PSI - Issue 72

G. Gusev et al. / Procedia Structural Integrity 72 (2025) 464–469

466

Fig. 1. (a) Plan of pile-soil system; (b) Photo of the upper part of the pile and the attached mass (protective cover removed).

A digital MEMS accelerometer ADXL355 (produced by Analog Devices) was used to measure the vibrational response. It converts the three components of vibration acceleration into a digital signal. The x and y axes of the sensor lie in the horizontal plane, while the z axis is vertical, aligned with the pile (see Fig. 1a). The accelerometer has a frequency range from 0 to 1000 Hz. Its high resolution (20 bits over a ±2g range) and low noise floor allow detection of both relatively weak vibrations and the response to an impact load. The accelerometer was connected to the Raspberry Pi 3b+ via an SPI bus, and three DS18b20 digital temperature sensors were connected via a 1-wire bus: one measured the ambient air temperature, another measured the soil surface temperature, and a third measured the pile temperature at a depth of about 0.2 meter On the Raspberry Pi 3b+, a schedule-based algorithm was implemented to activate the actuator and simultaneously record data from the accelerometer and the temperature sensors. The experiment data were stored in files and transferred to a server via the FTP protocol. Data processing- extracting the signal parameters was carried out on the server. 3. Experimental data processing The vibrational signals were processed, and their parameters were extracted as follows. First, a Fourier transform was used to obtain the vibration spectrum (Fig. 2a), and the natural frequencies were determined from the peaks in the spectrum (in the example in Fig. 2a, there are two frequencies at about 20 and 100 Hz). Next, for each frequency, the damping ratio was estimated. A Fourier filter was applied to the signal in a narrow band around the natural frequency, and then a fragment was selected where the signal decayed. Next, the Hilbert transform was used to obtain the signal envelope, which was approximated by an exponential function of the form exp( - α t ). Hence, the decay rate α was obtained (see Fig. 2b, where the red curve shows the exponential). As is well known, the equation describing damped oscillations can be written in the form:

2

(1)

2 x x      n n

0

x

Damping ratio ς c an be expressed in terms of α and ω n = 2 π f n as follows:

n 

(2 )







The values of ς and f n were estimated for the measured data over the entire duration of the experiment.