Methodology for the Generation of High-Quality Ultrasonic Images of Complex Geometry Pieces Using Industrial Robots.

Industrial robotic arms integrated with server computers, sensors and actuators have revolutionized the way automated non-destructive testing is performed in the aeronautical sector. Currently, there are commercial, industrial robots that have the precision, speed and repetitiveness in their movements that make them suitable for use in numerous non-destructive testing inspections. Automatic ultrasonic inspection of complex geometry parts remains one of the most difficult challenges in the market. The closed configuration, i.e., restricted access to internal motion parameters, of these robotic arms makes it difficult for an adequate synchronism between the movement of the robot and the acquisition of the data. This is a serious problem in the inspection of aerospace components, where high-quality images are necessary to assess the condition of the inspected component. In this paper, we applied a methodology recently patented for the generation of high-quality ultrasonic images of complex geometry pieces using industrial robots. The methodology is based on the calculation of a synchronism map after a calibration experiment and to introduce this corrected map in an autonomous, independent external system developed by the authors to obtain precise ultrasonic images. Therefore, it has been shown that it is possible to establish the synchronization of any industrial robot with any ultrasonic imaging generation system to generate high-quality ultrasonic images.

Design of Ultrasonic Synthetic Aperture Imaging Systems Based on a Non-Grid 2D Sparse Array.

This work provides a guide to design ultrasonic synthetic aperture systems for non-grid two-dimensional sparse arrays such as spirals or annular segmented arrays. It presents an algorithm that identifies which elements have a more significant impact on the beampattern characteristics and uses this information to reduce the number of signals, the number of emitters and the number of parallel receiver channels involved in the beamforming process. Consequently, we can optimise the 3D synthetic aperture ultrasonic imaging system for a specific sparse array, reducing the computational cost, the hardware requirements and the system complexity. Simulations using a Fermat spiral array and experimental data based on an annular segmented array with 64 elements are used to assess this algorithm.

Design of 2D Planar Sparse Binned Arrays Based on the Coarray Analysis.

The analysis of the beampattern is the base of sparse arrays design process. However, in the case of bidimensional arrays, this analysis has a high computational cost, turning the design process into a long and complex task. If the imaging system development is considered a holistic process, the aperture is a sampling grid that must be considered in the spatial domain through the coarray structure. Here, we propose to guide the aperture design process using statistical parameters of the distribution of the weights in the coarray. We have studied three designs of sparse matrix binned arrays with different sparseness degrees. Our results prove that there is a relationship between these parameters and the beampattern, which is valuable and improves the array design process. The proposed methodology reduces the computational cost up to 58 times with respect to the conventional fitness function based on the beampattern analysis.

The Signal Projection Method: A General Approach to Active Imaging With Arrays.

Using a generic model of how active array imaging systems operate, a formal definition of the image as a projection of the reflected wave field onto the wave field emitted to probe the medium is proposed. This definition is applied to the case of imaging in homogeneous and isotropic media to which, by using the frequency-domain solution of the wave equation, the relationship between the calculated image, the emitted and received signals, the positions of the transducers, the speed of the waves used, and the reflectivity of the medium is shown at each point. Then, a general algorithm for the formation of images of a region in this kind of media is derived. Finally, using real and simulated ultrasound signals, images obtained with the proposed signal projection method are compared with those produced by the "de facto" standard delay and sum-based algorithms.

Defect detection in anisotropic plates based on the instantaneous phase of signals.

Anisotropic materials are widely employed in industry and engineering, and efficient nondestructive testing techniques are important to guarantee the structural integrity of the involved parts. A simple technique is proposed to detect defects in anisotropic plates using ultrasonic guided waves and arrays. The technique is based on the application of an objective threshold to a synthetic aperture image obtained from the instantaneous phase (IP) of the emitter-receiver signal combinations. In a previous work the method was evaluated for isotropic materials, and in this paper it is shown that with some considerations the technique can also be applied to anisotropic plates. These considerations, which should be taken into account in beamforming, are (1) group velocity dependence with propagation direction, and (2) elastic focusing, which results in energy concentration in some propagation directions, with the practical consequence that not all aperture signals effectively contribute to the image. When compared with conventional delay-and-sum image beamforming techniques, the proposed IP technique results in significant improvements relative to defect detection and artifacts/dead zone reduction.

Instantaneous phase threshold for reflector detection in ultrasonic images.

A method for reflector detection, based on the instantaneous phase of the aperture data for ultrasonic images, is proposed. The instantaneous phase (IP) image is obtained by replacing the amplitude information by the instantaneous phase in the delay-and-sum (DAS) beamforming. From the analysis of the IP image, a threshold level is defined in terms of the number of signals used for imaging. This threshold is applied to the IP image, resulting in a two-level image which gives a statistical indication of whether the pixels of a region in the image are related to a reflector or noise/artifacts. Because the proposed method uses only the instantaneous phase of the signals, it is less sensitive to attenuation than conventional DAS amplitude images. The point spread function of a 32-element array with half-wavelength pitch at 5 MHz in water is simulated and the reflector is detected for signal-to-noise ratio values larger than -29.6 dB. A phantom and an aluminum plate with artificial defects are tested with the proposed technique, using linear arrays of 64 and 16 elements, respectively. When compared with DAS amplitude images and with two-level images obtained by thresholding the amplitude images using empirical threshold values, the proposed technique reduced artifacts and dead zone, and detected all reflectors, increasing reflectors' detectability and decreasing the occurrence of false indication of reflectors. The proposed technique can be used as additional information for amplitude image analysis, with the advantages that it does not need time-gain compensation and that it considers an objective threshold value.

An ultrasonic imaging system based on a new SAFT approach and a GPU beamformer.

The design of newer ultrasonic imaging systems attempts to obtain low-cost, small-sized devices with reduced power consumption that are capable of reaching high frame rates with high image quality. In this regard, synthetic aperture techniques have been very useful. They reduce hardware requirements and accelerate information capture. However, the beamforming process is still very slow, limiting the overall speed of the system. Recently, general-purpose computing on graphics processing unit techniques have been proposed as a way to accelerate image composition. They provide excellent computing power with which a very large volume of data can easily and quickly be processed. This paper describes a new system architecture that merges both principles. Thus, using a minimum-redundancy synthetic aperture technique to acquire the signals (2R-SAFT), and a graphics processing unit as a beamformer, we have developed a new scanner with full dynamic focusing, both on emission and reception, that attains real-time imaging with very few resources.

2D array design based on Fermat spiral for ultrasound imaging.

The main challenge faced by 3D ultrasonic imaging with 2D array transducers is the large number of elements required to achieve an acceptable level of quality in the images. Therefore, the optimisation of the array layout, in order to reduce the number of active elements in the aperture, has been a research topic in the last years. Nowadays, array technology has made viable the production of 2D arrays with larger flexibility on elements size, shape and position, allowing to study other configurations different to the classical matrix organisation, such as circular, archimedes spiral or polygonal layout between others. In this work, the problem of designing an imaging system array with large apertures and a very limited number of active elements (N(e)=128 and N(e)=256) using the Fermat spiral layout has been studied. As summary, a general discussion about the most interesting cases is presented.

Coarray synthesis based on polynomial decomposition.

Synthetic aperture (SA) techniques have been frequently used to reduce the volume and complexity of the imaging systems. A useful tool for designing synthetic aperture configurations is the coarray. This is the virtual aperture that produces in one way the same beam pattern as the SA system in emission and reception. In this correspondence, we propose a new algorithm, based on the polynomial decomposition, that allows to obtain any wanted coarray on a linear array using whatever synthetic aperture configuration. With this fast and simple algorithm, the desired coarray is decomposed into a set of sub-apertures, whose length is determined by the requirements and resources of the system. The result is the set of weights that have to be applied on the sub-apertures to get the desired coarray, and consequently, the wanted beam pattern.

Damage characterization using guided-wave linear arrays and image compounding techniques.

In this work, a high-resolution imaging method for the inspection of isotropic plate-like structures using linear arrays and Lamb waves is proposed. The evaluation of these components is limited by the low dynamic range resulting from main lobe and side lobe field patterns, and from the narrowband nature of the Lamb waves. Based on a full matrix array, synthetic aperture technique using all emitter-receiver combinations, different images from the same object are obtained by using different apodization coefficients, which are related to a trade-off between main lobe width and relative side lobe level. Several image compounding strategies have been tested and a new algorithm, based on apodization and polarity diversities between signals, is proposed. However, some effects, such as the dead zone close to the array and reverberations caused by interactions of the wavefront and defects, still limit the quality of the images. The use of spatial diversity, obtained by an additional array, introduces complementary information about the defects and improves the results of the proposed algorithm, producing high-resolution, high-contrast images. Experimental results are shown for a 1-mm-thick isotropic aluminum plate with artificial defects using linear arrays formed by 30 piezoelectric elements, with the low dispersion symmetric mode S0 at the frequency of 330 kHz.