My DIY BB-8: Problems, Solutions, Lessons Learned

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.

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Ask Hackaday: How Do You Convert Negative Voltages to Positive?

I have a good background working with high voltage, which for me means over 10,000 volts, but I have many gaps when it comes to the lower voltage realm in which RC control boards and H-bridges live. When working on my first real robot, a BB-8 droid, I stumbled when designing a board to convert varying polarities from an RC receiver board into positive voltages only for an Arduino.

Today’s question is, how do you convert a negative voltage into a positive one?

In the end I came up with something that works, but I’m sure there’s a more elegant solution, and perhaps an obvious one to those more skilled in this low voltage realm. What follows is my journey to come up with this board. What I have works, but it still nibbles at my brain and I’d love to see the Hackaday community’s skill and experience applied to this simple yet perplexing design challenge.

The Problem

RC toy truck and circuit with no common
RC toy truck and circuit with no common

I have an RC receiver that I’ve taken from a toy truck. When it was in the truck, it controlled two DC motors: one for driving backwards and forwards, and the other for steering left and right. That means the motors are told to rotate either clockwise or counterclockwise as needed. To make a DC motor rotate in one direction you connect the two wires one way, and to make it rotate in the other direction you reverse the two wires, or you reverse the polarity. None of the output wires are common inside the RC receiver, something I discovered the hard way as you’ll see below.

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Ottawa Maker Faire: Droids And Pick And Place Machines

Three things that I love about participating in Maker Faires are seeing all the awesome stuff people have done over the past year, spending time with all my maker friends in one big room over two days and the reactions to what I made. The 2016 Ottawa Maker Faire had all this in spades.

BB-8 – Droid With Magnetic Personality

There’s just something about BB-8 that touches people. I once heard of a study that showed that when buying kid’s toys, adults were attracted to circles, that that’s the reason teddy bears often have round heads with big round eyes. Similar reactions seem to happen with BB-8, the droid from last year’s Star Wars movie. Adults and kids alike pet him, talk baby-talk to him, and call to him with delight in their voice. I got those reactions all throughout the Maker Faire.

But my favorite reaction happened every time I removed the head and lifted the top hemisphere of the ball to expose the electronics inside. Without fail the reaction of adults was one of surprise. I don’t know if it was because of the complexity of the mechanism that was revealed or because it was just more than they expected. To those whom I thought would understand, I gave the same speech:

“This is the remote control receiver taken from a toy truck, which puts out negative and positive voltages for the different directions. That goes to this ugly hack of a board I came up with that converts it all to positive voltages for the Arduino. The Arduino then does pulse width modulation to these H-bridge driver boards, for speed control, which then talk to these two drill motors.”

Bowie and BB-8
Bowie and BB-8

Those I wasn’t sure would understand were given a simpler overview. Mine’s a hamster drive (we previously covered all the possible ways to drive a BB-8) and so I showed how it sits on two Rollerblade wheels inside the ball. I then flipped it over to show the heavy drill batteries underneath, and then explained how the magnets at the top of the drive mechanism attracted the magnets under the head, which got another look of revelation. All went away satisfied.

But BB-8 sometimes needs a break from human interaction and seeks out its own kind, like Bowie which you can read about below along with more awesome Maker Faire exhibits.

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How BB-8 Works Revealed At Star Wars Celebration Europe

Finally the workings of the official BB-8 that you’ve seen rolling around at various events have been revealed. Its makers [Matt Denton] and [Josh Lee] participated in an hour-long presentation at Star Wars Celebration Europe 2016 just this past week where the various views of its internals were shown in action. It’s since had BB-8 builders (yours truly included) analyzing the workings for new ideas. We also now have the official name for it, red carpet BB-8.

For the first half of their talk they went over how BB-8 was implemented for Star Wars: The Force Awakens. As we’ve long known this was done using 7 puppeted BB-8’s, though it was revealed that only 4 were actually used, including a stationary one called the wiggler whose purpose you can guess. Another thing we didn’t know is that they did consider building a working BB-8 for filming but decided they needed something bullet proof, that would work right every time without making a film crew wait for repairs, and so went with the puppets instead.

The second half of their talk contained the big reveal, the mechanism inside red carpet BB-8’s ball. It turns out to be pretty close to what many builders have been doing. If you’ve seen the DIYer’s guide to the different BB-8 drive systems then you’ll understand when we say it’s a pendulum drive (aka axle drive). That is, there’s a motorized axle that crosses the middle of the ball and the ball rotates on that axle. Meanwhile a large mass suspended below the axle acts as the pendulum mass.

BB-8 builders have known the importance of keeping as much mass as possible as low down as possible for stability, but it was revealed the great extent to which that has been done in the red carpet version. Motors for the head’s pitch and yaw are located at the bottom and their motion is transferred up to the center using what are maybe best known as bicycle brake cables. Another big reveal was a linear actuator for the body roll, tilting the center stuff with respect to the mass lower down. The actuator itself is located in the lower section. Also, BB-8 builders have been mounting the drive motors for rotating the ball with respect to the axle, in line with the axle. However, in red carpet BB-8 the motor is also at the bottom and its motion appears to be transferred up to the axle via belt and worm gears. You may mistake the gold cylinders on either side of the central gimbal system to be motors but they’re actually Moflon slip rings.

Those are just a few of the insights gained so far from analyzing the video below. Doubtless people will be noticing a lot more in the weeks to come.

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LEGO Monowheel Corners Like It’s On Rails

[Jason]’s at it again. This time the LEGO maestro is working on a LEGO BB-8 droid. As a first step he’s made a motorized monowheel that not only races along hallways and through living rooms at the peril of any passing people, but turns as well.

To drive it forward there’s an axle that runs across the center of the wheel and a motor that rotates that axle. He’s also included some weight bricks. Without the mass of those bricks for the rotation to work against, the motor and axle would just spin in place while the friction of the floor keeps the wheel from rotating. If you’ve seen the DIYer’s guide to making BB-8 drive systems, you’ll know that this is classified as an axle drive system.

LEGO monowheel interior shown while leaning to turn
LEGO monowheel interior shown while leaning to turn

For steering the monowheel left or right he has another mass located just above the axle. Shifting the mass to the left causes the monowheel to lean and move in that direction. Shifting the mass to the right makes the wheel move to the right in the same fashion. Being ever efficient, [Jason] has the motor that shifts the mass doubling as the mass itself.

As with any proof-of-concept, there are still some issues to work out. When turning the wheel left or right it can tip onto its side. Ridges on both sides of the wheel’s circumference reduce the chances of that happening but don’t eliminate it altogether. Also, the steering mass/motor doesn’t yet have a self-centering mechanism; after a turn it’s up to the person holding the remote control to find center. If the mass isn’t correctly centered after a turn, there tends to be some wobble.

As always, we’re looking forward to seeing how [Jason] solves those issues but first he’ll have to put it back together since, as you can see from the video below, it didn’t quite pass the stair test.

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Driving BB-8: More Than One Way to Move this Bot

BB-8 is the much loved new droid introduced in the 2016 movie Star Wars: The Force Awakens, though in my case from the very first trailer released in 2014 I liked it for the interesting engineering problems it posed. How would you make a robot that’s a ball that rolls along, but with a head that stays on top while the ball rolls under it?

To make the ball roll, the answer most people found obvious at first was to use the analogy of a hamster wheel. The hamster running inside makes the wheel turn. In the BB-8 building world, which is quite large, the drive mechanism has come to be called a hamster drive, or just a hamster.

Magnets holding the head on
Magnets holding the head on

For the head, it seemed obvious that there would be magnets inside the ball, perhaps held in place near the top of the ball by a post extending up from the hamster. Corresponding magnets in attraction would then be attached to the underside of the head, and balls (also mounted under the head) would keep the head moving smoothly over the ball.

The magnet approach for the head has turned out to be the method used by all BB-8 builders that I’ve seen. However, the hamster has turned out to be only one of multiple solutions. Since the original debut many different methods have been used in builds and we’re going to have a lot of fun looking at each separate approach. It’s almost like revealing a magic trick; but really it’s all just clever engineering.

Note that for the actual movie, a combination of 7 or 8 props and CGI were used. The official working BB-8s that are shown at various promotional events were built after the movie was made and as of this writing, few details of their construction have been released. One notable detail, however, is that they aren’t using hamster drives.

Below are details of all the different BB-8 drive systems I’ve seen so far that have been built along with how they work.

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Perfecting The DIY BB-8

Until about a year ago, the Droid Builder’s Club had just about everything figured out to build any sort of robot from Star Wars. Building an R2D2 clone was easy, and even R5 and R6 droids were common. There were even a few attempts to clone IG-88. Then Disney happened, The Force Awakens was released, and the world was introduced to the hero of the third trilogy, BB-8. Several people have gone to incredible lengths to replicate BB-8 as a unique homebrew robot, but no one has put in more effort than [James Bruton]. He’s wrapping up his third DIY version of BB-8.

[James]’ third version of the BB-8 droid has two older brothers we’ve seen before. [James] started the construction of his earliest BB-8 not long after the trailer for The Force Awakens, and long before we knew the makers of Sphero robot toys weren’t behind this hero puppet. Since then, a number of improvements have been made to the drive system, allowing the third version of [James]’ BB-8 to turn on a dime and roll just like its on-screen counterpart.

Right now, [James] is about 80% done with his newest droid, with just a bit extra circuitry to have all the functionality seen on the ‘real’ stage droid. Like most of the R2D2 builds out there, there might be enough room inside this droid for some additional capabilities. There appears to be enough space behind one of the body panels for an extending arm, making the possibility of a flamethrower thumbs up very real.

[James] is also one of the judges for this year’s Hackaday Prize, and will (hopefully) be at this year’s Hackaday Prize award ceremony and Hackaday SuperConference in San Francisco. If a set of highly likely probabilities pans out, [James]’ BB-8 will also be at the con, and we’ll see it careening down that one weird block of Lombard Street. Awesome.

Entire playlist for the build of BB-8 v.3 below. Pictures are available here.

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