EP1256763B1 - Method and device for long-term safe flame monitoring - Google Patents

Description

The invention relates to a method for flame monitoring on one or more burners, in particular blower burners, as well as a monitoring device for flame monitoring on such burners. Such methods or monitors are e.g. from EP 0 953 805 and US Pat. No. 4,701,624.

For burners operated with gaseous or liquid fuels, it must be monitored during operation of the burner for safety reasons whether the fuel actually used burns. These are different monitoring devices in use. For burners with a blue flame so-called ionization probes are often used. In addition, flame detectors are in use that detect the invisible or visible radiation of the flame.

The safety of the flame detection depends on whether the corresponding sensor element works correctly for the radiation to be detected. The sensor element generates an electrical signal indicative of the strength or power of the received radiation. In this case, long-term (creeping) changes in the properties of the sensor element are dangerous, especially in the case of burner operation. If the sensor, which may for example be formed by a semiconductor, is affected by temperature, combustion gases or other wear influences, the switching thresholds and detection thresholds for the radiation intensity, which should be considered as a characteristic for burning a flame or extinguishing it, may be shifted ,

On this basis, it is an object of the invention to provide a method and a monitoring device for flame monitoring, which avoids malfunction due to wear of the sensor element.

This object is achieved by the monitoring method according to claim 1 and the monitoring device according to claim 6.

According to the method of the invention, the electrical signal delivered by the radiation-sensitive detection device becomes parallel through two filter devices with different characteristics. Both filter output signals are checked to see if they are within an expected range. Only if this is the case for both filter output signals will the presence of a flame with a corresponding output signal be indicated. The filters may be analog filters or computational blocks of a microcomputer program.

The filtering of the electrical signal of the detection device with two different filters makes it possible to determine creeping changes in the electrical properties of the detection device. This is due to the fact that the radiation emitted by the flame is not constant over time. Rather, there is usually a certain flickering of the flame. This is especially true for forced air burners, especially oil-fired burners. The flickering of the flame produces a proportion of radiation that fluctuates sporadically. The fluctuations are in a frequency range between 10 and 60 hertz - depending on the burner. On the other hand, when the flame is constantly burning, even if it flickers slightly, there is a fixed average radiation level. The two-channel evaluation of the radiation signal according to the invention now makes it possible to separately detect and examine different frequency components of the radiation signal.

For example, a time-weighted mean value of the radiation signal can be evaluated in the first channel. This is obtained, for example, by low-pass filtering or numerical averaging. The signal thus filtered, when the detector is operating properly, maps the received radiation power. Will be an optical sensor for visible light as detection device used, the signal corresponds to the detected mean flame brightness.

In the other channel, for example, the alternating components of the radiation signal can be filtered out, which characterize the flickering of the flame. This can be done, for example, with a high pass or with a bandpass. The bandpass can also serve to drastically reduce the influence of stray light sources, which can also produce alternating light. This is especially true if the bandpass is sufficiently detuned against normal line frequencies (50 hertz), so that light fluctuations, such as occur in 50 or 100 hertz rhythm to fluorescent lamps, are irrelevant. The bandpass is preferably tuned to a frequency below the line frequency. Instead of the bandpass, a calculation block can also be used which determines the sum of the amounts of differences of a number of consecutive samples of the luminance signal. If the sum falls below a limit, the sporadic brightness fluctuation (flickering) is too low. Thus, either the sensor is defective or the flame extinguished; an error is displayed.

If the radiation-sensitive detection device (eg a photoresistor) is slowly destroyed by the action of heat, the influence of combustion gases or other wear influences, it gradually goes into the high-resistance or low-resistance state starting from a resistance value corresponding to the actual radiation effect (illuminance). In other words, the current resistance of the photoresistor or other sensing element moves from its desired value to another value, with the influence the illuminance gradually decreases with increasing destruction of the detection element (eg, a semiconductor). If the resistance value due to destruction of the detection device is also in the event of flame failure in a valid range for the flame message, no flame failure can be detected solely by evaluating the mean value of the radiation signal. However, as explained above, the destruction of the detection device was accompanied by a reduction in its sensitivity, so that the flicker content in the radiation signal continued to decrease even with the flame still burning. This is registered in the channel provided for detecting the stochastic alternating component. If the detected flicker falls below a minimum value, the output signal of this channel is no longer within the expected range. Accordingly, an output signal for indicating the presence of a flame is no longer generated, and thus a trouble message occurs.

With the presented concept, a corresponding evaluation device or monitoring device responds to the creeping destruction of the sensor element of the detection device even before a real error occurs by a corresponding failure message. In practical operation, this would cause the burner to stop, i. the system falls to the safe side. Dangers for humans and material are excluded thereby - the security is increased.

The presented concept is especially important for long-running oil burners. A check of the functionality of the detection device of the monitoring device is not only at the start of operation when igniting the flame, but by the different signal processing in both parallel channels, during operation of the burner constantly taking place.

The gradual destruction of a sensor component (detection device) can also lead to an electrical resistance being shifted to particularly low or particularly high values. In a preferred embodiment of the invention, the average of the radiation signal indicative of the resistance of the sensor device is examined to determine whether the current resistance of the sensor device is below a minimum value. This minimum value serving as limit value is preferably set to a value which corresponds to an excessively bright illumination (radiation intensity) which can not be applied by the flame. This measure brings both security against short circuits on the sensor element or in the supply lines, as well as against a gradual resistance shift. Another safety measure may be to evaluate the failure of a detector when the electrical signal before ignition of a flame is in an area where it is expected in the presence of a flame.

If a flame monitoring according to the presented method, destruction of the sensor element (photoresistor) occurs, which causes the resistance to drift to a value that is still in the valid range for a flame message, but the resistance is insufficient or not at all is affected by the flame image, ie the flickering of the flame, this is detected via the alternating signal detection (second channel). The detection device may be due to destruction of their semiconductor detect no or only insufficient flicker signal from the flame. By this method, it is thus possible to detect transition states of the sensor element (photoresistor), which arise from a slow and steady destruction of the sensor element, and render harmless.

The monitoring device according to the invention has two mutually parallel channels with different signal evaluation devices, one of which, for example, evaluates the signal average and the other the alternating component of the signal. The evaluation can be done via filtering devices hardware or software. Following this, thresholds or window discriminator circuits can be used to examine whether the signals are in the desired and expected range. The threshold value switches, window discriminators, possibly required rectifiers for signal rectification and the like can be realized both by hardware or by software.

The monitoring device according to the invention is particularly suitable for flame monitoring by detecting the visible light with a photoresistor as a sensor element. This makes it possible to realize simple, cost-effective and, in the event of continuous operation of the burner, safe monitoring devices.

Advantageous details of embodiments of the invention will be apparent from the drawings, the following description or the subclaims.

Fig. 1

einen Gebläsebrenner mit optischer Flammenüberwachung in schematischer Darstellung,

Fig. 2

die Helligkeit-Widerstands-Kennlinie eines Fotowiderstands, in intaktem Zustand und in verschiedenen Verschleißzuständen,

Fig. 3

einen beispielhaften Zeitverlauf für ein von dem Fotowiderstand erzeugtes Signal,

Fig. 4

ein Ausführungsbeispiel einer Überwachungseinrichtung für einen Gebläsebrenner, als Blockschaltbild, und

Fig. 5

eine alternative Ausführungsform einer Überwachungseinrichtung als schematisiertes Schaltbild.

In the drawings, embodiments of the invention are illustrated. Show it:

Fig. 1

a fan burner with optical flame monitoring in a schematic representation,

Fig. 2

the brightness-resistance characteristic of a photoresistor, in intact condition and in different states of wear,

Fig. 3

an exemplary time profile for a signal generated by the photoresistor,

Fig. 4

an embodiment of a monitoring device for a fan burner, as a block diagram, and

Fig. 5

an alternative embodiment of a monitoring device as a schematic diagram.

In Fig. 1, a monitoring device 1 for the flame 2 of a fan burner 3 is illustrated schematically. The blower burner 3 is connected to a blower 4 and a fuel supply line 5. The fuel is, for example, heating oil. The flame 2 has a turbulent flame image. Their brightness changes around an average. Time variations of the brightness L around the mean value M correspond to the flickering of the flame, as illustrated in FIG. The fluctuations are stochastic. They are often in the range of 10 - 60 hertz.

The monitoring device 1 monitors the visible light emitted by the flame 2 by means of a radiation-sensitive detection device. This is formed by a photoresistor 6, which is connected to a monitoring circuit 7. This is, for example, part of a higher-level control device and thus serves, as schematically indicated in Fig. 1, for controlling the fan burner 3 and in particular for direct and indirect shutdown of the fan 4, and the shut-off of the fuel supply by means of a corresponding controlled valve. 8

The photoresistor 6 is arranged to catch a part of the visible light emitted from the flame 2. At its output terminals, it thus generates a signal which reproduces the brightness curve illustrated in FIG. The internal resistance of the photoresistor 6 is dependent on the illuminance. This relationship is illustrated in FIG. As the illuminance L increases, the resistance R decreases more and more. In darkness or low illuminance, the resistance takes its rest resistance R ₀ . At very high illuminances, which are higher than any of the flame 2 can be generated, the resistance R approaches its minimum value R _M. FIG. 2 illustrates as curve I the dependence of the resistance R on the illuminance L for an intact photoresistor 6. Due to temperature influences, aging and the action of combustion gases, the characteristic of the photoresistor 6 may change over time. Any damage is usually accompanied by the fact that the steepness of the characteristic decreases in the range between the quiescent resistance R _o and the minimum value R _M. The dashed line in Fig. 2 illustrated curve II illustrates such a case. The resting resistance R ₀ has decreased; the minimum value has increased and the slope of the characteristic is reduced.

To an even greater extent, this is the case for a still further damaged photoresistor 6, as illustrated in FIG. 2 by a third curve III. This can also be shifted overall to higher resistance values or to lower resistance values. However, as the curves are always shifted - they have in common that the photosensitivity of the photoresistor 6 decreases.

The monitoring circuit 7 is illustrated separately in FIG. 4. The monitoring circuit 7 has a first channel 11 for evaluating the DC component of the signal generated by the photoresistor 6 and a second channel 12 for evaluating the AC signal component. Both channels 11, 12, receive the same input signal, which is output from an R / V converter 14, which outputs a voltage corresponding to the resistor R. The R / U converter is the input side connected to the photoresistor 6 and is in the simplest case by a voltage divider (one with the photoresistor 6 in series ohmic resistance) is formed.

The first channel 11 contains for signal evaluation (as Signalauswerteinrichtung) a low-pass filter 15, which serves to determine the average value of the R / U converter output signal. For this purpose, the low pass 15 forms the time-weighted average. Its corner frequency is, for example, at 20 hertz. By this dimensioning is achieved that an omission of the flame, i. a change in the average value of the signal detected very quickly and the burner 3 can thus be stopped very quickly.

At the output of the low-pass filter 15, an analog / digital converter 16 is connected (A / D converter), which digitalizes the filter output signal to a microcontroller 17 passes. Alternatively, the low pass 15 can be dispensed with. The A / D converter then passes to the microcontroller 17 samples of the current time signal. The microcontroller can computationally form the mean value of the time signal by adding up in each case a defined number of the last measured values. The sum thus obtained corresponds to the mean value.

The channel contains, as a signal evaluation device 12, a bandpass filter 18 (or alternatively a high-pass filter). The center frequency of the bandpass 18 is, for example, at 30 hertz, wherein the bandwidth can be sized relatively large. For example, the 3dB cutoff frequencies are 10 and 40 hertz. Network-correlated alternating light components of extraneous light sources can thus be excluded, the flickering of the signal (see FIG. 3) being recorded in broadband. The filter output signal can be sent directly to the microcontroller 17 be transferred. If it samples the input signal periodically (optionally via an A / D converter 19), stochastic values are obtained here as long as a flicker signal is present. Thus, the individual samples are within a specified range of variation. If this value is undercut, the alternating component is below a specified limit. The microcontroller 17 can check this by continuously forming differences between successive signal values and only recognizing a flickering signal when the individual differences exceed a minimum value. If smaller differences occur several times in succession, it can be assumed that the alternating component of the signal according to FIG. 3 is below a predetermined limit. Alternatively, the sum over the amounts of several successive differences can be formed and compared to the limit.

The microcontroller 17 is programmed to emit a valid flame signal (indicating a burning flame) only when the DC signal detected across the channel 11 is within a predetermined range and at the same time the flicker signal detected in the channel 12 exceeds a minimum value. The predetermined range for the DC signal corresponds to a resistance range B for the current resistance of the photoresistor 6 (FIG. 1). The flicker signal provided by the channel 12 must be above a limit value G. This is illustrated in FIG. 3.

The monitoring device 1 described so far operates as follows:

When operated properly, the photovidiant 6 detects the light emitted by the flame 2. The brightness varies according to the diagram of Fig. 3. Corresponding to the time course of the electrical signal at the output of the transducer 14. The channel 11 determines the short-term average of this signal by low-pass filtering. If the flame 2 has such a brightness that the resistance value of the photoresistor 6 fluctuates around the value P illustrated in FIG. 2, which lies in the region B, this is recognized by the microcontroller 17. At the same time, the bandpass 18 in the channel 12 becomes the flicker component of the channel filtered out. The microcontroller 17 checks whether the flicker proportion is greater than predetermined by the limit G (FIG. 3). If so, the microcontroller registers this. If both conditions (channel 11, average in area B, channel 12 flicker rate greater than limit G) are met, the microcontroller outputs a flame signal indicating the presence of a flame or internally generates a corresponding signal for further processing.

If the photoresistor 6 or the electrical properties change over time such that, for example, it obtains a characteristic according to curve II or III, its resistance value does not drift out of the range. However, the height of the bandpass 18 of the extracted flicker signal decreases with decreasing slope (curve II or curve III). Therefore, the flicker signal falls immediately below its limit G, whereby the microcontroller 17 no longer recognizes both signals as valid. Thus, it reports flame failure and thus enables shutdown of the burner. 3

In addition, it can be provided that the microcontroller, by determining that the signal of the channel 11 is in the validity range B, but the flickering signal of the channel 12 has failed, generates a signal which indicates the failure of the photoresistor 6.

5, a modified embodiment of the monitoring device 1 is illustrated. The monitoring circuit 7 assumes here functions that have been taken over by the microcontroller 17 in the monitoring circuit 1 of FIG. 4.

As far as functional equality, reference is made in the following description to the same reference numerals, as in connection with the above description, and generally made to the above description.

The photoresistor 6 is connected with a connection to an operating voltage U _b and with its other connection to the R / V converter 14, which generates a voltage output signal which corresponds to the current flowing through the photoresistor 6 current. As the resistance of the photoresistor 6 decreases, the output voltage of the transducer 14 increases. The output voltage is transmitted to the low-pass filter 15, which determines the time average of the converter output signal. The output signal of the low-pass filter 15 is passed to a window discriminator 21 which checks whether the low-pass output signal lies within a predetermined switching range which corresponds to the region B according to FIG. 2. The limits of the switching range are monitored by two trigger circuits 22, 23 which have setting inputs 24, 25 for determining the trigger thresholds. The trigger outputs are connected to an exclusive-OR gate 26, which only provides a valid output signal at its output when only one of the two trigger circuits 22, 23 detects border crossings.

The control inputs 24, 25 serve to adjust the switching thresholds of the trigger circuits 22, 23 as needed and to adapt them to the respective operating mode of the burner 3. For example, in particular the lower switching threshold responsible for the low incidence of light during ignition operation can be set differently (lower) than after ignition during burner operation (this is called a negative switching differential).

The channel 12 may include a signal rectifier 27 following the bandpass 18, which converts the flicker signal into a DC signal. A connected trigger circuit 28 serves to check whether the alternating signal (flickering signal) exceeds a predetermined limit G. The two channels 11, 12 are the output side via a logic circuit 29, which is formed, for example, as an AND circuit, linked together to produce a flickering signal.

A continuous-duty monitoring device 1, which is provided in particular for flame monitoring on oil-operated blower burners, has a photoresistor 6, which is connected to a monitoring circuit 7. This evaluates the output from the photoresistor 6 signal from two channels. A first channel 11 is used to detect the average brightness. A second channel 12 is used to detect alternating parts resulting from the flickering of the flame. The flame will only be considered proper burning recognized, if at both channel outputs of the channels 11, 12, a signal is present, or the signal is in each case in a predetermined range. In particular, creeping changes in the characteristics of the photoresistor, as they occur in the case of continuous combustion operation and are dangerous, can be detected in this way. It is ensured that the flame monitoring is not performed or attempted with a defective photoresistor.

Claims (15)

1. Process for flame monitoring on one or more burners, in particular blower burners,
   by means of a radiation-sensitive detection unit (6), which emits an electric signal identifying the radiation output,
   wherein during the process the electric signal is directed parallel through at least two filter units (15, 18) with different characteristics, and
   the two filter output signals are checked to determine whether they lie in an expected range (B, G),
   wherein an output signal identifying the presence of a flame is only generated when both filter output signals lie in their respective expected ranges (B, G).
2. Process according to Claim 1, characterised in that signal fluctuations of the electric signal are detected by means of one filter unit and a signal mean value is detected with the other filter unit.
3. Process according to Claim 1, characterised in that the signal fluctuations of the electric signal are applied as identification for the operability of the radiation-sensitive detection unit (6).
4. Process according Claim 1, characterised in that it is evaluated as identification for an outage of the detection unit (6) when the signal value thereof lies below a predetermined limit (R_k).
5. Process according to Claim 1, characterised in that it is evaluated as identification for the outage of the detection unit (6) when prior to ignition of a flame the electric signal lies in a range, in which it is expected to be in the presence of a flame.
6. Monitoring unit (1) for flame monitoring at one or more burners (3), in particular blower burners,
   by means of a radiation-sensitive detection unit (6), which emits an electric signal identifying the recorded radiation output,
   with a first channel (11), which is connected to the detection unit (6) and includes a signal evaluation unit (15), to which the electric signal of the detection means (6) is directed, and which emits a flame signal at its output when the electric signal corresponds to a first criterion,
   with a second channel (12), which is connected to the detection unit (6) and includes a signal evaluation unit (15), to which the electric signal of the detection means (6) is directed, and which emits a flame signal at its output when the electric signal corresponds to a second criterion,
   wherein the signal evaluation units (15, 18) of the two channels (11, 12) are arranged for monitoring different criteria, and
   with a logic evaluation unit (17, 21, 29), which has two inputs, which are connected to the channels (11, 12), and which only emits a flame recognition signal when both channels (11, 12) respectively emit a flame signal.
7. Monitoring unit according to Claim 6, characterised in that the signal evaluation units (15, 18) are filter units.
8. Monitoring unit according to Claim 6, characterised in that one channel (11) as signal evaluation unit (15) is a means (15) for forming a time mean value and the other channel (12) as signal processing unit (18) is a means for detecting signal changes.
9. Monitoring unit according to Claim 6, characterised in that one channel (11) as signal evaluation unit (15) includes a low-pass filter (15) for detecting the radiation output.
10. Monitoring unit according to Claim 9, characterised in that the channel (11) has a window discriminator (21) adjoining the low-pass filter (15).
11. Monitoring unit according to Claim 10, characterised in that the window discriminator (21) has at least one control input (25) for defining its switching limits.
12. Monitoring unit according to Claim 6, characterised in that one of the signal evaluation units (15, 18) is arranged for selection of flashing signals.
13. Monitoring unit according to Claim 12, characterised in that the signal evaluation unit (18) is a filter unit with high-pass capability.
14. Monitoring unit according to Claim 12, characterised in that the signal evaluation unit (18) is a filter unit (18) with low-pass capability.
15. Monitoring unit according to Claim 6, characterised in that the detection unit (6) is a photo-resistance.

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