Piezoelectric sensors are well known. They are used for sensing material properties such as viscosity and density, for detecting the presence of certain materials in an environment, for measuring purity of fluid substance, and the like. Structures known for acoustic sensing range from the simple crystal resonator, crystal filters, acoustic plate mode devices, Lamb wave devices, and the like. Briefly, these devices comprise a substrate of piezoelectric material such as quartz, langasite or lithium niobate, or thin films of piezoelectric material, such as aluminum nitride, zinc oxide, or cadmium sulfide, on a non-piezoelectric substrate. The substrate has at least one active piezoelectric surface area, which is highly polished. Formed on the surface are input and output transducers for the purpose of converting input electrical energy to acoustic energy within the substrate and reconverting the acoustic energy to an electric output signal. These transducers may consist of parallel plate (bulk wave) or periodic interdigitated (surface-generated wave) transducers. It is noted that a single transducer may act both as the input and the output transducer.
Each of the afore-mentioned sensors can be designed to operate while being fully immersed in the fluid. However the sensitive electronics are then subjected to, in the least, noise signals and reading errors and, in the extreme, to corrosion or even explosive hazards. Passivation of the electronics surface is well known and is suitable in some limited applications, as seen for the Love Wave and surface transverse wave (STW) sensors, and such are described for example by R. L. Baer, C. A. Flory, M. Tom-Moy and D. S. Solomon, “STW Chemical Sensors,” Proc. 1992 Ultrasonics Symp., pp. 293-298 (1991). However passivation is not complete and electrical components of the circuit are still exposed to capacitive loading and noise injection. Moreover, most passivation methods require the use of material having poor acoustic characteristics compared to single crystal materials. Finally, these passivated STW sensors exhibit undesirably high shear rate for many liquid phase measurements. While such sensors potentially address many sensor applications, they are not ideal, for instance, in measuring fluids in oil production, especially in down-well environments.
In most applications the surface opposite the transducers is in direct or indirect contact with the fluid being measured and interfaces acoustic energy to and from it. In addition to the interface function, the piezoelectric material forms a protective membrane between the fluid and a cavity containing electrical components of the sensor. As the volume behind the piezoelectric material is commonly not pressurized to the same level of the fluid, the piezoelectric material is exposed to the pressure difference between the fluid and the pressure within the cavity. Therefore, the finite strength of the material limits the operating pressure to which the sensor may be exposed. Even if the material is sufficiently strong to withstand the pressure, the nonlinear effect on the sensor of membrane flexure will severely affect the sensor characteristics.
On the other hand many technology areas may benefit from measuring fluid with low sensitivity to pressure variations or at high pressure levels. Examples of such technologies include by way of a non-limiting example, gas production, oil well and oil pipes, hydraulic systems, injection molding equipment, anti terror detection system for detection of biological and chemical substances, and the like. Therefore there is a long felt and heretofore unanswered need in the industry for an electro acoustic sensor capable of operating with low sensitivity to pressure variations, and/or in high ambient pressure environments. The present invention is directed to a solution to that need.