System architecture

System decomposition

• Motion control:

  • Duet 3 6HC

  • Responsible for:

    • kinematics

    • motion planning

    • step generation

    • G-code execution

• Host system:

  • Raspberry Pi

  • Runs:

    • Duet services and web interface

    • plugins

    • logging and visualization (e.g. Grafana)

• Real-time supervision:

  • ECU (master PCB)

  • Responsible for:

    • sensor fusion

    • machine state estimation

    • feed and spindle supervision

• Distributed sensing:

  • Multiple sensor PCBs in zonal architecture

  • Local preprocessing and event-based reporting

• Safety:

  • Hardware-only safety chain

  • Independent of Duet, Pi, and ECU firmware

Communication topology

• Duet ↔ Raspberry Pi:

  • Native Duet architecture (services, UI, plugins)

• ECU ↔ Raspberry Pi:

  • SPI or UART

  • Configuration, data logging, visualization

• ECU ↔ Duet:

  • GPIO signals

  • Low-latency macro triggering

• ECU ↔ Sensor PCBs:

  • CAN-FD

  • Event-driven, heartbeat-monitored

• ECU ↔ Spindle servo drive:

  • RS-485

• Safety chain:

  • Electrically independent

  • Directly actuates power and pneumatics

Responsibility boundaries

• Duet:

  • Motion execution only

  • No supervision or state estimation

• ECU:

  • Process supervision only

  • No trajectory planning

• Raspberry Pi:

  • Configuration, logging, visualization

  • No real-time control

• Sensor PCBs:

  • Local signal processing

  • No global decision-making

• Safety system:

  • Hardware-only

  • Overrides all other systems

System architecture diagram

        graph TD
%% Core control
Duet[Duet 3 6HC
Motion control
G-code
Kinematics]
Pi[Raspberry Pi
Duet services
Plugins
Logging
Grafana]
ECU[ECU / Master PCB
Real-time supervision
Adaptive logic]

%% Sensors
Piezo[Piezo surface microphones
Chatter detection]
Accel[Accelerometers
Directional vibration
Tilt estimation]
Temp[Temperature sensors]
Strain[Strain gauge
X carriage]

%% Distributed sensing
SensorPCBs[Distributed sensor PCBs
FFT / envelopes
Event-based reporting]

%% Actuators
Spindle[Spindle servo drive
AC servo]
Steppers[Stepper drives
Axes]

%% Safety
Safety[Safety chain
Contactors
E-stop
Pneumatic venting]

%% Machine
Machine[CNC machine
Frame, axes, spindle]

%% Connections
Pi <-->|Internal Duet link| Duet
Pi <-->|SPI / UART| ECU

Piezo --> SensorPCBs
Accel --> SensorPCBs
Temp --> SensorPCBs
Strain --> SensorPCBs

SensorPCBs <-->|CAN-FD| ECU

ECU <-->|RS-485| Spindle
ECU <-->|GPIO macros| Duet

Duet --> Steppers
Steppers --> Machine
Spindle --> Machine

Safety --> Spindle
Safety --> Steppers
Safety --> Machine