Abstract:
In a method of controlling a sensor for determining an oxygen concentration in a gas mixture, in particular in the exhaust gas of internal combustion engines, a detection voltage supplied by a Nernst measurement cell and corresponding to the oxygen concentration is transformed by a circuit arrangement into a pump voltage for a pump cell, and an anodic or cathodic limit current flows over the pump cell, depending on the oxygen content of the gas mixture. In stable operation of the sensor, during which an anodic limit current flows for a selectable period of time, the pump cell and/or the Nernst measurement cell receives at least one voltage pulse supplied independently by the measured detection voltage or the pump current thus established, so that the sensor is depolarized.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method of controlling a sensor for determining an oxygen concentration in a gas mixture, in particular in the exhaust gas of internal combustion engines. 
     BACKGROUND INFORMATION 
     Sensors are used to determine the adjustment of a fuel-air mixture for operation of an internal combustion engine by determining the oxygen concentration in the exhaust gas of the internal combustion engine. The fuel-air mixture may be in the rich range, i.e., the fuel is present in stoichiometric excess, so that only a small quantity of oxygen is present in the exhaust gas in comparison with other partially unburned components. In the lean range, where there is more oxygen than air in the fuel-air mixture, the oxygen concentration in the exhaust gas is high accordingly. 
     Lambda probes are known for determining the oxygen concentration in the exhaust gas; these probes detect a lambda value of &gt;1 in the lean range or &lt;1 in the rich range, and a lambda value=1 in the stoichiometric range. In a known way, a Nernst measurement cell of the sensor supplies a detection voltage which is sent to a circuit arrangement. With the help of this circuit arrangement, the detection voltage is transformed to a pump voltage for a measurement probe (pump cell) which is also a part of the sensor. The measurement probe functions as a pump cell, where oxygen ions are pumped from a first electrode to a second electrode of the pump cell or vice versa, depending on the prevailing oxygen concentration in the gas mixture on which the measurement is to be performed. Depending on whether the lambda probe detects a rich range, i.e., a lambda value&lt;1, or a lean range, i.e., a lambda value&gt;1, the circuit arrangement determines whether an electrode of the pump cell connected to an active input of the circuit arrangement is switched as an anode or a cathode. The second electrode of the pump cell is connected to ground, so that either a cathodic limit current is set at the pump cell with a rich measurement gas or an anodic limit current is set with a lean measurement gas. 
     With a known sensor design, an electrode of the Nernst measurement cell and an electrode of the pump cell are each arranged in a joint cavity of the sensor, which is exposed to the exhaust gas through a diffusion barrier. If the fuel-air mixture to be monitored is in the lean range for a long period of time, oxygen ions diffuse out of the exhaust gas through the diffusion barrier into the joint cavity of the Nernst electrode of the Nernst measurement cell and the one pump electrode of the pump cell. According to the higher oxygen content in the lean range, an anodic limit current is applied to the pump cell by the circuit arrangement. In this way, additional oxygen ions are pumped into the joint cavity through the pump cell. One disadvantage of this is that if the internal combustion engine operates under lean conditions for a long period of time, e.g., several hours, fewer oxygen ions are pumped into the joint cavity of the Nemst electrode and the one pump electrode through the pump cell than would be necessary to maintain λ=1 in the cavity. This is due to falsification of the voltage of the Nernst measurement cell due to the participation of the Nernst electrode in the function of the internal pump electrode. This is the case when the internal pump electrode has become inactive due to long-lasting cathodic operation or due to manufacturing tolerance. However, due to the increasing concentration of oxygen ions in the joint cavity, the Nernst measurement cell determines that the fuel-air mixture is becoming richer, so that the sensor is subject to a rich drift leading to inaccuracies in the output signal. 
     SUMMARY OF THE INVENTION 
     The method according to the present invention for controlling a sensor offers the advantage that such a rich drift can be compensated. Due to the fact that the polarity of the pump voltage is reversed or the Nernst voltage is increased in selectable intervals after a selectable period of time during which sensor operation has been exclusively lean, it is advantageously possible to pump oxygen ions from the joint cavity of the Nernst electrode and the one pump electrode through the pump cell or the Nernst measurement cell, so that rich drift of a measurement probe can be compensated. Furthermore, CO coverage of the electrode can be eliminated. This activates the Nernst electrode, so that a difference in oxygen concentration between the Nernst electrode and a reference electrode again corresponds to the actual oxygen content in the gas mixture on which the measurement is to be performed. Brief evacuation of oxygen ions can be set according to the choice of the frequency and duration of pulses. The frequency and duration of pulses can be varied by an analysis and control circuit arrangement of the sensor as a function of an oxygen content detected in the gas mixture on which the measurement is to be performed. This ensures that only rich drift of the sensor will in fact be compensated, and reverse signal corruption due to a disturbance in adjustment of λ=1 in the cavity is prevented. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The Figure shows a sectional diagram through a head of a sensor. 
    
    
     DETAILED DESCRIPTION 
     The Figure shows a sensor  10  in a sectional diagram through a measurement head. Sensor  10  is designed as a planar broad-band sensor having a number of individual layers arranged one above the other, optionally structured, for example, by film casting, punching, screen printing, lamination, cutting, sintering, or the like. Production of the layer structure will not be discussed further here as part of the present description because it is already known. 
     Sensor  10  is used to determine an oxygen concentration in the exhaust gases of internal combustion engines to obtain a control signal for adjusting a fuel-air mixture with which the internal combustion engine is operated. Sensor  10  has Nernst measurement cell  12  and a pump cell  14 . Nernst measurement cell  12  has a first electrode  16  and a second electrode  18  between with there is a solid electrolyte  20 . Electrode  16  is exposed to exhaust gas  24  to be measured through a diffusion barrier  22 . Sensor  10  has a measurement orifice  26  which can receive exhaust gas  24 . Diffusion barrier  22  extends at the base of measurement orifice  26 , forming a cavity  28  withing which electrode  16  is arranged. Electrode  18  of Nernst measurement cell  12  is arranged in a reference air channel  30  and is exposed to a reference gas such as air which is applied to reference air channel  30 . Solid electrolyte  20  is preferably made of zirconium oxide stabilized with yttrium oxide, while electrodes  16  and  18  are made of platinum, for example. 
     Sensor  10  is connected to a circuit arrangement  32 , which is used to analyze signals of sensor  10  and to control the sensor. Electrodes  16  and  18  are connected to inputs  34  and  36  to which a detection voltage U D  of Nemst measurement cell  12  is applied. 
     Pump cell  14  is composed of a first electrode  38  and a second electrode  40  between which is arranged a solid electrolyte  42 . Solid electrolyte  42  is itself made of zirconium oxide stabilized with yttrium oxide, while again, electrodes  38  and  40  may be made of platinum. Electrode  38  is also arranged in cavity  28  and is thus also exposed to exhaust gas  24  through diffusion barrier  22 . Electrode  40  is covered with a protective layer  44  which is porous so that electrode  40  is exposed directly to exhaust gas  24 . Electrode  40  is connected to an input  46  of circuit arrangement  32 , while electrode  38  is connected to electrode  16  and is jointly connected with it to input  34  of circuit arrangement  32 . 
     Sensor  10  also includes a heating device  49  formed by a wave-form heater. Heating device  49  receives a heating voltage U H . 
     Sensor  10  functions as follows: 
     Exhaust gas  24  is in cavity  28  above measurement orifice  26  and diffusion barrier  22  and is thus in contact with electrodes  16  of Nemst measurement cell  12  and electrode  38  of pump cell  14 . Because of the oxygen concentration present in the exhaust gas on which the measurement is to be performed, an oxygen concentration difference is established between electrode  16  and electrode  18 , which is exposed to the reference gas. Electrode  16  is connected by terminal  34  to a current source of circuit arrangement  32  which supplies a constant current. Because of an oxygen concentration difference prevailing at electrodes  16  and  18 , a certain detection voltage U D  is established. Nernst measurement cell  12  operates here as a lambda probe, which detects whether there is a high oxygen concentration in exhaust gas  24  or a low oxygen concentration. It is clear on the basis of the oxygen concentration whether the fuel-air mixture with which the internal combustion engine is operating is a lean or rich mixture. When there is a change from the rich range to the lean range or vice versa, detection voltage U D  drops or increases accordingly. 
     With the help of circuit arrangement  32 , detection voltage U D  is used to determine a pump voltage U P  which is to be sent to pump cell  14  between its electrodes  38  and  40 . Pump voltage U P  is negative or positive, depending on whether detection voltage U D  signals that the fuel-air mixture is in the rich or lean range, so that electrode  40  is switched either as a cathode or as an anode. Accordingly, a pump current I P  is established and can be measured by a measurement device of circuit arrangement  32 . With the help of pump current I P , oxygen ions are pumped either from electrode  40  to electrode  38  or vice versa. Measured pump current I P  is used to control a device for adjusting the fuel-air mixture with which the internal combustion engine is operated. 
     In addition, it is assumed that the fuel-air mixture with which the internal combustion engine is operated is in a lean range for a long period of time. Therefore, a high oxygen content is established in exhaust gas  24  accordingly and is detected by sensor  10 . A corresponding detection voltage U D  is applied over the period of lean operation in accordance with the high oxygen content. Circuit arrangement  32  here includes a timer  50 , with which detection voltage U D  is sampled and a determination is made regarding the period of time over which this has been at a certain height. Timer  50  supplies a signal  52  when detection voltage U D  is within a certain value range corresponding to lean operation of the internal combustion engine for a definable period of time, which may be, for example, several minutes, hours, or the like. During lean operation of the internal combustion engine, a cathodic pump current I P  flows. Due to this cathodic pump current I P , oxygen ions are pumped out of cavity  28  via electrode  38 , so that over a long period of time fewer oxygen ions are pumped out of cavity  28  than enter cavity  28  from exhaust gas  24  through diffusion barrier  22  by cathodic pump current I P . Due to the declining pump current of the pump cell, Nernst measurement cell  12  detects a fuel-air mixture which is becoming richer. Sensor  10  is thus subject to a rich drift. The reason for this is the faulty detection of the oxygen concentration in the cavity. The distribution of the pump current to the internal pump electrode and Nemst electrode  38 ,  16  changes over time to the detriment of the internal pump electrode, so detected Nemst voltage U D  no longer corresponds to the concentration ratio between cavity  28  and reference air channel  30 , but instead is falsified by a superimposed polarization voltage. It seems to be increased. Therefore, the system establishes a higher oxygen concentration than λ=1 in the cavity. 
     A switching means  54  which causes a pulse-like reversal of pump current I P  is driven by signal  52  generated by timer  50 . Thus, although pump current I P  is flowing as an anodic current in accordance with the actual measurement of the oxygen concentration in exhaust gas  24 , switching device  54  reverses it briefly to a cathodic pump current I P  in a pulsed manner. This causes oxygen ions to be pumped from electrode  38  of pump cell  14  to electrode  40  and thus out of cavity  28  in accordance with this pulse-like reversal. A frequency and a duration of the pulses with which pump current I P  is reversed briefly depends on signal  52 , which in turn depends on detection voltage U D . It is thus possible to supply different signals  52  at different oxygen concentrations in exhaust gas  24  and in a different time range within which detection voltage U D  is in a certain value range. Thus, the frequency and/or pulse duration with which pump current I P  is reversed can be made variable. The frequency and pulse duration are adjusted so that only the rich drift of sensor  10  is compensated. 
     According to another embodiment, in particular with a pumped reference, it is possible to provide for brief voltage pulses, which are above the measured Nernst voltage and have the same polarity, to be applied to Nernst measurement cell  12 . According to detection voltage U D  which is then impressed on the Nernst measurement cell, a great transport of oxygen ions out of cavity  28  into reference air channel  30  through electrode  16  is established. This also eliminates the polarization on electrodes  16  and  38  due to a declining oxygen ion content in cavity  28  during long-term lean operation. Due to the fact that oxygen ions in exhaust gas  24  cannot diffuse subsequently through diffusion barrier  22  as rapidly or cannot be pumped through pump cell  14  into cavity  28  as are pumped out through electrode  16 , there is an activation of electrodes  16  and  38  which compensates for the rich drift. The pump status of the pump cell prevailing in lean operation supports this activation. 
     Thus, on the whole, the rich drift during long-term lean operation is eliminated by brief, defined rich operation of sensor  10 .