contact task simulation issues with gazebo and ROS

[jacknlliu]

• physics engine in gazebo for contact simulation
  dart with contact dynamic model

• exact joint and link state feedback after contact
  /joint_states will provide wrong data which is very large and exceeds the joint limit after the robot has a collision with an object in gazebo, see reset /joint_states and gazebo plugins state in gazebo  #27 and wrong and very large joint position after robot had a collision with objects.

• joint velocity or force controller which needs controller with gravity compensation(gazebo is working on general gravity compensation)

• reset

  • correctly reset ros controllers (plugin) for gazebo (with or without reset time)
  • correctly reset gazebo without reset time!( We need continuous time for ros controllers)
  • correctly reset gazebo with world state and some models state
  • (optional) correctly reset gazebo with link velocity or force

Reference

• Contact and gazebo_ros_controller issue
• dartsim tutorials_manipulator
• Gazebo Multi-Physics Engine Support
• Gazebo Manage physics profiles
• Existing Physics API
• drcsim_gazebo_ros_plugins

[jacknlliu]

For /joint_states published by joint_state_publisher which providing wrong data after contact, we can use gazebo plugin joint_state_publisher .

For using gazebo plugin joint_state_publisher, we should add the following lines to your URDF

  <gazebo>
      <plugin name="gazebo_joint_state_publisher" filename="libgazebo_ros_joint_state_publisher.so">
        <robotNamespace></robotNamespace>
        <updateRate>200.0</updateRate>
        <jointName>elbow_joint, shoulder_pan_joint, shoulder_lift_joint, wrist_1_joint, wrist_2_joint, wrist_3_joint</jointName>
      </plugin>
  </gazebo>

NOTE: it seems can't get the velocity correctly from universal_robot, and it only gets the should_pan_joint velocity, others are all zero. But it can get the velocity correctly from iiwa robot. It is very strange!

Some useful things to deep into joint state controller issue, see namespaceangles, /joint_states from joint state controller and gazebo plugin joint state publisher.

[jacknlliu]

We have three main issues in this process:

• reset ros controllers failed/ /gazebo/set_model_config failed with gazebo ros simulation hardware interface.
• compile gazebo9 with dart from source, but when compile failed with gazebo_ros_pkgs
• dart physics engine can't work with ros controllers in gazebo9

[jacknlliu]

Physics engine will give a very large force(impulse) when robot end-effector has a collision with some fixed object, the magnitude of the force is about 1e4 (also can reference to [1]).

And the contact force will keep vibrating when has collisions, and it is not realistic in the real physical world.

We should solve the contact impulse issue if we want to simulate the contact and make the learned policies more robust.

Some tricks to solve this, for example

• when contact occurs (a contact signal should be got), reset the force as 0.
• and using F_render = k * F_sim, where k from 0 increasing to 1 along with the update time.
• using mujoco model-based method

Reference

• S. Alrasheed, “Impulse, Momentum, and Collisions,” in Principles of Mechanics: Fundamental University Physics, S. Alrasheed, Ed. Cham: Springer International Publishing, 2019, pp. 73–85.
• How to retrieve contact forces leggedrobotics/raisimLib#38
• E. Todorov, T. Erez, and Y. Tassa, “MuJoCo: A physics engine for model-based control,” 2012, pp. 5026–5033.
• Chai3d contact model example ODE-tool
• Chai3d bullet dynamic spring

[jacknlliu]

Research about gazebo contact simulation

Reference

• Gazebo Supported Physics Engines
• OpenAI open sources a high-performance Python library for robotic simulation
• simbody
• simbody contact community
• simbody large contact force
• Elastic Foundation model
• gazebo benchmark
• awesome-robotics-libraries/COMPARISONS