Giuseppe Silipigni

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.

Reducing pulse compression sidelobes by means of a reactance transformation

Pulse compression is a well-known technique widely used within the Non-Destructive Testing community [1]. Although pulse compression helps in increasing Signal-to-Noise ratio, the presence of side-lobes in the impulse response retrieved by its use, hampers the range resolution performance. A widely-used approach to face this problem consists in tapering the signal excitation by windowing it in time domain. Several different kinds of windows can be found in literature, and they optimize different aspects related to the sidelobes reduction [2].