EP3767174B1 - Method and device for recalibrating a measuring system for regulating a fuel-air mixture in a heating device - Google Patents

Description

The invention is in the field of controlling a fuel gas-air mixture for a combustion process in a heating device, in particular a combustion chamber in a heating device for hot water preparation or heating a building. To measure the quality of combustion, which mainly depends on the ratio of air to fuel gas during combustion (lambda value, also known as air ratio), an ionization measurement is carried out in a flame area, particularly in many heating devices. Such measurements are intended to enable stable control over long periods of time, which is why it may be necessary to detect slow changes in the measuring system and to carry out recalibration.

According to the state of the art, the respective actual value of the ionization in the flame area is determined by means of an ionization electrode, which is proportional to the lambda value currently present, so that this can be derived from the ionization measurement. An alternating voltage is applied to the ionization electrode, whereby the flame area ionized in the presence of flames has a rectifying effect, so that an ionization signal mainly only flows during one half-wave of the alternating current. This current or a proportional voltage signal derived from it, hereinafter referred to as the ionization signal, is measured and, if necessary, processed as an ionization signal after digitization in an analog/digital converter. The lambda value can be measured via calibration and regulated to a target value using a control loop. The supply of air and/or fuel gas is changed by suitable actuators until the desired target value for lambda is reached. In general, a lambda value > 1 (1 corresponds to a stoichiometric ratio) is aimed for, e.g. lambda = 1.3, to ensure that enough air is supplied for clean combustion with essentially no production of carbon monoxide. However, lambda must remain small enough to ensure stable combustion. The control can be carried out in particular via a valve for the supply of fuel gas and/or a fan for the supply of ambient air.

The basic structure of such heaters, measuring systems for ionization measurement and their use for control are known, for example, from the EP 0 770 824 B1 and the EP 2 466 204 B1 It is also described there that the control accuracy can change over time due to various influences, in particular due to influences on the condition or shape of the ionization electrode. Various methods for recalibration are specified if necessary, but all of these require a relatively high level of effort and/or can have the disadvantage that during recalibration the heater must be operated at lambda values of 1 or even lower at times, which can lead to the temporary generation of undesirable carbon monoxide. In addition, very high flame temperatures occur in this area, which can cause additional damage to the ionization electrode during calibration.

From the EP 2 014 985 B1 A control system is already known that can be operated and calibrated without shifting the combustion to a range close to lambda = 1, so that even during calibration little carbon monoxide is produced. However, it is not always possible to maintain an optimal lambda value.

Furthermore, it is from the EN 2 103 189 A1 It is known to use an ignition electrode in a burner after ignition for an ionization measurement and thus to monitor a flame.

Finally, it is from the EP 1 750 058 A2 It is also known to operate two ionization electrodes in one burner in order to enable mutual calibration. However, this requires additional installation work in the burner and cannot simply prevent both ionization electrodes from exhibiting a similar drift, for example, which cannot then be determined.

The present invention aims to remedy this situation by providing a quick method for recalibrating an existing control system or correcting a calibration curve on which this control system is based, which can be carried out with little additional equipment.

A method and a device according to the independent claims serve to solve this problem. Advantageous embodiments and further developments of the invention are specified in the respective dependent claims. The description, in particular in connection with the figures, illustrates the invention and gives further embodiments.

The method according to the invention is used for recalibrating calibration data of a first measuring system for measuring a first ionization signal in a flame region of a heating device operated with combustion air and fuel gas, wherein the first measuring system measures an ionization signal which is derived from a first ion current flowing from an ionization electrode to a counter electrode through the flame region, and from this the ratio of combustion air to fuel gas (lambda) at a Combustion in the heater is determined and regulated on the basis of calibration data, wherein the first measuring system is recalibrated at least according to predeterminable criteria or at predeterminable time intervals, and wherein the recalibration is carried out by means of an ignition electrode present in the heater for igniting the combustion, which is operated to generate a second ionization signal. In some heaters, a second ionization signal is measured anyway in order to determine and monitor the presence of a flame.

Independently of this known function as a flame detector, such a second ionization signal can also be used for other tasks during operation, in particular for recalibrating calibration data for the actual control system. This allows the first ionization signal to be corrected from time to time by comparing it with the second ionization signal, if necessary by taking into account (stored) empirical values for both ionization signals and their changes over time.

For this purpose, the ignition electrode is operated in a second measuring system to measure the second ionization signal. In this way, not only changes in the ionization electrode itself, but also changes in its electronics, in particular long-term drift, can be detected and corrected. The second measuring system, which is also typically used for flame monitoring, works according to the following principle:

An alternating voltage without a direct voltage component from a voltage source with a high output impedance is applied between the ignition electrode and ground. Due to a rectifying effect of a flame plasma when the flame is burning, an ionization current flows to ground during each positive half-wave of the alternating voltage. The voltage amplitude of each positive half-wave is the voltage source is reduced, while the negative half-wave remains unchanged. This imposes a negative DC voltage component on the AC voltage. The amplitude of this negative DC voltage component is converted as an average value by means of an amplifier circuit into a voltage signal which, due to its characteristic curve with constant gas supply and increasing air supply, can be used as a second ionization signal for the purposes described here. Typically, this signal is digitized using an analog/digital converter (e.g. into values between 0 and 1023) so that it can be further processed in a microprocessor.

The characteristic curve of the signal results from a combination of different effects. On the one hand, ionization in the flame area is strongest when combustion is carried out in a stoichiometric ratio of combustion gas and combustion air; on the other hand, as the gas velocity increases (larger amount of gas per unit of time), the flames move away from the gas outlet openings, which electronically form the mass in the system, which reduces the ion current. The temperature of the flames and the ignition electrode may also play a role in the rectifying effect mentioned above. The result is a curve with an easily reproducible minimum, which is close to a lambda value typical for continuous operation.

In combination with different load levels, an entire characteristic map of the first measuring system can be recalibrated in this way, whereby calibration data for the relation of ionization signal to the ratio of combustion air to fuel gas at different load conditions are stored in the first and second measuring systems and a comparison is made between the values determined by both measuring systems at each recalibration. measured values, whereby the first measuring system is recalibrated with the data from the second measuring system in case of deviations.

In a preferred embodiment, the combustion in the heater is brought to at least one constant combustion state that can be specified by the first measuring system and then the second ionization signal of the second measuring system is measured while maintaining or deliberately changing this state. This gives two measured values for the same state that can be compared with each other. If the second ionization signal is considered more reliable, the measured value calculated from the first ionization signal can be provided with an appropriate correction for future measurements. At least deviations between the two measurements can be determined and measures can be derived from them.

It is particularly advantageous if the combustion is brought into several different constant states one after the other by the first measuring system and the second measuring system is switched on in each of these states, the second ionization signal is measured in this state and/or when this state changes and any deviation from the first ionization signal is determined. In this way, the curve for the dependence of the air ratio on the first ionization signal, i.e. the calibration curve of the first measuring system, can be checked with a suitable level of accuracy that depends on the number and distances between the states (the so-called support points) and completely corrected if necessary.

In a preferred embodiment, the different constant states are different load levels of the heater, which are achieved by different constant speeds of a fan and/or different constant amounts of supplied fuel gas per unit of time are determined by different constant settings of a fuel gas valve.

Furthermore, a device is proposed, set up to carry out the method described here, which has a combustion chamber with an air supply and a fuel gas supply, which are controlled by a control unit, and with a first measuring system, comprising an ionization electrode, a counter electrode, a first AC voltage source and first evaluation electronics for determining a first ionization signal, which can be fed to the control unit, wherein a second measuring system is present for measuring a second ionization signal, which can be generated between an ignition electrode provided for igniting a combustion and the counter electrode by the second measuring system and wherein the first and the second system are each set up to determine a lambda value.

The second measuring system is constructed independently of the first measuring system, namely in that it has no common parts apart from the counter electrode. This allows, at least within certain limits, errors in the electronics of the first measuring system to be detected and corrected.

Preferably, the second measuring system is constructed in a diverse manner to the first measuring system, namely by having as few or no identical components as possible. By using other electronic components and/or a different structure and/or a different measuring principle, even systematic errors such as slow drift of amplification, resistance or other components can be detected and compensated.

In a preferred embodiment, a comparator is provided to which measured values of the first and second measuring systems can be fed, and a correction unit is used to recalibrate calibration data of the first measuring system when deviations between the two measured values are detected.

Also to be described here is a computer program product comprising instructions that cause the described device to carry out the method proposed here. Modern heating devices typically contain an electronic control that contains at least one programmable microprocessor that can be controlled by such a computer program product.

Fig. 1:

schematisch eine erfindungsgemäße Vorrichtung,

Fig. 2:

ein Diagramm zur Veranschaulichung des Verlaufs des ersten lonisationssignales bei einer bestimmten Luftzahl (Lambda) in Abhängigkeit von der Gebläsedrehzahl (normal und gedriftet),

Fig. 3:

eine schematische Schaltung zur Erzeugung eines lonisationssignals im zweiten Messsystem S2,

Fig. 4:

ein Diagramm zur Veranschaulichung eines Messvorganges mit dem zweiten Messsystem S2, und

Fig. 5:

ein Diagramm zur Veranschaulichung einer korrigierten Kalibrierkurve für das erste Messsystem S1.

A schematic embodiment of the invention, to which it is not limited, and the functioning of the method according to the invention will now be explained in detail with reference to the drawing. They show:

Fig.1:

schematically a device according to the invention,

Fig. 2:

a diagram to illustrate the course of the first ionization signal at a certain air number (lambda) depending on the fan speed (normal and drifted),

Fig. 3:

a schematic circuit for generating an ionization signal in the second measuring system S2,

Fig.4:

a diagram illustrating a measuring process with the second measuring system S2, and

Fig.5:

a diagram illustrating a corrected calibration curve for the first measuring system S1.

Figure 1 shows a schematic of an embodiment of a device proposed here. In a heater 1 for burning a fuel gas with air, a flame region 2 is formed during operation. Air enters the combustion chamber 1 via an air supply 3 and a fan 5. Fuel gas is mixed with the air via a fuel gas supply 4 and a fuel gas valve 6. An ignition electrode 7 ignites the mixture at the start of the combustion process and is then used, for example, as part of a flame monitor. A first ionization signal is measured in the flame region 2 by means of an ionization electrode 8. A first measuring system S1 is used for this purpose, from which the ionization electrode 8 is subjected to an alternating voltage from a first alternating voltage source, with first evaluation electronics 13 measuring the resulting ionization signal and converting it into a lambda value, i.e. a mixture ratio of air to fuel, according to stored calibration data (control curve). Using this value as the actual value, a control unit 17 can control the fan 5 and/or the fuel gas valve 6 so that a desired target value for lambda is set. In order to achieve safe long-term operation, the control curve must be corrected at certain intervals, whereby the intervals can be selected, for example, according to the operating time of the heater 1 and/or other parameters. This is due, on the one hand, to the fact that the ionization electrode 8 can change during operation, for example due to thermal bending and/or an increasing oxide layer on the surface. The electronics can also change. Unfortunately, it is difficult to carry out an absolute calibration retrospectively and automatically with existing devices, which is why the present invention uses the ignition electrode 7 to generate a second ionization signal as a new approach to recalibration. For this purpose, a second measuring system S2 is put into operation by means of a switching unit 10, which has a second An alternating voltage source 12 is connected to the ignition electrode 7 instead of ignition electronics (if this has not already been done for flame monitoring), with a second evaluation electronics 13 measuring and evaluating a second ionization signal that also supplies an actual value for lambda. Ideally, both actual values supplied by the measuring systems S1 and S2 are the same or are in a ratio that has not changed since the last calibration, so that the control curve in the first measuring system S1 can remain unchanged. However, if deviations between the two measured values or their ratio are detected in a comparator 15, a correction unit 16 is used to determine a correction factor with which the control curve is corrected, so that the control unit 17 can carry out further control based on the corrected control curve with the first measuring system. It is assumed that the second measuring system S2 measures more reliably than the first measuring system S1, which is why S1 is corrected to the actual value of S2. However, based on experience and/or theoretical considerations, this correction can be weakened by a damping value if the entire calculated correction is not to be applied or not to be applied immediately.

Fig.2 shows in a diagram how the first ionization signal (and in a similar form the second ionization signal) depends on the speed of the fan 5. The speed is typically in the range between 1000 and 10 000 revolutions per minute [rpm] and certain speeds can be used as reference points i1, i2, ... i10 for checking and recalibration. The upper curve A shows the dependency for a new ionization electrode 8, while the lower curve B illustrates the dependency for a used and already somewhat aged (e.g. oxidized or bent) ionization electrode 8. If the ionization signal 11 were converted into a lambda value from the calibration curve A, an incorrect actual value would result for an aged ionization electrode 8, which would lead to suboptimal control.

A typical procedure according to the invention for recalibrating the control curve in the first measuring system is described below as an example, but the invention is not limited to this specific procedure, since there are many ways of using the ignition electrode 7 for recalibration. In the embodiment chosen here, the heater initially works in normal operation with a certain supply of fuel gas and an associated speed of the fan 5, whereby the ionization signal 11 is regulated by the first measuring system S1 to a value specified as a target value for this state, e.g. 100 µA [microAmpere], by adjusting the speed of the fan and/or the fuel supply. With valid calibration data (characteristic map, control curve), this type of regulation ensures that a desired lambda value is maintained over a large load range. However, if a certain number of operating hours of the heater has been exceeded or a restart has taken place or there are other reasons, a recalibration can be triggered. In the embodiment described here, a simple control curve is corrected using so-called support points i1, i2, ... i10 on the x-axis (fan speed), so that the values between the support points i1, i2, ... i10 can be obtained by interpolation if required. For a speed range between 1000 and 10000 rpm [revolutions per minute], for example, a maximum of 10 support points at suitable intervals are sufficient for sufficiently accurate recalibration, but more or fewer can of course be used. For each support point, here at 3000 rpm as an example, a recalibration is carried out at a suitable time, for example when a load of the corresponding magnitude is needed or can at least be removed. For this purpose, a transition is made from normal operation, in which the measuring system S1 is used to control a constant lambda value, e.g. B. Lambda = 1.3, is used and this is adjusted exactly as the actual value for the relevant speed according to the valid calibration curve, switched to the recalibration procedure if a condition for recalibration is met. In this case, the gas supply is kept constant for the entire period of recalibration and firstly the fan speed. Then the first measuring system S1 is switched to the second measuring system S2. At the beginning of the recalibration, this determines a second ionization signal I2 determined by means of the ignition electrode 7. If it is assumed that the desired lambda value is no longer achieved precisely due to changes in the measuring system S1, this can now be determined and corrected using the measuring system S2. To do this, the gas supply is left unchanged, while the fan speed is reduced in a defined manner until a value below the desired lambda value is reached, but which is still significantly above a stoichiometric ratio of air to fuel gas, so that hardly any carbon monoxide is produced during this process and there is no excessively high flame temperature (see point "1" in Fig.4 ). Starting from this lambda value, the speed of the fan 5 is increased until the second ionization signal detects a strong increase in the flame lifting off the burner 9 (see point "2" in Fig.4 ). From this point, the speed of the fan 5 is reduced again, while the ionization signal is observed in order to determine the exact position of the (absolute) minimum of the ionization signal and to adjust the setpoint to the minimum or close to it (see point "3" in Fig.4 ). At this point, it is checked whether the actual speed of the fan 5 corresponds to the expected speed, for example approximately 6,000 rpm.

From a control point of view, it is easier to use a value in a flank close to a minimum as a setpoint (here especially in the flank towards the richer mixture, i.e. between points "1" and "3" in Fig.4 ), because if the actual value changes (positive or negative), it is then clear in which direction a correction must be made. In any case, a desired lambda value close to 1.4 can be set without carbon monoxide being produced in the process.

The speed found in this way can now be compared with the original speed resulting from the control and its calibration can be corrected at this point (interpolation point) of the calibration curve. This can also be carried out in other load conditions (interpolation points) at suitable times so that the calibration can be corrected accordingly there too.

To determine the required correction, one considers, for example, the ratio of I2/I1. For example, one increases the fan speed (an increase increases the air ratio and reduces the risk of unintentional carbon monoxide emissions) until the ratio of I2/I1 has increased by 5 percentage points and determines at which fan speed this increase is achieved, whereby the initial speed (here 3000 rpm) and the final speed (here e.g. 4000 rpm) are put into relation (result 0.75) and can be compared with a previously stored reference value (e.g. 0.7). The reference value in relation to the newly measured ratio provides a correction factor (here 0.7/0.75 = 0.93) with which the original value of the calibration curve (100 µA) must be corrected at this point, resulting in the new recalibrated value of the calibration curve (93.3 µA). If you do not want to apply a single recalibration to a calibration curve immediately, you can use a so-called damping factor between 0 and 1. Each recalibration then has a correspondingly smaller effect on the calibration curve, which also dampens the adverse effects of any errors during recalibration and only results in a correct new calibration curve after several recalibrations.

Fig.3 shows a schematic circuit that can be used for the measuring system S2. A second AC voltage source 12 with a high output resistance 18 initially supplies an AC voltage without DC voltage component to the ignition electrode 7 and the counter electrode 9 (ground). If a flame occurs between the two (shown here as equivalent circuit 19), the voltage only drops in one half-wave due to the rectifying effect of the flame (shown as a diode in the equivalent circuit), so that an alternating voltage with a negative direct voltage component is present at the input of the second evaluation electronics 14 (amplifier and converter), which becomes the second ionization signal in the evaluation electronics 14 and can be converted in an analog/digital converter 20 and then further processed.

Fig.4 qualitatively illustrates what happens during the recalibration process using the second measuring system S2. In the diagram shown, the second ionization signal I2 is plotted on the Y axis (in digital form, e.g. as a number between 0 and 1023) against the fan speed on the X axis with a constant gas supply. The resulting characteristic diagram shows an almost constant initial range, a drop to a minimum (point "3") and then an increase. Experience has shown that the minimum is approximately at a usually desired lambda value of 1.3 to 1.4, but the constant range to the left of this, e.g. at point "1", is still far from lambda = 1. In the rise at around point "2", the flame begins to separate, which can then become unstable as the air supply increases. However, between points "1" and "2", the air supply can be varied without generating carbon monoxide or instabilities in order to find the minimum at point "3" and use it for recalibration. Instead of the fan speed, the lambda value could also be used as the unit on the X-axis because of the relationship described.

Fig.5 shows the result of the recalibration for a support point at a fan speed of 3000 rpm. Due to the recalibration at this support point, the desired constant lambda value of 1.3 is no longer achieved with an ionization signal of 100 µA, but already with an ionization signal of 93.3 µA. This value is after the Recalibration therefore requires the new setpoint at this point with corresponding adjustment of the values in the vicinity of this fan speed. Recalibration at several support points results in a new calibration curve for the desired lambda value, which corresponds to the Fig.2 takes into account the drift of the measuring system S1 shown.

The present invention makes it possible to set up a reliable recalibration of an existing standard control system without making any changes to the heater itself, using only additional electronics, by using the ignition electrode to generate a second ionization signal in the flame area, with which any long-term drift of the existing control system can be corrected at predeterminable intervals. A second ionization signal is used in many applications for flame monitoring anyway, so that only a few additional electronic components are required to use it to recalibrate the standard control system, in particular to correct a long-term drift.

List of reference symbols

1

heater

2

Flame area

3

Air supply

4

Fuel gas supply

5

fan

6

Fuel gas valve

7

Ignition electrode

8th

ionization electrode

9

Burner / Counter electrode

10

Switching unit

11

first alternating voltage source

12

second AC voltage source

13

first evaluation electronics

14

second evaluation electronics

15

Comparator

16

Correction unit

17

Control unit

18

Output resistance

19

Equivalent circuit of a flame

20

Analog/digital converter

S1

first measuring system

S2

second measuring system

I1

first ionization signal

I2

second ionization signal

A

Calibration curve new ionization electrode

B

Calibration curve used ionization electrode

Claims (8)

1. Method for recalibrating calibration data of a first measuring system (51) for measuring a first ionization signal in a flame region (2) of a heater (1) operated with combustion air and fuel gas, wherein the first measuring system (51) measures an ionization signal (I1), which is derived from a first ion current flowing from an ionization electrode (8) to a counter electrode (9) through the flame region (2), and from which the ratio of combustion air to fuel gas (lambda) during combustion in the heater (1) is determined and controlled on the basis of calibration data, wherein the first measuring system (51) is recalibrated at least according to predefinable criteria or at predefinable time intervals, wherein recalibration is carried out by means of an ignition electrode (7) located in the heater (1) for igniting combustion, which is operated in a second measuring system (S2) for generating a second ionization signal (I2), and wherein calibration data for the relationship between ionization signals (I1 and/or I2) and the ratio of combustion air to fuel gas under different load conditions is stored in the first (51) and in the second (S2) measuring system and a comparison is performed between the measured values determined by the two measuring systems (S1, S2) on each recalibration, wherein the first measuring system (51) is recalibrated in the event of divergences from the data for the second measuring system (S2).
2. Method according to claim 1, wherein combustion in the heater (1) is achieved in at least one constant state of combustion, which can be predefined by the first measuring system (51) and then, while maintaining or selectively altering this state, the second ionization signal (I2) of the second measuring system (S2) is measured.
3. Method according to any of the preceding claims, wherein combustion is achieved in succession by means of the first measuring system (51) in a plurality of different constant states and, in each of these states, the second ionization signal (I2) is measured at least in this state or in the event of a change in this state.
4. Method according to claim 3, wherein different constant states are different load stages of the heater (1), which are determined at least by different constant rotational speeds of a fan (5) or by different constant quantities of fuel gas supplied per unit of time by different constant settings of a fuel gas valve (6).
5. Method according to any of the preceding claims, wherein, after a constant state has been established, the ratio of combustion air to fuel gas (lambda) is first reduced by a predefinable value and then continuously or gradually increased, in order to record the profile of the second ionization signal (I2) as a function of the lambda value and to detect predefinable characteristic points of this profile.
6. Device, configured to implement a method according to any of the preceding claims, with a combustion chamber (1), having an air supply (3) and a fuel gas supply (4), which are regulated by a control unit (17), and with a first measuring system (51), comprising an ionization electrode (8), a counter electrode (9), a first alternating current source (11) and a first electronic evaluation system (13) for determining a first ionization signal (I1), which can be fed to the control unit (17), wherein there is a second measuring system (S2) for measuring a second ionization signal (I2), which can be produced between an existing ignition electrode (7) for igniting combustion and the counter electrode (9) of the second measuring system (S2) and wherein the first (51) and the second (S2) system are each configured to determine a lambda value and the second measuring system (S2) is set up independently of the first measuring system (51), that is to say, having no common components with the latter, apart from the counter electrode (9).
7. Device according to claim 6, wherein the second measuring system (S2) is set up differently from the first measuring system (51), that is to say, having the fewest possible or none of the same components as the latter and/or operating on a different principle.
8. Device according to any one of claims 6 or 7, wherein there is a comparator (15), to which measured values of the first (51) and the second (S2) measuring systems relating to a predefinable lambda value can be fed, and a correction unit (16) for recalibrating calibration data of the first measuring system (51) when divergences are detected in the two measured values.

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