Why?

Because it is the 'hello world!' of modern robotics.

Videos

Testing with training wheels
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Just standin' 'round
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Components

	Arduino Nano	EBay	Lightweight, 5V tolerant, easy to program, cheap as silicon chips, and no tears when clumsy roboticists fry them
	Bluetooth receiver	EBay	If you want to control it remotely from the PC or a phone
	BNO055	EBay	IMU which does the orientation calculation on-chip.
	Stepper Motors	EBay	An odd choice for this kind of robot
	Stepper Motor Controllers	AliExpress	The ZD-M42s work well with an Arduino, but will not work well with a 3V3 source like a Raspberry Pi.
	Robot Wheels	EBay	These wheels are designed for a RC model car. There are many types of fittings. They have to fit onto the shaft of the steppers - an adaptor might be needed.
	Aluminium Bar	Bunnings	Easy to bend, drill and fashion - for the frame.
	Mini Caster.	Bunnings	These are just for the training wheels. Actually you don't even need the wheels on the ends of the ruler - just a ruler to stop the robot falling over. Maybe some soft buffers or something.
	7.4 Li-ion battery	EBay or local hobby shop	Should try out an 11.1V one of these.

Why Steppers

Because they can be controlled easily, and don't have unpredictable torque issues.
Because it's winter, and indoor robots can be built next to the fireplace.

Circuit diagram

The TX from the HC-06 is 3.3V, but it's high enough to trigger Arduino RX without a level converter. It ain't MILSPEC but it works.
7.4V (from two Li-Pos) is a good voltage to run the Nano from. A 6V SLA was tried, and it sagged when the motors started, crashing the Arduino. SLAs are fine for wheelchair robots, but lithium has so many advantages.

Software libraries

BNO055 Arduino library from Adafruit

Software

There really isn't much software to make this go. This is a pretty simple program.

Unfortunately, it's kind of tricky to do the timing on an Arduino to do this well - reliably pulsing a stepper motor controller faster than 1ms is a bit messy on the Arduino when you want to do other things along the way (like interrogate the IMU via I2C). DC motors would seem to be a better match for this, but the torque seems to be unpredictable.

The future

DC Motor Version.
Write a interrupt/timing Arduino program to control the steppers, and get around the 1ms issues.

Cool links

Get Allegrobotics updates on twitter
A whole bunch of people have done similar things.

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Something I'm doing wrong? Solved my problems? Got a better idea? Got a similar problem?
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