Ultra-Low-Cost Passive 3D-Printed Vibration Transducers for Condition Monitoring by means of Wireless Chipless Transponders

This paper presents a novel displacement transducer, based on a 3-dimensional coupled-line directional coupler, suitable for wireless passive vibration sensing. The 3-D printed transducer is based on two quarter-wave microstrip lines, which are aligned one on top of the other so that part of the signal flowing in one line is coupled to the other. The four ports of the directional coupler are realized on the same layer by adopting two transmission-line transitions. The circuit is manufactured with stereolithography 3-D printing. Selective metallization is achieved by embossing the desired patterns on the 3-D printed dielectric and then applying silver nanoparticle ink to the protruding areas. Vibrations change the distance between the lines, thereby modulating the coupling coefficient of the directional coupler. This way, the sensed vibration can be encoded in the amplitude modulation (AM) of a backscattered RF carrier. This information can be transmitted in real time and recovered by the reader using a simple AM demodulator without the need for any additional electronic component on the tag, that, thus, results in an ultra-low-cost chipless sensor.