EP1413728B1 - Controller and method for controlling a NOX-sensor arranged in an exhaust gas channel of an internal combustion engine - Google Patents

Description

The invention relates to a method and a controller for operating an arranged in an exhaust passage of an internal combustion engine exhaust gas sensor with the features mentioned in the preamble of claims 1 and 19.

It is known that, for example, to determine the NO _x content in the exhaust gas of an internal combustion engine usually employed NO _x sensors. Such NO _x sensors are known in various designs, so that can be dispensed with a more detailed description here. Especially with a lean executable engine it is important to comply with the NO _x limit to know the _NOx content in the exhaust system behind a _NOx storage catalytic converter in order to, for example, determine the time at which a storage of _NOx in the NO _x storage is no longer possible, that is, the NO _x storage is filled, so that a regeneration phase is necessary.

For this purpose, usually located behind the catalyst in the exhaust system of an at least temporarily lean-running internal combustion engine, a measuring device which is able to measure the NO _x concentration in the exhaust gas, so for example a NO _x sensor. The measuring accuracy of exhaust gas sensors such as NO _x sensors or lambda probes usually depends on external parameters. Therefore, it is important to keep these parameters within the optimum ranges for accurate control, and readjust if necessary. For example, with NO _x sensors, the temperature must be kept constant in order to ensure high accuracy of the signal. The smaller the deviations from the desired temperature of the sensor, the more accurate the sensor signal.

GB 2345142 shows a similar system to that described above.

So far, a PI controller based on voltage without exhaust gas mass flow-dependent feedforward control is used in this exhaust gas temperature sensor, which operates with a resistance difference as a controller input variable and a voltage as a controller output variable. The weak point of this system is the great inaccuracy in the regulation of the sensor temperature in the event of fluctuations in the exhaust gas mass flow due, among other things, to a missing model of the path to be controlled.

The invention is therefore based on the object to provide a controller and a method for controlling an arranged in an exhaust passage of an internal combustion engine exhaust gas sensor, which eliminate the disadvantages mentioned, and in particular to provide a robust controller or a robust method for controlling an exhaust gas sensor.

This object is achieved by a controller and a method having the features mentioned in claims 1 and 19.

A particular advantage of the method for operating an exhaust gas sensor arranged in an exhaust duct of an internal combustion engine is that the measurement accuracy of exhaust gas sensors is increased by a control of operating parameters of the exhaust gas sensor by a model-based controller and / or an exhaust gas flow dependent pilot control of operating parameters of the exhaust gas sensor.

To control operating parameters of an exhaust gas duct of an internal combustion engine arranged exhaust gas sensor, therefore, a controller is used, which is characterized in that the controller is designed as a model-based controller and has a predetermined by a model of the controlled system.

In a preferred embodiment of the method according to the invention, it is provided that the operating parameter temperature is controlled and / or controlled during operation of a NO _x sensor. It is advantageous if the temperature is determined by measuring an electrical resistance value. It is also advantageous if the control evaluates the difference between the current and the setpoint temperature of the sensor as a controller input variable and the heating power for heating the sensor varies as a controller output variable. This achieves a linear relationship between controller input and output quantity.

In a preferred embodiment of the method according to the invention, it is provided that the exhaust gas flow-dependent pilot control is set up so that the exhaust gas flow-dependent pilot control becomes active in the case of a very rapidly decreasing or increasing exhaust gas flow. Specifically, this can be realized in that the exhaust gas flow-dependent feedforward control is active when the exhaust gas flow changes faster than graded to the sampling time gradient.

In a further preferred embodiment of the invention, it is provided that the current value of the measured sensor resistance and / or the current value of the measured sensor temperature are taken into account in the release condition of the exhaust-gas-dependent pilot control.

It proves to be an advantage if, with rapidly falling exhaust gas mass flow, the precontrol briefly reacts for a short time with a reduced heating power and with rapidly increasing exhaust gas mass flow the pilot control reacts briefly with an increased heating power. This compensates for unwanted heating or cooling of the sensor.

In a further preferred embodiment of the method according to the invention, it is provided that a plurality of temporally successive differences of the measured exhaust gas mass flow values are evaluated in a pilot control of the exhaust gas sensor. In particular, good results are achieved if the number of temporally successive differences of the measured exhaust gas mass flow values to be evaluated for a precontrol is determined as a function of the sampling time, a higher number being evaluated for short sampling times and a smaller number of differences being evaluated for long sampling times.

It has also proved to be advantageous that a pilot control matched to a sampling time of 50 ms becomes active with a low heating power value if three time-sequential differences of the measured exhaust gas mass flow values are greater than the gradient to be applied. Whereas a feedforward control with a high heating power value tuned to a sampling time of 50 ms advantageously becomes active if four time-sequential differences of the measured exhaust-gas mass flow values are smaller than the gradient to be applied. These two approaches are each a compromise between the avoidance of a faulty control and a short reaction time.

In order to prevent unwanted cooling of the exhaust gas sensor, a preferred embodiment of the method according to the invention provides that at least at the beginning of deceleration phases, a high heating power precontrol takes place.

It proves to be advantageous that the model-based controller is determined by means of a linear time-continuous method. It has proven to be particularly practical if the model-based controller using the LQG / LTR method (LQG / LTR = Linear Quadratic Gaussian / Loop Transfer Recovery) is designed.

A particularly robust controller is obtained if the model of the controlled system is based on the parameters time constant T, damping d and static gain k _SR .

lautet, wobei

hno0y

die aktuelle Ausgangsgröße,

hno1y

die im vorangegangenen Zeittakt berechnete Ausgangsgröße,

hno2y

die zwei Zeittakte zuvor berechnete Ausgangsgröße,

hno0u

die aktuelle Eingangsgröße,

hno1u

die im vorangegangenen Zeittakt gemessene Eingangsgröße,

hno2u

die zwei Zeittakte zuvor gemessene Eingangsgröße,

k_SR

die statische Verstärkung und

HNOij

die Regelparameter (i = 0, 1, 2 und j = U, YK)

darstellen.In a further preferred embodiment of the method according to the invention, it is provided that the difference equation underlying a time-discrete realization $$\begin{array}{ll}\mathrm{ent}\mathrm{0}\mathrm{y}=& {\mathrm{k}}_{\mathrm{SR}}\left(\mathrm{ENT}\mathrm{0}\mathrm{U}\cdot \mathrm{ent}\mathrm{0}\mathrm{u}+\mathrm{ENT}\mathrm{1}\mathrm{U}\cdot \mathrm{ent}\mathrm{1}\mathrm{u}+\mathrm{ENT}\mathrm{2}\mathrm{U}\cdot \mathrm{ent}\mathrm{2}\mathrm{u}\right)\\ & -\left(\left(-\mathrm{ENT}\mathrm{1}\mathrm{YK}\right)\cdot \mathrm{ent}\mathrm{1}\mathrm{y}+\left(-\mathrm{ENT}\mathrm{2}\mathrm{YK}\right)\cdot \mathrm{ent}\mathrm{2}\mathrm{y}\right)\end{array}$$

is, where

hno0y

the current output,

hno1y

the initial value calculated in the previous cycle,

hno2y

the two clocks previously calculated output,

hno0u

the current input,

hno1u

the input measured in the preceding cycle,

hno2u

the two clocks previously measured input,

k _SR

the static amplification and

HNOij

the control parameters (i = 0, 1, 2 and j = U, YK)

represent.

Particularly advantageous is when the controller is designed as a linear regulator. When using a linear controller, good control results are obtained even if the controlled system has a linear relationship, that is, if there is a linear relationship between controller input and controller output variables.

A preferred embodiment of the regulator according to the invention is characterized in that the controller comprises a Kalman filter and an optimal state feedback. To avoid an undesired "run-up" of the integrator, it is advantageous if the controller has an integrator windup avoiding integrator.

In a further preferred embodiment of the regulator according to the invention provided that the controller comprises a means for measuring the exhaust gas mass flow value, whereby an exhaust gas mass flow-dependent feedforward control is made possible.

Further preferred embodiments of the invention will become apparent from the remaining, mentioned in the dependent claims characteristics.

Figur 1

eine Darstellung der in einem Sprungversuch ermittelten Kennlinien und

Figur 2

eine Darstellung von Amplituden- und Phasengang (Bode-Diagramm) der offenen Kette bestehend aus dem erfindungsgemäßen Regler und einem ein Integrator-Windup vermeidenden Integrator.

The invention will be explained in more detail in embodiments with reference to the accompanying drawings. By way of example, an NO _x sensor is used as the exhaust gas sensor. Show it:

FIG. 1

a representation of the determined in a jump test curves and

FIG. 2

a representation of the amplitude and phase response (Bode diagram) of the open chain consisting of the controller according to the invention and an integrator windup avoiding integrator.

The controller according to the invention is characterized in that it is based on a new controller structure, which includes a model of the controlled system. Due to this additional information about the controlled system, a higher control accuracy is achieved. In addition to the new controller, the relationship between controller input and output has been linearized to achieve better control results. The controller now varies the heating power and thus controls the heating of the sensor to its setpoint temperature. Regulator input is a temperature difference, which results from the current and the target temperature of the sensor. The new controller structure includes a special integrator whose advantage consists in avoiding an "integrator windup", ie an undesired "startup" of the integrator when the manipulated variable has reached the saturation value of the control loop input. In addition, the new controller structure can be combined with an exhaust gas mass flow-dependent pilot control in order to compensate for the dead time due to the position of the NO _x sensor far back in the exhaust gas tract.

mit den oben definierten Größen.An exemplary embodiment of the model-based controller was designed according to the LQG / LTR (Linear Quadratic Gaussian / Loop Transfer Recovery) method. The LQG / LTR method is a linear continuous-time design method. The exemplary controller designed thereby consists of a Kalman filter as observer and an optimal state feedback. The underlying difference equation for a discrete-time realization, as implemented, for example, in the software of an engine control unit, is: $$\begin{array}{ll}\mathrm{ent}\mathrm{0}\mathrm{y}=& {\mathrm{k}}_{\mathrm{SR}}\cdot \left(\mathrm{ENT}\mathrm{0}\mathrm{U}\cdot \mathrm{ent}\mathrm{0}\mathrm{u}+\mathrm{ENT}\mathrm{1}\mathrm{U}\cdot \mathrm{ent}\mathrm{1}\mathrm{u}+\mathrm{ENT}\mathrm{2}\mathrm{U}\cdot \mathrm{ent}\mathrm{2}\mathrm{u}\right)\\ & -\left(\left(-\mathrm{ENT}\mathrm{1}\mathrm{YK}\right)\cdot \mathrm{ent}\mathrm{1}\mathrm{y}+\left(-\mathrm{ENT}\mathrm{2}\mathrm{YK}\right)\cdot \mathrm{ent}\mathrm{2}\mathrm{y}\right)\end{array}$$

with the sizes defined above.

Since the order of the controlled system model dictates the order of the model-based controller, it was necessary to determine a suitable yet simple model of the controlled system. For this jump tests were carried out with the track, with performance jumps were given to the track such that the track output, the sensor temperature, jumps around its operating point, the target temperature performs, as expected in normal operation. In FIG. 1 is the result of a jump attempt performed. The voltage jump quantity 10 U_sprung, is converted into a power quantity via the heating resistor. The intermediate quantity 11 R _PVS , which is the internal resistance of the sensor, and the desired step response 12 T_Tip, the temperature of the sensor element, are additionally plotted in the diagram. With the aid of a suitable auxiliary means, the measured step response 12 was superimposed on the simulated behavior of a PT ₂ element 13 T_model. As in FIG. 1 It became clear that a PT _{2 member} closely approximates the measured behavior. The parameters of the exemplary model are: time constant T = 4.2s, damping d = 1.2s and static gain k _SR = 17 ° C / W.

FIG. 2 shows the amplitude and phase response (Bode diagram) of the open chain 20 consisting of the controller according to the invention and the special integrator described above. With a phase margin of around 73 °, the structure has a high degree of robustness.

mit der Temperaturerhöhung ΔT in [K] und der Masse m in [kg] des Körpers sowie der ihm zugeführten Wärmemenge Q in [Ws]. Die Umrechnung vom bisher verwendeten Innenwiderstandswert des Sensors in seine Temperatur erfolgt mit Hilfe einer vom Sensorhersteller ermittelten nichtlinearen Kennlinie.Since it is a linear controller, a linear relationship between controller input and output is required for a good control result. This is the reason for changing the controller input from the measured sensor resistance value to a sensor temperature value and from the heater voltage value of the controller output to a heating power value. The linear relationship between the temperature rise of a body and the amount of heat supplied to it is: $$\mathrm{\Delta }T~\frac{1}{m}Q$$

with the temperature increase ΔT in [K] and the mass m in [kg] of the body as well as the amount of heat Q supplied to it in [Ws]. The conversion of the previously used internal resistance value of the sensor into its temperature takes place with the aid of a nonlinear characteristic curve determined by the sensor manufacturer.

In order to be able to better take into account fluctuations in the exhaust gas mass flow in the temperature control of the NO _x sensor, the proposed new controller structure can be combined with an exhaust gas mass flow-dependent pilot control. The pilot control becomes active when the exhaust gas mass flow increases or decreases very rapidly. The underlying idea is that there is information about an imminent cooling or heating of the sensor located farther back in the exhaust tract via the exhaust gas mass flow. If the mass flow changes faster than the gradients matched to the sampling time, the feedforward will be activated by either a high heat-up to prevent the impending cooling or a lowering of the heating power to a low level due to impending heating. The current value of the measured sensor resistance is included in the release condition of the feedforward control in order to avoid too low a heating power precontrol when the sensor temperature is too low, and vice versa. As an alternative to the sensor resistance, it is also possible to measure the sensor temperature, the temperature and the resistance being converted into one another on the basis of the abovementioned characteristic curve.

It has been determined by measurements that, when the exhaust gas mass flow drops rapidly, an undesired increase in the sensor temperature takes place at the first moment, to which the precontrol reacts for a short time compensatingly with a reduced heating power value. Conversely, when the exhaust gas mass flow increases rapidly, cooling of the sensor takes place at the first moment, to which the precontrol briefly reacts with increased heating power.

The measurement of the exhaust gas mass flow is affected by a measurement noise. In order to differentiate from this measurement noise, it is usually not appropriate to respond to any (small) change in the exhaust gas mass flow with an adjustment of the heating power. Instead, it will be useful, depending on the sampling time, to take into account a higher or lower number of chronologically successive differences of the measured exhaust gas mass flow values for a precontrol. The number of temporally successive differences must be chosen so that a compromise is found between the demarcation against the measurement noise and too long a reaction time. For shorter sampling times, it may be advantageous to increase the number of to consider, taking into account, time-sequential differences of the measured exhaust gas mass flow values; For longer sampling times, it may be necessary to reduce this number.

For a precontrol with a low heating power value and the special sample sampling time of 50 ms, for example, the criterion "three successive differences of the measured exhaust gas mass flow values are greater than the gradient to be applied (corresponds to a response time of 3 * 50 ms)" represent a compromise between avoiding a wrong feedforward and a short reaction time.

For a pilot control with a high heating power value, the compromise in a special embodiment with the sampling time 50 ms, for example, by the criterion "four temporally successive differences of the measured exhaust gas mass flow values are smaller than the gradient to be applied (corresponds to a reaction time of 4 * 50ms)" formed become.

At the beginning of a fuel cut-off phase, the sensor usually cools something out. Therefore, it is also possible to switch to maximum heating for overrun phases already via the pilot control, even before this is the case via the control deviation and the controller itself.

The invention is not limited to the embodiments shown here. Rather, it is possible 'to realize by combining and modifying the means and features mentioned further variants, without departing from the scope of the invention.

LIST OF REFERENCE NUMBERS

10

Voltage step size

11

between size

12

Step response

13

simulated behavior of a PT ₂ -element

20

open chain

Claims (22)

1. Method for operating an exhaust gas sensor which is arranged in an exhaust gas duct of an internal combustion engine, characterized in that

   - close-loop control of operating parameters of the exhaust gas sensor is carried out by means of a model-based closed-loop controller, and

   - if the exhaust gas mass flow increases or decreases more quickly than an applied gradient which is matched to a sampling time of the exhaust gas sensor, an exhaust-gas-flow-dependent pilot control of the operating parameters becomes active.
2. Method according to Claim 1, characterized in that during the operation of an NO_x sensor, closed-loop and/or open-loop control of the temperature as operating parameter is carried out.
3. Method according to Claim 2, characterized in that the temperature is determined by measuring an electrical resistance value.
4. Method according to Claim 2, characterized in that the closed-loop control evaluates the difference between the current temperature and the setpoint temperature of the sensor as a closed-loop controller input variable, and the heating power of the heating of the sensor is varied as a closed-loop controller output variable.
5. Method according to one of the preceding claims, characterized in that
   in a condition for enabling the exhaust-gas-flow-dependent pilot control,

   - the current value of the measured sensor resistance and/or

   - the current value of the measured sensor temperature

   is/are taken into account.
6. Method according to one of the preceding claims, characterized in that in the case of a rapidly dropping exhaust gas mass flow the pilot control reacts temporarily with a reduced heating power.
7. Method according to one of the preceding claims, characterized in that in the case of a rapidly rising exhaust gas mass flow the pilot control reacts temporarily with an increased heating power.
8. Method according to one of the preceding claims, characterized in that in the case of pilot control of the exhaust gas sensor a plurality of chronologically successive differences between the measured exhaust gas mass flow values are evaluated.
9. Method according to one of the preceding claims, characterized in that the number of chronologically successive differences between the measured exhaust gas mass flow values, which differences are to be evaluated for pilot control, is determined as a function of the sampling time, wherein in the case of short sampling times a relatively high number, and in the case of long sampling times a relatively low number, of differences is evaluated.
10. Method according to one of the preceding claims, characterized in that a pilot control which is matched to a sampling time of 50 ms becomes active with a low heating power value, if three chronologically successive differences between the measured exhaust gas mass flow values are larger than the gradient to be applied.
11. Method according to one of the preceding claims, characterized in that a pilot control which is matched to a sampling time of 50 ms becomes active with a high heating power value, if four chronologically successive differences between the measured exhaust gas mass flow values are smaller than the gradient to be applied.
12. Method according to one of the preceding claims, characterized in that high heating power pilot control takes place at least at the start of overrun phases.
13. Method according to one of the preceding claims, characterized in that the model-based closed-loop controller is designed using a linear method which is continuous over time.
14. Method according to one of the preceding claims, characterized in that the model-based closed-loop controller is designed using the LQG/LTR method (LQG/LTR = Linear Quadratic Gaussian/Loop Transfer Recovery).
15. Method according to one of the preceding claims, characterized in that the model of the closed-loop controlled system is based on the parameters

    - time constant (T),

    - attenuation (d) and

    - static amplification (k_SR).
16. Method according to one of the preceding claims, characterized in that the difference equation on which a time-discrete implementation is based is $$\mathrm{hno}\mathrm{0}\mathrm{y}={\mathrm{k}}_{\mathrm{SR}}\cdot \left(\mathrm{HNO}\mathrm{0}\mathrm{U}\cdot \mathrm{hno}\mathrm{0}\mathrm{u}+\mathrm{HNO}\mathrm{1}\mathrm{U}\cdot \mathrm{hno}\mathrm{1}\mathrm{u}+\mathrm{HNO}\mathrm{2}\mathrm{U}\cdot \mathrm{hno}\mathrm{2}\mathrm{u}\right)-\left(\left(-\mathrm{HNO}\mathrm{1}\mathrm{YK}\right)\cdot \mathrm{hno}\mathrm{1}\mathrm{y}+\left(-\mathrm{HNO}\mathrm{2}\mathrm{YK}\right)\cdot \mathrm{hno}\mathrm{2}\mathrm{y}\right)$$

    

    where

    hno0y is the current output variable,

    hno1y is the output variable which is calculated in the preceding clock cycle,

    hno2y is the output variable which is calculated to two clock cycles before,

    hno0u is the current input variable,

    hno1u is the input variable which is measured in the preceding clock cycle,

    hno2u is the input variable which is measured two clock cycles before,

    k_SR is the static amplification, and

    HNOij is the closed-loop control parameters (i = 0, 1, 2 and j = U, YK).
17. Closed-loop controller for operating an exhaust gas sensor which is arranged in an exhaust gas duct of an internal combustion engine, characterized in that the closed-loop controller is configured to carry out the method according to one of Claims 1 to 16 and is embodied as a model-based closed-loop controller and has a structure which is predefined by a model of the controlled system.
18. Closed-loop controller according to Claim 17, characterized in that the closed-loop controller is embodied as a linear closed-loop controller.
19. Closed-loop controller according to one of Claims 17 and 18, characterized in that there is a linear relationship between the closed-loop controller input variables and closed-loop controller output variables.
20. The closed-loop controller as claimed in one of Claims 17 to 19, characterized in that the closed-loop controller comprises a Kalman filter and optimum state feedback.
21. Closed-loop controller according to one of Claims 17 to 20, characterized in that the closed-loop controller has an integrator-windup-avoiding integrator.
22. Closed-loop controller according to one of Claims 17 to 21, characterized in that the closed-loop controller comprises means for measuring the exhaust gas mass flow value.

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