Abstract:
A method for regulating the temperature of a sensor for determining an oxygen concentration in gas mixtures, in particular in exhaust gases of internal combustion engines, where a detection voltage that corresponds to the oxygen concentration and is supplied by a Nernst measurement cell is analyzed, the sensor is adjusted to an operating temperature by a heating device, and the instantaneous operating temperature is determined from a measurement of an internal a.c. resistance of the Nernst measurement cell. In starting and/or restarting operation of the sensor, an internal a.c. resistance of a lead of electrodes of the Nernst measurement cell is determined, and the instantaneous internal a.c. resistance thus determined is taken into account in determining the operating temperature.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method of regulating the temperature of a sensor for determining an oxygen concentration in gas mixtures, in particular in exhaust gases of internal combustion engines. 
     BACKGROUND INFORMATION 
     Sensors are used to preset a fuel-air mixture for operation of an internal combustion engine by determining the oxygen concentration in the exhaust gas of the 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 amount of oxygen is present in the exhaust gas in comparison with other partially unburned components. In the lean range, where more oxygen than air is present 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, detecting a lambda value&gt;1 in the lean range or &lt;1 in the rich range and lambda=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. The detection voltage is determined here by a difference in oxygen concentration at an electrode exposed to the gas for measurement and at an electrode of the Nernst measurement cell exposed to a reference gas. The detection voltage increases or decreases according to the oxygen concentration in the exhaust gas. A solid electrolyte body which is conductive for oxygen ions is arranged between the electrodes of the Nernst measurement cell. 
     Such sensors must be heated to temperatures above approximately 300° C. in the active range in order to achieve the necessary ion conductivity of the solid electrolyte. To achieve an increase in measurement accuracy of the sensor, it is known that the operating temperature of the sensor can be controlled and regulated as necessary. It is known, in addition, that a heating device may be provided for the sensor and can be turned on or off in accordance with an operating temperature measured by the sensor. 
     To determine the operating temperature, it is known that an alternating voltage can be applied to the Nernst measurement cell and an a.c. resistance of the sensor can be determined with a measurement device. 
     A disadvantage of the known method is that the temperature-dependent a.c. resistance is determined by starting with a constant a.c. resistance of the electrodes, the solid electrolyte and the leads to the electrodes. The leads here have approximately 50% of the total resistance of the Nernst measurement cell in the operating state. Due to a manufacturing scattering, the lead resistance is subject to a relatively great scattering, so the measurement device determining the a.c. resistance of the Nernst measurement cell has an error corresponding to this scattering. The measurement device adds this scattering error to a temperature-induced fluctuation in the a.c. resistance and supplies a corresponding faulty control signal for the heating device of the sensor. This regulates the sensor at an incorrect operating temperature. 
     SUMMARY OF THE INVENTION 
     The method according to the present invention has the advantage that the operating temperature of the sensor can be regulated accurately. Due to the fact that an internal a.c. resistance of a lead of electrodes of the Nernst measurement cell is determined in starting or restarting operation of the sensor, and the instantaneous internal resistance of the lead thus determined is taken into account in determinating the operating temperature, manufacturing fluctuations in the resistance value can be eliminated. The internal a.c. resistance of the Nernst measurement cell then measured during operation of the sensor in fact fluctuates only because of a change in temperature, so that the control signal supplied by the measurement device for the heating device can be supplied with a great accuracy. In particular, it is also advantageous that a change in resistance due to aging can be taken into account in resuming operation of the sensor due to the repeated measurement of the internal resistance of the lead. 
     In another preferred embodiment of the present invention, the instantaneous internal a.c. resistance of the lead is determined by a brief overheating phase of the heating device, while the total internal a.c. resistance is being measured. A constant value for the resistance component of the electrodes and the resistance component of the solid electrolyte between the electrodes is subtracted from this measured internal a.c. resistance. This yields the exact internal resistance of the lead of the sensor. Furthermore, it is preferable if a temperature coefficient of the electrodes is taken into account in the determination of the instantaneous internal resistance of the lead, so that the accuracy in determination of the actual internal resistance of the lead can be increased. 
     The method according to the present invention also offers the advantage that overheating of the sensor is prevented. Due to the fact that an internal a.c. resistance of a lead of the electrodes of the Nernst measurement cell is determined during operation of the sensor, in particular during a shut-down phase of the sensor, and the instantaneous internal a.c. resistance thus determined is taken into account in determining the operating temperature, fluctuations in the internal a.c. resistance can be taken into account to advantage during operation of the sensor. This makes it possible to turn the heating device of the sensor off and on in a controlled manner, preventing overheating of the sensor which could lead to heat stress cracks in the sensor. In particular since the internal resistance of the solid electrolyte body of the Nernst measurement cell is very small at the operating temperature of the sensor, fluctuations in the lead resistance to the Nernst measurement cell have strong effects accordingly. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a sectional diagram through a sensor. 
     FIG. 2 shows an equivalent circuit diagram of a Nernst measurement cell of the sensor. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows a sensor  10  in a sectional diagram through a measurement head. Sensor  10  is designed as a planar broad-band sensor and is composed of a number of individual layers arranged one above the other, optionally structured, for example, by film casting, punching, screen printing, laminating, cutting, sintering or the like. Production of the layered structure will not be discussed in further detail in 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 setting a fuel-air mixture with which the internal combustion engine is operated. Sensor  10  has a Nernst measurement cell  12  and a pump cell  14 . Nernst measurement cell  12  has a first electrode  16  and a second electrode  18  between which there is a solid electrolyte  20 . Electrode  16  is exposed through a diffusion barrier  22  to exhaust gas  24  on which the measurement is to be performed. Sensor  10  has a measurement opening  26  which can receive exhaust gas  24 . Diffusion barrier  22  extends at the base of measurement opening  26 , forming a cavity  28  within 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 in reference air channel  30 . Solid electrolyte  20  is made of zirconium oxide stabilized with yttrium oxide, for example, while electrodes  16  and  18  are made of platinum and zirconium oxide, for example. 
     Sensor  10  is connected to a circuit arrangement  32 , which is just indicated here and is used to analyze signals of sensor  10  and to control sensor  10 . Electrodes  16  and  18  are connected here to inputs  34  and  36  to which a detection voltage U D  of Nernst measurement cell  12  is applied. 
     Pump cell  14  has a first electrode  38  and a second electrode  40  between which there is a solid electrolyte  42 . Solid electrolyte  42  is in turn composed of a zirconium oxide stabilized with yttrium oxide, for example, while electrodes  38  and  40  may be made of platinum and zirconium oxide. 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 one input  46  of circuit arrangement  32 , while electrode  38  is connected to electrode  16  and is switched together with it to input  34  of circuit arrangement  32 . 
     Sensor  10  also includes a heating device  50  formed by a heating wave form and connected to inputs  52  and  54  of circuit arrangement  32 . A heating voltage U H  can be applied to inputs  52  and  54  by a control circuit  56 . 
     The function of sensor  10  is as follows. 
     Exhaust gas  24  enters cavity  28  through measurement opening  26  and diffusion barrier  22  and is thus applied to electrodes  16  of Nernst 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  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 a prevailing oxygen concentration difference at electrodes  16  and  18 , a certain detection voltage (Nernst voltage) U D  is established. Nernst measurement cell  12  operates here as a lambda probe which detects whether there is a high oxygen concentration or a low oxygen concentration in exhaust gas  24 . It is clear on the basis of the oxygen concentration whether the fuel-air mixture with which the internal combustion engine is operated is a rich or lean mixture. Detection voltage U D  drops or increases in changing from the rich range to the lean range or vice versa. 
     With the help of circuit arrangement  32 , detection voltage U D  is used to determine a pump voltage U P  to be applied 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 anode. Accordingly, a pump current l P  which is established can be measured by a measurement device of circuit arrangement  32 . With the help of pump current I P  either oxygen ions are pumped from electrode  40  to electrode  38  or vice versa. Measured pump current I P  is used to control a device for setting the fuel-air mixture with which the internal combustion engine is operated. 
     Heating voltage U H  can be applied to outputs  54  and  52  of circuit arrangement  32  by control equipment  56 , so that heating device  50  can be turned on and off. Sensor  10  can be brought to an operating temperature of more than approximately 300° C. by heating device  50 . Sensor  10  is exposed to a certain varying thermal energy through exhaust gas  24  because of the fluctuations in speed of exhaust gas  24  and/or temperature fluctuations in exhaust gas  24 . Heating device  50  must be turned on and off depending on the heating of sensor  10  by exhaust gas  24 . To determine the instantaneous operating temperature of sensor  10 , circuit arrangement  32  has a measuring circuit  58  by which an internal a.c. resistance of Nernst measurement cell  12  including its leads to circuit arrangement  32  can be measured. Internal a.c. resistance of Nernst measurement cell  12  is known to be temperature dependent, so that the operating temperature can be deduced from the measured internal a.c. resistance of Nernst measurement cell  12 . Measuring circuit  58  supplies a signal  60  for heating control  56  depending on the measured operating temperature. 
     Determination of internal a.c. resistance of Nernst measurement cell  12  will be discussed in greater detail on the basis of the equivalent circuit diagram of Nernst measurement cell  12  shown in FIG.  2 . 
     A total internal a.c. resistance R i  of Nernst measurement cell  12  is composed of partial resistances R 1 , R 2 , R 3 , R 4  and R 5 . Resistance R 1  is obtained from the internal resistance of solid electrolyte body  20 , resistance R 2  is obtained from the internal a.c. resistance of electrode  16 , resistance R 3  is obtained from the internal a.c. resistance of electrode  18 , resistance R 4  is obtained from the internal a.c. resistance of the lead of electrode  16  to terminal  34 , and resistance R 5  is obtained from the internal a.c. resistance of the lead of electrode  18  to terminal  36 . 
     Internal a.c. resistances R 1 , R 2  and R 3  are known on the basis of the structural design of sensor  10 . Resistances R 4  and R 5  depend on the structuring of the leads, which are usually formed by printed conductors applied by screen printing and are subject to manufacturing fluctuations. The value of the sum of resistances R 1 +R 2 +R 3  amounts to 10Ω, for example, at the operating temperature, while the value of the sum of resistances R 4 +R 5  may be between 40Ω and 80Ω, for example. Thus, different internal a.c. resistances of Nernst measurement cell  12  of 50 to 90Ω, for example, may occur at sensors  10  with identical designs. 
     Sensor  10  is overheated briefly by heating device  50  when starting up or resuming operation of sensor  10 . During this overheating phase, the a.c. resistance of Nernst measurement cell  12  is determined by measuring circuit  58 . Then an a.c. voltage is applied to Nernst measurement cell  12  in a known manner and is superimposed on actual detection voltage U D . Determination of an a.c. resistance is generally known, so that it need not be discussed in detail here as part of the present description. 
     The sum of the known resistances R 1 +R 2 +R 3  is subtracted from a.c. resistance R i  determined by measuring circuit  58  during the brief heating of sensor  10 , so that an instantaneous a.c. resistance R i =R 4 +R 5  of the leads of Nernst cell  12  can be determined. This is thus determined individually for sensor  10 , with manufacturing fluctuations in lead resistances now being taken into account. 
     The total internal a.c. resistance of Nernst measurement cell  12  which is now known is thus derived from R i =R 1 +R 2 +R 3 + instantaneously measured R 4 +R 5 . In the subsequent stabilization of a.c. resistance R i  of Nernst measurement cell  12  at 100Ω, for example, which is accomplished by turning heating device  50  on and off, the actual internal resistance of the leads can thus be taken into account. By eliminating the manufacturing tolerances in stabilization of the operating internal a.c. resistance of Nernst measurement cell  12 , sensor  10  can be operated at a “correct” operating temperature. 
     Instantaneous lead resistance R 4 +R 5  of Nernst measurement cell  12  can be determined, for example, in a definable interval, i.e., instantaneous internal resistance R 4 +R 5  is not determined with each restart of sensor  10 , usually on starting the motor vehicle, but instead only with every n-th restart, e.g., with every hundredth start. This prevents excessive aging of heating device  50  and sensor  10  due to repeated overheating to determine the actual internal a.c. resistance of the lead. Thus, it is possible on the whole in operation of sensor  10  to adjust control signal  60  of measuring device  58  to the actual internal a.c. resistance of Nernst measurement cell  12 , eliminating manufacturing tolerances in lead resistance of Nernst measurement cell  12 . Thus, heating device  50  is turned on and off by heating circuit  56  in accordance with this corrected signal  60  to regulate the operating temperature of sensor  10 .