Abstract:
A device for detecting chemical substances includes a plurality of sensors arranged in an array. The sensors are connected to respective oscillator circuits which drive the sensors, and the oscillator circuits are coupled to a power multiplexer which provides the circuits with power according to a timing pattern such that not all of the oscillator circuits are activated at any one time. Preferably, only one oscillator circuit is activated at any given time. This multiplexing arrangement saves power and substantially eliminates cross talk between the oscillator circuits. The oscillator circuits are preferably application specific integrated circuits (ASICs), and the sensors are preferably surface acoustic wave (SAW) devices. In use, the SAW sensors are exposed to a gas, such as air, containing the chemical substance to be detected. Signals from the SAW sensors are analyzed to identify the chemical substance.

Description:
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&#39;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. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic representation of a preferred embodiment of the invention, in which a plurality of chemical detection sensors are arranged in an array and power is multiplexed individually to the sensors. 
     FIG. 2 shows the chemical detection sensors and the respective circuits that drive them located on opposite sides of a circuit board. 
     FIG. 3 shows a configuration similar to that of FIG. 2, except that the respective driving circuits are integrated into a single chip. 
     FIGS. 4A and 4B are schematic representations showing the air flow path through the sensor device when the device is operated in the detection and purge modes, respectively. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The preferred embodiment of the present invention comprises a chemical detection unit having an array of individual sensors powered by a battery. Electrical power requirements are reduced by multiplexing the battery power to the individual sensors. In addition to reducing power requirements, the multiplexing also eliminates crosstalk between the individual sensors. Impedance matching between the sensors and the driver electronics is enhanced by the component layout. 
     As shown in FIG. 1, detection sensors comprised of surface acoustic wave (SAW) devices  101 ,  102 ,  103 ,  104 ,  105 ,  106 , are arranged in an array  100  to detect the presence of chemical substances such as airborne trace contaminants. Preferably, the chemical substances to be detected comprise particles, such as molecules in the form of a gas vapor or an aerosol. While the preferred embodiment incorporates six SAW detection sensors  101 - 106 , more or fewer detection sensors may be used. The operation and characteristics of SAW sensors are described in U.S. Pat. Nos. 5,645,608 to Lokshin et al., 5,476,002 to Bowers et al., 5,469,369 to Rose-Pehrsson et al., and 5,488,866 to Ravel et al., all of which are hereby incorporated by reference herein. Each SAW detection sensor  101 - 106  comprises a piezoelectric crystal, with the top few layers of the piezoelectric surface being driven to oscillate in a surface acoustic mode by a respective driving circuit such as an oscillator  111 ,  112 ,  113 ,  114 ,  115 ,  116 . An electric field is applied parallel to the surface of the crystal, and Rayleigh waves are generated which move along the surface of the crystal. The resonant frequency of the crystal changes as a function of the mass of the trace contaminant adsorbed onto the crystal surface. The ability of the array to identify selected chemicals is enhanced by applying coatings to the sensing surfaces of the sensors  101 - 106 , as is well known in the art. The coatings increase adsorption rates onto the surface of the SAW sensors  101 - 106 . A different coating (or coatings) is preferably applied to each of the sensors  101 - 106 , so that each SAW sensor has an affinity for a particular chemical or class of chemicals. The response of the SAW sensors  101 - 106  depends on the types and amounts of chemicals deposited on their respective sensory surfaces. Each mixture of chemicals will yield a unique response, and thus, detection of any particular mixture will provide an associated “fingerprint” which is indicative of that chemical substance. 
     In the preferred embodiment, identification of chemicals is accomplished using a signal processing unit  155 , which includes a reference sensor  107 , a mixer  120 , a reference driving circuit  117 , such as an oscillator and a microprocessor  150 . The reference sensor  107  preferably comprises a SAW device that is connected to the oscillator circuit  117 . The reference SAW  107  is covered so that it is shielded and sealed against the sample of air being sensed. In response to driving signals from the oscillator  117 , the reference SAW  107  generates an output signal at its resonant frequency. Such output signal passes through the reference oscillator  117  to the mixer  120  which sequentially mixes such output signal with each of the respective outputs (resonant frequencies) from the detection sensors  101 - 106 . 
     For any particular sensor  101 - 106 , the signal from the mixer  120  is indicative of the particular chemical substance loaded onto (in general, absorbed onto) that sensor, and the characteristics (e.g., the beat or difference frequency) of this signal are analyzed by the microprocessor  150 . By deconvolving the signal from the mixer  120  using various algorithms known in the art and correlating the results with known “fingerprints” particular to various chemicals, the identification and concentration of the chemicals adsorbed onto the surfaces of SAW sensors  101 - 106  can be determined. 
     The sensor oscillators  111 - 117  are preferably comprised of individual application specific integrated circuits (ASICs) that incorporate impedance matching circuits. The ASICs also include amplifiers which amplify the signals output by the sensors  101 - 107 . Although these ASICs are identical in the preferred embodiment, they may alternatively be individually designed and tailored for processing specific sensor responses. As shown in FIG. 2, the individual ASICs  111 - 116  for the sensors  101 - 106  may each comprise a separate chip, or alternatively, as shown in FIG. 3, the individual ASICs  111 - 116  may be integrated on a single chip. In either case, only one of the sensor ASICs  111 - 116  is dedicated to each of the SAW sensors  101 - 106 . 
     Referring again to FIG. 1, power to the sensor oscillators  111 - 116  is controlled by a multiplexer  130 , such as a digital switch, which sequentially applies voltage to each oscillator  111 - 116 . Power is supplied to the multiplexer  130  by a power supply  135  which may be connected to the multiplexer via a power switch  137 . The power supply preferably consists of one or more storage batteries such as alkaline batteries or lithium batteries. The mixer  120  and the multiplexer  130  are controlled by the microprocessor  150 . In multiplexing power to the sensor ASICs  111 - 116 , power is applied so that not all of the sensor ASICs  111 - 116  are powered at any instant in time. Preferably, the multiplexer  130  supplies power to the sensor ASICs  111 - 116  according to a predetermined timing pattern in which only one of the sensor ASICs  111 - 116  is powered at any instant in time. This ensures that there is no crosstalk between the sensor ASICs  111 - 116 , thereby facilitating low noise operation of the device. In addition, by multiplexing power to the individual sensor ASICs  111 - 116 , the overall power consumption is reduced. Power from the power supply  135  is preferably applied continuously to the reference ASIC  117 . Alternatively, power may be distributed intermittently to the reference ASIC by the multiplexer  130  so that it is only on when one of the sensor ASICs  111 - 116  is on. 
     Use of the ASICs  111 - 116  also allows the microprocessor to rapidly interrogate each sensor  101 - 106  with low noise and high stability. Because the circuitry of each ASIC is integrated onto a common silicon substrate with short conductive paths, such circuitry does not require a long stabilization time and stabilizes quickly when power is applied, thereby enhancing system performance. 
     As shown in FIG. 2, the detection sensors  101 - 106  and the reference sensor  107  are mounted on one side of a circuit board  200 , while the sensor ASICs  111 - 116  and the reference ASIC  117  are mounted on the opposite side of the board  200 . The ASICs  111 - 117  are electrically coupled to their respective sensors  101 - 107  by respective electrical conductors  201 - 207  which extend through the board. The ASICs  111 - 117  and sensors  101 - 107  are arranged with each ASIC  111 - 117  directly opposite the sensor  101 - 107  that it drives, so that the length of each conductor  201 - 207  is approximately equal to the thickness of the board. The electrical conductors  201 - 207  provide respective electrical paths that are each preferably no longer than 1 inch, and more preferably no longer than one half inch, and still more preferably no longer than one quarter inch. 
     As mentioned above, the individual ASICs  111 - 117  can be embodied in respective separate chips as in FIG. 2, or such individual ASICs  111 - 117  can be embodied in fewer chips, such as the single chip shown in FIG.  3 . The components discussed in FIG. 2 are shown in FIG. 3 with the corresponding referenced numbers primed. As in the embodiment of FIG. 2, the respective electrical paths  201 ′- 207 ′ of FIG. 3 which pass through the circuit board  200 ′ are preferably no longer than 1 inch, and more preferably no longer than one half inch, and still more preferably no longer than one quarter inch. 
     The chemical substances to be detected are introduced into the device by drawing in ambient air containing the chemical substances to be detected. As illustrated in the schematic diagram of FIG. 4A, a three way valve  400  such as a magnetically latched solenoid valve is positioned in a detection mode to admit ambient air represented by arrow  405  through a first ambient air inlet  410  and onto the array  100  of detection sensors  101 - 106 . Air is drawn past the sensor array  100  by a pump  420 . As the air passes over the surfaces of the SAW sensors  101 - 106 , the substances in the air to be detected are loaded (e.g., through absorption) onto the respective coated sensing surfaces of the detection sensors  101 - 106 . 
     Alternatively, a gaseous sample may be obtained from a surface by utilizing a sample acquisition device such as that disclosed in the copending application of William D. Bowers, Ser. No. 09/151,743, filed on the same date as the present application, entitled “Pulsed Air Sampler,” which is hereby incorporated herein by reference. 
     As soon as the response from the SAW sensors  101 - 106  is processed to detect the chemical substance in the air sample, the microprocessor  150  immediately switches the unit from a detection mode to a purge mode. Such immediate switching minimizes the loading on the polymer coated surfaces and allows the detection sensors  101 - 106  to be cleaned quickly, so that a new sample can be drawn in to confirm the results of the previous sample. The microprocessor automatically switches back and forth in this manner between detection mode and purge mode until the chemical substance is no longer detected (i.e. the ambient air is free of the substance). 
     To efficiently purge the detection sensors  101 - 106 , a source of clean air is required, so that the adsorbed chemical remaining on the detection sensors  101 - 106  can be desorbed. Such clean air is created in the disclosed detection unit by drawing ambient air (which may contain the substance(s) to be detected) through a second ambient air inlet  430  which leads to a scrubber  440 . The scrubber  440  contains charcoal or another substance, such as Tenax®, that absorbs, deactivates, or otherwise neutralizes chemicals from ambient air. Suction for drawing the air through the scrubber is provided by activating the pump  420  and positioning the three way valve  400  so that air from the scrubber  440  is admitted into the sensor array  100 . The stream of clean, purified air  406  from the scrubber is thus drawn past the sensor array  100 , thereby allowing the chemicals loaded onto the array to desorb into the clean air. After the sensor array  100  has been purged in this manner for typically 1-3 minutes, the chemical which had been adsorbed onto the sensors  101 - 106  will be sufficiently desorbed back into the stream of clean air  406  that another sample of unpurified ambient air can be admitted through the air inlet  410  to the sensor array  100  for analysis. 
     The invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is therefore indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within that scope.