CargoBot
Remote-controlled cargo robot with Bluetooth telemetry
Author: Andrei Rusanescu
GitHub Project Link: link_to_github
Description
CargoBot is a remote-controlled four-wheeled robot that carries cargo and navigates surfaces with imperfections. It is controlled from a laptop keyboard via Bluetooth and uses an STM32 Nucleo-U545RE-Q microcontroller programmed in Rust with Embassy-rs. It sends real-time telemetry data to a PC dashboard via Bluetooth, while simultaneously displaying information on an onboard OLED.
The central idea of the project is measuring and visualizing the impact of cargo load on motor performance: when the robot carries something heavy, the PID controller automatically increases the PWM duty cycle to maintain a constant speed. This compensation is observable live on the robot's OLED, on the PC dashboard, and through 3 LEDs (green/yellow/red) that visually indicate the effort level.
Motivation
I am particularly interested in cars and networking. This project combines multiple peripherals studied in the lab (PWM, GPIO, I2C, UART, Bluetooth) into a functional system. It is a challenge that clearly demonstrates technical effort - the difference in motor effort with and without cargo, and how to compensate for the load.
Architecture
The system is organized around five main subsystems:
1. MCU (STM32 Nucleo-U545RE-Q) The central unit running all Embassy-rs async tasks. Coordinates all subsystems and shared state protected by Mutex.
2. Motor Subsystem The L298N dual H-bridge receives PWM signals from the STM32 and drives 4 DC motors (2 per channel, left/right side in parallel - skid steering). The two IR LM393 optical sensors read encoder disc pulses on GPIO interrupts and compute RPM per side.
3. Sensing Subsystem
- MPU-6500 (I2C, address 0x68): reads accelerometer + gyroscope data, combined via complementary filter to get a stable tilt angle
- 2x HC-SR04 (GPIO trigger/echo): front and rear obstacle detection
- 2x IR LM393 encoders: RPM feedback for PID
4. Communication Subsystem HC-06 Bluetooth module connected to STM32 via UART. Bidirectional: laptop sends movement commands (forward/backward/left/right + speed), robot sends back JSON telemetry at 5Hz.
5. Display & Indicators Subsystem
- OLED SSD1306 128x64 (I2C, address 0x3C, shared bus with IMU): displays RPM, tilt angle, obstacle distance, state, load level
- 3x LEDs (green/yellow/red) on GPIO: visual indicator of motor effort based on PWM duty cycle
- Passive buzzer on PWM: horn and audio feedback
Embassy-rs Async Tasks:
| Task | Frequency | Responsibility |
|---|---|---|
| sensor_task | 20 Hz | Read IMU (I2C) + ultrasonic sensors |
| motor_task | 50 Hz | Apply PWM to L298N, read encoders |
| navigation_task | 10 Hz | State machine: FORWARD / COMPENSATE / AVOID / STOP |
| telemetry_task | 5 Hz | Serialize JSON and send over UART to HC-06 |
| display_task | 4 Hz | Update OLED |
Navigation State Machine:
| State | Entry Condition | Action |
|---|---|---|
| FORWARD | No obstacle | Move at target speed, PID active |
| COMPENSATE | Pitch > 5 deg (ramp) | Increase PWM proportional to tilt angle |
| AVOID | Obstacle < 30cm | Stop, rotate, resume forward |
| STOP | Manual command / error | Motors off, telemetry continues |
Peripheral Usage:
| Peripheral | Component | Usage |
|---|---|---|
| PWM | STM32 -> L298N | Motor speed control (0–100% duty cycle) |
| PWM | STM32 -> Buzzer | Horn |
| GPIO Output | STM32 -> HC-SR04 trigger | 10 micro-s pulse to trigger ultrasonic |
| GPIO Input Interrupt | HC-SR04 echo -> STM32 | Measure echo duration -> distance |
| GPIO Input Interrupt | LM393 encoders -> STM32 | Count pulses -> compute RPM |
| GPIO Output | STM32 -> LEDs R/Y/G | Load indicator: green (<35%), yellow (35–65%), red (>65%) |
| GPIO Output | STM32 -> L298N IN1-IN4 | Motor direction control |
| I2C (shared bus) | STM32 -> MPU-6500 (0x68) | Accelerometer + gyroscope for tilt angle |
| I2C (shared bus) | STM32 -> SSD1306 (0x3C) | OLED telemetry display |
| UART | STM32 -> HC-06 | Bidirectional Bluetooth: commands in, telemetry out |
Log
Week 6 - 12 Apr
Ordered most of the components needed.
Week 13 - 19 Apr
Assembled the mechanical parts (wheels, motors, car platform).
Week 20 - 26 Apr
Working on the Schematic in KiCad. Tested individual components: bluetooth module, display, motors, distance sensors, LM393 speed sensors.
Week 27 Apr - 3 May
Ordered Li-Ion Samsung 18650 3.6V 3450mAh 8A batteries, a charger for the batteries, more male-female and female-female jumpers and a smaller breadboard (400 points).
Week 4 - 10 May
Soldered IMU and OLED display in the lab. Assembled final product. Started to write the software for the cargobot and tested it carrying another car. The HC-SR04 sensors are not so precise as the car sometimes crashes into walls if driven into.
Week 11 - 17 May
Hardware
The robot is built on a 4WD chassis with 4 DC motors (3–6V) driven through an L298N dual H-bridge using skid steering (left/right side in parallel). Speed and direction are controlled via PWM from the STM32. Two IR optical sensors read encoder discs on the motors to compute RPM. An MPU-6500 IMU over I2C measures tilt angle using a complementary filter. Two HC-SR04 ultrasonic sensors handle obstacle detection front and rear. An SSD1306 OLED displays live telemetry. An HC-06 Bluetooth module provides bidirectional communication with the laptop.
Schematics
KiCad Schematic:
Bill of Materials
| Device | Usage | Price |
|---|---|---|
| STM32 Nucleo-U545RE-Q | Main microcontroller (Cortex-M33), runs all Embassy-rs tasks | 0 RON (provided by university) |
| 4x DC motors (3-6V), encoder discs, wheels | Wheels and motors for the CargoBot | 40 RON |
| 2x IR Speed Sensor LM393 x2 | Optical encoder reading, counts pulses from encoder discs to compute RPM | 16.22 RON |
| L298N Dual H-Bridge | Motor driver, controls speed (PWM) and direction of both motor sides | 8.96 RON |
| MPU-6500 Accelerometer & Gyroscope | 6-axis IMU, measures tilt angle via complementary filter (I2C, 0x68) | 9.92 RON |
| HC-06 Bluetooth Module | Bidirectional wireless UART, receives keyboard commands, sends telemetry JSON | 25.13 RON |
| OLED SSD1306 0.96" I2C | On-board display, shows RPM, tilt, obstacle distance, state, load level (I2C, 0x3C) | 14.01 RON |
| 2x HC-SR04 Ultrasonic Sensor x2 | Obstacle detection, front and rear, GPIO trigger/echo | 18.80 RON |
| 18650 Battery Holder 2S | Holds 2x 18650 cells in series, 7.4V output for L298N and STM32 | 5.74 RON |
| 2x Li-Ion Samsung 18650 3.6V 3450mAh 8A | Batteries to provide voltage for the motors and for the board | 63 RON |
| Battery charger | Charges the Li-ion batteries | 35 RON |
| 3x LED (red/yellow/green) + resistors | Visual motor effort indicator | 0 RON (owned) |
| Female-Female jumper wires | Wires for connections | 7.73 RON |
| Male-Female jumper wires | Wires for connections | 7.73 RON |
| Male-Male jumper wires | Wires for connections | 0 RON (owned) |
Software
| Library | Description | Usage |
|---|---|---|
| embassy-stm32 | Async HAL for STM32U5 | PWM, I2C, UART, GPIO, Timer drivers |
| embassy-executor | Async task executor | Spawning and running concurrent tasks |
| embassy-time | Async timers and delays | Task scheduling at fixed frequencies |
| embassy-sync | Synchronization primitives | Mutex and Channel for inter-task shared state |
| ssd1306 | OLED SSD1306 driver | I2C display rendering |
| embedded-graphics | 2D graphics library | Drawing text and shapes on OLED |
| heapless | No-alloc data structures | String/Vec without heap allocation |
| defmt + defmt-rtt | Logging framework | Debug output via probe |
| libm | Math functions (no_std) | atan2, sqrt for complementary filter |
| Python pyserial | Serial communication | PC-side script to receive telemetry and send commands over Bluetooth |
| Python matplotlib / Flask | Data visualization | Live dashboard with RPM, PWM, tilt, distance graphs |