Y. Yañez

Ultrasonic Lamb wave NDE system using an air coupled concave array transducer

Rapid and non contact ultrasonic NDE techniques are of great industrial interest. This paper describes an air-coupled ultrasonic inspection system based on concave linear arrays working on pitch-catch configuration, The system has been designed for real-time characterisation of sheet and plate manufactured materials such as paper and resin-fibre composites. The proposed system is based on air-coupled Lamb wave excitation and reception that performs a rapid measurement of the optimum input angle of the incident beam impinging the material surface. No mechanical parts are used for tuning the plate wave excitation with the angle, doing that electronically by steering the acoustic beam. This solution increases the exploration velocity and the measurement repeatability and system reliability. The main contributions are related to the utilization of a 0.8 MHz ultrasonic air-coupled concave array transducer. This transducer, using only 32 elements, is able to generate a 2.5 square cm size flat wavefront, steering up to /spl plusmn/26 degrees with 1.6 degree resolution, keeping the distance of wave flight and the impact point constant. The angular resolution can be improved up to 0.2 degrees using a novel micro-deflection technique, without any increment of the system complexity.

Wide dynamic range electronics for air-coupled Lamb-waves ultrasonic NDT using piezoelectric transducer arrays

The excitation and front-end electronics for an ultrasonic transducer array used in an air-coupled NDT system is presented. The main characteristic of the whole system is its wide dynamic range, making possible the inspection of thin and porous materials. The system is composed of two 32 elements concave array with a resonant frequency of 780 kHz. To excite the transducers, an array of pulsers has been designed. The transducers are excited by 500 ns and 500 V electrical pulses. The receivers are ultra low noise amplifiers with a 10 MHz bandwidth and 80 dB of gain.

Study of the time-dependence of the mechanical properties of doughs for flour strength evaluation

Both flour strength and dough processing affect the dough consistency that determines its potential for breadmaking purposes. Quick identification of poor dough quality would reduce problems with dough handling during further stages of the process and maximise productivity. Maintaining consistent production would contribute to better control of product quality and consequently lead to high levels of customer satisfaction. Ultrasonic measurements have already been carried out to characterise dough properties. The ultrasonic wave parameters generally measured include the velocity of propagation and the attenuation of the acoustic wave traveling through the sample. These measurements can be related to both viscoelastic and physical properties of the sample, providing the flour strength. However, due to the time-dependent nature of dough accurate measurements of the ultrasonic velocity and attenuation are sometimes difficult to attain especially in highly attenuating materials like dough. Furthermore, due to viscoelastic properties of dough when a sample is placed between both transducers it slowly flows away from the transducer surface producing changes in the values of the ultrasonic velocity and attenuation with time. The greater these changes are the softer the dough is. This makes necessary a settling time in order to get an accurate measurement. In this work, an alternative method for evaluating the flour strength using low intensity ultrasound is shown. The evolution with time of both velocity and attenuation is monitored and then related to the flour strength. Main advantage of this novel approach is that changes in time of ultrasonic velocity and attenuation are easy to monitor than carry out accurate measurements of them after a settling time. Experimental results on doughs with different flour strength are presented, compared and discussed. Automation of the food industry requires fast and reliable measurements of the physical properties of materials during processing. The mixture of wheat flour, water, yeast and other ingredients produces a dough with specific viscoelastic characteristics capable of retaining gas and producing aerated goods. Within the baking industry, the control of dough properties is required to achieve final product quality and consistency. Traditional methods for dough testing are slow and off-line and do not provide fundamental rheological information. There is therefore a need for the development of fast and on-line instruments capable of providing relevant data for baking. The ultrasonic wave parameters generally measured include the velocity of propagation and the attenuation of the acoustical wave traveling through the sample. These can be related to various of its physical properties. Sensors and instruments based on non-contact methods are especially attractive to the food industry to be employed in quality assurance, process control and non-destructive inspection (1-3), for being both hygienic and easy to maintain. However, there is still a need to develop new techniques that can perform precise evaluations of dough and flour quality. Currently, there are only a few studies using ultrasound for characterising flour-water systems (4-9). In this paper, the ultrasonic measurements of velocity and attenuation are used for the classification of flours intended for different purposes and are compared with conventional flour testing methods. The time-dependence of the mechanical properties of dough is also studied and the results are related to flour strength. The purpose of this study is to determine the potential of ultrasound for use to predict flour and dough quality by millers and bakers by means the determination of flour strength. Section II gives a short outline of the experimental procedure and set-up used during the experiments. In Section III the experimental results are explained and discussed in detail. Finally, conclusions are made in Section IV.

P3F-1 Lamb Wave Generation with an Air-Coupled Piezoelectric Array Using Different Square Chirp Modulation Schemes

This paper describes a new Lamb wave excitation technique using square chirp signals for air-coupled inspection of laminate materials in a non destructive testing system. The airborne ultrasonic waves are generated with an air-coupled ultrasonic plane array transducer with 15 active elements. Each array element is excited with a square chirp signal in order to apply pulse compression techniques. The advantages of using chirp signals are a greater SNR when using pulse compression (compared with burst or single pulse), a more precise time of flight calculation and a greater use of the transducer bandwidth. However, a decrease in the signal amplitude is expected due to the dispersion of the Lamb waves.

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 systems 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).