## 3 Experimental Set-Up and Electronic Equipment
### Electronic Instrument Design for SHM Evaluation  
The main architecture and design of the electronic instrument developed as part of this work has been presented in [21]. The instrument consists of a 16-channel device fully programmable in active or passive modes of operation. One of the main characteristics of this electronic system is the real-time acquisition and processing of ultrasonic signals received by 16 channels, which is an advantage in research of sensor layouts with more  
Figure 1: The 1.4 mm thick aluminum plate with 8-sensor arrangement (red squares). The picture also indicates the ROI (yellow area) and the area outside the ROI (black area) that corresponds to the aluminum laminate’s dimensions. than 3 or 4 channels; the latter being quite a common experimental situation made available with use of a digital oscilloscope. For the passive operation mode treated in this work, we designed 16 new receiver boards that easily replaced the previous ones thanks to the modular architecture of the system. A block-scheme of the new board is shown in Figure 2. The input attenuator is a fundamental block to adjust for different sensor sensitivities and impact energies. For example, impacts with 10 to 100 mJ of energy can be monitored with the 6.5 mm PVDF circular type (see [16; 17]), producing voltage signals in the range of 100 mV to 1 V, while the highest sensitivity of piezoceramic sensors, such as the Physik Instrumente P-876.SP1, allows monitoring of signals up to 300 mVpp from the low-energy impact hammer or free-falling sphere typically used during non-destructive laboratory tests. The piezoceramic material of these types of sensors has a high electromechanical coupling factor and high sensitivity. The two different types of piezoelectric sensors used are shown in Figure 2, and their signal output amplitude was measured by direct connection to a digital oscilloscope; the comparison of the amplitudes is reported in Figure 3 for different impact energies from 0 to 45 mJ. In Figure 3, we can observe different energy values, U = m \(\times\) g \(\times\) h, corresponding to the same value of mass, m, and acceleration due to gravity, g, of the impacting steel sphere: these different values were obtained according to the above relationship by varying the falling height, h.  
These amplitudes are relative to the peak amplitude of the large A\({}_{0}\) mode which is generated by the impact and has low-frequency components, typically below 50 kHz as predicted by [PERSON] in [22]. The data reported in Figure 3 are important in making a decision as to the programmable threshold value depending on the expected impact energy. In this way, the real-time electronics capture the signals with a predefined minimum energy and, thanks to the programmed pre-trigger time, the signal trace also contains the information of signals preceding the large A\({}_{0}\) mode. This is a relevant feature of the electronic system because higher frequency components of the A\({}_{0}\) and S\({}_{0}\) propagate at higher velocities, as explained in Section 4. The electronic analog front-end (AFE) (see Figure 4) is mainly composed of a programmable input attenuator (from 0 to \(-\)20 dB), a unity gain buffer based on an LF347 op amp, a non-inverting amplifier with voltage gain of 25 dB based on an LM6172 op amp, and a band pass filter (BPF). The BPF is designed to obtain an active (op amp LM6172) second-order Bessel HPF with the cut-off frequency (fc1) equal to 270 kHz at \(-\)3 dB, and a passive first-order LPF with the cut-off frequency (fc2) equal to 800 kHz at \(-\)3 dB. The last filtering block was inserted to limit the in-band noise; the noise measurements of the output of the analog chain with a standard 50-ohm input impedance for two different prototype boards are reported in Table 1.  
Figure 2: **(Left)** Piezoceramic sensor PI_876.SP1. Piezopolymer (PVDF) sensor protected by a white adhesive label and electrical connections on a PCB. (Right) Circular element of 6.45 mm diameter shown by the red circle on the CAD drawing.