ros_scxml

Lightweight finite state machine library that uses the SCXML standard

---

Prerequisites

QT 5

The QScxml library is only available from version Qt 5.7 and higher. This implementation currently uses Qt 5.12.2 or above.

Recommended Download (PPA)

• This PPA provides binary distributions of various versions of Qt for various operating systems
  • On the PPA site, select the QT (5.12 or above) version for your system
    • Qt 5.13.x is recommended for Ubuntu 18.04
• Add the PPA

  sudo add-apt-repository ppa:beineri/opt-qt-5.13.2-bionic
  sudo apt-get update

  Note: Edit command above to match the version of qt that you'd like to install

• Install the full Qt library

  sudo apt install qt513-meta-full
  

  Note: Edit command above to match the version of qt that you'd like to install

Alternative Download (Qt Installer)

• The library can be downloaded from here. Run the installation script with root access and follow the on screen instructions.

Setup environment variables

In order to make this library accessible to cmake the CMAKE_MODULE_PATH and LD_LIBRARY_PATH environment variables must be set. Locate your Qt installation directory (usually in the /opt directory) and set the environment variables as follows:

export CMAKE_PREFIX_PATH=<path>/<to>/<qt>/lib/cmake:$CMAKE_PREFIX_PATH
export LD_LIBRARY_PATH=<path>/<to>/<qt>/lib:$LD_LIBRARY_PATH
export LD_LIBRARY_PATH=<path>/<to>/<qt>/plugins:$LD_LIBRARY_PATH  

For the version installed above qt513 it would be as follows:

export CMAKE_PREFIX_PATH=/opt/qt513/lib/cmake:$CMAKE_PREFIX_PATH
export LD_LIBRARY_PATH=/opt/qt513/lib:$LD_LIBRARY_PATH
export LD_LIBRARY_PATH=/opt/qt513/plugins:$LD_LIBRARY_PATH

---

Demo program

Description

• Demo State Machine Workflow (Produced from the scxml file with QT Creator)

• Implementation: The demo_scxml_state_machine ROS node shows how to use the State Machine library with ROS and process specific code. A key part of this code is in the section that adds custom functions which get invoked when specific states are entered or exited; inspect this section in order to understand how this is accomplished:

    // adding application methods to SM
    MockApplication process_app(nh);
    bool success = false;
    std::vector<std::function<bool()> > functions = {
  
      // custom function invoked when the "st3Reseting" state is entered
      [&]() -> bool {
        return sm->addEntryCallback(
            "st3Reseting",
            [&](const Action& action) -> Response {
              // checking passed user data first
              if (!action.data.empty())
              {
                try
                {
                  double secs = boost::any_cast<double>(action.data);
                  ROS_INFO("State received time value of %f seconds", secs);
                }
                catch (boost::bad_any_cast& e)
                {
                  ROS_WARN_STREAM(e.what());
                }
              }
  
              process_app.resetProcess();
              ros::Duration(3.0).sleep();
              // queuing action, should exit the state
              sm->postAction(Action{ .id = "trIdle" });
              return true;
            },
            false);  // false = runs sequentially, use for non-blocking functions
      },
  
      // custom function invoked when the "st3Execute" state is entered
      [&]() -> bool {
        return sm->addEntryCallback(
            "st3Execute",
            [&process_app](const Action& action) -> Response { return process_app.executeProcess(); },
            true);  // true = runs asynchronously, use for blocking functions
      },
  
      // custom function invoked when the "st3Execute" state is exited
      [&]() -> bool {
        return sm->addExitCallback("st3Execute", [&process_app]() {
          process_app.pauseProcess();
          ROS_INFO_STREAM("Process counter at " << process_app.getCounter());
        });
      },
  
      // custom function invoked prior to entering the "st3Completing" state
      [&]() -> bool {
        return sm->addPreconditionCallback("st3Completing", [&process_app](const Action& action) -> Response {
          Response res;
          static const int REQ_VAL = 16;
          if (process_app.getCounter() >= REQ_VAL)
          {
            return true;
          }
          res.msg = boost::str(boost::format("Counter less than %i") % REQ_VAL);
          res.success = false;
  
          return res;
        });
      },
  
      // custom function invoked when the "st3Suspending" state is entered
      [&]() -> bool {
        return sm->addEntryCallback(
            "st3Suspending",
            [&](const Action& action) -> Response {
              ROS_INFO("Suspending process");
              process_app.haltProcess();
              ros::Duration(3.0).sleep();
  
              // queuing action, should exit the state
              sm->postAction(Action{ .id = "trSuspending" });
              Response res;
              res.success = true;
              return std::move(res);
            },
            true);  // true = runs asynchronously, use for blocking functions
      },
  
      // custom function invoked when the "st3Completing" state is entered
      [&]() -> bool {
        return sm->addEntryCallback(
            "st3Completing",
            [&process_app](const Action& action) -> Response {
              Response res;
              res.success = true;
              res.data = ros::Time::now().toSec();
              return std::move(res);
            },
            false);  // true = runs asynchronously, use for blocking functions
      },
  
      // custom function invoked when the "st2Clearing" state is entered, it will exit after waiting for 3 seconds
      [&]() -> bool {
        return sm->addEntryCallback(
            "st2Clearing",
            [&](const Action& action) -> Response {
              ROS_INFO("Clearing to enable process, please wait ...");
              ros::Duration(3.0).sleep();
  
              // queuing action, should exit the state
              sm->postAction(Action{ .id = "trStopped" });
              return true;
            },
            true);  // true = runs asynchronously, use for blocking functions
      },
  
      // custom function invoked when the "st2Clearing" state is exited
      [&]() -> bool {
        return sm->addExitCallback("st2Clearing",
                                   [&process_app]() { ROS_INFO("Done Clearing, Process is now good to go ..."); });
      },
    };
  

---

Workspace Setup

catkin (ROS1)

1. Build the workspace

   catkin build
   

colcon (ROS2)

1. Build colcon environment

   colcon build --symlink-install
   

---

RUN Demo

ROS 1

1. Start the roscore

2. Go to the resource directory:

   roscd roscpp_scxml_demos/resource/
   

3. Open the demo_packml_sm.scxml file with QTCreator to display a graphical depiction of the SM workflow.

4. Run the node with the demo scxml file:

   rosrun roscpp_scxml_demos demo_scxml_state_machine _state_machine_file:=demo_sm.scxml
   

5. Echo the current state:

   rostopic echo /current_state
   

6. Call the service to print the available actions at the current state:

   rosservice call /print_actions "{}"
   

   The SM terminal should display something as follows:

   SM Actions:
     -userClear
     -trAborted
   

   This means that the actions available at the current state are trAborted and userClear

7. Publish an action from the list to the state machine, for instance the userClear action will be requested as follows:

   rostopic pub -1 /execute_action std_msgs/String "data: 'userClear'"
   

   This should cause the state machine to go into the st2Clearing state. A few seconds after that it should go into the st2Stopped state as the custom entry callback function automatically posts an action.

8. Repeat steps 6 and 7 to experiment with other states and actions, use the SM graph shown in QTCreator to verify the workflow.

9. Additionally, the SM will publish a message to the /process_msg topic only when it's in the st3Execute state. When the SM is in that state echo the topic as follows:

   rostopic echo /process_msg
   

ROS 2

1. From the rclcpp_scxml_demos directory run the demo node as follows

   ros2 run rclcpp_scxml_demos demo_scxml_state_machine __params:=resource/params.yaml
   

2. From another terminal echo the current state

   ros2 topic echo /current_state
   

You should get the name of the current state printed in the terminal

3. Print current actions

   ros2 service call /print_actions std_srvs/srv/Trigger
   

In the node terminal you should see a list of available actions within the current state

4. Execute action

   ros2 topic pub -1 /execute_action std_msgs/msg/String '{data: trAborted}'
   

If the action is valid you should see the following confirmation message in the node terminal "Action trAborted successfully executed"