DE102008004015B4 - Method for detecting contact resistances in leads of a probe - Google Patents

Abstract

Method for detecting a contact resistance on a plug-in connection in a line of a probe, comprising the steps:
a) heating the probe with a first heater voltage (step S1);
b) detecting and storing at least a first value of a parameter (Vh_high_T) at a first heater voltage (step S2),
c) lowering the first heater voltage to a second lower heater voltage (step S3),
d) detecting and storing a second value of the parameter (Vh_low_T) at the second lower heater voltage (step S4),
e) determining the contact resistance on the basis of an assignment of a difference of the two stored values of the parameter (Vh_low_T, Vh_high_T) to a contact resistance by means of a previously stored map (step S5), wherein the parameter is a probe temperature or an internal resistance of the probe.

Description

The invention relates to a method for detecting contact resistances in lines of a probe, for example a lambda probe.

From the DE 10 2007 034 251 A1 , of the DE 10 2006 012 461 A1 , of the DE 100 29 795 C2 , of the WO 2007/074021 A1 and the DE 38 36 045 A1 Various lambda probes for determining the concentration of a gas component in a gas mixture are known. Out DE 4227812 A1 a method for controlling the heating power of a continuous annealing plant for metallic strand is known.

A linear lambda probe typically has six electrical connections. Through these connections, the linear lambda probe is connected to the engine control unit (ECU). The electrical cables between lambda probe and ECU contain at least one plug connection. Especially in plug-in connections, contact problems may cause electrical contact resistances to form in the individual lines, for example due to moisture penetrating into the plug-in connections.

High electrical contact resistance in the lines can affect the function of the lambda probe. In order to ensure the proper functioning of the lambda probe, it is in principle desirable to compensate for possible effects of contact resistances by suitable measures and / or to monitor them by suitable diagnoses.

In an example of possible effects of contact resistance on the lines VN, VG, d. H. Lines which are connected to a Nernst cell, a lambda probe to a line VN (Nernst cell) and a line VG (virtual ground) is connected. In the case of a contact resistance in the respective line, the following effects may occur. The contact resistance can lead to errors in internal resistance measurements and as a result to an increased operating temperature of the probe. The increased operating temperature, in turn, poses a risk, since it can lead to a probe damage and also to a signal inaccuracy at lambda less than or greater than 1 because of a cross-sensitivity to the probe temperature.

Currently, the following methods are in use to compensate for the effects described.

A resistance in the line VN or VG generally leads to a falsification of the internal resistance measurement (Ri measurement). As a result, some types of probes (with a strong heating element) are at risk of overheating the probe. There are currently no known measures to compensate for this adulteration.

With the existing plausibility diagnosis of the probe temperature only very large contact resistance can be detected, which lead to such a strong falsification of the temperature measurement, that the falsified measured temperature is no longer in the vicinity of the required operating temperature even at maximum heat output. However, the actual probe temperature in this case is already well above the operating temperature and there is a risk of damage to the probe. A diagnosis that is able to detect smaller contact resistances (which just do not lead to overheating) is currently unknown.

The inaccuracy due to a contact resistance for lambda greater than or less than 1 can be compensated by adjusting the probe signal during coasting (gain adaptation). Plausibility diagnostics for monitoring the signal accuracy are available, for example by diagnoses during overrun fuel cutoff.

The technical explanation for the risk of overheating in contact resistances in the lines VN and / or VG is the following.

The internal resistance of the lambda probe ceramic is temperature-dependent. A high temperature means low resistance and vice versa. The internal resistance of the probe is measured via the Nernst cell. The contacts of the Nernst cell are VN and VG. From the measured resistance is closed back to the temperature of the probe. A controller (heating controller) sets the introduced heating power so that the temperature or the resistance corresponds to a specific setpoint or operating temperature, eg. B. 830 ° C or 75 ohms for a typical sensor.

If there is a contact resistance in the line VN or VG, this resistance is inevitably also measured. However, a distinction as to what proportion of the measured resistance is caused by the probe and which proportion by the contact resistance is not possible. If a contact resistance exists, the probe is heated by the heating controller to a lower resistance than would be the case without the contact resistance. A lower resistance of the probe means a higher temperature. If the contact resistance is not correctly determined and, for example, an excessively low contact resistance is determined to be present, there is a risk that the probe will be overheated.

In the following, an exemplary calculation based on the special specification for a typical sensor shows how a probe is heated when a contact resistance is present.

For example, the setpoint of the example probe is 830 ° C. This corresponds to 75 ohms (nominal probe). Furthermore, a transfer resistance of 25 ohms is present in the line VN. In this case, the heating controller adjusts the heating power so that the resistance of the probe (75 ohms-25 ohms) = 50 ohms. 50 ohms correspond to about 900 ° C (nominal probe). The probe is thus operated 70 ° C above the setpoint.

In another example, the measurement is shown in the vehicle with a typical probe, the probe having a contact resistance in the line VN. In the following table, a comparison of measured and actual probe temperature of the probe is shown. Contact resistance in line VN System temperature (controlled variable) Applied heating power (set by controller) Actual probe temperature (thermocouple) 0 ohms 830 ° C 10.26 V 840 ° C 10 ohms 830 ° C 10.77 v 870 ° C 20 ohms 830 ° C 11:17 v 895 ° C 30 ohms 830 ° C 11.88 v 945 ° C

The maximum permitted operating temperature for this example probe is 1000 ° C. The resistance of a nominal probe at 1000 ° C is about 30 ohms. In the present case, this means that overheating from a contact resistance of 45 ohms on a nominal probe is imminent.

However, the correlation between the internal resistance (Ri) and the temperature (T _tip ) of the probe is tolerant. A limit probe (with respect to Ri-T _tip correlation) can have a temperature of 950 ° C (nominal 830 ° C) with an internal resistance of 75 ohms. In the case of a boundary probe, even minor transitions lead to overheating. The exact value of the critical contact resistance is currently unknown.

In conclusion, it can be stated that contact resistances in the lines of a linear lambda probe can lead to undesired effects. An example of this is the possible overheating of the probe or the sensor, which can be caused by contact resistances, for example in the lines VN and VG, as described above. There is currently no solution to this problem.

The object of the invention is therefore to provide an improved method for detecting contact resistance in the lines of a probe, such as a linear lambda probe.

According to the invention this object is achieved according to claim 1 and claim 2.

Such a method has the advantage that a contact resistance can be better determined, since the observation is used that a contact resistance in a line leads to an increased Heizleistungsbedarf and the amount of this increase is highly dependent on the measured probe temperature. Furthermore, the greater the contact resistance, the greater the difference in the required heating power between the different temperatures.

According to the invention the object is also achieved according to claim 4.

The method has the advantage that a contact resistance can be determined easily and quickly. In this case, the observation is used that a contact resistance in the line of the probe in comparison to a line of a probe without contact resistance has an increased heating power requirement to achieve a certain (falsified) measured probe temperature. If, therefore, the heating power demand exceeds the heating power requirement for a line of a probe without a contact resistance by a predetermined amount, it can be concluded that there is a contact resistance.

Advantageous embodiments and modifications of the invention will become apparent from the dependent claims and the description with reference to the drawings.

In a further embodiment according to the invention, the two methods according to the invention are carried out in a predetermined operating state of the vehicle, for example in an idling phase or fuel cut-off phase of the vehicle. This has the advantage that in such an operating state relatively well-defined conditions are given.

In a further embodiment according to the invention, the parameters which are influenced by the heater voltage or the probe temperature and allow a conclusion about the contact resistance are recorded, for example, in a predetermined period of time, and the mean value is formed therefrom. In this way, the parameter can be determined more accurately, with fluctuations being less significant than if the parameter is recorded and stored only once, which is basically possible.

The invention will now be described with reference to the accompanying drawings. It shows:

1 a flowchart of an embodiment of the invention.

2 a diagram in which the temperatures of lambda probes available in the engine control at different heat output and different contact resistance in the line VN are shown; and

The invention is based on measurements carried out under the following conditions. These conditions are purely exemplary in order to explain the principle of the invention. The invention is not limited to these conditions.

• – einem Testfahrzeug (Benzinmotor, 4 Zylinder Reihenmotor, 2,4 l Hubraum)
• – Linearen Lambdasonden, wobei alle Sonden mit einem Thermoelement versehen sind
• a) nominale Sonde bezüglich Ri-T_tip Korrelation (d. h. die Sonde weist bei 75 Ohm eine Temperatur von ca. 830°C auf)
• b) Sonde nahe minimaler Grenze* bezüglich Ri-T_tip Korrelation (d. h. die Sonde ist bei 75 Ohm kälter als 830°C)
• c) Sonde nahe maximaler Grenze bezüglich Ri-T_tip Korrelation (d. h. die Sonde ist bei 75 Ohm heißer als 830°C)
• – eine Breakoutbox und eine Widerstandsdekade zum Einbringen von Übergangswiderständen in die Sondenleitung
• – Erfassung von ECU-Größen mit einem System, das beispielsweise zur Visualisierung von Variablen in der ECU dient
• – ein Dual-Scan zum Auslesen der Thermoelemente (für die tatsächliche Sondentemperatur)
• * Für den Sensor nahe minimaler Grenzlage wurde zusätzlich noch ein Übergangswiderstand von 0,5 Ohm in der Leitung Vh-eingebracht.

The measuring equipment initially consists of the following components:

• - a test vehicle (gasoline engine, 4 cylinder inline engine, 2.4 l displacement)
• - Linear lambda probes with all probes fitted with a thermocouple
• a) nominal probe with respect to Ri-T _tip correlation (ie the probe has a temperature of about 830 ° C at 75 ohms)
• b) probe near minimum limit * with respect to Ri-T _tip correlation (ie the probe is colder than 830 ° C at 75 ohms)
• c) probe near maximum limit with respect to Ri-T _tip correlation (ie the probe is hotter at 75 ohms than 830 ° C)
• A breakout box and a resistor decade for introducing contact resistances into the probe lead
• - Acquire ECU variables with a system that is used, for example, to visualize variables in the ECU
• - a dual-scan to read the thermocouples (for the actual probe temperature)
• * For the sensor near minimum limit position, a contact resistance of 0.5 ohms in the line Vh-was additionally introduced.

The basic procedure consists of the following steps:
First, a certain resistance is introduced, for example, in the line VN, z. B. a resistance (contact resistance) of 0 ohms, 10 ohms, 20 ohms and 30 ohms. Subsequently, the vehicle is operated at idle while the engine is heated. The probe temperature is adjusted by the heater controller to a setpoint or an operating temperature. Subsequently, the necessary heater power is determined. The heater controller is then deactivated and switched to manual specification, for example via an application system. Thereafter, we gradually reduce the heater power until a system temperature of, for example, near 660 ° C is reached, or, for example, a deactivation threshold for the lambda sensor. This procedure is performed for the nominal sensor.

For the two borderline sensors, only measurements with adjusted state (temperature at setpoint 830 ° C) and 0 Ohm contact resistance are present.

As in 2 is shown, the Y axis denotes the probe temperature T _tip measured by the system. This measured probe temperature T _tip does not correspond to the actual probe temperature, except for 0 Ohm contact resistance. The X-axis again indicates the introduced heating power V_EFC.

• 1) Ein Übergangswiderstand in der Leitung VN führt zu einem erhöhten Heizleistungsbedarf zum Erreichen einer bestimmten (verfälscht) gemessenen Sondentemperatur T_tip. Der erste nominale Sensor 10 weist einen Übergangswiderstand von 0 Ohm auf, während der zweite, dritte und vierte nominale Sensor 20, 30, 40 jeweils einen Übergangswiderstand von 10 Ohm, 20 Ohm bzw. 30 Ohm aufweist. Mit steigendem Übergangswiderstand steigt auch der Heizleistungsbedarf um eine Sondentemperatur von ca. 830°C zu erreichen. So wird ein mehr an Heizleistung V_EFC von ca. Δ1.68 V benötigt, um den vierten nominalen Sensor 40 mit einem Übergangswiderstand von 30 Ohm auf eine Sondentemperatur von 830°C zu bringen gegenüber dem ersten nominalen Sensor 10 mit einem Übergangswiderstand von 0 Ohm. Die beiden Limit-Sensoren 50, 60 die beide einen Übergangswiderstand von 0 Ohm aufweisen zeigen jedoch, dass es bei den Sensoren Schwankungsbereiche gibt. So gibt der erste Limit-Sensor 50 einen Sensor mit einem Übergangswiderstand von 0 Ohm an, der mit einem verhältnismäßig geringen Heizleistungsbedarf auf die gewünschte Sondentemperatur von 830°C gebracht werden kann, während der zweite Limit-Sensor 60 mit einem Übergangswiderstand von 0 Ohm wiederum einen sehr hohen Heizleistungsbedarf hierfür aufweist.
• 2) Der Betrag dieser Erhöhung für den Heizleistungsbedarf ist stark von der gemessenen Temperatur abhängig. Die technische Erklärung hierfür ist, dass der Übergangswiderstand in der Leitung immer gleich bleibt. Der Widerstand in der Sonde dagegen ändert sich mit der Temperatur. Der relative Fehler in der Widerstandsmessung durch den Übergangswiderstand ist daher stark von der Sondentemperatur abhängig. Beispiel: Erster nominaler Sensor 10 mit 0 Ohm: 10.2 V für 830°C Vierter nominaler Sensor 40 mit 30 Ohm: 11.88 V für 830°C Unterschied in der Heizleistung bei 830°C: 1.68 V Erster nominaler Sensor 10 mit 0 Ohm: 8.73 V für 710°C Vierter nominaler Sensor 40 mit 30 Ohm: 8.95 V für 710°C Unterschied in der Heizleistung bei 710°C: 0.22 V
• 3) Der Unterschied der benötigten Heizleistung zwischen verschiedenen Temperaturen ist umso größer, je größer der Übergangswiderstand ist. Beispiel: Erster nominaler Sensor 10 mit 0 Ohm: 10.2 V für 830°C Erster nominaler Sensor 10 mit 0 Ohm: 8.73 V für 710°C Unterschied in der Heizleistung: 1.47 V Vierter nominaler Sensor 40 mit 30 Ohm: 11.88 V für 830°C Vierter nominaler Sensor 40 mit 30 Ohm: 8.95 V für 710°C Unterschied in der Heizleistung bei 710°C: 2.93 V

The following observations can be made using the diagram in 2 be taken on which the invention is based:

• 1) A contact resistance in the line VN leads to an increased heating power requirement to reach a certain (falsified) measured probe temperature T _tip . The first nominal sensor 10 has a 0 Ohm contact resistance, while the second, third, and fourth nominal sensors 20 . 30 . 40 each has a contact resistance of 10 ohms, 20 ohms and 30 ohms. With increasing contact resistance, the heating power requirement increases to reach a probe temperature of about 830 ° C. Thus, a more heating power V_EFC of about Δ1.68 V is needed to get the fourth nominal sensor 40 with a transfer resistance of 30 ohms to a probe temperature of 830 ° C compared to the first nominal sensor 10 with a contact resistance of 0 ohms. The two limit sensors 50 . 60 however, both of which have 0 Ohm contact resistance show that there are fluctuation ranges in the sensors. So gives the first limit sensor 50 a sensor with a contact resistance of 0 ohms, which can be brought to the desired probe temperature of 830 ° C with a relatively low heating power requirement, while the second limit sensor 60 With a contact resistance of 0 ohms again has a very high heating power requirement for this.
• 2) The amount of this increase for the heating power requirement is highly dependent on the measured temperature. The technical explanation for this is that the contact resistance in the line always remains the same. The resistance in the probe changes with temperature. The relative error in the resistance measurement by the contact resistance is therefore strongly dependent on the probe temperature. Example: First nominal sensor 10 with 0 ohms: 10.2 V for 830 ° C Fourth nominal sensor 40 with 30 ohms: 11.88 V for 830 ° C Difference in heat output at 830 ° C: 1.68 V First nominal sensor 10 with 0 ohms: 8.73 V for 710 ° C Fourth nominal sensor 40 with 30 ohms: 8.95 V for 710 ° C difference in heating power at 710 ° C: 0.22 V
• 3) The difference in the required heating power between different temperatures is greater, the greater the contact resistance. Example: First nominal sensor 10 with 0 ohms: 10.2 V for 830 ° C First nominal sensor 10 with 0 ohms: 8.73 V for 710 ° C Difference in heat output: 1.47 V Fourth nominal sensor 40 with 30 ohms: 11.88 V for 830 ° C Fourth nominal sensor 40 with 30 ohms: 8.95 V for 710 ° C difference in heat output at 710 ° C: 2.93 V

The invention now attempts to exploit the observed effects 2) and 3) by lowering the probe temperature under well-defined conditions, such as a hot engine at idle. After lowering, the required heating power V_EFC after lowering is compared with the heating power V_EFC required before the lowering. If only a slight difference is detected, there is no suspicion of a contact resistance in the lines VN or VG. If, on the other hand, a large difference is found, contact resistance is suspected. The height of the contact resistance can be estimated from the observed difference in the required heater power V_EFC for the high and low probe temperature.

If a contact resistance is detected, this information can be used, for example, either to compensate for the effect, for example, by correcting the internal resistance measurement (Ri measurement) by the determined amount of contact resistance, or it can be an error entry, the Driver causes a workshop to visit.

The effect described under 1) can in principle also be used to detect contact resistances, wherein no lowering of the temperature is necessary, as in the method described above.

The following criterion for detecting contact resistance is used: If the heating power required to keep the probe at operating temperature, for example 830 ° C., exceeds a certain limit value, there is the suspicion of a contact resistance.

However, with the probe under investigation, recognition based solely on this criterion is only possible to a limited extent because a boundary probe without contact resistance, here the second limit sensor 60 , in the lines VN or VG requires a higher heating power than a nominal probe with 30 Ohm contact resistance, here the fourth nominal sensor 40. Alone on the basis of effect 1) can therefore for the examined nominal probes 10-40 so only very large contact resistance, ie contact resistances, for example greater than 30 ohms, are detected.

However, the criterion 1) described above (no lowering of the temperature necessary) can be used as a necessary preselection criterion for the probe being investigated in order to start the method described above, in which the effects 2) and 3) (lowering of the temperature) are utilized.

In principle, however, there are also probes in which a distinction alone based on the criterion 1) (no lowering of the temperature necessary) is quite possible.

A reliable distinction between a contact resistance in the lines VN and VG and another cause of failure, such as weak heater element, contact resistance in the heater lines, etc., but with the criterion 1) (no lowering of the temperature necessary) alone not possible.

In 1 a flow chart for carrying out the method according to the invention is shown.

The method begins with waiting in a step S1 (state 1) for a suitable condition a) to detect a heater voltage of a probe. Such a suitable condition a) is given, for example, when the vehicle is idling or a fuel cut-off phase and the engine is correspondingly "hot" or warmed up, so that the probe is heated accordingly or can be heated, for example to an operating temperature of about 830 ° C.

If such a condition a) is detected in step S1, then in a step S2 (state 2) the heater voltage is detected at a correspondingly high probe temperature, here at a probe temperature of about 830 ° C. In this case, for example, the heater voltage can be detected within a predetermined period of, for example, 2 s and from this the mean value can be formed. The heater voltage or its mean value at a high probe temperature is stored in a memory device as Vh_high_T. However, if the idle phase (condition a)) ends in-between (condition f)), no heater voltage is detected and the routine returns to step S1.

Following the step S2, when the heater voltage for the high probe temperature has been stored (condition b)), the target temperature for the probe temperature is changed in a step S3 (condition 3). In the present case, the probe temperature set point is lowered from its original value of, for example, 830 ° C to a lower value of, for example, 710 ° C. Again, if the idle phase (condition a)) is terminated in-between (condition f)), the target temperature for the probe temperature is not reduced, but the routine returns to step S1.

If the probe temperature has reached the new set value of 710 ° C (step S3, condition c)), then in a step S4 (condition 4), the heater voltage is detected at the lower probe temperature. In this case, the heater voltage can be detected within a predetermined period of, for example, 2 s and from this the mean value can be formed. The heater voltage or its mean value at a lower probe temperature is stored in the memory device as Vh_low_T (condition d)). Again, as before, if the idle phase (condition a)) is terminated in-between (condition f)), then the heater voltage is not sensed at the lower probe temperature, instead the routine returns to step S1.

Subsequent to step S4, it is determined in a next step S5 on the basis of the two stored values for a heater voltage at a high probe temperature Vh_high_T and at a lower probe temperature Vh_low_T, whether a contact resistance exists and, if so, how high the contact resistance is.

The contact resistance is estimated by means of a stored in the memory device of the engine control characteristic table, for example, as follows: R_line = f (Vh_high_T - Vh_low_T)

A map for determining the contact resistance from (Vh_high_T - Vh_low_T) is composed, for example, as follows. First, as described above, the following values are determined and stored in the memory device:
Vh_high T = required heater power for 830 ° C
Vh_low_T = required heater power for 710 ° C

From the map, it can be seen that, for example, for Vh_high_T - Vh_low_T = 2.45 V, the line has a contact resistance (R_line) of 20 ohms. (Vh_high T - Vh_low_T) 1.5 V 2 V 2.45 V 2.93 V Contact resistance R_leitung 0 ohms 10 ohms 20 ohms 30 ohms

If the contact resistance (R_line) in the line is greater than a predetermined resistance of, for example, 30 ohms, an error information is set. Furthermore, the setpoint for the probe temperature is again set high, for example, to 830 ° C. After the evaluation and determination of the contact resistance and after completion of the idling phase of the vehicle (condition e)), a new cycle can be started, i. H. it is again determined in a step S1 whether the vehicle has reached a suitable condition a).

In the following table, the states and transition conditions of the process according to the invention are described in detail again: Condition 1: "Wait for suitable condition" Action: none Condition a): transition from 1 to 2 Condition for transition: engine idling and (coolant temperature> 90 ° C) and (heater voltage> 10.5 V) and probe temperature near 830 ° C Action at transition: none Condition 2: "Detecting heater voltage at high probe temperature" Actions (1-time execution): - Detect heater voltage for 2 s - Form average - Save result in variable Vh_high_T Condition b): transition from 2 to 3 Condition for transition: Action for state 2 terminated Transition action: none State 3: "Change setpoint for probe temperature" Action (1-time execution): - Set the probe temperature set point to 710 ° C Condition c): transition from 3 to 4 Condition for transition: probe temperature has reached 710 ° C Transition action: none Condition 4: "Detecting heater voltage at low probe temperature" Actions (1-time execution): - Detect heater voltage for 2 s - Form average - Save result in variable Vh_low_T Condition d): transition from 4 to 5 Condition for transition: Action for state 4 terminated Transition action: none Condition 5: "Evaluation and measures" Actions (1-time execution): - Estimated contact resistance by means of characteristic table stored in motor control: R_line = f (Vh_high_T - Vh_low_T) - set offset to correct the resistance measurement = R_line - if R-line> 30 ohms, set fault information - set setpoint for probe temperature to 830 ° C Condition e); Transition from 5 to 1 Condition for transition: Action for state 4 completed and idle phase completed Transition action: none Condition f): transition from 2, 3, 4 to 1 Condition for transition: Idle phase completed Transition action: Set setpoint temperature for probe temperature to 830 ° C

However, the method according to the invention is not limited to the present embodiment but can be varied in many ways.

The previously described steps S1 to S5 and the associated diagram in FIG 1 are correspondingly applicable to the modifications of the method according to the invention, as will be described in more detail below. The various embodiments of the method according to the invention can also be combined with one another.

Instead of lowering the probe temperature to a specified value as described and observing and recording and storing the heating power, conversely, the heating power can be lowered to a certain value and the probe temperature can be observed.

In this case, the probe is first heated with a predetermined heating power. Subsequently, the probe temperature is detected and stored as Vh_high_T. Thereafter, the heating power is lowered to a predetermined value and the probe temperature for the lower heating power detected and stored as Vh_low_T. From a corresponding map table, a contact resistance can be taken from the two probe temperatures for a high and a low heat output.

Furthermore, instead of the heating power, the pulse width of a PWM output can be observed. In this case, the probe is first heated with a predetermined heating power or heater voltage and the pulse width of the PWM output detected and stored as Vh_high_T. Subsequently, the heater voltage is lowered to a predetermined lower value and detected the pulse width of the PWM output at the lower heater voltage and stored as Vh_low_T. Based on a corresponding previously stored map table then a contact resistance based on the two pulse widths of the PWM signal for a high and a low heat output can be removed.

Furthermore, instead of the probe temperature, the internal resistance of the probe can be used. In this case, the probe is first heated with a predetermined heater voltage. Subsequently, the internal resistance of the probe is detected and stored as Vh_high_T. Subsequently, the heater voltage is lowered to a predetermined value and the internal resistance of the probe for the lower heating power detected and stored as Vh_low_T. Using a corresponding map table previously stored, a contact resistance can be taken from the two internal resistances of the probe for a high and a low heater voltage.

As before with reference to 1 has been described, the probe is heated to a high probe temperature when idling the vehicle and lowered later. Instead of idling or the fuel cut-off phase, the method can also be used in other engine conditions. This applies to all embodiments of the invention as described above.

In addition, the comparison of the heater voltage with a threshold for a heater voltage as a prerequisite to the start of the process (condition a)) can be omitted. In principle, however, it is also possible to use the comparison of the heater voltage with a limit value for the heater voltage as a prerequisite for the start of the method (condition a)) and as a direct criterion for detecting contact resistances in the lines, such as lines VN and VG.

Instead of correcting the resistance measurement via an offset in the case of a contact resistance in the lines VG or VN, ie. H. the detected contact resistance is taken into account in the internal resistance measurement (Ri measurement) or in the heating of the probe, the corrective action can also take place elsewhere. For example, a change in the setpoint over the probe temperature or the setpoint for the probe resistance, the limitation of the maximum allowable heater voltage, the limitation of the maximum allowable engagement of the heating controller, etc .. These are only a few examples of measures that can be taken. The invention is not limited to these measures.

Furthermore, the temperature reduction carried out must not be performed in one step, but can be completed step by step, wherein after each step, an evaluation of the heater voltage can take place. At the end, for example, an average of all heater voltages of the individual stages can be formed.

The method can be applied to other lines in addition to lines VN or VG, as long as a resistance measurement can be performed on these lines and can be concluded from the result of this resistance measurement on the temperature of the probe. For example, so-called lines VIP connected to a pumping cell may also be taken into account, to name one example of many.

The invention can also be applied to other probes or sensors z. B. Binary lambda sensors, NOx sensors, are applied, if these sensors and the associated systems have the necessary technical characteristics, d. H. For example, an internal resistance measurement (Ri measurement) is possible, as well as, for example, a regulation of the heater power.

The inventive method, as described above, for detecting and determining contact resistance, for example in the lines of an exhaust gas sensor has the advantage that undesirable effects can be avoided by contact resistances such as damage to the lambda probe by overheating by z. B. a corrective intervention is performed. Furthermore, unwanted effects can be avoided by contact resistances, which can not be eliminated by a corrective intervention, such unwanted effects can at least be detected and reported to the driver, for example. It is thus possible to initiate an error entry as soon as an influence on the emissions and / or driveability of the vehicle must be feared. In this way, a driver can timely visit a workshop.

Claims (10)

A method of detecting a contact resistance on a connector in a conduit of a probe comprising the steps of: a) heating the probe with a first heater voltage (step S1); b) detecting and storing at least a first value of a parameter (Vh_high_T) at a first heater voltage (step S2), c) lowering the first heater voltage to a second lower heater voltage (step S3), d) detecting and storing a second value of the parameter ( Vh_low_T) at the second lower heater voltage (step S4), e) determining the contact resistance on the basis of an assignment of a difference of the two stored values of the parameter (Vh_low_T, Vh_high_T) to a contact resistance by means of a previously stored map (step S5), wherein the parameter is a probe temperature or an internal resistance of the probe. Method for determining a contact resistance on a plug-in connection in a line of a probe, comprising the steps: a) heating the probe to a first probe temperature (step S1) b) detecting and storing at least a first value of a heater voltage (Vh_high_T) at the first probe temperature (step S2), c) lowering the first probe temperature to a second lower probe temperature (step S3), d) detecting and storing a second value of the heater voltage (Vh_low_T) at the second lower probe temperature (step S4), e) determining the contact resistance on the basis of an assignment of a difference of the two stored values of the heater voltage (Vh_low_T, Vh_high_T) to a contact resistance by means of a previously stored map (step S5). Method according to at least one of claims 1 or 2, characterized in that the first and / or the second value of the parameter or the heater voltage is detected over a predetermined period of time and the mean value is stored therefrom. Method for determining a contact resistance in a line of a probe, comprising the following steps: A) heating the probe to a predetermined first probe temperature (step S1); B) detecting the first heater voltage (Vh_high_T) at the predetermined first probe temperature (step S2), C) If the first heater voltage (Vh_high_T) exceeds a predetermined limit, there is a contact resistance in the lead of the probe, the method further comprising the steps of: c1) lowering the first probe temperature to a predetermined lower second probe temperature (step S3), d) detecting and storing the second heater voltage (Vh_low_T) at the lower second probe temperature (step S4), e) determining the contact resistance on the basis of the two stored values (Vh_low_T, Vh_high_T) from a corresponding previously stored map table (step S5). Method according to at least one of claims 1 to 4, characterized in that the step a) or the step A) is carried out when a vehicle has reached a predetermined operating state, in particular an idling phase or Schubabschaltephase and / or the coolant temperature has reached predetermined value of in particular greater than 90 ° C and / or the heater voltage has reached a predetermined value, in particular greater than 10.5 V. Method according to at least one of claims 1 to 5, characterized in that the probe temperature in step d) or in step d1) is lowered in one step or stepwise, wherein determined at a stepwise reduction of the probe temperature, the heater voltage in each stage and evaluated becomes. Method according to at least one of claims 1 to 6, characterized in that the line of the probe is a line (VN, VG, VIP), which is connected to a Nernst cell or pump cell or another line, in which a resistance measurement is feasible and wherein based on the resistance measurement on the probe temperature can be concluded. Method according to at least one of claims 1 to 7, characterized in that the probe is a lambda probe, in particular a linear or binary lambda probe, or a NOx-probe. Method according to at least one of claims 1 to 8, characterized in that when in step e) or in step C) a contact resistance is detected, a suitable measure is taken to prevent, for example, overheating of the probe. A method according to claim 9, characterized in that the appropriate action comprises at least one or more of the following measures: correcting a resistance measurement of the lead of the probe based on the established contact resistance to prevent overheating of the probe; Adjusting the probe temperature setpoint or the probe resistance setpoint based on the established contact resistance; Limiting the maximum permitted heater voltage or the maximum allowable engagement of the heater controller based on the established contact resistance; Issuing a warning to a driver when the detected transient resistance exceeds a predetermined threshold.

Images