METHOD AND APPARATUS FOR DIAGNOSING AN EXHAUST GAS SENSOR

Abstract

A method and a device for performing diagnostics on an exhaust gas sensor includes a series circuit of a Nernst cell and a first other cell. The exhaust gas sensor has a first, a second, and a third connection where a voltage drop can be measured between the first connection and the second connection by the Nernst cell and a voltage drop between the second connection and the third connection can be measured by the first other cell. The diagnostic method has steps for feeding a direct or alternating current into the first, second, and third connections; for directly or indirectly sensing an electrical voltage potential at at least one of the first, second, and third connections; and for evaluating the voltage potential sensed to diagnose a short circuit or an open circuit in the Nernst cell and/or in the first other cell.

Claims

1-19. (canceled)

20. A diagnostic method for an exhaust gas sensor, the exhaust gas sensor having a series circuit of a Nernst cell and a further cell, and the exhaust gas sensor has a first connection, a second connection, and a third connection, and wherein a voltage drop across the Nernst cell is measurable between the first and second connections and a voltage drop across the further cell is measurable between the second and third connections, the diagnostic method comprising: feeding a direct current or an alternating current into the first, second, and third connections; directly or indirectly capturing an electrical voltage potential at at least one of the first, second, or third connections; and evaluating the captured electric voltage potential for diagnosing a short circuit or a line break in at least one of the Nernst cell or the further cell.

21. The method according to claim 20, wherein the further cell is a first further cell and the exhaust gas sensor has a second further cell connected in series with the Nernst cell and the first further cell, and wherein the exhaust gas sensor has a fourth connection such that a voltage drop across the second further cell is measurable between the fourth connection and one of the first, second, or third connections, and wherein the diagnostic method further comprises: feeding a direct or alternating current into the fourth connection; directly or indirectly capturing an electrical voltage potential at at least one of the first, second, third, or fourth connections; and evaluating the captured electrical voltage potential in order to diagnose a short circuit or a line break in at least one of the Nernst cell, the first further cell, or the second further cell.

22. The method according to claim 21, which comprises feeding the direct current or alternating current into the first, second, third, and fourth connections in a temporally staggered manner.

23. The method according to claim 21, which comprises connecting a voltage divider to at least one of the first, second, third, or fourth connections and capturing a voltage potential in the voltage divider in order to indirectly capture the voltage potential at the respective connection.

24. The method according to claim 20, which comprises using an analog/digital converter in capturing the voltage potentials and converting captured signals to digital signals with the analog/digital converter and evaluating the digital signals.

25. The method according to claim 24, which comprises supplying the voltage potentials of the first, second, and third connections, and optionally a fourth connection, in succession to the analog/digital converter by way of multiplexing.

26. The method according to claim 20, which comprises concluding that a short circuit exists at a given connection of a cell if the alternating current is fed in at the given connection and a DC voltage potential is captured at the given connection.

27. The method according to claim 26, which comprises capturing the short circuit at the given connection of the cell only when the direct or alternating current is additionally fed in at another connection assigned to the cell and an AC voltage potential or a lower DC voltage potential is captured at the given connection.

28. The method according to claim 20, which comprises concluding that a line break has been captured in a cell if the following two conditions are met: the direct or alternating current is fed in at a connection assigned to the cell and a substantially identical DC or AC voltage potential is then captured at the connection; and a lower DC voltage potential is captured at another connection assigned to the cell.

29. The method according to claim 28, wherein the line break is captured in the cell only when a DC or AC voltage potential is additionally captured at a connection of an adjacent cell.

30. The method according to claim 20, which comprises feeding in a direct current or an alternating current each having a current intensity of between 30 μA and 500 μA.

31. An apparatus for diagnosing an exhaust gas sensor having a series circuit with a Nernst cell and a further cell, the exhaust gas sensor having a first connection, a second connection, and a third connection, and a voltage drop across the Nernst cell is measurable between the first and second connections and a voltage drop across the further cell is measurable between the second and third connections, the apparatus comprising: a current feed device for feeding a direct current or an alternating current into the first, second, third connections; a device for directly or indirectly capturing an electrical voltage potential at at least one of the first, second, or third connections; and an evaluation device for evaluating the electrical voltage potential captured at one or more of the first, second, or third connections in order to diagnose a short circuit or a line break in one or both of the Nernst cell or the further cell.

32. The apparatus according to claim 31, further comprising a multiplexer configured to supply the voltage potentials of the first, second, and third connections in succession to said evaluation device.

33. The apparatus according to claim 31, wherein: the further cell is a first further cell and the exhaust gas sensor has a second further cell connected in series with the Nernst cell and the first further cell, the exhaust gas sensor has a fourth connection such that a voltage drop across the second further cell is measurable between the fourth connection and one of the first, second, and third connections; said current feed device is configured to feed a direct current or alternating current into the fourth connection; said device is configured for directly or indirectly capturing the electrical voltage potential at at least one of the first, second, third, or fourth connections; and said evaluation device is configured for evaluating the captured voltage potential in order to diagnose a short circuit or a line break in at least one of the Nernst cell, the first further cell, or the second further cell.

34. The apparatus according to claim 33, further comprising a multiplexer configured to supply the voltage potentials of the first, second, third, and fourth connections in succession to said evaluation device.

35. The apparatus according to claim 33, further comprising a voltage divider connected to at least one of the first, second, third, and fourth connections, wherein a voltage potential is measurable in said voltage divider in order to indirectly capture the voltage potential at the respective said connection.

36. The apparatus according to claim 31, wherein said evaluation device comprises an analog/digital converter for capturing the voltage potentials and is configured to evaluate digital signals from said analog/digital converter.

37. The apparatus according to claim 34, further comprising a second multiplexer configured to feed the direct current or alternating current from a single current source in succession into the first, second, and third connections, and optionally into a fourth connection.

38. The apparatus according to claim 33, wherein one of the first or second further cells is a pump cell or a trimming resistor.

39. The apparatus according to claim 33, wherein the first further cell is a pump cell and the second further cell is a trimming resistor.

Description

[0033] FIG. 1 shows an equivalent circuit diagram of an exhaust gas sensor and of an apparatus for diagnosing the exhaust gas sensor according to one exemplary embodiment of the present invention;

[0034] FIG. 2 shows diagnosis of a short circuit which is present between a Nernst cell and a pump cell of the exhaust gas sensor with respect to the battery voltage;

[0035] FIG. 3 shows diagnosis of the short circuit which is present between a Nernst cell and a pump cell of the exhaust gas sensor with respect to the battery voltage; and

[0036] FIG. 4 shows diagnosis of a line break within the pump cell.

[0037] It should be noted that the exemplary embodiments described below are merely a limited selection of possible variant embodiments of the invention.

[0038] FIG. 1 shows an equivalent circuit diagram of an exhaust gas sensor 1 and of an apparatus 100 for diagnosing the exhaust gas sensor 1 according to one exemplary embodiment of the present invention. The exhaust gas sensor 1 is a broadband lambda probe which is referred to below as a lambda probe 1 for simplification. The lambda probe 1 measures a concentration of a gas component in the exhaust gas of an internal combustion engine.

[0039] In a known manner, the lambda probe 1 has a combination of a Nernst cell 2 and a first further cell in the form of a pump cell 3. In FIG. 1, the electrical equivalent circuit diagram of the Nernst cell 2 is formed by a series circuit comprising a resistor with a resistance value RN.sub.ernst and a voltage source with a Nernst voltage V.sub.Nernst. In a corresponding manner, in FIG. 1, the electrical equivalent circuit diagram of the pump cell 3 is formed by a series circuit comprising a resistor with a resistance value RP.sub.ump and a voltage source with a pump voltage V.sub.Pump.

[0040] The Nernst cell 2 and the pump cell 3 are in turn connected in series, wherein a Nernst cell voltage V.sub.n is dropped across the Nernst cell 2 and a pump cell voltage V.sub.p is dropped across the pump cell 3.

[0041] The Nernst cell 3 acts as a galvanic cell, at which an electrical voltage V.sub.Nernst is generated, which voltage changes on the basis of a concentration of a redox pair in the exhaust gas which passes through the Nernst cell 2.

[0042] An electrical voltage is applied to the pump cell 3. If the voltage exceeds a threshold value, a so-called limit current occurs and behaves in a manner proportional to the difference in the oxygen concentration on both sides of the Nernst cell 2. Oxygen atoms or oxygen ions are transported—in a polarity-dependent manner—with the current values. An amount of oxygen from the exhaust gas is supplied to the Nernst cell 2 from the pump cell 3 via a very narrow diffusion gap by means of electronic pump current control in such a manner that the state λ=1 prevails at the Nernst cell. Oxygen is pumped out if there is excess air in the exhaust gas (lean range). Oxygen is supplied by reversing the pump voltage if there is a low residual oxygen content in the exhaust gas (rich range). The respective pump current forms the output signal.

[0043] In other words, a pump voltage V.sub.Pump is applied to the pump cell 3 of the lambda probe 1, which pump voltage is set on the basis of a Nernst voltage V.sub.Nernst, which corresponds to the oxygen concentration in the exhaust gas and is removed at the Nernst cell 2, with the aid of the pump current controller. The pump current controller is used to adjust the pump current in such a manner that the pump voltage V.sub.Pump is within predefinable limits.

[0044] The value of the pump current which is needed to set the state λ=1 is dependent on the oxygen concentration and therefore on the lambda value of the exhaust gas to be determined.

[0045] The Nernst cell 2 is connected between a first connection 11 and a second connection 12. The pump cell 3 is connected between the second connection 12 and a third connection 13. Accordingly, the node between the pump cell 3 and the Nernst cell 2 is connected to the second connection 12.

[0046] The lambda probe 1 has a trimming resistor 4 (R.sub.trim) which is connected in series with the pump cell 3 in the exemplary embodiment illustrated. A fourth connection of the trimming resistor 4 (R.sub.trim) which is not connected to the pump cell 3 is denoted using the reference sign 14. The trimming resistor 4 is used to compensate for production-related parameter variation of the lambda probe 1. The trimming resistor 4 may have a low resistance of the order of magnitude of approximately 40Ω. The trimming resistor 4 is referred to as a second further cell 4.

[0047] The drive circuit 100 comprises a direct or alternating current source 6 which is configured to generate a current with a current intensity of between 30 and 500 μA. The drive circuit 100 also has a multiplexer 7 which can be used to selectively introduce the generated direct or alternating current at the first to fourth connections 11 to 14. The drive circuit 100 also has four passive voltage dividers 21, 22, 23, 24 which are each accordingly assigned to one of the connections 11 to 14. A first voltage divider 21 is assigned to the first connection 11, a second voltage divider 22 is assigned to the second connection 12, a third voltage divider 23 is assigned to the third connection 13 and a fourth voltage divider 24 is assigned to the fourth connection 14. Within the respective voltage dividers 21, 22, 23, 24, an electrical voltage is respectively tapped off via a fifth to eighth connection 31 to 34. The fifth connection 31 is assigned to the first voltage divider 21, the sixth connection 32 is assigned to the second voltage divider 22, the seventh connection 33 is assigned to the third voltage divider 23 and the eighth connection 34 is assigned to the fourth voltage divider 24.

[0048] The fifth to eighth connections 31 to 34 are each connected, via a further multiplexer 8, to an analog/digital converter (ADC) 5 which selectively converts the respective analog voltages applied to the fifth to eighth connections 31, 32, 33, 34 into a digital signal.

[0049] For the procedure according to the invention, the current/voltage characteristics of the current source 6 and the voltages tapped off at the fifth to eighth connections 31, 32, 33, 34 are used.

[0050] In the method according to the invention, direct or alternating currents are fed into the first to third connections 11, 12, 13 and possibly into the fourth connection 14 in a temporally staggered manner. The direct or alternating currents fed into the different connections 11, 12, 13, 14 may be substantially identical or may differ slightly, for example within a range of 10%. The direct or alternating currents which are fed in each have a current intensity of between 30 and 500 μA.

[0051] A voltage potential is captured in the voltage dividers 21, 22, 23, 24 in order to indirectly capture the voltage potential at this connection 11, 12, 13, 14. The voltage potentials are also captured using the analog/digital converter 5, with the result that digital signals are captured from the analog/digital converter 5 and evaluated. In this case, the voltage potentials of the first to third connections and possibly the fourth connection 11, 12, 13, 14 are supplied in succession to the analog/digital converter 5 by means of multiplexing.

[0052] FIGS. 2 and 3 show diagnosis of a short circuit which is present between the Nernst cell 2 and the pump cell 3 of the exhaust gas sensor 1 with respect to the battery voltage.

[0053] The short circuit is captured at the connection 12 if the direct or alternating current is fed in at the connection 12 and a DC voltage potential is captured at this connection 12. The accuracy of the diagnosis can be increased if the short circuit is captured at the connection 12 only when the alternating current is additionally fed in at another connection 13 assigned to the pump cell 3 and an AC voltage potential or a lower DC voltage potential is captured at this connection 13.

[0054] FIG. 4 shows diagnosis of a line break within the pump cell 3. The line break at the pump cell 3 is captured if the following two conditions are met: the alternating current is fed in at a connection 12 assigned to the pump cell 3 and a substantially identical AC voltage potential is captured at this connection 12, and a lower DC voltage potential is captured at another connection 13 assigned to the pump cell 3.

[0055] The accuracy of the diagnosis can be increased if the line break in the pump cell 3 is captured only when an AC voltage potential is additionally captured at the connection 11 of an adjacent cell, here the Nernst cell 2.

[0056] The respective electrical voltage potential is preferably captured at a connection of a cell 2, 3, 4 while the alternating current is being fed only into one of the two connections of this cell 2, 3, 4. The respective electrical voltage potential is particularly preferably captured at a connection of a cell 2, 3, 4 while the alternating current is being fed only into a single connection of the exhaust gas sensor 1. As a result, clear conclusions with respect to a short circuit or a line break can be drawn from the behavior of the respective cell 2, 3, 4.

[0057] The following modifications of the exemplary embodiments are possible:

[0058] In the exemplary embodiments illustrated, a linear lambda probe which is usually arranged upstream of a catalytic converter and is used to adjust the exhaust gas mixture is used. The present invention can also be applied to a so-called binary probe which does not have a pump cell.

[0059] Instead of using a multiplexer, different current sources can be alternatively used to feed the currents into the connections 11, 12, 13, 14.