Week 12 — Mechanical Design, Machine Design

During the Mechanical Design and Machine Design week, our team developed ReFill, an automated filament rewinding machine designed to transfer filament from one spool to another in a controlled and efficient way.

The machine combines mechanical design, electronic control, and embedded programming to automate the rewinding process while improving filament organization and reducing material waste.

The complete development process, including the mechanical system, fabrication, assembly, testing, and team collaboration, is documented in the Group Assignment page.

My main contribution focused on the electronics and embedded control system. I was responsible for designing the electronic schematic, developing the custom PCB, generating fabrication files, programming the ESP32 firmware, integrating encoder feedback, and validating the interaction between hardware and software.

After completing the electronic design, I developed the embedded software responsible for coordinating all machine operations. The firmware was programmed using the Arduino framework running on the XIAO ESP32-C3 microcontroller.

The main objective was to create a reliable control system capable of managing user interaction, monitoring rotational feedback, controlling motor movement, and executing the rewinding sequence autonomously.

To simplify maintenance and improve scalability, the software was organized around a Finite State Machine (FSM). This approach separates machine behavior into clearly defined operational states, making the code easier to debug and expand.

Each state controls a specific phase of operation and defines the transitions required to move between them according to sensor inputs, user commands, and process completion conditions.

Finite State Machine used to coordinate machine behavior.

During startup, the firmware initializes all hardware resources, including communication interfaces, GPIO pins, interrupts, PWM channels, and the OLED display.

This configuration stage ensures that every peripheral starts in a known state before entering the main control loop.

The motor driver, Hall sensor, user encoder, and display are all configured inside the setup routine, while the Hall sensor uses hardware interrupts to guarantee accurate pulse acquisition.

void setup() {

  Wire.begin(SDA_PIN, SCL_PIN);

  display.begin(
    SSD1306_SWITCHCAPVCC,
    0x3C
  );

  attachInterrupt(
    digitalPinToInterrupt(HALL_PIN),
    contarPulsos,
    RISING
  );

}

volatile long pulsos = 0;

void IRAM_ATTR contarPulsos() {
  pulsos++;
}

One of the most critical parts of the system is the rotational feedback mechanism.

A Hall-effect encoder generates pulses every time the spool rotates. These pulses are captured using an interrupt service routine, ensuring that no measurements are lost even while other tasks are executing.

The pulse count is continuously converted into spool turns, allowing the machine to determine when the desired rewinding objective has been reached.

The rewinding motor is controlled through an L298N driver using PWM signals generated by the ESP32.

PWM control allows speed regulation while maintaining a simple hardware architecture. The firmware can start, stop, and adjust motor behavior according to the active machine state.

During normal operation the motor rotates continuously until the target number of turns is reached or a safety condition is detected.

digitalWrite(MOTOR_R, LOW);
ledcWrite(
  pwmChannel,
  velocidad
);

void leerEncoderUI() {

  bool A = digitalRead(ENC_UI_A);

  if (A != lastUIA) {

    if (digitalRead(ENC_UI_B) != A)
      menuIndex++;
    else
      menuIndex--;

  }
}

User interaction is performed through a rotary encoder with an integrated push button.

Rotational movement allows navigation through menu options, while the push button executes the selected action.

This approach reduces the number of physical controls required while providing an intuitive interface for machine operation.

Real-time feedback is presented through a 128x64 OLED display.

The interface continuously updates operational information such as the number of completed turns, target value, machine state, and available menu options.

This visualization system allows the user to monitor machine progress without requiring an external computer connection.

display.print("Vueltas: ");
display.println(vueltas);

display.print("Objetivo: ");
display.println(vueltasObjetivo);

Although my primary contribution focused on the electronic and embedded control system, the final validation of the project required the integration of these subsystems into the complete mechanical assembly developed by the team.

The custom control board was connected to the motor driver, Hall-effect encoder, user interface, limit switch, and power distribution system, allowing the machine to operate as a single coordinated platform.

Through this integration process, the firmware could interact directly with the mechanical transmission system, monitor spool rotation in real time, execute the programmed rewinding cycles, and provide feedback to the user through the OLED interface.

This stage was particularly valuable because it transformed the electronic prototype into a fully functional machine, validating both the hardware architecture and the software developed during the project.

The complete documentation of the mechanical design, fabrication, assembly process, transmission system, bearing arrangement, structural components, and final machine construction can be found in the Group Assignment documentation.

This project represented one of the most complete and rewarding experiences of the Fab Academy so far because it brought together knowledge acquired throughout multiple previous assignments into a single functional machine.

From my perspective, the most valuable aspect was understanding how individual disciplines such as electronics design, PCB fabrication, embedded programming, and machine integration must work together to create a reliable system. While each of these topics had been explored separately during previous weeks, this project required combining them into a cohesive solution.

One of the biggest challenges was developing a control system capable of interacting with both the user and the mechanical components in real time. Designing the PCB, implementing the firmware architecture, managing encoder feedback, and validating the communication between all subsystems required multiple iterations and continuous testing.

The implementation of the finite state machine helped me better understand how software architecture influences the reliability and scalability of a machine. Likewise, working with interrupts, PWM control, sensor feedback, and user interfaces strengthened my confidence in embedded systems development.

Beyond the technical achievements, this project reinforced the importance of collaboration. The final machine was only possible because each team member contributed expertise from different areas, and seeing all subsystems come together into a working prototype was one of the most satisfying moments of the assignment.

Looking back, this project demonstrated how digital fabrication extends beyond producing individual parts. The real challenge lies in integrating mechanical, electronic, and software components into a complete system capable of solving a real-world problem. That integration process is the lesson I will carry forward into future projects.