## 4 Conclusions  
In this paper, a master multi-slave system based on structural modal frequencies having high reliability on data integrity is presented. The proposed system is able to detect dynamic events, e.g., collision with external bodies, as well as static damages, leading to changes in the internal composition of the structure, hence, in its fundamental frequency. The SHM problem has been widely covered, different solutions have been reported, and the employment of the introduced system has been justified. The designed implementation has been deepened; its master multi-slave and single-slave node structures have been reported. The redundancy of this system can be identified from several points of view: the first point is the presence of CRC in every packet exchanged between master and slave nodes as long as the system is working, and the second point is the multiplicity of communication protocols between master and slave, as a fast but wired serial protocol is used in parallel to a long-range wireless one. Finally, the third point where redundancy is encountered is the structure of the single slave node, as it is composed of two accelerometer-based systems: a main high-resolution accelerometer and a less accurate unit that can be used as a backup if a failure occurs in the main element. System functionality is then described, reporting the working statuses and the possible operations that it can perform. The system was tested on a modular structure where the detachment of a part represented structural damage. Responses of the slave nodes were then analyzed considering two scenarios: the first with the intact structure and the second with its alteration. The reliability of the backup accelerometer over the main one was analyzed by observing the system's response to environmental stresses. Subsequently, periodic stimuli were induced in the first and second scenarios in order to observe the variation in the system response. In these situations, the output from the two accelerometers in the time and frequency domains was analyzed and compared. A difference in damping time, structure main frequency, and modal harmonic  
Figure 12: Time response retrieved from the main accelerometer installed on node 5 before (**a**) and after (**b**) the structural alteration. collocation were noticed. From this outcome, it was possible to observe and evaluate the system frequency response variation after a structural alteration in accordance with theory. The main limitation of our approach is the lack of methods for the interpretation of the results, motivated by the aim to detect changes in structure frequency response after its damage. An idea for further work is to use this system for multiple application fields, e.g., weigh-in-motion (WIM). In this sense, the sampling frequency of the system could be a limit as it decreases measurement resolution, especially in FFT and spectrogram setups; hence, the detection of minimum deviations in modal harmonics is not guaranteed. For this reason, in future works, we aim to improve the system both in terms of internal hardware and management of each slave node in order to reach plausible data sampling rates of up to 5 kSPs. The main idea behind this improvement is to integrate an external RAM memory which stores data after each mission as to lighten the workload of the internal flash memory and increase data sampling speed. Another important update to the system is the implementation of an event triggering internal mechanism which automatically activates a mission when an environmental stimulus is detected; this could lead to a better independence of the system in terms of measurements planning and real-time operation. Other factors to be considered for future works are the correlation between retrieved data, as well as several statistical analyses, in order to increase the reliability of our results. The implementation of this system in a real application scenario is an important topic, as it depends on the shape and composition of the structure.