ABSTRACT
The Total Focusing Method (TFM) yields a focused image in emission and in reception while Phased Array (PA) imaging provides Dynamic Depth Focusing (DDF) in reception only. Besides, most NDE applications have two propagation media, where refraction at the interface complicates time-of-flight (TOF) and focal law computations. This affects especially TFM, which must compute the TOFs from all elements to image pixels and use them to select the data for imaging. A new method with real-time Dynamic Depth Full Focusing (DDFF), in emission and reception, is proposed in this work. It is called Total Focusing Phased Array (TFPA) because it uses concepts of TFM and PA. Omnidirectional emissions are used to create a synthetic aperture as in TFM, while beamforming is carried out along scan lines as in PA, simplifying the delay calculation in the presence of interfaces and providing an efficient hardware implementation. Refraction at the interface between two media is eliminated by a Virtual Array (VA) that converts such scenario into a simple homogeneous medium. Propagation can be considered along scan lines from the virtual array at constant speed, as in homogeneous media. Strict dynamic focusing is performed in real-time, an important difference with other approaches that require iterative Fermat search to get the focal laws for every imaged point. With TFPA only 3 parameters per element and scan line are required to perform this task. Experiments are carried out to compare the three techniques, PA, TFM and TFPA. TFM and TFPA yield similar image quality, offering improved depth of field and resolution over PA. On the other hand, TFPA avoids most of the burden for computing TOFs and operates in real time with one or two media propagation.
ABSTRACT
Auto-focused virtual source imaging (AVSI) has been recently presented as an alternative method for synthetic aperture focusing through arbitrarily shaped interfaces with arrays. This paper extends the AVSI concept to the case of the total focusing method (TFM-AVSI) using several virtual receivers for each virtual source. This approach overcomes the known contrast limitation of AVSI, while preserving the advantage of performing synthetic focusing in the second medium only [no time-of-flight (TOF) calculations through the interface]. In contrast, equipment with more active channels must be used to digitalize the signals received by all the array elements after each focused emission. When compared with the conventional TFM, the proposed method reduces the processing complexity of the most time consuming task: TOF calculation in the presence of interfaces. This improvement could lead to more efficient real-time implementations of the TFM in non-destructive testing applications where water immersion or flexible wedges are used. In this paper, the mathematical formulation for the new method is given, accounting for the surface slope and the array angular sensitivity. Its performance is evaluated by numerical simulation, experimentally and compared with AVSI and the conventional TFM. It was found that the TFM-AVSI achieves the same resolution and contrast as that of the TFM, although it shows a wider blind zone below the interface due to focusing with normal incidence.
ABSTRACT
This work presents a new method, named auto-focused virtual source imaging (AVSI), for synthetic aperture focusing through arbitrarily shaped interfaces with arrays. First, the shape of the component surface is obtained by time-of-flight (TOF) measurements. Then, a set of virtual source/receivers is created by focusing several array subapertures at the interface normal incidence points. Finally, the synthetic aperture focusing technique (SAFT) is applied to the received signals to generate a high-resolution image. The AVSI method provides several advantages for ultrasonic imaging in a two-media scenario. First, knowledge of the probe-part geometry is not required, because all information needed for image formation is obtained from a set of ultrasonic measurements. Second, refraction complications in TOF calculations are avoided, because foci at the interface can be considered as virtual source/ receivers, and SAFT can be performed in the second medium only. Third, the signal-to-noise ratio is higher than with synthetic aperture techniques that use a single element as emitter, and fourth, resolution is higher than that obtained by phased-array imaging with the same number of active elements, which reduces hardware complexity. The theoretical bases of the method are given, and its performance is evaluated by simulation. Finally, experimental results showing good agreement with theory are presented.
ABSTRACT
In nondestructive evaluation (NDE) a coupling medium (wedge) is frequently inserted between the array probe and the object being evaluated. In this situation, focal law computing is complicated by the refraction effects at the interface. Furthermore, there are not known techniques to perform dynamic focusing by hardware in these conditions. This work addresses these problems by following a two-step procedure. First, a virtual array that operates in a single medium with nearly equivalent time-of-flight to the foci is obtained. Then, simple hardware is proposed to perform dynamic focusing in real-time. It operates with arrays of any geometry as required by the virtual array in presence of arbitrarily shaped interfaces. The paper describes the theory and evaluates the timing errors of the approximations made. These errors are low enough to allow use of the new technique in most NDE and some specific medical applications. The new technique is validated by simulation and experimentally.
