EP0770824A2 - Method and circuit for controlling a gas burner - Google Patents

Abstract

The regulation system uses an ionisation electrode (4) providing an ionisation signal dependent in the temp. or lambda value of the combustion, supplied to a regulation circuit (9) for comparison with a reference value, for adjustment of the air/fuel ratio for the burner (1). A calibration cycle is initiated at regular intervals or after a defined operating time, with the lambda value reduced and the corresponding ionisation signal measured. The max. values of the ionisation signal are stored for correction of the required value for the regulation.

Description

The invention relates to a method for controlling a gas burner, in particular a gas fan burner, with a measuring electrode, in particular an ionization electrode, which applies an electrical variable derived from the combustion temperature or the actual lambda value to a control circuit which compares this variable with a selected electrical setpoint and sets the gas-air ratio (lambda) to a corresponding lambda setpoint. The invention further relates to a corresponding control circuit.

Such a regulation is described in DE 39 37 290 A1. There the ionization electrode lies in a direct current circuit. The evaluation of the ionization current is problematic in practice if a proportional relationship between the ionization current and the lambda value is to be determined.

In the older patent application P 44 33 425 a control device for a gas fan burner is described. Through an alternating voltage superimposition, the Evaluate ionization current safely. The respective excess air (lambda value) of the respective combustion state is recorded via the ionization electrode and compared in the control circuit with a set target value. The composition of the gas-combustion air mixture is adjusted accordingly, so that the end result is always a desired lambda setpoint. An over-stoichiometric ratio of air to gas is desired, the lambda setpoint preferably being between 1.15 and 1.3. It is achieved in that with different gas qualities, for example natural gas and liquid gas, as well as with changing ambient conditions, an optimal combustion takes place in terms of emissions and the efficiency of the combustion technology.

During operation, the thermal coupling between the ionization electrode and the gas burner can change, for example due to bending, wear and contamination of the ionization electrode or sooting of the burner. It was found that this leads to the fact that the ionization current and thus the measured variable derived therefrom change despite the lambda value remaining the same. The proportionality factor therefore changes between the lambda value and the electrical variable derived therefrom. Since this changed measuring voltage is applied to the comparator of the control circuit, on which the - unchanged - setpoint also acts, the control circuit will adjust the gas-air mixture, i.e. the lambda value, causing a deviation of the actual lambda value from the lambda Setpoint comes, which is undesirable.

The object of the invention is to propose a method and a circuit of the type mentioned at the outset with which the influence of a change in the proportionality between the lambda value and that derived therefrom electrical measured variable on the control is compensated in such a way that the desired gas-air ratio (lambda setpoint) is maintained.

According to the invention, the above object is achieved in a method of the type mentioned at the outset by the features of claim 1 and with respect to the circuit by the features of claim 6.

After a certain operating time, which can be determined either by an operating hours counter or by counting the start-up processes of the burner, the control is switched off for a short time and a calibration cycle is carried out. In this, the gas-air mixture is forcibly enriched, i.e. the lambda value is reduced from> 1. The measured electrical quantity passes through a maximum at lambda = 1. This value is recorded. If it deviates from the set basic electrical setpoint, this is readjusted. Such a deviation occurs when the ionization electrode is bent, worn or sooty, which in itself would lead to an undesirable adjustment of the gas-air ratio. Such an adjustment is avoided by the invention, so that the desired lambda setpoint is also controlled when the proportionality factor existing between the combustion temperature and the electrical measured variable has changed.

After the calibration cycle, if necessary after evaluating one or more transfer criteria, the system switches back to "control". If the deviation lies outside a "window", an interference signal is triggered and / or the burner is forcibly switched off.

• Figur 1 ein Blockschaltbild einer Regelschaltung bei einem Gasgebläsebrenner,
• Figur 2 ein Kennliniendiagramm und
• Figur 3 ein Zeitdiagramm beim Start eines Kalibrierungsvorgangs.

Further refinements result from the subclaims and the following description of a Exemplary embodiment In the drawing:

• FIG. 1 shows a block diagram of a control circuit in a gas fan burner,
• Figure 2 is a characteristic diagram and
• Figure 3 is a timing diagram at the start of a calibration process.

A gas burner (1) has a speed-adjustable fan (2) which conveys combustion air. It is provided with a gas supply (3) in which a gas solenoid valve (3 ') is arranged. An ionization electrode (4) is arranged as the measuring electrode in the flame area of the gas burner (1). This measuring electrode (4) is common in gas burners. However, it is usually only used for flame monitoring. The measuring electrode (4) detects the ionization current which occurs in the respective combustion state. According to Richardson's equation, this depends on the electrode temperature and thus also on the respective lambda value of the respective gas-air mixture.

An AC voltage, in this case simply the AC mains voltage, is applied to the measuring electrode (4) via a capacitive coupling element (5). The coupling element (5) is connected to earth via a resistor (6), so that the ionization path (flame area) is electrically connected in parallel to the resistor (6).

A low-pass filter (8) is connected to the measuring electrode (4) via a voltage-impedance converter (7) and is connected on the output side to a control circuit (9).

The control circuit (9) according to FIG. 1 has a comparator (10) to which a setpoint generator (11) is placed. At the Setpoint generator (11) can be set to an electrical setpoint corresponding to the desired lambda value, for example 1.15 to 1.3. The output direct voltage of the low-pass filter (8) is applied to the comparator (10) and is proportional to the respective lambda value. On the output side there is a voltage / current converter (12) on the comparator (10), which is connected via a changeover switch (13) to a power driver (14) which controls the speed of the fan (2) and / or the position of the gas solenoid valve (3 ') controls.

In the control circuit (9) an automatic start (15) is integrated, which controls the changeover switch (13). A setpoint generator (16) for a starting speed is located on the changeover switch (13). A memory (17) is also provided for the current speed value and / or the current setting value of the gas solenoid valve (3 ').

A Schmitt trigger (18) is also connected to the output of the low pass (8) and is used for flame monitoring.

The control circuit described so far works as follows:

When the gas burner (1) starts, the automatic start (15) switches to the setpoint device (16). The fan (2) thus runs via the power driver (14) at a starting speed which results in a mixture which can be ignited safely.

After ignition and successful flame formation, the automatic start (15) switches the changeover switch (13) to the voltage / current converter (12). The ionization current detected by the ionization electrode (4) leads to a direct voltage being superimposed on the alternating voltage. This is proportional to the ionization in the flame area. It is proportional to the respective excess air (lambda). In practice, it is between 0 V and 200 V. For further processing, the voltage is reduced and a DC voltage between 0 V and 10 V occurs at the output of the low pass (8) in the example.

The voltage (ionization voltage Ui) embodying the excess air of the respective gas-air mixture is compared in the comparator (10) with a desired value. The difference between the two values is converted into a current that changes the state of charge of the storage capacitor (17), which corresponds to the instantaneous speed value, and thus controls the speed of the fan (2) accordingly until the respective excess air (actual lambda value ) is the same as the Lambda setpoint.

If there is a change in the combustion conditions afterwards, for example a change in the gas type, a change in the gas pressure, a change in the ambient temperatures or the like, and if the actual lambda value deviates from the target lambda value, these faults are corrected in the manner described.

When the flame goes out, the gas supply (3) is blocked by the gas solenoid valve (3 ') via the Schmitt trigger (18).

To adjust the excess air, the speed of the fan (2) or the gas supply (3) is regulated.

The control circuit (9) can also be constructed as a digital circuit with a microprocessor.

An activation circuit (21) is also provided. This counts the starting processes triggered by the automatic start (15) or records the operating hours of the gas burner (1). A ramp generator (22) is connected to the activation circuit (21) and is connected to a third Switch position of the switch (13) is connected.

At the output of the low pass (8) there is a detection circuit (23) which is also connected to the activation circuit (21) and which is followed by a storage circuit (24). The memory circuit (24) is connected to the setpoint generator (11).

The additional circuit functions in a calibration cycle as follows:

After a certain number of starting processes or operating hours, for example 100 starting processes or 10 operating hours, the activation circuit (21) brings the changeover switch (13) into its third switching position and activates the ramp generator (22). The control described above is switched off.

The ramp generator (22) now controls the blower (2) or the gas solenoid valve (3 ') in such a way that the gas-air mixture is "enriched", ie the gas content increases. The lambda value is continuously reduced from a value> 1, for example 1.3, to a value below 1. This results in a curve of the measurement voltage (ionization voltage Ui) derived from the ionization electrode (4) at the output of the low pass (8), as is shown by way of example in one of the curves I, II, III in FIG. 2. Which of the curves occurs depends on the state of the ionization electrode (4) or the gas burner (1); So it depends on how the ionization electrode (4) lies in the connection area of the burner flames. For example, if the ionization electrode (4) is bent, worn or sooty, a different voltage profile is produced than in the "good" condition.

All curves I, II, III run through at lambda = 1 Maximum. The maxima of curves I, II, III are designated A, B, C in FIG.

The detection circuit (23) detects the respective voltage maximum A, B, C, for example by evaluating the slope of the curve I, II or III. The respective maximum voltage is stored in the memory circuit (24). The memory circuit (24) sets the basic value (100%) of the setpoint generator (11) to this value.

If one assumes, for example, that I is the characteristic of a "good" condition of the ionization electrode (4) and assumes that the lambda setpoint should be 1.2, then the setpoint generator (11) has been set in this way that it was set to 90% of its basic value (100%) (cf. a in Fig. 2, where Fig. 2 is not to scale).

As long as nothing changes in the state of the ionization electrode (4) or the gas burner (1), nothing is changed in the calibration cycles on the basic value (100%) of the setpoint generator (11).

If the characteristic curve (II) with the maximum value (B) results in a calibration cycle, which is the result of a change in state of the ionization electrode (4), then this voltage value (B) is stored in the memory circuit (24) as the basic value for the setpoint generator ( 11) saved. The setpoint generator (11) remains set to 90% of a basic value, which b shows in Fig.2. From Fig. 2 it can be seen that at voltage (b) (90% of maximum voltage B) via comparator (10), when the control is switched on again after the calibration cycle by means of the switch (13), a control to the lambda Setpoint of 1.2.

It is thus achieved that the control circuit (9) always depends on the respective state of the ionization electrode (4) is readjusted in such a way that the control circuit (9) regulates the actual lambda value to the desired lambda setpoint during control operation. Operational changes in the state of the ionization electrode (4) or the gas burner (1) are thus balanced.

There are limits to the adjustment of the setpoint generator (11) described. These are indicated in Fig. 2 by the window (F). As long as the maxima of the voltage profiles, such as A, B, are within the window (F) in the calibration cycles, the described adjustment of the setpoint generator (11) takes place. If there is a voltage maximum, such as C, which lies outside the window (F), then the detection circuit (23) detects this and triggers an interference signal and / or a forced shutdown of the gas burner (1).

The calibration cycles are very short compared to the times in which the gas burner (1) operates in normal control mode, so that the combustion occurring during the calibration cycles with a lambda value that deviates from the lambda setpoint can be accepted. The combustion improves in each of the regular operations following a calibration process.

Further developments of the calibration processes described above are explained below.

The described control function is switched off during calibration. The calibration is preferably carried out when the speed of the fan (2) does not change in order to suppress the influence of the fan (2) on the combustion. It is expedient to carry out the calibration at a medium speed so that the calibration signal does not hit the modulation limits of the control signal (J) which is applied to the gas solenoid valve (3 '). The calibration can also be done during of switching the blower (2) from one power level to the other power level because the speed change compared to the calibration process is slow, so that the speed during the calibration process is quasi constant.

The calibration process is started at time (t1) (see Fig. 3) by the event or operating hours counter during the transition from the full load stage to the partial load stage of the blower (2) when the decreasing modulation current (J) reaches a low value (Jk). The modulation current (J) and thus the gas supply via the gas solenoid valve (3 ') is then increased by the control circuit (9), as a result of which the ionization voltage (Ui) increases accordingly. At time (t2), the ionization voltage (Ui) reaches a predetermined value, for example 0.9 Uimax. The time period (t1 to t2) serves to start the preheating of the ionization electrode (4). From time (t2) up to time (t3) the modulation current (J) is kept constant. During this period (t2 to t3), the ionization electrode (4) heats up to a stable temperature and thus ensures reproducible measured values.

After the time (t3), the modulation current (J) is increased by the control circuit (9) so that the maximum value (Uimax) of the ionization voltage (Ui) is exceeded. This - new - maximum value (Uimax) and / or the measured values resulting in the time period (t3 to t4) is / are stored for further processing in the calibration process.

The modulation current (J) is increased further until the ionization voltage (Ui) is again approximately 10% below the Uimax value, which is the case in FIG. 3 at time (t4). In the period (t3 to t4), the lambda value of the combustion itself is unfavorable, but this is not important since this period lasts at most a few seconds.

After the time (t4), the control circuit (9) switches back to the control process described above. This starts when the ionization voltage (Ui), the modulation current (J) and the gas pressure (p) have stabilized at time (t5).

The control circuit (9) derives a correspondingly adapted new setpoint for the ionization voltage from the stored - new - maximum value of the ionization voltage or from the measurement values obtained in the period (t3 to t4).

Because of the short sampling period of the control circuit (9), a series of measured values will also result in the time span (t3 to t4). Measured values that differ greatly from the other measured values in the series are suppressed because they can be based on external electrical interference pulses.

In order to reduce the influence of only temporarily occurring, but unusual, but still tolerable calibration measurement series, an average can be formed between the new measurement series and the measurement series of previous calibration processes.

Before the new calibration value, which can be derived from the new maximum value of the ionization voltage or from the measured value series, is actually used to recalibrate the target value of the ionization voltage, two transfer criteria are checked by the control circuit (9).

The first transfer criterion detects a sudden change in all components of the control loop. It is fulfilled if the deviation of the new calibration value from the previous calibration values is sufficiently small.

The second transfer criterion detects a "creeping drift" of the system (burner control), which is sufficiently small in the event of a deviation from the values provided by the manufacturer.

The burner operation will only continue with the recalibration if both transfer criteria are met. If one of the transfer criteria is not met, the burner operation is first interrupted by a control shutdown and after repeated repetition by a lockout.

Claims (7)

Method for controlling a gas burner, in particular a gas fan burner, with a measuring electrode, in particular an ionization electrode, which applies an electrical variable (ionization signal) derived from the combustion temperature or the lambda value to a control circuit which compares this variable with a selected electrical setpoint and sets the gas-air ratio (lambda value) to a corresponding lambda setpoint,
characterized,
that after a certain operating time or at regular intervals, a calibration cycle is forcibly carried out, in which the lambda value is reduced from a value> 1 and in which the resulting electrical variable (ionization signal) is measured and its maximum value (A, B, C ) is stored and that the electrical setpoint is adjusted with this maximum value so that the control circuit regulates to the same lambda setpoint. Method according to claim 1,
characterized,
that a calibration cycle is initiated after a certain number of operating hours or switch-on of the gas burner. The method of claim 1 or 2,
characterized,
that if the maximum value (A, B, C) is outside a predetermined window (F), a fault signal occurs. Method according to one of the preceding claims,
characterized,
that the lambda value is traversed from a value> 1 to a value below 1 in the calibration cycle. Method according to one of the preceding claims,
characterized,
that the lambda value> 1 in the calibration cycle is at least as large as the adjustable lambda setpoint. Method according to one of the preceding claims,
characterized,
that in each calibration cycle the control signal (J) for a gas solenoid valve (3 ') is first brought to a value suitable for preheating the ionization electrode (4) and then the control signal (J) is increased until the maximum value of the ionization signal (Ui) passes through and the resulting value is evaluated for calibration. Circuit for regulating a gas burner, in particular a gas fan burner with a measuring electrode, in particular an ionization electrode, which applies an electrical measured variable (ionization signal) corresponding to the combustion temperature (lambda value) to the control circuit, a comparator in the control circuit comparing the respective electrical measured variable (ionization signal) compares with a setpoint generator and regulates the gas-air ratio to a lambda setpoint,
characterized,
that a changeover switch (13) interrupts the control and a ramp generator (22) reduces the gas-air ratio from a lambda value> 1, the electrical measured variable (U) passing through a curve (I, II, III), and that a detection and storage circuit (23,24) the The value of the measured variable in the maximum (A, B, C) of the curve (I, II, III) is recorded and stored and the setpoint generator (11) is adjusted to this value as the basic value.

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