Strain sensing using flexible surface acoustic wave sensor

Surface acoustic wave (SAW) sensors offer overwhelming advantages over other competitive sensing technologies due to its small size, cost-effectiveness, fast response time, passive and wireless capabilities. Development of SAW sensors allows investigation of their potential not only for measuring less-time dependent parameters, such as pressure and temperature, but also dynamic parameters like mechanical strains. The concept behind this work is to develop a passive flexible SAW sensor with optimized materials selection that can be used in harsh environments to measure mechanical strains occurring in aerospace applications. A flat 0-3 composite thin substrate is fabricated using a hot-press, an interdigital transducer (IDT) finger deposition is made through additive manufacturing. The sensor substrate comprises polyvinylidene fluoride as a polymer matrix, lead zirconate titanate powders as well as carbon nanotubes as nanoparticle fillers, exhibiting favorable flexibility and piezoelectric properties. The electromechanical property is enhanced using a non-contact corona poling technique with high electric field. IDT fingers are printed using direct printing additive manufacturing technique of conductive paste. Design parameters of SAW IDTs are optimized using a second-order transmission matrix approach. Rayleigh waves, generated on the fabricated substrate by an RF excitation signal, travel through the substrate and can provide useful information for desired parameters. In this work the sensing mechanism is based on the radio frequency scattering parameters response of the device. Results show a correlation between the amplitude and phase frequency response of the scattering parameters, and the mechanical strain. Experimental study on SAW substrate fabrication and analysis of sensed results with phase shift in wave speed due to strains are discussed.