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Medical grade BLDC motor FOC Controller with LCD and fiber optic wire control

  • Writer: Rotem Segev
    Rotem Segev
  • Nov 9
  • 3 min read

We designed a high-precision embedded control platform designed to drive a BLDC motor using Field-Oriented Control (FOC), control the position of a fiber-optic light delivery wire for medical therapy, and provide a user-programmable motion interface via a full TFT touch display.


The architecture uses two dedicated STM32 MCUs for deterministic motor control and robust system management.

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System Architecture

Main Control Unit – STM32H743

  • Runs the UI, system logic, scripting engine, safety supervision, logging, and communications

  • Drives the 2.4" TFT LCD capacitive touch screen (240×320)

  • Loads files, animation assets and logs from SD card and external SPI Flash

  • Monitors power domains (battery/charger/Vin) and system health

  • Issues motion commands to the motor MCU over UART

  • User defines motor movement scripts via touch UI

  • Manages:

    • RGB status LED

    • Touch input, buttons, external sensors

    • Error logging to SD

    • USB/UART communication with host device


Motor Control Unit – STM32H750

  • Dedicated FOC motor controller

  • Implements closed loops for:

    • Position

    • Speed

    • Current (torque)

  • Interfaces with:

    • Magnetic encoder (12-bit, absolute)

    • Hall sensors (internal + external)

    • Friction wheel IR encoders

    • Light interrupters for safety zones

    • Shunt current sensors (INA240)

  • Executes motion trajectories received from main MCU

  • Reports status, errors, and feedback in real-time

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Unique Mechanism – Fiber Optic Wire Positioning

A medical fiber optic wire is mechanically moved by the BLDC motor. To determine its precise position:

Sensor

Purpose

IR1 / IR2 / IR3

Detect wire motion and motion validity (Go/No-Go safety)

Friction wheel IR encoder

Tracks actual wire displacement and direction

Absolute Hall sensor

Reference Point B – guarantees repeatable absolute stop

Motor internal encoder

Reference Point A – relative position

STM32 FOC loops

Maintain accurate motion, torque, and velocity

This allows the system to always locate and return the wire to the exact same position, even after power cycles.

The wire emits strobed light for medical therapy, synchronized to the movement script.


Motor & FOC Capabilities

The system incorporates advanced motor control features:

  • Field-Oriented Control (FOC) for maximum torque efficiency

  • 4-quadrant operation

  • Low-speed torque stability

  • Velocity and position loops with PID

  • 5.5V – 24V motor supply

  • 3A continuous / 5A peak output

  • Integrated 12-bit magnetic encoder

  • Script-driven motion profiles

  • C++ / Python controllable through library


Motor driver: L6234 3-phase bridge

Current sense: INA240 high-precision bidirectional amplifier


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Power System

Multi-stage protected power design supporting:

Mode

Behavior

Vin primary

Charges battery, powers system, disables battery load

Battery mode

Runs system from 18650 pack

Sleep mode

Partial shutdown, wake from touch/button

Components include:

  • Hot-swap and protection: TPS25983 (2.7–26V, 18A, transient protected)

  • Battery charger: BQ25308 (1-cell 3A charging)

  • Buck 5V, Boost 12V, Multiple LDO rails

  • Over-current, over/under voltage, load monitoring

  • Clean ground segmentation and EMI filtering



Physical Interfaces

Interface

Function

TFT LCD + Capacitive Touch

UI for setting motion scripts

SD Card

Logs, images, configs, motion programs

External SPI Flash

Failsafe boot storage

UART to host

Diagnostics, control

Multiple sensor JST/Pico connectors

Wire location, safety, encoders

Trigger button

Start motion

Motor connector

BLDC + Hall + encoder

FOC Control block diagram


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And this is the complete PID loop:

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photos of the assembled system:


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How did we compute the FOC algorithm parameters:


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Outputs a torque or current reference.


Clark Transform:


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Park Transform:

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Current PI gains:


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Inverse Park:


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Inverse Clark to drive voltages (SVPWM):


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Stage

Equation Purpose

Encoder

Compute θe​,ω

Position PID

Convert position error → speed ref

Speed PI

Convert speed error → torque/current ref

Clarke

Convert 3-phase currents → iα,iβ

Park

Convert to id,iq using θe

PI Current loops

Generate vd​,vq​ to correct error

Inverse Park

Convert back to vα,vβ

SVPWM / Inverse Clarke

Generate 3-phase PWM signals


 
 
 

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