Strip Transducer Research Articles

Defect sensitivity of piezoelectric strip transducers based on planar Lamb waves

Long narrow piezoelectric fiber composite transducers that can generate planar ultrasonic guided waves are investigated for damage detection in plates. These strip transducers function well as actuators because the ultrasonic energy does not spread out and the signal remains strong. However, their suitability as sensors must be investigated because the received signal is integrated over the entire length of the transducer, potentially making them less sensitive to localized damage since the voltage is averaged over the length of the sensor. A segmented sensing scheme is introduced here and studied both numerically and experimentally. The strip transducer is subdivided into n segments, where n varies from 1 to 11, and the sensitivity to a through-thickness slot is assessed. The results indicate that there is a considerable improvement in defect sensitivity provided by segmented sensing. For a 130-mm-long strip transducer and a 15-mm-long slot, the transmission coefficient decreased from 0.86 to 0.70 to 0.59, while the reflection factor increased from 0.05 to 0.30 to 0.36 for the full 130-mm strip, a 31-mm-long segment, and a 7-mm-long segment, respectively.

Modified synthetic transmit aperture algorithm for ultrasound imaging

The modified synthetic transmit aperture (STA) algorithm is described. The primary goal of this work was to assess the possibility to improve the image quality achievable using synthetic aperture (SA) approach and to evaluate the performance and the clinical applicability of the modified algorithm using phantoms. The modified algorithm is based on the coherent summation of back-scattered RF echo signals with weights calculated for each point in the image and for all possible combinations of the transmit–receive pairs. The weights are calculated using the angular directivity functions of the transmit–receive elements, which are approximated by a far-field radiation pattern of a narrow strip transducer element vibrating with uniform pressure amplitude over its width. In this way, the algorithm takes into account the finite aperture of each individual element in the imaging transducer array. The performance of the approach developed was tested using FIELD II simulated synthetic aperture data of the point reflectors, which allowed the visualization (penetration) depth and lateral resolution to be estimated. Also, both simulated and measured data of cyst phantom were used for qualitative assessment of the imaging contrast improvement. The experimental data were obtained using 128 elements, 4 MHz, linear transducer array of the Ultrasonix research platform. The comparison of the results obtained using the modified and conventional (unweighted) STA algorithms revealed that the modified STA exhibited an increase in the penetration depth accompanied by a minor, yet discernible upon the closer examination, degradation in lateral resolution, mainly in the proximity of the transducer aperture. Overall, however, a considerable (12 dB) improvement in the image quality, particularly in the immediate vicinity of the transducer’s surface was demonstrated. The modified STA method holds promise to be of clinical importance, especially in the applications where the quality of the “near-field” image, that is the image in the immediate vicinity of the scanhead is of critical importance such as for instance in skin- and breast-examinations.

Piezoelectric Fiber Composite Strip Transducer Design Considerations for Generating Lamb Waves

Lamb waves have great promise among the various active monitoring schemes being researched for Structural Health Monitoring. Planar Lamb wave excitation using surface-bonded piezoelectric strips to function as comb transducers is investigated numerically for both the fundamental symmetric S0 and the antisymmetric A0 wave modes. This comb-type transducer is comprised of multiple piezoelectric elements, which are piezoelectric fiber composites sandwiched between electrodes. These transducer strips reduce the wiring requirements, can conform to curved surfaces, and the properties can be tailored to suit the application. Transducer variables are many; here fiber orientation, volume fraction, poling directions, and electrode locations are investigated. Specifically, piezocomposite properties are estimated through micromechanical modeling for input to finite element analysis of Lamb wave generation and propagation. For this multiphysics problem, the finite element analysis employs co-simulation using ABAQUS Standard and ABAQUS Explicit to simulate transient wave propagation from a strip transducer on a plate. In the finite element results, the amplitude of the Lamb wave at a prescribed distance from the transducer is evaluated to characterize the effectiveness of the composite comb transducer. Furthermore, results show that there are a number of cases where composite actuators provide a large amplitude response. After considering piezoelectric fiber direction, poling direction, and electric field direction, it appears that fiber, polarization, and activation electric fields through the thickness of the transducer represent the optimal design.

A tunable oscillator using magnetostatic forward‐volume wave resonator with wide strip transducer

AbstractIt is shown theoretically and experimentally that a tunable oscillator without spurious oscillations can be obtained by using a magnetostatic forward‐volume wave resonator with wide strip transducers. First, the equivalent circuit of a rectangular resonator is presented and the unloaded Q and the external Q are expressed in terms of the dimensions and the material constants of the resonator. By means of this expression, the condition is derived for selective excitation of the dominant mode and for suppression of the higher‐order modes. A 5‐GHz tunable oscillator using this resonator was fabricated. It was demonstrated that stable oscillation characteristics without spurious oscillations can be obtained.

Open circuit parameters of a magnetostatic forward volume wave straight-edge resonator coupled by orthogonal transducers

An impedance matrix is derived for a magnetostatic forward volume wave straight-edge resonator (MSFVW-SER) coupled by orthogonal strip transducers. This approach provides a theoretical method to predict the MSFVW-SER properties together with the conditions of the operating frequency, the geometrical structure, the saturation magnetization and the linewidth of YIG film. The validity of the method is supported by experiment.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Predictions of the Gauss-Hermite beam model and finite element methodf or ultrasonic propagation through anisotropic stainless steel

The predictions of the Gauss-Hermite beam model are compared to those obtained by the finite-element method for a model problem. This is motivated by the desire to examine the trade-offs between computational speed and accuracy in the Gauss-Hermite model. In the model problem, a contact strip transducer radiates through an isotropic layer of ferritic steel into an anisotropic layer of austenitic stainless steel with various directions of the preferred axis of columnar grain alignment. Comparisons are made of time-domain waveforms in a common observation axis in the austenitic material. The predictions of the two models are found to be in good agreement near the center of the beam, with deviations developing as one moves away from the central ray. These are interpreted to be a consequence of the Fresnel approximation made in the Gauss-Hermite model. However, the region that contains most the energy is in the vicinity of the central ray, where there is excellent agreement between the two models. This loss in accuracy is accompanied by a several orders of magnitude increase in computation time.

Tunable magnetostatic surface-wave-oscillators

A tunable S-band delay line oscillator using magnetostatic surface-waves is described in this paper. This oscillator is made of two monolithic magnetostatic surface wave delay lines in the feedback loop of a solid-state amplifier. Experimental data are presented. This device is compared to an oscillator using only one delay line and multibar parallel - strip transducers in order to select an oscillating single mode.

The signal/noise gain of ideal receiving arrays

A study is made of the theoretical signal/noise gain of an ideal strip transducer; this gain is the actual signal/noise ratio compared with that for a ‘point’ receiver (i.e. one whose dimensions are small compared with the wavelength at the lowest significant frequency). The results are not restricted to narrow-band systems and relate generally to noise fields which may be non-isotropic and have non-uniform frequency spectra. The theory is used to investigate the correlation between the noise outputs of the various sections of a subdivided transducer, and it is shown that, at least in the narrow-band case, the usual assumption of negligible correlation is generally justified in practice. Multiplicative systems are also considered, and it is shown that, in certain circumstances, the overall signal/noise ratio of such a system may be better than that of either of its component groups. The results obtained are also generally applicable to ideal radio receiving arrays.