UPDATE: MABEL is now able to drive around wirelessly using a Wiimote as a controller. (I'll be publishing the code later on in the main project details for anyone building along.) The Pi can now communicate to the Arduino over a USB serial connection, although I had to solder the wires directly onto the pi and Arduino solder joints, as MABELs slim frame doesn't allow a USB cable to fit. In the future I'd like to move away from the WiiMote in exchange for a newer controller like the PS4 one. Using the joysticks I will be able to have velocity control over the robot for more precise movement and control. The next step for MABEL is to get the servos to move up and down whilst balancing, by using the handy PCA9865 Servo controller and the Adafruit ServoKit python library. MABEL is still a bit wobbly, but I've found that reducing the Kd PID constant helps to reduce oscillations when balancing. If you have ant questions about the build and want to build your own, feel free to contact me on raspibotics@gmail.com, I'm happy to help!