Introduction

The goals of this lab are to gain experience working with PID controllers, launch files, and the ROS parameter server.

Turtlebot PID

• If you haven't already, run the account configuration script (ros_config_account.sh)
• Accept the PID lab assignment invitation posted to Piazza. The repository page for this lab already contains the starter code that you will be using. Clone the repository into your workspace folder:

  $ cd ~/catkin_ws/src
  $ git clone GIT_URL pid_chase      
  

• Take a minute to read over the file pid_chase.py in the scripts directory, and the file pid_chase.launch in the launch directory. Make sure you understand the contents of those files.
• Build your workspace and source ~/catkin_ws/devel/setup.bash.
• Connect a Turtlebot to your computer. For this lab you will need to connect both the Kinect and the robot itself. Launch the Turtlebot using pid_chase.launch. Point the robot toward a wall and run pid_chase.py. The robot should reach the target distance, but the process isn't very pretty.
• Modify pid_chase.py so that it reads the PID gain parameters from the parameter server and uses the PID controller to set the velocity. (You'll need to modify your script to use pid_control instead of bang_control.)
• Use the rosparam command line utility to set the three parameters expected by pid_chase.py to some initial values and re-execute pid_chase.py.

• Experiment with different values for your gain parameters until you find settings that work well. You want the Turtlebot to smoothly approach the target distance then stop at the appropriate point without excessive oscillations. The p_error term should be driven to a value that is very close to 0. The pid_chase.py node publishes all three error terms. You can echo them to a terminal:

  rostopic echo /p_error
  

  or you can visualize them using rqt_plot:

  rosrun rqt_plot rqt_plot /p_error/data /i_error/data
  

• Once you are satisfied with your gain parameters, modify pid_chase.launch to set the parameters appropriately. The launch file XML format is described on this wiki page. Test the new launch file. Once everything is working, commit and push your changes. Make sure all members have joined the Github group.
  

If You Have Extra Time

• One weakness of the current version of this code is that the robot makes fast velocity changes when the distance changes: for example, when someone steps in front of the depth sensor. This results in jerky motions that are hard on the robot's motors.

  We can force the velocity to change smoothly by taking a weighted average between the previous velocity and the desired velocity:

        new_velocity = α × desired_velocity + (1 - α) × previous_velocity

  Values of α near 1 will lead to fast velocity changes, while values of α near 0 will lead to slow changes in velocity.

• Update pid_chase.py to implement velocity smoothing. (You may need to re-tune your PID gain terms. In effect, smoothing the velocity changes the dynamics of the system that the PID controller is trying to control.)