## 2 Equipping a Vibration-Based NDE Method onto a Drone  
The vibration-based NDE method introduced in this study uses a single piezoelectric (PZT) material to act as both actuator and sensor. The method is known as the EMI technique, first proposed by [PERSON] et al. [4]. Currently, this technique requires one to permanently attach a PZT transducer onto the surface of the target structure, usually in 10 mm square sizes for performing the NDE method [5; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17]. Electrical impedance of the attached PZT transducer is usually measured using equipment such as 4294A (Agilent Technologies, Santa Clara, CA, USA) costing up to 50,000 USD, or the AD5933 evaluation board (Analog Devices, Norwood, MA, USA) costing less than 100 USD, which is light enough for a drone to carry. The attached PZT excites the structure (thickness and radial vibration modes) within a selected frequency range (usually between 20 and 400 kHz), creating a standing wave at each frequency, whilst also acquiring data. By observing the 1-D equation introduced by [PERSON] et al. [4], the measured electrical admittance (inverse of impedance) of the PZT, \(Y(\omega)\) is directly related to the mechanical impedance of the host structure, \(Z_{s}(\omega)\). Thus, any changes in the structure can be detected by measuring the electrical impedance. The rest of the variables, including \(\omega\), \(a\), \(\varepsilon_{33}^{T}\), \(\delta\), \(d_{3x}^{2}\) and \(\overline{Y}_{xx}^{E}\) represent the excitation frequency geometric constant, dielectric constant, dielectric loss tangent, coupling constant, and the [PERSON] Young's modulus. \[Y(\omega)=i\omega a\bigg{(}\varepsilon_{33}^{T}(1-i\delta)-\frac{Z_{s}(\omega )}{Z_{s}(\omega)+Z_{a}(\omega)}d_{3x}^{2}\overline{Y}_{xx}^{E}\bigg{)}. \tag{1}\]  
Data analysis of the EMI method is achieved by comparing two impedance signatures, before and after damage measurements have been recorded. A statistical technique known as the root mean square deviation (RMSD) is used to quantify the variations for the two impedance signatures into a single number. The RMSD equation is shown in Equation (2), where \(Re(Z_{k})_{i}\) is the real part of the reference impedance signature, and \(Re(Z_{k})_{j}\) is the real part of the corresponding impedance signature for each measurement time at the \(k\)th measurement step. In general, the real part of an impedance signature is used as it has been experimentally proven by the authors of this study that it performs better than the imaginary part for damage identification [18]. Here, large variations in the two impedance signatures will result in a higher RMSD value. \[\text{RMSD}=\bigg{(}\sum_{k=1}^{N}\Big{[}Re(Z_{k})_{j}-Re(Z_{k})_{i}\Big{]}^{ 2}\Big{/}\sum_{k=1}^{N}[Re(Z_{k})_{i}]^{2}\bigg{)}^{1/2}. \tag{2}\]  
One of the advantages of the EMI method is the use of the high frequency range, allowing one to detect small defects. However, the sensing area at this frequency range is relatively small and a large number of PZT transducers are required for monitoring large infrastructures. The concept introduced here, for using a drone to monitor structures, requires the modification of the PZT transducer so that it is re-attachable. This means that it should be able to be temporarily attached to a structure, with a connection strong enough for a successful damage detection. To achieve this, the conventional method of permanently attaching the PZT is modified so that it allows temporary attachment to a host structure using magnetic force. This concept is shown in the red box in Figure 1, where the PZT (5A4E, 0.5 mm thickness type purchased from piezo.com) is attached on top of a magnet (size 25 mm \(\times\) 15 mm \(\times\) 3 mm). This allows the PZT device to be easily attached and detached from a structure, as long as the target structure is ferromagnetic. However, if the host structure is non-metallic (e.g., concrete, composite, wooden structure etc.