Range sensor measurements do not have any effect on the produced costmap

edit

I have two ultrasound range finders attached to my robot from which i'd like to produce a costmap. I instantiate the costmap_2d_node with the range_sensor_layer::RangeSensorLayer plugin but the resulting costmap does not show any values when visualized in rviz.

This is my launch config:

<launch>
    <node pkg="tf2_ros" type="static_transform_publisher" name="robotPos_broadcaster1" args="2.0 0.0 0.0 0 0 0 1 map base_link" />

    <node pkg="tf2_ros" type="static_transform_publisher" name="proximity_broadcaster1" args="0.3 0.5 0.0 0 0  0.70710808 0.70710548 base_link proximity_front_right" />
    <node pkg="tf2_ros" type="static_transform_publisher" name="proximity_broadcaster2" args="-0.3 0.5 0.0 0 0 0.70710808 0.70710548 base_link proximity_front_left" />
    <node pkg="learning_tf2" type="readSensors.py" name="readSensors1" />


    <node pkg="costmap_2d" type="costmap_2d_node" name="costmap_node">
            <rosparam file="$(find learning_tf2)/params/minimal.yaml" command="load" ns="costmap" />
    </node>

</launch>

This is the file params/minimal.yaml:

footprint: [[-0.325, -0.325], [-0.325, 0.325], [0.325, 0.325], [0.46, 0.0], [0.325, -0.325]]

plugins:
  - {name: sonar, type: "range_sensor_layer::RangeSensorLayer"}

sonar:
  topics: ["/ultrasound_front_left", "/ultrasound_front_right"]

The output of rosnode info /costmap_node is:

--------------------------------------------------------------------------------
Node [/costmap_node]
Publications: 
 * /costmap_node/costmap/costmap [nav_msgs/OccupancyGrid]
 * /costmap_node/costmap/costmap_updates [map_msgs/OccupancyGridUpdate]
 * /costmap_node/costmap/footprint [geometry_msgs/PolygonStamped]
 * /costmap_node/costmap/parameter_descriptions [dynamic_reconfigure/ConfigDescription]
 * /costmap_node/costmap/parameter_updates [dynamic_reconfigure/Config]
 * /costmap_node/costmap/sonar/parameter_descriptions [dynamic_reconfigure/ConfigDescription]
 * /costmap_node/costmap/sonar/parameter_updates [dynamic_reconfigure/Config]
 * /rosout [rosgraph_msgs/Log]

Subscriptions: 
 * /costmap_node/costmap/footprint [geometry_msgs/PolygonStamped]
 * /tf [unknown type]
 * /tf_static [tf2_msgs/TFMessage]
 * /ultrasound_front_left [sensor_msgs/Range]
 * /ultrasound_front_right [sensor_msgs/Range]

Services: 
 * /costmap_node/costmap/set_parameters
 * /costmap_node/costmap/sonar/set_parameters
 * /costmap_node/get_loggers
 * /costmap_node/set_logger_level


contacting node http://HI-Z0GR8:35510/ ...
Pid: 22374
Connections:
 * topic: /rosout
    * to: /rosout
    * direction: outbound
    * transport: TCPROS
 * topic: /tf_static
    * to: /proximity_broadcaster1 (http://HI-Z0GR8:36481/)
    * direction: inbound
    * transport: TCPROS
 * topic: /tf_static
    * to: /robotPos_broadcaster1 (http://HI-Z0GR8:33215/)
    * direction: inbound
    * transport: TCPROS
 * topic: /tf_static
    * to: /proximity_broadcaster2 (http://HI-Z0GR8:43631/)
    * direction: inbound
    * transport: TCPROS
 * topic: /ultrasound_front_left
    * to: /readSensors1 (http://HI-Z0GR8:38483/)
    * direction: inbound
    * transport: TCPROS
 * topic: /ultrasound_front_right
    * to: /readSensors1 (http://HI-Z0GR8:38483/)
    * direction: inbound
    * transport: TCPROS

When visualizing the the map in rviz (topic /costmap_node/costmap/costmap) it just displays a plain gray area. However, when visualizing the ultrasound sensors (topics /ultrasound_front_left, /ultrasound_front_right) in rviz, the readings from the sensors are visualized properly.

Do I do anything wrong or is it not possible to visualize the map in rviz?