Abstract:
A chemiresistor sensor system that compensates for changes in resistance caused by changes in ambient temperature, thereby increasing the accuracy of the sensor system&#39;s ability to detect target analytes. The sensor system generally includes a first resistor, a second resistor, and a load regulator or switch that is sensitive to changes in ambient temperature. At least one of the first resistor and the second resistor is a sensing element having a resistance that changes in response to the presence of one or more of the analytes. The switch manages an electrical load across the first resistor and the second resistor. The switch prevents passage of the electrical load across the first and/or second resistor when the ambient temperature is at a first value. The switch permits passage of the electrical load across the first and/or second resistor when the ambient temperature is at a second value.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to chemiresistor sensors. In particular, the present invention relates to a chemiresistor sensor system having multiple resistive elements that can be selectively incorporated into a sensor circuit to change the overall resistance of the circuit in response to changes in ambient temperature.  
       BACKGROUND OF THE INVENTION  
       [0002]     Detecting the presence of specific chemical compounds in the atmosphere is important in a variety of different applications. For example, it is often important to detect the presence and concentration of potentially flammable compounds in the atmosphere. Chemical compounds of interest are often referred to as target analytes.  
         [0003]     A variety of different sensor systems known in the art can be used to detect the presence and concentration of different analytes. For example, conductiometric sensor systems, optical sensor systems, and surface acoustic wave sensor systems can all be used.  
         [0004]     One type of conductiometric sensor is a polymer-absorption chemiresistor sensor. Polymer-absorption chemiresistor sensors include a sensor probe having a pair of electrodes and a sensing element. The probe is part of a sensor circuit.  
         [0005]     The sensing element typically takes the form of a polymeric sensor film that spans the two electrodes. The sensor film is exposed to the surrounding atmosphere. The exact composition of the polymeric sensor film varies depending on the target analyte, as is known in the art, such that the sensor film absorbs the target analyte when it is present in the surrounding atmosphere.  
         [0006]     A load is applied across the sensor film via the electrodes. Upon exposure to and absorption of the target analyte, the sensor film swells and undergoes a volumetric change. The change in volume changes the electrical resistance of the film.  
         [0007]     A processor or control unit is typically coupled to the sensor circuit. The processor monitors the resistance of the sensor film to determine the absence, presence, and concentration of the target analytes. The processor can be coupled to a user interface. The user interface typically includes an indicating device that generates a signal when the concentration of the target analyte exceeds a predetermined threshold value.  
         [0008]     The resistance of the sensor film changes not only in response to absorption of the target analytes, but also in response to changes in ambient temperature. If the sensor film has a positive temperature coefficient of resistance, the resistance of the sensor film increases as ambient temperature increases. If the sensor film has a negative temperature coefficient of resistance, the resistance of the sensor film decreases as ambient temperature increases. Whether the sensor film has a positive or negative temperature coefficient of resistance depends on the composition of the sensor film and the application.  
         [0009]     Because detection of target analytes is based on changes in the resistance of the sensor film that occur when the sensor absorbs target analytes, changes in ambient temperature that change the resistance of the sensor film can negatively affect the sensor system&#39;s ability to accurately detect the presence of target analytes. For example, if the sensor film has a positive temperature coefficient of resistance and increases in resistance upon the absorption of target analytes, increases in ambient temperature might cause the sensor system to generate a false signal indicating that target analytes are present when they are not.  
         [0010]     While conventional chemiresistor sensor systems perform adequately for their intended uses, they are subject to improvement. Specifically, there is a need for a chemiresistor sensor system that can modify its overall resistance in response to changes in ambient temperature to increase the accuracy of the sensor system.  
       SUMMARY OF THE INVENTION  
       [0011]     The sensor system improves upon the prior art by providing a chemiresistor sensor system that compensates for changes in resistance caused by changes in ambient temperature, thereby increasing the accuracy of the sensor system&#39;s ability to detect target analytes.  
         [0012]     The sensor system generally includes a first resistor, a second resistor, and a load regulator or switch. At least one of the first resistor and the second resistor is a sensing element having a resistance that changes in response to the presence of one or more of the analytes. The resistors also change resistance in response to changes in ambient temperature.  
         [0013]     The switch manages an electrical load across the first resistor and the second resistor. The switch prevents passage of the electrical load across the first and/or second resistor(s) when the ambient temperature is at a first value. The switch permits passage of the electrical load across the first and/or second resistor when the ambient temperature is at a second value.  
         [0014]     The sensor system advantageously incorporates the first and/or second resistors into a sensing circuit in response to changes in ambient temperature. By selectively incorporating the first and/or second resistors in a variety of different configurations, such as electrically in parallel or in series, the sensor system is able to change its overall resistance to compensate for changes in resistance of the first and second resistors caused by changes in ambient temperature. The sensor system&#39;s ability to compensate for changes in resistance caused by changes in ambient temperature increases the system&#39;s ability to accurately detect the presence of target analytes.  
         [0015]     Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]     The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein:  
         [0017]      FIG. 1  is a block diagram of a chemiresistor sensor system according to the present invention;  
         [0018]      FIG. 2A  is a simplified schematic diagram showing circuitry of a sensor probe of the sensor system of  FIG. 1  according to a first embodiment;  
         [0019]      FIG. 2B  is a simplified schematic diagram showing circuitry of a sensor probe of the sensor system of  FIG. 1  according to a second embodiment; and  
         [0020]      FIG. 2C  is a simplified schematic diagram showing circuitry of a sensor probe of the sensor system of  FIG. 1  according to a third embodiment. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0021]     The following description of the preferred embodiments is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses.  
         [0022]      FIG. 1  generally depicts the major components of an exemplary chemiresistor sensor system at  10 . The sensor system  10  is generally comprised of a chemiresistor sensor probe  12 , a control unit  14 , and a user interface  16 . The sensor probe  12  includes temperature compensating elements  20 .  
         [0023]     The sensor probe  12  interacts with an external environment  17  to detect the presence of chemical compositions of interest, or target analytes  18 . The sensor probe  12  generates a raw output signal  19   a  based on continuous detection of analytes  18  in the external environment  17 . The raw output signal  19   a  is processed by the control unit  14 . The control unit  14  transmits a calculated output signal  19   b  to the user interface  16  to relay analysis of the raw output signal  19   a  from the sensor probe  12 . The control unit  14  supplies operating commands and a load, both represented at  22 , to the probe  12 .  
         [0024]     The user interface  16  provides information to a user regarding the status of the sensor system  10 , such as whether or not the system  10  detects the presence of the target analytes  18 . The user interface  16  can be of a variety of different forms known in the art and can range from a simple alarm signal to a sophisticated computerized display.  
         [0025]     The sensor probe  12  can take the form of a variety of different sensor probes. For example, the sensor probe  12  can take the form of any of the sensor probes described in U.S. patent application Ser. No. 10/412,602, titled Robust Chemiresistor Sensor and filed Apr. 11, 2003. U.S. patent application Ser. No. 10/412,602 is hereby incorporated by reference.  
         [0026]     The sensor probe  12  includes a conductive sensor element or film. The sensor film can be any suitable sensor film known in the art, such as those described in U.S. patent application Ser. No. 10/411,805, which was filed on Apr. 11, 2003 and is titled “Vapor Sensor and Materials Therefor.” The sensor film absorbs the target analytes  18  and changes resistance upon absorbing the target analytes.  
         [0027]     With additional reference to  FIG. 2 , a simplified schematic diagram showing the circuitry of the sensor probe  12  is illustrated at  100 .  
         [0028]      FIG. 2A  illustrates the circuitry of the sensor probe  12  according to a first embodiment at  100 A. In the embodiment at  100 A, the sensor probe  12  includes a first resistor R 1 , a second resistor R 2 , and a load regulating device or switch SW. The second resistor R 2  and the switch SW are part of the temperature compensation element  20 A.  
         [0029]     The first resistor R 1  is provided by the sensor film. The second resistor R 2  can be a second sensor film or any conventional resistor known in the art. The first resistor R 1  and the second resistor R 2  are arranged electrically in parallel. The resistance of the circuit  100 A is monitored by the control unit  14 .  
         [0030]     The switch SW is movable between an open position and a closed position. The switch SW opens and closes in response to changes in ambient temperature. The switch SW can be any suitable switch known in the art, such as a stand-alone thermostatically activated switch or a switch controlled by external means. In some embodiments, the switch SW is a bimetal temperature control, such as any one of the 36T series of bimetal temperature controls from Therm-O-Disc Inc. of Mansfield, Ohio, for example.  
         [0031]     When the switch SW is in the closed position the second resistor R 2  is connected electrically in parallel with the first resistor R 1 . When the switch SW is in the open position the resistor R 2  is removed from the circuit, leaving only the first resistor R 1  in the circuit.  
         [0032]     An additional embodiment of the circuitry of the sensor probe  12  is illustrated in  FIG. 2B  at reference numeral  100 B. The circuit  100 B includes the same elements as the circuit  100 A. As with the circuit  100 A, the temperature compensation elements  20 B include the second resistor R 2  and the switch SW. The only substantial difference between the circuit  100 A and the circuit  100 B is the manner in which the different elements are arranged. Therefore, the general description of the resistors R 1  and R 2  and the switch SW set forth in connection with the description of the circuit  100 A equally applies to the circuit  100 B.  
         [0033]     In the circuit  100 B, the first resistor R 1  and the second resistor R 2  are arranged electrically in series. The switch SW is positioned to provide a low resistance bypass around the resistor R 2 . When the switch SW is open the load passes through both the first resistor R 1  and the second resistor R 2  in series to increase the overall resistance of the circuit  100 B. When the switch SW is closed the load bypasses the second resistor R 2  to remove the second resistor R 2  from the circuit  100 B and lower the overall resistance of the circuit  100 B.  
         [0034]      FIG. 2C  illustrates an additional embodiment of the circuitry of the sensor probe  12  at reference numeral  100 C. The circuit  100 C includes all of the same elements as the circuit  100 A. However, in the circuit  100 C the second resistor R 2 , like the first resistor R 1 , is provided by the sensor film of the probe  12  with the first resistor R 1  and the second resistor R 2  having different resistances. The temperature compensation elements  20  include the switch SW and both resistors R 1  and R 2 . The general description of the resistor R 1  and the switch SW set forth in connection with the description of the circuit  100 A equally applies to the circuit  100 C.  
         [0035]     In the circuit  100 C, the first and second resistors R 1  and R 2  are on independent load paths. Actuation of the switch SW places either the first resistor R 1  or the second resistor R 2  in the circuit  100 C. Specifically, the switch can be moved between a first position in which it contacts the load path of the first resistor R 1  to include the first resistor R 1  in the circuit  100 C and a second position in which it contacts the load path of the second resistor R 2  to include the second resistor R 2  in the circuit  100 C. The first resistor has a first resistance and the second resistor has a second resistance that is lower than the first resistance.  
         [0036]     The circuit  100  may take the form of numerous other embodiments in addition to those provided at reference numerals  100 A,  100 B, and  100 C. For example, the circuit  100  can include a combination of series and parallel circuits as well as combinations of positive, negative, and zero temperature coefficient resistors. Any suitable circuit having a plurality of resistors operable to compensate for changes in resistance due to changes in ambient temperature can be used.  
         [0037]     Operation of the sensor system  10  will now be described. The target analytes  18  are absorbed by the sensor film of the probe  12  when the analytes  18  are present in the external environment  17 . The sensor film swells upon absorption of the analytes  18 . As the film swells, the distance between conductive particles embedded in the sensor film increases, thus changing the resistance R 1  (or R 2  in the circuit  100 C) of the film as measured by the control unit  14 .  
         [0038]     Upon detecting a change in resistance, the control unit  14  transmits a calculated output  19   b  to the user interface  16  instructing the user interface  16  to alert the user that the target analytes  18  have been detected by the probe  12 . The user interface  16  may be any appropriate interface capable of providing an alert to the user. The interface  16  may range in complexity from a simple alarm to a complex computer providing audio and visual alerts.  
         [0039]     Operation of the sensor probe  12  outfitted with some of the different sensor circuits  100  set forth herein will now be described.  
         [0040]     With respect to the circuit  100 A, the switch SW opens and closes in response to changes in ambient temperature. If the first and second resistors R 1  and R 2  both have a positive temperature coefficient of resistance, such that the resistance increases as temperature increases, the switch SW remains in an open position when the ambient temperature is at or below a predetermined temperature value or threshold. When the switch SW is in the open position only the resistor R 1  is in the circuit. Operation of the switch SW can be controlled by the control unit  14  and/or the switch SW can be a stand-alone thermostatically activated switch.  
         [0041]     When the ambient temperature rises above the predetermined temperature threshold, the switch SW closes to place the second resistor R 2  in parallel with the first resistor R 1 . Having the first and second resistors R 1  and R 2  in parallel lowers the overall resistance of the circuit  100 A below the individual resistance of the first and second resistors R 1  and R 2  to take into account the increased resistance of the resistors R 1  and R 2  caused by the increase in ambient temperature.  
         [0042]     When the ambient temperature drops back to or below the predetermined temperature, the switch opens to return the resistance of the circuit  100 A to its optimal resistance for the predetermined temperature.  
         [0043]     If the first and second resistors R 1  and R 2  both have a negative temperature coefficient of resistance, such that the resistance decreases as the temperature increases, the operation of the switch SW is reversed. Specifically, the switch SW opens when the ambient temperature rises above the predetermined temperature and closes when the ambient temperature is at or below the predetermined temperature.  
         [0044]     With respect to the circuit  100 B, the switch SW remains open at or below a predetermined ambient temperature threshold or value so that the first resistor R 1  and the second resistor R 2  are electrically in series to maintain the overall resistance of the circuit  100 B at an acceptable resistance. If the first and second resistors R 1  and R 2  both have a positive temperature coefficient of resistance, such that the resistance increases as temperature increases, the switch SW closes when the ambient temperature increases above a predetermined temperature threshold. Closing the switch effectively removes the second resistor R 2  from the circuit and lowers the overall resistance of the circuit  100 B to counteract the increase in resistance caused by the increase in ambient temperature.  
         [0045]     If the first and second resistors R 1  and R 2  both have a negative temperature coefficient of resistance, such that the resistance decreases as the temperature increases, the operation of the switch SW is reversed. Specifically, the switch SW remains closed at standard or lower ambient temperatures and opens when the ambient temperature rises above a predetermined temperature threshold.  
         [0046]     With respect to the circuit  100 C, if the first and second resistors R 1  and R 2  each have a positive coefficient of resistance, the switch SW is placed in the first position at or below a predetermined ambient temperature value or threshold to include the first resistor R 1  in the circuit  100 C. When the ambient temperature rises above the predetermined temperature threshold the switch moves to the second position to remove the first resistor R 1  from the circuit and include the second resistor R 2  in the circuit. Replacing the first resistor R 1  with the second resistor R 2  lowers the overall resistance of the circuit  100 C because the second resistor R 2  has a lower resistance than the first resistor R 1 .  
         [0047]     If the first and second resistors R 1  and R 2  have a negative temperature coefficient of resistance, the operation of the switch SW in response to changes in ambient temperature is reversed. Specifically, the switch SW moves to the second position when the ambient temperature is at or below the predetermined temperature threshold. The switch moves to the first position when the temperature rises above the predetermined temperature threshold.  
         [0048]     The circuits  100  compensate for changes in the resistance of the first and second resistors R 1  and R 2  that occurs due to changes in ambient temperature. Therefore, the sensor system  10  is able to distinguish between changes in resistance caused by the presence of the target analytes  18  versus changes in resistance caused by changes in ambient temperature. As a result, the sensor system  10  can detect the presence of the analytes  18  with improved accuracy.  
         [0049]     The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.