What the heck was that?
This was my first impression of BB-8 when a 3-second glimpse of it was revealed during the first “teaser trailer” for Star Wars Episode 7. No one had seen a droid such as this. A dome bouncing along on a rotating ball.
I had to have one.
I reasoned that I could drive a ball with a remote control tank inside. The trick would be to keep the dome from falling off. Three weeks later, I had an outer shell with a finished Styrofoam dome. The dome had five cannibalized computer mice underneath, their rubber balls replaced with ⅞” steel bearings. The inner tank had a framework attached, supporting an array of five magnet columns, spring-loaded to keep them in contact with the inner ball while the tank crawled around the interior. This design failed for two reasons. Firstly, the tank only touched the ball on its four corners, so there was little traction. Secondly, the inertia of the droid when it stopped kept the lower ball rolling, causing the upper dome to tip off. I needed to increase the traction and come up with a way to stabilize the ball.
The solution I came up with was a custom drive system that puts the weight and traction directly in the bottom of the lower ball. Two rubber drive wheels, powered by two Power Wheels motors and gearboxes, and controlled by radio with a Sabertooth 2×25 motor controller, would give me the control and traction I needed. I would be able to turn the drive system 360° and have enough power to overcome the drag of the magnets at the top of the ball. I added a sliding platform so that I could move the magnet array forward and backward inside the lower ball. A 360° servo rotates the magnet platform and the upper dome.
For the enclosure I used 18″ polycarbonate globes from edee.com. (The community now feels 20″ is more accurate.) Because the globes come with a hole already cut out, and gaps occur when you cut out the outer shell pieces, three globes are needed.
To stabilize the droid, I purchased the only “old school” gyroscope I could find, but it didn’t have enough rotating mass to stabilize such a large ball. To add more rotating mass, I built four additional gyroscopes out of discarded computer hard drives, stacking four discs in each drive and spinning them with a control circuit, powered by a battery.
I also tried to replicate the different emblems on the sides of the body. And now that we have better pictures of BB-8, I plan to rework the outer shell to make it even more movie-accurate.
- The gyroscopes I used are practically useless, because the weight of the drive system grew too heavy to be steadied by such a small amount of rotating mass. Electronic gyroscopes and tilt sensors are the way to go, or build a larger old-school gyroscope.
- The only way to get the drive system to propel the lower ball and be able to drag the dome along with it is for the drive system to have some weight. Otherwise, you will not have enough traction to move the droid. My BB-8 weighs 36 lbs. The dome is less than one pound.
- My drive wheels are too close together to turn the drive system within the lower ball, as designed. They should be spaced farther apart.
- Using Power Wheels motors and gearboxes makes for one noisy droid!
Posted by Kurt Zimmerman on Monday, September 28, 2015
Stay tuned for more of this project build here on Make:.