
     Implementations of this aspect of the disclosure may include one or more of the following features. In some implementations, each of the plurality of swaths is substantially parallel to one another. In certain implementations, each of the plurality of swaths extends from a central region in a star pattern. The central region can be an area of the cleaning surface corresponding substantially to a local maximum of the received debris signal. The star pattern can radiate through an angle of about 360 degrees. 
     In certain implementations, at least a portion of at least some of the plurality of swaths overlap one another. In some implementations, the amount of overlap between swaths can be adjusted based at least in part on the magnitude of the debris signal. Additionally or alternatively, the number of swaths can be based at least in part on the signal from the debris sensor. In certain implementations, adjusting the number of swaths includes adjusting the number of swaths in proportion the magnitude of the debris signal. 
     In some implementations, the length of each swath is adjusted based at least in part on the signal from the debris sensor. Additionally or alternatively, each swath can be terminated when the debris signal falls below a threshold. In certain implementations, the debris sensor is an optical sensor disposed in a cleaning pathway of the cleaning apparatus. The debris sensor can include an optical sensor disposed on a waste receptacle releasably engageable with the cleaning apparatus. Additionally or alternatively, debris sensor comprises a piezoelectric sensor element. 
     In another aspect, a method of operating an autonomous cleaning apparatus includes controlling a drive system of the cleaning apparatus to move the cleaning apparatus over a cleaning surface, receiving a signal from a debris sensor of the cleaning apparatus, moving the cleaning apparatus along the heading in the direction of the detected debris. The signal corresponds to a heading in the direction of detected debris. 
     Implementations of this aspect of the disclosure may include one or more of the following features. In some implementations, the debris sensor includes a camera directed substantially forward of the cleaning apparatus. In certain implementations, the camera is movable to scan an area substantially forward of the cleaning apparatus. Additionally or alternatively, the size of the debris is determined and the cleaning apparatus is moved away from debris larger than a threshold size. 
     In another aspect, a method of navigating an autonomous coverage robot includes maneuvering an autonomous coverage robot over a surface, detecting a first change in a signal emitted from a maintenance station configured to receive the autonomous coverage robot, detecting a second change in the signal from the maintenance station, and determining the probability that the robot will find the maintenance station in a period of time. The determined probability is based at least in part on an elapsed time between the detected first change in the signal and the detected second change in the signal. 
     Implementations of this aspect of the disclosure may include one or more of the following features. In some implementations, determining the probability that the robot will find the maintenance station in the period of time includes updating a probability distribution based at least in part on the elapsed time. The probability distribution can be a non-parametric model (e.g., a histogram). Additionally or alternatively, the probability distribution can be a parametric model, such as a Poisson distribution in which the mean of the Poisson distribution is estimated (e.g., as an average). 
     In some implementations, the method of navigating an autonomous coverage robot further includes determining the probability that power available from a battery carried by the robot will be depleted before the robot can find the maintenance station. In certain implementations, a period of time is allotted for finding the maintenance station. The allotted period of time can be based at least in part on the determined probability that the robot will find the maintenance station in the allotted period of time. In some examples, the power to the robot is reduced during the allotted period of time. For example, reducing the power can include reducing power to a cleaning assembly carried by the robot. 
     In certain implementations, the method of navigating an autonomous coverage robot further includes detecting whether the robot has been removed from the surface and ignoring the detected first change in the signal occurring just prior to detection that the robot has been removed from the surface and ignoring the detected second change in the signal occurring just after detection that the robot has been removed from the surface. For example, detecting that the robot has been removed from the surface can include receiving a signal from one or more sensors (e.g., wheel drop sensors and/or cliff detectors) carried by the robot. 
     In certain implementations, releasable contact between the robot and the maintenance station is established. Upon establishing releasable contact between the robot and the maintenance station, a battery carried by the robot can be charged. 
     In yet another aspect, a method of navigating an autonomous coverage robot includes maneuvering an autonomous coverage robot over a surface, detecting a first structure disposed along the surface, detecting a second structure disposed along the surface, determining the probability that the robot will find the first structure in a period of time, wherein the determined probability is based at least in part on detecting the second structure and an elapsed time between the detecting the first structure and detecting the second structure. 
     Implementations of this aspect of the disclosure may include one or more of the following features. The first structure can be a maintenance station configured to receive the robot and the second structure is a lighthouse. Additionally or alternatively, the first structure can be a first lighthouse and the second structure is a second lighthouse. 
     In still another aspect, a system includes a maintenance station and an autonomous coverage robot. The maintenance station includes an emitter for emitting a signal. The autonomous coverage robot is configured to maneuver over a surface and includes at least one receiver for receiving the emitted signal and a controller. The controller configured to maneuver the robot across the surface, detect a first change in a signal emitted from the maintenance station and received by the at least one receiver, detect a second change in the signal from the maintenance station and received by the at least one receiver and determine the probability that the robot will find the maintenance station in a period of time. The determined probability is based at least in part on an elapsed time between the detected first change in the signal and the detected second change in the signal. 
     Implementations of this aspect of the disclosure may include one or more of the following features. The emitter can include an infrared emitter and the at least one receiver comprises an infrared receiver. In some implementations, the autonomous coverage robot further includes a battery. The maintenance station can be configured to releasably engage the autonomous coverage robot to transfer power to the battery. 
     In yet another aspect, a method of calibrating a debris monitoring system of a waste receptacle includes detecting an initiation condition, applying a first pulse width modulation duty cycle to an emitter array, measuring a first signal at a receiver in response to the first pulse width modulation cycle, applying a second pulse width modulation duty cycle to an emitter array, measuring a second signal at the receiver in response to the second pulse width modulation duty cycle, determining whether the difference between the measured first signal and the measured signal is greater than the threshold, and setting the measured second signal as a base brightness based at least in part on the determination of whether the difference between the measured first signal and the measured second signal is greater than the threshold. The second pulse width modulation duty cycle is less than the first pulse width modulation duty cycle. 
     Implementations of this aspect of the disclosure may include one or more of the following features. Detecting the initiation condition can include detecting insertion of the waste receptacle into the body of a debris collection device (e.g., an autonomous cleaning robot). Additionally or alternatively, detecting the initiation condition can include detecting applied power (e.g., detecting insertion of a battery and/or the position of a power switch). In some implementations, an indicator is activated based at least in part on detecting the initiation condition. 
     In certain implementations, the indicator is deactivated based at least in part on whether the difference between the measured first signal and the measured second signal is greater than the threshold. 
     In some implementations, activating and/or deactivating the indicator includes activating and/or deactivating one or more light-emitting diodes. 
     In certain implementations, applying the first pulse width modulation duty cycle to the emitter array includes applying a maximum pulse width modulation duty cycle to the emitter array. 
     In some implementations, the waste receptacle defines an opening to receive debris into the waste receptacle. The first emitter array can be arranged to emit a signal across at least a portion of the opening. Measuring the first and second signals at the receiver can each include receiving an unreflected portion of the signal emitted by the first emitter. Additionally or alternatively, measuring the first and second signals at the receiver can each include receiving a reflected portion of the signal emitted by the first emitter. 
     In certain implementations, applying the second pulse width modulation duty cycle to the emitter array includes determining whether the applied second pulse width modulation is greater than a limit value. 
     In still another aspect, a debris monitoring system includes a receptacle, a plurality of first emitters and a plurality of second emitters, a first receiver, and a second receiver. The receptacle includes a barrier extending horizontally across a width of the receptacle and extending vertically along at least a portion of a height of the receptacle, the barrier defining at least a portion of an opening to receive debris into the receptacle. The first emitters are vertically spaced apart from one another on a first side of the opening, and the second emitters are vertically spaced apart from one another on a second side of the opening. The emitters of the first and second emitters are arranged to emit a signals span the horizontal and vertical dimensions of the opening. The first receiver is proximate to the plurality of first emitters. The second receiver is proximate to the plurality of second emitters. 
     In some implementations, the at least a portion of the barrier is a door movable to allow access to debris stored in the receptacle. For example, the barrier can include a hinged door. Additionally or alternatively, the barrier can include a slidable door. 
     In certain implementations, a vertical dimension of the opening is substantially ½ or less of the combined height of the receptacle (e.g., substantially ½ or less of the combined height of the barrier and a vertical dimension of the opening). 
     In some implementations, a width of the opening can be about ⅔ or less of a width of the receptacle. In these implementations, the barrier can extend substantially across the entire width of the receptacle. Thus, for example, the width of the barrier can be at least ⅓ greater than the width of the opening. 
     The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG. 1A  is a top view of an autonomous robotic cleaner. 
         FIG. 1B  is a bottom view of an autonomous robotic cleaner. 
         FIG. 1C  is a side view of an autonomous robotic cleaner. 
         FIG. 2  is a block diagram of systems of an autonomous robotic cleaner. 
         FIGS. 3A-3B  are top views of autonomous robotic cleaners. 
         FIG. 3C  is a rear perspective view of an autonomous robotic cleaner. 
         FIGS. 3D-3E  are bottom views of autonomous robotic cleaners. 
         FIGS. 3F-3G  are perspective views of an autonomous robotic cleaner. 
         FIGS. 4A-4B  are perspective views of removable cleaning bins. 
         FIGS. 4C-4E  are schematic views an autonomous robotic cleaner. 
         FIG. 5A  is a top view of an autonomous robotic cleaner. 
         FIG. 5B  is a top view of a bin sensor brush. 
         FIGS. 6A-6C  are schematic views of autonomous robotic cleaners. 
         FIGS. 7A-7B  are front views of removable cleaning bins. 
         FIGS. 7C-7E  are perspective views of removable cleaning bins. 
         FIGS. 7F-7H  are front views of removable cleaning bins. 
         FIGS. 8A-8E  are front views of removable cleaning bins. 
         FIG. 9A  is process flow chart of a debris monitoring routine. 
         FIG. 9B  is a process flow chart of a debris quantifying routine. 
         FIG. 9C  is a process flow chart of a bin-full detection routine. 
         FIG. 9D  is a process flow chart of a threshold setting routine. 
         FIG. 9E  is a process flow chart of a calibration routine 
         FIG. 10A  is a schematic of a robot cleaning pattern. 
         FIG. 10B  is a schematic of a robot cleaning pattern. 
         FIG. 11  is a perspective view of a robot. 
         FIGS. 12A-12B  are schematic views of autonomous robotic cleaners. 
         FIG. 13A  is a perspective view of a cleaning bin. 
         FIGS. 13B-13D  are schematic views of cleaning bin indicators. 
         FIG. 14A  is a schematic view of a cleaning bin indicator system. 
         FIGS. 14B-14C  are schematic views of remote cleaning bin indicators. 
         FIG. 14D  is a schematic view of an autonomous robotic cleaner and an evacuation station. 
         FIG. 15A  is a schematic view of an autonomous robotic cleaner and an evacuation station. 
         FIG. 15B  is a schematic view of an autonomous robotic cleaner moving relative to an evacuation station. 
         FIG. 16  is a process flow chart of a seeking routine. 
         FIG. 17  is a schematic view of an autonomous robotic cleaner moving relative to an evacuation and a second structure. 
         FIG. 18  is a process flow chart of a seeking routine. 
         FIG. 19A  is a partially exploded, top perspective view of an autonomous robotic cleaner. 
         FIG. 19B  is a partially exploded, bottom perspective view of the autonomous robotic cleaner of  FIG. 19A . 
         FIG. 19C  is a cross-sectional front view of the autonomous robotic cleaner of  FIG. 19A  in an unexploded configuration, taken along the line  19 C- 19 C. 
         FIG. 19D  is a perspective view of the dust bin of the autonomous robotic cleaner of  FIG. 19A . 
         FIG. 19E  is a side view of the dust bin of the autonomous robotic cleaner of  FIG. 19A . 
         FIG. 19F  is a cross-section of the dust bin of the autonomous robotic cleaner of  FIG. 19A , taken along the line  19 F- 19 F. 
         FIG. 19G  is a cross-section of the dust bin of the autonomous robotic cleaner of  FIG. 19A , taken along the line  19 G- 19 G. 
     
    
    
     Like reference symbols in the various drawings indicate like elements. 
     DETAILED DESCRIPTION 
     Referring to  FIGS. 1A-1C , an autonomous robotic cleaner  11  includes a robot body  31  (e.g., a chassis and/or housing) which carries an outer shell  6  connected to a bumper  5 . The robot body  31  also carries a control panel  10  and an omnidirectional receiver  15 , which has a 360 degree line of vision for detection of signals emitted towards the robot  11  from substantially all directions. 
     Referring to  FIG. 1B , installed along either side of the robot body  31  are differentially driven wheels  45 , each rotatable about a transverse axis, to mobilize the robot  11  and provide two points of support. The differentially driven wheels  45  may move the robot  11  in forward and reverse drive directions such that the robot body  31  has a corresponding forward portion  31 A forward of the differentially driven wheels  45  and a rear portion  31 B rear of the differentially driven wheels  45 . 
     Cliff sensors  30 A (e.g., infrared sensors) are installed on the underside of the robot  11 , along the forward portion  31 A of the robot body  31 , to detect a potential cliff forward of the robot  11  as the robot  11  moves in the forward drive direction. Cliff sensors  30 B are installed on the underside of the robot  11 , along the rear portion  31 B of the robot body  31 , to detect a potential cliff rear of the robot  11  as the robot  11  moves in the reverse drive direction. At least one of the cliff sensors  30 B is disposed on a debris bin  50  in fluid communication with a cleaning head  40  to receive debris removed from a cleaning surface. The cliff sensor  30 B disposed on the cleaning bin  50  can be in communication with one or more components on the robot body  31  and/or powered by a source on the robot body  31  through a communication and/or power channel, each described below, established between the cleaning bin  50  and the robot body  31 . The cliff sensors  30 A,B are configured to detect sudden changes in floor characteristics indicative of an edge or cliff of the floor (e.g. an edge of a stair). As described in further detail below, cliff sensors  30 A and  30 B can facilitate execution of a cleaning pattern including back and forth motion of the robot  11  over an area containing debris. For example, cliff sensors  30 A,B disposed forward and rear of the robot  11  can reduce the likelihood that the robot  11  would move over a cliff forward or rearward of the robot  11  as the robot moves back and forth during execution of a cleaning pattern. 
     The forward portion  31 A of the chassis  31  includes a caster wheel  35  which provides additional support for the robot  11  as a third point of contact with the floor and does not hinder robot mobility. Located proximate to and on either side of the caster wheel  35  are two wheel-floor proximity sensors  70 . The wheel-floor proximity sensors  70  are configured to detect sudden changes in floor characteristics indicative of an edge or cliff of the floor (e.g. an edge of a stair). The wheel-floor proximity sensors  70  provide redundancy should the primary cliff sensors  30 A fail to detect an edge or cliff. In some implementations, the wheel-floor proximity sensors  70  are not included, while the primary cliff sensors  30 A remain installed along the bottom forward portion  31 A of the chassis  31 . In certain implementations, the caster wheel  35  is not included and additional support for the robot  11  is provided by at least a portion of the cleaning head assembly described in detail below. 
     A cleaning head assembly  40  is disposed generally between the forward portion  31 A and the rear portion  31 B of the robot  11 , with at least a portion of the cleaning head assembly disposed within the robot body  31 . The cleaning head assembly  40  includes a main  65  brush and a secondary brush  60 . A battery  25  is carried on the robot body  31  and, in some implementations, is proximate the cleaning head assembly  40 . In some examples, the main  65  and/or the secondary brush  60  are removable. In other examples, the cleaning head assembly  40  includes a fixed main brush  65  and/or secondary brush  60 , where fixed refers to a brush permanently installed on the robot body  31 . 
     A side brush  20  is supported on one side of the robot body  31  such at least a portion of the side brush  20  extends beyond the robot body  31 . In some implementations, the side brush  20  is configured to rotate 360 degrees, about an axis substantially perpendicular to the cleaning surface, when the robot  11  is operational. The rotation of the side brush  20  may improve cleaning in areas adjacent the robot&#39;s side, and areas (e.g., corners) otherwise unreachable by the more centrally located cleaning head assembly  40 . 
     A removable cleaning bin  50  is supported towards the back end  31 B of the robot  11 , with at least a portion of the removable cleaning bin disposed within the outer shell  6 . In certain implementations, the cleaning bin  50  is removable from the chassis  31  to provide access to bin contents and an internal filter  54 . Additionally or alternatively, access to the cleaning bin  50  may be provided via an evacuation port  80 , as shown in  FIG. 1C . In some implementations, the evacuation port  80  includes a set of sliding side panels  55  which slide along a side wall of the chassis  31  and under side panels of the outer shell  6  to open the evacuation port  80 . The evacuation port  80  is configured to mate with corresponding evacuation ports on a maintenance station or other device configured to evacuate debris from the bin  50 . In other implementations, the evacuation port  80  is installed along an edge of the outer shell  6 , on a top most portion of the outer shell  6 , on the bottom of the robot body  31 , or other similar placements where the evacuation port  80  has ready access to the contents of the cleaning bin  50 . 
       FIG. 2  is a block diagram of systems included within the robot  11 . The robot  11  includes a microprocessor  245  capable of executing routines and generating and sending control signals to actuators within the robot  11 . Connected to the microprocessor  245  is memory  225  for storing routines and sensor input and output, a power assembly  220  (e.g., a battery and/or a plurality of amplifiers able to generate and distribute power to the microprocessor  245 ), and other components included within the robot  11 . A data module  240  is connected to the microprocessor  245  which may include ROM, RAM, an EEPROM or Flash memory. The data module  240  may store values generated within the robot  11  or to upload new software routines or values to the robot  11 . 
     The microprocessor  245  is connected to a plurality of assemblies and systems, one of which is the communication system  205  including an RS-232 transceiver, radio, Ethernet, and wireless communicators. The drive assembly  210  is connected to the microprocessor  245  and includes right and left differentially driven wheels  45 , right and left wheel motors, and wheel encoders. The drive assembly  210  is operable to receive commands from the microprocessor  245  and generate sensor data transmitted back to the microprocessor  245  via the communication system  205 . A separate caster wheel assembly  230  is connected to the microprocessor  245  and includes a caster wheel  35  and a wheel encoder. The cleaning assembly  215  is connected to the microprocessor  245  and includes a primary brush  65 , a secondary brush  60 , a side brush  20 , and brush motors associated with each brush. Also connected to the microprocessor is the sensor assembly  235  which may include infrared proximity sensors  75 , an omnidirectional detector  15 , mechanical switches installed in the bumper  5 , wheel-floor proximity sensors  70 , stasis sensors, a gyroscope  71 , and infrared cliff sensors  30 . 
     Referring to  FIGS. 3A-3E , example locations of the cleaning bin  50  and a filter  54  disposed on the chassis  31  and the outer shell  6  are shown.  FIG. 3A  displays a robot  300 A with an evacuation port  305  disposed on the top of the robot  300 A, and more specifically installed on the top of a cleaning bin  310 A. The cleaning bin  310 A may or may not be removable from the chassis  31  and outer shell  6 , and if removable, is removable such that the bin  310 A separates from (e.g., is releasably engageable with) a back portion  312 A of the robot  300 A. 
     Referring to  FIG. 3B , a cleaning bin  310 B is installed towards the rear portion of a robot  300 B and includes a latch  315 . In some implementations, a portion of the cleaning bin  310 B slides toward the forward portion of the robot  310 B when the latch  315  is manipulated, providing access to the contents of the cleaning bin  310 B for removal. Additionally or alternatively, the cleaning bin  310 B is removable from a back portion  312 B of the robot  310 B to provide access to the contents of the cleaning bin  310 B for removal and/or to provide access to a filter (e.g., filter  54 ) disposed substantially within the cleaning bin  310 B. In this implementation, the cleaning bin latch  315  may be manipulated manually by the operator or autonomously by a robotically driven manipulator. 
     Referring to  FIG. 3C , a robot  300 C including a cleaning bin  310 C located on a rearmost side wall  320  of the outer shell  6 . The cleaning bin  310 C has a set of movable doors  350 , each slidable along the side of the robot body  31  and each recessable under the outer shell  6 . In some implementations, with the doors  350  recessed under the outer shell  6 , the cleaning bin  310 C is configured to accept and mate with an external evacuation port. 
       FIG. 3D  provides a bottom view of a robot  300 D and the bottom of the cleaning bin  310 D located on the bottom, rear portion of the robot  300 D. The cleaning bin  310 D has a latch  370  allowing a door  365  located on the bottom of cleaning bin  310 D to slide towards the forward portion of the robot  300 D so that contents of the cleaning bin  310 D may be removed. In certain implementations, the cleaning bin  310 D supports a filter (e.g., filter  54  shown in  FIG. 1C ) and the cleaning bin  310 D is removable from a back portion  312 D of the robot  300 D to facilitate cleaning and/or replacing the filter. The cleaning bin  310 D and latch  370  may be manipulated manually by an operator or autonomously by a robotically driven manipulator. 
       FIG. 3E  provides a bottom view of a robot  300 E and the floor of the cleaning bin  310 E located on the bottom, rear portion of the robot  300 E. The cleaning bin  310 E includes a port  380  for accessing contents of the cleaning bin  310 E. An evacuation hose may be attached to the port  380  to evacuate the cleaning bin  310 E. In certain implementations, the cleaning bin  310 E is removable from a back portion  312 E of the robot  300 D to access and clean a filter disposed within the cleaning bin  310  (e.g., filter  54  shown in  FIG. 1C ). 
     Referring to  FIG. 3F , a robot  300 F includes a cleaning bin  310 F disposed along a rear robot portion  312 F. In some implementations, the cleaning bin  310 F includes at least one evacuation port  380  on a rear side (three are shown). The evacuation ports  380  may be configured to receive an evacuation hose for removing debris from the bin  310 F. Additionally or alternatively, the evacuation ports  380  may be configured to facilitate manual removal of debris (e.g., by holding the bin  310 F to allow debris within the bin to fall out of the bin under the force of gravity). 
     Referring to  FIG. 3G , a robot  300 G includes a cleaning bin  310 G located on a rear robot portion  312 G The cleaning bin  310 G includes one or more evacuation ports  380  on a side portion (e.g. left and/or right sides). The evacuation ports  380  are configured to receive an evacuation hose for removing debris from the bin  310 G. 
     The robotic cleaner  11  may receive a number of different cleaning bins  50 . For example, referring to  FIG. 4A , a cleaning bin  400 A is configured to mate with external vacuum evacuation ports. The vacuum bin  400 A defines a main chamber  405 A having a sloped floor  410 A that aids movement of debris towards evacuation ports  415 ,  420 ,  425 . A first side evacuation port  415  is located adjacent a center evacuation port  420  which is located between the first side evacuation port  415  and a second side evacuation port  425 . Located on the side walls of the bin  400 A are two evacuation outlets  430  that are installed to further aid a vacuum in its evacuation operation. 
     Referring to  FIG. 4B , a bin  400 B includes teeth  450  along a mouth edge  452  of the bin  400 B. The teeth  450  reduce the amount of filament build up on the main brush  60  and/or the secondary brush  65  (see  FIG. 1B ) by placing the bin  400 B close enough to the brush  60 ,  65  such that the teeth  492  slide under filament build up on the brush  60 ,  65  and pull off filament build up as the brush  60 ,  65  rotates. In some examples, the bin  400 B includes between about 24-36 teeth. In the example shown, the bin  400 B defines a sweeper bin portion  460  and a vacuum bin portion  465 . The comb or teeth  450  are positioned between the sweeper bin portion  460  and the vacuum bin portion  465  and arranged to lightly comb the sweeper brush  60  as the sweeper brush  60  rotates. The comb or teeth  450  remove errant filaments from the sweeper brush  60  that accumulate either on the teeth  450  or in the sweeper bin portion  460 . The vacuum bin portion  465  and the teeth  450  above it do not interfere with each other. The bin  400 B carries a vacuum assembly  480  (e.g. a vacuum motor/fan) configured to draw debris through a channel such as the channel defined a pair of squeegees  470 A and  470 B in the vacuum bin portion  460 . 
     The bin  400 B includes electrical contacts  482 A,  482 B, which are releasably engageable with corresponding electrical contacts on the robot body  31  such that power is supplied to the bin  400 B when the bin  400 B is engaged with the robot body  31 . In some implementations, the power is provided to the vacuum assembly  480 . In certain implementations, the electrical contacts  482 A,  482 B can provide communication to a bin microprocessor  217 . The filter  54  (shown in  FIG. 1C ) can separate the vacuum bin portion  460  from the vacuum assembly  480 . In some examples, the filter  54  pivots open along a side, top, or bottom edge for servicing. In other examples, the filter  54  slides out of the vacuum bin portion  460 . 
     In some instances, the bin  50  includes a bin-full detection system for sensing an amount of debris present in the bin  50 . For example, referring to  FIGS. 5A-5B , the bin-full detection system includes an emitter  755  and a detector  760  housed in the bin  50 . A housing  757  surrounds each of the emitter  755  and the detector  760  and is substantially free from debris when the bin  50  is also free of debris. In one implementation, the bin  50  is detachably connected to the robotic cleaner  11  and includes a brush assembly  770  for removing debris and soot from the surface of the emitter/detector housing  757 . The brush assembly  770  includes a brush  772  mounted on the robot body  31  and configured to sweep against the emitter/detector housing  757  when the bin  50  is removed from or attached to the robot  11 . The brush  772  includes a cleaning head  774  (e.g. bristles or sponge) at a distal end farthest from the robot  11  and a window section  776  positioned toward a base of the brush  772  and aligned with the emitter  755  or detector  760  when the bin  50  is attached to the robot  11 . The emitter  755  transmits and the detector  760  receives light through the window  776 . In addition to brushing debris away from the emitter  755  and detector  760 , the cleaning head  774  reduces the amount of debris or dust reaching the emitter  755  and detector  760  when the bin  50  is attached to the robot  11 . In some examples, the window  776  comprises a transparent or translucent material and is formed integrally with the cleaning head  774 . In some examples, the emitter  755  and the detector  760  are mounted on the chassis  31  of the robot  11  and the cleaning head  774  and/or window  776  are mounted on the bin  50 . 
     Referring to  FIG. 6A , in some implementations a sweeper robot  11  includes a brush  60  and a flap  65  that sweep or otherwise agitate debris from the cleaning surface for movement into a bin  700 A having an emitter  755  and a detector  760 , each positioned near a bin mouth  701  (e.g., an opening defined by the bin  700 A). 
     Referring to  FIG. 6B , in certain implementations a bin  700 B includes a vacuum/blower motor  780 , and an emitter  755  and a detector  760  located near an inlet  782  of a vacuum flow path into the bin  700 B. The robot body  31  of the robot  11  includes a robot vacuum outlet  784  that engages (e.g., fits flush with) the vacuum inlet  782  of the bin  700 B. By placing the emitter  755  and the detector  760  near the debris inlet  782 , debris can be detected along the intake flow path rather than within the debris chamber  785 . Therefore, a bin-full condition may be triggered when either the amount of debris swept or vacuumed along the flow path is extremely high (which may typically be a rare scenario), or when the debris chamber  785  is full (e.g. debris is no longer deposited therein, but instead backs up along the intake flow path near the inlet  782 ). 
     Referring to  FIG. 6C , in some implementations, a combined vacuum/sweeper bin  700 C includes an emitter  755  and a detector  760  pair positioned near a sweeper bin inlet  782 A and a vacuum bin inlet  782 B. The emitter  755  and detector  760  mounted near the sweeper bin inlet  782 A are supported on the robot body  31  of the robot  11 . Additionally or alternatively, the inlet sensors  755 ,  760 , several emitter arrays  788  are positioned on an interior surface of the bin  700 C (e.g., a bottom interior surface of the bin  700 C) and one more detectors  760  are positioned on a substantially opposite interior surface of the bin  700 C (e.g., a top interior surface of the bin  700 C). As described in further detail below, signals from the detectors  760  located along the intake flow path, as well as the container of the bin  700 C, may be compared for detecting the presence of debris and/or for determining bin fullness. For example, when a heavy volume of debris is pulled into the bin  700 C by the brush  60 , flapper  65 , and/or vacuum motor  780 , the detectors  760  located along the flow path may generate a low detection signal. However, detectors  760  located on the top interior surface of the bin  700 D will not detect a full bin  700 C, if it is not yet full. Comparison of the detector signals avoids a false bin-full condition. 
       FIGS. 7A-7E  illustrate a transmissive optical debris-sensing system for detecting debris within the bin  50 . As shown in  FIG. 7A , in some examples, the bin  50  includes emitters  755  located on a bottom interior surface  51  of the bin  50  and detectors  760  located on an upper interior surface  52  of the bin  50 . The emitters  755  emit light that traverses the interior of the bin  50  and which may be detected by the detectors  760 . When the interior of the bin  50  is clear of debris, the transmitted light from the emitters  755  produces a relatively high signal strength in the detectors  760 , because very little of the transmitted light is diverted or deflected away from the detectors  760  as the transmitted light passes through the empty interior of the bin  50 . By contrast, when the interior of the bin  50  contains debris, at least some of the light transmitted from the emitters  755  is absorbed, reflected, or diverted as the light strikes the debris, such that a lower proportion of the emitted light reaches the detectors  760 . The degree of diversion or deflection caused by the debris in the interior of the bin  50  correlates positively with the amount of debris within the bin  50 . 
     By comparing the signals generated by the detectors  760  when the bin  50  does not contain debris to subsequent signal readings obtained by the detectors  760  as the robot  11  sweeps and vacuums debris into the bin  50  during a cleaning cycle, the presence of debris within the bin  50  may be determined. For example, when the subsequently polled detector signals are compared to initial detector signals (e.g., signals taken when the bin  50  is substantially empty), a determination can be made whether the debris accumulated within the bin  50  has reached a level sufficient to trigger a bin-full condition. 
     One example bin configuration includes one emitter  755  and two detectors  760 . Another configuration includes positioning one or more emitters  755  and detectors  760  in the bin  51  and cross-directed in mutually orthogonal directions. The robot  11  may determine that heavy debris has accumulated on the bottom of the bin  50  but has not filled the bin  50 , when signals generated by a first detector  760  on the inner top surface  52  is relatively low and signals generated by a second detector  760  on an inner side wall (which detects horizontally-transmitted light) does not meet a bin-full threshold. Additionally or alternatively, when both detectors  760  report a relatively low received-light signal, it may be determined that the bin  50  is full. 
     Referring to  FIG. 7B , in some implementations, the bin  50  includes a detector  760  proximate a calibration emitter  805 , both disposed behind a shield  801  on the top interior surface  52  of the bin  50 . An emitter  755  is disposed on the bottom interior surface  51  of the bin  50 . A calibration signal reading is obtained by emitting light from the calibration emitter  805  which is then detected by the detector  760  as a first reading. The translucent or transparent shield  801  prevents emission interfere between the transmission of light from the calibration emitter  805  to the detector  760  with dust or debris from the bin  50 . The emitter  755  then transmits light across the interior of the bin  50  and the detector  760  takes a second reading of received light. By comparing the second reading to the first reading, a determination may be made whether the bin  50  is full of debris. In some examples, the robot  11  includes sensors  755 ,  760  positioned along a debris flow path prior to a mouth  53  of the bin  50 . The bin full sensors  755 ,  760  may detect debris tending to escape from the bin  50 . 
     Referring to  FIG. 7C , in some implementations, the bin  50  includes two emitter arrays  788  and two detectors  760 . Each emitter array  788  may include several light sources. The light sources may each emit light frequencies that differ from one another within the same emitter arrays  788 . For example, varying frequencies of light emitted by the light sources exhibit various levels of absorption by debris of different sizes. A first sub-emitter within the emitter array  788  may emit light at a first frequency, which is absorbed by debris of very small particle size, while a second sub-emitter within the emitter arrays  788  may emit light at a second frequency which is not absorbed by small-sized debris particles. The robot  11  may determine whether the bin  50  is full even when the particle size of the debris varies by measuring and comparing the received light signals from the first and second sub-emitters. Undesirable interference with the optical transmissive detection system may be avoided by employing sub-emitters emitting light at different frequencies. 
     Multiple emitter arrays  788  and detectors  760  may provide more accurate and reliable bin fullness detection as compared to, for example, a single emitter and detector pair. In the example shown, the multiple emitter arrays  788  provide cross-bin signals to detect potential bin blockages. One possible blockage location is near an intruding vacuum holding bulkhead  59 , which partially divides the bin  50  into two lateral compartments. Additionally or alternatively, a blockage may occur when received debris of a large enough size (e.g. paper or hairball) blocks and compartmentalizes the bin  50  at least temporarily. In certain implementations, a blockage occurs when shifting, clumping, moving, vibrated, or pushed debris within the bin creates one or more compartments in the bin  50  (e.g., via systematic patterns of accumulation). If debris accumulates in one lateral compartment, but not another, a single detector pair may not detect such accumulation. A single detector pair may also provide a false-positive signal from a large debris item or clump (e.g., indicating that the bin  50  is full when it is not). Multiple emitter arrays  788  located on the bottom interior surface  51  of the bin  50  and multiple detectors  760  located on the top interior surface  52  of the bin  50  in two different lateral or front-to-back locations covers more potential volume of the bin  50  for more accurate and reliable bin fullness detection as compared to a single detector pair in the same or similar orientation. A histogram or averaging of the bin detector signals or using XOR or AND on the results of more than one break-beam may be used to get more true positives (even depending on the time since accumulation began). 
     Referring to  FIG. 7D , in certain implementations, the bin  50  includes a transmissive optical detection system including two emitter arrays  788 , each having a diffuser  790  diffusing emitted infrared light. The diffuse light transmitted to the interior of the bin  50  provides a steadier detection signal generated by the detectors  760  relative to a detection signal generated from a concentrated beam of light from a non-diffuse light source at least because the diffuse light provides a type of physical averaging of the emitted signal. The detectors  760  receiving diffused infrared light signals can measure an overall blockage amount versus interruption of only a line-of-sight break beam from one emitter. 
     Referring to  FIG. 7E , in certain implementations, the bin  50  includes a light pipe or fiber-optic pathway  792  disposed on the bottom interior surface  51  of the bin  50 . Light from a light source  793  in the bin  50  travels along the fiber-optic pathway  792  and is emitted from distributor terminals  794 . This bin configuration centralizes light production to the single light source  793 , rather than supplying power to several independent light sources, while distributing light across the bin  50 . The distributor terminals  794  may also include a diffuser  790 , as discussed above with respect to  FIG. 7D . 
     Referring to  FIGS. 7F-7H , in some implementations, the bin  50  includes optical debris detection by reflective light transmission. In one example, as shown in  FIG. 7F , the bin  50  includes a shielded emitter  756  located near a detector  760 . Light emitted by the shielded emitter  756  does not travel directly to the detector  760  because of the shielding. However, light emitted from the emitter  756  is reflected by the interior surface  55  of the bin  50 , and traverses an indirect path to the detectors  760 . The attenuation of the reflected light caused by debris within the bin  50  may be comparatively greater than in a direct transmissive configuration, because the path the reflected light must travel within the bin  50  is effectively doubled, for example. Although the shielded emitter  756  and detector  760  are illustrated as being proximate to each other, they may be additionally or alternatively spaced apart from each other. The emitter  756  and detector  760  may be positioned on the same surface, or on different surfaces. 
     Referring to  FIG. 7G  in certain implementations, two sets of shielded emitters  756  and detectors  760 , each located on opposite horizontal sides of the interior of the bin  50 . In this configuration, light received by each detector  760  may be a combination of light directly transmitted from the shielded emitter  756  located on the opposite side of the bin  50 , as well as light reflected off the interior surface  55  by the proximal shielded emitter  756 . In some examples, a first set of shielded emitters  756  and detectors  760  is located on a bin surface adjacent to a second set of shielded emitters  756  and detectors  760 . In one example, a single shielded emitter  756  and detector  760  pair is located on a bottom surface  51  of the bin  50 . 
       FIG. 7H  illustrates a configuration in which the bin  50  includes a diffusive screen  412  placed along the transmission path of the shielded emitter  756  disposed on a bottom surface  51  of the bin  50 . The diffusive screen  790  diffuses light emitted from the shielded emitter  756  that reflects off various surfaces of the interior  55  of the bin  50  before reaching the detector  760 , thereby providing a detection signal that reflects a broad area of the interior of the bin  50 . 
     Referring to  FIGS. 8A-8E , in some implementations, the bin  50  includes an optical detection system  800  that detects debris moving through a combination of reflective and transmissive signals in the bin  50 . The optical detection system  800  includes a first receiver  802 A, a second receiver  802 B, a first emitter array  804 A, and a second emitter array  804 B. During use, debris  48  enters the bin  50  through the mouth  53  and forms an accumulation  49  extending from the bottom surface  51  of the bin. As debris  48  continues to enter the bin  50 , the accumulation  49  can increase in size in a direction defined from the bottom surface  51  to the top interior surface  52  (compare  FIGS. 8A, 8B, and 8C ). As described in further detail below, the emitter arrays  804 A,B are sequentially enabled and disabled (e.g., pulsed at a substantially constant frequency) while the receivers  802 A,B are synchronously sampled to measure reflected and transmissive signals and further processed to detect the debris  48  moving past the optical detection system  800  and to determine whether the bin  50  is full of debris (e.g., whether accumulation  49  of the debris  48  has size and/or density characteristics indicative of a “bin full” condition). 
     When the bin  50  is empty (as shown in  FIG. 8A ) or contain an accumulation  49  of debris below the receivers  802 A,B and emitters  804 A,B (as shown in  FIG. 8B ), the transmissive signal received at each receiver  802 A,B is greater than (e.g., substantially greater than) the reflected signal received at the respective receiver. As the bin  50  fills with debris  48  (e.g., during operation), the magnitude of the reflected signal can increase relative to the magnitude of the transmissive signal measured by each respective receiver  802 A,B. When the accumulation  49  of debris has filled the bin  50  (as shown, for example, in  FIG. 8C ), the reflective signal is about equal to or greater than the transmissive signal measured at the respective receiver  802 A,B. As discussed in further detail below, a comparison of the reflected signal measured at the receiver  802 A with the reflected signal measured at the receiver  802 B can provide an indication of whether the accumulation  49  of debris in the bin  50  is symmetrical ( FIG. 8C ) or axisymmetrical ( FIGS. 8D and 8E ). 
     The first and second receivers  802 A,B are disposed on substantially opposite sides of the mouth  53  of the bin and separated from one another along the largest dimension of the mouth  53 . The first and second receivers  802 A,B are generally directed toward one another such that each receiver may measure light originating from a source proximate to the other receiver, as described in further detail below. In some implementations, the first and second receivers  802 A,B are supported on substantially opposing side walls  57  of the bin  50 . The mouth  53  can be an opening in a substantially vertical plane perpendicular to the cleaning surface when the bin  50  is mounted on the robot body  31 . For example, the mouth  53  can be a substantially rectangular opening, with the side walls  57  define the short sides of the substantially rectangular opening and the bottom surface  51  and the top portion  52  define the long sides of the substantially rectangular opening. 
     In some implementations, the first and second receivers  802 A,B supported on substantially opposing side walls  57  of the bin  50  can reduce the likelihood of false positive signals by providing redundant measurements that may be compared to one another to determine a bin-full condition or an anomaly in debris accumulation in the bin. For example, if the reflected signals received by the first and second receivers  802 A,B are substantially similar, this can be an indication that the bin is full. Additionally or alternatively, if the reflected signal received by the first receiver  802 A is larger (e.g., substantially larger) than the reflected signal received by the second receiver  802 B, this can be an indication of axisymmetric debris accumulation in the portion of the bin closest to the first receiver  802 A (as shown, for example, in  FIG. 8D ). Similarly, if the reflected signal received by the second receiver  802 B is larger (e.g., substantially larger) than the reflected signal received by the first receiver  802 A, this can be an indication of axisymmetric debris accumulation in the portion of the bin closest to the second receiver  802 B (as shown, for example, in  FIG. 8E ). In certain implementations, the redundant measurements afforded by the first and second receivers  802 A,B can detect an anomaly such as a piece of paper or other obstruction in the area of a respective one of the first and second receivers  802 A,B. 
     The first and second receivers  802 A,B and the first and second emitter arrays  804 A,B are disposed toward the top interior surface  52  of the bin  50  to bias the sensing area toward the top of the bin  50 , where most of the debris enters the bin  50  in certain implementations. Additionally or alternatively, positioning the first and second receivers  802 A,B and the first and second emitter arrays  804 A,B toward the top interior surface  52  of the bin  50  facilitates bin-full detection (e.g., reduces the likelihood of false positive signals) in implementations in which the bin  50  fills from the bottom surface  51  to the top surface  52 . In certain implementations, positioning the receivers  802 A,B and emitter arrays  804 A,B toward the top interior surface  52  can reduce deterioration of the receivers  802 A,B and emitter arrays  804 A,B resulting from the accumulation of debris on the receivers  802 A,B at least because the top portion of the bin  50  is typically the position of least debris accumulation. 
     The first and second emitter arrays  804 A,B are disposed proximate to and below the respective first and second receivers  802 A,B such that each emitter array  804 A,B emits a signal substantially diagonally across at least a portion of the mouth  53 . Each emitter array  804 A,B is oriented to emit a signal across the mouth  53  of the bin  50 , toward a respective opposing receiver  802 A,B. For example, the first emitter array  804 A emits a signal toward the second receiver  802 B such that the second receiver  802 B receives a transmissive (e.g., unreflected) portion of a signal from the first emitter array  804 A and the first receiver  802 A receives a reflected portion of a signal from the first emitter array  804 A when there is no debris in the bin  50 . The second emitter array  804 B and the first receiver  802 A are arranged relative to one another in an analogous manner. 
     Each emitter array  804 A,B is substantially unshielded and may include one or more light sources  806  (e.g., two light sources). In implementations in which the emitter arrays  804 A,B include more than one light source  806 , light sources  806  of each array are arranged one above the other and spaced apart from one another. In these implementations, such spacing of multiple light sources  806  can facilitate emission of signals that cover all or a substantial portion of the mouth  53  without requiring custom lensing of the light sources  806 . The light sources  806  may be arranged to emit signals substantially covering the mouth  53  (e.g., covering more than about 50% of the area of the mouth  53 ) when all of the light sources  806  emit a signal. In certain implementations, the first receiver  802 A and the first emitter array  804 B is substantially identically arranged as the second receiver  802 A and the second emitter array  804 B such that, for example, the signals emitted by the first emitter array  804  intersect (e.g., criss-cross) the signals emitted by the second emitter array  804  along an axis substantially bisecting the mouth  53 . 
     In some implementations, the receivers  802 A,B and the emitter arrays  804 A,B are supported on the robot body  31 , just upstream of the mouth  53  of the bin  50  such that the receivers  802 A,B and the emitter arrays  804 A,B remain disposed on the robot body  31  when the bin  50  is disengaged from the robot body  11 . In some implementations, at least some of the receivers  802 A,B and the emitter arrays  804 A,B are mechanically coupled to the bin  50  and, thus, move with the bin  50  when the bin  50  is disengaged from the robot body  11 . The receivers  802 A,B and the emitter arrays  804 A,B may be in wireless communication with the microprocessor  245  and/or the bin microprocessor  217  (see  FIG. 2 ). The wireless communication between the microprocessor  245  and/or bin microprocessor  217  and the optical detection system  800  can include one or more of the following: infrared communication, electromagnetic communication, and radiofrequency communication. 
     Referring to  FIG. 9A , the optical detection system  800  includes a debris monitoring routine  900  to monitor passage of debris into the bin. The debris monitoring routine  900  may be implemented through communication between the optical detection system  800  and one or more of the bin microprocessor  217  and the microprocessor  245 . 
     The first emitter array  804 A and the second emitter array  804 B are activated and deactivated  902  to emit respective signals across the mouth  53  of the bin  51 . The activation and deactivation  902  is done sequentially such that the first emitter array  804 A and the second emitter array  804 B are each deactivated during a first time step, the first emitter array  804 A is activated and the second emitter array  804 B is deactivated during a second time step, and the first emitter array  804 A is deactivated and the second emitter array  804 B is activated during a third time step. In some implementations, the activation and deactivation  902  of the first and second emitter arrays  804 A,B is cycled at a substantially constant frequency of about 0.5 kHz to about 20 kHz (e.g., about 1 kHz). 
     The first receiver  802 A is measured  904 . The measurement can be taken at a substantially constant rate of about 0.25 kHz to about 10 kHz (e.g., about 4 kHz). In some implementations, the second receiver  802 B is measured in an analogous manner. The measured signals from the first receiver  802 A and the second receiver  802 B can reduce the likelihood of false positive measurements by, for example, comparing the measured signals from the first receiver  802 A and the second receiver  802 B. Additionally or alternatively, the measured signals from the first receiver  802 A and the second receiver  802 B can be used to determine whether the debris is entering the bin  50  from the right side or from the left side. 
     The movement of debris through the mouth  53  is detected  906  based at least in part on a first measurement obtained when the first and second emitter arrays  804 A,B are each deactivated, a second measurement obtained when the first emitter array  804 A is activated and the second emitter array  804 B is deactivated, and a third measurement obtained when the first emitter array  804 A is deactivated and the second emitter array  804 B is activated. For example, detecting  906  the movement of debris through the mouth  53  can include comparing an instantaneous value of a measurement to its respective average value. The impact of ambient light can be filtered out by adjusting the magnitudes of second and third measurements based at least in part on the first measurement, taken with both emitter arrays  804 A,B deactivated. Additionally or alternatively, as described in further detail below, a base brightness can be determined through a dynamic calibration routine initiated, for example, based at least in part upon detection of an initiation condition. 
     In some implementations, the first, second, and third measurements are processed as a function of time and changes in at least one of the processed measurements (e.g., at least one of the processed second and third measurements) are detected. For example, processing as a function of time may include a low pass filter to baseline the measured value to an average value. Such low pass filtering can reduce sensor-to-sensor variation and, thus, for example, improve the robustness of the debris detection using the optical debris detection system  800 . 
     The detected  906  debris through the mouth  53  of the bin  51  can include generating a signal to initiate a spot coverage routine to move the robot  11  over an area corresponding to the detected debris, as described in detail below. In certain implementations, the initiation of such a spot coverage routine is based at least in part on a quantified amount of debris. For example, the spot coverage routine can be initiated and/or adjusted if a large amount of debris is detected in a given area. 
     For the sake of clarity of description, the debris monitoring routine  900  has been described as monitoring passage of debris into a debris bin based on measuring signals at the first receiver  802 A. However, it should be noted that the debris monitoring routine  900  can additionally or alternatively include analogous measurements of signals at the second receiver  802 B. 
     In some implementations, referring to  FIG. 9B , the optical detection system  800  includes a debris quantifying routine  975 . The debris quantifying routine  975  may be implemented through communication between the optical detection system  800  and one or more of the bin microprocessor  217  and the microprocessor  245 . 
     The debris quantifying routine  975  includes periodically assigning  978  a score to the debris passing through the mouth  53 . The score can be based, at least in part, on the amount of light determined to be blocked by the debris, which can be substantially quantified based on one or more of the following: the magnitude of the measured debris signal (indicative of the size of the debris) and the duration of the measured debris signal (indicative of the concentration of debris). The assigned debris score is added  980  to previous debris scores. The adding  980  of the present debris score to the previous debris scores can include regularly decrementing  988  the running sum of the debris scores by a fixed amount. Such regular decrementation is sometimes referred to as “leaky” integration and can reduce the likelihood that small and light debris (e.g., loose carpet fibers or other “ambient” debris that is part of the surface being cleaned) will be detected as debris while still allowing large pieces of debris and high concentrations of small debris to be detected. The amount of decrementation can be a fixed value. Additionally or alternatively, the amount of decrementation can be adjusted (e.g., manually adjusted) based on the surface being cleaned such that surfaces that shed (e.g., carpet) will have a generally higher decrement than surfaces that do not shed (e.g., hardwood floors). 
     If the summed debris score is greater than a threshold  982 , a dirt detection signal is generated  984  and the summed debris score is reset  986  (e.g., reset to zero). If the summed debris score is not greater than the threshold  982 , periodic debris scores will continue to be assigned  978  and added  980  to previous debris scores. The threshold for determining the generation of the debris signal can be a fixed value stored in the bin microprocessor  217 . In certain implementations, the threshold can be lower at the beginning of the cleaning cycle (e.g., when the detected debris signal is more likely to be indicative of debris on the floor) than at the end of the cleaning cycle. Additionally or alternatively, the threshold can increase the more often debris is detected. This can reduce the likelihood that the robot  11  will run a spot coverage pattern too many times. 
     Referring to  FIG. 9C , in some implementations, the optical detection system  800  includes a bin-full detection routine  990  to determine whether the bin  50  is full of debris. The bin-full detection routine  990  may be implemented through communication between the optical detection system  800  and one or more of the bin microprocessor  217  and the microprocessor  245 . 
     The first emitter array  804 A and the second emitter array  804 B are activated and deactivated  992  to emit respective signals across the mouth  53  of the bin  51 , and the first receiver  802 A is measured  994 . The activation and deactivation  992  and the measurement  994  is analogous to the activation and deactivation and measurement described above with respect to the debris monitoring routine  900  such that, in some implementations, the same set of measurements is used as part of the debris monitoring routine  900  and the bin-full detection routine  990 . 
     The amount of debris in the bin is determined  996  based at least in part on comparing a first reflective signal to a first transmissive signal, where the reflective signal is derived from a measurement by the first receiver  802 A when the first emitter array  804 A is activated and the second emitter array  804 B is deactivated and the transmissive signal is derived from a measurement by the first receiver  802 A when the first emitter array  804 A is deactivated and the second emitter array  804 B is activated. 
     For the sake of clarity of description, the bin-full detection routine  990  has been described as determining whether the bin is full based on measuring signals at the first receiver  802 A. However, it should be noted that the debris monitoring routine  900  can additionally or alternatively include analogous measurements of signals at the second receiver  802 B. 
     Referring to  FIG. 9D , determining  996  whether the bin  50  is full of debris may include a threshold setting routine  1050 . The threshold setting routine  105  may be implemented through communication between the optical detection system  800  and one or more of the bin microprocessor  217  and the microprocessor  245 . 
     The threshold setting routine  1050  includes comparing  1052  a measured reflective signal to a measured transmissive signal (e.g., the reflective and transmissive signals measured by the first receiver  802 A and/or the second receiver  802 B). In some implementations, the comparison  1052  of the measured reflective signal to the measured transmissive signal is based on an average (e.g., time-averaged) value of each signal. Such averaging can reduce the likelihood of false positive bin-full results by, for example, reducing the impact of spurious and/or transient conditions on bin-full detection. In certain implementations, the measured reflective signal and the measured transmissive signal are compared  1052  at a rate of 1 Hz to 100 Hz (e.g., about 60 Hz). 
     If the measured reflective signal is less than the measured transmissive signal  1054 , the threshold setting routine  1050  continues to compare the measured reflective signal to the measured transmissive signal. Such a condition represents a bin that is relatively empty since light emitted by an emitter array (e.g., emitter arrays  804 A,B) generally reaches a receiver (e.g., receivers  802 A,B) disposed across the mouth  53  of the bin. If the measured reflective signal is greater than or equal to the measured transmissive signal  1054 , the reflective signal is compared to the transmissive signal to determine  1066  whether both signals are less than a minimum target value (e.g., equal to zero or about equal to zero). This reflects an anomalous condition, such as extremely rapid filling of the bin. If both signals are equal to zero, a bin-full signal is generated  1062 . 
     The value at which the reflective signal becomes greater than or equal to the transmissive signal is referred to as the crossover value and generally represents an indication that the bin is becoming full since light emitted by an emitter array is transmitted and scattered in approximately equal amounts as it is directed across the mouth  53  of the bin. In general, setting the threshold value as a function of the crossover value of the receiver can serve to self-calibrate the bin-full detection. 
     In some implementations, setting  1056  the threshold includes multiplying the crossover value by a fixed multiple (e.g., doubling the crossover value). In certain implementations, setting  1056  the threshold includes multiplying the crossover value by a value proportional (e.g., directly proportional, inversely proportional) to the value of the crossover point. Additionally or alternatively, setting  1056  the threshold can include multiplying the crossover value by a value proportional (e.g., directly proportional, inversely proportional) to the amount of time in which the crossover point was reached and/or to the peak transmissive signal. 
     The set threshold value can be reduced  1058  in a regular decrement over time. This can ensure that a bin-full condition will eventually be reached and, thus, reduces the likelihood that the robot  11  will continue to attempt to clean in the event of an error or an anomalous condition. 
     The reflective signal is compared  1060  to the set threshold. Given that the bin-filling process is generally slow, this comparison can be done at a relatively frequency of about 1 Hz to about 100 Hz (e.g., about 60 Hz). 
     If the reflective signal is greater than or equal to the set threshold, a bin-full signal is generated  1062 . In some implementations, the threshold value is set as an average of the signals measured by the first and second receivers  802 A,B. Additionally or alternatively, the generation  1062  of a bin full signal can be based at least upon a comparison of the threshold to an average of the reflected signals measured by the first and second receivers  802 A,B. As described in further detail below, this bin-full signal can be used to alert the user to the bin-full condition. In certain implementations, the bin-full signal is used to initiate a navigation routine to find a docking station (e.g., maintenance station  1250 ). Additionally or alternatively, the generation  1062  of the bin-full signal can disable at least a portion of the cleaning head  40  such that additional debris is not drawn into the bin  50 . 
     The reflective signal continues to be compared to the transmissive signal to determine  1064  whether the reflective signal has become less than or equal to the transmissive signal after having been greater than the transmissive signal (this is sometimes referred to as becoming “uncrossed”). If the reflective signal is greater than or equal to the transmissive signal and the threshold value is set, the threshold value continues to be reduced  1058  until the reflective signal is greater than or equal to the threshold. If the reflective signal becomes less than the transmissive signal after the threshold value has been set, the threshold value is reset  1067  (e.g., set to a large value and/or resetting a flag) and the reflective signal continues to be compared to the transmissive signal  1054  to determine  1054  a new crossover point and set  1056  a new threshold. Such dynamic resetting of the threshold reduces the likelihood of false-positive bin full detection resulting from, for example, debris becomes lodged and then dislodged in the debris bin  50 . 
     Although the optical detection system  800  has been described as being implemented in an autonomous, robot cleaning device, the optical detection system  800  can be additionally or alternatively incorporated into a non-autonomous cleaning device (e.g. a conventional vacuum cleaner). 
     The debris signal from a debris detection system (e.g., an optical detection system such as the optical detection system  800  or a piezoelectric debris detection system) can be used to alter operation of the robot  11 , including selecting a behavioral mode (such as entering into a spot cleaning mode), changing an operational condition (such as speed, power or other), steering in the direction of debris (particularly when spaced-apart left and right debris sensors are used to create a differential signal), or taking other actions. For example, based at least on a detected debris signal, the robot  11  can substantially immediately begin movement through a spot coverage pattern, including the spot coverage patterns described in further detail below. The microprocessor  25  can move the robot  11  through one or more of the spot coverage patterns below by controlling the drive assembly  210  based at least in part on a signal received from the gyroscope  71 . For example, the signal received from the gyroscope  71  can allow the robot  11  to move in a direction relative to the sensed debris and/or to return to the position of the sensed debris. 
     Referring to  FIG. 9E , in some implementations, the optical detection system  800  includes a dynamic calibration routine  1100  to set  1116  base brightness used for debris detection (e.g., through the debris monitoring routine  900  shown in  FIG. 9A  and described above). As indicated above, the base brightness can be subtracted from subsequent signals received at receivers  802 A,B to improve, for example, the accuracy of debris detection. In some implementations, the calibration routine  1100  can activate and/or deactivate a bin-full indicator (e.g., bin full indicator  1015  in  FIG. 12A ) based at least in part on determining whether the bin is full. The dynamic calibration routine  1100  may be implemented through communication between the optical detection system  800  and one or more of the bin microprocessor  217  and the microprocessor  245 . 
     The dynamic calibration routine  1100  includes applying  1104  a first pulse width modulation duty cycle to the first emitter array  804 A if an initiation condition is detected  1102  and measuring  1106  the signal from the first emitter array  804 A at the second receiver  802 B. If the duty cycle of the first emitter array  804 A is determined  1110  to be greater than a limit, a second pulse width modulation duty cycle is applied  1108  to the first emitter array  804 A and a second signal is measured  1112  at the second receiver  802 B. If the difference between the first measured signal and the second measured signal is greater than a threshold, the measured  1112  second signal is set  1116  as the base brightness. As used herein, a pulse width modulation refers to controlling the average value of power supplied to a load (e.g., the first emitter  804 A) by turning the power to the load on and off at a fast pace, and the duty cycle describes the proportion of “on” time to the regular interval. Thus, as compared to a lower pulse width modulation duty cycle, a higher pulse width modulation duty cycle corresponds to higher power provided to the load since the power is “on” for a longer period of time. 
     Detecting  1102  the initiation condition can include detecting insertion of the bin  50  into the robot body  31 . Additionally or alternatively, detecting  1102  an initiation condition can include detecting application of power (e.g., insertion of a battery  25  into robot body  31  and/or position of a power switch) to the autonomous robotic cleaner  11 . In some implementations, detecting  1102  the initiation condition can include activating a bin-full indicator based at least in part on detecting the initiation condition. For example, upon detection  1102  of insertion of the bin  50  into the robot body  31  a bin full indicator can be activated. As used herein, a bin full indicator can include a visual indicator (e.g., a light emitting diode and/or a text message on a user interface) and/or an audible indicator (e.g., an alarm). 
     Applying  1104  the first pulse width modulation duty cycle to the first emitter  804 A can include applying a maximum pulse width modulation duty cycle to the first emitter  804 A. 
     Measuring  1106  the first signal at the second receiver  802 B can include measuring the unreflected portion of the signal from the first emitter array  804 A. For example, as described above, the first emitter array  804 A can be arranged to emit a signal across at least a portion of the mouth  53  of the bin  50 . Additionally or alternatively, measuring  1106  the first signal at the second receiver  802 B can included measuring a reflected portion of the signal from the second emitter  804 B proximate to the second receiver. 
     Applying  1108  the second pulse width modulation duty cycle to the first emitter array  804 A includes lowering the pulse width modulation duty cycle from the first pulse width modulation duty cycle. In some implementations, the second pulse width modulation duty cycle is lowered by a fixed percentage from the previous pulse width modulation duty cycle. Additionally or alternatively, the second pulse width modulation duty cycle can be lowered by progressively larger percentages with each iteration of applying  1108  the second pulse width modulation duty cycle to the first emitter  804 A. 
     Determining  1110  whether the pulse width modulation duty cycle of the first emitter array  804 A is greater than a limit can include comparing the pulse width modulation duty cycle of the first emitter array  804 A to a limit stored in one or more of the bin microprocessor  217  and the microprocessor  245 . For example, the limit can be less than 90 percent (e.g., less than 50 percent, less than 40 percent) of the maximum pulse width modulation duty cycle of the first emitter array  804 A. Additionally or alternatively, the limit can be any value greater than zero. 
     If the determination  1110  is that the pulse width modulation duty cycle of the first emitter array  804 A is less than the limit while the difference between the first measured signal and the second measure signal is less than the threshold, the dynamic calibration routine  1100  can end. Such termination of the dynamic calibration routine  800  indicates that the measured signal at the first emitter array  804 A is not changing sufficiently with a corresponding change in the first and second measured signals. This insufficient change in the measured signal at the first emitter array  804  can indicate that debris was present in the bin  50  during the initiation condition. For example, an insufficient change in the measured signal at the first emitter array  804 A can indicate that debris was present in the bin  50  when the bin  50  was inserted in the robot body  31 . Additionally or alternatively, an insufficient change in the measured signal at the first emitter array  804 A can indicate that debris was present in the bin  50  when a battery was inserted into the robot body  31  and/or when power was provided to the optical detection system  800 . Accordingly, in implementations in which the bin full indicator is activated based at least in part on the detection of the initiation  1102  condition, the bin full indicator can remain activated upon termination of the dynamic calibration routine  1100 . 
     Measuring  1112  the second signal at the second receiver  802 B can be analogous to measuring  1106  the first signal at the second receiver  802 B. 
     Determining  1114  whether the difference between the first measured signal and the second measured signal is greater than a threshold can include comparing the first measured signal to the second measured signal after each signal has been processed. For example, each of the first and second measured signals can be processed through a low band pass filter. The threshold used in the determination  1114  can be a constant stored in one or more of the bin microprocessor  21  and the microprocessor  245 . 
     If the determination  1114  is that the difference between the first measured signal and the second measured signal is less than or equal to the threshold, the second pulse width modulation duty cycle is decreased  1115  from the second pulse width modulation duty cycle from the previous iteration. In some implementations, the second pulse width modulation duty cycle is decreased  1115  by between about 1 percent to about 30 percent (e.g., about 10 percent) in each successive iteration. In certain implementations, the second pulse width modulation duty cycle is decreased  1115  by progressively larger amounts in each successive iteration. 
     If the determination  1114  is that the difference between the first measured signal and the second measured signal is greater than the threshold, the second measured signal is set to the base brightness (e.g., through storage in one or more of the bin microprocessor  21  and the microprocessor  245 ). Additionally or alternatively, a bin-full indicator can be deactivated based at least in part on the determination  1114  that the difference between the first measured signal and the second measured signal is greater than the threshold. For example, the determination  1114  of a difference greater than the threshold can be an indication that the bin  50  is not full upon the initiation condition and, thus, the bin-full indicator can be deactivated. 
     While the dynamic calibration routine  1100  is described herein as being based on signals emitted from the first emitter array  804 A and received at the second receiver  802 B, it should be appreciated that the dynamic calibration routine  1100  can additionally or alternatively be based on signals emitted from the second emitter array  804 B and received at the first receiver  802 A. 
     Referring to  FIG. 10A , the robot  11  can include a spot cleaning mode (sometimes referred to as a spot coverage mode) including a star pattern  1150  having pairs  155  of outward swaths  1152  and inward swaths  1153  emanating from a central region  1151 . Each pair  155  of swaths  1152 ,  1153  defines an included angle α and is angularly stratified from an adjacent pair  155  of swaths  1152 ,  1153  by an external angle β. The repeated back and forth pattern of the star pattern  1150  can approximately mimic the cleaning pattern commonly used by operators of handheld vacuum cleaners. 
     To maneuver through the star pattern, the robot  11  moves in a forward direction of travel from a central region  1151  along an outward swath  1152  and reverses direction to return to the central region  1151  along an inward swath  1153 . This process can be repeated such that the robot  11  traces the star pattern  1150  corresponding to the plurality of pairs  155  of swaths  1152 ,  1153 . The star pattern  1150  can extend 180 degrees about the central region  1151 . In certain implementations, the central region  1151  is substantially centrally oriented relative to an area of detected debris  1154 . In some implementations, the central region  1151  is substantially peripherally oriented relative to an area of detected debris  1154 . 
     The robot  11  can move through the star pattern  1150  in a clockwise or counterclockwise direction. For example, the direction of movement of the robot  11  through the star pattern  1150  can be at least partly based on a determination of the direction of debris (e.g., based on a comparison of measured signals at the first and second receivers  802 A,B of the optical detection system  800 ). 
     The length of the outward swath  1152  can be a fixed length. For example, the length of the outward swath  1152  can be between 0.5 and 5 (e.g., 1) times a dimension of the robot  11  (e.g., the fore-aft dimension of the robot). As another example, the length of the outward swath  1152  can be a function of a quantity of debris detected by the debris detection system in the central region  1151  such that the length of the outward swath  1152  is inversely proportional to the quantity of debris detected by the debris detection system in the central region  1151  such that the robot  11  moves through a smaller star pattern  1150  in areas of higher debris concentration. 
     In certain implementations, the length of the outward swath  1152  can be a variable length. For example, the robot  11  can proceed along the outward swath  1152  until a detected quantity of debris falls below a threshold amount (e.g., indicating the perimeter of a high-debris area) 
     The included angle α between each outward swath  1152  and a corresponding inward swath  1153  is 0 to 45 degrees. In certain implementations, the included angle α is swept by turning the robot  11  (clockwise or counterclockwise) substantially in place at the end of the outward swath  1152  before reversing the direction of the robot  11  to move along the inward swath  1153 . In some implementations, the value of the included angle α is at least partly based on a quantity of debris detected by the debris detection system (e.g., optical detection system  800 ). For example, the angle α can be at least partly determined by the amount of debris detected as the robot  11  moves from the central region  1151 , along the outward swath  1152 . In such an implementation, the detection of a relatively large amount of debris along the outward swath  1152  can result in a small included angle α such that there is significant overlap in the paths cleaned by the robot along the outward and inward swaths  1152 ,  1153 . 
     In certain implementations, the external angle β between adjacent swath pairs  1155  is greater than 0 degrees and less than about 90 degrees. The external angle β can be fixed relative to the included angle α. For example, the external angle β can be substantially equal to the included angle α. Additionally or alternatively, the external angle β can be set according to one or more of the criteria described above with respect to the included angle α. 
     In some implementations, the external angle β is between about −90 degrees and about 90 degrees. In such implementations, the robot  11  can move along the star pattern  1150  by moving both clockwise and counterclockwise such that adjacent swath pairs  1155  can partially and, in some instances, completely overlap. 
     In certain implementations, cliff sensors  30 A and  30 B (shown in  FIG. 1B ) disposed along the respective forward and rear portions  31 A,B of the robot  11  can reduce the likelihood that the robot  11  will maneuver over a cliff while executing the star pattern  1150  or another cleaning pattern including repeated backward and forward motion. For example, cliff sensors  30 A disposed along the forward portion of the robot  31 A can detect a potential cliff forward of the robot  11  as the robot moves in the forward direction and cliff sensors  30 B disposed along the rear portion of the robot  31 B can detect a potential cliff rear of the robot  11 . In response to a potential cliff detected by the cliff sensors  30 A and/or cliff sensors  30 B, the robot  11  can abort the spot coverage pattern and, for example, initiate avoidance and/or an escape behavior. Thus, as compared to a robot with cliff sensors along only a forward portion, the robot  11  can execute a wider array of cleaning patterns including, for example, cleaning patterns that do not require the robot  11  to be in a specific forward orientation. 
     In certain implementations, referring to  FIG. 10B , the robot  11  includes a spot cleaning mode including a “cornrow” pattern  1180  having repeated adjacent rows  1182 . The robot  11  can initiate movement through the cornrow pattern  1180  at least partially based on the detection of debris  1184  on the cleaning surface. Additionally or alternatively, each row  1182  can extend substantially perpendicular to a detected direction of debris  1184  (e.g., as detected by first and second receivers  802 A,B of the optical detection system  800 ). 
     The robot  11  can move along the cornrow pattern  1180  by moving along a row  1182   a  until a quantity of detected debris (e.g., as determined by the optical detection system  800 ) falls below a threshold and then moving the robot  11  in a substantially opposite direction along an adjacent row  1182   b  and repeating this pattern for a set period of time or until the robot  11  moves through one or more rows without detecting a quantity of debris above the threshold. 
     In some implementations, the robot moves along the adjacent rows  1182   a,b  such that the adjacent rows  1182   a,b  overlap. The amount of overlap can be a fixed amount such as, for example, a fixed multiple (e.g., one half) of the size of the cleaning head. Additionally or alternative, the amount of overlap between certain adjacent rows  1182   a,b  can be based at least in part on the quantity of debris  1184  detected by the robot  11 , with the degree of overlap being directly proportional to the quantity of debris  1184  detected. 
     While the robot  11  has been described as operating in a spot coverage mode to move through the star pattern  1150  and the cornrow pattern  1180  based at least in part on a detected debris signal, other types of patterns are additionally or alternatively possible. For example, the robot  11  can move through an inward spiral pattern, an outward spiral pattern, and/or a zig-zag pattern. 
     Referring to  FIG. 11 , in some implementations, the robot  11  includes a camera  1190  disposed toward the forward portion of the robot  11 , with a field of view beyond the perimeter of the robot  11 . This camera  1190  can be in communication with the microprocessor  245  such that the movement of the robot  11  over the cleaning surface can be based at least in part on the detection of debris and/or an obstacle by the camera  1190 . For example, the microprocessor  245  can process the signal from the camera  1190  to recognize debris on the cleaning surface and maneuver the robot  11  toward the debris. 
     Additionally or alternatively, the microprocessor  245  can process the signal from the camera  1190  to recognize obstacles and/or debris in the vicinity of the robot  11  and maneuver the robot  11  to avoid obstacles and/or debris larger than a specific size threshold (e.g., a value less than about the smallest opening defined by the cleaning head). 
     Referring to  FIGS. 12A-12B , in some implementations, the robot  11  includes robot communication terminals  1012  and the bin  50  includes bin communication terminals  1014 . Information regarding bin-full status is communicated from the bin  50  to the robot  11  via the communication terminals  1012 ,  1014 , for example. Additionally or alternatively, a cliff detection signal from one or more rear cliff sensors  30 B disposed on the bin  50  is communication from the bin to the robot  11  via the communication terminals  1012 ,  1014 . In some implementations, the bin communication terminals  1014  contact the corresponding robot communication terminals  1012  when the bin  50  is attached to the robot  11 . In some examples, the communication terminals  1012 ,  1014  include serial ports operating in accordance with an appropriate serial communication standard (e.g. RS-232, USB, or a proprietary protocol). 
     In some examples, the robot  11  includes a demodulator/decoder  29  through which power is routed from the battery  25  through the communication terminals  1012 ,  1014  and to the bin  50 . Bin power/communication lines  1018  supply power to a vacuum motor  780 , a bin microcontroller  217 , and the rear cliff sensor  30 B. The bin microcontroller  217  monitors the bin-full status reported by the debris detection system  700  in the bin  50 , and piggybacks a reporting signal onto the power being transmitted over the bin-side lines  1018 . The piggybacked reporting signal is then transmitted to the demodulator/decoder  29  of the robot  11 . The microprocessor  245  of the robot  11  processes the bin full indication from the reporting signal piggybacked onto the power lines  1018 , for example. 
     In certain implementations, the bin microcontroller  217  monitors the bin-full status reported by the debris detection system  700  in the bin  50  (e.g., independently of a robot controller), allowing the bin  50  to be used on robots without a debris detection system  700 . A robot software update may be required for the bin upgrade. 
     In some implementations, as shown in  FIG. 12A , the bin  50  includes a bin power source  1013  (e.g., a battery) in electrical communication with the bin microcontroller  217 , the vacuum motor  780 , a bin-full indicator  1015 , and/or a rear cliff sensor  30 B disposed on the bin  50 . The bin microcontroller  217  may control power to the vacuum motor  780 , based at least in part on the bin-full status reported by the debris detection system  700 . For example, the bin microcontroller  217  may disable power to the vacuum motor  780  upon detection of a bin-full condition reported by the debris detection system  700 . Additionally or alternatively, the bin microcontroller  217  may control the status of the bin-full indicator  1015  (e.g., an LED) to provide the user with a visual indication of the status of the bin (e.g., the bin is full if the bin-full indicator  1015  is illuminated). Powering the bin-full indicator  1015  with the bin power source  1013  allows the bin-full indicator  1015  to remain illuminated while the bin  50  is disengaged from the robot  11  (e.g., while the bin  50  is being emptied). 
     Referring to  FIG. 12B , in some implementations, the robot  11  includes a receiver  1020  (e.g., an infrared receiver) and the bin  50  includes a corresponding emitter  1022  (e.g., an infrared emitter). The emitter  1022  and receiver  1020  are positioned on the bin  50  and robot  11 , respectively, such that a signal transmitted from the emitter  1022  reaches the receiver  1020  when the bin  50  is attached to the robot  11 . For example, in implementations in which the receiver  1020  and the remitter  1022  are infrared, the emitter  1022  and the receiver  1020  are positioned relative to one another to facilitate line-of-sight communication between the emitter  1022  and the receiver  1020 . In some examples, the emitter  1022  and the receiver  1020  both function as emitters and receivers, allowing bi-directional communication between the robot  11  to the bin  50 . In some examples, the robot  11  includes an omni-directional receiver  13  on the chassis  31  and configured to interact with a remote virtual wall beacon  1050  that emits and receives infrared signals. A signal from the emitter  1022  on the bin  50  is receivable by the omni-directional receiver  13  and/or the remote virtual wall beacon  1050  to communicate a bin fullness signal. If the robot  11  was retrofitted with the bin  50  and received appropriate software, the retrofitted bin  50  can direct the robot  10  to return to a maintenance station (e.g., maintenance station  1250  in  FIGS. 15A ,B) for servicing when the bin  50  is full. While infrared communication between the robot  11  and the bin  50  has been described, one or more other types of wireless communication may additionally or alternatively be used to achieve such wireless communication. Examples of other types of wireless communication between the robot  11  and the bin  50  include electromagnetic communication and radiofrequency communication. 
     Referring to  FIGS. 13A-13D , in some implementations, the bin  50  includes a bin-full indicator  1130 . In some examples the bin-full indicator  1130  includes visual indicator  1132  such as an LED ( FIG. 13B ), LCD, a light bulb, a rotating message wheel ( FIG. 13C ) or a rotating color wheel, or any other suitable visual indicator. The visual indicator  1132  may steadily emit light, flash, pulse, cycle through various colors, or advance through a color spectrum in order to indicate to the user that the bin  50  is full of debris, inter alia. The indicator  30  may include an analog display for indicating the relative degree of fullness of the bin  50 . For example, the bin  50  includes a translucent window over top of a rotatable color wheel. The translucent window permits the user to view a subsection of the color wheel rotated in accordance with a degree of fullness detected in the bin  50 , for example, from green (empty) to red (full). In some examples, the indicator  30  includes two or more LEDs which light up in numbers proportional to bin fullness, e.g., in a bar pattern. Alternatively, the indicator  1030  may be an electrical and/or mechanical indicator, such as a flag, a pop up, or message strip, for example. In other examples, the bin-full indicator  1130  includes an audible indicator  1134  such as a speaker, a beeper, a voice synthesizer, a bell, a piezo-speaker, or any other suitable device for audibly indicating bin-full status to the user. The audible indicator  1134  emits a sound such as a steady tone, a ring tone, a trill, a buzzing, an intermittent sound, or any other suitable audible indication. The audible indicator  1134  modulates the volume in order to draw attention to the bin-full status (for example, by repeatedly increasing and decreasing the volume). In some examples, as shown in  FIG. 13D , the indicator  1130  includes both visual and audible indicators,  1132  and  1134 , respectively. The user may turn off the visual indicator  1132  or audible indicator  1134  without emptying the bin  50 . In some implementations, the bin-full indicator  1130  is located on the robot body  31  or shell  6  of the robot  11 . 
     Referring to  FIGS. 14A-14B , in some implementations, the bin  50  wirelessly transmits a signal to a remote indicator  1202  (via a transmitter  1201 , for example), which then indicates to a user that the bin is full using optical (e.g. LED, LCD, CRT, light bulb, etc.) and/or audio output (such as a speaker  1202 C). In one example, the remote indicator  1202  includes an electronic device mounted to a kitchen magnet. The remote indicator  1202  may provide (1) generalized robot maintenance notifications (2) a cleaning routine done notification (3) an abort and go home instruction, and (4) other control interaction with the robot  10  and/or bin  50 . 
     An existing robot  11 , which does not include any communication path or wiring for communicating with a bin-full sensor system  700  on the bin  50 , is nonetheless retrofitted with a bin  50  including a bin-full sensor system  700  and a transmitter  1201 . “Retrofitting” generally means associating the bin with an existing, in-service robot, but for the purposes of this disclosure, at least additionally includes forward fitting, i.e., associating the bin with a newly produced robot in a compatible manner. Although the robot  11  cannot communicate with the bin-full sensor system  700  and may possibly not include any program or behavioral routines for responding to a bin-full condition, the bin  50  may nonetheless indicate to a user that the bin  50  is full by transmitting an appropriate signal via the transmitter  1201  to a remote indicator  1202 . The remote indicator  1202  may be located in a different room from the robot  11  and receives signals from the bin  50  wirelessly using any appropriate wireless communication method, such as IEEE 801.11/WiFi, BlueTooth, Zigbee, wireless USB, a frequency modulated signal, an amplitude modulated signal, or the like. 
     In some implementations, as shown in  FIG. 14B , the remote indicator  1202  is a magnet-mounted unit including an LED  1204  that lights up or flashes when the bin  50  is full. In some examples, as shown in  FIG. 14C , the remote indicator  1202  includes an LCD display  1206  for printing a message regarding the bin full condition and/or a speaker  1208  for emitting an audible signal to the user. The remote indicator  1202  may include a function button  1210 , which transmits a command to the robot  11  when activated. In some examples, the remote indicator  1202  includes an acknowledge button  1212  that transmits an appropriate command signal to the mobile robot  20  when pushed. For example, when a bin-full signal is received, the LCD display  1206  may display a message indicating to the user that the bin is full. The user may then press the button  1212 , causing a command to be transmitted to the robot  11  that in turn causes the robot  11  to navigate to a particular location. The user may then remove and empty the bin  50 , for example. 
     In some examples, the remote indicator  1202  is a table-top device or a component of a computer system. The remote indicator  1202  may be provided with a mounting device such as a chain, a clip or magnet on a reverse side, permitting it to be kept in a kitchen, pendant, or on a belt. The transmitter  1201  may communicate using WiFi or other home radio frequency (RF) network to the remote indicator  1202  that is part of the computer system  1204 , which may in turn cause the computer system to display a window informing the user of the bin-full status. 
     Referring to  FIG. 14D , when the optical detection system  800  determines that the bin  50  is full and/or when the microprocessor  245  determines that a state-of-charge of the battery  25  has fallen below a threshold, the robot  11 , in some examples, maneuvers to a maintenance station  1250  (e.g., a dock) for servicing. Maneuvering the robot  11  to the maintenance station  1250  is described in further detail below. 
     The robot  11  releasably engages with the maintenance station  1250 . In some examples, the maintenance station  1250  automatically evacuates the bin  50  (e.g. via a vacuum tube connecting to an evacuation port  80 ,  305 ,  380 ,  415 ,  420 ,  425 ,  430  of the bin  50 ). Additionally or alternatively, the maintenance station  1250  charges the battery  25 . For example, the maintenance station  1250  can charge the battery  25  through releasable engagement with at least one charging terminal  72 . In some examples, the charging terminal  72  is disposed along a bottom portion of the robot  11 . Additionally or alternatively, the charging terminal  72  can be disposed along a top portion and/or a side portion of the robot  11 . The at least one charging terminal  72  can be a contact terminal. 
     If the cleaning head  40  is full of filament build up, the robot  11  may automatically discharge the cleaning brush/flapper  60 ,  65  for either automatic or manual cleaning. The brush/flapper  60 ,  65  may be fed into the maintenance station  1250 , either manually or automatically, which strips filament and debris from the brush/flapper  60 ,  65 . 
     Referring to  FIGS. 15-16 , in some examples, the maintenance station  1250  emits a signal  1252  (e.g., a single signal, multiple signals, or multiple overlapping signals). The signal  1252  can be, for example, one or more optical signals (e.g., infrared) and/or acoustic signals. The robot  11  includes a receiver  15  for receiving the signal  1252 . Other details and features of signal emission by the maintenance station  1250  and signal reception by the robot  11  are disclosed in U.S. Pat. No. 7,332,890, entitled “Autonomous Robot Auto-Docking and Energy Management Systems and Methods,” the entire contents of which are incorporated herein by reference. 
     As the robot  11  moves over a cleaning surface  1 , the receiver  15  can receive the signal  1252  emitted by the maintenance station  1250  as the robot  11  moves along a path  1254  (e.g., in a bounce mode). The robot  11  can detect the time t 1 -t 7  associated with each change in the signal  1252 , with each change in the signal  1252  representing respective movement of the robot  11  into and out of the signal  1252 . For example, the robot  11  detects movement out of the signal  1252  at t 1  and detects movement into the signal  1252  at t 2 . Similarly, the robot  11  detects movement out of the signal  1252  at t 3  and detects movement into the signal  1252  at t 4 . As described below, the microprocessor  245  of the robot  11  can seek the maintenance station  1250  based at least in part on the elapsed time between t 1  and t 2 , t 3  and t 4 , etc. For the sake of clarity of explanation, seven times associated with change in the signal  1252  are shown in  FIG. 15B . However, it should be appreciated that the robot can detect any number of times. 
     In some implementations, seeking  1300  the maintenance station  1250  can include maneuvering  1302  the robot  11  over the cleaning surface  1  along path  1254 , detecting  1304  a first change in a signal emitted from the maintenance station  1250 , detecting  1306  a second change in the signal emitted from the maintenance station  1250 , and determining  1308  the probability that the robot will find the dock in a period of time. The determination  1308  of the probability that the robot will find the dock in a period of time is based at least in part on the elapsed time between the detected  1304  first change in the signal and the detected  1306  second change in the signal. This determination  1308  can reduce, for example, the likelihood that the robot  11  will become stranded on the cleaning surface  1  without enough power to return to the maintenance station  1250 . In certain implementations, the robot  11  seeks  1300  the maintenance station  1250  continuously. In some implementations, the robot  11  seeks  1300  the maintenance station  1250  periodically. Additionally or alternatively, the robot  11  can seek  1300  the maintenance station  1250  upon detection that a state-of-charge of the battery  25  is below a threshold (e.g., below about 50 percent). 
     Maneuvering  1302  the robot  11  over the cleaning surface can include maneuvering the robot  11  while one or more other behaviors are being executed. For example, maneuvering  1302  can include moving the robot  11  over the cleaning surface  1  in a bounce mode, a spot coverage mode, an escape mode, a migration mode, etc. Additionally or alternatively, maneuvering  1302  the robot  11  over the cleaning surface  1  can be determined by an arbiter. Details and features of such an arbiter are described in U.S. Pat. No. 7,388,343, entitled “Method and System for Multi-Mode Coverage for an Autonomous Robot,” the entire contents of which are incorporated herein by reference. 
     Detecting  1304  the first change in the signal emitted from the maintenance station  1250  includes receiving (e.g., by receiver  15 ) the signal  1252  emitted from the maintenance station  1250 . The detected  1304  first change in the signal can include detecting a change from receiving no signal to receipt of a signal and/or detecting a change from receipt of a signal to receipt of no signal. In some implementations, detecting  1304  the first change in the signal includes detecting an encoded signal. For example, the signal can be encoded to identify the maintenance station  1250  associated with the robot  11  such that the robot  11  does not seek a maintenance station  1250  that is not associated with the robot  11 . 
     Detecting  1306  the second change in the signal emitted from the maintenance station  1250  includes receiving (e.g., by receiver  15 ) the signal  1252  emitted from the maintenance station  1250 . Detecting  1306  the second change in the signal  1252  temporally follows detecting  1304  the first change in the signal such that there is an elapsed time between the detected  1304  first change in the signal and the detected  1306  second change in the signal. 
     Determining  1308  the probability that the robot will find the maintenance station  1250  is based at least in part on the elapsed time between detecting  1304  the first change in the signal and detecting  1306  the second change in the signal. The elapsed time between detecting  1304  the first change in the signal and detecting  1306  the second change in the signal represents the time between maintenance station  1250  sightings by the robot  11 . In some implementations, the elapsed time is used to update a probability distribution based at least in part on the elapsed time and/or previously determined elapsed times. For example, the elapsed time between t 6  and t 5  can be used to update a probability distribution including the elapsed time between t 4  and t 3  and the elapsed time between t 2  and t 1 . 
     The probability distribution can be used to estimate the probability that the robot  11  will reach the maintenance station  1250  within a period of time (e.g., a specified period of time or a variable period of time). For example, the probability distribution can be used to estimate the probability that the robot  11  will reach the maintenance station  1250  within five minutes. 
     Additionally or alternatively, the probability distribution can be used to determine the amount of time required for the robot  11  to reach the maintenance station  1250  with a certain probability. For example, the probability distribution can be used to estimate the amount of time required for the robot  11  to reach the maintenance station  1250  with greater than 75 percent probability. In some examples, the amount of time required for the robot  11  to reach the maintenance station  1250  with a certain probability can be the time allotted to allow the robot  11  to find the maintenance station  1250 . Thus, in one example, if the estimated time required for the robot to reach the maintenance station  1250  with greater than 95 percent probability is five minutes and a 95 percent success rate in finding the maintenance station  1250  is desired, the robot  11  will begin attempting to find the maintenance station  1250  when the remaining battery life  25  is five minutes. To allow for a further margin of safety, the robot  11  can reduce power consumption of the battery  25  by reducing, for example, the amount of power to the cleaning head  40  during the allotted time. 
     In some implementations, the probability distribution of elapsed times is a non-parametric model. For example, the non-parametric model can be a probability distribution histogram of probability as a function of elapsed time. The elapsed time ranges used for resolution of the histogram can be fixed values (e.g., about 5 second to about two minute intervals). 
     In certain implementations, the probability distribution of elapsed times is a parametric model. For example, the parametric model can be a Poisson distribution in which a successful outcome is an outcome in which the robot  11  reaches the maintenance station  1250  within a period of time and a failure is an outcome in which the robot  11  does not reach the maintenance station  1250  within a period of time. The mean of the Poisson distribution can be estimated, for example, as the arithmetic mean of a plurality of elapsed time measurements. From the Poisson distribution, the probability that the robot  11  will reach the maintenance station  1250  within a period of time can be determined. For example, the Poisson distribution can be used to determine the probability that the robot  11  will reach the maintenance station  1250  within five minutes. As an additional or alternative example, the Poisson distribution can be used to determine the amount of time required for the robot  11  to reach the maintenance station  1250  with a certain probability (e.g., a probability of greater than 75 percent). 
     In some implementations, determining  1308  the probability that the robot  11  will find the maintenance station  1250  can include determining the probability that power available from the battery  25  carried by the robot  11  will be depleted before the robot  11  can find the maintenance station  1250 . For example, the amount of time corresponding to the remaining power available from the battery  25  can be estimated based on the rate of power consumption of the robot  11  in the current mode of operation. The probability that the robot  11  will reach the maintenance station  1250  within the remaining battery time can be determined, for example, using the non-parametric and/or the parametric models discussed above. 
     If the robot  11  is removed from the cleaning surface  11 , the elapsed times between successive sightings of the maintenance station  1250  may not be representative of the amount of time required for the robot  11  to find the maintenance station  1250 . Thus, in some implementations, seeking  1300  the maintenance station  1250  includes ignoring a change in the detected signal following detection that the robot  11  was removed from the surface  1 . For example, if the robot  11  was removed from the surface  1  between t 1  and t 2 , the detected  1304  first change in the signal  1252  corresponding to t 1  is ignored and the detected  1306  second change in the signal  1252  is also ignored such that the next elapsed time is determined as the difference between t 4  and t 3 . In certain implementations, detecting that the robot has been removed from the surface includes receiving a signal from one or more sensors (e.g., cliff sensors  30 A and  30 B and/or proximity sensors  70 ) carried by the robot  11 . Additionally or alternatively, wheels  45  can be biased-to-drop and detecting that the robot has been removed from the surface can include detecting that the wheels  45  have dropped. Details of such biased-to-drop wheels  45  and detection of dropped wheels is disclosed in U.S. Pat. No. 7,441,298, entitled “Coverage Robot Mobility,” the entire contents of which are incorporated herein by reference. 
     Referring to  FIGS. 17-18 , the maintenance station  1250  emits a first signal  1252 ′ (e.g., a single signal, multiple signals, or multiple overlapping signals) and a second structure  1258  emits a second signal  1258 . The second structure  1258  can be a lighthouse (e.g., a navigation beacon), a gateway marker, a second maintenance station, etc. The robot  11  moves on the cleaning surface  1 , along a path  1260  such that the robot  11  intersects the signal  1252 ′ emitted by the maintenance station  1250  and intersects the signal  1258  emitted by the second structure  1256 . The robot  11  intersects the signal  1252 ′ at t 1 ′, t 4 ′, and t 5 ′, and the robot  11  intersects the signal  1258  at t 2 ′ and t 3 ′. The second structure  1256  can act as a landmark to assist in the prediction of finding the maintenance station  1250 . For example, as described below, the time between sighting the second structure  1256  and sighting the maintenance station  1250  can be used to predict the amount of time needed to find the dock given that the second structure  1256  was just seen. 
     In some implementations, seeking  1400  the maintenance station  1250  includes maneuvering  1402  the robot over the cleaning surface  1 , detecting  1404  the maintenance station  1250 , detecting  1406  the second structure  1256 , and determining  1408  the probability that the robot will find the maintenance station  1250  within a period of time. In some implementations, the signal  1252 ′ from the maintenance station  1250  differs from the signal  1258  emitted from the second structure  1256  (e.g., encoded differently and/or having different wavelengths). Seeking  1400  can allow the robot  11  to navigate by choosing actions that provide the best chance of moving from one landmark to the next, stringing together a path that ends at a goal location, such as the maintenance station  1250 . 
     Detecting  1404  the maintenance station  1250  includes detecting a change in the received signal  1252 ′ emitted by the maintenance station  1250 . At time t 1 ′, for example, the change in the received signal  1252 ′ is a change from receiving the signal  1252 ′ to not receiving the signal  1252 ′. As another example, at time t 4 ′, the change in the received signal  1252 ′ is a change from not receiving the signal  1252 ′ to receiving the signal  1252 ′. 
     Detecting  1406  the second structure  1256  includes detecting a change in the received signal  1256  emitted by the second structure  1256 . At time t 2 ′, for example, the change in the received signal  1258  is a change from not receiving the signal  1258  to receiving the signal  1258 . As another example, at time t 3 ′, the change in the received signal  1258  is a change from receiving the signal  1258  to not receiving the signal  1258 . 
     Determining  1408  the probability that the robot  11  will find the maintenance station  1250  within a period of time is based at least in part upon the elapsed time between detecting  1404  the maintenance station  1250  and detecting  1406  the second structure  1256 . For example, the elapsed time is the difference between t 2 ′ and t 1 ′ and the probability determination is the probability that the robot  11  will find the maintenance station  1250  given that the second structure  1256  has just been detected. The determination  1408  of the probability that the robot  11  will find the maintenance station  1250  within a period of time can be analogous to the determination  1308  discussed above. 
     In some implementations, the maintenance station  1250  is a first lighthouse (e.g., when the battery  25  is fully charged) and the second structure  1256  is a second lighthouse such that the robot  11  moves along the cleaning surface  1  based on relative positioning to the maintenance station  1250  and/or to the second structure  1256 . 
       FIGS. 19A-G  show another implementation of an autonomous robotic cleaner. Features identified by reference symbols including a prime are analogous to features identified by corresponding unprimed reference symbols in the implementations described above, unless otherwise specified. Thus, for example, robot  11 ′ is analogous to robot  11  and bin  50 ′ is analogous to bin  50 . 
     A bin guide  33  defines at least a portion of a receiving volume  37  defined by the robot body  31 ′. Bin  50 ′ is movable (e.g., slidable) along bin guide  33  to lock into place (e.g., as described below) such that mouth  53 ′ of bin  50 ′ aligns with a top portion of the receiving volume  37 . For example, such alignment is shown in  FIG. 8A ,  FIGS. 19C, and 19F .  FIG. 19C  is a cross-sectional view taken through robot  11 ′, along the receiving volume  37 , with bin  50 ′ inserted in the receiving volume  37 . Accordingly, as shown in  FIG. 19C , for example, debris moves past infrared array assemblies  810  disposed along a top portion of receiving volume  37  and into mouth  53 ′ defined by bin  50 ′. Such movement is shown schematically in  FIG. 19F , for example, in which the position of infrared array assembly  810  (which is disposed along the receiving volume  37  of the robot  11 ′ and, thus, represented as a dashed line in  FIG. 19F ) is shown relative to mouth  53 ′ defined by bin  50 ′. 
     Each infrared array assembly  810  includes an emitter array (first emitter array  804 A′ or second emitter array  804 B′, as shown in  FIG. 19C ), with each respective emitter array including two light sources  806 ′. Each infrared array assembly  810  also includes a receiver (first receiver  802 A′ or a second receiver  802 B′, as shown in  FIG. 19C ) and a filter  812  disposed between the receiving volume  37  and the respective emitter array and receiver of the infrared array assembly  810 . Each filter  812  can be an infrared transparent daylight filter. 
     Although each infrared array assembly  810  is shown as disposed along receiving volume  37  defined by robot body  31 ′, each infrared array assembly  810  can be disposed on bin  50 ′. Whether the infrared array assembly  810  is disposed on the receiving volume  37  or the bin  50 ′, the first and second receivers  802 A′,  802 B′ and the first and second emitter arrays  804 A′,  804 B′ can be substantially evenly spaced across the mouth  53 ′ on each horizontal side of the mouth  53 ′ to substantially span horizontal and vertical dimensions of the mouth  53 ′ with emitted light from the array assemblies  810 . 
     Robot  11 ′ includes a dust bin  50 ′ for collecting debris while the robot  11 ′ is in operation. The dust bin  50 ′ is releasably detachable from the robot  11 ′ (e.g., releasably detachable from the robot body  31 ′) to allow debris to be removed from the dust bin  50 ′ and/or to allow a filter  811  carried by the dust bin  50 ′ to be replaced. The dust bin  50 ′ can be removed from robot  11 ′ by moving a release  819  (e.g., depressing the release  819  and/or lifting the release  819 ) that moves a latch  809  such that the dust bin  50 ′ can be slidably removed from the robot  11 ′. In some implementations, release  819  can include one or more lights (e.g., lights indicative of an operating mode of the robot  11 ′) and/or one or more proximity sensors. In certain implementations, release  819  senses the position of the latch  809  such that release  819  provides an indication of the position of the bin  50 ′ (e.g., an indication that the bin  50 ′ is not fully engaged with the robot  11 ′). 
     The bin  50 ′ includes a barrier  55  which extends horizontally across the width of the bin  50 ′ and extends vertically along at least a portion of the bin  50 ′ such that the barrier  55  defines at least a portion of a horizontal bottom portion of the mouth  53 ′. In some implementations, barrier  55  defines at least a portion of a compartment that retains debris settled at the bottom of the bin  50 ′ when the bin is in situ in the robot  11 ′. In certain implementations, at least a portion of the barrier  55  is a door (e.g., a hinged door and/or a slidable door) that is movable to allow access to debris stored in the bin  50 ′. In some implementations, the barrier  55  is rigidly fixed relative to the mouth  53 ′ and access to debris is obtained through one or more doors forming part of a side wall, a bottom wall, or a rear wall of the bin  50 ′. 
     In some implementations, the vertical dimension of the mouth  53 ′ is substantially ½ or less of the combined height of the barrier  55  and the vertical dimension of the mouth  53 ′. Accordingly, in implementations in which the height of the bin  50 ′ is defined approximately by the combined vertical dimensions of the mouth  53 ′ and the barrier  55 , the vertical dimension of the barrier  55  can be greater than the vertical dimension of the mouth  53 ′. These relative dimensions of the barrier  55  to the mouth  53 ′ can facilitate storage of a large amount of debris in the bin  50 ′ while retaining the profile of the robot  11 ′ during use. 
     Although the mouth  53 ′ and the barrier  55  are shown as extending substantially across the entire width of the bin  50 ′, other configurations are also possible. For example, the mouth  53 ′ can extend about ⅔ of the width of the bin  50 ′ or less while the barrier  55  extends substantially across the entire width of the bin  50 ′ such that the width of the barrier  55  is at least ⅓ greater than the width of the mouth  53 ′. These relative dimensions of the barrier  55  to the mouth  53 ′ can facilitate storage of a large amount of debris in the bin  50 ′ while retaining the profile of the robot  11 ′ during use. 
     Although the bin  50 ′ is shown as defining a mouth  53 ′ having a single opening, other implementations are also possible. For example, the bin  50 ′ may define a mouth having multiple openings which can facilitate increasing turbulence along the flow path  819  ( FIG. 19F ) and/or facilitate breaking up large pieces of debris as it moves along the flow path  819 . For example, the bin  50 ′ may define a mouth having two openings horizontally spaced apart from one another. More generally, as used herein, the term mouth refers to the total open area through which debris passes into the bin  50 ′ during operation. 
     The bin  50 ′ includes a protrusion  807  disposed toward an end portion of the bin  50 ′ that engaged with the robot  11 ′. The protrusion  807  can engage with robot  11 ′ to reduce the likelihood of damage to portions of the bin  50 ′ as the bin  50 ′ is slid into engagement with the robot  11 ′. For example, the protrusion  807  can reduce the likelihood of damage to the door  54 ′ and/or to the release  819  as the bin  50 ′ is slid into the robot  11 ′. Additionally or alternatively, the protrusion  807  can facilitate alignment of the latch  809  for securing the bin  50 ′ to the robot  11 ′. 
     The bin  50 ′ further includes a filter  811 , a motor  815 , and an impeller  817 . During use, a fluid stream  819  (e.g., debris carried in air) is drawn into the bin  50 ′ by negative pressure created by rotation of the impeller  817  driven by the motor  815 . The fluid stream  819  moves past the optical detection system  800 ′ such that debris detection and bin-full detection can be carried out as described above. The fluid stream  819  moves through a filter  811  such that the debris is separated from the air, with the debris remaining in the bin  50 ′ (e.g., in a portion of the bin  50 ′ at least partially defined by barrier  55 ) and the air exiting the bin  50 ′ through an exhaust  813  defined by the bin  50 ′. 
     An optical detection system  800 ′ is similar to optical detection system  800  and operates to detect debris and bin-full conditions in a manner analogous to the debris and bin-full detection described above with respect to  FIGS. 8A-8E . In general, the views shown in  FIGS. 8A-8E  correspond to the front view of the bin  50 ′ shown in  FIG. 19C . As shown in  FIG. 19C , the mouth  53 ′ defined by the bin  50 ′ extends along only part of the vertical dimension of the bin  50 ′. Thus, to further illustrate the correspondence in structure between the bin  50  shown in  FIG. 8A  and the bin  50 ′ shown in  FIG. 19C , the position of the mouth  53 ′ is shown as dashed line in  FIG. 8A . 
     Accordingly, it should be appreciated that the detection of the debris  48  shown in  FIG. 8B  is analogous to the debris detection of debris entering bin  50 ′ through mouth  53 ′, along path  819 . Similarly, it should be appreciated that the bin-full detection as a result of the accumulation  49  of debris shown in  FIG. 8C  is analogous to the bin-full detection of an accumulation of debris in a compartment defined by the bin  50 ′. Likewise, it should be further appreciated that the detection of asymmetric debris accumulation shown in  FIGS. 8D and 8E  is analogous to the detection of asymmetric debris accumulation in a compartment defined by the bin  50 ′. 
     Other details and features combinable with those described herein may be found in U.S. patent application Ser. No. 11/751,267, filed May 21, 2007, entitled Coverage Robots and Associated Cleaning Bins, and U.S. patent application Ser. No. 10/766,303, filed Jan. 28, 2004, entitled Debris Sensor for Cleaning Apparatus, now U.S. Pat. No. 6,956,348. The entire contents of each of the aforementioned applications are hereby incorporated by reference in their entirety. 
     A number of implementations have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the disclosure. Accordingly, other implementations are within the scope of the following claims.