Step 7: And now, the hard part.
Usually, the controller is some kind of microprocessor. In some rare cases, one might be compelled for unknown reasons to do it entirely with analog circuitry. Segstick instead uses a more conventional digital controller implemented on everybody's favorite: the Arduino. Specifically, it uses the Arduino Nano 3.0 from Gravitech. But any Arduino will do.
For sensing, Segstick uses the Sparkfun Razor 6DOF IMU. Really, you only need one accelerometer axis and one gyro axis, as will be discussed in the next step, but this package is so convenient and allows you to mount the board any way you want. Since it uses all 3.3V sensors, it will need to be powered from the Arduino's 3.3V output. Each sensor outputs an analog voltage which is read in by the Arduino's analog to digital converter (ADC). More on that in the next step.
For controlling the motors, Segstick uses two Polulu High-Power 18v25 motor drivers. These are tiny but powerful reversible motor controllers capable of providing up to 25A each. I decided to add a heat sink to them, but they seem to run cool enough without it. They do require large-gauge wiring to the battery and to the drill motor leads.
The circuit schematic is shown below. How you implement it (breadboard, protoboard, printed circuit board) is up to you. I did it on a vector board that I later cut to size. Soldered connections will generally be more reliable than breadboard-type connections on a moving vehicle, so I'd recommend using sockets for the components but directly soldering wired connections.