Design Method of Interdigital Transducer Structure Based on Archimedean Spiral Structure and Acoustic Field Characteristics Analysis

Interdigital transducers (IDTs) are important for structural health monitoring (SHM) applications in plate-like structures because of their electromechanical designability and guided-wave excitation capability. To improve the broadband and directional actuation performance of flexible guided-wave transducers, this study proposes an Archimedean spiral interdigital transducer (AS-IDT) based on a bilayer composite substrate composed of polyvinylidene fluoride (PVDF) and polydimethylsiloxane (PDMS). The electrode spacing of the AS-IDT was designed according to the wavelength of the A₀ Lamb-wave mode in a 1-mm-thick aluminum plate. Finite element simulations, device fabrication, frequency-response tests, acoustic-field measurements, and comparison experiments with a conventional annular IDT were carried out to evaluate the proposed structure. The experimental results show that the AS-IDT has a center frequency of approximately 505 kHz and an effective bandwidth of approximately 610 kHz, corresponding to a relative bandwidth of 120.8%. The scanning laser Doppler vibrometer (SLDV) measurements show that the normalized maximum out-of-plane velocity amplitude attenuates at an average rate of 0.929% per mm along the 90° direction. The angle–amplitude–frequency results further indicate that the AS-IDT can produce a directional guided-wave field within the designed radiation sector. Compared with the conventional annular IDT fabricated using the same PVDF/PDMS substrate, the AS-IDT shows a wider effective bandwidth and more concentrated directional actuation under the same flat-plate test conditions. These results demonstrate the potential of the proposed AS-IDT for directional guided-wave actuation in plate-like structural health monitoring.