Ultrasonic monitoring allowing the evaluation of the performance of muscles under training has been developed. The
monitoring scheme is suitable to determine muscle movement and is based on the measurement of the transit time of
longitudinally polarized ultrasound propagating across the observed muscle. Variations of the length of the muscle lead
to variations of the lateral extension since the volume of the muscle is conserved. The corresponding variations of the
observed time-of-flight result dominantly from the variation of the path length. This allows the time-resolved detection
of the movement of the muscles in the path of the ultrasonic beam. In this way not only the degree of contraction or
relaxation, but also the speed of these processes can be quantitatively monitored. The muscle thickness has been
determined with a resolution of ± 0.02 mm corresponding to about ± 0.2 % of the thickness of the relaxed muscle. This
resolution is already in the range of unavoidable uncertainties caused by the surface structure of the individual muscles.
Similarly, the already obtained resolution in time corresponds to a fraction 1/750 of the time of the fastest known human
muscle movement of 7.5 ms, observed for the full contraction of the eye lid muscle. The time of flight is measured along
a line between two electro-acoustic transducers positioned on the skin on opposite sides of the monitored muscle. The
transducers can be placed at any desired position but should be positioned such, that no bones or intestines are
obstructing the path between them. The time-of-flight from which all other data is derived is observed with the aid of a
computer-controlled arbitrary function generator and a synchronized transient recorder. Even in the demonstrated
developmental state the equipment is already rather compact (lap-top size) and can be battery operated.
For applications involving the determination of variations of the time-of-flight in pulsed echo or transit experiments a
method has been developed based on Fourier transformation with forced optimized compression of the reference signal
to an only bandwidth limited approximation of a Dirac-function. The respective transformation of time shifted response
signals allows the effective separation of otherwise overlapping signals and the detection of differences in the time-of-flight
for the individual components with high resolution. The developed processing scheme corrects for dispersion and
attenuation in the electronics, the transmission lines, and the transducers. The method is presented and applications are
demonstrated.
Conducting micro-spheres approximating point probes have been employed to piezoelectrically excite and detect
ultrasonic wave packages in anisotropic single crystals. Imaging based on the detection of magnitude and phase is
performed in transmission. The experimental data can be used for the determination of the elastic constants of the
material. Here we compare this approach with imaging using conventional ultrasonic lenses and water as a coupling
fluid. The large bandwidth and the absence of internal lens echoes in the Coulomb excitation and detection scheme
permit unperturbed monitoring of multiple echoes in plane-parallel samples and the detailed investigation of mode
conversion processes of longitudinal and transverse waves at the surfaces of the crystal. Due to differences in the
coupling between the probes and the ultrasound in the sample, excitation of ultrasound by an acoustic lens or an
electrical point contact, respectively, result in noticeably different phonon focusing patterns. This is illustrated for
lithium niobate single crystals.
The determination of the velocity of sound for small objects suffers from limited resolution concerning as well the
determination of the extension along the path of the sound waves as the determination of the time of flight. Imaging of
planar objects with a wedge shaped boarder allows imaging in transmission with no object and the full object in the path
of the acoustic waves in a continuous manner. Such phase tracking available by PSAM can be used to determine the
variation of the time-of-flight with ultimate resolution. Furthermore, for a coupling fluid with a speed equal to the speed
of the object under study, the extension of the object does not contribute to the result. Similarly for coupling fluids with
sound velocities close to the one of the object under study the error concerning the extension which can be substantial for
microscopic objects is reduced and can be minimized by selection of suitable fluids. The method is demonstrated and
application involving different objects and fluids are demonstrated.
For several years guided waves have been used for pipe wall defect detection. Guided waves have become popular for
monitoring large structures because of the capability of these waves to propagate long distances along pipes, plates,
interfaces and structural boundaries before loosing their strengths. The current technological challenges are to detect
small defects in the pipe wall and estimate their dimensions using appropriate guided wave modes and to generate those
modes relatively easily for field applications. Electro-Magnetic Acoustic Transducers (EMAT) can generate guided
waves in pipes in the field environment. This paper shows how small defects in the pipe wall can be detected and their
dimensions can be estimated by appropriate signal processing technique applied to the signals generated and received by
the EMAT.
Narrowband excitation at 86 MHz with vector detection and wideband excitation in the range of 2 to 20 MHz have both
been used for tomographic imaging in transmission. A line-shaped point spread function has been realized by temporal
apodization selecting from a pulsed signal observed in transmission only the contribution traveling the path connecting
the transducer foci. By this method a pair of scanned focusing transducers mounted in a defocused arrangement was
employed for tomographic imaging. The technique relates to shadowing of a point source in transmission as used in X-ray
tomography, but, in addition, variations of the time-of-flight are measured else by phase contrast or a cross-correlation
procedure with high resolution. From these data an image with velocity contrast can be derived in addition to
the conventional image representing the extinction in the samples under investigation. Examples presented include
resolution test samples and biomedically relevant materials. It is also demonstrated that the coherent detection scheme
can be used to enhance the resolution by the synthesis of an enlarged aperture. Respective procedures are implemented
for image reconstruction.
Coulomb excitation and detection of ultrasonic waves in piezoelectric crystals by spherical electrical probes is discussed
in view of the opening angle of the cone of longitudinal waves coupling to such a probe. The electric field distribution in
the piezoelectric crystal under the probe is modeled by means of finite elements in order to determine the effective size
of the probe normalized to the sphere radius. The dynamic impedance of the probe is estimated, and it is shown that a
probe of a size appropriate to illuminate or detect from the piezoelectric half space has a frequency-independent
impedance of about 3 k&OHgr; under idealizing assumptions. Measurements of the directionality of ultrasound emission and
detection at a frequency of about 100 MHz are presented for three probes with different tip radii, varying from about
30 &mgr;m to 2.5 mm. As expected, larger probes yield a higher directionality. A relatively large forward contribution is
observed even for small spheres.
Surface focused acoustic transmission microscopy is employed for projection (tomographic) imaging of bonded
materials including wafers. Short pulse excitation with apodized focusing transducers operated in transmission and two
channel quadrature transient detection are employed for multiple contrast imaging. The achievable contrast schemes are
based on mode selection for longitudinal, transverse, mode converted, and scattered modes. The identification of the
involved modes including conversion schemes is experimentally accessible by time-gating of the recorded signal and by
observation of spatially selected holograms. Perfect bonding, disbonding, and weak bonding can be studied and
characterized by the developed mode selective imaging scheme. The characteristic features of weak bonding phenomena
are demonstrated and characterized.
Ultrasonic monitoring schemes for the detection of the solid-liquid interface during directional solidification have been developed including electronic equipment for the Material Science Laboratory (MSL) of the International Space Station (ISS). Special signal and data processing suitable for automatic monitoring, on board signal averaging, and operation under a limited data transfer condition is discussed. The achievable resolution in the micrometer regime as well as post experimental processing and evaluation for high resolution monitoring are presented and exemplified for typical applications.
Electric surface excitation of ultrasound in the Coulomb field of scanned electrically conductive spherical local probes and similar detection has been employed for imaging of the transport properties of acoustic waves in piezoelectric materials including singlecrystalline wafers. The employed Coulomb scheme leads to a fully predictable and almost ideal point excitation and detection. In combination with two-channel quadrature transient detection it allows high precision spatially and temporally resolved holographic imaging. Via modeling of the excitation and propagation properties, the effective elastic tensor and the piezoelectric properties of the observed materials can be determined with high resolution from a single measurement. The generation and detection scheme as well as the theoretical background are demonstrated and applications are exemplified.
Acoustic Micro Imaging (AMI) has long been established as a method of NDT of the wafer-to-wafer bonding quality in directly bonded wafers. In conventional imaging systems a C-Mode Scanning Acoustic Microscope operating in reflection is utilized. In this paper a non-confocally adjusted Phase Sensitive Acoustic Microscope (PSAM) operating in transmission at a frequency of 85 MHz is employed for imaging. This mode of operation results in a time-dependent point spread function, which together with full-transient data acquisition allows for its optimization in terms of resolution in the post-processing stage. Furthermore, the information contained in the images produced by varying time-dependent PSF is used for identification of bonding defects in directly bonded wafers. Both completely disbanded and weak bond regions in the wafer-to-wafer interface are identified. These latter areas are present, e.g. at the rim of the entirely disbanded regions as an intermediate interface condition between fully bonded and completely disbanded states. Mode conversion of the ultrasound waves at the solid-solid and solid-liquid wafer boundaries has been exploited to excite shear waves that are sensitive to weak bonds. A short burst transducer excitation and time-selective post-processing of the acquired data is employed to prevent overlap with the direct transmission signal or its echo sequences and in this way making visible the amplitude variation induced by the interface bond degradation.
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