Measuring and testing – Gas analysis – By vibration
Patent
1995-01-03
1997-09-16
Williams, Hezron E.
Measuring and testing
Gas analysis
By vibration
73 3106, 73 5441, 73 6175, 73 6179, 73 6453, 310324, G01N 2924, G01N 2902
Patent
active
056683030
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND AND SUMMARY OF THE INVENTION
The invention relates to a sensor and, more particularly, to a sensor provided with a thin membrane, capable of storing electrical charges, whose surface is in contact with the chemical, biological, and/or other physical parameters to be measured. The membrane is part of an electromechanical resonance circuit, operating with propagating acoustic waves. Provision is made opposite the membrane for converters, equipped with interdigital structures, to act as transmitter and receiver for the propagating acoustic plate waves. Each of the converters is equipped with interdigital structures that act as narrow band filters. The transmitter converter and the receiver converter is arranged with a distance between them. The signal from the receiver converter is detected capacitively. A suitable electronic control and evaluation circuit or feedback is provided in addition between the output signal of the receiver converter and the transmitter converter. Finally, a DC voltage is applied between the membrane on the one hand and the transmitter converter and the receiver converter on the other. The voltage has an alternating voltage superimposed on it in the transmitter converter.
Technical advancement has made it necessary to detect the presence of chemical or biochemical substances and to determine their concentrations, and to measure other physical parameters and their changes in the form of electrical signals. Sensors are used for this application. The sensor's theoretical purpose consists in the conversion of chemical and biochemical reactions, or the detection of the existence of or change in other physical parameters, into electrical signals, frequently accomplished with electronic means. The areas of application for such sensors include, for example, process monitoring, detection of impurities, performance of analyses, exhaust emissions monitoring, waterways monitoring, gas alarm systems, and medical technology, etc.
Mass-sensitive converters, also called gravimetric sensors, have been discussed for use as high-sensitivity sensors. This group of sensors reacts, for example, to the accumulation of, or a change in the accumulation of, the desired substance on the sensor on which a chemically active coating is provided for this purpose. The sensors are designed as an electromechanical resonance circuit. By using electrical oscillations and suitable feedback, the mechanical components of the sensor are set oscillating. If continuous waves are used in the resonator, the resonant frequency is determined by the phase velocity of the waves and possibly by the impressed wavelength.
Thus, for example, sensors are known that work with acoustic surface waves. A piezoelectric layer is used as the mechanical part of the resonator. Interdigital structures are evaporated onto the piezoelectric layer. The structures then, generate continuous acoustic surface waves under electric excitation that are emitted along the piezoelectric layer. Another interdigital structure applied to the piezoelectric layer acts as a receiver, receiving the acoustic surface waves emitted by the transmitter and converting them into an electrical signal. An electronic amplifier is provided between the transmitter and the receiver. The amplifier provides feedback between the receiver and the transmitter as well as compensation for losses. This produces an oscillating system that is free of damping.
The mechanical oscillator can be provided on one or both sides with a chemically active coating that is selected for the desired substance. The desired substance binds on the chemical coating of the substrate. This causes a change in mass or a change in the surface properties of the substrate. This in turn leads to a change in the resonant frequency or a change in the propagation rate of the waves. This causes a frequency shift in the resonant circuit, so that the deposition of the desired substance can be evaluated by an electrical signal.
The propagation rate of the acoustic surface waves is first determined by the mat
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Sensors and Actuators B Chemical, entitled "Acoustic Love-wave sensor for concentration in H.sub.2 O solutions", vol. 87, No. 1/3, Mar. 1992, pp. 602-605.
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Giesler Thomas
Meyer J.-Uwe
Finley Rose M.
Forschung e.V Fraunhofer-Gesellschaft zur Foerderung der angewan
Williams Hezron E.
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