On the Optimization of Spread Spectrum Chirps Into Arbitrary Position and Width Pulse Signals. Application to Ultrasonic Sensors and Systems

Abstract

This work presents in detail a new method for the optimization of rectangular spread spectrum excitation signals for its use in simple and low-cost ultrasonic pulsers, but that could be extended to other applications based on spread spectrum signals. Starting from rectangular linear frequency modulated (RLFM) chirps, it uses the transfer function of the transmitted signal and received echoes from reference specimens to iterate through a recursive algorithm to obtain arbitrary position and width pulse (APWP) signals with the desired bandwidth, maximizing the energy and the flatness of the spectrum, and enhancing the resolution and dynamic range of conventional chirp excitation signals. This optimization procedure can be repeated for any transducer and material, so that it achieves the best performance in each experimental environment. Such characteristics are ideal for Time-of-Flight estimation, imaging, and applications in which spectral regularity is needed, such as the Split Spectrum Processing (SSP) algorithm, which is used as example to test the performance of the proposed excitation signals. It also allows to specify and change the bandwidth reducing the need to change the transducers. The method is tested with different transducers (2 and 5 MHz focused transducers) and complex composite materials (aluminum-carbon aviation composite and high porosity GFRP composite) in immersion setup for imaging applications using SSP algorithm.INDEX TERMS Ultrasound, Spread Spectrum, chirp, arbitrary position and width pulses, Split Spectrum Processing, non destructive testing.

Coded Excitation for Ultrasonic Testing: A Review

“…A. Rodríguez-Martínez et al used optimized linearly chirped coded pulses combined with split-spectrum processing algorithms to detect defects in carbon fiber-reinforced composites and glass fiber-reinforced composites with significant acoustic attenuation [74]. The results are shown in Figure 22, where a noticeable improvement in the SNR can be observed.…”

“…For most devices, they are only capable of transmitting rectangular pulses with either a high or low level. Therefore, as a compromise, the option is to transmit rectangular linear frequency-modulated (RLFM) signals [74][75][76][77], also known as pseudo-chirp signals, as shown in Figure 10. However, transmitting such alternative signals does come with certain drawbacks.…”

Selection and Design of Excitation Signal in Test System

“…Zhai Guofu proposed in [2] that the multi-mode phenomenon of electromagnetic ultrasonic sensors can be suppressed by changing the frequency bandwidth of the excitation signal, and a single SH-guided wave can be generated for crack detection and corrosion detection technology. Rodriguez-Martinez proposed a new model to introduce an optimized rectangular pulse excitation signal [3], which improves the resolution and dynamic range of the traditional chirped excitation signal and can optimize the performance of the sensor. Meng Deyong proposed a new model to use the code density histogram method in [4], that is, the test excitation amplitude slightly larger than the ADC input range to verify the static parameters of ADC.…”