Patent Publication Number: US-2022236739-A1

Title: System and method for optical localization

Description:
TECHNICAL FIELD 
     The present disclosure relates to autonomous mobile robots, particularly a localization system for mobile robots using optical devices. 
     BACKGROUND 
     Robotic vehicles may be configured for autonomous or semi-autonomous operation for a wide range of applications including product transportation, material handling, security, and military missions. Autonomous mobile robotic vehicles typically have the ability to navigate and to detect objects automatically and may be used alongside human workers, thereby potentially reducing the cost and time required to complete otherwise inefficient operations such as basic labor, transportation and maintenance. An important part of robotic autonomy is robot&#39;s ability to reliably navigate within a workspace. Numerous positioning system approaches are known that attempt to provide accurate mobile robot positioning and navigation without the use of GPS. Some autonomous vehicles track movement of driven wheels of the vehicle using encoders to determine a position of the vehicle within a workspace. Other autonomous vehicles use other approaches such as GPS-pseudolite transmitters, RF beacons, ultrasonic positioning, active beam scanning and landmark navigation. 
     In particular, a landmark navigation system uses a sensor, usually a camera, to determine a vehicle&#39;s position and orientation with respect to artificial or natural landmarks. Artificial landmarks may be deployed at known locations and certain systems contemplate artificial landmarks that involve the use of a high contrast bar code or dot pattern. A sensor device can observe both the orientation and distance relative to the landmark, so that only two landmarks need to be viewed in order to compute the vehicle&#39;s position. The challenge in a landmark navigation system is in reliably identifying the landmarks in cluttered scenes. The accuracy of the position computation is dependent on accurately determining the camera orientation to the landmark. Also, sufficient illumination is necessary with existing landmark navigation solutions. 
     Nevertheless, landmark navigation is attractive because of its potential for accuracy, high reliability, low cost and relative ease of deployment. There is, therefore, a need for an improved landmark navigation positioning system that can achieve the reliability and accuracy that current positioning system solutions for robotic or unmanned vehicles cannot. 
     The proposed optical system of localization for mobile robots can provide additional accuracy and reliability over existing methods of localization (such as those relying on Ultra Wideband (“UWB”) localization), and additionally can potentially use the same sensors for obstacle detection and avoidance, for example. 
     SUMMARY 
     In accordance with one disclosed aspect, there is provided a system for optical localization. The system includes a plurality of movable stationary landmarks defining an operating space and an autonomous mobile robot located in and operating within the operating space. The mobile robot includes a self-propelled mobile chassis, an optical sensor assembly disposed on a raised portion vertically spaced apart from the chassis and configured to detect at least one of the plurality of landmarks, and a controller configured to determine the position and orientation of the chassis based at least on information from the optical sensor assembly. The optical sensor assembly may include a LiDAR sensor or an optical camera. Each landmark of the plurality of landmarks may be in the form of a structure having an elevated portion extending vertically from the ground surface to a height level which is equal to or higher than a horizontal plane parallel to the surface and extending from the optical sensor assembly of the mobile robot, wherein the elevated portion is optically detectable by the optical sensor assembly. Each landmark of the plurality of landmarks may have one or more of: a characteristic cross-sectional feature for determining orientation (of the optical sensor assembly/mobile robot) relative to the landmark; a characteristic visually distinct portion for determining orientation (of the optical sensor assembly/mobile robot) relative to the landmark; and an identifier uniquely identifying the landmark from other landmarks. The optical sensor assembly may be mounted on an actuated column vertically movable between an extended portion where the optical sensor assembly is vertically spaced apart from the chassis and a retracted position where the optical sensor assembly is held relatively near the ground. 
     In accordance with another disclosed aspect, there is provided a method for optical sensor-based localization of an autonomous mobile robot. The method involves detecting, by an optical sensor assembly, an optical reference, determining, by a processing unit, based on the detected optical reference—a distance to the optical reference, a relative angle to the optical reference, and an orientation of the optical reference; and calculating, by the processing unit, the orientation and position of the mobile robot based on the detected distance, orientation, and relative angle of the optical reference using a known relationship between the mobile robot, the optical sensor assembly, and the detected optical reference. The method may further include moving the optical reference, while keeping the optical sensor assembly stationary or moving the optical sensor assembly, while keeping the optical reference stationary; tracking, by the processing unit, the relative movement of the optical reference to the optical sensor assembly and information regarding which of the optical reference or optical sensor assembly was moved, and determining, by the processing unit, a new position and orientation of the mobile robot based on the detected distance and relative angle of the optical reference using a known relationship between the mobile robot, the optical sensor assembly, and the detected optical reference, the tracked relative movement of the optical reference the sensor assembly, and the information regarding which of them was moved. The known relationship may be either a static relationship defined at initialization, or a dynamic relationship which may change during operation and be communicated to the processing unit. 
     In accordance with a further disclosed aspect, there is provided a method for optical sensor-based localization of an autonomous mobile robot during operation. The method involves detecting, by an optical sensor assembly of a mobile robot located at a first position, a first optical reference and a second optical reference, determining, by a processor, based on the detected optical references—a distance to each optical reference, and a relative angle to each of the detected optical references; calculating, by the processor, the orientation and position of the mobile robot based on the detected distances and relative angles of the optical references, detecting, by the optical sensor assembly, further optical references, calculating, by the processor, the position of each further optical reference with respect to the first and second optical references, moving, by the mobile robot, from the first position to a second position, detecting, by the optical sensor assembly, at least two previously detected optical references, and calculating, by the processor, the orientation and position of the mobile robot based on the detected distances and relative angles of any two of the detected optical references. 
     The method may further involve establishing, by the processor, a global coordinate system based on the detected optical references. The method may then include detecting, by a second sensor of the mobile robot, one or more objects, calculating, by the processor, the position of each of the detected objects with respect to the optical references by—determining, by the processor, the relative position of the second sensor to the mobile robot, determining, by the second sensor, the position of each object relative to the robot, and transforming, by the processor, the position of each object relative to the second sensor to the global coordinate system; and storing, by the processor, the calculated positions with respect to the global coordinate system in a memory. The method may also involve storing, by the processor, the relative positions of each of the detected optical references in a memory, and determining, by the processor, the identity of features detected by the optical sensor assembly as optical references based on at least the stored relative positions of the optical references stored in the memory. The method may additionally involve detecting, by the optical sensor assembly, an optical feature of a second mobile robot, determining, by the processor, based on the detected optical feature one or more of a distance to the second mobile robot and an orientation of the second mobile robot, calculating, by the processor, the orientation and position of the second mobile robot relative to the optical references based on the detected distances and relative angles of the optical feature, and maintaining, by the mobile robot, a minimum distance of separation to the second mobile robot. The method may then also involve communicating, by the processor of the mobile robot through a communication device on the mobile robot, with the processor of the second mobile robot through a communication device on the second mobile robot, and transmitting, by the processor of the mobile robot, the orientation and position of the second mobile robot relative to the optical references. 
     In accordance with yet another disclosed aspect, there is provided a method for initializing a system for optical localization of an autonomous mobile robot. The method involves placing at least three optical references, the placement of the optical references forming a predetermined angle, concealing two optical references defining a width of an operating space from an optical sensor assembly of a mobile robot, detecting, by the optical sensor assembly, an environment of the operating space, unmasking the two optical references to the optical sensor assembly and detecting, by the optical sensor assembly, the two optical references, and determining, by a processor of the mobile robot, the width of the operating space based on the distance between the two detected unmasked optical references. The method then involves rotating, by the mobile robot, searching for and detecting, by the optical sensor assembly, the third optical reference, selected based on the relative angle of the location of the third reference with respect to the line formed by the two detected unmasked optical references, and defining, by the processor of the mobile robot, the length of the operating space as a perpendicular distance between the detected third optical reference and the line formed by the two detected unmasked optical references. 
     In accordance with another aspect, also disclosed herein is a method for expanding an operation space of a mobile robot. This method includes determining, by a processing unit, that the mobile robot has completed a work task in the operating space followed by assigning, by the processing unit, a relocation task to the mobile robot, the relocation task comprising moving one or more landmarks of a plurality of landmarks from a first position of each of the one of more landmarks to a second position of each of the one or more landmarks. The method then includes executing, by the mobile robot, the relocation task, the task involving navigating, by the mobile robot, to a first landmark of the one or more landmarks located at a first position using the disclosed optical localization system comprising the plurality of landmarks, transporting, by the mobile robot, the first landmark to a second position for the landmark, comprising navigating using the optical localization system, and repeating from the navigating step for each other landmark of the one or more landmarks to be moved. The method then includes assigning, by the processing unit, a new work task to the mobile robot in the operating space defined by new landmark positions. In this manner, once the work task (e.g. a method of transportation of articles) has been completed for one operating space, the mobile robot can automatically define a new operating space, and perform the work task in the new operating space, without requiring human intervention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the following, embodiments of the present disclosure will be described with reference to the appended drawings. However, various embodiments of the present disclosure are not limited to arrangements shown in the drawings. 
         FIG. 1  is a plan view of a system for optical localization. 
         FIG. 2  is a perspective view of an embodiment of an autonomous mobile robot using the system for optical localization of  FIG. 1 . 
         FIG. 3  is a perspective view of an embodiment of a landmark used in the system for optical localization of  FIG. 1 . 
         FIG. 4  is a side view of an alternative embodiment of an autonomous mobile robot using the system for optical localization of  FIG. 1 . 
         FIG. 5  is a block diagram view of a method for optical localization. 
         FIG. 6  is a block diagram view of an alternative method for optical localization. 
         FIG. 7  is a plan view of a system implementing a method of optical localization. 
         FIG. 8  is a block diagram view of a method of initializing a system for optical localization. 
         FIG. 9  is a schematic plan view illustrating a system implementing a method for expanding the operating space of a mobile robot. 
         FIGS. 10A and 10B  are schematic plan views of an alternative embodiment of a system implementing a method for expanding the operating space of a mobile robot. 
         FIG. 11  is a perspective view of an alternative embodiment of a landmark operable with the disclosed system. 
         FIG. 12  is a block diagram illustrating a method for expanding the operating space of a mobile robot. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , a system for optical localization of an autonomous mobile robot is shown generally at  100 . The system  100  includes a plurality of movable stationary landmarks  101 ,  102 ,  103 ,  104 ,  105 , and  106  defining a work field  107 . The work field  107  is defined by defining a base line  170  and a field boundary  172 . The base line  170  may define the start and end positions for moving work for example, and provides a reference line for an axis for an x-y coordinate system, for example, in which a mobile robot is being localized. The field boundary  172  determines the area a mobile robot  110  may freely move in. The work field  107  may be defined at initialization in a variety of ways—for example, the mobile robot  100  may be provided the dimensions of the work field  107  by an operator and the size of work field  107  is defined by these parameters, the robot  110  using odometry to stay within the boundaries and only using landmarks  101 ,  102 ,  103 ,  104 ,  105 ,  106  to correct odometry drift. In another example, the work field  107  may be defined by providing the robot  110  with configuration information regarding the system such as the landmarks  101  and  102  defining one end of the work field  107  and the landmarks  105  and  106  defining the opposite end, with the base line  107  defined as the line between landmarks  105  and  106 , and the field boundary between landmarks  101  and  105 , running through landmark  103  as well as between landmarks  102  and  106  running through landmark  104 . In yet another example, the robot  110  may be provided with configuration information that the work field  107  is defined by three pairs of landmarks, with the base line  170  defined by the line running through third pair of landmarks (in this case landmarks  105  and  106 ), and the field boundary running through the landmark of each pair on the same side. The system  100  may additionally include a plurality of articles  108  (such as plant pots being transported by the mobile robot) located in the work field  107 . The system  100  includes the autonomous mobile robot  110  also located in the work field. 
     The robot  110  includes a raised optical sensor  112  (sometimes also referred to herein as an optical sensor assembly) mounted on a raised portion of the robot  110  and having a field of view  113 , and may include a manipulator  111  for interacting with articles  108 . The robot  110  may also include a storage space  114  for storing articles, and a second optical sensor  116  mounted on the robot  110  and having a different field of view from the elevated optical sensor  112 , such as the complementary field of view  117  shown in  FIG. 1 . The field of view  113  of the raised optical sensor  112  is preferably around 270 degrees or greater, allowing the sensor  112  to see two or more of the stationary landmarks  101 ,  102 ,  103 ,  104 ,  105 , and  106  at any given time. For example, in  FIG. 1 , landmarks  103 ,  104 ,  105  and  106  are within the field of view  113 . For a work field  107  of a different size, there may be additional landmarks which extend along the lines formed by landmarks  101 ,  103 ,  105  and by landmarks  102 ,  104 ,  106 , for example. 
     Referring to  FIG. 2 , an embodiment of the mobile robot  110  of  FIG. 1  is shown in greater detail. In other embodiments the mobile robot may be unmanned aerial vehicles or other unmanned ground vehicles or any other mobile robot. The raised optical sensor  112  can be seen attached to the top of a tower structure  118  of the mobile robot  110 . The tower structure  118  may additionally house additional components, such as a communication system  119  allowing the mobile robot  110  to communicate over a wireless network, for example. If present as in the depicted embodiment, the second sensor  116  may be mounted at a different elevation on the mobile robot  110  than the raised optical sensor  112 , and may be useful in detecting obstacles at different heights, or for detecting objects such as articles  108  while the plane of view of the raised optical sensor  112  goes over such objects. Each of sensors  112  and  116  may be a Light Detection and Ranging (LiDAR) sensor, an optical camera, or a combination of the two. Both sensors  112  and  116  may also be used for other purposes, such as pathfinding, object avoidance, safety, and data gathering for example. 
     Referring to  FIG. 3 , an embodiment of a movable stationary landmark such as landmarks  101 ,  102 ,  103 ,  104 ,  105  or  106  of  FIG. 1  is shown generally as  300 . The landmark  300  includes an elevated portion  310 , which extends vertically so that the raised optical sensor  112  retains line of sight to the elevated portion  310  even if intervening objects are on the surface between the robot  110  and the landmark  300 . The elevated portion  310  generally extends at least to a height level which is equal or higher than a horizontal plane parallel to the surface extending from the raised optical sensor  112 . The elevated portion  310  may have a characteristic cross-sectional geometry feature  311  so that a LiDAR or other optical sensor operating at the horizontal plane parallel to the surface extending from the raised optical sensor  112  can distinguish the landmark  300  from other objects having a cross-section at that plane. The characteristic feature  311  may additionally provide information on the relative angle of the detecting sensor  112  to the landmark  300 , such as in this case being a feature (the chamfered edge) that exists only on a single edge of the cone, meaning for a given known orientation of the landmark  300 , the relative angle to the landmark can be determined by finding the chamfered edge, for example. The landmark  300  may additionally include a visually distinct portion  312 , such as a striped face. The striped face may contain material with different (enhanced) reflectivity compared to the rest of the landmark and the surrounding environment, for example, to produce a distinct increase in reflective intensity in a particular wavelength—under either or both of optical lighting and LiDAR. The visually distinct portion  312  serves a similar purpose as the characteristic feature  311 , for either or both of an optical camera version of sensor  112  or the LiDAR version of sensor  112 . The visually distinct portion  312  may assist the processing algorithm of the sensor  112  in distinguishing the landmark  300  from background objects. Similarly, the visually distinct portion  312  may additionally provide information on the relative angle of the detecting sensor  112  to the landmark  300 , since like the characteristic feature  311 , the portion  312  may exist only on one face of the landmark  300 , and the relative angle to the landmark can be determined by finding the striped face, for example. Aspects of landmark  300  such as feature  311  or portion  312  may also be used to supplement other methods of determining the orientation of robot  110 , such as an Inertial Measurement Unit (IMU), odometry, global mapping, or any other orientation determination method. The landmark  300  also includes a unique identifier  320 . The unique identifier  320  is a feature of landmark  300  which uniquely identifies it from other instances of landmark  300 , such as uniquely identifying landmark  101  from  102 , for example. The identifier  320  is shown in  FIG. 3  as a circle with a pattern, but may be any other type of identifier, such as an alphanumeric character, a color, a shape, a pattern, a QR code, any combination of the above or any other method of uniquely identifying the landmark  300  detectable by the optical sensor  112 . Uniquely identifying the landmark  300  allows the system a further method of determining the orientation of the mobile robot  110 , and also allows for an additional method in determining absolute positioning which may improve accuracy. 
     Referring to  FIG. 4 , an alternative embodiment of mobile robot  110  of  FIG. 1  is shown. In this embodiment, the manipulator  111  is a Selective Compliance Assembly Robot Arm (SCARA) manipulator, and the optical sensor  112  is attached to telescoping column  115  of the manipulator  111 . The telescoping column  115  is extendable and collapsible along a range of heights  140 , moving the optical sensor  112  between a raised position  142  and a lowered position  144 , along with an end effector of manipulator  111 . In this embodiment, when the optical sensor  112  is in the raised position  142 , it has sufficient height to clear the articles  108  in the work field  107 , allowing the optical sensor  112  to detect landmarks  101 - 106 , for example. Conversely, when the optical sensor  112  is in the lowered position  144 , it can act as the second optical sensor  116  of  FIG. 2 , providing an alternative elevation more suitable for detecting obstacles near the ground such as articles  108 . As the robot  110  does not necessarily require continuous uninterrupted detection of either landmarks  101 - 106  or articles  108  for effective navigation, the optical sensor  112  can be raised and lowered according to the navigational needs of robot  110 . For example, when the robot  110  is manipulating articles  108 , the robot  110  is likely to be stationary, and the optical sensor  112  can be directed to detecting articles  108  in the lowered position  144 , the robot  110  remembering its localization from when the landmarks  101 - 106  were last detected. Conversely, when the robot  110  is moving long distances, the robot  110  can remember the location of articles  108  and navigate with the optical sensor  112  in the raised position  142  to maintain line of sight on landmarks  101 - 106  for accurate localization. In combination with stored memory, a height adjustable optical sensor  112  can effectively replace the second optical sensor  116  in certain applications, reducing the cost and complexity of the robot  110 . 
     Referring to  FIG. 5 , a method for optical sensor-based localization of an autonomous mobile robot is shown generally at  500 . The method  500  includes a detecting step  502  followed by a determination step  504  and finally a calculating step  506 . In the detecting step  502 , an optical sensor assembly detects an optical reference. The optical sensor assembly may be a sensor assembly disposed on the autonomous mobile robot, detecting the optical reference. The optical reference in this case may be a static, passive, stationary landmark, or the optical reference may be a mobile landmark such as another mobile robot capable of self-relocation, for example. Alternatively, the optical sensor assembly may be external to the mobile robot, and the optical reference may be one or more features of the mobile robot itself which can be detected by the external optical sensor assembly. The optical sensor assembly in this case may be attached to a stationary object such as a tower, or may be disposed on a mobile base, such as another mobile robot, for example. In either case, at least one of the optical sensor assembly or the optical reference should remain stationary to provide a fixed reference point for the other. in the determination step  504 , a processing unit determines, based on the detected optical reference, a distance to the optical reference, a relative angle to the optical reference, and an orientation of the optical reference. The distance to and the relative angle to the optical reference may be acquired through the detection process itself, such as with a LiDAR sensor which can simultaneously acquire both sets of information from operation. In alternative systems, such as using an optical camera, the distance to the reference may be determined using methods such as stereoscopic triangulation, for example. The orientation of the optical reference may be determined using one or more optical features of the optical reference, such as through detecting multiple points of the optical reference and determining its orientation by calculating its facing based on the relative angle to each of the detected points, for example. The processing unit may be located on the mobile robot, or may be external to the mobile robot, being located for example on a stationary tower which may also have the optical sensor assembly, or the processing unit may be a local server or cloud server in communication with the mobile robot. The calculating step  506  involves calculating, by the processing unit, the orientation and position of the mobile robot based on the detected distance, orientation, and relative angle of the optical reference, as determined in determining step  504 , using a known relationship between the mobile robot, the optical sensor assembly, and the detected optical reference. Examples of the known relationship may include the location of the optical sensor assembly with respect  10  to the mobile robot such as whether it is on the robot or external, or a particular detected geometry of the optical reference with respect to the mobile robot such as the position of the optical reference on the mobile robot if the reference is attached to the robot. 
     Referring to  FIG. 6 , a method for optical sensor-based localization of an autonomous mobile robot during operation is shown generally at  600 . The method  600  includes a first detecting step  602 , a determining step  604 , a first calculating step  606 , a second detecting step  608 , a second calculating step  610 , a moving step  612 , a third detecting step  614 , and a third calculating step. In the first detecting step  602 , an optical sensor assembly disposed on the autonomous mobile robot located at a first position detects a first optical reference and a second optical reference. The first and second optical references may be special landmarks configured for the method for optical sensor-based localization, such as special cones (artificial landmarks) or self-propelled mobile robots (mobile landmarks), or may be a features natural to the environment (natural landmarks), which may be modified to increase detectability by the optical sensor assembly, for example. In the determining step, a processor determines based on the detected optical references a distance to each optical reference and a relative angle to each of the detected optical references. The distance to and the relative angle to the optical reference may be acquired through the detection process itself, such as with a LiDAR sensor which can simultaneously acquire both sets of information from operation. In alternative systems, such as using an optical camera, the distance to the reference may be determined using methods such as stereoscopic triangulation, for example. The first calculating step  606 , involves calculating, by the processor, the initial orientation and position of the mobile robot based on the detected distances and relative angles of the optical references. The processor may calculate the distance and relative angle to the first optical reference and the distance and relative angle to the second optical reference, defining the line between the two optical references as one coordinate axis and/or the width of the operating space, and a line perpendicular to the two detected optical references as the orthogonal axis, then calculates the position of the mobile robot based on the coordinate axes, for example. The method  600  then involves detecting further optical references by the optical sensor assembly in the second detecting step  608  and calculating, by the processor, the position of each further optical reference with respect to the first and second optical references in the second calculating step  610 . The processor may calculate the detected positions of the additional optical references based on the coordinate axes, for example, and one or more of the detected additional optical references may be used to define the lengthwise boundary of the operating space of the mobile robot, for example. 
     While the mobile robot is operating, the mobile robot will generally move from its initial position, the first position, to a second position, as in the moving step  612 . During and after this process, the mobile robot needs to be continuously “localized”. The method  600  does so by continually detecting, as in the third detecting step  614 , at least two of the previously detected optical references through the optical sensor assembly, allowing the processor to continue to accurately calculate the position and orientation of the mobile robot in the third calculating step  616  based on the detected distances and relative angles of the two detected optical references. The processor may keep track, in a memory, identities of each of the optical references such that the mobile robot remains localized in the coordinate axes, for example. 
     Referring to  FIG. 7 , a system implementing a method of optical localization is shown generally at  700 . The system  700  includes a number of elements similar to system  100  described above with reference to  FIG. 1 , such as a plurality of movable stationary landmarks  101 ,  102 ,  103 , and  104  defining a work field  107  with a base line  170  and boundary line  172  constraining the operating space of a mobile robot  110  with a mounted optical sensor assembly  112 . In the system  700 , however, there is a second mobile robot  710  with its corresponding optical sensor assembly  712 . As shown in  FIG. 7 , the optical sensor assembly  112  of the first robot  110  may have two optical references  103  and  104  within its rearward-facing field of view (as shown in  FIG. 1 ), as the sensor assembly  112  has direct line of sight  702  to each optical reference  103  and  104 . Additionally, the optical sensor assembly  112  is able to detect the second mobile robot  710  shown in this case by detecting its optical sensor assembly  712  which is asymmetrical allowing the first robot  110  to determine both the position and orientation of the second robot  710 , for example, but, in other embodiments, the first robot may be able to detect the second robot  710  through some other means such as detecting the second robot  710  with its second optical sensor assembly, or detecting the second robot  710  by detecting its manipulator, for example. However, the optical sensor assembly  712  of the second robot  710  is unable to detect two optical references (in this case,  101  and  102 ) with its rearward-facing field of view (as shown in  FIG. 1 ), due to an obstacle  708  occluding line of sight  707  to the optical reference  102 , the optical sensor assembly  712  only being able to detect the obstacle  708  with its line of sight  706  and not the optical reference  102 . Being able to only detect one optical reference  101 , the second robot  710  is unable to localize itself accurately. However, as the first robot  110  is able to detect two optical references  103  and  104 , and can detect the second robot  710  through having line of sight  704 , the first robot  110  can accurately localize the second robot  710 , and can do so collaboratively by determining the position and orientation of the second robot  710  and communicating the information with the second robot  710 . While, as shown in  FIG. 7 , the reason for the inability of the second robot  710  to localize itself is due to the presence of an obstacle  708 , the described method of localizing the second robot  710  using the first robot  110  also applies to any other case where the second robot  710  cannot localize itself, but the first robot  110  can localize itself and can detect and determine the relative position and orientation of the second robot  710 , such as when the second robot is too far from any optical reference, but the first robot  110  is within range. This can further be extrapolated to a third, fourth, etc. mobile robot allowing a chain of mobile robots to extend the radius of accurate localization without requiring additional landmarks, for example. Furthermore, this is applicable even if each mobile robot is itself moving, as long as one mobile robot can detect two stationary landmarks, allowing the chain or mobile robots to operate relatively far from stationary landmarks. 
     Referring to  FIG. 8 , a method for initializing a system for optical localization of an autonomous mobile robot is shown generally at  800 . The method  800  includes a placing step  802 , an identifying step  804 , a determining step  806 , a searching step  808  and a defining step  810 . The placing step  802  involves placing at least three optical references. The three optical references are placed at a known predetermined angle, which is ideally approximately 90 degrees for a rectangular operating space, but may be any other angle. The identifying step  804  involves identifying, by a processor of the mobile robot, two optical references that are detected by an optical sensor assembly of the mobile robot. 
     The identifying step  804  may involve concealing the two optical references, the two concealed references defining a first length of an operating space, from an optical sensor assembly of a mobile robot, followed by detecting, by the optical sensor assembly, an environment of the operating space. These steps are done to map the background features which can then be ignored by the localization system in order to remove potential outliers that may otherwise confuse the system in identifying the optical references. Finally, the two masked (concealed) optical references are unmasked to the optical sensor assembly and detected by the optical sensor assembly by comparing the detected features of the optical references with the background, the optical references can be clearly identified to the system despite the presence of outliers (the outliers may be additional optical references of other work spaces for other robots, for example. 
     In another embodiment, the identifying step  804  may involve detecting a plurality of potential optical references by the optical sensor assembly. The processor then ranks each potential optical reference according to a predetermined criteria, such as reflectivity, relative position to the mobile robot, size, shape, or any other detectable feature. The processor then selects two of the potential optical references as the identified optical references based on the criteria—for example, the processor may select the most intensely reflective references which are within the expected range of positions of the optical references in the predetermined shape. 
     After identifying the first two optical references, the method  800  proceeds to determining step  806 , which involves determining, by a processor of the mobile robot, the width of the operating space based on the distance between the two identified optical references. The two optical references may form one axis of the coordinate system, for example. The method  800  then proceeds to searching step  808 , which involves searching for and detecting, by the optical sensor assembly, the third optical reference, selected based on the relative angle of the location of the third reference with respect to the line formed by the two detected optical references. in this step  808 , the robot may be instructed to rotate or move, by a predetermined angle or distance sufficient for the optical sensor assembly to detect at least the third optical reference, or may be instructed to rotate or move until the third optical reference is detected in a predefined search pattern. In some embodiments, the searching for and detecting step  808  may involve detecting and identifying one or more intermediary optical references which do not define the operating space (such as optical references  103  and  104  of  FIG. 1 , for example) and the third optical reference may additionally be selected based on an expected distance from the first and second optical references. The robot may record and use the positions of the intermediary optical optical references with respect to the first, second and third optical references for determining the position of the robot within the operating space, such as when one of the first, second, or third optical references cannot be detected due to field of view or obstruction, for example. 
     Finally, the initialization method  800  concludes with defining step  810 , which involves defining, by the processor of the mobile robot, the length of the operating space as a perpendicular distance between the detected third optical reference and the straight line formed by joining the two detected optical references. The perpendicular direction of the perpendicular distance may form the orthogonal axis of the coordinate system, for example. With the robot localized and the operating space defined both lengthwise and widthwise, the initialization method  800  is now concluded and the robot may now operate in the operating space, using, for example, localization method  600  to localize itself during operation. The method  800  may then optionally include searching for and detecting further optical references, such as a fourth optical reference, which does not define the operating space. The further optical references can be used in place of the first, second, or third optical reference in determining the position of the robot within the operating space by knowing the relative position and angle of the further optical reference with respect to the first, second, and third optical references, such as when one of the first, second, or third optical references cannot be detected due to field of view or obstruction, for example. 
     In other embodiments, the operating space may not be a rectangular shape, but may be any polygonal shape. in such embodiments, the method of initialization can be used in a similar manner with respect to the first two optical references, and then detecting additional defining optical references in order to define the work field of the robot. The total number of defining optical references (including the first two optical references) is 3 for a n-sided regular polygon, and n for an n-sided irregular polygon. The expected angles of the vertices of the polygon should be predefined, and the robot searches for optical references along the predefined heading. For a regular polygon, the dimensions of the operating space can be defined by 3 optical references, extrapolating with the equal side lengths determined by the distance to the third optical reference. For an irregular polygon, each side length is defined by the distance from the previous optical reference to the next detected closest optical reference based on an expected angle dictated by the predefined heading. 
     The method for initializing a system for optical localization of an autonomous mobile robot  800  may be repeated with another set of optical references and/or predefined parameters to redefine or expand the operating space of the mobile robot, for example. 
     Referring to  FIG. 9 , this illustrates how the previously described system  100  of  FIG. 1  may be refined to incorporate a method for expanding the operating space of the mobile robot. The system  900  includes a mobile robot  901  and four landmarks  902 ,  903 ,  904 , and  905 , which define an operating space  910  within which the robot  901  may carry out tasks, using the landmarks  902 - 905  for optical localization during carrying out the tasks. In the embodiment shown, the task may be moving articles  920  such as potted plants from one side of operating space  910  (such as near landmarks  903  and  905 ) to the opposite side (such as near landmarks  902  and  904 ), for example. In this embodiment, operating space  910  may be a single bay in a plant nursery, and there may be other bays adjacent to the operating space  910  such as additional bays  912  and  914 . The bays  910 ,  912  and  914  may all be aligned and flanked by access pathways  916  and  918 , which are generally kept free of obstacles. Additional bays  912  and  914  may each have corresponding sets of articles  922  and  924  such as pots which are to be moved to the opposite end of their respective bays and arranged in an orderly fashion. In this scenario, once the robot  901  has completed the initial task of moving and arranging articles  920  in the operating space  910 , the robot is now idle. 
     Usually, an external agent such as a human operator must then manually move one or more of the landmarks  902 - 905  to new positions so as to define a new operating space, such as bay  912 , and manually move the robot to bay  912 . However, in the disclosed embodiment, the robot  901  recognizes that it has completed all available tasks assigned to it within operating space  910 , and additionally has tasks in additional bays  912  and  914  assigned to it. Upon completion of the tasks in operating space  910 , the mobile robot  901  then begins the process of moving the operating space  910  from its initial bay to bay  912 . To move the operating space  910 , the robot  901  moves landmark  902  to a first new position  906 , and landmark  903  to a second new position  907 . (Although not described in detail, the orientation of each repositioned landmark may also be taken into account when it is repositioned). New positions  906  and  907  are on the opposite side of, and substantially equally distant to, landmarks  904  and  905  compared to initial positions of landmarks  902  and  903 . Ideally, the landmarks  902  and  903  are moved one at a time, with the robot  901  relying on the remaining three landmarks to remain “localized”. To the extent that the effective optical range between the mobile robot (more precisely, the optical sensor on the mobile robot) and the landmarks might be a relevant consideration, it may be preferable to move landmarks  902  and  903  across the positions of landmarks  904  and  905 , so that the mobile robot  901  can move within a space where it remains within effective optical range of the localization system provided by the remaining three landmarks. For example, when the robot  901  is moving landmark  902 , it first moves from operating space  910  into the adjacent bay  912 , but staying relatively near landmarks  904  and  905  such that landmark  903  remains in effective optical range (to the extent that the optical range may be an issue). The robot  901  then moves into access pathway  916  and moves to pick up landmark  902 . The robot  901  then moves landmark  902  to new position  906  following path  930 . However, it is possible that when the robot  901  is moving along path  930 , it may reach a point where landmark  903  is out of effective optical range of the robot. The robot  901  can still carry out navigation based on the two remaining landmarks  904  and  905 . For example, while the landmark  903  may be out of effective optical range of the robot  901 , the landmark  903  may still be in functional range of the robot  901 . In such a case, the robot  901  may still be able to detect landmark  903 , but the distance/relative angle information may be relatively less accurate. However, the robot  901  remains within effective optical range of landmarks  904  and  905  at all times and is able to accurately detect distance and relative angle information from these two landmarks. Thus, through triangulation or trilateration, the robot  901  can at least narrow down its position/orientation. When landmark  902  is placed in new position  906 , the robot  901  may then navigate back to pick up landmark  903 , using landmarks  902  (at  906 ),  904  and  905  when the robot  901  is in bay  912 , and landmarks  903 ,  904  and  905  when it is in space  910 , for example. When landmark  903  is picked up, the robot  901  again uses the accurate information from landmarks  904  and  905  coupled with possibly less accurate information from landmark  902  (at  906 ) to navigate along path  932 , and place landmark  903  at new position  907 . The operating space  910  is now redefined as bay  912 , and the robot  901  can then carry out the task of moving and arranging articles  922  in bay  912  using the landmarks  904 ,  905 ,  902  (at  906 ), and  903  (at  907 ) for localization. 
     When the robot  901  has completed all tasks in the operating space  910  (now  912 ), it can repeat the process, this time moving landmarks  904  and  905  to new positions  908  and  909  along paths  934  and  936  respectively, redefining the operating space  910  as bay  914  in order to allow the robot  901  to move and arrange articles  924 . In this manner, the robot  901  can effect horizontal operating space expansion as the robot  901  can continuously move into adjacent operating spaces to continue operation. 
     Referring to  FIGS. 10A and 10B , an alternative system implementing a different method for expanding the operating space of a robot is shown generally at  1000 . The system  1000  includes a mobile robot  1001  and four landmarks  1002 ,  1003 ,  1004 , and  1005  located within a field  1010 . The robot  1001  and landmarks  1002 - 1005  are similar to the landmarks  92 - 95  of  FIG. 9 . 
     As seen in  FIG. 10A , to the extent that the effective optical range for the mobile robot may be an issue, the effective range of the mobile robot vis-a-vis the landmarks  1002 - 1005  determines an operating space  1014 , defined by border line  1015 . The operating space can be further divided into a drop-off area  1012 , defined by border line  1013 , and a pick-up area  1016 , defined by border line  1017 , on either side of the landmarks  1002 - 1005 . In this embodiment, the robot  1001  is tasked with moving a plurality of articles  1022 , such as potted plants, from the pick-up area  1016  to the drop-off area  1012 . In such a case, it may be desirable for the robot  1001  to autonomously expand the operating space  1014  such that additional articles  1022  may be accessed, so that the robot  1001  may complete its task of moving articles  1022  entirely autonomously without the need for an external party such as a human operator to monitor and/or assist the robot  1001  in redefining its operating space  1014 , for example. 
     Referring now to  FIG. 10B , the robot  1001  has completed its initial task of moving and arranging articles  1020  placed into what was drop-off area  1012  of  FIG. 10A , and what was pick-up area  1016  of  FIG. 10A  is now vacant. In order to access further articles  1022 , the robot  1001  now proceeds to expand the operating space  1014  vertically, within the same field  1010 . The robot  1001  first approaches landmark  1002 , and then transports it along path  1030  to a new position  1006 . The robot  1001  then repeats the process except with landmark  1003 , transporting it along path  232  to a new position  1007 . With the landmarks  1002 - 1005  now located at  1004 ,  1005 ,  1006 , and  1007 , the robot  1001  has now redefined the operating space  1014 . The region which was previously empty between the landmarks  1002 ,  1003  and landmarks  1004 ,  1005  in  FIG. 10A  is now defined as new drop-off area  1012 B by border line  1013 B. The robot can now repeat the task of moving and arranging articles  1022  from new pick-up area  1016 B to new drop-off area  1012 B, placing them next to the previously-placed articles  1020 . 
     The field  1010  may continue to extend for any length, and the robot  1001 , by following this method, will be able to eventually access and move all articles  1022  in field  1010 . For example, as seen in  FIG. 10B , there is a single row of articles  1022  not included in new pick-up area  1012 B. If the robot  1001  needs to also move these articles  1022 , the robot  1001  may repeat the above procedure, instead moving landmarks  1004 ,  1005  to new positions adjacent to the last row, thereby again redefining new pick-up and drop-off areas, for example. If there are even more articles  1022 , the robot  1001  may continuously repeat this process, by alternatively moving landmark sets  1002 ,  1003  and  1004 ,  1005  in a staggered manner to continuously redefine and effectively expand the operating space  1014  of the mobile robot  1001  to accommodate a vertically-extending field  1010  of any length. 
     Furthermore, the vertical operating space expansion of  FIGS. 10A and 10B  may be coupled with the horizontal operating space expansion of  FIG. 9  if the adjacent fields follow a specific configuration. If adjacent fields or bays are arranged in alternating fashion with articles clustered at alternating opposite ends, the robot can expand the operating space vertically along a first field according to the system shown in  FIGS. 10A and 10B , then expand the operating space horizontally into an adjacent field according to the system shown in  FIG. 9  once it has reached the end, then expand the operating space vertically in the opposite direction for the second field, expanding horizontally again, and repeating to cover a field arrangement of any size. 
     Referring to  FIG. 11 , an alternative embodiment of a robot-movable landmark is shown generally at  1100 . (The landmark  1100  may also be a UWB-based beacon (originally intended for use in system relying on localization using UWB), that is co-opted and repurposed for use in the present optical localization system of the present invention). In this embodiment, the landmark  1100  may comprise a base  1102 , a robot-interaction region  1104 , and an elevated portion  1106 . The base  1102  may optionally include various ports such as power and signal interfaces for charging or configuring the landmark. The landmark  1100  or the base  1102  may also include indicator lights for displaying the status of the landmark. The robot-interaction region  1104  preferably has a substantially similar shape to the articles, such that the robot can easily interact with the landmark  1100  using the same end effector of a manipulator that is used to interact with articles. In the disclosed embodiment, the articles may be cylindrical pots, and the landmark  1100  has a cylindrical robot-interaction region  1104  of similar dimensions to the pots (articles), such that the robot can easily interact with and transport the landmark  1100 . The elevated portion  1106  extends above the robot-interaction region  1104 . The additional height provided by the elevated portion  1106  may provide clearance over the articles and assists in providing an unobstructed line of sight with the raised optical sensor of the mobile robot. The elevated portion  1106  comprises one or more of: a characteristic cross-sectional geometry feature; a visually distinct portion; and a unique identified (as previously shown and described in  FIG. 3 , but which are not specifically depicted here so as not to obscure other details). The elevated portion  1106  may also provide other functionality, such as assisting human operators in identifying the operating space, for example. 
     Referring to  FIG. 12 , a method for expanding the operating space of a robot is shown generally at  1200 . The method includes a determining step  1202 , an assigning step  1203 , and executing step  1204  and a second assigning step  1209 . In the determining step  1202 , a processing unit determines that the mobile robot has completed a work task in a current operating space. The work task may be the last task assigned to the robot such that there are no further tasks to do in the operating space, and the robot may become idle without additional tasks assigned. In the assigning step  1203 , the processing unit assigns a relocation task to the mobile robot. In the executing step  1204 , the mobile robot executes the relocation task, the relocation task including a navigating step  1205 , and interacting step  1206 , a transporting step  1207 , and a repeating step  1208 . The executing step  1204  begins with the navigating step  1205 , which involves the mobile robot navigating to a first landmark of the one or more landmarks located at a first position using a localization system comprising the plurality of landmarks. The executing step  1204  then proceeds to the interacting step  1206  where the mobile robot interacts with the first landmark to ready the first landmark for transport, such as engaging the first landmark with the end effector of a manipulator on the mobile robot, for example. The executing step  1204  then involves transporting the first landmark to a second position for the landmark by the mobile robot, including navigating the mobile robot using the localization system, in the transporting step  1207 . If there are still other landmarks in the one or more landmarks to be moved, the executing step  1204  then proceeds to the repeating step  1208 , which involves repeating the steps of the executing step  1204  starting from the navigating step  1205  for each other landmark of the one or more landmarks to be moved. If all the landmarks have been moved, the method  1200  instead proceeds to the assigning step  1209 , where the processing unit assigns a new work task to the mobile robot in the operating space defined by new landmark positions. 
     While specific embodiments have been described and illustrated, such embodiments should be considered illustrative of the invention only and not as limiting the invention as construed in accordance with the accompanying claims.