Measuring and testing – Vibration – By mechanical waves
Reexamination Certificate
1998-11-19
2001-02-13
McCall, Eric S. (Department: 2855)
Measuring and testing
Vibration
By mechanical waves
C073S292000, C310S31300R, C310S31300R
Reexamination Certificate
active
06186005
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a liquid sensor and in particular to a liquid sensor which is composed of a surface-wave component.
2. Description of Prior Art
Surface-wave components have been used for decades in electronics, e.g. as bandpass filters for the high-frequency region, and manufactured in large quantities. Depending on the choice of material used for the substrate, different wave-forms are excited. For example, for gas sensor applications, Rayleigh-wave components with a deflection perpendicular to the surface are normally used. Such components, provided with a selective layer, are employed as bio/chemosensors.
If surface-wave components are to be used as sensors for the measurement of physical liquid properties, e.g. the density, the viscosity, the shear rigidity, the conductivity, the dielectric constant, etc., Rayleigh-wave modes lead to the generation of compression waves and thus to a high energy loss. For liquid sensors, components which generate purely shear waves are used. This type of wave can be excited in various piezoelectric materials, among others in all y-rotated quartz crystal cuts.
In the y-rotated quartz crystal cuts, surface shear waves (SSWs) and bulk shear waves, in particular the so-called ‘surface skimming bulk waves’ (SSBWs), can, however, also be excited. Surface-wave components for which these wave types can be excited are commercially available, e.g. as bandpass filters in the high-frequency region, sometimes above 1 GHZ.
When surface-shear-wave components of this type are used as sensors for the measurement of liquid properties, problems arise that play no part or only a minor part when they are used as filters. The gravest of these problems arises from the simultaneous excitation of surface shear waves (SSWs) and bulk shear waves (SSBWs) and the interference of these waves. Another problem, that can lead to considerable disturbances in the measurement effect, is the so-called ‘triple transit echo’. These effects, which will be explained in more detail in the subsequent description of the present patent application, give rise to problems which render these known components unsuitable for liquid sensor applications.
Surface-wave sensors of the above type are described in H. Drobe, A. Leidl, M. Rost, I. Ruge, “Acoustic sensors based on surface-localized HPSWs for measurements in liquids”, in: Sensors and Actuators A, 37-38 (1993), pp. 141-148. Piezoelectric resonators on the basis of surface waves are also known from E. Benes, M. Gröschl, “Piezoelektrische Resonatoren als Sensorelemente”, in: e & i, 112. Jahrgang 1995, Heft 9, S. 471 bis 483.
SUMMARY OF THE INVENTION
It is the object of the present invention to provide surface-wave sensors for measurements in liquids which exhibit a high sensitivity to liquid properties and small design-induced disturbances.
This object is achieved by a liquid sensor consisting of comb-shaped electrodes, applied to a piezoelectric substrate, as interdigital transducers for generating an electroacoustic wave from a transmitting transducer to a receiving transducer, where the transmitting transducer generates a surface shear wave and a bulk shear wave, where the number of finger pairs of the interdigital transducers and the material out of which the electrodes are made are so chosen that a surface shear wave generated by the transmitting transducer and a bulk shear wave generated by the transmitting transducer have different frequencies.
The material chosen for the electrodes preferably has a small acoustic propagation speed. In addition, a material with a high mass per unit area is preferably used for the electrodes. The surface-wave liquid sensor is preferably so constructed that two fingers of an electrode are arranged next to one another in each case and form a pair, a pair of one electrode forming a finger quartet with a neighbouring pair of the second electrode. The number of finger quartets is preferably chosen to be greater than twice the ratio of the excitation frequency to the frequency difference between the surface shear wave and the bulk shear wave.
To reduce the disturbances created by the so-called ‘triple transit echo’, the insertion loss of an electroacoustic wave between the transmitting transducer and the receiving transducer is set to a loss between 10 and 30 dB, preferably between 15 and 25 dB.
In the surface-wave sensor according to the present invention, interference between the surface shear wave, which constitutes the actual useful signal, and bulk shear waves, e.g. the ‘surface skimming bulk waves’, is prevented. By a suitable choice of the insertion loss, the influence of the so-called ‘triple transit echo’ and an electromagnetic crosstalk between transmitting transducer and receiving transducer can also be reduced considerably. At the same time, the sensor according to the present invention provides increased measurement sensitivity to the viscosity of liquids. Disturbances in the measurement effect are thereby greatly reduced.
Further developments of the present invention are disclosed in the dependent claims.
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patent: 2802946 (1978-08-01), None
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Drobe, et al., Acoustic Sensors Based on Surface-Localized HPSWs for Measurements in Liquids, Elsevier Sequoia, (1993).
Benes et al., Piezoelektrische Resonatoren als Sensorelemente, e & i, (1995).
Mackerras, High-Frequency Operation of Surface-Acoustic-Wave Multielectrode Transducers, Electronic Letters, (1974).
Aw-Musse Abdullahi
Fraunhofer-Gesellschaft Zur Forderung der Angewandten Forschung
Glenn Michael A.
McCall Eric S.
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