A. Bulletti

Surface Resistivity Characterization of New Printed Circuit Board Materials for Use in Spacecraft Electronics

New dielectric materials have been introduced for printed circuit board applications, such as Thermount and polyimide with the aim to match the requirements for high speed and high density of electronic devices that are planned for new spacecraft electronic boards. Before these newer substrate can fully replace the well-known space-approved material epoxy FR-4, it is necessary to investigate more deeply their electrical and mechanical properties. The scope of this study is to report quantitative characterization of the surface resistivity for the different material samples under various testing conditions that include relative humidity, temperature, solder flux contamination, and corona discharge. The surface resistivity results are reported for sets of samples measured under a combination of testing conditions.

An Integrated Acousto/Ultrasonic Structural Health Monitoring System for Composite Pressure Vessels

This paper describes the implementation of a structural health monitoring (SHM) method for mechanical components and structures in composite materials with a focus on carbon-fiber-overwrapped pressure vessels (COPVs) used in the aerospace industry. Two flex arrays of polyvinylidene fluoride (PVDF) interdigital transducers have been designed, realized, and mounted on the COPV to generate guided Lamb waves (mode A0) for damage assessment. We developed a custom electronic instrument capable of performing two functions using the same transducers: passive-mode detection of impacts and active-mode damage assessment using Lamb waves. The impact detection is based on an accurate evaluation of the time of arrival and was successfully tested with low-velocity impacts (7 and 30 J). Damage detection and progression is based on the calculation of a damage index matrix which compares a set of signals acquired from the transducers with a baseline. This paper also investigates the advantage of tuning the active-mode frequency to obtain the maximum transducer response in the presence of structural variations of the specimen, and therefore, the highest sensitivity to damage.

Acoustoseismic Method for Buried-Object Detection by Means of Surface-Acceleration Measurements and Audio Facilities

An experimental setup for acoustoseismic detection of shallow buried objects is presented. The seismic (Rayleigh) waves were generated by exciting an acoustic airborne source by a series of sine-wave bursts, which were designed to cover a frequency range (100-1000 Hz) large enough to distinguish the vibrational characteristics of buried compliant objects. The signals were recorded by means of contact-acceleration microelectromechanical-system sensors moved above different buried objects (compliant and rigid). Signal acquisitions on only sandy soil revealed the natural variability of the outdoor test bed. This variability of soil parameters pointed out the difficulties of buried-object detection based on the amplitude thresholding of the signal spectrum. For this aim, the signals were processed in both time and frequency domains. An audio-channel output was devised to avail of the human hearing apparatus in distinguishing the buried objects according to spatial variations of the acceleration signals obtained by scanning the soil surface.

Silicon micromachined accelerometers for the detection of compliant anti-personnel landmines

Acoustic methods have been recently investigated for the detection of shallow landmines. Some plastic landmines have a compliant case which can made to vibrate by an airborne excitation like a loudspeaker. Our study is based on the possibility to detect landmines by contact or non-contact sensors like accelerometers or phonometers. Phonometers can provide sufficient seismic sensitivity but their response is influenced by direct acoustic wave coupling from the driving source. On the contrary accelerometers are much less influenced by the direct acoustic wave coupling and they have high sensitivity to acquire soil surface vibrations. In our experiments we can measure typical accelerations in order of 2 m/s2 with sensitivity of 800 mV/g. The acceleration signal elaboration and visualization developed in this work demonstrates the suitability of these sensors for acoustic landmine detection study and allows a fast analysis to evaluate the presence of a buried landmine.

Theoretical and experimental analysis of an equivalent circuit model for the investigation of shallow landmines with acoustic methods

Acoustic methods have been recently investigated for the detection of shallow landmines. Some plastic landmines have a flexible case which can made to vibrate by an airborne excitation like a loudspeaker. The soil-mine system shows a resonant behavior which is used as a signature to discriminate from other rigid objects. The mechanical resonance can be detected at the soil surface by a remote sensing systems like a laser interferometer. An equivalent physical model of the mine-soil system has been investigated having the known physical characteristics of mine simulants. The authors designed and built a test-object with known mechanical characteristics (mass, elasticity, damping factor). The model has been characterized in laboratory and the results compared with the classic mass-spring loss oscillator described by Voigt. The vibrations at the soil surface have been measured in various positions with a micro machined accelerometer. The results of the simulations for the acceleration of the soil-mine system agree well with the experiment. The calibrated mine model is useful to investigate the variation of the resonance frequency for various buried depths and to compare the results for different soils in different environmental conditions.

Piezopolymer Interdigital Transducers for a Structural Health Monitoring System

During the last 10 years, the structural health monitoring (SHM) technique based on Lamb waves, involving intelligent sensors spread on the structure like biological receptors on the human body, achieved good and promising results showing broad-based economic benefits. There are a few who demonstrated SHM systems based on the use of a network of piezoceramic sensors applied directly to the structure for monitoring and surveying, controlled by a dedicated control unit, the health of the structure itself (detection of cracks, delaminations, disbonds, and corrosion to monitoring the mechanical impacts on metal and composite structures, etc.). The work describes a new approach for the development of this SHM system based on flexible piezopolymer transducers (PPTs) which have some advantages over the piezoceramic thin disk transducers in space applications.

A Single Display for RASCAN 5-frequency 2-polarisation Holographic Radar Scans

The RASCAN holographic radar system has been developed by the Remote Sensing Laboratory of Bauman Moscow Technical University. The present design uses flve frequencies and two polarisations to give 10 distinct images of scan from buried objects. Because of the sinusoidal phase variation of the interference signals, all displays show a complex picture of dark and light phases which vary in a complicated way between difierent frequencies and polarizations. This is a preliminary investigation into the optimal presentation of the 10 images as a single composite image. The objective is to display as much as possible of the information present in the original image. The solution presented here is to sum the absolute values of the background-corrected amplitude over both the flve frequencies and the two polarizations. The method is justifled using an experiment in which nine US pennies, and 9 metal washers, were buried in sand at increasing depths in the range 0 to 56mm. The method is illustrated by example images from the flelds of civil engineering and mine detection.

A Novel Differential Time-of-Arrival Estimation Technique for Impact Localization on Carbon Fiber Laminate Sheets

Composite material structures are commonly used in many industrial sectors (aerospace, automotive, transportation), and can operate in harsh environments where impacts with other parts or debris may cause critical safety and functionality issues. This work presents a method for improving the accuracy of impact position determination using acoustic source triangulation schemes based on the data collected by piezoelectric sensors attached to the structure. A novel approach is used to estimate the Differential Time-of-Arrival (DToA) between the impact response signals collected by a triplet of sensors, overcoming the limitations of classical methods that rely on amplitude thresholds calibrated for a specific sensor type. An experimental evaluation of the proposed technique was performed with specially made circular piezopolymer (PVDF) sensors designed for Structural Health Monitoring (SHM) applications, and compared with commercial piezoelectric SHM sensors of similar dimensions. Test impacts at low energies from 35 mJ to 600 mJ were generated in a laboratory by free-falling metal spheres on a 500 mm × 500 mm × 1.25 mm quasi-isotropic Carbon Fiber Reinforced Polymer (CFRP) laminate plate. From the analysis of many impact signals, the resulting localization error was improved for all types of sensors and, in particular, for the circular PVDF sensor an average error of 20.3 mm and a standard deviation of 8.9 mm was obtained.

A study of acoustic methods for compliant landmines detection by using the surface acceleration parameter

This paper presents the experimental results for an acousto-seismic buried landmine detection method. The excitation source used was a woofer loudspeaker and lightweight silicon MEMS accelerometers were adopted to measure the ground surface acceleration. The experiments were performed in an outdoor unconsohdated sand test-bed with several kinds of compliant and rigid test-objects, all buried at depth of 4cm and placed at distances ranging from 10cm to 50cm. The different responses of the compliant and rigid objects was processed in both time and frequency domains. A novel audio representation of the acquired signals was developed for exploiting the human-hearing capabilities for the identification of buried objects.

Multifunctional Piezopolymer Film Transducer for Structural Health Monitoring Applications

This paper describes the design of a multifunctional transducer for structural health monitoring (SHM) applications that integrates an interdigital piezoelectric transducer, used for Lamb wave generation and reception, a circular piezoelectric sensor, used for the detection and localization of low-velocity impacts, and a resistance temperature detector (RTD) in a single device. The three elements were fabricated on the same metallized piezoelectric polyvinylidene fluoride (PVDF) film using a laser etching process. Characterization of the resulting device proved the advantages of having different transducers on the same device, providing information useful to implement advanced SHM algorithms. Testing of the RTD highlighted some criticalities of the metallized PVDF film that prevented accurate temperature measurements due to spurious behavior under strain.