bumper sensor stuck at the end of range after few iterations

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Hi everybody,

I'm using the lib gazeborosbumper.so library to get sonar data from my simulated pioneer 2DX robot. Everything seems fine with the readings, but after a certain number of steps (during which I kill and then restart gazebo many times) each bumper sensor start to give wrong readings. Specifically, it starts to give always then end of range, constant up to the last digit. Here my sdf file:

<sensor name="laser0" type="ray">
    <pose>0.3 0.0 0.3 0 0 0</pose>
    <ray>
      <scan>
        <horizontal>
          <samples>8</samples>
          <resolution>1</resolution>
          <min_angle>-0.7853</min_angle>
          <max_angle>0.7853</max_angle>
        </horizontal>
      </scan>
      <range>
        <min>0.05</min>
        <max>4.20</max>
        <resolution>0.01</resolution>
      </range>
      <noise>
        <type>gaussian</type>
        <mean>0.0</mean>
        <stddev>0.0</stddev>
      </noise>
    </ray>
    <plugin name="laser_controller" filename="libgazebo_ros_laser.so" >
      <alwaysOn>true</alwaysOn>
      <updateRate>0</updateRate>
      <visualize>true</visualize>
      <topicName>scan</topicName>
      <frameName>chassis</frameName>
    </plugin>
    <visualize>true</visualize>
  </sensor>

any idea?

Thanks