Method for self-diagnosis of a particle sensor

Abstract

Method for the self-diagnosis of a particle sensor (20) for determining a particle content in a gas stream, wherein the particle sensor (20) has on its surface at least two interlocking interdigital electrodes (22, 23) and a heating element (26) separated from the electrodes (22, 23) by an insulating layer, with which the particle sensor (20) can be heated in a regeneration phase and soot loading on the particle sensor (20) can be removed, wherein a semiconducting layer (28) is formed in the insulating layer directly below the electrodes (22, 23) by means of external or self-doping, which consists of aluminium oxide with a sodium ion doping of 100 ppm to 10000 ppm and under which an insulating layer of pure aluminium oxide is introduced, and for self-diagnosis a measuring voltage is applied at least temporarily between the electrodes (22, 23) and a self-diagnostic current (31) is measured,wherein a DC voltage is applied between the electrodes (22, 23) for self-diagnosis as a measuring voltage and the particle sensor (20) is heated to temperatures > 500°C by means of the heating element (26) before being connected to the DC voltage, wherein the heating of the particle sensor (20) to temperatures > 500°C is carried out for at least 30 seconds and wherein the temperatures > 500°C are monitored and controlled by means of a platinum resistor integrated in the particle sensor (20) which has the form of a measuring meander, and wherein the connection of the electrodes (22, 23) to the DC voltage is carried out only for a short time in the range of 1 ms to 100 ms.

Description

State of the art

[0001] The invention relates to a method for the self-diagnosis of a particle sensor for determining a particle content in a gas stream, wherein the particle sensor has on its surface at least two interlocking interdigital electrodes and a heating element separated from the electrodes by an insulating layer, with which the particle sensor can be heated in a regeneration phase and soot loading on the particle sensor can be removed.

[0002] Soot particle sensors are used today, for example, to monitor soot emissions from internal combustion engines and for on-board diagnostics (OBD), such as monitoring the function of particulate filters. Collecting, resistive particle sensors are known that evaluate changes in the electrical properties of an interdigital electrode structure due to particle accumulation. Two or more electrodes can be provided, preferably interlocking in a comb-like fashion. An increasing number of particles adhering to the particle sensor short-circuits the electrodes, resulting in a decreasing electrical resistance, a decreasing impedance, or a change in a parameter related to resistance or impedance, such as voltage and / or current, as particle accumulation increases.For evaluation, a threshold value, for example a measuring current between the electrodes, is generally set, and the time until the threshold value is reached is used as a measure of the amount of particles deposited. Alternatively, the rate of change of the signal during particle deposition can also be evaluated. If the particle sensor is fully loaded, the deposited particles are burned off in a regeneration phase using a heating element integrated into the particle sensor.

[0003] Such a resistive particle sensor is described in DE 101 33 384 A1. The particle sensor consists of two interlocking, comb-like electrodes, which are at least partially covered by a trap sleeve. If particles from a gas stream are deposited on the particle sensor, this leads to a measurable change in the impedance of the particle sensor, from which the amount of deposited particles and thus the amount of particles carried in the exhaust gas can be determined.

[0004] DE 101 49 333 A1 describes a sensor device for measuring the humidity of gases, comprising a resistance measuring structure arranged on a substrate, wherein the measuring structure interacts with a carbon black layer and a temperature measuring device is provided. This sensor device can also be used to determine the carbon black concentration in the exhaust gas of an internal combustion engine.

[0005] From DE 10 2004 028 997 A1, a method for controlling particle deposition on a sensor element is known. The sensor element has a first electrode and a second electrode, and a first voltage U1 and a second voltage U2 can be applied to voltage terminals. The method provides that the sensor element can be operated with an increased voltage U1 for a first time interval t1 and, after exceeding a trigger threshold AP of the sensor element, can be operated with a lower voltage U2, which is less than the increased voltage U1. This method makes it possible to shorten the time after regeneration of the sensor element, during which no measurement signal is available, until a usable signal is obtained through the deposition of a sufficient quantity of particles, by operating the sensor element with an increased operating voltage during this phase.The increased operating voltage leads to an increased rate of particle deposition on the sensor element. Once a sufficiently large quantity of particles has accumulated on the sensor element to produce a usable measurement signal, the sensor element is operated at a lower voltage, resulting in a correspondingly lower particle deposition rate. This extends the measurement time until the next necessary regeneration of the sensor element. The procedure therefore comprises two successive operating phases: a first phase with an increased operating voltage, during which a sufficient measurement signal is not yet available, and a second phase with a reduced voltage, during which the actual measurement of the particle concentration takes place. During both phases, the resistance or impedance of the sensor element is determined by measuring the current: once to detect the trigger threshold and once to determine the particle deposition rate.In both phases, a defined particle deposition is necessary. The selected voltages therefore represent a compromise in both phases between optimized particle deposition and precise resistance or impedance measurement.

[0006] From DE 103 19 664 A1, a sensor for detecting particles in a gas stream, in particular soot particles in an exhaust gas stream, is known. The sensor has at least two measuring electrodes arranged on a substrate made of an electrically insulating material. The measuring electrodes are coated with a protective layer. This protective layer protects the electrodes from corrosion at harsh ambient temperatures. The protective layer can be electrically conductive or an electrical insulator. A conductive protective layer allows the particle concentration to be determined by resistive direct current measurement, resulting in a parallel connection between the electrodes via the protective layer and the adhering particles. With an insulating protective layer, an impedance measurement using an alternating voltage is required.

[0007] To regenerate the particle sensor after particle accumulation, the sensor element must be cleaned using an integrated heating element. This must be done at specific intervals to prevent inaccuracies in particle concentration measurements.

[0008] For self-diagnosis, for example, in another application of the applicant (DE 10 2007 046 096 A1) it is provided that the particle sensor has an additional flat test electrode, and in several process steps different test voltages are applied between the measuring electrodes and the test electrode and a current or a capacitance is measured in each case and a conclusion is drawn on the determined values ​​as to whether the particle sensor is functioning properly.

[0009] EP 1 925 926 A1 describes a device and an evaluation method for verifying the functionality or plausibility of a sensor based on an electrode system, in particular a particle sensor, wherein the device comprises at least one reference electrode system. The evaluation method involves comparative measurements on the measuring electrodes and on the reference electrodes, whereby the proper functioning of the sensor is determined based on the values ​​obtained.

[0010] Since, in an on-board diagnostics application, the particle sensor is located downstream of the particulate filter in the exhaust stream, there should be no particles, especially soot particles, in the exhaust gas at the sensor's location when the particulate filter is fully functional, as these could otherwise generate a sensor signal. However, the absence of a sensor signal can also indicate a defective particle sensor, meaning that a potentially faulty particulate filter might not be detected.

[0011] DE 10 2006 053 100 A1 relates to a method for determining the temperature prevailing at a resistive particle sensor. It also proposes performing a self-diagnosis of the resistive particle sensor, in which the intrinsic conductivity of insulating layers below the electrode system of the resistive particle sensor is determined during a regeneration phase of the resistive particle sensor.

[0012] German patent application DE 10 2007 046 097 A1 relates to a sensor element for detecting particles in a gas stream. It comprises an interdigital electrode system with two interdigital electrodes connected to each other or to a test element by means of a semiconducting or low-conductivity material. DE 10 2007 046 097 A1 also proposes a method for the self-diagnosis of this sensor element.

[0013] DE 41 33 426 A1 discloses a circuit arrangement used for evaluating and testing a capacitive sensor. Two switches allow selective switching between a measurement mode and a test mode.

[0014] DE 10 2004 036 388 A1 proposes to use the conductivity between the electrodes of a resistive particle sensor during heating for an assessment of the condition of the particle sensor.

[0015] US patent 2002 / 0118027A1 describes a nanostructured ceramic platform for gas sensors that enables high sensitivity and robustness through dense nanopores. This platform is suitable for chemical and physical sensors as well as sensor arrays that remain stable even under harsh conditions.

[0016] It is therefore an object of the invention to provide a method which allows reliable self-diagnosis of the particle filter, particularly in this installation situation.

[0017] It is also an object of the invention to provide a device suitable for carrying out the method. Disclosure of the invention

[0018] The problem relating to the method is solved by the features of claims 1 and 2.

[0019] The method according to the invention provides that a semiconducting layer is formed in the insulating layer directly below the electrodes by means of external or self-doping, and that a measuring voltage is applied between the electrodes and a self-diagnostic current is measured at least temporarily for the purpose of self-diagnosis.

[0020] This approach has the advantage of allowing for a diagnostic check of the particle sensor's measuring electrodes. Such self-diagnosis makes it possible to identify defective particle sensors whose electrodes were damaged during manufacturing or over the sensor's lifetime. The error can then be compensated for by correcting the measured particle signal, or, in the event of a complete sensor failure, the driver / operator of the vehicle / system can be notified via visual and / or audible warnings. This is particularly advantageous for particle sensors installed downstream of a particulate filter (in the direction of exhaust gas flow), where it is unclear whether the particle sensor is functioning correctly and the exhaust gas contains little to no soot, or whether the particle sensor is faulty and the exhaust gas is currently heavily contaminated with soot.Advantageously, the functional test of the sensor according to the invention is carried out both during the final inspection before delivery to the customer and during operation “soot-free”, i.e. in an operating phase of the vehicle / system in which no soot particles are released, which could possibly distort the measured values.

[0021] The particle sensor has a semiconducting layer directly beneath the electrodes within an insulating carrier layer. This layer is created by self-doping and / or external doping of the insulating layer with sodium ions, which are present in concentrations ranging from 100 ppm to 10,000 ppm within the insulating layer. This doping allows for the generation of a certain conductivity in the semiconducting layer, particularly at high temperatures, by increasing the mobility of the ions. The temperature-dependent resistance of the semiconducting layer can be controlled by adjusting the concentration and selection of the ions.

[0022] Layers produced in this way exhibit electrical resistances in the range of > 100 MΩ between the electrodes during measurement operation (soot collection) up to approximately 400°C. If this layer is heated for diagnostic purposes, for example to approximately 850°C, the resistance drops to 0.2 to 1 MΩ, which corresponds to a current of approximately 10 to 50 µA at a measurement voltage of 10 V DC. This current can be used as a measure of the particle sensor's accuracy.

[0023] In one embodiment, the doping of the insulating carrier layer to create the semiconducting layer beneath the electrodes is achieved through sodium impurities from the electrodes applied during the manufacturing process. In this case, this rather undesirable effect is deliberately used for doping. An additional doping process is thus unnecessary.

[0024] In another embodiment, the self-doping of the insulating carrier layer to create the semiconducting layer beneath the electrodes is achieved by the targeted introduction of sodium ions and / or other highly mobile ions into the insulating carrier layer. While this is more complex than the first embodiment, it offers advantages with regard to a defined doping profile.

[0025] To isolate the semiconducting layer of the particle sensor from below within the layer structure, an insulating layer of pure aluminum oxide is introduced beneath the semiconducting layer according to the invention. This achieves an effective separation from functional layers located deeper within the particle sensor structure, which may also possess semiconducting properties.

[0026] In addition, an extra layer of barium-doped aluminum oxide can be introduced under the insulating layer of pure aluminum oxide, thus forming a particularly effective barrier layer, especially for the easily mobile sodium ions.

[0027] According to the invention, a DC voltage is applied between the electrodes for self-diagnosis as a measuring voltage, and the particle sensor is heated to temperatures > 500°C by means of the heating element before being connected to the DC voltage. According to the invention, the particle sensor is heated to these temperatures for at least 30 seconds, typically for about one minute.

[0028] Applying a DC voltage, which can be particularly advantageous in vehicles, typically results in a polarization effect, causing the conductivity of the semiconducting layer to decrease steadily. Heating can regenerate this conductivity, as this process redistributes the mobile ions evenly within the semiconducting layer, thus neutralizing the polarization. Additionally, a recharge pulse of 1 to 1000 ms duration at approximately -10 V DC can be applied to achieve "active" regeneration. In this method, the maximum value of the measured self-diagnostic current, obtained immediately upon application of the measurement voltage, can be used as a measure of the particle sensor's performance with its electrodes.

[0029] According to the invention, it is further provided that the electrodes are only subjected to a DC voltage for a short time in the range of 1 ms to 100 ms. This offers the advantage that the semiconducting layer is only slightly polarized during these short time intervals, and therefore several measurement cycles (for example, up to 1000 cycles) are possible over longer periods before regeneration is necessary.

[0030] A preferred application of the method variants described above involves the regeneration of the particle sensor as part of on-board diagnostics in a diesel engine. In this application, the precise and reproducible diagnosis of the particle load in a diesel particulate filter (DPF) located in the exhaust system of the diesel engine is particularly important.

[0031] The invention will be explained in more detail below with reference to an embodiment illustrated in the figures. The figures show: Fig. 1. In a schematic representation, the technical environment in which the method can be applied, Fig. 2 schematically a particle sensor in an exploded view and Fig. 3 a measurement diagram for the self-diagnosis of the particle sensor.

[0032] Fig. Figure 1 schematically shows the technical environment in which the method according to the invention can be applied. An internal combustion engine 10, which can be a diesel engine, receives combustion air via an air supply 11. The amount of combustion air can be determined by means of an air mass meter 12 in the air supply 11. This air quantity can be used to correct for the probability of particles adhering to the exhaust gas of the internal combustion engine 10. The exhaust gas of the internal combustion engine 10 is discharged via an exhaust system 17, in which an exhaust gas purification system 16 is arranged. This exhaust gas purification system 16 can be designed as a diesel particulate filter. Furthermore, an exhaust gas sensor 15, designed as a lambda sensor, and a particle sensor 20 are arranged in the exhaust system 17, the signals of which are supplied to an engine control unit 14.The engine control unit 14 is still connected to the air mass meter 12 and determines, based on the data supplied to it, a quantity of fuel which can be supplied to the internal combustion engine 10 via a fuel metering unit 13.

[0033] The particle sensor 20 can also be arranged downstream of the exhaust gas purification system 16, which offers advantages in terms of homogenizing the exhaust gas flow at this point and is particularly relevant when used for on-board diagnostics. The devices shown enable monitoring of the particle emissions from the internal combustion engine 10 and prediction of the fouling level of the exhaust gas purification system 16, which is designed as a diesel particulate filter (DPF).

[0034] Fig. Figure 2 shows a schematic representation of a particle sensor 20 in an exploded view according to the state of the art.

[0035] A first electrode 22 and a second electrode 23 are applied to insulating carrier layers 21, for example made of aluminum oxide. The electrodes 22, 23 are designed in the form of two interdigitating, interlocking comb electrodes. A first connection 24 and a second connection 25 are provided at the end faces of the electrodes 22, 23, via which the electrodes 22, 23 can be connected to a sensor control unit (not shown) for power supply and measurement.

[0036] In addition, in the example shown, a heating element 26 is integrated between the insulating carrier layers 21, which is connected to the sensor control unit via additional connections.

[0037] If such a particle sensor 20 is operated in a gas stream containing particles 29, for example in the exhaust gas duct of a diesel engine or a combustion plant, particles 29 from the gas stream are deposited on the particle sensor 20. In the case of the diesel engine, the particles 29 are soot particles with a corresponding electrical conductivity. The deposition rate of the particles 29 on the particle sensor 20 depends, among other things, on the particle concentration in the exhaust gas and also on the voltage applied to the electrodes 22, 23. The applied voltage generates an electric field that exerts a corresponding attraction on electrically charged particles 29 and on particles 29 with a dipole charge. Therefore, the deposition rate of the particles 29 can be influenced by appropriately selecting the voltage applied to the electrodes 22, 23.

[0038] In this embodiment, the electrodes 22, 23 and the uppermost insulating carrier layer 21, on which the electrodes 22, 23 are located, are coated with a protective layer 27. This optional protective layer 27 protects the electrodes 22, 23 from corrosion at the typically high operating temperatures of the particle sensor 20. In this embodiment, it is made of a material with low conductivity, but it can also be made of an insulator.

[0039] Particles 29 from the gas stream have deposited on the protective layer 27 in the form of a layer. Due to the low conductivity of the protective layer 27, the particles 29 form a conductive path between the electrodes 22, 23, resulting in a change in resistance between the electrodes 22, 23, depending on the amount of particles 29 deposited. This change can be measured, for example, by applying a constant voltage to the terminals 24, 25 of the electrodes 22, 23 and determining the change in current due to the deposited particles 29.

[0040] If the protective layer 27 is constructed as an insulator, the deposited particles 29 lead to a change in the impedance of the particle sensor 20, which can be evaluated by a corresponding measurement, preferably with an alternating voltage.

[0041] If the particle sensor 20 is so covered with a layer of particles 29 that additionally deposited particles 29 do not lead to any further change in the resistance or impedance of the particle sensor 20, the particle sensor 20 is regenerated within a regeneration phase. For this purpose, the particle sensor 20 is heated with the aid of the heating element 26 until the adhering particles 29 are burned off. In an initial phase after regeneration, when only a few particles 29 are present on the particle sensor 20, no meaningful resistance or impedance measurement is possible. Only after a sufficiently long time are enough particles 29 present on the particle sensor 20 again to form a closed current path between the electrodes 22, 23 via the particles 29, thus enabling a measurement.Known evaluation methods determine the time after regeneration until a predefined threshold of the measurement signal is reached, for example, a predefined current value, in order to determine the particle concentration in the gas stream. Alternative methods use the signal change rate after reaching a minimum signal to determine the particle concentration.

[0042] According to the invention, the uppermost insulating carrier layer 21, on which the electrodes 22, 23 are located, is additionally doped so that a semiconducting layer 28 is formed. This can be configured, for example, as follows: Option 1: Below the electrodes 22, 23 is the insulating carrier layer 21, which consists of pure aluminum oxide. This layer becomes contaminated by sodium impurities when the electrodes 22, 23, which are typically platinum measuring electrodes, are applied. This leads to the formation of the semiconducting layer 28 in the area of ​​the electrodes 22, 23. The electrodes 22, 23 can be applied particularly easily and cost-effectively using a screen printing process and then fixed to the insulating carrier layer 21 by a subsequent firing process. During this process, the sodium ions can diffuse into the insulating carrier layer 21 and form the semiconducting layer. Option 2: Below electrodes 22 and 23 is the insulating support layer 21, which consists of pure aluminum oxide and into which sodium ions are introduced by targeted doping. The concentration of sodium ions in the insulating support layer 21, within the doping zone that forms the semiconducting layer 28, ranges from 100 ppm to 10,000 ppm. Typically, this value is around 1,000 ppm. In principle, other readily mobile ions, such as lithium, can also be introduced as dopants. Option 3: As described in variant 2, the insulating carrier layer 21 is enriched with sodium ions by targeted doping, thus forming the semiconducting layer 28. To insulate the semiconducting layer 28 from other functional layers of the particle sensor 20 below, a further layer of aluminum oxide is introduced directly beneath the semiconducting layer 28 according to the invention. Additionally, a layer of barium-doped aluminum oxide can be introduced beneath this layer, which acts as a barrier layer, particularly for the more mobile sodium ions.

[0043] The method according to the invention provides that, for self-diagnosis, a measuring voltage is applied at least temporarily between the electrodes 22, 23 and a self-diagnosis current 31, which flows through the electrodes 22, 23 and the semiconducting layer 28, is measured, wherein the self-diagnosis current 31 is a measure of the functionality of the particle sensor 20 or of its quality.

[0044] For self-diagnosis, a DC voltage is applied between electrodes 22 and 23 as a measuring voltage. Due to polarization effects, the self-diagnostic current 31 decreases steadily after the measuring voltage is applied in this configuration. Therefore, the particle sensor 20 is heated to temperatures > 500°C, typically to approximately 850°C, using the heating element 26, either before or after the DC voltage is applied. This allows the semiconducting layer 28 to regenerate, enabling the sodium ions to redistribute themselves evenly within the semiconducting layer 28 and thus eliminating the polarization. This regeneration heating is typically performed for at least 30 seconds, usually for about 1 minute.This regeneration temperature can be monitored and controlled, for example, by means of a measuring meander (platinum resistor), which is usually integrated into the particle sensor 20 for temperature monitoring. Alternatively, the temperature-dependent resistance of the heating element 26 can be used for temperature monitoring or control.

[0045] The invention provides for the electrodes 22, 23 to be connected to a DC voltage, which is only applied to the electrodes 22, 23 for a short time, in the range of 1 ms to 100 ms, typically for 10 ms.

[0046] Fig. Figure 3 shows an example measurement diagram 30, in which the time course of the self-diagnostic current 31 is depicted. A current 32 is plotted against time 33, which flows through the electrodes 22, 23 and through the semiconducting layer 28 when the measuring voltage is applied.

[0047] In the example shown, a DC voltage of 10 V is applied as the measuring voltage. The self-diagnostic current 31 initially rises, depending on the doping of the semiconducting layer 28, to values ​​between 10 µA and 50 µA and then steadily decreases to zero within approximately 10 s due to polarization effects. The initial value of the self-diagnostic current 31 can be used as evidence of the functionality of the particle sensor 20.

Claims

[1] Method for the self-diagnosis of a particle sensor (20) for determining a particle content in a gas stream, wherein the particle sensor (20) has on its surface at least two interlocking interdigital electrodes (22, 23) and a heating element (26) separated from the electrodes (22, 23) by an insulating layer, with which the particle sensor (20) can be heated in a regeneration phase and soot loading on the particle sensor (20) can be removed, wherein a semiconducting layer (28) is formed in the insulating layer directly below the electrodes (22, 23) by means of external or self-doping, which consists of aluminium oxide with a sodium ion doping of 100 ppm to 10000 ppm and under which an insulating layer of pure aluminium oxide is introduced, and for self-diagnosis a measuring voltage is applied at least temporarily between the electrodes (22, 23) and a self-diagnostic current (31) is measured,wherein a DC voltage is applied between the electrodes (22, 23) for self-diagnosis as a measuring voltage and the particle sensor (20) is heated to temperatures > 500°C by means of the heating element (26) before being connected to the DC voltage, wherein the heating of the particle sensor (20) to temperatures > 500°C is carried out for at least 30 seconds and wherein the temperatures > 500°C are monitored and controlled by means of a platinum resistor integrated in the particle sensor (20) which has the form of a measuring meander, and wherein the connection of the electrodes (22, 23) to the DC voltage is carried out only for a short time in the range of 1 ms to 100 ms. [2] Method according to claim 1, characterized by , that an additional layer of barium-doped aluminum oxide is introduced beneath the insulating layer of pure aluminum oxide.

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