A pressure sensor was developed based on a piezoelectric bending resonator. The resonator is covered and mechanically coupled with a sealed enclosure. The impedance spectrum of the resonator changes with the deformation of the enclosure induced by pressure or force applied to the enclosure. The changes in the impedance can be mapped to exchanges in the external environment, and the shifts in the resonance can be used to track the pressure.
The passive, downhole, high-pressure sensor is readable remotely from the surface using an electromagnetic system that also has a pressure range up to 20 to 200 MPa, with a minimum electric impedance at resonance frequency
To accomplish this, a mechanically coupled high-pressure cylindrical enclosure is used to seal a piezoelectric bending resonator with a resonance in low-RF frequency range. The mechanical coupling makes the resonance frequency of the resonator sense the pressure outside of the enclosure. By using high-performance piezoelectric ceramic material, and by maintaining low-pressure air in the enclosure, the resonator maintains high mechanical Q and low electric impedance at resonance.
The circular bending resonator has two piezoelectric disks mounted on a circular plate. The disks are poled in the same direction, from bottom to top. An electric terminal connects to two outside electrodes on the disks. The other two electrodes on the disks are grounded to the circular plate and to the enclosure. The plate and the cylinder tub of the enclosure are made of one piece.
Finite element analysis shows the feasibility of the design concept. This analysis was performed for sensors with PZT-8 disks and a titanium plate. The enclosure is 25 mm in diameter and 60 mm in height. The minimum electric impedance is predicted to be 30 ohms at resonance.