Fiber Bragg Grating-Based Sensor System for Strain and Angle Assessment in Passive Orthosis

In this letter, we present the development and application of a fiber Bragg grating (FBG) sensor system for the instrumentation of a lower limb passive orthosis for gait assistance. The sensors include FBG strain sensors attached to the orthosis structure for monitoring the strains in the structure during gait. In addition, another FBG is used as angle sensor positioned on the user lumbar for angle monitoring of the trunk in the frontal plane. The sensors were characterized as a function of strain and angle resulting in root-mean-squared errors of 6.15<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\;\mu \epsilon$</tex-math></inline-formula> and 0.30 ° for strain and angle, respectively. Then, in the application tests, the strain sensor demonstrate its feasibility by means of strain estimation within the range of 10–200<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\;\mu \epsilon$</tex-math></inline-formula> as well as the periodic strain pattern following the ground reaction force variation during the stance phase of the gait. Furthermore, the angle measurement during the wearable gait tests indicated a measurement range of −15 ° to 30 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^{\circ }$</tex-math></inline-formula> with an estimated linear velocity of 4.0 km/h, which is the reference one used on the treadmill. Therefore, the proposed sensor system is an integrated solution for in-situ gait analysis, which can extent the functionalities not only of the passive orthosis, but also in active rehabilitation robots.