DELIVERWHOA two-wheeled delivery robot for the campus. No kickstand, no third wheel: it stays upright because the code says so.

The robot has to cross indoor and outdoor ground, fit through standard doors and lifts, carry a 5 kg parcel and run two hours. Two driven wheels keep it compact and agile, and turn balance into the central engineering problem.

Chosen: a 57BLR50-24-01 brushless motor (84 W, 24 V, 0.23 N·m at 3500 rpm) behind a 10:1 MG23-G10-D8 gearbox, so 2.3 N·m at 350 rpm with a 400 N load rating.

The Simulink block diagram became a single C++ loop on the ESP32. Read the IMU, fuse it into one angle, run the right PID, write PWM to both drivers, handle the controller. Same structure as the model, so the gains transferred with only re-tuning.

• Two PIDs, one output. The main loop tracks the tilt setpoint; inside ±2° a softer fine loop takes over so the robot stops hunting.
• Filter before control. The gyro integrates cleanly but drifts; the accelerometer is noisy but absolute. 98 / 2 keeps the best of both.
• Calibrate every boot. 1 000 samples averaged on the ground so the zero angle is the real zero.
• Deadbands and toggles. ±12 on the sticks; ✕ disarms the loop, △ recalibrates.

// simplified from the ESP32 firmware
void loop() {
  float gyro  = mpu.gyroX() * dt;
  float accel = atan2(mpu.accY(), mpu.accZ());
  angle = 0.98 * (angle + gyro) + 0.02 * accel;

  float target = map(ps5.LStickY(), deadband 12);
  float err = target - angle;
  float out = fabs(err) < 2.0
    ? finePID.step(err)   // P 3.0  I 0    D 0.12
    : mainPID.step(err);  // P 10.0 I 0.4  D 1.5

  float turn = map(ps5.RStickX(), deadband 12);
  driveL.pwm(out + turn);  // BLD-4055, sign = direction
  driveR.pwm(out - turn);

  if (ps5.Cross())    stabilise = !stabilise;
  if (ps5.Triangle()) calibrate(1000);
}