Bristlebot Shape and Motion Tracking

This project follows the Scientific Method. Review the steps before you begin.

Data Collection

1. Set up an area to do your experiment (Figure 3).
   1. You will need a flat, level, smooth surface. Bristlebots will not work well on carpet, rough surfaces, or wood floors with gaps between the pieces of wood. 
   2. Make a ring to keep your bristlebots contained. You can use a hula hoop or build one with strips of paper taped together end-to-end.
   3. Mount a phone or camera directly above the center of the ring, facing straight down. If you do not have a clamp available, you can try holding the phone or camera by hand, but you will need to hold it as still as possible for the video.

Image Credit: Ben Finio / Science Buddies

Figure 3. Experiment setup. A ring made from strips of paper taped together end to end is taped to the table. A gooseneck phone clamp is attached to a shelf (not pictured) above the table, so a phone can be mounted facing down at the ring.

2. Build two bristlebots with the same mass but different moments of inertia.
   1. You can find basic instructions to build a bristlebot in this activity or in the following video.
   2. For one bristlebot, tape an entire popsicle stick onto the toothbrush before you attach the battery and motor (Figure 2, left side).
   3. For the other bristlebot, cut a popsicle stick into three equal-length pieces. Tape them side-by-side onto the toothbrush head before you attach the battery and motor (Figure 2, right side).
   4. Disconnect one pair of motor/battery wires on each bristlebot to turn them off for now.

https://www.youtube.com/watch?v=Q1zToREgV0c

3. Record three videos of each bristlebot moving in your arena (six videos total). Each video should have at least 10 seconds of the bristlebot moving with an unobstructed camera view (not blocked by your hand).
   1. Important: Before you record, make sure to read the "Supported Video Formats" section of the Tracker website. 
   2. Re-connect one of the bristlebot's wires.
   3. Place it in the center of the arena, let go, and immediately start recording a video. It may be easier if you have a second person to help operate the camera.
   4. Wait at least 10 seconds to stop the video. If the bristlebot falls over or gets stuck during this time, delete the video and start over. 
   5. After 10 seconds of unobstructed movement, stop the video.
   6. Move the bristlebot back to the center and start a new video. Repeat until you have three videos for each bristlebot. 
   7. Disconnect one pair of wires for each bristlebot to turn them off for now.
4. Transfer the video files to your computer. You may want to rename the files to make them easier to keep track of, such as "high moment of inertia trial 1.mp4" and so on.

Motion Tracking

1. Install the Tracker Video Analysis software on your computer.
2. If this is your first time using the software, follow some of the tutorials available on the website.
3. Select File→Open→File Chooser to open the video file for your first trial.
4. File→Save→Tab to save a .trk file with your tracking information. Remember to save your file frequently as you work. Note: if you want to easily share your entire project with someone else, you can save a "project" instead (.trz file), which will include a copy of the video file itself. This will result in a larger file on your computer, and is not necessary if you are only analyzing the data yourself.
5. Instead of just tracking a point (like your bristlebot's center of mass), you will track a vector as it moves. A vector has both magnitude and direction. This will allow you to track both your bristlebot's position and its angle so you can see how it rotates. 
6. Advance the video until the bristlebot is moving with an unobstructed camera view. You may need to skip the first few seconds of footage if your hand is in the way or you have not put the bristlebot down yet.
7. Select Track→New→Vector.
8. Think about two points on your robot that are easily visible and you can consistently identify in each frame of the video. The points where the wires connect to the motor and battery are good examples.
9. Add a vector to the video frame.
   1. Hold down the shift key.
   2. Click on the first point you decided to track.
   3. While still holding down the shift key, drag your mouse to the second point.
   4. Release the mouse button. The video should automatically advance to the next frame.
10. You may wish to adjust what is visible on the screen as you track. Click the eyeball button in the top toolbar to see some of the options.
    1. Change "Trails" from "Short trails" to "All steps" to see every frame you have tracked.
    2. Turn off "Labels" to remove the text labels and make the screen less cluttered.
11. Adjust the data shown in the graphs and table on the right side of the screen.
    1. Click the "Plots" drop-down menu and change it to 3 plots.
    2. Click the y-axis label of each plot to change what is displayed. 
       1. Change the top plot to "xtail" (the horizontal position of the tail end of your vector in pixels).
       2. Change the middle plot to "ytail" (the vertical position of the tail end of your vector in pixels).
       3. Change the bottom plot to "θ" (the angle of the vector in degrees, where 0° is to the right, +90° is up, -90° is down, and 180° is to the left).
    3. Adjust the "Columns" menu of the data table to show the same data that is plotted in the graphs. 
    4. You can drag the slider bars to change the relative sizes of the tracking window, graphs, and data table.
12. After you have set the screen the way you like it (you can always change it again later), return to tracking.
    1. Repeat the shift-click-drag process to add a vector to each frame until you have tracked at least 10 seconds of movement.
    2. Note: this might take a while! If your video was recorded at 30 frames per second, then 10 seconds of video is 300 frames.
13. Figure 4 shows an example of what your screen might look like after completing this process.  
    1. Look at the vectors in the video frame. Can you identify areas where the bristlebot was spinning tightly in circles versus where it was moving in more gradual curves?
    2. Look at the graph for the angle. Note how the angle "wraps around" from +180° to -180°. This lets you count how many complete revolutions the bristlebot completed in a 10-second window (each time the line goes off the bottom of the graph and comes back around the top).

Image Credit: Ben Finio / Science Buddies

Figure 4. Screenshot of Tracker software after tracking 10 seconds of bristlebot movement.

14. Make a data table like Table 1.
15. Count the number of times your bristlebot went through a full revolution in a ten-second period and record the value for Trial 1. 
16. Repeat steps 3-14 for each additional trial and record the results in your data table.
17. Calculate the average number of revolutions in 10 seconds for each bristlebot. 
18. Which bristlebot spins faster on average, the one with a higher moment of inertia, or lower? Does this match your prediction?
19. Can you use what you learned to design a bristlebot that travels (on average) more straight instead of spinning in tight circles?