Browsing by Author "Xu, B."
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Item Open Access FEM-based analysis on sensing out-of-plane displacements of low-order Lamb wave modes by CMUTs(AIP Publishing LLC, 2022-11-23) Lu, W.; Zhang, S.; Wang, R.; Yang, Y.; Zhang, G.; Zhang, W.; Xu, B.; Yılmaz, MehmetIt is well known that acoustic emission (AE) signals, generated by external impacts or damages such as crack initiation, mainly propagate in the form of Lamb waves in plate-like structures. In this work, MEMS-based resonant capacitive micro-machined ultrasonic transducers (CMUTs), which are designed for sensing out-of-plane displacements, have been verified by finite element method (FEM) modeling and theoretical analysis for their feasibility of detecting low-order Lamb waves (A0 and S0). First, combining the propagation theory of Lamb waves and the “spring-mass-damper” model of CMUTs, the out-of-plane sensing mechanism has been explained, together with the analytical expression of sensitivity. Then, simulations based on FEM have been carried out to show that the designed CMUTs are sensitive to out-of-plane displacements, while extremely insensitive to in-plane displacements. Meanwhile, a transient analysis has found the potential abilities of CMUTs for sensing A0 and S0 lamb waves. Besides, the sensing characteristics of CMUTs have also been investigated, including the influence of squeezed-film damping, the amplitude of the input signal, the cell number, and cell space. Finally, the ball drop impact is simulated to show the potential of identifying the location of the AE source by CMUTs. Our studies reveal the out-of-plane sensing behaviors of CMUTs for Lamb waves and may have the potential in promoting the miniaturization and integration of AE sensors.Item Open Access High efficiency 1.9 Kw single diode laser bar epitaxially stacked with a tunnel junction(IEEE, 2021-04-21) Zhao, Y.; Wang, Z.; Demir, Abdullah; Yang, G.; Ma, S.; Xu, B.; Sun, C.; Li, B.; Qiu, B.We report on the development of a 940-nm diode laser bar based on epitaxially stacked active regions by employing a tunnel junction structure. The tunnel junction and the device parameters were systematically optimized to achieve high output and power conversion efficiency. A record quasi-continuous wave (QCW) peak power of 1.91 kW at 25 °C was demonstrated from a 1-cm wide bar with a 2-mm cavity length at 1 kA drive current (200 μs pulse width and 10 Hz repetition rate). Below the onset of the thermal rollover, the slope efficiency was as high as 2.23 W/A. The maximum power conversion efficiency of 61.1% at 25 °C was measured at 300 A. Reducing the heatsink temperature to 15 °C led to a marginal increase in the peak power to 1.95 kW.Item Open Access In-plane-sensing analysis of comb-like capacitive micro-machined ultrasonic transducers (cmuts) using analytical small-signal model and fem(Institute of Electrical and Electronics Engineers, 2023-04-18) Zhang, S.; Lu, W.; Yang, Y.; Wang, R.; Zhang, G.; Xu, B.; Yılmaz, Metin; Zhang, W.In this work, capacitive micro-machined ultrasonic transducers (CMUTs) were developed into comb-like shapes to make these comb-like shaped structures work for sensing in-plane vibrations of ultrasonic guided waves. On this basis, an analytical small-signal model, which is mainly a combination of the forced vibration theory and the simplified parallel-plate capacitor model, was proposed to satisfy the requirements of theoretical design. Through the proposed model, the in-plane-sensing behaviors of a comb-like CMUT cell can be predicted, including vibrating velocity, output current, and sensitivity. Compared with the results calculated from the finite element method (FEM) simulation, it was found that the static state and the frequency-domain results of the analytical small-signal model agree well with those of FEM simulations if the used first natural frequencies of these two methods are identical. Considering the fringing field capacitance could further improve the accuracy of the analytical small-signal model. At last, influences of some external parameters, i.e., dc bias voltage, air damping, and input in-plane displacement, on the sensitivity of a comb-like CMUT cell were discussed by the analytical small-signal model and FEM simulation. Relevant results reveal the way to design a comb-like CMUT and indicate the conditions when the analytical small-signal model is accurate. Our work develops the theory on the in-plane-sensing comb-like CMUT and is expected to be combined with the theory on the previous out-of-plane-sensing CMUT to realize 3-D-CMUT for sensing 3-D guided waves.Item Open Access Shortest unique substring query revisited(Springer, Cham, 2014) Ileri, Atalay Mert; Külekci, M.O.; Xu, B.We revisit the problem of finding shortest unique substring (SUS) proposed recently by Pei et al. (ICDE'13). We propose an optimal O(n) time and space algorithm that can find an SUS for every location of a string of size n and thus significantly improve their O(n 2) time complexity. Our method also supports finding all the SUSes covering every location, whereas theirs can find only one SUS for every location. Further, our solution is simpler and easier to implement and can also be more space efficient in practice, since we only use the inverse suffix array and the longest common prefix array of the string, while their algorithm uses the suffix tree of the string and other auxiliary data structures. Our theoretical results are validated by an empirical study that shows our method is much faster and more space-saving. © 2014 Springer International Publishing Switzerland.Item Open Access Theoretical and simulation studies on designing a phase-reversal-based broadband CMUT with flat passband and improved noise rejections for SHM(Institute of Electrical and Electronics Engineers, 2022-11-15) Lu, W.; Zhang, S.; Wang, R.; Xu, B.; Yılmaz, Mehmet; Zhang, W.In the past two decades, capacitive micromachined ultrasonic transducers (CMUTs) have been greatly explored for applications in structural health monitoring (SHM); however, relevant theories about their broadband sense have not been investigated systematically. Therefore, broadband CMUTs have been specifically developed from the aspects of theory and simulation in this work. Based on these theoretical developments, we propose a new design of phase-reversal-based CMUT, which has a flat passband for broadband sensing and two stopbands at both sides for improved noise rejections. First, the expressions for the evaluation of the total output current and the sensitivity of a CMUT constituted of multiple cells are deduced from the theoretical spring–mass–damping model. Then, theoretical and simulation analysis on a CMUT combined with two different cells have revealed that reversing the current phase of one of these two cells can produce significant stopbands for rejecting the low- and high-frequency noises, which are useful not only for a CMUT in coarse vacuum (low pressure) but also a CMUT in the air (atmospheric pressure). Especially, for a CMUT in a coarse vacuum, this design can effectively build a passband among the resonant frequencies of each cell instead of compensating each other to zero. Finally, the genetic algorithm is adopted to design a broadband CMUT with a given passband in air, the results of which are verified by the frequency- and time-domain simulations concurrently. Our research work may produce a theoretical way for the design of broadband CMUTs with noise rejections.