CNC Mill Concept: Control Electronics, Motion & Electrical

Introduction

This post is part of the CNC Mill Concept hub. It covers the control electronics architecture, distributed sensor PCB capabilities, the motion system, and the electrical installation and pneumatics.

Control Electronics and PCB Architecture

Purpose

• Provide a clear separation between:
  • motion control
  • process supervision
  • sensing and data acquisition
  • safety
• Enable extensible, modular sensing and supervision
• Avoid tight coupling between motion execution and adaptive logic

ECU (Master PCB)

• Acts as real-time supervisory controller
• Responsibilities:
  • sensor fusion
  • machine state estimation
  • process supervision decisions
• Interfaces:
  • CAN-FD:
    • communication with distributed sensor PCBs
  • RS-485:
    • communication with spindle servo drive
  • GPIO:
    • low-latency triggering of Duet macros
  • SPI or UART:
    • communication with Raspberry Pi
• Does not:
  • generate motion trajectories
  • directly drive axes
  • participate in safety chain

Distributed Sensor PCBs

• Zonal architecture:
  • multiple PCBs placed near sensors
• Connected to ECU via CAN-FD
• Responsibilities:
  • sensor signal acquisition
  • local filtering
  • FFT and envelope extraction
  • threshold detection
• Communication behavior:
  • event-driven data transmission
  • periodic heartbeat messages
• Fault handling:
  • ECU detects missing heartbeats
  • missing data treated as sensor failure
• Debug and validation:
  • raw data access via USB or SPI

Communication Architecture

• CAN-FD used for:
  • robust, deterministic sensor data exchange
• SPI / UART used for:
  • configuration
  • logging
  • visualization
• GPIO used for:
  • low-latency supervisory actions
  • feed and spindle-related macros

Raspberry Pi Integration

• Runs Duet services and web interface
• Hosts plugins for:
  • configuration of ECU and sensor PCBs
  • data logging
  • visualization (e.g. Grafana)
• Not used for:
  • real-time control
  • safety-critical functions

Sensor PCB Capabilities

General Architecture

Two identical distributed sensor PCBs are used.

• Mounted close to structural measurement locations
• Based on an STM32G4 (CAN-FD capable)
• CAN-FD communication to the main ECU
• Optional synchronization line for deterministic simultaneous sampling
• Careful separation of analog and digital domains

All nodes share the same CAN-FD bus. The ECU is located at one physical end of the bus.

CAN Topology

Fig. 1 shows the CAN-FD bus topology with both sensor nodes and their connected sensors.

Strain Gauge Interface

• Supports multiple full-bridge configurations
• External 24-bit ADC recommended for dynamic strain measurement
• Differential low-noise instrumentation front-end
• Bridge excitation provided by PCB
• Shielded differential wiring to remote DMS

Typical usage:

• 2 × X-beam full bridges

Accelerometer Interface

• SPI interface for digital 3-axis accelerometers
• Deterministic sampling capability
• Remote mounting via short shielded cable or rigid daughterboard
• Optional synchronization between PCBs

Typical usage:

• 1 × spindle plate accelerometer
• 1 × tower accelerometer

Microphone Interface (Piezo Surface Microphone)

• High-impedance charge amplifier front-end
• Anti-alias filtering
• Analog input to ADC
• Shielded cable required

Typical usage:

• 1 × spindle housing microphone
• 1 × tower microphone

Temperature Sensor Interface

• SPI/I²C thermocouple frontend or RTD interface
• Alternatively precision analog temperature input
• Slow sampling rate sufficient

Typical usage:

• 1 × spindle housing temperature
• 1 × tower plate temperature

Piezo Communication Interface

• SPI or I²C master interface
• Used to communicate with external piezo shunt PCBs
• No high-voltage circuitry on the sensor PCB
• Digital control and monitoring only

Motion System

Purpose

• Provide precise, repeatable axis motion
• Keep motion execution simple and robust in the initial build
• Allow future upgrades without redesigning the machine structure

Current Motion Configuration

• Motion controller:
  • Duet 3 6HC
• Motors:
  • stepper motors on all linear axes
• Motor drive:
  • steppers driven directly by Duet
• Mechanical transmission:
  • HTD belt between motor and leadscrew
• Purpose of belt coupling:
  • mechanical filtering of motor vibration
  • increased effective steps per millimeter
  • flexibility in gear ratio selection
• Leadscrews:
  • directly coupled to axis motion
• Feedback:
  • open-loop stepper operation in current phase

Design Considerations

• Motion system prioritized for:
  • simplicity
  • predictability
  • compatibility with supervision layer
• Motion execution kept independent from:
  • adaptive supervision logic
  • compliance estimation
• Any feed or spindle adaptation performed via:
  • Duet macros
  • supervisory requests only

Deferred Motion Upgrades

• Closed-loop servo drives
• Dual-loop control using external feedback
• Glass scale integration
• Custom servo inverter development

Scope Limitations

• No real-time modification of trajectories by ECU
• No active damping or compensation within motion controller
• Motion system behavior assumed deterministic for supervision purposes

Electrical Installation and Pneumatics

Purpose

• Ensure operator and bystander safety
• Provide deterministic, fail-safe shutdown behavior
• Comply with applicable DIN / EN machine safety principles
• Keep safety independent from software and firmware

Electrical Installation

• Electrical system designed in accordance with:
  • DIN / EN 60204-1 principles
• Separate grounded electrical cabinet
• Segregation of:
  • mains power
  • motor power
  • control signals
  • sensor signals
• Use of:
  • main contactors
  • appropriate fusing
  • protective earth bonding
• All safety-relevant wiring implemented in hardware

Emergency Stop (E-Stop)

• Emergency stop system implemented as hardware-only
• Normally-open main contactor
• E-stop directly interrupts:
  • mains power to drives
  • control power where required
• No software involvement in E-stop behavior
• E-stop overrides:
  • Duet
  • ECU
  • Raspberry Pi

Pneumatics

• Pneumatic system used for:
  • tool clamping
  • auxiliary machine functions
• Depressurization valve:
  • normally open
  • wired directly into shutdown circuit
• Behavior on:
  • E-stop
  • power loss
• Result:
  • automatic venting of pneumatic system
  • loss of pressure as safe state
• Pneumatic control logic not software-dependent for safety

Fail-Safe Behavior

• Loss of electrical power results in:
  • drives de-energized
  • pneumatic system depressurized
• Safety chain behavior is deterministic and testable
• No single software fault can inhibit safe shutdown

Scope Limitations

• No software-based safety logic
• No safety functions implemented in Duet, ECU, or Raspberry Pi
• Safety system not affected by firmware updates

License

This project (with exceptions) is published under the CC Attribution-ShareAlike 4.0 International License.