My Code:

- In main()

ros::Subscriber sub0 = nh.subscribe("panda_link0_contact_sensor_state", 1000, bumperCallback);

- In bumperCallback()

void bumperCallback(const gazebo_msgs::ContactsStateConstPtr& cs)
{
    ROS_INFO("Header:"); 
    //uint32 seq
    //time stamp
    //string frame_id
    ROS_INFO_STREAM(cs->header);

    ROS_INFO("states:"); 
    ROS_INFO_STREAM(cs->states[0].collision1_name);
    ROS_INFO_STREAM(cs->states[0].collision2_name);
    //ROS_INFO_STREAM(cs->states[0]);

}

- In Model Description file

<gazebo reference="${arm_id}_link0">
      <sensor name="${arm_id}_link0_contact_sensor" type="contact">
        <visualize>true</visualize>
        <always_on>true</always_on>
        <update_rate>30.0</update_rate>
        <contact>
          <geom>panda_link0_geom</geom>
          <collision>panda_link0_collision_collision</collision>
        </contact>
        <plugin name="${arm_id}_link0_bumper" filename="libgazebo_ros_bumper.so">
          <bumperTopicName>panda_link0_contact_sensor_state</bumperTopicName>
          <frameName>link0</frameName>
        </plugin>
      </sensor>
      <selfCollide>true</selfCollide>
    </gazebo>

References

- the discription of bumper from gazebo webseit

http://gazebosim.org/tutorials?tut=ros_gzplugins#collapseOne

<gazebo>
  <plugin name="${name}_gazebo_ros_bumper_controller" filename="libgazebo_ros_bumper.so">
    <alwaysOn>true</alwaysOn>
    <updateRate>${update_rate}</updateRate>
    <bumperTopicName>${name}_bumper</bumperTopicName>
    <frameName>world</frameName>
  </plugin>
</gazebo>

• bumper provides contact feedback

http://docs.ros.org/api/gazebo_msgs/html/msg/ContactsState.html

Header header                          # stamp
gazebo_msgs/ContactState[] states      # array of geom pairs in contact

uint32 seq
time stamp
string frame_id

string info                                   # text info on this contact
string collision1_name                        # name of contact collision1
string collision2_name                        # name of contact collision2
geometry_msgs/Wrench[] wrenches               # list of forces/torques
geometry_msgs/Wrench total_wrench             # sum of forces/torques in every DOF
geometry_msgs/Vector3[] contact_positions     # list of contact position
geometry_msgs/Vector3[] contact_normals       # list of contact normals
float64[] depths                              # list of penetration depths

• Some discussion inspired me

1. Super good for learning Gazebo in franka robot!!!
2. https://answers.ros.org/question/29158/how-do-i-use-force-sensor-bumper-sensor-in-gazebo/
3. https://answers.ros.org/question/244380/how-to-subscribe-to-a-topic-whom-type-is-gazebo_msgcontactstate/
4. Here I found the “selfCollide” tag!!!
5. About the priority of the tag in SDF
6. About C++ in ROS, I found this code is very important, because without it the system always stop and will not run the continuous subsribers. In Terminal will show that:

Segmentation fault (core dumped)

So the following code can jump the uncollided link.

if (0 == contacts->states.size()) {	return;	}

rqt_plot

At last, I should plot the $(q, q', q'', \tau)$ There is a topic /joint_states, I can use

$ rostopic echo /joint_states

to know the order of my joints. (Reference)
So I can plot the position, velocity and effort by:

• joint (what we want to run)

 rqt_plot /franka/joint1_position_controller/command /franka/joint2_position_controller/command /franka/joint3_position_controller/command /franka/joint4_position_controller/command /franka/joint5_position_controller/command /franka/joint6_position_controller/command /franka/joint7_position_controller/command

or (what we actually run in simulator)

rqt_plot /joint_states/position[2] /joint_states/position[3]  /joint_states/position[4] /joint_states/position[5] /joint_states/position[6] /joint_states/position[7] /joint_states/position[8]

• velocity

rqt_plot /joint_states/velocity[2] /joint_states/velocity[3]  /joint_states/velocity[4] /joint_states/velocity[5] /joint_states/velocity[6] /joint_states/velocity[7] /joint_states/velocity[8]

• effort

rqt_plot /joint_states/effort[2] /joint_states/effort[3]  /joint_states/effort[4] /joint_states/effort[5] /joint_states/effort[6] /joint_states/effort[7] /joint_states/effort[8]

PS.
/joint_states is the feedback from Gazebo simulator, which is what robot actually run in simulation environment.