HARNode: A Time-Synchronised, Open-Source, Multi-Device, Wearable System for Ad Hoc Field Studies

TL;DR

HARNode is an open-source, multi-device wearable system designed for flexible, scalable, and synchronized human activity recognition field studies, enabling rapid deployment and high-accuracy data collection with minimal hardware expertise.

Contribution

The paper introduces HARNode, a fully open-source, time-synchronized wearable platform with scalable sensor placement and high accuracy, addressing key limitations of existing HAR systems.

Findings

Achieved approximately 98% accuracy with seven nodes in activity classification.

System setup takes under five minutes per person in field conditions.

Sensor-overprovisioning improves classification performance with fewer nodes.

Abstract

Human activity recognition (HAR) research often lacks accessible, comprehensive field data. Commercial systems are rarely open source, hard to expand, and limited by issues like node synchronisation, data throughput, unclear sensor placement, complexity, and high cost. As a result, researchers typically use only a few intuitively placed sensors and conduct limited field trials. HARNode overcomes these challenges with a fully open-source hardware and software platform. Each node includes an ESP32-S3 module (AtomS3), a 9-axis IMU (Bosch BMX160), pressure and temperature sensors (Bosch BMP388), a display, and an I2C port. Data is streamed via Wi-Fi, with NTP-based time synchronisation achieving roughly 1 ms accuracy. The system runs for up to 8 hours and is built using off-the-shelf parts, a simple online PCB service, and a compact 3D-printed housing with Velcro straps, enabling flexible and scalable body placement while requiring little hardware knowledge. In a study with ten subjects wearing eleven HARNodes each, setup took under five minutes per person. A random forest classifier distinguished walking from stair-climbing transitions, showing the benefits of sensor-overprovisioning: Seven nodes achieved approx. 98% accuracy, matching the performance of all eleven. These findings confirm HARNode's value as a fast-deploying, scalable tool for field-based HAR research and optimised sensor placement.