Passive, wireless SAW OFC strain sensor

Surface acoustic wave (SAW) technology, when paired with orthogonal frequency coding (OFC), has proven to be a versatile platform for the design of passive, wireless sensors. This paper presents the design and demonstration of a passive, wireless SAW strain sensor that is uniquely identifiable in a multi-sensor system. A cantilever fabricated out of the SAW substrate acts as the strain sensing mechanism. Force applied at the end of the cantilever causes a strain distribution at the root of the beam. In turn, a measurable change in the SAW propagation delay occurs between the SAW transducer and OFC reflector bank. Relating SAW propagation delay changes to strain is accomplished by developing a simple 1-D model. This model assumes strain in only the Z direction (YZ-LiNbO₃) which allows the stiffness matrix to be ignored and an effective stiffness constant to be used instead. Derivations for strain coefficient as well as the strain and force equations needed to measure strain are given. The sensor test setup and experimental results are detailed and discussed. Additionally, a magnetic field sensor is demonstrated as an application for this design.