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Detection and sizing of corrosion in pipelines and pressure vessels over large, partially accessible areas is of growing interest in the petrochemical industry. Low-frequency guided wave diffraction tomography is a potentially attractive technique to rapidly evaluate the thickness of large sections of partially accessible structures. Finite element simulations of a 64-element circular array on a plate show that when the scattering mechanism of the object to be reconstructed satisfies the Born approximation, the reconstruction of the thickness is accurate. However, the practical implementation is more challenging because the incident field is not known. This paper describes the baseline subtraction approach commonly used in structural health monitoring applications and proposes a new approach in which the measurement of the incident field is not required when using a circular array of transducers. Experimental results demonstrate that ultimately the scattering from the array of transducers is a major source of error in the tomographic reconstruction, but when there is no scattering from the array of transducers the reconstructions are very similar to the finite element simulations.

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The unique performance features of harmonic drives, such as high gear ratios and high torque capacities in a compact geometry, justify their widespread industrial application. However, harmonic drive can exhibit surprisingly more complex dynamic behavior than conventional gear transmission. In this paper a systematic way to capture and rationalize the dynamic behavior of the harmonic drive systems is developed. Simple and accurate models for compliance, hysteresis, and friction are proposed, and the model parameters are estimated using least-squares approximation for linear and nonlinear regression models. A statistical measure of variation is defined, by which the reliability of the estimated parameter for different operating condition, as well as the accuracy and integrity of the proposed model is quantified. By these means, it is shown that a linear stiffness model best captures the behavior of the system when combined with a good model for hysteresis. Moreover, the frictional losses of harmonic drive are modeled at both low and high velocities. The model performance is assessed by comparing simulations with the experimental results on two different harmonic drives. Finally, the significance of individual components of the nonlinear model is assessed by a parameter sensitivity study using simulations.

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