EP1719947B1 - Method and device for flame monitoring - Google Patents

Description

The invention relates to a method and a device for flame monitoring according to the subject matter of claims 1 and 5.

A method and a device for flame monitoring, for example, from EP 617 234 A1 known. This document discloses a Ionisationsflammenwächter with a capacitor which is connected to a reference voltage source and a coupling element with the secondary circuit of a Zündübertragers. As long as there is no flame between the ignition electrode and the ground line, the capacitor is charged via a resistor to an operating voltage. As soon as an ionization current flows as a result of the formation of flames, the capacitor is discharged. The capacitor is connected to a monitoring circuit which, when falling below a predetermined threshold value, generates an output signal which indicates the presence of a flame.

The EP 1 256 763 A2 discloses a flame monitoring method in which the radiation generated by the flame is detected by a photoresistor and the sensor signal is evaluated in two channels. The first channel is used to detect the average brightness and the second channel is used to detect alternating parts resulting from the flickering of the flame. The flame is only recognized as burning if the signal is within a specified range at both channel outputs.

The document JP-62255729 discloses a circuit and method for flame monitoring in which a reference voltage source is connected via a switch to a capacitor which is closed to charge the capacitor or to discharge the capacitor (18) is opened, wherein the capacitor is charged during the charging phase with a reference voltage and the Capacitor is discharged via a during the discharge phase connected to a flame sensor coupling member. In this case, the time duration is evaluated until falling below a threshold value provided for the discharge phase of the capacitor.

The invention has for its object to provide a method and a device for flame monitoring, which is versatile and allows a simple signal evaluation.

This object is achieved by the features specified in claims 1 and 5.

In the method according to the invention, a capacitor connected to a voltage source during a charging phase is charged to a voltage value, and during a discharge phase the capacitor is discharged via a coupling member connected to the flame sensor. The period of time for the charging and discharging phase of the capacitor is chosen as a function of the characteristic, in particular the impedance of the flame sensor. The charging or discharging of the capacitor is repeated cyclically and the resulting voltage signal is evaluated for flame monitoring single-channel, the evaluation of the voltage signal on the capacitor is synchronous with the mains frequency.
For signal evaluation, a threshold value which is uniform for different sensor impedances is preferably used.

By the invention, various flames, for. B. pilot flame or flame at maximum load of an oil, gas, or solid fuel burner are monitored, with a variety of different flame sensors, eg. As photoresistor, ionization current electrode, UV tubes, etc. can be used for flame monitoring.

The invention does not require active signal amplification for evaluation. As a result, the monitoring circuit can be constructed with a small number of components. For example, the capacitor provided for flame monitoring also performs the function of signal filtering with a low-pass character.

The method according to the invention can be used in continuous operation or in the intermittent operation of a burner, wherein different fault scenarios can be taken into account in the signal evaluation. For example, the impedance of the flame sensor can assume a static value in the event of a fault or when exposed to daylight. This can be detected at the end of the charging phase by evaluating the voltage signal obtained at the capacitor. Also may be component failure of the circuit or the sensor For example, a short circuit of the flame sensor or a line break are detected to the flame sensor.

By the inventive method, extraneous light can be detected. If the flame sensor is irradiated with a fluorescent lamp or light bulb, this changes the impedance of the flame sensor in the rhythm of the mains frequency or their multiples. The network-harmonic changes of the sensor impedance caused by the extraneous light source lead to no signal dynamics in a network-synchronous evaluation of the voltage signal. For detection of extraneous light in continuous operation, and the Flackeranteil the flame, the z. B. in the frequency range of 8-30 Hertz, be monitored and evaluated.

Fig.1

ein prinzipielles Blockschaltbild einer nicht erfindungsgemässen Überwachungsschaltung

Fig. 2

Spannungssignalverlauf in Abhängigkeit von der Sensorimpedanz

Fig.3

eine gemäss der Erfindung dargestellte Schaltung zur Erkennung von Fremdlicht

Fig.4

Spannungssignalverlauf mit Fremdlichtsignal

Fig.5

bis 8 jeweils eine weitere Ausführungsform der erfindungsgemässen Überwachungsschaltung

Further advantages will become apparent from the following description of the invention with reference to the embodiments and the figures. Show it:

Fig.1

a schematic block diagram of a non-inventive monitoring circuit

Fig. 2

Voltage waveform as a function of the sensor impedance

Figure 3

a circuit according to the invention for detecting extraneous light

Figure 4

Voltage waveform with extraneous light signal

Figure 5

to 8 each show a further embodiment of the inventive monitoring circuit

Fig. 1 shows the basic structure of a non-inventive circuit for flame monitoring, which can be adapted with little modification to different flame sensors for detecting the flame and flame existence of oil, gas and solid fuel burners.

The flame sensor is z. B. a photoresistor 1, which has a radiation sensitivity in the spectral range to be monitored. The radiation sensitivity manifests itself by different impedance values upon irradiation of the flame sensor, with an increase in the intensity of the flame radiation resulting in a decrease in the impedance value of the photoresistor.

The photoresistor 1 is connected via a coupling member 19 to a capacitor 18 provided for evaluation. The capacitor 18 is connected via a switch 12 to a reference voltage source 13, which has an internal resistance 11.

For charging, the capacitor 18 is connected via the internal resistance 11 by means of the switch 12 to the reference voltage source 13. Characterized the capacitor 18 is charged to a voltage value which is dependent on the internal resistance 11 of the reference voltage 13, the impedance of the coupling member 19 and the photoresistor 1. After a defined charging time, a measured value dependent on the impedance of the flame sensor 1 is obtained by an A / D converter 20. The A / D converter 20 can be connected to the capacitor 18 via a switch 17 and a resistor 16. However, the A / D converter 20 may also be directly connected to the capacitor 18. The switches 12 and 17 may, for. B. field effect transistors.

In the discharge phase, the connection to the reference voltage source 13 is interrupted by means of the switch 12 and the capacitor 18 is discharged via the coupling impedance 19 through the photoresistor 1. After a defined discharge time, the A / D converter 20 supplies a measured value which is dependent on the impedance of the flame sensor 1 and filtered by the capacitor 18. The control of the charging and / or discharge phase is carried out by a control unit 21, which z. B. is designed as a microprocessor or logic device with comparator.

FIG. 2 shows the waveform for the voltage Uc obtained at the capacitor as a function of the impedance of the flame sensor and the time. The increase in impedance is shown by an arrow 33. With increasing impedance, the voltage Uc obtained at the end of the charge phase 31 or discharge phase 32 at the capacitor assumes a higher value. By a cyclic repetition of charging or discharge phase, a characteristic of the respective sensor impedance voltage signal 30 is obtained, which is evaluated for flame monitoring. For evaluation of the sensor impedance-dependent voltage signal 30, a uniform threshold value 34 is preferably used. The definition of the threshold value 34 and the time duration for charging or discharging phase can be carried out by a control unit. The period of time for the charging or discharging phase is selected as a function of the respective impedance or characteristic of the flame sensor. By evaluating the voltage signal 30 at the end of the charging phase 31 and / or at the end of the discharge phase 32, z. B. component failure of the monitoring circuit or error of the flame sensor can be detected.

FIG. 3 shows an inventive development of in FIG. 1 shown monitoring circuit, which additionally has a voltage divider 27, which serves to return the network phase to the control unit 21. The voltage across the capacitor 18 is thereby detected synchronously with the mains frequency. The charging phase is preferably chosen so long that after charging of the capacitor 18, the switch 12 remains closed for at least one network period. During this time, by monitoring the mains phase and closing the switch 17, the voltage obtained at the capacitor 18 is detected by the A / D converter 20 cyclically and synchronously with the mains frequency. If the flame sensor is irradiated by a fluorescent lamp, for example, then the sensor impedance changes in the rhythm of the mains frequency or its multiples.

In Fig. 4 the voltage Uc obtained at the capacitor is shown together with a mains-synchronous extraneous light signal 50 as a function of time. Through a cyclic repetition of the charging or discharging phase, a voltage signal 40, characteristic of the respective sensor impedance, is obtained, which can be detected and evaluated in a network-synchronous manner at the times t1, t2, t3, etc. In this case, identical voltage values Uc are obtained for one and the same sensor impedance in this exemplary embodiment. From these voltages z. B. an average can be formed, which is evaluated for extraneous light detection. If the mean value lies below a defined threshold value 34, then this is recognized as extraneous light error.

FIG. 5 shows a circuit in which the sampling can be done at arbitrary times. The samples supplied by a sample-and-hold device 28 in synchronism with the line frequency are buffered in a capacitor 30. A pulse shaper 29 generates from the mains frequency a control pulse which, for a short time, closes the sample-and-hold circuit 28, thereby causing the capacitor 30 to be charged with the samples.

FIG. 6 shows a circuit that is used for two different flame sensors 1 and 2. In the case of a gas flame 3, a chemical reaction takes place during combustion, as a result of which free ions occur. These cause the flame 3 to become conductive and a current to flow when a voltage is applied. The ions move only in the direction of the flame. If an AC voltage is applied between the burner mass and the ionization electrode 2, then a rectification effect occurs due to the ionization.
A serial link 22 shows a simplified equivalent circuit for the rectification effect by flame ionization. An alternating voltage is applied to the ionization electrode 2 via a capacitor 25 and a resistor 26. Due to the flame ionization, a rectification of the ionization current takes place, which leads to a Potential shift on the capacitor 25 leads. Via a coupling resistor 23 and a low-pass filter 24, the charge transfer from the capacitor 25 to the capacitor 18 is coupled. During the discharge phase, the capacitor 18 is then discharged in dependence on the ionization current.

Fig. 7 shows a further education in FIG. 6 shown circuit which additionally has a voltage divider 27, which serves to return the network phase to the control unit 21. The detection of the voltage across the capacitor 18 is thereby synchronous to the mains frequency. The evaluation can be done in the same manner as described above in connection with a photoresistor.

FIG. 8 shows a monitoring circuit for a UV sensor. In this circuit, a pulsating voltage is applied to a UV sensor 4 via a capacitor 25, a resistor 26 and a diode 5. When irradiated with UV light, the W tube is then ignited. The cyclic firing of the W-tube drives a pulsed current through the diode 5 and leads to a potential shift at the capacitor 25. Via a coupling resistor 23 and a low-pass filter 24, the charge transfer at the capacitor 25 is coupled to the capacitor 18. The charge transfer at the capacitor 25 is polarized such that this leads to a discharge of the capacitor 18 during the discharge phase. The evaluation of the voltage signal on the capacitor 18 for flame monitoring can be carried out in the same manner as has been described in connection with a photoresistor or ionization.

Claims (7)

1. Method for flame monitoring in which a capacitor (18) is charged with a reference voltage (13) during a charging phase (31) and the capacitor is discharged via a coupling element (19) connected to a flame sensor (1) during a discharging phase (32), characterised in that the duration for the respective charging or discharging phase (31, 32) of the capacitor (18) is selected as a function of the characteristic of the flame sensor used (1) and that, for flame monitoring, the respective charging or discharging of the capacitor is repeated cyclically, whereby a voltage signal (30, 40) is received at the capacitor (18) which is subject to single-channel evaluation with the aid of a threshold value (34), with the evaluation of the voltage signal at the capacitor (30, 40) being undertaken synchronously with the ac mains frequency.
2. Method according to claim 1, characterised in that the duration of the respective charging or discharging phase (31, 32) is dependent on the impedance of the flame sensor (1).
3. Method according to claim 2, characterised in that the voltage signal (30, 40) at the capacitor (18) is evaluated with a uniform threshold value (34) for different impedances of the flame sensor (1).
4. Method according to claim 2 or 3, characterised in that evaluation of the voltage signal (30, 40) at the capacitor (18) at the end of the charging and/or discharging phase (31, 32) detects component faults or flame sensor faults.
5. Device for flame monitoring with a capacitor (18) which is connected for charging to a reference voltage source (13) and for discharging via a coupling element (19) to a flame sensor (1), with the reference voltage source (13) being connected via a switch (12) to the capacitor which, at the instigation of a control unit (21) is closed for charging the capacitor or opened for discharging the capacitor (18), characterised in that the control unit (21) features an A/D converter (20) that is connected via a switch (17) or directly to the capacitor (18), with the time for respectively charging or discharging (31, 32) the capacitor (18) being determined by the control unit (21) as a function of the characteristics of the flame sensor (1) used and that, at the instigation of the control unit for flame monitoring (21), the respective charging or discharging of the capacitor is repeated cyclically, whereby a voltage signal (30, 40) which is subject to single-channel evaluation by means of a threshold value (34) is received at the capacitor (18), with a voltage divider (27) being provided for feeding back the ac mains phase to the control unit (21).
6. Method according to claim 5, characterised in that a sample-and-hold element (28) is provided for grid-synchronous sampling of the voltage signal (30, 40).
7. Method according to claim 6, characterised in that a pulse shaper stage (29) creates a control pulse for buffering the sample values in a capacitor (30).

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