Joints Ros-Gazebo

edit

Hello,

I want to know how the joints work with gazebo. My problem is that I don't know what kind of physics laws are used for gazebo.

I am using one plugin for control the wheels on (4 wheel) differential car. I am using one class in the pkg pr2_controller/manager and one of my question is the next:

In this plugin, there are 3 states, star, update and stop. I configure update to 50 Hz. Every time to run this state, I send efforts to joints by this way:

std::string joint_blw;
  if (!n.getParam("joint_blw", joint_blw))
  {
    ROS_ERROR("No joint BLW given in namespace: '%s')",
              n.getNamespace().c_str());
    return false;
  }
  joint_state_blw_ = robot->getJointState(joint_blw);
  if (!joint_state_blw_)
  {
    ROS_ERROR("summit_xl_ctrl could not find joint named '%s'",
              joint_blw.c_str());
    return false;
  }
  ROS_DEBUG("summit_xl_ctrl found joint named '%s'", joint_blw.c_str());
...
...
joint_state_blw_->commanded_effort_ = ...

This joint is continuous and has this urdf:

.. ..

  <joint name="joint_back_left_wheel" type="continuous">
    <parent link="base_link"/>
    <child link="back_left_wheel"/>
    <origin xyz="-0.223 0.154 0" rpy="0 0 0" /> 
    <axis xyz="0 1 0" rpy="0 0 0" />
    <limit effort="100" velocity="100"/>
    <joint_properties damping="0.0" friction="0.0"/>
  </joint>

  <transmission name="joint_blw_trans" type="pr2_mechanism_model/SimpleTransmission">
     <actuator name="joint_blw_motor" />
     <joint name="joint_back_left_wheel" />
     <mechanicalReduction>1</mechanicalReduction>
     <motorTorqueConstant>1</motorTorqueConstant>
  </transmission>

Finally, I don't know where is this effort apply and what is the performance of this effort, like I send torque to make par or whatever in N or Nm?. Where can I find any documentation or tutorial about this?

By the Way this is for the Uc3m University