J.L. San Emeterio

Influence of thresholding procedures in ultrasonic grain noise reduction using wavelets

Wavelet transform based techniques are used for signal-to-noise ratio (SNR) enhancement in ultrasonic non-destructive testing and evaluation of strong sound scattering materials. The overall denoising performance of a wavelet signal processor is conditioned by several processing parameters, including the type of wavelet, thresholding method, and threshold selection rules. Different thresholding procedures and threshold selection rules are analysed in this paper using the discrete wavelet transform and decomposition level dependent thresholds. Global performance is evaluated by means of the SNR enhancement using synthetic grain noise registers with an incrusted flaw signal, with different values of the input SNR, and experimental ultrasonic traces acquired from a carbon fibre reinforced plastic composite block.

Improvement in transient piezoelectric responses of NDE transceivers using selective damping and tuning networks

NDE ultrasonic applications for quality control purposes, based on piezoelectric devices working as pulsed ultrasonic probes, usually include some tuning circuit either across the pulser output connectors or close to the piezoelectric probe electrodes. Classic criteria to determine tuning parameters in ultrasonic transducers do not perform very well in transient regime under the typical electrical conditions and peculiar output impedances of the required high-voltage pulsers. In most practical situations, the tuning values are manually adjusted in specific circuits for each NDE case, testing each channel of a multiprobe inspection system to find the best sensitivity. In this paper, the positive influences of certain selective damping and tuning networks on the time and frequency behaviors of NDE piezoelectric transceivers are analyzed in detail. Different waveforms and spectra of received ultrasonic signals are comparatively evaluated for different real tuning conditions, after passing through two ultrasonic processes linked in cascade: a transmitter of industrial type and a broad-band ultrasonic receiver. Piezoelectric transducer impedances, transduction transfer functions, and HV output spikes from a piezoelectrically loaded NDE pulser, are computed, to identify separately the influence of each stage. In order to improve amplitude and axial resolution in the received transient responses, relative contributions from emission and reception tuning networks are individually evaluated for a particular NDE two-stage piezoelectric channel. Also shown are the final experimental results relative to the ultrasonic test pulse and detected in a through-transmission NDE configuration, gradually improved by using distinct tuning levels.

Dependence of pulser driving responses on electrical and motional characteristics of NDE ultrasonic probes

Acoustic performance in ultrasonic transmitters can be improved by means of a suitable electrical driving response and matching/tuning networks. It is important to predict this electrical response, but doing so is not easy because it departs notably from the nominal pattern with the loading probes. In practice, the analysis of HV pulser spikes in NDE applications requires fairly complex models in the transient regime and, in addition, non-linear problems could arise, especially in the case of tuned transmitters. In this paper, the most relevant influences of loading characteristics of NDT ultrasonic probes on the pulser electrical driving responses are evaluated in time and frequency domains. Conventional pulse generators and typical NDE pulsers are considered. Driving responses are analysed across commercial ultrasonic probes and, alternatively, across similar purely electrical loads. Distinct influences on pulser responses from electrical and motional sections of the probes are identified. All these aspects are studied on the basis of experimental and computer results.

Electrical matching effects on the piezoelectric transduction performance of a through-transmission pulsed process

Abstract Many applications of non destructive testing employ piezoelectric transducers operated in through-transmission mode, to perform C-scan ultrasonic imaging. Normally a high efficiency and a pulsed regime for the ultrasonic process are required. The enlargement of both the amplitude and bandwidth in the testing signals permits to improve the resolution and sensitivity of the imaging systems. This can be done by means of external electric circuits. In this paper, the effects of some electrical matching components over the global emitter-receiver piezoelectric transduction performance, are analyzed under transient operating conditions, considering the signal waveforms and frequency spectra. In particular, a broadband configuration with inductive tuning circuits individualized for both transmitter and receiver subsystems is evaluated, by means of equivalent circuit analysis, computer simulation, and experimental assessment.

FOCUSED HIGH-POWER ULTRASONIC TRANSDUCER WITH STEPPED-PLATE RADIATOR FOR INDUSTRIAL APPLICATION IN GASES

Flexural axisymmetric vibrating plates with an extensive surface area and stepped profile have shown to be very useful high-power ultrasonic radiators. The stepped profile of these plates provided, together with a rather uniform distribution of displacement, a highly-directional radiation. In addition, if the profile of the non-stepped face is modified, by shifting adequately the different internodal zones, a remarkable focusing effect can be obtained. This paper deals with the characteristics and performance of a high-power transducer constructed with this new stepped-focused radiator vibrating with seven nodal circles. A similar transducer, but with the previously studied stepped-directional radiator, is also presented.

IGH axisymmetric modes of vibration of stepped circular plates

Flexurally vibrating stepped plates of extensive area have been shown to be very effective directional ultrasonic radiators [1]. Thus, there is a need to study these vibrating structural elements. A number of papers on this subject have been published in the past few years. Nevertheless, almost all of them have been restricted to the study of plates with one step and/or to the first vibration mode. This paper deals with a study of axisymmetric vibrating stepped plates without any restriction either in the number of steps or in the order of the vibration mode. The only condition, which has been imposed due to the design of the radiators, is that the number of step discontinuities has to be equal to the number of nodal circles. The analytical approach is based on the Rayleigh method. Iterative numerical procedures have been developed to find the solution of the derived set of equations. Experimental tests on plates vibrating with up to seven nodal circles have been performed.

Some non-linear aspects of the electronic stages in time-domain modelling of NDE pulse-echo ultrasonic systems.

Electronics interfacing with NDE probes frequently include non-linear switching devices and semiconductor networks, which influence the excitation pulses and detected echo signals. Classical approaches to modelling a pulse-echo process use ideal assumptions for the electronics and do not consider these influences on the echoes, which can be very relevant in HF cases. This paper proposes new ways to consider these non-linear effects in a time-domain simulation process, extending previous approaches by including new elements in the modelling. Specific electrical models covering the pulse-echo process are applied in the evaluation of echo-graphic signals. They include semiconductor devices and other non-ideal elements. From these models, and using SPICE as a simulation tool, strong non-linear effects on pulsed responses, computed for both E/R stages of typical NDE transceivers, are analysed.

Determination of electromechanical coupling factors of low Q piezoelectric resonators operating in stiffened modes

The electromechanical coupling coefficient is usually determined from the relative spacing of the frequencies of resonance and antiresonance. The conventional formula is derived from equations describing the electrical behavior of an ideal piezoelectric resonator in the absence of losses. In this paper, the influence of the intrinsic material losses on the shift of the resonance/antiresonance frequencies, and therefore on the accuracy of the standard formula to determine k, is analyzed. The exact one-dimensional model of the piezoelectric resonator vibrating in a pure stiffened mode, with a rigorous account of the internal mechanical losses, has been taken as a reference, instead of the frequently used lumped approximate equivalent circuit (Butterworth-Van Dyke). It is shown that the coupling coefficient determined from the frequencies f/sub s/ and f/sub p/ is less than the intrinsic coupling coefficient, and that the error increases for highly attenuating materials with weak electromechanical coupling. The error due to the effect of attenuation, which increases with the decrease of the product Q/sub m/k of the resonator intrinsic parameters, has been systematically evaluated and plotted for 0.5<Q/sub m/k<15. The accuracy and the field of application of the conventional formula to determine the coupling factor by a non-iterative method is therefore established.

An acoustic transducer system for long-distance ranging applications in air

Abstract Transducers for long-distance pulse-echo ranging applications should combine high efficiency and directivity with a relatively wide bandwidth. Since the air has a very low acoustic impedance, the efficient transmission of sound from a solid transducer is one of the major problems. A means to improve the impedance matching between the source and the propagation medium is to increase the radiation impedance by using extensive vibrating plates as radiators. Nevertheless, these extensive radiators will vibrate at a flexural mode resonance, producing a poor directivity due to phase cancellation. In this paper we present a transducer system, based on a stepped-plate radiator, which is able to generate efficiently high-directional radiation in air. To be used for pulse-echo ranging applications, the stepped-plate radiator has been specially adapted in order to increase the bandwidth of the transducer system. In particular, three main features have been introduced: the development of very thin stepped plates; the addition of a matching layer; and the backing of the plate with a damping material. Electroacoustical and pulse-echo characterizations of the transducer system have shown its good performance for use as a level or distance sensor, specially for long-range applications (10–100 m).

Influence of Internal Mechanical Losses on the Fundamental Frequencies of Thickness Extensional Piezoelectric Resonators

The six different fundamental frequencies fn, fs, fr and fa, fp, fm, defined according to the IEEE Standard on Piezoelectricity (Std 176, 1987), are evaluated for different values of the electromechanical coupling coefficient kt and the mechanical loss tangent (tan δm). The exact one-dimensional expression for the electrical input impedance of thickness extensional piezoelectric resonators is taken as a reference. Mechanical losses are introduced by means of a complex elastic constant (cD 33)* = (cD 33) (1 + j tan δm). The shifts of the six characteristic frequencies caused by internal mechanical losses are investigated around the fundamental resonance. The experimental values measured for a 1–3 piezocomposite plate are compared with the theoretical values.