Acoustic emission sensor system using a chirped fiber-Bragg-grating Fabry–Perot interferometer and smart feedback control

Significance Statement

Acoustic emission is transient elastic waves over a broad frequency range within a material. Of particular interest are AE events in the 100kHz-1MHz range, whose generation is normally associated with irreversible structural changes, for instance, crack initiation and its propagation, and other damage related changes. Therefore, acoustic emission detection becomes a critical non-destructive technique in monitoring structural health and durability. Compared to other electronic sensors, acoustic emission sensors based on fiber-Bragg gratings are desirable for their light weight, superior multiplexing capacity, small size, corrosion resistance, and immunity to electromagnetic forces.

The functioning of acoustic emission sensors is based on detecting the acoustic emission induced spectral movement of the Bragg wavelength of the gratings through a demodulation mechanism. In this intensity based demodulation method, a narrow-linewidth laser is locked to the liner slope of the reflection spectrum. However, the operation of the sensors is in most cases subject to a low frequency strain stemming from temperature changes and structural deformations. The developed strain knocks the laser wavelength out of the linear range of the gratings.

In a recent paper published in Optics Letters researchers led by professor Ming Han from the University of Nebraska-Lincoln developed a fiber-optic acoustic sensor based on a chirped fiber-Bragg Grating interferometer enhanced with laser intensity demodulation and a smart control for acoustic detection under a background strain.

The laser for the proposed acoustic sensor could be locked to one of the notches within its tuning range in order to achieve the acoustic detection. If the spectral notch moved out of the tuning range owing to low-frequency background strain, a nearby notch moved into the tuning range. The laser is, therefore, tuned to that notch and then locked at its spectral slope. This is achieved by a smart feedback control unit, so the acoustic emission is monitored with minimal disruptions. This unlocking and relocking process calls for a smart control system.

The authors bonded the sensor with an aluminum cantilever beam in a bid to apply the large background strain. They attached the free end to a mechanical shaker, which initiated tensile and compressive strain to the sensor. They selected a narrow line-width laser diode because of its fast wavelength tuning.

The smart feedback control, consisting of a microprocessor and a proportional integral controller, provided a signal control to the laser controller. The authors realized that the laser was initially locked to a notch within the tuning range set to a value that corresponded to an injection current of about 70mA. They verified the locking process by a zero error signal. The microprocessor helped the authors to read out the laser power and equally adjust the locking voltage.

The authors observed that when the sensor stretched, the wavelength as well as the injection current shifted to the upper limit of the tunable range. At this time, the neighboring notch shifted into the tuning range. The authors observed that the microprocessor sent high logic signals to the logic input that opened the feedback loop, consequently unlocking the laser.

This study demonstrated the successful operation of a fiber-optic acoustic emission sensor which was based on a chirped fiber-Bragg grating sensing unit and enhanced with a feedback control that enabled acoustic emission detection at low frequency and a large background strain. They adopted a semiconductor-based laser, which had a high-speed tuning capacity through current injection.

The research was supported by the Office of Naval Research (ONR).

Acoustic emission sensor system using a chirped fiber-Bragg-grating Fabry–Perot interferometer and smart feedback control - advances in engineering

About The Author

Dr. Qi Zhang is the post-doc optical design engineer in LI-COR Biosciences. She earned her B. E. in Electronic Information Engineering, B. A. in English from Tianjin University (2006), and M.S. in Optics from Nankai University (2009) in Tianjin, China. Later she continued her Ph.D. study in Electrical and Computer Engineering in University of Nebraska-Lincoln (UNL), USA, and obtained her Ph.D. degree in Dec. 2015. She was the postdoctoral associate working in Dr. Ming Han’s lab in UNL from 2016-2017.

Her Ph.D. study focused on fiber-optic phase-shifted Bragg grating sensor design. Since the last year of her Ph.D. study, she embarked on fiber-optic adaptive acoustic emission sensor for structural health monitoring. Dr. Zhang currently involved in the optical design of ultra-high-sensitivity IR trace gas analyzer in LI-COR Biosciences.

About The Author

Dr. Ming Han received his PhD degree in Electrical Engineering from Virginia Tech in 2006 and his MS and BS degrees in Electronic Engineering from Tsinghua University in 2000 and 1998, respectively. After working as a research associate at Virginia Tech for two years, he joined the University of Nebraska-Lincoln (UNL) as an assistant professor in the Department of Electrical and Computer Engineering in 2008, and then was promoted to associate professor in 2014. He is the Associate Chair of the department since 2015, and was selected as a William E. Brooks Engineering Leadership Fellow in 2014.

Dr. Han is a senior member of SPIE, a member of OSA, IEEE, and ISA, and serves as an associate editor for “Optical Engineering”. He has received several awards including the Alan Berman Research Publication Award from NRL in 2015, the Edgerton Innovation Award and the Faculty Research & Creative Activity Award from UNL College of Engineering, both in 2014, and the Chief of Naval Research (CNR) Challenge Award from ONR in 2011.

His current research mainly focuses on fiber-optic sensors and sensor instrumentation for structural health monitoring and applications in extreme environment.

Reference

Qi Zhang, Yupeng Zhu, Xiangyu Luo, Guigen Liu, and Ming Han. Acoustic emission sensor system using a chirped fiber-Bragg-grating Fabry–Perot interferometer and smart feedback control. Optics Letters, volume 42, No. 3 / 2017 /, pages 631-634.

Department of Electrical and Computer Engineering, University of Nebraska-Lincoln, Lincoln, Nebraska 68588-0511, USA.

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