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Note: This tutorial assumes that you have completed the previous tutorials: Writing a realtime controller, Starting Gazebo, Adding Objects To The World, Teleop Arm Using move_arm.
(!) Please ask about problems and questions regarding this tutorial on answers.ros.org. Don't forget to include in your question the link to this page, the versions of your OS & ROS, and also add appropriate tags.

Incrementally develop and test a controller in simulation.

Description: This tutorial walks you through the process of creating s simple custom controller, incrementally adding functionality to it and testing each step in simulation.

Tutorial Level: INTERMEDIATE

/!\ work in progress

Get Gazebo Up and Running

Make sure you complete StartingGazebo before continuing. The AddingObjectsToTheWorld and Teleop Arm Using move_arm tutorials will provide you with further insights into what is going on.

A Word about Launch and Parameter Files

In order to make the cycle of development and testing easier, we create (and update) two launch files, one for starting up Gazebo with a robot in it, and one for spawning the controller we are testing. There also is one parameter file that tells pr2_controller_manager about the controller plugins to launch, and later a configuration file for the tutorial controller.

Launch file for Gazebo

This is essentially copy-pasted from two launch files referenced in the Gazebo tutorials, and it stays constant throughout the incremental development steps further down.

 $ cat `rospack find gazebo`/launch/empty_world.launch
 $ cat `rospack find pr2_gazebo`/pr2.launch

If you have trouble with this launch file, you should have a look at gazebo and pr2_gazebo to see if anything has changed there.

   1 <launch>
   2   <!-- launch gazebo with empty world -->
   3   <param name="/use_sim_time" value="true" />
   4   <node name="gazebo" pkg="gazebo" type="gazebo"
   5         args="$(find gazebo_worlds)/worlds/empty.world"
   6         respawn="false" >
   7     <env name="LD_LIBRARY_PATH"
   8          value="$(find pr2_gazebo_plugins)/lib:$(find gazebo)/gazebo/lib:$(optenv LD_LIBRARY_PATH)" />
   9     <env name="GAZEBO_RESOURCE_PATH"
  10          value="$(find pr2_ogre):$(find ikea_ogre):$(find gazebo_worlds):$(find gazebo)/gazebo/share/gazebo" />
  11     <env name="OGRE_RESOURCE_PATH" value="$(find ogre)/ogre/lib/OGRE" />
  12   </node>
  13   
  14   <!-- spawn PR2 -->
  15   <include file="$(find pr2_description)/robots/upload_pr2.launch" />
  16   <node pkg="gazebo" type="spawn_model"
  17         args="-urdf -param robot_description -model pr2"
  18         respawn="false" output="screen" />
  19   <node pkg="robot_state_publisher" type="state_publisher"
  20         name="robot_state_publisher" output="screen">
  21     <param name="publish_frequency" type="double" value="50.0" />
  22     <param name="tf_prefix" type="string" value="" />
  23   </node>
  24 </launch>

Launch files for testing the controller

Looking at an example for inspiration:

 $ cat `rospack find pr2_gazebo`/controllers/l_arm_position_controller.launch

we see that

Incremental Development

Step zero: create package directory and files

  1. Create a directory to contain the controller code. In the following, we will call it controller_tutorial. Change the names in the manifest.xml and other files accordingly if you chose another name.

    •    $ mkdir controller_tutorial
         $ cd controller_tutorial
  2. If necessary, add the direcory to your ROS_PACKAGE_PATH environment variable. E.g.

    •    $ export ROS_PACKAGE_PATH=${ROS_PACKAGE_PATH}:`pwd`
  3. Populate the direcory with files. You can use step0.tar.gz to do so, which contains just an empty shell that compiles, links, and registers itself as a plugin:

    •    $ tar xfvz step0.tar.gz
         CMakeLists.txt
         Makefile
         controller_plugins.xml
         controller_tutorial.cpp
         manifest.xml
  4. Build and check if the plugin has been correctly registered:
    •    $ make
         $ rospack plugins --attrib=plugin pr2_controller_interface

      The output should contain a line with controller_tutorial (probably the last line).

The controller_tutorial.cpp from step zero does not actually do anything. So let's start by just having it list the available robot links. For this, we add code to the TutorialController::init() which actually uses the pr2_mechanism::RobotState passed as parameter. The relevant snippet uses documentation from the pr2_mechanism_model package:

   1 bool TutorialController::
   2 init(pr2_mechanism::RobotState *robot, const ros::NodeHandle &n)
   3 {
   4   fprintf(stderr, "TutorialController::init()\n");
   5   
   6   typedef std::vector<pr2_mechanism::Joint*> jointvec_t;
   7   jointvec_t const & jvec(robot->model_->joints_);
   8   for (jointvec_t::const_iterator ij(jvec.begin());
   9        ij != jvec.end();
  10        ++ij)
  11     {
  12       fprintf(stderr,
  13               "  n: %s\tt: %d\tmin: %g\tmax: %g\n",
  14               (*ij)->name_.c_str(),
  15               (*ij)->type_,
  16               (*ij)->joint_limit_min_,
  17               (*ij)->joint_limit_max_);
  18     }
  19   
  20   return true;
  21 }
  1. Unpack the step1.tar.gz tarball to get the code into the controller_tutorial directory and build it:

    •    $ tar xvfz step1.tar.gz
         CMakeLists.txt
         Makefile
         controller_plugins.xml
         controller_tutorial.cpp
         manifest.xml
         start_pr2_gazebo.launch
         test.launch
         test.yaml
         $ make
  2. Launch Gazebo:
    •    $ roslaunch start_pr2_gazebo.launch
  3. In another terminal, launch the controller:
    •    $ roslaunch `rospack find controller_tutorial`/test.launch
      You should see some output like the following in the terminal where you started Gazebo:
      TutorialController::init()
        n: base_laser_joint   t: 4    min: 0  max: 0
        n: bl_caster_l_wheel_joint    t: 2    min: 0  max: 0
        n: bl_caster_r_wheel_joint    t: 2    min: 0  max: 0
        n: bl_caster_rotation_joint   t: 2    min: 0  max: 0
        n: br_caster_l_wheel_joint    t: 2    min: 0  max: 0
        n: br_caster_r_wheel_joint    t: 2    min: 0  max: 0
        n: br_caster_rotation_joint   t: 2    min: 0  max: 0
        n: double_stereo_frame_joint  t: 4    min: 0  max: 0
        n: fl_caster_l_wheel_joint    t: 2    min: 0  max: 0
        n: fl_caster_r_wheel_joint    t: 2    min: 0  max: 0
        n: fl_caster_rotation_joint   t: 2    min: 0  max: 0
        n: fr_caster_l_wheel_joint    t: 2    min: 0  max: 0
        n: fr_caster_r_wheel_joint    t: 2    min: 0  max: 0
        n: fr_caster_rotation_joint   t: 2    min: 0  max: 0
        n: head_pan_joint     t: 1    min: -2.92      max: 2.92
        n: head_plate_frame_joint     t: 4    min: 0  max: 0
        n: head_tilt_joint    t: 1    min: -0.55      max: 1.047
        n: high_def_frame_joint       t: 4    min: 0  max: 0
        n: high_def_optical_frame_joint       t: 4    min: 0  max: 0
        n: l_elbow_flex_joint t: 1    min: -2.3       max: 0.1
        n: l_forearm_cam_frame_joint  t: 4    min: 0  max: 0
        n: l_forearm_cam_optical_frame_joint  t: 4    min: 0  max: 0
        n: l_forearm_joint    t: 4    min: 0  max: 0
        n: l_forearm_roll_joint       t: 2    min: 0  max: 0
        n: l_gripper_float_joint      t: 3    min: -0.05      max: 0.001
        n: l_gripper_joint    t: 3    min: 0  max: 0.09
        n: l_gripper_l_finger_joint   t: 1    min: 0  max: 0.548
        n: l_gripper_l_finger_tip_joint       t: 1    min: 0  max: 0.548
        n: l_gripper_motor_accelerometer_joint        t: 4    min: 0  max: 0
        n: l_gripper_palm_joint       t: 4    min: 0  max: 0
        n: l_gripper_r_finger_joint   t: 1    min: 0  max: 0.548
        n: l_gripper_r_finger_tip_joint       t: 1    min: 0  max: 0.548
        n: l_gripper_tool_joint       t: 4    min: 0  max: 0
        n: l_shoulder_lift_joint      t: 1    min: -0.5236    max: 1.3963
        n: l_shoulder_pan_joint       t: 1    min: -0.714602  max: 2.2854
        n: l_upper_arm_joint  t: 4    min: 0  max: 0
        n: l_upper_arm_roll_joint     t: 1    min: -0.8       max: 3.9
        n: l_wrist_flex_joint t: 1    min: -0.1       max: 2.2
        n: l_wrist_roll_joint t: 2    min: 0  max: 0
        n: laser_tilt_joint   t: 4    min: 0  max: 0
        n: laser_tilt_mount_joint     t: 1    min: -0.785     max: 1.57
        n: narrow_stereo_frame_joint  t: 4    min: 0  max: 0
        n: narrow_stereo_l_stereo_camera_frame_joint  t: 4    min: 0  max: 0
        n: narrow_stereo_optical_frame_joint  t: 4    min: 0  max: 0
        n: narrow_stereo_r_stereo_camera_frame_joint  t: 4    min: 0  max: 0
        n: plug_holder_joint  t: 4    min: 0  max: 0
        n: r_elbow_flex_joint t: 1    min: -2.3       max: 0.1
        n: r_forearm_cam_frame_joint  t: 4    min: 0  max: 0
        n: r_forearm_cam_optical_frame_joint  t: 4    min: 0  max: 0
        n: r_forearm_joint    t: 4    min: 0  max: 0
        n: r_forearm_roll_joint       t: 2    min: 0  max: 0
        n: r_gripper_float_joint      t: 3    min: -0.05      max: 0.001
        n: r_gripper_joint    t: 3    min: 0  max: 0.09
        n: r_gripper_l_finger_joint   t: 1    min: 0  max: 0.548
        n: r_gripper_l_finger_tip_joint       t: 1    min: 0  max: 0.548
        n: r_gripper_motor_accelerometer_joint        t: 4    min: 0  max: 0
        n: r_gripper_palm_joint       t: 4    min: 0  max: 0
        n: r_gripper_r_finger_joint   t: 1    min: 0  max: 0.548
        n: r_gripper_r_finger_tip_joint       t: 1    min: 0  max: 0.548
        n: r_gripper_tool_joint       t: 4    min: 0  max: 0
        n: r_shoulder_lift_joint      t: 1    min: -0.5236    max: 1.3963
        n: r_shoulder_pan_joint       t: 1    min: -2.2854    max: 0.714602
        n: r_upper_arm_joint  t: 4    min: 0  max: 0
        n: r_upper_arm_roll_joint     t: 1    min: -3.9       max: 0.8
        n: r_wrist_flex_joint t: 1    min: -0.1       max: 2.2
        n: r_wrist_roll_joint t: 2    min: 0  max: 0
        n: sensor_mount_frame_joint   t: 4    min: 0  max: 0
        n: torso_lift_joint   t: 3    min: 0  max: 0.2
        n: wide_stereo_frame_joint    t: 4    min: 0  max: 0
        n: wide_stereo_l_stereo_camera_frame_joint    t: 4    min: 0  max: 0
        n: wide_stereo_optical_frame_joint    t: 4    min: 0  max: 0
        n: wide_stereo_r_stereo_camera_frame_joint    t: 4    min: 0  max: 0
      As you can see, there are quite a few joints on PR2...

Step two: commanding sinusoidal torques

Now it is time to actually command some torques. To achieve this, we will simply send some sinusoidal torques to some of the joints. The names of the joints to command and parameters of the sine waves will be read from a YAML file. The name of that file will be read from the parameter server when the controller starts up.

  1. Unpack step2.tar.gz into the controller_tutorial directory:

    •    $ tar xfvz step2.tar.gz
         CMakeLists.txt
         Makefile
         controller_plugins.xml
         controller_tutorial.cpp
         manifest.xml
         start_pr2_gazebo.launch
         test.launch
         test.yaml
         wave.yaml
  2. Look at the relevant changes:
    • Our dependence on the YAML parser has been added to manifest.xml

    • controller_tutorial.cpp now uses a simple data structure for storing the waveform parameters for each of the joints that is being commanded by the controller. The mapping from joints to waveforms happens in TutorialController::init(), and the update of the command torques in TutorialController::update(). Also, TutorialController::starting() is used to record the start time of the controller, such that at each tick we can determine the number of seconds (wallclock) since we started.

    • A private parameter has been added to the test.launch file, such that the TutorialController::init() can get the name of the waveform file name from the parameter server.

    • The new wave.yaml file maps joint names to the parameters of the sinusoidal torque that we are going to command them with.

  3. Build
    •    $ make
  4. Launch Gazebo
    •    $ roslaunch start_pr2_gazebo.launch
  5. Launch test
    •    $ roslaunch test.launch
      The terminal where you started Gazebo should now spew out 8 columns of changing numbers, and the simulated robot should be moving its arms (in a not so useful manner).


2019-10-19 13:02