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Ok I've solved this problem and I would like share it since a lot of people are complaining. The errors come usually as

1- Controller Spawner couldn't find the expected controller_manager ROS interface.
2- Failed to initialize the controller
3- Initializing controller 'joint1_position_controller' failed

This error pops up from different sources. First, make sure you installed all related control packages

ros-<rosdistro>-ros-control  ros-<rosdistro>-ros-controllers ros-<rosdistro>-gazebo-ros ros-<rosdistro>-gazebo-ros-control

Second, if you are using namespace as in the following line which is located in .urdf file

<gazebo>
        <plugin name="gazebo_ros_control" filename="libgazebo_ros_control.so">
            <robotNamespace>/rrbot</robotNamespace>
            <robotSimType>gazebo_ros_control/DefaultRobotHWSim</robotSimType>
        </plugin>
</gazebo>

Make sure you are using the same namespace in .launch and .yaml files which appears as ns="/rrbot"

 <node name="control_spawner" pkg="controller_manager" type="spawner" respawn="false"
      output="screen" ns="/rrbot" args="joint1_position_controller"/>

and the .yaml may look like this

rrbot:
  # Publish all joint states ------------------------
  joint_state_controller:
    type: joint_state_controller/JointStateController
    publish_rate: 50

  # Position Controllers ----------------------------
  joint1_position_controller:
    type: my_controller/MyPositionController
    joint: joint1
    gain: 0.1

If you are using your own plugin controller, you better check it is registered which can be done as (i.e. source your catkin_ws folder first)

rospack plugins --attrib=plugin controller_interface | grep my_controller

you should except to get something like this

my_controller /home/xxx/Desktop/RosControllers/catkin_ws1/src/my_controller/controller_plugins.xml

In my case none of the above solutions solved my problem. The interesting thing I found getParam() is not getting the correct joint name which in the source file as

bool MyPositionController::init(hardware_interface::EffortJointInterface* hw, ros::NodeHandle &n)
{
    // retrieve the joint object to control
    std::string joint_name;
    if( !n.getParam("joint1", joint_name) ){
        ROS_ERROR("No joint name specified");
        return false;
    }
     ....
}

when I type "joint" rather than "joint1" which represents the joint's name in my .urdf, the error gone. I've seen some books use "joint_name" which yields an error as well. I'm not sure if this is a new update or these books use old ros version which uses joint_name rather than joint. I'm using Gazebo 9 and Melodic in Ubuntu 18.04.