Abstract:
A temperature sensor, in particular for a gas sensor, has at least one printed conductor, where a temperature-dependent change in a resistance of the printed conductor is detected and analyzed. The printed conductor has at least one section made of a solid electrolyte.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a temperature sensor, in particular for a gas sensor. 
     BACKGROUND INFORMATION 
     Temperature sensors are known. They are used in gas sensors to detect and regulate an operating temperature of the gas sensor. Such gas sensors are needed, for example, to determine the oxygen partial pressure in the exhaust gas from an internal combustion engine. They are usually constructed in layers, with the individual layers containing the elements needed to ensure functionality. 
     The individual layers are made of a ceramic material which is permanently applied by film casting, punching, screen printing, laminating, cutting, sintering or the like. Individual ceramic layers have a printed conductor made of a cermet or they are made of a solid electrolyte and thus have the ability to conduct electricity. Such gas sensors have at least one area where a solid electrolyte is applied between at least two cermet electrodes. The conductivity of the solid electrolyte varies as a function of the oxygen partial pressure, for example, and thus is a direct measure of the oxygen concentration in the exhaust gas. However, for an exact determination of the measured value, the temperature must be above 300° C. because the solid electrolyte has the required basic conductivity only above such a temperature, and gas sensors therefore have at least one heating device. 
     On the other hand, the temperature of the exhaust gas varies greatly during operation of an internal combustion engine. Since the conductivity of the solid electrolyte depends on temperature, this change in temperature must be determined. 
     It is known that such gas sensors may be equipped with temperature sensors to detect and analyze a temperature-dependent change in resistance of a printed conductor. 
     First, the resistance of the printed conductor of the heating device may be measured. Printed conductors are usually made of a cermet, using platinum, for example, as the metal. The resistance is then dominated by the metallic conducting platinum. One disadvantage of this is that the resistance temperature coefficient is low, and this low dependence of resistance on temperature is often inadequate to construct a suitable control device. 
     In addition, temperature can be detected on the basis of the resistance of the solid electrolyte, in which case the solid electrolyte with the known sensors has a double function. The solid electrolyte has a much higher resistance temperature coefficient. A disadvantage of the known methods is that the resistance also depends on various external factors such as the oxygen partial pressure, activation, aging and poisoning, so that accurate determination of the measured value is no longer possible. 
     SUMMARY OF THE INVENTION 
     The temperature sensor according to the present invention offers the advantage that it permits much more accurate determination of measured values. Due to the fact that the printed conductor has at least one section made of a solid electrolyte, the favorable resistance temperature coefficient can be utilized while, on the other hand, this section is not exposed to the various interfering external factors such as poisoning, activation, aging and oxygen partial pressure. 
     It is especially advantageous to cover the printed conductor of the temperature sensor with an insulation layer. Such an insulation layer may be made of aluminum oxide, for example. 
     It is also advantageous to integrate the temperature sensor into a layer carrying the heating device, because it is delineated from the other elements of the gas sensor by insulation layers. In this case, to simplify the production of the gas sensor, a partial section of the printed conductor of the heating device may correspond to a partial section of the printed conductor of the temperature sensor. The section of the temperature sensor having the solid electrolyte is in contact with the printed conductor of the heating device and is also in contact with a printed conductor of the temperature sensor. 
     It is also preferable for the temperature-dependent change in resistance to be detected by an alternating voltage acting on it. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a schematic exploded view of a gas sensor. 
     FIG. 2 shows a schematic top view of an embodiment of a temperature sensor. 
     FIG. 3 shows a schematic top view of an alternative embodiment of a temperature sensor. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows a schematic exploded view of the structure of a gas sensor  10 . The individual layers are formed by film casting, punching, screen printing, laminating, cutting, sintering or the like. Gas sensor  10  illustrated here by way of an example is composed of a carrier layer  12  to which a heating device  14  is applied. Heating device  14  usually has a printed conductor  16  made of a cermet, for example, and shielded by two insulation layers  18 . 
     In addition, such a gas sensor  10  has a layer  20  with a recess  22  in which a reference gas is enclosed. Above recess  22  there is a cermet electrode  24  which is covered by a layer of a solid electrolyte  26 . A second cermet electrode  28  follows and is covered by a porous protective layer  30 . 
     The functioning of a gas sensor  10  is known and will not be explained further within the scope of this description. The necessary prerequisite for operation is to first reach a temperature of more than 300° C. which is made possible by heating device  14 , and it is also necessary to detect an actual temperature for determination of a temperature-dependent correction factor for determination of the measured value. 
     FIG. 2 shows a top view of a temperature sensor  40  according to the present invention accommodated in the area of heating device  14 . Of course, temperature sensor  40  may also be arranged in other areas of gas sensor  10 . However, it is especially advantageous due to manufacturing considerations to arrange it in the area of heating device  14  because the necessary insulation layers  18  are present there. 
     Temperature sensor  40  has a printed conductor  42  having at least one section  44  of a solid electrolyte. Printed conductor  42  of temperature sensor  40 , like printed conductor  16  of heating device  14 , is arranged between two insulation layers  18  and is thus protected from interfering external influences. By applying an alternating voltage to the contact points  48 , a conductivity is measured. Because of the great resistance temperature coefficient of printed conductor  42  in section  44 , a sufficient dependence of the resistance on temperature is achieved for suitable temperature control. The components of gas sensor  10  needed for analysis and control of the temperature are not shown for reasons of simplicity. 
     FIG. 3 shows an alternative embodiment of temperature sensor  40 . A partial section  15  of printed conductor  16  of heating device  14  is also at the same time a partial section  15  of printed conductor  42  of temperature sensor  40 . In this way, the number of contact points is smaller and the material costs are also reduced. During the measurement, printed conductor  42  receives an alternating voltage at contact points  17 ,  48 . The alternating voltage may have superimposed on it a d.c. voltage which is necessary for operation of heating device  14 , or the voltage for heating operation is interrupted during the measurement. 
     Stabilized zirconium dioxide, for example, may be used as the solid electrolyte for section  44  because it can be incorporated especially easily by conventional production methods. As an alternative, however, other materials having marked changes in resistance in the temperature range of operation would also be suitable.