Abstract:
A measuring device for determining a gas concentration includes a gas-sensitive element, a sensing device, a stimulation unit, and a processing unit. The gas-sensitive element is configured to absorb a gas. The sensing device is configured to determine a parameter of the gas-sensitive element in a predetermined time period, where the parameter depends on an absorbed quantity of the gas. The stimulation unit is configured to stimulate the gas-sensitive element and accelerate desorption of the gas out of the gas-sensitive element. The processing unit is configured to determine a rate of change of the parameter, to control the stimulation such that a concentration of the gas in the gas-sensitive element lies outside of an equilibrium state, and to determine the gas concentration based on the rate of change.

Description:
[0001]    This application claims priority under 35 U.S.C. §119 to patent application no. DE 2016 201 950.3, filed on Feb. 10, 2016 in Germany, the disclosure of which is incorporated herein by reference in its entirety. 
         [0002]    The disclosure relates to a gas sensor. In particular, the disclosure relates to a device and a method for the improved determination of the concentration of a gas. 
       BACKGROUND 
       [0003]    A gas sensor is set up to determine the concentration of a predetermined gas, for example ozone, in a fluid medium such as air. For this purpose, the gas sensor follows the indirect measuring principle, in which a gas-sensitive element is provided, on which it is possible to determine a parameter that can be influenced by the gas concentration. Ideally, the parameter depends only on the concentration of the gas to be measured (selectivity). Even small changes in the gas concentration are intended to measurably influence the parameter (sensitivity). There should be a defined relationship between the gas concentration and the measurable parameter (accuracy). The property to be measured should change as quickly as possible with the gas concentration (short measuring time). In addition, these changes should be reversible (service life of the sensor). The concentration should be measurable with little outlay (costs and ability to be miniaturized) and the measurement method should need only little energy, in order to be suitable for mobile application, for example in a Smartphone. 
         [0004]    A small and indirectly measuring gas sensor is usually based on a field effect transistor (FET) or a metal oxide layer (MOX layer). In the FET-based sensors, the gas to be measured normally influences the work function of a gas-sensitive layer applied to the gate electrode and therefore normally causes a change in the source-drain current. Examples of such sensors are described in the applications DE 10 2008 048 715 or EP 1 104 884 82. 
         [0005]    In the gas sensors which are based on an MOX layer, the electrical resistance of a heatable MOX layer is measured. This resistance changes as a result of the presence of specific gases and their chemical interaction with the MOX layer (oxidation and reduction processes). In order to control the sensitivity and selectivity of the sensor for a predetermined gas, the MOX layer can be heatable. By means of thermal stimulation, the sensor can be kept in a defined state in order to ensure the desired measuring accuracy. The thermal stimulation is also called regeneration. Other types of regeneration are likewise possible, for example optical. 
       SUMMARY 
       [0006]    The disclosure is based on the object of specifying an improved indirectly measuring gas sensor. The disclosure achieves this object by means of the subjects of the claims, detailed description, and drawings. 
         [0007]    A method for determining a gas concentration comprises steps of absorbing the gas by means of a gas-sensitive element; determining a rate of change of a parameter of the element over a predetermined time period, wherein the parameter depends on the quantity of absorbed gas; stimulating the element in order to accelerate desorption of the gas out of the element, wherein the stimulation is carried out in such a way that the concentration of the gas in the element lies outside an equilibrium state; and determining the gas concentration on the basis of the rate of change. 
         [0008]    It has been recognized that it is not necessary to wait until the sensor is in an equilibrium state with the gas in order to determine the gas concentration. Times in the range of up to several minutes usually elapse until the equilibrium state is assumed. In order to permit a faster measurement, it may be sufficient to determine the rate of change of the parameter of the element. In this case, the sensor is deliberately kept in a state outside the equilibrium. For this purpose, the sensor can be stimulated alternatively as a function of the rate of change or in a time-controlled manner, in particular periodically. As a result, the concentration of the gas can be determined quickly and economically and the power consumption of the method can be reduced. The predetermined time period in which the rate of change is determined is preferably as immediately as possible after the beginning or ending of the stimulation of the element. At these times, the rate of change is advantageously high, so that the measurement error can be relatively small. In addition, the size of the time window can be chosen to be smaller as a result, so that the determination can demand less time. 
         [0009]    Preferably, the stimulation is carried out periodically, in order to keep the concentration of the gas in the element in a predetermined range. In other words, it is preferred to keep the element within a predetermined range of the non-equilibrium state by means of alternating stimulation and removal of the stimulus. For example, it can be attempted to keep the element in a range in which the parameter is about 50 to 80% of the value in the equilibrium state. As a result, the rate of change can also be determined repeatedly successively in a targeted manner. 
         [0010]    It is particularly preferred for the determination of the gas concentration to be based on the rate of change immediately after the stimulus has been switched off. It has been shown that the parameter is particularly stable directly after the stimulus has been switched off and permits low-noise determination of the parameter. This can be attributed partly to the fact that the measured values when the stimulus is switched off do not depend on the stimulation and therefore not on possible fluctuations or possible noise. The gas concentration can be determined in an improved manner on the basis of the rate of change. 
         [0011]    It is particularly preferred for the gas concentration K to be determined on the basis of the relationship 
         [0000]    
       
         
           
             
               K 
               = 
               
                 A 
                 · 
                 
                   
                     ( 
                     
                       dR 
                       dt 
                     
                     ) 
                   
                   B 
                 
               
             
             , 
           
         
       
     
         [0000]    where R is the parameter and B is approximately 2. In other words, the gas concentration depends on the rate of change via the power law, wherein the power of the rate of change virtually always approximately assumes a value of 2. Then, in the formula indicated above, only A exists as a free parameter. This one free parameter can permit simple and economical calibration of the method with only one measured value. Such a calibration is particularly suitable for products in the entertainment electronics sector, since the calibration can be carried out automatically as soon as a current gas concentration in the area of the gas-sensitive element is known. Regular calibrations make it possible to counteract possible sensor drift effectively. In further embodiments, more complex models for adjustment and calibration can also be used. 
         [0012]    It is particularly preferred for the parameter to be determined electrically. In particular, the parameter can relate to the crossover behavior of a transistor or the electrical resistance of the element. In other embodiments, the parameter can also be read optically, for example. Here, physical contact with the gas-sensitive element is not required for the measurement. 
         [0013]    The stimulation can likewise be carried out in different ways. In a first embodiment, the gas-sensitive element is heated for stimulation, in a second embodiment is irradiated by means of light of a predetermined wavelength and, in a third embodiment, is exposed to an electric field. Other stimuli which in each case have the object of removing absorbed gas from the gas-sensitive element are likewise possible. 
         [0014]    A measuring device for determining a gas concentration comprises a gas-sensitive element for absorption of the gas; a sensing device for determining a parameter of the element, wherein the parameter depends on the absorbed quantity of the gas; a stimulation unit for stimulating the element, in order to accelerate desorption of the gas out of the element; and a processing unit. Here, the processing unit is set up to determine a rate of change of the parameter in a predetermined time period, to control the stimulation in such a way that the concentration of the gas in the element lies outside an equilibrium state, and to determine the gas concentration on the basis of the rate of change. 
         [0015]    The measuring device can be constructed to be small and compact and, for example, find application in a mobile device such as a Smartphone. Response and reaction time of the measuring device can be shortened substantially as compared with a known measuring device. Power consumption of the measuring device can be reduced. 
         [0016]    It is particularly preferred for the gas-sensitive element to comprise a metal oxide. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    The disclosure will now be described in more detail with reference to the appended figures, in which: 
           [0018]      FIG. 1  shows a measuring device for determining a gas concentration; 
           [0019]      FIG. 2  shows a curve of a parameter dependent on the gas concentration on the measuring device from  FIG. 1 ; 
           [0020]      FIG. 3  shows curves of a measured signal with different gas concentrations according to different measuring principles; 
           [0021]      FIG. 4  shows the curve of the parameter on the gas-sensitive element during the measurement according to  FIG. 3 ; 
           [0022]      FIG. 5  shows gas concentrations which were determined on the basis of the relationship from  FIG. 5 ; 
           [0023]      FIG. 6  shows measured values which were determined on the basis of the relationship from  FIG. 5 ; and 
           [0024]      FIG. 7  shows a flowchart of a method for determining a gas concentration. 
       
    
    
     DETAILED DESCRIPTION 
       [0025]    Although the technique according to the disclosure can be implemented in different ways, in the following text, purely by way of example, the basis will be the determination of a concentration of ozone by means of a metal oxide. Other gases can likewise be detected and, instead of the metal oxide, another element can also be used, in particular a field effect transistor. 
         [0026]      FIG. 1  shows a measuring device  100  for determining a concentration of a gas  105  which, in particular, can be present in surrounding air. The measuring device  100  comprises a gas-sensitive element  110 , a sensing device  115  for determining a parameter of the element  110 , a stimulation unit  120  and a processing unit  125 . In a preferred embodiment, the gas-sensitive element  110 , the sensing device  115  and the stimulation unit  120  are combined with one another to form an integrated sensor  130 . 
         [0027]    The gas-sensitive element  110  has the property of absorbing some of the gas  105  out of the surroundings and, depending on the absorption that has taken place, of changing a parameter, which can be determined by means of the sensing device  115 . In one embodiment, the element  110  comprises a metal oxide, the resistance of which changes, the more of the gas  105  is absorbed in the element  110 . Whether the parameter rises or falls with rising concentration of the gas usually depends on the gas and in particular on its oxidation properties. If the element  110  is exposed to a predetermined gas concentration of the gas  105 , then it usually takes a time interval in the minute range until so much of the gas  105  is absorbed in the element  110  that the parameter no longer changes. This state is called the equilibrium state. The element  110  attempts to assume the equilibrium state by absorbing or desorbing gas  105 , depending on the concentration of the gas  105  in the surroundings. If more or less of the gas  105  is absorbed in the element  110  than corresponds to the concentration of the gas  105  in the surroundings, then the element  110  is in the non-equilibrium state. 
         [0028]    The desorption of gas  105  out of the element  110 , that is to say the expulsion of gas particles out of the element  110 , can be promoted by means of the stimulation unit  120 . The stimulation unit  120  can, for example, comprise a light source, in particular a light-emitting diode, the light emitted from which has a predetermined wavelength. This wavelength can comprise about 450 nm, for example. In other exemplary embodiments, the stimulation unit  120  can also be set up to heat the element  110  or to produce an electric field in the area of the element  110 . 
         [0029]    The processing unit  125  is set up to control the stimulation unit  120  on the basis of the parameter of the element  110  that is determined by means of the sensing device  115 , in such a way that the element  110  is in a predetermined non-equilibrium with respect to the surrounding gas concentration. Here, the process is to be carried out in particular periodically or intermittently by the stimulation unit  120  being alternately activated and deactivated. How long the individual activation or deactivation phases last can depend in particular on the parameter of the element  110 . Furthermore, the processing unit  125  should be set up to determine a rate of change of the parameter of the element  110  and the concentration of the gas  105  in the area of the element  110  on the basis of the rate of change. Preferably, an interface  135  is provided, via which the processing unit  125  can provide a result of the concentration determination of the gas  105  externally. 
         [0030]    The measuring principle will be described in more detail below with reference to  FIG. 2 .  FIG. 2  shows a curve  225  of a parameter of the element  110  on the measuring device  100  from  FIG. 1 . In the embodiment illustrated, the parameter of the element  110  is intended to be a resistance, which is plotted in the vertical direction in the graph illustrated and which rises with a rising quantity of absorbed gas  105  in the element  110 . In other embodiments, however, another parameter, for example a generated voltage or a current, of the element  110  can be involved, and the relationship between the parameter and the concentration can also be inverted with respect to the illustration of  FIG. 2 . Plotted in the horizontal direction is a time. Illustrated in a left-hand area is an equilibrium measurement  205  (equilibrium: GG) and, in a right-hand area, a non-equilibrium measurement  210  (non-equilibrium: NGG). 
         [0031]    Also illustrated are a first equilibrium  215  without the influence of the stimulation unit  120  and a second equilibrium  220  under the influence of the stimulation unit  120 . If the stimulation unit  120  is active, then the curve  225  initially falls more quickly and then more and more slowly and adheres to the second equilibrium  220 . In a corresponding way, when the stimulation unit  120  is switched off, the curve  225  initially rises quickly and then more and more slowly and adheres to the first equilibrium  215 . 
         [0032]    Within the context of the equilibrium measurement  205 , in a first phase the gas  105  absorbed in the element  110  can be desorbed under the influence of the stimulation unit  120 , so that, after the stimulation unit  120  has been switched off, the first equilibrium  215  is reached by the curve  225  after a predetermined time and the parameter can be determined. The equilibrium measurement  205  is relatively time-consuming and energy-intensive. 
         [0033]    It is therefore proposed, within the context of a non-equilibrium measurement  210 , by means of alternating activation and deactivation of the stimulation unit  120 , deliberately to bring about a non-equilibrium state, which lies between the equilibria  215  and  220 . Preferably, the parameter of the curve  225  is kept in a predetermined range between the equilibria  215  and  220 , for example by maintaining a predetermined range from relative equilibria, for example between 20% and 80%. The form of the curve sections of the curve  225  with and without activated stimulation unit  120  is known and usually follows an inverse e-function. To determine the concentration of gas  105  on the element  110 , it may therefore be sufficient to determine a characteristic influencing factor of the curve segment. This influencing factor can in particular comprise the slope of the curve  225  at a predetermined time or in a predetermined time period, in particular at the start of a curve section. In each case immediately after the stimulation unit  120  has been switched off, these slopes are plotted as rates of change in the illustration of  FIG. 2 . 
         [0034]    On the basis of a rate of change, the associated gas concentration can be determined by means of the following formula in accordance with the power law: 
         [0000]    
       
         
           
             
               
                 
                   K 
                   = 
                   
                     A 
                     · 
                     
                       
                         ( 
                         
                           dR 
                           dt 
                         
                         ) 
                       
                       B 
                     
                   
                 
               
               
                 
                   ( 
                   
                     equation 
                      
                     
                         
                     
                      
                     1 
                   
                   ) 
                 
               
             
           
         
       
     
         [0000]    with:
 
K=gas concentration
 
A, B=constants
 
R=parameter.
 
         [0035]    Usually, the constant B is about 2 so that, by means of correctly choosing the constant A, the mapping of the rate of change of the determined parameter of the curve  225  onto the gas concentration K can be carried out. In particular, it may be sufficient for any desired, known gas concentration K to determine the constant A by using the determined rate of change of the parameter in order to calibrate the measuring device  100 . If the measuring device  100  is arranged in a mobile device, for example, then the concentration of ozone can be interrogated by means of a trustworthy web service in order to carry out this adjustment. 
         [0036]      FIG. 3  shows exemplary curves of a measured signal with different gas concentrations in accordance with different measuring principles. In an upper illustration, the determination is carried out by means of equilibrium measurement  205  and, in the lower area, by means of non-equilibrium measurement  210 , as explained in more detail above with reference to  FIG. 2 . The numerical values specified, in particular for gas concentrations, resistances and times, are exemplary. The gas to be determined is ozone here, although this is likewise only an example which can represent many different gases. 
         [0037]    For the measured signals illustrated, the gas-sensitive element  110  was exposed to different concentrations of ozone, which are plotted in the illustrations, at intervals of 30 minutes. In each graph, it is possible to see four measured curves, which are assigned to four identical gas-sensitive elements  110 . The differences between the measuring curves indicate the scatter between the gas-sensitive elements  110 . 
         [0038]    It can be seen that, after the equilibrium measurement  205 , in each case several minutes are required in order to determine the correct concentration, whereas a considerably faster determination is possible by means of the non-equilibrium measurement  210 . It can also be seen that the curves of the four gas-sensitive elements can be brought into coincidence with one another by means of a simple adjustment. 
         [0039]      FIG. 4  shows the variation of the parameter of the curve  225  from  FIG. 2  that can be determined on the gas-sensitive element  110  over time during the determination according to  FIG. 3 . The stimulation unit  120  was switched on and off in a five-second cycle. In other embodiments, a mark-space ratio differing from 1:1 or a period length other than 10 seconds can also be used. 
         [0040]      FIG. 5  shows a mathematical relationship between the rate of change of the parameter on the measuring device  100  from  FIG. 1  and a gas concentration K. The numerical values illustrated are once more exemplary and four different measuring curves are assigned to four identical gas-sensitive elements  110 . The curves illustrated were created with reference to specific rates of change, which were each determined immediately after the stimulation unit  120  had been switched off. It is obvious that the relationship illustrated follows the power law. 
         [0041]      FIG. 6  shows exemplary gas concentrations which were determined on the basis of the relationship of  FIG. 5 . 
         [0042]      FIG. 7  shows a flowchart of a method  300  for determining a gas concentration. The method  300  is set up in particular to be performed by means of the measuring device  100  from  FIG. 1 . 
         [0043]    In a step  305 , gas  105  is absorbed by the gas-sensitive element  110 . This process lasts for a predetermined time, wherein the gas  105  is initially enriched quickly and then more and more slowly on the gas-sensitive element  110  (cf.  FIG. 2 ), until finally an equilibrium state  215  between the absorbed gas  105  and the gas  105  present in the surroundings of the element  110  is reached. 
         [0044]    In a step  310 , the rate of change of a parameter of the element  110  is determined. The parameter indicates the quantity of gas  105  absorbed on the gas-sensitive element  110  and can be determined in particular on the basis of the crossover behavior of a transistor which comprises the element  110 , or the electrical resistance of the element  110 . 
         [0045]    In an optional step  315 , the element  110  is stimulated in order to reduce the quantity of gas  105  accumulated on the element  110 . Preferably, the stimulation comprises heating the element  110 , for example by means of an external heating element or by an electrical current through the element  110  being brought about. The simulation effects expulsion of accumulated gas  105  out of the element  110 , wherein the quantity of bound gas initially falls quickly and then more and more slowly, until a second equilibrium state  220  is reached. The intensity and duration of the stimulation is preferably managed in such a way that the concentration of the gas  105  in the element  110  lies between the two equilibrium states  215  and  220 . 
         [0046]    In a step  320 , the gas concentration in the area of the element  110  on the basis of the rate of change 
         [0047]    In a step  320 , the gas concentration in the area of the element  110  is determined on the basis of the rate of change of the parameter. Here, the determination preferably relates to a predetermined time period which lies as immediately as possible after the end of the stimulation in step  315 , when the rate of change is still high. In a further embodiment, the gas concentration can also be determined on the basis of the rate of change of the parameter during the stimulation. It is also possible for both rates of change to be used as a basis for determining the gas concentration. 
         [0048]    Then, in a step  325 , an optional pause can be inserted in order to permit the enrichment of gas  105  on the element. The method  300  can then return to step  305  and run through again.