Measuring and testing – Gas analysis – By vibration
Reexamination Certificate
1998-09-11
2001-11-27
Williams, Hezron (Department: 2856)
Measuring and testing
Gas analysis
By vibration
C073S592000, C073S335020, C073S024050, C073S061490, C073S610000, C073S626000, C324S727000, C422S088000, C422S069000
Reexamination Certificate
active
06321588
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to a real time contamination monitor, and more specifically, to an array of chemical sensors which is capable of measuring chemical contamination at the molecular level.
Environmental hazards are becoming more commonplace, with release of airborne chemicals often posing risks over a widespread area. Rapid and accurate detection of such chemicals is necessary to safeguard workers and the population at large. Chemical detectors for detecting at the molecular level commonly comprise polymer coated surface acoustic wave (SAW) sensors that detect, identify, and quantify the substance. A SAW sensor operates in effect as a microbalance through the de-tuning of the crystal's resonant frequency as mass is added to its surface. When a SAW sensor is used as part of an oscillator, changes in the characteristics of acoustic waves propagating through the SAW sensor may be used to determine the nature of one or more substances that has adsorbed onto the sensor.
While such chemical sensor arrays can be battery powered, and therefore portable, a significant amount of power is required to run the array of sensors. The power requirements necessitate frequent battery changes or recharging which impair the usefulness of such portable sensor arrays. Additionally, if the sensor array is miniaturized for enhanced portability, crosstalk can result between the individual sensors in the array. Such crosstalk can degrade the signal-to-noise ratio and affect the detection capability of the array. Sensor performance can also be adversely affected by poor impedance matching between the individual sensors and the drive electronics, as well as by circuitry that requires long stabilization times.
Accordingly, there is a need for a compact, real time, battery operated, low power, low noise, high stability, miniaturized chemical sensor array which can detect changes in mass due to molecular contamination on the order of 10
−11
to 10
−13
g-cm
−2
or less, and chemical concentrations in the parts per million to parts per trillion range.
SUMMARY OF THE INVENTION
According to one aspect of the invention, a sensor array for detecting chemical substances includes a plurality of chemical detection sensors, a plurality of driver circuits for driving the plurality of detection sensors, respectively, and a power multiplexer electrically coupled to the plurality of driver circuits, in which the power multiplexer receives power from a power supply. By way of example, the driver circuits may comprise oscillator circuits. The sensor array further includes a signal processing unit (SPU) electrically coupled to the plurality of driver circuits and to the multiplexer, in which the multiplexer is responsive to the SPU to electrically couple the plurality of driver circuits to the power supply according to a predetermined timing pattern such that less than all of the detection sensors are powered at any instant in time to substantially eliminate cross talk between adjacent circuits. In a preferred embodiment of the invention, the timing pattern is such that no more than one of the detection sensors is powered at any instant in time.
The SPU preferably comprises a reference sensor, a microprocessor, a reference driver circuit for driving the reference sensor, and a mixer, which mixes the output of the reference driver circuit with the output from at least one of the sensor driver circuits to provide a signal to the microprocessor, the signal being indicative of chemical loading on at least one of the detection sensors. The reference sensor and the detection sensors preferably comprise respective surface acoustic wave (SAW) devices. Each of the plurality of driver circuits preferably comprises an oscillator circuit in the form of an integrated circuit such as an application specific integrated circuit (ASIC). The detection sensors are joined to their respective driver circuits by electrical paths that extend through a circuit board and are preferably no longer than one inch, more preferably no longer than one half inch, and still more preferably no longer than one quarter inch.
Another aspect of the invention comprises a method of operating a sensor array to sense a chemical substance, in which the method includes exposing a plurality of SAW sensors to a gas which may contain the chemical substance, using a plurality of oscillator circuits to drive the plurality of SAW sensors, respectively, providing power to only one of the plurality of oscillating circuits at a time, and processing signals from the sensors to detect the chemical substance. The chemical substance may comprise, for example, particles in the form of a vapor or an aerosol. In a preferred embodiment, the method further includes purging or degassing the sensing surfaces of the SAW sensors by providing a flow of air through a scrubber material to provide a flow of clean air, and directing the flow of clean air across the sensing surfaces to permit the chemical substance to desorb from the surfaces into the clean air.
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Acoustic TSMSAWFPWAPM Wave Microsensors. Jay W. Grate, Stephen J. Martin, Richard M. White, Reprinted from Anal. Chem. 1993, vol. 65, 940A-948A.
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Bahrami Frank
Bowers William D.
Tran John
Femtometrics, Inc.
Knobbe Martens Olson & Bear LLP
Wiggins David J.
Williams Hezron
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