Alignment-Free, Self-Calibrating Elbow Angles Measurement Using Inertial Sensors

TLDR

Inertial measurement unit (IMU) joint angle measurements are widely used in sports, gait analysis, and rehabilitation because they are easy to handle and inexpensive, but current algorithms require calibration motions or precise sensor alignment. The study proposes an alignment‑free, self‑calibrating elbow‑angle measurement method that uses arbitrary user movements and an initial zero‑reference arm pose. The method employs real‑time optimization to determine the elbow’s two dominant rotation axes and is evaluated by comparing IMU‑derived angles to marker‑based optical tracking in a motion‑capture laboratory. Self‑calibration converged in under 9.5 s on average, yielding RMS errors of 2.7° for flexion/extension and 3.8° for pronation/supination, making the method particularly suitable for rehabilitation settings where precise alignment and calibration are difficult.

Abstract

Due to their relative ease of handling and low cost, inertial measurement unit (IMU)-based joint angle measurements are used for a widespread range of applications. These include sports performance, gait analysis, and rehabilitation (e.g., Parkinson's disease monitoring or poststroke assessment). However, a major downside of current algorithms, recomposing human kinematics from IMU data, is that they require calibration motions and/or the careful alignment of the IMUs with respect to the body segments. In this article, we propose a new method, which is alignment-free and self-calibrating using arbitrary movements of the user and an initial zero reference arm pose. The proposed method utilizes real-time optimization to identify the two dominant axes of rotation of the elbow joint. The performance of the algorithm was assessed in an optical motion capture laboratory. The estimated IMU-based angles of a human subject were compared to the ones from a marker-based optical tracking system. The self-calibration converged in under 9.5 s on average and the rms errors with respect to the optical reference system were 2.7° for the flexion/extension and 3.8° for the pronation/supination angle. Our method can be particularly useful in the field of rehabilitation, where precise manual sensor-to-segment alignment as well as precise, predefined calibration movements are impractical.