Why joint position controller is not sensitive to changing PID gains?

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Dear Ros Users

I'm going to use ziegler-nichols method to tune PID gains of an simple inverted pendulum model. I'm wondering why my controller whom type is "position_controllers/JointPositionController", is not sensitive to changing PID gains. Even in the case of setting all k_p, k_i and k_d to zero in following YAML configuation file, controller works well as before.

inverted_pendulum:
  # Publish all joint states -----------------------------------
  joint_state_controller:
    type: joint_state_controller/JointStateController
    publish_rate: 50  

  # Position Controllers ---------------------------------------
  joint1_position_controller:
    type: position_controllers/JointPositionController
    joint: shoulder_pan_joint
    pid: {p: 0.0, i: 0.0, d: 0.0}

And this is the snippets of the urdf model:

  <joint name="shoulder_pan_joint" type="revolute">

    <parent link="Link1"/>

    <child link="Rod"/>

    <!--<origin xyz="0 0.15 0.65" rpy="0 0 0" />-->

    <origin xyz="0 -0.015 0.65" rpy="0 0 0" />

    <axis xyz="0 1 0" />

    <limit effort="300" velocity="1" lower="-2.61799387799" upper="1.98394848567"/>

    <dynamics damping="5" friction="0.1"/>

  </joint>

      <transmission name="tran1">

        <type>transmission_interface/SimpleTransmission</type>

        <joint name="shoulder_pan_joint">
          <hardwareInterface>PositionJointInterface</hardwareInterface>
        </joint>

        <actuator name="motor1">
          <hardwareInterface>PositionJointInterface</hardwareInterface>
          <mechanicalReduction>1</mechanicalReduction>
        </actuator>

      </transmission>

<gazebo reference="shoulder_pan_joint">
    <provideFeedback>true</provideFeedback>
    <disableFixedJointLumping>true</disableFixedJointLumping>
</gazebo>

  <!-- ros_control plugin -->
  <gazebo>
    <plugin name="gazebo_ros_control" filename="libgazebo_ros_control.so">
      <robotNamespace>/inverted_pendulum</robotNamespace>
    </plugin>
  </gazebo>

Would you please help me to resolve this issue?

I'm using Ubuntu 14.01 and ROS indigo.