## 6 Energy Autonomous Failure Detection System
The first is by increasing the system losses by the association of diodes that may drop the input voltage [28]. Secondly, it influences the DC-DC converter efficiency, which is related to the input voltage [28]. The procedure to reach fast-charging was investigated experimentally by testing the time to recharge the super capacitor after communication with the lower acceleration of 0.2 g. The results show that the system can work properly within VM stage numbers higher than five. Besides, for five stages of VM, the recharge time is about 2 min and 6 s, whereas, for six VM, the required time for re-charging the super capacitor is around 31.9 s (Figure 13). This is related mainly to the used DC-DC converter, which works with higher efficiency in the case of higher input voltage. The system efficiency directly affects the required time to charge the super capacitor in the initial phase and the recharging time. The proposed failure detection system was evaluated under two different working conditions. In the first case, a gradual acceleration change was realized to follow the system's response to progressive changes. The second case involved triggering an abrupt change between two acceleration levels of the vibration converter to study the response time and adaptability to changes in the system. Specifically, we aimed to study the response time and output voltage of the complete proposed solution in both cases. In the first scenario, the applied acceleration varied from 0.1 g to 0.25 g with a small step of approximately 0.015 g. During the increase in the applied acceleration, the measured acceleration at the harvester output was collected along with the probability of failure, as shown in Figure 15. The experimental acceleration presents some noise in comparison with the expected acceleration variation, which can result from the associated variations from the devices in proximity. In this case, an unexpected change in the acceleration is not regarded as a failure; since the failure needs to occur for a certain time other than this, it is regarded as erroneous acceleration due to a real working environment. Therefore, the transmitted message of failure is sent only when the real failure occurs with acceleration above 0.25 g after 10 min, as shown in Figure 15a. At the same time, as the acceleration increases, the recharging time decreases, which is reduced due to the increase in the input voltage of the energy-management circuit. In Figure 15b, the ADC input of the energy-management circuit is presented. The ADC shows a gradual variation of the voltage, which follows the increase of the applied acceleration. This proves that the EM converter can provide a continuous and sustainable power supply for the WSN. The response time of the proposed failure detection system was determined based on the comparison between the real failure existence and the declaration of the failure at the base station. In Figure 16, a representation of the real acquisition data between the real acceleration changes and the identification of failure is plotted. The failure occurred after 644 s from the initialization of the system, which was declared at the base station after approximately 669 s from the initial state. Hence, failure detection was realized after 25 s from its actual occurrence. Figure 15: Failure detection for different (**a**) acceleration levels and (**b**) ADC input voltages. Besides, some errors can typically occur when the usual communication happens simultaneously with the error caused by the acceleration. This effect can be shown by alternating between failure and normal conditions several times. In the second scenario, an abrupt change of the acceleration between 0.2 and 0.25 was realized to study the fast response of the system. Figure 17 shows an alternation between 0.2 g and 0.25 g acceleration. When the acceleration was less than 0.2 to 0.23, the ADC input voltage increased from about 1.3 V, where a message that indicated that there was no failure was shown (Figure 17a).