## 2 Equipping a Vibration-Based NDE Method onto a Drone
), a support magnet must be attached to the host structure, as shown in Figure 1. The experiment setup is also shown in the figure, where a laptop is connected to the AD5933 evaluation board with the positive and negative wires connected to the PZT device. The drone used for the study is the commercial product model SYMA X5 HW, costing 60 USD. The package provides an application for a smart phone for real time visualization through a Wi-Fi connection. The wires of the PZT device are attached to the drone, where it is controlled by the user. Although the PZT device is connected to the laptop through the AD5933 evaluation board, it is possible for one to convert the board into a wireless system, eliminating the USB data cable shown in the figure. Such a system has been investigated by several authors for attempting to create a wireless impedance measuring device using the AD5933 chip. Some of the first wireless impedance measuring devices using this chip were created by [PERSON] et al. (2007) and [PERSON] et al. (2008). [PERSON] et al. presented a wireless system using the AD5933 impedance measuring chip with an Xbee 2.4 GHz radio (MaxStream, Lindon, Utah, USA) and an ATmega128L microcontroller (Atmel, San Jose, CA, USA). The performance of the proposed system was demonstrated by comparing the results with those from the data acquisition system (National Instruments, Austin, TX, USA), measuring the impedance up to 90 kHz [19]. [PERSON] et al. created an extremely compact wireless impedance measuring system with dimensions of 52 mm \(\times\) 3.7 mm, using an ATmega128L microcontroller. The performance of the system was compared to an Agilent 4294A impedance analyzer protecting against corrosion, loosening and tightening bolts. Due to its small size, the system has a very low power consumption, which would be suitable for the drone monitoring concept shown in this study [20]. First, since the EMI method is a vibration-based technique, it is vital to investigate whether vibrations caused from the drone (wind or motor spinning) have any effect on the impedance signatures. In order to check this, five impedance signatures were measured at hovering state in order to evaluate  
Figure 1: Vibration-based drone structural health monitoring concept. the repeatability performance of the drone equipped with the EMI method. Figure 2 shows the results of measuring the impedance signatures in the frequency range from 30 to 70 kHz, during which virtually no change in the signatures can be seen. This is an expected result as the EMI method uses a high frequency range over 20 kHz and vibrations from the drone (e.g., motor blade, wind) are assumed to be much lower than this value. Here, the largest difference between any two impedance signatures did not exceed the value of 0.66%. Thus, this value can be defined as the threshold value for the study and any values below 0.66% should be ignored when identifying damage. Having established that the impedance measurements are barely affected by the vibrations created from the drone, the next step was to evaluate the repeatability of attaching and reattaching the PZT device onto a structure. Since the impedance signatures are highly sensitive, reattaching the PZT device can have a significant impact on the impedance signature. In order to investigate this matter, the PZT device is detached and reattached to a metal plate with dimensions of 100 mm \(\times\) 100 mm \(\times\) 0.15 mm, with impedance signatures being measured each time in the 55 kHz to 70 kHz range. For the purpose of the test, the reattachment was conducted in two ways. First, the reattachment was achieved by manually controlling the drone when placing the PZT device at the corner of the metal plate, where the magnetic force of the device allowed it to be temporarily attach to the surface. This process is shown in Figure 3a where the impedance signatures were measured 10 times for every reattachment of the PZT device.