KR20180105585A - Method for correcting an offset of an ammonia sensor - Google Patents

Abstract

The present invention relates to a method for correcting at least an offset of an ammonia sensor disposed at downstream of an SCR catalytic converter in an SCR system. The method comprises: detecting an ammonia slip by using an ammonia sensor; setting a modeled ammonia charge level (FNH3mod) to a maximum ammonia charge level (FNH3max); inserting a small amount of reducing agent for the SCR system; stopping inserting the small amount of reducing agent when the modeled ammonia charge level (FNH3mod) decreases to a nominal ammonia charge level (FNH3max); detecting an actual offset; and correcting an offset by using a difference between the actual offset and a predicted offset.

Description

METHOD FOR CORRECTING AN OFFSET OF AMMONIA SENSOR < RTI ID = 0.0 >

The present invention relates to a method for correcting an offset of an ammonia sensor in an SCR system. The present invention also relates to a computer program for executing each step of the method when executed on a computer, as well as a machine-readable storage medium storing the computer program. Finally, the present invention relates to an electronic control device configured to execute the method.

Recently, the (S elective- C atalytic- R eduction), especially SCR catalytic converter for the reduction of nitrogen oxides in automobile exhaust gas (NOx) is used. In this case, when ammonia (NH ₃ ) is present as a reducing agent, the nitrogen oxide molecules on the surface of the SCR catalytic converter are reduced to elemental nitrogen. The reducing agent is also commercially available as AdBlue ^® and is provided in the form of urea water from which ammonia is separated and is injected into the exhaust gas line by a dosing module upstream of the SCR catalytic converter. The determination of the desired metering feed rate is performed in an electronic control device in which a strategy for operation and monitoring of the SCR system is stored.

Recently, known SCR catalytic converters store ammonia on the catalytic converter surface. The storage capacity is critically dependent on the temperature of the catalytic converter surface and decreases with increasing temperature. The greater the amount of ammonia that can be coupled to the surface of the catalytic converter to be used for reduction, the higher the nitrogen oxide conversion rate. As long as the storage capacity of the SCR catalytic converter is not used up, excessively charged reducing agent is stored. On the other hand, if the input unit supplies a smaller amount of reducing agent than is necessary for the complete reduction of the nitrogen oxides present in the exhaust gas, the ammonia charge level is reduced by the subsequent reduction of the nitrogen oxides on the surface of the catalytic converter. Currently available metering feed strategies for SCR systems usually involve a slight overdose to achieve maximum nitrogen oxide conversion. In this case, it should be taken into account that excessively charged ammonia passes through the surface of the catalytic converter without being used. Amount of passing ammonia is also called ammonia slip.

From the relationship between the ammonia charge level and the nitrogen oxide conversion rate, on the one hand, an excessive increase of the reducing agent causes an increase in the amount of ammonia stored in the SCR catalytic converter, an increase in the nitrogen oxide conversion rate appears. On the other hand, when the SCR catalytic converter is already driven optimally, the nitrogen oxide conversion rate is maintained. In this case, if the measured nitrogen oxide conversion rate decreases, that is, if the above-mentioned sensor signal rises, the cause is retrained to ammonia upstream of the SCR catalytic converter. In this case, it can be assumed that as the maximum storage capacity of the SCR catalytic converter is used, ammonia slip occurs through the surface of the catalytic converter without excess use of ammonia. The excess amount of reducing agent is always corrected through control.

DE 10 2010 002 620 A1 describes the metering feed rate control using the adaptation factor which indicates the ratio of the nominal metering supply to the actual metering supply. The adaptive coefficient directly varies the pilot control amount of the reducing agent and is used to regulate the nitrogen oxide concentration measured by the sensor downstream of the SCR catalytic converter to the modeled nitrogen oxide value. The control is adjusted using the I-controller to suit each system and long-lasting environmental impact, thereby reducing the number of adaptive interventions required in the event of a system failure. The control can also take into account very large and spontaneous fluctuations, for example, when charging the wrong reducing agent. In this case, the I-controller works fine, but correspondingly slow.

DE 10 2012 221 574 A1 discloses a charge level observer. This high speed P controller continuously compares the modeled NOx concentration downstream of the SCR catalytic converter with the corresponding signal of the corresponding NOx sensor. If there is a deviation, the P-controller can perform the level adjustment until it reaches the nominal efficiency again within a few seconds.

For thorough control, it is important to accurately measure the NOx concentration downstream of the SCR catalytic converter using a nitrogen oxide sensor. Commercially available nitrogen oxide sensors show ammonia cross-sensitivity, that is, the sensor signal of ammonia transverse sensitivity not only contains nitrogen oxide concentration, but also shows the sum signal of nitrogen oxide and ammonia. Therefore, in all the controls based on the nitrogen oxide sensor, the ammonia slip occurs as the metered amount is selected so small that the nitrogen oxide has not been converted or the metered amount is selected too large and the free ammonia passes through the SCR catalytic converter It can not be judged whether or not it is. The latter case occurs mainly with a slight excess of NOx conversion, which is important for maximum nitrogen oxide conversion.

Additional ammonia sensors are used at the location of the nitrogen oxide sensor to distinguish the sum signal of nitrogen oxide and ammonia. This ammonia sensor measures the ammonia concentration and the resulting ammonia concentration can be subtracted from the sum signal, which makes the SCR system robustly prepared to yield the maximum nitrogen oxide conversion. In addition, the ammonia sensor can be used to confirm that ammonia slip has occurred and correspondingly reduce the metering feed of the reducing agent.

To be able to utilize ammonia sensors, monitoring the offset of the ammonia sensor over the lifetime of the ammonia sensor and shifting the offset are essential. Such monitoring is already known in the case of nitrogen oxide sensors. However, in nitrogen oxide sensors, operating points where nitrogen oxide emissions are discontinued are used. If this is dedicated to the ammonia sensor, monitoring or movement of the offset means that it is only executed when there is no ammonia slip. As previously described, ammonia sensors are often used in SCR systems where weak overpouring is constantly adjusted, where no ammonia slip can be excluded at all.

The present invention relates to a method for correcting the offset of an ammonia sensor disposed at least downstream of an SCR catalytic converter in an SCR system. The method includes the following steps: First, an ammonia slip is detected using a signal from the ammonia sensor. As a result, at this point, the ammonia charge level of the SCR catalytic converter must reach a maximum value (hereinafter referred to as maximum ammonia charge level). In order to take this into account, the modeled ammonia charge level to be used, in particular, to regulate the metering feed of the reducing agent in the SCR catalytic converter is set to the maximum ammonia charge level. Subsequently, the metering feed of the reducing agent is reduced until an undercurrent of the reducing agent is achieved, i.e., less than the metering feed required to fully convert the nitrogen oxide presently entering the SCR catalytic converter through the SCR. Correspondingly, the ammonia charge level drops, which is also expressed as a reduction in the modeled ammonia charge level.

When the modeled ammonia charge level reaches the nominal ammonia charge level for a given operating condition, the undercharge is terminated and the metering feed is regulated again. The ammonia charge level was reduced to such an extent that, through under-dosing, it could be assumed that there was no ammonia slip at least during the calibration time. As a result, the signal of the ammonia sensor only reproduces the actual offset during the correction time. This actual offset is detected and then used for offset correction of the ammonia sensor. The correction of the offset is performed using the difference between the actual offset and the prediction offset. Preferably, the difference may be added to an existing offset to obtain a corrected offset. In repetition of the method, the corrected offset can preferably be used as a prediction offset. This method has the advantage that the shift of the offset is quickly detected and corrected accordingly.

These ammonia sensors are mainly used in SCR systems with high nitrogen oxide emissions. This SCR system is operated at least temporarily with a slight overloading of the reducing agent to achieve the maximum nitrogen oxide conversion rate, unless undue input is performed. The reason why the above-described method is preferable in such an SCR system is that the ammonia slip must be premised on the case of an excessive input.

Preferably, as the modeled ammonia charge level reaches the nominal ammonia charge level, the under-injection is terminated and then is queued for a debouncing time before the actual offset is detected. The ammonia charge level does not decrease to an even distribution throughout the entire SCR catalytic converter, but begins to descend first at the exhaust gas inlet of the SCR catalytic converter. Due to inertial and gas SCR catalytic converter transit times, delays occur until the ammonia charge level decreases through the entire SCR catalytic converter. This debouncing time is formed to compensate for the delay.

To lower the risk of ammonia slip, at least the following conditions for release of the offset correction, preferably release of offset detection and more preferably release of debouncing time, may be examined: In the first condition, the temperature of the SCR catalytic converter is measured, and the temperature gradient calculated therefrom is compared with the first threshold value. If the temperature slope of the SCR catalytic converter is greater than the first threshold value, correction of at least offset, preferably correction of offset detection, and more preferably correction of debouncing time, may also be blocked. The comparison of the temperature slope and the first threshold value can be continuously performed at any time, preferably until the offset is corrected. As the storage capacity of the SCR catalytic converter is temperature dependent, there is a risk of ammonia slip during detection of the actual offset if the temperature rises dramatically.

In the second condition, the slope of the ammonia sensor signal of the ammonia sensor is calculated during or after the under-injection, and the slope of the ammonia sensor signal can be compared with the second threshold value. If the slope of the ammonia sensor signal is greater than the second threshold value, correction of at least offset, preferably correction of offset detection, and more preferably correction of debouncing time, may also be blocked. If the slope of the ammonia sensor signal is too large, it can be inferred that there is an ammonia slip during or after under-injection, even though the ammonia slip should be regarded as non-existent after detection of the actual offset.

In the third condition, after the under-injection is completed, the integration of the exhaust gas mass flow is performed, and the integrated exhaust gas mass flow can be compared with the third threshold value. If the integrated exhaust gas mass flow is greater than the third threshold value, correction of at least offset, preferably correction of offset detection, and more preferably correction of debouncing time, may also be blocked. If the integrated exhaust gas mass flow falls below the first threshold value over this period, it can be expected that there will be no continuing ammonia slip through a small exhaust gas mass flow and concomitant metering feed. Also, the period represents a maximum debouncing time and, in some cases, a correction time.

The ammonia sensor signal can be filtered over the correction time, in particular to detect the actual offset. This provides the advantage that undesirable artifacts to the actual offset are eliminated. Then, the actual offset is detected within the correction time.

Preferably, a weighting function can be used in correcting the offset of the ammonia sensor. The weighting function is converted, for example, into a curve or a data set of the curve using the difference between the actual offset and the prediction offset. In this way, each offset correction of the offsets does not cause an imbalance, and as a result does not negatively affect the offset.

A computer program is designed to perform each of the method steps, particularly when executed on a computer or control device. The computer program makes it possible to implement a method without structural modification of the electronic control unit in a conventional electronic control unit. To this end, the computer program is stored in a machine-readable storage medium.

By installing the computer program in a conventional electronic control device, an electronic control device designed to perform offset correction of the ammonia sensor is obtained.

Embodiments of the present invention are illustrated in the drawings and described in detail below.
FIG. 1A is a graph showing the ammonia charge level according to temperature in a conventional SCR catalytic converter. FIG.
FIG. 1B is a graph showing the nitrogen oxide conversion rate and the ammonia slip according to the ammonia charge level of the conventional SCR catalytic converter shown in FIG. 1A.
2 is a graph illustrating the relationship between the modeled ammonia charge level, the nominal ammonia charge level, the temperature, and the signal of the ammonia sensor during five International Worldwide Harmonized Transient Cycles (WHTC) over time, in accordance with one embodiment of the present invention. Fig.
3 is a graph showing in more detail the fourth WHTC in section III of the graph of FIG.
4 is a flow chart of one embodiment of a method according to the present invention.
Figure 5 is a graph showing the transition region from fourth WHTC to fifth WHTC in another section (V) of the graph of Figure 2, to which one embodiment of the method according to the present invention is applied.

FIG. 1A is a graph showing a relationship between a temperature T and an ammonia charge level FNH3 in a conventional SCR catalytic converter. FIG. The graph shows the minimum ammonia charge level (FNH3min), the nominal ammonia charge level (FNH3nom), and the maximum ammonia charge level (FNH3max), respectively, as a function of temperature (T). If the ammonia charge level (FNH3) is lower than the minimum ammonia charge level (FNH3min), the nitrogen oxides passing through the SCR catalytic converter are incompletely converted through the SCR. If the ammonia charge level FNH3 exceeds the maximum ammonia charge level FNH3max, an ammonia slip will occur, which will be specified again in FIG. Therefore, the control of the SCR system should maintain the ammonia charge level (FNH3) between the minimum ammonia charge level (FNH3min) and the maximum ammonia charge level (FNH3max). The marked temperature (T ₁ ) to be used in FIG. 1B can be seen.

1B is a graph showing the nitrogen oxide conversion rate (NOxKonv) according to the ammonia charge level FNH3 at the temperature T ₁ marked in FIG. 1A in the case of the conventional SCR catalytic converter. The higher the ammonia charge level (FNH3), the greater the nitrogen oxide conversion rate (NOxKonv), because more reactants are provided for the SCR. With respect to the nitrogen oxide conversion rate (NOxKonv), the operating point 10 for the nominal ammonia charge level (FNH3nom), the minimum ammonia charge level (FNH3min) for operation of the SCR catalytic converter as the nitrogen oxide conversion rate (NOxKonv) And the operating point 12 for the maximum ammonia charge level FNH3max are shown. If the ammonia charge level (FNH3) exceeds the maximum ammonia charge level (FNH3max), ammonia is no longer stored in the SCR catalytic converter.

The amount of ammonia 20, which corresponds to the ammonia slip, passing through the SCR catalytic converter is also shown in the graph. However, it can be seen that as the nitrogen oxide conversion rate (NOxKonv) further increases in this region, the NOx is no longer converted by the SCR. In order to comply with the law, the SCR catalytic converter is operated at an operating point 13 that is above the maximum ammonia charge level (FNH3max), in particular when the nitrogen oxide emissions are high, in other words, overcharging. The amount of ammonia 20 to be passed through, ie the ammonia slip, should be kept below the limits stipulated by law. As a result, the SCR system ensures that the highest ammonia charge level (FNH3) is realized when the amount of ammonia 20 passing through is otherwise as low as possible, so as to achieve the highest possible nitrogen oxide conversion rate (NOxKonv) while lowering the ammonia slip Should be controlled. The SCR system used in this embodiment is operated upon an overloading of the reducing agent, unless otherwise stated, that is, unless otherwise provided in a method aspect of the method according to the present invention. Also shown is an operating point 15 at the time of undercurrent 32 to be used subsequently. At the time of under-injection 32, as less reducing agent is added than necessary for the minimum ammonia charge level (FNH3min), the ammonia charge level FNH3 decreases.

In FIG. 2, a graph of five World Harmonized Transient Cycles (WHTC1 to WHTC5) run continuously for the SCR catalytic converter is shown over time t. In this embodiment, the ammonia storage capacity of the SCR catalytic converter is significantly reduced by the aging effect. The upper part of the graph shows the temperature (T) of the SCR catalytic converter, averaged for better availability, and the middle part of the graph shows the modeled ammonia charge level (FNH3mod) and the nominal ammonia charge level (FNH3nom) And a signal YNH3 of the ammonia charge level FNH3 disposed downstream of the SCR catalytic converter is displayed at the lower end of the graph. The nominal ammonia charge level (FNH3nom) is inversely proportional to the behavior of temperature (T) over five WHTCs (WHTC1 to WHTC5). The signal YNH3 of the ammonia sensor is located in the low level region during the first WHTC1. As the storage capacity of the SCR catalytic converter decreases, the metering feed rate increases through control to fully convert the nitrogen oxides. Correspondingly, the signal YNH3 of the ammonia sensor increases over both the subsequent WHTC2 and WHTC3 until it rises strongly enough to exceed the threshold 21 of ammonia slip in the fourth WHTC4.

The section labeled "III" in FIG. 2 represents the fourth WHTC4 shown in more detail in FIG. 3, wherein the graph of FIG. 3 corresponds to the graph of FIG. As can be clearly seen, the signal YNH3 of the ammonia sensor exceeds the threshold 21 of ammonia slip at about 6,800 seconds. Various methods for the detection of ammonia slip are known, in which, in addition to the excess of the threshold value 21, an exhaust gas flow rate which must at least flow through the SCR catalytic converter is taken into account. Thus, the ammonia slip is detected at about 6,880 seconds after the ammonia sensor signal YNH3 has exceeded the threshold value 21. At this point, as the modeled ammonia charge level FNH3mod is set to the maximum ammonia charge level FNH3max and the under-injection is performed, the modeled ammonia charge level FNH3mod again decreases. The fourth WHTC4 is terminated and the signal YNH3 is reduced to almost zero in the fifth WHTC5.

Of course, ammonia sensors already have ammonia sensors, and more specifically, the temperature (T) of the SCR catalytic converter is both high and low. As a result, offset correction of the ammonia sensor based solely on temperature (T) is not helpful.

4 shows a flowchart of one embodiment of an offset correction method of an ammonia sensor according to the present invention. In the first step, the signal YNH3 of the ammonia sensor is recorded (30). From the signal YNH3, for example, the detection 31 of the ammonia slip 31 is executed as described above. To reduce the ammonia charge level FNH3 again, the modeled ammonia charge level FNH3mod is set to the maximum ammonia charge level FNH3max (32). Correspondingly, the ammonia charge level (FNH3mod) is then lowered as a result of the under-injection (33) being carried out, as already mentioned in the description of Figure 1b. The resulting modeled ammonia charge level (FNH3mod) behavior is already shown in Fig. 2, and more particularly in Fig.

If the modeled ammonia charge level (FNH3mod) is lowered to the nominal ammonia charge level (FNH3nom), this is checked in query step 34 and then the underinjection 33 is terminated (35). The probability of ammonia slip is minimized as the ammonia charge level is lowered. In addition to these conditions, in the present embodiment, three additional conditions 40, 50, and 60 are checked before the method continues by release 36. [ These conditions (40, 50 and 60) relate to the risk factors of ammonia slip which should be minimized.

)가 계산된다(43). 온도 기울기(

)가 제1 비교(44)에서 온도(T)의 제1 임계값(S_T)보다 클 경우, 그 다음 방법 단계가 차단된다(37). 그렇지 않은 경우에는 릴리스(36)를 위한 제1 조건(40)이 충족된 것으로 간주된다. The first condition 40 is related to the temperature T of the SCR catalytic converter. During the continuation of the previous method, a measurement 41 of temperature T and filtering 42 by a PT1 filter, e.g. a low-pass filter, is performed. From the measured temperature (T) the temperature gradient (

) Is calculated (43). Temperature slope (

Is greater than the first threshold value (S _T ) of temperature (T) in the first comparison (44), the next method step is blocked (37). Otherwise the first condition 40 for release 36 is considered to be satisfied.

)가 계산된다(52). 제2 비교(53)에서 신호(YNH3)의 기울기(

)가 신호(YNH3)의 제2 임계값(S_Y)과 비교된다. 상기 신호(YNH3)의 기울기(

)가 제2 임계값(S_Y)보다 클 경우, 그 다음 방법 단계가 차단된다(37). 그렇지 않은 경우에는 릴리스(36)를 위한 제2 조건(50)이 충족된 것으로 간주된다. In the second condition 50, which is the basis for judging the implementation of the undercurrent 33, the recorded ammonia sensor signal YNH3 is filtered through a combination of DT1 filter (s) and PT1 filter (s) 51 ), From which the slope of the signal YNH3 (

) Is calculated (52). In the second comparison 53, the slope of the signal YNH3 (

Is compared with the second threshold value (S _Y ) of the signal YNH3. The slope of the signal YNH3 (

) Is greater than the second threshold value (S _Y ), the next method step is blocked (37). Otherwise the second condition 50 for the release 36 is considered to be satisfied.

In the query step 33, if it is ascertained that the modeled ammonia charge level FNH3mod has been lowered to the nominal ammonia charge level FNH3nom, then the exhaust gas flow rate through the SCR catalytic converter at the third condition 60 is checked. To this end, the exhaust gas mass flow (q _m) is recorded first, 61, is then carried out the integral (62). Then, the integrated exhaust gas mass flow ∫q _m , which in principle reflects the exhaust gas mass flow, is added to the integrated exhaust gas mass flow ∫q _m or the exhaust gas flow rate 3 threshold (S _m ). If the integrated exhaust gas mass flow ∫q _m is greater than the third threshold S _m , then the next method step is blocked 37. Otherwise, the third condition 60 for release 36 is deemed to be satisfied.

If all the conditions (40, 50 and 60) are satisfied in the release step 36, it is assumed that there is no ammonia slip. Subsequently, before the signal YNH3 of the ammonia sensor is filtered 71 through the PT1 filter, for example the low-pass filter, over the correction time t _K , the ammonia charge level FNH3 is adjusted via the entire SCR catalytic converter And waits for the bouncing time t _E (70). In the next step, the actual offset (O _a ) of the ammonia sensor from this is detected (72). The difference D between the actual offset O _a and the prediction offset O _v is calculated 73. The prediction offset is the unique offset of the sensor of use, provided by the manufacturer or detected or learned by any method, in the first round of the method according to the invention. In yet another embodiment, as selected as the actual offset (O _a) a prediction offset (O _v) in the first proceeding of the method, the difference (D) to be used for correction of the offset in the first proceeding is derived as zero. In the iteration of the method according to the invention, the corrected offset O _k in the preceding progression of the method is selected as the prediction offset O _v . Too large deviations due to erroneously detected actual offsets (O _a ) may be mitigated, for example, through a weighting function 74 in the form of a curve or data set, without adversely affecting the offset (O _k ) So that the difference D is weighted. Eventually, the offset correction 75 is performed in such a manner that the difference D is added to the prediction offset O _v so that the corrected offset O _k is maintained.

Fig. 5 shows a section labeled "V" in the graph of Fig. 2, which refers to the transition region between the fourth WHTC4 and the fifth WHTC5, which is related to the correction of the offset. In the lower part of this graph, a signal (YNOx) of the nitrogen oxide sensor disposed downstream of the SCR catalytic converter is also displayed, which has a transverse sensitivity to ammonia. Similar to FIGS. 2 and 3, the detection 31 of ammonia slip is performed at about 6,880 seconds and the modeled ammonia charge level FNH3mod is set to the maximum ammonia charge level FNH3max (32). By performing the under-put 33, the modeled ammonia charge level FNH3mod is lowered from about 7,070 seconds to the nominal ammonia charge level FNH3nom again. Due to the inertia and exhaust gas SCR catalytic converter transit time, the ammonia sensor signal YNH3 decreases with delay. The signal (YNOx) of the nitrogen oxide sensor rises again within the range of about 7,150 seconds, and finally again before it decreases with delay. The debounce time t _E shown here is the time from when the modeled ammonia charge level FNH3mod falls back to the nominal ammonia charge level FNH3nom (about 7,070 seconds), when the signal YNH3 of the ammonia sensor goes to zero (About 7,240 seconds). As the nitrogen oxide sensor continues to record a small concentration of nitrogen oxide that has not been converted by the SCR catalytic converter, the signal (YNOx) of the nitrogen oxide sensor does not fall to zero. From the last mentioned point, it is assumed here that there is no ammonia slip over the correction time (t _K ), which extends to about 7,320 seconds. Correspondingly, the detection 72 of the actual offset O _a and the correction 75 of the offset during the correction time t _K are performed.

Claims (13)

A method for correcting (75) an offset in an SCR system of an ammonia sensor disposed at least downstream of an SCR catalytic converter, comprising the steps of:
I. detecting (31) ammonia slip using an ammonia sensor,
II. Setting (32) a modeled ammonia charge level (FNH3mod) to a maximum ammonia charge level (FNH3max)
III. Performing (33) an undercurrent injection of a reducing agent for the SCR system,
IV. If the modeled ammonia charge level (FNH3mod) is reduced to the nominal ammonia charge level (FNH3nom), then terminate the undercurrent (33) (35)
V detecting (72) the actual offset (O _a ), and
VI. The actual offset (O _a), and the predicted offset (O _v), the offset compensation method of the ammonia sensor including a step 75 to correct an offset using the difference (D) between. 2. The method of claim 1, wherein the SCR system is operated at least temporarily with a weak overloading of the reducing agent when underinjection (33) is not performed. Method according to claim 1 or 2, characterized in that after the method step (IV), the step (70) is waited for the debouncing time (t _E ) prior to the method step (V). 4. The method according to any one of claims 1 to 3, further comprising the steps of:
- measuring (41) the temperature (T) of the SCR catalytic converter,
- Temperature slope of SCR catalytic converters (

(Step 43),
- Temperature slope of SCR catalytic converters (

) To a first threshold (S _T )
- Temperature slope of SCR catalytic converters (

(37) at least blocking the method step (VI) if the first threshold value (S _T ) is greater than the first threshold value (S _T ). 5. The process according to any one of claims 1 to 4, further comprising the following steps after method step III:
- the slope of the signal (YNH3) of the ammonia sensor

(Step 52),
- the slope of the signal YNH3 (

) To a second threshold value (S _Y ) (53),
- the slope of the signal YNH3 (

Is greater than a second threshold value (S _Y ), at least a step (37) of blocking the method step VI is executed. 6. The method according to any one of claims 1 to 5, further comprising the following steps after method step IV:
- integrating (62) the exhaust gas mass flow (q _m )
- integrating the step of comparing the exhaust gas mass flow (∫q _m) with the third threshold value (S _m) (63),
Characterized in that step (37) of at least blocking the process step VI is carried out if the integrated exhaust gas mass flow (∫q _m ) is greater than the third threshold value (S _m ) . Method according to claim 4, 5 or 6, characterized in that, if method step VI is blocked (37), method step V is also blocked (37). The method according to any one of the preceding claims, for the detection (72) of the actual offset, according to claim 71, which is filtered over a compensation period (t _K), the signal (YNH3) defined in the ammonia sensor , Offset correction method of ammonia sensor. 9. A method according to any one of claims 1 to 8, characterized in that the correction (75) of the offset of the ammonia sensor is performed using a weighting function (74). 10. A method according to any one of the preceding claims, characterized in that in the iteration of the method, the corrected offset (O _k ) is used as the prediction offset (O _v ). A computer program designed to perform each step of the method according to any of the claims 1 to 10. 12. A machine-readable storage medium having stored thereon a computer program according to claim 11. An electronic control device designed to perform an offset correction (75) of an ammonia sensor using a method according to any one of the preceding claims.

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