## 6 Energy Autonomous Failure Detection System  
Failure detection was carried out based on changes in the measured acceleration of the EM converter. In this case, when the measured acceleration exceeds \(\pm\)0.25 g, the system declares the current case as a failure and sends alert messages to the control unit. In practice, an increase in the acceleration refers to an increase in the generated voltage level by the electromagnetic converter to reach a voltage above the voltage threshold noted as Vthr. The designed EM converter presents a voltage threshold of 2 V. The threshold is defined by a priori vibration measurements on the machine during operation. The complete execution scenario of the proposed failure detection is illustrated in Figure 12. The wireless sensor node was connected to the voltage output of the EM converter. The node continuously measures the voltage and compares the obtained results to the defined Vthr. In the beginning, the \"status\" register of the WSN is initiated as \"0\" to indicate the normal working mode of the harvester. In case the obtained voltage exceeds the Vthr, the WSN updates its register's status to \"1\". The voltage data and the status information are transmitted to the base station for processing and failure identification. Considering the characteristic behavior of the vibration converter, the obtained voltages are converted to acceleration values, whereas the status information is used to print out the existence of a failure or not. The proposed system works in two major modes, where the first is starting with initially a fully charged super capacitor and the second mode is the recharge of the super capacitor after each established communication by the WSN that consumes the energy stored. The wireless sensor node requires a supply voltage between 3 V and 3.3 V, with a total energy consumption of 25 mA on each measurement and communication cycle. For three consecutive messages, the power consumption of the node is about 225 mW. For this, the super capacitor is calculated based on Equation (5), and it should be higher than 0.6 E. The selected super capacitor has a size of 0.69 F and is composed of dual parallel capacitors with capacities of 0.49 F and 0.2 F. Figure 13 shows the charging of the super capacitor process for an acceleration of 0.3 g. The super capacitor is fully charged in around 14 min, where the recharging time is estimated to be 0.41 min. The voltage of the LTC3588 \(P_{\text{Goad}}\) pin can be activated during the charging process when a high current injection by the fully charged voltage multiplier capacitors causes an increase in the super capacitor voltage to 3 V for a short time. When the voltage of \(P_{\text{Goad}}\) reaches 3.3 V, the UVLO circuit connects the charged super capacitor to the WSN that can send a message to the base station. When communication is established, the super capacitor voltage decreases and the voltage of \(P_{\text{Goad}}\) reduces until the super capacitor gets charged again (Figure 13b). The time of the recharging of the super capacitor after the communications is related to the input voltage level that is generated by the electromagnetic converter. The time is meas  
Figure 12: Flow-chart of the failure detection system based on the voltage output of the EM converter. different accelerations from 0.195 g to 0.3 g (Figure 14). For a low acceleration level of 0.2 g, the startup time of the system is about 45 min, where the recharging time is around 28 s, whereas, for a high acceleration of 0.25 g, the recharging time is about 24 s. Typically, the system can deliver the current situation information within a few seconds. Figure 14: Time to recharge of the super capacitor for different acceleration levels. Figure 13: (**a**) Charging of the super capacitor and (**b**) re-charging of the super capacitor after communication. The selection of the number of VM stages influences the ability to charge the super capacitor under different conditions. The increase in the number of VM stages influences the system in two different ways.