Development of a High-Precision and Large-Range FBG-Based Sensor Inspired by a Crank-Slider Mechanism for Wearable Measurement of Human Knee Joint Angles

Abstract

This article proposes a fiber Bragg grating (FBG) based angle sensor with an extensive measurement range and high precision for human knee joint measurement. The proposed sensor mainly comprises an angle-linear displacement conversion cam, a crank-slider mechanism-inspired conversion flexure, an optical fiber embedded with an FBG element, and a sensor package. The cam transforms the wide-range knee angle input into vertical linear displacement output. The conversion flexure further converts such vertical displacement into a reduced horizontal displacement/stretching applied to the optical fiber with a motion scale ratio of 6:1. The flexure design features a symmetrical structure to improve stability and depress hysteresis. The fiber is suspended on the flexure’s output beams with a two-point pasting configuration. Both theory analysis and finite element method (FEM)-based simulations revealed the linear relationship between the input angle and the fiber strain. Static and dynamic experiments have verified the performance of the proposed sensor, demonstrating a sensitivity of 62.03 pm/° with a small linearity error of 1.36% within [0, 140°]. The root mean square errors (RMSE) were 0.72° and 0.84° for angle velocities of 80°/s and 350°/s, respectively. Wearable experiments during sitting and walking have been performed to validate the effectiveness of the proposed sensor.