*2.2. Laboratory Setup*

The laboratory setup consisted of a signal generator (40 MHz DDS Function/Arbitrary Generator, TG4001, Thurlby Thandar Instruments Ltd., United Kingdom), in which an excitation was defined in the form of a 2.5 sine wave modulated by a Hanning window. The operational frequency was set to 52 kHz. Then, the signal was amplified and split into actuators and synchronization channels. Two digital oscilloscopes (WaveSurfer 424 and WaveRunner 104MXi, LeCroy, NY, USA) were used to store the signals

received from all the sensors. Piezoelectric transducers (CMAP6, plate 5 × 5 × 2 mm, Noliac) were mounted on the bolt heads (excitation) and at the ends of their shanks (response). Sensor wax was used to attach them (this enabled trouble-free recovery of all the sensors), while their cables were fixed to a single spot with a weak adhesive to hold them during the test, allowing for non-invasive removal.

As usual, the measuring system used (piezoelectric transducer, amplifier, signal generator) has its limitations. This applies to both the excitation frequency and the signal amplification. This is one of the most important factors that affects the obtained accuracy of identification. In this case, the frequency and gain were selected in such a way that the response signal had the highest SNR at the highest possible frequency.

A set of static tests was carried out using the testing machine Instron J1D 1200 kN (Figure 3). To evenly load the oscilloscopes that store the measurement data (saving data at the same time), it was assumed that each one would register the pattern of the excitation signal (in order to synchronize the recorded signals in time) and the response from the two screws.

**Figure 3.** Laboratory setup: (**a**) Piezoelectric transducers mounted at the bolt ends (a washer-type force sensor is also visible in this case). (**b**) Digital oscilloscopes, function generator, and amplifier. (**c**) Static test machine Instron J1D 1200 kN.
