Instructables
Picture of Motorized Uniwheel Toy Thing (Mutt)
Mut 1.JPG
Mut 4.JPG
Mut 5.JPG
Mut 6.JPG
Mut 2.JPG
Mutt2.JPG
Mutt2 2.JPG
Warning: cd rotors are dangerous, one of mine exploded on another project as it was cracked. Wear safety glasses.
After watching the animated movie Robots for the nth time I thought it would be cool to have one of those speedy unicycle robots and after much ruminating came up with the following project.

For a step-by-step instructable on how to build this toy, go here: http://www.instructables.com/id/Build-a-Motorized-Uniwheel-Toy-Thing-Mutt/ 
Build : It is essentially a motorized wheel with two out-riggers with wheels and which support a battery and motor. Since I do not have the expertise, money or motivation for a complex project, I decided to keep it very simple : a wheel, motor, battery and axle. I suppose technically this is just a tricycle but once under power it should theoretically balance only on the primary wheel. As it stands it has no directional control except to overweight one of the outriggers to make it move in that direction or balance the two outriggers equally to make it go straight.
Inexpensive : made this for less than five dollars in stuff.  

Important Note: Please realize that when the Mutt is not moving and is at rest, it is only balanced on the primary wheel and one of the outrigger wheels. It can't really "fall over" because the center of gravity is so near the ground that it just naturally sits on two wheels. The motor is hanging really low but not touching the ground. The Mutt in effect holds itself up. It is kind of counter intuitive.

Photo 1 : front side view. Basically this is just a dvd used as the primary wheel and a coathanger used as an axle. For a bearing I inserted a plastic wheel into the center hole of the DVD. Metal strips epoxied to the coat hanger axle serve as the battery holder and motor mount. The metal strips also serve to keep the primary wheel centered on the axle - a primitive sort of bearing retainer. The nut hanging off the axle is just a simple counterweight to keep Mutt going straight; the battery side being a little heavier.
Photo 2 : top view.
Photo 3 : side view motor.
Photo 4 : side view motor. The motor is equivalent to one of the small motors in a dvd player. I got mine from a solar hat fan toy. I just fabricated a motor mount which is much more complex and ugly than it needs to be. It is actually just kind of stuck in there with one bolt to hold it tight. The motor pulley and drive belt also came from the dvd player. I used a slightly larger washer epoxied to the dvd plastic bearing to create the larger axle pulley.  There is also a simple on and off slide switch wired in there for the motor. Note that the motor never touches the ground. The Mutt's weight is resting on the dvd and one of the outrigger wheels. You can actually run it with one of the outrigger wheels missing.
Photo 5 : close up of bearing. Lots of slop between the coat hanger axle and the plastic bearing. A real bearing would be sweet.
Photo 6 : front view. Just route the wires up over the top and out of the way.

Results : this was just a kind of proof of concept to see how well it would work. I was surprised how much power the little motor generates yet it is underpowered and needs a more torquey motor . The only way to stop it is catch it or let it crash into something. It tends to wobble a lot because it is unbalanced laterally. There is a lot of friction between the outrigger wheels and the coat hanger. Stripping the insulation off the axle or adding wheel bearings would make this thing cruise really easy.
Though it is entertaining, it would be nice to control the speed and direction.

Todo : I would like to build a larger Mutt with a more powerful motor and use a two channel radio to control the motor and some sort of steering mechanism. Since leaning is the only reasonable way to get the main wheel to turn, I have thought about using a motor or servo to extend/retract a counterweight on one side to make the Mutt turn. 

Latest: I have begun building a Mutt 2 and am taking photos as I build it so that I can show a step by step process. I will be using a much larger wheel than a dvd and a more poweful motor along with a two channnel radio that I hacked from a cheap radio controlled helicopter. 

Mutt 2: Photo 7 is current status of my Mutt 2 construction. The wheel is a hideous pink exercise wheel and the outriggers are plywood with bolts for axles. My next task is to mount the motor, then battery, and radio receiver. Will be all downhill at that point. 

Mutt 2 Update 1: I have built Mutt 2 using the exercise wheel. It uses a single channel radio to control the motor speed so there is still no directional input. Once I have a working chassis that won't fall over I will use it to build a Mutt 2 in which I can achieve directional input through two servos, one on each side, that will drop a wheel to the ground and steer the Mutt in the opposite direction by tilting the Mutt in that direction. Pretty simple but effective I think. I just need to find a 3 channel airplane kit to hack for motor control and two servos.

Mutt 2 Update 2: Have received 4 channel radio, mounted servos for steering and just need to attach wheels to the servos, mount the radio and battery packs and test drive the Mutt 2. School is out so now I have time to finish this little project.

Photo 8: This is the completed but not tested Mutt 2 with 1 channel radio to control speed. It has 7.2 battery pack, radio, motor, axles with counterweight washers and bearing wheels. It has pretty good stability but still no directional ability. 

Update: Here is link: http://gscustoms.wordpress.com/2009/09/21/uniwheel-an-introduction/ to an interesting uniwheel robot project. This is a true self-balancing uniwheel. Not sure of the final status of this but it looks awesome.
triggur3 years ago
I'm pretty sure uni means two less than three.

Still, it's a neat toy that reminds me of the pull-stick derby cars toys they used to have that were designed to scatter apart when they hit each other.

:)
JimRD (author)  triggur3 years ago
I guess what I mean by uni is that it has one powered wheel and when it gets going, and if it is really balanced, the two training wheels should swing back and up and off the ground. In principle anyway. I remember having a windup toy that would break apart whenever it hit something.
a.baz3 years ago
thanks
Please post a step by step on this
JimRD (author)  a.baz3 years ago
I have created a step-by-step instructable to build a Mutt.
http://www.instructables.com/id/Build-a-Motorized-Uniwheel-Toy-Thing-Mutt/
JimRD (author)  a.baz3 years ago
I have decided to create a step by step for this one as my Mutt 2/3 is on hold, burned up my receiver.
a.baz JimRD3 years ago
Thank you Because you answered my request
JimRD (author)  a.baz3 years ago
I plan on posting a step by step of the Mutt 3 design which will incorporate a steering mechanism. I think from the photos of Mutt you should be able to make one of your own as it is simply a bent coathanger for the axle, make some sort of wheel bearing to fit inside the dvd center hole, hang some sort of homemade motor bracket off one side, 2 wheels and a battery. Thats it, your design will probably work better than mine. I used epoxy to glue the center wheel bearing to the dvd and also to hold the motor bracket onto the coathanger.
Keeping the weight as low as possible gives it stability. Also you can get the motor and pully from an old computer cd/dvd player.
thats pretty neat :)
JimRD (author)  x8ATT0SA1x23x3 years ago
Thanks. I am working on Mutt 2 and have it working (sort of). It is bigger, heavier and right now a little unbalanced - it falls over a lot. It still has no directional control but using Mutt 2 I can now build a directional version (Mutt 3) once I get a new radio transmitter and receiver. I will make a video and post an instructional on Mutt 2 in a week or two, once I get its balance worked out. It ain't pretty, but its fun.
jacketcat3 years ago
brief and awesome ! it's a pity that cant reach the video
ElvenChild3 years ago
in the video at 16 seconds what does that sign say? Thanks in advance!
those who risk the most reap the sweetest reward
JimRD (author)  birdman4723 years ago
Correct.
nmvb3 years ago
something goes flying at second 22
JimRD (author)  nmvb3 years ago
The counterweight, which is just a nut hanging off the axle. To make the Mutt go straight I just hang weights on one of the axles. Not a very aesthetically engineered solution.
Bosun Rick3 years ago
"" Todo : I would like to build a larger Mutt with a more powerful motor and use a two channel radio to control the motor and some sort of steering mechanism. Since leaning is the only reasonable way to get the main wheel to turn, I have thought about using a motor or servo to extend/retract a counterweight on one side to make the Mutt turn. ""
If you are going to use a 2 channel R/C radio, just reposition the outrigger wheels into a Tricycle arrangement, this way it will stand on its own, and steering can be done with a servo moving the front (steering) wheel Left & Right. You will have to tinker with the arm swing of the servo so straight ahead is the center of the swing. The motive power can be sent either to the front or rear wheels as desired (I'd suggest rear wheel drive for ease of construction)

SHIFT!3 years ago
Awesome! Please post a step by step on this!
JimRD (author)  SHIFT!3 years ago
Thank you. Yes, a step by step would be great but knowing myself, it probably won't get done. I really hope someone out there, who likes to document stuff, builds a much better one. I have this vision of remote control versions racing around a running track. I recently saw a webarticle on Gyrover from the University of Hong Kong, which is a robot monowheel - very sophisticated. Check it out here: http://arl.acae.cuhk.edu.hk/node/341