Abstract:
A gas detector with a compensated electrochemical sensor exhibits altered sensitivity in response to decreasing stochastic noise in an output thereof. A gain parameter can be adjusted to alter sensitivity. A life-time estimate can be made based on sensitivity.

Description:
FIELD OF THE INVENTION  
       [0001]     The invention pertains to gas detectors. More particularly, the invention pertains to gas detectors having age-compensated electrochemical sensors.  
       BACKGROUND OF THE INVENTION  
       [0002]     Depending on the circumstances it can be desirable and/or particularly important to be able to sense the presence of various gases which might be dangerous or explosive. These include carbon monoxide, carbon dioxide, propane, methane, as well as other potentially explosive gases.  
         [0003]     A variety of sensors are known which can detect various gases. These sensors are based on different technologies and have different performance characteristics and different cost characteristics. One technology of ongoing interest is represented by electrochemical sensors. This class of sensors is potentially reliable and inexpensive.  
         [0004]     Electrochemical sensors can be designed so as to be responsive to a gas of interest and to be highly sensitive. They respond to a gas of interest with a respective output current. However, such sensors have a zero output current failure mode and zero output current in the absence of the selected gas. Because there is no specific failure indicator, external circuits have to be designed to supervise these types of sensors.  
         [0005]     It has been known to use electrical stimulus to apply a current to such sensors, to measure the sensor&#39;s signal over time, and calculate a capacitance value. This capacitance value can indicate that the sensor(s) has (have) degraded beyond a predetermined threshold, or, it can be an indication the sensor has been removed from the circuit. However, by itself, it does not indicate the sensitivity of the respective electrochemical sensor.  
         [0006]     Another prior art method measures an electrical noise in a sensor output signal. A trouble condition or indication can be output if the noise level falls below a predetermined fixed threshold. This method is based in a known characteristic; that as gas concentration increases, the sensor(s) not only output a signal indicative thereof, they also exhibit increased noise.  FIG. 1A  is a graph of output noise vs. gas concentration in parts per million which illustrates this characteristic.  FIG. 1B  illustrates exemplary response of an electrochemical sensor to a pulse of 100 ppm of CO.  FIG. 1C  illustrates increasing noise in response to exposure to the CO. However, this method does not teach maintaining the sensitivity. It only provides an indication of a failed sensor relative to a fixed threshold. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]      FIG. 1A  is a graph illustrating variations in sensor noise as a function of parts per million of a selected gas;  
         [0008]      FIG. 1B  is a graph illustrating increase of sensor output signal in response to the presence of a selected gas;  
         [0009]      FIG. 1C  illustrates high frequency noise variations as the sensor responds to increasing concentrations of a selected gas;  
         [0010]      FIG. 2  is a graph illustrating noise as a function of mass of electrolyte of a sensor;  
         [0011]      FIG. 3  is a block diagram of an exemplary detector in accordance with the invention;  
         [0012]      FIG. 4  is a flow diagram of one aspect of the present invention; and  
         [0013]      FIG. 5  is a flow diagram of another aspect of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS  
       [0014]     While embodiments of this invention can take many different forms, specific embodiments thereof are shown in the drawings and will be described herein in detail with the understanding that the present disclosure is to be considered as an exemplification of the principles of the invention and is not intended to limit the invention to the specific embodiment illustrated.  
         [0015]     A disclosed embodiment of the invention overcomes the problems with monitoring the sensitivity of an electrochemical sensor over time. There are at least four active components that can be used to determine the condition of the sensor. These include the noise level in the sensor&#39;s output signal, the drift in the signal over time, the internal capacitance of the sensor, and the internal impedance of the sensor.  
         [0016]     The sensor noise level will increase as the signal increases relatively proportionate to ambient gas concentration. When the sensor is detecting ambient gas, the increase in noise can be correlated against the signal increase from the electrochemical gas sensor. A function that combines the noise level in the absence of gas and the noise level with gas can be used to calculate a sensitivity adjustment factor that is applied to the gas signal to determine the local levels of ambient gas.  
         [0017]     The electrical noise in the sensor can be combined with other electrical signals from the electrochemical sensor to determine the sensitivity thereof. A prediction can be made as to remaining lifetime of the sensor.  
         [0018]     The electrical signals from an electrochemical sensor exhibit noise that is related to the level of sensed gas, see  FIGS. 1A-1C . If the sensor electrolyte dries out, there is less electrical activity to generate noise and the noise level will fall.  FIG. 2  illustrates an exemplary relationship between the mass of electrolyte and the noise level with no gas present. However, before the characteristics in this graph are exhibited, the noise level may actually rise during the final stages of drying before decreasing. Algorithms in the processor can use the increase in noise above a normal expected value to anticipate a pending fault condition.  
         [0019]     Relative to  FIG. 3 , a gas detector  10  which embodies the present invention includes an electrochemical sensor  12  which has an output, line  12   a , that is coupled to a pair of operational amplifiers  14 ,  16 . The amplifier  14  provides a buffered output of the signal from sensor  12  and is configured as a relatively low pass filter and current-to-voltage converter, see  FIG. 1 , which is associated with the output signal from the sensor  12 . An output  14   a  from operational amplifier  14  can be coupled to a sensor signal input port  18   a  of a programmable processor  18 .  
         [0020]     Operational amplifier  16  is configured as a high pass filter with additional gain and responds only to the high frequency noise in the signal from the operational amplifier  14 , line  14   a . The combination of the low pass characteristics of amplifier  14  and the high pass characteristics of amplifier  16  create a band pass for the noise. That signal is coupled, via line  16   a , to a noise input port  18   b  of the processor  18 . Processor  18  thus has access to a concentration signal, line  14   a , and an associated noise signal, line  16   a.    
         [0021]     Processor  18  can in turn be coupled via output port  18   c  to interface circuits  20  as would be understood by those of skill in the art. Circuitry  20  can include an rf antenna, indicated in phantom,  22  for wireless configurations. Alternately, interface circuits  20  can couple signals to and from a wired medium  24 . Detector  10  can thus communicate with an external alarm system, for example, as disclosed in Tice et al. U.S. Pat. No. 6,320,501 entitled “Multiple Sensor System for Alarm Determination with Device-to-Device Communications”, assigned to the assignee hereof and incorporated herein by reference. It will be understood that neither of the detailed configurations of the interface circuits  20  nor the type of medium, wired or wireless, are limitations of the present invention.  
         [0022]     Processor  18  operates in accordance with prestored control software  26  which could be stored, for example, in electrically eraseable read only memory EEPROM  26   a . The detector  10  can be contained within and carried by a housing  30  as would be understood by those of skill in the art.  
         [0023]     The processor  18  in combination with control software  26  can carry out signal processing in response to signal inputs, port  18   a  and noise inputs, port  18   b . Exemplary processing is discussed subsequently relative to  FIGS. 4, 5 .  
         [0024]     In order to make a meaningful measurement of the noise level, it is important to remove transients from the signal. The transients can be removed signal processing carried out by processor  18 . One exemplary form of such processing selects the smaller of two time sequential signal values and uses the smaller value in place as the signal value.  FIG. 4  illustrates this processing. Other methods can be used such as averaging or selecting the smaller of more than two time sequential signals.  
         [0025]     The processor  18  can now establish the noise level in the signal of line  14   a . Different methods can be used. The preferred method is to determine an average of the maximums of the signal (AvgMAX) and an average of the minimums (AvgMIN) of the signal over an extended period of time. The noise level (NL) can then be established: NL=AvgMAX−AvgMIN.  
         [0026]     The extended period of time would be selected as would be understood by those of skill in the art such to follow drifting in the sensor signal without significantly changing NL measurements. If a gas is sensed, the signals will increase rapidly. However, this can cause an error in the calculation of NL. The calculation of the NL is the temporarily disabled until the signal again stabilizes within expected levels. The NL is then used subsequently to determine the sensitivity of the sensor.  
         [0027]     The sensor will normally drift over time as the conditions change. The drift range (DR) is calculated over a long period of time to detect when the detector is deviating from normal. Normal expected range of drifting can be stored in memory, such as EEPROM  26 a and the drift range compared to the expected range. Variations from the expected range can be used in the determination of the sensitivity.  
         [0028]     Another indication of the sensor condition can be established by measuring the capacitance of the electrochemical sensor  12 . This can be accomplished by coupling an electrical current to the sensor  12  and measuring the response signal over time. A capacitance value (C) can then be calculated and applied later in determining the sensitivity. It should be noted that C by itself is not a direct indicator of sensitivity of the electrochemical sensor. It is an indirect indicator that is factored into the functions for sensitivity calculation and sensor life-time.  
         [0029]     The determinations of NL, DR, and C can be affected by environmental conditions such as temperature. Therefore, these values can be compensated according to predetermined relationships as would be understood by those of skill in the art. The measurement of humidity and time can also be used to predict drying of the electrolyte and thus factored into the function.  
         [0030]     The sensitivity of the electrochemical sensor  12  can be determined as a function of NL, DR, C, and TIME. The addition of the manufacturer provided sensitivity (FS) information can be used to calculate an sensitivity adjustment factor (SA) such that SA=f{NL, DR, C, TIME, FS}. As the detector degrades in performance, the SA value will increase as a non-linear function.  
         [0031]     The SA can be applied to compensate the sensor signals (CSS) back towards the original factory calibration. One of the TIME relationships can include a normal expected degrading of the electrochemical sensor over time due to the eroding of the electrode surfaces. This may be in the range of 5% change per year and would normally be established by a manufacturer of sensor  12 . Another TIME relationship can be the averaging time of the routines such that transient conditions are further reduced and these TIME relationships can range from short term for NL to long term for DR.  
         [0032]     As the SA value increases, it is also be an indication that the remaining sensor life time (SLT) is decreasing. If SLT decreases such that it is below a dynamic threshold based upon NL, DR, C, and TIME, then a trouble indication can be generated so the sensor can be replaced. In the meantime, the processor  18  and control program  26  will continue to attempt to maintain the original factory calibration.  
         [0033]     As noted above, the signal, line  14 a, will increase with the specified gas being present. An alarm can be established based upon predetermine routines as a function of CSS, gas alarm threshold(s), and TIME as illustrated in  FIG. 5 .  
         [0034]     In accordance with the above,  FIG. 5  illustrates steps of an exemplary method  100 . In a step  102  the output from sensor  12  is obtained via processor  18 . In a step  104  the transient signals, as discussed above, are removed therefrom.  
         [0035]     In a step  106  a running average of maximum noise signals is established. In a step  108 , a running average of minimum noise values is established.  
         [0036]     The noise level NL is calculated in step  110  as discussed above. In step  112  the average CO signal is established.  
         [0037]     A maximum average CO signal over a predetermined time interval is established in step  114 . In step  116  the minimum of the average CO signal value over the time period is established.  
         [0038]     A drift value is established in step  118 . The capacitance of the sensor  12  can be established by any one of a variety of known methods, step  120 .  
         [0039]     The sensitivity adjustment for the signal could be established as a function of noise level, drift and capacitance in step  122 . In step  124  the sensitivity adjustment is established. Compensated sensor signals can be established in step  126 .  
         [0040]     In step  128  a determination can be made of remaining sensor lifetime as a function of maximum lifetime under ideal conditions and the previously determined sensitivity adjustment, step  124 . If the sensor lifetime is less than a predetermined value, step  130 , then a trouble indicator can be communicated from processor  18  via interface circuits  20  to an alarm system wherein detector  10  is installed. Finally, in a step  132 , the processor  18  can output an alarm indication if a function which is based on compensated sensor signals and time crosses a gas alarm threshold.  
         [0041]     This method can also supervise the connection of circuitry to the electrochemical sensor. If the sensor is removed from the circuitry, the NL will immediately fall to the level of the circuitry noise. The new measured NL level can cause a re-calculation of the SA that then is applied to the SLT prediction that would likely end up below a function value resulting from N, DR, C, and TIME. This will result in a trouble indication so that the sensor can be serviced to restore operation.  
         [0042]     One of the dynamic aspects of the present invention is that multiple factors can be used to determine the sensitivity adjustment over time. These values of NL, DR, and C can combine differently over time according to a predetermined function to adjust the sensitivity and also determine the SLT so sensor maintenance can be performed when required. A significant change in the value of NL, DR, or C can cause an immediate re-calculation of the SA and SLT. A period of time can also trigger this re-calculation.  
         [0043]     Because the equations are dynamic, no internal predetermined SLT threshold is used. Rather, new SLT thresholds are calculated on demand after sensor values are received by the processing routines. The result is a more robust detector that adjusts itself to the present condition of the sensor.  
         [0044]     The generation of alarm and trouble separately enables the system to which the detector, such as detector  10 , is coupled to exhibit a proper response to the detector&#39;s condition. The output indications can be transmitted in communication messages, different wireless patterns, or different audio patterns which are emitted from the detector  10 .  
         [0045]     From the foregoing, it will be observed that numerous variations and modifications may be effected without departing from the spirit and scope of the invention. It is to be understood that no limitation with respect to the specific apparatus illustrated herein is intended or should be inferred. It is, of course, intended to cover by the appended claims all such modifications as fall within the scope of the claims.