Embedded systemsDecember 2024 - April 2025

BLDC Motor FOC Control

Advanced Motor Control with STM32 and Field-Oriented Control

FOC Algorithm
Industry-standard Field-Oriented Control
Timed Control Loop
2.5 kHz timer interrupt and 25 kHz PWM
Industrial Protocol
Modbus RS485 communication

System Overview

This STM32 controller receives PC speed commands over Modbus RS485 and drives a BLDC motor with Field-Oriented Control.

The published firmware runs the open-loop mode, with the report's closed-loop velocity controller left commented out; an SPI encoder reports angle and speed, and an OLED displays motor state. Interrupt-based communication and timer-driven control are integrated with a stop input, manually etched PCB, and custom printed enclosure.

How it actually works

Inside the Modbus BLDC controller

Follow one speed command through the published STM32 firmware: the Modbus frame, the 1-per-100 ms ramp, the 2.5 kHz timer tick, sine PWM, the encoder and the OLED.

Things to try

Move the slider, press Send frame and watch only bytes 4 and 5 change.
Open 2 Ramp and E-stop and press E-stop during a ramp.
Compare the duty cycles in 4 Sine PWM with the OLED in 6 OLED.

Built from the project's own code and documents.from code: A constant or rule copied from the project's repositoryfrom report: A value from the project's report or presentationsimulated: Computed in your browser by running the project's logic

Target speed to send (rad/s)20

Sent as the 16-bit register value. The firmware accepts 0 to 65535; this slider stops at 40 so the drawn rotor stays readable.

data (last frame)

speed (ramped, rad/s)

20.0

is_stopping

false

MAX485

Receive

The PC sends a Modbus RTU Write Single Register frame (function 06, slave 1, register 40001) at 9600 baud. The firmware never checks the address, function code or CRC: it copies the frame, echoes 8 bytes back and reads only bytes 4 and 5 as the target. For function 06, an echo is also the normal reply.

01

[0] Slave ID

06

[1] Function

00

[2] Register hi

00

[3] Register lo

00

[4] Value hi

14

[5] Value lo

89

[6] CRC lo

C5

[7] CRC hi

Frame 01 06 00 00 00 14 89 C5your input

Firmware result: data = 0x00 << 8 | 0x14 = 20. Only the gold bytes reach the motor code.

Last frame on the RS-485 linereal duration 8.33 ms (8 bytes x 10 bits at 9600 baud), slowed here

Receive to idle: MAX485 listening, DE (PA12) and RE (PA11) low
RX callback: memcpy, switch to transmit, data = RxData[4..5]
Echo 8 bytes back with DE and RE high
TX complete: back to receive, re-arm reception

Transceiver now: Receive (DE and RE low)

Core/Src/main.c, RS-485 callbacksView on GitHub

void MAX485_EnableTransmit(){
HAL_GPIO_WritePin(MAX485_DE_PIN_GPIO_Port,MAX485_DE_PIN_Pin, GPIO_PIN_SET);
HAL_GPIO_WritePin(MAX485_RE_PIN_GPIO_Port,MAX485_RE_PIN_Pin, GPIO_PIN_SET);
}
void MAX485_EnableRecieve(){
HAL_GPIO_WritePin(MAX485_DE_PIN_GPIO_Port,MAX485_DE_PIN_Pin, GPIO_PIN_RESET);
HAL_GPIO_WritePin(MAX485_RE_PIN_GPIO_Port,MAX485_RE_PIN_Pin, GPIO_PIN_RESET);
}
void HAL_UARTEx_RxEventCallback(UART_HandleTypeDef *huart, uint16_t Size) {
memcpy(TxData, RxData, Size);
MAX485_EnableTransmit();
HAL_UART_Transmit_IT(&huart1, TxData,8);
data = RxData[4] << 8 | RxData[5];
}
void HAL_UART_TxCpltCallback(UART_HandleTypeDef *huart) {
if (huart->Instance == USART1) {
MAX485_EnableRecieve();
HAL_UARTEx_ReceiveToIdle_IT(&huart1, RxData, sizeof(RxData));
}
}

Simply Modbus Master Write settings: RTU, 9600 baud, 8 data bits, 1 stop bit, no parity, slave ID 1, register 40001, function code 6, command 01 06 00 00 00 00 89 CA — The team's Modbus master: function 6, register 40001, frame 01 06 00 00 00 00 89 CASlides p. 8

Constants, strings and pins are from the firmware at the pinned commit; the 2.5 kHz and 25 kHz rates are worked out from its timer settings. The ramp, E-stop and angle run as a simulation in your browser using the firmware's rules; there is no measured motor data in the report. The published code runs open loop; the closed-loop gains appear only in the report.

How it works

Connect PC commands to motor feedback and local control.

1
Receive a speed command
The MAX485 passes PC Modbus data to a UART receive buffer, and the callback extracts the requested speed from two data bytes.
2
Adjust the setpoint
The firmware increases or decreases the speed setpoint in steps toward the target, subject to the emergency-stop state.
3
Run the motor feedback loop
A timer callback measures elapsed time and calls the FOC and motor-control functions using encoder feedback and the current setpoint.
4
Expose motor state
Show measured angle, speed, and target speed on the OLED, or display the emergency-stop state when the stop input is activated.

Project Demo

The project's own demonstration footage.

Watch BLDC Motor FOC Control on YouTube

Engineering features

Developed a BLDC motor control system using the Modbus protocol over RS485, with an STM32 microcontroller handling real-time communication and speed control.
Implemented Field-Oriented Control on a 2.5 kHz timer interrupt with sine PWM, an SPI magnetic encoder and interrupt-driven Modbus processing; a closed-loop PI velocity mode is documented in the report.
Designed and fabricated a custom PCB to house all the components into a single board.

Inside the build

The control logic, integration and design decisions, one note at a time.

Interrupt-based RS485 direction switching

UART reception uses the STM32 HAL receive-to-idle interrupt interface. The receive callback switches the MAX485 into transmit mode and echoes the received frame; the transmit-complete callback restores receive mode. The target-speed value is assembled from the high and low bytes in the received buffer.

Timed FOC with encoder feedback

The firmware initializes an AS5048A SPI encoder and the motor driver in open-loop mode with 7 pole pairs; the report lists a closed-loop velocity controller (P 5, I 30) that the published code leaves commented out. Timer 4 triggers control updates at 2.5 kHz, while Timer 1 supplies elapsed time. The callback invokes loop_foc and bldc_move, with encoder-derived angle and velocity also available to the display.

Fabricated board and accessible enclosure

The team transferred a printed PCB layout to copper, corrected traces manually, and chemically etched the board. The enclosure divides component mounting and wiring into a base and top holder, with openings for the motor, front-facing OLED, and stop button so the operator can access controls without opening the case.

Build artifacts

Figures taken from the project's own report or presentation.

CAD rendering of the BLDC controller PCB assembly with the STM32 board, interfaces, and control components — PCB Design, Figure 3, extracted from PDF page 4. This is a CAD rendering of the assembly.

Evidence & limitations

What the project documents show, and where each claim comes from.

Reported integrated motor operation

The report includes the board design, schematic, firmware listing, communication and speed-control diagrams, and a result-video reference. Its results section reports PC-commanded motor speed control, live OLED feedback, and operation of the stop button; it does not provide numerical speed-tracking or stop-latency measurements.

BLDC controller project report
PDF pages 2-4: hardware and PCB fabrication; 5-15: firmware and control; 16, 18: results and conclusion.
Motorpilot source repository
Project demonstration
Linked from the supplied portfolio.

CAD & mechanical models

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BLDC Motor

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PCB

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Project Resources

The original documents behind this case study.

PCB Schematics
EPRO file
View Download
Presentation Slides
PDF file
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Detailed Report
PDF file
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3D CADs
ZIP file
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BLDC Motor FOC Control

FOC Algorithm

Timed Control Loop

Industrial Protocol

System Overview

Inside the Modbus BLDC controller

How it works

Receive a speed command

Adjust the setpoint

Run the motor feedback loop

Expose motor state

Project Demo

Engineering features

Inside the build

Interrupt-based RS485 direction switching

Timed FOC with encoder feedback

Fabricated board and accessible enclosure

Build artifacts

Evidence & limitations

Reported integrated motor operation

CAD & mechanical models

BLDC Motor

Housing Top

Housing Bottom

PCB

Project Resources

Hiring for robotics or embedded work?