Imagine trying to make a ball-shaped robot that rolls in any direction but with a head that stays on. When I saw the BB-8 droid doing just that in the first Star Wars: The Force Awakens trailer, it was an interesting engineering challenge that I couldn’t resist. All the details for how I made it would fill a book, so here are the highlights: the problems I ran into, how I solved them and what I learned.
The Design Criteria: Portable and Inexpensive
I had two design criteria in mind. The first was to keep it low-cost. Some spend $1000 to $1500 on their BB-8s. I wanted to spend as little as I could, so as many parts as possible had to come from my existing stock and from online classifieds and thrift stores. Not counting the parts that were discarded along the way, the cost came in just shy of $300.
The second design criteria was to make it portable. It had to be something I could take on the bus or carry while walking a reasonable distance (I once carried it twenty-five minutes to a nearby school).
Both of these criteria meant that it had to be smaller than full size. A full size ball is 20″ in diameter. Mine has a 12″ ball which makes it 3/5 scale. Also, the larger it is, the more powerful and costly the motors, batteries, motor controllers, magnets, and so on.
Version One: Quick And Easy
First I tried a minimalist approach. For the ball, I found my 12″ cardboard globe on kijiji.ca. I bought an RC toy truck at a yard sale and attached a pole to it for holding magnets near the top of the globe. I then made a head with corresponding magnets under it. The head magnets attract to the pole magnets, keeping the head on. Meanwhile, the truck rolling inside the ball makes the ball move.
Making the ball roll around was easy. Making it roll around while keeping the head on was very hard. The magnets at the top of the pole attract the magnets under the head, pulling them down hard onto the surface of the globe. That essentially glues the truck to the top of the globe.
To overcome that the truck needs sufficient traction. That also means the truck needs to be heavy. And lastly, the truck’s motor needs to be powerful enough to overcome its own weight and the grip of the magnets on the ball. The alternative is to make the magnetic attraction weaker, but if it’s too weak the head falls off. It’s a tricky balancing act, in both senses of the word.
But the most dome-like head I could make stay on was just a cardboard skeleton. Anything more filled out would be heavier and require a stronger magnetic attraction. The toy truck’s motor would not be up to it.
Version Two: Drill Motors And Drill Batteries
For more powerful motors and more mass I figured I could kill two birds with one stone by going with drill motors and drill batteries in a hamster drive configuration. Using drill parts kept costs down as the batteries and one drill came from yard sales while the other drill was free through freecycle.org. Meanwhile, both are heavy.
To make sure it all fit, I drew up a 3D model in Blender, the free 3D modelling and animation software that I use a lot. In fact, finding out how to make the batteries and motors fit was the first step. They had to be as low as possible. Their large mass low down is what keeps the droid vertical, with the much lighter head at the highest point.
The drill batteries had to be easily removable for recharging. To hold them under the drive plate I simply used Velcro. Meanwhile, the drill battery stems went up through a hole in the drive plate. I made a connector to electrically connect to the battery terminals. It is a plastic rectangle with thin copper sheet metal for the contacts. Once the battery was Velcroed in place, this plastic and metal piece was lowered down onto the stem, the copper metal making contact with the battery terminals.
For the brains I used an Arduino UNO. To drive the motors I had all the parts for making two H-bridge driver boards, with the exception of 4 MOSFETs and some fuses. The Arduino does pulse width modulation (PWM) to the driver boards for speed control, as well as playing sounds at certain times when the motors are turned on.
For remote control, I hacked the RC receiver from the toy truck and added an extra set of AA batteries in parallel for more runtime. A problem I ran into right away though, was that the RC receiver put out voltages of both polarities based on which direction the motors should rotate, whereas the Arduino’s pins take only positive voltages. To solve that I came up with a converter board to go between them.
Getting all that to work reliably took a while. Before I added fuses, I burned a few MOSFETs. At one point I’d put an N-type MOSFET where a P-type should have gone and vice versa. That resulting problem alone took a few days of spare time to figure out.
The wheels were old Rollerblade wheels — I keep a small bucket of these in my shop. I decided I wanted the ball to roll at around 1 foot per second and doing the math, that meant the wheels would have to rotate at around 2 rotations per second or 120 RPM. I found a PWM value that would give something close to that and started blowing fuses. I started with 1 amp fuses, then 2, 5, and finally settled on 10 amp fuses.
My final hurdle was that the motors would behave oddly when the motors were told to turn in opposite directions but were fine when they were told to turn in the same direction. This turned out to be a bad assumption on my part about how the RC receiver was wired internally — none of the output wires were common inside. After some changes to the circuit, I now had stable electronics.
I had basically been treating the RC receiver as a black box, but when I asked for help about my converter board here on Hackaday, it was pointed out that the receiver likely contained H bridges. Opening it up, that’s exactly what I found. The converter board works fine for now, but in the near further I’ll use one of the suggestions from that Hackaday post to eliminate the board altogether. I might even try all of the suggestions, just for fun.
The Drive System
The motors were too long to fit in line between the wheels and so had to be mounted off to the sides. To transfer rotation to the wheels, I drew up some gears in Blender and 3D printed them at our local University of Ottawa Makerspace. In the print settings I used 2 shells and only 50% infill. The gears are held firmly onto the shafts solely using nuts and washers on either side. They’ve held up amazingly well, even with slipping and grinding during development.
For the bearings for the center gears and the wheels I used an old trick of making bearing blocks from hardwood.
I wanted to keep as much room as possible on the drive plate available for adding things later and so initially I’d mounted the motors at only three points. But this allowed the motors to move a little causing the gears to slip. To fix that I later added a fourth mounting point and haven’t had any slipping since.
At the end of the shaft for the drill motors is a hole that a screw goes into. That’s part of how the chuck is kept on a drill motor, and that’s how one gear was kept on. However, this screw had a tendency to get loose. Putting a little Loctite on the threads fixed that.
Given that I was trying to fit a lot in a small droid, I had to mount some things higher than I’d have liked to. When the droid stops, mass high up causes the droid to wobble. In the BB-8 droid used for promotional events they’ve gone to a great extent to keep the majority of the mass as low as possible. Originally I had the Arduino batteries and the RC receiver with its extra batteries fairly high up. I later mounted them much lower. When holding the internals in my hands I could tell the difference but it didn’t make a noticeable difference with the wobble.
Instead, for that I added Adafruit’s BNO055 inertial measurement unit (IMU) board. With it I could tell what angle the droid was at when stopping and experimented with PID loops and other algorithms of my own to minimize wobble. That helped.
As I said, I used a 12″ cardboard globe. To increase the traction of the wheels inside the globe I sprayed the globe’s interior with an anti-slip spray from a hardware store. This made a huge difference. However, over time the anti-slip coating vanished, and so I’m looking for another, more permanent coating, perhaps urethane or something. If anyone has any suggestions, please let me know in the comments. It also has to stop off-gassing eventually. The anti-slip spray had an odor for a long time.
But the main problem with the cardboard globe was its thinness. With the huge weight of the internals pushing down on it where the wheels are, it warped significantly and required a lot of effort and a copious amount of tape to keep the two hemispheres together. This large amount of tape also made an uneven ridge for the head to slide over, making it get stuck. At that point either the drive system continued to move while the head fell off or the drive system couldn’t move at all.
The solution was to carefully coat a new globe in three layers of fiberglass. I took my time doing this, over a month and a half, applying one piece at a time and then sanding before putting the next piece. The result was a fantastic improvement. It no longer deformed and now it takes a minimal effort to attach the two hemispheres with only eight narrow strips of transparent duct tape.
The Never-ending Saga
My BB-8 is now at the point that I can call it finished. At least it’s finished as far as all the engineering is concerned, and that’s usually where I call it quits.
While the paint job worked out well, up close you can see that the details are painted on. It’d be nice to have at least the lines on the head be actual grooves. Also, these drill motors are brushed motors and I’ve since learned that doing high frequency PWM to brushed motors damages them over time. I’d like to replace them with brushless motors. And as anyone who’s use cordless drills a lot knows, drill batteries don’t last long, and so it’d be great to switch to LiPos.
But for now, the reactions I get from both kids and adults is beyond my wildest expectations. Kids threat it like a friend while adults have petted it and called to it like it was a baby or a dog wagging its tail. I’d call that a success.
18 thoughts on “My DIY BB-8: Problems, Solutions, Lessons Learned”
Great build! I hope to build a BB-8 of my own some day.
For the traction issue try spray on undercoating and/or spary on truck bed liner. Both are essentially spray on rubber. I’d due a test spray on something first though.
I was also going to suggest truck bed liner. … undercoating, not so much, may remain tacky and oily. There’s a stone chip guard like stuff that’s more like bedliner, but that’s intended as a layer over and under something else.
I looked up undercoating and it looks affordable, though if it remains tacky and oily then that’s not good. Spray on bed liner seems out of BB-8’s budget range though.
Yah, problem is you might have to test multiple kinds of undercoating to get the right stuff, then not so affordable. Or get some stuff that seems okay at room temp, then turns to gloop when sphere interior hits 30C
Plastidip would work…
Would plastidip stick to the inside of a cardboard bowl? From reading their website it sounds like it shrinks and is meant to shrink around an object, like the handles of a pair of pliers. I’ve never worked with it so I could be wrong.
You may also want to try liquid latex. This is often used on non-slip socks. Hardest part will be to get a completely even layer. I would try pooring and letting the excess drip off.
“While adults have petted it and called to it like it was a baby or a dog wagging its tail.”
Thats just weird.
Well it’s whole point was to be robo-Jar-Jar
If that was the point, I’ll have to give props to Disney for failing spectacularly.
one of the guys at the Adelaide Hackerspace has a full size K9, I was tending the stall at the Royal Show(state fair) and the attention K9 got was amazing!
How about a nice spray of contact cement and a handful of sand?
Because the sand wears off and will get into everything inside.
The anti-slip spray seems to be a similar concoction. The coating includes small, hard particles, not unlike sand or salt in size. It vanishes over time.
Rather than looking at lining the inside of the sphere (although I was thinking something more like felt that could be glued) could you not just score the inside with a knife blade in a grid pattern to roughen it up? Also put some rubber (rubber band) around the wheels. I did this with some cheap plastic wheels and it made a world of difference!
Interesting ideas. I’m not sure I’d want to roughen up the inside of the sphere given the month it took me to put the three layers of fiberglass on it – though I can’t see that there’s much risk. Your rubber band idea would work, except that they won’t hold on where these rollerblade wheels contact the globe – unless they were very wide rubber bands that stretched partway down the sides. Too bad, that would certainly be big, quick improvement.
You can always cut a bicycle inner tube (find one the right diameter) into sections and use that in whatever width oyu need. We used to use these in place of rubber bands in the field as they last longer. Search for “ranger bands” if you just want to buy some already cut but it’s cheaper if you need a lot of them to just get the inner tube.
Please be kind and respectful to help make the comments section excellent. (Comment Policy)