🤖 Multi-Axis Robotic Arm Control (v1.0)

📌 Project Overview

This project focuses on the development of a lightweight robotic arm controlled by an STM32L476RG (Cortex-M4) microcontroller. The architecture is designed to transition from direct hardware PWM control to an I2C-based expansion system, ultimately integrating with ROS 2 for advanced motion planning (and manipulation).

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🛠 Progress & Milestones

🟢 Phase 1: Core Hardware Abstraction (Completed)

• C++ Hardware Wrapper: Successfully encapsulated STM32 HAL (C-based) libraries into a modular, object-oriented Servo class.
• Direct Timer Control: Implementation of PWM generation via Timer 2 (TIM2) using multiple channels on pins PA0 and PA1.
• Synchronized Motion Profiling: Developed non-blocking update() methods to allow smooth, simultaneous movement of multiple joints without CPU "stalling."

🟡 Phase 2: Expansion & I2C Integration (In Progress)

• PCA9685 Driver Implementation: Developing a custom C++ driver to offload PWM generation for up to 16 servos via the I2C protocol ($SDA$/$SCL$).
• Bus Management: Shifting from local MCU timers to an external 12-bit PWM controller to reduce interrupt overhead and simplify cable management.
• Power Distribution: Using a dedicated AC-DC power converter (output 6V/5A) to handle the high inrush current of multiple simultaneous servo movements, protecting the nucleo logic board.

⚪ Phase 3: Mechanical Design & Simulation

• Structural Engineering: 3D modeling a lightweight, high-rigidity chassis optimized for low-torque MG90S servos.
• Kinematics Optimization: Calculating center of mass and leverage points to maximize effective payload capacity.

⚪ Phase 4: ROS 2 & High-Level Control

• micro-ROS Integration: Bridging the STM32 with a ROS 2 workspace via Serial/USB transport.
• Joint State Broadcasting: Exporting commanded joint trajectories to RViz2 for real-time kinematic digital-twin synchronization.
• Inverse Kinematics (IK): Implementing coordinate-based movement to allow the end-effector to reach specific $X, Y, Z$ targets.

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📐 System Architecture

Component Selection

Component	Specification	Role
MCU	STM32L476RG (80MHz)	Main Logic & Real-time Processing
PWM Driver	PCA9685 (I2C)	16-Channel 12-bit PWM Expansion
High-Torque Actuator	MG996R (Standard Size)	Primary Shoulder Rotation
Precision Actuators	MG90S (Metal Gear)	Primary base Articulation
Lightweight Actuators	SG90 (Plastic Gear)	Other joints / Gripper Mechanism
Interface	UART / micro-ROS	External Command & Telemetry

Logic Flow

1. Command: User or ROS 2 node sends target joint angles.
2. Processing: STM32 calculates the duty cycle scaling for the 12-bit PCA9685 registers.
3. Transmission: I2C Bus transmits data packets to the 0x40 address.
4. Actuation: PCA9685 provides stable, independent 50Hz PWM signals to all connected servos.

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🔌 Schematic & Wiring

(Section to be updated with circuit diagrams)

Current Pinout (Phase 1):

• PA0: Elbow Servo (TIM2_CH1)
• PA1: Wrist Servo (TIM2_CH2)
• VCC: 5V From USB Port
• GND: Common Ground