There is a long history of putting a tethered insect on a ball to measure its movements in response to controlled sensory stimuli. As mentioned in the publication, Karl Götz and Erich Buchner had fly-on-a-ball systems working almost four decades ago. Our work merely uses modern image processing technology to acquire, with high temporal resolution, velocity about all axes of rotation of the ball.
Between the published paper and these files, we want to provide everything you need to build a complete fly-ball-tracker system. We apologize for any missing information, but will be happy to fix that if you let us know. We also hope to improve the organization of all this information in the coming weeks.
Zipped Autodesk Inventor CAD files for the tracker system are available here. TreadmillSystemFinal.iam is the top-level assembly.
Software, Gerber files, and documentation to assemble the camera/optic chip system part of the ball tracker/Fly treadmill are available in this zipped file.
MATLAB code to calibrate the ball tracker using Camera3 (see paper) is available here.
The Janelia Farm Drosophila-on-a-ball team 
Standing (l-r): Michael Reiser, Gus Lott, Vivek Jayaraman. Sitting (l-r): Johannes Seelig and Eugenia Chiappe. Key contributors missing in this picture: Nir Dutta, Jason Osborne.