the robot is rotating by iteself without any forces or input. why and how can i stop it ?

edit

Try setting friction parameters to the wheel links and ensuring that your wheels have contact with the ground. Otherwise, there is nothing to stop your wheels from rotating. It would be helpful to visualize the wheel contacts in Gazebo and make sure that they are not jittery. If they are, you need to tune your minDepth, kp, and kd tags.

For example:

<gazebo reference="left_wheel">
    <mu1>0.9</mu1>
    <mu2>0.9</mu2>
  <minDepth>0.001</minDepth>
    <kp>100000000</kp>
    <kd>1</kd>
</gazebo>

<gazebo reference="right_wheel">
    <mu1>0.9</mu1>
    <mu2>0.9</mu2>
  <minDepth>0.001</minDepth>
    <kp>100000000</kp>
    <kd>1</kd>
</gazebo>

Guide on Friction: http://gazebosim.org/tutorials?tut=fr...

I would also recommend setting <implicitSpringDamper> to true to get better performance.

<gazebo reference="left_wheel_hinge">
       <implicitSpringDamper>true</implicitSpringDamper>
</gazebo>

<gazebo reference="right_wheel_hinge">
       <implicitSpringDamper>true</implicitSpringDamper>
</gazebo>