1. Automatic dynamic focusing through interfaces
J. F. Cruza, J. Camacho, J. M. Moreno, C. Fritsch
UMEDIA Group, Consejo Superior de Investigaciones Científicas (CSIC)
La Poveda (Arganda del Rey), Madrid, Spain
Background, motivation and objectives
An interface between the coupling medium and the inspected part is frequently found in
Non Destructive Testing (NDT). When phased array technology is used, the focal laws
must be computed taking into account the refraction at the interface. This requires
computing intensive iterative procedures, since no closed formulae exist for the general
case. Furthermore, these focal laws must be stored in the equipment, which may require
a considerable amount of memory. These problems become harder with dynamic
focusing, since the process must be followed for a large amount of foci. Therefore, there
is a great interest on finding methods that address these problems.
Statement of contribution / methods
This work presents a new approach based on two steps. In the first step, the two
propagation media scenario is converted into a single homogeneous medium by
computing a virtual array with equivalent flight times to the foci. In the second step, a
focusing hardware conveniently initialized evaluates in real-time the sampling instants
that correspond to the focal laws.
The main idea is to avoid the cumbersome processes associated to computing the focal
laws through the interface. By removing the interface, step 1 provides an array that
operates in a homogeneous medium, so that focusing law computing is faster, in fact as
fast as for in-contact inspections. But, furthermore, step 2 provides a method that yields
a few values to initialize a specialized focusing hardware. This hardware automatically
performs the dynamic focusing at all the output samples.
The work addresses the applicability limits of the proposed technique (minimum range,
focusing errors, etc.) for arbitrarily shaped interfaces and several propagation media.
Focusing errors are obtained by comparison of the time-of-flight estimated by the new
technique with the theoretical values obtained by numerical procedures using double
precision arithmetic.
The focusing hardware was implemented in our own technology. It requires very few
FPGA resources. Then, the full procedure has been experimentally tested. Images of an
aluminium block with side-drilled holes using a plastic wedge are compared with those
obtained with conventional off-line focal law computing procedures.
Results, discussion and conclusions
Focusing errors are small enough to validate the new technique for a wide range of
applications. In fact, for active apertures currently used in NDT, the resolution and
dynamic range are almost not affected, as it is experimentally shown. Furthermore, the
computing time is dramatically reduced by evaluating a few parameters instead of
focusing delays for every output sample, element and steering direction. The instrument
storage requirements become significantly reduced as well.