ROS Resources: Documentation | Support | Discussion Forum | Index | Service Status | ros @ Robotics Stack Exchange
Ask Your Question
1

Gazebo vacuum gripper not rotating gripped model

asked 2022-01-12 07:05:35 -0500

pap-x gravatar image

Hello all! I'm using the vacuum gripper plug in with the ur5 robot in a similar way to this. Although I can grip the part and move it around with no problems, I can't rotate it If I rotate just the end effector. You can see the same behavior in the gif of the simulation using the above link. It doesn't make sense if the plug in creates a rigid joint between the model and the end effector. Where could be the problem? TIA!

edit retag flag offensive close merge delete

1 Answer

Sort by » oldest newest most voted
1

answered 2022-01-12 12:34:18 -0500

osilva gravatar image

updated 2022-01-14 04:50:55 -0500

HI @pap-x

In the urdf: https://github.com/lihuang3/ur5_ROS-G...

Wrist3 is connected to EE with a fixed joint

<joint name="${prefix}ee_fixed_joint" type="fixed">
  <parent link="${prefix}wrist_3_link" />
  <child link = "${prefix}ee_link" />
  <origin xyz="0.0 ${wrist_3_length} 0.0" rpy="0.0 0.0 ${pi/2.0}" />
</joint>

Vacuum gripper is connected to EE with a revolute joint

<joint name="gripper_joint" type="revolute">
  <axis xyz="1 0 0" />
  <parent link="ee_link" />
  <child link="vacuum_gripper" />
  <origin rpy="0 1.5708 0" xyz="0.01 0.0125 0" />
  <limit effort="50" velocity="50" lower="0" upper="0" />
  <dynamics damping="0.0" friction="10"/>
</joint>

The vacuum grippers are small cylinders attached to EE link.

A crude sketch:

image description

EDIT: Disregard this: If you want to modify the behaviour, you will need to modify the URDF

Looking at the plugin one can appreciate how the force for each link in contact:

    ignition::math::Vector3d force = norm_force * diff.Pos().Normalize();
    links[j]->AddForce(force);

The force direction applied by the cylinders vacuum won’t change with the EE rotation.

For more information, please check this thread discussion: https://github.com/ros-simulation/gaz...

edit flag offensive delete link more

Comments

The problem has not to do with the robot itself since the end effector is rotating properly. The problem is between the gripped object and the cylinders (vacuum grippers) where the cylinders rotate but the object remains in the same orientation as before.

pap-x gravatar image pap-x  ( 2022-01-13 04:56:12 -0500 )edit

Thank you for the clarification. I wasn’t very happy with the answer yesterday either. If you take a look at the plugin it may be modified to change this behaviour. I guess you will need to track orientation of every possible child at all times.

osilva gravatar image osilva  ( 2022-01-13 05:10:49 -0500 )edit

Check this interesting thread on the discussion of the plugin: https://github.com/ros-simulation/gaz...

You need to consider tracking direction of the forces. Not an easy problem to solve.

osilva gravatar image osilva  ( 2022-01-13 05:39:26 -0500 )edit

The force direction applied by the cylinders vacuum won’t change with the EE rotation.

osilva gravatar image osilva  ( 2022-01-13 05:46:28 -0500 )edit

I'm afraid it falls outside the scope of my capabilities, but I have a much clearer understanding of the factors involved! Thanks for your help!

pap-x gravatar image pap-x  ( 2022-01-13 06:20:25 -0500 )edit
1

I’ll edit my answer for future users and draw a sketch later today. I learned too. Initially I thought this shouldn’t be too hard but the more I think it’s not a trivial problem to solve.

osilva gravatar image osilva  ( 2022-01-13 06:25:54 -0500 )edit

Question Tools

2 followers

Stats

Asked: 2022-01-12 07:05:35 -0500

Seen: 222 times

Last updated: Jan 14 '22