Design of a printable multi-functional sensor for remote monitoring

This paper proposes a novel printable multi-functional passive sensor for remote monitoring. The sensor mainly consists of a series of pairs of transmission lines and sensing resistors whose resistances vary with one physical parameter. A short-duration radio frequency pulse as interrogation signal travels along the transmission line and is partially reflected at each resistor due to impedance mismatch. By measuring the energies of the discrete reflected pulses in time domain, all the physical parameters could be detected simultaneously. This design not only saves complex circuitry but also enables easy adaptation for detecting multiple parameters. We have theoretically analyzed the sensor assuming it has an arbitrary number of sensing resistors. The introduced algorithm between the pulse energies and resistances is verified by simulation. As a prototype, an inkjet-printed sensor on polyimide foil is presented. The experimental measurement has successfully proven the design concept.