Abstract:
A gas sensor is provided for determining the concentration of a gas component in a measuring gas, in particular for determining the oxygen concentration in the exhaust gas of internal combustion engines, which has an electrode pair situated on a solid-state electrolyte and made up of an outer pump electrode and an inner pump electrode, which is accessible to the measuring gas supplied via a diffusion barrier, the electrode pairs being triggered in a clocked manner and having a potential of varying polarity applied in each clock-pulse period. To improve the measuring precision of the gas sensor without additional electrodes, a cavity is situated between the diffusion barrier and the inner pump electrode, the cavity serving as storage volume for the oxygen pumped through the solid-state electrolyte.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention is based on a gas sensor for determining the concentration of a gas component in a measuring gas, in particular for determining the oxygen concentration in the exhaust gas of internal combustion engines. 
       BACKGROUND INFORMATION 
       [0002]    In a gas-measuring probe or gas sensor for determining the λ-value in exhaust gases of internal combustion engines (M. Oshuga &amp; Y. Ohyama “A study on the oxygen-biased wide range air-fuel ratio sensor for rich and lean air-fuel ratios”, Sensors and Actuators, 9 (1986), pages 287-300), the outer pump electrode of the electrode pair situated on the solid-state electrolyte is exposed to the atmosphere, and the inner pump electrode is covered by a diffusion barrier, which has an adapted thickness and is acted upon by the exhaust gas. The electrode pair is controlled in a clocked manner, and the inner pump electrode and the outer pump electrode are alternately connected to a potential of varying magnitude, which causes oxygen from the atmosphere to be pumped into the diffusion barrier (pump-in phase) and from the diffusion barrier to the atmosphere (evacuation phase) in alternation. In so doing, a current—referred to as bias current—is flowing from the inner pump electrode to the outer pump electrode in the pump-in phase, and a pump current—referred to as sensing current or measuring current—is flowing from the outer pump electrode to the inner pump electrode in the evacuation phase. The last current value in each evacuation phase is detected with the aid of a sample and hold circuit and supplies the measure for the oxygen concentration as λ-value of the exhaust gas. The last current value in each evacuation phase is likewise detected with the aid of a sample and hold circuit and supplies a control signal for an electrical heater with the aid of which the temperature of the solid-state electrolyte is controlled to a constant value. 
         [0003]    For the clocked control of the electrode pair, the electrode pair is situated in the bridge branch of a switch bridge made up of four electronic switches, of which two switches that are lying in two diagonal branches are triggered by the clock pulses of a clock-pulse generator, and two switches that are lying in the two other diagonal branches are triggered by the inverted clock pulses, which are shifted by half a clock-pulse period. Due to the alternate biasing into conduction of the individual switch pairs, two potentials that vary in polarity are applied to the electrode pair in each clock-pulse period, a potential difference of, for example, 0.3 V existing between inner pump electrode and outer pump electrode in the pump-in phase, and a potential difference of 0.1 V, for instance, existing between outer pump electrode and inner pump electrode in the evacuation phase. 
       SUMMARY OF THE INVENTION 
       [0004]    The gas sensor according to the exemplary embodiment and/or exemplary method of the present invention has the advantage of a considerably higher measuring accuracy in determining the concentration of the gas components in the measuring gas, in particular in determining the oxygen concentration or the air/fuel ratio (λ) in exhaust gases. The provision of the cavity between diffusion barrier and the inner pump electrode situated on the solid-state electrolyte produces a region having a constant oxygen concentration, which serves as storage volume. In contrast to the gas sensor described in the introduction, the oxygen must therefore not be stored in the diffusion barrier, which shortens the diffusion barrier and falsifies the measuring signal due to the shortened diffusion barrier. Like the known gas sensor described in the introduction, the gas sensor according to the exemplary embodiment and/or exemplary method of the present invention has only two electrodes, which allows a cost-effective production and a wide λ-measuring range, which has been considerably expanded into the enriched range of the exhaust gas. Operating the electrodes with a varying potential improves their pumping ability for oxygen. 
         [0005]    Advantageous further refinements and improvements of the gas sensor are rendered possible by the measures specified in the additional claims. 
         [0006]    According to one advantageous specific embodiment of the present invention, the pump current, which was measured over one clock-pulse period or a plurality of clock-pulse periods and averaged, is utilized as a measure for the concentration of the gas component, i.e. the λ-value of the exhaust gas. As an alternative, the pump current, measured in the pump-in and evacuation phase and filtered by a time constant that is considerably greater than the cycle duration of the clocking, is used as measure for the concentration of the gas component. 
         [0007]    According to one advantageous specific embodiment of the present invention, the oxygen quantity delivered into the cavity in the pump-in phase is controlled as a function of the instantaneous oxygen concentration in the measuring gas. This not only results in the desired broadening of the measuring range in the rich range of an exhaust gas, but also greatly reduces the pumped-in oxygen quantity in the lean range of the exhaust gas so as not to stress the pump electrodes unnecessarily. 
         [0008]    To control the fed-in oxygen quantity, the pump current—the so-called bias current—flowing in the pump-in phase may be adjusted as a function of the measured concentration of the gas component. Instead of specifying the bias current, it is also possible to specify a certain charge quantity using which an equivalent oxygen quantity is pumped into the cavity. This is advantageous in those cases where the constancy of the bias current is able to be achieved only with difficulty. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  shows in a cutaway, schematized view, a longitudinal section of a sensor element of a gas sensor for measuring the exhaust gas of internal combustion engines. 
           [0010]      FIG. 2  shows a diagram to elucidate the functionality of the gas sensor in lean-gas operation. 
           [0011]      FIG. 3  shows a diagram to elucidate the functionality of the gas sensor in rich-gas operation. 
           [0012]      FIG. 4  shows a block circuit of the gas sensor including sensor element and control device. 
           [0013]      FIG. 5  shows a longitudinal section of a sensor element of the gas sensor in various modifications, in a separate cutaway view. 
           [0014]      FIG. 6  shows a longitudinal section of a sensor element of the gas sensor in various modifications, in another separate cutaway view. 
           [0015]      FIG. 7  shows a longitudinal section of a sensor element of the gas sensor in various modifications, in another separate cutaway view. 
           [0016]      FIG. 8  shows a longitudinal section of a sensor element of the gas sensor in various modifications, in another separate cutaway view. 
           [0017]      FIG. 9  shows a block diagram of the gas sensor including the sensor element according to  FIG. 8  as well as control device. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    The gas measuring probe or gas sensor described here is used to determine the concentration of a gas component in a measuring gas, and may be employed as Lambda sensor for determining the oxygen concentration in the exhaust gas of internal combustion engines by which the air/fuel ratio in the exhaust gas of internal combustion engines, which is indicated as so-called λ-value, is ascertained. The following description therefore relates to such a gas sensor for determining the λ value. 
         [0019]    The gas-measuring probe or gas sensor has a sensor element  11 , shown in  FIG. 1  in schematized form in longitudinal section, which is usually accommodated in a housing and exposed to the exhaust gas by its gas-sensitive portion. Situated in the gas-sensitive portion is an electrode pair, which is connected to a connector cable routed out of the housing and to a control device  10  ( FIG. 4 ) with the aid of circuit traces and a connector plug. The electrode pair encompasses an outer pump electrode  12  and an inner pump electrode  13 , both of which are situated on a solid-state electrolyte  14 , which is made of Yttrium-stabilized zirconium-oxide (ZrO 2 ), for example. As is not shown further in  FIG. 1 , solid-state electrolyte  14  is made up of a number of laminated solid-state electrolyte layers or foils, in which an electric resistance heater  15  is disposed, which is embedded in an insulation layer (not shown here). Resistance heater  15  is used to set a constant operating temperature of solid-state electrolyte  14 . Outer pump electrode  12  is situated on the outside of solid-state electrolyte  14  and therefore directly exposed to the exhaust gas, whereas the exhaust gas is able to reach inner pump electrode  13  only via a diffusion barrier  16 . In the exemplary embodiment of  FIG. 1 , a cavity  17  is formed inside solid-state electrolyte  14 , from which a channel  18  commences, which discharges on the outside of solid-state electrolyte  14 . Inner pump electrode  13  is affixed inside cavity  17  on solid-state electrolyte  14 , and channel  18  is completely filled with a porous ceramic material such as, for example, ZrO 2  or Al 2 O 3  so as to form diffusion barrier  16 . 
         [0020]    Electrode pair  12 ,  13  is triggered by a clock-pulse generator  27  of control device  10  ( FIG. 4 ) using a selected clock-pulse frequency and selected pulse-duty factor, a potential of varying polarity being applied to the two pump electrodes  12 ,  13  in each clock-pulse period T. In the diagrams of  FIGS. 2 and 3 , the voltage applied to the electrode pair using a 50% pulse-duty factor is illustrated by the dotted line (curve a). In a so-called pump-in phase A, which corresponds to half a clock-pulse period in the example, a positive potential is applied to inner pump electrode  13  (and a negative potential is applied to outer pump electrode  12 ). This causes negatively charged oxygen ions to diffuse through diffusion barrier  16  to inner pump electrode  13 . Due to the oxygen-ion flow, a pump current I p , also referred to as bias current and understood as the movement of positive charge carriers, is flowing from inner pump electrode  13  to outer pump electrode  12  in the pump-in phase. In the following evacuation phase B, which once again extends over half a clock-pulse period in the example, a positive potential is applied at outer pump electrode  12  (and a negative potential at inner pump electrode  13 ). As a result, negatively charged oxygen ions diffuse through diffusion barrier  16  to the outer pump electrode, and a pump current +I p  is flowing from outer electrode  12  to inner electrode  13 . It should be noted that both pulse-duty ratio and clock-pulse frequency as well as the applied voltage are variable. The characteristic of pump current I p  as movement of positive charge carriers from outer pump electrode  12  to inner pump electrode  13  is shown in  FIGS. 2 and 3  in the form of a solid line (curve b). Due to this clocked triggering of electrode pair  12 ,  13 , oxygen is pumped through the solid-state electrolyte into cavity  17  in pump-in phase A, and oxygen is pumped out of cavity  17  via the solid-state electrolyte in evacuation phase B. In addition to the oxygen pumped through the solid-state electrolyte, various exhaust-gas components reach cavity  16 , which either discharge oxygen or bind oxygen through an electro-chemical reaction at inner pump electrode  13 . The oxygen-equivalent concentration C o2  produced in cavity  17  overall is shown in  FIG. 2  for lean-gas operation, and in  FIG. 3  for rich-gas operation as a dot-dash line (curve c) in each case. It should be noted that idealized conditions were assumed in the diagrams in  FIGS. 2 and 3  in order to clarify the mechanism taking place. In reality, the transitions in curves b are not quite as abrupt as shown but have a continuous characteristic with a lower gradient. 
         [0021]    In lean-gas operation ( FIG. 2 ), oxygen-equivalent concentration C o2  is generated in cavity  17  in pump-in phase A (rising flank of curve c). In evacuation phase B, oxygen present in cavity  17  is first pumped out (falling flank of curve c). If no further oxygen remains in cavity  17 , then pump current I p  drops (falling flank of curve b). The lean gas diffusing through diffusion barrier  16  into cavity  17  carries oxygen that is likewise evacuated (horizontal portion of curve b). An average pump current I p  comes about (curve d) over the clock-pulse period, which is equivalent to the gas flow flowing through diffusion barrier  16  and provides a measure for the oxygen concentration and thus the air-fuel ratio in the exhaust gas. Pump current I p  flowing in every clock-pulse period is measured in a measuring stage  28  made up of a shunt  29  and a differential amplifier  30  of control device  10  ( FIG. 4 ) and averaged over a plurality of clock-pulse periods (block  19  in  FIG. 4 ). As an alternative, pump current I p  measured in measuring stage  28  is filtered using a time constant that is considerably greater than cycle duration T of the clocking (block  19  in  FIG. 4 ). 
         [0022]      FIG. 3  shows the conditions that were described for lean operation with the aid of  FIG. 2 , for rich-gas operation. In pump-in phase A, the oxygen pumped into cavity  17  reacts with the rich gas, which either is already present in cavity  17  or which diffuses into cavity  17  via diffusion barrier  16 , i.e., with gas component CH 4 , which, through bonding of oxygen, lets CO 2  and H 2 O come about. The oxygen remaining from the reaction generates an oxygen-equivalent concentration C o2  in cavity  17  (rising flank of curve c), which is considerably lower than during lean-gas operation. In evacuation phase B, oxygen present in cavity  17  is first pumped out (falling flank of curve c). If no further oxygen remains in cavity  17 , then pump current I p  returns to zero (falling flank of curve b). Inwardly diffusing rich gas collects in cavity  17 , which leads to a need for oxygen (slanted negative portion of curve c). The resulting average value of pump current I p  (curve d) once again corresponds to the gas flow through diffusion barrier  16  and constitutes a measure for the λ lambda value that is smaller than 1. 
         [0023]    Due to pump current −I p —the so-called bias current—flowing in pump-in phase A, an expansion of the measuring range of the gas sensor in rich-gas operation (fuel excess) is achieved. In lean-gas operation (air excess), this bias current has a disadvantageous effect since it further increases the oxygen quantity forming in cavity  17  by the electrochemical reaction, and thereby further increases the oxygen flow to be pumped, so that electrodes  12 ,  13  are subjected to unnecessary loading and age faster. As a counter measure, the oxygen quantity pumped into cavity  17  in pump-in phase A is adjusted as a function of the oxygen concentration in the exhaust gas, i.e., the air/fuel ratio. This may be achieved by varying the pulse-duty factor or by variable dimensioning of bias current −I p , so that, in lean-gas operation, bias current −I p  becomes sufficiently small to relieve the stress on electrodes  12 ,  13 . The lambda signal able to be picked up at block  19  in control device  10  and fed into a filter  20 , such as a PID filter, is used to set the bias current. The output of filter  20  determines the magnitude of the bias current. 
         [0024]    As an alternative, it is not a specific constant bias current that is defined in control device  10  as a function of the lambda signal, but a specific charge quantity, which is pumped into cavity  17  as equivalent oxygen quantity. This is advantageous in those cases where the constancy of the bias current is able to be achieved only with difficulty. 
         [0025]    For the continuous measurement of the inner resistance of sensor element  11 , a sample and hold circuit  31  is connected to the output of differential amplifier  30  in control device  10 , which samples bias current −I p  once every clock-pulse period and holds the sampling value until the next measurement. Using the measured resistance value, the temperature of the solid-state electrolyte is able to be constantly controlled to the operating temperature with the aid of resistance heater  15  provided in sensor element  11 . 
         [0026]      FIGS. 5 ,  6  and  7  show three sensor elements  11  in longitudinal, part-sectional view, these sensor elements being modified with regard to the placement of pump electrodes  12 ,  13 , cavity  17  and diffusion barrier  16 . In the sensor element according to  FIG. 5 , cavity  17  and diffusion barrier  16  are designed as concentric rings, and diffusion barrier  16  encloses a gas-entry channel  21  discharging on the outside of solid-state electrolyte  14 , and cavity  17  encloses diffusion barrier  16 . The two pump electrodes  12 ,  13  are embodied as ring electrodes, which are separated by the solid-state electrolyte, outer pump electrode  12  once again being affixed on the outside of solid-state electrolyte  14  so as to concentrically enclose gas-entry channel  21 , and inner pump electrode  13  being disposed inside cavity  17  and likewise being affixed on solid-state electrolyte  14 . 
         [0027]    In the exemplary embodiment of  FIG. 6 , outer pump electrode  12  and inner pump electrode  13  are situated on the outside of solid-state electrolyte  14 , i.e., on the same large surface. Forming cavity  17 , inner pump electrode  13  is covered by diffusion barrier  16 , which has a box-like design for this purpose and is resting on the outer side of solid-state electrolyte  14  via its box edges. Diffusion barrier  16  may also be covered by a protective layer  22 . 
         [0028]    Sensor element  11  according to  FIG. 7  differs from sensor element  11  in  FIG. 6  only in that outer pump electrode  12  is situated on the other large surface of solid-state electrolyte  14 , which faces away from the large surface that supports inner pump electrode  13  having diffusion barrier  16 . 
         [0029]      FIG. 8  shows a sensor element  11 , whose design is identical design to that of  FIG. 1 , but which has an additional electrode on the outside of solid-state electrolyte  14 , which forms a so-called Nernst electrode  24  and is covered by a porous protective layer  23 . Using this additional outer electrode, a voltage-jump or λ=1 sensor may additionally be realized by sensor element  11 . Inner pump electrode  13  disposed within cavity  17  is used as reference electrode, which may readily be realized by suppressing evacuation phase B, so that cavity  17  remains filled with oxygen at all times. As illustrated in the block diagram of  FIG. 9 , the sensor element is switched in the control device from the “broadband sensor” operating mode to the “voltage jump sensor” operating mode for this purpose, which is symbolically indicated by throwing switch  25  in  FIG. 9 . This causes clock-pulse generator  27  to be switched off in control device  10 , and a reference current source  26  to be applied to electrode pair  12 ,  13 . The λ value, which is tapped at lower λ output of control device  10  in  FIG. 9 , is derived from the potential of Nernst electrode  24 . After switch  25  has been moved back into the lower switch position in  FIG. 9 , the “broadband sensor” operating mode has been reset, and the λ value is available at upper λ output of control device  10  in  FIG. 9 . 
         [0030]    In the measuring probe described in different variants of an embodiment, it is also possible to expose outer pump electrode  12 , which is situated on the outside of solid-state electrolyte  14  and exposed to the measuring or exhaust gas, to a reference gas, which may be atmospheric air, without this causing a change in the function of the measuring sensor. 
         [0031]    The measuring sensor may also be used to determine the concentration of nitrogen oxides in the exhaust gas of internal combustion engines. 
         [0032]    In the exemplary embodiment described, the pump voltage applied to electrode pair  12 ,  13  is predefined in control device  10  ( FIG. 4 ). Instead of the pump voltage, it is also possible to specify the pump current.