Patent Publication Number: US-2023150137-A1

Title: Controlling multiple robots to cooperatively unload a truck or other container

Description:
CROSS REFERENCE TO OTHER APPLICATIONS 
     This application claims priority to U.S. Provisional Patent Application No. 63/274,464 entitled CONTROLLING MULTIPLE ROBOTS TO COOPERATIVELY UNLOAD A TRUCK OR OTHER CONTAINER filed Nov. 1, 2021 which is incorporated herein by reference for all purposes. 
    
    
     BACKGROUND OF THE INVENTION 
     Robots have been provided to perform a variety of tasks, such as manipulating objects. For example, a robotic arm having an end effector may be used to pick and place items. Examples of commercial applications of such robots include sortation, kitting, palletization, depalletization, truck or container loading and unloading, etc. 
     In some contexts, the objects to be handled vary considerably in size, weight, packaging, and other attributes. Typically, a robotic arm is rated to handle up to a maximize size, weight, etc. of object. In some contexts, the conventional approach may require a robotic arm able to handle the largest, heaviest, and/or otherwise most difficult object that may be required to be handled. 
     In some contexts, such as loading or unloading a truck or other container, the workspace constrains robot movement. For example, the system must limit the range of movement of the robotic arm to avoid having any part of the robot collide with the inner walls of the truck or container or other obstacles in the workspace. In addition, to work inside a truck or other container a robot must be able to fit and move within the constrained interior space, limiting the size and lifting capacity of an individual robot deployed to work in such a space. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various embodiments of the invention are disclosed in the following detailed description and the accompanying drawings. 
         FIG.  1    is a block diagram illustrating an embodiment of a robotic system configured to control a plurality of robots to perform a task cooperatively. 
         FIGS.  2 A- 2 C  illustrate an example of a cooperative pick and place task performed in an embodiment of a robotic system as disclosed herein. 
         FIG.  3    is a block diagram illustrating an embodiment of a robotic control system. 
         FIG.  4    is a state diagram illustrating an embodiment of a robotic system configured to control a plurality of robots to perform a task cooperatively. 
         FIG.  5 A  is a flow diagram illustrating an embodiment of a process to cooperatively perform a task as a “leader” robot in an embodiment of a robotic system as disclosed herein. 
         FIG.  5 B  is a flow diagram illustrating an embodiment of a process to cooperatively perform a task as a “follower” robot in an embodiment of a robotic system as disclosed herein. 
         FIG.  6 A  is a diagram illustrating an embodiment of a system to control multiple robots to cooperatively load/unload a truck or other container. 
         FIG.  6 B  is a diagram illustrating an embodiment of a system to control multiple robots to cooperatively load/unload a truck or other container. 
         FIG.  6 C  is a diagram illustrating an embodiment of a system to control multiple robots to cooperatively load/unload a truck or other container. 
         FIG.  7 A  is a diagram illustrating an example of robots working independently in an embodiment of a system to control multiple robots to cooperatively load/unload a truck or other container. 
         FIG.  7 B  is a diagram illustrating an example of robots working cooperatively in an embodiment of a system to control multiple robots to cooperatively load/unload a truck or other container. 
     
    
    
     DETAILED DESCRIPTION 
     The invention can be implemented in numerous ways, including as a process; an apparatus; a system; a composition of matter; a computer program product embodied on a computer readable storage medium; and/or a processor, such as a processor configured to execute instructions stored on and/or provided by a memory coupled to the processor. In this specification, these implementations, or any other form that the invention may take, may be referred to as techniques. In general, the order of the steps of disclosed processes may be altered within the scope of the invention. Unless stated otherwise, a component such as a processor or a memory described as being configured to perform a task may be implemented as a general component that is temporarily configured to perform the task at a given time or a specific component that is manufactured to perform the task. As used herein, the term ‘processor’ refers to one or more devices, circuits, and/or processing cores configured to process data, such as computer program instructions. 
     A detailed description of one or more embodiments of the invention is provided below along with accompanying figures that illustrate the principles of the invention. The invention is described in connection with such embodiments, but the invention is not limited to any embodiment. The scope of the invention is limited only by the claims and the invention encompasses numerous alternatives, modifications and equivalents. Numerous specific details are set forth in the following description in order to provide a thorough understanding of the invention. These details are provided for the purpose of example and the invention may be practiced according to the claims without some or all of these specific details. For the purpose of clarity, technical material that is known in the technical fields related to the invention has not been described in detail so that the invention is not unnecessarily obscured. 
     A robotic system is disclosed to control and use multiple robots, e.g., two or more robotic arms, to cooperatively unload or load a truck or other container. In various embodiments, robotic arms mounted on a frame or other structure are positioned in a truck or other container. To unload, the robots pick items from the truck and place them on a robotically controlled conveyor or other conveyance. For heavy or bulky items, two or more robots are used cooperatively to pick the item and place it on the conveyor or other destination. As unloading of a region of the truck or other container is completed, e.g., items within reach of the robots have all been unloaded but more items remain, the frame and robotic arms mounted thereon, and the conveyor, are extended (robotically) further into the truck or other container, and items comprising a next set of items within reach of the robotic arms are moved. To load, the frame/robots and conveyor are extended into the truck or other container, to a position near a far/back end of the truck/container, and the robots work cooperatively to load items into the truck/container as they arrive via the conveyor. 
     In various embodiments, a system as disclosed herein includes two robots with a conveyor or conveyors where packages are placed [picked] between them. 
     Most general robot arms that are mounted near a wall are prone to have a part of their arm (e.g., robot elbow) collide with the wall when moving the robot endpoint/load to a location on the opposite side of the robot from the wall. The problem is especially bad when using the most common type of industrial robots with kinematics with the first three joints in a roll-pitch-pitch configuration. The problem is exacerbated when using two of these arms such that they are placed side by side with an intervening conveyor and are therefore much closer to the walls of the truck than a single robot/conveyor system. The below description matches the robot locations and tasks that are part of this system as implemented in various embodiments. 
     In various embodiments, each robot base is mounted on a mounting surface that is angled away from the nearby wall of the truck or other container. An angle of the mounting surface 10°-20° from level, in various embodiments, greatly decreases the potential collision between the robot elbow and the truck wall. This greatly increases the usable workspace to pick packages in front of the robot and to place boxes on a conveyor or conveyors near the midline of the truck. In various embodiments, this placement of each robot&#39;s workspace greatly increases the usable (non-wall-collision) joint workspace of two robots to cooperatively pick packages as described above. 
     In various embodiments, robots move, or are moved, into the container/truck, as they unload packages and clear out space, they move, or are moved, further into the truck. In some embodiments, they are attached to a conveyor belt which moves along with the robots, and onto which the robots place the picked packages to remove them from the truck. Alternatively, there may be a central aisle way between the robots where a series of robot vehicles may park to receive packages placed on them to be carried out of the truck and/or in which a robotically controlled conveyor may be positioned and controlled. 
     A vision system identifies pickable packages in the scene, which it provides to the robots. The robots then prioritize picking the packages so they do not get too close to one another, to prevent collisions between the robots. Motion planning allows the robots to plan paths that avoid colliding the robots. In some embodiments, a hierarchical planner/scheduler enables each robot to plan its movements with knowledge of what the other robots plan to do, and potential collisions can be avoided and/or identified to trigger replanning by one or both robots. 
     In cases with only one output belt to place on, the robots alternate placing, to prevent collisions. In cases where each robot has its own output belt, they are allowed to operate at their own speed and do not need to synchronize their place motions. 
     In some embodiments, for objects stacked to the top of the truck/container, the robots may pick them from the side. Lower objects are generally picked from the top. Large or irregular objects are picked collaboratively with both robots. 
     In the case of truck or container loading, in various embodiments, initially robots move, or are moved, fully into the container/truck. As they load packages and build out the load, they move, or are moved, further out of the truck. In various embodiments, the robots may be attached to and/or otherwise work in conjunction with a robotically controlled conveyor belt which moves along with the robots, and from which the robots pick the packages to load into the truck. Packages may be fed to the conveyor located between the robots from a conveyor or other conveyance structure that extends from the truck/container into a loading dock or other area outside the truck. Packages may be fed into the input conveyance structures by human workers or other robots, for example. 
     Picking from a disordered jumble/pile of packages, as may occur when unloading a trucking having a variety of items packed therein, may cause other packages to move/tumble/fall. In some embodiments, a vision system is used to identify the new configuration and target a next package before the next pick is made. 
     In some embodiments, to get access to grasp a package that is out of reach, such as one at the top of a stack of packages, the system may identify, plan, and pick packages not on the top to cause a higher, out of reach package(s) to move (e.g., due to gravity) to a lower, pickable location. 
       FIG.  1    is a block diagram illustrating an embodiment of a robotic system configured to control a plurality of robots to perform a task cooperatively. In the example shown, system and environment  100  includes a first robotic arm  102  equipped with a suction-type end effector  104  and a second robotic arm  106  equipped with a suction-type end effector  108 . In the state shown, robotic arm  102  and robotic arm  106  are positioned to perform cooperatively a pick and place task with respect to a large box  110 . A control computer  112  is configured to communicate wirelessly with one or more of the robotic arm  102 , robotic arm  106 , and one or more cameras or other sensors  114  in the workspace. Image data received from camera  114 , for example, may be used by the control computer  112  to generate a three-dimensional view of the workspace and to send commands and information to the robotic arm  102  and robotic arm  106 , as appropriate, to facilitate the cooperative pick and place task. 
       FIGS.  2 A- 2 C  illustrate an example of a cooperative pick and place task performed in an embodiment of a robotic system as disclosed herein. In the example shown, in  FIG.  2 A  robotic arm  202  with suction type end effector  204  and robotic arm  206  with suction type end effector  208  are positioned to begin to perform cooperatively a pick and place task with respect to large box  210 , similar to the starting state shown in  FIG.  1   . In various embodiments, robotic arm  202  may be the “leader” and robotic arm  206  the “follower” in a cooperative pick and place as disclosed herein. The “leader” may be selected by any suitable method, such as by assigning the “leader” role to the robot that initiated the cooperative task, by assigning the role randomly to one or the other of the participating robots, by an “election” or other selection method. 
     To initiate the operation, in various embodiments, as “leader” robotic arm  202  would move its end effector  204  to the position shown and would then grasp the box  210 , e.g., by moving the end effector  204  into a position in contact or nearly in contact with the side of box  210  and applying suction. A signal may be sent to the other robot (and/or a process to control the other robot) to indicate that the leader has completed its grasp. The follower, e.g., robotic arm  206  in this example, would then grasp the box  210 , e.g., at a side opposite from the side at which the leader (i.e., robotic arm  202 ) had grasped the box  210 . The follower would record a transform based on the position and orientation of the leader&#39;s end effector  204  and the relevant dimension of box  210 . For example, the vision system and/or other sensors may be used to measure the dimension, or to recognize the box  210  (e.g., specifically and/or by type) and to use the item and/or type information to determine the dimension, e.g., by look up. 
     As shown in  FIG.  2 B , once both robots ( 202 ,  206 ) have grasped box  210 . The leader, robotic arm  202  in this example, computes and moves the box along a trajectory determined by robotic arm  202  (and/or a control process associated therewith) independently of the follower robotic arm  206 . In various embodiments, the follower robot, robotic arm  206  in this example, receives (e.g., periodically, continuously, etc.) position and orientation information for the end effector  204  of leader robotic arm  202 . The follower robotic arm  206  (and/or a control process associated therewith) uses the position and orientation information of the leader robot ( 202 ,  204 ) and the previously-determined and recorded transform to compute a new target position and orientation for the follower&#39;s end effector  208 , and computes and applies torques to motors comprising robotic arm  206  as needed to minimize the error (difference) between the current position and orientation of the follower&#39;s end effector  208  and the (most recently updated) target. 
     Once the object (box  210 ) has been placed in the destination position, as shown in  FIG.  2 C  for example, the leader robot (robotic arm  202 ) releases its grasp and informs the follower that the pick and place task has been completed. In response, the follower (robotic arm  206 ) releases its grasp and both robots ( 202 ,  206 ) are free to perform other work, such as (returning to) independently picking and placing smaller/lighter objects and/or cooperatively performing a next pick and place task for another large or heavy object. 
       FIG.  3    is a block diagram illustrating an embodiment of a robotic control system. In various embodiments, the robotic control system  302  of  FIG.  3    includes or is included in the control computer  112  of  FIG.  1   . In various embodiments, one or more modules or subsystems comprising the robotic control system  302  of  FIG.  3    may be distributed across multiple computing nodes, such as computers and/or processors comprising one or more of control computer  112 , robotic arm  102 , and/or robotic arm  106  of  FIG.  1   . 
     In the example shown, robotic control system  302  includes a hierarchical planner, scheduler, and/or control module comprising a robot cooperation facilitation module  304  configured to facilitate cooperative performance of tasks by two or more robots, as disclosed herein, and robot-specific controllers  306  and  308 . For example, robot  1  controller  306  may be associated with robotic arm  102  of  FIG.  1    and/or robotic arm  202  of  FIGS.  2 A through  2 C , while robot  2  controller  308  may be associated with robotic arm  106  of  FIG.  1    and/or robotic arm  206  of  FIGS.  2 A through  2 C . 
     In various embodiments, the respective robots associated with robot  1  controller  306  and robot  2  controller  308 , respectively, each may operate independently, e.g., to pick and place objects the robot is able to handle singly. In various embodiments, cooperative tasks using two or more robots may be initiated and/or performed by one or more of communications sent between robot  1  controller  306  and robot  2  controller  308 ; bilateral communications between robot cooperation facilitation module  304 , on the one hand, and the respective robot  1  controller  306  and robot  2  controller  308 , on the other; and/or communications among all three (or more) entities. 
     In the example shown, robotic control system  302  further includes a computer vision subsystem  310  configured to receive image and depth data from one or more 3D cameras and/or other sensors, such as camera  114  of  FIG.  1   , and to use the received data to generate and/or update a three-dimensional view of the workspace. The output of the computer vision subsystem  310  may be provided to one or more of the robot cooperation facilitation module  304 , robot  1  controller  306 , and robot  2  controller  308 , to enable them to initiate and perform cooperatively a task to pick and place an item. For example, image data may be used to determine that a box or other object is too large and/or too heavy for a single robot to pick and place. The three-dimensional view of the workspace and objects within may also be used to determine respective grasp strategies and/or locations for each robot, to determine collision-free trajectories to move each robot&#39;s end effector to its corresponding pick location, and to determine a collision-free trajectory through which to cooperatively move the object to the destination location at which it is to be placed, for example. 
       FIG.  4    is a state diagram illustrating an embodiment of a robotic system configured to control a plurality of robots to perform a task cooperatively. In various embodiments, the state diagram  400  of  FIG.  4    may be implemented by and/or with respect to a robot configured to cooperatively perform an operation using two or more robots. In some embodiments, the state diagram  400  of  FIG.  4    may be implemented by control computer  112  of  FIG.  1    and/or one or more of robot cooperation facilitation module  304 , robot  1  controller  306 , and robot  2  controller  308  of  FIG.  3   . 
     In the example shown, in state  402  a robot works independently to perform tasks. For example, the robot may independently pick and place items, such as to fill a box or other receptacle in a kitting operation, place items on a conveyer belt or other conveyance in a sortation operation, stack items on a pallet, etc. Upon receiving an indication that help is needed to perform a task ( 404 ), such as an indication that an item that has been perceived and which needs to be picked and placed is too large to grasp and move with one robot, the robot and/or controller transitions to a state  406  in which cooperative performance of the task is initiated. For example, a communication may be sent to another robot (e.g., from robot  1  controller  306  to robot  2  controller  308  of  FIG.  3   ) or to a higher-level planner/scheduler (e.g., robot cooperation facilitation module  304  of  FIG.  3   ), or the higher-level planner/scheduler may recognize the need for cooperative performance of the task and may initiate the transition to state  406 . 
     In the example shown, the robot and/or controller may transition back to working independently in state  402 , via a “cancel help” transition  408 . For example, the robot/controller and/or a higher-level planner/scheduler may determine that the task has already been performed by and/or assigned to one or more other robots. 
     In some embodiments, in the “initiate cooperation” state  406 , the robot/controller that is initiating cooperative performance of the task communicates directly or indirectly with a helper robot, e.g., by requesting help. Another robot may be assigned to help and/or may agree to help. The robot may be assigned and/or agree to help at a future time or upon occurrence of a future condition, such as completion of a task the helper robot has already started and/or a task that has higher priority. For example, a task to clear other objects from around the large or heavy object, to facilitate the cooperative task, may have a higher priority and therefore may be completed first. Once the helper robot is ready to perform the cooperative task, the helper robot informs the task initiator, directly or indirectly (e.g., via a higher-level planner/scheduler, such as robot cooperation facilitation module  304  of  FIG.  3   ), that the helper robot is ready, prompting a transition  410  to “start cooperation” state  412 . The helper may transition directly from working independently, in state  402 , to “start cooperation” state  412 , via the “give help” transition  414  in the example shown. 
     Once all participating robots are ready in the “start cooperation” state  412 , a “leader” is determined, if needed, and the leader transitions ( 416 ) to “do leader” state  418  while the follower(s) transition ( 420 ) to “do follower” state  422 . In the “do leader” state  418  and “do follower” state  422 , the leader and follower(s) cooperate as disclosed herein to cooperative perform the task, such as to pick and place a large or heavy object, as in the example illustrated in  FIG.  2 A through  2 C . Once the task has been completed, the leader and follower(s) transition ( 424 ,  426 ) back to the “work independently” state  402  and resume working independently. 
       FIG.  5 A  is a flow diagram illustrating an embodiment of a process to cooperatively perform a task as a “leader” robot in an embodiment of a robotic system as disclosed herein. In various embodiments, process  500  of  FIG.  5 A  may be implemented by a robot controller associated with a robot that is participating as the “leader” in cooperative performance of a task by two or more robots as disclosed herein. 
     In the example shown, at  502  an indication to begin a cooperative task (with one or more other robots) in the role of “leader” is received. For example, an indication to cooperatively perform a pick and place task may be received. At  504 , the leader determines a location at which to grasp the object and plans a trajectory to safely move its end effector into position to grasp the object and at  506  the leader moves its end effector along the trajectory to the grasp position. At  508 , the leader determines (independently of any other robot) a trajectory to move the object to an associated destination. For example, a model of the robot and its kinematics and image and/or other information about the workspace (e.g., configuration data, CAD files, etc.), one or more attributes of the object (e.g., dimensions, rigidity, etc.), and image/sensor data may be used to plan the trajectory. At  510 , an indication is received from the “follower” robot(s) with which the robot implement process  500  is to cooperate that the follower robot(s) is/are ready to begin cooperative performance of the task. In response, at  512  the “leader” robot moves its end effector (and the object in the joint grasp of the leader and follower(s)) to the destination along the trajectory determined by the leader. At  514 , upon placing the object at the destination the leader robot releases its grasp and informs the follower robot(s) that the task has been completed. In various embodiments, the leader then resumes operating independently. 
       FIG.  5 B  is a flow diagram illustrating an embodiment of a process to cooperatively perform a task as a “follower” robot in an embodiment of a robotic system as disclosed herein. In various embodiments, process  520  of  FIG.  5 B  may be implemented by a robot controller associated with a robot that is participating as the “follower” in cooperative performance of a task by two or more robots as disclosed herein. 
     In the example shown, at  522  an indication is received to begin performing a task cooperatively with one or more other robots in the “follower” role, as disclosed herein. At  524 , the follower determines a grasp point—e.g., one on an opposite side of the object from the side at which the “leader” has indicated it will grasp the object—and plans a trajectory to move into position to grasp the object at that point. At  526 , the follower moves its end effector to the determined grasp position and grasps the object, e.g., in response to receiving an indication that the leader has completed its grasp. At  528 , the leader&#39;s end effector position and orientation information are received, and the follower uses this information along with information about the object (e.g., the size of the object in the dimension that separates the leader&#39;s end effector and the follower&#39;s end effector) and computes a transform. In various embodiments, the transform comprises a matrix or other mathematical construct that can be applied to the position and orientation of the leader&#39;s end effector, typically expressed in the leader&#39;s frame of reference, to provide a corresponding position and orientation for the follower&#39;s end effector that would maintain the relative position and orientation of the follower&#39;s end effector with respect to the leader&#39;s end effector as the end effectors and the object grasped between them are moved through the workspace to the destination at which the object is to be placed. At  530 , the follower robot informs the leader that the follower is “ready”, e.g., the follower has grasped the objected, computed the transform, and is ready to maintain the position of its end effector relative to (e.g., opposite) the leader&#39;s end effector. 
     At  532 , as the leader robot begins to move along the trajectory determined independently by the leader, the follower uses the transform it computed and successively received position and orientation information for the leader&#39;s end effector, as it is moved through the workspace. For example, for each of at least a subset of the received positions and/or orientations of the leader&#39;s end effector, the follower computes a new goal position and/or orientation for its own end effector and applies torques to it motors as determined to be needed to minimize the error (e.g., difference) between the current position and/or orientation of its end effector and the current goal. 
     At  534 , the follower receives an indication (e.g., from the leader) that the cooperative task is “done”, in response to which the follower releases its grasp and the process  520  ends. 
       FIG.  6 A  is a diagram illustrating an embodiment of a system to control multiple robots to cooperatively load/unload a truck or other container. In the example shown, system and environment  600  includes a truck or other container  602  with many boxes stacked inside. While the boxes as shown in  FIG.  6 A  are uniform in size and regular in their dimensions, in various embodiments the payload may include objects of varying sizes, weight, rigidity, packaging type, and other attributes. While the boxes shown in  FIG.  6 A  are neatly stacked, in various embodiments the payload may include a disorganized pile of items or a mix of relatively more organized and less organized sets of items. 
       FIG.  6 A  includes at bottom a rear view (bottom left) and side view (bottom right) of a robotic truck loading/unloading system  604 . In the example shown, robotic truck loading/unloading system  604  includes a frame  606  dispose on a moveable and/or mobile chassis  608 . In some embodiments, chassis  608  enables the robotic truck loading/unloading system  604  to be moved manually into position, such as by being pushed into position by human or robotic workers. In some embodiments, the chassis  608  is self-propelled and may be positioned (e.g., advanced into or backed out of a truck or other container) under robotic control. For example, once positioned manually in the deepest part of a truck or container (for loading) or at the rear access opening (for unloading), the chassis  608  may be propelled under robotic control to advance as needed to continue unloading or back out to a new position to continue loading. 
     Referring to  FIG.  6 A , in the example shown a first robot  610  and second robot  612  are positioned on opposite sides of an aperture (opening) defined by the frame  606 . In various embodiments, the robots  610 ,  612  are controlled to cooperatively load/unload a truck or other container, as disclosed herein. A 3D camera  614  mounted on the top cross member of frame  606  provides image and depth information usable by a computer vision system, such as computer vision subsystem  310  of  FIG.  3   , to provide a three-dimensional view of the interior workspace in which the robotic truck loading/unloading system  604  is currently using the robots  610 ,  612  cooperatively to load or unload the truck. 
       FIG.  6 B  is a diagram illustrating an embodiment of a system to control multiple robots to cooperatively load/unload a truck or other container. In the example and state shown, the robotic truck loading/unloading system  604  of  FIG.  6 A  has been positioned in the rear opening of a fully loaded truck  602 . For example, truck  602  may have arrived at and backed into a loading dock or bay, such as at a warehouse or distribution center. A robotically controlled conveyor  620  is positioned to feed to a downstream location boxes unloaded from the truck  602  and placed on conveyor  620  by the robotic truck loading/unloading system  604 . In some embodiments, the conveyor  620  comprises a component and/or system separate from the robotic truck loading/unloading system  604  and which is moved into a position between the robots  610 ,  612 , e.g., after the robotic truck loading/unloading system  604  has been moved into position. In other embodiments, the conveyor  620  comprises an integral part of the robotic truck loading/unloading system  604  and remains in position between the robots  610 ,  612  as the chassis  608  is moved to position the robotic truck loading/unloading system  604 , e.g., as shown. 
     In the example and state shown in  FIG.  6 B , conveyor  620  feeds boxes onto a further conveyor  622 , which carries the boxes (e.g., boxes  624 ,  626 ) to a further downstream position. At the further downstream position, other workers (e.g., humans, other robots) may perform further tasks with respect to boxes unloaded from truck  602 , such as to unpack them, further move them to storage or staging locations, send them on for transportation to other destinations, etc. 
       FIG.  6 C  is a diagram illustrating an embodiment of a system to control multiple robots to cooperatively load/unload a truck or other container. In  FIG.  6 C , the outline of the truck  602  is shown in a dashed line and robotic truck loading/unloading system  604  is shown in a position deep within the truck  602 , continuing an unloading operation. As shown, boxes of varying sizes remain stacked within truck  602 . Boxes  624  and  626  have been picked from within the truck by the robots  610 ,  612  and placed on conveyor  620 , which carries them out of the truck  602  and deposits them onto conveyor  622 , in this example. 
     In various embodiments, as the robotic truck loading/unloading system  604  continues to unload the truck  602 , once boxes within reach have been unloaded if further boxes remain to be unloaded the robotic truck loading/unloading system  604  advances, e.g., under robotic control and self-propulsion, further into the truck  602  to put robots  610 ,  612  in position to reach a next layer or other range or set of boxes or other objects. 
     In various embodiments, a robotic truck loading/unloading system as disclosed herein, such as robotic truck loading/unloading system  604 , is configured to use two or more robots to cooperatively perform a task in connection with a truck (or other container) loading/unloading operation. For example, the robots  610 ,  612  may work independently to load or unload objects that can be handled safely by a single robot but may be used to cooperatively perform a task to load or unload a large, bulky, and/or heavy box or other object. 
       FIG.  7 A  is a diagram illustrating an example of robots working independently in an embodiment of a system to control multiple robots to cooperatively load/unload a truck or other container. In the example shown, the robots  610 ,  612  are being used to independently load/unload objects, such as boxes  702 ,  704 , in an independent mode of operation, e.g., as in state  402  of  FIG.  4   . In various embodiments, the robots  610 ,  612  work independently but in a cooperative way when operating in the “independent” work mode. For example, in various embodiments, the robots  610 ,  612  alternate picking and placing items, to reduce the risk of collision or having to wait to avoid collision. As one robot (e.g.,  610 ) is placing an item on the conveyor  620 , the other robot (e.g.,  612 ) may be reaching to grasp a next item to be grasped by that robot. By the time the latter robot is ready to place, the former robot has moved clear of the place region of the conveyor. In some embodiments, each robot  610 ,  612  takes the other robot&#39;s planned trajectories into consideration in planning its own trajectory. For example, each may plan a trajectory that avoids intersecting a trajectory another robot plans to traverse. In some embodiments, if a risk of collision is detected, one or both robots may enter a short wait period, each of a separately determined random length, before resuming operation. 
       FIG.  7 B  is a diagram illustrating an example of robots working cooperatively in an embodiment of a system to control multiple robots to cooperatively load/unload a truck or other container. In the example shown, the robots  610 ,  612  are being used to cooperatively load/unload a large box  706 . In the example and state shown, the robots  610 ,  612  have grasped the box  706  on opposite sides. Techniques disclosed above are used by one or more of robotic truck loading/unloading system  604 , robot  610 , and robot  612 , to move box  706  to perform the required task, e.g., stacking box  706  on one or more other boxes to load truck  602  or placing box  706  on conveyor  620  (not shown in  FIG.  7 B , for clarity) to unload. For example, robot  610  may operate as the “leader”, implementing process  500  of  FIG.  5 A , while robot  612  may serve as the “follower”, implementing process  520  of  FIG.  5 B , or vice versa. 
     In various embodiments, techniques disclosed herein may be used to control multiple robots to cooperatively load/unload a truck or other container. 
     Although the foregoing embodiments have been described in some detail for purposes of clarity of understanding, the invention is not limited to the details provided. There are many alternative ways of implementing the invention. The disclosed embodiments are illustrative and not restrictive.