DE102006060636A1 - Method for balancing sensor element for probe to determine concentration of gas component in gas mixture, involves arranging pump cell with two pump electrodes on ion-conducting solid electrolyte - Google Patents

Abstract

The method involves arranging a pump cell with two pump electrodes (140,150) on an ion-conducting solid electrolyte (110). A gas mixture with oxygen air coefficient lambda value of 1 is pumped or evacuated in the hollow space (130) from the outer pump electrode. The resulting composition is derived between the pump flow (I-pump) and the nernst voltage (U-Nernst) that belongs to a gas mixture of lambda 1 for determining the diffusion resistance of the diffusion barrier (120). Independent claims are also included for the following: (1) a computer program (2) a computer program product.

Description

State of the art

The The invention is based on a method for calibrating a sensor element according to the preamble of claim 1.

object The present invention also relates to a computer program and a Computer program product suitable for carrying out the method are.

electrochemical Gas sensors in the form of lambda probes are in large numbers used in exhaust systems of internal combustion engines in motor vehicles, around for the engine control signals over the Be able to provide exhaust composition. This way you can The engine can be operated so that the exhaust gases have an optimal composition for the Aftertreatment with catalysts usually present in the exhaust system today exhibit.

In 1 a generic gas sensor known from the prior art is shown. The sensor element 100 has a gas entry hole 115 through which exhaust gas flows in and through a diffusion barrier 120 in a measuring room 130 arrives. A first electrode, also as outer electrode or outer pump electrode 150 is on the outside of the solid electrolyte 110 and under a porous protective layer 155 arranged exposed to the exhaust gas of an internal combustion engine (not shown). In the measuring space is a second electrode, also as an inner electrode or inner pumping electrode 140 designated arranged.

Between the inner pumping electrode 140 and the outer pumping electrode 150 a _pump voltage U _{pump is} applied so that a pump current I _pump flows. In the solid electrolyte 110 is also a in an insulating layer 162 embedded heating 160 arranged. Through this heater 160 the sensor element is heated to a temperature which is an optimal function of the sensor element 100 allowed.

The core element of such a broadband lambda probe is the measuring space, also referred to as a cavity 130 passing through the gas-permeable diffusion barrier 120 is limited. The amount of oxygen or fat gas diffusing through this barrier is measured either by means of the limiting current (single cell) or by means of the pumping current (double cell) necessary for controlling the void concentration to lambda = 1. The flowing measuring current is proportional to the oxygen or fat gas content in the exhaust gas. The proportionality factor is a function of the porosity and geometry of the diffusion barrier and thus its diffusion resistance.

Because of variations in printing technology, each such sensor has an individual proportionality factor. Nevertheless, to obtain the same characteristic for all probes, ie the same measuring current for a specific exhaust gas composition, is a balancing resistor 180 provided that the measuring resistor 170 is connected in parallel and is arranged in the housing of the probe. The setting of an adjustment resistor assigned to each probe 180 means a high procedural effort. In addition, this results in costs due to an additional required connection cable.

Of the proportionality an unbalanced probe is determined using so-called "thrust balance" in the vehicle. Here, the internal combustion engine is rinsed with air and thus at a known gas concentration proportionality factor determined and saved. This can compensate for aging effects be achieved, since the adjustment can be renewed as often as desired.

Disclosure of the invention

Advantages of the invention

The inventive method with the characteristics of the independent Claim 1 has the opposite the advantage of that for the adjustment of the sensor element no trim resistor required is. This is eliminated the additional high procedural effort, also arise no additional costs for the provision and installation of the balancing resistor and the associated connecting cable.

The basic idea of the invention is to carry out the adjustment process in the presence of a lambda = 1 gas in the exhaust gas, wherein the pumping current is controlled so that a defined Nernst voltage is established between the inner pumping electrode and another electrode exposed to a gas mixture of lambda = 1. The pumping current required by a defined Nernst voltage, that is defined gas concentration in the cavity 130 is a function of diffusion properties of the diffusion barrier.

By those in the dependent Claims listed measures are advantageous developments, refinements and improvements of the independent Claim specified method possible.

To the Nernstzel formed in this way It is advantageous to set the Nernst voltage by setting the pump current to values between 10 and 400 mV, in particular 100 mV or to values between -10 and -400 mV, in particular close to the jump region and thus sensitive enough to small changes in the oxygen concentration in the cavity -300 mV. By setting the Nernst voltage to these values, the probe is far enough away from the plateau of the Nernst voltage curve as a function of the pump current for this stationary measurement, so that the pump current can be set very precisely.

Conveniently, is the arranged on the solid electrolyte, a gas mixture of lambda = 1 exposed electrode the exhaust electrode itself. In another Embodiment, this electrode forms the reference electrode, the is arranged in a reference gas of lambda = 1.

Around the Nernst voltage as possible capture precisely to be able to provides an embodiment of the method, for the correction of himself Nernst voltage setting the voltage drop across the internal resistance the pump cell and the decomposition voltage of the water to be considered.

The Decomposition voltage of the water is known per se and is 1.2 V. The internal resistance of the pump cell can by applying two different constant pumping voltages between the outer pumping electrode and the inner pumping electrode and the detection of this adjusting streams according to the ohmic Law to be determined.

• – Einstellen eines konstanten Pumpstroms,
• – Öffnen des Stromkreises und
• – Erfassen des Spannungsverlaufs an der Pumpzelle zwischen äußerer und innerer Pumpelektrode mit sehr hoher Abtastrate.

In another advantageous embodiment of the method, it is provided to determine the internal resistance of the pump cell by the following steps:

• Setting a constant pumping current,
• - opening the circuit and
• - Detecting the voltage curve at the pumping cell between the outer and inner pumping electrode with a very high sampling rate.

the lies the consideration on the basis that the resulting discharge of the double layer itself in a relatively slow exponential decay, while the ohmic voltage drop immediately with the stopping of the pumping current disappears. The height this initial one rectangular voltage jump after opening the circuit is determined and evaluated.

at another turn of the method, which in the Basically a transient measurement is, the pump current is applied in a pulsed manner and the between the outer pumping electrode and Nernst voltage building up the inner pumping electrode in each case to one defined time after opening of the circuit measured and from this to the diffusion resistance closed.

at a further embodiment of the method is provided, the Time constant on decay of the Nernst voltage between outer pumping electrode and the inner pumping electrode after turning off the pumping current to determine and from this to conclude the diffusion resistance.

Short description of the drawing

embodiments The invention is illustrated in the drawings and in the following Description closer explained.

It demonstrate:

1 a sectional view of a known from the prior art sensor and

2 schematically a gas sensor in the form of a single cell, in which the inventive method is used.

Embodiments of the invention

One known from the prior art, in 1 illustrated gas sensor (protozoa) has a sensor element 100 on, by a trained in layered solid electrolyte 110 is formed. On the outside of the sensor element 100 is exposed to the exhaust gas a first electrode, also as an outer pumping electrode 150 designated, arranged by an open-pored protective layer 155 is covered. In the solid electrolyte 110 is a measurement volume 130 formed in which a second electrode, also as an inner pumping electrode 140 designated, is arranged. The exhaust gas of an internal combustion engine (not shown) flows through a gas inlet hole 115 via a diffusion barrier 120 into the measuring volume 130 , By an electronic circuit, not shown, between the outer pumping electrode 150 and in the measurement volume 130 arranged inner pumping electrode 140 generates a constant pumping voltage U _pump , so that a _pumping current I _pump flows. The in the cavity or in the measuring volume 130 through the diffusion barrier 120 diffusing oxygen or the fat gas is measured on the basis of the limiting current or in a so-called Doppelzeller (not shown), in which a reference volume with a reference electrode disposed therein are additionally determined on the basis of the regulation of the cavity concentration to lambda = 1 pumping current. The measuring current I _pump is proportional to the oxygen or the fat gas content in the exhaust gas. The proportionality factor in turn represents a function of the porosity and geometry of the diffusion barrier ere or short of the diffusion resistance.

Since such probes each have an individual proportionality factor due to manufacturing-related scattering, an adjustment must be provided for these probes in order to obtain the same characteristic, ie the same measuring current, for a particular exhaust gas composition for different probes. Such an adjustment can, for example, in a known per se by the in 1 shown, parallel to a measuring resistor R _M 170 switched and with this a current divider forming balancing resistor R _Abg 180 be made. The adjustment takes place in that the balancing resistor R _Abg 180 is set, which, however, is associated with a great effort. In addition, in each case a balancing resistor R _Abg and another supply line is required, which not only increases the manufacturing cost, but also the cost.

The method according to the invention described below makes such a balancing resistor and the associated disadvantages superfluous. The basic idea of the invention is to determine the proportionality factor of an unbalanced gas sensor in that in the cavity 130 by regulating a pumping current between the inner pumping electrode 140 and outer pumping electrode 150 a defined gas concentration is generated. This is determined by the resulting Nernst voltage between the inner pumping electrode 140 and either the outer pumping electrode 150 in the case of a single cell or in the case of a two-cell cell, in which a reference electrode is arranged in the solid electrolyte, between the inner pumping electrode 140 and the reference electrode measured. In the case of the two-cell, it is also possible, the resulting Nernst voltage also between the inner pumping electrode 140 and the outer pumping electrode 150 to eat. A prerequisite for a usable voltage signal is that the outer pumping electrode 150 or the reference electrode is in a lambda = 1 - gas atmosphere. This comparison also has the decisive advantage that it can be repeated as often as desired and, to that extent, is not performed once only, as is the case when using a balancing resistor 180 the case is.

• a) Mithilfe des nicht abgeglichenen Gassensors oder mittels einer anderen Sonde, beispielsweise einer Hinter-Kat-Sonde, wird das Abgas auf ein festes Lambda, vorzugsweise Lambda = 1 eingeregelt bzw. der Moment detektiert, in dem dies der Fall ist;
• b) vom Abgas wird von der äußeren Pumpelektrode 150 zur inneren Pumpelektrode 140 Sauerstoff in den Hohlraum 130 gepumpt, der aus der Zersetzung von Wasser stammt;
• c) im Hohlraum reichert sich dieser Sauerstoff an;
• d) bei genügend hoher Sauerstoffkonzentration entsteht zwischen der inneren Pumpelektrode 140 im Hohlraum 130 und der abgasseitigen äußeren Pumpelektrode 150 eine Nernstspannung U_Nernst, die gemessen wird;
• e) wie groß der für eine vorgebbare definierte Nernstspannung U_Nernst benötigte Pumpstrom I_pump ist, hängt davon ab, wie viel Sauerstoff durch die Diffusionsbarriere 120 aus dem Hohlraum 130 ins Abgas diffundiert. Anhand des Betrages dieses Pumpstroms I_pump, der ebenfalls gemessen wird, kann der Proportionalitätsfaktor und damit der Diffusionswiderstand der Diffusionsbarriere ermittelt werden.

The adjustment is made by the following steps:

• a) With the aid of the unbalanced gas sensor or by means of another probe, for example a rear-cat probe, the exhaust gas is adjusted to a fixed lambda, preferably lambda = 1 or detects the moment in which it is the case;
• b) from the exhaust gas is from the outer pumping electrode 150 to the inner pumping electrode 140 Oxygen in the cavity 130 pumped, which comes from the decomposition of water;
• c) this oxygen accumulates in the cavity;
• d) at sufficiently high oxygen concentration arises between the inner pumping electrode 140 in the cavity 130 and the exhaust-side outer pumping electrode 150 a _Nernst voltage U _Nernst , which is measured;
• e) How large is the pumping current I _pump required for a predefinable defined _Nernst voltage U _Nernst depends on how much oxygen through the diffusion barrier 120 from the cavity 130 diffused into the exhaust. On the basis of the amount of _pumping current I _pump , which is also measured, the proportionality factor and thus the diffusion resistance of the diffusion barrier can be determined.

Preferably, the _Nernst voltage U _Nernst is adjusted to values between 10 mV and 400 mV. In this case, the Nernst cell thus formed is close to the jump region and sensitive enough to small changes in the oxygen concentration in the cavity 130 , In principle, it is also possible to adjust the Nernst voltage to 450 mV. In this case, the _pumping current I _{pump is} exactly equal to the normal pumping current in air with lambda = 1 in the cavity. In this so-called plateau region of the probe, however, the _Nernst voltage U _Nernst reacts only relatively weakly to small changes in the _pumping current I _pump .

This Matching procedure has compared to Thrust adjustment has the advantage that it is carried out in normal driving can. However, the condition of the internal combustion engine must be for a specifiable Time must be constantly adjustable in the millisecond range, so that the Pumping can proceed. This is preferably at idle Motors possible.

In addition to the mentioned range between 10 and 410 mV, the Nernst voltage can also be regulated between -10 and -400 mV, in particular adjusted to -300 mV. This oxygen is from the cavity 130 pumped out. Since this comes from the decomposition of water, hydrogen remains in the cavity 130 back. Its concentration now determines the Nernst voltage and depends on how much hydrogen can escape through the diffusion barrier from the cavity.

Also in this case the probe is far enough away from the plateau and allows a very precise Adjustment of pumping current.

In one embodiment of the method is provided to correct the Nernst voltage the voltage drop across the internal resistance of the pump cell and the decomposition voltage of the water to be into account.

The Decomposition voltage of the water is approximately equal to the voltage, the necessary is to generate a pump current up to 0.1 mA.

The internal resistance can be determined in two ways. On the one hand, he can by applying two different sized constant pumping voltages between the outer pumping electrode 150 and the inner pumping electrode 140 and the thereby adjusting pump currents are determined. If these pumping currents are sufficiently large, the internal resistance of the pumping cell can be determined by means of Ohm's law.

at another, so-called current interrupt method is provided, after setting a constant pump current to open the circuit and the voltage curve at the pumping cell with a very high sampling rate to eat. The after opening the circuit undergoes discharge of the double layer in a relatively slow exponential decay, while the ohmic voltage drop immediately after drop of the pump current disappears. The height the initially rectangular Voltage jump after opening the Circuit is determined and closed on the internal resistance.

In addition to the above-mentioned stationary measurement, in which a constant pumping current is adjusted, a transient measurement is possible in which the pumping current is applied in a pulse-like manner. The Nernst voltage between the outer pumping electrode 150 and the inner pumping electrode 140 is measured in the blanking phases after the decay of the polarization and the elimination of the ohmic voltage drop. For this purpose, the voltage is measured at a predeterminable time after opening the electric circuit.

Yet another embodiment of the method provides for the measurement of the cooldown. In this case, the time constant on decay of the _Nernst voltage U _Nernst between the outer pumping electrode 150 and the inner pumping electrode 140 determined after switching off the pump voltage.

In a gas sensor with a reference electrode, which is arranged in a reference gas space (not shown), it is also possible to proceed in such a way that a constant value of lambda = 1 is set in the exhaust gas. After this value is set, a large, preferably 3 to 5 mA amount of oxygen flow (= pumping current of the nominal characteristic in air) in the cavity 130 pumped and thus an oxygen-rich atmosphere in the cavity 130 created. Then, from the reference channel oxygen in the cavity 130 pumped out until a lambda = 1 gas is present in the reference channel. In this case, a reference pump current is controlled so that between the reference electrode and the inner pumping electrode 140 a Nernst voltage of 450 mV can be tapped. This reference pump current is maintained. Then, the pumping current between the outer pumping electrode 150 and the inner pumping electrode 140 reduced, until only so much oxygen in the cavity 130 exists that the voltage between the reference electrode and the inner pumping electrode 140 preferably 100 or -100 mV. This method has the advantage over that described above that no correction of the Nernst voltage is required. Again, the diffusion resistance can be determined.

The The method described above can be used, for example, as a computer program on a computing device, in particular a control unit of a Internal combustion engine can be implemented and run there. The program code may be stored on a machine-readable medium that the controller reads can.

Claims (11)

Method for adjusting a sensor element for a probe for determining the concentration of a gas component in a gas mixture, in particular for a broadband planar lambda probe for determining the oxygen concentration in the exhaust gas of internal combustion engines, comprising a pump cell with two on an ion-conducting solid electrolyte ( 110 ) arranged pumping electrodes ( 140 . 150 ), of which an outer pumping electrode ( 150 ) exposable to the gas mixture and an inner pumping electrode ( 150 ) through a porous diffusion barrier ( 120 ) separated from the gas mixture and in a cavity ( 130 ) is arranged, characterized in that in a gas mixture with the air ratio lambda = 1 oxygen from the outer pumping electrode ( 150 ) in the cavity ( 130 ) is pumped or pumped out of it and the resulting relationship between the _pumping current (I _pump ) and between the inner pumping electrode ( 140 ) and one on the solid electrolyte ( 120 _Nernst voltage (U _Nernst ) arranged to measure the diffusion resistance of the diffusion barrier ( 120 ) is used. Method according to claim 1, characterized in that that the Nernst voltage by adjusting the pumping current to values between 10 and 400 mV, in particular 100 mV or to values between -10 and -400 mV, especially -100 mV is regulated. Method according to claim 1 or 2, characterized in that the particles attached to the solid electrolyte ( 120 ), a gas mixture of lambda = 1 exposed electrode, the outer pumping electrode ( 150 ) or a reference electrode arranged in lambda = 1 reference electrode. Method according to one of the preceding claims, characterized in that the voltage drop at the internal resistance of the pump cell and the decomposition voltage of the water are taken into account for the correction of the self-adjusting _Nernst voltage (U _Nernst ). A method according to claim 4, characterized in that the internal resistance of the pump cell by applying two different constant pumping voltages (I _pump ) between the outer pumping electrode ( 150 ) and the inner pumping electrode ( 140 ) and detection of the self-adjusting currents (I _pump ) is determined. A method according to claim 4, characterized in that the decomposition voltage of water on the basis of the pumping voltage (U _pump ) is determined, which is required for a lambda = 1 - exhaust gas atmosphere, a _pumping current (I _pump ) of preferably 0.1 mA produce. Method according to claim 4, characterized in that the internal resistance of the pump cell is determined by the following steps becomes: - To adjust a constant pumping current, - opening the circuit and - To capture the voltage curve at the pumping cell between the outer and inner pumping electrode with a very high sampling rate. A method according to claim 1, characterized in that the pumping current is applied in a pulse-like manner and that between the outer pumping electrode ( 150 ) and the inner pumping electrode ( 140 ) _Nernst voltage (U _Nernst ) each measured at a defined time after opening the circuit and from this on the diffusion resistance is closed. Method according to Claim 1, characterized in that the time constant for the decay of the Nernst voltage (U _Nernst ) between the outer pumping electrode ( 150 ) and the inner pumping electrode ( 140 ) is determined after switching off the pump current (I _pump ) and from this it is concluded that the diffusion resistance. Computer program that shows all the steps of a procedure according to one of the claims 1 to 9, if it's on a computing device expires. Computer program product with program code based on a machine readable carrier is stored to carry The method according to one of claims 1 to 9, when the program is up running a computer or a controller.