Luca Senni

Exploiting Pseudorandom Sequences to Enhance Noise Immunity for Air-Coupled Ultrasonic Nondestructive Testing

The application of a pulse-compression procedure based on pseudorandom m-sequences to air-coupled ultrasonic nondestructive testing is presented. The signal-to-noise ratio enhancement assured by this technique is theoretically analyzed and experimentally verified, particularly focusing on the positive effect of the procedure to reduce quantization noise. The possibility of faithfully reconstructing signals whose amplitudes are lower than the quantization step of the analog-to-digital converter (ADC) board is analyzed theoretically and experimentally verified.

Exploiting Non-Linear Chirp and sparse deconvolution to enhance the performance of pulse-compression ultrasonic NDT

A pulse-compression procedure based on Non-Linear Chirp for Ultrasonic Non Destructive Testing is presented. Non-Linear Chirp signals can be tailored to reproduce any desired continuous spectrum. Here, such capability is exploited to take into account the features of the specific hardware set-up in use, and particularly to adapt the excitation signal to the probes. An application of the proposed procedure to Air-Coupled Ultrasound Imaging is also presented. In order to enhance the imaging resolution, L1-norm Total Variation deconvolution is exploited and compared with standard L2-norm Wiener deconvolution. Although the usage of Air-Coupled probes entails a high attenuation of the ultrasonic signals due to transmission in air and reflections at the air-sample interface, the experimental results show that the combination of Pulse Compression technique and of advanced signal processing guarantees the effectiveness of the inspection.

Nonlinear convolution and fourier series coefficients estimate

In the present the paper an original description of the procedure for the modeling of nonlinear systems based on the socalled non linear convolution approach [1, 2, 3] is reported. This approach relies on the modeling of a nonlinear system by means of the linear convolution and we show how, in the case of memoryless nonlinear systems, the method is related to the Fourier Series representation of nonlinear systems output. Based on these observations a Fourier Series implementation of the model is proposed, which allows a significant reduction in the associated computational cost. Experimental results confirm the validity of the proposed Fourier Series implementation of the nonlinear convolution based modeling technique.

From chirps to random-FM excitations in pulse compression ultrasound systems

Pulse compression is often practiced in ultrasound Non Destructive Testing (PDF) systems using chirps. However, chirps are inadequate for setups where multiple probes need to operate concurrently in Multiple Input Multiple Output (PDF) arrangements. Conversely, many coded excitation systems designed for PDF miss some chirp advantages (constant envelope excitation, easiness of bandwidth control, etc.) and may not be easily implemented on hardware originally conceived for chirp excitations. Here, we propose a system based on random-FM excitations, capable of enabling PDF with minimal changes with respect to a chirp-based setup. Following recent results, we show that random FM excitations retain many advantages of chirps and provide the ability to frequency-shape the excitations matching the transducers features.

Frequency modulated continuous wave ultrasonic radar

Pulse compression techniques relying on chirp excitation are largely adopted in both diagnostic and Non-Destructive Testing applications for ultrasound. In the present work, the adoption of the Frequency Modulated Continuous Wave Radar technique to ultrasonic Non-Destructive Testing is considered. Numerical simulations on both synthetic and experimental data are performed and the results are compared with those obtained by means of the standard matched-filter approach. Moreover, in order to improve the resolution of the technique, the Chirp-Z-Transform is exploited instead of the Fourier Transform. By proper design of the excitation signal, the Frequency Modulated Continuous Wave Radar approach retrieves the envelope of the pulse-compression output signal without the need of any correlation step at the heart of the matched filter approach. This feature allows for an integrated hardware elaboration, relaxing both the computational and the ADC resources.

A pulse compression procedure for the measurement and characterization of non-linear systems based on exponential chirp signals

Ultrasonics diagnostic and non-destructive evaluation techniques exploiting non-linear phenomena such as harmonics and sub-harmonics generation, non-linear propagation, etc., are more and more diffused due to the benefits they assure in terms of spatial resolution and SNR for specific noise components. Nonetheless some issues still hamper a widespread diffusion of non-linear ultrasonics applications mainly due to the low energy associated to the harmonics or sub-harmonics generation or to the high excitation power needed to trigger the non-linearities. Despite the better resolution, the resulting inspection range of such techniques is typically lower than that of common procedure relying on the assumption of “linearity” of the system. Strategies able to increase the inspection capability are therefore desirable. At the same time it is worth trying to exploit all the information enclosed in the linear and non-linear response instead of using only a part of it. For this reason, in the present paper we review a pulse-compression procedure based on exponential chirp developed for the characterization of the acoustic distortion that could be extremely useful in non-linear ultrasonics applications. The measurement procedure provides the benefit of Pulse Compression in term of SNR enhancement and contextually allows the responses associated to the different harmonics to be easily separated. A detailed theoretical discussion of the technique is reported, and it is also explained how to extend the procedure to deal with sub-harmonic generation.

Comparative study between linear and non-linear frequency-modulated pulse-compression thermography

Pulse-compression thermography is an emerging non-destructive technique whose effectiveness strictly depends on the choice of the coded excitations used to modulate the heating stimulus. In this paper, the features of frequency-modulated coded signals, i.e., chirps, have been tested for imaging thin Teflon defects embedded within a carbon fiber composite specimen. With the aim of maximizing the heat transferred within the sample, the use of several optimized non-linear chirp signals has been also investigated and their defect detection capability compared in terms of the maximum achievable signal-to-noise ratio.

Reactance transformation to improve range resolution in pulse-compression detection systems

The presence of side-lobes in the impulse response retrieved after pulse compression is a limitation in the range resolution performance of detection systems; in order to limit this effect the signal is often shaped by means of windows. In this paper we analyse the causes that produce side-lobes and propose a technique for their reduction, based on time-domain constraints which, in the case of base-band signals, turn in the definition of a particular low-pass filter. In real world applications, the pulse compression is performed by using bandpass signals as excitation, and among these the chirp signals are largely adopted: the useful bandwidth is thus shifted from the base-band. We consequently modify the filtering procedure defined for processing base-band signals, and propose a reactance transformation of the frequency axis to define the filter for the reduction of side-lobes in the case of band-pass chirp excitations. The paper shows that the application of the proposed filtering technique to chirp signals is particularly simple. The effectiveness of the technique is validated on experimental test data.

Chirp design in a pulse compression procedure for the identification of non-linear systems

The Hammerstein model of a nonlinear systems can be efficiently identified by means of a technique based on pulse compression. The procedure relies on the properties of the exponential chirps, adopted as excitation signals. The present paper proposes to include the initial phase of the chirp, together with its time duration and frequency extremes, among the parameters to design the excitation signal. Considering this widened set of design parameters, we identify the constraints that the excitation signal must meet to carry out an accurate identification. The paper shows that introducing the initial phase as further degree of freedom, adds flexibility to the design process and allows for the use of much shorted chirp signals, making measurements faster and reliable. The experimental results reported demonstrate the validity of the constraints identified and show that, choosing appropriate combinations of the parameters, very short chirp accomplish the phase constraint needed for an accurate modelling of the non-linear system.

Detection of rebars in concrete using advanced ultrasonic pulse compression techniques

HighlightsUltrasonic testing has been used to inspect concrete samples.The use of pulse compression technique and coded signals has been proposed.A novel post processing of the acquired signals has been proposed.The proposed combination has allowed signals from reinforcement bar to be detected. ABSTRACT A pulse compression technique has been developed for the non‐destructive testing of concrete samples. Scattering of signals from aggregate has historically been a problem in such measurements. Here, it is shown that a combination of piezocomposite transducers, pulse compression and post processing can lead to good images of a reinforcement bar at a cover depth of 55 mm. This has been achieved using a combination of wide bandwidth operation over the 150–450 kHz range, and processing based on measuring the cumulative energy scattered back to the receiver. Results are presented in the form of images of a 20 mm rebar embedded within a sample containing 10 mm aggregate.