Bluetooth IMU Beacon

This project was the creation of a Bluetooth-based IMU device that was placed on an athletes body for positional tracking.

The goal was to use a series of IMUs placed on various "hotspots" of a golfers body to track their body during swings. This would provide a cheap and accurate solution for discovering swing characteristics that could be used by both golfers and coaches alike.

For my portion of the project, I helped decide the components used on the IMU and which protocols we should use for data communication.

To test the boards once they were assembled, I wrote some test firmware using ChibiOS to verify the correct operation of each board component. This was done by writing the test firmware for an extra STM32 board I had. To get the test firmware running on our IMU, I wrote a board part description for ChibiOS for the IMU we built. This allowed the firmware to be directly run on the IMU board without any changes.

Board Components

The board was made up of two primary components, an NRF52832 microcontroller and Bluetooth radio, paired with an ICM42688 over SPI.

Firmware

My primary task in the project was writing the firmware for these IMU boards. I decided to use Zephyr RTOS to write the firmware for the boards. Zephyr was chosen for multiple reasons:

• Stable, well tested codebase
• Very flexible
• Full hardware support for NRF52
• Great development environment

To write the firmware for the boards multiple things were required:

• Custom board definitions for Zephyr
  • I wrote a board definition for Zephyr, so I could take advantage of the IMU and NRF52 within Zephyr
  • Custom device tree for the board
  • Kconfig/menuconfig-like entries for board settings
• ICM42688 Driver
  • There was no driver for the ICM42688 written for Zephyr
  • Ported ICM42605 firmware to ICM42688
  • Patched bugs in ICM42605 driver when porting to ICM42688
    • Contained bug in trigger thread that performed an incorrect cast, causing a firmware fault.
    • Missing work queue implementation was added.
    • Added ability to change trigger settings and stack size in Kconfig.
• Virtual IMU Driver
  • I created a virtual IMU driver, so the firmware could be tested on other boards that didn't contain an IMU
  • The virtual IMU driver generated false movement data that simulated the IMU being moved in a circular motion.
• Efficient Data Transfer Routine
  • This caused the biggest headache during development
  • Our system was designed such that there would be one or two Bluetooth capturing stations nearby the golfer to collect the data from the IMUs wirelessly.
  • The design of the system dictates that 8-10 IMUs should be placed on the golfers body. This means there are 8-10 different IMU data streams being recorded at once. Recording all of these streams over a single BLE connection is very difficult.
    • Many BLE radios can only reliably maintain 4-5 active connections at once before dropping packets (This is what I found during my testing)
    • The IMU data was originally placed in the IMUs advertisement packet, but since the advertisement packet can only contain 37 bytes it was hard to fit the required data in it.
  • To reduce the amount of data transferred, a custom IMU sample format was created. It cut the size of each IMU measurement from 64-bits down to 32-bits without losing any data. This was done by trimming padding bytes and packing the integer and fixed point values into a single uint32_t value.
  • To reduce to amount of active connections for the capture stations, the capture stations only connected to an IMU when it needed data transferred off of it.
    • To determine how much data was on an IMU, the IMU board's BLE advertisement packet contained a section with the amount of IMU readings stored onboard.
    • The capture stations scanned for the IMU boards, and would connect to whichever one contained the most IMU readings.
    • Once connected, the IMU readings were transferred wirelessly until the amount of packets requested were sent.

If you'd like to use the firmware or know more specifics about the protocols used, the README in the GitHub repo contains instructions for installing and operating the firmware.

View my code on GitHub