# [CNC Mill Concept: Control Electronics, Motion & Electrical](https://blog.hirnschall.net/cnc-mill-concept/electronics-motion-and-electrical/) author: [Sebastian Hirnschall](https://blog.hirnschall.net/about/) meta description: ECU and distributed sensor PCBs on STM32G4 with CAN-FD, Duet 3 6HC open-loop stepper motion with HTD belts, and a DIN/EN 60204-1 electrical installation. meta title: CNC Mill Concept — Control Electronics, Motion & Electrical date published: 25.03.2026 (DD.MM.YYYY format) date last modified: 19.05.2026 (DD.MM.YYYY format) --- Introduction ------------ This post is part of the [CNC Mill Concept hub](https://blog.hirnschall.net/cnc-mill-concept/). 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. ![CAN-FD bus topology: ECU at one end, Sensor Node A (left X-beam) and Sensor Node B (right X-beam), with strain gauge, accelerometer, microphone, and temperature sensor connections per node.](https://blog.hirnschall.net/cnc-mill-concept/electronics-motion-and-electrical/resources/img/can-topology-overview.jpg) Figure 1: CAN-FD bus topology: ECU at one end, Sensor Node A (left X-beam) and Sensor Node B (right X-beam), with strain gauge, accelerometer, microphone, and temperature sensor connections per node. ### 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