Hi,

I am trying to model three revolute joints in series using an sdf file (below you can find the sdf snippet for link and joint definition) with the following configuration:

1. Revolute 1 with axis (0 0 1): Main_body->Link1 (zero mass)
2. Revolute 1 with axis (0 1 0): Link1 (zero mass)->Link2(zero mass)
3. Revolute 1 with axis (1 0 0): Link2 (zero mass) -> Link3

Here link 1 and link 2 are of zero mass.

The problem is when I am trying to assign a force using SetForce function to main body at its center of mass, it is not moving. I guess this is because Gazebo is unable to solve the forces through massless link to be transferred to the link 3 which is not mass less. I am however able to set any acceleration, velocity and position to any of the link. I want to apply force to the main body. Is there a way around? I would also want to apply torques to the link 3 from mainbody.

Also I am trying to apply force on the joints between main_body and link 1 and that also seems to not be working. Please suggest the needful. Thank you.

Massless link defination

  <link name='RW2'>
    <pose frame=''>0 0 0 0 0 0</pose>
    <inertial>
      <pose frame=''>0.030076 -1e-06 -0.015425 0 -0 0</pose>
      <mass>0</mass>
      <inertia>
        <ixx>0</ixx>
        <ixy>0</ixy>
        <ixz>0</ixz>
        <iyy>0</iyy>
        <iyz>0</iyz>
        <izz>0</izz>
      </inertia>
    </inertial>
    <self_collide>0</self_collide>
    <enable_wind>0</enable_wind>
    <kinematic>1</kinematic>
    <visual name='RW2_visual'>
      <pose frame=''>0 0 0 0 -0 0</pose>
      <geometry>
        <mesh>
          <scale>0.1 0.1 0.1</scale>
          <uri>/home/suyash/urdf_main/src/cleaner/meshes/RW.STL</uri>
        </mesh>
      </geometry>
      <material>
        <shader type='pixel'/>
      </material>
      <transparency>1</transparency>
      <cast_shadows>0</cast_shadows>
    </visual>
  </link>

JOINT definition:

 <joint name='RW1_to_RW2' type='revolute'>
    <parent>RW1</parent>
    <child>RW2</child>
    <pose frame=''>0 0 -0.015 0 -0 0</pose>
    <axis>
      <xyz>0 1 0</xyz>
      <use_parent_model_frame>0</use_parent_model_frame>
      <limit>
        <lower>-1</lower>
        <upper>1</upper>
        <velocity>100</velocity>
      </limit>
    </axis>
  </joint>