Device for temperature measurement at an oxygen probe

A device for temperature measurement at an oxygen probe, in particular a lambda probe, having a solid electrolyte arranged between two electrodes. At least one of the electrodes is provided with two spaced terminals for measuring an electrical resistance of the electrodes.

REFERENCE TO RELATED APPLICATIONS 
This application claims the priority of German application Ser. No. P 44 15 
980.3, filed May 6, 1994, which is incorporated herein by reference. 
BACKGROUND OF THE INVENTION 
The invention relates to an arrangement for temperature measurement at an 
oxygen probe, in particular a lambda probe, having a solid electrolyte 
arranged between two electrodes. 
Oxygen probes are known. They have an ionically conducting solid 
electrolyte located between two electrodes. The two electrodes are in this 
case gas-permeable and a measurement voltage can be applied to them. 
Depending on the oxygen content in the gas to be measured, a limit current 
or a Nernst voltage is set, which are dependent on the difference in the 
oxygen concentrations at the electrodes. Oxygen probes of this type are 
used, for example, as lambda probes in motor vehicles, in order to measure 
a specific oxygen content of the exhaust gas of internal combustion 
engines. 
In the active range, oxygen probes must be heated to temperatures above 
approximately 300.degree. C. in order to obtain the necessary ionic 
conductivity of the solid electrolyte. Since the signal from the oxygen 
probe is dependent, amongst other things, on the temperature of the oxygen 
probe, temperature and velocity fluctuations of the gas to be measured are 
frequently so large that the temperature of the measurement probe must be 
monitored and, if necessary, regulated in order to obtain an increase in 
the measurement accuracy. 
In order to regulate the temperature at the measurement probe, it is known 
to assign to the measurement probe a probe heater which can be switched on 
or off as a function of a temperature measured at the oxygen probe. In 
this case, use is made of the effect that the internal resistance of the 
oxygen probe is temperature-dependent and the temperature of the oxygen 
probe can be deduced using the magnitude of this internal resistance. For 
this purpose, it is known to load the probe signal with a defined 
resistance and to calculate the internal resistance from the resulting 
load voltage. It is further known to impress an AC voltage on the probe 
signal using a known resistance and to calculate the AC impedance from the 
voltage drop across the oxygen probe. Furthermore, an alternating current 
of known amplitude can be superimposed on the probe signal and the AC 
impedance can be calculated from the reaction of the probe voltage 
amplitude. 
In the known devices for temperature measurement, it is disadvantageous 
that they can be produced only at great expense using a multiplicity of 
components and in the end only a temperature of the solid electrolyte can 
be determined. However, a signal voltage from the oxygen probe is, in 
addition to the temperature of the solid electrolyte, dependent on a 
temperature difference between the measurement electrodes. This later 
temperature difference cannot be determined using devices known to date. 
SUMMARY OF THE INVENTION 
The above problems generally are solved according to the invention, by 
providing an oxygen probe, in particular a lambda probe, having a solid 
electrolyte arranged between two electrodes, wherein at least one of the 
two electrodes has two spaced terminals for measuring an electrical 
resistance of the electrodes. The oxygen probe according to the present 
invention has the advantage in comparison with the above known probe 
arrangements that determination of the temperature at the active solid 
electrolyte and determination of the temperature differences between the 
two measurement electrodes are possible. As a result of the fact that at 
least one of the electrodes consists of a material whose electrical 
resistance depends on the temperature, and this electrode has two spaced 
terminals for measuring an electrical resistance of the measurement 
electrodes, it is possible in a simple manner to arrange the electrode as 
part of a bridge circuit which is known per se, so that the temperature of 
the corresponding measurement electrode can be determined by balancing of 
the bridge circuit. By virtue of a corresponding geometrical arrangement 
of the measurement electrode, that is to say, by choice of the connection 
points and/or the size of the contact surface of the measurement 
electrode, the temperature can be determined at each position on the 
surface of the solid electrolyte. This temperature measurement in no way 
impairs operation of the oxygen probe. 
According to a particularly advantageous embodiment of the invention 
provision is made for each of measurement electrodes to be provided with 
two respective spaced terminals, so that these electrodes are in each case 
a component or arm of a bridge circuit. A temperature difference between 
the measurement electrodes can thereby be determined. 
Further advantageous configurations of the invention are found in the 
remaining features described in the application.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The FIGURE shows an exploded representation of a limit-current probe given 
the overall reference numeral 10. Only the functional parts of the 
limit-current probe 10 are shown in the FIGURE, while the housing and 
housing terminals are left out for reasons of clarity. The limit-current 
probe 10 has a planar, elongated body of a solid electrolyte 12 which 
conducts using oxygen ions and consists, for example, of stabilized 
zirconium dioxide. A first electrode 16 is arranged on that surface 14 of 
the solid electrolyte 12 which faces upward in the FIGURE. The electrode 
16 consists, for example, of a porous layer of platinum. The electrode 16 
is in this case formed at a front end 18 of the solid electrolyte 12, so 
that this electrode 16 is directly exposed to the gas to be measured. 
As can be seen, the electrode 16 has two spaced terminals 20 and 22 with 
which contact is made by respective conductor tracks 24 and 26 extending 
along the remainder of the length of the solid electrolyte 12. The 
electrode 16 and the conductor tracks 24 and 26 may in this case be 
applied directly onto the surface 14 of the solid electrolyte 12 by means 
of a known printing technique, for example. The electrode 16 is enclosed 
by a cover 28. The cover 28 consists of a porous material and forms a 
defined diffusion resistance to the oxygen molecules in the gas to be 
measured. The cover 28 may, for example, be applied by pressing on 
zirconium dioxide, aluminum oxide or magnesium spinel and subsequent 
annealing. The cover 28 is arranged over the entire solid electrolyte 12 
and therefore covers the electrode 16 and the conductor tracks 24 and 26. 
At the end remote from the exhaust gas, the cover 28 has recesses which 
allow contact to be made with the conductor tracks 24 and 26. 
A second measurement electrode 30 is arranged opposite the electrode 16 on 
a surface 29 of the solid electrolyte 12 which faces downward in the 
FIGURE. As shown, the measurement electrode 30 likewise has two spaced 
terminals 32 and 34 with which contact is made by respective conductor 
tracks 36 and 38 extending along the remainder of the surface of the solid 
electrolyte 12. The measurement electrode 30 likewise consists, for 
example, of a porous platinum layer. An electrical insulating layer 40 is 
arranged between the conductor tracks 36 and 38 and surface 29 of the 
solid electrolyte 12. The insulating layer 40 prevents short-circuit 
currents between the conductor tracks 24 or 26, respectively, and the 
conductor tracks 36 or 38, respectively. The insulating layer 40 may, for 
example, consist of zirconium dioxide or aluminum oxide. The measurement 
electrode 30 and the conductor tracks 36 and 38 are covered by an 
additional insulating layer 42 which likewise consists, for example, of 
zirconium dioxide or aluminum oxide, but allows at least one lateral 
access to the measurement electrode 30 for the oxygen-containing gas to be 
measured. At the end remote from the gas to be measured, the insulating 
layer 42 has recesses which allow contact to be made with the conductor 
tracks 36 and 38. A heating element 44 is applied, for example, by 
printing, onto the insulation layer 42, with which heating element contact 
is made by conductor tracks 46 and 48 extending over the insulating layer 
40 along the layer of the solid electrolyte 12. The heating element 44 is 
designed, for example, as a meandering conductor track 50 which is 
connected to the conductor tracks 46 and 48. The heating element 44 is 
provided with another cover 52 which consists of a porous material, for 
example zirconium dioxide or aluminum oxide. The conductor tracks 46 and 
48 and the cover 52 are shortened in order to avoid a short-circuit 
between the conductor tracks 36 and 46 or 38 and 48, respectively and in 
order to make it possible to make contact with the conductor tracks 36, 
38, 46 and 48. The FIGURE illustrates, solely by way of example, a 
limit-current probe 10 which has a height of approximately 1 mm and a 
width of approximately 5 mm when assembled. 
During operation of the limit-current probe 10 shown in the FIGURE, it is 
exposed in the oxygen-containing gas to be measured, for example, to an 
exhaust gas of a motor vehicle. An electric voltage is applied to the 
electrodes 16 and 30, e.g., via the respective conductor tracks. The 
oxygen molecules in the gas to be measured diffuse through the electrodes 
16 and 30 and the solid electrolyte 12 and are converted into oxygen ions 
because of the applied electric voltage. The limit current which then 
flows is measured by means of a measuring instrument, not shown, and 
provides a measure of the oxygen concentration in the gas to be measured 
and therefore, for example, of the adjustment of a fuel/air mixture of an 
internal combustion engine of a motor vehicle. 
According to the invention, the electrical resistance of the electrode 16 
or 30 is simultaneously measured using the conductor tracks 24 and 26 or 
36 and 38, respectively. The electrodes 16 and 30 consist of a material, 
for example, platinum, whose resistance value changes depending on the 
temperature. By integrating the electrodes 16 and 30, in a simple manner, 
in a generally known bridge circuit (not shown), e.g., by connecting each 
of the electrodes in a respective arm of the bridge circuit, a change in 
the resistance value of the electrodes 16 or 30, respectively, can be 
deduced on the basis of the unbalancing of the bridge circuit. Since the 
resistance/temperature behavior of the electrodes 16 and 30 is known, the 
actual temperature existing in immediate proximity to the solid 
electrolyte 12 can be deduced therefrom. For sufficiently exact 
temperature measurement, it is per se necessary to equip one of the 
electrodes 16 and 30 with the two terminals and conductor tracks 24 and 26 
or 36 and 38, respectively, intended for resistance measurement. However, 
both electrodes 16 and 30 are preferably connected, in each case to two 
conductor tracks, so that the temperature of the electrode 16 and the 
temperature of the electrode 30 can be determined independently of one 
another. As a result of a temperature difference occurring between the 
electrodes 16 and 30, which affects the signal voltage of the entire 
limit-current probe 10, a deviation of the signal voltage can very 
accurately be determined by the temperature difference, so that this 
deviation can accordingly be taken into account when determining the 
oxygen content of the gas to be measured. A very much more accurate 
determination of the oxygen content in the gas to be measured is therefore 
overall possible. 
The invention now being fully described, it will be apparent to one of 
ordinary skill in the art that any changes and modifications can be made 
thereto without departing from the spirit or scope of the invention as set 
forth herein.