Multicoil reader for wireless measurements of strain

This thesis presents a new multicoil reader to interrogate and locate an embeddable wireless sensor through electromagnetic induction to detect small strain. The passive sensor employs a piezoelectric crystal oscillator with a resonant frequency of ~1.8 MHz to detect strain and a pair of planar coils to communicate wirelessly through inductive coupling. Operating at these low frequencies limits the effective distance of wireless communication to ~2 inches due to the quasi-static electromagnetic field generated by the reader coil. The accuracy of the passive strain sensor decreases when there exists an offset in the relative position between the pair of inductively coupled coils. This work presents a new multicoil reader capable of locating the sensor coil to remove measurement error associated with misaligned coils. The multicoil reader consists of four rectangular reader coils that independently induce electric potentials/currents into the sensing coil and measure the frequency response. From the frequency response, the multicoil reader calculates the mutual inductance between each pair of inductively coupled coils. To locate the sensor coil, this work employs high-fidelity multiphysics simulations to solve the “forward problem” of determining the mutual inductance between a pair of inductively coupled coils as a function of relative position. From the multiphysics simulations and the kinematics of the multicoil reader, this research creates backward conversion libraries that correlate the location of the sensor coil as a function of the simulated mutual inductances in each coil of the multicoil reader. Using MATLAB scripts, the multicoil reader imports the measured mutual inductances, assembles the backward conversion libraries of simulated data, and searches libraries using Simulink’s n-D lookup tables that match the measured mutual inductances to the simulated and the corresponding sensor coil location.M.S.Includes bibliographical reference