Domo Kun WobblyBot – Simple Self Balancing Two-Wheel Robot

A self balancing two-wheel robot that wobbles, hence the name WobblyBot.

Quite possibly the simplest design for a robot that could (sort of) balance itself  on two wheels, without the use of accelerometer, gyroscope or microcontroller.

A great weekend project, and do check out the video to see it in action. And why Domo Kun? Because my other half think it's cute :) You can pretty much change the WobblyBot into any character you like.

The WobblyBot - as seen on Hacked Gadget, Engadget and Hack-A-Day

How It Works


It's about simplifying things and getting back to basics. the goal is to build a balanced robot instead of a robot that balance itself.

The robot is essentially a simple pendulum, with the pivot at the wheel axle. The bottom part of the robot’s body is significantly heavier than the upper part of it. This serves as a counter weight, keeping the entire body upright, hence the balancing act.

The design of the cradle.

The design of the cradle and the upper body frame. You can download my SketchUp file for reference. The build and the diagram is 1:1 scale.



Putting Things Together



The cradle is made from MDF plate riveted to aluminum L bars. No special reason for such selection of materials, they are just stuff I have aplenty. The cradle can be made a whole lot simpler with other materials such as Acrylics.

The length of the cradle is really up to your choosing, but the width is design to fit a D sized battery holder with ease. The exact placement of the batteries are critical since it doubles up as the counterweight for the robot.

The batteries must be placed dead center, length-wise and width-wise. If not the robot will either not able to balance straight up, or it will not be able to move straight forward and backward (because one wheel is carrying more load than the other).

DC Geared Motors used are rated at 12V, 100mA, 130RPM and 58.8mN.m torque. The motor are driven at half the power (around 5.5V and 50mA). I’ll explain why later.

DC Geared Motors are used instead of the a normal DC Motors. They are better suited for this project due to the fact that it produces low RPM, and have enough torque to drive the robot. You will later find out that the robot could end up becoming quite heavy.


The motors are mounted directly to the cradle, note that there are room underneath the motor, this is the space where we will add more counter weight if need be.



The completed cradle. The circuit that drives the motors is mounted on top of the batteries. I initially planned to make my own circuits before laziness strikes and end up hacking an RC Car for the circuit :)



The wheel diameter is 11cm to clear the cradle width. Tried looking looking around for a pair of wheels that fit, unfortunately none were found.

The wheels are cut from 1cm thick MDF. For grip, rings are cut from motorcycle’s tire inner tube and stretched over the circumference of the wheels.



The completed lower part of the robot. The wheels are mounted directly to the motor shaft. At this point you can give a test drive and see if you balance everything just right.


Test Drive

Depending on the rating of the DC Geared Motor used and the distribution of weight, there are three possible outcome of the test:

• The robot stalls because the motors do not produce enough torque to drive it.
• Or motor’s torque is too high that cradle starts making full spins, particularly when the robot starts to move from a stationary position.
• Or the robot moves but there are minor swings of the cradle particularly when the robot starts to move from a stationary position, or when it changes direction.

If the robot stalls, than reduce the weight by changing the batteries to size C or even AA, but i would personally suggest to get a motor with larger torque.

If the cradle makes full spins, either reduce the power to the motors by adding limiting resistors, and/or add more counter weight to the cradle (more tips below).  I used both approaches, resistors to limit the power to 5.5V 50mA, and the adding of more counter weight.

The minor swings of the cradle everytime the robot starts to move or change direction is perfectly normal (it’s a WobblyBot after all). The force exerted by the motor’s torque is opposed by the weight of the cradle and both forces will eventually balance out after a few wheel turns.


The Upper Body Frame


The upper body frame is constructed from balsa wood and it is really up to your creativity. The goal is to keep the upper side of the body as lightweight as possible, hence the use of balsa. Glue is used liberally for the joints and staple guns provide the reinforcements.

The frame is held in place on the cradle with brackets on four ends, and top four edges of the frame are slightly tapered to make it easy to slot in the robot’s facade later.



Tuning The WobblyBot Further



The robot should be able to stand straight up when stationary. If it’s leaning either way, simply add/shift more weight to the opposing side. Balance is key. If not the robot will either not able to balance straight up, or it will not be able to move straight forward and backward (because one wheel is carrying more load than the other).

You can also tune the amount of wobble this way, the more counter weight added to the cradle, the less wobbly the robot gets. Keeping in mind that weight must be kept evenly distributed.

Also take note that some more weight will be added to the upper body frame of the robot when you add the facade, so compensate the counter weight accordingly.

It may take several attempts before you hit the right balance between just the right amount of wobble and the right amount of counter weight.

All this tuning and the adding of counter weight will make the robot quite heavy. Be sure to not overdo it and end up straining the motor.

The easiest way to add weight and balance them out is by using these stick-on car wheel balancing lead. Simply stick them onto the cradle.





Making The Facade

This is the part where you can get really creative. The facade is nothing more than a box made from thin cardboards.

Come to think of it, a bunch WobblyBots looking like Pac-Man and Ghosts (Blinky, Pinky, Inky and Clyde) would make a really cool swarm.

The dimensions

Constructing the cardboard facade

The cardboard facade is created in a manner that it can be slotted snugly over the balsa upper body frame. There it sits without any gluing or mounting.

This is for ease of access to the component inside, i.e when you want to switch ON/OFF the robot or when you need to change the batteries.

This work by Chein is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported License.

Iron Man Inspired Repulsor Beam Blaster V1.0

Homebrew repulsor beam blaster  that’s designed to produce extremely intense burst of light that can be used to repulse your archnemesis, girlfriend /wife and pet cat.

UPDATE: This hack's featured on  Hack A Day!!, and Gizmodo and Engadget and Slashgear and TechCrunch. Many thanks to the folks there, and to everyone that took the trouble to visit.

WARNING: The build involves high voltage and intense light so please take the necessary precautions. And please be warned that , walking around with one of this strapped to your hand may seriously jeopardize your chances with the opposite sex. 

 

Assembly 


 

The repulsor is essentially two circuits that are switched ON/OFF using a DPST switch.

The source of the DC Step-up Charging Module is the good old analog cameras. When extracting the module, mark the wires so you don’t mix them up. If you have one lying around, then don’t waste time reinventing the wheel. If not, then google is your friend.

The function of this circuit is to gradually step up the 3V source up until 330V, and releasing the stored charges very rapidly when triggered. High current and voltage are needed to energize the Xenon gas inside the tube bulb yielding intense but short burst of light.

Do not short the lead of a charged capacitor, it will pop in your face, literally. I learnt my lesson the hard way a while back. Play safe, always have a large resistor handy, discharge the capacitor via the resistor before you begin to fiddle with it.

The purpose of the discharge switch in the schematic is to short and safely discharge the capacitor when it’s no longer in use.

Assembling The Lamp 

The 8 LEDs are connected in parallel and soldered to the board forming a circle. The Xenon tube bulb with it’s reflector is attached to the center.

The lamp reflector is a bit tricky to source. The challenge is trying to find the right size that fits or at least can be modified. Finally achieved a perfect fit after extensive searching, followed by some cutting and sanding.

The diffuser is whole lot easier to make. It is cut to shape from semi-translucent polypropylene (resin identification code 5). The cylindrical casing is salvaged from a broken eye-ball lamp.

    

Metal Works And Mounting

The arm mounting frame is made from aluminum flat bars bended to size. The underside is layered with velcro straps. Everything riveted in place, protruding rivets are sanded.

The circuit and the batteries are mounted on flat plates riveted to the above mounting frame. 





Lamp And Switch Box Mount 



Where the switch box and the lamp are mounted. The switching box is made from a project box, with an attached limit switch. This is the designated “Fire” switch. The fire switch is align to the ring finger. When the finger is outstretched, it triggers the capacitor’s rapid release of charges resulting in a flash burst.
The rest of the switches are the:


1) Power – Push ON, push OFF, DPST switch. Turns on both the LED lamp and the DC step-up charging circuit.

2) Charge – Push ON, Release OFF switch. This triggers the charging of the capacitor. It will start to whine, gradually increasing in pitch as it charges.

3) Discharge – Push ON, push OFF switch. The switch shorts the circuit between the capacitor leads.


When the discharge switch is turned on, the orange LED lights up indicating that there are charges stored in the storage capacitor. It will gradually dim and die off as the capacitor drains.

  


Finishing Touches

The photos are pretty self explanatory. Aluminum strips, zip ties, cable organizers, spacers, hot glue and some bolt and nuts for the finishing touches. The small white brick is the 10 watt discharge resistor mounted on top of the battery holder. 

 




Fire It Up

1. Turn the power switch ON. The LED lamp lights up.
2. Press the charge switch and capacitor will start to charge with a whining sound. Once fully charged the charge indicator will start to blink.
3. Outstretch the fingers to fire.
4. Press the charge switch to recharge. Fire and repeat.
5. When done, turn ON the discharge switch to drain the capacitor.