From Chirps to Random-FM Excitations in Pulse Compression Ultrasound Systems

Pulse compression is often practiced in ultrasound Non Destructive Testing (NDT) systems using chirps. However, chirps are inadequate for setups where multiple probes need to operate concurrently in Multiple Input Multiple Output (MIMO) arrangements. Conversely, many coded excitation systems designed for MIMO miss some chirp advantages (constant envelope excitation, easiness of bandwidth control, etc.) and may not be easily implemented on hardware originally conceived for chirp excitations. Here, we propose a system based on random-FM excitations, capable of enabling MIMO with minimal changes with respect to a chirp-based setup. Following recent results, we show that random-FM excitations retain many advantages of chirps and provide the ability to frequency-shape the excitations matching the transducers features.Comment: 4 pages, 4 figures. Post-print from conference proceedings. Note that paper in conference proceedings at http://dx.doi.org/10.1109/ULTSYM.2012.0117 has some rendering issue

Laisvai pasirenkamos trukmės ir pozicijos impulsų sekos ultragarsinėms vizualizacijos ir matavimo sistemoms

The quality of the ultrasonic measurements is determined by the received signal energy, bandwidth and correlation properties. Ultrasonic transducers and signal propagation alters the spectral content of signals, the signal-to-noise ratio and correlation properties decrease. Conventional signals do not allow these losses to be corrected or inefficiently exploit the amplitude-time range dedicated for the excitation, excitation electronics are complex. New rectangular spread spectrum excitation signals have been proposed: arbitrary position and width pulse sequences (APWP). The novelty of the proposed APWP approach is that the optimization of the APWP sequence accounts the system transmission function, thus enhancing the desired signal properties. Signals combine the useful properties of rectangular pulses and spread spectrum signals, allow to control the correlation properties and spectral shape, do not require complex excitation electronics, and efficiently utilize the amplitude-time range dedicated for the excitation. The proposed signals provide an opportunity to improve the measurement quality when measuring flow, distance or thickness. The results of the work are also applicable in imaging, because the wider spectrum yields a better resolution, while smaller sidelobes and a higher signal-to-noise ratio allow to increase the contrast. Signals are extremely effective in spectroscopy when seeking to maximize the spectral coverage, its smoothness and uniform signal-to-noise ratio over the frequency range

Monitoring system for long-distance pipelines subject to destructive attack

In an era of terrorism, it is important to protect critical pipeline infrastructure, especially in countries where life is strongly dependent on water and the economy on oil and gas. Structural health monitoring (SHM) using acoustic waves is one of the common solutions. However, considerable prior work has shown that pipes are cylindrical acoustic waveguides that support many dispersive, lossy modes; only the torsional T(0, 1) mode has zero dispersion. Although suitable transducers have been developed, these typically excite several modes, and even if they do not, bends and supports induce mode conversion. Moreover, the high-power transducers that could in principle be used to overcome noise and attenuation in long distance pipes present an obvious safety hazard with volatile products, making it difficult to distinguish signals and extract pipeline status information. The problem worsens as the pipe diameter increases or as the frequency rises (due to the increasing number of modes), if the pipe is buried (due to rising attenuation), or if the pipe carries a flowing product (because of additional acoustic noise). Any system is therefore likely to be short-range. This research proposes the use of distributed active sensor network to monitor long-range pipelines, by verifying continuity and sensing small disturbances. A 4-element cuboid Electromagnetic Acoustic Transducer (EMAT) is used to excite the longitudinal L(0,1) mode. Although the EMAT also excites other slower modes, long distance propagation allows their effects to be separated. Correlation detection is exploited to enhance signal-to-noise ratio (SNR), and code division multiplexing access (CDMA) is used to distinguish between nodes in a multi-node system. An extensive numerical search for multiphase quasi-orthogonal codes for different user numbers is conducted. The results suggest that side lobes degrade performance even with the highest possible discrimination factor. Golay complementary pairs (which can eliminate the side lobes completely, albeit at the price of a considerable reduction in speed) are therefore investigated as an alternative. Pipeline systems are first reviewed. Acoustic wave propagation is described using standard theory and a freeware modeling package. EMAT modeling is carried out by numerical calculation of electromagnetic fields. Signal propagation is investigated theoretically using a full system simulator that allows frequency-domain description of transducers, dispersion, multi-mode propagation, mode conversion and multiple reflections. Known codes for multiplexing are constructed using standard algorithms, and novel codes are discovered by an efficient directed search. Propagation of these codes in a dispersive system is simulated. Experiments are carried out using small, unburied air-filled copper pipes in a frequency range where the number of modes is small, and the attenuation and noise are low. Excellent agreement is obtained between theory and experiment. The propagation of pulses and multiplexed codes over distances up to 200 m are successfully demonstrated, and status changes introduced by removable reflectors are detected.Open Acces

Fatigue crack detection on structural steel members by using ultrasound excited thermography = Erkennung von Ermüdungsrissen in Stahlbauteilen durch ultraschallangeregte Thermografie

Ultrasound excitation of structural steel members leads to localised energy dissipation at existent fatigue cracks and thus allows for thermographic flaw detection. Essential effects on the defect-selective heating such as flaw size, plate thickness, crack mouth opening or static preload are systematically investigated. Laser vibrometry measurements of the crack edges, theoretical modelling of frictional heating and numerical simulations contribute to the understanding of the involved physics