Robot architecture

Zig has four brains: one computer that choreographs, and three microcontrollers that each own a body part and keep it safe on their own. This page is the map — every detail lives one click away.

Figure 1: the three body parts of Zig.

The Head is composed of the blue parts and the neck with it's 3 motors in black. The torso is in white with the battery on the back. The legs are composed of the grey parts and the wheels in red and black.

The four brains

Brain	Part	Role	Why a dedicated brain	Contract
Raspberry Pi 5	Main Computer	choreographer: command arbitration, head trajectories, CoG feedforward, gateway to controller & AI station	needs Linux (network, audio, vision) — therefore never trusted with real-time	System links
Pico 2	Legs & Balance	two-wheel balance, locomotion, leg height	hard real-time control loop; must survive alone if the Pi dies	bus-pi-pico
XIAO RP2350	Head	3-axis neck + 2 antennas, head sensors	local smoothing of streamed targets; freezes safely on link loss	bus-pi-xiao
Nano Every	Torso	power monitoring, fan, LED matrix, torso sensors	slow local automations, isolated from critical loops	bus-pi-nano

Topology

---
config:
  look: neo
  layout: dagre
---
flowchart TB
    ext["Controller · Station & AI PC"] -.-> pi5["Raspberry Pi 5<br><i>choreographer</i>"]
    pi5 <--> i2c{"I2C bus"}
    i2c <--> xiao["XIAO RP2350 — <br>head<i>local: 5 servos, head sensors and actuators</i>"] & nano["Nano Every — <br>torso<i>local: fan, LED matrix, sensors and power switch</i>"]
    n1["UART bus"] <-- <br> --> pico2["Pico 2 — balance<br><i>local: IMU, 2 encoders,<br>2 BLDC, 2 leg servos, 2 drivers</i>"]
    pi5 --> n1

    n1@{ shape: diam}
    style ext fill:#e6e6e6
    style pi5 fill:#FFD600
    style xiao fill:#FFCDD2
    style nano fill:#E1BEE7
    style pico2 fill:#FFE0B2
    linkStyle 4 stroke:#000000,fill:none

*Figure 2: the four brains and their buses.

Full message maps live in the contract pages; local wiring lives in each part's Electronics page.*

Design principles

The Pi choreographs but never balances: everything time-critical lives in a microcontroller. Every brain is safe alone — each link has a defined loss-of-link behavior, consolidated in Safety & Degraded Modes. Each piece of information has exactly one home: buses are documented once, in their contract page, and everything else links there. Territories follow wiring, not mounting position: a sensor wired to the Pico belongs to Legs even if it is bolted inside the torso.

Going further

Lifecycle & Modes · Motion & Balance · Electronics · Firmware