## 5 Results and Discussion
### Discussion of Results
#### 5.3.1 Measurement Accuracy
The sensor's underlying technology may introduce limitations that must be considered when selecting the sensors. In this case, the sensor's ultrasonic emitter-receiver has a nearby area where the signal is not correctly registered. This feature must be taken into account when positioning the sensor to avoid adverse effects. Figure 12 shows values of the measurements taken by the wireless temperature sensor and the official test sensor. In this case, a better performance of the wireless sensor is obtained, with much lower noise levels than the hardwired sensor. The tendency of the measured temperatures is very similar, with an average difference in values of 1.8%, due to the high noise levels. Applying a moving average to the official values shows that the difference in measurements is in the order of 0.05%. These values demonstrate the ability of the proposed AIoT network to efficiently and accurately monitor the temperature of the specimen in the official test. The high measurement accuracy and low noise levels of the temperature sensor make the sensor model selected for this test also valid for other tests with more stringent requirements, such as thermal tests with low temperature gradients, where the measurement frequency does not exceed 1 Hz. Figure 12: Graphical representation of the data recorded with the temperature sensor and the official sensor of the test. The trend of the plotted values is increasing since they correspond to the initial zone of the day where the temperature of the environment increases slightly, affecting the temperature of the specimen, and then decreases again at dusk. This thermal effect is tightly controlled by laboratory environmental conditions, minimizing its effect on specimen behavior. The temperature variation due to this effect is less than 1 \({}^{\circ}\)C. Temperature variations due to rolling and friction between moving parts of the specimen must be negligible to maintain its structural integrity. Otherwise, significant temperature increases could occur, potentially reducing the strength and stability of the component. Figure 13 shows the inclination values measured in one of the test cycles. It is observed how the inclination varies from an initial position until reaching a minimum value, and then returns to the initial position, completing the loading cycle. The values measured by the wireless sensor and the wired sensor are very similar, with an average difference of 1.2%, validating the measurement capability of the AIoT architecture in low-frequency structural fatigue tests. These measurements show that the specimen undergoes a rotation in each cycle of approximately 13\({}^{\circ}\). The tilt sensor used allows much larger ranges of tilt to be measured, so its applicability to other fatigue tests with larger rotation angles is guaranteed. The rotational acceleration allowed by the sensor also ensures its applicability to other structural fatigue tests. However, this sensor would not be valid for dynamic testing of vibration modes, where accelerations typically reach 100 g and test frequencies exceed 1000 Hz, the latter being the main limitation. It can be noticed that the local variations in the tilt value follow the same evolution measured by both sensors, but that the wireless sensor has a slight offset with respect to the official one. Different measurement technologies could provide different values under specific conditions. However, in this case the measurement principle of both sensors was the same, so this gap of values is due to differences in the positioning of the sensors, i.e., slight differences in the directions of the measurement axes, which have a cumulative effect and whose initial value is carried over the entire measurement cycle. This fact shows the high variability of inclination measurements with the initial direction of the axis of rotation. To mitigate this effect, a fixture can be used as a guide to position the tiltmeter. Figure 13: Graphical representation of the data recorded with the inclination sensor and the official sensor of the test.