Real-Time Control Framework

The Real-Time Control Framework (RTCF) provides an easy way to develop modular and real-time safe controller architectures, that can be seamlessly integrated into a Robot Operating System (ROS) environment. Minimum learning effort is required for users that are familiar with ROS.

About the Project

In recent years, ROS evolved to one of the de-facto standards in robotics research. Although control systems are an inherent part of any robotic application, their implementation in ROS is somewhat limited due to its design that is not real-time safe and tuned for asynchronous operation. In the past, several attempts have been made to mitigate those issues. A very popular among them is ros_control. However, this library has some shortcomings, including missing modularity as well as a steep learning curve.

For this reason, we developed a new software package based on the OROCOS toolchain and rtt_ros_integration that we call the Real-Time Control Framework (RTCF). The features include:

• High modularity (reusable components and configurable at run-time, compatible with roslaunch)
• High-performance controller execution (> 1 kHz possible)
• Fundamentally real-time safe design with low overhead
• Real-time safe logging (compatible with rosconsole)
• Real-time safe dynamic parameters (compatible with dynamic_reconfigure)
• Seamless integration into ROS
• Almost no learning required for ROS-developers

Getting Started

This section describes how you can obtain and build a copy of the RTCF.

Prerequisites

Currently, the RTCF is developed in the following environment:

• ROS Melodic
• Ubuntu 18.04

Chances are good that the RTCF also works on ROS Noetic and ROS Kinetic without modifications, although this is not tested.

For proper real-time execution, the PREEMPT_RT kernel patch is required. Please keep in mind, that the real-time performance of your controller will heavily depend on the proper setup of PREEMPT_RT, which is not covered here. However, your controllers can also run on a non-real-time system if reduced performance (e.g., jitter, missed deadlines) is acceptable.

Installation

The main dependencies of this project are the OROCOS toolchain and rtt_ros_integration. For a convenient setup of those, a script is provided in this repository. Simply create an empty ROS workspace and execute the following in a terminal:

./setup_orocos path_to/created_workspace/

If you are worried about typing your password, just have a look at the script file. sudo is only required for installing missing packages and dependencies. It takes 5-30 minutes to build everything on the computers we tested. Don't forget to follow the instruction in the last output line of the script.

Now, the RTCF can be built. To do this, this repository must be cloned into your project's ROS workspace:

git clone https://github.com/KIT-ISAS/RTCF path_to_project_workspace/src/RTCF

If everything is set up correctly, your preferred build command for ROS workspaces (catkin_make or catkin build) will now build RTCF like any other ROS package.

Publication

If you find this work useful, please cite the following paper:

M. Fennel, S. Geyer, U. D. Hanebeck, "RTCF: A framework for seamless and modular real-time control with ROS", Software Impacts, vol. 9, August 2021.

@article{RTCF,
author = {Michael Fennel and Stefan Geyer and Uwe D. Hanebeck},
title = {RTCF: A framework for seamless and modular real-time control with ROS},
journal = {Software Impacts},
volume = {9},
pages = {100109},
moth = {August},
year = {2021},
doi = {10.1016/j.simpa.2021.100109},
url = {https://doi.org/10.1016/j.simpa.2021.100109},
}

Usage

The RTCF is designed to be used as simple as possible by ROS users. However, some new terminology appears due to the usage of OROCOS behind the scenes. Here is some translation guide from OROCOS to ROS:

• component: node or nodelet
• input port: subscriber
• output port: publisher
• connection: topic

In the following, the basic concepts of the RTCF, which are required for component development and deployment, are introduced. Furthermore, it is highly recommended to have a look at the code in the rtcf_examples-package.

Components

Each module of a complex control architecture is reflected by a component in the RTCF. A component is basically a library that lives in a ROS package and that is loaded at runtime and connected with other components or ROS topics to perform its task.

To create a component called SimpleComponent in the package simple_component the following code is needed:

CMakeLists.txt:

cmake_minimum_required(VERSION 3.0.2)
project(simple_component)

find_package(OROCOS-RTT REQUIRED)
include(${OROCOS-RTT_USE_FILE_PATH}/UseOROCOS-RTT.cmake)
orocos_use_package(ocl-logging)

find_package(catkin REQUIRED COMPONENTS rtcf std_msgs)

include_directories(${catkin_INCLUDE_DIRS})
orocos_component(simple_component src/component.cpp)
target_link_libraries(simple_component ${catkin_LIBRARIES})

orocos_generate_package()
catkin_package()

package.xml:

<package>
  <name>simple_component</name>
  <version>0.0.1</version>
  <description>RTCF tutorial</description>
  <maintainer email="[email protected]">Michael Fennel</maintainer>
  <license>MIT</license>

  <buildtool_depend>catkin</buildtool_depend>
  <build_depend>rtcf</build_depend>
  <build_depend>std_msgs</build_depend>
  <run_depend>rtcf</run_depend>
  <run_depend>std_msgs</run_depend>

  <export>
    <rtt_ros>
      <plugin_depend>rtt_std_msgs</plugin_depend>
    </rtt_ros>
  </export>

</package>

src/component.hpp:

#ifndef COMPONENT_HPP
#define COMPONENT_HPP

#include <std_msgs/Float64.h>

#include <rtcf/macros.hpp>
OROCOS_HEADERS_BEGIN
#include <rtt/Port.hpp>
#include <rtt/RTT.hpp>
OROCOS_HEADERS_END

class SimpleComponent : public RTT::TaskContext {
  public:
    SimpleComponent(std::string const& name);
    bool configureHook();
    bool startHook();
    void updateHook();
    void stopHook();
    void cleanupHook();

  private:
    RTT::OutputPort<std_msgs::Float64> port_out_;
    RTT::InputPort<std_msgs::Float64> port_in_;
    std_msgs::Float64 msg_;
};
#endif

src/component.cpp:

#include "component.hpp"

#include <rtcf/macros.hpp>
OROCOS_HEADERS_BEGIN
#include <rtt/Component.hpp>
OROCOS_HEADERS_END

#include <rtcf/rt_logging_macros.hpp>

SimpleComponent::SimpleComponent(std::string const& name) : TaskContext(name), port_out_("out_port"), port_in_("in_port") {}

bool SimpleComponent::configureHook() {
    this->ports()->addPort(port_in_);
    this->ports()->addPort(port_out_);
    return true;
}

bool SimpleComponent::startHook() { return true; }

void SimpleComponent::updateHook() {
    // just forward the message
    if (port_in_.read(msg_) == RTT::NewData) {
        port_out_.write(msg_);
    }
}

void SimpleComponent::stopHook() {}

void SimpleComponent::cleanupHook() {}

ORO_CREATE_COMPONENT(SimpleComponent)

This component simply reads each message from its input port and forwards it to the output port. To accomplish that, there are several methods (configureHook(), startHook(), updateHook(), stopHook(), and cleanupHook()), which are called automatically during the component life-cycle. The most important one is the updateHook() as this is supposed to do the heavy lifting, i.e., the actual module function, in each iteration. Note, that updateHook() is called in every controller iteration independent of whether there is new data or not.

Deploying

In the previous chapter, the creation of a simple component was presented. Now, these components need to be loaded at runtime. Here, the RTCF follows an approach very similar to nodelets from ROS, where multiple modules (nodelets) are loaded into a single process. In contrast to nodelets, the RTCF goes further and executes all components in a single thread with a fixed order. This allows almost zero overhead and very high controller frequencies. For the determination of the order, the necessary resolution of dependencies arising from the data exchange between the components is performed automatically by the RTCF as far as possible.

The launch-file for the previous example looks like this:

<launch>
    <node name="rt_runner" pkg="rtcf" type="rt_runner" output="screen">
        <rosparam param="mode">"wait_for_components"</rosparam>
        <rosparam param="ros_mapping_blacklist">".*tmp.*"</rosparam>
        <rosparam param="ros_mapping_whitelist">".*mapped.*"</rosparam>
        <rosparam param="num_components_expected">2</rosparam>
        <rosparam param="frequency">1.0</rosparam>
    </node>
    <node name="simple1" pkg="rtcf" type="rt_launcher" args="simple_component SimpleComponent" output="screen">
        <remap from="out_port" to="/mapped/tmp"/>
        <remap from="in_port" to="/mapped/in"/>
        <rosparam param="is_first">True</rosparam>
    </node>
    <node name="simple2" pkg="rtcf" type="rt_launcher" args="simple_component SimpleComponent" output="screen">
        <remap from="out_port" to="/mapped/out"/>
        <remap from="in_port" to="/mapped/tmp"/>
        <rosparam param="topics_ignore_for_graph">".*something"</rosparam>
    </node>
</launch>

The first node tag creates a so-called rt_runner, which is responsible for executing and organizing all components. It has several settings that can be given as parameters:

• frequency: The control loop frequency in Hz.
• ros_mapping_whitelist: A regular expression for connections between components that shall be linked to a ROS topic with the same name.
• ros_mapping_blacklist: A regular expression for connections between components that shall be excluded from linking to ROS topics. Can be combined with ros_mapping_whitelist.
• mode: The mode of the runner:
  • wait_for_components: Do not start the control loop until the expected number of components in num_components_expected is reached.
  • no_wait: Start the control loop immediately after the first component is loaded. When additional components are loaded, the control loop is paused briefly.
  • wait_for_trigger: The control loop does not start and stop automatically, but it is triggered using the services provided under /rt_runner/activate_rt_loop and /rt_runner/deactivate_rt_loop.
• wait_policy: The policy of the wait-command used in the real-time loop. The default value is absolute. This option is ignored in simulation mode (parameter /use_sim_time=True).
  • absolute: If the runtime of the components exceeds the cycle period, the following wait times are shortened to catch up with missed wake-up times. See also ORO_WAIT_ABS in OROCOS documentation.
  • relative: If the runtime of the components exceeds the cycle period, the following wait times are not shortened. See also ORO_WAIT_REL in OROCOS documentation. Due to the implementation, slightly high cycle periods (a few microseconds)
• lock_memory: Lock the application memory using mlockall(). This option can reduce or even prevent page-faults. If it is enabled following additional parameters become available:
  • safe_heap_size_kB: The heap size in kilobytes that is pre-faulted (i.e. allocated, accessed, and then released). A value of zero means that no pre-faulting of the heap takes place. The default value is zero.
  • safe_stack_size_kB: The stack size in kilobytes that is pre-faulted (i.e. allocated, accessed, and then released). A value of zero means that no pre-faulting of the stack takes place. The default value is zero. Internally, the stack allocation is done using alloca().
• cpu_affinity_mask: A bitmask specifying on which CPUs the real-time thread is going to be run. The default value is 1. Examples: 1 means CPU0, 8 means CPU3, 3 means CPU0 and CPU1.

The other node tags of type rt_launcher create an instance of the component SimpleComponentfrom the package simple_component. Therefore, the rt_launcher program is used with the following options:

• topics_ignore_for_graph: A regular expression for all connections or topics (after remapping and name resolution) that are ignored for dependency resolution among the loaded components.
• is_first: Marks a component as to be executed first. This is useful for components that talk to the robot hardware.

In comparison with a standard launch-file, the main differences are

1. the necessity for the rt_runner-node and
2. the shift of the information given in the pkg- and type-attributes of the node-tag towards the args-attribute.

All other launch-file features, such as parameters, remaps, and namespaces, stay intact and are transparently handled by the RTCF.

Introspection

ROS provides small helper tools to analyze its runtime behavior such as rostopic or rosnode. The RTCF offers similar tools to allow easy debugging and runtime inspection. These are:

• rosrun rtcf component_list: List all the components currently loaded similar to rosnode list.
• rosrun rtcf component_info: Obtain information about a single component. Usage is identical to rosnode info.
• rosrun rtcf connection_list: List all internal real-time connections similar to rostopic list. Note, that ports which are not connected to other ports are not listed here.
• rosrun rtcf connection_info: Show information about a connection with a given name. Usage is similar to rostopic info.
• rosrun rtcf component_order: Print the determined real-time execution order after dependency resolution. This has no ROS equivalent.

Real-Time Safe Code

Writing real-time safe code is not black magic, but it requires some discipline. Here are some things that should NEVER be done in a real-time context:

• Usual logging or anything that directly prints to the console (std::cout, ROS_INFO and friends).
• Writing/reading files.
• Other potentially blocking operations
  • Waiting for threads
  • ros::Service::call()
  • ...
• Allocations. This includes data types where the allocation is not obvious. However, pre-allocations, before real-timeness is required, are fine. Examples for allocations are:
  • std::string()
  • std::vector()
  • std::map()
  • new
  • ROS message types with variable field size
  • ...

Note, that the attribute real-time safe does not necessarily imply efficient or fast. It just means that the code execution does not block for an indefinite time because it is waiting for something else.

Advanced Features

The RTCF is shipped with many more useful features, which are explained in the next sections.

Custom Message Packages

Custom ROS message packages can be easily used within the RTCF, thanks to rtt_roscomm.

To generate such an RTCF/OROCOS-compatible message package, execute the following command in your workspace's src-directory:

rosrun rtt_roscomm create_rtt_msgs your_custom_msgs

To use these message in your component, add the following lines to your package.xml:

  <run_depend>your_custom_msgs</run_depend>
  <build_depend>your_custom_msgs</build_depend>
  <export>
    <rtt_ros>
      <plugin_depend>rtt_your_custom_msgs</plugin_depend>
    </rtt_ros>
  </export>

Only use a single <export> and <rtt_ros> tag, if multiple message packages are required. In your CMakeLists.txt, add rtt_your_custom_msgs to your find_package()-command. For a working example, refer to the packages rtcf_examples_msgs, rtt_rtcf_example_msgs, and the Identity-component in rtcf_examples.

Parameter Handling

ROS parameters can be used in an RTCF component using the same API as in ROS nodes. Therefore, the component class must include rtcf/rtcf_extension.hpp and additionally inherit from RtcfExtension. Then, the methods getPrivateNodeHandle() and getNodeHandle() are available to obtain a private node handle or a node handle in the node's namespace. Refer to rtcf_examples/src/parameter_handling/ for an example.

Do not use this in updateHook() as it will break real-time properties. If access to parameters is really a requirement in the real-time context, refer to the next section.

Dynamic Reconfigure

The RTCF wraps dynamic_reconfigure from ROS to facilitate components that can be adjusted during runtime, e.g., for the dynamic tuning of controller parameters. To implement this in a custom component for real-time safe datatypes (see above), a dynamic_reconfigure-config-file as known from ROS must be created. Furthermore, a struct with a conversion function must be provided. The struct has the following form, where both, the ConfigType-typedef and the converting constructor with the ConfigType parameter, are required.

struct ExampleParameters {
    using ConfigType = rtcf_examples::ExampleConfig;
    // list all fields of the .cfg-file
    int int_param;
    double double_param;
    bool bool_param;
    int size;

    // converting constructor
    ExampleParameters(const ConfigType& config) :
        int_param(config.int_param),
        double_param(config.double_param),
        bool_param(config.bool_param),
        size(config.size) {}
    // standard constructor
    ExampleParameters() {}
};

If the header rtcf/rt_dynamic_reconfigure.hpp is included and the component class has a member such as RTDynamicReconfigure<ExampleParameters> dynamic_config;, the dynamic_reconfigure backend is set up using dynamic_config.configure(getPrivateNodeHandle()); in the configureHook(). After that, the current parameters can be queried real-time safe using:

ExampleParameters config_data;
dynamic_config.getValue(config_data);

A complete example is given in rtcf_examples/src/parameter_handling/.

Logging

The RTCF provides two types of logging statements in rt_logging_macros.hpp. The use of ROS_*-style logging is discouraged as it breaks rqt_logger_level as well as real-timeness. For the logging in non-real-time sections, the following macros can be used:

NON_RT_DEBUG(...);
NON_RT_DEBUG_STREAM(...);
NON_RT_INFO(...);
NON_RT_INFO_STREAM(...);
NON_RT_WARN(...);
NON_RT_WARN_STREAM(...);
NON_RT_ERROR(...);
NON_RT_ERROR_STREAM(...);
NON_RT_FATAL(...);
NON_RT_FATAL_STREAM(...);

For real-time safe logging, the following macros can be used:

RT_DEBUG(...);
RT_DEBUG_STREAM(...);
RT_INFO(...);
RT_INFO_STREAM(...);
RT_WARN(...);
RT_WARN_STREAM(...);
RT_ERROR(...);
RT_ERROR_STREAM(...);
RT_FATAL(...);
RT_FATAL_STREAM(...);

Again, real-time safe only means that your logging statements will still have deterministic execution times, but there is still a significant overhead for logging. Furthermore, real-time logging statements can be dropped without notice if necessary resources are not available when the logging statement is called.

Both logging styles are fully compatible with rosconsole, including rqt_logger_level, /rosout ,as well as ROSCONSOLE_CONFIG_FILE. For a complete example, refer to rtcf_examples/src/logging/

Runtime Analysis

To monitor the performance of the deployed controllers, it is vital to measure the calculation time as well as the jitter of the updateHook()-invocations. For this reason, the rt_runner publishes a timestamp of the update invocation time as well as the calculation time on the topic /rt_runner/iteration_info after the completion of every iteration.

Additionally, some timing statistics are printed to the console each time the control loop is stopped. If a more detailed analysis is desired, a rosbag can be recorded using

rosbag record /rt_runner/iteration_info -o bagname --duration 3h

or a similar command. The resulting rosbag can then be analyzed conveniently with an in-built script that plots the latencies as well as the calculation durations. The necessary command for this is:

rosrun rtcf evaluate_performance_rosbag path_to_rosbag frequency

Miscellaneous

Some controllers might be interested in information about the current execution cycle (e.g., for stamping messages or for scaling the gains of a PID controller). To serve these needs, the component-class can inherit from the RtcfExtension-class, which is included using rtcf/rtcf_extension.hpp. This gives access to the following methods:

this->getTime();      // current ROS-timestamp
this->getFrequency(); // control loop frequency in Hz
this->getPeriod();    // control loop period in s

Usage in Simulations

Code that can be used in simulation as well as on real hardware is always preferred. For this reason, the RTCF implements the /use_sim_time-parameter that is used in ROS.

Normally, the RTCF uses CLOCK_MONOTONIC to perform its wait and time measurement operations. If /use_sim_time is set to true, the /clock-topic is used as a time source.

Due to the implementation of the underlying rtt_rosclock-library, care must be taken when the simulation clock frequency is chosen: The control loop will only wake up when the control period has expired and a new clock tick arrives. This means a clock that runs slightly slower than the control frequency will result in a slightly reduced control frequency. However, a clock that runs slightly faster than the control frequency will almost half the resulting control frequency! For this reason, it is ideal if the clock frequency is much higher than the control frequency.

Contributing

Contributions are what make the open-source community such an amazing place to learn, inspire, and create. Any contributions you make are greatly appreciated.

1. Fork the Project
2. Create your Feature Branch (git checkout -b feature/AmazingFeature)
3. Commit your Changes (git commit -m 'Add some AmazingFeature')
4. Push to the Branch (git push origin feature/AmazingFeature)
5. Open a Pull Request

Also, feel free to open an issue at any time.

License

Distributed under the MIT License. See LICENSE for more information.

Contact

• Michael Fennel - [email protected]
• Stefan Geyer - [email protected]

Project Link: https://github.com/KIT-ISAS/RTCF

Acknowledgements

• Funded by: Intelligent Sensor-Actuator-Systems Laboratory (ISAS), Karlsruhe Institue of Technology (KIT)
• OROCOS: https://docs.orocos.org/
• rtt_ros_integration: https://github.com/orocos/rtt_ros_integration
• ROS: https://www.ros.org/
• Readme Template: https://github.com/othneildrew/Best-README-Template