  Monitoring of civil infrastructures is critically needed to track aging,
damages and ultimately to prevent severe failures which can endanger many
lives. The ability to monitor in a continuous and fine-grained fashion the
integrity of a wide variety of buildings, referred to as structural health
monitoring, with low-cost, long-term and continuous measurements is essential
from both an economic and a life-safety standpoint. To address these needs, we
propose a low-cost wireless sensor node specifically designed to support modal
analysis over extended periods of time with long-range connectivity at low
power consumption. Our design uses very cost-effective MEMS accelerometers and
exploits the Narrowband IoT protocol (NB-IoT) to establish long-distance
connection with 4G infrastructure networks. Long-range wireless connectivity,
cabling-free installation and multi-year lifetime are a unique combination of
features, not available, to the best of our knowledge, in any commercial or
research device. We discuss in detail the hardware architecture and power
management of the node. Experimental tests demonstrate a lifetime of more than
ten years with a 17000 mAh battery or completely energy-neutral operation with
a small solar panel (60 mm x 120 mm). Further, we validate measurement accuracy
and confirm the feasibility of modal analysis with the MEMS sensors: compared
with a high-precision instrument based on a piezoelectric transducer, our
sensor node achieves a maximum difference of 0.08% at a small fraction of the
cost and power consumption.
