can't find specific joint when using gazebo model plugin libgazebo_ros_ft_sensor.so

edit

I am trying to attach a Force/Torque sensor to the end-effector of an industrial robot. I think I should attach it on joint link6-tool0 to measure ft. However, I wrote whatever except for joint_1~6 in the <jointname> tag, the terminal says:

[FATAL] [1460706723.091181613, 198.988000000]: gazebo_ros_ft_sensor plugin error: jointName: link6-tool0 does not exist

usage:

<gazebo>    
    <plugin name="FTsensor_controller"    filename="libgazebo_ros_ft_sensor.so">
       <updateRate>100.0</updateRate>
       <topicName>FTsensor_topic</topicName>
       <gaussianNoise>0.0</gaussianNoise>
       <jointName>link6-tool0 </jointName>
    </plugin>    
</gazebo>

I checked gazebo_ros_ft_sensor.cpp , relative codes are as follow:

this->joint_ = this->model_->GetJoint(this->joint_name_);
   if (!this->joint_)
   {
     ROS_FATAL("gazebo_ros_ft_sensor plugin error: jointName: %s does not exist\n",this->joint_name_.c_str());
     return;
   }

Since joint_1~6 and joint link6-tool0 are defined in the same XACRO, I have no idea why the plugin can find joint_1~6 but can not find link6-tool0. It confuses me a lot. What is actually passed by the Load function of the plugin? And how can I get the ft measurement of the end-effector?