The impedance response of the PZT sensors was collected with a Newtons4th PSM 3750 frequency analyser (Newtons4th Ltd., Leicester, UK) interfaced with an impedance analyser. The impedance analyser was connected to the PZT sensors through 50 Ω coaxial cables, which were connected at one end to the impedance analyser, and the other end connected to the wires which were soldered to the PZT sensors.