Let's Make Robots!

CoAxial Wheeled Balancing Robot

Steerable radio controlled robot whcih balances autonomously on two co-axial wheels.



My other holy grail. The co-axial wheeled balancing robot. I've tried this several times. I didn't alwasy keep a photographic record, but here's one of my earlier ones. (And probably the one which came closest to working.)

Here is it stripped down. The two MFA motors are brilliant with their built in gearboxes, but they ain't cheap. Thay also work from an irritating 18V, so I controlled them by 3/4 PWM from a pair of 12V home alarm system batteries.

The two M8 threaded rods show how high up the centre of balance the batteries were located.

The chassis is made from bent sheet aluminium.


In the second photo, it has the PCBs and wheels added. There are two 2-layer PCBs here. One is teh MCU (the brains) and the other is the motor controller (298D based). As always, my PCBs are home-made. (Yes, fritzl, walkthrough to follow.)

At the bottom left of the PCBs ( at the top right of the leftmost tyre) you can see the accelerometer and gyroscope.

The accelerometer is one I speak about in http://letsmakerobots.com/node/100 and theComponentsentry for the R/C helicopter gyro will follow.

Since it never really balanced on its own (I think there was too much backlash in the drive train) I'm not sure if "completed" is the word I'd use, but I'm done with this version.

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i love ballanced bots, it have really a nice look, the references of the acelerometer and the gyroscope? what micrcocontroler do you use and if you are using kalman filter or something else!?

i wold really love to see the code =P

I'm thinking in make one, but code for something like that is not simple!!

Two mentions of Kalman. I have to put my hands up and say "Who?"

Re the code. Well, Mechanical engineering is not my thing. CODE is. The code is big but not complicated. Perhaps the hardest part is doing everything in integer maths. (I coded the whole lot in 8-bit RISC assembler.)

"Give me a platform on which to balance and a long enough chunk of EEPROM and I shall write code to move the Earth." - BOA, 2008  

God created the integers; all else is the work of man - Leopold Kronecker

lolz, nice that you are a good programer.. you should take a look at kalman filter i wold love to understand it enuf to implement on a balancing robot.

anyway.. gratz

Very nice :) I'm interested in details too.. Did you implemented a Kalman filter ?


OKay, NOW I've looked Kalman filter up on that internet thing. I have to say I don't think that what I implemented was a Kalman filter. In fact, I made the assumption that my signals were complete and non-noisy! Maybe that's where I went wrong. I guess that's what all the smart people are for.


OKay. I knew that the accelerometer alone wouldn't be enough: how would it know which way was "down" if it was leaning forward or if it was accelerating? I thought that a gyroscope MIGHT be enough, but when I tried it I discovered "gyroscope drift". So, I used both and discovered that the acceleromter was an acurate but slow way of finding which way is down and a gyro is a fast, but slightly inaccurate way.


The BOA (BaseOverApex) filter integrates the gyro output over time to give an angle. The acceleromter "down" is used to calibrate this on a ragular basis. Maybe it will work. Maybe it won't. Once I find a suitably powerful, suitably cheap drive train I'll find out.


 ...and tell the rest of you. 

God created the integers; all else is the work of man - Leopold Kronecker

We want the video :)

Yeah, mea too. Sorry the video is so crap. The original is a quicktime movie and I can't find any softare to modify it. I would make it brighter. The 'bot is actually balancing here. I'm holding a ribbon cable so I can watch the LCD while it's wiggling about all over the place.

God created the integers; all else is the work of man - Leopold Kronecker


I've been thinking I'd like to try a ball bot .   You said the motors were "brilliant with their built in gearboxes", but was the gearing causing the "backlash in the drive train"?  Looking forward to updates and other articles as always.


Nope. The gearboxes attached to the motors were perfect. I added an extra 1:4 reduction which was crap.

God created the integers; all else is the work of man - Leopold Kronecker

awsome . i tried to make one like that with lego. its way hard good work.