gazebo ros bumper not working with fused links

edit

Dear all, I'm struggling with the gazebo_ros_bumper when applied to a link which is statically fused to another one.

My system is Ubuntu 16.04 with Kinetic.

My robot has a leg composed by four components: hip assemply, upper leg, lowel leg and foot (full description is here).

Between lower leg and foot there is a fixed joint:

    <joint name="${name}_foot_joint" type="fixed">
        <origin xyz="${lowerleg_length} 0 0" rpy="${PI/2} 0 ${-PI/2}"/>
        <parent link="${name}_lowerleg"/>
        <child  link="${name}_foot"/>
    </joint>
    <!-- Lower leg link -->
    <link name="${name}_lowerleg">
        <inertial>
            <origin xyz="${xcom_lowerleg} ${reflect_front*ycom_lowerleg} ${reflect_front*zcom_lowerleg}"/>
            <!-- adding half kg to lf lowerleg -->
                <mass value="${m_lowerleg}"/>
            <inertia ixx="${ixx_lowerleg}" iyy="${iyy_lowerleg}" izz="${izz_lowerleg}"
                     ixy="${reflect_front*ixy_lowerleg}" ixz="${reflect_front*ixz_lowerleg}" iyz="${iyz_lowerleg}"/>
        </inertial>
        <visual>
            <geometry>
                <mesh filename="package://hyq_description/meshes/leg/lowerleg.dae" scale="1 1 1"/>
            </geometry>
            <material name="white"/>
        </visual>
        <collision>
            <origin xyz="${lowerleg_length/2.0} 0 0" rpy="0 ${-PI/2}  0"/>
            <geometry>                  
                <cylinder length="${lowerleg_length}"  radius="0.02"/>
            </geometry>
        </collision>
    </link>
    <!-- Foot link -->
    <link name="${name}_foot">
        <collision>
            <origin xyz="0 0 0" rpy="0 0 0"/>
            <geometry>
                <sphere radius="0.02175"/>
            </geometry>
        </collision>
        <visual>
            <geometry>
                <sphere radius="0.02175" />
            </geometry>
            <material name="black" />
        </visual>
    </link>

To both the lower leg and foot I have attached a bump sensor:

        <gazebo reference="${name}_lowerleg">
        <kp>1000000.0</kp>
        <kd>100.0</kd>
        <mu1>1.0</mu1>
        <mu2>1.0</mu2>
        <maxVel>1.0</maxVel>
        <maxContacts>1</maxContacts>
        <sensor name="${name}_shin_contact_sensor" type="contact">
            <always_on>true</always_on>
            <update_rate>250.0</update_rate>
            <contact>
              <collision>${name}_lowerleg_collision</collision>
            </contact>
            <plugin name="${name}_shin_bumper" filename="libgazebo_ros_bumper.so">
              <bumperTopicName>/${name}_shin_sensor</bumperTopicName>
              <frameName>base_link</frameName>
            </plugin>
        </sensor>   
    </gazebo>

    <gazebo reference="${name}_foot">
                    <kp>1000000.0</kp>
                    <kd>100.0</kd>
                    <mu1>1.0</mu1>![image description](/upfiles/1518629691790135.png)
                    <mu2>1.0</mu2>
                    <maxVel>1.0</maxVel>
        <maxContacts>1</maxContacts>
        <sensor name="${name}_foot_contact_sensor" type="contact">
            <always_on>true</always_on>
            <update_rate>250.0</update_rate>
            <contact>
                <collision name="${name}_foot_collision">${name}_foot_collision</collision>
                <topic>/${name}_foot_collision_topic</topic>
            </contact>
            <plugin name="${name}_foot_bumper" filename="libgazebo_ros_bumper.so">
                <bumperTopicName>/${name}_bumper</bumperTopicName>
                <frameName>base_link</frameName>
            </plugin>
        </sensor>
        <material>Gazebo/Black</material>
    </gazebo>

However, only the shin sensor outputs contacts, while the foot sensor outputs on the topic /${name}_bumper an empty list contacts, even if I can clearly see the contacts in Gazebo.

I suspect this is related to the fact that the lowerleg get fused inside gazebo.

From the interface I see the collision has a strange name (${name}_lowerleg_fixed_joint_lump__${name}_foot_collision_1)

I even tried to put this name in the <collision> tag, either in the name attribute or within the tag, with no success.

What can I do? What am I missing?