Digital processing system for an air-coupled concave array NDT system

This work presents a processing subsystem for an air- coupled concave array NDT system, which uses Lamb waves. This processing system is able to equalize, delay and add the signals provided by a circular concave array used to receive Lamb waves in laminate materials, such as paper. A Field Programmable Gate Array (FPGA) is used in order to achieve shorter processing times and more flexibility than with DSP systems. The used array has circular concave shape and hence the delays used to get a plain wavefront are not lineal, thus a table with the delay values must be stored inside the FPGA. The Non-Destructive Testing is a growing field of interest for those industries where the high value of the developed components implies the need of a better quality control. In most of these cases the materials cannot be destroyed or altered, therefore a Non Destructive Test is needed. Ultrasonic testing is widely used to test a great variety of materials, but ultrasonic waves need a couplant to get a good transmission coefficient between the inspected material and the ultrasonic transducer. When air is used as couplant, new problems appear, like the higher attenuation that ultrasound suffers in this element. However, the greatest problem is the huge acoustic impedance mismatch that exists between air and the transducers. This mismatch, which is 40(PZT) to 0.006(Air), impedes a good transmission of the energy from the transducer to the inspected material. To improve the bad transmission coefficient, matching layers are included in the transducer (1) and improvements in the system's dynamic range are achieved (2). Lamb waves (3) are also widely used to inspect laminate materials. The main benefit which Lamb waves contribute in is the ability to inspect great portions of material in a short time, making them ideal for inspections where a great area has to be inspected. Moreover, the usefulness of Lamb waves in industrial applications where paper is tested has been demonstrated (4), (5). The usual way to produce a Lamb wave in a plate is to impact a plain wavefront with a certain angle respect to this plate. Lamb wave ultrasonic techniques are mostly based on the application of single element ultrasonic transducer with specific configurations, where the single element is specifically oriented to excite specific Lamb wave modes, see (6). The angular range where Lamb waves can be excited is very narrow in air (7), therefore a system that provides high angular resolution is needed. An array system would be a good solution to obtain this high angular resolution. It also has two main advantages, a transducer array will allow accurate beam steering, and the signal - to - noise ratio will improve. Furthermore, the array has a circular concave shape, so that it raises the angular steering margin of the array. The array used in this paper and its characteristics are deeply described in (8).