WO2012034761A1

WO2012034761A1 - Method for determining a property of a gas in a measurement gas space

Info

Publication number: WO2012034761A1
Application number: PCT/EP2011/062816
Authority: WO
Inventors: Lothar Diehl; Thomas Seiler
Original assignee: Robert Bosch Gmbh
Priority date: 2010-09-15
Filing date: 2011-07-26
Publication date: 2012-03-22
Also published as: DE102010040821A1

Abstract

The invention relates to a method for determining at least one property of a gas in a measurement gas space (112), in particular for detecting at least one gas component in the gas. At least one sensor element (114) is used, wherein the sensor element (114) comprises at least one pump cell (125) having at least one first electrode (120), at least one second electrode (124) and at least one solid electrolyte (123) connecting the first electrode (120) and the second electrode (124). The property is ascertained from a pump flow through the pump cell (125). In at least one measurement step of the method, the pump cell (125) is operated in a controlled manner by way of closed-loop or open-loop control using at least one guide variable (148). In at least one control step of the method, the guide variable (148) is varied and a variance of the pump flow is ascertained.

Description

Description title

Method for determining a property of a gas in a sample gas chamber Prior art

Numerous different devices and methods for determining the properties of gases in a sample gas space are known from the prior art. In the following, the invention will be described essentially with reference to devices and methods which use at least one solid electrolyte material, that is to say a material which has ion-conducting properties, for example a ceramic solid electrolyte material. Examples of such ceramic

Solid electrolyte materials are yttria-stabilized zirconia (YSZ) and / or scandium-doped zirconia (ScSZ). Examples of devices of the type mentioned are described in Robert Bosch GmbH: Sensors in motor vehicles, edition 2007, pages 154-159. The at least one property of the gas may in particular be a fraction of a gas component of the gas, which can be determined qualitatively and / or quantitatively. In particular, a

Oxygen content can be determined. The share can be in the form of a

Percentage and / or in the form of a partial pressure. Alternatively or additionally, however, other properties of the gas can also be determined, for example physical and / or chemical properties.

In devices according to the above-described prior art, which are used to determine an oxygen content in a gas, which are commonly referred to as lambda probes, exist in addition to so-called

Jump probes also called broadband lambda probes. Such broadband lambda probes exist in a single-cell configuration or in multicellular

Design. In unicellular lambda probes is usually one in one

Measuring cavity diffusing amount of 0 ₂ and / or fat gas based on a

Limit current determines which is established by the cell. In multi-cell broadband lambda probes, as described for example in Robert Bosch GmbH: Sensors in motor vehicles, edition 2007, pages 158-159, the oxygen content is calculated on the basis of Control of a cavity concentration measured on λ = 1 necessary pump current. The pumping current is at least approximately proportional to the 0 ₂ - or fat gas content in the exhaust gas. The measurement of the cavity concentration is usually based on the determination of a Nernst voltage between a Nernst electrode in the cavity and an oxygen-purged reference electrode in a reference space. From the prior art, it is generally known to influence the target Nernst voltage, for example to lower permanently or to set a higher Nernst voltage. From DE 10 2005 056 515 A1, a method is also known in which a

Gas composition or a gas type of a lambda probe supplied gas is detected by means of a modulated measurement gas change. In this case, the sensitivity of the probe for various gases is periodically changed by periodically adjusting the air ratio in the cavity of the lambda probe.

However, a technical challenge of known devices and methods basically consists in the fact that it can lead to a passivation of the inner pumping electrode arranged in the cavity. Such passivations can be caused, for example, by chromium deposition and / or by platinum segregation and / or by other processes which increase the activity and / or the number of

Can reduce three-phase positions at the electrode. In this case, at a certain pumping current I _p falls from the inner pumping electrode (IPE) to the outer one

Pumping electrode (APE) from a significant overvoltage at the interface between the electrode and the solid electrolyte from. As a result, the nominal Nernst voltage, which is typically set at 450 mV, can already be achieved with smaller currents than would be necessary for the adjustment of λ = 1 in the electrode cavity. The result is too small a pumping current, ie a drop in the characteristic Ι _ρ (λ).

Disclosure of the invention

Accordingly, a method and a sensor device for detecting at least one property of a gas in a measurement gas space are proposed, which at least largely avoid the disadvantages of known methods and sensor devices. The method and the sensor device can be used, in particular, to qualitatively and / or quantitatively detect a proportion of at least one gas component in the gas, for example a percentage and / or a partial pressure of the gas component, which in particular can be oxygen. In the proposed method, at least one sensor element is used, wherein the sensor element comprises at least one pump cell with at least one first electrode, at least one second electrode and at least one solid electrolyte connecting the first electrode and the second electrode. The first electrode and / or the second electrode may, for example, be designed as cermet electrodes, for example as platinum cermet electrodes.

The solid electrolyte can basically be any body with ion-conducting

Be properties. In particular, this may be a solid.

In principle, however, as an alternative or in addition to a solid, but also bodies in a different state of aggregation are conceivable, for example liquid electrolytes.

In particular, the solid electrolyte may be a ceramic solid electrolyte or may comprise a ceramic solid electrolyte. Alternatively or additionally, however, the solid electrolyte may also comprise one or more other materials having ion-conducting properties, for example one or more polymers. The solid electrolyte can

For example, be designed on a zirconia-based solid electrolyte, for example, YSZ and / or ScSZ. The method is performed such that the property is deduced from a pumping current through the pumping cell, for example, the oxygen content in the

Measuring gas chamber. The measuring gas chamber may in particular be an exhaust gas tract of an internal combustion engine, and the gas may be an exhaust gas of the

Internal combustion engine. However, other embodiments and / or purposes are in principle possible.

In at least one measuring step of the method, the pumping cell is operated or controlled using at least one reference variable. In particular, as will be explained in more detail below, the second electrode may be arranged in an electrode cavity. For example, it is possible to detect a Nernst voltage between a measuring electrode which is likewise arranged in the electrode cavity and which may be configured as identical with the second electrode or which may be designed as a separate measuring electrode, and a reference electrode arranged in a reference gas space, for example a reference gas channel, and Actual value can be used. This actual value of the Nernst voltage can with a

predetermined setpoint of a Nernst voltage are compared as a reference variable, and the pumping current through the pumping cell can be adjusted such that a

Control difference between the actual value and the reference variable disappears. For this process variant, for example, Robert Bosch GmbH: Sensors in

Motor vehicle, edition 2007, pages 158-159 and the regulation described there of a broadband lambda probe be referenced. Alternatively or additionally, the

Pump cell, however, also controlled using other control variables, control variables or reference variables and / or controlled operated. For example, a pumping current can be regulated by the pumping cell to a predetermined value. In addition to the described measurement step, which can be carried out simply, repeatedly or even over a longer period of time, the method furthermore comprises at least one control step, which can likewise be carried out simply, repeatedly or even over a relatively long period of time.

For example, at least one control step can be followed by at least one measuring step, after which a second measuring step can again follow. For example, measuring step and control step can be carried out alternately. However, other embodiments are possible in principle.

In the control step, the reference variable is changed. This change can take place, for example, by a predetermined amount, for example by changing a setpoint Nernst voltage U NS _O II by, for example, a predetermined amount, in particular increasing it. Alternatively or additionally, a desired pumping voltage Up.soii and / or a desired pumping current I _P , _S oii can also be changed.

For example, also be increased by a predetermined amount, in particular.

The change in the reference variable can be done once or several times. In particular, the change can also be repeated, with the same change or an adapted change. In particular, the change in the

Reference variable also oscillating done, for example periodically oscillating. The adaptation can then take place, for example, such that the mean value of the oscillation of the reference variable is set in such a way, for example by a

Control device that the resulting change in the pumping current, for example, the resulting pumping current amplitude or amplitude of the change in the pumping current, a predetermined threshold, for example, 20 μΑ, does not exceed. Furthermore, in the control step, a change in the pumping current I _p is detected by the pumping cell. In other words, the change in pumping current in

Reaction to the change in the reference variable can be detected. This change can be detected as an absolute change or in the form of a temporal evolution of the pumping current. Also other ways of quantifying the

Change is possible, such as detecting a change over a period of time.

In particular, the method can be carried out in such a way that the reference variable in the measuring step is adapted, in particular increased, if the change in the pumping current in the control step no longer satisfies at least one threshold condition. For example, it can be queried in this control step, whether the pumping current reaches at least a predetermined threshold, exceeds or falls below. For example, it is possible to query whether the pumping current or a characteristic variable derived therefrom or the change in the pumping current or the magnitude thereof are smaller than a predefined threshold value, smaller than or equal to a predefined threshold value

Threshold, or similar threshold conditions can be set. The threshold condition can be adapted in particular such that a in the

In the context of customary changes of the sensor element, a still tolerable change in the pump current still satisfies the threshold condition, whereas changes which indicate a strong electrode polarization and / or contamination of the electrodes and / or passivation of the electrodes, in particular of the first electrode, do not affect the threshold condition to fulfill more. In particular, the method can be carried out in such a way that an on-board

Compensation of a Elektrodenpolarisation is performed, for example, in the above described broadband lambda probes with multiple cells. The method can be carried out in particular such that a nominal Nernstspannung in the

Control step, which is also referred to as a test phase, by a defined amount, for example by 20 to 400 mV, in particular by 100 to 300 mV and particularly preferably by 200 mV, briefly increased, for example by a period of 0.5 to 10 s, in particular a period of 0.8 to 2 s and more preferably for a period of 1 s. If, in this case, the pump current I _P increases by at least one threshold value or by more than one threshold value, a new regulation setpoint value of the

Nernst voltage U NS _O II are stored in a control electronics. For example, the threshold may be 50 μΑ to 400 μΑ, in particular 100 μΑ to 300 μΑ and more preferably 200 μΑ. The control setpoint of the Nernst voltage can be raised, for example, by 20 mV to 400 mV, in particular by 100 mV to 300 mV and particularly preferably by 200 mV. In general, the adjustment of the reference variable can be simple or iterative. If an iterative adaptation, for example, the reference variable can be increased or decreased so many times by one or more predetermined amounts, which can be adapted to the number of steps and / or, for example, increasingly smaller, until the change in the pumping current in the Control step that meets at least one threshold condition.

If it is detected in the control step that the change in the pumping current in the control step no longer fulfills the at least one threshold condition, one or more further measures can be taken, alternatively or in addition to a change in the reference variable or an adjustment of the reference variable. In a preferred embodiment of the method, for example, if it is determined that the change in the pumping current no longer satisfies the at least one threshold condition, a regeneration step can be initiated. such

Regeneration steps may include various measures involving the

Reversing one or more of the electrodes at least partially. For example, a regeneration step may include a pump flow reversal by the pump cell. Such pumping current reversal can be initiated for example for a certain period of time. Alternatively or additionally, for example, temperature increases or other measures for the regeneration of one or more of the electrodes come into consideration, for example, by the sensor element is temporarily exposed to a rich exhaust gas.

The method can be carried out in particular such that the first electrode and / or the second electrode are acted upon with gas from the measurement gas space or can be acted upon. For example, the sensor element may be configured such that the first electrode and / or the second electrode are in communication with the measurement gas space directly or via a gas-permeable protective layer. Alternatively, however, the first electrode and / or the second electrode may also be connected to the measuring gas space by at least one diffusion barrier. For example, the first electrode and / or the second electrode may be disposed on a surface of the

Sensor element may be arranged or in an electrode cavity in the interior of the Sensor element, which via the at least one diffusion barrier with the

Sample gas space communicates. The at least one diffusion barrier can, for example, protect the first electrode and / or the second electrode from contamination and / or set a limiting current of the pump cell.

Alternatively or additionally, one or more of the electrodes may be in communication with a reference gas space. For example, the second electrode may be an electrode in communication with a reference gas space, while, for example, the first electrode may be supplied with gas from the measurement gas space. Accordingly, the second electrode can for example be arranged directly in the reference gas space or communicate with this reference gas space in another way. The reference gas space is a gas space in which a defined gas atmosphere can be established. For example, this can be a closed cavity, which can be acted upon by a pumping process with a defined atmosphere. Alternatively or additionally, however, the reference gas space may also comprise at least one reference gas channel, for example an air reference channel, which may be connected to an environment of the sensor element, for example, which may be formed separately from the sample gas space. In this way, for example, a defined air atmosphere can be established in the reference gas space. The proposed

Embodiment in which the second electrode is in communication with at least one reference gas space can be used in particular in a single-cell sensor structure. However, other sensor structures are basically possible. As an alternative or in addition to the arrangement in which the second electrode is arranged in a reference gas space, the second electrode or at least one of the second electrodes (if a plurality of second electrodes are provided) can also be arranged in an electrode cavity. This electrode cavity can be formed, for example, separated from the sample gas space by the solid electrolyte.

For example, the electrode cavity may be configured as a cavity in which a defined by the above-described control or regulating method

Gas atmosphere or gas composition to be set, for example, a defined lambda value. For this purpose, in the electrode cavity

For example, at least one measuring electrode may furthermore be arranged, for example at least one Nernst electrode. This at least one measuring electrode may be wholly or partially component identical to the second electrode or electrically to the second Electrode be connected, which is particularly preferred in the context of the present invention and as described for example in Robert Bosch GmbH: sensors in

Motor vehicle, edition 2007, pages 158-159 is described. In such second electrodes, which at the same time as a pumping electrode for a pumping cell and as

Nernstelektrode be used for a Nernstspannungsmessung, makes the inventive prevention or the inventive compensation of

Electrode polarization particularly advantageous noticeable. Alternatively, however, the at least one measuring electrode in the electrode cavity may also be formed separately from the second electrode. Other embodiments are possible.

The pumping cell can be operated in the measuring step, in particular in a limiting current operation. In this case, a limiting current is to be understood as meaning the saturation current for a specific composition of the gas which sets in the pump current-voltage characteristic. The pumping voltage can in particular be selected such that the pumping current is in the saturation region, as is common in many broadband probes.

The command variable can, as described above, be configured in various ways. In particular, the reference variable can be at least one nominal value of a

Potentials and / or a voltage at the second electrode. As described above, in particular a desired Nernst voltage can be specified as a reference variable, which is measured, for example, between the second electrode and a reference electrode, which can be arranged, for example, in a reference gas space in the manner described above.

The first electrode can in particular be charged with gas from the measuring gas space, as stated above. The sensor element may further comprise at least one reference electrode in at least one reference gas space, wherein a

Nernst voltage U _N between the second electrode and the reference electrode can be detected. In the measuring step, as stated above, in particular the

Pumping current through the pumping cell using a nominal Nernst voltage U N.Soll 3lS reference variable. In the control step can, as stated above, in particular the desired Nernst voltage U NS _O II be changed by a value ÄUN.S _O II, in particular be increased, the change ΔΙ _{Ρ of} the pump current is detected and compared with at least one threshold. The nominal Nernststpannung can for example be changed, in particular be increased if the change ΔΙ _Ρ reaches or exceeds the threshold. For example, the desired Nernst voltage U NS _O II can be changed by one or more predetermined amounts, wherein the amounts can be fixed or can for example be adapted to the iteration of the method, for example, iteratively smaller can be selected when the change ΔΙ _{Ρ of} the pump current approaches the threshold.

Alternatively or additionally, the reference variable may also include a pump voltage U _P , for example a desired pump voltage. In the control step, this pumping voltage can be changed by a value ΔΙΙ _Ρ , wherein the change ΔΙ _Ρ of the pumping current can be detected in response to this change in the pumping voltage and can be compared with at least one threshold. In the measuring step, the pump voltage can then be changed, in particular increased, if the change ΔΙ _Ρ at least reaches or exceeds the threshold value. In this way, for example, unicellular sensor elements can be operated with the proposed method.

Alternatively or additionally, the reference variable may also include a pumping current I _p . In the control step, the pumping current can be changed by a value ΔΙ _Ρ . In this case, a change AU _{P of} a pump voltage and / or a change AU _{N of} a Nernst voltage can be detected and compared with at least one threshold value. For an intact sensor element, for example, it may be required that the resulting AUp or AU _N exceeds or does not fall below the threshold value. For example, in the measuring step, the pumping current can be changed, in particular increased, if the changes AU _P or AU _N exceed or do not fall below the threshold value.

In another aspect of the present invention, a sensor device is provided for detecting at least one property of a gas in a sample gas space

which may be particularly adapted to perform a method according to one or more of the embodiments described above.

Accordingly, for possible embodiments of the sensor device, reference may be made to the above description of the proposed method. The

Sensor device may include corresponding devices to perform one, several or all of the substeps of the method described above. The

Sensor device comprises at least one sensor element, in particular at least one ceramic sensor element. The sensor element comprises at least one pump cell at least one first electrode, at least one second electrode and at least one connecting the first electrode and the second electrode

Solid electrolyte. For further possible embodiments of the sensor element, reference may be made to the above description. The sensor device further comprises at least one controller connected to the sensor element or at least connectable. The controller can be set up centrally or else decentrally and can include, for example, one or more data processing devices. The control can also be completely or partially integrated in a central motor control or also be integrated in whole or in part in the sensor element and / or a plug.

The controller is arranged to close the property from pumping current through the pumping cell. For example, the controller may for this purpose comprise at least one application device, for example at least one voltage source and / or current source, with at least one pump cell

Apply pumping voltage and / or at least one pumping current.

For example, from the pumping current to the gas composition, for example, the proportion of the gas of the gas component in the measuring gas space, be closed, for example, an oxygen content. For this purpose, the controller may, for example, further comprise at least one measuring device, for example at least one current measuring device and / or one

Voltage measuring device, for example, to measure the pumping current through the pumping cell. The controller is furthermore set up to operate or control the pumping cell in at least one measuring step using at least one reference variable. For this purpose, the controller may, for example, at least one

Voltage control and / or at least one current control and / or at least one voltage control and / or at least one current control include, which may be connected, for example, with the pumping cell. The controller is further configured to change in at least one control step, the reference variable and to detect a change in the pumping current. For this purpose, for example, the controller can be set up in terms of programming in order to carry out the checking step and to carry out the corresponding changes or measurements. To carry out the measurements, the controller can, for example, comprise at least one measuring device, as stated above. The proposed method and the proposed sensor device have numerous advantages over known methods and devices. Thus, these can be used in particular for reliable on-board compensation of a change in one or more of the electrodes of the sensor element. In this way it is possible to reliably prevent or at least reduce a change in the measured value and / or a signal distortion which can occur as a result of aging.

BRIEF DESCRIPTION OF THE DRAWINGS Exemplary embodiments of the invention, which are illustrated by way of example in the figures, are explained below. The invention is not on the

Embodiments limited.

In detail show:

Figure 1 shows an embodiment of a sensor device according to the invention;

FIG. 2 shows pump current-pump voltage characteristic curves and Nernst voltage

Pump voltage characteristics for various sensor elements; and

Figure 3 pumping current nominal Nernstspannungs characteristics for proper

Sensor elements and for sensor elements with electrode polarization.

Embodiments of the invention

FIG. 1 shows by way of example an embodiment of an inventive device

Sensor device 1 10 for detecting at least one property of a gas in a measuring gas chamber 1 12 shown. In particular, an oxygen content in the

Measuring gas chamber 1 12 can be determined. The sensor device 110 includes a

Sensor element 114 and a controller 118 connected to the sensor element 114 via an interface and / or another connection 116.

The sensor element 1 14 comprises in the illustrated embodiment, a first electrode 120, which is also referred to below as APE (outer

Pump electrode) and which may be arranged on an outer side of the sensor element. The first electrode 120 may, for example, by a gas-permeable Protective layer 122 to be separated from the sample gas space 112. Furthermore, the sensor element 1 14 comprises a second electrode 124, which in the illustrated

Embodiment is disposed in an electrode cavity 126 and which accordingly can also be referred to as inner pumping electrode (IPE). The electrode cavity 126 is connected via a gas inlet hole 128 and a diffusion barrier 130, which limits subsequent diffusion of oxygen and / or gas, with the

Measuring gas chamber 112 in conjunction and is thus acted upon with gas from the sample gas space. The second electrode 124 is connected to the first electrode 120 via a solid electrolyte 123. The first electrode 120, the solid electrolyte 123 and the second electrode 124 together form a pumping cell 125. Furthermore, the sensor element 1 14 in the illustrated embodiment comprises a third electrode 132, which is also referred to as a reference electrode (RE) and which in a

Reference gas space 134 is arranged. The reference gas space 134, which may be designed to be open or which may also be completely or partially filled with a gas-permeable, porous material 136 as shown in FIG. 1, may be configured, for example, as a reference gas channel 138, for example as an air reference channel. The second electrode 124, the solid electrolyte 123 and the third electrode 132 together form a Nernst cell 139. The interface 116 preferably comprises electrode leads 140, 142 and 144 to the first electrode 120, the second electrode 124 and the third electrode 132 In the illustrated embodiment, a control device 146. About the control device 146, a Nernst voltage between the IPE 124 and the RE 132 can be set or adjusted to a nominal Nernstspannung UN, S _O II 3ls reference variable 148. For example, the controller 1 18 for this purpose include an adjustable voltage source 150 for setting the target Nernstspannung. For example, one end of the voltage source 150 and the electrode lead 142 of the second electrode 124 may be connected to electrical ground 152 such that the one provided via the electrode lead 144 of the third electrode 132

Nernst voltage U _N (actual value) and the target Nernst voltage U _N, soii (setpoint) in a comparison device 154 (for example, a comparator) can be compared with each other to a control difference from a control variable and / or manipulated variable 156 in the form of a pumping voltage U _P and or to generate a pumping current I _P , which in turn is applied to the APE 120 to adjust an atmosphere corresponding to the reference variable 148 in the electrode cavity 126. The specification of the reference variable 148, for example, by a

Data processing device 158 take place in the controller 118, which, for example, affects the adjustable voltage source 150 and / or may include all or part thereof. Furthermore, the controller 1 18 may include one or more measuring devices 160, which may detect, for example, the pumping current I _P and / or the pumping voltage U _P. The measured values of this measuring device 160 can be provided to the data processing device 158, for example. Alternatively or additionally, the measuring device 160 may also be wholly or partially inserted in the

Data processing device 158 be integrated. The data processing device 158 may, for example, be set up in terms of programming in order to carry out a method in accordance with one or more of the method variants described above. The

Data processing device 158 may include, for example, a microcontroller. Alternatively or additionally, the controller 118 may also be configured in whole or in part as an application-specific integrated circuit (ASIC). Various other configurations are possible.

With reference to Figures 2 and 3, the problem of conventional devices 1 10 and methods will be explained. In conventional sensor devices 1 10 of

described type, which are not set up to perform a method according to the invention, the control setpoint of the Nernst voltage U NS _O II is usually fixed. This serves to provide a void concentration of λ = 1 in the

Adjust or regulate electrode cavity 126. In typical

Sensor devices, the control setpoint over the entire life of the sensor device 1 10 is typically 450 mV. However, this leads, as will be described with reference to Figure 2, in particular in sensor elements 1 14, in which the second electrode 124 at the same time for a pump current measurement and for a

Nernstspannungsmessung is used, aging effects, which affect the

Affect measurement. Thus, over the lifetime, a passivation of the inner pumping electrode 124 arranged in the electrode cavity 126 may occur,

for example, by chromium deposition or by platinum segregation or by other processes that can reduce the activity and / or the number of three-phase sites on the electrode 124. In this case, falls at a certain pump current I _P from the IPE 124 to the outer pumping electrode 120, a significant

Transient overvoltage at the interface IPE 124 / solid electrolyte 123 from. As a result, the desired Nernst voltage U _N, soii of, for example, 450 mV even at lower currents be reached than would be necessary for the adjustment of λ = 1 in the electrode cavity 126. The result is a too small pumping current I _P , ie a drop in the characteristic curve Ι _Ρ (λ).

In Figure 2, the pumping voltage U _{P is applied} to the pumping cell 125 on the horizontal axis. On the left vertical axis of the pumping current l _{P is} plotted. The curve 162, which is assigned to this left vertical axis, denotes a pumping current pumping voltage characteristic of an intact sensor element 14, at which no electrode polarization has yet occurred at the second electrode 124. The dashed curve 164, however, denotes a pumping current of a polarizing sensor element 114, that is, a sensor element 114, in which at the second electrode 124, the polarization effects described above have occurred. It can be seen that this curve is much flatter and later the

Saturation pump current l _s approaches. Furthermore, in FIG. 2, the Nernst voltages U _{N are} plotted on the right vertical axis as a function of the pumping voltage U _P. Again, the solid line 166 indicates a Nernst voltage of an intact sensor element 114, whereas the dashed line 168 indicates a Nernst voltage of a polarizing one

Sensor element 1 14 denotes, that is, a sensor element 114, wherein the second electrode 124 has the above-described polarization effects. Starting from these curves 166, 168, the effect outlined above can be described. Typically, in sensor devices 110 of the type described in FIG. 1, the Nernst voltage is set to a value U _N, soii of 450 mV, which corresponds to the pump voltage U _P, i for an intact sensor element 14. As can be seen from FIG. 2, the pump current characteristic 162 of such an intact sensor element 1 14 is already in the saturation region at this pump voltage U _P, i and has at least approximately the value l _s . In the case of a polarizing sensor element 14, on the other hand, a high voltage across the polarization of the second electrode 124, which is used both as an electrode for the pumping current measurement by the pumping cell 125 and as a Nernst electrode for the Nernst cell 139, decreases at the Nernst voltage added. As a result, the effectively measured voltage U _{N reaches} the

Electrode lead 144 in Figure 1 already at smaller pump currents l _P the value U _N, soii of 450 mV. This is illustrated in FIG. 2 in that the Nernst voltage curve 168 of the polarizing sensor element 114 rises considerably earlier than the one

Nernst voltage curve 166 of the intact sensor element 114. Accordingly, the curve 168 reaches the value of 450 already at a considerably smaller pumping voltage U _{P, 2} mV. At this pumping voltage U _{P, 2, however} , the pumping current characteristic curve 164 of the polarizing sensor element 114 is not yet in saturation, but at a value I _P * at which the pumping current characteristic curve still rises sharply. While for an intact

Sensor element 1 14 thus essentially applies: ^ü '^ ^p (U _px ) ~ 0, applies to a

d U _p polarizing sensor element 114 with curves 164, 168: (U _{p 2} )> 0. In other d U _P

In the case of an intact sensor element 1 14 _{, the} pumping current l _P _that arises is very little dependent on the reference variable U _N, soii, whereas in the case of polarizing sensor elements 14, there is a marked increase in the pumping current l _P with the reference variable UN, S _O II. This is shown in FIG. Here, the curve 170 denotes the pump current I _P of an intact sensor element NS as a function of the reference variable U _O II, whereas curve 172 represents the pumping current of a polarizing the sensor element 1 fourteenth

However, precisely this relationship is preferably used in a method according to the invention. In such a method, in at least one measuring step, the pump cell 125 is controlled or controlled using the reference variable U NS _O II and closed from the pumping current on the property of the gas in the sample gas space 1 12, for example, the oxygen partial pressure. In contrast, in at least one control step of the method, the reference variable U NS _O II is changed, for example, by a predetermined amount ΔΙΙ N, S _O II- The change in the pump current ΔΙ _{Ρ is} detected. In curve 170 in Figure 3, this change is nearly 0 or very small. In contrast, in a non-intact sensor element 114 with the curve 172, a very steep rise of l _P with the reference variable UN, S _O II, at least at the operating point of 450 mV, can be observed. In other words, ΔΙ _Ρ divided by ΔΙΙ NS _O II is greater than 0. This ΔΙ _Ρ (or, which is equivalent to ΔΙ _Ρ divided by ΔΙΙ NS _O II or, which is also equivalent to l ' _P ) may be of at least one threshold be compared. If this at least one threshold is at least reached or exceeded, then it can be concluded that the sensor element 1 14 is no longer intact. In this case, for example, a warning can be issued or the reference variable U NS _O II can be adjusted. For example, the reference variable I NS _O II can be increased, for example by a predetermined amount. Thereafter, a further control step can be run through and, if appropriate, the setpoint Nernst voltage can be further increased, as long as the above-described threshold condition is still not met and the change ΔΙ _{Ρ is} still too large. In this way it can be achieved that U _{N is increased} in Figure 2 such that U _{P, 2} so far to the right shifts in Figure 2, that the pumping current l _P * approaches the saturation pump current l _s .

The method can for example be carried out in such a way that in solid

Time intervals, for example, every 5 hours and / or preferably at a lean exhaust gas condition, preferably - but not necessary - in overrun, the change of the pump current l _P detected with an increase of U NS _O II and then optionally the target Nernst voltage U NS _O II is increased. Then another test cycle, ie a further control step, can be run through and, if appropriate, the setpoint Nernst voltage U _N , soii can be further increased. Optionally, further control steps may follow.

Preferably, the change ΔΙ _Ρ in a U N.soii increase and subsequent decrease is measured to provide redundancy of the data. Alternatively or additionally, the desired Nernst voltage U NS _O II in this or other

Embodiments of the method according to the invention, as well as one or more other command variables, permanently oscillate, for example with an amplitude of 10 mV and, for example, with a frequency of 100 Hz. The mean of this

Oscillation can be regulated by a control device to a minimum value such that a resulting pump current amplitude does not exceed a certain threshold value, for example 20 μΑ.

The method according to the invention was described above using the example of a multicellular

Sensor element 1 14 described. Alternatively or additionally, however, the method described above can also be applied to a single-cell lambda probe, for example to a sensor element 1 14, which has only the pumping cell 125. In this case, for example, the target Nernst voltage UN _, S _O II can be replaced by a desired pumping voltage U _P, S ₀ II. Since the curve 164 in FIG. 2 reaches the saturation l _s much later than the intact curve 162, it is likewise possible by means of a

Threshold method can be detected in the control step, when the pump cell 125 is no longer running in the limiting current operation. In this case, for example, the desired pumping voltage U _{P, S0} II can be increased, which in turn can also take place in one or more steps, optionally also iteratively and / or oscillatingly.

By the method described above and the sensor device 10 described above, a high robustness against an electrode polarization can be achieved.

In addition, the method described above allows a continuous operation of the sensor element 1 14 at a lowered temperature, which for example by means of a heating element, not shown in Figure 1 can be adjusted. For example, with the proposed method in one or more of the variants presented above, a temperature of the pumping cell 125 can be set to a value of 500 to 800 ° C, in particular to a value of 600 to 700 ° C. Due to this lowered temperature of the sensor element 1 14 and in particular the pumping cell 125, a heating power requirement can be significantly reduced. Furthermore, the amount of catalytically active noble metal in one or more of the electrodes 120, 124 and 132, in particular in the IPE 124, can be significantly reduced with the proposed method and the resulting robustness with respect to electrode polarization. Furthermore, the detection of an electrode polarization in the at least one control step, for example by varying the reference variable and in particular the desired Nernst voltage, also in one or more of the above

Method as a trigger for a regeneration process, for example by means of a temporary pumping current reversal and / or a temporary or permanent

Temperature increase and / or a temporary application of grease gas can be used. Such measures generally have a regenerating effect.

The change in the reference variable can be carried out in any method according to the invention both in the positive and in the negative direction. So were above in the

Substantial increases in the reference variable are described. As can be seen in FIG. 2, however, in principle it is also possible to reduce the reference variable,

for example, by not only increasing a nominal Nernst voltage, but, alternatively or additionally, also lowering it to detect a polarization. The method described above and the sensor device 110 described above have been illustrated using the example of sensor devices 110 for detecting oxygen in the measurement gas space 112. However, the proposed method and the proposed sensor device 1 10 are also suitable for other applications. Thus, the proposed method can in principle also be applied, for example, to NOx sensors, so that the sensor device 110 can be used, for example, for detecting a proportion of NOx in the measurement gas space 112. Such sensor elements are basically also known from the prior art.

Claims

Method for determining at least one property of a gas in a measurement gas space (112), in particular for detecting at least one

Gas component in the gas, wherein at least one sensor element (1 14) is used, wherein the sensor element (114) at least one pump cell (125) having at least a first electrode (120), at least one second electrode (124) and at least one of the first electrode (120) and the second electrode (124) connecting solid electrolyte (123), wherein from a pumping current through the pumping cell (125) is closed on the property, wherein in at least one measuring step of the method, the pumping cell (125) below

Using at least one reference variable (148) controlled or controlled operated, wherein in at least one control step of the method, the reference variable (148) is changed and a change in the pumping current is detected.

Method according to the preceding claim, wherein the reference variable (148) in the measuring step is adapted, in particular increased, if the change of the pumping current in the control step no longer fulfills at least one threshold condition.

Method according to the preceding claim, wherein the adaptation of the

Command variable (148) iteratively until the change in pumping current in the control step satisfies the at least one threshold condition.

Method according to one of the preceding claims, wherein at least one regeneration step is initiated if it is determined in the control step that the change in the pumping current no longer fulfills the at least one threshold condition, in particular a regeneration step with at least one pumping current reversal in the pump cell (125) and / or a temporary or permanent increase in temperature and / or with a temporary application of grease gas. Method according to one of the preceding claims, wherein the second electrode (124) is selected from: an electrode (124) in communication with at least one reference gas space (134), in particular an electrode (124) connected to at least one reference gas channel (138) ; an electrode (124) disposed in an electrode cavity (126).

Method according to one of the preceding claims, wherein the pumping cell (125) is operated in a limiting current operation in the measuring step.

Method according to one of the preceding claims, wherein the reference variable (148) comprises at least one desired value of a potential and / or a voltage at the second electrode (124).

Method according to the preceding claim, wherein the second electrode (124) with gas from the sample gas chamber (1 12) is acted upon, wherein the sensor element (1 14) further comprises at least one reference electrode (132) in at least one reference gas space (134), wherein a Nernst voltage U _N between the second electrode (124) and the reference electrode (132) is detected, wherein in the measuring step, the pumping current through the pumping cell (125) using a nominal Nernstspannung UN _, S _O II 3ls command variable (148) is controlled.

Method according to the preceding claim, wherein in the control step, the desired Nernst voltage U NS _O II is changed by a value AUN.S _O II, wherein the

Change ΔΙ _{Ρ of} the pump current detected and compared with at least one threshold, wherein the target Nernstspannung is changed, in particular, is increased when the change ΔΙ _Ρ reaches or exceeds the threshold.

Method according to one of the preceding claims, wherein the reference variable (148) comprises a pumping voltage U _p , wherein in the control step the

Pumping voltage is changed by a value ΔΙΙ _Ρ , the change ΔΙ _{Ρ of} the pumping current is detected and compared with at least one threshold, wherein in the measuring step, the pump voltage is changed, in particular is increased when the change ΔΙ _Ρ reaches or exceeds the threshold. 1 1 A method according to any one of the preceding claims, wherein the reference variable (148) comprises a pumping current l _p , wherein in the control step, the pumping current to a value ΔΙ _{Ρ is} changed, whereby a change AU _{P of} a pump voltage and / or a change AU _{N of} a Nernst voltage are detected and compared with at least one threshold value, wherein in the measuring step the pump current is changed, in particular increased, if the changes AU _P or AU _N exceed the threshold or not fall below.

Sensor device (1 10) for detecting at least one property of a gas in a measuring gas space (1 12), in particular using a method according to any one of the preceding claims, comprising at least one

Sensor element (1 14), wherein the sensor element (1 14) at least one pump cell (125) with at least one first electrode (120), at least one second electrode (124) and at least one of the first electrode (120) and the second electrode (124 ) connecting solid electrolyte (123), wherein the

Sensor device (1 10) further comprises at least one connected to the sensor element (1 14) control (1 18), wherein the controller (1 18) is adapted to close from a pumping current through the pumping cell (125) on the property the controller (1 18) is further configured to operate or control the pumping cell (125) in at least one measuring step using at least one reference variable (148), wherein the controller (1 18) is further configured to perform in at least one control step the

Control variable (148) to change and detect a change in the pumping current.