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Tutorial 7: Adding a robot arm and a camera to the Turtlebot4

Development of Inteligent Systems, 2024

This tutorial contains a modified robot, with an added robot arm with a camera. You can move the arm as you wish, placing the camera at a convenient position. This is mainly meant for the parking task and for reading the QR on top of the cylinder, but you are welcome to use it as you wish.

You should see the robot in this configuration after starting the simulation

Install packages

All relevant control packages are most probably already installed on your computer. If not, we need the following packages in order for this modified version of the Turtlebot to work:

sudo apt install ros-humble-ros2-control ros-humble-ros2-controllers ros-humble-ign-ros2-control

Start the simulation with the upgraded Turtlebot

After you have downloaded and built this package you can start the simulation with:

ros2 launch dis_tutorial7 sim_turtlebot_nav.launch.py

Once the simulation is running you can notice there is an additional camera on top of the robot, as in the above image. This camera can be accessed the same way the oakd camera is accessed (the same topics are available), but the prefix is top_camera. The arm is controlled by the arm_controller controller, which should be started after the diffdrive_controller. You can verify that the arm controller is working if, after unpausing the simulation, you see a printout in the terminal [spawner_arm_controller]: Configured and activated arm_controller as in the below image:

You can modify the launch file sim_turtlebot_nav.launch.py the same as the one in dis_tutorial3. The main difference is that this one loads a lot of the launch files from this package, instead of the system installed ones. This is necessary in order to load all the things we need to load for the robot arm to work.

Setting the arm position

The node arm_mover_actions.py is a node that you can use to set the arm positon in a simple way. The node is communicating with the arm_controller through an action interface, just like we set a goal for the robot in the Nav2 stack (tutorials 3 and 4). You can include this node in some launch file, or start it with ros2 run:

ros2 run dis_tutorial7 arm_mover_actions.py

This node listens for a command of type String on the /arm_command topic. Currently, there are four approximate positions that are hard-coded:

self.arm_poses = {'look_for_parking':[0.,0.4,1.5,1.2],
                  'look_for_qr':[0.,0.6,0.5,2.0],
                  'garage':[0.,-0.45,2.8,-0.8],
                  'up':[0.,0.,0.,0.],
                  'manual':None}

This positions are approximate positions that might be suitable for different tasks. The "garage" position is for packing the arm so we do not hit something while driving the robot. The "up" position just sets all the joints to 0. The "look_for_parking" is a configuration that might be suitable for parking the robot, as seen in the below image. The "look_for_qr" position is a position that might be suitable for reading the QR code on top of the cyllinder. You are highly encouraged to modify these postions as you see fit! When you start the simulation the arm is in the 'garage' configuration.

Configuration 'look_for_parking' The image from the topic

The position of the arm should be set from your code, and you can test the positions by using the 'ros2 topic' interface:

ros2 topic pub --once /arm_command std_msgs/msg/String "{data: garage}"

or

ros2 topic pub --once /arm_command std_msgs/msg/String "{data: look_for_parking}"

or

ros2 topic pub --once /arm_command std_msgs/msg/String "{data: look_for_qr}"

or

ros2 topic pub --once /arm_command std_msgs/msg/String "{data: up}"

You can add configurations by simply updating the self.arm_poses dictionary.

Setting the arm position 'manually'

The last key in the dictionary - "manual" is there for debugging purposes, so that you are able to quickly test a configuration of the arm.

Be careful when using the "manual" keyword, as you can easily crash the node with the wrong format message. To set a manual position send a String message in the format 'manual:[pos1, pos2, pos3, pos3]' where pos1-4 are floats (the numbers should contain a . ). For example:

ros2 topic pub --once /arm_command std_msgs/msg/String "{data: 'manual:[0.,0.6,0.5,2.0]'}"

Some notes

This is a 4DOF arm. All the joints are rotational. The commands set the rotation of each joint in sequence. The first practically rotates about the Z axis of the robot (same as the robot). The other three joints rotate about a vector which is normal to the Z axis of the robot. The axis of each joint are visualized in Rviz, you can also take a look at the arm definition in /urdf/arm/arm.urdf.xacro.

The arm does not respect the laws of physics. In order to computationally simplify the simulation and reduce the planning complexity, the arm is not simulated realistically. It has no collision geometry, its mass/inertia is negligible, and its joints have no angle limitations. This can enable you to put the arm in some weird configurations. You are welcomed to play with it, but it has not be extensively tested and some configurations might break the simulation. In evaluation phase, it is best to stick to tested configurations.

The controller has more capabilities then used. The joint_trajectory_controller/JointTrajectoryController used for controlling the rotations of the joints make the arm follow trajectories, not only reach a single position. The arm_mover_actions.py script only sets a single position in the trajectory, but you can modify it to follow a set of consequtive positions. Additionally, you can also change the time in which the controller should reach the position. This is set in the line point.time_from_start = rclpy.duration.Duration(seconds=3.).to_msg() in the arm_mover_actions.py script. `

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