Design, Fabrication and Calibration of Compliant, Multi-Axis, Fiber-Optic Force/Torque Sensors for Biomechanical Measurements

Abstract

This thesis presents the design, fabrication and characterization of various prototypes of multi-axis, compliant force and torque sensors based on fiber-optic sensing technology, the novel calibration methodologies and the experimental results. A compliant 3-axis, intensity modulated-based, fiber-optic force sensor that simultaneously measures normal and shear forces was designed, prototyped and successfully calibrated. A nonlinear Hammerstein-Weiner model (NLHW) was able to characterize the linear and nonlinear behaviour of this prototype. The optimized results have shown a reduction of over 40% in the Root Mean Square Errors (RMSE) in comparison with the linear estimation models. For biomechanical applications such as ground reaction force and gait measurements, the sensor must be able to measure the complete degree of freedom of any force or torque applied at a certain point. Therefore, a wearable compliant 6-axis force and torque sensor was developed and prototyped. It combines two different force sensing technologies: the 3-axis fiber-optic based force sensor and a pressure sensor matrix. The sensor was capable of accurately measuring the full ground reaction force and moment in real-time with minimal gait disturbance. To enhance the durability, avoid the necessary multi-stage conditioning circuits and their resulting extra electronic components, a 6-axis force and torque sensor that is fully optical has been developed and characterized. The sensor is cost-effective, lightweight and flexible with a large force and torque measurement range suitable for biomechanics and rehabilitation systems. A novel calibration methodology which splits the calibration procedure into two estimation models that work simultaneously as a single calibration system named Least Squares Decision Trees (LSDT). Using LSDT, the estimation speed increased by 55.17% and the RMSE reduced to 0.53%. To improve sensor portability, further reduce size and eliminate electromagnetic interference effects as well as enhance sensor biocompatibility, a non-conductive, electrically passive, fiber Bragg grating (FBG) based normal and shear force sensing elements were designed, fabricated and calibrated. The sensing elements are small size, lightweight and compliant. The results achieved from the proposed calibration method have revealed an improvement from an R-squared value of 93% to 100% when compared to a data obtained using a linear least squares method.