Quadruped Spider Robot
A 4-legged spider robot controlled via Bluetooth, capable of walking in all directions.
Author: Vlad-Ștefan DAMIAN
GitHub Project Link: https://github.com/UPB-PMRust-Students/fils-project-2026-vladduu
Description
A quadruped spider robot with 4 legs and 3 degrees of freedom per leg (12 servo motors total). Each leg has an ankle, knee and hip joint driven by SG90 servo motors through a PCA9685 16-channel PWM driver. The robot is controlled wirelessly using an HC-05 Bluetooth module and can get up from a lying position, walk forward, walk backward, turn left, turn right and it is controlled from the Arduino Bluetooth Control Application. The firmware runs on an STM32 NUCLEO-F446RE microcontroller written in Rust using the embassy-rs async framework.
Motivation
I have always been fascinated by robots, especially biomimetic robots, machines that move like living creatures. With that idea in mind, i wanted to build a quadruped spider robot cause it was one of my childhood dreams. Also, i knew that the challenge of making 12 motors work together in a coordinated gait, all in real time on a microcontroller will push me to deeply understand both hardware and software design.
Architecture
The system is composed of:
- STM32 NUCLEO-F446RE — the main microcontroller that runs the Rust/embassy-rs firmware. It communicates with the PCA9685 over I2C and receives Bluetooth commands over UART.
- PCA9685 PWM Driver — generates PWM signals for all 12 servo motors simultaneously. Powered by a dedicated 5V external power supply.
- HC-05 Bluetooth Module — receives single-character commands (F/B/L/R/S) from a mobile RC app and forwards them to the MCU over UART.
- DC-DC Step Down module XL4005 - used for the current regulation of the PCA9685 PWM Driver.
- DC-DC Step Down module LM2596 - used for the current regulation of the microcontroller.
- 18650 Li-Ion batteries - to power the circuit.
The gait algorithm uses diagonal leg pairs (FL+RB, FR+LB) stepping one leg at a time. Each step consists of: lift knee → swing hip forward → plant foot → push hip back → reset. While a leg steps, the opposite hips shift slightly for balance stabilization.
Log
Week 1 - 3
Bought all the components needed.
Week 4 - 7
Started building the main frame and installig the SG90 servo motors, batteries, regulators, to test the idea and also the weight. First i made it just as a prototype.
Week 8 - 10
Seeing that the prototype failed(it was too heavy because of the construction materials, and the SG90 servo motors broke down very quickly and didn't have enough power for that weight), i decided to rebuild it using 3d printed parts and upgrading to mg90s metal gear servo motors.
Hardware
The robot frame is built using PLA and AMS 3D printed parts and metal brackets. 12 MG90s servo motors are mounted across 4 legs (3 per leg: ankle, knee, hip). All servos are connected to a PCA9685 PWM driver which is controlled by the STM32 NUCLEO-F446RE over I2C. An HC-05 Bluetooth module connects to the MCU over UART for wireless control. The servos are powered by a dedicated external power supply composed of two 18650(a total of 7.4V) rechargeable batteries with a discharge of 30A and regulated to 6V using the XL4005 module.
Servo Channel Mapping:
| Channel | Joint |
|---|---|
| 0 | Right Back Ankle |
| 1 | Right Back Knee |
| 2 | Right Back Hip |
| 3 | Left Back Ankle |
| 4 | Left Back Knee |
| 5 | Left Back Hip |
| 6 | Front Left Ankle |
| 7 | Front Left Knee |
| 8 | Front Left Hip |
| 9 | Front Right Ankle |
| 11 | Front Right Knee |
| 12 | Front Right Hip |
| 13 | Front Right Ankle (remapped - ch10 damaged) |
Photos
Schematics
Bill of Materials
| Device | Usage | Price |
|---|---|---|
| STM32 NUCLEO-F446RE | Main microcontroller | 131 RON |
| PCA9685 PWM Driver | 16-channel I2C PWM servo driver | 32 RON |
| MG90s Servo Motor | Leg joints (x12) | 19,33 RON x12 = 232 RON |
| HC-05 Bluetooth Module | Wireless RC control | 46 RON |
| 18650 Batteries | External servo power | 44 RON |
| Building Materials | Robot frame | 100 RON |
| Jumper wires | Connections | 15 RON |
| 18650 Batteries Case | Used for conecting the batteries in series (7.4V) | 20 RON |
| DC-DC Step Down module LM2596 | Regulating the current to 5V for the microcontroller | 15 RON |
| DC-DC Step Down module XL4005 | Regulating the current to 6V for the servo driver | 15 RON |
| Total | 650 RON |
Software
| Library | Description | Usage |
|---|---|---|
| embassy-executor | Async task executor for Cortex-M | Runs concurrent tasks for the gait sequence and Bluetooth handling |
| embassy-stm32 | STM32 Hardware Abstraction Layer | Hardware drivers for I2C (servo), UART (Bluetooth), and GPIO |
| embassy-time | Time management utilities | Handles asynchronous time delays between servo movements |
| embassy-sync | Synchronization primitives | Sharing state and passing Bluetooth commands between async tasks |
| pwm-pca9685 | PCA9685 PWM driver | Independent control of all 12 servo motors over I2C |
| cortex-m | Low-level Cortex-M processor access | Core processor operations and register access |
| cortex-m-rt | Cortex-M minimal runtime | Device boot sequence and interrupt vector routing |
| defmt | Deferred formatting logging framework | Highly efficient console logging and formatting |
| defmt-rtt | RTT transport for defmt | Transmitting log messages to the host PC via Real-Time Transfer |
| panic-probe | Panic handler | Catches system errors/panics and prints them via defmt |