Knee Joint Curvature Detection System Based on Fiber Optic Mach-Zendler Interferometric Curvature Sensor

In the monitoring process of human motion and posture, the movement information of the knee joint is useful for diagnosis and rehabilitation evaluation. This paper presents a knee directional bending measurement device (KDBMD) using a Mach–Zehnder interferometer-based directional curvature (MZI-BDC) sensor that was fabricated by fusion-splicing a section of three-core fiber between two single-mode fibers with a core offset. Knee flexion and extension induce bending of the MZI-BDC sensor, changing its modal interference, which makes the sensor suitable for monitoring knee directional bending. Frequency-based modulation and swept source-based techniques are used in the KDBMD. The measurement range of the KDBMD is 0°–90° in both the forward and backward bending directions. For forward bending (bending direction of 0°), the sensitivity and resolution of the proposed KDBMD are 5.29 nm/
$\text{m}^{-{1}}$
and 0.11
$\text{m}^{-{1}}$
, respectively. For backward bending (bending direction of 180°), the sensitivity and resolution of the KDBMD are −3.11 nm/
$\text{m}^{-{1}}$
and 0.12
$\text{m}^{-{1}}$
, respectively. The temperature sensitivity of the KDBMD is 0.043 nm/°C. Furthermore, the performance of the KDBMD for knee-joint movement detection is compared with that of an inclinometer. The experimental results indicate that the data acquired from the KDBMD and the photoelectric encoder are consistent with regard to the accuracy and responsiveness. The proposed device, which is versatile and inexpensive to fabricate and can achieve temperature compensation, has potential for applications in gait analysis.