turntable

The purpose of this ROS package is to evaluate the creation, excitation, and simulation of a simple URDF describing a two-link, two-joint robotic actuator. This package contains two scripts designed to 1) generate joint angles to direct the actuator to follow a circular trajectory and 2) publish markers at points along the path of the actuator. If desired, the package components can be run individually, however, it is strongly recommended to use one of the provided launch files to run this package.

At runtime, the launch files will set the robot description on the parameter server, run the required nodes, and launch rviz with a given configuration file. Nominally, all nodes and topics needed will be inside the turntable1 namespace. This namespace can be configured by modifying the demonstration launch file.

A description of the interfaces in use, an overview of the robot "DJ", a quick breakdown of the code, and helpful tips for using this package immediately follow.

Interfaces

Subscribers

• run_turntable.py

  • The node in this script does not subscribe to any topics.

• crank_tracker.py

  • The node in this script does not subscribe to any topics.

Publishers

• run_turntable.py

  • This node publishes joint angles for revolute joint 1 (J1) and joint 2 (J2) on the joint_states topic in this node's namespace. The data format is of type sensor_msgs/JointState.

• crank_tracker.py

  • This node publishes Marker information for spawning markers viewable in rviz. The relevant marker data is published on the path topic in this node's namespace. The data format is of type visualization_msgs/Marker.

tf Listeners

• run_turntable.py

  • The node in this script does not listen for any TF data.

• crank_tracker.py

  • The node in this script listens uses tf2 to listen for transform data from the world frame to the EE frame. This transform data is of type geometry_msgs/TransformStamped.

Robot Overview

The robot in this package is nicknamed "DJ". The URDF for DJ is found here. You can see the graphviz representation of DJ's construction by looking at this Graphviz PDF.

Since DJ is fairly simple, his construction can be essentially represented as...

[base] <--J1--> [L1] <--J2--> [L2] <--EE--> [EE]

...where [links] and <--joints--> are represented as shown.

Code Description

Run_turntable.py handles publishing joint angle solutions for the arm to follow a circular trajectory on the aforementioned joint_states topic. This script will perform some intial ROS setup, establish the update rate at 50 Hz, and loop through a process which entails 1) determining new joint angles for J1 and J2 by using the included function IK_2R_2L, and 2) forming the JointState message using the newly found angles, current system time, and a sequence ID. Note that the frame_ID called out is for the end effector (EE).

This package does not have any specific state publishing mechanism included. Instead, it leverages the robot_state_publisher package (see the included launch files for an example of starting a node for this) to make tf data available to other potential consumers.

Crank_tracker.py requires that valid tf data be available; if so, it will take the relevant pieces of the most recently available transform information to create markers. These markers are published at a rate of 20 Hz at the end of DJ's arm. Much of the code in this file is actually formatting the marker icons that you will see when using rviz.

Helpful Inclusions

Launch Files

• To run a demo of the 2R robotic arm in action, enter: roslaunch turntable demo.launch

• To debug the functionality of the 2R robotic arm in rviz, enter: roslaunch turntable debug_urdf.launch

References

None