Printed Coil on Quartz Crystal Resonator Sensor for Electromagnetic Contactless Interrogation

This paper presents a novel quartz crystal resonator (QCR) sensor, which embeds a conductive planar coil connected to the QCR electrodes that enables electromagnetic contactless interrogation techniques. The planar coil and the QCR electrodes are printed on a 330 μm-thick bare piezoelectric quartz crystal exploiting the aerosol-jet printing technology. The printed coil acts as the secondary coil of a pair of coupled inductors, allowing for a self-contained sensing element which avoids the need to connect an external coil for interrogation. The adopted interrogation technique, which advantageously enables distance-independent operation, is based on the measurement of the reflected impedance of the QCR sensor through the planar primary coil of the coupled inductors. The resonant frequency of the QCR has been measured without contact using the primary coil connected to an impedance analyser. The measured resonant frequency is 4.790260 MHz, and has a maximum deviation of about 50 Hz, i.e. 10.5 ppm, with respect to reference measurements taken via contact probes. Contactless operation distances up to 12.2 mm have been obtained.