Low-frequency Sensor Research Articles

Health monitoring of reinforced concrete slabs subjected to earthquake-type dynamic loading via measurement and analysis of acoustic emission signals

This paper discusses the applicability of Acoustic Emission (AE) to assess the damage in reinforced concrete (RC) structures subjected to complex dynamic loadings such as those induced by earthquakes. The AE signals recorded during this type of event can be complicated due to the arbitrary and random nature of seismicity and the fact that the signals are highly contaminated by many spurious sources of noise. This paper demonstrates that by properly filtering the AE signals, a very good correlation can be found between AE and damage on the RC structure. The basic experimental data used for this research are the results of fourteen seismic simulations conducted with a shake table on an RC slab supported on four steel columns. The AE signals were recorded by several low-frequency piezoelectric sensors located on the bottom surface of the slab. The evolution of damage under increasing values of peak acceleration applied to the shake table was monitored in terms of AE and dissipated plastic strain energy. A strong correlation was found between the energy dissipated by the concrete through plastic deformations and the AE energy calculated after properly filtering the signals. For this reason, a procedure is proposed to analyze the AE measured in a RC structure during a seismic event so that it can be used for damage assessment.

Enhanced magnetoimpedance and field sensitivity in microstructure controlled FeSiCuNbB ribbons

Fe73.5Si13.5Cu1Nb3B9 and Fe77.2Si11.2Cu0.8Nb3.3B7.5 nanocomposite materials consisting of nanocrystalline phase in an amorphous matrix were obtained by heat-treatment of their precursor amorphous ribbons. The influence of structural modifications induced during the heat-treatment on soft magnetic properties and magnetoimpedance (MI) effect have been studied. The structural investigations on both these ribbons revealed the presence of two phases, fine grained Fe3Si phase and a residual amorphous phase on heat-treatment. The maximum MI ratio obtained in the present study is 95% at f = 4 MHz, for the optimized heat-treated Fe77.2Si11.2Cu0.8Nb3.3B7.5 ribbon. This is ascribed to the increase in magnetic permeability and decrease in coercive force and intrinsic resistivity. Moreover, a maximum magnetic field sensitivity (ξ) of 8.3%/Oe at f = 2.5 MHz is obtained, for the optimized nanocrystalline Fe73.5Si13.5Cu1Nb3B9 ribbon. This suggests that tailoring of the nanocrystalline microstructures induced by optimum heat-treatment conditions can result in obtaining excellent combinations of the magnetic permeability and resistivity. Our results indicate that these Fe-based nanocrystalline materials can be ideally used for low magnetic field and high frequency sensor applications.

Designing of RFID-Based Sensor Solution for Packaging Surveillance Applications

In this work, a two-chip battery assisted Radio Frequency Identification (RFID) based sensor platform is presented. The radio frequency communication interface is based on the EPC Gen 2 standard. A laboratory setup of the platform has been shown and characterized for a moisture content sensor application. The laboratory setup of the sensor platform has a reading range of 3.4 meters which is in comparison to commercial available Gen 2 tags. The laboratory platform has an average power consumption of 2.1 μW operating at 3 V, which together with a printed battery gives an estimated lifetime for data logging of several years. The proposed RFID platform provides a tradeoff between, communication performance, compatibility with international standards, and flexibility in on-package customization including type and number of sensors. The proposed architecture separates the high-performance communication circuit and the low-frequency sensor interface logic. In the future, the sensor interface maybe integrated using printed logics to further enhance the flexibility and low-cost customization features of the architecture.

Magnetoelectric interactions in ferromagnetic-piezoelectric layered structures: Phenomena and devices

Layered magnetostrictive-piezoelectric structures are multifunctional due to their dual-responsiveness to mechanical and electromagnetic forces. Here, we discuss studies of magnetoelectric (ME) interactions in ferrite-lead zirconate titanate (PZT) and terfenol-PZT material couples. Key findings include: (1) the observation of a giant low-frequency ME effect in the layered systems; (2) data analysis based on our model for low frequency ME effects; (3) observation and theory of enhanced ME coupling at the electromechanical resonance (EMR); and (4) theory and measurements of microwave ME effects, at the ferromagnetic resonance of ferrites. The layered structures are potential candidates for sensors, gyrators and microwave devices. Low frequency sensors are feasible with excellent sensitivity to minute magnetic field variations. One could also realize composite based ferromagnetic resonance devices, such as resonators, filters and phase shifters with electric field tunability for use at 1–70 GHz.

Robust pi controller design satisfying sensitivity and uncertainty specifications

This paper presents a control design method for determining proportional-integral-type controllers satisfying specifications on gain margin, phase margin, and an upper bound on the (complementary) sensitivity for a finite set of plants. The approach can be applied to plants that are stable or unstable, plants given by a model or measured data, and plants of any order, including plants with delays. The algorithm is efficient and fast, and as such can be used in near real-time to determine controller parameters (for online modification of the plant model including its uncertainty and/or the specifications). The method gives an optimal controller for a practical definition of optimality. Furthermore, it enables the graphical portrayal of design tradeoffs in a single plot, highlighting the effects of the gain margin, complementary sensitivity bound, low frequency sensitivity and high frequency sensor noise amplification.

Optimal design of immersed SAW devices for high- and low-frequency sensor applications

In many sensor applications surface acoustic wave (SAW) devices may have to be immersed in a liquid. Attenuation of the surface acoustic wave modes and leakage of energy into the liquid have to be considered in designing appropriate devices. To design an immersible SAW device, the attenuation of the SAW must be decreased to get the highest SAW generation efficiency. Through numerical simulation, the optimal geometry of underwater SAW devices is studied. Parameters such as optimal piezoelectric crystal cut, SAW propagation direction and non-dimensional wavenumber (ka), are determined for all the modes of surface wave propagation to get the maximum SAW excitation efficiency, the minimum attenuation in propagation and pure mode propagation. In this paper we have studied SAW propagation in a thin piezoelectric film on a semi-infinite substrate which is exposed to a liquid. The assumption of finite thickness for the piezoelectric film allows us to consider both low- (< 50 MHz) and high-frequency devices.

Three-channel low-frequency fiber-optic voltage sensor

We demonstrate a frequency-multiplexed three-channel fiber-optic interferometric dc and low-frequency voltage sensor based on the electrostrictive effect in lead magnesium niobate transducers. The sensor relies on the nonlinear transduction mechanism in electrostrictive materials for frequency-division multiplexing by use of a single interferometer and a single unmodulated laser. To our knowledge this is the first direct measurement of three low-frequency voltage signals with a single fiber-optic interferometer. Demonstrated cross talk between channels is less than −75 dB. The cross talk is shown to be caused by the nonlinear output of the interferometer at large applied strains.

Acoustic diagnoses of GIS; field experience and development of expert system

The authors report on the practical aspects of acoustic diagnoses of gas insulated substations (GIS). Thirty-eight tests have been carried out during factory testing, commissioning testing, and service. The number of flaws revealed in the tests varies considerably. The flaw frequency in a GIS probably reflects the care taken during assembly. The results from the field tests reported verify that the acoustic emission method is suited for identifying and locating flaws in GIS. Using low-frequency (20-80 kHz) sensors, the method is sensitive to the most important flaws in the insulation system. The test procedures as well as criteria for source recognition, localization, and evaluation are discussed. An expert system assisting operators in diagnosing flaws in GIS is described. The system creates a database containing a representation of the GIS and the observations and conclusions from the acoustic test. >

Fiber-optic dc and low-frequency electric-field sensor

We demonstrate an interferometric fiber-optic dc and low-frequency electric-field sensor based on the electrostrictive effect. The quadratic relationship between the applied electric field E and the induced strain e in electrostrictive ceramics (e.g., Ba:PZT) allows for the detection of low-frequency signals as sidebands of a high-frequency carrier. A preliminary investigation of the sensor has yielded a minimum detectable E field of 0.35 (V/m)/ Hz at 5 Hz for a sensor with 3 cm of fiber.

On acoustic sensor sensitivity

Acoustic sensor sensitivity expressed as frequency change per unit of measurand can result in the erroneous conclusion that higher-frequency sensors are superior to lower-frequency ones. It is argued that, when compared on the bases of reproducibility and resolution capability, good low-frequency sensors are superior to good high-frequency ones.

Use of predetection mixing of intensity modulated optical beams

Predetection mixing of the intensity modulation on optical beams is shown to have performance advantages in certain laser radar applications. The method allows detection of signals with high frequency intensity modulation using a low frequency sensor. The method can lead to a signal-to-noise ratio improvement compared to a system using a high frequency sensor. The technique is demonstrated by using a sensor with a 5-kHz cutoff frequency to receive a laser beam intensity modulated at 4 MHz.

Acoustic positioning for an array of freely drifting infrasonic sensors

Initial testing of the prototype element of a freely drifting infrasonic sensor array is described. The intent of this measurement system is to gather wide aperture data sets which will be used both to characterize ambient noise in the region 1-10 Hz and to assess the gains possible from beam forming utilizing a collection of very low frequency (VLF) sensors. Coherent processing (beam forming) of the infrasonic sensor data is made possible by relative position measurements derived from mutual acoustic interrogation of the elements at a higher frequency. Surface echo data from a recent sea test of the prototype buoy are used to illustrate the type of pulse processing which will be implemented as a first step in the localization procedure.

Space applications of superconductivity

This is the first of a seven part series on the potential applications of superconductivity in space. The series considers six classes of superconducting instrumentation for space applications; high field magnets, low frequency sensors, digital electronics, microwave and infrared detectors, instruments for gravitational studies, and high-O cavities. This introductory article provides background information for the study and briefly summarizes the contents of the articles which follow. This series is derived from a study supported by NASA and, as such, contains some speculation about ultimate instrument performance levels and future space missions.