Abstract:
A system and method for controlling motion interference avoidance for a plurality of robots are disclosed, the system and method including a dynamic space check system wherein an efficiency of operation is maximized and a potential for interference or collision is minimized.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Patent Application Ser. No. 60/791,724 filed on Apr. 13, 2006, hereby incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to a system for controlling a plurality of robots and a method for controlling motion interference avoidance for the plurality of robots. 
     BACKGROUND OF THE INVENTION 
     Movement of objects in space is a necessary task in a typical manufacturing environment. Robotics have increasingly been employed to effect the necessary movement. However, when multiple objects are being moved, a potential for interference between the objects exists. An interference exists if the at least two objects share the same space at the same time. That is, the objects have the same coordinates with respect to a common frame of reference. 
     With modern industrial robots moving at considerable velocities, interferences between robots can result in a collision and undesirable damage to the robots and work pieces being handled by the robots. The collisions may lead to costly down time in the manufacturing process. Accordingly, it is desirable to avoid such collisions. 
     Prior art systems and methods have been used in an attempt to minimize interferences and collisions. However, there are several shortcomings of the prior art systems and methods. Typically, a tool center point (TCP) is only checked relative to a predetermined interference area (static space). For multiple robots, it is difficult to directly or effectively prevent the collision or interference thereof. Further, it is difficult to specify an interference space in respect of a static coordinate system for multiple moving robots. Any interference space is not only a function of the robot motion path, but also a function of the motion speed. Difficulty also exists in attempting to handle a deadlock situation when two or more robots request to move to a common space at the same time. 
     Prior art systems also attempt to prevent a TCP for a robot from colliding in a fixed space relative to its world coordinate system. When multi robots (with multiple controllers) share common spaces or so called interference spaces during a task execution, each controller has to wait until no robot is in the common spaces, and then the controller can issue the motion control commands to allow the robot to move. This process is also called a wait and move process, which generally increases working cycle time. Draw backs exist when using the prior art systems with multiple robots. It is difficult to effectively specify an interference space in terms of a fixed coordinate system, because the interference space is not only the function of the robot motion path but also the motion speed. When more than one robot requests to move to a common space at the same time, it creates a deadlock situation where none of the robots can move because they are waiting for one another. 
     It would be desirable to have a system and method for controlling motion interference avoidance for a plurality of robots, wherein an efficiency of operation is maximized and a potential for interference or collision is minimized. 
     SUMMARY OF THE INVENTION 
     Consistent and consonant with the present invention, a system and method for controlling motion interference avoidance for a plurality of robots, wherein an efficiency of operation is maximized and a potential for interference or collision is minimized, has surprisingly been discovered. 
     In general terms, the invention relates to a system and method of controlling motion of a plurality of robots on the rail and a plurality of the controllers, wherein robots controlled by each controller will only work on a specified dynamic space, thus avoiding a collision. Control objectives are realized by defining a set of dynamic spaces for each controller, an ability to monitor the dynamic spaces in real time, and control mechanisms that are able to hold or unhold a motion of the robot controlled by each individual controller to militate against interference between robots within the dynamic space. 
     In one embodiment, the system for controlling motion interference avoidance for a plurality of robots comprises a plurality of controllers adapted for connection to a source of power; a sequence of instructions residing on the controller for execution thereon, the sequence of instructions including a dynamic space check control method; a plurality of robots in communication with and controlled by the controllers; and a programmable logic device in communication with the controllers. 
     The invention also provides methods for controlling motion interference avoidance for a plurality of robots. 
     In one method according to the invention comprises the steps of providing a plurality of controllers adapted for connection to a source of power, a plurality of robots in communication with and controlled by the controllers, and a programmable logic device in communication with the controllers; and providing a sequence of instructions including a dynamic space check control method residing on the controller for execution thereon, the dynamic space check control method further comprising the steps of setting up dynamic spaces for each of the robots; updating the dynamic spaces; detecting the dynamic spaces; measuring interferences; a motion hold command; a motion release command; and a deadlock decision. 
     In another method according to the invention comprises the steps of providing a plurality of controllers adapted for connection to a source of power, a plurality of robots in communication with and controlled by the controllers, and a programmable logic device in communication with the controllers; and providing a sequence of instructions including a dynamic space check control method residing on the controller for execution thereon, the dynamic space check control method further comprising the steps of setting up dynamic spaces for each of the robots including one of a spherical method, a cylindrical method, and a box method; updating the dynamic spaces including sending updated reference information from one of the controllers to other of the controllers via a network media at an interpolated interval; detecting the dynamic spaces including receiving the updated reference information from neighboring controllers at the interpolated interval; measuring interferences including reconstruction of the dynamic space of the neighboring controllers and calculation of a minimal distance between the dynamic space of the neighboring controller and the dynamic space of the one of the controllers, wherein the step of calculation of a minimal distance between the dynamic space of the neighboring controller and the dynamic space of the one of the controllers incorporates time, distance, and one of mechanical inertia of acceleration and deceleration; a motion hold command; a motion release command; and a deadlock decision. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       The above, as well as other advantages of the present invention, will become readily apparent to those skilled in the art from the following detailed description of a preferred embodiment when considered in the light of the accompanying drawings in which: 
         FIG. 1  is a schematic diagram showing an example of a plurality of robots on a rail with a network media and PLC connected to each controller; 
         FIG. 2  shows three examples of modeling method for dynamic spaces that are relative to the TCP position; 
         FIG. 3  shows a typical example of setting up the dynamic spaces for a series of the top loader robots on the rail; 
         FIG. 4  illustrates the concept of the TRACE of the dynamic space; it also shows how to calculate the minimal distance between the two dynamic spaces in terms of the trice of the dynamic space; 
         FIG. 5  illustrates an example of automatic motion hold and unholds control flow with two controllers; 
         FIG. 6  shows the deadlock prevention in different situations; and 
         FIG. 7  is a schematic flow diagram showing a dynamic space check method according to an embodiment of the invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The following description is merely exemplary in nature and is not intended to limit the present disclosure, application, or uses. It should be understood that throughout the drawings, corresponding reference numerals also indicate like or corresponding parts and features. In respect of the methods disclosed, the steps presented are exemplary in nature, and thus, are not necessary or critical. 
       FIG. 1  is a schematic diagram showing a robotic system  10  known as a rail application. The robotic system  10  includes a plurality of robot controllers  12  for controlling at least one of a plurality of robots  14 , the controllers in electrical communication with a source of electrical power. In the embodiment shown, each of the controllers  12  includes an associated motion system (not shown) to control two attached top loader robots  14  designated as R 1  and R 2 , although other types and configurations can be used as desired. The controllers  12  are in communication with one another by a computer communication network  16  with a programmable logic device (PLC)  18  and are connected by network media  20 . Each of the controllers  12  can execute a sequence of an instruction or program residing within the controller  12 . In the rail application, the robots  14  move along a rail  22  and can perform independent or in a coordinated motion. 
       FIG. 7  shows a dynamic space check control method  24  according to an embodiment of the invention. In general, the control method  24  involves several steps. One step of the dynamic space check control method  24  involves setting up a dynamic space  26  for each controller  12 . There are a variety of ways to set up a dynamic space in terms of a geometric model and a reference point. In  FIG. 2 , three methods for setting up a dynamic space  26  are shown: a spherical method  28 ; a cylindrical method  30 ; and a box method  32 . The methods  28 ,  30 ,  32  shown are for dynamic spaces that are relative to a tool center point (TCP) position, although other methods can be used as desired. Additionally, other shapes and configurations can be used without departing from the scope and spirit of the invention. Each modeling utilizes a set of parameters. For the spherical method  28 , TCP, an offset OFFSET from the TCP, and a radius R of the sphere are used. For the cylindrical method  30 , TCP, an offset OFFSET from the TCP, a height of the cylinder H, and a radius of the cylinder R are used. TCP, an offset OFFSET from the TCP, a height of the box H, a length of the box L, and a width of the box are used for the box method  32 . For purposes of the method disclosed herein, it is assumed that all of the methods  28 ,  30 ,  32  have the same orientation as the TCP position. It is understood that other configurations can also be used. 
       FIG. 3  illustrates a method of setting up the dynamic spaces  26  for the robots  14 . For purposes of illustration, it is assumed that all of the robots  14  have a common world coordinate frame and the world coordinate Y-axis is aligned with a longitudinal axis of the rail  22 . Each controller  12  is controlling the pair of robots  14  designated by R 1  and R 2 . A first axis of each robot  14  is aligned with the rail  22 . A set of dynamic spaces for each controller  12  is defined by a low boundary LB and a high boundary HB relative to a Y-axis position of the robot  14  designated by R 1  represented by an offset  1  OFS 1  and an offset  2  OFS 2 , respectively. The low boundary LB and the high boundary HB are defined as planes that are substantially perpendicular to the rail  22  and move along the first axis of the robot  14  designated by R 1 . Therefore, the low boundary LB and the high boundary HB specify a space region for each controller  12 . A minimal distance between the high boundary HB (or the low boundary LB) to the low boundary LB (or the high boundary HB) of a neighboring controller is designated DIST. A motion of the robots  14  is controlled by the different controllers  12  so that DIST is always greater than a margin distance, thus militating against an interference among the dynamic spaces. 
     Another step of the dynamic space check control method  24  involves updating the dynamic spaces  34  during a task execution. Because the set up dynamic space  26  is attached to a reference frame or a position, the set up dynamic space  26  dynamically changes during movement of the robot  14 . Each controller  12  sends updated reference information to the other controllers  12  via the network media  20 . The updated reference information is sent in a selectable interval of time, normally at an interpolated interval ITP. For the top loader robot  14  application, the reference information is typically a Y coordinate position (a position on the rail) of one of the robots  14  designated by R 1  for each of the controllers  12 . The reference information is sent to a neighboring controller  12  via the network media at every interpolated interval ITP. 
     Next, the dynamic spaces are detected  36  across the controllers  12 . Each controller  23  receives the updated reference information from other controllers at a desired interval. Typically, the desired interval is a multiple interval of the interpolated interval ITP, depending on a capacity of the network media  20 . For the top loader robot  14  application, each controller receives the updated reference information of the neighboring controllers  12  at every interpolated interval ITP. Based on the updated reference information, each controller  12  calculates the dynamic spaces, the low boundary LB, and the high boundary HB for the neighboring controllers  12  with respect to its own reference frame. 
     A measurement of interferences  38  of the dynamic spaces is then determined between the controllers  12 . The measurement of interferences  38  step further involves reconstruction of the dynamic space of the neighboring controller  12  and a calculation of the minimal distance DIST between the dynamic spaces. The distance measured between the dynamic spaces incorporates mechanical inertia of acceleration or deceleration, time, and distance in order to effectively militate against interference between the dynamic spaces. In order to illustrate the measurement, the following terminologies are used:
         Current position (cur_pos) refers to an instantaneous measurement of a position at the present time.   Current frame (cur_frm) refers to an instantaneous measurement of a frame at the present time.   Command position (cmd_pos) refers to an interpolated position at the present time that the robot intents to move to.   Command frame (cmd_frm) refers to a coordinate frame at the present time, derived from a set of cmd_pos.   Current dynamic space (cur_ds) refers to a space occupation at present time derived from one or set of cur_pos or cur_frm.   Command dynamic space (cmd_ds) refers to a space occupation at present time derived from one or set of cmd_pos or cmd_frm.   Trace of the dynamic space (tr_ds) refers to a total dynamic space occupation from cur_ds to the cmd_ds.   Minimal distance of two dynamic spaces is defined by minimal distance of their traces of the dynamic spaces.       

       FIG. 4  illustrates how the distance DIST of the dynamic space is calculated in terms of a TRACE of dynamic space for different configurations, where C 1  and C 2  represent respective first and second controllers  12 , and each cube represents a TRACE of a dynamic space. Left and right cubes for each configuration represent respectively the dynamic space HB for C 1  and LB for C 2 .
         Configuration  1 , R 1  for C 1  and C 2  moving in +Y (move same direction), DIST equals the distance from cmd_ds of C 1  to cur_ds of C 2 .   Configuration  2 , R 1  for C 1  and C 2  move toward each other. DIST equals the distance from cmd_ds of C 1  to cmd_ds of C 2 .   Configuration  3 , R 1  for C 1  and C 2  moves away from each other. DIST equals the distance from cur_ds of C 1  to cur_ds of C 2 .       

     A motion hold command  40  is issued by the controllers  12  to stop the robots  14  at command positions whenever the measured minimal distance DIST between the dynamic spaces equals or is less than a specified marginal distance. The hold command militates against the minimal distance DIST being less than the marginal distance. 
     A motion unhold command or motion release command  42  is issued by the controllers  12  once the measured minimal distance DIST is greater than the specified marginal distance. Accordingly, all robots  14  being held resume motion.  FIG. 5  illustrates an example of how the motion hold command and the motion unhold command control flow between two controllers  12 . Each controller  12  independently conducts the dynamic space calculation  36  and the distance measurement  38 , and makes a motion control decision. However, each controller  12  sends or receives reference information to or from the other controller  12  such that dynamic space is calculated based on updated reference information. Data sending or receiving is accomplished via the network  16  connected between the controllers  12 . 
     The dynamic space check control method  24  includes a step of making a deadlock decision  44 . Typically, when a potential interference is detected, the motion hold command  40  is issued to the robots  14 . To prevent a deadlock, where the robots will not resume motion, the deadlock decision step  44  is activated. The deadlock decision step  44  checks the measured distance DIST of the dynamic space and checks a rate of change of the measured distance DIST. If the motion of the robot  14  will result in an increase of the measured distance DIST from the previous measured distance DIST, then the robot  14  will be issued the motion release command  42 , even if the current measured distance DIST is less than the marginal distance.  FIG. 6  depicts the control logic.
         The trailing robot in case A and B of  FIG. 6  will be held until the interference is cleared due to the motion of the leading robot.   The leading robot in case A and B of  FIG. 6  will not be held because the motion of the leading robot will clear the dynamic space interference for the trailing robot.   If neighboring robots move toward each other as in C of  FIG. 6 , the system will hold both robots (or both pairs) upon the detection of the space interference. Manual intervention is required to clear this situation.   If one robot is stationary while the other robot (or robot pair) is moving toward the stationary robot as in D of  FIG. 6 , the system will hold the moving robot (or robot pair) only.       

     The deadlock decision step  44  allows an operator to get out the deadlock situation as quickly as possible. In the meantime, the dynamic space check control method  24  to militate against the deadlock situation also maximizes a production cycle efficiency. 
     The dynamic space check control method  24  can be easily defined in terms of a specified moving frame or reference point of interest relative to the robot  14  within the controller  12 . The dynamic space check control method  24  is dynamically updated with its reference frame or reference point during task execution. Additionally, the dynamic space check control method  24  can be easily represented across controllers  12  because it is easier to pass a reference frame or a reference point from one controller  12  to another controller  12  via network media  20  than to pass a geometric space. Interference avoidance is also facilitated for multiple moving robots  14 . The motion hold command and the motion release command of the dynamic space check control method  24  also optimizes a cycle time for the robots  14 . The dynamic space check control method  24  also voids using hardware to achieve the required functionality. 
     For coordinated motion between groups of robots  14  controlled by a common controller  12 , a control system known as a space control manager system is provided within each common controller  12 . The space control manager system controls traffic or priority for a common space accessed by one of the robots  14  or group of robots  14 . The traffic or priority can be controlled for each common spaced accessed, within or across controllers  12 , group motion, or independent motion. This facilitates additional functionality for multiple robots to access a common space. The additional functionality includes support for multiple robot  14  groups dynamic space check control within one controller  12 , multiple robot  14  groups across the controllers  12 , multiple robot  14  groups teach pendent (TP) program execution, concurrent single robot  14  group TP program execution, multiple robot  14  group TP program and single robot  14  group TP program execution, coordinated motion, and jog operation for multiple group motion. 
     In order to facilitate the additional functionality, additional control methods are provided within the controllers  14 . For multiple robot  14  groups dynamic space check control within one controller  12 , the common space control manager controls the priority for involved groups dynamically for each common space Priority of the group motion is determined according to the following rules. Priority is assigned on a first come, first served basis. Thus, the group arriving the earliest according to a waiting queue is assigned the highest priority. For group motion or coordinated motion, all the robots  14  within the groups have the same priorities when the groups are in the waiting queue waiting for a space to be cleared. 
     For multiple robot  14  groups across the controllers  12  accessing a common space, the controller  12  whose group or groups of robots  14  occupy the space has a higher priority than another controller  12 . The space control manager determines a priority of its own controller  12  based on the status of the other controller  12 , as well as the status of its own controller  12 . When its own controller has a higher priority, dynamic space check control within one controller  12  is followed. When no group occupies the common space and no group is in the waiting queue, the space control manager&#39;s own controller surrenders control, which allows the other controller  12  to take control if a group or groups are waiting in the queue to access the common space. If two robots  14  from different controllers  12  intend to access the common space at the same time, the controller with a higher priority assignment will be permitted to access the common space first. 
     The space control manager controls a state transition for each of multiple robot  14  groups that access a common space. For each state transition, the common space manager takes an action within motion sub-systems interrupt (INTR) or filter (FLTR) tasks to verify that group motion requirements will not be violated. For movement OUT→IN, access to the space is permitted, and the space is designated as occupied. In respect of movement OUT→WAIT, access to the space is not permitted, motion is held, and the waiting priority for the motion group is updated in the queue. For WAIT→IN, access is permitted into the space, the group motion is released, and the waiting priority for the motion group in the queue is updated. An occupation designation for the space is cancelled for IN→OUT movement. For movement IN→IN and OUT→OUT, motion is not restricted since the space status is not changed. 
     For concurrent single robot  14  group TP program execution, the space control manager assigns the same priority to each group member in the group to permit access to the common space. When one of the groups has to wait due to an occupied space, all of the groups within the group are held simultaneously. The space control manager allows the holding group or groups to complete an interpolated position for a current interpolation period (ITP) to maintain the group motion relationship while all groups stop at a held position. 
     When releasing groups when the space becomes available, the space control manager releases the motion simultaneously. This allows the group motion relationship to be maintained while motion release is occurring. 
     When using an interpolated position to check the space, the TCP is caused to wait inside of the space because the space control manager allows the interpolated point to complete. Causing the TCP to wait inside of the space may produce an ambiguous state, and result in a race condition after power is restored. It is more desirable to cause the TCP to wait outside of the space. At each interpolated stage, the TCP is determined by the actual command position of the TCP and the delta position calculated by the speed and a predication rate. Accordingly, the TCP can be stopped prior to actually entering the space using the TCP calculated according to this method. 
     The primary deadlock control involves prevention as well as easy handling if it does occur. Ideally, the motion control manager will not allow the deadlock to occur in normal operation condition. However exception may occur under following conditions: the space is enabled dynamically while multiple robots are in the same space; a program is aborted due to an error while multiple robots are in the same space; and a TP program is started while multiple robots are in the same space. The space control manager facilitates a recovery if a deadlock occurs. Any deadlock groups can be jogged if inside of a common space. Once the the group is jogged outside of the deadlocked space, the group cannot be jogged inside the space again unless the space is cleared. Non-deadlocked groups outside of the space can be jogged, but the non-deadlocked groups cannot be jogged into a deadlocked space until the space is cleared. A TP program involving groups not deadlocked can be executed. However, these groups involved in the TP program cannot enter the deadlocked space during execution of the TP program. Additionally, if an attempt is made to execute a TP program with groups involved in the deadlock, the system will abort the program and return to deadlock. Finally, if the deadlock involves groups from two controllers, recovery of the deadlock from either controller will recover the other controller from deadlock as well. 
     In accordance with the provisions of the patent statutes, the present invention has been described in what is considered to represent its preferred embodiment. However, it should be noted that the invention can be practiced otherwise than as specifically illustrated and described without departing from its spirit or scope.