Method for Aligning a Robotic Arm

Abstract

A method for aligning a robotic arm in a superordinate reference pose is specified, —where the robotic arm includes a plurality of robotic joints, each of which includes a drive device which enables rotation about an associated axis of rotation, where an associated eccentric lever element is formed for at least two selected robotic joints by one or more other partial elements of the robotic arm, the method including: aligning the drive device of a first selected robotic joint in an automated manner in a first target position in which the associated first eccentric lever element is disposed in a reversal position, aligning the drive device of a second selected robotic joint in an automated manner in a second target position in which the associated second eccentric lever element is disposed in a reversal position, where the sub-steps are repeated in an iterative loop until the change in angle effected in each sub-step falls below a predetermined limit value.

Claims

1. A method for aligning a robotic arm in a superordinate reference pose, where said robotic arm comprises a plurality of robotic joints, each of which comprises a drive device which enables a rotation about an associated axis of rotation, where an associated eccentric lever element is formed for at least two selected robotic joints by one or more other partial elements of said robotic arm, said method comprising the sub-steps of: i) aligning said drive device of a first selected robotic joint in an automated manner in a first target position in which the associated first eccentric lever element is disposed in a reversal position ii) aligning said drive device of a second selected robotic joint in an automated manner in a second target position in which the associated second eccentric lever element is disposed in a reversal position, where sub-steps i) and ii) are repeated one after the other in an iterative loop until the change in angle effected in each sub-step falls below a predetermined limit value.

2. The method according to claim 1, in which said drive devices of said at least two selected robotic joints each comprise an electric drive machine, comprise an output member which is rotatable relative to the associated axis of rotation by way of said drive machine, comprise a current sensor for measuring an operating current flowing within said electric drive machine, and where said automated alignment in the first target position can be effected for each of said drive devices by the following sub-steps a) successively moving to a plurality of predetermined angular positions of said output member, measuring an associated current value for each predetermined angular position by way of said current sensor, b) determining a target position from the pair of values thus determined, such that the associated current value of the target position comes as close as possible to a zero-crossing.

3. The method according to claim 2, in which step a) comprises the following sub-steps: a1) successively actuating a first sequence of predetermined angular positions such that said output member is continuously rotated in a fixed first direction of rotation, a2) successively actuating a second sequence of predetermined angular positions such that said output member is continuously rotated in an oppositely directed second direction of rotation, and where in step c), a first reference angle is determined from the pairs of values of the first sequence determined, and a second reference angle is determined from the pairs of values of the second sequence determined, a superordinate target position is determined by averaging said first and said second reference angles.

4. The method according to claim 1, where the total number of robotic joints is between 3 and 7.

5. The method according to claim 1, in which an associated eccentric lever element is formed for a number n of three or four selected robotic joints by one or more other partial elements of said robotic arm, where the method comprises an iterative loop in which the following sub-step is carried out in each run of the loop one after the other for all n selected robotic joints: s-i) aligning said drive device of said respective selected robotic joint in an automated manner in an associated target position in which said associated eccentric lever element is disposed in a reversal position, where the loop is to be run until the change in angle effected in each sub-step falls below a predetermined limit value.

6. The method according to claim 1, in which the sequence in each run of the iterative loop in which said individual selected robotic joints are aligned runs continuously from the outside to the inside.

7. The method according to claim 1, in which the sequence in each run of the iterative loop in which said individual selected robotic joints are aligned runs continuously from the inside to the outside.

8. The method according to claim 1, in which either all existing robotic joints or all with the exception of the innermost and/or outermost robotic joints are selected robotic joints which are aligned in an automated manner within the iterative loop.

9. The method according to claim 1, in which said selected robotic joints each comprise a rotary position sensor as part of said drive device for determining an angular position of said output member.

10. The method according to claim 9, in which said individual rotary position sensors are calibrated with the local angular reference position derived from said superordinate reference pose of said robotic arm.

11. The method according to claim 1, in which said selected robotic joints each comprise a torque measuring device as part of said drive device for measuring a torque acting within said drive device.

12. The method according to claim 11, in which said individual torque measuring devices are calibrated by measuring the torque in said superordinate reference pose of said robotic arm.

13. The method according to claim 1, in which, after reaching said superordinate reference pose, a selected robotic joint is moved to an angular position in which said associated eccentric lever element causes a maximum torque.

14. The method according to claim 1, in which the entire alignment of said robotic arm in said superordinate reference pose is repeated several times at intervals.

15. The method according to claim 14, in which the repeated alignment of said robotic arm in said superordinate reference pose is used to monitor incorrect settings of individual elements of said robotic arm.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0048] The invention shall be described hereafter by way of a few preferred embodiments with reference to the appended drawings, in which:

[0049] FIG. 1 shows a schematic perspective illustration of a robotic arm,

[0050] FIG. 2 shows a sequence of several poses of a simplified sketched two-axis robotic arm, and

[0051] FIG. 3 shows a sequence of several poses of a simplified sketched three-axis robotic arm.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0052] Same elements in the figures or those with the same function are denoted with the same reference characters. FIG. 1 shows a schematic perspective illustration of a robotic arm 100 which can be aligned using the method according to the invention. This is a robotic arm with seven robotic joints J1 to J7, each of which enables a rotation about an associated axis of rotation R1 to R7. This is therefore a robotic arm with seven rotational degrees of freedom. “Innermost” joint J1 is connected to a base B which serves as a superordinate mechanical mass. “Outermost” joint J7 can carry an end effector (not shown in detail) at location TCP. A drive device each is arranged within individual joints J1 to J7. These are rotary drives for rotating the individual joints, the basic structure of which and their mechanical mode of operation are known from prior art. Two or more of the rotary joints shown can be aligned with the method according to the invention. They are therefore referred to as “selected joints” in the context of the invention. A superordinate reference pose has been reached at least for these selected joints after having run through the method, as shall become more evident in the context of FIGS. 2 and 3.

[0053] The so-called selected joints are those with respect to which an eccentric lever element, which can develop an angle-dependent torque due to gravity, is formed with other partial elements of the robotic arm. The selected joints can therefore be made to assume a reversal position in an automated manner with respect to this lever using the method according to the invention. For this purpose, the associated axes of rotation should be able to be aligned with at least one horizontal directional component. Both requirements are presently fulfilled at least in terms of joints J2, J3 and J5: An eccentric lever arm is respectively formed by the more outer parts of the robotic arm, and the associated axis of rotation can assume a horizontal direction in a vertically extended pose (along the z-axis). A superordinate reference pose Pr can therefore be defined such that joints J2, J3 and J5 cause a vertical extension. The angular positions of remaining joints J1, J4, J6 and J7 can there either remain freely adjustable or made to assume a target position in some other way.

[0054] FIG. 1 serves as an example of how both “selected robotic joints”, which can be aligned in an automated manner using the method, as well as other joints can come together in a robotic arm 100. In principle, however, robotic arms 100 can also be implemented in which at least the majority of joints or even all of the joints are so-called selected joints. Joint J1 close to the base could in principle also be a selected joint, for example, if it were to be in a horizontal axial position due wall mounting or an L-shaped connection member on the base. The outermost joint (presently J7) could also be a selected joint, for example, if a tool were mounted at the tool center point (TCP) which would form an eccentric lever element. In principle, it would also be possible for all joints therebetween to be selected joints, for example, if the joint connections (i.e., the linkages) were shaped in such a way that all joints J2 to J6 could assume a horizontal axis position within the x-y plane or at least with a significant directional component in the x-y plane. For the illustrative examples for carrying out the method in FIGS. 2 and 3, respective robotic arms are therefore assumed in which all the joints present are implemented as “selected joints”.

[0055] The inner structure of joint J3 is sketched in somewhat greater detail in FIG. 1 By way of example as one of the “selected joints”: This robotic arm J3 comprises a drive device 1 which enables rotation about axis of rotation R3. In this example, drive device 1 comprises an electric drive machine 5 which in principle can be operated both in the motor mode as well as in the generator mode. The rotor of this drive machine is coupled to an output member 40 via a drive shaft 7 for transmitting torque. Also provided optionally in the region between drive machine 5 and output member 40 can be a gear (presently not shown) which translates the rotation and indirectly imparts the coupling. It is only essential that a rotation of machine 5 is transmitted to output member 40. The elements which are disposed further outwardly as seen from joint J3 together form an eccentric lever element 200. By rotating the output member, this lever element 200 can be made to assume an upper reversal position. This position can in particular be recognized by a zero-crossing of the machine current and assumed in an automated manner. For this purpose, a control device 25 is arranged, for example, in the region of end cap 22 of the joint and comprises a current sensor (presently not shown) and, in addition to the functionality of a control device, also covers the functionality of an evaluation device. Individual angular positions of the output member can then be actuated in an automated manner with this control device 25, and the zero run of the current curve (possibly with averaging the values for the forward and reverse directions) can also be determined in an automated manner.

[0056] FIG. 2 shows a sequence of several poses P0 to Pr of a simplified sketched robotic arm with two rotary joints Ga and Gb. Both joints are to be selected joints, the respective axis of rotation of which is perpendicular to the vertical z-direction. Joint Ga is the joint near the base and is connected to base B by a linkage Ka. The two joints are connected to one another by a linkage Kb. An external linkage Kc carries an end effector, presently not shown. P0 is the starting pose from which the superordinate reference pose has been reached in several iterative steps with the method according to the invention. In reference pose Pr, all linkages are extended vertically in the z-direction and angular positions α and β of the two joints are each (by definition) at 180°.

[0057] In order to reach this reference pose Pr, starting from the original pose P0, several changes in angle are performed in an iterative process in individual joints Ga and Gb. In the first run of loop L1, two sub-steps i) and ii) take place. In first sub-step i), for example, outer joint Gb is adjusted. The associated change in angle from 0 to 1 is carried out in an automated manner in such a way that associated eccentric lever element 200b is made to assume an upper reversal position. For outer joint Gb, eccentric lever element 200b is formed by outer linkage Kc. A gravitational force Fg acts at center of mass 201 of this lever element pulling the lever element downwardly and causing a torque in the region of joint Gb. In the first partial step, this lever element 200b is now made to assume an upper reversal position in an automated manner according to pose P1 in which the torque of the lever is zero This reversal position can be determined in particular by determining (and possibly averaging) zero-crossings of the machine current in the associated drive device. The robotic arm is made to assume pose P1 in an automated manner in which center of mass 201 of lever element 200b is located on the z-axis.

[0058] In second sub-step ii), a corresponding alignment is carried out for joint Ga disposed further inside: Associated eccentric lever element 200a is there formed by two linkages Kb and Kc and joint Gb located therebetween. In sub-step ii) inner joint Ga is moved in such a way that this entire lever element 200a comes to lie with its associated center of mass on the z-axis. The angular position in joint Ga is there changed from α.sub.0 to α.sub.1, and pose P2 has been reached. For the present example with only two selected joints Ga and Gb, the first run of loop L1 of the iterative process is completed. For the second run of loop L2, only first sub-step i) for pose P3 is only still indicated in FIG. 2: Here as well, similar to the very first motion, only lever element 200b is rotated with its center of gravity on the z-axis. As indicated by the further arrows, corresponding sub-step ii) follows for inner joint Gb and these sub-steps are alternately repeated in several further iterative loops until a termination criterion for the remaining changes in angle has been reached. At this point in the method, completely vertically extended reference pose Pr with the defined accuracy has been reached.

[0059] FIG. 3 shows an example of how the method described can be applied to a larger number of joints to be aligned in an automated manner. A simplified sketch of a robotic arm is shown with three selected rotary joints Ga, Gb and Gb, corresponding joint angles α, β, γ, and four linkages Ka, Kb, Kc and Kd which connect the joints to one another or to base B and the tool center point. Again, a first run of loop L1 is shown in which these three joints are aligned one after the other in an automated manner in three sub-steps s-1), s-2) and s-3). Here as well, a sequence from the outermost joint to the innermost joint is shown by way of example, although this is not mandatory. In each sub-step s-i), the relevant eccentric lever arm is made to assume the upper reversal position. Each of the joints is adjusted once during each loop. Similarly to the example in FIG. 2, the limit values for the changes in angle are also undercut after a certain number of runs of the loops, and reference pose Pr has been reached with the predefined accuracy.

LIST OF REFERENCE CHARACTERS

[0060] 1 drive device [0061] 3 drive housing [0062] 5 drive machine [0063] 7 drive shaft [0064] 21 elevated portion [0065] 22 end cap [0066] 25 control device (including evaluation device and current sensor) [0067] 40 output member (output shaft) [0068] 100 robotic arm [0069] 200 eccentric lever element [0070] 200a eccentric lever element for joint Ga [0071] 200b eccentric lever element for joint Gb [0072] 201 center of mass [0073] B base of the robotic arm [0074] Fg gravitational force [0075] Ga selected robotic joint [0076] Gb selected robotic joint [0077] Gc selected robotic joint [0078] α angle at selected robotic joint Ga [0079] β angle at selected robotic joint Gb [0080] γ angle at selected robotic joint Gc [0081] i first sub-step [0082] ii second sub-step [0083] s-i step no. i [0084] J1 first robotic joint with axis R1 [0085] J2 second robotic joint with axis R2 [0086] J3 third robotic joint with axis R3 [0087] J4 fourth robotic joint with axis R4 [0088] J5 six robotic joint with axis R6 [0089] J6 sixth robotic joint with axis R6 [0090] J7 seventh robotic joint with axis R7 [0091] Ka linkage [0092] Kb linkage [0093] Kc linkage [0094] Kd linkage [0095] L1 first run of the loop [0096] L2 second run of the loop [0097] Pi pose no. i [0098] Pr reference pose (rest pose) [0099] TCP end effector (tool center point) [0100] x, y, z cartesian spatial directions