Piezoelectric thick-film fluid density sensor based on resonance vibration

This paper presents the first thick-film resonant fluid density sensor. The sensor is based on a ceramic diaphragm system which is forced to vibrate in a selected flexural mode. The resonant frequency changes with the density of the fluid due to the change of the inertial mass of the vibrating 96% alumina substrate. A simplified analytical modelization demonstrates that the resonance vibration frequency depends on the weighted density ρ*, as ≈√1/ρ*, where ρ* is the density of the ceramic diaphragm and the fluid weighted in accordance to their area in the plane of motion. The sensor was fabricated using standard screen-printing and firing procedures typical in the development of thick-film prototype. The device has been connected in a close loop circuit making use of a PLL structure that allows the system to impose and to maintain the resonance condition under different load cases. The design of the sensor and its classic analysis are reported together with experimental characterization in seven different fluids