Burner controller and adjusting method for a burner controller

Abstract

A burner controller (15) regulates the air index, with a combustion sensor. In accordance with the invention the reference value of the sensor signal is determined not only with the current burner power output but also on the basis of a measure for the current fuel energy content.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a burner controller which ensures inter alia that in a burner the ratio of the amount of air to the amount of fuel, referred to as the air index or lambda, is properly set over the entire power range.

2. Description of Prior Art

In general lambda is to be slightly over the stoichiometric value of 1, for example 1.3.

Advantageously the air index is regulated, for which purpose the arrangement needs a sensor which directly or indirectly observes the combustion process. As is known that purpose is served by using through-flow sensors in the feed passages, a gas sensor in the exhaust gas passage, a temperature sensor at the combustion chamber wall, a radiation sensor in the combustion chamber, or an ionization electrode in the flame. Under some circumstances however the level of regulating accuracy is adversely affected because of changes in the flame shape and size which can occur even with a constant burner power output.

In particular ionization electrodes are sensitive in relation to such changes in flame. The attempt has been made to resolve that problem by means of specific electrode design configurations. In 1983 a spiral-shaped monitoring electrode was disclosed in JP-A-58 099614. DE-C-195 02 900 showed in 1996 various electrode shapes which in addition are dedicated to air index regulation.

Recently a burner controller has been disclosed in EP-A-1 154 202. An ionization electrode is used as the combustion sensor. The ionization signal is compared to its reference value, which corresponds to an ionization signal that is desired at the prevailing power output. To establish a reference value curve, a behavior which is desired on the part of the ionization signal over the entire power range is established during an adjustment procedure, and stored in the burner controller. With the intention of regulating the ionization signal to its reference value the burner controller sets a setting member, for example a modulating valve in the gas feed passage.

That burner controller permits accurate air index regulation which can react particularly quickly to dynamic alterations. In certain burner installations however which are mostly equipped with an atmospheric burner the level of regulating accuracy is adversely affected by the above-mentioned problem of changes in flame shape and size.

EP-A-1 293 727 published in March 2003 describes some methods of calibrating such a burner controller. The calibration operation effects that the burner controller adapts the reference value curve of the ionization signal to possibly altered circumstances such as an undesired contamination or bending of the ionization electrode. Optionally the adaptation of the reference value curve involves a value which alters over the power range, which value is determined by the burner controller as a function of measurement results. The function constants required for that purpose have been previously ascertained in an adjusting method and stored in the burner controller. Accordingly the reference value curve of the ionization signal occurs afresh.

Although calibration corrects the above-mentioned adverse effects on accuracy at regular intervals, it cannot do that continuously.

BRIEF SUMMARY OF THE INVENTION

The object of the invention is to propose a burner controller which permits reliable and accurate regulation of the air index.

According to the invention, there is provided a burner controller which evaluates a signal of a combustion sensor and sets a setting member for a fuel feed or for an air feed by comparison of the signal with a reference value,

wherein the burner controller determines the reference value on the basis of a measure for the current burner power output, and

the burner controller determines the reference value at least also on the basis of a measure for the current fuel energy content.

It has been found that the prevailing energy content of the fuel mixture crucially determines the adverse effects on accuracy, as referred to in the opening part of this specification, although it is thought that other circumstances also play a part. Supposedly, the flow characteristics change with the energy content.

In accordance with the invention the burner controller firstly ascertains the current fuel energy content. This is for example effected by means of an additional sensor. In an advantageous configuration of the invention however a measure for the current setting of the setting member is used, which is conveniently already known to the burner controller in any case as a regulating parameter. A comparison with previously stored settings of the setting member for different fuel energy contents then approximately results in the current energy content. Moreover those stored settings of the setting member for the burner type in question or the burner installation in question have been ascertained once in an adjusting method, as is already shown in EP-A-1 154 202, more specifically in such a way that, with the different fuel energy contents, the respective desired air index occurred.

This operation of ascertaining the current fuel energy content on the basis of the current setting of the setting member has proven to be substantially independent of frequently changing conditions such as power output, air pressure and air humidity content.

Once the burner controller approximately knows the prevailing fuel energy content, in accordance with the invention, it determines the reference value for the signal of the combustion sensor that then applies. This can be effected in various ways. For example the burner controller iteratively adapts the reference value in small predefined steps until an additional combustion sensor again detects the optimum air index. In an advantageous configuration of the invention however data about desired signals of the combustion sensor at different fuel energy contents have been detected in advance and stored in the burner controller. In normal operation the burner controller processes those data for example such that those desired signals, suitably weighted to comply with the ascertained current fuel energy content, are continuously added. The result is the reference value for the signal of the combustion sensor.

In this way, a high level of regulating accuracy is maintained, even if at constant power output the flame shape and size change. Although some combustion sensors types referred to in the opening part of this specification can be used, an advantageous configuration of the invention lies in the combination with an ionization electrode. As has already been described above, the problem has long been dealt with by using specific electrode configurations. Those special electrodes have not yet met with great acclaim. The present invention thus follows a fresh path, which permits using rather conventional ionization electrodes.

The invention also concerns an adjusting method, preferably for such a burner controller, in which a burner is equipped with a combustion sensor, a setting member, a burner controller and a test sensor for establishing the quality of combustion. In burner operation using two or more kinds of fuel, the desired signals of the combustion sensor at the respective fuel energy content are ascertained. Those data are then stored in a burner controller. In an advantageous configuration, the method according to the invention is implemented at differing values of the feed that is not influenced by the setting member. That air or fuel feed in normal operation forms a precise measure for burner power output. Thus, for one or more kinds of fuel, some characteristics of the combustion sensor, which are dependent on the power output, can be stored in a burner controller.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a block circuit diagram of an ionization evaluation device in a burner controller according to the invention, and

FIG. 2 shows a block circuit diagram of a burner controller according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 diagrammatically shows the operating principle of ionization evaluation device 14 in a burner controller according to the invention. In an equivalent circuit a flame 1 is represented by a diode 1a and a resistor 1b. An ac voltage of for example 230V is applied across L and N. If a flame 1 is present, a greater current flows through a blocking capacitor 3 in the positive half-wave than in the negative half-wave, by virtue of the flame diode 1a. As a result, a positive dc voltage U_B is produced at the blocking capacitor 3 between L and a resistor 2 which is provided for the purposes of electric-shock contact protection.

Therefore a direct current flows through a decoupling resistor 4 from N to the blocking capacitor 3. In that situation the magnitude of the direct current depends on U_B and thus directly on the flame resistor 1b. The flame resistor 1b also influences the alternating current through the decoupling resistor 4, but to a differing degree from the direct current. Therefore a direct current and an alternating current flow through the resistor 4 as described above.

Now, connected on the output side of the resistor 4 are a high pass filter 5 and a low pass filter 6. The alternating current is filtered out by the high pass filter 5 and the dc voltage component blocked. The dc voltage component which is dependent on the flame resistor 1b is filtered by the low pass filter 6 and the alternating current substantially blocked. In an amplifier 7 the alternating current flowing out of the high pass filter 5 is amplified and a reference voltage U_Ref added. In an amplifier 8 the direct current flowing out of the high pass filter 6 is amplified with possibly slight alternating current components and the reference voltage U_Ref is added.

The reference voltage U_Ref can be selected to be of any value, for example U_Ref=0, but preferably it is so selected that the amplifiers and comparators require only one supply.

At a comparator 9 the ac voltage from the amplifier 7 and the dc voltage from the amplifier 8 are compared to each other and a pulse width-modulated (PWM) signal is produced. If the amplitude of the mains voltage changes the ac voltage and the dc voltage change in the same ratio and the PWM signal does not change. The deviation in the PWM signal can be set by means of the amplifiers 7 and 8 in a wide range between τ=0 and τ=50% pulse duty factor.

The dc voltage component U₌ is compared in a comparator 10 to the reference voltage U_Ref. If a flame is present the dc voltage component is greater than the reference voltage (U₌>U_Ref) and the comparator output of the comparator 10 switches to 0. If no flame is present the dc voltage component is approximately equal to the reference voltage (U₌≈U_Ref). Because of the small ac voltage which is superimposed on the dc voltage component and which the low pass filter 6 does not filter out the dc voltage component briefly falls below the reference voltage and pulses appear at the comparator output of the comparator 10. Those pulses are applied to a post-triggerable monoflop 11.

The monoflop 11 is triggered in such a way that the pulse sequence outputted fiom the comparator 10 arrives more quickly than is the pulse duration of the monoflop. As a result, if no flame is present, a 1 constantly appears at the output of the monoflop. If a flame is present the monoflop is not triggered and a 0 permanently appears at the output. The monoflop 11 thus forms a “missing pulse detector” which converts the dynamic on/off signal into a static on/off signal.

Both signals, the PWM signal and the flame signal, can now be subjected to further processing separately or can be interlinked by means of an or-member 12. When the flame is present, a PWM signal appears at the output of the or-member 12, the pulse duty factor of that PWM signal being a measure for the flame resistor 1b. The PWM signal forms an ionization signal 13 which is fed to a regulator 26 shown in FIG. 2. If there is no flame the output of the or-member 12 is permanently at 1. The ionization signal 13 can be transmitted by way of an optocoupler (not shown) in order to achieve protective separation between the mains side and the protective low voltage side.

FIG. 2 diagrammatically shows a block circuit diagram of a burner controller 15 according to the invention which can be designed substantially as a program portion for execution in a microprocessor.

A power demand 22 which corresponds to a certain air feed is sent to the burner installation. During burner operation the air feed is a measure for the burner output. In order to provide that air feed precisely, a regulating circuit (not shown) measures the air volume flow with a differential pressure sensor over an air resistance in the exhaust gas passage of the burner installation. On the basis of a resulting differential pressure signal 20, the regulating circuit sets the motor speed of an air blower 19. Because the differential pressure signal 20 forms an accurate measure for the current air feed, it is also used as an input parameter for air index regulation. It would also have been possible to select a controlled rotary speed for the blower motor, a measured valve position, or another regulating parameter. Finally the feed of a combustion gas to the burner is so adapted to the current air feed, that the air index is right. For that purpose the burner controller 15 produces a setting signal 18 which directly or indirectly, for example by way of a motor, sets a gas valve 17. Usually a mechanical pressure regulator is also provided in the gas feed passage.

The differential pressure signal 20 is fed by way of a filter 21 to a control unit 23. Stored there are characteristic data which establish the characteristic curves of two control signals 24 and 25, for a lean and a rich gas respectively. A regulator 26 weights and adds the two control signals and thus ascertains the setting signal 18. That procedure for processing the control signals depends on the ionization signal 13, which is compared with its reference value, which as much as possible corresponds to the desired air index.

The ionization signal 13 of an ionization electrode 16 which sticks in the flame 1 has been produced by the ionization evaluation device 14. It is firstly smoothed by the regulator 26 by means of a low pass filter 27 in order to suppress interference pulses and flicker. A comparison unit 28 then subtracts a reference value signal 30 which is delayed by a correction unit 29. In the next step a downstream-connected proportional regulator 31 and a parallel integration unit 32 produce an internal regulating value x for weighting of the two control signals 24 and 25. By way of the setting signal 18 the internal regulating value x effects that the difference between the ionization signal 13 and its reference value signal 30 is regulated down towards zero.

In a stable steady-state condition the regulating value x also represents a good measure for the energy content of the gas which is being fed in at that time, which content depends on the gas composition and pressure, wherein the lean and the rich gas of the two control signals 24 and 25 form the reference. Data about the desired ionization signals for the same reference gases are stored in the control unit 23, also in the form of two characteristic curves, as a function of the differential pressure signal 20. On the basis of those data the control unit 23 with the present differential pressure produces two reference signals from which the reference value signal 30 is produced. In order that the reference value signal 30 corresponds to the type of gas which is being fed in at the present time, the reference signals are weighted by a proportional measure for the energy content thereof, namely the regulating value x, and added together. Equivalently, the two reference signals can firstly be subtracted from the ionization signal 13 by a respective comparison unit 28 and only then be weighted by the regulating value x and added together. Here the reference value is furnished in the complex configuration of two reference signals and a weighting factor for comparison with the ionization signal 13. Further alternatives are possible.

The air index regulation includes two feedback effects and for that reason must involve dynamic damping by virtue of a suitable choice of the settings of the proportional regulator 31 and the integrating unit 32 so as to avoid an oscillation-susceptible characteristic.

The regulating value x is also fed to a calibration unit 36. The calibration unit 36 includes a clock which triggers calibration operations at regular periods. When the time is again ready the calibration unit 36 firstly brings the power demand 22 and therewith the input parameter of air index regulation to fixed, predetermined values. Then in a first step it increases the reference value signal 30 in order to bring the system into a sensitive working region slightly closer to the stochastic combustion point with the air index equal to 1. Thereafter the calibration unit 36 detects the value x1 of the steady-state regulating value x.

In a second predetermined step the calibration unit 36 increases the reference value signal 30 once again. Consequently the regulator 26 regulates the setting signal 18 by reduction of the regulating value x to a still somewhat richer combustion condition. After for example 16 seconds, when the regulating value x is again in a steady-state condition, its new value x2 is detected. The calibration unit 36 however also calculates an expectation value 40 for the new value x2, more specifically from a third-order polynomial development of the value x1, the constants of which were ascertained in an adjusting method for the type of burner and stored as characteristic data in the burner controller. The expectation value 40 is subtracted from the value x2 which is actually detected. Any difference is an indication that the air index in normal operation was not at its desired value and combustion was excessively lean or excessively rich.

If the difference ascertained in that way should exceed certain threshold values, the calibration unit 36 will indicate emergency operation or even shut down operation of the burner. In addition, the calibration unit 36 averages by exponential weighting the differential value with the mean value of the differential values from the earlier calibration procedures, more specifically so that later ones weigh more heavily than earlier ones. If the newly produced mean value exceeds a low threshold value then the calibration unit 36 adapts the production of the reference signals in the control unit 23 by adding or instead subtracting a respective small value to both characteristic curves over the entire differential pressure range. The result is, that the higher characteristic curve is displaced upwardly, respectively downwardly, more than the lower one is. Afterwards, combustion in normal operation should be a little richer or leaner, respectively. By repeated calibration the air index in normal operation is iteratively moved towards its desired value.

Claims

1. A burner controller which evaluates a signal of a combustion sensor and sets a setting member for a fuel feed or for an air feed by comparison of the signal with a reference value,

wherein the burner controller determines the reference value on the basis of a measure for the current burner power output, and

the burner controller determines the reference value at least also on the basis of a measure for the current fuel energy content, the current fuel energy content being derived from a measure for the current setting of the setting member and from data about desired settings of the setting member for fuels having different fuel energy contents.

2. A burner controller as set forth in claim 1, wherein the burner controller determines the reference value at least also by processing data about desired signals of the combustion sensor at different fuel energy contents on the basis of the measure for the current fuel energy content.

3. (canceled)

4. A burner controller as set forth in claim 1, wherein the combustion sensor is an ionization electrode which sticks in the flame.

5. An adjusting method for a burner controller, comprising the steps of:

equipping a burner with a combustion sensor, a setting member, a burner controller and a test sensor for establishing quality of combustion,

operating the burner with a first fuel of a certain energy content at a first plurality of setting member states,

determining a desired signal of the combustion sensor by means of the test sensor results for the first plurality of setting member states with the first fuel,

establishing first data about the desired signal for the first plurality of setting member states with the first fuel,

operating the burner with a second fuel of a different energy content at a second plurality of setting member states,

determining a desired signal of the combustion sensor by means of the test sensor results for the second plurality of setting member states with the second fuel,

establishing second data about the desired signal for the different setting member states with the second fuel, and

storing the established first and second data in the burner controller.

6. An adjusting method as set forth in claim 5, wherein the burner operations are repeated at least partially at different values of the feed that is not influenced by the setting member.

7. An adjusting method as set forth in claim 5, wherein the specific energy content of one of the first and second fuels is at least 7% higher than that of the other.

8. An adjusting method as set forth in claim 5, wherein the burner operations are at least partially so effected that a desired setting value of the setting member too is obtained by means of the test sensor results and data about the desired setting value are established.

9. The adjusting method of claim 5, further comprising:

operating the burner with a third fuel, and

controlling the setting member state based upon the stored first and second data.

10. A method of controlling a burner comprising:

storing a desired burner condition for each of a plurality of burner output settings using a plurality of fuel types, each of the plurality of fuel types having an energy content different from the energy content of the other of the plurality of fuel types;

establishing a desired output for the burner;

providing a first combustion fluid to the burner based upon the established desired output;

comparing a signal indicative of a condition of the burner with a signal based upon the stored desired burner conditions; and

controlling the supply of a second combustion fluid to the burner based upon the comparison.

11. The method of claim 10, wherein:

the first combustion fluid comprises oxygen; and

the second combustion fluid comprises a fuel.

12. The method of claim 11, further comprising:

determining a first desired flame condition for a first burner output setting using a fuel type having a first energy content;

determining a second desired flame condition for a second burner output setting using the fuel type having the first energy content;

determining a third desired flame condition for a third burner output setting using a fuel type having a second energy content;

determining a fourth desired flame condition for a fourth burner output setting using the fuel type having the second energy content, and wherein storing a desired burner condition comprises:

storing the first desired flame condition, the second desired flame condition, the third desired flame condition and the fourth desired flame condition.

13. The method of claim 12, wherein determining a first desired flame condition comprises;

sensing the condition of the burner flame using an ionization electrode.

14. The method of claim 10, wherein comparing a signal indicative of a condition of the burner with a signal based upon the stored desired burner conditions comprises:

generating an ionization signal;

subtracting a reference signal from the ionization signal; and

generating a regulating signal based upon the result of the subtraction, and wherein controlling the supply of a second combustion fluid to the burner based upon the comparison comprises:

controlling the supply of the second combustion fluid based upon the regulating signal.

15. The method of claim 14, wherein the reference signal is generated based upon a desired burner condition for the established desired output.

16. The method of claim 14, wherein controlling the supply of the second combustion fluid based upon the regulating signal comprises:

weighting, based upon the regulating signal, a first signal indicative of a determined desired flame condition for the established desired output of the burner using a fuel type having a first energy content;

weighting, based upon the regulating signal, a second signal indicative of a determined desired flame condition for the established desired output of the burner using a fuel type having a second energy content; and

controlling the supply of the second combustion fluid based upon the weighted first signal and the weighted second signal.