P.T. Sanz

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.

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.

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).

Definition and measurement of the normalized electrical impedance of lossy piezoelectric resonators for ultrasonic transducers

Relevant equivalent circuit parameters and values of material constants of a piezoelectric resonator can be determined from measurements of its electrical input impedance as a function of frequency. The complex electrical impedance curves and the associated critical frequencies are the basis of this characterization by the piezoelectric resonance method. In this paper, the previously introduced concept of normalized electrical impedance of the lossy resonator, extended to include piezoelectric losses, is applied to the analysis of the effects of different types of intrinsic losses on peak values, bandwidths and characteristic frequencies. The resulting impedance patterns depend solely on the electromechanical coupling coefficient and the loss tangents, providing a useful tool for the analysis of low-Q resonators. The normalized impedance is experimentally evaluated from the basic data provided by an HP 4194A impedance analyser by means of specifically developed ASP programs.

Influence of dielectric losses on the shift of the fundamental frequencies of thickness mode piezoelectric ceramic resonators

Abstract The influence of dielectric losses on the shifts of the characteristic frequencies f s and f p , defined as the frequencies of maximum electrical conductance and resistance respectively, is investigated at the fundamental resonance bands of thickness extensional piezoelectric ceramic resonators. The more general expression for the complex electrical input impedance is used as a reference instead of the usual Butterworth–Van Dyke equivalent circuit. The concept of normalised electrical impedance of the lossy resonator is introduced and applied to the computation of the departs of the characteristic frequencies f s and f p , from the associated lossless critical frequencies f 1 and f 2 , as a function of k t , tan δ m , and tan δ e . It is shown that, according to the adopted model, the effects of dielectric losses on the critical frequencies are different than those produced by mechanical losses: on the one hand, intrinsic mechanical losses increase f s and lower f p ; on the other hand, dielectric losses lower f s and augment the decrease of f p . The frequency displacements have been computed over a wide range of the fundamentals parameters, which include typical values of ceramic materials used for ultrasonic imaging applications.

A 5 MHz high-voltage demultiplexed ultrasonic array system for rapid-scan testing of advanced materials

Abstract The most common techniques in non-destructive ultrasonic testing applications make use of the mechanical or manual scanning of the sensor devices. An important limitation of these procedures is the time required to acquire the inspection data when only a single transducer is scanned over the whole area of interest. To overcome this problem, a multichannel operation must be introduced. In this paper a 5 MHz array system of eight channels with electronic scanning through a high-voltage demultiplexer is presented. The system has been specially developed for rapid-scan purposes, needing only one commercial monochannel flaw detector as external equipment. The sensor device is a broad-band eight-element array transducer of multilayer type with one matching layer. A new analog demultiplexer has been designed, which is able to control the pass of broad-band pulses containing large peaks of voltage and current.

Different tuning contributions in piezoelectric transceivers improving transient signals for ultrasonic imaging

Ultrasonic applications based on piezoelectric devices usually introduce some kind of electrical tuning circuits which are normally fitted in an experimental way. In this paper, the influences of these tuning elements on piezoelectric transceivers are analysed in transient regime, by sensing the ultrasonic signals received through two piezoelectric processes linked in cascade: a transmitter of echographic imaging type, and a broad-band piezoelectric ultrasonic receiver. The relative contributions, in order to improve the image resolution, from two parallel tuning circuits in emission and reception, are separately evaluated. Signal spectra and transducer transfer functions are obtained for different tuning conditions.

Distortions on the output waveforms of high-voltage spike generators induced from piezoelectric loads

Abstract Piezoelectric ultrasonic transducers usually present a relatively low input electrical impedance at high frequencies, as well as a strong reactive character, making it difficult to achieve their efficient electrical excitation in transient regime as HF ultrasonic emitters driven by conventional generators. For this reason, special pulser units delivering high-voltage spikes are normally used, for instance in ultrasonic evaluation (NDE) of the inside of structures and materials. Even with this special type of driving equipment, if sharp excitation pulses, as required in many applications, are applied across transducer electrodes, strong interactions appear between piezoelectric loads and generators. These electronic-piezoelectric interactions cause marked perturbations in the output waveforms from the pulser, which are more pronounced in multichannel applications. In this paper, the limitations of electronic pulse generators to supply “nominal” signals when driving piezoelectric probes in ultrason...