Interpretation of Force/Torque Sensor using gazebo_ros_ft_sensor plugin

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I've successfully added a force torque/sensor to my model by adding following codes in my urdf file:

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


<gazebo>
    <plugin name="ft_sensor" filename="libgazebo_ros_ft_sensor.so">
        <updateRate>10.0</updateRate>
        <topicName>ft_sensor_topic</topicName>
        <jointName>shoulder_pan_joint</jointName>
    </plugin>
</gazebo>

Although the model is static and nothing is moving in gazebo gui, the outputs of force/torque sensor published by "ft_sensor_topic" is fluctuating. For example the force value that I expect to be -100 is fluctuating between -55 N and -143N. Other 5 components of torque and force values are also fluctuating in a similar range.

I also tried to reduce Gaussian Noise of the sensor by adding <gaussianNoise>0.0</gaussianNoise> to the plugin tag. but this didn't solve the problem.

I'm using gazebo 2.2.3 + Ros indigo running in ubuntu 14.04.