## 3 Description of the Measurement System and Experimental Test
### Wireless Sensor Nodes Description  
A rise in the development of wireless sensors has been observed in recent years, since they allow to perform monitoring activities on mechanical structures where the installation of wired devices is not feasible, and the power supply for their operativity can be harvested from environmental sources without the need of a wired electrical supply. Examples can be found in applications developed for vibration and pressure monitoring on freight wagons [20,21], and the monitoring of wind turbines [22] and bridges [23]. Some applications of wireless sensors can also be found in the context of active control [24,25].  
In the framework of HVTL, the authors developed a system composed of wireless sensor nodes to perform accelerometric measurements on HVTL conductors with the aim of obtaining, in real time, from the line, the fymax parameter (Figure 3a), which is a fatigue indicator standardly employed in laboratory fatigue tests. Through a suitable logic, the sensor measurements can also be used to identify the different wind-induced phenomena occurring on the conductors, namely aeolian vibrations, subspan oscillations, and galloping [26]. This system is therefore employed in the field of HVTL for the estimation of the residual life of fittings and conductors and to study countermeasures for limiting the impact of wind-induced phenomena and aeroelastic effects on the structure [27,28,29].  
The realized sensors are compact and light devices composed of a plastic box, a custom printed circuit board (PCB), and a rechargeable lithium battery. To reduce the measurement noise, the sensor and the clamping system have been developed with a total mass target below 1 kg. The electronic part accounts for approximately 200 g, meaning that further weight reduction can be achieved through optimizing the clamp shape. Thanks to the low power consumption and the energy inflow coming from a mini photovoltaic panel mounted on the top of the enclosure, the sensors are autonomous from the power supply point of view and can therefore be adopted for continuous monitoring in long-term campaigns.  
On the PCB, several components are present. Among them, the most significant ones are a microcontroller, a Bluetooth transceiver, a temperature sensor, and a MEMS triaxial accelerometer. Since the accelerometer is a triaxial MEMS type, it allows the measurement of the acceleration static components on the three axes. This feature allows to use the accelerometer as an inclinometer, and it is therefore possible to compute the inclination angle of the device through Equation (21).  
\[\theta=\arctan\left(\frac{a_{x}}{\sqrt{{a_{y}}^{2}+{a_{z}}^{2}}}\right)\frac{1 80}{\pi}, \tag{21}\]  
where \(a_{x}\), \(a_{y}\), and \(a_{z}\) are the static acceleration components with the reference system shown in Figure 3b. As can be observed, the angle to be measured for the sag estimation lays in the x-z plane (vertical plane).  
The accelerometer's main features are listed in Table 1, while a complete description of the sensors can be found in [30; 31]. The idea at the basis of this paper is to take advantage of the versatility of these devices already employed in the field of HVTL for monitoring sag with continuity in time.  
Figure 3: Wireless sensors developed for vibration monitoring of HVTL conductors: (**a**) Three sensors mounted on conductors of an operative HVTL; (**b**) Reference system of the triaxial MEMS accelerometer equipping wireless sensors.