EuRoC Forum

[AM-Robotics]

While testing Task2_v1_1 or Task2_v1_3 we got some strange error regarding the torque limit of Joint 6

e.g.
LWR is in emergency stop due to joint errors: joint lwr::kuka_lwr::joint6 exceeded maximum torque of 30.0Nm at step 421.752s! Restart Simulation!

One configuration of the LWR was: [1.579, -0.263, -1.440, 0.497, 1.856, 0.842, 1.565] while carrying no load. Furthermore no contacts or collisions are visible.

What is your guess?

[fschmidt]

the only way i can imagine why this could happen is that you command a too high acceleration to the robot.

did you move the robot while this was happending (either by set_servo_target or move_along_joint_path)?

from a log-file i could check which acceleration-values you used.

[jwienke]

We are also experiencing weird things with joint 6 in servo mode. While in servo-mode it is not possible to exceed thresholds, what we see is that sometimes the whole last segment including the gripper starts shaking like crazy without commanding a changed target pose. We only resend the position we have already reached long time before without any issues (servo...). Usually, this shaking in servo mode happens in case of collisions, however, the arm is far away from any visible self-collision and we already limited the possible angles for this joint way beyond what is specified.

[fschmidt]

can you please send a logfile to support-challenge02@robotics-challenges.eu.

call the save_log service when the robot starts to shake and then find the generated logfile within the simservVM in the folder /home/simguest/euroc_c2s1_logs/euroc_c2_s1_*_USERID.log

thanks.

[fschmidt]

concerning this topic i have received log files which clearly show self collisions causing such problems.
please make sure your path planner uses the correct collision geometries!
we are using simplified boundary volumes (cylinders and spheres) to define the collision shape of the LWR. those are not really close to the actual and rendered geometries! the exact collision geometries can be extracted from various model files:

The mechanics of the LWR are also available in multiple model formats in the provided ROS package
/opt/euroc_c2s1/ros/install/share/euroc_c2_description:
• URDF (ROS) format: /euroc_c2_description/urdf/kuka_lwr.urdf
• SDF (Gazebo) format: /euroc_c2_description/kuka_lwr/kuka_lwr.sdf
• XML (OpenRAVE) format: /euroc_c2_description/openrave/kuka_lwr.xml

you can enable the display of transparent collision geometries in the gazebo viewer:
Menu: View/Collisions.

you can also enable the display of contact-points:
Menu: View/Contacts.

with this it should be pretty easy to see self collisions. compare attached images.

closeup

self_collision_closeup.jpg (47.25 KiB) Viewed 545 times

self_collision_shape.jpg (48.08 KiB) Viewed 545 times

self_collision_shape_and_cp.jpg (48.28 KiB) Viewed 545 times