Robot Operating System
This page serves as a short introduction to ROS for the new or potential user. Although ROS is a tremendously complex and multifaceted software package, this page endeavors to outline the basic uses and functionality provided by ROS's framework. This is done through example by discussing the high level design of a ROS system developed by Jake Ware and Jarvis Schultz in 2011 for the puppeteer robot system. There is also a short "highlights" section that directs new users towards some useful ROS features that might not be readily apparent.
Above all else, ROS should be seen as a tool to create and manage complex electromechanical systems. Originally developed by the Stanford Artificial Intelligence Laboratory in 2007, the ROS project was adopted by Willow Labs in 2008 and remains in their care. The following is Willow Labs' description of ROS:
"ROS is an open-source, meta-operating system for your robot. It provides the services you would expect from an operating system, including hardware abstraction, low-level device control, implementation of commonly-used functionality, message-passing between processes, and package management. It also provides tools and libraries for obtaining, building, writing, and running code across multiple computers." [Source: ROS Intro]
All of this is true, but the underlying message of all this technical sophistication is still that ROS enables groups of people to work on complex projects by providing a common and well organized framework, while adding a minimal amount of overhead.
A presentation was given to the LIMS lab by Jake Ware and Jarvis Schultz in the Spring of 2011. Although it is not comprehensive, it covers the overall structure and use of ROS, some of the utilities provided, discusses some applications, and goes over pros and cons of using it in a project.
Willow Garage ROS Compilation: Three Years
Currently, ROS is only fully supported in Ubuntu Linux. The full list of supported systems can be found here. A detailed installation walkthrough can be found here. The installation time can take anywhere from 45 minutes to several hours depending on the speed of your internet connection.
If you are planning on using ROS for a long term project, it is absolutely worth investing the time to work through the tutorials provided on the ROS website. Although there are many more tutorials focused on specific stacks and packages, the introductory tutorials are the best place to start and can be found here. If you need a quick refresher, or are having trouble remembering some of the command line tools, you can use this cheat sheet.
In the interest of demonstrating some basic techniques and good practices in ROS, the following example system is presented and described. First, the overall layout and structure of the system will be explained and justified. This will be followed by a short description of each of the packages and a link to download the actual code. The purpose of this system is to perform open and closed-loop control of a mass hung from a winch on a puppeteer robot using the Microsoft Kinect for object tracking. The majority of this code was written by either Jarvis Schultz or Jake Ware, but credit will be given for the parts that were not original.
As mentioned above, the purpose of this system is to perform open and closed-loop tracking on a mass hanging from a puppeteer robot's winch. For the purposes of this article, we will not discuss the robot's code or the code in the Kinect stack. It is particularly important to treat the Kinect as a black box because that software is updated and maintained by Willow Garage. Assuming these two systems perform as they should, we can focus on the six nodes that were written specifically for this system. These six nodes are as follows:
Serial Node (C++): Interfaces with the robot through a serial port assigned to the FTDI cable that attaches to the XBee Wireless Chip. As a safety fail safe, It also watches stdin for any key strike and executes an emergency stop when it sees one.
Estimator Node (C++): Collects state information about the hanging mass from the object tracker and about the robots current position from robot's encoder-based odometry calculations. Although the current version of the Control Node does this, it will eventually be responsible for calculating string length and robot and mass velocities.
Control Node (Python): Uses the current state, last state, and time to calculate the proper gains and control inputs for the next time step.
Object Tracker Node (C++): Find the hanging mass location from the point cloud data generated from the openni_camera node developed by WIllow Garage.
Marker Node (C++): Generates 3D visuals for the robot and mass and displays them in the proper orientation and position in rviz, ROS's visualization software.
Keyboard Node (C++): This node watches stdin for keyboard input and modifies the operating condition of the system according to a defined command set.
Puppeteer Messages (NA): A collection of all the message definitions for the topics and services used in this system.
Kinect Nodes: This is a black box for us and consists of openni_camera and several other ROS packages. This software must be started before the system can function.
rviz: ROS's visualization software that is extremely useful for debugging and working with 3D data.
The following block diagram illustrates the flow of information between these nodes. The timing of the system is driven by the 30Hz rate of the Kinect. That is, the Kinect drives the openni (Kinect) nodes, the openni nodes drive the object tracker, the object tracker drives the estimator, the estimator drives the control node, the control node sends a new command to the robot, and process is repeated. In the long run, the system won't be driven off the 30Hz rate of the Kinect, but by an independent timer that will get robot position updates more frequently.
It is helpful to have some background information on the Kinect to understand how this system operates.
Kinect Hardware: -1 RGB camera (640x480), 1 infrared camera (640x480 with 2048 depth levels), 1 infrared emitter (iFixit - Teardown) -Structured Light approach to measuring depth -Emits a grid pattern in infrared light and the infrared camera measures the deformation from another point of view and generates a grey scale depth image -30 Hz update rate