Orocos/ROS topics tutorial

This tutorial will teach how to set up an Orocos component to communicate with ROS through Orocos ports / ROS topics.


If you don’t have a working version of Orocos + ROS installed in your system, please refer to the first tutorial Installation.

These tutorials assume you have a working knowledge of Orocos components, if not read The Orocos Component Builder’s Manual or follow the basic RTT tutorials.

In this tutorial we will show you how to connect Orocos ports to ros topics. We will create a component with an InputPort and an OutputPort over which std_msgs are transported. We will need the help of rtt_std_msgs to create the correct transports.

Component header file:


#include <rtt/RTT.hpp>
#include <rtt/Component.hpp>
#include <std_msgs/Bool.h>
#include <std_msgs/Int32.h>

class StandardComponent:
    public RTT::TaskContext
    /// Constructor
    StandardComponent(std::string const& name);

    /// Destructor

    RTT::InputPort<std_msgs::Bool> input;
    RTT::OutputPort<std_msgs::Int32> output;

    int counter;
    std_msgs::Bool incoming_msg;

    /// Configure
    bool configureHook();

    /// Update
    void updateHook();


#endif // StandardComponent_H

Component cpp file:

#include <orocos_ros_communication/StandardComponent.h>

StandardComponent::StandardComponent(std::string const& name)
    : RTT::TaskContext(name)
    , input("input")
    , output("output")
    , counter(0)
    this->ports()->addEventPort("input", input).doc("Input port which is read in the updateHook");
    this->ports()->addPort("output", output).doc("Data producing output port");

bool StandardComponent::configureHook()
    return input.connected();

void StandardComponent::updateHook()

    RTT::log(RTT::Info) << "Waking up!" << RTT::endlog();

    if(input.read(incoming_msg) == RTT::NewData)

        std_msgs::Int32 out_msg;
        out_msg.data = counter;


This component has two ports, an input port for std_msgs::Bool messages, and an output port that writes std_msgs::Int32 messages. The input port was added as an event port such that an incoming message will trigger the updateHook of our component. In the updateHook a counter is incemented and written to the output port.

We can deploy, as always, with an start.ops file:


loadComponent("my_component", "StandardComponent")

stream("my_component.input", ros.comm.topic("some_namespace/component_input"))
stream("my_component.output", ros.comm.topic("some_namespace/component_output"))


The first line just imports our component library. The rtt_rosnode import will create a ros_node for the deployer, you can check with rosnode list in the terminal.

The rtt_roscomm import gives us the tools to connect the ports to topics, and the rtt_std_msgs provides the required typekits.

We can then connect the input and output ports to ros topics as follows:

stream("my_component.input", ros.comm.topic("component_input"))
stream("my_component.output", ros.comm.topic("component_output"))

In order for this to work, there must be a ros master running. You can also use a launch file to deploy this start script, making use of the rtt_ros package, for example:

  <include file="$(find rtt_ros)/launch/deployer.launch">
    <arg name="DEPLOYER_ARGS" value="-s $(find orocos_ros_communication)/start/start.ops"/>
    <arg name="LOG_LEVEL" value="info"/>
    <arg name="DEBUG" value="false"/>

Defining custom messages

In the example above we have used the std_msgs, and their rtt typekits from rtt_std_msgs. You can also use custom ROS messages. Fox example, when you have a package my_msgs with .msg and .srv files, you can make their types available using create_rtt_msgs of the rtt_roscomm package:

rosrun rtt_roscomm create_rtt_msgs my_msgs

This will create a rtt_my_msgs package with a CMakeLists.txt, and package.xml file. When you want to connect the ports to ros topics with the deployer, just import the package in the .ops file: