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
---------------------------

.. mermaid::

   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