Fiber Bragg Gratings (FBG) for Force Sensing in Biomechanics Applications

Abstract

Fiber Optic (FO) sensors are robust, cheap, immune to electromagnetic interference (EMI) and compact, resulting in their high usability in harsh environments. Fiber Bragg Grating (FBG) based sensors have emerged as a popular subclass of fiber optic (FO) sensors finding applications in the fields of Structural Health Monitoring (SHM), oil/gas field monitoring mainly due to its strong sensitivity to strain and easy integration into complex systems. Although FBGs have found industrial applications in several fields, Biomechanics, being a field largely depending on measurement of forces, pressure strain etc. remains widely unexplored and there has been little work on developing FBG based force sensors, e.g. a force platform for application in biomechanics labs. FBGs are not directly sensitive to transverse force, but longitudinal strain. Transverse force can induce strain via a transducer, but it would also induce challenges such as birefringence (peak splitting). Practical limits of fibers e.g. strain tolerance, transducer deformation also put constrains in designing such systems. Hence, a quantitative study was needed in this area. In this thesis, we have quantitatively proved that it is possible to develop an FBG-based force platform despite the challenges. We have derived suitable FBG parameters and simulated a mechanical transducer that would enable us to do so. The contribution of this thesis is mainly twofold; first: the quality of the reflected spectra from an FBG was systematically analyzed with respect to FBG length and modulation depth leading us to find optimum ranges for a biomechanical force sensing application. Second: a simplified Finite Element Method (FEM) model of a force platform is developed and a static analysis is performed to estimate its behavior under realistic range of forces. Results from the FEM simulation were used to simulate the characteristic of the reflected spectra from an FBG with parameters derived from the previous step. Distortion due to transverse force causing birefringence was also analyzed.