Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices
Reexamination Certificate
2000-06-08
2001-10-16
Dougherty, Thomas M. (Department: 2834)
Electrical generator or motor structure
Non-dynamoelectric
Piezoelectric elements and devices
Reexamination Certificate
active
06304021
ABSTRACT:
PRIOR ART
The invention is directed to a method and apparatus for operating a microacoustic sensor arrangement, in particular for eliminating impurities in the sensor arrangement, according to the preamble of the main claim.
Microacoustic sensor arrangements are used with so-called acoustic SAW or surface wave components (SAW=Surface Acoustic Wave) as sensors for a wide range of physical quantities, especially in liquids. In this connection, an important field is that of the measurement of electrical quantities such as the dielectric constant and/or conductivity, the measurement of mechanical quantities such as density and/or viscosity and the examination of chemical properties, e.g., the presence of specific substances in liquids.
A known sensor arrangement is based on a measurement principle described, for example, in the article “A Study of Love-wave Acoustic Sensors”, J. Du, G. L. Hardling, P. R. Ogilvy and M. Lake, in the technical journal Sensors and Actuators A56 (1996), pages 211 to 219. In the measurement construction described in this article, a sensor is realized which operates with horizontally polarized acoustic shear waves, so-called leaky waves or surface skimming bulk waves (SSB waves) or Love waves. These acoustic wave modes are generated and also detected by interdigital transducers, as they are called, which are also known per se from the above-mentioned prior art, so that the desired sensor signal can be obtained from the propagation behavior on a propagation or measurement path.
Depending on the required measurement construction, different materials and arrangements are used for the sensor elements, e.g., a determined substrate material for the sensor elements, a given wave propagation direction, possibly also a specific layer construction on the substrate material and a determined arrangement of the sensor elements formed as electroacoustic transducers. In this connection, one or more of the above-mentioned acoustic wave modes, known per se, occur; these acoustic wave modes differ from one another with respect to possible measurement sensitivity, propagation speed, an acoustoelectric coupling factor and susceptibility to transverse effects, etc. and therefore determine the suitability of a specific sensor type for a specific measurement task.
As was mentioned, the acoustic wave modes described above and known from the prior art have to do with horizontally polarized acoustic shear waves in which a wave propagating along the surface of the substrate on which the electroacoustic transducer is located is utilized.
Aside from measurement sensitivity, other boundary conditions such as contamination, aggressive media in a liquid and cross-sensitivities must be taken into account when using SAW components as sensors, especially for examining liquids. When used in liquid media such as those applied in the automotive field (e.g., oils, fuels, combustion liquid, etc.) or in biological sensor equipment, susceptibility to contamination plays a central role because a depositing of particles on the surface of the sensor leads directly to corruption of the measurement signal due to the increased mass. Therefore, as regards practical use of these sensors, where regular exchange of the sensor element can often not be tolerated, the risk that components of the measurement fluid will be deposited on the surface of the sensor or substrate constitutes an important problem.
ADVANTAGES OF THE INVENTION
The above-mentioned method for operating a microacoustic sensor arrangement, in particular for eliminating contamination in the sensor arrangement, and an apparatus for carrying out the method are advantageously further developed according to the invention by the characterizing features of the main claim and apparatus claim. According to the invention, a method is advantageously provided by which a cleaning of sensor arrangements of the generic type can be achieved during operation without external auxiliary means.
The invention makes use of the characteristic that, aside from the shear modes, other acoustic wave modes with predominantly sagittal polarization (for example, Rayleigh modes, as they are called) occur in many substrate materials for the electroacoustic transducers, e.g., in certain quartz sections, lithium tantalate sections and lithium niobate sections which can be used for the above-described SAW sensors with acoustic shear modes.
Because of the particle movement in these wave modes which extend vertical to the substrate surface, pressure waves are directly radiated in a liquid layer located on the surface so that, although it is impossible to use these wave modes for purposes of sensors, they can be utilized for cleaning purposes on the surface of the substrate. The radiation of ultrasonic waves of this type in a liquid causes transport phenomena, known per se, in the liquid, which transport phenomena can be used for accelerated dissolution of particles deposited on the surface of the substrate.
Apart from cleaning the sensor surface, desorption processes of molecules adsorbed on the sensor surface can also be reinforced by the utilized sound waves, which can be made use of especially in chemical sensors which are mostly based on a reversible adsorption of chemical substances.
These acoustic wave modes with sagittal polarization frequently occur in a propagation direction that is rotated by 90° in relation to the above-mentioned shear modes for the sensors, so that particularly simple arrangements are possible.
For this purpose, it is possible to make use of the effect that an increase in the frequency of the ultrasonic wave generated with the sagittal wave modes generally has a positive effect on the material removal and cleaning processes. In the microsensors used according to the invention with the interdigital transducers, which are known per se from the prior art mentioned above and are used as electroacoustic transducers, ultrasonic frequencies of several-times ten to several hundred megahertz can be realized in a simple manner. Since these transducers can be arranged directly on the surface of the substrate to be cleaned, this surface having a typical size of a few mm
2
, the outputs required for achieving an extensive cleaning effect are relatively low.
A determined embodiment form of the sensor arrangement with SH-APM sensors (SH-APM=Shear Horizontal Acoustic Plate Modes) makes use of the horizontally polarized plate shear modes in the thin plates of suitable piezoelectric materials, which plate shear modes are capable of propagation. These wave modes can be excited by the metallic interdigital transducers arranged on the plate surfaces. These wave modes are accordingly shear waves that are reflected repeatedly between the two plate surfaces. When one or both surfaces are wetted with a measurement liquid, a viscous coupling is brought about, so that the wave damping and the propagation speed depend on the density-viscosity product of the measuring liquid.
Further, these sensor arrangements with the SH-APM sensors are suitable as gravimetric chemical or biological sensors and, when the acoustoelectrical effect is utilized, for examining conductive liquids. As a rule, the measurement medium is applied to the surface of the substrate remote of the sensor transducers for this purpose, since the sensitive sensor transducers are protected in this way. The cleaning transducers can accordingly be arranged on the wetted side of the substrate in this embodiment form.
To summarize, all of the embodiment forms result in advantageous sensor arrangements which allow a simple and advantageous cleaning process for the measurement-sensitive areas of the sensor arrangement. To achieve a cleaning effect at the sensor surface, an integration of cleaning transducers for exciting sagittally polarized acoustic surface waves can advantageously be carried out in connection with arrangements of the sensor transducers for generating horizontally polarized acoustic shear modes for measuring purposes. The arrangement of the cleaning transducers is carried out in sensor
Hecht Hans
Herrmann Falk
Wolf Konrad
Dougherty Thomas M.
Robert & Bosch GmbH
Striker Michael J.
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