WO2012076029A1 - Method for safe robot motion with hazardous work piece - Google Patents

Abstract

The invention is related to a method to specify a movement of an end-effector (12, 32, 52, 86, 96, 98, 100, 102) of a robot (14a-f), whereas the movement is starting from a first point (20, 64) and proceeding along a predetermined curved movement path (18, 62) to a second point (22, 76) within a workspace of the robot (14a-f), whereas the end-effector (12, 32, 52, 86, 96, 98, 100, 102) comprises an interaction side (16) and whereas the alignment (24, 38) of the end-effector (12, 32, 52, 92, 86, 98, 100, 102) is defined to be approximately perpendicular thereto. The end-effector (12, 32, 52, 86, 96, 98, 100, 102) is at least predominately orientated antiparallel (68, 74, 80) to a belonging straightened tangent (66, 72, 78) of each position (64, 70, 76) along the curved movement path (18, 62). The invention is also related to a method to move an end-effector (12, 32, 52, 86, 96, 98, 100, 102) of a robot (14a-f) by the robot

Description

Method for Safe Robot Motion with Hazardous Work Piece

Description

The invention relates to a method to specify a movement of an end-effector of a robot, whereas the movement is starting from a first point and proceeding along a predetermined curved movement path to a second point within a workspace of the robot, whereas the end-effector comprises an interaction side and whereas the alignment of the end-effector is defined to be approximately perpendicular thereto.

It is known that the purpose of collaborative robots is to assist the human co-worker in manufacturing tasks that are executed in sequence, simultaneously or in conjunction. Each of the two focuses on their respective strengths; the robot on precision, repeatability, large forces on demand; the human on complex steps, frequently varying steps and flexibility. This means that the robot must manipulate parts and subassemblies in a safe manner in the presence of its human co-worker. Such parts can have hazardous edges and/or corners and must be handled with proper procedure in the presence of human workers, just as a human colleague would be careful about his manipulations.

Protection systems for robots normally do not allow any presence of a human worker within a certain safety zone around the robot. If the worker enters such a zone, an emergency stop will be the consequence. An approach for a safety system, which allows a collaboration of a robot with a human worker is disclosed for example in the patent document DE 10 2007 006 708.

Disadvantageously in the state of the art is that it is only foreseen to predict and avoid a dangerous situation based on a more or less fixed movement path of the robot, which is not intrinsically optimized concerning the security of a collaborating worker. Based on this state of the art it is the objective of the invention to provide a method for the specification of the movement of a robot respectively a method to move a robot, which intrinsically reduces the risk of a dangerous situation of the worker while collaborating with the robot.

This problem is solved by a method to specify a movement of an end-effector of a robot of the aforementioned kind respectively a belonging robot movement. This is characterized in that the end-effector is at least predominately orientated antiparallel to a belonging straightened tangent of each position along the curved movement path.

The basic idea of the invention is to use the several and possibly redundant degrees of freedom in the motion of the robot to specify a more safe motion. The path chosen for the motion trajectory, the speed the robot moves at, and the orientation of the end-effector can be specified variously.

Thus, the details of the motion that the robot carries out with a potentially hazardous work piece held by its end-effector can be arranged so that the potential injury hazard to the human colleague is reduced. A typical robot comprises six or seven degrees of freedom in movement, whereas six degrees of freedom in movement are sufficient to reach every coordinate within the working range of the robot in every desired orientation. A robot foreseen for gripping purposes is normally provided with an end effector comprising the capability to grip or hold a work piece, for example a finger gripper or a suction gripper. While attaching such types of end-effector with the work piece, usually both a certain coordinate and a certain orientation of the gripper are required, so that there is no redundancy of a degree of freedom in movement for a robot with six degrees of freedom in movement.

On the other hand, while moving the coupled or gripped work piece from a first point to a second point, a certain orientation of the end effector respectively a work piece coupled therewith is not required. The basic boundary condition is that a collision of the end-effector or the work piece with the collaborating human or with other objects in the working environment must be avoided. Three of the degrees of freedom in movement are required to move along a movement path, which normally is defined by a set of subsequent coordinates. Thus the remaining degrees of freedom in movement can be used to adapt the whole movement of the robot in that way that the risk for a dangerous situation for the collaborating human is intrinsically reduced.

The invention aims at reducing the risk associated with a work piece gripped by the end-effector or with the gripper. The risk of a collision of a collaborating human with a work piece held by the end effector of the robot is highest if the work piece is moved directly toward the collaborating human, and it is lowest if the work piece is moved away from the collaborating human. Thus, according to the invention, the end effector must be oriented opposite to the actual direction of motion along the whole movement path. Furthermore, not the work piece but the robot arm itself is supposed to be moved towards the collaborating human. However, in this case the risk for a collaborating human is lower, since the robot is rather large, so that it cannot be overlooked and also because the robot arm can be shaped in such a way that its external contour is rather softly shaped. Thus, a movement of the robot is specified that reduces in an advantageous way the risk of a dangerous situation for a human worker while collaborating with the robot.

In a variant of the invention the end-effector is a gripper whereas the gripper fingers usually are aligned approximately in parallel to the open gripper fingers. A gripper with gripper fingers is on one the one hand a rather flexible tool, which is suitable for a large variation of different shaped objects. On the other hand, the gripper fingers partly encapsulate some areas of the work piece when it is gripped, so that also a hazardous edge of a work piece, which provides a high risk for a collaborating human, might be shielded by the gripper fingers. The alignment of the gripper usually is defined to be parallel to the gripper fingers. This is based on the assumption that a gripped work piece - if it is not shielded by the gripper fingers - extends column-like in the backward direction out of the gripper along a belonging tangent of the movement path. As described before, this assures the maximum security for a collaborating human since the work piece is straight moved away from the collaborating human.

According to another embodiment of the invention, the gripper is assumed to have gripped a work piece between its gripper fingers, whereas a first part of the work piece, which lies not directly between the gripper fingers, is exposed, so that it might be contacted by a cooperating human worker. Thus a second part of the work piece is encapsulated by the gripper fingers and not exposed.

In the case that the gripped work piece comprises in its exposed first part a hazardous edge the alignment of the gripper is defined according to the orientation of the hazardous edge of the gripped work piece. Hence the alignment of the workpiece determines the alignment of the gripper and is defined by a vector between the hazardous edge and a rear center point of the gripper. Preferably the rear center point is in the middle of a flange of the robot arm, where the gripper flange is mounted on. Dependent on how the gripper respectively has gripped the workpiece, this might cause some smaller differences to the definition of the alignment of the gripper without gripped work piece, which is assumed to be approximately in parallel to the gripper fingers. Thus the end-effector is orientated in such a way, that the alignment of the work piece is at least predominately orientated antiparallel to a belonging straightened tangent of each position along the curved movement path.

Background for defining the alignment of the gripper equal to the orientation of the hazardous edge of a gripped work piece is that exactly the hazardous edge shall be moved along the belonging tangent of the actual position of the gripper on the movement path. The hazardous edge is the most dangerous part for a collaborating human, so it has always to be oriented in that way, that it is pointing directly away from the collaborating human. Even this definition of the alignment of the gripper might differ from the definition of the alignment of an empty gripper, namely in parallel to the gripper fingers, both definitions are covered by the same idea. So the alignment according to the orientation of the hazardous edge relative to the gripper must be seen as sub-case of the aforementioned definition of the alignment of the gripper.

The problem is also solved by a method to move an end-effector of a robot by the robot, comprising the step performing a movement from a first point to a second point specified according to the invention. The advantages of this method to move an end- effector concur with the aforementioned advantages to specify such a movement. According to a special embodiment of the invention, the method to move an end- effector of a robot by the robot comprises the additional step of determining the orientation of the hazardous edge of the gripped work piece before performing a movement from a first point to a second point. This might be required, if a work piece with hazardous edge is gripped, whereas it is not known, how the hazardous edge is oriented. So this must be evaluated at first, for example by using a vision system with camera. Afterwards the orientation of the gripper must be chosen in such a way, that the orientation of the hazardous edge is at least predominately orientated antiparallel to a belonging straightened tangent of each position along the curved movement path.

According to other variants of the invention, the steps of moving from the second point to an end point with a predetermined end-orientation of the end-effector and/or of moving from a start point with a predetermined start-orientation of the end-effector to the first point are included. As mentioned before, a gripping or releasing of the work piece might require six degrees of freedom in movement of the robot, so there is no degree of freedom left for an intrinsically risk-minimizing orientation of the gripper at the start- and endpoints of the movement path. The start point represents the point of gripping the work piece and the endpoint represents the point of releasing the work piece. So such a start- and end-movement might be in the simplest case a pure re-orientation whereas the belonging point on the movement path stays the same.

It might be advantageous to continuously supervise the movement of the robot and/or the end-effector by a supervision system. If risk of a collision is detected, the robot movement must be stopped or modified, for example slowed down. This increases once again the security for a collaborating human.

Since the overall speed of the robot with gripper might be slowed down due to the increased time for re-orientation, it is useful to perform the antiparallel orientation of the end-effector respectively the gripper only in those sections of the curved movement path, where a human worker cooperating with the robot is exposed to a risk or a dangerous situation caused by the robot movement. Thus, if for some movement tasks no human collaborator is present within the working area of the robot or within a protected safety zone around the robot, all tasks for protecting a human worker are only required if there is any risk of a human worker entering the area.

Thus it might be sufficient to activate the protective measures, for example the anti- parallel orientation of the gripper, only at movement speeds of the robot, which exceed a certain limit, for example 100mm/s. A lower speed can be assumed not to pose a hazard for a human collaborator. Faster movement speeds can lie in the range of 500mm/s and higher.

According to another variant of the invention, it is foreseen to perform an initial offline verification of the planned movement and not to perform the planned movement, with the actual robot when above verification fails or if a collision is predicted. This might be useful for rather large work pieces that can be gripped in a multitude of orientations relative to the gripper. Here the risk of a collision of the work piece with surrounding objects, such as other robots or other equipment, might be rather high. So collisions can be avoided by such a preceding initial off-line verification, which can be done automatically by using an off-line simulation program which is known by a person skilled in the art, whereas either a separate computing device or a belonging robot controller can be utilized. Such an off-line verification should be performed automatically and might require only a few seconds.

Further advantageous embodiments of the invention are mentioned in the dependent claims.

The invention will now be further explained by means of an exemplary embodiment and with reference to the accompanying drawings, in which:

Figure 1 shows a robot with end-effector,

Figure 2 shows a robot wrist with first gripper,

Figure 3 shows a robot wrist with second gripper and gripped work piece and Figure 4 shows exemplary steps of a gripper movement.

Fig. 1 shows an arrangement 10 of a robot 14a-f with end-effector 12. The robot itself is indicated to comprise a kinematic chain of six members, so that it has six degrees of freedom in movement. Those members are the robot base 14a, which can be rotated around its axis of rotation 26. Connected therewith in a pivotable way is a first part 14b of a robot arm, which is connected on its other side in a rotatable way with a second part 14c of the robot arm. At the tip of the robot arm a robot wrist is mounted, comprising the parts 14d, 14e and 14f. The robot wrist 14d-f itself provides three degrees of freedom of motion. Thus the robot wrist is an important part of the robot for changing the orientation of the end-effector in such a way that the risk for a collaborating human is reduced intrinsically. The workspace around the robot 14a-f is the area which can be reached by the tip of the robot arm, for example an area with a radius of 3m around the robot base 14a. The end-effector 12 is mounted on the outer side of the kinematic chain of the robot members. An end-effector as such is a device or a tool, which is suitable for performing some working steps such as welding or gripping. In this case the end-effector is assumed to provide some gripping ability. Thus it could be a suction device, an electromagnetic device or a gripper with gripper fingers for example.

The robot is controlled by a robot controller 29, which has stored a robot program within its internal memory. The robot program comprises a set of coordinate values, which define the course of a curved movement path 18, starting at a first point 20 (A) and leading to a second point 22 (B). The robot controller 29 provides signals to the robot 14-f respectively to the different drives for each of its members, to perform a desired movement. In the case that the robot 14a-f has to grip known work pieces which can only be gripped in a clearly defined position relative to the end-effector 12, the robot program might comprise also the desired orientation of the end-effector along the curved movement path 18. In this example, the alignment 24 of the end- effector 12 is antiparallel to the tangent on the actual position on the curved movement path 18. For the case that there are several variants of work pieces to be held, a supervision system comprising for example a camera 28 is useful. Hence the gripping process itself might be controlled or supervised in such a way that a hazardous edge of a work piece is either shielded within the gripper fingers of a gripper or - if this is not possible - that at least the orientation of an exposed hazardous edge relative to the end-effector respectively the gripper is determined. Such a supervision system can be integrated within the robot controller 29 or provided as separate component. Fig. 2 shows a robot wrist 40 with a first gripper 32 in an illustration 30. The robot wrist 40 consists of three members who are turnable each to each other so that the robot wrist can change its orientation between both ends by 180° or even above. So the robot wrist 40 is an important component to perform a movement of the gripper according to the invention. The gripper 32 comprises two gripper fingers 34, 36, whereas of course also a larger number of gripper fingers or differently shaped gripper fingers 34, 36 are conceivable. The gripper fingers 34, 36 can be opened and closed as indicated with the arrow 42, so that a work piece can be clamped between the gripper fingers 34, 36. The alignment 38 of the gripper 32 is in parallel to its gripper fingers 34, 36, assuming that a work piece clamped between the fingers extends along the alignment axis 38.

Fig. 3 shows a comparable robot wrist with a second gripper 52 in an illustration 50. In-between the gripper fingers 54, 56 a work piece 58a+b is clamped. A second part 58a of the work piece is shielded between the gripper fingers, so it is not exposed, whereas a first part 58b is exposed, since it sticks out of the gripper. The first part 58b of the work piece comprises a hazardous edge 55, which is a potential risk for a collaborating human to be injured. The orientation of the hazardous edge 55 relative to the gripper 52 is defined by a vector 59 from a rear center point 53 at a central flange for the connection to a robot and pointing to the hazardous edge 55 itself. The orientation of the hazardous edge 55 of the work piece 58a+b is not in parallel to the gripper fingers, so in this case the gripper 52 will be moved at a slight angle along and relative to a curved movement path to provide a high degree of intrinsic safety. The alignment of the gripper is defined to be identical to the vector 59.

Fig. 4 is showing some exemplary steps of a gripper movement along a second curved movement path 62 in an illustration 60, whereas a complete robot is not shown. At a first point 64 of the curved movement path 62 a third gripper 86 connected to a robot wrist 92, has gripped a work piece 94 with hazardous edge between its gripper fingers 88, 90. The orientation of the gripper 86 in this stage of the movement is not optimized for an increased intrinsic safety. Moreover, it depends for example on the requirements for a gripping procedure and possibly no redundant degree of freedom in movement is available. In the next step from P1 to P2 a reorien- tation 82 of the gripper 86 into another orientation position takes place. Now the gripper 96 is orientated antiparallel 68 to the straightened tangent 66 at the first point, whereas the gripper is still at the same first point 64 of the curved movement path 62. Now the gripper is moved according to the invention along the curved movement path from P2 to P3 in such a way, that the orientation of a hazardous edge of the work piece 94 is predominantly in parallel to the belonging tangents at the movement path, as shown in exemplary manner for the intermediate point 70, the belonging straightened tangent 72 and the antiparallel orientation 74. Thus it is ensured that the hazardous edge of the work piece 94 is always moved directly away from a possibly collaborating human. The opposed side which might be moved directly to a collaborating human consists mainly of the robot wrist 92 respectively other members of the not shown robot. Therefore those members should be shaped in a rounded and soft manner to ensure that a possible collision with a collaborating human causes only a minor risk. At the final point 76 the movement along the curved movement path 62 is finished, whereas the orientation of the gripper 100 is antiparallel 80 to the straightened tangent 78. A final reorientation movement 84 around the second respectively final point 76 from P3 to P4 brings the gripper into a final position, at which the gripped work piece 94 can be released.

List of reference signs robot with end-effector

end effector

a-f robot

interaction side of end effector

first curved movement path

first point

second point

alignment of the end-effector

rotation axis of robot base

camera

robot controller

robot wrist with first gripper

first gripper

first gripper finger of first gripper

second gripper finger of first gripper

alignment of gripper in parallel to gripper fingers

components of robot wrist

movement direction of gripper fingers

robot wrist with second gripper and gripped work piece

second gripper

rear center point of gripper

first gripper finger of second gripper

hazardous edge of work piece

second gripper finger of second gripper

a non-exposed second part of work piece

b exposed first part of work piece

orientation of hazardous edge of work piece relative to second gripper exemplary steps of a gripper movement

second curved movement path

first point

straightened tangent at first point antiparallel orientation of end effector at first point intermediate point

straightened tangent at intermediate point

antiparallel orientation of end effector at intermediate point second point

straightened tangent at second point

antiparallel orientation of end effector at second point re-orientation movement at first point

re-orientation movement at second point

third gripper in first orientation at first point

first gripper finger of third gripper

second gripper finger of third gripper

robot wrist

gripped work piece

third gripper in second orientation at first point

third gripper at intermediate point

third gripper in first orientation at second point

third gripper in second orientation at second point

Claims

Claims

1. Method to specify a movement of an end-effector (12, 32, 52, 86, 96, 98, 100, 102) of a robot (14a-f), whereas the movement is starting from a first point (20, 64) and proceeding along a predetermined curved movement path (18, 62) to a second point (22, 76) within a workspace of the robot (14a-f), whereas the end-effector (12, 32, 52, 86, 96, 98, 100, 102) comprises an interaction side (16) and whereas the alignment (24, 38) of the end-effector (12, 32, 52, 86, 96, 98, 100, 102) is defined to be approximately perpendicular thereto, characterized in that

the end-effector (12, 32, 52, 86, 96, 98, 100, 102) is at least predominately orientated antiparallel (68, 74, 80) to a belonging straightened tangent (66, 72, 78) of each position (64, 70, 76) along the curved movement path (18, 62).

2. Method according to claim 1 , characterized in that the end-effector (12, 32, 52, 86 96, 98, 100, 102) is a gripper (32, 52, 86, 96, 98, 100, 102) with gripper fingers (34, 36, 54, 56, 88, 90).

3. Method according to claim 2, characterized in that the gripper (32, 52, 86, 96, 98, 100, 102) has gripped a work piece (58a+58b, 94) in between its gripper fingers (34, 36, 54, 56, 88, 90), whereas a first part (58b) thereof is exposed and whereas a second part (58a) is shielded between the gripper fingers (34, 36, 54, 56, 88, 90).

4. Method according to claim 3, characterized in that the gripped work piece (58a+58b, 94) comprises in its exposed first part (58b) a hazardous edge (55), whereas the alignment (68, 74, 80) of the end-effector (12, 32, 52, 86, 96, 98, 100, 102) is defined by a vector (59) between a rear center point (53) of the gripper and the hazardous edge (55).

5. Method to move an end-effector (12, 32, 52, 86, 96, 98, 100, 102) of a robot (14a- f) by the robot, comprising the steps • performing a movement from a first point to a second point specified according to any of the claims 1 to 4

6. Method to move an end-effector of a robot by the robot, comprising the steps

• determining the orientation (59) of the hazardous edge (55) of the gripped work piece (58a+58b, 94)

• performing a movement from a first point (20, 64) to a second point (22, 76) specified according to claim 4

7. Method according to claim 5 or 6, comprising the step

• movement from the second point (22, 76) to an end point with a predetermined end-orientation of the end-effector (12, 32, 52, 86, 96, 98, 100, 102)

8. Method according to any of the claims 5 to 7, comprising the step

• movement from a start point with a predetermined start-orientation of the end- effector (12, 38, 52, 86, 96, 98, 100, 102) to the first point (20, 64)

9. Method according to any of the claims 5 to 8, comprising the step

• continuous supervision of the movement of the robot (14a-f) and/or the end- effector (12, 38, 52, 86, 96, 98, 100, 102) by a supervision system (28)

• modify or stop movement, when a risk of a collision is detected

10. Method according to any of the claims 5 to 9, characterized in that the antiparailel orientation of the end-effector respectively the gripper is only performed in those sections of the curved movement path, for which a human worker cooperating with the robot is exposed to a risk or a dangerous situation caused by the robot movement.

11. Method according to claim 10, characterized in that the risk is caused by a movement speed of the robot, which exceeds a certain limit.

12. Method according to claim 10, characterized in that the risk is caused by a presence of the human worker within a safety zone around the robot.

13. Method according to any of the claims 5 to 12, comprising the additional proceeding step

• off-line verification of the planned movement

• not performing the planned movement, if the off-line verification fails