## 2 Result and Discussion
The data in Figure 3e were also from the mean of five sets of experimental data, and the error bar was taken from the standard deviation. It can be seen that the SAVWS could monitor the accelerations from 1.6 m/s\({}^{2}\) to 3.7 m/s\({}^{2}\) by changing the mass. The load was a transmitter module, which was composed of a voltage monitoring circuit and a wireless transmitter. The circuit diagram of the transmitter module is shown in Figure S7. The voltage monitoring circuit could be regarded as an electronic switch, which could monitor the voltage of C\({}_{2}\) when the module was woken up by the MEMS switch. When the voltage of C\({}_{2}\) exceeded 2.9 V, the wireless transmitter would start working. After the voltage of C\({}_{2}\) fell below 2 V, the voltage monitoring circuit would turn off the wireless transmitter. This process could prevent the voltage of C\({}_{2}\) from being too low to affect the operation of the transmitter. As shown in Figure 3f, when the transmitter module sent out an alarm signal, the voltage of C\({}_{2}\) dropped from 2.9 V to 2 V. Because the capacitance C\({}_{2}\) was 330 uF, it could be concluded that only 0.64 mJ energy was needed for the wireless transmitting module to send an alarm signal. Because of the protection of the voltage monitoring circuit, the voltage of C\({}_{2}\) should be at least 2.9 V before the system could start  
Figure 3: Working principle and performance of SAVWS. (**a**) The circuit schematic diagram of the E-TENG and PMS. (**b**) The voltage waveform of C\({}_{2}\) and the waveform of I\({}_{0}\). (**c**) The circuit schematic diagram of the control circuit of the MEMS switch. (**d**) The voltage waveform of the C\({}_{0}\). (**e**) The change in acceleration threshold at different mass. (**f**) The voltage change in the transmitter module when it works, and the power-on time. (**g**) The change in power-on time under different accelerations. sending alarm signals. The time that C\({}_{2}\) took to increase from 0 to 2.9 V was called the power-on time. Furthermore, the change in power-on time under different accelerations was shown in Figure 3g. The power-on time decreased from 60 s to 8.4 s as the acceleration increased from 1 m/s\({}^{2}\) to 4.5 m/s\({}^{2}\). Since the abnormal acceleration in a machine is an occasional event, the maximum power-on time of 60 s could fully meet the performance requirements of the system. ### Working Principle and Performance of SAVWS  
The SAVWS can be installed in mechanical equipment and use the energy harvested from the vibration of the machines to monitor the working state of machines (Figure 4a). When the acceleration of the machine is abnormal, the SAVWS sends alarm signals, and the receiver transfers the alarm to the computer through the serial port. To further explore the working process of SAVWS, the working environment of the SAVWS was simulated in the laboratory. A shaker was used to provide vibration with a frequency of 30 Hz to simulate the vibration of machines during the experiment. The mass was 700 g, which meant the acceleration threshold of the system was about 2.5 m/s\({}^{2}\). The experimental environment is shown in Figure 4b. The photos of the transmitter unit and receiver unit are shown in Figure 4c,d. Figure 4: Application of the SAVWS for the sustainable and autonomous wireless monitoring system. (**a**) The diagram of the SAVWS for a wireless monitoring system. (**b**) Experimental environment and  
photos of SAVWS components. (**c**) Photos of the transmitter unit. (**d**) Photos of the receiver unit. (**e**) The stored and regulated voltage waveforms in the working processing. During the working process, the voltage waveform of the C\({}_{2}\) indicated the working state of SAVWS (Figure 4e). In the initial state, the E-TENG had no output and the voltage of C\({}_{2}\) was zero. When the acceleration arrived at 2 m/s\({}^{2}\), the E-TENG started charging the capacitor C\({}_{2}\) through the PMS.