Development of a Hybrid Force-Displacement Sensor Based on Fiber Bragg Grating for Radial Artery Pulse Waveform Measurement

This paper proposes a novel Fiber Bragg Grating (FBG)-based hybrid force and displacement sensor with a high sensitivity to achieve accurate measurement of radial artery pulse waveforms. The proposed sensor prototype mainly consists of a force-sensitive orthogonal planar spring structure, an FBG optical fiber, and a sensor frame. The side-type orthogonal planar spring flexure has been utilized and optimized to achieve an excellent linear force-displacement relationship along the axial direction. It offers other merits in terms of equivalent functions as a parallel mechanism configuration, compactness, and convenient assembly. The employed optical fiber has been tightly stretched to form a suspended configuration vertical to the flexure's central line. This two-point pasting configuration and the use of the transverse movement property of the suspended fiber can achieve improved sensitivity and resolution, and avoid shortcomings associated with the traditional and direct pasting for the whole FBG element. The theoretical model that corresponds to the sensing principle has been derived. The simulation based on finite element method (FEM) has been adopted to conduct design optimization to determine detailed structural parameters and achieve enhanced sensitivity, as well as performance investigation. The optimized prototype has demonstrated an excellent force resolution of 0.47mN with a small linearity error of 0.98% within the sensing range [0.9N, 2.7N] and a high displacement sensitivity of $0.103\mu \text{m}$ with a linearity error of 4.08% within the sensing range [0.3mm, 0.7mm], and the R ² values are determined as 99.99% and 99.30%, respectively. The experiments for artery pulse waveform measurement have been performed to validate the effectiveness of the proposed sensor design.