Closed kinematic chain in ISAAC GYM

Robotics & Edge Computing Robotics - Isaac Isaac Gym
,
1

Hi all,

I am trying to import my custom robot which has a closed kinematic chain. The problematic part in the urdf file is:

   ...

  <joint name="joint1" type="revolute">
    <origin rpy="0 0 0" xyz="0 0 0.92"/>
    <parent link="link0"/>
    <child  link="link1"/>
    <axis xyz="0 0 1"/>
    <limit effort="87" lower="-3.14" upper="3.14" velocity="2.1750"/>
  </joint>
  <link name="link2">
    <visual>
      <geometry>
        <mesh filename="package://my_description/meshes/visual/link1.dae"/>
      </geometry>
    </visual>
    <collision>
      <geometry>
        <mesh filename="package://my_description/meshes/visual/link1.dae"/>
      </geometry>
    </collision>
	<inertial>      
		<origin rpy="0 0 0" xyz="0 0 0"/>      
		<mass value="250"/>      
		<inertia ixx="1" ixy="0" ixz="0" iyy="1" iyz="0" izz="1"/>    
	</inertial> 
  </link>
  <joint name="joint2" type="prismatic">
    <origin rpy="0 -0.0498132 0" xyz="3.22 0 18.58"/>
    <parent link="link1"/>
    <child link="link2"/>
    <axis xyz="1 0 0"/>
    <limit effort="87" lower="0" upper="10" velocity="2.1750"/>
  </joint>
  <link name="link3">
    <visual>
      <geometry>
        <mesh filename="package://my_description/meshes/visual/link2.dae"/>
      </geometry>
    </visual>
    <collision>
      <geometry>
        <mesh filename="package://my_description/meshes/visual/link2.dae"/>
      </geometry>
    </collision>
	<inertial>      
		<origin rpy="0 0 0" xyz="0 0 0"/>      
		<mass value="50"/>      
		<inertia ixx="1" ixy="0" ixz="0" iyy="1" iyz="0" izz="1"/>    
	</inertial> 
  </link>

  <joint name="joint3Psi" type="continuous" >
    <parent link="link2" />
    <child link="link3_theta" />
    <axis xyz="0 0 1" />
    <dynamics damping="0.005" />
  </joint>

  <link name="link3_theta" />

  <joint name="joint3Theta" type="continuous" >
    <parent link="link3_theta" />
    <child link="link3_alpha" />
    <axis xyz="0 1 0" />
    <dynamics damping="0.005" />
  </joint>

  <link name="link3_alpha" />

  <joint name="joint3Alpha" type="continuous" >
    <parent link="link3_alpha" />
    <child link="link3_psi" />
    <axis xyz="1 0 0" />
    <dynamics damping="0.005" />
  </joint>

  <link name="link3_psi" />

  <joint name="joint3" type="prismatic" >
    <origin xyz="0.08 0 -3" />
    <parent link="link3_psi" />
    <child link="link3" />
    <axis xyz="0 0 -1"/>
    <limit effort="87" lower="0" upper="10" velocity="2.1750"/>
  </joint>


  <joint name="joint3_2Psi" type="continuous" >
    <parent link="link2" />
    <child link="link3_2_theta" />
    <axis xyz="0 0 1" />
    <dynamics damping="0.005" />
  </joint>

  <link name="link3_2_theta" />

  <joint name="joint3_2Theta" type="continuous" >
    <parent link="link3_2_theta" />
    <child link="link3_2_alpha" />
    <axis xyz="0 1 0" />
    <dynamics damping="0.005" />
  </joint>

  <link name="link3_2_alpha" />

  <joint name="joint3_2Alpha" type="continuous" >
    <parent link="link3_2_alpha" />
    <child link="link3_2_psi" />
    <axis xyz="1 0 0" />
    <dynamics damping="0.005" />
  </joint>

  <link name="link3_2_psi" />

  <joint name="joint3_2" type="fixed" >
    <origin xyz="-0.08 0 -3" />
    <parent link="link3_2_psi" />
    <child link="link3" />
    <axis xyz="0 0 -1"/>
    <limit effort="87" lower="0" upper="10" velocity="2.1750"/>
  </joint>

Since the error occurs in the joint3_2 part, I assume that ISAAC GYM does not support this closed kinematic chain… Could you guys confirm this and also suggest any walk around solution?

Best

2

Hi,

To my knowledge URDF does not support closed chain structures as it has a tree-based architecture.
Maybe you can resolve this by adding an external constraint forces in the simulator to keep links attached