KEYWORDS: Phased arrays, Imaging systems, Digital signal processing, Transducers, Ultrasonography, Silicon carbide, Analog electronics, Human-machine interfaces, Nondestructive evaluation, Control systems
Phased array imaging systems provide the features of electronic beam steering and dynamic depth focusing that cannot
be obtained with conventional linear array systems. This paper presents a system design of a digital ultrasonic imaging
system, which is capable of handling a 64-element 35MHz center frequency phased array transducer. The system
consists of 5 parts: an analog front-end, a data digitizer, a DSP based beamformer, a computer controlled motorized
linear stage, and a computer for post image processing and visualization. Using a motorized linear stage, C-scan images,
parallel to the surface of scanned objects may be generated. This digital ultrasonic imaging system in combination a 35
MHz phased array appears to be a promising tool for NDT applications with high spatial resolution. It may also serve as
an excellent research platform for high frequency phased array design and testing as well as ultrasonic array signal
algorithm developing using system's raw RF data acquisition function.
PMN-PT single crystal 1-3 composite high frequency phased arrays with center frequency of 35 MHz were fabricated
and characterized for silicon carbide (SiC) NDE imaging applications. The 35 MHz 64-element array was successfully
prototyped using PMN-PT single crystal and PC-MUT technology. The broad bandwidth > 90% and high sensitivity
(echo amplitude > 500 mV from the impulse response with 0 gain) was observed with reasonably high uniformity. These high frequency phased arrays are promising for ceramic NDE imaging.
In this paper, the development of a 35 MHz 64-channel Piezoelectric Composite based Micromachined Ultrasound
Transducer (PCMUT) phased array for NDE ultrasound application is presented. A 35 MHz PMN-PT single crystal 1-3
composite based PC-MUT phased array was designed with extensive acoustic field and 1D modeling. The initial
modeling results demonstrated that the focused detection resolution (10% of -3 dB beam width) could be as small as 30
μm in the azimuth direction. The maximum imaging depth for ceramic samples is around 20 mm. The PC-MUT array
being developed will extend the state-of-art NDE phased array technology from approximately 20 MHz to 35 MHz,
which will greatly enhance the imaging resolution for a broad range of NDE ultrasound applications.
Single crystal piezoelectric composite transducers including 75 MHz PC-MUT (piezoelectric composite micromachined
ultrasound transducers), diced 10 MHz and 15 MHz 1-3 composite transducers were successfully demonstrated with
broad bandwidth and high sensitivity. In this paper, the design, fabrication and characterization of composite transducers
are reported. C-scan experiments for SiC ceramic samples were performed using these composite transducers as well as
some commercial NDE transducers. The results suggest that significant improvements in resolution and penetration
depth can be achieved in C-scan NDE imaging using single crystal composite broadband transducers.
In this paper, the design, fabrication and characterization of PMN-PT single crystal/epoxy composites are reported for
NDT ultrasound transducers. Specifically, 1-3 PMN-PT/epoxy composites with center frequencies of 5 MHz - 40 MHz
were designed and fabricated using either the dice-and-fill method or a photolithography based micromachining process.
The measured electromechanical coefficients for composites with frequency of 5 MHz - 15 MHz were about 0.78-0.83,
and the coupling coefficients for composites with frequencies of 25 MHz- 40 MHz were about 0.71-0.72. The dielectric
loss remains low (< 0.05). These properties hold promise for advanced NDT ultrasound applications.
Transducers incorporating single crystal piezoelectric Pb(Mg1/3Nb2/3)x-1TixO3 (PMN-PT) exhibit significant advantages over ceramic piezoelectrics such as PZT, including both high electromechanical coupling (k33 > 90%) and piezoelectric coefficients (d33 > 2000 pC/N). Conventional <001> orientation gives inherently larger bandwidth and output power than PZT ceramics, however, the anisotropy of the crystal also allows for tailoring of the performance by orienting the crystal along different crystallographic axes. This attribute combined with composition refinements can be used to improve thermal or mechanical stability, which is important in high power, high duty cycle sonar applications.
By utilizing the "31" resonance mode, the high power performance of PMN-PT can be improved over traditional "33" mode single crystal transducers, due to an improved aspect ratio. Utilizing novel geometries, effective piezoelectric constants of -600 pC/N to -1200 pC/N have been measured. The phase transition point induced by temperature, pre-stress or field is close to that in the "33" mode, and since the prestress is applied perpendicular to the poling direction in "31" mode elements, they exhibit lower loss and can therefore be driven harder.
The high power characteristics of tonpilz transducers can also be affected by the composition of the PMN-PT crystal. TRS modified the composition of PMN-PT to improve the thermal stability of the material, while keeping the loss as low as possible. Three dimensional modeling shows that the useable bandwidth of these novel compositions nearly equals that of conventional PMN-PT. A decrease in the source level of up to 6 dB was calculated, which can be compensated for by the higher drive voltages possible.
TRS is developing new transducers based on single crystal piezoelectric materials such as Pb(Mg1/3Nb2/3)x-1TixO3 (PMN-PT). Single crystal piezoelectrics such as PMN-PT exhibit very high piezoelectric coefficients (d33 ~ 1800 to >2000 pC/N) and electromechanical coupling factors (k33 > 0.9), respectively, which may be exploited for improving the performance of broad bandwidth and high frequency sonar. Apart from basic performance, much research has been done on reducing the size and increasing the output power of tonpilz transducers for sonar applications. Results are presented from two different studies.
"33" mode single crystal tonpilz transducers have reduced stack lengths due to their low elastic stiffness relative to PZTs, however, this produces non-ideal aspect ratios due to large lateral dimensions. Alternative "31" resonance mode tonpilz elements are proposed to improve performance over these "33" designs. d32 values as high as 1600 pC/N have been observed, and since prestress is applied perpendicular to the poling direction, "31" mode Tonpilz elements exhibit lower loss and higher reliability than "33" mode designs.
Planar high power tonpilz arrays are the optimum way to obtain the required acoustic pressure and bandwidth for small footprint, high power sensors. An important issue for these sensors is temperature and prestress stability, since fluctuations in tonpilz properties affects power delivery and sensing electronic design. TRS used the approach of modifying the composition of PMN-PT to improve the temperature dependence of properties of the material. Results show up to a 50% decrease in temperature change while losing minimal source level.
A 50 MHz annular array with six equal-area elements was designed. A novel fine-grain, 1-2 micron particle size, lead titanate was used as the piezoelectric material. The full set of electromechanical properties was calculated using resonance techniques. The low permittivity ((epsilon) s33/(epsilon) 0 equals 180) of the lead titanate allowed a larger overall diameter when matching to 50 ohm electronics, which kept the aspect ratio (width/height) of all the elements above 3. Laser micromachining was used to fully separate the array elements, and a double matching layer scheme was used to acoustically match the device. The array was modeled using both one-dimensional (KLM) and finite element modeling (PZFlex), and good agreement between the two was obtained. Using a broadband excitation, a center frequency of 49 MHz was obtained on the echo reflected from a flat reflector, with an inerstion loss of 20 dB and a bandwidth of 50%. Maximum calculated cross-talk values were below -30 dB between elements.
An exposimetry system for characterization of high frequency ultrasound fields has been developed and built. By extrapolating the recommendations of the AIUM and IEC standards to higher frequencies, an exposimetry system operating above 15 MHz was outlined. The system incorporates a five degrees-of-freedom positioning system, including three automated translational motors that provide 0.5 micron resolution. Two manual rotational axes utilize a worm-gear and concentric cylinder arrangement to insure orthogonal rotational adjustment. Overall bandwidth of the system is 100 MHz and is limited by the type of hydrophone used. Using a calibrated 0.04 mm diameter needle-type hydrophone, measurements of single element transducers of 25-50 MHz have been made. LiNbO3 and PVDF transducers of f-numbers from 2-3 have been tested and 2D intensity beam profiles plotted. Results from a 50 MHz LiNbO3 transducer show good agreement between empirical (8.6 mm) and theoretical (9.0 mm) focal points. The -3 dB beamwidth was also measured (108 micron) to be comparable to that of the calculated value (86 micron). It is shown that this system provides a good means for characterization and analysis of the beam profiles of high frequency transducers.
Polyvinylidene fluoride (PVDF) transducers have been developed for ultrasonic backscatter microscopy (UBM) applications. Single element devices encompassed the frequency range of 40 to 65 MHz and incorporated a variety of backing materials and tuning circuits. Pulse-echo testing was performed using the 1 (mu) J energy setting on a Panametrics 5900 pulser/receiver, configured with +40dB gain and 20 dB attenuation. A broad spectrum of performance was observed depending on the choice of backing and tuning, with -6dB bandwidths of 55% to 116% and echo amplitudes from 100mVpp to 940mVpp. It was observed that electrical impedance matching increased sensitivity and tuned the center frequency at the expense of reduced bandwidth. For imaging applications the tuned devices provided superior performance. Although single element transducers in this frequency range provide axial resolution on the order of 50 micrometer, a tradeoff exists between the lateral resolution and the depth of field. To obtain high lateral resolution and a long depth of field an annular array design was adopted. Laser dicing was used to fabricate the elements of the array from a sheet of PVDF. Interconnect to each element was achieved using pin contacts molded into the backing. The design incorporated a 5 mm aperture with six equal area elements focused at 10 mm. At a center frequency of 50 MHz this design will achieve a lateral resolution of 58 micrometer, comparable to the axial resolution, over the entire 7.5 mm depth of field.
A wide variety of fabrication techniques and materials produce ultrasound transducers with very different performance characteristics. High frequency (50 MHz), focused single element transducers using lead zirconate titanate (PZT) fiber composite, lead titanate (PbTiO3) ceramic, polyvinylidene fluoride (PVDF) polymer and lithium niobate (LiNbO3) single crystal are compared in design and performance. The transducers were all constructed with a 3 mm aperture and f- number of 2 - 3. Design considerations discussed include optimization of designs using different lens, backing and matching materials for acoustic matching and the use of several electrical tuning techniques to match the transducers to the 50(Omega) circuitry. Transducers were tested for insertion loss and -6dB bandwidth using a quartz flat- plate target. Insertion loss measurements between transducers were -20dB to -50dB with bandwidths in the range of 50 - 120%. Through the use of an ultrasound backscatter microscope (UBM), the transducer were compared using in vitro images of the human eye. Images of a wire phantom were also made for comparison of lateral and axial resolution of each device.
Very high frequency (VHF) ultrasound (> 20 MHz) has recently gained much attention as an effective non-invasive means to diagnose ocular and dermatological lesions. These ultrasonic backscatter microscopy systems have advanced rapidly with the electronics industry; however the VHF transducers are often the limiting factor in the overall image quality. This overview examines a number of the issues facing the high frequency transducer designer, including active material selection, passive components, acoustic and electrical matching and accurate characterization. Modified lead titanate was used as an active material to examine the use of a transmission line transformer to improve the electrical match between 25 and 40 MHz transducers and 50 (Omega) electronics. At 40 MHz, the transformer was shown to improve response only modestly, increasing bandwidth a few percent and insertion loss a few dB. At 25 MHz, -6 dB bandwidth improved almost 60%; however, peak sensitivity increased only 1 dB. PZT fiber composites were investigated to determine the effect of various volume fractions and passive backings on high frequency response. Parylene C was shown to be an effective matching layer for these composites, improving bandwidth of the 30% volume fraction transducers by 20% and sensitivity by 5 dB. Finally, a test setup, taking advantage of state of the art in linear positioners, is presented, addressing the issues pertinent to high frequency transducer characterization.
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