A high-sensitivity fiber Bragg grating sensor for displacement measurement in structural health monitoring.

Displacement measurement is of great significance to monitor the crack variation and ensure the health of building structures. Aiming at the problems of low sensitivity and high temperature error of fiber Bragg grating (FBG) displacement sensors in displacement monitoring, this paper presents an adjustable cantilever beam displacement sensor with the FBGs as the sensing element. The sensor adds double FBGs on the relative surfaces of the equal-strength cantilever beam, which increases the bending deformation on the FBG of the beam surface to improve the sensitivity and realize the temperature compensation of the sensor. By adding an adjustable external rod structure between a flexible spring and a fixed foot stand, the sensor can regulate the range of initial crack width for different occasions. A theoretical analysis of the displacement sensor is performed, and the simulation analysis and optimization design for the structural parameters of the cantilever beam elastic sensitive element are implemented by adopting SolidWorks and ANSYS software. Finally, a displacement testing platform is constructed to test its performance. Experimental results show that this design has a high sensitivity coefficient of 39.47 pm/mm and a temperature coefficient of 1.04 pm/°C in the range of initial crack width from 0 to 110 mm or from 0 to 130 mm depending on different monitoring situations. Furthermore, good linearity, hysteresis delay, repeatability, and temperature compensation performance have also been demonstrated.

Bandwidth extension of seismometer by using a novel topology leaf spring.

A novel topology leaf spring as a substitute for the traditional spring was designed to expand the effective bandwidth of a seismometer. The finite element analysis showed that the seismometer with the new topology spring has better performance than that with the traditional spring. Compared with the traditional pendulum, the proposed topology structure was featured with lower natural frequency and higher spurious frequency. The bandwidth of the proposed seismometer was enhanced to [2 s, 72 Hz] with an increase of 43% over the traditional seismometer bandwidth of [1 s, 51 Hz] as well as the signal distortion of the seismometer was also reduced with the proposed topology spring.

Improving the magnetic field homogeneity by varying magnetic field structure in a geophone.

The magnetic field structure is a key factor that affects performance of the magneto-electric geophone. In order to enhance the magnetic field homogeneity and magnetic induction intensity of the magnetic field structure, this paper proposes a new magnetic field structure. It consists of two cylindrical permanent magnets: an H-type magnetic boot and an external magnetic yoke. The proposed magnetic field structure can broaden the range of a uniform magnetic field and increase the magnetic field intensity of working air-gap. To confirm the validity of the design, the finite element analysis and real measurement experiments were conducted. The finite element simulations using the ANASYS Electromagnetics Suite 17.2.0 showed that the air-gap magnetic induction intensity is increased and the work space with a uniform magnetic field is broadened. Meanwhile, the output voltage of the coil is increased, and the harmonic distortion rate of output voltage is reduced. According to the real measurement experimental results, compared with the traditional magnetic field structure, the uniform range of the magnetic field is improved 23% in the entire air-gap path, and the magnetic induction intensity enhances 24% over the proposed new magnetic field structure.

Improvement in the accuracy of estimating the time-of-flight in an ultrasonic ranging system using multiple square-root unscented Kalman filters.

The square-root unscented Kalman filter (SRUKF) is applied to identify the shape parameters of an ultrasonic echo envelope. The SRUKF has better stability than the normal unscented Kalman filter (UKF) because the square-root of the error covariance matrix used in the SRUKF guarantees positive semidefiniteness. Considering the effect of the initial state on the convergence speed of filters, the multi-SRUKF is used to estimate the time-of-flight (TOF). Each SRUKF has a different initial state. The result estimated in a limited time with minimum mean square error is finally adopted. Simulation experiments for various couples of shape parameters and signal-to-noise ratios validate the improvement in the TOF accuracy. Real experiments using the echo signals of a SensComp 600 ultrasonic transducer show that the relative means and standard deviations of the TOF error obtained using the multi-SRUKF method are less than 0.2% and 0.15%, respectively.

Improvement of energy efficiency via spectrum optimization of excitation sequence for multichannel simultaneously triggered airborne sonar system.

Both the energy efficiency and correlation characteristics are important in airborne sonar systems to realize multichannel ultrasonic transducers working together. High energy efficiency can increase echo energy and measurement range, and sharp autocorrelation and flat cross correlation can help eliminate cross-talk among multichannel transducers. This paper addresses energy efficiency optimization under the premise that cross-talk between different sonar transducers can be avoided. The nondominated sorting genetic algorithm-II is applied to optimize both the spectrum and correlation characteristics of the excitation sequence. The central idea of the spectrum optimization is to distribute most of the energy of the excitation sequence within the frequency band of the sonar transducer; thus, less energy is filtered out by the transducers. Real experiments show that a sonar system consisting of eight-channel Polaroid 600 series electrostatic transducers excited with 2 ms optimized pulse-position-modulation sequences can work together without cross-talk and can measure distances up to 650 cm with maximal 1% relative error.