## 5 Conclusions  
This paper has presented a combined hardware and software solution capable of performing highly accurate measurements in a WiFi-based meshed configuration of sensing nodes. The main focus was the coherent software framework developed for off-the-shelf and cost-effective microcomputer hardware such as RPi, turning them into sensing nodes that also perform network communication. A smartphone app that facilitates the initiation and management of the measurement process and allows for data processing and storage is presented. Further, the current implementation, by polling their internal sensors, allows for measurement with smartphone devices integrated into the mesh. Several test measurements that focus first on the specific data acquisition characteristics of the systems in a meshed configuration are reported. The results confirm the high performance of the measurement system in terms of stable sampling at high sampling rates up to 1 kHz and an accurate time synchronization between nodes, with time shifts being reliably under 0.2 ms. A second set of tests showed that the systems can perform well in typical SHM tasks such as the frequency measurement and force computation of stay cables, velocity measurement based on acceleration sensing, and modal identification of structures. Limitations seen in the results provided are mostly due to the use of very cheap MEMS-based sensors, where the costs of a single node are below 50 E, not accounting for power supply and enclosure.  
In addition to the quality of measurements, workflows are relatively simple and user-friendly due to the highly integrated nature of hardware and software components. The RPi-based monitoring system presented is a flexible, highly mobile, cost-effective, and robust system that has been found to  
Figure 15: Comparison of the identified mode shapes obtained by reference system () and RPi-based measurement system (). The first five mode shapes are displayed along with the theoretical modes shapes ().  
offer measurement characteristics in a meshed configuration that is sufficient for a wide range of SHM applications and that are superior to many standard WSN systems. Finally, the solution presented here is not based on any proprietary technology and lends itself well to teaching purposes.  
Conceptualization: G.M.; data curation: J.F.E. and S.R.; investigation: G.M.: J.F.E. and S.R.; methodology: G.M.; project administration: G.M.; software: J.F.E. and J.T.; supervision: G.M.; validation: G.M., J.F.E., and S.R.; visualization: J.F.E.; writing--original draft: [PERSON]; writing--review & editing: [PERSON].  
This research received no external funding.  
The authors declare no conflict of interest.  
The following abbreviations are used in this manuscript:  
ADC Analog-to-Digital Converter  
AP Access Point  
API Application Programming Interface  
DHCP Dynamic Host Configuration Protocol  
FFT Fast Fourier Transform  
GPIO General Purpose Input Output  
HNA Host and Network Association  
HTTP Hypertext Transfer Protocol  
IP Ingress Protection  
IP Internet Protocol  
I2C Inter-Integrated Circuit  
MAC Media Access Control  
MAC Modal Assurance Criterion  
mDNS Multicast Domain Name System  
MEMS Micro-Electro-Mechanical Systems  
NTP Network Time Protocol  
OLSR Open Link State Routing  
PZT Piezoelectric lead zirconate Titanate  
QFC Queensferry Crossing  
RPC Remote Procedure Call  
RPi Raspberry Pi  
SHM Structural Health Monitoring  
SPI Serial Peripheral Interface