Ball & Beam PID Control

A real-time ball-balancing system built on the STM32 Nucleo F446RE microcontroller. A VL53L1X Time-of-Flight laser sensor measures the ball's position on a beam, and a discrete PID controller drives a servo motor to keep the ball centered.

Demo video: ball_beam_PID_vid.mp4

---

Overview

The ball and beam is a classic control theory problem: a ball rolls freely along a beam whose angle is controlled by a servo motor. Without feedback, the ball rolls off the end. This project implements a discrete PID (Proportional-Integral-Derivative) feedback loop that continuously adjusts the beam angle to stabilize the ball at a fixed setpoint.

System Architecture

flowchart LR
    A["VL53L1X\nToF Sensor"] -->|"Ball distance (mm)"| B["PID Controller\n(STM32F446RE)"]
    B -->|"PWM signal"| C["MG996R\nServo Motor"]
    C -->|"Beam angle"| D["Ball Position"]
    D -->|"Feedback"| A

Loading

Hardware

Component	Model	Role
Microcontroller	STM32 Nucleo F446RE (ARM Cortex-M4, 180 MHz)	Processing & PWM generation
Servo Motor	Miuzei MG996R	Beam actuation
Distance Sensor	VL53L1X 4 m ToF (CJMCU-531 breakout)	Ball position measurement
Mechanical Frame	3D-printed — GrabCAD model	Structure

Peripheral Mapping

Peripheral	Function
I2C1	VL53L1X sensor communication
TIM2 CH1 (PWM)	Servo motor control (700–1250 µs pulse width)
UART2	Debug output at 115 200 baud

PID Controller

Parameter	Value	Description
Setpoint	280 mm	Target ball distance from sensor
Kp	0.3	Proportional gain
Ki	0.001	Integral gain
Kd	150.0	Derivative gain

Implementation notes:

• Error signal is filtered (53% current + 47% previous sample) to reduce ToF sensor noise
• Proportional output is clamped to ±25 to respect physical servo travel limits
• Control effort is reduced when the ball is within 10 mm of setpoint to prevent oscillation

Project Structure

Ball_Beam_PID_Control/
├── Core/
│   ├── Src/
│   │   ├── main.c                # PID algorithm, servo driver, control loop
│   │   ├── VL53L1X_api.c         # ST Time-of-Flight sensor driver
│   │   ├── VL53L1X_calibration.c # Sensor offset calibration
│   │   └── ...                   # STM32 HAL / system files
│   └── Inc/                      # Corresponding header files
├── Drivers/
│   ├── CMSIS/                    # ARM Cortex-M core support
│   └── STM32F4xx_HAL_Driver/     # ST Hardware Abstraction Layer
├── ball_beam_PID.ioc             # STM32CubeMX project configuration
└── ball_beam_PID_vid.mp4         # Demo recording

Getting Started

Prerequisites

• STM32CubeIDE v1.x or later
• STM32 Nucleo F446RE board with ST-Link USB cable
• VL53L1X sensor and MG996R servo wired per ball_beam_PID.ioc

Build & Flash

1. Clone the repository

   git clone https://github.com/<your-username>/Ball_Beam_PID_Control.git

2. Import into STM32CubeIDE File → Open Projects from File System → select the cloned folder

3. Build Project → Build All (Ctrl+B)

4. Flash to board Connect the Nucleo via USB, then Run → Run (or F11 for debug mode)

Pin Configuration

Full pin assignments are defined in ball_beam_PID.ioc and can be viewed in STM32CubeMX. Open the .ioc file in STM32CubeIDE to see the pinout diagram.

Software Dependencies

Library	Source
STM32F4xx HAL	Included — generated by STM32CubeMX
CMSIS	Included — ARM Cortex-M core support
VL53L1X ULD API	Included — ST STSW-IMG009

License

The VL53L1X ULD API and STM32 HAL drivers are distributed under ST's SLA0044 license. See Drivers/STM32F4xx_HAL_Driver/LICENSE.txt and Drivers/CMSIS/LICENSE.txt for details.