Transducer Loading Research Articles

Transmission Sensitivities of Contact Ultrasonic Transducers and Their Applications.

In all ultrasonic material evaluation methods, transducers and sensors play a key role of mechanoelectrical conversion. Their transduction characteristics must be known quantitatively in designing and implementing successful structural health monitoring (SHM) systems. Yet, their calibration and verification have lagged behind most other aspects of SHM system development. This study aims to extend recent advances in quantifying the transmission and receiving sensitivities to normally incident longitudinal waves of ultrasonic transducers and acoustic emission sensors. This paper covers extending the range of detection to lower frequencies, expanding to areal and multiple sensing methods and examining transducer loading effects. Using the refined transmission characteristics, the receiving sensitivities of transducers and sensors were reexamined under the conditions representing their actual usage. Results confirm that the interfacial wave transmission is governed by wave propagation theory and that the receiving sensitivity of resonant acoustic emission sensors peaks at antiresonance.

An experimental study of winding vibration of a single-phase power transformer using a laser Doppler vibrometer

Transformer tank vibration has been used for the condition monitoring and fault diagnosis of power transformers, as well as for the evaluation of radiated sound power from such power transformers. In both applications, seeking a correlation between transformer tank vibration and the vibration of the transformer windings and core becomes a challenge for fault identification and noise control at the sources of vibration and noise. The purpose of this paper is to experimentally investigate the winding vibration of an electrically live power transformer and characterize the changes in the spatial and frequency features of the vibration as various mechanical faults are introduced to the transformer winding. To avoid the effects of transducer loading and electromagnetic fields on the measurement results, a laser Doppler vibrometer was used to make non-contact measurements of the winding vibration.

Phased annular array transducers for omnidirectional guided wave mode control in isotropic plate like structures

Ultrasonic guided waves are fast emerging as a reliable tool for continuous structural health monitoring. Their multi-modal nature along with their long range propagation characteristics offer several possibilities for interrogating structures. Transducers commonly used to generate guided waves in structures excite multiple modes at any frequency; their complex scattering and reflection from defects and boundaries often complicates the extraction of useful information. Often it is desirable to control the guided wave modes propagating in a structure to take advantage of their unique properties for different applications. Earlier attempts at guided wave mode control involved developing fixed wavelength linear and annular array transducers. Their only disadvantage is that the transducer is limited to a particular wavelength and a change in wavelength necessitates a change in the transducer. In this paper, we propose the development of an annular array transducer that can generate mode controlled omnidirectional guided waves by independently controlling the amplitude and phase of the array elements. A simplified actuator model that approximates the transducer loading on the structure to a constant pressure load under the array elements is assumed and an optimization problem is set up to compute the excitation voltage and phase of the elements. A five element annular array transducer is designed utilizing 1–3 type piezocomposite materials. The theoretical computations are experimentally verified on an aluminum plate like structure by exciting A0 and S0 guided wave modes.

Transducer loading effect on the performance of PZT-based SHM systems

In the last years, the piezoelectric transducers of lead zirconate titanate (PZT) have been widely used in structural health monitoring (SHM) systems based on the electromechanical (E/M) impedance technique. Although many studies indicate the successful use of PZT in damage detection, some practical considerations still have to be considered in real applications. In this work, the effect of the transducer loading caused by mechanical impedance of the host structure is analyzed using an equivalent electromechanical circuit. Tests were performed on structures with various sizes and the results show that the transducer loading significantly reduces the sensitivity of the system for detecting structural damages.

A New Broadband Microcurrent Transducer for Insulator Leakage Current Monitoring System

In this paper, a type of high bandwidth microcurrent transducer designed for online monitoring of an insulator is described. According to the equivalent circuit model, the effects of transducer winding number, stray capacitance, and loading resistance on passbandwidth and sensitivity are discussed. The academic lower limit of the transducer is nearly zero and the upper limit is infinity, which are proven by the analysis of the electronic circuit and the simulation by saber. The test of the current transducer shows that the transducer can be used in most high-voltage insulations systems with broad passbandwidth, high linearity, high sensitivity, and noncontact.

Microscopic magnetic resonance elastography (μMRE)

Magnetic resonance elastography (MRE) was extended to the microscopic scale to image low-frequency acoustic shear waves (typically less than 1 kHz) in soft gels and soft biological tissues with high spatial resolution (34 micromx34 micromx500 microm). Microscopic MRE (microMRE) was applied to agarose gel phantoms, frog oocytes, and tissue-engineered adipogenic and osteogenic constructs. Analysis of the low-amplitude shear wave pattern in the samples allowed the material stiffness and viscous loss properties (complex shear stiffness) to be identified with high spatial resolution. microMRE experiments were conducted at 11.74 T in a 56-mm vertical bore magnet with a 10 mm diameterx75 mm length cylindrical space available for the elastography imaging system. The acoustic signals were generated at 550-585 Hz using a piezoelectric transducer and high capacitive loading amplifier. Shear wave motion was applied in synchrony with the MR pulse sequence. The field of view (FOV) ranged from 4 to 14 mm for a typical slice thickness of 0.5 mm. Increasing the agarose gel concentration resulted in an increase in shear elasticity and shear viscosity. Shear wave motion propagated through the frog oocyte nucleus, enabling the measurement of its shear stiffness, and in vitro shear wave images displayed contrast between adipogenic and osteogenic tissue-engineered constructs. Further development of microMRE should enable its use in characterizing stiffer materials (e.g., polymers, composites, articular cartilage) and assessing with high resolution the mechanical properties of developing tissues.

Acoustic emission from single carbon fibres and model microcomposites

It is difficult to interpret acoustic emissions generated within stressed fibre composites. To obtain a better understanding of the characteristic features of acoustic emission at source in carbon fibre reinforced plastics, a number of experiments have been performed to measure the emission from the fracture of single carbon fibres alone and of single fibres in microcomposites. An exponentially shaped steel cone was used as a waveguide to couple a single fibre to a transducer and tensile loading was achieved by slowly adding water to a container suspended from the sample. The microcomposites consisted of a single carbon fibre surrounded by a small number of glass fibres, all bonded by acrylic or epoxy resin. Measurements were made on five different types of carbon fibre. For freely mounted single fibres the amplitude of the acoustic emission produced at fracture was high, and it increased approximately linearly with strain energy in the fibre prior to fracture. A similar carbon fibre in a microcomposite suffered a succession of failures, and the amplitudes of the emission were lower, in qualitative agreement with the strain energy being lower at fracture. Results are also given for the acoustic emission produced during torsion tests on a microcomposite. In these samples the fibres remained intact and emissions were produced by debonding and/or matrix cracking.

The use of piezoelectric film and ultrasound resonance to determine the complete elastic tensor in one measurement.

The ultrasonic and elastic properties of materials are conventionally measured using quartz, lithium niobate, etc., transducers and a pulse-echo technique with the transducer driven at resonance. Problems with the technique include transducer ringing, transducer-sample coupling, parallelism of sample faces, beam diffraction, and the necessity of remounting transducers in order to measure all of the elastic constants. Usually, these problems can be minimized, but with samples that are only a fraction of a millimeter in size, conventional ultrasound measurements become difficult if not impossible. However, nearly all of these problems may be avoided if a resonance technique is used, and all of the elastic constants may be determined with a single measurement. For the broadband response and minimum transducer loading required for a resonance measurement in a small sample, polyvinylidene fluoride (PVDF) piezoelectric film (as thin as 9 microns) is ideally suitable.

Resonant ultrasound measurements in single crystals of intrinsic and superconducting copper-oxygen plane compounds

Resonant ultrasound measurements in very pure single crystals of La2CuO4 have been made. The material was prepared such that its resistivity at 300 K is extremely high (170 Ω cm), and untwinned crystals of about 0.1×0.1×0.1 cm were used. Oxygen was then introduced, eventually leading to superconductivity. Here, data on the three longitudinal sound velocities and attenuations as a function of temperature for different oxygen dopings are presented and the effects of magnetic and superconducting interactions are discussed. All data were taken with a resonant ultrasound method developed by us in which the transducers are flexible and are less than 1% of the sample mass, minimizing transducer loading effects.

Strain gauge transduction of lip and jaw motion in the midsagittal plane: Refinement of a prototype system

This paper describes the design refinements made in an earlier prototype system for transducing lip and jaw movements in the midsagittal plane. Major improvements over the prototype system included: reduced transducer loading, improved two-dimensional separation and resolution, elimination of possible artifacts due to lateral-medial or rotational movement, electrical isolation of the subject from the transducer, increased capability to transduce movements of children as well as adults, and an accessory instrument for orienting the transducers with reference to defined anatomical planes.

Capacitance Detector for Loss Measurements

A capacitance pickup is applied to loss measurements of rotating mass-loaded rods in the frequency range 0.001–10 cps. Variations of loss and modulus are studied as a function of time and frequency. The same pickup is useful for measuring any specimen where loading by the receiving unit is undesirable. An apparatus which eliminates transducer loading is described for decay or bandwidth measurements of thin film strips at frequencies up to 10 kc. [This work is supported by the Bureau of Naval Weapons and by the U. S. Atomic Energy Commission.]